Transport in dispersed ionic conductors: Effect of insulating particle size

Dispersed ionic conductors are random mixtures of a solid salt, e.g. AgI, LiI, with fine particles of an insulating second phase, like Al₂O₃ or SiO₂. These composites can show a dramatic increase in ionic conductivity compared to the pure homogeneous system. Generally, this observation is attributed to an increased conductivity along the internal interface between the conducting salt and the insulating material. In this work a three-component random resistor network (RRN) model for dispersed ionic conductors is reviewed. In the model, the ionic conductor is represented by normally conducting bonds, the insulating material by non-conducting bonds and the interface between the two phases by highly conducting bonds. A special feature of the model is the existence of two critical concentrations of the insulating phase, p′_c and p″_c , for interface percolation and bulk conduction, respectively, where critical transport properties corresponding to conductor/superconductor and conductor/insulator networks are predicted. The model describes satisfactorily the dependence on composition of the conductivity and activation energy of dispersed ionic conductors. Furthermore, the observed effect on the conductivity of the size of dispersed particles can be described qualitatively well by a generalized version of the RRN model, which in addition predicts a sensitive dependence of the critical thresholds on particle size. Non-universality features in the critical exponents for the conductivity are also discussed within a continuum percolation analog of the model.     

Sputter-deposited network glasses

Thin films of lithium borate, sodium borate, rubidium borate, and lithium silicate glasses are prepared by ion beam sputter deposition in a thickness range between 70 and 1,400 nm. The chemical, structural, and electrical properties of the films are determined by transmission electron microscopy and impedance spectroscopy. A comparison between the thin glass layers and the corresponding bulk glasses, prepared from the melt, shows that both have similar chemical compositions and atomic short range orders. In contrast, the ionic conductivities of the borate glass films are found to be significantly increased compared to the corresponding bulk materials, while the increase depends on the chemical composition of the glasses and is not observed in the case of the silicate system. We propose a modified network structure of the sputter-deposited glass layers, namely, an increased nonbridging oxygen concentration, to be responsible for the elevated ionic conductivity.     

Low temperature specific heat and thermal conductivity of bulk metallic glass (Cu50Zr50)94Al6

The low temperature specific heat and thermal conductivity of (Cu₅₀Zr₅₀)₉₄Al₆ bulk metallic glass have been studied experimentally. A low temperature anomaly in the specific heat is observed in this alloy. It is also found that in addition to Debye oscillators, the localized vibration modes whose vibration density of state has a Gaussian distribution should be considered to explain the low temperature phonon specific heat anomaly. The phonon thermal conductivity dependence on temperature for the sample does not show apparent plateau characteristics as other glass materials do; however, the influence of the resonant scattering from the localized modes on the lattice thermal conductivity is prominent in the bulk metallic glass at low temperatures.     

两相复合材料有效热导率的理论推导

复合材料有效热导率的解析表达式一直是传热问题中人们想要解决的问题.本文选用合适的单元体,采用热阻模型和积分平均方法,分别对分散相为球体和圆柱体的两相复合材料的有效热导率进行了推导.对于多孔复合材料,当孔隙率较大或温度较高时辐射换热的影响不能忽略,本文分析了气孔为球体或圆柱体时辐射换热对其有效热导率的影响.将计算所得有效热导率与相关实验数据进行了比较,结果表明两者吻合较好,证明了公式的准确性.     

Thermal conduction through loose and granular two-phase materials at normal pressure

The integrated theory derived for the lattice-type dispersions is modified and extended to estimate the effective thermal conductivity of loose and granular two-phase materials at normal pressure assuming an effective continuous media approximation. A comparison of calculated values ofλ _e with the reported experimental results over a wide range of loose and granular two-phase materials shows a good agreement.     

