Nanocrystalline alloys VITROPERM® are materials on the basis of Fe, Si and B with additions of Nb and Cu. They are produced via Rapid Solidification Technology as a thin ribbon, initially in the amorphous state and then crystallized in a subsequent heat treatment around 500 – 600 °C. This gives rise to an extremely fine grained microstructure with grain sizes of 10 nanometers – hence the name nanocrystalline.
These nanocrystalline alloys combine low magnetic anisotropy and low magnetostriction, both prerequisites for high magnetic permeability, with high magnetic flux density Bs and good thermal stability.
The fact that an extremely fine grained structure leads to good magnetic properties comes as a surprise since in conventional soft magnetic alloys the coercivity Hc increases with decreasing grain size D 1/D law of Mager. Yet excellent soft magnetic properties are re-established in the grain size regime below about 20 nm.
Soon after the discovery of these material VAC’s engineers were able to give fundamental contributions to the understanding of the relations between composition, microstructure, grain size and the resulting magnetic properties such as anisotropy, magnetostriction, coercivity and permeability. The results of VAC’s investigations are published in numerous scientific papers.
The figure shows the relation between coercivity Hc and grain size D. In the range of conventional materials (D >1 µm) Mager’s 1/D-law holds, whereas a steep decrease of coercivity Hc results when grain size is reduced below 100 nm D6-Law (Herzer’s D6law).
Based on the know how on materials and applications VAC was the pioneer promoting nanocrystalline soft magnetic materials in various innovative applications
- High permeability VITROPERM cores with flat hysteresis loop and good temperature constancy for dc-tolerant Ground Fault Circuit Interrupters (GFCI)
- Low loss and light-weight VITROPERM Transformers in Switched Mode Power Supplies in the power range up to 100 kW
- VITROPERM Cores with very high magnetic flux density swing for compact Magnetic Amplifier Chokes (MagAmps)