<p>Throughout the last century, energetic materials have been subject to drop weight impact tests to measure their sensitivity, with which material’s properties are correlated to their impact sensitivity. However, there is little research that focuses on utilizing the piezoelectric effect to control the sensitivity of energetics. Piezoelectricity is the effect of an electric charge accumulating due to an applied mechanical stress. It is demonstrated in previous work that fluoropolymers such as polyvinylidene fluoride (PVDF) contribute to higher sensitivity in nanocomposite energetic materials through their piezoelectric properties. This property can be amplified in fluoropolymers in the beta (β) phase through polling methods and can be quantitatively analyzed by the piezoelectric coefficient (d₃₃). This research is focused on characterizing the effect of piezoelectricity on the impact sensitivity and ignition delay of nAl/PVDF-TrFE composites through the presence of varied d₃₃ coefficients. The composite films were fabricated with the tape casting method with 85 μm thickness. The content of nAl was limited to 10 wt% in order to sustain feasible poling. Poling was achieved without any further manipulation of the composition so that a direct comparison could be observed. The magnitude of effect that the piezoelectric coefficient has on an energetic composite was discovered. The samples that had no d₃₃ value were 8% less sensitive and experienced longer ignition delay times compared to the poled samples. This work proved that impact sensitivity and ignition delay can be manipulated through poling methods. This concept of controlling the sensitivity of energetic materials can be used to develop more customizable composites in the future.</p>

The work presented in this dissertation focuses on the study of post deposition spatial and temporal evolution of adatom islands and molecules on surfaces using ab initio and semiemperical methods. It is a microscopic study of the phenomena of diffusion and reaction on nanostructured surfaces for which we have developed appropriate computational tools, as well as implemented others that are available. To map out the potential energy surface on which the adatom islands and molecules move, we have carried out ab initio electronic structure calculations based on density functional theory (DFT) for selected systems. For others, we have relied on semiempirical interatomic potentials derived from the embedded atom method. To calculate the activation energy barriers, we have employed the "drag" method in most cases and verified its reliability by employing the more accurate nudged elastic band method for selected systems. Temporal and spatial evolution of the systems of interest have been calculated using the kinetic Monte Carlo (KMC), or the more accurate (complete) Self Learning kinetic Monte Carlo (SLKMC) method in the majority of cases, and ab initio molecular dynamics simulations in others. We have significantly enhanced the range of applicability of the SLKMC method by introducing a new pattern recognition scheme which by allowing occupancy of the "fcc" and "hcp" sites (and inclusion of "top" site in the pattern recognition as well) is capable of simulating the morphological evolution of iii three dimensional adatom islands, a feature not feasible via the earlier - proposed SLKMC method. Using SLKMC (which allows only fcc site occupancy on fcc(111) surface), our results of the coarsening of Ag islands on the Ag(111) surface show that during early stages, coarsening proceeds as a sequence of selected island sizes, creating peaks and valleys in the island-size distribution. This island size selectivity is independent of initial conditions and results from the formation of kinetically stable islands for certain sizes as dictated by the relative energetics of edge atom detachment/attachment processes together with the large activation barrier for kink detachment. On applying the new method, SLKMC-II, to examine the self diffusion of small adatom islands (1-10 atoms) of Cu on Cu(111), Ag on Ag(111) and Ni on Ni(111), we find that for the case of Cu and Ni islands, diffusion is dominated by concerted processes (motion of island as a whole), whereas in the case of Ag, islands of size 2-9 atoms diffuse through concerted motion whereas the 10-atom island diffuses through single atom processes. Effective energy barriers for the self diffusion of these small Cu islands is 0.045 eV/atom, for Ni it is 0.060 eV/atom and for Ag it is 0.049 eV/atom, increasing almost linearly with island size. Application of DFT based techniques have allowed us to address a few issues stemming from experimental observations on the effect of adsorbates such as CO on the structure iv and stability of bimetallic systems (nanoparticles and surfaces). Total energy calculations of Ni-Au nanoparticles show Ni atoms to prefer to be in the interior of the nanoparticle. CO molecules, however, prefer to bind to a Ni atom if present on the surface. Using ab initio molecular dynamics simulations, we confirm that the presence of CO molecule induces diffusion of Ni atom from the core of the Ni-Au nanoparticle to its surface, making the nanoparticle more reactive. These results which help explain a set of experimental data are rationalized through charge transfer analysis. Similar to the case of Ni-Au system, it is found that methoxy (CH3O) may also induce diffusion of inner atoms to the surface on bimetallic Au-Pt systems. Our total energy DFT calculations show that it is more favorable for methoxy to bind to a Pt atom in the top Au layer than to a Au atom in Au-Pt system thereby explaining experimental observations. To understand questions related to the dependence of product selectivity on ambient pressure for ammonia decomposition on RuO2(110), we have carried out an extensive calculation of the reaction pathways and energy barriers for a large number of intermediate products. On combining the reaction energetics from DFT, with KMC simulations, we show that under UHV conditions, selectivity switches from N2 ( ∼ 100 % selectivity) at T = 373K to NO at T = 630K, whereas under ambient conditions, N2 is still the dominant product but maximum selectivity is only 60%. An analysis based on thermodynamics alone shows a contradiction between experimental data at UHV with those under ambient pressure. Our calculations of the reaction rates which are essential for KMC simulations removes this apv parent inconsistency and stresses the need to incorporate kinetics of processes in order to extract information on reaction selectivity.

