Acceptor-doped material

In acceptor-doped material, the diagram is virtually unchanged for dopant levels below that of the intrinsic defects, that is, less than 1020 defects m-3 in the examples given above. When the concentration of acceptors passes this quantity, the [e ] = [h ] plateau lengthens and divides with [h ] greater than [e ]. The separation of these... 

Intentionally donor-doped BaTiOj The effect of po2 on the conductivity of a donor-doped system has been studied for lanthanum-doped BaTiC>3 as shown in Fig. 2.14 for 1200 °C. The behaviour differs from that shown in Fig. 2.12 for acceptor-doped material. Firstly, there is a shift of the curves towards higher oxygen pressures. Secondly, at intermediate po2 there is a region, particularly at higher lanthanum contents, where the conductivity becomes independent of p0l. At sufficiently low pressures the curves coincide with those of the pure ceramic. 

Donor-doped PZTs have higher permittivities and d coefficients than acceptor-doped materials and are therefore more suitable for converting mechanical into electrical vibrations. They have higher dissipation factors than acceptor-doped materials and are therefore not as suitable for wave filters. If this were not the case, their low ageing coefficients would be an advantage. 

Thus, the higher fullerene is expected to exhibit organic electron donor properties, and acceptor-doped materials should be stable. This exciting possibility is currently being actively pursued. 

In some acceptor-doped materials, electron holes may also become dominating defects at high oxygen activities, so that we may consider a more complicated defect structure and electroneutrality ... 

An analogous expression can be obtained for acceptor-doped materials. The energy levels for the various dopants are illustrated in Fig. 2.6. Deep dopants can act as optically active centers or as catalytically active surface sites, as given in the following paragraphs. 

Recorded membrane performances of a range of different materials systems ceria are summarized in Table 12.6. The materials have been grouped into four classes (i) composite membranes, (ii) materials with redox active dopants, (iii) acceptor-doped materials, and (iv) materials with additional co-substitution. Selected results are either discussed above (Sections 12.6.4.2 and 12.6.4.3) or in the following sections and in Section 12.6.4.5. 

Under certain conditions acceptor-doped ceria itself exhibits significant electronic conductivity and addition of an electronic conducting second phase is not necessary. This is for instance the case with the classical acceptor-doped materials Cei yGdy02 y/2 and Cei y Smy02 y/2 under wide p02 membrane applications (with strong reduction on the low p02 side). Also, other redox active elements may be substituted in ceria to enhance further either the p-type or n-type conductivity. The former can be achieved with e.g. the addition of Pr or Tb, and numerous studies exist for Pr substitution - - - - or Pr and Gd substitution (see also Section 12.4.2.2). Fewer studies are available where enhanced n-type conduction has been introduced by substitution with redox active elements, but recently... 

As expected from the discussion in the previous section, the electronic conductivity in acceptor-doped ceria (e.g. Sm-doped or Gd-doped ceria) and ceria doped with redox active elements such as Pr or Tb, as well as for materials with multiple substituents (e.g. Gd and Pr substitution) may be sufficient for use as an oxygen membrane. Several studies give measurements of the oxygen fluxes through dense samples of doped ceria. A subset of measured oxygen fluxes as well as the experimental conditions under which they were measured is collected in Table 12.6. The performance of the purely acceptor-doped materials (materials 17,18, and 19) was discussed in Section 12.6.4.3. The performance of membrane materials vdth redox active dopants (materials 15 and 16) and with redox active dopants -l- Co addition (materials 20 and 21) is discussed in this section. 

For donor-doped semicondnctors, this region is sometimes called the saturation region for acceptor-doped materials, it is often termed the exhaustion region. 

Acceptor doping, as in lithium oxide doping of nickel oxide, produces p-type thermistors. The situation in nickel-oxide-doped Mn304 is similar but slightly more complex. This oxide has a distorted spinel structure (Supplementary Material SI), with Mn2+ occupying tetrahedral sites and Mn3+ occupying octahedral sites in the crystal, to give a formula Mn2+[Mn3+]204, where the square parentheses enclose the ions in octahedral sites. The dopant Ni2+ ions preferentially occupy... 

Insertion of the equilibrium constants will result in equations that can be solved for [h ] as a function of px2, which, in turn, can be used to determine the values of the other defects as a function of pXl (Section 7.10.2). The diagrams for donor and acceptor doping of MX in which electronic defects dominate over Schottky equilibrium (Fig. 8.2a and 8.2c) can be compared to that for undoped material (Fig. 7.11), redrawn here (Fig. 8.2b). 

Acceptor doping in perovskite oxides gives materials with a vacancy population that can act as proton conductors in moist atmospheres (Section 6.9). In addition, the doped materials are generally p-type semiconductors. This means that in moist atmospheres there is the possibility of mixed conductivity involving three charge carriers (H+, O2-, and h ) or four if electrons, e, are included. 

The magnetic structure becomes more complex when doped materials are considered. Acceptor doping of LaCoCL by incorporation of an alkaline earth cation, Ca, Sr, Ba, in place of La, as in La SpCoCL now introduces holes into the system. These are generally located on the Co3+ ions to form Co4+. The Co3+ ions are thought to be mainly in the IS state and Co4+ in the HS state, with an electron configuration for this 3d5 ion of t2g e2. Further studies are needed to completely resolve the spin and charge distribution. 

As already stated in Section V.2., materials such as acceptor doped Zr02, Ce02 and SrTi03 show a space charge behavior that is characterized by a depletion of oxygen vacancies leading to a depression of the ionic conductivity.158... 

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