CH5716

Processing of Materials

Ceramic Processing

Lecture MC8 –Microstructural Control

Defining Some Boundaries

Shaping

&

Forming

Micro-

structure

Raw

Materials

•Grain size, shape and distribution

in dense ceramics •Mainly considered with respect to

sintering processes

•Here pores are generally bad news

•However porous structures can

often have useful applications

•The ability to control or tailor the

properties of these can aid certain

functionality

Understanding your Requirements

What is it you need to achieve in your layer?

•Dense or porous

•What thickness

•Conductivities

•Firing temperature

•Firing sequences

•Cofired or separate firings

•Any reduction steps

•What is the overall geometry of your prototype

•How will it be tested

•Where does your layer fit in to the device

•Are there possible interactions with adjacent layers

LSCM Layer in IP-SOFC

Bulk of Process development effort went into

stabilising ACC layer to prevent delamination

Porous Structures

Porous structures come in many types

All with varying characteristics

•Porosity Length scales •Micro porosity d <2nm

•Meso porosity 2nm < d <50nm

•Macro porosity d >50nm

•We will be looking at macro porosity which we can control or

influence through ceramic processing

Process Induced Porosity

•Isolated closed pores

•Tortuous interconnected pathways

•More open interconnected structures

•Skeletal structures

•Closed cell foams

•Open cell foams

De

cre

asin

g %

TD

(IUPAC 1994)

Particle Size & Sintering •Well dispersed fine particles will sinter to

a dense body

•Generally this is what we discuss as our

aim when talking about processing

•However this can be modified to give

various porous morphologies

Lowering Firing temperature •Reduction in the amount of sintering that takes

place

•Can result in a network of fine porosity

•Can retain fine grain size with fairly high surface

area

•Good for catalysis or other surface effects

Increasing Particle Size •Reducing level of densification at a given

temperature

•Coarser porosity

•Larger grains, fewer grain boundaries

•Good for mass transport through pores and bulk

ceramic effects (conductance)

Not an excuse for allowing agglomerated systems

These are still bad as they are difficult to predict and control

Can also result in poor rheology

Calcination to Increase Size •Thermal process with the aim of increasing particle size

•Driving force the same as with densification

•Reduction of surface area

•In this case a wide or bimodal size distribution can be useful •Fines will be consumed into larger particle

•Ripening effect

•Overall distribution will move to larger sizes (specially d10 and d50)

•Lower temperature process than sintering •Aim to give enough energy to dive fines to ripen

•Avoid too much sintering of larger particles

•Some hard agglomerates may form between large particles •Gentle mill post calcination will sort this

•Loose packed powder in crucible or on setter plate sufficient

Pore Formers •Fugitive material added to green body

•May be extra binder to reduce green density and so extent of densification •This can impact rheology of tape slips or inks (reduction in viscosity)

•Limit to how much can be added

•Other options are to introduce additional materials that will burn out on firing •Must burn out cleanly with no residue or contamination

•Hydrocarbon polymers ( eg polystyrene or PMMA) often used •Well controlled particle morphologies predictable pore structure

•No good in organic systems - will dissolve

•Other materials use in organic media •Graphite – platelet

•Glassy carbon – spherical

•Starch based materials also used depends on form and solvent system

•Any pore former will impart specific pore structures depending on their original

morphologies (templating)

•Volume effects must be accounted for in slurry formulations •Additional solids fractions

Spherical glassy carbon used in this layer

Graphite (platelet) used in this layer

Templating polymer ceramic

Ceramic coating

polymer

mixing

Forming & drying

Burn out &

Firing

Final porous

Structure

SEM Image of Templated Structure

•Utilises the pore former in a precise fashion •Replicates morphology

•Highly organised structures can be created

•Templating can take place on different length

scales •eg sol-gel and colloidal suspensions can be used for

mesoporous templating

•Can be combined with macro templating in single

structure

•Good dispersion of ceramic and template

essential

Reticulated Structures •Forms an open cellular structure via a

polymer replica technique

•Based on coating an existing polymeric

sponge type structure (eg PU foam) •Template can be immersed in slurry to saturate

•Removed and excess material drained

•Leaves coated template

•Template burns off during firing •Open structured ceramic remains

•Cellular struts hollow due to template burn out

Applications Catalyst supports

Filters

High stiffness low density structures

Bio ceramics – artificial bone

Reticulated ceramic foam

Cross section of cross member showing

hollow structure A.R.Studart, V.T. Gonzenbach, E. Tervoort, L.J. Gauckler J.Am.Ceram.Soc. 89, 6, 1771, (2006)

Ceramic Foams •Forms a cellular structure through the

introduction of gas into a liquid •Chemical blowing agents (thermal)

•Reactive outgassing

•Physical mixing

•Stability of cell walls important •Control of surface tension

•Use of colloidal surfactants at liquid - air

interface

•This form of foam formation avoids burn

out of template material

A.R.Studart, V.T. Gonzenbach, E. Tervoort, L.J. Gauckler J.Am.Ceram.Soc.

