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STRUCTURAL HEALTH MONITORING OF FILAMENT WOUND GLASS FIBER/EPOXY COMPOSITES WITH CARBON BLACK FILLER VIA ELECTRICAL IMPEDANCE TOMOGRAPHYAkshay Jacob Thomas (7026218) 02 August 2019 (has links)
<div>
<p>Fiber reinforced polymer
composites are widely used in manufacturing advanced light weight structures
for the aerospace, automotive, and energy sectors owing to their superior
stiffness and strength. With the increasing use of composites, there is an increasing
need to monitor the health of these structures during their lifetime.
Currently, health monitoring in filament wound composites is facilitated by
embedding piezoelectrics and optical fibers in the composite during the
manufacturing process. However, the incorporation of these sensing elements
introduces sites of stress concentration which could lead to progressive damage
accumulation. In addition to introducing weak spots in the structure, they also
make the manufacturing procedure difficult. </p>
<p> </p>
<p>Alternatively,
nanofiller modification of the matrix imparts conductivity which can be
leveraged for real time health monitoring with fewer changes to the
manufacturing method. Well dispersed nanofillers act as an integrated sensing
network. Damage or strain severs the well-connected nanofiller network thereby
causing a local change in conductivity. The self-sensing capabilities of these
modified composites can be combined with low cost, minimally invasive imaging
modalities such as electrical impedance tomography (EIT) for damage detection.
To date, however, EIT has exclusively been used for damage detection in planar
coupons. These simple plate-like structures are not representative of
real-world complex geometries. This thesis advances the state of the art in
conductivity-based structural health monitoring (SHM) and nondestructive
evaluation (NDE) by addressing this limitation of EIT. The current study will
look into damage detection of a non-planar multiply connected domain – a
filament-wound glass fiber/epoxy tube modified by carbon black (CB) filler. The
results show that EIT is able to detect through holes as small as 7.94 mm in a
tube with length-to-diameter ratio of 132.4 mm-to-66.2 mm (aspect ratio of
2:1). Further, the sensitivity of EIT to damage improved with decreasing tube
aspect ratio. EIT was also successful in detecting sub-surface damage induced
by low velocity impacts. These results indicate that EIT has much greater
potential for composite SHM and NDE than prevailing work limited to planar geometries
suggest.</p>
</div>
<br>
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Manufacturing and Testing of Composite Hybrid Leaf Spring for Automotive ApplicationsHimal Agrawal (7043354) 12 August 2019 (has links)
Leaf
springs are a part of the suspension system attached between the axle and the
chassis of the vehicle to support weight and provide shock absorbing capacity
of the vehicle. For more than half a century the leaf springs are being made of
steel which increases the weight of the vehicle and is prone to rusting and
failure. The current study explores the feasibility of composite leaf spring to
reduce weight by designing, manufacturing and testing the leaf spring for the
required load cases. An off the shelf leaf spring of Ford F-150 is chosen for making
of composite hybrid spring prototype. The composite hybrid prototype was made
by replacing all the leaves with glass fiber unidirectional laminate except the
first leaf. Fatigue tests are then done on steel and composite hybrid leaf
spring to observe the failure locations and mechanism if any. High frequency
fatigue tests were then done on composite beams with varying aspect ratio in a displacement-controlled
mode to observe fatigue location and mechanism of just glass fiber composite
laminate. It was observed that specimens with low aspect ratio failed from
crack propagation initiated from stress concentrations at the loading tip in
3-point cyclic flexure test and shear forces played a dominant role in
propagation of crack. Specimens with high aspect ratio under the same loading did
not fail in cyclic loading and preserved the same stiffness as before the
cyclic loading. The preliminary fatigue results for high aspect ratio composite
beams predict a promising future for multi-leaf composite springs.
