Browsing by Subject "Microstructure"
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Item Influence of compatibility conditions on the microstructure at phase transformation(2013-07) Chen, XianThe purpose of this research is to study systematically compatibility conditions and their implications for the microstructure of a phase-transforming material. The phase transformation in this thesis is restricted to crystalline solid-to-solid phase transformation. The conditions of compatibility refer to compatibility in the sense of the nonlinear elastic theory of martensite. Different versions of these conditions of compatibility are studied in this thesis, ranging from "weak compatibility'' (continuity along lines aligned with precipitates) to very strong conditions of compatibility as expressed by the "cofactor conditions''. In the case of a diffusionless, reversible martensitic phase transformation, the free energy of the undistorted body is described as the volume integral of the free energy density function, which depends on the temperature and deformation gradient of the continuous body. This free energy at continuum level describes the elastic and chemical energy stored in the lattice. Macroscopic deformations are related to lattice deformation by the Cauchy-Born rule.This rule yields a deformation gradient F relating a sublattice of the austenite phase to the primitive lattice of the martensite phase. We derive a heuristic algorithm to find F directly from X-ray diffraction measurement for both phases. For such a transformation both the lattice parameters and the symmetry of the crystal structure change. We assume that the free energy is invariant under rigid rotations and symmetry operations. The transformation stretch matrix U is calculated from the deformation gradient F by polar decomposition. The associated crystallographically equivalent variants U1, ..., Un are determined by symmetry arguments (We can choose U1 = U.). The matrices I (austenite) and U1, ..., Un (variants of martensite) determine the_para>energy wells of the free energy density. The formation of microstructure arises from the simultaneous requirements of energy minimization, i.e., being near the energy wells, and compatibility. The Widmanstatten type precipitation process produces a microstructure of elongated precipitates. For this microstructure we propose a weaker condition of compatibility than is used in the study of martensite. This weaker condition implies a rank-two connection between energy wells and predicts directions of elongation for the precipitates. This condition can be interpreted as a mathematical condition of semi-coherence. The transformation stretch matrix is calculated by the same algorithm mentioned above. The weak compatibility condition is equivalent to the statement that the smallest and largest eigenvalues of U satisfy &lambda1 &le 1 &le &lambda3, which in turn implies that there is an undistorted direction e. We study this condition in the thermoelectric material PbTe/Sb2Te3, which consists of Sb2Te3 precipitates in a PbTe matrix. This material shows typical Widmanst"atten microstructure. The satisfaction of the rank-two condition for this material implies that the undistorted directions of the precipitates lie on the lateral surface of a cone determined by the eigenvalues of U. By symmetry, there are four crystallographically equivalent cones, that all together restrict the spacial distribution of the Widmanstatten precipitates Sb2Te3. A 3D image reconstructed from a set of SE images of the precipitates by means of slice-and-view technique shows a good agreement with this theory. For the martensitic phase transformation, we discuss the cofactor conditions. These are currently the strongest achievable conditions of compatibility for the formation of microstructure of austenite and twinned martensite. The satisfaction of the cofactor conditions implies the existence of infinitely many compatible ways that twinned martensite laminates of any volume fraction can coexist with austenite at a low-energy interface. In this thesis we show that, in fact, many of these energy minimizing microstructures have zero elastic energy at all length scales. Experimentally, we have successfully achieved the first example Zn45Au30Cu25 whose lattice parameters closely satisfy the cofactor conditions for both Type I and Type II twin systems. This material shows enhanced reversibility and extremely low hysteresis upon cyclic phase transformation. Strikingly, the martensitic microstructure has no reproducibility from cycle to cycle.This phenomenon contrasts sharply with the traditional martensite for a polycrystalline solid, which shows a detailed martensite memory effect for cyclic phase transformation. The zero elastic energy microstructures can be used as the building blocks of a set of compatible triple junctions between a pair of Type I twin/austenite and quad junctions consisting of a pair of Type I twin/Type II twins. From X-ray diffraction measurements, we calculate these building blocks for Zn45Au30Cu25, which are then used to construct a complex mosaic of microstructure. This microstructure is apparently observed under optical microscopy, but this awaits detailed confirmation by Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM), currently underway by the author in collaboration with researchers at Carnegie Mellon University and the University of Antwerp.Item