Grain Boundary Diffusion Technology

With the wide application of sintered NdFeB materials in modern industry and electronic technology, especially in the fields of new energy vehicles, industrial robots and mobile intelligence, it is required that NdFeB products be developed towards lightness, thinness, miniaturization and high energy. The requirements for energy product, coercive force and thermal stability of magnets are getting higher and higher. However, under the traditional preparation process, the high coercive force and high magnetic energy product of sintered NdFeB have always had a trade-off.

The coercive force of the magnet can be improved by doping heavy rare earths such as Dy or Tb into the alloy, but due to the large amount of dysprosium and terbium entering the main phase grains, the remanence will be significantly reduced. Moreover, this process consumes a large amount of expensive heavy rare earth resources, resulting in a substantial increase in the manufacturing cost of the magnet. In the industry, (BH)max (MGOe) +Hcj (kOe) is usually used to represent the comprehensive magnetic properties of magnets. How to further increase the coercive force of magnets under the premise of maintaining high remanence, so as to prepare magnets with high comprehensive magnetic properties has become a Hotspots of industry research and development.

Grain boundary diffusion technology is a technical means developed in recent years that can effectively improve the magnetic properties of sintered NdFeB magnets. By forming a layer of heavy rare earth film on the surface of the magnetic steel, after vacuum heat treatment, the heavy rare earth enters the interior of the magnet along the grain boundary, and at the same time, the heavy rare earth atoms replace the Nd atoms around the main phase grains to form a high coercivity shell. The microstructure can greatly increase the coercive force of the magnet on the basis of extremely low remanence drop value. In general, for sintered NdFeB magnets with a thickness less than 8mm, Dy diffusion HcJ is used to increase 4kOe~7kOe; Tb diffusion HcJ is used to increase 8 kOe~11 kOe, and the two kinds of diffusion Br are reduced within 0.3kGs.

Grain Boundary Diffusion Technology

Grain Boundary Diffusion Technology

Compared with traditional non-diffusion magnets, the main advantages of the grain boundary diffusion process are as follows:

1. For magnets of the same brand, the usage of Dy and Tb can be greatly reduced by using the grain boundary diffusion method, and the cost is lower; 2. The high comprehensive magnetic performance magnets such as 50EH and 52UH can be prepared by using the grain boundary diffusion method, which cannot be achieved by traditional techniques.

We know that everything has many aspects, and the grain boundary diffusion process is no exception, and there are also some shortcomings. For example, due to the existence of concentration gradients, the diffusion depth is limited, and the thickness of mass-produced diffusion magnets is usually within 10 mm, which limits the application range of this process.

The main process form of grain boundary diffusion

The main process form of grain boundary diffusion

After years of development, the process of grain boundary diffusion has developed various methods such as sputtering, evaporation, coating, and electrodeposition. The diffusion sources include heavy rare earth elements and alloys, hydrides, fluorides, oxides, etc.

1. Sputtering

Schematic diagram of magnetron sputtering process

1-1 Magnetron sputtering

It is a method in which glow discharge is generated by voltage, argon gas is ionized, and argon ions bombard the target to sputter out heavy rare earth particles such as dysprosium and terbium, which are deposited on the surface of the magnetic steel to form a film. The products suitable for the magnetron sputtering method include square sheets, tiles, discs, rings, etc., and the diffusion source is mostly a pure metal target of dysprosium or terbium. Grain Boundary Diffusion Technology

Magnetron sputtering has the advantages of good coating uniformity, dense film layer, and stable coercivity enhancement effect. The disadvantages are: 1. The equipment is more expensive, and the price of a single continuous equipment is between 3.5-5 million; 2. The utilization rate of the target is low, so the relative cost is relatively high.

1-2 Multi-arc sputtering

It is a coating in which field discharge produces local high-temperature sputtering. It has the characteristics of high coating efficiency and target utilization rate, good binding force and so on. But the disadvantages are: 1. The coating particles are large and the surface of the coating is rough; 2. The uniformity of the coating is average; 3. The stability of the coating is not ideal; 4. The local temperature rises.

