Although nanocrystalline silver is used commercially to treat burns and wounds, the mechanisms of action (MOA) for its activity are not clear. The purposes of this work were to determine if nanocrystalline silver has anti-inflammatory activity, determine physicochemical properties critical for its MOA, and develop nanocrystalline silver-derived solutions for use in the treatment of lung diseases, including ARDS and pneumonia. In a porcine contact dermatitis model, nanocrystalline silver had anti-inflammatory activity independent of antimicrobial activity, with increased apoptosis induction in inflammatory cells, but not keratinocytes; decreased expression of TNF-, TGF-, IL-8, and MMPs; and increased expression of IL-4, EGF, KGF, and KGF-2. Treatment with AgNO3 (Ag+) increased inflammation, and caused apoptosis induction in keratinocytes. Thus, nanocrystalline silver releases additional species, perhaps Ag^(0)-containing clusters, resulting in anti-inflammatory activity. SIMS analysis showed significant deposition of Ag-clusters after nanocrystalline silver, but not AgNO3, treatment. Nanocrystalline silver had a systemic effect, despite SIMS analysis showing minimal skin penetration by silver, suggesting that nanocrystalline silver interacts with cells near tissue surfaces that release signals altering the inflammatory cascade. Relative to various Ag+-releasing dressings, nanocrystalline silver had significantly enhanced antimicrobial activity, Ag+-resistant bacteria kill, and was not prone to development of resistant bacteria, indicating that nanocrystalline silver releases antimicrobial species additional to Ag+, and has multiple bactericidal MOA. Single silver nanocrystals are inactive, and heat treatment of nanocrystalline silver resulting in crystallites over ~30 nm caused loss of antimicrobial activity, soluble silver, silver oxide, and oxygen. This indicates a poly-nanocrystalline silver structure is necessary for optimal antimicrobial activity, as is having silver oxide to pin the nanostructure, preventing its growth. While oxygen is necessary during sputtering to produce silver oxide, too much oxygen reduces antimicrobial activity, as silver oxide is predominantly deposited. Sufficient total silver, modifiable with current and time, is also important for activity. Nanocrystalline silver-derived solution properties vary significantly with dissolution conditions. Solutions generated at pH 4-6 have stronger antimicrobial activity, and solutions generated at pH 9 have stronger anti-inflammatory activity. Overall, nanocrystalline silver-derived solutions have biological properties similar to nanocrystalline silver, indicating that they may be useful in a variety of medical applications.

Philosophiae Doctor - PhD / Nanocrystalline silicon (nc-Si:H) is an interesting type of silicon with superior electrical properties that are more stable compared to amorphous silicon (a-Si:H). Silicon nitride (SiNₓ) thin films are currently the dielectric widely applied in the microelectronics industry and are also effective antireflective and passivating layers for multicrystalline silicon solar cells. Research into the synthesis and characterization of nc-Si:H and SiNₓ thin films is vital from a renewable energy aspect. In this thesis we investigated the film growth mechanisms and the filament stability during the hot-wire chemical vapour deposition (HWCVD) of nc-Si:H and SiNₓ thin films. During the HWCVD of nc-Si:H, electron backscatter diffraction (EBSD) revealed that the tantalum (Ta) filament aged to consists of a recrystallized Ta-core with Ta-rich silicides at the hotter centre regions and Si-rich Ta-silicides at the cooler ends nearer to the electrical contacts. The growth of nc-Si:H by HWCVD is controlled by surface reactions before and beyond the transition from a-Si:H to nc-Si:H. During the transition, the diffusion of hydrogen (H) within the film is proposed to be the reaction controlling step. The deposition pressure influenced the structural, mechanical and optical properties of nc-Si:H films mostly when the film thickness is below 250 nm. The film stress, optical band gap, refractive index and crystalline volume fraction approached similar values at longer deposition times irrespective of the deposition pressure. Filament degradation occurred during the HWCVD of SiNₓ thin films from low total flow rate SiH₄ / ammonia (NH₃) / H₂ gas mixture. Similar to the HWCVD of nc-Si:H, the Ta-core recrystallized and silicides formed around the perimeter. However, Tanitrides formed within the filament bulk. The extent of nitride and silicide formation, porosity and cracks were all enhanced at the hotter centre regions, where filament failure eventually occurred. We also applied HWCVD to deposit transparent, low reflective and hydrogen containing SiNₓ thin films at total gas flow rates less than 31 sccm with NH₃ flow rates as low as 3 sccm. Fluctuations within the SiNₓ thin film growth rates were attributed to the depletion of growth species (Si, N, and H) from the ambient and their incorporation within the filament during its degradation.

