<p>Over the past two decade, there have been renewed interests in the use of continuum models of dislocation to predict the plastic strength of metals from basic properties of dislocations. Such interests have been motivated by the unique self-organized dislocation microstructures that develop during plastic deformation of metals and the need to understand their origin and connection with strength of metals. This thesis effort focuses on the theoretical development of a vector-density based representation of dislocation dynamics on the mesoscale accounting for the kinematics of finite deformation. This model consists of two parts, the first is the development of the transport-reaction equations governing dislocation dynamics within the finite deformation setting, and the second focuses on the computational solution of the resulting model. The transport-reaction equations come in the form of a set of hyperbolic curl type transport equations, with reaction terms that nonlinearly couple these equations. The equations are also geometrically non-linear due to finite deformation kinematics and by their constitutive closure. The solution of the resulting model consists of two parts that are coupled in a staggered fashion, the crystal mechanics equations are lumped in the stress equilibrium equations, and the dislocation transport-reactions equations. The two sets of equations are solved by the Galerkin and First-Order System Least-Squares (FOSLS) finite element methods. A special attention is given to the accurate modeling of glissile dislocation junctions using de Rahm currents and graph theory ideas. The introduction of these measures requires the derivation of further transport relations. Using homogenization theory, we specialize the proposed model to a mean deformation gradient driven bulk plasticity model. Lastly, we simulate bulk plasticity behavior and compare our results against experiments.</p>

<p>  </p> <p>With the advent of femtosecond lasers in the early 1990s, ultrafast laser processing has proven to be an imperative tool for micro/nanomachining. Two-photon lithography (TPL) is one such unique microfabrication technique exploiting the nonlinear dependency of the polymerization rate on the irradiating light intensity to produce true three-dimensional structures with feature sizes beyond the diffraction limit. This characteristic has revolutionized laser material processing for the fabrication of micro and nanostructures. This research first gives a general overview of TPL, including its operating principle, experimental setup, compatible materials, and techniques for improving the final resolution of the fabricated structure. Insights to improve throughput and speed of fabrication to pave a way for the industrialization of this technique are provided.</p> <p>Following that, the report delves into the process of fabricating two true three-dimensional mechanical metamaterials via the stereolithography technique. This chapter encompasses the design, fabrication, and experimental analysis of a three-dimensional axisymmetric structure with elliptical perforations distributed periodically on the walls of the structure with varying thicknesses. Furthermore, this study discusses the significance of the elliptical perforations in terms of auxetic behavior and load-bearing capacity against a so-called plain structure under quasistatic compression loading.</p> <p>Finally, the report explores the technique of fabricating a true three-dimensional cylindrical auxetic structure via two-photon polymerization. This section of the report examines the optical setup utilized, the sample preparation procedure, and calibration experiments performed in order to fabricate a three-dimensional thin-walled right cylinder with bowtie like perforations arranged on the walls to promote the exhibition of symmetric negative Poisson’s ratio under uniaxial quasistatic compression loading.</p>

