DC Experiments in Quantitative Scanning Thermal Microscopy

This paper presents experimental results of quantitative DC measurements carried out by the use of a scanning thermal microscope equipped with nanofabricated thermal probes, and their numerical simulations done by finite element analysis. In the proposed method, the probe resistance variations are measured for the sample-to-air transition. It is shown that taking the signal measured in air as a reference makes the measurement less sensitive to instabilities of ambient conditions. This paper also presents a simple theoretical model describing the phenomena associated with heat transfer in the probe–sample system. Both experimental and numerical results confirm the theoretical findings. The registered signal can be related to the thermal conductivity of different materials, which makes the method useful for determining the local thermal conductivity.     

Analysis of Possibilities of Application of Nanofabricated Thermal Probes to Quantitative Thermal Measurements

A steady-state thermal model of the nanofabricated thermal probe was proposed. The resistive type probe working in the active mode was considered. The model is based on finite element analysis of the temperature field in the probe-sample system. Determination of the temperature distribution in this system allows calculations of relative changes in the probe electrical resistance. It is shown that the modeled probe can be used for measurements of the local thermal conductivity with the spatial resolution determined by the probe apex dimensions. The probe exhibits the maximum sensitivity to the changes in the thermal conductivity of the sample between 2 W·m⁻¹ ·K⁻¹ and 200 W·m⁻¹ ·K⁻¹. The influence of the thermal conductivity of the probe substrate on metrological characteristics of the probe as well as the thermal resistance of the probe-sample contact on the determination of the sample thermal conductivity were also analyzed. The selected results of numerical analysis were compared with data of preliminary experiments.     

High Thermal Conductivity of Sandwich-Structured Flexible Thermal Interface Materials

Thermal interfaces are vital for effective thermal management in modern electronics, especially in the emerging fields of flexible electronics and soft robotics that impose requirements for interface materials to be soft and flexible in addition to having high thermal performance. Here, a novel sandwich-structured thermal interface material (TIM) is developed that simultaneously possesses record-low thermal resistance and high flexibility. Frequency-domain thermoreflectance (FDTR) is employed to investigate the overall thermal performance of the sandwich structure. As the core of this sandwich, a vertically aligned copper nanowire (CuNW) array preserves its high intrinsic thermal conductivity, which is further enhanced by 60% via a thick 3D graphene (3DG) coating. The thin copper layers on the top and bottom play the critical roles in protecting the nanowires during device assembly. Through the bottom-up fabrication process, excellent contacts between the graphene-coated CuNWs and the top/bottom layer are realized, leading to minimal interfacial resistance. In total, the thermal resistance of the sandwich is determined as low as ~0.23 mm² K W⁻¹. This work investigates a new generation of flexible thermal interface materials with an ultralow thermal resistance, which therefore renders the great promise for advanced thermal management in a wide variety of electronics.     

Imaging of Thermal Conductivity with Sub-Micrometer Resolution Using Scanning Thermal Microscopy

With the development of new emerging technologies, many objects in scientific research and engineering are of sub-micrometer and nanometer size, such as microelectronics, micro-electro-mechanical systems (MEMS), biomedicines, etc. Therefore, thermal conductivity measurements with sub-micrometer resolution are indispensable. This paper reports on the imaging of various micrometer and sub-micrometer size surface variations using a scanning thermal microscope (SThM). The thermal images show the contrasts indicating the differences of the local thermal conductivity in the sample. Thermal resistance circuits for the thermal tip temperature are developed to explain the heat transfer mechanism between the thermal tip and the sample and to explain the coupling between the local thermal conductivity and the topography in the test results.     

Non-destructive Thermal Diagnostics of Porous Materials

Developed mathematical models of apparent thermal conductivity of porous materials are applied to non-destructive methods of thermal diagnostics. The non-destructive thermal diagnostics of porous materials can be used to estimate the size of pores and cracks in the range 10⁻⁹ to 10⁻³ m. A fractal model of porous structure and dependences of thermal conductivity/diffusivity on (experimental) gas pressure are used as a basis for structure parameter calculations. The measuring element (sensor) in this method is the mean free path of gas molecules in pores and cracks (Knudsen number) that is very sensitive to changes in gas pressure. Possible applications of the developed methods include non-destructive thermal diagnostics (NDTD) of nano- and micro-crack sizes; opening, closing and size changes of the cracks at high temperatures in a wide temperature range; evaluation of interfacial and contact heat barrier resistance for coatings; remote laser thermal diagnostics of the cracks; as well as obtaining data on strength, thermal shock behavior, failure and fatigue behavior of coatings and other structures. Examples of several applications of the NDTD method are presented. Invited paper presented at the Fifteenth Symposium on Themophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

