Insulation and semiconducting polymers

Multi-pin edge connectors are used extensively in electronic circuits. The thermoplastic housing must position the pins accurately for correct mating with the connector, and withstand momentary high temperature peaks when soldered connections (at 300 °C) are made to the pins. Glass-fibre filled polycarbonate or polybutylene terephthalate mouldings survive ageing tests at 125 °C. 

Geometries for the determination of electric strength (a) Sphere on film (b) sphere recessed into sheet (c) cylinders embedded in plastic and (d) the corresponding Weibull plot of the results of many tests on LDPE at 20°C (from Seanor D. A, Ed., Elearical Properties of Polymers, Academic Press, 1982). 

Variation of electric strength of recent XLPE cable with the duration of the voltage application, tested at 90 °C, with 6h on, 18 h off. (From Ishibashi, A. et o/., IEEE Trans. Dielectrics Electrical Insuial, 5, 695, 1998) 1998 IEEE 

Electrical breakdown is associated with the growth of trees., named after the structures that grow from charged metal needles in laboratory tests. Bow tie shaped trees grow in both directions from voids in the XLPE of high voltage DC cables (Fig. 12.7). The void acts as an electrical stress concentration, which initiates the electrical or electrochemical breakdown process. 

In the former, corona discharges occur in voids, causing hollow channels, lined with decomposed polymer, grow in the polymer. The field strength to cause corona discharge should be an inverse function of the void diameter, so breakdown should be avoidable if voids are smaller than 25 xm. 

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Metallic polymers can be obtained by (a) pyrolysis of insulating or semiconducting polymers (b) incorporation of metallic particles (c) action of electron donors and acceptors on conjugated polymers and (d) producing half-filled band structures, (a) and (b) are the principal routes to commercial products (see Sections 22.5.1 and 22.5.2). The discovery of metallic conductivity in doped conjugated polymers is relatively recent, but has been subject to intense activity (see Section 22.4). So far, a linear carbon-backbone polymer with intrinsic metallic behaviour has not been reported. This stems from the fact that some structural deformation can apparently always produce a semiconducting state of lower total energy. 

The most unusual and interesting feature of these polymers is their capacity to switch between insulating and conducting (or semiconducting) states. All other materials, with the only additional exception of some intercalation compounds, are normally found only as conductors or semiconductors or insulators, without the facility to switch between these states. 

Conductivity also varies with temperature generally decreasing for metallic materials such as silver and copper, but increasing as temperature is increased for semiconductive materials such as insulator, semiconductive, and conductive polymers (Section 19.1). 

The use of polymers with a low relative permittivity, see Section 2.7, as the FET gate insulator has been found to result in higher carrier mobility and improved device performance (Veres et al., 2003). The authors developed stable semiconductive polymers, poly(triarylamines), and used them to fabricate FETs with silicon dioxide and poly(methylmethacrylate) gate insulators. Values of the carrier mobilities measured in the FETs were about 10 times smaller than those determined by TOF experiments. A dramatic increase in FET mobility was observed when a low permittivity fluoropolymer (e = 2.1) was employed as gate insulator. The FET mobilities for devices with gate insulators with relative permittivities in the range 2-18 are shown in Fig. 10.12(a). The devices were made with two different poly(triarylamines)... 

The dimensional hierarchy of silicon-based polymers is summarized in Figure 6. The right circle corresponds to the saturated systems already discussed. The left circle corresponds to the unsaturated systems. The electronic properties of silicon-based polymers vary from conducting (metallic) and semiconducting to insulating. This figure shows that silicon atoms can form many kinds of materials with various properties. However, the study of silicon-based materials has been concentrated in a very small area of this figure. 

Silicon-based polymers form a dimensional hierarchy from disilanes, to crystal silicon, and through polysilanes, ladder polymers, siloxenes, polysilane alloys, clusters, and amorphous silicons and include unsaturated systems, such as polysilenes, hexasilabenzenes, and so on. Their properties depend basically on the network dimensions and can vary from conducting (metallic) and semiconducting to insulating. 

Unlike (SN), most polymers correspond to closed-shell systems where all the electrons are paired. Such a configuration leads to insulating or semiconducting properties as noted previously. Polyacetylenes and related conjugated polymers, for example, have conductivities that classify them as semiconductors. The carbon atom in polyacetylene is sp hybridized, which leaves one p electron out of the bond-forming hybrid orbitals. In principle, such a structure might be expected to give rise to extended electronic states formed by overlap of the p (tt) electrons and thus provide a basis for metallic behavior in polymers. 

Figure 1.2 illustrates the typical design of a printed transistor the source and drain electrodes are mounted on a polyester foil, followed by the semiconducting layer of polymer (i.e. polythiophenes), the insulating layer of polymer insulators is on top and, as the final layer, the gate electrode. 

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