A study on sodium nitrate passivation as a new insulating technology for soft magnetic composites created by a team led by Professor Mi Yan and Dr. Chen Wu was recently published in Engineering.
The insulating coating exhibits a two-layered structure containing an oxide barrier and a hydroxide precipitation layer. The growth and the dissolution rate of the coating varies according to the pH value of the NaNO3 passivation solution, resulting in different coating thicknesses that are correlated to the magnetic activity of the corresponding soft magnetic compounds. Image credits: Prof. Mi Yan
Soft magnetic composites, which are made on the basis of metal magnetic powders through insulating coatings, compaction, bonding and annealing, serve as important basic materials in different fields, which include transportation, energy, national defense and aerospace.
Due to the low-electrical-resistivity nature of the soft magnetic alloys, it is difficult to control the eddy current loss, which has turned out to be a bottleneck for high-frequency applications. Phosphorization technology is generally used to produce insulating coatings for industrial production and scientific research.
The resulting phosphate coating, however, tends to decompose at more than 600 ℃ and miss the insulation effect at higher temperatures. It is important to form a new insulation technology to create coating layers with strong adhesion, along with suitable thermal stability and electrical resistivity for high frequency applications of soft magnetic compounds.
For soft magnetic composites, the team of Professor Yan and Dr. Wu suggested passivation of sodium nitrate as a new insulation technology. Depending on methodical compositional and microstructural examinations, developments of the coating in various pH values were revealed, together with the growth mechanisms of the coatings revealed by thermodynamic and kinetic analyses.
The study reveals that the insulating coating achieved with acidic NaNO3 passivation solution with pH = 2 consists of Fe2oh3SiO2To2oh3, and AlO(OH). The result of the large growth rate of the coating layer is due to the strong oxidation competence of NO3− in an acidic environment—at the same time, the dissolution rate of the passive layer is also high due to the high H.+ concentration, which leads to the small thickness of the passive layer at pH = 2.
Using increased pH to 5, the Fe2oh3 transforms into Fe3oh4 with a weakened ability to oxidize NO3−. Despite the slightly reduced growth rate of the passive layer, the H+ concentration reduction also prevents its dissolution well, leading to the maximum thickness of the insulating coatings for considerably increased electrical resistance and ideal alternating current (AC) magnetic performance (μe = 97.2, pcv = 296.4 mW/cm3 below 50 kHz and 100 mT).
Increasing the pH to 8 considerably worsens the oxidizability of the NO3−leading to only Al2oh3AlO(OH), and SiO2 in the passive layer with delayed growth and much reduced thickness. In addition, corrosion occurs in some areas of the magnetic powder surface, resulting in slow performance.
The NaNO3 Passivation technology formed in this study is expandable to other magnetic alloy systems and also sets the concrete basis for the establishment of new and advanced insulating coatings with the use of oxidizing agents 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/
