Electrically Triggered Domain Wall Movement in Cu2Se Semiconductor.

The ion-conductive α-Cu2Se is found to possess antipolar dipoles, and the movement of the domain boundary under the applied voltage causes change of resistance, showing promising application in memristors. However, due to the complex ordering of Cu ions in the α-Cu2Se, there are multiple types of domain wall structure. Here, we show that two typical domain walls in α-Cu2Se can be formed, by controlling the voltage during phase transition from high-temperature cubic ß-Cu2Se to α-Cu2Se. We also show by in situ transmission electron microscopy that the formed [01̅0]/[101̅] domain wall performs a reversible movement under the applied external voltage, while the [010]/[01̅0] domain wall does not move. We further demonstrate that pinning of the [010]/[01̅0] domain wall could be due to the formed dislocations in the interface. This study shows that applying preprocess conditions is important to obtain the designed microstructure and resistive properties of α-Cu2Se.

Excellent Stability of Ga-Doped Garnet Electrolyte against Li Metal Anode via Eliminating LiGaO2 Precipitates for Advanced All-Solid-State Batteries.

Ga-doped garnet-type Li7La3Zr2O12 (Ga-LLZO) ceramics have long been recognized as ideal electrolyte candidates for all-solid-state lithium batteries (ASSLBs). However, in this study, it is shown that Ga-LLZO easily and promptly cracks in contact with molten lithium during the ASSLB assembly. This can be mainly ascribed to two aspects: (i) lithium captures O atoms and reduces Ga ions of the Ga-LLZO matrix, leading to a band-gap closure from >5 to <2 eV and a structural collapse from cubic to tetrahedral; and (ii) the in situ-formed LiGaO2 impurity phase has severe side reactions with lithium, resulting in huge stress release along the grain boundaries. It is also revealed that, while the former process consumes hours to take effect, the latter one is immediate and accounts for the crack propagation of Ga-LLZO electrolytes. A minute SiO2 is preadded during the synthesis of Ga-LLZO and found effective in eliminating the LiGaO2 impurity phase. The SiO2-modified Ga-LLZO solid electrolytes display excellent thermomechanical and electrochemical stabilities against lithium metals and well-reserved ionic conductivities, which was further confirmed by half-cells and full batteries. This study contributes to the understanding of the stability of garnet electrolytes and promotes their potential commercial applications in ASSLBs.

Enhancing the Thermoelectric and Mechanical Properties of Bi0.5Sb1.5Te3 Modulated by the Texture and Dense Dislocation Networks.

Bi2Te3-based materials are dominating thermoelectrics for almost all of the room-temperature applications. To meet the future demands, both their thermoelectric (TE) and mechanical properties need to be further improved, which are the requisite for efficient TE modules applied in areas such as reliable micro-cooling. The conventional zone melting (ZM) and powder metallurgy (PM) methods fall short in preparing Bi2Te3-based alloys, which have both a highly textured structure for high TE properties and a fine-grained microstructure for high mechanical properties. Herein, a mechanical exfoliation combined with spark plasma sintering (ME-SPS) method is developed to prepare Bi0.5Sb1.5Te3 with highly improved mechanical properties (correlated mainly to the dislocation networks), as well as significantly improved thermoelectric properties (correlated mainly to the texture structure). In the method, both the dislocation density and the orientation factor (F) can be tuned by the sintering pressure. At a sintering pressure of 20 MPa, an exceptional F of up to 0.8 is retained, leading to an excellent power factor of 4.8 mW m-1 K-2 that is much higher than that of the PM polycrystalline. Meanwhile, the method can readily induce high-density dislocations (up to ∼1010 cm-2), improving the mechanical properties and reducing the lattice thermal conductivity as compared to the ZM ingot. In the exfoliated and then sintered (20 MPa) sample, the figure-of-merit ZT = 1.2 (at 350 K), which has increased by about ∼20%, and the compressive strength has also increased by ∼20%, compared to those of the ZM ingot, respectively. These results demonstrate that the ME-SPS method is highly effective in preparing high-performance Bi2Te3-based alloys, which are critical for TE modules in commercial applications at near-room temperature.

