Materials Transactions, Vol. 44, No. 4 (2003) pp. 641 to 644
#2003 The Japan Institute of Metals
Hydrogen Isotope Effects on Absorption Properties of Ti–Cr–V Alloys*1
Takuya Tamura*2 , Atsunori Kamegawa, Hitoshi Takamura and Masuo Okada
Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
Ti–Cr–V alloys are known to absorb protium (hydrogen atom) up to H/M = 2, while Cr-rich alloys absorb up to H/M = 1 because of the
formation of mono-protides (mono-hydrides). However, few hydrogen isotope effects of the Ti–Cr–V alloys have been reported. This paper aims
to clarify the hydrogen isotope effects on the absorption properties of the Ti–Cr–V alloys in a composition range near the boundary between the
appearance regions of mono- and di-protides. It was found that the appearance region of the mono-dueteride was Cr-richer composition than that
of the mono-protides. The absorption plateau pressure for deuterium was lower than that for protium in the appearance region of the di-protide
and the di-deuteride, but higher in the appearance region of the mono-protides and the mono-deuteride. The hydrogen isotope effects on the
plateau pressure were found to cause the difference in the appearance regions of the protides and the deuterides.
(Received November 29, 2002; Accepted February 5, 2003)
Keywords: hydrogen storage alloy, titanium–chromium–vanadium alloy, deuterium, isotope effect, hydride, deuteride
1.
Introduction
Protium (hydrogen atom) storage alloys are considered to
be used as hydrogen storage tanks for fuel-cell automobiles.
Ti–Cr–V alloys with a BCC structure are regarded as one of
the candidates for hydrogen storage tanks.1) Since the Ti–Cr–
V alloys show the capacity of nearly 3 mass%at 373 K, a lot
of works, such as the additions of other elements,2–7) the
effects of heat-treatment and composition,8–12) V free
alloys,13–16) cyclic properties17,18) and the effects on hysteresis,19) have been reported for improving the properties.
Ti–Cr–V alloys absorb protium up to H/M = 2 (about
3.7 mass% of protium), but desorb about 2.4 mass% from the
viewpoint of protium desorption capacity (9-0.01 MPa) by
forming stable protides in low hydrogen pressure regions.
The protide is defined as a compound of protium, namely, a
hydride hereafter. For further increasing the desorption
capacity of the Ti–Cr–V alloys, it is quite important to know
how to decrease the regions of stable protides. The present
authors reported that decreasing the V content and increasing
the Cr content shifted the beginning of the plateau regions to
the low protium concentration, and higher protium desorption capacity was achieved by increasing the Cr content
and controlling the measurement temperature in order to
control the desorption plateau pressure near atmospheric
pressure in the alloys containing less than 40 at%V (at% is
abbreviated hereafter).20–23) However, the Cr-rich alloys
were found to absorb up to H/M = 1 because of the formation
of mono-protides.22,23) On the basis of these results, the
present authors reported the region with the higher protium
desorption capacity in the Ti–Cr–V alloys.23)
Vanadium is known to have large hydrogen isotope
effects.19,24) However, no hydrogen isotope effects of the
Ti–Cr–V alloys were reported except for the works by
Cho.25,26) Therefore, the purpose of this study is to clarify the
hydrogen isotope effects on the absorption properties of the
*1This Paper was Presented at the Autumn Meeting of the Japan Institute of
Metals, held in Osaka, on November 3, 2002.
address: Institute for Structural and Engineering Materials,
National Institute of Advanced Industrial Science and Technology,
Nagoya 463-8560, Japan.
*2Present
Ti–Cr–V alloys in a composition range near the boundary
between the appearance regions of the mono- and di-protides.
2.
