Texture development in Bi1.5Pb0.5Sr1.7Y0.5Co2O9−δ layered cobaltite by high-temperature compression deformation and its effect on thermoelectric properties

Texture development in Bi_1.5Pb_0.5Sr_1.7Y_0.5Co₂O_9−δ (BPSYO) layered cobaltite by plastic deformation is examined experimentally by high-temperature uniaxial compression, in order to develop a method to reduce the resistivity of thermoelectric cobaltite by controlling the spatial arrangement of the (0 0 1) conductive CoO₂ layers. It is found that the present oxide can be plastically deformed up to a true strain of −2.2 at 1113 K with strain rates in the order of 10⁻⁵ s⁻¹. It is concluded that high-temperature compression is effective both for densification and texture development. Evolution of (0 0 1) (compression plane) texture is experimentally confirmed as expected from the crystal structure, where a dislocation that has a small Burgers vector may move in the (0 0 1) plane. The formation of a sharp (0 0 1) texture results in a reduction in resistivity to about one-tenth of that of the specimen without texture. The estimated value of the dimensionless thermoelectric figure of merit is 0.11 at 973 K, which is close to the practical level of a thermoelectric material.     

Interlayer materials for diffusion bonding of Bi2Sr2Ca2Cu3Ox superconducting wire

It is established that resistance welding machine's transformer winding discs thicknesses growth higher than certain critical value causes loss increase and excessive consumption of the winding copper. An analytical calculation of this value is given. Relative units' application in analytic expressions considerably simplifies disc critical thickness calculation.     

Microstructural evolution during high-temperature oxidation of Ti2AlC ceramics

Microstructural development during high-temperature oxidation of Ti₂AlC below 1300 °C involves gradual formation of an outer discontinuous TiO₂ layer and an inner dense and continuous α-Al₂O₃ layer. After heating at 1400 °C, an outer layer of mixed TiO₂ and Al₂TiO₅ phases and a cracked α-Al₂O₃ inner layer were formed. After heating to 1200 °C and cooling to room temperature, two types of planar defect were identified in surface TiO₂ grains: twins with (2 0 0) twin planes, and stacking faults bounded by partial dislocations. Formation of planar defects released the thermal stresses that had generated in TiO₂ grains due to thermal expansion mismatch of the phases (TiO₂, α-Al₂O₃ and Al₂TiO₅) in the oxide scale. After heating to 1400 °C and cooling to room temperature, crack propagation in TiO₂ grains resulted from the thermal expansion mismatch of the phases in the oxide scale, the high anisotropy of thermal expansion in Al₂TiO₅ and the volume changes associated with the reactions during Ti₂AlC oxidation. An atomistic oxidation mechanism is proposed, in which the growth of oxide scale is caused by inward diffusion of O^2? and outward diffusion of Al³⁺ and Ti⁴⁺. The weakly bound Al leaves the Al atom plane in the layered structure of Ti₂AlC, and diffuses outward to form a protective inner α-Al₂O₃ layer between 1100 and 1300 °C. However, the α-Al₂O₃ layer becomes cracked at 1400 °C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation.     

硅纳米线复合Mg2Si基热电材料的制备与性能研究

利用溶液法混合粉体并通过电场激活压力辅助烧结(FAPAS)方法制备了不同硅纳米线含量的Mg_2Si基复合热电材料,研究了硅纳米线的掺入及含量对基体材料热电性能的影响。结果表明:硅纳米线掺入后材料电导率大幅降低,塞贝克系数基本不变,热导率小幅降低。随着硅纳米线掺量增加,材料电导率降低,塞贝克系数稍有提高,热导率有升高趋势。硅纳米线掺量为0.1at%的样品在800 K时ZT值达到最高值0.5。     

Effect of TiO2 on high-temperature thermoelectric properties of ZnO

The as-sintered Zn_1−xTi_xO (0.01 ≤ x ≤ 0.05) samples contained a solid solution of Zn_1−xTi_xO with a wurtzite structure and a small amount of the cubic spinel Zn₂TiO₄. The amount of Zn₂TiO₄ increased with an increase in TiO₂ content. The density and grain size increased with the small TiO₂ content (≤0.01), and then they decreased gradually by further increasing the TiO₂ content. The addition of TiO₂ to ZnO led to a significant increase in the electrical conductivity and a decrease in the absolute value of the Seebeck coefficient, resulting in an increase in the power factor. The highest value of power factor (7.6 × 10⁻⁴ W m⁻¹ K⁻²) was attained for Zn_0.98Ti_0.02O at 1073 K. It is demonstrated that the TiO₂ addition is fairly effective for enhancing thermoelectric properties.     