<p>Adhesives  are  found  in  almost  every  aspect  of  the  modern  world.  They  are  found  in plywood used in buildings, electronics, shoes, plumbing and in almost every facet of your daily life. Nature also has an abundance of these adhesives that are used fora multitude of applications. Some  animals, like  the  blue  mussel, use  their  adhesive  for  protection  against  ocean  waves  and predators  while other animals, such  as  the  spider, use  it  to  trap  prey. Investigation  of  theses adhesives has led to the identification of several different proteins that allow for these animals to make  their  adhesive.  Some  of  them  are  composed of rare  amino  acids that  while  other  animals use  a combination  of  inorganic  and  organic  components.  Understanding  of  these  unique adhesives  can be  a  boon  for designof future  adhesives  that  do  not  have  the disadvantagesof current day commercialized glues.</p> <p>Increasing interest  in  the  restoration  of  natural  oyster  reefs  and  the  cement  that  holds them  together  has  resulted  in  the  identification  of  the  Shelk2  protein  that  is  found  both  in  the mantle  of  the  oyster’s  shell  as  well  as  the  cement  that  holds  the  reefs  together. Gaining  an understanding  of  how  this  protein  functions  and  its  part  in  the  oyster  reef  could  be  quite beneficial  for  projects  investing  in  reef  restorations  as  well  as  underwater  adhesion.  Gathering protein  from  the  animal  for  experimentation  and  characterization  can  be  labor  intensive  and extremely challenging. Luckily, cloning technology has become a useful tool for the expression of large quantities of proteins that can be difficult or impossible to gather from the native animal. Using <em>E. coli</em>, it  is  possible  to design  and express  this protein  in  hopes  of  gaining  a  better understanding of its impact on oyster settlement and adhesion.</p> <p>Sustainability is a major downside to current day adhesives that current technologies have not  been  able  to  solve. Most adhesives  that  are  on  the  market  today  are  primarily  derived  from petroleum. Current  research  has  begun  investigating  alternatives  to  the  large   epoxy   and formaldehyde  adhesive  market,  but  the  barrier  of  entry  is  hard  to  overcome.  To  replace  these glues  the  new  material  must  be  affordable,  non-petroleum  derived,  and  available  on  a  massive scale.  These  requirements  are  hard  to  meet  for  many  materials  and  due  to  that  the  current  bio-adhesive are generally very low strength.</p> <p>The work presented here will detail the characterization, and expression of some of these natural  adhesives that  have  been  found  in  the  Eastern  oyster. Another  aspect of  this  work includes the synthesis of a new bio-based adhesive system. Utilizing biomimetic chemistry along with  sustainably  sourced  materials  a  new  adhesive  has  been  formulated that  has  comparable adhesive strength to current day commercial adhesives.</p>

<p>Functional oxide-based thin films have attracted much attention owing to their broad applications in modern society. The multifunction tuning in oxide thin films is critical for obtaining enhanced properties. In this dissertation, four new nanocomposite thin film systems with highly textured growth have been fabricated by pulsed laser deposition technique. The functionalities including ferromagnetism, ferroelectricity, multiferroism, magnetoelectric coupling, low-field magnetoresistance, transmittance, optical bandgap and dielectric constants have been demonstrated. Besides, the tunability of the functionalities have been studied via different approaches.</p> <p>First, varies deposition frequencies have been used in vertically aligned nanocomposite BaTiO₃:YMnO₃ (BTO:YMO) and BaTiO₃:La_0.7Sr_0.3Mn₃(BTO:LSMO) thin films. In both systems, the strain coupling effect between the phases are affected by the density of grain boundaries. Increasing deposition frequency generates thinner columns in BTO:YMO thin films, which enhances the anisotropic ferromagnetic response in the thin films. In contrast, the columns in BTO:LSMO thin films become discontinuous as the deposition frequency increases, leading to the diminished anisotropic ferromagnetic response. Coupling with the ferroelectricity in BTO, the room temperature multiferroic properties have been obtained in these two systems.</p> <p> Second, the impact of the film composition has been demonstrated in La_0.7Ca_0.3MnO₃ (LCMO):CeO₂thin film system, which has an insulating CeO₂in ferromagnetic conducting LCMO matrix structure. As the atomic percentage of the CeO₂increases, enhanced low-field magnetoresistance and increased metal-to-insulator transition temperature are observed. The thin films also show enhanced anisotropic ferromagnetic response comparing with the pure LCMO film.</p> <p> Third, the transition metal element in Bi₃MoM_TO₉(M_T, transition metals of Mn, Fe, Co and Ni) thin films have been varied. The thin films have a multilayered structure with M_T-rich pillar-like domains embedded in Mo-rich matrix structure. The anisotropic magnetic easy axis and optical properties have been demonstrated. By the element variation, the optical bandgaps, dielectric constants as well as anisotropic ferromagnetic properties have been achieved. </p> <p> The studies in this dissertation demonstrate several examples of tuning the multifunctionalities in oxide-based nanocomposite thin films. These enhanced properties can broaden the applications of functional oxides for advanced nanoscale devices.</p><br>

