Potting Compound for High Voltage Tube (X-Ray...) - Page 1
Forum section: electronics / projects

author: sixtimesseven
author_rank: Frequent Contributor
timestamp: December 02, 2019, 07:47:35 pm
content: HiI'm looking for a potting solution to get rid of the (vegetable) oils I used so far to keep the 55kv creeping all over ... well everywhere. Sneaky bastards those high powered electronsI want to build a case around a small 50W Microfocus XRay tube with either a steel case, aluminium and some lead reinforcement or a cast lead housing if I want to be really sure... However sealing everything + having room for expansion of the oil is a pain.First idea was something like paraffin but that will melt and potentially get messy. Also it burns. Normal potting would be good but I'm worried about a hot expanding glass cylinder under vacuum pressing against somewhat hard potting compound.So I'm looking for these clear, reenterable, self healing type (silicone?) potting compunds like they use in IGBT's, high voltage multimeters etc. which should be very flexible and still stay in place inside the case.Something like:https://www.intertronics.co.uk/product/optically-clear-silicone-potting-compound-opti-tec-7020/But available in 100ish gram quantities.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 02, 2019, 08:52:23 pm
content: You can't use any old potting compound at 55kV - it needs to be exceptionally inert and have a very high dielectric strength. You also need to vacuum degas it after immersion to eliminate any trapped air.I wouldn't think the re-enterable silicone would work here, but maybe. The thing is, do you want to find out it isn't up to the job the hard way? Paraffin wax is a safe choice, and so is mineral oil - the former is less messy but much more difficult to degas - otherwise, two-part silicone that releases methanol and ammonia during curing, rather than acetic acid, might work.For small quantities of silicone potting compounds I like MG Chemicals -https://www.mgchemicals.com/products/rtv-silicones/potting-compounds/- but I don't know if their stuff is available in Switzerland.

author: sixtimesseven
author_rank: Frequent Contributor
timestamp: December 02, 2019, 10:07:23 pm
content: Quote from: MagicSmoker on December 02, 2019, 08:52:23 pmDon't worry, I will test that before I put anything of value in there. The arc protection on my hv supply has to be tested from time to time...The 55kv anode is seperated by >62mm. That should be plenty "even" with an air gap at ~3kv/mm for air. However on glas even slight contamination leads to creepage/breakdown along the gas.Regarding potting: Even the "worst" compounds I have used for work had at dielectric strength in the order of kv/mm.For example the first compound I picked randomly on digikey claims up to 20kv/mm:https://media.digikey.com/pdf/Data%20Sheets/MG%20Chemicals%20PDFs/RTV11.pdf, that would be plenty.My thought was that gel / jelly like with maximum flexibility could be better and would allow the glass to expand without stresses.Any thoughts on how paraffin would behave. I mean it would start to melt around the tube - which is good since that would provide some cooling - But also the pressure of the molten wax inside the solid paraffin block would rise. My guess is that the glass tube is much stronger than the paraffin block which would break first but...

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 02, 2019, 11:11:22 pm
content: Quote from: sixtimesseven on December 02, 2019, 10:07:23 pmOkay, that's plenty of separation. I was thinking you might have to pot a Cockcroft-Walton multiplier or transformer secondary or the like where there might not be much separation between extreme potential differences.Quote from: sixtimesseven on December 02, 2019, 10:07:23 pmUh-huh... The problem here is the high electric field strength causes chemical reactions to occur that otherwise wouldn't (or only at an infinitesimal rate). Just because a material is rated to withstand 20kV/mm doesn't mean it can withstand itindefinitely.That said, properly cured silicone - usually requiring addition of a curing agent or catalyst in potting applications - has a good combination of high dielectric strength and chemical inertness. But, you have to vacuum degas it otherwise the dielectric strength in the void(s) is merely that of air...Quote from: sixtimesseven on December 02, 2019, 10:07:23 pmNo idea. Paraffin isn't particularly rigid or strong even when solid, but your concern is valid. FWIW, low to moderate power commercial x-ray tubes are usually potted in silicone while higher power tubes are almost always immersion-cooled in a high dielectric strength oil.

author: sixtimesseven
author_rank: Frequent Contributor
timestamp: December 02, 2019, 11:34:02 pm
content: Quote from: MagicSmoker on December 02, 2019, 11:11:22 pmHmm.. I did not know that. Thanks!

