Micro and nano-fabricated sensors (e.g., accelerometers and gyroscopes) and actuators (e.g., light valve chips for projection and cell-phone displays) have become commonplace in recent years. Some of these devices must operate in a hermetically sealed, lowpressure ambient, a need that motivated the development of low-cost, wafer-scale vacuum encapsulation technologies. In this talk, I’ll identify a promising direction for nanotechnology, in which vacuum is more than simply the ambient surrounding a microstructure, but rather is a critical element in device operation.
Thermionic energy converters were conceived in 1915, demonstrated in 1939, and were the focus of astronomical investments during the space race by NASA and the Soviet Union. A 6 kW thermionic converter, fabricated using precision machining and vacuumtubetechnology, was flown in the late 1980s by the Soviets. Thermionic converters can be fabricated using extensions of MEMS technology, in which advances in materials, micromachining, and vacuum encapsulation processes can be used to enhance performance and reduce fabrication costs. Potential commercial applications include topping cycles in small-scale co-generation. Recently, a new conversion concept has been demonstrated at Stanford, in which a semiconductor photocathode replaces the conventional metal cathode. This photon-enhanced thermionic energy (PETE) converter harvests photon energies above the bandgap, as well as broad-spectrum radiation through heating of the photocathode. It is attractive as the high-temperature topping cycle for solar-thermal power stations. Micro- and nano-structured, high-temperature materials and micromachining processes are also essential to fabricating wafer-scale, cost-effective PETE converters. I will conclude by summarizing the research directions that are needed to bring thermionic and PETE conversions into the mix of energy conversion options.
Birck Nanotechnology Building, Room 1001, Purdue University, West Lafayette, IN