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Effect of nanocellulose reinforcement on the properties of polymer compositesShikha Shrestha (6631748) 11 June 2019 (has links)
<div>
<p><a>Polymer
nanocomposites are envisioned for use in many advanced applications, such as
structural industries, aerospace, automotive technology and electronic
materials, due to the improved properties like mechanical strengthening,
thermal and chemical stability, easy bulk processing, and/or light-weight
instigated by the filler-matrix combination compared to the neat matrix. In
recent years, due to increasing environmental concerns, many industries are
inclining towards developing sustainable and renewable polymer nanocomposites.
Cellulose nanomaterials (CNs), including cellulose nanocrystals (CNCs) and
cellulose nanofibrils (CNFs), have gained popularity due to their excellent
mechanical properties and eco-friendliness (extracted from trees, algae, plants
etc.). However, to develop CN-reinforced nanocomposites with industrial
applications it is necessary to understand impact of hygroscopic swelling
(which has very limited </a>quantitative study at present),
aspect ratio, orientation, and content of CNs on the overall performance of
nanocomposites; and overcome the low dispersibility of CNs and improve their
compatibility with hydrophobic matrix. In this work, we attempt to understand
the influence of single nanocrystals in the hygroscopic and optical response
exhibited by nanostructured films; effect of CNCs on the properties of PVA/CNC
fibers by experimental evidence with mathematical modeling predictions; and
hydrophobized CNFs using a facile, aqueous surface modification to improve
interfacial compatibility with epoxy. </p><p><br></p>
<p>To evaluate the effect of CNC
alignment in the bulk response to hygroscopic expansion, self-organized and
shear-oriented CNC films were prepared under two different mechanisms. The coefficient of hygroscopic swelling (CHS)
of these films was determined by using a new contact-free method of Contrast
Enhanced Microscopy Digital Image Correlation (CEMDIC) that enabled the
characterization of dimensional changes induced by hygroscopic swelling of the
films. This method can be readily used for other soft materials to accurately
measure hygroscopic strain in a non-destructive way. By calculating the CHS
values of CNC films, it was determined that hygroscopic swelling is highly
dependent on the alignment of nanocrystals within the films, with aligned CNC
films showing dramatically reduced hygroscopic expansion than randomly oriented
films. Finite element analysis was used to simulate moisture sorption and kinetics
profile which further predicted moisture diffusion as the predominant mechanism
for swelling of CNC films. </p>
<p><br></p><p>To study the effects of different types
and aspect ratios of CNCs on mechanical, thermal and morphological properties
of polyvinyl alcohol (PVA) composite <a>fibers, CNCs
extracted from wood pulp and cotton were reinforced into PVA to produce fibers
by dry-jet-wet spinning. The fibers were collected as-spun and with first stage
drawing up to draw ratio 2. </a>The elastic modulus and tensile strength of the
fibers improved with increasing CNC content (5 – 15 wt. %) at the expense of
their strain-to-failure. The mechanical properties
of fibers with cotton CNC were higher than the fibers with wood CNC when the
same amount of CNCs were added due to their higher aspect ratio. The degree of orientation along the spun fiber axis
was quantified by 2D X-ray diffraction. As expected, the
CNC orientation correlates to the mechanical properties of the composite fibers.
