Doctor of Philosophy, The Ohio State University, 2023, Electrical and Computer Engineering

Beta-phase gallium oxide (β-Ga2O3), with its ultrawide band gap energy (~4.8 eV), high predicted breakdown field strength (6-8 MV/cm), controllable n-type doping and availability of large area, melt-grown, differently oriented native substrates, has spurred substantial interest for future applications in power electronics and ultraviolet optoelectronics. The ability to support bandgap engineering by alloying with Al2O3 also extends β-(AlxGa1-x)2O3 based electronic and optoelectronic applications into new regime with even higher critical field strength that is currently unachievable from SiC-, GaN- or AlxGa1-xN- (for a large range of alloy compositions) based devices. However, the integration of β-(AlxGa1-x)2O3 alloys into prospective applications will largely depend on the epitaxial growth of high quality materials with high Al composition. This is considerably important as higher Al composition in β-(AlxGa1-x)2O3/Ga2O3 heterojunctions can gain advantages of its large conduction band offsets in order to simultaneously achieve maximized mobility and high carrier density in lateral devices through modulation doping. However, due to the relative immaturity of β-(AlxGa1-x)2O3 alloy system, knowledge of the synthesis and fundamental material properties such as the solubility limits, band gaps, band offsets as well as the structural defects and their influence on electrical characteristics is still very limited. Hence, this research aims to pursue a comprehensive investigation of synthesis of β-(AlxGa1-x)2O3 thin films via metal organic chemical vapor deposition (MOCVD) growth methods, building from the growth on mostly investigated (010) β-Ga2O3 substrate to other orientations such as (100), (001) and (-201), as well as exploring other polymorphs, such as alpha (α) and kappa (κ) phases of Ga2O3 and (AlxGa1-x)2O3 to provide a pathway for bandgap engineering of Ga2O3 using Al for high performance device applications. Using a wide range of material characterization techniqu (open full item for complete abstract)

PHD, Kent State University, 2016, College of Arts and Sciences / Chemical Physics

Photoalignment is a well-established technique for surface alignment of the liquid crystal director. Previously, chrome masks were necessary for patterned photoalignment but were difficult to use, costly, and inflexible. To extend the capabilities of photoalignment we built an automated maskless multi-domain photoalignment device based on a DMD (digital multimirror device) projection system. The device is capable of creating arbitrary photoalignment patterns with micron-sized features. Pancharatnam-Berry phase (PB-phase) is a geometric phase that arises from cyclic change of polarization state. By varying the azimuthal anchoring angle in a hybrid-aligned liquid crystal cell we can control the spatial variation of the PB-phase shift. Using our automated photoalignment device to align the liquid crystal arbitrary wave front manipulations are possible. The PB-phase shift effect is maximized when the cell is tuned to have a half-wave retardation and disappears at full-wave retardation, so the cell can be switched on and off by applying a voltage. Two wavefront controlled devices developed using this technique will be discussed: A switchable liquid crystal phase shift mask for creating sub-diffraction sized photolithographic features, and a transparent diffractive display that utilizes a switchable liquid crystal diffraction grating.

PHD, Kent State University, 2009, College of Arts and Sciences / Department of Physics

The study of quantum phase transitions (QPT) provides a new route to find and understand unconventional phases in condensed matter physics. The presently studied alloy, Ni(1-x)Vx, offers an opportunity to investigate a ferromagnetic quantum phase transition, a transition from a ferromagnetic ordered state into a paramagnetic state at T = 0 K, by varying the vanadium concentration, x. Magnetization and transport measurements are used to probe the critical behavior of the phase transition and characterize the onset of “unconventional behavior” such as non-Fermi liquid behavior, which signals a deviation from Fermi liquid theory, a fundamental concept in metals. Towards 11.2 % vanadium, the Curie temperature (Tc) is reduced to zero from its pure nickel value of Tc = 627 K. The critical behavior of the phase transition in samples with the higher nickel content (x < 11%) at a finite Tc essentially follows theories as expected for weak itinerant magnets. The samples with more vanadium (x > 11.2%) do not show a conventional ferromagnetic transition or the typical properties of an ordinary paramagnet. Instead, we see evidence for power laws with unusual exponents in the temperature dependence of the magnetization and the resistivity due to an inhomogeneous magnetic moment distribution. We compare our data findings with recent theories addressing a new critical scenario, quantum phase transitions with disorder. One signature is a Quantum Griffiths' phase which is observed as power laws with non-universal exponents heading towards a T → 0 instability. At very low temperatures, the quantum Griffiths phase in Ni-V leads to the formation of a frozen cluster glass phase. To our knowledge, our compound is the first to experimentally show all signatures of a quantum Griffiths phase in an extended regime, and therefore provides an ideal model system for a disordered itinerant 3-d Heisenberg system.

