Insulation characteristics

For many electronic and electrical appHcations, electrically conductive resias are required. Most polymeric resias exhibit high levels of electrical resistivity. Conductivity can be improved, however, by the judicious use of fillers eg, in epoxy, silver (in either flake or powdered form) is used as a filler. Sometimes other fillers such as copper are also used, but result in reduced efficiency. The popularity of silver is due to the absence of the oxide layer formation, which imparts electrical insulating characteristics. Consequently, metallic fibers such as aluminum are rarely considered for this appHcation. 

Thermal Conductivity and Heat Capacity. Most fibers have similar thermal conductivities and heat capacities. The insulating characteristics of textiles are more related to fabric geometry than they are dependent on fiber thermal characteristics. 

Pure and almost stoichiometric NiO shows, therefore, a very low conductivity of about 10 (Hem) at 25°C but, as illustrated in Figure 7, this value can be increased to about 1 (Hem) by the addition of lithium (11). This stabilizes the formation of the Nfi" states at a higher concentration, resulting in higher and more reproducible conductivities. Similarly, the insulating characteristics of NiO can be improved by the addition of a stable trivalent ion such as Cr " in soHd solution. This addition decreases the fraction of Nfi" ions formed. Because electron transfer between Ni " and Cr " is not favorable, the overall conductivity is substantially decreased. 

Many plastics are superb electrical insulators including, in many instances, good insulation characteristics at high frequencies. 

Finally mention may be made about the influence of humidity on the electrical insulating properties of plastics. Once again the polymers may be classified into two groups, those which do not absorb water and those which do. The nonabsorbent materials are little affected by humidity whereas the insulation characteristics of the absorbent materials deteriorate seriously. These latter materials are generally certain polar materials which all appear capable of forming some sort of bond, probably a hydrogen bond, with water. Three reasons may be given for the deleterious effects of the water. 

In addition to the presence of stable C—F bonds, the PTFE molecule possesses other features which lead to materials of outstanding heat resistance, chemical resistance and electrical insulation characteristics and with a low coefficient of friction. It is today produced by a number of chemical manufacturers such as Du Pont (Teflon), ICI (Fluon), Hoechst (Hostaflon TF), Rhone-Poulenc (Soreflon), Montecatini (Algoflan), Nitto Chemical-Japan (Tetraflon) and Daikin Kogyo-Japan (Polyflon). 

In 1930 BASF, then part of IG Farhen, installed a plant for producing 100 tonnes of polystyrene per annum and in 1933 the first injection moulded articles were produced. In the US semi-plant-scale work at the Dow Chemical Company showed promise of commercial success in 1934. As a consequence there became available shortly before World War II a material of particular interest because of its good electrical insulation characteristics hut otherwise considerably inferior to the polystyrene available today. Because of these excellent electrical characteristics prices were paid of the order of several dollars per pound for these polymers. 

Because of the large quantities of soap left in the polymer, which adversely affects clarity, electrical insulation characteristics and problems in agitation and densification, this process is used only for making latices. 

The pure hydrocarbon nature of polystyrene gives it excellent electrical insulation characteristics, as a result of both the fundamentally good characteristics of the material and to the low water absorption of such a hydrocarbon polymer. The insulation characteristics are therefore well maintained in humid conditions. 

The electrical insulation characteristics of polystyrene are extremely good. Typical figures are given in Table 16.3. 

As mentioned earlier, unmodified polystyrene first found application where rigidity and low cost were important prerequisites. Other useful properties were the transparency and high refractive index, freedom from taste, odour and toxicity, good electrical insulation characteristics, low water absorption and comparatively easy processability. Carefully designed and well-made articles from polystyrene were often found to be perfectly suitable for the end-use intended. On the other hand the extensive use of the polymers in badly designed and badly made products which broke only too easily caused a reaction away from the homopolymer. This resulted, first of all, in the development of the high-impact polystyrene and today this is more important than the unmodified polymer (60% of Western European market). 

The electrical insulation properties of the acetal resins may be described as good but not particularly outstanding. There are available alternative materials which are better insulators and are also less expensive. There are, however, applications where impact toughness and rigidity are required in addition to good electrical insulation characteristics, and in these instances acetal resins would be considered. Table 19.3 lists some of the more important electrical characteristics of Delrin acetal resin. Data for the trioxane-based copolymer resin (e.g. Celcon) are virtually identical. 

The electrical insulation characteristics of bis-phenol. A polycarbonates are in line with those to be expected of a lightly polar polymer (see Chapter 6). 

Electrical insulation characteristics, although also surpassed by some other polymers such as PTFE and polyethylene. 

The heat and flame resistance coupled with good electrical insulation characteristics, which includes in some grades good arcing and arc tracking resistance, has led to PPS replacing some of the older thermosets in electrical parts. These include connectors, coil formers, bobbins, terminal blocks, relay components, moulded bulb sockets for electric power station control panels, brush holders, motor housings, thermostat parts and switch components. 

With a Tg of 273°C and excellent resistance to hydrolysis these films may be used continually at temperatures up to 160°C and for short periods up to 260°C. Their excellent electrical insulation characteristics are well maintained up to temperatures quite close to the Tg as illustrated in Figure 21.12. Some other typical properties are given in Table 21.7. 

Compared with the major vinyls cellulose acetate plastics have a high water absorption, poor electrical insulation characteristics, limited aging resistance and... 

In contrast to the caustic soda-catalysed resols the spirit-soluble resins have good electrical insulation properties. In order to obtain superior insulation characteristics a cresol-based resol is generally used. In a typical reaction the refluxing time is about 30 minutes followed by dehydration under vacuum for periods up to 4 hours. 

Asbestos may be used for improved heat and chemical resistance and silica, mica and china clay for low water absorption grades. Iron-free mica powder is particularly useful where the best possible electrical insulation characteristics are required but because of the poor adhesion of resin to the mica it is usually used in conjunction with a fibrous material such as asbestos. Organic fillers are commonly used in a weight ratio of 1 1 with the resin and mineral fillers in the ratio 1.5 1. 

Electrical properties Relating to the resistance, electrical capacity, and insulating characteristics of a conductor or electrical device. 

The toughness of a material is a design driver in many structures subjected to impact loading. For those materials that must function under a wide range of temperatures, the temperature dependence of the various material properties is often of primary concern. Other structures are subjected to wear or corrosion, so the resistance of a material to those attacks is an important part of the material choice. Thermal and electrical conductivity can be design drivers for some applications, so materials with proper ranges of behavior for those factors must be chosen. Similarly, the acoustical and thermal insulation characteristics of materials often dictate the choice of materials. 

During 1930s, the commercial interest of polystyrene started due to its good electrical insulation characteristics. During the second world war, two companies, namely, I.G. Farben-industries (Germany) and Dow Chemical Company (USA) started the large scale production of polystyrene. 

It is also having good electrical insulation characteristics. It is having low moisture absorption and the electrical properties are maintained in humid conditions. Due to the low cost, good mouldability together with its transparency and colourability this material is having widespread application. 

A damaged layer between 1 to 10 nm thick according to the material and the polishing conditions is generated by the CMP process. This layer would seem to have to be removed as it presents poorly defined physical properties, for example, in terms of contamination, internal stress, insulating characteristics, and the like. Nevertheless the detrimental effects of this layer still have to be clearly demonstrated. 

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