New insulation technology developed for soft magnetic compounds

It was recently published in Engineering.

The insulating coating exhibits a two-layer structure containing an oxide barrier and a hydroxide precipitation layer. The growth and dissolution rate of the coating vary with the pH value of the NaNO.3 passivation solution, resulting in different coating thicknesses, which correlate with the magnetic performance of the corresponding soft magnetic compounds. Image Credits: Prof. Mi Yan

Made from metal magnetic powders by insulating coating, compaction, bonding, and annealing, soft magnetic compounds serve as important basic materials in a variety of fields including transportation, energy, national defense, and aerospace.

Due to the low electrical resistivity nature of soft magnetic alloys, it is difficult to control eddy current loss, which has proven to be an obstacle for high frequency applications. Phosphorization technology is generally used to produce insulating coatings for industrial production and scientific research.

However, the resulting phosphate coating tends to break down above 600℃ and loses insulation impact at higher temperatures. Forming a new insulation technology is important to create coating layers with robust adhesion, along with adequate thermal stability and electrical resistivity for high-frequency applications of soft magnetic compounds.

For soft magnetic compounds, Prof. Yan and Dr. Wu’s team have suggested passivation with sodium nitrate as a new isolation technology. Based on methodical compositional and microstructural examinations, the evolutions of the coating at various pH settings have been revealed, along with the growth mechanisms of the coatings revealed through thermodynamic and kinetic analyses.

The study reveals that the insulating coating achieved with an acidic NaNO3 passivation solution with pH = 2 contains FetwoEITHER3YesOtwoTo thetwoEITHER3and AlO(OH). The result of the large growth rate of the coating layer is due to the strong oxidation capacity of NO3 in an acidic environment, at the same time, the dissolution rate of the passivation layer is also high due to the high H+ concentration, which leads to the small thickness of the passive layer at pH = 2.

As the pH increases to 5, the FetwoEITHER3 is transformed into Faith3EITHER4 with weakened oxidation capacity of NO3. Despite the slightly reduced growth rate of the passivation layer, the H+ Reducing the concentration also inhibits their dissolution well, leading to maximum insulation coating thickness to greatly increase electrical resistivity and ideal alternating current (AC) magnetic performance (mme = 97.2, PCV = 296.4 mW/cm33 below 50 kHz and 100 mT).

Increasing the pH to 8 considerably impairs the oxidizability of NO3which leads to only AltwoEITHER3AlO(OH) and SiOtwo in the passivation layer with retarded growth and very reduced thickness. Also, corrosion occurs in some areas of the magnetic powder surface, resulting in slow performance.

Nano3 The passivation technology formed in this study can be expanded to other magnetic alloy systems and also lays the concrete foundation for the establishment of new and advanced insulating coatings with the use of oxidizing agents such as superoxide, nitrite, and permanganate.

Magazine reference:

Yan, M., et al. (2022) Sodium nitrate passivation as a new insulation technology for soft magnetic compounds. Engineering.


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