Fracture mode,microstructure and temperature-dependent elastic moduli for thermoelectric composites of PbTe–PbS with SiC nanoparticle additions

Twenty-six (Pb_0.95Sn_0.05Te)_0.92(PbS)_0.08–0.055% PbI₂–SiC nanoparticle (SiC_np) composite thermoelectric specimens were either hot pressed or pulsed electric current sintered (PECS). Bloating (a thermally induced increase in porosity, P, for as-densified specimens) was observed during annealing at temperatures >603?K for hot-pressed specimens and PECS-processed specimens from wet milled powders, but in contrast seven out of seven specimens densified by PECS from dry milled powders showed no observable bloating following annealing at temperatures up to 936?K. In this study, bloating in the specimens was accessed via thermal annealing induced changes in (i) porosity measured by scanning electron microscopy on fractured specimen surfaces, (ii) specimen volume and (iii) elastic moduli. The moduli were measured by resonant ultrasound spectroscopy. SiC_np additions (1–3.5 vol.%) changed the fracture mode from intergranular to transgranular, inhibited grain growth, and limited bloating in the wet milled PECS specimens. Inhibition of bloating likely occurs due to cleaning of contamination from powder particle surfaces via PECS processing which has been reported previously in the literature.     

纳米Ag颗粒表面等离子激元对上转换材料光致发光性能影响的研究

本文采用共烧结工艺将纳米Ag颗粒引入Yb³⁺, Er³⁺共掺的NaYF₄上转换材料中, 利用X射线衍射及扫描电子显微镜技术对制备的NaYF₄材料进行结构特性和表面形貌的表征, 通过吸收谱及荧光光谱测试技术对NaYF₄材料光吸收及光发射特性进行表征. 通过对纳米Ag颗粒引入量的优化, 获得了Yb³⁺, Er³⁺共掺的NaYF₄上转换材料荧光发射峰的增强, 300—800 nm全光谱范围内增益达28%, 在544 nm处获得最大增益55%, 具有显著的荧光增强效果. 同时分析了不同数量纳米Ag颗粒的引入对NaYF₄材料吸收谱及光致发光特性影响, 指出了表面等离子激元的光猝灭及共振吸收增强作用机理.     

Thermal expansion of SOFC materials

A short overview is given for the thermal expansion of solid oxide fuel cell materials. The thermomechanical compatibility of state-of-the-art materials is compared with alternative, new materials. With these alternatives a better adjustment of the thermal expansion coefficients of the various fuel cell components is possible and fuel cells based on the newly developed materials are proposed.     

Fabrication and characterization of Ni-SSZ/SSZ/LSM-SSZ anode-supported SOFCs by tape casting and single-step co-sintering techniques

In this work, Ni-10 % Sc₂O₃-stabilized ZrO₂ (SSZ)/SSZ/La_0.8Sr_0.2MnO_3-δ (LSM)-SSZ anode-supported solid oxide fuel cells (SOFCs) have been successfully prepared by tape casting and single-step co-sintering procedures. The structure contains Ni-SSZ anode substrate and Ni-SSZ anode functional, dense SSZ electrolyte, LSM-SSZ cathode functional, and LSM-SSZ cathode layers were successfully prepared at 1250, 1300, and 1350 °C, respectively. The microstructures of the single cells were examined by SEM. There were some close pores in electrolyte of Cell-1250, and the cathode particle size obviously increased in Cell-1350. Therefore, Cell-1300 showed the optimal cell performance, the maximum power density attained 920 mW cm^?2 at 800 °C. The impedance analysis demonstrated that the co-sintered temperatures have effects on not only the polarization resistance R _P of a single cell but also its overall ohmic resistance R _S . The results indicate that the tape casting and single-step co-sintering methods are both time saving and feasible for the development of anode-supported SOFCs.     

Development of Co-Sintering Process for Effective Fabrication of Direct-Write Planer Waveguide

We propose a co-sintering process for effective fabrication of three-layered structure for direct-write planar waveguide. Processing parameters and optical properties of the waveguide was investigated.     

