Inversion Boundaries Research Articles

Influence of TiO2 doping on the grain growth and electrical properties of ZnO–Cr2O3-based varistor ceramics

The influence of TiO2 doping in the range 0–1 mol% on the microstructure and electrical characteristics of ZnO–Cr2O3-based varistor ceramics was studied. Exaggerated grain growth is observed owing to the presence of TiO2, which triggers the formation of inversion boundaries (IBs) in only a limited number of grains. TiO2 acts as the controller of ZnO grain growth, adjusting the grain size of the ZnO from 5.6 to 9.3 μm, and thus controlling the breakdown voltage. Meanwhile, the TiO2 doping decreased the height of the Schottky barrier, resulting in a lower nonlinear coefficient and a higher leakage current. The results are important for the control of the ZnO grain size with ZnO–Cr2O3-based varistors (having different nominal breakdown voltages) to obtain a suitable thickness for applications.

An Ultralow Concentration of Cr2O3 Dopants‐Driven Lower Temperature Sintering ZnO‐Based Varistor Ceramics

Low working voltage‐driven ZnO‐based varistor ceramics play an important role in the multilayer chip varistors, which require a low sintering temperature of ZnO varistor for its low energy consumption. Herein, a remarkable reduction of the sintering temperature from the usual 1100–1300 °C to 950 °C is successfully achieves in the ZnO ceramics via a certain 0.05 mol% of Cr2O3 dopants. The underlying mechanism is found to be involved with the formation of basal‐plane inversion boundaries between the ZnO grains, which can promote the rapid grain growth within the ceramics. Furthermore, the ZnO varistors with 0.05 mol% Cr2O3 dopant exhibit excellent performance. A low breakdown voltage of 416 V mm−1, a high nonlinear coefficient of 39, and a low leakage current of 3.4 μA are obtained simultaneously. This work presents an effective and promising approach for the cost‐efficient preparation of high‐performance ZnO‐based varistors, which have particular significance for the application of multilayer chip varistors with low working voltage.

Deciphering multi-phase inversion tectonics in the southern Dezful Embayment: insights from the Rag-e-Sefid Oilfield, SW Iran

ABSTRACT Identification of inversion tectonics and inverted structures is very important in petroleum exploration and production due to the positive or negative impacts they can exert on the hydrocarbon traps. In order to characterize the occurrence of multi-phase inversion tectonics and its effect in the Rag-e-Sefid Oilfield, located in the southern Dezful Embayment (SW Iran), this research describes the structural style and deformation history through structural and tectono-sedimentary analyses. The results obtained from the interpretation of seismic profiles and the investigation of the geometry of the sedimentary layers across the growth structures indicate that the Rag-e-Sefid anticline is created from multiple fault-propagation folds. The final shape and geometry of the Rag-e-Sefid anticline are affected by the Rag-e-Sefid and Nourooz-Hendijan-Izeh Faults activity. The deformation history in the north-western part of the Rag-e-Sefid anticline is older than the south-eastern part of it. The Rag-e-Sefid Fault set in the north-western and south-eastern parts of the Rag-e-Sefid Oilfield experienced activity at least during seven and four tectonic phases, respectively. Multiple fault-propagation folding and the fold axis rotation in the north-western part of the Rag-e-Sefid anticline controlled by the presence of several décollement surfaces and the occurrence of inversion tectonics along the pre-existing basement structures. Three-phase inversion tectonics along the Nourooz-Hendijan-Izeh Fault and the Rag-e-Sefid Fault set in the north-western part of the Rag-e-Sefid Oilfield occurred at the Late Devonian-Carboniferous (positive inversion), Permian-Early Triassic (negative inversion), and Late Cenomanian-Early Turonian (positive inversion) boundaries and modified the evolutionary history of the sedimentary basin. The inversion affected hydrocarbon trap development at the Late Cretaceous and controlled their current conditions in the southern Dezful Embayment. Based on the hydrocarbon migration in these regions from the Late Miocene to the present day, the N-S trending (Pan Africa trend) north-western part of the Rag-e-Sefid anticline has a larger share of oil and gas compared to the NW-SE trending (Zagros trend) south-eastern part. The results of this study could add data to worldwide examples of the positive impact of tectonic inversion on the hydrocarbon traps development in the foreland of a collisional orogen.

