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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.
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An insufficient understanding of the corrosion, especially the internal corrosion, of diluted bitumen (referred to as “dilbit”) pipelines exacerbates the debate around new pipeline construction. This thesis elucidates the effects of important parameters, such as the presence of bitumen, chloride droplets, steel deformation, and the size of silica particle deposits, on the internal corrosion of pipeline steel. The inclusions and matrix of API-X100 pipeline steel before and after 30 days of exposure to bitumen were characterized by ex-situ scanning electron microscopy and the results showed that no obvious corrosion can be observed to occur at the inclusions or the matrix at 60 and 120°C. In order to understand their role in pipeline corrosion, chloride droplets on API-X100 pipeline steel were covered by a non-corrosive oil (paraffin oil), and used to simulate the corrosive environment encountered in dilbit pipelines. The corrosion under a paraffin-covered chloride droplet is distinct from the traditional Evans droplet in that the active corrosion is governed by the initial stochastic nature of pit density rather than differential aeration of oxygen. Further, bitumen was diluted with paraffin oil to simulate dilbit with a 30:70 paraffin oil to bitumen volume ratio. The areas covered by simulated dilbit did not show any corrosion, confirming the non-corrosive nature of simulated dilbit. The chloride droplet corrosion penetration into pre-deformed API-X100 pipeline steel and ASTM A106B pipeline steel beneath simulated dilbit was 1.17 ± 0.06 and 1.18 ± 0.09 times higher than that for the same steels without pre-deformation, respectively. The corrosion behavior of API-X100 pipeline steel under large (80% cumulative distribution = 540 µm) and small (80% cumulative distribution = 43 µm) silica particles and submerged beneath chloride droplets covered by simulated dilbit was also studied. The maximum penetration rate of localized corrosion under the larger-sized silica particles was 1.4 ± 0.1 times that of the localized corrosion beneath the smaller-sized silica particles. Chloride droplets originating from upstream processing are a threat to the integrity of dilbit pipelines. This threat increases if the steel is deformed and if internal deposits, which often carry the water, consist of larger particles.
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TECK and Vale operate their medium temperature copper and nickel sulfide concentrate leaching processes at 150°C. “Medium temperature” is used to describe a variety of processes that operate above the melting point of sulfur (119°C) but below the temperature where sulfur become highly viscous (159°C). During leaching, and depending upon various process parameters, iron (Fe) may precipitate as hematite, goethite, jarosite or other oxyhydroxide compounds. Hematite is the favored precipitate because it is the most environmentally (thermodynamically) stable and does not ad/absorb as much copper (Cu), nickel (Ni), or other solution constituents during precipitation. A better understanding of the formation and structure of these iron precipitates may elucidate key factors that would ultimately result in lower valuable metal losses and more stable leach residues.This thesis details the experimental work performed to clarify the conditions under which the precipitation of highly crystalline hematite occurs during medium temperature leaching of copper sulfide concentrates. Various process parameters at the lab scale were studied and classical, as well as newly developed, methods to identify the optimal conditions for hematite precipitation were employed. Higher acid concentrations resulted in increased copper extractions and favor the formation of hematite during concentrate leaching, rather than other metastable phases. Seeding with synthetic hematite resulted in more crystalline residues. Furthermore, commercially available water displacement formula ‘WD40®’ and other novel reagents (benzene sulfonic acid, phenyl phosphonic acid, decane, mineral oil) affect Fe precipitation and sulfur chemistry, leading to very different process outcomes such as improved extractions (from 98.0 to 99.2%) and larger, more easily separated, sulfur particles (from 20 µm to 1 mm).The solubility of ferrihydrite (and its main transformation product, hematite) increased with increasing acid concentration. The solid-state transformation of ferrihydrite to hematite was found to be the major mechanism. These results indicate that the ferrihydrite formed in the CESL process will eventually transform into hematite, but that solution potential will play an important role in the nature of iron oxide residue. Ferrihydrite transformation was not complete within the time (60 min) that is typically used in medium temperature leaching for the simple system studied here.
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Hot-dip galvanized steel is widely used in the electric power utility industry for power transmission and distribution structures because of its suitable service life in many atmospheric and underground conditions. The Zn coating protects the carbon steel substrate via the formation of a passive layer and by providing a sacrificial anode. Transmission towers are designed to last decades and the slow corrosion rate of the Zn coating plays an important role in assuring this service. However, under certain conditions, the corrosion rate of Zn coatings is considerable, and it may threaten the integrity of the galvanized steel structure, thus limiting its service life. Therefore, evaluating the below-grade corrosion of galvanized steel transmission towers is necessary.In this work, different experimental and analytical techniques coupled with thermodynamic and mathematical modeling were employed to evaluate the soil corrosion of hot-dip galvanized steel transmission towers. Results showed that corrosion of the Zn coated steel takes place in three successive stages, mostly in the form of localized corrosion, likely arising from the complex nature of the soil environment. Mechanistic studies of the pitting corrosion of Zn revealed that it has very different behavior than other metals, such as Fe and stainless steel. While stable pit growth of other metals strongly depends on the presence of a metal salt film, stable Zn pits can grow at potentials well below the salt film precipitation potential, most likely due to Zn’s high tendency for complexation. Once a corrosion pit is formed in the Zn coating, which can penetrate the coating and expose the steel substrate to the environment, its growth is independent of the presence of the salt film. This distinct dissolution behavior of Zn, along with the localized nature of soil corrosion of hot-dip galvanized steel, were used to develop a mathematical model to predict the soil corrosion of galvanized steel. This new model, which is based on fundamental electrochemical equations, considers all three stages of the corrosion process. The model, which dynamically considers environmental variables to calculate the corrosion rate, was validated using existing models from the literature as well as in-situ corrosion testing.
