The phase composition and thermoelectric properties of a beta-Zn4Sb3 based material before and after thermocycling tests at 273-723 K have been studied. Specimens with stoichiometric Zn and Sb molar ratios and with additions of different excess Zn quantities have been synthesized by direct component smelting followed by spark plasma sintering. The phase composition and structure of the materials have been studied using X-ray diffraction and transmission electron microscopy. The thermal conductivity and specific heat of the materials have been measured with laser flash and differential scanning calorimetry techniques. The phase composition of the as-synthesized, as-sintered and as-thermocycled Zn4+xSb3 thermoelectric material has been found to vary depending on excess Zn content in the charge. Excess Zn has been shown to dissolve in the beta-Zn(4)Sb(3)phase upon spark plasma sintering and thermocycling. beta-Zn4+xSb3 solid solution depletion of interstitial zinc atoms or ZnSb phase precipitation upon thermocycling increase the thermal conductivity and reduce the thermoelectric efficiency of the material. The Zn4.1Sb3 composition has shown high thermoelectric efficiency, the 715 K ZT being similar to 1.23 and 1.18 before and after thermocycling, respectively.

Получение термоэлектрических материалов на основе халькогенидов висмута и сурьмы в виде прутков больших диаметров методом горячей экструзии связано с проблемой сохранения текстуры деформации в них как по длине, так и по сечению. В работе впервые представлены поликристаллы на основе Bi2Te3 n- и p-типа проводимости диаметром до 35 мм, полученные этим методом. Проведены детальные исследования структурных свойств. Измерены механические (предел прочности) и термоэлектрические характеристики (коэффициент термоэдс, электропроводность и термоэлектрическая добротность) экструдированных образцов. Полученные поликристаллы n- и p-типа проводимости диаметром 35 мм по термоэлектрическим и механическим свойствам не уступают промышленно выпускаемым поликристаллам диаметром 25 мм. Ключевые слова: термоэлектричество, Bi2Te3, экструзия, термоэлектрические свойства, механические свойства.

Hot extrusion of thermoelectric materials of large diameters is associated with the problem of preserving the deformation texture in them both in length and in cross-section. The paper shows for the first time the technology of obtaining polycrystals based on Bi2Te3 n- and p-type conductivity with a diameter of up to 35 mm, by hot extrusion. Detailed studies of the structural properties have been carried out. The mechanical properties (strength) of the crystals were measured. The thermoelectric properties of polycrystals of various diameters (the coefficient of thermal EMF α, electrical conductivity sigma and figure of merit Z by the Harman method) were measured. It is shown at the optimal technology polycrystals based on Bi2Te3 n- and p-type conductivity with a diameter of 35 mm were produced, which are comparable in thermoelectric and mechanical properties with polycrystals of traditional diameter (25 and 30 mm), which are currently commercially produced. Keywords: thermoelectricity, Bi2Te3, extrusion, thermoelectric properties, mechanical properties.

The phase composition and thermoelectric properties of Cu2 – xSe samples with different deviations from the stoichiometric composition (x = 0.03, 0.08, 0.13, 0.18, and 0.23) are studied. Bulk thermoelectric materials based on Cu2 – xSe are synthesized by spark plasma sintering. The phase composition is studied using the X-ray diffraction method. Depending on the chemical composition, the materials based on Cu2 – xSe at room temperature can be either a single-phase material containing only the cubic β-Cu2Se phase or a two-phase material containing a mixture of monoclinic α-Cu2Se and cubic β-Cu2Se phases. It is shown that the deviation of the stoichiometric composition has a substantial effect on the electrophysical characteristics of the material. With an increase in the deviation of the stoichiometric composition, the concentration of the main charge carriers in Cu2 – xSe increases because of the formation of copper vacancies. The change in the thermal conductivity substantially contributes to the thermoelectric figure of merit of Cu2 – xSe compounds. In the investigated range of chemical compositions, the maximum values of thermoelectric figure of merit around ZT ~ 1.3 at 600°C are observed in samples with the Cu1.97Se composition.

