Thermal Conductivity of Amorphous Materials

Thermal conductivity is one of the most fundamental properties of solid materials. The thermal conductivity of ideal crystal materials has been widely studied over the past hundreds years. On the contrary, for amorphous materials that have valuable applications in flexible electronics, wearable electrics, artificial intelligence chips, thermal protection, advanced detectors, thermoelectrics, and other fields, their thermal properties are relatively rarely reported. Moreover, recent research indicates that the thermal conductivity of amorphous materials is quite different from that of ideal crystal materials. In this article, the authors systematically review the fundamental physical aspects of thermal conductivity in amorphous materials. They discuss the method to distinguish the different heat carriers (propagons, diffusons, and locons) and the relative contribution from them to thermal conductivity. In addition, various influencing factors, such as size, temperature, and interfaces, are addressed, and a series of interesting anomalies are presented. Finally, the authors discuss a number of open problems on thermal conductivity of amorphous materials and a brief summary is provided.     

用导热性半固化片制造印制板

导热性半固化片对制造导热性电路板,近来变得通用了。实际上,印制电路板是用部分固化的(B-阶段)环氧树脂片或膜,即众所周知的半固化片加工的。半固化片在许多环氧,聚酰亚胺、氰酸脂和PTFE/玻璃纤维组成中是通用的,是为满足工业对高温稳定性、低介电常数和低热膨胀的要求而设计的。 然而,直到现在才能够提供导热的半固化片,同时对制造的PCB保持必要的电气绝缘。现在有效应用的导热的和电气绝缘的半固化片比标准FR-4半固化大10倍以上的导热率和1.5倍以上的介电强度。这些双重性质使之作为电路板组成部分之金属散热器的实际应用     

高热导率陶瓷材料的进展

叙述了在电子器件上常用高热导率陶瓷材料的性能和应用，主要包括BeO，BN，AIN等三种陶瓷材料。特别介绍了AIN陶瓷的发展前景及其最新应用。     

Processing and Thermal Conductivity of Lead Telluride Microwires

We present a unique method for producing continuous PbTe microwires which may be spooled for commercial use, and provide evidence that such wires tend to form single crystals with diameters less than 10 μm. Because PbTe microwires can, potentially, be used to fabricate miniature thermoelectric generators, a commercial fiber optic draw tower was used to produce such wires. In general, the thermoelectric figure of merit of a semiconductor is expected to decrease if its thermal conductivity increases. Because PbTe microwires with diameters less than 10 μm contain no grain boundaries and have fewer other defects, for example cracks and pores, their thermal conductivity was expected to be high, as a result of reduced phonon scattering. Accordingly, it was of interest to determine how the thermal conductivity of PbTe microwires behaved in two diameter ranges, i.e., those with diameters <10 μm and those with substantially larger diameters. We report thermal conductivity data for such microwires, at 725 K, determined by use of the 3ω method. Special attention was paid to minimizing conduction and radiation losses which are sources of error at high temperatures. The results prove that the thermal conductivity of PbTe microwires remains invariant in the diameter range 1.6–60 μm.     

热导式氢气纯度分析仪的设计

热导检测器是利用被测组分和参考气的热导系数不同而响应的浓度型检测器,是一种非破坏性检测器,可与其他检测器联用。本文介绍了一种利用热导检测器测量氢气纯度的方法,提出了热导式氢气纯度分析仪的总体设计方案。本设计的特点是将参考气(氮气)直接密封于热导检测器中,测量时不用再向仪器持续输入参考气,再利用数字温度传感器DS18B20检测温度,对恒流源及运放的温漂进行补偿。主要介绍了氢气热导池检测器、信号处理电路、键盘显示等电路的工作原理及部分程序的设计思想。并给出了样机的应用测试数据。     

Size-Dependent Lattice Thermal Conductivity of Nanostructured Bulk Semiconductors

Recently, nanostructuring of bulk semiconductors has emerged as an effective approach to develop high-efficiency thermoelectric materials for large-scale device applications, where the thermal conductivity reduction predominates in the enhanced figure of merit of these materials. In this work, a quantitative nanothermodynamic model was established to calculate the lattice thermal conductivity of semiconductor nanocomposites considering the interface scattering effects. It is found that the lattice thermal conductivity can be significantly reduced in nanostructured bulk semiconductors compared with their bulk counterparts. The findings in this work may provide new insights into the fundamental understanding of phonon transport in nanocomposites and also the development of high-performance thermoelectric materials.     

