# Effect of the Insulating Layer on the Properties of SMC Inductors
## 3 Method and Preparation of SMC Samples  
As mentioned in the previous chapter, four different SMC materials were examined and produced for high-frequency testing. Two of the samples produced were composed of very pure iron powder and epoxy resin (organic binder), while the other two were made with inorganic coatings. One was based on ferrite nano-powders and another on phosphoratized ferromagnetic powder. The latter was a typical commercial product. The adopted water-atomized iron was a high-purity powder with a minimum content of oxygen and other elements. The epoxy resin used was a specific polymeric product used in automotive sectors and can be applied to 300 \({}^{\circ}\)C. The nano-ferrite powder is used as a dye in several industrial sectors; its particle dimensions are tens of nm. Figure 2: Types of SMC insulating coatings: (**a**) organic layer, (**b**) thick organic layer, (**c**) inorganic layer, (**d**) phosphoratized inorganic layer or similar techniques, (**e**) organic layer filled with nanoparticles, and (**f**) multilayers with the possible hybrid nature of coatings. Epoxy SMC 1-800 denotes the sample containing 1% in weight of epoxy and compacted at 800 MPa, while epoxy SMC 2-800 denotes the sample containing the same epoxy resin in 2 wt.%, pressed at the same hydraulic pressure level. Nano-ferrite SMC 1-800 refers to the 1 wt.% of the ferrite insulating layer. Phosphatized SMC-800 is a product available on the market; it is already adopted to produce SMC inductors. The constant weight percentage content does not allow an easy comparison between different binders, since the binder density affects the final layer volume. However, both the nano-ferrite and the epoxy resin begin to reach insulation suitable for their use in inductor cores starting from around 1% of content by weight. It is possible that different effective binder volumes are compensated by other physical properties. As an example, resin is a dense liquid or gel that is well distributed, but highly deformable, while ferrite is a nano-sized solid powder that is discrete and hard to displace during the molding phase, but is less sensitive to deformations. The samples of pure iron and all proposed binders were produced by mixing the two constituents by hand, followed by a mixing process in an industrial mixer for 30 min, as seen in Figure 3. All the samples were then compacted into discs using a hydraulic press at 800 MPa, as shown in Figure 4. The organic binder samples were then cured at 150 \({}^{\circ}\)C for six hours. Instead, the phosphatized SMC-800 was thermally treated at 530 \({}^{\circ}\)C in air, while the nano-ferrite SMC was left to be evaluated as a green sample. A milling process was then executed to remove the center material in order to produce the toroid geometry seen in Figure 4. After machining, each discoidal sample was divided into two parts: the toroidal ring and the smaller central disk. Next, the toroidal rings were wound with one layer of electrical tape to protect the wire insulation of the secondary coils from accidental micro-cuts. With the secondary coils and ultimately with the primary coils, the windings are completed, as shown in Figure 5.  
Figure 4: The manufacturing process of the toroid: compacting and milling. Figure 3: Different steps of the mixing process, for instance, the epoxy coating procedure. The latter was well suited to obtain a further machined sample in the shape of a small rod. Finally, a further rod made of soft ferrite completed the sample set. The SMC rods were machined at the final diameter (4.5 mm) and then brought to the same length (11.5 mm). Each rod equested a winding made of 29 coils with a 0.25 mm diameter enameled copper wire, as shown in Figure 6.  
Choosing the suitable materials to perform a comparison was a challenging task, which started from the considerations of the properties of a ring sample made from a part of a commercial SMC inductor.