Rare-earth doped complex oxide materials have a number of interesting applications in the clean energy economy. By tailoring the materials composition, the electronic, thermal, and ionic transport properties can be tuned for the intended application. In this study, three examples of such applications are considered. First, a crystal structure known as sodium super ionic conductors (Na1+3xZr2(P1-xSixO4)3 ) was modified to synthesize a cerium super ion conductor [Ce.1Zr.9)40/39Nb(PO4)3]. Tuning the composition resulted in a complex oxide that has been shown to have selective cerium ion conductivity, making it a candidate as a solid electrolyte in a real time rare earth ion sensor. These sensors were successfully applied to demonstrate real time monitoring of cerium concentrations in aqueous solutions and could potentially be modified to detect other rare earth elements, eliminating the time intensive chemical assay testing currently used in rare earth recycling and refining. Another application of complex oxides includes the use of lanthanum doped strontium manganese oxide (La.8Sr.2MnO3) as an electronically conductive diffusion barrier. This material was applied to 2205 stainless steel to test its potential as an interconnect in solid oxide fuel cells (SOFCs). These barrier layers can reduce high temperature oxidation and spallation in SOFCs interconnects, potentially increasing the lifetime of SOFC stacks, and allowing for cheaper ferrous alloys to be used in operating conditions up to 800°C, further lowering the overall cost of SOFCs. Lastly, gadolinium doped strontium titanate (Gd.08Sr.92TiO3) is considered as a thermoelectric material. Specifically the addition of a second phase of gadolinium doped cerium oxide (Gd.2Ce.8O3-δ) was used to achieve enhancements in the thermoelectric properties. / 2016-10-27T00:00:00Z

OBJECTIVES: To investigate the effect of various finishing procedures and repeated firings on color stability and translucency of different ceramics. Also, to examine the mechanical properties after finishing procedures. MATERIALS & METHODS: Four glass-ceramics were investigated; IPS e.max CAD, IPS e.max Press, Celtra Duo, and Celtra Press. For each material 87 disc-shaped specimens, 2 mm in thickness and 12 mm in diameter were fabricated. Specimens were divided into 3 groups (n=29) as no treatment, polishing, and glazing group. Five samples from each group were submitted to 1, 3, and 5 simulated “stain” firings using a ceramic furnace according to the manufacturer’s instructions. Color coordinates were recorded in the CIE L*a*b* system using an X-rite Ci7600 spectrophotometer and Vita EasyShade. Color differences relative to the control and translucency by contrast ratio were recorded. The 3-point flexural strength test was performed using an Instron universal testing machine. Hardness test was performed using Vickers microhardness and surface roughness was measured using a contact profilometer. Data were analyzed using multiple ANOVA (α=0.05). RESULTS: In general, glazed materials showed higher flexural strength than polished and no treatment groups. Celtra Duo showed the highest mean Vickers hardness followed by e.max CAD, e.max Press then Celtra Press. Surface roughness is lowered significantly when the glaze was applied to all materials. The contrast ratio of Celtra Duo is significantly lower than e.max CAD, e.max Press, and Celtra Press. Firing cycles displayed a significant effect in lowering the translucency of e.max CAD, and in raising the translucency of Celtra Duo and Celtra press, but no significant effect on Emax press. Surface treatment showed a non-significant effect on contrast ratio in all materials tested. Repeated firing cycles significantly affected color change for all materials tested, except for Celtra Duo. Also, polishing and glazing significantly affected color change for all tested materials. CONCLUSION: Type of material, surface treatments and firing cycles have a significant effect on color change and translucency. Surface treatment have a significant effect on the mechanical properties of glass ceramics; glazing can improve the flexural strength and reduce surface roughness of the ceramic materials.

