Optimal microstructure

First of all, let us discuss titanates. If we are interested primarily in the piezoelectric properties of the titanate, then we are concerned with the ability of the tetragonal domains to line up with one another. These domains, which occur below the Curie point, are about 1 ja in width. Since the crystallites are hemmed in by their neighbours, they can only partially submit to the polarization-induced shape change (electrostriction). However, if the crystallites are large enough ( 10 ju) so that different domain systems can occur within one crystallite, then a partial compensation for the changes in domain shape is possible. This is not possible for crystallites 1 /i which consist of only one domain. Optimal piezoelectric properties are thus expected in dense specimens with grain sizes between 10 and 20 p. 

Titanates can also be used as dielectrics in condensers to give relative dielectric constants as high as 4000. For this application, we seek to suppress piezoelectric effects and ferroelectric hysteresis losses. This can best be achieved, according to the ideas above, by producing high density, very fine-grained material with grain sizes 1 jU. 

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Bernd, R. Uwe, J. LiCo02 cathodes with optimized microstructure. One of the keys for high performance and long life MCFC. Denki Kagaku 1996, 64 (6), 519-525. 

Figure 8. Polarization curves (IR-free. measured in humidified Hj) at various SDC anodes at Tca = 800 C.O original one (sintered at 1050 °C, without polymer) O the optimized microstructure (sintered at 1150°C with 0.5 wt% polymer) Ru-catalyzed (0.1 mg-Ru ern loaded on the optimized one) Ni-catalyzed [0.75 mg-Nicnf (8 vol%) loaded on the optimized one, see Section 11.3).

The desired main crystal phase of the phlogopite type was formed at temperatures above 850°C, entirely consuming the norbergite. Fluorborite, Mg3(B03)F3 developed as a secondary crystal phase. Optimal microstructure formation, however, occurred at 950 C. 

The critical current density Jc (T, B) obtained by applying optimal microstructural pinning of the flux lines in conductors of different composition and at different temperatures is shown in Fig. 4.2-6. Two factors of influence may be applied to obtain a higher critical current density Jc (T, B) through effects of the intrinsic properties either a decrease in temperature of application, e.g., from 4.2 to 1.8 K, or an increase of Bc2 by alloying as shown for (Nb, Ta, Ti)3Sn in both Table 4.2-8 and Fig. 4.2-6. It should be noted that the intrinsic properties are affected only marginally by differences in processing of the conductors. 

The developments of new processing and microstmctural characterization techniques led to a increasing of mechanical properties of DSEs and, as a result, ceramies with optimized microstructures for structural and high-temperature applications have been manufactured. In particular, the strength some oxide and boride DSEs with submicron interphase spacing can reach 1.5-2 GPa at ambient temperature, and most of the strength is retained up to 1900 K. 

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