Thermoelectric Properties of Off-Stoichiometric Ti-Ni-Sn Half-Heusler Systems

The Ti-Ni-Sn half-Heusler compound exhibits a high power factor but relatively low figure of merit due to its high thermal conductivity. In this paper, we propose an effective and inexpensive way to reduce the thermal conductivity, and independently increase the power factor. To this end, we have systematically synthesized off-stoichiometric Ti-Ni-Sn half-Heusler compounds and introduced Y-Sb dilute co-doping at Ti-Sn sites. Excess Ni introduces interstitial defects in the half-Heusler crystal and results in significant reduction of the thermal conductivity, which drops below 3?W/mK at room temperature. In addition, the Y-Sb dilute co-doping at Ti-Sn sites improves the power factor while the thermal conductivity remains reasonably small. As a result, the figure of merit at room temperature is eight times larger than that of nondoped Ti-Ni-Sn.     

Thermoelectric Properties and n- to p-Type Conversion of Co-Doped ZrNiSn-Based Half-Heusler Alloys

Half-Heulser thermoelectric materials ZrNi_1?y Co _y Sn (y?=?0, 0.02, 0.04, 0.08, 0.12) were prepared by a time-efficient levitation melting and spark plasma sintering procedure. X-ray diffraction analysis and electron probe microanalysis showed that single-phase half-Heusler compounds without compositional segregation have been obtained. The effects of Co doping on the electrical conductivity, Seebeck coefficient, and thermal conductivity of ZrNiSn-based half-Heusler alloys have been investigated from 300?K to 900?K. The Seebeck coefficient displayed a change from negative to positive values above nominal Co doping content of y?=?0.02, indicating a transition in the conduction behavior from n-type to p-type. The maximum dimensionless figure of merit ZT of undoped ZrNiSn sample reached 0.5 at 870?K.     

Thermoelectric Properties of Organic Charge-Transfer Compounds

We measured the thermoelectric (TE) properties of compressed pellets of various organic charge-transfer (CT) complexes, such as (TTF)(TCNQ), (BO)(TCNQ) and (ET)₂(HCNAL), where TTF, TCNQ, BO, ET, and HCNAL represent tetrathiafulvalene, tetracyanoquinodimethane, bis(ethylenedioxy)-tetrathiafulvalene, bis(ethylenedithio)tetrathiafulvalene, and 2,5-dicyano- 3,6-dihydroxy-p-benzoquinone, respectively. The metallic (TTF)(TCNQ) and semiconducting (BO)(TCNQ) complexes showed Seebeck coefficients (S) of −18 μV/K and −30 μV/K at 300 K, respectively. On the contrary, the Mott insulator (ET)₂(HCNAL) was found to show a rather high absolute S (−116 μV/K at 300 K), the magnitude of which is comparable to those of the conventional inorganic TE materials. With increasing temperature (170 K to 300 K), the electrical conductivity was increased about two orders of magnitude while the S value was nearly constant. These results suggest that S values could be determined mainly by spin entropy (configurations) of carriers in the Mott insulator (ET)₂(HCNAL). The magnitude of the observed S value was compared with that derived from a theoretical model (generalized Heikes formula).     

Electronic Structure and Thermoelectric Properties of Si-Based Clathrate Compounds

Thermoelectric properties of Au-substituted Si-based clathrates, Ba₈AuGa₁₃Si₃₂ and Ba₆A₂AuGa _x Si_45−x (A = Sr, Eu, x = 13, 14), were experimentally and theoretically investigated. The polycrystalline samples of the Au-substituted Si-based clathrates were prepared by using the spark plasma sintering technique. The electronic structure of Ba₆A₂AuGa₁₃Si₃₂ was theoretically calculated by ab initio calculations, and the thermoelectric properties of Ba₆A₂AuGa _x Si_45−x were estimated through the calculated electronic structure. The effective mass of Ba₆A₂AuGa _x Si_45−x was experimentally estimated to be greater than that of Ba₈AuGa₁₃Si₃₂. Experimentally observed electronic properties agree with the calculations for Ba₆A₂AuGa _x Si_45−x . The maximum ZT value of Ba₆Sr₂AuGa₁₄Ge₃₁ is about 0.5 at 850 K. The calculated thermoelectric properties agree very well with the experimental results in the range from room temperature to 900 K.     

