Structure, Homogeneity and Properties of Thermoelectric Materials Based on Ternary Solid Solutions of Bismuth and Antimony Chalcogenides

Ternary solid solutions Bi_xSb_1-xTe and especially Bi₂Te_1-ySe_y grown by vertical zone melting have the tendency for dendrite liquation. It causes considerable chemical inhomogeneity of the material and multicomponent structure. The effect of long-term annealing at 380–400°C on microstructure, phase composition and properties of structural components has been studied. It was shown that proper growth regime provides ingots with oriented column structure and good texture, which maximizes thermoelectric properties along the growth axis.     

Preparation and Thermoelectric Properties of Polycrystalline Si<Subscript>0.4</Subscript>Ge<Subscript>0.6</Subscript>

Polycrystalline Si_0.4Ge_0.6 was prepared by electron-beam float-zone melting, and its thermoelectric properties were studied in the intrinsic region. The samples were found to be p-type, with a room-temperature resistivity from 5 to 20 Ωcm. The thermoelectric figure of merit of the alloy in the intrinsic region (500–1000 K) is lower than that of heavily doped alloys. Above 1000 K, a sharp rise in its thermoelectric figure of merit would be expected.     

Enhanced thermoelectric figure of merit of p-type half-Heuslers

Half-Heuslers would be important thermoelectric materials due to their high temperature stability and abundance if their dimensionless thermoelectric figure of merit (ZT) could be made high enough. The highest peak ZT of a p-type half-Heusler has been so far reported about 0.5 due to the high thermal conductivity. Through a nanocomposite approach using ball milling and hot pressing, we have achieved a peak ZT of 0.8 at 700 °C, which is about 60% higher than the best reported 0.5 and might be good enough for consideration for waste heat recovery in car exhaust systems. The improvement comes from a simultaneous increase in Seebeck coefficient and a significant decrease in thermal conductivity due to nanostructures. The samples were made by first forming alloyed ingots using arc melting and then creating nanopowders by ball milling the ingots and finally obtaining dense bulk by hot pressing. Further improvement in ZT is expected when average grain sizes are made smaller than 100 nm.     

制备工艺对n型Bi_2Te_3基材料热电性能和抗压强度的影响

以商用区熔(ZM)n型Bi2Te3基材料为原料,采用简单研磨结合放电等离子烧结技术(ZM+SPS)和熔体旋甩(MS)结合放电等离子烧结技术(MS+SPS)制备了n型Bi2Te3基块体热电材料.对三种不同工艺制备出样品的微结构、热电性能和力学性能进行了研究.FESEM微结构表征结果表明:区熔样品的晶粒粗大,有较强的取向性;经SPS烧结后,晶粒细化,取向性大为降低;而区熔样品经MS+SPS后,晶粒得到进一步细化,且没有明显的取向性.对三组样品进行的热电性能和抗压强度测试,结果表明:区熔原料最大ZT值为0.72(430K),抗压强度仅为40MPa;经SPS后,样品的最大ZT值为0.68(440K),抗压强度为110MPa,相比区熔样品提高了175%;MS+SPS样品的最大ZT值为0.96(320K),其室温ZT值相比区熔样品提高了64%,抗压强度相比区熔样品提高了400%,达到200MPa.     

Analysis of anisotropy of properties on the basis of studies of texture of coarse-grained ingots of thermoelectric materials

An analysis of anisotropy of the properties of a Bi₂Se_0.3Te_2.7 solid solution was carried out using construction of demonstrative surfaces for thermoelectric effectiveness and thermal expansion coefficients. It is shown that the texture is an important factor forming anisotropy of properties and technological fitness of ingots for manufacturing modules. Anisotropy of properties based on the studies of the ingot textures obtained using the float-zone method and Bridgman method (growing thermoelectric plates in a flat cavity) was studied.     

熔速对氩气保护GH4169G合金电渣锭组织及夹杂物的影响

研究了熔速对氩气保护GH4169G电渣锭宏微观组织及非金属夹杂物的影响。结果表明:适当增加熔速有利于缩短铸锭的局部凝固时间,减小二次枝晶间距,从而细化枝晶组织,但对Nb、Ti等易偏析元素沿径向的宏观分布影响不大。熔速对GH4169G铸锭中的夹杂物类型影响较小,主要为氧化物、氟化物和氮化物三类。夹杂物在铸锭表面最多,向内部迅速减少并趋于稳态。铸锭内部夹杂物多以氧化物为核心,氮化物为次外层,碳化物为最外层的双层或三层结构。采用MeltFlow-ESR模拟方法,分析了熔速对重熔过程中夹杂物运动轨迹的影响,发现提高熔速有利于夹杂物向铸锭表面运动,降低铸锭表面夹杂物富集区的厚度和铸锭内部夹杂物的数量。此外,提高熔速有利于缩短夹杂物析出长大的时间,降低夹杂物尺寸。     

