3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Development of titanium alloys for hydrogen storage(2016-10-11) Abdul, Jimoh MohammedThe thesis investigated the effect of partial substitution of Cr or Ti with 2-6 at.% Fe, or 0.05-0.10 at.% Rh/Pd on the structure, hardness, corrosion behaviour and hydrogen storage characteristics of an arc-melted Ti35V40Cr25 at.% alloy. The effects of an annealing and a quenching heat treatment on the properties were also investigated. Melting of the eight alloys was done in a water-cooled, copper-hearth arc melting furnace under an argon atmosphere. Each of the eight ingots was cut into three: one as the as-cast sample and the other two separately quartz-sealed and loaded in two batches in a heat treatment oven and heated to 1000 °C for 1 hour. The first set of quartz tubes were immediately removed and broken in cold water to quench the alloy, hence locking the microstructure. The second batch was loaded into the furnace, heated to 1000 °C for 1 hour and then slowly furnace-cooled. The alloys (as-cast and heat treated) were characterised for phase identification using optical microscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) with Electron Diffraction X-ray Spectroscopy (EDS) using an Oxford system. Thermo-Calc software was used to model the phases using the Solid Solution 4 and Titanium 3 Databases. The hardness values (under a 2 kg load) of all samples were recorded. Potentiodynamic corrosion tests were performed in 6M KOH at 25 °C, and Tafel curves were recorded from -1.4V to -0.2V with a scanning rate of 1mV/sec. A Sievert’s apparatus was used for pressure composition temperature (PCT) measurements at 30, 60 and 90 °C. All the alloys contained a primary bcc (V) phase. The secondary phases were a combination of αTi, Ti(Cr,V)2 Laves phases (C14, C15 or C36) and a minor ωTi phase. The cell volume of the primary (V) phase decreased with addition of Fe and 0.05 Rh but increased with 0.1 Rh and Pd. The hardness of the base alloy increased with additions of Fe and 0.10 at.% Pd, but decreased with additions of Rh and 0.05 at.% Pd. Additions of Rh, Pd and 2 at.% Fe decreased the corrosion rate, while additions of 5 and 6 at.% Fe increased the corrosion rate. The reversible hydrogen storage capacity (RHSC) of the base alloy, otherwise known as useful capacity, was enhanced with addition of Pd and Rh, but decreased with Fe addition. Both annealing and quenching increased the hardness of the 0.05 at.% Rh and all the Fe containing alloys. Heat treatment decreased the hardness of the base alloy, both Pd alloys and v the 0.10 at.% Rh samples. Quenching decreased the hardness of the 0.10 at.% Rh and both Pd-containing alloys. The corrosion rate of the 0, 5 and 6 at.% Fe, 0.05 at.% Rh and the Pd-containing alloys decreased after annealing.at.% FeThe rate increased after annealing the 2 at.% Fe and 0.10 at,% Rh samples. The as-cast sample containing 2 at.% Fe had the lowest corrosion rate (0.0004 mm/y) and the quenched 6 at.% Fe was the least corrosion resistant sample with a corrosion rate of 0.037 mm/y. The quenched 5% Fe alloy had the highest hardness (460 MPa), while the annealed 0.10 at.% Rh sample had the lowest (388 MPa). The quenched 0.05 at.% Pd sample had the highest RHSC (2.28 wt%) while the lowest RHSC of 0.44 wt% was observed in the as-cast 2 at.% Fe sample. Annealing improved the RHSC of all samples except the base Ti35V40Cr25 and 6 at.% Fe alloys, while quenching was detrimental to RHSC of all the samples but the 2 at.% Fe, 0.05 at.% Pd and 0.10 at.% Rh alloys. Increasing the addition of palladium from 0.05 to 0.10 at.% Pd showed no significant improvement on RHSC of the base alloy, thus addition of 0.05 at.% Pd would be sufficient. The RHSC of the annealed 0.05 Rh alloy (2.25 wt% H) was close to the value of the 0.10 at.% Pd, so rhodium could be considered as an alternative to the quenched 0.05 at.% Pd. The RHSC was 1.56, 0.44, 0.75 and 0.68 wt% for 0, 2, 5 and 6 at.% Fe as-cast alloys respectively. Although the 2 at.% Fe alloy had the lowest RHSC, it could find its application as electrode in 6M KOH solution electrolyte because of its low corrosion rate.Item Comparative analysis of biohydrogen producing bacterial consortia in three thermophilic anaerobic fluidised bed bioreactors(2012) Sebola, Keneilwe MmuleGlobal warming has stimulated research into alternative energy carries and fuels. Hydrogen is one of these alternative fuels that are recognized as a promising future energy source. Historically, it is produced by water electrolysis and the gasification of coal. Hydrogen is a natural though transient by-product of several microbial driven biochemical reactions, including anaerobic digestion and fermentation. Microorganisms degrade complex molecules to produce butyrate and alcohols with CO2 and hydrogen as the only by- products. Hydrogen produced by microorganisms is known as biohydrogen. This study aimed to identify biohydrogen- producing bacteria in three Anaerobic Fluidised Bed Bioreactors (AFBRs), which are capable of producing hydrogen under anaerobic and thermophilic conditions, by using PCR-DGGE analysis of the 16 S rDNA genes. Sewage sludge from anaerobic digester and cow- dung were used as inoculum to isolate potential H2- producing organisms. The operational conditions were manipulated to removing mesophilic bacteria and non- spore forming bacteria by gradually increasing the temperature to thermophilic conditions and the pH maintained at acid conditions to allow acidotolerant bacteria to survive. The bioreactors were operated for a period of forty days for each research cycle. Bioreactor One was operated at 55°C for eight days and then the temperature kept at a constant 65°C for the remainder of the research cycle. Bioreactor Two was operated for 8 days at each temperature ranging from 45°C - 65°C, with increments of 5 °C. Bioreactor Three was operated at each temperature for ten days ranging from 55- 70°C, also with increments of 5°C. Samples for microbial community identification were taken at 55°C and 65°C. The bacterial morphologies and structural properties were evaluated by examining the hydrogen- producing granules, isolated at 65°C, using scanning electron microscopy. Species of the families Bacillus, Enterobacteria, Actinomyces, Clostridium and Veillonella were identified. Clostridium thermopalmarium, Bacillus coagulans and Bacillus thermoamylovorans were the culturable species at the desired operational temperature of 65°C. C. thermopalmarium and B. coagulans are major H2 producers with theoretical values of 4mol H2/ g glucose and 3mol H2/ mol sugar, respectively. Sewage sludge is the best source of biohydrogen producing bacteria in comparison to cow dung as an inoculum in AFBRs. Bioreactor 1 with a constant operational temperature of 65°C and at a HRT of 6.5 allowed for the desired Clostridium sp. to be the predominant H2 producer.