Segregation in the MgO-MgAl 2 O 4 system processed from nitrate precursors
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Segregation in the MgO–MgAl2O4 system processed from nitrate precursors Tania Bhatia,a) K. Chattopadhyay, and Vikram Jayaram Center for Advanced Study, Department of Metallurgy, Indian Institute of Science, Bangalore, 560012, India (Received 12 June 1998; accepted 26 April 1999)
The occurrence of segregation and its influence on microstructural and phase evolution have been studied in MgO–MgAl2O4 powders synthesized by thermal decomposition of aqueous nitrate precursors. When the nitrate solutions of Mg and Al were spray-pyrolyzed on a substrate held at 673 or 573 K, homogeneous mixed oxides were produced. Spraying and drying the nitrate solutions at 473 K resulted in the formation of compositionally inhomogeneous, segregated oxide mixtures. It is suggested that segregation in the dried powders was caused by the difference in solubility of the individual nitrate salts in water which caused Mg-rich and Al-rich salts to precipitate during dehydration of the solutions. The occurrence of segregation in the powders sprayed at 473 K and not 573 or 673 K is ascribed to the sluggish rate at which the early stages of decomposition occurred during which the cations segregated. The phase evolution in segregated and segregation-free MgO–MgAl2O4 powders has been compared. The distinguishing feature of the segregated powders was the appearance of stoichiometric periclase grain dimensions in excess of 0.3 m at temperatures as low as 973 K. By comparison, the segregation-free powders displayed broad diffraction peaks corresponding to fine-grained and nonstoichiometric periclase. The grain size was in the range 5–30 nm at temperatures up to 1173 K. The key to obtaining fine-grained periclase was the ability to synthesize (Mg Al)O solid solutions with the rock salt structure. In the temperature range 973–1173 K, spinel grain size varied from 5 to 40 nm irrespective of its composition and did not appear to be influenced by segregation.
I. INTRODUCTION
The usefulness of liquid precursor pyrolysis as a technique to study metastable effects in ceramics was illustrated by Wefers and Bell1 who showed that a variety of metastable phases of Al2O3 could be produced on thermal decomposition of Al(OH)3. More recently, metastability of products synthesized through pyrolysis of liquid precursors has been studied in several systems.2–6 In a recent review, Levi7 has explained the thermodynamics of metastability accessed through the pyrolysis of inorganic precursors. One of the most attractive features about the techniques involving pyrolysis is the low temperature (T/Tm < 0.5, where T/Tm is the homologous temperature) at which it is possible to synthesize metastable and nanophase materials. The work assumes im-
a)
Address all correspondence to this author. Present address: Department of Metallurgy and Materials Engineering, Institute of Materials Science, Box U-136, University of Connecticut, Storrs, Connecticut 06269-3136. J. Mater. Res., Vol. 14, No. 8, Aug 1999
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