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THE FABRICATION OF POLYMER-DERIVED SICN/SIBCN CERAMIC NANOSTRUCTURES AND INVESTIGATION OF THEIR STRUCTURE-PROPERTY RELATIONSHIP
Title
THE FABRICATION OF POLYMER-DERIVED SICN/SIBCN CERAMIC NANOSTRUCTURES AND INVESTIGATION OF THEIR STRUCTURE-PROPERTY RELATIONSHIP.
Creator
Sarkar, Sourangsu, Zhai, Lei, University of Central Florida
Abstract / Description
Polymer-derived Ceramics (PDCs) represent a unique class of high-temperature stable materials synthesized directly by the thermal decomposition of polymers. This research first focuses on the fabrication of high temperature stable siliconcarbonitride (SiCN) fibers by electrospinning for ceramic matrix composite (CMC) applications. Ceraset™ VL20, a commercially available liquid...
Show morePolymer-derived Ceramics (PDCs) represent a unique class of high-temperature stable materials synthesized directly by the thermal decomposition of polymers. This research first focuses on the fabrication of high temperature stable siliconcarbonitride (SiCN) fibers by electrospinning for ceramic matrix composite (CMC) applications. Ceraset™ VL20, a commercially available liquid cyclosilazane, was functionalized with aluminum sec-butoxide in order to be electrospinnable. The surface morphology of the electrospun fibers was investigated using the fibers produced from solvents. The electrospun fibers produced from the chloroform/N,N-dimethylformamide solutions had hierarchical structures that led to superhydrophobic surfaces. A ÂÂÂÂ"dry skinÂÂÂÂ" model was proposed to explain the formation of micro/- and nanostructures. The second objective of the research is to align the multiwalled carbon nanotubes (MWCNTs) in PDC fibers. For this purpose, a non-invasive approach to disperse carbon nanotubes in polyaluminasilazane chloroform solutions was developed using a conjugated block copolymer synthesized by ATRP. The effect of the polymer and CNT concentration on the fiber structure and morphology was also examined. Detailed characterization using SEM and TEM was performed to demonstrate the orientation of CNTs inside the ceramic fibers. Additionally, the electrical properties of the ceramic fibers were investigated. Finally, the structural evolution of polymer-derived amorphous siliconborocarbonitride (SiBCN) ceramics with pyrolysis temperatures was studied by solid-state NMR, Raman and EPR spectroscopy. Results suggested the presence of three major components: (i) hexagonal boron nitride (h-BN), (ii) turbostratic boron nitride (t-BN), and (iii) BN2C groups in the final ceramic. The pyrolysis at higher temperature generated boron nitride (BN3) with a simultaneous decomposition of BN2C groups. A thermodynamic model was proposed to quantitatively explain the conversion of BN2C groups into BN3 and ÂÂÂÂ"freeÂÂÂÂ" carbon. Such structure evolution is believed to be the reason that the crystallization of Si4.0B1.0 ceramics starts at 1500 °C, whereas Si2.0B1.0 ceramics is stable upto 1600 °C.
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Date Issued
2010
Identifier
CFE0003446, ucf:48408
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0003446
The effect of carbon nanotube/organic semiconductor interfacial area on the performance of organic transistors
Title
The effect of carbon nanotube/organic semiconductor interfacial area on the performance of organic transistors.
Creator
Kang, Narae, Khondaker, Saiful, Leuenberger, Michael, Zhai, Lei, University of Central Florida
Abstract / Description
Organic field-effect transistors (OFETs) have attracted tremendous attention due to their flexibility, transparency, easy processiblity and low cost of fabrication. High-performance OFETs are required for their potential applications in the organic electronic devices such as flexible display, integrated circuit, and radiofrequency identification tags. One of the major limiting factors in fabricating high-performance OFET is the large interfacial barrier between metal electrodes and OSC which...
