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ItemCharacterization of open celled metal foams(Georgia Institute of Technology, 2011-01-26) Lin, Stephanie JanetOpen cell metal foams are a type of engineered material can be characterized by high porosity, high strength to weight ratio, tortuous flow paths and high surface area to volume ratio. It is the structure that gives the metal foams the characteristics that make them well suited for many application including heat exchangers. In this work, the structure of open celled metal foams is quantitatively characterized using an image analysis based method in order to predict the evaporative heat transfer of the metal foam using the fluid permeability. Several image processing algorithms were developed to quantitatively characterize the porosity, surface area per unit volume and the tortousity of metal foams from digital images of the cross sections of the material, and an expression was used to calculate the fluid permeability. An algorithm was developed to partion the pore space in the digital images so that individual cells within the structure could also be quantitatively characterized. Tools were also developed to predict the structure of open celled foam processed using the sacrificial template method by digitally constructing microstructures based the particle packing of the sacrificial templating material.
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ItemZnO nanocones and nanoplatelets: synthesis and characterization(Georgia Institute of Technology, 2010-08-12) Chang, YanlingNanowire structure plays an important role in the development of nanotechnology. However, further study shows that the shape of nanowires may not be the ideal morphology for some applications such as solar cells and sensors. Thus, the purpose of this thesis is to find a low cost and high yield approach to the synthesis of other morphologies of nanostructures in order to further improve the performance of these nanodevices. To this end, a chemical approach has been extended to the synthesis ZnO nanocones and platelets. With UV illumination, the synthesis of ZnO nanocones was achieved on GaN films on sapphire and gold films on silicon substrates. Both TEM and XRD results show that as-grown ZnO nanocones are single crystals. The formation of ZnO nanocones could be explained by the absorption process of photons. The UV light induced thermal gradient modifies the heat distribution as well as the reagent transport. The chemical reaction system is kinetically limited and results in ZnO nanocones. If the UV light is blocked, the ZnO nanowires result. In addition, the density of ZnO nanocones is higher than ZnO nanowires grown without UV illumination. By this chemical approach, ZnO platelets could also be obtained on GaN films deposited by PLD, whose c-axis is parallel to the surface of the substrate. The diameters and the thickness of the platelets depend on the quality and thickness of GaN film. TEM results illustrate that the obtained ZnO platelets are single crystals grown along the <0 1 1 0> direction within the {0 0 0 1} planes. Relative growth rates of various facets were altered by the presence of [1 0 0] textured GaN film. The suppression of the growth along c axis can also be achieved by citrate anions as a structure-directing agent to adsorb selectively on ZnO basal planes. Electrical measurement shows that the resistance of ZnO platelets is about 20-40 GΩ¸ and it is higher than that of ZnO nanowires. Piezoelectric potential calculation results also indicate that the piezoelectric potential is higher than for ZnO nanowires with the same external applied stress. These procedures and results demonstrate an easy and low cost way to fabricate ZnO nanocones and platelets, which may aid the utilization of nanostructures in solar cells, sensors and other applications to further improve their performance.
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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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ItemPulsed Laser Deposition of Hydroxyapatite Thin Films(Georgia Institute of Technology, 2005-01-17) Johnson, ShevonPulsed laser deposition (PLD) was used to deposit hydroxyapatite (HA) thin films on various substrates, including silicon (100) and titanium (Ti-6Al-4V) alloy. Thin films of amorphous HA were deposited at room temperature and then annealed over a range of temperatures. The microstructure and composition of the films were determined using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and X-ray diffraction (XRD). The HA films were found to achieve total crystallinity at 350㮠The mechanical properties of the films were studied by means of nanoindentation and scratch adhesion testing. Crystalline and adherent HA thin films prepared using PLD and post deposition annealing have many potential medical and dental applications.