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Thermal Conductivity

Polymers are inherently thermally insulating, with thermal conductivity values always lower than 0.5 W/mK. The main mechanism of heat conduction in polymers is thermal vibrations of the atoms (phonons), since free movement of electrons is not possible. Therefore, the crystallinity of the polymer strongly affects its thermal conductivity, which ranges from 0.1 to 0.2 W/mK for amorphous polymers (e.g., PS) to 0.4-0.5 W/mK for highly crystalline polymers (e.g., HDPE). For some applications, thermally conductive fillers have to be added in order to provide higher values of thermal conductivity. Traditional thermally conductive fillers include metal powders (e.g., silver, copper, aluminum, nickel) or ceramic powders (e.g., boron nitrides, aluminum nitride). However, due to the high density of metals and high cost of ceramic fillers, carbon-based fillers seem to be the most promising thermally conductive additives (Ebadi-Dehaghani and Nazempour 2012). Diamond is electrically insulating but has a very high thermal conductivity (>2000 W/mK); however, diamond powders are very expensive and highly abrasive and rarely used as additives in polymers. Graphite is well known as a good thermally conductive additive. The thermal properties of graphite are highly anisotropic, since phonons propagate very quickly along the graphene planes but are slower to travel from one plane to another (>2000 W/mK in plane, ^10 W/mK through plane). In particular, expanded graphite, due to the particular morphology, high aspect ratio of the particles, and fair dispersion in the polymer matrix, is outperforming in terms of thermal conductivity compared to other carbon-based fillers like tubular and fibrous carbon (Debelak and Lafdi 2007). Compared to metallic powders, graphite has a much lower density, therefore allowing a much higher thermal conductivity at the same weight percent in the polymer. Moreover, metallic powders are quite abrasive and can lead to a higher wear of the equipment, whereas graphite has low hardness and is a chemically inert solid lubricant. For applications where high electrical resistivity is required, ceramic powders (e.g., boron nitride, aluminum oxide, and nitride) can be employed, but the thermal conductivity is lower compared to graphite at the same loading. For loading levels below the electrical percolation threshold or in combination with ceramic powders, the use of graphite allows to manufacture thermally conductive and electrically insulating polymer compounds. For loading levels above the electrical percolation threshold, graphite remains the best solution for making polymers thermally conductive when electrical conductivity is also wished or tolerated. Recently, silica-coated graphite has been suggested as a possible alternative to ceramic fillers (Choi et al. 2013). The silica coating on the graphite

Illustration of the retardation of phonon transfer and electron blocking effect of silica-coated graphite

Fig. 8 Illustration of the retardation of phonon transfer and electron blocking effect of silica-coated graphite. The size of the red arrow indicates the heat flux (reprinted from Choi et al. (2013) with permission from Elsevier)

particles acts has an electron blocking effect, whereas the phonon transfer is only slightly retarded, as illustrated in Fig. 8. As a consequence, silica-coated graphite polymer composite remains electrically insulating up to very high loadings, while maintaining good thermal conductivity.

Clearly, high thermal conductivity values can be obtained at high filler loadings (see Fig. 6). However, highly loaded compounds bring problems in terms of processability and strongly modify other properties of the compound like density, viscosity, and mechanical properties. The effect of particle shape on thermal conductivity has been investigated for copper (Tekce et al. 2007). Copper fibers are better performing compared to copper platelets and spherical Cu particles. Similar effect is observed also for graphite, where high thermal conductivities can be achieved at much lower loadings with high aspect-ratio expanded graphite compared to standard graphite (Fukushima et al. 2006). It has been reported that large graphite flakes have lower percolation threshold compared to small graphite flakes (Debelak and Lafdi 2007). Synergistic effects between different carbon fillers (graphite, carbon black, carbon fibers) have been also studied (Hauser et al. 2008). The use of synthetic graphite as main filler and carbon black and carbon fibers as minor fillers has been suggested in order to create thermally conductive pathways between graphite particles.

