The thermoelectric figure of merit “ZT”, is a dimensionless quantity that measures the efficiency of a material for thermoelectric applications. It is defined as [latex]ZT = \frac{S^2 \sigma T}{\kappa}[/latex], where S is the Seebeck coefficient, [latex]\sigma[/latex] is electrical conductivity, T is absolute temperature, and [latex]\kappa[/latex] is thermal conductivity. A higher ZT value indicates a more efficient thermoelectric material.
The figure of merit encapsulates the essential properties a material must possess to be effective in thermoelectric energy conversion. The numerator, [乳胶]S^2 \sigma[/latex], is known as the power factor. A high Seebeck coefficient (S) is needed to generate a large voltage from a given temperature difference, and high electrical conductivity ([latex]\sigma[/latex]) is required to minimize resistive (Joule) heating losses. The denominator, thermal conductivity ([latex]\kappa[/latex]), must be as low as possible. A low [latex]\kappa[/latex] helps maintain a large temperature difference across the device, which is essential for both power generation (Seebeck effect) and cooling (Peltier effect).
The primary challenge in thermoelectric material science is that these properties are often interdependent and conflicting. For instance, materials with high electrical conductivity (like metals) also tend to have high thermal conductivity due to the Wiedemann-Franz law. The quest for high ZT materials has 引领 to advanced strategies like nanostructuring. By creating structures with features on the nanoscale, it is possible to scatter phonons (which carry heat) more effectively than electrons (which carry charge), thereby reducing [latex]kappa[/latex] without significantly harming [latex]sigma[/latex]. This ‘phonon-glass electron-crystal’ concept has led to significant improvements in ZT values over the last few decades.
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