Thermal diffusivity/conductivity of MgAlON-BN composites
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THERMOPHYSICAL properties such as thermal diffusivity, thermal conductivity, and heat capacity have both theoretical and practical importance. In many engineering situations, a knowledge of their thermophysical properties is essential to analyzing heat-transfer data in order to calculate the ability of a structural component to conduct heat or to dissipate a large quantity of locally generated heat.[1] For example, thermal conductivities of refractories are important parameters that underlie the design of furnaces, metallurgical reactor, boilers, etc.[2,3] In recent years, there has been increased interest in the area of ceramic-ceramic composites. The primary reason is that ceramic-ceramic composites have better properties than monolithic ones.[4,5,6] One such system that has been shown to have improved high-temperature mechanical properties, thermal shock durability, and wetting characteristics is magnesium aluminate spinel-boron nitride (MgAlON-BN) composite.[7,8,9] It is expected that this composite could find applications as high performance refractory, which can be used, for example, in special refractory nozzles, tubes, and break rings for the continuous casting of steel. To the best knowledge of the present authors, no systematic study of the thermal diffusivity/conductivity of MgAlON and MgAlON-BN composites has been carried out so far. The present work is thus motivated. Our goal in this work was to study the thermal diffusivity/conductivity of MgAlON-BN composites of variable composition, prepared by hot pressing technology and intended for operation under hostile environments. Z.T. ZHANG, Graduate Student, and S. SEETHARAMAN, Professor, are with the Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden. Contact e-mail: [email protected] W.C. LI, Professor, is with the Department of Physical Chemistry, University of Science and Technology, Beijing 100 083, People’s Republic of China. Manuscript submitted December 23, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
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EXPERIMENTAL PROCEDURE
A. Sample Preparation Single-phase MgAlON samples were prepared by mixing Al2O3, AlN, and MgO powders. Based on previous works in the present laboratory,[10] a mixture with a molar ratio of Al2O3/AlN/MgO 5 7/2/1 was selected to synthesize MgAlON. Then, different amounts of BN were added to the mixture. The weighed powders were mixed in a ball mill using an ethanol medium for 12 hours and then dried. The mixtures were then pressed into cylindrical compacts. These cylindrical compacts, covered with boron nitride, were hot pressed at 1800 °C under 20 MPa for 2 hours. X-ray diffraction analysis of the sample was carried out to confirm the formation of the MgAlON-BN. It should be pointed out that a small amount of MgO, AlN, and Al2O3 volatilized under the reaction conditions. Scanning electron microscopy (SEM) was used to examine the fracture samples. Disks with about 12-mm diameter and 1- to 2-mm thickness were prepared for thermal diffusivity measurements in directi
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