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Thermo-mechanical Stability and Strengthening Mechanisms of Ti/Ni Multilayer Thin Films
Get PDFThesis (Ph.D.)--University of Washington, 2016-03In this work, systematic studies on mechanical and microstructural properties of Ti/Ni multilayer thin films were carried out to understand the coupled size and thermal effects on strengthening mechanisms of metallic multilayer thin films. The size effect comes from the individual layer thickness and total film thickness, and the thermal effect comes from either post-deposition annealing, in-situ high temperature deposition, or localized surface annealing by pulse-laser irradiation. For as-deposited metallic multilayer thin films, the deformation behavior was found to follow the traditional trend from dislocation mediated motions with dislocation pile up along interfaces with layer thickness from microns down to a few tens of nanometers, and single dislocation bowing between interfaces with layer thickness from a few tens of nanometers to a few nanometers, to grain boundary mediated motions with further reduced layer thickness. In addition, strong orientation dependent hardness was observed in as-deposited multilayer thin films. The anisotropic hardness is attributed to dominant deformation mechanism switch from dislocation pile-up against the interfaces to confined layer slip within the layers as the loading direction changes from perpendicular to parallel to the interfaces. A systematic study on Ti/Ni multilayers with layer thickness from 200 nm to 6 nm and annealing temperature up to 500 °C led to the establishment of a coupled layer-thickness and annealing-temperature dependent strengthening mechanism map. For annealed films, grain boundary relaxation is considered to be the initial strengthening mechanism with higher activation temperature required for thicker layers. Under further annealing, solid solution hardening, intermetallic precipitation hardening, and fully intermixed alloy structure continue to strengthen the thin layered films, while recrystallization and grain growth lead to the eventual softening of thick layered films. For films with intermediate layer thickness, a strong orientation dependent hardness behavior is exhibited under high temperature annealing due to mechanism switch from grain growth softening to intermetallic precipitation hardening when changing the loading orientation from perpendicular to parallel to the layer interfaces. Furthermore, deposition temperature induced texture evolution and mechanical strengthening were studied for Ti/Ni multilayer thin films deposited with elevated substrate temperatures up to 500 °C. An obvious substrate-temperature dependent texture strengthening was observed. After low temperature deposition, preferred crystallographic texture were detected for both Ti and Ni layers, and columnar structure was observed to extend through layers, leading to initial strengthening. The columnar structure became more distinct and complete with the increase of substrate temperature, and meanwhile more atomic diffusion and intermixing occur along the Ti/Ni interfaces, promoting the formation of Ti-Ni intermetallic precipitates, and the subsequent mechanical strengthening. After high temperature deposition, columnar Ti-Ni alloys were observed with disintegration of layered structure. Ti-Ni intermetallic was detected with preferred crystallographic texture. Recrystallization was observed with even higher deposition temperature, leading to even higher material strength. Finally, a picosecond pulse laser was utilized to treat Ti/Ni multilayer thin films to induce desired microstructure change and surface strengthening. It was observed that with the increase of pulse-laser energy, the surface morphology evolves from homogeneous grain surface, to a cross-hatched pattern surface, and then to a rough melted surface covered with bubbles, voids and cracks. And the cross-section morphology evolves from a multilayered structure to partially intermixed and eventually fully intermixed structure. Due to the precipitation of Ti-Ni intermetallic phase, laser treatment with high pulse-energy led to surface strengthening on Ti/Ni multilayer thin films. In addition, the film and layer thickness effects on microstructural and mechanical properties were investigated using layer thicknesses of 20 nm and 50 nm, and film thicknesses 500 nm and 1 µm. It was found that thinner film and larger layer thickness requires the least energy to produce the intermixing effect while thicker film and smaller layer thickness requires a lot more energy to produce the desired intermix and mechanical strengthening
High Strain-Rate Material Behavior by Short Pulse Laser-Generated Stress Waves
Get PDFThesis (Ph.D.)--University of Washington, 2014The response of materials when subjected to high strain-rate, high pressure loading can vary drastically from static conditions. Dynamic material behavior arises whenever loading is suddenly applied to a material or structure and it is essential that this behavior is well understood for qualitative and quantitative predictability. Stress waves generated from tabletop short pulse lasers allows the study of material behavior in the pressure range of 10's GPa and strain-rates up to 10<super>8</super> s<super>-1</super>. In this dissertation, a parametric study was first carried out to understand the effect of various parameters on the stress wave generation by laser pulse absorption. The technique was then applied to several material systems ranging from conventional solids such as Si and aluminum, to biological cells and further to nanoporous oxides, to understand their unique response under high strain-rate loading. To better understand the applicability of the technique and the role of the loading parameters on stress generation, a parametric study of the laser fluence, pulse duration, and confinement of the laser absorption volume was performed. Similar stress profiles are observed under high fluence, confined conditions for lasers pulse durations spanning picoseconds to nanoseconds. Low fluence and unconfined absorption with picosecond laser pulses allows an order of magnitude enhancement in strain-rate, while still generating peak stresses relevant to dynamic material behavior. Using the knowledge gained from the parametric study, high strain-rate compressive yielding of aluminum alloys subjected to picosecond and nanosecond pulsed laser loading reveals an elastic-plastic response consistent with observations from other experimental techniques at lower strain-rates. Stress wave evolution as a function of propagation was studied, and no additional rate dependence from picosecond to nanosecond loading was observed. Previous application of the laser spallation technique was applied to measure the adhesion strength of cells on inorganic substrates. Due to the time scale of the interaction, post mortem observations were used to determine the loading required to remove the cells. Transient finite element analysis was carried out to investigate the detachment mechanism responsible for cell decohesion. Failure is driven by large interfacial strains experienced due to the very rapid acceleration of the substrate into the cell. Stress, strain, and interface failure evolution exhibit no rate or cell geometry dependence, indicating the laser spallation techniques ability to measure an intrinsic, undisturbed, and short time scale adhesion strength. The final study conducted was the determination of the dynamic behavior of a novel material system, synthetic nanoporous zeolite, measured by laser-induced loading. High strain-rate loading prompts a ductile to brittle transition for zeolite. Compression experiments demonstrate brittle fracture under moderate loading of several hundred MPa. Spallation measurements were performed with peak stresses up to 3 GPa. A spallation strength of 215 MPa for bulk zeolite was measured, with no dependency on peak stress. The low spallation strength, as well as scanning electron microscope observations of the spallation morphology, confirms that bulk zeolite behaves as the brittle solid at high strain-rates
