The manufacture of special purpose thermionic valves operating at relatively high temperatures and under conditions of high vacuum necessitates the use of brazing alloys specifically suited to these requirements. Such alloys must be free from impurities that might hinder wetting and flow of the molten brazing material, must have low vapour pressures at high service temperatures, high melting points and, where needed, good mechanical properties at high temperatures.

While the well known silver-copper eutectic alloy melting at 778°C can satisfy a good many of these requirements when produced under careful conditions, alloys containing palladium show distinct advantages in terms of both vapour pressure requirements and mechanical properties, while also giving improved wetting characteristics on molybdenum, tungsten and nickel alloys. In addition they offer reduced risk of failure of the joints due to intergranular penetration.

The Pallabraze range of high purity palladium brazing alloys developed by Johnson Matthey was described in this journal some years ago by M. H. Sloboda (Platinum Metals Rev., 1963, 7, 8), but a revised Data Sheet just published by the company brings up to date the detailed properties and uses of these alloys. As will be seen from the table, a range of brazing materials is available to provide melting points spaced at convenient intervals so that complex assemblies can be fabricated by the step-by-step technique.