The discovery of carbon nanotubes in 1990s popularized a new area of research in materials science called Nanoscience. In the following decades, several carbon based nanoparticles were discovered or engineered and with the discovery of Graphene nanoplatelets (GNP) in 2010, carbon based nanoparticles were propelled as the most promising class of nanoparticles. High mechanical strength and stiffness, excellent electrical and thermal conductivity, and high strength to weight ratios are some of the unique abilities of CNTs and GNPs which allow their use in a wide array of applications from aerospace materials to electronic devices. In the current work presented herein, CNTs and GNPs are added to polymeric materials to create a nanocomposite material. The effects of this nanoparticle addition (a.k.a reinforcement) on the mechanical properties of the nanocomposite polymer materials are studied. Specifically, efforts are focused on studying fracture toughness, a material property that describes the material's ability to resist crack growth. Relative to the conventional metals used in structures, epoxy-based composites have poor fracture toughness. This has long been a weak link when using epoxy composites for structural applications and therefore several efforts are being made to improve their fracture toughness. In the first, second and third chapters, the enhancement of fracture toughness brought about by the addition of carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) was investigated. CNT-Epoxy and GNP-Epoxy Compact Tension (CT) samples were fabricated with 0.1% and 0.5% nanofiller weight concentrations. The potential synergistic effects of dual nanofiller reinforcements were also explored using CNT/GNP-Epoxy CT samples at a 1:3, 3:1 and 1:1 ratio of CNT:GNP. Displacement controlled CT tests were conducted according to ASTM D5045 test procedure and the critical stress intensity factor, $K_{IC}$, and the critical fracture energy, $G_{IC}$, were calculated for all the material systems. Significant enhancements relative to neat epoxy were observed in reinforced epoxies. Fracture surfaces were analyzed via scanning electron microscopy. Instances of CNT pullouts on the fracture surface were observed, indicating the occurrence of crack bridging. Furthermore, increased surface roughness, an indicator of crack deflection, was observed along with some crack bifurcations in the GNP-Epoxy samples. In the fourth chapter of Part I, the influence of pre-crack characteristics on the Mode-I fracture toughness of epoxy is investigated. Pre-crack characteristics such as pre-crack length, crack front shape, crack thickness and crack plane profile are evaluated and their influence on the peak load, fracture displacement, and the critical stress intensity factor, $K_{IC}$ is studied. A new method of razor blade tapping was used, which utilized a guillotine-style razor tapping device to initiate the pre-crack and through-thickness compression to arrest it. A new approach of quantitatively characterizing the crack front shape using a two-parameter function is introduced. Surface features present on the pre-crack surface are classified and their effects on the post crack initiation behavior of the sample are analyzed. This study aims to identify and increase the understanding of the various factors that cause variation in the fracture toughness data of polymeric materials, thus leading to more informed engineering design decisions and evaluations. Chapters six and seven of Part II investigate the SHM capabilities of dispersed MWCNTs in mock, inert, and active energetics. In these experimental investigations, the strain and damage sensing abilities of multi-walled carbon nanotube (MWCNT) networks embedded in the binder phase of polymer bonded energetics (PBEs) are evaluated. PBEs are a special class of particulate composite materials that consist of energetic crystals bound by a polymer matrix, wherein the polymer matrix serves to diminish the sensitivity of the energetic phase to accidental mechanical stimuli. The structural health monitoring (SHM) approach presented in this work exploits the piezoresistive properties