ZrB2陶瓷的自蔓延高温合成和热压烧结

采用自蔓延高温合成(SHS)技术研究了Zr-B2O3-Mg体系反应原料的不同粒度和掺量对反应产物的影响规律,并采用热压烧结方法烧结得到ZrB2陶瓷.先用XRD对材料的相组成进行了分析;再通过化学分析测定精确的相组成;由SHS研究装置测量燃烧温度;由SEM观察材料的显微结构;用排水法测定烧结体的密度.研究结果表明:Zr粉粒径为50μm和Mg过量15%(摩尔分数)时的反应体系是最理想的SHS反应体系,SHS产物粒径为2～5 μm.酸洗产物粒径为0.5～2 μm;其中含有ZrB2(94.59%,质量分数,下同),ZrO2(3.87%),H3BO3(1.54%).烧结体是单相的ZrB2陶瓷;粒径为2～10μm;相对密度为95.4%.     

Bi0.5Sb1.5Te3/Bi2Te2.4Se0.6 thin film thermoelectric sensors

In order to make thin film thermoelectric sensors, p-type (Bi_0.5Sb_1.5Te₃) and n-type (Bi₂Te_2.4Se_0.6) thermoelectric thin films were deposited on Pyrex and Teflon substrates by the flash evaporation technique. Their thermoelectric properties were investigated for annealed (473 K, 1 h) films. Thin film temperature sensors were fabricated and their thermal emf, sensitivity, and time constant were measured and evaluated. The sensor deposited on Teflon substrate showed better performance than that on the Pyrex substrate. The sensitivity and time constant were 2.54 V/W and 13.7 s, respectively, for the sensor with a leg size of 3 mm (w)×16mm (l)×4μm (t).     

High-temperature deformation of Sr(FeCo)1.5Ox ceramics

Compressive creep of polycrystalline SrFe_1.2Co_0.3O_x and SrFeCo_0.5O_x ceramics has been investigated at 940–1000°C in constant-load and constant-displacement-rate experiments. At low stresses, the stress exponent was ≈1 and the activation energy was ≈110–135 kJ/mol. At higher stresses, a transition occurred and the stress exponent became ≈2.4–3.1 and the activation energy became ≈425–453 kJ/mol. At higher stresses, there was no dependence of the steady-state flow stress on oxygen partial pressure from 10–10⁵ Pa. The creep parameters and scanning and transmission electron microscopy observations of the deformed samples suggested that deformation was controlled by diffusion at low stresses and dislocation glide at high stresses.     

Effects of high-pressure high-temperature treatment on the thermoelectric properties of PbTe

Changes in the thermoelectric properties of nominally undoped stoichiometric PbTe caused by high-pressure high-temperature (HPHT) treatment near 6.5 GPa and 900 °C are reported. Comparison of the electrical resistivity, thermopower, and thermal conductivity suggest that the carrier concentration and lattice defect density are decreased by HPHT treatment. Annealing the treated samples at intermediate temperatures (500–600 °C) reversed the changes. These observations are consistent with Pb vacancies being “squeezed out” by the applied pressure, and subsequently reintroduced by annealing.     

Enhanced thermoelectric performance of Bi_2Se_3/TiO_2 composite

Bi₂(Te,Se)₃ alloys are conventional commercial thermoelectric materials for solid-state refrigeration around room temperature.In recent years,much attention has been paid to various advanced thermoelectric composite materials due to the unique thermoelectric properties.In this work,Bi₂Se₃/TiO₂ composites were prepared by hot pressing the plate-like Bi₂Se₃ powders coated in situ with hydrolyzed hytetabutyl-n-butyl titana...     

Effect of powder mixing on thermoelectric properties in Bi2Te3-based sintered compounds

The effect of powder mixing on thermoelectric properties was studied in n-type Bi₂Te_2.85Se_0.15 compounds and p-type Bi_0.5Sb_1.5Te₃ compounds. The figure-of-merit of the n- and p-type sintered compounds was strongly affected by carrier mobility. The use of coarse powders (200–300 μm) was beneficial to improve the crystallographic orientation of sintered compound. However, voids in the compound decreased the carrier mobility. As the fine powders (below 45 μm) were blended with coarse powders up to about 30%, the carrier mobility was increased due to the reduction of the voids. The addition of fine powders over 30% degraded the carrier mobility due to the decrease of crystallographic orientation and the increase of particle boundary. When the fine powder content was 20%, the n- and p-type compounds exhibited the maximum figure-of-merit of 2.31 × 10⁻³ K⁻¹ and 2.89 × 10⁻³ K⁻¹, respectively.     

