Overview
The coercivity of soft magnetic materials is very low and can be repeatedly magnetized in a magnetic field. When the external electric field is removed, all or most of the magnetism obtained will disappear.
Soft ferrite is produced by powder metallurgy. There are several types of Mn-Zn, Cu-Zn, Ni-Zn, among which Mn-Zn ferrite has the largest output and consumption.
Soft ferrite is divided into the following nine types: pure iron and low carbon steel, iron-silicon alloy, iron-aluminum alloy, iron-silicon-aluminum alloy, nickel-iron alloy, iron-cobalt alloy, Soft ferrite, amorphous soft magnetic alloy, superfine crystalline soft magnetic alloy.
Characteristic requirements
Requirements: Four highs-µi, Q, fr, stability (M, DF);
Features: easy to obtain Magnetism is also easy to lose, mainly used for high-f weak field
classification
According to crystal structure
Spinel type; cubic crystal system;
Garnet type: body-centered cubic type
Magnetolite type: hexagonal crystal system
According to material application performance
(1), high magnetic Conductivity material (µi = 2000--4104): low frequency, broadband transformer and small pulse transformer
(2), low loss material: power core, high power occasion;
(3), low-loss and high-temperature qualitative materials: communication filter cores;
(4), high-frequency and large magnetic field materials: cavity resonators, high-power transformers, etc.
< p>(5), power ferrite (high Bs) material: switching power supply and low-frequency power transformer(6), high-density recording material: used as recording and video head;
(7) Wave absorber material: absorb electromagnetic wave energy, widely used in anti-interference electronic technology
Magnetic characteristic parameters and improvement measures
Magnetic characteristic parameters
1. Initial permeability
µI=lim_(△H→0){△B/△H}
2. Magnetic lossQuality factor: Q=ωL / R;
Loss tangent: tanδ=1/Q;
Specific loss coefficient: tan /µi =1/µi·Q
General material µi·Q = constant.
3. Temperature stability Temperature coefficient: αμ
Specific temperature coefficient: αu/µi
4. Falling reflects the stability of the material over time
5. Magnetic aging
6. Cutoff frequency fr The frequency point corresponding to the rapid drop due to the domain wall or natural resonance, which measures the upper limit of the material application frequency.
Related theories and methods for improving magnetic permeability
I. Theoretical overview of initial permeability:
Microscopic mechanism: reversible domain rotation, reversible domain Wall displacement
µi = µi turn + µi position
For general sintered ferrite:
1. If there are many internal pores, low density, wall displacement Difficult, µi conversion is dominant;
2. If the crystal grains are large, the pores are few, and the density is high, the wall shift is the main factor.
The difficulty of magnetization depends on the magnetization power ( The ratio of MsH) to retardation is higher, it is easy to magnetize; otherwise, it is difficult to magnetize.
< p>Two, theoretically improve the magnetic permeability conditions:1. Necessary conditions:
1>. Ms should be high (∝Ms2 );
2>.k1, λs→0;
2. Sufficient conditions:
1>. Less raw material impurities, ;
2>. The density should be increased (P ↓), that is, the material grain size should be large (D↓);
3>. The structure should be uniform (grain boundary block ↓);
4>. Eliminate internal stress s·σ ↓ ;
5>. Stoma ↓, another phase ↓ (demagnetizing field↓)
Three, improve µi’s method
(1) Improve the Ms of the material
Spinel ferrite Ms = | MB-MA|
1. Select unit ferrite with high Ms
Such as: MnFe2O4 (4.6--5 µB); NiFe2O4 (2.3 µB)
2. Add Zn to reduce MAs
In addition:
CoFe2O4 (3.7 µB) magnetocrystalline anisotropy
Fe3O4 (4 µB) low resistivity and high K
Li0.5Fe2.5O4(2.5 µB)
Poor sinterability, 10000C, Li volatilizes
(2). Reduce k1 and s
1. Unit ferrite with L=0; MnFe2O4, Li0.5Fe2.5O4, MgFe2O4
2. Select L to be quenched; NiFe2O4, CuFe2O4
3. Ion substitution reduces k1, λs
1>. Add Zn2+ to dilute the magnetic anisotropy of magnetic ions
2>. Add Co2+: general ferrite k1<0, k1>0 of Co2+, positive
Negative k compensation;
3>. Introducing Fe2+, Fe2+ is positive k in MnZn, which can be positive and negative compensation
Compensation adjustment k;
4>. Add Ti4+, 2Fe3+ Fe2++Ti4+;
5>High permeability composition range
(3). Microstructure:
5> p>
1. Crystalline state: grain size, completeness, uniformity;
2. Grain boundary state: thickness, pores, other phases;
3. Crystal Intra-grain pores, another phase: size, number and distribution;
4. High µ material: large grains, uniform and complete grains, thin grain boundaries, no pores and other phases
(四). The influence of internal stress on µ:
1>. It is caused by magnetostriction during the magnetization process, which is proportional to s;
2>. After sintering, the cooling rate is too fast, the lattice strain and ion, Distortion due to uneven distribution of vacancies;
3>. The stress caused by pores, impurities, other phases, lattice defects, uneven crystallization, etc., is related to the purity of the raw materials and the process.
