Papers by Keyword: Thermoelectrics

Abstract: Heusler alloys have been considered as one of the most promising thermoelectric materials for electrical power generation in a temperature range of 500–800 °C. Establishment of phase diagrams allows one to predict formation, equilibria, and stability of phases in of these ternary alloys. In this work we report on the simulation and investigation of phase diagram and phase equilibria in ternary Ti-Fe-Sb system which is of considerable interest for thermoelectric applications. The simulation was carried out using the CALPHAD method in Pandat software. The existence of the thermoelectric Heusler TiFe_1.5Sb phase was revealed in a temperature range from 970 to 1070 K. The equilibria between TiFe_1.5Sb and other phases were determined. The entropy of formation was calculated for the phases existing at 970, 1020 and 1070 K using a fitting approach. A narrow equilibrium region containing pure body centered cubic Fe and TiFe_1.5Sb was found.

Abstract: The recent revitalization of Ioffe plots (entropy conductivity versus electrical conductivity) reminds us that Isotan (Cu55Ni44Mn1) is an outstanding thermoelectric material with a power factor of up to 60 W cm^-1 K^-2 at a specific electrical conductivity of almost 20,000 S cm^-1 at elevated temperature. Even though, Isotan is widely used in thermoelements for temperature measurement, its high open-circuited thermal conductivity of approximately 70 W cm^-1 K^-2 [1] hindered further research as a promising thermoelectric material. Isotan was chosen as a starting composition. Influence of partial substitution of Cu and Ni with heavy elements (Sn,W) on the thermoelectric properties was studied. The alloys were fabricated by arc-melting and microstructurally characterized for grain size and elemental composition by scanning electron microscope (SEM) combined with energy-dispersive X-ray (EDXS). Lattice symmetry and parameters were estimated by X-ray diffraction (XRD). Functional properties as Seebeck coefficient, electrical conductivity and power factor were used to evaluate the thermoelectric performance.

Abstract: Skutterudites are an important class of thermoelectric p- and n-type materials and they have already achieved fair efficiencies for the conversion of heat to electricity. Nevertheless researchers try to further enhance the figure of merit, ZT, by various ways. In this work we study microstructure and mechanical properties of two thermoelectric materials: an industrial n-type (Mm,Sm)_yCo₄Sb₁₂ skutterudite and an industrial p-type DD_yFe₃CoSb₁₂ skutterudite, both mixed with 1 wt.% of Ta_0.8Zr_0.2B. Thin lamellae were prepared from the compacted materials using a focused ion beam. Analytical transmission electron microscopy was used on lamellae to study details of microstructure. A fine dispersion of precipitates was found both at nanograin boundaries and in their interiors. Quasistatic and dynamic nanoindentation tests were carried out on planar polished sections in the range of applied loads from 0.01 to 10 mN. The results were complemented with quantitative modulus mapping of local mechanical properties with 10-nm resolution.

Abstract: High-pressure studies on thermoelectric materials allow the study of the relationship between structural, elastic, and electronic properties. The High Pressure Science and Engineering Center (HiPSEC) at UNLV performs interdisciplinary research on a wide variety of materials at high pressures. One such system, CrSi₂ is an indirect band gap semiconductor that has potential applications in solar cells.

Abstract: We investigate thermoelectric characteristics of SiGe nanostructures for realizing high-sensitive infrared photodetector applications. In this paper, for future Ge and SiGe nanowires, we fabricate p-type Ge-on-insulator (GOI) substrates by a direct wafer bonding process. We discuss the annealing effect on the GOI substrate in the process and measure its Seebeck coefficient in the temperature range of 290-350 K. The Seebeck coefficient of the GOI layers is almost identical with the reported values for Ge. This result confirms that the measured Seebeck coefficient of GOI layers is not influenced by the buried oxide (BOX) layer and the Si substrate.

