SiGe Thermoelectric Materials Research Articles

Modeling and property study of thermoelectric converter based on subwavelength photothermal absorption structure

Achieving light absorption and conversion in a wide wavelength region and large angles of incidence is an effective way to improve the efficiency of thermoelectric devices. In this work, the thermoelectric converter device with a sub-wavelength photothermal absorption structure is designed, and the thermoelectric device is adapted to medium and high-temperature working environments by introducing PbS and SiGe thermoelectric materials. By changing the material of the surface heat-absorbing layer and studying the adjustment effect of the shape and size of the sub-wavelength photothermal absorption structure on reflectivity, the perfect absorption of sunlight with a wide wavelength region and large angles of incidence is realized, and the maximum photothermal conversion is achieved. Furthermore, the qualitative and quantitative relationships between the thermal absorption and thermoelectric conversion performance of PbS and SiGe thermoelectric materials and the physical parameters of surface subwavelength photothermal absorption structures are studied, so as to maximize the absorption and conversion of thermal energy into electrical energy. This work provides research ideas for the design of perfect thermoelectric devices.

Nanostructured Si–Ge thermoelectric material for 1200 V/W highly sensitive infrared thermopile sensor device

In this paper, we report the characteristics of a thermopile infrared sensor in which a nanostructured Si–Ge thermoelectric material is used. Although the thermopile infrared sensor is capable of being operated without power consumption, a challenge has been improving its sensitivity. With the aim of improving its sensitivity, we have realized a nanostructured Si–Ge thermoelectric material with low thermal conductivity (0.8 W/m K or less) by forming nanocrystals through heat treatment of amorphous Si–Ge. The thermopile infrared sensor composed of this material exhibited a high sensitivity of 1200 V/W at a pressure of 1.0 × 10−1 Pa or less.

Thermoelectric properties of SiGe thin films prepared by laser sintering of nanograin powders

Silicon-germanium is an important high-temperature thermoelectric material for thermal to electric energy conversion. This paper focuses on the study of nanograin thermoelectric Si80Ge20 thin films. The nanograin Si80Ge20 thin films were prepared by a novel method of ball milling to generate nanoparticles and sintered by a quasi-continuous wave, near-infrared laser of wavelength 1070 nm. The electrical and thermal properties variations with different laser sintering power were measured, and the sintering mechanism is discussed. The Si/Ge wetting to substrate for a percolation morphology and the control of carrier concentration are found to be the key factors for good electrical conductivity. The stoichiometric ratio of Si and Ge was found to be unchanged in the ball milling and laser sintering process. We demonstrate that the laser sintering was capable of creating well alloyed thermoelectric materials while minimizing grain growth. The thermoelectric parameters like electrical conductivity, thermal conductivity, and figure of merit were measured, and the results are compared with other reported bulk and thin-film studies, and the advantages of our method are discussed. We demonstrate that low-cost nanograin SiGe thin films prepared by ball milling and laser sintering process have a comparable thermoelectric figure of merit to other SiGe thermoelectric materials and which can be further improved.

Effect of Synthesis Procedure on Thermoelectric Property of SiGe Alloy

SiGe thermoelectric material has been synthesized by ball milling combined with hot pressing (HP) or spark plasma sintering (SPS). Effects of ball milling time, powder to ball weight ratio and sintering method on microstructure and thermoelectric properties of SiGe are studied. The results show that longer ball milling time leads to decreased density and worse electrical properties. In the sintering process, SPS results in much larger density and better electrical properties than HP. The Si0.795Ge0.2B0.005 sample prepared by 2 h ball milling combined with SPS obtains a maximum power factor of 3.0 mW m−1 K−2 at 860 K and ZT of 0.95 at 1000 K.

Fabrication and contact resistivity of W–Si3N4/TiB2–Si3N4/p–SiGe thermoelectric joints

The design and fabrication of silicon germanium (SiGe) thermoelectric elements, typically including the selection of electrode and intermediate materials, the process of joining electrode and intermediate layer onto thermoelectric materials, are the major challenge for SiGe thermoelectric device technology. In this study, W–Si3N4 and TiB2–Si3N4 composites are designed as the electrode and intermediate layer, respectively, and the W–i3N4/TiB2–Si3N4/p–Si80Ge20B0.6 joints are fabricated by a one-step spark plasma sintering process. The influences of the composition of TiB2–Si3N4 intermediate layer on the interfacial structure, contact resistivity and interfacial thermal stability are investigated. The interfacial thermal stability is improved with increasing Si3N4 content in TiB2–Si3N4 intermediate layer due to the reduced mismatch of coefficients of thermal expansion between the intermediate layer and SiGe. On the other hand, the contact resistivity increases with the rising of Si3N4 content due to the weakened TiB2/SiGe ohmic contact, which degrades the device efficiency. As the balanced point, the intermediate layer with the composition of 80vol% TiB2+20vol% Si3N4 provides good interfacial thermal stability and moderately small contact resistivity (~75μΩcm2 after aging at 1000°C for 120h) simultaneously, which is an optimized intermediate layer composition for W–Si3N4/TiB2–Si3N4/p–Si80Ge20B0.6 thermoelectric element. The TiB2–Si3N4 intermediate layer has excellent chemical stability to both W–Si3N4 electrode and SiGe thermoelectric material at high temperatures, which contributes to the sharp interface of the joint and effectively prevents the inter-diffusion between the electrode and the SiGe.

