Medium density fiberboard (MDF) production uses thermomechanically refined fiber processed under shear with high pressure steam. The industry evaluates fiber quality with visual and tactile inspection, emphasizing fiber dimensions, morphology, and bulk density. Considering wood reactivity, the hypothesis is that a variety of chemical and physical changes must occur that are not apparent in visual/tactile inspection. An industry/university cooperation, this work studies effects of refining energy (adjusted by refiner-plate gap) on fiber: size, porosity, surface area, surface and bulk chemistry, fiber crystallinity and rheology, and fiber interaction with amino resins. The intention is to reveal novel aspects of fiber quality that might impact MDF properties or process control efficiency, specific to a single industrial facility. In cooperation with a North American MDF Douglas fir plant, two refining energies were used to produce resin and additive-free fibers. Refining reduced fiber dimensions and increased bulk density, more so at the highest energy. Thermoporosimetry showed increases in sub-micron scale porosity, greatest at the highest energy. Mercury intrusion porosimetry (MIP) revealed porosity changes on a higher dimensional scale. Brunauer-Emmett-Teller gas adsorption and MIP showed that refining increased specific surface area, more so at the highest energy. Inverse gas chromatography showed that the lowest refining energy produced surfaces dominated by lignin and/or extractives. The highest energy produced more fiber damage, revealing higher energy active sites. A novel rheological method was devised to study fiber compaction and densification; it did not distinguish fiber types, but valuable aspects of mechano-sorption and densification were observed. Refining caused substantial polysaccharide degradation, and other degradative effects that sometimes correlated with higher refining energy. Lignin acidolysis was detected using nitrobenzene oxidation, conductometric titration of free phenols, and formaldehyde determination. Formaldehyde was generated via the C2 lignin acidolysis pathway, but C3 cleavage was the dominant lignin reaction. Observations suggested that in-line formaldehyde monitoring might be useful for process control during biomass processing. According to rheological and thermogravimetric analysis, lignin acidolysis was not accompanied by repolymerization and crosslinking. Lignin repolymerization must have been prevented by the reaction of benzyl cations with non-lignin nucleophiles. This raises consideration of additives that compete for lignin benzyl cations, perhaps to promote lignin crosslinking and/or augment the lignin network with structures that impart useful properties. Fiber/amino resin interactions were studied with differential scanning calorimetry (DSC) and X-ray diffraction (XRD). All fiber types, refined and unrefined, caused only a slight increase in melamine-urea-formaldehyde (MUF) resin reactivity. Generally, all fiber types decreased the enthalpy of MUF cure, suggesting fiber absorption of small reactive species. But DSC did not reveal any dependency on fiber refining energy. According to XRD, all fiber types reduced crystallinity in cured MUF, more so with refined fiber, but independent of refining energy. The crystallinity in cured urea-formaldehyde resin was studied with one fiber type (highest refining energy); it caused a crystallinity decrease that was cure temperature dependent. This suggests that resin crystallinity could vary through the thickness of an MDF panel. / PHD