高优值系数In4Se3多晶的制备及其热电输运特性

分别采用不同的熔炼、退火工艺, 结合放电等离子烧结方法制备了块状多晶In₄Se₃热电材料。研究了熔炼时间和退火时间对材料物相、成分、显微结构及热电性能的影响。熔炼后铸锭中存在In及InSe杂相, Se缺失量随熔炼时间的延长而增加, 使得样品载流子浓度增大, 电导率有所提高, 熔炼48 h样品ZT值相对较高。在确定熔炼工艺的基础上, 进行不同时间的退火处理后, InSe相消失, 显微结构中分布有较大尺寸的台阶状结构, 这种台阶状结构有利于降低热导率, 而对电导率无明显影响。实验结果表明: 一定程度延长熔炼时间、退火时间对提高样品的热电性能有积极作用, 其中熔炼48 h再退火96 h后的样品ZT值最高, 在702 K达到0.83, 比文献值提高约32%。     

等离子旋转电极雾化法制备高品质Ti-6.5Al-1.4Si-2Zr-0.5Mo-2Sn合金粉末

旨在制备高品质Ti-6.5Al-1.4Si-2Zr-0.5Mo-2Sn粉末,为后续粉末高温钛合金构件的制备奠定基础。首先采用真空自耗电弧熔炼(VAR)技术制备Ti-6.5Al-1.4Si-2Zr-0.5Mo-2Sn合金铸锭,对铸锭进行化学成分检测,并分析其合金元素损耗、成分均匀性以及显微组织和物相组成。利用制得棒料,采用等离子旋转电极雾化法(PREP),选取不同转速制备得到钛合金粉末,将粉末筛分成不同粒度范围。研究了棒料转速与粉末理化性能间的关系。采用X射线衍射分析仪(XRD)、扫描电镜(SEM)、金相显微镜(OM)分别分析了粉末的物相组成、形貌和微观组织。研究表明:通过独特的压制电极设计,可制得成分均匀、元素损耗小的钛合金铸锭,且各合金元素含量满足国标的要求。铸锭微观组织为层片状结构,基体中存在少量大小不均的Ti5Si3硅化物相。PREP法制得的钛合金粉末呈正态分布,且球形度好,无空心球和卫星球。随着转速增加,小颗粒粉末占比增加,大颗粒粉末占比大幅度降低。粉末颗粒以胞状组织为主,存在少量的枝晶。合金粉末主要由α′马氏体相组成。相比合金铸锭,粉末中各合金元素略有损耗,O元素质量分数小于0.1%,有利于制得高性能的粉末钛合金。     

Effect of ultrasonic processing of molten metal on structure formation and improvement of properties of high-strength Al-Zn-Mg-Cu-Zr alloys

The ultrasonic treatment of molten and solidifying metal used along with other physical methods, such as thermal treatment, filtration, etc., enables one to obtain a nondendritic structure and uniformly disperse intermetallic particles in aluminum ingots. The structure and properties of ingots and extruded shapes from highstrength Al-Zn-Mg-Cu alloys (containing up to 0.3% Zr) are shown to be considerably improved after ultrasonic treatment.Abbreviations F fine filtration - HT heat treatment - UST-1 ultrasonic treatment in a trough - T undercooling of melt in a water-cooled tray - UST-2 combination of undercooling with ultrasonic treatment - UST-3 ultrasonic treatment in the liquid ingot bath - K reference experimental results     

Processing of Fe-Al-C-Ce alloys through air induction melting with flux cover (AIMFC) and electroslag remelting (ESR)

The effect of process parameters on recovery of reactive element, cerium, during air induction melting with flux cover (AIMFC) and electroslag remelting (ESR) of Fe-10.5 wt% Al-0.8 wt% C-(0.1 and 0.3) wt% Ce alloys and also the effect of melting techniques on hot workability, structure and tensile properties (at room temperature and at 873 K) of Fe-10.5 wt% Al-0.8 wt% C-0.3 wt% Ce alloy have been investigated. Good recovery of cerium was obtained by AIMFC. While conventional fluxes were found to be unsatisfactory, modified flux containing CeO₂ gave better recovery of cerium during ESR. The best recovery of cerium was achieved by using calcium as a deoxident during ESR. The combination of AIMFC and ESR yields a sound ingot of Fe-Al-C-Ce quaternary alloys free from gas and shrinkage porosity with very low oxygen, nitrogen and sulphur contents. Processing of AIMFC ingots through ESR has resulted in improved hot-workability. The ESR processed and hot-rolled alloy exhibited superior tensile elongation as compared to hot-rolled AIMFC alloy. This may be attributed to the comparatively sound, homogeneous and clean ingot, with a refined microstructure and fine uniform distribution of precipitates observed in hot-rolled ESR ingots.     

