Influence of annealing on the distribution of thermoelectric figure of merit in bismuth-telluride ingots

The p-type (Bi_0.25Sb_0.75)₂Te₃ doped with 8 wt% excess Te alone and n-type Bi₂(Te_0.94Se_0.06)₃ codoped with 0.017 wt% Te and 0.068 wt% I were grown by the Bridgman method and were cut into a parallelepiped of 5 × 5 × 15 mm³, where the length of 15 mm is parallel to the freezing direction. The specimen is mounted on an X-Y stage and the temperature difference between two probes set at an interval of 1 mm was approximately 2.6 K. The local Seebeck coefficient α_ℓ and local electrical resistivity ρ_ℓ along the freezing direction were measured at a scan step of 1 mm before and after annealing at 673 K for 2 h in vacuum. The local thermal conductivity κ_ℓ was calculated from ρ_ℓ using the relation between κ and ρ obtained previously for a series of bismuth-telluride compounds. As a result, α_ℓ ρ_ℓ and κ_ℓ before and after annealing changed significantly from place to place, so that the effect of annealing on the local thermoelectric figure of merit (ZT)_ℓ was never uniform throughout the specimen surface. The maximum (ZT)_ℓ of the as-grown p- and n-type specimens reached surprisingly great values of 1.88 and 1.59 at 298 K, respectively, which correspond to about twice as large as those observed macroscopically by the conventional Seebeck coefficient apparatus with an interval (between two probes) of 8 mm. Probably, these maximum values of (ZT)_ℓ would be an upper limit of ZT for the p- and n-type bismuth- telluride bulk compounds, at least in the present fabrication method.     

Enhanced thermoelectric figure of merit of p-type half-Heuslers

Half-Heuslers would be important thermoelectric materials due to their high temperature stability and abundance if their dimensionless thermoelectric figure of merit (ZT) could be made high enough. The highest peak ZT of a p-type half-Heusler has been so far reported about 0.5 due to the high thermal conductivity. Through a nanocomposite approach using ball milling and hot pressing, we have achieved a peak ZT of 0.8 at 700 °C, which is about 60% higher than the best reported 0.5 and might be good enough for consideration for waste heat recovery in car exhaust systems. The improvement comes from a simultaneous increase in Seebeck coefficient and a significant decrease in thermal conductivity due to nanostructures. The samples were made by first forming alloyed ingots using arc melting and then creating nanopowders by ball milling the ingots and finally obtaining dense bulk by hot pressing. Further improvement in ZT is expected when average grain sizes are made smaller than 100 nm.     

Ultra-low lattice thermal conductivity and promising thermoelectric figure of merit in borophene via chlorination

Monolayer boron-based materials are of current interests due to its polymorphism.Herein,motivated by the recent experimental synthesis of semiconducting hydroge...     

Laser synthesis of copper oxides 2D structures with high Seebeck coefficient and high thermoelectric figure of merit

Semiconductors are considered promising materials of thermo-converters and thermo-sensors. The photons generated by a KrF laser source (λ?=?248 nm, τ_FWHM?≤?25 ns) were used for the synthesis of stable crystalline phases of copper oxides 2D single-layered semiconductor structures via the reaction of ablated copper atoms with oxygen molecules by reactive pulsed laser deposition (RPLD). Obtained semiconductor 2D single-layered structures of (23–75) nm thickness were deposited on 293, 600 or 800 K?<?100?>?Si substrates in oxygen atmosphere at 1.0, 3.0 and 5.0 Pa. X-ray diffraction analysis evidenced polycrystalline structures of the deposits reviled of two crystalline semiconductor phases CuO (002) and CuO(111). Semiconductor temperature trend was detected with variable energy band gap (E_g) in the range of (0.10–1.5) eV depending on substrate temperature, oxygen pressure and structure thickness. The optimum conditions were found out when the S coefficient was being homogeneously increased from 2.0 mV/K up to 10.5 mV/K and thermoelectric figure of merit (ZT) from 0.0035 up to 9.0 in the range of (290–340) K. The interpretation of behaviour for these 2D single-layered semiconductor structures of thermoelectric properties was provided. Obtained 2D single-layered structures based on copper oxides with such high S coefficient and high ZT are exceptionally strong candidates for a new effective thermo-sensors and thermo-converters operating at moderate temperature. Moreover, RPLD serves as an up to date method for the synthesis of 2D structures with such superior thermo-sensor and thermo-converter properties. These 2D single-layered structures based on copper oxides are among the most promising candidates based on non-toxic precursors for “green” technology fabrication of efficient thermo-sensors and thermo-converters.

