Nanocrystalline alloys are now widely used in high frequency electromagnetic applications such as high frequency transformers and chokes, while it’s two-dimensional properties are seldom investigated. The nanocrystalline sample can be extremely brittle after annealed and crystallized which is very difficult to measure the properties by traditional method in 2D cases. The magnetic properties of nanocrystalline alloys under rotating field are measured and discussed in this paper. By combining the needle probe technique and the H-coil method, the magnetic flux density B and the magnetic field strength H in the sample can be measured. Finally, the modified B-H sensor is calibrated by the Helmholtz coils and the sensitivity of measuring B and H can achieve 4.3 mV/T and 30 μV/A/m respectively.

To meet the demand of energy saving and emission reduction, high frequency and high power density devices have attracted significant attention in recent years.1,2 Magnetic characterization of such materials helps engineers design and develop efficient electrical machines and devices as well as scientists work in this field. As the magnetic properties of nanocrystalline alloys are very sensitive to their annealing process, the detailed properties of such materials have not be fully examined. When annealed by a transverse field, the nanocrystalline tapes can show excellent broadband magnetic properties and even lower energy loss than the ferrite at high frequencies.3 But the characterization of nanocrystalline alloys is restricted in the so-called one-dimensional method, standardized in the IEC60404-6, which can measure the magnetic properties only along the casting direction of the nanocrystalline sample while it can’t reflect any information about the anisotropy properties. Several researchers have moved to the topic of measurements of 2D magnetic properties of electrical steels for decades of years.4 In recent years, the vector magnetic properties of Fe-based amorphous sheet have been tested under the alternating and rotating fields while the maximum testing frequency is 750 Hz far from the working condition of the high frequency applications.5 But such measurement of nanocrystalline sample is restricted because the tapes become very brittle after the annealing process. By measuring the vector magnetic flux density B and the magnetic field strength H, the specific loss per cycle can be calculated through the field-metric method.6 However, localized flux density detection is limited in only two methods: The B-coil method and the needle probe technique.7 The B-coil method is most commonly used because it is more accurate based on the Faraday’s law. But unfortunately, the B-coil method needs to drill holes to warp the search coil, which is not suitable when the sample is extremely thin and brittle. For measuring magnetic field strength H, a flat coil called H-coil is commonly used since it is easy to prepare and has a better frequency response up to Kilo-Hertz.8 In this paper, a new B-H sensor structure by combining the needle probes with the H-coil has been developed.

To characterize the 2D magnetic properties of nanocrystalline alloys, a Rotational Single Sheet Tester(RSST) is developed in our laboratory. As the nanocrystalline sample is very brittle after the crystallization process, a 50mm×50mm square sample was cut before annealing. As shown in Fig. 1, a compact B-H vector sensing structure is proposed. The upper part of the structure is used to detect the flux density both in the x and the y directions and the magnetic field strength in the x direction, while the lower part is only used to detect magnetic field strength in the y direction. The magnetization homogeneity of the sample has great influence on the design parameters of the B-H vector sensor. By applying a magnetic shielding lamination, the magnetic stray field from the poles can be largely reduced. This lead to the desired 2D homogeneous field distribution in the sample. To improve the sensibility of the H-coil, 0.03 mm enameled wires are warped on the 14mm×14mm×0.5mm flat board. The magnetic field strength H in the sample can be deduced by:

H=1μ0N𝐻SH=1μ0𝐾HTVHdt
(1)

where NH, SH, VH are the number of turns, effective cross sectional area and output voltage of the H-coil respectively. Since the SH is very hard to be measured, the coefficient KH of the H-coil is introduced, which can be calibrated in a Helmholtz coils up to 1 kHz. During the measurement of flux density B, two pairs of needled probes are maintained contact with the nanocrystalline sample surface. The flux density B can be calculated through the induced voltage caused by the eddy current on the surface of the sample:9 

B=2Sb=KBTVBdt
(2)

where Sb is the effective area of the loop formed by the spacing between the needles and the thickness of the conductive sheet, and the coefficient KB which is similar to the KH can be calibrated by measuring the distance between the needles.

FIG. 1.

