Nano-powders of Na0.5K0.5NbO3 made by a sol–gel method

7/30/2019 Nano-powders of Na0.5K0.5NbO3 made by a solgel method

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R E S E A R C H P A P E R

Nano-powders of Na0.5K0.5NbO3 made by a solgel method

Anirban Chowdhury Jonathan Bould

Yifan Zhang Craig James Steven J. Milne

Received: 30 September 2008 / Accepted: 21 January 2009 / Published online: 14 February 2009

Springer Science+Business Media B.V. 2009

Abstract Sodium potassium niobate (NKN) nano-

particle powders were synthesised through the thermal

decomposition of a solgel NKN precursor. Powders

and gels were characterised by X-ray diffraction

(XRD), Fourier transform infrared spectroscopy

(FTIR), thermogravimetric analysis (TGA) and trans-

mission electron microscopy (TEM). Hydrated

carbonate phases formed as a result of reaction with

evolved vapours during organic decomposition, and by

reaction of NKN powders with H2O and CO2 on

exposure to air. The primary particle size of thepowders increased from\50 to\250 nm as decom-

position temperatures were raised from 500 to 950 C.

Keywords Nano-powders Solgel

Sodium potassium niobate X-ray diffraction

Fourier transform infrared spectroscopy

Transmission electron microscopy

Synthesis method

Introduction

Over the past few years, environmental concerns

have stimulated interest in developing lead-free

ferroelectric and piezoelectric ceramic compositions

as replacements for lead zirconate titanate. One of the

most promising candidates in this category is a solid

solution series based on sodium potassium niobate

(NKN), Nax

K1-xNbO3, modified by lithium and

tantalum ions (Saito et al. 2004; Guo et al. 2005).For the Na

xK1-xNbO3 system (NKN), composi-

tions around x = 0.5, Na0.5K0.5NbO3, lie in the

vicinity of one of the systems morphotropic phase

boundaries and show the most favourable ferroelec-

tric and piezoelectric parameters (Shirane et al. 1954;Haertling 1967; Egerton and Dillon 1959; Jaeger and

Egerton 1962; Tennery and Hang 1968). Given the

technological potential of NKN-based piezoceramics,

it is important to develop appropriate ceramic fabri-

cation techniques. Bulk ceramics are prepared

traditionally using powders obtained from milling

and calcining mixtures of oxides or compounds such

as carbonates that decompose into oxides at high

temperatures. These calcined powders are then com-

pacted and sintered to form high-density ceramics.

However, in the case of NKN, it is difficult to achievehigh densities using conventional powder processing

methods. There are also problems in avoiding loss of

volatile Na2O and K2O vapours during both calcina-

tion and sintering.

Over recent years, a number of solution-based

powder synthesis routes have been developed as

alternatives to the mixed-oxide route, including co-

precipitation and solgel methods (Smart and Moore

1996). These soft chemistry methods can result in

A. Chowdhury (&) J. Bould Y. Zhang

C. James S. J. Milne

Institute for Materials Research, Houldsworth Building,

University of Leeds, Leeds LS2 9JT, UK

e-mail: [email protected]

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DOI 10.1007/s11051-009-9595-0


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smaller particle sizes and improved chemical unifor-

mity compared to mixed-oxide routes. When applied

to NKN, they could offer reductions in processing

temperatures which would be expected to reduce the

tendency for loss of alkali metal oxides, and to

produce smaller particle size thereby enhancing

densification kinetics.Sodium niobate, NaNbO3, nanopowders have been

reported from the reaction of hydrogen peroxide

solution with sodium and niobium ethoxides (Cheng

et al. 2006). Transmission electron micrographs of

the powder heat treated at 500 C for 1 h revealed

primary particles around 1530 nm in size. Lithium

modified NaNbO3 has been made using Na2CO3 and

Li2CO3 precursors along with ammonium niobium

oxalate, NH4H2[NbO(C2O4)3] 3H2O (Franco et al.

