Refrigeration by combined tunneling and thermionic emission in vacuum: Use of nanometer-scale design

As microelectronics become faster and their densities increase, there is an increasing need for efficient, compact, reliable, low-noise cooling and temperature stabilization. Thermoelectric refrigeration is a solid state active cooling method which requires no moving parts or fluid. However, the efficiency of thermoelectric coolers is rather low: the efficiency is limited by the difficulty of finding a material with high electrical conductivity, high thermal power and low thermal conductivity.; Thermionic emission across a vacuum gap overcomes two major problems of thermoelectric cooling: lattice thermal conduction and joule heating. At present, however, there is no successful example of refrigeration based on thermionic emission in vacuum. To obtain a sufficient amount of current at room temperature, a work function of <0.4 eV is necessary.; Currently, there are no known materials with such a low work function. Surfaces with work functions as low as ∼1.0 eV can be prepared by evaporating alkali metal onto refractory metals. Applying a strong electric field can further lower the barrier: our calculations show that the barrier can be lowered significantly by bringing the emitter and collector to within nanometer range and applying few volts. Calculations show that it is possible to cool the emitter significantly using existing materials combined with an established technique for controlling electrode spacing in the nanometer range.; To demonstrate thermionic cooling at room temperature, the following experiment was conducted. A microfabricated cantilever with built-in thermistor was used as an emitter. Resistance changes in the thermistor indicate temperature changes due to electron emission from the tip located at the end of the cantilever. A feedback mechanism, similar to that of STM, maintained a nanometer gap between the emitter and collector. As Cs was evaporated onto the emitter, the temperature change was measured.; A temperature change of ∼0.3 mK was observed upon Cs evaporation onto an Au emitter for an emission current of 15 nA. The measurement was taken with different emission currents and bias voltages, and it was observed that the measurement had the dependence on these parameters as predicted by theory.; To circumvent the need for low work function surface, a semiconductor-coated emitter with graded electron affinity is also suggested. Finally, the feasibility of thermionic cooler is discussed from a practical standpoint.