P-type Thermoelectric Materials Research Articles

Formation of the Thermoelectric Candidate Chromium Silicide by Use of a Pack-Cementation Process

Transition-metal silicides are reported to be good candidates for thermoelectric applications because of their thermal and structural stability, high electrical conductivity, and generation of thermoelectric power at elevated temperatures. Chromium disilicide (CrSi2) is a narrow-gap semiconductor and a potential p-type thermoelectric material up to 973 K with a band gap of 0.30 eV. In this work, CrSi2 was formed from Si wafers by use of a two-step, pack-cementation, chemical diffusion method. Several deposition conditions were used to investigate the effect of temperature and donor concentration on the structure of the final products. Scanning electron microscopy and x-ray diffraction analysis were performed for phase identification, and thermal stability was evaluated by means of thermogravimetric measurements. The results showed that after the first step, chromizing, the structure of the products was a mixture of several Cr–Si phases, depending on the donor (Cr) concentration during the deposition process. After the second step, siliconizing, the pure CrSi2 phase was formed as a result of Si enrichment of the initial Cr–Si phases. It was also revealed that this compound has thermoelectric properties similar to those reported elsewhere. Moreover, it was found to have exceptional chemical stability even at temperatures up to 1273 K.

High-temperature thermoelectric properties of Cu1.97Ag0.03Se1+y

Copper selenide is of recent interest as a high- performance p-type thermoelectric material. Adding Ag to copper selenide increases the dimensionless figure-of-merit zT up to 780 K, with Cu1.97Ag0.03Se reaching a zT of 1.0 at 870 K. The increase compared with Cu2Se can be explained by analyzing the Hall carrier concentration and effective mass. Addition of Ag reduces the carrier con- centration to a nearly optimum value at high temperature. If the Hall carrier concentration were to be further reduced to 6.6 9 10 20 cm -3 at 750 K, the average zT would be substantially improved for waste heat recovery applications.

Shifting up the optimum figure of merit of p-type bismuth telluride-based thermoelectric materials for power generation by suppressing intrinsic conduction

The abundance of low-temperature waste heat produced by industry and automobile exhaust necessitates the development of power generation with thermoelectric (TE) materials. Commercially available bismuth telluride-based alloys are generally used near room temperature. Materials that are composed of p-type bismuth telluride, which are suitable for low-temperature power generation (near 380 K), were successfully obtained through Sb-alloying, which suppresses detrimental intrinsic conduction at elevated temperatures by increasing hole concentrations and material band gaps. Furthermore, hot deformation (HD)-induced multi-scale microstructures were successfully realized in the high-performance p-type TE materials. Enhanced textures and donor-like effects all contributed to improved electrical transport properties. Multiple phonon scattering centers, including local nanostructures induced by dynamic recrystallization and high-density lattice defects, significantly reduced the lattice thermal conductivity. These combined effects resulted in observable improvement of ZT over the entire temperature range, with all TE parameters measured along the in-plane direction. The maximum ZT of 1.3 for the hot-deformed Bi0.3Sb1.7Te3 alloy was reached at 380 K, whereas the average ZTav of 1.18 was found in the range of 300–480 K, indicating potential for application in low-temperature TE power generation. Thermoelectric materials, which convert temperature differences and electric voltage into each other, serve in refrigeration or power generation applications. Currently, bismuth telluride (Bi2Te3) and its alloys are the most widely used thermoelectric materials. Tie-Jun Zhu, Xin-Bing Zhao and co-workers from Zhejiang University, China, have now investigated the effect of antimony (Sb) alloying on bismuth tellurides through a series of polycrystalline solid solutions of Bi2-xSbxTe3—where x varies between 1.4 and 1.8—prepared by hot deformation. Systematic tuning of the alloy composition showed that higher antimony content raised the material's optimal conversion temperature by repressing undesirable conduction. This effect arises from an increase in both the hole concentration and the band gap in the material. For a composition where x is 1.7, the alloy showed optimal performances at 380 kelvin—a suitable temperature for low-temperature power generation from the waste heat generated by industry or vehicles. The p-type bismuth telluride-based polycrystalline materials suiting for low-temperature power generations (near 380 K) have been obtained through Sb-alloying and HD, which suppresses the detrimental effect of intrinsic conduction at elevated temperature via increasing the hole concentration and band gap. The hot-deformed Bi0.3Sb1.7Te3 alloy, not usual composition Bi0.5Sb1.5Te3, shows a maximum ZT of 1.3 at 380 K, indicating a bright application potential in low-temperature power generations.

