fivemack | All other nations have inferior potassium

I recently bought some samples of rare-earth elements from elementsales.com - gadolinium, terbium and dysprosium - to play with their magnetic properties. They're supplied as coins inside plastic discs, since they're reasonably reactive.

The gadolinium behaves roughly as I was expecting it to; it's quite strongly attracted to a magnet when cold, and less so when hot. I thought the Curie point was a sharp phase transition and the material would be non-magnetic above 19C, but the material sticks to a magnet even if I've freshly taken it out of hot water. I've been a bit wary since the Curie point of NdFeB magnets is only about 80C; I should get hold of a more-robust magnet. eBay has a very limited range of SmCo2 magnets (most hits for samarium-cobalt are guitar pickups); possibly I just want a large iron bar magnet, but I'm not quite sure where to buy those in the real world.

The terbium and dysprosium, however, are also attracted to the magnet (the Dy less so than the Tb) at room temperature. It's a fairly fearsome magnet, so I suppose that the Tb and Dy have some traces of Gd left in them and that's what's being picked up; in which case I should try boiling them and seeing how the magnetism goes away. I need to think more about how to measure the forces here; I can't think of a setup with magnet, element, spring-balance and bits of string where I can just read off the force, and a model where I pull on a spring balance until the element comes free of the magnet seems impossible to get good readings from.

I imagine a note to the element supplier saying that they are supplying inferior gadolinium-laced terbium would not be useful; separating adjacent rare earth elements is proverbially hard.

Any advice on better magnets, better terbium, or better experimental setup?

Flat | Top-Level Comments Only

A brief thought: www.periodicvideos.com often talks about the purity of its metal samples. I haven't watched those elements yet, but maybe they talk about the purity of their samples...

Thanks for the suggestion. They admire how nice their dysprosium sample is, but manage to have less fun with gadolinium than I did last night.

I wonder if they could use magnetism to separate adjacent rare earth elements. At least, if the material were a powder, the magnetic separation would separate the powder grains with less gadolinium in than the ones with more gadolinium.

Perhaps they did that, sold the ultra-refined terbium at a premium price, and then sold you the tailings.

It seems that ion-exchange is the separation method of choice - considering how much of a pain all of the other chromatography methods I've encountered are, that probably explains the expense of purification.

I wouldn't count on powder grains being small enough for the ratios of rare earth elements therein to vary very much.

I would have thought that terbium would have been one of the easier ones to separate, due to having a +4 oxidation state - however, on a little reading it isn't as nice as the +4 oxidation state of cerium.

Dysprosium, of course, is named after the fact that it's hard to get.

Does praseodymium do anything interesting? I just think it's the element with the funkiest-sounding name; it sounds like it should be a name for a captain of an alien spaceship on Star Trek.

That's 'has a +4 oxidation state' in the sense 'with hot enough fluorine at a high enough pressure you can make TbF4, which is then a useful perfluorinating agent'. I have a healthy fear of even low-pressure, cold fluorine, nor do I want to be even slightly perfluorinated.

Can you use a weaker magnet, or put a layer of foam rubber (or similar) underneath the magnet so that not-cold Gd won't stick to it?

The force experiment - something like this?


Anchor        coin glued to trolley
  ________           _
X-\/\/\/\/------======                NN===SS
  --------       O  O
   spring  non-ferrous trolley      magnet slowly approaching trolley

For an experimental setup - pendulum displacement? You should be able to come up with force by measuring angle off of vertical. You might need to add some non-reactive weight to the pendulum, but, IIRC, the math isn't bad at all.

I have measured magnetic forces on small paramagnetic beads. The magnet in question is a 17T helium-cooled superconducting magnet (usually used for levitation). I had a sensitive (mg) balance on a bracket attached to the ceiling, and a plastic frame around the bracket. The bead was suspended from the frame by a thread, adjustable in length. The bead was directly above the magnet, on the axis (field vertical). I tared the balance to read zero when the string was short and the bead far away from the magnet. As you lengthen the string, the bead hangs closer to the magnet and the reading on the balance is the magnetic force. It's quite easy to set up, and you can plot force as a function of distance (or of field if you know the magnet's field profile). I have a diagram in my thesis, which probably makes more sense than my explanation - if you like I'll email it to you.

I'm assuming that a commercial gauss meter is not going to have the necessary sensitivity or will have a higher price tag than you want to pay. Or I may be entirely mis-understanding what you want ot accomplish. In any case if you think it merits attention google has many listings and even plans for building your own.

As far as an iron bar magnet you might have good luck talking to a local machine shop. Many places have magnetic particle inspection to find cracks in ferrous parts that have been straightened. An inspection firm that performs MPI testing may also be able to help you. In either case the technique works by magnetizing the part with either a circular or a linear field and coating it with a flourescent magnetic powder. Discontinuities in the magnetic filed cause the powder to have a different orientation there which then appear under a black light. Typically the part is demagnitized after inspection, but for your purpose it is trivial to leave off the powder and leave the bar magnetized.

I did my final-year Physics project on the gadolinium phase transition; as I remember it the temperature--heat capacity curve was a cusp rather than a step, and fairly broad. (They took my handwritten report away to copy afterwards and never gave it back, sadly.)

Any advice on better magnets, better terbium, or better experimental setup?

No, but you can have some unsolicited finding-that-ever-so-endearing. :)

All other nations have inferior potassium

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