DRUM Community: Materials Science & Engineering

Nano-engineering and Simulating Electrostatic Capacitors for Electrical Energy Storage

Sun, 01 Jan 2012 00:00:00 GMT

Title: Nano-engineering and Simulating Electrostatic Capacitors for Electrical Energy Storage Authors: Haspert, Lauren Abstract: Electrical energy storage solutions with significantly higher gravimetric and volumetric energy densities and rapid response rates are needed to balance the highly dynamic, time-variant supply and demand for power. Nanoengineering can provide useful structures for electrical energy storage because it offers the potential to increase efficiency, reduce size/weight, and improve performance. While several nanostructured devices have shown improvements in energy and/or power densities, this dissertation focuses on the nanoengineering of electrostatic capacitors (ESC) and application of these high-power electrostatic capacitors in electrical energy storage systems. A porous nano-template with significant area enhancement per planar unit area coated with ultra-thin metal-insulator-metal (MIM) layers has shown significant improvements in areal capacitance. However, sharp asperities inherent to the initial nano-template localized electric fields and caused premature (low field) breakdown, limiting the possible energy density (E = ½ CV2/m). A nanoengineering strategy was identified for rounding the template asperities, and this showed a significant increase in the electrical breakdown strength of the device, providing rapid charging and discharging and an energy density of 1.5 W-h/kg - which compares favorably with the best state-of-the-art devices that provide 0.7 W-h/kg. The combination of the high-power ESC with a complementary high-energy-density electrochemical capacitor (ECC) was modeled to evaluate methods resulting in the combined power-energy storage capabilities. While significant improvements in the ESC's energy density were reported, the nanodevices display nonlinear leakage resistance, which directly relates to charge retention. The ECC has distinctly different nonlinearities, but can retain a greater density of charge for significantly longer, albeit with slower inherent charging and discharging rates than the ESC. The experimentally derived dynamic model simulating the nonlinear performance of the ESC and ECC devices indicated this hybrid-circuit reduces the time required to charge the ECC to near-maximum capacity by a factor of up to ~ 12.

Nanomechanical Properties and Buckling Instability of Plasma Induced Damaged Layer on Polystyrene

Sun, 01 Jan 2012 00:00:00 GMT

Title: Nanomechanical Properties and Buckling Instability of Plasma Induced Damaged Layer on Polystyrene Authors: Lin, Tsung-Cheng Abstract: In this thesis we report on an investigation of an elastic buckling instability as a driving force for the roughening of polystyrene, a model resist, during Ar+ plasma etching. Polystyrene films etched by pure Ar+ plasma with different ion energies were characterized using both atomic force microscopy topography and force curve measurements. By using height-height correlation function in analyzing the AFM measured topography images, we find that surface corrugation of etched polystyrene film surfaces all display a dominant wrinkle wavelength (ë), which is a function of ion energy. Next, we characterized the mechanical properties of these samples using AFM force curve measurements in an controlled ambient environment. We analyzed the measured force curves using a systematic algorithm based on statistical fitting procedures, and taking into account the adhesive interaction, in order to determine the effective elastic modulus of the films. We find that the effective elastic modulus (EBL) of the etched samples increases monotonically with increasing ion energy, but the changes are rather subtle as compared to the elastic modulus (EPS) of the unetched one. In order to test the validity of a buckling instability as the mechanism for surface roughening in our polystyrene-Ar plasma system, the elastic modulus of individual layer (i.e. ion-damaged layer plus unmodified foundation) needs to be determined. We present a determination of the damaged layer elastic modulus (EDL) from the effective elastic modulus of the damaged layer/polystyrene bilayer structure (EBL), based upon a finite element method simulation taking into account the thickness and elastic modulus of the damaged layers. We extract the damaged layer elastic modulus versus etching ion energy initially within the approximation of a spherical tip in contact with a flat sample surface. We next extend our model, by considering a periodic corrugated film surface, with its amplitude and wavelength determined by AFM, to take into account the effect of roughness induced by plasma exposure. The damaged layer elastic modulus extracted from these two approximations gives of quantitative agreement, and thus evidence for the correlation between buckling instability and plasma-induced roughening.