In this study, Ingots of (Bi, Sb)2Te3 thermoelectric material with p-type conductivity have been obtained by hot extrusion. The main regularities of hot extrusion of 30 mm rods have been analyzed with the aid of a mathematical simulation on the basis of the joint use of elastic-plastic body approximations. The phase composition, texture and microstructure of the (Bi, Sb)2Te3 solid solutions have been studied using X-ray diffraction and scanning electron microscopy. The thermoelectric properties have been studied using the Harman method. We show that extrusion through a 30 mm diameter die produces a homogeneous strain. The extruded specimens exhibit a fine-grained structure and a clear axial texture in which the cleavage planes are parallel to the extrusion axis. The quantity of defects in the grains of the (Bi, Sb)2Te3 thermoelectric material decreases with an increase in the extrusion rate. An increase in the extrusion temperature leads to a decrease in the Seebeck coefficient and an increase in the electrical conductivity. The specimens extruded at 450 °C and a 0.5 mm/min extrusion rate have the highest thermoelectric figure of merit (Z = 3.2 × 10−3 K−1).

In this work we explored the possibility of obtaining single-phase β-Zn4Sb3 material by direct melting and spark plasma sintering (SPS). The effect of the SPS conditions and synthesized material composition on the structure and thermoelectric properties of the material was studied. X-ray diffraction was used for phase analysis, and scanning electron microscopy was used for characterizing the morphology of the bulk samples. We found electro-migration of Zn due to the exposure to DC pulses during SPS at 40 MPa. Zinc electro-migration produced a zinc-excess layer on the sample surface on the cathode side and a zinc-depleted layer, i.e., ZnSb, on the anode side of the sample. We show that an increase in the sintering pressure, and hence a decrease in the electric current passing through the material, produces single-phase β-Zn4Sb3 material in the entire sample bulk, provided the synthesized material contains excess zinc. The highest ZT of ∼ 1.28 at 673 K was obtained for the material synthesized with Zn excess and sintered by SPS at a pressure of 100 MPa.

Bismuth antimony telluride is the most commonly used commercial thermoelectric material for power generation and refrigeration over the temperature range of 200 - 400 K. Improving the performance of these materials is complected balance of optimizing thermoelectric properties. Decreasing the grain size of Bi0.5Sb1.5Te3 significantly reduces the thermal conductivity due to the scattering phonons on the grain boundaries. In this work, it is shown the advances of spark plasma sintering (SPS) for preparation of nanocrystalline p-type thermoelctrics based on Bi0.5Sb1.5Te3 at different temperatures (240, 350, 400 degrees C). The complex study of structural and thermoelectric properties of Bi0.5Sb1.5Te3 were presented. The high dimensionless thermoelectric figure of merit ZT similar to 1 or some more over 300 - 400 K temperature range for nanocrystalline p-type Bi0.5Sb1.5Te3 was obtained.

The method of two-crystal X-ray diffractometry is used to control the quality and perfection of monocrystalline silicon obtained by implantation of hydrogen ions and subsequent thermal annealing, which is used in a number of semiconductor technologies. The principal feature of this approach is the ability to quickly obtain reliable experimental results, which was confirmed in this paper by the use of X-ray topography. The presented data provide information on the state of the disturbed layer of silicon crystals of n-type conductivity (ρ = 100 Om ⋅ cm) by orientation (111), 2 mm thick, implanted by protons with energy E = 200, 300, 100 + 200 + 300 keV, dose D = 2 ⋅ 1016cm-2 and subjected to subsequent thermal treatment in the temperature range T from 100 to 900 °С. We have established a non-monotonic dependence of the integral characteristics of the disturbed layer, namely the average effective thickness Leff and the average relative deformation ∆а/а, on annealing temperature, with the maximum level of distortion in the field of temperature ∼300 °С, using the method of integral characteristics. Obtained data allowed to assess the general condition of disturbed layer during thermal treatment.

