Magnetic ferrites introduction
Ferrite is a metal oxide with ferromagnetism. In terms of electrical properties, ferrite has a much higher electrical resistivity than metal and alloy magnetic materials, and has a higher dielectric property. The magnetic properties of ferrite also exhibit high magnetic permeability at high frequencies. Therefore, ferrite has become a non-metallic magnetic material widely used in the field of high frequency and low voltage. Since the magnetic energy stored in the unit volume of ferrite is low, the saturation magnetization is also low (usually only 1/3 to 1/5 of pure iron), thus limiting its low-frequency power and high-energy density. Applications in high power applications.
Ferrites are sintered from iron oxides and other ingredients. Generally, it can be divided into three types: permanent ferrite, soft ferrite and spin ferrite.
The ferrite magnet is also called ferrite magnet, which is the small black magnet that we usually see. The constituent raw materials are mainly iron oxide, barium carbonate or barium carbonate. After magnetization, the strength of the residual magnetic field is high and the residual magnetic field can be maintained for a long time. Usually used as a permanent magnet material. For example: speaker magnet.
The soft ferrite is formed by sintering and sintering iron oxide and one or more other metal oxides (for example, nickel oxide, zinc oxide, manganese oxide, magnesium oxide, cerium oxide, cerium oxide, etc.). It is called soft magnetic because the residual magnetic field is small or almost absent when the magnetizing magnetic field disappears. Usually used as a choke, or core of an intermediate frequency transformer. This is completely different from the permanent ferrite.
Spin ferrite refers to a ferrite material having a gyromagnetic property. The gyromagnetic material of a magnetic material refers to a phenomenon in which a plane-polarized electromagnetic wave propagates in a certain direction during the propagation of a plane-polarized electromagnetic wave in a certain direction under the action of two mutually perpendicular DC magnetic fields and electromagnetic wave magnetic fields. Rotating ferrite has been widely used in the field of microwave communication. According to the type of crystal, the ferrite can be classified into a spinel type, a garnet type, and a magnetoplumbit type (hexagon type) ferrite.
Different in magnetic properties and applications, ferrite can be divided into five types: soft magnetic, permanent magnet, gyromagnetic, moment magnetic, and piezoelectric.
Soft magnetic material
Such materials are easily magnetized and demagnetized under weak magnetic fields, such as zinc-chromium ferrite and nickel-zinc ferrite. Soft ferrite is a kind of ferrite material with wide application, large variety, large quantity and high output value. It is mainly used as a variety of inductive components, such as filter cores, transformer cores, wireless electromagnetic cores, as well as tape recording and video heads, and is also a key material for magnetic recording components.
A compound having a uniaxial anisotropy of a hexagonal structure. It is mainly composed of three ferrites of lanthanum, cerium and lead and their composite solid solution. There are points of the same magnetic and heterogeneous magnetic. Since such a ferrite material retains a strong constant residual magnetic property for a long time after the external magnetization field disappears, it can be used to generate a constant magnetic field to the external space. It is widely used, for example, as a constant magnet in various types of meters, generators, telephones, speakers, televisions and microwave devices.
Hard magnetic material
The ferrite hard magnetic material is not easily demagnetized after being magnetized, and is therefore also referred to as a permanent magnet material or a constant magnetic material. Such as barium ferrite, steel oxygen and so on. It is mainly used for recorders, pickups, speakers, magnetic cores of various instruments in telecommunication devices.
The gyromagnetic material of a magnetic material refers to a phenomenon in which a plane-polarized electromagnetic wave propagates in a certain direction inside a material under the action of two mutually perpendicular stable magnetic fields and electromagnetic wave magnetic fields, but its polarization plane continuously rotates around the propagation direction. Metals and alloy materials have a certain degree of gyromagnetic properties. However, since the electrical resistivity is low and the eddy current loss is too large, electromagnetic waves cannot penetrate into the interior, so they cannot be used. Therefore, the application of ferromagnetism of ferrite gyromagnetic materials has become a unique field of ferrite. Most of the gyromagnetic materials are combined with waveguides or transmission lines for transmitting microwaves to form various microwave devices. Mainly used in radar, communication, navigation, telemetry and other electronic equipment.