Experimental Procedures
The alloys were prepared by arc-melting on a water-cooled
copper hearth under an Ar atmosphere. The purities of the
raw materials were as follows: Ti, Cr > 99.99 mass%; V >
99.95 mass%. Samples were remelted five times to ensure
their homogeneity. Heat-treatment was carried out under an
Ar atmosphere. The samples were heated at a rate of 600 K/h
to 1673 K, annealed for 1 h, and then quenched into icedwater. Crystal structures were studied by X-ray diffractometer (XRD) using Cu-K radiation. The XRD samples for
studying the structure of deuterides were prepared by treating
in acetone at 173 K to keep the conditions for fully absorbing
deuterium at 244 K under 3 MPa. The pressure composition
isotherms (PCT curves) were measured at 273 K or 244 K
using a Sieverts-type apparatus. The purities of the hydrogen
and deuterium gases were 99.99999 mass% and 99.6 mass%,
respectively. The PCT curves of hydrogen were measured
from 0.01 MPa to 10 MPa, and those of deuterium were
measured from 0.01 MPa to 3 MPa. The zero points of PCT
curves were the points before activation and the PCT curves
presented in this study were taken from the first cycle data.
3.
3.1
Results and Discussion
Deuterium storage properties and deuteride structures in the Ti–Cr–20V alloys
Since the phase of forming protide depends on the ratio of
Ti and Cr largely,22,23) the V content was fixed to 20 at% in
this study in order to examine the hydrogen isotope effects in
a composition range near the boundary between the appearance regions of the mono- and di-protides. It has been found
by XRD that all the Ti–Cr–20V alloys have a BCC single
phase.
Figure 1 shows PCT curves (absorption process) for
protium at 244 K for the Ti–xCr–20V (x ¼ 54 to 64) alloys.
As can be seen, the alloys containing more than 56 at%Cr
absorb protium up to H/M = 1 because of the formation of
642
T. Tamura, A. Kamegawa, H. Takamura and M. Okada
Fig. 3 XRD patterns of the deuterides for the Ti–xCr–20V (x ¼ 58 and 62)
alloys. The deuterides for the Ti–58Cr–20V alloy and the Ti–62Cr–20V
alloy were measured at the (A)-point and the (B)-point shown in Fig. 2,
respectively.
Fig. 1 PCT curves (absorption process) for protium at 244 K for the Ti–
xCr–20V (x ¼ 54 to 64) alloys.
sample at the (A)-point for the 58 at%Cr alloy has a FCC
phase. This FCC phase is considered to be a di-deuteride,
judging from the di-protide with the FCC structure.22,23) The
sample at the (B)-point for the 62 at%Cr alloy consists mostly
of a HCP phase and has a small quantity of the FCC phase of
the di-deuteride. This HCP phase is considered to be a monodeuteride, judging from the mono-protide with the HCP
structure.22,23) Therefore, the appearance region of the monodueteride was Cr-richer composition than that of the monoprotides.
3.2
Hydrogen isotope effects on the plateau pressure of
the Ti–Cr–20V alloys
Figure 4 shows PCT curves for protium and deuterium at
273 K for the Ti–52Cr–20V alloy. The Ti–52Cr–20V alloy
Fig. 2 PCT curves (absorption process) for deuterium at 244 K for the Ti–
xCr–20V (x ¼ 54 to 64) alloys.
mono-protides.22,23)
Figure 2 shows PCT curves (absorption process) for
deuterium at 244 K for the Ti–xCr–20V (x ¼ 54 to 64)
alloys. It was found that the alloys containing more than
60 at%Cr absorb deuterium up to D/M = 1. This result differs
from that for protium shown in Fig. 1.
Figure 3 shows XRD patterns of the deuterides for the Ti–
xCr–20V (x ¼ 58 and 62) alloys. The deuterides for the Ti–
58Cr–20V alloy and the Ti–62Cr–20V alloy were measured
at the (A)-point and the (B)-point shown in Fig. 2, respectively. Each sample has diffraction peaks from the BCC
phase with the structure before the plateau region. Thus, the
diffraction peaks from the BCC phase are not indexed. The
Fig. 4 PCT curves for protium and deuterium at 273 K for the Ti–52Cr–
20V alloy.
Hydrogen Isotope Effects on Absorption Properties of Ti–Cr–V Alloys
643
Fig. 5 PCT curves for protium and deuterium at 244 K for the Ti–62Cr–
20V alloy.
forms the di-protide and the di-deuteride in absorbing
protium and deuterium, respectively. The hysteresis for
deuterium is as large as that for protium. This phenomenon
was considered in our previous work.19) The absorption
plateau pressure for deuterium is lower than that for protium.