<div>Despite the advances of analytical and numerical methods for composite materials, it is still challenging to predict the onset and evolution of their different failure mechanisms. Because most failure mechanisms are irreversible processes in thermodynamics, it is beneficial to model them within a unified thermodynamic framework. Noting the advantages of so-called generalized standard models (GSMs) in this regard, the objective of this work is to formulate constitutive models for several main failure mechanisms: brittle fracture, interlaminar delamination, and fatigue behavior for both continuum damage and delamination, in a generalized standard manner.</div><div><br></div><div>For brittle fracture, the numerical difficulties caused by damage and strain localization in traditional finite element analysis will be addressed and overcome. A nonlocal damage model utilizing an integral-type regularization technique will be derived based on a recently developed ``local'' continuum damage model. The objective is to make this model not only rigorously handle brittle fracture, but also incorporate common damage behavior such as damage anisotropy, distinct tensile and compressive damage behavior, and damage deactivation. A fully explicit integration scheme for the present model will be developed and implemented.</div><div><br></div><div>For fatigue continuum damage, a viscodamage model, which can handle frequently observed brittle damage phenomena, is developed to produce stress-dependent fatigue damage evolution. The governing equation for damage evolution is derived using an incremental method. A class of closed-form incremental constitutive relations is derived. </div><div><br></div><div>For interlaminar delamination, a cohesive zone model (CZM) will be proposed. Focus is placed on making the associated cohesive elements capable of displaying experimental critical energy release rate--mode mixture ratio relationships. To achieve this goal, each cohesive element is idealized as a deformable string exhibiting path dependent damage behavior. A damage model having a path dependence function will be developed, which will be constructed such that each cohesive element can exhibit designated, possibly sophisticated mixed-mode behavior. The rate form of the cohesive law will be subsequently derived.</div><div><br></div><div>Finally, a CZM for interlaminar fatigue, capable of handling brittle damage behavior, is developed to produce realistic interlaminar crack propagation under high-cycle fatigue. An implicit integration scheme, which can handle complex separation paths and mixed-mode delamination, is developed. Many numerical examples will be utilized to clearly demonstrate the capabilities of the proposed nonlocal damage model, continuum fatigue damage model, and CZMs for quasi-static and fatigue delamination.</div>

<div>Accelerating the development of novel materials is one of the central goals of the Materials Genome Initiative and improving the predictive power of computational</div><div>material science methods is critical to attain this goal. Molecular dynamics (MD) is one such computational technique that has been used to study a wide range of materials since its invention in the 1950s. In this work we explore some examples of using and increasing the predictive power of MD simulations to understand materials phenomena and provide guidelines to design tailored materials. We first demonstrate the use of MD simulations as a tool to explore the design space of shape memory alloys, using simple interatomic models to identify characteristics of an integrated coherent second phase that will modify the transformation characteristics of the base shape memory alloy to our desire. Our approach provides guidelines to identify potential coherent phases that will achieve tailored transformation temperatures and hysteresis. </div><div><br></div><div>We subsequently explore ideas to enhance the length and time scales accessible via MD simulations. We first discuss the use of kinetic Monte Carlo methods in MD simulations to predict the microstructure evolution of carbon fibers. We ?find our approach to accurately predict the transverse microstructures of carbon fibers, additionally predicting the transverse modulus of these fibers, a quantity difficult to measure via experiments. Another avenue to increase length and time scales accessible via MD simulations is to explore novel implementations of algorithms involved in machine-learned interatomic models to extract performance portability. Our approach here results in significant speedups and an efficient utilization of increasingly common CPU-GPU hybrid architectures.</div><div><br></div><div>We finally explore the use of machine learning methods in molecular dynamics, specifically developing machine learning methods to discover interpretable laws directly from data. As examples, we demonstrate the discovery of integration schemes for MD simulations, and the discovery of melting laws for perovskites and single elements. Overall, this work attempts to illustrate how improving the predictive capabilities of molecular dynamics simulations and incorporating machine learning ideas can help us design novel materials, in line with the goals of the Materials Genome Initiative.</div>