archives@tulane.edu / Metal whisker formation has been an unwanted byproduct in electronics for the past 70 years. However, in that period, no one has come up with one mechanism that can explain all the collected data. This dissertation focuses on copper whisker formation in the presence of a sulfur-containing compound. Pulsed laser deposition is used to create a molybdenum disulfide thin film, and the resulting whiskers are analyzed with scanning electron microscopy and x-ray photoelectron spectroscopy. The copper whiskers are proven to be pure copper and can be several millimeters in length. The collected data supports a recrystallization-based mechanism, where the copper vapor on the surface recrystallizes at the base of the copper whisker, therefore, adding to the length of the whisker. / 1 / Megan Elizabeth Woods

Nanomaterials are small structures that have at least one dimension less than ~100 nanometers. Depending on the number of dimensions that are not confined to the nanoscale range, nanomaterials can be classified into 0D, 1D and 2D types. Due to their small sizes, nanoparticles possess exceptional physical and chemical properties which opens a unique possibility for the next generation of strain sensors that are cheap, multifunctional, high sensitivity and reliability. Over the years, thanks to the development of new nanomaterials and the printing technologies, a number of printing techniques have been developed to fabricate a wide range of electronic devices on diverse substrates. Nanomaterials based thin film devices can be readily patterned and fabricated in a variety of ways, including printing, spraying and laser direct writing. In this work, we review the piezoresistivity of nanomaterials of different categories and study various printing approaches to utilize their excellent properties in the fabrication of scalable and printable thin film strain gauges. CNT-AgNP composite thin films were fabricated using a solution based screen printing process. By controlling the concentration ratio of CNTs to AgNPs in the nanocomposites and the supporting substrates, we were able to engineer the crack formation to achieve stable and high sensitivity sensors. The crack formation in the composite films lead to piezoresistive sensors with high GFs up to 221.2. Also, with a simple, low cost, and easy to scale up fabrication process they may find use as an alternative to traditional strain sensors. By using computer controlled spray coating system, we can achieve uniform and high quality CNTs thin films for the fabrication of strain sensors and transparent / flexible electrodes. A simple diazonium salt treatment of the pristine SWCNT thin film has been identified to be efficient in greatly enhancing the piezoresistive sensitivity of SWCNT thin film based piezoresistive sensors. The coupled mechanical stretching and Raman band shift characterization provides strong evidence to support this point of view. The same approach should be applicable to other types of carbon based strain sensors for improving their sensitivity. The direct laser writing (DLW) method has been used for producing flexible piezoresistive sensor and sensor arrays on polyimide film substrates. The effect of CO2 laser irradiation conditions on the morphology, chemical composition and piezoresistivity of the formed graphitic line features were systematically studied to establish the related processing-structure-property relationship. The DLW generated sensors have been demonstrated for their use as strain gauges for structural health monitoring of polymeric composites, and as flexible and wearable sensors of gesture recognition for human-machine interactions. The versatility of the DLW technique demonstrated in this work can be highly valuable in different industrial sectors for developing customized flexible electronics. / A Dissertation submitted to the Department of Industrial and Manufacturing Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2016. / October 25, 2016. / Includes bibliographical references. / Zhibin Yu, Professor Co-Directing Dissertation; Tao (Ted) Liu, Professor Co-Directing Dissertation; Jim P. Zheng, University Representative; Changchun (Chad) Zeng, Committee Member; Mei Zhang, Committee Member.

