富山大学水素同位体科学研究センター研究報告 27 : 27 – 36, 2007.

論文
ZrC の水素吸収放出
水素吸収放出挙動
吸収放出挙動波多野 雄治， 野崎 天生, 本間 啓史, 松山 政夫
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

Absorption and Release of Hydrogen Isotopes by ZrC
Yuji Hatano, Teo Nozaki, Hirofumi Homma and Masao Matsuyama
Hydrogen Isotope Research Center, University of Toyama
Gofuku 3190, Toyama 930-8555, Japan(Received January 10, 2008; accepted March 13, 2008)

Abstract
Interaction between hydrogen isotopes and ZrC powder was examined at 873 Kand pressure from 0.89 to 99 Pa. Reversible absorption and release of hydrogen isotopes
were observed. The equilibrium concentrations of hydrogen isotopes in the above
pressure range were from 82 to 120 mol ppm; the pressure dependence observed wasmuch weaker than that expected from Sieverts’ law. The diffusivity of hydrogen wasevaluated to be 3×10-16 m2·s-1 by assuming that powder particles were spherical withdiameters of 1 µm.

1. Introduction
Zirconium carbide, ZrC, is a potential candidate of coating material for fuelparticles in high temperature gas-cooled reactors [1]. Hydrogen production from water27波多野雄治・野崎天生・本間啓史・松山政夫

by thermo-chemical processes is one of the main objectives of the development of thistype of nuclear reactor [2]. The contamination of product hydrogen by tritium produced
by ternary fission of uranium is a critical issue for this application [3], and hence thepermeability of hydrogen isotopes through coating materials has to be carefullyevaluated. It is known that ZrC in carbon deficient composition, ZrC1-X, absorbs
hydrogen [4]. The solubility of hydrogen isotopes at/near stoichiometric composition,
however, has been scarcely reported. In addition, no data on diffusivity is available inliterature.
The authors have previously carried out hydrogen absorption experiments for
TiC having near-stoichiometric composition and successfully evaluated diffusion
coefficients of hydrogen in a temperature range from 773 to 973 K [5]. In the present
study, the absorption and desorption of hydrogen and deuterium by ZrC were examinedby the similar technique at 873 K and pressure from 0.89 to 100 Pa. The diffusion
coefficient was evaluated from absorption curves.

2. Experimental
Specimen used in the present study was powder of ZrC supplied by Kojundo
Chemical Laboratory Co., Japan. The nominal purity of the specimen was 95%.
According to the supplier, the main impurity was Hf. The specimen powder wasanalyzed by X-ray diffraction (XRD) before and after hydrogen absorption experiments.
The size distribution of powder particles were examined with a scanning electron
microscope (SEM).
The hydrogen absorption experiments were carried out at 873 K using avacuum apparatus which was similar one used in the previous study [6]. The specimen28ZrC の水素吸収放出挙動

powder of 2 g was installed in a quartz tube attached to the apparatus and heated invacuum for degassing. Elevation of specimen temperature resulted in significant releaseof gas whose mass number was 28, i. e. CO and/or N2. The specimen temperature wasraised gradually to keep the pressure in the apparatus lower than 10-3 Pa. The degassing
was stopped when the pressure in the apparatus became lower than 10-5 Pa at 1373 K.
Then, absorption of hydrogen (or deuterium) was examined in a closed vacuum system,
in which the hydrogen absorption was indicated by pressure drop [6]. The initialpressure of H2 (or D2) gas was adjusted to be 5.2 or 105 Pa, and the specimen
temperature to be 873 as described above. The rate of hydrogen absorption wasevaluated from the change in pressure with time. After the attainment of equilibrium,
the specimen was cooled down to room temperature, and H2 gas remaining in gas phase
was evacuated by a turbo-molecular pump. Then the apparatus was isolated again fromthe pump, and the specimen was heated to 873 K in a closed system to examine
desorption.

