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.

Committee: Jay Mathews (Advisor); Jonathan Slagle (Committee Member); Mariacristina Rumi (Committee Member); Partha Banerjee (Committee Member) Subjects: Materials Science; Optics; Physics

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)

Committee: Gang Chen (Advisor); David Drabold (Committee Member); Martin Kordesch (Committee Member); Hao Chen (Committee Member) Subjects: Chemistry; Condensed Matter Physics; Materials Science

Doctor of Philosophy (Ph.D.), University of Dayton, 2024, Electro-Optics

Tunable optical filters play a crucial role in various applications including telecommunications, spectroscopy, and sensing. Among different approaches for achieving tunability, phase change materials (PCMs) have garnered significant attention due to their reversible transition between amorphous and crystalline states in response to external stimuli such as temperature, electric field, or optical irradiation. The unique optical properties of PCMs, such as changes in refractive index, transmission, and absorption, during phase transition enable the creation of dynamically reconfigurable optical devices. By integrating PCMs into photonic structures such as Fabry-Perot cavities, photonic crystals, or metamaterials, researchers have demonstrated tunable filters with capabilities ranging from spectral filtering to wavelength-selective switching. In this work we have developed the fundamental principles underlying the operation of PCM-based thin film tunable optical filters, and highlight the key considerations in material selection, device design, and integration. Additionally, we systematically explore the incorporation of PCMs in optical thin film designs, and analyze the performance and conceptual gaps in conventional approaches. This dissertation addresses the design, fabrication, and testing of tunable filters for a few select applications to showcase their utility. It also addresses the initial implementation of electrical switching for multilayer thin-film stacks.

Committee: Andrew Sarangan (Advisor); Partha Banerjee (Committee Member); Arka Majumdar (Committee Member); Keigo Hirakawa (Committee Member); Imad Agha (Committee Member) Subjects: Engineering; Optics

Doctor of Philosophy, University of Akron, 2023, Polymer Engineering

One-way shape memory polymers (SMPs) possess the unique ability to remember a programmed 'temporary shape' and revert to its original shape when exposed to an external stimulus. Typically, SMPs contain two structure-spanning, solid networks; a permanent elastic network that is strained during programming to drive shape recovery; and a temporary network that fixes the programmed shape. The shape-shifting features of SMPs make them useful for a wide range of potential applications, including 4D printing, soft robotics, flexible electronics, soft aeronautical engineering, and biomedical devices. An interesting pathway to develop SMPs is by blending an elastomer and a crystalline small molecule, where the elastomer forms the permanent network (that promotes shape recovery), and the small molecule crystal forms the temporary networks (that promotes shape fixity). Typical examples of these systems include crosslinked elastomers (natural rubber) swelled in fatty acids (lauric acid, stearic acid, and palmitic acid), straight-chain alkanes (eicosane, tetracosane) or synthetic waxes (paraffin wax). However, a drawback of this approach is the blooming and expulsion of the small molecule during shape programming and recovery. This dissertation attempts to focus on semi-crystalline shape memory elastomers developed from blends of high cis 1,4 polybutadiene and reactive monomers (octadecyl acrylate and benzyl methacrylate) or molecular crystals (n-eicosane and n-tetracosane) with the aim being reducing the effect of blooming while keeping a simple fabrication route to develop these SMPs. The synthetic, network, mechanical, thermal, and morphological properties of a series of polybutadiene-based semicrystalline or glassy blends were studied to understand the structureproperty relationships between their permanent and reversible networks. Furthermore, it will be shown that thermally annealed high cis 1,4 polybutadiene also demonstrates thermoresponsive actuatio (open full item for complete abstract)

Committee: Kevin Cavichhi (Advisor); Fardin Khabaz (Committee Chair); Qixin Zhou (Committee Member); Li Jia (Committee Member); Weinan Xu (Committee Member) Subjects: Chemistry; Materials Science; Plastics

