Search Inside the UNT Theses and Dissertations
  •  

  You limited your search to:

 Department: Department of Materials Science and Engineering
  Remove
 Collection: UNT Theses and Dissertations
Morphological properties of poly (ethylene terephthalate) (PET) nanocomposites in relation to fracture toughness.
The effect of incorporation of montmorillonite layered silicate (MLS) on poly (ethylene terephthalate) (PET) matrix was investigated. MLS was added in varying concentration of 1 to 5 weight percent in the PET matrix. DSC and polarized optical microscopy were used to determine the crystallization effects of MLS addition. Non isothermal crystallization kinetics showed that the melting temperature and crystallization temperature decrease as the MLS percent increases. This delayed crystallization along with the irregular spherulitic shape indicates hindered crystallization in the presence of MLS platelets. The influence of this morphology was related with the fracture toughness of PET nanocomposites using essential work of fracture coupled with the infra red (IR) thermography. Both the essential as well as non essential work of fracture decreased on addition of MLS with nanocomposite showing reduced toughness. digital.library.unt.edu/ark:/67531/metadc4845/
Orientation, Microstructure and Pile-Up Effects on Nanoindentation Measurements of FCC and BCC Metals
This study deals with crystal orientation effect along with the effects of microstructure on the pile-ups which affect the nanoindentation measurements. Two metal classes, face centered cubic (FCC) and body centered cubic (BCC, are dealt with in the present study. The objective of this study was to find out the degree of inaccuracy induced in nanoindentation measurements by the inherent pile-ups and sink-ins. Also, it was the intention to find out how the formation of pile-ups is dependant upon the crystal structure and orientation of the plane of indentation. Nanoindentation, Nanovision, scanning electron microscopy, electron dispersive spectroscopy and electron backscattered diffraction techniques were used to determine the sample composition and crystal orientation. Surface topographical features like indentation pile-ups and sink-ins were measured and the effect of crystal orientation on them was studied. The results show that pile-up formation is not a random phenomenon, but is quite characteristic of the material. It depends on the type of stress imposed by a specific indenter, the depth of penetration, the microstructure and orientation of the plane of indentation. Pile-ups are formed along specific directions on a plane and this formation as well as the pile-up height and the contact radii with the indenter is dependant on the aforesaid parameters. These pile-ups affect the mechanical properties like elastic modulus and hardness measurements which are pivotal variables for specific applications in micro and nano scale devices. digital.library.unt.edu/ark:/67531/metadc6050/
Phase Separation and Second Phase Precipitation in Beta Titanium Alloys
The current understanding of the atomic scale phenomenon associated with the influence of beta phase instabilities on the evolution of microstructure in titanium alloys is limited due to their complex nature. Such beta phase instabilities include phase separation and precipitation of nano-scale omega and alpha phases in the beta matrix. The initial part of the present study focuses on omega precipitation within the beta matrix of model binary titanium molybdenum (Ti-Mo) alloys. Direct atomic scale observation of pre-transition omega-like embryos in quenched alloys, using aberration-corrected high resolution scanning transmission electron microscopy and atom probe tomography (APT) was compared and contrasted with the results of first principles computations performed using the Vienna ab initio simulation package (VASP) to present a novel mechanism of these special class of phase transformation. Thereafter the beta phase separation and subsequent alpha phase nucleation in a Ti-Mo-Al ternary alloy was investigated by coupling in-situ high energy synchrotron x-ray diffraction with ex-situ characterization studies performed using aberration corrected transmission electron microscopy and APT to develop a deeper understanding of the mechanism of transformation. Subsequently the formation of the omega phase in the presence of simultaneous development of compositional phase separation within the beta matrix phase of a Ti-10V-6Cu (wt%) alloy during continuous cooling has been investigated using a combination of transmission electron microscopy and atom probe tomography. The results of these investigations provided novel insights into the mechanisms of solid-state transformations in metallic systems by capturing the earliest stages of nucleation at atomic to near atomic spatial and compositional resolution. digital.library.unt.edu/ark:/67531/metadc67975/
Piezoresistive Polyvinylidene Fluoride/Carbon Filled Nanocomposites
This thesis examines the value of using dispersed conductive fillers as a stress/strain sensing material. The effect of the intrinsic conductivity of the filler on the ability to be effective and the influence of filler concentration on the conductivity are also examined. To meet these objectives, nanocomposites of polyvinylidene fluoride (PVDF) with carbon nanofibers (CNFs) and carbon nanotubes (CNTs) were prepared by melt-blending using a twin screw extruder. Since PVDF has a potential to be piezoresistive based on the type of crystalline phase, the effect of CNFs on PVDF crystallinity, crystalline phase, quasi static and dynamic mechanical property was studied concurrently with piezoresponse. Three time dependencies were examined for PVDF/CNTs nanocomposites: quasi-static, transient and cyclic fatigue. The transient response of the strain with time showed viscoelastic behavior and was modeled by the 4-element Burger model. Under quasi-static loading the resistance showed negative pressure coefficient below yield but changed to a positive pressure coefficient after yield. Under cyclic load, the stress-time and resistance-time were synchronous but the resistance peak value decreased with increasing cycles, which was attributed to charge storage in the nanocomposite. The outcomes of this thesis indicate that a new piezoresponsive system based on filled polymers is a viable technology for structural health monitoring. digital.library.unt.edu/ark:/67531/metadc68059/
Polyethylene-layered double hydroxide and montmorillonite nanocomposites: Thermal, mechanical and flame retardance properties.