Moiré patterns and inversion boundaries in graphene/hexagonal boron nitride bilayers

In this paper a systematic examination of graphene/hexagonal boron nitride (g/hBN) bilayers is presented, through a recently developed two-dimensional phase field crystal model that incorporates out-of-plane deformations. The system parameters are determined by closely matching the stacking energies and heights of g/hBN bilayers to those obtained from existing quantum-mechanical density functional theory calculations. Out-of-plane deformations are shown to reduce the energies of inversion domain boundaries in hBN, and the coupling between graphene and hBN layers leads to a bilayer defect configuration consisting of an inversion boundary in hBN and a domain wall in graphene. Simulations of twisted bilayers reveal the structure, energy, and elastic properties of the corresponding moir\'e patterns and show a crossover as the misorientation angle between the layers increases from a well-defined hexagonal network of domain boundaries and junctions to smeared-out patterns. The transition occurs when the thickness of domain walls approaches the size of the moir\'e patterns and coincides with the peaks in the average von Mises and volumetric stresses of the bilayer.

Mineral Exploration Potential Estimation Using 3D Inversion: A Comparison of Three Different Norms

Gravity data have been frequently used in researching the subsurface to map the 3D geometry of the density structure, which is considered the basis for further interpretations, such as the estimation of exploration potential in mineral exploration. The gravity inversion, practically employed to map the density structure, can be achieved by different methods. The method based on Tikhonov regularization is the most commonly used among them. Usually, the subsurface is discretized into a set of cells or voxels. To recover a stable and reliable solution, constraints are introduced into the Tikhonov regularization. One constrained inversion introduces a quadratic penalty (L2 norm) into the inversion, which imposes smooth features on the recovered model. Another gravity inversion, known as sparse inversion, imposes compactness and sharp boundaries on the recovered density structure. Specifically, the L1 norm and L0 norm are favored for such a purpose. This work evaluates the merits of the gravity data inversion in cooperation with different model norms and their applicability in exploration potential estimation. Because these norms promote different features in the recovered models, the reconstructed 3D density structure reveals different geometric features of the ore deposit. We use two types of synthetic data for evaluating the performances of the inversion with different norms. Numerical results demonstrate that L0 norm-based inversion provides high-resolution recovered models and offers reliable estimates of exploration potential with minimal deviation from theoretical mass compared to inversions equipped with the other two norms. Finally, we use the gravity data collected over the iron ore deposit at the Dida mining area in Jilin province (Northeast China) for the application. It is estimated that the exploration potential of the iron ore deposits is about 3.2 million tons.

Understanding and controlling inversion boundaries in ZnO

In this paper, we report a study of the head-to-head (-O-Zn-O-) inversion boundaries (IB) in zinc oxide. We investigate the effect of impurities (Fe, Bi, Pr, Sn, Ti, and Sb) on the IB formation using ab-initio calculations in the framework of density-functional theory (DFT) for different atomic coverages. In agreement with experiment, DFT predicts spontaneous formation of IBs in Sb-, Sn-, and Fe-doped ZnO, with coverages of 1/3, 1/2, and 1 monolayer, respectively. Half a monolayer of Ti also lowers the IB formation energy, but brings it to about 1 J/m2, which is not enough to trigger IB formation. Bi and Pr do not sufficiently promote IB formation at conditions relevant for ZnO sintering. Inspired by the remarkable success of DFT for evaluation of IB energies, we discuss the implications for the IB design and control.

