Advanced Materials Research Vols. 55-57 (2008) pp 185-188 online at http://www.scientific.net © (2008) Trans Tech Publications, Switzerland
Preparation of Barium Strontium Titanate Ceramics via Combustion Method T. Bongkarna* and C. Wicheanratb Department of Physics, Faculty of Science, Naresuan University, Phitsanulok, 65000, Thailand a
[email protected],
[email protected]
Keywords: Barium Strontium Titanate, Phase Formation, Microstructure, Combustion
Abstract. This study concentrated on the crystal structure and microstructure of [(Ba0.75Sr0.25)TiO3; (BST)] ceramics at different firing temperatures. The BST powders were prepared by a combustion method. (CO(NH2)2) was used as a fuel. Crystallinity of the calcined powders was improved by increasing the calcining temperature, as indicated by the increase in intensity of the X-ray diffraction peak. The pure perovskite phase of BST powders was obtained with a calcinations condition of 1200 oC. The a axis lattice constant of BST calcined powders and sintered ceramics were calculated and it was found that the crystal structure is a cubic phase. The microstructure of BST calcined powders and sintered ceramics were analyzed by a scanning electron microscope (SEM). The SEM result indicated that the average particle size and average grain size increased with the increase of calcinations and sintering temperatures, respectively. The apparent density of the samples was measured by the Archimedes method. Introduction Barium strontium titanate, BaxSr1-xTiO3 compounds, are the continuous solid solution between BaTiO3 and SrTiO3 over the whole composition range. They are promising candidates for microelectronic devices that can be integrated with capacitor, turnable microwave and semiconductor technologies [1]. There are reports that BaxSr1-xTiO3 [2] has been synthesized using an established solid-state reaction method. The intensity of the X-ray diffraction peak indicated that crystallinity and lattice parameters of the calcined powders were improved by increasing the calcining temperature. The particle size distribution also increased at higher calcining temperatures. Fu et al [3], reported that the crystal structures of BaxSr1-xTiO3 (0.45 ≤ x ≤ 0.9) ceramics are in a tetragonal phase when x ≥ 0.65 at room temperature. The values of the coercive electric field (E c) and remanent polarization (Pr) increases as the Ba/Sr ratio of BaxSr1-xTiO3 ceramics increases except for x≤ 0.5. The conventional method of synthesizing Ba xSr1-xTiO3 relies on a solid-state reaction between BaCO3, SrCO3 and TiO2 at a high temperature. Un-fortunately, this method has a tendency to produce coarse BaxSr1-xTiO3 powder with compositional inhomegeneity, partial particle agglomeration and the completion of the reaction requires a very high temperature [4]. The combustion method has tremendous present interest; it helped to decrease the temperature in the preparation and has a small particle size [5]. The combustion reaction method also has interesting characteristics such as its simplicity, its relatively low cost and the fact that it usually results in products with the desired structure and composition [6]. Thus, the aim of this study was to prepare BaxSr1-xTiO3 (BST) ceramics via the combustion method and also to investigate the effect of firing temperatures on its phase formation and microstructure. Experimental Ba0.75Sr0.25TiO3 (BST) was fabricated using an established combustion method. BaCO3, SrCO3 and TiO2 powders were used as starting materials. The powders were mixed by ball milling for 24 h with ethanol as a solution media. The mixed powders were dried at 100 – 120 oC for 6 h. Prior to sieving, the powders and urea were mixed in agate motor and calcined at 600 – 1200 oC for 4 h. The calcined powders were reground by wet ball milling with 1 wt% binder for 24 h. The obtained All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 202.28.21.4-20/08/08,07:20:19)
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powders were pressed into pellets with a diameter of 15 mm prior to sintering at 1200 - 1400 oC for 2 h. X-ray diffraction (XRD) was performed to examine the phase constitution of the specimens at room temperature. The microstructures of the BST samples were examined using a scanning electron microscopy (SEM). Densities of the sintered ceramics were measured by the Archimedes’s method and the average grain size was determined by using a mean linear intercept method. Results and Discussion XRD patterns of BST powders calcined at 600 - 1200 oC are shown in Fig.1. For the powders calcined at below 1200 oC, X-ray peaks of precursors and impurities, BaCO3, BaO and TiO2, appeared, while the high purity of the cubic perovskite phase was discovered in powders calcined above 1200 oC, which could be matched with JCPDS file number 39-1395 [7]. The XRD pattern showed a major X-ray reflection peak in the perovskite BST phase, indicating the polycrystalline nature of the powder with (011) as the major peak. Crystallinity of the calcined powders was improved by increasing the calcination temperature, as indicated by the increase in intensity of the X-ray diffraction peak. The percent perovskite phase of BST compounds are calculated and listed in Table 1. The purity of the perovskite phase rose with increasing temperatures and 100% of the perovskite phase was found above 1200 o C. The a axis lattice constant of BST calcined powders are calculated in Table1. The lattice parameter of the powders decreased with an increase in the calcining temperature. This indicates that at low calcining temperatures, the powders exist in a more strained form with the atomic entities in non-equilibrium positions, which relax to an equilibrium position at higher temperatures. Another possible reason may be that the domain mobility is restricted due to pinning of Figure 1. XRD patterns of BST the domain boundaries by crystal defects [2]. The SEM photomicrographs of BST calcined powder: powders at 1000 and 1200 oC are illustrated in Fig. 2. With the increasing of calcinations temperatures, the average particle size increased. These powders exhibited an almost spherical morphology and have a porous agglomerated form. The average particle size was discovered to be about 0.43, 0.37, 0.39, 0.52 and 0.55 µm for the samples calcined at 600, 800, 1000, 1100 and 1200 oC, respectively.