颗粒线度和非谐效应对传感器电极材料导电性能及其热稳定性的影响

本文考虑到原子的非简谐振动,应用固体物理理论和方法,研究了氧传感器多孔电极材料导电性能及热稳定性随温度和颗粒线度的变化规律,探讨原子非简谐振动和材料颗粒线度的影响.结果表明:(1)多孔Pt电极材料的电导率随温度的升高而非线性减小;电导率随颗粒半径的增大而非线性增大;电导率随时间增长而减小,但变化极小;(2)多孔Pt电极材料的电导率远小于Pt纳米材料的电导率,也小于块状Pt电极材料的电导率,且颗粒越小,颗粒线度效应越显著;(3)电导率的温度稳定性系数随温度的升高和颗粒线度的减小以及表面层参数的增大而非线性减小,即温度越高、颗粒线度越小、表面层参数越大,电极材料导电性的热稳定性越好;(4)表面层的存在使电导率降低,且降低情况与温度和颗粒线度有关,即颗粒越小,温度越低,电导率下降越明显;非简谐效应对电极材料的电导率几乎没有影响.     

A lower bound on the variance of conductance in random resistor networks

We study the conductance of random resistor networks ind2 dimensions. It is shown (under some technical assumptions) that if a network exhibits a nonzero conductivity in the infinite-volume limit, then the variance of a finite-volume conductance grows at least like the volume.     

Thermal conduction in a homogeneous two-phase system

Assuming a regular geometry of dispersed phase (λ ₂) an integrated theory for the effective thermal conductivityλ _e of all kind of two-phase materials (including loose materials) is developed. The flux modification is carried out by considering the effective neighbouring interactions in the solution of Poisson’s equation. A comparison of calculatedλ _e values with the reported experimental results over a wide range of two-phase materials shows a good agreement.     

Dielectric relaxation and ion transport in silver–boro-tellurite glasses

Glass systems of composition xAg₂SO₄–20Ag₂O–(80?x) [0.50 B₂O₃–0.50 TeO₂], where x = 5, 10, 15, 20, 25 and 30 mol% have been prepared by melt-quenching technique. Frequency- and temperature-dependent conductivity measurements have been carried out in the frequency range 10 Hz to 10 MHz and at a temperature range of 303–353 K, respectively. DC conductivities exhibit Arrhenius behavior over the entire temperature range with a single activation barrier. Addition of Ag₂SO₄ expands the glass network and, consequently, conductivity increases. This suggests that the structure and network expansion are the key parameters for enhancing conductivity. Impedance spectra of these glasses show a single semicircle, indicating one type of conduction. AC conductivity behavior of the glasses was analyzed using both single power law and Kolhrauh–William–Watts (KWW) stretched exponential relaxation function. The power law exponent (s) is temperature-dependent, while the stretched exponent (β) is insensitive to temperature. Scaling behavior has also been carried out using reduced plots of conductivity with frequency, which suggests the ion transport mechanism remains unaffected by temperature and composition.     

Physical studies on composites Ag7I4VO4-Al2O3

The electrical properties of the solid electrolytes Ag₇I₄VO₄-Al₂O₃ (0-40 mol% Al₂O₃) are investigated. The electrical conductivity, dielectric constant and dielectric loss are increased by increasing the concentration of Al₂O₃; showing a maximum at 30 mol% Al₂O₃. The conductivity is found to be increased by decreasing the particle size of Al₂O₃. The results are explained using the random resistor network model (RRN). This is due to the formation of a highly conducting interface layer along the matrix-particle interface. This layer is destroyed at concentrations higher than 30 mol% Al₂O₃.     

Thermal conductivity of GexSb(As)ySe100‐x‐y glasses measured by Raman scattering spectra

We systematically measured thermal conductivity of Ge_xSb(As)₁₀Se_90−x, Ge_xSb₁₅Se_85−x, and Ge_xSb(As)₂₀Se_80−x chalcogenide glasses by measuring their Stokes and anti‐Stokes Raman scattering spectra and estimating the temperature raised by laser irradiation via the ratio of Stoke and anti‐Stokes scattering cross‐section. We aimed at demonstrating the viability of Raman scattering method for thermal conductivity measurements, and understanding the role of chemical composition in determining thermal conductivity of the chalcogenide glasses. We found that, while the values of the thermal conductivity measured in the paper are in a range from ~0.078 to 1.120 Wm^‐1K^‐1 that are in agreement with those reported data in the literatures, thermal conductivity increases before it reaches a maximum at the glass with chemically stoichiometric composition, and then decreases with increasing Ge content. We ascribed the threshold behavior of the thermal conductivity to the demixing of the structural units like GeSe₂, As₂Se₃ and Sb₂Se₃ from the main glass network. The present study demonstrated that Raman scattering method is simple and easy to measure thermal conductivity of the material. Copyright © 2014 John Wiley & Sons, Ltd.     