Micromechanical models were used to predict the fiber performance and compare
with experimental results. Finally, surface and cross-sectional morphologies of
fibers were analyzed by scanning electron microscopy and optical microscopy.</p><p><br></p>
<p>To improve the
dispersibility and compatibility of CNFs with epoxy, CNFs were modified by
using a two-step water-based method where tannic acid (TA) acts as a primer
with CNF suspension and reacts with hexadecylamine (HDA), forming the modified
product as CNF-TA-HDA. The modified (-m) and unmodified (-um) CNFs were filled
into hydrophobic epoxy resin with a co-solvent (acetone), which was
subsequently removed to form a solvent-free two component epoxy system,
followed by addition of hardener to cure the resin. Better dispersion and
stronger adhesion between fillers and epoxy were obtained for m-CNF than the
um-CNF, resulting in better mechanical properties of nanocomposites at the same
loading. Thermal stability and the degradation temperature of m-CNF/epoxy improved
when compared to neat epoxy. </p>
</div>
<br>
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ENGINEERING NANOCOMPOSITES AND INTERFACES FOR CONDUCTION AND RADIATION THERMAL MANAGEMENTXiangyu Li (5929961) 17 January 2019 (has links)
<p>The thesis covers the following topics:</p>
<p>1. aggregation and size effect on metal-polymer nanocomposite thermal interface materials</p>
<p>2. diffusion limited cluster aggregation lattice simulation on thermal conductivty</p>
<p>3. thermal interfacial resistance reduction between metal and dielectric materials by inserting an intermediate metal layer</p>
<p>4. absence of coupled thermal interfaces in al2o3/ni/al2o3 sandwich structure</p>
<p>5. ultra-efficient low-cost radiative cooling paints</p>
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Performance Informed Technical Cost Modeling for Novel ManufacturingRobin Joseph Glebes (7443716) 17 October 2019 (has links)
<p>Inaccurate cost
estimates contribute to lost implementation opportunity of novel manufacturing
technologies or lost revenue due to under-bidding or loss of an over-bid
contract. High-volume, long-term orders, such as those the automotive industry
begets, are desired as they lock in revenue streams for months into years.
However, high-rate composite materials and their manufacturing processes are
novel among the industry and traditional costing methods have not advanced at a
proportional rate. This research effort developed a method to reduce the
complex composite manufacturing systems to fungible, upgradable, and linkable
individual processes that derive their manufacturing parameters from the
performance part design process. Employing technical cost modeling, this method
accurately quantifies the value of pursuing composite manufacturing by
integrating impregnation, solidification, heat transfer, kinetics, and
additional technical data from computer-aided part design simulation tools to
deliver an accurate cost estimate. </p>
<p>Cost modeling provides a
quantitative result that weighs heavily in the decision making process for adoption
of a new manufacturing method. In this dissertation, three case studies were
investigated for three different management decision cases: part production
management, in-house manufacturing management, and global manufacturing
management. </p>
<p>Part production management
is the decision making process for selecting a certain manufacturing method. A
case study with a Tier 1 Part Producer was conducted to provide a comparison of
two emerging novel preforming systems versus their in-use, metals based high-rate
manufacturing line in manufacturing a structural automotive part. Determining
material usage was the primary cost driver focus. Equipment Supplier A’s
process operated by seaming single layers of thermoplastic tape into rolls and
then stacking prior to consolidation and resulted in a scrap rate of 23-28%
with a cost of $32.87-36.01 per kilogram saved depending on the input tape
width. Equipment Supplier B’s layup process, essentially a multi-head automatic
tape layup machine, resulted in scrap rate of 20-27% with a cost of
$34.48-36.67 per kilogram saved depending on the input tape width. This
exceeded the Tier 1 Part Producer’s requirement of $6.6-11
per kilogram saved and led to them to abandon this
application as a feasible project and instead look for a different part with a
higher return regarding cost for weight saved.</p>
<p>In-house manufacturing
management is the decision making process governing manufacturing operating
procedures. A case study for the Manufacturing Design Laboratory’s (MDLab)
hybrid molding line was undertaken to determine the manufacturing cost for a
composite test coupon. Processing parameters were obtained from three sources:
performance design computer aided engineering (CAE), common industry transfer
estimation times, and a calculated preform layup time. Compared to a similarly shaped
test coupon made of aluminum, highly-automated manufacturing realizes weight
savings of 46.25% and cost savings of 16.5%. Low-automation manufacturing
captures the same weight savings, but has a cost for weight saved penalty, cost
increase, of $9.89 per kilogram, showing how influential the labor contribution
is to manufacturing cost. </p>
<p>Global manufacturing
management is the decision making process governing manufacturing location.
