James P. Harbison

Anomalous increase in strength of in situ formed Cu-Nb multifilamentary composites

Cu‐Nb wire composites with 0.105, 0.148, and 0.182 volume fraction of Nb filaments were produced in situ and their mechanical properties measured as a function of filament size and interfilament spacing. The yield stress and the ultimate tensile strength increased with both niobium volume fraction and overall composite reduction. At room temperature, the ultimate tensile strength of the Cu–18.2 vol% Nb composite reduced by 99.999% in cross‐sectional area (100–200 A filament thickness) reached the value of 2230 MN/m2 (323 ksi) and further increased to 2850 MN/m2 (413 ksi) when measured at 77u2009°K. These values are higher by a factor of 4 than the values predicted by the rule of mixtures based on the highest reported strength of both niobium and copper. The composite strength is as high as that of the best copper whiskers and is shown to closely approach the theoretical strength of the material. The anomalous increase in strength despite the low volume fraction of reinforcing filaments suggests that the filam...

Superconducting and mechanical properties of in situ formed multifilamentary Cu‐Nb3Sn composites

A systematic experimental study of the variations of superconducting transition temperature and critical‐current density is reported for in situ formed multifilamentary Cu‐Nb3Sn composites containing 10 at.% Nb and 2–3 at.% Sn annealed at either 650 or 700u2009°C. A particular emphasis was placed on the evaluation of uniformity, thermal stability, and mechanical strength. Critical‐current density was measured as a function of transverse magnetic field and was found to increase in samples measured in a bent position. The overall critical‐current performance is comparable to that of reinforced stabilized conventional composites. Large residual resistivity ratios are indicative of a clean high‐conductivity matrix surrounding each individual filament, an important requirement for thermal stability. High resistance to plastic flow in these composites is attributed to strong filament‐to‐matrix bonding and small interfilament spacing. The ultimate tensile strength at 77u2009°K reached a value of ∼100 ksi (690 MPa). The ...

Superconducting properties of in situ formed Cu‐V3Ga composites

Cu‐V3Ga composites were prepared from in situ formed multifilamentary Cu‐V wires containing 20 vol.% of vanadium filaments. The highest value of the upper critical field at 4.2 K of the reacted composites was found to be 22.4 T, with a corresponding midpoint superconducting transition of 15.5 K. The overall critical‐current density compares favorably with commercial V3Ga composites over the entire field range (2×105 A/cm2 at 4 T, 104 A/cm2 at 18 T).

Critical currents of in situ formed multifilamentary Cu-Nb3Sn composites

Critical current densities of in situ formed Cu-Nb3Sn composites with discontinuous filaments were measured as a function of superconducting volume fraction, matrix resistivity, area reduction ratio, and applied magnetic field. In agreement with recent modelling by Tinkham and co-workers, the effective superconducting volume fraction in a given composite was found to be field-dependent, necessitating the distinction between microstructural and electrical percolation. In composites with a low filament volume fraction, proximity effect coupling, controlled by matrix resistivity, was found to be the dominant factor determining both the composite remnant resistivity and the critical current density. For sufficiently high filament volume fractions and area reduction ratios, the remnant resistivities fall below the level of detection, as predicted by theory, and critical current densities become comparable to those of continuous filament composites. SEM, TEM, and STEM analysis reveal a dense distribution of submicron, ribbon-like Nb3Sn filaments in relatively pure Cu matrix. The microstructure of the filaments is equi-axed with an average grain size of ∼ 400Å, ensuring effective flux pinning.

Superconducting critical properties and AC losses in a large sample of in situ formed Cu‐Nb3Sn composite

Critical‐current density, upper critical field, and hysteresis ac losses have been evaluated for a large (100‐m long) sample of in situ formed Cu‐Nb3Sn composite with superconducting volume fraction λ≃0.23. The magnetic field dependence of the critical current, measured in wire and tape samples of different reductions, cannot be explained by a model based on a single pinning mechanism. The enhancement of the pinning force in highly reduced tape composites is attributed to surface flux pinning at the filament‐matrix interfaces. Hysteretic ac losses obtained by calorimetric and electronic measuring methods confirmed the results reported previously for small in situ samples.

Strain‐induced lateral carrier confinement in quantum wells grafted onto nonplanar substrates

We remove a thin semiconductor film from its growth substrate and reattach it to a nonplanar host substrate. The film is under a large, localized bending stress. In a GaAs/AlGaAs film with a quantum well near one surface where the bending strain is greatest, carriers are laterally confined by the strain to regions where the band gap is red‐shifted by up to 62 meV.

VOLTAGE TUNABLE POTENTIAL WELLS FOR WIRE CONFINEMENT OF EXCITONS

We use applied voltage to modulate lateral strain in a quantum well, thereby achieving tunable confinement of excitons. A capacitor structure is fabricated by attaching a flexible film containing a quantum well and a conducting layer to a doped Si substrate patterned with raised ribs. The film conforms to the ribs, creating a potential well for excitons where the quantum well is in tension. The film is pulled toward the substrate through electrostatic force, increasing the confinement. Modulation of strain confinement by over 50% is reported. The effect is reversible with voltage and polarity independent, as expected for electrostatic attraction.

Superconducting and mechanical properties of insitu formed multifilamentary Cu‐Nb3Sn composites

A systematic experimental study of the variations of superconducting transition temperature and critical‐current density is reported for in situ formed multifilamentary Cu‐Nb3Sn composites containing 10 at.% Nb and 2–3 at.% Sn annealed at either 650 or 700u2009°C. A particular emphasis was placed on the evaluation of uniformity, thermal stability, and mechanical strength. Critical‐current density was measured as a function of transverse magnetic field and was found to increase in samples measured in a bent position. The overall critical‐current performance is comparable to that of reinforced stabilized conventional composites. Large residual resistivity ratios are indicative of a clean high‐conductivity matrix surrounding each individual filament, an important requirement for thermal stability. High resistance to plastic flow in these composites is attributed to strong filament‐to‐matrix bonding and small interfilament spacing. The ultimate tensile strength at 77u2009°K reached a value of ∼100 ksi (690 MPa). The ...

Superconducting and mechanical properties ofinsituformed multifilamentary Cu‐Nb3Sn composites

A systematic experimental study of the variations of superconducting transition temperature and critical‐current density is reported for in situ formed multifilamentary Cu‐Nb3Sn composites containing 10 at.% Nb and 2–3 at.% Sn annealed at either 650 or 700u2009°C. A particular emphasis was placed on the evaluation of uniformity, thermal stability, and mechanical strength. Critical‐current density was measured as a function of transverse magnetic field and was found to increase in samples measured in a bent position. The overall critical‐current performance is comparable to that of reinforced stabilized conventional composites. Large residual resistivity ratios are indicative of a clean high‐conductivity matrix surrounding each individual filament, an important requirement for thermal stability. High resistance to plastic flow in these composites is attributed to strong filament‐to‐matrix bonding and small interfilament spacing. The ultimate tensile strength at 77u2009°K reached a value of ∼100 ksi (690 MPa). The ...