Tom Troczynski


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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2019)
Process engineering of functional metakaolin based geopolymers (2018)

Geopolymers (GPs) are a class of inorganic materials which can be used as construction and refractory cements and as functional materials for environmental applications. GPs are low CO₂ emissions binders with high durability that can replace traditional cementitious materials. However the effects and interactions of processing parameters on the different stages of GP setting (“geopolymerization”) are still under scrutiny and the molecular mechanisms and rate limiting steps controlling the setting kinetics are unknown. The crystallization in GPs, which ultimately controls their performance in advanced applications such as water purification and toxic waste encapsulation, is a poorly investigated topic.This dissertation provides new experimental evidences on the role of chemical composition and curing process on metakaolin-based GPs. Steady state and dynamic rheological studies, contact angle tests, microstructural (SEM), structural (XRD and FTIR) and mechanical analyses lead to better understanding of the fundamental transformations occurring during geopolymerization. GPs were seeded with different oxides and zeolites to determine the rate limiting step, increase the reaction rate and control the crystallization. This work contributes to clarification the complex effects of soluble silica on the geopolymerization process. It is shown that soluble and colloidal silicates (such as Na₄SiO₄ and Na₂SiO₃) can act as seeding agents, changing the geopolymerization rate limiting step at temperatures T≥35°C. However, they also slow down the reaction rate, possibly by forming passivation layers on the metakaolin particles, thus producing a more chemically stable and mechanically stronger amorphous gel. Silicates also decrease the water requirement in GPs and thus the porosity. Under certain conditions silicates can increase the percentage of crystalline Faujasite in GPs, but the crystallization process requires higher curing temperatures and times (T>40°C and t>4 days, depending on the amount of silicates). The alkali metals have also a structure-directing role in crystallization of GPs in the form of zeolite, favoring faujasite structure. Water has a templating effect in GPs, favoring the structure of zeolite LTA-type over hydrosodalite. This work also illustrates the compromises that need to be made when selecting appropriate processing parameters to tailor the rheology, structure and properties of geopolymers for specific applications.

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Bio-inspired calcium phosphate/biopolymer nanocomposite fibrous scaffolds for hard tissue regeneration (2015)

This study discloses an original process for making calcium phosphate (CaP)/biopolymer nanocomposite fibrous scaffolds by biomimetic in-situ synthesis and electrospinning. Electrospinning (ES) produces non-woven nanofibrous mesh structure with 3D interconnected pores and a high surface area by applying electrostatic force to polymer-based solution. The resulting topography of the scaffolds mimics the natural extracellular matrix of human tissues, with the potential application in tissue engineering, drug delivery, and wound dressing. We have demonstrated that the possibility of inclusion of CaP into biopolymer nanofibers, inspired by mineralized collagen fibrils in bone tissue, makes ES an attractive processing route for preparation of the nanocomposites for bone tissue regeneration. Two different nanocomposite fibers were explored; i) poly(lactic acid) (PLA) with dicalcium phosphate anhydrate (DCPA) and ii) alginate with hydroxyapatite (HAp). In-situ synthesized DCPA in non-aqueous PLA solution were electrospun into self-fused and intra-nano porous networks. Homogeneous dispersion of DCPA nanocrystallites in the PLA nanofibers was induced by controlling the interaction of Ca²+ ions and the carbonyl groups in PLA, providing nucleation sites for DCPA during the in-situ synthesis. It is shown that the nucleation and growth of HAp on electrospun alginate nanofibers was generated at the [–COO‾]–Ca²+–[–COO‾] linkage sites on electrospun alginate nanofibers impregnated with PO₄³‾ ions during cross-linking treatment of alginate. This novel in-situ synthesis developed in this work resulted in the uniform distribution of the CaP nanophases and avoided agglomeration of the inorganic nanoparticles fabricated by the conventional mechanical blending method. Rat calvarial osteoblasts were stably attached and proliferated faster on the CaP/biopolymer nanocomposites fibrous scaffolds than the pure polymer scaffolds, respectively. Mineralized bone-like nodules deposited after 6 weeks of seeding on DCPA/PLA scaffolds. The unique nanofibrous architectures combined with the CaP nanophases were engineered using ES and the novel biomimetic in-situ synthesis. It is anticipated that the nanocomposite systems mimicking the mineralized collagen fibrils in bone tissue could be advantageous in bone tissue regenerative medicine applications.

