Neodymium Rare Earth Magnets

[gary350]

WHY are some rare earth magnets N52 some are N48 and some N35 being the weakest?

 

[JonSea]

Strength vs cost. Not a great mystery.

 

[dknguyen]

It's a tradeoff between strength and temperature rating.

 

[gophert]

WHY are some rare earth magnets N52 some are N48 and some N35 being the weakest?

Why are some people worth $8/hr, some $12 and some more than $100/hr.

 

[gophert]

It's a tradeoff between strength and temperature rating.

Temp? I don't think so. Temp has to do with the binder technology for the powdered magnetic materials. There is a separate code for temp rating on these magnets.

 

[gary350]

I was thinking it might have something to do with the chemical make up of the magnet or density of the magnetic material or the power used to charge the magnets. N52 is stronger than N48. Also maybe magnets are ordered by the user to have a specific strength. There has to be quality control to get each rating.

 

[dknguyen]

Temp? I don't think so. Temp has to do with the binder technology for the powdered magnetic materials. There is a separate code for temp rating on these magnets.

Yeah you're right. But your reasoning about the binder technology sounds a bit strange to me. How come the concerns I always hear about magnets at higher temperatures is demagnetization first, and never the disintegration of the binder material?

 

[gophert]

Yeah you're right. But your reasoning about the binder technology sounds a bit strange to me. How come the concerns I always hear about magnets at higher temperatures is demagnetization first, and never the disintegration of the binder material?

Because you are confusing alloy magnets with NdFeB magnets. The Curie Temp (demagnetization temp) of a powdered NdFeB magnet is way above the max operating temp of the binder. 400°F difference for the more standard binders.

 

[Les Jones]

This table does show that N52 magnets have a lower working temperature than the N35 &N48 magnets.
Here is a link to more information on neodymium magnets.
Les.

 

[gophert]

This table does show that N52 magnets have a lower working temperature than the N35 &N48 magnets.
Here is a link to more information on neodymium magnets.
Les.

Your first table must have a typo - your second table clearly states that the "N" class (lowest temp rating) means 80°C max working temp. I don't know why they show an N52 to be <65°C. If it is less than 80, it is not an N52.

 

[dknguyen]

Because you are confusing alloy magnets with NdFeB magnets. The Curie Temp (demagnetization temp) of a powdered NdFeB magnet is way above the max operating temp of the binder. 400°F difference for the more standard binders.

No, I know we're talking about NdFeB magnets here. By alloy magnets I assume you mean something like Samarium Cobalt which is more expensive and used when temperatures exceed what NdFeB can handle.

I have never heard of the binder being the temperature limiting factor before demagnetization for rare earth magnets. What is the actual temperature at the binder fails at? The Curie temperature might be 310C/590F, but that's irrelevant because that is when the magnet is completely de-magnetized and no one is ever going to operate a magnet anywhere near that point. Loss of magnetic strength occurs far earlier starting at 80C/176F which is considered to be the maximum operating temperature for common grades.

 

[gophert]

...

 

[gary350]

WHY is a 52 stronger than 48?

 

[gophert]

The powders used to make them are sintered to a higher density and less oxidation during the processing. A fraction of a percent of void space make a huge difference.

 

[gary350]

The powders used to make them are sintered to a higher density and less oxidation during the processing. A fraction of a percent of void space make a huge difference.

That is a good place to start the investigation. A 52 will be heaver than a 48 if both magnets are the same physical size.

 

[shortbus=]

Have a look here - https://www.jobmastermagnets.com/The-Difference-Between-Neodymium-Samarium-Cobalt-Ceramic While some don't give the composition of the magnets, and just call them "Rare Earth", they may have different 'rare earth' in them.

 

[gary350]

This is how magnets are made

 

[gophert]

Yup, a few things not covered are,
The powder needs to be mixed in the V-blender cause binder material is added to help keep the powder in the compressed shape. Usually a wax or oil.

Also, the guy explaining the process seems much more impressed with the high voltage to magnetize and then says "at 12.5 amps". If you look at the magnetizer, it says "12.5 kA" which explains a lot.

To achieve the highest possible grades,
1) after the jet milling process, the particle sizes are separated and then recombined to achieve the maximum possible density (e.g. combine 30% of a small particles with 70% large particles - large particles are 10x diameter of small ). Large particles must be same size or smaller than typical particles used for lower grade magnets so micron-scale powders must be used for "small" particle sizes. Everything must be done under inert gases once micron-scale particles are used to avoid oxidation.

2) the optimized packing of particle sizes makes compressing the powders into a shape much more difficult because the packed material does not flow well. Simple, single-hit compression does not work as well to get a dense "green" part. Additional tricks are used.

3) shrinking the little voids as much as possible after sintering involves a second, high pressure heat-treatment process (known as "HIPPING") to squeeze out the voids and achieve maximum density.

4) even higher magnetic field is used to magnetize the sintered + HIPPED parts which takes more time to charge and cool.