Brooks Instrument Blog

Did you ever look at a large expensive building covered in glass? That glass is usually coated with some exotic materials to reflect heat in either direction. Some glass manufacturers are even shipping self cleaning glass. If you look hard, some of those large glass panels have stripes on them. Builders and architects hate that; so do I.

The stripes are usually caused by variations in the thickness of the coating material causing light to reflect non-uniform angles. A glass coating company I recently visited was coating sample pieces and was struggling with that same problem. This glass company was vaporizing a super secret liquid precursor for coating glass. In this process the vapor traveled through a very hot slot (as wide as the glass) over hot moving glass where a surface reaction takes place causing it to bond with the glass.

The natural thought is that the glass is speeding up or slowing down when passing under the slot causing variations in the coating thickness. A lot of money was spent trying to correct this to no avail. The actual problem turned out to be incomplete vaporization of the liquid. When a vapor containing large droplets of liquid hits a hot surface it rapidly expands causing a sudden increase in pressure. These pressure spikes basically blow the vapor away causing thin areas in the coating as wide as the slot. These pressure spikes were happening at surprisingly regular intervals causing beautifully striped glass, (if you like that sort or thing).

So, we did the math on flow rates, vapor pressure, temperature and partial pressure and built a new Brooks direct liquid injection vaporizer that wouldn’t spit liquid. We installed the new vaporizer and coated some glass. Yikes no more stripes! High fives all around! I love seeing happy customers.