Development of a Highly Densified Magnetic Sheet for Inductors and Advanced Processes through Silane Surface Treatment of Fe Nanopowder

Abstract

For developing subminiature and highly integrated multilayer inductors, soft magnetic powder was used; however, its ferrite magnetic component is characterized by high resistivity and reduced direct current saturation, leading to the deterioration of the inductor under high currents. Therefore, herein, to improve the electromagnetic properties of thin-film inductors, Fe nanopowder was used to increase the volume fraction of magnetic sheets. Surface treatment was performed by using silane coupling agents, which improved the bonding strength and dispersibility of the Fe nanopowder with a heterogeneous epoxy binder. For uniform surface treatment on the nanopowder, the silane-treated powder was aged for 24 h, at a temperature of 3 °C. The surface-treated Fe nanopowder was used with a mixing ratio of the soft magnetic powder (coarse:fine:nano) of 7:2.5:0.5 wt.%; this was successful in producing a flexible and highly densified magnetic sheet. As a result, the volume fraction of the magnetic sheet for thin-film inductors to which a low-temperature aging-treated nanopowder was applied was significantly improved.

1. Introduction

Miniaturization of electronic components, including smartphones and TVs, has been accelerating in recent years. Smartphones, in particular, require miniaturization and high characterization of many electronic components to simultaneously realize slimness, as well as various convenience functions. More than 100 power inductors are used in smartphones as filters in combination with capacitors [1,2]. Additionally, as the mounting area decreases and functions increase, electronics component companies are focusing on the development of subminiature and highly integrated multilayer inductors [3,4,5,6,7]. To improve the electromagnetic properties of the inductor, many researchers have made an effort to develop a new composition of magnetic powders, change the inductor’s design, and optimize the internal electrode design [8,9].

Soft magnetic powder used as a material for multilayer power inductors is divided into ferrite and metal composites. Ferrite magnetic materials have high resistivity (10²–10¹⁰ Ωcm) and very low eddy current loss, resulting in inductor materials. In addition, the DC saturation characteristics are reduced due to its low saturation magnetization properties. For this reason, the high current stability of the multilayer power inductor is deteriorated. However, metal composites show low hysteresis losses, with low coercive force and high stability at high currents.

To address this, in this study, a highly densified magnetic sheet for use as a power inductor was fabricated by using Fe-6.5 wt.% Si coarse powder, fine iron powder, and nano iron powder [10,11,12,13,14,15]. However, the use of nano iron powder with a high specific surface area is difficult to implement with the binding system used in the current mass production process. This problem was solved through nanopowder surface treatment with a silane coupling agent, and its mechanism was identified [16,17,18,19].

2. Materials and Methods

Three kinds of powders were used to prepare a magnetic sheet. Fe-6.5 wt.% Si powder (average particle size = 40 µm), manufactured by Atmix Inc., and fine iron powder (average particle size = 3 µm) were used as raw materials, and Fe nanopowders (average particle size = 100 nm) were used to effectively fill the void space between the coarse Fe-6.5 wt.% Si powder and fine iron powder. Nanopowders were surface-treated by using an epoxy-based silane coupling agent from Shinetsu Inc(Tokyo, Japan). To achieve uniform surface treatment, a silane solution using ethanol and distilled water was prepared (Figure 1). The prepared silane solution was added to the nanopowder for a mixture of 1% silane solution, which was then mixed with a pre-mixer for 1 h. The surface-treated nanopowders were subjected to dehydration condensation at 3 °C for 24 h, thereby preparing surface-treated nanopowders for producing a magnetic sheet [20].

After mixing each powder at a constant mixing ratio, a magnetic slurry was prepared by using an epoxy binder and a dispersant. The prepared slurry was dispersed and mixed at a frequency of 64 Hz for 3 min, using a resonance acoustic mixer. The magnetic slurry produced was made of a magnetic sheet, using a tape casting process (Figure 2).

The microstructures of the magnetic sheets were analyzed by scanning electron microscope (SEM, JSM-7100F, JEOL Ltd., Akishima City, Japan). The surface characteristics of the magnetic sheets were analyzed by using a surface profiler (Dektak 150, Veeco Tucsom Inc., Oyster Bay, NY, USA). The density and volume fraction of the magnetic sheets were evaluated by electric balance (OHAUSS Adventure, Mettler Toledo Inc., Columbus, OH, USA), using the Archimedes method. Transmission electron microscope images were taken on a JEOL 4010 (TEM, JEOL Ltd., Akishima City, Japan). Permeability of toroidal was evaluated by LCR meter (Agilent E4980A, Agilent Inc., Santa Clara, CA, USA).

