1. Introduction
Using ultrafast cooling technology to cool the liquid metal to room temperature, and to form a solid alloy with a disordered arrangement of atoms, amorphous samples can be obtained. Amorphous alloys have received much attention since their inception; their intrinsic quality [
1] and glass formation [
2] have been explored. Amorphous materials exhibit special properties due to the disordered arrangement of atoms, e.g., strength, hardness, and corrosion resistance [
3,
4].
Soft magnetic materials have excellent magnetic properties and usability, and they are widely used in high-tech fields such as aviation and aerospace. With the development of human society and technology, the requirements for lightweight and miniaturized soft magnetic material equipment are increasing. Therefore, the development of excellent soft magnetic materials with high saturation magnetic induction, high permeability, low coercivity, and low iron loss has become one of the important goals of current and future research [
5].
Amorphous soft magnetic materials emerged in the late 1970s. Due to the long-range disorder of atoms, the anisotropy of the material is greatly reduced, which is beneficial to obtain high permeability and low coercivity. As a soft magnetic material with excellent performance, iron-based amorphous alloys can be used in various electronic and power components [
6,
7,
8], such as switching power supplies [
9], transformers, transducers, filters [
10], and sensors. Due to the low no-load loss, transformers with iron-based amorphous alloys have been widely used in rural power grids with long no-load time and low power efficiency, which has significant energy-saving effects. Due to the excellent corrosion resistance, iron-based amorphous alloys can be applied to new and powerful sensors to meet the needs of new technology fields such as automobiles, biological robots, power motors, chemicals, and medical electronics. In addition, due to the high initial magnetic permeability, iron-based amorphous alloys can be used to develop various devices for high-sensitivity occasions to ensure precision and stability [
11].
Although iron-based amorphous alloys have high saturation magnetic induction and low coercivity, the effective permeability at the high frequencies is much lower than that of cobalt-based amorphous alloys, which limits the application of iron-based amorphous alloys in high-frequency magnetic components. Due to the high cost of Co raw materials, people have been trying to improve the high-frequency permeability of iron-based amorphous alloys to replace cobalt-based amorphous alloys in some special applications. In 1988, the Finemet alloy developed by Yoshizawa from Hitachi Metals exhibited both high saturation magnetic flux density and effective magnetic permeability, which was a breakthrough in the research of soft magnetic materials [
12]. On the basis of this research, a series of nanocrystalline soft magnetic materials were developed, with the more famous ones being Nanoperm [
13] and Hitperm [
14] alloys. Nanocrystalline materials require 10–15 nm crystal grains dispersed in the amorphous matrix; hence, there is a process of nucleation and growth during the annealing process. The atomic nucleation mechanism and retention of nanocrystalline phases in materials have important effects on the properties [
15,
16,
17].
In the production of the steel industry, rare earth elements have been widely used as impurity scavengers in steel melting and casting processes. Because the reactions between rare earth elements and harmful impurities such as oxygen or sulfur are thermodynamically more favored compared to those between impurities and Fe [
18], a small addition can achieve good results; accordingly, it has been called “the vitamin of modern industry”. Adding a small amount of rare earth elements to steel has the functions of removing, deoxidizing, and refining grains. Rare earth elements have magnetism because their internal
f-electron shells are not completely filled. Therefore, the addition of small amounts of magnetic rare earth elements will affect not only the GFA (glass-forming ability) of bulk glass-forming alloys, but also their magnetic properties.
The magnetic properties of rare earth elements mainly originate from the
f orbitals which are not filled. From a microscopic point of view, the magnetism is not due to the coupling of the cooperative exchange phenomenon (direct exchange or super exchange); it is because of the coupling between the orbital momentum and the spin of each atom. The rare earth element gadolinium (atomic number 64) has a special outer electron arrangement, with one electron in each of the seven orbitals of the 4
f sublayer, which means that it has the largest number of unpaired electrons among all rare earth elements and, thus, the highest “natural” spin value. In this way, gadolinium has the largest magnetic moment of the unpaired electron and has a special spin dynamic mechanism [
19,
20]. Gadolinium has a wide range of applications in alloys.