Measurement of the Intrinsic Thermal Conductivity of a Multiwalled Carbon Nanotube and Its Contact Thermal Resistance with the Substrate

The intrinsic thermal conductivity of an individual carbon nanotube and its contact thermal resistance with the heat source/sink can be extracted simultaneously through multiple measurements with different lengths of the tube between the heat source and the heat sink. Experimental results on a 66‐nm‐diameter multiwalled carbon nanotube show that above 100 K, contact thermal resistance can contribute up to 50% of the total measured thermal resistance; therefore, the intrinsic thermal conductivity of the nanotube can be significantly higher than the effective thermal conductivity derived from a single measurement without eliminating the contact thermal resistance. At 300 K, the contact thermal resistance between the tube and the substrate for a unit area is 2.2 × 10^?8 m² K W^?1, which is on the lower end among several published data. Results also indicate that for nanotubes of relatively high thermal conductance, electron‐beam‐induced gold deposition at the tube–substrate contacts may not reduce the contact thermal resistance to a negligible level. These results provide insights into the long‐lasting issue of the contact thermal resistance in nanotube/nanowire thermal conductity measurements and have important implications for further understanding thermal transport through carbon nanotubes and using carbon nanotube arrays as thermal interface materials.     

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.     

Experimental Measurements and Theoretical Prediction of the Thermal Conductivity of Two- and Three-Phase Water/Olivine Systems

The thermal conductivity of olivine, dry and mixed with water, up to saturation, has been measured with a thermal probe, using the step heating method. The olivine is composed of solid particles with dimensions in the range from 0.8 to 1 mm. Dry olivine has been measured in the range of temperatures between –17° to +50°C. Olivine mixed with water has been measured at +50°C. The cubic cell model has been used to make predictions to compare with the measured data. Comparisons of the experimental thermal conductivities and the predicted values of dry and water-mixed olivine show good agreement. The cubic cell model can be used to evaluate the porosity of olivine and the thermal conductivity of the solid particles, from the values measured at dryness and saturation, with reasonably good agreement. In this way, it is not necessary to measure the mineral composition of the particles of the porous media. Also, the porosity of the medium is predicted with reasonable agreement, which takes into account the phenomenon of the porosity increase near the probe, since the diameter of the probe is smaller than that of the solid particles.     

Thermal Diffusivity Measurements on Insulation Materials with the Laser Flash Method

An iterative approach is adopted to determine the thermal diffusivity of the xonotlite-type calcium silicate insulation material with very low thermal conductivity. The measurements were performed with a conventional laser flash apparatus by rear-face detection of the temperature response of the three-layered sample, where the insulating material is sandwiched between two iron slices. In the evaluation of the thermal conductivity, the theoretical curve is fitted to the complete temperature–time curve, instead of just using the t _1/2 point. The theoretical model is based on the thermal quadrupole method. The nonlinear parameter estimation technique is used to estimate simultaneously the thermal diffusivity, heat transfer coefficient, and absorbed energy. Based on experimental results, the optimal thickness range of the insulation material in the sample is indicated as 1.6 to 1.9 mm. The effects of the uncertainties of the thicknesses, contact resistance, and thermophysical properties of the three layers on the measurement uncertainty are estimated, giving an overall uncertainty in the thermal conductivity of approximately 7.5%.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

新一代高导热金属基复合材料界面热导研究进展

热物理性质不同的材料之间存在界面热阻,界面热阻对热传输过程产生极大的影响,并在很大程度上决定了复合材料的导热性能。金刚石颗粒增强金属基复合材料(Metal matrix composites,MMCs)充分发挥了金刚石的高热导率和低热膨胀系数的优点,有望获得高的热导率以及与半导体相匹配的热膨胀系数,可满足现代电子设备在散热能力上提出的越来越高的要求,作为新一代电子封装材料已引起广泛关注。界面热导(界面热阻的倒数)既是决定复合材料导热能力的关键因素,也是研究的难点,复合材料制备工艺、界面改性方式(金属基体合金化或金刚石表面金属化)以及改性金属种类均会影响界面热导。详细论述了界面热导理论及实验研究的最新成果,并对金刚石/金属复合材料在未来研究中面临的主要问题进行探讨。     

