Rui M. Almeida
article posted 21 March 2016
Rui M. Almeida is Professor of Materials Science and Engineering at Instituto Superior Técnico (IST), University of Lisbon, Portugal. He received his MS and PhD degrees from the University of California, Los Angeles (UCLA). He is the head of a research group at the Centro de Química Estrutural (CQE), at IST. His main research interests are in glass structure (studied by spectroscopic methods) and photonic materials, including glasses, photonic crystals and the processing, structure and properties of glasses and nanocrystalline materials in bulk and sol-gel derived thin film form, with special emphasis on the photoluminescence of lanthanide-doped photonic bandgap materials and structures. Materials processing by sol-gel methods include also bioactive scaffold materials for hard tissue regeneration.
He has been a Visiting professor at international universities like Penn State, UCLA and Lehigh (in USA), he is a member of the Advisory Editorial Boards of the Journal of Non-Crystalline Solids and Journal of Sol-Gel Science and Technology and an Associate Editor of the International Journal of Applied Glass Science and he is a Fellow of the American Ceramic Society and the Society of Glass Technology. He has published over 200 papers in international journals, he is the author/co-author of 10 book chapters and is the editor of four books. He is also on the organizing committee for several international conferences.
The atomic composition of silicate glass surfaces
Rui M. Almeida*1, Himanshu Jain2 & Carlo G. Pantano3
1 Instituto Superior Técnico, Lisboa, Portugal
Low-energy ion scattering spectroscopy (LEIS), a technique which detects only the top atomic monolayer of a solid surface, has been utilized to investigate the relationship between the composition of the as-cast glass surface, that of the glass fractured under vacuum and the nominal glass (batch) stoichiometry. LEIS measurements of binary and ternary silicate glasses (modified with Na2
O, CaO, SrO and BaO) have shown that the as-cast and the as-fractured glass surface compositions are different. While the former surface is usually depleted of alkali ions (Na+
) compared to the nominal bulk glass composition, probably due to their preferential evaporation from the melt, there is frequently a strong accumulation of the mobile (monovalent) modifier cations on the fresh fracture surface, due to their preferential shielding of oxygen and migration to the glass surface during fracture. Unexpectedly, in the presence of Cs+
ions appear as less mobile, despite the size factor. The Si/O ratio, however, does not usually deviate much from stoichiometry.
If the alkali ions and associated non-bridging oxygen (NBO) ions are not randomly distributed, a segregated distribution of alkali–NBO bonds may also have fundamental repercussions on how we treat the fracture of such model glasses. So, in particular, we have performed experiments to verify the validity of Greaves’ modified random network model of glass structure.
Depth profiles, obtained via prolonged sequences of short sputtering followed by LEIS analysis, suggest elemental concentration gradients normal to both the cast and fracture glass surfaces, although possible preferential sputtering phenomena complicate the compositional analysis. Once the interplay between ion scattering and preferential sputtering becomes better understood, these very effects will bring new knowledge about the chemical structure of glass surfaces.
Lehigh University, Bethlehem, USA
Pennsylvania State University, State College, USA