Nanocrystalline Cores: Material Properties and Application Guide

What Is a Nanocrystalline Core

A nanocrystalline core is a magnetic component made from an iron-based alloy that has been rapidly solidified into a thin ribbon and then annealed so that the structure consists of nanoscale grains embedded in an amorphous matrix. The typical alloy family is iron-silicon-boron-niobium-copper, and the resulting material is marketed under designations such as 1K107.

The fine grain structure suppresses the coercivity of the material while keeping a high saturation flux density. The result is a soft magnetic material with permeability, losses and thermal behaviour that fill the gap between amorphous metal and ferrite.

Magnetic Properties in Numbers

The saturation flux density of iron-based nanocrystalline material is about 1.25 T, the initial permeability reaches 60,000 to 80,000 after field annealing, the coercivity is below 1 A/m, and the Curie temperature is about 570 degrees Celsius. The ribbon thickness of 18 to 22 microns keeps the eddy-current loss low up to frequencies of tens of kilohertz.

Because the permeability is high and the loss is low, a nanocrystalline core can handle the same inductance and power as a much larger ferrite core, or the same core size at a higher switching frequency with lower temperature rise.

Comparison with Amorphous and Ferrite Cores

Amorphous metal has a higher saturation of about 1.56 T but a lower permeability and higher loss than nanocrystalline material at high frequency, which makes it the preferred choice for 50 Hz distribution transformer cores. Ferrite has a very low saturation of 0.4 to 0.5 T and needs a large cross-section for power applications, but it is cheap and easy to produce in many shapes.

Nanocrystalline material combines a saturation of 1.25 T with the highest permeability of the three and very low losses at medium and high frequencies. It is the material of choice for precision current transformers, common-mode chokes and high-frequency power transformers where size and accuracy matter.

Core Types and Forms

Nanocrystalline cores are available as toroidal ring cores wound from continuous ribbon, as cut C-cores with ground mating faces for easy winding, and as block cores assembled for high-power medium-frequency transformers. Each form is annealed and coated, and can be cased for protection.

For DC-biased circuits, the ring can be ground to introduce a defined air gap, which lowers the effective permeability and prevents saturation. The permeability class is selected by the annealing process and by the grinding depth.

Main Applications

The dominant applications are common-mode chokes for EMC filtering in inverters and switched-mode power supplies, precision current transformers for metering and protection, high-frequency transformers for DC-DC converters, and pulse transformers for gate drive circuits.

In solar inverters, electric vehicle chargers and industrial drives, nanocrystalline chokes and transformers reduce the size and the copper loss of the magnetic components while meeting the electromagnetic compatibility limits of the product.

Frequently Asked Questions

Q: How is a nanocrystalline core different from an amorphous core?
A: Amorphous metal has a completely disordered atomic structure and a saturation of about 1.56 T, while nanocrystalline material contains nanoscale grains and offers a higher permeability and lower loss at high frequency with a saturation of about 1.25 T. For 50 Hz distribution transformers amorphous cores are preferred, while for high-frequency components nanocrystalline material is better.

Q: At which frequency are nanocrystalline cores most useful?
A: The material performs well from a few kilohertz up to about 100 kilohertz. Above this range the eddy-current losses of the conductive ribbon grow and ferrite or other materials may be more practical.

Q: Can a nanocrystalline core be used in a power transformer?
A: Yes, in high-frequency power transformers for switch-mode converters, where the high saturation and low loss allow a small core cross-section. At 50 Hz, however, amorphous or grain-oriented silicon steel cores are normally more economical for the same power.

Q: Why are the cores so sensitive to mechanical stress?
A: The magnetic properties depend on the perfectly annealed microstructure, and bending or impact disturbs the domain structure and reduces the permeability. Cores must be handled carefully and are usually supplied cased or taped for protection.

Q: What determines the price of a nanocrystalline core?
A: The price is set mainly by the alloy raw material, the ribbon width and thickness, and the processing steps of winding, annealing and coating. Cased and gapped versions cost more than bare rings, and small batch sizes increase the unit price.

Q: How should the core be specified for a new design?
A: Specify the operating frequency, the peak current or power, the required inductance or turns ratio, and the ambient temperature. From these values the supplier recommends the core size, the permeability class and the number of turns, and confirms the saturation margin at the working point.