I need a solution for making transparent doors and windows opaque.
Curtains are not an option. I have seen windows and doors with gas
trapped between the panes. When a switch is flipped the gas is charged
and the glass becomes opaque.

I would like to know what these windows are called and if there are
companies that provide them.

While not using gas, I know of SwitchLite Privicy glass that has the
same characteristics.  SwitchLite alternates between clear and
translucent with a flip of a switch, but uses a LCD interlayer between
the glass panes to acomplish this.  SwitchLite is a subdivision of
Pulp Studios in Los Angeles.  You can view their website at

http://www.switchlite.com/home.html

This might interest you:

http://www.polytronix.com/privacyglass.htm

I've found a name for the type of gas-containing windows you
described: gasochromic windows.

http://www.commercialwindows.umn.edu/materials_tech3.html

http://www.solgel.com/articles/Sept01/ren_gaso.htm

I don't know of a commercial source for this type of window, however.

Thanks! That could be exactly what we need.