A. Driessen

Pressure dependence of the T/sub c/ of YBa/sub 2/Cu/sub 3/O/sub 7/ up to 170 kbar

The superconducting onset temperature T/sub co/ of single-phased YBa/sub 2/Cu/sub 3/O/sub 9-//sub delta/ (with deltaapprox. =2) measured resistively in a diamond anvil cell is found to increase at a rate dT/sub co//dp = 0.043 K kbar/sup -1/ up to 170 kbar. This is much weaker than for La-Ba-Cu-O for which dT/sub co//dp = 0.64 K kbar/sup -1/. The pressure dependence of the high-T/sub c/ superconductors measured so far cannot be explained within a standard electron-phonon Bardeen-Cooper-Schrieffer theory. Predictions of resonating-valence bonds and bipolaronic theories are discussed.

Search for new metal-hydrogen systems for energy storage☆

Abstract The heats of formation of the transition metal hydrides A5BHx A2BHx, ABHx, AB2Hx and AB5Hx (A ≡ Sc, Ti, V, Y, Zr, Nb, La, Hf, Ta, Ni, Pd; B ≡ transition metal) are calculated by means of a semiempirical model based on the electronic structure of the host alloy. Out of the 1380 possible compounds A5B, A2B and AB, only 44 are predicted by the model to react with hydrogen with a heat of formation between −12 and −25 kJ. Approximately half of these potential hydrogen storage systems have already been investigated.

Heat of formation and pressure-composition isotherms of disordered transition metal hydrides

Abstract A semiempirical model for the heat of hydride formation based on the electronic structure of the host metal is used to calculate local enthalpies of formation in disordered transition metal hydrides. These local enthalpies, which depend on both the atomic fraction of the hydrogen nearest-neighbour metal atoms and on the atomic volume of the alloys, are used to calculate the pressure-composition isotherms of alloys A 1− y B y H c . With increasing y the density-of-sites function of the alloy widens and the critical temperature is lowered. These theoretical predictions are in agreement with experimental data on Nb 1− y V y H c , Nb 1− y Mo y H c , V 1− y Cr y H c , Pd 1− y Ni y H c and Pd 1− y Pt y H c .

Thermal expansion, compressibility and Grüneisen parameter of YBa2Cu3Ox

Abstract The thermal expansion of YBa2Cu3Ox has been measured between 4 K and 1100 K. The bulk isothermal compressibility has been determined from direct measurement of the pressure-volume relation on compacted samples by means of a simple powder model which takes into account the porosity of the sample. From these data the average Gruneisen parameter 〈γ〉 ≡ -d ln 〈ω〉 / d ln V is found to be 3.1.

Diamond anvil cell for measurements in high magnetic fields

Abstract We constructed a diamond anvil cell for pressures up to 100 GPa in magnetic fields up to 12 T. The cell can be operated at any temperature between 10 and 350 K. Loading by condensation of gases is possible.

Superconducting Pd-Hydrides and -Deuterides Synthesized under High Pressure in a Diamond-Anvil-Cell*

A diamond anvil cell for pressures up to 1 Mbar has been developed for the synthesis of metal-hydrides in thermodynamic equilibrium with compressed molecular hydrogen gas. As example of an application of this new technique results on the critical temperature and its pressure dependence for stoichiometric palladium hydrides and deuterides are presented and discussed within the context of the standard BCS-theory of superconductivity. The behaviour of PdH and PdD under pressure is compared to that of the recently discovered high-Tc superconducting metal-oxides and fundamental differences are found.

Thermodynamic properties of hydrogen between 100 and 1000 K at pressures up to 1 Mbar

Abstract Using the available data from the literature in the fluid and solid phase we have constructed a consistent equation of state for molecular hydrogen. The relevant thermodynamic properties can be derived from the equation of state.

The formation of metal hydrides under very high hydrogen pressure

Abstract The formation of metal hydrides is described in terms of a mean field lattice gas model. The parameters of this model are derived for AgH x , which is expected to become a high T c superconductor for x ∼0.8. Two alternative complete sets of pressure-composition isotherms are given for pressures up to 1 Mbar and temperatures up to 1000 K.

Volume dependence of high-Tc superconductors

Abstract The pressure dependence of Tc is found to be large for the superconductors (La Ba Cu O, La Sr Cu O) with a relatively low Tc and small for Y Ba Cu O. The implications of this trend for the theories of high-Tc superconductivity (especially those based on a Hubbard-like model) are discussed.

Pressure dependence of the Tc of superconducting metal-hydrides, measured in a diamond anvil cell

Abstract For the preparation of high concentration metal-hydrides a very large chemical potential of molecular hydrogen is needed. This can be achieved by means of a high pressure diamond anvil cell. We developed a diamond anvil cell technique for filling the sample space at cryogenic temperatures with molecular hydrogen, loading metal samples with hydrogen at high pressures and in-situ measurements of the resistance and superconducting transition of the sample.