Millennial Special Leading Papers on Ceramics in the 20<sup>th</sup> Century: the Best of JCerSJ Effects of Additives on Thermal Conductivity and Electrical Resistivity of SiC Ceramics

Abstract

In the early 1980s, SiC ceramics with unique properties were developed by researchers in the laboratory of Hitachi, Ltd. These SiC ceramics have high thermal conductivity (>250W/mK) comparable to that of metals, as well as high electrical resistivity (about 4×1011Ωm) close to that of insulators such as alumina. This development was epoch-making, since at that time, no other materials exhibiting both high thermal conductivity and high electrical resistivity were known except BeO and the development of SiC ceramics exhibiting these properties could not be expected because SiC is naturally a semiconductor. In addition, as the thermal expansion coefficient of SiC ceramics (4×10-6/K) is nearly equal to that of Si, the application of this material in the field of semiconductors as a substrate and heat sink was anticipated.This material is commercially used at present as a heat sink for semiconductor lasers globally and commands a high share of the market.The period when this material was developed was the most active for the ceramics industry and development. Materials called “New Ceramics” or “Fine Ceramics” were attracting attention worldwide. The fact that high thermal conductivity and high electrical resistivity could be compatible in a popular material propelled ceramics researchers to try to find materials similar to these SiC ceramics. As a consequence, a remarkable increase of the thermal conductivity in AlN, which is more manageable than SiC for semiconductor packages, was subsequently achieved. The role of these SiC ceramics in promoting the development of these new materials is a major one.These SiC ceramics are also important from the point of view of grain boundary phenomena unique to ceramics research. The SiC is produced by sintering SiC powders with small amounts of BeO additives. High thermal conductivity and high electrical resistivity result from the change of the physical and chemical states of grains and grain boundaries caused by the addition of BeO. Because of the low solubility of Be into SiC grains, the purity of SiC grains is enhanced as the impurities are emitted to grain boundary regions. The resulting decrease of phonon scattering is thought to be the cause of the high thermal conductivity. At the same time, the low solubility of Be into SiC grains results in a low carrier concentration in SiC grains and the formation of high potential barriers for the carriers at the grain boundaries. This is the reason for the high electrical resistivity. ZnO varistor is an example of “grain boundary controlled ceramics” and the varistor property of SiC ceramics has also been recognized. The dramatic changes of grain boundary characteristics exhibited by these SiC ceramics have led to the recognition of the importance of grain boundary control. Technological advances for grain boundary analysis and control, including chemical composition distribution analysis and direct measurement of electrical properties at grain boundaries, were hastened. SiC contributed to these advances.Many papers have been written on these SiC ceramics. This paper details the effects of additives on thermal conductivity and electrical resistivity. The characteristics of SiC ceramics change markedly depending on the type of chemical additive. Of all the additives, only BeO results in anomalous thermal conductivity and electrical resistivity of SiC ceramics. This is due to the low solubility of Be into SiC grains. These are essential properties of SiC ceramics. This paper, describing these properties, deserves to be considered as one of “the leading papers on ceramics of the 20th century.”SiC ceramics with high thermal conductivity and high electrical resistivity are said to be the product of “serendipity.” The effect of BeO addition was discovered in the course of finding effective additives to develop SiC ceramics