Thin insulation for 400v plates?

Question

In a design of mine, I need to place two plates on top of each other as close as possible. With a 400v potential difference possible between the plates, what is the best material that can be placed in a very thin layer between the plates to keep them electrically isolated? I understand that there will be a degree of capacitance induced by such a setup, but it will be negligible for me.

I have thought about KAPTON tape. All of this is very small (< 5mm plates).

I found the following quote on DuPont’s website about their KAPTON polyimide tape:

Excellent dielectric strength makes Kapton® polyimide film a perfect material for thin electrical insulation applications. With a dielectric strength rating of 7700 V/mil for a .001”-thick film, no other material can provide such protection in such a small space.

Answer 1

In a design of mine, I need to place two plates on top of each other as close as possible. With a 400v potential difference possible between the plates, what is the best material that can be placed in a very thin layer between the plates to keep them electrically isolated? I understand that there will be a degree of capacitance induced by such a setup, but it will be negligible for me

If 0.002" is not too thck:

Per mil - ~= mica 3000 V, Teflon 1500 - 4000 V

So for 5 x 5mm square plates Mica seems "useful".

Digikey mica insulator
0.002" thick (6000 V nominal) - $0.42 in stock. MAkes maybe 4 OF 5 X 5 mm spacers.

Thinner may be available.

Answer 2

I ended up going with Anindo Ghosh’s suggestion to use PTFE. I had some lying around and it was a perfect solution for my temperature range and small size.

Source: Wikimedia Commons

The plumbers tape available from my local store claims a dielectric strength of 1.4kv/mil at 1Mhz. This is more than enough for my me, and at the lofty price of $1.99 it really can’t be beat.

Answer 3

If you make the plates out of aluminum you can use rubies or sapphires to separate the plates!

Fairly economical too, have the plates (at least one, but both might be safer) "hard anodized". The anodic oxide Al2O3 is the same material as the aforementioned gemstones and is very hard and an excellent insulator. It's also the insulator used in aluminum electrolytic capacitors. It can withstand high temperatures without degradation compared to plastics and is much less prone to chemical damage. From this design guide:

Anodic coatings typically exhibit a voltage breakdown of 900 to 1000 volts/mil, a dielectric constant of 6.4 to 6.6, and resistivity from 1014 ohm cm to 200 degrees centigrade. This resistivity is of the same order of magnitude as glass and porcelain. These properties make hard anodized aluminum an excellent insulation mounting for electronic components. In power transistor mountings, for example, anodized aluminum provides a lower operating temperature and longer life.

They mention a standard tolerance of +/-.0005 on a coating thickness of .002, which would yield a dielectric strength of 1350 to 2500V on a single layer and double that on two. Relative permittivity is a bit high, but you said you don't care much about that.