Abstract: The Seebeck coefficient of P-doped ultrathin Si-on-insulator (SOI) layers is investigated for the application to a highly-sensitive thermopile infrared photodetector. It is found that the Seebeck coefficient originating from the phonon drag is significant in the lightly doped region and depends on the carrier concentration with increasing carrier concentration above ~5×10¹⁸ cm^-3. On the basis of Seebeck coefficient calculations considering both electron and phonon distribution, the phonon-drag part of SOI Seebeck coefficient is mainly governed by the phonon transport, in which the phonon-phonon scattering process is dominant rather than the crystal boundary scattering even in the SOI layer with a thickness of 10 nm. This fact suggests that the phonon-drag Seebeck coefficient is influenced by the phonon modes different from the thermal conductivity.

Abstract: In order to increase the electrical conductivity greatly but maintain a large Seebeck coefficient and a low thermal conductivity simultaneously, the binary-phased LaCeFe₃CoSb₁₂-Sb nanocomposites composed of LaCeFe₃CoSb₁₂ skutterudite nanospheres and semimetal Sb microsized ribbons were fabricated via a hydro/solvo thermal route. The results suggest that the Sb powders result in a disordered structure during a hot-press process at its melting-point temperature and the disordered structure has been partly preserved into the room-temperature materials successfully. The Sb microsized ribbons enhance the electrical conductivity of the binary-phased materials largely, meanwhile the disordered structure increases the Seebeck coefficient obviously even though the thermal conductivity is also increased slightly. Consequently, the figure of merit of the binary-phased materials is improved significantly and the maximum value of 1.54 at 773 K has been realized for the LaCeFe₃CoSb₁₅ material.

Abstract: The SrTiO₃ is a ceramic with thermoelectric properties, i.e., it can generate electrical voltage between its terminals when subjected to a temperature gradient, and this behavior is known as Seebeck effect. One parameter for quantifying the efficiency of converting thermal energy into electric energy is through the value of the Figure of Merit (ZT). STO nanostructured powders were synthesized by ultrasonic irradiation (USS), and the effects of USS in STO synthesis were evaluate, and it was compared with the conventional route of solid-state reaction (SSR). USS powders were produced by solubilization of Ti (C₄H₉O)₄ and SrCO₃ in acid solution and following the posterior precipitation by adding NH₄OH solution. After drying process, the precipitated was added to a KOH solution and was subjected to ultrasonic irradiation to promote the STO formation. DRX analysis revealed that from USS method it was possible to obtain exclusive STO phase. SEM characterization showed that the USS powder presented particle size about 100nm, which was much lower than that obtained by SSR, about 250nm.

Abstract: Most of refrigerators commonly use the conventional refrigeration system known as Vapor Compression Refrigeration System becoming a big issue lately due to ozone depleting substance it uses as the refrigerant. This paper will shows step by step of an experiment with the objective of constructing a refrigeration system based on thermoelectric which is reliable and compete able with the Vapor Compression Refrigeration System. The designing of this refrigeration system shows attention to the environment that is combined with the knowledge so the environmental friendly technology can be applied. The performance of thermoelectric refrigerator was conducted under variation of input power (40W, 72W, and 120W) and operated in ambient temperature and cooling load of water 1000mL to investigate the characteristic of system, the performance, and also the COP. The COP is decrease by increasing of cooling load, QL. The best actual COP is 0.182 reached when the refrigerator operated at input power 40W. The result, it shows that decreasing of ambient temperature affects the decreasing of cabin temperature. Thermoelectric and heat pipe refrigerator cooling system can reach cabin temperature with power 120Watt (8.73A, 14V) produces temperature of compartment is 10.63°C indicates effective performance work-based thermoelectric applications.

Abstract: nfluence of physical boundary conditions on the thermoelectric transportation coefficients has been analyzed starting form Onsager equations. Four boundary conditions have been considered: electric short, i.e, the chemical potential difference is zero; electric open, or electric current free; isothermal, i.e., no temperature difference; adiabatic, or heat flux free. Four kinds of thermoelectric equations have been derived with different boundary conditions. It was found that the influence of boundary cannot be ignored when figure-of-merit is near and larger than 1.0. This results could be useful in designing thermoelectric device with high performance thermoelectric materials.