富山大学水素同位体科学研究センター研究報告 21：13−26，2001.

表面制御透過実験装置の製作と性能試験
波多野 雄治、能村 衛、渡辺 国昭
富山大学水素同位体科学研究センター〒930-8555 富山市五福 3190
Alexander I. Livshits, Andrei O. Busnyuk
Bonch-Bruyevich University of Telecommunications
61 Moika, St. Petersberg 191186, Russia
中村 幸男、大藪 修義
核融合科学研究所〒509-5292 岐阜県土岐市下石町 322-6

Construction and Performance Test of Apparatus for Permeation Experimentswith Controlled Surfaces
Yuji Hatano, Mamoru Nomura and Kuniaki Watanabe
Hydrogen Isotope Research Center, Toyama University
Gofuku 3190, Toyama 930-8555, Japan
Alexander I. Livshits, Andrei O. Busnyuk
Bonch-Bruyevich University of Telecommunications
61 Moika, St. Petersberg 191186, Russia
Yukio Nakamura, Nobuyoshi Ohyabu
National Institute for Fusion Science
Oroshi-cho, Toki 509-5292, Japan(Received March 30, 2002; Accepted December 20, 2002)
Abstract
A new apparatus was constructed to examine gas-, atom- and plasma-driven permeation ofhydrogen isotopes through group VA metal membranes with precisely controlled surface states.
Absorption and desorption experiments are also possible. The new apparatus consists of two
vacuum chambers, an upstream chamber and a downstream chamber, separated by a specimen
membrane. Both chambers are evacuated by turbo-molecular pumps and sputter-ion pumps. The
upstream chamber is equipped with Ta filaments serving as atomizers in atom-driven permeation
experiments and cathodes in plasma-driven permeation experiments. The specimen membrane isformed into a tubular shape and electrically isolated from the chamber. Hence, ohmic heating of13波多野雄治･能村衛･渡辺国昭･中村幸男･大藪修義

the membrane is possible, and this feature of the membrane is suitable for surface cleaning by
high-temperature heating and impurity doping for the control of surface chemical composition
through surface segregation. Both chambers were evacuated to 1×10-7 Pa after baking. The main
component of residual gas was H2, and the partial pressures of impurity gases other than H2 wereca. 1×10-8 Pa. Gas- and atom-driven permeation experiments were successfully carried out withhydrogen gas for Nb membrane activated by heating in vacuum at 1173 K. Superpermeation wasobserved in the atom-driven permeation experiments. Absorption experiments with a clean surface
were also carried out. The surface was, however, cleaned only partially, because the temperature
distribution was not uniform during high-temperature heating. Nevertheless, surface cleanliness
was retained during absorption experiments under the present vacuum conditions. A new
membrane assembly that will enable a uniform temperature distribution is now under construction.

1. Introduction
Surfaces of metals are usually covered by thin layers of non-metallic impurities such as
oxygen and carbon. Hence, the investigation on interaction between hydrogen isotopes and metal
surfaces covered by impurity layers are important in various fields of science and technology suchas surface physics and chemistry, catalysis, hydrogen energy and nuclear fusion systems. The
precise control of surface chemical composition, however, is required to carry out the systematic
study. This kind of study is carried out in ultra-high vacuum (UHV) apparatuses in most cases, but
the control of surface composition is generally difficult even in UHV. This is because dissolutionof impurities into metal bulk can take place as well as evaporation from surface to gas phase.
Recently, the present authors have found out that the recombination rate constant of
deuterium at Nb surface systematically decreases with increasing oxygen content in the bulk
because of the surface segregation of oxygen [1]. The control of surface composition by surface
segregation is promising technique, because impurity coverage is determined thermodynamicallyby the impurity concentration in the bulk and the heat of segregation.

