GB/T 29628-2013Guides for methods of measurement of the magnetic properties of permanent (magnetically hard) materials by pulsed field magnetometry (English PDF)

6.1 Software and hardware conditions for testing

- Magnetic field generator; including.

- Capacitive discharge power supply; Magnetized solenoid.

- Magnetic polarization strength and magnetic field strength testing coil;

- Data collection and digitization equipment; including.

- integrator;

- Digital devices.

- Data processing software with the following functions.

- "Zero signal" processing;

- Positioning of J (H) hysteresis loop;

- Self demagnetization correction;

- Low pass filtering;

- Eddy current correction.

6.2 Magnetic field generator

6.2.1 Overview

Under conditions where the energy is sufficient to saturate the specimen magnetization, the repeatability of the capacitor charging voltage is relatively unimportant. The only sensitivity to voltage repeatability is the correction of the "zero signal" (see 6.5.2). ±1% voltage repeatability is relatively easy to achieve. Reaching this level shall not affect the correction of the 'zero signal'. In general, the voltage repeatability can reach ±0.1%.

6.2.3 Magnetized solenoid

6.2.3.1 Peak magnetic field strength

The impedance of a magnetized solenoid can affect the strength of the magnetic field.

The peak magnetic field strength generated by the magnetized solenoid shall cause the specimen to reach magnetization saturation in both positive and negative directions, with a margin of+10%. The required magnetic field strength depends on the material, orientation, and self-demagnetization factor of the specimen.

6.2.3.2 Dimensions

The magnetized solenoid shall have a certain volume to accommodate the specimen and test coil system.

6.2.3.3 Magnetic field uniformity

The magnetic field uniformity within the entire specimen area shall be within ± 1%.

6.2.3.4 Magnetic field frequency

The magnetic field shall have a low time change rate dH/dt to avoid generating large eddy currents in conductive samples and to facilitate the correction of eddy current effects.

6.3 Magnetic polarization strength and magnetic field strength test coil

6.3.1 Overview

The permanent magnet pulse measurement method uses the values of the magnetic polarization intensity of the specimen induced by the test coil and the magnetic field intensity of the specimen. The test coil includes.

- Magnetic polarization strength testing coil (J coil);

- Magnetic field strength testing coil (H coil).

6.3.2 J coil

The J coil is composed of a test coil and a compensation coil connected in series and reversed. The test coil and the compensation coil have equal ampere turns, but the degree of coupling with the specimen is different, so the measured magnetic polarization strength of the specimen does not include the magnetic field signal. The J to become measurement results.

After the above processing, the J signal plots the synchronized H signal to obtain the J (H) hysteresis loop. The B (H) hysteresis loop can be obtained by converting the J (H) hysteresis loop.

For the measurement results of conductive permanent magnet specimens, the influence of eddy currents needs to be corrected.

6.5.3 Positioning of J (H) hysteresis loop

Although the external magnetic field increases from zero and eventually returns to zero, the initial magnetic polarization strength of the specimen is unknown (the initial magnetic polarization strength signal is related to the magnetization state of the specimen). Therefore, it is necessary to perform positioning processing on the return line. If the specimen reaches magnetization saturation in both positive and negative directions, the midpoint between the positive and negative residual magnetic polarization values can be considered as J=0.

6.5.4 Self demagnetization correction

The permanent magnet pulse measurement method adopts open magnetic circuit measurement. In order to obtain the intrinsic magnetic properties of the specimen, a self-demagnetization factor shall be used to correct the test results. This is also a common problem faced by all open circuit measurement methods. The self-demagnetization factor depends on the geometric dimensions of the specimen and the generating heat, the temperature of the specimen can increase. On the other hand, the heating of the magnetized solenoid shall also increase the temperature of the specimen inside it. The design of a magnetized solenoid shall consider both the maximum magnetization field and the factors causing heat transfer to the specimen due to the heating of the solenoid. The magnetized solenoid circulating cooling system can effectively reduce the temperature rise of the specimen caused by the heat conduction of the magnetized solenoid.

The permanent magnet pulse measurement method shall have the following software and hardware conditions.

7.2 Temperature measurement

The temperature of the specimen can be determined by various methods. The method of direct contact temperature measurement between thermocouple and specimen can be used. Laser temperature measurement method can also be used. Laser is directed towards the magnet through the bottom opening of the magnetized solenoid, enabling non-contact temperature measurement.

8.1 H channel calibration

Place the H coil in a uniform, static magnetic field calibrated by a nuclear magnetic resonance magnetometer. Quickly remove this coil to determine the overall accuracy of the test coil and integrator.

8.2 J channel calibration

The pure nickel standard specimen has a determined theoretical saturation magnetic polarization strength. The eddy currents generated internally are basically the same as those in pure copper specimens. The total eddy currents in the magnetic saturation region of pure nickel specimens are independent of their relative magnetic permeability.

Therefore, the saturation magnetization state of pure nickel standard specimens can be used for calibration of the J channel.

Under identical magnetic fields, two pure nickel and copper specimens with identical shapes and sizes are measured using a permanent magnet pulse measuring instrument.

Using an iterative method, by adjusting the eddy current scaling factor of the pure copper standard specimen, the eddy current in the pure copper standard specimen is subtracted from the measurement results of the pure nickel standard specimen until the eddy current influence in the saturation region of the hysteresis loop of the pure nickel standard specimen is completely removed. After the above processing, the theoretical saturation magnetic polarization intensity of the pure nickel standard specimen can be used for calibration of the J channel. […]