Metal additive manufacturing stands poised to disrupt multiple industries with high material use efficiency and complex part production capabilities, however many technologies deposit material with sub-optimal properties, limiting their use. This decrease in performance largely stems from porosity laden parts, and asymmetric solidification-based microstructures. Solid-state additive manufacturing techniques bypass these flaws, using deformation and diffusion phenomena to bond material together layer by layer. Among these techniques, Additive Friction Stir Deposition (AFSD), stands out as unique for its freeform nature, and thermomechanical conditions during material processing. Leveraging its solid-state behavior, optimized microstructures produced by AFSD can reach performance levels near, at, or even above traditionally prepared metals. A strong understanding of the material conditions during AFSD and the phenomena responsible for microstructure evolution. Here we discuss two works aimed at improving the state of knowledge surrounding AFSD, promoting future microstructure optimization. First, a parametric study is performed, finding a wide array of producible microstructures across two material systems. In the second work, a stop-action type experiment is employed to observe the dynamic microstructure evolution across the AFSD material flow pathway, finding specific thermomechanical regimes that occur within. Finally, multiple conventional alloy systems are discussed as their microstructure evolution pertains to AFSD, as well as some more unique systems previously limited to small lab scale techniques, but now producible in bulk due to the additive nature of AFSD. / Doctor of Philosophy / The microstructure of a material describes the atomic behavior at multiple length scales. In metals this microstructure generally revolves around the behavior of millions of individual crystals of metal combined to form the bulk material. The state and behavior of these crystals and the atoms that make them up influence the strength and usability of the material and can be observed using various high fidelity characterization techniques. In metal additive manufacturing (i.e. 3D printing) the microstructure experiences rapid and severe changes which can alter the final properties of the material, typical to a detrimental effect. Given the other benefits of additive manufacturing such as reduced costs and complex part creation, there is desire to predict and control the microstructure evolution to maximize the usability of printed material. Here, the microstructure evolution in a solid-state metal additive manufacturing, Additive Friction Stir Deposition (AFSD), is investigated for different metal material systems. The solid-state nature of AFSD means no melting of the metal occurs during processing, with deformation forcing material together layer by layer. The conditions experienced by the material during printing are in a thermomechanical regime, with both heating and deformation applied, akin to common blacksmithing. In this work specific microstructure evolution phenomena are discussed for multiple materials, highlighting how AFSD processing can be adjusted to change the resulting microstructure and properties. Additionally, specific AFSD process interactions are studied and described to provide better insight into cumulative microstructure evolution throughout the process. This work provides the groundwork for investigating microstructure evolution in AFSD, as well as evidence and results for a number of popular metal systems.

Additive friction stir deposition (AFSD) is an emerging solid-state metal additive manufacturing technology that uses deformation bonding to create near-net shape 3D components. As a developing technology, a deeper understanding of the processing science is necessary to establish the process-structure relationships and enable improved control of the as-printed microstructure and material properties. AFSD provides a unique opportunity to explore the friction stir fundamentals via direct observation of the material during processing. This work explores the relationship between the processing parameters (e.g., tool rotation rate Ω, tool velocity V, and material feed rate F) and the thermomechanical history of the material by process monitoring of i) the temperature evolution, ii) the force evolution, and iii) the interfacial contact state between the tool and deposited material. Empirical trends are established for the peak temperature with respect to the processing conditions for Cu and Al-Mg-Si, but a key difference is noted in the form of the power law relationship: Ω/V for Cu and Ω2/V for Al-Mg-Si. Similarly, the normal force Fz for both materials correlates to V and inversely with Ω. For Cu both parameters show comparable influence on the normal force, whereas Ω is more impactful than V for Al-Mg-Si. On the other hand, the torque Mz trends for Al-Mg-Si are consistent with the normal force trends, however for Cu there is no direct correlation between the processing parameters and the torque. These distinct relationships and thermomechanical histories are directly linked to the contact states observed during deformation monitoring of the two material systems. In Cu, the interfacial contact between the material and tool head is characterized by a full slipping condition (δ=1). In this case, interfacial friction is the dominant heat generation mechanism and compression is the primary deformation mechanism. In Al-Mg-Si, the interfacial contact is characterized by a partial slipping/sticking condition (0<δ<1), so both interfacial friction and plastic energy dissipation are important mechanisms for heat generation and material deformation. Finally, an investigation into the contact evolution at different processing parameters shows that the fraction of sticking is critically dependent on the processing parameters which has many implications on the thermomechanical processing history. / Doctor of Philosophy / Additive manufacturing or three-dimensional (3D) printing technologies have been lauded for their ability to fabricate complex geometries and multi-material parts with reduced material waste. Of particular interest is the use of metal additive manufacturing for repair and fabrication of industrial and structural components. This work focuses on characterizing the thermomechanical processing history for a developing technology Additive Friction Stir Deposition (AFSD). AFSD is solid-state additive manufacturing technology that uses frictional heat and mechanical mixing to fabricate 3D metal components. From a fundamental materials science perspective, it is imperative to understand the processing history of a material to be able to predict the performance and properties of a manufactured part. Through the use of infrared imaging, thermocouples, force sensors, and video monitoring this work is able to establish quantitative relationships between the equipment processing parameters and the processing history for Cu and Al. This work shows that there is a fundamental difference in how these two materials are processed during AFSD. In the future, these quantitative relationships can be used to validate modeling efforts and improve manufacturing quality of parts produced via friction stir techniques.

