Nanopowders of Na0.5K0.5NbO3 Prepared by the Pechini Method

Anirban Chowdhury,w Samantha O’Callaghan,z Thomas A. Skidmore, Craig James, and Steven J. Milne

Institute for Materials Research, Houldsworth Building, University of Leeds, Leeds LS2 9JT, U.K.

A Pechini-based chemical synthesis route was used to producepowders of Na0.5K0.5NbO3 (NKN). The thermochemistry of thegel was investigated using thermogravimetric analysis-fouriertransform infrared (TGA-FTIR) evolved gas analysis; in addi-tion, powder FTIR was used to analyze the gel residues afterdifferent heat treatments. The final decomposition of the organiccomponents occurred at B6501C. However, hydrated–carbon-ated secondary phase(s) were detected by FTIR in powders thathad been heated at 7001C, indicating that the NKN nanopow-ders are susceptible to a reaction with atmospheric moisture andcarbon dioxide. The NKN particle sizes were in the range 50–150 nm after decomposition at 7001C.

I. Introduction

LEAD zirconate titanate (PZT) and related compositions havedominated the piezoelectric ceramic market for several de-

cades. However, the difficulty in removing lead during compo-nent recycling is a serious environmental issue. Consequently,there has been a drive to develop environmentally benignlead-free piezoceramics with properties comparable to those oftraditional lead-based materials. One of the most promisingalternatives is a solid solution series based on sodium potassiumniobate. The NaxK1�xNbO3 system has a perovskite structure forcompositions, xo0.973. Compositions around Na0.5K0.5NbO3

(NKN) lie in the vicinity of one of the system’s morphotropicphase boundaries, and show the most favorable ferroelectricparameters.1–5 Compositions of NKN modified by lithium andtantalum ions constitute one of the most promising known lead-free piezoelectric systems.6,7

Given the technological potential of NKN-based lead-freepiezoceramics, it is important to develop suitable ceramic fab-rication strategies. In the case of NKN, it is difficult to achievehigh densities using conventional processing, involving powdersobtained from calcination of mixtures of oxides or carbonates.There are also problems in avoiding loss of volatile Na2O andK2O vapors during both calcination and sintering. A number ofsolution-based powder synthesis routes have been developedand applied to other ceramic systems as alternatives to the mixedoxide route.8 These ‘‘soft chemistry’’ methods offer the potentialfor smaller particle sizes, lower processing temperatures and im-proved chemical uniformity compared with mixed-oxide routes.In addition to anticipated improvements in NKN ceramic dens-ification kinetics, the consequent reductions in processing tem-peratures, should reduce the tendency for loss of alkali metaloxides, and result in tighter compositional control.

Solution-based powder synthesis routes include methodsbased on coprecipitation, hydrothermal synthesis, and sol–gel.The Pechini method is another widely used solution route9: a

mixture of metal salts is dissolved in citric acid, followed by areaction with ethylene glycol to induce esterification reactionsleading to the formation of a polymeric gel. An inorganic pow-der is then obtained by thermally decomposing the gel. Thisapproach has the advantage over precipitation reactions in thatall the components are retained in the intermediate polymericproduct, thereby promoting chemical homogeneity in the finalpowder. Over the years, the Pechini method, originally devisedfor BaTiO3, has been successfully applied to a range of ferro-electric systems, for example Pb(Zr0.52Ti0.48)O3,

10 PbTiO3,11,12

(Pb,La)TiO3,13 and (Pb,La)(Zr,Ti)O3.

14 Powders of Na1�xLixNbO3,

15 NaNbO3,16 KNbO3, and KNbO3–BaTiO3

17

have also been reported using the Pechini route. A Pechini routehas been reported for the deposition of thin films of NKN,using sodium and potassium acetates, together with niobiumchloride.18

In this study, NKN powders have been synthesized from anaqueous precursor solution of ammonium niobate oxalate, so-dium nitrate, and potassium nitrate, using the Pechini method.The thermal decomposition of the polymeric resin has beencharacterized using thermogravimetric analysis-fourier trans-form infrared (TGA-FTIR) evolved gas analysis and normalpowder FTIR. Information on phase development and particlesize is also reported.

