超低温下金纳米薄膜导电和导热性质的实验研究

受到晶界和表面对电子散射的影响,金属纳米薄膜在导电和导热方面表现出与体材料不同的性质。实验研究了厚度为76 nm的金薄膜在不同温度(3～240K)下的导电和导热特性。结果表明薄膜电导率和热导率均大大低于体材料值;薄膜和体材料的电导率随温度变化趋势相同,热导率的变化趋势相反;薄膜的Lorenz数大于体材料值,Wiedemann-Franz定理不成立。     

氩晶体薄膜法向热导率的分子动力学模拟

结合卫星“微型核”的特点，研究电介质薄膜中的导热机理以及薄膜厚度对导热系数的影响.以结构较为简单、具有可靠势能函数，实验数据较为丰富和可靠的氩的(fcc)晶体为模型,采用平衡分子动力学方法(EMD)和各向异性非平衡分子动力学方法(NEMD)计算了氩晶体及其法向薄膜的热导率,并与实验结果进行比较.模拟结果表明，氩晶体纳米薄膜的热导率显著小于对应大体积晶体的实验值，具有明显的尺寸效应.在氩薄膜厚度为2.124—5.310nm的模拟范围内,薄膜的法向热导率随着薄膜厚度的增加而呈近似线性增加. 关键词： 热导率 纳米薄膜 尺寸效应 平衡分子动力学 非平衡分子动力学     

氮化硅纳米薄膜非平衡热导率实验研究

3ω实验方法是一种可以对薄膜热导率进行瞬时测量的方法。根据3ω方法测试原理,搭建了薄膜导热系数测试平台,并且分别测试低频率段和高频段薄膜与基底的温升以及薄膜热导率。测试结果表明:Si3N4薄膜的热导率随温度的升高而增大;高频段下,热导率受频率影响大,误差大;在低频段下薄膜热导率与频率变化基本无关;基于电子与声子的局部热平衡运输方程假设,S i3N4薄膜的热导率具有极度非平衡性;通过比较电阻、热导率与温度的关系可以看出加热器的尺寸大小会影响薄膜的热导率,最佳加热器的宽度选用20μm左右。     

铂薄膜导热特性的分子动力学模拟

采用非平衡态分子动力学(NEMD)方法模拟分析了纳米铂(Pt)薄膜的导热性能与脉冲激光作用下的温度响应特性.结果表明,100～500 nm铂薄膜的法向导热系数比体材料值低很多,而且低于其面向导热系数;微米铂薄膜的温度响应时间在纳秒量级;在脉冲加热的初始阶段,有一快速非傅立叶热波沿铂薄膜的厚度方向传递.     

硅纳米薄膜中声子弹道扩散导热的蒙特卡罗模拟

通过建立声子散射概率函数描述声子在输运过程中的散射,提出了一种模拟声子弹道扩散导热的蒙特卡罗方法,并将其应用于硅纳米薄膜中的稳态和瞬态弹道扩散导热过程的研究. 提出的蒙特卡罗方法对边界发射的声子束进行跟踪,根据散射概率函数模拟声子束在传播区域内经历的散射过程,并通过统计声子束的分布得到温度分布. 稳态导热过程的模拟发现,尺寸效应会引起边界温度跳跃,其值随着Knudsen数的增大而增大;计算的硅纳米薄膜的热导率随着厚度的增大而增大,与文献中的实验数据和理论模型相符. 通过瞬态导热过程的模拟得到了纳米薄膜内的温度分布随时间的变化,发现瞬态导热过程中的热波现象与空间尺度相关,材料尺寸越小,弹道输运越强,薄膜中的热波现象也越显著. 关键词： 纳米薄膜 弹道扩散导热 蒙特卡罗模拟 尺寸效应     

化学浴沉积法制备CdS多晶薄膜

利用化学浴沉积法制备适合于铜铟镓硒薄膜太阳能电池缓冲层材料的CdS多晶薄膜,研究了在不同温度和不同时间下沉积薄膜的性质．薄膜生长开始由ion-by-ion机制控制,随着时间的进行,cluster-by-cluster机制占据主导．薄膜的生长速度随着沉积温度的升高而快速增加,直到达到饱和厚度．并且饱和厚度随温度升高而相应降低．SEM表明随沉积时间增加以及温度升高,薄膜表面形貌从多孔到粗糙的不均匀转变．XRD结果显示,薄膜由立方和六方两相结构组成,控制沉积时间对薄膜的主要晶相结构很关键．所有温度下沉积的CdS     

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.     

