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  Partner: UNT Libraries
 Department: Department of Materials Science
 Degree Level: Doctoral
 Collection: UNT Theses and Dissertations
Analysis of Thermoplastic Polyimide + Polymer Liquid Crystal Blends

Analysis of Thermoplastic Polyimide + Polymer Liquid Crystal Blends

Date: May 1998
Creator: Gopalanarayanan, Bhaskar
Description: Thermoplastic polyimides (TPIs) exhibit high glass transition temperatures (Tgs), which make them useful in high performance applications. Amorphous and semicrystalline TPIs show sub-Tg relaxations, which can aid in improving strength characteristics through energy absorption. The a relaxation of both types of TPIs indicates a cooperative nature. The semicrystalline TPI shows thermo-irreversible cold crystallization phenomenon. The polymer liquid crystal (PLC) used in the blends is thermotropic and with longitudinal molecular structure. The small heat capacity change (ACP) associated with the glass transition indicates the PLC to be rigid rod in nature. The PLC shows a small endotherm associated with the melting. The addition of PLC to the semicrystalline TPI does not significantly affect the Tg or the melting point (Tm). The cold crystallization temperature (Tc) increases with the addition of the PLC, indicating channeling phenomenon. The addition of PLC also causes a negative deviation of the ACP, which is another evidence for channeling. The TPI, PLC and their blends show high thermal stability. The semicrystalline TPI absorbs moisture; this effect decreases with the addition of the PLC. The absorbed moisture does not show any effect on the degradation. The addition of PLC beyond 30 wt.% does not result in an improvement ...
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Epoxy + liquid crystalline epoxy coreacted networks

Epoxy + liquid crystalline epoxy coreacted networks

Date: December 2000
Creator: Punchaipetch, Prakaipetch
Description: Molecular reinforcement through in-situ polymerization of liquid crystalline epoxies (LCEs) and a non-liquid crystalline epoxy has been investigated. Three LCEs: diglycidyl ether of 4,4'-dihydroxybiphenol (DGE-DHBP) and digylcidyl ether of 4-hydroxyphenyl-4"-hydroxybiphenyl-4'-carboxylate (DGE-HHC), were synthesized and blended with diglycidyl ether of bisphenol F (DGEBP-F) and subsequently cured with anhydride and amine curing agents. Curing kinetics were determined using differential scanning calorimetry (DSC). Parameters for autocatalytic curing kinetics of both pure monomers and blended systems were determined. The extent of cure for both monomers was monitored by using Fourier transform infrared spectroscopy (FT-IR). The glass transitions were evaluated as a function of composition using DSC and dynamic mechanical analysis (DMA). The results show that the LC constituent affects the curing kinetics of the epoxy resin and that the systems are highly miscible. The effects of molecular reinforcement of DGEBP-F by DGE-DHBP and DGE-HHC were investigated. The concentration of the liquid crystalline moiety affects mechanical properties. Tensile, impact and fracture toughness tests results are evaluated. Scanning electron microscopy of the fracture surfaces shows changes in failure mechanisms compared to the pure components. Results indicate that mechanical properties of the blended samples are improved already at low concentration by weight of the LCE added into ...
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Materials properties of hafnium and zirconium silicates: Metal interdiffusion and dopant penetration studies.

Materials properties of hafnium and zirconium silicates: Metal interdiffusion and dopant penetration studies.

Date: August 2002
Creator: Quevedo-Lopez, Manuel Angel
Description: Hafnium and Zirconium based gate dielectrics are considered potential candidates to replace SiO2 or SiON as the gate dielectric in CMOS processing. Furthermore, the addition of nitrogen into this pseudo-binary alloy has been shown to improve their thermal stability, electrical properties, and reduce dopant penetration. Because CMOS processing requires high temperature anneals (up to 1050 °C), it is important to understand the diffusion properties of any metal associated with the gate dielectric in silicon at these temperatures. In addition, dopant penetration from the doped polysilicon gate into the Si channel at these temperatures must also be studied. Impurity outdiffusion (Hf, Zr) from the dielectric, or dopant (B, As, P) penetration through the dielectric into the channel region would likely result in deleterious effects upon the carrier mobility. In this dissertation extensive thermal stability studies of alternate gate dielectric candidates ZrSixOy and HfSixOy are presented. Dopant penetration studies from doped-polysilicon through HfSixOy and HfSixOyNz are also presented. Rutherford backscattering spectroscopy (RBS), heavy ion RBS (HI-RBS), x-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HR-TEM), and time of flight and dynamic secondary ion mass spectroscopy (ToF-SIMS, D-SIMS) methods were used to characterize these materials. The dopant diffusivity is calculated by modeling ...
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Polymer hydrogel nanoparticles and their networks