Loss of soft ferrite
Causes of magnetic loss
In a weak alternating field, soft magnetic materials will be magnetized and store energy on the one hand, and on the other On the one hand, due to various reasons, B lags behind H and causes loss, that is, the material absorbs energy from the alternating field and dissipates it in the form of heat energy.
Magnetic loss classification
Non-resonant zone (smaller loss):
1>. Eddy current loss;
It is caused by eddy current caused by electromagnetic induction.
Generally, when the ρ of ferrite is very high, the eddy current loss can be ignored; for high μ materials, due to the high content of Fe^2+ (ρ=10^-2～10Ωm), the eddy current loss is relatively large .
The effective way to reduce eddy current loss is to increase ρ (ρ inside the grain, ρ at the grain boundary)
2>. Hysteresis loss;
It means that the soft magnetic material has irreversible magnetization in the alternating field to form a hysteresis loop, which causes material loss, and the size is proportional to the loop area.
Reason: the irreversible wall shift makes B lag behind H.
Methods to reduce loss:
1) Under low field, to prevent irreversible magnetization process, reducing loss is consistent with the method of increasing µi; but at the same time, attention should be paid to prevent irreversible wall displacement The appearance of
2) Under the high field, the irreversible magnetization process is completed as soon as possible, and the area of the hysteresis loop is reduced.
3>. Residual loss;
Yes All losses of soft magnetic materials except eddy current loss and hysteresis loss are mainly the magnetic aftereffect loss in the low-frequency weak field. In the high-frequency field, the resonance tail extends to the low-frequency field;
The magnetic aftereffect determines Yu: Diffusion ion and vacancy concentration; related to working temperature and frequency;
Diffusion relaxation time: τ = 1 / (9.6 ρ·f·exp(-θ/T))
< p>where f: lattice vibration frequency; ρ: diffuse ion concentration; θ: activation energy;The ion activation energy θ is high, and the ambient temperature T is low, so τ is much longer than the corresponding τ of the application frequency. Low loss;
Resonance area (large loss):
4>. Size loss;
5>. Domain wall Loss;
6>. Natural resonance
Methods to improve the performance of soft ferrite materials
1. Raw materials: high purity, good activity, less impurities, For MnZn materials
, the particle size is best in the range of 0.15 to 0.25 µm. Pay special attention to the mixing of large impurities with a relatively large radius;
2. In addition to meeting high Ms, the formula is more important to meet k1≈0, λs≈0;
Generally, when the µi is required to be below 5000 When necessary, you can add necessary additives such as CaO, TiO2, LaO, CuO, Bi2O3, B2O3, BaO, V2O5, ZrO2, etc. to improve the loss characteristics and other performance effects;
3. Ensure high density And excellent microstructure, the magnetization process is dominated by wall shift. The secondary reduction sintering method and the balanced atmosphere sintering method are indispensable conditions for obtaining stable and excellent performance;
4. Use appropriate heat treatment process to further improve the microstructure performance, promote homogenization, and eliminate internal stress , Adjust the stable distribution of ions and vacancies.
