Fabricación de núcleos magnéticos amorfos mediante melt spinning y CDEC

Abstract

Materials used to manufacture transformer cores, as well as other electromagnetic devices, greatly determine their operation. The initial use of Fe or texturized Fe-Si alloys has evolved to the possibility of using amorphous alloys, with lower losses involved in the electric current transformation. Nevertheless, obtaining amorphous alloys is not immediate, because such structure must persist at room and the use temperature of these devices. In this Doctoral Thesis, the aim is to sinter amorphous or nanocrystalline powders by means of the electrical discharge of a capacitor (the technique known as Capacitor Electrical Discharge Consolidation, CDEC). It consists in a sintering technique that simultaneously applies, to a powder mass, pressure and the heat coming from the Joule effect of a high intensity electric current. The idea of using this technique lies in its speed, allowing to maintain the initial structure of the powders, and in the intention of attaining the geometry decided in the sintering die. It is initially necessary to set up a melt spinning equipment, a technique used to obtain alloys in the form of ribbons in amorphous state. A simple Fe-Si-B alloy will be prepared, guaranteeing an amorphous microstructure by a very quick cooling from the liquid state. The amorphous alloy, obtained in form of ribbons, must be milled to obtain powders, for which different mills have been tested. Finally, the sintering process of the amorphous or nanocrystalline powder will be carried by using a capacitor. The objective of the work will be the analyze the effect of the variables involved in the processes, as well as defining standard operating conditions of the different used equipments. The variables to be considered include those of the process of obtaining the initial alloy in an arc melting furnace and, mainly, those of the melt spinning process, that is, rotation speed of the wheel, crucible-wheel distance, and ejection temperature and pressure. Once the amorphous ribbons are obtained, the milling variables will be analyzed for different equipments: attritor mill, balls vibratory mill and rings vibratory mill, comparing the granulometry and amorphous character as a function of milling time and working frequency in each one of the equipments. Finally, for the material sintering, a discharge equipment with a capacity of 132 μF and a voltage of 200 V is used, adapting the optimum operating conditions. It has also been started the setup of a high voltage equipment with the same purpose. On the ribbons obtained, their quality will be controlled, and mainly their amorphous microstructure by means of X-ray diffraction and SEM and TEM microscopy. Moreover, on the powder obtained after the mechanical milling, and the sintered compacts by CDEC, the persistence of the amorphous state will be controlled by X-ray diffraction and TEM microscopy. In addition, the powder granulometry will be controlled by laser diffraction, and on the compacts, the magnetic characteristics, among other properties.