We have studied the structure, phase composition and mechanical properties, e.g. microherdness and impact toughness of partially stabilized zirconia crystals in the 2-4 mol.% stabilizing yttria concentration range. We have shown that with increasing concentration of stabilizing yttria the microhardness increases while the impact fracture toughness varies nonmonotonically and reaches the maximum for the partially stabilized zirconia crystal with 3 mol.% Y2O3. The anisotropy of the microhardness and impact fracture toughness was studied for wafers perpendicular to the < 100 >, < 110 > and < 111 > directions with different indenter diagonal orientations. We have shown that the microhardness of the partially stabilized zirconia crystals depends but slightly on crystallographic orientation whereas the impact fracture toughness differs between crystallographic faces of the crystals. The highest impact toughness was obtained for the {100} plane and the indenter diagonal aligned parallel to the < 100 > direction, while the lowest impact toughness was obtained for the {110} plane and the indenter diagonal aligned parallel to the < 110 > direction. Local Raman study of the monoclinic phase distribution in the vicinity of the indentation shows that the maximum quantity of the monoclinic phase forms along the < 110 > direction regardless of indenter diagonal orientation. We have shown that the strain anisotropy during the phase transition has variable effect on crack impedance. The effect of transformation hardening mechanism depends on crack plane orientation and transition induced stress. (C) 2019 Elsevier B.V. All rights reserved.

Abstract We have studied the dependence of the thermoelectric properties of the bulk (Bi,Sb)2Te3 material on the temperature of spark plasma sintering (SPS). For analysis of the experimental results we took into account the regularities of structure formation in the material. The average crystallite size decreases with an increase in the SPS temperature to above 400 °C. Transmission electron microscopy showed that at above 400 °C the bulk and boundaries of the initial grains that are several micrometers in size contain a large number of nanosized grains (approx 10 – 20 nm) having the same composition. We show that the dependence of the thermoelectric properties of the material on SPS temperature correlates with changes in the fine structure of the material which is controlled by the redistribution of the intrinsic point defects. Our results suggest that, along with the well-known nanostructure formation processes, there is one more high-temperature self-organizing process of the formation of the nanostructural material based on the redistribution and change in the form of occurrence of the nonequilibrium point defects. This information can be used to broaden the possibilities of controlling the properties of the bulk thermoelectric material on the basis of (Bi,Sb)2Te3.

Phase stability and transport characteristics of (ZrO2)(1-x)(Sc2O3)(x)(CeO2)(y )(x = 0.08-0.10; y= 0.005-0.015) and (ZrO2)(1-x-y-z)(SC2O3)(x)(CeO2)(y)(Y2O3)(z) (x = 0.08-0.10; y = 0.005-0.010; z = 0.005-0.020) melt-grown crystals have been studied after air heat treatment at 1000 degrees C for 400 h. Annealing of the (ZrO2)(1-x)(Sc2O3)(x)(CeO2)(y) crystals caused the formation of a rhombohedral phase and reduced the electrical conductivity of the specimens in the entire test temperature range. Yttrium co-doping of the (ZrO2)(1-x)(Sc2O3)(x)(CeO2)(y) crystals stabilized the cubic phase for some compositions. In two-phase (ZrO2)(1-x-y-z)(Sc2O3)(x)(CeO2)(y)(Y2O3)(z) crystals containing a cubic and a tetragonal phases exhibited conductivity degradation. Annealing of cubic crystals caused the formation of the t' phase and increased their conductivity. (C) 2019 Elsevier B.V. All rights reserved.

Major advantage of extruded Bi2Te3 based thermoelectric materials is high mechanical strength compared with that of melt-crystallized materials. Mechanical properties are of special importance for thermogenerator module applications where thermogenerator branches may undergo elevated thermal stresses due to large temperature differences at the modules. Since extrusion is typically a high-temperature process the structure of extruded materials is controlled by the plastic deformation in multiple slip systems resulting in the formation of a final deformed structure. The grain orientations are predominantly such that the most probable cleavage plane orientation is parallel to the extrusion axis. Recovery processes occur simultaneously and different recrystallization stages may take place. In the latter case the deformed texture may be destroyed. Structure evolution along the extruded rod of Bi2Se0.3Te2.7 ternary solid solution was studied with metallography and X-ray diffraction. Extrusion was interrupted for the study and so the specimen was a whole rod the initial part of which was the extrusion billet and the final part was the as-extruded material. The structure of the material is formed by competitive processes of dislocation generation and annealing. The plastic deformation energy is the highest in the extruder zone of the rod. Both the hardening processes and the texture are controlled by the plastic deformation mechanism. Plastic deformation is accompanied by generation of defects that are most likely vacancy type ones.

