Introduction
Experiments have shown that any substance can be magnetized more or less in an external magnetic field, but the degree of magnetization is different. According to the characteristics of substances in an external magnetic field, substances can be divided into five categories: paramagnetic substances, diamagnetic substances, ferromagnetic substances, ferrimagnetic substances, and diamagnetic substances.
According to the molecular current hypothesis, matter should show roughly similar properties in a magnetic field, but this tells us that the properties of matter in an external magnetic field are very different. This reflects the limitations of the molecular current hypothesis. In fact, there are differences in the microstructure of various substances, and this difference in the structure of the substance is the cause of the difference in the magnetic properties of the substance.
We call paramagnetic and diamagnetic materials as weakly magnetic materials, and ferromagnetic materials as strong magnetic materials.
Generally speaking, magnetic materials refer to ferromagnetic materials. Magnetic materials can be divided into soft magnetic materials and hard magnetic materials according to the difficulty of demagnetization after magnetization. The material that is easy to demagnetize after magnetization is called soft magnetic material, and the material that is not easy to demagnetize is called hard magnetic material. Generally speaking, the remanence of soft magnetic materials is small, and the remanence of hard magnetic materials is larger.
Basic characteristics
1. Magnetization curve of magnetic materials
Magnetic materials are composed of ferromagnetic materials or ferrimagnetic materials, which are applied to the magnetic field H Below, there must be a corresponding magnetization intensity M or magnetic induction intensity B, their change curve with the magnetic field intensity H is called the magnetization curve (M～H or B～H curve). Generally speaking, the magnetization curve is nonlinear and has two characteristics: magnetic saturation and hysteresis. That is, when the magnetic field intensity H is large enough, the magnetization M reaches a certain saturation value Ms, and continues to increase H, and Ms remains unchanged; and when the M value of the material reaches saturation, the external magnetic field H decreases to zero, M and It does not return to zero, but changes along the MsMr curve. The working state of the material is equivalent to a certain point on the M～H curve or B～H curve, and this point is often called the working point.
2. Common magnetic performance parameters of soft magnetic materials
Saturation magnetic induction intensity Bs: Its magnitude depends on the composition of the material, and its corresponding physical state is the orderly arrangement of the magnetization vectors inside the material.
Residual magnetic induction Br: is the characteristic parameter on the hysteresis loop, the value of B when H returns to 0.
Rectangle ratio: Br∕Bs
Coercivity Hc: It is the amount that indicates the difficulty of magnetization of the material, which depends on the composition and defects of the material (impurities, stress, etc.).
Permeability μ: is the ratio of B to H corresponding to any point on the hysteresis loop, which is closely related to the working state of the device.
Initial permeability μi, maximum permeability μm, differential permeability μd, amplitude permeability μa, effective permeability μe, pulse permeability μp.
Curie temperature Tc: The magnetization of a ferromagnetic substance decreases with increasing temperature. When a certain temperature is reached, the spontaneous magnetization disappears and turns into paramagnetism. The critical temperature is the Curie temperature. It determines the upper limit temperature of the magnetic device.
Loss P: Hysteresis loss Ph and eddy current loss Pe P = Ph + Pe = af + bf2+ c Pe ∝ f2 t2 /, ρ is reduced, the way to reduce the hysteresis loss Ph is to reduce the coercive force Hc ; The method to reduce the eddy current loss Pe is to thin the thickness t of the magnetic material and increase the resistivity ρ of the material. The relationship between the loss of the magnetic core and the temperature rise of the magnetic core in free still air is: total power dissipation (mW)/surface area (cm2)
3. Conversion between the magnetic parameters of the soft magnetic material and the electrical parameters of the device
When designing the soft magnetic device, the voltage-current characteristics of the device must first be determined according to the requirements of the circuit. The voltage-current characteristics of the device are closely related to the geometry and magnetization state of the magnetic core. The designer must be familiar with the magnetization process of the material and master the conversion relationship between the magnetic parameters of the material and the electrical parameters of the device. The design of soft magnetic devices usually includes three steps: correct selection of magnetic materials; reasonable determination of the geometric shape and size of the magnetic core; according to the requirements of magnetic parameters, simulate the working state of the magnetic core to obtain the corresponding electrical parameters.