The modified B-H sensing structure for measuring vector magnetic properties of nanocrystalline sample.

FIG. 1.

The modified B-H sensing structure for measuring vector magnetic properties of nanocrystalline sample.

Close modal

As is mentioned above, the coefficients of the sensors need to be calibrated in a standard field. To calibrate the H-coil in a relatively high frequency and a large homogeneity field, a three-coil Helmholtz coil has been analyzed and realized in our lab, which can achieve about 1mT at a frequency of 1 kHz. As shown in Fig. 2, the three-coil Helmholtz coil with the special turns arrangement has a very large homogeneity area.10 The round table in the central of the coils has a 0.2-degree resolution to rotate the H-coil at the experiment. Fig. 3 presents the results of calibration with different flux densities and frequencies. After the calibration, the KH is 1.92×10-3m2 in the x-direction and 1.96×10-3m2 in the y-direction with the error 1.303% and 0.355% respectively. Similarly, the coefficient of the B sensor KB is finally determined as 2.67×106m-2 for the x direction and 2.72×106m-2 for the y direction by measuring the distance between the two needles when measuring the flux density. The final design parameters of the newly developed B-H sensor are list in Table I.

FIG. 2.

Designing of the Helmholtz coil: (a) Realization of the three-coil Helmholtz coils; (b) Calculation results of the field homogeneity.

FIG. 2.

Designing of the Helmholtz coil: (a) Realization of the three-coil Helmholtz coils; (b) Calculation results of the field homogeneity.

Close modal
FIG. 3.

Calibration of the H-coils: (a) Calibrating the coefficient KHx; (b) Calibrating the coefficient KHy.

FIG. 3.

Calibration of the H-coils: (a) Calibrating the coefficient KHx; (b) Calibrating the coefficient KHy.

Close modal
TABLE I.

Parameters of the B-H sensor (1kHz).

t (mm)ad(mm)bϕ(mm)cn(turns)Sd
Needle probes 0.025 30 4.3 mV/T 
H-coils 0.5 12 0.03 350 30 μV/(A/m) 
t (mm)ad(mm)bϕ(mm)cn(turns)Sd
Needle probes 0.025 30 4.3 mV/T 
H-coils 0.5 12 0.03 350 30 μV/(A/m) 
a

t is thickness of nanocrystalline sample for needles probe, and the thickness of flat frame of H coil.

b

d is the distance between two needle probes, and the width for the H coil.

c

ϕ is the diameter of the probe for needle probes, and the diameter of enameled wires for H coil.

d

S is the sensitivity of the needle probes and the H coil.

A single square sheet was put in the central of the magnetizing apparatus. Both alternating and rotating flux conditions can be achieved by a control program. Since the signals from the sensors are very small, low noise SR 560 is used to pre-amplifies the voltages from the output of the sensor. A FT-3M type Fe based nanocrystalline sample is tested and analyzed in this experiment. Since it is hard to keep the circular loci of flux density B of the nanocrystalline sample at high flux density levels in relatively high frequencies, Fig. 4 gives series circular rotating loci of flux density B under 0.5 T and the corresponding H loci at 8 kHz. It can be seen in Fig. 4 (a) and (b), we can see that under the rotating flux condition, the sample exist a strong anisotropy. The magnetic field strength H in the y direction is nearly two times larger than that in the x direction which will cause a large loss in the y direction.

FIG. 4.

Series of circular B loci and corresponding H loci at 8 kHz.

FIG. 4.

Series of circular B loci and corresponding H loci at 8 kHz.

Close modal

By combining the needle probe technology with the H-coil method, a new B-H sensing structure is proposed in this paper. To have a better accuracy, two magnetic shielding sheets are inserted to the sensor to improve the magnetic field homogeneity. What’s more, a PCB board with a printed circuit has been implemented which can make the structure more compact and easier to get the signal out from the sensor. Finally, the 2D magnetic properties of a FT-3M type nanocrystalline sample are tested and discussed at 8 kHz, which is seldom studied but important to the application of the nanocrystalline alloys.

This work is supported by the National Natural Science Foundation of China under Grant No. 51377042.

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