14表面制御透過実験装置の製作と性能試験

On the other hand, metals covered by thin impurity layers have enormous capabilities forpermeation and absorption for suprathermal atoms and ions of hydrogen isotopes [2,3]. Hence,
metallic membranes and panels covered by such impurity layers can be applied for particle control
and separation of D/T from He in fusion devices [3-9]. Such high capabilities for permeation andabsorption are due to the suppression of reemission of hydrogen isotopes from incident surfaces by
non-metallic impurities. Reduction in the impurity coverage by sputtering degrades the absorption
and permeation capabilities, and hence compensation for removed impurities is necessary tomaintain the performance of membranes and panels under sputtering. Surface segregation of
impurities from the bulk is promising also for the compensation of sputtered impurities.
The group VA metals (V, Nb and Ta) are candidates of pumping membrane and panel
materials in fusion energy systems [5-9], hydrogen storage materials [10, 11] and membranematerials for hydrogen purification in hydrogen energy systems [12-14]. Hence, the present
authors have started the systematic study on the interaction between the hydrogen isotopes and thesurfaces of group VA metals covered by non-metallic impurities such as oxygen and sulfur by
surface segregating technique. The main apparatuses used for this study are a surface analysis
system (X-ray and ultraviolet photoelectron spectroscopy) and a permeation apparatus constructed
recently. This permeation apparatus has capability to carry out gas-, atom- and plasma-driven
permeation experiments with well-controlled surface chemical compositions as well as
absorption/desorption experiments. One of the interesting features of the apparatus is the shape of
specimen membrane; the membrane is in tubular form and not in sheet form as usual permeation
apparatuses. This shape of membrane is beneficial for surface treatment. The present paper
describes the characteristics of the permeation apparatus in detail as well as the results ofperformance tests.

2. Characteristics of permeation apparatus
A schematic description of the constructed permeation apparatus is shown in Fig. 1. This

15

波多野雄治･能村衛･渡辺国昭･中村幸男･大藪修義

TMPapparatus consists of two UHV chambers,
Linear motion
feedthroughan upstream chamber and a downstream
chamber, separated by a tubular specimen
The

upstream

QMA
QMA

membrane.

Downstream

chambercomprises the main discharge chamber andauxiliary chamber, and analysis instrumentsIPsuch as pressure gauges are installed in thelatter. Both upstream and downstream
chambers

wereturbo-molecular

evacuated

pumps(TMPs)

View
port

1

FilamentbyandMembrane

sputter-ion pumps (IPs). The conductance to

TMP

IP

the upstream IP can be adjusted to 0.1, 20
and 7000 L⋅s-1 by moving diaphragms

Upstream

connected to the linear motion feedthrough.

Fig. 1 Schematic description of constructed
permeation apparatus.

The total pressure is measured by Pirani
gaugesathighpressure

region(0.13-1.3×105 Pa) and ionization gauges at
low pressure region (10-8-1.0 Pa). The
partial pressure of each gas species aremeasured by quadrupole mass analyzers
Filament(QMAs). These pressure gauges and mass
analyzers were calibrated with diaphragmNb membrane

Mo shield

Nb strip

Mo plate

gauges whose full scales are 1.33×105 and13.3 Pa. The maximum baking temperature
is 673 K for the main discharge chamber
and 473 K for the auxiliary and downstream

Fig. 2

16

Photo of upper flange of main dischargechamber.

表面制御透過実験装置の製作と性能試験

chambers. Hydrogen, deuterium and other gases for surface modification such as O2 are
introduced into both chambers through variable-leak valves.
Figure 2 shows the upper flange of the main discharge chamber. This chamber isequipped with the specimen membrane and 9 Ta filaments whose diameter is 0.8 mm; 4 filaments
are mounted on the upper flange and 5 filaments on the lower flange. Molybdenum shields are
installed between the filaments and the specimen membrane to avoid the deposition of Ta on to themembrane. The filaments are used for two purposes: dissociation of hydrogen and deuterium
molecules for atom-driven permeation experiments and electron emission for plasma-driven
permeation experiments. In the latter case, the discharge is carried out by applying voltage (<120
V) between the filaments serving as cathode and the chamber wall serving as anode. The kinetic
energy of incident ions onto the membrane was controlled by the bias voltage between thechamber wall and the specimen membrane. Magnets made of SmCo alloy are mounted on theoutside of the chamber wall as shown in Fig. 3 to provide the multicusp magnetic field (see Fig.
4); the magnetic flux density in the chamber is ca. 0.01 T. The chamber wall is cooled by water
during operation to avoid the increase in background pressure.

SmCo magnet

S
N

N

S

S

N

N

AuxiliarychamberS

S

Ta filament

N
SmCo magnet
Tubular membrane

Water jacket

Fig. 3

Photo of main discharge chamber(right) and auxiliary chamber (left).

Fig. 4

17

Schematic description of multicusp
magnetic field.

