Electromigration of hydrogen in tantalum

Scripta

METALLURGICA

V o l . 15, p p . Printed in

101-103, 1981 the U.S.A.

Pergamon Press Ltd. All rights reserved

ELECTROMIGRATION OF HYDROGEN IN TANTALUM C.L.Jensen and D.E.Field Dept. of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455 (Received

November

3,

1980)

Introduction Although the electromigration of hydrogen in tantalum has been investigated several times considerable disagreement exists regarding the concentration dependence of the effective valence of hydrogen. The initial electromigration study of the tantalum-hydrogen system was done by Mareche et al. (I). Their data can be used to calculate an effective valence for hydrogen of +1.4 at 870 K (2). No concentration dependence for the effective valence was reported. Ivashina et al. (3) later reported that at 313 K the effective valence of hydrogen ranges from +5 at O.IO H/Ta to -5 as the hydrogen concentration approaches zero. They indicated that the effective valence is zero at O.O13 H/Ta and suggested that electromigration of hydrogen along dislocations and grain boundaries is responsible for the reversal in the sign of the effective valence at low hydrogen contents. An investigation by Erckmann and Wipf (4) did not confirm Ivashina's results, but rather showed a slight increase in the effective valence with decreasing hydrogen concentration. They reported an effective valence of +0.44 at 320 K. The general form of the concentration dependence of the effective valence observed by Jensen (5) was similar to that reported by Ivashina et al (3). However, the effective valence reported by Jensen is an order of magnitude smaller than that observed by Ivashina. Although the data of Jensen suggest that the effective valence of hydrogen is near zero at O.O15 H/Ta, it could not be determined if an actual reversal of migration direction occurred at lower hydrogen concentrations. The purpose of the present investigation was to determine if mechanically induced defects affect the observed concentration dependence of the effective valence as suggested by Ivashina et al. (3) and to determine if the concentration dependence of the effective valence observed in previous investigations could be reproduced. Experimental

Procedure

Tantalum rod of 99.96% purity was swaged from a diameter of 6.4mm to 1.7mm and cut to 5Omm lengths. Sixteen specimens were annealed in an argon atmosphere at 2000 K for I hour. This heat treatment was sufficient to produce a nearly continuous bamboo structure. The remaining specimens were left in the cold worked state. All specimens were electropolished and then electrolytically charged to hydrogen concentrations ranging from O.OO1 H/Ta to 0.09 H/Ta. Direct electric current was passed through the specimens which were connected in series by copper rods which had been secured to the ends of each sample by threading. Great care was taken to ensure that each copper-tantalum junction had low electrical resistance as temperature gradients resulting from ohmic heating would cause thermomigration of hydrogen toward cooler regions of the specimen. All specimens were submerged in a mineral oil bath at 298+ 1 K. Copper leads were spot welded to the samples for electric field and in situ resistivity measurements. The average electric field was determined by measuring the voltage between copper leads which were attached near the ends of each specimen. Eight copper leads were spot welded to each of four samples. The electrical resistivity of seven segments of each sample was measured as a function of time. These data were used to infer when steady-state conditions had been achieved. For a sample length of 5Omm, an average electric field of 3 x 10 -I volts/m and a temperature of 298 K, it was found that steady-state conditions were achieved in approximately 120 days. The samples were then sheared into 20 segments which were analyzed for hydrogen content by hot vacuum extraction. The position of each segment with respect to the end of the original specimen was determined from the length and weight of the original specimen and the weight of the segments assuming the rod to be a perfect cylinder.

i01 0036-9748/81/010101-03502.00/0 Copyright (c) 1 9 8 1 P e r g a m o n Press

Ltd.

102

ELECTROMIGRATION OF H IN T a

Vol.

1S,

No.

1

Results and Discussion The natural logarithm of the hydrogen concentration is plotted against the product of the average electric field and segment position for several cold worked samples in Figure i. Such a plot should yield a straight line with slope Z*e/kT when the activity of hydrogen in the metal is linearly proportional to the hydrogen concentration and the effective valence, Z*, is constant (4,5). T is the absolute temperature, k is Boltzmann's constant and e is the fundamental unit of charge. Boes and Z~chner (6) have found that the activity of hydrogen is a linear function of the hydrogen concentration for hydrogen concentrations extending up to at least 0.03 H/Ta from 300 K to 400 K. This fact in conjunction with the linearity of the data in Figure 1 indicate the assumption regarding the activity of hydrogen which is necessary for calculation of Z* is appropriate. Because of the relatively small electric field employed in this research, the magnitude of hydrogen redistribution within a single sample is small. This allows the assignment of a single value of Z* for hydrogen in each sample even though Z* shows a considerable concentration dependence over the range of hydrogen concentration spanned by all samples.

