DIFFUSION OF NICKEL43 IN NICKEL OXIDE (NiO) BY R. LINDNER AND A. AKERSTROM Laboratory for Nuclear Chemistry, Chalmers University of Technology and The Swedish Institute for Silicate Research, Gothenburg, Sweden Received 12th April, 1957 The diffusion of nickel-63 in NiO in form of crystals, metal oxidation layers and sintered pellets has been measured in the temperature range 740-1400" C by the surface decrease method. The temperature function of the self-diffusion coefficient is represented by the equation : D = 1.7 x 10-2 exp (56,00O/RT) cm2 sec-1. Preliminary experiments concerning the self-diffusion of nickel in nickel oxide sintered pellets have previously been published.1 The results and especially the high activation energy found were rather doubtful and it had to be assumed that the short-lived radioactive tracer nickel-65 (half-life 2-56 h) was not suitable for such experiments.Consequently the investigation has been repeated using the long lived nickel-63 (85 years half-life) which, because of its soft P-radiation of 65 kV maximum energy, has to be measured in a flow gas counter. The investigations were initiated by one of us in co-operation with W. J. Moore during a stay at Indiana University (U.S.A.), but it was decided, considering the importance of the investigation for the knowledge of nickel metal oxidation, that two separate and independent sets of experiments should be performed. In spite of practically identical material and equipment the preliminary results ob- tained differed considerably : Moore 2 gave a preliminary value for the activation energy of self-diffusion of 43 kcal whereas our preliminary results 3 yielded a value of 62 kcal/mole.The latter value has been practically confirmed by this in- vestigation. There should be a close connection between the activation energy for self-diffusion of nickel and that for the formation of nickel oxide by metal oxidation, as nickel oxide is considered to be a typical p-conductor and the diffusion of nickel via vacancies should determine both processes. The commonly accepted value for the activation energy of metal oxidation is about 41 kcal/mole as stated by Gulbransen and Andrew.4 In a later report 5 the same authors report a higher activation energy of about 68 kcal for temperatures above 900" C but assume that this value has no real significance as the oxidation process is disturbed by periodical cracking of the oxide layer.EXPERIMENTAL The following nickel oxide preparations were used in the diffusion experiments. Pure nickel oxide crystals, which were practically black, cleaved from a boule obtained by the Verneuil process. The composition of this material was checked by measurements of electric conductivity which extrapolated to room temperature showed values of the order of 10-7 ohm-1 cm-1, which corresponds to a deviation from stoichiometric composition of about 10-6 according to Morin.6 The sintered samples were prepared from nickelous carbonate (Bakers A.R.) which had been heated to about 700" C to decomposition. After pressing, the pellets were fired at 1300" C and obtained a density of 6.2 (85 % of theoretical density).Their electrical conductivity was comparable to that of the crystals mentioned above. Spectroscopically pure nickel foil (Johnson and Matthey), identical with the material used by Gulbransen was oxidized at 1000" C. The radioactive tracer nickel-63 was ob- tained from Harwell and its purity checked by complexing elution from a Dowex 50 ion 133134 DIFFUSION IN NICKEL OXIDE exchange column with EDTA in presence of cobalt carrier. The absorption curve of this tracer and all other measurements were taken in a flowing methane proportional counter of about 40 % efficiency. No deviating absorption characteristics were found if the 90 % argon + 10 % methane " Q "-gas was used. The absorption curve, obtained with thin absorbers of plastic (Pliofilm 0.6 mg cm-2) and aluminium (1.1 mgcm-2) is shown in fig.1 and is practically identical with that ob- tained by Brosi, Borkowski, Conn and Griess.7 As can be seen from the figure, ab- sorption occurs according to two distinct groups of energy corresponding to absorption coefficients of 4950 cm2 g-1 and 1150 cm2 g-1. These values are confirmed by evaluation of the self-absorption curve in nickel metal as given by Brosi, Borkowski, Conn and m9 cm-' FIG. 1.-Absorption curve of Ni63. Griess in which case a composite absorption coefficient of 4200 cm2 g-1 can be stated. This has to be considered when evaluating the diffusion experiments according to the surface decrease method.