Materials Transactions, Vol. 48, No. 5 (2007) pp. 1012 to 1016
#2007 The Japan Institute of Metals
Hydrogen Sorption in Zirconium and Relevant Surface Phenomena
Jeshin Park1 , Wonbaek Kim1; * and Misook Won2
1
2
Korea Institute of Geoscience and Minerals Resources, Daejeon 305-350, Korea
Korea Basic Science Institute Busan Branch, Busan 609-735, Korea
The hydrogen sorption of pure zirconium was investigated as a function of activation temperature. The sorption speed increased with
activation temperature up to 750 K and decreased at higher temperatures. The X-ray photoelectron spectroscopy study revealed that oxygen peak
virtually diminishes on heating at temperatures above 750 K. The cross-sectional TEM showed that the surface amorphous layer crystallizes on
heating at 873 K. The structural transition from amorphous to crystalline state is attributed to the decrease in hydrogen sorption.
[doi:10.2320/matertrans.48.1012]
(Received September 29, 2006; Accepted February 15, 2007; Published April 25, 2007)
Keywords: non-evaporation getter (NEG), sorption rate, activation, deactivation
1.
Introduction
Non-evaporable getters (NEG) have been widely used in
many fields as displays, accelerators and ultra high vacuum
systems.1) Zr-based alloys are typical non-evaporable getter
(NEG) materials due to their high sorption properties for
various active gases. In order for a getter to absorb the active
gases, it should be heated in high vacuum or in inert
environment to reveal a clean metallic surface. The process is
recognized as activation. The activation temperature is
considered as a temperature at which the getter material
gains its full sorption capacity and thus is important for
effective use of a specific getter material. However, the way
to determine the activation temperature (or time) have not
been documented well. One suggested method is to examine
the disappearance of oxygen peaks during in-situ heating of
getter material in XPS4) (X-ray photoelectron spectroscopy)
chamber.
A previous study reported a maximum of hydrogen
sorption speed with activation temperature for Zr57 V36 Fe7
alloy.5) The alloy, commercially known as ST707, has
relatively low activation temperature ranging from 673 K
and 773 K.2) The alloy is composed of two phases i.e. AB2
type cubic Laves and hexagonal -Zr solid solution.3) During
series of research on getter materials we also noticed similar
decrease in sorption speed for pure Zr as well as various Zrbased alloys including ST 707. We, however, could not
isolate reasons for the reduced sorption since the alloy
systems were too complicate to analyze properly. Instead, we
have chosen pure Zr to elucidate the reasons for the activation
behavior in Zr alloys. In this paper, the phenomenon was
investigated using XPS and TEM (Transmission Electron
Microscopy).
2.
Experimental Procedure
Pure Zr powders were prepared by HDH (Hydride
DeHydride) method. The raw material was Zr sponge
(99.5%, Sejong Inc.). The average particle size of Zr powers
was about 45 microns. The activation behavior of pure Zr was
studied by the ultimate pressure measurement of the speci*Corresponding
author, E-mail: [email protected]
men chamber at room temperature after heating at 373, 473,
573, 673, 773, 873 and 973 K. To minimize the effect of
chamber heating, the sample was heated inductively. The
duration of heat treatment for activation was 10 minutes in all
cases.
The surface structure was studied by X-ray photoelectron
spectroscopy(Escalab 250 XPS Spectrometer) equipped with
a ceramic heating stage. The XPS spectrum was obtained
using a monochromatic AlK X-ray. The temperature of the
samples on a ceramic heating stage was controlled up to
873 K with a temperature controller (RHC, VG Scientifics,
UK). The chamber vacuum was maintained at 6:7 106 Pa
with an ion pump (MidiVac, Varian, USA). The composition
of each element was calculated using the sensitivity factors
from a library of Scofield Al anode. Since the powder
compact was not suitable for the XPS study, Zr sheet was
used instead. The sheet sample was heated from 323 to 973 K
at intervals of 50 K in the XPS system. The XPS spectrum
was recorded after the sample reached to a programmed
temperature. The peak intensity of the elements was divided
by the corresponding sensitivity factors and plotted as a
function of the activation temperature.
The hydrogen sorption rate was measured at the molecular
flow regime under 5 103 Pa as described in detail in
ASTM-F798.6) The basic principle of the method is based on
the measurement of total pressure difference between gas
inlet and sample chambers and the sorption rate of getter is
calculated as
S ¼ Q=P2 and Q ¼ CðP1 P2 Þ
Here, S is sorption speed of sample, Q sorption quantity, C
conductance of orifice, P1 and P2 are the pressure of gas inlet
and sample chambers, respectively.
The sample chamber was made of quartz (diameter 40 mm,
length 160 mm) and was evacuated with a scroll pump and a
turbo pump. The sample was about 1.5 gram and compressed
uniaxially to a pellet of 10 mm diameter. The hydrogen
sorption rate was measured at room temperature after
activation for 10 minutes at a given temperature. After the
measurement, the microstructure was observed by HTEM
(High-resolution Transmission Electron Microscopy) and
oxygen and nitrogen contents were measured by a gas
analyzer (LECO, EF-400).
Hydrogen Sorption in Zirconium and Relevant Surface Phenomena
1013
(a)
Fig. 1 Initial hydrogen sorption rates of pure Zr with activation temperature.
(b)
Fig. 2 Ultimate chamber pressure after activation heating of Zr powder at
each temperature.
3.
