- Email: [email protected]
Reversible disproportionation of ZrCo under high temperature and hydrogen pressure
ELSEVIER
Journal of Nuclear Materials 223 (1995) 294-299
jou.nnlof ggclgr materials
Reversible disproportionation of ZrCo under high temperature and hydrogen pressure S. Konishi *, T. Nagasaki, K. Okuno Japan Atomic Energy Research Institute, Tokai, lbaraki 319-11, Japan Received 3 June 1994; accepted 5 December 1994
Abstract
Chemical behavior of an intermetallic compound ZrCo with hydrogen at high temperature region was studied with respect to application utilizing the reversible hydrogenation. While ZrCo compound reacts with hydrogen to form ZrCoH(0_3) below 400°C, the disproportionation reaction, 2ZrCo + H 2 ~ Z r H 2 + ZrCo 2 occurs above 400°C under hydrogen pressure higher than the equilibrium decomposition pressure of ZrCoH(0_3). X-ray diffraction analysis showed that the reaction is completed in 5 h at 500°C and it takes more than 40 h at 400°C. No sign of generation of Z r H 2 + ZrCo 2 phase was observed by the X-ray diffraction in 10 h at 400°C, however. The disproportionated product completely returned to single-phase ZrCo when evacuated at or above 500°C for several hours, thus this reaction is reversible. Kinetics of the disproportionation reaction was expressed by an Avrami equation R = 1 - e x p { - ( t / z ) ' } . The reaction speed was expressed by an Arrhenius form. For practical application of ZrCo, the results suggest some operational precaution and limitation, and recovery treatment when the material is suspected to have lost some capacity by disproportionation reaction.
1. Introduction
An intermetallic compound ZrCo was proposed as an alternative of uranium for recovery, storage and supply of tritium [1,2] and is obtaining increasing attention for applications to fusion reactor fuel systems [3-5]. Z r C o c o m p o u n d shows an attractive hydriding/dehydriding property as a substitute of uranium, and has less pyropholicity and easier handling because it is not a nuclear fuel material [2,5]. Several types of ZrCo beds up to 700 l capacity were fabricated and demonstrated for practical services in tritium applications. Typically, ZrCo absorbs hydrogen isotopes down to ca. 10 -4 Torr (10 -2 Pa) at room temperature for tritium recovery, and is heated to approximately 400°C
* Corresponding [email protected]
author,
e-m ail:
to desorb hydrogen at around atmospheric pressure. Fig. 1 shows the pressure-composition isotherm for Z r C o - H system previously reported by the authors [1]. It is a disadvantage of ZrCo that it has a minor amount of dissolved hydrogen in metal as a-phase. The amount of residual hydrogen depends on the pressure and temperature as described in the isotherm. When evacuated at 450°C, hydrogen concentration in the metal is reported to be reduced to as low as in the order of 10 -4 [5]. It has been noticed that the amount of residual hydrogen in ZrCo metal sometimes increases accompanied by reduction of capacity of hydrogen storage, in tens of times of use for supply, or when it is overheated under hydrogen atmosphere. Since it is not desirable for practical use of ZrCo for storage of hydrogen, the nature of this phenomenon was investigated. This report describes a reversible disproportionation reaction in the Z r C o - H system under high temperature and pressure region.
0022-3115/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD! 0022-3115(95)00007-0
S. Konishi et aL ~Journal of Nuclear Materials 223 (1995) 294-299 2. Experimental An intermetallic compound of ZrCo was made by arc melting in argon atmosphere from an equimolar mixture of Zr and Cx~. A powder sample was prepared by several times of hydriding-dehydriding cycle. X-ray analysis indicated little trace of other phases in the sample. The experimental apparatus for measurement of pressure-composition isotherms shown in Fig. 2 was used to observe the reaction. Detailed apparatus and procedure are described in a previous report [1]. Sample of either ZrCo o r ZrCoH2.6_ 7 was exposed to pure hydrogen at 400 to 600°C in a closed volume. In a typical experiment, ZrCoH x sample is immediately equilibrated with hydrogen according to the isotherm. Formation of different phase was measured by the change of pressure in the volume over relatively long time until the pressure stabilizes. After this change, pressure-composition isotherms for this system were taken to ,;ee the change of the hydrogen storage capacity. Diffraction spectra of the cooled samples were taken with C u K a X-ray in air to identify the reaction product. Reaction kinetics was investigated with this pressure change. Some ZrCo beds equipped with an internal heater
6/
'
,
400oc 5~ / / 6 5 ° 0
4l/~j~
I/f
295
used in previous works [2] and larger models with similar configuration were used for the test under practical conditions.
