New insulation technology developed for soft magnetic composites

A study on sodium nitrate passivation as a new insulation technology for soft magnetic composites by a team led by Professor Mi Yan and Dr. Chen Wu was created recently published in mechanical engineering.

The insulating coating has a two-layer structure that includes an oxide barrier and a hydroxide precipitate layer. The growth and dissolution rate of the coating varies with the pH of the NaNO3 Passivation solution, resulting in different layer thicknesses that correlate with the magnetic performance of the corresponding soft magnetic composites. Photo credit: Prof. Mi Yan

Soft magnetic compounds, which are manufactured on the basis of metallic magnet powders through insulating coating, compaction, bonding and annealing, function as important base materials in various 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 make insulating coatings for industrial production and scientific research.

However, the resulting phosphate coating tends to decompose above 600°C and fails to insulate at higher temperatures. It is important to develop new isolation technologies to create coating layers with robust adhesion together with appropriate thermal stability and electrical resistance for high frequency applications of soft magnetic composites.

For soft magnetic composites, Professor Yan and Dr. Wu proposed sodium nitrate passivation as a new insulation technology. Dependent on methodological compositional and microstructural investigations, evolutions of the coating at different pH settings were revealed, together with the growth mechanisms of the coatings revealed by thermodynamic and kinetic analyses.

The study shows that the insulation coating is achieved with an acidic NaNO3 Passivation solution with pH = 2 contains Fe2O3SiO2Al2O3and AlO(OH). The result of the large growth rate of the coating layer is due to the strong oxidizing ability of NO3 in an acidic environment – but at the same time the dissolution rate of the passivation layer is high due to the high H+ concentration, which leads to the low thickness of the passive layer at pH = 2.

Using a pH raised to 5, the Fe2O3 turns into Fe3O4 with weakened oxidizing ability of NO3. Despite the slightly reduced growth rate of the passivation layer, the H+ Reducing the concentration also inhibits its dissolution well, resulting in maximum insulating coating thickness for greatly increased electrical resistance and ideal magnetic performance at alternating current (AC) (µe = 97.2, PCV = 296.4mW/cm3 below 50 kHz and 100 mT).

Increasing the pH to 8 significantly worsens the oxidizability of the NO3which only leads to Al2O3AlO(OH) and SiO2 in the passivation layer with delayed growth and greatly reduced thickness. In addition, corrosion occurs in some areas of the magnet powder surface, resulting in slow performance.

The NaNO3 The passivation technology developed in this study is extendable to other magnetic alloy systems and also forms the concrete basis for the establishment of new and advanced insulating coatings using oxidants such as superoxide, nitrite and permanganate.

Magazine reference:

Yan, M., et al. (2022) Sodium nitrate passivation as a novel insulation technology for soft magnetic composites. Mechanical engineering.



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