Controlling Composite TiO₂ Powder Characteristics in the Solid-State Synthesis of BaTiO₃ Powders for Improved Sintering and Permittivity: A Comparative Study

Abstract

In this study, the anatase–rutile phase fraction of TiO₂, which would play a vital role in the solid-state synthesis of BaTiO₃, was controlled to form a composite phase. The composite TiO₂ was applied to a solid-state synthesis reaction, and the tetragonality, dielectric properties, and microstructure of the resulting BaTiO₃ powders were analyzed under different synthesis conditions of TiO₂. Furthermore, a comparative analysis of solid-state-synthesized BaTiO₃ powders prepared using anatase, rutile, and mixed (anatase + rutile) TiO₂ was performed to elucidate the mechanism of improvement in the sintering behavior and dielectric properties of the BaTiO₃ powder synthesized using the composite TiO₂. As a result of applying composite TiO₂, BaTiO₃ powder with excellent tetragonality was synthesized. In addition, it is effective in powder growth and the control of powder morphology, so sintering and dielectric properties can be improved at relatively low temperatures.

1. Introduction

Recently, research and development (R&D) related to future mobility solutions, including electric vehicles (EVs), have received increasing attention to foster clean, sustainable mobility and the development of electronic equipment components for next-generation communication technologies, such as 5G and 6G. As a result, the demand for passive components in electronic devices with a high energy density and high power density has increased. In particular, active R&D has been focused on the miniaturization of multi-layer ceramic capacitors (MLCCs), which account for the largest proportion of passive components [1,2]. The miniaturization of electronic products leads to the issue of limited mounting space for the electronic components. To address this limitation, the development of ultra-thin MLCCs is essential. BaTiO₃, a typical ferroelectric, has a high dielectric constant at room temperature, low dielectric loss, and excellent insulation resistance, so it is used as a main raw material for MLCC. Generally, perovskite-structured small-sized ferroelectric BaTiO₃ powders are first synthesized, and then, dielectric slurries are prepared for fabricating a few micrometer-thick dielectric sheets. These sheets are then stacked to form hundreds of layers to produce a compact, high-capacitance MLCC [3,4]. Typically, BaTiO₃ powders for application in miniature MLCCs are prepared via hydrothermal synthesis. The hydrothermal synthesis method is advantageous for the preparation of homogeneous BaTiO₃ particles with a narrow size distribution [5,6,7,8,9]. In particular, nanopowder synthesis using this method is simple, as the process does not involve any heat treatments. However, in the case of the BaTiO₃ powder synthesized through the hydrothermal method, the lattice hydroxyl groups (–OH) attack the oxygen lattice, forming oxygen vacancies through a charge compensation mechanism. The hydroxyl defects develop into intragranular pores, which results in a decrease in the dielectric constant [10,11]. Furthermore, during BaTiO₃ grain growth via Ostwald ripening at temperatures of 800 °C or higher, internal pores coalesce and grow. Consequently, defects are inevitably present in BaTiO₃ synthesized through the hydrothermal method [12]. Recently, significant efforts have been devoted to the R&D of MLCCs for diverse applications, such as electric/electronic parts for the automotive industry. This has led to a growing interest in the R&D of the solid-state synthesis of BaTiO₃ rather than the hydrothermal synthesis of BaTiO₃, as the former is conducive to particle size atomization [13,14]. In the solid-state synthesis of BaTiO₃, powders of carbonates and oxides, such as BaCO₃ and TiO₂, are mixed, and solid-state reaction with heat treatment at appropriate high temperatures is subsequently performed. During this process, Ba²⁺ ions diffuse into the TiO₂ surface and form an initial BaTiO₃ layer at the BaCO₃/TiO₂ grain boundaries. Continuous reaction of the as-formed layer with BaCO₃ results in the formation of a Ba₂TiO₄ intermediate phase. Subsequently, with a continuous supply of thermal energy through heat treatment, BaTiO₃ is produced through the reaction between Ba₂TiO₄ and TiO₂. The diffusion of Ba²⁺ ions at the TiO₂ interface is the key mechanism of the solid-state synthesis reaction [1,13,15,16,17,18,19,20,21].

