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Material selection for thin film thermocouples operating at extremely high temperatures


Hecht, G. ; Societe Francaise du Vide -SFV-, Paris; Deutsche Vakuum-Gesellschaft:
Thin Films. Proceedings of the joint 4th International Symposium on Trends and New Applications in Thin Films TATF '94 and the 11th Conference on High Vacuum, Interfaces and Thin Films HVITF '94
Oberursel: DGM-Informationsgesellschaft, 1994
ISBN: 3-88355-199-6
International Symposium on Trends and New Applications in Thin Films (TATF) <4, 1994, Dresden>
Conference on High Vacuum, Interfaces and Thin Films (HVITF) <11, 1994, Dresden>
Fraunhofer IST ()
adhesion; alumina; boron nitride; cover layer; crack; glassy carbon; high temperature material; insulator layer; molybdenum; short circuit; thermal expansion coefficient; thin films; vacuum annealing

For correct temperature measurement at different positions on small cartridges heated up to high temperatures (1500 degree Celsius and more) High Temperature Thin Film Thermocouple (HTTFT) arrangements shall be used. If electrically conducting cartridge materials are chosen, insulating intermediate layers must be deposited before the thin film thermocouple array can be prepared. As promising insulator layer materials boron nitride and alumina were selected. For these materials even at elevated temperatures a sufficiently high electrical resistivity can be expected. Moreover a reliable adhesion of the coatings, even after several heating cycles, requires nearly similar thermal expansion coefficients of layer and substrate. Considering this viewpoint, as promising cartridge materials tungsten, molybdenum and glassy carbon were selected. The coated samples were heated several times in a furnace under high vacuum conditions (residual pressure smaller than 10 high -3 Pa) up to 1500 degree C elsius. The samples were investigated by different techniques (scanning electron microscopy, X-ray diffraction). From our experiments many information on the high temperature behaviour of different substrate and layer materials were derived. The combination molybdenum substrate - alumina layer was selected as most promising. Even after six heating runs the layers showed a good adhesion. However, during the first heating run the sputtered alumina layers undergo phase transformations to the stable corundum structure. Because of the higher density of the corundum phase small cracks and therefore electrical short circuits were aenerated. For HTTFT applications completely insulating layers are necessary. Obviously this requires high temperature deposition techniques for the alumina preparation. For the thermocouple films the combination W - W/Re was used. For HTTFT operation at temperatures bigger than 1000 degree Celsius a cover layer is necessary to avoid re-evaporation of W- W/Re films.