Abstract

1. Introduction
Zirconium alloys are very much active to inorganic gases and adsorb the gases quite rapidly. In addition, they show sufficiently low equilibrium pressure of hydrogen isotopes at room temperature. Because of these properties, they have been used as materials of bulk getter pumps for oil-free ultra-high vacuum systems ¹ -³ . They also have potential for fuel processing in controlled thermonuclear fusion devices. For this purpose, however, they will have to be improved to meet various requirements, which depend on their applications such as storage 6 - 3 , supply 3 , 10 , recovery 11 -15 , separation 16 and so on 1 7 - 2 1 . The applicability of them, however, is rather limited at present. This is due to their high reactivity against impurity gases ²²-² 6 , especially to oxygen containing mol- ecules as HzO, COz, CO and Oz, which appear most often in vacuum systems. Serious problems arises from those gases owing to the formation of stable overlayers of oxide and carbide 3 2 - 2 4 . Namely, the active materials are deactivated against hydrogen ab- sorption and desorption. On the other hand, contamination of fuel gases is unavoidable owing to leakage and out-gassing from the wall of the fuel processing system. Therefore, one has to develop new materials inactive to impurity gases but active to hydrogen isotopes. One solution of this problem is to prepare surface protective layers on active materials which are free from contamination by the impurity gases and act as perme- window for hydrogen isotopes. To develop such protective layers, we have ation studied physicochemical properties of a Pd-overlayer prepared with electroless plating on ZrsAls. This will be denoted as Pd/ZrsAls below. ZrsAls was selected as a model system of hydrogen storage materials for the fuel processing. Parts of the study have been reported previously from view points of surface states and some characteristic features of deuterium desorption ²¹. In this paper, absorption kinetics of deuterium will be compared in detail between Pd/Zr 3 Al 1 and bare Zr 3 Al 2 surfaces.
2. Experimental
2. 1. Sample preparation
Zr , Al, alloy was purchased from Japan Metals & Chemicals Co. It had been pre- pared with argon arc melting of the stoichiometric mixture of Zr and Al powder. A lump of the alloy was annealed at 1100°C for 150 min in vacuum, cooled to room tem- perature with a rate of 150°C/h, and then supplied to sample preparation. First, grains of the alloys were prepared by hydride formation and sieved to 0.2 - 0.5mm in diameter. Subsequently, Pd-overlayers were grown over those grains with the electro- less plating method. The detailed procedures have been described elsewhere 17 . The bare and Pd-coated ZrsAl 2 grains were used as samples in the present study.
2. 2. Absorption measurement
Effects of the Pd-overlayers on the deuterium absorption were observed with the constant volume method using a high vacuum system. Figure 1 shows the schematic illustration of the system used in the present study. It was equipped with a quadru- pole mass spectrometer, B-A type ionization vacuum gauge, capacitance manometer and sample tube made of quartz. The system was evacuated with a sputter ion pump and turbomolecular pump backed with an oil-sealed rotary pump. The residual pressure of the system was routinely below 3×10- Pa. Each sample was activated by vacuum heating at 800°C for 30min and cooled to a given temperature before the absorption measurements. Deuterium gas was intro- duced into the closed part of the system enclosed by the valves, v-3, 4 and 5. Sub- sequently, the valve v-4 was opened quickly, and the time course of pressure decay due to the absorption was measured using the capacitance manometer. The absorption ki- netics was analyzed through the decay curves for various absorption temperatures.
2. 3. Durability test
CO 2 and N 2 were used as impurity gases. Figure 2 shows the procedures of the durability tests of Zr s Al s and Pd/Zr s Al s . First, a sample was activated at 800°C for 30 min, and the absorption rate of deuterium, v o , was measured at room temperature. Subsequently, the sample was exposed to CO, or N, under given exposure con- ditions, and the system was evacuated. Lastly, the absorption rate of deuterium, vexp, was measured under the same conditions as those before the impurity exposure. Changes in the absorption rate with the exposure to impurity gases were evaluated as the normalized absorption rate, v exp/v o , for each impurity gas. In the present study, the exposures of CO, and N, were varied in ranges of 10*-10* and 10*-10*L, respectively, where L is Langmuir, a unit of gas exposure equivalent to 10-* Torr*s.
