STATEMENT OF PROBLEM. Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramic material has been reported to possess remarkably high flexural strength; however, after sintering zirconium dioxide (ZrO2) frameworks and fixed partial dentures, some may not fit properly when trying them on their master cast. PURPOSE. The objective of this study was to investigate joining fully sintered YTZ-P by various methods including adhesive cements or fusion with intervening materials. MATERIALS AND METHODS. 216 specimens were used in this study; bars were fabricated from Vita In-Ceram YZ blocks and three bars were joined using dental cements; RelyX Unicem 2 Clicker and Multilink Automix, or fusing them with VM9, Infusion Glass, Boron Oxide, Aluminum Oxide, LUDOX, Colloidal Zirconia and Mullite Zirconia. The load to failure in Newtons was determined using a 3-point bending test. RESULTS. Sandblasted bars for RelyX (64.38N, RelyX-A-Sb) and Multilink (48.58N, M-Sb) wasn’t significantly higher compared to as-sintered groups (45.87N, RelyX-A) and (34.69N, M-A). Coating the bars with Monobond-Plus had a significantly higher load to failure 123.22N, RelyX-A-MP and 136.06N, M-MP compared to as-sintered and sandblasted groups. VM9 at an end firing temperature (EFT) of 1200 °C was significantly higher (228.97N, V-5) compared to lower fusing temperatures. Glass at an end firing temperature of 1200 °C was significantly higher (460.32N, G-6) compared to lower fusing temperatures. Frit at an end firing temperature of 1200 °C was significantly higher (214.52N, F-6) compared to lower fusing temperatures. Optical microscopy showed that Glass group G-6 (1200°C, EFT) had a thickness range from .09 to 0.03 mm while for Glass-A (1050°C, EFT) it was 0.5 mm. CONCLUSIONS. Within the limitations of this study, we concluded that sandblasted bars didn’t significantly increase the means load to failure values compared to as-sintered groups. Coating the bars with Monobond-Plus had a significantly higher load to failure compared to as-sintered and sandblasted groups. VM9, Glass and Frit at an end firing temperature of 1200 °C showed significantly higher means load to failure compared to lower fusing temperature groups. / 2018-09-28T00:00:00Z

Magister Scientiae Dentium - MSc(Dent) / Introduction: The evolution of Dentistry witnessed an increase in fixed prostheses as opposed to removable ones. Zirconia (ZrO2) and Lithium disilicate (LDS) are becoming the material of choice in implant or tooth retained prostheses. Polyetheretherketone (PEEK) is a recent alternative as it is lighter and causes less wear of opposing retained teeth. Biofilm formation is a permanent daily struggle for patients as it can be found in nearly all surfaces exposed to the natural environment. Therefore, the interest in a new device capable of removing or reducing oral biofilm from fixed prostheses is increasing. Aquaflosser (AQ) and Waterpik (WP) are examples of these oral irrigating devices that were introduced to the dental market recently. They can be effective in removing dental biofilm from different surfaces. Purpose of study: The purpose of this study is to evaluate biofilm formation on three fixed dental substructures and to evaluate the efficacy of two oral irrigating devices on biofilm removal from these three substructures.

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The gradual depletion of and dependence on fossil fuels, air pollution and global warming have all accelerated the development of alternative energy systems which use hydrogen as an energy carrier. The hybrid sulphur cycle (HyS) is the foremost electrothermochemical process that can produce hydrogen as the energy carrier. The HyS cycle consists of two units, namely the sulphuric acid decomposition reactor and the sulphur dioxide electrolyser (SDE). The SDE is responsible for the SO2 electrooxidation to sulphuric acid and protons at the anode and the electro–reduction of protons to hydrogen at the cathode. This research study focuses on the kinetic data collected from the prepared catalysts for SO2 electro–oxidation at the anode. Platinum dispersed on carbon, niobium pentoxide, tantalum pentoxide and zirconium dioxide as electrocatalysts were prepared using sodium borohydride as a reducing agent. These electrocatalysts were characterized using transmission electron microscopy and x–ray diffraction. Cyclic voltammetry was used to study the electrochemical active surface area (EAS) and the results showed that Pt/ZrO2–C had a higher EAS area than Pt/Ta2O5–C, Pt/Nb2O5–C and Pt/C. The high EAS of Pt/ZrO2–C can be explained by the low crystal size however after a series of linear polarisation scans Pt/ZrO2–C experiences a much greater area loss than all the other catalysts. Linear polarisation scans for each of the catalysts revealed that the influence of increased temperature and sulphuric acid concentration were showed improved results. Levich and Koutecky–Levich plots revealed that the SO2 oxidation is a multistep reaction on all the prepared catalysts and that there are regions which are kinetic and diffusion controlled and diffusion–only controlled. Pt/Ta2O5–C catalysts exhibited superior catalytic activity and stability compared Pt/Nb2O5–C, Pt/ZrO2–C and Pt/C. The Pt/ZrO2–C exhibited the most inferior catalytic activity and stability. / Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2011.

