Preparation of bonded magnets (2) Isotropic and Anisotropic Magnetic Powder

In the previous issue, we have learned about the process classification and corresponding advantages and disadvantages of bonded magnets. In this issue, we will talk about bonded magnetic powder.

Isotropic rapid quenching rare earth permanent magnet powder

The production method of bonded NdFeB magnetic powder is completely different from sintered NdFeB, because the alloy ingot or sintered body of sintered NdFeB does not have practical coercive force after being broken to the particle size for bonded magnets, and the preparation method for mass production is inert. In a gas environment, the molten alloy condenses into a microcrystalline or even amorphous structure at a cooling rate of 105~106°C/s, and then undergoes crystallization heat treatment to make the grain grow to tens or hundreds of nanometers, which is smaller than the critical size of Nd2Fe14B single domain High intrinsic coercive force is obtained in submicron grains.

Generally, it is difficult for magnetic powder to be broken into such fine single crystal particles, and the technology of rapid quenching and directional growth of submicron grains is not yet mature, so the melting and spinning quenching method produces polycrystalline powder, and the easy magnetization axis of each crystal grain has no Strong alignment tendency, magnetic powder is isotropic. Such a high cooling rate is achieved by pouring or spraying hot molten alloy liquid onto a water-cooled rotating copper wheel with a line speed of 16-30m/s. The liquid alloy is thrown out along the tangential direction under the acceleration of the rotating copper wheel and It is condensed into a thin strip with a thickness of ~100μm, and the cooling rate sensitively determines the grain size of the alloy thin strip, thereby sensitively affecting the shape of the magnetic powder demagnetization curve and the intrinsic coercive force.

Preparation of bonded magnets (2) Isotropic and Anisotropic Magnetic Powder

Preparation of bonded magnets (2) Isotropic and Anisotropic Magnetic Powder

Since the cooling rate depends on many factors such as the temperature of the alloy liquid, the flow rate, the rotational speed and temperature of the copper wheel, and the argon atmosphere, it is difficult to strictly control and optimize simultaneously. It is easy to cause the grain size distribution to be too wide, and the corresponding grain intrinsic coercive force distribution is also very wide, resulting in poor squareness of the demagnetization curve of the magnetic powder. Therefore, in actual production, the magnetic powder is usually rapidly condensed to a partially amorphous state at a wheel speed that is appropriately higher than the optimum line speed, and then crystallized at 630°C to adjust the crystallization state to an average grain size of 40nm. In order to achieve stable and good permanent magnet properties, Magnequench Magnetics Corporation of the United States, which dominates the isotropic quenching Nd-Fe-B market, adopts such a technical route to produce.

Anisotropic quick-quenching rare earth permanent magnet powder prepared by HDDR process

HDDR process is a very effective technical means to produce anisotropic NdFeB magnetic powder. HDDR process includes hydrogenation – disproportionation – dehydrogenation – recombination (Hydrogenation – Disproportionation – Desorption – Recombination, HDDR for short) four stages. The essence of the HDDR process is that the rare earth intermetallic compound absorbs hydrogen and undergoes disproportionation decomposition. In the subsequent forced dehydrogenation process, the disproportionation products recombine into the original compound phase with fine grains, so as to realize the refinement of the material grains and produce along the The crystal structure in the C-axis direction of the main phase can prepare magnetic powder with excellent magnetic properties and magnetic anisotropy.

Aichi Steel utilizes the dynamic HDDR process (d-HDDR) that reasonably controls the temperature and hydrogen pressure over time to realize the anisotropic texture of the magnetic powder more conveniently, and can achieve the state of industrialized stable production, so that HDDR each Anisotropic NdFeB magnetic powder was introduced to the market.

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