THERMOELECTRICS

The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. Simply put, a thermoelectric (TE) module generates power when there is a different temperature on each side, and when a voltage is applied to it, it creates a temperature difference.

Possible applications for TE modules are for waste heat recovery from the transportation sector, as well as industrial processes.  TE modules can also be used for self-powered appliances, auxiliary power supplies, and heat exchange for hot spots on PC chips. Recent advances in thermoelectric conversion, in the area of QW thin film materials, have demonstrated the potential for achieving the desired high-efficiencies and power densities that can be used for fabricating future power supplies, self powered appliances, and space power supplies. A 14% conversion efficiency on 1 in² areas with a thin film (11 µm thick QW films on a 5 µm Si substrate) thermoelectric couple composed of N-type Si/SiGe and P-type B₄C/B₉C has been already demonstrated.

The cooling property of these devices is due to the Peltier Effect, while the electrical power generating property is due to the Seebeck Effect.  A thermoelectric module can be used as either a cooler or a power generator, but not with the best efficiency. Peltier Effect coolers are almost always constructed with Bismuth Telluride (Bi₂Te₃) and used around room temperature and below. Seebeck Effect power generators are often constructed of PbTe or, SiGe as well as Bi₂Te₃ and are used at much higher temperatures.

The dimensionless figure of merit (ZT) measures the performance of a thermoelectric material.  Present bulk materials with ZT=1 at room temperature give efficiencies of around 8%.  Utilizing nanotechnology, E2TAC plans to achieve efficiencies up to 20%.  Energy transport in nanostructures take into account quantum size effects, and there by using new processes we can create more interfaces which will scatter phonons, but not electrons, which in turn will increase the materials ZT. 

The goal at E2TAC is to achieve the best possible efficiency for the thermoelectric device by: 

• Fabricate superlattice quantum well structures
  • Up to 16 in² area QW devices with up to 14% efficiency at 2 W/cm² power and delivering 1 kW
  • Reproducibly achievement of films for devices fabrication
  • Atomic layer accuracy leading to tight control over film thickness and excellent uniformity in film thickness and composition over large areas
  • Thick multilayer stacks (up to 11 mm)
• Decrease parasitic losses from substrate