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This work comprehensively investigated the corrosion behaviour of titanium in the presence of the solids in simulated copper sulfide leaching solutions. The effect of inert solid deposits on the corrosion behaviour of titanium grade 2 (Ti-2) was investigated. The solid deposits effectively slowed the mass transfer of the corrosion process, thus resulted in a decreased corrosion rate of the underlying Ti-2 in sulfuric acid-chloride solution. This effect, however, resulted in an increased corrosion rate of Ti-2 in simulated leaching solution (acid plus dissolved metals) since its barrier effect slowed the formation and repair of the protective oxide film by passivating species from the bulk solution. The deposit-covered Ti-2 showed a significant increase in corrosion rate at high temperatures compared with the bare Ti-2. The barrier effect of the four deposits for the investigated condition was in the sequence of gypsum > S3 (d₀.₅: 20.4 μm) > S2 (d₀.₅: 193 μm) > S1 (d₀.₅: 425 μm).Titanium hydrides—important titanium acid corrosion products—were investigated due to their close relationship to titanium substrate cracking. Titanium hydrides generated by electrolysis catalysed the hydrogen evolution reaction and facilitated subsequent passivation of Ti-2. X-ray diffraction analysis revealed that these hydrides were primarily composed of TiH₁.₅, with small amounts of TiH₁.₇ and TiH₂. The effect was investigated of Fe(III) and Cu(II) species in leaching solutions on Ti-2 passivation. The critical concentrations of Fe (III) and Cu(II) species required to induce passivation increased linearly with temperature from 30 to 80°C. The mechanism associated with the passivation was acceleration of cathodic reactions due to the introduction of Ti-2 oxidants. Cu(II) was more effective than Fe(III) at passivating Ti-2 under experimental conditions.Erosion-corrosion of Ti-2, which pertains to mineral slurries in acidic leaching conditions, was investigated using electrochemical techniques. Erosion-corrosion of Ti-2 was caused by solid particle impingement. Electrochemical noise revealed that solid particle impacts resulted in localised fracture of the passive film, and erosion-corrosion proceeded in the form of current transients. The investigation based on the current transients revealed that erosion-corrosion is a threat to titanium equipment exposed to acidic slurries.
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Two hybrid energy storage systems, i.e., a fixed bed flow cell (FBFC) and a tri-functional battery (TFB) are introduced, which use either synthetic CuFeS₂ or a mineral concentrate (MC) as electrode materials and a source of Cu. In the FBFC, the composite negative electrode is CuFeS₂ or MC sandwiched in graphite felt (GF). The Fe^II^/Fe^III^ redox reaction (in the presence of Cu^II^) occurs on a GF positive electrode. Under optimized conditions, the presence of CuFeS₂ resulted in a continuous increase in the specific capacity of the FBFC from 9 to 48 mAh g-¹ and in the specific energy from 2 to 6.3 Wh kg-¹ in 500 galvanostatic charge/discharge (GCD) cycles. However, in the same setup, the MC had an increase in the specific energy from 3.5 to 8.5 Wh kg-¹ in 400 GCD cycles. Advantageously, 10.3 and 12.7% Cu is extracted from the synthetic CuFeS₂ and MC, respectively.In the TFB, two energy intensive processes, Cu extraction from CuFeS₂ and Zn electrowinning, are integrated for energy storage. In this setup, the positive slurry electrode (PSE) composed of CuFeS₂ or MC mixed with activated carbon (AC) in H₂SO₄ was separated by a membrane from the circulating Zn²⁺ solution in the negative compartment. The Zn deposition/re–dissolution and commencement of reversible reactions in the PSE during GCD cycles are responsible for energy storage akin to a battery. The maximum 388 Wh kg-¹ specific energy (1.13 Wh l-¹) during the 1st discharge cycle decreased to ≈50 Wh kg-¹ over the subsequent 14 GCD cycles. The low coulombic (≈50%) and energy (~40%) efficiencies are offset by ~23% Cu extraction from CuFeS₂ in 100 GCD cycles. The cell potential of ~0.95 V and potential efficiency (>70%) imply that the TFB can be used as a hybrid energy storage device.Using MC in the TFB-M, a monotonic increase in energy density from 2.6 to 36.2 mWh l-¹ at low energy efficiency (between 14–43%) was obtained for the initial 14 GCD cycles. On the other hand, in 100 GCD cycles, ~16.1% Cu was also extracted from the MC.
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Aluminum-copper casting alloys have relatively high strength and hardness, fatigue and creep resistances and good machinability, all of each are dependent on the copper content of the alloy. The Al-Cu casting alloy (4.2-5.0 wt% Cu), known as B206, is a potential candidate material for use in marine applications where good mechanical properties and high strength to weight ratio is desired. These properties are ideal for components of tidal-based energy generating systems. However, corrosion continues to be an issue. This dissertation presents and discusses the results of several electrochemical and microstructural investigations conducted on B206, contributing to a further understanding of the fundamental corrosion processes. Applications of this research are strongest within the marine industry field, yet are extendable to other infrastructural and engineering applications such as aerospace and military.Results of this work elucidate the mechanism of localized corrosion of B206 alloy in seawater. Focused ion beam (FIB) used to determine the subsurface microstructure at local attack sites within the corroded area reveals that localized corrosion is propagated where continuous particles are buried beneath the surface. Propagating away from the initiation sites, corrosion develops preferentially along the grain boundary network beneath the alloy surface. Retrogression and re-aging (RRA) of the alloy to modify the grain structure and render uniform the distribution of the second phase is revealed not to have a substantial effect on the corrosion susceptibility of the alloy. However, Electrochemical Impedance Spectroscopy (EIS) and Mott-Schottky tests support the feasibility of implementing anodizing and possibly anodic protection systems for B206 in specific service environments. EIS was also used to determine the effect of cathodic protection (CP) on coated B206 and reveals that its corrosion resistance with CP is superior to the situation without CP and, therefore, that the coating is compatible with CP. Due to its use in the as-cast state, the effect of casting porosity on the corrosion of B206 was investigated using a pencil electrode method. Results reveal that the corrosion can be attributed to the local chemistry inside the pores (conductivity and potential at the bottom of pores).
