Diffusion and segregation along grain boundary at the electrolyte-anode interface in IT-SOFC

The atomic level chemical and microstructural features of grain boundaries in gadolinium-doped ceria (GDC) electrolyte thin film supported by Ni-GDC cermet anode were characterized by high resolution transmission electron microscope (HR-TEM) and scanning TEM (STEM). It was found that metallic Ni can diffuse from the anode into the thin film electrolyte along grain boundaries. In addition, Ce and Gd can also diffuse and segregate at grain boundaries between Ni grains in the anode substrate. HR-TEM observations revealed that Ni diffusion and segregation at grain boundaries between GDC grains enhanced the inhomogeneity and led to microstructural changes at grain boundary regions, i.e. the formation of superstructure. The observations also indicated that enhanced inhomogeneity at grain boundaries might play a significant role in the conductivity of GDC electrolyte film in solid oxide fuel cells.     

Transport characteristics of thermally degraded ZnO films doped with Al

The thermal degradation characteristics of Al doped ZnO (AZO) films, which were prepared by radio frequency magnetron sputtering at room temperature and subsequently annealed in air, were examined by making comparisons with those of the as-deposited and the vacuum-annealed AZO films. AZO films annealed in air at temperature higher than 300 °C exhibited severely degraded conductivity due to the decrease of carrier concentration and Hall mobility, which could be interpreted as grain boundary scattering from the thermally activated mobility behavior. Experimental observations, that vacuum-annealing of the as-deposited AZO film led to substantial reduction in carrier concentration and that subsequent vacuum-annealing of the air-annealed AZO film could not restore the carrier concentration unlike the restoration of Hall mobility up to 80%, led to a perception that substantial amount of free carriers in the as-deposited AZO film were generated from oxygen vacancies. It was proposed that ratios of the barrier height to the Fermi level, which were determined by taking into account the non-parabolic nature of the conduction band, could be used as a measure for the influence of grain boundary scattering on the overall electron transport mechanism in AZO films.     

考虑界面散射的金属纳米线热导率修正

理论分析了声子和电子输运对Cu, Ag金属纳米线热导率的贡献. 采用镶嵌原子作用势模型描述纳米尺寸下金属原子间的相互作用, 应用平衡分子动力学方法和Green-Kubo函数模拟了金属纳米线的声子热导率; 采用玻尔兹曼输运理论和Wiedemann-Franz定律计算电子热导率; 并通过散射失配模型和Mayadas-Shatzkes模型引入晶界散射的影响. 在此基础上, 考察分析了纳米线尺度和温度的影响. 研究结果表明: Cu, Ag纳米线热导率的变化规律相似; 电子输运对金属纳米线的导热占主导地位, 而声子热导率的贡献也不容忽视; 晶界散射导致热导率减小, 尤其对电子热导率作用显著; 纳米线总热导率随着温度的升高而降低; 随着截面尺寸减小而减小, 但声子热导率所占份额有所增加. 关键词： 纳米线 热导率 表面散射 晶界散射     

Effect of phonon scattering mechanisms on the lattice thermal conductivity of skutterudite-related compound

We have prepared the skutterudite-related compounds FeCo_3Sb_{12} and La_{0.75}Fe_3CoSb_{12} with different average grain sizes (about 0.8 and 3.9μm) by hot pressing. Samples were characterized by XRD, EPMA and SEM. The lattice thermal conductivity was investigated in the temperature range from room temperature to 200℃. Based on the Debye model, we analyse the change in lattice thermal conductivity due to various phonon scattering mechanisms by examining the relationship between the weighted phonon relaxation time τ(ω/ω_D)^2 and the reduced phonon frequency ω/ω_D. The effect of grain boundary scattering to phonon is negligible within the range of grain sizes considered in this study. The large reduction in lattice thermal conductivity of FeCo_3Sb_{12} compound contributes to the electron－phonon scattering. As for La_{0.75}Fe_3CoSb_{12} compound, the atoms of La filled into the large voids in the structure of the skutterudite produce more significant electron－phonon scattering as well as more substitute of Fe at Co site at the same time. Moreover, the point-defect scattering appears due to the difference between the atoms of La and the void. In addition, the scattering by the rattling of the rare-earth atoms in the void is another major contribution to the reduced lattice thermal conductivity. Introducing the coupling of the electron－phonon scattering with the point-defect scattering and the scattering by the rattling of the rare-earth atom is an effective method to reduce the lattice thermal conductivity of the skutterudite-related compounds by substitution of Fe for Co and the atoms of La filled in the large voids in the skutterudite structure.     

