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Palmer, Benjamin CliveSensitization Effects on Environmentally Enhanced Cracking of 5XXX Series Alloys: Macro and Mesoscale Observations
Master of Sciences (Engineering), Case Western Reserve University, 2017, Materials Science and Engineering
The focus of this study was on the tensile behavior and damage development in 5083- H131 Al-Mg alloy sensitized to different levels. Samples were tested in the as-received state, after sensitization at 175°C for 100hrs, or 80°C for >500hrs. Tensile testing was conducted under moderate (50%RH) or low (<1%RH) humidity environments to determine the environmental effects on the mechanical behavior of the material. Three different deformation/fracture modes were present depending on the sensitization level and testing environment. Interrupted tensile tests and microscopy revealed that strain was more heterogeneously distributed in the highly sensitized specimens compared to the as-received ones. Differential scanning calorimetry was also performed as a means of determining the degree of sensitization of specimens thermally exposed at temperatures from 60-175°C. This technique was able to detect the presence of Mg-rich phase(s) at thermal exposures as low as 60°C, though it has quantitative limits due to the resolution limit.

Committee:

John Lewandowski, Dr. (Advisor); David Schwam, Dr. (Committee Member); Clare Rimnac, Dr. (Committee Member)

Subjects:

Materials Science; Mechanical Engineering

Keywords:

Environment-enhanced-cracking; Stress corrosion cracking; 3-D tomography; Aluminum-magnesium alloys; Differential scanning calorimetry

Ghods, MasoudEffect of Convection Associated with Cross-section Change during Directional Solidification of Binary Alloys on Dendritic Array Morphology and Macrosegregation
Doctor of Engineering, Cleveland State University, 2017, Washkewicz College of Engineering
This dissertation explores the role of different types of convection on macrosegregation and on dendritic array morphology of two aluminum alloys directionally solidified through cylindrical graphite molds having both cross-section decrease and increase. Al- 19 wt. % Cu and Al-7 wt. % Si alloys were directionally solidified at two growth speed of 10 and 29.1 µm s-1 and examined for longitudinal and radial macrosegregation, and for primary dendrite spacing and dendrite trunk diameter. Directional solidification of these alloys through constant cross-section showed clustering of primary dendrites and parabolic-shaped radial macrosegregation profile, indicative of “steepling convection” in the mushy-zone. The degree of radial macrosegregation increased with decreased growth speed. The Al- 19 wt. % Cu samples, grown under similar conditions as Al-7 wt. % Si, showed more radial macrosegregation because of more intense “stepling convection” caused by their one order of magnitude larger coefficient of solutal expansion. Positive macrosegregation right before, followed by negative macrosegregation right after an abrupt cross-section decrease (from 9.5 mm diameter to 3.2 mm diameter), were observed in both alloys; this is because of the combined effect of thermosolutal convection and area-change-driven shrinkage flow in the contraction region. The degree of macrosegregation was found to be higher in the Al- 19 wt. % Cu samples. Strong area-change-driven shrinkage flow changes the parabolic-shape radial macrosegregation in the larger diameter section before contraction to “S-shaped” profile. But in the smaller diameter section after the contraction very low degree of radial macrosegregation was found. The samples solidified through an abrupt cross-section increase (from 3.2 mm diameter to 9.5 mm diameter) showed negative macrosegregation right after the cross-section increase on the expansion platform. During the transition to steady-state after the expansion, radial macrosegregation profile in locations close to the expansion was found to be “S-shaped”. This is attributed to the redistribution of solute-rich liquid ahead of the mushy-zone as it transitions from the narrow portion below into the large diameter portion above. Solutal remelting and fragmentation of dendrite branches, and floating of these fragmented pieces appear to be responsible for spurious grains formation in Al- 19 wt. % Cu samples after the cross-section expansion. New grain formation was not observed in Al-7 wt. % Si in similar locations; it is believed that this is due to the sinking of the fragmented dendrite branches in this alloy. Experimentally observed radial and axial macrosegregations agree well with the results obtained from the numerical simulations carried out by Dr. Mark Lauer and Prof. David R. Poirier at the University of Arizona. Trunk Diameter (TD) of dendritic array appears to respond more readily to the changing growth conditions as compared to the Nearest Neighbor Spacing (NNS) of primary dendrites.

Committee:

Surendra Tewari, Ph.D. (Advisor); Jorge Gatica, Ph.D. (Committee Member); Orhan Talu, Ph.D. (Committee Member); Rolf Lustig, Ph.D. (Committee Member); Kiril Streletzky, Ph.D. (Committee Member)

Subjects:

Aerospace Materials; Automotive Materials; Chemical Engineering; Condensed Matter Physics; Engineering; Fluid Dynamics; High Temperature Physics; Materials Science; Metallurgy

Keywords:

Directional Solidification; Natural Convection; Fluid Flow; Binary Alloys; Macrosegregation; Dendritic Array; Dendrite Morphology; Solutal Remelting; Thermosolutal Convection; Aluminum Alloy; Cross section Change

Holcombe, Evan W.Multi-Scale Approach to Design Sustainable Asphalt Paving Materials
Master of Science (MS), Ohio University, 2017, Civil Engineering (Engineering and Technology)
The continuous use of recycled material in asphalt pavement mixtures, specifically Reclaimed Asphalt Pavement (RAP), Recycled Asphalt Shingles (RAS) and Re-Refined Engine Oil Bottoms (REOB), have developed an increasing need to further evaluate the performance of these mixtures at the micro and macro-levels, as the use of such materials reduces cost of virgin materials and energy consumption. Although asphalt binder, including recycled or additive materials, may meet a desired performance grade (PG) using macro-scale tests, they may lack critical nano-mechanical properties that largely affect long-term performance, such as adhesion and diffusive efficiency between virgin and recycled binders. These commonly overlooked properties can correlate with performance behaviors such as fatigue and low temperature cracking during field performance. This study was conducted in two major parts. Part one was performed with the intent to evaluate the nano-mechanical and blending-diffusive efficiency of toluene and trichloroethylene extracted RAP and RAS binder using tapping mode imagery and force spectroscopy using Atomic Force Microscopy (AFM). Furthermore, this study was set to correlate the findings from micro-testing to macro-scale laboratory performance tests including Semi-Circular Bending (SCB) to evaluate fatigue cracking resistance at intermediate temperatures, Asphalt Concrete Cracking Device (ACCD) to evaluate low temperature cracking and AASHTO 283 ITS to study moisture damage susceptibility of intermediate course mixtures with high RAP and RAS contents. Results showed that tear-off RAS material have a significant effect on fatigue and low temperature cracking performance, primarily at long-term aged conditions. Neither tear-off nor manufactured waste RAS binder blend well with virgin binder, whereas RAP shows a zone of blending. AFM imaging indicated all extracted RAS binder had a much rougher surface texture than RAP or virgin binders and did not contain any “bee” structures. The procedure of splitting RAP material for sampling during the volumetric mix design process has a significant effect on the optimal virgin binder content design, which in turn has a large effect on performance properties. Part two of this thesis summarizes the results of laboratory tests that were conducted to evaluate the microstructure, adhesion and other mechanical properties of asphalt binders meeting the same Performance Grade (PG) but produced using different processes and modifiers. Atomic Force Microscope (AFM) tapping mode imaging and force spectroscopy experiments were conducted on different straight run and modified asphalt binders meeting the same performance grade. In addition, Bitumen Bond Strength (BBS) and Semi-Circular Beam (SCB) tests were conducted on the different binders evaluated and mixes prepared using those binders, respectively, for comparison. The AFM images indicated that the microstructure of the modified binders was different than those of the straight run binders. The AFM force spectroscopy test results showed that binders with same PG grade could have significantly different adhesion properties. The results of the SCB tests indicated that the fatigue performance was affected by the adhesion properties of the binders evaluated. The AFM bonding energy had a very good correlation with the flexibility index parameter obtained from SCB test results. The results of this part suggests that the adhesion properties of asphalt binders should be included in their evaluation process and specifications.

