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ItemConformal sol-gel coatings on three-dimensional nanostructured templates(Georgia Institute of Technology, 2007-12-19) Weatherspoon, Michael RaymondA custom-built surface sol-gel pumping system was built for applying conformal sol-gel based coatings with controlled thicknesses on three-dimensional (3-D) nanostructured templates. The 3-D templates utilized in this work were derived from biological species, such as diatoms and butterfly wings, as well as a synthetic photoresist polymer (SU-8). Tin oxide coatings were applied on silica-based diatom frustules using the automated surface sol-gel pumping system. An organic dendrimer method was developed for amplifying hydroxyl groups on the silica-based frustule surfaces to enhance the surface sol-gel deposition process. Conformal tin oxide coatings with controlled thicknesses were obtained on the hydroxyl amplified frustule surfaces; however, little if any deposition was observed on the frustules that were not subjected to the hydroxyl amplification process. The automated surface sol-gel system was also utilized to apply multicomponent tin oxide-doped titania alkoxide chemistries on the wing scales of a blue Morpho butterfly. The alkoxide solutions reacted directly with the OH functionalities provided by the native chitin chemistry of the scales. The tin oxide served as a rutile nucleating agent which allowed the titania to completely crystallize in the high refractive index rutile titania phase with doping concentrations of tin oxide as low as 7 mol % after annealing at 450oC. The tin oxide-doped titania coatings were both nanocrystalline and nanothick and replicated the nanostructured scales with a high degree of precision. Undoped titania coatings applied on the scales required a heat treatment of 900oC to crystallize the coating in the rutile titania phase which led to adverse coarsening effects which destroyed the nanostructed features of the scales. Tin oxide-doped titania coatings were also deposited on 3-D SU-8 photonic crystal structures. The coating was crystallized in an acidic solution at 80oC which led to the formation of rutile titania inverse opal photonic crystal structures which maintained the overall structure and ordering of the template. Barium titanate and europium-doped barium titanate coatings were applied on diatom frustules using a conventional reflux/evaporation deposition process. The silica-based diatom frustules had to first be converted into magnesia/silicon composite replicas using a gas/solid displacement reaction to render the template chemically compatible with the barium titanate-based coating. Conformal titanate-based coatings were obtained on the magnesia frustule replicas possessing uncontrolled thicknesses and excess inorganic particles using the reflux/evaporation deposition process. The europium-doped barium titanate coated frustules exhibited bright red photoluminescent properties upon stimulation with an ultraviolet light source.
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ItemHydrothermal conversion of diatom frustules into barium titanate based replicas(Georgia Institute of Technology, 2007-07-10) Ernst, Eric MichaelNumerous organisms produce ornately detailed inorganic structures (often known as shells) with features on length scales from the nanoscale to the microscale. One organism, commonly referred to as a diatom, originates from algae and is found throughout the oceans on Earth. These diatoms possess skeletal structures, frustules, made from silicon dioxide. This chemical makeup limits the number of possible applications for which these structures can be used. Using a series of gas displacement reactions, these frustules can be converted to other useful materials, such as magnesium oxide and titanium dioxide, while maintaining the features of the frustule template. In the current research, silicon dioxide frustules were converted to titanium dioxide replicas using method previously devised by our group. The titanium dioxide replicas were subjected to a hydrothermal reaction by exposing the replicas to an aqueous basic solution containing barium hydroxide to form barium titanate and barium strontium titanate replicas. The effects of reaction temperature, time, and solution composition on extent of conversion were examined. The conventional method of converting titanium dioxide to barium titanate, using a convection heating oven, was compared with a microwave assisted heating method to study the advantages of using microwave heating over convection heating.
