Determination of the Effect of Variability of the Thermal Properties of Materials When Measuring Thermal Conductivity Using Steady-State Thermal Methods

The corrections for distortion of the temperature field are determined by solving a nonlinear differential equation under conditions when the thermal properties of the material are variable, and the thermal conductivity is calculated for calorimeters operating under steady-state thermal conditions. The condition under which these corrections are zero is obtained.     

金属反射型保温材料导热系数的测定

根据稳态法测量导热系数原理,设计了一套稳态导热系数测量装置,利用自动控制技术实现实验过程的可控性。通过与导热系数参比块的测量校对,该装置测量结果的准确度达到标准要求。并利用此测量装置对压力容器金属反射型保温层试样的导热系数进行了测定,测定温度在80～220 ℃范围内,测量误差小于1.3%,导热系数值由0.040 W/(m·K)变化到0.053W/(m·K)。     

Thermal Nondestructive Testing of the Thermal Properties of Materials Using Multifactor Transformation Functions

A solution of the direct problem of heat conduction when a linear pulsed heat source acts in the plane of contact of two semibounded bodies, obtained by the method of finite differences, is presented. The results of modeling are given. Using a discrete model, a method of determining (identifying) the thermal properties of materials using multifactor transformation functions is considered. __________ Translated from Izmeritel'naya Tekhnika, No. 7, pp. 44–49, July, 2005.     

Measurements of the Thermal Conductivity and Thermal Diffusivity of Polymer Melts with the Short-Hot-Wire Method

In this paper, the thermal conductivity and thermal diffusivity of four kinds of polymer melts were measured by using the transient short-hot-wire method. This method was developed from the hot-wire technique and is based on two-dimensional numerical solutions of unsteady heat conduction from a wire with the same length-to-diameter ratio and boundary conditions as those in the actual experiments. The present method is particularly suitable for measurements of molten polymers where natural convection effects can be ignored due to their high viscosities. The results have shown that the present method can be used to measure the thermal conductivity and thermal diffusivity of molten polymers within uncertainties of 3 and 6%, respectively. Further, the thermal conductivity and thermal diffusivity of solidified samples were also measured and discussed.     

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。     

Experimental Study on the Effective Thermal Conductivity and Thermal Diffusivity of Nanofluids

This paper reports measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids using the transient short-hot-wire technique. To remove the influences of the static charge and electrical conductance of the nanoparticles on measurement accuracy, the short-hot-wire probes are carefully coated with a pure Al₂O₃ thin film. Using distilled water and toluene as standard liquids of known thermal conductivity and thermal diffusivity, the length and radius of the hot wire and the thickness of the Al₂O₃ film are calibrated before and after application of the coating. The electrical leakage of the short-hot-wire probes is frequently checked, and only those probes that are coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Al₂O₃/water, ZrO₂/water, TiO₂/water, and CuO/water nanofluids are measured and the effects of the volume fractions and thermal conductivities of nanoparticles and temperature are clarified. The average diameters of Al₂O₃, ZrO₂, TiO₂, and CuO particles are 20, 20, 40, and 33 nm, respectively. The uncertainty of the present measurements is estimated to be within 1% for the thermal conductivity and 5% for the thermal diffusivity. The measured results demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancement and can be predicted accurately by the model equation of Hamilton and Crosser, when the spherical nanoparticles are dispersed into fluids.     

Anisotropic Thermal Diffusivities of Plasma-Sprayed Thermal Barrier Coatings

Thermal barrier coatings (TBCs) are used to shield the blades of gas turbines from heat and wear. There is a pressing need to evaluate the thermal conductivity of TBCs in the thermal design of advanced gas turbines with high energy efficiency. These TBCs consist of a ceramic-based top coat and a bond coat on a superalloy substrate. Usually, the focus is on the thermal conductivity in the thickness direction of the TBC because heat tends to diffuse from the surface of the top coat to the substrate. However, the in-plane thermal conductivity is also important in the thermal design of gas turbines because the temperature distribution within the turbine cannot be ignored. Accordingly, a method is developed in this study for measuring the in-plane thermal diffusivity of the top coat. Yttria-stabilized zirconia top coats are prepared by thermal spraying under different conditions. The in-plane and cross-plane thermal diffusivities of the top coats are measured by the flash method to investigate the anisotropy of thermal conduction in a TBC. It is found that the in-plane thermal diffusivity is higher than the cross-plane one for each top coat and that the top coats have significantly anisotropic thermal diffusivity. The cross-sectional and in-plane microstructures of the top coats are observed, from which their porosities are evaluated. The thermal diffusivity and its anisotropy are discussed in detail in relation to microstructure and porosity.     

Influence of Thermal Properties on the Sensitivity of Thermal Wave Interferometry for the Characterization of Plasma-Sprayed Coatings

Thermal wave interferometry (TWI) is used as a means for measuring the thermal properties of plasma-sprayed coatings. The solution of the inverse problem is influenced by the magnitude of the signal-to-noise ratio and the amount of data available. This is investigated using a sensitivity analysis. The critical factors that determine the accuracy and the limitations of the technique are discussed. Numerical and experimental thermal wave tests confirm the range of applicability and the accuracy of TWI for measuring thermal properties.     