Committee:

Munir Nazzal, Dr. (Advisor)

Subjects:

Civil Engineering; Materials Science

Keywords:

reclaimed asphalt pavement; recycled asphalt shingles; re-refined engine oil bottoms; atomic force microscopy; fatigue cracking; adhesion; diffusion, moisture damage; thermal cracking

Razgoniaev, AntonDesign, synthesis, and characterization of photoresponsive materials using coordination bonds and other supramolecular interactions
Doctor of Philosophy (Ph.D.), Bowling Green State University, 2017, Photochemical Sciences
When designing light-responsive, healable materials and adhesives, these materials need to include controllable reversible, bonding interactions. Such dynamic interactions are difficult to control, however. In this work, we present how these interactions can be controlled by incorporating photoactive metal ions into supramolecular polymer network what allow the tuning of optical and mechanical properties of the polymers with light. Utilizing this approach, we created a series of supramolecular polymer melts and studied their mechanical and photo physical properties. We have shown that the photochemistry and photophysical properties of the metal center can be used to control the mechanical properties of the materials, and introduce new optical and mechanical properties not seen in the traditional covalent polymers. In particular, photo-induced metal-ligand bond labilization led to partial depolymerization of the supramolecular assembly, and softening of the materials. When the light stimulus was removed, the material recovered the initial stiffness back. We also investigated structure-property relationships in such systems where mechanical properties of the supramolecular polymers are controlled by coordination environment around metal cross-linking center. We also considered how polymer host matrix impacts on the photophysical and photochemical properties of chromophores that undergo molecular motion in the exited state. In particular, change in excited state dynamics of [Cu(dmp)2]+ can be used to sense viscosity of various polymers. A linear dependence of excited state lifetime and emission wavelength on viscosity was correlated with restricted photoinduced structural distortion of Cu(I) complex in more flow-resistance media.

Committee:

Alexis Ostrowski, Ph.D. (Advisor); Scott Rogers, Ph.D. (Committee Member); Alexander Tarnovsky, Ph.D. (Committee Member); R. Marshall Wilson, Ph.D. (Committee Member)

Subjects:

Chemistry; Materials Science; Polymer Chemistry; Polymers

Keywords:

Photochemistry; Chemistry; Materials Science; Polymer Chemistry; Polymers

Riyad, M Faisal Simultaneous analysis of Lattice Expansion and Thermal Conductivity in Defected Oxide Ceramics
Master of Science, The Ohio State University, 2017, Mechanical Engineering
Objective of this thesis is to investigate the impact of point defects on thermal conductivity and lattice expansion in uranium dioxide ceramic. Specific emphasize is on light ion irradiation induced point defects which causes the degradation of thermal conductivity of oxide ceramics. Radiation induced defects include vacancies and interstitials hosted by the anion and cation sub lattice of the structure. A crystallographic structure is assumed for each defect and is used to model defect impact on lattice parameter. In ceramic materials, thermal conductivity is governed by phonon modes determined by crystalline structure. The irradiation induced point defects limit thermal transport by acting as phonon scattering centers. The point defects scattering originates from both the mass and ionic radius mismatch between the impurity atom and the host lattice. We present a model to estimate the phonon scattering parameter for different types of point defects and implement it in classical phonon mediated thermal transport model to estimate the thermal conductivity reduction in light ion irradiated UO2. The results are compared to results of experimental measurements. Laser based modulated thermoreflectance (MTR) technique was used to measure the thermal conductivity model in ion irradiated UO2 samples. Unlike laser flash analysis, traditionally used for measuring thermal conductivity in nuclear materials, MTR method has a sensitivity to a few micron thick thin damage resulting from ion beam irradiation. In this technique, the irradiated sample, coated by a thin metallic film, is heated by a harmonically modulated laser pump and a probe beam measures the temperature induced changes in reflectivity. In this work, experimentally measured thermal wave phase profiles obtained from UO2 samples irradiated with 2.6 MeV H+ ions were analyzed using different multilayer approximations of the damaged region. An infinite damage layer approximation model that neglects undamaged layer and peak damage region characteristic to light ion irradiations is discussed. The limitation of the approach and demonstration of its applicability range was analyzed. Finally, measured conductivities of the ion irradiated samples using a thermal conductivity model for point defects was examined. Previously reported XRD measurements on same proton irradiated UO2 samples show the lattice expands linearly as a function of atomic displacements (dpa). The defect concentration can be defined as a function of dpa and the defect production rate. The estimation of defect concentration is validated by accounting their overall contribution to the change in lattice parameters and comparing them with the measured values by XRD. Finally, their overall contribution to the reduction in thermal conductivity is compared with the experimentally measured values to determine the concentration of defects in the lattice structure of UO2.

Committee:

Marat Khafizov, Dr. (Advisor); Sandip Mazumder, Dr. (Committee Member)

Subjects:

Materials Science; Mechanical Engineering; Nuclear Engineering; Nuclear Physics

Keywords:

Lattice Expansion; Thermal Conductivity; Defected Oxide Ceramics

Rezaei, Seyed EmadDefect Engineering: Novel Strengthening Mechanism for Low- Dimensional Zinc Oxide Nanostructures
Master of Science in Materials Science and Engineering (MSMSE), Wright State University, 2018, Materials Science and Engineering
The advent of nanomaterials has opened a new avenue for designing and fabricating materials with unique properties, e.g., superior mechanical properties. Based on a common notion, the perfect structures are assumed to exhibit better mechanical properties, such as higher yield strength and Young’s modulus. Therefore, researchers have devoted an extensive amount of time to decrease defect concentration by fabricating materials with the micro/nanoscale, e.g., nanowires (NWs) and nanobelts (NBs), to enhance the mechanical characteristics of the system. However, defects are a part of the fabrication process and precise control over synthesizing procedure is needed to eliminate them from the material. In this work, we showed, with the help of the classical molecular dynamics method, that these inherited defects can be employed as a microstructural feature to improve the mechanical properties of low dimensional nanomaterial, i.e., defect engineering. Our results indicate that the NWs with a high density of I1 stacking faults (I1-SFs) show higher compressive/tensile critical stress (14% increase), as well as Young’s Modulus (37% increase), in comparison to the perfect structure over a wide range of temperature: ranged from 0 K to 500 K. Such an improvement is in agreement with the in-situ experimental measurements of highly defective GaAs NWs, and can be justified by interplay between surface stresses and the intrinsic stress field of locked SFs. The SF-induced stresses are partially relaxed by raising the temperature for this non-trivial strengthening. Moreover, a specific stress relaxation mechanism, twin boundary formation, was found to take place in highly defected NWs, which further postponed the phase transition from hexagonal (HX) to cubic and subsequently boosted the toughness of NWs; this phenomenon appears as a stress plateau in highly defected NWs. Numerous parametric studies on the system variables, such as cross-section geometry, aspect ratio, width, and SF distribution, were performed to find the optimum design. Our results demonstrated the promise and applicability of this strengthening method over a wide temperature range and geometrical features. This novel method, defects engineering, adds a new parameter to the design-space of materials and also paves the way to the fabrication of a new class of materials with superior mechanical properties, including higher stiffness, strength, and ductility.

