Thermoelectric Properties Of P-type Research Articles

Defect engineering and alloying strategies for tailoring thermoelectric behavior in GeTe and its alloys

GeTe exhibits excellent p-type medium-temperature thermoelectric properties with low toxicity and good mechanical characteristics, making it highly promising for development in the thermoelectric field. However, GeTe is prone to producing Ge vacancies, leading to high p-type carrier concentration, which results in elevated electronic thermal conductivity and a low Seebeck coefficient. This study systematically analyzes intrinsic and extrinsic defects in GeTe and its alloys, focusing on reducing p-type carrier concentration through first-principles calculations. The results reveal that substituting Ge-sites with Bi (BiGe) yields lower donor defect formation energy, effectively reducing p-type carrier concentration of GeTe and its alloys compared to other elemental doping. Additionally, alloying with certain elements, such as Pb, proves favorable for decreased p-type carrier concentration due to lowered energy levels of valence band maximum (VBM). Inspired by this, screening divalent elements for alloying on Ge-sites reveals that Sr, Ba, Eu, and Yb substantially reduce the VBM of GeTe. Further calculations for Ba and Yb-alloyed GeTe confirm changes in formation energies for donor (favorable) and acceptor (unfavorable) defects. Our work provides a systematic investigation of intrinsic and various extrinsic doping defects in GeTe and its alloys, shedding light on possible strategies of optimizing carrier concentration in these compounds.

High-throughput deformation potential and electrical transport calculations

The deformation potential plays an important role in electrical transport properties, but in the context of high-throughput searches requires a consistent and readily computable reference level. Here, we design a high-throughput method for calculating the deformation potential for semiconductors in the MatHub-3d database. The deformation potential is obtained from the volume-dependent band edge (valence band maximum and conduction band minimum) variations with respect to the reference level. In order to adapt the calculation of reference level to a high-throughput manner, the average value of the first valence electron energy band associated with each compound is used as the reference level. Deformation potentials for 11,993 materials are calculated using the above-mentioned method. By considering the calculated deformation potentials, electronic structures, and bulk moduli, 9957 compounds with converged electrical transport properties are obtained. 332 promising p-type thermoelectric materials, and 321 n-type entries are then proposed. Among them, 156 compounds are screened to having both potentially good n- and p-type thermoelectric transport properties. The band structures and chemical bonding information for several typical compounds are further presented to reveal favorable band and bonding features for thermoelectrics.

Opening the Bandgap of Metallic Half-Heuslers via the Introduction of d-d Orbital Interactions.

Half-Heusler compounds with semiconducting behavior have been developed as high-performance thermoelectric materials for power generation. Many half-Heusler compounds also exhibit metallic behavior without a bandgap and thus inferior thermoelectric performance. Here, taking metallic half-Heusler MgNiSb as an example, a bandgap opening strategy is proposed by introducing the d-d orbital interactions, which enables the opening of the bandgap and the improvement of the thermoelectric performance. The width of the bandgap can be engineered by tuning the strength of the d-d orbital interactions. The conduction type and the carrier density can also be modulated in the Mg1- x Tix NiSb system. Both improved n-type and p-type thermoelectric properties are realized, which are much higher than that of the metallic MgNiSb. The proposed bandgap opening strategy can be employed to design and develop new half-Heusler semiconductors for functional and energy applications.

Saturation limit and p-type thermoelectric properties of RuAs2−xGex

Ruthenium arsenide is made p-type by the addition of germanium, and it exhibits a large substitution range without affecting the stability. A series of RuAs2−xGex with x = 0.02, 0.04, 0.08, 0.16, 0.32, and 0.64 shows the saturation limit of Ge to be between 0.16 and 0.32. The electrical contribution to the thermoelectric performance is greatly improved with a power factor of 1.03 mW/(m K2). However, the substitution does not affect the rigidity of the lattice, as the Debye temperature remains around 420 K, which means that the thermal conductivity remains high resulting in a modest maximum zT of 0.11.

