Study on Hermetic EDGE Sealing of Anodic Bonding for Vacuum Glazing
|School||Harbin Institute of Technology|
|Keywords||Vacuum glazing Heat transport mechanism Anodic bonding Electrostatic fore|
Even fill gases such as Krypton and xenon are used, thermal transmittance U value due to convection and conduction cannot be reduced to below 1Wm-2K-1. One attractive possibility to essentially improve the insulation properties of a glazing is to evacuate the space between the glass panes. A vacuum glazing of two glass panes, connected by an airtight edge seal, is evacuated to a pressure of below 10-1Pa. The panes, each coated with a highly infrared-reflecting layer to minimize thermal radiation, are supported by a matrix of spacers to prevent collapse.In vacuum glazing, four distinct heat transfer mechanisms contribute to the total thermal transmittance of the glazing: thermal conduction through residual gas, spacers, edge seal as well as radiation heat transfer between the two panes. Because of the high-temperature edge sealing technique which imposes limitation of using low-emissivity coating, the current commercial vacuum glazing cannot attain a U value lower than 0.9Wm-2K-1. In this paper, anodic bonding was used as an airtight edge seal for vacuum glazing. Such process will allow the use of highly efficient low-emissivity-coatings (0.04) to reduce thermal radiation, which would provide a U value of vacuum glazing approximately 0.5Wm-2K-1 calculated here.Conductive materials (such as metal and semiconductor) and insulators (such as glass and ceramic) could be anodically bonded together under the function of voltage and heated temperature. Anodic bonding is a fast and low-temperature bonding technique, what’s more, the materials could be anodically bonded strongly without any media between. This motivated us to further look into the possibility of using anodic bonding as a sealing method for vacuum glazing. An initial study was designed to determine its viability for the application under the standard atmosphere,it turned out a promising solution. Trials of anodically bonded pairs of Al and glass, Sn-Ag based solder and glass, Sn-0.5Al solder and glass were conducted in vacuo (～5?10-2Pa). Bonding assessment of the specimens such as leakage rate, ultrasonic testing, toughness of interface, SEM and EDS analysis were conducted. The results demonstrated that only when provided a voltage and temperature morn than a given value, anodic bonding would happen, the current in the bonding circuit increase with the increase of voltage and temperature. Oxygen anions in glass and cations in metal diffused to each other and an alkaline depletion layer was found in the glass adjacent to the anode. Al can form strong bonds in conjunction with glass. However, Al-sheets of thicknesses which are relevant for vacuum glazing edge sealing cannot produce leak-tight joints. Sn-3.5Ag and Sn-3.0Ag-0.5Cu alloy bond to glass weakly and had many defects. Sn-0.5Al alloy melt at 300°C in vacuo, the squeezed out liquid solder adapted to glass well and formed strong bond. For the sake of the squeezed out“fresh area”, a solder dispenser was designed to distribute the liquid Sn-0.5Al solder on glass surface. A leak-tight anodically bonded specimen was fabricated using this solder dispenser (leakage rate better than 2?10-10mbar l-1 s-1).In addition, the electrostatic fore generated between the two materials during anodic bonding was investigated. The polarization of glass under the function of electric filed is a key process during the bonding. The electrostatic pressure having a magnitude of several MPa pulls two materials into intimate contact.