Thermoelectric properties of Tl-filled Co-free p-type skutterudites: Tlx(Fe,Ni)4Sb12

Our group has discovered that Tl-filled CoSb3-based skutterudites (TlxCoSb3) show an excellent thermoelectric (TE) figure of merit, ZT, of 0.9 [A. Harnwunggmoung et al., Appl. Phys. Lett. 96, 202107 (2010)]. However, all TlxCoSb3 studied so far show n-type characteristics. Since a powerful TE module requires a similar performance in both n- and p-type TE materials, p-type skutterudites filled by Tl need to be developed. Furthermore, from the viewpoint of cost-effectiveness, Co should be replaced by cheaper elements. Therefore, we attempted to develop Tl-filled Co-free p-type skutterudites with high ZT. We selected the sample compositions to be TlxFe2.5Ni1.5Sb12 and studied the high-temperature TE properties. It was found that the samples with x ≤ 0.6 exhibited p-type characteristics and the sample with x = 0.6 exhibited the best TE performance. The maximum ZT, obtained at 422 K, was 0.16, which is comparable to those of other Tl-filled p-type skutterudites.

Highly efficient (In₂Te₃)x(GeTe)(3-3x) thermoelectric materials: a substitute for TAGS.

GeTe is a versatile base compound to produce highly efficient p-type thermoelectric materials such as the TAGS materials (AgSbTe2)1-x(GeTe)x and GeTe-PbTe nanocomposites. The pure GeTe composition shows a very high power factor, ~42 μW cm(-1) K(-2), between 673 K and 823 K, which is among the highest power factors that have ever been reported in this temperature range. However, its relatively high thermal conductivity limits the dimensionless figure of merit ZT to values of only unity. In this paper, we present an efficient approach to reduce the thermal conductivity by preparing (In2Te3)x(GeTe)(3-3x) solid solutions. In spite of a slight degradation of the electronic properties, the drastic reduction of the thermal conductivity due to a synergistic combination of reduced electronic thermal conductivity, strong alloy scattering, and vacancy phonon scattering leads to ZT values as high as 1.35 at 823 K for the x = 0.05 sample. Our results show that (In2Te3)x(GeTe)(3-3x) is a prospective substitute for TAGS as a p-leg element for high-temperature power generation.

The Effect of Te Substitution for Sb on Thermoelectric Properties of Tetrahedrite

We present the study of the effect of Te substitution on the thermoelectric properties for Sb in Cu12Sb4−xTexS13 tetrahedrite compounds with x ranging from 0.2 to 1.5 in the temperature range of room temperature to 723 K. Powder x-ray diffraction and electron microscopy results indicate a successful homogenous substitution without the alteration of the crystal structure or the introduction of secondary phases. Thermoelectric property measurements show that the excess electrons from Te during the substitution fill the unoccupied levels near the top of the valence bands in pure Cu12Sb4S13 compound, moving the Fermi level closer to the top of the valence bands. This leads to an enhancement in thermopower but also to an increase in electrical resistivity. Overall, the reduction in total thermal conductivity gives rise to improved ZT values in all substituted samples. The highest ZT value obtained in this study is 0.92 at 723 K for x = 1, which is comparable to that of other p-type bulk thermoelectric materials.

Evaluation of the Structure and Transport Properties of Nanostructured Antimony Telluride (Sb2Te3)

Antimony telluride, (Sb2Te3), and its doped derivatives are considered to be among the best p-type thermoelectric (TE) materials for room temperature (300–400 K) applications. However, it is still desirable to develop rapid and economical routes for large-scale synthesis of Sb2Te3 nanostructures. We report herein a high yield, simple and easily scalable synthetic method for polycrystalline Sb2Te3 nanostructures. Prepared samples were compacted into dense pellets by use of spark plasma sintering. The products were characterized by x-ray diffraction and scanning electron microscopy. To investigate the anisotropic behavior of Sb2Te3 TE transport property measurements were performed along and perpendicular to the direction of compaction. Thermal conductivity, electrical conductivity, and Seebeck coefficient measurement over the temperature range 350–525 K showed that the anisotropy of the material had a large effect on TE performance.