The gradual depletion of and dependence on fossil fuels, air pollution and global warming have all accelerated the development of alternative energy systems which use hydrogen as an energy carrier. The hybrid sulphur cycle (HyS) is the foremost electrothermochemical process that can produce hydrogen as the energy carrier. The HyS cycle consists of two units, namely the sulphuric acid decomposition reactor and the sulphur dioxide electrolyser (SDE). The SDE is responsible for the SO2 electrooxidation to sulphuric acid and protons at the anode and the electro–reduction of protons to hydrogen at the cathode. This research study focuses on the kinetic data collected from the prepared catalysts for SO2 electro–oxidation at the anode. Platinum dispersed on carbon, niobium pentoxide, tantalum pentoxide and zirconium dioxide as electrocatalysts were prepared using sodium borohydride as a reducing agent. These electrocatalysts were characterized using transmission electron microscopy and x–ray diffraction. Cyclic voltammetry was used to study the electrochemical active surface area (EAS) and the results showed that Pt/ZrO2–C had a higher EAS area than Pt/Ta2O5–C, Pt/Nb2O5–C and Pt/C. The high EAS of Pt/ZrO2–C can be explained by the low crystal size however after a series of linear polarisation scans Pt/ZrO2–C experiences a much greater area loss than all the other catalysts. Linear polarisation scans for each of the catalysts revealed that the influence of increased temperature and sulphuric acid concentration were showed improved results. Levich and Koutecky–Levich plots revealed that the SO2 oxidation is a multistep reaction on all the prepared catalysts and that there are regions which are kinetic and diffusion controlled and diffusion–only controlled. Pt/Ta2O5–C catalysts exhibited superior catalytic activity and stability compared Pt/Nb2O5–C, Pt/ZrO2–C and Pt/C. The Pt/ZrO2–C exhibited the most inferior catalytic activity and stability. / Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2011.

Purpose: The purpose of this study was to evaluate the fracture strength of monolithic FDPs of multi-layered zirconia (YML) with different connector dimensions and placement in the disc. Material and method: Forty 3-unit monolithic anterior FDPs were produced, 212223, and divided in four groups (n=10) based on the different connector dimensions and strategies for placement in KATANATM Zirconia Yttria Multi-Layered (YML)-disc. The groups were as follows: EB12, produced with 12 mm  connector dimension and EB7, with 7 mm  connector dimension which were both placed in Enamel layer and Body layer 1 (flexural strength: 750 and 1000 MPa, respectively) in the disc, BB12, with 12 mm  connector dimension and BB7, produced with 7 mm  connector dimension both placed in Body layer 2 and 3 (both with 1100 MPa) in the disc. The specimens were artificially aged by 10 000 cycles in thermocycling. Then, one random specimen was selected from each group (4 in total) and cemented onto the model, subsequently loaded to fracture using a universal testing machine. The result was compiled and reported descriptively. Result: The FDP in group BB12 remained intact and fracturing only occurred in this model at 893 N. The FDP in group BB7 demonstrated the highest fracture strength (972 N), followed by EB12 (842 N) and EB7 which showed the lowest fracture strength (465 N). All these three FDPs broke mesially on the pontics. Conclusion: Within the limitations of this pilot study, the following conclusions can be drawn: the fracture strength of monolithic FDPs of multi-layered zirconia is affected by the connector dimension. Placement of FDPs with a small connector area should be avoided in the layers with lower strength. The choice of model material affects the result. / Syfte: Syftet med föreliggande pilotstudie var att undersöka brotthållfastheten påmonolitiska broar av flerskiktad zirkonia (YML) med olika konnektordimensioneroch placering i disken. Material och metod: Totalt framställdes 40 stycken 3-leds monolitiska anteriorabroar, 212223. Broarna delades in i 4 grupper (n=10) efter olikakonnektordimensionering och placering i KATANATM Zirconia YttriaMulti-Layered (YML)-disken. Grupperna var enligt följande: EB12 med 12 mm²konnektordimension och EB7 med 7 mm² konnektordimension där broarnaplacerades i både Enamel Layer och Body Layer 1 (med 750 respektive 1000 MPai böjhållfasthet) i disken. Broarna i grupperna BB12 med 12 mm²konnektordimension och BB7 med 7 mm² konnektordimension placerades i BodyLayer 2 och Body Layer 3 (med 1100 MPa böjhållfasthet för respektive lager) idisken. Broarna genomgick termocykling, 10 000 cykler. Därefter valdes fyrabroar slumpmässigt ut, en bro från varje grupp, och cementerades på modell.Slutligen genomfördes ett brotthållfasthetstest med en universaltestmaskin tillsfraktur uppstod. Resultatet sammanställdes och redovisades deskriptivt. Resultat: Bron i grupp BB12 förblev intakt och fraktur skedde enbart i modellen(893N). Bron i grupp BB7 visade högst brotthållfasthet: 972N, följt av bron igrupp EB12 som visade brotthållfasthet på 842N och bron i grupp EB7 påvisadeden lägsta brotthållfastheten: 465N. Dessa tre broar frakturerade mesialt på dethängande ledet. Slutsats: Inom ramen för föreliggande pilotstudies begränsningar kan följandeslutsatser dras: brotthållfastheten på monolitiska broar i flerskiktad zirkoniapåverkas av konnektordimensioneringen. Placering av broar med mindrekonnektorarea än rekommendationen ska undvikas i skikten med lägre hållfasthet.Valet av modellmaterial har betydelse och påverkar resultatet.