钛酸铝材料的结构、热膨胀及热稳定性

详细回顾了关于铁酸铝材料结构、热膨胀和热稳定性的研究。A12TiO5属于正交晶系假板钛矿结构。其晶格热膨胀各向异性行为，导致多晶铁酸铝陶瓷材料的微裂纹化，从而具有低膨胀、低热导率和优良的抗热震性等特性。但Al2TiO5低温下属于动力学稳定态，当温度低于1280℃易于分解为α-A12O3和金红石型TiO2。引入异质同构的化合物MgTi2O5或Fe2TiO5固熔于A12TiO5晶格，可以降低热力学分解温度和增加结构熵，有效地抑制Al2TiO5的分解。Al2TiO5在还原气氛下的分解机理上不明确，需要进一步的研究。     

Thermal fatigue resistance characterization and ranking of materials using the V‐shape specimen testing method

Thermal fatigue resistance of materials is an extremely important criterion for the long‐term durability and reliability performance of very high‐temperature components and systems, such as advanced auto engine and exhaust systems. There is a broad range of material choices for thermal fatigue resistance applications. The final selection of the materials depends on the balance of engineering performance of the materials and the cost. To optimize the thermal fatigue resistance and cost of those materials, a reliable testing procedure for material thermal fatigue characterization and a material evaluation/selection matrix must be established. In this paper, the V‐shape specimen testing method in evaluating thermal fatigue resistance performance is introduced first. The influence of several factors, such as the thickness of specimens, operating temperature and hold time, on the thermal fatigue resistance is experimentally investigated. Subsequently, the statistical and probabilistic characteristics of the thermal fatigue failure data are analysed to reveal the possible failure mechanisms. Finally, a general rational approach for thermal fatigue resistance characterization and ranking is demonstrated, and a simple parameter λ = kσ_f/Eα, which combines the material strength, thermal conductivity and thermal expansion, is found to be the new breakthrough parameter, correlating to V‐shape thermal fatigue test results. Results on four currently used stainless steels verify the correlations and indicate the validity of this approach.     

Thermal Conductivity Behavior of Carbon Nanotubes Reinforced Copper Matrix Composites

Carbon nanotubes (CNT) exhibit excellent thermal conductivity.Therefore they are potential reinforcements in composites materials for thermal management applications,where high thermal conductivity and low coefficient of thermal expansion (CTE) are required.In the present study,CNT/Cu composites containing CNTs varying from 0 vol.％ to 15 vol.％ were prepared,and their thermal conductivity behavior was studied in detail.The results indicated that the thermal conductivity of the composites shows no enhancement by the incorporation of CNTs.The presence of interfacial thermal resistance and high level of porosity are the main reasons for this low thermal conductivity.The well dispersed 0-10 vol.％ CNTs composites show a very close to the thermal conductivity of Cu.However,the addition of 15 vol.％ CNTs results in a rather low thermal conductivity of CNT/Cu composites due to the presence a high level of porosity induced by the formation of CNT clusters.The present paper also claims that a further substantial enhancement in thermal conductivity is only possible if the nanotubes are randomly oriented in the plane or if they are all aligned in one direction,for which the processing of CNTs-aligning in metal matrix should be developed.     

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.     

A Robot Sensor for Measuring Thermal Properties of Gripped Objects

Robot manipulators will require a wide range of sensory systems if they are to become more widely useful. In this communication we describe a novel robot sensor designed to measure some of the thermal properties of a gripped object. A mathematical model of the sensor is proposed and validated by comparing predicted and measured sensor responses for a range of differing materials. The mathematical model can be used to analyze the sensor output and determine the thermal conductivity and thermal diffusivity of an unknown object held by the robot manipulator. This information will enable robot systems to discriminate between objects made of different materials and to aid recognition of unknown objects by providing information about their material structure.     