Impact of Insertion Sequences and RNAs on Genomic Inversions in Pseudomonas aeruginosa

In this article, a bioinformatics pipeline is proposed that focuses on two types of elements, namely the mobile genetic elements (MGE) and Ribonucleic acids (RNAs). The MGEs are called insertion sequences (ISs) in the prokaryotic domain. The objective of this research work is to study the behaviour of RNAs and MGEs genes, and the effects of their presence around inversions in genome sequences. The proposed pipeline finds the relation between the transposase gene types (e.g., DDE and DEDD) located within insertion sequences according to their IS family and sub-family, and RNAs (tRNA and rRNA) on the one hand, and genomic inversion on the other hand. More precisely, we wonder whether the insertion sequences and RNAs have any effects on the occurrence of inversions. To investigate this, we start by extracting all inversions between every two close isolates of P. aeruginosa from a high supported phylogenetic tree, and we investigate inversion boundaries, to determine the impact of IS elements and RNAs around these inversions. For RNAs genes, we noticed that a high occurrence of these elements are found in what we called InBoundary, while few to zero occurrences have been recorded in OutBoundary. We remark that the NCGM2.S1 genome has the largest amount of tRNAs and inversions comparing to other genomes. For RNA genes, the tRNAs genes (Arg, Ala, Leu) recorded the highest occurrences around the inversions, while there was few occurrences of rRNA genes, within all genomes. For IS elements, we noticed that the family of IS3 has the highest impact on inversions, but sometimes other type of IS elements can also be found with IS3 around the inversions. In this case, these types of ISs shared the same type of transposase gene (DDE). We finally conclude that the distribution of RNA and IS genes has a big impact on the inversions in bacterial genomes.

Impact of inversion tectonics on the spatial distribution of hydrocarbon traps in the NW Persian Gulf and the southern Dezful Embayment, SW Iran

Inversion tectonics has a major impact on distribution of fault-controlled hydrocarbon traps in the fold-and-thrust belts. In order to characterize the spatial distribution of several fault-controlled hydrocarbon traps adjacent to the Nourooz-Hendijan-Izeh Fault, this study describes the structural and tectono-sedimentary evolution of the fault based on the geological interpretation and tectono-sedimentary analysis of seven seismic sections in the Bahregansar, Hendijan, Tangu, and Rag-e-Sefid oilfields in the NW Persian Gulf and the southern Dezful Embayment. The results indicate that the Nourooz-Hendijan-Izeh Fault is a segmented pre-existing basement fault with steep dip, which reactivated to at least seven tectonic phases. Three-phase inversion tectonics along the Nourooz-Hendijan-Izeh Fault occurred at the Late Devonian-Carboniferous (positive inversion), Permian-Early Triassic (negative inversion), and Late Cenomanian-Early Turonian (positive inversion) boundaries and modified the evolutionary history of the sedimentary basin. Inversion affected hydrocarbon trap development at the Late Cretaceous and controlled their current conditions in the NW Persian Gulf and the southern Dezful Embayment along the northeastern margin of African-Arabian plates. Considering the hydrocarbon migration at the Late Miocene in these regions up to now, the N–S trending anticlines (Arabian trend) have a larger share of oil and gas in their hydrocarbon traps and lower hydrocarbon exploration risk than the NW-SE trending anticlines (Zagros trend). Mild to moderate inversion structures can be considered as preferred petroleum exploration targets.

TEM and DFT study of basal-plane inversion boundaries in SnO2-doped ZnO

In our recent study (Ribic et al. 2020) we reported the structure of inversion boundaries (IBs) in Sb2O3-doped ZnO. Here, we focus on IBs that form in SnO2-doped ZnO. Using atomic resolution scanning transmission electron microscopy (STEM) methods we confirm that in SnO2-doped ZnO the IBs form in head-to-head configuration, where ZnO4 tetrahedra in both ZnO domains point towards the IB plane composed of a close-packed layer of octahedrally coordinated Sn and Zn atoms. The in-plane composition is driven by the local charge balance, following Pauling's principle of electroneutrality for ionic crystals, according to which the average oxidation state of cations is 3+. To satisfy this condition, the cation ratio in the IB-layer is Sn4+: Zn2+=1:1. This was confirmed by concentric electron probe analysis employing energy dispersive spectroscopy (EDS) showing that Sn atoms occupy 0.504 ? 0.039 of the IB layer, while the rest of the octahedral sites are occupied by Zn. IBs in SnO2-doped ZnO occur in the lowest energy, IB3 translation state with the cation sublattice expansion of ?IB(Zn-Zn) of +91 pm with corresponding O-sublattice contraction ?IB(O-O) of -46 pm. Based on quantitative HRTEM and HAADF-STEM analysis of in-plane ordering of Sn and Zn atoms, we identified two types of short-range distributions, (i) zigzag and (ii) stripe. Our density functional theory (DFT) calculations showed that the energy difference between the two arrangements is small (~6 meV) giving rise to their alternation within the octahedral IB layer. As a result, cation ordering intermittently changes its type and the direction to maximize intrinsic entropy of the IB layer driven by the in-plane electroneutrality and 6-fold symmetry restrictions. A long-range in-plane disorder, as shown by our work would enhance quantum well effect to phonon scattering, while Zn2+ located in the IB octahedral sites, would modify the bandgap, and enhance the in-plane conductivity and concentration of carriers. Keywords