3. Results
3.1 Characteristics of Specimen Powder
Fig. 1 shows XRD pattern of powder particles after hydrogen absorption
experiments as an example. No significant difference was observed between the powder
under as-received conditions and after hydrogen absorption experiments. All peaks in
the XRD pattern could be assigned to ZrC (NaCl structure) and Si added as an internalstandard; no peaks of other phases such as metallic Zr and Zr oxides were observed. The
lattice parameter of ZrC was evaluated to be 0.4696 nm. Fig. 2 shows SEM micrographof the specimen powder. Majority of particles was in a range of size from 0.5 to 3 µm.

29

波多野雄治・野崎天生・本間啓史・松山政夫

The size of 55 particles was measured randomly, and the average value of particle size
was evaluated to be 1.6 µm.

3.2 Hydrogen Absorption
Figs. 3 (a) and (b) shows typical examples of the absorption curves obtained atinitial H2 pressure of 5.2 and 105 Pa, respectively. In the former case, the majority ofintroduced H2 was absorbed by the specimen powder, while only small portion wasIntensity (a.u.)

1400
ZrC
Si

1200
1000800600
4002001 µm

02040Fig. 1XRD pattern of ZrC powder.

60801002θ (deg.)

120140Fig. 2 SEM micrograph of ZrC powder.
110(b)(a)

5H2 Pressure (Pa)H2 pressure (Pa)6432105
10095

10900Fig. 3

246Time (ks)80100 200 300 400 500 600 700 800
Time (s)Typical examples of H2 absorption curves at 873 K and initial pressures of5.2 Pa (a) and 105 Pa (b).

3010ZrC の水素吸収放出挙動

dissolved in the latter case. The equilibrium pressures of H2 in these absorption
experiments were 1.1 and 99 Pa, respectively. The extent of pressure reduction
corresponded to H concentration in ZrC of 82 and 120 mol ppm ([H]/[ZrC] = 8.2×10-5
or 1.2×10-4).
The correlation between hydrogen concentration in ZrC and gas pressure isshown in Fig. 4 together with the data obtained by absorption experiments with D2 anddesorption experiments. The results of desorption experiments agreed well with those ofabsorption experiments. This observation suggests that ZrC reversibly absorbs andreleases hydrogen isotopes. In other words, ZrC can permeate hydrogen isotopes under
gradient of chemical potential of hydrogen isotopes. The solubility of deuterium wascomparable to that of hydrogen. The number of data points, however, is not enough toderive decisive conclusion
theisotope

Further

H2 absorption

effect.

investigation

isnecessary to understandH2 desorption[H] or [D] / [ZrC]

on

-310

D2 absorption-410

the solubility of tritium.
Oneof

the-5

100.1

1

10

1001000

Pressure (Pa)importantfeaturesofFig. 4 Pressure dependence of H or D concentrations
in ZrC.
31

波多野雄治・野崎天生・本間啓史・松山政夫

Fig. 4 is very weak pressure dependence of hydrogen concentration. Although thepressure of hydrogen was varied by two orders of magnitude, the concentration ofhydrogen in ZrC increased only by factor 1.5. Such weak pressure dependence can beexplained by the saturation of the sites for hydrogen dissolution at high pressure (99 Pa).
Results of neutron diffraction analysis for ZrC0.62D0.28 showed that both deuterium andcarbon atoms occupied octahedral sites of FCC type arrangement of Zr atoms [7]. In
other words, deuterium occupies carbon vacancies in this system. Samsonow andMorosow [4] examined the hydrogen absorption of ZrC1-X and reported that theconcentration of absorbed hydrogen roughly corresponded to the amount of carbon
vacancy. For examples, ZrC0.64 and ZrC0.81 absorbed hydrogen up to ZrC0.64H0.31 andZrC0.81H0.12, respectively. Their observations also indicate the occupancy of carbon
vacancies by hydrogen atoms. Hence, it is appropriate to consider that hydrogen anddeuterium occupied carbon vacancies also in the present specimen, and theconcentration of hydrogen under 99 Pa H2 (120 mol ppm) roughly correspond to that ofcarbon vacancies. Measurements of pressure-composition isotherms of TiC-H system
are currently under progress. In this system, hydrogen concentration under equilibriumobeyed Sieverts’ law at H2 pressure below 10 Pa but ceased to increase in higher
pressure range (~ 1000 Pa) at the concentration close to that of carbon vacancy32