Entropy engineering: A simple route to both p- and n-type thermoelectrics from the same parent material

Ternary half-Heusler (HH) semiconductors have been identified as excellent high-temperature thermoelectric (TE) materials, but the high thermal conductivity restricts the development and application. In this work, we report the design of the HH alloy Ti(Fe/Co/Ni)Sb with both p-type and n-type conductions from the same parent compound TiCoSb by entropy engineering. High-throughput (HTP) experiments have been first conducted to verify the formation of Ti(Fe1/3+xCo1/3Ni1/3-x)Sb solid solution and screen the range of compositions with the best p-type and n-type thermoelectric properties by regulating the Fe/Ni ratio. The random mixing of Fe/Co/Ni on the 4c site of the HH lattice induces a large lattice distortion, leading to a significantly reduced lattice thermal conductivity. Adjusting the sample composition (Fe/Ni ratio) changes the carrier concentration, carrier type and electronic band structure simultaneously, contributing to improved TE power factors for both p- and n-type materials. As a result, a peak TE figure of merit zT of up to ∼0.56 for p-type and ∼0.49 for n-type are achieved. Our experimental results demonstrate that entropy engineering is a promising strategy to extend the composition range and tune the TE properties for HH materials, which might be a generally applicable route to improve the thermoelectric performance of other TE materials.

Influence of carbon nanotubes on thermoelectric properties of p- and n-type Heusler alloys

This paper presents the results of studying the effect of carbon nanotubes on thermoelectric properties of p-type (Nb0.6Ta0.4)0.8Ti0.2FeSb and n-type Ti0.5Zr0.25Hf0.25NiSn half Heusler alloys. The experimental data obtained indicate a strong effect of the carbon nanotubes on electrical conductivity and Seebeck coefficient of the n-type compound, while the changes in these properties in the p-type compound were significantly less. It is suggested that a possible reason for this difference is the formation of a conducting cluster of carbon nanotubes in the sample of the n-type Heusler alloy. Keywords: carbon nanotubes, thermoelectric properties, Heusler alloys, electrical conductivity, Seebeck coefficient.

Fabrication of ultrafine powder using processing control agent, and investigation of its effect on microstructure and thermoelectric properties of p-type (Bi, Sb)2Te3 alloys

Low-dimensional materials can significantly enhance the efficiency of thermoelectric devices for power generation and cooling applications. In the present work, ultra-fine powders of p-type (Bi, Sb)2Te3 alloys are fabricated through high energy ball milling using stearic acid as a process control agent (PCA). The influence of the PCA addition on powder characteristics, microstructure and thermoelectric transport properties are studied. Further, the ultra-fine powder is subjected to calcination (Cal-PCA) and subsequently consolidated all powders using spark plasma sintering (SPS). The PCA, Cal-PCA, and non-PCA powder morphological effects on the microstructure and thermoelectric properties are systematically investigated and elucidated thoroughly. The electron beam scattering diffraction (EBSD) results confirmed that the PCA sample exhibited very fine grains (average grain size of ∼800 nm) compared to the non-PCA (average grain size of about 2.6 µm), while the grains were distributed randomly for all samples. Formation of fine grains and partial existence of the stearic acid (carbon and oxygen phases) in the matrix were strongly inhibiting the transport of the carriers that severely decreased the carrier mobility, reflecting the severe reduction in electrical conductivity for PCA sample compared to Cal-PCA and non-PCA. The lowest thermal conductivity (κ) of 0.745 W/mK was achieved for the PCA sample, which is 19%, 12% lower than that of non-PCA, and Cal-PCA samples. The strong reduction in κ was mainly attributed to the dramatic decrease in the phonon thermal conductivity owing to phonon scattering at numerous grain boundaries and oxide phases. The obtained high electrical conductivity with balanced thermal conductivity in Cal-PCA sample is attributed to the significant improvement in ZT of 1.1, which is 27%, and 47% higher than that of the Non-PCA sample at room temperature, and 350 K, respectively.

Thermoelectric performances for both p- and n-type GeSe.

In this paper, the thermoelectric properties of p-type and n-type GeSe are studied systematically by using first principles and Boltzmann transport theory. The calculation includes electronic structure, electron relaxation time, lattice thermal conductivity and thermoelectric transport properties. The results show that GeSe is an indirect band gap semiconductor with band gap 1.34 eV. Though p-type GeSe has a high density of states near Fermi level, the electronic conductivity is relative low because there is no carrier transport pathway along the a-axis direction. For n-type GeSe, a charge density channel is formed near conduction band minimum, which improves the electrical conductivity of n-type GeSe along the a-axis direction. At 700 K, the optimal ZT value reaches 2.5 at 4 × 1019 cm−3 for n-type GeSe, while that is 0.6 at 1 × 1020 cm−3 for p-type GeSe. The results show n-type GeSe has better thermoelectric properties than p-type GeSe, indicating that n-type GeSe is a promising thermoelectric material in middle temperature.

