Doping and modification of properties

Much of the work on chemical modification is best understood in terms of phase diagrams. An important system, in this respect, is the PbZrOj-PbTiOj (PZT) system 

The properties of phases in the KNbOj-NaNbOj system can be adjusted following the classical methods described previously, namely. A- and B-type doping, and solid solution formation with other perovskites such as BaTiOj and Ba(Zr, TijOj. A-site and B-site doping has a considerable effect on the position of the phase boundaries. For example, the substitution of 5% of the A-site ions with Li is sufficient to drop the orthorhombic to tetragonal phase boundary from close to 200°C to room temperature. The same is true for reaction with other perovskite phases, such as Ba(Ti, ZrjOj, which not only modify transition temperatures but also the spontaneous polarisation and piezoelectric coefficients. 

Many ferroelectric materials show interesting and potentially useful properties, but not at the temperature or pressure required for a particular application. It is then necessary to change or tune the property to the fit the application. This change is frequently brought about by the replacement of one or more of the constituents of the compound, or the deliberate addition of impurities. As in the case of semiconductors and other materials, the deliberate addition of impurities to change the physical properties of a solid is known as doping. 

Ferroelectric oxides, for example, are of interest as capacitor materials because of their high permittivity levels, but usually the sharp maximum in dielectric constant at the Curie point must be broadened and moved to room temperature. Consider BaTiOa, which has a high dielectric constant at the Curie temperature, about 393 K. The Curie temperature can be increased by the replacement of some of the Ba ions by Pb ions. These ions are softer (i.e. more easily polarised) than the Ba ions, as they have a lone pair of electrons and so are more easily affected by an applied electric field. The resultant compound retains the crystal structure of BaTiOa but has a formula Bai cPb cTi03. The compound Bao,6Pbo.4Ti03 has a Curie temperature of approximately 573 K, an increase of 200 K. In a similar way, the Curie temperature can be lowered by the substitution of Ba ions by Sr . These ions are smaller than Ba ions and can be considered to be harder and more difficult to polarise. The compound Bao,6Sro.4Ti03 has a Curie temperature of 0°C. The Curie temperature can also be lowered by the replacement of 

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Various results to furtiier develop these materials as TE materials have been obtained such as the densification of these materials through usage of sintering additives, doping with transition metals,and modification of the [B]/[C] composition, which previously had led to significant improvement in the TE properties of boron carbide. 

K. Sharma, A S. Al-Kabbi, G.S.S. Saini, S.K. Tripathi, Indium doping induced modification of the structural, optical and electrical properties of nanocrystalline CdSe thin films, J. Alloy. Compd. 564 (2013)42-48. 

The structure-property relations of fullerenes, fullerene-derived solids, and carbon nanotubes are reviewed in the context of advanced technologies for carbon-hased materials. The synthesis, structure and electronic properties of fullerene solids are then considered, and modifications to their structure and properties through doping with various charge transfer agents are reviewed. Brief comments are included on potential applications of this unique family of new materials. 

Modifications of the conduction properties of semiconducting carbon nanotubes by B (p-type) and N ( -type) substitutional doping has also been dis-cussed[3l] and, in addition, electronic modifications by filling the capillaries of the tubes have also been proposed[32]. Exohedral doping of the space between nanotubes in a tubule bundle could provide yet an-... 

Next, let us look at modification of CNTs. There are many approaches to modifying the electronic structure of CNTs oxidation [39], doping (intercalation) [69], filling [70] and substitution by hetero elements like boron and nitrogen atoms [71,72]. There have been few studies on the application of these CNTs but it will be interesting to study applications as well as electronic properties. 

The diffusion of hydrogen in highly or lightly silicon doped GaAs induces a modification of the electrical properties of the material a reduction of the free electron concentration (Fig. 2) and a significant increase of the electron mobility up to values close to the mobility in nonhydrogenated materials with the same net carrier concentration (Jalil et al., 1986 Pan... 

Haase and co-workers investigated electro-optic and dielectric properties of ferroelectric liquid crystals doped with chiral CNTs [495, 496]. The performance of the doped liquid crystal mixture was greatly affected even by a small concentration of CNTs. The experimental results were explained by two effects (1) the spontaneous polarization of the ferroelectric liquid crystal is screened by the 7t-electron system of the CNT and (2) the CNT 7i-electrons trap ionic impurities, resulting in a significant modification of the internal electric field within liquid crystal test cells. 

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