The effect of incorporation two clays; layered double hydroxides (LDH) and montmorillonite layered silicates (MLS) in linear low density polyethylene (PE) matrix was investigated. MLS and LDH were added of 5, 15, 30 and 60 weight percent in the PE and compounded using a Brabender. Ground pellets were subsequently compression molded. Dispersion of the clays was analyzed using optical microscopy, SEM and XRD. Both the layered clays were immiscible with the PE matrix and agglomerates formed with increased clay concentration. The thermal properties were studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Both clays served as nucleation enhancers increasing recrystallization temperatures in the composites. Flame retarding properties were determined by using the flammability HVUL-94 system. LDH indicated better flame retarding properties than MLS for PE. The char structure was analyzed by environmental scanning electron microscopy. Mechanical properties were studied by tensile testing and Vickers microhardness testing apparatus. digital.library.unt.edu/ark:/67531/metadc6087/
Polymer Liquid Crystal (PLC) and Polypropylene Interlayers in Polypropylene and Glass Fiber Composites: Mechanical Properties
In recent developments of composite materials, scientists and engineers have come up with fibers as well as matrices for composites and techniques of blending high cost components with low cost materials. Thus, one creates cost effective composite materials that are as efficient as space age components. One of the major breakthroughs in this area is the innovation of molecular composites, specifically polymeric liquid crystals (PLCs). These materials have excellent mechanical properties such as tensile impact and bending strength. They have excellent chemical resistance, low thermal expansivity, and low flammability. Their low viscosity leads to good processability One major setback in using space age composite technology in commercial applications is the price. Due to the complexity of processing, the cost of space composite materials is skyrocketing. To take the same concept of space age composite materials to create a more economical substitute has become a serious concern among scientists and engineers around the world. The two issues that will be resolved in this thesis are: (1) the potential impact of using PLCs (molecular reinforcement) can have on macro reinforced (heterogeneous composite, HC) long fiber systems; and (2) how strategic placement of the reinforcing layers can affect the macromechanical properties of the laminates. digital.library.unt.edu/ark:/67531/metadc5838/
Processing, Structure, and Tribological Property Interrelationships in Sputtered Nanocrystalline ZnO Coatings
Solid lubricant coatings with controlled microstructures are good candidates in providing lubricity in moving mechanical assembly applications, such as orthopedics and bearing steels. Nanocrystalline ZnO coatings with a layered wurtzite crystal structure have the potential to function as a lubricious material by its defective structure which is controlled by sputter deposition. The interrelationships between sputtered ZnO, its nanocrystalline structure and its lubricity will be discussed in this thesis. The nanocrystalline ZnO coatings were deposited on silicon substrates and Ti alloys by RF magnetron sputtering with different substrate adhesion layers, direct current biases, and temperatures. X-ray diffraction identified that the ZnO (0002) preferred orientation was necessary to achieve low sliding friction and wear along with substrate biasing. In addition, other analyses such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) were utilized to study the solid lubrication mechanisms responsible for low friction and wear. digital.library.unt.edu/ark:/67531/metadc12207/
Processing, structure property relationships in polymer layer double hydroxide multifunctional nanocomposites
Dan Beaty (1937-2002) was a prolific composer, pianist, researcher, educator, and writer. His large compositional output included chamber works, choral works, songs, orchestral pieces, electronic music, and keyboard works. Beaty was well versed in traditional Western music as well as the more avant-garde and perplexing idioms of the twentieth century. Beaty's compositions reflect the many fascinating, if not always popular, musical trends of his time. His music encompasses styles from serial to jazz, shows compositional influences from Arnold Schoenberg to Indonesian music, and demonstrates thought-provoking and highly intellectual craftsmanship. This document explores several of Beaty's songs through a discussion of the composer's life and compositional process. Songs included in this document are Three Weeks Songs, October, November, A Sappho Lyric, Love Song, That Night When Joy Began, and War Lyrics. This document was written to accompany the author's DMA Lecture-Recital at the University of North Texas. Unfortunately, Beaty's vocal music was never published and is mostly unknown. One goal of the project was to initiate interest in Beaty's songs. Through this document, Lecture-Recital, and additional performances, considerable strides have been made to bring Beaty's songs to new audiences throughout the United States. In addition, the author has received permission from the Beaty family to publish Dan Beaty's songs. digital.library.unt.edu/ark:/67531/metadc12174/
Saturation and foaming of thermoplastic nanocomposites using supercritical CO2.
