Thermoelectric Power Generation Applications Research Articles

Enhancement of thermoelectric properties of sulphurized CZTS nano-crystals by the engineering of secondary phases

In this manuscript, we have explored the potential of Copper Zinc Tin Sulfide (CZTS) nano-crystals for thermoelectric power generation application by the engineering of secondary phases. CZTS nano-crystals were grown on various substrates (crystalline silicon (c-Si), multi-crystalline silicon (mc-Si), Soda Lime Glass (SLG) and Indium Tin Oxide (ITO) coated glass) using sol-gel solution technique. The grown samples were sulphurized in tube furnace at 500C for 20 min under vacuum. It was observed that sample grown on mc-Si has highest Seebeck coefficient of 667 μV/0C while sample grown on ITO substrate has the lowest value of Seebeck coefficient (70 μV/0C) at measurement temperature 100 °C. The reported high Seebeck coefficient of CZTS/mc-Si sample is related to the emergence of secondary phases. To verify this argument, we have performed XRD, Raman spectroscopy and UV/Visible measurements. XRD data revealed the presence of (112), (103) and (220) phases of CZTS in all samples but CZTS/mc-Si and CZTS/c-Si samples demonstrated additional phases related to secondary phases. Raman spectroscopy was also used to verify the presence of secondary phases in CZTS/mc-Si. Raman measurements showed that all samples consisted of CZTS vibration mode at 329 cm−1 but CZTS/mc-Si and CZTS/c-Si have additional secondary phases due to CuxS, CuSnS and SnxS vibration modes. UV–Vis data also verified our argument that Seebeck coefficient was enhanced due to secondary phases engineering using various substrates for CZTS growth.

Optimizing the electrical transport properties of ZnSnO thin films by post growth annealing in air

In this manuscript, we have successfully enhanced the thermoelectric properties of ZnSnO thin films by post growth annealing method. Thin films of ZnSnO were grown on Si substrate by evaporating the Zinc and Tin metal powders in the tube furnace. After growth, post growth annealing was performed in open environment using a temperature range of 600−800 °C. The XRD results confirmed the formation of hexagonal closed packed structure of ZnSnO and the crystanality was found to be improved with post growth annealing temperature. The vibrational mode of ZTO at 435 cm−1 was verified by Raman spectroscopy measurements along with other Raman active modes related to single crystal Si and SnO2 as well. Seebeck data demonstrated that the values of Seebeck coefficient were increased from 204 to 1032 μV/°C as the annealing temperature increased from 600 to 800 °C. This increasing behavior of Seebeck coefficient was related with the mobility enhancement of the charge carriers by post growth annealing process. The Hall data supported our argument that mobility of carrier increased with annealing temperature due to high thermal energy. The giant enhancement of Seebeck coefficient and power factor values makes this material a potential candidate for thermoelectric power generation applications in near future.

Synergetic Approach for Superior Thermoelectric Performance in PbTe-PbSe-PbS Quaternary Alloys and Composites

Thermoelectric power generation is an energy conversion technology from heat to electric energy, which can be applied to waste heat power conversion. Among thermoelectric materials (TE), PbTe-PbSe-PbS quaternary alloys and composites are promising candidates for thermoelectric power generation applications in the mid-temperature operating range from 500 to ~850 K. Besides, the thermoelectric performance of quaternary alloys and composites is not fully optimized regarding its composition and synthesis process. In the quaternary system, PbTe-PbSe-PbS, it was found that PbS will form nanoprecipitation in the matrix of quaternary alloy for a small content of PbS (≤0.07), which reduces the lattice thermal conductivity. The power factor of PbTe-PbSe-PbS quaternary alloys can be significantly enhanced by using a band convergence in PbTe1−xSex. The band structure modifications, with the result of simultaneous PbS nanoprecipitation, give rise to a high Z T value of 2.3 at 800 K for (PbTe)0.95−x(PbSe)x(PbS)0.05. The chemical potential tuning by effective K-doping ( x = 0.02) and PbS substitution reveals a high power factor and low thermal conductivity, resulting in a comparatively high Z T value of 1.72 at 800 K. The combination of a high Seebeck coefficient and low thermal conductivity results in a very high Z T value of 1.52 at 700 K as n-type materials for low Cl-doped ( x = 0.0005) (PbTe0.93−xSe0.07Clx)0.93(PbS)0.07 composites. Therefore, this review presents the simultaneous emergence of effective chemical potential tuning, band convergence, and nanoprecipitation, giving rise to a significant enhancement of the thermoelectric performance of both p - and n -type PbTe-PbSe-PbS quaternary alloy and composite TE materials.

