Natural thermal radiation of heat-absorbing thermal vacuum chamber shields

A simplified method for computing the natural thermal radiation of heat-absorbing shields of vacuum chambers is elucidated; computational dependences are presented for shields of herringbone outlines and the influence of the geometric profile characteristics on the magnitude of the natural radiation is shown.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 30, No. 2, pp. 317–321, February, 1976.     

Interpretation of the thermal curve in differential thermal analysis

On the basis of an analysis of the heat transfer between samples and the surrounding medium, principles are formulated for the correct interpretation of the thermal curves in DTA. The influence of the type of phase transformation and the test conditions on the form of the thermal curve is shown.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 36, No. 3, pp. 480–486, March, 1979.     

热传导对双陶瓷热障涂层隔热效果影响研究

为了更好地设计双陶瓷热障涂层结构,考察在制备和服役过程中热导率的变化对隔热效果的影响,建立了双陶瓷热障涂层半透明数学模型,采用有限元ANSYS软件模拟了稳态隔热效果.结果表明:顶层陶瓷层的热导率增大降低了隔热效果,且随顶层厚度增加隔热效果降低幅度增大;第2层陶瓷层的热导率增大降低了隔热效果,且随顶层厚度增加隔热效果降低幅度减小;陶瓷层半透明且衰减系数很小时,顶层厚度增加,隔热效果先快速后缓慢增加至不变甚至略有降低,且远低于相同条件下不透明时.顶层陶瓷层热导率变化对隔热效果影响大于第2层陶瓷层.     

Measurements of the thermal gradient over EB-PVD thermal barrier coatings

Conventional two-layered structure thermal barrier coatings (TBCs), graded thermal barrier coatings (GTBCs) and graded thermal barrier coatings with micropores were prepared onto superalloy DZ22 tube by electron beam physical vapor deposition (EB-PVD). Thermal gradient of the TBCs was evaluated by embedding two thermal couples in the surfaces of the tube and the top coat at different surrounding temperatures with and without cooling gas flowing through the tube. The results showed that higher thermal gradient could be achieved for the GTBCs with micropores compared to the two-layered structure TBCs and GTBCs. However, after the samples were heated at 1050°C, the thermal gradient for the GTBCs with or without micropores decreased with the increase of heating time. On the other hand, the thermal gradient for the TBCs increased with the increase of heating time. Cross-section observations by scanning electron microscopy showed that the change in microstructure was the main reason for the change of the thermal gradient.     

On the thermal expansion of composite materials and cross-property connection between thermal expansion and thermal conductivity

The paper focuses on the quantitative characterization of heterogeneous microstructures from the point of view of the material’s thermal expansion. First, we derive expression for the second rank thermal expansion contribution tensor of an inhomogeneity and specify it for various inhomogeneity shapes. Case of a spheroidal inhomogeneity in an isotropic material is discussed in detail. Thermal expansion contribution tensor is used as a basic building block to calculate effective thermal expansion of a heterogeneous material and to derive explicit cross-property connection between thermal expansion and thermal resistivity of a composite. We compare our results with experimental data available in literature and with other approaches.     

Simultaneous measurement of the thermal conductivity and thermal diffusivity of liquids

In theory, the hot-wire technique for measuring the thermal conductivity of liquids can be used simultaneously to determine the thermal diffusivity. In practice, however, the latter property has so far been determined only with moderate accuracy because of (a) inaccurate bridge balancing due to drift problems, (b) parasitic capacities that delay the heating, and (c) poor precision in the determination of the time. A new measurement procedure has been developed which features (a) a short measuring time, (b) a reduced significance of the balancing technique, (c) a good reproducibility, and (d) a low sensitivity to most error sources. Thermal conductivity and thermal diffusivity results using this procedure, for toluene and n-heptane, which are the generally accepted standards for thermal conductivity, are presented and compared with results from other sources.     

From thermal conductive to thermal insulating:Effect of carbon vacancy content on lattice thermal conductivity of ZrCx

Lattice thermal conductivities of zirconium carbide(ZrCx,x=1,0.75 and 0.5)ceramics with different car-bon vacancy concentrations were calculated using a combination of first-principles calculations and the Debye-Callaway model.The Grüneisen parameters,Debye temperatures,and phonon group velocities were deduced from phonon dispersions of ZrCx determined using first-principles calculations.In addi-tion,the effects of average atomic mass,grain size,average atomic volume and Zr isotopes on the lattice thermal conductivities of ZrCx were analyzed using phonon scattering models.The lattice thermal con-ductivity decreased as temperature increased for ZrC,ZrC0.75 and ZrC0.5(Zr2C),and decreased as carbon vacancy concentration increased.Intriguingly,ZrCx can be tailored from a thermal conducting material for ZrC with high lattice thermal conductivity to a thermal insulating material for ZrC0.5 with low lattice thermal conductivity.Thus,it opens a window to tune the thermal properties of ZrCx by controlling the carbon vacancy content.     