89, 6, 1771, (2006)

Microstructures of a) closed and b) open cell

ceramic foams

Layered Structures •The simplest method of grading a structure

•Although not true grading

•Reducing severity of transition by spreading over several steps

•Layers can be deposited many ways •Tape casting

•Screen printing

•Spraying

•Thinner layers better •Spraying preferred as some level of diffuse interface

•Can be messy

•More layers with reduced step change between each

improves level of grading •Deposition of many layers can become cumbersome

•Practical applications limited to 3-4 layers

•Layers of different materials can also be built up and

cofired to create device architecture

Schematic of layer grading

Layered device architecture built

up by screen printing and tape

casting

SEM of layered graded structure by

plasma sparaying Nanyang Tecn University web page

Graded Structures •Allow the smooth transition from one structure or material to

another •In bulk materials this can accommodate changes in CTE between adjacent

materials

•In porous media it allows transition from a fine structure to coarse structure

with minimal sharp interfaces

•Minimised stress and possible failure across material thickness

•Allows multiple functionality in a single structure

•Beneficial for overall performance

•Creating truly graded porous structures can be tricky

•Work on compositional grading also widespread

Schematic of graded cellular structure

Electrophoretic Deposition EPD

•Movement of charged particles suspended in a liquid

under an applied electrical field

•Liquid can be organic or aqueous

•Can deposit thick film layers onto conductive

substrates

•Coating speed and properties function of field

properties and electrical behaviour of liquid and

particles

•Porosity of coating can be altered

by changing conditions of the

electrical field •Illustration shows effect of current pulse

•Similar effects observed for changes in

magnitude of current and voltage

•Suggests that dynamic changes

may lead to graded structures

Freeze Casting

J.D. McCrummen MSc Thesis Montana State Univ. 2008

•Based on standard tape casting process

•Aqueous based solvent system

•Caster modified to include freezing bed

•Ice crystals grow through tape acting as a

pore former

•Once fired creates a skeletal structure for

infiltration of catalytic species

Attraction of microstructure immediately

apparent Natural grading due to the nucleation and growth

of ice crystals

Current area of research

Interest in effects of process variables on

morphology of ice crystals Freezing temperature, casting speed, tape

thickness, solvent systems,

Infiltration •Infiltration is proving a useful tool for

catalytic applications •Relies on a porous skeleton for support

•This can be fabricated by any number of the

ways already discussed

•Can allow for cofiring of complete dense

and porous structural units •Promotes good bonding at interface

•Avoids possible detrimental reactions with

catalytic materials

•Cam allow the use of materials together that

would normally be incompatible

•The porous skeleton is saturated with

solution of metal salts •Complex oxides can be formed insitu from

appropriate mixed salts

•eg LaSrCrMn perovskite from stoichiometric

mixed nitrates

•Formed at relatively low temperature

<1000°C

•Maintains very fine structure (nm scale)

•Excellent catalytic properties

Surface Precipitated Structures

Porous tube wall

•Infiltration not always appropriate for creation of

nanostructures •Where multiple porous materials are in close proximity

infiltrating solution may be too mobile

•In IP-SOFC anode infiltration would lead to cell

shorting through material unintentionally deposited

through porous support tube

•Possible answer is to prewash anode particles in

appropriate solutions •Apply low temperature heat treatment to decompose salts

•Resulting metal oxides will either incorporate into host

particle lattice or coat surface

•On reduction metal will precipitate onto surface as

nanometric scale catalyst particle •Maintains excellent catalytic activity of infiltration

•If catalyst particle ripens, nanostructure can be refreshed by

redox cycle

•Recent developments have concentrated on B-site

doping on A-site deficient provskites •On reduction B-site dopant exsolves onto surface as

nanoparticle

Ni precipitates on surface of LSCM anode particles

Anode

Nickel nanoparticle exsolution observed on surface

of La0.4Ca0.34Ce0.06Ni0.06Ti0.94O3 after reduction at

900°C in 5% H2/Ar for 30 h.

Summary -1 •Reviewed a number of different ceramic process routes •Give you an idea of main process methods

•Focus on Thick film methods •However only scratches the surface- many techniques not covered

•Ink jet printing •Dip coating

•Stereo Lithography (3D printing) •Spin coating •Gel casting

•Slurry spraying •Thermal and plasma spraying

•+ others •Background reading always useful.

•Give more detail •Keep up to date with process developments

Summary -2 This part of lecture course only scratches the surface of green processing

•Many inter-related variables leading to complex systems

•Care and attention to detail central to reliable and reproducible processing

•Experiment & experience still key •Consider your requirements and materials

•One size will not fit all – Solutions will be specific to individual systems

•Learn from any mis-steps

•You must understand the fundamentals of the process its variables and use

this understanding to make sensible adjustments to obtain best results

•Think about the whole process •Issues in final stages may be solved by early stage changes

•Further reading to get information and background is important

•Best source of information journal review and current research papers •Journal of the American Ceramics Society

•International Journal of Applied Ceramic Technology

•Advanced Materials

•Advanced Functional Materials

•Journal of the European Ceramic Society

•Good microstructure vital to exploit your chemistry

•Good route to manufacture vital to getting your discovery out of

the lab