Show moreOrganic field-effect transistors (OFETs) have attracted tremendous attention due to their flexibility, transparency, easy processiblity and low cost of fabrication. High-performance OFETs are required for their potential applications in the organic electronic devices such as flexible display, integrated circuit, and radiofrequency identification tags. One of the major limiting factors in fabricating high-performance OFET is the large interfacial barrier between metal electrodes and OSC which results in low charge injection from the metal electrodes to OSC. In order to overcome the challenge of low charge injection, carbon nanotubes (CNTs) have been suggested as a promising electrode material for organic electronic devices. In this dissertation, we study the effect of carbon nanotube (CNT) density in CNT electrodes on the performance of organic field effect transistor (OFETs). The devices were fabricated by thermal evaporation of pentacene on the Pd/single walled CNT (SWCNT) electrodes where SWCNTs of different density (0-30/um) were aligned on Pd using dielectrophoresis (DEP) and cut via oxygen plasma etching to keep the length of nanotube short compared to the channel length. From the electronic transport measurements of 40 devices, we show that the average saturation mobility of the devices increased from 0.02 for zero SWCNT to 0.06, 0.13 and 0.19 cm2/Vs for low (1-5 /(&)#181;m), medium (10-15 /(&)#181;m) and high (25-30 /(&)#181;m) SWCNT density in the electrodes, respectively. The increase is three, six and nine times for low, medium and high density SWCNTs in the electrode compared to the devices that did not contain any SWCNT. In addition, the current on-off ratio and on-current of the devices are increased up to 40 times and 20 times with increasing SWCNT density in the electrodes. Our study shows that although a few nanotubes in the electrode can improve the OFET device performance, significant improvement can be achieved by maximizing SWCNT/OSC interfacial area. The improved OFET performance can be explained due to a reduced barrier height of SWCNT/pentacene interface compared to metal/pentacene interface which provides more efficient charge injection pathways with increased SWCNT/pentacene interfacial area.
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Date Issued
2012
Identifier
CFE0004558, ucf:49252
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0004558
FINITE ELEMENT METHOD MODELING OF ADVANCED ELECTRONIC DEVICES
Title
FINITE ELEMENT METHOD MODELING OF ADVANCED ELECTRONIC DEVICES.
Creator
Chen, Yupeng, Wu, Thomas, University of Central Florida
Abstract / Description
In this dissertation, we use finite element method together with other numerical techniques to study advanced electron devices. We study the radiation properties in electron waveguide structure with multi-step discontinuities and soft wall lateral confinement. Radiation mechanism and conditions are examined by numerical simulation of dispersion relations and transport properties. The study of geometry variations shows its significant impact on the radiation intensity and direction. In...
Show moreIn this dissertation, we use finite element method together with other numerical techniques to study advanced electron devices. We study the radiation properties in electron waveguide structure with multi-step discontinuities and soft wall lateral confinement. Radiation mechanism and conditions are examined by numerical simulation of dispersion relations and transport properties. The study of geometry variations shows its significant impact on the radiation intensity and direction. In particular, the periodic corrugation structure exhibits strong directional radiation. This interesting feature may be useful to design a nano-scale transmitter, a communication device for future nano-scale system. Non-quasi-static effects in AC characteristics of carbon nanotube field-effect transistors are examined by solving a full time-dependent, open-boundary Schrödinger equation. The non-quasi-static characteristics, such as the finite channel charging time, and the dependence of small signal transconductance and gate capacitance on the frequency, are explored. The validity of the widely used quasi-static approximation is examined. The results show that the quasi-static approximation overestimates the transconductance and gate capacitance at high frequencies, but gives a more accurate value for the intrinsic cut-off frequency over a wide range of bias conditions. The influence of metal interconnect resistance on the performance of vertical and lateral power MOSFETs is studied. Vertical MOSFETs in a D2PAK and DirectFET package, and lateral MOSFETs in power IC and flip chip are investigated as the case studies. The impact of various layout patterns and material properties on RDS(on) will provide useful guidelines for practical vertical and lateral power MOSFETs design.