     

Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4-ethylenedioxythiophene)

Thermoelectric generators (TEGs) transform a heat flow into electricity. Thermoelectric materials are being investigated for electricity production from waste heat (co-generation) and natural heat sources. For temperatures below 200 °C, the best commercially available inorganic semiconductors are bismuth telluride (Bi(2)Te(3))-based alloys, which possess a figure of merit ZT close to one. Most of the recently discovered thermoelectric materials with ZT>2 exhibit one common property, namely their low lattice thermal conductivities. Nevertheless, a high ZT value is not enough to create a viable technology platform for energy harvesting. To generate electricity from large volumes of warm fluids, heat exchangers must be functionalized with TEGs. This requires thermoelectric materials that are readily synthesized, air stable, environmentally friendly and solution processable to create patterns on large areas. Here we show that conducting polymers might be capable of meeting these demands. The accurate control of the oxidation level in poly(3,4-ethylenedioxythiophene) (PEDOT) combined with its low intrinsic thermal conductivity (λ=0.37 W m(-1) K(-1)) yields a ZT=0.25 at room temperature that approaches the values required for efficient devices.     

Enhancement of the thermoelectric figure of merit in p- and n-type Cu/Bi-Te/Cu composites

The resultant thermoelectric properties of welded Cu/Bi-Te/Cu composites were measured at 298 K as a function of relative thickness x of Bi-Te compound by changing the interval s between two thermocouples and compared with those calculated as a function of x by treating it as an electrical and thermal circuit. These composites were prepared by welding with eutectic solder of Pb-Sn, after one end surface of the as-grown p- and n-type Bi-Te ingots were plated with Ni. It was found that the observed ZT of composites has a local maximum at an optimum x even when s was changed, as in the case of Cu/Bi-Sb/Cu and Ni/Bi-Sb/Ni composites with various relative thicknesses. Appearance of a local maximum in ZT is owing to the barrier thermo-emf generated by a sharp temperature drop at the interface between Bi-Te compound and copper. It may be caused by the separation of non-equilibrium carriers at the interface between them. The observed maximum ZT values at 298 K of the p- and n-type composites reached surprisingly great values of 1.53 and 1.66 at x=0.98, which correspond to about twice as large as those of commercially utilized Bi-Te compounds. This enhancement of ZT is available for generators, but may be not utilizable as a Peltier module. The composite materials were thus found to be utilizable as useful means of further increase in ZT of macroscopic bulk materials.     

High-performance and highly stable 0.3-nm-full-width-at-half-maximum interference optical filters

Fabrication of very-narrow-bandpass optical tunable filters [(0.3 nm full width at half-maximum) (FWHM)] is reported. To improve the film densities, the O(2) ion-assisted deposition-method is used in the fabrication. In the succession of high- and low-refractive-index layers, the commonly used TiO(2) material is replaced by Ta(2)O(5), which suits the ion-assisted fabrication technique. The relative thicknesses of the filter multilayer structure of 1/2/1 are modified to 0.998/2.007/0.998, which reduces the shift difference in the central wavelengths with regard to the p and s polarizations when the filter is tilted. These improvements enabled fabrication of 0.3-nm-FWHM optical tunable filters with improved stability characteristics.     

Integrated figure of merit of public sector research evaluation

An approach for evaluation of research is described that integrates output indicators of four stages downstream the innovation process: immediate, intermediate, pre- ultimate and ultimate outputs. Indexes of leading output indicators are constructed. The indexes are integrated cumulatively to form an overall index of key output indicators, which is the integrated figure of merit (IFM). Data for the indicators are obtained from records and key informants, and the indicators are grouped by normalized weights. The paper also discusses the limitations and the methodological, conceptual and political/organizational issues of such an approach to research evaluation.     

High-performance flat-panel solar thermoelectric generators with high thermal concentration

The conversion of sunlight into electricity has been dominated by photovoltaic and solar thermal power generation. Photovoltaic cells are deployed widely, mostly as flat panels, whereas solar thermal electricity generation relying on optical concentrators and mechanical heat engines is only seen in large-scale power plants. Here we demonstrate a promising flat-panel solar thermal to electric power conversion technology based on the Seebeck effect and high thermal concentration, thus enabling wider applications. The developed solar thermoelectric generators (STEGs) achieved a peak efficiency of 4.6% under AM1.5G (1 kW m(-2)) conditions. The efficiency is 7-8 times higher than the previously reported best value for a flat-panel STEG, and is enabled by the use of high-performance nanostructured thermoelectric materials and spectrally-selective solar absorbers in an innovative design that exploits high thermal concentration in an evacuated environment. Our work opens up a promising new approach which has the potential to achieve cost-effective conversion of solar energy into electricity.     

A figure of merit for optimization of nanocrystal flash memory design

Nanocrystals can be used as storage media for carriers in flash memories. The performance of a nanocrystal flash memory depends critically on the choice of nanocrystal size and density as well as on the choice of tunnel dielectric properties. The performance of a nanocrystal memory device can be expressed in terms of write/erase speed, carrier retention time and cycling durability. We present a model that describes the charge/discharge dynamics of nanocrystal flash memories and calculate the effect of nanocrystal, gate, tunnel dielectric and substrate properties on device performance. The model assumes charge storage in quantized energy levels of nanocrystals. Effect of temperature is included implicitly in the model through perturbation of the substrate minority carrier concentration and Fermi level. Because a large number of variables affect these performance measures, in order to compare various designs, a figure of merit that measures the device performance in terms of design parameters is defined as a function of write/erase/discharge times which are calculated using the theoretical model. The effects of nanocrystal size and density, gate work function, substrate doping, control and tunnel dielectric properties and device geometry on the device performance are evaluated through the figure of merit. Experimental data showing agreement of the theoretical model with the measurement results are presented for devices that has PECVD grown germanium nanocrystals as the storage media.     