波多野雄治･能村衛･渡辺国昭･中村幸男･大藪修義

The details of Nb membrane assembly are shown in Fig. 5 as an example. The specimen
membrane is formed into a tubular shape by laser welding in Ar atmosphere and electricallyisolated from the chamber. This feature of membrane allows ohmic heating. One-end of the tubular
membrane is closed by welding and connected to an electric feedthrough through Nb strip and Mo
plate. The electric resistance of the Nb strip is the same as the membrane. The other end is kept
open and connected hermetically to the downstream chamber through a stainless steel joint tube,
larger stainless steel tube and ceramic insulation tube. Strips of Nb and Ni are connected to theupper part of membrane and the larger stainless steel tube as shown in this figure as electric leads.
The stainless steel joint tube is heated ohmically to minimize the temperature gradient in themembrane near the connection part, and the temperature of joint tube is measured by W-Re
thermocouple. The temperature of the membrane is measured also by W-Re thermocouple inserted
in the membrane from the downstream side (not shown in the figure). Typical thickness of themembrane is 0.1 mm.
The capability for ohmic
heating is very important feature forsurface treatment. It has beenreported that clean surfaces of Nb

Water

and Ta are available by heating in
UHV up to 2300 K [15, 16]. The
membrane can be easily heated up
to this temperature at current around
240 A. In addition, only thetemperature of the membrane iselevated except the Nb strip beneaththe membrane, and hence gases forsurface modification reacts only

Fig. 5 Details of membrane assembly.
18表面制御透過実験装置の製作と性能試験

with Nb membrane and strip. Therefore, the amount of doped impurity can be estimated accurately.
The high temperature heating (above 2000 K) for surface cleaning is carried out by an alternating
current power supply and high capacity transformer (6.5 kW). A direct current power supply isused for temperature control during permeation and absorption/desorption experiments.
The expected flux of atoms and ions to the membrane during discharge are 1017
atoms·cm-2·s-1 and 1015 ions·cm-2·s-1, respectively. The flux of atoms by thermal dissociation is
expected to be 1016 atoms·cm-2·s-1.

3. Results of performance tests
3.1. Vacuum conditions
The residual pressure after baking at the maximum temperatures was 1×10-7 Pa in both
upstream and downstream chambers. The mass spectrum of residual gas in the downstream
chamber is shown in Fig. 6. The main residual gas was H2, and small peaks of CO and H2O wereobserved; the peak height of CO and H2O was ca. 1/5 and 1/10 of that of H2, respectively. It is
plausible that the source of CO is the filament of QMA, and water vapor is the dominant residualgas other than H2 when QMA is switched off. No significant difference was observed in the mass
spectrum of residual gas in the upstream chamber.
The partial pressures ofand O2 were comparable to thebackground after introducing
hydrogen to pressures below
1×10-3Pa

purification

without
system.

any
Above

QMA ion current (arb. unit)

impurities such as H2O, N2

0.35
0.30

Total pressure: 1×10-7 Pa

0.25
0.20
0.15
0.10
0.05
0.00
-0.05

0

1×10-3 Pa , those impurity gases
Fig. 6
were detected, and their partial

19

5

10

15

20

25 30
M/e

35

40

45

50

Typical example of mass spectrum of residual
gas in downstream chamber.

波多野雄治･能村衛･渡辺国昭･中村幸男･大藪修義

pressures increased to ca. 3×10-7 Pa after introducing hydrogen to 10-2 Pa. A hydrogen purification
system is now under development for reduction in the partial pressures of impurities at highpressure operations.

3.2. Gas- and atom-driven permeation after activation of membrane at 1173 K
A niobium membrane was used for the performance test. The specimen membrane wasactivated by heating in vacuum up to 1173 K, and gas-driven permeation experiments were carriedout with H2 at 0.02 Pa of the upstream pressure. Figure 7 shows the steady state permeation
probability χ of H2 molecules striking the upstream surface together with corresponding
downstream pressure. The permeation probability χ increases with temperature, and the activationenergy of permeation EP was estimated to be 42 kJ·mol-1. This value of EP indicates thatpermeation rate is controlled by surface processes, i.e. sticking and recombination processes, andnot by the diffusion process in the bulk. This is because EP should take a negative value,-22 kJ·mol-1, in the diffusion-limited case. Namely, EP is determined by the activation energy ofdiffusion ED and the heat of solution ES as EP = ED + ES provided that the permeation rate is

The sticking coefficient of
H2 molecules αm can be easily
estimated

from

the

permeation

probability shown in Fig. 7. In thesteady state, the flux of hydrogen
penetrating into the membrane is

Permeation probability, χ

for Nb-H system.
10-3

10-3

Atom-driven permeation (H)
10-4
10-4

10-5

10-5

Gas-driven permeation
(H2(H
) 2)
Gas-driven
permeation
42 kJ·mol-1-1

42 kJ⋅mol
0.8

equal to the sum of release flux
Fig. 7from upstream and downstream
surfaces:

20

0.9

1.0

1.1

1.2

1.3

1.4

10-6
1.5

Downstream pressure (Pa)controlled by the bulk diffusion process, where ED = 10 kJ·mol-1 [17] and ES = -34 kJ·mol-1 [18]1000 / T (K-1)
Results of gas- and atom-driven permeation
experiments after activation of Nb membraneat 1173 K.