Heated to 70°C for 2 h

Ammonium hydroxidesolution slowly added until pH ~ 4

Mixed withconstant stirring

Mixed withconstant stirring

2 mol of ANO in 15 ml distilled water

20 mol of CA in distilled water

Resultant clear solution 1 mol of sodium nitrate and

1 mol of potassium nitrate

in 15 ml distilled water

Resultant clear solution

13 mol of EG added to the resultant solution

White gel

Fig. 1. Schematic preparation of the Na0.5K0.5NbO3 precursor.ANO, ammonium niobate oxalate hydrate (C4H4NNbO9 � xH2O),CA, citric acid (HOC(COOH)(CH2COOH)2); EG, ethylene glycol(HOCH2CH2OH).

J. Nino—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: preac@leeds.ac.ukzPresent address: Department of Materials Science and Metallurgy, University of Cam-

bridge, Cambridge CB2 3QZ, U.K.

Manuscript No. 24931. Received July 2, 2008; approved December 19, 2008.

Journal

J. Am. Ceram. Soc., 92 [3] 758–761 (2009)

DOI: 10.1111/j.1551-2916.2009.02950.x

r 2009 The American Ceramic Society

758

II. Experimental Procedure

Precursors from Sigma-Aldrich of sodium nitrate (NaNO3,� 99%), potasium nitrate (KNO3, � 99%), ammonium nio-bate oxalate hydrate (C4H4NNbO9 � xH2O, 99.99%), citric acid[HOC(COOH)(CH2COOH)2, 99.5%], and ethylene glycol(HOCH2CH2OH, 99%), were used to prepare the precursorgel. The key steps are summarized in Fig. 1.

TGA of the white gel intermediate was conducted in air(Stanton & Redcroft TGA 1000, London, UK). The TGA fur-nace was run at 201C/min till 9001C followed by a holding pe-riod of 20 min at 9001C. The gases that evoled from the sampleat different time/temperature combinations were analyzed byFTIR spectroscopy (NICOLET FTIR 560 spectrometer, Madi-son, WI) over the wavenumber range, 4000–400 cm�1. In sep-arate experiments, FTIR spectra of powders were recorded forgel samples heated at different temperatures (Perkin Elmer Spec-trum One FTIR spectrometer, CT). The phase content wasmonitored using X-ray powder diffraction (Philips APD 1700,Almelo, the Netherlands) with CuKa radiation (l5 1.5418 A).The particle size and shape of NKN powder were observed us-ing scanning electron microscopy (SEM) (LEO 1530 FEGSEM,Cambridge, U.K.).

III. Results and Discussion

A standard TGA plot of the dried NKN gel heated in air, Fig. 2,shows the main mass loss as occuring below 6501C, equating to

90 mass % of the original gel sample. The mass loss stages, fol-lowing the 701C pretreatment, occurred over the approximatetemperature ranges of 1201–2401C, 2401–5201C, and 5201–6501C, corresponding to 32, 33, and 25 mass% loss, respectively.

The corresponding Gram–Schmidt plot, showing the varia-tion in the total absorbance of evolved gasses as a function oftime/temperature, Fig. 3, indicated peaks at 2251, 3851, 4601,

100 200 300 400 500 600 700 800 9000

20

40

60

80

100

mas

s%

Temperature (°C)

Fig. 2. Thermogravimetric analysis of the Na0.5K0.5NbO3 precursor gelprepare by the Pechini route.

0 10 20 30 40 50 60 700.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce

Time (min)

200 400 600 800 900 900

Temperature (°C)

11.3

min

, 225

°C

19.3

min

, 385

°C

23

min

, 460

°C

26.8 min, 535 °C

Fig. 3. Gram–Schmidt plot of the NKN precursor gel prepare by thePechini route.

0

1

2

3

4

3730

3590

(d) 26.8 min, 535°C

675

2360

0.0

0.1

0.2

0.3

0.4

(b) 19.3 min, 385°C

2180

17502105

Abs

orba

nce

(arb

. uni

ts)

37303595

2355

2310

1510

670

4000 3500 3000 2500 2000 1500 1000 500

0.0

0.1

0.2

0.3

0.4

0.5

1750

21052180

151037303595

Wavenumber (cm–1)

2355

2310

670

(a) 11.3 min, 225°C

0.0

0.1

0.2

0.3(c) 23 min, 460°C

35953730

2355

2310

1750 1510

670

Fig. 4. Fourier transform infrared spectrum of the gases evolved fromthe Na0.5K0.5NbO3 precursor gel at time/temperatures corresponding topeak temperatures in Fig 3: (a) 2251C, (b) 3851C, (c) 4601C, and (d)5351C. Note: Order of magnitude difference in the absorbance scale for5351C.