Although current trends in global warming are of great concern, energy demand is still increasing, resulting in increasing pollutant emissions. To address this issue, we need reliable renewable energy sources, lowered pollutant emissions, and efficient and profitable processes for energy conversion. We also need to improve the use of the energy, produced by existing infrastructure. Consequently, the work presented in this thesis aims at investigating current scientific and technological challenges in energy conversion through biomass gasification and the alternative use of fossil fuels, such as diesel, in the generation of cleaner electricity through auxiliary power units in the transport sector. Production of chemicals, syngas, and renewable fuels is highly dependent on the development and innovation of catalytic processes within these applications. This thesis focuses on the development and optimization of catalytic technologies in these areas. One of the limitations in the commercialization of the biomass gasification technology is the effective catalytic conversion of tars, formed during gasification. Biomass contains high amounts of alkali impurities, which pass on to the producer gas. Therefore, a new material with alkali tolerance is needed. In the scope of this thesis, a new catalyst support, KxWO3 – ZrO2 with high alkali resistance was developed. The dynamic capability of KxWO3 – ZrO2 to store alkali metals in the crystal structure, enhances the capture of alkali metals "in situ". Alkali metals are also important electronic promoters for the active phase, which usually increases the catalysts activity and selectivity for certain products. Experimental results show that conversion of 1-methylnaphathalene over Ni/KxWO3 – ZrO2 increases in the presence of 2 ppm of gas-phase K (Paper I). This support is considered to contribute to the electronic equilibrium within the metal/support interface, when certain amounts of alkali metals are present. The potential use of this support can be extended to applications in which alkali "storage-release" properties are required, i.e. processes with high alkali content in the process flow, to enhance catalyst lifetime and regeneration. In addition, fundamental studies to understand the adsorption geometry of naphthalene with increasing temperature were performed in a single crystal of Ni(111) by STM analyses. Chapter 9 presents preliminary studies on the adsorption geometry of the molecule, as well as DFT calculations of the adsorption energy. In relation to the use of clean energy for transport applications, hydrogen generation through ATR for FC-APUs is presented in Papers II to V. Two promoted RhPt bimetallic catalysts were selected in a previous bench scale study, supported on La2O3:CeO2/d – Al2O3 and MgO : Y2O3/CeO2 – ZrO2. Catalyst evaluation was performed in a fullscale reformer under real operating conditions. Results showed increased catalyst activity after the second monolithic catalyst due to the effect of steam reforming, WGS reaction, and higher catalyst reducibility of the RhxOy species in the CeO2 – ZrO2 mixed oxide, as a result of the improved redox properties. The influence of sulfur and coke formation on diesel reforming was assessed after 40 h on stream. Sulfur poisoning was evaluated for the intrinsic activity related to the total Rh and Pt area observed after exposure to sulfur. Sulfur concentration in the aged catalyst washcoat was observed to decrease in the axial direction of the reformer. Estimations of the amount of sulfur adsorbed were found to be below the theoretical equilibrated coverage on Rh and Pt, thus showing a partial deactivation due to sulfur poisoning. / <p>QC 20150213</p>

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Синтезированы мемристорные сэндвич-структуры Zr/ZrO2-nt/Au диаметром 140 мкм на основе нанотубулярного слоя диоксида циркония толщиной 1.7 мкм и внутренним диаметром нанотрубок 55 нм. Проведена аттестация образцов методами сканирующей электронной и конфокальной микроскопии. Исследованы вольт-амперные характеристики полученных устройств в статическом и импульсном режимах резистивного переключения. Определены параметры резистивного переключения. Установлены механизмы проводимости, доминирующие в различных состояниях структуры. Продемонстрирована возможность формирования квантовых филаментов, состоящих из кислородных вакансий, в оксидном слое. Показана перспективность применения данных структур в качестве мемристорных элементов памяти. / Memristor Zr/ZrO2-nt/Au structure based on the zirconium oxide nanotubular layer with the thickness of 1.7 μm and the nanotubes inner diameter of 55 nm was synthesized. Attestation of the samples was performed with the methods of scanning electron and confocal microscopy. Current-voltage curves of the fabricated devices in static and pulsed modes of resistance switching were studied. Conduction mechanisms that dominate in different structure states were established. The formation of quantum filaments which consist of oxygen vacancies was shown to be possible in the oxide layer. The perspective of using these structures as memristor memory elements was shown.