Ultrahigh thermoelectric performance of 2H–MoS2 nanosheets with incorporated conducting secondary phase

The semiconducting molybdenum disulfide (MoS2) is a promising candidate for the thermoelectric power generation applications because of its inherently low thermal conductivity. However, the low electrical conductivity of 2H–MoS2 limits its practical usage. In this report, we have synthesized in situ MoS2 nanostructures with conducting secondary phase (Mo2S3) by hydrothermal method and demonstrated an ultrahigh enhancement in the figure–of–merit of MoS2. A ∼70 times enhancement in the electrical conductivity is observed for the secondary phase (Mo2S3) incorporated MoS2 sample. The presence of optimum barrier height (72 meV) for Mo2S3 incorporated MoS2 is attributed to the enhancement in the Seebeck coefficient due to low energy carrier filtering. The reduction in thermal conductivity of MoS2 in the presence of Mo2S3 is due to more phonon scattering at the interfaces in Mo2S3–MoS2 nanocomposite sample.

Geometric structural design for lead tellurium thermoelectric power generation application

High thermoelectric figure of merits have been achieved in lead telluride (PbTe) alloys recently, which are beneficial for future thermoelectric applications. In this study, a PbTe-based thermoelectric module is constructed by eight pairs of p-type PbTe–8%SrTe and n-type PbTe0.998I0.002–3%Sb legs with initial sizes of 3 × 3 × 3 mm3. The relationship between multiple geometric parameters and performance of module has been studied by finite-element simulation. The maximum output power (Pmax) of 7.6 W and efficiency (ηmax) of 15.3% have been achieved at ΔT = 500 K for ideal contacted PbTe-based thermoelectric module. Contact resistance must exist in experimental thermoelectric module, so it has been considered in simulation about geometry optimization. The cross-sectional area ratio (Ap/An) and height (H) show strong dependent relationship to optimize module performance. The larger Ap/An and relatively lower H are propitious to enhance the output power. Moreover, the efficiency is improved at an optimum Ap/An and relatively higher H. In this PbTe-based module, the Pmax = 4 W is achieved at Ap/An = 1.78 and H = 0.65 mm, while ηmax = 14% is achieved at Ap/An = 1.64 and H = 13 mm under ΔT = 500 K. The Pmax and ηmax are 20% and 65% larger than those of initial design sizes. Thus, geometric structure modification is a good way to improve the performance of thermoelectric applications.

Effect of secondary phases on the thermoelectric properties of Zn2GeO4 nano-crystals grown by thermal evaporation on Au coated Si substrate

This study reported the growth of Zn2GeO4 nano-crystals on Au coated Si substrate by thermal evaporation technique using vacuum tube furnace for thermoelectric power generation applications. The thermoelectric properties of nano-crystals were improved by generating the secondary phases in the grown samples using the post-growth annealing at various temperatures (600–900 °C). The low energy carriers filtered out at the interface of secondary phases and caused an enhancement in Seebeck coefficient and power factor. The emergence of secondary phases with annealing temperature was confirmed by various characterization techniques. XRD data suggested that samples annealed at 800 and 900 °C consisted of three extra secondary phases along with the phases related to Zn2GeO4, ZnO, Si and Au. Raman spectroscopy data demonstrated the presence of strong peak due to Si substrate for all samples but two weak peaks appeared at 375 and 800 cm−1 for samples annealed at 800 and 900 °C. The SEM images shows the presence of randomly oriented nano structures as annealing temperature increase up to 900 °C. The Hall measurements were also performed to strengthened our argument.

