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How to choose magnetic material? - Neodymium Magnet, SmCo, AlNiCo, Ferrite

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                       How to choose magnetic material ? - Neodymium Magnet, SmCo, AlNiCo, Ferrite

                           

When you choose permanent magnetic material, it’s necessary to consider the following aspects:

• Magnetic stability required;
• Maximum working temperature;
• Availability;
• Corrosion resistance;
• Cost;
• Size and/or weight limitations;
• Flux requirement for the particular application.

There are many different types of magnet material, but we will just choose four major types in use today to make comparisons. They are NdFeB, AlnicoSmCo and Ferrite magnets.

Most magnets are anisotropic, and can only be magnetized in the orientation direction. Although isotropic magnets can be magnetized in any direction, they are generally lower in performance than anisotropic magnets. 

1. Magnetic Performance of Different Magnets

BHmax is the point where a magnet provides most energy for the minimum volume. If you want to compare the magnetic performance of different types and grades of permanent magnets, the most convenient method is to consider their BHmax.


NdFeB(N38H)306  kJ/m³                   38 MGO                      
Alnico(anisotropic Alcomax III)                           42 kJ/m³5.2 MGO
SmCo(2:17) 208 kJ/m³26 MGO
Ferrite(anisotropic)26 kJ/m³3.3 MGO

Another parameter that should be taken into consideration is the flux density on the pole face of a magnet. This figure is often mistaken for the Br, but actually it is purely the induction in a closed circuit. The following table shows typical pole face flux densities of the four grades when working at approximately their BHmax points.


NdFeB(N38H)450 mT (4500 Gauss)             
Alnico(anisotropic Alcomax III)130 mT (1300 Gauss)
SmCo(2:17)350 mT (3500 Gauss)
Ferrite(anisotropic)100 mT (1000 Gauss)

2. Effects of Temperature

Effects of Temperature can be classified into two categories, reversible and irreversible. The reversible changes with temperature have nothing to do with the shape, size or the working point on the demagnetization curve. They are dependant upon material composition. When a magnet is returned to its initial temperature, reversible losses will disappear completely without remagnetization. Irreversible losses won’t come up if a certain temperature isn’t exceeded. In addition, they can also be limited by operating at as high a working point as possible. But when the external temperature exceeds the Curie temperature of a magnet, metallurgical changes occur within the magnet and there will be unrecoverable losses.

Temperature Coefficient of Br (20ºC-150 ºC)


NdFeB(N38H)- 0.12% ºC                           
Alnico(anisotropic Alcomax III)- 0.02% ºC
SmCo(2:17)- 0.03% ºC
Ferrite(anisotropic)- 0.19% ºC


Maximum Working Temperature (No Irreversible Losses)

The working point in the circuit determines the maximum working temperature of a magnet. The higher the working point the higher the temperature the magnet can operate.


NdFeB(N38H)120 ºC                                 
Alnico(anisotropic Alcomax III)550 ºC
SmCo(2:17)300 ºC
Ferrite(anisotropic)250 ºC
Irreversible losses may be restored by remagnetizing the magnet.


Curie Temperature (Unrecoverable Losses Occur)

When the Curie temperature is reached, metallurgical changes occur within the magnet structure and the individual magnetic domains break down. Once these losses come up they cannot be reversed by remagnetizing.


NdFeB(N38H)320 ºC                                 
Alnico(anisotropic Alcomax III)860 ºC
SmCo(2:17)750 ºC
Ferrite(anisotropic)460 ºC


Effects of Sub-zero Temperature

Different material groups are affected by low temperature differently. In addition, the influence is closely connected with the magnet shape as well as its working point on the demagnetization curve.


NdFeB(N38H)                                                     No irreversible losses down to 77K
Alnico(anisotropic Alcomax III)

Permanent losses of no more than 10% are to be expected down to 4K

SmCo(2:17)Minimal losses down to 4K
Ferrite(anisotropic)Large irreversible losses below - 60 ºC


3. Effects of Exposure on Magnetic Stability

Although high temperature is the biggest threat to magnetic stability, exposure to high external fields also has an effect on certain types of magnets. The following table shows different degree of effects:


NdFeB(N38H)Very Low                              
Alnico(anisotropic Alcomax III)High
SmCo(2:17)Very Low
Ferrite(anisotropic)Low


Effects of Shock and Vibration

The earliest magnets were always affected by shock and vibration but now it has little effect on modern magnet materials, except for the most closely calibrated devices. However, mechanical impact will cause magnet materials to be brittle and fractured. SmCo is the most brittle one.

