Papers by Author: Cornel Miclea

Abstract: Soft ferrites in the CuZnTi system, having the chemical composition Cu1-x-yZnxTiyFe2O4 with 0.5£x£0.7 and 0.00£y£0.05 were investigated as a function of composition, sintering temperature and cooling speed, in order to obtain materials with different Curie temperatures, between 30 and 180 oC and very high change rate of permeability with temperature around their Curie points. Such materials are well suited to use as high sensitive magnetic temperature sensors and transducers for temperature control. Zn and Ti additions to copper ferrite change the Curie temperature in a controllable manner, thus being possible to produce ferrite material with a fine control of the Curie point at any desired temperature. Most important was the behavior of magnetic permeability with temperature around the Curie point, where it may change with about 60 %/oC by a proper choice of the cooling speed of samples from the sintering temperature to room temperature. This makes such materials extremely attractive as magnetic temperature sensors of high sensitivity. Two applications of such materials as temperature sensors, namely an ultrathermostat and an on-off switch type relay were designed. Their functionality and performances are presented and discussed.

Abstract: The displacement devices using piezoceramic sensors play an important role in many fields. Soft type piezoelectric materials used for such sensors are more appealing because of their lower coercive field, relatively low modulus of elasticity, high electromechanical coupling factors and piezoelectric strain coefficients. We designed a PZT type material doped with niobium and nickel to increase the strain coefficient and still retaining a high coupling coefficient. The chemical formula was Pb0.97Ba0.08Nb0.03Ni0.06Zr0.46Ti0.40O3. The main piezoelectric parameters were: d33=680 pm/V, d31=-235 pm/V, kp=0.64 and k31=0.42. A unimorph transducer was made from a thin sheet of such a piezoceramic material, tightly bonded on a thin optically polished glass plate and mounted as a cantilever. The performance of this transducer was evaluated by means of a new optical method which makes use of the Moiré deflectometry and provides high precision. The method is rather simply and consists mainly in the following: the Moiré fringes rotate when the bimorph bends under the influence of the electric field and by measuring the rotating angle of the fringes one can easily determine the bimorph curvature and the free end displacement.

Abstract: Strontium hexaferrite nanopowders were prepared by mechanochemical synthesis from strontium and iron oxides using a high energy ball mill after 50 hours of milling. The synthesis process was checked by X-Ray diffractograms on powders milled for different times. The magnetic properties of hexaferrite nanopowder, both compacted and dispersed in a nonmagnetic matrix were determined. Severe stresses and structural deformations were introduced by mechanical processing, but they were eliminated, to a great extent, by a suitable heat treatment of the milled powder at 1000 oC for one and a half hour. Coercivities as high as 6600 Oe and specific magnetization of 65 emu/g were found for annealed noninteracting nanopowders. Such values are very near to the theoretical values for strontium ferrite. The magnetic behavior of such powders can be rather well described by the coherent rotation model of Stoner-Wohlfarth for an assembly of single domain particles oriented at random. Sintered bodies of such powders produced magnets with a high HC of 4600 Oe, a Br of 2100 Gs and an energy product maxim of approximately 1.85 MGOe.

Abstract: The influence of uranium doping up to 2 at. % on grain size, electrical resistivity and dielectric constant of barium titanate ceramics has been investigated. The samples were prepared by the conventional ceramic technique using pure raw materials. The mixed oxides were calcined at 1100 oC. The pressed pellets were sintered at temperatures between 13001500 oC in controlled atmosphere of argon and oxygen. The electrical resistivity drastically decreased, from 109 to 103 cm, with increasing uranium content up to 1.5 at. % and increased thereafter to about 107 cm, for 2 at. % U. The dielectric constant slightly increased with increasing U-content, up to about 1 at. % U and then shows a sudden increase with about two orders of magnitude, having a sharp maximum at 1.5 at. % U, when sintered in argon atmosphere compared to only half an order of magnitude when sintered in oxygen atmosphere. The results are discussed in terms of the possible A and B sites occupied by uranium as well as the boundary layer enriched with vacancies.