P-type Thermoelectric Materials Research Articles

Simultaneous blocking of minority carrier and high energy phonon in p-type skutterudites

Skutterudites are among the most promising thermoelectric materials. However, the strong bipolar effects and a lack of high energy phonon scattering make the thermoelectric performance of p-type skutterudites far below that of n-type skutterudites. Here, we reveal that the distribution of cations spatially fluctuate in p-type skutterudites, leading to a band edge fluctuation with small valence-band offsets, multi-scale strain field fluctuation, and formation of nanostructures. It further results in a significant suppressed bipolar effect, and wide-frequency spectrum phonon scattering. A 38% enhanced power factor and reduced lattice thermal conductivity approaching to the theoretical minimum at 723 K lead to a remarkable theoretical conversion efficiency of 13.3%, reaching the highest records in p-type lead-free thermoelectric materials. Our results provide a new thought to realize high performance in polynary thermoelectric materials.

Excellent ZT achieved in Cu1.8S thermoelectric alloys through introducing rare-earth trichlorides

Digenite (Cu1.8S) as a potential p-type thermoelectric (TE) material has attracted extensive attention due to its environmental-friendliness and low toxicity.

High thermoelectric performance of n-type Bi2Te2.7Se0.3via nanostructure engineering

BiSbTe has been realized as an ideal p-type thermoelectric material near room temperature; however, its commercial applications are largely restricted by its n-type counterpart that exhibits relatively inferior thermoelectric performance.

Facile synthesis of earth-abundant and non-toxic p-type Si96B4/SiCp nanocomposites with enhanced thermoelectric performance

One of the impediments in the development of thermoelectric devices for power generation is that they mostly contain toxic and expensive elements and/or are synthesized using expensive or time-consuming material processing methodology. We report the synthesis of Silicon–Boron (Si96B4) alloy employing earth abundant constituent element using a facile single-step reactive sintering using spark plasma sintering technique. In order to enhance its mechanical properties, the synthesized Si96B4 alloy was dispersed with SiC nanoparticles and the effect of its addition on the thermoelectric and mechanical properties in the resulting Si96B4/SiC nanocomposite has been investigated. A thermoelectric figure-of-merit ZT ~ 0.27 at 1123K was realized at an optimized composition of Si96B4/1wt% SiC nanocomposite. This enhancement in ZT primarily originates from a noticeable reduction in the thermal conductivity on SiC dispersion in Si96B4 alloy, owing to the scattering of heat-carrying phonons by nanoscale SiC particles and mesoscale SiB3 precipitates, formed in-situ. The synthesized samples were characterized using X-ray diffraction and field emission scanning electron microscopy, based on which the enhancement in their thermoelectric and mechanical properties are discussed. Considering the low-cost and non-toxicity of the constituent elements coupled with facile and up-scalable one-step processing employed in its synthesis, Si96B4/SiC nanocomposites could be a potential p-type thermoelectric material for high-temperature power generation applications.

Phase Transitions of Thermoelectric TAGS-85.

The alloys (GeTe)x(AgSbTe2)100–x, commonly known as TAGS-x, are among the best performing p-type thermoelectric materials for the composition range 80 ≤ x ≤ 90 and in the temperature range 200–500 °C. They adopt a rhombohedrally distorted rocksalt structure at room temperature and are reported to undergo a reversible phase transition to a cubic structure at ∼250 °C. However, we show that, for the optimal x = 85 composition (TAGS-85), both the structural and thermoelectric properties are highly sensitive to the initial synthesis method employed. Single-phase rhombohedral samples exhibit the best thermoelectric properties but can only be obtained after an annealing step at 600 °C during initial cooling from the melt. Under faster cooling conditions, the samples obtained are inhomogeneous, containing multiple rhombohedral phases with a range of lattice parameters and exhibiting inferior thermoelectric properties. We also find that when the room-temperature rhombohedral phase is heated, an intermediate trigonal structure containing ordered cation vacancy layers is formed at ∼200 °C, driven by the spontaneous precipitation of argyrodite-type Ag8GeTe6 which alters the stoichiometry of the TAGS-85 matrix. The rhombohedral and trigonal phases of TAGS-85 coexist up to 380 °C, above which a single cubic phase is obtained and the Ag8GeTe6 precipitates redissolve into the matrix. On subsequent cooling a mixture of rhombohedral, trigonal, and Ag8GeTe6 phases is again obtained. Initially single-phase samples exhibit thermoelectric power factors of up to 0.0035 W m–1 K–2 at 500 °C, a value that is maintained on subsequent thermal cycling and which represents the highest power factor yet reported for undoped TAGS-85. Therefore, control over the structural homogeneity of TAGS-85 as demonstrated here is essential in order to optimize the thermoelectric performance.