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Ferric iron precipitation is an important integral step of the hydrometallurgical processes. The precipitation product is often amorphous and leads to a significant amount of valuable metal loss to the residue along with residue disposal issues.Characterization of the leach residue samples from CESL and Vale medium temperature (150 °C) hydrometallurgical processes revealed that the precipitation of iron in the form of amorphous iron oxide phases results in approximately 2-4 times higher metal loss compared to the crystalline phases.To address the issue, simulated process solutions were used to study the effect of process parameters and their relative importance on batch precipitation conditions with the aim of obtaining a stable iron oxide phase i.e. hematite, while minimising associated metal loss to the precipitation product. It was found that the factors: initial ferric, H2SO4 and seed concentrations play an important role in the iron precipitation step. Mathematical models were developed for the iron precipitation and metal loss to the precipitates using statistical data analysis techniques.Results from this study show that the presence of low ferric or high acid concentrations and moderate amounts of seed are required to minimize metal loss to the precipitation product with moderate to high levels of iron precipitation. The supersaturation and the nucleation to growth ratios were found to determine the final product quality i.e. the particle size and associated metal loss.The presence of various anions or cations was also found to play an important role on the iron precipitation rate and product quality/nature. For example, the presence of chloride in the solution accelerated precipitation kinetics. The sulphate salts of the metals such as Mg and Cu increased the extent of precipitation, while aluminum sulphate decreased the extent of precipitation. Presence of the sodium ion in the system accelerated the precipitation kinetics but changed the nature of the product to sodium jarosite. The presence of low levels of arsenic (As:Fe ≤ 0.08) in the system were found to severely retard the precipitation rate. Adsorption of sulphate and incorporation of OH‒ into the hematite structure were responsible to produce a poorly crystalline hematite product.
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The corrosion characteristics of titanium (ASTM Grade 2) in copper pressure leaching environments are determined from room temperature and pressure up to high temperatures and pressures (230 °C, 430 psi). Anodic oxidation and controlled chemical oxidation methods are used to improve the corrosion resistance of Ti. Electrochemical and mass loss measurements are performed to evaluate the corrosion resistance of pre-oxidized titanium, compared to that of titanium with no prior oxidation, to generate a best practices guide for the hydrometallurgical industry. The results at low temperature showed that H₂SO₄ solution is very corrosive for Ti with a freshly polished surface. The corrosion rates (CRs) of Ti are obtained using mass loss and electrochemical measurements in H₂SO₄ with Cl-, Cu²⁺ and Fe³⁺ additions up to 175 °C. It is found that the CRs of Ti are unaffected by the presence of Cl− ions in H₂SO₄ solutions. CRs obtained from mass loss and electrochemical measurements confirm that Cu²⁺ and Fe³⁺ ions are good corrosion inhibitors for Ti. Iso-corrosion diagrams, with 0.1, 0.5 and 1 mm yr−1 lines for Ti in 3-50 wt.% H₂SO₄ solutions with Cu²⁺ and Fe³⁺ additions from room temperature to 175 °C are constructed from immersion test data. The effects of temperature (100-230 °C) and SO₄²− concentration (0-0.5 mol L−¹) on the pitting corrosion of Ti are studied in neutral Cl− containing solutions using cyclic potentiodynamic polarization and linear-sweep thermammetry measurements. A metastable pitting temperature threshold (MPTT) is defined for Ti as a function of sulfate to chloride mole ratio using linear-sweep thermammetry measurements.iiiAnodic oxide films (AOFs) are potentiostatically formed on Ti in 0.5 M H₂SO₄ solutions at various anodizing voltages (up to 80 V) at 25 °C. A new method is developed to fabricate chemically oxidized films (COFs) with high corrosion resistance by controlled chemical oxidation with H₂O₂ solutions at 90 °C. The corrosion behavior of the as grown AOFs and COFs is investigated in copper sulfide leaching solutions. It is confirmed that chemical oxidation with 2 M H₂O₂/0.1 M HCl solution leads to the best improvement of the corrosion resistance of Ti.
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Hydrometallurgical oxidation methods are increasingly being considered for the extraction of Cu from chalcopyrite. However, the kinetics of cathodic ferric ion reduction are poorly understood. This thesis investigates the kinetics of cathodic ferric ion reduction on chalcopyrite and its influence on the leaching process in acidic iron sulfate solution, with an emphasis first placed on the development of a speciation model for the H₂SO₄-Fe₂(SO₄)₃-FeSO₄-H₂O system from 25°C-150°C. Speciation results show that most Fe(III) is distributed as complexes or precipitates and the free Fe³+ accounts for only a minor percentage (up to 5.2% of total ferric) whereas a large amount of Fe(II) exists in the form of free Fe²+. The Nernst equation was used to study the redox potential of Fe³+/Fe²+ couple. The speciation model explains the change of redox potential with temperature for all nominal Fe³+/Fe²+ ratios. This model was validated by reliable prediction of measured redox potential, comparison of previously published results of ferric solubility, together with an analysis of the calculated pH and ionic strength.A novel expression was also developed to predict the redox potential of the ferric/ferrous couple. It seems that the redox potential can be easily and accurately determined only based on the variables of temperature and nominal Fe³+/Fe²+ ratio. The calculated free Fe³+ concentration allowed for a detailed investigation of the reduction kinetics of ferric ion on chalcopyrite by cathodic potentiodynamic polarization. The exchange current densities of the Fe³+/Fe²+ couple are on the order of 10-⁷–10-⁵ A/cm² in the range of 25–150°C, substantially less than that on platinum. Calculated rate constants can be well described by the Arrhenius equation. The transfer coefficient increases linearly with temperature (rather than being constant).The importance of the cathodic ferric ion reduction reaction on the overall leaching process is progressively increased when increasing the nominal Fe³+/Fe²+ ratio and temperature. Leaching is under anodic control when the nominal Fe³+/Fe²+ ratio is around 1:1, whereas at higher nominal Fe³+/Fe²+ ratios and temperatures it is under mixed control.These findings provide the basis for mechanistic analyses and attendant optimization studies of industrial leaching processes of chalcopyrite and other sulfide minerals.
