We report the impact of Ba sources in the sonochemically activated solid-state synthesis of BaTiO₃ powders. A water-soluble Ba(CH₃COO)₂ and TiO₂ powders were sonochemically mixed in aqueous medium with controlled pH values and subsequently calcined following a drying process. For a comparative study, solid-state BaCO₃ powders were also used to synthesize BaTiO₃ powders under the same ultrasonication, drying, and calcination conditions. Phase conversion ratio to BaTiO₃ and structural properties of the resulting powders for each case were characterized and compared in detail. We expect that this study will be helpful in understanding the process for synthesizing and doping BaTiO₃ powders using water-soluble raw materials.

Keywords: BaTiO₃, Powder, Sonochemical, Solid-state reaction.

Solid-state synthesis of BaTiO₃ has been carried out conventionally by using BaCO₃ and TiO₂ raw powders, as described in (1) [1-4].



In this process, the reaction takes place by coupled diffusion of Ba²⁺ and O^2- ions into the TiO₂ lattice from the contact point between the raw materials [3-5]. Therefore, the intimate and uniform mixing of BaCO₃ and TiO₂ to increase the initial contact surface area have been considered critical factors for the process. Accordingly, efforts have been made to promote the solid-state reaction of BaTiO₃ by utilizing nanometer-sized ultrafine raw materials or by reducing the particle size through high-energy milling processes [6-17].
Recently, it has been reported that the mixing process of raw materials can be effectively performed by using ultrasonication process within only a few minutes and that the solid-state reaction in (1) can be accelerated by the sonochemical activation effect [18-21]. We reported that when the raw materials were dispersed in aqueous medium, the sonochemical mixing process was strongly affected by pH of the aqueous medium due to the pH-dependent partial solubility of BaCO₃ in water [18].
Therefore, it is also intriguing to study the solid-state reaction using a water-soluble source instead of the solid-state source of BaCO₃ with a partial solubility since the solubility of Ba source was proved to be a critical factor for the sonochemically activated solid-state reaction in our previous study [18].
In this study, the effect of Ba-sources for sonochemically activated solid-state synthesis of BaTiO₃ powders is presented. We ultrasonicated the mixtures of TiO₂ powder with different Ba sources of water-soluble Ba(CH₃COO)₂ and solid-state BaCO₃ in aqueous medium with adjusted pH values and compared the properties of resulting powders. The results revealed that the phase conversion ratio through the solid-state reaction was critically dependent on the type of Ba source. Detailed microstructural and crystal structural characterizations were conducted on each resulting BaTiO₃ powder and compared.

Sample preparation
Barium acetate (Ba(CH₃COO)₂, ≥99%, Alfa Aesar, USA) and titanium dioxide (TiO₂, ≥99%, Sigma-Aldrich, USA) were used as raw materials, with the pH of distilled water controlled using potassium hydroxide (KOH) and hydrochloric acid (HCl) solutions. 150 ml of distilled water with controlled pH values (3, 5, 7, 9 and 11) was placed in a 250 ml beaker, and Ba(CH₃COO)₂ and TiO₂ were introduced stoichiometrically into the solvent as shown in Fig. 1. Sonochemical mixing process was carried out for 5 min at an ultrasonic power of 900 W using an ultrasonic homogenizer (Boshi Electronic Instrument, Hangzhou, China). The ultrasonicated mixtures were oven-dried at 120 °C for 12 hours, and the resulting dried powder was ground using a mortar and pestle. The powder was calcined in air at 900 °C (heating rate=5 °C/min) for 3 h for the solid-state synthesis of BaTiO­₃. The synthesized powder with the water-soluble Ba-source was referred to as W-BT. In addition, for a comparative study with the conventional solid-state synthesis of BaTiO₃, the same experimental procedure was carried out using barium carbonate (BaCO₃, 99%; Sigma-Aldrich, USA) and TiO₂ as raw materials. The synthesized powder was referred to as solid-state source BaTiO₃ (S-BT).
Characterizations
Crystal structures of calcined powders were investigated using an X-ray diffractometer (XRD, Ultima IV, Rigaku Corp, Tokyo, Japan) and Rietveld refinements were performed using Profex software (ver. 3.14.3). microstructural characterization was carried out using a field emission scanning electron microscope (FESEM, Mira 3, Tescan, Brno, Czech) and a transmission electron microscope (TEM, Titan Themis Z, FEI Company, USA). In addition, particle size analysis of the powder was conducted using a particle size analyzer (PSA, LA-950, Horiba Ltd., Japan).

