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.
Nature Materials 7, 105 - 114 (2008)
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 ( h max ) for power generation is given by:





A greater
zT indicates a greater thermodynamic efficiency, therefore, zT is 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.






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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, k L and electrons (or holes) transport heat leading to electronic thermal conductivity through Wiedeman- Franz law ( k E = L s T , where L is Lorenz factor).

Since all the electronic terms (
S , r and k E ) 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 k L through increasing the phonon scattering by heavy atoms, disorder, large unit cells, clusters, rattling atoms and nanostructures.
power factor
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
热电效应
热电效应能将温差直接转换成电压,反之亦然。当热电器件两端存在温差则会产生电压。反过来,当对热电器件两端施加电压则会产生温度差。这种效应可以应用于发电,制冷及温度测量。
塞贝克系数
材料的塞贝克系数亦称作热电势,用 S 表示,表征了材料对于给定温差后所产生的热电势的电压大小。塞贝克系数的单位是V/K,通常用μV/K。无论正负(正号代表p型而负号代表n型材料),塞贝克系数的值在几百μV/K被通常认为是好的热电材料。

从微观上讲,所施加的温差将会导致材料内部带电荷的载流子从高温端向低温端扩散。移动的载流子迁移到冷端后会形成载流子浓度差,从而在材料两端产生热电电压。


热电性能
热电发电机的性能不仅取决于所产生电能的大小,而且取决于在热端施加的热能的大小。热能输入对热电能源转换过程和常规的热传导都是必须的,而该热能输入同时会被工作时产生的焦耳热所补偿。由于从高温端到低温端的傅立叶热传导作用,施加温差后热电材料会发生热传导并导致一定的热量损失,从而降低其效率。

热电材料的最大转换效率由两个因素共同决定。第一个因素是卡诺循环效率,所有的热机都不可能高于这个效率极限;第二个因素是由热电材料本身的性质决定,包含材料的塞贝克系数( S )、电阻率( r )和热导率( k )。这些材料性质参数共同决定了新的材料性能——材料的热电优质,用 zT 表示。对于温差很小的情况下,热电发电的最大转换效率( h max )可表示如下:





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。











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热电材料物理
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。

热电材料同时应具备低的热导率。热导率来自两方面的贡献,首先是由声子传播(晶格振动)导致的晶格热导率(
k L );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( k E = L s T ,其中 L 是洛仑兹因子)。

由于所有与
zT 值相关的电性能参数( S , r k E )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( k L )。
热电应用
塞贝克效应可以用作热电发电,其功能和传统的热机相似,但它更加小巧、无传动部件及任何排放物、无需维护、同时效率一般也较低。它在废热发电利用(能量回收)方面,以及在汽车尾气热量回收发电(ATGs)以提升燃油效率方面有着广泛的应用。相同的发电机制,太空探测器也经常用到放射性同位素热电发电器(RTG)作为电源,其温差的产生是通过放射性同位素的衰变。

珀耳帖效应则可以用作热电制冷,其结构紧凑且无循环液体工作介质及传动部件;这种制冷方式在对转换效率要求不高的情况下有着重要的应用。

热电偶以及热电堆也是利用塞贝克效应的器件用于测试两个待测点之间的温度差,其工作时将其中的一个待测点连接伏特表而另一个待测点连接传感器探头。利用冷接点补偿技术,伏特表的温度以及传感器探头反馈的温度可以被分别测量。
More about transport (输运性质)
Thermoelectric is Cool , Thermoelectric is Hot !