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Na0.5K0.5NbO3 and 0.9Na0.5K0.5NbO3–0.1Bi0.5Na0.5TiO3 nanocrystalline powders synthesized by low-temperature solid-state reaction
Advanced Powder Technology 24 (2013) 908–912
Contents lists available at SciVerse ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Paper
Na0.5K0.5NbO3 and 0.9Na0.5K0.5NbO3–0.1Bi0.5Na0.5TiO3 nanocrystalline powders synthesized by low-temperature solid-state reaction Laijun Liu ⇑, Shaoying Zheng, Ruijing Huang, Danping Shi, Yanmin Huang, Shuangshuang Wu, Yunhua Li, Liang Fang, Changzheng Hu State Key Laboratory Breeding Base of Non-ferrous Metal and Characteristic Materials Processing, School of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China
a r t i c l e
i n f o
Article history: Received 4 November 2011 Received in revised form 7 September 2012 Accepted 3 January 2013 Available online 4 February 2013 Keywords: Powders Solid-state reaction Calcination Niobates Grain size
a b s t r a c t Nanocrystalline powders of K0.5Na0.5NbO3 (KNN) and 0.9Na0.5K0.5NbO3–0.1Bi0.5Na0.5TiO3 (KNN–BNT) have been prepared using a low-temperature solid-state reaction. Phase development of the powders incurred during various calcination temperatures was examined by X-ray diffraction (XRD). Crystallite size and particle morphology of KNN powders were examined by XRD and transmission electron microscopy, respectively. Perovskite phase was formed at the temperature as low as 500 °C, and the average crystallite size of KNN powders depended on calcination temperature. In addition, the crystalline structure of KNN powders tended to change from tetragonal symmetry to orthorhombic symmetry with increase in crystallite size. Similar results were obtained in KNN–BNT system. The developed method is well suited for the mass production of niobate nanocrystalline powders due to its simplicity and low cost. Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Alkaline niobates, such as KxNa1xNbO3 based ceramics, are considered to be promising lead-free piezoelectric materials [1]. However, KxNa1xNbO3 based ceramics present many drawbacks, for example, volatility of alkaline elements [2], high sensitivity of electrical properties to stoichiometry, difficulty to control the material composition, and complex densification processes [3]. In order to reduce the influence of these problems, nanocrystalline powders of alkaline niobates are needed to improve the sinterability and to prepare dense piezoelectric ceramics. Nanocrystalline K0.5Na0.5NbO3 (KNN) based compositions have been obtained through various soft chemistry routes, such as co-precipitation, sol–gel [4], Pechini [5] and microemulsion [6]. However, most of the chemical methods are complex and expensive. Solid-state reaction technique is a low-cost technique and requires a simple processing and is well suited for mass production, which also has been extensively used to prepare KNN-based ceramic powders. However, the calcined temperature of KNN-based ceramic powders as high as 900 °C are required [1–3]. Therefore, development of a low-temperature solid-state reaction method for the production of KNN-based ceramic powders should be very important on economizing energy and reduction cost. ⇑ Corresponding author. Tel.: +86 7735896290; fax: +86 7735896671. E-mail address: [email protected] (L. Liu).