In this study, TiO₂, an important component in the solid-state synthesis reaction of BaTiO₃, was controlled, and the corresponding changes in various properties of solid-state-synthesized BaTiO₃ powders were examined. Anatase- and rutile-phase TiO₂ have different diffusion rates and reaction rates due to their different crystal structures. Therefore, BaTiO₃ powder was prepared by controlling the ratio of the two phases with different properties of TiO₂, and the change in properties was examined. The composite TiO₂ was synthesized by heat-treating anatase TiO₂ to control the anatase–rutile phase ratio. In order to assess the effect of composite TiO₂ on the solid-state synthesis of BaTiO₃, mixed TiO₂ with the same phase ratio and a reference TiO₂ (anatase, rutile) were also prepared. The prepared TiO₂ was applied to the solid-phase synthesis of BaTiO₃, and the effect of the composite TiO₂ with a controlled anatase–rutile phase ratio on the properties of BaTiO₃ was investigated in detail.

2. Experimental Section

2.1. Preparation of Composite TiO₂

For the preparation of composite TiO₂, 80 nm anatase TiO₂ (purity > 99.8%, Ditto Technology Co., Ltd., Gunpo, Republic of Korea) was used as the starting material. To control the anatase–rutile phase ratio, heat treatment was performed at a temperature range of 800 to 900 °C for 1 to 9 h. In addition, anatase TiO₂ and 60 nm rutile TiO₂ (purity > 99.8%, Armstec Ind. Co., Ltd., Hwaseong, Republic of Korea) were weighed and mixed such that the anatase–rutile phase ratio matched with that of the heated composite TiO₂.

2.2. Solid-State Synthesis of BaTiO₃

For the solid-state synthesis of BaTiO₃, 150 nm BaCO₃ (purity > 99.8%, Qingdao Terio Corporation, Qingdao, China) and TiO₂ (anatase, rutile, composite, and mixed TiO₂) were mixed at a ratio of 1:1 in moles. To ensure uniform dispersion, a dispersing agent (DISPERBYK-111, BYK Additives & Instruments, Wesel, Germany) and ethanol (ethyl alcohol 99.5%, Daejung Chemicals & Metals Co., Ltd., Siheung, Republic of Korea) were added, and ball milling was performed for 24 h using 3 Φ beads. After ball milling, the powder was dried in an exhaust-fan-type oven at 100 °C for 24 h to remove ethanol. For the synthesis of BaTiO₃ powder using the dried mixed powder, calcination was performed for 2 h at a heating rate of 5 °C/min at temperatures of 1000 to 1100 °C in an air atmosphere.

2.3. Fabrication of Sintered Specimens

To the calcinated powder, 10 wt% polyvinyl alcohol (PVA 1500, Daejung Chemicals & Metals Co., Ltd., Siheung, Republic of Korea) was added, and the mixture was compressed at a pressure of 127 MPa for 1 min. Twenty round disk-shaped specimens of 10 Φ size were fabricated under uniaxial compression for each condition. After loading the fabricated disk specimens into a furnace, the specimens were sintered at 1300 °C for 2 h in an air atmosphere at a heating rate of 5 °C/min. Ag paste was applied to the sintered disk specimens, which were heated at 700 °C for 10 min at a heating rate of 1 °C/min to form an external electrode.

2.4. Evaluation

2.4.1. Dielectric Properties

Dielectric properties were evaluated under the conditions of 1 kHz and 1 V at ambient temperature using an LCR meter (E4980A, KEYSIGHT TECHNOLOGIES, Santa Rosa, CA, USA). In addition, the change in the value of the dielectric constant was observed by varying the temperature (from −55 to 160 °C).

2.4.2. Determination of the Crystal Structure

The crystal structures of composite TiO₂ prepared through heat treatment and the synthesized powder were determined via X-ray diffraction (XRD) using Cu Kα radiation (λ = 1.54 Å) in the 2θ range of 20° to 80° (D8 Advance A25, BRUKER, Billerica, MA, USA). The following equation was used, considering the intensity of the main XRD peak (101/110), to obtain the ratio of the anatase and rutile phases of the composite TiO₂ under each heat treatment condition [22]:

$$R\left(T\right)=0.679\frac{I_R}{I_R+I_A}+0.312({\frac{I_R}{I_R+I_A})}^2$$

where R(T) indicates the content (%) of the rutile phase at temperature T, whereas I_A and I_R denote the intensity of the main peak of the anatase (main peak = (101), 2Θ = 25.3°) and rutile phases (main peak = (110), 2Θ = 27.44°), respectively. In XRD analysis, Rietveld refinement was used to determine the tetragonality (c/a) of the BaTiO₃ powder.

