J. Töpfer

An oxide-based thermoelectric generator: Transversal thermoelectric strip-device

A special design of an oxide-based transversal thermoelectric device utilizing thermoelectric oxides in combination with a ceramic multilayer technology is proposed. Metal strips within the ceramic matrix replace the tilted stack of alternating layers used in artificial anisotropic transversal thermoelectric devices. Numerical three-dimensional simulations of both device types reveal better thermoelectric performance data for the device with metal stripes. A monolithic transversal strip-device based on the material combination La1.97Sr0.03CuO4/Ag6Pd1 was prepared and electrically characterized. A maximum power output of 4.0 mW was determined at ΔT = 225 K for the monolithic device. The observed results are in remarkable agreement with three-dimensional numerical simulations utilizing the transport parameters of the two materials and the geometry data of the device.

Co2Z hexagonal ferrites for multilayer inductors

Abstract Z type Ba3Co1·4Fe24·6O41 for multilayer inductor applications at high frequency (up to 2 GHz) was prepared by a standard ceramic route. Single phase Z type ferrite is obtained at 1300°C. Dense samples sintered at 1300°C display a permeability of μ=30 throughout the megahertz range and a maximum μ″ at 0·5 GHz. The addition of Bi2O3 shifts the temperature of maximum shrinkage down to 900°C. Dense samples were obtained after sintering at 900°C; however, their permeability is only μ=5, but the maximum μ″ is at 2 GHz. It is shown that Co2Z ferrite is not stable at T<1300°C and that partial decomposition causes a reduction of permeability. Ferrite tapes were prepared by tape casting, and multilayer structures were fabricated by screen printing, stacking, lamination and cofiring. Firing was performed at 1300, 1000 and 900°C, i.e. at high and low temperature ceramic cofiring conditions respectively. It is shown that Co2Z hexagonal ferrites are suitable materials for multilayer inductors, although those prepared at 900°C suffer from lower permeability.

Electron spin resonance (ESR) of magnetic sublattices in Sc-substituted barium hexaferrite

The partial substitution of Fe3+ by Sc3+ in barium hexaferrite has shown to be an effective method to tailor anisotropy for many novel microwave applications. Some basic studies have revealed that this substitution leads to unusual interactions among the magnetic sublattices of the ferrite. In order to investigate these interactions, samples with formula BaScxFe12-xO19 (1 ≤x ≤ 2) were prepared by sintering (1300°C, 6h). After structural characterization by x-ray diffraction, their ferromagnetic resonance spectra were measured in the X-band (9.4 GHz), in the 100-500 K temperature range. For x = 2, a single, broad resonance peak was observed at the low temperatures (103 K), exhibiting a progressive splitting into two peaks for increasing T, to finally coalesce again into a single (paramagnetic) narrow peak at 473 K. These results are interpreted in terms of a substitution of Fe3+ by Sc3+ ions in the 4fvi and 2b sublattices; the diamagnetic cations disrupt the superexchange interactions and produce a splitti...