(a) (b) Figure 2. SEM micrographs of BST powders calcined at: (a) 1000 oC and (b) 1200 oC.
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The phase formation behavior of the sintered ceramics was revealed by an XRD method. The XRD patterns of the BST ceramics sintered at sintering temperatures (1200-1400 oC) are shown in Fig. 3. From the pattern, a single perovskite cubic phase BST could be obtained in all sintered samples. This indicates that the conventional combustion technique is a simple and effective method to produce BST ceramics. Note that the crystal structure of BST at room temperature, which was prepared via a combustion and mixed oxide method ,[3] was tetragonal and cubic. It suggested that the Curie temperature of BST, prepared by combustion temperature, shifted when below room temperature. However, the structural reasons for this are uncertain at this stage. The lattice parameter of the ceramics also decreased with an increase in the sintering temperature (Table 1). This result was the same with the calcined process. The SEM photographs of BST ceramics sintered at 1200 and 1400 oC are shown in Fig. 4. The results indicated that grain size tends to increase with increasing sintering temperature (Table 1). The average grain size was discovered to be about 0.87, 0.93, 0.96, 5.40 and 7.12 µm for the samples sintered at 1200, 1250, 1300, 1350 and 1400, respectively. In addition, the porosity was obviously decreased with an increase of sintered temperature.
Figure 3. XRD patterns of BST ceramics sintered at various temperatures
(a) (b) Figure 4. SEM micrographs of BST ceramics sintered at: (a) 1200 oC and (b) 1400 oC.
The variation of the measured density and shrinkage of BST ceramics with different sintering temperatures are also shown in Table 1. The density and shrinkage increased with an increased sintering temperature. The density increased from 90.5% to 97.6% when the sintering temperature increased from 1200 oC to 1400 oC.
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Table 1. Percent perovskite phase, lattice parameter a, density and shrinkage of BST Calcination Temperature (°C) 600 800 1000 1100 1200
Calcined powders %perovskite Lattice phase Parameter a (%) (Å) 6.4 3.984 44.9 3.982 94.0 3.980 98.9 3.979 100.0 3.978
Sintering Temperature (°C) 1200 1250 1300 1350 1400
Sintered ceramics Lattice Density Parameter a (g/cm3) (Å) 3.954 5.24 3.947 5.33 3.947 5.34 3.946 5.55 3.929 5.64
Shrinkage (%) 10.67 11.56 15.33 15.33 15.56
Summary Perovskite (Ba0.75Sr0.25TiO3) powders and ceramics can be obtained successfully by a conventional combustion technique. The pure perovskite cubic phase has been discovered at calcinations temperature above 1200 oC. The lattice parameter a was decreased when calcinations and sintering temperature increased. The average particle size and average grain size was observed to increase at higher calcining and sintering temperature. The highest density and shrinkage was obtained in the sample that sintered at 1400 oC. Acknowledgements This work was financial supported by the Thailand Research Fund (TRF), Commission on Higher Education (CHE), Faculty of Science, Naresuan University. Thanks are also to Mr. Don Hindle for his help correcting the manuscript. References [1] [2] [3] [4] [5] [6] [7]
Y.B. Khollam, S.V. Bhoraskar, S.B. Deshpande, H.S. Potdar, N.R. Pavaskar, S.R. Sainkar and S.K. Date: Mater. Lett. Vol. 57 (2003), p. 1871. S. Kongtaweelert, D.C. Sinclair and S. Panichphant: Curr. Appl. Phys. Vol. 6 (2006), p. 474. C. Fu, C. Yang, H. Chen, Y. Wang and L. Hu: Mat. Sci. Eng. B. Vol. 119 (2005), p. 185. F. Zhang, T. Karaki, M. Adachi: Powder. Technol. Vol. 159 (2005), p. 13. K.C. Patil, S.T. Aruna and T. Mimani: Curr. Opim. Solid. St. M. Vol. 2 (1997), p. 158. K.C. Patil, S.T. Aruna and T. Mimani: Curr. Opim. Solid. St. M. Vol. 6 (2002), p. 507. Powder Diffraction File No. 39-1395, International Center for Diffraction Data, Newton Square, PA, 2003.