Microstructural Engineering and Phase Transformations Study of α″-Fe16N2 for Rare-Earth-Free Permanent Magnets(2018-08) Mehedi, MdPermanent magnets are one of the most important building blocks of motors, generators, sensors, hard disks and more. High-energy product permanent magnets contain a significant amount of rare-earth elements. The extraction process of rare-earths are expensive and energy intensive, and also hazardous for human health and the environment. Moreover, supply chain issues of the rare-earths added instability in the permanent magnet market. Additionally, we are observing an increased demand for permanent magnets in past couple of years because of the increase in vehicle electrification, and increasing demand for renewable energy sources. In these regards, there is a necessity to produce a rare-earth-free permanent magnet to reduce the detrimental effect on human health and the environment and have a stable supply chain. A promising rare-earth-free permanent magnet should possess a high remanent magnetic flux density (Br), a large coercivity (Hc), and consequently, a large energy product ((BH)max). α″-Fe16N2 has been emerging as one of the promising candidates because of its large magnetocrystalline anisotropy of 1.8x107 erg/cm3 and high saturation flux density of 2.6-2.9 T and the abundant availability of iron and nitrogen on the earth. In this dissertation, a method was demonstrated of fabricating α″-Fe16N2 ribbons with an optimized microstructure to obtain high coercivity. We developed a coercivity model based on the microstructure analysis and refined the microstructure using a different alloy system. We have designed a new alloy based on B and Cu doped into the Fe to obtain the suitable microstructure for high coercivity, hence high-energy product. The alloy design and development have been analyzed using a scanning electron microscope, and x-ray diffraction. The solid-state phase transformations are the key to forming the thermodynamically metastable martensitic FeN phases, such as α′-Fe8N and α″-Fe16N2. In this dissertation, we demonstrated nitrogen diffusion kinetics in the Cu and B doped Fe and found that the N diffusion coefficient is two magnitudes lower in the FeCuB matrix than the undoped nanocrystalline Fe. We also found the activation energy for N diffusion in FeCuB matrix as of 76 kJ/mol. The activation energy is important to understand the iron nitride phases in the Cu and B doped Fe. The martensitic phase transformation of FeN was also studied and optimized to obtain the α″-Fe16N2 phase. The change in microstructure due to solid-state phase transformations was also analyzed to understand the N and other alloying element’s behavior on the Fe matrix and their relation to magnetic properties. Finally, we also demonstrated a newly developed soft magnet with C doping in the FeN compound, Minnealloy, with an ultra-high saturation magnetization of 2.8±0.15 T, which is 27% higher than pure iron, and almost five times higher than Ferrite, the most used soft magnetic material. High saturation flux density will be helpful in reducing the machine size and weight.Item Shipboard turbulence and temperature profiles from the Near Inertial Coastal Experiment (NICE), 2017-2019(2023-01-27) Kelly, Samuel M; smkelly@d.umn.edu; Kelly, Samuel M; University of Minnesota Duluth, Large Lakes ObservatoryThe Near Inertial Coastal Experiment (NICE) observed the physical properties of western Lake Superior from 2016-2020. The aim was to observe near-inertial internal waves and vertical mixing due to turbulence. Using the R/V Blue Heron as an observing platform during the summers of 2017, 2018, and 2019, a Rockland Scientific VMP-250 recorded 5,813 profiles of temperature, salinity, fluorescence, turbidity, and the turbulent kinetic energy dissipation rate, and an RBR fastDuet recorded 3,974 profiles of temperature. These profiles are archived here. Additional underway data from the R/V Blue Heron is available at the Rolling deck 2 Repository (R2R; https://www.rvdata.us/search/vessel/Blue%20Heron). Moored data from the NICE experiment was documented by Austin and Elmer (2022).Item Stress-driven melt segregation and reactive melt in ltration in partially molten rocks deformed in torsion with applications to melt extraction from Earth's mantle.(2010-10) King, Daniel S. H.Melt extraction from Earth's upper mantle requires transport of magma from regions of partial melting at depth to the Earth's surface. During its ascent, melt interacts chemically and mechanically with the rock matrix. Melt reduces the viscosity of the partially molten rock compared to that of a melt-free rock. This weakening is a potential mechanism of strain localization that could have significant geodynamical implications. Magma interacts chemically with mineral phases during its ascent, dissolving phases in which it is undersaturated and precipitating phases in which it is oversaturated. Such melt-rock reaction can be a driving force for melt migration. Water and other volatiles also partition into the melt from minerals and are then expelled to Earth's oceans or atmosphere. This process leaves behind stronger dehydrated rocks, and it could be the mechanism by which the oceanic lithosphere (mechanical boundary layer) is formed. The work presented here is an experimental investigation of several mechanisms that influence the distribution of melt within a viscously deformable partially molten