Multilayer ceramic capacitor (MLCC) production and sales figures are the highest among fine-ceramic products developed in the past 30 years. The total worldwide production and sales reached 550 billion pieces and 6 billion dollars, respectively in 2000. In the course of progress, the development of base-metal electrode (BME) technology played an important role in expanding the application area. In this review, the recent progress in MLCCs with BME nickel (Ni) electrodes is reviewed from the viewpoint of nonreducible dielectric materials. Using intermediate-ionic-size rare-earth ion (Dy2O3, Ho2O3, Er2O3, Y2O3) doped BaTiO3 (ABO3)-based dielectrics, highly reliable Ni-MLCCs with a very thin layer below 2 µm in thickness have been developed. The effect of site occupancy of rare-earth ions in BaTiO3 on the electrical properties and microstructure of nonreducible dielectrics is studied systematically. It appears that intermediate-ionic-size rare-earth ions occupy both A- and B-sites in the BaTiO3 lattice and effectively control the donor/acceptor dopant ratio and microstructural evolution. The relationship between the electrical properties and the microstructure of Ni-MLCCs is also presented.

https://iopscience.iop.org/article/10.1143/JJAP.42.1/pdf

Base-Metal Electrode-Multilayer Ceramic Capacitors: Past, Present and Future Perspectives

A century of diligent R&D has resulted in a wide range of ceramic dielectrics and processing technologies. The technology used to manufacture an MLCC (multilayer ceramic capacitors) that costs pennies was unimaginable 30 years ago. The present trends of enhanced mobility, connectivity, and reliability in consumer, industrial, and military electronics will continue to drive future innovations in ceramic capacitor technology. In addition, power electronics applications are an emerging market in which ceramic capacitors will play an increasing role through improved breakdown strength, enhanced dielectric stability in harsh environments, and innovative packaging. The investment made by the US government to develop high energy density and high temperature capacitor technology will also contribute to the advancement of dielectric materials technology for pulse and power electronic capacitors.

A brief introduction to ceramic capacitors

The impedance of a BaTiO3 (BT)-based multilayer ceramic capacitor with a nickel internal electrode (Ni-MLCC) was investigated by measuring the frequency domain at various temperatures. All the obtained impedance data could be successfully fitted to a 4-RC section electrical equivalent network. The 4-RC section electrical equivalent network was successfully correlated to the microstructure: the core, the shell, the grain boundary, and the ceramic/internal electrode interface regions. Based on this electrical equivalent network, the electrical properties including the Curie-Weiss law, the current-voltage characteristics, and dc electrical degradation, were well explained. A model for the degradation behavior for BT-based Ni-MLCC with thin active layer thickness was proposed.

https://iopscience.iop.org/article/10.1143/JJAP.40.5624/pdf

dc-Electrical Degradation of the BT-Based Material for Multilayer Ceramic Capacitor with Ni internal Electrode: Impedance Analysis and Microstructure

The oxygen vacancy concentration of BaTiO3 doped with acceptors (Cr to Ni) is determined gravimetrically as a function of the O2 partial pressure during and after annealing at 700° to 1300°C. The oxygen vacancy concentration of these materials is larger than that of undoped and donor-doped BaTiO3. The oxygen vacancies are doubly ionized and they compensate the acceptors of lower valence. Both the vacancy concentration and the valence of the acceptor dopants depend on the annealing conditions. The electronic energy levels of the acceptors within the BaTiO3 band gap are derived from the gravimetric measurements. The electrical properties of the acceptor-doped ceramics are favorable for base-metal-electrode multilayer capacitors, which require sintering in reducing atmospheres.

Reversible Weight Change of Acceptor‐Doped BaTiO3

The Gerson–Marshall (1959) relationship predicts an increase in dielectric breakdown strength (BDS) and therefore, recoverable energy density (Wrec) with decreasing dielectric layer thickness. This relationship only operates however, if the total resistivity of the dielectric is sufficiently high and the electrical microstructure is homogeneous (no short circuit diffusion paths). BiFeO3–SrTiO3 (BF–ST) is a promising base for developing high energy density capacitors but Bi-rich compositions which have the highest polarisability per unit volume are ferroelectric rather than relaxor and are electrically too conductive. Here, we present a systematic strategy to optimise BDS and maximum polarisation via: (i) Nb-doping to increase resistivity by eliminating hole conduction and promoting electrical homogeneity and (ii) alloying with a third perovskite end-member, BiMg2/3Nb1/3O3 (BMN), to reduce long range polar coupling without decreasing the average ionic polarisability. These strategies result in an increase in BDS to give Wrec = 8.2 J cm−3 at 460 kV cm−1 for BF–ST–0.03Nb–0.1BMN ceramics, which when incorporated in a multilayer capacitor with dielectric layers of 8 μm thickness gives BDS > 1000 kV cm−1 and Wrec = 15.8 J cm−3.

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Superior energy density through tailored dopant strategies in multilayer ceramic capacitors

A barium titanate ceramic containing 13.5 mol % calcium zirconate was doped with up to 3 mol % of oxides of various metals that were considered likely on the basis of ionic size and valence to enter the small cation lattice with charge less than 4+. It was hoped in this way to compensate electrically for loss of oxygen during sintering in CO-CO2 mixtures, so as to obtain high resistivity dielectrics and allow the use of base metal electrodes in a monolithic capacitor. Doping with approximately 0.5 mol % Mn, Co, or Mg produced the highest resistivities, and dielectrics with nickel electrodes and relative permittivity up to 10 000 were obtained with resistivity in excess of 1012 Ω cm at room temperature. When the doped ceramic sintered in contact with nickel, the grain structure and permittivity-temperature characteristics depended on the oxygen partial pressure of the sintering atmosphere, apparently influenced by dissolution of Ni into the ceramic.

G. H. Maher

Papers

High resistivity BaTiO3 ceramics sintered in CO-CO2 atmospheres