Modeling and Optimization of Thermoelements by a Combined Analytical and Numerical Method

A combined analytical and numerical process has been developed to model and optimize thermoelements. In this way, the performance of commercial n- and p-type thermoelectric materials can be optimized to deliver the maximum output power and conversion efficiency. The validity of the method is demonstrated using a silicon germanium unicouple.

High-Performance Dispenser Printed MA p-Type Bi0.5Sb1.5Te3 Flexible Thermoelectric Generators for Powering Wireless Sensor Networks

This work presents a novel method to synthesize p-type composite thermoelectric materials to print scalable thermoelectric generator (TEG) devices in a cost-effective way. A maximum ZT of 0.2 was achieved for mechanically alloyed (MA) p-type Bi0.5Sb1.5Te3 (8 wt % extra Te additive)-epoxy composite films cured at 250 °C. A 50% increase in Seebeck coefficient as a result of adding 8 wt % extra Te in stoichiometric Bi0.5Sb1.5Te3 contributed to the increase in ZT. To demonstrate cost-effective and scalable manufacturing, we fabricated a sixty element thermoelectric generator prototype with 5.0 mm × 600 μm × 120 μm printed dimensions on a custom designed polyimide substrate with thick metal contacts. The prototype TEG device produced a power output of 20.5 μW at 0.15 mA and 130 mV for a temperature difference of 20 K resulting in a device areal power density of 152 μW/cm(2). This power is sufficient for low power applications such as wireless sensor network (WSN) devices.

Thermopower and chemical stability of Na0.77CoO2/Ca3Co4O9 composites

Layered cobaltates, including Na0.77CoO2 and Ca3Co4O9, can be viewed as the foundation of p-type oxide thermoelectric materials. Here, we demonstrate the successful fabrication of (1−x)Na0.77CoO2/xCa3Co4O9 (x=0, 0.3, 0.5, 0.7, 1) composites by means of spark plasma sintering. A significant enhancement of the Seebeck coefficient (thermopower) has been achieved in the composites, approximately 17% higher than that of Ca3Co4O9 at 680°C. The increases in Seebeck coefficient and electrical resistivity are mostly likely to be associated with the compressive strain in Ca3Co4O9 grains due to the mismatch of thermal expansion coefficients between Na0.77CoO2 and Ca3Co4O9. Most importantly, the chemical stability of Na0.77CoO2 could be improved by adding up to a 30vol.% fraction of Ca3Co4O9 without deteriorating its thermoelectric performance. This observation may open a pathway to practical applications of Na0.77CoO2-based thermoelectric materials.

Prediction of a large number of electron pockets near the band edges in type-VIII clathrate Si46 and its physical properties from first principles

The material design of type-VIII clathrate Si46 is presented based on first principles. The structural, electronic, elastic, vibrational, and thermodynamic properties of this hypothetical material are presented. Our results predict that type-VIII clathrate Si46 is an indirect semiconductor with a bandgap of 1.24 eV. The band structure revealed an interestingly large number of electron pockets near both conduction and valance band edges. Such a large density of states near the band edges, which is higher than that of the best thermoelectric materials discovered so far, can result in a large thermoelectric power factor (>0.004 W m−1 K−2) making it a promising candidate for thermoelectric applications. The elastic properties as well as the vibrational modes and the phonon state densities of this material were also calculated. Our calculations predict that the heat capacity at constant volume (isochoric) of this clathrate increases smoothly with temperature and approaches the Dulong–Petit value near room temperature. The electronic band structure shows a large number of valleys closely packed around the valance band edge, which is rare among the known semiconducting materials. These valleys can contribute to transport at high temperature resulting in a possibly high performance (ZT > 1.5) p-type thermoelectric material.

TRANSPORT PROPERTIES IN MISFIT-LAYERED Ca2Co2O5 COMPOUND

The low temperature transport and magnetic properties were investigated in the misfit-layered Ca 2 Co 2 O 5 compound. The compound exhibits incommensurate spin-density-wave (SDW) state in the dχ-1/dT curve at 20 K, which is confirmed by the resistivity (ρ) characterization. The resistivity shows an upturn from metallic to insulating behavior at TM-I (130 K) and strong Fermi liquid behaviors with ρ ~ T2 relation between TM-I and T*(225 K ). The Seebeck coefficient shows abnormal temperature dependence at Tp (40 K), which is suggested to origin from the long order of atomic in [ Ca 2 CoO 3] sublayer and spin fluctuation of Co ions. Moreover, large Seebeck coefficient (120 μV/K) and low thermal conductivity (2.5 W/mK) were obtained at room temperature, indicating that it is a promising p-type thermoelectric material for energy conversion.