Polymer hydrogel nanoparticles and their networks

Date: August 2002
Creator: Lu, Xihua
Description: The thermally responsive hydroxypropyl cellulose (HPC) hydrogel nanoparticles have been synthesized and characterized. The HPC particles were obtained by chemically crosslinking collapsed HPC polymer chains in water-surfactant (dodecyltrimethylammonium bromide) dispersion above the lower critical solution temperature (LCST) of the HPC. The size distributions of microgel particles, measured by dynamic light scattering, have been correlated with synthesis conditions including surfactant concentration, polymer concentration, and reaction temperature. The swelling and phase transition properties of resultant HPC microgels have been analyzed using both static and dynamic light scattering techniques. By first making gel nanoparticles and then covalently bonding them together, we have engineered a new class of gels with two levels of structural hierarchy: the primary network is crosslinked polymer chains in each individual particle, while the secondary network is a system of crosslinked nanoparticles. The covalent bonding contributes to the structural stability of the nanostructured gels, while self-assembly provides them with crystal structures that diffract light, resulting in colors. By using N-isopropylacrylamide copolymer hydrogel nanoparticles, we have synthesized nanoparticle networks that display a striking iridescence like precious opal but are soft and flexible like gelatin. This is in contrast to previous colored hydrogels, which were created either by adding dyes or fluorescent, ...
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The Electron Emission Characteristics of Aluminum, Molybdenum and Carbon Nanotubes Studied by Field Emission and Photoemission.

The Electron Emission Characteristics of Aluminum, Molybdenum and Carbon Nanotubes Studied by Field Emission and Photoemission.

Date: December 2002
Creator: Sosa, Edward Delarosa
Description: The electron emission characteristics of aluminum, molybdenum and carbon nanotubes were studied. The experiments were setup to study the emission behavior as a function of temperature and exposure to oxygen. Changes in the surface work function as a result of thermal annealing were monitored with low energy ultra-violet photoelectron spectroscopy for flat samples while field emission energy distributions were used on tip samples. The change in the field emission from fabricated single tips exposed to oxygen while in operation was measured using simultaneous Fowler-Nordheim plots and electron energy distributions. From the results a mechanism for the degradation in the emission was concluded. Thermal experiments on molybdenum and aluminum showed that these two materials can be reduced at elevated temperatures, while carbon nanotubes on the other hand show effects of oxidation. To purely reduce molybdenum a temperature in excess of 750 ºC is required. This temperature exceeds that allowed by current display device technology. Aluminum on the other hand shows reduction at a much lower temperature of at least 125 ºC; however, its extreme reactivity towards oxygen containing species produces re-oxidation. It is believed that this reduction is due to the outward diffusion of aluminum atoms through the oxide. Carbon nanotubes ...
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Structure property and deformation analysis of polypropylene montmorillonite nanocomposites.

Structure property and deformation analysis of polypropylene montmorillonite nanocomposites.

Date: May 2003
Creator: Hernandez-Luna, Alejandro
Description: Nanocomposites with expandable smectites such as montmorillonite layered silicates (MLS) in polymer matrices have attracted extensive application interest. Numerous MLS concentrations have been used with no particular justification. Here, we investigate the effects of MLS dispersion within the matrix and on mechanical performance. The latter is resolved through a three-prong investigation on rate dependent tensile results, time dependent creep results and the influence of a sharp notch in polypropylene (PP) nanocomposites. A fixed concentration of maleated polypropylene (mPP) was utilized as a compatibilizer between the MLS and non-polar PP. Analysis of transmission electron micrographs and X-ray diffraction patterns on the surface and below the surface of our samples revealed a unique skin-core effect induced by the presence of clay. Differential scanning calorimetric and polarized optical microscopic examination of spherulites sizes showed changes in nucleation and growth resulting from both the maleated PP compatibilizer and the MLS. These structural changes resulted in a tough nanocomposite, a concept not reported before in the PP literature. Nonlinear creep analysis of the materials showed two concentrations 3 and 5 % wt of PP, which reduced the compliance in the base PP. The use of thermal wave imaging allowed the identification of ductile failure among ...
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Characterization of methyltrimethoxysilane sol-gel polymerization and the resulting aerogels.

Characterization of methyltrimethoxysilane sol-gel polymerization and the resulting aerogels.

