Oxygen Transport Properties of Highly-Doped Perovskite-Type Ferrite Oxides Under High and

Low Oxygen Chemical Potentials

by

Nader Bayani

A thesis submitted in conformity with the requiremements for the degree of Masters of Applied Science

Graduate Department of Chemical Engineering and Applied Science University of Toronto

O Copyright Nader Bayani 2001

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Oxygen Transport Properîies of Hi@@-Doped Perovskite-Type Ferrite Oxides Under High and Low Oxygen Chernical Potentials Nader Bayani, Department of Chernical Engineering, University of Toronto Masters of Applied Science (M.A.Sc.)

Abstract

Diffiision coefficients and surface activation coefficients for samples of La&rogFe03d

and L@,zSro.8Cro,2Fen.803-d were detemined with the Isotope Exchange Depth Profihg /

Secondary [on Mass Spectrometry (EDWSiMS) method for high oxygen partial pressure

atmospheres (20% O2 in Ar) low oqgen partial pressure atmospheres (CO:C02 = 1 . 5%

each in Ar. Ne). A rnethod for distinguishing surface reaction pathways from gas

composition and depth profile data in CO/C@ abnospheres is presented. Results show no

significant gain in the transport piopenies at low partial pressures. indicating offsetting

effects of increased oxygen vacancies versus vacancy ordering. Addition of chromium to

the B-site resulted in no significant change in the transport properties. Under reducing

atmospheres the sampies showed hi& levels of Sr sepgation to the surface. possibly

lowering the surface activation rate despite the increase in surface area Sodium was also

observed on the sinface but its source was unknown.

Acknowlcdgements

This thesis wouid have not been possibIe without the assistance and guidance offered by

Dr. Charles Mims. With his timely suggestions and dedication he has made this research

possible.

1 am aiso indebted to ail those people at the University of Houston that have offered great

help and assistance in the preparation of the samples and the subsequent analysis of them.

Dr. Allan Jacobson and his tearn have dedicated considerable time to answering many of

the questions that posed to them dun'ng the process of research. Paul van der Heide (who

never got my name spelled right!) and Corina Lupu donated their time and effort towards

obtaining the profiles that this research depends on. and without their help this thesis

would have not been possible. Yuemei Yang has aiso been instrumental providing me

with much needed information. Dr- Harlan Anderson at the University of Missouri and

his team are also to be thanked for providing some of the samples.

1 must thank Dr. Rana Sodhi for his eXPeditiouS handihg of al1 my Il' hour requests

and for dedicahng his time and effort to this cause.

1 am aiso very gatetùl to ai1 those people here who have supported me in the lab and

during the process of research. namely Chris Bertole. Greg Vovk. Xiaohua Chen. David

Schmyer! Chiming Lau. Nathan Joos. Helena Malmberg, Ram Paul and Linjie Hu.

Table of Contents

1 . introduction

1 . 1 . Motivation

t 2. Theory

1 2.1 . Perovskite Structure

1.2.2. Bulk Transport (Oxygen Dimision)

1 .U. Surface Exchange (Oxygen Activation)

1.2.4. The Challenge

2. Literature Review

2.1. Mixed Ionic Electronic Conductors

2.2. The Ferrites

2.2.1. High Oxygen Partial Pressure

2.2.2. Low Oxygen Partial Pressure

2.2.3. The Structure of Femtes

2 . . Isotope Exchange Depth Profilmg 1

Secon@ Ion Mass Spectrometry jlEDP/SIMS)

3 Eqxrimentai Procedure

3 1 - Sample Preparation

3.1.1. Sintering of Powders

3.12. Cutting and Polishing

Page

1

1

3

4

7

10

11

3.2. Experimental Procedure and Apparatus

3.3. SlMS analysis

3.4. Method For Distinguishing Between

Surface Exchange Pathways

4. Results and Discussion

4.1. High Oxygen Partial Pressure (20% O?)

4.1.1. LaSrFeO

4.1 2. LaScCrFe0

4.2, Low Oxygen Partial Pressure

4.2.1. LaSrFeO

4.2.2. LaSrCrFeO

42-21. LaSrCrFeO at 750 O C

4.2.2.2. LaScCrFe0 at 850 OC

4.2.3. Changes in Surface Composition

5. Conclusions

7. Recommendations

1. Introduction

1.1, Motivation

Oxygen ranks among the most used elements in modem chemicai processes [II . Despite

its abundance in the earth's atmosphere, oxygen has proven IO be expensive and

troublesome to produce in pure form. oflen requiring energy intensive steps. Therefore.

many processes have relied on atmospheric air as their main source of oxygen:

consequently they need to deai with other gases that accompany such a mixture. These

impurities c m ofien resuit in decreased eficiency and unwanted by-products that can

sometimes be poisonous (e.g. NOx production in high temperature combustion). It is

therefore of interest to procure new alternatives for the separation of oxygen from air.

In cecent years considerable effort has k e n invested in the study and design of solid

oxide matenais that have the ability to conduct oxygen ions within their Iattice. When

these materials are used as membranes that separate two atmospheres of differing oxygen

potentiai. they wiil transport the oxygen across the potentiai gradient. To be usefid for

these applications. the membrane materials must posses a stable crystalline structure in

addition to k ing capable of transporthg oxygen. Two groups of metal oxide structures

that have been the subject of m t i n y are: the fluorite structure and the perovskite

structure. The detaiis of their lattice structure and the nature of oxygen conductivity d l

be discussed Iater. Of the perovskite structure materiais the ones that are able to conduct

etectrons in addition to oxygen ions have garnered great interest for theù ability to serve

as membrane material as well as electrodes for fuel cells. These materials b e been

iabeled Mïxed Ionic Electronic Conductors (MIEC). A typical configuration of a MEC

membrane is shown in Figure 1.1.

Figure 1.1 - Typical arrangement of M E C membrane.

When deding with an MIEC membrane three regions are of prime importance for oxygen

transport across the membrane. These three regions are labeled A, B and C in Figure 1.1.

Regions A and C correspond to the surface of the material. Oxygen is incorporated at

surface A. on which adsorption/dissociation reactions occur. resulting in the

incorporation of the oxygen ion into the Iattice (keeping in mind that the material is an

oxide). Region B is the buik of the material and through this region the difflision of the

ovgen ion occurs. At surface C the recombination/desorption reactions occur. which

could either be the pattiai oxidation of a fuel. or the evolution of moiecular oxygen. The

key parameters that dictate the oxygen transport capabilities of these materials are their

d a c e activity levels and the diffusivity of oxygen through the buik.

Potentiai uses of these materials now exceed the simple separation of oxygen that has

already been discussed. MEC membranes can be used in the partiai oxidation of Light

hydrocarbons, e.g. syngas production [2 J,4], They can be used as electrode materiai in

Solid Oxide Fuel Cells (SOFC). since they must be good electron conductors as well a s

oxygen ion conductors [SI. Whiie there are many factors that determine the suitability of

a certain composition for its intended application, the one factor that is most cruciai, is

obviously the materid's ability to conduct oxygen ions, namely, the two parameters

relating to the surface activity and the bulk difiùsivity and their dependence on oxygen

chernical potentiai, Despite the great effort that has been invested in studying these

materiais. a deep understanding of the nature of ion conduction still remains an elusive

eoai. It is to this effort and to the purpose of chacacterizing new materiais for potential "

use as MIEC membranes that this research is dedicated.

12. Theoy

Perovskite-type compounds posses a flexibility in their structure that allows for the

formulation of a great variety of stabIe compositions and for this reason they have k e n

the subject of numerous studies. Potentiaily high oxygen ion conductivities in perovskite-

type structures were k t reported in rare earth ahminates in the mid 1960's [6.71. in the

years since extensive research has d t e d in the emergence of theoreticai models for this

phenomenon. But in order to understand these models, a working knowledge of the

crystai structure is needed. Our focus wiil be the perovskite structure and the nature of

oxide ion mobility in its lattice.

1 2.1 The Perovskite Strucrure

Figure 1.2 - Two views of the perowkite structure AB03.

The perovskite stmcture gets its name h m the mineral Perovskite (Caicium Titaniurn

Oxide CaTi03). This minerai was first discovered by Gustav Rose in 1839 fiom samples

he had gathered h m the Ural mountains. he later m e d it "Perovskite" after the farnous

Russian mineraiogist Count Lev Aleksevich von Perovski ( h m St. Petersburg) [8]. The

ideal structure of a perovskite can be seen in Fig. 1.2; here the generai f o d a AB03 is

used, From view 1 of Figure 12 it cm be seen that the B cation is associated with 6

oxygen atoms, while view il shows that the A cation is associateci witb 12 oxygen atoms.

The A site cation may con& of an alkaIi, an aIlcaline earth or a rare earth ion [SI. Since

the A cation is generaily larger than the B cation, in many cases it will cause a distortion

of the BO6 octahedra. This size difference introduces a limiting factor in the ionic radius

distribution of the cations (if the perovskite structure is to be preserved). Too large of a

distortion will make other structures. such as orthorhombic or rhombohedral a more

stable alternative. in generai the toterance Limit of the ionic radii of the A and B site

cations is given by the Goldschmidt factor [Il:

where r ~ , r~ and ro are the radii of the respective ions. The perovskite structure is

considered to be stable in the ranges 1.0 < t < 0.75. white the ideal perovskite structure

exists only for values very close to unity. This substantial tolerance gives the perovskite a

stability that allows for modifications in thek composition. These modifications cm

result h m either aliovalent substitution of the A or B cations andlor from redox

reactions resuiting h m the different oxidation states of transition metals. if these are

used. The existence of stable nonstoichiometnc perovskite stnicnrres has interesting

consequences with respect to the ionic conductivity of these materiais.

in ment years considerable focus has been placed on perovskites h m the transition

metal series; Cr. Mn. Fe. and Co to be specific [10,1 I. 121. The A site of these materials is

~enerally h m the Lanthanide senes, with A-site substitutions. The materials studied in - this research are ferrites (ie. Fe predomhates the B site) and can be generally represented

as LaxSrt-x(Cr.Co)FeOM , to explain the nature and cunsequences of nonstoichiometry by

substitution, La,Sri,,FeOM will be used as an example.

When then A-site cation is parîidly substituted for a diffêrently charged one defects in

the crystai Iamce are formed, to illustrate this process let us consider LaFeO; as the

perovskite targeted for doping with SE

Here the KrBger-Vink [13] notation is used. The introduction of sr' in the A-site

normaliy occupied by the ~ d ' causes a charge imbalance of -1. which results in the ~ e ' -

changing its oxidation m e to 4+ (denoted by Fe:, ) to regain charge neurrality. This

charge compensation is in accordance with the Verwey prhciple of controtleci ionic

valency [14]. The formation of ~ e " can result in two teactions:

Here 5; denotes a doubly ionized oxygen vacancy. essentidly an oxygen hole formed to

achieve charge neutrality. ïhese oqgen vacancies are fie to move among energeticaIIy

quivalent crystallographic sites as Iong a s the structure is cubic qmmetric [SI. It is the

existeuce of these vacancies within a siabte structure and th& abiIity to move (difhse)

tint d10m for the transport of oqgen ions in the perovskite lattice.