Reactivity and diffusion between La0.8Sr0.2MnO3 and ZrO2 at interfaces in SOFC cores by TEM analyses on FIB samples

In this paper, we investigate the diffusion and reactivity at the interface between the electrolyte (YSZ) and the cathode (LSM) of planar SOFC single cells by Analytical and High Resolution Transmission Microscopy. Cells were obtained via an aqueous tape-casting process allowing casting the three layers (cathode, electrolyte, anode [Ni-YSZ]) in a single operation. To allow the cell to function at intermediate temperatures (750–850 °C), the final electrolyte thickness after co-sintering at 1350 °C must range between 40 μm and 50 μm. As-sintered cells as well as cells that have been operated at 850 °C have been considered here; it is shown that the electrical performances were one order of magnitude less than expected and that they deteriorated quickly under operating conditions.In order to explain this behavior, we carried out analytical transmission electron microcopy. To obtain precisely located chemical and structural information, we used the “H-Shape” as well as the Lift Out FIB (Focused Ion Beam) techniques to extract 5 × 10 × 0.1 μm TEM samples; additional High Resolution characterization was carried out at interfaces between LSM and YSZ grains on standard ion-milled samples. We showed that the co-sintering temperature (1350 °C) was responsible for some diffusion of manganese through the electrolyte and the cathode, leading then to the rise of a significant electronic conduction and to the drop off of the ionic conductivity, and accounts for the germination and growth of the resistive pyrochlore phase La₂Zr₂O₇. Operating the cell at 850 °C do not aggravate these phenomena, but rather alters the anode microstructure.     

The influence of pore and Ni morphology on the electrical conductivity of porous Ni/YSZ composite anodes for use in solid oxide fuel cell applications

A wide range of porous Ni–YSZ composite microstructures was produced by conventional tape casting and co-sintering using a variety of starting powders including Ni, NiO, graphite and Ni-coated graphite. The graphite additions were added to produce controlled levels of porosity in the final sintered and reduced anode. All materials indicated classical conductivity percolation behaviour with increasing Ni loadings. However, the percolation threshold at which electrical conduction became measurable was lowest for anodes made with Ni-coated graphite and highest for anodes containing large amounts of porosity introduced by large additions of graphite. Sintered and reduced anodes possessed large scale porosity introduced by the graphite additions and a finer scale porosity generated by the incomplete sintering of the Ni/YSZ powder network. A model was developed for predicting the influence of large scale porosity on conductivity and agreed well with the experimental results. The analysis indicates that fine scale porosity will have a more detrimental impact on conductivity compared to a coarse porous structure.     

Compatibility study of oxide and olivine cathode materials with lithium aluminum titanium phosphate

The compatibility of the solid electrolyte Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP) with the cathode materials LiCoO₂, LiMn₂O₄, LiCoPO₄, LiFePO₄, and LiMn_0.5Fe_0.5PO₄ was investigated in a co-sintering study. Mixtures of LATP and the different cathode materials were sintered at various temperatures and subsequently analyzed by thermal analysis, X-ray diffraction, and electron microscopy. Oxide cathode materials display a rapid decomposition reaction with the electrolyte material even at temperatures as low as 500 °C, while olivine cathode materials are much more stable. The oxide cathode materials tend to decompose to lithium-free compounds, leaving lithium to form Li₃PO₄ and other metal phosphates. In contrast, the olivine cathode materials decompose to mixed phosphates, which can, in part, still be electrochemically active. Among the olivine cathode materials, LiFePO₄ demonstrated the most promising results. No secondary phases were detected by X-ray diffraction after sintering a LATP/LiFePO₄ mixture at temperatures as high as 700 °C. Electron microscopy revealed a small secondary phase probably consisting of Li₂FeTi(PO₄)₃, which is ionically conductive and should be electrochemically active as well.     

Conductivity degradation of NiO-containing 8YSZ and 10YSZ electrolyte during reduction

Yttria-stabilized zirconia (YSZ) electrolyte containing dissolved nickel as a result of co-sintering in air with NiO was shown by TEM and magnetic measurements to develop isolated, nano-sized metallic nickel precipitates during treatments in hydrogen-containing atmosphere. During exposure to hydrogen containing atmosphere at 1000 °C for 500 h, an exponential decrease in oxygen ion conductivity was observed in Ni-doped 8YSZ, while no conductivity change was observed in Ni-doped 10YSZ.     

