Explore the vast range of titles available.

In many advanced materials production processes, the analysis must be non-invasive, rapid, and, if possible, operando. The Raman signal of the various forms of alumina, especially transition alumina, is very weak due to the highly ionic nature of the Al-O bond, which requires long exposure times that are incompatible with monitoring transitions. Here, we explore the use of the fluorescence signal of chromium, a natural impurity in alumina, and the Rayleigh wing to follow the crystallization process up to alpha alumina. To clarify the assignment of the fluorescence components, we compare the transformation of beta alumina single crystals into transition (gamma and theta) alumina and then into alpha alumina with the transformation of optically transparent alumina xerogel and glass, obtained by very slow hydrolysis-polycondensation of aluminum sec-butoxide, into alpha alumina. Vibrational modes are better resolved in thermally treated single crystals than in thermally treated xerogels. Measurements of the Rayleigh wing, the Boson peak, and the fluorescence signal are easier than those of vibrational modes for studying the evolution from amorphous to alpha alumina phases. The fluorescence spectra allow almost instantaneous (<1 s) quantitative control of the phases present.

This study examined the influence of the crystallinity of added nano-alumina on the sulfate resistance of ordinary Portland cement (OPC) paste. Two crystalline types of nano-aluminas (α-and γ-phase) were incorporated in cement pastes, which were exposed to sulfate solution. In the results, both paste samples having α- and γ-phase aluminas had accelerated compressive strength loss and increased length expansion compared to the sample without alumina addition. In particular, the rapidly decreased dynamic elastic modulus of the nano-alumina added samples postulates the greatly increased internal stress likely by the increased formation of volume expansive reaction products, such as ettringite, which was supported by the XRD and TG results. The greater ettringite formation in the nano-alumina added samples was likely due to reactive AH3 (=Al(OH)3) gel formation as the higher consumption degree of portlandite in the alumina added samples indirectly indicates the active AH3 gel formation, resulting in additional ettringite formation from the reaction of AH3 with Na2SO4 solution. A further degree of sulfate attack was observed in the γ-alumina added sample for the long-term Na2SO4 exposure (180 days) mainly due to the greater degree of gypsum formation inducing more internal expansive stress compared to the α-alumina added sample.

Today, cancer treatment is an important issue in the medical world due to the challenges and side effects of ongoing treatment procedures. Current methods can be replaced with targeted nano-drug delivery systems to overcome such side effects. In the present work, an intelligent nano-system consisting of Chitosan (Ch)/Gamma alumina (γAl)/Fe3O4 and 5-Fluorouracil (5-FU) was synthesized and designed for the first time in order to influence the Michigan Cancer Foundation-7 (MCF-7) cell line in the treatment of breast cancer. Physico-chemical characterization of the nanocarriers was carried out using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), vibrating sample magnetometry (VSM), dynamic light scattering (DLS), and scanning electron microscopy (SEM). SEM analysis revealed smooth and homogeneous spherical nanoparticles. The high stability of the nanoparticles and their narrow size distribution was confirmed by DLS. The results of the loading study demonstrated that these nano-systems cause controlled, stable, and pH-sensitive release in cancerous environments with an inactive targeting mechanism. Finally, the results of MTT and flow cytometry tests indicated that this nano-system increased the rate of apoptosis induction on cancerous masses and could be an effective alternative to current treatments.