Various manufacturing cost drivers are location dependent, thus a dataset was
developed to alter these parameters for the U.S. states. Global comparisons are
accomplished through indexing of global cost of living allowances and labor
rates. Within the U.S., high-automation
manufacturing costs in the West Coast/Pacific are 20.1% greater compared to the
Midwest and similarly, low-automation costs are 21.2% greater. Globally,
high-automation manufacturing costs in North America are 52.1% greater compared
to Asia while low-automation costs are 116.5% greater. These variations
highlight why we see geographically clustered manufacturing centers within the
states and major manufacturing relocations due to cost sensitive and labor sensitive
production. </p>
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Failure Prediction for Composite Materials with Generalized Standard ModelsZhenyuan Gao (7481801) 17 October 2019 (has links)
<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>
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FABRICATION OF SOLID, POROUS, AND MAGNETIC CERAMIC MICROPARTICLES VIA STOP-FLOW LITHOGRAPHYAlejandro Manuel Alcaraz Ramirez (7469432) 30 April 2020 (has links)
<p>Microparticles
have been investigated not only as feedstock spherical or amorphous bulk
materials used for shape molding, but also as agents that can perform work in
the micron scale. The fabrication of microparticles with active properties of
self-propulsion, self-assembly, and mobility with enhanced mechanical, thermal,
and chemical properties is of particular interest for emerging technologies
such as drug delivery, micro-robotics, micro energy generation/harvesting, and
MEMS. Conventional fabrication methods can produce several complex particle
shapes in one fabrication session or hundreds of spheroid shaped particles per
second. Innovative techniques, as flow lithography, have demonstrated control
over particle form and composition for continuous fabrication cycles. In recent years predefined shape polymer microparticles have been
fabricated as well as ceramic microparticles through suspension processing with
these set of techniques. Even though ceramic materials have been fabricated,
there is still a strong need to increment the palette of available materials to
be processed via flow lithography. We have pioneered the production of shaped
ceramic microparticles by Stop-Flow Lithography (SFL) using
preceramic polymers, providing control of particle size and shape in the range
of 1 – 1000
μm. The principal arranged technique (SFL) combines aspects of
PDMS-based microfluidics and photolithography for the continuous cyclable fabrication
of microparticles with predefined shapes. The PDMS microchannel devices used
were fabricated with vinyl film molds in a laminar hood avoiding the need for a
cleanroom, procedure that reduced fabrication costs. After a fabrication
session, the preceramic polymer microparticles were collected, washed, and
dried before entering an inert atmosphere furnace for pyrolysis. Additionally, by treating the material initially as liquid polymer,
special properties can be added by converting it into an emulsion or a
suspension. Microparticles
were functionalized by introducing porosity and magnetic nanoparticles in the
preceramic polymer matrix. The porous characteristic of a particle leads to an
increase in surface area, allowing the particle to be infiltrated with a
catalyzer or act as a chemical/physical carrier, and the magnetic behavior of
the particles allows a controllable trajectory with defined external magnetic
fields. These two properties can be used to fabricate bifunctional
microparticles to serve as drug carriers through human arteries and veins for drug delivery purposes.
We
successfully fabricated solid and functional ceramic microparticles in the 10 – 50
μm range with predefined shapes as hexagons, gears, triangles, and ovals. This
system is an economical route to fabricate functional defined shape particles
that can serve as microrobots to perform tasks in liquid media.</p>
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Mechanical behaviors of bio-inspired composite materials with functionally graded reinforcement orientation and architectural motifsDi Wang (8782580) 01 May 2020 (has links)
<p>Naturally-occurring biological
materials with stiff mineralized reinforcement embedded in a ductile matrix are
commonly known to achieve excellent balance between stiffness, strength and
ductility. Interestingly, nature offers a broad diversity of architectural
motifs, exemplify the multitude of ways in which exceptional mechanical
properties can be achieved. Such diversity is the source of bio-inspiration and
its translation to synthetic material systems. In particular, the helicoid and the
“brick and mortar” architectured materials are two key architectural motifs we
are going to study and to synthesize new bio-inspired materials. </p>
<p>Due to geometry mismatch(misorientation)
