Developing thermoelectric materials with superior performance means tailoring interrelated thermoelectric physical parameters – electrical conductivities, Seebeck coefficients, and thermal conductivities – for a crystalline system. High electrical conductivity, low thermal conductivity, and a high Seebeck coefficient are desirable for thermoelectric materials. Therefore, knowledge of the relation between electrical conductivity and thermal conductivity is essential to improve thermoelectric properties. In general, research in recent years has focused on developing thermoelectric structures and materials of high efficiency. The importance of this parameter is universally recognized; it is an established, ubiquitous, routinely used tool for material, device, equipment and process characterization both in the thermoelectric industry and in research. In this paper, basic knowledge of thermoelectric materials and an overview of parameters that affect the figure of merit ZT are provided. The prospects for the optimization of thermoelectric materials and their applications are also discussed.

A review on thermoelectric renewable energy: Principle parameters that affect their performance

TOPOLOGICAL defects formed during a rapid symmetry-breaking phase transition in the early Universe1,2 could be responsible for seeding large-scale structure, for the anisotropy of the microwave background radiation, and for the predominance of matter over antimatter3,4. The theory describing this cosmological phase transition is formally analogous to that describing the transition to the superfluid state in liquid 3He, so that in principle the process of cosmological defect formation can be modelled in the laboratory. Here we report the results of an experiment in which the 'primordial fireball' is mimicked using a neutron-induced nuclear reaction (n + 3He → p + 3He + 0.76 MeV) to heat small regions of superfluid 3He above the superfluid transition temperature. These bubbles of normal liquid cool extremely rapidly, and we find that their transition back to the superfluid state is accompanied by the formation of a random network of vortices (the superfluid analogue of cosmic strings). We monitor the evolution of this defect state by rotating the superfluid sample, allowing vortices to escape from the network and thus be probed individually. Our results provide clear confirmation of the idea that topological defects form at a rapid second-order phase transition, and give quantitative support to the Kibble–Zurek mechanism5,6 of cosmological defect formation.

Vortex formation in neutron-irradiated superfluid 3He as an analogue of cosmological defect formation

Ge–Sb–Te alloys are widely used for data recording based on the rapid and reversible amorphous-to-crystalline phase transformation that is accompanied by increases in the optical reflectivity and the electrical conductivity. However, uncertainties about the optical band gaps and electronic transport properties of these phases have persisted because of inappropriate interpretation of reported data and the lack of definitive analytical studies. In this paper we characterize the most widely used composition, Ge2Sb2Te5, in its amorphous, face-centered-cubic, and hexagonal phases, and explain the origins of inconsistent or unphysical results in previous reports. The optical absorption in all of these phases follows the relationship αhν∝(hν−Egopt)2, which corresponds to the optical transitions in most amorphous semiconductors as proposed by Tauc, Grigorovici, and Vancu [Tauc et al., Phys. Status Solidi 15, 627 (1966)], and to those in indirect-gap crystalline semiconductors. The optical band gaps of the amorpho...

Investigation of the optical and electronic properties of Ge2Sb2Te5 phase change material in its amorphous, cubic, and hexagonal phases

Graphene and related two-dimensional materials provide an ideal platform for next generation disruptive technologies and applications. Exploiting these solution-processed two-dimensional materials in printing can accelerate this development by allowing additive patterning on both rigid and conformable substrates for flexible device design and large-scale, high-speed, cost-effective manufacturing. In this review, we summarise the current progress on ink formulation of two-dimensional materials and the printable applications enabled by them. We also present our perspectives on their research and technological future prospects.

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Functional inks and printing of two-dimensional materials.

Nanostructured magnetic systems have many applications, including potential use in cancer therapy deriving from their ability to heat in alternating magnetic fields. In this work we explore the influence of particle chain formation on the normalized heating properties, or specific loss power (SLP) of both low- (spherical) and high- (parallelepiped) anisotropy ferrite-based magnetic fluids. Analysis of ferromagnetic resonance (FMR) data shows that high particle concentrations correlate with increasing chain length producing decreasing SLP. Monte Carlo simulations corroborate the FMR results. We propose a theoretical model describing dipole interactions valid for the linear response regime to explain the observed trends. This model predicts optimum particle sizes for hyperthermia to about 30% smaller than those previously predicted, depending on the nanoparticle parameters and chain size. Also, optimum chain lengths depended on nanoparticle surface-to-surface distance. Our results might have important implications to cancer treatment and could motivate new strategies to optimize magnetic hyperthermia.

