Contact resistance, polymer thermal conductivity

Other models take into consideration the effects of shape, size, and interfacial resistance on thermal conductivity. However, these models are unable to predict the effective thermal conductivity accurately if contact among filler particles exists. The Cheng and Vachon (Tavman 2003) model assumes a parabolic distribution of disperse phase (spheres or fibers) in a solid matrix. When k, > kp, thermal conductivity of the polymer composite is given by equation (11.7) ... 

There are a number of difficulties that may be encountered in measuring thermal conductivity in the solid and melt states. Problems such as thermal contact resistance and shrinkage are intrinsic to the polymer system and appear regardless of the method used for the measurements. 

The above equation should be used with caution, however, because it does not account for the quality of interfacial contact between the plastic and the filler system. Poor interfacial contact has the same effect as a thermal contact resistance and can result in a significant lowering in the ability of the highly conducting filler particles to transmit heat to the low-conductivity polymer matrix. What complicates the matter further is that these systems may possess good interfacial contact while the polymer matrix is molten but then become lower in thermal conductivity as interfacial contact resistance develops between the filler and the now-solidified polymer. This can be particularly confusing in the case of some filled semicrystalline polymers, where the appearance of the crystalline phase upon solidification should result in increased thermal conductivity, while the actual value appears to decrease. For this reason, it is considered safer to measure the thermal conductivity of filled materials. 

Contact resistance between matrix and particles (known as Kapitza resistance) was investigated by Hasselman (1987), who developed formulas for spherical, cylindrical, and flat-plate geometry to estimate the effective thermal conductivity of polymer composites with interfacial thermal barrier resistance—equation (11.9). 

The polymer performance and production efficiency can be enhanced as a function of the basic features of the reinforcement fillers. In the attempt to achieve fillers with increased performance, the following features must be monitored density, flame retardancy, mechanical resistance, thermal conductivity, and magnetic properties. Nanoparticles of carbides, nitrides, and carbonitrides can be used to reinforce polymer matrix nanocomposites with desirable thermal conductivity. However, current trends in the design of these materials reveal that is not enough to choose a wellperforming material for each component of the heat dissipation path. In addition, careful attention must be paid to the manner in which these materials interact with each other. A filler that conducts heat well but does not wet the matrix may lead to poor results compared to a lower conductivity filler that does wet the matrix. In other words, a major fact that leads to interfacial resistance is faulty physical contact between filler and matrix, which primarily depends on surface wettability (Han and Fina2011). 

Abstract. The context of this work is the enhancement of the thermal conductivity of polymer by adding conductive particles. It will be shown how we can use effective thermal conductivity models to investigate effect of various factors such as the volume fraction of filler, matrix thermal conductivity, thermal contact resistance, and inner diameter for hollow particles. Analytical models for lower bounds and finite element models will be discussed. It is shown that one can get some insights from effective thermal conductivity models for the tailoring of conductive composite, therefore reducing the amount of experimental work. 

Another mode of operation derived from contact AFM that is relevant to polymer studies is the scanning thermal microscope (SThM) [118, 146], In SThM the tip is a special device that has a resistive element. If a current is passed through this resistive element, its temperature depends on the heat transferred to the specimen. It thus acts as a thermal probe, as seen for example in Fig. 5.36. During scanning, thermal control of the probe can be used to generate images based on variation of either sample temperature or thermal conductivity. Filled polymers and polymer blends are candidates for this kind of study, but the resolution is relatively poor. Microfabricated thermal probes can give a resolution of 100 nm. 

Thermal impedence was affected by crosslinker and filler amount in a statistically significant way. As the filler particles are more conductive than the polymer, the greater the amount of filler present, the higher the composite conductivity. Crosslinker amount affected the hardness of the samples. The hardness of the composite affected its conformability, which in turn affected the contact resistance between the composite and test device. Additionally, the test method for thermal impedence was responsible for some of this effect. The test fixture... 

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