Studies of Hydrogen atom configurations in selected metal hydrides in view of repulsive interactions

The main objective of the present thesis was to gain more insight into the nature of H–H separations in transition metal based metal hydrides in view of H atom configurations and repulsive H–H interactions. In some metallic hydrides anomalies with respect to typically used geometrical criteria (Switendick rule about minimum hydrogen atom separation (not less than 2.1 Å) and Westlake rule about minimum size of interstitial hole suitable for hosting hydrogen atoms (radius at least 0.4 Å)) are observed and likely to influence their sorption properties by changing the hydrogen storage capacity or/and thermal stability. The studies were partly inspired by work on metallic hydrides/deuterides revealing the directional metal-hydrogen bonding effect (e.g. CeNi3D2.8, HoNi3Dx, ErNi3Dx, Ce2Ni7D~4) and by reports on extraordinarily short D–D (H–H) contacts (1.635(8) Å for LaNiInD1.22) in deuterides/hydrides of RENiIn (RE – rare earth element). Considering the results of this work, the present thesis attempts to answer the question whether the (short) D–D (H–H) contacts in transition metal based metal hydrides were related to preferred configurations of hydrogen atoms in the lattice and if so, how

Studies of the effect of melt spinning on the electrochemical properties of the AB2 Laves phase alloys

A comparative study of the effect of melt spinning on the electrochemical properties of the C14 and C15 AB2 alloys has been performed. The wheel speeds of 630, 2100, and 4100 cm/s were applied during the rapid solidification of both alloys. The structural analysis of the formed phases was performed by X-ray powder diffraction (XRD), while their microstructural morphology was studied by scanning electron microscopy (SEM). In both alloys a tremendous grain refinement due to the melt spinning process was observed: In addition, melt spinning also significantly contributed to the morphological variation of the microstructural changes in C14 alloys which showed changes from the equiaxed grain at lower speed to the small dendrites at higher speed. In contrast to the C14 alloys, the morphological variation was not observed for the C15 alloys. Furthermore, for both C14 and C15 alloys melt-spun at 2100 cm/s the maximum capacities of 435 and 414 mAh/g were achieved, respectively. As both alloys revealed the significant grain refinement due to the melt spinning, an increase in electrochemical capacity was achieved. However, the melt spinning parameters need to be further optimized to improve poor activation behavior of the rapidly solidified alloys

The role of grain boundary scattering in reducing the thermal conductivity of polycrystalline XNiSn (X=Hf, Zr, Ti) half-Heusler alloys

Thermoelectric application of half-Heusler compounds suffers from their fairly high thermal conductivities. Insight into how effective various scattering mechanisms are in reducing the thermal conductivity of fabricated XNiSn compounds (X = Hf, Zr, Ti, and mixtures thereof) is therefore crucial. Here, we show that such insight can be obtained through a concerted theory-experiment comparison of how the lattice thermal conductivity κLat(T) depends on temperature and crystallite size. Comparing theory and experiment for a range of Hf0.5Zr0.5NiSn and ZrNiSn samples reported in the literature and in the present paper revealed that grain boundary scattering plays the most important role in bringing down κLat, in particular so for unmixed compounds. Our concerted analysis approach was corroborated by a good qualitative agreement between the measured and calculated κLat of polycrystalline samples, where the experimental average crystallite size was used as an input parameter for the calculations. The calculations were based on the Boltzmann transport equation and ab initio density functional theory. Our analysis explains the significant variation of reported κLat of nominally identical XNiSn samples, and is expected to provide valuable insights into the dominant scattering mechanisms even for other materials