Committee:

Hamed Attariani, Ph.D. (Advisor); Nikolai Priezjev, Ph.D. (Committee Member); James Menart, Ph.D. (Committee Member)

Subjects:

Aerospace Engineering; Materials Science; Mechanical Engineering

Keywords:

Defect engineering; Nanowires; Nanobelts; Zinc oxide; Stacking fault

Morrison, Rachel LouiseUnderstanding the Interaction between Grain Boundaries and Precipitates in Ni-Al Using Molecular Dynamics
Master of Sciences, Case Western Reserve University, 2018, Materials Science and Engineering
This thesis investigates the interaction between gamma' precipitates and grain boundaries in a Ni-Al system during deformation. This interaction is investigated using molecular dynamics, and gamma'/boundary configurations were built to investigate how the orientation, size, and interaction of gamma' change the deformation behavior of the grain boundary. The gamma' aided in nucleating defects (i.e., dislocations) that contributed to the boundary sliding mechanism. By increasing the size of precipitates that bisect the boundary, the boundary becomes stronger, whereas increasing the size of precipitates adjacent to the boundary makes the boundary weaker. Additionally, the interaction of multiple gamma' plays a role in grain boundary sliding behavior. Low concentrations of gamma' produce sliding dominated by atomic shuffling, whereas high concentrations of gamma' produce sliding dominated by dislocation emission. More work is needed to investigate the effects of temperature, initial defects, and different grain boundary configurations on sliding behavior.

Committee:

Jennifer Carter (Committee Chair); John Lewandowski (Committee Member); David Matthiesen (Committee Member)

Subjects:

Materials Science

Keywords:

grain boundary sliding; nickel superalloys; molecular dynamics

Moffett, UtahInvestigation of a Heat Spreading Layer for Wing De-icing
Master of Sciences (Engineering), Case Western Reserve University, 2018, EMC - Aerospace Engineering
A heat spreading layer is investigated for use with the HeatCoat TM anti-icing system. Carbon-based materials such as graphite, carbon fibers, and carbon nanotubes were investigated for their favorable thermal properties, characterized with XRD, Raman spectroscopy, and thermal conductivity testing. Synthetic graphite was found to have the highest thermal conductivity and is therefore the most desirable heat spreading material for the antiicing system application. A finite element thermal analysis of the HeatCoat TM and heat spreading layer was performed to determine the effect of the heat spreading layer in- and through-plane thermal conductivity on the anti-icing performance. Through-plane thermal conductivity proved to be unimportant, but the in-plane thermal conductivity should be higher than 200 W/m-K to minimize the appearance of cold spots and maximize energy savings. In addition, the system performed best when the thermal interface resistance between the heaters and the heat spreader was at most 10^6 m^2K/W.

Committee:

Vikas Prakash (Committee Chair); Alexis Abramson (Committee Member); Yasuhiro Kamotani (Committee Member); James T'ien (Committee Member)

Subjects:

Aerospace Materials; Engineering; Materials Science; Mechanical Engineering

Keywords:

Carbon nanotubes; de-icing; thermal conductivity

Gbur, Janet LInclusion Effects on the Lifetime Performance of Superelastic Nitinol Wires
Doctor of Philosophy, Case Western Reserve University, 2018, Materials Science and Engineering

Miniaturization of Nitinol-based medical devices requires increased performance from less material, and therefore necessitates investigation of the connection between material processing and performance. Moreover, it is imperative to understand how impurities that result from processing affect its lifetime performance. Research on the microcleanliness (i.e. presence of nonmetallic inclusions, intermetallic particles, etc.) of fine Nitinol wires and its effect on fatigue performance are not frequently found in literature.

This study compared a manufacturer-defined standard (SP) and high (HP) purity, Nitinol superelastic (SE) fine (< 140 μm) wire. Nonmetallic inclusion (NMI) chemistry was determined to be TixNiyOz with varied morphology, occurring with/without pores. Combined NMI/pore area percentages ranged from 0.08% ± 0.04% (HP) to 1.44% ± 0.26 (SP) when measured with scanning electron microscopy (SEM). Plasma focused ion beam (PFIB) serial sectioning was also used characterize the combined NMI/pore volume percentages yielding 0.09% (HP) and 0.47% (SP).

Differential scanning calorimetry was used to identify SE transformation temperatures. Microindentation hardness measurements showed gradients with the center of the wire measuring 359 ± 6.4 HK (HP) and 329 ± 7.2 HK (SP) while mid-radius values were similar for both at 366 ± 5.1 HK and 361 ± 6.0 HK, respectively. Higher tensile and upper plateau strengths were observed in SP (1573 MPa and 515 MPa, respectively) than HP (1420 MPa and 392 MPa, respectively) with greater reduction in area for HP (92.2%) vs. SP (80.2%). Fatigue enhancement was observed in HP wires in flex bending fatigue at strain amplitudes (εa) from 0.67-11% with a run-out at 106 cycles and rotating bending fatigue (εa= 1.2-1.5%) though run-out at 108 cycles. Fractography showed crack initiation attributed to NMIs/pores at the surface or sub-surface with feature areas ranging from submicron to 20.4 μm2.

Characterization techniques showed that HP contained less large-dimension NMIs/pores than SP leading to less potential stress concentrations and points for crack initiation. These differences in relative purity were reflected in the fatigue results where the HP exhibited enhanced fatigue resistance in both low cycle fatigue and high cycle fatigue regimes.

Committee:

John Lewandowski (Advisor); Sunniva Collins (Committee Member); Ina Martin (Committee Member); Matthew Willard (Committee Member)

Subjects:

Materials Science

Keywords:

Non-metallic inclusion; fatigue; wire; biomedical; Nitinol

Yu, JiayiTunable Biodegradable Polymers for Regenerative Medicine
Doctor of Philosophy, University of Akron, 2018, Polymer Science
Since the early 1960s, synthetic biodegradable polymers have been widely used in biomedical applications due to their large chemical diversity and the reproducible properties. However, the local acidification during degradation has shown to cause significant inflammation that can lead to device or implant failure. It is necessary to design new biodegradable polymer systems that do not cause local acidosis during degradation. To facilitate this requirement, Becker group has developed the amino acid-based poly(ester urea)s. These polymers are semi-crystalline. Their hydrolysis byproducts are non-toxic and can be self-buffered by the presence of the urea linkage at each repeat unit. In addition, there is a tremendous physical and chemical landscape that is available for exploration by using different natural amino acids with different pendant groups and different diols. This dissertation outlines our efforts to develop biodegradable polymers with tunable mechanical properties, degradation rates, and bioactivity. We varied the diol chain length (Chapter 3), branch density (Chapter 4), bioceramic contents (Chapter 5) in the poly(ester urea) system; cis/trans ratio (Chapter 6) in the biodegradable elastomer system and studied how these subtle structural differences would influence the mechanical properties and water uptake ability. Based on their tunable physical properties, these materials can be selected and used for various biomedical applications (Chapter 7).

