The sol/gel process has recently received attention because it enables the production of pure and homogeneous glass without external heating, eliminating the high processing temperatures needed in conventional methods. A disadvantage of the process is that if the rate of evaporation during syneresis is too high, cracking due to mechanical shrinkage can be extensive. A slower evaporation rate produces less cracking, but results in a longer processing time. The key to decreasing processing time while simultaneously eliminating shrinkage cracks lies in monitoring the change of the mechanical properties during the evaporation process. Ultrasonic techniques are ideal for this purpose.
To monitor the process, an ultrasonic signal was passed through the material. Spectrum changes were measured and then converted to changes in modulus and viscosity, while shrinkage and density changes were monitored using an air-type transducer and an electronic balance. The results show no observable change up to the beginning of the gelation process, followed by rapid increase in the modulus and viscosity after the onset of shrinkage. This finding was confirmed using a viscometer and laser light scattering measurements. Because of the effectiveness of this ultrasonic technique in detecting changes in the material, such as the onset of cracking, it may be used in controlling and improving the process.