1 2.2. Bulk Transport of Oxygen (Oxygen Difi ion)

To address ihe flux of oxygen ions in the bulk of the pemvskite structure. we fÙst need to

d e h e a reaction by which the oxygen in the atmosphere can be incorporated into the

lattice (a more detailed analysis of the surface reaction will be presented later):

While this reaction is overly simple in addressing the complexity (and ambipity) of the

m e surface reaction(s) it suffices as a starting point for the treatment of buk dihivity.

The consequence of this incorporation into the lattice is that a chemical potential gradient

is generated in the membrane material (if one assumes that one side is exposed to a lower

oxygen potential) and because of the mobility of the charge c a r ~ n g species a net flux of

oxygen will occur towards the lower potential side. Consideration of the conductivity of

the charge carrying species (oxygen vacancies. free electrons and electron holes) and the

chemical potential gradient of the oxygen results in the term for oxygen flux (Wagner

equation) [13.16]:

wbere the conductivities denoted by a are those of the ionic and electronic charge carrier.

L is the membrane thickness, F is the Faraday constant and the Iirnits of integration are

the oxygen partial pressures on either side of the membrane.

If we assume that the oxygen vacancies are fitiiy ionized and al1 contribute to the ionic

conductivity (an assumption that is valid for the most part at low vacancy concentrations)

the ionic conductivity can be expressed with the Nernst-Einstein equation:

where D, is the vacancy diffusion coefficient, and V, the perovskite molar volume. [n the

materials considered here (and in perovskites in general) the electronic transport is very

fast (ie. a,! > ai,) [5] so that the conductivity terms in the Wagner equation wilI involve

ionic conmbutions primarily and can be simplified by using Eqn, 1.7:

At low vacancy concentrations the term D, GUI be assumed to be constant. however as

the vacancy concentration increases there is a slight decrease in the difhivity coefficient

arising h m the fact that the vacancy diffusion tenn relates to tcie probability of vacancy

hopping [ 171-

The expression for the oxygen flux poses the problem of detennining the vacancy

diffusion coefficient. This can be achieved by experïmental measurernents of chemical

diffusion coefficient and tracer diffusion coefficient. The chemical diffiision coefficient

relates to the diffusion occurring under oxygen chernicd potential gradients. whiIe the

tracer d i f i i on coefficient relates to the selfsiaision of oxygen under no oxygen

chemicd potential gradients.

If the interna1 defect ceactions are assumeci to be fast. it can be said that the transport of

oxygen is driven by the gradient in the 'heutrain oxygen species (Cc&) [5 ] - a

hypotheticai species that simplifies the mathematicai modeling - which alIows for the

expression of the chemical diffusion in terms of Fick's first law.

where ?cd& = - &v/& and jot = L/t jo.

For perovskites the chernical diffusion coefficient can be expressed by (as derived h m

the Wagner equation) [5 ] ,

where 6 = [Vol.

If s e l f d i o n of labeled oxygen atoms ('%O) under no chemical potential gradients (no

net flux) is studied by examining the depth profile of the oxygen isotope ratio. the tracer

diffusion coefficient @*) can be obtained. This coefficient is related to the vacancy

diffusion coefficient by [SI,

where f i s the correlation factor for d i o n of oxygen vacancies in the ideal perovskite

anion sublattice and is given as: H.69 (for small oxygen vacancy concentrations) [18].

1.2.3 Surface Oxygen Exchange (Oxygen Activation)

As was mentioned earlier the u.aasport of oxygen in the buik depends first on the

incorporation of said oxygen into the perovskite. This pmcess occurs at the surface ot' the

perovskite material and involves a series of reaction steps. with each one capable of being

the rate-determining step [19]. S o m of the possible steps include: adsorption.

dissociation, charge transfer. surface diffusion of intermediate species and incorporation

into the bulk. Furthemore. additional seps may be involved when one considers ail the

possible gas compositions (CO. C a , CI%, ... etc.) that the surface can be exposed to.

Other complicating factors become apparent when the electrostatic nature of the surface

is considered. The ionosorbed adsorbate layer on the surface mates an elecûicai double

layer througb which the electrons must travel in order for the charge transfer reactions to

occur. making that reaction a possible rate determining step [20]. Another charged layer

c m exist in the ionic crystal lattice near the surface region. This space charge region will

cause a redistribution of the lattice components making the surface and near-surface

region adopt a composition that might ciiffer signincautiy h m the bulk [2132.23].

The consequeme of this king that there might be considerable segregation of one

elernent to the surface. and even the formation of a second phase.

The surface presents a challenging environment and determinhg its chernical properties

cati be a daunting task, for this reason there is littie solid know[edge of the processes that

occur at the surface of pemvskite materiah.

By observing a trend in the expmimentally obtained surface activation and bulk diffusion

coefficients of various perovskite materials, Kilner et al. [34] were abk to postdate that

the surface oxygen vacancy is the site at which oxygen exchange occurs. Cn theù review

of published values for both D* and k*, the authors noticed a linear reiationship between

the bufk diffusion and the d a c e exchange. together with previous observation on the

dependence of the surface reactions on the vawicy concentration [El. they were able to

denve a term reIating the surface exchange coefficient to the bulk vacancy concentration.

where a, is the ce11 parameter for the perovskite, m is the number of oxygen sites per ceII.

and v is the thermally activated exchange frequency.

I2.4. The Chatknge

It can be seen that the oxygen vacancy concentration pIays a very important d e in both

the surface reactions and the buik diffusion process. This yields to the conclusion that

materiah that exhibit the largest concentrations of oxygen vacancies wiU perform the

best. However, as wiii be discussed later, increased oxygen vacancy concentration wi

have a negative impact on the very reactions that depend on it, When considering gas

compositions other than molecular oxygen, more pathways for oxygen exchange with the

d a c e become possi'ble, which can interfere with efforts to masure the reaction

parameters. Because of these factors, among many others, the study of cornplex materials

such as the perovskite-type oxides is hught with challenges that often result in

unexpected outcornes.

2. Literature Review

2.1. Mirred Ionic Electronic Conductor Perovskites

Since the early work of Takahashi [6.71 numerous other researchers have studied high

oxygen ion conductivity in solid oxide matenals. but the MIEC perovskites only started

to be the subject of research in the early 1980's. Teraoka et ai. [26?738?9] conducted

pioneering work in the study of new compositions for very high oxygen conductivity.

From measurements of oxygen flux in the series Lai-,Sr,C0i-~Fe~03d [26]. they noticed

that oxygen flux increased by increasing the Co and Sr content of the perovskite. and the

highest flux reported was that for the composition SrCo~.gFeo,zO~4 at 800 - 900 OC. tn a

subsequent study Teraoka et al, [27j expanded their composition range and studied

various B-site cations as well as A-site cations for their effect on oxygen flux. In the end

they had subjected materials in the composition range of L ~ o . ~ & . J C O O . S F ~ O , ~ O ~ ~ (A = L a

Na, Ca, Sr. and Ba) and L ~ & ~ ~ Q , ~ C O ~ , ~ B ~ . ~ O ~ ~ (B = Cr, M n Fe. Co, Ni. and Cu) to

expimental scmtiny. Their observations indicated that oxygen permeation increased in

the following order:

La < Na < Sr -= Ca < Ba for the A-site

Mn < Cr < Fe < Co <Ni < Cu for the B-site.

AIthough cobaltite compositions exhibit hi& iooic conductivity, they suffer h m the

drawback of decreased stabiüty at Iow oxygen partial pressures. Studies by Tejuca et al.

[30] and Yokokawa et ai. [31] found that compositions with high cobalt concentrations

decomposed at bigber partial pressures than some other compositions. Yokokawa et al.

were able to categorize the stability of a series of perovskite compositions based on their

stability at tow oxygen partial pressures. They found that the stabitity of undoped

perovskites decreased in the order,

LaCrOM (1 O-" atm) > LaFe0~ ( 1 O-" atm) > LaMnON ( 1 0-15 atm) > Lac0034 ( 1 O-' atm)

with the pressure values correspondmg to the oxygen partiai pressure (Pm) that is the

b i t of the perovskite's stability. As can be seen here the cobaltite perovskite has a poor

tolerance for low oxygen partial pressures and tends to decompose at the conditions that

are typicai for the use of MEC membranes. The most stable composition is the

chromium containing one and is able to withstand oxygen partial pressures that are 13

orders of magnitude lower than the cobaitite oxygen partial pressure limit. This would

make the chromium perovskites ideal for typicai applied environments if they are good

ionic conductors.

in studies of Lai-,A,Cr03d (A = Sr. Ba Ca) and cither simihr compounds Hammou [31]

and Minh [33] reporteci very low oxygen ion conductivity in the chromiurn containing

perovskite materiah and suggested that they be used as interconnect materiais for SOFC,

since they exhiiited similx thermai expansion (and other mechanicd charactenstics) but

were not involved in the oxidation prucess. Suzuki et ai. [34] studied thin fih of

L i ~ ~ ~ S r ~ . ~ ~ C r ~ . ~ ~ 0 ~ and also reported Iow ionic conductivities, however, they noticed that

ionic conductivity exhibited a dual behavior under varying oxygen partial pressure. They

noticed that the conductivity increased with increasing oxygen partial pressure at above

IO-' atm 02 and increased with decreasing oqgen partial pressure at below 104 atm Oz.

They attributed the former to grain bomdary control of oxygen d i i i o n and the latter to

increased oxygen vacancy concentration at lower oxygen partial pressures. The low

oxygen ion conductivity of the chromium containmg perovskites discounts their

usefulness for use as membrane materials and teads us to the iron containing perovskites

as the next suitable contenders.

23. The Ferrites

Teraoka et al. [35] and Zhang et al. [36] have reported that the substitution of Co by Fe in

Lai.,Sr,Coi,Fev03d tends to stabilize the perovskite structure; an observation that is in

accordance with the trend observed by Yokokawa et ai. This leads to the question of how

well will a perovskite of 100% Fe in the B-site behave, since this composition shouid

yield the most stable perovskite structure (excluding the Cr family.)

in a comparative study Ishigaki et al. [37.38] researched the diffusion of oxygen in single

crystals of Lac003 and LaFe03 to determine how the two would differ in their ionic

conductivity. Previous studies [39,40] had determineci that the oxygen vacancy

concentration of the ferrite species was three orders of magnitude lower than that of the

cobaltite at the sarne temperature and oxygen partial pressure. To see how this wouid

affect the ability of the ferrite to conduct oxygen they measured the tracer diffusion

coefficient in the two samples. Results teveaied that the tracer diffusion coefficient was

about 2.5 orders of magnitude Iower in the ferrite (at a coincident temperature and partial

pressure), however. the vacancy diffusion coefficient revealed to be the same for the

f i t e as for the cobaitite. What this meant is that the oxygen vacancy concentration was

determining the diffusion tate of oxygen, whiïe the vacancy diffusion coefficient (b)

remained the same. One can now expect to improve the oxygen d i h i o n by increasing

the vacancy concentration; which in turn can be accomplished by doping the A-site with

Sr (as previously shown).

