Controlled double-jet precipitation of uniform colloidal crystalline particles of Zr- and Sr-doped barium titanates
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Controlled double-jet precipitation of uniform colloidal crystalline particles of Zr- and Sr-doped barium titanates Yie-Shein Her and Egon Matijevi´c Center for Advanced Materials Processing, Clarkson University, Box 5814, Potsdam, New York 13699-5814
Min Che Chon Chon International, 128-27 Tangju–dong, Chongno–ku, Seoul 110-071, Korea (Received 21 September 1995; accepted 12 June 1996)
The synthesis of uniform colloidal crystalline particles of Zr- and Sr-doped barium titanates at a low temperature of 85 ±C by the controlled double-jet precipitation (CDJP) technique is described. The stoichiometry of the powders can be precisely controlled by adjusting the compositions of the starting reactant solutions. Barium titanate with 20% Zr substitution, sintered at 1275 ±C, satisfied the requirements for the Y5V multilayer capacitors application. The grain sizes are uniform and small, ranging from 1 to 3 mm. Solids with an extremely sharp change in the dielectric constant as a function of temperature, which are suitable for thermal IR detectors application, can be obtained when both Sr and Zr are incorporated as dopants.
I. INTRODUCTION
Barium titanate (BaTiO3 ) is an important and commonly used base material for the manufacture of ceramic dielectrics. At the Curie point of 120 ±C, the crystal lattice undergoes a structure change from tetragonal to cubic and exhibits a sharp peak in the dielectric constant. In order to achieve the phase change and, therefore, the maximum dielectric constant at an appropriate temperature, it is necessary to incorporate into BaTiO3 Curie point shifters, such as BaZrO3 and SrTiO3 . Normally, such powders have been prepared by mixing barium carbonate or oxide, titanium dioxide, strontium carbonate or oxide, and/or zirconium oxide, and calcining the solid mixture at temperatures .1000 ±C, followed by wet pulverizing, filtering, and then drying the product. The so-obtained titanates consist, as a rule, of aggregates of irregular morphology, which usually require high sintering temperatures, resulting in large grain sizes. The Curie temperature is often a strong function of the sintering conditions, causing problems in practical applications. It is assumed that molecular inhomogeneity in the powders is one of the main reasons for the described difficulties.1 In order to eliminate these deficiencies, a number of wet chemical synthesis techniques have been developed, including homogeneous precipitation,2,3 sol-gel,4–8 pyrolysis,9 or hydrothermal10–12 processes, and techniques using metal citrate,13 oxalate,14 or polyacrylate15 precursors. However, even these procedures suffer from various disadvantages, including inferior particle morphology, dielectric properties, sinterability, and microstructure. The preparations are J. Mater. Res., Vol. 11, No. 12, Dec 1996
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also uneconomical due to the cost of the starting materials, high reaction temperatures and pressures, and low yields. Furthermore, th
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