Doctor of Philosophy (Ph.D.), University of Dayton, 2023, Mechanical Engineering

Paraffin phase change materials (PCMs) are often used for thermal energy storage due to their high gravimetric latent heat values and low cost. However, they are not well suited for high heat transfer rate applications where their low thermal conductivities limit use. Additionally, applications in which there are volume restrictions may drive material selection toward metallic phase change materials, where the volumetric latent heat can be higher than those of paraffins with comparable melting points. Gallium, for example, has over twice the volumetric latent heat and a thermal conductivity that is two orders of magnitude greater than that of octadecane. The use of gallium is not without issue. Gallium is known to experience supercooling which is often viewed as a detrimental property. Thus, an improved understanding of supercooled gallium nucleation is useful. One purpose in this study is to explore the effects of the thermal history and mass on the supercooling of gallium. In this work, differential scanning calorimetry and a thermal cycling chamber were used to characterize these effects. Material overheating was found to have the largest impact. Additionally, an active control methodology was created to successfully activate solidification without significantly affecting the bulk material temperature. While the previously mentioned active control methodology demonstrated successful nucleation of supercooled gallium using a vortex-tube/cold-finger design, the time (>10 min) and level of support equipment required (e.g., compressor) allows for improved design. Thus, a thermoelectric cooling approach, using electric current to rapidly remove heat from the supercooled gallium, was investigated. In this work, a two-stage thermoelectric cooler and cold-finger design was implemented to decrease the required nucleation time, and increase the effectiveness of the nucleation process. An order of magnitude decrease in time needed for nucleation (~10 s) was achieved for al (open full item for complete abstract)

Committee: Jamie Ervin (Advisor); Larry Byrd (Committee Member); Jun-Ki Choi (Committee Member); Andrew Chiasson (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy (Ph.D.), University of Dayton, 2021, Electro-Optics

Reconfigurable photonics has been at the forefront of modern optics research, especially as optics and electronics merge into proper single-device integration. Within that umbrella, we can identify a series of critical devices commonly used in free-space applications adaptive optical elements, such as liquid crystal on silicon spatial light modulators (SLMs) and micromechanical adaptive beam steering mirrors. Light modulating devices play an essential role in spatio-temporal beam shaping, image processing, and display technologies for their role in converting intensity patterns into phase or amplitude light modulation. At the core, the physical idea remains the same: locally controlling the refractive index of the constituent material to affect the amplitude, phase, and polarization of incident light. Critical issues common to many reconfigurable devices are bulkiness, low speeds, and large voltage requirements or power consumption. However, phase change materials (PCMs) such as Ge2Sb2Te5 (GST) offer an alternative path for high-speed light modulation circumventing each of these issues. In general, PCMs alter their atomic structure via a thermal stimulus which yields large electrical, thermal, and optical contrast. Moreover, the nanosecond transition between the amorphous and metastable rock salt phase state leads to a substantial difference in the complex refractive index. In this work, an investigation is conducted on GST as a solid-state material for light modulators in the visible and infrared regimes. A holistic approach is taken to investigate the design, fabrication, and performance of each device. This work addresses critical issues in each design such as mitigating the electrical contact resistance, maximizing amplitude modulation, and improving phase transition speeds. Additionally, this work investigates the optical properties of nanopatterned GST using both top-down and bottom-up fabrication approaches that incorporate the necessary thermal mana (open full item for complete abstract)

Committee: Imad Agha (Advisor); Thomas Searles (Committee Member); Jay Mathews (Committee Member); Andrew Sarangan (Committee Member) Subjects: Optics; Physics

Doctor of Philosophy (Ph.D.), University of Dayton, 2022, Electro-Optics

Photonics is really the study of the manipulation of light. In order to manipulate light, the materials the light is propagating through need to be understood. One very important group of materials used for the manipulation of light are what are known as phase change materials (PCMs) and in particular chalcogenide PCMs. Chalcogenide materials are materials that include at least one element from the VIth row of the periodic table. These materials are so important because of their amazing ability to switch between material states in either a volatile or non-volatile fashion. Each of these different states has very different optical, electrical, and or thermal properties. By learning how to manipulate these materials we can create all types of photonic devices such as, spatial light modulators, accumulator memories, optical computing circuits, variable resistors, and even passive temperature control windows. This work covers various experimental setups, measurement techniques, and devices for the characterization and implementation of these fascinating materials, in particular Ge2Sb2Te5 or GST which has been known about for over 50 years, but is still not completely understood. Utilizing the power of electro-optic modulators I have created an original setup that has full control over the number of pulses as well as pulse shape for 1550 nm and 775 nm light. A pump-probe measurement scheme was implemented to achieve in situ reflection measurements before, during, and after a phase change event. An interferometer was employed to add phase accumulation measurements to the setup's capabilities as well. All this information is vital for the understanding of these materials.