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     

金属有机物化学气相沉积生长GaN薄膜的室温热电特性研究

采用金属有机物化学气相沉积技术生长了不同掺杂浓度的GaN薄膜, 并且通过霍尔效应测试和塞贝克效应测试, 表征了室温下GaN薄膜的载流子浓度、迁移率和塞贝克系数. 在实验测试的基础上, 计算了GaN薄膜的热电功率因子, 并且结合理论热导率确定了室温条件下GaN薄膜的热电优值(ZT). 研究结果表明: GaN薄膜的迁移率随着载流子浓度的增加而减小, 电导率随着载流子浓度的增加而增加; GaN 薄膜材料的塞贝克系数随载流子浓度的增加而降低, 其数量级在100–500 μV/K范围内; GaN薄膜材料在载流子浓度为1.60×10¹⁸ cm^-3时, 热电功率因子出现极大值4.72×10^-4 W/mK²; 由于Si杂质浓度的增加, 增强了GaN薄膜中的声子散射, 使得GaN薄膜的热导率随着载流子浓度的增加而降低. GaN薄膜的载流子浓度为1.60×10¹⁸ cm^-3时, 室温ZT达到极大值0.0025.     

二维材料XTe_2(X=Pd,Pt)热电性能的第一性原理计算

利用密度泛函理论结合玻尔兹曼输运方程,预测了二维层状热电材料XTe_2 (X=Pd, Pt)的热电性质.两种材料都具有较低的热导率,材料的晶格热导率随温度的升高而降低,且表现出各向异性.而电子热导率随温度的升高而升高.在较低温时,晶格热导率对总热导率的贡献占据主导地位.较高的载流子迁移率、电导率及塞贝克系数也对材料的热电转换效率产生极大的影响,展现出较为优异的电输运性能.对比分析PdTe_2和PtTe_2两种材料的ZT值,发现两种材料的热电性能以p型掺杂为主. PtTe_2单层的ZT值高于PdTe_2单层,并且PtTe_2单层在常温下的ZT峰值可达到2.75,是一种极具潜力的热电材料.     

Different effects of grain boundary scattering on chargeand heat transport in polycrystalline platinum and goldnanofilms

The in-plane electrical and thermal conductivities of several polycrystalline platinum and gold nanofilms with different thicknesses are measured in a temperature range between the boiling point of liquid nitrogen (77K) and room temperature by using the direct current heating method. The result shows that both the electrical and thermal conductivities of the nanofilms reduce greatly compared with their corresponding bulk values. However, the electrical conductivity drop is considerably greater than the thermal conductivity drop, which indicates that the influence of the internal grain boundary on heat transport is different from that of charge transport, hence leading to the violation of the Wiedemann--Franz law. We build an electron relaxation model based on Matthiessen's rule to analyse the thermal conductivity and employ the Mayadas & Shatzkes theory to analyse the electrical conductivity. Moreover, a modified Wiedemann--Franz law is provided in this paper, the obtained results from which are in good agreement with the experimental data.     

硅纳米薄膜法向热导率的进一步分析

应用非平衡分子动力学方法进一步研究了平均温度为300K、厚度为2.715nm-43.44nm的单晶硅薄膜的法向热导率,模拟结果表明,薄膜热导率低于同温度下单晶硅的实验值,存在显著的尺寸效应,当膜厚度在20nm以下时,法向热导率随尺度减小而线性减小,当膜厚度大于20nm时法向热导率随尺度呈现二阶多项式变化。法向热导率的变化规律与面向热导率的变化规律类似,表明薄膜厚度和表面晶格结构对声子传热影响的复杂性。     

Ballistic–diffusive phonon transport and size induced anisotropy of thermal conductivity of silicon nanofilms

The effective thermal conductivity of nanofilms is size dependent due to the diffusive–ballistic transport of phonons. In this paper, we investigate the cross-plane phonon transport from the viewpoint of the phonon Boltzmann equation. A predictive model for the size dependent thermal conductivity is proposed and agrees well with the results of molecular dynamics simulation for silicon nanofilms. The ballistic transport has different effects on the heat conduction in the in-plane or cross-plane directions, which causes the anisotropy of thermal conductivity of nanofilms. Such anisotropy is also size dependent and vanishes with the increase of film thickness.     

