Thermoelectric properties of n-type nanocrystalline bismuth-telluride-based thin films deposited by flash evaporation

The thermal conductivity of n-type nanocrystalline bismuth-telluride-based thin films (Bi2.0Te2.7Se0.3) is investigated by a differential 3 method at room temperature. The nanocrystalline thin films are grown on a glass substrate by a flash evaporation method, followed by hydrogen annealing at 250 °C. The structure of the thin films is studied by means of atomic force microscopy, x-ray diffraction, and energy-dispersive x-ray spectroscopy. The thin films exhibit an average grain size of 60 nm and a cross-plane thermal conductivity of 0.8 W/m K. The in-plane electrical conductivity and in-plane Seebeck coefficient are also investigated. Assuming that the in-plane thermal conductivity of the thin films is identical to that of the cross-plane direction, the in-plane figure of merit of the thin films is estimated to be ZT=0.7. As compared with a sintered bulk sample with average grain size of 30 µm and nearly the same composition as the thin films, the nanocrystalline thin films show approximately a 50% reduction in the thermal conductivity, but the electrical conductivity also falls 40%. The reduced thermal and electrical conductivities are attributed to increased carrier trapping and scattering in the nanocrystalline film

Structure and thermoelectric properties of boron doped nanocrystalline Si0.8Ge0.2 thin film

The structure and thermoelectric properties of boron doped nanocrystalline Si0.8Ge0.2 thin films are investigated for potential application in microthermoelectric devices. Nanocrystalline Si0.8Ge0.2 thin films are grown by low-pressure chemical vapor deposition on a sandwich of Si3N4/SiO2/Si3N4 films deposited on a Si (100) substrate. The Si0.8Ge0.2 film is doped with boron by ion implantation. The structure of the thin film is studied by means of atomic force microscopy, x-ray diffraction, and transmission electron microscopy. It is found that the film has column-shaped crystal grains ~100 nm in diameter oriented along the thickness of the film. The electrical conductivity and Seebeck coefficient are measured in the temperature range between 80–300 and 130–300 K, respectively. The thermal conductivity is measured at room temperature by a 3 method. As compared with bulk silicon-germanium and microcrystalline film alloys of nearly the same Si/Ge ratio and doping concentrations, the Si0.8Ge0.2 nanocrystalline film exhibits a twofold reduction in the thermal conductivitity, an enhancement in the Seebeck coefficient, and a reduction in the electrical conductivity. Enhanced heat carrier scattering due to the nanocrystalline structure of the films and a combined effect of boron segregation and carrier trapping at grain boundaries are believed to be responsible for the measured reductions in the thermal and electrical conductivities, respectively

Effect of grain size on thermoelectric properties of n-type nanocrystalline bismuth-telluride based thin films

The effect of grain size on the thermoelectric properties of n-type nanocrystalline bismuth-telluridebased thin films is investigated. We prepare the nanocrystalline thin films with average grain sizesof 10, 27, and 60 nm by a flash-evaporation method followed by a hydrogen annealing process. Thethermoelectric properties, in terms of the thermal conductivity by a differential 3 method, theelectrical conductivity, and the Seebeck coefficient are measured at room temperature and used toevaluate the figure of merit. The minimum thermal conductivity is 0.61 W m−1 K−1 at the averagegrain size of 10 nm. We also estimate the lattice thermal conductivity of the nanocrystalline thinfilms and compare it with a simplified theory of phonon scattering on grain boundaries. Fornanosized grains, the lattice thermal conductivity of nanocrystalline thin films decreases rapidly forsmaller grains, corresponding to the theoretical calculation. The figure of merit is also decreased asthe grain size decreases, which is attributed to the increased number of defects at the grainboundaries

Strong enhancement of phonon scattering through nanoscale grains in lead sulfide thermoelectrics

We present nanocrystalline PbS, which was prepared using a solvothermal method followed by spark plasma sintering, as a promising thermoelectric material. The effects of grains with different length scales on phonon scattering of PbS samples, and therefore on the thermal conductivity of these samples, were studied using transmission electron microscopy and theoretical calculations. We found that a high density of nanoscale grain boundaries dramatically lowered the thermal conductivity by effectively scattering long-wavelength phonons. The thermal conductivity at room temperature was reduced from 2.5 W m1K 1 for ingot-PbS (grain size 4200 lm) to 2.3 W m1K 1 for micro-PbS (grain size 40.4 lm); remarkably, thermal conductivity was reduced to 0.85 W m1 K 1 for nano-PbS (grain size B30 nm). Considering the full phonon spectrum of the material, a theoretical model based on a combination of first-principles calculations and semiempirical phonon scattering rates was proposed to explain this effective enhancement. The results show that the high density of nanoscale grains could cause effective phonon scattering of almost 61%. These findings shed light on developing high-performance thermoelectrics via nanograins at the intermediate temperature range.This contribution was supported primarily by the startup of the South University of Science and Technology of China, supported by the Shenzhen government, and the national 1000 plan for young scientists. This work was also partially supported by a grant-in-aid of ‘985 Project’ from Xi’an Jiaotong University, the National Natural Science Foundation of China (Grant No. 21201138 and 11204228), the National Basic Research Program of China (2012CB619402 and 2014CB644003) and the Fundamental Research Funds for the Central UniversitiesS