表面制御透過実験装置の製作と性能試験

αmφm = kruCu2 + krdCd2,(1)where φm is the flux of H2 molecules striking the upstream surface, kr the recombination rate
constant, C the hydrogen concentration in subsurface layer, and u and d indicate upstream anddownstream sides, respectively. The permeation flux φP can be expressed as φP = krdCd2. The
concentration gradient of hydrogen in the bulk is negligibly small in the surface-limited regime,
and hence φP can be rewritten as
φP = αmφm ⋅krd / (kru + krd).

(2)

By assuming that kr on upstream and downstream surfaces are equal to each other (kru = krd), φP
can be derived as φP = αmφm / 2. The permeation probability χ is expressed as χ = φP /φm = αm / 2.
The values of αm thus obtained ranged from 2.7×10-5 (690 K) to 2.6×10-4 (1000 K). The sticking
probability αm takes a value around 0.1 provided that the surface of Nb membrane is clean [1, 16].
The values obtained are significantly smaller than that for clean surface, indicating that the surface
is contaminated by impurity. This impurity appeared to come from metal bulk and not from gas
phase. This is because the values of αm obtained at various H2 pressures were comparable to eachother, while the partial pressures of impurities in the gas phase were dependent on H2 pressure as
described in 3.1. In the previous paper [1], the present authors found that oxygen segregates to thesurface of Nb by vacuum heating. Hence, it is plausible that oxygen is the dominant surface
impurity.
The sticking probability αm is generally expressed as αm = αm0 exp( -EC / RT) where αm0 is
the pre-exponential factor, EC the potential barrier against dissociative chemisorption, R the gas
constant and T temperature. The potential barrier EC is negligibly small for the clean surfaces oftransition metals and putted up by surface impurities [19]. It is widely accepted that both αm0 and
EC are independent of temperature, and the temperature dependence of αm is determined only by
this “constant” EC. This idea is not correct when the impurity coverage at the surface is dependent
on temperature through segregation/dissolution of impurities from/into the bulk. One of the main
objectives of the present research project is to develop the phenomenological model to describe αm21

波多野雄治･能村衛･渡辺国昭･中村幸男･大藪修義

and kr by taking account of such temperature dependence of impurity coverage.
The atom-driven permeation experiments were also carried out by thermal atomization by
Ta filaments and the results are shown in Fig. 7. In this case, only the downstream pressures during
permeation experiments are indicated, because flux of atoms has not been measured andpermeation probability of atoms could not be estimated. It is, however, clear that the downstreampressure increased by atomization. In addition, no temperature dependence was observed in thepermeation rate in the case of atom-driven permeation, whereas the permeation rate increased withtemperature in the case of gas-driven permeation as mentioned above. These observations
correspond to the phenomenon called superpermeation. The mechanism of superpermeation can be
explained as follows. In the case of atom-driven permeation, Eq. (2) can be rewritten as
φP = (1/2) αaφa ⋅krd / (kru + krd),(3)

where αa and φa are the sticking coefficient and the flux of hydrogen atom, respectively. Factor 1/2
is necessary to adjust the mass balance between atomic incidence and molecular release. In thiscase, φP can be expressed as φP = αaφa / 4 at kru = krd. The internal energy of hydrogen atom ishigher than that of molecule by 2.24 eV, and hence atoms easily overcome EC. Namely, αa is close
to unity and independent of temperature even at the surfaces covered by impurities [2,3,19].
Therefore, larger and temperature independent permeation (superpermeation) takes place underatomization.

3.3. Measurement of αm after surface cleaning at high temperature
The membrane was heated gradually up to a high temperature (2323 K) and kept at thistemperature for 15 s. During temperature elevation, significant release of CO was observed. It is
plausible that oxygen and carbon present in the bulk as impurities were released mainly as CO.
After the high temperature heat treatment, the sticking coefficient of hydrogen molecules
αm was obtained by two different methods. In low temperature region where T< 673 K, αm wasestimated from the absorption rate of H2 by the membrane measured by the following procedure.