Effects of Microstructural Evolution on the Thermoelectric Properties of Spark-Plasma-Sintered Ti0.3Zr0.35Hf0.35NiSn Half-Heusler Compound

The MNiSn (M = Ti, Zr, Hf) half-Heusler semiconducting compounds are widely investigated due to their good potential for thermoelectric (TE) power generation applications. In the current work, the evolution of the transport and structural properties of the Ti_0.3Zr_0.35Hf_0.35NiSn compound upon various thermal treatments was studied. The nominal composition was arc melted, ball milled, and spark plasma sintered (SPS). Following SPS, large Hf-rich domains were found by scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS). Subsequently, the samples were subjected to homogenization treatments at 1163 K for 480 h and 610 h under argon atmosphere. Following these thermal treatments, the relative amount of the Hf-rich domains was reduced and they became smaller in size, with increasing thermal treatment duration. Nevertheless, no uniphased structure was reached. The dissolution of the Hf-rich domains in the half-Heusler matrix resulted in increase of both the Seebeck coefficient and electrical resistivity values and a decrease of the carrier concentration, attributed to elimination of these metallic domains. Resulting from the high atomic disorder caused by substitution at the M site, low thermal conductivity values of ~3.8 W m^?1 K^?1 were obtained leading to high ZT values of up to 0.82 following SPS.     

Effects of Ball-Milling Atmosphere on the Thermoelectric Properties of TAGS-85 Compounds

Inspired by the high ZT value lately attained in Ar-protected ball-milled nanocrystalline p-BiSbTe bulk alloy, we report herein an investigation of the effects of ball-milling atmosphere on the thermoelectric (TE) properties of the traditional TE material (GeTe)₈₅(AgSbTe₂)₁₅ (TAGS-85). TAGS-85 samples were prepared via a melting–quenching–annealing process, and then ball-milled in different atmospheres and subsequently densified using a spark plasma sintering technique. The Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient were measured as a function of temperature from 10 K to 310 K. It was found that different ball-milling atmospheres, i.e., air, liquid N₂ (LN₂), and Ar, profoundly affected the TE properties. A state-of-the-art figure of merit ZT ≈ 0.30 was attained at 310 K in the Ar-ball milled sample. The results are discussed in terms of the carrier concentration, mobility, crystallinity, and the grain boundary scattering.     

Reduced Grain Size and Improved Thermoelectric Properties of Melt Spun (Hf,Zr)NiSn Half-Heusler Alloys

Half-Heusler thermoelectric materials Hf(/Zr)NiSn were prepared by levitation melting followed by melt-spinning to refine the boundary structures, and then they were consolidated by spark plasma sintering. X-ray diffraction analysis and scanning electron microscopy showed that single phased half-Heusler compounds without compositional segregation had been obtained. It was found that the thermoelectric properties, especially the thermal conductivity, depended strongly on the boundary structures. The melt-spinning samples with refined boundary structures had a lower thermal conductivity but a power factor comparable to that of the sample prepared by levitation melting, thus providing good thermoelectric properties.     

p型方钴矿化合物BayFeCo3Sb12的热电性能研究

用两步固相反应法合成了单相的p型BayFeCo3Sb12化合物,并系统地研究了Ba不同填充分数对方钴矿化合物热电性能的影响:化合物载流子浓度强烈地依赖于填充原子的填充分数,随Ba填充分数y的增加,载流子浓度及电导率降低;塞贝克系数随温度T的上升而增加,比CoSb3的塞贝克系数有一定程度的提高,尤其是在中温部分有大幅度提高,得到的最大塞贝克系数由CoSb3的107μVK-1提高到Ba1.0FeCo3Sb12的235μVK-1晶格热导率随Ba的填充分数y的增加而进一步下降,Ba08FeCo3Sb12甚至降到2.2 Wm1K1;Ba08FeCo3Sb12化合物显示最大热电性能指数,在850K左右其最大无量纲热电性能指数ZT值达0.75.     

