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 conductivity and thermoelectric power measurements as a function of temperature have been carried out on Ag2Te thin films of different thicknesses prepared on glass substrates at room temperature in a vacuum of 5×10−5 Torr. It is found that the phase transition temperatures (located by a steep change in resistance and thermoelectric power with temperature) during heating and cooling are different, thereby showing a thermal hysteresis during the phase transition. It is also found that the magnitude of the hysteresis is a function of thickness, increasing with decreasing thickness. This can be due to the large surface‐to‐volume ratio, small grain size, and a large density of dislocations in thin films.

Thermal hysteresis during phase transition in Ag2Te thin films: Thickness effect

The aging of island silver films deposited on glass at room temperature and at a pressure of 2×10−5 Torr was studied by monitoring the dc electrical resistance of the films. Films in the resistance range 1–11 MΩ/⧠ were studied under different conditions to ascertain the role of the residual gases and surface contaminants on the agglomeration rate. It was found that residual gases do not significantly affect the agglomeration rate, but a film of water vapor adsorbed on the glass surface retards the agglomeration to a great extent. An expression is arrived at for the functional dependence of the interisland spacing with time for the different conditions of study, assuming that the conduction in these island films is by quantum mechanical tunneling.

Influence of residual gases and surface contaminants on the aging behavior of island silver films

A study of substrate variation effects on the properties of n-CdSe0.7Te0.3 thin film/polysulphide photoelectrochemical solar cells

PbTe thin films of thicknesses ranging from 400 to 4000 A have been prepared by vacuum evaporation at a pressure of 5×10−5 Torr on clean glass substrates held at room temperature. The thermoelectromotive force of these films has been measured in the temperature range 300–600 K. It is found that thermoelectric power, SF varies anomalously with temperature, being constant at lower temperatures, and rapidly decreasing at higher temperatures. SF is found to be positive indicating that the samples are p type. The anomalous behavior is explained by assuming that at higher temperatures additional donor levels are generated due to creation and ionization of defects in the system.

Anomalous temperature dependence of thermoelectric power of PbTe thin films

In situ electrical conductivity measurements have been made on vacuum‐deposited amorphous thin films of various thicknesses of a Se50Te50 alloy in the temperature range 300–430 K. From the electrical conductivity, x‐ray studies, and electron diffraction studies it is found that the as‐grown films are amorphous and undergo an irreversible amorphous‐crystalline transition on heating in situ. The amorphous‐crystalline transition takes place in the temperature range 320–360 K for the different films. However, no systematic variation of the transition temperature with the thickness of the films is observed. All the films except the thinnest ones have a sharp transition temperature. X‐ray and electron diffraction analyses show that the Se50Te50 films above 360 K are polycrystalline. Above 360 K the electrical conductivity of the polycrystalline Se50Te50 films varies as an exponential function of reciprocal temperature.

V. Damodara Das

Papers

Thermal hysteresis during phase transition in Ag2Te thin films: Thickness effect

Influence of residual gases and surface contaminants on the aging behavior of island silver films

A study of substrate variation effects on the properties of n-CdSe0.7Te0.3 thin film/polysulphide photoelectrochemical solar cells

Anomalous temperature dependence of thermoelectric power of PbTe thin films

In situ electrical conductivity and amorphous‐crystalline transition in vacuum‐deposited amorphous thin films of a Se50Te50 alloy