thermally conductive | Technical feature
Figure 3: Data showing the effect of processing technique thermal conductivity of a graphite loaded HDPE compound Source: Imerys Graphite & Carbon
Figure 4: In-plane and through-plane thermal conductivity of injection molded HDPE plaques loaded with different graphite grades Source: Imerys Graphite & Carbon
particle. In fact, the graphite particles cannot be considered as single crystals but as single crystal domains of various sizes and orientation. The lower aspect ratio of the KS grade, and the higher
random orientation of the crystals in the particle, can also explain the higher through-plane conductivity observed. In this case, the platelets are less oriented, decreasing the phonon path in the through-plane direction. Particle size is also a fundamental parameter as it is directly related to the number of thermal contacts. As already mentioned, the main barrier to phonons is the contact region between the particles. It is then understandable that limiting the number of these thermal barriers is fundamental to achieving high thermal conductivity. This explains why compounds made with very large fl ake particles such as the natural graphite Timrex 80x150/96 have high in plane thermal conductivity (low number of thermal contact resistance points) when compared to materials containing the smaller Timrex PP44. Recently, the high aspect ratio C-Therm graphite has
been developed to give high thermal conductivity at lower loading. It should be noted that polymer com- pounds at the same thermal conductivity level made with either graphite or C-Therm display similar mechanical and rheological properties[7]. However, due to the lower loading needed to achieve high conductiv- ity, C-Therm is a good additive choice for applications that require weight saving[8] such as, automotive parts, or in formulations that require the addition of other mineral reinforcing additives or glass fi bres.
The choice of graphite grade depends on the fi nal
application and heat sink design. Injection moulded heat sinks, due to the presence of ribs in their design, will need compounds containing large graphite particles with high aspect ratio to enhance the in-plane conductivity, while for extruded heat exchangers, such as cooling pipes, compounds containing low aspect ratio graphite are preferred. Geothermal pipes and pressure pipes in general are a special class of extruded heat exchanger where mechanical properties are very important if the burst pressure level is to be kept high. Graphite, unfortunately, quickly decreases impact properties and elongation at break of a compound which indicates a reduction in burst stability so a graphite grade must be found that provides a suitable compromise between mechanical properties and thermal conductivity. Where low conductivity is the target, conductive
carbon black can be an alternative to graphite. The high structure of a conductive carbon black is important in terms of thermal conductivity as the covalently bound primary particles keep the number of thermal contacts (which are detrimental to fi nal thermal conductivity) at a low level. Thermal conductivity is also improved by the crystalline microstructure of conductive carbon black, while its high structure also facilitates dispersion in the polymer melt. Good dispersion is essential in pressure
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