Construction of Gallium Point at NMIJ

Two open-type gallium point cells were fabricated using ingots whose nominal purities are 7N. Measurement systems for the realization of the melting point of gallium using these cells were built. The melting point of gallium is repeatedly realized by means of the measurement systems for evaluating the repeatability. Measurements for evaluating the effect of hydrostatic pressure coming from the molten gallium existing during the melting process and the effect of gas pressure that fills the cell were also performed. Direct cell comparisons between those cells were conducted. This comparison was aimed to evaluate the consistency of each cell, especially related to the nominal purity. Direct cell comparison between the open-type and the sealed-type gallium point cell was also conducted. Chemical analysis was conducted using samples extracted from ingots used in both the newly built open-type gallium point cells, from which the effect of impurities in the ingot was evaluated.     

Melt‐Centrifuged (Bi,Sb)2Te3: Engineering Microstructure toward High Thermoelectric Efficiency

Microstructure engineering is an effective strategy to reduce lattice thermal conductivity (κ_l) and enhance the thermoelectric figure of merit (zT). Through a new process based on melt‐centrifugation to squeeze out excess eutectic liquid, microstructure modulation is realized to manipulate the formation of dislocations and clean grain boundaries, resulting in a porous network with a platelet structure. In this way, phonon transport is strongly disrupted by a combination of porosity, pore surfaces/junctions, grain boundaries, and lattice dislocations. These collectively result in a ≈60% reduction of κ_l compared to zone melted ingot, while the charge carriers remain relatively mobile across the liquid‐fused grains. This porous material displays a zT value of 1.2, which is higher than fully dense conventional zone melted ingots and hot pressed (Bi,Sb)₂Te₃ alloys. A segmented leg of melt‐centrifuged Bi_0.5Sb_1.5Te₃ and Bi_0.3Sb_1.7Te₃ could produce a high device ZT exceeding 1.0 over the whole temperature range of 323–523 K and an efficiency up to 9%. The present work demonstrates a method for synthesizing high‐efficiency porous thermoelectric materials through an unconventional melt‐centrifugation technique.     

Properties of Sn-doped Bi2Te3 − x Sex single crystals

The effect of Sn doping (0.2 and 0.4 at %) on the properties of Czochralski-grown single crystals of n-type Bi₂Te_2.85Se_0.15 solid solutions is studied. Thermoelectric power, electrical conductivity, thermal conductivity, and Hall effect measurements in the range 77–400 K demonstrate that Sn doping has a significant effect on the transport properties of the solid solutions. Between 300 and 370 K, the thermoelectric figure of merit of Bi_1.996Sn_0.004Te_2.85Se_0.15 single crystals is higher than that of the Sn-free solid solution. In addition, hot-microprobe thermoelectric power measurements, highly sensitive to variations in carrier concentration, indicate that the Sn-doped single crystals are very uniform in electrical properties, both along the growth direction and radially.     

The influence of a galvanomagnetic stationary vortex current on the magneto-thermoelectric figure of merit of graded inhomogeneous bismuth-antimony alloys

We have studied the influence of a longitudinal graded inhomogeneity in crystals of bismuth-antimony solid solutions on their effective magnetoresistance, which is one of the parameters determining the efficiency (thermoelectric figure of merit) of a thermoelement operating in the presence of a transverse magnetic field. It is shown that, in a thermoelement operating under the action of an external magnetic field, the negative effect of the crystal inhomogeneity caused by the temperature gradient can be compensated for by its graded composition so as to provide for an increase in the thermoelectric figure of merit of the thermoelement.     

Investigation of spitting behaviour of NiCoCrAlY type materials during electron beam evaporation

ABSTRACT

Spitting behaviour of four casted and powder metallurgy ingots used in the fabrication of NiCoCrAlY-type environmental protective coatings by electron beam physical vapour deposition was investigated and correlated with ingot microstructures and compositions. Spits found in the coatings mainly consisted of the Ni₅Y phase precipitated in the β-NiAl matrix while their typical size exceeded 50?µm. Spitting was less pronounced for ingots having a higher Cr content, larger size of γ-Ni(Co, Cr) precipitates formed within β-NiAl grains and narrower transitional zone formed at the interface between molten and unmolten ingot regions and consisting of coarsened β grains. Large (above 100?µm) agglomerates of the Ni₅Y phase detected below the zone were identified as the main spit precursors.     

