The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. A thermoelectric device creates voltage when there is a different temperature on each side(Seebeck effect). Conversely, when a voltage is applied to it, it creates a temperature difference(Peltier effect). This effect can be applied to electricity generation, temperature measurement or heating/cooling system.
ZT is an index for measuring the properties of these thermoelectric elements. ZT value is proportional to the temperature difference, the electrical conductivity and the square of the seebeck coefficient(the magnitude of an induced thermoelectric voltage in response to a temperature difference) and inversely proportional to thermal conductivity. Hence thermal conductivity is an one of important thermo physical property and a material with lower thermal conductivity will maximize the thermal effects. In recent years, several groups have experimentally shown that the thermal conductivity of the silicon nanowire is lowered 1/100 to 1/200 compared with that of bulk silicon.
This is because the thermal conductivity is decreased as the number of phonon mode(lattice vibration in solid) is reduced.
We investigate the thermoelectric properties of electron –phonon interactions and calculate Seebeck coefficient based on the Non-equilibrium Green’s function. To further improve the performance of thermoelectric devices, we develop the in-house simulation tool to find the way to maximize the Seebeck coefficient and optimize the design of the thermoelectric systems for high efficiency. Applications of thermoelectricity include chip level electronics cooling, power generators for remote telecommunications, temperature control system etc.
The figure shows SEM image of a typical device with the nanowires shown as a red to white gradient by heat from the both-end heaters. The dark blue lines under the nanowire are electrodes for temperature measurement and light blue lines are for resistance measurement. Both factors are used for the heater power calculation.