Current research efforts have focused on the synthesis of novel, segmented, cross-linked networks and thermoplastics for emerging technologies. Tailoring macromolecular structures for improved mechanical performance can be accomplished through a variety of synthetic strategies using step-growth polymerization. The synthesis and characterization of novel Michael addition networks, ionene families, and ion-containing polyurethanes are described, with the underlying theme of fundamentally investigating the structure-property relationships of novel, segmented macromolecular architectures. In addition, it was discovered that both covalent and electrostatic crosslinking play an important role in the mechanical properties of all types of polymers described herein. Novel cross-linked networks were synthesized using quantitative base-catalyzed Michael chemistry with acetoacetate and acrylate functionalities. These novel synthetic strategies offer unique thermo-mechanical performance due to the formation of a multiphase morphology. In order to fundamentally elucidate the factors that influence the kinetics of the Michael addition reaction a detailed analyses of model compounds were conducted in the presence of an in-situ IR spectrometer to optimize reaction conditions using statistical design of experiments. Networks were then prepared based on these optimized conditions. The mechanical performance was evaluated as a function of molecular weight between crosslink points. Furthermore, the incorporation of hydrogen bonding within the monomer structure enhanced mechanical performance. The changes in morphological, thermal, and mechanical properties evaluated using dynamic mechanical analysis (DMA) and tensile behavior are described. In addition, the use of preformed urethane segments provides a safer method for incorporating hydrogen bonding functional groups into macromolecules. In order to compare the thermomechanical and morphological properties of ion-containing polyurethanes to non-charged polyurethanes, poly(tetramethylene oxide)-based polyurethanes containing either a novel phosphonium diol or 1,4-butanediol chain extenders were prepared using a prepolymer method. The novel phosphonium polyurethane was more crystalline, and it was presumed that hydrogen bonding in the non-charged polyurethane restricted polymer mobility, and reduced PTMO crystallinity, and hydrogen bonding interactions were significantly reduced due to the presence of phosphonium cations. These results correlated well with mechanical property analysis. The phase separation and ionic aggregation were demonstrated via wide-angle X-ray scattering, small-angle X-ray scattering, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy during STEM imaging, as described herein. In addition, a novel polyurethane containing imidazolium cations in the hard segment was synthesized and behaved very similarly to the phosphonium cation-containing polyurethane. Ammonium ionenes, which contain quaternary nitrogen in the macromolecular repeating unit, have many potential uses in biomedical applications. They offer interesting coulombic properties, and the charge density is easily controlled through synthetic design. This property makes ionenes ideal polyelectrolyte models to investigate the influence of ionic aggregation on many physical properties. Ammonium ionenes were prepared via the Menshutkin reaction from 1,12-dibromododecane and 1,12-bis(N,N-dimethylamino)dodecane. The absolute molecular weights were determined for the first time using an on-line multi-angle laser light scattering (MALLS) in aqueous size exclusion chromatography (SEC). Tensile testing and DMA were used to establish structure-property relationships between molecular weight and mechanical properties for a series of 12,12-ammonium ionenes. Furthermore, degradation studies in the presence of base support the possibility for water-soluble coatings with excellent mechanical durability that are amenable to triggered depolymerization. A novel synthetic strategy was utilized to prepare chain extended 12,12-ammonium ionenes containing cinnamate functional groups. In the presence of UV light, the polymers chain extended, and the resulting ionenes possessed enhanced thermomechanical properties and increased molecular weight. In addition, the novel synthesis of imidazolium ionenes was demonstrated, and the charge density was tuned for appropriate applications using either low molecular weight segments or oligomeric precursors. The change in charge density had a profound role in imidazolium ionene thermal and mechanical behavior. / Ph. D.