Mechanism of Ga2O3 addition on the enhancement of electrical and mechanical properties of ZnO based varistor

ZnO based varistors added with minor content of Bi2O3, Sb2O3, Co2O3, SiO2, MnO2 et al. with high voltage gradient, excellent mechanical and thermal properties were fabricated by Ga2O3 doping and using raw ZnO nanoparticles. The compositions and microstructure of the varistors, as well as their electrical, mechanical, and thermal properties were investigated via XRD, XPS, SEM, E-J, C–V, mechanical, and thermal expansion measurements. Also, the mechanism of Ga2O3 addition on the electrical and mechanical properties of the varistors was discussed in detail. Results showed that the added Ga2O3 preferentially occupied the lattice position of ZnO crystal through the formation of a substitutional solid solution (Donor doping), it then occupied the void position through the formation of an interstitial solid solution (Acceptor doping), in which the residual Ga2O3 existed in the grain boundary and served as inversion boundaries. The formation of the substitutional and interstitial solid solutions helped to improve the electrical properties, when Ga2O3 content was 0.40 mol%, values of E1mA, α, and K were 1235.00 V mm−1, 46.0, and 1.37, respectively, being due to the small particle size and the related content of donor, acceptor and grain boundary in ZnO grain; The increased content of inversion boundaries stimulated the abnormal growth of ZnO grain, and the formed plate-like grain helped to improve the mechanical properties and thermal expansion coefficient of the varistors, values of σf, Ef, and KIC could reach 147.43 MPa, 213.61 GPa and 2.05 MPa m1/2, showing improvements of 25.29%, 47.67%, and 38.51%, respectively, compared with those of ZnO based varistors without Ga2O3.

Inversion Boundary Annihilation in GaAs Monolithically Grown on On‐Axis Silicon (001)

AbstractMonolithic integration of III–V materials and devices on CMOS compatible on‐axis Si (001) substrates enables a route of low‐cost and high‐density Si‐based photonic integrated circuits. Inversion boundaries (IBs) are defects that arise from the interface between III–V materials and Si, which makes it almost impossible to produce high‐quality III–V devices on Si. In this paper, a novel technique to achieve IB‐free GaAs monolithically grown on on‐axis Si (001) substrates by realizing the alternating straight and meandering single atomic steps on Si surface has been demonstrated without the use of double Si atomic steps, which was previously believed to be the key for IB‐free III–V growth on Si. The periodic straight and meandering single atomic steps on Si surface are results of high‐temperature annealing of Si buffer layer. Furthermore, an electronically pumped quantum‐dot laser has been demonstrated on this IB‐free GaAs/Si platform with a maximum operating temperature of 120 °C. These results can be a major step towards monolithic integration of III–V materials and devices with the mature CMOS technology.