The present paper describes a solid-state reaction technique for the preparation of nanocrystalline powders of KNN and 0.9Na0.5K0.5NbO3–0.1Bi0.5Na0.5TiO3 (KNN–BNT) solid solution. The subject of this work is to study the influence of calcination temperature on particle characteristic of the compositions, thus to develop an optimized process to obtain a low–cost production. 2. Experimental procedure Solid-state reaction method was used to prepare K0.5Na0.5NbO3 (KNN) and 0.9K0.5Na0.5NbO3–0.1Bi0.5Na0.5TiO3 (KNN–BNT) compositions. Na2CO3 (99.9%), K2CO3 (99.9%), Nb2O5 (99.5%) TiO2 (99.5%) and Bi2O3 (99.9%) were used as starting materials. All of the materials were dried at 200 °C to remove any moisture. The raw materials were milled individually by a planetary ball mill in an anhydrous alcohol for 8 h. Then these powders were weighted and mixed and milled again using 1–5 mm size ZrO2 balls in ethanol medium for 8 h. After drying the mixture, a thermal treatment at temperatures between 500 and 1000 °C for 2 h with a heating rate of 5 °C/min was followed. Simultaneous thermogravimetric and differential thermal analyses were carried out on the initial mixture before the calcination process using Netzsch STA 449C analyzer. Around 50 mg of powder was placed in a Pt/Rh crucible and heated up to 1000 °C with a heating rate of 5 °C/min. The measurements were performed in a flowing air atmosphere. Infrared spectra were recorded using
0921-8831/$ - see front matter Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2013.01.001
L. Liu et al. / Advanced Powder Technology 24 (2013) 908–912
a NEXUS 470FT-IR spectrometer. A few milligrams of samples were mixed in an agate mortar, placed between KBr pellets and recorded from 4000 to 400 cm1. Samples was analyzed by X-ray diffraction (XRD), carried out with a PANalytical X’Pert PRO X-ray diffractometer with Cu Ka (k = 0.15418 nm) incident radiation. Raman spectra were recorded for the powders at room temperature using a Jobin Yvon T64000 spectrometer. An Ar+ laser with 514.5 nm wavelength and <50 mW power at the samples (BeamLok 2080, Spectra-Physics) was used for sample excitation. Particle size and morphology of the calcined powders were evaluated using secondary electrons images of Field Emission Scanning Electron Microscopy, FE-SEM (Hitachi S-4800) and Transmission Electron Microscopy (TEM, Hitachi H-7100 175) with an accelerating voltage of 120 kV. Powders were suspended in isopropanol, and a drop of this suspension was deposited on a holey carbon-coated film supported on a 400-mesh copper grid.
3. Results and discussion Fig. 1 shows the results of TG/DSC for the KNN initial mixture. Four weight loss peaks can be observed on the derivative of the TG curve at 50, 80, 160, and 680 °C, associated with endothermic peaks. The low temperature weight losses (7.5 wt.%) occurring at 50 and 80 °C are attributed to the removal of environmental moisture absorbed gas and water. The weight loss at 160 °C is related to simultaneous losses of H2O and CO2 [7]. The decomposition of AHCO3 to A2CO3 (where A are K, Na) occurs at 255 °C; 2 wt.% of AHCO3 is formed after the milling from the H2O and CO2 present in the atmosphere. The endothermic effects at 300 and 480 °C are consistent with the polymorphic transition of A2CO3 [8]. The main weight losses occur in a narrow temperature range between 400 and 750 °C. At temperatures above 700 °C, the weight loss produces by decomposition of carbonates has been completed and no further weight losses are observed, indicating that the final composition of the samples correspond to the nominal one. The TG/DSC results indicate that the calcination should be performed at temperatures 700–800 °C. In order to evaluate the effect of calcination temperature on the characteristic of KNN powders, calcination temperatures between 500 and 1000 °C also have been selected. FT-IR spectra of the KNN initial mixture on the region between 4000 and 400 cm1 are shown in Fig. 2. They show seven main absorption bands at 3462, 1640 (double peak), 1464, 1328, 843, 700 and 505 cm1. The spectral gap in the region of 3460– 3520 cm1 is caused by the depletion of the IR laser power by water impurities in the DFG crystal [9]. The absorption band at
Fig. 1. TG–DTA curves of the KNN initial mixture.
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1640 cm1 can be attributed to the in-plane bending vibration of H2O [4,5]. The peak appears in every IR spectrum due to adsorbed water on the surface of every sample from environmental moisture. The band at 1464 cm1 can be assigned to the C–O asymmetrical stretching and the group of bands at 843 cm1 due to the CO2 out-of-plane symmetric deformation of the carbonate 3 group [8]. The band at 1328 cm1 can be attributed to HCO3 ions [10].The bands appearing at 700 and 505 cm1 could be ascribed to the characteristic vibration of Nb–O. IR spectra of the KNN initial mixture treated thermally between 500 and 800 °C show a complete disappearance of the absorption band at 1464 cm1 (in Fig. 2). The broad strong band centered at 650 cm1 indicates the formation of the perovskite phase [11]. The IR results are not in good agreement with the ones observed by TG/DSC, where the weight loss corresponding to the decomposition of carbonates completes at 680 °C. It is suggested that the increase in dwelling time can significantly improve the content of KNN. XRD patterns of the KNN initial mixture calcined at different temperatures are shown in Fig. 3a. The main phase corresponds to perovskite structure (JCPDS-ICDD 77-0038), but a trace of secondary phase K2CO3 (JCPDS-ICDD 87-0730) can be observed at 500 °C due to decomposition reaction has not completed. Singlephase perovskite can be obtained by increasing the calcination temperature. However, impurity phase K5.75Nb10.85O30 (JCPDSICDD 38-0297) presents again for heat treatment at 1000 °C due to volatilization of sodium. XRD patterns show a coexistence of tetragonal and orthorhombic, which is revealed by the relative intensities of (1 0 0) and (0 0 1) peaks diffraction peaks (seen in Fig. 3b). Moreover, with the increase in calcination temperature, an increase of the orthorhombic phase in relation to the tetragonal phase is observed. Because of the small grain size in nano-region, it can be concluded that a size-induced structure phase transition is responsible for the appearance of tetragonal symmetry. The similar feature of size-induced structure transition can be observed in KNN derived by wet-chemical method [11,12] or other perovskite systems [13,14]. Crystallite size of the KNN powders is calculated from the full width at half maximum of the diffraction peaks by using the Scherrer’s equation:
D¼
kk B cos h
where D is the crystallite size, k is the X-ray wavelength, B is the full width at half maximum of the diffraction line, h is the angle of diffraction, k is a constant (having the value 0.9 in our case). Fig. 3c
Fig. 2. FT-IR spectra of the KNN initial mixture and the KNN initial mixture calcined at different temperatures.