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Date: August 2003
Creator: Dong, Hanjiang
Description: Methyl-functionalized porous silica is of considerable interest as a low dielectric constant film for semiconductor devices. The structural development of these materials appears to affect their gelation behaviors and impact their mechanical properties and shrinkage during processing. 29Si solution NMR was used to follow the structural evolution of MTMS (methyltrimethoxysilane) polymerization to gelation or precipitation, and thus to better understand the species that affect these properties and gelation behaviors. The effects of pH, water concentration, type of solvents, and synthesis procedures (single step acid catalysis and two-step acid/base catalysis) on MTMS polymerization were discussed. The reactivity of silicon species with different connectivity and the extent of cyclization were found to depend appreciably on the pH value of the sol. A kinetic model is presented to treat the reactivity of both silicon species involved in condensations separately based on the inductive and steric effects of these silicon species. Extensive cyclization in the presence of acid, which was attributed to the steric effects among numerous reaction pathways for the first time, prevents MTMS gelation, whereas gels were obtained from the two-step method with nearly random condensations. The experimental degree of condensation (DC) at the gel point using the two-step procedure was determined ...
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Mechanical behavior and performance of injection molded semi-crystalline polymers.

Mechanical behavior and performance of injection molded semi-crystalline polymers.

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Date: August 2003
Creator: Simoes, Ricardo J. F.
Description: I have used computer simulations to investigate the behavior of polymeric materials at the molecular level. The simulations were performed using the molecular dynamics method with Lennard-Jones potentials defining the interactions between particles in the system. Significant effort was put into the creation of realistic materials on the computer. For this purpose, an algorithm was developed based on the step-wise polymerization process. The resulting computer-generated materials (CGMs) exhibit several features of real materials, such as molecular weight distribution and presence of chain entanglements. The effect of the addition of a liquid crystalline (LC) phase to the flexible matrix was also studied. The concentration and distribution of the second phase (2P) were found to influence the mechanical and tribological properties of the CGMs. The size of the 2P agglomerates was found to have negligible influence on the properties within the studied range. Moreover, although the 2P reinforcement increases the modulus, it favors crack formation and propagation. Regions of high LC concentration exhibit high probability of becoming part of the crack propagation path. Simulations of the tensile deformation under a uniaxial force have shown that the molecular deformation mechanisms developing in the material depend on several variables, such as the magnitude of ...
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Characterization and mechanical properties of nanoscale precipitates in modified Al-Si-Cu alloys using transmission electron microscopy and 3D atom probe tomography.

Characterization and mechanical properties of nanoscale precipitates in modified Al-Si-Cu alloys using transmission electron microscopy and 3D atom probe tomography.

Date: May 2007
Creator: Hwang, Junyeon
Description: Among the commercial aluminum alloys, aluminum 319 (Al-7wt%Si-4wt%Cu) type alloys are popularly used in automobile engine parts. These alloys have good casting characteristics and excellent mechanical properties resulting from a suitable heat treatment. To get a high strength in the 319 type alloys, grain refining, reducing the porosity, solid solution hardening, and precipitation hardening are preferred. All experimental variables such as solidification condition, composition, and heat treatment are influence on the precipitation behavior; however, precipitation hardening is the most significant because excess alloying elements from supersaturated solid solution form fine particles which act as obstacles to dislocation movement. The challenges of the 319 type alloys arise due to small size of precipitate and complex aging response caused by multi components. It is important to determine the chemical composition, crystal structure, and orientation relationship as well as precipitate morphology in order to understand the precipitation behavior and strengthening mechanism. In this study, the mechanical properties and microstructure were investigated using transmission electron microscopy and three dimensional atom probe tomography. The Mn and Mg effects on the microstructure and mechanical properties are discussed with crystallographic study on the iron intermetallic phases. The microstructural evolution and nucleation study on the precipitates in the ...
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Formation and Quantification of Corrosion Deposits in the Power Industry

Formation and Quantification of Corrosion Deposits in the Power Industry

Date: May 2007
Creator: Namduri, Haritha
Description: The presence of deposits on the secondary side of pressurized water reactor (PWR) steam generator systems is one of the main contributors to the high maintenance costs of these generators. Formation and transport of corrosion products formed due to the presence of impurities, metals and metallic oxides in the secondary side of the steam generator units result in formation of deposits. This research deals with understanding the deposit formation and characterization of deposits by studying the samples collected from different units in secondary side system at Comanche Peak Steam Electric Station (CPSES). Fourier transform infrared spectrophotometry (FTIR), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) have been used for studying the phases, morphologies and compositions of the iron oxides formed at Unit 1 and Unit 2 of secondary side of steamgenerator systems. Hematite and magnetite were found to be the dominant phases of iron oxides present in the units. Fe, Cr, O, Ni, Si, Cl and Cu were found in samples collected from both the units. A qualitative method was developed to differentiate iron oxides using laser induced breakdown spectroscopy (LIBS) based on temporal response of iron oxides to a high power laser beam. A quantitative FTIR technique was ...
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