author: sixtimesseven
author_rank: Frequent Contributor
timestamp: December 04, 2019, 04:09:04 pm
content: I asked some vendor for recommendations and I got two compounds which I liked:1.https://res.cloudinary.com/electrolube/image/upload/v1573043823/SC4003E_t6gp9w.pdf2.https://www.mgchemicals.com/downloads/tds/tds-834fx-2parts.pdfFor availability reasons I went with the first option. That one is also slightly less hard, has better thermal conductivity and costs a bit less.

author: james_s
author_rank: Super Contributor
timestamp: December 04, 2019, 04:40:44 pm
content: You don't want to pot an xray tube. Electrical insulation is only one of multiple functions the oil has. The other major function is cooling, oil circulates via convection and cools the anode, absorbing heat and transferring it to the housing.

author: sixtimesseven
author_rank: Frequent Contributor
timestamp: December 04, 2019, 04:44:47 pm
content: Quote from: james_s on December 04, 2019, 04:40:44 pmCommerial counter example:https://microxray.com/products/siliconpottedxraytube/Edit: Ok, only 15W, mine has 50W. But that is for the continuous rating. I only want to do imaging to start with so short intervalls with time to cool down...

author: HighVoltage
author_rank: Super Contributor
timestamp: December 04, 2019, 04:49:12 pm
content: Quote from: james_s on December 04, 2019, 04:40:44 pmVery true!For high voltage insulation that also needs heat transfer for cooling, a silicone based potting is not a good choice.I would at least watch the temperature closely.Polyurethane or epoxy can be a much better option.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 04, 2019, 04:50:49 pm
content: Quote from: james_s on December 04, 2019, 04:40:44 pmHmm... I assumed that potting was okay for this tube, but after actually looking at the datasheets in the OP is states that this tube is supposed to be immersion cooled.

author: sixtimesseven
author_rank: Frequent Contributor
timestamp: December 04, 2019, 05:03:39 pm
content: Quote from: HighVoltage on December 04, 2019, 04:49:12 pmPolyurethane / Epoxy can be had with much better thermal conductivity. I've even seen some with added ceramic powders which have great conductivity. However, I wanted to go for a soft material because I was worried about the different temperature expansion coefficients and the stresses on the glas tube. Also that was what seems to work from the commercial examples I found...

author: sixtimesseven
author_rank: Frequent Contributor
timestamp: December 04, 2019, 05:09:36 pm
content: Quote from: MagicSmoker on December 04, 2019, 04:50:49 pmSure, that's the recommended way and probably necessary for continuous operation. However, I do not plan to run continuous and I plan to put a thermal sensor of some description in there to monitor what I get.Also, as stated building a housing which is oil tight - In particular along the feed troughs and making sure the oil expansion can be compensated is doable but orders of magnitude more complicated. And if I don't have to, why should I bother?

author: james_s
author_rank: Super Contributor
timestamp: December 04, 2019, 10:42:57 pm
content: Well, give it a try then, just beware of the limitations, there's a reason commercial xray heads invariably are filled with oil or SF6. Watch out for differing expansion coefficients too, I remember someone tried potting some nixie tubes years ago in clear resin and they all broke.

author: MadTux
author_rank: Frequent Contributor
timestamp: June 28, 2025, 09:22:16 pm
content: Oil expansion compensation is easy, just add a slightly larger, air filled rubber/silicone tube, sealed on both ends, to a non critical area and make sure, it can't move into a HV critical area.It's somewhat tempting, if you have old dentist x-ray head, where the oil has degraded, so it's arcing over, just to think about filling it with molten paraffin wax.But that darn melting expansion likely would crack the xray tube, if everything around is solid aluminum and the molten wax has nowhere to expand.On HV multipliers and other less delicate stuff, paraffin potting works just fine.So I'll fill my old xray head with some expensive transformer oil from ebay....

author: TimFox
author_rank: Super Contributor
timestamp: June 28, 2025, 09:35:14 pm
content: I found out the hard way that the expansion/contraction of wax (such as paraffin) as it goes through its melting point (increasing or, in my case, decreasing) is huge.

author: amyk
author_rank: Super Contributor
timestamp: June 30, 2025, 05:17:55 am
content: Quote from: TimFox on June 28, 2025, 09:35:14 pmThat's why it's used inhttps://en.wikipedia.org/wiki/Wax_motors.