A new insulation technology has been 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 recently published in Engineering.

The insulating coating exhibits a two-layer structure containing an oxide barrier and a layer of hydroxide precipitation. The growth and dissolution rate of the coating varies with the pH value of NaNO3 passivation solution resulting in different coating thicknesses that are related to the magnetic characteristics of the respective soft magnetic composites. Image credits: Prof. Mi Yang

Soft magnetic compounds, which are made on the basis of metal magnetic powders through insulating coating, densification, bonding and annealing, act as important core materials in various fields, including transportation, energy, national defense and aerospace.

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

However, the resulting phosphate coating tends to decompose above 600℃ and loses its insulating effect at higher temperatures. It is important to establish a new insulation technology to create coating layers with stable adhesion along with suitable thermal stability and electrical resistance for high frequency applications of soft magnetic composites.

For soft magnetic composites, Professor Yan and Dr. Wu’s team proposed sodium nitrate passivation as a new isolation technology. Depending on the methodical compositional and microstructural studies, the evolution of the coating at different pH settings is revealed, together with the growth mechanisms of the coatings revealed by thermodynamic and kinetic analyses.

The study revealed that the insulating coating achieved with acidic NaNO3 passivating solution with pH = 2 contains Fe2O3SiO2Al2O3and AlO(OH). The result of the large growth rate of the capping layer is due to the strong oxidation competence 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, resulting in a small thickness of the passive layer at pH = 2.

Using increased pH to 5, Fe2O3 is transformed into Fe3O4 with weakened NO oxidizing ability3. Despite the slightly reduced growth rate of the passivation layer, H+ decreasing the concentration also inhibits its dissolution well, resulting in a maximum insulating coating thickness for significantly increased electrical resistance and ideal alternating current (AC) magnetic performance (me = 97.2, Pcv = 296.4 mW/cm3 below 50 kHz and 100 mT).

Increasing the pH to 8 significantly worsens the oxidizability of NO3leading only to Al2O3AlO(OH) and SiO2 in the passivation layer with slow growth and much reduced thickness. In addition, corrosion occurs in some areas of the surface of the magnetic powder, which slows down the operation.

NaNO3 The passivation technology established in this study can be extended to other magnetic alloy systems and also lays the concrete foundation for the creation of new and advanced insulating coatings using oxidants such as superoxide, nitrite, and permanganate.

Journal reference:

Yan, M. et al. (2022) Sodium Nitrate Passivation as a New Insulation Technology for Soft Magnetic Composites. Engineering. doi.org/10.1016/j.eng.2022.01.016.

Source: https://engj.org/

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