2.4.3. Microstructure

The microstructure and crystal structure of the composite TiO₂ powder prepared through heat treatment were characterized using high-resolution transmission electron microscopy (HRTEM, JEM-4010, JEOL Ltd., Tokyo, Japan). The morphology and size of particles and grains in the microstructure of the as-synthesized BaTiO₃ powder and sintered specimens were analyzed using field-emission scanning electron microscopy (FE-SEM, CLARA, TESCAN, Brno, Czech Republic).

3. Results and Discussion

Anatase TiO₂ with a particle size of 80 nm was pre-annealed at a temperature range of 800 to 900 °C for 1 to 12 h. Characterization of the anatase TiO₂ powder pre-annealed at 800 °C for 1 h revealed the typical crystal structure of anatase TiO₂. When the pre-annealing time was varied in the range of 3 to 5 h, the fraction of the rutile phase increased, as confirmed via XRD analysis. After pre-annealing at a temperature of 800 °C for 5 h, anatase–rutile composite TiO₂ powder with 18% rutile TiO₂ was obtained. When the temperature was increased to 850 °C and the annealing time was increased, a smooth transformation of the anatase to the rutile phase was achieved. After annealing at 900 °C, an analysis of the resulting crystal structure showed that most of the TiO₂ underwent phase transformation to form rutile TiO₂ within a short annealing time (Figure 1a,b). Therefore, the crystal structure analysis results reveal that controlling the anatase–rutile phase fraction of the composite TiO₂ to form different crystal structures is possible by altering the pre-annealing conditions (Figure 1c). In the subsequent solid-state synthesis of BaTiO₃, TiO₂ samples obtained under pre-annealing conditions #1 to #3 and #5 were used (Figure 1d).

The anatase phase of TiO₂ has an unstable crystal structure, and under specific temperature conditions, it spontaneously transforms into rutile TiO₂, which is a stable phase [23,24,25,26,27]. The crystal structure analysis showed that the phase transformation of the anatase TiO₂ to the crystal structure exhibiting a composite phase with approximately 18% rutile phase occurred under the conditions of annealing at 800 °C for 5 h (Figure 1). Figure 2 shows the results of the HRTEM analysis of the annealed TiO₂ powder. Grains of larger and smaller sizes were observed in the powder. Fast Fourier transformation (FFT) analysis of the HRTEM images confirmed that the larger grains possessed the anatase crystal structure, whereas smaller grains were rutile phase TiO₂. These observations indicate that when anatase TiO₂ was annealed at the given temperature, small grains could transform to the rutile phase.

Mixed powders were prepared by mixing TiO₂ synthesized under different annealing conditions (#1 to #3; #5) with BaCO₃ with a molar ratio of 1:1. In addition, anatase and rutile TiO₂ powders were mixed to ensure that the anatase–rutile phase ratio was the same as that of the heated TiO₂ powder prepared under respective annealing conditions. The prepared TiO₂ powder was mixed with BaCO₃ at a molar ratio of 1:1 to obtain a BaCO₃–TiO₂ powder. Furthermore, powders were prepared by separately mixing 100% anatase TiO₂ and 100% rutile TiO₂ with BaCO₃ at a molar ratio of 1:1 (

Table 1. Composition of prepared powders for the solid-state synthesis of BaTiO₃.

		TiO2
Heated TiO2	BT#1	#1
	BT#2	#2
	BT#3	#3
	BT#4	#5
Mixed TiO2	BT#5	A:R = 82:18
	BT#6	A:R = 65:35
	BT#7	A:R = 41:59
	BT#8	A:R = 13:87
Ref.	BT(A)	Anatase TiO2
	BT(R)	Rutile TiO2

).

The BaCO₃–TiO₂ powders, mixed as described above, were calcined at 1000 and 1100 °C for 2 h. XRD analysis of the powders obtained via solid-state synthesis at 1000 °C under respective mixing conditions revealed the presence of typical BaTiO₃ crystal structures. By analyzing the diffractions from the (002) and (200) planes of the synthesized BaTiO₃ powders, a structure closely resembling a cubic structure, without distinct peak splitting of the (002) and (200) planes, was confirmed for the BaTiO₃ powder obtained via solid-state synthesis using 100% anatase TiO₂ and 100% rutile TiO₂, respectively (Figure 3a,b). In addition, observing clear 002/200 peak splitting was difficult in the case of the BaTiO₃ powder synthesized using a simple mixture of anatase and rutile TiO₂ powders. However, in the XRD patterns of the BT#1 to BT#4 powders synthesized using the composite TiO₂ powder prepared under the annealing conditions of #1 to #4, apparent 002/200 peak splitting appeared. In the case of BaTiO₃ powders BT#1 to BT#4 synthesized at a temperature of 1100 °C, apparent peak splitting was observed on the 002/200 planes. However, 002/200 peak splitting was not apparent in the case of the BT#5 to BT#8 powders, which used the simply mixed TiO₂ powder.

