Hareesh Mavoori

Bonding nature of rare-earth-containing lead-free solders

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.

Universal solders for direct and powerful bonding on semiconductors, diamond, and optical materials

The surfaces of electronic and optical materials such as nitrides, carbides, oxides, sulfides, fluorides, selenides, diamond, silicon, and GaAs are known to be very difficult to bond with low melting point solders (<300 °C). We have achieved a direct and powerful bonding on these surfaces by using low temperature solders doped with rare-earth elements. The rare earth is stored in micron-scale, finely-dispersed intermetallic islands (Sn3Lu or Au4Lu), and when released, causes chemical reactions at the interface producing strong bonds. These solders directly bond to semiconductor surfaces and provide ohmic contacts. They can be useful for providing direct electrical contacts and interconnects in a variety of electronic assemblies, dimensionally stable and reliable bonding in optical fiber, laser, or thermal management assemblies.

Enhanced thermal and magnetic actuations for broad-range tuning of fiber Bragg grating–based reconfigurable add–drop devices

We demonstrate two new approaches to broad-range tuning of fiber Bragg grating devices: amplified thermal tuning and programmable magnetic tuning. The thermal-strain tuning approach employs a novel configuration to amplify thermally induced wavelength shifts by use of a negative thermal-expansion component. The magnetic-strain tuning approach allows programmable and latchable wavelength shifts through magnetic interactions that induce controlled strain on the fiber grating. The advantages and disadvantages of these two techniques are contrasted.

Significantly enhanced creep resistance in low-melting-point solders through nanoscale oxide dispersions

Nanosized, nonreacting, and noncoarsening oxide dispersoids have been incorporated into solder alloys to create an improved solder structure with an ultrafine grain size of ∼2000–5000 A and significantly enhanced mechanical properties. As much as three orders of magnitude reduction in the steady-state creep rate has been achieved. These solders also exhibit improved (4–5 times higher) tensile strength at low strain rates and improved ductility under high strain rate deformation. It is demonstrated that with a dispersion of TiO2 particles, the Pb–Sn eutectic solder with a low melting point of 183 °C can be made more creep resistant than the 80Au–20Sn eutectic solder with a much higher melting point of 278 °C.

Broad-range, latchable reconfiguration of Bragg wavelength in optical gratings

Tunability of optical signal filters such as fiber Bragg gratings is important for high volume data communications. A magnetically tunable and latchable optical fiber Bragg grating structure has been devised, and a fine wavelength control as well as a broad-range wavelength tunability has been demonstrated. A spinodally decomposed, anisotropically elongated microstructure was induced in Fe–Cr–Co alloys to fabricate square-loop, programmable magnets. The adjustable attractive force between the magnetic poles was used to controllably strain the grating and induce a very large shift in the Bragg reflection wavelength by as much as 15 nm (which is equivalent to more than 36-channel span in a high density optical communication system). This novel approach can be useful not only for a variety of optical communication networking applications, but also for imparting latchable reconfiguration of structural periodicity and functional properties in a wide class of materials and devices.

Low-field magnetostriction in an annealed Co–30% Fe alloy

Magnetostriction properties of a deformed and annealed Co–Fe alloy have been investigated. A significant magnetostriction value greater than ∼110×10−6 was obtained in a cold-rolled and recrystallized sample at a practical, low field of ∼100 Oe, while the as-cold-rolled sample gave a small magnetostriction of only ∼15×10−6 for the same applied field. Cold rolling and recrystallization produces much smaller magnetostriction in the longitudinal direction than the transverse direction in the low-field regime. The saturation magnetostriction of the recrystallized Co–Fe alloy was ∼140×10−6. The drastic dependence of the magnetostriction behavior on the alloy processing is tentatively attributed to the microstructure and texture changes brought about by the heat-treatment processing. The availability of such a high magnetostriction in low applied fields using a ductile and low-cost alloy material can be useful for various technical applications.