Yue, Yuanzheng

Zhang, Y.F.

Zhao, D.H.

English

    Aalborg UniversitetSub-Tg Enthalpy Relaxation in a Hyperquenched SiO2-Al2O3 SystemZhang, Y.F.; Zhao, D.H.; Yue, YuanzhengCreative Commons LicenseUnspecifiedPublication date:2017Document VersionPublisher's PDF, also known as Version of recordLink to publication from Aalborg UniversityCitation for published version (APA):Zhang, Y. F., Zhao, D. H., & Yue, Y. (2017). Sub-Tg Enthalpy Relaxation in a Hyperquenched SiO2-Al2O3System. Abstract from 8th International Discussion Meeting on Relaxations in Complex Systems, Wisla , Poland.General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            ? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            ? You may not further distribute the material or use it for any profit-making activity or commercial gain            ? You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access tothe work immediately and investigate your claim.Downloaded from vbn.aau.dk on: December 25, 2020Sub-Tg Enthalpy Relaxation in a Hyperquenched SiO2-Al2O3 System  Y. F. Zhang1,2*, D. H. Zhao2, Y. Z. Yue1,2,4 1 School of Materials Science and Engineering, Qilu University of Technology, Jinan 250353, China 2State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China 3Unifrax I LLC, Tonawanda, NY 14150, USA 4Department of Chemistry and Bioscience, Aalborg University, 9220 Aalborg, Denmark *E-mail: zhang-yanfei@hotmail.com   Abstract: We study the sub-Tg enthalpy relaxation in an extremely unstable binary 69SiO2–31Al2O3 (SA) glass against crystallization.1 We reveal the structural origin of the extremely unstable feature of the studied glass by exploring the change of the medium-range structure during the relaxation process. This is done by using hyperquenching-annealing-differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) and high-resolution transmission electron microscopy (HRTEM). The DSC results show a non-symmetric energy release peak with a long tail at the lower temperature starting from the onset temperature for the hyperquenched SA glass. With increasing the degree of sub-Tg annealing, the tail gradually evolves into a peak and the peak becomes broader. This phenomenon is quite different from that of many other hyperqenched glass systems, in which the long tail gradually disappears with increasing sub-Tg annealing. Using NMR, we find that the contents of both the 5- and 6- fold coordinated Al species in the hyperquenched SA glass decrease with sub-Tg annealing, while the 4-fold coordinated Al species increases. This leads to the formation of the oxygen triclusters, which can act as the mullite nucleation sites and facilitate the formation of mullite. This is confirmed by the ordered structural domains at nano-scale as shown in the HRTEM images. These ordered domains can lower the activation energy for further structural ordering during the sub-Tg annealing, and thereby lowers the energy barrier for crystal growth. In addition, the extremely unstable behavior of the studied glass could also be attributed to the higher content of the 5- fold coordinated Al (~ 63%). The triclusters also indicate the structural heterogeneity of the supercooled liquid. This work provides insights into not only the structural evolution during the sub-Tg relaxation but also reveal the structural origin of an unstable glass system in terms of the structural heterogeneity. REFERENCES 1. Zhang Y, Zhao D, Yue Y. Phase transitions and glass transition in a hyperquenched silica-alumina glass. J. Am. Ceram. Soc. (2017) DOI: 10.1111/jace.14880. 

Sub-Tg Enthalpy Relaxation in a Hyperquenched SiO2-Al2O3 System

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Sub-Tg Enthalpy Relaxation in a Hyperquenched SiO2-Al2O3 System

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