Thermoelectric materials which can convert energies directly between heat and electricity are used for solid state cooling and power generation. There is a big challenge to improve the efficiency of energy conversion which can be characterized by the figure of merit (ZT). In the past two decades, the introduction of nanostructures into bulk materials was believed to possibly enhance ZT. Nanocomposites is one kind of nanostructured material system which includes nanoconstituents in a matrix material or is a mixture of different nanoconstituents. Recently, nanocomposites have been theoretically proposed and experimentally synthesized to be high efficiency thermoelectric materials by reducing the lattice thermal conductivity due to phonon-interface scattering and enhancing the electronic performance due to manipulation of electron scattering and band structures. In this review, we summarize the latest progress in both theoretical and experimental works in the field of nanocomposite thermoelectric materials. In particular, we present various models of both phonon transport and electron transport in various nanocomposites established in the last few years. The phonon-interface scattering, low-energy electrical carrier filtering effect, and miniband formation, etc., in nanocomposites are discussed.

An analytic model is presented to estimate the performance of the thermoelectric refrigeration constructed by the functional graded thermoelectric materials with the spatial-and temperature-dependent transport coefficients, such as the Seebeck coefficient, the thermal conductivity and the electrical conductivity along the transport direction. In the case of varied carrier concentration, the temperature-dependent transport coefficients used in our analytic model are calculated by utilizing the Boltzmann transport equations. We find that higher maximum cooling power and larger maximum temperature difference could be achieved in by using graded thermoelectric materials compared to that with homogeneous materials. The optimal distribution of carrier concentration is also discovered to effectively enhance the maximum coefficient of performance.

Thermoelectric nanocomposites (TENCs) with inorganic nanostructures embedded in an organic matrix have attracted much attention in recent years. There is hardly any theory to guide the design of such TENC although various combinations of inorganic fillers and organic matrices are reported. We recently proposed a concept of "electron−percolation phonon−insulator" to provide a guiding ideology for designing inorganic−organic TENC. In this work, we systematically exemplify this theory by measuring the transport properties of TENC with a sequence of one-, two-, and three-dimensional nanostructured fillers embedded in an insulating poly(vinylidene fluoride) matrix. The topological structure of the connected fillers is found to be a key factor to achieve high thermoelectric performance. The intrinsic Seebeck coefficient of filler materials and the contact resistance between fillers also play important roles.

We use the Boltzmann transport equation under the relaxation time approximation to investigate the effect of minority blocking on the transport properties of nanocomposites (NCs). Taking p-type Bi0.5Sb1.5Te3 NCs as an example, we find that the thermally excited minority carriers can be strongly scattered by engineered interfacial potential barriers. Such scattering phenomena suppress the bipolar effect, which is helpful to enhance the Seebeck coefficient and reduce the electronic thermal conductivity, especially at high temperatures. Further combining with the majority carriers low-energy filtering effect, the power factor and the figure of merit (ZT) can be significantly enhanced over a large temperature range from 300 K to 500 K. Such an improvement of ZT is attributed to the majority carriers low-energy filtering effect at low temperatures and to the minority carriers blocking effect at high temperatures. A principle that is helpful to provide guidance on the thermoelectric device design is identified: (1) blocking the minority carriers as often as possible and (2) filtering the majority carriers whose energy is lower than 2–3kBT near the cold end.