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Calcium Phosphate Silicate Cement for Risedronate Drug Delivery (2015)

The effectiveness of bone cements in treating bone fracture is impaired by osteoporosis, which not only delays the osseointegration but also compromises the stability of implants. As a result, further fractures are not unusual after bone cement implantation in osteoporotic patients. This dissertation reports the investigation of the novel calcium phosphate silicate cement (CPSC) as a possible drug delivery system (DDS) for risedronate (RA) to treat osteoporosis and to restore bone fracture. Risedronate belongs to the family of bisphosphonate and, as the 3rd generation of bisphosphonate, can effectively suppress osteoclast activities and treat osteoporosis. In this work, the CPSC material properties were characterized as a function of RA content. High performance liquid chromatography was used to detect RA release profiles from cements and the Higuchi’s Law was employed to explain its release mechanisms. In vitro biocompatibility of RA-added CPSC (CPSC-R) was evaluated by MTT assays, flow cytometry, and real-time polymerase chain reaction. In the tibia implantation model from osteoporotic rabbits, biomarkers, X-rays, computed tomography, histology and PCR arrays were used to evaluate CPSC-R in vivo performance.It has been found that RA greatly affected CPSC setting time and compressive strength in a concentration-dependent manner. It was also found that RA disrupted CPSC hydration and delayed calcium silicate hydrate gel formation. RA was progressively adsorbed onto the unreacted calcium silicate and formed calcium-RA complexes. RA release kinetics from cement was controlled by the implant degradation and was in a good agreement with the theoretical calculations. CPSC-R was biocompatible and improved osteoblast proliferation and differentiation. Biomarker studies showed that CPSC-R significantly reduced osteoclast activities as compared to the sham control (p
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Process engineering, characterization and self-healing assessment of toughened calcium phosphate silicate composite bone cements (2014)

Self-healing, the ability to repair defects without external assistance, is one of the most magnificent characteristics of natural tissues. Achieving similar characteristics in biomaterials substituting natural tissues is highly desirable. As ceramic bone cements are designed to substitute bone tissues, the knowledge of their self-healing processes and characteristics is of vital importance for the advancement of bioceramics in orthopedic applications. In this work we have studied self-healing mechanisms of polyvinyl alcohol (PVA) fiber toughened Tri-Calcium Silicate (C₃S) cements, with and without calcium phosphate additions. The C₃S-PVA samples were partially fractured in three-point-bending, and then soaked in Simulated Body Fluid (SBF) for 7 days at 37°C. The variations in the morphology and width of the healed cracks were tracked by optical and electron microscopy. Chemical composition and phase analysis were determined using EDX, XRD and FTIR. The energy absorbed before the failure of C₃S-PVA samples, determined through the area under load-displacement curves, was nearly two orders of magnitudes higher than the pure C₃S samples. Most of the cracks in the previously fractured C₃S-PVA samples soaked in SBF were visually eliminated in 7 days, also resulting in partial restoration of their load-carrying capacity. Based on the EDX, XRD and FTIR results, a healing mechanism was proposed, including preferential precipitation of calcium phosphates and calcium carbonate phases within the cracks. The same healing treatment was applied to the new composite cement, wherein the C₃S matrix included 10wt% of Mono Calcium Phosphate (MCP) for improved cement biocompatibility and bioactivity. The toughness of C₃S-10MCP-PVA samples was also almost two orders of magnitudes higher than the pure C₃S-10MCP. C₃S- 10MCP-PVA samples had higher damage tolerance (deflection at maximum load) than C₃S-PVA samples. Self-healing studies of the C₃S-10MCP-PVA showed better restoration of the load_carrying capacity than C₃S-PVA. Such evidence emphasizes the effective role of calcium phosphate in the healing process of the toughened bioceramic cements. While such successful SBF-induced healing does not guarantee similar mechanisms operating in vivo, this pioneering research opens up avenues for further improvements of the cementitious ceramic composites in medical applications, as well as in broader engineering applications, e.g. in construction industry.

