When a variety of materials that makes up the vacuum insulated glass, such as glass, stainless steel, getter, and glass solder, are exposed to air, the surface of the material will absorb and dissolve a certain amount of gas. It later becomes the source of gas released by the materials sealed between the layers of the vacuum insulated glass. Vacuum insulated glass is the type of small volume vacuum unit that has a large surface area. A piece of 1 m2 vacuum insulated glass has an inner cavity with a volume equivalent only to the size of a ping-pong ball. Therefore, the gas released in the chamber of the vacuum insulated glass will have a great impact on the level of vacuum. For vacuum insulated glass degassed and sealed under room temperature, its level of vacuum will degrade rapidly within hours. The degassing process of vacuum insulated glass requires not only to fully discharge the gas inside the chamber, but also to bake the entire piece of glass to a certain temperature to release the gas absorbed by the surfaces and dissolved in the materials to the cavity for degassing.
As glass is the main material that makes up the vacuum insulated glass, the degassing temperature must first consider the degassing requirements of the glass material. The gas sources of glass during baking come primarily from its surface, surface layer, and inner layer.
1. Glass surface
There is a large amount of OH- existing on the surface of the glass, which has a very strong affinity to water. As such, the glass surface attracts a large number of water molecules and a small amount of CO2. Some gas is bonded with the glass surface only through physical attraction or weak chemical attraction. Heating it in vacuum to approximately 150℃ will release the gas molecules in a couple of minutes.
2. Weathered surface
The sodium calcium glass used in vacuum insulated glass contains a considerable amount of alkaline oxide which has poor chemical stability and is susceptible to moisture corrosion and weathering. The weathered layer is generally a few microns thick. The gas contained is mainly H2O. When the glass is heated to a certain temperature in a vacuum environment, the water will be released smoothly from the Si-OH-OH-Si structure to form Si-O-Si+H2O. The weathered layer of the glass is then restored. The gas in the glass surface can be removed by baking in a vacuum state for about 1 hour. The weathered layer can also be removed by baking in dry air with a humidity level less than 60% for about 1 hour. The effect is comparable to that of vacuum degassing.
3. Inside of glass body
The body of the glass contains a large amount of gas, mainly H2O and a small amount of CO2, O2 and SO2. It takes certain amount of time to bake and degas.
After the degassing process, as long as the working temperature of the glass does not exceed 300℃, the discharge of gas in the glass has minimal effect on the level of vacuum of the device and can therefore be neglected.