Time:9/7/2022 2:47:13 PMauthor:source:
Thin-thick permanent magnets in smart devices
Current technological updates have made devices thinner and thinner, putting pressure on every industry involved to manufacture components that use as little volume as possible. This presents many challenges to magnetism due to the fabrication and physical properties of permanent magnets. The biggest concern with magnets is demagnetization. Keeping permanent magnets magnetized is very difficult if the intended circuit is not designed with in mind.
Three main components contribute to the demagnetization of a magnet: temperature, shape, and an opposing external magnetic field. All three of these come down to the pressure exerted by the magnet's coercive force on the magnet. The characteristic that determines the magnet's ability to resist demagnetization. The higher the coercivity, the easier it is to keep the magnet magnetized. We probably don't have a lot of control over the magnet's temperature and reverse magnetic field (environment and circuitry around the magnet), but hopefully we can control its shape and choose material grades to ensure we're in a safe working area for coercivity.
Typically, in electronics, the ambient temperature is assumed to be between 65 and 80 degrees Celsius. Usually there isn't any opposing external field, but if there is another magnet, an electrical coil, or a changing nearby electric field is possible and cannot be ignored. Opposing magnetic fields (motors, actuators, resonant or vibrating motors, transformers, charging coils) are occasionally present intentionally.
Once we have considered the temperature and reverse field of the circuit, it is time to consider the shape of the magnet. The magnets run on a curve, much like the power or torque curve of a motor. Magnets are stronger in better circuits and weaker (and closer to demagnetization) in poorer circuits. We call the operating point in an open-air circuit without a reverse magnetic field the permeability of the magnet (average B/H of the μ magnet), which allows us to calculate the magnet's internal diamagnetic field. Magnets are always fighting themselves internally to stay magnetized. We'll skip over the mathematical details of the permeability (Pc or load line), but it depends heavily on the ratio of the pole face area to the thickness of the magnet (for anisotropic magnets, a magnet with a set magnetization axis, e.g. Sintered NdFeB (new magnet) or Samarium Cobalt). Thinner magnets will also not shrink the pole faces significantly, which will greatly reduce Pc, causing the magnet to weaken or demagnetize.
In electronics, we typically see magnets that are less than 1mm thick, but many times larger in other directions (forming large pole faces and thin magnets). This is the worst case for a magnet PC. We have to try to make the magnet as thick as possible (balanced with the pole faces of the intended use case) to increase the Pc (slightly increase the strength of the magnet, but greatly reduce its chance of demagnetization). We can determine these risks with simple calculations for simple shapes (see Magnetic Calculator for some examples) and use FEA for more complex shapes and circuits. We can choose material grades with greater coercivity to address some of these issues, but we can't increase the coercivity infinitely, it only allows us to do so, which does increase cost and negatively affects strength (flux density). The shape and circuitry of the magnets are critical in permanent magnet design.
Some other considerations are the challenges of making small magnets. Sintered NdFeB (new magnet) is very brittle. They rupture easily under pressure, especially when they have large surface areas and thin cross-sections. NdFeB is easy to oxidize, so it must be coated to prevent corrosion. To avoid the dog-bone effect of magnet edge plating, they are tumbled to round the edges and provide a smooth coating transition. This tumbling in the grinding media is generally safe, but when the magnet is thin and has a large area, it is easy to break the magnet. Cutting very thin magnets is also a challenge. New magnets are cut from larger blocks of sintered material, but this challenge can often be overcome. Due to physical limitations of coercivity and structure, processability is much less of a consideration than demagnetization and magnetic properties. Carefully consider the mechanical tolerances of thin magnets. The Pc of 0.5mm magnet and 0.4mm magnet is very different!
One aspect that is often not considered for magnets of this size is our effect on the surface of magnets with different processes. These effects are negligible and can be safely ignored in magnets with very small surface area to volume ratios (most magnets). But once we started producing magnets that were thin or small compared to the surface area, there wasn't a lot of untouched magnetic mass in the center. Large magnets will work as expected if you do some damage to their surface area (tens of microns of damage), but if the magnet is only a few hundred microns thick, there is little material inside that is unaffected. This seems like an obvious conclusion, but the effect is magnified as the magnet volume is reduced and is usually not taken into account. When the magnet is thinned, the surface area decreases at a much lower rate than the volume. This damage comes from multiple sources, and we'll briefly cover the biggest considerations. Grinding damages the exterior of the magnet, often disrupting its crystal structure, resulting in low flux output or low coercivity (which makes it easier to demagnetize the surface). Deoxidation (acid bath) prior to electroplating can remove some of the intergranular material that imparts extra coercivity to these crystals. Finally, the coating itself is not part of the magnet. At best, it increases the magnet's air gap and reduces the useful magnetic volume. For example, a 0.5mm thick magnet with a 10 micron coating will only be 0.48mm (coated on both sides) useful magnets (some of which are damaged by other processes), anything the magnet attracts is farther away by default due to the thickness of the coating 0.01mm. Distance has a very large effect on magnetic properties. The coating itself may also be ferromagnetic (in the case of the nickel coating), which takes the magnetic flux away from your intended circuit and gives it a path to the other side of the magnet (effectively increasing the magnetic circuit's leakage).
Copyright © Huizhou Zhiyan Magnetic Materials Co., Ltd. ICP1702657
Magnet:Magnet Sitemap(sitemap.html sitemap.xml)
Links