Fig. 3 X-ray photoelectron spectra during in-siut heating of Zr sheet
sample.
Results and Discussion
3.1 Getter Property
Figure 1 shows the initial sorption rate for hydrogen after
activation at various temperatures. The hydrogen sorption
rate of Zr increases with activation temperature. It suggests
that the degree of activation increases with temperature.
However, the rate starts to decline at 873 K revealing a
maximum. A similar behavior was previously reported for
Zr57 V36 Fe7 alloy by C. Benvenuti et al.3) The sorption rate
maximum with activation temperature can be expected if the
surface area reduces by sintering. The temperature range,
however, was too low for any significant sintering to occur.
Our study revealed no signs of macroscopic sintering
throughout the experiment. Therefore, it is obvious that
other reasons should be found.
When a getter is activated by heating in a sealed vacuum
chamber, residual gases are removed by absorption reducing
or at least suppressing the pressure rise from the chamber
outgassing. Once it is activated fully, no further reduction in
chamber pressure is expected to occur. Therefore, the
completion of activation would be identified by the minimum
in ultimate pressure. It usually took more than 10 hours to
reach at a stable ultimate pressure. Figure 2 shows the
ultimate chamber pressure measured at room temperature
after heating at each temperature. It shows that the activation
of Zr occurs at temperatures between 573 and 773 K.
Fig. 4
Amounts of Zr and oxygen with activation temperature.
However, after heating at 873 K, the chamber pressure rises
abruptly. Therefore, significant changes of Zr should occur
which acts to retard the hydrogen sorption. One easy
explanation would be the surface area reduction by sintering
at higher temperature. However, no sign of macroscopic
sintering or surface area change by BET (Brunauer-EmmettTeller, Micrometrics, Model ASAP2400) was observed.
The activation of a getter has been investigated using Xray photoelectron spectroscopy. The activation corresponds
to the observation that the surface oxide peak disappears
while metallic peaks prevails. Figure 3 shows XPS peaks of
1014
J. Park, W. Kim and M. Won
(a)
(b)
(c)
Fig. 5 HTEM images and oxygen content at each location of pure Zr before activation (a) and after activation at 773 K for 10 min (b), and
at 873 K (c) for 10 min.
oxygen and zirconium. The activation of Zr is identified by
the disappearance of oxygen peak after heating at 573 K
(Fig. 3(b)). The variation of O1s and Zr3d spectra with
activation temperature was plotted in Fig. 4. It shows that
oxygen on surface diminishes with concurrent increase in Zr.
At temperatures higher than 575 K virtually no oxygen could
be identified which indicated the completion of activation.
Nevertheless, no further information on the sorption rate
maximum could be obtained from the XPS study.
3.2 Relevance of Structural Transition
Figure 5(a)(b)(c) is the high-resolution TEM image and
EDS profile near the surface. The surface of Zr was covered
with about 20 nm thick oxide (298 K) (a). After activation at
773 K, the surface oxide layer was thinned to about 15 nm
(b). It was about 20 nm after activation at 873 K (c). The
oxygen content of original sample was 22.82 at% near
surface. It decreased to 16.58 at% after activation at 773 K
for 10 minutes. It increased to 20.26 at% after heating at
873 K.
Figure 6 shows the cross-section HTEM micrographs of
the surface structure of non-activated (a), after activation at at
773 K (b) and at 873 K (c). The oxide layer of original sample
was amorphous. It was similar after the activation treatment
Hydrogen Sorption in Zirconium and Relevant Surface Phenomena
1015
(a)
(c)
(b)
Fig. 6
HTEM images of pure-Zr before activation (a) and after activation at 773 K for 10 min (b), and at 873 K (c) for 10 min.
at 773 K. However, after heating at 873 K, the amorphous
structure is seen to crystallize.
Hydrogen molecules are adsorbed on the surface and
dissociated into hydrogen atoms. These atoms are absorbed
on surface layer of the sample and diffuse into the bulk. Each
process is the rate-limiting step for the absorption of
hydrogen. It is reasonable to expect the structural change in
surface layer as crystallization may have an effect on the
absorption of hydrogen.
The solubility and diffusivity of hydrogen in amorphous
and crystalline phases has been the subject of several
research. They utilized electrochemical hydrogen permeation
and P-C-T (Pressure-composition-temperature) diagram to
evaluate the effect of structure. In this study, the hydrogen
sorption rate was determined by measuring pressure drop in
closed vacuum chamber containing sample which is analogous to the PCT measurement. It is reported that at same
temperature and pressure, the solubility of hydrogen in some
amorphous alloys was substantially higher than in crystalline
phase.7–10) The higher solubility of amorphous alloys was
explained that hydrogen dissolves in larger sites with lower
ground state energy and thus more interstitial sites are
available for hydrogen compared to crystalline phase.9) It
may be that the decrease in hydrogen sorption at high
temperature is due to the lowered hydrogen solubility in
crystallized phase.
4.
Summary
The activation and sorption properties of pure Zr were
investigated. The hydrogen sorption rate of Zr increased with
activation temperature reaching at a maximum at 773 K.
Above 773 K, the sorption rate decreased again. In-situ XPS
measurement showed the disappearance of oxygen peak at all
temperatures above 773 K. TEM study revealed that the
surface amorphous layer crystallized during heating at above
773 K. The decrease in hydrogen sorption after heating at
above 773 K is due to the lower solubility of hydrogen in
crystalline phase.
1016
J. Park, W. Kim and M. Won
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