Preparation of soft ferrite material powder
The preparation of soft ferrite powder mostly adopts fire method and wet chemical method. The preparation of ferrite powder mainly adopts wet Chemical method, the preparation of soft ferrite powder mainly adopts wet chemical methods such as co-precipitation method, sol-gel method, and hydrothermal method. The following takes the wet process to prepare Mn-Zn ferrite powder as an example to describe.
1 Preparation of fine ferrite powder by co-precipitation method
The preparation of fine ferrite powder by chemical co-precipitation method is to select a suitable water-soluble metal salt and prepare it according to the preparation method. The material composition is measured, the metal salt is dissolved and mixed evenly in an ionic state, and then a suitable precipitant is selected to uniformly precipitate or crystallize the metal ions, and then the precipitate is dehydrated or thermally decomposed to obtain fine ferrite powder. Therefore, the chemical co-precipitation method is the most economical method for preparing ferrite powder. Due to the characteristics of high purity, uniform particle size distribution, and good activity of the prepared powder particles, it has been deeply studied and widely used in recent years. The co-precipitation method can be divided into several methods according to the different precipitants: carbonate, oxalate and hydroxide.
1) Hydroxide co-precipitation method This method can be divided into neutralization method and oxidation method. The neutralization method is to neutralize the ferric ions and other metal salt solutions that make up the ferrite material with an alkali, and under certain conditions, directly form spinel ferrite in the aqueous solution. The ionic reaction equation is: 2Fe3++ M 2++ OH----- MO- Fe2O 3↓ The main influencing factors for the formation of ferrite in the neutralization method are the pH value and temperature of the solution (generally pH is 10-13, and the temperature is near boiling) .
The main process of the oxidation method is to first prepare an aqueous sulfate solution containing ferrous ions and other divalent metal ions, add an excess of strong alkali solution, and maintain the pH at a certain value to form a suspension, and then Air is introduced into this solution to oxidize and gradually generate ferrite precipitates. The formation of ferrite and its grain size are affected by factors such as solution pH and temperature. At pH>10, the size of ferrite particles increases with the increase of metal cation concentration, and decreases with the decrease of temperature. To prepare a sediment with practical value, perfect structure, and a certain particle size, it is necessary to select appropriate conditions to achieve.
2) Carbonate co-precipitation method
The carbonate co-precipitation method is to add an appropriate precipitant carbonic acid to the metal salt solution Salt, the precursor precipitate is obtained, and then calcined into a powder. In the co-precipitation, in order to prevent the pollution of sodium ions, NH3-NH4HCO 3 is used as the precipitating agent, which can eliminate the difficulty in filtration and post-sintering caused by the use of a single precipitating agent. This method has simple process, easy operation, low cost, and good economic value.
2Sol-gel method
Sol-gel method is a new wet chemical synthesis method that emerged in the 1990s and is widely used in various inorganic functions In the synthesis of materials. This method is to dissolve metal organic compounds such as alkoxides in organic solvents, hydrolyze, polymerize, and form sols by adding pure water, etc., and then take appropriate methods to form gels, and then dry them at low temperature in a vacuum state. The loose dry gel is then calcined at high temperature to obtain nano-scale oxide powder. The structure and properties of the gel depend to a large extent on the subsequent drying and densification process, and ultimately determine the properties of the material.
The powder prepared by this method has high purity, good uniformity, and small particle size. Especially for multi-component systems, its uniformity can reach the molecular or atomic level.
The sintering temperature is lower than the high-temperature solid-phase reaction temperature, and the grain size increases with the increase of temperature and time. The complete crystallization temperature is about 750 ℃. Compared with the co-precipitation method, the nanopowder synthesized by this method agglomerates only during sintering, and crystallizes completely at a low temperature (700-800 ℃). This saves energy and avoids the introduction of impurities from the reactor due to the high sintering temperature. At the same time, it is easy to partially form gel before firing, which has a large surface area, which is conducive to the formation of products. It is a better method for preparing ultrafine powder.
