Industry new

Magnetic Materials and Their Magnetic Engineering

Magnetic materials refer to materials that have a strong magnetic and engineering application value. It can be roughly divided into three categories: "permanent magnetism," "temporary magnetism," and "semi-permanent magnetism." They are widely used in electronics, electrical machinery, information, machinery and transportation industries. This article describes the origin of magnetic, various types of magnetic materials, features and functions. Magnetic materials are materials that you and I acquiesce. More noticeable, such as magnets on whiteboards, magnets under magnetic checkers, compasses, tapes, magnetic heads, floppy diskettes, etc.; and magnetic materials such as motors and televisions that are packed in larger amounts in some devices. Internal transformers, cars, etc. It can be said that magnetic materials have been closely related to the lives of modern people. In the area of materials science, it is reclassified as "electronic material" (coinciding with conductive materials, insulators, semiconductors, etc.). However, magnetic materials include metal materials, ceramic materials, and even polymer materials. Its morphology also includes blocks (b1uk), powders, and thin films. Therefore, the magnetic material itself is a material with a diversified role. From a physics point of view, any material is a magnetic material, that is, each material has a certain magnetic phenomenon. Some will cancel out a small part of the magnetic field in the magnetic field, presenting “diamagnetism” (such as copper); some will have a slight positive induction in the magnetic field, and appear “paramagnetism” (eg, air); In the magnetic field, it will induce a very strong magnetic quantity - called magnetization, and it will be ferromagnetic (also known as ferromagnetic) or ferrimagnetism (also known as ferromagnetism). In industry, only materials with strong magnetic properties or sub-strong magnetic properties can be utilized. But in the physical, chemical and medical fields, other types of magnetism also have a great function. The most interesting example is the medical use of magnetic resonance of human organ molecules. It can quickly perform a full-body health examination. The "magnetic property" of an organ molecule can detect the presence or absence of lesions. The equipment used is called MRI (magnetic resonance imaging). Here, we only intend to introduce ferromagnetic and sub-strong magnetic materials (permanent and temporary magnetic materials) with large industrial application value; semi-permanent types and applications are less, and space is limited. The Origin of Magnetics Until the twentieth century, people (including scientists) did not understand the magnetic properties of materials better than our hundreds of thousands of years ago. For more than 70 years now, relying on the continuous efforts of many physicists, chemists and mathematicians who have undergone rigorous scientific practice, they have finally been able to unravel their mysterious veil and see the whole picture. Let us follow the path of sages to understand the origin of magnetism. It is known from experiments that there is a force of attraction or repulsion between two magnetic poles, which is called magnetic force. Therefore, from the measurement of force, the size of "magnetic" can be known. Force will have a moment, because the magnetic moment is called "magnetic moment" (magnetic moment). Early scientists (for example, Faraday, Curie, et al.) tried to measure the magnitude of the magnetic moment contained in a material and its relationship with temperature in a magnetic field, and discovered different reactions of different substances. The amount of magnetic moment contained in an object is called the "magnetization amount." The amount of magnetization that can be caused by a unit magnetic field is called "magnetic susceptibility". From the quantitative relationship of magnetic susceptibility to temperature, we can define the difference between diamagnetic, paramagnetic and ferromagnetic. But why? Still no answer. First, what is the magnetic moment? If the magnet is divided again, each new particle is a new magnet with north and south (N, S) poles. The magnetic moment is the smallest segment that still holds the N and S poles. At present, we know that electron spins or revolutions cause such a minimum unit (such as a magnetic field caused by a current flowing around a coil). In other words, the magnetic moment is the net amount of electronic motion (revolution, rotation) that has not been cancelled out, that is, the net value of magneticspin. In addition to diamagnetic materials, all other materials have more or less magnetic moments in the magnetic field and can be quantitatively measured. Obviously they all contain magnetic atoms (molecules). How come strong magnetism comes from? Why do we have the same magnetic atoms and strong magnetism, but some do not? In 1907, Weiss repeated the experiment in 1895 and matched the theory of the mathematician Langeuim, assuming that there was an interaction between the magnetic "molecules" (when the numerator was the smallest unit of matter). It is a molecular field and boldly infers that the molecular field of a non-ferromagnetic substance is small, and the ferromagnetic substance has a very large molecular field large enough to align the magnetic moments of the molecules to saturation. When the temperature rises