Showing papers on "Thermal contact conductance published in 1989"

Thermal conductivity of dielectric thin films

Abstract: A direct reading thermal comparator has been used to measure the thermal conductivity of dielectric thin‐film coatings. In the past, the thermal comparator has been used extensively to measure the thermal conductivity of bulk solids, liquids, and gases. The technique has been extended to thin‐film materials by making experimental improvements and by the application of an analytical heat flow model. Our technique also allows an estimation of the thermal resistance of the film/substrate interface which is shown to depend on the method of film deposition. The thermal conductivity of most thin films was found to be several orders of magnitude lower than that of the material in bulk form. This difference is attributed to structural disorder of materials deposited in thin‐film form. The experimentation to date has primarily centered on optical coating materials. These coatings, used to enhance the optical properties of components such as lenses and mirrors, are damaged by thermal loads applied in high‐power las...

Refrigerated MR magnet support system

Abstract: A magnet cartridge (17) and thremal shield (25) are supported by three concentric nested thin wall tubes (33,35,37) from a vacuum vessel (11). The innermost tube (33) of thermal inulating material is affixed to magnet cartridge at one end and supports the first stage heat station (57) at the other. The intermediate tube (35) of heat conducting material transfers the load from the inner tube (33) to the outer tube (37), which is affixed to the vacuum vessel. The intermediate tube also thermally connects the first stage heat station to the thermal radiation shield. The outer tube also supports the thermal radiation shield. All of the tube joints rely on epoxy-bonded threads to provide good mechanical strength, low motion thermal contact resistance, and no relative motion.

Non-Fourier Heat Conduction in a Semi-infinite Solid Subjected to Oscillatory Surface Thermal Disturbances

Abstract: Many investigators have explored the effect of non-Fourier conduction in transient processes in recent years. Based on a relaxation model for heat conduction in solids and liquids, the traditional heat diffusion equation is replaced with a hyperbolic equation that accounts for the finite thermal propagation speed. The use of the hyperbolic equation removes the nonphysical phenomenon of the diffusion equation analysis that predicts instantaneous temperature disturbances at all points in the medium for a step heat flux at the boundary. It further removes the peculiarity of an infinite temperature gradient at the boundary as time goes to zero. The objective of this work is to show that the non-Fourier conduction effect can be important even at a long-time after the initial transient if the thermal disturbance is oscillatory with the period of oscillation of the same order of the magnitude as the thermal relaxation time. In particular, the thermal response of a semi-infinite solid subjected to a sinusoidal boundary heat flux condition is generated. The present solution illustrates readily that, in many practical situations such as the repeated irradiation of a solid by a laser with very short pulse width, heat transfer analyses using the traditional Fourier heat diffusion equationmore » can result in significant errors. The current results also suggest that the thermal relaxation time of a solid can be determined by measuring the thermal response of the solid can be determined by measuring the thermal response of the solid irradiated by a high-frequency heat flux.« less

Predicting the thermal conductivity and temperature distribution in aligned fiber composites

Abstract: A numerical model capable of calculating the effective thermal conductivity and temperature distribution in aligned fiber composites, where the ratio of the fiber to matrix conductivities is an order of magnitude greater than unity, was developed. To determine the validity of this model, an experimental investigation was conducted utilizing cylindrical samples whose fiber direction relative to the cylinder axis varied from Odeg to 90deg in 15deg increments. The temperatures, as predicted by the numerical model, were within five to eight percent of the experimentally measured values for all cases considered. Comparison of sample geometry and effective thermal conductivity indicated that the numerical model more accurately predicted the temperature distribution in samples whose fiber angle with respect to the cylinder axis was large and that accurate prediction of the edge effects, particularly in corner regions where these effects dominate, would require a three-dimensional numerical model.

