**24. Hydrothermal synthesis**

The hydrothermal (HT) method, which uses autogenous pressure developed at temperatures above the boiling point of water, has been used especially in the synthesis of various ceramic oxide powders. The advantage of the hydrothermal method is that ceramic materials can be synthesized at relatively low temperatures (100-300C) without milling or calcinations [11]. The particle size and shape can also be controlled by various processing variables such as temperature, pH, and the addition of surfactants ormineralizers. The reaction is controlled by dissolution/precipitation of reactants in an aqueous medium. Therefore, the above processing variables are thought to have a significant influence on the dissolution or precipitation behavior, even though the exact roles or effects are not fully understood and differ in various systems.

For the hydrothermal synthesis, stoichiometric amounts of Gd2O3 and Eu2O3 were dissolved in 100 ml of distilled water acidified by the addition of nitric acid. After the complete dissolution of these oxides, NH4OH solution was added drop wise until the pH of the solution reached 9. White precipitates were instantaneously formed and so-obtained precipitates were washed several times with distilled water by centrifugation. The washed precipitates were mixed with 50 ml solution of H3BO3 and distilled water. The amount of H3BO3 in the solution was adjusted so as to have an appropriatemole ratio with respect to Gd and Eu to give GdBO3:Eu3+. NH4OH was used to adjust pH of the precursor solution to 7–10. After vigorous stirring, the precursor solution was placed in a Teflon-lined stainless steel autoclave with a volume of 100 ml. The solution was heated at 200-240C for 3–10 h" and cooled to room temperature. The resulting powders were filtered and washed several

The Role of Sintering in the Synthesis of Luminescence Phosphors 355

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times with distilled water by centrifugation, and finally suspended in ethanol then dried at 60°C for 5 h. An outline of the experimental procedure is shown in **Fig. 28.** For comparison, the bulk GdBO3:Eu3+ samples are prepared by a solid-state (SS) reaction at 500 and 11000C each for 2 h, respectively, from the mixture of Gd2O3, Eu2O3, and H3BO3 (20 wt.% excess).A high pH and excess H3BO3 promote the growth of GdBO3 in a preferred orientation i.e., (2 0 0).

Fig. 28. XRD patterns of Gd0.9BO3:Eu0.13+ powders synthesized with (a) stoichiometric, (b) excess amount of H3BO3, (c) by hydrothermal method (at 2000C, 10h, pH8) and (d) by solid state reaction [25].

#### **25. References**


times with distilled water by centrifugation, and finally suspended in ethanol then dried at 60°C for 5 h. An outline of the experimental procedure is shown in **Fig. 28.** For comparison, the bulk GdBO3:Eu3+ samples are prepared by a solid-state (SS) reaction at 500 and 11000C each for 2 h, respectively, from the mixture of Gd2O3, Eu2O3, and H3BO3 (20 wt.% excess).A high pH and excess H3BO3 promote the growth of GdBO3 in a preferred orientation i.e., (2 0 0).

Fig. 28. XRD patterns of Gd0.9BO3:Eu0.13+ powders synthesized with (a) stoichiometric, (b) excess amount of H3BO3, (c) by hydrothermal method (at 2000C, 10h, pH8) and (d) by solid

[1] Chun-Chieh Lin, Wei-Ren Liu, and Nae-Lih Wu. Effects of Sintering Fluxes on Structure

[2] L. Guanghuan , L. Tao , S. Yanhua, G. Guimei, X. Jijing, AN Baichao, G. Shucai and H.

Na, K) Phosphors. J. Rare Earths, Vol. 28, No. 1, Feb. 2010, p. 22

Meeting, © 2010 The Electrochemical Society

and Luminescence of BaMgAl1O17:Eu2+ Phosphor. Abstract #2345, 218th ECS

Guangyan. Preparation and luminescent properties of CaAl2O4:Eu3+,R+ (R=Li,

state reaction [25].

**25. References** 


**16** 

*Malaysia*

**Sintering and Characterization of Rare Earth** 

**Soft Combustion Synthesis** 

*Universiti Sains Malaysia, USM, Pulau Pinang* 

Khairunisak Abd. Razak and Srimala Sreekantan

**Doped Bismuth Titanate Ceramics Prepared by** 

Umar Al-Amani Azlan, Warapong Krengvirat, Ahmad Fauzi Mohd Noor,

Nowadays, ferroelectric ceramics and thin films have attracted much attention for various studies which are generally used for numerous potential applications in ferroelectric random access memory (FRAM), in microelectronic mechanical system (MEMS), non-linear optical devices, surface acoustic wave devices, tunable capacitors, sensing applications or pyroelectric detectors (Besland et al., 2006, Yang et al., 2008). The main focus to develop the ferroelectric thin films has started in 1980s (Besland et al., 2006). Several methods were initially used for deposition of thin films such as conventional dipping, sputtering and spin coating techniques. Typically, the deposition of thin films will be more complex to obtain a good layer that consists of several ferroelectric compounds on substrate. Furthermore, high precision deposition technique is essential to control the desired thickness and surface layer of ferroelectric compounds. As far as our concern, the up to date technique such as physical vapor deposition (PVD), RF sputtering, chemical vapor deposition (CVD) and metal-organic chemical vapor deposition have been frequently used in many studies to obtain a better ferroelectric thin films condition. Nevertheless, a major concern on expensive equipment and experience user limit this technique in many studies. In order to develop the ferroelectric materials, the preparation in the form of bulk ceramics has been extensively studied. Up to now, several methods, including solid state reaction, hydrothermal synthesis, mechanical activation technique, sol-gel method, co-precipitation method were used for the preparation of bulk ceramics. Recently, the soft combustion synthesis is used as alternative route since it offers several beneficial points to the processing element and the properties of ceramics (Yan ,Razak, 2010). Bismuth titanate, Bi4Ti3O12 or BTO has received a lot of attention as dielectric and ferroelectric materials. Many studies have been conducted in various processing route to improve the microstructure that has a significance effect on dielectric and ferroelectric properties (Hardy et al., 2004, Pookmanee et al., 2004, Zhi-hui et al., 2010). In addition, a modification on basic compound is essential to enhance those properties. Since BTO is also sought as a potential material for dielectric application, in this chapter the effect of Sm3+ and Pr3+ doped-BTO was prepared and characterized by soft combustion technique. In order to investigate a possible application as wireless dielectric

antenna, the dielectric study at different frequencies was carried out.

**1. Introduction** 

