Thermal tunneling

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Thermotunnelung (engl. Thermoelectric tunneling ) is a postulated physical principle for heat engines and heat pumps, which on the tunnel effect is based, and in common with a thermocouple having.

It is comparable with the Seebeck or the Peltier effect . The estimated possible efficiency for cooling was initially around 55% of the Carnot efficiency . This would make the process somewhat more efficient than a heat pump or a compressor cooling machine in a refrigerator. However, prototypes only achieved 5% efficiency and an article was published as early as 2001 according to which the effect is even more inefficient than the Peltier effect with all known materials.

principle

Instead of bringing two different metals or semiconductors into contact like a thermocouple , the thermotunneling process uses a vacuum between the materials. Due to the vacuum, they are well insulated with regard to heat conduction, although they are only about 7 nm (70  Å ) apart.

The electrons can tunnel across the vacuum . To make this easier, microstructures or special doping can be applied to the interfaces. The microstructures forbid various low-energy quantum states . As a result, the electrons are automatically at higher energy levels and tunnel across the gap with a greater probability.

Efficiency

It is true that heat conduction is completely prevented by lattice vibrations through the vacuum gap. However, this may only be so wide that individual electrons can tunnel quantum mechanically through this gap . At first glance, this interruption of the phononic heat conduction, i.e. the heat conduction via lattice vibrations, appears to be extremely efficient. With a gap size that enables quantum mechanical tunneling, however, the electromagnetic forces are so great that the lattice vibrations are passed on almost unhindered due to electromagnetic coupling.

An efficient decoupling of the lattice vibrations only takes place when the gap size is larger than the wavelength of the thermal radiation of the surfaces. At room temperature , the majority of blackbody radiation is in the range of a few hundred nanometers to a few micrometers. A gap that exceeds this size, however, largely prevents quantum mechanical tunneling. A distance that is sufficiently small to allow an appreciable electrical current to flow when there is a temperature difference is so small that a significant heat flow also flows over it. The enthalpy corresponding to this heat is lost to the system without being converted into electrical energy . That makes the whole process inefficient.

history

The process was already known in the 1970s . At that time, however, it was not pursued any further because the very small gaps between the metal plates could not be created.

The process is currently being developed further by Borealis Exploration Limited and General Electric . Borealis is developing various manufacturing processes: One of them uses porous materials to create the tunnel. The electrons in these materials must have a mean free path that is greater than the layer thickness so that the tunnel probability is high enough. Another method uses a "lost" thin layer between the contacts, which is subsequently removed and leaves a thin gap.

General Electric is working on the active control of the distance with piezo actuators , as is also used in the tunnel microscope . The aim is to develop a refrigerator or freezer that was subsidized in 2005 from the US research and development program Energy Efficient Building Technologies .

literature

  • Hui Tong Chua, Xiaolin Wang, Jeffrey M. Gordon: Thermionic and tunneling cooling thermodynamics , Applied Physics Letters , May 17, 2004, Volume 84, No. 20, pp. 3999-4001, abstract
  • Langley Research Center: Thermo-Electron Ballistic Coolers or Heaters .
  • Avto Tavkhelidze et al .: Observation of New Quantum Interference Effect in Solids. PDF
  • Y. Hishinuma, TH Geballe, BY Moyzhes, TW Kenny: Refrigeration by combined tunneling and thermionic emission in vacuum: use of nanometer scale design. Applied Physics Letters, 2001, 78 (17): pp. 2572-2574
  • DEVELOPMENT OF A HIGH-EFFICIENCY SOLID-STATE COOLING CHIP Speech by RT Cox (Cool Chips) at the IMAPS in Palo Alto, California, October 25, 2004 (PDF file; 86 kB)
  • Artemy Martinovsky, Avto Tavkhelidze, Isaiah Watas Cox: Thermotunnel converter with spacers between the electrodes. , U.S. Patent No. US 6876123 B2 , August 28, 2002
  • Stanton Earl Weaver Jr .: Heat transfer device and method of making and operating the same. , European Patent No. EP 1612492 A1 , June 16, 2005

Web links

Individual evidence

  1. ↑ Marc D. Ulrich, Peter A. Barnes, Cronin B. Vining: Comparison of solid-state thermionic refrigeration with thermoelectric refrigeration. In: Journal of Applied Physics, August 2001, Volume 90, No. 3, pp. 1625-1631 doi : 10.1063 / 1.1380996