Principle and technology of relative detection of magnetic properties of NdFeB products
Sintered neodymium iron boron permanent magnetic material products should provide the technical magnetic parameters of the product, including remanence Br, magnetic induction coercive force Hcb, intrinsic coercive force Hcj, magnetic energy product (BH)m, demagnetization curve squareness and BH Demagnetization curve recovery magnetic material rate μrec and so on.
Today we will focus on the principle and technology of relative testing (or comparative testing) of the magnetic properties of NdFeB products. Sintered NdFeB products come in a variety of shapes and sizes. In addition to strict requirements on product performance grades, some users also require relative testing of the magnetic properties of the samples. For example, some users require sampling of the surface magnetic field of the product. Some users request the magnetic flux value of the product, and some users request the magnetic moment of the product. In order to meet these requirements, correspondingly developed:
- 1. Teslameter detects the surface field of the magnet;
- 2. Magnetic flux meter measures the magnetic flux of the magnet;
- 3. Helmholtz coil + flux meter measures the magnetic moment of the magnet. Today I will give you a brief introduction to these instruments.
Teslameter measures the surface magnetic field of a magnet
Tesla meters include Hall-effect Tesla Meter, coil method Tesla meter and rotating coil method Tesla meter. The Hall-effect Tesla meter is more commonly used in the inspection of permanent magnet materials. The Hall effect is a phenomenon in which a chip semiconductor carrying current is placed in a magnetic field perpendicular to the direction of the current, and the semiconductor will generate a transverse magnetic field, that is, an electromotive force is generated in the direction perpendicular to the magnetic field and the current. (Related reading: The working principle of the Hall element) When the Hall device and the passing current are constant, the Hall voltage is proportional to the measured magnetic field, so the Hall effect can measure the magnetic field.
To measure the surface magnetic field of a magnet pole with a Hall-effect Tesla meter, you need to pay attention to the following issues:
- 1. The plane of the Hall probe must be perpendicular to the direction of the magnetic field, so the plane of the Hall probe must be close to the surface of the magnetic pole;
- 2. The surface field at different positions of the magnetic poles in the open magnetic field is not uniform, so the surface magnetic field at the specified fixed position should be measured.
Fluxmeter measures the magnetic flux of a magnet
The magnetic flux cannot be directly measured, but it can be calculated and determined by the electromotive force of the measuring coil through Maxwell’s equation. The flux meter measures the magnetic flux of the sample and has been widely used as a comparative measurement method in the detection of permanent magnet products. The disadvantage of this method is that when measuring small-sized samples, the degree of the magnetic flux meter is related to the position of the sample in the measuring coil.
Permanent magnets with ultra-high coercivity are either small in size or complex in shape. For some thin or tile-shaped magnets, it is very important to measure the surface field with a Tess meter or use a flux meter to measure the magnetic flux value. Difficult because the measured values are very scattered and have poor repeatability. For such thin magnets or special-shaped magnets (such as tile-shaped or sector-shaped), only the method of measuring the magnetic moment can be used.
The method of measuring the magnetic moment of the permanent magnet is a Helmholtz coil and a flux meter (as shown below). This method has the characteristics of high speed, high accuracy and small error, and is suitable for measuring products with complex shapes.
Permanent magnet density measurement
The mass of a certain substance per unit volume is called its density. The density of sintered neodymium iron boron permanent magnet is one of the important indexes to measure its quality. The theoretical density of Nd2Fe14B compound is 7.62g/cm3, and the density of pure ternary sintered NdFeB permanent magnet should reach 7.5-7.55 g/cm3. When a small amount of dysprosium, terbium and other elements are added, the density of the sintered NdFeB permanent magnet may be higher.
There are many methods for measuring density, and the hydrostatic balance method is commonly used. According to Archimedes’ principle, an object is reduced in mass due to buoyancy in a fluid. Therefore, the mass of the sample in the air and the mass in the fluid are weighed, and the volume of the sample can be calculated when the density of the fluid is known, thereby calculating the density of the sample.
In order to accurately measure the density of sintered NdFeB permanent magnets, it is recommended to use a fluid with an accurate density, such as absolute ethanol (0.989g/cm3) or distilled water (1.003 g/cm3).
Source: China Permanent Magnet Manufacturer – www.ymagnet.com