and incompatibilities of mechanical properties between fiber and matrix
materials, it is acknowledged that misoriented stiff fibers would rotate in
compliant matrix beneath uniaxial deformation. However, the role of fiber reorientation inside the flexible
matrix of helicoid composites on their mechanical behaviors have not yet been
extensively investigated. In the present project, fiber reorientation values
of single misoriented laminae, mono-balanced laminates and helicoid architectures
under uniaxial tensile are calculated and compared. In the present work, we introduce a Discontinuous Fiber
Helicoid (DFH) composite inspired by both the helicoid microstructure in the
cuticle of mantis shrimp and the nacreous architecture of the red abalone
shell. We employ 3D printed specimens, analytical models and finite
element models to analyze and quantify in-plane fiber reorientation in helicoid
architectures with different geometrical features. We also introduce additional architectures, i.e.,
single unidirectional lamina and mono-balanced architectures, for comparison
purposes. Compared with
associated mono-balanced architectures, helicoid architectures exhibit less
fiber reorientation values and lower values of strain stiffening. The
explanation for this difference is addressed in terms of the measured in-plane
deformation, due to uniaxial tensile of the laminae, correlated to lamina
misorientation with respect to the loading direction and lay-up sequence.</p>
<p>In addition to fiber, rod-like,
reinforced laminate, platelet reinforced composite materials, “brick and
mortar” architectures, are going to be discussed as well, since it can provide in-plane
isotropic behavior on elastic modulus that helicoid architecture can offer as
well, but with different geometries of reinforcement. Previous “brick and mortar” models available in the
literature have provided insightful information on how these structures promote
certain mechanisms that lead to significant improvement in toughness without
sacrificing strength. In this work, we present a detailed comparative analysis that
looks at the three-dimensional geometries of the platelet-like and rod-like
structures. However, most of these previous analyses have been focused on
two-dimensional representations. We 3D print and test rod-like and tablet-like
architectures and analyze the results employing a computational and analytical
micromechanical model under a dimensional analysis framework. In particular, we
focus on the stiffness, strength and toughness of the resulting structures. It
is revealed that besides volume fraction and aspect ratio of reinforcement, the
effective shear and tension area in the matrix governs the mechanical behavior
as well. In turns, this
leads to the conclusion that rod-like microstructures exhibit better
performance than tablet-like microstructures when the architecture is subjected
to uniaxial load. However, rod-like microstructures tend to be much weaker and
brittle in the transverse direction. On the other hand, tablet-like
architectures tend to be a much better choice for situations where biaxial load
is expected.</p>
<p>Through varying the geometry of
reinforcement and changing the orientation of reinforcement, different
architectural motifs can promote in-plane mechanical properties, such as strain
stiffening under uniaxial tensile, strength and toughness under biaxial tensile
loading. On the other hand, the various out-of-plane orientation of the
reinforcement leads to functionally graded effective indentation stiffness. The
external layer of nacre shell is composed of calcite prisms with graded orientation
from surface to interior. This orientation gradient leads to functionally
graded Young’s modulus, which is confirmed to have higher fracture resistance
than homogenous materials under mode I fracture loading act.</p>
<p>Similar as graded prism
orientation in calcite layer of nacre, the helicoid architecture found in
nature exhibits gradients on geometrical parameters as well. The pitch distance
of helicoid architecture is found to be functionally graded through the thickness
of biological materials, including the dactyl club of mantis shrimp and the
fish scale of coelacanth. This can be partially explained by the long-term evolution
and selection of living organisms to create high performance biological
materials from limited physical, chemical and geometrical elements. This
naturally “design” procedure can provide us a spectrum of design motifs on
architectural materials. </p>
<p>In the present work, linear
gradient on pitch distance of helicoid architectures, denoted by functionally
graded helicoid (FGH), is chose to be the initial pathway to understand the
functionality of graded pitch distance, associated with changing pitch angle.
Three-point bending on short beam and low-velocity impact tests are employed in
FEA to analyze the mechanical properties of composite materials simultaneously.