2. Evaporation

Evaporation is a method of depositing and forming a film by heating and vaporizing the coating material in a vacuum state. The evaporation method is divided into static evaporation and rotary evaporation, which are similar to rack plating and barrel plating in the electroplating process, and are suitable for large and small magnets respectively. Grain Boundary Diffusion Technology

Static evaporation diagram

2-1 Static evaporation

The advantage is that the evaporation temperature is high, the sublimation of the evaporation source, the deposition on the surface of the magnetic steel and the diffusion process in the magnet are carried out simultaneously, so the diffusion effect of the heavy rare earth is better. Since the quality of the heavy rare earth film after evaporation is related to the evaporation temperature, vacuum degree and plating distance (the distance between the heavy rare earth target and the magnetic sheet), the magnetic steel sheet needs to be placed in a specific tooling to ensure the plating distance, resulting in a relatively slow process. Complex, low production efficiency, and poor film consistency, so static evaporation is rarely used in the domestic magnetic material industry.

Rotary evaporation process image source: Gao Jingyuan, rotation of Nd-Fe-B permanent magnet material

2-2 Rotary Evaporation

It is a way to continuously mix the small and micro magnet steel and the heavy rare earth target through rotation, and the metal Dy/Tb volatilized after high temperature is deposited on the surface of the product. This process is suitable for small products, especially electroacoustic products with a single weight below 1g; the cost of diffusion materials is low, the weight gain is small, and the HcJ is significantly improved; the consistency of the improved HcJ is high. The disadvantage is that the separation process of the product and dysprosium and terbium metal is relatively labor-intensive; and this process is rarely used in the industry, and the equipment usually needs to be customized, which has certain technical barriers.

3. Coating

Automatic spraying process diagram

3-1 Automatic spraying

Put the single-layer magnetic sheet into the workpiece disk, and use a pneumatic spray gun to spray reciprocatingly to form a film. When the diffusion source is fluoride or oxide, it can be sprayed in the atmosphere. If it is a hydride or heavy rare earth alloy powder, it needs inert gas protection. .

The advantages are: 1. There are many suitable diffusion sources, including hydrides, fluorides, oxides and alloys; 2. Products of various shapes and sizes can be diffused, such as tiles, squares, discs, and rings; 3. The degree of equipment automation 4. Low investment in equipment and high utilization rate of heavy rare earths. Grain Boundary Diffusion Technology

Main disadvantages: patent risks exist when the diffusion source is fluoride, safety hazards exist when hydrides and alloys are used, and poor consistency of oxide properties.

Screen printing process diagram

3-2 Screen printing

It is to prepare heavy rare earth powder into ink. When printing, pour ink into one end of the screen printing plate, apply a certain pressure on the ink part on the screen printing plate with a scraper, and at the same time move toward the other end of the screen printing plate at a constant speed, and the ink During the movement, it is squeezed from the mesh by the scraper to the magnetic sheet to form a film.

Due to the package of organic reagents in the ink, it prevents hydrides or heavy rare earth alloys from contacting the air and avoids oxidation. Therefore, heavy rare earth hydrides can be used as a diffusion source to coat in an atmospheric environment, effectively avoiding the risk of fluoride patents. At the same time, the screen printing process has the characteristics of high production efficiency, high utilization rate of heavy rare earths, and good weight gain consistency, which is very suitable for the grain boundary diffusion of square magnets. In the past two years, the screen printing process has been rapidly promoted in the industry and has become one of the mainstream processes.

The disadvantages are: 1. The screen and scraper are not suitable for tile-shaped products with uneven surfaces; 2. Products with different sizes and weight gain ratios usually require corresponding inks, screen printing plates and fixtures, which are suitable for large batches and stable Order. For small batch orders, frequent replacement of screens and fixtures will result in reduced efficiency, increased costs, and low flexibility.

4. Electrodeposition

In electrophoretic deposition, the heavy rare earth diffusion source and alcohol are configured into a suspension, and the particles of the heavy rare earth diffusion source are deposited on the surface of the magnetic steel sheet through a direct current electric field to form a heavy rare earth film. The advantages are high production efficiency, simple process and low production cost. The disadvantage is that the coating has low adhesion, uniformity, and poor performance consistency during mass production. Therefore, this process is rarely used.

With the gradual increase of market pressure, the cost competition has already changed from the comparison of diffusion and non-diffusion processes to the comparison of different diffusion processes, and even the comparison of the utilization rate of heavy rare earths and manufacturing costs under the same diffusion method. Different diffusion processes have their own characteristics. In actual production, it is necessary to select a suitable diffusion process based on product characteristics, performance requirements, and production costs. It is believed that more new diffusion processes will be developed and used in the future. At the same time, the development of easy-diffusion substrates, the ultimate utilization of heavy rare earth diffusion sources, the breakthrough of diffusion depth and the development of non-rare earth diffusion sources will also attract the attention and efforts of the majority of scientific research and production personnel.

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