Doctor of Philosophy, University of Akron, 2007, Polymer Engineering

A free energy functional has been formulated based on an order parameter approach to describe the competition between liquid-liquid phase separation and solid-liquid phase separation. In the free energy description, the assumption of complete solvent rejection from the crystalline phase that is inherent in the Flory diluent theory was removed as solvent has been found to reside in the crystalline phase in the form of intercalates. Using this approach,we have calculated various phase diagrams in binary blends of crystalline and amorphous polymers that show upper or lower critical solution temperature. Also, the discrepancy in the χ values obtained from different experimental methods reported in the literature for the polymer blend of poly(vinylidenefluoride) and poly(methylmethacrylate) has been discussed in the context of the present model. Experimental phase diagram for the polymer blend of poly(caprolactone) and polystyrene has also been calculated. Of particular importance is that the crystalline phase concentration as a function of temperature has been calculated using free energy minimization methods instead of assuming it to be pure. In the limit of complete immiscibility of the solvent in the crystalline phase, the Flory diluent theory is recovered. The model is extended to binary crystalline blends and the formation of eutectic, peritectic and azeotrope phase diagrams has been explained on the basis of departure from ideal solid solution behavior. Experimental eutectic phase diagram from literature of a binary blend of crystalline polymer poly(caprolactone) and trioxane were recalculated using the aforementioned approach. Furthermore, simulations on the spatio temporal dynamics of crystallization in blends of crystalline and amorphous polymers were carried out using the Ginzburg-Landau approach. These simulations have provided insight into the distribution of the amorphous polymer in the blends during the crystallization process. The simulated results are in (open full item for complete abstract)

Doctor of Philosophy (PhD), Wright State University, 2022, Electrical Engineering

5th generation wireless systems (5G) have expanded frequency band coverage with the low-band 5G and mid-band 5G frequencies spanning 600 MHz to 4 GHz spectrum. This dissertation focuses on a microelectronic implementation of CMOS 65 nm design of an All-Digital Phase Lock Loop (ADPLL), which is a critical component for advanced 5G wireless transceivers. The ADPLL is designed to operate in the frequency bands of 600MHz-930MHz, 2.4GHz-2.8GHz and 3.4GHz-4.2GHz. Unique ADPLL sub-components include: 1) Digital Phase Frequency Detector, 2) Digital Loop Filter, 3) Channel Bank Select Circuit, and 4) Digital Control Oscillator. Integrated with the ADPLL is a 90-degree active RC-CR phase shifter with on-line amplitude locked loop (ALL) calibration to facilitate enhanced image rejection while mitigating the effects of fabrication process variations and component mismatch. A unique high-sensitivity high-speed dynamic voltage comparator is included as a key component of the active phase shifter/ALL calibration subsystem. 65nm CMOS technology circuit designs are included for the ADPLL and active phase shifter with simulation performance assessments. Phase noise results for 1 MHz offset with carrier frequencies of 600MHz, 2.4GHz, and 3.8GHz are -130, -122, and -116 dBc/Hz, respectively. Monte Carlo simulations to account for process variations/component mismatch show that the active phase shifter with ALL calibration maintains accurate quadrature phase outputs when operating within the frequency bands 600MHz-930MHz, 2.4GHz-2.8GHz and 3.4GHz-4.2GHz.