Predicting, Measuring, and Tailoring the Transverse Thermal Conductivity of Composites from Polymer Matrix and Metal Filler

The addition of conductive filler in a polymer matrix is an effective way to increase the thermal conductivity of the plastic materials, as required by several industrial applications. All quantitative models for the thermal conductivity of heterogeneous media fail for heavily filled composites. The percolation theory allows good qualitative predictions, thus selecting a range for some qualitative effects on the thermal conductivity, and providing a way to choose a range for some experimental parameters. The design of such composite materials requires a study of its thermal features combined with different mechanical, ecological, safety, technical, and economical restrictions. A specific small guarded hot plate device with an active guard, conductive grease layer, and controlled variable pressure was used for measurement of the transverse thermal conductivity on 15 mm sided samples of composite parts. Extensive thermal and composition measurements on filled thermoplastics show that the conductivity of the filler, its size and shape, and its local amount are, with the degree of previous mixing, the main factors determining the effective conductivity of composites. For injection-molded polybutylene terephtalate plates, the best filler is the short aluminum fiber. With fibers of 0.10 mm diameter, it is possible to obtain conductivities larger by factors of 2, 6, and 10 than those of polymer for aluminum contents of 20, 42, and 43.5 vol%, respectively.     

热辐射对碳纳米管纱名义热导率的影响

碳纳米管纱是完全由碳纳米管构成的宏观材料,因其类似“气凝胶”的结构,具有较大的比表面积,相对于普通宏观材料,会有更多的热量通过表面热辐射耗散出去。本文详细地推导了电加热时,准一维热传播材料表面温度分布和热导率计算公式,并引入热辐射项对上述传热过程进行修正,获得表面真实温度分布,进而结合碳纳米管纱的物理性质,分析了测试样品长度对碳纳米管纱名义热导率(指测试获得的热导率)的影响。对于长度为5 mm的碳纳米管纱测试样品,名义热导率约为真实热导率的4倍。     

导热高分子材料在包装印刷领域的研究进展

目的综述导热高分子材料在包装印刷领域的应用及研究现状,拓展导热高分子材料的应用领域。方法首先介绍2类导热高分子材料的制备方法,即本征型和填充型导热高分子材料;其次全面综述用于包装印刷领域的导热膜/纸、导热胶黏剂和导热油墨;最后总结常用的各类导热机理模型。结果与本征型导热高分子相比,填充型导热高分子具有加工简单、成本低廉、应用面广等优点,是目前研究最多的导热高分子材料。导热膜/纸、导热胶黏剂和导热油墨具有广泛的研究基础,市场需求旺盛。导热预测模型虽能够有效预测复合材料的热导率,但会受到填料含量和粒子形貌的影响。结论导热高分子材料在包装印刷领域拥有巨大的应用需求,开展导热高分子的研究具有重要的现实和理论意义。     

The Thermal Conductivity, Thermal Diffusivity, and Heat Capacity of Gaseous Argon

This paper presents new absolute measurements for the thermal conductivity and thermal diffusivity of gaseous argon obtained with a transient hot-wire instrument. Six isotherms were measured in the supercritical dense gas at temperatures between 296 and 423 K and pressures up to 61 MPa. A new analysis for the influence of temperature-dependent properties and residual bridge unbalance is used to obtain the thermal conductivity with an uncertainty of less than 1% and the thermal diffusivity with an uncertainty of less than 4%. Isobaric heat capacity results were derived from measured values of thermal conductivity and thermal diffusivity using a density calculated from an equation of state. The heat capacities presented here have a nominal uncertainty of 4% and demonstrate that this property can be obtained successfully with the transient hot wire technique over a wide range of fluid states. The technique will be useful when applied to fluids which lack specific heat data.     

Thermal Conductivity of Saturated Liquid Toluene by Use of Anodized Tantalum Hot Wires at High Temperatures

Absolute measurements of the thermal conductivity of a distilled and dried sample of toluene near saturation are reported. The transient hot-wire technique with an anodized tantalum hot wire was used. The thermal conductivities were measured at temperatures from 300 K to 550 K at different applied power levels to assess the uncertainty with which it is possible to measure liquid thermal conductivity over wide temperature ranges with an anodized tantalum wire. The wire resistance versus temperature was monitored throughout the measurements to study the stability of the wire calibration. The relative expanded uncertainty of the resulting data at the level of 2 standard deviations (coverage factor k = 2) is 0.5 % up to 480 K and 1.5 % between 480 K and 550 K, and is limited by drift in the wire calibration at temperatures above 450 K. Significant thermal-radiation effects are observed at the highest temperatures. The radiation-corrected results agree well with data from transient hot-wire measurements with bare platinum hot wires as well as with data derived from thermal diffusivities obtained using light-scattering techniques.