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

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

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

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

Power Generation in Slope-Type Thin-Film Thermoelectric Generators by the Simple Contact of a Heat Source

To conveniently generate electric energy for next-generation smart network monitoring systems, we propose the design and fabrication of slope-type thin-film thermoelectric generators by the simple contact of a heat source. N-type Bi2Te3 films and p-type Sb2Te3 films were formed on a stainless-steel substrate employing potentiostatic electrodeposition using a nitric acid-based bath, followed by a transfer process. In order to naturally induce a temperature difference (ΔT) between the ends of the generator, slope blocks made by polydimethylsiloxane (PDMS) were prepared and then inserted between the generators and heat sources. The performance of the generators, the open circuit voltage (Voc), and the maximum output power (Pmax), were measured using PDMS slope angles as the temperature of the heat source was increased. The ΔT of the generators increased as the slope angle was increased. The generator with the highest slope angle (28°) exhibited a Voc of 7.2 mV and Pmax of 18.3 μW at ΔT of 15 K for a heat source temperature of 42 °C. Our results demonstrate the feasibility of slope-type thin-film thermoelectric generators, which can be fabricated with a low manufacturing cost

Dual-Type Flexible-Film Thermoelectric Generators Using All-Carbon Nanotube Films

The long-term stability of n-type single-walled carbon nanotubes (SWCNTs) in air makes all-carbon thermoelectric generators (TEGs) viable. To increase the performance of TEGs, we developed a dual-type flexible-film thermoelectric generator (DFTEG). The vacuum filtering was used to form p- and n-type SWCNT films from ethanol-based dispersion and water-based solutions with cationic surfactant, respectively. DFTEGs were fabricated as follows: strip-shaped p- and n-type SWCNT films were attached on the top and back sides of a polyimide substrate, respectively, and were connected alternately in series by bending copper tapes on the edge of the polyimide substrate. The thermoelectric performance was measured after attaching the DFTEG outside a beaker full of water, where the water surface reached the center of the DFTEG. For a 10 mm long film and 15 p-n pairs, the DFTEG had an output voltage of 40 mV and a maximum power of 891 nW at a temperature difference of 25 K. The measured thermoelectric performance was significantly higher than that of the single-type TEG for almost the same SWCNT films. This result demonstrates that thermoelectric performance can be improved by using DFTEGs that are fabricated with optimum structural designs