The present trend is to build more of the circuitry directly into the chips and to package many interconnected chips within single modules, cards, or boards.

One can readily translate this trend into specific needs for new materials. Increasing the density of devices on chips requires development of submicron metal lines and an increase in the depth of circuitry from two or three levels to five or six. These developments will require, respectively, new photoresists with strong sensitivities at short wavelengths, and insulating polymers that planarize and have unusually low dielectric constants. Increasing the complexity of the package causes problems of reliability associated with mechanical stresses at the interconnections between the various devices. These stresses arise from the mismatch in thermal expansion coefficients between the silicon of the chip, the alumina of the module, and the epoxy fiberglass in the card or board. Reducing these stresses requires development of polymeric substrates for modules, cards, and boards that better match the thermal expansion coefficient of the silicon chip.

Transportation Industry

Several needs for synthesis of advanced materials are common to both the aircraft and the automobile industries. For example, lightweight structural materials are needed for fuel efficiency. Significant progress has been made over the past 15 years in the development of lightweight graphite-fiber-reinforced epoxy composites. The synthesis of tougher matrix resins might allow such composites to be used more widely in structural elements such as aircraft wings. Graphite fiber composites are relatively expensive for use in automobiles, and thus the primary emphasis has been on fiberglass-reinforced resins. In the future, we might look for the synthesis of inexpensive, self-reinforcing, injection-moldable liquid crystalline polymers to bring about major advances in structural materials for automobiles. In fact, certain types of liquid crystalline polymers retain mechanical properties up to about 350°C and might even be used in parts of engines.

Another need in the transportation industry is for improved flame-resistant materials for use in the interiors of aircraft and automobiles. Currently, too little effort is directed at designing new kinds of textiles that are intrinsically flame resistant and that do not produce smoke or toxic gases in a fire. The challenge is not only to achieve the key feature of flame resistance, but also to provide comfort and wearability, and to make such textiles available at costs competitive with those of currently used materials.

The search continues for inexpensive, high-temperature ceramics that can be used to increase the efficiency of combustion engines. The intrinsic brittleness of ceramics remains a major stumbling block, notwithstanding recent progress in the preparation of composites reinforced with stabilized zirconia and fiber-reinforced composites. Success in this area will require synthesis



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