March 2009 Communications of the American Ceramic Society 759

and 5351C. Hence, the TGA mass loss between 2401 and 5201Cis in fact a two-stage process. Details of the vapor compositionat key stages of the decomposition reaction, corresponding topeak temperatures in the Gram–Schmidt plot (Fig. 3), are indi-cated in the evolved gas FTIR spectra shown in Fig. 4. Peakassignments were carried out with reference to standard texts19–21

and instrument software (Nicolet Vapour Phase Library,Madison, WI). The principal features are described below.

The FTIR spectrum for the first Gram–Schmidt peak tem-perature at 2251C indicates CO2 formation, with peaks at 2355and 2310 cm�1 (stretching mode) and around 670 cm�1 (bend-ing mode). There is also an indication of some carbon monoxideevolution (faint peaks at 2180 and 2105 cm�1). Absorbance inthe range B3600–3700 cm�1 is consistent with the O–H stretchof H2O, with bending mode absorbance at B1510 cm�1. Or-ganic vapors are indicated by the faint broad absorbance in thecarbonyl C5O stretch region (B1750 cm�1), implying, for ex-ample, organic esters, ketones, or aldehydes. The FTIR spectrafor temperatures of 3851 and 4601C are similar to 2251C. Thevapor sample corresponding to the fourth Gram–Scmidt peak,at 5351C, shows the most intense CO2 and water absorbance ofany spectrum: no organic vapors were detected. There was avery faint CO2 absorbance (only) in the 6201C vapor sample, butno vapors were detected from a sample heated at 6501C, inagreement with the TGA results (Fig. 2). Hence, decompositionof the polymeric citric acid–ethylene glycol gel was completed attemperatures r6501C. The combination of CO2 and H2O at alltemperatures indicates combustion to be the principal gel de-composition mechanism.

Further information on the decomposition process was ob-tained by carrying out FTIR analysis on the solid residue afterdifferent heat treatments (30-min dwell time) as shown in Fig. 5.The plot at 4501C shows a small peak at 1050 cm�1 probablydue to the C–O stretching vibration typical of C–OH groups, forexample of an ester or alcohol. The peak around 1340 cm�1 istypical of CH3 bending modes, while the broad peak around1600 cm�1 may comprise alkene C5C stretching, carbonylstretching (at its upper wavenumber range). It could also in-clude absorbance due to the metal carbonate species present inthe partially decomposed gels as both Na2CO3 and K2CO3 showstrong absorbance in the B1600 cm�1 region. A broad O–H

stretching peak is also present. Heating the gel at 5001C pro-duced little change in the FTIR pattern. By 7001C, the FTIRpowder spectrum showed no evidence of organic residues. Therewas a weak absorbance in the O–H stretch region, and alsoaround 1600 and 1410 cm�1 for the 7001C sample, which, at thistemperature, could indicate inorganic hydrated–carbonated(hydoxycarbonate) phase(s). The preceding FTIR evolved gasanalysis did not show evolution of any vapors from the sampleat this temperature (7001C). Hence, the probable source of hy-drate–carbonate formation is a reaction of the inorganic (fullydecomposed) NKN powder with atmospheric H2O and CO2 onexposure to air during cooling and/or storage. Heating the pow-ders at 11001C did not fully eliminate the hydrated carbonateFTIR peaks, although they were much less intense that in the7001C powder. The lower intensity is attributable to an increasein the particle size and a decrease in the surface area in powdersheated at 11001C.

In a separate experiment, a sample of ammonium oxalateniobate was heated in air and the powder residue was analyzedby FTIR. The powder spectrum, after heating at 6001C for 30min, also showed evidence of hydrates and carbonates. Similarpeaks, of reduced intensity, were present in a sample heated at9501C. Again, full decomposition of organic (oxalate) groupscan be expected at such a temperature. This result demonstratesthat Nb2O5 is itself prone to hydration and carbonation on ex-posure to air.

4000 3500 3000 2500 2000 1500 1000 500

Tra

nsm

ittan

ce

3430

3430

13401605

1315

141016153430

Wavenumber (cm–1)

450°C

500°C

700°C

1100°C

1605

1050

Fig. 5. Fourier transform infrared spectra of the powders produced byheat treatments between 4501 and 11001C.

