Active metal brazing of various metals on nitride ceramics

The development of new materials for thermoelectric generators (TEG) with higher operating temperatures requires improved metallised substrates. Commonly used alumina substrates with copper metallisation show low thermal conductivity and low stability against thermal cycles. Aluminum nitride (AlN) and silicon nitride (Si3N4) are very attractive alternative substrate materials due to their high thermal conductivity, electrical resistivity and mechanical strength. However, it is important to realise reliable bonds between the nitride ceramics and metals like copper, nickel or refractory metals to ensure that AlN and Si3N4 can be practically used as substrates for the hot or cold side of a TEG. In this work, the interactions between two silver based active braze filler metals, nitride c eramics and nickel, tungsten and molybdenum have been examined. The microstructures of the ceramic-metal composites and the elemental distributions of the reaction products at the ceramic/braze and metal/braze interfaces were analysed using scanning electron microscopy and energy dispersive X-ray spectroscopy. In all cases the formation of a TiN reaction layer at the ceramic/braze interface was observed. Active metal brazing of nickel to AlN and Si3N4 was possible with both brazes, whereas the metallisation of the nitride ceramics with the refractory metals worked only in combination with AlN.