3. Results and discussion 3.1. Disproportionation of ZrCo Fig. 3 shows an example of the isotherm for dehydriding ZrCoH x at 500°C following exposure to hydrogen above 2000 Tort at 500°C for 6 h. The equilibrium hydrogen pressure decreases sharply at the apparent ratio of hydrogen atom to Zr atom or ZrCo molecule, nn/n(zrCo) of around 1.0, similar to that for Z r H 2. In the X-ray diffraction patterns shown in Fig. 4, the spectrum of the sample exposed to hydrogen at 500°C was identified for the mixture of Z r H 2 and ZrCo 2. These results indicate that the disproportionation reaction 2ZrCoHx --} Z r H 2 + ZrCo 2
(1)
occurred. Some decrease of hydrogen storage capacity and existence of more strongly bound hydrogen were noticed on the ZrCoH~ kept in hydrogen for several hours at 450°C above 1000 Torr. From the results of X-ray analysis for the samples exposed to pressurized hydrogen at high temperatures, the disproportionation reaction proceeds slower at lower temperature. Because of the difficulty in the quantitative measurement by X-ray analysis, the reaction speed could not be obtained. Thermochemical data [6,7] suggest that Z r H 2 and ZrCo 2 are more stable compounds than ZrCoH~ and ZrCo and disproportionation reaction (1) could occur at lower temperatures. However the reaction rate at the temperature lower than 400°C is extremely slow. In the above experiments, hydrogen pressure was always kept to maintain to apparent nH/n(zrco) exceeds unity, or to be sufficient to form Z r H 2. Reaction occurred either, from 13-phase ZrCo hydride 2ZrCoHo_y ~--} ZrH 2 + ZrCo 2 + (2 - y ) H 2 ( y < 2),
(2)
2 or from a-phase ZrCo with dissolved hydrogen
1
130oc |
/ 0
I
2ZrCoH x + 2(1 - x ) H 2 ~ Z r H 2 + ZrCo 2 ( x < 1).
oAov hydrogenating ,Awv dehydrogenating I
1
2 I3 H/nzrco
Fig. 1. Pressure-composition isotherms of the Z r C o - H 2 system.
(3) No peaks that indicate disproportionation were observed for the samples kept in the hydrogen pressure lower than the equilibrium pressure of the Z r C o - H 2 system by continuous evacuation, in which the normal dehydriding reaction ZrCoH~ --, ZrCo + ( x / 2 ) H 2 ,
(4)
296
S. Konishi et al. /Journal of Nuclear Materials 223 (1995) 294-299
Hydrogen purifier
Thermocouple Sheathed ~.,Jk-~.~n Samplecell heater~ ~ Sample~
k
" "
I
I
l[ G
Schulz gauge
(I)- -(I)
Turbomolecular pump
.eservoirs
Capacitance manometers
Fig. 2. Experimental apparatus used for the quantitative measurement of isotherm and disproportionation reaction.
that is much faster than reaction (2) or (3) can proceed. It is understandable that the disproportionation reaction (1) that requires movement and reconfiguration of metal atoms proceeds slower than the dehydriding of ZrCo hydride where much faster diffusion of hydrogen atoms occur.
3.2. Reverse reaction
The sample that completed disproportionation was then evacuated at 500°C for 12 h. By this treatment, the X-ray diffraction spectrum that showed the mixture of ZrH 2 and ZrCo 2 completely changed back to the
i
2. 500°C, disproportionation :3.500°C, dehydriding
/
•
7,
S
3
ZrCo2
•
ZrH2
/
4
o
•
,, ,! ,i ........
Z...
20
60
40
80
26
(b) After Disproportionation
after regeneration D///
I
~ O
"-:',
4. 500°C,regeneration
-
O ZrCo
5
1.100°C,hydriding I
1
I
nH/nzrco
2
3
Fig. 3. Examples of the apparent isotherm for the ZrCo experienced disproportionation.
20
40
60
80
20 (a~ ZrCo
Fig. 4. X-ray diffraction spectra of the ZrCo and the product of disproportionation.
297
S. Konishi et aL /Journal of Nuclear Materials 223 (1995) 294-299
cess occurs under the condition that ZrH x dehydrides, that is, heating the sample and evacuate it below equilibrium pressure at a temperature above 450°C. It could not be concluded whether this reaction occurs at lower temperature because of the very low dissociation pressure of ZrH x. The X-ray spectra obtained with the reproportionated sample showed no trace of Zr-metal phase that indicates somewhat permanent dissociation of ZrCo.