Polystyrene (PS) nanocomposite foams were prepared using supercritical fluid (SCF) CO2 as a solvent and blowing agent. PS was first in-situ polymerized with a range of concentrations of montmorillonite layered silicate (MLS). The polymerized samples were then compression molded into 1 to 2mm thick laminates. The laminates were foamed in a batch supercritical CO2 process at various temperatures and pressures from 60°-85°C and 7.6-12MPa. The resulting foams were analyzed by scanning electron microscopy to determine effect of MLS on cellular morphology. Differential scanning calorimetry was used to determine the impact of nanocomposite microstructure on glass transition of the foamed polymer. X-ray diffraction spectra suggested that the PS/MLS composite had an intercalated structure at both the 1% and 3% mixtures, and that the intercalation may be enhanced by the foaming process. digital.library.unt.edu/ark:/67531/metadc4745/
Scratch Modeling of Polymeric Materials with Molecular Dynamics
It is impossible to determine the amount of money that is spent every replacing products damaged from wear, but it is safe to assume that it is in the millions of dollars. With metallic materials, liquid lubricants are often used to prevent wear from materials rubbing against one another. However, with polymeric materials, liquid lubricants cause swelling, creating an increase in friction and therefore increasing the wear. Therefore, a different method or methods to mitigate wear in polymers should be developed. For better understanding of the phenomenon of wear, scratch resistance testing can be used. For this project, classic molecular dynamics is used to study the mechanics of nanometer scale scratching on amorphous polymeric materials. As a first approach, a model was created for polyethylene, considering intramolecular and intermolecular interactions as well as mass and volume of the CH2 monomers in a polymer chain. The obtained results include analysis of penetration depth and recovery percentage related to indenter force and size. digital.library.unt.edu/ark:/67531/metadc149608/
State accountability ratings as related to district size and diversity.
All Texas school districts were examined to determine the relationship of district size and diversity to the accountability ratings of selected Texas school districts and the implications of including all data in the accountability rating system. Eight large districts and 12 small districts were matched demographically utilizing data from the 2003-2004 school year. Information from the Texas Education Agency was accessed over 2003-2004 and 2004-2005. The ratings were found to be lowered from Recognized to Academically Acceptable with the inclusion of these groups 6 out of 20 times. These findings indicate that the Texas accountability system, in its current structure, excludes certain students based upon race and economic status and is not in compliance with what the law intended. This study should be replicated on a larger scale to assess its validity for a larger sample of small districts. Equity among states should be examined to provide information for a nationwide accountability system. digital.library.unt.edu/ark:/67531/metadc6049/
Stimuli-responsive microgels for self-assembled crystalline structures and controlled drug release.