High thermoelectric performance of few-quintuple Sb2Te3 nanofilms

Antimony telluride (Sb2Te3) has a very low ZT value at room temperature, even though the alloys of Bi2Te3 and Sb2Te3 are the most widely used thermoelectric materials. Taking advantage of nano-structures and topological insulators (TIs), we report that high ZT values of larger than 2 can be achieved in one quintuple-layer (QL, ZT≥ 2.1) and 4 QLs (ZT≥ 2.2) of Sb2Te3 at room temperature, while the ZT value for bulk is around 0.5. For 1 QL, Seebeck coefficient has been greatly enhanced due to the increase in the number of band extrema by virtue of the twelvefold valley degeneracy from surface states. For 4 QLs, the high ZT value is attributed to the much longer surface relaxation time associated with the robust nature of the topological surface states. Moreover, the ZT values and the TI character of few-quintuple Sb2Te3 nanofilms demonstrate thickness(layer)-dependent behaviors. Our results offer significant clues for future applications and investigations of Sb2Te3-related quintuple layers as promising p-type thermoelectric materials and quasi-2D topological insulators for nanoelectronics.

Achieving high power factor of p-type BiSbTe thermoelectric materials via adjusting hot-pressing temperature

Achieving high power factor values for BiSbTe alloys is important for further elevation of their thermoelectric performance. In this report, Bi0.4Sb1.6Te3 bulk samples were prepared by a fusion method followed by hot pressing at different temperatures (523 K, 573 K, 623 K and 663 K), and their thermoelectric properties were investigated from 300 K to 500 K. It was found that the power factor and thermal conductivity increased significantly with increasing hot-pressing temperature. A record power factor of 53.8 μW/(cm K2) at 300 K was achieved for the sample hot pressed at 663 K. In addition, a maximum ZT of 1.3 at 413 K and average ZTave of 1.2 at 300–500 K were achieved for the sample hot pressed at 623 K.

Prediction for electronic, vibrational and thermoelectric properties of chalcopyrite AgX(X=In,Ga)Te2: PBE + U approach.

The electronic, vibrational and thermoelectric transport characteristics of AgInTe2 and AgGaTe2 with chalcopyrite structure have been investigated. The electronic structures are calculated using the density-functional theory within the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof functional considering the Hubbard-U exchange correlation. The band-gaps of AgInTe2 and AgGaTe2 are much larger than previous standard GGA functional results and agree well with the existing experimental data. The effective mass of the hole and the shape of density of states near the edge of the valence band indicate AgInTe2 and AgGaTe2 are considerable p-type thermoelectric materials. An analysis of lattice dynamics shows the low thermal conductivities of AgInTe2 and AgGaTe2. The thermoelectric transport properties' dependence on carrier concentration for p-type AgInTe2 and AgGaTe2 in a wide range of temperatures has been studied in detail. The results show that p-type AgInTe2 and AgGaTe2 at 800 K can achieve the merit values of 0.91 and 1.38 at about 2.12 × 1020 cm−3 and 1.97 × 1020 cm−3 carrier concentrations, respectively. This indicates p-type AgGaTe2 is a potential thermoelectric material at high temperature.

Realizing p-Type MoS2 with Enhanced Thermoelectric Performance by Embedding VMo2S4 Nanoinclusions.

Two-dimensional transition-metal dichalcogenide semiconductors (TMDCs) such as MoS2 are attracting increasing interest as thermoelectric materials owing to their abundance, nontoxicity, and promising performance. Recently, we have successfully developed n-type MoS2 thermoelectric material via oxygen doping. Nevertheless, an efficient thermoelectric module requires both n-type and p-type materials with similar compatibility factors. Here, we present a facile approach to obtain a p-type MoS2 thermoelectric material with a maximum figure of merit of 0.18 through the introduction of VMo2S4 as a second phase by vanadium doping. VMo2S4 nanoinclusions, confirmed by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) measurements, not only improve the electrical conductivity by simultaneously increasing the carrier concentration and the mobility but also result in the reduction of lattice thermal conductivity by enhancing the interface phonon scattering. Our studies not only shed new light toward improving thermoelectric performance of TMDCs by a facile elemental doping strategy but also pave the way toward thermoelectric devices based on TMDCs.

