Solar reflecting mirror having a protective coating and method of making same

Abstract

A solar reflecting mirror includes a shaped glass substrate having a focal area, a reflective coating over its convex surface and a sodium ion barrier layer over its concave surface. The shaped substrate has a strain pattern having a radial tension strain at the bottom area, and circumferential compression strain at the periphery of the substrate. As the distance from the periphery of the shaped substrate increases, the circumferential compression strain decreases to a “transition line” where circumferential tension strain begins. As the distance from the transition line in a direction toward the bottom area of the glass substrate increases, the circumferential tension increases. To compensate for the strain pattern in the shaped glass substrate to avoid buckling of, and surface cracks of, the barrier layer, the barrier layer including an oxide of silicon and aluminum thickness, among other things is varied on. A method of making the solar mirror from shaped sections is also discussed.

1 . A solar reflecting mirror having a curved reflective surface, comprising:
 a transparent substrate having a convex surface and an opposite concave surface, and   a reflective coating over the convex surface and an alkali barrier layer over the concave surface wherein the reflective coating reflects selected wavelengths of the electromagnetic spectrum.   
     
     
         2 . The solar mirror according to  claim 1  wherein the alkali barrier layer has mechanical and chemical protective properties. 
     
     
         3 . The solar mirror according to  claim 1  wherein the barrier layer is on the concave surface of the substrate and comprises an oxide of silicon and aluminum. 
     
     
         4 . The solar mirror according to  claim 3  wherein the barrier layer has a weight percent of silicon greater than a weight percent of aluminum. 
     
     
         5 . The solar mirror according to  claim 4  wherein the barrier layer comprises 15 atomic percent aluminum and 85 atomic percent silicon, and the film is deposited by magnetron sputtering vacuum deposition. 
     
     
         6 . The solar mirror according to  claim 5  wherein the barrier layer has a thickness in the range of 700-950 nanometers. 
     
     
         7 . The solar mirror according to  claim 1  wherein the transparent substrate is a soda-lime-silica shaped glass substrate having a focal area and the barrier layer is a sodium ion barrier layer. 
     
     
         8 . The solar mirror according to  claim 7  wherein the barrier layer has a first surface and an opposite second surface, and the first surface of the barrier layer is in surface contact with the concave surface of the shaped glass substrate and the second surface of the barrier layer is facing away from the concave surface of the shaped glass substrate. 
     
     
         9 . The solar mirror according to  claim 8  wherein the barrier layer comprises an oxide of silicon and aluminum, and the first surface of the barrier layer has a first weight percent of silicon and the second surface of the barrier layer has a second weight percent of silicon, wherein the first weight percent of silicon is different than the second weight percent of silicon. 
     
     
         10 . The solar mirror according to  claim 7  wherein the shaped glass substrate comprises at least two shaped glass segments maintained together to provide the shaped glass substrate. 
     
     
         11 . The solar mirror according to  claim 10  wherein each segment comprises (1/(total segments of the parabolic shaped glass substrate)) part of the parabolic shaped glass substrate. 
     
     
         12 . The solar mirror according to  claim 7  wherein perimeter of the shaped glass substrate comprises four corners and four sides. 
     
     
         13 . The solar mirror according to  claim 7  wherein the shaped glass substrate has a strain pattern comprising a radial tension strain at a bottom area of the shaped glass substrate, and circumferential compression strain at a periphery of the shaped glass substrate; wherein as the distance from the periphery of the shaped glass substrate increases in a direction toward the bottom area of the shaped glass substrate, the circumferential compression strain decreases to an area designated as a “transition line” where circumferential tension strain and the radial tension strain are present in the glass, and as the distance from the transition line in a direction toward the bottom area of the shaped glass substrate increases, the circumferential tension strain increases. 
     
     
         14 . The solar mirror according to  claim 13  wherein the barrier coating covers the concave surface of the glass shaped substrate and has a constant thickness. 
     
     
         15 . The solar mirror according to  claim 14  wherein the barrier layer has a thickness in the range of 60 to 100 nanometers, and a composition comprising an oxide of silicon and aluminum, and the reflective coating is a silver coating. 
     
     
         16 . The solar mirror according to  claim 13  wherein the barrier coating increases in thickness as the distance from the periphery of the shaped glass substrate toward the bottom area of the shaped glass substrate increases 
     
     
         17 . The solar mirror according to  claim 16  wherein the barrier coating is in the thickness range of 40 to 100 nanometers. 
     
     
         18 . The solar mirror according to  claim 13  wherein the barrier coating has a first constant thickness from the perimeter of the shaped glass substrate to the transition line of the shaped glass substrate, and a second constant thickness from the transition line of the shaped glass substrate to the bottom area of the shaped glass substrate, wherein the first constant thickness is different from the second constant thickness. 
     
     
         19 . The solar mirror according to  claim 18  wherein the first constant thickness of the barrier coating is in the range of 40 to 60 nanometers, and the second constant thickness is in the range of greater than 60 to100 nanometers. 
     
     
         20 . A method of making a solar reflecting mirror having a curved reflective surface, comprising:
 providing a flat transparent sheet;   shaping the sheet to provide a shaped transparent substrate having a convex surface and an opposite concave surface and a focal area;   applying a reflective coating over the convex surface of the substrate, and   providing an alkali barrier layer over the concave surface of the substrate.