A Method for Thermal Diffusivity Determination of Thermal Insulators

A so-called “three-point” (3P) method has been developed for thermal diffusivity measurements of thermal insulating materials. One side of a cylindrical specimen, sandwiched between two thin metal plates, is subjected to intense light from an incandescent lamp to generate a thermal perturbance. The temperature response is measured in three locations along the test specimen. Thermocouples are located at the front and rear faces of the specimen, and the third is placed inside the specimen at a known location. The two outside temperatures are used as boundary conditions, and the unknown thermal diffusivity is calculated from the third temperature versus time curve. The method combines the advantages of rapid transient non-contact heating methods with the well-defined boundary conditions of steady-state methods. The results of the 3P method are compared with those from steady-state methods for a micro-porous insulation material and for a honeycomb structure.     

Monitoring the Thermal Parameters of Different Edible Oils by Using Thermal Lens Spectrometry

Several vegetable edible oils (sunflower, canola, soya, and corn) were used to study the thermal diffusivity of edible oils. Thermal lens spectrometry (TLS) was applied to measure the thermal properties. The results showed that the obtained thermal diffusivities with this technique have good agreement when compared with literature values. In this technique an Ar⁺ laser and intensity stabilized He–Ne laser were used as the heating source and probe beam, respectively. These studies may contribute to a better understanding of the physical properties of edible oils and the quality of these important foodstuffs.     

导热型高性能树脂微电子封装材料之二:封装材料的导热和热膨胀性能

微电子行业迫切需要新一代优异性能的封装材料。采用聚酰亚胺/氮化铝复合材料作为封装材料，可以组合聚酰亚胺和氮化铝的优点，具有高导热、低热胀、低介电、电绝缘等优异的性能，应用于现代微电子领域前景良好。文章讨论了这种封装材料的导热性能和热膨胀性能。PMR聚酰亚胺/氮化铝复合封装材料导热系数随氧化铝的加入量而显著提高，Bruggeman方程最适合于描述该封装材料的导热系数。PMR聚酰亚胺与氮化铝复合后热膨胀系数显著减小。     

Thermal performance of the CrIS passive cryocooler

The configuration, performance, and test validation of a passive radiant cooler for the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Crosstrack Infrared Sounder (CrIS) Instrument are presented. The cooler is required to provide cryogenic operation of IR focal planes. The 11 kg device, based on prior ITT Industries Space Systems Division coolers, requires virtually no power. It uses multiple thermally isolated cooling stages, each with an independent cryoradiator, operating at successively colder temperatures. The coldest stage, with a controlled set point at 81 K, cools a longwave IR (LWIR) focal plane. An intermediate stage, with a 98 K control point, cools detectors operating in MWIR and SWIR spectral regions. The warmest stage includes a fixed, integral earth shield that limits the thermal load from the earth in the NPOESS Operational Low-earth Orbiting (LEO) orbit. A study of the thermal balance and loads analysis used to evaluate the predicted cooler performance is discussed. High performance margins have been retained throughout the cooler development, fabrication and test phases of the program. The achievable in-orbit temperatures for this cooler are anticipated to be 73 K for the LWIR cooling stage and 91 K for the midwave IR (MWIR)/shortwave IR (SWIR) stage. Test results from two iterations of thermal vacuum verification testing are presented. Lessons learned from the first test, which failed to produce the predicted performance are included. The thermal model of the cooler and test configuration was used to identify deficiencies in the test targets resulting in unexpected heat loads. Corrective action was implemented to remove the heat leaks and a second test verified both the cooler performance and the correlation of the detailed thermal model.     

梯度热障涂层热冲击性能研究

采用等离子喷涂的方法,分别获得了２ｍｍ ＺｒＯ２－ＮｉＣｒＡｌ和ＺｒＯ２－Ｎｉ／Ａｌ系梯度热障涂层。热冲击实验结果表明,两种涂层具有不同的失效机理。ＺｒＯ２－ＮｉＣｒＡｌ系的失效是由于基于基体氧化引起的涂层整体脱落;而ＺｒＯ２－Ｎｉ／Ａｌ系的失效是支径向裂纹扩展到Ｎｉ／Ａｌ底层氧化共同作用的结果。     

Thermal Resonance at the Microscale in AC Scanning Thermal Microscopy with a Thermal-Resistive Probe

Investigation of the thermal-resistive probe response in an AC scanning thermal microscope (SThM) as a function of the distance probe–material surface under ambient conditions and the current excitation frequency is presented. The analysis of temperature experimental results points out a phenomenon which can be interpreted as a thermal wave resonance. The modeling of the thermal response with the finite element method considering the surrounding medium as a thermally conducting medium tends to confirm this. This phenomenon is independent of the sample nature and is related to the thermal diffusion length of the air medium between the probe and the sample. An equivalent resonance factor is defined: it shows a linear dependence with the characteristic distance for which the thermal resonance phenomenon is an extremum. The system—probe/sample surface—behaves as a resonant cavity at the microscale. This configuration is not specific to a SThM and can occur within electronic devices.     

