Solution and Diffusion of Hydrogen in Tungsten

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Abstract
Using mass spectroscopic and ultrahigh vacuum techniques, solution and diffusion of hydrogen in tungsten was investigated for pressures between 600 and 10−8 Torr and temperatures between 1100 and 2400 K. Solubility and diffusion constants are derived from degassing rates of a solid cylinder which was pre-loaded with hydrogen at ≈600 Torr. The solubility constant, S=2.9×10−1×exp (−24000/RT) Torr liter/cm3Torr1/2, and the diffusion constant, D=4.1×10−3×exp (−9000/RT) cm2/sec, are obtained, which in conjunction with literature values for the permeation constant P are consistent with the equation P=SD. Comparison to theory indicates that the solubility and diffusion constants are characteristic of interstitially dissolved hydrogen. Expressions are derived for the concentration of interstitial hydrogen as a function of pressure and temperature. Semiquantitative values for the total hydrogen concentration at low pressures are derived from H2 pressure changes which result when a sample is flashed between selected, high temperatures. Below 10−4 Torr, the total hydrogen concentration appears to be several orders of magnitude higher than the concentration of interstitial hydrogen, indicating that hydrogen is held in, and diffuses via both interstitial sites and a second kind of site of unknown nature. A semiquantitative analysis of diffusion is given for the case of the diffusing species held in, and diffusing via, two different kinds of sites.

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