Power generation and thermoelectric cooling enabled by momentum and energy multiband alignments.

Thermoelectric materials transfer heat and electrical energy, hence they are useful for power generation or cooling applications. Many of these materials have narrow bandgaps, especially for cooling applications. We developed SnSe crystals with a wide bandgap (E g ≈ 33 k B T) with attractive thermoelectric properties through Pb alloying. The momentum and energy multiband alignments promoted by Pb alloying resulted in an ultrahigh power factor of ~75 µW cm-1 K-2 at 300 K, and an average figure of merit ZT of ~1.90. We found that a 31-pair thermoelectric device can produce a power generation efficiency of ~4.4% and a cooling ΔT max of ~45.7 K. These results demonstrate that wide-bandgap compounds can be used for thermoelectric cooling applications.

Strong Anisotropy and Bipolar Conduction-Dominated Thermoelectric Transport Properties in the Polycrystalline Topological Phase of ZrTe5.

ZrTe5 has unique features of a temperature-dependent topological electronic structure and anisotropic crystal structure and has obtained intensive attention from the thermoelectric community. This work revealed that the sintered polycrystalline bulk ZrTe5 possesses both (020) and (041) preferred orientations. The transport properties of polycrystalline bulk p-type ZrTe5 exhibits an obvious anisotropic characteristic, that is, the room-temperature resistivity and thermal conductivity, possessing anisotropy ratios of 0.71 and 1.49 perpendicular and parallel to the pressing direction, respectively. The polycrystalline ZrTe5 obtained higher ZT values in the direction perpendicular to the pressing direction, as compared to that in the other direction. The highest ZT value of 0.11 is achieved at 350 K. Depending on the temperature-dependent topological electronic structure, the electronic transport of p-type ZrTe5 is dominated by high-mobility electrons from linear bands and low-mobility holes from the valence band, which, however, are merely influenced by valence band holes at around room temperature. Furthermore, external magnetic fields are detrimental to thermoelectric properties of our ZrTe5, mainly arising from the more prominent negative effects of electrons under fields. This research is instructive to understand the transport features of ZrTe5 and paves the way for further optimizing their ZTs.

Realizing High Thermoelectric Performance in Sb-Doped Ag2Te Compounds with a Low-Temperature Monoclinic Structure.

To recover the low-grade waste heat (300-500 K), it is of urgent importance to develop and improve the thermoelectric performance at a low-temperature region. Herein, we have realized a record high ZT value of 1.4 at 410 K and a record high average ZT value of 0.6 in the temperature interval from 300 to 400 K for Sb-doped Ag2SbxTe1-x (x = 0-0.03) compounds, which show an improvement of 180 and 120% compared to pristine Ag2Te, respectively. Sb doping increases the carrier concentration and electrical conductivity, leading to a remarkable improvement of electrical transport properties. The Ag2Sb0.015Te0.985 sample obtains the maximal power factor of 1.07 × 10-3 W m-1 K-2 at 410 K, which is increased by 80% in comparison to that of pristine Ag2Te. Moreover, as a result of the intensified alloying phonon scattering by Sb doping, Ag2Sb0.01Te0.99 possesses the minimum lattice thermal conductivity of 0.35 W m-1 K-1 at 300 K, which demonstrates a decline of 57% compared to that of pristine Ag2Te. All of these produce a great enhancement on the thermoelectric performance of Ag2Te materials, which shows great potential in the application of recycling the low-grade waste heat at a low-temperature region.

Identifying the Origins of High Thermoelectric Performance in Group IIIA Element Doped PbS.

