Neodymium Magnets
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Neodymium Magnets
Rare earth magnets are the most powerful magnets ever made. They are in the forefront of today’s technological revolution. These magnets are made from alloys containing rare earth elements. The rare earth elements are a group of 17 metallic elements found in the middle of the periodic table. Scandium (Sc), Yttrium (Y), Lanthanum (La) and the 14 Lanthanides constitute this group.
Due to their applications in varied fields like defense technology, green energy, consumer electronics, and chemical catalysis, rare earth elements have become very important, technologically and strategically. Nations are competing with each other to secure their supply of rare earth minerals.
The rare earth elements Neodymium (Nd), Dysprosium (Dy), and Samarium (Sm) are particularly valued for their magnetic properties. Magnetism arises due to the alignment of unpaired electrons in the atoms. The presense of many unpaired electrons make rare earth elements most suitable for high power magnets. Neodymium magnets are rapidly becoming the most preferred magnets in a number of applications including electric vehicles, wind turbines, guidance systems of aircraft and missiles, computer hard drives, and consumer electronics. A neodymium magnet can store about 18 times more magnetic energy than an iron magnet of the same volume.
In their pure form, rare earth elements lose their magnetism at very low temperatures, so even at room temperature they fail to retain their magnetism. To remedy this, they are combined with transition metals like iron or cobalt. Magnets made from alloys neodymium–iron–boron (NdFeB) and samarium–cobalt (SmCo) can retain excellent magnetic properties well above room temperature.
Neodymium (atomic number 60) is a lanthanide series rare earth metal which oxidizes quickly in air. It has high density and high melting point. Neodymium is rarely found in nature as a free element, but occurs in ores such as monazite and bastnäsite.
Since their introduction in the 1980s, neodymium(Nd) magnets have steadily increased their share of the market and high-end applications. The strongest magnets of today are produced from an alloy of neodymium, iron, and boron that also contains some additional transition metals (NdFeB magnets), they have remarkably high magnetic flux density reaching up to 1.4 Tesla.
Neodymium magnets are produced through two different manufacturing processes; sintering and bonding. Sintering is a process that binds metal or ceramic powder into solid dense parts by applying heat below the material’s melting point. In this process, the atoms/molecules of the material diffuse across the boundaries of the particles, fusing them together and forming a solid piece. Sintering reduces porousity and increases strength. It is particularly suited for making complex shapes with precision. Sintered Neodymium magnets are considered as marvels of material science due to their enhanced field strength, energy density, and other magnetic properties like high remanence and coercivity.
Bonded neodymium magnets are produced by mixing neodymium powder with polymer binders and shaping them via compression or injection molding. The resulting magnets are corrosion resistant, but they have lower magnetic strength than sintered magnets. Bonded magnets have other advantages like performance consistency and high material utilization.
Neodymium magnets have the strongest magnetic fields and compact sizes. The exceptionally high field strength is due to the alignment of atomic dipoles under an external field. Even after the external field is removed, a significant portion of this alignment is retained. Neodymium magnets come in different grades. Maximum operating temperatures of different grades range from 80°C to 230°C.
Remanence
Remanence is the value of the magnetic flux density within the magnet that remains when the external field is decreased from saturation to zero. Neodymium magnets have remarkably high remanence values. Sintered Nd magnets have remanence values ranging from 1.0 Tesla to 1.4 Tesla, making them the most powerful permanent magnets available. Bonded Neodymium magnets have lower remanence values, in the range of 0.6 T to 0.7 T.
Coercivity
Coercivity is the minimum value of external magnetic field that can destroy the magnetism of a magnet. It is a measure of a magnet’s resistance to demagnetization. Coercivity is measured in Oersteds(Oe) or ampere turns per meter (A/m). Sintered neodymium magnets offer exceptional coercivity in the range of 750 Oe to 2000 Oe, while bonded Nd magnets have a range of coercivity range of 600 Oe to 1200 Oe.
Maximum Energy Product (BHmax)
BHmax represents the maximum amount of magnetic energy stored per unit volume of a magnet. It is the maximum value of the product of the flux density B and the applied field strength H. It is the primary measure for evaluating the power of magnets. Higher BHmax values indicate stronger magnetic fields and higher efficiency in magnetic circuits. It is measured in Joules /m³ or MGOe(Mega Gauss Oersted). Neodymium magnets typically have BHmax values in the range of 30 to 55 MGOe. Due to these very high values of the energy product, Nd magnets of very small sizes can store considerable amounts of magnetic energy. Ferrite magnets have BHmax values only in the range of 1.0 to 5.3 MGOe., and the values for AlNiCo magnets are in the range 1.35 to 9.0 MGOe.
Curie Temperature (Tc)
Curie temperature is the critical temperature at which a magnet loses its ferromagnetic properties and becomes paramagnetic. Knowledge of Curie temperature is important when selecting magnets for high temperature environments. Neodymium magnets have Curie temperature in the range 310°C to 400°C. Working temperature limits are much below this, in the range 80°C to 230°C.
In NdFeB magnets, other elements like cobalt, copper, gadolinium or dysprosium are added in trace amounts to improve thermal stability, coercivity and corrosion resistance.
Neodymium magnets are susceptible to corrosion and oxidation. Therefore, these magnets are coated with protective layers such as nickel, copper, zinc or tin. Additional epoxy resin or plastic coating may also be applied.
Recent innovations in radial sintered Nd-magnets have significantly advanced magnet engineering. Sophisticated techniques are developed to align the microstructure for improved magnetic features.
A key feature of radial magnets is their ability to direct the magnetic field around the circumference of ring magnets, with the south pole on the inner diameter and the north pole on the outer edge. Radially oriented Nd-magnets offer higher energy density and improved performance and are best suited for applications requiring consistent magnetic strength throughout the component. They are widely used in synchronous motors, stepper motors and DC brushless motors.
Invention of Neodymium Magnets
NdFeB magnets were developed by two researchers working independently of each other. The inventions of Japanese engineer Masato Sagawa and American metallurgist John Croat created today’s most powerful NdFeB magnets.
Before the discovery of Nd-magnet, the strongest permanent magnet was the samarium-cobalt magnets which was expensive due to the high cost of samarium. Scientists and corporations were searching for new material combinations for a cost-effective yet powerful magnet.
Masato Sagawa working at Sumitomo Special Metals of Japan invented the sintered NdFeB magnet in 1982. It replaced the expensive samarium with cheaper neodymium and iron. Sagawa’s sintering process produced NdFeB magnets with twice the strength of samarium cobalt magnets. Today these magnets have achieved great economic success due to wide range of applications in cars, air conditioners, medical imaging equipment and wind turbine generators.
In 1982 itself, John Croat, at the General Motors Research Laboratories, developed bonded neodymium magnets with the same NdFeB configuration. Both Sagawa and Croat announced their inventions at the Magnetism and Magnetic Materials Conference in Pittsburgh, USA in November 1983.
Croat’s bonded magnets are cheaper to produce and his manufacturing process is suitable for making a wide range of magnet shapes, especially thin-walled ring magnets which are ideal for use in small motors.