O f-
-i-
3.3. Reaction kinetics
1
10
100
1000
10000
time / min
Fig. 5. Typical rate of disproportionation of ZrCoH x for x < 1 and x > l .
single phase of cubic ZrCo. Measured isotherms of this sample and hydrogen shown in the Fig. 3 indicate formation of only ZrCoH(0_3) when hydrided, and the same hydrogen absorption capacity as before the disproportionation. These results suggest that the disproportionation reactions (1)-(3) are reversible. This pro-
In the experiment of the disproportionation of ZrCo kept in hydrogen atmosphere in a closed volume, change of the pressure reflects the progress of reaction. Increase of the pressure in a closed system shown in Fig. 2 for reaction (2) or decrease of the pressure for reaction (3) were analyzed to determine the reaction kinetics. Fig. 5 shows the examples of the change of the H / Z r C o value under high hydrogen pressure and temperature. Both reaction (2) and (3) are observed. The ratio of the product phase increases slowly at the beginning and end of the reaction, showing sigmoids. In order to analyze this kinetics, the AvramiErofeev equation a = 1 - exp{-(t/r)n},
(5)
where a represents the ratio of the normalized product phase estimated from the pressure change, was attempted to fit. The time constant ~" was obtained
10 1
(a) 0.8 10 o
°.6f j
10.1
e-
0.4
• 400°0 • 500°C
10.3
slopen:=3 ,
.1
,
i
,
i
1
10
0
.
1
J 1
/
° tyf
z
ill
experimental
n2
~
• 600°C i
/~'
I
[] 550°C 0
/
0 55°°c
0
t
n=4 ~ 1
Normalized Time
10
t/x
t/x Fig. 6. Normalized disproportionation kinetics of ZrCo using the Avrami-Erofeev equation. (a) Logarithmic plot for estimation of n. (b) Comparison between the experimental data and fitted equation.
298
S. Konishi et aL /Journal of Nuclear Materials 223 (1995) 294-299
from the time that gives a -- 0.632. Eq. (5) was rewritten as
-In(1
- a) = (t/z)".
Temperature/°C
To obtain the value of n, the slopes of the logarithmic plot of - l n ( 1 - a ) against t/'r was measured in the range of 0.2 < t/'r < 2 where errors in reading time and pressure are not significant and change of the value of a is large. Fig. 6 shows some examples of this logarithmic plot for reaction (3) under 1000 Torr of hydrogen at various temperatures. Between 400 and 600°C, all the disproportionation kinetics were normalized to Eq. (5) and the value of n = 3 gave a good agreement for most of the measurements. Eq. (5) is known to describe the phase transition process controlled by the growth of a new phase from nuclei in the solid. The equation with n = 3 represents a two-dimensional growth of the product phase and a constant generation rate of nuclei, or a three-dimensional growth and a constant number of nuclei. The observed similarity of this reaction with phase transition is understood as the disproportionation (1) proceeds without major movement of reactant, except for hydrogen that migrates much faster than metal atoms in the lattice. Thus, this disproportionation process can be well described by using Eq. (5). The reversible nature of the reaction also supports the similarity of this process and phase transition. The previously reported differences in the disproportionation speed of the identical sample with different history [8] may be explained using this equation that formulates the apparent incubation period. Same measurements for reaction (2) for the previously hydrided sample kept under 3000 Torr of hydrogen gave higher n values around 4. This seems to suggest a faster generation of reaction nuclei during the disproportionation, probably due to much higher hydrogen content in the ZrCo hydride phase. Using the time constant ~- in Eqs. (5) and (6), it is possible to normalize and summarize the disproportionation speed under a given condition regardless of the n value. An Arrhenius plot 'of the reciprocal time constant 1/~" is shown in Fig. 7. A linear relation can be seen in the temperature range between 400 and 550°C, although deviation is observed at 600°C where inevitable error was suspected to have affected. It seems that the rate of reaction (2) from hydrided ZrCo showed different temperature dependence from that of reaction (3) from ZrCo, probably owing to different crystal structure, or generation rate of reaction nuclei. When the 'kinetics is expressed by Eqs. (5) and (6), ~" is equivalent to the estimated time for 63% disproportionation under a given condition. From this figure, it is possible to estimate the allowable time for the ZrCo at high temperature and hydrogen pressure for practical use. For the system with n = 3 or higher, the degree of disproportionation a stays very low until ca.
600
10
(6)
400
5O0
I
I
I
I.-'
._= E "E tO
o
10
•
© ©
10
00
.E_ i-
•
1000 T o r r H2, started from ZrCo
8
•
2000 T o r r H2, started from Z r C o
0
C) 3000¥orrH2, startedfrom ZrCoH2
10
l
1.1
I
1.2
t
I
1.3
©
| i
I
i
1.4
1.5
Reciprocal temperature lO00/k Fig. 7. Arrenius plot of the time constant of the disproportionation reaction of ZrCo using the Avrami-Erofeev equation.
