This paper studies the structure, temperature dependences of electric resistance, and adsorption properties of nanoporous three-component V-(N, He) coatings. The coatings were produced using the technique of ion beam assisted deposition, in particular, deposition of vanadium onto a titanium substrate simultaneously bombarding it with
Presently scientists believe that solid-state hydrogen storage devices used for the vehicles should have high net capacity, short fueling time and low desorption temperature. In this regard proper consideration is given to metal hydrides and chemical hydrides that are used for these purposes though have definite drawbacks due to properties degradation, heavy weight and problems of waste disposal.
Alongside with hydride candidates for hydrogen storage much attention is given to highly porous structures, in particular, carbon-based nanomaterials, fullerenes, glass microspheres and powder materials. Besides, the microporous structures can be created in a form of coatings using plasma technique.
Actually, the presence of a gas component within a deposited flow and a
high degree of process nonequilibrium are required to form a porous coating. It
was shown recently [
On the other hand, many papers are available that study the hydrogen behavior
in materials suitable for the first wall of thermonuclear reactor [
Thus, the above analysis shows that the fine-dispersed admixtures that are present in a coating in the form of soluble and insoluble gases can create in combination with open nanoporosity the conditions for capturing the hydrogen in large amounts.
The purpose of this research is to obtain a porous nanocrystalline vanadium nitride-based coating, to study its structure, electrophysical characteristics, and hydrogen adsorption capacity.
The coatings were created using ion beam assisted deposition plant
ARGO-1 [
Prior to applying a coating in order to create vacancy porosity in
titanium foil it was bombarded with
The studies of the initial stage of formation of VN composites including
temperature coefficient of resistance (TCR)
[
(a) Structure V-N composite; (b) resistivity as a function of temperature.
The structure of vanadium films that were deposited at bombardment with
mixed
(a) Structure V-(N, He) composite; (b) resistivity as a function of temperature.
The increase in the amount of equiaxed internal pores can be explained
by presence of helium in a bombarding ion beam. The energy of helium migration
in vanadium is not high. It is equal to 0.13 eV [
Due to the high energy of bombarding gas ions the formation of a structure
and phase composition of a coating has a double-stage character. At the stage
of nucleation when the surface of grain nuclei contacts the deposited metal atoms
the change in their sizes, distribution density and phase composition takes
place. At the stage of coating growth whose thickness is within the depth of
ion pass (50 to 80 nm) the bombardment is continued and due to this the
component composition of a composite also continues to change [
The availability of gas pores in V-(N, He) also leads to the change in
dependences of a value of electric resistance on annealing temperature (Figure
A hopping mechanism of conductivity prevails in a
The calculations based on the results of our previous work [
The measurement data showed that specific adsorption surface of studied
samples was equal to
A phase composition of a titanium foil that was subjected to a
preliminary bombardment with
(a) Titanium structure irradiated by
Figure
Pressure in the chamber as a function of temperature for (a) Ti; (b) Ti with V-(N, He) coating for different quantity of hydrogen injection: 1–38.6 ncm3; 2–57.9 ncm3; 3–95.5 ncm3; 4–112.3 ncm3, 5–155.7 ncm3, 6–185.2 ncm3.
Proceed from the
Isotherms of hydrogen adsorption of V-(N, He) coatings on titanium substrate: (a) not irradiated previously, (b) irradiated.
The figures show that the porous structure plays main role at
The obtained data show that the active hydrogen accumulators are the
result of the porosity of the coating obtained through the ion beam assisted
deposition but also that preliminary created by ion implantation. In addition
it should be noted that the activity of the preliminary created porosity
depends both on the hydrogen temperature and pressure. At low temperatures the
implantation based porosity contributes more to hydrogen absorption than at
high temperatures (compare Figures
For the comparison the absorption of hydrogen by a titanium substrate at hydrogen pressure of 0.25 MPa is no more then 4.4 ncm3/g. It is seen that the hydrogen-related activity of titanium is much lower in comparison with that of the created porous structures. This means that the porosity of a coating and substrate play a key role in absorption of hydrogen at a room temperature. At a higher temperature the increase in the hydrogen absorption may take place due to two factors, in particular, hydride formation in the intermetallide TiV phases and hydrogen capture with nitride traps that were formed during ion beam assisted deposition of coatings.
The results of the research show that the ion beam assisted deposition
can be used to obtain nanostructural porous structures. The bombardment of the
deposited vanadium coating with the mixed beam of nitrogen and helium ions
stimulates the formation of nitride phases whose intercrystal spaces contain
equiaxial pores. It is quite possible that the volume of pores is occupied by the
helium-vacancy clusters. The electric resistance of such materials exceeds that
of powder vanadium nitrides by factor of 10. A sign of the temperature coefficient of
resistance changes from a negative to the positive one in the temperature range
of 250 to
The results of adsorption studies showed that the obtained structures
have a rather developed specific surface. It has been established that the main
contribution to the process of hydrogen storage at low temperatures is made by
the created porous structure. At elevated temperatures the absorption capacity
of the intermetallide and nitride phases may increase. The preliminary ion
bombardment with the mixed