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Particulate fouling as a result of corrosion product sedimentation is known to be a significant issue in the heat exchanger of nuclear power plants and thus the development of monitoring technologies for the detection of fouling is important. Since electrochemical processes are usually very sensitive to water chemistry, they provide an opportunity for the development of accurate and online sensors. The main aim of this work is to develop an electrochemical sensor to detect particulate fouling at various temperatures and pressures up to 200 ºC. In order to develop such an electrochemical sensor, knowledge of the interfacial chemistry and electrochemistry of both the suspended particles and the sensing probe are required. Potentiometric titration was used to measure the pH of zero charge (PHZC) of magnetite and hematite (both known particulate foulants) from 25 ºC to 200 ºC. Electrochemical impedance spectroscopy (EIS) was used to measure the minimum differential capacitance of a glassy carbon electrode (GC) as a function of electrode potential i.e. the potential of zero charge (PZC). The obtained results clarified the oxide particle-electrode interaction since a GC electrode was used as a detector probe. A sensor for particulate fouling detection was then investigated and a new experimental method for the detection of magnetite particles at temperatures up to 200 ºC was developed. An electromagnetic GC electrode was employed to collect the magnetite particles from the suspension solution and it was observed that changes in double-layer capacitance could be used to detect deposition at different conditions. Finally, the impact of particulate fouling on water chemistry was studied. A novel electrochemical method was employed to accurately measure the kinetics of H2O2 decomposition on the surface of magnetite at temperatures up to 200 ºC. This work provides an experimental methodology for the prediction of failures due to particulate fouling processes in heat exchangers by providing a means to estimate the extent of fouling, the interactions between colloidal foulants and their corresponding impact on water chemistry.
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Hydrometallurgy may be an alternative to the currently practiced smelting process forcopper extraction from chalcopyrite (CuFeS₂). However, the low temperaturehydrometallurgical processes for chalcopyrite continue to face challenges, mostly relating totheir slow dissolution rates or high sulfuric acid production. The slow dissolution rate of themineral is strongly linked to the formation of the passive film on its surface. However,despite 40 years of research on this topic, there is still not a complete agreement betweenresearchers about the composition and stability of chalcopyrite’s passive film in sulfuric acidsolutions. In this work, the nature of chalcopyrite’s passive film and its stability were studiedby application of a variety of electrochemical techniques. Additionally, the electrochemicalresults of the chalcopyrite study were compared to those obtained for a pyrrhotite electrode(Fe₁₋xS), as pyrrhotite electrochemistry represents a simplified case of the chalcopyritesystem. X-ray photoelectron spectroscopy (XPS) was used to analyze the composition of theproduct layers formed on the surface.It is shown that the chalcopyrite electrode is passive for potentials up to 0.90 VSHE.Above this potential, transpassive dissolution occurs. Results of XPS studies have suggestedthat a metal-deficient sulfide film (Cu₁₋xFe₁₋yS₂₋z) is the most plausible copper and ironcontaining sulfide phase which passivates the surface of chalcopyrite. In addition, an outerlayer of iron oxyhydroxide (FeOOH) forms on the passive film. FeOOH forms via oxidationof the passive film’s ferrous sulfide phases. The thickness of the sulfide passive film wascalculated to be approximately 6.7 nm. It is demonstrated that the transpassive dissolution ofchalcopyrite is significantly linked to oxidation of sulfur (from sulfide in the passive film toelemental sulfur and maybe sulfur species with higher oxidation states, e.g. thiosulfate). Noelemental sulfur or polysulfide species were detected on the surface for potentials below 0.90VSHE.
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Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
The current work investigates the dissolution of chalcopyrite (CuFeS₂) under two different leaching scenarios using scanning electrochemical microscopy (SECM): (1) leaching in the presence of ferric-ferrous ions (“ferric leaching”) and (2) leaching by the application of a potential without ferric-ferrous ions (“potentiostatic leaching”). Ferric leaching processes have reported a solid elemental sulfur product layer that impedes chalcopyrite dissolution. However, potentiostatic leaching tests have reported a copper sulfide layer as the product that forms during dissolution in the absence of ferric-ferrous.In this work, we used the SECM in two ways: (1) to detect the species released during dissolution in the two scenarios using tip CVs and (2) map the conductivity of the surface under the different conditions. Four ferric-ferrous molar ratios in sulfuric acid solution were used in the ferric leaching scenario: 1:1, 10:1, 100:1 and 1000:1, and the chalcopyrite mixed potentials from these tests were applied to a separate sample potentiostatically in pure sulfuric acid. The tip CVs identified Cu²⁺ and Fe²⁺ released in both leaching scenarios. However, in the potentiostatic leaching case, a CuS product was determined to form at the tip, which was not observed in the ferric leaching case. Diagnostic tests determined that the CuS formed from a copper-thiosulfate complex, with the thiosulfate ion being the intermediate soluble sulfur species released during chalcopyrite dissolution. Once, the CuS-type layer forms on chalcopyrite, in the presence of ferric, it is further oxidized to form a sulfur-rich copper-deficient layer.Contact angle measurements show that the chalcopyrite surfaces produced during ferric leaching are relatively more hydrophobic compared to the potentiostatically leached samples. This is because the sulfur-rich layer in the ferric leaching scenarios is hydrophobic in nature. CuS, from the potentiostatic case, has more holes than electrons and is perceived to have a net positive charge on its surface. These holes attract the water dipole with the negatively charged end oriented toward the surface to flatten the droplet, yielding a lower contact angle and enhancing hydrophilicity. These results support the understanding that mechanistic outcomes from amperometric techniques cannot ultimately be used to explain the results from industrial ferric leaching processes.