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Sintering studies of magnesia-chromite refractory composites (2014)

The magnesia-chromite refractory composites are the best candidates for the lining of non-ferrous metal smelting and refining furnaces, due to their high melting temperature, chemical inertness, and excellent thermal shock resistance. However, their high sintering temperatures (>1700°C) increase the processing complexity and costs. In this investigation, the primary goal was to study the sintering of these composites, with the long-term engineering goal to reduce their sintering temperature to Al₂O₃>SnO₂. The enhanced MgCr₂O₄ densification was attributed to the cation distribution in spinel structure (inversion phenomenon), caused by the inherent affinity of Fe⁺³ and Al⁺³ to tetrahedral sites. Fe₂O₃ and Al₂O₃ showed to form inverted spinel, while SnO₂ resulted in the formation of normal spinel solid solutions.Twelve magnesia-chromite composites were synthesized to study the effects of Al₂O₃, Fe₂O₃ and Cr₂O₃ on their sintering conditions; Cr₂O₃ decreased the density, while Fe₂O₃ and Al₂O₃ enhanced the densification of composites. The microstructural studies revealed that Fe₂O₃ and Al₂O₃ reduced the dihedral angle between MgO and spinel, while Cr₂O₃ increased it. The increased densification by Fe₂O₃ and Al₂O₃ was attributed to the decreased dihedral angle and formation of inverted solid solutions.The optimized composition [MgO6.9Cr₂O₃6.9Al₂O₃2.7Fe₂O₃]mol% (MK) reached nearly full density in air at 1475°C for 70minutes; 1700°C is currently used for magnesia-chromite refractories.In order to study the effects of the particle size on densification, magnesia-chromite composites (NMK) with average particle size of ~20 nm were synthesized via Pechini's method. Reducing the particle size from 1.2 um for MK to 20 nm for NMK reduced the onset sintering temperature by 200°C to 1000°C.The densification results were evaluated using master sintering curve theory for the first time for this kind of composites. The sintering activation energy was 443.7 and 302.6 kJ/mol for MK and NMK respectively. It was hypothesized that the oxygen diffusion through lattice and grain boundaries was rate controlling mechanism for MK and NMK respectively.

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Synthesis and characterization of calcium phosphage silicate bio-cements (2014)

Calcium phosphate silicate cement (CPSC) describes a family of materials in which the powder component is composed of the mixture of hydraulic calcium silicates and calcium phosphates. CPSC was developed and characterized in this work with the broad goal to address, and possibly overcome, the disadvantages of calcium silicate and calcium phosphate cements used in medical and dental fields. The main objective of this work was to synthesize and characterize CPSC, focusing particularly on the hydration process of CPSC. The cements consisting of various amounts of triclacium silicate (C₃S) and calcium phosphate monobasic (CPM), were synthesized by the sol-gel process, followed by heat treatment at 1550ºC and planetary ball-milling. It has been determined that after mixing with water, C₃S hydrates to calcium silicate hydrates (C-S-H) and calcium hydroxide (CH); within 10 min CPM reacts with CH to form dicalcium phosphate dihydrate (DCPD), which further reacts with CH and precipitates hydroxyapatite (HAP). It is proposed that the phosphate ions incorporate into C-S-H to form another type of hydrates C-S-P-H. The morphology of the hydrates depends on the process of hydration and the composition of CPSC. At the early stages of hydration, the hydration products form “almond-shaped” particles that serve as a nucleation site for the hydrates. The hydrates take tubular shape and form bundles clustered along the radial direction of the tubes. CPM influences the hydration kinetics of C₃S by increasing the duration of the hydration acceleration period rather than increasing the hydration rate, especially for the higher content of CPM in CPSC. CPM also increases the porosity of CPSC and reduces the content of CH, thought to be the “weak link” in the set CPSC. As a compromise between the two effects, the optimal content of CPM appears to exist at about 10 wt% of CPM in CPSC. After immersion in simulated body fluid, HAP forms on the surface of CPSC indicating that CPSC is bioactive in vitro. Cytotoxicity assay and cell adhesion assay against human gingival fibroblast indicated that the biocompatibility of CPSC is significantly enhanced.

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Hydrogen-induced damage of lead-zirconate-titanate (PZT) (2013)