Doctor of Philosophy (PhD), Wright State University, 2020, Electrical Engineering

Synthetic aperture ladar (SAL) is an emerging remote sensing technology capable of providing high-resolution, interpretable, and timely imagery. SAL and synthetic aperture radar (SAR) are similar in that they provide high-resolution imagery suitable for a wide-variety of applications beyond the diffraction limit of the real aperture. Several advantages of SAL are; realistic imagery resulting from diffuse scattering of optically-rough objects, fine directionality of laser beam making the technology inherently low probability-of-detect, and shorter synthetic aperture collection times, all of which result from operating at optical as opposed to RF wavelengths. With the dramatic decrease in wavelength, SAL systems become more susceptible to phase errors induced by platform motion, vibration, and atmospheric turbulence. In this research effort, we focus on mitigating the detrimental effects of atmospheric turbulence on SAL image quality. We show that traditional autofocusing algorithms; Phase Gradient Autofocus (PGA), Sharpness-based Autofocus, and Sparsity Driven Autofocus (SDA), are unable to mitigate atmospheric phase errors due to their spatially-variant nature. We overcome the challenge imposed by spatially-variant atmospheric phase errors through the use of a model-based image reconstruction framework. Utilizing this framework we implement three different spatially-variant model error correction algorithms; Moving Target Autofocus (MTA), Spatially-variant Phase Correction (SVPC), and Model-based Atmospheric Phase Correction (MBAPC) algorithms. The MTA algorithm is a spatially-variant phase error estimation algorithm originally designed for focusing moving targets in SAR. We develop an image-quality metric (IQM) based parameter tuning algorithm that enables the success of the MTA algorithm for the unique challenges presented by atmospheric phase errors. Both SVPC and MBAPC are spatially-variant model error correction algorithms developed to handle atmospheric pha (open full item for complete abstract)

Master of Science, The Ohio State University, 2020, Electrical and Computer Engineering

Baud rate clock and data recovery circuits are critical to high speed serial links since these require only one sample per data period thereby requiring low speed samplers and comparators. This work models and discusses the backend of one particular Baud rate CDR – Mueller Muller, and analyses some of the building blocks of the CDR – Phase Detector, Phase Interpolator and the Quadrature Phase Generator. Firstly, a PAM-4 Quadrature Phase Detector operating at 80Gb/s is discussed. The challenges associated with designing a Mueller-Muller PD for an asymmetric channel are discussed and one way to resolve this issue is proposed. Then the underlying digital blocks that make up the Phase detector are expanded upon. Secondly, a 64-step digitally controlled Phase Interpolator running at 16GHz clock rate is analyzed and its design challenges with regards to achieving linearity and ensuring duty cycle fidelity are explored. Finally, a Quadrature Phase Generator with digital delay control is analyzed. It is modeled at 16GHz clock rate and the range/resolution problem and its impact on clock jitter is explored.

Doctor of Philosophy, University of Akron, 2018, Polymer Science

Giant shape amphiphiles with isobutyl polyhedral oligomeric silsesquioxane (BPOSS) cages as the periphery at two discotic trisubstituted derivative of benzene core were specifically designed and synthesized. Depending upon the number of BPOSS cages, these molecules first assembled into either cylindrical or spherical units via p-p interactions among the core unites. The packing of the molecules is mandated by the steric hindrance of the BPOSS cages at the periphery with hydrogen bonding interactions. If the space-packing is allowed, the cylindrical building block can form. Otherwise, the cylindrical building block will be forced to interrupt periodically and to form spherical building blocks. These units can further modular assemble into supramolecular structures. The cylindrical units form columnar structures with both hexagonal and rectangular packing, while the spherical units construct a Frank-Kasper A15 phase, similar to the metal alloy structures. In addition, based on the mechanism proposed by this work, five more giant shape amphiphiles with high steric hindrance on the periphery were synthesized, these giant shape amphiphiles successfully formed A15 phases with precisely size control, validating the reliability of this strategy. Formation of A15 phase base on nano-spherical building blocks offers a new pathway to design and construct new supramolecular phases and further functionalizes these structures.

