Herbert Schreinemacher

Hydrothermal preparation of barium titanate from barium-titanium acetate gel precursors

Abstract Clear, transparent gels of BaTi acetate have been prepared from titanium tetrabutylate (TBT), Ba acetate aqueous solution, isopropanol and acetic acid. Infrared spectroscopy and transmission electron microscopy (TEM) of such gels revealed linked clusters of polymer Ti oxyacetate with surface adsorbed Ba acetate. Spray dried Ti oxyacetate has been found to have the stoichiometric composition Ti 2 O 3 [CH 3 COO] 2 . Calcination of BaTi acetate gels to BaTiO 3 in air turned out to be a less favourable preparative route, because stable intermediate carbonate phases were formed. Corresponding to the high surface area and fine dispersion of Ba and Ti, BaTi acetate gels are higly reactive precursors for hydrothermal synthesis of BaTiO 3 . Nearly quantitative (> 99·7%) chemical reactions were obtained without using Ba excess at processing. TEM provided interesting information about the chemical mechanisms of hydrothermal processing.

High-permittivity dielectric ceramics with high endurance

Abstract Donor-doped BaTiO3 ceramics show very high endurance under DC field stress and are there-fore suitable candidates for the manufacture of multilayer capacitors with thin dielectric layers. CeO2 additives were found to be excellent donor dopants. The donor effect results from Ce3+ ions incorporated on Ba sites of the perovskite lattice. Defect chemistry, solubility of CeO2 in BaTiO3 and the ferroelectric phase transition of Ce-modified BaTiO3 have been studied by several methods.

Ca-acceptors in dielectric ceramics sintered in reducive atmospheres

Dielectric ceramics of the system (Ba,Ca)(Ti,Zr)O3 (BCTZ) have been fired with BaO-excess in reducive atmosphere. By the influence of BaO-excess the Ca2+-ions are shifted from the Ba- to the Ti-sites of the perovskite lattice. The solubility of Ca2+ on Ti-sites is largely independent of the firing atmosphere, but shows a pronounced temperature dependence. At 1400 °C ≅ 2 at.% Ca can be dissolved on the Ti-sites. Ca-ions on Ti-sites are strong electron acceptors. Highly insulating materials can be thus prepared in reducive atmospheres. The Ca-acceptors are electrically compensated by large numbers of ionized oxygen vacancies. Electromigration of the oxygen vacancies gives rise to ionic conductivity, resulting in low life stability of multilayer capacitors, sintered in reducive atmosphere.

DIELECTRIC PROPERTIES OF ACCEPTOR-DOPED (BA, CA)(TI, ZR)O3 CERAMICS

The dielectric properties of (Ba,Ca)(Ti,Zr)O3 ceramics containing various acceptor and donor dopants on the B sites have been studied. Formation of charge compensating “complexes” between acceptors, donors and oxygen vacancies has been observed. All acceptors cause a reduction of the Curie point. The maximum dielectric constant increases with the average grain size almost independent of the acceptor incorporated. Ceramics fired in reducive atmospheres reveal pronounced changes of the dielectric properties after annealing in oxidizing atmosphere. In particular the losses and the maximum dielectric constants are significantly affected by the annealing time, temperature and partial pressure of oxygen.

Dysprosium Doped Dielectric Materials for Sintering in Reducing Atmospheres

Substitution of Dy rare earth ions was studied in Ba(Ti,Zr)O3 dielectric materials, using thermogravimetry, X-ray diffraction and dielectric measurements. Dy3+ ions enter both the A- and the B-sites of the perovskite structure, however, the solubility on B-sites is up to 9 mol %, whereas it is only 2.5 mol% on A-sites. Dy can be easily shifted from A- to B-sites and back, using Ba or Ti excess in the material. Dy3+ on B-sites is a strong electron acceptor. Dy doped dielectric materials are cofired with Ni electrodes in reducing atmosphere to highly insulating BME multilayer capacitors.