Lead-Zirconate-Titanate Pb(Zr,Ti)O₃ (PZT) based actuators are evaluated by automotive industry for advanced fuel-injection systems, including hydrogen injection. However, hydrogen can have deleterious effect on the PZT's functionality and properties. The general objective of this work is to study the interactions between PZT and hydrogen. The results of long-term (200-1200 hours) high-pressure (10 MPa) hydrogen exposure on the PZT microstructure show that hydrogen has only superficial effects on the microstructure of bare PZT. However, when an electrode is attached to PZT, the hydrogen damage increased; a porous layer developed immediately adjacent to the electrodes on the PZT surface due to hydrogen spillover. The kinetics of the PZT structural modifications due to hydrogen was investigated by online monitoring of the electrical properties of PZT above the Curie temperature, up to 650°C. The results show that the structural changes can be described by the classical nucleation and growth theory. The growth of the new structure appears to be limited by the diffusion of protons into PZT, with a calculated activation energy of 0.44± 0.09 eV, at 450-650°C. Two relaxation peaks were observed in the dissipation factor curves of the hydrogen-treated PZT. While the kinetics of one of the relaxation peaks obeys the classical Arrhenius law with the activation energy of 0.66 eV, the other peak shows an unusual relaxation kinetic. The mechanisms for the formation of these relaxation peaks are determined. Low temperature (20°C) diffusion of hydrogen into the PZT was also studied, using the water electrolysis technique. Based on the microstructural observations, the diffusion coefficient of hydrogen in PZT was calculated as 9×10-¹¹ cm²/sec. The Maxwell-Wagner polarization mechanism is determined to be responsible for the changes in the hydrogen-affected PZT capacitance. In the last part of the project, alumina coatings were applied to PZT plates using the sol-gel technique, to explore the possibilities of decreasing H₂ damage to PZT. The functionality of the coating against hydrogen damage was evaluated by the water electrolysis technique. Significant decrease of hydrogen damage was observed even for highly porous coatings. The mechanisms by which the alumina coating decreases the hydrogen damage were tentatively proposed.

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Synthesis and sintering of chromium-free complex spinels in the MgO-Al2O3-FeOx-Me4+O2 systems (2013)

Magnesia-chrome refractory ceramics are used in non-ferrous industry because of their corrosion resistance against fayalite-type slags, rich in FeO. Unfortunately, Cr³⁺ may oxidize to Cr⁶⁺ during the smelting and converting processes, thus making their spent pot-lining hazardous. Two strategies are explored in this present work to develop chrome-free refractories sinterable at lower temperatures. The first is to replace Cr³⁺ with tetravalent cations (Me⁴⁺) that form complex spinel with MgO and Al₂O₃ at 1350-1550 ºC. These ions could promote simultaneous synthesis and sintering through formation of non-stoichiometric spinel at high temperatures and re-precipitation of the complex spinel during cooling. The second objective is to study the contribution of nano-size powders of the spinel formers to achieve both synthesis and sintering of Cr-free spinel solid solutions.The current research proved that synthesis and sintering of such spinels, or their solid solutions, could be performed at much lower temperatures (1350-1550 ºC) than the Cr³⁺-bearing version, which requires firing at 1750-1850 ºC to achieve equivalent properties. It is hypothesised that the defect super-structure due to Me⁴⁺ ions inducing the spinel phase formation at relatively low temperatures (1350-1550 ºC) results in the simultaneous synthesis and sintering of these complex spinels. The defect super-structures formed at lower temperatures have a higher lattice parameter than the final spinel solid solution phase. The volume expansion due to spinel formation that retards densification is also overcome due to the shrinkage in volume of the complex spinel phase. The results indicate that addition of Fe₂O₃ in MgO-Al₂O₃-Me⁴⁺O₂ systems results in improved reaction sintering with an open porosity of ~4% at 1450 ºC. However, the narrow particle size distribution for preparation of aggregates makes it necessary to fire the pressed specimens at higher temperature or in two stages in order to achieve similar open porosity to the co-clinkered aggregates for basic refractory ceramics. This research work contributes to the preparation of chrome-free aggregates and binding systems and show that synthesis and sintering of magnesia-spinel phase led to development of chromium-free binding phase for basic castables with comparable flexural and compressive strength to magnesia-chrome bricks with firing temperatures below 1500 ºC.

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Degradation of silicon nitride glow plugs in electric field - experiments and modeling (2011)