2.2.1. High Oxygen Partiai Pressure

Minisaki et ai [41,42] undertook the task of determinhg how the oxygen vacancy

concentration changed in La1-,Sr,Fe0~~ with differing levels of A-site substitution.

oxygen partial pressure and temperature. The results suggested that oxygen vacancy

concentration (or nonstoichiometry) increased with A-site substitution and temperature

and decreased with oxygen partial pressure. Ishigaki et d [43] and Kim et al. [44]

followed with a study of oxygen diffusion in the same materiai with varying Sr doping (x

= O - 1). Their studies were carried at high temperatures (800 - 1050 "C) and at high

oxygen partial pressures. Both researchers reported an increase in the apparent difision

coefficient (D* in Ishigaki's case) with increasing Sr content. However. Kim et aI.

reported a decrease in the vacancy diffusion activation enthdpy. while ishigaki et al. did

not see any significant change in the activation enthalpy with increasing Sr content,

Despite analyzhg simiIar materials the two researcher's actuai values differed

considerably; ishigaki's results indicated higher diffusion rates than ~ i r n ' s . ~

As was mentioned before, the oxygen vacancy concentration increases at lower oxygen

partial pressures, a phenornenon that is particdarly helpfd for the femte pecovskites

since tbey are able to withstand lower oxygen partial pressures (and the m o n for which

t This discrepancy serves to illustrate the difficulties in obtaiaiig consistem experimental conditions when ddng widi high temperama and sample p-on techniques.

they are king considemi over the cobaltites). This tolerance should theoreticaily heIp the

ferrites impmve their oxygen diffusion rate as well as to make them more suitable for the

typicai reai-life conditions they are intended for.

2.2.2. Low Oxygen Partial Pressure

In a series of experiments geared towards detennining the performance of Lai.,Sr,Fe03d,

under hi& and low oxygen partial pressures. ten Elshof et al. 145.461 investigated the

flux of oxygen through membranes constructed of these materiais (x = 0.1 - 0.4). in their

studies the authors noticed that the activation enthaipy decreased in atmospheres of

CO/C02 gas mixtures (Iow Po?). They M e r noticed that the tatedetermining step

changed fram buik diffusion to the oxidation of CO at the gadsolid interface. When they

reused the sample exposed to low oxygen partial pressures. they noticed that the samptes

had k o m e better oxygen conductors even when placed in high oxygen partial pressures.

They theorized that a reaction with the solid had occurred that had changed the surface of

the oxide. thus increasing the surface area. XPS studies on used samples revealed that Sr

had segregated to the surface and had "roughened" the surface to increase the surface

area. this in turn facilitated the oxygen flux. Van Hassel et al. [4q noticed a similar effect

when they exposed Lai-,Sr,FeOj4, which had the surface modified by addig Pt. to low

oxygen partial pressures. Their results also indicated that the surface CO oxidation was

the rate-âetermining step. These results indicate that the stmcture of the perovskite

materials piays a crucial role in the overall process of oxygen conduction when these

materials are exposed to low oxygen partial pressures.

2.2.3. The Stnichue of Femtes

The fïrst structurai studies of the perovskites in the series Lai,Sr,FeOM were carried out

by Waugh [48] at the Massachusetts Institute of Technology. Observations in that study

reveded complex phase changes in the structure as the Sr level was changed (at low Sr

doping levels). As the nonstoichiometry (oxygen vacancy) was reduced the sample

changed fiom orthorhombic to cubic. Takano et ai. [49,50] later determined the lattice

constants for the compositions with x = 0.4,0.5,0.7.0.8, and 0.9. The authors proposed

that the ~e'" ion undergoes a disproportionation that produces a non-inte@ oxidation

state.

A previous study [SI] had also hinted at the existence of an intermediate ~ e j - l ~ e ~ '

oxidation state at low Sr doping levels. while higher doping (larger x) had shown ~ e ~ '

oxidation states. Battie et ai. [52] investigated the structure and oxidation states of

LaSr2Fe;Os (ie. x = 0.66) and proposeci the disproportionation of ~ e " into the mixed

valence ~e~'/Fe* state.

Dam et al. [53] later expanded that research to include a variety of compositions in the

range of O 5 x 5 1. Their resuits at very low temperatures (T = 4.2 K , d = O) revealed the

existence of three structura1 region with dation to the Sr IeveI,

0 5 ~ 5 0 . 2 -. ûrthorhombic, Fe3+ and Fe5+. magnetic order.

0.4 5 x 10.7 -. Rhombohedral, ~ e ~ ' and Fep. paramagnetic order.

0.8 5 x 5 1.0 -. Cubic. Fe3' and ~e"*. paramagnetic order.

It was aiso determined that the ~ e ' " content increased with Sr doping. At doping levels of

x = 0.8 - 0.9 they observeci a complex system of structures that dependeci on the

annealing attnosphere in which the sarnpIes where prepared. Upon reduction with

hydrogen the x = 0.8 samples pduced a compIex mixnire of SrtLaFe30s and SriFo_Oc.

Li et al. [54] reported evidence for charge ordering in the structure of the Sr doped

femtes (at low temperatures). They determined two regimes under which the iron ions

ordered themselves.

x 5 0.4 -. ~ e ~ - and ~ e ~ ' ordering.

0.5 5 x 50.7 + ~ e ~ ' and ~ e - ordering dong C 1 1 1 > mis.

with the latter ordering in the following partem.

These orderings in the Uon atoms and oxygen vacancies wili have consequences for the

transport of oxygen. the detaiIs of which will be elaborated later.

23. Isotope Exchange Depth Profilhg / Seconda y Ion Mass Spectromety (IEDPJSIMS)

It bas already been deterrnined that the c i f i i on of oxygen in the M E C pemvskites

depends rnainiy on two factors: the oxygen vacancy concentration and the oxygen

vacancy diffusion coefficient D,. To determine the latter various methods can be

employed, as was mentioned in section 12.2. Essentiaily these experiments can be

divided into two categories: flux measurement under an oxygen chemicai potential

@ent and self-diffusion of oxygen (no oxygen chernical potential gradient). Of these

two types. the one that gives a more direct understanding of the vacancy diffusion

coefficient is the self-diffusion coefficient (Eqn. 1.1 1). This requires the measurement of

oqgen movement in the material when there is no net flux. To distlliguish benveen the

oxygen atoms without disturbing the chemicai potentiai. it is possible to employ the

stable isotopes of oxygen ("0 and "0). These isotopes are chemically equivalent to the

"0 oxygen atom and they will not disturb the oxygen chemicai potential in the system

whiie d l being able to manifest themselves in analysis. The diffusion coefficient that

corresponds to the transport of these oxygen isotopes will then be caikd the tracer

diffusion coefficient (D*) and can be assumed to be the same as the s e l f a i o n

coefficient (51.

One method that is used to measure the tracer diffusion coefficient is calleci Isotope

Exchange Depth Profihg (IEDP). In this method the sample. having been equiIibrated

with a given oxygen atmosphere, is suddedy switched to another atmosphere containing

the same oxygen partiai pressure. however. with a different isotope of oxygeu (typicdly

"0). The labeled oxygen is then alIowed to exchange with the unlabeled oxygen in the

sample (no net flux) for a known time, following which it is quenched rapidiy. The

resulting sample will then have a profile of differently labeled oxygen atoms w i î b its

lattice. This profile, when measured, will yield values for the diffusion and surface

activation coefficient of the material (Appendix A). While this technique has a relatively

long history of usage [SI, only within the last two decades has it been possible to obtain

high quality profile data; mainly due to the improvements in an anaiysis rnetliod called

Secondary Ion Mass Spectroscopy (SIMS) [SI.

When a solid surface is bombarded by a beam of energetic particles (ions. atoms or

molecules) it will inevitably eject materiai fiom near the point of impact. this

phenomenon is cailed sputtering [56]. SIMS uses this effect to analyze material from the

surface as contained in the ejecta ( s e Fig. 2.1). A smail portion of the ejected material

will be ionized. therefore. it is possible to use mass spectrometric methods to determine

theu nam.

Figure 2.1 - Secondary Ion Mass Spectrometry (SIMS)

Since the primary beam removes materid h m the surface. it will mate a Crater, if given

sufficient time. This yields the possibility of anal-g the contents of the sample as a

function of depth. By dynamically measuring the mass of the ejecta and the time. one is

capable of correlating the m a s composition to the depth of the sample (this method is

called "dynamic SIMS")[56]. Together, EDP and SIMS allow for the measurement of

oxygen diffusion and surface activation of perovskite materials under v a ~ n g conditions.

Kilner et al. [55] proposeci a method of EDPISIMS analysis that monitors the negative

ions (as opposed to the positive ions) within the ejecta. whose advantages and

disadvantages they ouiiined.

Ions Monitored Advantage Disadvantage

Low yields of O'

Positive (+) Hi& yields of matrk cations 8 Possibility of Electron (M-) Stimulated Desorption

IESW

Negative (-) High yields of O- r No ESD signal

Low yields of M' Electron background.

Table 2.1 - Advantages and disadvantages of Negative Ion SiMS [55]

Since the oxygen profile in the soiid is of prime importance. it can be seen that for the

d y s i s of oqgen seifdiffusion in perovskites iEDP/SIMS (Negative Ions) is the most

promising method. Using this technique the srnailest penetration depth that can be

detected is around 30A per datapoint (so if we want 25 points. then the depth would be

ca 0.08 mimns) [551. One drawback of the dynamic SIMS technique is that once we

have sputtered away a deep crater, the waiis will start to interfere with the signal while

the condition worsens as the crater gets deeper [ S I ; Wermore. the tirne needed to

sputter a crater deep enough to measure a relatively deep profile will be extremely long.

To address this issue Chater et al [57] developed a technique that they called Linescan

SiMS. in this method the sample is cut in such a way as to expose a cross-section of the

profiie. The sample is then analyzed with static SIMS at specilïed intervals dong the

profile thus producing a set of datapoints that correspond to the oxygen content at varying

depths (see Fig. 2.2.)

EXCHANGE ANNEAL S lMS LINESCAN PROFILING

Fi,% 2.2 - Linescan SIMS method. (Adapted fiom [57])

With the L-SMS method it is now possiile to analyze the oxygen isotope depth profile

in perovskite materials with high dihivity and at high temperatures

3. Experimental Procedure

3.1. SampIe Preparation

n i e sample preparation involved two major steps: first the samples were sintered from

powders and formed into discs or rods, and then they were cut into pmper sized discs and

polished.

3.1.1. Sintering of Powders

The samples were made by sintering pellets or rods fiom powders obtained h m Praxair

(Lao.tSro.8Cro.2Feo,80M made fiom Lot#: 03-P3 102DM) at the University of Houston and

from constituent oxide powders (in the case of La&ro,gFeO34) at the University of

Missouri. The conditions of the sintering are outlined in Table 3.1.

Composition Temperature 1 Time Rate of Heating h2sr0.8Feo3d 1300 "C I 12 ~ r s 2 "C I min

Table 3.1 - Sample sintering conditions.

The density of thus sintered samples were then measured by the respective laboratones

and found to be within 97 - 98 % of the theoretical density.

3.1 3. Cutting and Polishing of Samples

Once the samples had been formed and tested for density they were cut down into disc

shaped pucks of roughly 5 mm diameter and 2 mm thickness. These sampIes were then

polished down to a 0.2 mimn finish on one side of the puck The polish was done by

stepwise mechanical grinding with abrasive sheets followed by mechanical polishing

with a fine diamoud suspension, The samples were then tested with Hitachi Mode1 S4500

Scanning Electron Microscope (SEM) to characterke the surface roughness (set: Fig.

3.1).

Figure 3-1 - SEM image of the surface of a potished sample.

Small craters on the surface were observed and attributed to perovskite grains that had

been plucked out fiom the surface by the polishing step. These craters are expected to be

present ody at the inmediate d a c e and are not indicative of bubbles in the mateciai,

which codd mean pomsity within the buik. From the SEM images it was established that

the 0 2 micron finish was indeed obtained. notwithstanding the pIucked grain craters.