Enhancement of thermoelectric figure of merit by doping Dy in La0.1Sr0.9TiO3 ceramic

Ceramic samples of La_0.1Sr_0.9−xDy_xTiO₃ (x = 0.01, 0.03, 0.07, 0.10) have been prepared by the solid-state reaction method. Characterization from the powder X-ray diffraction indicates that their crystal structure changes from cubic to tetragonal phase. Their electrical and thermal transport properties are measured in the temperature range of 300-1100 K. n-Type thermoelectric is obtained with large Seebeck coefficient. The figure of merit is markedly improved, due to relatively lower electrical resistivity and thermal conductivity by Dy doping effect. A much lower electrical resistivity of 0.8 mΩ cm at room temperature is obtained in La_0.1Sr_0.8Dy_0.1TiO₃, and with a relatively lower thermal conductivity of 2.5 W/m K at 1075 K. The maximum figure of merit reaches ∼0.36 at 1045 K for La_0.1Sr_0.83Dy_0.07TiO₃, which is the largest value among n-type oxide thermoelectric ceramics.     

Modified thermoelectric figure of merit estimated from enhanced mobility of [100] oriented beta-FeSi<Subscript>2</Subscript> thin film

The modification of thermoelectric figure of merit was estimated from enhanced mobility of [100] oriented beta-FeSi₂ film. beta-FeSi₂ on Si(001) substrate was prepared by molecular beam epitaxy method using an Fe source. The crystallographic orientation of beta-FeSi₂ film on Si(001) substrate was characterized by using X-ray diffraction. Using scanning electron microscopy, surface morphology and film thickness of samples were observed and estimated, respectively. The mobility of beta-FeS₂ film on Si(001) substrate were also characterized by Hall measurement at room temperature. A part of the enhancement of figure of merit was evaluated as the functions of mobility and crystallographic orientation of samples.     

A figure of merit for overall performance and value of AFDD tools

A figure of merit, V, to characterize the value of an AFDD maintenance tool is presented. The AFDD tool is fed data representing air-conditioners operating with and without faults across a range of conditions. The calculation of V considers the probability of each scenario occurring, the tool's response, and the resulting implications. These results are summed to give a typical average value for deploying the AFDD tool, as compared to maintenance being performed without AFDD. Case studies illustrate the calculation of V. Six AFDD tools and two idealized tools are evaluated. Five of the six real tools give negative V, meaning that their use causes more harm than good. The sixth shows approximately $10 benefit per nominal ton of air-conditioner capacity per typical maintenance visit. This represents about half of the maximum potential value. These results demonstrate the importance of measuring AFDD performance and the potential monetary benefits of AFDD.     

Surface morphology and thermal figure of merit of a new compound thin film

The third nonlinear optical properties of a new compound 4,4′-bis(3-methoxy benzylidene amino) biphenyl doped poly-methyl methacrylate (PMMA) have been studied using Z-scan technique. Experiments are performed using a continuous waveguide (cw) diode laser at 532 nm wavelength and 0.68 kW/cm² laser intensity. The optical power limiting behavior of sample doped PMMA was also investigated. It also shows a very good optical limiting behavior with a limiting threshold of 4.7 mW. We attribute the nonlinear absorption and optical limiting property of the sample film to two photon absorption effect at 532 nm. The experimental evidences of observing diffraction pattern in compound 4,4′-bis(3-methoxybenzylideneamino) biphenyl doped PMMA has been present. The refractive index change, Δn, and nonlinear refractive index, n ₂ determined from the number of observed ring. We obtained good values of Δn = 105.154 × 10^?4and n ₂ = 154.154 × 10^?7 cm²/W. Variation of refractive index with temperature, dn/dT, and figure of merit, H, are found to be 8.858 × 10^?6 1/°C and 5.316 × 10^?6, respectively. This large nonlinearity is attributed to a thermal effect resulting from linear absorption. Theoretical diffraction pattern that agree well with experimental one are generated using a wave theory.     

Solid-state precipitation of stable and metastable layered compounds in thermoelectric AgSbTe<Subscript>2</Subscript>

The precipitation of Sb₂Te₃ in Sb-rich AgSbTe₂ is studied by X-ray diffraction and electron microscopy. The results indicate that Sb₂Te₃ does not form directly, but rather through the precipitation of an intermediate metastable phase. Diffraction, energy-dispersive spectroscopy, and high-resolution transmission electron microscopy indicate that this intermediate phase has a nominal composition (Ag,Sb)₃Te₄ and a structure with a seven-layer stacking sequence rather than a five-layer one as in Sb₂Te₃. Two mechanisms based on experimental observations are proposed for the conversion of (Sb,Ag)₃Te₄ to Sb₂Te₃: evaporation–condensation and individual step motion. The microstructural evolution and mechanisms of the transformation are discussed in detail.