A brief history
China is the first country in the world to discover material magnetism and apply magnetic materials. There are records of natural magnetic materials (such as magnetite) as early as the Warring States Period. The method of manufacturing artificial permanent magnet materials was invented in the 11th century. In 1086, "Mengxi Bi Tan" recorded the making and use of the compass. From 1099 to 1102, there was a description of the compass used for navigation, and the phenomenon of geomagnetic declination was also discovered. In modern times, the development of the power industry has promoted the development of metallic magnetic materials-silicon steel sheets (Si-Fe alloys). Permanent magnet metals developed from carbon steel in the 19th century to later rare earth permanent magnet alloys, and their performance improved more than 200 times. With the development of communication technology, soft magnetic metal materials have changed from flakes to filaments and then to powders, which still cannot meet the requirements of frequency expansion. In the 1940s, the Dutch J.L. Snowyk invented ferrite soft magnetic materials with high resistivity and good high-frequency characteristics, and then low-cost permanent ferrites appeared. In the early 1950s, with the development of electronic computers, Chinese-American Wang An first used rectangular magnetic alloy components as the internal memory of the computer, and was soon replaced by rectangular magnetic ferrite memory cores. Development has played an important role. In the early 1950s, it was discovered that ferrite had unique microwave characteristics, and a series of microwave ferrite devices were made. Piezomagnetic materials have been used in sonar technology during the First World War, but due to the appearance of piezoelectric ceramics, the use has been reduced. Later, a rare earth alloy with strong pressure and magnetism appeared. Amorphous (amorphous) magnetic materials are the result of modern magnetic research. After the invention of rapid quenching technology, the ribbon-making process was solved in 1967, which is a positive transition to practical use.
Classification
Magnetic materials have magnetically ordered, ferromagnetic materials, and broadly include weakly magnetic and antiferromagnetic materials that can apply their magnetism and magnetic effects. Magnetism is a basic property of matter. Substances can be divided into diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic substances according to their internal structure and their properties in the external magnetic field. Ferromagnetic and ferrimagnetic materials are strong magnetic materials, while diamagnetic and paramagnetic materials are weakly magnetic materials. Magnetic materials are divided into metal and non-metallic materials according to their properties. The former mainly includes electrical steel, nickel-based alloys and rare earth alloys, while the latter is mainly ferrite materials. According to the use, it is divided into soft magnetic materials, permanent magnetic materials and functional magnetic materials. Functional magnetic materials mainly include magnetostrictive materials, magnetic recording materials, magnetoresistance materials, magnetic bubble materials, magneto-optical materials, gyromagnetic materials, and magnetic thin film materials. The magnetization curve and hysteresis loop reflect the basic magnetic properties of magnetic materials. And magnetic loss.
Permanent magnet material
After being magnetized by an external magnetic field, even under the action of a considerable reverse magnetic field, it can still maintain a part or most of the original magnetization direction. The requirements for this type of material are high residual magnetic induction intensity Br, high coercivity BHC (ie anti-demagnetization ability), and magnetic energy product (B< /i>H) (that is, the magnetic field energy provided to space) is large. Compared with soft magnetic materials, it is also called hard magnetic materials. There are three types of permanent magnet materials: alloys, ferrites and intermetallic compounds. ①Alloys: including casting, sintering and machinable alloys. The main varieties of casting alloys are: AlNi (Co), FeCr (Co), FeCrMo, FeAlC, FeCo (V) (W); sintered alloys are: Re-Co (Re stands for rare earth elements), Re-Fe and AlNi (Co) ), FeCrCo, etc.; Machinable alloys include: FeCrCo, PtCo, MnAlC, CuNiFe and AlMnAg, etc. The latter two are also known as semi-permanent materials with the lower BHC. ②Ferrites: the main component is MO·6Fe2O3, and M represents Ba, Sr, Pb or SrCa, LaCa and other composite components. ③Intermetallic compounds: mainly represented by MnBi.
Permanent magnetic materials have many uses. ①The main applications based on the principle of electromagnetic force are: speakers, microphones, meters, buttons, motors, relays, sensors, switches, etc. ②Applications based on the principle of magnetoelectric action mainly include: microwave electron tubes such as magnetrons and traveling wave tubes, picture tubes, titanium pumps, microwave ferrite devices, magnetoresistive devices, Hall devices, etc. ③Applications based on the principle of magnetic force mainly include: magnetic bearings, concentrators, magnetic separators, magnetic suction cups, magnetic seals, magnetic blackboards, toys, signs, password locks, photocopiers, thermometers, etc. Other applications include: magnetic therapy, magnetized water, magnetic anesthesia, etc.
According to the needs of use, permanent magnet materials can have different structures and shapes. Some materials are different between isotropy and anisotropy.
Soft magnetic material
Its main function is the conversion and transmission of magnetic and electromagnetic energy. Therefore, high permeability and magnetic induction are required for this type of material, and the area of the hysteresis loop or the magnetic loss should be small. Contrary to permanent magnetic materials, the smaller the Br and BHC, the better, but the larger the saturation magnetic flux density Bs, the better.
