Powder Processing

High porosity Nb, Mo-Re, and Ir metals with controlled pore size have been produced in various shapes by a powder metallurgy method. Production of these materials involves tailoring the size of powders to attain the correct green pore size for a given packing density through various combinations of presintering, milling, or sieving. Powders were slurry cast into soft tooling to create complex shapes and test pellets, which were then debound and vacuum sintered to the desired density and pore size. With these techniques, densities as low as 35 % may be attained. In the case of spherical powders, the pore size can be accurately predicted using a particle packing equation. In the case of non-spherical powders, the equation predicts a pore size larger than measured. Background: The green pore size of a powder compact is a function of particle size, particle shape, particle size distribution, and packing density. Given the particle size D, and packing density f, the mean pore size d can be estimated by the following equation [1]: