ISSN 1027-4510, Journal of Surface Investigation. X-ray, Synchrotron and Neutron Techniques, 2008, Vol. 2, No. 3, pp. 440–443. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © N.N. Nikitenkov, Yu.I. Tyurin, I.P. Chernov, A.M. Lider, A.V. Skirnevskii, 2008, published in Poverkhnost’. Rentgenovskie, Sinkhrotronnye i Neitronnye
Issledovaniya, No. 6, pp. 21–24.
Radiation-Enhanced and Thermostimulated Hydrogen Release
from Palladium and Zirconium
N. N. Nikitenkov, Yu. I. Tyurin, I. P. Chernov, A. M. Lider, and A. V. Skirnevskii
Department of Natural Sciences and Mathematics, Tomsk Polytechnic University, Tomsk, 634059 Russia
Received September 10, 2007
Abstract—The results of a mass-spectrometric analysis of hydrogen release from palladium and zirconium
samples under the action of accelerated electrons (with an energy of 40 keV and a current density of 3 to
30 µA/cm2) and x-rays (with energies of 40 and 120 keV) are presented. The amount of hydrogen removed from
these samples and the residual hydrogen content are monitored via the methods of mass-spectrometry and thermodesorption. The conclusion is made that substantial removal of hydrogen (up to 90% of the initial content)
from the analyzed materials can be achieved under the action of electrons and x-rays. It is found that x-ray irradiation can ensure more efficient removal of hydrogen than electron bombardment.
DOI: 10.1134/S102745100803021X
INTRODUCTION
Metals can accumulate a large amount of hydrogen.
Being captured by various defects of a material’s structure, hydrogen affects its mechanical, electrical, and
radiation properties. The aim of this work is to study the
action of ionizing radiations on hydrogen release from
palladium and E-125 zirconium binary alloy, which differ substantially in chemical composition and structure.
The necessity of such investigations is caused by practical needs: palladium has the highest hydrogen solubility and is the best catalytic agent and accumulator of
hydrogen among all metals, and zirconium alloys are
the basic constructional materials of the elements of
reactor cores and heat-generating systems in nuclear
power plants. The investigations described in this paper
are a continuation of works [1–3].
METHODS OF INVESTIGATION
Palladium and zirconium samples were electrolytically hydrogenated in an acid electrolyte (1I H2SO4) at
the current density Jhyd = 0.5 A/cm2 for 6 min and at
Jhyd = 0.2 A/cm2 for 4 h, respectively. Palladium foil
samples with an area of ~2 cm2 and thickness d ~ 100 µm
were used repeatedly after the measurements of thermostimulated gas release (TSGR) spectra under gradual heating up to 900°ë. Before hydrogenation, zirconium alloy samples with dimensions of 3 × 20 × 1 mm
were annealed in vacuum of 10–5 Pa during which temperature slowly increased up to 900°ë and then
decreased to room temperature without reduction of
vacuum. The samples were bombarded by electrons
with energy Öe = 40 keV at a current density on the
sample surface from 3 to 30 µA/cm2 and P = 10–4 Pa. In
TSGR measurements, the initial pressure was 10–6 Pa.
The samples were also irradiated by x-rays with energies Öx = 40 and 120 keV. During electron bombardment, the intensity of hydrogen isotope release was
analyzed via mass-spectrometry. Before and after irradiation, the level of hydrogen concentration in samples
was monitored by the TSGR method calibrated against
gravimetric measurements. The intensities of hydrogen
release under electron bombardment and TSGR spectra
were recorded using a hardware–software complex that
controlled scanning of an MX-7304 mass spectrometer,
accomplished gradual heating of samples during TSGR
measurements, and recorded temperature and intensity
measurements for the selected mass-spectrum lines
(from one to six masses) with a rate ranging from one
to ten measurements per second. The final processing
of measurements was performed using the OriginLab
Corporation OriginPro 7.0 software package. The
details of the experiment have been described in [1–3].
RESULTS AND DISCUSSION
Figure 1 shows the TSGR spectra obtained for palladium samples before and after electron bombardment
at different values of current densities Jt in the electron
beam. It can be seen that an increase in the electron current density substantially decreases the amount of
residual hydrogen in the samples. Note that spectrum 3
remains virtually unchanged under the second heating
cycle, which was performed without hydrogenation of
a sample, i.e., corresponds to that of the initial sample.
It can also be seen from Fig. 1 that the spectra obtained
before and after electron bombardment exhibit a different behavior of intensity: the maxima of thermal gas
release from the hydrogenated sample and the samples
acted upon by electrons are observed at ím ~ 200°ë and
440
RADIATION-ENHANCED AND THERMOSTIMULATED HYDROGEN RELEASE
Y, arb. units
441
Y, arb. units
101
1
100
100
1
2
10–1
10–1
3
10–2
10–2
3
2
10–3
0
0
100
200
300
400
500
600
700
50
100
150
800
T, °C
Fig. 1. TSGR spectra obtained for (1) the initial hydrogenated sample, (2) the sample after the action of electrons
with Je = 10 µA/cm2 and electron bombardment time t =
600 s, and (3) the sample after the action of electrons with
Je = 30 µA/cm2 and t = 600 s.