Structural and thermoelectric properties of fine-grained Bi0.4Te3.0Sb1.6 thin films with preferred orientation deposited by flash evaporation method

Structural and thermoelectric properties of p-type fine-grained Bi0.4Te3.0Sb1.6 thin filmsare investigated. The films are deposited by a flash evaporation method and exhibit apreferred orientation in the c-axis direction. By optimizing deposition conditions, weachieve thin films with clean surfaces. Then, in order to enhance the crystallinity withpreferred orientation and the thermoelectric properties of the thin films, they are annealedin hydrogen ambient at atmospheric pressure and temperatures ranging from 200 to 350 oC.The cross-sectional microstructure and crystallinity of the thin films are investigated byscanning electron microscopy and x-ray diffraction, respectively. The electricalconductivity, Seebeck coefficient, and thermoelectric power factor are measured at roomtemperature. We confirm that the grain growth and the crystallization along the c-axis areenhanced as the annealing temperature increases. The highest performance of p-typeBi0.4Te3.0Sb1.6 thin films observed in this study have an annealing temperature of 300 oC,resulting in a thermoelectric power factor of 34.9μW cm-1 K-2 at the average grain size of88 nm. We consider that the synthesis conditions reduce the number of potentialscattering sites at grain boundaries and defects, thus improving the thermoelectric powerfactor

Fabrication and characterization of bismuth-telluride-based alloy thin film thermoelectric generators by a flash evaporation method

Bismuth-telluride-based alloy thin film thermoelectric generators are fabricated by a flash evaporation method. We prepare Bi0.4Te3.0Sb1.6 (p-type) and Bi2.0Te2.7Se0.3 (n-type) powders for the fabrication of the flash evaporated thin films. The overall size of the thin film thermoelectric generators, which consist of 7 pairs of legs connected by aluminum electrodes, is 20mm by 15mm. Each leg is 15mm long, 1mm wide and 1μm thick. We measure the output voltage and estimate the maximum output power near room temperature as a function of the temperature difference between hot and cold junctions of the thin film thermoelectric generators. In order to improve the performance of the generators, a hydrogen annealing process is carried out at several temperatures from 25 oC to 250 oC. The highest output voltage of 83.3 mV and estimated output power of 0.21 μW are obtained from a hydrogen annealing temperature of Ta =250 oC and a temperature difference of ΔT = 30K. The hydrogen annealing temperature of Ta = 250 oC also results in the best electrical performance for both p-type thin film (Seebeck coefficient = 254.4 μV/K, resistivity = 4.1 mΩ cm, power factor = 15.9 μW/cm K2) and n-type thin film (-179.3 μV/K, 1.5 mΩ cm, 21.5 μW/cm K2)

Preparation and characterization of Bi0.4Te3.0Sb1.6 nanoparticles and their thin films

In this article, we perform a preliminary study for assembling micro-size thermoelectric devices with low-cost fabrication by a preparation of nanoparticles and their thin films. Bi0.4Te3.0Sb1.6 nanoparticles with an average size of approximately 50 nm are fabricated by a beads-milling method. The nanoparticle solution is prepared by mixing with toluene and a surfactant, and thin films with 1 μm thick are deposited on Al2O3 substrates by a printing method. The thin films are sintered at temperatures ranging from 300 to 500 oC for 60 minutes in hydrogen ambient. We investigate the thin film structures and the thermoelectric properties at room temperature. As the sintering temperature increases, hexagonal-shaped crystals are grown on the film surface while the atomic composition is almost constant throughout all the sintering temperatures. The XRD patterns indicate that all the nanoparticle thin films are found to mostly exhibit the same XRD intensities and have multiple peaks correspond to each other. The in-plane electrical conductivity of the thin films increases but the Seebeck coefficient decreases as the sintering temperature increases. As a result, the best performance of the thermoelectric power factor of 1.3 μW/(cm K2) is achieved at a sintering temperature of 350 oC