3Hydrothermal method
Hydrothermal method is also a new synthetic method for preparing ultrafine powder developed in the past 10 years. This method uses water as a solvent to chemically react substances in a solution at a certain temperature and pressure to prepare inorganic functional material micropowders. This method can achieve the doping of multivalent ions. These characteristics are new materials for research Provides favorable conditions. In the hydrothermal reaction, the formation of micropowder crystal grains undergoes a dissolution-crystallization process. The prepared micropowder crystals have small particle size and relatively uniform particle size, and do not need high-temperature calcination pretreatment. The synthesis temperature is about 900 ℃, and the crystals formed It is relatively complete, has high purity, and has high activity. Studies have shown that the temperature and time of the hydrothermal reaction have a greater impact on the purity, particle size, and magnetic properties of the product, and the prepared micropowder crystal grains are generally only tens of nanometers.
4Supercritical method
Supercritical method refers to a method of preparing fine powder under supercritical conditions in a hydrothermal reactor with organic solvents, etc. instead of water as solvent . The disappearance of the liquid phase during the reaction is more conducive to the uniform growth and crystallization of the particles in the system. It is superior to the hydrothermal method and is a method worthy of further study. The particle size distribution of the micropowder prepared by the supercritical fluid drying method is relatively uniform, the crystal is complete, the specific surface energy is small, and it is not easy to agglomerate.
The history of soft ferrite materials research
The first country in the world to start research-China
China was the first country in the world to discover material magnetism and Countries where magnetic materials are applied. 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.
The application of soft magnetic materials in industry
The application of soft magnetic materials in industry began at the end of the nineteenth century. It appeared with the rise of electric power and telecommunications technology. Its application range is extremely wide. Soft magnetic materials are not only used in the field of home appliances, information technology, automobiles and other supporting fields, but more importantly, soft magnetic materials as the main raw materials for the production of electronic components have brought continuous demand for them. In recent years, its market demand has increased year by year, and its product categories have also increased, which has become a bright spot in the development of the magnetic materials industry. According to statistics from authoritative organizations, the output of soft magnetic materials in China in 2004 exceeded 100,000 tons, and sales revenue was about 7 billion yuan. Its output accounted for about 33% of the world’s total output of magnetic materials, and the realized sales revenue accounted for the world’s magnetic materials. About 40% of the total sales revenue of materials.
The demand for domestic high-performance permanent ferrite magnetic materials (equivalent to the FB4 and FB5 and above series of Japanese TDK products) will account for about 40% of the total demand for permanent ferrite magnetic materials in 2000 (Less than 60,000 tons) increased to more than 70% (approximately 150,000 tons) of high-performance soft ferrite magnetic materials (equivalent to PC40 and H5C2 and above series of Japanese TDK products) in 2005 accounted for the demand for soft ferrite magnetism The proportion of total material demand will increase from less than 10% in 2000 to more than 30% in 2005 (PC40 and above 20,000 tons, H5C2 and above 10,000 tons)
Ferrite soft magnetic materials in the 20th century Research results
Human research on ferrite began in the 1930s.
In the 1940s, Holland J.L. Snowyk invented a ferrite soft magnetic material with high resistivity and good high frequency characteristics.
The 1940s to the 1960s were a period of rapid development of science and technology. The invention of radar, television broadcasting, integrated circuits, etc. had higher requirements for soft magnetic materials, and soft magnetic alloy ribbons were produced. And soft ferrite materials. The 1950s was a period of vigorous development of ferrite. In 1952, the magnetoplumbite hard ferrite was successfully developed; in 1956, a planar ultra-high frequency ferrite was developed in this crystal system, and a garnet-type ferrite with rare earth elements was discovered at the same time, thus forming a spinel There are three major crystal system ferrite material systems: stone type, magnetoplumbite type and garnet type. In the 1970s, with the development of telecommunications, automatic control, and computer industries, soft magnetic alloys for magnetic heads were developed. In addition to the traditional crystalline soft magnetic alloys, another type of material-amorphous soft magnetic has emerged. It should be said that the advent of ferrite is an important milestone in the development history of strong magnetism and magnetic materials. So far, ferrite magnetic materials have been widely used in many high-tech fields.
