The fabrication of near net-shaped spinel bodies by the oxidative transformation of Mg/Al 2 O 3 precursors
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The fabrication of near net-shaped spinel bodies by the oxidative transformation of MgyyAl2 O3 precursors P. Kumar and K. H. Sandhage Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210 (Received 19 February 1998; accepted 20 July 1998)
The feasibility of transforming shaped Mg–Al2 O3 -bearing precursors into monolithic spinel (MgAl2 O4 ) bodies with a retention of shape and dimensions has been demonstrated. Dense, shaped precursors (disks, bars) were fabricated by the pressureless infiltration of molten Mg into porous Al2 O3 preforms. After solidification (and machining, in the case of bar-shaped specimens), the Mg-bearing precursors were oxidized in flowing O2 (g) at 430–700 ±C. Postoxidation annealing at 1200 ±C resulted in the conversion of MgO and Al2 O3 into MgAl2 O4 . After sintering at 1700 ±C, spinel bodies that retained the precursor dimensions (to within 0.65%) were produced. Phase and microstructural analyses at various stages of processing are discussed.
I. INTRODUCTION ±
Owing to a high melting point (ø2100 C), high hardness (16.1 GPa), and excellent resistance to chemical attack, MgAl2 O4 (spinel) has found use in a variety of refractory applications (e.g., for steel, glass, and cement production).1–9 Spinel is chemically compatible with zirconia, alumina, and mullite, so that these latter oxides can be used as reinforcements in spinel-matrix composites (e.g., for use in oxidizing atmospheres at elevated temperatures).10–20 The high resistivity and optical isotropy of spinel have also resulted in the use of this oxide in electronic and optical devices (e.g., electrical insulators, windows for pressure vessels, and infrared sensors).1,21–24 Spinel powders have been synthesized by a variety of methods, including nitrate pyrolysis,25 sol-gel processing,26–28 co-precipitation,29 and the firing of solid salt mixtures.30–35 A spinel green body can be produced by blending spinel powder with a malleable organic binder and then compacting and forming the mixture (e.g., by extrusion, forging, tape casting/calendering, or pressing) into a desired shape.2,36–38 Subsequent pyrolysis of the organic material at a modest temperature yields a porous spinel body that must then be exposed to a high-temperature anneal (typically in excess of 1600 ±C, in the absence of a sintering aid or the application of external pressure31,32,36,37,39,40 ) to allow for sintering to a high relative density. Due to sinteringinduced shrinkage, however, the final spinel body does not retain the dimensions of the formed green body and, if such shrinkage is nonuniform, the green body shape will also be lost. An alternative method for fabricating a shaped spinel-bearing body is the oxidation of a solid, Mgbearing precursor (e.g., a Mg–Al2 O3 precursor).41,42 Magnesium is a ductile metal that can serve a role J. Mater. Res., Vol. 13, No. 12, Dec 1998
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similar to that of organic binders used in co
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