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Temperature, since sintered NdFeB possess a negative temperature coefficient, the instantaneous or the continuous highest temperature of the working environment will give demagnetization in different and certain percentage, including reversible and irreversible, recoverable and unrecoverable demagnetization.
Humidity, Sintered NdFeB itself is easy to be corroded, rusted and oxidized, we usually take the method of surface treatment to protect it, but we can't keep away the influence to magnet caused by ambient humidity fundamentally. The drier of the working environment, the longer of the working life of the magnet.
Sintered Nd-Fe-B, as the strongest permanent magnets discovered up to now, is preferred by permanent magnetic devices, due to its highly cost-effective performance based on rich rare-earth resources, manufacture in large scale and technology development.
Ⅰ Sintered Nd-Fe-B magnets are popular for their merits as follows:
1.High energy product，commonly referred to as maximum value (BH) max. Sintered Nd-Fe-B magnets have high magnetic energy per unit volume, which makes it possible for modern permanent magnetic devices to become smaller and lighter and, therefore, cost greatly reduced.
2. Great intrinsic coercive force, denoted as Hcj. Since Hcj is easy to reach up to 12 kOe, magnets applied in high demagnetizing field could be designed thin enough to keep devices compact.
3. Demagnetization curve with good rectangularity. Magnetic polarization strength is almost invariable in a wild range of demagnetizing field. Shown in rectangular coordinates, the B-H demagnetization curve is a straight line in the second quadrant and extending to the third quadrant. That means, in a magnetic circuit, polarization strength isin constant. Accordingly, magnetic circuit design work is simplified and development cycle of new permanent magnetic devices is greatly shortened.
4，Low flux leakage. On the one hand, sintered Nd-Fe-B magnets have an extremely high anisotropy field and, flux lines, thereby, are confined to the two pole faces, resulting in a low leakage. On the other hand, the great intrinsic coercive force permits magnets of a small dimension in the magnetization direction and naturally reducing the flux leakage. To save time and cost for design and development of permanent magnetic devices, finite element method could be used to simulate magnetic circuit and calculate the exact field.
5，High energy efficiency. Thanks to the good rectangularity of demagnetization curve, recoil permeability denoted as rec could be lower than 1.2, while the lower rec is, the better magnet flux get back to original magnetic state.,
6，Low eddy current loss. Sintered Nd-Fe-B magnets have higher resistivity than Sm-Co magnets, let alone ordinary metallic materials. In alternating magnetic field such as that in electric motors, magnets have lower eddy current loss, and therefore devices work with less heat and lower temperature rise.
Ⅱ There are some key points in search of good sintered Nd-Fe-B magnets:
1. Grade
The operating point of a permanent magnet should be higher above the knee point Bk, Hk at the highest working temperature. In another word, the maximum reverse applied field (self-demagnetizing field included) denoted as Hr and the apparent flux density Bd should meet the following inequality:
Bk, Hk refer to flux density and magnetic field at knee point.
As the picture shows, Bk=0.2 T at 80C and Bk=0.4 T at 100C. Then Bd needs to be >0.2 T at 80C and >0.4 T at 100C, or at 100C, B-H curve should cross the operating line at a point where Bd>0.4 T. Otherwise an irreversible demagnetization will occur at the high temperature.
For this reason, we should determine the highest working temperature of magnets before selecting grade. From the picture, we find both magnetic remanence and coercive force decrease as the temperature rises, thus the knee point goes up. Since the operating line of a magnet is determined once circuit structure is designed, what we need to do, in fact, is to find the minimum coercive fore.
B-H curves of different grades are available. Feel free to contact us for details.
2. Dimension
Sintered Nd-Fe-B magnets are brittle materials with significant surface effect, so generally we suggest the dimension is above 1.0 mm. But do not hesitate to contact us for special solution.
Residual internal stress always exists in sintered Nd-Fe-B magnets for its brittleness. So magnets need to be chamfered to prevent edge chips.
3. Surface treatment
Sintered Nd-Fe-B magnets are sensitive to the environment and surface treatment is usually required. We provide various metallic and non-metallic coatings to prevent corrosion, the major cause of field failure except for high temperature.
L/D is a ratio to analyze the shape of a magnet, and it is a very important parameter to judge the temperature resistance ability of the magnet basically.
Normally, we advise L/D ratio＞0.5 (≥0.7 will be even better )
L/D ratio was calculated as follow:
（1） e g, D10*6 disc magnet, magnetized direction is 6, its L/D ratio： 6/10 = 0.6, it has a good L/D ratio, it would stand the high temperature as we designed.
（2） e g, 15*15*3 block magnet, magnetized direction is 3, its L/D ratio: 3 /（2* = 3 / 16.92 = 0.17, L/D ratio is very low，so it would not stand the high temperature as we designed, we have to revise something to improve it.
For more details, please consult our professional engineers.
Sintered NdFeB magnets are relatively inferior on corrosion resistance comparing with the traditional magnets such as Sm-Co, Alnico and ferrite. For general applications, sintered NdFeB magnets may need to be coated or surface treated depending on their intended uses. Coating treatment for the sintered NdFeB magnets not only improves their corrosion resistance, but also appearance, wear resistance as well as appropriate for applications in clean room conditions.
There are various coatings suitable for the sintered NdFeB magnets, ranging from plating coating layers such as Ni, Zn, Au and organic electro-deposition (epoxy for example), physical vacuum deposition (PVD) of Al, galvanic tin, chemical vapor deposition (CVD), or any combinations of these.
Besides of coating protection, the sintered NdFeB magnets can also be surface treated by chromatation, phosphatization, and other chemical passivation treatments to improve their corrosion resistance.
Not all types of coating or passivation treatment will be suitable for every magnet, and the final choice will depend on their applications and environment. For example, Ni coating has good corrosion resistance for a variety of atmospheres, but it shields part of the magnetic flux because nickel is a ferromagnetic material. Furthermore, the affinity of Ni coating glued to other metals is inferior to that of Zn coating. Comparing with Ni coating, Zn coating has advantages of good gluing ability, less magnetic flux shielding so that the magnet has good homogenous apparent flux features from piece to piece. But the corrosion resistance of Zn coating is inferior to Ni coating when the magnet is used in stronger corrosive environment. If the magnet is intended to be used in high humid and strong corrosive surroundings, it is suggested that the magnet be coated with multiple layers such as Ni+Cu+Ni, Ni+epoxy, Zn+epoxy et al. If the magnet is intended to be used in dry atmosphere and weak corrosive environment, Zn coating is recommended.
It will affect the working life whether the coating is stand or fall.
The main detections are as follows:
Visual inspection. Check the thickness of coatings. Drop test (mainly among the Zinc plated products), crosscut test (mainly among the Nickel plated products), PCT test, SST salt spray test, constant temperature and humidity test etc.
NdFeB permanent materials are based on the inter-metallic Nd2Fe14B which has a very high Br and Hcj.
High energy density makes NdFeB widely used in the modern industry and electronic applications, which makes instruments, generators, magnetic separators etc, smaller, lower weight, and thinner.
The advantages of NdFeB are high cost/performance ratio and good mechanical properties, while the disadvantage is the low Curie temperature with bad temperature resistance. We have to improve its chemical composition and do surface treatment to achieve actual applications.
The process of sintered NdFeB magnets is powder metallurgy, the smelted alloy powders are pressed into mould in the magnetic fields, the compression block was sintered into densified products. We usually do thermal effect processing in order to promote the Hcj of the magnets.
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