Thermal conductivity of high temperature superconductor substrate materials: Lanthanum aluminate and neodymium aluminate
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The thermal conductivities of LaA10 3 and NdA10 3 have been measured for the first time between 10 and 300 K. These substrates can be classified as intermediate thermal conductors. The measured room temperature values of 9 W m"1 K"1 for LaAlO3 and 7.5 W m"1 Kr1 for NdA103 are close to those predicted from the crystal structure of these materials, despite the fact that the crystals are twinned.
I. INTRODUCTION Since the discovery of superconductivity at elevated temperatures in copper-oxide perovskite systems, two broad areas of research in this field have developed. On the one hand, studies of the basic properties of these structures have been carried out with the aim of understanding the mechanism of Cooper-pairing in these materials. On the other hand, much effort has already been expended in preparing very high quality thin film superconductors for device applications. With regard to the growth of high-r c superconductor thin films, the first and perhaps most important question a researcher must ask is, what material do I use for my substrate? There are many factors that govern the choice of substrate, and the selection depends to some extent on what application one has in mind for the proposed thin film device. An obvious restriction is that one must choose a substrate that is conducive to growth of a high quality film. Generally it is desirable to select a substrate whose lattice constant is near (say within a few percent) that of the high-Tc superconductor that is to be grown on it; a good lattice match between film and substrate is less likely to strain unduly the structure of the thin film and degrade its superconducting properties. For a similar reason, it is desirable to choose a substrate that has a thermal expansion coefficient close to that of the superconductor. These are probably the two most important criteria for choosing a proper substrate for thin film growth, and they significantly reduce the number of available options. On the other hand, for specific applications of these films, other substrate parameters may play an important role. For instance, in high frequency situations, rf losses and dielectric constant clearly must be considered. In any application in which the substrate/film system may be exposed to large heat loads, the nature of the thermal conductivity of the substrate clearly becomes an important criterion; for instance, a substrate must conduct heat away fast enough to maintain the superconducting film below its transition temperature. While the heat 2492
http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 9, Sep 1992
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conduction properties of common substrates such as silicon and sapphire are well known, the same cannot be said for some of the more exotic materials which have cropped up as substrates for high-Te superconductors. It is the purpose of this study, therefore, to investigate the heat conduction properties of two such substrates, LaA10 3 and NdA10 3 , in order to provide a guide for crystal growers who have in mind applications in whic
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