Insulating fillers

A soft, gritty amorphous silica composed of minute siliceous skeletons of small aquatic plants. Used in filtration and decoloration of liquids, insulation, filler in dynamite, wax, textiles, plastics, paint, and rubber. Calcined and flux-calcined diatomaceous earth contains appreciable amounts of cristobalite, and dust levels should be the same as for cristobalite. 

For certain products, skill is required to estimate a product s performance under steady-state heat-flow conditions, especially those made of RPs (Fig. 7-19). The method and repeatability of the processing technique can have a significant effect. In general, thermal conductivity is low for plastics and the plastic s structure does not alter its value significantly. To increase it the usual approach is to add metallic fillers, glass fibers, or electrically insulating fillers such as alumina. Foaming can be used to decrease thermal conductivity. 

Some applications, however, must conduct heat but not electricity. In these applications the adhesive must permit high transfer of heat plus a degree of electrical insulation. Fillers used for achieving thermal conductivity alone include aluminum oxide, beryllium oxide, boron nitride, and silica. Table 9.9 lists thermal conductivity values for several metals as well as for beryllium oxide, aluminum oxide, and several filled and unfilled resins. 

Some of the insulating fillers found in the first row of Table 5.19 are used in... 

The most important additive in an adhesive formulation is the filler. Fillers such as metal particles are used to render adhesives both electrically and thermally conductive while electrically insulative fillers such as metal oxides are used to provide thermal conductivity alone. Regardless of whether they are electrically conductive or not, fillers provide numerous simultaneous benefits, including the following ... 

The major industrial applications of hexagonal boron nitride rely on its high thermal conductivity, excellent dielectric properties, self-lubrication, chemical inertness, nontoxicity, and ease of machining. These are, for instance, mold wash for releasing molds, high-temperature lubricants, insulating filler material in composite materials, as an additive in silicone oils and synthetic resins, as filler for tubular heaters, and in neutron absorbers. On the other hand, the industrial applications of cubic boron nitride rely on its high hardness and are mainly as abrasives. 

The specific heat of amorphous plastics increases with temperature in an approximately linear fashion below and above Tg, but a steplike change occurs near the Tg. No such stepping occurs widi crystalline types. The high degree of die molecular order for crystalline TPs makes their values tend to be twice those of the amorphous types. The TSs has the highest values. To increase TC the usual approach is to add metallic fillers, glass fibers, foamed structure, or electrically insulating fillers such as alumina. 

Okamoto et al. studied the proton conductivities for SPI/Si02 hybrid membranes at different humidification conditions at 60°C. They found that under 100% RH, the hybrid membrane showed a decrease in proton conductivity compared with that of pristine SPI (from 178 to 124 mS cm" ) however, the conductivity at low RHs of 30%-50% hardly changed. It is considered that in the highly swollen hybrid membranes, the doped silica particles act as an insulating filler to reduce the conductivity by the tortuosity effect. On the other hand, under the low humidification of 30%-50% RH, the silica particles may act as water adsorbate to compensate the conductivity. 

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