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The corrosion properties and passive film behaviour of wrought (WR) and electron beam melted (EBM) Ti-6Al-4V (Titanium Grade 5 or TG5) were compared in simulated physiological solution.Microstructural characterization was performed using field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) to correlate the microstructural characteristics to corrosion resistance. The "bimodal" and "basket-weave" morphologies were observed for WR and EBM samples, respectively. Electrochemical measurements were also carried out for the evaluation of corrosion and passive film properties. Open circuit potential (OCP) and potentiodynamic polarization (PDP) measurements showed that the EBM sample has lower corrosion current densities. After choosing three potentials in the passive region from the PDP curves (300, 500, and 700 mVAg/AgCl), a passive film was formed on the EBM and WR TG5 samples using potentiostatic polarization (PS). Electrochemical impedance spectroscopy (EIS) and Mott-Schottky (M-S) analysis were used to assess the passive film properties in Ringer’s physiological solution at 310.15 K (37±1 °C). A more compact and protective passive film developed on the EBM sample at each film formation potential. M-S measurements revealed that the passive film on both WR and EBM samples were n-type semiconductors with fewer donor densities (ND) for the EBM sample. Furthermore, the spontaneously formed passive layers on the surface after immersion for 1 hour (at OCP) were compared. EIS results showed that the passive layers have duplex characteristics with higher corrosion resistance for the EBM sample. Moreover, the naturally formed passive films on the WR and EBM samples showed n-type characteristics, and the donor density was lower for the EBM sample. Immersion tests (IT) were also carried out at a controlled temperature and time to measure the weight loss due to corrosion of both WR and EBM alloys. The EBM sample showed a lower corrosion rate by weight loss after 3 weeks; however, weight gain was observed after 6 and 14 weeks of immersion for both WR and EBM samples. Overall, microstructural characterization and electrochemical measurements confirmed the better corrosion behaviour and passive film properties for the EBM alloy in Ringer’s physiological solution at body temperature (37±1 °C).
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The current work deals with the development of scanning electrochemical microscopy (SECM) as a product selective catalyst screening tool for CO₂ electroreduction to formate reaction (CO2RF) in aqueous media. CO2RF is a proposed route for utilizing CO₂ and surplus electrical energy produced by intermittent renewable sources. In CO2RF devices, CO₂ is electrochemically converted to formic acid or to a solution of formate ions. The resulting product can be either sold on the market, used for H₂ storage, or oxidized back to CO₂ to recover the electricity in direct liquid fuel cells. Our goal is to develop SECM for the simultaneous characterization of multiple CO₂ electroreduction (CO2RR) catalysts, probing their electrocatalytic activity in a product selective manner. To illustrate our method, we studied tin oxide-based catalyst, one of the most promising CO2RF catalyst. We submitted these catalysts to an electroreduction pre-treatment at −1.25 and −3 VAg/AgCl, respectively yielding a smooth oxide rich surface and an oxide poor surface covered in nanoparticles (30 to 70 nm diameter). The creation of spherical nanoparticles by electroreduction of tin oxide at −3 VAg/AgCl in an aqueous carbonate solution has, to our knowledge, never been reported. These two pre-treated catalysts, in conjunction with the un-pretreated surface, were simultaneously characterized by SECM, determining their CO2RF electrocatalytic activity relative to each other. The cyclic voltammetry (CV) tip detection mode provided product selective activity data for three different reaction products: formate, CO and H₂. We demonstrated that pre-electroreduction at −1.25 VAg/AgCl formed a surface with improved selectivity toward CO2RF. We also discussed the effect of SECM experimental parameters on the success of CO2RR catalyst characterization, as well as the challenges and limitations of the method. Our development in SECM opens the door to automated screening of CO2RR catalyst, which dovetails with the recent advances in model-based computational catalyst screening. While high screening throughput can be achieved using these various theoretical methods, they depend on experimental catalyst characterization and screening to validate their models. By automating the slowest step of the process, the experimental bottleneck on catalyst screening can hopefully be alleviated.
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This thesis analyzed and investigated the premature failures of pipes made from type 316L austenitic stainless steels. Multiple leaks were observed in scattered locations of a piping network of around 10 km after only 4 months in service transferring ammonium sulfate solution. The initial investigation indicated that the piping network was constructed 3 years earlier. After the construction, the stainless steel pipes were hydrotested to ensure the joints integrity. However, the piping network was not properly drained and dried after the hydrotest which resulted in water stagnation for the complete idle period between construction and commissioning. Therefore, an electrochemical, chemical, mechanical and metallurgical testing and analyses were conducted to determine the damage mechanism which consequently caused these failures. I have conducted electrochemical tests on a 316L stainless steel electrode in chloridized ammonium sulfate solution to determine its corrosivity. The electrochemical tests showed that the corrosion rates of 316L SS in ammonium sulfate solution is very low. This conclusion was supported by other laboratory studies at higher temperature and by the industrial corrosion tables published online. Also, two spools from the piping network that experienced the failures were analyzed using stereoscope, optical microscope, scanning electron microscopy/energy dispersive spectrometry, X-ray fluorescence and carbon/sulfur analyzer, tensile testing and microhardness testing. The results of these tests indicated that the pipes were leaking at the 6 O’clock position near the weld and heat affected zone areas. The morphology of the attack illustrated a narrow opening with large sub-surface cavity and tunneling initiated from the internal surface of the pipes. The weld joints displayed weld defects in terms of root concavity and lack of penetration. The metallurgical investigation strongly suggests that the pipes failed due to Microbiologically Influenced Corrosion (MIC). During the idle period of 3 years, the stagnant untreated water in the closed system was an appropriate environment for bacterial growth leading to severe damage at the welding joints and the base metal.
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In the field of hydrometallurgy, the industrial uptake of leaching models has been overlooked partially due to the lack of universal models. A model developed for one plant cannot easily be transferred for the application of a different plant without redesigning the leaching kinetics in the code. The Multiple Convolution Integral (MCI)-based model developed in this thesis has the ability to be universally applied by user-controlled inputs. Chemical reactions can be selected while the modeling software calculates the mass and energy balances. Residence times, operating conditions, and the rate-limiting reagents can also be defined to calculate a precise fraction reacted (leach extent) for sulphide minerals. The ability of the using the MCI model for predicting sphalerite leaching is examined in comparison to hydrometallurgical plant data collected from a Canadian pressure leach operation. The results are promising, showing that the model can predict plant Zn extraction data to within an error of 1.5 %. The model is further verified through bench scale pressure leaching experiments where 94 % of the zinc is extracted within 90 minutes using a concentrate sample from the same industrial plant. The effect of temperature is analyzed and the activation energy is calculated to be 40.8 kJ/mol. Interesting discoveries with respect to the reagent concentrations and their effect on the overall fraction reacted are also explored from the model results. In addition, the limitations of the MCI model are explained along with suggestions for improvement.