During the formation of amorphous alloys, intermetallic compounds can be regarded as competing phases of the amorphous phase. The acquisition of the amorphous alloy depends on the mutual interference and hindrance between the competing phases [
21]. Gd element has stronger affinity toward oxygen, i.e., Gd and oxygen react preferentially in thermodynamics. Oxygen can significantly reduce the formation ability of amorphous alloys; thus, in molten liquid alloy, a small amount of Gd element also plays the role of oxygen adsorbent during the amorphous formation process, which suppresses the adverse effects of oxygen in the process of melting and casting, suppresses heterogeneous nucleation, and improves the ability of amorphous formation [
22,
23].
With the development of smart grids, the requirements for the accuracy of the upgraded smart meter transformer cores have increased. The main requirement is that the
μe remains constant over a wide frequency range and wide external fields. The traditional process is to use transverse magnetic annealing to reduce the low-frequency
μe value of Finemet, and the material can maintain the stability of the
μe value in a wide frequency range. However, transverse magnetic annealing is limited by the size of the external magnetic field of the equipment, and tension annealing is more effective than transverse magnetic annealing [
24]. In this experiment, a sample with rare earth element added was treated by constant tension annealing, and the change in its magnetic properties was tested.
In this paper, Gd element was added to the Finemet alloy; processes in actual industrial production were selected to prepare the magnetic cores, and the structures and magnetic properties were compared with the Finemet alloy without Gd.
2. Experimental Procedures
Using the processes of vacuum melting and rapid cooling, Fe
73.5−xCu
1Nb
3Si
13.5B
9Gd
x (x = 0, 0.5, 1.0, and 1.5) amorphous alloy ribbons were prepared, and their formulas are shown in
Table 1.
In this experiment, the alloy Fe73.5−xCu1Nb3Si13.5B9Gdx (x = 0, 0.5, and 1.0) was made into amorphous ribbons with a thickness of 24–26 μm, while the Fe72Cu1Nb3Si13.5B9Gd1.5 alloy had poor fluidity, and only a small amount of ribbons were prepared, without magnetic cores.
Since the samples of this experiment were prepared on a small scale, in order to facilitate the comparison with existing products, the annealing treatment was processed together with mass-produced products. Therefore, an existing annealing process and magnetic core size were selected. The amorphous alloy ribbons were wound into toroidal cores of Φ25 mm (outer diameter) × Φ16 mm (inner diameter) × 10 mm (height). Six samples of each formula were produced, and they were annealed in a vacuum furnace at 525 °C or 557 °C, with three samples for each annealing temperature. After annealing, the corresponding plastic shells were selected for packing. In addition, part of the as-cast Fe72.5Cu1Nb3Si13.5B9Gd1 alloy and original Finemet alloy ribbons were subjected to constant tension annealing at 600 °C and 630 °C, respectively, using self-made equipment from Dayou-Tech Company (Yichun, Jiangxi, China). After tension annealing, all ribbons were wound into Φ18.3 mm (outer diameter) × Φ12.8 mm (inner diameter) × 10 mm (height) toroidal magnetic cores and packaged into corresponding plastic shells.
The static soft magnetic performances were tested using an MATS-2010SD soft magnetic DC measuring device (Hi = 0.08 A/m, Hj = 0.8 A/m, Hs = 40 A/m, N1:N2 = 5:2); a Tonghui Electronics (Changzhou, Hunan, China) TH2829C precision LCR meter was used to test the inductance values at different temperatures and frequencies. In the magnetic performance test results obtained, the average value of the three samples was taken as the final performance value.
A Bruker D8 Advance XRD (X-ray diffractometer, Berlin, Germany) was used to analyze the crystallization (CuKα, 40 kV, 30 mA, 2θ = 20–90°, 8°/min); an NETZSCH differential thermal analyzer (Bavaria, Germany) was used to measure differential thermal data (heating rate 10 °C/min, Ar protection, gas flow 100 mL/min); a Lakeshore 7410 VSM (vibrating sample magnetometer, Westerville, USA) was used to test coercivity and magnetization values. A tungsten filament scanning electron microscope from Hitachi (Tokyo, Japan) was used to observe microstructure, and EDS (energy-dispersive spectroscopy) was used to analyze the element contents. The instrument used to observe microscopic morphology was a Tecnai G2 F30 (FEI Company, Hillsboro, OL, USA) field-emission TEM (transmission electron microscope) operated at 300 kV; 3DAP (CAMECA Instruments LEAP 5000XR, Madison, WI, USA) was used to detect the content and distribution of each element.