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Diffusion of hydrogen and deuterium in Zr-Al
645
Journal of Nuclear Materials 111 & 112 (1982) 645-647 North-Holland Publishing Company
DIFFUSION R.J. KNIZE
OF HYDROGEN
AND DEUTERIUM
IN Zr-Al
and J.L. CECCHI
Plasma Physics Laboratory,
Princeton
University, Princeton,
New Jersey 08544, USA
1. Introduction The control of impu_rities and hydrogenic recycling [l] in the tokamak fusion test reactor will be facilitated by a Zr-Al nonevaporable bulk getter system [2]. The getter utilizes the alloy ST lOl@ (84% Zr-16% Al) [3] in the form of a powder press bonded to a constantan heating substrate. After a 700°C activation, an operation temperature of 200-400°C is maintained, providing adequate impurity (C, N, 0) pumping. Hydrogenic isotopes are pumped reversibly. Optimization of getter performance necessitates a detailed understanding of the getter operation for a variety of experimental parameters, including pressure, temperature, getter mass, and getter thickness. For pumping at low pressures or during slow desorption, the getter operation will be dependent primarily on surface kinetics, which have been described previously [4]. During pumping at higher pressures or during rapid desorption, bulk diffusion may play an important role [5]. In this paper we present the first measurements of the diffusivities of hydrogen (DH) and deuterium (Do) in Zr-Al as determined from detailed analysis of desorption.
-0026-Y z 0024--” a 00220.020
0
2
4
6
0
IO
Fig. 1. Desorption of the getter as a function of time. The getter was heated to 700°C and initially loaded with 0.66 Torr 1 g-’ of H,. P’/’ is proportional to the surface concentration 4,.
speed
(at low pressures),
Kq*),
p is the mass density,
K
is Sievert’s constant (P = A is the surface area, and D is the diffusivity. Fig. 1 shows the square root of the hydrogen pressure [which is proportional to q&t)] during a typical desorption. The initial time behavior exhibits the t’i2 dependence predicted by eq. (1). Eventually, the diffusion gradient stabilizes and the desorption becomes primarily dependent on surface kinetics, so that qs( t) is given by [5],
(F+-$‘,
2. Getter desorption
4,(t)=
Hydrogenic regeneration of Zr-Al is effected by increasing the Zr-Al temperature with a concomitant increase in the hydrogenic equilibrium pressure. The desorbed gas is pumped by a back-up system. The initial time behavior of the desorption from the 120 pm thick Zr-Al layer will be similar to that for a semi-infinite solid. For this condition, the hydrogenic surface concentration at time t, qs( t), is described by (51,
where M is the getter mass and q’ is a constant which depends on the initial loading and the initial diffusionlimited desorption.
where q,, is the initial concentration, S is the pumping speed of the back-up system, So is the getter pumping
0022-33 15/82/0000-0000/$02.75
0 1982 North-Holland
3. Experimental procedure and results The experimental apparatus has been described previously [6]. The diffusion of hydrogen and deuterium was measured using a standard-size getter with a mass of 120 g. The initially desorbed getter was heated to a constant temperature 7’ over the range 723-1023K (450-750°C) and loaded to an initial concentration of
R.J. Knize, J.L. Cecchr / Dqf usion of hydrogen and deuterrum in Zr
646
Al
lattice of size a using the Zener approximation: Do=&.
10-S'
I
1.0
I
1
/
/
/
1.2
I.1
I
1
1.3
I/T(IO-"K-I)
fig. 2. Hydrogen diffusivity D, temperature T.
as a function
of the alloy
(3)
The predicted vibration frequency P - 8 X 1O’j s ’ is in good agreement with the vibration frequency inferred from measurements of the isotopic solubility dependence [6]. We also note that the resultant diffusivities predict that the characteristic time for diffusion through the alloy layer is about 2 min at 700°C and 280 min at 400°C. By equating the average diffusion flux to the incident adsorbed flux, the pressure P, where diffusion starts to limit the pumping speed can be estimated as P,=
D 2p2A2 (4)
S; KL* ’
where L is the getter thickness. This formula predicts that the pumping speed at 400°C will become diffusion-limited and fall as the pressure is increased above about 10e3Torr. This simple argument is in agreement with a more detailed numerical analysis [5] and previous measurements [7]. Since the getters will be used in regions where the pressures may exceed this value (such as near the TFTR limiters or inside a pumped limiter), the effect of diffusion on the pumping speed may be important. Operation of the getter at higher temperatures will raise P,.
