Fractional factorial design applied to optimize experimental conditions for preparation of ultrafine lanthanum-doped str
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Fractional factorial design applied to optimize experimental conditions for preparation of ultrafine lanthanum-doped strontium titanate powders Wein-Duo Yanga) and Ching-Shieh Hsieh Department of Chemical Engineering, National Kaohsiung Institute of Technology, Kaohsiung 80807, Taiwan, Republic of China (Received 12 October 1998; accepted 7 May 1999)
A fractional factorial design was implemented to optimize the experimental conditions for the preparation of ultrafine lanthanum-doped strontium titanate from titanyl acylate precursors. The effects of preparation conditions such as the molar ratio of acetic acid to titanium alkoxide, the water to titanium alkoxide ratio, pH value, the reaction temperature, and stirring speed were systematically studied by using Taguchi orthogonal array design. Results indicated that the effects of the reaction temperature and stirring speed on the reaction were the key variables influencing the average particle size of powders obtained. By combining the optimal settings of the two influential processing variables, it was possible to obtain an ultrafine powder with a particle size of about 340 Å. This was put to a test in the laboratory, and a polycrystalline, narrow size distribution ultrafine SrTiO3 powder that had a particle size of about 380 Å and readily sintered at 1150–1250 °C was obtained.
I. INTRODUCTION
In the preparation of strontium titanate from the expensive alkali earth alkoxide, a high molecular weight gel is easily formed by reaction with moisture.1,2 This gel is less soluble and affects the quality of the product.3 The titanyl acylate precursor, made from chelating titanium alkoxide with acetic acid, can be dissolved in water and reacts readily with the alkali earth metal ion to form a clear and homogeneous solution.4,5 Strontium titanate powder is then produced by adding a strong alkaline solution (pH > 11).6 The process of forming the perovskite powder directly in a strong alkaline solution has the advantages of low temperature and excellent compositional homogeneity of the powder, making it an ideal method. However, Kao and Yang7 proposed a modified sol-precipitation method to prepare ultrafine strontium titanate powder from a titanyl acylate precursor in a strong alkaline solution. The powder obtained had an average particle size of 400–500 Å and sintered readily at 1150–1250 °C. Kao and Yang8 also found that during the sol-precipitation process, the strontium ion diffused into the gel formed by the hydrolysis of the titanyl acylate precursor to form strontium titanate powder. The mechanism of this process is the same as the one Diaz-Guemes et al.9 suggested for the reaction of tetraethyl orthotitanate with strontium hydroxide. The alkali earth ion dif-
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J. Mater. Res., Vol. 14, No. 8, Aug 1999 Downloaded: 29 Nov 2014
fuses into the gel to form SrTiO3 powder. The physical and chemical properties of the SrTiO3 powder formed are contr
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