New inversion boundary structure in Sb-doped ZnO predicted by DFT calculations and confirmed by experimental HRTEM

Today, ab-initio calculations are becoming a powerful tool to perform virtual experiments that have the capacity to predict and to reproduce experimentally observed non-periodic features, such as interfaces, that are responsible for quantum properties of materials. In our paper we investigate 2D quantum-well structures, known as inversion boundaries (IB). Combining atomistic modeling, DFT calculations and HRTEM analysis we provide a new fundamental insight into the structure and stability of Sb-rich basal-plane IBs in ZnO. DFT screening for potential IB model was based on the known stacking deviations in originating wurtzite structure. The results show that the model with Aβ−Bα−AβC−γB−βC sequence (IB3) is the most stable translation for Sb-doping, as opposed to previously accepted Aβ−Bα−AβC−γA−αC (IB2) model. The key to the stability of IB structures has been found to lie in their cationic stacking. We show that the energies of constituting stacking segments can be used to predict the stability of new IB structures without the need of further ab-initio calculations. DFT optimized models of IBs accurately predict the experimentally observed IB structures with lateral relaxations down to a precision of ~1 pm. The newly determined cation sublattice expansions for experimentally confirmed IB2 and IB3 models, ΔIB(Zn-Zn) are +81 pm and +77 pm, whereas the corresponding O-sublattice contractions ΔIB(O-O) are –53 pm and –57 pm, respectively. The refined structures will help to solve open questions related to their role in electron transport, phonon scattering, p-type conductivity, affinity of dopants to generate IBs and the underlying formation mechanisms, whereas the excellent match between the calculations and experiment demonstrated in our study opens new perspectives for prediction of such properties from first principles.

Segregation and properties at curved vs straight (000) inversion boundaries in piezotronic ZnO bicrystals

AbstractTEM and SEM investigations of ZnO bicrystal interfaces were undertaken with an aim to study the correlation of local grain‐boundary structure, segregation, and electrical transport perpendicular to the interface. To this end, varistor‐like ZnO bicrystals with piezotronic characteristics were chosen with (000)║(000) tail‐to‐tail orientation with respect to the c‐axis. In order to contrast different local grain‐boundary structures with different coherency and segregation of bismuth, but identical macroscopic polarization state, two complementary processing techniques were applied. A diffusion‐bonded bicrystal with an intermediate thin film containing Zn–Bi–Co–O provided a straight interface as reference. In contrast, a ZnO bicrystal prepared by epitaxial solid‐state transformation was manufactured by bonding two ZnO single crystals with a 100 µm thick polycrystalline ZnO varistor material with a typical dopant composition including bismuth and cobalt. This structure was annealed to the point that a bicrystal was formed with the varistor concentration at the boundary, which was strongly curved due to the polycrystalline microstructure still providing a shadow image at the interface. The results highlight a distinct correlation between local interfacial morphology, degree of segregation of bismuth, and degree of nonlinearity of the electrical transport across the interface.

Enhancement of Electrical Conduction and Phonon Scattering in Ga2O3(ZnO)9-In2O3(ZnO)9 Compounds by Modification of Interfaces at the Nanoscale

The Ga2O3(ZnO)9 and In2O3(ZnO)9 homologous phases have attracted attention as thermoelectric (TE) oxides due to their layered structures. Ga2O3(ZnO)9 exhibits low thermal conductivity, while In2O3(ZnO)9 possesses higher electrical conductivity. The TE properties of the solid solution of Ga2O3(ZnO)9-In2O3(ZnO)9 were explored and correlated with changes in the crystal structure. High-quality (1−x)Ga2O3(ZnO)9-(ZnO)9 (x = 0.0 to 1.0) ceramics were prepared by the solid-state route using B2O3 and Nd2O3 as additives. The crystal structures were analysed by x-ray diffraction, high-resolution transmission electron microscopy and atomic resolution scanning transmission electron microscopy–high-angle annular dark field imaging–energy dispersive x-ray spectroscopy (STEM–HAADF–EDS) techniques. A layered superstructure with compositional modulations was observed in all samples in the (1−x)Ga2O3(ZnO)9-xIn2O3(ZnO)9 system. All the ceramics exhibited nanoscale structural features identified as Ga- and In-rich inversion boundaries (IBs). Substitution of 20 mol.% In (x = 0.2) in the Ga2O3(ZnO)9 compounds generated basal and pyramidal indium IBs typically found in the In2O3(ZnO)m system. The (Ga0.8In0.2)2O3(ZnO)9 compound does not exhibit the structural features of the $$ Cmcm $$CmcmGa2O3(ZnO)9 compound, which is formed by a stacking of Ga-rich IBs along the pyramidal plane of the wurtzite ZnO, but features that resemble the crystal structure exhibited by the $$ R\overline{3} m $$R3¯mIn2O3(ZnO)m with basal and pyramidal indium IBs. The structural changes led to improved TE performance. For example, (Ga0.8In0.2)2O3(ZnO)9 showed a low thermal conductivity of 2 W/m K and a high power factor of 150 μW/m K2 giving a figure of merit (ZT) of 0.07 at 900 K. This is the highest ZT for Ga2O3(ZnO)9-based homologous compounds and is comparable with the highest ZT reported for In2O3(ZnO)9 homologous compounds.