10-8 1.0
1.2
I.1 I/T
1.3
5. Summary
(IO-3K-‘)
Fig. 3. Deuterium diffusivity D, temperature T.
as a function of the alloy
0.1 to 5 Torr 1 g ‘. The getter was desorbed by opening a fast gate valve to a turbomolecular pump. The diffusivity was determined from the measured variables (S, K, qo, S,) and the t ‘1’ slope of the time dependence of p’/*. Figs. 2 and 3 show the measured hydrogen ( DH) and deuterium (DD) diffusivities as functions of temperature. The diffusivities were fitted to Arrenhius functions with the results: D, = exp[-2.9(0.7) and D, = exp[ - 0.4( I .O) 9900(600)/T] cm* s-’ 12 100(900)/T] cm* s-‘.
We have measured the hydrogen and deuterium diffusivities in Zr-Al using the time dependence of the desorption. The measured pre-exponential factor Do predicts a hydrogen atom-lattice vibration frequency which agrees with results obtained by other measurements. The magnitude of the diffusivities is such as to limit the pumping at pressures exceeding lop3 Torr.
Acknowledgements We wish to thank H.F. Dylla for useful discussions, and E. Pinelli and F. Egan for their excellent technical support. This work was supported by the Department of Energy Contract No. DE-AC02-76-CHO-3073.
4. Discussion References The measured pre-exponential factor Do in the diffusivity [D = Do exp( -E/kT)] can be used to estimate the vibration frequency Y of hydrogen atoms in the
[ 11J.L. Cecchi, J. Nucl. Mater. 93/94 (1980) 28. [2] J.J. Sredniawski,
in: Proc. 8th Symp. on Engineering
Prob-
R.J. Knize, J.L. Cecchi / Diffusion of hydrogen and deuterium in Zr - Al lems of Fusion Research, San Francisco, CA, 1979 (IEEE, New York, 1980) p. 505. [3] The Zr-Al getter is manufactured by SAES Getters, S.P.A., Milan, Italy. [4] J.L. Cecchi, S.A. Cohen and J.J. Sredniawski, J. Vat. Sci. Technol. 17 (1980) 294.
647
[5] R.J. Knize and J.L. Cecchi, to be published. [6] R.J. Knize, J.L. Cecchi and H.F. Dylla, J. Vat. Sci. Technol. 20 (1982) 1135. [I] L. Rosai, B. Ferrario and P. della Porta, J. Vat. Sci. Technol. 15 (1978) 746.
Journal of Nuclear Materials 111 & 112 (1982) 645-647 North-Holland Publishing Company
DIFFUSION R.J. KNIZE
OF HYDROGEN
AND DEUTERIUM
IN Zr-Al
and J.L. CECCHI
Plasma Physics Laboratory,
Princeton
University, Princeton,
New Jersey 08544, USA
1. Introduction The control of impu_rities and hydrogenic recycling [l] in the tokamak fusion test reactor will be facilitated by a Zr-Al nonevaporable bulk getter system [2]. The getter utilizes the alloy ST lOl@ (84% Zr-16% Al) [3] in the form of a powder press bonded to a constantan heating substrate. After a 700°C activation, an operation temperature of 200-400°C is maintained, providing adequate impurity (C, N, 0) pumping. Hydrogenic isotopes are pumped reversibly. Optimization of getter performance necessitates a detailed understanding of the getter operation for a variety of experimental parameters, including pressure, temperature, getter mass, and getter thickness. For pumping at low pressures or during slow desorption, the getter operation will be dependent primarily on surface kinetics, which have been described previously [4]. During pumping at higher pressures or during rapid desorption, bulk diffusion may play an important role [5]. In this paper we present the first measurements of the diffusivities of hydrogen (DH) and deuterium (Do) in Zr-Al as determined from detailed analysis of desorption.
-0026-Y z 0024--” a 00220.020
0
2
4
6
0
IO
Fig. 1. Desorption of the getter as a function of time. The getter was heated to 700°C and initially loaded with 0.66 Torr 1 g-’ of H,. P’/’ is proportional to the surface concentration 4,.
speed
(at low pressures),
Kq*),
p is the mass density,
K
is Sievert’s constant (P = A is the surface area, and D is the diffusivity. Fig. 1 shows the square root of the hydrogen pressure [which is proportional to q&t)] during a typical desorption. The initial time behavior exhibits the t’i2 dependence predicted by eq. (1). Eventually, the diffusion gradient stabilizes and the desorption becomes primarily dependent on surface kinetics, so that qs( t) is given by [5],
(F+-$‘,
2. Getter desorption
4,(t)=
Hydrogenic regeneration of Zr-Al is effected by increasing the Zr-Al temperature with a concomitant increase in the hydrogenic equilibrium pressure. The desorbed gas is pumped by a back-up system. The initial time behavior of the desorption from the 120 pm thick Zr-Al layer will be similar to that for a semi-infinite solid. For this condition, the hydrogenic surface concentration at time t, qs( t), is described by (51,
where M is the getter mass and q’ is a constant which depends on the initial loading and the initial diffusionlimited desorption.
