Electrical Properties of the Cerium and Gadolinium Hydrogen Systems

Abstract

Conductivities of polycrystalline solid samples of cerium hydride and gadolinium hydride were measured over the composition ranges H/Ce=0–2.5 and H/Gd=0–2.13 at room temperature and at elevated temperatures (400°—750°C). Conduction is metallic and the results for the two systems are very similar. The conductivity in the variable composition dihydride phase of each system decreases with increasing hydrogen content and is explained assuming that the hydrogen is anionic, taking its extra electron from the system's conduction band. Measurements of absolute Seebeck coefficients (CeH2.37, —17 μV/°C; GdH2.12, —6 μV/°C) support this model. The measured conductivity curves extrapolate to zero at CeH2.7 and GdH2.3. For the dihydride phases the composition dependencies of the conductivities are used to calculate carrier mobilities. The values obtained are in the range 1 to 10 cm2/V-sec. The conductivities of compositions H/M=0 to 2 show that the dihydrides can exist with hydrogen-to-metal ratios much less than 2.0. Ionic conduction has been found not to be important to the conduction mechanism from the results of a concentration cell measurement.

An electrical resistance analysis of the erbium-hydrogen system

“…However, the general shape of his conductivity curve was similar to part BC of our isotherms; his measurements did not reach point D because they were performed up to H/Er=2.1 maximum. Heckman (1964) suggested that the /3/(p+y) phase boundary could be obtained by the linear extrapolation of his conductivity data from H/Me=2-0-2-1 to zero conductivity (see also Libowitz 1972). We think such an extrapolation has no physical justification, and it is contradicted by our data for AR/Ro and its temperature dependence round about point D on the curve.…”

“…Measurements of the electrical conductivity of the heavy rare earth metals (Gd, Ho, Er) in hydrogen have been reported only by Heckman (1964Heckman ( , 1969 as far as we know. His experiments were performed mainly at room temperature with bulk metal samples cleaned in a conventional way (-10-6 Torr) and he was unable to determine the solubility limit of hydrogen in the 01 phase from his bulk conductivity measurements (he did not see point A on the curve).…”

The Rare-Earth Hydrides

“…48 As for the other rare-earth metals, the conductivity increased linearly in the two-phase metal-plus-dihydride region reaching a maximum value of about 27,500 ohm _ 1 cm _ 1 (p = 36 μοηιη-cm) at GdHi. The electrical conductivity of the gadolinium-hydrogen system from gadolinium metal to GdH 2 .…”

“…More recently, Heckman 48 has measured the conductivities of hydrided bars of cerium in the composition range H/Ce = 0 to 2.5 and the temperature range room temperature to 750°C. His results, like Daou's results, showed an approximate linear increase in conductivity from cerium metal (p = 8 X 10" 5 ohm-cm) to CeHi.…”

Quenching effect in gadolinium — influence of hydrogen

“…The behaviour is similar after quenching from 350 K. This phenomenon may arisc from the resistivity of the Gd-H compounds which is lower than that of the pure metal (140 pQcm). The resistivity goes to a minimum of 57 pQcm for the compound Gd-H1 [3].…”