Tetragonality (c/a ratio) means the ratio of the a-axis lattice constant to the c-axis lattice constant. Tetragonality is the source of the high dielectric properties of BaTiO₃ and is one of the important criteria used to evaluate ferroelectric BaTiO₃ powder. The a-axis and c-axis lattice constants were obtained by calculating the (200) and (002) peaks between 44 and 46° (2θ) of the measured XRD pattern using Rietveld analysis [28,29]. Rietveld refinement of the XRD results revealed that the BaTiO₃ powders synthesized using composite TiO₂ powders at 1000 and 1100 °C had excellent tetragonality values of 1.0094–1.01 (1000 °C) and 1.0102–1.0104 (1100 °C), respectively (Figure 4). However, when mixed TiO₂ was used for the synthesis, the resulting powders exhibited low tetragonality values of 1.0048–1.0076 (1000 °C) and 1.0077–1.0087 (1100 °C).

First, a solid-state synthesis reaction was performed at 1000 °C under the mixing conditions of BT(A) and BT(R). Microstructure observations conducted after the synthesis under the respective conditions revealed grain growth of the BaTiO₃ powders (Figure 5). The low tetragonality, as indicated by the XRD analysis results, is attributed to the lack of sufficient grain growth [30,31]. Furthermore, grain growth of the BaTiO₃ powders was confirmed when the solid-state synthesis was performed at 1000 °C under the mixing conditions of BT#1 to BT#4. However, under the mixing condition of BT#4, with the highest ratio of the rutile phase, atomized powders were observed to be agglomerated. In the case of powders synthesized under the mixing conditions of BT#5 to BT#8 at 1000 °C, grain growth was suppressed, compared to that of the powders synthesized using composite TiO₂.

Next, a solid-state synthesis reaction was performed at 1100 °C under the mixing conditions of BT(A) and BT(R). Observation of the resulting microstructure shows that grain growth progressed to a higher extent compared to that of the powders synthesized at 1000 °C (Figure 6). In the case of BT(A), the synthesized powders comprised spherical-shaped particles. However, in the case of BT(R), morphology analysis of the synthesized powder revealed high anisotropy at the edges and in the uniaxial direction. In the case of BaTiO₃ powders synthesized at 1100 °C under the mixing conditions of BT#1 to BT#4, the growth of grains with sizes in the range of 268–446 nm was observed. However, numerous grains suppressed for growth were observed, as in the case of the powder synthesized using BT#4 at 1000 °C. In the case of BaTiO₃ powders synthesized at 1100 °C under the mixing conditions of BT#5 to BT#8, no grain growth was observed. Furthermore, the powder morphology of edges and anisotropic shapes were similar to those observed for the powders synthesized under different synthesis conditions at 1000 °C. The microstructure analysis of BaTiO₃ powders synthesized via the solid-state method at each calcination temperature under the respective TiO₂ conditions demonstrated that isotropic BaTiO₃ powders were formed when annealed TiO₂ powders were used. In addition, grain growth of the powders could be easily achieved using annealed TiO₂.

The calcination of BaCO₃ and TiO₂ resulted in the formation of BaTiO₃ through a solid-state synthesis reaction involving interfacial contact and diffusion between the materials (Figure 7). The solid-state synthesis reaction can be represented using the following equations.

$${\mathrm{B}\mathrm{a}\mathrm{C}\mathrm{O}}_3+{\mathrm{T}\mathrm{i}\mathrm{O}}_2\to {\mathrm{B}\mathrm{a}\mathrm{T}\mathrm{i}\mathrm{O}}_3+{\mathrm{C}\mathrm{O}}_2$$

(1)

$${\mathrm{B}\mathrm{a}\mathrm{T}\mathrm{i}\mathrm{O}}_3+{\mathrm{B}\mathrm{a}\mathrm{C}\mathrm{O}}_3\to {\mathrm{B}\mathrm{a}}_2{\mathrm{T}\mathrm{i}\mathrm{O}}_4+{\mathrm{C}\mathrm{O}}_2$$

(2)

$${\mathrm{B}\mathrm{a}}_2{\mathrm{T}\mathrm{i}\mathrm{O}}_4+{\mathrm{T}\mathrm{i}\mathrm{O}}_2\to {2\mathrm{B}\mathrm{a}\mathrm{T}\mathrm{i}\mathrm{O}}_3$$

