THE INFLUENCE OF N-IMPURITIES ON THE DIFFUSION COEFFICIENT OF H AND D IN Nb AND Ta

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THE INFLUENCE OF N-IMPURITIES ON THE DIFFUSION COEFFICIENT OF H AND D IN Nb

AND Ta

Zh. Qi, J. Völkl, H. Wipf, G. Alefeld

To cite this version:

Zh. Qi, J. Völkl, H. Wipf, G. Alefeld. THE INFLUENCE OF N-IMPURITIES ON THE DIFFUSION

COEFFICIENT OF H AND D IN Nb AND Ta. Journal de Physique Colloques, 1981, 42 (C5), pp.C5-

763-C5-768. �10.1051/jphyscol:19815118�. �jpa-00220987�

CoZZoque C5, supplkment aLc nOIO, Tome 42, octobre 1982 page C5-763

THE INFLUENCE OF N-IMPURITIES ON THE DIFFUSION COEFFICIENT OF H AND D I N Nb AND Ta

Zh.

q i f ,

Physik-Depnrtrnenl; der Technischen (lniversittft btiinchen, 0-3045 Garchiizy b . !~Yu'r~cherr, F. 3.

(;.

Abstract.- Gorsky-effect measurements of the diffusion coeffici- ent of H and D in Nb and Ta in the presence of N interstitials are reported. The influence of the N interstitials on the diffu- sion coefficient is correlated with the low-temperature diffusion anomaly of H in the pure metals. For H in pure Nb and pure Ta the activation enthalpy decreases strongly below about 250 K which suggests a change of the microscopic diffusion mechanism. For these two systems the reduction of the diffusion coefficient due to N interstitials is particularly effective in the region of the low activation enthalpy leading to a suppressionof the low-tempe- rature diffusion process at the largest N concentrations. On the other hand, for D in Nb and Ta for which no decrease of the acti- vation enthalpy is observed in the pure metals only a minor re- duction of the low-temperature diffusivity by the N interstitials is found. The results cannot solely be explained by trapping ef- fects. The low-temperature diffusion mechanism is particularly strongly influenced by the N impurities.

1. Introduction.- Interstitial impurity atoms modify the behavior of metal-hydrogen systems /I-19/. The most direct effect is trapping, which is commonly used as explanation for many experimental results.

Such results are deviations from Sievert's law /I-4/, changes in the phase diagram

and especially in the solubility limit of the

phase, resistivity differences between trapped and free H

and elastic relaxation effects caused by trapped H /9-14/. Recently, also the tunneling of trapped

was observed in low-temperature specific heat /15/ and neutron-scattering

measurements. In this study the influence of N interstitials on the diffusion coefficient of H and D in Nb and Ta is investigated. The experiment,?, are a continuation of a pre- vious work of Miinzing et al. /17/ which was confined to the system Nb- N-H. For Nb the influence of interstitial impurities on H diffusion was also studied by Matusiewicz et al. /la/ and by Richter and Springer

2. Experimental.- Our experiments were Gorsky-effect measurements /20- 22/ on Nb or Ta foils of 0.25 or 0.1 mm thickness. The foils were puri- fied by annealing at temperatures close to the melting point in a va- cuum of several 10-lo mbar. The resistivity ratio of samples measured after this annealing was better than 1000, which indicates that the

*on leave from the Institute of Metal Research, Academia Sinica, -. snenyang, China

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19815118

C5-764 JOURNAL DE PHYSIQUE

content of residual N, 0 , or C impurities was less than 30 at-ppm. For N doping, the samples were exposed to appropriate N,-gas pressures at about 2000 K /23/ (immediately after the purification procedure). The exact N-concentrations achieved were determined from the residual re- sistivity ratios. The samples were loaded with H or D electrolytically, and in some cases also from the H,-gas phase.

