X-band transparent stick for DIY electron spin resonance spectrometer - Page 1
Forum section: electronics / rf-microwave

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 18, 2017, 06:17:39 pm
content: Hi!I've been messing around with building an NMR spectrometer myself these last years, and now I've gone and started buying materials for the magnet.I've decided that step 1 is to get a ESR/EPR spectrometer working first, it's essentially the same, you're just kicking electrons instead of nuclei.So I'll have a cavity (lenght of WR90 waveguide) in the center of my magnet, a gunn diode in one end (driver, detector) and the sample in the other.I'd like to be able to move the sample a bit, as in the linked article, and I'm going to put a turnable plug in the bottom of the waveguide, and mount the sample on a small 'stick' to get it in the center-ish of the field.What could I make that 'stick' out of, so that it doesn't mess up my cavity?Beryllium oxide would probably be the 'proper' thing to use, but I reaaallly don't want to work with that.. The article below suspends the sample in a magnesium oxide pellet, it could be that MgO would work, if I can sinter a small rod, or maybe just compress one hydraulically?Thanks in advance!http://www.pci.tu-bs.de/aggericke/Lehre/Spektroskopie/ESR_NMR/ESR.htm

author: ConKbot
author_rank: Super Contributor
timestamp: May 18, 2017, 09:20:43 pm
content: MgO and BeO have a similar loss tangent, but a higher dielectric constant on MgO. If you can calibrate it out or your process would be insensitive to the mismatch, pressing a slug from high purity powder is an option, if the lack of binder isn't a problem.Polystyrene has a lower loss tangent and dielectric constant than both oxides however. If you can get a pure enough source, RF wise it would work. Compatibility with the NMR and ESR is beyond me though.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 18, 2017, 09:59:17 pm
content: BrO??Polystyrene might be a good option! I was also concidering PTFE or nylon but that's probably too high-density.If the pin was stationary, you could of course tune the cavity with it in place, but it's going to do a concentric movement around in there, I'm not so sure you could tune away from that.Pressing a slug definately sounds more human-friendly that scintering one!

author: ConKbot
author_rank: Super Contributor
timestamp: May 19, 2017, 01:23:21 am
content: Whoops, MgO, phone keyboard thought it knew better than me again.Also check to see if you can find dielectric constants and loss tangents near the frequency you're working at. The quick guide I was looking at didn't give any specifics as it was a comparison.Fused quartz also had very low loss, perhaps you can find a surplus lens, grind it into a rod and grind a small sample cup into the end. (Diamond cutoff wheel, flood coolant and lots of patience?)

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 21, 2017, 01:59:30 pm
content: Fused quartz sounds fancy! It's just not very easy to machine, so yeah, you'd have to find something surplus and adapt.PTFE doesn't look too bad, actually, from the RF side of things.If I machine a flush circular plug in the bottom plate of the cavity, and mount the 'stick' on that, what would mess up the cavity least: drilling a shallow blind hole, in which to mount the stick, or just epoxy it on top, and hope it stays put?

author: SeanB
author_rank: Super Contributor
timestamp: May 21, 2017, 04:33:06 pm
content: Drill the hole, as the epoxy will invariably have a high loss at RF, just because of the inherent properties of the resin. You can put a copper shim over the hole to minimise the error from it.Otherwise consider finding an old LNB and doing some rather nasty work to the ceramic substrate of it as a holder. Just etch off all of the traces with some aqua regia, and cut into strips to make the holder.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 21, 2017, 10:17:11 pm
content: QuoteThe article I'm sorta being 'inspired' by uses transition metal ions. Fe3+ should be good, along with some others. I think you can also see some larger pi-orbital systems, like conjugated double bonds, carbonyl groups and aromatic rings, but I don't know to what extent.You're right, the waveguide is just because it'd make a good cavity, and it just so happened that: - the frequency's around 12 GHz, I had a section lying around, and it fits snugly between my magnets, as seen in the image. I'm going to put a gunn diode in one end (either pick up a WR90 mount, or mount it IN the section shown), use that as both freq. gen, and receiver, and thus all the microwave stuff is neatly contained.I'm still working on how to mount a movable disc in the bottom waveguide blind, but that'll be next week's headache..Using a thin strip of pcb might be good, it's just tough to mount the sample in. unless you drill a hole, and then press the sample into a disc fitting that exactly..

author: LaserSteve
author_rank: Super Contributor
timestamp: May 22, 2017, 11:40:54 am
content: I've had good luck buying small samples of pure, powdered, polymers from a company called "Scientific Polymer Products, INC. ". Costs are reasonable and the quality has been good..Steve

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 22, 2017, 12:19:58 pm
content: Quote from: evb149 on May 22, 2017, 01:37:38 amThanks for the reading material! I'll give it a look! I'm not sure aqueous is the best way to go; I'd rather evaporate it down to a dry powder and then press it into a little rod or disc. Garden centres sell iron (II) sulphate (that's ferrous) for dealing with moss in grass, and I'm going to start by percipitating that as the carbonate, then let it oxidize in air (non-acidic ferrous solutions does so so fast it looks like they're rusting!"Thanks! The magnet U is a ferrous permanent magnet from a 1920's telephone dynamo that I've heat treated to be non-magnetic. The magnets are just neodidlium bought online, and the waveguide just happened to fit tightly in the gap! Complete coincidence. Field strenght will be around 0.7T, if i remember correctly. The waveguide is either copper, brass, bronze, or beryllium copper. Hard to tell, but red, atleast. I'm not completely sure how the gunn diode will couple, I honestly haven't worked with them before..

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 22, 2017, 09:41:35 pm
content: Oh, and by the way, there's a great section on constructing NMR/ ESR stuff in ' Experiments in modern physics ' :https://archive.org/details/ExperimentsInModernPhysics.I'm still unsure about how I'm going to do the modulation coil, though.This is the general idea, I'm thinking 10 turns on one magnet, and a simple low (maybe 24V) voltage AC supply.. Can always adjust and attenuate if necessary.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 29, 2017, 06:51:18 pm
content: I'm actually unsure how much modulation oompf I need. 10 windings is a bit low, I might try something a tad higher, and then just start low and raise power slowly untill I see the resonance. I'm also considering doing away with the oscilloscope and AC modulation alltogether; it's just a matter of sweeping a range of magnetic field, so if I used a slow chart recorder as my output device, I could just have a triggerable slow DC ramp gen sweeping once over the desired range, isn't that correct?I'm also unsure how neodymium behaves as a core material with a coil around it. Putting the modulation directly on the iron yoke, or Helmholtz'ing it might be beneficial. I think I'm going to build a simple relative hall-effect gaussmeter, so that I can verify that I can actually sweep across a range of magnetic field.The audio amplifier is a good idea, yeah, that looks like it's sorta the same conditions as a low-Z woofer. Maybe I should get a tube amp for it, you know, to get that warm tube electron spin resonance sound