Committee:

Matthew L. Becker, Ph.D. (Advisor); Bryan Vogt, Ph.D. (Committee Chair); Yu Zhu, Ph.D. (Committee Member); Amis J. Eric, Ph.D. (Committee Member); Chrys Wesdemiotis, Ph.D. (Committee Member)

Subjects:

Biomedical Engineering; Materials Science; Plastics; Polymer Chemistry; Polymers

Keywords:

biodegradable polymers; poly ester urea; diol chain length; branch density; composite; stereochemistry; mechanical property; biodegradation rate; application, processing; characterization; structure property relationship; amino acid; tunable property

Cardanini, Alisha AnnFinite Element Analysis of Bi-Metallic Structures with Adhesive Delamination
Master of Science, The Ohio State University, 2017, Welding Engineering
Bi-metal structures made of aluminum and steel are increasingly used for light-weighting applications. Replacing steel parts with aluminum in the body in white can reduce the weight of a vehicle up to 30%. The coefficient of thermal expansion (CTE) of aluminum is almost twice that of steel. Due to such large CTE mismatch, thermal buckling can become a concern when the bi-metal structure is exposed to elevated temperature. When adhesive is added between the aluminum and steel, its curing process can be affected due to buckling of the dissimilar metals. Moreover, stress in the structure developed at high temperature can be permanently locked in when the adhesive fully cures. This can lead to a higher residual gap between the aluminum and the steel than in structures without adhesive. The objective of this research is to quantitatively understand the stress and strain evolution in a bi-metallic Al / adhesive / steel structure exposed to a thermal cycle representative of that used in automotive paint bake process, including delamination of adhesive between the substrates. To achieve this objective, it is essential to first capture the behavior of the bi-metallic structure without adhesive and validating such models. Once validated, addition of cured adhesive and its delamination behavior is then incorporated. Delamination behavior relies on the fracture energy release rate of the adhesive material, which is determined through fracture toughness testing. Specially, the research consists of the following two main tasks. First, preliminary finite element models have been developed to capture the behavior of thermal induced buckling, including its deflection profile and stress close to the fasteners. These studies revealed that for a maximum paint bake temperature of 180°C residual stress is only found within the fastening region. This indicates that paint bake process itself does not produce enough heat to exceed elastic strain limits of the bulk structure. Several geometric factors are studied, including plate thickness, fastener pitch, and flange height. These factors reveal the effect of the geometry on the maximum deflection in buckling. Second, adhesive fracture toughness testing is conducted to measure the mode I fracture energy release rate. The fracture toughness is then incorporated in to both lap shear and thermal buckling model. Both use cohesive zone method incorporating linear traction-separation law for modeling the adhesive damage behavior. A hybrid continuum-cohesive element is created to incorporate both delamination effects as well as cured stress locking. The understanding established in this research is essential to optimize the design of bi-metallic structure to control distortion and residual stress in the structure, two important performance properties. Taken as a whole, the research results reported in this thesis represent a first step toward improving the quantitative understanding of adhesive deformation and failure behaviors in Al-steel bi-metallic structure. Future work includes (1) incorporation of non-linear traction-separation behavior in the cohesive elements, and (2) testing of adhesive fracture toughness as a function of temperature. Addressing the future work can further improve the accuracy of the computational model.

Committee:

Wei Zhang (Advisor); Avraham Benatar (Committee Member)

Subjects:

Engineering; Materials Science; Mechanics

Keywords:

FEA; Adhesive; Bi-Metallic; Buckling; Traction-Separation; Automotive; Epoxy

Bekele, SelemonStructural and Dynamical Properties of Water and Polymers at Surfaces and Interfaces: A Molecular Dynamics Investigation
Doctor of Philosophy, University of Akron, 2018, Polymer Science
All-atom molecular dynamics simulations have been carried out to study the wetting of model atactic polystyrene thin films by water droplets as a function of surface polarity, the structure and dynamics at the surface of polymer films, the properties of interfacial water molecules in contact with atactic polystyrene surfaces of varying polarity, and the spreading dynamics of water droplets of varying sizes on a completely wetting surface. The simulated contact angle of a water droplet on atactic polystyrene thin films is found to decrease monotonically with increasing degree of surface oxidation which is used as a measure of surface polarity. The number of hydrogen bonds between water molecules and the polystyrene at the interface is found to monotonically increase with polarity. The contribution of the non-dispersion interactions to the interfacial energy at the polystyrene/water interface has been determined as a function of surface polarity. The roughness of the polystyrene surface and the orientational ordering of the surface phenyl rings are found to be independent of surface polarity when the polystyrene is exposed to vacuum. Surface roughness appears to increase and orientational ordering of surface rings appears to decrease slightly with polarity when the polystyrene is in contact with water. Density profiles of polymer films as a function of distance relative to an instantaneous surface exhibit a structure indicative of a layering at the polymer/vapor interface similar to the typical layered structure observed at the polymer/substrate interface. Interfacial molecules at the polymer/vapor interface have a higher mobility compared to that in the bulk while the mobility of the molecules is lower at the polymer/substrate interface. Time correlation of the instantaneous polymer/vapor interface shows that surface fluctuations are strongly temperature dependent and are directly related to the mobility of polymer chains near the interface. Interfacial water molecules on a bare atactic polystyrene surface and those which do not make hydrogen bonds with the oxidized surfaces are found to have a faster dynamics and appear to have a universal water-water hydrogen bond relaxation time of about 5 ps. Diffusion coefficients and the relaxation times of the water molecules involved in hydrogen bonding with the surface show strong dependence on surface polarity with a hydrophobic to hydrophilic transition regime with contact angle in the range 40-50o. The spreading of water droplets on completely wetting surface is characterized by a bulk part of normal density sliding over a monolayer of high density water. The monolayer has a molecular dimension and moves ahead of the bulk part of the droplet. The monolayer motion exhibits two spreading regimes, each following a power law in time. The bulk part of the droplet initially spreads over the monolayer with increasing radius until a characteristic time t* where the monolayer changes from one power law behavior to another. For times after t* ~ 0.1- 0.5 ns, it shrinks while maintaining what appears to be a constant contact angle until it disappears altogether and only a monolayer of water remains on the substrate.

Committee:

Mesfin Tsige (Advisor); Ali Dhinojwala (Committee Chair); Toshikazu Miyoshi (Committee Member); Hunter King (Committee Member); Jie Zheng (Committee Member)

Subjects:

Condensed Matter Physics; Materials Science; Nanoscience; Polymers

Keywords:

MD simulation;molecular dynamics;oxidized surfaces; water contact angle; interfacial water;surfaces and interfaces;droplet spreading;

Zhan, XunCrystallization Micro-mechanism of Amorphous Ni-P
Doctor of Philosophy, Case Western Reserve University, 2017, Materials Science and Engineering
The crystallization of near-eutectic amorphous Ni–P can be significantly retarded by alloying a small fraction of tungsten. Complimentary characterization techniques are applied to understand this phenomenon. DSC (differential scanning calorimetry) reveals the isochronal and isothermal crystallization kinetics. XPS (X-ray photoelectron spectroscopy) provides core-level electronic signatures of Ni, phosohorus and tungsten, which reflect SRO (short-range order) evolution during crystallization. FEM (fluctuation electron microscopy) provides the MRO (medium-range order) evolution during crystallization. TEM (transmission electron microscopy) provides high-spatial-resolution information on phase nucleation and spatial distribution of atom species. Physical theory has been developed by combining results of these techniques to explain the role of tungsten: Macroscopic aspect (energetics and kinetics), the presence of tungsten reduces the driving force and increases the activation energy for crystallization. Microscopic aspect (micro-mechanistics), the presence of tungsten probably reduces the free volume (hypothesis) due to large atomic radius ratios of rW/rNi and rW/rP; introduces tungsten atoms diffusion to segregate due to chemical potential difference of tungsten in different crystalline phases; involves the breaking of all of W–P bonds with high bond energy. Moreover, theoretical criteria of an effective metal X alloying to improve the thermal stability of M–ML (metal–metalloid) amorphous systems has been proposed. The criteria are: (1) Large negative heat of mixing among X, M and ML. (2) Minimum amorphous free volume by appropriate combination of rX, rM and rML. (3) Large chemical potential difference of X in minor than in major crystalline phase. (4) Large X–ML and X–M bond energy. The criteria conclude on what other potential alloying elements will do, which has implications for fundamental science and technologies. In addition, magnetization curves of as-plated and tempered Ni80P20 and Ni76W4P20 were measured by VSM (vibrating sample magnetometer). Alloying tungsten does not change the paramagnetism of amorphous Ni80P20, but decreases the saturation magnetization of Ni80P20 after crystallization.

