**5. Observation during the combustion process**

**Figure 1** shows the actual condition before and after combustion on BTO and Pr3+ doping. The Bi-Ti precursor was observed in clear-yellowish solution (**Figure 1a**) whereas the Pr3+ precursor was found in clear-greenish solution (**Figure 1b**). The Sm3+ precursor was observed in transparent solution (not shown here). The Bi-Ti precursor was then stirred at 40°C for 2 hours and the colour of the solution changed slightly milky-yellowish as shown in **Figure 1c**. The solution was then continuously evaporated and temperature maintained at ~90°C. The colour of the solution changed. It was found that higher Pr3+ doping tends to prolong hydrolysis process. After that, the temperature increased rapidly to ~120°C and the solution completely evaporated, resulting dark-yellowish gel (**Figure 1e**). The gel started to induced ignition at ~150°C. Metal nitrates were decomposed to form metal oxides and nitrogen. The compound was eventually acting as an oxidizer to continue the combustion synthesis, which was accompanied by the releasing of voluminous gases. At the end of this stage, flaming occurred and resulted foamy-like structure as shown in **Figure 1f**. The flaming temperature was found to be approximate 230°C. In order to remove the carbon

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

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

**Figure 1** shows the actual condition before and after combustion on BTO and Pr3+ doping. The Bi-Ti precursor was observed in clear-yellowish solution (**Figure 1a**) whereas the Pr3+ precursor was found in clear-greenish solution (**Figure 1b**). The Sm3+ precursor was observed in transparent solution (not shown here). The Bi-Ti precursor was then stirred at 40°C for 2 hours and the colour of the solution changed slightly milky-yellowish as shown in **Figure 1c**. The solution was then continuously evaporated and temperature maintained at ~90°C. The colour of the solution changed. It was found that higher Pr3+ doping tends to prolong hydrolysis process. After that, the temperature increased rapidly to ~120°C and the solution completely evaporated, resulting dark-yellowish gel (**Figure 1e**). The gel started to induced ignition at ~150°C. Metal nitrates were decomposed to form metal oxides and nitrogen. The compound was eventually acting as an oxidizer to continue the combustion synthesis, which was accompanied by the releasing of voluminous gases. At the end of this stage, flaming occurred and resulted foamy-like structure as shown in **Figure 1f**. The flaming temperature was found to be approximate 230°C. In order to remove the carbon

750oC for 1 hour to achieve a sample of high density of 97.2% (Kan et al., 2002).

**4.6 Soft combustion synthesis** 

be discussed in the following section.

**5. Observation during the combustion process** 

content in as-combusted powders, the calcination is necessary to enhance the degree of crystallinity with high purity of BTO content.

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.