ím ~ 600–800°ë, respectively. Moreover, at í > 600°C,
the intensity of ç2 release from the samples subjected
to electron bombardment can be higher than the intensity of release from the initial hydrogenated palladium
sample. The reason is that, during bombardment,
hydrogen is removed from traps with weak bonds to
vacuum and an insignificant amount of hydrogen is
captured by traps with stronger bonds that arise under
the action of electrons.
Figure 2 shows the time-dependences of the intensity of ç2 release obtained at three values of the current
density in the electron beam. It can be seen that the
intensity maxima and the shape of kinetic curves
strongly depend on the current density. As the current
density increases, intensity of gas release grows and
falls over shorter periods of time. For example, maxima
in curves 1, 2, and 3 are observed, respectively, within
14, 11, and 9 s of electron bombardment. When samples are substantially heated due to an increase in the
current densities, a curve shape characterized by more
than one maximum (curve 3) is observed, and the intensities corresponding to these maxima can be different.
It is natural to attribute such behavior of curves to at
least two mechanisms responsible for radiationenhanced hydrogen release. One of these mechanisms
is related to electron action, the other to temperature
changes. Under the action of electrons on the zirconium
samples, the intensity of hydrogen release exhibits
qualitative variations similar to those shown in Fig. 2.
The difference is that the intensity maximum is related
to the sample temperature and can be observed only
200
250
300
t, s
Fig. 2. The intensity of hydrogen release from the palladium
samples vs. the electron bombardment time at the following
values of Je: (1) 3, (2) 15, and (3) 30 µA/cm2. Maximum
temperatures of sample heating under the action of the electron beam are (1) 30, (2) 70, and (3) 250°C.
when an electron beam strongly heats the sample (up to
~500°C), i.e., at long time intervals.
Figure 3 shows the dependence between the integral
intensity of hydrogen release and the electron current
density which was constructed according to the results
of our experiments. Here, experimental points are
approximated by the curve
Y = A exp(J/B) + C,
(1)
where A = 0.29, B = 4.74, and C = 113.29.
It follows from (1) that BdY = (Y – C)dJ; i.e., the
increment of the intensity of molecular hydrogen
release (dY) is proportional to the increment of the electron beam current density (dJ) and the amount of
molecular hydrogen in the near-surface region. This is
due to the fact that, in the near-surface region, the electron beam stimulates neutralization of ç+ ions and heterogeneous recombination of H atoms, leading to formation of ç2 molecules and subsequent desorption of
molecular hydrogen.
Under electron bombardment, hydrogen release
proceeds via several mechanisms. Such behavior is
likely to be explained by the excess of electrons and the
deficiency of protons (hydrogen nuclei) in the near-surface region whose thickness is equal to the electron free
path. As a result, gradients of electrostatic, magnetic,
and temperature fields (apart from the concentration
gradient) arise in the sample. They initiate the corresponding diffusion fluxes with diffusion coefficients
that can substantially exceed those inherent to concentration diffusion. Furthermore, electron bombardment
eliminates the surface potential barrier for hydrogen
release from the sample.
JOURNAL OF SURFACE INVESTIGATION. X-RAY, SYNCHROTRON AND NEUTRON TECHNIQUES Vol. 2 No. 3 2008
442
NIKITENKOV et al.
tions and subsequent actions of accelerated electrons
and x-rays. It is seen that these actions substantially
change the shape of spectrum. Electrons act mainly on
low-temperature traps; x-rays affect high-temperature
ones. Moreover, the higher the energy of the x-rays, the
more efficient hydrogen removal from high-temperature traps becomes. As is seen in Fig. 4, at temperatures
higher than 800°ë, the intensity of hydrogen release
after the action of electrons and x-rays with an energy
of 40 keV is higher than after the action of x-rays with
an energy of 120 keV. Thus, under ionizing radiations,
the intensity of hydrogen release from the zirconium
alloy depends on the type and energy of radiation. Such
behavior is apparently related to the penetration depth
of radiation. Therefore, it may be stated that the curves
in Fig. 4 clearly indicate the predominantly bulk character of processes responsible for hydrogen release.
Y, arb. units
280
240
200
160
120
0
10
20
30
Je, µA/cm2
Fig. 3. The integral intensity of hydrogen release from the palladium sample vs. the values of electron current density Je. The
bombardment time is t = 600 s, and the electron energy is
Ee = 40 keV.
Figure 4 shows the TSGR spectra obtained for the
E-125 zirconium sample subjected to hydrogenation
and for the same sample after preliminary hydrogena-
In order to determine the amount of hydrogen
removed from the samples during irradiation, we have
developed the following procedure. The TSGR spectra
obtained before and after irradiation were integrated,
and the amount of removed hydrogen was determined
from the relation ä = (I1 – I2)/I1, where I1 and I2 are the
values of integrals (the areas under the TSGR spectra).
