15 advanced concepts related to magnetic materials
Hard magnetic materials (such as NdFeB) have two distinct characteristics: one is that they can be strongly magnetized under the action of an external magnetic field, and the other is hysteresis, that is, the hard magnetic materials remain magnetized after removing the external magnetic field. The following figure shows the relationship between the magnetic induction intensity B and the magnetization field intensity H of the hard magnetic materials, which is called hysteresis loop curve.
When the reverse magnetic field gradually changes from O to – HC, the magnetic induction intensity B disappears, which indicates that the reverse magnetic field must be applied to eliminate the remanence. HC is called coercivity, which reflects the ability of magnetic materials to maintain the remanence state. The purple line is called demagnetization curve.
Intrinsic curve and rectangularity / squareness
The intrinsic magnetic induction of permanent magnetic material magnetized by external magnetic field is called intrinsic magnetic induction Bi, also known as magnetic polarization J. The curve describing the relationship between the intrinsic magnetic induction intensity Bi (J) and the magnetic field intensity H is the curve reflecting the intrinsic magnetic properties of permanent magnetic materials, which is called intrinsic demagnetization curve, or intrinsic curve for short.
When the magnetic polarization J on the intrinsic demagnetization curve is 0, the corresponding magnetic field strength is called intrinsic coercivity Hcj.
Surface treatment phosphating
Sintered NdFeB magnets will be oxidized and corroded in the air. When the NdFeB magnet turnover and storage time is too long, and the subsequent surface treatment method is not clear, the phosphating process is generally used for simple anti-corrosion treatment. The process of phosphating treatment of magnet surface is as follows: degreasing → washing → pickling → washing → surface adjustment → phosphating treatment → sealing and drying. At present, commercial phosphating solution is mainly used. After phosphating, the product has uniform color and clean surface, and can be sealed in vacuum, which greatly prolongs the storage time.
Surface treatment electrophoresis
The electrophoretic coating is to immerse the parts in the water-soluble electrophoretic bath solution, insert the positive electrode and the negative electrode in the bath solution at the same time, reduce the direct current at the two poles, so as to produce the electrochemical reaction, so that the water-soluble coating (generally high molecular resin, such as epoxy resin) is evenly deposited on the parts, forming an anti-corrosion coating composed of resin particles, or forming an anti-corrosion coating of high molecular polymer Rotten layer. The electrophoretic coating not only has good adhesion with the surface of porous magnet, but also has excellent corrosion resistance, such as salt spray resistance, acid resistance, alkali resistance, etc.
Surface treatment: Perrin parylene
Parylene is a kind of protective polymer material, Chinese name, poly-p-xylene, perrelin. It can be vapor deposited in vacuum. The good penetration of parylene active molecules can form a pinhole free and uniform thickness transparent insulating coating inside, bottom and around the component, providing a complete high-quality protective coating for the component to resist acid-base, salt spray, mold and various corrosive gases The infringement of human rights. Parylene’s unique preparation process and excellent properties make it possible to fully coat small and ultra small magnetic materials without weak spots. The magnetic materials can be immersed in hydrochloric acid for more than 10 days without corrosion. At present, many small and ultra small magnetic materials in the world use parylene as insulation and protective coating.
Dimensional tolerance is referred to as tolerance, which refers to the allowable variation of part size in machining. The magnetic material with certain dimensional difference is allowed. It refers to the absolute value of the difference between the maximum limit size of tolerance and the minimum limit size, or the difference between the allowable upper deviation and the lower deviation.
Geometric tolerance is also called geometric tolerance, including shape tolerance and position tolerance. Any part is composed of points, lines and surfaces, which are called elements. After machining, the actual elements of parts have errors relative to the ideal elements, including shape error and position error. This kind of error affects the function of mechanical products, so the corresponding tolerance should be specified in the design and marked on the drawing according to the specified standard symbol.
Neutral salt spray test (NSS)
Salt spray test is a kind of environmental test which mainly uses the artificial simulated salt spray environmental conditions created by the salt spray test equipment to evaluate the corrosion resistance of products or metal materials. It is divided into neutral salt spray and acid salt spray. The difference lies in the different standards and test methods, also known as “NSS” and “Cass” test. The neutral salt spray test of sintered Nd-Fe-B was carried out. According to the national standard, the continuous spray test method was adopted. The test conditions were as follows: The results show that the pH value of the collected salt spray settling solution is between 6.5 and 7.2 under the conditions of 35 ℃± 2 ℃, 5% ± 1% NaCl solution (mass fraction), and the sample placing angle has an effect on the test results. The sample surface is placed in the salt spray box with an inclination angle of 45 °± 5 °.
The hygrothermal test of sintered NdFeB is an accelerated test method to evaluate the hygrothermal degradation effect of samples. The samples are subjected to high unsaturated steam pressure for a long time. The test conditions are as follows: distilled water ± 85 ℃ or distilled water ± 5%. The severity level is 1, that is 168 hours.
High pressure accelerated aging test (PCT)
High pressure accelerated aging test is generally called pressure cooker cooking test or saturated steam test. It is mainly used to test the high humidity resistance of samples under severe temperature, saturated humidity and pressure environment.
The high pressure accelerated aging test of sintered NdFeB is to put the samples into the high pressure accelerated aging test equipment containing distilled water or deionized water with resistivity greater than 1.0 m Ω· cm.
Hardness and strength
Hardness refers to the ability of a material to resist local hard objects pressing into its surface. It is an index to compare the hardness of various materials. The higher the hardness, the stronger the ability of metal to resist plastic deformation.
Strength refers to the maximum ability of a material to resist external forces. According to the different forms of external force, the strength can be divided into three types:
- Tensile strength (tensile strength) refers to the strength limit when the external force is tensile force.
- Compressive strength refers to the strength limit when the external force is pressure.
- Bending strength refers to the strength limit when the external force is perpendicular to the axis of the material and makes the material bend after the action.
It usually reflects the strength of material with crack propagation, and the unit is MPa · M1 / 2. Tensile testing machine, stress sensor, extensometer, signal amplification dynamic strain gauge, etc. should be used to test the fracture toughness of materials. In addition, the samples should be made into thin sheets.
Impact strength (impact fracture toughness)
The unit of energy absorbed in the fracture process of reactive material under impact stress is J / m2. The measured value of impact strength is too sensitive to the size, shape, processing accuracy and test environment of the sample, so the measured value will be more dispersed.
Three point bending method is used to measure the bending fracture strength of materials. Because the sample is easy to process and measure, it is most commonly used to describe the mechanical properties of sintered NdFeB magnets.
Source: China Permanent Magnet Manufacturer – www.rizinia.com