where q,, is the initial concentration, S is the pumping speed of the back-up system, So is the getter pumping
0022-33 15/82/0000-0000/$02.75
0 1982 North-Holland
3. Experimental procedure and results The experimental apparatus has been described previously [6]. The diffusion of hydrogen and deuterium was measured using a standard-size getter with a mass of 120 g. The initially desorbed getter was heated to a constant temperature 7’ over the range 723-1023K (450-750°C) and loaded to an initial concentration of
R.J. Knize, J.L. Cecchr / Dqf usion of hydrogen and deuterrum in Zr
646
Al
lattice of size a using the Zener approximation: Do=&.
10-S'
I
1.0
I
1
/
/
/
1.2
I.1
I
1
1.3
I/T(IO-"K-I)
fig. 2. Hydrogen diffusivity D, temperature T.
as a function
of the alloy
(3)
The predicted vibration frequency P - 8 X 1O’j s ’ is in good agreement with the vibration frequency inferred from measurements of the isotopic solubility dependence [6]. We also note that the resultant diffusivities predict that the characteristic time for diffusion through the alloy layer is about 2 min at 700°C and 280 min at 400°C. By equating the average diffusion flux to the incident adsorbed flux, the pressure P, where diffusion starts to limit the pumping speed can be estimated as P,=
D 2p2A2 (4)
S; KL* ’
where L is the getter thickness. This formula predicts that the pumping speed at 400°C will become diffusion-limited and fall as the pressure is increased above about 10e3Torr. This simple argument is in agreement with a more detailed numerical analysis [5] and previous measurements [7]. Since the getters will be used in regions where the pressures may exceed this value (such as near the TFTR limiters or inside a pumped limiter), the effect of diffusion on the pumping speed may be important. Operation of the getter at higher temperatures will raise P,.
10-8 1.0
1.2
I.1 I/T
1.3
5. Summary
(IO-3K-‘)
Fig. 3. Deuterium diffusivity D, temperature T.
as a function of the alloy
0.1 to 5 Torr 1 g ‘. The getter was desorbed by opening a fast gate valve to a turbomolecular pump. The diffusivity was determined from the measured variables (S, K, qo, S,) and the t ‘1’ slope of the time dependence of p’/*. Figs. 2 and 3 show the measured hydrogen ( DH) and deuterium (DD) diffusivities as functions of temperature. The diffusivities were fitted to Arrenhius functions with the results: D, = exp[-2.9(0.7) and D, = exp[ - 0.4( I .O) 9900(600)/T] cm* s-’ 12 100(900)/T] cm* s-‘.
We have measured the hydrogen and deuterium diffusivities in Zr-Al using the time dependence of the desorption. The measured pre-exponential factor Do predicts a hydrogen atom-lattice vibration frequency which agrees with results obtained by other measurements. The magnitude of the diffusivities is such as to limit the pumping at pressures exceeding lop3 Torr.
Acknowledgements We wish to thank H.F. Dylla for useful discussions, and E. Pinelli and F. Egan for their excellent technical support. This work was supported by the Department of Energy Contract No. DE-AC02-76-CHO-3073.
4. Discussion References The measured pre-exponential factor Do in the diffusivity [D = Do exp( -E/kT)] can be used to estimate the vibration frequency Y of hydrogen atoms in the
[ 11J.L. Cecchi, J. Nucl. Mater. 93/94 (1980) 28. [2] J.J. Sredniawski,
in: Proc. 8th Symp. on Engineering
Prob-
R.J. Knize, J.L. Cecchi / Diffusion of hydrogen and deuterium in Zr - Al lems of Fusion Research, San Francisco, CA, 1979 (IEEE, New York, 1980) p. 505. [3] The Zr-Al getter is manufactured by SAES Getters, S.P.A., Milan, Italy. [4] J.L. Cecchi, S.A. Cohen and J.J. Sredniawski, J. Vat. Sci. Technol. 17 (1980) 294.
647
[5] R.J. Knize and J.L. Cecchi, to be published. [6] R.J. Knize, J.L. Cecchi and H.F. Dylla, J. Vat. Sci. Technol. 20 (1982) 1135. [I] L. Rosai, B. Ferrario and P. della Porta, J. Vat. Sci. Technol. 15 (1978) 746.