22表面制御透過実験装置の製作と性能試験

First, pumping of the chambers was stopped overnight, and the membrane surfaces weredeactivated by the exposure to residual gas of rather high pressure (10-4 Pa) at room temperature.
The chambers were evacuated to UHV again, and H2 gas was introduced into the upstream
chamber to 1×10-5 Pa. After the establishment of very stable gas flow, the hydrogen introduction
was stopped by pumping the backside of the variable-leak valve (VLV), in which the opening of
VLV was kept constant. Then, the membrane was activated by heating at 1273 K for 1 min in
UHV and cooled down to a given temperature. Hydrogen gas was introduced into the upstream
chamber in stepwise by supplying H2 gas of the same pressure as before to the backside of VLV.
The H2 pressure in the upstream chamber did not reach 1×10-5 Pa but 2-3×10-6 Pa rapidly. Thispressure difference is due to pumping by the membrane. The sticking coefficient αm was estimatedfrom this pressure difference. In this case, not only the membrane but Nb strip below themembrane should contribute to the H2 absorption, and hence αm was estimated from the total
surface area of membrane and Nb strip.
In high temperature region where T>1273 K, H2 gas was introduced into the upstream
chamber up to 4×10-4 Pa, and the membrane was heated up to a given temperature quickly. The
pressure of H2 dropped sharply
owing to the atomization at themembranesurface10-2

andafter heat treatment at 2323 K

the chamber wall. The sticking
coefficient αm was estimatedαmsubsequent pumping of atoms by
10-3

before10-4from the extent of pressure dropby assuming that αm is equal to

10-5
0.4

the atomization rate and all
atoms produced are adsorbed on

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

1000 / T (K )-1

Fig. 8the chamber wall. The Nb strip

23

Temperature dependence of αm before and after
heat treatment at 2323 K.

波多野雄治･能村衛･渡辺国昭･中村幸男･大藪修義

should contribute to this atomization, too. Hence surface area of the strip is considered in theestimation of αm also in this case.
The sticking coefficient αm thus obtained is plotted in Fig. 8 together with that measured
before high temperature heat treatment in the previous section. It should be emphasized that αm
increased significantly by high temperature heating. In addition, the values of αm determined by
two different methods are on the same straight line parallel to the horizontal axis. Namely, notemperature dependence was observed in αm after high temperature heating. This observation
indicates that the potential barrier against dissociative chemisorption EC is almost zero. Hence, thesurface is considered to be clean. The values of αm shown in Fig. 8, however, are still smaller thanthat of clean Nb surface (ca. 0.1). This discrepancy was ascribed to the overestimation of surface
area contributing to the H2 absorption and atomization. Such overestimation was caused by
non-uniform temperature distribution during high temperature heating. The temperature of a partof Nb strip just beneath the membrane was slightly higher than other part of the strip and themembrane. Therefore, the surface of this part is considered to be cleaner than other part. It is
plausible that this particular part played the dominant role in H2 absorption and atomization. The
new membrane is under construction at the present by taking account of this problem. It is,however, clear that the surface cleaned by high temperature heating was not contaminated by
impurities during absorption and atomization experiments. Therefore, it can be safely concluded
that the present apparatus has capability to carry out permeation and absorption/desorption
experiments with clean surfaces.

4. Conclusions
A new apparatus was constructed to examine gas-, atom- and plasma-driven permeationof hydrogen isotopes through group VA metal membranes with precisely controlled surface states.
Absorption/desorption experiments are also possible. This apparatus consists of two vacuum
chambers, an upstream chamber and a downstream chambers, separated by a specimen membrane.

24表面制御透過実験装置の製作と性能試験

Both chambers are evacuated by turbo-molecular pumps and sputter-ion pumps. The upstream
chamber is equipped with Ta filaments serving as atomizers in atom-driven permeation
experiments and cathodes in plasma-driven permeation experiments. The specimen membrane isformed into a tubular shape and electrically isolated from the chamber. Hence, ohmic heating of
the membrane is possible, and this feature of membrane is suitable for surface cleaning by hightemperature heating and impurity doping for the control of surface chemical composition throughsurface segregation. Both chambers were evacuated to 1×10-7 Pa after baking. The main
component of residual gas was H2, and the partial pressures of impurity gases other than H2 wereca. 1×10-8 Pa. Gas- and atom-driven permeation experiments were successfully carried out withhydrogen gas for Nb membrane activated by heating in vacuum at 1173 K. Superpermeation wasobserved in the atom-driven permeation experiments. The absorption experiments with cleansurface were also carried out. The surface was cleaned only in part, because the temperature
distribution was not uniform during high temperature heating. Nevertheless, vacuum conditions
were enough good to retain surface cleanliness during absorption experiments. Design of new
membrane assembly was started to obtain uniform temperature distribution.

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26