abstract: In this dissertation, micro-galvanic corrosion effects and passivation behavior of single-phase binary alloys have been studied in order to formulate new insights towards the development of “stainless-like” lightweight alloys. As a lightweight material of interest, Mg-xAl alloys were studied using aqueous free corrosion, atmospheric corrosion, dissolution rate kinetics, and ionic liquid dissolution. Polarization and “accelerated” free corrosion studies in aqueous chloride were used to characterize the corrosion behavior and morphology of alloys. Atmospheric corrosion experiments revealed surface roughness and pH evolution behavior in aqueous environment. Dissolution in absence of water using choline-chloride:urea ionic liquid allowed for a simpler dissolution mechanism to be observed, providing additional insights regarding surface mobility of Al. These results were compared with commercial alloy (AZ31B, AM60, and AZ91D) behavior to better elucidate effects associated with secondary phases and intermetallic particles often present in Mg alloys. Aqueous free corrosion, “accelerated” free corrosion and ionic liquid dissolution studies have confirmed Al surface enrichment in a variety of morphologies, including Al-rich platelet and Al nanowire formation. This behavior is attributed to the preferential dissolution of Al as the more “noble” element in the matrix. Inductively-coupled mass spectroscopy was used to measure first-order rate reaction constants for elemental Mg and Al dissolution in aqueous chloride environment to be kMg= 9.419 x 10-6 and kAl = 2.103 x 10-6 for future implementation in kinetic Monte Carlo simulations. To better understand how “stainless-like” passivation may be achieved, Ni-xCr alloys were studied using polarization and potential pulse experiments. The passivation potential, critical current density, and passivation current density were found to decay with increasing Cr composition. The measured average number of monolayers dissolved during passivation was found to be in good agreement with percolation theory, with a fitted 3-D percolation threshold of p_c^3D=0.118 compared with the theoretical value of 0.137. Using these results, possible approaches towards achieving passivation in other systems, including Mg-Al, are discussed. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2018

The potential use of sodium propionate as a corrosion inhibitor for mild steel in aqueous media is investigated using a range of electrochemical and surface analytical techniques. The use of sodium propionate for the inhibition of mild steel corrosion is discussed, and the effective pH range of sodium propionate using various buffers is investigated. The effectiveness of sodium propionate as an inhibitor for mild steel pitting corrosion in the presence of various concentrations of CI- is studied. The effect of some oxidants, IO3-, BrO3-, NO32- on the anodic behaviour of mild steel in deaerated 0.01M carboxylate solutions of acetate, propionate, formate, succinate and salicylate is investigated. The critical temperature for effective inhibition of mild steel corrosion with sodium propionate is established, and the chemical composition of the film formed on mild steel surface in sodium propionate solution is studied using surface sensitive Fourier transform infrared spectroscopy FTIR. The efficiency of sodium propionate is compared to that of conventional inhibitors and a mechanism for the inhibition of mild steel corrosion with sodium propionate is proposed. / Doctor of Philosophy (PhD)

In this thesis, silicon oxide thin films fabricated on silicon substrates by reactive radio frequency (rf) magnetron sputtering and supercritical CO2 (SCCO2) treatment at room temperature were investigated. The electrical properties including I-V and C-V of the films prepared at different processing conditions were discussed. Using the Transmission Electron Microscope (TEM), the thickness of silicon oxide thin films were measured. The results suggested that the film quality can be significantly improved by the SCCO2 treatment after reactive sputtering. The leakage current of the films at an electrical field of 1 MV/cm is 1¡Ñ10-8A/cm2 with a hysteresis voltage of 0.01V. The silicon oxide thin films can be used as a barrier layer for Al/SiO2/Si silicon solar cells. The energy conversion efficiency of a single crystal silicon solae cell is 10.2% under AM1.5 (965W/m2) radiation. After rapid thermal annealing(RTA) at 500¢J, the measured short-circuit current, open- circuit voltage, fill factor are 53mA, 0.54V and 0.53, respectively.

<p>Silicon carbide rectifiers are commercially available since 2001, and MESFET switches are expected to enter the market within a year. Moreover, three inch SiC wafers can be purchased nowadays without critical defects for the device performance and four inch substrate wafers are announced for the year 2005. Despite this tremendous development in SiC technology, the reliability issues like device degradation or high channel mobility still remain to be solved.</p><p>This thesis focuses on SiC surface passivation and termination, a topic which is very important for the utilisation of the full potential of this semiconductor. Three dielectrics with high dielectric constants, Al2O3, AlN and TiO2, were deposited on SiC with different techniques. The structural and electrical properties of the dielectrics were measured and the best insulating layers were then deposited on fully processed and well characterised 1.2 kV 4H SiC PiN diodes. For the best Al2O3 layers, the leakage current was reduced to half its value and the breakdown voltage was extended by 0.5 kV, reaching 1.6 kV, compared to non passivated devices.</p><p>As important as the proper choice of dielectric material is a proper surface preparation prior to deposition of the insulator. In the thesis two surface treatments were tested, a standard HF termination used in silicon technology and an exposure to UV light from a mercury lamp. The second technique is highly interesting since a substantial improvement was observed when UV light was used prior to the dielectric deposition. Moreover, UV light stabilized the surface and reduced the leakage current by a factor of 100 for SiC devices after 10 Mrad γ ray exposition. The experiments indicate also that the measured leakage currents of the order of pA are dominated by surface leakage.</p>

