, membrane coating

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Andy Tomlinson2004-02-23 22:23

What conductive coating did Dayton Wright use on the mylar membrane after liquid soap failed?


Paul Young2004-02-23 22:23

After the membrane was stretched and bonded to one of the polystyrene stator frames (using a ”welding” technique - the plastic was soaked in acetone until the surface softened and then it was pressed against hard the stretched membrane while the acetone evaporated and the plastic solidified) the coating was applied using a silk-screen method combined with a custom blended colloidal carbon based ”paint”. We went to this paint due to problems with the soap coating just migrating over time off the membrane to somewhere (the stators?). A silk-screen was used to coat the membrane in thin stripes because we needed to keep the total resistance of the cell high enough that there would be close to zero charge migration across the surface at the lowest frequencies used (20Hz). If the surface resistance is too low, distortion is created as the membrane surface charge moves in response to the combination of the modulating electrostatic fields from the stators and the deflections of the membrane. However, we also wanted (with the XG-10) to change the coating from ”soap” to make it far more reliable and resistant to changes over a target operating life of 20+ years. We tried quite a variety of designs and chemicals to achieve the goals of: high ohms-per-square uniformity of resistance over both the surface and from cell-to-cell very high bonding strength to the membrane. In the end we had to blend our coating to a lower ohms-per-square resistance value than was desired - to achieve the degree of uniformity over the surface and from cell-to-cell. My memory isn't great on this part, but I think we were trying for a +/- 10 or 20 percent variability in both at max.. Had we used this coating over the entire surface, the cell resistance would have been too low and the forces on it would modulate at low frequencies causing waveform distortion. So we designed a simple silk-screen that put stripes of coating on the membrane. At each end of the cell (the short sides) there was a ”buss bar” of coating deposited that ran the width of the cell and was about 1 cm. wide. These coating buss bars were connected to each other along the long dimension of the cell using a series of parallel stripes about 1 mm wide separated by about 1 mm (a 50% coating density in effect). Using this method of coating we could achieve the target range of cell resistance. The raw material colloidal graphite coating that we used was purchased from a company called Atcheson (Atchison?) Colloids in the USA. We used some pretty evil solvents with it to both dilute it down to the correct viscosity for silk screening as well as to supply the chemical aggressiveness required to attack the mylar membrane to achieve a good bond strength after it evaporated. I will do my best remember the chemicals and resistance values - but since I have no notes left on these - don't blame me if they are not correct. They will be approx. at least: Solvents used with the colloidal graphite - hexanone(?), and Methyl Ethyl Ketone (M.E.K.) that is a base solvent for many commercial adhesives. NOTE - these things are very bad to inhale - be extremely careful - work outdoors if possible, and use an expensive organics suited air mask or separate clean breathing air supply!! Resistance values - basic coating (no stripes) was approx. 100K ohms/square, cell resistance (end-to-end) was approx. 100 Meg. ohms - not counting the series fixed carbon ”safety” resistor (re catastrophic arcing) that was connected between the EHT supply and one end of the cell at the bus bar strip. The other end of the cells stripes where they met the bus bar stripe was left floating. The design was a success re coatings because they were uniform, the distortion was very low, and nothing could remove that coating once it dried. I have a friend who is still using his XG-10s today and I built them for him in 1977. Hope this helps you.

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