PhD Thesis, Kenny L. Alvarez
Kenny L. Alvarez
Title: Novel amorphous and nanocrystalline Fe-based soft magnetic powders produced by gas atomisation.
Defense date: 3/07
Director: José Manuel Martín
President: José Manuel Sánchez
Industry is currently facing increasing challenges related to resources and the environment, and some sectors have been going to great lengths to control them. Two of the biggest concerns are using materials more efficiently while simultaneously reducing the amount of energy consumed and the amount of pollutant gases emitted. Within this context, soft magnetic materials have been broadly studied because they are widespread in many industrial sectors and can be found in numerous applications. In fact, this group of materials comprise about 40 % of the total market of magnetic materials and they are present in electrical and electronic devices, telecommunication industry, energy conversion and transportation, and the automotive sector, among other. In the last decade, the electrical and electronic industry has exponentially increased its production due to the massive use of technological devices and the worldwide tendency to be fully connected, triggering a high demand for soft magnetic materials. This high demand, together with a tendency to miniaturise devices, have led researchers and industries to search for novel and more efficient soft magnetic materials.
Fe-based amorphous and nanocrystalline alloys are one of the major soft magnetic materials, because they exhibit superior soft magnetic properties, including high saturation magnetisation, extremely low coercivity and excellent power loss performance. However, the preparation of Fe-based amorphous materials is challenging; it consists of a rapid solidification process, requiring in some cases cooling rates higher than 104 K/s. For that reason, for many years the main fabrication process has been melt-spinning. This process produces thin ribbons that are extensively used to manufacture electronic devices, as well as other components. Nevertheless, there are some difficulties that must be addressed; the geometry of thin ribbons makes difficult the manufacturing of finished or semi-finished part, especially when complex geometries are required. This problem has led several researchers to explore novel materials and new fabrication processes. One of the promising technologies that has been recently used to produce amorphous materials is the gas atomisation process. Gas atomisation is a mass production process that turns a metallic molten stream into small droplets, which later solidify in a spherical form. The high cooling rates achieved during the solidification process, the elevated production rates and the product characteristics make this process a very attractive way to produce amorphous materials. In fact, it is now being considered as a substitute for melt-spinning in some applications.
In order to address the problem described above and increase the knowledge in this field, this thesis presents interesting results with regard to the production of novel Fe-based amorphous alloys. Based on amorphous forming ability studies, various amorphous Fe-based compositions are produced by gas atomisation. The
as-quenched powders are submitted to different heat treatments in order to study relaxation and crystallisation phenomena. In addition, after optimising the annealing process, the as-quenched, relaxed and nanocrystalline powders are used as raw materials in the manufacturing of soft magnetic cores, which are characterised in their working conditions.
The as-quenched powders exhibit excellent soft magnetic properties, which are further improved by proper heat treatments. The soft magnetic cores reveal extraordinary behaviour at high-frequency, which in other words means that the power loss at high-frequency is comparatively lower than for other reported materials. Furthermore, the analysis of the results allow the author of this thesis to establish three conditions for the formation of amorphous structure of Fe-Si-B alloys. All these results are presented and proposed with the aim of considering gas atomisation as an excellent alternative to produce amorphous materials.