A kind of soft magnetic material-iron powder core
Soft magnetic materials can be roughly divided into four categories. ① Alloy thin strip or sheet: FeNi (Mo), FeSi, FeAl, etc. ②Amorphous alloy ribbon: Fe-based, Co-based, FeNi-based or FeNiCo-based, etc., with appropriate Si, B, P and other doping elements, also known as magnetic glass. ③Magnetic medium (iron powder core): FeNi (Mo), FeSiAl, carbonyl iron, ferrite and other powder materials, which are coated and bonded by an electrical insulating medium, and then pressed into shape as required. ④Ferrite: including spinel type──MO·Fe2O3 (M stands for NiZn, MnZn, MgZn, Li1/2Fe1/2Zn, CaZn, etc.), magnetoplumbite type──Ba3Me2Fe24O41 (Me stands for Co, Ni, Mg, Zn, Cu and their composite components). Soft magnetic materials are widely used, mainly for high-frequency acceleration of magnetic antennas, inductors, transformers, magnetic heads, earphones, relays, vibrators, TV deflection yokes, cables, delay lines, sensors, microwave absorbing materials, electromagnets, and accelerators Cavity, magnetic field probe, magnetic substrate, magnetic field shielding, high-frequency quenching energy accumulation, electromagnetic chuck, magnetic sensitive element (such as magneto-caloric material as switch), etc.
Moment magnetic andMagnetic recording material
Mainly used for information recording, contactless switch, logic operation and information amplification. The characteristic of this material is that the hysteresis loop is rectangular.
Gyromagnetic material
It has unique microwave magnetism, such as the tensor characteristic of permeability, Faraday rotation, resonance absorption, field shift, phase shift, birefringence and spin wave, etc. Effect. The devices designed accordingly are mainly used for the transmission and conversion of microwave energy. Commonly used are isolators, circulators, filters (fixed or ESC), attenuators, phase shifters, modulators, switches, limiters and Delay lines, etc., there are still developing surface magnetic wave and magnetostatic wave devices (see microwave ferrite devices). Commonly used materials have formed series, including Ni series, Mg series, Li series, YlG series, BiCaV series and other ferrite materials; and can be made into different structures such as single crystal, polycrystalline, amorphous or thin film according to the needs of the device And form.
Piezomagnetic materials
This type of material is characterized by mechanical deformation under the action of an external magnetic field, so it is also called magnetostrictive material. Its function is to act as magneto-acoustic or magnetic force. Energy conversion. It is commonly used in the vibrating head of ultrasonic generators, mechanical filters of communication machines and electrical pulse signal delay lines, etc. It can be combined with microwave technology to make micro-acoustic (or rotary acoustic) devices. Due to the high mechanical strength of the alloy material, vibration resistance and no explosion, the vibrating head mostly uses Ni series and NiCo series alloys; for use under small signals, Ni series and NiCo series ferrites are mostly used. A new type of amorphous alloy with stronger piezomagnetism is suitable for making delay lines. The production and application of piezomagnetic materials are far less than the previous four materials.
Application of magnetic materials-transformers
Magnetic materials are widely used materials in production, life, and national defense science and technology. Such as the manufacture of various motors and transformers in power technology, various magnetic components and microwave tubes in electronic technology, filters and intensifiers in communication technology, magnetic mines, electromagnetic guns, and various household appliances in national defense technology, etc. . In addition, magnetic materials have also been widely used in geological and mineral exploration, ocean exploration, and new technologies in information, energy, biology, and space. Magnetic materials have a wide range of uses. Mainly use its various magnetic properties and special effects to make components or devices; used to store, transmit and convert electromagnetic energy and information, or generate a certain intensity and distribution of magnetic fields in a specific space; sometimes directly in the natural form of the material Utilization (such as magnetic liquid). Magnetic materials play an important role in the field of electronic technology and other fields of science and technology.
Characteristics of magnetic materials
Materials with ferromagnetic properties have the following characteristics:
①Even if there is no external magnetic field, in each small area (magnetic domain ) There is still a permanent magnetic moment inside. However, when there is no external magnetic field in an unmagnetized magnetic material, the magnetic moment direction of each magnetic domain is arbitrarily distributed, and its vector sum is zero, so the material as a whole has no magnetism.
②Easy to magnetize. This is because under the action of an external magnetic field, the direction of the magnetic moment of each magnetic domain is trying to turn to the direction of the magnetic field, so a large magnetic induction intensity B can be obtained. According to the formula B=μrB0 (B0 is the magnetic induction intensity in vacuum), the relative permeability of the magnetic material is μr is huge. In fact, the μr of magnetic materials reaches 10-10, while the μr≈1 of non-magnetic materials.
③There is a phenomenon of magnetic saturation, that is, B increases with the increase of H, but after increasing to a certain value Bs, it will no longer increase with H. BS is the saturation magnetic flux density of the magnetic material. The reason for the saturation phenomenon is that the magnetic moments of all magnetic domains turn to the direction of the magnetic field after H reaches a certain value. For this reason, B and H are not linear, so the permeability is not constant, but is related to the magnetic field strength.
④There is hysteresis. That is, the change of magnetic induction intensity lags behind the change of magnetic field.