Committee: Imad Agha (Committee Chair); Joshua Hendrickson (Committee Member); Mehdi Asheghi (Committee Member); Chenglong Zhao (Committee Member); Andrew Sarangan (Committee Member) Subjects: Optics

MS, University of Cincinnati, 2021, Engineering and Applied Science: Mechanical Engineering

In effort to both save operating expenses and be environmentally friendly, thermal energy storage provides a means for companies to handle daytime HVAC requirements while using off-peak (nighttime) electrical power. This paper sets out to compare three of the most common techniques used for thermal energy storage, by comparing both the analytical modeling of their energy storage and actual experimental data for their energy storage, using the same exact test apparatus for each of the techniques. The results of this experiment show that using normal HVAC temperatures, sensible water chilled to its maximum value after only about two hours, while PCM would take nearly six hours to achieve “linkage,” or solidified material merging between the helix coils. Ice building, done with -7° coolant, took 4.5 hours to achieve linkage. Initial heat transfer was proportional to the difference between initial tank temperature and the coolant temperature, and went asymptotically towards zero for sensible as the temperature of the tank and coolant reach equilibrium. For ice, the heat transfer rate was always more than twice that of PCM during latent storage, which is attributed to the difference between coolant temperatures and freezing points for the respective materials. Sensible water cooldown would require 232.8% of the tank volume to store the same energy relative to the environment compared to ice building, and 126.3% of the tank volume compared to phase change material. This is to be weighed with the benefit of using existing HVAC condensing units to chill the water, and the fact that water itself is inexpensive. The high latent heat of freezing for water meant it held more energy than both the water sensible cooldown and PCM freezing, but with the downside of requiring medium temperature condenser units in order to be efficient (instead of the high temperature units used in typical HVAC). After 4.5 hours, PCM would surpass the energy stored in the same volume as water sensi (open full item for complete abstract)

Committee: Michael Kazmierczak Ph.D. (Committee Chair); Ahmed Elgafy Ph.D. (Committee Member); Sang Young Son Ph.D. (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2021, Mechanical Engineering

With the advantages of high bandwidth and abilities to see through opaque materials, millimeter wave (mmW) band (30 to 300 GHz) has been intensively explored in recent years. Although there are increasing demands for reconfigurable mmW systems for their potential applications in defense, switching, imaging, and sensing, overcoming the limitations such as high losses and large power consumption in mmW systems is still a challenge. Phase change materials (PCM) like vanadium dioxide (VO2), which have novel and tunable physical properties such as electrical resistivity and optical transmittance, are appealing choices for mmW reconfiguration to provide faster operation speed and lower loss microsystems. One aspect of VO2 thin film that is not fully exploited is the metal-insulator transition (MIT) region, where the electrical resistivity changes about four orders of magnitude with external stimuli. In this work, we present a highly sensitive antenna-coupled VO2 microbolometer for mmW imaging. The proposed microbolometer takes advantage of the large thermal coefficient of resistance (TCR) of VO2 at the non-linear region. The thermal resistance of the device is significantly improved by micro-electro-mechanical systems (MEMS) techniques to suspend the device above the substrate, compared with non-suspended microbolometers. The finite element method is employed to analyze the electrothermal and electromagnetic performance of the device. The frequency range of operation is 65 to 85 GHz, and the realized gain at broadside is > 1.0 dB. Simulation results indicate a high responsivity of 1.72x10^3 V/W and a low noise equivalent power (NEP) of 33 pW/√Hz. Targeting for broader applications, it is highly desired to deposit VO2 thin films on silicon (Si) substrate. Here, we employ the annealed alumina (Al2O3) buffer layers to obtain high-contrast VO2 thin films. The fabrication details for the Al2O3 buffer layers using atomic layer deposition (ALD) and VO2 thin films using DC sput (open full item for complete abstract)

Committee: Nima Ghalichechian (Advisor); Hanna Cho (Committee Member); Renee Zhao (Committee Member) Subjects: Electrical Engineering; Materials Science; Mechanical Engineering; Nanotechnology

