NdFeB magnets

In the application field of NdFeB magnets, there is a close relationship between the magnetism and temperature. When the temperature of the magnet exceeds a certain threshold, permanent demagnetization will occur, and the maximum operating temperature that different grades of NdFeB magnets can withstand varies.   Curie temperature   When studying the effect of temperature on magnetism, "Curie temperature" is a key concept. The naming of this term is closely related to the Curie family. In the early 19th century, the famous physicist Pierre Curie discovered in his experimental research that when a magnet is heated to a certain temperature, its original magnetism will completely disappear. Later, people named this temperature Curie point, also known as Curie temperature or magnetic transition point.   From a professional definition, Curie temperature is the critical temperature at which magnetic materials achieve the state transition between ferromagnetic and paramagnetic materials. When the ambient temperature is lower than the Curie temperature, the material exhibits ferromagnetic properties; when the temperature is higher than the Curie temperature, the material turns into a paramagnet. The height of the Curie point mainly depends on the chemical composition and crystal structure characteristics of the material.   When the ambient temperature exceeds the Curie temperature, the thermal motion of some molecules in the magnet intensifies, the magnetic domain structure is destroyed, and a series of ferromagnetic properties such as high magnetic permeability, hysteresis loop, magnetostriction, etc. associated with it will disappear, and the magnet will undergo irreversible demagnetization. Although the demagnetized magnet can be re-magnetized, the required magnetization voltage is much higher than the first magnetization voltage, and after re-magnetization, the magnetic field strength generated by the magnet is usually difficult to restore to the initial level.   Material Curie temperature Tc (℃) Maximum operating temperature Tw (℃) NdFeB 312 230   Working Temperature   Refers to the temperature range that the neodymium magnet can withstand during actual use. Due to the differences in thermal stability of different materials, the corresponding operating temperature range is also different. It is worth noting that the maximum operating temperature of neodymium is significantly lower than its Curie temperature. Within the operating temperature range, as the temperature increases, the magnetic force of the magnet will decrease, but after cooling, most of the magnetic properties can be restored.   There is an obvious positive correlation between Curie temperature and operating temperature: Generally speaking, the higher the Curie temperature of a magnetic material, the higher its corresponding upper limit of operating temperature, and the better its temperature stability. Taking sintered NdFeB material as an example, by adding elements such as cobalt, terbium, and dysprosium to the raw materials, its Curie temperature can be effectively increased, which is why high coercivity products (such as H, SH, etc. series) generally contain dysprosium.   Even for the same type of magnet, different grades of products have different temperature resistance due to differences in composition and microstructure. Taking NdFeB magnets as an example, the maximum operating temperature range of different grades of products is roughly between 80℃ and 230℃.   Working temperature of sintered NdFeB permanent magnets Coercivity Level Max Working Temperature N Normal 80 ℃ M Medium 100 ℃ H High 120 ℃ SH Super High 150 ℃ UH Ultra High 180 ℃ EH Extremely High 200 ℃ AH Aggressively High 230 ℃   Factors affecting the actual working temperature of NdFeB magnet   Shape and size of neodymium magnets: The aspect ratio of the magnet (i.e., the permeability coefficient Pc) has a significant impact on its actual maximum operating temperature. Not all H-series NdFeB magnets can work normally at 120°C without demagnetization. Some magnets of special sizes may even demagnetize at room temperature. Therefore, for such magnets, it is often necessary to increase their actual maximum operating temperature by increasing the coercivity level.   The degree of closure of the magnetic circuit: The degree of closure of the magnetic circuit is also an important factor affecting the actual maximum operating temperature of the magnet. For the same magnet, the higher the degree of closure of its working magnetic circuit, the higher the maximum operating temperature it can withstand, and the more stable the magnet performance. It can be seen that the maximum operating temperature of the magnet is not a fixed value, but will change dynamically with the change of the degree of closure of the magnetic circuit.

