Jörg Töpfer

Chemical and structural effects on the high-temperature mechanical behavior of (1−x)(Na1/2Bi1/2)TiO3-xBaTiO3 ceramics

Bismuth sodium titanate–barium titanate [(1−x)(Na1/2Bi1/2)TiO3-xBaTiO3, NBT-100xBT] is one of the most well studied lead-free piezoelectric materials due in large part to the high field-induced strain attainable in compositions near the morphotropic phase boundary (x = 0.06). The BaTiO3-rich side of the phase diagram, however, has not yet been as comprehensively studied, although it might be important for piezoelectric and positive temperature coefficient ceramic applications. In this work, we present a thorough study of BaTiO3-rich NBT-100xBT by ferroelastic measurements, dielectric permittivity, X-ray diffraction, and Raman spectroscopy. We show that the high-temperature mechanical behavior, i.e., above the Curie temperature, TC, is influenced by local disorder, which appears also in pure BT. On the other hand, in NBT-100xBT (x < 1.0), lattice distortion, i.e., tetragonality, increases, and this impacts both the mechanical and dielectric properties. This increase in lattice distortion upon chemical substitution is counterintuitive by merely reasoning on the ionic size, and is due to the change in the A-O bond character induced by the Bi3+ electron lone pair, as indicated by Raman spectroscopy.

Investigations on the charge transport in LaMnO3 +δ at low temperatures

Samples of LaMnO3 +δ with 0.08 ⩽δ⩽ 0.16 have been synthesized in order to study the effect of non-stoichiometry, δ, which is related to the concentration of MnIV ions, on the physical properties of the material. For δ⩽ 0.10 the compound crystallizes in the orthorhombic perovskite-type structure, while for δ > 0.10 a rhombohedral cell is formed. All LaMnO3 +δsamples with δ⩽ 0.12 are paramagnetic, or, below TC ferromagnetic semiconductors, while samples with larger values of δ exhibit a discontinuity in the temperature dependence of the resistivity, indicating a transition to a ferromagnetic metallic-like state at low temperatures. Upon further lowering of the temperature the charge carriers become localized again. These results are consistent with thermopower measurements. All samples are ferromagnetic at low temperatures. The magnetic properties can be interpreted on the basis of Goodenoughs superexchange model.

Nonstoichiometry, point defects, and magnetoresistance of Sr2FeMoO6−δ

The phase stability, nonstoichiometry, point defects, and magnetoresistance (MR) of polycrystalline Sr2FeMoO6−δ (SFMO) were studied. Thermogravimety at 1200 °C in combination with x-ray diffraction shows that single-phase SFMO exists between −10.2≤log pO2≤−13.7 at 1200 °C. At lower oxygen partial pressure mass loss signals reductive decomposition; at higher pO2 a mass gain indicates oxidative decomposition into SrMoO4 and SrFeO3−x. The nonstoichiometry δ at 1200 °C was measured as function of pO2 and oxygen vacancies were found to represent majority defects. The vacancy concentration increases with decreasing pO2; a maximum nonstoichiometry of δ=0.086 is observed close to the lower phase boundary. Samples with different δ were prepared at 1200 °C and various pO2. The variation of structural parameters, magnetization, and MR is discussed in relation to oxygen nonstoichiometry δ. Maximum MR=6.5% at 298 K and 1 T is observed for samples with large δ.

LTCC-Modules with Integrated Ferrite Layers-Strategies for Material Development and Co-Sintering

The integration of passive components (resistors, capacitors, inductors) into LTCC modules is a challenging task in multilayer ceramics technology. We report on multilayer assemblies consisting of ...

Integration of Ni-Cu-Zn Ferrite in LTCC-Modules

Integration of magnetic functional components in LTCC circuit boards calls for co-firing of dielectric and ferrite tapes. Ni-Cu-Zn ferrites with permeability of μ =900 were developed which can be fully densified at the standard LTCC sintering temperature of 900 °C. Successful co-firing of this ferrite with dielectric tapes requires the adaptation of the shrinkage behavior of the materials as well as the thermal expansion during the cooling period - especially in the temperature range below the transformation point of the glassy phase of the dielectric tape. To match these preconditions, a new dielectric LTCC material with steep sintering curve and high thermal expansion coefficient was designed. Sintered multilayer composed of Ni-Cu-Zn ferrite and tailored dielectric tapes are free of cracks and possess no open porosity. No significant interdiffusion between the ferrite and dielectric tapes was found by EDX measurements. Compared to pure ferrite laminates the permeability of co-sintered Ni-Cu-Zn ferrite l...

Preparation, thermal stability and permeability behaviour of Z-type hexagonal ferrites for multilayer inductors

Co 2 Z hexaferrite Ba 3 Co 2 Fe 24 O 41 with iron excess Ba 3 Co 2-x Fe 24+x O 41 (0 ≤ x ≤ 0.8) and deficiency Ba 3 Co 2+y Fe 24-y O 41 (0 ≤ y ≤ 0.6) were prepared by a mixed oxalate co-precipitation route and the standard ceramic technology. XRD studies showed that both synthesis routes lead to an almost single phase Z-type ferrites for x = 0 after calcination and sintering at 1330°C. For iron deficient samples, 0 ≤ y ≤ 0.6, the multi-phase mixtures were observed. The permeability of Z-type ferrites varied with composition x. The maximum permeability of µ’ = 37 was observed for the sample with x = 0.6. Addition of Bi 2 O 3 as sintering aid shifts the temperature of maximum shrinkage to lower temperatures and dense samples were obtained after sintering at 950°C. However, the permeability is only µ’ = 3. It is shown here that Co 2 Z ferrites are not stable under 1300°C and that partial thermal decomposition causes a reduction of the permeability.