abstract: Materials have been the backbone of every major invention in the history of mankind, e.g. satellites and space shuttles would not exist without advancement in materials development. Integral to this, is the development of nanocrystalline (NC) materials that promise multitude of properties for advanced applications. However, they do not tend to preserve structural integrity under intense cyclic loading or long-term temperature exposures. Therefore, it is imperative to understand factors that alter the sub-features controlling both structural and functional properties under extreme conditions, particularly fatigue and creep. Thus, this dissertation systematically studies the tensile creep and fatigue behaviour of a chemically optimized and microstructurally stable bulk NC copper (Cu)-3at.% tantalum (Ta) alloy. Strategic engineering of nanometer sized clusters of Ta into the alloy’s microstructure were found to suppress the microstructure instability and render remarkable improvement in the high temperature tensile creep resistance up to 0.64 times the melting temperature of Cu. Primary creep in this alloy was found to be governed by the relaxation of the microstructure under the applied stress. Further, during the secondary creep, short circuit diffusion of grain boundary atoms resulted in the negligible steady-state creep rate in the alloy. Under fatigue loading, the alloy showed higher resistance for crack nucleation owing to the inherent microstructural stability, and the interaction of the dislocations with the Ta nanoclusters. The underlying mechanism was found to be related to the diffused damage accumulation, i.e., during cyclic loading many grains participate in the plasticity process (nucleation of discrete grain boundary dislocations) resulting in homogenous accumulation rather than localized one as typically observed in coarse-grained materials. Overall, the engineered Ta nanoclusters were responsible for governing the underlying anomalous high temperature creep and fatigue deformation mechanisms in the alloy. Finally, this study presents a design approach that involves alloying of pure metals in order to impart stability in NC materials and significantly enhance their structural properties, especially those at higher temperatures. Moreover, this design approach can be easily translated to other multicomponent systems for developing advanced high-performance structural materials. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2019

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To prevent clotting during dialysis, heparin is used to line the tubing which blood flows through. Unfortunately, many side effects arise from taking heparin, especially when it is used for an extended period of time. As such, long-term exposure for individuals undergoing dialysis every day is unavoidable. To prevent the solubilized heparin from entering the bloodstream, a polymer-based natural material is being investigated. This materials properties include reduction of coagulation and elimination of the long-term effects of heparin such as heparin induced thrombocytopenia and osteoporosis. Cellulose nanocrystals (CNCs) contain the same 1,4 linked pyranose backbone structure as heparin along with desirable mechanical properties, like high stiffness and anisotropic shape. By altering the functionalization on the surface of CNCs to closely mirror that of heparin, it should be possible to make a biomimetic material that counteracts blood clotting, while not introducing soluble small molecule anti-coagulants into the body. Through blood assays and platelet fixing analysis, we have been able to show that this change in functionalization does reduce coagulation. Surface chemistry of CNCs were modified from 'plain' CNCs (70 mmol SO3-/kg residual from hydrolysis) to 500 mmol COO-/kg (TEMPO oxidized) and 330 mmol SO3-/kg CNC (sulfonated CNCs). We will show that by utilizing CNCs reactive functional groups and incredible mechanical properties we are able to create a material that reduces clotting while maintaining the tubing's mechanical strength as well as eliminating heparin's side effects associated with it being a soluble anticoagulant. / MS / To prevent clotting during dialysis, heparin is used to line the tubing which blood flows through. Heparin, an anticoagulant, is more commonly known as a ‘blood thinner’ which is a misnomer because it does not actually thin blood. Heparin works by inhibiting clotting factors in the coagulation cascade pathway which in turn limit the formation of blood clots and create the ‘thinning’ effect mentioned earlier. When dialysis is performed the interaction between blood and the dialyzer tubing initiates the formation of a blood clot. This is where heparin use comes in. Unfortunately, many side effects arise from taking heparin, especially when it is used for an extended period of time. As such, long-term exposure for individuals undergoing dialysis every day is unavoidable. To prevent heparin or its mimics from entering the bloodstream, a polymer-based natural material is being investigated. The properties of this material will include reduction of coagulation and elimination of the long-term effects of heparin. The polymer-based natural material being investigated is cellulose nanocrystals (CNCs). CNCs contain the same ring structure and chemical linkage sites as heparin along with desirable mechanical properties. By altering the surface chemistry on the CNCs to closely mirror that of heparin, it should be possible to make a biomimetic material that counteracts blood clotting, while not introducing a solution based small molecule anticoagulant to the body. Through blood assays and platelet fixing analysis, we have been able to show that this change in functionalization does reduce coagulation. The ‘plain’ CNCs used contained an initial charge density of 70 mmol SO₃⁻ /kg. This residual charge density was a result from the acid hydrolysis performed to acquire CNCs from cellulose. Chemically modified CNCs contained many more negatively charged functional groups with TEMPO oxidized and sulfated CNCs having 500 mmol COO⁻/kg and 330 mmol SO₃⁻ /kg, respectively. We will show that by utilizing CNCs reactive functional groups and incredible mechanical properties we are able to create a material that reduces clotting while maintaining the tubing’s mechanical strength as well as eliminating heparin’s side effects associated with it being a soluble anti-coagulant.

