**1. Introduction**

436 Sintering of Ceramics – New Emerging Techniques

Mullins, W.W.,(1956), Two-dimensional motion of idealized grain boundaries. J. Appl. Phys.

Fathi, M.H.; Kharaziha, M. (2009). Two-step sintering of dense, nanostructural forsterite.

Fradkov, V. E. and Shvindlerman, L. S., (1988) Structure and Properties of Interfaces in

McFadden, S.X. ; Mishra, R.S. ; Valiev, R.Z. ; Zhilyaev, A.P. & Mukherjee, A.K. (1999). Low-

Robert, C.L. ; Ansart, F. ; Degolet, C.L. ; Gaudon, M. & Rousset, A. (2003). Dense yttria

Shahraki, M.M.; Shojaee, S.A.; Sani, M.A.D.; Nemati, A. & Safaee, I. (2010). Two-step

Shvindlerman, L.S., Gottstein, G., (2001), Grain boundary and triple junction migration

Verhasselt, J.C., Gottstein, G., Molodov, D.A., Shvindlerman, L.S., (1999) Acta Mater. 47,

Von Neumann, J., (1952) in: Metal Interfaces, American Society for Testing Materials,

Wang, J.C.; Huang, C.Y.; Wu, Y.C. (2008) Two-step sintering of fine alumina-zirconia

Wang, X.H. ; Chen, P.L. & Chen, I.W. (2006). Two-step sintering of ceramics with constant grain-size, I Y2O3. Journal American Ceramics Society. Vol. 89. Pp. 431-437. Wang, X.H.; Deng, X.Y.; Bai, H.L.; Zhou H.; Qu, W.G. & Li, L.T. (2006). Two-step sintering of

Winning, M., Gottstein,G., Shvindlerman, L.S., in: T. Sakai, G. Suzuki (Eds.), (1999),

Wright, G.J. (2008). Constrained sintering of yttria-stabilized zirconia electrolytes: The

International Journal Applied to Ceramic Technologies. Vol. 5. Pp. 589-596.

ceramics with constant grain-size, II BaTiO3 and Ni-Cu-Zn ferrite. Journal

Recrystallization and Related Phenomena, The Japan Institute of Metals, pp. 451–

influence of two-step sintering profiles on microstructure and gas permeance.

temperature superplasticity in nanostructured nickel and metal alloys. Nature. Vol.

stabilized zirconia: sintering and microstructure Ceramics International. Vol. 29.

27, 900–904.

396. Pp. 684-686.

Pp. 151-158.

887–892.

456.

Cleveland, OH, P. 108.

ceramics. Ceramics international.

American Ceramics Society.Vol. 89. 438-443. Washburn, J., Parker,E.R., J. (1952), Journal of Metals. 4, 1076–1078.

Materials Letters - 63, Pp. 1455–1458.

sintering of ZnO varistors. Solid State Ionics.

Materials Science and Engineering, A302, 141–150.

Soraes, A., Ferro, A. C. and Fortes, M. A., (1941) Scripta metallurgycal., 1985, 19. Swinkels, F.B. & Ashby M.F. (1981). Acta Metallurgycal. Vol. 29. Pp. 259.

Wakai, F. et al. (1990) A superplastic covalent crystal composite. Nature. Vol. 344.

Metals. Nauka, Moscow, p. 213.

All materials respond to stimulus, whether it be an electric field, mechanical stress, heat or light. The manner and degree to which they respond varies and is often what determines which material is selected for a given application. On the most basic level, elastic materials deform in response to mechanical stress and return to their original form when the load is removed. Other materials conduct electricity in response to an applied voltage. Both of these are well-known phenomena, and materials with such behaviors are sometimes called "trivial". On the other hand are pyroelectric and piezoelectric materials, which generate an electric field with a stimulus of heat or mechanical stress, respectively (unexpected phenomenon) and are called "smart" or "functional" materials. Ferroelectric materials are materials that exhibit piezoelectricity and pyroelectricity, as well as the phenomenon which gives them their name (ferroelectricity).

Due to their unique properties, ferroelectric materials are widely used in all areas of electronics and microelectronics, such as cellular phones, computers, cars, airplanes and satellites [KENJI, BUCHANAN]. They have a high discharge dielectric constant (ε) [SHEPARD, RADHESHYAM ,ZHIN], which allows them to be used in high permitivity dielectric devices. Their pyroelectric behaviour is used in heat sensors [PADMAJA, YOO, WHATMORE], and their piezoelectricity is applied in devices like resonadores, sonars, horns, and actuators [GURURAJA, YAMASHITA 1997, YAMASHITA 1998, CHEN]. A combination of their properites are applied in electro-optical devices such as controlable diffraction grids, waveguides, etc. [HEIHACHI, HAMMER, BLOMQVIST]. They are also used in dynamic random access memory (DRAM) [KINGON 2000, KINGON 2006, KOTECKI] and non-volatile memory (NVRAM) [MASUI, KOHLSTEDT].