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Fig. 4. SEM micrographs of the calcined KNN powders: (a) 500 °C, (b) 800 °C.
Fig. 3. (a) XRD patterns of the KNN initial mixture calcined for 2 h at different temperatures. (b) Detail of the XRD on the region corresponding to (0 0 1) and (1 0 0) diffraction peaks. (c) Evolution of the crystallite size as a function of the calcination temperature.
shows the crystallite size as a function of calcination temperature. The average crystallite size of KNN powders varied in the range of 20–80 nm. Fig. 4 shows the morphology of the KNN powders calcined at 500 °C (a) and 850 °C (b) during 2 h. The calcined powders are composed of small plate-like particles with sizes of 50–150 nm that form strongly bound agglomerates of 500 nm (no sintering necks can be observed between individual particles). It indicates that nanocrystallline powder is obtained by the solid-state reaction process at low temperatures (600 °C). Fig. 5 shows representative TEM image of KNN powders calcined at different temperatures. The images show that the powders are agglomeration, consisting of a large amount of crystallites. The change of particle size is in good agreement with the results calculated by XRD patterns. Therefore, the observed particles are composed of crystallites forming polycrystalline parcels due to the calculated crystallite size is considerable smaller than the observed particle size. The Raman spectroscopy is very sensitive to the octahedral tilting associated with the phase structure deformation of perovskite induced by the variation of particle size. The Raman spectra of the KNN powders calcined at temperatures between 500 and 1000 °C are shown in Fig. 6a. The main vibrations are associated to the BO6 perovskite-octahedra for the related system KNN. The small grain size differences do not produce noticeably differences on
the Raman spectra, as experimentally corroborated. The vibrations of the BO6 octahedron consist of 1A1g (m1) + 1Eg (m2) + 2F1u (m3, m4) + F2g (m5) + F2u (m6). Of these vibrations, 1A1g (m1) + 1Eg (m2) + 1F1u (m3) are stretching modes and the rest are bending modes. In particular, A1g (m1) and F2g (m5) are detected as relatively strong scatterings in KNN because of a near-perfect equilateral octahedral symmetry. The Raman bands are quite broad, which is mainly due to the small crystallite size and to the cation disorder on the 12-fold coordinate site [15], as the perovskite structure is not completely formed. A detail of the region between 420 and 740 cm1 is shown in Fig. 6b1, where the spectrum is fitted to the sum of two Lorentzian functions centered at 535 and 611 cm1, ascribed to Eg (m2) and A1g (m1) Raman modes (see Fig. 6b1), respectively. The peak A1g (m1) shifts to higher frequency when the calcination temperature increases above 700 °C (see Fig. 6b2) due to an increase in the strength constant caused by the shortening of the distance between B5+ type ions and their coordinated oxygens. The structure of KNN transforms from tetragonal to orthorhombic symmetry due to the crystallite size effect. These results are in good agreement with the evolution of the crystalline structure observed by XRD. In addition, the full width at half-maximum (FWHW) of A1g (m1) mode is sensitive to the crystallite size. The FWHM values of the calcined KNN powders decrease with the calcination temperature increase (see Fig. 6b3) from 95 cm1 for the sample calcined at 500 °C to 55 cm1 for calcinations at 1000 °C. The narrowing induced by the increase of the calcination temperature is consistent with the increase of particle size during the calcination step, which is evidenced by SEM and TEM micrographs. In order to extend the method to niobate solid solution, a niobate-titanate 0.9K0.5Na0.5NbO3–0.1Bi0.5Na0.5TiO3 (KNN–BNT) sys-
L. Liu et al. / Advanced Powder Technology 24 (2013) 908–912
Fig. 5. TEM micrographs of KNN powders obtained by different thermal treatments at (a) 600 °C and (b) 800 °C.