In this study, the thermoelectric properties of group IIIA element (Al, Ga, In) doped PbS are systematically investigated. Al shows a low solubility limit (<1 mol %) in PbS, whereas Ga and In are soluble up to 2 mol %. Both experimental results and theoretical calculations suggest that Ga or In doping introduces strong gap states in PbS, which are the physical origins of enhanced effective mass and Seebeck coefficients. Meanwhile, a subtle simulation of carrier-concentration-dependent mobilities under single Kane band model clearly reveals that Ga doping significantly lowers the deformation potential of n-type PbS, whereas In does not. This lower deformation potential yields higher electrical conductivities at the same doping levels. The weakened electron phonon coupling phenomenon by Ga doping in PbS is further verified by our first-principles calculations. The rare combination of large effective mass and low deformation potential in Ga-doped PbS contributes to a high ZT value of ∼0.9 at 723 K, ∼50% higher than that of Cl-doped PbS control sample.

Enhanced Mechanical Properties of Na0.02Pb0.98Te/MoTe2 Thermoelectric Composites Through in-Situ-Formed MoTe2.

Lead telluride (PbTe) is one of the best thermoelectric materials in the intermediate temperature range, which shows great potential for waste heat recycling. However, its low strength and high brittleness limit its large-scale application because the thermoelectric device usually undergoes mechanical vibration, mechanical and/or thermal cycling, and thermal shock in service. In this study, the enhanced mechanical properties and thermoelectric properties of PbTe are realized simultaneously through introducing dispersive transition-metal dichalcogenide MoTe2 (molybdenum telluride). The in-situ-formed MoTe2 precipitations with a size in the range from 2 to 5 µm and the tight and smooth interface between the PbTe matrix and precipitates contribute to the obvious crack deflection, crack bridging, and pull-out of long grains, dissipating more energy during crack propagation and resulting in a tortuous propagation path. Because of the toughening and the dispersion strengthening effect, the compressive strength, bending strength, and fracture toughness of the sample with a composite amount of 1% are 109 MPa and 50 MPa and 0.65 MPa·m1/2, respectively, which are increased by about 37, 117, and 67% compared to the Na0.02Pb0.98Te matrix. Additionally, the in situ MoTe2 precipitates intensify the interface phonon scattering and thus decrease the lattice thermal conductivity. As a result, the Na0.02Pb0.98Te-1%MoTe2 sample achieves a maximum ZT value of 1.46 at 700 K, which is 11% higher than that of Na0.02Pb0.98Te without any MoTe2 nanoprecipitation.

High Figure of Merit in Gallium-Doped Nanostructured n-Type PbTe-xGeTe with Midgap States.

PbTe-based thermoelectric materials are some of the most promising for converting heat into electricity, but their n-type versions still lag in performance the p-type ones. Here, we introduce midgap states and nanoscale precipitates using Ga-doping and GeTe-alloying to considerably improve the performance of n-type PbTe. The GeTe alloying significantly enlarges the energy band gap of PbTe and subsequent Ga doping introduces special midgap states that lead to an increased density of states (DOS) effective mass and enhanced Seebeck coefficients. Moreover, the nucleated Ga2Te3 nanoscale precipitates and off-center discordant Ge atoms in the PbTe matrix cause intense phonon scattering, strongly reducing the thermal conductivity (∼0.65 W m-1 K-1 at 623 K). As a result, a high room-temperature thermoelectric figure of merit ZT ∼ 0.59 and a peak ZTmax of ∼1.47 at 673 K were obtained for the Pb0.98Ga0.02Te-5%GeTe. The ZTavg value that is most relevant for devices is ∼1.27 from 400 to 773 K, the highest recorded value for n-type PbTe.

Thermoelectric power generation: from new materials to devices.

Thermoelectric technology offers the opportunity of direct conversion between heat and electricity, and new and exciting materials that can enable this technology to deliver higher efficiencies have been developed in recent years. This mini-review covers the most promising advances in thermoelectric materials as they pertain to their potential in being implemented in devices and modules with an emphasis on thermoelectric power generation. Classified into three groups in terms of their operating temperature, the thermoelectric materials that are most likely to be used in future devices are briefly discussed. We summarize the state-of-the-art thermoelectric modules/devices, among which nanostructured PbTe modules are particularly highlighted. At the end, key issues and the possible strategies that can help thermoelectric power generation technology move forward are considered. This article is part of a discussion meeting issue 'Energy materials for a low carbon future'.

