Message-ID: <39AD45B9.1A256B2E@bellsouth.net> From: John De Armond Newsgroups: rec.crafts.glass Subject: Dichroic glass Date: Wed, 30 Aug 2000 13:34:49 -0400 "Peter A. Neary" wrote: > > Sorry if this is a bit hair-brained. But my father seems to think he should > be able to make dichoic glass using a tungsten filament to vapourize the > coating material in a vaccuum chamber. Is he right? If so any information > would be greatly appreciated. What coating material/ oxides? Vacuum > pressure? Does the vacuum chaber need to be heated? Why? > Thanks, Peter. Tungsten is a poor choice. Hard to control. A good metal for your first experiment would be titanium. Wire is available from Strem chemicals (http://www.strem.com). Titanium yields very pretty rainbow coloration, even in one coating. Of course, it will take some refinement and experimentation to achieve the nice solid colors that commercial thin film outfits produce but it's doable. I've done thin film deposition in my shop using little more than a modified pressure cooker, a low voltage power supply and a high vacuum system. The pressure cooker is modified by sealing all openings using high vacuum techniques (welding preferred), substituting a Viton O-ring for the gasket (lower outgassing at vacuum), adding a view port (cut a hole, high vac epoxy a hunk of thick glass over it.) and bringing two power leads in through homemade vacuum feedthu fittings (compression fittings with wire potted in the opening using high vac epoxy.) The high vacuum stuff (mechanical pump, diff pump, ion or cold cathode gauge, fittings, etc) can be had used at Duniway Stockroom, http://www.duniway.com. Figure $2k for this stuff. Here's roughly the procedure. Attach some titanium wire to the electrode on the pressure cooker lid. Chemically clean the glass using lab detergent (alconex recommended), then caustic soda followed by conc nitric acid. Bake @ 250 deg F for a couple of hours. Place in pressure cooker, held off the bottom a half inch or so with glass beads or some other heatproof, non-porous substance. Close up the chamber and start the vacuum system. Optionally, heat the chamber on a hot plate to oh, 200 deg or so. Not too high or the O-ring outgases. You should achieve 10 E-5 Torr vacuum. If not, look for leaks, outgassing components, etc. Valve off the vacuum chamber and bring the pressure up to about 3 Torr using dry argon. Establish a plasma discharge in the chamber to further clean the glass. I do this by coupling a magnetron from a microwave oven through the viewport. The entire chamber will light up with a blue glow. You should see the pressure rise a bit. After several minutes of plasma cleaning, open the vacuum valve and bring the chamber back down. Valve off the vacuum to keep vaporized metal out of the vac system. Apply current to the terminals so that the titanium wire heats enough to have a significant vapor pressure. This takes "a few" (less than 12) volts at several tens of amps. I use a large unregulated DC power supply and a variac. The proper temperature must be achieved experimentally unless you have an optical pyrometer (and used a sapphire window instead of glass.) The brightness level of a light bulb operating about 30% undervoltage is a good starting place. When the titanium is hot enough, it evaporates and the vapor travels to all corners of the chamber. It condenses to a thin film whenever it touches a cool surface. It coats all surfaces in the cooker but most of the heavy vapor coats the glass right under the wire. Again, experimenting is necessary to determine how much time is needed for each deposition. A dichroic coating is made up of up to 30 or so layers of thin films separated by dielectric (oxide) layers in between. (Very greatly simplified) The spacing is 1/4 wavelength of the color you want to reflect. In this simplified procedure, you're laying down metal layers. You have to repeat the vaporization procedure for each layer you desire to lay down. With titanium, only a few layers yield interesting effects. To achieve an oxide layer in this application, you can admit some oxygen to the chamber and then establish another plasma discharge. This will form atomic oxygen which oxidizes the surface of the titanium film. Again, much experimentation is required to perfect. If you're really interested in this, I suggest that your first step be to buy the book "Handbook of Thin Film Technology" ISBN, 0-07-039742-2. Sucker cost me $104 back in '92. Lastly, Several years ago, I briefly chatted with a couple in Atlanta who were doing Dichroic glass in their basement (or at least had the equipment - not sure how well it was going.) I have the name of Dave & Sherry Moser in Norcross, Ga. Sorry, don't have a phone number. If you want to do some digging, you might look them up and have a chat. John |