Effects of Radiation

Magnets are used within particle beam deflection applications and those with a higher Hci are more suitable to be used in such environments. According to some tests, SmCo has significant losses when it is exposed to high levels of radiation (109 to 1010 rads). For NdFeB, it losses 50% at 4x106 rads and 100% at 7x107 rads. Losses at low levels of radiation are basically the same as temperature losses. It’s remarkable that some magnet materials have Cobalt in them, and Cobalt can retain radiation after exposure.

Effects of Shape

The performance and stability of a magnet are also affected by its shape. The shape of the magnet determines its working point along the demagnetization curve. The higher the working point is the more difficult for the magnet to be demagnetized. Magnets that have a longer length or are used in a closed magnetic circuit have better performance and magnetic stability.

Some methods can be adopted to improve magnetic stability in performance, such as local demagnetization and high temperature aging treatment. After expose the magnet in advance to any possible detrimental influences, the unstable texture and magnetic domains disappear and the magnet can be magnetically more stable.

The total breakdown of composition will also cause loss of performance. Corrosion can break the magnet structure down, and for NdFeB, exposure to Hydrogen will lead to structural breakdown as well.

Effects of Time

Time has little effect on magnets and it is negligible. There is only a loss of less than 1 x 10 -5 per annum at 200 ºC on average. Actually, a period of 100,000 hour (11.4 years) causes no loss for SmCo while it just cause a loss of less than 3% for Alcomax III at low permeance coefficients.

4. Corrosion Resistance without Coating

Coating can prevent the magnets form being corroded. There are many protective coatings available. NdFeB magnets often have Nickel, Zinc, Lacquer, Epoxy or Parylene as a protective coating. Usually Alnico magnets don’t need coating, but powder coating and electroplating can be used when required.


NdFeB(N38H)Poor                                     
Alnico(anisotropic Alcomax III)Fair
SmCo(2:17)Excellent
Ferrite(anisotropic)Excellent


5. Price Comparison

There are several factors that affect the price of a magnet, such as shape, tolerances and quantity. However, the most significant effect is the cost of the basic raw material. When new sizes and volume production of magnets are required, tooling should be considered sometimes. Besides, fixtures are sometimes required for close tolerance machining.


NdFeB(N38H)High (x10)                             
Alnico(anisotropic Alcomax III)Medium (x5)
SmCo(2:17)Very High(x20)
Ferrite(anisotropic)Low(x1)


6. Properties of Magnetic Lines of Force

  1. Magnetic lines of force form closed curve which outside the magnet is directed from north pole to south pole and inside the magnet from south pole to north pole.

  2. Magnetic lines of force always seek the path of least resistance between opposite magnetic poles.

  3. Magnetic lines of force can never cross. They repel one another when they travel in the same direction.

  4. Normally magnetic lines of force always move along curved paths.

  5. Magnetic lines of force will always follow the shortest path through any medium.

  6. Magnetic lines of force always enter or leave a magnetic material at right angles to the surface.

  7. All Ferromagnetic materials have limited ability to carry lines of force. When they have reached their limit, they behave as if they were not there, like an air gap or similar.

7. Useful Design Suggestion

  1. Always pay attention to the working temperature of the material you need.
    Temperature has the most significant effect on magnetic stability, so always take it into consideration as part of your design and your material/grade choice.

  2. The strongest one may not be the best one.
    Beside flux strength, there are still many other factors in magnetic design to be considered.

  3. Magnet performance can be improved with a steel pole piece. 
    Sometimes it is useful to use a steel pole piece to help divert the flux to a more useful part of the magnetic circuit.

  4. Choose two poles instead of one pole for holding/attracting applications.
    As the flux won’t travel a long distance, the air gap between the magnet and the object to be attracted should be kept as small as possible.

  5. Replicating the application is the best way to test a magnetic device.
    There is not a simple single test that tells you everything about a magnet, so reusing the magnet in its application would be the best way for you know more and more about its performance.

  6. When you use rare earth magnets, focusing flux lines will be a challenge.
    Steel poles are not useful in this case.

8. Typical Applications of Permanent Magnets

Permanent magnets have wide and various applications in lots of industries. However, they can all be divided into several categories as the following:

  1. Converting Electrical Energy into Physical Motion
    Motors, speakers, actuators, meters and other instrumentation

  2. Converting Physical Motion into Electrical Energy
    Generators, sensors and microphones

  3. Producing Mechanical Energy
    Holding, attracting, lifting, 
    driving, conveying, repelling and separating 

  4. Mechanical to Heat
    Eddy Current and hysteresis drives

  5. Controlling Fields
    Annealing, plasma control, sputtering and NMR



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