炭化烧蚀材料热解膨胀和线膨胀对内部温度场影响研究

本文根据炭化烧蚀材料的防热机理,认为防热材料表面化学烧蚀后退、热解膨胀和内部温度升高引起线膨胀是造成防热层厚度变化的因素,国内外学者主要研究了表面化学烧蚀后退对飞行器外形变化和温度场的影响,而对于后两者的具体影响却少有研究。本文数值模拟了高硅氧-酚醛炭化材料的烧蚀热响应过程,分析和解释了计算结果,说明了防热材料的线膨胀和热解膨胀对外形和温度场有较大的影响。     

Preparation techniques and materials for long term stable SOFC - Single cell membranes

Chemical reactions and thermal expansion mismatch between electrodes and electrolyte may reduce the long term stability of SOFC-single cells and can cause undesirable thermomechanical stresses. In solid electrolyte cells the formation of MnAl₂O₄ was detected between the air electrode (La_0.5Ca_0.5MnO₃) and the electrolyte (YSZ/Al₂O₃) in a 5 μm diffusion zone within the electrolyte. The electronically conducting spinel MnAl₂O₄ is thought to be the main factor for delamination of the air electrode under anodic current (electrolysis). The performance and long term stability of the air electrode/electrolyte interface can be improved for electrolysis conditions by an additional intermediate YSZ-layer made by sol-gel technique. The mismatch in thermal expansion between the electrode materials and the electrolyte have been eliminated via optimized doping and by adding small amounts of a silicate-based substituent with a very low thermal expansion co-efficient for the cathode and anode, respectively. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

折射率与电声子相互作用的关系

从微观出发研究电声子相互作用对折射率的影响。用半经典方法导出一个适用于晶态和无序度小的介质折射率公式和折射率温度系数公式。首次指出电声子相互作用引起的折射率温度系数,高温时为一常数,低温时与T~3成正比。并与实验作了对比,理论曲线与实验符合得很好。用得到的公式计算了好几种材料的温度系数。计算值与实验值相符合。     

Zero thermal expansion in a nanostructured inorganic-organic hybrid crystal

There are very few materials that exhibit zero thermal expansion (ZTE), and of these even fewer are appropriate for electronic and optoelectronic applications. We find that a multifunctional crystalline hybrid inorganic-organic semiconductor, beta-ZnTe(en)(0.5) (en denotes ethylenediamine), shows uniaxial ZTE in a very broad temperature range of 4-400 K, and concurrently possesses superior electronic and optical properties. The ZTE behavior is a result of compensation of contraction and expansion of different segments along the inorganic-organic stacking axis. This work suggests an alternative route to designing materials in a nanoscopic scale with ZTE or any desired positive or negative thermal expansion (PTE or NTE), which is supported by preliminary data for ZnTe(pda)(0.5) (pda denotes 1,3-propanediamine) with a larger molecule.     

Molecular dynamics study of the role of material properties on nanoparticles formed by rapid expansion of a heated target

Rapid expansion of a heated target and its decomposition into fragments is investigated by using molecular dynamics simulations. Particular attention is focused on the void formation and nucleation that governs the target disintegration. The cluster formation process is investigated as a function of material properties (initial temperature, interaction potential and composition). Calculation results demonstrate the influence of these properties on void nucleation and growth and on the characteristic parameters of nanoparticles to be formed. In particular, larger initial temperature and expansion rate lead to the formation of smaller fragments. These effects are found to be similar for three different materials (silicon, nickel and metal alloy). In addition, the stoichiometrical cluster composition obtained in the expansion of a binary alloy is found to be fairly well preserved. The calculation results can be used for the interpretation of the experimental findings showing the formation of nanoparticles by short and ultra-short pulse laser ablation of both simple and more complex materials.     