t = 0.3~" where a begins to rise rapidly. This apparent incubation time is attributed to the slower increase of volume of the product of the disproportionation when the nuclei are few and small. 3.4. Practical implication For the application of ZrCo as storage media of hydrogen isotopes, the results suggests some operational precaution and limitation, and recovery treatment when the material is suspected to be degraded. The relationship shown in Fig. 7 indicates the expected life or suggested operational limit of the metal at high temperature and hydrogen pressure. For instance, at 400°C, contact of ZrCo with high pressure hydrogen isotopes should be limited to ca. 50 h in total to prevent possible capacity loss by disproportionation. This limit allows a number of repeated heating of the ZrCo beds f o r p r a c t i c a l supply of hydrogen isotopes without significant loss of capacity. Small ZrCo bed that contains 50 g of ZrCo hydride powder has been tested for thermal cycle between room temperature and 400°C for 25 times and slight decrease of the capacity was observed [2]. It should be noted that according to the Eq. (5), in the incubation period, the early stage of the disproportionation (or the generation of nuclei) can proceed without any notable loss of capacity in repeated or prolonged heating. Then in a practical use, a "sudden"
S. Konishi et al. ~Journal of Nuclear Materials 223 (1995) 294-299
capacity loss might be sometimes experienced for the bed that have previously developed the disproportionation nuclei begun to grow. With a larger bed with 2 kg of ZrCo for 300 l hydrogen capacity, approximately 30% capacity reduction was found in the latest use over the 5 years of actual tritium service in the Tritium Process Laboratory at the Japan Atomic Energy Research Institute. Regeneration of the bed was tested by vacuum pumping with a Normetex Scroll pump at elevated temperature, and treatment at 500°C for approximately 10 h has completely regenerated the bed. Based on the reversible nature of the reaction, it is understandable that the reverse process, reproportionation, is also much easier and faster for the material that has not completed the disproportionation. Particularly within the incubation period, vacuum treatment will remove reaction nuclei and will recover the incubation time, and thus prevent the rapid disproportionation. It is recommended to equip the bed with a vacuum pump to extract hydrogen from metal, and to regenerate it when the disproportionation occurs. In practice, vacuum pumps are often equipped for a tritium storage system even with uranium, for faster supply, periodical complete regeneration, or measurement of physical inventory. Supply of hydrogen isotope by heating of the bed under pumping will completely avoid the condition that leads to the disproportionation. Periodical complete regeneration, for instance to measure the inventory, will provide a 'virtually limitless life for ZrCo beds. Overheating of the storage material in a small closed volume is an incident that should be strictly avoided with any of the storage media. The interlocks to prevent heating of the storage bed with outlet valves closed, or to prevent overheating that are often considered for practical ZrCo or U beds are also very effective to avoid dispropo:rtionation of ZrCo. Therefore in practical applications, the disproportionation is possible but not a major drawback of ZrCo as a storage media of hydrogen isotopes.
4: Conclusion
Some features of the reversible disproportionation reaction of ZrCo under high temperature and hydro-
299
gen pressure were understood. Although the detailed mechanism of the reaction needs to be investigated, some implications for practical application were drawn. The results suggest that 1) In the heating of ZrCoH~ for supply of hydrogen, temperature should be limited to lower than 400°C and the time and hydrogen pressure should be minimized. 2) Periodical complete regeneration is recommended to prevent disproportionation and to keep the incubation period. 3) Vacuum treatment at high temperature regenerates the capacity of ZrCo when the disproportionation is suspected. 4) The disproportionation of ZrCo is not usually a problem for practical recovery and supply of hydrogen isotopes. However, the use of hydride media for the recovery, storage and supply of hydrogen isotopes, particularly tritium, is increasingly required to respond to various demands. It is desired to study and develop various storage materials based on the recent improvements in understanding on intermetallic materials. For a prolonged use of the bed for pressurized supply of hydrogen gas, materials that require lower temperature for dehydriding are needed, not only to avoid disproportionation, but also to reduce temperature related problems such as tritium permeation and heat load in gloveboxes.
References
[1] T. Nagasaki, S. Konishi, H. Katsuta and Y. Naruse, Fusion Technol. 9 (1986) 506. [2] S. Konishi, T. Nagasaki, N. Yokokawa and Y. Naruse, Fusion Eng. Des. 10 (1989) 355. [3] E. Willin, M. Sirch, R.-D Penzhorn and M. Devillers, Fusion Teehnol. 14 (1988) 756. [4] ITER Fuel Cycle, ITER Documentation Series No. 31 (IAEA, Vienna, 1991). [5] R.-D. Penzhom, M. Devillers and M. Sirch, J. Nucl. Mater. 170 (1990) 217. [6] J.C: Gachon and J. Hertz, CALPHAD 7 (1983) 1. [7] D.R. Fredrickson, L.R. Nuttall, H.E. Flotow and W.N. Hubbard, J. Phys. Chem. 67 (1963) 1506. [8] M. Devillers, M. Sirch and R.-D, Penzhorn, Chem. Mater. 4 (1992) 631.
