Technically useful bulk superconductors must have high transport critical current densities, Jc, at operating temperatures. They also require a normal metal cladding to provide parallel electrical conduction, thermal stabilization, and mechanical protection of the generally brittle superconductor cores. The recent discovery of superconductivity at 39 K in magnesium diboride (MgB2) presents a new possibility for significant bulk applications, but many critical issues relevant for practical wires remain unresolved. In particular, MgB2 is mechanically hard and brittle and therefore not amenable to drawing into the desired fine-wire geometry. Even the synthesis of moderately dense, bulk MgB2 attaining 39 K superconductivity is a challenge because of the volatility and reactivity of magnesium. Here we report the successful fabrication of dense, metal-clad superconducting MgB2 wires, and demonstrate a transport Jc in excess of 85,000 A cm-2 at 4.2 K. Our iron-clad fabrication technique takes place at ambient pressure, yet produces dense MgB2 with little loss of stoichiometry. While searching for a suitable cladding material, we found that other materials dramatically reduced the critical current, showing that although MgB2 itself does not show the 'weak-link' effect characteristic of the high-Tc superconductors, contamination does result in weak-link-like behaviour.

Because of the high homologous operation temperature of solders used in electronic devices, time and temperature dependent relaxation and creep processes affect their mechanical behavior. In this paper, two eutectic lead-free solders (96.5Sn-3.5Ag and 91Sn-9Zn) are investigated for their creep and stress relaxation behavior. The creep tests were done in load-control with initial stresses in the range of 10-22 MPa at two temperatures, 25 and 80~ The stress relaxation tests were performed under constant-strain conditions with strains in the range of 0.3-2.4% and at 25 and 80~ Since creep/relaxation processes are active even during monotonic tensile tests at ambient temperatures, stressstrain curves at different temperatures and strain rates provide insight into these processes. Activation energies obtained from the monotonic tensile, stress relaxation, and creep tests are compared and discussed in light of the governing mechanisms. These data along with creep exponents, strain rate sensitivities and damage mechanisms are useful for aiding the modeling of solder interconnects for reliability and lifetime prediction. Constitutive modeling for creep and stress relaxation behavior was done using a formulation based on unified creep plasticity theory which has been previously employed in the modeling of high temperature superalloys with satisfactory results.

Nano-sized, nonreacting, noncoarsening oxide dispersoids have been incorporated into solder alloys to create a new, improved solder structure with an ultrafine grain size of ~200-500 nm. The new solders exhibit significantly enhanced creep resistance combined with increased strength. The well-known thermal instability problem with ultrafine-grained structure appears to have been overcome in these solder alloys and the microstructure was seen to be quite stable upon high temperature exposure (e.g. 120°C). This is attributed to the presence of very fine dispersoid particles which impede grain boundary sliding and dislocation movement. The dispersions are seen to have a profound effect on the mechanical deformation characteristics of the solders with respect to creep. As much as three orders of magnitude reduction in the steady state creep rate has been achieved. The new solders also exhibit improved ductility under high strain rate deformation and improved strength (4-5 times higher tensile strength) at low strain rates. It is demonstrated that with a dispersion of TiO 2 particles, the Pb-Sn eutectic solder with a melting point of 183°C can be made more creepresistant than the 80Au-20Sn eutectic solder with a much higher melting point of 278°C. The new creep-resistant solders can be useful for optical and optoelectronic packaging in which dimensional stability of the assembled structure is essential.

The ability of rare-earth-containing lead-free solders to wet and bond to silica was investigated. Small additions of Lu (0.5–2 wt. %) added to eutectic Sn–Ag or Au–Sn solder render it directly solderable to a silicon oxide surface. The bonding is attributed to the migration of the rare-earth element to the solder–silica interface for chemical reaction and the creation of an interfacial layer that contains a rare-earth oxide. It was found that additions of rare-earth materials did not significantly modify the solidification microstructure or the melting point. Such oxide-bondable solders can be useful for assembly of various optical communication devices.

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