Table 1 shows the particle size distribution of the Fe-Si-Cr alloy powder according to D90, D50 and D10 measurements. Figure 1 shows the result of PSD for each group of the sieved Fe-Si-Cr alloy powders. We can see that the distribution in most samples is uniform. However, S5 shows quite a wide distribution, this is because all powder particles over 63 µm were included in S5 leading to a wider distribution.
Table 1
Particle size distribution of the sieved Fe-Si-Cr alloy powders
Sample
|
S1
|
S2
|
S3
|
S4
|
S5
|
D90 (µm)
|
35.31 µm
|
50.90 µm
|
70.64 µm
|
92.98 µm
|
140.05 µm
|
D50 (µm)
|
19.72 µm
|
35.23 µm
|
50.97 µm
|
63.89 µm
|
98.12 µm
|
D10 (µm)
|
10.03 µm
|
26.23 µm
|
39.32 µm
|
49.55 µm
|
68.52 µm
|
X-ray diffraction (XRD) patterns are shown in Figure 2. A Cu-Kα source was used for measurements taken from 20° to 90° at 0.01° intervals. After the Fe-Si-Cr alloys were divided into five groups according to fraction sizes and the XRD data was recorded, it was found that the XRD peaks appear in almost the same places as the characteristic peaks of Fe. The majority of the content in the Fe-Si-Cr alloy powders is Fe, while Si and Cr contents are relatively smaller, as such, it is natural that the XRD peaks are close to those expected from pure Fe.
Figure 3 shows the morphology of the Fe-Si-Cr alloy powder in each group. The shapes of the particles in the powders turned out to be spherical, this is because the powder was fabricated using the gas atomizing method. The S2 to S5 SEM images were recorded at ⋅300 and ⋅1,500 magnification, while the S1 was recorded at ⋅300 and ⋅2,000 magnification. Figure 4 show the energy dispersion spectroscopy (EDS) data for the sieved Fe-Si-Cr alloy powders. The EDS data shows that each sample contains about 87 wt% of Fe, about 11 wt% of Si and about 2 wt% of Cr.
In Figure 5, the field dependence of magnetization for the Fe-Si-Cr alloy powders is represented. As shown in Figure 1, S5 powder has an extremely broad distribution compared to other samples. So, we decided to exclude the S5 data because of the possibility of not enough accuracy of the data. Figure 5(a) shows the magnetic hysteresis loop, Figure 5(b) shows the enlarged hysteresis loop data from -15 to 15 Oe and Figure 5(c) shows the relationship between the magnetization saturation and coercivity for each particle size. When measuring the VSM for each powder size group, Fe-Si-Cr alloy was found to have soft magnetic the characteristics with low coercivity. The sample showing the highest magnetization saturation was S2 with a saturation value of 169.38 emu/g and a size of 25-38 µm, while the sample with the lowest coercivity was the 38-53 µm sample with a value of 0.93543 Oe. Generally, higher magnetization saturation induces good DC-bias characteristic, which means the inductance maintains as the current increases.[22] However, in order to induce good properties of electronic devices like powder-inductors, enhanced properties are required for not only magnetization saturation, but also permeability, resistivity, and core-loss.
Figure 6 shows the frequency dependence of each group’s permeability over a frequency range from 100 kHz to 110MHz. Figure 6(a) and 6(b) show the real and imaginary parts of each group’s permeability, respectively, at 1 MHz and 3 MHz. Figure 6(c) shows the quality factor for each group of the sieved Fe-Si-Cr alloy powders. The relationship between permeability and quality factor is expressed in equation (2).
$$\text{Q}=\frac{{{\mu }}^{{\prime }}}{{{\mu }}^{{\prime }{\prime }}}=\frac{\frac{{\text{B}}_{0}}{{\text{H}}_{0}}\text{cos}{\delta }}{\frac{{\text{B}}_{0}}{{\text{H}}_{0}}\text{sin}{\delta }}=\frac{1}{\text{tan}{\delta }}$$
2
Quality factor is inversely proportional to the loss tangent (\(\text{tan}\delta\)), which means the lower loss a sample has, the higher the quality factor is. [23] Figure 6(d) shows the permeability for each group at frequencies of 1 MHz and 3 MHz. The impedance analyzer shows that S1 has the highest permeability in the 1 MHz and 3 MHz bands, these are the bands mainly used by portable electronic devices. [24] Therefore, it seems that S1 would be more suitable in high frequency applications because it maintains its permeability stable in high frequency. Also, if we check the quality factor, we can see that the quality factor and resonance frequency both increase as the powder particle size decreases.
The electrical resistivity of the powder is an important physical quantity that directly affects the intra-particle eddy-current loss from the SMC core the powder is used to form. The resistivity of each powder was measured by a powder resistivity measurement system. The resistivity was measured by pressing the powder increments from 0 to 2.0 ton, with the press increasing 0.2 ton in each step. The final data expressed in Figure 7 is the data collected from the final 2.0 tons press, this is when the powder compaction density is at its greatest, this data is the closest to the actual resistivity of each powder. The S1 powder showed the highest resistivity at 0.3352 Ω∙cm. So, S1 is expected to have the lowest core-loss based on highest resistivity, influenced by decrease of eddy-current loss.
In Figure 8, the core-losses from each group of Fe-Si-Cr powders according to frequency is presented. The core-losses of the Fe-Si-Cr cores were measured at a fixed value of Bm = 10 mT. Figure 8(a), (b) and (c) show how the core-loss, hysteresis loss and eddy-current loss increase linearly with the particle size of the Fe-Si-Cr alloy powders. Hysteresis loss and eddy-current loss were separated from the total core-loss using curve fitting. Figure 8(d) shows the core-loss, hysteresis loss and eddy-current loss of each sample measured at 1 MHz. The S1 powder, which has the smallest particle size, showed the lowest core-loss of 187.26 kW/m3 at 1 MHz. The graph shows that the decrease in hysteresis loss is smaller than the decrease in eddy-current loss as the particle size decreases. This means that the total core-loss of the samples is dominated by the eddy-current loss rather than the hysteresis loss. These results come from the intrinsic properties of Fe based materials, where increased eddy-current loss is a major factor in the increased total core-loss at high frequency. As such, the increased core-loss in the large size powders is because of an increase of eddy-current loss caused by intra-particle eddy currents. [25, 26] The relationship between the resistivity and core-loss shows that group with high resistivity also had low core-loss because of the reduction in intra-particle eddy-current loss.