Characterization of the interface between an Fe–Cr alloy and the p-type thermoelectric oxide Ca3Co4O9

A customized Fe–Cr alloy that has been optimized for high temperature applications in oxidizing atmospheres has been interfaced via spark plasma sintering (SPS) with a p-type thermoelectric oxide material: calcium cobaltate (Ca3Co4O9). The properties of the alloy have been analyzed for its compatibility with the Ca3Co4O9 in terms of its thermal expansion and transport properties. The thermal and electrical contact resistances have been measured as a function of temperature, and the long term electronic integrity of the interface analyzed by measuring the resistance vs. time at an elevated temperature. The kinetics of the interface have been analyzed through imaging with scanning electron microscopy (SEM), elemental analysis using energy dispersive spectroscopy (EDS), and phase identification with X-ray diffraction (XRD). The results reveal the formation of an intermediate phase containing calcium and chromium in the interface that is highly resistive at room temperature, but conducting at the intended thermoelectric device hot-side operating temperature of 800 °C. As the alloy is well matched in terms of its thermal expansion and highly conducting compared to the Ca3Co4O9, it may be further considered as an interconnect material candidate at least with application on the hot-side of an oxide thermoelectric power generation module.

Increasing the thermoelectric power generated by composite films using chemically functionalized single-walled carbon nanotubes

We prepared and characterized flexible thermoelectric (TE) materials based on thin films of single-walled carbon nanotube (SWCNT) composites with polyvinylalcohol. While pristine SWCNTs incorporated in a polymer matrix generated a p-type TE material, chemical functionalization of SWCNTs by using polyethyleneimine produced an n-type TE material. TE modules made of both p- and n-type composite were fabricated to demonstrate TE voltage and power generation. A single p–n junction made of two composite strips containing 20wt.% of SWCNTs generated a high TE voltage of 92μV per 1K temperature gradient (ΔT). By combining five electrically connected p–n junctions an output voltage of 25mV was obtained upon the applying ΔT=50K. Furthermore, this module generated a power of 4.5nW when a load resistance matched the internal module resistance of 30kΩ. These promising results show the potential of TE energy conversion provided by the SWCNT composite films connected in scalable modules for applications that require light weight and mechanical flexibility.

Effects of Spark Plasma Sintering Temperature on Thermoelectric Properties of Higher Manganese Silicide

Higher manganese silicide (HMS) is a promising p-type thermoelectric material. HMS samples were synthesized by a vacuum induction melting process and sintered by spark plasma sintering (SPS) at various temperatures to obtain a single phase of HMS and investigate the effect of the SPS temperature on the thermoelectric properties. A single phase of HMS was obtained, and the appearance and the amount of Mn2O3 and MnSi as secondary phases could be controlled via the SPS temperature. The effects of the SPS temperature on the electrical conductivity and Seebeck coefficient of the HMS samples were investigated. The changes in the electrical conductivity and Seebeck coefficient were attributed to changes in the density and the amount of Mn2O3 secondary phase.

Interfacial reaction between n- and p-type thermoelectric materials and SAC305 solders

Abstract Sn96.5Ag3.0Cu0.5 (SAC305) is one of the most promising lead-free solder alloys and bismuth telluride-based compounds are the best known thermoelectric materials. Interfacial reactions of the SAC305/Bi0.5Sb1.5Te3 and SAC305/Bi1.8Sb0.2Se0.15Te2.85 couples at 260 °C are studied to investigate the effect of reaction on the efficiency and reliability of thermoelectric device. Initially, the major products in SAC305/Bi0.5Sb1.5Te3 couple reaction for a short time are a strip-type Ag-rich IMC layer and a dense SnTe–SbSn mixture layer. With the reaction time increasing, a two-phase layer, which is SnTe–SbSn mixture with pores with 200–500 nm size which are occupied by solder originally, appears next to the dense layer. However, the reaction products do not change significantly. It is observed that the growth rate of reaction phase layer is about 4.05 μm/min. As for the SAC305/Bi1.8Sb0.2Se0.15Te2.85 couple reaction, besides SnTe phase, Bi–Te phase is also formed at the solder/Bi1.8Sb0.2Se0.15Te2.85 interface. As the reaction time increases, the morphology of IMC layer becomes rough, and then the stable scallop-type IMC appears; the growth rate of reaction phase layer is about 0.39 μm/min.