The thermoelectric performance of new structure SnSe studied by quotient graph and deep learning potential

The structure predicting and structure screening of new structure SnSe are studied by quotient graph and deep learning potential. Using the quotient graph, we predict the new structure based on the feature of the experimental structure. Facing many structures generated by the quotient graph and reducing computation cost, the deep learning potential is used to optimize the structure. Through the electronic fitness function, it is found that the new structure with space group no. 64 shows a good electrical property of p-type and n-type. Based on the new structure, the n-type and p-type thermoelectric properties are studied by First-principles calculations and Boltzmann transport theory. The mechanical stability and thermodynamic stability of the new structure are checked, indicating that the new structure is stable. Due to the cyclic structure, the new structure shows large values of bulk modulus (B ~ 62.82 GPa), shear modulus (G ~ 43.78 GPa), Young’s modulus (E ~ 106.59 GPa), Debye temperature (θ = 291 K), sound velocity (Va ~2890 m/s), and low value of Gruneisen parameter γ ~2.2, implying that strong Sn–Se atomic interactions and large lattice thermal conductivity. At 800 K, the figure of merit of p-type (1.83) is superior to that of n-type (1.68). The main scattering process of both charge transport and thermal transport is dominated by optical phonon branches. This work not only demonstrates the excellent property of the new structure but also provides insights to understand the transport performance of the cyclic structure.

Large thermoelectric power factor in whisker crystals of solid solutions of the one-dimensional tellurides Ta4SiTe4 and Nb4SiTe4

One-dimensional tellurides Ta4SiTe4 and Nb4SiTe4 were found to show high thermoelectric performance below room temperature. This study reported the synthesis and thermoelectric properties of Ta4SiTe4-Nb4SiTe4 solid solutions and Mo- or Ti-doped (Ta0.5Nb0.5)4SiTe4. Thermoelectric power of the solid solutions systematically increased with increasing Ta content, while their electrical resistivity was unexpectedly small. Mo- and Ti-doped (Ta0.5Nb0.5)4SiTe4 showed n- and p-type thermoelectric properties with large power factors exceeding 40 μW cm−1 K−2, respectively. The fact that not only Ta4SiTe4 and Nb4SiTe4 but also their solid solutions showed high performance indicated that this system is a promising candidate for thermoelectric applications at low temperatures.

Promising room temperature thermoelectric conversion efficiency of zinc-blende AgI from first principles

The theoretical investigation on structural, vibrational, and electronic properties of zinc-blende (ZB) AgI were carried out employing first principles density functional theory calculations. Thermoelectric properties then were predicted through semi-classical Boltzmann transport equations within the constant relaxation time approximation. Equilibrium lattice parameter, bulk modulus, elastic constants, and vibrational properties were calculated by using generalized gradient approximation. Calculated properties are in good agreement with available experimental values. Electronic and thermoelectric properties were investigated both with and without considering spin–orbit coupling (SOC) effect which is found to have a strong influence on p-type Seebeck coefficient as well as the power factor of the ZB–AgI. By inclusion of SOC, a reduction of the band-gap and p-type Seebeck coefficients as well as the power factor was found which is the indication of that spin–orbit interaction cannot be ignored for p-type thermoelectric properties of the ZB–AgI. By using deformation potential theory for electronic relaxation time and experimentally predicted lattice thermal conductivity, we obtained a ZT value 1.69 (0.89) at 400 K for n-type (p-type) carrier concentration of 1.5 × 1018 (4.6 ×1019) cm−3 that makes ZB–AgI as a promising room temperature thermoelectric material.

Improved thermoelectric performance of flexible p-type SiGe films by B-doped Al-induced layer exchange

The Al-induced layer exchange is a useful method for the low-temperature synthesis of high-performance SiGe thermoelectric thin films. Because B has a high solid solubility in SiGe, we examined the self-organized doping of B to SiGe during layer exchange to further improve p-type thermoelectric properties. The hole concentration and the electrical conductivity were improved in the whole SiGe composition range when an appropriate amount (~1%) of B to Al was added. A Si0.4Ge0.6 layer fabricated on a polyimide substrate at 350 °C exhibited a power factor of 240 µW m−1 K−2, which is best recorded as environmentally-friendly inorganic semiconductors formed on flexible substrates. The low-temperature Fermi-level-control technology for semiconductor films will exploit various flexible devices including thin-film thermoelectrics.