Journal of Nuclear Materials 223 (1995) 294-299
jou.nnlof ggclgr materials
Reversible disproportionation of ZrCo under high temperature and hydrogen pressure S. Konishi *, T. Nagasaki, K. Okuno Japan Atomic Energy Research Institute, Tokai, lbaraki 319-11, Japan Received 3 June 1994; accepted 5 December 1994
Abstract
Chemical behavior of an intermetallic compound ZrCo with hydrogen at high temperature region was studied with respect to application utilizing the reversible hydrogenation. While ZrCo compound reacts with hydrogen to form ZrCoH(0_3) below 400°C, the disproportionation reaction, 2ZrCo + H 2 ~ Z r H 2 + ZrCo 2 occurs above 400°C under hydrogen pressure higher than the equilibrium decomposition pressure of ZrCoH(0_3). X-ray diffraction analysis showed that the reaction is completed in 5 h at 500°C and it takes more than 40 h at 400°C. No sign of generation of Z r H 2 + ZrCo 2 phase was observed by the X-ray diffraction in 10 h at 400°C, however. The disproportionated product completely returned to single-phase ZrCo when evacuated at or above 500°C for several hours, thus this reaction is reversible. Kinetics of the disproportionation reaction was expressed by an Avrami equation R = 1 - e x p { - ( t / z ) ' } . The reaction speed was expressed by an Arrhenius form. For practical application of ZrCo, the results suggest some operational precaution and limitation, and recovery treatment when the material is suspected to have lost some capacity by disproportionation reaction.
1. Introduction
An intermetallic compound ZrCo was proposed as an alternative of uranium for recovery, storage and supply of tritium [1,2] and is obtaining increasing attention for applications to fusion reactor fuel systems [3-5]. Z r C o c o m p o u n d shows an attractive hydriding/dehydriding property as a substitute of uranium, and has less pyropholicity and easier handling because it is not a nuclear fuel material [2,5]. Several types of ZrCo beds up to 700 l capacity were fabricated and demonstrated for practical services in tritium applications. Typically, ZrCo absorbs hydrogen isotopes down to ca. 10 -4 Torr (10 -2 Pa) at room temperature for tritium recovery, and is heated to approximately 400°C
* Corresponding [email protected]
author,
e-m ail:
to desorb hydrogen at around atmospheric pressure. Fig. 1 shows the pressure-composition isotherm for Z r C o - H system previously reported by the authors [1]. It is a disadvantage of ZrCo that it has a minor amount of dissolved hydrogen in metal as a-phase. The amount of residual hydrogen depends on the pressure and temperature as described in the isotherm. When evacuated at 450°C, hydrogen concentration in the metal is reported to be reduced to as low as in the order of 10 -4 [5]. It has been noticed that the amount of residual hydrogen in ZrCo metal sometimes increases accompanied by reduction of capacity of hydrogen storage, in tens of times of use for supply, or when it is overheated under hydrogen atmosphere. Since it is not desirable for practical use of ZrCo for storage of hydrogen, the nature of this phenomenon was investigated. This report describes a reversible disproportionation reaction in the Z r C o - H system under high temperature and pressure region.
0022-3115/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD! 0022-3115(95)00007-0
S. Konishi et aL ~Journal of Nuclear Materials 223 (1995) 294-299 2. Experimental An intermetallic compound of ZrCo was made by arc melting in argon atmosphere from an equimolar mixture of Zr and Cx~. A powder sample was prepared by several times of hydriding-dehydriding cycle. X-ray analysis indicated little trace of other phases in the sample. The experimental apparatus for measurement of pressure-composition isotherms shown in Fig. 2 was used to observe the reaction. Detailed apparatus and procedure are described in a previous report [1]. Sample of either ZrCo o r ZrCoH2.6_ 7 was exposed to pure hydrogen at 400 to 600°C in a closed volume. In a typical experiment, ZrCoH x sample is immediately equilibrated with hydrogen according to the isotherm. Formation of different phase was measured by the change of pressure in the volume over relatively long time until the pressure stabilizes. After this change, pressure-composition isotherms for this system were taken to ,;ee the change of the hydrogen storage capacity. Diffraction spectra of the cooled samples were taken with C u K a X-ray in air to identify the reaction product. Reaction kinetics was investigated with this pressure change. Some ZrCo beds equipped with an internal heater
6/
'
,
400oc 5~ / / 6 5 ° 0
4l/~j~
I/f
295
used in previous works [2] and larger models with similar configuration were used for the test under practical conditions.