Improved Thermoelectric Properties in Ga2Te3-GaSb Vacancy Compounds

Thermoelectric materials with high figure of merit, which requires large Seebeck coefficient, large electrical conductivity, and low thermal conductivity, are of great importance in solid-state cooling and power generation. Solid-solution formation is one effective method to achieve low thermal conductivity by phonon scattering due to mass and strain field fluctuations. This type of scattering is maximized in structures containing vacancies. The thermoelectric properties of Ga₂Te₃-GaSb vacancy compounds were studied in this work. We find that the lattice thermal conductivity is reduced by over an order of magnitude with the addition of only very moderate amounts of Ga₂Te₃. Additionally, both the carrier type and concentration can be modified. While the vacancy structure induced by Ga₂Te₃ addition to GaSb can effectively reduce phonon conductivity, carrier mobility is also degraded, and optimized thermoelectric properties require careful control of the vacancy content in these solid solutions.     

Thermal Cycling Effects on the Thermoelectric Properties of n-Type In,Ce-Based Skutterudite Compounds

n-Type In-filled CoSb₃ is a known skutterudite compound that has shown promising thermoelectric (TE) properties resulting in high dimensionless figure of merit values at elevated temperatures. Use in various waste heat recovery applications will require survival and operation after exposure to harsh thermal cycling environments. This research focused on uncovering the thermal cycling effects on TE properties of n-type In_0.2Co₄Sb₁₂ and In_0.2Ce_0.15Co₄Sb₁₂ skutterudite compositions as well as quantifying their temperature-dependent structural properties (elastic modulus, shear modulus, and Poisson??s ratio). It was observed that the Seebeck coefficient and resistivity increased only slightly in the double-filled In,Ce skutterudite materials upon thermal cycling. In the In-filled skutterudites the Seebeck coefficient remained approximately the same on thermal cycling, while the electrical resistivity increased significantly after thermal cycling. Results also show that the thermal conductivity marginally decreases in the case of In-filled skutterudites, whereas the reduction is more pronounced in In,Ce-based skutterudite compounds. The possible reason for this kind of reduction can be attributed to grain pinning effects due to formation of nanoinclusions. High-temperature structural property measurements (i.e., Young??s modulus and shear modulus) are also reported. The results show that these structural properties decrease slowly as temperature increases and that the compounds are structurally stable after numerous (up to 200) thermal cycles.     

A Comparison Between the Effects of Sb and Bi Doping on the Thermoelectric Properties of the Ti0.3Zr0.35Hf0.35NiSn Half-Heusler Alloy

We deal here with Sb and Bi doping effects of the n-type half-Heusler (HH) Ti_0.3Zr_0.35Hf_0.35NiSn alloy on the measured thermoelectric properties. To date, the thermoelectric effects upon Bi doping on the Sn site of HH alloys have rarely been reported, while Sb has been widely used as a donor dopant. A comparison between the measured transport properties following arc melting and spark plasma sintering of both Bi- and Sb-doped samples indicates a much stronger doping effect upon Sb doping, an effect which was explained thermodynamically. Due to similar lattice thermal conductivity values obtained for the various doped samples, synthesized in a similar experimental route, no practical variations in the thermoelectric figure of merit values were observed between the various investigated samples, an effect which was attributed to compensation between the power factor and electrical thermal conductivity values regardless of the various investigated dopants and doping levels.     