Doctor of Engineering, University of Dayton, 2021, Mechanical Engineering

One way to reduce conventional energy consumption is through the use of a vertical ground-coupled heat pump (GCHP) systems where heat is charged/discharged to/from the ground by an array of grouted vertical borehole heat exchangers. Although this technology is promising to increase the efficiency of heat-pumps, the main obstacle is the high initial cost. This work examines the viability of one possibility means to overcome the first cost challenge, which is to add micro-encapsulated, paraffin-based phase-change material (PCM) to the borehole grout to dampen the borehole heat exchanger (BHE) peak fluid temperatures. As with any thermal energy storage scheme, its purpose is to reduce the size of equipment and devices required to meet peak loads, and thus the purpose of PCM in this study is to dampen peak temperature response of the borehole, and potentially allow for reduction in design borehole length, and therefore cost, of the borehole array. A numerical analysis of the heat transfer characteristics of a GCHP systems is performed to investigate the effects of adding micro-encapsulated PCM into the borehole grout. The numerical model was completed in COMSOL, where the apparent heat capacity method is used, and validated against experimental data. A parametric study of the PCM thermal properties was conducted to establish design recommendations for the vertical heat exchange borehole grout. Results of this study show that adding PCM into the borehole does not always improve the overall performance of the GCHP system; rather, it could deteriorate the system performance if the PCM thermal properties and melt temperature are not correctly chosen. An optimum mass of PCM exists for borehole grout due to the competing factors of PCM thermal conductivity and its latent heat capacity, but to be effective, the PCM thermal conductivity should be approximately equivalent to that of the grout material. Further, the optimal melt temperature of the PCM was found to be that which (open full item for complete abstract)

Committee: Andrew Chiasson (Advisor); David Myszka (Committee Member); Muhammad Usman (Committee Member); Gilbert Robert (Committee Member) Subjects: Energy; Geophysics; Mechanical Engineering

Master of Science (M.S.), University of Dayton, 2021, Materials Engineering

VO2 is a type of phase change material (PCM) that can switch between a metallic state and a semiconducting state at a temperature of around 68 °C. This produces a large change in electrical resistance (almost three orders of magnitude) and large optical changes. Since this phase change occurs close to room temperature, VO2 has a large number of potential applications, such as thermally activated switches, optical modulators and optical limiters. Due to the multiple oxidation states of vanadium, VO2 thin films are typically difficult to produce. Traditionally, they are produced by reactive physical vapor deposition on heated substrates. In our research group, we have developed a different method where VO2 thin films are fabricated by thermal oxidation of PVD-deposited metallic vanadium films. Due to the high reactivity of vanadium, even small changes in the oxidation conditions will result in significant variations in the oxide films. In this thesis, we have examined the uniformity of VO2 films using stylus profiling, SEM, and 4-point probe measurements. The thickness expansion of the films due to oxidation was calculated and verified against experimental data. We also characterized the temperature profile inside the oxidation furnace. In addition, following an approach similar to the thermal oxidation of silicon, a vanadium oxidation model combining multiple oxidation states has been proposed and developed.

Committee: Andrew Sarangan Ph.D., M.A., P.E. (Advisor); Terrence Murray Ph.D. (Committee Member); Christopher Muratore Ph.D. (Committee Member); Robert Wilkens Ph.D., P.E. (Other); Eddy Rojas Ph.D., M.A., P.E. (Other) Subjects: Engineering; Materials Science; Optics

Master of Science (M.S.), University of Dayton, 2018, Electrical Engineering

The project explores the vastly different opto-electronic properties of GST in two different phases: amorphous and FCC crystalline. The eventual goal is to design and fabricate photonic devices whose functionality depends on the distinct material properties of the two phases of GST. A prototypical device structure was designed with a lattice of GST nanorods grown on a Silicon substrate. The GST nanorods are surrounded by a thermally conductive material, such as Boron Nitride, that rapidly quenches the nanorods during the phase change. An electrical contract on top of the device is used to initiate the GST phase transition. Simulations for this device design are used to explore the range of values needed for nanorod dimensions and applied voltages to control the phase transitions, as well as determine the effectiveness of the material surrounding the nanorod. Preliminary experiments are conducted to characterize the resistivity and sheet resistance of the GST samples and contact resistance between different GST phases and the contact metals, Tungsten and Molybdenum. The measured contact resistances and calculated sheet resistances for the two metals are comparable.

Committee: Joseph Haus Dr. (Committee Chair); Imad Agha Dr. (Committee Member); Andrew Sarangan Dr. (Committee Member); Joshua Hendrickson Dr. (Committee Member) Subjects: Electrical Engineering; Optics

Master of Science (M.S.), University of Dayton, 2018, Electro-Optics

In the last 10 years, the field of integrated photonics has gained prominence due to the need for higher bandwidths for communications, at lower cost. To have fully integrated devices on-chip, silicon compatibility becomes a necessity. This thesis explores two different devices that are compatible with silicon and therefore lend themselves to full on-chip integration. The first device is a p-i-n structure photodiode created with Germanium Tin (GeSn). The device shows a promising extended infrared range compared to a device made from pure Germanium (Ge). Time-resolved power dependent measurements on the responsivity were taken and reveal the different recombination mechanisms occurring in the device. The second device is a non-volatile memory created from Germanium Antimony Tellurium (GST). Here it is shown that by laser illumination, the GST film can be switched from a highly reflective/conducting crystalline state, to a less reflective/resistive amorphous state and back again. Ellipsometry was done on the individual states to show that they have drastically different indices of refraction and extinction coefficients, which can be utilized to create many novel silicon compatible devices.