2. Results.- The results for the diffusion coefficients are compiled in Figs. 1 to 4. In both metals, Nb and Ta, the presence of N impuri- ties reduces the diffusivity of H and D. The reduction is the more pro- nounced the lower the temperature.

For

in both Nb and Ta, the diffusion coefficient is especially strongly reduced below'L250 K , where a decrease of the activation enthal- py is observed in the pure metals. This low-temperature anomaly, which is yet theoretically unexplained, is an indication for a change of the predominant microscopic diffusion mechanism. The particularly large influence of the N below

K leads, at the largest N-concentration, to a complete suppression of the low-temperature diffusion mechanism;

the temperature dependence of the data can now be described by a single activation enthalpy over the whole temperature range of the measure- ments.

Contrary to the results for H, the diffusion of D in pure Nb and

~ a * does not show a low-temperature anomaly so that the data can be described by a single activation enthalpy. This behavior is also ob- served for the N-doped samples, for which the value of the activation enthalpy becomes slightly larger with increasing N concentration.

A remarkable result of our data is that the change in the diffu- sion coefficient due to N doping is independent of the H 0r.D concen- tration. This behavior is particularly well demonstrated by the results for D in Nb (Fig. 3) where, for samples with 0.26 at% N, the D concen- trations vary by a factor of almost 8 from 0.11 up to 0.82 at% without noticeable changes in the diffusivity.

Discussion.- Diffusion in the presence of impurities is frequently dis- cussed within trap models like that of Oriani

In this model, it

* A previously reported experiment

showing a low temperature de-

crease of the activation enthalpy also for D diffusion in Ta was

spoiled by the accidental presence of several 100 at-ppm H as a con-

sequence of a recrystallisation process which was applied in order to

remove cold-work dislocations. Since the D concentration was of the

same order of magnitude the slow Gorsky relaxation due to D remained

hidden behind the fast Gorsky relaxation of H.(In the present experi-

ments such a recrystallisation was not necessary).

Fig. 2 Diffusion coefficient of H in undoped and N-doped Ta. The

figure includes also previous results of Ruckel /24/

C5-766 JOURNAL DE PHYSIQUE

Fig. 3 Diffusion coefficient of D in undoped and N-doped N b

Fig.

4

-

ficient o f

D

N-

Ta

and (ii) the H diffusivity between traps remains unchanged. The diffu- sion in the presence of traps can then be described by an effective dif- fusion coefficient Deff given by

In this equation, D is the diffusion coefficient measured in the ab- sence of traps. The differential quotient describes how much - ^under

equilibrium conditions - the concentration cfree of free, untrapped H increases for a differentially small increase of the total H concentra- tion ctot.

In agreement with the predictions of this theory, the experimen- tal diffusion coefficient Deff is indeed smaller than or at least equal to the diffusion coefficient D in the absence of traps. However, it is not possible to achieve a quantitative description of our data by the trapping model, except by considering a multiplicity of trap sites. The main reason for this is that the data show, for a given N content, no dependence on the H(D) concentration, whereas in the trap model such a dependence is predicted for our experimental range of H(D) and N con- centrations. Further, within the trap model, it must be considered to happen purely fortitiously that in both Nb and Ta N impurity atoms sup- press so effectively the low-temperature diffusion mechanism of H. The model leads finally to a convex curvature of the diffusion coefficient

in an Arrhenius plot (larger activation enthalpies at lower tempera- tures), a behavior certainly not substantiated by the present data.

(Especially for D a convex curvature is expected since no low-tempera- ture anomaly is observed in pure Nb and Ta).

We conclude that the trapping model discussed above solely cannot quantitatively explain all the features of H(D) diffusion in the pre- sence of impurity atoms.

likely reason for this failure is changes in the bulk diffusivity also between the impurity atoms, caused for instance by far-reaching lattice strains around these impurities. Our results show that the low-temperature diffusion mechanism (observed for H but not for

is particularly effectively suppressed by impurities.

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