**2. Experimental procedures**

Titanium oxide (TiO2 - 99.9% purity, anatase phase, Aldrich), barium carbonate (BaCO3 - 99.9% purity, Aldrich) and strontium carbonate (SrCO3 - 99.9% purity, Aldrich) powders were weighed according to the stoichiometric proportion of Equation 1. The mixture was ball-milled in a high energy vibratory mill (SPEX 8000) for 6 h using a nylamid vial with 10 toughened zirconia balls (10 mm diameter) and a ball-to-powder weight-ratio of 10:1. The milled powders were uniaxially pressed in a stainless steel cylindrical die (10 mm inner diameter) using 1 GPa pressure. The green compacts were placed on top of TiO2 substrates inside alumina crucibles (to avoid the reaction of barium oxide with the crucibles) and reactively sintered using a two-step heating program (Figure 1). The compacts are heated from room temperature up to 1273 K (reaction temperature) at 5 K/min and held there for 1 h. The temperature is then increased to between 1523 and 1573 K (sintering temperature) at 3.0 K/min and held there for 2 h. Finally they are cooled to room temperature at 3.0 K/min. The resulting BSTx sintered ceramics with the perovskite ABO3 structure complied with the formula Ba(1-X)SrXTiO3 where X = 0, 0.1, 0.2 … 1 and were named accordingly, e.g. BST3 refers to the X = 0.3 composition. The starting powders, milled powders and sintered ceramics were characterized by x-ray diffraction (XRD) using a Rigaku Dmax-2100 diffractometer equipped with Co Kα radiation; scanning electron microscopy (SEM) using a

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

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

Titanium oxide (TiO2 - 99.9% purity, anatase phase, Aldrich), barium carbonate (BaCO3 - 99.9% purity, Aldrich) and strontium carbonate (SrCO3 - 99.9% purity, Aldrich) powders were weighed according to the stoichiometric proportion of Equation 1. The mixture was ball-milled in a high energy vibratory mill (SPEX 8000) for 6 h using a nylamid vial with 10 toughened zirconia balls (10 mm diameter) and a ball-to-powder weight-ratio of 10:1. The milled powders were uniaxially pressed in a stainless steel cylindrical die (10 mm inner diameter) using 1 GPa pressure. The green compacts were placed on top of TiO2 substrates inside alumina crucibles (to avoid the reaction of barium oxide with the crucibles) and reactively sintered using a two-step heating program (Figure 1). The compacts are heated from room temperature up to 1273 K (reaction temperature) at 5 K/min and held there for 1 h. The temperature is then increased to between 1523 and 1573 K (sintering temperature) at 3.0 K/min and held there for 2 h. Finally they are cooled to room temperature at 3.0 K/min. The resulting BSTx sintered ceramics with the perovskite ABO3 structure complied with the formula Ba(1-X)SrXTiO3 where X = 0, 0.1, 0.2 … 1 and were named accordingly, e.g. BST3 refers to the X = 0.3 composition. The starting powders, milled powders and sintered ceramics were characterized by x-ray diffraction (XRD) using a Rigaku Dmax-2100 diffractometer equipped with Co Kα radiation; scanning electron microscopy (SEM) using a

the detection and characterization of ferroelectric domains in the BSTx samples.

between SrTiO3 and BaTiO3 [BERBECARU, YUN].

**2. Experimental procedures** 

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.

$$\text{(1-X)}\\\text{BaCO}\_3 + \text{XSrCO}\_3 + \text{TiO}\_2 \rightarrow \text{Ba}\_{\text{(1-X)}}\\\text{Sr}\_{\text{X}}\text{TiO}\_3 + \text{CO}\_2\text{(g)}\tag{1}$$

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 fabrication of high-density ceramics with very low porosity.

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

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

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

Ba1-XSrXTiO3 Ceramics Synthesized by an Alternative Solid-State Reaction Route 441

The x-ray diffraction patterns for the milled BaCO3, TiO2 and SrCO3 powder mixtures are presented in Figure 5. The main BaCO3 peak, located at approximately 2θ = 28° is present up to X = 0.9, and the small peak at approximately 30º belongs to TiO2. As expected, increasing the SrCO3 content increases the height of the SrCO3 peak located at approximately 30º. There is a degree of amorphization due to the creation of defects in the crystal structure. Figure 6 presents the thermogravimetric analyses (TGA) curves for the same collection of samples as analyzed by x-ray diffraction (Figure 5). All the curves are similar in their key characteristics. The weight loss behavior for a single sample, with X = 0.65, is shown in Figure 8. There are four stages of weight loss centered at 403, 773, 973 and 1273 K. In the first stage, from room temperature (RT) up to 403 K, a weight reduction of approximately 1.3% occurs due to evaporation of water from the material surface. In the second stage, from 403 to 773 K, the weight loss of about 5.2% is related to the loss of chemically bound water in the form of OH groups from the BaOH that was formed when BaO combined with water during milling [BALÁZ]. This loss typically occurs between 473 and 873 K [ASIAIE]. The phenomenon of water loss has also been reported to occur in other carbonates between 593 and 723 K [DING]. The third stage, between 773 and 973 K, is not related to any structural modification of the BSTx samples. According to the literature, the decomposition of strontium and barium carbonates to form CO2 occurs at higher temperatures, between 1023 and 1273 K [JUDD, L'VOV, MAITRA]. However, in this case, the generation of CO2 begins as early as at 833 K and runs up to 1273 K. The x-ray difffraction patterns of Figure 7 show the appearance of a peak (2θ ≈ 36°) at 873 K, which corresponds to the formation of the perovskite structure of BaTiO3/SrTiO3. Therefore, the weight loss from 773 to 1323 K can be considered a single stage that varies with Sr content. It is directly related to the CO2 excess from the carbonates used as starting powders (see Equation 1). It is the difference between the weight of the (1-X)BaCO3 + XSrCO3 + TiO2 starting powders and that of the resulting Ba(1-X)SrXTiO3. Table 1 lists the total weight loss, the loss in the different stages, and the weight loss expected from CO2 liberation. For the group of samples as a whole, an approximate 5% weight loss was observed between RT and 773 K, and the weight loss from

773 to 1323 K corresponds closely to the stoichiometric CO2 loss.

**SrCO3**

**Intensity (a. u.)**

**BaCO3**

**30 40 50 60**

**2**θ

Fig. 5. X-ray diffraction patterns for (1-X)BaCO3 + XSrCO3 + TiO2 milled powders.

**(1-X)BaCO3**

**milled powders** 

 **+ XSrCO3**

**X = 1.0 X = 0.9 X = 0.7 X = 0.5 X = 0.3**

**X = 0.0**

 **+ TiO2**

conventional attrition, planetarium or automatic-agate milling. The milled powders were compacted using much higher pressure (~1.0 GPa) than applied using the conventional approach (~0.1 GPa). This resulted in uniform green compacts which can conduct an homogeneous chemical reaction. Finally, the thermal treatment induces simultaneous reaction and sintering (reaction-sintering, Figure 1) as opposed to the conventional manufacturing process where the starting powders are milled, thermally-treated to react, a second milling process is performed, and then the twice-milled powders are pressed into compacts. Finally, a second thermal treatment (sintering) densifies the compacts.
