Crystallization kinetics and magnetic properties of Fe_63.5Co₁₀Si_13.5B₉Cu₁Nb₃ nanocrystalline powder cores

Abstract

Amorphous ribbons of the alloy Fe_63.5Co₁₀Si_13.5B₉Cu₁Nb₃ were prepared by the standard single copper wheel melt spinning technique in air and their crystallization kinetics was analyzed by non-isothermal differential scanning calorimetry (DSC) measurements. The crystallization activation energies (E_x, E_a1 and E_a2) of amorphous ribbons calculated from Kissinger model were 448, 385 and 396 kJ/mol for the first and the second crystallizations, respectively. The Avrami exponent n was calculated from the Johnson–Mehl–Avrami (JMA) equation and was used to identify the crystallization mechanism for the amorphous ribbons. The ribbons were milled into different sized flakes, which were molded subsequently to cores using 3 wt.% epoxy as a binder. The effective permeability of the cores showed high frequency stability and increased with the size of the flakes. For the cores made from small sized flakes (− 75 μm), the quality factor was increased at high frequencies, which was attributed to the reduction in the eddy current loss.

Introduction

The magnetic properties of Fe-based nanocrystalline alloys have been extensively studied since Yoshizawa et al. [1] reported the excellent soft magnetic behaviors of these nanocrystalline alloys. In particular, it was found that adding small amounts of Cu and Nb to the Fe–Si–B amorphous alloys can significantly improve their soft magnetic properties after partial devitrification through isothermal annealing [2]. The alloy elements Cu and Nb play an important role in the formation of nanostructure in the Fe–Si–B–Cu–Nb alloys. Cu segregates at the initial stage of crystallization to form Cu-rich clusters that facilitate nucleation of α-Fe (Si) grains. Rejection of insoluble and slow diffusing Nb atoms to the grain interfaces hinders the grain growth, which leads to a decrease in crystallization kinetics and reduction in grain size [2], [3], [4]. The excellent soft magnetic properties are attributed to the ultrafine structure composed of α-Fe (Si) nanocrystallites in a residual amorphous matrix [5]. In such a structure, the positive magnetostriction of the amorphous phase can be compensated by the negative magnetostriction of the nanocrystalline α-Fe (Si) phase. Furthermore, since the size of nanocrystallites (~ 10–15 nm) is smaller than the ferromagnetic exchange interaction length (about 35 nm for FeSiBCuNb amorphous/nanocomposite system), ferromagnetic exchange coupling between the nanocrystalline α-Fe (Si) grains and amorphous matrix evens out of the magnetocrystalline anisotropy [6]. As a result, ultrasoft magnetic properties with high saturation, very high permeability, low frequency losses, low coercivity and near zero magnetostriction are obtained [7], [8].

In recent years, many studies have been made on the fabrication of Fe-based nanocrystalline and amorphous alloy powder cores [9], [10], [11]. Most of these studies have been centered on applying the melt spinning and in-rotating-water quenching techniques to produce soft magnetic materials which are mostly in the form of ribbons or wires. Ribbons and wires are satisfactory forms for a range of applications such as sensors and pulse generators, but are not suitable for applications where large or complex shaped components are required. The ball milling technique combines the possibility of phase formation with a product material which is in powdered form, and therefore is suitable for compaction and densification in a variety of shapes. The soft magnetic properties of nanocrystalline alloys are significantly changed during the fabrication process of consolidated cores. These changes include crystallization heat treatment, fragmentation of the crystallized ribbons by milling, ball milling, compacting and stress relief annealing, during which processes the magnetic properties such as permeability and core loss will be changed. The soft magnetic behavior is directly related to the size and shape of fragmented particles and the size of grains within the particles [12], [13]. The crystallization kinetics is very important for the nanocrystalline/amorphous matrix structure materials, as it provides the ability to tailor the microstructure. The amount of the nanocrystalline phase formed within the matrix can be controlled to achieve the desired magnetic performance. The aim of this paper is to investigate the effects of substitution of small amounts of Fe with Co in Fe_63.5Co₁₀Si_13.5B₉Cu₁Nb₃ alloy on the crystallization kinetics and to study the effect of permeability and quality factor of the nanocrystalline alloy powder cores.