Thermal Conductivity and ZT in Disordered Organic Thermoelectrics

For decades, continuous attempts have been made to improve the figure of merit (ZT) of thermoelectrics. The theory behind the Seebeck effect itself is well researched, but the problem with ZT is related to materials properties that offset one another. This work analyzed the link between the site energy distributions and thermal conductivity of oxidized poly(3,4-ethylenedioxythiophene-tosylate) (PEDOT:Tos), which was reported to be a good organic thermoelectric. To understand how heat flow was affected by “disorder” in PEDOT:Tos and the associated electron–phonon interactions, we computed the values of the thermal conductivity κ and ZT using materials parameters extracted from the open literature. By varying the values of the parameters separately, we were able to identify their individual influence on κ and ZT. Our results suggest that ZT is most sensitive to changes in σ, the bandwidth of the density of states (DOS) of the transport sites, and less so to changes in n _eff, the effective carrier density. Our simulations also suggested that ZT could become exceptionally large (approaching a value of ~20) if σ were lowered to 1 meV to 2 meV. This would be a tremendous approach to increase ZT in oxidized PEDOT:Tos.     

Thermal Conductivity of Exfoliated p-Type Bismuth Antimony Telluride

A bulk p-type thermoelectric compound with nominal composition Bi_0.5Sb_1.5 Te₃ has been exfoliated using dimethyl sulfoxide as a solvent. Samples have been prepared from the exfoliated platelets by pressing followed by sintering or hot pressing. The exfoliated nanoplatelets have been characterized for size distribution and composition using scanning electron microscopy and energy-dispersive spectrometry. The smallest size platelet was 40 nm, and the maximum in the size distribution was near 80 nm. The exfoliated platelets and sintered sample showed significant deficiency in Sb and Te. The nonstoichiometry in the composition of the exfoliated platelets indicates that the mechanism of exfoliation may not be between quintuplets only, with other layers also being active. The composition of the hot-pressed sample remained closer to that of the bulk. Results of x-ray diffraction indicated the presence of Bi₂Te₃ and Bi_0.5Sb_1.5Te₃ phases and pure Te and Sb. Residual porosity was observed in the hot-pressed and sintered samples. The thermal conductivity of the samples was measured by transient thermoreflectance. The results showed that the thermal conductivity of the hot-pressed sample was reduced by a factor of two compared with that of the bulk as a result of the presence of a high density of interfaces and residual porosity. The thermal conductivity of the sintered sample showed an increase above that of the bulk sample, which is explained by the change in composition due to loss of Sb and Te.     

金刚石/硅复合膜的导热特性研究

用金刚石粉,用不同时间在两片镜面抛光的(111)硅基片上分别打磨.两片硅基片打磨后都仍保持镜面特征.采用微波等离子体化学气相沉积系统,利用氢气、甲烷和氧气为前驱气体,在同样参数条件下,在基片上制备了直径5 cm的金刚石薄膜.用扫描电镜和X射线衍射分析两片薄膜结构.分析结果表明其表面形貌基本相同都为(111)择优取向的金刚石薄膜;但X射线衍射分析表明打磨时间较长的薄膜中含有一定量在非晶成分.用热导测试仪测试两薄膜和硅基片的热导率约为:241.7 W/mK,192.9 W/mK和169.3 W/mK.结合扫描电镜和X射线衍射分析结果我们讨论了基底处理对金刚石/硅复合膜的导热特性的影响.     