Committee:

Frank Ernst, Dr. (Advisor); John Lewandowski, Dr. (Committee Member); Matthew Willard, Dr. (Committee Member); Rohan Akolkar, Dr. (Committee Member)

Subjects:

Chemical Engineering; Materials Science

Jain, DharamdeepHumidity Driven Performance of Biological Adhesives
Doctor of Philosophy, University of Akron, 2018, Polymer Science
Biological adhesives are sticky secretions or structures produced by several organisms in nature to serve roles such as locomotion, prey capture and defense. These adhesives stick in a variety of environmental conditions and can maintain their adhesion exceptionally well. The present work focuses on understanding one such environmental factor, `humidity’ and presents its correlation with the material composition in influencing the adhesion mechanism in two diverse biological attachment systems: Capture silk and Gecko setae. Understanding adhesion in these natural systems is essential with respect to humidity since many synthetic materials including glues fail in presence of water. The first and second studies focus on the glue laden capture silk produced by web building spiders. In the first study, we explored the capture silk of cobweb weaver `black widow spider’ known as `gumfoot glue’. We first investigated the chemical composition of the glue and for the first time reported that it is majorly a combination of hygroscopic organic salts (low molecular mass compounds, LMMCs) and novel glycoproteins, apart from previously known peptides. Next, we correlated the glue composition with humidity based macro and molecular level studies and showed the synergistic role of LMMCs and glycoproteins in adhesion across the range of humidity conditions. Based on the first study which showed the presence and importance of diverse LMMCs in capture silk adhesion, we designed our second study in understanding the role of LMMCs in the capture silk. Based on hypothesis that LMMC’s compositions control the maximum adhesion and viscosity trends across species, we designed the study in which by using Solution-State NMR, we first analyzed the water-soluble extract of glues for four different spider species from diverse habitats and found extract belonging to each species is a distinct combination of organic LMMCs present in varied proportions. Next, we studied the water uptake of glues and their isolated LMMCs compositions. The results showed that hygroscopic strength of LMMCs alone can’t explain the adhesion response of glues. We believe it is the chemical interactions of diverse LMMCs with glycoproteins that controls the adhesion mechanism of capture silks in presence of humidity. In the third, fourth and fifth studies, we switch to a different adhesive system and present investigations based on the hairs present on gecko feet, known as `setae’. In the third study, we first time established the chemical composition of hairs by characterizing molts from gecko feet and showed the presence of ß-keratin and unbound lipids. Also, we showed lipids in hairs were more mobile as compared to lipids in epidermal skin based on which we proposed structural arrangement of lipids and keratin in the setal hairs. The fourth study focused on understanding the role of surface lipids detected in the third study. By means of shear adhesion and contact angle experiments, we found those lipids do not affect adhesive and anti-adhesive properties respectively. The existing hypothesis of ß-keratin softening and leading to higher adhesion in presence of humidity was tested in our fifth study. By series of water uptake and NMR measurements, we found ß-keratin absorbs water and gets soft at a macro and molecular level. Friction cell based shear adhesion measurements on setae supported the hypothesis and showed an increase in adhesion with increase in humidity. The research studies presented provides a detailed account of correlation of environmentally relevant parameter, `humidity’ with the building blocks of capture silk and gecko setae and their adhesion performance. The results provide design insights in developing synthetic materials such as adhesives that can work in different humidity environments.

Committee:

Ali Dhinojwala, Dr. (Advisor); Mesfin Tsige, Dr. (Committee Chair); Todd A. Blackledge, Dr. (Committee Member); Miyoshi Toshikazu, Dr. (Committee Member); Joy Abraham, Dr. (Committee Member)

Subjects:

Biology; Biophysics; Materials Science; Polymers

Keywords:

Biomimicry, Adhesion, Spider Silk, Capture Silk, Geckos, Setae, Water, Lipids, Keratin, Glycoproteins, Hygroscopic Compounds

Amonson, Michael D.Multiple Charge Carrier Species and Their Effects in Photorefractive Two-Beam Coupling in Potassium Niobate
Master of Science (M.S.), University of Dayton, 2017, Electro-Optics
This thesis reports on an experiment to measure charge carrier contributions from different Fe species and their effects on beam coupling efficiency using self-pumped counter-propagating two-beam coupling in iron-doped potassium niobate KNbO3:Fe. We used multiple continuous wave lasers operating across the visual spectrum to explore charge carrier creation from various transitions. Photorefractive grating formation data was acquired and analyzed using a new theoretical model which incorporates multiple charge carrier species. Initial analysis provides supporting evidence of a multiple charge carrier model and presents new insights about the effects of various charge carriers on the photorefractive periodic space-charge fields.

Committee:

Dean Evans (Advisor)

Subjects:

Electromagnetism; Materials Science; Optics; Physics

Keywords:

Potassium Niobate; Iron Doped Potassium Niobate; Photorefractive; Multiple Charge Carriers; Two-Beam Coupling

Liu, ZhihuiProperties of 3D Printed Continuous Fiber-Reinforced CNTs and Graphene Filled Nylon 6 Nanocomposites
MS, University of Cincinnati, 2017, Engineering and Applied Science: Materials Science
Nanomaterials have attracted much attention due to the excellent properties they possess and their promising applications. The combination of 3D printing and composite materials has redefined the mechanical properties of 3D printed products. In this research, nylon (PA) 6 nanocomposites filled with either carbon nanotubes(CNTs), graphene or graphene-NH2 were 3D printed together with Kevlar fibers into specimens for mechanical tests and other characterizations. Different weight percentages of CNTs and graphene were used to produce the nanocomposites, in order to figure the properties of each nanoparticle reinforced PA 6. The melt mixed CNTs or graphene nanocomposites were extruded into filaments and used in the 3D printer. A Markforged printer allowed the production of continuous Kevlar fiber reinforced nanocomposites. The tensile and flexural tests revealed that the best weight percentage of CNTs is 0.5wt%, where the entanglements and agglomerates of CNTs were not so obvious. Surprisingly, the CNTs filled PA 6 nanocomposites did not show as significant improvements in mechanical properties as graphene filled PA 6, due to the weak interfacial interactions between the CNTs and the PA 6 matrix. The addition of Kevlar fibers increased the tensile strength and flexural modulus of PA 6 by 526% and 1388%. Also, the tensile fatigue results showed that 1%CNT/PA 6+Kevlar specimens have the longest fatigue life among the materials tested. Graphene filled PA 6 presented much better improvements in mechanical properties. With only 0.1wt% of graphene, the tensile modulus improved by 101% and with 1wt% of graphene the modulus improved by 153%. Additionally, although Kevlar fibers dominate the main mechanical properties of these composite materials, the existence of graphene also contributes to the enhancement of strengths and moduli, unlike CNTs. Strong interfacial bonding allows efficient load transfer between matrix and reinforcement. Therefore, graphene-NH2/PA 6 showed significant improvements in both tensile and bending strengths. The tensile modulus of 0.1% graphene-NH2/PA 6 and 1% graphene-NH2/PA 6 are increased by 212% and 253%. Flexural tests showed obvious difference between different nanoparticle fillers. However, the anisotropic specimens did not show much difference between different weight percentages of the same kind of nanocomposite. It is found that the well-dispersion of nanoparticles in the matrix and strong interfacial bonding between the filler and the matrix are the main reasons for the enhancement of mechanical properties of nanocomposites. The addition of Kevlar fibers improved the stiffness and strength of the composites significantly.

