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

Thin films of Bi2Te2Se1 of various thicknesses have been deposited on clean glass plates using the flash evaporation technique. Electrical resistance and thermoelectric power measurements have been carried out on these films in the temperature range 300–485 K. The thickness dependences of electrical resistivity and thermoelectric power of the films have been analyzed using the effective mean-free path model. The thickness dependence of activation energy of the films is explained by Seto’s polycrystalline model. Various material parameters such as mean-free path and Fermi energy have been calculated from the analysis of experimental data. The thermoelectric power factor of the films has been calculated using the measured electrical resistivity and thermoelectric power values.

Size and temperature dependence of electrical resistance and thermoelectric power of bi2te2se1 thin films

Band gap and intergrain barrier activation energies in Bi90Sb10 thin films

Presented are the results of investigations on the effect of an applied DC electric field on the post-deposition resistance changes of island copper films on glass. Based on the functional dependence of the film resistance on time, an agglomeration rate is defined with the theory of mobility coalescence being invoked to explain the resistance increase of the films after deposition. Repeated deposition was employed until the film resistance became steady, after which measurements of resistance versus temperature were made. The agglomeration rate for film of a particular resistance decreased as the number of deposition cycles was increased, due to formation of large, less mobile islands. For one of the higher-resistance films, the agglomeration rate showed a drastic fall for a particular value of deposition cycle number indicating that large-scale coalescence (LSC) had occurred. It was found that the presence of a field does not alter the post-deposition coalescence process in island copper films, possibly due to the relative immobility of the copper islands. The initial resistances of the film with and without the field were, however, vastly different. The agglomeration rates of films of different initial resistances showed an oscillatory behaviour corroborating earlier findings on copper films. The resistance-temperature curves showed a minimum below which the film showed a negative temperature coefficient of resistance (TCR) and a positive TCR above the temperature of transition. The transition temperature is found to shift to higher values as the films approach the discontinuous-semi-continuous structure transition. The resistance-temperature behaviour is explained if one considers that the negative TCR region is due to an increase in the number of thermally activated charged islands while the positive TCR region is due to enhanced mobility of the copper islands at elevated temperatures leading to coalescence and thus to an increase in the average inter-island spacing and the resistance of the film.

Effect of applied field and temperature on the aging of copper discontinuous films studied by the repeated deposition technique

We have used the melt-quenching technique to prepare the bulk material and vapor-quenching technique to prepare the thin films of Bi93Sb7 alloy. The Bi93Sb7 alloy thin films of different thicknesses were grown onto well-cleaned glass and silicon substrates. The films were annealed at 150 ° C for 4 h in a vacuum of the order of 10−6torr in order to remove the defects and to increase the grain size. The bulk and thin-film x-ray diffraction results agree with the transmission electron microscopy results and the compositional analysis of bulk by particle-induced x-ray emission and of thin films by Rutherford backscattering. The thickness and temperature dependences of thermoelectric power and electrical resistivity have been analyzed. The negative temperature coefficient of resistivity confirmed that the material is semiconducting in nature. The negative thermoelectric power confirmed that the present bismuth-rich material is a n type. In this paper we have made an attempt to study the thermoelectric properti...

Study of structural-, compositional-, and thickness-dependent thermoelectric and electrical properties of Bi93Sb7 alloy thin films

Three hundred-nanometre thick films of n-InSe have been vacuum-deposited by flash evaporation on conducting indium oxide film coated clean glass substrates held at 373 K at 6.6 × 10−3 Pa pressure. These InSe films have been used as photoelectrodes in a photoelectrochemical (PEC) cell with graphite as the counter electrode, the saturated calomel electrode as the reference electrode and aqueous polyiodide electrolyte (with different pHs) as the redox electrolyte. PEC cells have been characterised by making I–V and C-V measurements under different experimental conditions, i.e., using the as-grown InSe films as photoelectrodes before and after surface treatment and after annealing and/or etching and for different pHs of the electrolyte. The optimum pH has been found to be about 6.2. It is also found that surface treatment and annealing leads to increase in the efficiency of the PEC solar cell. The open circuit voltage and photo current density are found to be of the order of 580 mV and 3000 μA/cm2, respectively under AM1 condition.

V. Damodara Das

Papers

Size and temperature dependence of electrical resistance and thermoelectric power of bi2te2se1 thin films

Band gap and intergrain barrier activation energies in Bi90Sb10 thin films

Effect of applied field and temperature on the aging of copper discontinuous films studied by the repeated deposition technique

Study of structural-, compositional-, and thickness-dependent thermoelectric and electrical properties of Bi93Sb7 alloy thin films

Effect of annealing and surface treatment on the efficiency of photoelectrochemical (PEC) solar cells with vacuum-deposited n-InSe thin film electrode