ZrC の水素吸収放出挙動

evaluated by chemical analysis. Detailed results in TiC-H system will be reported in aseparate paper.
The present results lead to two important predictions. First, solubility of tritium,and consequently permeability, should be reduced by the reduction in the carbon
vacancy concentration. Second, the measurement of solubility of hydrogen isotopes is asensitive method to determine the concentration of carbon vacancy, especially in a low
concentration range where precise measurement by chemical analysis is impossible. It is
known that ZrC contains certain concentration of carbon vacancies even undercoexistence of elemental carbon [8,9]. The concentration of carbon vacancy may
influence not only on the transport of tritium but also on that of other hazardous fission
products. Hence, it is important to develop the techniques to control and evaluate theconcentration of carbon vacancies in ZrC. Further investigation is necessary to derivedecisive conclusions on the correlation between carbon vacancy concentration andsolubilities of hydrogen isotopes in ZrC.
The rate-limiting step of hydrogen absorption was evaluated in the manner
described in the previous paper [5]. All absorption curves indicated that the rate-limiting
step is the bulk diffusion process. The diffusion coefficient of hydrogen D in ZrC wasevaluated by fitting the solution of diffusion equation to the absorption curves obtained33

波多野雄治・野崎天生・本間啓史・松山政夫

at initial pressure of 105 Pa by assuming that the powder particles are spherical withdiameters of 1 µm for simplicity. The surface concentration of hydrogen was assumed tobe constant during the absorption since the extent of pressure change was small asmentioned above. Under such conditions, the amount of absorbed hydrogen at time t, Mt,
is described as [10]Mt
6 ∞ 12 2
2= 1−∑ 2 exp(− Dn π t / a ) ,2M∞
π n =1 n(1)where a is the radius of powder particle. The result of fitting is shown in Fig. 5. The
experimentally-obtained absorption curve was well reproduced at D = 3×10-16 m2·s-1.
Fig. 6 shows the comparison with the values of hydrogen diffusivity in TiC obtained in
the previous study. The obtained value of diffusion coefficient of hydrogen in ZrC is
close to that in TiC. It is necessary to examine the dependence of diffusivity on thecarbon vacancy concentration.
-141.210◆ ZrC-15

0.8-2 -1D (m s )Mt /M∞

1.0

0.60.40.20.0D = 3 ×10-16 m-2s-10Fig. 5200400Time (s)

600

800

Example of fitting of Eq. (1) (line)
to absorption curve correspondingto Fig. 3 (b) (dots).

3410

○ TiC

-1610-1710

1.0

1.1
1.2-1
1000 / T (K )1.3

Fig. 6 Comparison between diffusion
coefficients of hydrogen in ZrC
and TiC.

ZrC の水素吸収放出挙動

4. Conclusions

The absorption and release of hydrogen isotopes by ZrC powder was examinedat 873 K and pressure from 0.89 to 99 Pa. Reversible absorption and release ofhydrogen isotopes were observed. The equilibrium concentrations of hydrogen isotopes
in the above pressure range were from 82 to 120 mol ppm; the pressure dependence
observed was much weaker than that expected from Sieverts’ law. Such weak pressure
dependence was attributed to the saturation of carbon vacancies by hydrogen isotopes athigh pressure. The diffusivity of hydrogen was evaluated to be 3×10-16 m2·s-1 fromabsorption curves.

Acknowledgements

This study was supported in part by a Grant-in-Aid for Scientists Research (C)of Ministry of Education, Culture, Sports, Science and Technology of Japan, No.
17560613.
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