(a) citrate (b) itaconate

Fig. 6. The proposed structure of (a) barium titanium citrate and (b)the itaconate derivative formed by the thermal decomposition of bariumtitanium citrate at 2301C, after Hennings and Mayr.22

10 20 30 40 50 60

(220)(211)

(210)

Inte

nsity

(ar

b.un

its)

° 2θ

(100)

(110)

(111)

(200)

(221)

Fig. 7. X-ray diffraction pattern of powder produced at 7001C.

Fig. 8. Scanning electron micrograph of Na0.5K0.5NbO3 powder pro-duced at 7001C.

760 Communications of the American Ceramic Society Vol. 92, No. 3

It has been suggested in the literature that citrate salts of Baand Ti, on heating, decompose by initial dehydration atB2101C, followed by loss of CO2 at 2301C to convert the ci-trate to an itaconate complex, Fig. 6.22 It is plausible that acomparable citrate decomposition step occurs in the more com-plex NKN citrate–glycolate–oxalate gel over the temperaturerange 1201–2401C. The accompanying evolution of organic va-pors (e.g., esters, ketones, and aldehydes) over this temperaturerange in the NKN systemmay be linked to decomposition of theglycolate and oxalate components of the gel. The FTIR dataindicate that further molecular changes take place in the NKNgel between 2251 and 4601C, which also release CO2, H2O, andorganic vapors. The precise decomposition reactions of the mul-ticomponent NKN gel are likely to be complex and involve anumber of side reactions. Based on FTIR data, it is not possibleto identify the full molecular structure of the species present inthe gels at different stages of thermal decomposition. How-ever, the present results illustrate the general trends, and confirmthat the final decomposition to inorganic NKN powder is com-pleted by 6501C.

An example of an XRD pattern of an NKN powder calcinedat 7001C is shown in Fig. 7. It confirms that the powders are wellcrystallized; the pattern is indexed on the basis of a pseudo-cubicsystem. There was no evidence of a secondary phase; hence, thehydrate-carbonate indicated in the FTIR pattern of this powderwas present in a quantity below the XRD detection limit (a few%) and/or is amorphous. SEM indicated the size range of theprimary particles in the 7001C powder to be around 50–150 nm,Fig. 8, but particles are agglomerated into micrometer-sizedunits exhibiting significant neck growth. The presence of strongagglomerates would distract from the full potential of the nano-particles in terms of enhanced sintering behavior. A number ofparticles exhibited a cuboid morphology, Fig. 8, which has beenpreviously observed in NKN powders synthesized by mixed-ox-ide processing.23

IV. Conclusion

Powders of NKN have been prepared by a Pechini-based routeusing a water-soluble ammonium oxalate niobium complex, to-gether with sodium and potassium nitrates. Thermal analysis,along with FTIR of the intermediate gel product, indicated fourstages in the thermolysis reaction, each involving oxidation re-actions to produce CO2 and H2O vapors. The final decompo-sition stage in air was completed atB6501C. NKN particle sizeswere 50–150 nm for a sample heat treated at 7001C. However,analysis of the NKN powders by FTIR indicates that they aresusceptible to hydration and carbonation on exposure to air.

References

1G. Shirane, R. Newnham, and R. Pepinsky, ‘‘Dielectric Properties and PhaseTransitions of NaNbO3 and (Na,K)NbO3,’’ Phys. Rev., 96, 581–8 (1954).

2G. H. Haertling, ‘‘Properties of Hot-Pressed Ferroelectric Alkali Niobate Ce-ramics,’’ J. Am. Ceram. Soc., 50 [6] 329–30 (1967).

3L. Egerton and D. M. Dillon, ‘‘Piezoelectric and Dielectric Properties of Ce-ramics in the System of Potassium–Sodium Niobate,’’ J. Am. Ceram. Soc., 42,438–42 (1959).

4R. E. Jaeger and L. Egerton, ‘‘Hot Pressing of Potassium Sodium Niobates,’’ J.Am. Ceram. Soc., 45, 209–13 (1962).

5V. J. Tennery and K.W. Hang, ‘‘Thermal and X-Ray Diffraction Studies of theSodium Niobate(V)-Potassium Niobate(V) System,’’ J. Appl. Phys., 39 [10] 4749–53 (1968).

6Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T.Nagaya, and M. Nakamura, ‘‘Lead-Free Piezoceramics,’’ Nature, 432 [7013] 84–7(2004).

7Y. Guo, K.-I. Kakimoto, and H. Ohsato, ‘‘(Na0.5K0.5)NbO3-LiTaO3 Lead-Free Piezoelectric Ceramics,’’ Mater. Lett., 59 [2–3] 241–4 (2005).

8L. Smart and E. Moore, Solid State Chemistry: An Introduction, 2nd edition,Nelson Thornes Ltd., London, 1996, 374pp.

9M. Pechini U.S. Patent number 3.330.697, 1967.10A. Abreu, S. M. Zanetti, M. A. S. Oliveira, and G. P. Thim, ‘‘Effect of Urea

on Lead Zirconate Titanate-Pb(Zr0.52Ti0.48)O3-Nanopowders Synthesized by thePechini Method,’’ J. Eur. Ceram. Soc., 25 [5] 743–8 (2005).

11E. C. Paris, E. R. Leite, E. Longo, and J. A. Varela, ‘‘Synthesis of PbTiO3 byUse of Polymeric Precursors,’’ Mater. Lett., 37 [1–2] 1–5 (1998).

12E. R. Leite, E. C. Paris, E. Longo, and J. A. Varela, ‘‘Direct Amorphous-to-Cubic Perovskite Phase Transformation for Lead Titanate,’’ J. Am. Ceram. Soc.,83 [6] 1539–41 (2000).

13J. H. G. Rangel, P. R. G. Goncalves Jr., M. M. Oliveira, M. I. B. Bernardi, E.Longo, L. E. B. Soledade, I. M. G. Santos, and A. G. Souza, ‘‘NanometricPb1�xLaxTiO3 (x5 0, 0.13 and 0.27) Powders Obtained by the Polymeric Precur-sor Method,’’ Mater. Res. Bull., 43 [4] 825–35 (2008).

14B. D. Stojanovic, M. A. Zaghete, C. O. Paiva-Santos,M. Cilense, R.Magnani,E. Longo, and J. A. Varela, ‘‘Hot-Pressed 9.5/65/35 PLZT Prepared by the Poly-meric Precursor Method,’’ Ceram. Int., 26 [6] 625–30 (2000).

15R. C. R. Franco, E. R. Camargo, M. A. L. Nobre, E. R. Leite, E. Longo, andJ. A. Varela, ‘‘Dielectric Properties of Na1�xLixNbO3 Ceramics from PowdersObtained by Chemical Synthesis,’’ Ceram. Int., 25 [5] 455–60 (1999).

16M. A. L. Nobre, E. Longo, E. R. Leite, and J. A. Varela, ‘‘Synthesis andSintering of Ultrafine NaNbO3 Powder by Use of Polymeric Precursors,’’ Mater.Lett., 28 [1–3] 215–20 (1996).

17I. Pribosic, D. Makovec, and M. Drofenik, ‘‘Chemical Synthesis of KNbO3

and KNbO3–BaTiO3 Ceramics,’’ J. Eur. Ceram. Soc., 25 [12] 2713–7 (2005).18F. Soederlind, P.-O. Kaell, and U. Helmersson, ‘‘Sol–Gel Synthesis and Char-

acterization of Na0.5K0.5NbO3 Thin Films,’’ J. Cryst. Growth, 281 [2–4] 468–74(2005).

19G. Socrates, Infrared and Raman Characteristic Group Frequencies: Tables andCharts, 3rd edition, Wiley, Chichester, 2001.

20R. M. Silverstein and F. X. Webster, Spectrometric Identification of OrganicCompounds, 6th edition, Wiley, New York, 1997.

21D. H. Williams and I. Fleming, Spectroscopic Methods in Organic Chemistry,4th edition, McGraw-Hill, London, 1990.

22D. Hennings and W. Mayr, ‘‘Thermal Decomposition of Barium–TitaniumCitrates into Barium Titanate,’’ J. Solid State Chem., 26 [4] 329–38 (1978).

23P. Bomlai, P. Wichianrat, S. Muensit, and S. J. Milne, ‘‘Effect of CalcinationConditions and Excess Alkali Carbonate on the Phase Formation and ParticleMorphology of Na0.5K0.5NbO3 Powders,’’ J. Am. Ceram. Soc., 90 [5] 1650–5(2007). &

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