3. Results and discussion 3.1. Disproportionation of ZrCo Fig. 3 shows an example of the isotherm for dehydriding ZrCoH x at 500°C following exposure to hydrogen above 2000 Tort at 500°C for 6 h. The equilibrium hydrogen pressure decreases sharply at the apparent ratio of hydrogen atom to Zr atom or ZrCo molecule, nn/n(zrCo) of around 1.0, similar to that for Z r H 2. In the X-ray diffraction patterns shown in Fig. 4, the spectrum of the sample exposed to hydrogen at 500°C was identified for the mixture of Z r H 2 and ZrCo 2. These results indicate that the disproportionation reaction 2ZrCoHx --} Z r H 2 + ZrCo 2
(1)
occurred. Some decrease of hydrogen storage capacity and existence of more strongly bound hydrogen were noticed on the ZrCoH~ kept in hydrogen for several hours at 450°C above 1000 Torr. From the results of X-ray analysis for the samples exposed to pressurized hydrogen at high temperatures, the disproportionation reaction proceeds slower at lower temperature. Because of the difficulty in the quantitative measurement by X-ray analysis, the reaction speed could not be obtained. Thermochemical data [6,7] suggest that Z r H 2 and ZrCo 2 are more stable compounds than ZrCoH~ and ZrCo and disproportionation reaction (1) could occur at lower temperatures. However the reaction rate at the temperature lower than 400°C is extremely slow. In the above experiments, hydrogen pressure was always kept to maintain to apparent nH/n(zrco) exceeds unity, or to be sufficient to form Z r H 2. Reaction occurred either, from 13-phase ZrCo hydride 2ZrCoHo_y ~--} ZrH 2 + ZrCo 2 + (2 - y ) H 2 ( y < 2),
(2)
2 or from a-phase ZrCo with dissolved hydrogen
1
130oc |
/ 0
I
2ZrCoH x + 2(1 - x ) H 2 ~ Z r H 2 + ZrCo 2 ( x < 1).
oAov hydrogenating ,Awv dehydrogenating I
1
2 I3 H/nzrco
Fig. 1. Pressure-composition isotherms of the Z r C o - H 2 system.
(3) No peaks that indicate disproportionation were observed for the samples kept in the hydrogen pressure lower than the equilibrium pressure of the Z r C o - H 2 system by continuous evacuation, in which the normal dehydriding reaction ZrCoH~ --, ZrCo + ( x / 2 ) H 2 ,
(4)
296
S. Konishi et al. /Journal of Nuclear Materials 223 (1995) 294-299
Hydrogen purifier
Thermocouple Sheathed ~.,Jk-~.~n Samplecell heater~ ~ Sample~
k
" "
I
I
l[ G
Schulz gauge
(I)- -(I)
Turbomolecular pump
.eservoirs
Capacitance manometers
Fig. 2. Experimental apparatus used for the quantitative measurement of isotherm and disproportionation reaction.
that is much faster than reaction (2) or (3) can proceed. It is understandable that the disproportionation reaction (1) that requires movement and reconfiguration of metal atoms proceeds slower than the dehydriding of ZrCo hydride where much faster diffusion of hydrogen atoms occur.
3.2. Reverse reaction
The sample that completed disproportionation was then evacuated at 500°C for 12 h. By this treatment, the X-ray diffraction spectrum that showed the mixture of ZrH 2 and ZrCo 2 completely changed back to the
i
2. 500°C, disproportionation :3.500°C, dehydriding
/
•
7,
S
3
ZrCo2
•
ZrH2
/
4
o
•
,, ,! ,i ........
Z...
20
60
40
80
26
(b) After Disproportionation
after regeneration D///
I
~ O
"-:',
4. 500°C,regeneration
-
O ZrCo
5
1.100°C,hydriding I
1
I
nH/nzrco
2
3
Fig. 3. Examples of the apparent isotherm for the ZrCo experienced disproportionation.
20
40
60
80
20 (a~ ZrCo
Fig. 4. X-ray diffraction spectra of the ZrCo and the product of disproportionation.
297
S. Konishi et aL /Journal of Nuclear Materials 223 (1995) 294-299
cess occurs under the condition that ZrH x dehydrides, that is, heating the sample and evacuate it below equilibrium pressure at a temperature above 450°C. It could not be concluded whether this reaction occurs at lower temperature because of the very low dissociation pressure of ZrH x. The X-ray spectra obtained with the reproportionated sample showed no trace of Zr-metal phase that indicates somewhat permanent dissociation of ZrCo.
