Joseph S. Cantrell

Crystal structure and hydriding behavior of LaNi5−ySny

The phase composition and structures of several LaNi 5-y Sn y alloys with y<0.3 have been studied using X-ray diffraction. These alloys are found to retain the hexagonal symmetry of LaNi 5 with increases in both lattice parameters. Since the Sn atoms are larger than the Ni atoms which are being replaced, small distortions are likely to arise in the positions of the Ni atoms located at the midplane 3g sites that are above and below the Sn atoms. Effects of variations in alloy stoichiometry and hydrogen absorption/desorption under different conditions are also reported

Thermodynamic and degradation studies of LaNi5 (closely stoichiometric)-H and LaNi5−xMnx-H with x = 0.5−2.0

Abstract Plateau pressures and reaction calorimetric enthalpies have been measured for activated (3×) LaNi5−xMnx-H for x = 0.5, 1.0, 1.5, 2.0 and for LaNi5 of rather exact stoichiometry. The calorimetric enthalpies for hydride formation increase in exothermicity and the plateau pressures decrease with x. There are extended regions of hydrogen solubility in the hydride phase where |ΔHH| falls linearly with H content for intermetallics with larger values of x. Degradation of the x = 1.5 alloy does not occur appreciably during cycling at 573 or 623 K but does occur significantly when it is allowed to remain at these temperatures at large hydrogen contents.

Thermodynamic properties and the degradation of ZrNiHx at elevated temperatures

Abstract The hydrogen absorption and desorption behavior of ZrNi was measured over the temperature range from 525 K to 700 K. The isotherms demonstrate that the (β + γ) two-phase region extends to these temperatures, in contrast to a recently published phase diagram. The hysteresis ratio of the absorption to desorption plateau pressures decreases with temperature and approaches unity just above 700 K. The pressure-composition data were reproducible, even at temperatures greater than 680 K, provided the isotherm measurements were completed within a few hours. If the hydrided ZrNiHx sample was held above 680 K for more extended periods, significant changes in the pressure and isotherms were observed. The extent of these changes is dependent on both the temperature and hydrogen content. Only minor changes were seen after 24 h at 700 K for x 1.5. X-ray diffraction studies of several ZrNiHx samples revealed that the degradation in the isotherms can be attributed to the formation of ZrHx and Ni via disproportionation.

Boron Modifications Produced in an Induction-Coupled Argon Plasma

Most of the small particles (50 to 100 micrometers in diameter) of microcrystalline β-rhombohedral boron that quickly transit an argon plasma maintained within a radio-frequency induction-coupled torch emerge as better crystallized spheroids of the same crystalline form and nearly the same size as the starting material. A few crystals of each of four distinctive, well-faceted habits are formed along with the general product. Three of these types are monocrystals of the β-rhombohedral polymorph, of the tetragonal-III modification, and of an unreported cubic form of boron. Specimens of the fourth type are polycrystals of another unreported form of boron, apparently consisting of many hexagonal platelets stacked in an imprecise fashion.

X-ray diffraction, neutron scattering and NMR studies of hydrides formed by Ti4Pd2O and Zr4Pd2O

Abstract The oxygen-stabilized Ti 4 Pd 2 O and Zr 4 Pd 2 O alloys with the cubic E9 3 -Ti 2 Ni type structure have been reacted with hydrogen gas. Powder X-ray diffraction (XRD) has shown that the hydride phase retains a face-centered-cubic unit cell with an expansion of lattice parameter that is dependent on hydrogen content. The crystal structure has been solved for Zr 4 Pd 2 OD 3.5 by neutron diffraction. NMR measurements of proton relaxation times were made on Ti 4 Pd 2 OH 5.0 to assess diffusion behavior. Although the activation energy of 0.39 eV derived for Ti 4 Pd 2 OH 5.0 was nearly identical to the value found for hydrogen diffusion in oxygen-free Ti 2 PdH 1.47 , the spin-lattice ( T 1d ) and rotating-frame ( T 1ρ ) relaxation times minima were at much lower temperatures for Ti 4 Pd 2 OH 5.0 , which implies more rapid hydrogen diffusion rates in the oxygen-stabilized phase. The nearest neighbor coordination sites that are involved in proposed hydrogen diffusion path ways are discussed and compared to the deuterium sites of Zr 4 Pd 2 OD 3.5 .

Phase composition and the effect of thermal cycling for VHx, V0.995C0.005Hx, and V0.975Zr0.020C0.005Hx

Abstract X-ray diffraction (XRD) studies were performed on hydride phases formed by vanadium and its carbon substituted alloys. It was previously found that thermal cycling of VH x across the β–γ mixed phase region changed the reversible hydrogen storage capacity and other properties. The present materials were compared where annealing was one prior treatment and the materials were cycled from 3 cycles to over 1000 cycles and one sample was cycled 6182 times. The effects on phase composition, c / a unit cell axial ratios, unit cell volume and crystallite particle size and BET surface area measurements were studied. Earlier studies placed the β–γ phase boundary at x =0.82 for XRD studies. The phase boundaries of the substituted alloys are found to be virtually the same. Thermal cycling produced similar behavior for all the materials.

Some physical properties of Zr2CuH4.2 in relation to those observed for other MoSi2-type hydrides

Abstract The intermetallic compound Zr 2 Cu with the tetragonal MoSi 2 -type structure produces a metastable ternary hydride Zr 2 CuH x . Powder X-ray diffraction indicated a monoclinic unit cell for this phase when x = 4.2. Nuclear magnetic resonance of the proton rotating frame relaxation times gave an activation energy of 0.28 eV for hydrogen diffusion in Zr 2 CuH 4.2 . Irreversible changes in the proton relaxation times were noted after the Zr 2 CuH 4.2 sample had been heated above 500 K. X-ray diffraction confirmed the decomposition of the original ternary phase into a mixture of ZrH x and other components, including metallic copper.

Reactions of lithium hydride and beryllium hydride: thermal studies and identification of products

Abstract Using simultaneous differential thermal analysis and thermogravimetric analysis, and powder X-ray diffraction, we determined that the reactions of lithium hydride and beryllium hydride follow the pathway for reaction mixtures containing less than 66.6 mol.% LiH, and for reaction mixtures containing 66.6 mol.% LiH or more. LiBeH 3 undergoes fusion at 430 K. Li 2 BeH 4 has two thermal transitions at 550 K and 660 K; the first may be intramolecular rearrangement and the second may be fusion. The two products are indistinguishable by powder X-ray diffraction.

Determination of deuterium site occupation in Zr4Pd2OD4.5

Abstract Powder neutron diffraction analysis has been used to determine hydrogen site occupation at room temperature in the lattice of the oxygen-stabilized Zr 4 Pd 2 O phase. For the composition Zr 4 Pd 2 OD 4.5 it was established that only two types of interstices, designated as D7 and D2, were occupied. Hydrogen in the D7 site is octahedrally surrounded by 6Zr. This site is nearly filled. In the D2 site hydrogen is tetrahedrally co-ordinated to 3Zr and Pd and is 1/3 filled. The distribution of hydrogen on the D2 sites is random.