Y, arb. units
0.20
0.15
1
0.10
0.05
2
3
1
0
4
0
100
200
300
400
500
600
700
800
900
T, °C
Fig. 4. TSGR spectra obtained for (1) the hydrogenated zirconium sample, (2) the hydrogenated sample subjected to the action of
accelerated electrons, and (3, 4) the hydrogenated sample subjected to x-ray irradiation. Irradiation parameters: (curve 2) Öe =
40 keV, Je ~ 20 µA/cm2, t = 900 s, and ímax ~ 300°C; (curve 3) Öx = 40 keV, t = 900 s, and íroom; and (curve 4) Öx = 120 keV, t =
900 s, and íroom.
JOURNAL OF SURFACE INVESTIGATION. X-RAY, SYNCHROTRON AND NEUTRON TECHNIQUES Vol. 2 No. 3 2008
RADIATION-ENHANCED AND THERMOSTIMULATED HYDROGEN RELEASE
The values of ä obtained at the first stages of analyses
were compared with the values äm = (ëm1 – ëm2)/ëm1,
where ëm1 and ëm2 are the mass concentrations of
hydrogen before and after irradiation of samples. It was
found that the values of ä and äm differ by no more
than 5%. Such close agreement allowed us to determine
the amount of hydrogen removed from samples subjected to irradiation without resorting to the gravimetric
method.
With the use of the procedure described above, we
have obtained the following values of integrals corresponding to curves 1–4 in Fig. 4: I1 = 18.2, I2 = 9.3, I3 =
3.6, and I4 = 3.8. From these values, it is not difficult to
estimate the amount of hydrogen (ä) removed from the
corresponding samples after each type of the actions.
For example, after electron bombardment, äe = (18.2–
9.3)/18.2 ≈ 0.49; after the action of x-ray quanta with
two values of energy, äx ≈ 0.79. Thus, under the action
of electrons and x-rays, the amounts of hydrogen
removed from the zirconium sample are, respectively,
~50% and ~80%. Similar calculations of TSGR spectra
obtained for the palladium sample (Fig. 1) have
revealed that the amount of removed hydrogen is more
than 90% under electron bombardment (Jel = 30 µA/cm2
and t = 600 s), hydrogen removal from the samples is
not observed under the action of x-ray quanta (Öx = 40
and 120 keV), and the entire TSGR spectrum shifts
(this shift is not shown in the figures) to higher temperatures without a decrease in intensity.
CONCLUSIONS
The obtained results lead to the following conclusions:
1. Under electron bombardment, the most intense
release of hydrogen from the palladium and zirconium
samples is observed over the first two minutes. The
intensity reaches its maximum within 5–15 s after the
onset of bombardment.
2. Before and after electron bombardment, the maxima of thermostimulated release of hydrogen from the
443
palladium samples is observed, respectively, at ím ~
200–230°ë and ím ~ 600–800°ë.
3. The TSGR spectra obtained for the hydrogenated
zirconium samples exhibit several maxima due to the
presence of various traps of hydrogen whose identification is reported in [2]. Under the action of electrons and
x-ray quanta with an energy of 40 keV, hydrogen leaves
mainly low bond energy traps; under the action of x-ray
quanta with an energy of 120 keV, hydrogen is removed
from traps of all types.
4. Under electron bombardment (Öe = 40 keV and
Je = 30 µA/cm2), the amount of hydrogen removed
from the Pd sample is more than 90%. When the palladium samples are acted upon by x-rays (under the given
conditions of our experiment), hydrogen is not removed
from the samples. Residual hydrogen is captured by the
traps that were not observed before electron bombardment (i.e., these traps arise during electron bombardment). When the zirconium samples are acted upon by
electrons (Öe = 40 keV and Je = 20 µA/cm2) and x-ray
quanta (Öe = 40 and 120 keV), the amounts of removed
hydrogen are, respectively, ~50% and up to 80%.
5. The efficiency of hydrogen removal from metals
and alloys subjected to ionizing radiation substantially
depends on the chemical composition and structure of
alloys and, simultaneously, on the type, energy, and
density of radiation.
ACKNOWLEDGMENTS
This study was supported by the Russian Foundation for Basic Research, projects nos. 06-08-00662 and
07-08-00300.
REFERENCES
1. Yu. I. Tyurin, I. P. Chernov, M. Krening, and Kh. Baumbakh, Radiation-Enhanced Release of Hydrogen from
Metals (Tomskii Univ., Tomsk, 2000) [in Russian].
2. N. N. Nikitenkov, Yu. I. Tyurin, I. P. Chernov, and
A. V. Skirnevskii, Izv. TomPU 4, 52 (2006).
3. A. V. Skirnevskii, N. N. Nikitenkov, Yu. I. Tyurin, et al.,
Izv. VUZov, Ser. Fiz. 49 (10), 269 (2006).
SPELL OK
JOURNAL OF SURFACE INVESTIGATION. X-RAY, SYNCHROTRON AND NEUTRON TECHNIQUES Vol. 2 No. 3 2008