Any subsurfke damage introduced by the polishing proces is expected to %ai" durhg

the hi& temperature anneal that wiil occur pnor to the experiments 1551.

33. Experimental Procedure and Apparatus

To remove any remaining residue h m the poiishing step. the samples were first washed

in an ultrasonic bath of acetone, followed by rinsing with a few drops of distilleci water.

This linal step removed any remaining organic and inorganic contamination h m the

surface of the samples. Any other organic residue wouid simpiy burn out in the hi&

temperature anneal.

The cleansed sample was then placed in a reactor consisting of a 20 cm quartz tube (ID 7

mm, OD 9 mm) with a flat bottom on one end. in which a second quartz tube (5 mm OD.

Imrn ID) was placed for gas delivery (see Fig. 3.2). The sample rested on the flat bottom

of the outer quartz tube with the polished side facing the delivery gas tube. The

temperature was monitored with a Type S thennocouple with a bead of less than 1 mm in

diameter to ensure fast response tirne. The thennocouple was placed on the underside of

the outer quartz tube's flat bottom in order to measure the temperature of the point that is

closest to the sample. without actualIy king present in the reactor. A drop of hi&

temperature cerarnic adhesive was placed on top of the thennocouple bead to hold it in

place.

Once the sample was in position and the reactor assembted. the unit was connected to the

gas exchange assembly. The exchange appatanis consisted of an electrical furnace in

which the sample was placeci. and an exchange network through which the gases

flowedfmixed. Two different atmospheres were used in the experiments: 20 % Oxygen in

Argon (Matheson). and 5 % CO. 5 % Ca. with Neon and Argon [CO 50 % in Neon

(Matheson), CO: IO % in Argon (Matheson)] king the balance gas. The assembly for the

latter is shown in Figure 33.

Gar Flow I

ncmiocouplc w

Pmvskirt Samplc

Outer Q u m Tube

Inira Q u m Tube

Gas D r l i v q Holt

C m i c Cap

Figure 3.2 - Reactor assembiy.

CwIAr Cûiiïe 'CO2iCûi Fumace ArNe

Figure 3.3 - Gas exchange assembly for CO/C@ gas mixtures.

in the COIC@ experiments !he COz passed through an oxygen trap to remove any

molecular oxygen (@) h m the stream. while the CO gas passed through a carbonyl trap

(T = 300t OC) to remove any nickel carbonyl that might be in the gas stream (which

would have precipitated on the sample/reactor if not removed). The oxygen assembly was

similar to the CO/COt assembly except no &mg or cleaning of the gas was required.

The gas flows were controlled with AaIborg Mode1 DFC 2600 digital m a s flow

controllers. The gas switch was operated pneumatically (He at 100 PSI. < 1 sec switch)

and remotely via cornputer. The furnace was controlled with an Omega CNlOOO

controller and the temperature was rnaintained at T + 0.2 OC. The gas outflow was

monitored with an MT1 Modet lOOC quadrupoIe Mass Spectrometer (MS). whose control

sofhvare aiso logged the temperature of the sample as a h c t i o n of tirne.

Once the samples were in position in the reactor and the assembly had been readied. the

annealing gas (either 20% 1602 or CO/C"@ = 1) was circulated at the desired tlowrate

(10 cdmin or 30 cclmin) and the b a c e was turned on. The gas outflow was then

monitored with the MS. Once the temperature had equilibrated. the MS data was

monitored to make sure the oxygen potential had equilibrated. the anneal then was carried

out at approximateiy ten rimes the tracer exposure time (or 1 hr+). When the anneal was

completed the gas was rapidly switched to the tracer composition at the same flowrate.

During the tracer exposure the temperature as well as the MS signature was logged. At

the completion of the tracer exposure the sample was quickly quenched with liquid

nitrogen. The sample cooled to a nondevant temperature (T-1001- OC) in less than 5

seconds, which effectively stops the exchange of tracer gas and '%eezesV the oxygen

profile at the specified the . The sample was then retrieved. cut and polished in

preparation for the SiMS anaiysis.

33. SIMS Anaiysis

Readiigs of oxygen isotope ratios were obtained at 100 to 300 pm intervais using a

Physicai Electronics Mode1 6600 quadrupie rnass spectrorneter with a 3 keV Cs'

primary ion beam incident at 60' with respect to the sample normal that was rastered over

100 .u 100 pm areas. Negative secondary ion popdations were anaiyzed over a 1 eV

energy window centered at an emission energy of 2 eV. Dead time effects associated with

electron multiplier detectors were minimueci by ensuring the count rates rernained below

5x10~ counts per second (cps). Crater edge effets were removeci by analyzing the central

30% of the Crater. Charge neutralization was induced through electron irradiation of the

sputtered area Also. prior anaiysis of an AIz03 wafer revealed (a) the effectiveness of the

neutralization. and (b) '6~:'80 iniensity ratios which were in agreement with the natural

abundance of these isotopes. This aiso reveds that molecdar interference in the form of

H a is not affecting the 180 signal (a vacuum of bener than 2x10") torr prevailed

throughout).

3.4. Method for Distingaishing Between Surface Ercbange Pathways

WhiIe the analysis of data for the samples annealed in is well established (as

mentioned previously). the anaiysis of the data for the samples anneaied in the CO/COt

mixture requires a new approach if the d a c e reactions are to be distinguished. The

presence of CO and C@ in the gas resuits in two possiile surfice ceaction pathways:

"Simplen exchange with the solid and a REDOX exchange reaction with the solid. These

pathways are explained schematicalty in Figure 3.4 and in Eqns. 3 3 and 3.3. In the

"simple" exchange model, the ~ " 0 2 mo1ect.de is adsorbed to the surface of the oxide on

the oxygen site, possibly forming an intermediate carbonate species. An exchange of

labeled oxygen from the gas for th: d a c e oxygen results in a transfer of 180 to the

solid. This exchange process cm be tracked by obserwig the CO2 MS data While an

analogous "simple" reaction is possible for the Oz case, it has been shown that is

negligible [24]. The other process. the REDOX exchange, meanwhile can result in the

oxidation of a surface oxygen vacancy site with COa forming CO. or the reverse

reduction reaction abstracting an oxygen atom h m the solid with CO. The equilibrium is

then dictated by the species concentration. To track this reaction we would look for CI80

in the MS data

Figure 3.4 - Two possi%Ie pathways for the exchange of labeled oxygen atoms. (The circles with dots represent 180)

"Simple" Exchange : + l6oS + ~ ' ~ 0 ~ ~ 0 + lEoS 3.2

REDOX Exchange : ci802 + vos + cI8o + I8oS 3 -3

The key surface process is the REDOX reaction. since this is the oniy step that results in

the transfer (not exchange) of oxygen atoms fiom the gas to the materiai and is one of the

reactions participating in the partiai oxidation membranes for syngas production.

However, since both reactions contribute to the iabeled isotope fraction in the sample. we

need to mode1 the reactions in order to differentiate the individual contributions. We

approximate the gas phase reactions by modeling the reactor as a Constantly Stined Tank

Reactor (CSTR),

WhiIe the surface concentration is.

&da = [(ko0 + k0,)( xc02- x,) + D* &J&] (at x = 0)

Findy. the bulk diffusion is modeled,

Ôxit/& = -D* 8xJÔx2

where, Fco and Fco2 are the flow rates for CO and CO?_ respectivety, xcoz and xco

represent the '*O hction in each of those gases and x, is the hct ion in the surface, ka,, is

the REDOX reaction coefficient, po is the oxygen density in the solid and A is the unit

area This system can now be used to mode1 the isotope profile in the solid as well as the

isotope hction in the effluent gas.

4. Results and Discussion

4.1. EIigh Osygen Partial Pressure (20 % a) 4.1.1. LaSrFe0

Since there was no data on the diffusion and surface activation coefficients of the

materiais in literature, the first tests were tailoreci to "brackef' the oxygen profile to

within haif the thickness of the samples. Once that was accomplished it was then possible

to set the time and temperame without overshoocing (or under exposing) the sample and

profile. To this effect the first set of experiments were carried out at 20 % 01 in Ar at

750°C and 850 O C and for various time durations. The results then ailowed for subsequent

eqerirnents to measure the tracer diffusion coefficient and the surface activation

coefficient of the samples. A sample of the data for Lao.&*Fe034 and the

corresponding mode1 fit is show in Figures 4.1 and 4.2.

O 1 Dcpm (ml

Figure 4.1 - Oxygen profle for La&r&eOu in 20 % 01, 750°C and 10 minutes.

Figure 4.2 - Oxygen profile for Lao.zSri~.&eO~~ in 20 % Oz, 850°C and 10 minutes.

The solid lines indicate the mode1 fit using non-linear least squares regression (Appendix

A) while the dots represent the S M S data The ordinate represents the normalized

isotope fiaction, which corresponds to the isotope fraction that has been adjusted to the

background isotope concentration. From the two graphs it can be seen that a close tit

between the two is obtained, The effect of temperature is apparent in the depth of the

profiles. since no other condition was change& The conesponding tracer diffusion and

surface activation coefficients are show in Tabte 4.1.

Temperature Surface Activation (k*) cm/s Bulk Diffusion (D*) cm'ls 750 OC 4 9 x 106 1.1 10-~ 850 OC 9 3 x 1o4 5.6 x 10-7

TabIe 4.1 - Oxygen transport coefficients for i . a ~ & r ~ . ~ F e O ~ ~ in 20 % Oz.

It can be seen that the diffusion coefficient increased by a factor of 5 when the

temperature increased by 100 O C . On the other hand the surface activation only increased

by a factor of approximately 2 when the same increase of temperature was made. To

further evaiuate the e f f i t of temperature and to see where these results fa11 when

compared to published results for similar compounds we look at the Arrhenius plot of the

diffusion coefficient (D*) in Figure 4.3.

Figure 4 3 - Arrhenius plot of diffusion coefficient for Lar.,SrxFe034 (Hi@ Pol).

There is a distinct trend visible in Figure 4.3. in which the diffusion coefficient increased

with increasing Sr content, as seen in Isbigaki et aL's redts [4338]. The resuits for

La&ro reporteci here a h fall within îhe observed trend if one can extrapolate

the vatues to the higher temperatures th& Ishigaki et al. used. Moreover. and more

notably, the activation enthalpies for the oxygen diffusion coefficient are simiiar to those

of Ishigaki et al. The activation enthalpy obtained h m the reported diffusion coefficients

for L~2Sro.8FeOs4 (High Paz) is 140 Wmol while Ishigaki et ai. reported activation

enthalpies of 164 kl/mo[ for La&ro.~FeOs4 [43], 177 Id/mol for h-~~~Sro.&!03d [43]

and 214 Umol for LaFeû3 (Single Crystal) [38]. In the absence of tracer diffiision

coefficient for higher Sr doped ferrites in literature. we will consider the chemical

diffusion coeKcient for as reported by Bredesen et al. [58]. However, it

must be kept in mind that since this d i f i o n coeflicient is obtained at oxygen chemical

potentiai gradients it is dependent on the partial pressure of the oxygen in the gas phase

but is related to Dv and hence D* via the equations 1. IO and 1. I I . Yet if we look at the

trend we cari see that the previous observations stiil hold.