Single layer In-O atomic sheets as phonon and electron barriers in ZnO-In2O3 natural superlattices: Implications for thermoelectricity

The phases in the ZnO half of the ZnO-In2O3 binary system are natural superlattices consisting of a periodic stacking of single InO2 sheets separated by indium doped ZnO blocks with a spacing that depends on the composition according to the relationship In2O3(ZnO)k. Characterization by combined, atomic resolution, aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy analysis indicates that the atomic structure of each InO2 layer consists of a single continuous sheet of octahedrally coordinated InO2. The sheets are also crystallographic inversion boundaries. Analysis of the electrical conductivity, thermal conductivity, and Seebeck coefficient data at 800 °C indicates that the InO2 sheets not only decrease thermal conductivity by phonon scattering but also create potential barriers to electron transport. The origin of the potential barriers, the role of piezoelectric effects, and their dependence on superlattice spacing are discussed qualitatively. It is also argued that the crystallographically aligned InO2 sheets within individual grains are also transport barriers in randomly oriented polycrystalline materials.

Utilising unit-cell twinning operators to reduce lattice thermal conductivity in modular structures: Structure and thermoelectric properties of Ga2O3(ZnO)9

The Ga2O3(ZnO)m family of homologous compounds have been identified as potential thermoelectric materials, but properties are often limited due to low densification. By use of B2O3 as an effective liquid phase sintering aid, high density, high quality ceramic samples of Ga2O3(ZnO)9 have been synthesised. The atomic structure and local chemical composition of Ga2O3(ZnO)9 have been determined by means of high resolution X-ray diffraction and atomic resolution STEM-HAADF, EDS and EELS measurements. X-ray analysis showed that the compound crystalizes in the Cmcm orthorhombic symmetry. Atomically resolved HAADF-STEM images unambiguously showed the presence of nano-sized, wedge-shaped twin boundaries, parallel to the b-axis. These nano-scale structural features were chemically investigated, for the first time, revealing the exact distributions of Zn and Ga; it was found that Ga ions occupy sites at the junction of twin boundaries and inversion boundaries. HAADF-EDS analysis showed that the calcination step has a significant impact on crystal structure homogeneity. By use of a sintering aid and optimization of processing parameters the ceramics achieved a low thermal conductivity of 1.5–2.2 W/m.K (for the temperature range 300–900 K), a power factor of 40–90 μW/K.m2, leading to a ZT of 0.06 at 900 K. The work shows a route to exploit nanoscale interface features to reduce the thermal conductivity and thereby enhance the thermoelectric figure of merit in complex thermoelectric materials.

Phonon Scattering and Electron Doping by 2D Structural Defects in In/ZnO.

In/ZnO bulk compounds have been synthesized using a simple solid-state process. In this study, both the structural features and thermoelectric properties of the Zn1-xInxO series with ultralow indium content (0 ≤ x ≤ 0.02) have been studied. High-angle annular dark-field scanning transmission electron microscopy analyses highlight that indium has the ability to create multiple basal plane and pyramidal defects that produce ZnO domains with inverted polarity starting from dopant concentrations as low as 0.25 atom %. Interestingly, the formation of parallel inversion boundaries consisting of InO6 octahedra in the ZnO4 tetrahedra matrix is responsible for phonon scattering while increasing electrical conductivity, thereby enhancing the thermoelectric properties. This effect of multiple extended two-dimensional defects on the thermoelectric properties of ZnO is reported for the first time with such low indium doping. On the chemistry side, the present results point toward a lack of In solubility in the ZnO structure. Moreover, this study is a step forward to the synthesis of other thermoelectric compounds where dopant-induced planar defects in bulk transition metal compounds have the potential to enhance both phonon scattering and electronic conductivity.

