**3. Rare-earth doping and other properties**

Recent studies revealed that ion substitution in perovskite BTO structure is an efficient technique for improving the drawbacks of BTO as ferroelectric ceramics (Cui ,Hu, 2009, Santos et al., 2009, Simões et al., 2008). It was reported that the fatigue free films with excellent ferroelectric properties are obtained by substitution of Bi-site ions in BTO films for La3+ ions using a pulsed laser deposition (Park et al., 1999). This ion substitution resulted in large remanent polarization value over 20µC/cm2, which was considerably higher than that of BTO films. Furthermore, it has good fatigue resistance, low leakage current at 10-7 A/cm2 at 5V and low processing temperature approximately in the range of 650 to 700oC. Recently, it has been reported that the substitution of Nd3+ in BTO thin films was more effective for improving the ferroelectric properties than La substitution (Kim ,Kim, 2005). This result can be explained by the fact that the substitution of Bi3+ by rareearth ions with a smaller ionic radius for the Bi3+ site is effective in improving the ferroelectric properties. In this case, the ionic radius of Nd3+ is much smaller than those of Bi3+ and La3+. According to both studies, it is necessary to find out more rare-earth elements with smaller ionic radius in other to enhance the ferroelectric properties of BTO ceramics. Besides that, a small amount of rare-earth elements is important to tailor the microstructures of BTO. Recently, it was reported that Nd doping into BTO ceramics act as a grain-growth inhibitor whereby a remarkable decrease in the grain size with fine and homogeneous microstructure (Kan et al., 2008). It is well known that typical BTO powders

Considerable attention has recently been paid to bismuth layer-structured ferroelectric (BLSF) as ferroelectric materials instead of unfriendly lead (Pb)-based ferroelectrics because of its excellent fatigue resistance and Pb-free chemical composition (Algueró et al., 2006, Subbarao, 1961, Xue et al., 2009, Yang et al., 2003). The general formula is given by (Bi2O2)2+ (Am-1BmO3m-1)2- where A = Bi3+, Pb2+ Sr2+, Ba2+, etc. and B = Nb5+, Ta5+, Ti4+, etc. m = 1, 2, 3, 4, 5, etc. (Bi2O2)2+ is the bismuth oxide layer and (Am-1BmO3m+1)2- is the pseudo perovskite layer (Armstrong ,Newnham, 1972, Newnham et al., 1971, Yan et al., 2006). BLSF is expected to have various numbers of pseudo perovskite blocks in unit cells. BLSF, bismuth titanate, Bi4Ti3O12 (m = 3) or BTO has three pseudo perovskite blocks in half-unit cells. In simple words, its structure can be described as formed by three unit cells of (Bi2Ti3O10)2- with perovskite like structure interleaved with (Bi2O2)2+ layers (Ng et al., 2002). BTO is an attractive material that has low processing temperature (700-750oC) than other BLSF (e.g. SrBi2Ta2O9) and strong anisotropy of the spontaneous polarization (Ps) along the *a*-axis ( ~ 50 µC/cm2) and *c*-axis ( ~ 4 µC/cm2) (Wang et al., 1999, Zhi-hui et al., 2010). However, the low remanent polarization (Pr = 5 µC/cm2), low fatigue resistance and high dielectric loss of BTO would limits its application in FRAM applications. The reduction in remanent polarization and fatigue with high dielectric losses become more serious issues due to defects in perovskite structure whereby the Bi ions volatile during sintering process and create the Bi vacancies accompanied by oxygen vacancies. Nevertheless, there are advantages on BTO whereby it has high Curie temperature at 675oC and high dielectric permittivity ( ~ 200), making this material for other possible applications such as capacitors,

Recent studies revealed that ion substitution in perovskite BTO structure is an efficient technique for improving the drawbacks of BTO as ferroelectric ceramics (Cui ,Hu, 2009, Santos et al., 2009, Simões et al., 2008). It was reported that the fatigue free films with excellent ferroelectric properties are obtained by substitution of Bi-site ions in BTO films for La3+ ions using a pulsed laser deposition (Park et al., 1999). This ion substitution resulted in large remanent polarization value over 20µC/cm2, which was considerably higher than that of BTO films. Furthermore, it has good fatigue resistance, low leakage current at 10-7 A/cm2 at 5V and low processing temperature approximately in the range of 650 to 700oC. Recently, it has been reported that the substitution of Nd3+ in BTO thin films was more effective for improving the ferroelectric properties than La substitution (Kim ,Kim, 2005). This result can be explained by the fact that the substitution of Bi3+ by rareearth ions with a smaller ionic radius for the Bi3+ site is effective in improving the ferroelectric properties. In this case, the ionic radius of Nd3+ is much smaller than those of Bi3+ and La3+. According to both studies, it is necessary to find out more rare-earth elements with smaller ionic radius in other to enhance the ferroelectric properties of BTO ceramics. Besides that, a small amount of rare-earth elements is important to tailor the microstructures of BTO. Recently, it was reported that Nd doping into BTO ceramics act as a grain-growth inhibitor whereby a remarkable decrease in the grain size with fine and homogeneous microstructure (Kan et al., 2008). It is well known that typical BTO powders

**2. Bismuth titanate and other properties** 

antennas, sensors and piezoelectric (Golda et al., 2011).

**3. Rare-earth doping and other properties** 

are attributed to high anisotropic grains, in which the ferroelectric properties are grain orientation dependent. Thus, it can be said that the homogeneity in microstructure is strongly influenced by rare-earth content. In addition, the corresponding microstructure can produce better ferroelectric properties. The summary of doping studies in BTO and their properties are listed in **Table 1**. Based on this summary, the selection of processing route is important to determine the grain orientation and microstructure as well as dielectric and ferroelectric properties. Besides that, the doping studies can improve the dielectric and ferroelectric properties of BTO.


Table 1. Summary of doping studies in BTO and their properties.

Sintering and Characterization of Rare Earth Doped

**4.3 Sol-gel synthesis** 

technique.

**4.4 Hydrothermal synthesis** 

the electrical properties (Yan ,Razak, 2010).

**4.5 Co-precipitation method** 

Bismuth Titanate Ceramics Prepared by Soft Combustion Synthesis 361

the impact energy or milling intensity. Zdujic et al. (Zdujić et al., 2006) reported that a mixture of *α*-Bi2O3 transformed to Bi2O2CO3 at a milling intensity of ~ 0.49 W/g, which in turn was converted directly into a nanocrystalline BTO phase when the intensity was increased to ~ 2.68 W/g. Thus, it can be concluded that the parameter of mechanical milling

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

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

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

has a significance result on the particle morphology and sinterability.