Microwave-assisted and thermal stepwise expansion of oxidized graphites

The results of studies of the thermal and microwave-assisted expansion of graphites oxidized with acids are presented. The expansion of oxidized graphites heated to 180–300°C for up to 30 min or exposed to microwave radiation for 3–60 s is demonstrated to be of stepwise character. The bulk density of oxidized graphites as a function of thermal expansion conditions and microwave irradiation duration is determined. The results of studies on the thermal expansion of polymer-oxidized graphite composite materials are reported. Optical microscopy is used to monitor changes in the mean diameter of the filler particles and the mean distances between them in composite materials based on PVC plastigel and low molecular silicone rubber upon heating to the point of emergence of electric conduction.     

Analytical and Numerical Studies of the One-Dimensional Spin Facilitated Kinetic Ising Model

The one-dimensional spin facilitated kinetic Ising model is studied analytically using the master equation and by simulations. The local state of the spins (corresponding to mobile and immobile cells) can change depending on the state of the neighbored spins, which reflects the high cooperativity inherent in glassy materials. The short-time behavior is analyzed using a Fock space representation for the master equation. The hierarchy of evolution equations for the averaged spin state and the time dependence of the spin autocorrelation function are calculated with different methods (mean-field theory, expansion in powers of the time, partial summation) and compared with numerical simulations. The long-time behavior can be obtained by mapping the one-dimensional spin facilitated kinetic Ising model onto a one-dimensional diffusion model containing birth and death processes. The resulting master equation is solved by van Kampen's size expansion, which leads to a Langevin equation with Gaussian noise. The predicted autocorrelation function and the global memory offer in the long-time limit a screened algebraic decay and a stretched exponential decay, respectively, consistent with numerical simulations.     

Glow and bulk expansion of oxidized graphites stimulated by electromagnetic radiation

The results of studies on the interaction of oxidized graphites and electroconductive materials with the electromagnetic radiation of a microwave oven are presented. The effects of glow and expansion of oxidized graphites stimulated by microwave radiation in air and an inert atmosphere are studied. Differences in the character of expansion of oxidized graphites and conductive materials on a dielectric and metal substrate under the influence of electromagnetic radiation are examined.     

简易测量柔性材料线胀系数的方法

介绍了一种简易测定柔性材料线胀系数的实验装置和测量方法.利用原有FD-LE线膨胀系数测定仪,使用圆珠笔中的弹簧、笔芯和笔头制作了材料和千分表的连接装置,并利用改进后的仪器测量了几种柔性材料的线胀系数.     

Thermal expansion properties of Lu2-xFexMo3O12

The structures and thermal expansion properties of Lu2-xFexMo3O12 have been investigated by X-ray diffraction(XRD).XRD patterns at room temperature indicate that compounds Lu2-xFexMo3O12 with x≤1.3 exhibit an orthorhombic structure with space group Pnca;compounds with x=1.5 and 1.7 have a monoclinic structure with space group P21/a.Studies on thermal expansion properties show that the linear thermal expansion coefficients of orthorhombic phase vary from negative to positive with increasing Fe content.Attempts to make zero thermal expansion materials indicate that zero thermal expansion can be observed in Lu1.3Fe0.7Mo3O12 in the temperature range of 200-400℃.     

Zero thermal expansion in metal-organic framework with imidazole dicarboxylate ligands

Exploring new abnormal thermal expansion materials is important to understand the nature of thermal expansion. Metal-organic framework (MOF) with unique structure flexibility is an ideal material to study the thermal expansion. This work adopts the high-resolution variable-temperature powder x-ray diffraction to investigate the structure and intrinsic thermal expansion in Sr-MOF ([Sr(DMPhH₂IDC)₂]_n). It has the unique honeycomb structure with one-dimensional (1D) channels along the c-axis direction, the a-b plane displays layer structure. The thermal expansion behavior has strong relationship with the structure, ZTE appears in the a-b plane and large PTE along the c-axis direction. The possible mechanism is that the a/b layers have enough space for the transverse thermal vibration of polydentate ligands, while along the c-axis direction is not. This work not only reports one interesting zero thermal expansion material, but also provides new understanding for thermal expansion mechanism from the perspective of the structural model.