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Thick Film Heater for Aluminium Nitride Ceramic
Keywords: thick film technology, heater, aluminium nitirde
Heater systems for isolated steel are commercial available. Those systems consist of a dielectric paste, heater pastes with different sensitivities and contact paste. The requirements differ for the different pastes. The dielectric paste needs high isolation resistivity and dielectric strength, as well for room temperature as for higher temperatures. The solid components of the heater paste are palladium (Pd) alloyed silver (Ag) and glass. The ratio of Ag to Pd and the amount of the metallic phase influence resistivity and its temperature coefficient. In the last years the demand for compatible paste systems for aluminum nitride (AlN) substrates is increased. AlN shows excellent thermal conductivity and good resistance to thermal shock, making these substrates interesting for heater applications. However, the available paste systems cannot be used for AlN because the thermal expansion coefficient (TEC) of the used glass is too high. Steel substrates have higher TEC than that of the glass, which induce compressive stress into the glass after the firing process. On the other hand, AlN has a lower TEC which result in tensile stress. The tensile stress can caused cracks into the glass and no adhesion of the film. The investigations of these work will show the development of compatible glass, which can be used as dielectric. Especially the thermal expansion coefficient of the glass should be suited to AlN. A heater paste will be developed based on the new glass. The investigations of the conductive components are done for silver and palladium. The influence of the silver palladium ratio on the thermal coefficient of the resistance (TCR) is investigated. Furthermore the heater structures are characterized by their temperature distribution with IR-camera. The maximum of electrical load of the heater structure will be determined with the STOL (short time overload) test. Originally submitted to Advanced Materials.
Melanie Hentsche, Scientist
Fraunhofer Institute for Ceramic Technologies and Systems
Dresden 01277,
Germany


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