PNNL

Abstract

The feasibility of joining porous alumina (Al2O3) bodies to monolithic Al2O3 using Ag-CuO and Ag-V2O5 alloys via reactive air brazing (RAB) was examined for a nanoporous filter application. Brazing for these systems is complicated by the conflicting requirements of satisfactory wetting to fill the braze gap, while minimizing the infiltration of the porous body. By varying the firing time, temperature, and initial powder size, porous bodies with a range of pore microstructures were fabricated. The wettability was evaluated via sessile drop testing on monolithic substrates and porous body infiltration. Porous Al2O3/monolithic Al2O3 brazed samples were fabricated, and the microstructures were evaluated. Both systems exhibited satisfactory wetting for brazing, but two unique types of brazing behavior were observed. In the Ag-CuO system, the braze alloy infiltrated a short distance into the porous body. For these systems, the microstructures indicated satisfactory filling of the brazed gap and a sound joint regardless of the processing conditions. The Ag-V2O5 alloys brazed joints exhibited a strong dependence on the amount of V2O5 available. For Ag-V2O5 alloys with large V2O5 additions, the braze alloy aggressively infiltrated the porous body and significantly depleted the Ag from the braze region resulting in poor bonding and large gaps within the joint. With small additions of V2O5, the Ag infiltrated the porous body until the V2O5 was exhausted and the Ag remaining at the braze interlayer bonded with the Al2O3. Based on these results, the Ag-CuO alloys have the best potential for brazing porous Al2O3 to monolithic Al2O3.