Finding of the generalized equation of thermal conductivity for porous heat-insulating materials
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Abstract:
The object of this research is the process of the heat transfer through porous heat insulating materials. The problematic place of research is the absence of a generalized equation of thermal conductivity, which makes unable to predict the effective thermal conductivity of the material at the structure formation stage. The reason of it is lack of complex entrance independent factors of porous structure that influence on the effective thermal conductivity. For determining of this factors the computer simulation was used, it includes three dimensional samples and simulation of thermal process. After it, obtained computer modeling results were confirmed by laboratory experiment with using of the thermal conductivity meter ITP-MH4 of the company «SKB Stroyprybor».The regression equation of thermal conductivity for porous heat-insulating materials was found by the experimental design method, the analysis of it was showed that the most influence (80 %) on coefficient of effective thermal conductivity have the pore diameter along to the heat flow and the total impact of the pore diameter perpendicular to the heat flow with temperature gradient. The thermal conductivity of initial material without pores λmat in investigated range of 0,05 to 0,95 W/(m·K) isn't a significant factor. The temperature gradient doesn't linear and not directly proportional impact on the thermal conductivity of the final material.The generalized equation of thermal conductivity and the main factors, which influence on the coefficient of effective thermal conductivity, allow improving the thermal conductivity of new insulation materials and making it possible to develop a complete theory of thermophysical parameters control of porous heat insulating materials by changing the porous structure.Keywords:
Temperature Gradient
Heat equation
This paper discusses the improvement of operational reliability and lifetime of power electronic modules due to the reduction of the temperature gradient in the semiconductor structures. High temperature gradient in the power electronic modules having a large area of the semiconductor structure is a more affecting issue than the junction temperature. The improvement in the temperature gradient is achieved by varying/degrading the thermal resistance along the heat sink length. A conventional cooling system for three semiconductor modules based on a forced air heat sink was modelled and analysed to derive a reduction rate of the appropriate thermal resistances. Implementation of the forced air heat sink having non-uniformed thermal resistances along the heat sink length ensures the uniform temperature distribution across the power semiconductors and, therefore, the improved thermal gradient.
Temperature Gradient
Sink (geography)
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The increasingly stringent requirements of thermal engineering standards lead to ever greater thicknesses of thermal insulating layers in the structures, which makes the insulation of the manufacturer to improve material parameters. Thermally reflective insulation currently occupies only a small volume of products in the building industry, but their use in buildings where a large temperature gradient is created for thermal insulation offers many advantages. The pressure on the thermal insulation efficiency is increasing, so that their thickness is as small as possible.
Thermal bridge
Pipe insulation
Dynamic insulation
Temperature Gradient
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To predict the value of thermal contact resistance, the concept of detached thermal contact resistance was introduced. The whole thermal contact resistance in the article is viewed as the connecting in series. The model of a heat channel was established and the distributed equations of the temperature were obtained in order to study the three dimensional phenomenon of heat transfer in the interface of solid. The temperatuer distribution is achieved by applying numerical method to the model. So the detached thermal contact resistance could be calculated and the fitting formula of the thermal contact resistance was obtained. The study shows the formula accords with the result well in most cases. If the model is applied to the equivalent heat channel of the interface, the whole thermal contact resistance of the interface could be predicted.
Thermal contact
Contact resistance
Interface (matter)
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This paper describes the thermal contact resistance and its effect on the performance of thermal interface material. An ASTM D 5470 based apparatus is used to measure the thermal interface resistance. Bulk thermal conductivity of different interface material is measured and compared with manufacturers’ data. Also, the effect of grease void in the contact surface is investigated using the same apparatus. The flat type thermal interface tester is proposed and compared with conventional one to consider the effect of lateral heat flow. The results show that bulk thermal conductivity alone is not the basis to select the interface material because high bulk thermal conductivity interface material can have high thermal contact resistance, and that the center voiding affects the thermal interface resistance seriously. On the aspect of heat flow direction, thermal impedance of the lateral heat flow shows higher than that of the longitudinal heat flow by sixteen percent.