Moment magnetic material
This refers to a ferrite material having a rectangular hysteresis loop. Its characteristic is that when there is a small external magnetic field, it can be magnetized and saturated, and after removing the external magnetic field, the magnetic properties remain the same as when saturated. Such as magnesium manganese ferrite, lithium manganese ferrite and so on. Such ferrite materials are mainly used for memory cores of various electronic computers and the like.
Such materials refer to ferrite materials that are mechanically elongated or shortened in the direction of the magnetic field during magnetization, such as nickel-zinc ferrite, nickel-copper ferrite, and nickel-chromium ferrite. The piezoelectric material is mainly used as a transducer for mutual conversion of electromagnetic energy and mechanical energy, and is used as a magnetostrictive element for ultrasonic.
Magnetic materials are widely used in electroacoustics, telecommunications, electric meters, motors, memory devices, microwave components, etc. It can be used to record language, music, image information, magnetic storage devices for computers, vouchers for passengers, and magnetic cards for fare settlement. The following focuses on the magnetic materials used on the tape and the principle of action.
After the hard magnetic material is magnetized, residual magnetism is left, and the strength and direction of the remanence are determined by the strength and direction of magnetism during magnetization. The audio tape consists of a tape base, an adhesive and a magnetic powder layer. The tape base is generally made of polycarbonate or vinyl chloride. The magnetic powder is a fine powder of r-Fe2O3 or CrO2 with a strong residual magnetism. When recording, the current corresponding to the sound change is amplified and sent to the coil of the recording head to generate a concentrated magnetic field in the gap of the head core. As the coil current changes, the direction and intensity of the magnetic field also change accordingly. As the tape passes through the gap of the head at a constant speed, the magnetic field passes through the tape and magnetizes it. Since the magnetic tape leaves the magnetic head and has a corresponding remanence, its polarity and intensity correspond to the original sound. As the tape moves, the sound is continuously recorded on the tape.
When playing a sound, place the recorded tape close to the gap of the playback head at the same speed as the recording. The head core is made of a high permeability ferrite soft magnetic material, which has little resistance to magnetic flux. Therefore, the residual magnetic flux of the audio recorded on the magnetic tape easily forms a loop through the magnetic core of the magnetic head. The residual magnetic flux on the tape induces an induced electromotive force on the playback head coil that is the same as the residual flux. After being amplified by the amplifier, it is sent to push the speaker, and the audio signal recorded on the tape is restored to the original sound.
The video tape and the audio tape are basically the same in material and function, but the recording records the electrical signal representing the sound, and the recording records the television signal representing the scene. There are not only sound signals but also image signals in the television signal. Video tape Compared with audio tape, video tape recording density is very high, because the recording tape recording wavelength is on the order of micrometers, in order to have sufficient sensitivity and signal-to-noise ratio in this wavelength range, the magnetic particle size must be small, the magnetic layer surface must be smooth. Moreover, the wear resistance of the surface of the magnetic layer must be good in order to be used under the high-speed friction of the same magnetic head and the friction of the fixed portion of the transport system of the magnetic tape. For this purpose, the adhesive used must be heat and abrasion resistant.
Fourth, computer magnetic storage device
The magnetic material used in computer magnetic storage devices and magnetic cards used as passengers' vouchers and fare settlements and the principle of action are basically the same as those used in magnetic tapes, and their functions are basically the same. There is a narrow strip of tape on the magnetic card. When you take the subway from station A to station B, the ticket (coin) from station A to station B is put into the instrument at station A, and then a magnetic card is thrown. In the process of throwing this magnetic card, the magnetic record of getting off at station B has been recorded. Take this magnetic card to the station B and put it into the instrument. The door opens and exits. If you don't get off at station B, but get off at station C, which is farther than station B, the coins you put in are not enough, and the exit door will not open. You must take the magnetic card to make up the ticket before you can leave the station.