Today, it is possible to restore hearing in congenitally deaf children and severely hearing-impaired adults through cochlear implants (CIs). A CI consists of an external sound processor that provides acoustically induced signals to an internal receiver. The receiver feeds information to an electrode array inserted into the fluid-filled cochlea, where it provides direct electrical stimulation to the auditory nerve. Despite its great success, there is still room for improvement, so as to provide the patient with better frequency resolution, pitch information for music and speech perception and overall improved quality of sound.  A better stimulation mode for the auditory nerves by increasing the number of stimulation points is believed to be a part of the solution. Current technology depends on strong electrical pulses to overcome the anatomical gap between neurons and the CI. The spreading of currents limits the number of stimulation points due to signal overlap and crosstalk. Closing the anatomical gap between spiral ganglion neurons and the CI could lower the stimulation thresholds, reduce current spread, and generate a more discrete stimulation of individual neurons. This strategy may depend on the regenerative capacity of auditory neurons, and the ability to attract and guide them to the electrode and bridge the gap. Here, we investigated the potential of cultured human and murine neurons from primary inner ear tissue and human neural progenitor cells to traverse this gap through an extracellular matrix gel. Furthermore, nanoparticles were used as reservoirs for neural attractants and applied to CI electrode surfaces. The nanoparticles retained growth factors, and inner ear neurons showed affinity for the reservoirs in vitro. The potential to obtain a more ordered neural growth on a patterned, electrically conducting nanocrystalline diamond surface was also examined. Successful growth of auditory neurons that attached and grew on the patterned substrate was observed. By combining the patterned diamond surfaces with nanoparticle-based reservoirs and nerve-stimulating gels, a novel, high resolution CI may be created. This strategy could potentially enable the use of hundreds of stimulation points compared to the 12 – 22 used today. This could greatly improve the hearing sensation for many CI recipients.

Thesis (M.Sc. (Physics)) -- University of Limpopo, 2015 / Nanocrystalline materials of defect free anatase and rutile SnO2 together with Ti and Y in anatase SnO2 have been modelled successfully using classical molecular dynamics simulations together with Buckingham potential. The structural properties of these SnO2 phases were analysed using radial distribution functions (RDFs). The effect of increasing temperature in pure SnO2 and doped SnO2 were studied. In both pure and doped materials, RDFs suggest phase transition at higher temperature, where anatase SnO2 transforms to rutile SnO2. Rutile SnO2 was found to be more stable than anatase SnO2. The results showed that the dopants have different effects on the SnO2 material. Ti defect is found to lower the transformation temperature of anatase to rutile SnO2. Y defect is found not to have any effect on the anatase to rutile SnO2 transformation. Thermodynamic properties such as volume thermal expansion coefficient and specific heat capacity were also calculated from above Debye temperature. Volume thermal expansion coefficient was obtained from volume versus temperature curves. Volume thermal expansion coefficient for rutile and Ti-anatase SnO2 were found to be not of the same order with the calculated results. Specific heat capacity calculated from energy versus temperature curves was found to be in agreement with the Dulong and Petit law of solids. Nanocrystalline Al/Y co-doped SnO2 powders were successfully synthesized using the sol-gel method. The samples were subjected to different temperatures 100 (as prepared) 200, 400, 600, 800 and 1000 oC. The effects of co-doping and temperature on the structural and optical properties of Al/Y co-doped SnO2 nanoparticles as well as morphology were investigated. The characterization techniques used were X-ray powder diffraction (XRD), Raman spectroscopy, Scanning electron microscopy (SEM) and UV-visible spectroscopy (UV-vis). The average particle sizes were found to be in the range between 2.5–8 nm and the strains were calculated to be 2.76–0.53 with increasing temperature for as prepared and the sample sintered at different tempe-ratures. The Raman bands were found to correspond with the literature. At a higher temperature of about 800 oC the materials were found to contain the second phase which is yttrium stannate. However no information about aluminium was found. The optical band gap were found to be between 3.3–3.99 eV in the temperature range 200–1000 oC.