Doctor of Philosophy (Ph.D.), University of Dayton, 2017, Aerospace Engineering

Future high performance aircraft will need non-conventional thermal management systems to remove challenging heat loads. Historically, aircraft thermal management systems have relied on a combination of air cycle machine and fuel for cooling heat sinks. Next generation tactical aircraft may rely on a vapor compression system as a method to manage thermal loads. Vapor compression systems (VCS) offer higher coefficient of performance (COP) values than aircraft cycle system (ACS). However, the design of a VCS system is more complex since two phases (liquid-vapor) are used to remove heat. In order to incorporate a VCS system in future tactical aircraft design, an understanding of two-phase flow behavior is needed. This experimental dissertation seeks to increase the understanding of two-phase flow behavior and is divided into three segments. In the first segment of this dissertation, electrical capacitance tomography and neural networks were used to identify two-phase flow patterns for refrigerant R-134a flowing in a horizontal tube. In laboratory experiments, high-speed images were recorded for human visual classification of liquid–vapor flow patterns. The corresponding permittivity data obtained from tomograms was then used to train feed-forward neural networks to recognize flow patterns. An objective was to determine which subsets of data derived from tomograms could be used as input data by a neural network to classify nine liquid–vapor flow patterns. Another objective was to determine which subsets of input data provide high identification success when analyzed by a neural network. Transitional flow patterns associated with common horizontal flow patterns were considered. A unique feature of this research was the use of the vertical center of mass coordinate in pattern classification. The highest classification success rates occurred using neural network input which included the probability density functions (in time) for both spatially averaged permittivity and (open full item for complete abstract)

Doctor of Philosophy (PhD), Ohio University, 2016, English (Arts and Sciences)

This document has been removed, as have many others, pending administrative updates to the University's publication system.

Master of Science in Electrical Engineering (MSEE), Wright State University, 2015, Electrical Engineering

This thesis presents wide tuning range in-phase and quad-phase (I/Q) output digital control oscillator (DCO) and voltage control oscillator (VCO) and a simple high performance phase frequency detector (PFD) in 90 nm CMOS technology with 1.2 V power supply. The designed I/Q output DCO has an operating frequency range from 1.1-8.2 GHz with high resolution and wide linearity by using a novel skew delay structure. At 2.02GHz oscillation frequency, the measured phase noise of I/Q output DCO is -90.43dBc/Hz at an offset of 1 MHz offset and has 5 mW of power dissipation. A complementary series coupled quadrature VCO (S-QVCO) is also implemented with measured phase noise of -114.1 dBc/Hz at an offset of 1 MHz from 3.5 GHz center frequency. The operating range of the designed S-QVCO is 3.05-4.62 GHz and has 2.6 mW power dissipation. A simple high resolution novel PFD is designed for high frequency signal detection and low jitter phase locked loop applications. The proposed PFD design can operate over a wide range of frequencies from 10 kHz to 6 GHz and can detect phase differences for inputs as small as 125 fs for all frequencies of operation and for all process corners.

Doctor of Philosophy (PhD), Ohio University, 2016, Physics and Astronomy (Arts and Sciences)

Size, speed and efficiency are the major challenges of next generation nonvolatile memory (NVM), and phase-change memory (PCM) has captured a great attention due to its promising features. The key for PCM is rapid and reversible switching between amorphous and crystalline phases with optical or electrical excitation. The structural transition is associated with significant contrast in material properties which can be utilized in optical (CD, DVD, BD) and electronic (PCRAM) memory applications. Importantly, both the functionality and the success of PCM technology significantly depend on the core material and its properties. So investigating PC materials is crucial for the development of PCM technology to realized enhanced solutions. In regards to PC materials, Sb-Te binary plays a significant role as a basis to the well-known Ge-Sb-Te system. Unlike the conventional deposition methods (sputtering, evaporation), electrochemical deposition method is used due to its multiple advantages, such as conformality, via filling capability, etc. First, the controllable synthesis of Sb-Te thin films was studied for a wide range of compositions using this novel deposition method. Secondly, the solid electrolytic nature of stoichiometric Sb2Te3 was studied with respect to precious metals. With the understanding of 2D thin film synthesis, Sb-Te 1D nanowires (18 – 220 nm) were synthesized using templated electrodeposition, where nanoporous anodic aluminum oxide (AAO) was used as a template for the growth of nanowires. In order to gain the controllability over the deposition in high aspect ratio structures, growth mechanisms of both the thin films and nanowires were investigated. Systematic understanding gained thorough previous studies helped to formulate the ultimate goal of this dissertation. In this dissertation, the main objective is to understand the size effect of PC materials on their phase transition properties. The reduction of effective memory cell size in conjunctio (open full item for complete abstract)

