Realizing the Accurate Measurements of Thermal Conductivity over a Wide Range by Scanning Thermal Microscopy Combined with Quantitative Prediction of Thermal Contact Resistance

Quantitative thermal performance measurements and thermal management at the micro-/nano scale are becoming increasingly important as the size of electronic components shrinks. Scanning thermal microscopy (SThM) is an emerging method with high spatial resolution that accurately reflects changes in local thermal signals based on a thermally sensitive probe. However, because of the unclear thermal resistance at the probe-sample interface, quantitative characterization of thermal conductivity for different kinds of materials still remains limited. In this paper, the heat transfer process considering the thermal contact resistance between the probe and sample surface is analyzed using finite element simulation and thermal resistance network model. On this basis, a mathematical empirical function is developed applicable to a variety of material systems, which depicts the relationship between the thermal conductivity of the sample and the probe temperature. The proposed model is verified by measuring ten materials with a wide thermal conductivity range, and then further validated by two materials with unknown thermal conductivity. In conclusion, this work provides the prospect of achieving quantitative characterization of thermal conductivity over a wide range and further enables the mapping of local thermal conductivity to microstructures or phases of materials.     

Thermal Conductivity of AlN and SiC Thin Films

The thermal conductivity of AlN and SiC thin films sputtered on silicon substrates is measured employing the 3ω method. The thickness of the AlN sample is varied in the range from 200 to 2000 nm to analyze the size effect. The SiC thin films are prepared at two different temperatures, 20 and 500°C, and the effect of deposition temperature on thermal conductivity is examined. The results reveal that the thermal conductivity of the thin films is significantly smaller than that of the same material in bulk form. The thermal conductivity of the AlN thin film is strongly dependent on the film thickness. For the case of SiC thin films, however, increased deposition temperature results in negligible change in the thermal conductivity as the temperature is below the critical temperature for crystallization. To explain the thermal conduction in the thin films, the thermal conductivity and microstructure are compared using x-ray diffraction patterns.     

稳态法测算导热系数的原理

分析了测量导热系数传统模型与相关文献中的修正模型的原理与不足,提出一个较为严密的三维传热学模型,利用MATLAB数值模拟验证了其合理性。推导出计算圆柱型试样导热系数的公式,并得到了适合实际应用的简化公式。以真空橡皮盘为例,定量分析了侧壁散热对导热系数测量值的影响程度,给出了试样规格与测算精度的关系曲线。当试样半径与厚度的比值大于28.9时,侧壁散热的影响将小于5%,故可忽略。     

聚丙烯腈基高模量碳纤维导热性能的影响因素

采用激光闪射法测试了6种不同强度和模量的聚丙烯腈(PAN)基碳纤维(CF)的热导率,探讨了不同温度下CF轴向热导率的变化及样品厚度对热导率测试结果的影响。采用X射线衍射(XRD)、拉曼等技术检测了CF的微晶尺寸、取向及有序度,考察了CF热导率与其微观结构的相关性。结果显示,实验范围内PAN基CF样品厚度对热导率测试结果略有影响,热导率随测试温度升高而降低,PAN基CF的致密性、晶体尺寸及结构有序度对其热导率有较大影响。     

高空间分辨率的热测试技术——扫描热显微技术

简述了用扫描热显微技术进行高空间分辨率热测量 试的技术特点和研究现状,介绍了实际测试的情况,初步分析了扫描热地和样品表面间的传热机理,提出了进一步研究的方向。     

Thermal conductivity of Inconel 718 and 304 stainless steel

The results of thermal conductivity measurements on Inconel 718 and 304 stainless steel by the comparative and flash diffusivity techniques are reported for the temperature range 0–700°C. For 304 stainless steel, excellent agreement with published data is found for the specific heat, thermal diffusivity, and thermal conductivity. In the case of Inconel 718, the measurements show that the conductivity depends critically on the sample thermal history and the metallurgical condition of the alloy. Measurements on a solution-treated sample indicated a conductivity function close to that reported previously, while precipitated samples showed a higher conductivity, similar to the conductivityvs-temperature function used for reduction of comparative thermal conductivity data with Inconel 718 references. These results indicate that Inconel 718 is not a suitable reference for high-accuracy comparative thermal conductivity measurements unless its thermal history and associated conductivity function are known.     

