Energy and environmental research emphasizing low-rank coal: Task 6.2. Joining of advanced structural materials

Abstract

Silicon carbide (SiC) is considered an attractive material for structural applications in fossil energy systems because of its corrosion and wear resistance, high thermoconductivity, and high temperature strength. These same properties make it difficult to sinter or join SiC. Conventional sintering techniques require applying pressure and heating to temperatures near 2000{degree}C, or the use of binders with lower melting temperatures, or pressureless sintering with the aid of carbon and boron to near full density about 2100{degree}C. The sintering temperature can be reduced to 1850{degree}--2000{degree}C if SiC is sintered with the addition of small quantities of Al{sub 2}O{sub 3} and Al{sub 2}O{sub 3} {plus} Y{sub 2}O{sub 3}. In addition, reaction sintering has been used by mixing Si and C with SiC powder and heating the mixture to 1400{degree}C to cause the Si and C to react and form SiC, which bonds the aggregate together. Work proposed for this year was to center on determining gas compositions that could be used to increase the sinterability of oxide binders and on using the binder and gas combinations to join bars of SiC, alumina, and mullite (3Al{sub 2}O{center_dot}2SiO{sub 2}). During the course of the year the focus was shifted to SiC joining alone, because it was felt that alumina and mullite are too prone to thermal shock for use in structural applications in fossil energy systems. Because of a thermal expansion mismatch between alumina and SiC, only SiC and mullite were investigated as joining aides for SiC. Therefore, the objectives of this work evolved into examining the sintering phenomena of SiC and mullite-derived binders at and below 1500{degree}C in various atmospheres and determining which conditions are suitable to form strong joints in monolithic SiC structures to be used at temperatures of 1000{degree}--1400{degree}C.