Ion Beam Induced Artifacts in Lead-Based Chalcogenides.

Metal chalcogenides have attracted great attention because of their broad applications. It has been well acknowledged that microstructure can alter the intrinsic properties and performance of metal chalcogenides. The structure-property-performance relationships can be investigated at atomic scale with scanning transmission and transmission electron microscopy (STEM and TEM). Nevertheless, careful specimen preparation is paramount for accurate analyses and interpretations. In this work, we compare the effects of a variety of well-established TEM specimen preparation methods on the observed microstructure of an ingot stoichiometric lead telluride (PbTe). Most importantly, from aberration corrected STEM and first principles calculations, we discovered that argon (Ar) ion milling can lead to surface irradiation damage in the form of Pb vacancy clusters and self-interstitial atom (SIA) clusters. The SIA clusters appear as orthogonal nanoscale features when characterized along the crystal orientation of the rock salt structured PbTe. This obfuscates the interpretation of the intrinsic microstructure of metal chalcogenides, especially lead chalcogenides. We demonstrate that with sufficiently low energy (300 eV) Ar ion cleaning or appropriate high-temperature annealing, the surface damage layer can be properly cleaned and the orthogonal nanoscale features are significantly reduced. This reveals the materials' intrinsic structure and can be used as the standard protocol for future TEM specimen preparation of lead-based chalcogenide materials.

Lattice Softening Significantly Reduces Thermal Conductivity and Leads to High Thermoelectric Efficiency.

The influence of micro/nanostructure on thermal conductivity is a topic of great scientific interest, particularly to thermoelectrics. The current understanding is that structural defects decrease thermal conductivity through phonon scattering where the phonon dispersion and speed of sound are assumed to remain constant. Experimental work on a PbTe model system is presented, which shows that the speed of sound linearly decreases with increased internal strain. This softening of the materials lattice completely accounts for the reduction in lattice thermal conductivity, without the introduction of additional phonon scattering mechanisms. Additionally, it is shown that a major contribution to the improvement in the thermoelectric figure of merit (zT > 2) of high-efficiency Na-doped PbTe can be attributed to lattice softening. While inhomogeneous internal strain fields are known to introduce phonon scattering centers, this study demonstrates that internal strain can modify phonon propagation speed as well. This presents new avenues to control lattice thermal conductivity, beyond phonon scattering. In practice, many engineering materials will exhibit both softening and scattering effects, as is shown in silicon. This work shines new light on studies of thermal conductivity in fields of energy materials, microelectronics, and nanoscale heat transfer.

Enhancement of Thermoelectric Performance for n-Type PbS through Synergy of Gap State and Fermi Level Pinning.

We report that Ga-doped and Ga-In-codoped n-type PbS samples show excellent thermoelectric performance in the intermediate temperature range. First-principles electronic structure calculations reveal that Ga doping can cause Fermi level pinning in PbS by introducing a gap state between the conduction and valence bands. Furthermore, Ga-In codoping introduces an extra conduction band. These added electronic features lead to high electron mobilities up to µH ∼ 630 cm2 V-1 s-1 for n of 1.67 × 1019 cm-3 and significantly enhanced Seebeck coefficients in PbS. Consequently, we obtained a maximum power factor of ∼32 µW cm-1 K-2 at 300 K for Pb0.9875Ga0.0125S, which is the highest reported for PbS-based systems giving a room-temperature figure of merit, ZT, of ∼0.35 and ∼0.82 at 923 K. For the codoped Pb0.9865Ga0.0125In0.001S, the maximum ZT rises to ∼1.0 at 923 K and achieves a record-high average ZT (ZTavg) of ∼0.74 in the temperature range of 400-923 K.

All-Scale Hierarchically Structured p-Type PbSe Alloys with High Thermoelectric Performance Enabled by Improved Band Degeneracy.