Doctor of Philosophy, Miami University, 2013, Chemistry and Biochemistry

This dissertation consists of four research chapters, which address first the synthesis and characterization of a universal solid phase extraction (SPE) stationary phase and then the use of an alkylammonium formate ionic liquid as the reversed phase liquid chromatography (RPLC) mobile phase modifier for the separation of native proteins. The last chapter outlines a new chemical education experiment for teaching ion exchange. A universal SPE sorbent Strata-Kel-F-p-aminobenzoic acid (Strata-Kel-PABA) was synthesized by a two-step crosslinking reaction followed by an ester hydrolysis. The universal SPE sorbent combines fluorinated reversed phase, cation- and anion-exchange sites together. Compared with the commercially available cartridge Strata-X AW, Strata-Kel-PABA showed better cation exchange and reversed phase capacity and fair anion exchange capacity. The long term goal of this research is equipping this universal SPE cartridge with the autosampler of a RPLC instrument sample preparation. Proteins are hard to separate in their native conformation using RPLC because the tertiary and quaternary structures can be seriously affected by hydrophobic interactions with both stationary and mobile phases in RPLC. The ionic liquid, ispropylammonium formate (IPAF) has been investigated to improve protein stability as a novel RPLC mobile phase modifier. Promising results were shown for non-subunit and two-subunit proteins. The possible mechanism of the IPAF effect on protein stabilization can be explained by the micelle-like structure of the ionic liquid and the Hofmeister series. For labile four subunit proteins, such as lactate dehydrogenase (LDH), polyethylene glycol (PEG) has been combined with IPAF as the mobile phase additive to stabilize the protein. Remarkable enhancements in LDH peak intensity and LDH activity were observed by incorporating 6% PEG 8000 in the organic mobile phase that contained either acetonitrile (MeCN) or IPAF.

Doctor of Philosophy in Clinical-Bioanalytical Chemistry, Cleveland State University, 2024, College of Arts and Sciences

This dissertation explores the influence of mobile phase (MP) polarity on the separation of steroid enantiomer pairs in reversed-phase high-performance liquid chromatography. Previous studies varied the concentration of a single organic modifier in binary MPs, affecting both the MP strength and polarity. This work isolates the effect of MP polarity on separation alone by using a ternary MP. In this work, MP strength is kept constant by varying the percent water, while the MP polarity is changed by varying the ratio of the two organic modifiers. The combined ET(30) and Kamlet-Taft polarity model for MP polarity components of acidity, basicity, and dipolarity/polarizability was studied. The bile acid enantiomer pairs analyzed were alpha-muricholic acid and beta-muricholic acid, and omega-muricholic acid and gamma-muricholic acid. The separation factor (α) did not correlate with the total MP polarity, but did with a specific combined component polarity. The α versus the summed MP dipolarity/polarizability and basicity plot for all non-acetonitrile (ACN) ternary MPs experiments considered as a whole (six different organic pairs of methanol, isopropanol, dioxane, tetrahydrofuran in water), showed good correlation, except for the dioxane-containing MPs. Examination of previously obtained data of five other steroid enantiomer pairs (3β5β+3α5β-, 3β5α+3α5α-, 5α+5β-abiraterones, 17α- + 17β-estradiol, 11α- +11β-hydroxyprogesterone) showed good correlation with component polarity, excluding different aberrant organic modifier (isopropanol or methanol) MPs for several of these pairs. ACN-containing MPs demonstrated different separation characteristics, showing no correlation with non-ACN phases. The dissertation also reports the separation versus component polarity of non-enantiomer compounds (theophylline, caffeine), revealing that changing the polarity of the non-ACN-containing MPs did not result in a significant change in separation. Plots for the ACN-containing MPs wer (open full item for complete abstract)