tem was selected. Fig. 6 presents XRD patterns of the KNN–BNT initial mixture calcined at different temperatures. Similar results have been obtained from pre-milled K2CO3, Na2CO3, Nb2O5, Bi2O3 and TiO2 mixture. A small amount of Bi2O3 (JCPDS-ICDD 51-1161) can be found in the pattern when the mixture is calcined at 500 °C. An impurity phase KTiNbO5 (JCPDS-ICDD 71-1747) presents for thermal treatment at 1000 °C due to volatilizations of bismuth and sodium. Insert in Fig. 7 shows the increase of the crystallite size as a function of the thermal treatment temperature. Average crystallite size of the KNN–BNT varied in the range of 20–70 nm, which is smaller than that of KNN. Generally, the formation temperature of BNT phase is higher than that of KNN, therefore, for the same calcination temperature, the crystallite growth of KNN– BNT is slower than that of KNN. On the other hand, KNN and BNT have different cell parameters, the valences of A site and B site are different, therefore, the atomic diffusion in KNN–BNT solid solution should be slower than that in pure KNN, then leads to a smaller crystallite size. The method of the low-temperature synthesized KNN and KNN–BNT is better than mechanical activation process [16]. Therefore, the developed method presented in this paper will ensure low-temperature, low-cost and high-purity in niobate products, and can be extended to the synthesis of niobate-titanate solid solution.
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Fig. 6. (a) Raman spectra of KNN powders calcined at different temperatures; (b1) Magnified Raman spectra in the wavenumber range from 420 and 740 cm1 as a function of the composition and Lorentzian fits of the individual peaks of the Eg (m2) and A1g (m1) Raman modes. Evolution of Raman shift (b2) and FWHM (b3) of A1g mode in function of calcination temperature are also shown.
Fig. 7. XRD patterns of the KNN–BNT initial mixture calcined for 2 h at different temperatures. Inset shows the evolution of crystallite size as a function of calcination temperature.
4. Conclusions Pure K0.5Na0.5NbO3 and 0.9K0.5Na0.5NbO3–0.1Bi0.5Na0.5TiO3 powders with a range of particle size from 20 nm to 80 nm were
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prepared through dividedly pre-milling raw materials followed by mixing and calcination. The mixture of pre-milling raw materials calcined in the range of 500–800 °C can produce well crystallized particles. The crystalline structure of KNN powders tended to change from tetragonal symmetry to orthorhombic symmetry with increase in crystallite size due to size-induced effect. The defined method is well suited for mass production for niobate or niobate solid solution. Acknowledgments This work was financially supported by the Natural Science Foundation of China (Grant Nos. 11264010, 51002036, 21061004 and 50962004), and by the Natural Science Foundation of Guangxi (Grant No. BA053007) and the Projects of Department of Science and Technology of Guangxi (Grant No. 12118017-13). References [1] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, Nature 432 (2004) 84.