The importance of permanent magnets with rare earth metals has grown as emerging markets increase their demand for technologies made with these magnets. However, the constraints of rare earth suppliers, the environmental impact of the refining process, and price volatility have left consumers seeking alternative magnets for their applications. This has led to the search for a non-rare earth containing bulk material that will achieve similar magnetic strength and energy product as its rare earth counterpart. Mn-Ga binary alloys have shown promising magnetic properties, even though these alloys contain no rare-earth metals. Both theoretical predictions and experimental work showed that nanoscaled Mn-Ga samples, such as thin films, could have remarkable magnetic properties. Although the prediction provides a useful guideline, and thin films supply us with materials in some application, bulk materials are required for the majority of applications that require a strong magnetic field. To achieve nanostructure as obtained in thin films, we have performed research on making bulk (Mn1-x-yMy)Gax or Mn1-xGax-yMy alloys, where M is a substitutional metallic element, by dynamic mechanically milling and heat treatments. We report efforts to create hard magnetic materials for permanent magnet use. Using this process, materials with high coercivity, up to 18.8 kOe, have been fabricated. We demonstrate that we could enhance the remanence in bulk Mn based materials by engineering both the chemistry and fabrication routes. Fabrication was followed by a study into the coercivity mechanisms in these materials. / A Dissertation submitted to the Program in Material Science and Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester 2016. / July 12, 2016. / Hard magnetic material, Material science, Rare-earth free / Includes bibliographical references. / Ke Han, Professor Co-Directing Dissertation; Eric Hellstrom, Professor Co-Directing Dissertation; Don Levitan, University Representative; Naresh Dalal, Committee Member; Theo Siegrist, Committee Member; William Oates, Committee Member.

The isothermal and non-isothermal crystallization kinetics of poly (propylene succinate) stereocomplex over a broad range of temperatures has been studied. Crystallization peaks from Differential Scanning Calorimeter are only observed when cooling (at 10°C/min) from melt temperatures ~15 degrees above the observed melting peak due to self-seeding (heterogeneous melt). Annealing in this range of melt temperatures leads to a faster crystallization. This behavior is not found in analogs stereocomplexes of PLA, and is attributed to the interplay between dipolar interactions and diffusion of the enantiomeric helical sequences with formation of more nuclei with time in the melt. From a heterogeneous melt at low undercooling, the polymer displays a faster nucleation confined to the spherulitic boundaries, and then a growth with rate similar to that from a homogenous melt temperature. Linear spherulitic growth rates of the stereocomplex from a homogenous melt follow the classical bell shape crystallization temperature dependence peaking at 61ºC. The analysis of the growth rates according to secondary nucleation theory indicate two regimes of crystallization conforming to a transition from regime II to III. From the analysis, the nucleus basal surface free energy is estimated as ~ 55 ergs/cm2, in close agreement to values obtained for similar polyesters. / A Thesis submitted to the Program in Materials Science and Engineering in partial fulfillment of the Master of Science. / Spring Semester 2017. / April 10, 2017. / Includes bibliographical references. / Rufina G. Alamo, Professor Directing Thesis; Eric Hellstrom, Committee Member; Hoyong Chung, Committee Member.

Functional nano or microparticles in solution can form stimuli-responsive smart fluids that exhibit drastic property changes in the presence of magnetic or electric fields that originate from the interparticle interactions. For example, the most commonly utilized type of particle-based smart fluid are magnetorheological fluids (MRF) that contain ferromagnetic microparticles that allow them to reversibly solidify when they experience a magnetic field. The tunable nature of these materials not only make them useful in a variety of industries, but also make them a versatile system in which to study the influence of interparticle interactions on emergent behaviors. In this dissertation, we explore methods for tuning interparticle interactions with applied fields, additives, and functionalized particles and develop, through both experimentation and modeling, design rules for realizing new classes of smart fluids. First, we address a common limitation on the performance of MRF, namely slip failure, through the use of a shear-thickening additive to reinforce MRF particle chains as slip begins. Through flow- and oscillation-mode rheology, we find that a shear-thickening MRF has 60% higher yield stress than a conventional shear-thinning MRF. The shear-thickening additive allows us to affect the microstructure of the fluid in order to increase bulk performance by changing its failure mode. Next, we explore the hypothesis that highly anisotropic 2D sheets can reinforce conventional MRF as an additive by supporting the particle chains. Interestingly, the 2D sheets affect the performance of the fluid minimally in a boundary-driven flow because of the alignment of the sheets in the fluid velocity profile. However, we find that the 2D sheets increase MRF performance in pressure-driven flows by up to 45%. We determine through modeling that this performance improvement stems from the anisotropic sheets physically reinforcing the particle chains. This work has consequences for the design of MRF for applications using pressure-driven flows, such as soft robotics. In addition to studying the additives as a path to strengthening MRF, we investigate whether the magnetic particles themselves can be modified to chemically adhere to one another, thus providing additional attractive forces to supplement the magnetic force between particles. Using flow- and oscillation-mode rheology, we quantify the performance using both the yield stress and chain stiffness as performance metrics. By developing two different adhesive MRF, we find that linked chains exhibit a 40% increase in yield stress and a 100% increase in stiffness. Using thymine-functionalized particles, we present a dynamic method for linking particles in an MRF for increased performance. Finally, a system of polarizable nanoparticles is investigated after it is observed to exhibit a macroscopic cellular phase with particle-poor voids and particle-rich walls in a fluid cell when applying an AC and DC field. By tuning the applied AC and DC fields, we identify the conditions necessary for the phase transition using fluorescence microscopy. We also find through Cahn-Hilliard analysis and additional experiments that the cellular phase is the result of various types of electrically-induced interactions. Specifically, electrophoresis causes the particles to accumulate on one electrode, then electroosmotic and electrohydrodynamic flows occur and exert attractive and repulsive forces on the particles. When the electrohydrodynamic flow dominates, voids nucleate at high field regions at which point spinodal decomposition into the cellular phase occurs. This understanding allows us to explore ways to tune this behavior such as using photolithography to control the location of the voids, and thus the structure of the material.