SimiIarIy we can compare the activation energy for the surface reaction with those

previously obtained for related materials ( s e Figure 44). An andogous trend for the

surface activation coefficient is observed here for Ishigaki et aL's data. with this study's

resuit obtaining the higher value. However, it can be noted that the effect of temperature

is noticeably srnalier in the high Sr content (La&ro.&Osa) material than those observed

for the lower Sr content perovskites. The correspondhg activation energy for k* in

L a 0 ~ S r ~ . ~ F e 0 ~ ~ in 20 % Cl2 is 55 kJ/mol. whereas the results obtained by Ishigaki et ai.

fail in the range of CO0 - 200 Wmol. The resuits obtained by Beresen et ai. [58] for

L ~ O J S ~ ~ . ~ F ~ O ~ ~ under oxygen chemicai potential gradients show the continued decrease

in activation energy at even highet Sr doping. in fact in their paper. Beresen et al, [58]

reported no noticeable change in the surface activation coefficient with increasing

temperature* an in eEeçt it remaineci the same. Tbey however were not able to explain

their results. Since the large difference in activation entfialpy for our results is seen in the

surface coefficient and not in the bufk diffusion coeficient, it c m be presumed to be due

to differences in the d a c e composition or morphoIogy. Larger surface areas will

increase the observed surface activation coefficient (461. Surface impurities (metailic)

rnight also contribute to the activation of oxygen at the surface [4TJ and lower the

activation enthalpy of the sirrface reaction(s). These changes couid aiso bc due to

depletion of Iabeled isotopes h m the gas atmosphere because of the fast diffusion rate,

which was not modeled here for lack of MS data.

Figure 4A - Arrhenius plot of d. activation coefficient for Lai_xSrxFe03d (Hi& P02).

4.2.2. LaSrCrFe0

Chromium is prùnarily added to the perovskite to improve its stability under reducing

conditions; therefore, it is necessary to investigate its influence in the transport properties

of the materid.

Figure 4.5

-

-

-

I I

O 0 6 0 08

O 1 Dqrh (cm)

- Profile for Lao.zSro.8Cro.2Fe~.8034 in 20 % 02, 750°C and minutes.

As was mentioned earlier the chromium containhg perovskites are not good conductors

of oxygen ions [271 when compared to Fe and especiaily Co: in fact they are used as

intercomect material in SOFC's because of their low ionic conductivity. Therefore. it is

expected that the addition of Cr to the perovskite Iattice of a ferrite would resuIt in

decreased oxygen conductivity. The materiai studied here had the composition of

L a 0 ~ S r ~ . ~ C r ~ ~ F e ~ . ~ 0 ~ Under the same experimental conditions and procedures the

samples were annealeci in 20 % Ch, exposed to "02 , quenched and anaiyad for oxygen

isotope ratio. Figure 4.5 and 4.6 show the SIMS profiles and the corresponding mode1 fit.

Figure 4.6 - Profile for L~.zSro.8Cro.lFeo.803d in 20 % 0,. 850°C and 5 minutes.

In order to elucidate differences between the two material's oxygen transport properties it

is necessary to look at the actual values of the tracer d i f i i o n and surface activation

coefficients. Table 4.2 summarizes these values'.

Temperature Surface Activation (kt) c d s Bulk Diffusion (D*) cm'ls 750 OC 1.6 x 1w5 1.8 x IO-'

Table 4.2 - Oxygen transport coefficients for Lao.rSro.RCro.2Feo.a03a in 20 % 02.

The first thing that stands out is that the observed coefficients are noticeably larger than

those obtained for the non-chrornium samples. this of course is different than the previous

* Two batches of the same materiai were useci they were prepared by the same method and the same ingredients, the table here shows the resuits for the tirst batch.

6nding that chromium hinders oxygen transport in the mate&'. Looking closely at the

numbers we can see that there is a much Iarger increase (approx, order of 0.6) in the

surface activation at the lower temperature than on the bulk d i f i i o n at the same

temperature, while the higher temperature demonstrated a similar increase in both

coefficients. This unexpected result can not be readily explained, however it is possible

that the enhancement is not due to any effects directly related to the chrornium but rather

due to increased diaision/activation at grain botindaries that might be more abundant in

these samples because of either the presence of chromium in the lanice andlor related to

the preparation conditions of the sample. Zhang et al. [59] in their study of this same

materiai (Lao.2Sro.sCro.rFeo,803~) noticed that the tlwc of oxygen increased with

decreasing grain sue (hence more @sin boundaries) and proposed that the boundaries

provide a path for f i e r diffirsion and that they might affect the surtàce activation of

oxygen. They postxiated that the d i f i i o n is enhanceci at the grain b o u n â a ~ ~ due to

hiefier oxygen vacancy concentration at those positions andfor because of a higher

vacancy diffusion coefficient at the boundaries. Funhermore. they aiso proposed chat the

surface activation rate might increase because of more interfaces k ing availabIe for

reaction. Another possible explanation is that the addition of chrorniurn disnrpts the

ordering of the iron atoms in the lanice (as demibed in section 2.2.3), thus preventing

oxygen vacancy ordering. The consequences of this are discussed later when the resuits

for the Iow partial pressure experiments are reporteci. Finally, going back to the data

reporteci here it can be seen that the second set of data for the 850 O C experïments shows

A sample h m a second batch of materid (Baich 2) was aIso and& by the same method at 750 OC. The diffusion coefficien~ however, for tùis sampte was not detennined to great accuracy due to the very weak prome that resuited h m a tow nntàce aanrity. The implications wiU be discussed dong with the results of the surface activarion coefficient in the foilowing pauagraphs.

slightly larger values. This difference wi either be due to materid inhomogeneity or due

to inaccumies in the measurement of the values themselves. Given the complex nature

of the material and the multiphase behavior seen in the simpler non-Cr ferrite [53]. it is

more plausible that inhomogeneities in the material are responsible for the differences in

Figure 4.7 - Arrhenius pIot of diffusion coefficient for La~.,Sr,Cr~,Fey03d (High PO2).

To M e r compare the performance of the Cr B-substituted femte with the non-Cr ferrite

and other B-substituted ferrites we look at the Arrhenius plot of the diffusion coefficient

and the surface activation coefficients (Figures 4.7 and 4.8). From the graph we can see

that the activation energy for the Cr ferrite is very sirnilar to the non-Cr femte mentioned

before, however. as observed previously there is a general enhancement of the diffusion

coefficient with the Cr. The only other Cr-substituted fenite that has b e n characterized

for oxygen conductivity is ~ao.8Sro.rCro.fFeo.803~ that was recentiy studied by Atkinson et

ai. [60]. They reported very low tracer diffusion coefficients. which they admitted to be

much lower than the one reported by Ishigaki et al. [43] for the non-Cr version with

composition Lao 7sSro.zsFe03d . The values reported here for the high strontium content

ferrites seem to compte with those reported for cobaitites with hi& strontium content

[61] and with Co-substituted ferrites with low Sr content [62].

The surface activation coeficient is characteristically higher for L@.2Sro,sCro,zFeo,8~j4

thau for h.8Sro.2Cro2Feo.s034 as seen in Figure 4.7. however. the effect of temperanw

on Lao.8Sro.zCro.zFeo.~03-(i seems to be more pronounced than on the higher strontium

content perovskite. The temperature-induced enhancement (Le. activation energy) in the

activation coefficient of the latter seems to be closer to the high strontium cobaitite rather

than the low strontium Cr-ferrite, indicating that the effect of strontium doping on the

surface activation coefficient is dotninant at these levels.

Figure 4.7 - Arrhenius plot of surf. activation coefficient for Lai.,Sr,Crl,Fe,OM (High Po?).

Results for a second batch of the same material that was made and tested under identical

conditions. however. yielded inconcIusive resuIts. The sampIe was tested at 750 OC. yet

the resulting strrface activation coeficient diffm notably with the ones obtained

previously. in this case the value is extremeIy low. which could point to an unknown

factor affecting the surface activation of oxygen. This discrepancy in the surface

activation coefficients could point to either intrinsic or extrinsic factors that have not

ken identified. For this reason the oxvgen supply to the buik was very limited resdting

in an extremely low profle (not in depth but in isotope hction), which prevented the

accurate measurement of the diaision coefficient. as well as the surface activation. It is

noteworthy to mention that the surface activation coefficient reported here is closer to the

ones reported by Atkuison, hinting ttiat whatever induced the low surface oxygen

activation in the resuit reported here might also have affecteci the resuits obtained by

them. However, this same observation is not true for the diffusion coefficient result

obtained for the same sarnple when it is compared to Atkinson's data. The effects of

sample preparation conditions on the oxygen transport properties of these materials

clearly needs more research.

4.2. Low Oxygen Partial Pressure

4.2.1. LaSrFe0

One sample h m the non-Cr material was prepared for analysis using a mk~ture of

H~/c"o~ = 1 (5% each, balance Ar) at 850 OC and at a f lomte of 10 cdmin. The

experimental procedure was sirnilar to the ones used in the previous experiments. The

sample however was annealed in a 20% Oz gas mixture (pcior to the switch) and not the

non-labeled equivaient of the isotope gas. The results of the SiMS analysis and mode1 fit

are shown in Figure 4.8 and Table 4.3.

O 1 Depth (a)

Figure 4.8 - Profile for Lao.tSro.sCro2F~.~O~~ in C W 2 = 1. 850°C and 5 minutes.

Table 4.3 - Oxygen transport coefficients for La&ro.8Fe034 in C@/H2 = 1.

Comparing these resuits with the ones obtained previously for high oxygen partial

pressures we notice a modest gain in the d a c e activation coefficient and a modest loss

in the bulk diffusivity. At first giance this pattern does not make sense. However. as

noted previously the sample was not pre-annealed at Low partial pressures in nonlabeled

gas pnor to the isotope-auneal. Therefore. once the switch to Iabeled gas was made, a

local reduction at the surface by the hydrogen is expected, which will abstract surface

oxygen species leaving behind more surface oxygen vacancies. As was mentioned in

Section 2 (Literature Review) an increase in oxygen vacancy promotes the surface

reaction with the oxygen carrying species. thus accelerating the incorporation of the

oxygen. However, since the bulk has not yet had a chance to reduce and equilibrate with

the gas it wiii not have increased its oxygen vacancy concentration. This however. does

not explain the decrease seen in the diffusion coefficient: at best it is expected to have

remained the same. Figures 4.9 and 4.10 show these coeEcients in relation to previously

detennined values and values reporteci in literature for reIated materials. Because of the

changing oxygen stoichiometry the values reported here are not significant. and the

figures are shown just for visual comparison.

4.2.2. LaSrCrFe0

The purpose of adding chromiurn to the femte m a t e d is to stabilize the structure under

reducing conditions, since chromiurn had demonstrated remarkable stability under these

conditions [3 11. Although EDP experiments cm generate Iow oxygen partial pressures

they are not ideal for measuring the reduction-stability of these mateciais, however, they

are excellent for the measurement of the diffusivity and surface activity of these

materials. The reduction-stabiiity measurements require prolonged (and possibly cycled)

experiments that simulate actual operaiional loads.