Effect of thermal coarsening on the thermal conductivity of nanoporous gold

The thermal conductivities of nanoporous gold (NPG) microwires annealed at different temperatures have been measured in the temperature range from 100 to 320 K. Considering the electron-surface scattering, the thermal conductivity is expected to increase with the increase of ligament diameter. However, the thermal conductivity of NPG microwire is found to decrease after thermal coarsening, and has a maximum value at around 250 K for the as-dealloyed sample. We suggest that the defects accumulating at a relatively high temperature and the reduction in defect spacing may cause these temperature behaviors of thermal conductivity. Taking into account the electron scattering on ligament surfaces and defects, a modified theoretical model for the thermal conductivity of nanoporous metal is proposed to agree with our experimental results.     

EB-PVD process and thermal properties of hafnia-based thermal barrier coating

Thermal barrier coatings (TBCs) are being developed for the key technology of gas turbine and diesel engine applications. In general, 8 mass% Y₂O₃–ZrO₂ (8YSZ) coating materials are used as the top coating of TBCs. The development of hafnia-based TBC was started in order to realize the high reliability and durability in comparison with 8YSZ, and the 7.5 mass% Y₂O₃–HfO₂ (7.5YSH) was selected for coating material. By the investigation of electron-beam physical vapor deposition (EB-PVD) process using 7.5YSH ceramic ingot, 7.5YSH top coating with about 200 µm thickness could be formed. The microstructure of the 7.5YSH coated at coating temperature of 850 °C showed columnars of laminated thin crystals. On the other hand, the structure of the 7.5YSH coated at coating temperature of 950 °C showed solid columnars. From the result of sintering behavior obtained by heating test of 7.5YSH coating, it was recognized that the thermal durability of 7.5YSH coating was improved up to about 100 °C in comparison with 8YSZ coating. This tendency was confirmed by the experimental result of the thermal expansion characteristics of sintered 7.5YSH and 8YSZ.

©2003 Elsevier Science Ltd. All rights reserved.     

Phase composition and thermal conductivity of zirconia-based thermal barrier coatings

Atmospheric plasma spraying of powder materials has been used to produce thermal barrier coatings (TBCs) based on ZrO₂ stabilized with 7 wt % Y₂O₃, including coatings doped with neodymium and samarium oxides, for state-of-the-art and next-generation high-temperature gas turbine engines. Doping with neodymium and samarium oxides has been shown to reduce the thermal conductivity of the TBCs by 10–20%. At the same time, changes in the phase composition, crystal structure parameters, and microstructure of the TBCs during heat treatment at the service temperature lead to an increase in the thermal conductivity of all the coatings by 50–70%.     

Accurate measurement of the thermal conductivity and thermal diffusivity of toluene andn-heptane

Accurate and simultaneous measurements of the thermal conductivity and thermal diffusivity of toluene andn-heptane were made with an improved transient hot-wire method by using a transfer function having a feedback loop, in the temperature range of 0 to 45°C at atmospheric pressure. The accuracy of the empirical equations as a function of temperature is estimated to be 0.4 to 0.5% for the thermal conductivity and about 4% for the thermal diffusivity. Paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

Numerical simulation of thermal conductivity of MMCs: effect of thermal interface resistance

Abstract

The thermal conductivity of metal matrix composites is investigated by computational simulations, in which the effect of a thermal barrier resistance between the constituent phases is explicitly taken into account. A numerical unit cell approach, which is based on the finite element method, an analytical mean field method of the Mori–Tanaka type and bounding techniques are employed. To predict the effective conductivities of fibre composites two different types of unit cell are utilised for the numerical studies. Two dimensional unit cells are developed which allow for investigations of aligned, continuous fibre reinforced composites while three dimensional unit cells are employed to study a large variety of different arrangements of non-staggered and staggered aligned short fibres. In the case of short fibres the thermal barrier resistances of the end faces and of the cylindrical surfaces are modelled independently, which allows one to study both their individual and their combined influences on the overall behaviour. Results are presented for carbon fibre/copper composites and their overall thermal conductivities are investigated in terms of interfacial thermal barriers and microtopologies.     

