Nanocrystalline Cores

Nanocrystalline soft magnetic material is a fairly new development. The material composition is 82% iron with the remaining balance silicon, boron, niobium, copper, carbon, molybdenum, and nickel. The raw material is manufactured and supplied in an amorphous state. It is recrystalized into a precise mix of amorphous and Nanocrystalline phases when annealed, giving the material it's unique magnetic properties


Nanocrystalline soft magnetic cores

Nanocrystaaline NbCuFeSiB is a soft magnetic material produced from amorphous NbCuFeSiB ribbon which is made through rapid solidification of molten metal at a cooling rate of about a million degrees per second. The amorphous ribbons have a thickness about 30 μm.  After special heat treatment, a -Fe(Si) nanocrystalline phase is formed at 20 nm size.  This nanosized structure exhibits excellent soft magnetic properties.

Before the nanocrystalline soft magnetic material is developed, the widely used soft magnetic materials were silicon steel、permalloy、ferrite and amorphous alloys, and they play an important role in switched mode power supplies, electric power system equipment, electric meter and electric apparatus. But soft magnetic properties for the mentioned material are not perfect, the magnetic properties of all these soft magnetic materials have disadvantages. The silicon steel has high saturation flux density, but the permeability is very low particularly in high operating frequency; the ferrite exhibits good high frequency properties, but the saturation flux density and permeability is low; permalloy shows high permeability and low coercivity(Hc), but the Bs is low and can not be used in high operating frequency; the Co-based amorphous presents high permeability and low loss in wide operating frequency, but the Bs is low and the raw material is expensive;the Fe-based amorphous has a advantage of high Bs and low price raw material, but the permeability is relatively low. With high permeability(μi>100000),high saturation flux density(Bs>1.0T) and low loss(P0.2/50kHz=15w/kg),the nanocrystalline alloy exhibits excellent soft magnetic properties. The nanocrystalline cores have bright future in the fields of switched mode power supplies、common mode chokes、current transformers、 voltage independent RCCBs、IGBT inverter electric welder.

Another factor effecting the mechanical stress magnetic stability of nanocrystalline alloy is that this kind of alloy is very fragile, slightly mechanical stress will make the material break, which leads to the significant enhancement of coercivity and the reduction of permeability. This is a magnificent difficult to hinder nanocrystalline alloy to find wide application in industry.

By means of special heat treatment process, we can produce nanocrystalline cores with low coercivity of 0.2-0.6A/m,high initial permeability of μi equaling to 160-260k. All cores are surface treated and aged at 120oC for 3 hour to eliminate the irreversible loss, consequently the cores show special properties as follows:
  1. With advanced surface coating technique made of epoxy resin, we can produce nanocrystalline cores with excellent mechanical stress magnetic stability. Cores weighed 50g are dropped down the ground from 1 meter high level, the cores will not break and the permeability and coercivity remain unchanged.
  2. Impulse current withstand: when the cores are tested in voltage independent RCCBs with 3000A impulse current, their irreversible loss for permeability is less than 3%.
  3. Because of the excellent mechanical stress magnetic stability for the cores, the use of cores in transformers for IGBT inverter electric welder will greatly reduce the operating noise and can serve more than 20 years.
  4. Temperature magnetic stability: after aging at -25oC for 100 hours and 120oC for 100 hours, the irreversible loss for permeability is less than 2% and less than 3%.
Nanocrystalline common mode choke cores have high permeability which makes them suitable for use in common mode chokes and superior to the material most commonly used, Mn-Zn ferrite. Advantages of Nanocrystalline over Mn-Zn ferrite cores:
  1. The real part of complex permeability (μr' .) at lOOkHz and 20oC reaches more than twice as high as that of Mn-Zn ferrite cores. The impedance relative permeability (μrz) including imaginary part of complex permeability (μr") reaches more than four times as high as that of Mn-Zn ferrite cores. Consequently, the cores shows following benefits when compared with a same size Mn-Zn ferrite core (Table 1, Figure 1, Figure 2).
    • It allows four times higher impedance when applied to the number of winding turns.
    • Nanocrystalline can be reduced to 1/2 winding turns to obtain same impedance value as a Mn-Zn ferrite core in low frequency. Consequently, it reduces stray capacitance significantly. As a result, impedance in high frequency range becomes large.
  2. Frequency dependence of initial permeability (μr',) is not significantly affected by temperature change. Consequently, common mode chokes with Nanocrystalline cores have low temperature dependence of inductance and impedance over a wide frequency and temperature range (Figure 3, Figure 4).
Table 1. Comparison of magnetic and physical properties between Nanocrystalline and Mn-Zn ferrite
  Nanocrystalline Mn-Zn ferrite
Initial Permeability at 100 kHz μr' 20oC 16,000 5,300
100oC 16,300 7,000
Imppeadance permeability at 100 kHz μrz 20oC 25,700 5,300
100oC 26,300 7,000
Saturation magnetic flux density Bs* (T) 20oC 1.35 0.44
100oC 1.30 0.27
Residual magnetic flux density Br* (T) 20oC 0.81 0.10
100oC 0.73 0.06
Corrective Force Hc* (A/m) 20oC 1.3 8.0
100oC 1.4 4.9
Curie temperature Tc(oC) 570 150
Saturation magnetostriction λs(x10-6) +2.3 -1.1
Electrical resistivity ρ(μΩ.m) 1.1 1.0X106
Density d(kg/m3) 7.4X103 4.85X103
* DC magentic properties at 800 A/m


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