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

Bismuth thin films of different thicknesses between about 20 and 225 nm are vacuum deposited at room temperature in a vacuum of 3 × 10−3 Pa. The films are heat-treated “in situ” and the resistances monitored. It is found that the resistivity-temperature plots of the films after heat-treatment are non-linear and exhibit a minimum whose position is a function of thickness. It is also found that the films behave as semiconductors, the band gap decreasing with increasing thickness. The observations are interpreted on the basis of quantum size effect and the limitation of the electronic mean free path by the grain size of the films.



Dunne Wismuth-Schichten verschiedener Dicken zwischen 20 und 225 nm werden bei Zimmer-temperatur im Vakuum von 3 × 10−3 Pa hergestellt. Die Schichten werden “in situ” getempert und der Widerstand verfolgt. Es wird gefunden, das die Widerstands-Temperatur-Verlaufe der Schichten nach der Warmebehandlung nichtlinear sind und ein Minimum aufweisen, dessen Lage von der Schichtdicke abhangt. Es wird ebenfalls gefunden, das sich die Schichten wie Halbleiter verhalten, wobei die Bandlucke mit zunehmender Schichtdicke abnimmt. Die Beobachtungen werden auf der Grundlage des Quantensize-Effekts und der Begrenzung der mittleren freien Weglange der Elektronen durch die Korngrose der Schichten erklart.

Variation of energy gap and resistivity minimum position with thickness in bismuth thin films

Time dependent electrical resistance of Bi2(Te0.4Se0.6)3 thin films in vacuum and on exposure to atmosphere

Structural and electrical properties of Bi2(Te0.4Se0.6)3 thin films

Pb0.2Sn0.8Te thin films of different thicknesses have been deposited on glass substrates by flash evaporation. Electrical resistivity and thermoelectric power measurements have been carried out in the temperature range 300-500 K as a function of film thickness. The thickness dependence of the electrical resistivity and thermoelectric power observed has been explained using the effective mean free path model of classical size effect theory. The thermoelectric power factor was calculated from the measured values of electrical resistivity and thermoelectric power. The thermoelectric power factor is found to be dependent on thickness and temperature. By taking thermal conductivity values from the literature, the thermoelectric figure of merit was also calculated and it was found that this too was dependent on the temperature and thickness.

Variation of electrical transport properties and thermoelectric figure of merit with thickness in 1% excess Te-doped Pb0.2Sn0.8Te thin films

The vacuum deposited thin films of n-CdSe0.7Te0.3 of 5000 A thickness have been characterized by X-ray diffraction (XRD) for structural analysis, by energy despersive analysis of X-rays (EDAX) for compositional analysis and the scanning electron microscopy (SEM) for surface studies. Similar films have been deposited on Indium Oxide precoated microslide glass plates and the photoelectrochemical properties of n-CdSe0.7Te0.3/polyiodide junction are investigated for suitability for solar energy conversion by I–V measurements in the dark. Also, the carrier concentration and flat band potential have been calculated from the space charge capacitance vs voltage measurements. The minority carrier diffusion length, Lp has been determined using Gartner's model. Using the Mott-Schottky plots, the energy band diagram at flat band condition has been constructed. The occurrence of high quantum efficiency and high power conversion efficiency have been explained.

V. Damodara Das

Papers

Variation of energy gap and resistivity minimum position with thickness in bismuth thin films

Time dependent electrical resistance of Bi2(Te0.4Se0.6)3 thin films in vacuum and on exposure to atmosphere

Structural and electrical properties of Bi2(Te0.4Se0.6)3 thin films

Variation of electrical transport properties and thermoelectric figure of merit with thickness in 1% excess Te-doped Pb0.2Sn0.8Te thin films

Characterization of n-CdSe0.7Te0.3/(aq) Polyiodide Energetic Interface and Photoelectrochemical Studies