Influence of Y2O3 on the structure of Y2O3-doped BaTiO3 powder and ceramics
Abstract
Barium titanate (BaTiO3) doped with rare-earth elements (REE) is used as dielectric in the manufacture of multilayer ceramic capacitors (MLCCs). The most common REE oxide employed as dopant for this application is Y2O3. The behavior of the Y3+ in the BaTiO3 structure depends on its concentration and the sintering conditions, among other factors, which can induce the formation of secondary phases that are a potential cause a detriment to the electrical properties of BaTiO3. The purpose of this work is to perform a phase characterization of BaTiO3 doped with different concentrations of Y2O3, validating its possible contribution to the formation of secondary phases. The role of Y2O3 was evaluated on two kinds of raw materials. The first one is pure BaTiO3 (< 100 ppm Y) and the second kind is a commercial formulation designed for MLCCs known as X7R (-55°C and 125°C, 15% tolerance), which among other elements, already contained 1 wt% of Y2O3. High concentrations of Y2O3 (1% up to 20 wt%) were used aiming to promote structural changes, and even the formation of secondary phases in amounts suitable to be detected by X-ray diffraction. Heat treatment of powder and sintering of ceramics (powder compacted at 2 MPa) were conducted in air (1310°C in air for 3 h, two steps: 1350°C then 1150°C 15 h). A phase transition from tetragonal to a mixture of tetragonal and cubic was observed as Y2O3 concentration increases in the thermally treated powder and in the corresponding ceramics. Commercially formulated powder showed higher densification than pure BaTiO3, and produced cubic structure at higher Y2O3concentrations. The phase Ba6Ti17O40is detected in the 20 wt% Y2O3-doped sample.
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Introduction
BaTiO3 presents interesting electromagnetic properties and has become the main component of the formulation of the dielectric material for multilayer ceramic capacitors (MLCCs) [1, 2]. The formulation used in this application must be designed to control the electromagnetic properties of the layer, especially at high temperature and under high electric field [3,4]. For this purpose, several additives and dopants are added to BaTiO3. They include cations such as Mn, Mg and Ca, that can partially compensate the electrons and holes that the system might contain,due to the presence of oxygen vacancies [3, 5]. They also include sintering aids, such as SiO2, which reduce the sintering temperature. Indeed, it has been reported that SiO2 leads to formation of a liquid phase from the ternary system BaO-TiO2-SiO2, diminishing the eutectic point from 1320°C to near 1260 °C [6,7]. Finally, REE are added, Dy3+, Ho3+, Sm3+, La3+, Yb3+or Y3+. They substitute Ba and Ti cations in the BaTiO3 structure [8, 9]. However, in particular, Dy3+, Ho3+ and Y3+, have shown an amphoteric behavior (occupying A- or B-site) and they are described as helpful for the lifetime of the MLCCs [1]. Y2O3 is commonly employed as dopant in the commercial formulation of powder for fabrication of MLCCs, because at industrial scale. It results in similar properties than adding Ho2O3, Er2O3 or Dy2O3, and it is less expensive[8]. Dopants also take part in the formation a so-called “coreshell” structure that is claimed to contribute to the temperature stability of the dielectric properties [8-11]. Y2O3 ionic radius (0.107 nm) is intermediate between that of the Ba2+ ion (0.161 nm) and the Ti4+ ion (0.06 nm). Therefore Y3+can take either Ba2+ or Ti4+ cation site in the BaTiO3 lattice [1, 2], and can behave as acceptor or donor according to the position in the lattice. The energy required to form a Ti4+ vacancy in the BaTiO3 lattice is 7.56 eV whereas it is only 5.94 eV to form a Ba2+ vacancy [12-14]. The partial pressure of oxygen and sintering temperature will also induce the formation of Ba2+or Ti4+ vacancies, leading Y3+ to occupy either one or both of them [12, 14]. This will be influenced also by the Ba/Ti ratio, the dopant concentration and its solubility, which varies according to Y3+ taking either the Ba- or the Ti-site. Zhi et al. [15]
Conclusion
Pure and commercially formulated BaTiO3 powder was doped with Y2O3 by traditional solid state reaction. The Y2O3 was added to pure BaTiO3 and a commercial formulation that already contained 1 wt% of Y2O3, so that final concentrations were 2.5, 5 and 20 wt%. The samples, as powder and as ceramics, were analyzed by XRD. It was observed that as the Y2O3 concentration increases, the effects over the BaTiO3 structure are more evident. The phase transition from tetragonal to a mixture of tetragonal and cubic was observed either in powder thermally treated as in the corresponding ceramics obtained from them. The results indicate that the additives in the commercial formulation have a strong interaction allowing a better densification and a change of phase more than in the powder and ceramics from pure BaTiO3. It was even observed the formation of a secondary phase identified as Ba6Ti17O40 in the Y2O3-doped (20 wt%) commercially formulated BaTiO3 thermally treated powder. No secondary phases formed by BaTiO3 and Y2O3 were found, free Y2O3 is evidence of surpassing its solubility limit. The changes in the lattice show that Y entered into the lattice, and induces the formation of a secondary phase without Y or other elements of the formulation. These tests were conducted in air, so the oxides were not reduced during the treatments.