Here the performance was determined in tems of oxygen ion transport for the candidate

material Lao.~Sro.gCr02Fe0.803~ which has k e n designed for use under low oxygen partial

prcssurcs. TErc experiments u.m carried out under similar conditions as previo~sly

mentioned, with the exception of the gas ahnospbere. la these runs the sarnples were

preanneaied in a gas mi- composed of CO and CO2 (I:1) 5% each balanced witb

noble gases (Ar and Ne) and followed by a switch to the tracer equivalent of the same gas

(c '~o~/c '~o). The flowrate was set to 10 cdmin (the sarne as the previous ones), To

obtain relevant data about the reaction process (as exphined in section 3.4) the MS data

was analyzed as well. The same two temperatures were used here: 750 OC and 850 "C.

Figure 4.9 - Plot of diffusion coefficient for Lai-,Sr,Fe03d (Hi& Pol vs. C021H2 = 1).

Figure 4.10 - Plot of surf. activation coefficient for Lat-xSr,FeO~ (High Po2 vs. C@lHt = 1 ).

The effluent gas was monitored with the mass spectrometer to investigate the interaction

60 O 50 100 150 200 250 300 3SO Time (8)

Figure 4.1 1 - MS data of effluent gss "O content during experiment at 750°C.

of the ~ ' ' 0 2 and the CO and look for evidence of REDOX reactions. Figure 4.1 1 shows

the "0 isotope fraction of the effluent gas species as a function of expriment tirne. At

the time of switch to IabeIed gas, a sharp increase in the COr Iine foiiowed by a steep

declme can be observed. This phenornenon is due to the backpressure in the gas

which causes a momentary surge in the flow of gas at the t h e of the mitch, cesuiting in

a sudden nse in the "0 fiaction in the Ca he. Once the flow has retuxned to the preset

Ievel, the isotope fraction drops to the level that corresponds to that of the

"Simplen/REDOX reactions. Meanwhiie the CO Iine increases steadiIy as the experiment

1 once progresses. At the t h e of quench as indicated by the temperature profile, the CO-

again increases wEde the CO üne decreases, indicating a cessation of the exchange and

REDOX reactions. Keeping in mind that Figure 4.1 1 represents the fiaction of labeled

isotope ("O) in the gas species (as processed h m MS data), we can see the mle that the

two surface reactïons, "simple" exchange and REDOX exchange. play in incorporating

oxygen into the surface structure. The two reactions cm be represented as:

"Simple" Exchange : ~ ' ~ 0 ~ + I6oS 3 + 180S 3 -3

REDOX Exchange : ~ ' ~ 0 2 + vas + clgo + 3 3

in the "Simple" exchange mechanism the doubly labeled carbon dioxide adsorbs to a

surface oxygen atorn (unlabelled in this case) and forms an intermediate species with

mobile oxygen atoms. subsequently one of the labeled oxygen atorns is placed on the

surface site of the intermediate, foIIowing which the c h n dioxide molecule is

separateci. This molecule, h a h g repiaced one of its labeled oxygen atoms with an

uniabeled one, will then be singly labeled jsee Figure 3.4). This reaction can of course

take place with a labeled oxygen on the d a c e or the oxygen lefl after the separation can

be the unlabeled one; in either case the net remit king that no isotope exchange has

occurred and therefore no noticeabk change.

in the REDOX exchange mechanism a doubly labeled carbon dioxide molecule is

adsorbed at or near a sinface oxygen vac~uicy! following which one of the labeled oxygen

atoms is incorporated into the vacancy site and dissociateci h m the parent molecule.

Thus a labeled oxygen atom is incorporateci into the surface and the doubly labeIed

carbon dioxide is reduced to a labeled carbon monoxide molecuie. The reverse of this

reaction is also possible, in which a labeled or unlabeled carbon monoxide molecule

reacts with a d a c e oxygen atom to produce a carbon dioxide molecuie.

The gas products of these two reactions can be detected with a mass spectrometer. B y

looking at the ratio of IabeIed and unlabeled oxygen in these products, an indication of

the extent of reaction can be obtained. The rise in the CO line in figure 4.1 1 is indicative

of the production of C"O by the aduction of a labeled carbon dioxide molecule in the

REDOX mechanism. Although the labeled carbon monoxide could have originated fiom

18 16 a singiy labeled carbon dioxide (C O O), this would be a minor contributor. especially

at the beginning of the experiment. simply because of the abundance of the doubly

labeled molecules. Another source for a ct80 signal in the MS data is related to the

instrument itself. Some of the molecules entering the MS will split into "daughter"

fragments, which will be detected by the instrument as separate molecules. In the case of

carbon dioxide one of the hgments is carbon monoxide. and if the carbon dioxide is

labeled the m e n t will also be IabeIed. The signai h m these fragments. however. will

be a smaii fiaction of the parent molect.de and should not have a noticeable effect on the

signal, especially since the concentration of the parent molecule is relatively low. The

veracity of tbis assumption cm be seen in Figure 4.11: had the cL80 signal been afTected

significantiy by the hgrnentation of C''OZ in the MS, ihw the peak in the CO z 1' me seen

in the graph would have correspondeci to a similar peak in the CO line, yet since no such

peak is seen, one can conclude that the fhgmentation was small enough not to atrect the

net signai.

Tuming our attention to the d y s i s of the depth profile of oxygen isotopes in the sample

(Figure 4.12), we can observe that the profile bears similarities in shape to the profile

obtained for the bigb partial pressure sample (Figure 4.9, although some minor

dflerences can be noticed, Since the isotope fraction is not constant throughout the

exchange process the surface concentration of the labeled isotope will also change as a

function of that concentration (if one assumes the surface reaction to be fast enough. as it

wiIl be shown later).

O 200 100 600 8Oa 1000

Depth (micrometer)

FIGURE 4.12 - Profile for L~.zS~o.sC~o,zFeo,803-d in CO/CO2= 1 and 75o0C.

As the bulk isotope hction is dependent on the supply coming h m the surface the buik

wiii also be affected by the changes in the gas isotope fraction. The net cesuit k ing that

the shape of the profile wiil also not correspond to our expectations based upon the

previous conceptions of timddepth relationships. in those assumptions the depth was

assumed to increase proportionally with time, however, in this case the depth is more

reIated to the isotope concentration in the gas phase (through the surface isotope

fraction). The fit s how in Figure 4.12 (continuous line) corresponds to the results of a

mathematical model that simulates the ceactions outlined in Equations 4.1 and 4.2

(Appendix B). in the deepest portion of the profile the model deviates a little fiom the

isotope profile obtained h m the SIMS anaiysis. This discrepancy can be due to

inaccuracies reiated to the SiMS analysis itself or due to inhomogeneities in the material.

ff the material is not totally homogeneous, then some parts of the sample might have

different compositions h m their neighbors and therefore different diffusion properties.

For example, if a grain of materiai in the bulk has a lower Sr doping level than other parts

of the sample. then the oxygen vacancy concentration will be lower in that grain.

resulting in lower diffusion rates. Since the model assumes the sample to be homogenous

then the fit will reflect the profile that corresponds to the presumed uniforni composition.

ïhere are of course other causes for ciifferences between model fits and actual data, but

most of them pertain to the accuracy levels of measurement or calculation. The noted

deviation of the model, however, is not large enough to significantly affect the final

r e d t

It was previously assumed that the d a c e reaction rate is fast enough that the d a c e

composition will quickIy reflect changes in the gas isotope composition. Evidence for

this assumption can be seen when the MS data in Figure 4.1 1 is comparai with the SLMS

data in Figure 4.12. Loobg at the isotope ûaction at the surface of the sample (zero

depth in Figure 4-12), we can see that the ratio is roughly 0.45. ifwe compare this ratio to

the ratio of labeled isotope in the COz line (Figure 4.1 1) at the time of the quench we cm

see that the two are roughly equal. This indicates three things: 1) As was mentioned

earlier, it indicates that the surface exchange rate is fast enough to reflect changes in the

gas composition. 2) It serves as a validation of the SIMS/MS data and 3) it shows that

the quench was effective in stopping the exchange at the desired time.

4.22.2. LaSrCrFe0 at 850 O C

The MS data for the experiments at 850 O C show a sirnilar trend to the one for the

expriment at the lower temperature (Figure 4.1 3).

Figure 4.13 - MS data of effluent gas 180 content during experiment ai 850°C.

Again we see the sharp peak correspondhg to the backpressure in the line followed by a

steady rise in the tracer isotope fraction in both gases (CO and COz). NoticeabIe is the

lower overalI LeveI of the isotope h t i o n in the CO? when cornpareci to the data for the

750 O C experiment. At higher temperatures the d i h i o n is expected to proceed faster,

thus depleting the isotope h m the gas phase at a faster rate, which will result in a lower

isotope fraction in the gas phase. In contrast the CO line is "highef in the higher

temperature case. However. since the isotope fraction in the CO indicates the product of

the REDOX exchange reaction, the "higher" line indicates an increase in the rate of that

reaction Simple interpretations of these trends cm be misleading. For exampIe. if we

assume that the decrease in the CO1 isotope fiaction at the hiber temperatures indicates

that the "simpl&' exchange is proceeding faster we codd be wrong, since depletion of the

labeled isotope fiom the CO? v i e s dso occurs by the REDOX exchange mechanism.

Therefore. in order to andyze the MS data properly it is necessary to consider the whole

exchange mechiuiism in concert, and only then we can see which reaction is favoured by

temperature and which m e is not. This coupling of reactions is accounted for in the

mathematicai mode1 that is used to fit the parameters to the SIMS and MS data

(Appendix B).

The first thing that is apparent in the SIMS profile of the 850 OC sample (Figure 4.14) is

that the profile intefsetts the y-axis at a Iower level than the profile for the 750 'C

sample. This indicates tùat the buIk d i i o n rate has increased more ttian the swfâce

activation rates (or that the latter has decreased), thus depleting the surface h m oxygea

isotopes. Of course the alternative is that the sd ice activation rates have decreased.

lowering the supply to the bulk. The otfier thing that becornes apparent is that the profile

goes in deeper into the sample than the previous lower temperature case. Here the depth

of penetration is a more clear indication that the bulk diffusion coefficient has increased

with temperature, while the dope will be affected by the supply of oxygen fiom the

surface. Finally, unlike the previous case, the d a c e isotope fraction does not coincide

with the isotope fraction in the COI line of the MS data. A clearer view of these

phenomena is obtained when the mathematical mode1 is applied to the data.

O 200 400 600 800 1000

Depai (micrometer)

FIGURE 4.14 - Profile for La&ro.&ro2Feo.&~ in COlC02 = 1 and 850°C.

The cesults of the modehg are outlined in Table 4.4.

Temperame REWX Exchange (k,,) cm/s "SimpIeU Exchange (u cm/s Bulk D i h i o n (D*) m 2 / s 750 O c 5.0 x 104 > 2.0 x 104 8.0 x IO-'

Table 4.4 - Transport coefficients for L ~ & ~ o . ~ C ~ ~ ~ F ~ ~ . ~ O ~ ~ in CO/C@ = 1.

It is seen hem that the REDOX exchange is favoured by temperature. The diffiision

coefficient meauwhile experîences a stight incrpase in this temperature range. To put

these &ts in perspective, we'll look at them together with previousiy obtauied results

in Figures 4-15 and 4.16.

Figure 4.15 -Surf, activation coefficient for L ~ O & ~ . ~ C ~ , & Q ~ (High Po2 vs. COr/CO=1).