We show an example of hierarchically designing electronic bands of PbSe toward excellent thermoelectric performance. We find that alloying 15 mol % PbTe into PbSe causes a negligible change in the light and heavy valence band energy offsets (Δ EV) of PbSe around room temperature; however, with rising temperature it makes Δ EV decrease at a significantly higher rate than in PbSe. In other words, the temperature-induced valence band convergence of PbSe is accelerated by alloying with PbTe. On this basis, applying 3 mol % Cd substitution on the Pb sites of PbSe0.85Te0.15 decreases Δ EV and enhances the Seebeck coefficient at all temperatures. Excess Cd precipitates out as CdSe1- yTe y, whose valence band aligns with that of the p-type Na-doped PbSe0.85Te0.15 matrix. This enables facile charge transport across the matrix/precipitate interfaces and retains the high carrier mobilities. Meanwhile, compared to PbSe the lattice thermal conductivity of PbSe0.85Te0.15 is significantly decreased to its amorphous limit of 0.5 W m-1 K-1. Consequently, a highest peak ZT of 1.7 at 900 K and a record high average ZT of ∼1 (400-900 K) for a PbSe-based system are achieved in the composition Pb0.95Na0.02Cd0.03Se0.85Te0.15, which are ∼70% and ∼50% higher than those of Pb0.98Na0.02Se control sample, respectively.

Enhanced Density-of-States Effective Mass and Strained Endotaxial Nanostructures in Sb-Doped Pb0.97Cd0.03Te Thermoelectric Alloys.

Here we report that CdTe alloying and Sb doping increase the density-of-states effective mass and introduce endotaxial nanostructuring in n-type PbTe, resulting in enhanced thermoelectric performance. A prior theoretical prediction for the presence of resonance states in the conduction band of this system, however, could not be confirmed. An amount of 3 mol % CdTe alloying widens the band gap of PbTe by 50%, leading to enhanced carrier effective mass and Seebeck coefficient. This effect is even more pronounced at high temperatures where the solubility of CdTe increases. At 800 K, when the carrier concentration is the same (4 × 1019 cm-3), the Seebeck coefficient of CdTe-alloyed PbTe is -195 µV K-1, 16% higher than that of the Cd-free control sample (-168 µV K-1). Sb doping considerably increases the electron concentration of Pb0.97Cd0.03Te, giving rise to optimized power factors of ∼17 µW cm-1 K-2 at 800 K. More importantly, Sb induces strained endotaxial nanostructures evenly distributed in the matrix. These Sb-rich nanostructures account for the ∼40% reduction in the lattice thermal conductivity over the whole measured temperature range. As a result, a maximum ZT of 1.2 is attained at 750 K in 0.5 mol % Sb-doped Pb0.97Cd0.03Te alloys.

One-step ultra-rapid fabrication and thermoelectric properties of Cu2Se bulk thermoelectric material.

Cu2Se is a promising material for high temperature thermoelectric energy conversion due to its unique combination of excellent electronic properties and low thermal conductivity owing to its ionic liquid characteristics at high temperature. In this paper, fully dense single-phase bulk Cu2Se material was prepared by the combination of self-propagating high-temperature synthesis (SHS) with in situ quick pressing (QP) for the first time. This new approach shortens the duration of the synthesis from days to hours compared to conventional preparation methods. SHS-QP technique is an ultra-fast preparation method, which utilizes the heat released by the SHS reaction and an external applied pressure to achieve the synthesis and densification of materials in one-step. The ultra-fast process of the SHS-QP technique enables the fabrication of single-phase Cu2Se bulk materials with relative density of over 98% and with precise control over the stoichiometry owing to the ability to suppress the Se vapor during the reaction. The SHS-QP prepared Cu2Se samples exhibit excellent thermoelectric figure of merit, ZT ∼ 1.5 at 900 K, which is comparable to those of Cu2Se materials prepared by conventional methods. This study opens a new avenue for the ultra-fast and low-cost fabrication of Cu2Se thermoelectric materials.

Optimizing the average power factor of p-type (Na, Ag) co-doped polycrystalline SnSe.