Effect of pH on the particle size of ultrasonicated raw materials
Fig. 2(a) shows a SEM micrograph of water-soluble Ba(CH₃COO)₂ raw powder before ultrasonication, and (b)-(d) represent their microstructures after ultrasonication in aqueous medium at various pH levels and the subsequent drying process. All dried powders after ultrasonication exhibited a smaller particle size than untreated powder and showed a tendency that the particle size increased with the increase in pH of solvent. It has been reported that the solubility of the barium compounds decreases with an increase in the pH [22]. Therefore, Ba(CH₃COO)₂ dissolved in high pH aqueous medium recrystallizes rapidly during drying, and the particles formed through this process will grow in size. This can be considered the factor influencing the pH-dependent particle size after dissolution with ultrasonication and subsequent recrystallization.
Fig. 3(a) shows a SEM micrograph of solid-state BaCO₃ raw powder before ultrasonication, and (b)-(d) represent their microstructures after ultrasonication in aqueous medium at various pH levels and the subsequent drying process. It has been reported that BaCO₃ is partially soluble in water and its partial solubility is dependent on the pH [18, 23, 27]. However, although the degree of the fragmentation in BaCO₃ powder decreased with increasing pH [18], the changes in size with respect to pH were not significant and all resulting powders exhibited a smaller particle size and a narrower size distribution compared to the Ba(CH₃COO)₂ powders in Fig. 2.
Fig. 4(a) shows a TEM micrograph of TiO₂ raw powder before ultrasonication, and (b)-(d) represent their microstructures after ultrasonication in aqueous medium at various pH levels and the subsequent drying process. Regardless of the pH, no significant change was observed in the particle size of TiO₂ before and after the sonochemical process and the particle size was ~80 nm for all samples. It has been reported that TiO₂ is chemically stable and does not dissolve in water [24], and the influence of pH on the TiO₂ particle size was negligible also in this experiment.
Effect of pH on the solid-state reaction for BaTiO₃
Fig. 5(a) shows XRD patterns of W-BT powders synthesized by the calcination of sonochemical mixtures of water-soluble Ba(CH₃COO)₂ and TiO₂ at 900 ^oC for 3 h. For pH = 3, the powder was composed of major BaTiO₃ phase and a secondary Ba₂TiO₄ phase. BaCO₃ phase started to appear from pH = 5, and multiphase of BaTiO₃, Ba₂TiO₄ and BaCO₃ were observed in W-BT powders up to pH = 11.
Rietveld refinements on the XRD patterns were performed to investigate the phase conversion ratio to BaTiO₃ in the powders depending on the starting materials and pH. As shown in Fig. 6, a high conversion ratio of ~95% was obtained in the solid-state synthesized W-BT only for pH = 3. For higher pH values (pH = 5-11), a relatively low BaTiO₃ phase conversion ratio of 40-50% was obtained in W-BT powders. On the other hand, S-BT, which uses conventional solid-state source of BaCO₃ and TiO₂, exhibited almost single perovskite phase regardless of pH as shown in Fig. 5(b) and Fig. 6.
Because the solid-state reaction takes place through the coupled diffusion of Ba²⁺ and O^2- ions into the TiO₂ lattice at the contact point between the starting materials [3-5], phase conversion ratio is determined by the properties of starting powders such as particle size, distribution, and surface area. Therefore, there have been reports on the enhancement of the solid-state reaction for BaTiO₃ by using nanosized raw materials to maximize the contacting area [6-17]. In Figs. 2 and 3, we observed the particle size of ultrasonicated Ba sources under different pH values. Regardless of pH values, the particle size of BaCO₃ source was much smaller than that of Ba(CH₃COO)₂ source and that is the reason for that the conversion ratio in S-BT was much higher than that in W-BT. Among the W-BT powders, the conversion ratio was much higher at pH = 3 than at pH ≥ 5 and this can be attributed to the fact that the particle size of Ba(CH₃COO)₂ was much smaller than the others, as shown in Fig. 2.
On the other hand, the conversion ratio was higher than 98% for all S-BT powders regardless of the pH in the aqueous medium. In this experiment, the calcination of the ultrasonicated mixtures for S-BT was carried out only at 900 ^oC for 3 h and this is relatively low temperature for the solid-state reaction for BaTiO₃ as compared to the conventional calcination temperatures in literature [1, 18, 19]. Therefore, the high conversion ratio of S-BT in this experiment shows the effect of the sonochemical activation on the enhancement of the solid-state reaction and this result is quite consistent with our previous reports [18, 19].
Structural characterizations of W-BT and S-BT
For microstructural characterizations, we selected W-BT and S-BT powders synthesized at pH = 3 as they both exhibited a sufficiently high conversion ratio for comparative analysis. Fig. 7(a) shows SEM micrographs of the W-BT powder and the enlarged micrograph is shown in the inset. Strongly agglomerated particles (> 10 µm) were easily observed in W-BT even though its primary particle size was ~160 nm as shown in the inset. On the other hand, Fig. 7(b) shows that the agglomeration in S-BT was much weaker than that in W-BT. Even in this case, the primary particle size was measured to be ~160 nm. It has been reported that the crystallite size of solid-state synthesized BaTiO₃ powder is predominantly affected by the initial TiO₂ particle size [25, 26]. Because the particle size of TiO₂ was not changed by the sonochemical process in aqueous medium as shown in Fig. 4, this result is consistent with the reported results. With this result, it was found that different barium sources only affect the degree of agglomeration of primary particles, not the primary particle size, in this set of BaTiO₃ powders. To investigate the degree of the agglomeration quantitatively, we also carried out the particle size analysis (PSA) for the powders and the results are shown in Fig. 8. The D50 of W-BT was measured to be 16.7 µm, while for S-BT, it was 2.2 µm. These quantitative results are quite consistent with the qualitative observations in Fig. 7.
The structural characterizations of W-BT and S-BT powders were conducted through Rietveld refinements using the XRD patterns shown in Fig. 5, and the summarized results are presented in Table 1. The lattice parameters, volume and tetragonality were almost similar in both powders. Therefore, it was found that Ba sources do not affect the crystallographic properties of the resulting primary BaTiO₃ particles, but rather only influence the degree of agglomeration.