above the Curie point (editor note: the temperature at which the ferromagnetic material changes from ferromagnetic to paramagnetic, known as the Curie point), thermal energy destroys the alignment of the molecular field and disrupts the magnetic "molecules," ie, Paramagnetic. However, why do most of the ferromagnetic elements such as iron, cobalt, and nickel do not attract other iron, cobalt, and nickel? Since they have been magnetized to saturation, they should be used as strong permanent magnets. Weiss put forward another bold assumption, that is, to reduce the level of free energy in order to achieve stability, will progress turbulence. The interior of a ferromagnetic material is automatically divided into many small areas called magnetic domains. In the same magnetic region, the magnetization directions are the same, and the magnetization directions of different magnetic regions are different and distorted. Therefore, they cancel each other out, and usually do not feel that they have magnetism. Only if they are magnetized within the magnetic field and break the chaos of the magnetic region, they can feel it. Its strong magnetism. Later experiments (1931) confirmed this “prophecy” (see Figure 1), which made Weiss famous for a long time. His bold assumptions and careful verification of his attitude towards scholarship are the principles he enjoys. In 1948, Néel, Weiss’s pupil, continued his research and found that the magnetic moments of certain material atoms are greatly influenced by the crystal lattice, and that the molecular field is very strong and negative, leading to the presence of neighboring atoms. The magnetization is in the opposite direction. If they are equal in size, they completely cancel out and present "antiferromagnetism". If it is not equal in size, it will appear as "sub-magnetism." At this point, the principle of "magnetism" of matter has been generally announced. Neil was thus awarded the Nobel Prize in physics in 1970. The magnetic phenomenon of magnetic materials is determined by the length of the magnetic domain. The interface between the magnetic region and the magnetic region is called the domain wall, and the inner magneto gradually turns from one direction to the other. It is very thin, only tens to hundreds of angstroms. If the wall of the magnetic material can move with the applied magnetic field, the material can be easily magnetized to saturation and easily degaussed; otherwise, if the idea hinders the movement of the magnetic wall, it is magnetized to saturation after the material It is not easy to be degaussed. The former exhibits a temporary magnetism and the latter exhibits a permanent magnetism. The work of magnetic materialists lies in the use of solid-state physics, materials engineering, physical metallurgy, mechanical metallurgy, and other scientific principles or techniques to control the composition and microstructure of magnetic materials to make them behave as desired. The advantages and disadvantages of permanent magnetic materials and their application to magnetic materials are often expressed in terms of hysteresis curves (see Figure 2). The OBs magnetization curve on the graph shows the tangent slope at the origin as the initial permeability (μo). The secant slope represents a specific ratio of B/H (magnetic induction/magnetic field strength), and the largest one is the maximum permeability ( Μm). The Bs point represents the saturation induction unit expressed in kG; the Br point is the residual magnetic induction; the Hc point is called coercive force or coercive force (Oe or kOe, 1Oe is equivalent to (1000/4π) A/m]. Any point on the hysteresis curve of the second quadrant represents a specific B×H value (the area enclosed by the projection lines of B and H), and the largest one is called the maximum energy product (BH)m. G.Oe, MGOe for millions of times.] Permanent magnetic materials stress Hc, Br, and (BH)m as much as possible, especially (BH)m, which represents the energy stored inside the magnet after magnetization, the greater the (BH)m value, the more it can work externally. , Like an inexhaustible battery, if the Hc is large enough (thousands of Oe or more) and the Curie temperature is high enough, it will not be easily eliminated. Engineering Hc> 200Oe, it can be called a permanent magnet. From the end of the nineteenth century to the early twentieth century, the only permanent magnet available was quenched carbon steel. When carbon steel is quenched and hardened, Hc is increased, and the harder is the higher Hc, so the permanent magnetism is also called "hard" magnetism. On the contrary, the annealing softening presents temporary magnetic or "soft" magnetism. Hardened steel Hc only has 50 to 70 Oe, and (BH)m only has 0.2 to 0.3 MGOe. In 1916, scientists added Cr, W, and Co to carbon steel to increase Hc to 145-250 Oe, and (BH)m was close to 1 MGOe, which was a big breakthrough at that time. In 1931, Mishima invented the Fe-Ni-Al ternary alloy magnet. Hc reached 500 Oe (BH)m and reached 1.4 MGOe, opening the door to the development of modern permanent magnet materials. Mainly composed of Fe-Al-Ni, Alnico alloys with the addition of elements such as Co, Cu, Si and Ti have been the mainstream of permanent magnets until 1970. Material scientists use the rules of alloy design to control their phase changes to produce a split-phase decomposition reaction and cool them in a magnetic field, so that the resulting phase grows along the direction of the magnetic field to produce excellent magnets with high anisotropy. 