1999). The powders were of a high surface area,

*10 m2 g-1, with equivalent spherical diameters of*130 nm. Powders of NaNbO3 (Nobre et al. 1996)

have also been reported using methods based on a

Pechini-type reaction route, involving citric acid and

ethylene glycol reagents, giving high crystallinity and

high surface area *28 m2 g-1, with equivalent

spherical diameters of*46 nm.

In the present study, we report the synthesis and

properties of Na0.5K0.5NbO3 (NKN) nanopowders

produced via a solgel method involving ethoxides of

sodium, potassium and niobium as precursors, and 2-

methoxyethanol as solvent.

Experimental

Precursor solutions were prepared from commercially

available ethoxides of sodium [CH3CH2ONa], potas-

sium [CH3CH2OK] and niobium [(CH3CH2O)5Nb]

(Aldrich). The ethoxides were stored and handled

under a dry N2 atmosphere in a re-circulating glove

box (Saffron, UK). Chemicals were weighed and

mixed in 2-methoxyethanol [CH3OCH2CH2OH](Aldrich), followed by stirring for 2 h to give a

yellow-coloured solution, referred to as the stock

solution, with a concentration of 0.34 M (in terms of

Nb content).

The stock solution was maintained at 6070 C,

with slow stirring for 4 h. The sample was exposed to

atmospheric moisture, but no deliberate addition of

water was carried out. A sticky resinous gel formed

after standing for a further 3 h at room temperature.

The gel was transferred to an oven and dried at

120 C for a period of 24 h to form a yellow powder.

For each batch, *0.5 g of dried gel powder was

produced; this was ground into a finer powder using

an agate mortar and pestle. The powder was calcined

at different temperatures in order to study phase

development using XRD (Philips APD 1700, Almelo,The Netherlands) with CuK

aradiation. Fourier

transform infrared spectroscopy (FTIR) was carried

out on samples of the NKN powder after calcination

at different temperatures for dwell times of 30 min

(Perkin Elmer Spectrum One FTIR spectrometer).

Spectra were recorded over the wavenumber range

4,0001,000 cm-1. The particle size and morphology

were evaluated using transmission electron micros-

copy (TEM, Philips CM 200 FEGTEM, Eindhoven,

the Netherlands) with an accelerating voltage of

200 kV. Unit cell parameters were calculated using aleast squares refinement programme. For TEM

investigations, powders were suspended in isopropa-

nol, and a drop of this suspension was deposited on a

holey carbon-coated film supported on a 400 mesh

copper grid. Thermogravimetric analysis (TGA) was

conducted in air (Stanton & Redcroft TGA 1000,

London, England). The gel for this purpose was

obtained by drying the sol at 60 C for 4 h. The TGA

furnace was run at 20 C min-1 until 950 C. This

was the maximum working temperature deemed to

avoid significant levels of alkali metal evaporationand consequent damage to the apparatus; the TGA

sample was held at 950 C for 20 min. Surface area

measurements were performed using a 3-point BET

technique (Quantachrome Instruments, Florida,

USA).

Results and discussion

A standard TGA plot of the NKN gel powder shows

decomposition and mass loss to occur in five discretestages, up to a temperature of 750 C, Fig. 1. The

approximate temperature ranges of these decomposi-

tion steps were as follows (the percentage mass loss

for each stage is shown in parenthesis): \130 C

(5.4%); 130350 C (2.0%), 350470 C (1.3%),

470560 C (1.7%) and 560750 C (0.8%). The

total mass loss up to 750 C was therefore around

11% of the original dried gel starting sample. A

small, gradual loss of a further 1 mass % occurred

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from *750 C up to the maximum temperature

studied, 950 C, Fig. 1.

The TGA loss at \130 C is consistent with

evaporation of: ethanol (boiling point = 79 C)

derived from the metal ethoxides; methoxyethanol

solvent (boiling point = 124 C); and water present

in the precursors, or absorbed from the atmosphere.