Committee: Imad Agha Ph.D. (Committee Chair); Qiwen Zhan Ph.D. (Committee Member); Joshua Hendrickson Ph.D. (Committee Member); Jay Mathews Ph.D. (Committee Member) Subjects: Optics

Doctor of Philosophy, The Ohio State University, 2016, Electrical and Computer Engineering

Millimeter-wave (mm-Wave) and Terahertz (THz) technology have become attractive due to emerging applications in wireless, satellite communication, imaging and sensing, and material characterization. Although, several THz applications have been proposed, the lack of electronic components is an ensuing issue. In this dissertation, we examine materials and mm-Wave/THz filters for sensor applications. Both bandpass and reconfigurable filters are considered and specific designs are provided. Specifically, we introduce metamaterials to improve spatial filters in the Ka (26.5 – 40 GHz) and THz (0.3 – 10 THz) bands for sensor and detector applications. Periodic structures are presented to design narrowband and broadband frequency selective surfaces (FSSs). For the Ka band design, both the narrowband and broadband FSS designs are studied. Experimental results show excellent agreements with the simulations for all the designs. In addition, a low loss FSS transparent window is designed, for the 1st time ever, to enhance the THz spectroscopic measurement system, especially for THz imaging of biological samples. The proposed metamaterial designs provide up to 136% of bandwidth hence the THz image quality was improved by 30%. Reconfigurability of mm-Wave and THz is also introduced to enhance the sensitivity of associated detectors and sensors. In contrast to traditional RF MEMS used for reconfiguration in THz applications, we introduced material-based switching: bimaterial and phase-change materials (PCMs) actuators. First, we develop reconfigurable filters operating in the THz band that subjected to the temperature gradient of the surrounding medium by employing bimaterial actuator. The filter was fabricated and the equivalent circuit was extracted and demonstrated. Measurement and simulation results show good agreement with excellent transmittance (>80%). Next, vanadium dioxide (VO2) is considered here as PCMs, which shows insulator-to-metal transition (IMT) properties w (open full item for complete abstract)

Committee: John L. Volakis (Advisor); Niru K. Nahar (Advisor); Siddharth Rajan (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy (PhD), Ohio University, 2013, Physics and Astronomy (Arts and Sciences)

In this dissertation, we present the experimental and theoretical investigation of extensive properties of chalcogenide materials and their potential application in solid electrolytes and phase change memory materials. Extended X-ray absorption fine structure (EXAFS) spectroscopy was employed to study the structural properties and the results were validated from the computer simulated models through ab-initio molecular dynamic (AIMD) simulations. EXAFS analysis on Ge-Sb-Te (GST) alloys, synthesized using electrodeposition and radio frequency sputtering methods confirmed the structural similarities in Ge-Te and Sb-Te bond pairs suggesting the possibility of utilizing the electrodeposition method to grow GST alloys in nanoporous materials and thus enabling miniaturizing the phase change memory devices. The analyses of structural, electronic and optical properties of computer generated amorphous and crystalline TiO2 confirmed the structural similarities of amorphous TiO2 with the anatase phase of crystalline TiO2 and hence recommending the possibilities of replacing the crystalline TiO2 by less processed thus cheaper form of amorphous TiO2. Moreover, the AIMD simulations of the ionic conductivity of transitions metals like Ag and Cu in Ge-Se glasses confirmed the superiority of Ag over Cu in terms of conductivity. Ag was found to be easily hopping around while Cu was often trapped. In addition, an experimental and computational investigation on Ag-doped Ge-Sb-Te alloys predicted an enhanced crystallization of Ge-Sb-Te alloys. The enhanced crystallization was related to the reduction of fraction of tetrahedral Ge relative to octahedral Ge as also reflected as the increased Ge-Te bond lengths on adding Ag. Finally, further investigation of dopant-induced modification of GST alloys with transition metals (Cu, Ag and Au) demonstrated the superiority of Ag over Cu and Au regarding crystalline speed while at  2% dopant level no significant structural modification was observ (open full item for complete abstract)