纳米结构碲化铋合金的制备及电热输运特性

采用惰性气体保护蒸发-冷凝法制备了纳米Bi及Te粉末, 结合机械合金化和放电等离子烧结技术, 在不同烧结温度下制备出了单一物相且具有纳米层状结构及孪晶亚结构的n型Bi₂Te₃块体材料, 并系统研究了块体材料的晶粒尺度、微结构及其对电热传输特性的影响. SEM, TEM分析结果表明, 以纳米粉末为原料, 通过有效控制工艺条件, 可以制备出具有纳米层状结构Bi₂Te₃合金块体材料, 同时纳米层状结构中存在孪晶亚结构; 热电性能测试结果表明, 具有纳米层状结构及孪晶亚结构的块体试样与粗晶材料相比, 热导率大幅度降低, 在423 K附近, 热导率由粗晶材料的1.80 W/mK降至1.19 W/mK, 晶格热导率从1.16 W/mK降至0.61 W/mK, 表明纳米层状结构与孪晶亚结构共存, 有利于进一步提高声子散射, 降低晶格热导率. 其中在693 K放电等离子烧结后的试样于423K附近取得最大值的无量纲热电优值(ZT), 达到0.74.     

Weakly nonlinear conductivity of random composites: A series expansion approach

We present a series expansion calculation of the bulk effective coefficient of weakly nonlinear behavior in some continuum composite conductors and in simple cubic random resistor networks. The expansion is in powers of the relative difference between the linear Ohmic conductivities of the components. It is carried up to third order for an independent random bond network and a diagrammatic scheme is used to aid in implementing the calculation. For continuum composites, only the first term of the expansion can be calculated explicitly without detailed information about the microgeometry. Such information is difficult to acquire and even more difficult to exploit.     

Determination of electrodynamical parameters of dielectric pipe-shaped materials using millimeter wave cavity

Employment ofE _0np modes of cylindrical cavity resonator for measurement of electrodynamical parameters of dielectric pipe-shaped materials in millimeter wavelength band is suggested. Dielectric permittivity and conductivity of a sample can be found by measuring the frequency shift and theQ-factor change of the resonator.Method of mode basis is used for the calculation of sample permittivity and conductivity . Measurements were carried out at 8mm wavelength and related calculations have demonstrated use and possibilities of the method in studying the dielectric properties of samples of cylindrical and other forms.     

Electrical properties of V2O5-doped TlI

This paper reports the effect of heterogeneously doped vanadia on the ionic conductivity of thallium iodide (TlI). These compounds have been prepared and studied by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy and electrical conductivity. The materials were found to be binary phase systems with the vanadia particles dispersed in the grains of TlI. The electrical conductivity shows an upward trend in the composition range x=0.1–0.5; however, with further increase in vanadia content, conductivity decreases. This behavior is attributed to disordering phenomena at interface boundaries and space-charge layers formed in the bulk grains of TlI. In addition to this, the increased content of vanadia in the system leads to the disappearance of the β?α phase transition of TlI, which is usually observed in the pure compound.     

Electrical properties of MWCNT-composite (Se80Te20)100−xAgx (0 ≤ x ≤ 4) chalcogenide glasses

This article describes the preparation of multi-walled carbon nanotube (MWCNT) chalcogenide glass composite by the melt-quenching technique. MWCNT composite (Se₈₀Te₂₀)_100?xAg_x (0 ≤ x ≤ 4) bulk samples are characterized by the XRD, SEM and EDX. The electrical measurements were carried out in the temperature range of the 308-388 K. Cole–Cole plot has been used to determine the electrical conductivity at room temperature. It has been observed that MWCNT chalcogenide composite have higher value of electrical conductivity than pure glass. The results have been discussed on the basis of increased ionic conductivity (Ag⁺ ions) in MWCNT doped (Se₈₀Te₂₀)_100?xAg_x (0 ≤ x ≤ 4) bulk samples.     