High-Throughput Screening for Advanced Thermoelectric Materials: Diamond-Like ABX2 Compounds.

High-throughput (HTP) calculations are a highly promising direction for the discovery of novel functional materials. Here we use an HTP framework to investigate the electronic structures and p-type thermoelectric properties of the ABX2 compounds with diamond-like structures. We show application of HTP both to identify compounds and also to identify underlying trends. A total of 65 entries out of 84 908 in the Materials Informatics Platform are selected for this study. The electronic structures and chemical-bonding analyses reveal that there exists a general conductive network consisting of the anion X sublattice, which dominates the electrical transport properties of the compounds. Electrical and thermal transport properties of the 41 pnictide and chalcogenide compounds with sufficient band gaps are studied. Pnictide compounds have relatively smaller Seebeck coefficients than the chalcogenide compounds. This is due to the smaller effective masses around the valence band maxima. The electrical conductivities and power factors, however, are better in pnictide compounds. This is because pnictide compounds have high electronic group velocities and electronic relaxation times. Combined with the predictions of lattice thermal conductivities based on the Slack model, 12 novel p-type and n-type ABX2 materials with high ZT values are predicted.

Direct observation of pseudo-gap electronic structure in the Heusler-type Fe2VAl thin film

We investigated the electronic structure of a thermoelectric material, Heusler-type Fe2VAl thin film, by hard X-ray photoemission spectroscopy and electronic band structure calculation to understand the improvement process of the thermoelectric properties of this material. Electronic structure of the Fe2VAl thin film showed a semi-metallic electronic structure with a pseudo-gap at the Fermi level. The observed electronic structure of the Fe2VAl thin film suggests that an optimal carrier doping exists to best control the n- and p-type thermoelectric properties and enhance the power factors.

Carbon Nanotube-Based Organic Thermoelectric Materials for Energy Harvesting.

Carbon nanotubes (CNTs) have attracted much attention in developing high-performance, low-cost, flexible thermoelectric (TE) materials because of their great electrical and mechanical properties. Theory predicts that one-dimensional semiconductors have natural advantages in TE fields. During the past few decades, remarkable progress has been achieved in both theory and experiments. What is more important is that CNTs have shown desirable features for either n-type or p-type TE properties through specific strategies. Up to now, CNT‒polymer hybrids have held the record for TE performance in organic materials, which means they can potentially be used in high-performance TE applications and flexible electronic devices. In this review, we intend to focus on the intrinsic TE properties of both n-type and p-type CNTs and effective TE enhanced strategies. Furthermore, the current trends for developing CNT-based and CNT‒polymer-based high TE performance organic materials are discussed, followed by an overview of the relevant electronic structure‒TE property relationship. Finally, models for evaluating the TE properties are provided and a few representative samples of CNT‒polymer composites with high TE performance are highlighted.

Electronic and Thermoelectric Properties of SnSe1−xS x (x = 0, 0.25, 0.5, 0.75, and 1) Alloys: First-Principles Calculations

Ab initio investigations of the electronic and thermoelectric (TE) properties of SnSe1−xS x (x = 0, 0.25, 0.5, 0.75, and 1) alloys are performed using density functional theory. The TE properties are calculated using the semi-classical Boltzmann transport theory within the constant relaxation time approximation. Band gap values are found to range between 0.94 eV and 1.02 eV in agreement with the experimental findings and previous calculations. All alloys tend to exhibit p-type TE properties, indicated by a sharp peak near the Fermi level that indicates a heavy carrier concentration. Electrical conductivity is found to decrease, whereas the Seebeck coefficient and the power factor increase for higher concentrations. The three alloys, SnS, SnSe and SnSe0.75S0.25 alloys exhibit the same power factor of 3.5 × 10−3 W/m K2, which is promising for thermoelectric applications.

