A collaborative effort is proposed to investigate the behavior of binary mixtures of disk-shaped clay particles and charged nanoparticles in aqueous solutions. The study is motivated by a unique sol-to-gel transition that was recently discovered by one of the PI's in mixtures of kaolinite clay particles and silica nanoparticles. The gels display a very open, porous structure in which the clay particles are arranged in edge-to-edge contact, while at the same time possessing significant yield stress. In addition, the gels show a remarkable ability to rejuvenate repeatedly and reproducibly after breakage by shear. These properties suggest a variety of beneficial applications. The project has two primary objectives. First, a comprehensive experimental investigation will be performed to determine the fundamental mechanism driving the gel transition. Specific issues that will be addressed include possible micro-scale phase separation between the nanoparticles and platelets, importance of deposition of the nanoparticles onto the faces and/or edges of the platelets, and the cause of the observed edge-to-edge arrangement of the clay particles in the gel. We will also explore the relationship between the measured rheological properties of the gel and the development of the microstructure. Major experimental tools to be used include field emission and environmental scanning electron microscopy, atomic force microscopy, and rheometry. Second, a variety of experimental tests will be conducted to measure the rheological, mechanical, and material properties of both the gels and the silica/clay composites obtained after drying and sintering. Knowledge of these properties is critical for the eventual development of applications for these unique materials. For the gels, their response to shear and normal stress, including their ability to repeatedly reform after breakage, will be probed. Drying and sintering these gels will produce silica/kaolinite composites with a very open, porous structure made of relatively inert materials. In addition to studying the actual drying and sintering process, measurements will be performed to determine the composite's microstructure, compression strength, surface area, and thermal properties. Completion of the proposed work will provide a thorough understanding of the gellation mechanism, as well as knowledge of the key microstructural and mechanical/functional properties of both the gel and the resulting silica/clay composite. This knowledge will also be valuable in understanding the behavior of other binary colloidal systems.
Broader Impacts of Proposed Work The project will be a collaborative effort involving one senior and one junior faculty from the Departments of Chemical Engineering, and Materials Science and Engineering. Each of the PI's has expertise in their specific tasks to be performed. There are a wide range of potential applications for the proposed materials, including catalyst supports, filters, membranes, and heat insulating materials. While the work will focus primarily on the silica/kaolinite system, the results would be applicable to any system displaying a similar type of gel transition or structure. The project will provide training to two graduate students, and undergraduate participation will be actively pursued. In addition, the PI's are heavily involved in increasing the enrollment of females and minorities in engineering, especially in "forefront" areas like nanotechnology. One example is our work with C-Tech2, a program focused on science and engineering that brings 30 to 40 high school female and minority students to the Virginia Tech campus for a two-week period each summer. The visiting students are provided with demonstrations and information on the importance of nanotechnology to society. The results of the proposed project will provide significant additional resources for this important program.
This project focused on developing a fundamental understanding of the properties and formation mechanism of a novel, porous, nano-composite material that had been discovered in the laboratory of one of the Principal Investigators. The material consisted of kaolinite clay particles and silica nanoparticles. Adding salt to an aqueous suspension of these components induced a sol-to-gel transition. This gel was then freeze-dried to produce a porous green body, which was then sintered at high temperature to produce a strong yet porous ceramic. An SEM image showing the microstructure of these composites is shown in Figure 1. This particular sample was created using an initial suspension containing 10% vol. kaolonite plus 8% vol. nanoparticles and then sintered for one hour at 1250oC. As seen the structure is quite open (porosity of roughly 70%) with pores that can be as large as 100 microns. It was found that part of the reason for the substantial increase in strength that occurred upon sintering was the formation of a new phase at the interface between the kaolinite and silica nanoparticles. While there are a number of potential applications for these materials (i.e., packaging supports, insulators), the one that was focused on in this project was as a scaffold to use in creating a separation membranes. Specifically, the porous and inert nature of the composite means that it should be possible to create surfaces with very controlled chemical and physical properties inside the scaffold which could be used, for example, to remove a specific contaminant from a liquid source. There were a number of significant advances to these basic materials that were made during the course of this grant period. The maximum strength of these composites occurred when the volume fraction of silica nanoparticles and kaolinite clay particles using in the initial suspension was approximately equal. This can be seen in Figure 2, which shows the flexural strength as a function of kaolinite concentration when the total volume concentration (kaolinite plus silica nanoparticles) was held constant at 18% vol. It can also been seen that a substantial increase in the strength is produced when the sintering temperature increases from 1000oC to 1250oC, which was found to result from the formation of a new interfacial phase. It was found that using silica rods instead of spherical silica nanoparticles when synthesizing these composites yielded materials with even higher mechanical strength without any reduction in porosity. One of the drawbacks of the basic scaffolds shown in Figure 1 is the relatively low surface area. To overcome this, a novel procedure was developed in which nanoparticles could be infiltrated into the finished scaffolds, followed by drying and a mild sintering to lock the nanoparticles into place. By repeating this procedure multiple times, composites with precisely controlled surface area could be produced. This can be seen clearly in Figure 3, which shows the specific surface area (surface area per mass of sample) versus the number of infiltration steps with either 12 or 22 nm nanoparticles. It was furthermore discovered that this surface area was maintained upon testing the composite as a water purification membrane, meaning that the nanoparticles were not displaced from the scaffold upon the application of a differential pressure. Overall, this project resulted in seven journal papers (including one review paper), two peer-reviewed conference proceedings, plus numerous technical presentations at international conferences. Significant progress was made in both our understanding of the properties of these materials as well as in development of modifications of the materials for specific applications.