Rapid preparation of Ge0.9Sb0.1Te1+x via unique melt spinning: Hierarchical microstructure and improved thermoelectric performance

Thermoelectric semiconductors based on GeTe has been proved to be an important materials for converting waste heat into electricity. In this study, we demonstrate an enhancement of thermoelectric performance for p-type Ge0.9Sb0.1Te1+x by a unique melt spinning method combined with hot pressing (MS-HP), through which increased Seebeck coefficients and reduced thermal conductivities are achieved simultaneously. Moreover, the MS technique greatly gives rise to a variety of microstructures, such as the fine Ge precipitates, twins and nano-domains, all of which contribute to the significantly reduced thermal conductivities. Finally, the Ge0.9Sb0.1Te1.03 reaches a maximum zT value of ∼1.9 at 760 K and a giant average zT of ∼1.2 between 300 K and 820 K, which is comparable to the reported results for GeTe based materials. Combined with the rapid fabrication, Ge0.9Sb0.1Te1+x prepared by melt spinning process is an attractive p-type material for thermoelectric power generation application.

Growth of Zn2GeO4 thin film by thermal evaporation on ITO substrate for thermoelectric power generation applications

In this paper, we have reported the growth of Zn2GeO4 thin film and investigated its potential for thermoelectric power generation applications. Zn2GeO4 alloy thin film was grown on Indium coated glass substrate by the evaporation of Zn and Ge metals with constant oxygen gas flow rate of 100 sccm in tube furnace. The grown film was cut into pieces and annealed at various temperatures from 500° to 700°C with a step of 100 °C in a programmable furnace for one hour. The structure of as grown and annealed thin films was verified by XRD and Raman spectroscopy measurements. The XRD data evident that Zn2GeO4 alloy hexagonal structure along with GeO2 and ZnO phases were observed at annealing temperatures 600 and 700 °C but below this temperature no alloy phase was detected by XRD and Raman Spectroscopy. To calculate the thermoelectric properties, temperature dependent Seebeck measurements were performed in the temperature range of 25–100 °C. It was observed that the value of Seebeck coefficient was increased from 91 to 847 μV/K as the annealing temperature increases from 500° to 700°C. This behavior was explained as; high temperature causes stress and cracks in the grown films which may induce electric and thermal discontinues at tips of cracks which cause high thermoelectric concentration. Scanning electron microscope images verified the development of cracks in the samples as annealing temperature increases. The behavior of Seebeck coefficient with the measurement temperature was also observed and explained in detail. The high value of Seebeck coefficient suggested that this material can be a potential candidate for thermoelectric power generation applications in near future.

Thermoelectric properties and thermal stress simulation of pressureless sintered SiC/AlN ceramic composites at high temperatures

This paper provides a detailed study of the thermoelectric properties of SiC/AlN composites in a wide temperature range. Evaluation of thermal stress durability of the composites was simulated and analyzed. SiC/AlN composites with 0–30 wt% of AlN were produced by the pressureless sintering method. Phase analysis, densification, and microstructure of the obtained composites were inspected. SiC/AlN ceramics had a porous structure with porosity ranging from 23.77% to 31.74%. Complete 2Hss SiC/AlN solid solution was formed and its content increased with the increment of AlN wt%. Addition of AlN to SiC ceramics gave a harmonic structure with elongated grains morphology. Electrical resistivity and thermoelectric properties of the investigated ceramics were evaluated in the temperature range of 25–1000 K. The results were essentially dependent on AlN content, microstructure and temperature. SiC/AlN composites gave identical behavior of the p-type semiconductors with outstanding thermoelectric properties at high temperatures. The lowest electrical resistivity of 1.5 × 105 µOhm × cm and the highest Seebeck coefficient of 370 µ V/K were recorded for 30% AlN composite at 1000 K. Figure of merit (ZT) increased dramatically with increasing the AlN content. 30% AlN composites recorded ZT value 130 times larger than that of the 0% AlN composite. Thermoelectric power factor was markedly enhanced by increasing AlN wt%. Thermal stress analysis of the different ceramics was simulated by finite element method up to a temperature of 1773 K. SiC/AlN composites gave splendid thermal stress durability with uniform transition and distribution of heat. SiC/AlN composites are strongly nominated to be used effectively in thermoelectric power generation and high-temperature applications.