Enhancing the thermoelectric performance of free solidified p-type Bi0.5Sb1.5Te3 alloy by manipulating its parent liquid state

In this work, an irreversible temperature induced liquid–liquid structural transition (TI-LLST) during the heating process was suggested by abnormal resistivity behaviors of Bi_0.5Sb_1.5Te₃ melt. Based on this information, by manipulating the parent liquid state in free solidification, series of explorations were carried out on the solidified structures and TE properties of Bi_0.5Sb_1.5Te₃ alloy. We found that, from the melt after TI-LLST, the solidified microstructures are evidently refined with more Te-rich eutectic strips precipitating on matrix boundaries. More significantly, hierarchical crystal defects are obviously increased, with tinier nanoprecipitates, more subgrains and boundaries, and much more densified lattice distortions. Consequently, the thermal conductivity, especially the lattice thermal conductivity, is decreased notably with the power factor increased moderately at high temperature. In addition, the ambipolar excitation temperature is shifted to higher value and the slope of thermal conductivity vs temperature is also declined. All these alterations contribute to a synergistic enhancement of the TE figure of merit (ZT) and the maximum figure shifts to higher temperature, with ZT_max = 0.78 at 442 K. The present work may provide a new hint for innovating preparation methods of bulk TE materials with simple free solidification.     

陶瓷及陶瓷基复合材料高温钎料的研究现状与进展

为充分发挥高温结构陶瓷材料的高温件能优势,针对陶瓷及陶瓷基复合材料的高温钎料的研究一直足陶瓷连接领域的发展方向.文章综述了SigN4、SiC陶瓷和C/C复合材料对应的高温钎料的研究难点和研究现状.虽然高温钎料的研究获得一些进展,但仍缺乏对陶瓷具有连接强度岛而且耐高温性能好的实用高温钎料.指出对于Si3N4陶瓷,以V为活性元素的高温钎料值得深入研究;而在设计和研制SiC连接用的高温新钎料时.应该允分考虑钎料与SiC之间的界面反应并予以控制.并对C/SiC陶瓷基复合材料用高温钎料的研究进行了简要的探讨.     

Mechanical spectroscopy of the high-temperature brittle-to-ductile transition in ceramics and cermets

In ceramics such as alumina, zirconia or silicon nitride, the high-temperature “brittle-to-ductile” transition is associated with grain boundary sliding often lubricated by an intergranular glassy film. This dissipative mechanism gives rise to a mechanical loss peak, which turns into an exponential background at higher temperature. When the amount of intergranular glassy phase is higher, the exponential background is globally higher and so is the creep rate. When the mechanical loss peak is due to amorphous pockets surrounded by a rigid skeleton as in silicon nitride, it may account for the toughness improvement by energy dissipation in these glassy pockets, the surrounding skeleton being responsible for creep resistance. Good examples of two-phase materials, in which one phase is responsible for mechanical strength, the other for toughness, are the cermets or the cemented carbides. For instance, in WC–Co, a relaxation peak has been found in the Co binder phase which can be correlated with an increase in toughness, the high-temperature exponential background accounting for the creep behavior of the material.     

Fe掺杂对Bi2Sr2Co2Oy热电性能和自旋关联的影响

采用固相合成法制备出系列Fe掺杂的Bi2Sr2Co2Oy，样品，并对样品进行XRD分析，电阻率（p）、热电势（固和顺磁共振研究。结果表明：Fe掺杂浓度x≤0．3时样品基本为单相。Fe掺杂使体系的电阻率略微增大，热电势显著升高，这可能与Fe掺杂降低了空穴载流子浓度有关。Fe掺杂浓度x=0．05样品获得最大的功率因子（power factor，S^2／P）。顺磁共振结果显示，不掺Fe的样品有着较强的顺磁共振（ESR）信号，随着Fe含量的增加，ESR信号向低频方向移动，并逐渐宽化减弱直至消失。这表明Fe掺杂改变了体系的自旋关联状态，占据了Co位参与了Co—O—Co之间的自旋关联。研究结果表明合适的元素掺杂可以有效地调整体系的自旋关联状态，改善材料的热电性能。     