In order to investigate the thermal decomposition

sequence in more detail, FTIR analysis of the gel

residue after different heat treatments was carried out.Peaks were assigned with reference to standard texts

(Socrates 2001; Williams and Fleming 1990). Helpful

background information on this specific NKN system

was obtained by running FTIR spectra of Na2CO3,

K2CO3 powders (both Aldrich) and a Nb2O5 nano-

sized powder formed from the thermal decomposition

of Nb(OC2H5)5 at 600 C. These are presented in

Fig. 2; the spectra indicate that each of the three

compounds shows a hydrated phase to be present. InNb2O5, this is indicated by FTIR peaks at 3,480 cm

-1

(broad) and 1,630 cm-1, Fig. 2. For NKN-based

ceramics made by conventional mixed-oxide pro-

cessing, it is normal practice to dehydrate the alkali

metal carbonate powders prior to use as they are

known to absorb H2O from the atmosphere (Fig. 2),

but this result demonstrates that it is also advisable to

dehydrate the starting Nb2O5 reagent for the purpose

of attaining accurate control of product composition.

The FTIR spectra of the NKN gel samples after

heating at progressively higher temperatures areshown in Fig. 3. Wavenumbers of key peaks are

listed in Table 1. The FTIR spectrum from the gel

sample dried at 250 C indicates weak, broad peaks

centred *3,300, 1,610, 1,430 and 1,310 cm-1. The

broad absorbance around 3,300 cm-1 is typical of O

H stretch from H2O. Based on the knowledge of other

metal-alkoxy solgel systems (Chowdhury et al.

2008), the TGA steps up to 470 C, are probably,

associated with the thermal decomposition of organic

residues (cleavage of covalent organic bonds). Hence,

organic groups are likely to be present in the 250 Csample. Any CC stretch would overlap with the

lower wavenumber range of the broad OH band. The

Fig. 1 A TGA plot for the dried sodium potassium niobate

(NKN) gel-powders (heated in air). The percentage values in

bracket show the mass change during the respective temper-

ature zones

4000 3500 3000 2500 2000 1500 1000

1775

1550 - 12002965

2855

1500 - 1200

2595

3480

1630

Transmitta

nce

Wavenumber (cm-1)

2488

2448

1775

1063

K2CO

3

Nb2O

5

Na2CO

3

Fig. 2 FTIR plots of (a)

Na2CO3 (Aldrich), (b)K2CO3 (Aldrich) and (c)

Nb2O5 (obtained by drying

the Niobium ethoxide

(Aldrich) precursor at

600 C)

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peaks at 1,430 and 1,310 cm-1 could arise from

combinations of absorbances due to organic func-

tional groups, e.g. asymmetric bending of CH3 groups

(around 1,430 cm-1) and symmetric CH3 bending (at

1,310 cm-1), but could also signify that inorganic

carbonates, or bicarbonates (HCO3-) are present in

the gel (Rojac et al. 2006; Busca and Lorenzelli

1982). Indeed, there is a very strong absorbance in

the range *1,5001,200 cm-1 in the Na2CO3 and

K2CO3 spectra in this region (Fig. 2). A very weakpeak at 1,770 cm-1 is consistent with C=O stretch, of

for example inorganic ester, Fig. 3, but alternatively,

both Na2CO3 and K2CO3 show strong absorptions in

this region (Fig. 2), suggesting the peak could arise

from an inorganic carbonate group.

The small 1,770 cm-1 peak also existed in the

350 C sample, Fig. 3; the other peaks were similar

to those of the 250 C sample. However, the

1,770 cm-1 peak was absent in the 450 C sample.

It is assumed that all organics had decomposed by

*450 C.

At 650 C there were several changes. The1,365 cm-1 peak disappeared and there was a peak

at 1,654 cm-1 as opposed to 1,630 cm-1 at lower

temperatures, Table 1. At 750 C, peaks at 1,520 and

1,215 cm-1 are developed. On increasing the decom-

position temperature to 850 C, the peak at

*1,630 cm-1 could not be distinguished, but 1,430

and 1,215 cm-1 peaks remained, together with OH

stretch at high wavenumbers, indicating a hydrated

phase. The 950 C sample showed no evidence of

secondary carbonated/hydrated phases.