No description available.

Ion bombardment can lead to a spontaneous formation of a range of nanopatterns on surfaces, including nanodots, nanoscale ripples, and nanoscale pits or holes. Research in this thesis is mainly focused on the behavior of ripple patterns on silicon surfaces as a function of the ion incidence angle. A preliminary study of ion beam nanopatterning of polymer thin films is also presented in this thesis. The research on ion beam nanopatterning of Si is largely motivated by a recent theory predicting the development of well ordered ripples when the ion incidence angle is close to the critical angle. For this study of silicon nanopatterning, initially flat samples were bombarded by a broad beam of 250 eV Ar+ ions with a range of angles (65° - 41°) close to the critical angle, leading to the spontaneous formation of nanoscale ripples. Atomic Force Microscopy (AFM) topographs show the change of ripples as the function of ion incidence angle. The ripple wavelength and crest length are measured from AFM topographs to quantify the degree of order. The ripple peak width in the AFM Power Spectral Density is also examined. In general, the ripples achieve higher degree of order close to critical angle. However, the behavior of ripple formation close to critical angle is more complicated than the theory’s prediction. More studies need to be done to further understand this complex behavior. For the preliminary study of polymer thin film nanopatterning, initially flat samples were bombarded by 250 eV Ar+ ions at the incidence angle of 0°, 45°, and 65° to study the interaction of ion beams with polymers based on different proposed pattern forming mechanisms. Scanning Electron Microscopy (SEM) and AFM topographs show that several types of interesting patterns formed on the surface of different polymer thin films. / 2022-05-15T00:00:00Z

Transition metal borides (labeled as MBenes) nanosheets, an emerging family of two dimensional (2D) materials, is considered promising in the fields of electrocatalysis, ion batteries and superconductors, despite only a few MBenes have been successfully prepared. Thus, expanding the landscape of 2D metal borides is of great importance. However, developing facile methods for ultrathin crystalline MBenes with satisfactory lateral dimension for electronic devices is still highly desirable due to the difficulties in the traditional synthesis approaches such as chemical vapor deposition (CVD) and mechanical exfoliation. Hereby, we focus on the preparation of metal borides with a simple atomic substitution method. In this project, we use ultrathin materials including Mo5N6 and exfoliated MoS2 as well as WS2 as precursor and B/B2O3 powder as boron sources. By this method, we obtain various ultrathin crystalline metal borides without damaging the original morphology. Additionally, we also demonstrate the achievement of B-doping metal dichalcogenides and nitrides by partial substitution through controlling the reaction time and temperature. We believe this method will give a new insight on how to obtain various ultrathin high crystalline metal borides by simply boronizing different precursors