Despite the achievable high thermoelectric properties in SnSe single crystals, the poor mechanical properties and the relatively high cost of synthesis restrict the large scale commercial application of SnSe. Herein, we reported that co-doping with Na and Ag effectively improves the thermoelectric properties of polycrystalline SnSe. Temperature-dependent carrier mobility indicates that the grain boundary scattering is the dominant scattering mechanism near room temperature, giving rise to low electrical conductivity for the polycrystalline SnSe in comparison with that of the single crystal. Co-doping with Na and Ag improves the electrical conductivity of polycrystalline SnSe with a maximum value of 90.1 S cm-1 at 323 K in Na0.005Ag0.015Sn0.98Se, and the electrical conductivity of the (Na, Ag) co-doped samples is higher than that of the single doped samples over the whole temperature range (300-773 K). Considering the relatively high Seebeck coefficient of 335 µV K-1 at 673 K and the minimum thermal conductivity of 0.48 W m-1 K-1 at 773 K, Na0.005Ag0.015Sn0.98Se is observed to have the highest PF and ZT among the series of samples, with values of 0.50 mW cm-1 K-2 and 0.81 at 773 K, respectively. Its average PF and ZT are 0.43 mW cm-1 K-2 and 0.37, which is 92% and 68% higher than that of Na0.02Sn0.98Se, 40% and 43% higher than that of Ag0.02Sn0.98Se, and 304% and 277% higher than that of the previously reported SnSe, respectively.

Thermal Stability of P-Type BiSbTe Alloys Prepared by Melt Spinning and Rapid Sintering.

P-type BiSbTe alloys have been widely implemented in waste heat recovery from low-grade heat sources below 600 K, which may involve assorted environments and conditions, such as long-term service, high-temperature exposure (generally 473-573 K) and mechanical forces. It is important to evaluate the service performance of these materials in order to prevent possible failures in advance and extend the life cycle. In this study, p-type Bi0.5Sb1.5Te3 commercial zone-melting (ZM) ingots were processed by melt spinning and subsequent plasma-activated sintering (MS-PAS), and were then subjected to vacuum-annealing at 473 and 573 K, respectively, for one week. The results show that MS-PAS samples exhibit excellent thermal stability when annealed at 473 K. However, thermal annealing at 573 K for MS-PAS specimens leads to the distinct sublimation of the element Te, which degrades the hole concentration remarkably and results in inferior thermoelectric performance. Furthermore, MS-PAS samples annealed at 473 K demonstrate a slight enhancement in flexural and compressive strengths, probably due to the reduction of residual stress induced during the sintering process. The current work guides the reliable application of p-type Bi0.5Sb1.5Te3 compounds prepared by the MS-PAS technique.

Homologous Series of 2D Chalcogenides Cs-Ag-Bi-Q (Q = S, Se) with Ion-Exchange Properties.

Four new layered chalcogenides Cs1.2Ag0.6Bi3.4S6, Cs1.2Ag0.6Bi3.4Se6, Cs0.6Ag0.8Bi2.2S4, and Cs2Ag2.5Bi8.5Se15 are described. Cs1.2Ag0.6Bi3.4S6 and Cs1.2Ag0.6Bi3.4Se6 are isostructural and have a hexagonal P63/mmc space group; their structures consist of [Ag/Bi]2Q3 (Q = S, Se) quintuple layers intercalated with disordered Cs cations. Cs0.6Ag0.8Bi2.2S4 also adopts a structure with the hexagonal P63/mmc space group and its structure has an [Ag/Bi]3S4 layer intercalated with a Cs layer. Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 can be ascribed to a new homologous family Ax[MmS1+m] (m = 1, 2, 3···). Cs2Ag2.5Bi7.5Se15 is orthorhombic with Pnnm space group, and it is a new member of the A2[M5+nSe9+n] homology with n = 6. The Cs ions in Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 can be exchanged with other cations, such as Ag+, Cd2+, Co2+, Pb2+, and Zn2+ forming new phases with tunable band gaps between 0.66 and 1.20 eV. Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 possess extremely low thermal conductivity (<0.6 W·m-1·K-1).