The 1% mass change highlighted by TGA above750 C is most probably due to the final residual

carbonate phase decomposing, but the FTIR spectra

showed hydrated carbonate phases persisted in a

NKN powder sample even after decomposition at

850 C. It is probable that some of the hydrated

carbonates detected in the high temperature FTIR

samples are a consequence of a reaction between the

NKN powders, after thermal decomposition, with

moisture and carbon dioxide in the air during sample

storage, prior to recording the FTIR spectra. The

absence of peaks in FTIR patterns in the 950 Csample may therefore be due to its larger particle

size, as described below, and consequent lower

surface area available to react with atmospheric

vapours.

The peak changes described above are considered

to mark a change from a system containing a mixture

of organic residues and co-existing hydrated carbon-

ates, \450 C, to one where NKN co-exists with

hydrated carbonate phases (450850 C), and finally

single-phase NKN is present (950 C). Variations in

peak positions at temperatures above 550 C signifyslight changes in the composition of the constituent

carbonate species are taking place.

X-ray diffraction (XRD) plots of the calcined

NKN powders are shown in Fig. 4. There was no

evidence of crystallisation in the 350 C powder.

However, a sample heated at 450 C (for 30 min)

was crystalline, exhibiting a pseodocubic XRD

pattern (the broad hump in the background intensity

is due the glass sample holder). Close inspection

4000 3500 3000 2500 2000 1500 1000

1520

13101610

1430

1450

3300

250C

350C

450C

550 C

650C

750 C

850C

950C

1770

1430 1120

1070

1770 1630 1310

1070

1630 1365

1070

1630 1365

1070

1654

1430

1070

16541430

1215

1430

1215

Transm

ittance

Wavenumber (cm-1)

Fig. 3 The Fourier transform infrared spectra of sodium

potassium niobate (NKN) gel-powders heated at various

temperatures

Table 1 Wavenumbers of key peaks of the NKN gel samples

after heating at progressively higher temperatures

Temp (C) Wavenumbers of the key peaks (cm-1)

250 *3,300 (very broad), 1,770, 1,610, 1,430,1,310, 1,120, 1,070

350 *3,300 (very broad), 1,770, 1,630, 1,450,

1,310, 1,070

450 *3,300 (very broad), 1,630, 1,365, 1,070

550 *3,300 (very broad), 1,630, 1,365, 1,070

650 *3,300 (very broad), 1,654, 1,430, 1,070

750 *3,300 (very broad), 1,654, 1,520, 1,430, 1,215

850 *3,300 (faint), 1,430, 1,215

950 *3,300 (faint)

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reveals faint extra peaks at 10.6, 27.5 and 48.2 2h,

in addition to the NKN peaks. In combination with

FTIR results (Fig. 3), the extra XRD peaks most

likely originate from hydrated carbonate phase(s),

although no matches could be found with any metal

carbonate or hydrated phases listed in standard XRD

data files. For samples heated at C550 C, no extra

XRD peaks were detected. Hence the hydrated

carbonate phase(s), indicated in the FTIR patterns,

were present at a level below the XRD detection limit(a few wt%), or present in an amorphous form.

The XRD pattern of the 650950 C samples

showed splitting of some peaks, e.g. the pair of

closely spaced peaks at *22 2h and also the peaks

at *46 2h. The Na0.5K0.5NbO3 phase is known to

be orthorhombic at room temperature (Tennery and

Hang 1968). Initial crystallisation occurred at 450 C,

but peaks in this sample, and the 550 C sample,

were broad and no clear peak splitting was observed.

The patterns for the 650950 C samples in Fig. 4

exhibited peak splitting, although continued peakbroadening made it difficult to resolve closely spaced

peaks.

A variation in the relative intensity of certain

peaks was observed in the temperature range 650

950 C. For alkali niobates, the relative intensity of

pairs of peaks at*22 2h and*45 2h (Fig. 4) can

be indicative of variations in the proportions of

orthorhombic and tetragonal phases. For single-phase

orthorhombic samples, the peak intensity ratio a may

be expressed as a = (I110/I001 ? I220/I002)/2, with a

value ofa = 1.8, whilst for a tetragonal NKN-based

composition, a * 0.5 (Skidmore and Milne 2007).