Recafling that the REDOX reaction hvolves the presence of a surface oxygen vacancy

site for reaction, we cm anticipate that this reaction will benefit h m more such sites

k ing available for reaction. It has already been discussed that increasing temperature

increases oxygen vacancy concentration [SI, therefore, it can be anticipated that increased

temperatures will benefit reactions that depend on oxygen vacancies. in Figure 4.15. the

effect of temperature on the surface activation coefficients for various conditions and

materiais are shown. Here it can be seen that the REDOX exchange coefficient (b)

increases with temperature and more important than that. the activation energy of this

reaction is vew close to that of the hi& PO? case for the same sample (k, Ea = 95 kJ for

Low PO2 cornpared to k* Ea = 79 - 101 kJ for High Po& This serves to reinforcc the

concept that the surface oxygen vacancy plays a similar role in both reactions. despite the

different gases and oxygen partial pressures.

As can be seen h m the relative magnitudes of the k, coefficients. the "simple" exchange

reaction seems to play an important role in the exchange of oxygen atoms under these

conditions. A decrease in this exchange rate could account for some of the decrease in the

surface oxygen isotope ratio with temperature.

Beyond the effect of temperature. one needs to look at how the lowered oxygen

pressure affects the oxygen transport properties of this matenai, after al1 they are intended

for use in these reducing conditions. While deducing the effects of the oxygen partial

pressure on the d a c e reactions could be cumbersome when the reactions themselves are

inherentïy d-erent. nevertheless, some cornparison cm be made between the two similar

teactions: the REDOX reaction of CO/C& and the surface dissociation of a, since both

these rely on the surface oxygen vacancy site for d o n . TKhile. as mentioued earlier,

the two reactions do show a very similar increase with temperature, they do however

differ sigdicantly in magnitude. Reducing conditions are expected to increase the

nwber of oxygen vacancies in the perovskite materid and therefore the rates of the

reactions that depend on them [SI, This phenornenon is exemplified by the data obtained

by Atkinson et ai. [60] and shown in the same plot. in theu resdts the surface reaction

rate increased when the sample was placed in a reducing environment. However. in our

case we see that for the higher Sr doped materiai a lower oxygen partial pressure

(reducing envùonment) d t s in a lower ovedl d a c e reactian rate. and yet under the

same conditions an increase in temperature resuited in an hcrease in the ceaction rate.

There seems to be a confiict in the effects of oxygen vacancy concentration and reaction

rate. Before discussing possible explanations for this conflict. we wiIl look at the plot of

the diffusion coeficient to see if the observed trends still hold. since the diffusion

reaction shouid be the same for either low or high partial pressures [oxygen vacancy

concentration nonvithstanding).

The Arrhenius plot of the diffusion coefficient for this sample and otherç is shown in

Figure 4.16. Were the trends that had ken obse~ed for the suface reactions are

seemingly reversed. Keeping in mind that repeatability of sample conditions have been a

constant concern in this fieid we will take a closer look at the trends. The low oxygen

partial pressure seems to enhance the diffusion, albeit very slightiy. while the temperature

increase seerns to have littIe effect on the enhancement of the diffusion coefficient. When

compared with the resdts obtained by Atkinson for a Iower doped sample we cm see the

modest gain we obtained at iow partiai pressures is dwarfed by the gargiillnian, multiple

orders of magnitude, gain in diffusion coefficient Atkinson obtained. However, the

activation energy (temperature effect) obtained here is very sirnilar to the one that is

observed for Atkinson's data (Ea = 35 Wmol for Sr = 0.8 and Ea = 45 Idlm01 for Sr =

0 3 [60]) suggesting similar energy barriers in the reaction. The differences in the

magnitude of the coefficients, however, are most ke ly due to vacancy conceniration

ciifferences in the bulk (more specificaiiy the avaiiable oxygen vacancy concentration.)

m < IM,

'.p C

1 1

Figure 4.16 -Dimion coefficient for La&rogCro.tF~.gO~ (Hi& Po2 vs. CWCO = 1).

The movement of oxygen vacaucies (vacancy diffusion) within the perovskite stmctwe is

dependant on the available neighboring sites for rnovement (ie. oxygen atoms), in other

words the diffuçion of oxygen vacancies can k aff'ted by the increase of the oxygen

vacancies themselves. Since the number avaiiable local sites for movement decreases

with increased neighboring vacancy sites, the diffusion itself can be lowered if at a local

level the vacancies themselves interfere with their movement. Under the assumption of

random dimiution of defects the local oxygen vacancy does not reach saturation levels

even at hi& doping levels. however if the assumption of random distribution is wrong

then we can have Iocai domains under which the vacancies will order and therefore

decrease the diffusion rate. The tendency to fom ordered structures p w s progressively

with increasing defect concentration. and in perovskites and related structures. ordering

seems to be more common than random distribution [5 ] . Therefore, at high doping levels

(ie, more v a c q concentration) we can expect the tendency for vamcy ordering to be

highest, hence negatively impacthg the gain in ionic conductivity expected h m the

i n c m in vacancy concentration.

43.3. Changes in Surface Composition

The Lal,Sr,FeOM (x = O - 1) system and its phase transitions were studied by Dam et

aI. [53] under various conditions. They noted several phase changes with increasing Sr

doping Ievels and for the region of x = 0.8 to 0.9 they noted that at those doping levels

the mataiai exhibited the most complex phase system of al1 compositions. They dso

noted that the phase of the material was dependant on the gas atmosphere it was annealecl

i~ with the hydrogen-reduced sample giving a complex mixture of phases. BattIe et al

[52] reported the presence of two oxygen vacancy ordering states in LatSr2Fe30M. Li et

d. 1541 have a h found evidence for ordering in samples of Lai,Sr,Fad (x = O - 0.7)

with the x = 0.7 sample ordering dong the CI 11> mis. These d t s point towards the

existence of a cornplex, gas atmusphere related, vacancy ordered phases in the high Sr

doped materials, wtuch could account for the observeci anemic increase in the diffusion

coefficient at Iow oxygen parcial pressures. A cornparison with the Atkinson data ia

Figure 4.16 revealed that despite the smal1 comparative gain in the magnitude in the

diffusion coefficient, the activation energies bear great sirniIarities. This magnitude refers

to the pre-exponentiai term in the Arrhenius equation. Bouwmeester and Burggraaf [5]

indeed have noted that this pre-exponential terrn is related in part to the density of mobile

oxygen anions and the availability of sites (vacancies) to which they rnight jump and that

its value is determined by the state of order in the oxygen sublattice. Therefore. we can

now see how the activation energy of the diffiision coefficient can be ço similar and yet

the actuai values of the coefficient differ so much beween ~ Ia t ed materiais.

The resutts for the d a c e reactions indicated that decreasing the oxygen partid pressure

decreased their rate. sometbing that was surprishg gîven the rise in surface oxygen

vacancy that is expected in such reducing environmem. An observation in the

appearance of the sample d e r the experiment might help in the understanding of these

results.

When the reduced samples were removed h m the reactor folIowing the temperame

quench, ~Iatïvely large black circdar areas were observed on the polished surface of the

saruples- These spots were observed on al1 the samples exposed to the CO/C@ gas

atmosphere and at both tempemtms. Howevet, they were absent in the samples that

wece exposed to the 20% oxygen atmosphere. Because of the ekvated temperatures of

the experiments these "deposits" are not expected to be wbon depositions, since at those

temperatures any carbon deposition would simply bum out. They spots were not expected

to be nickel deposits resulting h m carbonyl impurities in the carbon monoxide gas,

since these were removed by the wbonyt mp. The mysterious identity of these spots

needed to be determined in order to determine if they had infiuenced the d a c e reactions

in any way. Previous observations by ten EIshof et al. [46] indicated h t for low Sr

doped non-Cr ferrites (Lal&Fe034 , n = O - 0.4). similar surface phenornena was

observed on samples exposed to reducing COK02 gases. they also mled out carbon

deposition and foilowing XPS and EDX d y s i s concluded that the deposits were most

likely Sr containhg compounds (SrCO3 . SrO). Therefore a simiIar efYect is expected in

the simples w d here. aithough at a much higher level given the higher Sr doping levels

of these samples. Since the samples were rendered unusable for such EDX and XPS

anaiysis by the cutting/polishing in preparation for the SiMS analysis, samples of a very

simijar related compound (Lao.rSr0.&.3.~C~Feo.703-d, X = Undisclosed) that were

exposed to the same gas and on which h e same spots were seen. were used for analysis.

It is expected chat the resuits should hold true for the two samptes, given the great

similarity in composition between the two, especiaily the same high Sr doping Ievel.)

In Figure 4.17, the extent of the surface modification can be seen. A p t majority of the

d a c e seems to have been &ected by the reducing atmosphere, while the shape of the

spot reflects the geometry of the quartz delives, tube. A better view of the nrrfàce

anomaly c m be seen in Figure 4.18. Here the onset of the modified region can be seen.

On the Ieft of the image, correspondhg to an area closer to the center, "flakes" of some

material c m be seen, wtiile on the ri@ band side no such deposits are seen.

Figure 4.17 - Low magnifkation SEM view of modified surface.

Figure 4.18 - ûnset of modifieci surface region.

Figure 4.19 - Close-up of a "tlake" of surface deposit.

Figure 420 - EDX of the region outiined in Figure 420.

Figure 4.21 - EDX of non-modified region.

To analyze the composition of the obsewed regions. EDX analysis of the sample was

performed. As can be seen in figures 420 and 4.21 there is a notable increase in the

ievels of Sr between the "fiake" seen in the modified region and the surface

corresponding to a non-modified region. There is also a slight decrease in the Fe signal as

the surface is modified. This obsermion points towards a segregation of material at the

surface. Since the EDX analysis yields information about the composition of the materiai

averaged out to a depth of about 1 micron, the true d a c e composition is not revealed.

The fact that a change in the Sr level is observed despite this averaging out points

towards a strong presence of Sr in the surface and subsurface levels of the sample. To

better understand the composition of the srrrfwe it is necessary to investigate the sampIe

with techniques that yield information much doser to the surface, on such technique

king XPS. ïhe resuits of XPS analysis of reduced samples compared with their as-

poiished cornterparts are show in figure 42,423, and 4.24.

Figure 4.22 - XPS of a reduced sample compared to an as-polished sample.

Figure 4.23 - Sr signais iu the XPS analysis of reduced and as-polished samples.

Figure 4.24 - La signais in the XFS anaiysis of reduced and as-polished samples.

In Figure 4.22 a survey of the elements on the surface of the two samples is shown.