Thermally conductive aluminium nitride-filled epoxy resin (for electronic packaging)

Abstract: The thermal conductivity and the relative permittivity of an AlN particulate-filled epoxy resin have been investigated as a function of the volume loading ratio of AlN and the temperature. A thermal conductivity of 4 W/mK has been achieved. The observed thermal conductivities depend on the overall statistics of the particle arrangement in the epoxy matrix. An analysis led the authors to assumed the presence of clusters of AlN in the epoxy matrix. The temperature dependence of the thermal conductivity can be deduced from the room temperature of the thermal conductivity and the measured coefficients of thermal expansion. >

Effective Thermal Conductivity Within Packed Beds of Spherical Particles

Abstract: Five beds of different materials were evaluated to determine the effective thermal conductivity as a function of the mechanical load on the bed, the conductivity of the bed material, and the interstitial gaseous environment surrounding the bed particles. The effective thermal conductivity of the packed beds were found to be dependent upon the thermal conductivity of the bed material and the axial load

Thermal conductivity of inhomogeneous materials

Abstract: The effective thermal conductivity is calculated from the rate of entropy production per unit volume. Thermal conductivity and the temperature field are expressed in terms of Fourier components and these are related. The rate of entropy production is then obtained in terms of the volume-averaged thermal conductivity and the Fourier components of thermal conductivity. A simple expression for the effective thermal conductivity is found. In the case of striations it leads to well-known results. The formalism is applied to solids with inhomogeneously distributed solutes. It is shown that the thermal conductivity is less than the volume-averaged thermal conductivity and that homogenization by diffusion increases the thermal conductivity. Similar results would apply to the electrical conductivity of inhomogeneous alloys.

Thermal resistance of silicon point contacts

Abstract: We investigate thermal transport on microcontacts established between two silicon wedges. The measurements are carried out with a variable contact radius of 0.3--2 \ensuremath{\mu}m at temperatures from 2 to 300 K. The measured thermal resistance is used to calculate the contact radius with an accuracy better than 50%. Comparison of electrical and thermal transport indicates the existence of a tunnel barrier between the contact legs. Electrical measurements are therefore unreliable to determine the contact size.

Thermal conductivity, heat capacity, and thermal diffusivity of selected commercial AlN substrates

Abstract: The thermal transport properties of four commercially available AlN substrates have been investigated using a combination of steady-state and transient techniques. Measurements of thermal conductivity using a guarded longitudinal heat flow apparatus are in good agreement with published room temperature data (in the range 130–170 W · m−1 · K−1). Laser flash diffusivity measurements combined with heat capacity data yielded anomalously low results. This was determined to be an experimental effect for which a method of correction is presented. Low-temperature measurements of thermal conductivity and heat capacity are used to probe the mechanisms that limit the thermal conductivity in AlN.

Film resistance and constriction effect of current in a contact interface

Abstract: A computer simulation based on the finite-element method was used to study the conductance of a contact interface, and the property of conductance was investigated using a global model of contact. The contact interface of the model consists of real contact part with the area fraction f and noncontact part (gap) with the area fraction 1-f. This conductance was computed against f. It was made clear that a great deal of current can flow through a small real contact area (current constriction effect) and hence the conductance does not decrease until f becomes close to zero. The increase of film resistance of the real conducting area is taken into account and the behavior of the contact conductance is studied. The decrease of constriction effect is quantitatively shown and discussed with respect to the increase of film resistance. >

Quantification of thermal contact conductance in electronic packages

Abstract: Numerically obtained values of typical electronic package interface pressures are presented. Combining these predicted pressures and the results of previous investigations, the individual thermal contact resistances and overall package resistance were determined for an 18-lead plastic dual-in-line package constructed from typical commercially available materials. The results indicate that the summation of the contact resistances may make up as much as 43% of the total package thermal resistance for the packages investigated. The results also indicate that models in which these contact resistances are neglected may predict mean chip temperatures significantly below those actually present. The results also indicate that materials with thermal expansion coefficients below that of silicon tend to result in higher interface pressures and, hence, higher contact conductance than expansion matched materials. However, if placed incorrectly, these higher pressures can have a detrimental effect on the overall reliability of semiconductor devices. >