On the Use of the Transient Hot-Strip Method for Measuring the Thermal Conductivity of High-Conducting Thin Bars

The thermal conductivity of thin, high-conducting ceramic bars—commonly used in mechanical tensile testing—is measured using a variant of the short transient hot-strip technique. As with similar contact transient methods, the influence from the thermal contact resistance between the sensor and the sample is accurately recorded and filtered out from the analysis—a specific advantage that enables sensitive measurements of the bulk properties of the sample material. The present concept requires sensors that are square in shape with one side having the same width as the bar to be studied. As long as this requirement is fulfilled, the particular size of the thin bar can be selected at will. This paper presents an application where the present technique is applied to study structural changes or degradation in reinforced carbon–carbon (RCC) bars exposed to thermal cycling. Simultaneously, tensile testing and monitoring of mass loss are conducted. The results indicate that the present approach may be utilized as a non-destructive quality control instrument to monitor local structural changes in RCC panels.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Effects of Cation Impurities on Thermal Conductivity of Yttria-Doped Aluminum Nitride

Effects of cation impurities such as Fe and Si on thermal conductivity of yttria-doped aluminum nitride were investigated. Two types of aluminum nitride powders, Tokuyama F-grade and Denka AP-10, were compared. The powders have similar contents of oxygen but different levels of cation impurities. Since the thermal conductivity of sintered aluminum nitride is virtually affected by the residual oxygen contents in the grains, effects of yttria additions and prolonged firing time are also investigated. The resultant thermal conductivity is thought to be influenced by the sample size of sintering bodies because extracting oxygen from the aluminum nitride lattice in larger samples requires longer time resulting from a longer diffusion path. In comparing the AlN powders, impurity contents of Fe and Si are somewhat higher in Denka powder than Tokuyama powder. These impurity elements degrade the thermal conductivity of sintered aluminum nitride. Degradation effects in thermal conductivity due to presence of Fe and Si are evaluated by artificially introducing these elements to Tokuyama AlN powder. The difference in the thermal conductivity resulting from the two types of powders is discussed.     

纳米氧化锆涂层晶粒度与隔热性能的关系

将纳米热障涂层隔热性能与微观组织联系起来并进行定量描述的工作还处于起步阶段,采用纳米ZrO2团聚体粉末在不同的喷涂参数下制备了3种不同的纳米热障涂层,利用扫描电子显微镜、IA-32定量金相分析软件、X射线衍射仪对其进行了观察和分析,并结合非稳态激光脉冲法与差示扫描量热仪测量了涂层的热导率.研究表明:涂层中同时存在柱状组织和纳米级等轴组织,并以柱状组织为主;随着喷涂参数的改变,二者的相对比例发生变化;纳米级等轴组织含量对涂层的平均晶粒度和热导率都有重要影响;随着涂层晶粒度的减小,热导率随之降低.     

Thermal conductivity of CVD diamond films

Diamond films 60 and 170 µm in thickness were grown by PACVD (plasma-assisted chemical vapor deposition) under similar conditions. The thermal diffusivity of these freestanding films was measured between 100 and 300 K using AC calorimetry. Radiation heat loss from the surface was estimated by analyzing both the amplitude and the phase shift of a lock-in amplifier signal. Thermal conductivity was calculated using the specific heat data of natural diamond. At room temperature, the thermal conductivity of the 60 and 170 m films is 9 and 16 W-cm^–1. K^–1 respectively, which is 40–70% that of natural diamond, The temperature dependence of thermal conductivity of the CVD diamond films is similar to that of natural diamond, Phonon scattering processes are considered using the Debye model, The microsize of the grain boundary has a significant effect on the mean free path of phonons at low temperatures. The grain in CVD diamond film is grown as a columnar structure, Thus, the thicker film has the larger mean grain size and the higher thermal conductivity. Scanning electron microscopy (SEM) and Raman spectroscopy were used to study the microstructure of the CVD diamond films. In this experiment, we evaluated the quality of CVD diamond film of the whole sample by measuring the thermal conductivity.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Thermophysical Properties of Porous Sandstones: Measurements and Comparative Study of Some Representative Thermal Conductivity Models