The effect of processing method and conditions has been investigated for carbon-polymer composites (Haddadi-Asl and Mohammadi 1996). The degree of anisotropy of the thermal conductivity depends on the compounding and finishing process. For graphite-loaded polymer parts, the through-plane thermal conductivity at graphite loadings below 30% is rather low when the graphite compound is injection molded (high degree of orientation of graphite particles), while samples processed via compression molding are much more conductive (low degree of orientation). As illustrated in Fig. 9, the difference between compression and injection-molded samples is less important at very high graphite loadings (>50%), where high through-plane thermal conductivity can be achieved also via injection molding. On the other hand, in-plane thermal conductivity of injection-molded pieces is much higher, and graphite compounds show good thermal conductivity

Through-plane thermal conductivity of compression (squares) and injection-molded (circles)i HDPE samples loaded with TIMREX KS44 synthetic graphite

Fig. 9 Through-plane thermal conductivity of compression (squares) and injection-molded (circles)i HDPE samples loaded with TIMREX KS44 synthetic graphite

In-plane thermal conductivity of injection-molded HDPE loaded with TIMREX KS44 synthetic graphite

Fig. 10 In-plane thermal conductivity of injection-molded HDPE loaded with TIMREX KS44 synthetic graphite

already at low loadings and really excellent thermal conductivity at high loadings (>5 W/mK); see Fig. 10. The same conclusions are valid also for expanded graphite- loaded compounds but at much lower loadings; see Fig. 11. In this case, high thermal conductivities can be reached at low C-THERM loadings with weight-saving benefits.

In recent years, there is an increasing demand of thermally conductive polymers for applications that require heat dissipation (heat sinks) or heat exchange.

Through-plane thermal conductivity of compression-molded TIMREX C-THERM 001 (squares) and TIMREX KS44 (triangles) synthetic graphite-loaded HDPE

Fig. 11 Through-plane thermal conductivity of compression-molded TIMREX C-THERM 001 (squares) and TIMREX KS44 (triangles) synthetic graphite-loaded HDPE

Thermally conductive polymers are considered as a good alternative to metals, offering substantial advantages compared to standard metal-based technologies (Cevallos et al. 2012). Filled polymers will never reach the thermal conductivity values of pure metals (>200 W/mK). However, considering the advantages of plastics in terms of weight reduction, design flexibility, corrosion resistance, durability, and manufacturing cost reduction, the achieved thermal conductivity levels (1-30 W/mK) are often sufficient to fulfill the requirements for certain applications. There are several applications that can benefit from thermally conductive polymers, and the targets in terms of thermal conductivity can vary a lot, from 0.8 W/mK for geothermal pipes to 20 W/mK and more for LED heat sinks. Until now, thermally conductive polymers have been used in mass production only in few applications, mainly as heat sink for LED. In the near future, it is expected to use them also for other industrial and automotive sectors (including air-cooled and liquid-cooled systems for batteries, electric vehicle thermal management, and lightning modules). Thermally conductive pipes can be used for geothermal, solar, and floor heating systems. Typical examples are plastic pipes for geothermal applications, where polyethylene (HDPE)- based pipes have almost completely replaced metallic pipes (copper, stainless steel), thanks to their better properties in terms of flexibility, resistance to corrosion, ease of installation, and costs. However, standard PE pipes used in heat exchangers were originally intended for hot and cold water distribution and have low thermal conductivity (0.4 W/(mK)) which represents a limit to their potentialities. In order to build more efficient geothermal systems, pipe materials with better thermal conductivity are required, while keeping good flexibility and mechanical properties. Such pipes with increased thermal conductivity will reduce the thermal resistance of the borehole and decrease the length needed for geothermal heat exchangers, therefore lowering system costs. Calculations indicate that doubling the thermal conductivity of the pipes to 0.8 W/mK by the addition of graphite is sufficient to significantly reduce the borehole thermal resistivity and consequently of the borehole heat exchanger length (Gilardi and Bonacchi 2012).

 
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