(3)

At the initial stage of the reaction, BaCO₃ contacts the surface of TiO₂, and BaTiO₃ is formed via diffusion and an interface reaction (Equation (1)). The continuous adsorption of BaCO₃ and diffusion of ions result in the development of an intermediate-phase Ba₂TiO₄ (Equation (2)). Finally, Ba²⁺ ions diffuse to the inside of TiO₂, forming BaTiO₃ (Equation (3)). In particular, the solid-state synthesis reaction using two materials, BaCO₃ and TiO₂, involves the synthesis of BaTiO₃ through the unilateral diffusion of Ba²⁺ ions into TiO₂ [32].

In solid-state synthesis, the control of the surface characteristics, size, and morphology of TiO₂ is imperative for controlling the properties of BaTiO₃ [17]. The size effect has a significant impact on various properties of the powder [33,34]. In the case of TiO₂, for powder sizes of approximately 50 nm, the surface energy of rutile TiO₂ is high at 1.78–2.08 J/m², whereas that of anatase TiO₂ is considerably lower at 1.28–1.40 J/m². In particular, for small-size powders, rutile TiO₂ has a high surface energy, thereby showing superior reactivity at the surface and interface compared to that of anatase TiO₂ [35,36,37]. The application of heat treatment to TiO₂ powders results in the formation of powders with a composite phase comprising anatase and rutile phases, and TiO₂ powders with sizes of approximately 50 nm undergo phase transformation to the rutile phase (XRD, TEM). Specifically, the as-formed composite TiO₂ powders consist of larger-sized grains (~150 nm) in the anatase phase and smaller-sized grains (~50 nm) in the rutile phase. Among these TiO₂ powders, rutile TiO₂ powders with a grain size of 50 nm have a high surface energy, and after their surface contact with BaCO₃, the reaction rate could be faster than that of other large-sized powders during the formation of BaTiO₃. The rutile TiO₂ powders with smaller sizes react with BaCO₃ at the initial stages of the solid-state synthesis process. In contrast, anatase TiO₂ powders undergo the reaction with BaCO₃ at later stages of the synthesis process owing to their lower surface energy. During the solid-state reaction, the driving force for grain growth is determined by the mean grains size and grain size distribution. TiO₂ powders with smaller-sized grains react with BaCO₃ at the initial stages of synthesis, and grain growth progresses. Therefore, the average grain size of the synthesized BaTiO₃ could increase [38,39,40,41,42]. Such increases in the grain size are confirmed via the mean grain size analysis of BaTiO₃ synthesized at calcination temperatures of 1000 and 1100 °C using annealed TiO₂. A comparative analysis of the mean grain size of calcinated BaTiO₃ powders synthesized using composite TiO₂ and mixed TiO₂ was performed. The grain size of BaTiO₃ calcined at 1000 °C using composite TiO₂ was approximately 250–280 nm. In addition, the grain size increased further to 270–445 nm at a calcination temperature of 1100 °C. However, when mixed TiO₂ was used for the solid-state synthesis, the BaTiO₃ grain size was smaller than that obtained using composite TiO₂ (Figure 8).

Powder compacts obtained under different conditions of BT(A) to BT#8 and calcinated at 1100 °C were sintered at 1300 °C for 2 h. Based on the microstructure analysis of the fractured surface of the specimens, grains with a size of 3 μm were observed in the sintered specimen prepared using the BT(A) powder. Furthermore, numerous pores were observed in the fractured surface analysis. In the case of the specimen prepared using the BT(R) powder, grains with a size of 2.3 μm were observed. For the specimens obtained using powders prepared under the synthesis conditions of BT#1 to BT#4, grains with sizes of 27.7–31.9 μm were observed. Although sintering densification did not occur under BT(A) and BT(R) conditions, under the conditions of BT#1 to BT#4 that used composite TiO₂, sufficient grain growth was observed. However, under the conditions of BT#5 to BT#8 that used a simple mixture of anatase and rutile TiO₂, sufficient grain growth, which was confirmed in the case of composite TiO₂, was not observed. A certain extent of grain growth was observed in the BT#8 powder, which had a relatively high fraction of the rutile phase (Figure 9).

