The oxidative transformation of solid, barium-metal-bearing precursors into monolithic celsian with a retention of shape
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The oxidative transformation of solid, barium-metal-bearing precursors into monolithic celsian with a retention of shape, dimensions, and relative density Seyed M. Allameh and Kenneth H. Sandhage Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210 (Received 3 January 1997; accepted 28 January 1998)
The conversion of Ba–Al2 O3 –Si–SiO2 , Ba–Al–Al2 O3 –SiO2 , and Ba–Sr–Al–Al2 O3 –SiO2 precursors into monolithic, monoclinic celsian has been examined. The relative amounts of metal and oxide in each type of precursor were adjusted so that the overall stoichiometry and molar volume were similar to those of the desired product, celsian. Metal 1 oxide mixtures were mechanically alloyed and then uniaxially pressed to yield 84–92% dense precursor disks. The precursors were converted into celsian by exposure to a series of heat treatments from 300–1500 ±C in oxygen-bearing gases. Differences and similarities in the phase evolution of the various precursors are discussed. Celsian disks were produced that retained the precursor shape, dimensions, and relative (% theoretical) density.
I. INTRODUCTION
Celsian, BaAl2 Si2 O8 , is a high-melting feldspar (Tm 1760 ±C) with attractive thermochemical properties for high-temperature applications.1–6 BaAl2 Si2 O8 has excellent resistance to high-temperature reduction or oxidation (unlike SiC) and is chemically compatible with Al2 O3 and Al6 Si2 O13 (mullite) reinforcements below 1720 ±C and 1554 ±C, respectively.7–15 BaAl2 Si2 O8 is also reported to be chemically compatible with Si3 N4 at elevated temperatures in nonoxidizing atmospheres.16–21 Monoclinic celsian possesses a modest specific density (3.39 gycm3 ) and a low thermal expansion coefficient (2.3–3.5 3 1026y±C from 20–1200 ±C).23–26 Modest and thermally stable values of dielectric permittivity and loss at high frequencies also make monoclinic celsian attractive for use in radomes and other electromagnetic devices.24,26–28 A variety of methods have been used to produce celsian powder, including hydrothermal syntheses,29–31 ion exchange of alkali feldspars,32 and firing of solid salt mixtures.17–19,24,26–28,33–38 A shaped celsian green body can be produced by blending celsian powder with a malleable organic binder and then deforming the mixture into a desired shape by extrusion, forging, or pressing. The organic material can then be removed by pyrolysis, and the resulting porous ceramic body can be sintered to a high density at elevated temperatures (i.e., typically at >1550 ±C17,18,24,26–28,39 ). However, sintering-induced shrinkage will cause the fired precursor body to lose the dimensions and, if such dimensional changes are nonuniform, the shape of the formed precursor. While a glass-ceramic process can be used to produce a shaped J. Mater. Res., Vol. 13, No. 5, May 1998
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celsian body,40–44 temperatures in excess of 1760 ±C are required to melt a stoichiometric celsian compositi
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