Silicon nitride (Si₃N₄) based ceramic Glow Plugs (GP) are considered by the automotive industry as a reliable, long-term source of ignition for natural gas and hydrogen internal combustion engines. The commercial GPs investigated in this work comprised of an all-ceramic heater with two U-shaped tungsten carbide heating elements encased in an Yb₂O₃-doped silicon-nitride (Si₃N₄) insulating phase. Upon applying electric potentials of 10-14V, the temperature on the surface of ceramic heater rapidly raises to as high as 1500ºC. This work looks into various modes of deterioration of GPs, particularly resulting from interaction of high operating temperature and the electric field within the GP heaters. An extensive scanning electron microscopy and energy dispersive x-ray spectroscopy investigation was performed to determine the degradation mechanisms of GPs in natural gas-burning rig, electric rig and engine. GP testing has shown that under the influence of constant electric load (DC) the sintering aid (Yb₂O₃) cations continuously migrate away from the high potential side of the heating elements following the electric field pattern. A 2D mathematical model was developed to simulate the redistribution of the sintering additive (Yb₂O₃) cations as a function of time, temperature, and electric field. The damage pattern of the tested GPs suggests synergistic impact of temperature, voltage, and environment on GPs lifetime. For the GPs tested in the burner rig and in engine the internal joule heating, externally applied combustion heat, together with the corrosive nature of the combustion gases, synergistically contribute to the degradation of Si₃N₄-based heaters. The comparison of cross sections for aged GPs revealed an increase in Yb ions migration with increasing temperature, electric field, and test duration. This study confirms that the removal of just one of the failure stimuli may significantly improve the GP performance. For example, applying AC voltage provided a significant improvement of GP durability in electric rig, even without addressing any other damage phenomena.

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On The Degradation of Porous Stainless Steel in Low and intermediate Temperature Solid Oxide Fuel Cell Support Materials (2011)

Research on oxidation kinetics of stainless steel traditionally focuses on flat sheet material. Little is known about the oxidation of steel within porous structures or particles of different sizes. In cases where oxidation of porous materials is reported, the data are seldom related to the actual surface area of the material. Instead, the mass change is often reported as a percentage mass gain only. In some literature references, the oxidation mass gain is assumed to increase with increasing porosity, often without information of the surface area of the pores. If an area-normalized oxidation mass gain is calculated, it is often normalized to the outside dimensions of the investigated specimens, making comparisons between different microstructures difficult. In this work, oxidation of spherical stainless steel powders with different powder particle sizes and of porous sintered stainless steel specimens is analyzed. Oxidation kinetics are correlated to the powder particle size and initial metal surface area of spherical stainless steel powders, addressing this knowledge gap. For oxidation kinetics of spherical steel powders, the dynamic change in metallic surface area over time is taken into account in the model developed in this work. Maximum oxidation mass gain of stainless steel powder based on composition and changes in phase structure, microstructure, and composition of oxides growing under the influence of prolonged exposure to solid oxide fuel cell (SOFC) operating temperatures is analyzed.The oxidation mass gain of sintered porous stainless steel is influenced by microstructure. The oxidation mass gain correlated to the entire surface area of the 3-D structure of the sintered porous specimens indicates slightly lower oxidation rate kinetics per unit surface area at 1073 K than published kinetics of similar materials in dense form.Additionally, the chromium diffusion through four spinel coatings that have been proposed as protective coatings for stainless steels used in SOFCs is analyzed in this work. Al-Mg-type spinels have the lowest Cr-diffusion rate at the investigated conditions and among the investigated materials.

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Aqueous suspension plasma spraying of yttria stabilized zirconia solid oxide fuel cell electrolytes (2010)

In order to meet increasing world energy demand in a sustainable manner, clean andefficient new energy technologies need to be developed. Fuel cells have been proposed as a potential energy conversion technology to help facilitate this transition to cleaner energy. Solid oxide fuel cells (SOFC) in particular are thought to be a practical, near term clean energy technology; however, current state-of-the-art wet ceramic fabrication techniques make SOFC manufacturing labour-intensive, fairly expensive and difficult to automate, and the high firing temperatures required limit the usable materials sets and increase production times. Plasma spraying (PS) is a potential next generation SOFC fabrication process that can rapidly produce fully sintered ceramic layers without the need for post deposition heat treatments; however, it is difficult to produce the thin, fully dense layers required for SOFC electrolytes using conventional plasma spray techniques, as the carrier gas based feeding configurationstypically require large feedstock powders. Suspension plasma spraying (SPS) is amodification of conventional PS processes that uses micron or sub-micron sized feedstockpowders suspended in a carrier liquid. SPS has the potential to significantly improve coating quality and microstructural control. Thus plasma spray manufacturing methods may have the ability to both reduce cell fabrication and material costs and improve cell performance, making them an important step toward successful SOFC commercialization.This project investigated the properties of metal supported aqueous SPS yttria stabilizedzirconia (YSZ) layers that could be used as SOFC electrolytes and developed a thorough understanding of the relationships between the base layers (substrate and cathode),suspension and plasma spraying parameters and the resulting coating properties. Using this understanding, plasma sprayed full cells (cathode, electrolyte and anode) with optimized electrolyte microstructures with 96% density were produced andelectrochemically tested. The measured open circuit voltage values were approximately 90% of the Nernst voltages, and electrolyte area specific resistances below 0.1 Ω cm² wereobtained at 750⁰C for electrolyte thicknesses below 20 μm. Least-squares fitting was used to estimate the contributions of the YSZ bulk material, its microstructure, and the contact resistance to the measured series resistance values.

