Thermoelectric effect
The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice-versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference. This effect can be used to generate electricity, measure temperature or pump heat.
Seebeck coefficient
The Seebeck coefficient or thermopower, represented by S, of a material measures the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material. S has units of V/K, though μV/K is more common. Values in the hundreds of μV/K, regardless of sign (+ for p-type and - for n-type materials), are typical of good thermoelectric materials.
At the atomic scale, an applied temperature difference causes charged carriers in the material to diffuse from the hot side to the cold side. Mobile charged carriers migrating to the cold side to create a carrier concentration gradient thus giving rise to a thermoelectric voltage.

Thermoelectric performance
The performance of a  thermoelectric generator depends not only on the power produced but also how much heat is provided at the hot end. The heat input is needed for the thermoelectric process  as well as normal thermal conduction and is offset by the Joule heating. The Fourier thermal conduction of the thermoelectric materials add a thermal path from hot to cold that consumes some heat and reduces the efficiency.
It can be shown that the maximum efficiency of a thermoelectric material depends on two terms. The first is the Carnot efficiency, for all heat engines can not exceed Carnot efficiency. The second is a term that depends on the thermoelectric properties, Seebeck coefficient (
S), electrical resistivity (r) and thermal conductivity(k). These material properties all appear together and thus form a new material property called zT, the Thermoelectric Figure of Merit. For small temperature gradient, the maximum efficiency (hmax) for power generation is given by:

A greater zT indicates a greater thermodynamic efficiency, therefore, zTis a method for comparing the potential efficiency of different materials. Values of 1 are considered good; values of ~3 are essential for thermoelectrics to compete with mechanical devices in efficiency. To date, the best reported peak zT values in bulk materials are in the 1.5-2 range.

.

.
Physics of Thermoelectric Materials
A high zT material needs to have a high Seebeck coefficient (usually in low carrier concentration semiconductors or insulators) and a high electrical conductivity (s=1/r, found in high mobility semiconductors or high carrier concentration metals ). The thermoelectric power factor maximizes somewhere between metals and semiconductors. To ensure that the net Seebeck effect is large, there should only be a single type of carrier. Mixed n-type and p-type conduction will lead to opposing Seebeck effect and low absolute Seebeck coefficient. Thermoelectric devices need both n- and p-type materials, but one never wants to have both n- and p-type carriers in a single material.
Thermoelectric also needs a low thermal conductivity. Thermal conductivity comes from two sources. Phonons (
the crystal lattice vibrations) transport heat leading to lattice thermal conductivity, kL and electrons (or holes) transport heat leading to electronic thermal conductivity through Wiedeman- Franz law (kE = LsT, where L is Lorenz factor).
Since all the electronic terms (
S, r andkE) that affect zT are strongly coupled together, a net increase in zT requires precise control on band structure and scattering of carriers. Alternatively, an effective strategy  to enhance zT is to minimize kL  through increasing the phonon scattering by heavy atoms, disorder, large unit cells, clusters, rattling atoms and nanostructures.

Thermoelectric Applications

The Seebeck effect is used in the thermoelectric generator, which functions like a heat engine, but is less bulky, has no moving parts or emissions, needs no maintenance, and is typically less efficient. It has a wide use in power plants for converting waste heat into additional power (a form of energy recycling), and in automobiles as automotive thermoelectric generators (ATGs) for increasing fuel efficiency. Space probes often use radioisotope thermoelectric generators with the same mechanism but using radioisotopes to create the required heat difference.

The Peltier effect can be used to create a refrigerator which is compact and has no circulating fluid or moving parts; such refrigerators are useful in applications where their advantages outweigh the disadvantage of their very low efficiency.

Thermocouples and thermopiles are devices that use the Seebeck effect to measure the temperature difference between two objects, one connected to a voltmeter and the other to the probe. The temperature of the voltmeter, and hence that of the material being measured by the probe, can be measured separately using cold junction compensation techniques.
Apollo-12, 1969
Curiosity Rover, 2011
BMW TE car
Peltier cooler element

zT值越大意味著热力学效率越高，因此zT是衡量不同材料的潜在热电转换效率的方法。热电优质达到1会被认为是很好的热电材料；而达到3左右时，其转换效率可以和现有机械式热机媲美。目前为止，块体材料中所报道的最高zT峰值范围在1.5-2左右。

.

高zT值材料需具备高塞贝克系数(常出现在低载流子浓度半导体或绝缘体中)和高电导率(s=1/r, 常出现在高迁移率的半导体材料或高载流子浓度的金属中)。热电功率因子在介于金属和半导体之间的材料中获得最大值。为确保最终塞贝克效应的功效，材料中应该只含有一种载流子。n型和p型传导将会产生正负相反的塞贝克系数从而相互抵消，因此混合导电的材料其塞贝克系数的绝对值往往很低。

kL）；其次是电子(或空穴)输运带来的电子热导，电子热导的贡献可由Wiedeman- Franz定理来确定(kE = LsT，其中L是洛仑兹因子)。

zT值相关的电性能参数(S, rkE)之间强烈耦合，提升材料zT值需精确调控能带结构和载流子的散射。此外，提升zT值另一种可行途径是通过引入重原子、晶格无序、大晶胞尺寸、团簇结构、扰动原子及纳米结构等加强对声子的散射以降低晶格热导率(kL)。

More about transport (输运性质)
Thermoelectric is Cool, Thermoelectric is Hot !

< style="overflow:hidden; position:absolute; left:194px; top:25px; width:609px; height:79px; background:none; " id="wsp_60e09244" alt="" title="" />