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 Wave Resonator Cavity Applied to the Study of the Thermal Diffusivity of Coffee Infusions

Among the photothermal methods, the photopyroelectric technique, in its several experimental configurations, has been extensively used to measure the thermal properties of liquids, mainly the thermal effusivity and diffusivity. In this paper, the use of the so-called thermal wave resonator cavity method, in the cavity-length-scan mode, to measure the thermal diffusivity of commercial coffee infusions with samples at different concentrations and degrees of degradation induced by heating cycles is reported. A linear relationship between the logarithm of the pyroelectric signal amplitude and the sample thickness was observed, in agreement with the basic theory for the experimental configuration used here, from which the thermal diffusivity values of the samples were obtained. The thermal diffusivity was found to be almost independent of the coffee concentration in water but that this parameter is sensitive to sample modifications induced by degradation. This work represents another step to demonstrate the capability of the used method for characterization of the thermal properties of liquids.     

Study on the Thermal Expansion and Thermal Cycling of AlNp／Al Composites

The AIN particle reinforced aluminum matrix composites with 50% volume fraction were fabricated by squeeze-casting technology.The thermal expansion behavior and its response to thermal cycling were studied between 20℃ and 400℃.Compared with four theoretical models,the measured CTEs of the composite lie within the elastic bounds derived by Schapery′s analysis .Schapery′s model and Kerner′s model agree well with the CTEs of the composites at lower temperature and elevated temperature,respectively.Strain hysteresis was observed between heating and cooling curves during cycling.This was attributed primarily to the anelastic behavior of the matrix induced by matrix residual stresses.     

基于新型防热机理的防热材料的设计与试验验证

传统的防热材料大多是依靠材料自身的高熔点"忍受"热流或依靠缓慢烧蚀来被动地延长寿命的,这些材料因其密度大或耐氧化不足等问题已经不能满足飞行器设计者的期望,突破传统的被动式防热的思路从防热机理的源头上探索新的思路或许可以找到可行的技术途径。作者设计了一个新的材料体系——耗散防热材料,即在石墨中加入还原性金属,在烧蚀过程中还原性金属耗散热量,同时耗散外界的氧,自发生成氧化物陶瓷膜。新的材料设计的思想是"利用"热流而不是单纯"忍受"热流,初步试验验证表明,在廉价的石墨渗入耗散剂——铝制备的耗散防热材料,在2 900℃,4 MW/m2热焓值烧蚀下,线烧蚀率仅为传统C/C的1/10。其耗散防热原理包含了以往的汇热防热、辐射防热、烧蚀防热、发汗防热等防热形式,增加了相变反应防热,是一种新的防热原理,这种高效能、低成本的材料预计具有很好的应用前景,也将推动非平衡条件下的金属化学基础理论突破。本材料研究的科学问题,涉及高温、高压、高速气流冲刷等非平衡状态的化学反应问题、金属流动问题等,这些问题的研究必将推动材料科学与传热学、流体力学、燃烧化学、气动力学等学科的交叉互动和新的发展。     

Thermal Behavior of Ursodeoxycholic Acid in Urea: Identification of Anomalous Peak in the Thermal Analysis

The objective of this study was to clarify the thermal behavior of ursodeoxycholic acid (UDCA) in mixtures with urea. Physical mixtures of UDCA and urea in various ratios were prepared, and the thermal analysis of these sample mixtures was investigated using conventional differential scanning calorimetry (DSC) and variable-temperature powder X-ray diffractometry (VTXRD). The hot-stage microscopy (HSM) and powder X-ray diffractometry (PXRD) were used as complementary techniques. From the DSC results of all sample mixtures, it was found that there was no endothermic peak at the melting temperature of intact UDCA crystals. The DSC thermograms of each ratio showed only the endothermic peak at about 136°C due to the melt of urea and the anomalous endothermic peak at about 155°C-157°C. The VTXRD study revealed that the crystals of urea completely disappeared at a temperature of 140°C. At this temperature, it was identified that the VTXRD pattern obtained was of UDCA crystals. The crystalline peaks gradually decreased in intensity at a temperature of 150°C. When the temperature was up to 160°C, the identical crystalline peaks of UDCA crystals completely disappeared. It was concluded that the anomalous endothermic peak at 155°C-157°C was the peak due to the dissolution of UDCA crystals in the surrounding melted urea.     

Thermal conductivity of the hydrogen-helium mixture

Test data are presented pertaining to the thermal conductivity of the hydrogen-helium mixture over the 0–150C temperature range. Discussed is also the existence of a minimum in the composition characteristic of thermal conductivity within a wide temperature range.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 24, No. 4, pp. 657–662, April, 1973.