电子封装用AlSiC复合材料热导率影响因素探讨

采用模压成型和真空压力浸渗工艺制备了高体积分数SiC增强Al基复合材料(AlSiC)。物相和显微结构研究结果表明,此种方法制备的AlSiC复合材料,组织致密且大小两种粒径的SiC颗粒均匀分布于Al基质中,界面结合强度高;SiC增强颗粒与Al基质界面反应控制良好,未出现Al4C3等脆性相。在此基础上,研究了基体金属、粘结剂用量、粗细SiC颗粒比例对复合材料热导率的影响。结果表明,以1A90高纯铝为基体的复合材料的热导率高于以6061铝合金为基体的复合材料的热导率;粘结剂用量减少时,复合材料热导率提高;当SiC体积分数一定时,AlSiC复合材料的热导率随增强体中粗颗粒SiC比例增大而增大。     

Thermal Conductivity Measurement Methods for SiGe Thermoelectric Materials

A new technique to measure the thermal conductivity of thermoelectric materials at the microscale has been developed. The structure allows the electrical conductivity, thermal conductivity, and Seebeck coefficient to be measured on a single device. The thermal conductivity is particularly difficult to measure since it requires precise estimation of the heat flux injected into the material. The new technique is based on a differential method where the parasitic contributions of the supporting beams of a Hall bar are removed. The thermal measurements with integrated platinum thermometers on the device are cross-checked using thermal atomic force microscopy and validated by finite-element analysis simulations.     

Modeling of Thermal Conductivity of Graphite Nanosheet Composites

Recent experiments demonstrated a very high thermal conductivity in graphite nanosheet (GNS)/epoxy nanocomposites; however, theoretical analysis is lacking. In this letter, an effective medium model has been used to analyze the effective thermal conductivity of the GNS/polymer nanocomposites and has shown good validity. Strong influences of the aspect ratio and the orientation of the GNS are evident. As expected, interfacial thermal resistance still plays a role in determining the overall thermal transport in the GNS/polymer nanocomposites. In comparison with the interfacial thermal resistance between carbon nanotubes and polymers, the interfacial thermal resistance between GNS and polymers is about one order of magnitude lower, the reason for which is discussed.     

Micromachined Hot-Wire Thermal Conductivity Probe for Biomedical Applications

This paper presents the design, fabrication, numerical simulation, and experimental validation of a micromachined probe that measures thermal conductivity of biological tissues. The probe consists of a pair of resistive line heating elements and resistance temperature detector sensors, which were fabricated by using planar photolithography on a glass substrate. The numerical analysis revealed that the thermal conductivity and diffusivity can be determined by the temperature response induced by the uniform heat flux in the heating elements. After calibrating the probe using a material (agar gel) of known thermal conductivity, the probe was deployed to calculate the thermal conductivity of Crisco. The measured value is in agreement with that determined by the macro-hot-wire probe method to within 3%. Finally, the micro thermal probe was used to investigate the change of thermal conductivity of pig liver before and after RF ablation treatment. The results show an increase in thermal conductivity of liver after the RF ablation.     

Anomalous Low Thermal Conductivity of Atomically Thin InSe Probed by Scanning Thermal Microscopy

The ability of a material to conduct heat influences many physical phenomena, ranging from thermal management in nanoscale devices to thermoelectrics. Van der Waals 2D materials offer a versatile platform to tailor heat transfer due to their high surface-to-volume ratio and mechanical flexibility. Here, the nanoscale thermal properties of 2D indium selenide (InSe) are studied by scanning thermal microscopy. The high electrical conductivity, broad-band optical absorption, and mechanical flexibility of 2D InSe are accompanied by an anomalous low thermal conductivity (κ). This can be smaller than that of low-κ dielectrics, such as silicon oxide, and it decreases with reducing the lateral size and/or thickness of InSe. The thermal response is probed in free-standing InSe layers as well as layers supported by a substrate, revealing the role of interfacial thermal resistance, phonon scattering, and strain. These thermal properties are critical for future emerging technologies, such as field-effect transistors that require efficient heat dissipation or thermoelectric energy conversion with low-κ, high electron mobility 2D materials, such as InSe.     