The thermal conductivity and thermal diffusivity of porous consolidated sandstones have been measured simultaneously by the transient-plane source (TPS) technique in the temperature range from 280 to 330 K at ambient pressure using air as the saturant. The porosity and density parameters are measured using standard American Society for Testing and Materials (ASTM) methods at 307 ± 1 K. Data are presented for five types of samples ranging in porosity from 8 to 17 vol. %, taken from various positions above the baseline. The thermal conductivity and constituents of the minerals vary with porosity as well as with the position of the sample from the baseline. The thermal conductivity data are discussed in the framework of simple mixing laws and empirical models. Simple correlations between the effective density and porosity, and between the effective thermal conductivity and porosity, are also established     

Theoretical Modeling of Obliquely Crossed Photothermal Deflection for Thermal Conductivity Measurements of Thin Films

A three-dimensional theoretical model has been developed to calculate the normal probe beam deflection of the obliquely crossed photothermal deflection configuration in samples which consist of thin films deposited on substrates. Utilizing the dependence of the normal component of probe beam deflection on the cross-point position of the excitation and probe beams, the thermal conductivity of the thin film can be extracted from the ratio of the two maxima of the normal deflection amplitude, which occurs when the cross-point is located near both surfaces of the sample. The effects of other parameters, including the intersect angle between the excitation and the probe beams in the sample, the modulation frequency of the excitation beam, the optical absorption and thickness of the thin films, and the thermal properties of substrates on the thermal conductivity measurement of the thin film, are discussed. The obliquely crossed photothermal deflection technique seems to be well suited for thermal conductivity measurements of thin films with a high thermal conductivity but a low optical absorption, such as diamond and diamond-like carbon, deposited on substrates with a relatively low thermal conductivity.     

Thermal Conductivity of Suspensions of Carbon Nanotubes in Water

The enhancement of the thermal conductivity of water in the presence of carbon-multiwall nanotubes (C-MWNT) was investigated. Sodium dodecyl sulfate (SDS) was employed as the dispersant, and a 0.6 vol% suspension of C-MWNT in water was used in all measurements. The thermal conductivity was measured with a transient hot-wire instrument built for this purpose, and operated with a standard uncertainty better than 2 The maximum thermal conductivity enhancement obtained was 38 %. In an attempt to explain the experimental observations, a number of micro-structural investigations have been carried out and those results are presented here along with the analysis.     

Thermal Conductivity of Small Nickel Particles

The thermal conductivity of nanoscale nickel particles due to phonon heat transfer is extrapolated from thin film results calculated using nonequilibrium molecular dynamics (NEMD). The electronic contribution to the thermal conductivity is deduced from the electrical conductivity using the Wiedemann–Franz law. Based on the relaxation time approximation, the electrical conductivity is calculated with the Kubo linear-response formalism. At the average temperature of T=300 K, which is lower than the Debye temperature Θ_D=450 K, the results show that in a particle size range of 1.408–10.56 nm, the calculated thermal conductivity decreases almost linearly with decreasing particle size, exhibiting a remarkable reduction compared with the bulk value. The phonon mean free path is estimated, and the size effect on the thermal conductivity is attributed to the reduction of the phonon mean free path according to the kinetic theory.     

热障涂层热导率的研究进展

简要回顾了热障涂层体系的发展,讨论了氧化锆陶瓷材料的传热规律,包括涂层微观结构、陶瓷成分等因素的影响.同时指出了改进陶瓷涂层热导率的方法和开发适用于更高温度下的陶瓷涂层材料的指导原则,并详细介绍了改善热障涂层热导率的研究现状.     

