Electronic heat-transfer devices of a proposed type would exploit some of the quantum-wire-like, pseudo-superconducting properties of single-wall carbon nanotubes or, optionally, room- temperature- superconducting polymers (RTSPs). The devices are denoted thermo-electron ballistic (TEB) coolers or heaters because one of the properties that they exploit is the totally or nearly ballistic (dissipation or scattering free) transport of electrons. This property is observed in RTSPs and carbon nanotubes that are free of material and geometric defects, except under conditions in which oscillatory electron motions become coupled with vibrations of the nanotubes. Another relevant property is the high number density of electrons passing through carbon nanotubes — sufficient to sustain electron current densities as large as 100 MA/cm2. The combination of ballistic motion and large current density should make it possible for TEB devices to operate at low applied potentials while pumping heat at rates several orders of magnitude greater than those of thermoelectric devices. It may also enable them to operate with efficiency close to the Carnot limit. In addition, the proposed TEB devices are expected to operate over a wider temperature range.

A TEB Device would exploit thermionic emission, ballistic transport of electrons in carbon nanotubes, and Schottky barriers at nanotube/semiconductor interfaces. The thickness of each nanotube layer would be of the order of 5 ¼m.
A typical TEB device (see figure) would include an electrically and thermally conductive plate, denoted the hot plate, on the side from which heat is to be transferred; N layers (N = 3 in the figure) of bundled and aligned carbon nanotubes interspersed with N — 1 semiconductor layers; an electrically and thermally conductive plate, denoted the cold plate, on the side to which heat is to be transferred; and small batteries or other DC electric power sources and wiring connected to the hot and cold plates and to the semiconductor layers. Under the influence of the electric potential field applied by the DC sources, some of the thermally agitated electrons would be adiabatically swept away from the hot plate into the first layer of nanotubes. (Because of this mode of operation, the device could also be called a thermionic cooler or heater.) The applied electric field would accelerate the electrons moving in the nanotubes, giving them enough kinetic energy to overcome the Schottky barrier at the nanotube/ semiconductor interface. Consequently, the Schottky barrier would act as a one-way valve for energetic electrons.

In the same manner as described above, thermally agitated electrons in each semiconductor layer would be made to travel through the next nanotube layer to the next semiconductor layer, and so forth until the electrons reach the cold plate, from whence they would be removed via an ohmic contact into the wiring. The closed electric circuit would maintain charge neutrality, supplying electrons to the hot plate and semiconductor layers to replace those removed by thermionic emission and applied electric fields. In addition to a DC potential applied between the hot and cold plates, DC bias potentials could be applied to the semiconductor layers to control the quantum-mechanical tunneling of electrons through the Schottky barriers.

It will be necessary to perfect a number of techniques in order to fabricate TEB devices. Among these are techniques for (1) depositing RTSPs or growing highly pure, aligned single-wall carbon nanotubes on electrically and thermally conductive substrates that can serve as hot and cold plates; (2) cutting the nanotubes to make clean, flat planes on which semiconductor layers can be deposited; and (3) depositing RTSPs or growing highly pure, aligned single-wall carbon nanotubes on the semiconductor layers. The overall thickness of a TEB device would be determined largely by the number of carbon-nanotube layers, the length (≈5 mm) of the nanotubes in each layer, and the thicknesses of the semiconductor layers. It should be possible to make TEB devices so thin that they could be incorporated into or onto flexible structures.

This work was done by Sang H. Choi of Langley Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. LAR-16222.

NASA Tech Briefs Magazine

This article first appeared in the March, 2003 issue of NASA Tech Briefs Magazine.

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