Controlling Composite TiO2 Powder Characteristics in the Solid-State Synthesis of BaTiO3 Powders for Improved Sintering and Permittivity: A Comparative Study  
[PERSON]  
1 Electronic Convergence Division, Korea Institute of Ceramic Engineering and Technology, 101 Soho-Ro, Jinju S2851, Republic of Korea; [EMAIL_ADDRESS] (H.K.)1 Electronic Convergence Division, Korea Institute of Ceramic Engineering and Technology, 101 Soho-Ro, Jinju S2851, Republic of Korea; hkwak@kicet.  
using this method is simple, as the process does not involve any heat treatments. However, in the case of the BaTiO\({}_{3}\) 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 BaTiO\({}_{3}\) grain growth via Ostwald ripening at temperatures of 800 \({}^{\circ}\)C or higher, internal pores coalesce and grow. Consequently, defects are inevitably present in BaTiO\({}_{3}\) 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 BaTiO\({}_{3}\) rather than the hydrothermal synthesis of BaTiO\({}_{3}\), as the former is conducive to particle size atomization [13; 14]. In the solid-state synthesis of BaTiO\({}_{3}\), powders of carbonates and oxides, such as BaCO\({}_{3}\) and TiO\({}_{2}\), are mixed, and solid-state reaction with heat treatment at appropriate high temperatures is subsequently performed. During this process, Ba\({}^{2+}\) ions diffuse into the TiO\({}_{2}\) surface and form an initial BaTiO\({}_{3}\) layer at the BaCO\({}_{3}\)/TiO\({}_{2}\) grain boundaries. Continuous reaction of the as-formed layer with BaCO\({}_{3}\) results in the formation of a Ba\({}_{2}\)TiO\({}_{4}\) intermediate phase. Subsequently, with a continuous supply of thermal energy through heat treatment, BaTiO\({}_{3}\) is produced through the reaction between Ba\({}_{2}\)TiO\({}_{4}\) and TiO\({}_{2}\). The diffusion of Ba\({}^{2+}\) ions at the TiO\({}_{2}\) interface is the key mechanism of the solid-state synthesis reaction [1; 13; 15; 16; 17; 18; 19; 20; 21].  
In this study, TiO\({}_{2}\), an important component in the solid-state synthesis reaction of BaTiO\({}_{3}\), was controlled, and the corresponding changes in various properties of solid-state-synthesized BaTiO\({}_{3}\) powders were examined. Anatase- and rutile-phase TiO\({}_{2}\) have different diffusion rates and reaction rates due to their different crystal structures. Therefore, BaTiO\({}_{3}\) powder was prepared by controlling the ratio of the two phases with different properties of TiO\({}_{2}\), and the change in properties was examined. The composite TiO\({}_{2}\) was synthesized by heat-treating anatase TiO\({}_{2}\) to control the anatase-rutile phase ratio. In order to assess the effect of composite TiO\({}_{2}\) on the solid-state synthesis of BaTiO\({}_{3}\), mixed TiO\({}_{2}\) with the same phase ratio and a reference TiO\({}_{2}\) (anatase, rutile) were also prepared. The prepared TiO\({}_{2}\) was applied to the solid-phase synthesis of BaTiO\({}_{3}\), and the effect of the composite TiO\({}_{2}\) with a controlled anatase-rutile phase ratio on the properties of BaTiO\({}_{3}\) was investigated in detail.