The difference in the grain size of the sintered specimens according to the BaTiO₃ powder conditions can be explained using the step free energy related to the critical driving force for grain growth. When the shape of the grain is flat to show high anisotropy, the critical driving force for growth increases, and grain growth can be suppressed. However, when the grain surface is spherical, it forms an isotropic structure, and the step free energy decreases, resulting in a small critical driving force for grain growth. In particular, grain growth occurs more easily with a lower critical driving force. When the powders synthesized via the solid-state synthesis reaction using 100% anatase and 100% rutile TiO₂ are analyzed, flat-shaped grains with high anisotropy are observed (Figure 10). Moreover, in the case of the powders synthesized using the simple mixture of TiO₂, a similar flat and highly anisotropic morphology of the grains is observed. However, in the case of BaTiO₃ powders synthesized using the heated TiO₂ powder, the edges and vertices of the grains gradually change to form a spherical shape, creating an isotropic structure. Therefore, BaTiO₃ powders using annealed TiO₂ are transformed to a state with a lower critical driving force for grain growth; therefore, grain growth can occur easily during sintering [42,43,44].

For powders synthesized via solid-state synthesis using composite TiO₂ under BT#1 to BT#4 conditions, a stable dielectric constant value of approximately 2400 was observed (

Table 2. Results of tetragonality and permittivity according to BaTiO₃ conditions (calcination condition: 1000 °C × 2 h, 1100 °C × 2 h).

BaTiO3 Condition	1000 °C Calcination		1100 °C Calcination
	Tetragonality (c/a)	Permittivity (εr)	Tetragonality (c/a)	Permittivity (εr)
BT#1	1.0094	2195	1.0102	2446
BT#2	1.0097	2162	1.0104	2357
BT#3	1.0094	2093	1.0104	2334
BT#4	1.0100	2129	1.0102	2408
BT#5	1.0048	3494	1.0087	3584
BT#6	1.0076	2380	1.0085	2561
BT#7	1.0060	1993	1.0083	1980
BT#8	1.0060	1891	1.0077	1721
BT(A)	1.0070	4042	1.0098	16,780
BT(R)	1.0065	1782	1.0094	1867

). Moreover, when mixed TiO₂ was used for solid-state synthesis, the dielectric constant of the synthesized BaTiO₃ gradually decreased with an increasing rutile phase ratio (Figure 11). The dielectric constant is an essential piece of information when designing capacitors and in other circumstances where a material might be expected to introduce capacitance into a circuit. The dielectric constant is the ratio of the capacitance of the capacitor with test material as the dielectric to the capacitance of a capacitor with vacuum (or air) as the dielectric [45].

The changes in the dielectric constant were analyzed according to the temperature changes for the specimens sintered at 1300 °C with powders synthesized under the BT#1–BT#8 conditions, as shown in Figure 12. In the case of powders synthesized under the BT#1 to BT#4 conditions that used TiO₂ with a controlled anatase–rutile phase ratio through annealing, considerably high dielectric constant values were achieved near the Curie temperature (T_c). However, for the powders prepared using simply mixed TiO₂, relatively low dielectric constant values were confirmed at T_c. BaTiO₃ exhibits a tetragonal crystal structure in the temperature range of ambient temperature–T_c, and when the temperature increases above T_c, the BaTiO₃ crystal structure changes to a cubic phase. Therefore, BaTiO₃ prepared using composite TiO₂ requires high activation energy for transformation from a tetragonal structure to a cubic structure, whereas in the case of BaTiO₃ formed using mixed TiO₂, the crystal structure can be changed with relatively low activation energy.

4. Conclusions

To improve the properties of ferroelectric BaTiO₃ powders used as raw materials for MLCCs, TiO₂ powders with different composite phase ratios were used for their synthesis, and a comparative analysis was performed in terms of the crystal structure, powder morphology, and dielectric properties. The main aspects of this study are as follows.

(1)

Heat treatment was applied to anatase TiO₂ to achieve control of the composite phase structure with different anatase–rutile phase ratios.

(2)

Using composite TiO₂ in the solid-state synthesis of BaTiO₃ is highly effective in improving tetragonality.

(3)

Using composite TiO₂ in the solid-state synthesis of BaTiO₃ is effective in promoting grain growth of the powders. Furthermore, control of the powder morphology is achieved, and during sintering, the use of composite TiO₂ is advantageous for facilitating sintering densification at relatively low temperatures.

(4)

BaTiO₃ synthesized using composite TiO₂ via the solid-state method exhibits a stable dielectric constant of approximately 2400 and effectively retains a tetragonal structure at ambient temperature.

(5)

The use of composite TiO₂ is expected to be highly advantageous for fabricating MLCCs for use in the automotive and aerospace industries, where high-voltage applications with high reliability are of critical importance.