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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Why not try Panasonic pyrolytic graphite sheet 25μm, which you can buy from Farnell - and no doubt other major distributors - in the UK. The 25μm is the thinnest plain pyrolytic graphite sheet: the thinner 10μm grade comes laminated to some kind of plastic material.
I've been thinking of trying it out as a window for Grenz rays myself, and in theory it looks quite promising. It's not toxic - unlike Be - and the fact that it has been engineered to conduct heat away sounds ideal window-wise, since part of your beam is always going to be dissipated as heat in the window. Think edge cooling.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
jpsmith123 wrote ...
The thermal characteristics of that material sure look good. What worries me is the relatively low tensile strength .
When you find out how much a miserably small sheet of very fragile Be foil costs, you'll soon find a way to make things hum with the pyrolytic graphite.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
I'm beginning to think that it is within the realm of possibility for a serious amateur/hobbyist to make free-standing graphene films at home. The main challenge in this case seems to be the ability to make them strong enough to be used as window material to hold off atmospheric pressure on one side against vacuum on the other, over an appreciable surface area, while being very thin.
I found a few papers that shed some light on the subject of the variability of the as-made properties of the films, e.g., tensile strength, conductivity, etc. Maybe later I'll post links to some of them.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
jpsmith123 wrote ...
I'm beginning to think that it is within the realm of possibility for a serious amateur/hobbyist to make free-standing graphene films at home. The main challenge in this case seems to be the ability to make them strong enough to be used as window material to hold off atmospheric pressure on one side against vacuum on the other, over an appreciable surface area, while being very thin.
I found a few papers that shed some light on the subject of the variability of the as-made properties of the films, e.g., tensile strength, conductivity, etc. Maybe later I'll post links to some of them.
You also have to be able to identify areas without any nano holes. I suspect that graphene films produced by the mechanical exfoliation of graphite (Sellotape!) would be highly susceptible to this.
Although Be foil is fiendishly expense by the standards of amateur science, the price would still be trivial compared to the costs of chemical vapour deposition or production by solution chemistry as per here:
If you want to bombard or irradiate something with your beam, it is often cheaper and simpler to place the object inside the vacuum apparatus, so the problem of absorption by a window does not arise.
You also avoid the problem of bonding and bedding in the window. With Be this is usually done by fusing glass frit, which requires equipment, knowledge and experience in glass work. I have no idea how one would create vacuum seals for graphene windows, and expect you will have to discover that for yourself. Good luck with it all! :)
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Here's a handy chart illustrating Be window absorption below 3 keV.
Given that the dose rate immediately in front of the window can easily be 30 Gy/sec and more with a meaty water-cooled diffraction tube, you'll see that heat production in the window becomes a serious problem below 3 keV. Cranking up the current to get more Grenz rays will very quickly pop it.
Using as a rule of thumb the principle that two thirds of the output of continuous radiation will be in the lower one third of the spectral range, you will see that window heating by absorption remains a major issue even with 15kV on the anode, since two thirds of the photon output will be < 5 keV.
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Another Vacuum Pump Gamble (from ebay)
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