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Steady state hydrogen concentration as a function of applied electric potential at 298 K for several cold worked tantalum specimens.

o[o, ok o~, oo4 ' o~' o'~ 0[07 o'.o, 0'.09 HYDROGEN CONCENTRATION, H/To FIG. 2 Effective valence of hydrogen in cold worked and annealed tantalum at 298 K as a function of hydrogen concentration.

The effective valence of hydrogen at 298 K is plotted as a function of hydrogen concentration for the annealed and cold worked specimens in Figure 2. There is no significant difference in Z* for hydrogen in the cold worked and annealed tantalum. The data in Figure 2 are in agreement with those of Jensen (5). The experimental uncertainty in hydrogen concentration is near 0.0002 H/Ta. It can be seen from Figure 1 that this uncertainty will prevent determination of the sign of Z* at very low hydrogen concentrations. Thus, it is significant that the effective valence of hydrogen decreases at lower hydrogen concentrations but experimental uncertainty makes it possible to determine whether Z* changes sign or simply approaches zero at low hydrogen concentrations. The decrease in Z* at low hydrogen concentrations cannot be interpreted as a decrease in mobility resulting from hydrogen trapping by other interstitial elements. Peterson and Jensen (7) have measured the diffusivity of hydrogen in tantalum which was identical to the tantalum used in an electromigration study in which Z* data similar to those in Figure 2 were obtained (5). It was found that the diffusivity of hydrogen increased as the hydrogen concentration decreased and thus hydrogen trapping could not explain the decrease in Z* at low hydrogen concentrations. Indirect evidence for the lack of total hydrogen trapping in the present experiments can be obtained by noting the homogeneity of hydrogen distribution in each specimen. Electrolytic introduction of hydrogen does not produce a uniform hydrogen distribution and thus, long range hydrogen diffusion must have occurred during the course of the experiment.

Vol.

15,

No.

i

ELECTROMIGRATION

OF

H

IN T a

103

The electrical resistivity increase per unit hydrogen concentration in tantalum does not exhibit anomalous behavior in the low hydrogen concentration regime (5,8,9). The changes in the tantalum-hydrogen system which are responsible for the strong concentration dependence of the effective valence of hydrogen are apparently too subtle to be reflected in the electrical resistivity increment due to hydrogen. Although the existence of a reversal of migration direction at low hydrogen concentrations is uncertain, it is interesting to note that concentration dependent reversals have been observed in liquid metal systems (IO). Electromigration studies in the tantalum-hydrogen system will have to be extended to higher electric fields to determine if an actual reversal of migration direction occurs at low hydrogen concentrations. These experiments are underway. Acknowledsments This work was supported by the Graduate

School,University

of Minnesota.

References I. 2. 3. 4. 5. 6. 7. 8. 9. 10.

J. F. Mareche, J. C. Rat and A. Herold, J. Chimie Physique, 73, I (1976). H. Wipf, J. Less-Comd~on Metals, 49, 291 (1976). Yu. K. Ivashina, V. F. Nemchenko and V. G. Charnetskiy, Phys. Met. Metallography (Eng. Trans.) 40, 97 (1975). V. Erckmann and H. Wipf, Phys. Rev. Letters, 37, 341 (1976). C. L. Jensen, Thesis, Iowa State University, 1977. N. Boes and H. Z~chner, Ber. Bunsen-Gesellschaft, 80, 22, (1976). D. T. Peterson and C. L. Jensen, Met. Trans., IIA, 627 (1980). D. G. Westlake and S. T. Ockers, Met. Trans., 6A, 399 (1975). J. A. Pryde and I. T. Tsong, Acta Met., 19, 1333 (1971). J. L. Bough, D. L. Olson and D. A. Rigney, J. Appl. Phys., 43, 2476 (1972).