In summary, we reported the effect of Ba-sources on the sonochemically activated solid-state synthesis of BaTiO₃ powders. We ultrasonicated the mixtures of TiO₂ powder with different Ba sources of water-soluble Ba(CH₃COO)₂ and solid-state BaCO₃, and we compared the properties of resulting powders. When we used water-soluble source of Ba(CH₃COO)₂, the phase conversion to BaTiO₃ showed a strong pH dependence. Except for pH = 3, the conversion ratio of W-BT powders was quite low (40-50%) and this was mainly attributed to the change in the particle size of Ba source during the ultrasonication and drying process depending on pH in the aqueous medium. On the other hand, when using the solid-state BaCO₃ powder, the conversion ratio surpassed 98% for all S-BT powders under the same calcination condition (900 ^oC for 3 h in air). Considering that the BaCO₃ was micron-sized, this high conversion ratio indicates that the solid-state reaction to BaTiO₃ was significantly accelerated by the sonochemical mixing process compared with the conventional solid-state reaction employing a ball-mill process. Detailed microstructural and crystal-structural characterizations of W-BT and S-BT powders revealed that the properties of primary BaTiO₃ particles were not significantly affected by the Ba sources. However, the degree of agglomeration of primary particles was strongly associated with the type of Ba source. We expect that this study will be helpful in understanding the process for synthesizing and doping BaTiO₃ powders using water-soluble raw materials.

This work was supported by a Research Grant from Pukyong National University (2023).