Crystal Lattice Controlled SiGe Thermoelectric Materials with High Figure of Merit

ABSTRACTDirect energy conversion between thermal and electrical energy, based on thermoelectric (TE) effect, has the potential to recover waste heat and convert it to provide clean electric power. The energy conversion efficiency is related to the thermoelectric figure of merit ZT expressed as ZT=S2σT/κ, T is temperature, S is the Seebeck coefficient, σ is conductance and κ is thermal conductivity. For a lower thermal conductivity κ and high power factor (S2σ), our current strategy is the development of rhombohedrally strained single crystalline SiGe materials that are highly [111]-oriented twinned. The development of a SiGe “twin lattice structure (TLS)” plays a key role in phonon scattering. The TLS increases the electrical conductivity and decreases thermal conductivity due to phonon scattering at stacking faults generated from the 60° rotated primary twin structure. To develop high performance materials, the substrate temperature, chamber working pressure, and DC sputtering power are controlled for the aligned growth production of SiGe layer and TLS on a c-plane sapphire. Additionally, a new elevated temperature thermoelectric characterization system, that measures the thermal diffusivity and Seebeck effect nondestructively, was developed. The material properties were characterized at various temperatures and optimized process conditions were experimentally determined. The present paper encompasses the technical discussions toward the development of thermoelectric materials and the measurement techniques.

Fabrication and Evaluation of Graded(Si-MoSi2)/SiGe Thermoelectric Materials by HIP Sintering.

Based on the concept of functionally goaded materials, the thermoelectric conversion unit of SiGe with gaded electrodes was designed and fabricated by HIP sintering process in order to obtain a good electrical and mechanical contact. The composite of Si and MoSi2 was used as the electrode, and the effect of the MoSi2 volume fraction on the electrical resistivity was studied. The results show that the electrical resistivity decreases exponentially with increasing MoSi2 content and reaches the order of 10-4 Ω·cm when the volume fraction of MoSi2 was beyond 25%. The graded (Si-MoSi2) /SiGe obtained has dense microstructures and well bonded interfaces. The electrical resistivity continuously decreased from the SiGe to the surface electrode in the zone of gaded structure.

Effects of some rare earth oxides and/or SiO2 additions to a commercial ZrO2 paint for coating SiGe thermoelectric materials

The effects of some rare earth oxides (Y2O3, CeO2, Sm2O3 or Er2O3) and/or SiO2 additions to a commercial oxide paint (mainly ZrO2) were studied as coatings for SiGe thermoelectric materials. No spalling occurred in the samples prepared from coatings containing small amounts of Er2O3, SiO2 or both CeO2 and SiO2 while heating for 1322 h at 1080° C in vacuum, and the sublimation of silicon and/or germanium was reduced significantly. The improvements imparted by these oxide additions to the commercial oxide paint is thought to be due to decreased internal stress of the surface oxides. The addition of Y2O3 and Sm2O3 had a detrimental effect.

Commercial ZrO2 paints as coatings for SiGe thermoelectric materials

The use of ZrO2 paints to coat SiGe materials used in radioisotopic thermoelectric generators was studied. The best results were obtained when the SiGe alloys were double coated with a 200 h anneal at 1000° C after each coating. The thermoelectric properties of these coated samples were about the same as for the SiGe alloys coated by SiO2 or Si3N4. The vapoursupression properties of the best ZrO2 coatings fell between those of SiO2 and Si3N4. In the SiGe doped with GaP alloys, the interface between the oxide coating and alloy is enriched with Ga2O3.

The chemical characterization of silicon-germanium thermoelectric alloys

The performance of thermoelectric devices is sensitive to the chemical composition, the dopant concentration and the uniformity of the semiconductor materials used in their fabrication. Analytical methods for the determination of silicon, germanium, boron, and phosphorus in SiGe thermoelectric materials are described. These methods were developed because of the fragmentary data available in the literature regarding their chemical characterization. The methods are rapid, accurate, and, with the exception of boron, require a minimum of expensive apparatus. Germanium was determined acidimetrically by means of the mannitol complex after ion-exchange separation on Amberlite XE-243, silica by the conventional gravimetric method, phosphorus colorimetrically as the vanadomolybdophosphate, and boron by emission spectroscopy.