Rare Earth Oxides in MLCCs

As a crucial component of MLCCs, ceramic dielectric materials play a decisive role in determining MLCC performance. With the rapid development of MLCCs towards miniaturization, high capacitance, and thin layering, higher technical requirements have been placed on ceramic dielectric powders with high voltage resistance, high dielectric constant, and high reliability. Rare earth oxides are widely used in ceramics, including cerium oxide, lanthanum oxide, neodymium oxide, dysprosium oxide, samarium oxide, holmium oxide, erbium oxide, etc. Doping ceramics with small or trace amounts of rare earths can significantly alter the microstructure, phase composition, density, mechanical properties, physicochemical properties, and sintering behavior of ceramic materials. Therefore, modification through doping with rare earth oxides is one of the pathways to obtain high-end ceramic dielectric powders for MLCCs.

As important doping components in MLCC dielectric powders, rare earth oxides can effectively improve MLCC reliability and are indispensable raw materials in the development of high-end ceramic powders for MLCCs. Ceramic powders for MLCCs are mainly classified into three categories (Y5V, X7R, and COG). Among them, X7R materials are the specification with the most intense global competition and are also one of the varieties with the highest market demand and usage in electronic equipment. Their manufacturing principle is based on modifying nano-scale barium titanate (BaTiO₃) ceramic material.

Barium titanate is one of the main raw materials for manufacturing MLCCs. It exhibits excellent piezoelectric, ferroelectric, and dielectric properties. However, pure barium titanate has a large capacitance temperature coefficient, high sintering temperature, and relatively high dielectric loss, making it unsuitable for direct use in manufacturing ceramic capacitors.

Research shows that the dielectric properties of barium titanate are closely related to its crystal structure. By employing doping methods, the crystal structure of barium titanate can be controlled, thereby improving its dielectric properties. This is primarily because doped fine-grained barium titanate forms a core-shell structure, which plays a significant role in improving the temperature characteristics of capacitance.

Doping rare earth elements into the barium titanate structure is one of the methods used to improve the sintering behavior and reliability of MLCCs. Research on rare earth ion doping in barium titanate dates back to the early 1960s. The addition of rare earth oxides reduces oxygen mobility, which can enhance the dielectric temperature stability and electrical resistance of dielectric ceramics, thereby improving product performance and reliability. Commonly added rare earth oxides include yttrium oxide (Y₂O₃), dysprosium oxide (Dy₂O₃), and holmium oxide (Ho₂O₃), among others.

The ionic radius of rare earth ions has a crucial impact on the position of the Curie peak in barium titanate-based ceramics. Doping with rare earth elements of different radii alters the lattice parameters of crystals with a core-shell structure, thereby changing the internal stress within the crystals. Doping with larger-radius rare earth ions induces a pseudo-cubic phase in the crystal and generates residual stress within it. Conversely, introducing smaller-radius rare earth ions produces less internal stress and inhibits phase transitions in the core-shell structure. Even additives used in small quantities, the characteristics of rare earth oxides (such as particle size or shape) can significantly affect the overall performance or quality of the product. High-performance MLCCs continue to develop towards miniaturization, high layer count, high capacitance, high reliability, and low cost. The world's most advanced MLCC products have entered the nanoscale. As important doping elements, rare earth oxides should possess nanoscale particle size and good powder dispersibility.