The effect of contact geometry on the value of contact resistivity extracted from Kelvin structures

Abstract: A finite-element program that can model arbitrarily shaped contacts has been developed. It is used to evaluate the effects that changes in geometry, specific contact resistivity, sheet resistivity, and the modification of sheet resistivity under the contact have on contact systems. The Kelvin test structure is used to illustrate some of its capabilities. It is shown that the contact area of this test structure is of primary importance. The contact shape has little influence on the extracted value of contact resistance. >

Measurement of contact resistance in finned tube heat exchangers

Abstract: Experiments were conducted to measure and evaluate contact resistance in mechanically finned tube heat exchangers with different expansion interferences, fin densities, and initial assembly clearances between the tube and fin collar. Five different test cores were tested in a vacuum environment. The measured contact resistances were compared using a thermal model, and the agreement with reported data is good. Higher contact conductances were predicted by the contact conductance model because the experimental contact conductances were based on full contact ot the tube|FIN interface

A model for including thermal conduction in molecular dynamics simulations

Abstract: A technique is introduced for including thermal conduction in molecular dynamics simulations for solids. We develop a model to allow energy flow between the computational cell and the bulk of the solid when periodic boundary conditions cannot be used. Thermal conduction is achieved by scaling the velocities of atoms in a transitional boundary layer. The scaling factor is obtained from the thermal diffusivity, and the results show good agreement with the solution for a continuous medium at long times. We have investigated the effects of different temperature and size of the system, and of variations in strength parameter, atomic mass, and thermal diffusivity. In all cases, no significant change in simulation results has been found.

Thermal Diffusivity and Conductivity of Composites with Interfacial Thermal Contact Resistance

Abstract: Experimental data for three composite material systems are presented which indicate that the existence of an interfacial thermal barrier resistance can have a significant effect on the effective thermal conductivity of composites.

Variable thermal properties and thermal relaxation time in hyperbolic heat conduction

Abstract: Numerical solutions were obtained for a finite slab with an applied surface heat flux at one boundary using both the hyperbolic (MacCormack's method) and parabolic (Crank-Nicolson method) heat conduction equations. The effects on the temperature distributions of varying density, specific heat, and thermal relaxation time were calculated. Each of these properties had an effect on the thermal front velocity (in the hyperbolic solution) as well as the temperatures in the medium. In the hyperbolic solutions, as the density or specific heat decreased with temperature, both the temperatures within the medium and the thermal front velocity increased. The value taken for the thermal relaxation time was found to determine the 'hyperbolicity' of the heat conduction model. The use of a time dependent relaxation time allowed for solutions where the thermal energy propagated as a high temperature wave initially, but approached a diffusion process more rapidly than was possible with a constant large relaxation time.

Quantitative Thermal Wave Characterization of Coating Adhesion Defects

Abstract: Throughout the development of thermal wave NDE techniques the most common application has been the imaging of subsurface structure of coated and bulk materials [1]. The use of thermal wave imaging for the detection of coating adhesion defects was first proposed by Luukkala and Penttinen [2], and Busse and Ograbek presented the first experimental results on the detection of artificial adhesion defects of a graphite coating on aluminum [3].

