Papers by Author: A. Polar

Abstract: There is need for efficient energy conversion systems based on domestic fossil or biofuels. Solid Oxide Fuel Cells (SOFC's) are attractive in this case, however sealing is a critical issue in SOFC development. The purpose of this investigation is to find a procedure to seal yttria stabilized zirconia (YSZ) electrolyte to the stainless steel electrical interconnect or gas manifold. The seal is usually exposed to high temperatures in the range of 500 to 1000°C. Brazing by in-situ alloying of nickel and titanium foils was performed to braze zirconia to 444- stainless steel. Different combinations of nickel/titanium foils were used; brazing was done in vacuum at 6 x 10-6 torr at 960°C, 1010, and 1030°C for different brazing times. The braze and interfacial microstructures were characterized by optical microscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). This paper assesses the effect of process parameters on the development and stability of the braze metal and the interactions of the filler metal with the two substrates.

Abstract: This paper describes the metallurgical interfacial reactions at elevated temperatures between reactive zirconium metal and stable oxide ceramics, specifically beryllia, yttria, and magnesia- zirconia composite ceramic. The ceramic/metal systems were preheated at 600°C, and then heated to peak temperatures of 1800°C or above, depending of the system, in ultra pure Argon atmosphere. After a short stay at the peak temperature, each system was cooled to room. The interaction was monitored during heating by a video camera and the interfaces were microscopically examined after the thermal cycle. The microstructure and chemical changes at the interface were evaluated via SEM and EDS. During heating of the beryllia/Zr system, the ceramic was initially reduced and Be alloyed the Zr metal in solid solution, causing Zr to melt locally at the interface at about 1600°C instead of 1855°C. The alloy Zr-Be liquid is what later dissolved the beryllia and infiltrated partially into the ceramic substrate. It seems that there was no solid state reaction between the Zr metal and yttria since Zr melted at its melting temperature of 1855°C; it is evident, however, that the liquid Zr partially dissolved yttria at the interface; yttrium and oxygen segregated to the grain boundaries. The solidified metal tightly bonded to the ceramic substrate as the system cooled to room temperature. In the Zr-MgO/ZrO2 system, Zr melted at 1855°C and it reduced the magnesia, but at the same time the magnesium was volatilized.