Mechanical Properties of Gels; Stress from Confined Fluids

Scherer, George W.

Department of Energy

关键词: Leaching;    Origin;    Heating;    Sodium;    Fibers;   

DOI  :  10.2172/968308
RP-ID  :  DOE ER45642
RP-ID  :  FG02-97ER45642
RP-ID  :  968308

美国|英语

来源: UNT Digital Library

【 摘 要 】

Abstract for Grant DE-FG02-97ER45642 Period: 1997-2002 Mechanical Properties of Gels 2002-2008 Stress from Confined Fluids Principal investigator: Prof. George W. Scherer Dept. Civil & Env. Eng./PRISM Eng. Quad. E-319 Princeton, NJ 08544 USA Recipient organization: Trustees of Princeton University 4 New South Princeton, NJ 08544 USA Abstract: The initial stage of this project, entitled Mechanical Properties of Gels, was dedicated to characterizing and explaining the properties of inorganic gels. Such materials, made by sol-gel processing, are of interest for fabrication of films, fibers, optical devices, advanced insulation and other uses. However, their poor mechanical properties are an impediment in some applications, so understanding the origin of these properties could lead to enhanced performance. Novel experimental methods were developed and applied to measure the stiffness and permeability of gels and aerogels. Numerical simulations were developed to reproduce the growth process of the gels, resulting in structures whose mechanical properties matched the measurements. The models showed that the gels are formed by the growth of relatively robust clusters of molecules that are joined by tenuous links whose compliance compromises the stiffness of the structure. Therefore, synthetic methods that enhance the links could significantly increase the rigidity of such gels. The next stage of the project focused on Stress from Confined Fluids. The first problem of interest was the enhanced thermal expansion coefficient of water that we measured in the nanometric pores of cement paste. This could have a deleterious effect on the resistance of concrete to rapid heating in fires, because the excessive thermal expansion of water in the pores of the concrete could lead to spalling and collapse. A series of experiments demonstrated that the expansion of water increases as the pore size decreases. To explain this behavior, we undertook a collaboration with Prof. Stephen Garofalini (Rutgers), who has developed the best simulations of water ever reported by use of molecular dynamics. Simulated heating of water in small pores provided quantitative agreement with experiments, and showed that the origin of the high expansion is the altered structure of water in the first two molecular layers adjacent to the pore wall. The final focus of the project was to understand the damage done by crystals growing in small pores. For example, the primary cause of damage to ancient monuments in the Mediterranean Basin is growth of salt crystals in the pores of the stone. Salt may enter stone as a result of capillary rise of groundwater, by leaching of mortar joints, deposition of marine spray, or reactions with atmospheric pollutants (such as oxides of nitrogen or sulfur). As the water evaporates, the salt solution becomes supersaturated and crystals precipitate. Stress results, because the salt usually repels the minerals in the pore walls. Our goal was to identify the factors contributing to the repulsion, so that we could develop a chemical treatment to reduce the repulsion and hence the stress. (We have recently demonstrated an effective treatment as part of a separately funded study.) In collaboration with Prof. Garofalini, molecular dynamics simulations have been done that correctly reproduce the structure of water around dissolved ions of sodium and chloride. We simulated the interaction between crystals of sodium chloride and quartz, and found that this particular system exhibits attractive forces, in agreement with experiment. The origin of the attraction is the orientation of dipolar water molecules near the surfaces of the crystals. Similar calculations now must be done in systems, such as potassium chloride and quartz, where the interaction is repulsive. This grant supported the education of two doctoral students, Hang-Shing Ma (Ph.D., 2002) and Melanie Webb (Ph.D. expected 2010), three post-doctoral researchers, Joachim Gross, Gudrun Reichenauer, and Shuangyan (Sonia) Xu, and five undergraduates (for senior theses or independent projects), Shawn Ryan, Mary Yang, Matthew Gill, Lindsay Karfeld, and Greg Simmons.