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Leaching of a limonitic laterite (1.49% Ni, 0.33 % Co, 2.29% Mn, 51.8% Fe) using ferrous sulphate as reductant was studied in ammoniacal solutions. Studied parameters included temperature, agitation, ferrous sulphate concentration, solids content, ammonia concentration and a comparison between ammonium sulphate and chloride. Tests were performed in a batch cell with temperatures between 20 and 80ºC at atmospheric pressure. Leaching kinetics for nickel were different to cobalt and manganese. Nickel was favoured by temperature in ammonium sulphate reaching 66% extraction in 3 hours, whereas in ammonium chloride it showed slower but steady kinetics with an extraction of 75% in 24 hours. Cobalt extraction in ammonium sulphate was low at high temperature and solids content; however, at low solids content (4% w/w) and high temperature (80°C) cobalt extraction reached 80% in 4 hours. With 11% solids at the same temperature, extraction decreased to 20%. In presence of ammonium chloride, temperature had a positive impact on cobalt reaching 80% in 8 hours at 23% w/w solids. Cobalt appeared to re-precipitate in presence of ammonium sulphate. Manganese behaved similarly to cobalt, however, in presence of ammonium sulphate, it only reached 60% extraction after 8 hours at 80°C at low solids. In presence of ammonium chloride, manganese reached 65% in 3 hours while increasing solids was not detrimental. Manganese suffered adsorption and/or co-precipitation in ammonium sulphate and chloride. This decrease in extraction exhibited by cobalt and manganese was attributed to co-precipitation and/or adsorption in iron oxides/hydroxides. Nickel extraction appeared aided by ferrous sulphate. Cobalt and manganese improved in presence of higher ammonia/ammonium, agreeing with the observation that cobalt and manganese are more prone to co-precipitation/adsorption; for which they prefer lower ferrous sulphate and higher ammonia/ammonium in order to stabilize their ammines. Feed and solid residue analyses using chemical assays, x-ray diffraction, scanning electron microscopy and mineralogy, provided better understanding of limonite reduction. The main phase in the residue was magnetite. Differences in behaviour between nickel and cobalt/manganese suggest a two-stage process, and chloride solutions seem promising due to better stability of cobalt and manganese.
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Aluminium alloy B206 is one of the strongest and toughest alloys in the cast aluminium family. Although it is light and has excellent low cycle fatigue strength, AA B206 has been known to perform adversely due to its poor corrosion resistance. Thus corrosion has been identified as one of the major issues that jeopardizes the long-term use and performance of B206. The corrosion behaviour of B206 in seawater is studied through immersion testing and electrochemical techniques such as Potentiodynmaic Polarization, Potentiostatic Polarization, Cyclic Potentiodynmic Polarization and Linear Sweep Thermmametry in two different solutions, namely natural seawater and simulated seawater, at various temperatures. Techniques like Optical Microscopy, Energy Dispersive X-ray Spectroscopy and Scanning Electron Microscopy have been used to investigate the microstructure and surface morphology before and after the electrochemical tests. Heat treatment has been performed on the as-received samples using RRA and T7 heat treatment techniques to compare the corrosion behaviour of the former with the latter using electrochemical techniques and image analysis. Lastly, hardness tests have been performed on various heat treated and as-cast samples to establish a comparison in mechanical properties. This study shows that the extent of B206 corrosion depends on the oxidizing nature of the seawater environment i.e. low or high redox potential rather than on the temperature of the seawater. Natural seawater is more aggressive than simulated seawater. Also, heat treatment improved the corrosion resistance as compared to as-cast B206 which was determined by the values of corrosion current density and surface analysis. Furthermore, heat treatment has led to better mechanical properties as determined by hardness tests.
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Electrodeposition of metallic molybdenum from aqueous electrolyte has in most cases previously yielded poor results due to the extremely high rate of the secondary hydrogen evolution reaction occurring at the cathode. This results in low current efficiencies and thin brittle films. The use of a highly concentrated aqueous-acetate based electrolyte containing molybdate ions has been used to deposit thick (~50 μm) adhered, mirror like metallic molybdenum coatings. Plating variables were investigated to determine the optimum deposition conditions; it was seen that current density was the most influential factor for the successful deposition of the refractory metal. The coating surface was analysed using SEM and EDX. XRD analysis confirmed the deposits were amorphous in nature with broad peaks in the (110) orientation. The deposition mechanisms were studied through electrochemical techniques such as PDP and CV. It was concluded that metallic molybdenum is deposited in a two-step reduction process, with the formation of an intermediate coating of molybdenum oxide, requiring hydrogen gas to fully reduce. Corrosion studies have shown the coatings stability in a chlorinated environment however active uniform corrosion in alkaline conditions resulted in film failure. Exposure to strong acidic conditions result in oxidation and delamination of the coating.Up-scaling of the process was seen to be successful and large deposits of well adhered and uniform metallic molybdenum were formed under high applied currents.
