Different steps of the deforming procedure: (A) initial glass block, (B) sawn glass plate, (C) and (D) stretched glass plates. Credit: Barascu; JACerS, Wiley.
Stretch forming is a manufacturing process generally associated with metal forming, not with glass forming, but a new paper by a team from Germany could change that. (Sometimes the term “stretching’ is applied to glass, but usually in the context of drawing down the molten end of a glass rod or tube to make a fiber or narrower rod, not as a plastic deformation process the way metallurgists think of stretching—see video example here.)
The team was interested in making a porous glass with an aligned pore structure from a phase-separated alkali borosilicate composition. According to the paper, a Rapid Communication in the October issue of the Journal of the American Ceramic Society, these types of porous glasses are thermally, chemically and mechanically stable, which makes them attractive for applications as sensors, membranes, catalysis, chromatography and more.
The porous structure is created by spinodal decomposition followed by chemical removal by the sodium-rich borate phase. However, by its nature, spinodal decomposition creates an interwoven, three-dimensional, randomly oriented porosity.
If the porosity could be aligned, at the very least, opportunities for porous glass applications could be increased for processes such as membrane separation, catalytic separation and catalysis. Porous glasses used for sensors could have much quicker response times. And, with pore alignment, the glass loses its opacity, which would allow for applications where transparency is important.
The German team had a simple and elegant idea—why not stretch a piece of glass while it is exposed to the spinodal decomposition temperature regime so that the two-phase network would stretch as it forms? Then, when one phase is dissolved out, an aligned porous network should be left behind.
The idea worked.
They were able to achieve varying degrees of aligned porosity, depending on the process parameters. Glass plates were sliced from a larger block and subjected to temperatures that varied from 923-953˚C (the load was kept constant) and stretched until the sample broke. (As the picture shows, the samples exhibited the necking feature typically associated with plastic deformation of metals.)
As would be expected, the time it took to stretch to fracture was much shorter at higher temperatures because of the lower viscosity. However, even though stretching was faster at high temperatures, there was less time for alignment of the porosity. According to the paper, “It is important to recognize that the higher temperatures of stretching and, accordingly the shorter the period of times of the stretching process, lower the achieved degree of pore alignment.”
Also, pore diameter varied considerably, from 1-1,000 nanometers, which the authors attribute to the kinetics of phase separation, lower stretching temperature, more time to failure and more time for phase separation to take place.
The relationship between temperature and time to failure is far from linear. They note that even though they studied only a 30-degree temperature range, the time to failure ranged from 80 minutes to 50 hours, which explains why pore size distributions varied so much between the samples. It also means that pore size can be tuned with by selecting the right stretching temperature, and perhaps composition and load, too.
The paper is “Preparation of Porous Glass Monoliths with an Aligned Pore System via Stretch Forming,” A. Barascu, et al., JACerS, doi: 10.1111/j.1551-2916.2012.05394.x.