The quality and structural perfection of single crystal silicon have been studied using double-crystal X-ray diffraction after hydrogen ion implantation and thermal annealing used in a number of semiconductor technologies. The fundamental difference of this approach is the possibility to rapidly obtain reliable experimental results which were confirmed using X-ray topography. Data have been presented for the condition of the damaged layer in n-type silicon single crystals (r = 100 W × cm) having the (111) orientation and a thickness of 2 mm after proton implantation at energies E = 200, 300 and 100 + 200 + 300 keV and dose D = 2 × 1016 cm-2 and subsequent heat treatment in the T = 100–900 °C range. Using the method of integral characteristics we have revealed a nonmonotonic dependence of the integral characteristics of the damaged layer, i.e., the mean effective thickness Leff and the mean relative deformation Da/a, on the annealing temperature, the maximum deformation being observed for ~300 °C. The results have allowed us to make a general assessment of the damaged layer condition after heat treatment.

The thermal expansion of effective thermoelectric materials in the temperature range of 200-1200 K has been studied using quartz dilatometers. In present work the method and mathematical model for calculation of thermal linear expansion coefficients (TLEC) of thermoelectric materials were chosen and substantiated. As a result of the study, it was experimentally established that the TLEC of low temperature thermoelectric materials Bi2Te2.8Se0.2 and Bi0.5Sb1.5Te3 are 14.85.10(-6) K-1 and 14.98.10(-6) K-1 in the temperature range of 200-400 K respectively. Research of middle temperature materials was performed. It was found that the TLEC of Bi2Te2.4Se0.6 and Bi0.4Sb1.6Te3 weakly change in the working temperature range from 380 to 600 K and is in the range of (13.93-14.33).10(-6) K-1. Values of TLEC for PbTe is 23.07.10(-6) K-1, value of TLEC of GeTe is 24.47.10(-6) K-1 respectively. TLEC for high temperature nanostructural material Si0.8Ge0.2 is equal to 4.68.10(-6) K-1 in the temperature range 300-1200 K.

Gettering is defined as a process by which metal impurities in the device region are reduced by localizing them in predetermined, passive regions of the silicon wafer. Internal or intrinsic gettering is an effective way to reduce the contamination in active regions. The generation of internal getters is based on the decomposition of the supersaturated oxygen solid solution in silicon, which favours the formation of a complex defect system in silicon that consists of various precipitate/dislocation agglomerates. Regularities of microdefect formation during oxygen solid solution decomposition in silicon have been studied. We show that actual solid solution supersaturation, temperature and heat treatment duration determine the structure of the solid solution. Combining these factors, including heat treatment parameters, one can control solid solution decomposition rate and SiOx precipitate sizes and quantity. The methods of X-ray diffuse scattering and transmission electron microscopy have shown high efficiency for studying the effect of heat treatment in crystals. For annealing at 450 °C, solid solution decomposition occurs at high supersaturation degrees, and concentration inhomogeneity regions may form at an early decomposition stage over the actual annealing time (up to 40 h). With an increase in the temperature of subsequent annealing to 650 °C, local regions with above-average oxygen supersaturation degrees increase the efficiency of oxygen solid solution decomposition. Further, an increase in annealing temperature to T > 1000 °С results in a more intense generation of the largest precipitates at the expense of the smaller ones. Once the precipitate sizes become sufficiently large, the elastic stresses start to relax, leading to partial incoherence and the generation of dislocations around the particles. This type of defect structure seems to be the most efficient getter.

Polycrystalline thermoelectric elements of n -type Bi 2 (Te 0.95 Se 0.05 ) 3 and p -type (Bi 0.25 Sb 0.75 ) 2 Te 3 were fabricated by modified Bridgman technique, followed by electrospark cutting. A thermoelectric micromodule, consisting of 12 legs with an Ni anti-diffusion barrier and high-temperature SnSb solder between them, was assembled. To investigate thermoelements under conditions close to operating ones, the micromodule was annealed at 443 K for 1000 h. It was revealed that after annealing for more than 40 h, the near-contact zone of the n -type legs degraded, leading to a complete break of the micromodule junctions. Our results revealed that during annealing, the SnSb solder flowed into the unprotected side surface of the legs, and contacted with the cleavage planes along which tin can diffuse into the thermoelement volume. In contrast, the p -type legs were not affected by the contact with the solder. Different impacts of the solder on n - and p -type legs were explained in the framework of density functional theory (DFT) calculations. Substitution, diffusion and thermodynamic stability calculations showed that Bi to Sn substitution is energetically more beneficial than Sb to Sn substitution. Additionally, it was calculated that for the Bi 2 Te 3 + Sn system, it is more beneficial to form a TeSn and Bi phase, while an Sb 2 Te 3 + Sn system is thermodynamically stable.