Record high thermoelectric performance in bulk SrTiO3 via nano-scale modulation doping

Strontium titanite (SrTiO3), which is an experimentally-friendly thermoelectric material, could be a promising candidate for thermoelectric power generation applications. The theorectical study indicates the co-doping of La and Nb could enhance the thermoelectric performance, however, the thermoelectric figure of merits (ZTs) of SrTiO3 are still low because the co-doping process at nano-scale is experimentally difficult to control. Here we report a high performance SrTiO3 with La-Nb co-doping, which are prepared by a combination of hydrothermal method and high-efficiency sintering. Nano-scale co-doping is successfully modulated by hydrothermal method, and nano-inclusions precipitate during sintering process, to form complex microstructures. In this case, the electrical and thermal transport properties are optimized simultaneously by doping concentration and dopants type, resulting in a record-high ZT >0.6 at 1000−1100K in the 10mol% La and 10mol% Nb doped SrTiO3 bulk materials. The nano-scale modulation doping and microstructure controlling approach validated in the present study should be also applicable for other thermoelectric materials.

Super-fast preparation of Nd-filled p-type skutterudite compounds with enhanced thermoelectric properties

P-type filled skutterudite materials have been gained considerable research interest in recent years due to their promising thermoelectric power generation applications at intermediate temperature. Herein, we systematically investigated the influence of Nd filling on the thermoelectric properties of NdxFe2Co2Sb12 (x=0.4, 0.5 0.6, 0.7 and 0.8). Nd-filled skutterudites are synthesized using a simple and time-saving induction melt spinning technique followed by spark plasma sintering. The results show that Nd-filling leads to the significant reduction in the lattice thermal conductivity and enhancement of power factor over the entire temperature range. The most marked reduction in the lattice thermal conductivity is achieved with the value of 0.76W/mK for x=0.7 sample, due to strengthened phonon scattering. Meanwhile, the highest ZT=0.98 is attained at 740K for Nd0.7Fe2Co2Sb12. The rapid synthesis procedure provides an effective pathway for the fabrication of thermoelectric materials with high performance.

Observation and Characterization of the Thermoelectric Power Generation

Authors report methodical studies on some novel, alternative energy technologies, and produced results of a thermoelectric power generation (TEPG) system. For the sake of evaluating critical thermoelectric (TE) features, they have invented the state-of-the-art equipment that has important specification capability. The studies cover the efficiency and many aspects of TEPG features as follows. A thermoelectric module is measured in situ that includes TE efficiency, force response curve, current-voltage (I-V) and power-voltage (P-V) characterization, and power temperature (P-T) response curve. They have improved both the higher total output power and the more efficient TEPG efficiency than their comparison TE devices and systems in their studies. In addition, authors will present a series of design, construction, and characterization of a thermal electric generation system which aims to achieve a large power output and high efficiency for the energy harvesting application. Furthermore, their studies lead to important knowledge of TEPG systems in terms of multi-stack modules and of the optimization in TEPG applications. Finally, the prototypes are built for both the tabletop and other applications with new findings. Several sets of prototype TEPG are developed for experimental investigation and data analysis, followed by the summary and conclusions based on the data.

