Grain conductivity

Gurgel and Grenier s results showed the bed conductivity to increase from 0.14 to 0.17 W/mK as the pressure was raised from 4 mbar (evaporating pressure) to 110 mbar (condensing pressure). The principle reason stated for this small variation is the reduction in the gas conductivity with decreasing pressure (Knudsen effect) in the macropores. The solid grain conductivity varied linearly from 0.61 to 0.65 W/mK as the methanol concentration varied from 0 to 31%. 

Critoph and Turner [15] carried out similar direct measurements for ammonia and 208C (coconut shell based) carbon manufactured by Sutcliffe Speakman Carbons. The bed conductivity was found to be around 0.165 W/mK at concentrations less than 20% and to rise to 0.19 W/mK at 25% concentration. The corresponding grain conductivities rose from 0.85 to 1.25 W/mK respectively. The higher grain conductivity than that found by Gurgel and Grenier may reflect the different structures present within the extmded and nut shell carbons. 

Therefore, the composition and structure of the product often exhibits nonstoichiometry and inhomogeneity, which can lead to decrease in grain conductivity. 

Based on a number of studies it seems clear that the most important parameter for oxide ion conductivity in fluorites is the cation match with the critical radius, r, as defined in sub-section 15.3.2.3. This means that the highest conductivity is obtained when the ionic radius of the dopant is as close to r as possible. However, according to Steele the highest ionic single crystal (bulk grain) conductivity of... 

The Ag particles with about 10 nm disperse relatively homogeneously at the grain boundary or inside the P30T grains. Conductivity increased by more than 5 orders of magnitude with silver doping. 

Fig. 33.6. Grain-conductivity as a function of temperature for H30 -P"-Al20j (1) in saturated steam, (2) in open air, (3) in open air after 8.5 h annealing ( cross represents the conductivity data measured after rehydration in a saturated steam atmosphere.

High-current ohmic, where the resistance is controlled by the grain conductivity. 

Ma et al. [21] reported the influence of the sintering temperature on the grain conductivity as well as on the grain boundary conductivity for 20mol% Gd-doped Ce02. The effect of temperature is well correlated with that of the synthesis route. Examples can be given for more complex compositions such as Ceo.8- Gdo,2Sr t02 5, 0 <.v < 0.1. 

The total conductivity of a crystalline sample of the optimum composition, Lii.3Tii.7Alo.3(P04)3, of 7 X 10 S cm at room temperature shows a similar contribution from the intra-grain conductivity and the lithium conductivity through the grain boundaries. Interestingly, some authors have reported non-Arrhenius thermal dependence of the conductivity behaviour for this composition. Measurements between room temperature and 400 °C show a smooth evolution in apparent activation energy that reduces from ca 0.35 eV from 25 °C to 200 °C to ca 0.20 eV... 

The scope for application of these materials as solid state electrolytes is severely limited by both the limited conductivity achieved in polycrystalline samples and the ease of reduction of Ti and the associated increase in electronic conductivity. Attempts to overcome both of these limitations have resulted in a huge number of compounds being prepared that have either introduced new cations in order to modify the microstructure or the inter-grain conductivity or sought to replace titanium with more electropositive species such as Zr", Hf", Nb or Ta " " that will better withstand reducing conditions such as those in a rechargeable battery. 

A real capacitance behavior is observed only for frequencies less than 1/(2jiRC). A wide frequency range is obtained by increasing the grain conductivity as much as possible. This is the reason why strontium titanate was preferred to barium titanate, with the mobihty of electrons being greater (see Table 11.4). Typically, conductivity is 10 Q l cm i and the cut-off frequency 1 GHz. The apparent relative permittivity is of the order of 20,000. 

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