The development trend of soft ferrite materials
Ferrite absorbing materials
Due to the rapid development of science and technology, in the stealth technology of weapons and electronics In the computer information leakage prevention technology and the thermal effect in biology, the application of ferrite as a microwave absorbing material is particularly important. In recent years, researchers have focused on composite ferrite materials and nano-sized ferrites to control their electromagnetic parameters. Ferrite nano-magnetic materials are used as microwave absorbers, and the specific surface area of nano-scale particulate materials is larger than that of conventional coarse powders. 3-4 orders of magnitude, high absorption rate. On the one hand, it can absorb free molecules in the empty space or other molecules in the medium that are connected together by bonding, resulting in anisotropic changes. On the other hand, in the microwave field, the movement of active atoms and electrons is intensified, which promotes magnetization, and finally converts electromagnetic energy into heat energy, thereby increasing the absorption capacity of the absorber.
Application in information storage
Information storage ferrite material magnetic recording is a technology and device that uses ferromagnetic media to input, record, store and output information. The magnetic materials used for magnetic recording are divided into two categories: magnetic recording media, which are used as materials for recording and storing information, and belong to permanent magnetic materials. The other type is magnetic head materials, which are sensor materials used for input and output of information, and belong to soft magnetic materials.
Magnetic fluid
Magnetic fluid is a new type of functional material. It consists of three parts: magnetic particles, stabilizer (surface and active agent) and carrier fluid. It acts in a magnetic field. The following shows superior performance than other magnetic materials, so it is widely used. This is a synthetic colloidal system, including colloidal magnetic micro-materials (magnetite), dispersed in a continuous particle-carrying liquid with the assistance of surfactants, the diameter of magnetic particles is about 10mm. The magnetic fluid integrates the magnetizability of a solid and the fluidity of a liquid. Under the action of a magnetic field, the magnetic fluid can be magnetized, showing superparamagnetism. Magnetic fluid has a wide range of applications in the field of biomedicine. The magnetic drug carrier developed in recent years is a high and new technology that has been of great concern at home and abroad.
Green magnetic materials
With the opening of the 2010 World Expo . A new economic model based on low energy consumption, low pollution, and low emissions-the arrival of the low-carbon economy era. A sustained low-carbon and green economy will be the general trend of future world development, which is important for new energy, environmental protection, energy conservation and other emerging The industry will bring huge medium and long-term investment and development opportunities. Low-carbon economy involves a wide range of industrial and management fields, and is also closely related to the development of magnetic materials. It will also be an important development direction for the application and market of new high-tech magnetic materials in the future
China's magnetic materials market h2> Current status of China's magnetic material market
Current status of China's magnetic material market
Soft ferrite products, high-tech applications accounted for 22%, such as digital communications, electromagnetic compatibility (EMC), radio frequency broadband, anti-electromagnetic interference (EMI) , HD display, automotive electronics. The application of traditional middle and low-end products accounted for 78%, such as televisions, power adapters, electronic ballasts, ordinary switching power supply transformers, and antenna rods.
From an overall point of view, the performance of China's ferrite magnets is still in the middle and low grades. Although the output ranks first in the world, the output value is not ideal. The total output value of China's magnetic materials is about 26.5 billion yuan, the output value of permanent ferrite is 6.2 billion, and the average price is 15,000 yuan/ton; the output value of soft ferrite is 9.3 billion, and the average price is 31,000 yuan/ton, and the remaining samarium-cobalt magnets , Neodymium iron boron magnets and metal magnets account for 11 billion yuan in the market.
Prospects for the development of China's magnetic material market
1. Increasing labor and energy costs are a trend;
2. It is still inevitable that the price of raw materials will continue to rise amidst fluctuations. ;
3. Exports of magnetic products and electronic components will slowly recover in twists and turns
4. The fierce competition and price wars from domestic downstream users will inevitably force the prices of magnetic materials companies to fall, and profits will slow Step down
5. Companies that do not have a production scale and mass-produced low- and mid-range magnetic material products are struggling to survive
6. Companies with cost advantages and technological advantages will develop well
7. The overall level of China's magnetic material research and development and production will be in line with the international advanced level, and it will be a strong country from a major producer of magnetic materials.