Both static(three-point bending) and dynamic(low-velocity impact) tests reveal
that FGH with pitch angle increasing from surface to interior can provide multiple
superior properties at the same time, such as peak load and toughness, while
the helicoid architectures with constant pitch angle can only provide one
competitive property at one time. Specifically, helicoid architectures with
smaller pitch angle, such as 15-degree, show higher values on toughness, but
less competitive peak load under static three-point bending loading condition,
while helicoid architectures with middle pitch angle, larger than or equal to
22.5-degree and smaller than 45-degree, exhibit less value of toughness, but
higher peak load. The explanation on this trend and the benefits of FGH is
addressed by analyzing the transverse shear stresses distribution through the
thickness in FEA, combined with analytical prediction. In low-velocity impact
tests, the projected delamination area of helicoid architectures is observed to
increase when the pitch angle is decreasing. Besides, laminates with specific pitch angles, such as 45-degree,
classical quasi-isotropic laminate, 60-degree, specific angle ply, and 90-degree,
cross-ply, are designed to compare with helicoid architectures and FGH.</p>
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Nitride-Based Nanocomposite Thin Films Towards Tunable Nanostructures and FunctionalitiesXuejing Wang (9099860) 29 July 2020 (has links)
<p> Optical metamaterials have triggered extensive studies driven by their fascinating electromagnetic properties that are not observed in natural materials. Aside from the extraordinary progress, challenges remain in scalable processing and material performance which limit the adoption of metamaterial towards practical applications. The goal of this dissertation is to design and fabricate nanocomposite thin films by combining nitrides with a tunable secondary phase to realize controllable multi-functionalities towards potential device applications. Transition metal nitrides are selected for this study due to the inherit material durability and low-loss plasmonic properties that offer stable two-phase hybridization for potential high temperature optical applications. Using a pulsed laser deposition technique, the nitride-metal nanocomposites are self-assembled into various geometries including pillar-in-matrix, embedded nanoinclusions or complex multilayers, that possess large surface coverage, high epitaxial quality, and sharp phase boundary. The nanostructures can be further engineered upon precise control of growth parameters. </p><p> This dissertation is composed of a general review of related background and experimental approaches, followed by four chapters of detailed research chapters. The first two research chapters involve hybrid metal (Au, Ag) - titanium nitride (TiN) nanocomposite thin films where the metal phase is self-assembled into sub-20 nm nanopillars and further tailored in terms of packing density and tilting angles. The tuning of plasmonic resonance and dielectric constant have been achieved by changing the concentration of Au nanopillars, or the tuning of optical anisotropy and angular selectivity by changing the tilting angle of Ag nanopillars. Towards applications, the protruded Au nanopillars are demonstrated to be highly functional for chemical bonding detection or surface enhanced sensing, whereas the embedded Ag nanopillars exhibit enhanced thermal and mechanical stabilities that are promising for high temperature plasmonic applications. In the last two chapters, dissimilar materials candidates beyond plasmonics have been incorporated to extend the electromagnetic properties, include coupling metal nanoinclusions into a wide bandgap semiconducting aluminum nitride matrix, as well as inserting a dielectric spacer between the hybrid plasmonic claddings for geometrical tuning and electric field enhancement. As a summary, these studies present approaches in addressing material and fabrication challenges in the field of plasmonic metamaterials from fundamental materials perspective. As demonstrated in the following chapters, these hybrid plasmonic nanocomposites provide multiple advantages towards tunable optical or biomedical sensing, high temperature plasmonics, controllable metadevices or nanophotonic chips.</p><div><br></div>
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Novel Composites for Nonlinear Transmission Line ApplicationsAndrew J Fairbanks (10701090) 06 May 2021 (has links)
<p>Nonlinear transmission lines (NLTLs) provide a solid state
alternative to conventional vacuum based high power microwave (HPM) sources.