Extraordinary Thermoelectric Performance Realized in n-Type PbTe through Multiphase Nanostructure Engineering.

Lead telluride has long been realized as an ideal p-type thermoelectric material at an intermediate temperature range; however, its commercial applications are largely restricted by its n-type counterpart that exhibits relatively inferior thermoelectric performance. This major limitation is largely solved here, where it is reported that a record-high ZT value of ≈1.83 can be achieved at 773 K in n-type PbTe-4%InSb composites. This significant enhancement in thermoelectric performance is attributed to the incorporation of InSb into the PbTe matrix resulting in multiphase nanostructures that can simultaneously modulate the electrical and thermal transport. On one hand, the multiphase energy barriers between nanophases and matrix can boost the power factor in the entire temperature range via significant enhancement of the Seebeck coefficient and moderately reducing the carrier mobility. On the other hand, the strengthened interface scattering at the intensive phase boundaries yields an extremely low lattice thermal conductivity. This strategy of constructing multiphase nanostructures can also be highly applicable in enhancing the performance of other state-of-the-art thermoelectric systems.

On the Half-Heusler compounds Nb1-x{Ti,Zr,Hf}xFeSb: Phase relations, thermoelectric properties at low and high temperature, and mechanical properties

A re-investigation of phase equilibria, crystal structure and homogeneity region of the Half-Heusler (HH) phase in the ternary system Nb-Fe-Sb at 600 °C has solved controversies in the literature confirming the version of Melnyk et al. ([1] J. Phase Equilibria 20 (2) (1999) 113–118). For the first time transport properties of Half-Heusler (HH) compounds NbFeSb and Nb0.85M0.15FeSb (M = Ti, Zr, Hf) were studied in the full temperature range from 4.2 to 823 K. The semiconducting material NbFeSb has an electronic structure close to a metal-to-insulator transition, which leads to changes of the conductivity type with the composition as well as with increasing temperature. Ti, Zr and Hf doped NbFeSb alloys show metallic behavior and were confirmed to be high ZT p-type thermoelectric materials. Surprisingly, the lattice thermal conductivity for the Zr-doped composition was found to be higher than those of the Ti- and Hf doped materials; this effect can be explained in terms of mass and strain field fluctuations.For the first time we report experimental information on thermal expansion coefficients, specific heat and elastic moduli for these p-type compounds. The mechanical properties show a good compatibility with those previously reported for n-type HH alloys.

Solution-Processed Cu2Se Nanocrystal Films with Bulk-Like Thermoelectric Performance

Thermoelectric power generation can play a key role in a sustainable energy future by converting waste heat from power plants and other industrial processes into usable electrical power. Current thermoelectric devices, however, require energy intensive manufacturing processes such as alloying and spark plasma sintering. Here, we describe the fabrication of a p-type thermoelectric material, copper selenide (Cu2Se), utilizing solution-processing and thermal annealing to produce a thin film that achieves a figure of merit, ZT, which is as high as its traditionally processed counterpart, a value of 0.14 at room temperature. This is the first report of a fully solution-processed nanomaterial achieving performance equivalent to its bulk form and represents a general strategy to reduce the energy required to manufacture advanced energy conversion and harvesting materials.

Improved thermoelectric performance of n-type half-Heusler MCo1-xNixSb (M = Hf, Zr)

The MCoSb-based (M = Hf, Zr) half-Heusler compounds were recognized as a promising p-type thermoelectric (TE) material for more than 2 decades although the base compound is intrinsically n-type. Here we investigate the TE properties of Ni-substituted n-type MCoSb. The anomalous changes of carrier concentration and lattice thermal conductivity with higher amount of Ni indicate the presence of atomic disorder. Peak power factor of ∼33 μW cm−1 K−2 and peak ZT of 0.6 are obtained in ZrCo0.9Ni0.1Sb. Further substitute Zr by Hf suppresses the lattice thermal conductivity and yields a peak ZT exceeding 1 in the composition Zr0.5Hf0.5Co0.9Ni0.1Sb at 1073 K. Thus the MCoSb compounds possess promising TE properties by both n- and p-type doping, which is unique among the half-Heusler based TE materials.