Thermal grease
Interfacial thermal resistance
Thermal effusivity
Thermal contact
Contact resistance
Thermal bridge
Thermal transmittance
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The vertical carbon nanotube arrays (VACNT), as a result of its flexibility and axial high thermal conductivity, exert a huge potential and play an increasingly important role in thermal interface materials. This paper proposed a model which can predict the contact thermal resistance of VACNT. The contact thermal resistance of VACNT under different pressures is calculated and compared with the experimental data. Also, the effect of variations in the surface roughness and VACNT parameters on the contact thermal resistance is investigated. Results show that the theoretical results are in good agreement with the experimental data. The contact thermal resistance is composed of interfacial thermal resistance, constriction thermal resistance, and VACNT resistance. Among which the interfacial thermal resistance is the major thermal resistance. The variations in VACNT length and diameter can change the bending degree of VACNT under the same pressure, which presents important implications on contact thermal resistance and can be used to optimize the contact thermal resistance of VACNT. The surface roughness exerts little effect on contact thermal resistance.
Interfacial thermal resistance
Contact resistance
Contact area
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Inconel
Contact resistance
Thermal contact
Interfacial thermal resistance
Thermal grease
Thermal transmittance
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This paper describes the evaluation of relationship between thermal contact resistance and load with 3 types of TIM (Thermal Interface Materials), and the evaluation of performance of heat spreader. Experimental study was conducted for evaluation of thermal contact resistance, and the conditions of bare contact surface and TIM (copper paste and solder) in between were evaluated. For detection of spread angle in heat spreader, thermal analysis is performed. Experimental results showed contact thermal resistance with solder in between was lowest thermal contact resistance, and the resistance was not dependent on contact pressure. Numerical results showed that the spread angle is less than 45°, when the thickness of the heat spreader is 3 [mm].
Contact resistance
Thermal grease
Thermal contact
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For thermal management of electrical equipment, thermal contact resistance is one of the important parameters. However, thermal contact resistance is dependent on various factors, for example surface roughness, the contact pressure and the hardness of the material. Therefore, quantitative evaluation is difficult. Nowadays, CFD (Computational Fluid Dynamics) analysis is widely used in thermal design of electronics. However, unknown thermal contact resistance is always a problem for accurate temperature estimation. In this study, we examined surface roughness and material hardness dependence of thermal contact resistance and electrical contact resistance for simple estimation of thermal contact resistance. Measurement of thermal contact resistance takes a long time and electrical resistance measurement is much shorter. If thermal contact resistance can be estimated from electrical contact resistance, thermal contact resistance can be known in short time, and this method can support accurate CFD analysis. The materials to be measured are Al1070 and S45C, and three patterns (Ra = 0.2, 3.2, 12.5 μm) of surface roughness are examined. After the measurement of thermal and electrical contact resistance, we examined the ratio between electrical contact resistance and thermal contact resistance for the faster estimation of thermal contact resistance using the concept of Wiedemann-Franz law and Lorentz number like experimental constant.
Contact resistance
Electrical contacts
Thermal contact
Interfacial thermal resistance
Wiedemann–Franz law
Contact area
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Abstract 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 m 2 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.
Interfacial thermal resistance
Contact resistance
Thermal contact
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Citations (83)
This study targets to develop a 1-dimensional evaluation method of thermal conduction process around printing papers in DTP (Direct Thermal Printing) process. Our special attention was paid to investigate an evaluation method of thermal conductivity and contact thermal resistance of the printing papers. The evaluation of thermal conductivity of the printing papers is generally difficult because thermal conductivity of the papers is small like insulation. In addition, contact thermal resistance between the thermal head and the papers cannot be measured directly. Therefore, in this report, the clarification of the level of the thermal conductivity and the contact thermal resistance were targeted by using 1-dimensional thermal conductivity measurement system. From the measurement of the equivalent thermal conductivity, the level of the difference of the thermal conductivity and the contact resistance of the paper was investigated. In addition, the optimum pressing pressure of the platen roller in order to minimize thermal contact resistance is clarified.
Thermal effusivity
Thermal conductivity measurement
Thermal transmittance
Thermal contact
Contact resistance
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