Development of magnetic materials for transformers

SILVEYRA JOSEFINA M

San Carlos

Exposición; Advanced School on Materials Science & Engineering (SanCAS-MSE); 2012

The goal of my research project can be divided into three parts (even though they are not totally independent from each other):i) to study in depth the Finemet alloy. This is a very soft magnetic alloy with nanocrystalline structure, i.e. Fe-Si DO3 nanometric grains embedded in a ferromagnetic amorphous matrix. The chemical composition of the alloys is Fe73.5Si13.5B9Nb3Cu1.Although Finemet has been developed almost three decades ago, it was still source of uncertainties and controversy in the scientific community.In this framework, the article entitled ?Phase transformations in Mo-doped FINEMETs" intended to collaborate in the discussion of the boride phases that crystallize in these alloys when they are annealed at high temperatures.The article "Domain imaging in FINEMET ribbons" besides promoting a useful, cheap and forgotten experimental technique, i.e. the Bitter technique, shows interesting addition information on magnetic domains of FINEMET-like alloys.The article ?Structural and magnetic study of Mo-doped FINEMET? intended to be a clear guide of several models that can be applied to this system to obtain valuable information on the structure and magnetic properties of the samples. Our series of alloys was analysed following this procedure. The atomic (and mass) fractions and the chemical composition of the two phases of the nanocrystalline alloys were calculated. The presence of penetrating magnetic fields of each phase into the other one was evidenced.Many laboratories continued to synthesize FINEMET-like alloys with high purity raw materials for obtaining good properties. In the article in press ?High performance of low cost soft magnetic materials? we showed that if proper raw materials of commercial purity are used, properties as good as the former ones can be obtained with a remarkable reduction of costs.ii) to study a FINEMET-like series, in which Nb was gradually replaced by Mo.Chinese and Russian researchers had reported, in their native languages, that the magnetic permeability had a significant improvement when the Nb replacement for Mo was partial. The idea was to verify if that phenomenon actually existed, to explain it (it was not done before) and to spread it in the lingua franca of research; English. For this purpose, experimental equipments were set-up (including the programming of the software) for measuring hysteresis cycles, magnetoimpedance and magnetic permeability of FINEMET ribbons. The studies on structure, properties and performance of the series were reported in the articles mentioned above and in the following ones: ?Structure and soft magnetic properties of FINEMET type alloys: Fe73.5Si13.5B9Nb3-xMoxCu1 (x = 1.5, 2)?, ?Effects of substitution of Mo for Nb on less-common properties of Finemet alloys? and ?Dynamic coercivity of Mo-doped FINEMETs?.Actually, all the samples nanocrystallized by the traditional heat treatment in vacuum or controlled atmosphere revealed good properties and are suitable for technological applications ?but the expected great increase of the permeability was not observed. However the Mo-containing samples might be suitable for air annealing, which would be an important advantage in the fabrication process.iii) to fabricate a prototype of a particular application with the studied materials. Although this goal was not original at an international level, it was for our laboratory, where amorphous and nanocrystalline soft magnetic materials were studied for more than 15 years but from a fundamental/basic point of view. It was also an innovation at a national level and, most probably, regional level as well. The chosen application was common-mode choke. A poster was already presented on the scientific meeting SÓLIDOS 2011. In this study, cores of different materials were compared (nanocrystalline, ferrite and powder cores). The results showed that nanocrystalline cores had the best performance of the three types of materials in the measured frequency range: 1-1000 kHz.