NdFeB magnets, also known as neodymium magnets, are composed of neodymium, iron, and boron, forming a tetragonal crystal system with the chemical formula Nd2Fe14B. Discovered in 1982 by Masato Sagawa of Sumitomo Special Metals, these magnets boast a higher magnetic energy product (BHmax) than samarium-cobalt magnets, making them the magnets with the highest magnetic energy product known at that time.
The subsequent development of powder metallurgy by Sumitomo Special Metals and the melt-spinning process by General Motors enabled the production of NdFeB magnets. Today, they are the second most powerful permanent magnets after holmium magnets at absolute zero and are the most commonly used rare-earth magnets.
Chemical Composition and Manufacturing Process
NdFeB magnets primarily consist of the rare-earth element neodymium (Nd), iron (Fe), and boron (B). To achieve various performance characteristics, some neodymium can be replaced with other rare-earth metals such as dysprosium (Dy) and praseodymium (Pr), while iron can be partially substituted with cobalt (Co), aluminum (Al), and other metals. Boron, although present in a small amount, plays a crucial role in forming the tetragonal crystal structure of the intermetallic compound, contributing to high saturation magnetization, high uniaxial anisotropy, and a high Curie temperature.
The manufacturing process of NdFeB magnets involves powder metallurgy. Raw materials with a specific ratio, including neodymium, dysprosium, iron, cobalt, niobium, praseodymium, aluminum, and boron iron, are melted in a medium-frequency induction melting furnace to form alloy steel ingots. These ingots are then crushed into fine powders with a particle size of 3 to 5 micrometers. The powders are pressed into shape in a magnetic field and sintered to form a dense magnetic body in a vacuum sintering furnace, followed by tempering and aging to obtain a magnetic block with certain magnetic properties. After undergoing grinding, drilling, slicing, and surface treatment, the block is transformed into the final NdFeB product.
Magnetic Properties and Applications
NdFeB magnets exhibit exceptional magnetic properties, including high magnetic energy product and coercivity, enabling them to lift objects up to 640 times their own weight. This high energy density makes NdFeB magnets widely used in modern industrial and electronic technologies, enabling the miniaturization, lightweighting, and thinning of equipment such as instruments, electro-acoustic motors, magnetic separators, and magnetization devices.
NdFeB magnets are available in various grades, such as N35 to N52, 35M to 50M, 30H to 48H, 30SH to 45SH, and 28UH to 35UH. The magnetic properties of these magnets are characterized by parameters such as magnetic energy product (BH), remanence (Br), coercivity (Hcb, Hcj), and temperature coefficients.
- Magnetic Energy Product (BH): It is the product of magnetic flux density (B) and magnetic field intensity (H) at any point on the demagnetization curve of a permanent magnet. It represents the total energy stored per unit volume in the magnetic field produced by the magnet. The maximum value of B×H on the demagnetization curve is known as the maximum magnetic energy product.
- Remanence (Br): It is the residual magnetization left in the material after the external magnetic field is removed.
- Coercivity (Hcb, Hcj): It is the resistance of a material to becoming demagnetized. The intrinsic coercivity (Hcj) is the magnetic field intensity required to reduce the magnetization of the magnet to zero.
Testing Processes
Ensuring the quality and performance of sintered NdFeB magnets involves rigorous and precise testing of their magnetic properties. The testing process typically involves measuring magnetic flux density, coercivity, remanence, and other relevant parameters.
- Magnetic Flux Density: A gauss meter is commonly used to measure the strength of the magnetic field produced by the magnets.
- Coercivity: It measures the resistance of the material to magnetization.
- Remanence: It reflects the magnetization remaining in the material after the removal of an applied magnetic field.
In addition to these parameters, other tests such as temperature stability, corrosion resistance, and mechanical properties are also crucial for fully evaluating the performance of NdFeB magnets. Advanced equipment and methodologies are essential for obtaining reliable and consistent test results. Strict adherence to standardized testing procedures ensures the validity and comparability of the results.
Surface Treatment
To prevent corrosion, NdFeB magnets require surface treatment. Common methods include electroplating with gold, nickel, zinc, or tin, and surface coating with epoxy resin. NdFeB magnets are categorized into sintered and bonded types. Bonded NdFeB magnets are magnetically isotropic and corrosion-resistant, while sintered NdFeB magnets are prone to corrosion and require surface coating, such as zinc or nickel plating.
Recent advancements in surface treatment include the development of a nano-chelating film without plating. This technology creates a film with active chelating groups that are highly resistant to moisture, oxygen, chloride ions, and carbon dioxide. The film exhibits enhanced corrosion resistance and adhesion to organic resins, making it suitable for applications in harsh environments such as marine climates and high-performance electric motors.
Conclusion
NdFeB magnets are versatile and powerful permanent magnets with a wide range of applications. Their magnetic properties, combined with advancements in manufacturing and surface treatment technologies, make them essential components in various industries. Rigorous testing and adherence to standardized procedures ensure the reliability and performance of these magnets, making them indispensable in modern technology.