纳晶铜晶粒尺寸对热导率的影响

用热压烧结法制备得到纳晶铜块体. 用激光法测定了不同温度下制备得到的纳晶铜块体的热导率, 并建立卡皮查热阻模型对样品热导率进行模拟. 通过对比, 模拟结果与实验数据基本一致. 随着热压烧结温度的升高, 纳晶铜晶粒尺寸也随之增大. 在900和700 ℃其热导率分别达到了最大和最小值且所对应的热导率分别为200.63和233.37 W·m^-1·K^-1, 各占粗晶铜块体热导率的53.4%和60.6%. 验证了纳晶铜热导率在一定的晶粒尺寸范围内具有尺寸效应, 随着晶粒尺寸的减小, 热导率逐渐减小.     

Effect of grain-boundary electron scattering in magnetic layers on the magnetoresistive ratio of a polycrystalline sandwich under transverse charge transfer

For a polycrystalline magnetic sandwich, an analytical expression that relates the electron scattering by grain boundary roughness, the conductivity of ferromagnetic layers, and the grain size is derived for the case of a transverse charge transfer. The effect of this scattering mechanism on the giant magnetoresistance of the sandwich is studied with account taken for the dispersity of the ferromagnetic layers and the length of grain-boundary diffusion of nonmagnetic spacer atoms into the layers.     

Suppressed Thermal Conductivity in Polycrystalline Gold Nanofilm: The Effect of Grain Boundary and Substrate

We investigate the electrical conductivity and thermal conductivity of polycrystalline gold nanofilms,with thicknesses ranging from 40.5nm to 115.8 nm,and identify a thickness-dependent electrical conductivity,which can be explained via the Mayadas and Shatzkes(MS)theory.At the same time,a suppressed thermal conductivity is observed,as compared to that found in the bulk material,together with a weak thickness effect.We compare the thermal conductivity of suspended and supported gold films,finding that the supporting substrate can effectively suppress the in-plane thermal conductivity of the polycrystalline gold nanofilms.Our results indicate that grain boundary scattering and substrate scattering can affect electron and phonon transport in polycrystalline metallic systems.     

Properties of carbon films with high conductivity anisotropy

Carbon films with high conductivity anisotropy were obtained via sputtering a graphite target with an accelerated argon ion beam. The film structure was studied using Raman spectroscopy (combination scattering spectroscopy), transmission high-resolution electron microscopy, chemical analysis, X-ray photoelectron spectroscopy, and tunnel spectroscopy. We found conductivity anisotropy along and across the film chains of the order of 10⁴, which is associated with a high content of linear carbon chains with sp hybridization of bonding orbitals.     

The effect of (00l) crystal plane orientation on the thermoelectric properties of Bi2Te3 thin film

Highly (00l)-oriented pure Bi₂Te₃ films with in-plane layered grown columnar nanostructure have been fabricated by a simple magnetron co-sputtering method. Compared with ordinary Bi₂Te₃ film and bulk materials, the electrical conductivity and Seebeck coefficient of such films have been greatly increased simultaneously due to raised carrier mobility and electron scattering parameter, while the thermal conductivity has been decreased due to phonon scattering by grain boundaries between columnar grains and interfaces between each layers. The power factor has reached as large as 33.7 μW cm⁻¹ K⁻², and the out-of-plane thermal conductivity is reduced to 0.86 W m⁻¹ K⁻¹. Our results confirm that tailoring nanoscale structures inside thermoelectric films effectively enhances their performances.     

Electron scattering mechanisms in indium-tin-oxide thin films prepared at the various process conditions

The carrier concentrations and mobilities of indium-tin-oxide (ITO) thin films by DC magnetron sputtering at the various process conditions were measured by means of the Hall technique. The relationship between the carrier concentration and mobility showed two distinct features: (i) roughly up to the carrier concentration of 9.0 × 10²⁰/cm³, both the carrier concentration and mobility increased together; (ii) above the carrier concentration of 9.0 × 10²⁰/cm³, the carrier mobility decreased as the carrier concentration further increased. The distinct behavior of the carrier concentration and mobility was due to the transition of the dominant electron scattering mechanism. ITO thin film with a low degree of crystallinity was governed by the grain boundary scattering. However, the ionized impurity scattering was dominant in ITO thin film with a high carrier concentration over 9.0 × 10²⁰/cm³. The overall characterizations related to the carrier concentration and mobility were also performed using X-ray diffractometer, UV-vis-NIR spectrometer, scanning electron microscope, atomic force microscope.     