Tissue response to PNIPAM and HPC nanoparticles has been studied by implantation method. The results suggest that both PNIAPM and HPC nanoparticles possess good biocompatibility and they may serve as a good carrier for the applications of controlled delivery. Rheological properties of dispersions of IPN microgels composed of PNIPAM and PAAc have been studied. It is found that the IPN microgel dispersion can undergo a sol-gel transition at temperature above 33°C. In vivo drug release experiments suggest that the gelation procedure creates a diffusion barrier and thus leads to slow release. An emulsion method has been used to grow columnar crystals by mixing PNIPAM microgel dispersions with organic solvents. Effect of both temperature and microgel concentration on formation of columnar crystals has been studied. PNIPAM-co-NMA microgels have been used for the fabrication of crystalline hydrogel films by self-crosslinking microgels. The hydrogel film exhibits an iridescent. The thermally responsive properties and mechanical properties of this film have been studied. Melting temperature (Tm) of colloidal crystals self-assembled with PNIPAM-co-AAc microgels has been investigated as a function of pH, salt concentration and microgel concentration. It is revealed that Tm increases as pH value increases; Tm decreases with increase of salt concentration; Tm increases as microgel concentration increases. Phase behavior of PNIPAM-co-HEAc microgel dispersions has been investigated. It is observed that these microgel dispersions exhibit liquid, crystal, and glass phase. As microgel size increases, crystal phase shifts to low concentration range. As temperature increases, crystal phase shifts to high concentration ranges. These colloidal crystals can be stabilized by NaOH-induced gelation. Effect of NaOH concentration on formation of physical gelation has been investigated. digital.library.unt.edu/ark:/67531/metadc11001/
Structure and Low-temperature Tribology of Lubricious Nanocrystalline ZnO/Al2O3 Nanolaminates and ZrO2 Monofilms Grown by Atomic Layer Deposition
Currently available solid lubricants only perform well under a limited range of environmental conditions. Unlike them, oxides are thermodynamically stable and relatively inert over a broad range of temperatures and environments. However, conventional oxides are brittle at normal temperatures; exhibiting significant plasticity only at high temperatures (>0.5Tmelting). This prevents oxides' use in tribological applications at low temperatures. If oxides can be made lubricious at low temperatures, they would be excellent solid lubricants for a wide range of conditions. Atomic layer deposition (ALD) is a growth technique capable of depositing highly uniform and conformal films in challenging applications that have buried surfaces and high-aspect-ratio features such as microelectromechanical (MEMS) devices where the need for robust solid lubricants is sometimes necessary. This dissertation investigates the surface and subsurface characteristics of ALD-grown ZnO/Al2O3 nanolaminates and ZrO2 monofilms before and after sliding at room temperature. Significant enhancement in friction and wear performance was observed for some films. HRSEM/FIB, HRTEM and ancillary techniques (i.e. SAED, EELS) were used to determine the mechanisms responsible for this enhancement. Contributory characteristics and energy dissipation modes were identified that promote low-temperature lubricity in both material systems. digital.library.unt.edu/ark:/67531/metadc9741/
Study of Conductance Quantization by Cross-Wire Junction
Access: Use of this item is restricted to the UNT Community.
The thesis studied quantized conductance in nanocontacts formed between two thin gold wires with one of the wires coated by alkainthiol self assembly monolayers (SAM), by using the cross-wire junction. Using the Lorenz force as the driving force, we can bring the two wires in contact in a controlled manner. We observed conductance with steps of 2e2 / h. The conductance plateaus last several seconds. The stability of the junction is attributed to the fact that the coating of SAM improves the stability and capability of the formed contact. digital.library.unt.edu/ark:/67531/metadc5540/
Study of lead sorption on magnetite at high temperatures.
Lead's uptake on magnetite has been quantitatively evaluated in the present study at a temperature of 200°C and pH of 8.5 with lead concentrations ranging from 5 ppm to175 ppm by equilibrium adsorption isotherms. The pH independent sorption behavior suggested lead sorption due to pH independent permanent charge through weak electrostatic, non-specific attraction where cations are sorbed on the cation exchange sites. The permanent negative charge could be a consequence of lead substitution which is supported by increase in the lattice parameter values from the X-ray diffraction (XRD) results. Differential scanning calorimetry (DSC/TGA) results showed an increase of exothermic (magnetite to maghemite transformation) peak indicating substitution of lead ions due to which there is retardation in the phase transformation. Presence of outer sphere complexes and physical sorption is further supported by Fourier transformed infrared spectroscopy (FTIR). None of the results suggested chemisorption of lead on magnetite. digital.library.unt.edu/ark:/67531/metadc5445/
Supercritical CO2 foamed biodegradable polymer blends of polycaprolactone and Mater-Bi.