Master of Science in Electrical Engineering (MSEE), Wright State University, 2023, Electrical Engineering

A method for software-defined radio array calibration is presented. The method implements a matched filter approach to calculate the phase shift between channels. The temporal stability of the system and calibration coefficients are shown through the standard deviation over the course of four weeks. The standard deviation of the phase correction was shown to be less than 2 deg. for most channels in the array and within 8 deg. for the most extreme case. The standard deviation in amplitude scaling was calculated to be less than 0.06 for all channels in the array. The performance of the calibration is evaluated by the antenna gain and the difference from the ideal beam shape for the peak side lobe level and first null depth. For one example data collection, the gain was 61 dB for the array with a maximum difference of 0.2246 dB for the peak side lobe level and 0.3998 dB for the first null depth.

Master of Science (M.S.), University of Dayton, 2022, Electro-Optics

GeSbTe (GST) is a phase-change material that has proved useful for decades. It has seen use in optical disk memory as well as other forms of memory such as phase-change random access memory, or PCRAM, and it is still seeing use today in such devices. It is a material with extraordinary capabilities and these capabilities are further enhanced by doping GST with other materials. GST is a well-studied material in literature, and although it is extensively studied in its undoped form, it has also been extensively studied using different dopants. Dopants serve to improve the electrical, thermal, and optical properties of GST and they have been demonstrated to do so, yet not many have studied the optical emission of GST. Recently, it has been demonstrated that light emission can be achieved from GST by doping it with Ni. In this work, we fabricated layers of undoped, Ni-doped, and W-doped GST and characterized them using energy dispersive X-ray spectroscopy (EDX). The GST layers were subjected to annealing at temperatures of up to 440 °C, after which Raman spectroscopy was performed on each sample. The samples undergo a phase change from amorphous to face-centered cubic (FCC) and hexagonal close-packed (HCP) when annealed at different temperatures. Finally, photoluminescence measurements were performed on the fabricated layers. The undoped GST did not exhibit any luminescence,while the Ni-doped GST showed several luminescence peaks, with the most intense being at 988 nm and approximately 4 nm wide in the HCP phase. In the case of W-doped GST, no luminescence was observed in any state.

PHD, Kent State University, 2022, College of Arts and Sciences / Department of Physics

Strongly correlated electron systems are at the borderline of competing phases and can be tuned through different ground states by slight modifications. Therefore, they are good examples to study a quantum phase transition (QPT), to reveal how a quan tum critical point (QCP) at zero temperature is responsible for the unconventional properties observed at finite temperatures. QPTs are zero-temperature phase tran sitions, they are more complex and less understood than common phase transitions at finite temperatures. Examples are lacking, especially in the case where disorder is involved. Recent theories predict the possibility of an exotic quantum critical point in itinerant magnets with induced disorder that is accompanied by a quantum Grif- fiths phase. To explore such unconventional properties in close neighborhood to a magnetic phase, we aim to reveal the relevant quantum critical fluctuations with neutron scattering. We select systems with different magnetic order and choose as tuning parameter chemical substitution to study the effect of disorder. Such exper imental study aims to find key elements of a QCP with disorder. The two systems are the ferromagnetic (FM) alloy, Ni-V, tuned by the V-concentration into a para magnetic phase, and the non-magnetic Kondo semimetal, Ce(Cu,Ni)Sn, tuned by Cu concentration into an antiferromagnetic state. We apply different neutron scat- tering techniques and simple models to get essential characteristics of the magnetic correlations and fluctuations close to the QCP. Ni-V is a simple FM-alloy with a random atomic distribution that undergoes a quantum phase transition from a ferromagnetic to a paramagnetic state with sufficient substitution of Ni by V. First indication of a quantum Griffiths phase came from magnetization and μSR data, but the scale of the magnetic clusters remained elusive. Optimized small angle neutron scattering (SANS) data on different polycrystalline Ni- V samples close to the QCP (open full item for complete abstract)