DIRECTED SELF-ASSEMBLY OF NANOSTRUCTURES AND THE OBSERVATIONS OF SELF-LIMITING GROWTH OF MOUNDS ON PATTERNED CRYSTAL SURFACE DURING EPITAXIAL GROWTH

Sun, 01 Jan 2012 00:00:00 GMT

Title: DIRECTED SELF-ASSEMBLY OF NANOSTRUCTURES AND THE OBSERVATIONS OF SELF-LIMITING GROWTH OF MOUNDS ON PATTERNED CRYSTAL SURFACE DURING EPITAXIAL GROWTH Authors: Lin, Chuan-Fu Abstract: In this thesis I describe an approach toward investigating moving interfaces, surface stabilities and directing self assembly of nanostructures, using lithographic patterning to perturb a flat crystalline surface over a range of spatial frequencies, followed by epitaxial growth. GaAs(001) shows a transient instability toward topographical perturbations. We model this behavior using an Ehrlich-Schwoebel (ES) barrier which impedes the diffusion of atoms across steps from above. We show via both kinetic Monte Carlo (kMC) simulations and molecular beam epitaxial (MBE) growth experiments that patterning in the presence of an ES barrier can be used to direct the self assembly of mounds. Second, as we track the time evolution of mound formation, we find the evidence of "Self-Limiting Growth" on surfaces - we find that in the initial stage of growth, the pattern directs the spontaneous formation of multilayer islands at 2-fold bridge sites between neighboring nanopits along [110] crystal orientation, seemingly due to the presence of an Ehrlich-Schwoebel barrier and the effect of heterogeneous nucleation sites on the surfaces. However, as growth continues, the height of mounds at 2-fold bridge sites "self-limits": the mounds cease to grow. Beyond this point an initially less favored 4-fold bridge sites dominate, and a different pattern of self assembled mounds begins. The observation of self-limiting behavior brings us new understanding of mechanism for crystal growth. We also find that the transient amplification of pattern corrugation during growth is correlated with self-limiting behavior of mounds. We propose that a minimum, `critical terrace size' at the top of each mound is responsible for the observed self-limiting growth behavior. Finally, the observation of the sequence of the mounds forming on the patterned surfaces gives us rather direct evidence that the formation of growth mounds on the surface is a nucleated process, rather than an instability.

MICROSTRUCTURAL CHARACTERIZATION OF ULTRASONIC IMPACT TREATED AL-MG ALLOY

Sun, 01 Jan 2012 00:00:00 GMT

Title: MICROSTRUCTURAL CHARACTERIZATION OF ULTRASONIC IMPACT TREATED AL-MG ALLOY Authors: Tran, Kim Ngoc Thi Abstract: Aluminum 5456-H116 has high as-welded strength, is formable, and highly corrosion resistant, however, it can become sensitized when exposed to elevated temperatures for a prolonged time. Sensitization results in the formation of a continuous β phase at the grain boundaries that is anodic to the matrix. Thus the grain boundaries become susceptible to stress corrosion cracking (SCC) and intergranular corrosion cracking (IGC). Cracking issues on aluminum superstructures have prompted the use of a severe plastic deformation processes, such as ultrasonic impact treatment (UIT), to improve SCC resistance. This study correlated the effects of UIT on the properties of 5456-H116 alloy to the microstructural evolution of the alloy and helped develop a fundamental understanding of the mechanisms that cause the microstructural evolution. Ultrasonic impact treatment produces a deformed layer at the surface ~ 10 to 18 µm thick that is characterized by micro-cracks, tears, and voids. Ultrasonic impact treatment results in grain refinement within the deformation layer and extending below the deformed layer. The microstructure exhibits weak crystallographic texture with larger fraction of high angle grain boundaries. Nanocrystalline grains within the deformation layer vary in size from 2 to 200 nm in diameter and exhibit curved or wavy grain boundaries. The nanocrystalline grains are thermally stable up to 300°C. Above 300°C, grain growth occurs with an activation energy of ~ 32 kJ/mol. Below the deformation layer, the microstructure is characterized by submicron grains, complex structure of dislocations, sub-boundaries, and Moiré fringes depicting overlapping grains. The deformation layer does not exhibit the presence of a continuous β phase, however below the deformation layer; a continuous β phase along the grain boundaries is present. In general the highest hardness and yield strength is at the UIT surface which is attributed to the formation of nanocrystalline grains. Although the highest hardness and yield strength was observed at the UIT surface, the results were mixed with some lower values. The lower hardness and yield strength values at the UIT surface are attributed to the voids and micro cracking/micro voids observed in the deformation layer. The fracture mode was transgranular ductile fracture with micro void coalescence and dimples. Both UIT and untreated material exhibit similar levels of intergranular corrosion susceptibility. Corrosive attack was intergranular with slightly deeper attack in the untreated material. Numerical simulation modeling showed that the calculated residual stress under the tool, ~80 MPa, is of the same order of magnitude as the compressive residual stresses measured by XRD measurements near the surface. Modeling also showed that high effective strains were induced almost immediately. The UIT process also resulted in rapid localized heating to a maximum temperature of ~32°C during the first eleven pin tool cycles. The model also showed that during UIT processing, the material undulates as the pin tool impacts and retracts from the surface of the material. The undulations represent the elastic response of the surface to the compressive stresses built up during a pin tool cycle.