Thermal conductivities fillers affecting

The thermal conductivity of CNT composites is affected by the volume fraction of carbon nanotube filler. Yang et al. [40] studied the effect of CNT contents in the effective thermal conductivity. They display that the experimental results indicate that the thermal conductivity increase as MWCNT content increase (as shown in figure 7). The thermal conductivity can be increased by more than 100% by adding a small quantity of MWCNT. This result confirms that MWCNT are high thermal conductivity fillers which can be used to improve the thermal transport of composites. 

Heat conductivity of composite materials are severely and adversely affected by structural defects in the material. These defects are due to voids, uneven distribution of filler, agglomerates of some materials, unwetted particles, etc. Figure 15.18 shows the effect of filler concentration on thermal conductivity of polyethylene. Graphite, which is a heat conductive material, increases conductivity at a substantially lower concentration than does quartz. These data agree with the theoretical predictions of model. Figure 15.19 shows the effect of volume content and aspect ratio of carbon fiber on thermal conductivity. This figure should be compared with Figure 15.17 to see that, unlike electric conductivity which does depend on the aspect ratio of the carbon fiber, the thermal conductivity is only dependent on fiber concentration and increases as it increases. 

For a given polymeric structure, the morphology (crystallinity and orientation), formulation (additives, fillers and impurities), humidity (especially for polar polymers), temperature, and pressure, are the most important factors which affect the thennal conductivity. References [1-8] review many of these factors. In addition, see Bigg [14] and Ross et al [15] for detailed treatments of the effects of fillers and of pressure, respectively, on thermal conductivity. 

Electrical properties. Fillers and additives significantly increase the porosity of polytetrafluoroethylene compounds. Electrical properties are affected by the void content as well as the filler characteristics. Dielectric strength drops while dielectric constant and dissipation factor rise. Metals, carbon, and graphite increase the thermal conductivity of PTFE compounds. Tables 3.19 and 3.20 present electrical properties of a few common compounds. 

The incorporation of fillers can induce modifications in the thermal properties of the polymers. Factors that affect the thermal conductivity of composites are the dispersion and orientation of the filler particles, the filler aspect ratio, and the relative ratio of thermal conductivity of the filler and the matrix. The thermal conductivity was found to be increased when the Ti02 volume fraction increases. The measured values of thermal conductivity have been compared to different theoretical models. 

The thermophysical properties of multiphase systems are affected by matrix and filler characteristics. In the case of the polymer phase, the microstructure is the most important feature that influences thermal conduction ability. When discussing the filler, one must take into consideration filler physicochanical properties but also several microstructural parameters, such as the diameter, length, shape, distribution, volume fraction, the alignment, and the packing arrangement. Fillers may be in the form of fibers or particles uniformly or randomly placed in the polymer matrix material. Therefore, thermal conduction of particle-filled polymers is isotropic, although... 

In this work, researchers extruded various formulations and then injection molded these formulations into test specimens. The matrix material used was a liquid crystal polymer, which is a thermoplastic that can be lemelted and used again. Two carbon fillers were used carbon black and synthrtic graphite. Varying amounts of single fillers (2.5 to 15 wt% carbon black and 10 to 75 wt% synthetic graphite) were added to the liquid crystal polymer. The goal of this project was to drtermine how various amounts of these single carbon fillers affected the composite thermal and electrical conductivity. 

We also considered relationship between higher-order structure and thermal conductivity of molded products. Polymer orientation became higher near the gate than that at flow end, while thermal conductivity increased with increasing distance from gate. This tendency indicates that not only filler orientation but also polymer orientation affects thermal conductivity. It seemed that efficiency of heat transfer along polymer chain was greater than that of vertical direction of polymer chain. 

The effect of thermally conductive particles on the thermal properties of silicone rubber was studied. Different sized aluminum oxide was blended with addition cured silicone resin at various crosslink densities and filler loading levels. Thermal impedance of each sample was measured. Statistical analysis of the experimental data showed that hardness was not affected by filler type/size or filler amount however, the amount of crosslinker was statistically significant with respect to hardness. 

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. Deformation of the material during the test was found to influence the results of the testing. 

Classification by particle size is helpful in classification since particle size will affect performance but, by itself, falls short as a criterion when selecting fillers for applications which require certain levels of conductivity (thermal or electric) or of chemical interaction, etc. In one publication, materials were divided into particulates, fibers, and colorants. These distinctions are not helpful for a material designer. For a classification to be useful in filler applications, it must include the most important properties of fillers which affect the resultant material. The eight most important are as follows ... 

Applications. Optical microscopy finds several important applications in filled systems, including observation of crystallization and formation of spherulites and phase morphology of polymer blends. " In the first case, important information can be obtained on the effect of filler on matrix crystallization. In polymer blends, fillers may affect phase separation or may be preferentially located in one phase, affecting many physical properties such as conductivity (both thermal and electrical) and mechanical performance. 

Obviously, aggregate size distribution characterization in the mix is very delicate. Some transmission electron microscopy observations have been conducted on microtome thin cuts, but such characterizations are restricted to a small number of aggregates and can only lead to qualitative conclusions [23]. Direct characterization of object distribution in tie mix has also been conducted using x-ray [105] or neutron diffraction, but such approaches are strongly limited by the high concentration of filler objects and their refraction index, which is relatively close to that of rubber. One other way to characterize object size distribution is to extract the filler from the mix by thermal or catalyzed polymer decomposition these procedures probably greatly affect object size, because of possible reagglomerations. 

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