Supercritical CO2 foam processing of biopolymers represents a green processing route to environmentally friendly media and packaging foams. Mater-Bi, a multiconstituent biopolymer of polyester, starch and vegetable oils has shown much promise for biodegradation. The polymer, however, is not foamable with CO2 so blended with another polymer which is. Polycaprolactone is a biopolymer with potential of 4000% change in volume with CO2. Thus we investigate blends of Mater-Bi (MB) and polycaprolactone (PCL) foamed in supercritical CO2 using the batch process. Characterization of the foamed and unfoamed samples were done using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Micrographs of the samples from the SEM revealed that the cell size of the foams reduced and increased with increase in MB concentration and increase in the foaming temperature respectively. Mechanical tests; tensile, compression, shear and impact were performed on the foamed samples. It was noted that between the 20-25% wt. MB, there was an improvement in the mechanical properties. This suggests that at these compositions, there is a high interaction between PCL and MB at the molecular level compared to other compositions. The results indicate that green processing of polymer blends is viable. digital.library.unt.edu/ark:/67531/metadc5136/
Supercritical Silylation and Stability of Silyl Groups
Access: Use of this item is restricted to the UNT Community.
Methylsilsesquioxane (MSQ) and organosilicate glass (OSG) are the materials under this study because they exhibit the dielectric constant values necessary for future IC technology requirements. Obtaining a low-k dielectric value is critical for the IC industry in order to cope time delay and cross talking issues. These materials exhibit attractive dielectric value, but there are problems replacing conventional SiO2, because of their chemical, mechanical and electrical instability after plasma processing. Several techniques have been suggested to mitigate process damage but supercritical silylation offers a rapid single repair step solution to this problem. Different ash and etch damaged samples were employed in this study to optimize an effective method to repair the low-k dielectric material and seal the surface pores via supercritical fluid processing with various trialkylchlorosilanes. Fourier transform infrared spectroscopy (FTIR), contact angle, capacitance- voltage measurements, and x-ray photoemission spectroscopy, dynamic secondary ion mass spectroscopy (DSIMS), characterized the films. The hydrophobicity and dielectric constant after exposure to elevated temperatures and ambient conditions were monitored and shown to be stable. The samples were treated with a series of silylating agents of the form R3-Si-Cl where R is an alkyl groups (e.g. ethyl, propyl, isopropyl). Reactivity with the surface hydroxyls was inversely proportional to the length of the alkyl group, perhaps due to steric effects. Contact angle measurements revealed that heating the films in ambient diminished hydrophobicity. Depth and surface profiling using (DSIMS) and (XPS) were utilized to develop a model for surface coverage. digital.library.unt.edu/ark:/67531/metadc5216/
Surface Engineering and Characterization of Laser Deposited Metallic Biomaterials
Novel net shaping technique Laser Engineered Net shaping™ (LENS) laser based manufacturing solution (Sandia Corp., Albuquerque, NM); Laser can be used to deposit orthopedic implant alloys. Ti-35Nb-7Zr-5Ta (TNZT) alloy system was deposited using LENS. The corrosion resistance being an important prerequisite was tested electrochemically and was found that the LENS deposited TNZT was better than conventionally used Ti-6Al-4V in 0.1N HCl and a simulated body solution. A detailed analysis of the corrosion product exhibited the presence of complex oxides which are responsible for the excellent corrosion resistance. In addition, the in vitro tests done on LENS deposited TNZT showed that they have excellent biocompatibility. In order to improve the wear resistance of the TNZT system boride reinforcements were carried out in the matrix using LENS processing. The tribological response of the metal matrix composites was studied under different conditions and compared with Ti-6Al-4V. Usage of Si3N4 balls as a counterpart in the wear studies showed that there is boride pullout resulting in third body abrasive wear with higher coefficient of friction (COF). Using 440C stainless steel balls drastically improved the COF of as deposited TNZT+2B and seemed to eliminate the effect of “three body abrasive wear,” and also exhibited superior wear resistance than Ti-6Al-4V. digital.library.unt.edu/ark:/67531/metadc3618/
Synthesis and Characterization of Crystalline Assemblies of Functionalized Hydrogel Nanoparticles
Access: Use of this item is restricted to the UNT Community.