Not available / text

Computer processor chips of the last generation are based on silicon, modified to achieve maximum charge mobility to enable fast switching speeds at low power. III-V semiconductors have charge mobilities that are much higher than that of silicon making them suitable candidates for boosting the performance of new electronic devices. However, III-V semiconductors oxidize rapidly in air after oxide etching and the poor quality of the resulting oxide limits device performance. Our goal is to design a liquid-phase process flow to etch the oxide and passivate the surface of III-V semiconductors and to understand the mechanism of layer formation.Self-assembled monolayers of 1-eicosanethiol (ET) dissolved in ethanol, IPA, chloroform, and toluene were deposited on clean InSb(100) surfaces. The InSb passivated surfaces were characterized after 0 to 60 min of exposure to air. Ellipsometry measurements showed a starting overlayer thickness (due to ET, oxides, or both) of about 20 Å in chloroform and from 32 to 35 Å in alcohols and toluene. Surface composition analysis of InSb with X-ray photoelectron spectroscopy after passivation with 0.1 mM ET in ethanol confirmed the presence of ET and showed that oxygen in the Auger region is below detection limits up to 3 min after the passivation. Our results show that a thiol layer on top of a non-oxidized or low-oxide semiconductor surface slows oxygen diffusion in comparison to a surface with no thiol present, making this a promising passivation method of III-V semiconductors.

Environmental and economic factors demand radical advances in solar cell technologies. Organic and polymer photovoltaics emerged in the 1990's that have led to low cost per unit area, enabled in significant part by the convenient manufacturing of roll-to-roll-processible solution-cast semiconductors. Colloidal quantum dot solar cells dramatically increase the potential for solar conversion efficiency relative to organics by enabling optimal matching of a photovoltaic device's bandgap to the sun's spectrum. Infrared-absorbing colloidal quantum dot solar cells were first reported in 2005. At the outset of this study in 2007, they had been advanced to the point of achieving 1.8% solar power conversion efficiency. These devices degraded completely within a few hours’ air exposure. The origin of the extremely poor device stability was unknown and unstudied. The efficiency of these devices was speculated to be limited by poor carrier transport and passivation within the quantum dot solid, and by the limitations of the Schottky device architecture. This study sought to tackle three principal challenges facing colloidal quantum dot photovoltaics: stability; understanding; and performance. In the first part of this work, we report the first air-stable infrared colloidal quantum dot photovoltaics. Our devices have a solar power conversion efficiency of 2.1%. These devices, unencapsulated and operating in an air atmosphere, retain 90% of their original performance following 3 days’ continuous solar harvesting. The remarkable improvement in device stability originated from two new insights. First, we showed that inserting a thin LiF layer between PbS film and Al electrode blocks detrimental interfacial reactions. Second, we proposed and validated a model that explains why quantum dots having cation-rich surfaces afford dramatically improved air stability within the quantum dot solid. The success of the cation-enrichment strategy led us to a new concept: what if - rather than rely on organic ligands, as all prior quantum dot photovoltaics work had done - one could instead terminate the surface of quantum dots exclusively using inorganic materials? We termed our new materials strategy ionic passivation. The goal of the approach was to bring our nanoparticles into the closest possible contact while still maintaining quantum confinement; and at the same time achieving a maximum of passivation of the nanoparticles' surfaces. We showcase our ionic passivation strategy by building a photovoltaic device that achieves 5.8% solar power conversion efficiency. This is the highest-ever solar power conversion efficiency reported in a colloidal quantum dot device. More generally, our ionic passivation strategy breaks the past tradeoff between transport and passivation in quantum dot solids. The advance is relevant to electroluminescent and photodetection devices as well as to the record-performing photovoltaic devices reported herein.

No description available.

The probability of recombination of photogenerated electron hole pairs in crystalline silicon is governed by the density of surface defect states and the density of charge carriers. Depositions of intrinsic hydrogenated amorphous silicon (a-Si:H) in dc saddle field (DCSF) PECVD system and hydrogenated amorphous silicon nitride (SiNx) in rf PECVD system forms a dual layer stack on c-Si, which results in an excellent passivation of the surface and an anti-reflection coating. Response Surface Methodology is used in this work to optimize the deposition conditions of SiNx. Optimization of the response surface function yielded deposition conditions that materialized in a surface recombination velocity of less than 4cm/s. The BACH (Back Amorphous Crystalline silicon Heterojunction) cell concept makes use of this dual layer a-Si:H/SiNx stack to form a high efficiency photovoltaic device. The high quality passivating structure can result in the BACH solar cell device with more than 20% conversion efficiency.

The probability of recombination of photogenerated electron hole pairs in crystalline silicon is governed by the density of surface defect states and the density of charge carriers. Depositions of intrinsic hydrogenated amorphous silicon (a-Si:H) in dc saddle field (DCSF) PECVD system and hydrogenated amorphous silicon nitride (SiNx) in rf PECVD system forms a dual layer stack on c-Si, which results in an excellent passivation of the surface and an anti-reflection coating. Response Surface Methodology is used in this work to optimize the deposition conditions of SiNx. Optimization of the response surface function yielded deposition conditions that materialized in a surface recombination velocity of less than 4cm/s. The BACH (Back Amorphous Crystalline silicon Heterojunction) cell concept makes use of this dual layer a-Si:H/SiNx stack to form a high efficiency photovoltaic device. The high quality passivating structure can result in the BACH solar cell device with more than 20% conversion efficiency.