In this thesis, we build and solve various mathematical models to describe the interplay between alumina particles and a cryolite bath that takes place in a Hall-Héroult cell, which is the foundational process for aluminium production. We aim to understand the heat and mass transfer mechanisms taking place in order to gather insights into the aluminium production process and use these insights to suggest process improvements. First, we examine the problem of adding a cold alumina particle to a bath of hot cryolite. We develop a spherically symmetric model to describe heat and mass transfer between the particle and the surrounding material, allowing for freezing and melting of the cryolite close to the particle and its subsequent dissolution. We solve this model numerically using the method of lines using the early-time asymptotic solution to initiate the numerical scheme. We explore the behaviour as we vary the key dimensionless parameters. We also consider the model in the limits of small superheat and of small Stefan number. Using these asymptotic solutions, we find approximate expressions for the freezing, melting, and dissolution timescales, with freezing being the fastest effect and dissolution the slowest. Also, using the early-time solution, we determine constraints on the parameters for which shell growth is possible. Our results predict that increasing the superheat or decreasing the particle size should decrease the time it takes for a particle to dissolve, in accordance with practical experience. Furthermore, we note that the temperature changes are localised to a 5--10 particle radii. Second, we formulate a one-dimensional model for the phase-changing infiltration into a porous alumina raft placed on top of the cryolite. We simplify our model in the limit of small superheat. We solve the equations using the same approach as in Chapter 2 with the method of lines, using the early-time asymptotic solution to initiate the numerical scheme. We find that the early-time problem can exhibit a stable and an unstable solution and in certain parameter regions, these solutions disappear altogether. Furthermore, depending on the heat loss through the top of the raft, the model can undergo a discontinuous change in behaviour which corresponds to the clogging of the pore space. Additionally, near clogging, we find power-law behaviour of the frozen cryolite fraction and the location of the infiltration and solidification fronts. Third, we formulate a model for the disintegration of a spherical clump of alumina particles. We solve the problem numerically with the method of lines and see that, for certain parameters, the solution behaves like a travelling dissolution front, while in other regimes there is bulk dissolution. We investigate the travelling dissolution front by using the method of matched asymptotic expansions to reduce the full problem. The asymptotic expressions match well with the numerical solutions up to the order that would be expected. In Chapter 5 we formulate a model for the phase-changing infiltration into a spherical clump of alumina particles, which physically corresponds to a larger batch that is entirely enclosed by the cryolite. We find that many of the results in Chapter 3 carry over, such as the multiple solutions for early time, the late time clogging, or the fact that the phase-changing front is slower than the isothermal one. The presence of clogging in this model is especially interesting as it is caused by the slowing down of the front, as opposed to the cooling at the top, which was the case in Chapter 3. Finally, we summarize the findings and offer possible extensions to our models in Chapter 6 and present our mathematical findings in an approachable way in Chapter 7. The alumina feeding is a process with many interacting parts producing complex behaviour. In this thesis we shed light on a few problems, but the research is not over.

In the present study, alumina nanoparticles (nano-alumina) which were successfully fabricated by solvothermal method, were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Transmission Electron Microscopy (TEM), and Brunauer–Emmett–Teller (BET) methods. The removal of cationic dye, Rhodamine B (RhB), through adsorption method using synthesized nano-alumina with surface modification by anionic surfactant was also investigated. An anionic surfactant, sodium dodecyl sulfate (SDS) was used to modify nano-alumina surface at low pH and high ionic strength increased the removal efficiency of RhB significantly. The optimum adsorption conditions of contact time, pH, and adsorbent dosage for RhB removal using SDS modified nano-alumina (SMNA) were found to be 120 min, pH 4, and 5 mg/mL respectively. The RhB removal using SMNA reached a very high removal efficiency of 100%. After four times regeneration of adsorbent, the removal efficiency of RhB using SMNA was still higher than 86%. Adsorption isotherms of RhB onto SMNA at different salt concentrations were fitted well by a two-step model. A very high adsorption capacity of RhB onto SMNA of 165 mg/g was achieved. Adsorption mechanisms of RhB onto SMNA were discussed on the basis of the changes in surface modifications, the change in surface charges and adsorption isotherms.

Ceramic composites based on alumina and zirconia have found a wide field of application in the present century in orthopedic joint replacements, and their use in dentistry is spreading. The development of this class of bioceramic composites was started in the 1980s, but the first clinical applications of the total hip replacement joint were introduced in the market only in the early 2000s. Since then, several composite systems were introduced in joint replacements. These materials are classified as Zirconia-Toughened Alumina if alumina is the main component or as Alumina-Toughened Zirconia when zirconia is the main component. In addition, some of them may contain a third phase based on strontium exa-aluminate. The flexibility in device design due to the excellent mechanical behavior of this class of bioceramics results in a number of innovative devices for joint replacements in the hip, the knee, and the shoulder, as well in dental implants. This paper gives an overview of the different materials available and on orthopedic and dental devices made out of oxide bioceramic composites today on the market or under development.