Sintered NdFeB magnets are core functional components and are widely used in instruments and equipment such as motors, electroacoustics, magnetic attraction, and sensors. The magnets are subject to environmental factors such as mechanical force, hot and cold changes, and alternating electromagnetic fields. If the working environment is over the standard, it will seriously affect the function of the equipment and cause huge losses. Therefore, in addition to magnetic performance, we also need to pay attention to the mechanical, thermal, and electrical properties of magnets, which will help us better design and use magnet, and is also of great significance for improving their service stability and reliability.   Mechanical Properties   The mechanical properties of magnets include hardness, compressive strength, bending strength, tensile strength, impact toughness, etc. NdFeB is a typical brittle material. The hardness and compressive strength of magnets are high, but the bending strength, tensile strength, and impact toughness are poor. This makes it easy for magnets to lose corners or even crack during processing, magnetization, and assembly. Magnets are usually fixed in components and equipment by means of slots or adhesives, and shock absorption and buffering protection are also provided.   The fracture surface of sintered NdFeB is a typical intergranular fracture. Its mechanical properties are mainly determined by its complex multiphase structure and are also related to the formula composition, process parameters, and structural defects (voids, large grains, dislocations, etc.). Generally speaking, the lower the total amount of rare earth, the worse the mechanical properties of the material. By adding low-melting-point metals such as Cu and Ga in appropriate amounts, the toughness of neodymium magnet can be enhanced by improving the distribution of grain boundary phases. Adding high-melting-point metals such as Zr, Nb, and Ti can form precipitation phases at the grain boundaries, which can refine the grains and inhibit crack extension, helping to improve strength and toughness; but excessive addition of high-melting-point metals will cause the hardness of the magnetic material to be too high, seriously affecting processing efficiency.   In the actual production process, it is difficult to take both the magnetic properties and mechanical properties of magnetic materials into account. Due to cost and performance requirements, it is often necessary to sacrifice their ease of processing and assembly.   Thermal Properties   The main thermal performance indicators of NdFeB magnets include thermal conductivity, specific heat capacity and thermal expansion coefficient.   The performance of neodymium magnet gradually decreases with the increase of temperature, so the temperature rise of permanent magnet motor becomes a key factor affecting whether the motor can run under load for a long time. Good heat conduction and heat dissipation can avoid overheating and maintain the normal operation of the equipment. Therefore, we hope that the magnetic steel has a higher thermal conductivity and specific heat capacity, so that the heat can be quickly conducted and dissipated, and at the same time, the temperature rise will be lower under the same heat.   Electrical Properties   In the alternating electromagnetic field environment of the permanent magnet motor, the magnetic steel will produce eddy current loss and cause temperature rise. Since the eddy current loss is inversely proportional to the resistivity, increasing the resistivity of the NdFeB permanent magnet will effectively reduce the eddy current loss and temperature rise of the magnet. The ideal high-resistivity magnetic steel structure is to form an isolation layer that can prevent electron transmission by increasing the electrode potential of the rare earth-rich phase, so as to achieve the wrapping and separation of the high-resistance grain boundary relative to the main phase grains, thereby improving the resistivity of the sintered NdFeB magnet. However, neither the doping of inorganic materials nor the layering technology can solve the problem of magnetic performance deterioration. At present, there is still no effective preparation of magnets with both high resistivity and high performance.        

NdFeB magnets are produced by powder metallurgy process. They are a kind of powder material with strong chemical activity. There are tiny pores and cavities inside them, which are easily corroded and oxidized in the air. After the material is corroded or the components are damaged, the magnetic properties will be attenuated or even lost over time, thus affecting the performance and life of the whole machine. Therefore, strict anti-corrosion treatment must be carried out before use.   At present, the anti-corrosion treatment of NdFeB generally adopts electroplating, chemical plating, electrophoresis, phosphating and other methods. Among them, electroplating is the most widely used as a mature metal surface treatment method.   NdFeB electroplating uses different electroplating processes according to the different product use environments, and the surface coatings are also different, such as zinc plating, nickel plating, copper plating, tin plating, precious metal plating, etc. Generally, zinc plating, nickel plating + copper + nickel plating, nickel plating + copper + chemical nickel plating are the mainstream processes. Only zinc and nickel are suitable for direct plating on the surface of NdFeB magnets, so multi-layer electroplating technology is generally implemented after nickel plating. Now the technical difficulties of direct copper plating of NdFeB have been broken through, and direct copper plating and then nickel plating is the development trend. Such a coating design is more conducive to the thermal demagnetization index of NdFeB components to meet customer needs. The most commonly used coatings for NdFeB strong magnets are zinc plating and nickel plating. They have obvious differences in appearance, corrosion resistance, service life, price, etc.:   Polishing difference: Nickel plating is superior to zinc plating in polishing, and the appearance is brighter. Those who have high requirements for product appearance generally choose nickel plating, while some magnets are not exposed and the requirements for product appearance are relatively low. Generally, zinc plating is used.       Difference in corrosion resistance: Zinc is an active metal that can react with acid, so its corrosion resistance is poor; after nickel plating surface treatment, its corrosion resistance is higher.   Difference in service life: Due to different corrosion resistance, the service life of zinc plating is lower than that of nickel plating. This is mainly reflected in the fact that the surface coating easily falls off after a long time of use, causing oxidation of the magnet and thus affecting the magnetic properties.   Hardness difference: Nickel plating is harder than zinc plating. During use, it can greatly avoid collisions and other situations that may cause corner loss and cracking of NdFeB strong magnets.   Price difference: Zinc plating is extremely advantageous in this regard, and the prices are arranged from low to high as zinc plating, nickel plating, epoxy resin, etc.   When choosing NdFeB strong magnets, it is necessary to consider the use temperature, environmental impact, corrosion resistance, product appearance, coating bonding, adhesive effect, and other factors when choosing the coating.    