* For short diffusion times and/or low diffusion temperatures an approximation can be made using only the absorption of the very soft radiation, whereas at high temperatures and/or long diffusion times the absorption of the harder radiation may be used for evaluation.There is, however, a range corresponding to a decrease in surface activity between 40 % and 10 % in which none of these approximations is allowed. Because of this and because of the necessity of finding one equation for evalu- ation for the whole broad temperature range investigated by us, a two-term absorption formula had to be developed which fits the whole range of temperature and times. DIFFUSION EXPERIMENTS The deposition of the radioactive tracer on the diffusion specimen was accomplished by evaporation from a metal foil in waclco and condensation in an apparatus as described by Lindner and Parfitt.9 The metal foil was coated with radioactivity by electrolysis. Because of its higher melting point tungsten was used instead of tantalum which was used in the earlier experiments.A negligible amount of evaporation of foil material may occur, being higher with tantalum. Special experiments with nickel oxide with small additions of tantalum oxide did not show any appreciable effect on the self-diffusion of nickel by added tantalum, within ow limits of error. The radioactive nickel coating (less than 0.1 mg cm-2) was oxidized in air at 500" C during 10 min. During the diffusion run, no evaporation of radioactive nickel oxide occurred as confirmed by an experiment using two coated samples, the active sides of which were separated by a thin platinum ring. No measurable transfer occurred between the coated samples even at temperatures as high as 1100" C.R .LTNDNER AND A. ~ K E R S T R ~ M 135 RESULTS The results are shown in fig. 2. No systematic deviation between the different sets of samples could be found, meaning that diffusion was not affected by the state of the diffusion sample (crystal, sintered pellet or oxidized metal strip). Although there is some scatter the large range in temperature allows sufficient accuracy in determination of the activation energy, which according to the method of least squares is calculated to be 56,040 f 1280 cal/mole. The results of Gulbransen and Andrew for low- and high-temperature oxida- tion of nickel metal are also shown in fig.2 as dashed lines. The values coincide quite well although at lower temperatures the metal oxidation shows a markedly smaller activation energy. 107 T FIG. 2. DISCUSSION The constant value for the activation energy of self-diffusion as obtained from our measurements over a wide range of temperature makes it probable that the true activation energy of self-diffusion has been measured. The activation energy for metal oxidation, however, is markedly higher at higher temperatures and shows a value of 68 kcal, which according to Gulbransen and Andrew is without real physical significance. At temperatures below 1000" C the activation energy of 42 kcal has been commonly accepted, but we are forced to assume that a real difference exists between the activation energies for the two processes in this temperature range.Possibly the diffusion conditions in a growing metal film cannot be completely considered identical to those in a stabilized pure oxide even for p-conductors. A direct experimental investigation on an eventual mobility of oxygen in nickel oxide seems desirable.10 We wish to express our gratitude to Mr. W. Kebler, Speedway Research Laboratory, Linde Air Company, Indianapolis, U S A . , for the nickel oxide single crystals ; and Dr. E. A. Gulbransen, Westinghouse Research Laboratories, Pittsburgh, for the pure nickel metal foil. The support by the Swedish Council for Technical Research and Prof. C. Brossett, Head of the Swedish Institute for Silicate Research is gratefully acknowledged.136 DIFFUSION I N NICKEL OXIDE 1 Lindner and Akerstrom, 2. physik. Chem., 1956, 6, 162. 2 Moore, 6th Meeting Physical Chemistry (Paris 1956), paper 98. 3 Lindner, 3rd Int. Meeting on the Reactivity of Solids (Madrid, 1956). 4 Gulbransen and Andrew, J. Electrochem. Soc., 1954, 101, 128. 5 Gulbransen and Andrew, Westinghouse Scientific Paper, 1956, 60-94602-1 -P9. 6 Morin, Physic. Rev., 1954, 93, 1199. 7 Brosi, Borkowski, Conn and Griess, Physic. Rev., 1951, 81, 391. 8 Steigman, Shockley and Nix, Physic. Rev., 1939, 56, 13. 9 Lindner and Parfitt, J. Chem. Physics, 1957, 26, 182. 10 Ilschner and Pfeiffer, Nuturwiss., 1953, 40, 603.