<p dir="ltr">Conducting polymers have garnered enormous attention due to their unique properties, including tunable chemical structure, high flexibility, solution processability, and biocompatibility. They hold promising applications in flexible electronics, renewable energies, sensing, and healthcare. Despite notable progress in conducting polymers over the past few decades, most of them still suffer from complicated synthesis routes, limited processability, low electrical conductivity, and poor ambient stability compared to their inorganic counterparts. Additionally, the susceptibility of conducting polymers to high temperatures makes them not applicable in real-life electronics. To address the challenges of developing high-performance and stable conducting polymers, we present two key approaches: dopant innovation for polymer-dopant interaction engineering and the discovery of new conjugated polymer hosts. From the perspective of dopant design, we first utilize cross-linkable chlorosilanes (C-Si) to design thermally and chemically stable conductive polymer composites. C-Si can form robust siloxane networks and simultaneously<i> </i>dope the host conjugated polymers. Besides, we have introduced a new class of dopants, namely aromatic ionic dopants (AIDs). The use of AIDs allows for the separation of doping and charge compensation, two processes involved in molecular doping, and therefore leads to highly efficient doping and thermally stable doped systems. We then provide insights into the design of novel conjugated polymer hosts. Remarkably, we have developed the first thermodynamically stable n-type conducting polymer, n-doped Poly (3,7-dihydrobenzo[1,2-b:4,5-b′]difuran-2,6-dione) (n-PBDF). n-PBDF is synthesized from a simple and scalable route, involving oxidative polymerization and reductive doping in one pot in the air. The n-PBDF ink is solution processable with excellent ink stability and the n-PBDF thin film is highly conductive, transparent, patternable, and robust. In addition, precise control over the doping levels of n-PBDF has been achieved through chemical doping and dedoping. By tuning the n-PBDF thin films between highly doped and dedoped states, the system shows controllable conductivity, optical properties, and energetics, thereby offering potential applications in a variety of organic electronics. Overall, this research advances the fundamental understanding of molecular doping and offers insights for the development of high-conductivity, stable conducting polymers with tunable properties for next-generation electronics.</p>

<p>The Materials Genome Initiative (MGI) has established guidelines to accelerate the discovery, development, and implementation of advanced materials in order to address current and future challenges. A key area of interest is the pressing need for more efficient energy storage systems to support technologies such as electric vehicles and renewable energies. In this work, we present an Integrated Computational Materials Engineering approach for the development of novel solid-state electrolyte materials. In particular, we embark on a quest to unravel the potential of ceramic garnet lithium lanthanum zirconium oxide (LLZO) for next-generation battery technologies.</p> <p>Our exploration begins with an overview of the current state of the Materials Innovation Infrastructure (MII) and our rationale behind choosing LLZO. Through the use of machine learning techniques and molecular dynamics simulations, we aim for efficient material optimization. Our findings are reinforced through experiments by using these materials as inorganic fillers in composite polymer electrolytes. Our findings demonstrate that the combined use of these complementary techniques facilitates the discovery of potential alternative solid-state electrolytes. Finally, we propose future research directions in materials science for the design of advanced materials using these integrated approaches. </p>