Thermal contact conductance of various metals at elevated temperatures

Abstract: The measured values of thermal contact conductance far carbon steel 4130, stainless steel 304, and aluminum 2041-T6 are presented for mean interface temperatures up to 850°F snd moderate rates of heat flux up to 50,000 Btu/hr-ft?. All experiments were performed in air. The thermal contact conductance versus apparent pressure at constant is presented for carbon steel 4130. Results show a rising value of contact conductance with pPessure and a significant hysteresis between loading and unloading. Repeat experiments with the same specimens hut without surface refinishim suggest significant inlfuence of surface oxide. Thermal contact conductance as a function of temperature at constant pressure are presented for carbon steel 4130, stainless steel 304, and aluminum 2041-T6. value of contact conductance at higher temperatures. For carbon steel the increase occurs at temperatures greater than 550°F, while for aluminum the increase is evident for temperatures greater than 200°F . For both aluminum and stainless steel there appears to be an effect due to material creep at elevated temperatures. Elementary calculations suggest that the changes in contact conductance for the temperature range tested are due to thermal and property changes and not due to radiation. Results show a sharply rising

Influence of mechanical design parameters on electrical contact performance

Abstract: Long-time service data as well as data from current cycle testing on electrical contact resistance have been analyzed with respect to mechanical design parameters. The importance of the pressure distribution over the contact surface was studied for a constant surface force. The mechanical stresses acting over the contact surface have been calculated by the finite-element method for the type of contact investigated. It is found that a high specific contact pressure promotes lower contact resistance and favors their long-time stability. The pressure distribution on the contact surface has a significant influence on the aging properties of the connectors studied. It was confirmed that the aging behavior is strongly affected by the preparation methods. The aging mechanisms detected on the contact surfaces studied are fretting and formation of intermetallic phases. >

Thermal Conductivity of CuSbSe2 and CuBiSe2 in the Solid and Liquid State

Abstract: Thermal Conductivity of CuSbSe2 and CuBiSe2 have been studied in the solid and liquid state in a wide range of temperatures. Measurements of thermal conductivity were carried out using the concentric cylinder method which is based on the flow of heat through a cylindrical wall.

Irregularity of film resistivity in a contact interface and contact conductance

Abstract: The contact conductances are computed and the global behavior of contact phenomena is studied, using the same method and model as in previous papers, for the case where the film resistivity of a contact interface irregularly fluctuates against the position of the interface. The model of contact consists of two unit conducting cubes and a contact interface between the cubes. The interface is divided into a great number of square spots, and the resistivity of each spot is randomly determined, but the statistical distribution of the resistivities is assumed to be the normal (Gauss) distribution or error function. It is clarified that the contact conductance does not become low when the standard deviation of the normal distribution is large (i.e. the positional variation of the resistivity is large and spots with high and low resistivity coexist in the same interface) even if the average value of resistivity greatly rises. >

Changeover contact for power transformer screw leading-out terminals

Abstract: The utility model relates to a copper aluminium changeover contact which is connected with the screw of the power transformer. The contact conductance face is larger, the sealing performance is good, and the joint can not be burned.

Thermal dispersion in AC calorimetry of needle-shaped specimens

Abstract: The propagation of thermal waves in thin needle-shaped samples is described. The waves are generated by Joule power oscillation at one end of the sample. The amplitude and phase of the temperature oscillation are dependent upon thermal diffusivity. At low frequencies no phase shift appeared and only thermal conductivity was involved in the measurement. Specific heat was evaluated on the basis of thermal diffusivity and thermal conductivity.

Thermal conductance of cylindrical joints - A critical review

Abstract: Presented here is a review of the theoretical and experimental studies conducted over the past 30 years of the heat transfer characteristics of compound cylinders. For the purpose of this review, such work has been broadly classified into two categories—application oriented studies and fundamental studies. A majority of the application oriented studies deal with heat transfer in finned tubes as used in heat exchangers and heat transfer in nuclear fuel elements. In all cases, the heat flow is considered to be radially outward. The results of this review indicate that the contact resistance in heat exchangers is generally measured indirectly and is often assumed to be constant. The correlations proposed usually apply only to specific types of tubing and prescribed materials. The fundamental studies, in general, consider idealized surfaces with no large-scale irregularities such as waviness or. out-of-roundness. In most of the studies, the effect of the axial temperature gradient is not considered. Finally, it was found that there exists very little experimental data dealing with the fundamental aspects of heat transfer through cylindrical joints.