Two series monodispersed nanoparticles of hydroxylpropyl cellulose (HPC) and functionalized poly-N-isopropylamide (PNIPAM) particles have been synthesized and used as building blocks for creating three-dimensional networks, with two levels of structural hierarchy. The first level is HPC nanoparticles were made from methacrylated or degradable cross-linker attached HPC. These nanoparticles could be stabilized at room temperature by residual methacrylate or degradable groups are present both within and on the exterior of HPC nanoparticles. Controlled release studies have been performed on the particle and networks .The nearly monodispersed nanoparticles have been synthesized on the basis of a natural polymer of hydropropylcellulose (HPC) with a high molecular weight using the precipitation polymerization method and self-assembly of these particles in water results in bright colors. The HPC nanoparticles can be potential using as crosslinkers to increase the hydrogels mechanical properties, such as high transparency and rapid swelling/de-swelling kinetics. The central idea is to prepare colloidal particles containing C=C bonds and to use them as monomers - vinylparticles, to form stable particle assemblies with various architectures. This is accomplished by mixing an aqueous suspension of hydrogel nanoparticles (PNIPAM-co-allylamine) with the organic solvent (dichloromethane) to grow columnar crystals. The hydrogels with such a unique crystal structure behavior not only like the hydrogel opals, but also have a unique property: anisotropy. digital.library.unt.edu/ark:/67531/metadc4976/
Synthesis and characterization of crystalline assembly of poly Nisopropylacry-lamide)-co-acrylic acid nanoparticles.
In this study, crystalline poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAm-co-AAc) nanoparticle network in organic solvents was obtained by self assembling precursor particles in acetone/epichlorohydrin mixture at room temperature followed by inter-sphere crosslinking at ~98 °C. The crystals thus formed can endure solvent exchanges or large distortions under a temporary compressing force with the reoccurrence of crystalline structures. In acetone, the crystals were stable, independent of temperature, while in water crystals could change their colors upon heating or changing pH values. By passing a focused white light beam through the crystals, different colors were displayed at different observation angles, indicating typical Bragg diffraction. Shear moduli of the gel nanoparticle crystals were measured in the linear stress-yield ranges for the same gel crystals in both acetone and water. Syntheses of particles of different sizes and the relationship between particle size and the color of the gel nanoparticle networks at a constant solid content were also presented. Temperature- and pH- sensitive crystalline PNIPAm-co-AAc hydrogel was prepared using osmosis crosslinking method. Not only the typical Bragg diffraction phenomenon was observed for the hydrogel but also apparent temperature- and pH- sensitive properties were performed. The phase behavior of PNIPAm nanoparticles dispersed in water was also investigated using a thermodynamic perturbation theory combined with lightscattering and spectrometer measurements. It was shown how the volume transition of PNIPAM particles affected the interaction potential and determined a novel phase diagram that had not been observed in conventional colloids. Because both particle size and attractive potential depended on temperature, PNIPAM aqueous dispersion exhibited phase transitions at a fixed particle number density by either increasing or decreasing temperature. The phase transition of PNIPAm-co-AAc colloids was also studied. The results from the comparison between pure PNIPAm and charged PNIPAm colloids showed that the introducing of carboxyl (-COOH) group not only contributed to the synthesis of three-dimensional nanoparticle network but also effectively increased the crystallization temperature and concentration range. The phase transitions at both low and high temperatures were observed from the turbidity change by using UV-Vis spectrometer. Centrifugal vibration method was used to make crystalline PNIPAm-co-AAc dispersion at high concentration (8%). The turbidity test proved the formation of iridescent pattern. digital.library.unt.edu/ark:/67531/metadc4671/
Thermophysical, Interfacial and Decomposition Analyses of Polyhydroxyalkanoates introduced against Organic and Inorganic Surfaces
The development of a "cradle-to-cradle" mindset with both material performance during utilization and end of life disposal is a critical need for both ecological and economic considerations. The main limitation to the use of the biopolymers is their mechanical properties. Reinforcements are therefore a good alternative but disposal concerns then arise. Thus the objective of this dissertation is to investigate a biopolymer nanocomposite where the filler is a synthetically prepared layer double hydroxide (inorganic interface); and a biopolymer paper (organic interface) based coating or laminate. The underlying issues driving performance are the packing density of the biopolymer and the interaction with the reinforcement. Since the polyhydroxyalkanoates or PHAs (the biopolymers used for the manufacture of the nanocomposites and coatings) are semicrystalline materials, the glass transition was investigated using dynamic mechanical analysis (DMA) and dielectric spectroscopy (DES), whereas the melt crystallization, cold crystallization and melting points were investigated using differential scanning calorimetry (DSC). Fourier transform infrared (FTIR) spectroscopy was used to estimate crystallinity in the coated material given the low thermal mass of the PHA in the PHA coating. The significant enhancement of the crystallization rate in the PHA nanocomposite was probed using DSC and polarized optical microscopy (POM) and analyzed using Avrami and Lauritzen-Hoffman models. Both composites showed a significant improvement in the mechanical performance obtained by DMA, tensile and impact testing. The degradation and decomposition of the two composites were investigated in low microbial activity soil for the cellulose paper (to slow down the degradation rate that occurs in compost) and in compost. An in-house system according to the American Society for Testing and Materials ASTM D-98 (2003) was engineered. Soil decomposition showed that PHA coating into and onto the cellulose paper can be considered to be a useful method for the assessment of the degradability of the biopolymer. PHA nanocomposite showed enhanced compostability. digital.library.unt.edu/ark:/67531/metadc12111/
Topics in micro electromechanical systems: MEMS engineering and alternative materials for MEMS fabrication.