Giant dielectric behavior observed in Ca3Co4O9 ceramic

Giant dielectric properties of a Ca3Co4O9 ceramic, a p-type thermoelectric material, are reported for the first time. A very high dielectric permittivity of about 1.38 × 104 was observed at a temperature of 20°C and frequency of 1 kHz. A Debye-like relaxation behavior with a thermally activated mechanism was observed. The relaxation activation energy was 0.363 eV. It was found that aging time and thermal annealing of the ceramics with Au electrodes have remarkable influence on the value of the dielectric permittivity of this ceramic. Using combined -Z″ and M″ plots, it was found that the giant dielectric response observed in the Ca3Co4O9 ceramic is attributable to the sample-electrode contact effect.

High Temperature Thermoelectric Properties of Half-Heusler Compound PtYSb

Recent experimental examination of the thermoelectric properties of the half Heusler compound PtYSb has revealed that PtYSb has a relatively high thermoelectric figure of merit (ZT) of 0.2 at around room temperature. However, the thermoelectric properties have been examined only in the low-temperature region, and no high-temperature data has been reported to date. Here we present the high-temperature thermoelectric properties of polycrystalline bulk samples of PtYSb in the temperature range between room temperature and 973 K. The Seebeck coefficient was positive over the entire temperature range examined. A high power factor (2.1×10-3 W m-1 K-2) and low thermal conductivity (3.44 W m-1 K-1) were obtained at 973 K, which resulted in a relatively high ZT of 0.57 for PtYSb. Thus, PtYSb has the potential for application as a p-type thermoelectric material at high temperature.

Mechanical Alloying and Spark Plasma Sintering of Higher Manganese Silicides for Thermoelectric Applications

The present challenges in the energy crisis require finding new ways to reduce consumption of fossil fuels. Thermoelectrics can help reduce fuel consumption by producing electricity from waste heat. The higher manganese silicides (HMS) have shown promise in this field as inexpensive, nontoxic, and highly stable p-type thermoelectric materials. One of the production techniques for HMS is mechanical alloying by ball milling. In this research the effect of the ball-milling duration and speed on the phases produced was studied. Mn and Si powders were milled at speeds of 200 RPM to 800 RPM for 1 h to 7 h. X-ray diffraction (XRD) results of the samples prepared using mechanical alloying show deterioration into the MnSi phase. The sample that underwent 5 h of milling at 800 RPM showed the greatest amount of HMS phase and was subsequently spark plasma sintered. The sample showed insufficient thermoelectric properties (ZT ≈ 0.1 at 450°C), compared with either solid-state reaction samples showing ZT ≈ 0.4 or cast samples showing ZT ≈ 0.63 at 450°C. The reduced ZT values of the mechanically alloyed and spark-plasma-sintered samples were attributed to the high relative amount of MnSi phase. The correlation between the relative amount of MnSi and the transport properties is described in detail.

Significant ZT enhancement in p-type Ti(Co,Fe)Sb–InSb nanocomposites via a synergistic high-mobility electron injection, energy-filtering and boundary-scattering approach

It has been demonstrated that InSb nanoinclusions, which are formed in situ, can simultaneously improve all three individual thermoelectric properties of the n-type half-Heusler compound (Ti,Zr,Hf)(Co,Ni)Sb (Xie WJ, He J, Zhu S, Su XL, Wang SY, Holgate T, et al. Acta Mater 2010;58:4795). In the present work, the same approach is adopted to the p-type half-Heusler compound Ti(Co,Fe)Sb. The results of resistivity, Seebeck coefficient, thermal conductivity and Hall coefficient measurements indicate that the combined high-mobility electron injection, low energy electron filtering and boundary scattering, again, lead to a simultaneous improvement in all three individual thermoelectric properties: enhanced Seebeck coefficient and electrical conductivity as well as reduced lattice thermal conductivity. A figure of merit of ZT≈0.33 was attained at 900K for the sample containing 1.0at.% InSb nanoinclusions, a ∼450% improvement over the nanoinclusion-free sample. This represents a rare case that the same nanostructuring approach works successfully for both p-type and n-type thermoelectric materials of the same class, hence pointing to a promising materials design route for higher-performance half-Heusler materials in the future and hopefully will realize similar improvement in thermoelectric devices based on such half-Heusler alloys.