rock. Three mechanisms are considered, either alone or in various combinations. (1) An applied shear stress causes melt to align and segregate into melt-rich bands with a consistent geometrical relationship to the shear geometry. In Chapter 2, we investigate possible means of scaling the bands that form in experimental samples to Earth's mantle and explore the evolution of melt-rich bands at high shear strain. (2) Interfacial tension driven flow acts to homogenize the distribution of melt within a partially molten sample. In Chapter 3, we investigate the evolution of melt distribution during static annealing of a sample containing melt-rich bands. We compare the experimental results with models of interfacial tension driven flow to determine which mechanisms control the rate of melt redistribution. (3) A melt source that is undersaturated in some component, when coupled with a sink that is rich in that component, will infiltrate into the sink through reactive flow. This reactive flow can develop into an instability in which fingers of high melt fraction propagate into the sink. In Chapter 4 we investigate this process both under static conditions and in combination with stress-driven melt segregation.Item Synthesis, deposition, and microstructure development of thin films formed by sulfidation and selenization of copper zinc tin sulfide nanocrystals(2014-08) Chernomordik, Boris DavidSignificant reduction in greenhouse gas emission and pollution associated with the global power demand can be accomplished by supplying tens-of-terawatts of power with solar cell technologies. No one solar cell material currently on the market is poised to meet this challenge due to issues such as manufacturing cost, material shortage, or material toxicity. For this reason, there is increasing interest in efficient light-absorbing materials that are comprised of abundant and non-toxic elements for thin film solar cell. Among these materials are copper zinc tin sulfide (Cu2ZnSnS4, or CZTS), copper zinc tin selenide (Cu2ZnSnSe4, or CZTSe), and copper zinc tin sulfoselenide alloys [Cu2ZnSn(SxSe1-x)4, or CZTSSe]. Laboratory power conversion efficiencies of CZTSSe-based solar cells have risen to almost 13% in less than three decades of research. Meeting the terawatt challenge will also require low cost fabrication. CZTSSe thin films from annealed colloidal nanocrystal coatings is an example of solution-based methods that can reduce manufacturing costs through advantages such as high throughput, high material utilization, and low capital expenses. The film microstructure and grain size affects the solar cell performance. To realize low cost commercial production and high efficiencies of CZTSSe-based solar cells, it is necessary to understand the fundamental factors that affect crystal growth and microstructure evolution during CZTSSe annealing. Cu2ZnSnS4 (CZTS) nanocrystals were synthesized via thermolysis of single-source cation and sulfur precursors copper, zinc and tin diethyldithiocarbamates. The average nanocrystal size could be tuned between 2 nm and 40 nm, by varying the synthesis temperature between 150 °C and 340 °C. The synthesis is rapid and is completed in less than 10 minutes. Characterization by X-ray diffraction, Raman spectroscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy confirm that the nanocrystals are nominally stoichiometric kesterite CZTS. The ~2 nm nanocrystals synthesized at 150 °C exhibit quantum confinement, with a band gap of 1.67 eV. Larger nanocrystals have the expected bulk CZTS band gap of 1.5 eV. Several micron thick films deposited by drop casting colloidal dispersions of ~40 nm CZTS nanocrystals were crack-free, while those cast using 5 nm nanocrystals had micron-scale cracks. We showed the applicability of these nanocrystal coatings for thin film solar cells by demonstrating a CZTS thin film solar cell using coatings annealed in a sulfur atmosphere. We conducted a systematic study of the factors controlling crystal growth and microstructure development during sulfidation annealing of films cast from colloidal dispersions of CZTS nanocrystals. The film microstructure is controlled by concurrent normal and abnormal grain growth. At 600 °C to 800 °C and low sulfur pressures (50 Torr), abnormal CZTS grains up to 10 µm in size grow on the surface of the CZTS nanocrystal film via transport of material from the nanocrystals to the abnormal grains. Meanwhile, the nanocrystals coarsen, sinter, and undergo normal grain growth. The driving force for abnormal grain growth is the reduction in total energy associated with the high surface area nanocrystals. The eventual coarsening of the CZTS nanocrystals reduces the driving force for abnormal crystal growth. Increasing the sulfur pressure by an order of magnitude to 500 Torr accelerates both normal and abnormal crystal growth though sufficient acceleration of the former eventually reduces the latter by reducing the driving force for abnormal grain growth. For example, at high temperatures (700-800 oC) and sulfur pressures (500 Torr) normal grains quickly grow to ~500 nm which significantly reduces abnormal grain growth. The use of soda lime glass as the substrate, instead of quartz, accelerates normal grain growth. Normal grains grow to ~500 nm at lower temperatures and sulfur pressures (i.e., 600 °C and 50 Torr) than those required to grow the same size grains on quartz (700 °C and 500 Torr). Moreover, carbon