The distribution of second phase particles in the microstructure of composite ceramics affects the mechanical properties, and the intragranular structures often result in better properties compared to the intergranular structures. However, it is difficult to obtain composite ceramics with intragranular structure by conventional route. To produce composite ceramics with an intragranular structure in a simpler route. In this work, starting powders with different phase compositions were obtained by the co-precipitation method, and zirconia toughened alumina (ZTA) composite ceramics were prepared with these starting powders by spark plasma sintering (SPS). The results show that it is easier to fabricate ZTA composite ceramics with an intragranular structure by using composite powders containing amorphous or transition phase Al2O3 as starting materials. The phase composition of the powder prepared by the co-precipitation method after calcination at 1100 °C is θ-Al2O3 and t-ZrO2, and the average grain size after sintering at 1500 °C is 1.04 ± 0.28 µm, and the maximum Vickers hardness and fracture toughness of the specimens reach 19.37 ± 0.43 GPa and 6.18 ± 0.06 MPa·m1/2, respectively. The ZrO2 particles were the core of crystallization and grow together with the Al2O3 matrix, forming the intragranular structure of ZTA ceramics. This work may provide a new idea for preparing composite ceramics with intragranular structure.

Micron-sized near-spherical α-Al2O3 powders are widely used as thermal fillers due to their high thermal conductivity, high packing density, good flowability, and low cost. During the high-temperature calcination, the resulting α-Al2O3 powders often exhibit an aggregated worm-like morphology owing to limitations in solid-state mass transfer. Researchers have employed various mineralizers to regulate the morphology of α-Al2O3 powders; however, the preparation of micron-sized highly spherical α-Al2O3 powders via solid-state calcination is still a great challenge. In this work, micron-sized near-spherical α-Al2O3 powders were synthesized through high-temperature calcination using hydratable alumina (ρ-Al2O3) as precursor with water-soluble mineralizer ammonium fluoroborate (NH4BF4). ρ-Al2O3 can undergo a hydration reaction with water to form AlO(OH) and Al(OH)3 intermediates, serving as an excellent precursor. With the addition of 0.1 wt% NH4BF4, the product exhibits an optimal near-spherical morphology. Excessive addition (>0.2 wt%), however, significantly promotes the transformation of α-Al2O3 from a near-spherical to a plate-like structure. Further studies reveal that the introduction of NH4BF4 not only modulates the crystal morphology but also effectively reduces the content of sodium impurities in the powder through a high-temperature volatilization mechanism, thereby enhancing the thermal conductivity of the powder. It is shown that the thermal conductivity of the micron-sized α-Al2O3/ epoxy resin composites reaches 1.329 ± 0.009 W/(m·K), which is 7.4 times that of pure epoxy resin.

High purity hydrogen and solid-state byproducts are produced using a proposed plasma-activated aluminum and water reactions approach. These byproducts could be transformed into pure gamma Al2O3 powder material, while hydrogen can be used for electricity generation. Various chemical methods can be used for the synthesis of gamma alumina, but most could result in high levels of remaining impurities. Boehmite is a cost-effective starting material for the production of high-purity Al2O3. Herein, we present a novel method for the synthesis of boehmite and its transformation into high-specific-surface-area γ-alumina. Specifically, this method implicates the direct reaction between distilled water and plasma-treated aluminum powder. The results show the structural and morphological changes of the byproduct of the aluminum/water reaction to boehmite and γ-Al2O3 after a simple heating procedure (at 280 and 500 °C respectively). The high-purity hydrogen produced during the aluminum/water reaction can be used for the high-efficiency and environmentally friendly production of electrical energy.

High-temperature alloys are crucial to many important technologies that underpin our civilization. All these materials rely on forming an external oxide layer (scale) for corrosion protection. Despite decades of research on oxide scale growth, many open questions remain, including the crucial role of the so-called reactive elements and water. Here, we reveal the hitherto unknown interplay between reactive elements and water during alumina scale growth, causing a metastable ‘messy’ nano-structured alumina layer to form. We propose that reactive-element-decorated, hydroxylated interfaces between alumina nanograins enable water to access an inner cathode in the bottom of the scale, at odds with the established scale growth scenario. As evidence, hydride-nanodomains and reactive element/hydrogen (deuterium) co-variation are observed in the alumina scale. The defect-rich alumina subsequently recrystallizes to form a protective scale. First-principles modelling is also performed to validate the RE effect. Our findings open up promising avenues in oxidation research and suggest ways to improve alloy properties.