The process of putting a magnetic card into a station B or a station C is a process in which a magnetic recording becomes an electrical signal through a magnetic head. Then use the electrical signal to control the station door switch.
The magnetic material used for the iron core of the motor is generally hard ferrite. These materials are characterized by the fact that they are not easily demagnetized after magnetization. The resistance to magnetic flux is small.
According to the ferrite crystal structure and morphology, the preparation process is roughly divided into: polycrystalline ferrite production process; ferrite chemical process; single crystal ferrite manufacturing process and other special processes, such as ferrite polycrystalline film and non- Crystal ferrite and the like.
Polycrystalline ferrite production process
Similar to the sintering process commonly used in the ceramic industry, the following steps are included: a metal oxide or a carbonate or other compound which forms a ferrite by a solid phase reaction, and after being uniformly mixed, it is ball-milled, dried, and pressed into a specific shape. After calcination at a temperature of about 1000 ° C, it is thoroughly ground and mixed again. An appropriate amount of binder is added, pressed into a desired shape or extruded into a tubular shape, a rod shape or a strip shape as a plastic material. It is then sintered at a temperature of 1200 to 1400 ° C, and the exact temperature depends on the desired ferrite characteristics. The environmental conditions in the furnace play an important role in the final sintering process.
Ferrite chemical process
Also known as wet process, sometimes referred to as chemical coprecipitation. Processes for the preparation of higher performance ferrites can be divided into neutralization and oxidation processes. The process is as follows: firstly, the metal element required for preparing ferrite is formulated into a certain concentration of ionic solution, and then mixed according to the formula, and a ferrite powder is formed by chemical reaction such as neutralization or oxidation, and thereafter The process is the same as described above.
Single crystal ferrite manufacturing process
It is roughly the same as non-metallic single crystal growth. The growth of Mn-Zn and Ni-Zn ferrite single crystals is generally carried out by the Brizmann method, in which the polycrystalline ferrite is melted in a platinum crucible and then reduced in a suitable temperature gradient electric furnace. The bottom of the crucible slowly solidifies to form a single crystal. In order to balance the partial pressure of oxygen formed in the molten state, it is necessary to add an oxygen partial pressure of several or even 100 MPa in the furnace during crystal growth.
Preparation of ferrite polycrystalline film
For example, a vertically magnetized barium ferrite film is sputtered using a novel counter target sputtering apparatus. The garnet single crystal film is prepared by gas phase or liquid phase epitaxy on a single crystal substrate, and the specific process is very similar to the epitaxial method of the semiconductor single crystal film.
Preparation of amorphous ferrite
At present, an ultra-quenching method and a sputtering method are employed. The so-called ultra-quenching method is a method in which a ferrite material and an appropriate amount of a metal element are mixed, and a large temperature gradient is rapidly cooled in a high-temperature molten state. The research work in this area has just begun, and the performance of the products is still not satisfactory.
Powder preparation method
The preparation of the ferrite powder is to complete the manufacturing process from the raw material to the ferrite powder, and there are many methods for producing the ferrite powder.
(1) Sol-gel method
The sol-gel method is a method in which a metal organic or inorganic compound is solidified by a solution, a sol or a gel, and then heat-treated to form an oxide or other compound solid. The sol-gel method is a commonly used method in the wet chemical method for preparing materials and is widely used in the preparation of ferrite nanomaterials.
(2) Chemical coprecipitation method
Chemical coprecipitation is a common method for preparing ferrite. It uses a precipitant (such as OH-, CO32-, etc.) to co-precipitate the metal ions in the solution, and obtains the product through filtration, washing, drying, and burning.
(3) Oxide method
The main point of the ferrite preparation by the oxide method is that the raw materials are mixed and heated, and the ferrite powder is obtained by the reaction between the materials. In order to effectively promote the solid phase reaction to obtain a uniform, good ferrite powder, in addition to paying attention to the selection of raw materials, attention should also be paid to the determination of mixing, calcination and pulverization conditions. The oxide method is a means of large-scale industrial production.