Construction of Nonconductive Internal Components of Vacuum Systems

[ian_krase]

Simple fusors don't usually demand too much complexity in their internal construction. The grid goes on the end of the feedthrough stalk.

Things get more complicated if you're making ion guns, plasma diagnostics, accelerator systems such as BOTtles, or complicated ion/electron optics that don't really have much to do with fusors. If you do that, you need to have a bunch of metal parts that are mechanically connected but electrically insulated, all in the vacuum. And it appears to be a bit tricky to do this, and do it compactly.

Ways to do this as far as I know, include:

- Macor or Mycroy / Crystex machinable ceramic -- expensive, requires machine shop -- these are ceramic materials that machine somewhat like an even dustier and more brittle version of cast iron. Macor has a thermal expansion coefficient close to that of various 400-series stainless steels, which might be useful. All of them are quite expensive in any reasonable sized pieces -- there seems to be a bunch of off-brand Macor on Ebay though. Obviously these have all the advantages of ceramics in a vacuum. They also tolerate quite a bit of heat.

- PEEK, Ultem -- Insanely expensive -- PEEK is a very hard, strong, low-outgassing, chemical-resistant, high-temperature plastic. Ultem is much like PEEK but brown, translucent, and just less so, and also often much cheaper (but still expensive enough to discourage casual experimentation). And so they're usable in vacuums. Ultem sometimes comes up cheapish on Ebay. I wonder, though, how practical they are under plasma.

- 3D Printed PEKK or Ultem -- moderately costly, hard to do at home -- Stratasys Direct offers 3D-printed parts made out of PEKK, a very similar material to PEEK, or Ultem. Minimum cost for a part is about $40, and many parts of significant size and complexity can be manufactured for under $200. Make sure you don't have high definition mode turned on, it will triple the price. I have never tried this under vacuum, I worry that the somewhat porous FDM plastic construction might make for massive virtual leaks and/or enhanced outgassing. Also, NASA has released a paper on how to modify a hobbyist-grade 3D printer such as a Prusa to print Ultem.

- Ceramic or PEEK standoffs: --availability, cost, machining needs-- Threaded standoffs made of ceramic material or of vacuum-compatible plastics are available. Unfortunately, they are often very expensive, and making compact assemblies may require machining and/or cutting plate. Re-usability is a plus. You may also need to worry about virtual leaks from the trapped gas in the threaded holes, unless you drill or side-grind your screws.

- Glass Blobs -- skill, material constraints, strength, equipment -- If you're making a vacuum tube and already making lots of glass-metal seals, it does make some sense to just make more glass-metal seals to blobs of glass to hold your metal in place. Many small CRTs and vidicon tubes tend to have this.

- Epoxy blobs -- is this even a good idea -- We all know of Hysol 1C. If you add a bit of MgO powder to it, you can make it into a stiff paste rather than a soft semi-liquid glue. It's then possible to mold it around things and into shapes. I am extremely skeptical as to whether this is compatible with good vacuum quality.

- Terminal blocks -- You can get PEEK or ceramic bodied terminal blocks.





I am curious whether anybody has more experience with this, or better methods/ideas/materials.

[Dennis P Brown]

I've had great success machining Macor (I am a very poor machinist, too) and frankly, in many ways, this is easier to machine than aluminum (for me, at least) - making 0-ring groves is much easier, in fact. Macor isn't cheap - especially any larger pieces - but it is very easy to work with and handles o-ring seals and epoxy very well and its thermal properties are quite stable. When unsure, it is excellent to use as a first try if one knows machining and needs electrical insulation pieces or higher temp projects. If one can't use a lathe, then buying off-the-self and modifying or paying someone really is your only choices.

[Dan Knapp]

I’ve had some experience with Macor. It is easy to machine, but in my application I experienced arcing through above about 25 kV. I’ve been told that the machining introduces “microcracks” that need to be “healed” by firing after machining. Apparently this is also routine for machined (by grinding) alumina.

Another polymer insulator is Vespel polyimide. It machines easily, but is also very expensive. It can tolerate a rather high temperature for a polymer (I’ll find the actual number), but it won’t tolerate plasma exposure.

The ultimate material is boron nitride (sometimes referred to as “white graphite”). It is very easy to machine but very, very expensive (several hundred dollars for a 1” dia x 6” long piece). I’ve seen some mention of refiring after machining this material, but I don’t believe this is generally done. The Wisconsin group has used BN for feedthrough stalks.

Prior to using BN, the Wisconsin group used “blue nylon,” a MoS2 filled nylon normally used for its self lubricating properties.

[Mark Rowley]

No kidding on the expense of Macor. A surplus 3"x3" piece of rod stock seems to be about $200.

Mark Rowley

[John Futter]

Boron nitride is cheap if you get it from china
only catch is that you will spend around US$1000 minimum
this is due to minimum buy requirements
I got 100 x 5/8"rods 1 foot long for us$ 1200
you need to organise a group buy
I used these people https://www.innovacera.com/

[Bob Reite]

You can get boron nitride rods, sheets and bars from McMaster-Carr. Not cheap, a one inch by 12 inch long piece will set you back $376.79 plus shipping.

[ian_krase]

I'm pretty sure that Vespel is expensive enough to make PEEK look like polyethylene. I have never even considered using it.

One material I didn't' mention is Teflon, which is significantly cheaper than PEEK. However, its poor physical properties make it undesirable for many purposes. It might be better in the glass-filled version.

That's a huge quantity of rather cheap CBN -- what were you doing with it? Commercial project? Sadly I doubt that buying $1000 worth of white slippery stuff is a good plan for me.

Seriously, why do all the technical ceramics have to be so expensive?

Also, what's CBN like? Is it brittle? (i.e. in a more fragile, more glassy way than i.e. Macor is?) Or have delamination problems?

Is "Blue Nylon" actually a good vacuum material? Even for being a vacuum barrier?

[Jerry Biehler]

You use PVC if you machine off the outer surfaces where the mold releases are. It does not outgas much. There is polyamide (kapton) and polysulphone. I have used caption all over in vacuum systems.

There's alumina ceramic too. The stuff they make heat shrink out of, polyolefin, works pretty well too. And your various rubbers like nitrile.

A lot of things outgas but at the vacuum levels of fusors there is not much to worry about. I would be more concerned about the material handling the higher temps than anything else. If you are going UHV then things change a lot and everything become stainless and ceramic and you're pumping with ion pumps.

Hysol is fine for vacuum. Its been used for decades.

[John Futter]

When I was talking about boron nitride I was talking about the hexagonal variety, not the cubic
cubic is expensive and is not machinable
Hexagonal is very soft and machines like cheese you must protect bedways from it s another name for it is white graphite.
Things like lath back stops etc suddenly do not lock to bed if you put BN powder on bedways. Drill press tables jusr slide to bottom of pole etc
So I use it for insulators, miniature furnaces that turn co2 into graphite for our Accelerator mass spectrometer for C14 dating. I have also machined the rod into furnace boats. The uses go on nearly forever. If you drop a rod is usually does not break -- ie it is tough

[ian_krase]

Oops. I actually meant Hex BN, I just accidentally wrote 'CBN'. I guess it might not abrade bedways like Macor dust will.

Nice! That's awesome. I just wish it was easier to get in small quantity....

Kapton is of course a thing but I've never heard of any polyimide that was in billets thicker than thin film and wasn't crazy expensive.


I understand Hysol is fine but I'm skeptical about my Hysol/MgO mix clay and also the prospect of having so much of it exposed instead of in a bond gap. Also because I do things other than fusion Wear high vacuum is actually desirable.