Access: Use of this item is restricted to the UNT Community.
This paper deals with various topics in micro electromechanical systems (MEMS) technology beginning with microactuation, MEMS processing, and MEMS design engineering. The fabrication and testing of three separate MEMS devices are described. The first two devices are a linear stepping motor and a continuous rotary motor, respectively; and were designed for the purpose of investigating the frictional and wear properties of silicon components. The third device is a bi-stable microrelay, in which electrical current conducts through a secondary circuit, via a novel probe-interconnect mechanism. The second half focuses on engineering a carbon nanotube / SU-8 photoepoxy nanocomposite for fabricating MEMS devices. A processing method for this material as well as the initial results of characterization, are discussed. digital.library.unt.edu/ark:/67531/metadc4553/
Trapping of hydrogen in Hf-based high κ dielectric thin films for advanced CMOS applications.
In recent years, advanced high κ gate dielectrics are under serious consideration to replace SiO2 and SiON in semiconductor industry. Hafnium-based dielectrics such as hafnium oxides, oxynitrides and Hf-based silicates/nitrided silicates are emerging as some of the most promising alternatives to SiO2/SiON gate dielectrics in complementary metal oxide semiconductor (CMOS) devices. Extensive efforts have been taken to understand the effects of hydrogen impurities in semiconductors and its behavior such as incorporation, diffusion, trapping and release with the aim of controlling and using it to optimize the performance of electronic device structures. In this dissertation, a systematic study of hydrogen trapping and the role of carbon impurities in various alternate gate dielectric candidates, HfO2/Si, HfxSi1-xO2/Si, HfON/Si and HfON(C)/Si is presented. It has been shown that processing of high κ dielectrics may lead to some crystallization issues. Rutherford backscattering spectroscopy (RBS) for measuring oxygen deficiencies, elastic recoil detection analysis (ERDA) for quantifying hydrogen and nuclear reaction analysis (NRA) for quantifying carbon, X-ray diffraction (XRD) for measuring degree of crystallinity and X-ray photoelectron spectroscopy (XPS) were used to characterize these thin dielectric materials. ERDA data are used to characterize the evolution of hydrogen during annealing in hydrogen ambient in combination with preprocessing in oxygen and nitrogen. digital.library.unt.edu/ark:/67531/metadc5114/
Tribological Improvements of Carbon-Carbon Composites by Infiltration of Atomic Layer Deposited Lubricious Nanostructured Ceramic Oxides
A number of investigators have reported enhancement in oxidation and wear resistant of carbon-carbon composites (CCC) in the presence of protective coating layers. However, application of a surface and subsurface coating system that can preserve its oxidation and wear resistance along with maintaining lubricity at high temperature remains unsolved. To this end, thermodynamically stable protective oxides (ZnO/Al2O3/ZrO2) have been deposited by atomic layer deposition (ALD) to infiltrate porous CCC and graphite foams in order to improve the thermal stability and wear resistance in low and high speed sliding contacts. Characterization of microstructural evolution was achieved by using energy dispersive x-ray spectroscopy (EDS) mapping in scanning electron microscope (SEM) coupled with focused ion beam (FIB), x-ray tomography, high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD). Evaluation of the tribological properties of CCC coated with abovementioned ALD thin films were performed by employing low speed pure sliding tribometer and a high speed/frequency reciprocating rig to simulate the fretting wear behavior at ambient temperature and elevated temperatures of 400°C.It was determined with x-ray tomography imaging and EDS mapping that ALD ZnO/Al2O3/ZrO2 nanolaminates and baseline ZrO2 coatings exhibited excellent conformality and pore-filling capabilities down to ~100 μm and 1.5 mm in the porous CCC and graphite foam, respectively, which were dependent on the exposure time of the ALD precursors. XRD and HRTEM determined the crystalline phases of {0002} textured ZnO (wurtzite), amorphous Al2O3, and {101}-tetragonal ZrO2. Significant improvements up to ~65% in the sliding and fretting wear factors were determined for the nanolaminates in comparison to the uncoated CCC. A tribochemical sliding-induced mechanically mixed layer (MML) was found to be responsible for these improvements. HRTEM confirmed the presence of a high density of ZnO shear-induced basal stacking faults inside the wear tracks responsible for intrafilm shear velocity accommodation that mitigated friction and wear. digital.library.unt.edu/ark:/67531/metadc84254/