[2] P. Bomlai, P. Wichianrat, S. Muensit, S.J. Milne, J. Am. Ceram. Soc. 90 (2007) 1650–1655. [3] M. Matsubara, T. Yamaguchi, W. Sakamoto, K. Kikuta, T. Yogo, S. Hirano, J. Am. Ceram. Soc. 88 (2005) 1190–1196. [4] A. Chowdhury, J. Bould, Y. Zhang, C. James, S.J. Milne, J. Nanopart. Res. 12 (2009) 209–215. [5] A. Chowdhury, S. O’Callaghan, T.A. Skidmore, C. James, S.J. Milne, J. Am. Ceram. Soc. 92 (2009) 758–761. [6] C. Pithan, Y. Shiratori, J. Dornseiffer, F.H. Haegel, A. Magrez, R. Waser, J. Cryst. Growth 280 (2005) 191–200. [7] T. Rojac, M. Kosec, P. Segedin, B. Malic, J. Holc, Solid State Ion 177 (2006) 2987– 2995. [8] M.J. Harris, E.K.H. Salje, J. Phys. Condens. Matter 4 (1992) 4399–4408. [9] M. Miyazaki, A. Fujii, T. Ebata, N. Mikami, Science 304 (2004) 1134–1137. [10] V.P. Gavrilyuk, E.L. Vinograd, B.V. Grinyov, V.I. Goriletsky, Funct. Mater. 4 (1997) 540. [11] C. Wang, Y. Hou, H. Ge, M. Zhu, H. Wang, H. Yan, J. Cryst. Growth 310 (2008) 4635–4639. [12] Y. Shiratori, A. Magrez, C. Pithan, J. Eur. Ceram. Soc. 25 (2005) 2075. [13] T. Hoshina, H. Kakemoto, T. Tsurumi, S. Wada, M. Yashima, J. Appl. Phys. 99 (2006) 054311. [14] Q.F. Zhou, H.L.W. Chan, Q.Q. Zhang, C.L. Choy, J. Appl. Phys. 89 (2001) 8121. [15] Z.X. Shen, X.B. Wang, M.H. Kuok, S.H. Tang, J. Raman Spectrosc. 29 (1998) 379– 384. [16] L. Liu, M. Wu, Y. Huang, L. Fang, H. Fan, H. Dammak, M. Thi, Mater. Res. Bull. 46 (2011) 1467–1472.
Contents lists available at SciVerse ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Paper
Na0.5K0.5NbO3 and 0.9Na0.5K0.5NbO3–0.1Bi0.5Na0.5TiO3 nanocrystalline powders synthesized by low-temperature solid-state reaction Laijun Liu ⇑, Shaoying Zheng, Ruijing Huang, Danping Shi, Yanmin Huang, Shuangshuang Wu, Yunhua Li, Liang Fang, Changzheng Hu State Key Laboratory Breeding Base of Non-ferrous Metal and Characteristic Materials Processing, School of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China
a r t i c l e
i n f o
Article history: Received 4 November 2011 Received in revised form 7 September 2012 Accepted 3 January 2013 Available online 4 February 2013 Keywords: Powders Solid-state reaction Calcination Niobates Grain size
a b s t r a c t Nanocrystalline powders of K0.5Na0.5NbO3 (KNN) and 0.9Na0.5K0.5NbO3–0.1Bi0.5Na0.5TiO3 (KNN–BNT) have been prepared using a low-temperature solid-state reaction. Phase development of the powders incurred during various calcination temperatures was examined by X-ray diffraction (XRD). Crystallite size and particle morphology of KNN powders were examined by XRD and transmission electron microscopy, respectively. Perovskite phase was formed at the temperature as low as 500 °C, and the average crystallite size of KNN powders depended on calcination temperature. In addition, the crystalline structure of KNN powders tended to change from tetragonal symmetry to orthorhombic symmetry with increase in crystallite size. Similar results were obtained in KNN–BNT system. The developed method is well suited for the mass production of niobate nanocrystalline powders due to its simplicity and low cost. Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Alkaline niobates, such as KxNa1xNbO3 based ceramics, are considered to be promising lead-free piezoelectric materials [1]. However, KxNa1xNbO3 based ceramics present many drawbacks, for example, volatility of alkaline elements [2], high sensitivity of electrical properties to stoichiometry, difficulty to control the material composition, and complex densification processes [3]. In order to reduce the influence of these problems, nanocrystalline powders of alkaline niobates are needed to improve the sinterability and to prepare dense piezoelectric ceramics. Nanocrystalline K0.5Na0.5NbO3 (KNN) based compositions have been obtained through various soft chemistry routes, such as co-precipitation, sol–gel [4], Pechini [5] and microemulsion [6]. However, most of the chemical methods are complex and expensive. Solid-state reaction technique is a low-cost technique and requires a simple processing and is well suited for mass production, which also has been extensively used to prepare KNN-based ceramic powders. However, the calcined temperature of KNN-based ceramic powders as high as 900 °C are required [1–3]. Therefore, development of a low-temperature solid-state reaction method for the production of KNN-based ceramic powders should be very important on economizing energy and reduction cost. ⇑ Corresponding author. Tel.: +86 7735896290; fax: +86 7735896671. E-mail address: [email protected] (L. Liu).