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ItemThe protein and peptide mediated syntheses of non-biologically-produced oxide materials(Georgia Institute of Technology, 2007-07-09) Dickerson, Matthew B.The research detailed in this dissertation is focused on the use of biomolecules (i.e., peptides and proteins) to form non-biologically produced materials under mild reaction conditions (i.e, neutral pH, aqueous solutions, and room temperature). The peptides utilized in the studies detailed in this dissertation were identified through the screening of single crystal rutile TiO2 substrates or Ge powder with a phage-displayed peptide library. Twenty-one peptides were identified which possessed an affinity for Ge. Those peptides possessing a basic isoelectric point as well as hydroxyl- and imidazole-containing amino acid residues were found to be the most effective in precipitating amorphous germania from an alkoxide precursor. The phage-displayed peptide library screening of TiO2 substrates yielded twenty peptides. The titania formation activity of these peptides was found to correlate with the number of positive charges they carried. The titania materials generated by the library-identified and designed peptides were found to be composed of amorphous titania as well as <10 nm anatase and/or monoclinic TiO2 crystallites. Four recombinant proteins, derived from the amino acid sequences of proteins (silaffins) associated with biosilicification in diatoms, were also investigated for titania precipitation activity. The two most basic of these recombinant silaffins, rSil1L and rSilC, were able to induce the formation of titania. The titania precipitates generated by rSil1L were found to be similar to those produced by the phage-displayed library identified peptides. The second recombinant silaffin, rSilC, was found to produce hollow spheres of titania, which, following dehydration, were observed to transform into larger, solid spheres composed of radially aligned columns of rutile TiO2. The highly repetitive nature of the rSilC s amino acid sequence is believed to be responsible for the differences in TiO2 polymorph generated by the different recombinant silaffins and peptides. This dissertation also details research conducted on the formation of titania utilizing rSilC conjugated to synthetic and biogenic silica surfaces. These silica surfaces were functionalized with a newly developed drendritic growth technique. The dendritic functional-group amplification process was demonstrated to increase the loading of hexahisitidine tagged proteins on silica surfaces by more than 40%, as compared to traditional immobilization procedures.
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ItemThe kinetics of incongruent reduction between sapphire and Mg-Al melts(Georgia Institute of Technology, 2006-04-03) Liu, YajunThe kinetics of incongruent reduction between sapphire and oxygen-controlled Mg-Al melts was studied by measuring spinel-layer thickness, sample-weight change and sample-thickness change as a function of time at various temperatures. To eliminate the crucible contamination caused by impurities in commercial MgO crucibles, self-made high-purity MgO crucibles were achieved by gelcasting method, which is an attractive ceramic-forming technique for making high-purity ceramic parts. The oxygen-controlled alloys were obtained by the three-phase-equilibrium experiments at various temperatures. To avoid MgO formation, the oxygen-controlled alloys prepared at relatively lower temperatures were used for incongruent reaction at relatively higher temperatures. That is to say, the oxygen-controlled alloys prepared at 900°C, 1000°C, and 1100°C were used for spinel formation at 1000°C, 1100°C, and 1200°C, respectively. The experiments were conducted in a vertical furnace, and sapphire wafers were hung vertically in high-purity MgO crucibles so that the natural convection induced by the density change in the melt could be investigated. Experimental results obtained at 1000°C, 1100°C, and 1200°C showed that the spinel layer thickness on two kinds of sapphire wafers, namely {0001} and , followed orientation-independent parabolic kinetics, indicating the diffusion in spinel was one of the rate-limiting steps. In addition, the spinel layer thickness was not a function of position. The results of sample-thickness- change measurements also indicated that the effect of natural convection could be neglected. XPS, XRD, and TEM were also employed to characterize some samples in this study. Based on a simple model where the diffusion in spinel was the only rate-limiting step, the governing partial differential equations for diffusion and fluid dynamics were solved by the finite element method. The calculated theoretical parabolic constants at various temperatures were compared with these experimental results, and a good agreement was obtained. Some preliminary studies were also made on the morphologies of spinel particles at the nucleation stage. It was found that the triangular {111} faces of spinel particles were parallel to the surface of {0001} sapphire substrate. The product shape was consistent with the tetrahedron composed of {111} faces. The morphology of spinel particles on a sapphire substrate was more complicated in that the triangular {111} faces of spinel had to be inclined at a certain angle to the substrate in order to maintain the orientation relationship.
