Thermionic emission cathode covered by barium oxide. - Page 1
Forum section: electronics / projects

author: Drirr
author_rank: Regular Contributor
timestamp: April 28, 2022, 11:24:10 am
content: Hi,I'm trying to make barium vacuum tube. Everything works fine but is there any better way to stick Barium oxide to tungsten cathode then nitrocelulose?Please do not laugh,i now that my vacuum system is just basic. I'll fix it..ThanksDavid

author: CaptDon
author_rank: Super Contributor
timestamp: April 28, 2022, 01:57:12 pm
content: Almost looks like you have just re-created the Eimac 15E vacuum tube. If you remove the grid you would have the Eimac 15R. They were used in WWII aviation radars. Cute little buggers and very collectable!!

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 28, 2022, 02:51:06 pm
content: Maybe electron microscope folks could help? Although I don't understand, I thought oxide cathodes are indirectly heated so you need to put the oxide on a nickel sleeve? I thought thoriated tungsten is the directly heated cathode material?I'm not laughing, I'd like to be there and work on whatever it is you're cooking up!

author: paul@yahrprobert.com
author_rank: Regular Contributor
timestamp: April 28, 2022, 03:03:18 pm
content: Google for "dispenser cathode". This consists of some kind of porous tungsten filled with a barium containing substance. It allows the barium to be replenished after it evaporates. Not sure it the amateur can buy a small amount for experiments.

author: Wolfram
author_rank: Frequent Contributor
timestamp: April 28, 2022, 03:04:13 pm
content: Very nice setup, I especially like the ultracompact turbomolecular pump. What sort of baseline pressure are you getting?Oxide cathodes can be directly heated AFAIK. Nitrocellulose doesn't seem like the worst choice when it comes to binders. Ethyl cellulose has been used in the past at least.Are you coating the barium on the cathode in the form of carbonate or oxide? What's the particle size of the raw material?Quote from: paul@yahrprobert.com on April 28, 2022, 03:03:18 pmDispenser cathodes are nice but much harder to make. Due to the low resistivity of the bulk porous tungsten, they need to be indirectly heated, which means you need an electrically insulated heater that can operate up to 1000 celcius or so, with minimal offgassing. Directly heated oxide cathodes are likely the simplest way to get good emission at moderate temperatures. Did you already get the tube working using raw tungsten? It takes a lot more power and has lifetime issues, but also less uncertainties to deal with.Edit: There's some more info on cathode coatings here, including formulations:https://www.cathode.com/pdf/tb-160%20thru%20164.pdf. Most of them are mixed Ba/Sr/Ca, applied in the form of carbonates that are subsequently converted via an activation schedule. If you look at T-33C-131 for example, it's a 57/39/4 mix, percipitated (from chlorides?) using sodium carbonate, with a nitrocellulose binder.

author: TimFox
author_rank: Super Contributor
timestamp: April 28, 2022, 04:16:03 pm
content: When I needed dispenser cathodes (at work) for an experimental x-ray tube, I believe I bought assemblies fromhttps://www.spectramat.com/product/dispenser-cathodes/. After evacuating the chamber, there was a well-defined process for activating the cathodes.

author: trobbins
author_rank: Frequent Contributor
timestamp: April 29, 2022, 07:03:31 am
content: You may find some useful comments buried in RCA's 1962 Electron Tube Design (http://www.mcmlv.org/Archive/TubeTheory/RCA%201962%20Electron%20Tube%20Design.pdf).

author: Drirr
author_rank: Regular Contributor
timestamp: April 29, 2022, 07:31:54 am
content: @ CaptDon This could be real challengeI'll try it.@ Alex Eisenhut as I know, most electron microscopes have only directly heated tungsten cathode, other photos of my experiments:https://photos.app.goo.gl/wYbdb4sPQm2PqsQG7@ paul@yahrprobert.com I am not that advanced yet, maybe in future. Thanks for idea.@ Wolfram I do not have Penning gauge installed yet, only Pirani gauge. I'll let you now final pressure after I receive KF16 "T piece". Pirani is not albe to measure that "high vacuum". Coating material is BaCO3 dust. I do not know exact particle size. It looks like fine dust. >>https://photos.app.goo.gl/ef1JWqp454gfs7jC8Thanks for info on cathode coatings! I'll try mixtures mentioned in list.@ TimFox thanks for info@ trobbins this one is valuable for me. Thanks a lot!

author: axawire
author_rank: Contributor
timestamp: August 19, 2024, 06:32:19 pm
content: How did your Barium oxide coating work out? I am interested in your process that you did in the end?Thanks.

author: LaserSteve
author_rank: Super Contributor
timestamp: August 19, 2024, 10:05:33 pm
content: Mixed carbonates are the way to go, unless you want to try your hand at Indirect Heating.There are "direct" heated dispenser cathodes. The tungsten powder is mixed with a barium or scandate compound and a plastic binder, then the spiral or other shape is CNC machined. The tungsten is sintered at that point, having an additive to make sintering possible.You might want to talk to Heatwave. The owner is a great guy.This is one of my all time favorite videos.https://youtu.be/ZuhapGSexygThe plant WAS 40 minutes to the north of me and I always wanted a tour.For lamp filaments and tube cathodes, usually the W wire is wound on a molybdenum core and the core is selectively etched away.Flame heating of the W prior to winding is also used,There is also:https://ucfilament.com/cathode-heaters-and-electrode-device-filaments/I work on and repair Ion Lasers. While not for Hobbyists, we use directly heated dispensers @ 3.2 VAC 27-35 Amps, all the time.Steve