Barium titanate ceramics prepared from conventional and microwave hydrothermal powders
Introduction
Barium titanate (BaTiO3) is a well known electroceramic material used in the manufacture of thermistors, dielectric ceramic capacitors, etc. [1]. There is a considerable interest in evaluating new methods for the synthesis of very fine BaTiO3 powders with high purity and high sinterability 2, 3to improve its electrical properties [4]. Traditionally BaTiO3 is prepared by solid state reaction which involves ball milling of BaCO3 and TiO2 and the mixture is calcined at higher temperatures around 1100°C [5]. High-temperature BaTiO3 showed some drawbacks such as very large particle size, higher impurity content due to repetitive calcination and grinding treatments and also lower chemical activity and therefore not suitable for enhancing the dielectric properties of ceramics for high-performance uses. Various other low-temperature chemical methods such as oxalate process [6], hydrolysis of barium titanium alkoxides [7], homogeneous precipitation [8], sol–gel [9], combustion method [10], mixed citrates 11, 12and hydrothermal technique 13, 14have been used to prepare high-purity homogeneous and ultrafine powder of BaTiO3. The oxalate process based on solid-state reactions yields BaTiO3 upon calcination only at 800° to 1100°C and these high calcination temperatures give rise to particle aggregation and therefore reduce the sinterability of the BaTiO3 powder. In the case of sol–gel process, the metal organic precursors are very expensive and the reactions must be carried out in carbon-dioxide-free atmosphere to prevent BaCO3 formation. Also titanate powders obtained by wet chemical methods are frequently of submicron particle size and high chemical purity, but they are often highly agglomerated.
On the other hand, the conventional hydrothermal (CH) synthesis has enjoyed considerable success in which Ba(OH)2, TiO2 or gels of Ba–Ti acetate are heated in the temperature range of 80°–150°C in an aqueous environment in the presence of hydroxide ion 15, 16. Electrochemical methods under hydrothermal conditions have also been employed [17]. Hydrothermal method is a low-temperature process and it is environmentally friendly as the reactions are carried out under closed system conditions. The advantage of hydrothermal process is that conventional Teflon-lined bombs can be used which is very important for highly caustic solutions which are usually present in the synthesis of BaTiO3. There have been reports of hydrothermal synthesis of submicron powder (0.1 μm) that will densify with small grain size and high dielectric constant without the addition of dopants 18, 19. But, even for these samples, temperatures in excess of 1150°C are required for sintering.
Recent research in our laboratory has been focused on microwave-hydrothermal (MH) methods 20, 21, 22, 23, 24, 25, 26, 27, 28for fine powder preparation. Some of the primary reasons for the growing interest in using microwaves to ceramic materials synthesis are (i) cost savings due to rapid kinetics (time and energy), (ii) rapid internal heating and (iii) synthesis of new materials. In view of this, an attempt is made here to prepare BaTiO3 by MH method and evaluate its sinterability, microstructure and dielectric properties in comparison to those prepared by CH processing.
Section snippets
Experimental
Reagents of Ba(NO3)2, TiCl4 and KOH with 99.9% purity (of Aldrich Chemical) were used as starting chemicals. All the chemical reactions were conducted in a microwave digestion system MDS-2000 (CEM). The system operates at a 2.45-GHz frequency and can operate at 0–100% of full power (630+50 W). The solutions were treated in lined digestion vessels which are double-walled vessels consisting of a Teflon PFA inner liner and cover surrounded by higher strength vessel shell of ultem polyetherimide.
Results and discussion
Table 1 shows the results of both MH and CH synthesis. Cubic BaTiO3 resulted under all synthesis conditions used here. The mechanism of formation of BaTiO3 under hydrothermal conditions has been discussed previously in the literature 17, 29. Fig. 1 shows the XRD patterns for the as-prepared MH and CH powders at 138°C and their respective ceramics heat-treated at 1300°C for 2 h. From the XRD patterns it is obvious that both MH and CH as-prepared powders yielded cubic BaTiO3. It has been reported
Conclusions
Sintering behavior and dielectric properties of BaTiO3 ceramics derived from powders prepared by MH and CH methods were found to be similar. However, the former method may lead to energy savings because of rapid kinetics of synthesis.
Acknowledgements
This research was supported by The Materials Research Laboratory Consortium on Chemically Bonded Ceramics.
References (33)
- et al.
J. Solid State Chem.
(1978) - et al.
Mater. Res. Bull.
(1984) - et al.
Ceram. Int.
(1990) - et al.
Mater. Res. Bull.
(1992) - et al.
Mater. Lett.
(1996) - et al.
Powder Technol.
(1989) - et al.
Mater. Res. Bull.
(1985) Ceram. Bull.
(1993)- et al.
J. Mater. Sci.
(1990) Br. Ceram. Proc.
(1989)
Ferroelectrics
J. Mater. Sci.
Ceram. Bull.
J. Mater. Sci.
Advanced Ceramic Materials
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