3. Results and Discussion

In this experiment, toroidal bulk magnetic composites were first prepared to determine the optimum mixing ratio and electromagnetic properties between powders. Subsequently, the magnetic sheet was prepared at the optimum mixing ratio, to evaluate the volume fraction and physical properties of the sheet. The bulk magnetic material was manufactured in toroidal form, to confirm the optimum magnetic properties and volume fraction. The mixing ratio of Fe-6.5 wt.% Si powder (Figure 3a) and fine iron powder (Figure 3b) was adjusted from 10:0 to 7:3. Fe powder with a particle size of 10 nm (Figure 3c) was used for high volume fraction and improved permeability.

At this time, the molding pressure was increased from 1.8 to 6.25 ton/cm², to test its effect on permeability and volume fraction. When only Fe-6.5 wt.% Si powder was used, many voids were created between the coarse powders, resulting in low volume fraction and permeability. When bulk magnetic composites were prepared with the addition of 20% by weight of fine iron powder, the permeability and volume fraction were greatly improved. This testing confirmed that the volume fraction and permeability increase with increasing molding pressure (

Table 1. Results of evaluation of magnetic properties and volume fractions by molding pressure and mixing ratio of magnetic bulk composites.

	Ratio	Pressure (ton/cm2)	Permeability (μ)	Volume Fraction (%)
Fe-6.5Si	10:0	1.8	29.35	72.11
		2.5	32.1	73.45
		3.75	36.73	75.73
		6.25	45.08	79.29
	8:2	1.8	30.82	79.77
		2.5	32.42	80.89
		3.75	33.67	81.39
		6.25	35.17	82.37
	7:3	1.8	35.03	80.2
		2.5	35.66	80.75
		3.75	38.14	82.21
		6.25	40.33	83.27

).

It was confirmed that the mixing ratio of Fe-6.5 wt.% Si powder and fine iron powder showed high permeability when at a ratio of 7:3. This ratio allowed the relatively small-sized fine powders to fill the voids more effectively between the coarse powders and improved the volume fraction, compared to the other ratios tested (Figure 4).

Microstructure analysis of bulk composite by molding pressure and mixing ratio. Mixing ratio 10:0 (Figure 4a–d) with molding pressure 1.8 ton/cm² (Figure 4a,b) and 6.25 ton/cm² (Figure 4c,d). Mixing ratio 8:2 (Figure 4e–h) with molding pressure 1.8 ton/cm² (Figure 4e,f) and 6.25 ton/cm² (Figure 4g,h). Mixing ratio 7:3 (Figure 4i–l) with molding pressure 1.8 ton/cm² (Figure 4i,j) and 6.25 ton/cm² (Figure 4k,l)

In the case of bulk magnetic composites, the volume fraction and permeability are proportional to each other and can be explained by Ollendorf’s Equation [21]:

$$\mu =\frac{\eta {\mu }_0\left({\mu }_m-{\mu }_0\right)}{N\left(1-\eta \right)\left({\mu }_m-{\mu }_0\right)+{\mu }_0}$$

(1)

µ:relative permeability

n: volume packing fraction

N: coefficient of diamagnetic field (0 ≤ N ≤ 1)

µ_m: Intrinsic permeability of material(bulk)

µ₀ = 4π × 10^-7 H/m.

Using Ollendorf’s equation, we plotted the actual values (dotted lines) and the theoretical results (solid lines), taking into account the demagnetization fields (Figure 5) [21].

The experimental results and the results of Ollendorf’s equation are highly correlated. Therefore, to meet the demands of the IT device market requiring high performance/ultra-thin power inductors, an ultra-thin magnetic sheet with nano-sized soft magnetic powder should be developed (Figure 6).

The previous experiment showed that the addition of nanopowder of 0.5 wt.% or less did not affect volume fraction or permeability. Based on this result, a mixing ratio of 7:2.5:0.5 of this nanopowder was selected for the experiment, to improve volume fraction. Sample preparation for the selection of the magnetic sheet conditions was SPL2 with nanopowders, SPL3 with nanopowder surface treatment only, and SPL4 with aging treatment at 3 °C for 24 h after surface treatment. The surface treatment of nanopowder with a 1 wt.% silane coupling agent showed a 3.5% increase in volume fraction and a slight improvement in permeability (SPL3). Additionally, aging the nanopowder at 3 °C for 24 h resulted in a 6.6% increase in volume fraction and a 10% increase in permeability (Figure 7).