The three most common NLTL implementations are the lumped element, split ring
resonator (SRR), and the nonlinear bulk material based NLTLs. The nonlinear
bulk material implementation provides the highest power output of the three
configurations, though they are limited to pulse voltages less than 50 kV;
higher voltages are possible when an additional insulator is used, typically SF<sub>6</sub>
or dielectric oil, between the nonlinear material and the outer conductor. The
additional insulator poses a risk of leaking if structural integrity of the
outer conductor is compromised. The desire to provide a fieldable NLTL based
HPM system makes the possibility of a leak problematic. The work reported here develops
a composite based NLTL system that can withstand voltages higher than 50 kV and
not pose a risk of catastrophic failure due to a leak while also decreasing the
size and weight of the device and increasing the output power.</p>
<p>Composites with barium strontium
titanate (BST) or nickel zinc ferrite (NZF) spherical inclusions mixed in a
silicone matrix were manufactured at volume fractions ranging from 5% to 25%.
The dielectric and magnetic parameters were measured from 1-4 GHz using a
coaxial airline. The relative permittivity increased from 2.74±0.01 for the polydimethylsiloxane
(PDMS) host material to 7.45±0.33 after combining PDMS with a 25% volume
fraction of BST inclusions. The relative permittivity of BST and NZF composites
was relatively constant across all measured frequencies. The relative
permeability of the composites increased from 1.001±0.001 for PDMS to 1.43±0.04
for a 25% NZF composite at 1 GHz. The relative permeability of the 25% NZF
composite decreased from 1.43±0.05 at 1 GHz to 1.17±0.01 at 4 GHz. The NZF
samples also exhibited low dielectric and magnetic loss tangents from
0.005±0.01 to 0.091±0.015 and 0.037±0.001 to 0.20±0.038, respectively, for all
volume fractions, although the dielectric loss tangent did increase with volume
fraction. For BST composites, all volume fraction changes of at least 5%
yielded statistically significant changes in permittivity; no changes in BST
volume fraction yielded statistically significant changes in permeability. For
NZF composites, the change in permittivity was statistically significant when
the volume fraction varied by more than 5% and the change in permeability was
statistically significant for variations in volume fraction greater than 10%.
The DC electrical breakdown strength of NZF composites decreased exponentially
with increasing volume fraction of NZF, while BST composites exhibited no
statistically significant variation with volume fraction. </p>
<p>For composites containing both BST
and NZF, increasing the volume fraction of either inclusion increased the
permittivity with a stronger dependence on BST volume fraction. Increasing NZF
volume fraction increased the magnetic permeability, while changing BST volume
fraction had no effect on the composite permeability. The DC dielectric
breakdown voltage decreased exponentially with increased NZF volume fraction.
Adding as little as 5% BST to an NZF composite more than doubled the breakdown
threshold compared to a composite containing NZF alone. For example, adding 10%
BST to a 15% NZF composite increased the breakdown strength by over 800%. The
combination of tunability of permittivity and permeability by managing BST and
NZF volume fractions with the increased dielectric breakdown strength by
introducing BST make this a promising approach for designing high power
nonlinear transmission lines with input pulses of hundreds of kilovolts.</p>
<p>Coaxial nonlinear transmission
lines are produced using composites with NZF inclusions and BST inclusions and
driven by a Blumlein pulse generator with a 10 ns pulse duration and 1.5 ns
risetime. Applying a 30 kV pulse using the Blumlein pulse generator resulted in
frequencies ranging from 1.1 to 1.3 GHz with an output power over 20 kW from
the nonlinear transmission line. The output frequencies increased with
increasing volume fraction of BST, but the high power oscillations
characteristic of an NLTL did not occur. Simulations using LT Spice demonstrated
that an NLTL driven with a Blumlein modulator did not induce high power
oscillations while driving the same NLTL with a pulse forming network did. </p>
<p>Finally, a composite-based NLTL
could be driven directly by a high voltage power supply without a power
modulator to produce oscillations both during and after the formed pulse upon
reaching a critical threshold. The output frequency of the NLTLs is 1 GHz after
the pulse and ranged from 950 MHz to 2.2 GHz during the pulse. These results
demonstrate that the NLTL may be used as both a pulse forming line and high
power microwave source, providing a novel way to reduce device size and weight,
while the use of composites could provide additional flexibility in pulse
output tuning. </p>
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