Нові гідриди інтерметалічних сполук і сплавів на основі цирконію та титану

Zavaliy I.Yu. New hydrides of intermetallic compounds and alloys on the basis of zirconium and titanium. Manuscript.

Thesis for a degree of a Doctor of Sciences in Chemistry on speciality 02.00.01 inorganic chemistry; Ivan Franko Lviv National University; Lviv, 2006.

The dissertation is focused on studies of processes of hydrogen absorption/desorption (in gaseous phase and 6M KOH alkaline solution) of new and known intermetallic compounds and alloys on the basis of zirconium and titanium, namely, oxygen-modified з-phases with the Ti₂Ni (з-Fe₃W₃C) type of structure, oxygen-stabilised Zr₃Fe(Co,Ni)O_x compounds with the structure of the Re₃B type, the k-Zr₉V₄S phase with the Hf₉Mo₄B type structure, ternary Zr₆MX₂ (M=Fe,Co,Ni; X=Al,Ga,Sn,In) IMC with the Fe₂P type structure.

The single phase deuteride of the Zr₃V₃O_0.6D_9.6 composition has been synthesised for the first time and its crystal structure has been refined. A doubling of the hydrogen absorption capacity compared to the Zr₃V₃OD_4.8 deuteride has been shown to occur due to redistribution of oxygen atoms from the 16с position and filling of T1 interstices (triangular faces Zr₃). The distribution of D-atoms in the structure of the -Zr₃V₃O_0.6D_9.6 is compared with that in structure of other -phase hydrides in systems of Ti, Zr and Hf with transition d-metals. The study of hydrogenation properties of the (Zr,М 1)₃(V,М 2)O_x alloys, where М 1=Ti, Hf; М 2=Nb, Fe, Ni revealed that the highest hydrogen absorption capacity has been revealed by alloys with high level of Ti substitution for Zr - the H/M ratio for them rises up to ~2. However such alloys have slow hydrogenation kinetics. Although Fe-substituted alloys reveal better activation conditions, such a substitution is accompanied by substantial decrease of the capacity.

The oxide modification of the Zr_Fe alloys, particularly by oxides of rare-earth metals, was found to result in improvement of their hydrogenation characteristics. A number of the -Zr₄Fe₂O_xH_y hydrides has been synthesised and their hydrogenation capacity was shown to change with changes in oxygen content. The formation of saturated hydrides is shown to proceed with preservation of the structure of metallic matrix (although there exists small disordering of some metallic atoms); the lattice parameters the hydride phases and conditions of their thermal decomposition have been found. Magnetic and Mцssbauer studies have been performed for parent and hydrogenated -Zr₄Fe₂O_x and (Hf,Zr)₂Fe compounds. Saturated hydrides at low temperatures (up to 100...200 К) undergo magnetic ordering, whereas parent phases remain paramagnetic in the all temperature range.

The existence of the continuous solid solution in oxygen-modified Ті_4-хZr_xNi₂O_0.3 (0?х?4) alloys at 1250 К has been found for the first time. The zirconium substitution for titanium gives rise in hydrogenation capacity of the Ті_4-хZr_xNi₂O_0.3 alloys from 6 to 7.5 at./f.u., improves the hydrogenation kinetics and increases the stability of the synthesised hydrides. The crystal structure of the Ті₃ZrNi₂O_0.3D_6.7 deuteride has been refined on the basis of neutron diffraction analysis. The structure of the metallic matrix of the hydride was found to be disordered compared with the matrix of the parent alloy. Electrochemical testing of MH- electrodes prepared on the basis of the Ti₂Ni-type alloys indicated the positive influence of oxide modification and partial replacement of titanium by zirconium and vanadium on their discharge characteristics. Replacement of titanium by zirconium at small (up to ј) concentrations of the latter leads to the increase in the discharge capacity. The rise of oxygen concentration in the Ті_4-хZr_xNi₂O_y alloys the amount of hydrogen absorbed from the gaseous phase gradually decreases, however the amount of the electrochemically desorbed hydrogen substantially grows and for optimal compositions it exceeds 90 %. The cyclic stability of electrode materials on the basis of the (Ti,Zr)₂Ni alloy can be increased by partial replacement of titanium by vanadium, since the latter has higher resistance against corrosion in alkaline solutions, the oxygen-modified and vanadium-doped alloys being characterized by the best cyclic stability parameters, which could be explained by simultaneous rise of resistance toward disproportionation and formation of oxygen-stabilised phases. The influence of the HDDR processes on the charge-discharge characteristics of the MH-electrode have been studied for the first time. Such a chemical-technological treatment improves the microstructure homogeneity of Ti₄Ni₂O_x alloys and provides an increase of discharge capacity by 20-30 %, as well improves cyclic stability of the MH-electrodes based on the Ti₄Ni₂O_x alloy.

Saturated hydrides (deuterides) of oxygen-containing Zr₃MO_x compounds (M=Fe,Co, x = 0 1.0 і M=Ni, x = 0.4 1.0) with the Re₃B-type structures under 1...1.2 bar H₂ pressure have been synthesised for the first time and their structures have been studied by X-ray and neutron powder diffraction. The study of hydrogenation of these compounds revealed that the rise in oxygen content results in the decrease of hydrogenation capacity from 6.7 H/f.u. for the Zr₃Fe down to 5.35 H/f.u. for the Zr₃FeO_1.0 compound. The rise in oxygen content gives as well slight decrease of lattice parameters of parent intermetallics and their substantial increase for the saturated hydrides for all studied series. Hydrogen induced specific expansion of the unit cell V/at.H increases from 2.195 Е ³ for the Zr₃FeH_6.7 up to 3.378 Е ³ for the Zr₃FeO_1.0H_5.35. For all the studied compounds the influence of oxygen is less pronounced than for phases of the Ti₂Ni type. The study of crystal structure of the Zr₃NiO_xD_y saturated hydrides revealed the existence of redistribution of oxygen atoms upon deuteration. However after hydrogen desorption the refinement of the structure of the Zr₃NiO_0.6 alloy revealed the complete return of oxygen atoms into the initial positions. The hydrides of the zirconium-based k-phases, characterised by the Hf₉Mo₄B type of structure have been synthesised for the first time. Neutron diffraction analysis allowed refinement of the structure of the saturated Zr₉V₄SD_~23 deuteride.

The hydride (deuteride) of the Zr₆FeAl₂ intermetallic has been synthesised for the first time. The structure of the Zr₆FeAl₂D₁₀ deuteride has been refined with the use of neutron diffraction data. Hydrides of related intermetallics Zr₆MX₂H_x (M=Fe,Co,Ni; X=Al,Ga,Sn,In), have been synthesised and their structures were refined with the use of the X-ray diffraction. For most of hydrides the thermal stability has been studied in the 300...900 К temperature range. For all studied hydrides it has been found that hydrogenation leads to doubling of the unit cell and transition from the P 6m2 to P 62c space group. Quantum chemistry calculations indicated the change of electronic structure of the Zr₆FeAl₂ upon hydrogenation; the population of orbitals for the corresponding hydride and affinity of different interstices toward hydrogen have been calculated.

The studied materials of certain compositions can be used as efficient hydrogen storage media (for example, (Ti,Zr)₄Fe₂O_x), and those with low plateau pressure are promising as getters of hydrogen and other active gases (Zr₄Fe₂O_x, Zr₃MO_x, Zr₉V₄S). Oxygen modified Ti₂Ni-based alloys are proposed as negative electrode materials for Ni-MH chemical power sources, and the HDDR process are proposed for the treatment of MH-electrode materials to improve their characteristics.

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