The Influence of Mobile Copper Ions on the Glass-Like Thermal Conductivity of Copper-Rich Tetrahedrites

Tetrahedrites are promising p-type thermoelectric materials for energy recovery. We present here the first investigation of the structure and thermoelectric properties of copper-rich tetrahedrites, Cu12+xSb4S13 (0 0 consist of two tetrahedrite phases. In situ neutron diffraction data demonstrate that on heating, the two tetrahedrite phases coalesce into a single tetrahedrite phase at temperatures between 493 and 553 K and that this transition shows marked hysteresis on cooling. Our structural data indicate that copper ions become mobile above 393 K. Marked changes in the temperature dependence of the electrical and thermal transport properties of the copper-rich phases occur at the onset of copper mobility. Excess copper leads to a significant reduction in the total thermal conductivity, which for the nominal composition Cu14Sb4S13 reaches a value as low as 0.44 W m–1 K–1 at room temperature, and to thermoelectric properties consistent with phonon liqu...

Fabrication of Bi–Sb–Te Thermoelectric by Cold-Pressed Sintering for Motorcycle Exhaust.

This study was conducted on the Bi–Sb–Te thermoelectric material which is cold-pressed Sintering under 750 Mpa to make square thermoelectric pairs with size 8.2 mm × 8.2 mm and thicknesses 0.8 mm and 1.5 mm. The zone melting method was used to acquire P-type thermoelectric material Bi0.4Sb1.6Te3 and N-type thermoelectric material Bi2Te2.5Se0.5. At temperature 383 K, the measured Seebeck coefficient of Bi0.4Sb1.6Te3 is 222 μV/K, and its thermoelectric figure of merit ZT is 1.35. At temperature 400 K, the measured Seebeck coefficient of Bi2Te2.5Se0.5 is 210 μV/K, and its thermoelectric figure of merit ZT is 1.13. Using Solder paste Sn42Bi58 and copper electrode plate are in series connection with 16 pieces of P/N thermoelectric material to form thermoelectric modules. The thermoelectric module is actually pasted on the motorcycle waste heat source to be evaluated the performance, making the cold-end temperature dissipation heat can enhance the temperature difference between it so as to increase the output power. Increasing the leg thickness of thermoelectric module and making the about 35 °C temperature-difference of those can obviously enhance the performance of in terms of its voltage, its thermoelectric figure of merit ZT and output power of the thermoelectric modules.

Fabrications of Polyaniline Films by Pulse Electrodeposition in Acidic Solutions with Different Anions and Their Thermoelectric Performances

Polymerization of aniline was prepared by the pulse potentiostatic method in H3PO4, HClO4 and H2SO4 acidic solutions. The morphologies and thermoelectric performances were analyzed by scanning electron microscopy, Seebeck coefficient (S) and resistivity (R) measurements. The results show that flake polyaniline (PANI) films can be obtained in H3PO4 and HClO4 acidic solutions, and porous PANI films with nanofiber-overlapped structures can be prepared in H2SO4 solution under the same pulse parameters. PANI films prepared in the three solutions are all p-type thermoelectric materials. PANI films polymerized in H2SO4 solution possess the highest S (30.2 μV K−1) and lowest R (1.6 × 10−3 Ω m) compared with those prepared in H3PO4 and HClO4 solutions, indicating that nanofiber-overlapped structures formed in H2SO4 solution contribute better thermoelectric performance. In addition, the effects of pulse parameters (anodic potential φa, anodic pulse duration ta and cathodic pulse duration tc) on the surface morphologies and thermoelectric performances of PANI films were systematically investigated.