固相纳米复合材料的导热系数预测

研究了分散剂和纳米颗粒对固相纳米复合材料导热系数的影响。假设纳米颗粒在纳米流体中均匀分布,构建了一个考虑纳米颗粒尺度和分散介质影响的物理分析模型;并由此利用最小热阻力法则和比等效导热系数相等法则,建立了一个固相纳米复合材料的导热系数理论模型。计算结果表明,颗粒的体积分数和导热系数、以及分散剂导热系数的增大,都会引起复合材料导热系数的增大。     

Stellite12钴基合金热循环冲击前后拉伸断裂机理研究

对不同缺口的Stellite12钴基合金试样(700℃/20℃进行不同次数的热循环冲击和未冲击)进行原位拉伸,并结合试验数据的分析以及断口形貌的扫描电镜观察,分析了Stellite12钴基合金热循环冲击前后的拉伸断裂过程和断裂机理。结果发现:热循环冲击后不同半径试样的断裂过程略有不同,热循环冲击后的小圆弧缺口试样在缺口根部产生表面微裂纹,试样边缘及微裂纹两侧产生氧化微孔;原位拉伸时,该试样热冲击过程产生的裂纹先向试样厚度方向扩展,待厚度方向贯通,然后裂纹尖端的基体发生变形、黑相(白相)穿晶开裂、少量沿氧化微孔裂开,试样瞬间发生断裂;而经历热循环冲击后的大圆弧试样表面并未产生明显的裂纹,拉伸加载过程经历大圆弧根部基体变形、黑白相内开裂、边缘氧化微孔张开,试样突然断裂;对于未冲击试样,在加载过程中,试样的断裂过程经历基体变形、黑白相内部开裂,能量聚集到一定程度试样突然断裂。对于未热冲击的三种不同试样其断裂过程基本类似,仅仅是由于小圆弧半径的试样应力集中程度更大,从而使得其断裂应力低于平板以及大圆弧试样。     

Thermal Conductivity of Suspensions Containing Nanosized SiC Particles

Nanosized SiC suspensions were prepared, and their thermal conductivities were measured using a transient hot-wire method. The experimental results showed that the thermal conductivities of the studied suspensions were increased as expected, and the enhancement was proportional to the volume fraction of the solid phase, but the increasing ratio of the thermal conductivity was not significantly related to the base fluid. The effects of the morphologies (size and shape) of the added solid phase on the enhancement of the thermal conductivity of the nanoparticle suspension are reported for the first time.     

Modeling Thermal Conductivity for Alumina-Water Nanofluids

This article presents results of an ongoing investigation into the modeling of thermal conductivity for Alumina-Water nanofluids. In spite of having the promise of being an improved heat transfer medium, fundamental understanding and modeling of important thermo-physical properties of nanofluids (such as thermal conductivity) have remained a difficult task due to the possible influence of several particle and base-fluid properties on the behavior of nanofluids. The existing theories to explain the phenomenon of thermal conductivity augmentation have provided different and sometimes contrasting mechanisms. In this study, seven existing theoretical models for thermal conductivity of nanofluids have been evaluated for their accuracy by comparing the predicted versus experimental data for a wide range of test conditions. The existing models were found to provide inaccuracies (over/underpredictions) in the range of 3 to 58%. A new model has been developed using dimensionless analysis, which includes Prandtl number and a new dimensionless number that is a ratio of Reynolds number to the square root of Brinkman number for particle and fluids. The new model has been found to generally predict the thermal conductivity ratio (nanofluids to base fluids) within 5% accuracy range.     

Transient Characteristics of Thermal Conduction in Dispersed Composites

The effective thermal conductivity of dispersed composites with a hot-melt-adhesive matrix, measured using the steady-state method, is compared with the apparent thermal conductivity calculated from the average heat capacity and from the thermal diffusivity measured by the laser-flash method. The transient effect has been observed obviously at higher volume percentages for various dispersed particle sizes and ratios of the thermal conductivity values of dispersed and continuous phases. All of the experimental results are compared with those calculated by existing models and by the finite element method (FEM). An attempt has been made to show how the criterion for the homogeneity of dispersed composites under transient conditions is affected by the percentages of dispersed phase, dispersed particle size, and ratio of the thermal conductivity values of dispersed and continuous phases.