**7. Conclusion**

574 Sintering of Ceramics – New Emerging Techniques

Nanopowders, with grain diameter of about 27 nm, have been synthesized by the gaseous phase method and compacted via magnetic compaction process at Ural State Technical University, Russia (Kortov et al., 2008). Next, sintering of the compacts has been carried out at Institut National des Sciences Appliquées (INSA) of Lyon (France). The sintering temperature of 1473 K (dwelling time 60 min) was reached at a rate of 3 K/min. As expected, with such heating rate, a substantial grain growth occurred (i.e., from 27 to about 100 nm, cf. Fig. 14). Indeed, grain growth could have been reduced by using faster heating rates that are made possible by different sintering techniques such as spark plasma

Fig. 14. Microstructure of the sub-micrometric grain size of the impure polycrystalline alumina sample after sintering. This picture is the SEM observation of fracture surface. The average grain diameter has grown after sintering to about of 100 nm (the initial particle

The overall purity of this material is about the same as the one of impure polycrystalline alumina. In Fig. 15, the fraction R of charges removed from the irradiated volume as a function of temperature is given for the 0.1 and 1.2 µm samples. The manifest difference is the sharp enhancement of R between 600 and 663 K for the 0.1 µm sample (the value of R increases from 30 to 90 %), which contrasts with the continuous behaviour of the other polycrystalline samples. The activation energy that arises from the semi-logarithmic plot of the recovery parameter versus reciprocal temperature in the range 600-663 K is 0.53 eV (Moya et al., 2007). These results are an indication that detrapping occurs, at about 600 K, from a dominant efficient trap having a well-defined energy level in the gap as in the case of

diameter prior to sintering was about 27 nm).

silver doped single crystal (Zarbout et al. 2010).

sintering.

This chapter provides a method for the characterization of charge trapping and spreading in dielectrics. A quantitative recovery parameters reflecting the relative degree of the two competing processes is accurately derived. The experimental set up makes possible the assessment of the effect of temperature (in the range 300-700 K). The ability of polycrystalline alumina to trap or, conversely, to spread charges depends strongly on the grain size and segregation of impurities at interfaces. The results suggest that the grain boundary interfaces can be associated to shallow traps whereas the defects within the grains to deeper ones. The strong tendency for segregation of the main impurities implies that an internal gettering effect can also intervene. It appears therefore that the control of the microstructural development, during the conventional sintering process, is of importance as it provides ways to influence the insulator properties in technological applications of oxide ceramics, for instance, the breakdown strength. Further investigations dealing with the properties of nanostructured materials, processed by sintering techniques that reduce grain growth, could bring more understanding of the role of interfaces.
