Solid-state reaction assisted by sand milling for the preparation of BaTiO ₃ powders

To address the need for synergistic control over both particle size and tetragonal-tocubic ratio (c/a) of nano-barium titanate (BaTiO 3 ) powder in multilayer ceramic capacitors (MLCCs), this study employed a solid-phase reaction-assisted sand milling process to systematically investigate the effects of milling medium polarity, mechanical force application during milling, and calcination history on precursor dispersion, stoichiometric ratios, crystal phase evolution, and grain growth behavior. It is demonstrated that using deionized water as the milling medium leverages its high polarity and interfacial compatibility with hydrophilic particle surfaces to effectively suppress BaCO 3 /TiO 2 agglomeration and element segregation, optimizing the Ba/Ti molar ratio to 1.0019. By optimizing milling parameters (2500 rpm, 10 h), the average particle size reduces to 68.74 nm, the size distribution narrows to 40-90 nm, specific surface area significantly increases, and the solid-phase reaction temperature decreases from the conventional range above 1100 °C to 800-900 °C. Calcination temperature critically governs the synergistic evolution of cubic-to-tetragonal phase transformation and grain growth: at 800-900 °C, the powder maintains a cubic phase (c/a ≈ 1.000) with grain sizes of 87-109 nm; beyond 950 °C, grains exceeds the critical size of approximately 156.59 nm, tetragonal distortion emerges, and the c/a ratio monotonically increases from 1.00462 to 1.00881 with rising temperature (up to 1100 °C). The resulting powder exhibits a piezoelectric coefficient (d 33 ) of 219 pC/N, a mechanical quality factor (Q m ) ranging from 236 to 269, a planar electromechanical coupling coefficient (k p ) of 43.2%, a dielectric constant (ε r ) increasing from 1820 to 2374, and dielectric loss (tanδ) being below 0.025. This study determines the critical grain size for the cubic-to-tetragonal phase transition, achieving precise control over both the particle size and tetragonal structure of nano-BaTiO 3 powder, thereby providing a fundamental process basis for cost-effective and reproducible preparation of dielectric powders for high-end multilayer ceramic capacitors (MLCCs).