A study on the DC-electrical and thermal conductivities of epoxy/ZnO composites doped with carbon black

Thermo-electrical characterizations of hybrid polymer composites, made of epoxy matrix filled with various zinc oxide (ZnO) concentrations (0, 4.9, 9.9, 14.9, and 19.9 wt%), and reinforced with conductive carbon black (CB) nanoparticles (0.1 wt%), have been investigated as a function of ZnO concentration and temperature. Both the measured DC-electrical and thermal conductivities showed ZnO concentration and temperature dependencies. Increasing the temperature and filler concentrations were reflected in a negative temperature coefficient of resistivity and enhancement of the electrical conductivity as well. The observed increase in the DC conductivity and decrease in the determined activation energy were explained based on the concept of existing paths and connections between the ZnO particles and the conductive CB nanoparticles. Alteration of ZnO concentration with a fixed content of CB nanoparticles and/or temperature was found to be crucial in the thermal conductivity behavior. The addition of CB nanoparticles to the epoxy/ZnO matrix was found to enhance the electrical conduction resulting from the electronic and impurity contributions. Also, the thermal conductivity enhancement was mostly attributed to the heat transferred by phonons and electrons hopping to higher energy levels throughout the thermal processes. Scanning electron microscopy and energy-dispersive spectroscopy were used to observe the morphology and elements’ distribution in the composites. The observed thermal conductivity behavior was found to correlate well with that of the DC-electrical conductivity as a function of the ZnO content. The overall enhancements in both the measured DC- and thermal conductivities of the prepared hybrid composites are mainly produced through mutual interactions between the filling conductive particles and also from electrons tunneling in the composite's bulk as well.     

Experimental Studies on Thermal and Electrical Properties of Platinum Nanofilms

We experimentally studied the in-plane thermal and electrical properties of a suspended platinum nanofilm in thickness of 15 nm. The measured results show that the in-plane thermal conductivity, the electrical conductivity and the resistance-temperature coefficient of the studied nanofilm are much less than those of the bulk material, while the Lorenz number is greater than the bulk value. Comparing with the results reported previously for the platinum nanofilm in thickness of 28 nm, we further find that the in-plane thermal conductivity, the electrical conductivity and the resistance-temperature coefficient decrease with the decreasing thickness of the nanotilm, while the Lorenz number increases with the decreasing thickness of the nanofilm. These results indicate that strong size effects exist on the in-plane thermal and electrical properties of platinum nanofilms.     

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.     

Thermal conductivity of thermoelectric Al‐substituted ZnO thin films

ZnO:Al thin films with a low electrical resistivity were grown by magnetron sputtering on sapphire substrates. The cross‐plane thermal conductivity (κ = 4.5 ± 1.3 W/mK) at room temperature is almost one order of magnitude lower than for bulk materials. The thermoelectric figure of merit ZT at elevated temperatures was estimated from in‐plane power factor and the cross‐plane thermal conductivity at room temperature. It is expected that the thermal conductivity drops with increasing temperature and is lower in‐plane than cross‐plane. Consequently, the thin film ZT is at least three times higher than for bulk samples at intermediate temperatures. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrical conductivity of thick film YSZ

When yttria-stabilized zirconia (YSZ) electrolyte is coated and co-sintered on top of Ni–YSZ anode support, the measured conductivities of YSZ thick films (10–30 μm thick) are often lower than that of bulk YSZ. In this study, we found the observation by fabricating free-standing YSZ thick films and measuring and comparing in-plain and across-plain conductivities. The in-plane conductivity of free-standing YSZ film matched very well with the conductivity of mm-thick bulk sample. It was further shown that the conductivity decrease can be minimized by using better electrode morphology.Another factor that decreases the film conductivity was identified when the thick film was reduced. The conductivity decrease, ∼26% after reduction for 1h in humidified hydrogen gas, was due to Ni-doping into YSZ during sintering process.In order to minimize the conductivity drop of thick film YSZ during SOFC (solid oxide fuel cell) operation, an intermediate layer may be used between YSZ and anode support to prevent Ni-doping during co-sintering process in addition to the well-designed electrode morphology.     

类金刚石薄膜在硅基底上的沉积及其热导率

采用分子动力学方法模拟了碳在晶体硅基底上的沉积过程, 并分析计算了所沉积的类金刚石薄膜的面向及法向热导率. 对沉积过程的模拟表明, 薄膜密度及sp³杂化类型的碳原子所占比例均随沉积高度的增加而减小, 在碳原子以1 eV能量垂直入射的情况下, 在硅基底上沉积的薄膜密度约为2.8 g/cm³, sp³杂化类型的碳原子所占比例约为22%, 均低于碳在金刚石基底上沉积的情况. 采用Green-Kubo方法, 计算了所沉积类金刚石薄膜的热导率, 其面向热导率可以达到相同尺寸规则金刚石晶体的50%左右, 并且随着薄膜密度与sp³杂化类型碳原子所占比例的升高而升高.