New progress in the process technology of the hott

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New progress of permanent ferrite Technology Abstract: the latest La Co, La Zn addition technology, grain control technology, dispersant technology for the production of high-performance permanent ferrite at home and abroad, as well as the special mixing process for the production of fb9 series magnetic particles are introduced

key words: permanent ferrite; La CO and La Zn addition; Grain control; dispersant; Mixing

1 introduction

the factors that determine the performance of permanent ferrite can be expressed by the following formula [1~3]

where MS is the saturation magnetization, ρ Is the density, f is the degree of orientation, K1 is the magnetocrystalline anisotropy constant, FC is the existence rate of single domain particles. According to the above two formulas, there are five ways to improve the properties of permanent ferrite: (1) improve the orientation; (2) Increase the sintering density; (3) Increase MS and K1 of ferrite M phase; (4) Grain refinement and increasing single domain load will reduce the existence rate of particles; (5) The grain size of ferrite sinter is controlled to be uniform

in recent years, Japanese permanent ferrite manufacturers represented by TDK company, a new structure with more reasonable structural stress distribution, have made breakthrough progress in the research and production of permanent ferrite, and successively researched and produced fb6 and fb9 series permanent ferrite. China's permanent magnet ferrite industry has developed rapidly in recent years, but the overall technical level still lags far behind the international highest level represented by Japan. This paper introduces some of the latest foreign production technologies and processes of permanent ferrite, hoping to help the manufacturing enterprises of permanent ferrite

2 La CO and La Zn addition technology [4, 5]

represented by scientific researchers such as takahito Taguchi of TDK and Yasuhiro Ogata of Hitachi, the four coordinated (4F1) Fe ions in the ferrite are replaced by CO or Zn ions, and the SR ions in the ferrite are replaced by La ions for price compensation. The resulting ferrite can be represented by the formula sr1-x lax (fe12 ycoy) Zo 19, Where 0.04 ≤ x ≤ 0.45, 0.04 ≤ y ≤ 0.45, 0.7 ≤ Z ≤ 1.2, 0.8 ≤ X/Y ≤ 1.5. The MS of ferrite phase (M phase) obtained by this technology has been significantly improved, so that the magnetic properties of ferrite have been breakthrough improved. In recent years, most of the patents for permanent ferrite are related to the addition of La CO and La Zn. Fb6 series and above permanent ferrite are realized through the addition of La CO and La Zn. It is calculated at the transformation standard of 200 yuan per square meter

3 magnetic particle size distribution control technology

3.1 magnetic particle impurity removal technology [6]

in order to improve the existence rate of single domain particles, it is easy to think of grinding the magnetic particles as fine as possible during fine grinding. In practice, it is found that the magnetic performance decreases when the particles are too fine. The reason is that the particle size is less than 0.1 μ M (EBT measurement), part of the ferrite phase (M phase) is decomposed into Fe3O4 and SrCO3, etc. in addition, long-time grinding will make Fe of the steel ball enter the ferrite powder and affect the magnetic properties. In this technology, ground magnetic particles are heat treated at 600~900 ℃, Fe and fe2+ are oxidized to fe3+ after heat treatment, and then non-magnetic Fe2O3 and SrCO3 are removed by magnetic separation equipment; Or the Fe3O4 and SrCO3 of non-M phase with fine particle size in the top layer can be removed by slurry precipitation classification method. The above method can effectively control the distribution of ferrite grains and improve the existence rate of single domain particles. This technology can be used to produce sintered permanent ferrite with br>0.43t (4300g) and hcj>325ka/m (4100oe)

3.2 chemical synthesis methods

the chemical synthesis methods reported in literature [1, 7] include chemical coprecipitation method, chemical partial precipitation method, hydrothermal synthesis method, etc. these methods can effectively control the size and distribution of ferrite grains, but these methods are expensive and have little practical application value

3.3 graded grinding and circular grinding technology

some foreign manufacturers adopt the graded grinding method as shown in Figure 1. The key to this technology is to strictly control the average particle size of the feed coarse powder of 3~5m (air penetration method, the same below) and the standard deviation of particle size distribution δ= 0.14~0.16 μ m. This can be achieved by dry grinding and air separation and classification process of vibrating mill or ball mill; Second, the continuous sand mill with bottom feeding can grind coarse particles for a long time. This technique can effectively control the standard deviation of grain distribution δ= 0.14~0.16 μ m. The remanence and coercivity of permanent ferrite are increased by 4%

there is no continuous sand mill with bottom feeding as shown in Figure 1 in China. Some enterprises adopt the discontinuous graded grinding process as shown in Figure 2. This process is to put the coarse powder with fine particle size and narrow particle size distribution obtained through dry ball milling and air separation classification process into the ball mill for grinding to 0.8 ~ 1.0 μ m. Then use a sanding mill to grind it to 0.7 μ M or less. The standard deviation of grain distribution can also be well controlled by this technique δ= 0.14~0.16 μ m. Therefore, the magnetic properties can also be significantly improved

there is also a circulating grinding process in China as shown in Figure 3. This process is to grind a can of material of more than 5 tons. This process can grind the coarse particles for a long time by using the sedimentation principle. The fine particles float on the upper layer, and the grinding time will be shortened, which can effectively control the particle size distribution of the powder, Using this technology, the remanence and coercivity of sintered permanent ferrite can be increased by 0.01t (100g) and 8ka/m (100oe) respectively compared with that of single ball mill or sand mill