A tetrazole is a 5-membered ring containing 4 nitrogens and 1 carbon. Due to its energetic potential and structural similarity to carboxylic acids, this ring system has a wide number of applications. In this thesis, a new and safe sustainable process to produce tetrazoles was designed that acheived high yields under mild conditions. Also, a technique was developed to form a trityl-protected tetrazole in situ. The rest of this work involved the exploitation of the energetic potential of tetrazoles. This moiety was successfully applied in polymers, ionic liquids, foams, and gels. The overall results from these experiments illustrate the fact that tetrazoles have the potential to serve as a stable alternative to the troublesome azido group common in many energetic materials. Due to these applications, the tetrazole moiety is a very important entity.

M. Tech. Polymer Technology. / Studies the effects of incorporating epoxidised soybean oil (ESO) in a standard bisphenol A-type epoxy resin (EP) cured by both amine and anhydride hardeners. The EP/ESO ratio was set for 100/0, 75/25, 50/50, 25/75 and 0/100 (wt./wt.). The investigations performed covered the curing, rheology (gelling), and thermomechanical analysis and thermogravimetric analysis of the sample produced.

A vast array of high value parts in land- and air-based turbomachinery are subjected to non-isothermal cycling in the presence of mechanical loading. Crack initiation, growth and eventual failure more significantly reduce life in these components compared to isothermal conditions. More accurate simulation of the stress and strain evolution at critical locations of components, as well as test specimens, can lead to a more accurate prediction of remaining life to a structural integrity specialists. The focus of this thesis is to characterize the effects of thermomechanical fatigue (TMF) on generic turbomachinery alloy. An expression that can be used to estimate the maximum and minimum stress under a variety of loading conditions is formulated. Analytical expressions developed here are modifications of classic mechanics of materials methods (e.g. Neuber's Rule and Ramberg-Osgood). The novel models are developed from a collection of data based on parametric finite element analysis to encompass the complex load history present in turbine service conditions. Relevance of the observations and formulated solutions are also explored for the case of a tensile specimen containing a v-shaped notch. Accurate estimations of non-isothermal fatigue presented here endeavor to improve component lifing and decrease maintenance costs.

The first part of the dissertation focuses on understanding and modeling the conduction mechanism of conductive adhesives. The contact resistance is measured between silver rods with different coating materials, and the relationship between tunnel resistivity and contact pressure is obtained based on the experimental results. Three dimensional microstructure models and resistor networks are built to simulate electrical conduction in conductive adhesives. The bulk resistivity of conductive adhesives is calculated from the computer-simulated model. The effects of the geometric properties of filler particles, such as size, shape and distribution, on electrical conductivity are studied by the method of factorial design. The second part of the dissertation evaluates the reliability and investigates the failure mechanism of conductive adhesives subjected to fatigue loading, moisture conditioning and drop impacts. In fatigue tests it is found that electrical conduction failure occurs prior to mechanical failure. The experimental data show that electrical fatigue life can be described well by the power law equation. The electrical failure of conductive adhesives in fatigue is due to the impaired epoxy-silver interfacial adhesion. Moisture uptake in conductive adhesives is measured after moisture conditioning and moisture recovery. The fatigue life of conductive adhesives is significantly shortened after moisture conditioning and moisture recovery. The moisture accelerates the debonding of silver flakes from epoxy resin, which results in a reduced fatigue life. Drop tests are performed on test vehicles with conductive adhesive joints. The electrical conduction failure happens at the same time as joint breakage. The drop failure life is found to be correlated with the strain energy caused by the drop impact, and a power law life model is proposed for drop tests. The fracture is found to be interfacial between the conductive adhesive joints and components/substrates. This research provides a comprehensive understanding of the conduction mechanism of conductive adhesives. The computer-simulated modeling approach presents a useful design tool for the conductive adhesive industry. The reliability tests and proposed failure mechanisms are helpful to prevent failure of conductive adhesives in electronic packages. Moreover, the fatigue and impact life models provide tools in product design and failure prediction of conductive adhesives.