The present paper describes a solid-state reaction technique for the preparation of nanocrystalline powders of KNN and 0.9Na0.5K0.5NbO3–0.1Bi0.5Na0.5TiO3 (KNN–BNT) solid solution. The subject of this work is to study the influence of calcination temperature on particle characteristic of the compositions, thus to develop an optimized process to obtain a low–cost production. 2. Experimental procedure Solid-state reaction method was used to prepare K0.5Na0.5NbO3 (KNN) and 0.9K0.5Na0.5NbO3–0.1Bi0.5Na0.5TiO3 (KNN–BNT) compositions. Na2CO3 (99.9%), K2CO3 (99.9%), Nb2O5 (99.5%) TiO2 (99.5%) and Bi2O3 (99.9%) were used as starting materials. All of the materials were dried at 200 °C to remove any moisture. The raw materials were milled individually by a planetary ball mill in an anhydrous alcohol for 8 h. Then these powders were weighted and mixed and milled again using 1–5 mm size ZrO2 balls in ethanol medium for 8 h. After drying the mixture, a thermal treatment at temperatures between 500 and 1000 °C for 2 h with a heating rate of 5 °C/min was followed. Simultaneous thermogravimetric and differential thermal analyses were carried out on the initial mixture before the calcination process using Netzsch STA 449C analyzer. Around 50 mg of powder was placed in a Pt/Rh crucible and heated up to 1000 °C with a heating rate of 5 °C/min. The measurements were performed in a flowing air atmosphere. Infrared spectra were recorded using
0921-8831/$ - see front matter Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2013.01.001
L. Liu et al. / Advanced Powder Technology 24 (2013) 908–912
a NEXUS 470FT-IR spectrometer. A few milligrams of samples were mixed in an agate mortar, placed between KBr pellets and recorded from 4000 to 400 cm1. Samples was analyzed by X-ray diffraction (XRD), carried out with a PANalytical X’Pert PRO X-ray diffractometer with Cu Ka (k = 0.15418 nm) incident radiation. Raman spectra were recorded for the powders at room temperature using a Jobin Yvon T64000 spectrometer. An Ar+ laser with 514.5 nm wavelength and <50 mW power at the samples (BeamLok 2080, Spectra-Physics) was used for sample excitation. Particle size and morphology of the calcined powders were evaluated using secondary electrons images of Field Emission Scanning Electron Microscopy, FE-SEM (Hitachi S-4800) and Transmission Electron Microscopy (TEM, Hitachi H-7100 175) with an accelerating voltage of 120 kV. Powders were suspended in isopropanol, and a drop of this suspension was deposited on a holey carbon-coated film supported on a 400-mesh copper grid.
3. Results and discussion Fig. 1 shows the results of TG/DSC for the KNN initial mixture. Four weight loss peaks can be observed on the derivative of the TG curve at 50, 80, 160, and 680 °C, associated with endothermic peaks. The low temperature weight losses (7.5 wt.%) occurring at 50 and 80 °C are attributed to the removal of environmental moisture absorbed gas and water. The weight loss at 160 °C is related to simultaneous losses of H2O and CO2 [7]. The decomposition of AHCO3 to A2CO3 (where A are K, Na) occurs at 255 °C; 2 wt.% of AHCO3 is formed after the milling from the H2O and CO2 present in the atmosphere. The endothermic effects at 300 and 480 °C are consistent with the polymorphic transition of A2CO3 [8]. The main weight losses occur in a narrow temperature range between 400 and 750 °C. At temperatures above 700 °C, the weight loss produces by decomposition of carbonates has been completed and no further weight losses are observed, indicating that the final composition of the samples correspond to the nominal one. The TG/DSC results indicate that the calcination should be performed at temperatures 700–800 °C. In order to evaluate the effect of calcination temperature on the characteristic of KNN powders, calcination temperatures between 500 and 1000 °C also have been selected. FT-IR spectra of the KNN initial mixture on the region between 4000 and 400 cm1 are shown in Fig. 2. They show seven main absorption bands at 3462, 1640 (double peak), 1464, 1328, 843, 700 and 505 cm1. The spectral gap in the region of 3460– 3520 cm1 is caused by the depletion of the IR laser power by water impurities in the DFG crystal [9]. The absorption band at
Fig. 1. TG–DTA curves of the KNN initial mixture.