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Date Issued
2006
Identifier
CFE0001389, ucf:46987
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0001389
Nano-Particles in Multi-Scale Composites and Ballistic Applications
Title
Nano-Particles in Multi-Scale Composites and Ballistic Applications.
Creator
Gibson, Jason, Gou, Jihua, Raghavan, Seetha, Bai, Yuanli, Zhai, Lei, University of Central Florida
Abstract / Description
Carbon nanotubes, graphene and nano sized core shell rubber particles have all been extensively researched for their capability to improve mechanical properties of thermoset resins. However, there has been a lack of research on their evaluation for energy absorption in high velocity impact scenarios, and the fundamental mechanics of their failure mechanisms during highly dynamic stress transfer through the matrix. This fundamental research is essential for laying the foundation for...
Show moreCarbon nanotubes, graphene and nano sized core shell rubber particles have all been extensively researched for their capability to improve mechanical properties of thermoset resins. However, there has been a lack of research on their evaluation for energy absorption in high velocity impact scenarios, and the fundamental mechanics of their failure mechanisms during highly dynamic stress transfer through the matrix. This fundamental research is essential for laying the foundation for improvement in ballistic performance in composite armor. In hard armor applications, energy absorption is largely accomplished through delamination between plies of the composite laminate. This energy absorption is accomplished through two mechanisms. The first being the elongation of the fiber reinforcement contained in the resin matrix, and the second is the propagation of the crack in between the discreet fabric plies. This research aims to fundamentally study the energy absorption characteristics of various nano-particles as reinforcements in thermoset resin for high velocity impact applications. Multiple morphologies will be evaluated through use of platelet, tubular and spherical shaped nano-particles. Evaluations of the effect on stress transfer through the matrix due to the combination of nano sized and micro scale particles of milled fiber is conducted. Three different nano-particles are utilized, specifically, multi-walled carbon nanotubes, graphene, and core shell rubber particles. The difference in surface area, aspect ratio and molecular structure between the tube, platelet and spherical nano-particles causes energy absorption through different failure mechanisms. This changes the impact performance of composite panels enhanced with the nano-particle fillers. Composite panels made through the use of dispersing the various nano-particles in a non-contact planetary mixer, are evaluated through various dynamic and static testing, including unnotched cantilever beam impact, mixed mode fracture toughness, split-Hopkinson bar, and ballistic V50 testing.The unnotched cantilever beam testing showed that the addition of milled fiber degraded the impact resistance of the samples. Addition of graphene nano platelets unilaterally degraded impact resistance through the unnotched cantilever beam testing. 1.5% loading of MWCNT showed the greatest increase in impact resistance, with a 43% increase over baseline.Determining the critical load for mixed mode interlaminar shear testing can be difficult for composite panels that bend without breaking. An iterative technique of optimizing the coefficient of determination, R2, in linear regression is developed for objectively determining the point of non-linearity for critical load. This allows for a mathematical method of determination; thereby eliminating any subjective decision of choosing where the data becomes non-linear. The core shell rubber nano particles showed the greatest strain energy release rate with an exponential improvement over the baseline results.Synergistic effects between nano and micro sized particles in the resin matrix during transfer of the stress wave were created and evaluated. Loadings of 1% milled carbon fiber enhanced the V50 ballistic performance of both carbon nanotube and core shell rubber particles in the resin matrix. However, the addition of milled carbon fiber degrades the impact resistance of all nano-particle enhanced resin matrices. Therefore, benefits gained from the addition of micro-sized particles in combination with nano-sized particles, are only seen in high energy impact scenarios with micro second durations.Loadings of 1% core shell rubber particles and 1% milled carbon fiber have an improvement of 8% in V50 ballistic performance over the baseline epoxy sample for 44 mag single wad cutter gas check projectiles. Loadings of 1% multi-walled carbon nanotubes with 1% milled carbon fiber have an improvement of 7.3% in V50 ballistic performance over the baseline epoxy sample.The failure mechanism of the various nano-particle enhanced resin matrices during the ballistic event is discussed through the use of scanning electron microscope images and Raman spectroscopy of the panels after failure. The Raman spectroscopy data shows a Raman shift for the fibers that had an enhancement in the V50 performance through the use of nano-particles. The Raman band for Kevlar(&)#174; centered at 1,649 cm-1 stemming from the stretching of the C==O bond of the fiber shows to be more sensitive to the residual axial strain, while the Raman band centered at 1,611 cm-1 stemming from the C-C phenyl ring is minimally affected for the CSR enhanced panels due to the failure mechanism of the CSR particles during crack propagation.
