**4.4 Hydrothermal synthesis**

Another wet chemical technique is known as hydrothermal synthesis. In hydrothermal synthesis, the reaction mixture is heated above the boiling point of water in an autoclave or other closed system and the sample is exposed to steam at high pressures (Pookmanee et al., 2004, Shi et al., 2000, Yang et al., 2003). In addition, the parameter of Teflon-lined vessel such as temperature and reaction time are mainly important factor to determine the phase structure and particle morphology (Pookmanee et al., 2004). It was also reported that the hydrothermally powder was significantly influenced by different mineralizer KOH content and molar ratio of Bi/Ti (Shi et al., 2000). Recently, Xie et al. (Xie et al., 2007) reported that the concentration of KOH, reaction time and temperature had a significant effect on the phase composition and morphology of the resultant single crystals. Many authors reported that hydrothermal synthesis has several advantages including narrow particle size distribution, highly purity with fine powder, and low degree of agglomeration. In processing stand point, the hydrothermal synthesis is able to synthesize powder at a much lower temperature compared to other methods. Nevertheless, the synthesis in an aqueous environment causes water to be incorporated into the powder, thus causing deterioration in the electrical properties (Yan ,Razak, 2010).

### **4.5 Co-precipitation method**

In order to prepare the controlled morphology, narrow particle size distribution, high purity and high degree of crystallinity as well as possible reduction in sintering temperature, the co-precipitation method might be a promising route instead of other wet chemical route. Precipitation is the formation of a solid product or powder from a liquid solution which initiated by either changing the solution temperature, pressure, pH or using a chemical precipitation agent so as to exceed the solubility limit of the desired species (Pookmanee, Phanichphant, 2009, Thongtem ,Thongtem, 2004). In general, co-precipitation reaction relies

Sintering and Characterization of Rare Earth Doped

crystallinity with high purity of BTO content.

**6. Powder characterization** 

**6.1 Effect of calcination temperature on BTO** 

Bismuth Titanate Ceramics Prepared by Soft Combustion Synthesis 363

content in as-combusted powders, the calcination is necessary to enhance the degree of

Fig. 1. Evolution of (a) Bi-Ti precursor (b) Pr precursor (c) Bi-Ti precursor stirred at 40°C for 2 hour (d) Bi-Ti precursor heated at 90°C (e) viscous gel and (f) as-combusted powder.

X-ray diffraction (XRD) was conducted on the as-combusted powders which calcined at different temperatures and the result is shown in **Figure 2**. The study on different calcination temperature is essential in this work. The main reason is to determine the optimum calcination temperature, which will be used for the following characterization in particular dielectric study. As seen in **Figure 2**, the BTO phase was observed in as-combusted powder. The main peak corresponding to BTO was found at ~29o. However, the presence other intermediate phases such as BTO7.7 and BTO2 were also identified and marked in XRD pattern. At calcination temperature of 600oC, there was a tremendous increase at the main peak (~29o). This inedicates that the presence more BTO phase was observed after calcination process. In addition, the peaks corresponding to intermediate phases were decreased. Further increase calcination temperature, the intermediate phases were gradually eliminated. The intermediate phases completely disappeared at 750 and 800oC. In other words, the BTO phase was successfully formed as single phase at temperature as low as 750oC. It also suggest that the optimum calcination temperature for BTO is 750oC. This temperature is probably lower than other processing route such as conventional solid state reaction and some other wet chemical synthesis (Kan et al., 2003, Pookmanee, 2008). Moreover, as the calcination temperature was increased, the XRD peaks were sharper and the stable phase BTO powders with higher crystallinity could be obtained. **Table 2** presents the variation of lattice parameters and

on dissolving the metal salts, commonly metal chlorides, nitrates and hydroxides followed by a rapid pH change to form precipitate. The precipitate must be thoroughly washed to get rid of the impurities from the solutions prior to calcination. It was reported that the welldispersed particles of about 10 nm began to form a BTO phase at 470oC. The phase formation was complete after a 550oC for 30 minute heat treatment. It was finally sintered at 750oC for 1 hour to achieve a sample of high density of 97.2% (Kan et al., 2002).

### **4.6 Soft combustion synthesis**

The synthesis of BTO powders using combustion reactions, which provides good compositional control, is an alternative synthesis method which worth pursuing. The combustion synthesis enables synthesis at low temperatures and the products obtained are in a finely divided state with large surface areas. Furthermore, the nature of combustion synthesis is characterized by simple experimental set-up, short reaction time between the preparation of the reactants and the availability of the final product and less in external energy consumption (Aruna ,Mukasyan, 2008, Patil et al., 2002). Typically, the mixture of reactants consists of metal nitrate and a suitable organic fuel such as urea, glycine and citric acid. Additionally, the temperature is essential to boil the mixture until the ignition and selfsustaining reaction takes off. The large amount of gases formed can result in the appearance of a flame, which can reach temperatures in excess of 1000oC. In some cases, the external source like simple calcination is necessary to accomplish the synthesis of the appropriate phase. This is because the energy released from the exothermic reaction between the nitrate and the fuel is usually ignited at a temperature much lower than the actual phase formation. Thus, the single phase formation is not ease to produce. Recently, our group had performed a modification on soft combustion synthesis, whereby nitrate salts, Bismuth (Bi) and organic Titanium (IV) isopropoxide (Ti) were dissolved into 2-methaoxyethanol and acetylacetone. In addition, the organic fuel was not used in this work. To introduce the doping content, the Sm3+ and Pr3+ from nitrate salts were also used. The observation of the soft combustion will be discussed in the following section.