The results of investigating the crystal structure, ionic conductivity, and local structure of the (ZrO2)1 –x(Gd2O3)x and (ZrO2)1 –x(Y2O3)x (x = 0.04, 0.08, 0.10, 0.12, and 0.14) solid solutions are reported. The crystals are grown by directional crystallization of the melt in a cold container. The phase composition of the crystals is investigated by X-ray diffractometry and transmission electron microscopy. The transport characteristics are studied by impedance spectroscopy in the temperature range of 400 to 900°C. The local crystal structure is examined by optical spectroscopy. Eu3+ ions were used as a spectroscopic probe. The study of the local structure of the ZrO2–Y2O3 and ZrO2–Gd2O3 solid solutions revealed the features in the formation of optical centers, which reflect the character of localization of oxygen vacancies in the crystal lattice depending on the stabilizing oxide concentration. It is established that the local crystal environment of Eu3+ ions in the (ZrO2)1 –x(Y2O3)x and (ZrO2)1 –x(Gd2O3)x solid solutions is determined by the stabilizing oxide concentration and is practically independent of the stabilizing oxide type (Y2O3 or Gd2O3). The maximum conductivity at a temperature of 900°C is observed in the crystals with 10 mol % of Gd2O3 and 8 mol % of Y2O3. These compositions correspond to the t'' phase and are close to the interface between the cubic and tetragonal phase regions. It is found that in the ZrO2–Y2O3 system the highly symmetric phase is stabilized at a lower stabilizing oxide concentration than in the ZrO2–Gd2O3 system. The analysis of the data obtained makes it possible to conclude that, in this composition range, the concentration dependence of the ionic conductivity is mainly affected by the phase composition rather than the character of the localization of oxygen vacancies in the crystal lattice.

The anisotropy of the mechanical properties of crystalline ZrO2 – 2.8 mol.% Y2O3 solid solutions has been studied. The crystals have been grown by skull melting technique. The microhardness and fracture toughness for different crystallographic planes have been tested by indentation with different indenter diagonal orientations. We show that the microhardness of the material depends on the crystallographic orientation but slightly whereas the fracture toughness varies for different planes. The maximum fracture toughness has been observed in the specimen cut out from the crystal laterally to the <100> orientation. We have studied the microhardness anisotropy for different indenter diagonal orientations. The maximum fracture toughness has been obtained for the {100} plane and the <100> indenter diagonal orientation. The phase composition inside and outside the indents on the {100}, {110} and {111} surfaces for 20, 3 and 1 N loads has been studied using local Raman spectroscopy. The degree of the tetragonal-monoclinic transition has been assessed for different crystallographic planes and different indenter diagonal orientations. We show that the tetragonal-monoclinic transition is anisotropic, this affecting the transformation hardening mechanism. The maximum amount of the monoclinic phase has been detected in the vicinity of the indent in the {100} plane for the <100> indenter diagonal orientation. The highest fraction toughness has also been observed in {100} plane for the <100> indenter diagonal orientation. Probably, this indenter diagonal orientation provides for the maximum stress orientation along the coherent conjugation planes between the tetragonal and the monoclinic phases during the tetragonal-monoclinic transition, i.e. (100)t || (100)m and [001]t || [010]m.

In this work we report a study of (ZrO2)1−x−y(Sc2O3)x(СeO2)y solid solution crystals (x= 0.08–0.10; y= 0.005–0.015) grown by skull melting technique. The crystal structure of the material was studied using Xray diffraction, Raman spectroscopy and impedance spectroscopy. The study showed that optically homogeneous and transparent crystals cannot be grown from the melt in the experimental composition range. For overall stabilizing oxide concentrations of above 10mol% the crystals contained a cubic phase and a rhombohedral one, whereas at concentrations of lower than 10mol% a cubic and a tetragonal phases coexisted. Ceria introduction into the (ZrO2)1−x(Sc2O3)x system increases its high-temperature ionic conductivity. The highest ionic conductivity was observed in 0.5 mol% ceria containing crystals.