Master of Science (MS), Ohio University, 2022, Mechanical Engineering (Engineering and Technology)

Laser Powder Bed Fusion (L-PBF) is a branch of metal additive manufacturing technologies which has become increasingly more popular due to the geometric freedoms and strategic design methods which it allows. L-PBF produces metallic components to near net shape within a single process step while simultaneously allowing for the creation of complex geometries and internal structures which are not readily produced by other manufacturing techniques. Not without issues, L-PBF produces materials with preferential directions of growth in the underlying material microstructure as well as undesirable phase content in many cases. While techniques exist to change microstructure of L-PBF materials, many rely on post-processing or in situ control over the flow of heat. This thesis documents the development and analysis of a novel technique separate from previous methods which allows for in situ modification of grain structure produced in LPBF without the need of complex modification of the machine. Ultrasonic vibrations are introduced to the build process as an added parameter, hypothesizing that in situ ultrasonic cavitation will reduce grain size and modify the formation of secondary phases in a way that is beneficial to the as-manufactured material properties.

Doctor of Philosophy, The Ohio State University, 2022, Materials Science and Engineering

Since its discovery on early 2000's, Multi-principal element alloys (MPEAs) or High Entropy Alloys (HEAs) has gained a lot of research interest owing to the extensive opportunities for new materials discovery. MPEAs contain multiple elements at relatively high concentration. While the original efforts in MPEA development were focused on designing single-phase solid solutions, recent works are directed towards multi-phase MPEAs. The multi-phase MPEAs have precipitation hardening as an additional strengthening mechanism making it suitable for high temperature structural applications. Phase separation mechanism in many MPEA systems such as AlMo0.5NbTa0.5TiZr, Al0.5NbTa0.8Ti1.5V0.2Zr, and AlxCoCrCuFeNi are believed to occur through spinodal-mediated pathways. Understanding spinodal decomposition in multi-component alloys and the possible phase transformation pathways are crucial for the microstructural design of MPEAs. Firstly, a methodology was developed to calculate the spinodal driving force and initial concentration modulation in multicomponent alloys. Secondly, a phase-field model (PFM) with two order parameters was formulated to simulate the microstructural evolution for various free energy surfaces. Our PFM incorporates spinodal decomposition, order↔disorder transition, nucleation and growth, modulus anisotropy, modulus inhomogeneity, lattice misfit, and the effect of asymmetry in the free energy curves of the individual phases. Lastly, a software module (called PanPhasefield) was developed for simulating the microstructural evolution in MPEAs and other commercial alloys using the CALPHAD databases. The module was successfully commercialized and is available in the latest version of PandatTM, a CALPHAD software (https://computherm.com/).

PHD, Kent State University, 2022, College of Arts and Sciences / Department of Physics

The study of quantum phase transitions is a promising route for understanding the origin of unusual properties in strongly correlated electron systems. Recent theories predict a new quantum critical point (QCP) with exotic properties such as an observable quantum Griffiths phase in disordered itinerant systems. Previous studies on disordered ferromagnetic (FM) system Ni(1−x)V(x) showed only “signs” of magnetic clusters close to the critical concentration xc where FM is destroyed. This dissertation provides direct evidence of magnetic correlations at various length scales and fluctuations at different time scales close to the QCP using Ni-V and Ni-Cr samples. Polarized small-angle neutron scattering (SANS) measurements reveal short-range magnetic correlations in Ni-V samples close to QCP. This polarization study also supports long-range magnetic correlations within macroscopic domains. The structure characterization of Ni-Cr confirms a high-quality chemical structure with random atomic distribution after optimizing the annealing protocol. Muon spin rotation (µSR) measurements successfully show the advantage of Ni-Cr. The response is not dominated by any strong nuclear moment like Ni-V. The µSR asymmetry of Ni(1−x)Cr(x) close to xc reveals magnetic order at low temperatures with coexisting dynamic clusters. Both SANS and µSR measurements support that the long-range FM order and short-range fluctuations are present in FM alloys Ni-V and Ni-Cr close to QCP.