Conductivity drop and crystallites redistribution in gold film

Electrical conductivity of Au film deposited on Si(100), Si(111) and BK7 glass substrates, ranging from 20 nm to 1000 nm, was measured. Conductivity drop and a decrease of averaged crystallite size are observed around film thickness of 200 nm. Investigation of the conductivity drop led to the discovery that a redistribution of crystallites of {111} and {220} occurs in the conductivity drop region. By analyzing the relationship of distribution of crystallites, averaged crystallite size and conductivity ratios and together with the atomic force microscope pictures, leads to the conclusion that the thermal conductivity drop region is caused by the presence of many small crystallites. These small crystallites are responsible for yielding a lower averaged crystallite size and causing a higher grain boundary scattering. PACS 73.50; 68.47.De; 68.55.Jk     

Effect of microstructure on the thermoelectric properties of CSD-grown Bi2Sr2Co2Oy thin films

Three Bi2Sr2Co2Oy thin films with different microstructures have been prepared by chemical solution deposition on LaAlO 3(001) through varying the annealing temperature.With the decrease in the annealing temperature,both the size and c-axis alignment degree of grains in the film decrease as well,leading to an increase in the film resistivity.In addition,the decrease in the annealing temperature also results in a slight increase in the Seebeck coefficient due to the enhanced energy filtering effect of the small-grain film.The nanostructured Bi2Sr2Co2Oy film with an average grain size of about 100 nm shows a power factor comparable to that of films with larger grains.Since the thermal conductivity of the nanostructured films can be depressed due to the enhanced phonon scattering by grain boundary,a higher figure of merit is expected in Bi2Sr2Co2Oy thin film with grains in nanometer size.     

Grain boundary conductivity of yttria-doped zirconia: Influence of the microstructure and composite effect

Electrical conductivity measurements have been performed on yttrium-doped zirconia and on YSZ-alumina composites sintered from powders prepared by conventional techniques (commercial powder) or by freeze-drying. The results have been analyzed taking into account the microstructure of the interfaces which were characterized by electron microscopy. The samples sintered from freeze-dried powders show the highest conductivity values and this conductivity decreases in the presence of alumina. The microstructure of these polycrystals is clean, homogeneous with lens-shaped glassy pockets at triple-points and there is no evidence for continuous boundary films. Contrary, the samples prepared and sintered from commercial powders show a poor microstructure and the presence of a glassy film in a large number of grain boundaries. Furthermore, alumina leads to an increase in conductivity, which reaches a maximum at around 2 mol-% alumina. This result may be attributed to the influence of alumina on the viscosity and wetability of the glassy phase. However, all samples show the same grain boundary activation energy. This confirms that the transport mechanism is the same in all cases and that only clean grain boundaries contribute to the transport processes. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Thickness dependence of the anomalous Hall effect in disordered face-centered cubic FePt alloy films

The anomalous Hall effect in disordered face-centered cubic(fcc) FePt alloy films is experimentally studied. The longitudinal resistivity independent term of the anomalous Hall conductivity(AHC) increases and approaches saturation with increasing film thickness. The contribution of side jump scattering is suggested to decrease monotonically with increasing film thickness, which can be ascribed to the variation of the surface scattering with the film thickness. The sign of the skew scattering contribution to the AHC is opposite to that of the intrinsic contribution in the system.     

Influence of Electron–Phonon Interaction on the Lattice Thermal Conductivity in Single-Crystal Si

Lattice thermal conductivity can be reduced by introducing point defect, grain boundary, and nanoscale precipitates to scatter phonons of different wave-lengths, etc. Recently, the effect of electron–phonon (EP) interaction on phonon transport has attracted more and more attention, especially in heavily doped semiconductors. Here the effect of EP interaction in n-type P-doped single-crystal Si has been investigated. The lattice thermal conductivity decreases dramatically with increasing P doping. This reduction on lattice thermal conductivity cannot be explained solely considering point defect scattering. Further, the lattice thermal conductivity can be fitted well by introducing EP interaction into the modified Debye–Callaway model, which demonstrates that the EP interaction can play an important role in reducing lattice thermal conductivity of n-type P-doped single-crystal Si.     

