Design of unique composites based on aromatic thermosetting copolyesters

Aromatic thermosetting copolyester (ATSP) has promise in high-temperature applications. It can be employed as a bulk polymer, as a coating and as a matrix for carbon fiber composites (ATSP/C composites). This work focuses on the applications of high performance ATSP/C composites. The morphology of the ATSP matrix in the presence of carbon fiber was studied. The effect of liquid crystalline character of starting oligomers used to prepare ATSP on the final crystal structure of the ATSP/C composite was evaluated. Matrices obtained by crosslinking of both liquid crystalline oligomers (ATSP2) and non-liquid crystalline oligomers (ATSP1) tend to crystallize in presence of carbon fibers. The crystallite size of ATSP2 is 4 times that of ATSP1. Composites made from ATSP2 yield tougher matrices compared to those made from ATSP1. Thus toughened matrices could be achieved without incorporating any additives by just changing the morphology of the final polymer. The flammability characteristics of ATSP were also studied. The limiting oxygen index (LOI) of bulk ATSP was found to be 40% whereas that of ATSP/C composites is estimated to be 85%. Thus, ATSP shows potential to be used as a flame resistant material, and also as an aerospace reentry shield. Mechanical properties of the ATSP/C composite were characterized. ATSP was observed to bond strongly with reinforcing carbon fibers. The tensile strength, modulus and shear modulus were comparable to those of conventionally used high temperature epoxy resins. ATSP shows a unique capability for healing of interlaminar cracks on application of heat and pressure, via the Interchain Transesterification Reaction (ITR). ITR can also be used for reduction in void volume and healing of microcracks. Thus, ATSP resin systems provide a unique intrinsic repair mechanism compared to any other thermosetting systems in use today. Preliminary studies on measurement of residual stresses for ATSP/C composites indicate that the stresses induced are much lower than that in epoxy/C composites. Thermal fatigue testing suggests that ATSP shows better resistance to microcracking compared to epoxy resins.

Material characterization and process development for indium-arsenide / indium-gallium-antimonide channel, high electron mobility transistors for low power, high speed applications

Efforts to push the performance of transistors for millimeter-wave and microwave applications have borne fruit through device size scaling and the use of novel material systems. III-V semiconductors and their alloys hold a distinct advantage over silicon because they have much higher electron mobility which is a prerequisite for high frequency operation. InGaAs/InP pseudomorphic heterojunction bipolar transistors (HBTs) have demonstrated fT of 765 GHz at room temperature and InP based high electron mobility transistors (HEMTs) have demonstrated fMax of 1.2 THz. The 6.1 A lattice family of InAs, GaSb, AlSb covers a wide variety of band gaps and is an attractive future material system for high speed device development. Extremely high electron mobilities ~ 30,000 cm^2 V^-1s^-1 have been achieved in modulation doped InAs-AlSb structures. The work described in this thesis involves material characterization and process development for HEMT fabrication on this material system.