Many customers may have a question: do magnets of the same performance and volume have the same suction force? It is said on the Internet that the suction force of NdFeB magnets is 640 times its own weight. Is this credible?   First of all, it should be made clear that magnets only have adsorption force on ferromagnetic materials. At room temperature, there are only three types of ferromagnetic materials, they're iron, cobalt, nickel, and their alloys. They have no adsorption force on non-ferromagnetic materials.   There are also many formulas on the Internet for calculating suction. The results of these formulas may not be accurate, but the trend is correct. The strength of the magnetic suction is related to the magnetic field strength and the adsorption area. The greater the magnetic field strength, the larger the adsorption area and the greater the suction.   The next question is, if the magnets are flat, cylindrical, or elongated, will they have the same suction force? If not, which one has the greatest suction force?       First of all, it is certain that the suction force is not the same. To determine which suction force is the greatest, we need to refer to the definition of the maximum magnetic energy product. When the working point of the magnet is near the maximum magnetic energy product, the magnet has the greatest work energy. The adsorption force of the magnet is also a manifestation of work, so the corresponding suction force is also the greatest. It should be noted here that the object to be sucked needs to be large enough to completely cover the size of the magnetic pole so that the material, size, shape, and other factors of the object to be sucked can be ignored.   How to judge whether the working point of the magnet is at the point of maximum magnetic energy product? When the magnet is in a state of direct adsorption with the material being adsorbed, its adsorption force is determined by the size of the air gap magnetic field and the adsorption area.    Taking a cylindrical magnet as an example, when H/D≈0.6, its center Pc≈1, and when it is near the working point of maximum magnetic energy product, the suction force is the largest. This is also in line with the rule that magnets are usually designed to be relatively flat as adsorbents. Taking the N35 D10*6mm magnet as an example, through FEA simulation, it can be calculated that the suction force of the adsorbed iron plate is about 27N, which almost reaches the maximum value of magnets of the same volume and is 780 times its own weight.   The above is only the adsorption state of a single pole of the magnet. If it is multi-pole magnetization, the suction force will be completely different. The suction force of multi-pole magnetization will be much greater than that of single-pole magnetization (under the premise of a small distance from the adsorbed object).     Why does the suction force of a magnet of the same volume change so much after being magnetized with multiple poles? The reason is that the adsorption area S remains unchanged, while the magnetic flux density B value through the adsorbed object increases a lot. From the magnetic force line diagram below, it can be seen that the density of magnetic force lines passing through the iron sheet of a multi-pole magnetized magnet is significantly increased. Taking the N35 D10*6mm magnet as an example, it is made into a bipolar magnetization. The suction force of the FEA simulation adsorbing the iron plate is about 1100 times its own weight.     Since the magnet is made into a multi-pole magnet, each pole is equivalent to a thinner and longer magnet. The specific size is related to the multi-pole magnetization method and the number of poles.        

As a high-performance magnetic material in modern industry, NdFeB permanent magnets promote the progress of contemporary technology and society and are widely used in various fields. How to judge the advantages of permanent magnet products: 1. Magnetic properties; 2. The size of the magnet; 3. Surface coating.   1. Magnetic properties：First, the key to the decision is to control the magnetic properties of the raw materials during the production process.   Raw material manufacturers can choose mid-range or low-grade sintered NdFeB according to business needs. In accordance with the national standards for purchasing raw materials, our company only sells high-grade NdFeB.   The quality of the production process also determines the performance of the magnet.   Quality control during production is important.     2. Magnet shape, size and tolerance: Utilize various shapes of NdFeB magnets, such as round, special-shaped, square, arc-shaped, trapezoidal. Different sizes of materials are processed by different machine tools to cut rough materials, the technology and machine operator determine the accuracy of the product.   3. Surface coating treatment: The coating quality of the surface coating, zinc, nickel, nickel-copper-nickel electroplating copper and gold and other electroplating processes. The product can be electroplated according to customer requirements.   The quality of NdFeB products can be summarized as a good grasp of performance, dimensional tolerance control, and appearance inspection and evaluation of the coating. Tests such as the Gaussian surface of the magnetic flux of the magnet; dimensional tolerance, which can be measured with a vernier caliper; coating, coating color and brightness and coating bonding strength, and the appearance of the magnet surface can be observed to be smooth, with or without spots, and with or without edges and corners, to evaluate the quality of the product.  

