Plastic screws

It's just a thin, thermal-via-infested PCB of about 15 mm tall x 20 mm wide.

It'll sit on an aluminum spreader, which sits on an 9 x 11 mm TEC (Marlow NL1025T).

The issue is how to maintain a compressive preload on the TEC that will help prevent damage due to shock or fat fingers.

Cheers

Phil Hobbs

On Jul 17, 2019, Phil Hobbs wrote (in article):

keeping

How much vacuum does a SEM need? I was thinking bake-out being needed.

If FR-4 is good enough for that vacuum, then use FR-4 stepped washers, steel hardware, and belleville spring washers to maintain tension despite thermal variation.

Or use steel washers, and stagger them to make the thermal path long and windy.

Note that delrin is very slippery, and so delrin nut and bolt will tend to unscrew itself. A SS nut forced to thread onto a slightly oversize delrin rod should stay put. As will a press fit into an undersize hole.

The speed nuts are called Tinnerman nuts, as someone else also mentioned. Tinnerman invented this kind of nut.

Joe Gwinn

How about plastic screws and either coil springs or o-rings to control the squeezee force?

Fishing line and zip-ties are nylon, which is no good, by maybe polyester or linen thread would work

It would seem easier to just use a heat-pipe (if you can find a suitable fluid) and put all the TEC in atmosphere.

At least with the TEC in a good vacuum you'd get no moisture condensing on/inside it.

I was wondering if one can buy peltiers with the edges hermetically sealed (not with silicone rubber) and a vacuum inside. I'm guessing that sealing the edges with thin stainless foil brazed to the ceramic might work, and that sealing the edges with just frit and ceramic might be not flexible enough to withstand differential thermal expansion of the two ceramic plates.

Crazy idea: sandwich the TEC between big NdFeB magnets. Lots of compressive force and no heat leak. OTOH I find those magnets sometimes like to disintegrate to powder after a few years. I guess it might be due to poor plating quality. I presume that for the better suppliers it is a solved problem or they wouldn't use them in hard drives.

You could put just a piece of iron on the side of the TEC facing inside the vacuum, and a big magnet on the outside of the vacuum chamber.

bout 5

s a

abouts.)

SEMs use magnetic fields to focus (and scan) the electron beam.

Two big NdFeB magnets on either side of a TEC won't have a lot if external field, and - since it would be a dipole field, it would decay rapidly with distance - so it might be worth a try, but I wouldn't be too optimistic.

One could use spiderwire, which is carbon fiber? (but silly ideas)

George H.

For a one off, I could probably get what ever plastic you want to try and try using a die to cut threads on it.

I am in Delaware and there is a MSC and Grainger outlet near by so no shipping charge and no sales tax.

A friend has machine shop and we could try making a injection mold. He has a job machining PEEK so lots of free non virgin PEEK available. Dan

Some fishing line is uhmw pe.

Dan

Nah, SEMs work in the low microtorr range. Most don't even use a turbopump, because diffusion pumps are good enough and don't cause vibration.

UHV ( If FR-4 is good enough for that vacuum, then use FR-4 stepped washers, steel

Washers don't provide enough insulation, because they're short and fat.

Hard to get a factor of 100 that way.

Thanks!

Cheers

Phil Hobbs

The design is for a customer, so they probably wouldn't appreciate the Betsy Ross approach. ;)

Probably. Two-stage TECs are sort of disappointing because all but the last stage run at fairly high Q-dot.

I'm leery about using a heat pipe in a SEM--if it started leaking with the column valve open, the resulting glow discharge could damage the gun, which is super big bucks to fix.

Looks like OFHC copper will be required, but will be good enough.

Cheers

Phil Hobbs

It's easier than that. You just put a MEMS pressure sensor on the board, and when the pressure comes on-scale, you turn off the HV to the detector and run the cold plate up to room temperature.

Cheers

Phil Hobbs

Very nice, thanks!

Amazon swallowed Small Parts Inc. some years ago, and still seems to be in the biz.

Cheers

Phil Hobbs

At the moment I'm leaning towards a larger plastic piece held down by SS screws, clamping the PCB to the TEC.

Turns out, after quite a lot of math, that the best TEC for the job is an 80-junction Marlow one, the NL1025T. At 9x11 mm, it's quite a bit smaller than the board, so something more stable will be required to keep the preload even.

Cheers

Phil Hobbs

Interesting idea, I don't know.

Cheers

Phil Hobbs

Right, a 1T magnet inside an SEM chamber. That'd go over real big. ;)

Cheers

Phil Hobbs

Are M3 polycarbonite screws of any use?

A piece of rod or offcut PEEK with a threaded blind hole in it jammed on the end of a normal stainless steel bolt might be good enough.