M. Ashkin

Diffusion of vacancies and interstitials to edge dislocations

The steady‐state diffusion of radiation‐produced point defects in the stress field of an edge dislocation is solved by a perturbation method. The drift term entering the diffusion equation includes the size interaction and the inhomogeneity interaction as well as the effects of externally applied loads. By comparing the perturbation solution with the rigorous solution of Ham, we show that the perturbation solution is always adequate provided the drift term is proportional to the gradient of the interaction energy of the point defect with the dislocation. The steady‐state distributions of vacancies and interstitials is such that voids or vacancy clusters preferentially grow on the compressive side of the edge dislocation. The external stresses give rise to an orientation‐dependent bias of the edge dislocation which is shown to provide a possible mechanism for radiation‐induced creep.

Stress−induced diffusion of point defects to spherical sinks

Radiation damage in metals at elevated temperatures produces small dislocation loops and voids. The growth of these sinks is determined by the steady−state diffusion of point defects migrating in the stress field of these sinks. To obtain the steady−state current of point defects to these spherical sinks a perturbation method is developed to deal with the drift term of the diffusion equation. It is shown that the contribution of the drift term to the current can be expressed by a bias factor which differs from unity if the point defects interact with the spherical sink. Explicit expressions of the bias factors for voids and infinitesimal dislocation loops are given. If the metal is subject to external loads the bias factors of voids depend on the elastic dilatation, whereas the bias factors for dislocation loops depend on the deviatoric elastic strain. These results then provide the basis for stress−induced swelling and irradiation creep of metals. Both of these phenomena are briefly discussed.

Flux distribution and hysteresis loss in a round superconducting wire for the complete range of flux penetration

Flux distributions are determined numerically for quasistatic conditions within the Bean model. The field penetration ranges from 0 to 100%. The moving boundary of this distribution is calculated using an optimization technique to force the field component on the boundary to vanish, the variables being the parameters describing the boundary curve. The hysteresis loss is found using the flux distribution for a variable field superposed on a constant bias; the two fields are parallel to each other and transverse to the wire axis.

The upper critical field of NbN film

It is proposed and experimentally verified that the anomalously high superconducting critical field normal to the surface of NbN films possessing a column‐void microstructure is Hc3, the field appropriate for surface superconductivity. It is also proposed that because the coherence length is much less than the lateral column dimension that the resistivity of the column and not the film enters calculations of the Maki parameter α. A previously noted discrepancy in α is removed by these proposals.

Scaling laws for flux pinning in Nb and NbTi multifilamentary conductors

Magnetization measurements M(H,T) are made on several commercially drawn Nb–Cu and NbTi–Cu multifilamentary conductors as a function of applied magnetic field and temperature. The samples are distinguished by their filament sizes or the amount of cold work. From these measurements the pinning force Fp(H,T) and the critical current characteristics Jc(H) are deduced for all samples. Using this information Kramers scaling laws for flux pinning in multifilamentary conductors were tested. It is found that the model with some reservations seems to account for most of the observed behavior in the Nb–Cu and NbTi–Cu systems.

Comparison of calculated and measured hysteresis loss in multifilamentary superconducting wire

A previously developed theory for a single round filament of type II superconductor is applied to hysteresis losses in multifilamentary composite conductors. The single filament theory was extended for a wider range of time varying field amplitudes. Two contrasting wires were used to test the theory which predicts a universal curve for the loss in all conductors. A very close agreement was obtained in all details between theory and experiment for the wire with widely spaced (∼2.5 μm) large‐diameter (15 μm) filaments. The other wire had closely spaced (<1 μm) small‐diameter (2 μm) filaments and the shapes of the loss versus field plots for theory and experiment were in excellent agreement for this case. However, the effective filament diameter and superconductor volume fraction λ extracted from fitting the theory and measurements are much larger than the physical diameter and slightly larger than the physical λ, respectively. This is expected due to the proximity effect and possible interfilament contact. ...

THIN FILMS AND METASTABLE PHASES

Publisher Summary It is believed that all high-temperature superconducting phases are inherently unstable. This may then explain the current interest in growth techniques that might be capable of forming nonequilibrium, metastable phases of potentially high-T c superconductors. Among T c superconductors, attention is centered on techniques that involve growth from the vapor phase, such as sputtering, evaporation, and chemical vapor deposition (CVD). The result of those methods is the preparation of a compound called Nb 3 Ge. At present, that compound remains the highest temperature superconductor known with a maximum onset temperature of 23.6 K. This chapter reviews the mechanisms by which nonequilibrium phases are formed in thin films. The ability of sputtering, evaporation, and CVD to form phases is generally attributed to one or a combination of the following factors: (1) sputtering, evaporation, and CVD can all form phases at much lower temperatures than is possible by growing from the melt; (2) in each of these processes the impurities are introduced into the forming material; (3) sputtering and evaporation, in particular, are often considered to be very high-rate quenching methods by which high-temperature phases can be “frozen in” at lower temperatures.

Critical currents of superconducting conductors measured by a new technique

A magnetic torque technique for measuring critical current densities is described. Results obtained on Nb–Cu and (Nb–Ti)–Cu composite conductors by this technique are presented and compared with direct transport current measurements.

Microstructure and flux pinning in commercial Nb‐25% Zr superconducting wires

A detailed structural examination was carried out on commerical Nb‐25% Zr conductors which had been heavily cold worked and annealed in the temperature range 600–700 °C for times ranging from 1/4 to 10 h. The magnetization of some of these wires had previously been measured (Mathur et al.) as a function of applied magnetic field at temperatures ranging from 4.2 °K to the transition temperature and the results were interpreted in terms of a flux‐pinning model proposed by Kramer. Additional magnetization measurments on larger‐diameter wires were made to aid in the interpretation of the flux‐pinning mechanisms. Transmission electron microscopy, x‐ray analysis, and hardness measurements revealed that the predominant structural change in the annealed wires was recovery of dislocation structure. No βZr or αZr precipitates were detected. It is concluded that the optimum flux‐pinning properties in Nb‐25% Zr conductors for the processing schedules studied are associated with a highly recovered and small dislocatio...