Recent progress of thermoelectric devices or modules in Japan

The Japanese government has promoted the thermoelectric research and development as one possible technology for raising the energy usage efficiency since 1978. The 1st application of thermoelectric power generation in Japan was a candle radio using β-FeSi2 modules, which was commercialized in 1990. Development of thermoelectric devices or modules was reactivated in 2002, when the 5-year NEDO project titled by “Development of thermoelectric energy conversion system with high efficiency” started. The Bi-Te devices with high energy conversion efficiency of 7.2% under a temperature gradient of 303-553K was reported. Ministry of Economy, Trade and Industry (MITI) of Japan has launched the 10-year project of “Development on the innovative utilization technologies of unused heat energy” since 2013. The administrator of this project has been changed from MITI to NEDO since 2015. Prototype devices of Mg-Si, skutterudite and organic polymers have been developed for power generation. In this paper, the recent progress of thermoelectric devices or modules in Japan is reviewed.

Microstructure and thermoelectric properties of Sb doped Hf0.25Zr0.75NiSn Half-Heusler compounds with improved carrier mobility

Half-Heusler (HH) semiconductor alloys are being widely investigated due to their promising potential for thermoelectric (TE) power generation applications. Sb is an effective doping element for n-type ZrNiSn half-Heuslers alloys. HH thermoelectric materials Hf0.25Zr0.75NiSn1−xSbx (0 ≤ x ≤ 0.03) were synthesized by induction melting combined with plasma activated sintering (PAS) technique. X-ray diffraction concluded that single-phase HH compounds without compositional segregations were obtained. Presence of bended lamellar structures was revealed by the FESEM. Sb doping significantly enhanced the electrical conductivity, power factor and carrier concentration of the alloys. An increase in the carrier mobility was also observed. Consequently, optimum values of 4.36 × 10−3 W/mK2 and 4.7 × 1020 cm−3 were achieved for power factor and carrier concentration, respectively. As a result, a ZT value of 0.83 at 923 K was obtained which is about 67% improvement compared to the un-doped sample.

Development of Bi2Te2.4Se0.6 alloy for thermoelectric power generation applications

The quest for improving eco-friendly renewable energy conversion methods due to the declining resources of conventional energy reserves and a rise in environmental awareness is constantly growing. The thermoelectric method of directly converting thermal into electrical energy is attracting increased attention for such applications and thus enhancing its efficiency is of great importance. The most efficient n-type thermoelectric materials for temperatures up to 300 °C are currently Bi2TexSe3-x based. By optimizing the preparation process while considering the inherent crystallographic anisotropy, the efficiency of such materials can be further improved. In this study the Bi2Te2.4Se0.6 composition was optimized by CHI3 doping, preferred alignment of the crystallographic orientation, and lattice thermal conductivity minimization. The synthesis route included rocking furnace melting, energetic ball milling and hot pressing with optimal parameters for enhancement of the thermoelectric figure of merit, ZT, at temperatures higher than 200 °C, commonly applied in low temperature power generation applications. The transport properties in the directions parallel and perpendicular to the pressing direction were examined. In the direction perpendicular to the pressing axis, a maximal ZT of ∼0.9 was obtained at ∼175 °C, which is as far as we know among the highest ever reported for n-type Bi2TexSe3-x based alloys.

Role of Nanoscale Precipitates for Enhancement of Thermoelectric Properties of Heavily P-Doped Si-Ge Alloys

Thermoelectric (TE) devices can save energy by generating electricity from waste heat. For industrial application of TE power generation, TE materials with high conversion efficiency and that are non-toxic and inexpensive are required. The nanostructured silicon-germanium (Si-Ge) alloy is one possible candidate for such TE materials. Nanostructuring is an effective way to improve the conversion efficiency of materials because it dramatically reduces the lattice thermal conductivity (κlat). Here, we experimentally demonstrate effective phonon blocking without carrier mobility deterioration in bulk Si-Ge alloys containing phosphorous-rich nanoscale precipitates connected coherently or semi-coherently with the matrix phase. When the Ge content was less than 5%, the nanoscale precipitates effectively scattered heat-carrying phonons, leading to a sufficient reduction in κlat. However, at higher Ge content compositions, phonon scattering by Ge substitution with Si was more predominant than phonon scattering by nanoscale precipitates for the reduction of κlat. As a result, significant enhancement of zT was achieved at low Ge contents.