Committee: David Drabold Dist. Prof. (Advisor); Gang Chen Associate Prof. (Advisor) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy (PhD), Wright State University, 2012, Engineering PhD

Multi-scale hierarchical carbon structures have been developed by growing strongly attached carbon nanotubes (CNT) on high surface area substrates having open, interconnected porosity. This investigation was developed on cellular carbon foams but the process is equally suitable for other geometries including flat, fibers, and other porous substrates (interconnected). It is also adaptable to other substrate materials such as metals, alloys or ceramic compounds. Multiwalled carbon nanotubes are grown using a floating catalyst chemical vapor deposition (CVD) method after pre-coating the substrate with a silica nano-layer. The silica-coated graphitic substrates are seen to grow 280 times more nanotubes per unit area compared to bare graphite. Detailed spectroscopic and microscopic studies indicate that this significant improvement can be attributed to improved adhesion and distribution of the iron catalysts and enhanced catalytic activity from substrate interactions. Failure analysis of the nanotube layer under several types of loading demonstrates strong adhesion between CNT and substrate, with failure occurring in the underlying substrate. Attachment of carbon nanotubes can result in more than two orders of magnitude increase in specific surface area as independently confirmed by modeling the microstructure and direct surface area measurement using Brunauer-Emmett-Teller (BET) technique. These hierarchical materials are tested as encapsulation structures for phase change materials (PCM). The CNT can act as nanofin radiators enhancing energy exchange between the thermally conductive encapsulation and the PCM, hence improving thermal response time. A heat cell was designed to compare the response times of foam encapsulation with and without CNT. Encapsulation with CNT is found to have and significantly faster thermal response. DSC measurements demonstrate that CNT/foam hierarchical encapsulation provides 15% higher storage of latent heat. The improvements in thermal res (open full item for complete abstract)

Committee: Sharmila M. Mukhopadhyay PhD (Advisor); Raghavan Srinivasan PhD (Committee Member); H. Daniel Young PhD (Committee Member); P.Terry Murray PhD (Committee Member); Ajit K. Roy PhD (Committee Member) Subjects: Materials Science

MS, University of Cincinnati, 2010, Engineering : Electrical Engineering

Bulk alloy glasses of GexSbxSe100-2x composition were synthesized over the compositional range, 0% < x < 23%, and examined in modulated differential scanning calorimetric, Raman scattering and molar volume measurements. Raman vibrational density of states systematically evolve with glass composition ‘x' displaying normal modes of characteristic building blocks, including Sen polymeric chains, Corner-sharing (CS) Ge(Se1/2)4 , Edge sharing (ES)-Ge(Se1/2)4 and Pyramidal (PYR) Sb(Se1/2)3 units. Line shapes are deconvoluted in terms of requisite number of Gaussians, and mode scattering strengths and mode frequencies established. These data along with first principles cluster calculations are used to identify the mode assignments. Our data confirm that x = xct = 18.2% is the chemical threshold in these glasses, in harmony with the valence requirements of Ge and Sb, and with homopolar bonds proliferating above the threshold as expected. Calorimetric measurements yield variation in glass transition temperature, Tg(x), and show a global maximum near xct = 18.2%, the chemical threshold. Stochastic agglomeration theory is used to analyze Tg(x) variation and shows that backbones at x > xct are demixed into Ge-rich (ethanelike Ge2(Se1/2)6 ) and Sb-rich (ethylenelike Sb2(Se1/2)4 ) local structures. We find evidence of a reversibility window in the 14.9% < x < 17.5% range, or 2.45 < r < 2.55 mean coordination number range, wherein the non-reversing enthalpy (ΔHnr(x)) at Tg shows a square-well like global minimum with ΔHnr(x) term almost vanishing. Molar volumes independently show a local minimum in the reversibility window, confirming the space filling nature of networks formed in the window. Aging of glasses, studied at room temperature as a function of waiting time, shows the ΔHnr(x) term to age for glass compositions outside the reversibility window but not in the window. These data show that glass compositions at x < 14.9% are flexible, those at x > 17.5% stressed rigid and (open full item for complete abstract)

Committee: Punit Boolchand PhD (Committee Chair); Jason Heikenfeld PhD (Committee Member); Marc Cahay PhD (Committee Member); Bernard Goodman PhD (Committee Member); Darl McDaniel PhD (Committee Member) Subjects: Materials Science