Nanocrystalline Porous Thin Film VNx Hydrogen Absorbents: Method of Production, Structure and Properties

Abstract

Abstract. Vanadium and its alloy-based hydrides are extensively studied with regard to their use as hydrogen absorbents. The ion beam-assisted deposition method (IBAD) used for nanocrystalline VN x -H y thin-film hydrogen storages production is analyzed. The data of transmission and scanning electron microscopic studies of all stages of the film formation are considered. The main mechanisms of intergranular pores formation in nanograin structures have been established. The relation between the parameters of the ion beam-assisted deposition and those of film structure has been shown. The obtained data provide the explanation of the mechanisms of hydrogen absorption and desorption by thin films. It was suggested that the availability of branched network of intergranular pores allows accumulating the hydrogen by VN x -H y nano structures in large quantities and release it at the temperatures less than 275°C.Keywords: Nanocrystalline structures; hydrogen; storage; thin films; ion-beam assisted deposition. IntroductionVacuum deposition techniques, such as magnetron sputtering, physical and chemical vapor deposition, have been successfully used for thin films deposition for a long time. A characteristic feature of these techniques is that the resulting structures are formed under less than equilibrium conditions allowing the creation of materials with unique properties. B. Movchan and A.Dymchishin in 1969 [1] have shown the substrate temperature influence on the structure of Ti, Ni, W, ZrO 2 , and Al 2 O 3 films. Evaporation of the substances was carried out by the electron beam heating of the crucible. Three temperature zones with boundary temperatures T 1 and T 2 , that are respectively equal to 0.3 and 0.45 ... 0.5 of T m for metals, 0.22 ...0.26 and 0.45 ... 0.5 of T m for oxides were determined. The films with certain structure and properties have been formed inside each of these zones. These studies marked the beginning of the so-called structure zone model (SZM). Later J. Thornton [2] took into account the effect of the gas environment on the film structure additionally to the influence of the substrate temperature. It was shown that the film formation mechanism during ion bombardment was fundamentally changed not only by substrate temperature but also by pressure of the working gas (argon). Effect of the reactive gas (oxygen) on the formation of the microstructure of thermally evaporated metal (in that case, aluminum) has been studied in detail by P. Barna and M. Adamik [3]. Oxygen adsorbed on the surface during deposition reduces the mobility of grain nucleating centers and inhibits their coalescence. It results in the breaking of the columnar structure and nucleation of the grains with different texture. Eventually, at a high concentration of oxygen molecules, formation of aluminum oxide matrix with metallic inclusions is observed.The substrate temperature and gas concentration influence the formation of the film structure only at the initial stage. At this stage, the density of nucleation centers is determ...

Nanocrystalline Porous Hydrogen Storage Based on Vanadium and Titanium Nitrides

“…The experiments on the hydrogen absorption and desorption were carried out using the purpose made equipment. Examples of preparation and examination techniques were described in the original papers [19][20][21][22][23].…”

“…Our electron microscope studies [20][21][22]25] of VN films structure at the inhomogeneous phase of their growth Figure 3 shows SEM (a) and TEM (b) images of VN films deposited on silicon substrates. In the first case, the electron beam was directed at the film surface.…”

“…The measurements showed that the amount of hydrogen in VN sample was similar to the sample that was used for the adsorption analysis and was equal to 7.4 wt.% that confirmed the data from adsorption studies. (detailed description of hydrogen desorption experiment is in [22,23]). It is seen that the hydrogen desorption starts already at 30 ∘ b.…”

“…TiH 2 absorbs 4.0 wt.% H 2 . Therefore, the amount of absorbed hydrogen atoms comes to be 11.2 in VH 2 and 9.1 in TiH 2 (at/cm 3 , ×10 22 ). Amounts are essentially higher than, for example, in popular MgH 2 hydride (2.5 at/cm 3 , ×10 22 ) [1].…”

Synthesis of Nanopowders and Consolidation of Nanoceramics of Various Applications

“…The development of physical synthesis by gas condensation initiated relatively recent research in the field of purposely nanostructured ceramics [2]. In the last decades, nanostructured materials were found suitable in many advanced technologies such as thermomechanics, aerospace, nuclear power plant and biomedicine [3], especially in form of nanoceramic and nanocomposite coatings [4] or thin films [5]. Preparation of nanomaterials like nanoceramics is a complex process starting from the synthesis of oxide nanoparticles precursors and ending with the sintering [6].…”