Thin insulation for 400v plates?
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.