Although it might not be as apparent, the Sr and La signals do show some changes when

compared to an as-polished sample. What stands out the most in these XPS resuits is the

detection of an additional element on the surface of the reduced sample, analysis of

binding energies indicates that this element is sodium. The source of this new surface

species is unknown. Since the reduced sample was subject to more handling than its

couterpart, it is plausible that the sodium is contamination resulting h m various

sodium salts that are present in atmospheric moisture (including human sweat). The

presence of surface carbonates will greatly enhance the trapping of these species on the

surface and allow the reduced sample to exhibit such a signai for sodium. Surface

strontium carbonates are the species that ten Elshof et ai. [46] had conciuded to be the

form that the swface strontium existed as, therefore their presence is plausible. To better

understand the changes in the Sr and La in the surface we wilI take a closer look at these

signais in the data. Figures 4.23 and 4.24 show a more detailed analysis of these species

on the surfaces. Calculations of areas for the two signais reveals that the both the Sr atid

the La have a decreased signal strength in the reduced sample compared to its as-polished

counterpart. This lower signal could be due to either siightly dfierent conditions for the

XPS or due to the morphology of the surface, so to better compare the resuits we'lI look

at a comparative analysis of the Sr/La ratios. The reduced sample had a decrease in the Sr

signal of roughIy 28 % while the La was decreased by 69 %, which in h m wiU yield

différent S r h ratios for the two samples. In the as-polished sample the Sr/La ratio was

0.77:0.23 which is close to the bulk composition of 0.8:0.2. When the sarnple was

reduced the SrLa ratio of the surface changed to 0.87:0.13, clearly indicating that the

çurface was enriched by Sr at the expense of the La

In their studies, ten Elshof et al. [46] noticed an enhancement of the surface rate when the

samples where reduced, which they atûibuted to an increase in the d a c e area of the

sample. The samples they used, however, had a low level of Sr doping, while the

samples studied here are heavily Sr substituted. if the species formed at the surface have

a Iower activation coefficient for the surface reactions then they couid account for the

lowered observed surface rates, A heavy surface deposition of strontium carbonate or

strontium oxide could in fact account for the detrimental effect that the reduction has had

on the surface rate. By gradually srnothering the surface with these species the d a c e

wilI eventually exhibit lower surface oxygen isotope ratio. despite the increase in surface

area in a mdy of strontium carbonate as a cataiyst for the oxidative dehydrogenation of

ethane, Church et al. [631 noted that this materiai tends to be very selective and that the

high sektivity is related to the presence of sodium on the surface. They noticed sodium

on the surface of their materid. tvhich they did not expect skce there was none in uie

buik. They aiso noted that lauthana based catalysts were the most active compared to

strontium. If the source of the sodium was the materiai itself then the selectivity of the

Na-SKO3 system together with the decrease in the La on the surface could account for a

decrease in the oxygen activation on the surface, aithough more research into this effect

is needed.

The two ciBering behaviors of the d a c e and the bulk that was observed in the resuits

codd be due to a coupihg of the two phenomena described above. The anemic gain in

the diffusion coefficient couid be due to an ordering of the oxygen vacancies at the high

Sr doping levels, while the decrease in the surface oxygen concentration (surfie

activation of oxygen) couid be due to a decrease in the surface rates associateci with the

increased presence of strontium carbonate on the surface,

White these are prelimùiary results for the anaiysis of the modifieci surface, more detailed

analysis. which is under way. will reveai a better image of the nature and consequences

of this change.

5. Conclusions

The diffusion and surface activation coefficients for the materials La&r0.~FeO3-d and

L~lSro.aCro2F~.803d were determined under both hi& and low oxygen partial pressures

at two diEerent temperatures: 750 O C and 850°C. For L ~ , ~ S r ~ . $ e 0 ~ ~ at high oxygen

partiai pressures (20% a) it \vas determined that the bulk dimision coefficient (D*)

increased with temperature, correspondhg to an activation energy of 140 Wmol. The

observed activation energy a p e d well with those reported in literam for a lower Sr

content compound of the same formda The surface activation coeficicnt (k*) ais0

increased with temperature with an activation energy of 55 kJ/mol. The observed

activation energy for the surlàce reaction was considerably Iower than those reported in

literature for lower Sr doped equivalents of the same materid. The lowered activation

energy could not be readily explaineci. Factors that could have contributed towards

loweruig the activation energy include higher surface roughness (resulting in increased

surface ma) and surEdce impurities that might heIp the activation of oxygen on the

surface. However. because of the destructive nature of the profilhg method. after-the-fact

anaiysis of the surface was not possible. The same material. Lao.& fieO34, was dso

tested at a lower oxygen partid pressure corresponding to an atmosphere consisting of

C@/Ht = 1 gas (5% a). d t s of that experiment were inconclusive due to non-steady

oxygen stoichiometry during the exchange experiment.

Samples of a second compound La&ro.gCr~Feo.gO~d, were also subjected to the same

experimental procedure for high oxygen partial pressures. It was expected that for these

samples the oxygen transport wodd suffer a slight Ioss because of the addition of the less

ion-conductive chromium into the lattice. however. d t s showed a slight gain in the

diffiision and surface activation coefficients. For the diffusion coefficient the activation

energy (145 kJ/mol) was close to the one obtained for the non-chromium containine

sample. The surface activation exhibited a sUailar gain but the activation energy for the

surface reaction was considerably larger (79 kJ/mol). The overall gain in the values for

the diffusion and d a c e activation coefficients is probably due to smaller grain sizes,

which yield more grain boundaries. The same composition is reported in literature to

exhibit varying oxygen ionic conducîivity and surface activity at different grain sizes.

Duplicate analysis of sarnples made of the sarne composition~method but of a different

batck however. did not replicate the above values for diffusion and surface activation

coefficients, which M e r outiine the sensitivity of the matenai's oxygen transport

properties to the small variations in the preparation conditions.

Samples of La&ro 8Cr~.2Fe~.s0M exposeci to reducing conditions (CO/C02 = 1). which

had been allowed to equilibrate to the low oxygen partial pressures. yielded slightiy

higher diffusion coefficients while the surface reactions gave a lower surface oxygen

isotope ratio. The activation energy obtained (35 kJ/moI) for the d i f i ion coefficient at

the low oxygen partial pressures agreed well with that published in literature for an

analogous composition with Iower Sr levels (Sr = 0.2). However. the enhancement in the

diffusion coefficient (by the low PO2) was very much lower than the ones reported in

literature for the lower Sr doped materiai. The anemic gain can be explained by an

ordering of the oxygen vacancies. which has been obse~ed in iiterature. resulting in

fewer sites king available for mobiiity.

The sitrface reactions in this gas atmosphere are modeled as two exchange pathways:

-Simplew exchange and REDOX exchange. The REDOX exchange demonstrateci similar

gains (Ea = 95 Wmol) with temperature as the high Po* sample. The similarity is due to

h t h d a c e reactions relying on surface oxygen vacancy sites. Despite these two effects

of temperature. the surface reactions did not result in higher surface oxygen ratios at

lower partial pressures of oxygen, which was unexpected. Observations of surface

deposits on the reduced samples pointed at surface modifications that codd account for

the decrease in performance. XPS and EDX adysis of the sampies showed that Sr was

segregating to the surface at the expense of the La The Sr on surface is in the fonn of

carbonates, which at high concentrations could account for the decrease in the surface

reaction rates. Sodium was also observed on the surface of the reduced samples. although

its source is not known. if the sodium segregated h m the material. then it codd also

account for a Iowered surface reaction as part of the system Na-SrC03 as system that has

show selective behavior in ethylene oxidation reaction.

The results hint at an increased effect of sample properties such as grain size and surface

composition on the oxygen transport properties of highly doped perovskite-type

materials. They outline the need to characterize these properties in order to optimize the

materiais for their potential use as MEC membranes. if at high Sr doping levels grain

botmdaries become an important factor in the diffusion of oxygen. then proper

exploration of theic properties c m pave the way for new preparation techniques that yicld

berter performing materials, The observed high surface segregation of Sr in highiydoped

materiais is a h i t h g factor for the proper usage of these materiais in highly reducing

atmosphere. More research into the surface processes that bring about this segregation

and the conesponding effects on the surface reactions wiii help in the development of

new materiais for use as MlEC membranes for use in reducing atmospheres,

6. Recommendations

Based on observations made throughout the duration of this research the foliowing

recomrnendations are made:

AH variables of sample preparation shouid be documented to better mck and

identifj sources that cause variations in the materiai properties.

The grain boundaries of the matend shouid be better examined and if possible

SIMS imaging of the cross-section of an annealeci sample be done to observe the

variation in the isotope ratio with grains and grain boundaries.

Diffusion and surface activation data for intermediate temperatures is

recommended to better obtain activation energies.

ïhe surface of the materiais needs more analysis as to their composition prior and

after reduction. More notably the composition of the surface at various partial

pressures andor experiment times. to track the progress of the segregation

process.

The source of the observed surface sodium needs to be identified.

More dense SIMS analysis wi1l enable closer mode1 fits and hence more accurate

diffusion and surface activation coefficients.

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Appendk A - Mode1 for High Pot Experiments

At equilibrium the flux into and out of the sample are the same:

Jorn = JoOUt = JO

The flux into the sirrface is:

Jo(Cg-Cs) 180 atoms/(cmS)

The flux into the bulk is:

Cs is the concemion of oxygen in the gas. Cs is the concentration of oxygn at the surfxe, D is the self-diffiision coefficient and C(x) is the oxygen concentration as a fiincrian of depth.

The flux into the surface and away fkom the surface (ie. h o the bulk) mus be the same

so we have:

This equation has the bounctary conditiont:

K* is the surface exchange coefficient (cds)

Upon s o h g the following expression is obtained ':

Using this equation the depth profile data fC(x)] can be used to obtain the seIf d i f i i o n coeacient @*) and the srrrface exchange coefficient (k*).

' j, Crank, in: The lM&mafics ofD@tsion, 2"6 Ed. (Oxford University Ress, Oxford, 1975).

Appeiidix A - Coatinued

The following is an example of a program used for the non-iinear least squares fitting

of the S M &ta. It is based on the equations den'ved in the previous page. This code

is for the MathSofi Mathcad Rofessional softwm package (Versions 7 or newer).

Appendix B - Mode1 for CO/C02 Experiments

The following is a cornputer mode1 based on the equations mentioned in the text. It is

wrinen for the EZ Sorve software package*.

Courtesy of C.A. Mims Çimulates CO02 (X-o = 1) :CO(XtO = 01 exchange wrth semi-inf ini te s lab. Time dependence of d i sc re te depth leoe l s a r e tnregrated. Bach CO2 exchanqe 4HSt!npie" and X O O X a r e included) . These both contr ibute acd monicored n a m s s spec t ra l daci durrng the experiment. Uuts a r e mczometers fo: depth, square c e n t x e t e r s fo r surface area, seconds, and aicromales.

/ / ODE for surface -sxs = 180 content of t h i n surface layer t h x k n e s s :S.

SXS ' = ( iko+kxl' (xCO2-SXS~ -Do*<s%s-xO) '2/ ( z s - z O ) /:s

/ / ?as: zone xa20 = O

/ / CSTR equat i ons for qas phase

Z*X02'li-xCO2~ = ~ ~ ~ ' ( ~ C O ~ - ~ X ~ ) + ~ O * ( ~ ' X C O ~ - ~ C O - S X ~ ) ' A ' ~ O ECO'xCO = ko' (xC02-xCO) *A*ro

/ / Parameters Do = 56 / / um"2 s-1 ko = 1 e-6 / / exchanqe r a t e coeffrcrent by redox zeactron ï um s-L kx = -09 / / exchanqe r a t e coef f rc len t by CO2 exchanqe 1 üm s - l zs = 0.010 / / thlckness of surface layer ( t h n t o allow mass balance with gas) :O = 0.30 / / um -thickness of f r r s t l ayer under surface layer H = 1.5 / / expansron f a c t o r f o r succeeding layers (qeometrrc s e r r e s ) ECO2 = 10 / / L U W ~ S-i E C O = = 3 //mis-i A = L / / cm2 ro = 6.8 / / m o l O per (vclume u n i t !nucron*&)

included m Intmducn'on to Chernical Reaction Engineerinn and Kinetics, RW. Missen, C A M i and BA. SaviUe, John Wiley and Sons, Inc, 1999