of the distributed MWCNT networks. Major challenges faced during such implementation include the low binder concentrations of PBEs, presence of conductive/non-conductive particulate phases, high degree of heterogeneity in the PBE microstructure, and achieving the optimal MWCNT dispersion. In chapter seven, Ammonium Perchlorate (AP) crystals as the oxidizer, Aluminum grains as the metallic fuel, and Polydimethylsiloxane (PDMS) as the binder are used as the constituents for fabricating PBEs. To study the effect of each constituent on the MWCNT network's SHM abilities, various materials systems are comprehensively studied: MWCNT/PDMS (nBinder) materials are first evaluated to study the binder's electromechanical response, followed by AP/MWCNT/PDMS (inert nPBE) to assess the impact of AP addition, and finally, AP/AL/MWCNT/PDMS (active nPBE-AL) to evaluate the impact of adding conductive aluminum grains. Compression samples (ASTM D695) were fabricated and subjected to monotonic compression. Electrical resistance is recorded in conjunction with the mechanical test via an LCR meter. Gauge factors relating the change in normalized resistance to applied strain are calculated to quantify the electromechanical response. MWCNT dispersions, and mechanical failure modes are analyzed via scanning electron microscopy (SEM) imaging of the fracture surfaces. Correlations between the electrical behavior in response to the mechanical behavior are presented, and possible mechanisms that influence the electromechanical behavior are discussed. The results presented herein demonstrate the successful ability of MWCNT networks as structural health monitoring sensors capable of real-time strain and damage assessment of polymer bonded energetics. / Doctor of Philosophy / The discovery of carbon nanotubes in 1990s popularized a new area of research in materials science called Nanoscience. Carbon nanotubes (CNTs) are one of several forms of Carbon, meaning a differently structured carbon molecule in the same physical state similar to diamonds, graphite, and coal. In the following decades, several carbon based nanoparticles were discovered or engineered and with the discovery of Graphene (GNP) in 2010, carbon based nanoparticles were propelled as the most promising class of nanoparticles. High mechanical strength and stiffness, excellent electrical and thermal conductivity, and high strength to weight ratios are some of the unique abilities of CNTs and GNPs which allow their use in a wide array of applications from aerospace materials to electronic devices. In the current work presented herein, CNTs and GNPs are added to polymeric materials to create a nanocomposite material, where the term "composite" refers to a material prepared with two or more constituent materials. The effects of this nanoparticle addition (a.k.a reinforcement) on the mechanical properties of the nanocomposite polymer materials are studied. Specifically, efforts are focused on studying fracture toughness, a material property that describes the material's ability to resist crack growth. Fracture toughness is a critical material property often associated with material and structural failures, and as such it is very important for safe and reliable engineering design of structures, components, and materials. Moving from a single function (i.e. mechanical enhancement) to a more multi-functional role, taking advantage of the excellent electrical and mechanical abilities of CNTs, a structural health monitoring system is developed for use in polymer bonded energetics (eg. solid rocket propellants). When a material undergoes mechanical deformation or damage, the measured electrical properties of the material undergo some change as well. Using sensor networks built with multiple CNTs dispersed within a polymeric material, a whole structure can be made into an effective sensor where by simply monitoring the electrical properties, the extent of material deformation and damage can be known. Such a system is geared towards providing early warning of impending catastrophic material failures thus directly improving the safety during material handling and operations.

