They don't show a crystallization peak because the kinetics of nucleation and growth are so slow that you don't observe it in DTA/DSC within the time of your measurement. If you annealed right at the nose of the TTT diagram for a long enough time, eventually you would see signs of crystallization. This obviously doesn't apply to systems that fundamentally don't crystallize (some types of polymers).

I am more familiar with polymer glasses, where there are so-called semicrystalline (e.g. isotactic polypropylene) and amorphous (e.g. atactic polystyrene) polymers. In case of amorphous polymers you see only a small bump in the cp curve as you pass the glass transition from below. If the glass has undergone physical aging (volume and enthalpy relaxation) you will see a small endothermal overshoot. If the polymer is semicrystalline and cooled down very fast, you will see the cp step followed (later) by an exotherm ascribed to crystallization, followed by a melting curve.

I think the in inorganic glasses alsot there are compositions that cannot crystallize at all, and there are others which can (e.g. vitroceramics). I think that this depends on thes symmetry of the structur and on the temperature dependence of the viscosity curve.

Thanks Dr. Banhegi.... Do you mean symmetry of the structure of the crystallizing phase?? Some the conventional glasses like soda-lime-silica or borosilicate glass do not show any crystallization event....Those are "strong" glasses....their viscosity-temperature curve are different....than that of many other speciality glasses.............

Dear Dr Biswas,

As I said, I am not a silcate glass chemist, but a polymer scientists, so the bets thing I can do is to look for some papers realted to the crystalliation of borosilcate glasses. Here are some of them:

http://www.ceramics-silikaty.cz/2003/pdf/2003_02_056.pdf

http://link.springer.com/article/10.1007%2FBF00258167?LI=true

http://journals.cambridge.org/action/displayFulltext?type=1&fid=7942560&jid=JMR&volumeId=7&issueId=11&aid=7942558

http://www.wmsym.org/archives/1988/V2/71.pdf

http://onlinelibrary.wiley.com/doi/10.1002/9780470538371.ch7/summary

http://144.206.159.178/ft/1034/36743/633409.pdf

https://www.researchgate.net/publication/228090411_Advances_in_understanding_the_structure_of_borosilicate_glasses_A_Raman_spectroscopy_study

http://pdf.lookchem.com/pdf/05/2005_03_153.pdf

http://www.ingentaconnect.com/content/els/00406031/2001/00000373/00000001/art00456

http://connection.ebscohost.com/c/articles/77263416/synthesis-characterization-barium-borosilicate-glass-alo-composites

http://www.sciencedirect.com/science/article/pii/S0040603113000051

Unfortunately I cannot save your time to read these and other articles.

Please also find attached some papers on the relation between melt structure and crystallization in silicate glasses:

http://www.annualreviews.org/doi/abs/10.1146/annurev.ea.11.050183.000451?journalCode=earth

http://www.if.pwr.wroc.pl/~optappl/pdf/2005/no4/optappl_3504p829.pdf

You should identify the exact composition of the glass system you are interested in, then you shoulf look for papers o the DTA of similar glasses. Those probably contain relevan information.

I hope some experts of the field might give you closer answer.

Article Advances in Understanding the Structure of Borosilicate Glas...

They don't show a crystallization peak because the kinetics of nucleation and growth are so slow that you don't observe it in DTA/DSC within the time of your measurement. If you annealed right at the nose of the TTT diagram for a long enough time, eventually you would see signs of crystallization. This obviously doesn't apply to systems that fundamentally don't crystallize (some types of polymers).

Thanks Dr. Banhegyi for your effort.....Thanks Dr. Evenson....

Thanks Professor Toufik

Zach, Can you please provide some reference of the statements you made. I also found similar results and need an explanation.

Off the top of my head, there's an article called "A Kinetic Treatment of Glass Formation" by Uhlmann, Journal of Non-Crystalline Solids 7 (1972) 337-348. This can give you general idea of what I meant by my statement (although the article doesn't really talk about DSC/DTA). In general, however, you'd have to look into nucleation theory. "Phase Transformations in Metals and Alloys" by Porter and Easterling would be an absolute beginner's guide to some of the fundamentals. For something more advanced and comprehensive, you could look into "Nucleation in Condensed Matter" by Kelton and Greer - there is even discussion on devitrification, which would be more applicable to your question.

Also, looking back at my previous statement, I guess you wouldn't necessarily need to anneal right at the nose of the TTT-diagram in order to see crystallization - it's just that the kinetics would be the fastest there.

Some borosilicate glasses (Pyrex type for example) exhibit glass-glass phase separation and dont crystallize in a DTA run . Even if you use a slow heating rate.

However many glasses nucleate at the surface and if you want to HELP crystallization to occur you should use a smaller particle size powder. If you use a fine powder crystallization DTA peaks occur at lower temperature and the crystallized fraction is larger (at the same T).

Using powders doesnot assure crystallization to occur but can help. Many times you dont see a crystallization DTA peak in a bulk sample, but you have a crystallization peak using powder. (This indicates that crystallization is mainly SURFACE crystallization)

There are some works of Ralph Muller about it. (Müller, R. Journal of Thermal Analysis, Vol. 35 (1989) 823-835)

There is also an interesting work of Colmenero (JNCS) about using DTA to determine critical cooling rates of glasses using DTA.

Prof. Prado, thanks for your answer ...Do you expect any trace in the DSC thermograms related to the phase separation in case of borosilicates glasses which you have mentioned..............

Hello Kaushik Biswas

I would not expect an endothermic or exothermic event, although of course there is a change in free energy G. What you can see, perhaps, is a change in the baseline slope.

Thanks a lot Dr. Prado for your answer