Void Growth and Collapse in a Creeping Single Crystal
Aircraft engine components can be subjected to a large number of thermo-mechanical loading cycles and to long dwell times at high temperatures. In particular, the understanding of creep in single crystal superalloy turbine blades is of importance for designing more reliable and fuel efficient aircraft engines. Creep tests on single crystal superalloy specimens have shown greater creep strain rates for thinner specimens than predicted by current theories. Therefore, it is necessary to develop a more predictive description of creep processes in these materials for them to be used effectively. Experimental observations have shown that the crystals have an initial porosity and that the progressive growth of these voids plays a major role in limiting creep life. In order to understand void growth under creep in single crystals, we have analyzed the creep response of three dimensional unit cells with a single spherical void under different types of isothermal creep loading. The growth behavior of the void is simulated using a three dimensional rate dependent crystal plasticity constitutive relation in a quasi-static finite element analysis. The aim of the present work is to analyze the effect of stress traixiality and Lode parameter on void growth under both constant true stress and constant engineering stress isothermal creep loading. digital.library.unt.edu/ark:/67531/metadc84281/
A Wet Etch Release Method for Silicon Microelectromechanical Systems (MEMS) Using Polystyrene Microspheres for Improved Yield
Access: Use of this item is restricted to the UNT Community.
One of the final steps in fabricating microelectromechanical devices often involves a liquid etch release process. Capillary forces during the liquid evaporation stage after the wet etch process can pull two surfaces together resulting in adhesion of suspended microstructures to the supporting substrate. This release related adhesion can greatly reduce yields. In this report, a wet etch release method that uses polystyrene microspheres in the final rinse liquid is investigated. The polystyrene microspheres act as physical barriers between the substrate and suspended microstructures during the final liquid evaporation phase. A plasma ashing process is utilized to completely remove the polystyrene microspheres from the microstructure surfaces. Using this process, release yields > 90% were achieved. It is found that the surface roughness of gold surfaces increases while that of the silicon is reduced due to a thin oxide that grows on the silicon surface during the plasma process. digital.library.unt.edu/ark:/67531/metadc4457/
Wettability of Silicon, Silicon Dioxide, and Organosilicate Glass
Wetting of a substance has been widely investigated since it has many applications to many different fields. Wetting principles can be applied to better select cleans for front end of line (FEOL) and back end of line (BEOL) cleaning processes. These principles can also be used to help determine processes that best repel water from a semiconductor device. It is known that the value of the dielectric constant in an insulator increases when water is absorbed. These contact angle experiments will determine which processes can eliminate water absorption. Wetting is measured by the contact angle between a solid and a liquid. It is known that roughness plays a crucial role on the wetting of a substance. Different surface groups also affect the wetting of a surface. In this work, it was investigated how wetting was affected by different solid surfaces with different chemistries and different roughness. Four different materials were used: silicon; thermally grown silicon dioxide on silicon; chemically vapor deposited (CVD) silicon dioxide on silicon made from tetraethyl orthosilicate (TEOS); and organosilicate glass (OSG) on silicon. The contact angle of each of the samples was measured using a goniometer. The roughness of the samples was measured by atomic force microscopy (AFM). The chemistry of each of the samples were characterized by using X-ray photoelectron spectroscopy (XPS) and grazing angle total attenuated total reflection Fourier transform infrared spectroscopy (FTIR/GATR). Also, the contact angle was measured at the micro scale by using an environmental scanning electron microscope (ESEM). digital.library.unt.edu/ark:/67531/metadc12161/