1. The crystal structure of ferrite
(1) Spinel type ferrite
The ferrite whose crystal structure is similar to that of magnesium aluminum spinel (MgO·Al2O3) is called spinel type ferrite. It belongs to the cubic crystal system, and its chemical formula is generally expressed as (MeFe2O4). Among them, Me is usually a +2 ion, and natural ferrite-magnetite (Fe3O4) is a spinel structure, so it is called ferrite.
Me can also be replaced by Mg2+, Mn2+, Ni2+, Fe2+, Co2+, Cd2+, Cu2+, Li2+, etc. The corresponding ferrites are called magnesium ferrite, manganese ferrite, etc., and so on.
(2) Garnet type ferrite
The ferrite whose crystal structure is similar to that of natural garnet is called garnet type ferrite. It belongs to the cubic crystal system, and the molecular formula is Me3Fe5O12 or written as 3Me2O3·5Fe2O3. Among them, Me represents +3 rare earth metal ion. Garnet type ferrite has excellent magnetic and dielectric properties, high volume resistivity and low loss. Also has a certain light transmittance is an extremely important magnetic material in the fields of microwave, magnetic bubble, magneto-optical and so on.
(3) Magnetoplumbite type ferrite
The ferrite whose crystal structure is similar to that of natural magnetoplumbite is called magnetoplumbite type ferrite. It belongs to the hexagonal crystal system and the molecular formula is MeFe12O19, in which Me is a +2 metal ion, such as Ba2+, Pb2+, Sr2+, etc. This kind of magnetoplumbite ferrite containing Ba2+, Pb2+, and Sr2+ has a large coercivity Hc, and is a kind of hard magnetic material with strong magnetic properties.
2. Manufacturing of ferrite
Ferrite products are divided into volume form (three-dimensional) and film form (two-dimensional) according to their appearance, and they are divided into single crystal products and polycrystalline products according to their microstructure. Single crystal ferrite has excellent electrical, magnetic and optical properties, and is currently widely used in microwave devices, magneto-optical devices, magnetic bubble devices, magnetic recording heads, etc.
There are two types of ferrite films, single crystal films and polycrystalline films. Its development is the result of large-scale integrated circuits requiring miniaturization and integration of magnetic devices. At the same time, ferrite films have shown great potential in magneto-optical memory, surface wave devices, microwave devices, and magnetic recording devices. People are paying great attention. The following focuses on the production process of polycrystalline ferrite and ferrite film.
3. Production process of polycrystalline ferrite
According to whether it is calcined or not, the polycrystalline ferrite production process can be roughly divided into two types: calcined and non-calcined. The purpose of calcined is to reduce harvest shrinkage or to synthesize ferrite.
The following briefly introduces several ferrite powder preparation methods. Among them, oxide method, chemical precipitation method, electrolytic precipitation method, low temperature chemical method and partial salt decomposition method obtain fine and uniform ferrite, which can be formed and sintered without pre-sintering.
(1) Oxide method
Various oxides are directly used as raw materials, and fine powder is obtained by batching and grinding (often using ball milling). This method has simple process, convenient operation and the most widely used. However, due to the poor activity of the oxide raw materials, it is not easy to complete the reaction.
(2) Salt decomposition method
Use sulfate, nitrate, carbonate or oxalate such as manganese, zinc and iron as raw materials, mix according to a certain proportion and heat and decompose to obtain more active oxides. Sometimes the decomposition is completed at the same time as the ferrite synthesis, avoiding the pre-sintering process and directly entering the forming process.
(3) Chemical co-precipitation method
According to the calculation of the ingredients, the nitrate, carbonate or chloride solution containing various metal ions is mixed evenly according to the ratio. Then use a strong base such as NaOH, NH4OH, ammonium oxalate, ammonium carbonate, etc. as a precipitant to make it precipitate to obtain a uniformly mixed hydroxide or corresponding salt.
This kind of precipitation mixture is not only uniform, but also has fine particles, high activity, easy to carry out solid phase reaction, and can be sintered at a lower temperature. However, the precipitate is easy to adsorb alkalis, and it is difficult to remove impurities. In addition, the precipitation in the solution is often not completely complete, and the various components are not precipitated at the same time, so the solubility product should be considered and corrected during the batching. The operating conditions of this method are more complicated, so it is not suitable for mass production.
(4) Electrolytic co-precipitation method
Put the metal required by the formula in the electrolytic cell as the anode, and there is also a metal cathode. After electrolysis, when the metal equivalent to the formula is dissolved in the electrolyte, an oxidant is added to the appropriate position of the electrolytic cell to make the metal ion The oxide precipitates, resulting in a very homogeneous mixture. Then the precipitate is pumped out, washed, filtered, dried and pre-burned. The advantage of this method is that it can omit ball milling, mixing and other processes, facilitate continuous production, and greatly reduce the sintering temperature. In addition, due to electrolysis, impure raw materials can be used to obtain high-purity products.
(5) Spray calcination method
Various metal alkali salts are dissolved in alcohol according to the proportion, and sprayed into the combustion chamber through a high-pressure nozzle with high pressure. When the right amount of air and oxygen droplets are burnt instantaneously, they will react in contact with molecules to produce fine and light ferrite powder.
(6) Cryochemical Method
Namely freeze-drying method. The metal salt aqueous solution is sprayed on the low-temperature organic liquid to instantaneously freeze the fine droplets, and then sublimation and dehydration under low-temperature reduced pressure conditions, and then decomposition to obtain powder.