Committee:

Jing Shi, Ph.D. (Committee Chair); Gregory Beaucage, Ph.D. (Committee Member); Jude Iroh, Ph.D. (Committee Member)

Subjects:

Materials Science

Keywords:

3D printing;Nanocomposite;CNTs;graohene;Kevlar;Nylon 6

Yang, JianpingSynthesis and Characterizations of Lithium Aluminum Titanium Phosphate (Li1+xAlxTi2-x(PO4)3) Solid Electrolytes for All-Solid-State Li-ion Batteries
Master of Science in Materials Science and Engineering (MSMSE), Wright State University, 2017, Materials Science and Engineering
New-generation low-emission transportation systems demand high-performance lithium ion (Li-ion) batteries with high safety insurance at broad operable temperatures. Highly conductive solid electrolyte is one of the key components for such applications. The objective of this thesis is to synthesize and characterize aluminum doped lithium titanium phosphate, i.e. Li1+xAlxTi2-x(PO4)3 (LATP), one of the solid-state electrolytes for potential applications to all solid-state lithium-ion batteries. In this research, sol-gel method and one step solid-state reaction approaches were explored and critical processes were optimized towards maximizing lithium ion conductivities at room temperature. The impacts of the processing conditions on the structures, morphologies, compositions of the LATP products, and lithium ion conductions were presented. Particle growth kinetics and lithium ion conduction mechanism were briefly discussed. The highest conductivities of LATPs achieved via the sol-gel and solid-state synthesis are 1.24E-04 S/cm and 1.86E-04 S/cm, respectively, exhibiting the feasibilities of applying them to all-solid-state Li-ion batteries.

Committee:

Hong Huang, Ph.D. (Advisor); Allen Jackson, Ph.D. (Committee Member); Raghavan Srinivasan, Ph.D. (Committee Member)

Subjects:

Materials Science

Keywords:

materials science

Pradhan, PujaReal Time Spectroscopic Ellipsometry (RTSE) Analysis of Three Stage CIGS Deposition by co-Evaporation
Doctor of Philosophy, University of Toledo, 2017, Physics
Spectroscopic ellipsometry (SE) is a powerful tool to characterize multilayered thin films, providing structural parameters and materials optical properties over a wide spectral range. Further analyses of these optical properties can provide additional information of interest on the physical and chemical properties of materials. In-situ real time SE (RTSE) combines high surface sensitivity with fast data acquisition and non-destructive probing, thus lends insights into the dynamics of film growth. In this dissertation, the methods of SE have been applied to investigate the growth and properties of material components used in the CIGS thin film photovoltaic technology. Examples of RTSE data collection and analyses are demonstrated for the growth of selenium (Se), molybdenum diselenide (MoSe2) and copper selenide (Cu2-xSe), used in CIGS technology which can then be applied in complete analysis of three-stage CIGS deposition by co-evaporation. Thin film Mo deposited by sputtering is the most widely used back contact for solar cells using CIGS absorbers. In this study, in-situ and real time characterization have been utilized in order to investigate the growth as well as the structural, optical, and electronic properties of Mo thin films deposited by DC magnetron sputtering at different substrate temperatures. In these studies, the surface roughness on the Mo is observed to decrease with increasing substrate temperature. The growth rate, nucleation behavior, evolution of surface roughness and development of void structures in Mo show strong variations with deposition temperature. In depth analyses of (e1, e2) provide consistent estimates of void fraction, excited carrier mean free path, group speeds of excited carriers and intrinsic stress in the films. Complementary ex-situ characterization of the as deposited Mo films included XRD, resistivity measurements by four-point-probe, SEM, and profilometry. This dissertation describes the research performed on the (In1-xGax)2Se3 (IGS) thin films with different Ga composition (x) and different IGS bulk layer thicknesses. The (e1, e2) database for IGS was obtained at 400°C by RTSE starting from films deposited by co-evaporation from fluxes of In, Ga, and Se with different x as in the stage I of three-stage co-evaporation process. The goal of this study is to develop a dielectric function database of IGS films with different x, enabling composition monitoring and thickness control during IGS deposition. RTSE has also been applied successfully in this dissertation research for real time monitoring of Cu-poor to Cu-rich and Cu-rich to Cu-poor transitions during the growth of CIGS films by three-stage co-evaporation. RTSE analyses for all three stages of CIGS growth have been presented including new results for IGS-to-CIGS conversion throughout stage II, Cu2-xSe development at the end of stage II, and Cu2-xSe to CIGS conversion in stage III. Thus, it has been demonstrated that in-situ RTSE combines high thickness, phase, and compositional sensitivity with fast non-invasive data acquisition, thus providing unique insights into the dynamics of CIGS film growth. This non-destructive, high speed capability has the potential to supplement or replace existing monitoring techniques applied for multi-stage co-evaporation of CIGS in both laboratory and industry settings. For further insights into the effect of deposition temperature on device performance, a higher than standard substrate temperature was utilized for the growth of CIGS thin films. Elevation of the substrate temperature for stage II/III deposition from 570C to 620C has led to significant improvements in the efficiency of the CIGS solar cell. The highest efficiency CIGS solar cell obtained in this study is 17.4%.

Committee:

Robert Collins (Committee Chair); Nikolas J. Podraza (Committee Member); Bo Gao (Committee Member); Jacques G. Amar (Committee Member); Dean M. Giolando (Committee Member)

Subjects:

Materials Science; Physics; Solid State Physics

Keywords:

Spectroscopic Ellipsometry; RTSE; selenium; molybdenum diselenide ; copper selenide; Molybdenum; dielectric function;three-stage Copper Indium Gallium di-selenide deposition by co-evaporation

