The hydrogen permeation through pure niobium metal was investigated using a gas permeation technique, focusing mainly on the measurements in a highly soluble hydrogen state. The hydrogen diffusion was found to be the rate determining step for the hydrogen permeation reaction. However, the Sieverts law that the hydrogen solubility in a metal is proportional to the square root of hydrogen pressure, was no longer satisfied in the experimental condition of the temperatures of 473 K to 673 K and the applied pressures of 260 kPa at the inlet and 60 kPa at the outlet for hydrogen. This was mainly due to the presence of a large amount of soluble hydrogen in the metal under this condition. As a result, the measured apparent hydrogen permeability decreased monotonously with decreasing temperature in pure niobium. This was completely the reverse of the temperature dependence of the hydrogen permeability in the low solubility state where the Sieverts law was satisfied. It was stressed here that the permeation measurement of hydrogen in the high solubility state was indeed necessary for getting some performance data of metal membranes in practical use.

The deformation and fracture modes of pure niobium metal membrane were investigated by a series of hydrogen permeation tests in a highly soluble hydrogen state. The membrane underwent a large plastic deformation due to the lattice expansion and contraction during hydrogenation and dehydrogenation. A crack initiated on the membrane surface under the tensile stress condition, but not under the compressive stress condition. All the fractured sections in the membrane showed the characteristics of quasi cleavage fracture. However, even in the case of a large amount of hydrogen dissolved into the membrane, it never fractured as far as hydrogen keeps permeating through the membrane at a constant temperature and a constant hydrogen pressure difference between inlet and outlet surfaces. In other words, brittle fracture of pure niobium metal membrane was not caused by the amount of dissolved hydrogen, but by either a rapid change in the hydrogen density in the membrane or a continuous hydrogen dissolution reaction in progress into it.

The Lattice constants of pure niobium metal were measured by in situ X ray diffraction method at the temperature of 293 K to 673 K and the hydrogen pressure below 0.3 MPa. The deformation and fracture of niobium metal membrane during hydrogen permeation were treated by the measured lattice strain. The membrane was deformed and fractured easily when about 10 lattice contraction was introduced to it by lowering the hydrogen pressure at 673 K, at which the dehydrogenation reaction progressed smoothly. However, neither deformation nor fracture took place at the temperature lower than 473 K, because the dehydrogenation reaction hardly occurred in such a temperature range. Thus, the principal reason for the deformation or fracture of niobium metal membrane was attributable to the large volumetric contraction during dehydrogenation, the easiness of which, however, depended largely on the temperature.