**4.3 Sol-gel synthesis**

360 Sintering of Ceramics – New Emerging Techniques

As is known, synthesis process plays a crucial role to determine the microstructure of the ceramics as well as the control purity and stoichiometry. Different synthesis methods have been developed for the production of perovskite powders, like solid state reaction, sol-gel technique, hydrothermal synthesis, co-precipitation and combustion synthesis (Hardy et al., 2004, Kim, 2006, Kojima et al., 2009, Macedo et al., 2004, Pookmanee ,Phanichphant, 2009). It was reported that the ferroelectric properties of BTO can also be improved with a specific control in terms of microstructure, chemical homogeneity and its purity (Lu et al., 2005, Yang et al., 2008). Nevertheless, there are several merits and drawbacks of each synthesis process in order to control the ferroelectric domains through the preferred microstructure as well as crystal structure. Thus, the description of each synthesis process is discussed in the

The conventional solid state reaction is mostly used for preparation of bulk ceramics. It is an endothermic reaction used to produce simple oxide from carbonates, hydroxides and other metal salts. Such conventional reaction often results in high agglomeration and compositional inhomogeneity of powders because of high calcination temperature and repeated grinding. As a result, the sinterability of ceramics is fairly low subsequently a higher sintering temperature is required to enhance their properties. Subbarao (Subbarao, 1961, Subbarao, 1962) prepared the BTO ceramics using the solid state reaction and sintered from 1000 to 1250oC to achieve the theoretical density of about 80 %. In some cases, the sintering condition with longer soaking time is needed to enhance other properties. Watcharapasorn et al. (Watcharapasorn et al., 2010) studied the grain growth behavior of BTO ceramics using different sintering conditions. It was reported that the sintering of ceramics for longer time could render a material with more isotropic microstructure with reduced preferred orientation. Nevertheless, the increase in relative density (91 – 94 %) was

Mechanical activation technique was initially derived from mechanical alloying for synthesizing alloys and intermetallics. The corresponding technique is a common part of the powder preparation route in the field of ceramics where high-energy ball milling has become a conventional method for producing nanocrystalline materials. This technique uses low-cost and widely available oxides as starting materials and skips the calcination step at an intermediate temperature, leading to a simplified process (Stojanović et al., 2008). Furthermore, the mechanically derived powders have higher sinterability than those powders synthesized by the conventional ball milling. Kong et al. (Kong et al., 2001) obtained the large Pr (24 μC/cm2) and low Ec (11 kV/cm) for BTO ceramics with better density of 98 % after low temperature sintering at 850oC for the powder derived from mechanical activation technique. Stojanovic et al. (Stojanovic et al., 2006, Stojanovic et al., 2006) reported that the BTO powder can be directly synthesized using high impact milling for about 3 to 12 hours and then sintered at 1000oC for 2 h. Han et al. (Han ,Ko, 2009) stated the formation of BTO phase is highly dependent on the processing parameters particularly

**4. Processing route for preparation the BTO and rare-earth doping** 

following subsection.

**4.1 Conventional solid state reaction** 

very small with increasing soaking time.

**4.2 Mechanical activation technique** 

The most popular wet chemical technique like sol-gel synthesis is widely used since it offers excellent uniformity over a large area, easy composition control, short fabrication time, as well as a low temperature process at comparatively a low cost (Du et al., 2007). This technique can be used to prepare the samples in the form of bulk ceramics and thin films. Several factors that need to be considered in a sol-gel synthesis are solvent, precursors, catalyst, pH, additives and mechanical agitation (Du et al., 2007, Guo et al., 2007, Ke et al., 2010). These factors greatly influence the powder size and other properties. Du et al. (Du et al., 2007, Du et al., 2008) reported that a highly stable and homogeneous BTO powders was produced at calcination temperature as low as 550oC, which is fairly low in wet chemical technique.
