Browsing by Subject "Coatings"
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Item Accelerated aging of the digital micromirror device using full custom electrostatic control(Texas Tech University, 2004-08) Yu, TaoStiction and friction are integral to the dynamic operation of many Microelectromechanical(MEMS) and a comprehensive approach is needed to develop surface coating systems that take into account the fabrication, packaging and operational parameters. The operating environment is also vital for device performance and durability. Texas Instruments' Digital Micromirror Device™ (DMD™), one of the commercial MEMS, is used as a media to develop a testing system for quantitative understanding of stiction and friction properties. The DMD™ has several measurable parameters aiding in the determination of stiction and friction as a function of controllable variables. This paper discusses the development of custom testing system using the DMD™ for stiction studying. A custom control solution is designed and implemented to actuate the DMD™ and accelerate the aging of the lubricant coat. An optical system and allied software are also developed to analyze mirror states. Stiction quantification is carried out by supplying variable voltage reset pulses to initiate release in stuck mirrors. Measurement results indicate that the mirror performance degraded with scrubbing time. Critical parameters like the resonant operating frequency and hinge memory sag effects are also discussed. Knowledge gained through this study can be applied to better understand stiction in other MEMS.Item Design and fabrication of sub-millimeter scale gas bearings with tungsten-containing diamond like carbon coatings(2004) Kim, Daejong; Bryant, Michael D.Micro gas bearings of sub millimeter size (diameter of 500µm and length of 300µm) with micron clearances were designed and fabricated through X-ray lithography and Ni electroplating. Details of the fabrication processes for the micro gas bearings are described. Procedures to make X-ray mask and technical issues are discussed. Static and dynamic hydrodynamic characteristics of the micro gas bearings were assessed using Molecular Gas Lubrication (MGL) theory. Fabricated micro gas bearings had lower stability than plain circular gas bearings due to lower load capacity and poor gas damping. Improved bearing designs having non-symmetrical step geometry with deep axial groove were suggested. These bearings were predicted to have much higher load capacities and dynamic stabilities than the fabricated micro gas bearings. Micro gas bearings were tested via an air jet-driven micro turbine made of SU-8, press-fitted onto the shaft. A self-aligning assembly technique for the micro gas bearings (using capillary action of photo resist) was developed and proved to be very effective. Successful operation of micro gas bearings at 60,000 rpm was demonstrated under limited operating conditions. Tribological characteristics of Ni micro bearings and amorphous tungsten hydrocarbon (W-C:H) coated micro bearings were investigated in dry friction mode, using a newly designed micro tribo tester. 900nm thick uniform conformal coatings were deposited onto the inner surface of micro bearings. Chemical and mechanical microstructures were studied via X-ray Photoelectron Spectroscopy (XPS), Raman micro spectroscopy. Wear rates, mechanical and material properties, and other tribological characteristics of W-C:H coated Ni micro bearings were investigated and compared to those of uncoated Ni micro bearings. Uncoated Ni micro bearings, as deposited and annealed at 800o C, experienced severe wear and appeared inadequate for tribological applications. Micro bearings with low tungsten-containing (5% wt) hydrocarbon had higher wear resistance than micro bearings with high tungsten-containing (11% wt) hydrocarbon. During the wear test of the W-C:H coated micro bearings, a transfer layer formed on the counter steel shaft even under very small contact pressure, leading to low steady state friction and high wear resistance.Item PEG hydrogels as anti-fouling coatings for reverse osmosis membranes(2009-05) Sagle, Alyson Conner; Freeman, B. D. (Benny D.); Sharma, Mukul M.Water is becoming increasingly scarce as the demand for fresh water continues to rise. One potential new water resource is purified produced water. Produced water is generated during oil and gas production, and it is often contaminated with emulsified oil, high levels of salt, and particulate matter. Produced water purification using polymer membranes has been investigated, but its implementation is limited by membrane fouling. This study focused on the preparation and application of poly(ethylene glycol) (PEG) hydrogels as fouling-resistant coatings for commercial reverse osmosis (RO) membranes. To prepare fouling-resistant coatings for RO membranes, three series of copolymer hydrogel networks were synthesized using poly(ethylene glycol) diacrylate (PEGDA) as the crosslinker and acrylic acid (AA), 2-hydroxyethyl acrylate (HEA), or poly(ethylene glycol) acrylate (PEGA) as comonomers, and their transport properties were evaluated. The hydrogels have high water uptake and high water permeability, and crosslink density strongly influences water uptake and water permeability. For example, a 100 mol% PEGDA hydrogel contained 61% water by volume, but 80PEGA, which has essentially the same chemical composition but lower crosslink density, contained 72% water by volume. Hydrogel water permeability ranged from 10 to 26 (L [mu]m)/(m² hr bar) and correlates well with water uptake; high water uptake often leads to high water permeability. Additionally, the copolymers have hydrophilic surfaces with a low affinity for oil, based on contact angle measurements using n-decane in water. Commercial RO membranes (AG RO membrane from GE Water and Process Technologies) were coated with PEG hydrogels, and the desalination and fouling resistance properties of the coated membranes were tested. The water flux of coated membranes and a series-resistance model were used to estimate coating thickness; the coatings were approximately 2 [mu]m thick. NaCl rejection for both uncoated and coated membranes was 99.0% or greater. As determined by zeta potential measurements, both uncoated and coated RO membranes are negatively-charged, but coated membranes are less negatively-charged than uncoated RO membranes. Model oil/water emulsions, prepared with either a cationic or an anionic surfactant, were used to probe membrane fouling. In the absence of oil, surfactant charge, and therefore, electrostatic interactions play a significant role in membrane fouling. In the presence of DTAB, a cationic surfactant, the AG RO membrane water flux immediately dropped to 30% of its initial value, but in the presence of SDS, an anionic surfactant, its water flux gradually decreased to 74% of its initial value after 24 hours. However, in both cases, coated membranes exhibited less flux decline than uncoated membranes. Coated membranes also experienced little fouling in the presence of an n-decane/DTAB emulsion. After 24 hours, the water flux of a PEGDA-coated AG RO membrane was 73% of its initial value, while the water flux of an AG RO membrane fell to 26% of its initial value. Conversely, both coated and uncoated membranes fouled significantly in the presence of an n-decane/SDS emulsion, indicating that oil fouling is controlled both by electrostatic and hydrophobic interactions. Overall, this work provides answers to some of the fundamental questions posed regarding the viability of using modified membranes for produced water treatment.