The a values for the 850 and 950 C samples were

only slightly lower than the expected value for an

orthorhombic phase (Skidmore and Milne 2007), with

experimental values of 1.5 and 1.4 for the 850 and

950 C samples, respectively. Therefore, crystallisa-

tion to predominantly orthorhombic NKN phase is

indicated to occur on heating the precursor gels toC850 C. However, the peak ratio displayed by the

650 and 750 C samples, a * 1, suggests that a

significant amount of tetragonal phase may co-exist

with the orthorhombic phase at intermediate temper-

atures. The tetragonal phase of NKN is

thermodynamically stable above *200 C. Its pres-

ence in a metastable form in the 650 and 750 C

samples could be due to particle size effects.

Compositional non-uniformities in the samples may

also affect peak intensities at intermediate decompo-

sition temperatures.Estimated values of unit cell lattice parameters

were obtained using a least squares refinement

programme, giving values of: a = 5.660 A, b =

5.655 A and c = 3.946 A (calculated for a sample

decomposed at 850 C) with standard deviations of

B0.001 A. These values compare to a = 5.695 A,

b = 5.721 A and c = 3.974 A for a standard ortho-

rhombic NKN pattern (JCPDS, Joint Committee for

Powder Diffraction file number 32-0822).

10 20 30 40 50 60

(022)/(202)

(112)

(131)/(311)

(130)/(221)

(220)

(020)/(200)/(111)(002)

(001)

***

Intensity

(arb.units)

2 (degree)

9500C

8500C

7500C

6500C

5500C

4500C

3500C

(110)

(001

)

(110) (020)/(200)/(111) (2

2

0)

(002)

(130)/

(221)

(1

12)

(131)/(311)

(022)/(202)

Fig. 4 X-ray diffraction

plots for sodium potassium

niobate (NKN) powders

heated at various

temperatures; the *

symbol depicts the extra

phases in the 450 C

sample. The 850 and

950 C patterns are indexed

on the basis of an

orthorhombic system (Joint

Committee for Powder

Diffraction File no. 32-

0822)

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Figure 5 shows TEM images of the NKN pow-ders after heat treatments at 500950 C. As

expected, the particles are much smaller than those

produced by conventional mixed-oxide processing

(Bomlai et al. 2007). The 500 C micrograph

(Fig. 5a) shows most of the primary particles to be

\50 nm in size, but many of the individual primary

particles were clustered into agglomerates. Similar

particle sizes occurred in the 625 C sample,

Fig. 5b. Raising the temperature to 700 C produced

a large increment in size, with particle sizes

increasing to B170 nm, Fig. 5c. Particles up to*250 nm in size were present in the 950 C sample

(Fig. 5d). BET measurements indicated a surface

area of*11.5 m2/g for a powder decomposed at

550 C, decreasing to *2.4 m2/g for a 950 C

sample. Due at least in part to agglomeration

effects, the equivalent spherical diameters calculated

from these values of measured surface areas were

approximately twice the size of the primary particles

observed directly using TEM.

Conclusions

Thermal analysis and FTIR studies indicated that

Na0.5K0.5NbO3 (NKN) precursor gels made from an

ethoxide-based solgel route decomposed to produce

a NKN hydrated carbonate phase at process temper-

atures up to 850 C. There was also some evidence

that hydrated carbonate phases secondary phases

were produced by reaction of the NKN powders with

atmospheric moisture and carbon dioxide. The NKN

particle sizes varied from \50 nm in samples

decomposed at 500 C, to\250 nm for the highesttemperature studied, 950 C. The absence of second-

ary phases formed on exposure to air in powders

produced at 950 C is consistent with their increased

particle size and lower surface area.

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Fig. 5 TEM micrographs

of the sodium potassium

niobate (NKN) powders

obtained by heating the

precursor gel at a 500 C,

b 625 C, c 700 C and

d 950 C

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Nano-powders of Na0.5K0.5NbO3 made by a sol–gel method