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ItemNanocarving of Titania Surfaces Using Hydrogen Bearing Gases(Georgia Institute of Technology, 2005-05-18) Rick, Helene SylviaAn investigation of surface structures formed on polycrystalline and single crystal TiO2 (titania) samples having under gone various heat treatments in a controlled hydrogen bearing atmosphere was conducted. The study included the recreation and examination of the process discovered by Sehoon Yoo at Ohio State University to form nanofibers on the surface of polycrystalline TiO2 disks. Fibers were formed by heating samples to 700??in a 5%H2 95%N2 gas stream. The nanofibers formed during this processes are approximately 5-20 nanometers in diameter and can be 100??f nanometers long. The fibers do not actually grow on the surface, but are what remain of the surface as the material around them is removed by the gas stream V i.e., nanocarving. The mechanism of fiber formation and the effect of varying experimental parameters remained unknown and were explored within this study. This included changing gas composition, flow rate, and changes in sample preparation. The effect of isovalent doping and impurities within the starting powder were examined. Sintering temperature and time was investigated to determine the effect of grain size and surface morphologies prior to nanocarving. The effect of elevated temperature and 5%H2 95%N gas on the surface of TiO2 single-crystal wafers was also investigated. Test methods include Thermogravimetric Analysis (TGA), Mass Spectrometry (MS), Scanning Electron Microscopy (SEM), and X-ray diffraction (XRD) analysis.
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ItemReactive replacement and addition of cations in bioclastic silica and calcite(Georgia Institute of Technology, 2005-05-05) Allan, Shawn MichaelNumerous organisms produce ornately detailed inorganic structures (often known as shells) with features on length scales from 50 nm to several centimeters. One class of such organisms are the diatoms; microscopic algae that form silica frustules. Another group of algae, the coccolithophorids, produce similar calcium carbonate structures. Over 100,000 species comprise these two classes of algae, every one of which is endowed with a unique cytoskeleton structure. Using various types of displacement reactions, the chemistry of the original structure can be modified to produce a new material. Magnesium vapor has been found to displace the silicon in diatom frustules to yield an MgO structure. The conversion has been reported at temperatures from 650°C to 900°C. In the current work, the conversion and processing of silica frustules to MgO was examined in depth. The effect of reaction temperature on grain size and extent of conversion was evaluated. With the goal of obtaining high purity MgO structures, various methods for removing the silicon products of reaction were investigated. Wet chemistry and high temperature vapor etches were evaluated. The MgO reaction served as an intermediate step in the production of magnesium tungstate diatoms, which were imbued with photoluminescent properties. Reactions were identified to allow the conversion of calcium carbonate (calcite) structures to alternative chemistries. Calcite sand-dollars were converted to calcium tungstate or calcium molybdate by aqueous solution chemistry. In this process, sand dollar tests (shells) and coccolithophore frustules were reacted with ammonium para-molybdate or ammonium para-tungstate. The reactions were evaluated for shape preservation, phase purity, and photoluminescence of the structures.
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ItemDiatom Alchemy(Georgia Institute of Technology, 2004-12-03) Gaddis, Christopher StephenThis work resulted in the development of multiple distinct and novel methods of cheaply producing large numbers of biologically derived, complex, 3-dimensional microstructures in a multitude of possible compositions. The biologically derived structures employed in this work were diatoms, a type of single celled algae, which grow complex silica shells in species-specific shapes. Due to the wide diversity of naturally occurring diatom shapes (on the order of 105), and the flexibility in tailoring chemical compositions using the methods developed here, real potential exists for cheaply mass-producing industrially relevant quantities of controlled shape and size 3-d particles for the first time. The central theme of this research is the use of diatoms as a transient scaffold onto which a coating is applied. After curing the coating, and in some cases firing the coating to form ceramic, the diatom can be selectively etched away leaving a free standing replica of the original structure with the salient features of the pre-form intact, but now composed of a completely different material. Using this concept, specific methods were developed to suit various precursors. Dip coating techniques were used to create epoxy diatoms, and silicon carbide diatoms. The Sol-Gel method was used to synthesize zirconia diatoms in both the tetragonal and monoclinic phases. A multi step method was developed in which previously synthesized epoxy diatoms were used as a template for deposition of a silicon carbide precursor and then heat treated to produce a silicon carbide/carbon multi-component ceramic. A hydrothermal reaction was also developed to convert Titania diatoms to barium titanate by reaction with barium hydroxide. Finally, the device potential of diatom-derived structures was conclusively demonstrated by constructing a gas sensor from a single Titania diatom. Under suitable conditions, the sensor was found to have the fastest response and recovery time of any sensor of this type reported in the literature. Furthermore, this work has laid the groundwork for the synthesis of many other tailored compositions of diatoms, and provided several compositions for device creation.