Environmentally friendly BaxSr2−xTiFeO6double perovskite with enhanced thermopower for high temperature thermoelectric power generation

SrTiO3based double perovskites, BaxSr2−xTiFeO6(BSTF) compositions showed very large thermo-power (S) for the application of high temperature thermoelectric power generation.

Effect of Off-Stoichiometry on the Thermoelectric Properties of Heusler-Type Fe2VAl Sintered Alloys

Heusler-type Fe2V1−xAl1+x sintered alloys with micrometer-sized grains were fabricated by the powder metallurgical process using mechanical alloying and pulse-current sintering. Both positive (∼90 μV/K) and negative (∼−140 μV/K) Seebeck coefficients were obtained for the composition ranges of x > 0 and x < 0, respectively, resulting from a Fermi level shift caused by the change in the valence electron concentration. The electrical resistivity was reduced by the carrier doping effect, especially at lower temperatures, resulting in an increased thermoelectric power factor of 2.8 mW/m-K2 for the p-type alloy with x = 0.06 and 5.0 mW/m-K2 for the n-type alloy with x = −0.06. In addition, the lattice thermal conductivity decreased with |x| because of phonon scattering at crystal lattice defects induced by the off-stoichiometry. Consequently, the thermoelectric figure of merit, ZT, was enhanced and reached 0.07 for p-type alloys with 0.06 < x < 0.15 and 0.18 for n-type alloys with −0.15 < x < −0.10 around 500 K. The ZT value was especially enhanced at higher temperatures by the off-stoichiometric composition control, which could extend the range of heat source temperatures for thermoelectric power generation applications using this alloy.

Tin Incorporation in AgInSe2 Thin Films: Influence on Conductivity

Bipolarity with enhanced conductivity has been achieved by tin incorporation in AgInSe2 thin films. Structural and optical characterizations of these films prepared by reactive evaporation indicate that the incorporation neither distorts the tetragonal chalcopyrite structure nor affects the optical band gap of the parent compound. A detailed analysis of the low temperature conductivity of AgInSe2 (AIS) and tin incorporated AgInSe2 (AIS:Sn) suggests domination by variable range hopping, grain boundary effect, and thermal activation of carriers in different temperature regimes. The enhanced conductivity in AIS:Sn is attributed to donor and acceptor defect level formation, which has pushed the Fermi levels to ∼9 meV below the conduction band edge in n-type films and ∼3 meV above the valence band edge in p-type films. These improved characteristics of the films are likely to promote their suitability for photovoltaic and thermoelectric power generation applications.

An experimental study of a novel prototype for two-stage thermoelectric generator from vehicle exhaust

For the vehicle engine, about 30% of the primary energy is discharged as waste heat in the exhaust gases. The recovering of heat from that is a noticeable promising application of thermoelectric power generation (TEG). In this paper, a novel prototype for two-stage TEG from vehicle exhaust has been proposed. After system modeling, an experiment structure has also been built and tested for further study. Results of both theoretic analysis and experiment show the reasonability for exhaust heat recovery. The prototype can generate a maximum power output of about 250 W when operating at hot side temperature 473 K. The system thermal efficiency can reach 5.35%, it is improved by 32% compared to 4.04% of single stage TEG in the same operating conditions. System optimization and future improvement of the prototype is also discussed. Finally, based on a vehicle made by our research funder, economic value for commercialization in diesel vehicles has been analyzed.