Dopant levels

As indicated in Figure 4, the basic thermoelectric parameters are all functions of carrier concentration. Thus adjusting the dopant level to increase the output voltage generally also increases the electrical resistance. In addition, it affects the electronic component of the thermal conductivity. However, there are limitations on what can be accompHshed by simply varying the carrier concentration in any given material. 

Both anatase and mtile are broad band gap semiconductors iu which a fiUed valence band, derived from the O 2p orbitals, is separated from an empty conduction band, derived from the Ti >d orbitals, by a band gap of ca 3 eV. Consequendy the electrical conductivity depends critically on the presence of impurities and defects such as oxygen vacancies (7). For very pure thin films, prepared by vacuum evaporation of titanium metal and then oxidation, conductivities of 10 S/cm have been reported. For both siugle-crystal and ceramic samples, the electrical conductivity depends on both the state of reduction of the and on dopant levels. At 300 K, a maximum conductivity of 1 S/cm has been reported at an oxygen deficiency of... 

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... 

Each additional samaria formula unit creates one oxygen vacancy. The concentration of the vacancies is given by the electrical neutrality condition, 2[Sm Ce] = [V0 inferring that the vacancy concentration is linearly dependent on the dopant level. The ionic conductivity, a, can be calculated by... 

LaCo03 is rhombohedral from room temperature to 1000°C. For LSCF perovskite, the phase structure depends on the dopant level and temperature. In the La0 8Sr0 2CO, l ev03 system, the room-temperature phases are rhombohedral and orthorhombic for 0 < y < 0.7 and 0.8 < y < 1, respectively [79], For y = 0.8, i.e., I. a, xSrxCo0 2Fe0 sO3 the orthorhombic/rhombohedral transition at room temperature occurs at x = 0.3. When Sr content is increased to x > 0.6, the second phase was observed, while the main phases were rhombohedral for 0.6 < x < 0.8 and cubic for x > 0.8 [80],... 

In addition to the chemical interactions described above, microstructural changes in the electrode can lead to performance degradation after long operation time. For example sintering of the porous structure can degrade electrode performance. In the case of LSM cathodes, the sintering ability is found to be related to the strontium dopant level and stoichiometric composition of (La, Sr)xMn03. In addition, LSM with A-site deficient compositions (x < 1) sinters more readily than their B-site deficient counterparts (x > 1) [198, 199],... 

Davis et al. reported the successful etching of PTFE using single-photon energies in the quartz UV (308 nm and a pulse duration of 25 ns) by sensitizing the fluoropolymer with polyimide. 72 The number of pulses varied depending on fluence and material composition in order to achieve ablated features whose depths were reproducible as measured by a stylus-type profilometer. The pulse repetition rate was on the order of about 200 Hz. In that study, dopant levels... 

Ceria, particularly when doped with Gd203 or SmzOs," has received some attention for direct hydrocarbon conversion in SOFC. Dating back to Steele and co-workers,interesting properties have been demonstrated for ceria-based anodes in direct utilization of methane. Later work suggested that the performance of ceria-based anodes in hydrocarbons could be improved by the addition of precious-metal catalysts, at dopant levels,but the performance of these cells was still too low for practical considerations. The problem with doped ceria is likely that its electronic conductivity is not sufficient. In general, the electrode material should have a conductivity greater than 1 S/cm in order to be practical since a conductivity of 1 S/cm would lead to a cell resistance of 0.1 Q cm for an electrode thickness of 1 mm, even... 

A report on doping of polyacetylene by metal halides 462-463) shows that the interplanar spacing increases with the size of the anion and clustering is inferred to occur at low dopant levels as the dopant reflection appears at about 3 mol% while much of the material is still undoped. It is not totally clear whether similar effects might be the result of a combination of slow dopant diffusion and a diffusion coefficient which is dependent on dopant concentration this is discussed in more detail below. 

We have also estimated diffusion coefficients in Durham polymer using the electrochemical approach. Using this method the diffusion coefficient of Li+ was found to be 2.5 x 10 13 cm2 s-1 at a dopant level of 0.37 mol% and that of C104-1.3 x 10-14 cm2 s-1 at a level of 0.05 mol%. Increasing the doping level caused the diffusion coefficient to fall slightly. 

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