O f-
-i-
3.3. Reaction kinetics
1
10
100
1000
10000
time / min
Fig. 5. Typical rate of disproportionation of ZrCoH x for x < 1 and x > l .
single phase of cubic ZrCo. Measured isotherms of this sample and hydrogen shown in the Fig. 3 indicate formation of only ZrCoH(0_3) when hydrided, and the same hydrogen absorption capacity as before the disproportionation. These results suggest that the disproportionation reactions (1)-(3) are reversible. This pro-
In the experiment of the disproportionation of ZrCo kept in hydrogen atmosphere in a closed volume, change of the pressure reflects the progress of reaction. Increase of the pressure in a closed system shown in Fig. 2 for reaction (2) or decrease of the pressure for reaction (3) were analyzed to determine the reaction kinetics. Fig. 5 shows the examples of the change of the H / Z r C o value under high hydrogen pressure and temperature. Both reaction (2) and (3) are observed. The ratio of the product phase increases slowly at the beginning and end of the reaction, showing sigmoids. In order to analyze this kinetics, the AvramiErofeev equation a = 1 - exp{-(t/r)n},
(5)
where a represents the ratio of the normalized product phase estimated from the pressure change, was attempted to fit. The time constant ~" was obtained
10 1
(a) 0.8 10 o
°.6f j
10.1
e-
0.4
• 400°0 • 500°C
10.3
slopen:=3 ,
.1
,
i
,
i
1
10
0
.
1
J 1
/
° tyf
z
ill
experimental
n2
~
• 600°C i
/~'
I
[] 550°C 0
/
0 55°°c
0
t
n=4 ~ 1
Normalized Time
10
t/x
t/x Fig. 6. Normalized disproportionation kinetics of ZrCo using the Avrami-Erofeev equation. (a) Logarithmic plot for estimation of n. (b) Comparison between the experimental data and fitted equation.
298
S. Konishi et aL /Journal of Nuclear Materials 223 (1995) 294-299
from the time that gives a -- 0.632. Eq. (5) was rewritten as
-In(1
- a) = (t/z)".
Temperature/°C
To obtain the value of n, the slopes of the logarithmic plot of - l n ( 1 - a ) against t/'r was measured in the range of 0.2 < t/'r < 2 where errors in reading time and pressure are not significant and change of the value of a is large. Fig. 6 shows some examples of this logarithmic plot for reaction (3) under 1000 Torr of hydrogen at various temperatures. Between 400 and 600°C, all the disproportionation kinetics were normalized to Eq. (5) and the value of n = 3 gave a good agreement for most of the measurements. Eq. (5) is known to describe the phase transition process controlled by the growth of a new phase from nuclei in the solid. The equation with n = 3 represents a two-dimensional growth of the product phase and a constant generation rate of nuclei, or a three-dimensional growth and a constant number of nuclei. The observed similarity of this reaction with phase transition is understood as the disproportionation (1) proceeds without major movement of reactant, except for hydrogen that migrates much faster than metal atoms in the lattice. Thus, this disproportionation process can be well described by using Eq. (5). The reversible nature of the reaction also supports the similarity of this process and phase transition. The previously reported differences in the disproportionation speed of the identical sample with different history [8] may be explained using this equation that formulates the apparent incubation period. Same measurements for reaction (2) for the previously hydrided sample kept under 3000 Torr of hydrogen gave higher n values around 4. This seems to suggest a faster generation of reaction nuclei during the disproportionation, probably due to much higher hydrogen content in the ZrCo hydride phase. Using the time constant ~- in Eqs. (5) and (6), it is possible to normalize and summarize the disproportionation speed under a given condition regardless of the n value. An Arrhenius plot 'of the reciprocal time constant 1/~" is shown in Fig. 7. A linear relation can be seen in the temperature range between 400 and 550°C, although deviation is observed at 600°C where inevitable error was suspected to have affected. It seems that the rate of reaction (2) from hydrided ZrCo showed different temperature dependence from that of reaction (3) from ZrCo, probably owing to different crystal structure, or generation rate of reaction nuclei. When the 'kinetics is expressed by Eqs. (5) and (6), ~" is equivalent to the estimated time for 63% disproportionation under a given condition. From this figure, it is possible to estimate the allowable time for the ZrCo at high temperature and hydrogen pressure for practical use. For the system with n = 3 or higher, the degree of disproportionation a stays very low until ca.
600
10
(6)
400
5O0
I
I
I
I.-'
._= E "E tO
o
10
•
© ©
10
00
.E_ i-
•
1000 T o r r H2, started from ZrCo
8
•
2000 T o r r H2, started from Z r C o
0
C) 3000¥orrH2, startedfrom ZrCoH2
10
l
1.1
I
1.2
t
I
1.3
©
| i
I
i
1.4
1.5
Reciprocal temperature lO00/k Fig. 7. Arrenius plot of the time constant of the disproportionation reaction of ZrCo using the Avrami-Erofeev equation.