GaN薄膜的热导率模型研究

准确预测GaN半导体材料的热导率对GaN基功率电子器件的热设计具有重要意义.本文基于第一性原理计算和经典Debye-Callaway模型,通过分析和完善Debye-Callaway模型中关于声子散射率的子模型,建立了用于预测温度、同位素、点缺陷、位错、薄膜厚度、应力等因素影响的GaN薄膜热导率的理论模型.具体来说,对声...     

Drude's model calculation rule on electrical transport in Sb-doped SnO2 thin films, deposited via sol-gel

The evaluation of free carrier concentration based on Drude's theory can be performed by the use of optical transmittance in the range 800-2000 nm (near infrared) for Sb-doped SnO₂ thin films. In this article, we estimate the free carrier concentration for these films, which are deposited via sol-gel dip-coating. At approximately 900 nm, there is a separation among transmittance curves of doped and undoped samples. The plasma resonance phenomena approach leads to free carrier concentration of about 5×10²⁰ cm⁻³. The increase in the Sb concentration increases the film conductivity; however, the magnitude of measured resistivity is still very high. The only way to combine such a high free carrier concentration with a rather low conductivity is to have a very low mobility. It becomes possible when the crystallite dimensions are taken into account. We obtain grains with 5 nm of average size by estimating the grain size from X-ray diffraction data, and by using line broadening in the diffraction pattern. The low conductivity is due to very intense scattering at the grain boundary, which is created by the presence of a large amount of nanoscopic crystallites. Such a result is in accordance with X-ray photoemission spectroscopy data that pointed to Sb incorporation proportional to the free electron concentration, evaluated according to Drude's model.     

Influence of the quantum size effect for grazing electrons on the electronic conductivity of metal films

The thickness dependence of the electronic conductivity of thin (5–150 nm) single-crystal (100) films of refractory metals is investigated at different temperatures ranging from 4.2 K to room temperature. Regions of square-root, quasilinear, and quadratic dependences are observed. The quasilinear thickness dependence is explained by the influence of quantum effects on the transverse motion of electrons in the case when electron scattering by the film surfaces dominates. For macroscopic film thicknesses 30–50 nm, much greater than the Fermi wavelength of an electron, quantum corrections to the electronic conductivity reach values of the order of 50%. This is a consequence of the quantum size effect for grazing electrons, which leads to an anomaly in electron scattering by the film surfaces. The region of the quadratic thickness dependence corresponds to the quantum limit, and the square-root region corresponds to the classical limit. The effect is explained in a quasiclassical two-parameter model (the effective angle α* for small-angle electrons and the parameter γ, equal to the ratio of this angle to the diffraction angle) that takes into account the diffraction angular limits for grazing electrons. The effect occurs for parameters α*≪1 and γ∼1 and differs from the “ordinary” quantum size effect. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 693–698 (10 December 1997)     

Quantum transport in mesoscopic 3He films: experimental study of the interference of bulk and boundary scattering

We discuss the mass transport of a degenerate Fermi liquid ^{3}He film over a rough surface, and the film momentum relaxation time, in the framework of theoretical predictions. In the mesoscopic regime, the anomalous temperature dependence of the relaxation time is explained in terms of the interference between elastic boundary scattering and inelastic quasiparticle-quasiparticle scattering within the film. We exploit a quasiclassical treatment of quantum size effects in the film in which the surface roughness, whose power spectrum is experimentally determined, is mapped into an effective disorder potential within a film of uniform thickness. Confirmation is provided by the introduction of elastic scattering centers within the film. The improved understanding of surface roughness scattering may impact on enhancing the conductivity in thin metallic films.     

磷离子注入纳米金刚石薄膜的n型导电性能与微结构研究

系统研究了磷离子注入并在不同温度退火后的纳米金刚石薄膜的微结构和电学性能.研究表明,当退火温度达到800 ℃以上时,薄膜呈良好的n型电导.Raman光谱和电子顺磁共振谱的结果表明,薄膜中金刚石相含量越高和完整性越好,薄膜电阻率越低. 这说明纳米金刚石晶粒为薄膜提供了电导.1000 ℃退火后,薄膜晶界中的非晶石墨相有序度提高,碳悬键数量降低,薄膜电阻率升高.薄膜导电机理为磷离子注入的纳米金刚石晶粒提供了n型电导,非晶碳晶界为其电导提供了传输路径. 关键词： 纳米金刚石薄膜 n型 磷离子注入