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Trace amounts of electrolyte cobalt during zinc electrowinning (EW) significantly decrease the current efficiency of the EW process by accelerating the parasitic hydrogen evolution reaction. The maximum tolerable level of cobalt in zinc EW can be as low as 0.1- 0.3 mg/L. The typical method to remove cobalt from zinc electrolyte, which is based on cementation onto zinc dust at approximately 85°C, is not an efficient process. It suffers from long retention times (2-3 hours) and high consumption of reagents; especially zinc dust. The aim of the present research was to study cobalt cementation at high temperature and high pressure (HT/HP) to accelerate the rate of cobalt removal and reduce the consumption of the reagents (zinc dust and activators). Experimental variables included temperature (85-150°C), pressure (0-100 psig), zinc dust dosage, zinc dust particle size, and activators (copper and antimony).Based on this research, the following results were obtained:1. Increasing temperature had a significant effect on the rate of cobalt removal. The optimum temperature was found to be 125°C - temperature at which the target level of cobalt (0.1 mg/L) could be met in 20 min.2. At 125°C and in the presence of 2.5 mg/L Sb and 45 mg/L Cu, 3.5 g/L zinc dust was found as the optimum zinc dust addition to lower cobalt concentration from the initial level of 15 mg/L to below 0.1 mg/L.3. Smaller zinc particles showed better cobalt removal results, but the cement redissolution was also more severe with these particles.4. The role of Sb in the activation system was more important than Cu. However, the best result in terms of the rate and extent of cobalt removal was achieved when both of the activators were added to the solution together.5. As expected, increasing the overhead pressure of N₂ (tested at 85°C) did not alter the cobalt removal profile greatly. Also, the effect of increasing the partial pressure of H₂ (tested at 125°C, and above the amount generated in situ by the reaction) on cobalt removal was negligible.
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Residues of the CESL Cu and Vale Ni sulphide medium temperature pressure leach contain elevated levels of the interest Cu and Ni metals. Metal losses are disproportionately found within a poorly formed amorphous and semi-crystalline iron oxide phase analogous to ferrihydrite. This phase results from incomplete ferric hydrolysis and precipitation that occurs simultaneously with the leaching of the sulphide minerals. An investigation was conducted by ageing the residues at 95°C in water to assess the extraction of lost Cu and Ni and the transformation of ferrihydrite to the more crystalline goethite and hematite species. It was found that 22% of the Cu could be removed from the CESL residue in 24 hours. Repeated ageing was only capable of extracting 38% of the Cu. Extractions of 22% of the Ni and 6% of the Cu were achieved in 24 hours from the Vale residue. Recovery of the lost Cu and Ni was mainly attributed to the washing of entrained and adsorbed Cu and Ni sulphates evenly distributed throughout the residues. A small amount of Cu and Ni was gained from ferrihydrite dissolution during the ageing process. Further investigations by SEM-EDS revealed the presence of unleached Cu and Ni sulphides that also contribute notable metal losses. EMP analysis showed that all hematite precipitated in the Vale residue contained Cu, Ni and SO₄²−. The average hematite particle consisted of 66.1% Fe, 0.85% Ni, 0.73% Cu and 1.19% SO₄²− for an equivalent of 30% Ni and 25% Cu losses to the residue. QXRD analysis of the aged residue showed little evidence of ferrihydrite transformation though ageing stabilised the residues by reducing the amount of X-ray amorphous material present. Improved stability was confirmed by a reduction in mass loss following treatment by a sequential extraction procedure utilising acidified hydroxylamine hydrochloride. This treatment dissolves the ferrihydrite present in the residues and is used as a proxy to assess residue stability and losses of Cu and Ni to the non-crystalline iron oxide phase.
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Alkaline sulfide leaching (ASL) at approximately 100 ºC has been used to selectively extract arsenic and antimony from enargite and tetrahedrite concentrates. Sodium thio-arsenate has been postulated to crystallize from alkaline sulfide leaching solutions upon cooling. However, literature data on the solubility of sodium thio-arsenate as well as proof of its crystallization from ASL solutions is scant. In this thesis, the solubility of leach-produced and synthetic sodium thio-arsenate is studied.To determine arsenic solubility in ASL solutions, sodium thio-arsenate and sodium arsenic oxide sulfide complexes are synthesized by various means and characterized by EDX, QXRD, and ICP. The synthesis of amorphous As₂S₃, sodium arsenic oxy-sulfide complexes, and sodium thio-arsenate is first presented. For amorphous As₂S₃ synthesis, the effect of concentration of sodium sulfide (0.1 M) and hydrochloric acid (1 M), temperature (40 ~ 60 ºC), and aging time (48 hours) was optimized. The solubility of synthetic sodium arsenic oxy-sulfide complexes and sodium thio-arsenate in ASL solutions increases significantly as temperature is increased to 95 ºC. More importantly, the solubility of sodium thio-arsenate at certain temperatures is significantly affected by the concentration of sodium hydroxide and sulfide in solution. Due to the common ion effect, if NaOH and HS- concentrations are very high, the solubility of sodium thio-arsenate decreases. Enargite leaching tests were done to characterize the precipitate that occurred upon cooling and to verify the arsenic saturation point, which should be between 38.5 ~ 58 g/L (0.51 M ~ 0.78 M) As depending on the NaOH and HS- concentration. Comparison with solubility experiments of pure sodium thio-arsenate shows that arsenic solubility in ASL solutions is supersaturated. However, direct comparison of saturation in ASL solutions and the solubility as obtained by the synthetic solutions/crystallites prepared here is not possible given the complex nature of the ASL crystallites that appear not to contain the often discussed “sodium thio-arsenate”.
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The Caron process was developed in the 1920s. In this process, the ore is dried and milled, then roasted in a reducing atmosphere to convert nickel and cobalt to their metallic form. Nickel and cobalt are then leached in ammoniacal solution but some losses occur due to adsorption and co-precipitation with iron oxide or hydroxide precipitates. The effects of various parameters on iron precipitation in the Caron process leach were studied. The parameters studied include pH, temperature, contact time with iron and its precipitates and initial ferrous concentration. The adsorption of otherwise soluble Co onto freshly precipitated ferric hydroxide was measured at four different temperatures between 20ºC and 35ºC. The percentage of cobalt adsorbed onto iron precipitates was directly related to the initial iron concentration but the effect of temperature was less clear. The amount of cobalt adsorbed depended on the pH. Solution contact time with iron precipitates had a very small effect on cobalt adsorption (2-3% difference). The highest adsorption of cobalt (99%) was obtained with an initial concentration of 0.2 M ferrous sulfate at pH 7 after 2 hours of contact time. The main iron precipitate was ferrihydrite. X-ray analysis revealed the characteristic “2 line” ferrihydrite (Fe₅HO₈4H₂O) with no goethite being observed in any of the precipitates. TEM revealed an amorphous structure, which is also indicative of ferrihydrite. SEM showed a preponderance of poorly resolved precipitates, which did not appear to be crystalline. Traces of cobalt were measured in the ferrihydrite particulate by EDX.