3.4 crystallization control technology

literature [4] reports that the particle size is 0.7 ~ 0.9m, and the standard deviation of particle size distribution δ= 0.14~0.16 μ M of iron red (Fe2O3), 0.1 ~ 0.5% SiO2 and 0.05 ~ 0.2% H3BO3 as crystal size control agent are added during mixing, and they are pre burned at 1050 ~ 1250 ℃. Fig. 4 shows the metallographic photos of common pre fired materials and pre fired materials using crystallization control technology. The particle size shown in Figure 4b is 0.7 ~ 1.2 μ m. Standard deviation of particle size distribution δ= 0.14~0.16 μ m。 The most comprehensive result body of sintered permanent magnet oxygen output with br>0.42t (4200g) and hcj> 278ka/m (3500oe) can be produced by using this pre fired material

4 orientation improvement technology

there are two methods to improve the orientation, one is to increase the orientation field during molding, the other is to add dispersant into the slurry

4.1 high orientation field technology

the force on ferrite grains under the magnetic field can be expressed as [1] f H2A, h is the orientation field, and a is the area of the grain stress surface. It is reported that the particle size of magnetic powder is 0.95 μ M, the orientation field is 480ka/m (6000 OE), and the particle size is 0.67 μ M, the saturation orientation can be achieved only under the magnetic field of 915ka/m (11500oe). The fine grinding particle size of sintered permanent ferrite with high performance, especially fb6 series performance above, shall be 0.7 μ M, so the orientation field is required to be more than 800ka/m (10000oe). Foreign permanent ferrite manufacturers represented by Japan TDK used compressors with orientation field above 800ka/m (10000 OE) to produce fb6 series permanent ferrite before 1990. In China, about 2001, Sichuan West Group developed 80000 ~ 100000 ampere turn water-cooled strong magnetic field compressors. After 2002, they were gradually used by some domestic permanent ferrite manufacturers. In 2003, some enterprises produced fb6 series permanent ferrite in small quantities

4.2 dispersant technology

the agglomeration of ferrite grains in the slurry is an important reason affecting the directional arrangement of ferrite grains in the magnetic field. Reducing the agglomeration of ferrite powder in the slurry can be achieved by reducing the coercivity of ferrite powder and adding dispersant

foreign literatures [4, 8] reported that organic solvents such as ethanol, toluene, ethylbenzene and heptane were used as grinding media, carboxylic acids and carboxylic acid salts such as stearic acid, calcium stearate (barium, strontium, magnesium, aluminum and zinc), triglyceride, calcium triglyceride (barium, strontium, magnesium, aluminum and zinc) were used as dispersants, and sulfates and their salts, phosphates and their salts were also used as dispersants, When these dispersants are added at the weight ratio of 0.5 ~ 5% of the magnetic powder during fine grinding, the agglomeration of the magnetic powder can be effectively reduced. This technology can be used to produce high-performance sintered permanent ferrite with br=0.44t (4400g), hcj=318~335ka/m (4000~4200oe) and 98% orientation

it is also reported in foreign literature that [5, 9, 10] using water as the grinding medium, using carboxylic sugars containing multiple hydrophilic groups such as carboxymethyl cellulose, gluconic acid, calcium gluconate, sorbose, ascorbic acid, polycarboxylic acid and polycarboxylate as dispersants, sintered permanent ferrite with an orientation of 98% can also be obtained

5 special process technology

tdk's process for producing high-performance (fb9 Series) permanent ferrite not only includes the most advanced technologies such as La Co, La Zn addition technology, graded grinding technology, dispersant technology, but also adopts relatively special technologies. Here is a brief introduction to its production process

the traditional process flow for the production of sintered permanent ferrite is (this process is mostly adopted by Chinese permanent ferrite production units): mixing → pre sintering → coarse grinding → fine grinding → molding → sintering → grinding → inspection → packaging

the special process flow for producing high-performance permanent ferrite (fb9 Series) is [5, 11]: mixing → pre sintering → coarse crushing → fine grinding → drying or dehydration → mixing → molding → decarburization → sintering → grinding → inspection → packaging. Compared with the traditional process, the special process has three processes: drying or dehydration, mixing and decarburization. In order to obtain high-performance sintered permanent ferrite, the pre sintered ferrite should be finely ground to 0.4~0.6 μ m. The slurry with this particle size is difficult to form. In order to solve the forming problem, the fine abrasive slurry must be dried or dehydrated by drying equipment or centrifugal equipment, and then the dry powder shall be mixed by kneader. The first purpose of mixing is to disperse the condensed ferrite particles with the shear force of kneader to facilitate orientation; The other purpose is to add water in the mixing process to facilitate wet pressing; Another purpose is to add dispersant during mixing to make the dispersant fully contact with the magnetic powder, so as to reduce the agglomeration of the magnetic powder and improve the orientation. The magnetic powder mixed with water and dispersant shall be subject to decarburization (i.e. removal of dispersant) treatment at 100~500 ℃ after wet pressing under magnetic field. In addition to adding three processes in the special process, the raw materials, coarse crushing and fine grinding processes used for mixing are also different from the traditional process. The iron containing raw material used in the special process is SiO2

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