Microstructural features at different scales affect the constitutive stress-strain response and the fatigue crack initiation life in Ni-base superalloys. While numerous efforts have been made in the past to experimentally characterize the effects of these features on the stress-strain response and/or the crack initiation life, there is a significant variability in the data with sometimes contradictory conclusions, in addition to the substantial costs involved in experimental testing. Computational techniques can be useful tools to better understand these effects since they are relatively inexpensive and are not restricted by the limitations in processing techniques. The effect of microstructure on the stress-strain response and the variability in fatigue life were analyzed using two Ni-base superalloys; DS GTD111 which is a directionally solidified Ni-base superalloy, and IN100 which is a polycrystalline Ni-base superalloy. Physically-based constitutive models were formulated and implemented as user material subroutines in ABAQUS using the single crystal plasticity framework which can predict the material stress-strain response with the microstructure-dependence embedded into them. The model parameters were calibrated using experimental cyclic stress-strain histories. A computational exercise was employed to quantify the influence of idealized microstructural variables on the fatigue crack initiation life. Understanding was sought regarding the most significant microstructure features using explicit modeling of the microstructure with the aim to predict the variability in fatigue crack initiation life and to guide material design for fatigue resistant microstructures. Lastly, it is noted that crystal plasticity models are often too computationally intensive if the objective is to model the macroscopic behavior of a textured or randomly oriented 3-D polycrystal in an engineering component. Homogenized constitutive models were formulated and implemented as user material subroutines in ABAQUS, which can capture the macroscale stress-strain response in both DS GTD111 and IN100. Even though the study was conducted on two specific Ni-base superalloys; DS GTD111 and IN100, the objective was to develop generic frameworks which should also be applicable to other alloy systems.

ZnO nanowires and nanorods are a new class of one-dimensional nanomaterials with a wide range of applications in NEMS. The motivation for this work stems from the lack of understanding and characterization of their thermomechanical behaviors essential for their incorporation in nanosystems. The overall goal of this work is to develop a fundamental understanding of the mechanisms controlling the responses of these nanostructures with focus on: (1) development of a molecular dynamics based framework for analyzing thermomechanical behaviors, (2) characterization of the thermal and mechanical behaviors in ZnO nanowires and (3) development of models for pseudoelasticity and thermal conductivity. The thermal response analyses show that the values of thermal conductivity are one order of magnitude lower than that for bulk ZnO due to surface scattering of phonons. A modified equation for phonon radiative transport incorporating the effects of surface scattering is used to model the thermal conductivity as a function of wire size and temperature. Quasistatic tensile loading of wires show that the elastic moduli values are 68.2-27.8% higher than that for bulk ZnO. Previously unknown phase transformations from the initial wurtzite (WZ) structure to graphitic (HX) and body-centered-tetragonal (BCT-4) phases are discovered in nanowires which lead to a more complete understanding of the extent of polymorphism in ZnO and its dependence on load triaxiality. The reversibility of the WZ-to-HX transform gives rise to a novel pseudoelastic behavior with recoverable strains up to 16%. A micromechanical continuum model is developed to capture the major characteristics of the pseudoelastic behavior accounting for size and temperature effects. The effect of the phase transformations on the thermal properties is characterized. Results obtained show that the WZ→HX phase transformation causes a novel transition in thermal response with the conductivity of HX wires being 20.5-28.5% higher than that of the initial WZ-structured wires. The results obtained here can provide guidance and criteria for the design and fabrication of a range of new building blocks for nanometer-scale devices that rely on thermomechanical responses.