低维纳米材料热电性能测试方法研究

通过近几十年的研究,人们对于块体及薄膜材料的热电性能已经有了较全面的认识,热电优值ZT的提高取得了飞速的进展,比如碲化铋相关材料、硒化亚铜相关材料、硒化锡相关材料的最大ZT值都突破了2.但是,这些体材料的热电优值距离大规模实用仍然有较大的差距.通过理论计算得知,当块体热电材料被制作成低维纳米结构材料时,比如二维纳米薄膜、一维纳米线,热电性能会得到显著的改善,具有微纳米结构材料的热电性能研究引起了科研人员的极大兴趣.当块体硅被制作成硅纳米线时,热电优值改善了将近100倍.然而,微纳米材料的热电参数测量极具挑战,因为块体材料的热电参数测量方法和测试平台已经不再适用于低维材料,需要开发出新的测量方法和测试平台用来研究低维材料的热导率、电导率和塞贝克系数.本文综述了几种用于精确测量微纳米材料热电参数的微机电结构,包括双悬空岛、单悬空岛、悬空四探针结构,详细介绍了每一种微机电结构的制备方法、测量原理以及对微纳米材料热电性能测试表征的实例.     

Ag-ZnO纳米复合热电材料的制备及其性能研究

ZnO是一类具有潜力的热电材料, 但其较大声子热导率影响了热电性能的进一步提高. 纳米复合是降低热导率的有效途径. 本文以醋酸盐为前驱体, 溶胶-凝胶法制备了Ag-ZnO纳米复合热电材料. 扫描电镜照片显示ZnO颗粒呈现多孔结构, Ag纳米颗粒分布于ZnO的晶粒之间. Ag-ZnO纳米复合材料的电导率比未复合ZnO材料高出100倍以上, 而热导率是未复合ZnO材料的1/2. 同时, 随着Ag添加量的增加, 赛贝克系数的绝对值逐渐减小. 综合以上原因, 添加7.5%mol Ag的Ag-ZnO纳米复合材料在700 K时的热电优值达到0.062, 是未复合ZnO材料的约25倍. 在ZnO基体中添加导电金属颗粒有利于产生导电逾渗通道, 提高材料体系的电导率, 但同时导致赛贝克系数的绝对值减小. 总热导率的差异来源于声子热导率的差异. 位于ZnO晶界的纳米Ag颗粒, 有利于降低声子热导率. 关键词： 热电材料 ZnO 纳米复合 热导率     

Conductivity relaxation in zirconium fluoride glasses: effect of substitution of Zr by Y ions

The electrical conductivity and the conductivity relaxation of (55−x)ZrF₄–15BaF₂–xYF₃–30LiF glasses were studied in the temperature range from 300 K to just below the glass transition temperature and in the frequency range from 10 Hz to 2 MHz. No large changes in the conductivity were observed with the substitution of Zr⁴⁺ by the Y³⁺ ions. The activation energy remained almost constant up to 20 mol.% YF₃ content and increased for higher YF₃ content in the glass compositions. The frequency dependent conductivity was analyzed in terms of modulus formalism. The distribution parameter for the conductivity relaxation times remained almost unchanged with the substitution of YF₃ with an increase for 40 mol.% YF₃ content. The distribution of relaxation times of the present glasses was much broader than that for the YF₃-free zirconium fluoride glasses. The glass decoupling index decreased and the modulus relaxation rate increased with the increase of YF₃ content in the glass compositions with an anomaly for the composition having 20 mol.% YF₃ content.     

Bulk transport properties and exponent inequalities for random resistor and flow networks

The properties of random resistor and flow networks are studied as a function of the density,p, of bonds which permit transport. It is shown that percolation is sufficient for bulk transport, in the sense that the conductivity and flow capacity are bounded away from zero wheneverp exceeds an appropriately defined percolation threshold. Relations between the transport coefficients and quantities in ordinary percolation are also derived. Assuming critical scaling, these relations imply upper and lower bounds on the conductivity and flow exponents in terms of percolation exponents. The conductivity exponent upper bound so derived saturates in mean field theory.Research supported by the NSF under Grant No. DMR-8314625Research supported by the DOE under Grant No. DE-AC02-83ER13044