There has been a prevalence of long-distance running footwear incorporating carbon-fiber plates within their midsoles, effectively increasing their longitudinal bending stiffness (LBS). This modification of modern racing footwear has occurred concurrently with large improvements in running times (Bermon et al., 2021), putting into question how these footwear components affect performance (Muniz-Pardos et al., 2021). The current literature has investigated this at level running, but with the increasing popularity of trail running, it is of interest to investigate whether the benefits found during level running translate to graded running. Therefore, the overall aim of this study was to investigate the effects of increased footwear midsole longitudinal bending stiffness (i.e. carbon-fiber plates) on running energetics and biomechanics at various inclines. The effects of high LBS (Nike Vaporfly 4% with midsole intact) and low LBS (Nike Vaporfly 4% with mediolateral cuts made at the forefoot of the midsole through the carbon-fiber plate) footwear conditions were compared for running at 0°, 6°, and 12° inclines. Running energetics and biomechanics data were quantified by measuring metabolic rate and lower leg joint mechanics (from motion capture and ground reaction force measurements). Results from this study suggest that increasing longitudinal bending stiffness within the footwear midsoles has limited influence on running energetics (small non-significant improvements of metabolic power at all inclines), but has considerable effects on the biomechanics of the ankle and MTP joints. However, the most important between shoe differences were independent of grade, suggesting that the benefits of modern racing shoe observed for level running can be expected to translate to steep uphill running. Nevertheless, it should be noted that this study was only able to collect and use data for analysis from a limited number of participants (n=7), and therefore is underpowered, so there may be significant differences that go undetected

<p dir="ltr">Smart energetics are energetic materials (propellants, explosives, and pyrotechnics) with on/off capabilities or in real time modification of combustion behavior. Solid propellants are known for many positive qualities such as their simplicity and low cost but also their glaring lack of active burning rate control. Previous proposed methods of active control of solid propellants include pintle valve actuation and electronically controlled solid propellants, however there is a need for improved methods. Surface area modification is one proposed method and can be employed in real time to affect the burning behavior of solid propellants. To this end, derivations were conducted regarding a slot adjacent to a solid propellant strand and the pressure and slot width threshold conditions that allow for burning to occur inside of the adjacent slot. The derivations considered different modes of combustion (convective and conductive) and combustion threshold conditions. The derivations resulted in five equations that were curve fit to existing literature for validation resulting in high R squared values. A demonstration of the creation of an adjacent slot with a piezoelectric actuator, a mini case study of the adjacent slot proposal, and a discussion of methods to create an adjacent slot as well as the effect of propellant selection on convective burning in slots were all done to follow up on the promising results of the theoretical work. </p><p dir="ltr">Furthermore, flexoelectricity is the coupling between strain gradient and charge generation and has been considered to modify the combustion characteristics of energetic materials. This work measured the flexoelectric properties of polymers and their associated energetic composites including polyvinylidene fluoride (PVDF), micron aluminum (μAl)/PVDF, nano aluminum (nAl)/PVDF, poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)), nAl/P(VDF-TrFE), poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)), μAl/P(VDF-HFP), hydroxylterminated polybutadiene (HTPB), ammonium perchlorate (AP)/HTPB, μAl/AP/HTPB, polytetrafluoroethylene (PTFE), and polydimethylsiloxane (PDMS). The measurements made on PVDF, μAl/PVDF, P(VDF-TrFE), P(VDF-HFP), PTFE, and PDMS were all within or near to the range of measurements from the literature. Novel measurements were made on nAl/PVDF, nAl/P(VDF-TrFE), μAl/P(VDF-HFP), HTPB, AP/HTPB, and μAl/AP/HTPB. Additionally, the effect of porosity, particle additions (μAl, nAl, or AP), and manufacturing method (3D printing, casting, different 3D printers, etc.) on the flexoelectric performance of these samples was investigated. It was found that large pores (millimeter scale) added via the infill pattern of 3D printed PVDF and Al/PVDF samples decreased the effective flexoelectric effect relative to the near full density control samples. This contrasts with previous work showing that adding small (micron scale) pores increases the flexoelectric performance of various polymers and energetic materials. Mixed results were found with respect to the effect of particle additions (μAl, nAl, or AP) on the flexoelectricity of a variety of materials. This may be explained by the competing effect of particle additions adding extra local strain gradients which amplify flexoelectricity but also replace some polymer binder material (PVDF, P(VDF-TrFE), P(VDF-HFP), and HTPB) with the particle additions (μAl, nAl, and AP) which are typically less flexoelectric. Our work demonstrates that manufacturing method does affect the flexoelectric properties of polymers and energetic composites. Lastly, our flexoelectric measurements of P(VDF-HFP) and PTFE may help explain accidents related to Magnesium-Teflon®-Viton® (MTV) flare systems that have, in many cases, been attributed to electrostatic discharge.</p>

<p dir="ltr">Chemical reaction networks govern polymer degradation and contain critical design information regarding specific susceptibilities, degradation pathways, and degradants. However, predicting reaction pathways and characterizing complete reaction networks has been hindered by high computational costs because of the vast number of possible reactions at deeper levels of network exploration. In the first section, an exploration policy based on Dijkstra's algorithm on YARP using the reaction rate as a cost function was shown to provide a tractable means of exploring the pyrolytic degradation network of a representative commodity polymer, PEG. The resulting network is the largest reported to date for this system and includes pathways out to all degradants observed in earlier mass spectrometry studies. The initial degradation pathway predictions were validated by complementary experimental analysis of pyrolyzed PEG samples by ESI-MS. These findings demonstrate that reaction network characterization is reaching sufficient maturity to be used as an exploratory tool for investigating materials degradation and interpreting experimental degradation studies.</p>

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