Structural features and thermoelectric properties of Al‐doped (ZnO)5In2O3 homologous phases

AbstractIn this work, we investigated the influence of Al doping on the structure of the (ZnO)5In2O3 homologous phase and the thermoelectric characteristics of (ZnO)5(In1−xAlx)2O3 ceramics for x=0, 0.01, 0.03, 0.05, 0.1, and 0.2, prepared using a classic ceramic procedure and sintering at 1500°C for 2 hours. The Al substituted for In on both the primary sites in the Zn5(In1−xAlx)2O8 homologous phase, the octahedral sites in the basal‐plane inversion boundaries and the trigonal bi‐pyramidal sites in the zig‐zag inversion boundaries, which resulted in a uniformly increased shrinkage of the unit cell with the additions of Al. The a and c parameters were reduced for x=0.2 by a maximum 0.8%. All the samples had similar microstructures, so the differences in the TE characteristics mainly resulted from the effects of the substitution of Al for In, decreasing the charge‐carrier concentration and affecting their mobility. Slightly improved TE characteristics were only observed for Al additions with x=0.01‐0.05, while larger additions of Al only resulted in a reduced electrical conductivity and decreased ZT values in comparison to the un‐doped composition.

Inversion Boundaries and Phonon Scattering in Ga:ZnO Thermoelectric Compounds.

We investigated the high-temperature thermoelectric properties of Ga:ZnO bulk compounds, synthesized using a simple and scalable solid-state process. The effects of a low gallium content (x ≤ 0.04 in Zn1-xGaxO1+x/2) on the structural features and electrical/thermal properties are reviewed. Transmission electron microscopy analyses showed that 2D, nonperiodic defects had formed from a doping content as low as x = 0.01 Ga. The structural description of these nanoscale interfaces is, for the first time, carefully investigated in such low-Ga-content samples by HAADF-STEM analyses combined with structural modeling. It was found that the formation of head-to-head inversion twin (h-IT) boundaries and tail-to tail inversion boundaries (t-IB) in the bulk compounds is responsible for strong phonon scattering, while maintaining relatively good electrical conductivity and thereby enhancing the thermoelectric properties. The absolute value of the Seebeck coefficient decreases abruptly from 475 μV/K for x = 0 down to 60 μV/K for x = 0.005 at 350 K. At the same time, the electrical resistivity drops from 1 ohm cm for x = 0 to 1.7 × 10-3 ohm cm for x = 0.005. For higher Ga additions (x > 0.01), the increase in electrical resistivity is likely linked to the formation of interface defects at a larger extent in the wurtzite structure. The thermal conductivity also drops sharply with the increase in the Ga content from ∼33 W/m K for x = 0 to ∼8 for x = 0.04 at 350 K. This study is progress toward the synthesis of other thermoelectric materials where nanoscale interfaces in bulk compounds provide tremendous opportunities for further enhancing both the phonon scattering and the overall figure of merit.

High Thermoelectric Performance and Enhanced Mechanical Stability of p-type Ge1–xSbxTe

High thermoelectric figure of merit, zT, of ∼1.85 at 725 K along with significant cyclable temperature stability was achieved in Pb-free p-type Ge1–xSbxTe samples through simultaneous enhancement in Seebeck coefficient and reduction of thermal conductivity. Sb doping in GeTe decreases the carrier concentration due to the donor dopant nature of Sb and enhances the valence band degeneracy by increasing the cubic nature of the sample, which collectively boost Seebeck coefficient in the temperature range of 300–773 K. Significant thermal conductivity reduction was achieved due to collective phonon scattering from various meso-structured domain variants, twin and inversion boundaries, nanostructured defect layers, and solid solution point defects. The high performance Ge0.9Sb0.1Te sample shows mechanical stability (Vickers microhardness) of ∼206 Hv, which is significantly higher compared to other popular thermoelectric materials such as Bi2Te3, PbTe, PbSe, Cu2Se, and TAGS.