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1640 cm1 can be attributed to the in-plane bending vibration of H2O [4,5]. The peak appears in every IR spectrum due to adsorbed water on the surface of every sample from environmental moisture. The band at 1464 cm1 can be assigned to the C–O asymmetrical stretching and the group of bands at 843 cm1 due to the CO2 out-of-plane symmetric deformation of the carbonate 3 group [8]. The band at 1328 cm1 can be attributed to HCO3 ions [10].The bands appearing at 700 and 505 cm1 could be ascribed to the characteristic vibration of Nb–O. IR spectra of the KNN initial mixture treated thermally between 500 and 800 °C show a complete disappearance of the absorption band at 1464 cm1 (in Fig. 2). The broad strong band centered at 650 cm1 indicates the formation of the perovskite phase [11]. The IR results are not in good agreement with the ones observed by TG/DSC, where the weight loss corresponding to the decomposition of carbonates completes at 680 °C. It is suggested that the increase in dwelling time can significantly improve the content of KNN. XRD patterns of the KNN initial mixture calcined at different temperatures are shown in Fig. 3a. The main phase corresponds to perovskite structure (JCPDS-ICDD 77-0038), but a trace of secondary phase K2CO3 (JCPDS-ICDD 87-0730) can be observed at 500 °C due to decomposition reaction has not completed. Singlephase perovskite can be obtained by increasing the calcination temperature. However, impurity phase K5.75Nb10.85O30 (JCPDSICDD 38-0297) presents again for heat treatment at 1000 °C due to volatilization of sodium. XRD patterns show a coexistence of tetragonal and orthorhombic, which is revealed by the relative intensities of (1 0 0) and (0 0 1) peaks diffraction peaks (seen in Fig. 3b). Moreover, with the increase in calcination temperature, an increase of the orthorhombic phase in relation to the tetragonal phase is observed. Because of the small grain size in nano-region, it can be concluded that a size-induced structure phase transition is responsible for the appearance of tetragonal symmetry. The similar feature of size-induced structure transition can be observed in KNN derived by wet-chemical method [11,12] or other perovskite systems [13,14]. Crystallite size of the KNN powders is calculated from the full width at half maximum of the diffraction peaks by using the Scherrer’s equation:
D¼
kk B cos h
where D is the crystallite size, k is the X-ray wavelength, B is the full width at half maximum of the diffraction line, h is the angle of diffraction, k is a constant (having the value 0.9 in our case). Fig. 3c
Fig. 2. FT-IR spectra of the KNN initial mixture and the KNN initial mixture calcined at different temperatures.
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Fig. 4. SEM micrographs of the calcined KNN powders: (a) 500 °C, (b) 800 °C.
Fig. 3. (a) XRD patterns of the KNN initial mixture calcined for 2 h at different temperatures. (b) Detail of the XRD on the region corresponding to (0 0 1) and (1 0 0) diffraction peaks. (c) Evolution of the crystallite size as a function of the calcination temperature.
shows the crystallite size as a function of calcination temperature. The average crystallite size of KNN powders varied in the range of 20–80 nm. Fig. 4 shows the morphology of the KNN powders calcined at 500 °C (a) and 850 °C (b) during 2 h. The calcined powders are composed of small plate-like particles with sizes of 50–150 nm that form strongly bound agglomerates of 500 nm (no sintering necks can be observed between individual particles). It indicates that nanocrystallline powder is obtained by the solid-state reaction process at low temperatures (600 °C). Fig. 5 shows representative TEM image of KNN powders calcined at different temperatures. The images show that the powders are agglomeration, consisting of a large amount of crystallites. The change of particle size is in good agreement with the results calculated by XRD patterns. Therefore, the observed particles are composed of crystallites forming polycrystalline parcels due to the calculated crystallite size is considerable smaller than the observed particle size. The Raman spectroscopy is very sensitive to the octahedral tilting associated with the phase structure deformation of perovskite induced by the variation of particle size. The Raman spectra of the KNN powders calcined at temperatures between 500 and 1000 °C are shown in Fig. 6a. The main vibrations are associated to the BO6 perovskite-octahedra for the related system KNN. The small grain size differences do not produce noticeably differences on