The Effect of Polydispersivity on the Thermal Conductivity of Particulate Thermal Interface Materials

A critical need in developing thermal interface materials (TIMs) is an understanding of the effect of particle/matrix conductivities, volume loading of the particles, the size distribution, and the random arrangement of the particles in the matrix on the homogenized thermal conductivity. Commonly, TIM systems contain random spatial distributions of particles of a polydisperse (usually bimodal) nature. A detailed analysis of the microstructural characteristics that influence the effective thermal conductivity of TIMs is the goal of this paper. Random microstructural arrangements consisting of lognormal size-distributions of alumina particles in silicone matrix were generated using a drop-fall-shake algorithm. The generated microstructures were statistically characterized using the matrix-exclusion probability function. The filler particle volume loading was varied over a range of 40%–55%. For a given filler volume loading, the effect of polydispersivity in the microstructures was captured by varying the standard deviation(s) of the filler particle size distribution function. For each particle arrangement, the effective thermal conductivity of the microstructures was evaluated through numerical simulations using a network model previously developed by the authors. Counter to expectation, increased polydispersivity was observed to increase the effective conductivity up to a volume loading of 50%. However, at a volume loading of 55%, beyond a limiting standard deviation of 0.9, the effective thermal conductivity decreased with increased standard deviation suggesting that the observed effects are a tradeoff between resistance to transport through the particles versus transport through the interparticle matrix gap in a percolation chain.      

Spatially Resolved Thermal Conductivity Measurements Using a Thermoreflectance Microprobe

A transient thermoreflectance technique capable of creating maps of the out-of-plane thermal conductivity of materials with spatial resolution of 5 μm is presented. The applied noncontact optical microprobe uses a thin reflective metal film as temperature transducer and heat reservoir. The parameters of the experimental setup guarantee that the heat flow is one dimensional. Thus, relatively simple mathematical relations can be used to extract the thermal conductivity from the recorded signal shape, which is important for thermal conductivity maps with high pixel count. For reference materials (soda-lime glass and PbTe), the determined thermal conductivities were in good agreement with bulk values, indicating a typical measurement uncertainty of 10%. For low-conductivity substrates, the measuring scheme is relatively insensitive to thermal interface resistance. Thermal conductivity maps of lead-antimony-silver-tellurium surfaces showed distinct features which could be correlated with optical surface micrographs. The local conductivity fluctuations can possibly be attributed to local changes in chemical composition and secondary phase formation.     

一种聚酰亚胺超低热导红外热敏悬浮微桥

热导极为微小的热敏悬空微桥结构的制作是集成非致冷热成像探测阵列热敏感单元研究的核心。由于没有现成的将聚酰亚胺制成微桥的工艺,采用准分子激光微刻蚀技术,使用聚酰亚胺材料,并采用铂作为敏感材料来制作微桥。对所制作的铂/聚酰亚胺微桥原理性样件的初步测试表明,铂/聚酰亚胺微桥的热导约3.98×10-7W/K,与理论计算值接近。铂薄膜与聚酰亚胺微桥附着良好,工艺可行。在0～10V的驱动电压下可观察到铂/聚酰亚胺微桥样件明显的自热效应,其电加热响应时间小于100ms。     

一种测试多层膜系热导率的方法

提出了一种测试多层薄膜热导率的结构.用ANSYS有限元分析软件对该结构进行了仿真,分析了加热功率以及悬梁长度和宽度等因素对测试结构温度分布的影响.仿真结果与理论相吻合,验证了该方法的可行性.微桥结构多层膜系的热导率是影响器件性能的关键参数之一.该测量方法无须在真空中实施,结构简单,操作方便.