Steven J. Rothman

Self-diffusion in austenitic Fe-Cr-Ni alloys

The simultaneous lattice diffusion of 59Fe, 51Cr and 57Ni tracers has been measured in the alloys Fe-15Cr-20Ni, Fe-15Cr-45Ni, Fe-22Cr-45Ni and Fe-15Cr-20Ni-1.4Si between 960 and 1400 degrees C. In all four alloys at all temperatures, DCr>DFe>DNi, with DCr/DNi approximately=2.5 and DFe/DNi approximately=1.8. The activation enthalpies for diffusion of the major constituents in any one alloy are quite close to each other. No correlation is found between swelling under irradiation and the diffusion behaviour of the major constituents. The results are discussed on the basis of the random-alloy model. Grain-boundary diffusion is found in these alloys at surprisingly high temperatures.

Tracer diffusion in pure and boron-doped Ni3Al☆☆☆

Diffusion of 63Ni has been measured in pure and boron-doped polycrystalline Ni3Al intermetallic compounds as a function of temperature (692–1352°C), A1 concentration (24–26 at.% Al), and boron content (0–500 wt ppm). Volume and grain-boundary diffusion of 60Co and 68Ge have also been measured in pure Ni3, Al. Both conventional grinding and ion beam sputtering techniques have been used for the determination of the concentration profiles. The diffusivity of Ni, DNi∗, is independent of Al content above 1000° C, indicating that antisite defects are prevailing on both sides of stoichiometry. However, DNi∗ shows a minimum at the stoichiometric composition below 1000°C, and this trend becomes clearer with decreasing temperature. The diffusivities of60Co and68Ge are also independent of Al concentration in the temperature range between 880 and 1200°C, but their grain-boundary diffusion depends on Al concentration. The addition of boron linearly increases DNi∗, above 1000°C, but at lower temperatures,DNi∗, in off-stoichiometric compositions decreases to the value for stoichiometric Ni3Al on the addition of 100 wt ppm boron, and then increases as above with the further additions of boron to ~500 wt ppm. The present diffusion data suggest that a small concentration of vacancies, independent of temperature, is present on both sides of, and at, stoichiometry at low temperature.

Steady-state point-defect diffusion profiles in solids during irradiation

Abstract The steady-state distribution of mobile point defects in a monatomic solid under irradiation has been calculated for a semi-infinite solid and for a foil. The calculation was performed for defect annihilation by mutual recombination, at internal sinks, and by diffusion to the surface. Simple approximations, accurate to within 5%, have been developed for the analytical solutions to the differential rate equations.

Diffusion, correlation, and percolation in a random alloy

Abstract The relationship between physical correlation and tracer correlation in a random alloy and their bearing on the site percolation problem are discussed. Four Monte Carlo schemes are described for the calculation of the tracer and physical (vacancy) correlation factors. Results for the latter in the f.c.c. lattice are shown to be in excellent agreement with Mannings theory for concentrations greater than 0.4 of the mobile component. Below that concentration there is a slight deviation as the simulated and predicted vacancy correlation factors tend to zero at slightly different percolation thresholds. The site percolation threshold is determined to be 0·198 ± 0·004, in excellent agreement with previous Monte Carlo studies.

Tracer diffusion of Ba and Y in YBa sub 2 Cu sub 3 O sub x

Tracer self-diffusion of Ba and Y and the diffusion of Dy, Ho, and Gd, which substitute for Y, have been measured in polycrystalline YBa{sub 2}Cu{sub 3}O{sub {ital x}} over temperature and oxygen partial pressure ranges of 850 to 980 {degree}C and 10{sup 3} to 10{sup 5} Pa, respectively. The diffusion of Ba is slower than that of oxygen or copper, with a high activation energy of about 890{plus minus}80 kJ/mole. Large anisotropy has also been observed, with diffusion along the {ital c}-axis being more than three orders of magnitude slower than diffusion in randomly oriented polycrystals. Diffusion coefficients of Ba were, within experimental uncertainty, independent of oxygen partial pressure over the range measured. The diffusion coefficients of the Y-site species were nearly identical and an activation energy of about 1.0 MJ/mole was estimated, in agreement with that for high-temperature deformation. Attempts to speed up the kinetics through creation of point defects on the Y site by doping proved to be unsuccessful. These results are compared to cation diffusion in cubic perovskites and simple oxides.

Oxygen diffusion in La2− x Sr x CuO4− y

Diffusion of 18 O in the superconductor La 2− x Sr x CuO 4− y has been measured as a function of x (0 to 0.20) between 300 and 500°C at an oxygen partial pressure of ≍ 1 atm. Concentration profiles were obtained using a secondary ion mass spectrometer. The diffusion coefficient decreases with increasing Sr additions from 0.1 to 0.2, primarily because of an increase in activation energy. This result, which is contradictory to the expectation that the diffusion coefficient should increase with increasing vacancy concentration caused by the added Sr, can be explained if oxygen vacancies are bound to Sr clusters. Measurements on samples with x = 0 or 0.05 were unsuccessful, probably because of porosity.

Diffusion and conductivity in the NaCl-ZnCl2 system

Abstract The diffusion of Zn tracer and the ionic conductivity have been measured in pure and Zn-doped NaCl single crystals. Writing D 0 is 2 × 10−2 cm2 cm2/sec for pure and 1·5 × 10−4 cm2/sec for heavily doped crystals. Q is 23·8 kcal/mol for pure and 11·9 kcal/mol for heavily doped crystals. The free energy of association for the Zn++-cation vacancy pair is 11 kcal/mol at 976°K, which is much higher than predicted by theory; a lower free energy of association is obtained if next-nearest neighbour association is included in the calculation. Precipitation occurs at much higher temperatures and lower Zn concentrations than expected; the heat of solution of ZnCl2 in NaCl is 22·6 kcal/mol. The trapping of Zn++, probably by OH─ ions, was also noted.

Buildup of point-defect diffusion profiles in a foil during irradiation

Abstract The distribution of interstitials and vacancies in a foil under irradiation has been calculated as a function of both distance from the surface and irradiation time by solving the diffusion equation numerically on a computer. The defects were considered to annihilate at randomly distributed sinks, by mutual recombination, and by diffusion to the surface. Defect jump frequencies appropriate to silver at 125°C and foil thicknesses of 1 pm and 300 A were used. Large “humps” in the plot of vacancy concentration versus distance were found near the surface of the 1 pm foil at short irradiation times, unless the internal sink concentration was high. These humps may be responsible for some unusual void distributions observed near grain boundaries.

The tracer diffusion of Ge in Ni single crystals

The diffusion of 68Ge in single crystals of high-purity (over 99.99% pure) Ni has been measured using the sectioning technique. The plot of log D versus 1/T is straight from 939 to 1675K, over 61/2 orders of magnitude in D, with parameters D0=2.10+or-0.47 cm2 s-1, Q=2.74+or-0.03 eV (264+or-3 kJ mol-1). The data suggest a low Ge-vacancy binding enthalpy for Ni.

Effects of proton bombardment on self-diffusion in silver: A preliminary report☆

Abstract The radiation-enhanced self-diffusion coefficient D rad in silver under 270 keV proton bombardment has been measured over the temperature range from 240° to -60 °C. The flux dependence of D rad has also been studied at 106° and 179 °C with the flux varying from 2.5 × 10 12 to 6.3 × 10 13 protons/cm 2 sec. The experimental data are analyzed in the framework of the classical model involving single vacancies and interstitials as mobile defects.