the Raman spectra, as experimentally corroborated. The vibrations of the BO6 octahedron consist of 1A1g (m1) + 1Eg (m2) + 2F1u (m3, m4) + F2g (m5) + F2u (m6). Of these vibrations, 1A1g (m1) + 1Eg (m2) + 1F1u (m3) are stretching modes and the rest are bending modes. In particular, A1g (m1) and F2g (m5) are detected as relatively strong scatterings in KNN because of a near-perfect equilateral octahedral symmetry. The Raman bands are quite broad, which is mainly due to the small crystallite size and to the cation disorder on the 12-fold coordinate site [15], as the perovskite structure is not completely formed. A detail of the region between 420 and 740 cm1 is shown in Fig. 6b1, where the spectrum is fitted to the sum of two Lorentzian functions centered at 535 and 611 cm1, ascribed to Eg (m2) and A1g (m1) Raman modes (see Fig. 6b1), respectively. The peak A1g (m1) shifts to higher frequency when the calcination temperature increases above 700 °C (see Fig. 6b2) due to an increase in the strength constant caused by the shortening of the distance between B5+ type ions and their coordinated oxygens. The structure of KNN transforms from tetragonal to orthorhombic symmetry due to the crystallite size effect. These results are in good agreement with the evolution of the crystalline structure observed by XRD. In addition, the full width at half-maximum (FWHW) of A1g (m1) mode is sensitive to the crystallite size. The FWHM values of the calcined KNN powders decrease with the calcination temperature increase (see Fig. 6b3) from 95 cm1 for the sample calcined at 500 °C to 55 cm1 for calcinations at 1000 °C. The narrowing induced by the increase of the calcination temperature is consistent with the increase of particle size during the calcination step, which is evidenced by SEM and TEM micrographs. In order to extend the method to niobate solid solution, a niobate-titanate 0.9K0.5Na0.5NbO3–0.1Bi0.5Na0.5TiO3 (KNN–BNT) sys-
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Fig. 5. TEM micrographs of KNN powders obtained by different thermal treatments at (a) 600 °C and (b) 800 °C.
tem was selected. Fig. 6 presents XRD patterns of the KNN–BNT initial mixture calcined at different temperatures. Similar results have been obtained from pre-milled K2CO3, Na2CO3, Nb2O5, Bi2O3 and TiO2 mixture. A small amount of Bi2O3 (JCPDS-ICDD 51-1161) can be found in the pattern when the mixture is calcined at 500 °C. An impurity phase KTiNbO5 (JCPDS-ICDD 71-1747) presents for thermal treatment at 1000 °C due to volatilizations of bismuth and sodium. Insert in Fig. 7 shows the increase of the crystallite size as a function of the thermal treatment temperature. Average crystallite size of the KNN–BNT varied in the range of 20–70 nm, which is smaller than that of KNN. Generally, the formation temperature of BNT phase is higher than that of KNN, therefore, for the same calcination temperature, the crystallite growth of KNN– BNT is slower than that of KNN. On the other hand, KNN and BNT have different cell parameters, the valences of A site and B site are different, therefore, the atomic diffusion in KNN–BNT solid solution should be slower than that in pure KNN, then leads to a smaller crystallite size. The method of the low-temperature synthesized KNN and KNN–BNT is better than mechanical activation process [16]. Therefore, the developed method presented in this paper will ensure low-temperature, low-cost and high-purity in niobate products, and can be extended to the synthesis of niobate-titanate solid solution.
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Fig. 6. (a) Raman spectra of KNN powders calcined at different temperatures; (b1) Magnified Raman spectra in the wavenumber range from 420 and 740 cm1 as a function of the composition and Lorentzian fits of the individual peaks of the Eg (m2) and A1g (m1) Raman modes. Evolution of Raman shift (b2) and FWHM (b3) of A1g mode in function of calcination temperature are also shown.
Fig. 7. XRD patterns of the KNN–BNT initial mixture calcined for 2 h at different temperatures. Inset shows the evolution of crystallite size as a function of calcination temperature.
4. Conclusions Pure K0.5Na0.5NbO3 and 0.9K0.5Na0.5NbO3–0.1Bi0.5Na0.5TiO3 powders with a range of particle size from 20 nm to 80 nm were
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prepared through dividedly pre-milling raw materials followed by mixing and calcination. The mixture of pre-milling raw materials calcined in the range of 500–800 °C can produce well crystallized particles. The crystalline structure of KNN powders tended to change from tetragonal symmetry to orthorhombic symmetry with increase in crystallite size due to size-induced effect. The defined method is well suited for mass production for niobate or niobate solid solution. Acknowledgments This work was financially supported by the Natural Science Foundation of China (Grant Nos. 11264010, 51002036, 21061004 and 50962004), and by the Natural Science Foundation of Guangxi (Grant No. BA053007) and the Projects of Department of Science and Technology of Guangxi (Grant No. 12118017-13). References [1] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, Nature 432 (2004) 84.
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