# Electric and Adsorption Characteristics of Nanocrystalline V-(N, He) Coatings

Authors: Alexey G. Guglya; Viktor G. Kolobrodov; Ruslan L. Vasilenko
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

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 withN2+He ions of 30 keV. It is shown, in contrast to V-N composites that have negative TCR (temperature coefficient of resistance), that the V-(N, He) coatings have negative TCR only in the temperature range of 250 to 350C∘. A specific surface of pores and adsorption characteristics of V-(N, He) composites depend on preliminary treatment of a titanium surface. A coating deposited onto an untreated substrate has specific surface of 25.5 m2/g. The preliminary irradiation of titanium with N2+He beam up to a dose of 6 × 1017 ion/cm2 provides the increase in surface area up to 57.6 m2/g. The preliminary ion bombardment provides a considerable increase in hydrogen adsorption capacity of coatings. Especially, such impact is noticeable at the room temperature, when the amount of hydrogen absorbed by a coating that was applied onto a treated surface is more than three times of that absorbed by a coating that was deposited onto nonexposed titanium.


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## Body

## 1. Introduction

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 [1, 2] that in case of ion beam assisted deposition the composites with low phase formation Gibbs
energy have nanocrystalline structure
whose intercrystal space is filled with nanopores. It has also been established
that such (VN) structures have high heat-resistance and can withstand
temperatures up to 500°C and these are not subjected to blister
formation at irradiation with helium ions whose dose amounts to 2×1017ions/cm2 [3]. Moreover, such type of treatment provides transformation of a closed
porosity into the open one.On the other hand, many papers are available that study the hydrogen behavior
in materials suitable for the first wall of thermonuclear reactor [4–7]. In
particular it was established that presence of admixtures in a form of soluble
gases within a crystal lattice of structural materials creates conditions for
hydrogen capture. The bonding energy, for example for C–H pair reaches 2.8 eV
[7]. In addition it was shown that helium contained in such materials,
especially in the form of vacancy HenVm clusters, also
forms a network of active hydrogen bonding
centers [8–10].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.


## 2. Experimental Technique

The coatings were created using ion beam assisted deposition plant
ARGO-1 [11]. Vanadium was evaporated from electron beam module onto NaCl monocrystals
and onto titanium foil substrate 100μm thick. The films were deposited at
simultaneous bombardment with nitrogen and helium ions of 30 keV at gas
components ratio 1:1. The ion current
density was 1×1014ion/cm2⋅s. The substrate temperature
was 300°C. The metal evaporation rate was controlled using quartz resonator
removed from an ion bombardment zone and it was equal to 0.1–0.15nm/s. The
coatings were 100 nm thick. The resistance was controlled continuously at
depositing the coatings and at their further annealing. A coating structure was
studied using the electron microscope JEM-100CX.Prior to applying a coating in order to create vacancy porosity in
titanium foil it was bombarded withN2+He ions of the same energy
in a dose of up to 6×1017ions/cm2 at 300°C. A specific surface of the pores was measured using the method described
in paper [12]. The interaction of hydrogen with a studied material was
investigated through measurement of isochors adsorption. A dosed amount of
hydrogen was injected from a calibrated capacity into a chamber and dependence
of P on T was measured for each fixed amount of gas released into the chamber.