is removed by volatilization from films where normal crystal growth is fast. There are significant differences in the chemistry and in the thermodynamics involved during selenization and sulfidation of CZTS colloidal nanocrystal coatings to form CZTSSe or CZTS thin films, respectively. To understand these differences, the roles of vapor pressure, annealing temperature, and heating rate in the formation of different microstructures of CZTSSe films were investigated. Selenization produced a bi-layer microstructure where a large CZTSSe-crystal layer grew on top of a nanocrystalline carbon-rich bottom layer. Differences in the chemistry of carbon and selenium and that of carbon and sulfur account for this segregation of carbon during selenization. For example, CSe2 and CS2, both volatile species, may form as a result of chalcogen interactions with carbon during annealing. Unlike CS2, however, CSe2 may readily polymerize at room temperature and one atmosphere. Carbon segregation may be occurring only during selenization due to the formation of a Cu-Se polymer [i.e., (CSe2-x)] within the nanocrystal film. The (CSe2-x) inhibits sintering of nanocrystals in the bottom layer. Additionally, a fast heating rate results in temperature variations that lead to transient condensation of selenium on the film. This is observed only during selenization because the equilibrium vapor pressure of selenium is lower than that of sulfur. The presence of liquid selenium during sintering accelerates coarsening and densification of the normal crystal layer (no abnormal crystal layer) by liquid phase sintering. Carbon segregation does not occur where liquid selenium was present.Item Transverse shear microscopy: a novel microstructural probe for organic semiconductor thin films.(2010-08) Kalihari, VivekThe microstructure of ultrathin organic semiconductor films (1-2nm) on gate dielectrics plays a pivotal role in the electrical transport performance of these films in organic field effect transistors. Similarly, organic/organic interfaces play a crucial role in organic solar cells and organic light emitting diodes. Therefore, it is important to study these critical organic interfaces in order to correlate thin film microstructure and electrical performance. Conventional characterization techniques such as SEM and TEM cannot be used to probe these interfaces because of the requirement of conducting substrates and the issue of beam damage. Here, we introduce a novel contact mode variant of atomic force microscopy, termed transverse shear microscopy (TSM), which can be used to probe organic interfaces. TSM produces striking, high contrast images of grain size, shape, and orientation in ultrathin films of polycrystalline organic materials, which are hard to visualize by any other method. It can probe epitaxial relationships between organic semiconductor thin film layers, and can be used in conjunction with other techniques to investigate the dependence of thin film properties on film microstructure. In order to explain the TSM signal, we used the theory of linear elasticity and developed a model that agrees well with the experimental findings and can predict the signal based on the components of the in-plane elastic tensor of the sample. TSM, with its ability to image elastic anisotropy at high resolution, can be very useful for microstructural characterization of soft materials, and for understanding bonding anisotropy that impacts a variety of physical properties in molecular systems.Item Understanding particulate coating microstructure development.(2010-09) Roberts, Christine CardinalHow a dispersion of particulates suspended in a solvent dries into a solid coating often is more important to the final coating quality than even its composition. Essential properties like porosity, strength, gloss, particulate order, and concentration gradients are all determined by the way the particles come together as the coating dries. Cryogenic scanning electron microscopy (cryoSEM) is one of the most effective methods to directly visualize a drying coating during film formation. Using this method, the coating is frozen, arresting particulate motion and solidifying the sample so that it be imaged in an SEM. In this thesis, the microstructure development of particulate coatings was explored with several case studies. First, the effect of drying conditions was determined on the collapse of hollow latex particles, which are inexpensive whiteners for paint. Using cryoSEM, it was found that collapse occurs during the last stages of drying and is most likely to occur at high drying temperatures, humidity, and with low binder concentration. From these results, a theoretical model was proposed for the collapse of a hollow latex particle. CryoSEM was also used to verify a theoretical model for the particulate concentration gradients that may develop in a coating during drying for various evaporation, sedimentation and particulate diffusion rates. This work created a simple drying map that will allow others to predict the character of a drying coating based on easily calculable parameters. Finally, the effect of temperature on the coalescence and cracking of latex coatings was explored. A new drying regime for latex coatings was identified, where partial coalescence of particles does not prevent cracking. Silica was shown to be an environmentally friendly additive for preventing crack formation in this regime.