Air transportation has certain particularities. To ensure safety, both people and goods need to undergo security checks before boarding. If you carry magnetic materials, such as NdFeB magnets, or if customers are in a hurry to get the goods and hope that the manufacturer will ship them by air, can we bring the magnets on board?   Since weak stray magnetic fields can interfere with the aircraft's navigation system and control signals, the International Air Transport Association (IATA) has classified magnetic cargo as Class 9 dangerous goods, which must be restricted during transportation. Therefore, some air cargo with magnetic materials now needs to undergo magnetic testing to ensure the normal flight of the aircraft. Magnetic materials, audio materials, and other instruments with magnetic accessories must undergo magnetic testing.     Airlines or logistics companies that transport magnetic materials will force customers to undergo magnetic testing and issue an "Air Transport Conditions Identification Report" to ensure the normal flight of the aircraft. Air transport identification can generally only be issued by a qualified professional identification company recognized by the country's civil aviation administration, and it is generally necessary to send samples to the identification company for professional testing before issuing an identification report. If it is inconvenient to send samples, the identification company's professionals will conduct on-site testing and then issue an identification report. The validity period of the identification report is generally for the current year, and it is generally necessary to re-do it after the New Year.   During magnetic testing, customers are required to pack the goods according to air transport requirements. The testing will not damage the packaging of the goods. In principle, the goods will not be unpacked for testing, but only the stray magnetic field of the six sides of each piece of goods will be tested. If the goods fail the magnetic test, special attention should be paid. First, with the consent of the customer, the magnetic inspection staff will unpack the goods for inspection, and then make relevant reasonable suggestions based on the specific situation. If the shielding can meet the requirements of air transportation, the goods will be shielded according to the customer's entrustment, and relevant fees will be charged.  

Neodymium ndfeb materials have poor thermal stability, and their high temperature coefficient can easily cause irreversible demagnetization (also known as demagnetization) when permanent magnet motors are running.   On the one hand, the eddy current of permanent magnet motors generates heat on the surface of permanent magnets, and the heat dissipation conditions inside the motor are poor, which exceeds the working temperature of permanent magnets, causing permanent magnet demagnetization. Therefore, the temperature stability of permanent magnets is crucial to motor applications.   On the other hand, the unreasonable design of the working point of the permanent magnet motor magnetic circuit is also prone to irreversible demagnetization. When the motor encounters a large demagnetization during starting, reversing and stalling, the working point of NdFeB may drop below the inflection point of the demagnetization curve, causing irreversible demagnetization. Therefore, the working point of the permanent magnet motor magnetic circuit should be designed to be higher than the inflection point of the NdFeB material. When the motor stops running, the residual magnetic induction intensity Br of the permanent magnet material remains basically unchanged.   The design of permanent magnet motors must also understand the actual operating environment of the motor and take necessary measures in assembly to ensure that it is in a stable state without demagnetization at high temperatures. The SH grade NdFeB magnets used in motors that meet the standard requirements cannot guarantee that the motor will not lose magnetism during operation. Only by increasing the intrinsic coercive force and Curie temperature of the NdFeB magnets can the irreversible magnetic loss of the NdFeB magnets be reduced and the temperature stability of the permanent magnets be improved, thereby extending the service life of the permanent magnet motor.   

In daily life, magnets are a very common thing. From various special electronic devices to daily teaching aids and toys, magnets can often be seen.   We know that the main component of magnets is ferroferric oxide. An ordinary small magnet is made of black ferroferric oxide. However, due to the nature of ferroferric oxide itself, its attraction to iron objects is not too strong, and its magnetism will gradually weaken over time. In this case, how can we make a magnet with stronger attraction and less prone to decay? Under this premise, neodymium iron boron magnets came into being.     This kind of magnet with a shiny surface after anti-corrosion treatment is a neodymium boron magnets, and its chemical formula is Nd2Fe14B. The most commonly used neodymium iron boron magnet is made of neodymium, iron, and boron at high temperature sintering, and is the strongest artificial magnet to date. If the core element of traditional ferroferric oxide is iron, then the reason why neodymium iron boron magnets have such strong magnetism is the role of neodymium. The pieces of metal in the picture below are neodymium:     Neodymium is the fourth element of the lanthanide family of rare earth elements. Like iron, cobalt, nickel and the aforementioned gadolinium, it can also be attracted by magnets. In addition, neodymium is the most active of the lanthanide elements, so it is easily oxidized like iron, which is why there is a coating on the surface of the NdFeB magnet. If neodymium is used to enhance magnetism, then the role of boron should not be underestimated.    In the periodic table, boron is located to the left of carbon, so boron chemistry similar to carbon-centered organic chemistry has recently emerged. In NdFeB magnets, boron is the mediator between neodymium and iron. Boron greatly expands the maximum magnetism that a substance can produce while ensuring the stability of its molecular structure, making the neodymium magnetic properties of the entire magnet extremely high, and even allowing it to attract objects equivalent to 640 times its own weight.