t = 0.3~" where a begins to rise rapidly. This apparent incubation time is attributed to the slower increase of volume of the product of the disproportionation when the nuclei are few and small. 3.4. Practical implication For the application of ZrCo as storage media of hydrogen isotopes, the results suggests some operational precaution and limitation, and recovery treatment when the material is suspected to be degraded. The relationship shown in Fig. 7 indicates the expected life or suggested operational limit of the metal at high temperature and hydrogen pressure. For instance, at 400°C, contact of ZrCo with high pressure hydrogen isotopes should be limited to ca. 50 h in total to prevent possible capacity loss by disproportionation. This limit allows a number of repeated heating of the ZrCo beds f o r p r a c t i c a l supply of hydrogen isotopes without significant loss of capacity. Small ZrCo bed that contains 50 g of ZrCo hydride powder has been tested for thermal cycle between room temperature and 400°C for 25 times and slight decrease of the capacity was observed [2]. It should be noted that according to the Eq. (5), in the incubation period, the early stage of the disproportionation (or the generation of nuclei) can proceed without any notable loss of capacity in repeated or prolonged heating. Then in a practical use, a "sudden"
S. Konishi et al. ~Journal of Nuclear Materials 223 (1995) 294-299
capacity loss might be sometimes experienced for the bed that have previously developed the disproportionation nuclei begun to grow. With a larger bed with 2 kg of ZrCo for 300 l hydrogen capacity, approximately 30% capacity reduction was found in the latest use over the 5 years of actual tritium service in the Tritium Process Laboratory at the Japan Atomic Energy Research Institute. Regeneration of the bed was tested by vacuum pumping with a Normetex Scroll pump at elevated temperature, and treatment at 500°C for approximately 10 h has completely regenerated the bed. Based on the reversible nature of the reaction, it is understandable that the reverse process, reproportionation, is also much easier and faster for the material that has not completed the disproportionation. Particularly within the incubation period, vacuum treatment will remove reaction nuclei and will recover the incubation time, and thus prevent the rapid disproportionation. It is recommended to equip the bed with a vacuum pump to extract hydrogen from metal, and to regenerate it when the disproportionation occurs. In practice, vacuum pumps are often equipped for a tritium storage system even with uranium, for faster supply, periodical complete regeneration, or measurement of physical inventory. Supply of hydrogen isotope by heating of the bed under pumping will completely avoid the condition that leads to the disproportionation. Periodical complete regeneration, for instance to measure the inventory, will provide a 'virtually limitless life for ZrCo beds. Overheating of the storage material in a small closed volume is an incident that should be strictly avoided with any of the storage media. The interlocks to prevent heating of the storage bed with outlet valves closed, or to prevent overheating that are often considered for practical ZrCo or U beds are also very effective to avoid dispropo:rtionation of ZrCo. Therefore in practical applications, the disproportionation is possible but not a major drawback of ZrCo as a storage media of hydrogen isotopes.
4: Conclusion
Some features of the reversible disproportionation reaction of ZrCo under high temperature and hydro-
299
gen pressure were understood. Although the detailed mechanism of the reaction needs to be investigated, some implications for practical application were drawn. The results suggest that 1) In the heating of ZrCoH~ for supply of hydrogen, temperature should be limited to lower than 400°C and the time and hydrogen pressure should be minimized. 2) Periodical complete regeneration is recommended to prevent disproportionation and to keep the incubation period. 3) Vacuum treatment at high temperature regenerates the capacity of ZrCo when the disproportionation is suspected. 4) The disproportionation of ZrCo is not usually a problem for practical recovery and supply of hydrogen isotopes. However, the use of hydride media for the recovery, storage and supply of hydrogen isotopes, particularly tritium, is increasingly required to respond to various demands. It is desired to study and develop various storage materials based on the recent improvements in understanding on intermetallic materials. For a prolonged use of the bed for pressurized supply of hydrogen gas, materials that require lower temperature for dehydriding are needed, not only to avoid disproportionation, but also to reduce temperature related problems such as tritium permeation and heat load in gloveboxes.
References
[1] T. Nagasaki, S. Konishi, H. Katsuta and Y. Naruse, Fusion Technol. 9 (1986) 506. [2] S. Konishi, T. Nagasaki, N. Yokokawa and Y. Naruse, Fusion Eng. Des. 10 (1989) 355. [3] E. Willin, M. Sirch, R.-D Penzhorn and M. Devillers, Fusion Teehnol. 14 (1988) 756. [4] ITER Fuel Cycle, ITER Documentation Series No. 31 (IAEA, Vienna, 1991). [5] R.-D. Penzhom, M. Devillers and M. Sirch, J. Nucl. Mater. 170 (1990) 217. [6] J.C: Gachon and J. Hertz, CALPHAD 7 (1983) 1. [7] D.R. Fredrickson, L.R. Nuttall, H.E. Flotow and W.N. Hubbard, J. Phys. Chem. 67 (1963) 1506. [8] M. Devillers, M. Sirch and R.-D, Penzhorn, Chem. Mater. 4 (1992) 631.