In this experiment, silane coupling agents were prepared in a ratio of 4:1 with distilled water, for surface treatment of nanopowders. However, rapid surface reactions can form non-uniform coatings. To induce the formation of a uniform coating layer, a slow and uniform surface-treatment method was applied by aging for 24 h at a temperature of 3 °C. As a result, a uniform coating layer with a thickness of 5 nm was successfully formed on the surface of the nanopowder. Such a surface treatment works to fill the void space between the coarse powder and the fine powder when the magnetic slurry is made. As a result, the volume fraction is improved, and the permeability is increased (Figure 8).

In the previous experiment using the bulk magnetic composite, the best mixing rate was found at the magnetic powder mixing ratio of 7:2.5:0.5. Iron nanopowder was given the same aging treatment, 3 °C for 24 h. For the magnetic sheet using only Fe-6.5 wt.% Si powder with an average particle size of 40 µm, it can be seen that the inside of the sheet is not uniformly filled (Figure 9a). For the magnetic sheet produced with the coarse and the fine powder at 7:3, the empty spaces between the coarse powders were confirmed to be uniformly filled, as shown in Figure 9b. To improve the volume fraction, the magnetic sheet was manufactured by using nanopowder, without surface treatment. However, all the magnetic sheets cracked during sheet casting; this is because, due to its large specific surface area, the epoxy binder was not uniformly adsorbed on the surface of the nanopowder. Hence, the surface treatment process using a 1% silane solution is the key technology for casting flexible magnetic sheets. As a result of applying the process developed through this study, we have succeeded in developing highly densified magnetic sheets by using nanopowder (Figure 9c,d).

The characteristics of the magnetic sheet produced under each condition were analyzed. Analyzing the surface properties of the magnetic sheet shows that Ra = 2.01 µm and Rz = 12.7 µm for the mixing ratio 7:3. In the case of the magnetic sheet to which the surface-treated nanopowder (no aging process) was applied, it was confirmed that the surface characteristics were improved to Ra = 1.7 µm and Rz = 10.8 µm. For the magnetic sheet for which the surface was treated and the powder was subjected to the aging treatment at a temperature of 3 °C for 24 h, Ra = 0.8 µm and Rz = 5.2 µm; thus, the sheet properties were dramatically improved. The reason for this is that the silane coupling agent layer, which improves the bonding force between different materials, is uniformly formed on the surface of the powder and hence improves the bonding strength with the binder (

Table 2. Evaluation results of the surface characteristics of the magnetic sheet fabricated under each condition.

Condition	Ra (μm)	Rz (μm)	Density (g/cc)	Volume Fraction (%)
7:3	2.04	13.2	5.62	75.1
	2.05	12.4	5.54	77.4
	2.03	12.9	5.71	76.3
	1.92	12.8	5.69	77.8
	2.01	12.5	5.68	78.4
7:2.5:0.5 (non-aging)	1.74	11.4	5.91	79.8
	1.76	10.3	5.83	80.1
	1.67	10.4	5.79	80.3
	1.77	11.2	5.93	79.6
	1.69	10.8	5.75	80.4
7:2.5:0.5 (aging @ 3℃ for 24hr)	0.87	5.2	6.47	84.8
	0.81	5.0	6.54	84.5
	0.70	4.9	6.38	86.7
	0.95	5.7	6.58	84.1
	0.83	5.4	6.67	85.2

).

The volume fraction of the magnetic sheet produced under each condition was also evaluated. As shown in Figure 10, the magnetic sheet applied with the powder aged at 3 °C for 24 h showed the highest result, with a volume fraction of 85%.

Based on the volume fraction and permeability results of magnetic sheets with toroidal form and nanopowders, it is most important to develop an advanced process that can have a more effective dispersion method and a volume fraction similar to toroidal composites applied with molding pressure.

4. Conclusions

In order to manufacture a high-densification magnetic sheet, it is necessary to apply a nanopowder process that improves the coarse powder–fine powder filling method currently used. In this research, a highly densified magnetic sheet for a power inductor was successfully fabricated by a magnetic-sheet casting process, using Fe-6.5 wt.% Si coarse powder, fine iron powder, and nano iron powder. Using a surface treatment with a silane coupling agent, we effectively implemented Fe nanopowder with a high specific surface area with the binding system used in the current mass production process. The magnetic properties of the sheet fabricated by using trimodal powder were highly improved, compared to the magnetic sheet fabricated by using bimodal powder. The optimum conditions for the manufacture of high-filled magnetic sheets were determined through the experiments presented here and culminated in the study outcomes as follows.

• Nanopowder is receptive to surface treatment by epoxy-based silane coupling agents.
• The surface treatment condition of the nanopowder involves mixing the nanopowder with 1% silane coupling agent, followed by aging for 24 h at 3 °C.
• An effective dispersion process for effective dispersion of nanopowders was developed.
• A press-sheet casting process to fill powder effectively when forming a sheet was developed.