Refining of metallurgical silicon by directional solidification

The directional solidification of a typical and a previously refined metallurgical silicon was carried out in a vertical Bridgman furnace. The mold velocity out of the hot zone of the furnace changed from one experiment to another in the range between 5 and 110 μm s⁻¹. Samples were extracted from the cylindrical ingots obtained in the experiments to investigate the effects of the mold velocity on the micro and macrostructures and on the concentration profiles of impurities along the ingots. At the lowest mold velocity, the macrostructures consist of columnar grains oriented approximately parallel to the ingot axis. As velocity increases, grains become thinner and more inclined in the radial direction. Precipitated particles containing Si, Fe, Al, and Ti are observed at the top of all ingots and, as the mold velocity increases, they are also seen at the ingot bottom and middle. The concentration profiles of several impurities have been measured along the ingots by inductively coupled plasma atomic emission spectrometry (ICP), indicating an accumulation of impurities at the ingot top. Consequently, the bottom and middle of the ingots are purer than the corresponding metallurgical silicon from which they solidified. Slices from the ingot bottom have also been analyzed by the glow discharge mass spectrometry technique (GDMS), allowing measurement of impurity concentrations that were below the quantification limit of the ICP. The purification effect and the accumulation of impurities at the ingot top are more pronounced as the mold velocity decreases. In the ingots obtained from the typical metallurgical silicon at the lowest mold velocities (5 and 10 μm s⁻¹), except for Al, all impurities are in concentrations below an important maximum limit for the feedstock of solar grade silicon. At the same mold velocities, the concentrations of Fe, Ti, Cu, Mn, and Ni measured at the bottom of the ingots obtained from both types of metallurgical silicon (typical and previously refined) are even below some limits suggested directly for solar grade silicon.     

Composition variations in directionally solidified InSb-GaSb alloys

Melts containing 10%, 30% and 50% InSb were directionally solidified horizontally, vertically and in Skylab 3 and 4. The resulting concentration profiles were much more uniform in the ingots solidified horizontally. Fluctuations in composition occurred sooner as the InSb content increased and later in the horizontally processed ingots. In the 10% InSb ingot processed in SL-3 a large orientation-dependent segregation was observed at about 2 cm from the initial position of the interface. A radial variation in composition was also observed, and tentatively attributed to surface-tension driven convection.     

Effect of nonstoichiometry on the thermoelectric properties of Tl<Subscript>4</Subscript>TiS<Subscript>4</Subscript> crystals

Stoichiometric (1), Tl₂S-enriched (2), and TiS₂-enriched (3) Tl₄TiS₄ crystals (with deviations from stoichiometry within the homogeneity range of the ternary compound) have been grown by the Bridgman method, and their thermoelectric properties have been studied by the Harman technique. All of the crystals have a high thermoelectric figure of merit. The highest figure of merit is offered by crystal 1 at positive thermoelectric powers and by crystal 3 at negative thermoelectric powers. The thermoelectric properties of crystal 2 were found to irreversibly degrade over time.     

Thermoelectric properties of Bi2Te3-Sb2Te3 single crystals in the range 100–700 K

The electrical conductivity, thermoelectric power, and thermal conductivity of Bi₂Te₃-Sb₂Te₃ crystals grown by the floating-crucible technique were measured in the temperature range from 100 to 700 K. The thermoelectric figure of merit of the crystals was evaluated. The effect of crystal composition on these properties was analyzed.     

Composition and thermoelectric power factor variation of (Bi2Te3)0.96(Bi2Se3)0.04 crystal in growth direction

(Bi₂Te₃)_0.96(Bi₂Se₃)_0.04 crystal, which is an n-type thermoelectric semiconductor, has many applications in thermoelectric cooling systems. Single crystal of this composition was grown by Traveling Heater Method. A sensible gradient in thermoelectric power factor was observed in the first quarter length of the prepared crystalline ingot. Characterizing the crystallization procedure and ingot composition, the gradient was attributed to the variation of the Bi₂Se₃ concentration of Bi₂Te₃–Bi₂Se₃ quasi-binary solid solution system. The structural properties were characterized by means of XRD analyses. Results of composition variation (Bi₂Se₃ distribution function) were in good correlation with experimental thermoelectric power factor measured along the grown rod.