Glass - Back to the Future!

Presenting Author:

article posted 30 March 2016

Dr. Peter SundbergInorganic Chemist, Laboratory Manager at Glafo.

Use of reactive gases in the tempering process towards ultra-thin glasses for solar energy applications

Peter Sundberg*, Jerry Eriksson and Stefan Karlsson
Glafo the Glass Research Institute, PG Vejdes vg 15, SE-351 96 Vxj, Sweden

The LIMES project (Light Innovative Materials for Enhanced Solar Efficiency) is a fruitful collaboration to optimize many different properties in state-of-the-art solar glasses for photovoltaic (PV) modules. The bulk optical properties are optimized by adding fluorescent elements that absorbs harmful UV-radiation while increasing the overall efficiency in the visible part of the solar spectrum. Functional coatings that are both anti-reflective and self-cleaning are being studied.

In the current work package we are investigating routes to achieve thin thermally toughened cover glasses with enhanced mechanical and chemical properties. A promising route to achieve this is by thermal toughening in reactive gases.

Initially standard thermal toughening is optimized in a laboratory setting. Optimization is achieved by heating the glass specimens to temperatures above 660 C and quenching them while measuring the temperature profiles in both the glass center and near glass surface during this thermal cycling. Initial measurements are done using thermocouples placed in the glass center, while a methodology using pyrometers at different wavelengths will be presented. A set of measured temperatures a re shown in Figure 1.

Figure 1:
Measured temperature profiles by TC during thermal strengthening of glass:
thickness 8 mm
dummy sample
forced convection

In optimizing our laboratory settings the glass strength is evaluated using a combination of stress measurements using Scattered Light Polariscope ( SCALP) and ball drop tests. In Figure 2 an initial test shows compressive stresses at the surface of about 100 MPa, making the glass three-four times stronger than an ordinary annealed glass pane. Compressive stresses above 100 MPa are a requirement for safety glasses.

For thin glasses it is insufficient to toughen by thermal quenching due to difficulties in obtaining a sufficient thermal gradient in the glass. In our work we are using an innovative approach of heating and a subsequent rapid quenching in an atmosphere of reactive gases. A setup for heating and quenching in a reactive gas atmosphere will be presented. Initial tests have been made using aluminum precursors. The setup allows full heating and full quenching with the addition of a reactive gas atmosphere. The challenge is to establish a chemical gradient in the glass surface at these extremely short times at moderate to high temperatures for the glass treatments. The outcome from the reactive gas treatment is evaluated using a combination of analytical techniques.

Using the Surface Ablation Cell (SAC) is a suitable technique which is suitable for evaluating diffusion of all ions into a glass matrix. The analytical technique is straightforward and it can be used without expensive instrumentation for screening purposes and for quantitative work. In figure 3 we show the outcome from a measurement using the SAC technique to establish the aluminum gradient in the glass surface. The results obtained are from a first study of treating the glass for a few minutes in a reactive gaseous atmosphere made by use of an aluminum precursor. The Al2O3 content in the surface layer doubled, i.e. increased by almost 1 weight-%, during this treatment. The effect from the treatment with aluminum reactive species goes in 0.5 m into the bulk.

The influence on mechanical and chemical properties by these surface reactions will be studied in the project and presented in this communication.

Figure 2: The stress profile in the treated glass using SCALP.
Figure 3: The aluminum distribution in the outermost surface layer as measured by SAC.