Effect of Fillers on Thermal Conductivity

A is the shape parameter that increases with aspect ratio 02 is the volume fraction of the filler 0 is the maximum filler level while maintaining a continuous matrix phase (packing factor) 

B is calculated from the ratio of thermal conductivities of the two phases and as in Eq. (2.16). 

Graphs of the actual and predicted thermal conductivities for a silver-filled epoxy composition are shown in Fig. (2.16). 

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The higher thermal conductivity of inorganic fillers increases the thermal conductivity of filled polymers. Nevertheless, a sharp decrease in thermal conductivity around the melting temperature of crystalline polymers can still be seen with filled materials. The effect of filler on thermal conductivity for PE-LD is shown in Fig. 2.5 [22], This figure shows the effect of fiber orientation as well as the effect of quartz powder on the thermal conductivity of low density polyethylene. 

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. 

Electrically insulating and thermally conductive qualities are important in computer chips fabrication. One approach taken is based on boron nitride fillers which offers these two properties. There is also a need to develop materials which are thermally conductive but electrically insulating in high humidity conditions. Polyurethane composites filled with aluminum oxide or carbon fiber can be used for this application. Figure 19.15 shows the effect of the amount of filler on thermal... 

For many applications of filled polymers, knowledge of properties such as permeability, thermal and electrical conductivities, coefficients of thermal expansion, and density is important. In comparison with the effects of fillers on mechanical behavior, much less attention has been given to such properties of polymeric composites. Fortunately, the laws of transport phenomena for electrical and thermal conductivity, magnetic permeability, and dielectric constants often are similar in form, so that with appropriate changes in nomenclature and allowance for intrinsic differences in detail, a general solution can often be used as a basis for characterizing several types of transport behavior. Useful treatments also exist for density and thermal expansion. 

Polymer-matrix composites have been used as one of the most common packaging materials for encapsulating a variety of electronic components for dissipating heat [14]. In this section, 3D AlN nanowhiskers with brush-hke structure were filled into the polymer matrix to enhance its thermal conductivity. The 3D brush-hke AlN fiUers were fabricated by CS process [7a], as iUustrated in Section 3.2. The use of AlN as a filler candidate to enhance the thermal conductivity of the polymer is attributed to its attractive properties such as high thermal conductivity, high electrical resistivity, and good chemical stabihty with polymers [1]. To explore the promoting effect of the 3D brush-hke AIN fillers on thermal conductivity, three types of AIN fillers with different brush-hke filler aspect ratio were added into polymer matrix to fabricate a series of composites and their thermal conductivities were measured. The results demonstrated that the 3D brush-hke AIN nanowhiskers fillers can effectively enhance the thermal conductivity of the polymer composite. 

The effect of the detrimental exothermic reaction is assisted by the low thermal conductivity of the adhesive/resin itself and the FRP adherends. If the dimensions cannot be changed, the heat build-up should be limited, for example, by adding fillers into the adhesive/resin and thus reducing the mass of thermoset polymer in the adhesive/resin. Depending on the type of filler, the thermal conductivity may also be improved. In any case, the adverse impact of the above points should be reduced by the design or other appropriate precautions. 

Titanium or beryllium oxide also provides a degree of improvement in thermal conductivity to epoxy systems. Magnesium oxide and aluminum oxide have also been commonly used for this purpose, although the degree of improvement is not as great as with the fillers discussed above. The effect of various fillers on the thermal conductivity of cured adhesive is shown in Fig. 9.6. The incorporation of metal fibers with metal powders has been shown to provide synergistic improvement to the thermal conductivity of adhesive systems,... 

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. 

EFFECT OF THE TYPE AND AMOUNT OF FILLER ON THE THERMAL CONDUCTIVITY OF NONPOROUS RUBBERS. 

In this chapter, the study carried out on nanofillers reinforced natural/synthetic rubber has been discussed. After a description on the NR rubber and CaCOs as filler, the development of synthetic composites with the incorporation of micro and nano-CaC03 as a filler material has also been discussed for comparative study. In particular, the role of fillers on the property modification of rubber properties, such as surface properties, mechanical strength, thermal conductivity, and permittivity has been mentioned. The effectiveness of this coating was demonstrated. The importance of well-dispersed nanoparticles on the improvement of the mechanical and electrical properties of polymers is also emphasized. However, one of the problems encountered is that the nanoparticles agglomerate easily because of their high surface energy. 

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) ... 

In this expresdon, Cj is a term attributed to the effect of the filler on the cr taUinity and Ikikx, thermal conductivity of the polymer matrix. Since k can be measured as a function of polymer crystallinity, and the effect of filler type and amcentration on crystallinity can be measured independently of tiiermal conductivity me irements, the refinement of incorporating the term Cl is unnecessary. Agari determined Cj to be very dose to 1 for most of the compositions studied by curve fitting Eq. (20) to data. Making Cj = 1 simplifies Eq. (20), resulting in ... 

In this study, the main goal was the elaboration of a thermal conductive CPC (Conductive Polymer Composite) that will be used in thermal solar panel. In fact, the conductive polymer composite was obtained by blending bio-polymer matrix by different percentages of fillers (eGR/CNT). Effect of fillers percentage on thermal conductivity, solar absorbance and total emittance was investigated. 

A.5 Effect of CaCOs on the Thermal Properties of Polypropylene. CaCOs is added to polymers as a filler with the intent of lowering material costs. Calculate the thermal conductivity, the heat capacity, and the density of a polypropylene composite containing 40 wt% CaCOs. Take the following properties for PP ... 

The thermal properties of fillers differ significantly from those of thermoplastics. This has a beneficial effect on productivity and processing. Decreased heat capacity and increased heat conductivity reduce cooling time [16]. Changing thermal properties of the composites result in a modification of the skin-core morphology of crystalline polymers and thus in the properties of injection molded parts as well. Large differences in the thermal properties of the components, on the other hand, lead to the development of thermal stresses, which also influence the performance of the composite under external load. 

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