3. Results and discussion
3. 1. Activation process of samples
Vacuum heating is commonly applied to activate Zr-based alloy getters. Figures 3 and 4 show themal desorption spectra observed during activation of ZrsAlz and Pd/ ZrsAl 2 , respectively. Both of the samples were heated to 800°C with a temperature ramp of 7K/min. H x , H x O, CO and CO x were observed for both of the samples. However, the amounts of desorption and the temperatures of the desorption peaks differed from each other. As seen in Fig.3, H x was the main desorption species. It consisted of a single desorption peak which appeared around 350°C. The desorption peaks of H 2 O, CO and CO; were also observed around this temperature, but the amounts of desop- tion were one or two orders of magnitude smaller than that of H 2 . No other desop- tion peak was observed above 500°C. On the other hand, CO and CO 1 were main desorption species for Pd/Zr,Al, as shown in Fig.4. In addition, their desorption peaks were observed at higher tempera- tures around 600°C than those of H, and H, O, which appeared below 500°C. H 2 observed in Figs.3 and 4 is considered to arise from hydrogen atoms dissolved in those samples. HsO would be due to desorption of water adsorbed on the sample surface and/or inner wall of the sample tube. Presence of adsorbed water on those surfaces is plausible, because of the samples and sample tube were exposed to the air before themal desorption experiments. Concerning to CO and CO 2 , a part of them would arise from adsorbed CO and CO; from sample surface as
In the above, (a) and (g) denote adsorbed and gaseous state, respectively. Another part of CO and CO 2 is considered due to following reactions on the sample surfaces.
In the above, C(a), M.C. and M-O indicate carbonaceous contaminants and/or adsorbed hydrocarbons (being denoted as surface carbons below), metal carbides and metal oxides, respectively. Metal surfaces are known to be contaminated by surface carbons and other adsorbates 88 . Recent XPS (x-ray photoelectron spectroscopy) mea- surements ² ⁷ suggested that the reactions between surface carbons and metal atoms as indicated in reaction (1-c) and (1-d) took place at high temperatures above 500°C (be- ing denoted as high temperature region). Occurrence of those of surface reactions, of course, largely depend on surface metal elements and its states. Previous XPS mea- sure revealed that Zr-carbide was formed by the surface reaction such as (1-c) at the high temperature region. In addition, desorption of H 1 , CO, CO 2 and hydrocarbons was rather in small extent as shown in Fig.3. XPS measurements also revealed that Pd-carbide was not formed even at high temperature region, indicating that the reac- tion (1-d) would be proceeded. Indeed, CO and CO, were desorbed at high temperature region as shown in Fig.4. This would be due to the reaction between surface carbons and oxygen which was diffused to the surface from the inner part of the sample.
3. 2. Absorption rates of deuterium
The absorption rate would be exprssed as
where P is the pressure of hydrogen isotopes, k, the absorption rate constant, V the volume of the system, and n the order of reaction. For many Zr-based alloys, n is known to be unity 29-31 . In this case, equation (2) is rewritten as follows.
Namely, the plots of At, vs. In(AP,/AP,-,) give a straight line. Figures 5 and 6 show the plots of Ata vs. In(APa/APa-i) for ZraAla and Pd/ZraAla at various tem- peratures, respectively. Each of the plots in Figs 5 and 6 shows fairly well linearity, indicating that the absorption of deuterium obeys the first order kinetics. The rate equation can be expressed as
where v , is the frequency factor and E, the activation energy for absorption. The first order kinetics indicates that the rate determining step for absorption is the first order dissociation reaction of gaseous and/or adsorbed deuterium molecules to atomic state on the sample surface. From the slope of each line in figures 5 and 6, the rate constants, ka, for respec- tive temperatures were evaluated for Zrs Als and Pd/Zrs Als. Figure 7 shows the Arrhenius plots of the rate constants for Zr 3 Al 2 and Pd/Zr 3 Al 2 . From the slopes of the plots, the activation energies were evaluated as 7.4 and 8.9 kJ/mol(Ds) for Zr3 Al2 and Pd/Zr 3 Al 1 , respectively. The activation energies were quite low and closed each other, indicating that deuterium molecules are easily dissociate into atoms on those surfaces. The frequency factors for absorption could be also evaluated from the inter- sections of the straight line with vertical axis in the Arrhenius plot diagram. In the present study, however, the apparent values have no physical significance, because the specific surface areas of the samples were not measured. The desorption kinetics of deuterium for both the Zr 3 Al 1 and Pd/Zr 3 Al 1 has been confirmed to obey the second order kinetics ²¹ . The activation energies for desorption are 106.7 and 134.4 kJ/mol(D1) for Zr3Al; and Pd/Zr3Al;, respectively 8 T. Therefore, the potential energy diagram for the absorption and desorption of deuterium in/from the samples should be drawn schematically as shown in Fig.8. The heat of deuterium solution and/or deuteride formation, A H, can be approximated to the difference be- tween the activation energies of absorption and desorption. They were evaluated as 99.3 and 125.5 kJ/mol(D 2 ) for Zr3 Alz and Pd/Zr3 Alz, respectively. The value for Pd/Zr,Al, is close to the heat of solution of hydrogen in pure Pd32.
