Developments in soft magnetic power ferrites

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

Mn–Zn ferrites are very important ferromagnetic ceramics and widely used for high-frequency magnetic core materials because they have low power loss and high initial permeability up to several MHz range. Magnetic properties of Mn–Zn ferrites are strongly dependent on the processing. Small amounts of additives can greatly affect the properties of ferrites. Additives can be divided into three categories. The first group consists of additives that segregates to the grain boundaries and affect the grain-boundary resistivity. The second group consists of additives that affect microstructure development. The third group consists of additives that are dissolved into the grains and alter the magnetic properties. With expanded applications of soft magnetic materials expected in the future, it is appropriate to review the influence of additives (SiO2, CaO, V2O5, Bi2O3, MoO3, Nb2O5 and TiO2) on the magnetic properties and microstructure of these materials.

Influence of additives on the magnetic properties, microstructure and densification of Mn–Zn soft ferrites

Publisher Summary This chapter discusses the progress in spinel ferrite research. A large number of oxides with a metal–oxygen ratio of 0.75 as composition is known to crystallize into the spinel structure. Among these oxides, magnetite Fe 3 O 4 is an important compound, from which the spinel ferrites can be derived by partial substitution of the iron ions by other cations. Between most of the spinel oxides solid solutions can be formed, which means that a great variety of spinel oxides is possible. The simple spinel ferrites are those ferrites in which x = 0 and the bivalent metal ion is Mg or Cd or one of the bivalent transition metal elements Mn, Fe, Co, Ni, Cu, and Zn. Partial solid solutions of the simple ferrites are also possible with compounds that either do not exist or that do not have the spinel structure. A great variety in the chemical composition of spinel ferrites is possible. The diversity in composition results into a large range of physical properties that permits the tuning of the properties for specific applications and makes spinel ferrites of particular interest.

Chapter 3 Progress in spinel ferrite research

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.

Microstructure of SrTiO3 Boundary‐Layer Capacitor Material

AES analysis of grain boundaries of Mn-Zn ferrites, combined with ion etching, shows that CaO and SiO2 dopants are enriched in a layer of ∼2 nm; TEM showed no separate phase at the grain boundaries. Using TEM combined with X-ray microanalysis three types of secondary phases, one crystalline and two amorphous, were found at the multiple-grain junctions. Although the absence of a planar second phase has not been proved, these results argue against the presence of a high-resistivity glassy layer and support the hypothesis that the high resistivity is caused by segregation of Ca during cooling, causing oxidation of the grain boundaries.

P. E. C. Franken

Papers

Microstructure of SrTiO3 Boundary‐Layer Capacitor Material

Examination of Grain Boundaries of Mn‐Zn Ferrites by AES and TEM

Microstructure of strontium titanate boundary-layer capacitor material