Many "novel" materials known today were developed many decades ago. Ferroelectric materials are no exception, having been discovered more than seven decades ago. Valasek reported the first ferroelectric material, Rochelle salt (potassium or sodium tetrahydrate tartatre, KNaC4H4O6 • 4H2O) in 1921 [VALASEK]. Subsequently, potassium dihydrogen phosphate (KH2PO4) was identified by Busch and Scherrer in 1935 [BUSCH], and barium titanate (BaTiO3 or BT) was noted for its unusual dielectric properties by Wainer and

Ba1-XSrXTiO3 Ceramics Synthesized by an Alternative Solid-State Reaction Route 439

Philips XL30 ESEM; transmission electron microscopy (TEM) using a JEOL 2010; micro-Raman scattering using a DILOR unit; differential scanning calorimetry (DSC) using a Mettler Toledo; and thermogravimetric analysis (TGA) using a SDTA851 Mettler Toledo. The bulk density of the sintered ceramics was determined using the Archimedes method.

 (1-X)BaCO3 + XSrCO3 + TiO2 → Ba(1-X)SrXTiO3 + CO2 (g) (1) This fabrication method is a modification of the conventional route (solid state reaction) for the manufacture of Ba(1-X)SrXTiO3 ceramics. It requires less processing steps (Figure 2), is more straightforward than other chemical processes and, most important, leads to the

**<sup>1600</sup> ST 1573 K** 

**1h**

**1273 K**

**0 100 200 300 400 500 600 700**

**Time (min)**

**Milling Milling**

**Alternative AlternativeRoute**

**Uniaxial Uniaxial pressing**

**Heat treatment treatment (reaction reaction-sintering sintering)**

It must be noted that the high energy milling process used allows for homogenization and particle size reduction of the starting powders. It is a more efficient process than

Fig. 2. Stages of the proposed alternative route for the manufacture of BSTx.

**5 K/min**

**thermal cycle**

**2h**

**BT 1523 K**

fabrication of high-density ceramics with very low porosity.

Fig. 1. Heat treatment used for the manufacture of BSTx.

**Temperature (K)**

Salomon in 1942-43 [WAINER]. The discovery of ferroelectricity in ceramics from the BaO-TiO2 system was extremely important, as it was the first ferroelastic made from simple oxide materials. Since the discovery of BaTiO3, several other oxide-based ferroelectric materials have been developed, such as strontium titanate (SrTiO3 or ST), lead zirconate titanate (PZT), lead titanate (PbTiO3, PT), lithium tantalate (LiTaO3) phosphate, and potassium titanyl (KTiOPO4) to name a few [MESCHKE, KUGEL, HIDAKA, GOPALAN, ROSENMAN]. The study of BT-based ceramics with stoichiometric compositions different from pure BT has become one of the most important subjects of ferroelectrics in recent years. Particularly, substitution of Sr2+ ions in place of Ba2+ ions into BT leads to a solid solution, barium strontium titanate (BSTx or Ba(1-X)SrXTiO3, where 0 ≤ X ≤ 1). Ferroelectric materials have been synthesized by various techniques, the most commonly used today being the technical or sol-gel process for the production of powders or thin films [BOLAND, PARK, ZHU, KAMALASANAN]. Another technique used to obtain powders is hydrothermal process [XU, RAZAK, VOLD, CHENG]. Finally there is the conventional route solid-state reaction of mixed oxides [VITTAYAKORN, IANCULESCU, CHAISAN] to obtain powders and solid ceramics. The interest of processing highly dense BSTx ceramics is that the Curie temperature and thus the dielectric properties can be tuned using the chemical variations between SrTiO3 and BaTiO3 [BERBECARU, YUN].

This chapter provides the description of an alternative solid-state reaction route based on high energy ball milling and subsequent sintering for the synthesis and densification of BSTx bulk ceramics. It provides a more direct route than the conventional route of mixed oxides. In addition to presenting structural characterization and results of electrical measurements (dielectric constant versus temperature curves and ferroelectric hysteresis loops), a novel technique known as contact resonance piezoresponse force microscopy (CR-PFM) is applied in the detection and characterization of ferroelectric domains in the BSTx samples.
