Intrinsic conduction

Instead of depending on the thermally generated carriers just described (intrinsic conduction), it is also possible to deUberately incorporate various impurity atoms into the sihcon lattice that ionize at relatively low temperatures and provide either free holes or electrons. In particular. Group 13 (IIIA) elements n-type dopants) supply electrons and Group 15 (VA) elements (p-type dopants) supply holes. Over the normal doping range, one impurity atom supphes one hole or one electron. Of these elements, boron (p-type), and phosphoms, arsenic, and antimony (n-type) are most commonly used. When... 

Figure 5.4 The intrinsic conduction electron concentration as a function of temperature and band gap energy together with the values of the ionic diffusion coefficient which would provide an equal contribution to the conduction...

During the past 30 years considerable research has been undertaken that has led to electrically conducting polymers that do not rely on the use of fillers, the so-called intrinsically conductive polymers. Such polymers depend on the presence of particles which can transport or carry an electric charge. Two types may be distinguished ... 

The properties and applications of intrinsically conductive polymers have been reviewed (Frommer and Chance, 1986 Sauerer, 1991). The Important poly-pyrolles have been separately reviewed (Jasne, 1988). 

As future outlook, it would be of great relevance to be able to eventually enhanee the intrinsic conductivity of CNTs. In this respect, the latest development in alkali metal intercalated CNTs looks rather promising. In faet, Chauvet et al. recently sueeeeded to dope aligned CNT by potassium and found that the Pauli susceptibility increases a factor of 3 upon doping, indicating that K-doped tubes are still good eonductors [23]. 

A thin layer deposited between the electrode and the charge transport material can be used to modify the injection process. Some of these arc (relatively poor) conductors and should be viewed as electrode materials in their own right, for example the polymers polyaniline (PAni) [81-83] and polyethylenedioxythiophene (PEDT or PEDOT) [83, 841 heavily doped with anions to be intrinsically conducting. They have work functions of approximately 5.0 cV [75] and therefore are used as anode materials, typically on top of 1TO, which is present to provide lateral conductivity. Thin layers of transition metal oxide on ITO have also been shown [74J to have better injection properties than ITO itself. Again these materials (oxides of ruthenium, molybdenum or vanadium) have high work functions, but because of their low conductivity cannot be used alone as the electrode. 

Phthalocyanines with side groups which can undergo Diels-Alder reactions can be synthesized as precursors for ladder polymers (see pp772, 773).344-345 For these kinds of compounds an intrinsic conductivity is predicted.346 Both dienophilic and enophilic phthalocyanines have been prepared. The possibility to undergo a Diels-Alder reaction was tested with fumaronitrile and 2,3-dimethylbutadiene.345 Via a Diels-Alder reaction, a connection between a phthalocyanine and [60]fullercne was also achieved.336... 

Figure 20. Artificial muscle under work. In reduction (A) electrons are injected into the polymer chains. Positive charges are annihilated. Counter-ions and water molecules are expelled. The polymer shrinks and compaction stress gradients appear at each point of the interface of the two polymers. The free end of the bilayer describes an angular movement toward the left side. (B) Opposite processes and movements occur under oxidation. (Reprinted from T. F. Otero and J. Rodriguez, in Intrinsically Conducting Polymers An Emerging Technology, M. Aldissi, ed., pp. 179-190, Figs. 1,2. Copyright 1993. Reprinted with kind permission of Kluwer Academic Publishers.)...

H. Naarmann, in Intrinsically Conducting Polymers An Emerging Technology, M. Aldissi, ed., Kluwer, Dordrecht, Netherlands, 1993, p. 1-12. 

Conjugated polymers are generally poor conductors unless they have been doped (oxidized or reduced) to generate mobile charge carriers. This can be explained by the schematic band diagrams shown in Fig. I.23 Polymerization causes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the monomer to split into n and n bands. In solid-state terminology these are the valence and conduction bands, respectively. In the neutral forms shown in Structures 1-4, the valence band is filled, the conduction band is empty, and the band gap (Eg) is typically 2-3 eV.24 There is therefore little intrinsic conductivity. 

The actual utility of this discovery depends on the ability to go from hosts consisting of expensive, highly oriented, pyrolytic graphite to hosts composed of cheap graphite powders or fibers. Care must be taken on intercalation, because defects in such low-rank graphites may affect not only the intrinsic conductivity of the host (Z4) but may also serve as sites for oxidative reactions that may disrupt the host (Ell). 

From the mathematical standpoint, this model can be formulated as follows. Neglecting the intrinsic conductance, the condition of electrical neutrality of the semiconductor may be... 

Materials such as polypyrrole are exciting in terms of their future technological impact, not just because of the obvious applications of such a simple, cheap electrochromic but because it may be possible to develop them sufficiently to replace the more expensive, and often toxic, metallic conductors commonly employed in the electronics industry. This may not be such a distant dream since it has been calculated that the intrinsic conductivity of these materials, i.e. without the defects that are currently defeating attempts to increase their conductivity of c, < lOOOfl 1 cm", may be many times that of copper. 

Significant changes in electronic properties of a solid can result from composition variation. The examples chosen to illustrate this will mainly be drawn from oxides as these have been studied in most detail. In this chapter, pure (single-phase) solids will be described—intrinsic conductivity—while in the following chapter impurities and doping—that is, extrinsic conductivity—will be considered. Note that the principles described below apply equally well to doped crystals—the division into two chapters is a matter of convenience. 

The use of porphyrinic ligands in polymeric systems allows their unique physio-chemical features to be integrated into two (2D)- or three-dimensional (3D) structures. As such, porphyrin or pc macrocycles have been extensively used to prepare polymers, usually via a radical polymerization reaction (85,86) and more recently via iterative Diels-Alder reactions (87-89). The resulting polymers have interesting materials and biological applications. For example, certain pc-based polymers have higher intrinsic conductivities and better catalytic activity than their parent monomers (90-92). The first example of a /jz-based polymer was reported in 1999 by Montalban et al. (36). These polymers were prepared by a ROMP of a norbor-nadiene substituted pz (Scheme 7, 34). This pz was the first example of polymerization of a porphyrinic macrocycle by a ROMP reaction, and it represents a new general route for the synthesis of polymeric porphyrinic-type macrocycles. 

Intrinsically conducting polymers, 13 540 Intrinsic bioremediation, 3 767 defined, 3 759t Intrinsic detectors, 22 180 Intrinsic fiber-optic sensors, 11 148 Intrinsic magnetic properties, of M-type ferrites, 11 67-68 Intrinsic photoconductors, 19 138 Intrinsic rate expressions, 21 341 Intrinsic semiconductors, 22 235-236 energy gap at room temperature, 5 596t Intrinsic strength, of vitreous silica, 22 428 Intrinsic-type detectors, cooling, 19 136 Intrinsic viscosity (TV), of thermoplastics, 10 178... 

Intrinsic conductive polymers (ICP) obtained by polymerization of conductive macromolecules. This is a difficult route for industrial applications. 

Even when exposed to water, the conductivity of the doped SrTi03 at 973 K, 35 S/cm, would seem to be adequate for achieving good anode performance. However, it should be recognized that a high-performance anode will need to be porous and likely be in the form of a composite with an ionic-conducting oxide to enhance the TPB. For example, with cathodes based on Sr-doped LaMn03 (LSM) and YSZ, the conductivity of the composite can be more than a factor of 1(3 lower than that of dense LSM. Furthermore, the intrinsic conductivity of these oxides that are used for cathodes is well over 100 S/cm. 

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