Microstructure of ZnO Thin Films Deposited by High Power Impulse Magnetron Sputtering

High power impulse magnetron sputtering was used to deposit thin (~ 100 nm) zinc oxide (ZnO) films from a ceramic ZnO target onto substrates heated to 150 °C. The resulting films had strong crystallinity, highly aligned (002) texture and low surface roughness (root mean square roughness less than 10 nm), as determined by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and atomic force spectroscopy measurements. Deposition pressure and target–substrate distance had the greatest effect on film microstructure. The degree of alignment in the films was strongly dependent on the gas pressure. Deposition at pressures less than 0.93 Pa resulted in a bimodal distribution of grain sizes. An initial growth layer with preferred orientations (101) and (002) parallel to the interface was observed at the film–substrate interface under all conditions examined here; the extent of that competitive region was dependent on growth conditions. Time-resolved current measurements of the target and ion energy distributions, determined using energy resolved mass spectrometry, were correlated to film microstructure in order to investigate the effect of plasma conditions on film nucleation and growth

Improved Dehydrogenation Properties of Ti-Doped LiAlH₄: Role of Ti Precursors

The dehydrogenation properties of LiAlH₄ doped with different Ti precursors (Ti, TiO₂, and TiCl₃) via ball milling are investigated. The results not only show significant decreases in the decomposition temperatures (<i>T</i>_dec) and activation energies (<i>E</i>_A) of the first two dehydrogenation reaction steps of LiAlH₄ by doping with TiO₂ or TiCl₃, but also reveal how each Ti precursor affects the dehydrogenation process. Although doping LiAlH₄ with TiCl₃ induced the largest decrease in <i>T</i>_dec and <i>E</i>_A, TiO₂-doped LiAlH₄ produced a decrease in <i>T</i>_dec and <i>E</i>_A that is quite close to the TiCl₃-doped sample as well as superior short-term stability, suggesting that doping with TiO₂ has certain advantages over doping with TiCl₃. Further, the underlying mechanisms associated with the Ti precursors during the dehydrogenation reaction of LiAlH₄ have been studied using quasi in situ X-ray photoelectron spectroscopy. The results reveal that the Ti⁴⁺ and Ti³⁺ reduction processes and the segregation of Li cations to the surface of LiAlH₄ during ball milling play critical roles in the improved dehydrogenation properties observed

Effects of Titanium-Containing Additives on the Dehydrogenation Properties of LiAlH₄: A Computational and Experimental Study

Metal hydrides are attractive materials for use in thermal storage systems to manage excessive transient heat loads and for hydrogen storage applications. This paper presents a combined computational and experimental investigation of the influence of Ti, TiO₂, and TiCl₃ additives on the dehydrogenation properties of milled LiAlH₄. Density functional theory (DFT) is used to predict the effect of Ti-containing additives on the electronic structure of the region surrounding the additive after its adsorption on the LiAlH₄ (010) surface. The electron distribution and charge transfer within the LiAlH₄/additive system is evaluated. Electronic structure calculations predict covalent-like bonding between the Ti atom of the adsorbate and surrounding H atoms. The hydrogen (H) binding energy associated with the removal of the first H from the modified LiAlH₄ surface is calculated and compared with experimental dehydration activation energies. It is seen that the weaker H binding corresponds to the larger amount of charge transferred from the Ti atom to adjacent H atoms. A reduction in charge transfer between the Al atom and surrounding H atoms is also observed when compared to charge transfer in the unmodified LiAlH₄ surface. This reduction in charge transfer between Al–H weakens the covalent bond within the [AlH₄]⁻ tetrahedron, which in turn reduces the dehydrogenation temperature exhibited by LiAlH₄ when Ti-containing additives are used