Plastic High thermal conductivity

High-thermal-conductivity plastics are those that have been formulated to provide higher thermal conductivity while maintaining good electrical insulating... 

Heat pipes are used to perform several important heat-transfer roles ia the chemical and closely aUied iadustries. Examples iaclude heat recovery, the isothermaliziag of processes, and spot cooling ia the mol ding of plastics. In its simplest form the heat pipe possesses the property of extremely high thermal conductance, often several hundred times that of metals. As a result, the heat pipe can produce nearly isothermal conditions making an almost ideal heat-transfer element. In another form the heat pipe can provide positive, rapid, and precise control of temperature under conditions that vary with respect to time. 

Cases can be classified as either hermetic or nonhermetic, based on their permeabiUty to moisture. Ceramics and metals are usually used for hermetic cases, whereas plastic materials are used for nonhermetic appHcations. Cases should have good electrical insulation properties. The coefficient of thermal expansion of a particular case should closely match those of the substrate, die, and sealing materials to avoid excessive residual stresses and fatigue damage under thermal cycling loads. Moreover, since cases must provide a path for heat dissipation, high thermal conductivity is also desirable. 

Plastics are susceptible to brittle crack-growth fractures as a result of cyclic stresses in much the same way as metals. In addition, because of their high damping and low thermal conductivity, plastics are prone to thermal softening if the cyclic stress or cyclic rate is high. Examples of the TPs with the best fatigue resistance include PP and ethylene-propylene copolymers. 

Perforated, or matrix, heat exchangers are highly compact and consist of a stack of perforated plates made of high-thermal-conductivity material, such as copper or aluminum, alternating with spacers of low thermal conductivity, such as plastic or stainless steel. The pack of alternate low- and high-thermal-conductivity... 

Thermal conductivity of plastics is very low, which makes them excellent insulators against heat and cold. On the other hand, there are times when high thermal conductivity is preferred, for example, in processing, cooking, and heating equipment. Inorganic fillers can help, in proportion to their volume concentration. Perhaps this approach can be carried further or perhaps there are totally different mechanisms waiting to be discovered. 

Thermal conductivity Everyone knows that touching a metallic surface at room temperature produces a colder sensation than touching a piece of wood or plastic at the same temperature. The very high thermal conductivity of metals allows them to draw heat out of our bodies very efficiently if they are below body temperature. In the same way, a metallic surface that is above body temperature will feel much warmer than one made of some other material. The high thermal conductivity of metals is attributed to vibra-... 

In practice, the temperature-time curve schematically shown in Figure 7.16 does not only depend on the adhesive-related parameters, but also on the properties of the adherends, especially on their thermal conductivity. High thermal conductivity (e.g., metals) leads to shorter heating times than low thermal conductivity, as is to be found, for example, with plastics, glasses, wood. Even the dimensions of the adherends play a role. 

The thermal conductivity of solids varies considerably (Table 15.2). Metals have a high thermal conductivity, with silver having the highest room-temperature thermal conductivity, at 430 W m K . Alloys have lower thermal conductivities than pure metals. Ceramics are even lower, especially porous porcelains or fired clay products (Figure 15.3). The lowest thermal conductivities are shown by plastic foams such as foamed polystyrene. As would be expected, the thermal conductivity of crystals varies with direction. For example, the thermal conductivity of the hexagonal metal cadmium Cd, (A3 structure), is 83Wm K parallel to the c axis and 104 W m parallel to the a axis. At 25 °C, the oxide quartz, which has a hexagonal unit cell, has a thermal conductivity parallel to the c axis of 11 W m K , and 6.5 W m K paraUel to the a axis. 

A nozzle introduced into the insulating chamber 4.19(c) and with the gate in the mould cavity is a modification of the previous nozzle, and at the same time an example of the use of bimetallic inserts. The seal bushing needed to create a small insulating chamber is made of a material with low thermal conductivity (titanium alloy) and insulates the tip, which is made of a material with high thermal conductivity. The gate is substantially cooler, and therefore the nozzle 4.19(c) may be used for amorphous and slow-setting crystalline plastics. 

The high thermal conductivities of some PMCs has led to their increasing use in applications like spacecraft structures and electronic packaging components, e.g., printed circuit board heat sinks, heat spreaders, and heat sinks used to cool microprocessors. The addition of thermally conductive carbon fibers and ceramic particles to thermoplastics has opened the door to use of injection molded parts for which plastics previously could not be used because of their low thermal conductivities. We consider some examples in this section. 

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