Self-ignition route to Ag-doped Na1.7Co2O4 and its thermoelectric properties

Ag-doped Na_1.7Co₂O₄ thermoelectric oxides have been synthesized via a simple self-ignition route, which have different nominal compositions in the formula of Na_1.7Ag_yCo_2?yO₄ (y = 0, 0.1, 0.2, 0.3, 0.4, 0.5). The ceramic samples showed the property of high-crystallographic orientation. The electrical resistivity and Seebeck coefficient were measured using four silver probes method. Analysis showed that phase composition, thermoelectric performance and density of the Ag-doped Na_1.7Co₂O₄ ceramics were influenced by Ag dopant. In the whole measurement temperature, Na_1.7Ag_0.2Co_1.8O₄ performed the best thermoelectric properties. The power factor of the doped oxide reached 687.431 μW m^?1 K^?1 at 873 K.     

Electrical and magnetic properties of magnesium ferrite ceramics doped with Bi2O3

The effects of concentration of Bi₂O₃ and sintering temperature on DC resistivity, complex relative permittivity and permeability of MgFe_1.98O₄ ferrite ceramics were studied. The objective of the study was to develop magneto-dielectric materials, with almost equal values of permeability and permittivity, as well as low magnetic and dielectric loss tangent, for the design of antennas with reduced physical dimensions. It was found that the poor densification and slow grain growth rate of MgFe_1.98O₄ can be greatly improved by the addition of Bi₂O₃, because liquid phase sintering was facilitated by the formation of a liquid phase layer due to the low melting point of Bi₂O₃. It was found that 3% Bi₂O₃ can result in fully sintered MgFe_1.98O₄ ceramics. The DC resistivities of the MgFe_1.98O₄ ceramics were increased as a result of the addition of Bi₂O₃, except for 0.5%. The exceptionally low resistivities of the 0.5% samples were explained by a ‘cleaning’ effect of the small amount of liquid phase at the samples’ grain boundaries. The electrical and magnetic properties of the MgFe_1.98O₄ ceramics exhibited a strong dependence on the concentration of Bi₂O₃. The 0.5% samples were found to have the highest dielectric loss tangents, which can be understood similarly to the DC resistivity results. The 2–3% Bi₂O₃ is required to attain low dielectric loss MgFe_1.98O₄ ceramics for antenna application. Low concentration of Bi₂O₃ increased the static permeability of the MgFe_1.98O₄ ceramics owing to the improved densification and grain growth, while too high a concentration led to decreased permeability owing to the incorporation of the non-magnetic component (Bi₂O₃) and retarded grain growth. However, the addition of Bi₂O₃ alone is not able to produce magneto-dielectric materials based on MgFe_1.98O₄ ceramics, and further work is necessary to modify the permeability using cobalt (Co).     

Enhanced thermoelectric and mechanical performance of polycrystalline p-type Bi0.5Sb1.5Te3 by a traditional physical metallurgical strategy

In this paper, p-type Bi_0.5Sb_1.5Te₃ polycrystalline materials have been fabricated by a traditional vacuum melting method, and the effects of cooling rate and MoSi₂ addition on the microstructure, thermoelectric and mechanical performance of the polycrystalline materials have been studied detailedly. It shows that the amount of Te-rich eutectic phase increases and the lamellar microstructure has been refined with the increase of the cooling rate. Due to the combined effect of cooling rate on the carrier concentration and mobility, the air cooled sample has higher figure of merit than the furnace cooled, water cooled and liquid nitrogen cooled samples, and a maximal ZT of 1.02 at 50 °C was obtained for the air cooled polycrystalline sample. Under the same air cooling condition, the inhomogeneous nucleation sites increase with increasing the amount of MoSi₂ particles, therefore the amount of Te-rich eutectic phase increases and the lamellar microstructure get refined, and the thermal conductivity of the sample decreases significantly due to the extra phonon scattering by the refined microstructure and MoSi₂ particles. The resulted figure of merit ZT increases with increasing the amount of MoSi₂ particles, and it decreases with further increasing the MoSi₂ content after attaining the vertex of ZT = 1.33 at 100 °C at a content of 0.2 wt.% MoSi₂. The flexural strength of the air cooled polycrystalline sample also increases with the amount of MoSi₂ increasing from 0 to 0.3 wt.%, and a nearly 56% enhancement was achieved for the 0.2 wt.% MoSi₂ sample (28.0 MPa) compared with the MoSi₂ free sample. The improvement of flexural strength is in agreement with the Hall–Petch strengthening mechanism due to the lamellar microstructure refinement induced by MoSi₂.