Negative thermal expansion structures constructed from positive thermal expansion trusses

The negative thermal expansivity of a type of space frame structure is investigated herein. On the basis of space frame structure consisting of tetrahedral representative volume elements, a volumetric thermal strain, and a volumetric coefficient of thermal expansion (CTE) models are developed in this article for a special category of tetrahedron that is made from two types of materials, each for the three apex and the three base rods. Based on these models, the conditions for attaining negative volumetric thermal strain and negative coefficient of volumetric thermal expansion are established. Plotted results reveal a trend in which the extent of negative expansivity is increased for lower apex-to-base rod length and CTE ratios, and higher base rod CTE.     

Simultaneous measurements of thermal conductivity and thermal diffusivity of liquids under microgravity conditions

A transient short-hot-wire technique is proposed and used to measure the thermal conductivity and thermal diffusivity of liquids simultaneously. The method is based on the numerical evaluation of unsteady heat conduction from a wire with the same length diameter ratio and boundary conditions as those in the experiments. To confirm the applicability and accuracy of this method. Measurements were made for five sample liquids with known thermophysical properties and were performed under both normal gravity and microgravity conditions. The results reveal that the present method determines both the thermal conductivity and the diffusivity within 2 and 5%. respectively. The microgravity experiments clearly indicate that even under normal gravity conditions, natural-convection effects are negligible for at least l s after the start of heating. This method would be particularly suitable for a valuable and expensive liquid, and has a potential for application to electrically conducting and or corrosive liquids when the probe is effectively coated with an insulating and anticorrosive material. Paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

Determination of thermal diffusion coefficients using thermal field-flow fractionation and Mark-Houwink constants

In this paper, a new approach for determination of thermal diffusion coefficient D(T) values using thermal field-flow fractionation retention data and Mark-Houwink constants is reported. The method utilizes the availability of Mark-Houwink constants from the literature together with thermal field-flow fractionation retention data to calculate D(T) values for both narrowly and broadly dispersed polymeric samples. The proposed method was tested with thermal field-flow fractionation data from a number of published papers. In general, D(T) results obtained from the new approach agree well with those reported from the literature. Since Mark-Houwink constants have been extensively tabulated, the new method can be used to generate a broad database of D(T) values for use in the characterization of polymers and in studies of the thermal diffusion phenomenon.     

Effect of metal catalyst thermal expansion on thermal stability of synthetic diamonds

Thermal stability of synthetic diamonds grown in various metallic solutions in terms of their residual strength after thermal loading is studied. The micromechanical, cohesive zone model of crack initiation and propagation in the diamond crystal in a vicinity of metal inclusion has been developed. The inclusion-diamond thermal expansion mismatch is found to be a key factor responsible for the strength reduction of diamond crystals after high-temperature treatment. The results of computer simulation satisfactory correlate with the available experimental data.     

Effect of thermometer thermal conductivity and dimensions on accuracy of thermal flux measurements

Expressions are presented for evaluating errors in steady-state thermal flux measurements within massive objects with consideration of the effect of thermal conductivity and size of the active thermometer zone.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 55, No. 5, pp. 847–853, November, 1988.     

Approximation of inhomogeneous thermal lines via series connection of homogeneous thermal lines

A method is given for approximating an arbitrary inhomogeneous thermal line (ITL) as a series connection of homogeneous thermal lines (HTL). The method is compared with other approximation methods.     

An apparatus and comparative method for measuring the thermal conductivities of thermal insulators

A method and apparatus are considered for determining the temperature dependence of the thermal conductivity, mainly for thermal insulators. The method enables one to calibrate the apparatus on standard measures with given sizes and shapes.Translated from Izmeritel'naya Tekhnika, No. 4, pp. 35–37, April, 1994.     

Failure during thermal cycling of plasma-sprayed thermal barrier coatings

The thermal cycling behavior of plasma-sprayed ZrO₂?12wt.%Y₂O₃ coatings was studied. Coatings were produced with and without bond coats of Ni-Cr-Al-Zr and in some cases the substrates were heated to above the optimum temperature prior to spraying. The coatings (attached to the substrate) were thermal cycled to 1200 °C and their cracking behavior was followed by acoustic emission (AE) techniques. It was possible to examine the failure mechanisms by statistical analysis of the AE data and to evaluate the influence of preheating and bond coating. It is shown that the AE spectrum changes when a bond coat is used because of the presence of microcracks which, in turn, dissipate energy and improve the coating integrity. The preheating effect is reflected by a decrease in the peak count rate and an increase in the temperature at which AE activity is initiated.