Development of Flexible Thermoelectric Cells and Performance Investigation of Thermoelectric Materials for Power Generation

Abstract This paper presents experimental performance results for custom made thermoelectric generator. Two fabrication methods, powder compaction and painting over flexible substrate have been proposed and investigated. Experimental performance results have been presented for thermoelectric cells made from alloying Bismuth (Bi), Tellurium (Te) and Antimony (Sb). Powdered P-type (Bi0.4Sb1.6Te3) and N-type (Bi2Te3) thermoelectric materials were chosen for the construction of the thermoelectric cell. Experimental results showed that P-type Bi0.4Sb1.6Te3 had Seebeck coefficient of 211.77 μV/°C and while N-typeBi2Te3 had a Seebeck coefficient of 109.09 μV/°C. The flexible thermoelectric generator hasshown linear increase in open circuit voltage with increase in temperature difference across the cold and hot side.

Electrical Transport Properties of Single-Crystalline β-Zn4Sb3 Prepared Through the Zn-Sn Mixed-Flux Method

β-Zn4Sb3 is a promising p-type thermoelectric material for utilization in moderate temperatures. This study prepares a group of single-crystalline β-Zn4Sb3 samples using the Zn-Sn mixed-flux method based on the stoichiometric ratios of Zn4+x Sb3Sn y . The effect of Zn-to-Sn proportion in the flux on the structure and electrical transport properties is investigated. All samples are strip-shaped single crystals of different sizes. The actual Zn content of the present samples is improved (>3.9) compared with that of the samples prepared through the Sn flux method. Larger lattice parameters are also obtained. The carrier concentration of all the samples is in the order of over 1019 cm−3. With increasing Sn rate in the flux, this carrier concentration decreases, whereas mobility is significantly enhanced. The electrical conductivity and Seebeck coefficients of all the samples exhibit a behavior that of a degenerate semiconductor transport. Electrical conductivity initially increases and then decreases as the Sn ratio in the flux increases. The electrical conductivity of the x:y = 5:1 sample reaches 6.45 × 104 S m−1 at 300 K. Benefitting from the electrical conductivity and Seebeck coefficient, the flux proportion of the x:y = 7:1 sample finally achieves the highest power factor value of 1.4 × 10−3 W m−1 K−2 at 598 K.

One step synthesis and sintering of Ni and Zn substituted tetrahedrite as thermoelectric material

Tetrahedrite mineral family, one of the most widespread sulfosalts on Earth's crust, seems to meet the right features for an attractive sustainable p-type thermoelectric material, showing relatively high conversion efficiency. The sulphide powders were ball milled and then one-step simultaneous synthesis and sintering process was performed by open die pressing (ODP). This simple, fast, and easy scalable route was extensively discussed for the first time in this work and permitted to obtain thermoelectric pellets starting from sulphide precursors, with a whole process lasting less than 6 h. Both Zn and Ni cations were added as partial substituents of Cu in Cu12Sb4S13 permitting to considerably improve the thermoelectric performances of the undoped material prepared by ODP. The influence of precursors ratio on the tetrahedrite phase, secondary phase content and stoichiometry were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The density and mechanical stability of samples were evaluated as a function of the chemical composition and processing parameters. A complete thermoelectric characterization was carried out for samples with different Zn and Ni substitution up to 400 °C.

Facile Control of Interfacial Energy-Barrier Scattering in Antimony Telluride Electrodeposits

The augmented thermoelectric performance of nanocrystalline antimony telluride (Sb2Te3) films is investigated by introducing interfacial energy-barrier scattering (i.e., barrier heights), which occurs at both the grain boundaries and the interfaces with embedded second phases. It is postulated that the barriers created at both the interfaces and boundaries filter the low-energy carriers, thus favoring a high Seebeck coefficient. A facile, but high-precision composition-controlled electrodeposition technique is employed to synthesize single-phase nanocrystalline Sb2Te3 and nanocomposite Te/Sb2Te3. Both the initial composition of the Sb-Te solid solution and the post-annealing profiles are varied to control the grain size, as well as the formation of second-phase Te. The electrical and thermoelectric properties are measured and correlated with the physical properties, where an enhanced Seebeck coefficient at a fixed carrier concentration is interpreted as indicating that the energy-dependent carrier filtering effect is in force. On a promising note, modification of the Sb2Te3 film physical properties and formation of the second phase affect the interfacial energy-barrier scattering and yields an enhanced power factor. Thus, Sb2Te3 film is a promising p-type thermoelectric material for a room-temperature-operational micro-thermoelectric power generator.