Various donor dopants such as Y3+ and Nb5+ were incorporated in SrTiO3 in amounts ⩽25 mol % during sintering in air at 1450° C for ⩽ 15 h Dense ceramic materials with grains of optimum uniformity and largest size were obtained when the cation stoichiometry was adjusted to allow for charge compensation of the donor ions by strontium vacancies For donor levels ≳ 03 mol%, the dielectric constant measured at 25° C and 1 kHz was up to two orders of magnitude higher than that of undoped strontium titanate The increase in permittivity, however, was dependent on grain size, was influenced by the method of electrode application, and was suppressed by the presence of ≲ 01 mol% Mn in the ceramic These observations, together with data obtained from electrical measurements at other temperatures and frequencies, were consistent with interpretation of the anomalously high dielectric constant as a boundary-layer effect resulting from semiconducting grains and weakly insulating grain-surfaces It is suggested that donor doping influences the electrical properties of SrTiO3 mainly by increasing the volatility of oxygen from the grains during sintering, and by decreasing the rate of re-oxidation during cooling

Dielectric properties of donor-doped polycrystalline SrTiO3

The microstructure of strontium titanate internal boundary layer capacitors at various stages in their processing was studied by transmission electron microscopy of rapidly quenched and normally cooled samples. Compositions containing excess TiO2, Al2O3, and SiO2 have a completely wetting liquid phase at the sintering temperature; during cooling TinO2n−1, Magneli phases precipitate at multiple grain junctions. Diffused metal oxides and flux (Bi2O3, PbO, CuO, and B2O3) rapidly penetrate as a liquid phase along boundaries in postsintering heat treatment. This liquid phase disappears during slow cooling.

Microstructures of SrTiO3 Internal Boundary Layer Capacitors During and After Processing and Resultant Electrical Properties

https://ris.utwente.nl/ws/files/182215653/Winnubst1983aes.pdf

Aes/stem grain boundary analysis of stabilized zirconia ceramics

Grain boundary (GB) doped exhibits interesting electroceramic phenomena including varistor and barrier layer capacitor behaviour. We present here our investigation of GB acceptor-doped using analytical electron microscopy including electron holography. Mn was diffused into sintered polycrystalline to attain GBs which are rich in Mn. The presence and spatial extent of Mn at the GBs were analysed using x-ray emission spectroscopy (XES) and parallel electron energy loss spectroscopy (PEELS). The valence state of Mn was determined using PEELS to be predominantly +2. Finally, transmission high-energy electron holography was utilized to directly image and quantify the electrostatic potential and associated space-charge across the GBs directly. The holography results reveal a negatively charged GB with positive space-charge, indicating that Mn with a valence of +2 resides as an acceptor dopant on the Ti site at the GB core. The barrier height and local charge density distribution, including the Debye length, of the double Schottky barrier at the GB are derived from these holography results. This investigation demonstrates the usefulness of electron holography as a bulk-sensitive technique to probe the statics and dynamics of electrostatic field distribution and electrical charge across interfaces in technologically useful materials, and the need to employ diverse analytical techniques for such an investigation.

An investigation of acceptor-doped grain boundaries in ?

The bimodal distribution of grain size is frequently observed in calcium copper titanate (CaCu3Ti4O12 or CCTO) ceramics that exhibit high dielectric constant. In this study, we propose an analytical model composed of grains of two different sizes to estimate the dielectric behavior of CCTO based on microstructural features. Important assumptions are (1) CCTO is a barrier layer dielectric and (2) the insulating layer thickness is the same in both large and small grains. Explicit expressions for the effective dielectric constant of serial, parallel, and logarithmic mixture models are given. When compared with experimental results, the present bimodal model appears to be acceptable for describing the dielectric behavior of CCTO.

A Bimodal Grain Size Model for Predicting the Dielectric Constant of Calcium Copper Titanate Ceramics

SiTiO3 capacitor material with indiffused Bi2O3 was studied using SEM, ESCA combined with Ar+ ion-etching, and TEM equipped with EDX. The apparent thickness of a second-phase layer observed with SEM was found to be influenced by in-depth effects. ESCA and TEM results show that only a 10- to 100-nm thick layer of second phase is present between the SrTiO3 grains. In addition, it was found with TEM that the outer part of each grain contained Bi (at most 2 at.%), representing a diffusion layer. These results have implications for the boundary-layer model proposed to explain the dielectric properties.

M. P. A. Viegers

Papers

Microstructure of SrTiO3 Boundary‐Layer Capacitor Material

Microstructure of strontium titanate boundary-layer capacitor material