Nanocrystalline materials with grain size less than 100 nm have been receiving much attention because of their unparallel properties compared with their microcrystalline counterparts. Because of its high hardness, nanocrystalline nickel has been used for MEMS. Long term thermomechnical properties and deformation mechanism at both ambient and elevated temperatures need to be evaluated which is vital for reliability of its applications as structural material. In this thesis, nanoindentation creep of nanocrystalline nickel with an as-deposited grain size of 14 nm was characterized at elevated temperatures. The nanoindentation creep rate was observed to scale with temperature and applied load (or stress), and could be expressed by an empirical power-law equation for describing conventional crystalline solids. Creep activation energy was found to be close to that for grain boundary self-diffusion in nickel. The activation volume was also evaluated using a stress relaxation technique. The creep results were compared with those for fine-grained nickel in the literature. Possible mechanisms were discussed in light of the creep rate and temperature ranges. To provide a direct comparison, uniaxial creep tests were conducted on nanocrystalline nickel with an as-deposited grain size of 14 nm at 398 K. It was found that stress exponents under the two test conditions are almost the same, indicating a similar creep mechanism. However, the strain rate measured by nanoindentation creep was about 100 times faster than that by uniaxial creep. The rate difference was discussed in terms of stress states and the appropriate selection of Tabor factor. To further explore the time-dependent plastic behavior, multiple unload-reload tests were conducted on electrodeposited nanocrystalline nickel in both compression and tension. A hysteresis was observed during each unload-reload cycle, indicating irreversible energy dissipation. The dissipated energy was evaluated and the energy dissipation rate was found to increase with the flow stress to the third power and sensitive to the stress state (tension or compression). A mechanistic model based on grain boundary sliding was proposed to describe the unload-reload behavior. Experimental results were found to be in good agreement with the model predictions, suggesting the observed hysteresis was indeed caused by grain boundary sliding.

The structural quality of material (concentration and nature of defects) and optical properties (intensity and spectral emission range) of semiconductor materials are usually closely correlated. The idea of this work was to carry out a basic characterization of the structural (by X-ray diffraction technique and scanning electron microscopy) and optical (by micro photoluminescence measurements) properties of nanocrystalline ZnO samples and find a correlation. A number of ZnO samples prepared by atmospheric pressure metalorganic chemical vapor deposition at different regimes and on different substrates were investigated. According to the aim of the work the most important results can be summarized as following. The analysis of ZnO nanocrystalline structures deposited on Si (100) substrates have displayed a dependence of structural quality, morphology and microstructure as well as the optical spectral purity on the deposition temperature. The deposition at 500 ºС resulted in the massive of 1D ZnO nanopillars that demonstrated the best optical properties: a mono-emission in the ultraviolet spectral range was observed. Moreover, the results of microstructure investigation give a suggestion to the explanation of the ZnO nanopillars growth. The results obtained from ZnO on sapphire substrates revealed a moderate influence of the oxygen content during deposition on the structural quality of zinc oxide. However, a strong correlation between the oxygen content and deep-level emission intensity from ZnO nanostructures has been observed, which confirms the determinative role of oxygen for the defect emission from ZnO. It was shown that during the deposition of ZnO on specially prepared homoepitaxial template the substrate surface has not the major effect on the morphology of depositing ZnO structures. SiC was revealed to be the most appropriate substrate for hetero-deposition of textured ZnO nanostructures: the growth results in the massive of epitaxially related ZnO hexagons on the SiC (0001) plane. A number of factors - p-type conductivity of the substrate used, regular and uniform epitaxial growth of ZnO nanostructure, their excellent mono-spectral emission in short wavelength range of spectra, provides a strong background for further investigation of the electroluminescence properties of the obtained heterostructures and are of great importance for the progress of optoelectronics towards low-scaled elements.

Nano-grained nickel produced by electroforming technique was investigated for its microstructure evolution and mechanical properties upon annealing. It was found that during low temperature annealing (T<250 oC), electroformed nano-grained nickel showed scattered and isolated abnormal grain growth, followed by a major abnormal grain growth at 320 oC. A secondary abnormal grain growth, featuring faceted grain boundaries, was observed at a higher annealing temperature (T=528 oC). A semi-in-situ observation using optical microscopy was conducted to track the movement of the faceted grain boundaries, and it was found that these boundaries were mostly immobile. The mechanical properties under various annealing conditions were studies using hardness and tensile testing. The hardness was observed to decrease with increasing annealing temperature. The material became very brittle after annealing at 320 oC or higher temperatures. Fractography investigation showed that the brittleness is caused by intergranular fracture.