Figure 5 shows PCT curves for protium and deuterium at
244 K for the Ti–62Cr–20V alloy. The Ti–62Cr–20V alloy
forms the mono-protide and the mono-deuteride mostly in
absorbing protium and deuterium, respectively. The absorption plateau pressure for deuterium is higher than that for
protium. This result is the opposite of the result for the Ti–
52Cr–20V alloy shown in Fig. 4.
Effects of the Cr content on the plateau pressure of
the Ti–Cr–20V alloys
Figure 6 shows PCT curves (absorption process) for
protium at 273 K for the Ti–xCr–20V (x ¼ 48 to 62) alloys.
Figure 7 shows the absorption plateau pressures for
protium at 273 K for the Ti–xCr–20V (x ¼ 48 to 62) alloys
shown in Fig. 6 versus the Cr content. It is found that
logarithms of the absorption plateau pressures for the diprotide and the mono-protides change linearly to the Cr
content. The standard free energy change of the protide
formation GO can be expressed as GO ¼ RT ln Pab
where R, T and Pab are the gas constant, the measurement
temperature and the absorption plateau pressure, respectively. Thus, the absorption plateau pressure for the di-protide
and the mono-protides shown in Fig. 7 can be regarded as
apparent standard free energy changes of the protide
formations. Therefore, it can be said that the mono-protide
formation results from the lower GO (the absorption plateau
pressure) of the mono-protide formation than that of the diprotide formation, because a compound that has a lower GO
is more stable.
Figure 8 shows the absorption plateau pressures for
protium and deuterium at 244 K for the Ti–xCr–20V
Fig. 6 PCT curves (absorption process) for protium at 273 K for the Ti–
xCr–20V (x ¼ 48 to 62) alloys.
3.3
Fig. 7 The absorption plateau pressure for protium at 273 K for the Ti–
xCr–20V (x ¼ 48 to 62) alloys shown in Fig. 6 versus the Cr content.
(x ¼ 54 to 64) alloys shown in Figs. 1 and 2 versus the Cr
content. As can be seen, the changes of the absorption plateau
pressure for the di-deuteride, the mono-deuteride and the
mono-protides are almost linear to the Cr content. The line of
the absorption plateau pressure for the di-protide was drawn
referring to Figs. 4 and 7. The intersection of the plateau
pressure lines for the di-deuteride and the mono-deuteride
occurs at a Cr-richer composition than that for the di-protide
and the mono-protides. This result shows that the appearance
region of the mono-dueteride is Cr-richer composition than
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T. Tamura, A. Kamegawa, H. Takamura and M. Okada
Acknowledgments
This work has been supported in part by a Grant-in-Aid for
Scientific Research on Priority Area A of ‘‘New Protium
Function’’ from the Ministry of Education, Culture, Sports,
Science and Technology.
REFERENCES
Fig. 8 The absorption plateau pressure for protium and deuterium at 244 K
for the Ti–xCr–20V (x ¼ 54 to 64) alloys shown in Figs. 1 and 2 versus the
Cr content.
that of the mono-protides, because the mono-dueteride
formation results from the lower GO than that of the didueteride formation. Moreover, this result is caused by the
lower plateau pressure for the di-deuteride formation than
that for the di-protide formation and the higher plateau
pressure for the mono-deuteride formation than that for the
mono-protide formation. Thus, the hydrogen isotope effects
on the plateau pressure cause the difference between the
appearance regions of the protides and the deuterides.
4.
Conclusions
The hydrogen isotope effects on absorption properties of
the Ti–Cr–V alloys in a composition range near the boundary
between the appearance regions of the mono- and di-protides
have been investigated, and the following conclusions have
been derived.
(1) The appearance region of the mono-dueteride is Crricher composition than that of the mono-protides.
(2) The absorption plateau pressure for deuterium is lower
than that for protium in the appearance region of the diprotide and the di-deuteride, but higher in the appearance region of the mono-protides and the monodeuteride.
(3) The hydrogen isotope effects on the plateau pressure
cause the difference in the appearance regions of the
protides and the deuterides.
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