Sharpnack, Lewis LeeMesomorphism of Newly Synthesized Mesogens and Surface Morphology of Chalcogenide Glass Thin Films
PHD, Kent State University, 2017, College of Arts and Sciences / Department of Physics
This dissertation research describes three related projects. The first was an investigation of two de Vries smectic liquid crystal phases that exhibit lower thermal dependence of the smectic layer spacing than the corresponding conventional smectic phases and are well suited for use in electrooptical devices. The second project studied newly synthesized mesogens. This included investigations of several liquid crystalline semiconducting mesogens and a multitude of candidate de Vries smectic mesogens. The third was an investigation of a new non-contact alignment layer of Arsenic Sulfide (As2S3) to anchor the liquid director and use in electrooptical device. In additional to preliminary characterization methodologies such as polarizing optical microscopy and differential scanning calorimetry, two experimental techniques, X-ray diffraction (XRD) and X-ray reflectivity (XRR), were employed. The X-ray studies were conducted using the in-house spectrometers at Kent State University and the synchrotron X-ray source at the Brookhaven National Laboratory. XRR is used to investigate the structure of potential alignment layers. The results provide important insight into the challenges that need to be overcome to develop this alignment material into a viable commercial product. XRD is used to study the structural properties of several members of two new homologous series of liquid crystal compounds. The study of de Vries materials advances our understanding of the role of various molecular moieties on their phase behavior and, most importantly, their relatively temperature independent layer spacing in the Smectic A (SmA) and Smectic C (SmC) phases. This nearly constant layer spacing is critical for developing new fast ferroelectric and electroclinic effect based displays. The Stevenson research group at Queens University synthesized a multitude of new mesogens incorporating a siloxane tail at one end. This moiety is believed to enhance nano-segregation of the molecules and help form de Vries smectic A and C phases. The results indicate that some of the new mesogens exhibit low layer shrinkage that is indicative of the de Vries behavior. The effects of chain lengths and various moieties on the phase behavior is described in detail. These experiments identified several chiral mesogens as viable candidates for use in ferroelectric displays that are currently the subject of further investigations. Many of the non-chiral molecules studied exhibited de Vries or nearly de Vries layer shrinkage, however, these systems would require the addition of a chiral dopant to be used in ferroelectric applications. Three of the chiral siloxane based mesogens displayed ideal de Vries behavior. The smectic layer spacing changed by 1% or less of the total layer thickness for Si3OK11BPO*, Si3OK11BzPO*, and adpc042. These molecules are presently being investigated for device applications and modified with various terminal groups to enhance the miscibility of nano-particle dopants. Structural studies of novel triphenlyene based organic semiconductors mesogens synthesized by the Twieg group were performed. A desirable trait of organic semiconductors is for the ¿-electron orbitals to overlap and requires that carbon rings in adjacent molecules be parallel. Results of X-ray studies of a series of triphenylene molecules showed a hexagonal columnar (ColHex) phase. The diffraction patterns revealed that the lateral intermolecular distance was ~ 3.5 Å, consistent with the stacking of the triphenelene rings. The high-temperature ColHex phase of these materials at nearly 200 °C may also prove useful for high temperature applications. Films of As2S3 have recently been shown to align liquid crystals. This alignment technique, when fully developed, will eliminate the need for traditional mechanically buffed polymer films deposited on substrates, currently used in liquid crystal displays. Their surface roughness was determined in the two planar directions using x-ray reflectivity profiles to facilitate a comparison with other alignment layers that generate liquid crystal alignment primarily because of their anisotropic surface morphology. Our results reveal that As2S3 films develop anisotropic features under irradiation with polarized blue light that are consistent with the changes that occur in other alignment layers when they are “treated” either with mechanical buffing of polymer films or exposure to linearly polarized UV light. These studies also reveal the development of an extensive oxide layer and the ablation of the film under ambient conditions owing to the absorption of oxygen and moisture. This represents a significant barrier to their commercial applications.

Committee:

Satyendra Kumar, PhD (Advisor); Elizabeth Mann, PhD (Committee Member); Hamza Balci, PhD (Committee Member); Michael Fisch, PhD (Committee Member); Scott Bunge, PhD (Committee Member)

Subjects:

Chemistry; Materials Science; Physics

Keywords:

Reflectivity, liquid crystal, de Vries, ferroelectric, smectic, electrooptic device, display, x-ray diffraction, x-ray reflectivity

Caputo, Matthew P4-Dimensional Printing and Characterization of Net-Shaped Porous Parts Made from Magnetic Ni-Mn-Ga Shape Memory Alloy Powders
Doctor of Philosophy in Materials Science and Engineering, Youngstown State University, 2018, Department of Mechanical and Industrial Engineering
Ferromagnetic shape memory alloys (FSMA’s) are known to produce large strains in the presence of magnetic fields. Amongst the FSMA’s near stoichiometric Ni2MnGa alloys present copious conceivable applications such as actuators and sensors due to these large magnetic field induced strains (MFIS’s). Albeit, the large MFIS’s are often observed in single crystals; which are difficult to manufacture and possess limited ductility. Recent investigations of polycrystalline Ni-Mn-Ga foams are reported to exhibit comparable MFIS’s to those reported for single crystals. Therefore the ability to increase the MFIS in Ni-Mn-Ga alloys is envisioned through the introduction of pores in the microstructure. However, the manufacturing is difficult for all the above-mentioned Ni-Mn-Ga materials. Moreover, current techniques lack the ability to manufacture complex geometries. Additive Manufacturing (AM) via binder jetting is a method for producing porous near net shaped components utilizing micrometer sized material. This research investigates an additive manufacturing route of producing functional net shaped parts from pre-alloyed magnetic shape memory Ni-Mn-Ga powders. Three types of Ni-Mn-Ga powders were used in this investigation: spark eroded in liquid nitrogen (LN2), spark eroded in liquid argon (LAr), and ball milled (BM). Additive manufacturing via powder bed binder jetting, also known as 3D printing (3DP) was used in this research due to the ability to control part porosity and the possibility to obtain complex shaped parts from Ni-Mn-Ga alloys. The fourth-dimension (4D) is created by the predictable change in 3D printed part configuration over time as the result of shape-memory functionality. Powder characterization techniques including packing density measurements, size distribution analysis, and binder saturation experiments were conducted on the powders to obtain optimized printing parameters, respectively. The optimum layer thickness and binder saturation range was determined as 80 – 110 µm, and 110 – 250 %, respectively. Binder jetting of Ni-Mn-Ga powders followed by curing and sintering proved successful in producing net shaped porous structures (spring-like, 3-D hierarchical lattice structures, etc.) with suitable mechanical strength. Parts with porosities between 24.08 % and 73.43 % (1.164 g/cm3 to 6.35 g/cm3) have been obtained by using powders with distinct morphologies. The printed and sintered Ni-Mn-Ga parts undergo reversible martensitic transformations during heating and cooling, which is a prerequisite for the shape memory effect. Thermo-magneto-mechanical trained 3D printed parts obtained from ball milled Ni-Mn-Ga powders showed reversible magnetic-field-induced strains (MFIS’s) of up to 0.01%. Binder jetting additive manufacturing is a viable technology in solving the design issues of functional parts made of Ni-Mn-Ga magnetic shape-memory alloys (MSMA).

Committee:

C. Virgil Solomon, PhD (Advisor); Pedro Cortes, PhD (Committee Member); Tim Wagner, PhD (Committee Member); Donald Priour, PhD (Committee Member); Brett Conner, PhD (Committee Member)

Subjects:

Materials Science

Keywords:

Ni-Mn-Ga, Additive manufacturing, Shape memory alloy, metallic powder

Pandey, AnupModeling and Simulation of Amorphous Materials
Doctor of Philosophy (PhD), Ohio University, 2017, Physics and Astronomy (Arts and Sciences)
The general and practical inversion of diffraction data - producing a computer model correctly representing the material explored -is an important unsolved problem for disordered materials. Such modeling should proceed by using our full knowledge base, both from experiment and theory. In this dissertation, we introduce a robust method, Force-Enhanced Atomic Refinement (FEAR), which jointly exploits the power of ab initio atomistic simulation along with the information carried by diffraction data. As a preliminary trial, the method has been implemented using empirical potentials for amorphous silicon (a-Si) and silica ( SiO_2 ). The models obtained are comparable to the ones prepared by the conventional approaches as well as the experiments. Using ab initio interactions, the method is applied to two very different systems: amorphous silicon (a-Si) and two compositions of a solid electrolyte memory material silver-doped GeSe_3 . It is shown that the method works well for both the materials. Besides that, the technique is easy to implement, is faster and yields results much improved over conventional simulation methods for the materials explored. It offers a means to add a priori information in first principles modeling of materials, and represents a significant step toward the computational design of non-crystalline materials using accurate interatomic interactions and experimental information. Moreover, the method has also been used to create a computer model of a-Si, using highly precise X-ray diffraction data. The model predicts properties that are close to the continuous random network models but with no a priori assumptions. In addition, using the ab initio molecular dynamics simulations (AIMD) we explored the doping and transport in hydrogenated amorphous silicon a-Si:H with the most popular4 impurities: boron and phosphorous. We investigated doping for these impurities and the role of H in the doping process. We revealed the network motion and H hopping induced by the thermal fluctuations significantly impacts conduction in this material. In the last section of the dissertation, we employed AIMD to model the structure of amorphous zinc oxide (a-ZnO) and trivalent elements (Al, Ga and In) doped a-ZnO. We studied the structure and electronic structure of these models as well as the effect of trivalent dopants in both the structure and electronic structure of a-ZnO.

