Sintering studies on ultrafine ZrB 2 powder produced by a self-propagating high-temperature synthesis process
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A detailed study on the sintering behavior of zirconium diboride powder produced by the self-propagating high-temperature synthesis (SHS) process was carried out in the temperature range of 1500–1800 °C. The fine powder prepared by the SHS process exhibited excellent sinterability and could be sintered at 1800 °C for 1 h to approximately 94% of the theoretical density. The apparent activation energy of densification in the range of 1500–1800 °C was estimated to be 248 ± 4 kJ mol−1. A zirconium dioxide layer formed on the surface of the sintered body and was attributed to boron oxide formation during sintering and concurrent surface oxidation by the oxygen generated from the reduction of boron oxide in the carbonaceous atmosphere. Sintering aids like Fe and Cr appeared to help in densification of ZrB2 powder.
I. INTRODUCTION
Zirconium diboride is emerging as a potential advanced ceramic material because of its excellent properties: high melting point, hardness, elastic modulus, and electrical conductivity; and excellent chemical resistance to HCl, HF, and other nonferrous metals, cryolite, and nonbasic slags. The salient features of ZrB2 are given in Table I. These excellent properties make ZrB2 a potential material for use at high temperatures requiring wear, high-temperature oxidation, and corrosion resistance. Zirconium diboride has several applications, such as cathodes for electrochemical processing of aluminium (Hall–Heroult process), evaporation boats, crucibles for handling molten metals, thermowells, thermocouple sleeves for high-temperature use, wear parts, nozzles, armor, cutting tools, and as dispersoid in metal and ceramic matrix composites to improve mechanical properties. Several techniques have been established to prepare zirconium diboride powders. The different techniques include preparing powders from elements by melting, sintering, or hot pressing; borothermic reduction of metal-oxides and boric oxide; reduction of the metal oxide with carbon or boron carbide; aluminothermic, magnetiothermic, and silicothermic reduction of metal oxideboric oxide mixture; and self-propagating hightemperature synthesis (SHS) from elemental powders of boron and zirconium. In the SHS process, the exothermic heat generated during the chemical reaction is exploited to form useful materials. The essential feature of the process is that the heat required to drive the chemical reaction is supplied from the reaction itself. In recent years the SHS process J. Mater. Res., Vol. 15, No. 11, Nov 2000
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has found applications for preparing intermetallics and advanced high-temperature materials, particularly carbides, borides, silicides, and nitrides.1,2 Advantageous, fundamental, and technological aspects of SHS have been reviewed in literature.3,4 The SHS technique has inherent advantages over the other methods that require high-temperature furnaces and longer processing times. Materials produced by SHS have advantages such as high purity of product,5 low energy requirem
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