Preparação, análise microestrutural e propriedades magnéticas de nanocompósitos de CoFe2O4/CoFe2

• Ano de defesa: 2023
• Autor(a) principal: Ferreira, Edson Silva
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Mato Grosso; Brasil; Instituto de Física (IF); UFMT CUC - Cuiabá; Programa de Pós-Graduação em Física
• Palavras-chave em Português: Moagem ultrarrápida; Ferrita de cobalto; Alta coercividade; CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA; Ultrafast grinding; Cobalt ferrite; High coercivity
• Link de acesso: http://ri.ufmt.br/handle/1/5613

In this study, the preparation process of 24/2 nanocomposites will be carried out, followed by a detailed analysis of their internal structure. In addition, the magnetic properties will be investigated to optimise these characteristics in the samples. The synthesis of 24 was carried out using the gel-combustion method, and the substance was subjected to different calcination temperatures for a period of 2 hours under normal atmospheric conditions. Subsequently, the samples were submitted to milling processes at different periods to induce a high residual micro deformation inside the particles and increase the coercivity. After the synthesis of the cobalt ferrite nanocomposite, the next step is the reduction using hydrogen. In this process, the nanocomposite is exposed to a controlled environment containing gaseous hydrogen. At a temperature of 350°C and in time intervals of 10, 15, 20, 25 and 30 minutes, the chemical reduction reaction occurs, in which the hydrogen reacts with the oxides present in the nanocomposite. In addition to the 24/2 nanocomposite synthesis method with 2 gas, it is possible to obtain it with the high-energy milling method. In this way, ground cobalt ferrite is mixed with different concentrations of cobalt iron. Then, the sample is subjected to a ball mill for 1 minute to obtain a uniform mixture and a magnetic coupling between both phases, thus forming the 24/2 nanocomposite. In samples calcined at different temperatures, agglomeration of nanoparticles forms during heating at high temperatures, which contributes to the growth of crystallites and, consequently, to an increase in saturation magnetization. The redistribution of cations, specifically the 3+ ions, in octahedral sites is also responsible for the increase in saturation magnetization. Furthermore, the reduction in the magnetic coercivity of cobalt ferrite with increasing calcination temperature is caused by changes in the microstructure and distribution of cations in the ferrite crystal lattice. .The unground sample has the following characteristics: magnetic coercivity () of 0.4 KOe, magnetic saturation of 75 emu/g, remanence rate ⁄ of 0.38 and maximum magnetic energy () of 2.0 3 ⁄ . After grinding, there was a considerable increase in almost all attributes, which became = 3.7 kOe, = 66 emu/g, ⁄ = 0.56, and () = 9.6 3 ⁄ . The results indicate that this increase is related to the addition of stress and the density of microstructural defects. An increase in magnetic anisotropy is also observed with milling, which is attributed to anisotropic stress. Thus, it is concluded that ultrafast grinding brings several advantages over conventional grinding, especially in terms of efficiency, where you have a tenfold reduction in grinding time and a considerable increase in magnetic properties. Heat treatment in a hydrogen atmosphere is effective in transforming cobalt ferrite into cobalt iron. This process is fast and efficient, reducing cobalt ferrite by approximately 90% in just 20 minutes. Additionally, heat treatment increases the saturation magnetization of cobalt ferrite, making it more magnetic. On the other hand, the magnetic coercivity of cobalt ferrite decreases when reduced in iron-cobalt, which is expected since iron-cobalt is magnetically soft. The method of producing 24/2 nanocomposite through high-energy ball mill has several advantages. The high efficiency and speed of the process stands out, which is 60 times faster compared to other techniques.