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Date Issued
2013
Identifier
CFE0004849, ucf:49714
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0004849
Fabrication and Study of Graphene-Based Nanocomposites for Sensing and Energy Applications
Title
Fabrication and Study of Graphene-Based Nanocomposites for Sensing and Energy Applications.
Creator
McInnis, Matthew, Zhai, Lei, Yestrebsky, Cherie, Zou, Shengli, Blair, Richard, Chen, Quanfang, University of Central Florida
Abstract / Description
Graphite is an allotrope of carbon made up of atomically thin sheets, each covalently bound together, forming a ?-conjugated network. An individual layer, called graphene, has extraordinary electrical, thermal and physical properties that provide the opportunity for innovating new functional composites. Graphene can be produced directly on a metallic substrate by chemical vapor deposition or by chemical oxidation of graphite, forming a stable aqueous suspension of graphene oxide (GO), which...
Show moreGraphite is an allotrope of carbon made up of atomically thin sheets, each covalently bound together, forming a ?-conjugated network. An individual layer, called graphene, has extraordinary electrical, thermal and physical properties that provide the opportunity for innovating new functional composites. Graphene can be produced directly on a metallic substrate by chemical vapor deposition or by chemical oxidation of graphite, forming a stable aqueous suspension of graphene oxide (GO), which allows for convenient solution processing techniques. For the latter, after thermal or chemical reduction, much of the properties of the starting graphene re-emerge due to the reestablishment of ?-conjugation. The ?-conjugated basal plane of graphene has been shown to influence the crystallization of ?-conjugated polymers, providing thermodynamically strong nucleation sites through the relatively strong ?-? interactions. These polymers can homocrystallize into 1-D filaments, but when nucleated from graphene, the orientation and geometry can be controlled producing hierarchical structures containing an electrical conductor decorated with wires of semi-conducting polymer. The resulting structures and crystallization kinetics of the conjugated polymer, poly(3-hexylthiophene-2,5-diyl) (P3HT) nucleated by graphene was studied. Further, field-effect transistors were developed using graphene as both the electrodes and the polymer crystallization surface to directly grow P3HT nanowires as the active material. This direct crystallization technique lead to higher charge mobility and higher on-off ratios, and this result was interpreted in terms of the morphology and polymer-graphene interface.Besides these thin-film technologies, neat GO suspensions can be lyophilized to produce monolithic, free-standing aerogels and then reduced to produce an electrically conductive porous material with a surface area greater than 1000 m2/g. The present research focuses on functionalizing the aerogel surfaces with metal nanoparticles to increase electrical conductivity and to impart functionality. Functionalization was carried out by adding a metal salt as a precursor and a chelating agent to inhibit GO flocculation. The GO and metal salt were simultaneously reduced to form rGO aerogels homogeneously loaded with metal nanoparticles. The size and distribution of these nanoparticles was controlled by concentration and chelating agent identity and abundance. Optimum aerogel formulations were used as a functioning and reversible conductometric hydrogen gas sensor and as an anode in an asymmetric supercapacitor with excellent properties.
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Date Issued
2015
Identifier
CFE0006227, ucf:51066
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0006227

Pages