What is a Permanent Magnet?
Permanent magnet refers to a magnet that can retain high remanence for a long time in an open circuit state. [1]
- Chinese name
- Permanent magnets
- Foreign name
- permanent magnet
- nickname
- Hard magnet
- Features
- Not easily demagnetized or easily magnetized
- Permanent magnet refers to a magnet that can retain high remanence for a long time in an open circuit state. [1]
Basic concepts of permanent magnets
- The materials used as the magnet and electromagnet are mostly soft magnets. The polarity of the permanent magnet does not change, while the polarity of the soft magnet changes with the polarity of the applied magnetic field. They can all attract iron objects. We call this property magnetic.
Permanent magnet formation
- Steel or other materials can become permanent magnets because after they are properly processed and processed, the internal non-uniformity is in the best state and the coercive force is the largest. Iron's crystal structure, internal stress, and other non-uniformities are small, and the coercive force is naturally small. It does not require a strong magnetic field to make it magnetize or demagnetize, so it cannot become a permanent magnet. Materials that are easy to magnetize and demagnetize are often referred to as "soft" magnetic materials. "Soft" magnetic materials cannot be used as permanent magnets, and iron is such a material
- Just like the kind of magnetic steel bars you usually see. Permanent magnets are objects that can retain a certain residual magnetization after the external magnetic field is removed. In order to make such objects have zero residual magnetization and completely eliminate magnetism, a reverse magnetic field must be added. The magnitude of the reverse magnetic field required to completely demagnetize the ferromagnetic mass is called the coercive force of the ferromagnetic mass. Steel and iron are both ferromagnetic, but their coercive forces are different. Steel has a larger coercive force, while iron has a smaller coercive force. This is because in the steel making process, carbon, tungsten, chromium and other elements are added to the iron to make carbon steel, tungsten steel, chromium steel, and the like. The addition of carbon, tungsten, chromium and other elements causes steel to have various non-uniformities under normal temperature conditions, such as uneven crystal structure, uneven internal stress, and uneven magnetic strength. These non-uniform physical properties increase the coercive force of steel. And the greater the degree of unevenness within a certain range, the greater the coercive force. However, these inhomogeneities are not the best state that steel has or have achieved under any circumstances. In order to achieve the best state of internal heterogeneity of steel, proper heat treatment or machining must be performed. For example, in the smelting state of carbon steel, magnetic properties are similar to ordinary iron; after it is quenched from high temperature, the non-uniformity grows rapidly and it can become a permanent magnet material. If the steel is slowly cooled down from a high temperature, or the quenched steel is smelted at 600 to 700 degrees Celsius, the internal atoms have sufficient time to arrange into a stable structure, and various nonuniformities are reduced. The coercive force then decreases, and it no longer becomes a permanent magnetic material.
Classification of permanent magnets
- The first category is: alloy permanent magnet materials, including rare earth permanent magnet materials (Nd2Fe14B), samarium cobalt (SmCo), aluminum nickel cobalt (AlNiCo)
- The second category is: Ferrite permanent magnets (Ferrite)
- According to different production processes, they are divided into: sintered ferrite, bonded ferrite, and injection ferrite. These three processes are divided into isotropic and anisotropic magnets according to the orientation of the magnetic crystals.
- These are the main permanent magnetic materials on the market, and there are some that cannot be eliminated due to production process or cost reasons, such as Cu-Ni-Fe (copper nickel iron), Fe-Co-Mo (iron cobalt molybdenum) , Fe-Co-V (iron-cobalt-vanadium), MnBi (manganese-bismuth)
Permanent magnet applications
- There are a variety of applications for permanent magnets, including televisions, speakers, audio speakers, radios, bag buckles, data cable magnetic rings, computer hard drives, mobile phone shakers, and more. Permanent magnets such as speakers use the principle of energized coils to move in a magnetic field to produce sound. The permanent magnet on the horn uses the magnetic field generated by the current to interact with it when the current in the coil changes, causing the relative position of the coil and the magnet to change, driving the paper cone on the horn to vibrate, pushing the air and spreading this vibration. The ear thus hears the sound. In short, permanent magnets are omnipresent in people's lives, and it facilitates our production and life.
Permanent Magnet Development
- From the development history of permanent magnet materials, the carbon steel used at the end of the nineteenth century had a magnetic energy product (BH) max (a physical quantity that measures the magnetic energy density of a permanent magnet) of less than 1MGOe (mega-gao), while Nd-Fe produced in batches abroad -B permanent magnet material, magnetic energy product has reached more than 50MGOe. Over the past century, the material's remanence Br has increased very little. The increase in energy product can be attributed to the increase in coercive force Hc. The improvement of coercive force is mainly due to the understanding of its nature, the discovery of high magnetic crystal anisotropic compounds, and the advancement of preparation technology.
- In the early twentieth century, carbon steel, tungsten steel, chrome steel, and cobalt steel were mainly used as permanent magnetic materials. In the late 1930s, the successful development of AlNiCo permanent magnet materials made large-scale applications of permanent magnet materials possible. In the 1950s, the appearance of barium ferrite not only reduced the cost of permanent magnets, but also expanded the application range of permanent magnet materials to high-frequency fields. By the 1960s, the advent of rare earth cobalt permanent magnets opened up a new era for permanent magnet applications.
- In 1967, Strnat et al. Of the University of Dayton in the United States successfully made SmCo5 permanent magnets by powder bonding, marking the arrival of the rare earth permanent magnet era. So far, rare earth permanent magnets have experienced the first generation of SmCo5, the second generation of precipitation hardening Sm2Co17, and the development of the third generation of Nd-Fe-B permanent magnet materials.
- In addition, Cu-Ni-Fe, Fe-Co-Mo, Fe-Co-V, MnBi, A1MnC alloy, etc. have been used as permanent magnet materials in history. Due to their low performance and low cost, these alloys have been rarely used in most applications. And AlNiCo, FeCrCo, PtCo and other alloys are also used in some special occasions. At present, Ba and Sr ferrites are still the most used permanent magnet materials, but many applications are gradually being replaced by Nd-Fe-B materials. In addition, the current output value of rare earth permanent magnet materials has greatly exceeded that of ferrite permanent magnet materials, and the production of rare earth permanent magnet materials has developed into a large industry.