## 3. Research Results and Discussion

The studies of the initial stage of formation of VN composites including
temperature coefficient of resistance (TCR)
[1, 13] showed that the state of their grain boundary as well as TCR
sign depend on Gibbs energy value of a given nitride. The Gibbs energy for
vanadium nitride is equal to 70 cal/mol [14]. As a result the formation of the
given nitride is accompanied by the formation of an ample amount of
intercrystal nanopores (Figure 1(a)). The formed nanopores lead to the increase in
specific resistance of a nitride and to the appearance of a hopping mechanism
of conductivity and as a result of a negative TCR value (Figure 1(b)).(a) Structure V-N composite; (b) resistivity
as a function of temperature.
(a)(b)The structure of vanadium films that were deposited at bombardment with
mixedN2+He beam differs from that shown in Figure 1(a). As a result fcc
VN [13] structure formation also takes place. The main difference is that the
addition of helium leads to the formation of the equiaxed and fine-dispersed
porosity (Figure 2(a)). The average size of pores is equal to 15 nm, while the size
of pores in vanadium nitride coatings is equal to 50 nm [1]. The pores in
V-(N, He) composite were evenly distributed across the material thickness.(a) Structure V-(N, He) composite; (b) resistivity
as a function of temperature.
(a)(b)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 [15]. Due to this as the
formation of coatings progresses helium leaves the volume of grains and
accumulates in triple grain junctions. At the same time the implanted nitrogen
will form a vanadium nitride VN1−x lattice. The
availability of pressure inside the gas-filled pores will define their
equiaxiality.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 [16]. At the
stage of growth of V-(N, He) coating the dangling bonds in a VN1−x lattice are filled with nitrogen. The helium implanted at vanadium deposition leads
to the increase in concentration of gas components in the pores and increases
pressure in them.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 (Figure2(b))
in comparison with the analogous curves for V-N (Figure 1(b)). A special interest
presents the change of TCR sign in the temperature range of 250 to 350°C. A two-hour annealing at 500°C increases the specific resistance in contrast
to V-N composite. However, at cooling this value returns to the initial one.
The electron microscopic study revealed no changes in the material structure.
On the other part the change in TCR sign can be explained by the change of
electroconductivity mechanism in the studied temperature range.A hopping mechanism of conductivity prevails in aVN1−x composite that contains vacancy or near- vacancy pores. In this case the rise in temperature leads to
the reduction of electron barriers and to the decrease in electric resistance
of materials. As soon as the pores are filled with helium the rise in
temperature will inevitably lead to the increase in gas pressure. The pores of ~15 nm can be
described as “imperfect." The relation between a pore radius “r” and pressure “P”
in a pore can be described by the expression (see [17])  (1)P=μbr+2γr,where μis a shear modulus, b is a Burgers vector, and γis a
surface energy.The calculations based on the results of our previous work [11] show
that at depositing the V-(N, He) coating approximately 10 at.% of helium are
implanted in its lattice. Actually in the long run all helium is condensed in
the pores and therefore the internal pressure may reach 103-104MPa.
At annealing with increase in temperature up to 500°C the pressure may increase 3 times in comparison
with that at a room temperature [17, 18]. Such increase in pressure will
inevitably lead to the increase in stress in the area adjacent to a pore and to
the increase in specific electric resistance of a material on the whole, which
we observed in our experiments.The measurement data showed that specific adsorption surface of studied
samples was equal to25.5m2/g for a coating deposited onto
unirradiated titanium substrate (sample No1). The specific surface of a coating
deposited onto irradiated titanium substrate was 57.6m2/g (sample
No2).A phase composition of a titanium foil that was subjected to a
preliminary bombardment withN2+He ion beam is a titanium nitride
(TiN1−x). A crystalline lattice contains equispaced defective areas
that have a dark contrast (Figure 3(a)). It is assumed that these areas have helium
vacancy clusters of a small size (HenVm,n,m=2–10).
Figure 3(b) shows the simulated curves of distribution of the implanted nitrogen and
helium ions. The estimated level of damages in the investigated area is ~20 dpa [19].(a) Titanium structure irradiated byN2+He ions; (b) nitrogen and helium distribution.
(a)(b)Figure4(b) shows experimental P-T dependences at differed hydrogen inlet
doses for a sample No1. For the sake of comparison Figure 4(a) shows the analogous
dependences for a titanium substrate in its initial state. The P-T plots are
characterized by availability of inflection points for all lines of titanium
substrate with a coating and without it. It is seen that at interaction of
hydrogen with a titanium substrate the temperature at which two straight lines
are intersected for each curve is not dependent on pressure value (Figure 4(a)). For
substrates with a coating the slope of curves changes at 200°C (Figure 4(b)) in case of low pressure (≈0.1 MPa). With
increase of pressure in the chamber the intersection points shift to the area
of lower temperatures. From our point of view the presence of an intersection
point on P-T line implies the change in interaction mechanism of hydrogen and
studied material. Possibly, the titanium and porous film structure absorb
hydrogen in small amounts at the initial section of curves. Further this
absorption is intensified and this can be caused by onset of hydride formation
in the intermetallic phase of compounds. The attention should be paid to the
fact that with increase in pressure the angle between two straight-line
sections of the P-T lines becomes larger and it means that the pressure
produces definite impact on formation of these compounds.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.
(a)(b)Proceed from theP-T dependences the isotherms of hydrogen absorption by
the samples No1 and No2 (Figures 5(a) and 5(b)) can be easily calculated. In the studied
temperature and pressure ranges the hydrogen strictly obeys the ideal gas laws.
For the sake of comparison of different measurement data the value of absorbed
hydrogen was reduced to ncm3/g,
that is, to the volume of hydrogen at normal conditions (P=760mmHg,T=0°C).Isotherms of hydrogen adsorption of V-(N,
He) coatings on titanium substrate: (a) not irradiated previously, (b) irradiated.
(a)(b)The figures show that the porous structure plays main role at20°C. With increase in pressure up to ≈0.175 MPa the increase in absorption value is
not significant, however at ≈0.275 MPa the
absorption rate is essentially increasing. At pressure of ≈0.3 MPa the
hydrogen absorption isotherms reach a saturation level. It should be noted that
the isotherm of hydrogen absorption by the sample
No2 reaches a saturation level at absorption value that exceeds that for sample
No1 actually by factor of 2. The isotherms of hydrogen absorption by the samples
No 1 and 2 are characterized by a higher degree of absorption at temperatures
of 300°C and 500°C in comparison with 20°C in the whole pressure interval and the rate
of growth of absorption value is increasing in the interval of 0.1⋯0.125MPa.
The increase in the rate of growth is more pronounced for the sample No2 that
has a larger specific surface.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 Figures5(a) and 5(b)).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.


## 4. Conclusions

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 to350°C.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 mixedN2+He beam contributes to the formation
of a surface porous layer which is an additional factor contributing to the
increase in the adsorption capacity of porous materials.



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*Source: 912676-2009-03-26.xml*