600 ~ 2000Oe, (BH) m is between 3 ~ 12MGOe, can be adjusted by alloy composition and heat treatment and magnetic properties. Although this time, a large number of newer or cheaper permanent magnets have gradually replaced it, but it is extremely stable magnetic (applicable to the high temperature of 500 °C, making it in some specific applications (such as microwave communications), it is still not easy The Fe-Cr-Co permanent magnet alloy that was invented in the 1970s was designed using the principle of Alnico, and its magnetic properties are also similar to those of Alnico alloy. I have studied for many years. Figure 3 shows the use of magnetic field heat treatment to make Fe28Cr-12Co. The phase separation of the Ti alloy is arranged along the direction of the magnetic field, and the uniform diameter of the particles is about 300 & aring; In the period from 1932 to 1938, ferrites, the magnetic oxides that began to develop in Japan and Holland, are one of the main permanent magnetic materials in Japan.The main components of ferrites are Ba0.6Fe2O3 and Sr0.6Fe2O3. It belongs to hexagonal crystal system; its Hc is about 1.8-3.2kOe, Br is about 2.2-4.3, and (BH)m is about 1.0-4.0MGOe (depending on additives and equipment, etc.). It is widely used due to its low cost, easy preparation, and wide application. ,Currently Bay Month requires more than 2,000 tons, about 3/4 self-made. In 1969, material scientists developed a permanent magnet of rare earth-cobalt, opened up a new world for permanent magnets.In the past two decades, rare earth permanent magnets have made great progress. The earliest alloys of SmCo5 and Sm(Co,Fe,Cu,Zr)7.2-8.5 (namely Sm2CO17) alloys, to the nearest Nd2Fe14B alloys (since 1984), have a magnetic energy product from a record of 20 MGOe (SmCo5) to 30 MGOe (Sm2Co17). ) to 50MGOe (Nd-Fe-B alloy), showing the progress of the Pentium type, which is all back to the research and development of materials science. Domestic research and development work in this area has been synchronized with the international, industrial production The system has also been developed as a promising high-tech industry.Figure 4 is a high resolution electron microscope photograph of the Nd-Fe-B alloy studied by the author and shows that the grain boundary between the two Nd2Fe14B grains has a body-centered cubic ( Bcc) The structure of the phase, the parallel lines in the grains are the lattice images of the c plane, there are many other permanent magnet materials, such as Cu-Ni-Co alloy, Mn-Al-C alloy and Pt-Co alloy, etc. There are no less than ten kinds of species, and the space cannot be introduced one by one. Among the permanent magnetic materials, some are magnetic recording materials with small volume and high efficiency: powdery γ-Fe2O3, CrO2, Fe4N, metal powders such as Fe powder, Fe-Co alloy powder, etc. Used in a large number of industries such as audio tapes, video tapes, magnetic disks, etc.; it is also used as hard disk for film-like Fe-Ni, Fe-Ni-P, Fe-Ni-Cr, Fe-Ni-Co, etc. Cr is used for vertical recording, and Tb-Fe-Co and Gd-Co are used for readable and writable magneto-optical recordings. The permanent magnetic material is an energy storage device as described above. As long as it is properly designed, it can work. The above "record" is an example. Other applications include: horns, motors, generators, gauges, suction devices, magnetic separators and so on. Temporary Magnetic Material and Its Application The temporary magnetic material is a material that is strongly magnetized after it is magnetized (for example, when a current is wound on a coil wound on its outside), and the magnetic field is immediately demagnetized after the magnetic field is removed. Therefore, it can be used in AC motors, even high frequency and UHF applications. The application requirement is that the higher the magnetic permeability and Bs value are, the lower the Hc value is, the better (therefore, B*H value - represents the magnetic loss, the smaller). In terms of development, the temporary magnetic material (ie, soft magnetic material) is earlier than the permanent magnet material, and the result is more abundant. For example, pure iron itself is a very good soft magnetic material, and it has been used since the end of the 19th century. The current usage is still very large. In the 1910s, Fe-Ni alloys were invented by Bell Labs in the United States and later called permalloy; by the 1950s their μ0 values (see Figure 2) were as high as 100,000, called superconducting magnetic alloys ( Supermalloy). Its magnetic properties are greatly influenced by nickel content, rolling and annealing methods. Since it was first established around 1900, and since it was made of directional silicon steel sheets in 1930, it has become the mainstream of soft magnetic alloys for electric motors. Most of these flexible alloys are suitable for low-frequency applications due to their conductors. Ferrite soft magnetic materials are mainly spinel crystal systems; the general formula is MFe2O4, M is a divalent ion, such as Mn++, Zn++, Ni++, Cu++, Mg++, Co++, and even Fe++, etc., for example, the most common on the market. The (Mn, Zn)Fe2O4, (Ni,Zn).Fe2O4 and (Mn,Mg)Fe2O4 and so on. Due to ferrite soft magnetic material oxides, large resistance, suitable

PREVIOUS：Safety valve installation, use and maintenancNEXT：Hard soft magnetic alloy high temperature how