<p>Modern structural materials utilize tailored microstructures to retain peak performance within the most volatile operating conditions. Features such as grain size, grain boundary (GB) character and morphology and secondary phases are just a few of the tunable parameters. By tailoring these types of microstructural features, the deformation behavior of the material is also altered. The localization of plastic strain directly correlated to material failure. Thus, a systematic approach was utilized to understand the effect of microstructural features on the localization of plastic deformation utilizing digital image correlation (DIC). First, at the macroscopic scale, strain accumulation is known to form parallel to the plane of maximum shear stress. The local deviations in the deformation pathways at the meso-scale are investigated relative to the plane of maximum shear stress. The deviations in the deformation pathways are observed to be a function of the accumulated local plastic strain magnitude and the grain size. Next, strains characterized via DIC were used to calculate a value of incremental slip on the active slip systems and identify cases of slip transmission. The incremental slip was calculated based on a Taylor-Bishop-Hill algorithm, which determined a qualitative assessment of deformation on a given slip system, by satisfying compatibility and identifying the stress state by the principle of virtual work. Inter-connected slip bands, between neighboring grains, were shown to accumulate more incremental slip (and associated strain) relative to slip bands confined to a single grain, where slip transmission did not occur. These results rationalize the role of grain clusters which lead to intense strain accumulation and thus serve as potential sites for fatigue crack initiation. Lastly, at GB interfaces, the effect of GB morphology (planar or serrated) on the cavitation behavior was studied during elevated temperature dwell-fatigue at 700 °C. The resulting γ′ precipitate structures were characterized near GBs and within grains. Along serrated GBs coarsened and elongated <a>γ′ </a>precipitates formed and consequently created adjacent regions that were denuded of γ′ precipitates. Dwell-fatigue experiments were performed at low and high stress amplitudes which varied the amount of imparted strain on the specimens.<a> Additionally, the regions denuded of the γ′ precipitates were observed to localize strain and to be initial sites of cavitation.</a> <a>These results present a quantitative strain analysis between two GB morphologies, which provided the micromechanical rationale for the increased proclivity for serrated GBs to form cavities.</a></p>

<div>The global demand for energy of mankind, the ever-increasing cost of energy, and the expected depletion of fossil energy carriers within the next centuries urge the exploration of alternative and more sustainable ways to provide energy. The current quest for energy-efficient technologies for the replacement of existing cooling devices has made the magnetocaloric effect a field of current scientific interest. Cooling technologies based on magnetic refrigerants are expected to have a better environmental impact compared with those based on the gas compression-expansion cycle. This technology provides an alternative for refrigeration applications with advantages, such as high energy efficiency, environmental friendliness, and low power consumption. In search of promising magnetocaloric materials, several rare earth-depleted transition metal-based materials were designed and investigated.</div><div>In this work, RCrxAl2-x and RZnAl (R = Gd, Tb, Dy, Ho) belonging to the ternary rare-earth transition-metal Laves phases, were chosen as the starting point to establish the effect of valence electron concentration (VEC) on the magnetic behavior and magnetocaloric effect. Our result and the results from the previously studied RTAl phases (T = Cu, Ni, Co, Fe, Mn) shows that the perturbation of the valence electron concentration at the Fermi level is found to be the driving force that dictates the crystal structure, magnetocaloric and magnetic properties of these systems. Most notably, the decrease in the valence electron concentration at the Fermi level leads to an increase in the curie temperature.</div><div>In addition, we have further extended this theory to GdNiAl2 systems. GdNiAl2 is a known magnetocaloric material which exhibits an isothermal magnetic entropy change of ΔSM = 16.0 Jkg-1K-1 at TC = 28K under a magnetic field change from 0-5T. However, the low TC limits its application as a room temperature refrigerant. We, therefore, substituted Co for (Ni/Al) in the structure of GdNiAl2, intending to substantially perturb the position of the Fermi level of Ni since that will lead to a decrease in the VEC and hence elevate the TC. The study was also extended to another Icosagenides (Ga,), which saw the substitution of Ga for Al in GdNiAl2 and its Co substituted analogs. The Ga analogs exhibit complex magnetic behavior with a cascade (multiple) of magnetic transitions, as opposed to the rather simple magnetism of their Al congeners.</div>