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Enhancing aluminum corrosion for hydrogen generation (2009)

Aluminum powder when ball milled with secondary particles will corrode in water releasing hydrogen. With corrosion rates approaching 90% of available aluminum within 20 minutes, this process is of interest as a portable hydrogen source. Electrochemical polarization and hydrogen capture tests are used to study changes in solution pH and temperature during the reaction as well as calculate the activation energy for corrosion of the ball milled powder. Evidence that the ball milled aluminum is increasing the solution pH is presented along with tests indicating the pH shift is not sufficient to account for the increase in corrosion rate. The effect of solution temperature on reaction products and corrosion rate for aluminum powders is measured, and the hypothesis that the exothermic nature of the reaction combined with a deformed surface is creating high temperature micro-environments is discounted. Activation energy for the rate limiting step in the corrosion of ball milled aluminum is calculated between 72 – 74 kJ/mol, similar to that seen for aluminum disks at a similar pH. Finally BET measurements show an increase in surface area of the aluminum particles after ball milling between 10x-20x. The amount of hydrogen evolved in the first hour is seen to correlate almost exactly with the aluminum surface area.The addition of alumina powder without ball milling is shown to increase the corrosion rate of aluminum powders by an order of magnitude or greater and to delay or prevent passivation of the aluminum. Two models are proposed to explain this observation and tests run to support them. The high surface area (10m²/g) of alumina is thought to provide an alternative deposition site for hydroxide formed during aluminum corrosion and the positive surface charge alumina acquires at pH 7 to provide a source of protons for hydrogen evolution. The hydrogen exchange current densities on aluminum and platinum surfaces are shown to increase by over an order of magnitude in the presence of alumina particles. The acceleration of aluminum corrosion is only seen with electrical contact between the aluminum and alumina, but contact is not required to delay passivation.

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Master's Student Supervision (2010 - 2018)
Comparative studies of the oxidation of MoSi2 based materials (2015)

Molybdenum disilicide (MoSi₂) is a promising intermetallic material for high temperature applications (above 1000°C). However, it rapidly oxidizes at temperatures ranging from 400 to 600°C, which given enough time, can lead to its disintegration. Above 1000°C, MoSi₂ exhibits better oxidation resistance due to the formation of a continuous SiO₂ layer (or alumina layer for the materials doped with aluminum). The experiments in this study were divided into two main categories: low temperature oxidation (300 to 900°C; high oxidation rate expected) and high temperature oxidation (1000 to 1600°C; lower oxidation rate expected due to rapid formation of the protective oxide films). The isothermal exposure time in the low temperature oxidation experiments was from 4 to 240 hrs while it was from 2 to 144 hrs for the high temperature oxidation experiments. Five different, commercially available, MoSi₂ based heating elements, i.e. Kanthal Super (labelled by the manufacturer as KS-1700, KS-1800, KS-1900, KS-ER and KS-HT) were used in the experiments. It was found that the oxidation behavior of different materials under investigation depended strongly on their chemical and phase composition, exposure time and temperature. KS-ER and KS-1800 showed excellent resistance against the low temperature (300 to 900°C) degradation for up to 240 hrs, while KS-HT and KS-1900 underwent significant degradation after 240 hrs of air exposure within the same temperature range. In high temperature oxidation experiments, a dense barrier alumina film (1.5 µm thick at 1000°C to 50 µm thick at 1500°C for up to 144 hrs) formed on KS-ER samples. A dense glassy SiO₂ film (3 µm thick at 1000°C to 50 µm thick at 1600°C for up to 144 hrs) formed on the other types of samples. The glass scale on the surface of KS-1700, KS-1800 and KS-HT was significantly thicker (~3 times) than that on KS-HT over the temperature range of 1200°C to 1600°C after 144 hrs. The rate of alumina formation of KS-ER was relatively higher than the glass film formation of the other types of composite MoSi₂ materials. The differences in the oxidation behavior of various MoSi₂-based materials were linked to their chemistry and phase compositions.

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