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The electrochemical passivation of pure Fe in ammoniacal solution was investigated to determine the stability of both Fe-oxides and Fe-ammines during anodic polarization. The potentiodynamic experiments were done in 6M total NH₃[3M NH₄OH and 1.5M (NH₄4)₂CO₃] solution. Different experimental parameters such as temperature (15°, 25°, 35°, 45° and 55°C) and pH (6, 8, 10 and 12) were used. Pure oxygen and argon gas was sparged during the experiment to oxygenate or de-oxygenate the solution with no stirring in either case. Polarization plots show that both active anodic dissolution and passive regions are present for pure iron in ammoniacal solution. It also shows that as the temperature increased the dissolution rate increased in both anodic and passive regimes. At the same time, the active region for iron dissolution is present across a wider potential range. For pH 10 the highest dissolution rate is around 0.025A/cm² or 260 g/m²hr¹ at 55°C and passivation of iron generally occurs at ca. -0.36 V (SHE) irrespective of the temperature. The peak anodic dissolution rate (0.2 A.cm₋² or 2080 g/m²hr¹) surprisingly occurs at pH 6. Potentiostatic experiments were done at different fixed potentials at pH 9 and 25°C. The highest current density was registered at -0.6 V. The peak dissolution current observed for the potentiostatic tests is roughly two orders of magnitude lower than that observed in the potentiodynamic test for pH 9 and at 25°C. Solution and morphological analyses were done by ICP and SEM, respectively for pH 6 and 9 solutions. The current efficiency for pH 6 is far lower than at pH 9 which implies that the current registered at pH 6 is used for the formation of a product film. Speciation calculations indicate that this film may be siderite (FeCO₃) at low potential. From XPS analyses, it is believed that the passive layer formed at higher potentials (more than 0.40V) is Fe₂O₃. Speciation diagrams point to the stability of iron tetra-ammines at pH 10. It was shown that metastable Eh-pH diagrams for Fe/NH₃/CO₃/H₂O system can be generated through potentiodynamic measurements aimed at active and passive behavior of iron.
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Leaching of enargite samples containing approximately 12 % As, 0.5 % Sb and 38 % Cu was studied in alkaline sodium sulphide solutions. Samples were leached in the presence of sodium hydroxide and sodium sulphide, which is expected to hydrolyse and form sodium hydrosulphide. Kinetic parameters studied included temperature, particle size, reagent concentration and stoichiometry in high pulp density tests. Leaching behaviour of arsenic and antimony was very similar; it was enhanced as temperature and reagent dosage was increased and/or particle size decreased. Copper, iron, zinc, and silver were not extracted during the leaching procedure. Through chemical analysis, X-Ray Diffraction and Scanning Electron Microscopy leach solutions and residues were characterised. Arsenic and antimony were efficiently removed, leaving copper-sulphur compounds such as digenite, bornite and sodium copper sulphide (NaCu₅S₃). Some of the leaching results differ from those found in the literature, especially in regards to the nature of the solid residue and the leaching reaction given.Removal of arsenic from solution was analysed by acidification and crystallization. Acidification removed arsenic and antimony from solution to produce a mixture of oxides and sulphides; however, sulphide was removed from solution most likely as hydrogen sulphide, which would need to be scrubbed in a sodium hydroxide solution. Finally acid consumptions over arsenic plus antimony ratios were too large for a practical application. Crystallization on the other hand is a simpler alternative. The main requirement is to have high concentrations of arsenic and antimony in solution. In this case part of the arsenic and antimony would be recirculated to the leaching stage. Other aspects included behaviour of chalcopyrite and pyrite in alkaline solutions and the possibility of producing sulphide ions in situ. Unfortunately no considerable amounts of aqueous sulphide were produced. Also, the behaviour of arsenic and antimony (III) in sodium sulphide alkaline solutions was analysed using arsenic and antimony trioxide. These results are in an early phase of study and could be a relevant topic for further research. In both cases a black precipitate formed containing elemental antimony and oxides. However, no crystallization of thio-compounds seemed to have occurred.
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As aqueous processing moves to higher temperatures and pressures to take advantage of increased kinetics, there is a need to develop and test appropriate reactor materials to ensure that corrosion is minimized. Corrosion testing often requires an electrochemical approach for a comprehensive understanding of the range of behaviors exhibited from a corroding metal or alloy in different environments. Prior art of designs for electrodes, associated pressure vessels and sealing technology is presented. The development of an apparatus and methods for high temperature and high pressure electrochemical corrosion testing are discussed. The final flow-through electrochemical cell design, the Flow-Through External Pressure-Balanced Reference Electrode (FTEPBRE) design, working/counter electrode and other components, which were developed for temperatures and pressures in excess of 500ºC and 5000 PSI is presented. A two-electrode electrochemical testing method is presented, using Stainless Steel (SS 316) as both Quasi Reference Electrode (QRE) and Counter Electrode (CE), and Alloy 625 (Ni-062.8%, Cr-21.8%, Mo-7.35, Fe-3.97%, Nb-2.7%) as the Working Electrode (WE). The effects of pressure, and its combination with temperature on OCP and corrosion rate of alloy 625 (WE) in both naturally aerated and de-oxygenated environments in 0.1 M sodium sulphate (Na₂SO₄) solution with a flow rate of 7 mL/min were investigated and discussed. The effect of pressure represented as a change in activation volume and reaction volume for the homogenous and heterogeneous phases is also presented. The corrosion rate was observed to increase with both temperature and pressure: higher for naturally aerated conditions than the corresponding de-aerated ones. Results also show that the instability of the QRE affected the result and direction of the OCP tests. A reduction in the corrosion current was observed above 207 bar (3000 PSI) in the polarization tests and was attributed to the increasing stability of the passive film formed on the surface of the alloys.
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