The Production Process of Sintered NdFeB and the Importance of Oxygen, Nitrogen and Hydrogen Elements

Introduction to Sintered NdFeB

Sintered NdFeB is a significant rare earth permanent magnet material, known for its extremely high magnetic energy product and coercivity, earning it the title of "King of Permanent Magnets." It is a tetragonal crystal primarily composed of neodymium, iron, and boron, and is currently the second most magnetic permanent magnet after absolute zero holmium magnets. Sintered NdFeB permanent magnet materials are manufactured using powder metallurgy techniques, involving steps such as smelting, powder manufacturing, pressing molding, and sintering.

Sintered NdFeB permanent magnet materials possess excellent magnetic properties, including high remanence, high coercivity, and high maximum magnetic energy product. These features have led to their widespread use in various fields. For example, in new energy vehicles, sintered NdFeB is used in drive motors and generators. Due to its small size and high performance, it helps reduce motor weight and improve efficiency. Additionally, it is extensively applied in wind power generation, 3C products, medical devices, high-end CNC machine tools, and robotics, among other fields.

Production Process of Sintered NdFeB

The production process of sintered NdFeB is complex and involves multiple steps such as smelting, strip casting, hydrogen decrepitation, magnetic field orientation and pressing, sintering, tempering, surface treatment, and machining. The sintering process typically occurs under vacuum or inert gas protection to ensure material performance. To enhance the density and microstructural quality of the magnets, multiple rounds of heat treatment and aging treatment are usually required.

Oxygen, Nitrogen, and Hydrogen Elements in Sintered NdFeB Materials

During the production and preparation of sintered NdFeB, oxygen, nitrogen, and hydrogen elements are introduced. Oxygen primarily comes from the oxides in raw materials and exposure to air during molding and sintering. To control the oxygen content, a nitrogen or hydrogen atmosphere is typically used during molding and sintering to prevent oxidation reactions. For instance, during molding, protective agents such as boric acid or sodium borohydride may be mixed in to reduce the powder's exposure to oxygen, thereby lowering the oxygen content.

Nitrogen mainly originates from nitrogen gas in the air adsorbed on the NdFeB powder surface. During sintering, the large specific surface area of the powder leads to the adsorption of a significant amount of nitrogen molecules. If these nitrogen molecules are not promptly removed, they will react with the Nd-rich phase at high temperatures to form neodymium nitride, affecting magnet performance.

Additionally, during the production of sintered NdFeB, hydrogen primarily comes from the hydrogen decrepitation process. Hydrogen decrepitation involves placing NdFeB alloy strip castings into a sealed container and treating them in a high-purity hydrogen environment. The hydrogen pressure is around two atmospheres, and after some time, the alloy ingot will fracture, accompanied by a temperature rise, producing hydrogen gas. 

Furthermore, during the dehydrogenation stage, hydrogen in hydrogenated neodymium is released to form hydrogen gas, which is extracted and expelled by a vacuum pump. Hydrogen elements tend to diffuse into the central part of NdFeB magnets during sintering, causing hydrogen to be present in outer pores. This increases brittleness, reduces bending strength, and may lead to cracks.

In summary, precise control and analysis of oxygen, nitrogen, and hydrogen content in sintered NdFeB materials are crucial to ensure material quality and performance. By optimizing the content of these elements and their corresponding preparation processes, the application potential and market competitiveness of sintered NdFeB materials can be further enhanced.