What are Nd-Fe-B permanent Magnets?

What are Nd-Fe-B permanent Magnets?


Q: What are Nd-Fe-B permanent Magnets?

A: physicists of United States and Japan discovered Nd-Fe-B (neodymium–iron-boron) material in 1983. It is the strongest man-made permanent magnet so far. Nd-Fe-B material is mainly a type of intermetallic compound, Nd2Fe14B, which has a composition of two rare earth atoms, 14 iron atoms and a boron atom. Besides the main phase Nd2Fe14B in Nd-Fe-B material, there are small amounts of rare earth-rich phase (Nd-rich phase), boron-rich phase as well as other phases. The Nd-rich phase provides pinning of the domain walls so that Nd-Fe-B magnet has high coercive force. Today’s commercial Nd-Fe-B magnets have many combinations of partial alloying substitutions for Nd and Fe, leading to a wide range of available properties. The rare earth content of Nd-Fe-B magnet alloys is typically 30 to 35 weight percent.

Today, due to its superior magnetic properties and reasonable costs, Nd-Fe-B permanent magnet has found its extensive application in many fields such as: computer devices, communication tools, motors, medical treatment instruments, sensors and speakers etc.

There are two families of Nd-Fe-B permanent magnets commercially available, sintered and bonded magnet. Sintered Nd-Fe-B magnet is a fully dense and anisotropic material and provides the highest available magnetic energy products of any materials, ranging from 26-50 MGOe. While bonded Nd-Fe-B magnet is produced by bonding the rapidly quenched alloying powders with polymer bonders, and shows comparably low magnetic energy products. This is mainly because the commercially available bonded Nd-Fe-B magnets is isotropic on macroscopic, magnetic properties and it is not a fully densed material. But isotropic bonded Nd-Fe-B magnet has its special advantages. For example, it can be fabricated by compression and injection molded processes and offers complex shapes with higher size-precision than sintered magnets. Moreover, unlike the anisotropic magnet that must be magnetically oriented in a preferred direction during the molding process, the isotropic boned Nd-Fe-B magnet can be conveniently magnetized in any direction: axial, radial and multi-polar which implies complex magnetization capability over anisotropic magnets. Furthermore, isotropic bonded Nd-Fe-B magnet is made from rapidly quenched powders in which particles have fine microstructures and make it comparably stable in chemistry. We can also consolidate the rapidly quenched powders into a fully dense anisotropic magnet by plastic deformation to get high magnetic energy products comparable with sintered magnets.


Q: How to Comprehend the Differences of Operating Characteristics between Rare Earth Magnets and the Conventional Magnets?

A: Permanent magnets can be divided into two basic types with respect to their resistance to demagnetization measured by the intrinsic coercive force Hcj and their residual induction Br. For the Type I magnets, the intrinsic coercive force μ0Hcj<<Br, and for the Type II magnets, μ0Hcj>>Br, Rare earth permanent magnets are Type II magnets. This dissimilarity of the two characteristics means that the behavior of the two classes of magnets will be different, and they generally cannot be used interchangeably in the same application.

The first distinction between the two chasses of magnet concerns their resistance to demagnetization. For the Type II magnets, the point of irreversibility of induction moves from the second quadrant of the induction hysteresis loop B-H to the third quadrant. A magnet in this class can encounter levels of demagnetization greater than its own maximum self-demagnetizing influence without the need for remagnetization. Also, their recoil permeability is near unity, and the major and minor B-H hysteresis loops tend to have the same slope. When a Type II magnet is loaded and unloaded over a dynamic work cycle, as would happen in a torque drive or a latching magnet, it transforms energy efficiently. The magnet stores potential energy within its volume, and this energy can be converted back into mechanical work. For a Type I magnet, such as Alnico-S, most of the potential energy is stored in the leakage field external to the magnet volume, and only a small part of this energy can be converted back into mechanical work.

For the dynamic-work-cycle sort of service, the significant property of the magnet is its useful energy (BH)u, as opposed to its total available energy (BH)max. A comparison of the useful energy of common types of permanent, magnets shows that rare earth magnets are 8 to 10 times more efficient than Alnico or ceramic magnets.

The energy Wm stored in the magnetized permanent magnet and the energy Wg within the usable air gap are:

                            Wm= 1/ (2μ0   (2-1)

and                   Wg= (μ0/2)      (2-2)

Vm is in the volume of permanent magnet and Vg is air gap respectively. For rare earth permanent magnets (Type II magnet), their polarization J has almost no changes when a demagnetizing force is applied to a considerable extension. Equation (2-1) tells us that the energy stored in this type of permanent magnet is independent of the circuit wherever the magnet is placed.

A second point of distinction between the two classes is the greater disparity between the intrinsic and normal demagnetization curves for the type II magnets. This disparity means that a heavily self-demagnetized Type II magnet can be placed in an external field, and yet very large magnetic force or torque per unit mass may still be obtained. The torque is directly proportional to the intrinsic magnetization of the magnet and the field in which it is immersed. Rare earth magnets have high levels of intrinsic magnetization, making them practically immune to magnetization reversal when placed in very high demagnetizing fields. Therefore, multiple Type II magnets with different magnetization orientations can be purposely configured together to form an intended field. Such an approach is not practicable with conventional Type 1 magnet because of their susceptibility to demagnetization by the applied adverse fields of their neighborhood magnets.

For the reasons described above, rare earth magnets are also known as magneticall rigid material.



Q: How to Comprehend the Energy Stored in Permanent Magnets?

A: People are often confused by such an experience; when a magnet A is magnetized by a permanent magnet B has transported some part of energies to magnet A. Once magnet A is removed away, it turns magnetized and holds some energy. As the energy stored in magnet B is not weakened during such a cycle, then we can get infinite energies by many cycles like this .Is this true? Or else, where is this part of energies in magnet A from?

Here, we should not forget that during such a magnetizing-removing cycle, one has already fed back work to this magnet system when he is moving magnet A away. If the energy stored in magnet B is strictly unchanged during such a cycle, then the energy gained by magnet A must equal to the fed back energy from environment That is, the energy stored in permanent magnet B and its surrounding space cannot be taken away, it can simply do work along with the environment energies. This is an important characteristic of permanent magnet. In a word, perpetuum mechanism is impossible.