3. 3. Durability against impurity gases
Figure 9-(a) and (b) show plots of the normalized absorption rate of deuterium, voxo/vo, as a function of impurity gas exposure. In this figure, open symbols repre- sent results for Zr Al x and filled ones for Pd/Zr a Al x . Circles are the results for CO x exposure and squares for N 2 , respectively. Exposures of CO; caused the serious reduction of the absorption rate of deuteri- um for both the ZrsAls and Pd/ZrsAls. However, the extent of the reduction for Pd/ZrsAls was always smaller than that of bare ZrsAls. For example, 10'L of COs, exposure reduced the normalized absorption rates to 0.02 and 0.04 for 2rs Al; and Pd/Zr3Als, respectively. Pd/ZrsAls is 2.0 times more durable than bare ZrsAls to 104 L of CO 2 exposure. Namely, the Pd-overlayer acts as protective layer for CO 3 . On the other hand, N 1 exposure showed a smaller effect on the absorption rates for both of the samples than CO 1 exposure. However, the effectiveness of the Pd-overlayer was also observed. For example, 10'L of N 2 exposure reduced the normalized absorption rates of deuterium to 0.30 and 0.40 for Zr3 Als and Pd/Zr3 Als, respectively. Namely, Pd/Zr 3 Al 2 was only 1.3 times more durable than bare Zr 3 Al 2 . However, the effective- ness of the Pd-overlayer against N 1 increased with increasing N 2 exposure. For example, it was 4.1 times more effective in case of 4×10 8 L of N 1 exposure. It has been confirmed by XPS measurements that the activated Zr-alloy getter surfaces mainly consist of metallic Zr, which is easily deactivated by impurity gases owing to the formation of surface obstacle layers**. On the other hand, plated Pd kept pure metallic state in the range from room temperature to 800°C, although a fraction of the surface was covered with Zr diffused from the inner part of the sample. The extent of the reduction of absorption rate should largely depend on the ad- sorption properties of impurity gases. Table 1 shows the classification of metals and semi-metals based on adsorption  properties of gases ³³ . According to the table, the affinity of CO 3 for Zr and/or for Pd is comparable and that of N 4 is in the order of Zr>Pd. Those adsorption properties are in good agreement with the reduction of absorption rate observed in the present study, indicating that the durability of Pd- overlayer against impurity gases depends on the differences in the absorption properties between Pd and Zr 34. The change in the normalized absorption rate with increasing temperature will be important to practical applications of Pd/ZrsAls. Further investi- gation is required on this point.

4. Conclusions
Based on the above mentioned results and discussion, we concluded the followings: 1. The absorption of deuterium obeys the first order kinetics with respect to the deu¬ terium pressure for both ZrsAls and Pd/ZrsAls. The rate determining step for absorp- tion is the dissociation of deuterium molecules on the sample surface. 2. The activation energies for deuterium absorption were evaluated as 7.4 and 8.9 kJ /mol(Ds) for ZrsAls and Pd/ZrsAls. The heat of deuterium solution, AH, was also evaluated from the proposed potential energy diagram as 99.3 and 125.5 kJ/mol(Ds) for Zr3Als and Pd/Zr3Als, respectively. The value for Pd/Zr3Als is close to the heat of solution of hydrogen in pure Pd. 3. Exposure of CO 2 and N 2 caused the reduction of the absorption rate of deuterium. However, the extent of the reduction for Pd/ZrsAls was smaller than that of bare Zr Al 2 , indicating that the Pd-overlayer is effective against impurity gases. 4. The order of the effectiveness of the Pd-overlayer against impurity gases was N , > CO 2 . This could be explained in terms of the differences in the absorption properties of deuterium between Pd and Zr.
Acknowledgment
This work was partially supported by a grant-in-aid from the Ministry of Educa- tion, Science and Culture, Japan (#05650705).