Control of p-type and n-type thermoelectric properties of bismuth telluride thin films by combinatorial sputter coating technology

p- and n-type bismuth telluride thin films have been synthesized by using a combinatorial sputter coating system (COSCOS). The crystal structure and crystal preferred orientation of the thin films were changed by controlling the coating condition of the radio frequency (RF) power during the sputter coating. As a result, the p- and n-type films and their dimensionless figure of merit (ZT) were optimized by the technique. The properties of the thin films such as the crystal structure, crystal preferred orientation, material composition and surface morphology were analyzed by X-ray diffraction, energy-dispersive X-ray spectroscopy and atomic force microscopy. Also, the thermoelectric properties of the Seebeck coefficient, electrical conductivity and thermal conductivity were measured. ZT for n- and p-type bismuth telluride thin films was found to be 0.27 and 0.40 at RF powers of 90 and 120W, respectively. The proposed technology can be used to fabricate thermoelectric p–n modules of bismuth telluride without any doping process.

High-Temperature Thermoelectric Properties of Perovskite-Type Pr0.9Sr0.1Mn1−x Fe x O3 (0 ≤ x ≤ 1)

Polycrystalline samples of Pr0.9Sr0.1Mn1−x Fe x O3 (0 ≤ x ≤ 1) have been synthesized using a conventional solid-state reaction method, and the crystal structure studied at room temperature. The magnetic susceptibility was measured from 5 K to 350 K. The electrical resistivity, Seebeck coefficient, and thermal conductivity were investigated as functions of temperature below 850 K. For all samples, the perovskite structure at room temperature exhibited orthorhombic Pbnm phase. While the Pr0.9Sr0.1MnO3 (x = 0) sample exhibited ferromagnetic-like ground state below T C = 145 K (Curie temperature), the ferromagnetic transition temperature T C decreased with increasing x. The Seebeck coefficient of the samples with 0 ≤ x ≤ 0.8 decreased with increasing temperature because of double-exchange interaction of Mn ions. In fact, the carrier type for x = 0 changed from hole-like to electron-like behavior above 800 K. On the other hand, the samples with x ≥ 0.9 showed large positive Seebeck coefficient over the entire temperature range, indicating that the low-spin state of Fe ions dominated the electronic structure for this x range. In particular, the sample with x = 1 exhibited p-type thermoelectric properties with relatively high Seebeck coefficient, moderate electrical resistivity, and low thermal conductivity. Thus, the sample with x = 1 showed power factor of 20 μW m−1 K−2 at 850 K leading to ZT of 0.024 at this temperature, indicating that hole-doped perovskite-type iron oxide is a good candidate high-temperature thermoelectric p-type oxide.

Promising high temperature thermoelectric properties of dense Ba2Co9O14 ceramics

Layered cobalt oxides have shown high thermoelectric properties. The n=1 member of the Ban+1ConO3n+3(Co8O8) family, Ba2Co9O14, a new layered cobalt oxides family with Co(II) and Co(III) in the CdI2 layers, has been synthesized by solid state reaction and sintered as dense ceramics (relative density∼93%) by Spark Plasma Sintering. It presents promising p-type thermoelectric properties at high temperature. The dimensionless figure of merit ZT is 0.032 at 660K and 0.04 at 1000K, which is about one quarter to one third of the ZT value of Ca3Co4O9 ceramics.

Studies on Microstructure and Thermoelectric Properties of p-Type Bi-Sb-Te Based Alloys by Gas Atomization and Hot Extrusion Processes

p-Type Bi2Te3 + 75% Sb2Te3 based thermoelectric materials were fabricated via gas atomization and the hot extrusion process. The gas atomized powder showed a clean surface with a spherical shape, and expanded in a wide particle size distribution (average particle size 50 μm). The phase of the fabricated extruded and R-extruded bars was identified using x-ray diffraction. The relative densities of both the extruded and R-extruded samples were measured by Archimedes principle with ∼98% relative density. The R-extruded bar exhibited finer grain microstructure than that of single extrusion process, which was attributed to a recrystallization mechanism during the fabrication. The R-extruded sample showed improved Vickers hardness compared to the extruded sample due to its fine grain microstructure. The electrical conductivity improved for the extruded sample whereas the Seebeck coefficient decreases due to its high carrier concentration. The peak power factor, ∼4.26 × 10−3 w/mK2 was obtained for the single extrusion sample, which is higher than the R-extrusion sample owing to its high electrical properties.