Solution of hydrogen in niobium

Ultra-high vacuum techniques have been used to investigate the phase diagram of the hydrogen + niobium system by pressure and electrical resistance measurements over the ranges –78 to 300°C, H/Nb = 0–0.65, and 10–7–10 torr [1 torr = 1.33 × 103 N m–2]. Rapid sorption of gaseous hydrogen was observed at all temperatures down to –78°C. The boundary of the two-phase region is symmetrical about H/Nb = 0.256. The critical point is at 211°C, H/Nb = 0.256, 1 × 10–1 torr. The plateaux pressures p in the two-phase region are given by log10p(torr)=(7.87 ± 0.14)—(82.5 ± 1.1) kJ (mol 2.303 RT)–1. Isotherms of electrical resistivity plotted against hydrogen concentration show changes of slope at H/Nb ratios corresponding to the onset of hydride formation and reach maxima at H/Nb = 0.5. Matthiessen's rule is obeyed in the solid-solutions (α) phase while in the mixed-phase region a marked resistivity-temperature hysteresis occurs near 86°C. The hysteresis is thought to be associated with a change in lattice structure of the hydride.