Abstract
Traditional vacuum-based thermal energy conversion (VTEC) devices with “flat” metal emitters have been compromised by high working temperature mainly due to high work function and electron space charge effect. However, a VTEC system is inherently efficient due to unimpeded flow of electrons through vacuum and reduction of heat conduction by the vacuum medium. Consequently, VTEC devices are attractive in terms of space, weight and energy efficiency for waste heat recovery in a TEC system. Carbon nanostructured emitters including multiwalled carbon nanotubes (CNTs) and “ridge” nanodiamond thin films have shown relatively low threshold fields for electron field emission mainly due to their nanoscaled, high aspect ratio, emitting surfaces. In this study, the field-enhancing features of carbon nanostructures were investigated as thermal-field emitters for waste heat recovery. The turn-on field of CNTs was found to decrease from ~ 1.9 V/μm at room temperature to ~ 0.9 V/μm at 400 °C. A high emission current of ~ 26 μA was achieved at relatively low field of ~ 1.5 V/μm and low temperature of 400 °C. The maximum efficiency of the VTEC device is estimated to be ~ 15% at 300 °C with a current density of 54 mA/cm 2.
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