Multilayer ceramic capacitors (MLCCs) are the most widely used passive components in electronic products. They typically employ barium titanate (BaTiO₃), which has a high dielectric constant, as the dielectric material. However, the dielectric constant of pure BaTiO₃ varies dramatically with temperature, exhibiting a sharp dielectric peak near the Curie temperature (approximately 125 °C). This "temperature drift" characteristic makes pure BaTiO₃ unsuitable for direct use in practical electronic devices. Equally challenging is that nickel electrodes, now widely used, must be sintered in a reducing atmosphere to prevent oxidation, but this reducing environment generates a large number of oxygen vacancies in BaTiO₃, leading to decreased insulation resistance and increased leakage current.
Against this backdrop, material doping modification has become the core means of optimizing BaTiO₃ ceramic performance and addressing MLCC technical limitations. Among the various dopants, rare-earth elements-owing to their unique electronic structures, variable ionic radii, and amphoteric doping behavior-can precisely tune the BaTiO₃ lattice structure, passivate crystal defects, and optimize grain morphology. They are therefore the key modifying materials for improving the dielectric performance and service reliability of MLCCs.
Mechanisms of Rare-Earth Doping
Barium titanate has a typical ABO₃ perovskite structure, where the A-site is occupied by Ba²⁺ and the B-site by Ti⁴⁺. When rare-earth ions (R³⁺) enter the BaTiO₃ lattice, they can selectively occupy either the A‑site or the B‑site depending on their ionic radius, resulting in distinctly different doping effects.
A‑site doping: Large-radius rare-earth ions (e.g., La³⁺, Gd³⁺) tend to substitute for Ba²⁺ at the A‑site. Since their valence state is typically higher than that of the replaced Ba²⁺ host ion, this produces a donor‑doping effect, releasing electrons to maintain charge balance, reducing the insulation resistance, increasing the room‑temperature dielectric constant, improving grain uniformity, and lowering the room‑temperature dielectric loss.
B‑site doping: Small-radius rare-earth ions tend to substitute for Ti⁴⁺ at the B‑site, resulting in acceptor doping, which traps electrons and generates oxygen vacancies to compensate for charge. This helps suppress oxygen‑vacancy migration, thereby extending material lifetime.
Amphoteric doping: Rare-earth ions with intermediate ionic radii (e.g., Dy³⁺, Ho³⁺, Y³⁺) exhibit amphoteric doping behavior and can occupy either A‑ or B‑sites, depending on factors such as doping concentration, sintering temperature, oxygen partial pressure, and the Ba/Ti ratio. This type of doping can provide more comprehensive and balanced modification effects.
In addition to entering the lattice and causing lattice distortion, rare-earth elements also tend to segregate at grain boundaries, forming high‑resistance grain‑boundary barriers. This not only inhibits abnormal grain growth and refines grain size, but also increases the grain‑boundary activation energy, reducing oxygen‑vacancy migration under an electric field, thus improving the insulation reliability and high‑temperature stability of the material.
Types and Applications of Rare-Earth Dopants
Based on the above modification mechanisms, the selection of rare‑earth dopants for BaTiO₃ is mainly determined by the lattice site occupied (A‑ or B‑site) and specific application requirements (e.g., adjusting dielectric properties, improving reliability, optimizing sintering behavior). Commonly used rare‑earth dopants include:
01 Yttrium Oxide
Yttrium oxide is the most widely used basic rare‑earth dopant in MLCCs. The ionic radius of Y³⁺ depends on the coordination number; for example, at coordination number 6 (octahedral structure), its radius is about 0.89 Å, which lies between that of Ba²⁺ (1.35 Å) and Ti⁴⁺ (0.605 Å), giving it amphoteric site occupation in BaTiO₃. According to joint research by Southern University of Science and Technology and the National Key Laboratory of Fenghua Advanced Technology (Fenghua Hi‑Tech), when the doping concentration is controlled within a certain range (e.g., below 1.0 mol%), Y³⁺ preferentially substitutes for Ti⁴⁺ sites, forming acceptor doping. The resulting oxygen vacancies tend to segregate along grain boundaries during sintering, pinning the boundaries, hindering grain growth, and forming a fine‑grained "core–shell" structure (i.e., a high‑dielectric‑constant grain core encapsulated by a low‑dielectric‑constant shell). This ultimately increases the breakdown voltage, provides a flat temperature characteristic, and significantly extends the service life of MLCCs.
02 Dysprosium Oxide
The ionic radius of Dy³⁺ is 0.912 Å, intermediate between Ba²⁺ and Ti⁴⁺, making it a typical amphoteric dopant. During sintering, it can simultaneously substitute for both Ba²⁺ and Ti⁴⁺ in the lattice, providing charge compensation and suppressing oxygen vacancies. Dysprosium enrichment at grain boundaries refines the grains and promotes the formation of core–shell structures, yielding excellent temperature‑drift suppression. It is a key "performance regulator" for high‑end high‑capacitance, automotive‑grade, and AI‑server MLCCs that require thin layers, high capacitance, and high reliability.
03 Holmium Oxide
Holmium oxide (Ho₂O₃) is an auxiliary dopant for high‑temperature stability. It is often used in combination with dysprosium oxide to further broaden the temperature range and improve dielectric performance at elevated temperatures. It is suitable for high‑end MLCCs that demand stringent high‑temperature stability and reliable operation over a wide temperature range.
04 Terbium Oxide
Terbium oxide doping is mainly used to optimize sintering behavior and grain boundaries, thereby enhancing voltage‑withstanding capability. The Tb³⁺ ion has a radius close to that of Ba²⁺ in the BaTiO₃ lattice, so Tb³⁺ preferentially substitutes for Ba²⁺ at the A‑site, causing slight lattice contraction and distortion and forming a "shell–core" structure, which regulates the phase‑transition temperature and internal lattice stress. In addition, Tb³⁺ doping creates "electron traps" that effectively capture and suppress long‑range migration of oxygen vacancies under an electric field, thereby improving the insulation resistance and breakdown strength of the material.