Powder synthesis of barium titanate and barium orthotitanate via an ethylene glycol complex polymerization route
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Powder synthesis of barium titanate and barium orthotitanate via an ethylene glycol complex polymerization route S.J. Lee, M.D. Biegalski, and W.M. Kriven Department of Materials Science and Engineering, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801 (Received 25 January 1999; accepted 18 March 1999)
Pure and reactive barium titanate (BaTiO3) and barium orthotitanate (Ba2TiO4) powders have been synthesized by an ethylene glycol (EG), polymerizationcomplexation route. The EG content affected the crystallization behavior and powder morphology. The BaTiO3 powder, which had a particle size of approximately 100 nm, crystallized from amorphous to tetragonal phase on calcining at 700 °C for 1 h. Ball-milled BaTiO3 powder sintered to 97% relative density at 1200 °C after 2 h, with a grain size of approximately 200 nm. Ba2TiO4 powder required longer holding times or higher temperatures to be crystallized from the amorphous phase than did BaTiO3. In Ba2TiO4, the phase transformation between low-temperature monoclinic symmetry to high-temperature orthorhombic symmetry was observed by dilatometry and differential scanning calorimetry. A volume decrease of ∼0.5% accompanied the monoclinic-to-orthorhombic transformation on heating. The high-temperature orthorhombic phase could be retained down to room temperature by the addition of at least 6 wt% magnesia (MgO) stabilizer.
I. INTRODUCTION
Barium titanate (BaTiO3) has a large polarization and a high dielectric constant, whose value usually ranges from 650–8500, depending on the particle size and impurities.1– 6 Above 130 °C the crystal structure is cubic, and when the material is cooled below 130 °C, the material changes to tetragonal symmetry. At higher temperatures, above 1450 °C BaTiO 3 also displays a hexagonal crystal structure. 7 Barium orthotitanate (Ba2TiO4) has two crystalline structures that are orthorhombic and monoclinic.8 The orthorhombic form has been reported to exist at high temperatures and the monoclinic form at room temperature. The monoclinic form is suggested to be similar to – Ca2SiO4.9 As with the –Ca2SiO4 crystal structure, Ba2TiO4 could be expected to undergo a displacive transformation above room temperature. Bland9 surmised that the phase change from orthorhombic to monoclinic was near room temperature because the work of Todd and Lorenson10 found a peak in the heat capacity near room temperature. BaTiO3 powder is difficult to obtain by a conventional ceramic process.11 This is due to the formation of many intermediate compounds that reduce the purity of the BaTiO3 itself. Chemical methods are preferred because they can yield powders that are very pure. Previous experiments have yielded BaTiO3 powders of small particle size by use of various methods.2– 6,12,13 The sol-gel J. Mater. Res., Vol. 14, No. 7, Jul 1999
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methods based on complex polymerization produced 0.5–0.3 m particle sizes, with specific dielectric constants ranging
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