Approach to the Practical Use of Thermoelectric Power Generation

The results of research and development in the Japanese national project “Development for Advanced Thermoelectric Conversion Systems” are summarized, and the approaches to practical use of advanced thermoelectric modules and power generation systems are presented. The 5-year national project was successfully completed in March 2007. Three kinds of high- efficiency cascaded thermoelectric modules and two kinds of innovative Bi-Te thermoelectric modules were successfully developed. Heat cycle tests for three types of modules were also completed. Moreover, four types of advanced thermoelectric power generation systems were experimentally demonstrated for recovery of waste heat from the industrial and private sectors. In order to proceed further, thermoelectric power generation systems using practical heat sources were followed after installation of the developed modules. In parallel, various approaches for practical use by private companies, as well as plans for the next-phase project by the National Institute of Advanced Industrial Science and Technology (AIST) and the Engineering Advancement Association (ENAA), were also followed. The scenarios to proceed to the commercial phase of thermoelectric power generation are discussed on the basis of the results of the national project.     

Enhancing the Thermoelectric Properties via Modulation of Defects in P-Type MNiSn-Based (M = Hf,Zr, Ti) Half-Heusler Materials

The thermoelectric figure-of-merit (zT) of p-type MNiSn (M = Ti, Zr, or Hf) half-Heusler compounds is lower than their n-type counterparts due to the presence of a donor in-gap state caused by Ni occupying tetrahedral interstitials. While ZrNiSn and TiNiSn, have been extensively studied, HfNiSn remains unexplored. Herein, this study reports an improved thermoelectric property in p-type HfNi_1−xCo_xSn. By doping 5 at% Co at the Ni sites, the Seebeck coefficient becomes reaching a peak value exceeding 200 µV K⁻¹ that breaks the record of previous reports. A maximum power factor of ≈2.2 mW m⁻¹ K⁻² at 973 K is achieved by optimizing the carrier concentration. The enhanced p-type transport is ascribed to the reduced content of Ni defects, supported by first principle calculations and diffraction pattern refinement. Concomitantly, Co doping also softens the lattice and scatters phonons, resulting in a minimum lattice thermal conductivity of ≈1.8 W m⁻¹ K⁻¹. This leads to a peak zT of 0.55 at 973 K is realized, surpassing the best performing p-type MNiSn by 100%. This approach offers a new method to manipulate the intrinsic atomic disorder in half-Heusler materials, facilitating further optimization of their electronic and thermal properties.     

一种提高车载雷达的质量与可靠性水平的方法介绍

针对如何提高车载雷达质量与可靠性的问题,该文首先分析了车载雷达的应用环境特点和雷达自身的特点,然后结合某型车载雷达的研制过程,从质量与可靠性管理、可靠性设计、检验试验等几个方面对研制过程中需要贯彻的可靠性管理方法和设计理念进行了总结,并对几种具体方法进行了详细的描述。     

用系统指标占用系数分析法优选提高质量指标措施的方法

为实现数字电视平移转换而对有线电视系统所做的更新或改造中,有些地方可能与接下来的双向传输改造不相适应,需要重复改造,造成不同程度的浪费。简单分析指标占用系数和指标劣化量的关系,并提出在数字电视平移前,可用系统指标占用系数分析法分析核算系统各个单元器件的质量指标,找出系统中指标劣化量超大的器件,优选出指标劣化量小的器件,在此基础上总结几条减少指标劣化量的措施,对系统进行局部调整,即可提高系统的质量指标,满足数字电视平移转换的需要。     

一种提高λ/4相移光纤光栅开关性能的方法

基于耦合模理论，利用逆向递推传输矩阵方法，数值研究了光纤光栅透射输出端面存在反射时λ／4相移光栅的开关特性，以及啁啾的引入对其开关性能的影响。研究结果表明：对于反射相干叠加加强，可使λ／4相移光栅的阈值开关能量得到降低，但开关对比度将下降；在λ／4相移光纤光栅中引入负啁啾，能使开关对比度得到较大的提高，但双稳阈值开启能量会增加;引入正啁啾，可进一步降低开关阈值，但开关对比度会下降；啁啾的引入将对λ／4相移光栅的双稳环宽度产生较大影响。