Committee:

David A. Drabold (Advisor)

Subjects:

Condensed Matter Physics; Materials Science; Physics

Keywords:

FEAR; neutrons-diffraction data; EDOS; IPR; doping; amorphous Si; amorphous ZnO; trivalent elements doped ZnO

Elmushyakhi, AbrahamIn-Plane Fatigue Characterization of Core Joints in Sandwich Composite Structures
Doctor of Philosophy (Ph.D.), University of Dayton, 2017, Materials Engineering
In practice, adjacent preform sandwich cores are joined with a simple butt joint without special precautions. When molded, this gap is filled with resin and creates a resin rich area. Stress risers will be amplified under cyclic load, and consequently, the serviceability of the structure will be affected. Designers and researchers are aware of this problem; however, quantifying this effect and its intensity and consequence on the service life of the structures has not yet been developed. Despite pervious findings, limited experimental data backed by a comprehensive root cause failure analysis is available for sandwich under axial static, fatigue and post-fatigue. If such a comprehensive experimental characterization is conducted, specifically understanding the nature of the damage, intensity, and residual strength, then a valid multi-scale damage model could be generated to predict the material state and fatigue life of similar composite structures with/without core joints under in-plane static and fatigue load. This research study characterized the effect of scarf and butt core joints in foam core sandwich structures under in-plane static and fatigue loads (R=0.1 and R= -1). Post-Fatigue tensile tests were also performed to predict the residual strength of such structures. Nondestructive Evaluation Techniques were used to locate the stress concentrations and damage creation. A logical blend of experimental and analytical prediction of the service life of composite sandwich structures is carried out. The testing protocol and the S-N curves provided in this work could be reproducible and extrapolated to any kind of core material. This research study will benefit composite engineers and joint designers in both academia and industry to better apprehend the influence of core joints and its consequence on the functionality of sandwich structures.

Committee:

Elias Toubia (Advisor); Paul Murray (Committee Member); Thomas Whitney (Committee Member); Youssef Raffoul (Committee Member)

Subjects:

Aerospace Engineering; Aerospace Materials; Civil Engineering; Composition; Design; Engineering; Materials Science; Mechanical Engineering; Polymers

Keywords:

Sandwich Composite Structures; Design; Fatigue; Damage; Joints; Lightweight Materials; E-glass-vinyl ester; GFRP Laminate; Modeling; Prediction; Nondestructive Testing

Fischdick Acuna, Andres FabricioHybrid Laser Welding in API X65 and X70 Steels
Master of Science, The Ohio State University, 2016, Materials Science and Engineering
Hybrid laser welding presents an important advance in productivity due to high welding speeds. However, fast cooling rates are inherent to the process, affecting the resultant microstructures and joint performance. In this research, three API steels were welded using hybrid laser welding with three distinct preheating conditions. The specimens, which were obtained using one hybrid laser root pass and two other GMAW filling passes, were subjected to microstructural characterization and performance evaluation using hardness and toughness measurements. Incomplete joints with only the hybrid root pass and completed joints (root and filling passes) were evaluated. Hardness mapping revealed as the critical area the top portion of hybrid laser fusion zone, which was subsequently reheated by the GMAW filling pass. Optical and scanning electron microscopy revealed a bainitic-martensitic microstructure with the proportion of those two phases varying as a function of the preheating. Miniaturized Charpy V-notch testing was used to evaluate the local toughness and ductile-to-brittle transition of several regions within the joint. Fractographic analysis confirmed the abrupt transition from ductile-to-brittle behavior. The localized fracture toughness testing showed an adequate joint performance for all tested conditions. Nevertheless, the hardness values meet the requirements only for higher preheating temperature conditions.

Committee:

Antonio Ramirez (Advisor); John Lippold (Committee Member)

Subjects:

Engineering; Materials Science; Metallurgy; Petroleum Engineering

Keywords:

Hybrid Laser, Pipeline, GMAW, HLAW, Steel, Hardness, Toughness, KLST, MCVN, Miniaturized Charpy, Ductile-to-Brittle Transition Temperature, DBTT, Bainite, Martensite

Hehr, Adam JProcess Control and Development for Ultrasonic Additive Manufacturing with Embedded Fibers
Doctor of Philosophy, The Ohio State University, 2016, Mechanical Engineering
Ultrasonic additive manufacturing (UAM) is a recent additive manufacturing technology which combines ultrasonic metal welding, CNC machining, and mechanized foil layering to create large gapless near net-shape metallic parts. The process has been attracting much attention lately due to its low formation temperature, the capability to join dissimilar metals, and the ability to create complex design features not possible with traditional subtractive processes alone. These process attributes enable light-weighting of structures and components in an unprecedented way. However, UAM is currently limited to niche areas due to the lack of quality tracking and inadequate scienti c understanding of the process. As a result, this thesis work is focused on improving both component quality tracking and process understanding through the use of average electrical power input to the welder. Additionally, the understanding and application space of embedding fibers into metals using UAM is investigated, with particular focus on NiTi shape memory alloy fi bers.

Committee:

Marcelo Dapino, Professor (Advisor); Krishnaswamy Srinivasan, Professor (Committee Member); Blaine Lilly, Professor (Committee Member); Peter Anderson, Professor (Committee Member)

Subjects:

Materials Science; Mechanical Engineering

Keywords:

ultrasonic additive manufacturing, ultrasonic consolidation, 3D metal printing, dissimilar metal joining, process modeling, shape memory alloys

Deshpande, AnushreeSynthesis and Characterization of in-situ Nylon-6/Epoxy Blends
MS, University of Cincinnati, 2016, Engineering and Applied Science: Materials Science
Epoxy is a thermosetting polymer known for its excellent adhesion, thermal stability, chemical resistance and mechanical properties. However, one of the major drawbacks of epoxies is its inherent brittleness. In order to overcome this drawback, incorporation of a thermoplastic as a second phase has proven to improve the impact strength without affecting the mechanical properties of epoxy. Researchers in the past have studied polyamide/epoxy blends in terms of blend compatibility, thermo-mechanical properties and morphology via solution blending.

Committee:

Jude Iroh, Ph.D. (Committee Chair); Relva Buchanan, Sc.D. (Committee Member); Raj Manglik, Ph.D. (Committee Member)

Subjects:

Materials Science

Keywords:

polymer blending;in-situ polymerization;nylon-6;epoxy;glass transition temperature;cross-link density

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