Developing alternative sealing materials in fabrication of evacuated glazing at low temperature

Abstract

Very low heat transfer rates through multiple glass pane windows can be achieved by creating a vacuum between the glass panes and depositing low emittance coatings on the glass surface. This involves forming a vacuum tight seal around the periphery of the glazing, and creating a high vacuum (<10 𝑏 𝑟) within the glazing. Arrays of tiny support pillars are placed between the glass panes to maintain the separation between the panes which would otherwise touch under atmospheric pressure.

To date there are two principal methods reported in the literature which are widely used in the fabrication of functioning vacuum glazing i.e. solder glass and metal sealing techniques. Since the solder glass sealing process is usually a high temperature process (450ºC-530ºC) it restricts the use of tempered glass in the fabrication of vacuum glazing. Alternatively, metal sealing techniques are typically low temperature processes (≤200ºC) which enables the use of tempered glass panes in the fabrication of vacuum glazing. However, the high price and availability of these semi-precious metals makes this method of fabricating vacuum glazing less economically viable.

The aim of this project is to develop a novel method of fabrication for a new generation of vacuum glazing which have a high thermal performance, a potential life span of more than 20 years and a process capable of being exploited by industry in cost effective manner. In order to achieve this goal the main focus is to develop an alternative sealing system and the required fabrication methodology.

An analysis of the current sealing methodologies which have been adopted for vacuum glazing has been undertaken. Improving and optimizing the current metal based sealing methodology has been undertaken. Characterisation of the thermal performance of vacuum glazing samples using a hot box calorimeter was undertaken. An optical system was developed which provides a quick and easy way to study the internal pressure of vacuum glazing and characterise their thermal performance. A crucial step in this project was to develop a new reliable and effective pump out system and pump-out hole sealing methodology for vacuum glazing. A range of double, triple and hybrid vacuum glazing using indium sealing techniques and annealed glass panes were fabricated and their thermal performance was characterised. The hybrid vacuum glazing samples were tested in situ and subsequently re-characterised for thermal performance which showed some level of degradation.

Optical ageing and a long term indoor tests were carried out on a number of indium sealed vacuum glazing samples to test the durability of their thermal performance. After ageing tests the heat transfer coefficient of the samples increased averagely by 37.5%.

A new sealing system with a sealing temperature of less than 200oC was developed and a number of vacuum glazing prototypes using this method with tempered and annealed glass were fabricated and their thermal performance was characterised. In this fabrication method adhesives were used to bond glass panes around their periphery. To prevent the adhesive from outgassing to the evacuated space between the glass panes a metal barrier was created between the adhesive and the internal vacuum space. After the adhesive cured the glazing assembly was ready for evacuation through the developed pump-out system. Since the fabrication process was carried out at a relatively low temperature (150ºC) it was possible to take advantage of tempered glass properties and a wide range of low-e coatings. The prototype vacuum glazing exhibited an overall U-value of 0.86 Wm^-2K^-1 in the centre of glazing region. The fabrication method is reproducible and has potential to be exploited in a vacuum glazing production line.

Date of Award	May 2014
Original language	English
Supervisor	Trevor Hyde (Supervisor) & Yueping Fang (Supervisor)