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Effect of magnetic dipolar interactions on nanoparticle heating efficiency: Implications for cancer hyperthermia

Electrical resistivity and thermoelectric power measurements have been carried out as a function of thickness in the temperature range 300–480 K on flash evaporated Pb0.6Sn0.4Te thin films. Electrical resistivity and thermoelectric power variation with temperature of the films suggests that they have a near‐degenerate semiconductor behavior. The effective mean‐free‐path model of classical size effect has been used to analyze the thickness dependence of resistivity and thermoelectric power. Both are found to be near‐linear functions of inverse thickness. From the analysis of the experimental data, material parameters like mean free path and Fermi energy have been evaluated. Thermoelectric figure of merit has been calculated using the measured values of thermoelectric power and electrical resistivity and by taking thermal conductivity data from the literature. Thermoelectric figure of merit is also found to be thickness and temperature dependent.

Thickness and temperature effects on thermoelectric properties of Pb0.6Sn0.4Te thin films

Thin films of different thicknesses in the range 400–1600 A have been vacuum deposited on clean glass substrates held at room temperature by very fast evaporation of the Se10Sb10Te80 bulk alloy. The thermoelectric power of these films has been measured as a function of temperature during heating and cooling cycles by the integral method. It is found that the thermoelectric power of these films is slightly different during the heating and the cooling cycles which is ascribed to slight reorientation of microcrystallites as also evidenced by x‐ray diffraction. It is further found that the thermoelectric power (at a constant temperature) is a function of film thickness; it varies nearly linearly with reciprocal thickness. From these data, the nature of carrier scattering in the films has been ascertained. From the energy‐dispersive x‐ray analysis patterns of the films the semiquantitative content of the constituent elements Sb, Se, and Te has been determined.

Thermoelectric power of ternary semiconductor Se10Sb10Te80 thin films

Thin films of Pb0.8Sn0.2Te of thicknesses varying between 360 A and 3400 A have been prepared by flash evaporation on cleaned glass substrates held at room temperature. Thermoelectric power (TEP) of these films has been evaluated by measuring the thermal emf developed by the integral method in the temperature range 300 K to 500 K. It has been found that TEP of all the films is positive and increases with temperature in the low temperature region and tends to saturate at high temperatures, beyond 400 K. This has been attributed to the pinning of the Fermi level at high temperatures. It was also found that these films did not show the expected linear dependence of TEP on the inverse film thickness.

Thermoelectric Power Studies on 1% Excess Te Doped Pb$_{\bf 0.8}$Sn$_{\bf 0.2}$Te Thin Films

Bismuth thin films of various thicknesses between 15 nm and 350 nm were vacuum deposited at room temperature on to glass substrates, immediately after which they were twice heat treated at a uniform rate. During the heat treatment, the resistance changes were monitored and, using these data, the initial lattice distortion energy spectra of as-grown bismuth thin films have been evaluated. It is found that the defects have preferential activation energy values around 1.06 eV, 1.14 eV and 1.32 ev. It is also found that ∫F0 (E) dE oscillates with thickness, which is attributed to the quantum size effect.

Lattice distortion energy spectra of as-grown bismuth thin films and their thickness dependence

The electrical conductivity of thin films of Se80Te20 polycrystalline alloy vacuum-deposited at room temperature on glass substrates has been studied duringin situ heating and cooling cycles. From the electron diffraction of as-grown films it is seen that the studied films are amorphous at room temperature. The electrical conductivity and electron diffraction studies showed that the as-grown amorphous thin films undergo an amorphous-crystalline transition in the temperature range 340 to 360 K. Upon cooling, the films appear to undergo a crystalline crystalline transition around the same temperatures. There does not appear to be any dependence of the amorphous-crystalline transition temperature on the thickness of the films. However, high-resistance films (thinner films) have a well-defined transition temperature while the low-resistance films (thicker films) have a broader transition. The electrical conductivity of polycrystalline Se80Te20 films above 360 K appears to be an exponential function of reciprocal temperature.

V. Damodara Das

Papers

Thickness and temperature effects on thermoelectric properties of Pb0.6Sn0.4Te thin films

Thermoelectric power of ternary semiconductor Se10Sb10Te80 thin films

Thermoelectric Power Studies on 1% Excess Te Doped Pb$_{\bf 0.8}$Sn$_{\bf 0.2}$Te Thin Films

Lattice distortion energy spectra of as-grown bismuth thin films and their thickness dependence

In situ electrical conductivity and amorphous-crystalline transition in vacuum-deposited thin films of Se20Te80 alloy