Mechanical Characterization of Nanoporous Thin Films by Nanoindentation and Laser-induced Surface Acoustic Waves
No full textThesis (Ph.D.)--University of Washington, 2014Thin films represent a critical sector of modern engineering that strives to produce functional coatings at the smallest possible length scales. They appear most commonly in semiconductors where they form the foundation of all electronic circuits, but exist in many other areas to provide mechanical, electrical, chemical, and optical properties. The mechanical characterization of thin films has been a continued challenge due foremost to the length scales involved. However, emerging thin films focusing on materials with significant porosity, complex morphologies, and nanostructured surfaces produce additional difficulties towards mechanical analysis. Nanoindentation has been the dominant thin film mechanical characterization technique for the last decade because of the quick results, wide range of sample applicability, and ease of sample preparation. However, the traditional nanoindentation technique encounters difficulties for thin porous films. For such materials, alternative means of analysis are desirable and the lesser known laser-induced surface acoustic wave technique (LiSAW) shows great potential in this area. This dissertation focuses on studying thin, porous, and nanostructured films by nanoindentation and LiSAW techniques in an effort to directly correlate the two methodologies and to test the limits and applicabilities of each technique on challenging media. The LiSAW technique is particularly useful for thin porous films because unlike indentation, the substrate is properly accounted for in the wave motion analysis and no plastic deformation is necessary. Additionally, the use of lasers for surface acoustic wave generation and detection allows the technique to be fully non-contact. This is desirable in the measurement of thin, delicate, and porous films where physical sample probing may not be feasible. The LiSAW technique is also valuable in overcoming nanoscale roughness, particularly for films that cannot be mechanically polished, since typical SAW wavelengths are micrometers in scale whereas indentation depths are usually confined to the nanometer scale. This dissertation demonstrates the effectiveness of LiSAW on both thin porous layers and rough surfaces and shows the challenges faced by nanoindentation on the same films. Zeolite thin films are studied extensively in this work as a model system because of their porous crystalline framework and enormous economic market. Many types of zeolite exist and their widely varying structures and levels of porosity present a unique opportunity for mechanical characterization. For a fully dense ZSM-5 type zeolite with wear and corrosion resistance properties, nanoindentation was used to compare its mechanical properties to industrial chromium and cadmium films. Through tribological and indentation tests, it was shown that the zeolite film possesses exceptional resilience and hardness therefore demonstrating superior wear resistance to chromium and cadmium. This also highlighted the quality of nanoindentation measurements on thick dense layers where traditional nanoindentation excels. Nanoindentation was then performed on porous and non-porous MFI zeolite films with low-k (low dielectric constant) properties. These films were softer and much thinner than the ZSM-5 coatings resulting in significant substrate effects, evidenced by inflation of the measurements from the hard silicon substrate, during indentation. Such effects were avoided with the LiSAW technique on the same films where properties were readily extracted without complications. An alternative indentation analysis method was demonstrated to produce accurate mechanical measurements in line with the LiSAW results, but the non-traditional technique requires substantial computational intensity. Thus LiSAW was proven to be an accurate and efficient means of mechanical characterization for thin porous layers. The case for LiSAW was further supported by utilizing the technique on a porous nanostructured V2O5 electrode film. The surface roughness, on the same scale as indentation depths, created difficulty in obtaining consistent nanoindentation results. Since the film was too delicate for mechanical polishing, the nanoindentation results possessed a high level of uncertainty. It was demonstrated that the LiSAW technique could extract the mechanical properties from such layers without substrate effects and with higher accuracy than nanoindentation. The research in this dissertation directly demonstrates the areas where nanoindentation excels and the areas where it encounters difficulty. It is shown how the LiSAW technique can be an efficient alternative in the challenging areas through its dependence on bulk dispersive wave motion rather than localized deformation. Thus, LiSAW opens up many avenues towards the mechanical characterization of thin, porous, soft, or rough films. Nanoindentation remains an extremely useful technique for thin film characterization, especially with the alternative analysis adaptation. However, as films continue trending towards smaller length scales, more complex porous morphologies, and engineered nanoscale surfaces, LiSAW may well become an equally valuable and indispensable technique
Interface Engineering to Enhance the Mechanical Performance of Aluminum Thin Film Deposited on PET Substrate
No full textThesis (Master's)--University of Washington, 2020Al films magnetron sputtered on PET substrates with varying interfacial adhesion were produced by controlling the time of the plasma etching prior to the Al deposition. Uniaxial tension and laser-induced thin film spallation tests were performed on the Al film and PET substrate to investigate the effect of the adhesion on the ductility of the Al film. In addition, the effect of substrate thickness was studied using substrates with varying thicknesses. The laser spallation measurement revealed that the adhesion between the Al film and PET substrate increased from less than 5.3 MPa to 37 MPa by applying 10 min-plasma etching. The surface analysis indicated the chemical state of the surface is the dominant factor for the improved interfacial adhesion. The uniaxial tensile test of the PET supported Al film showed that the failure strain of Al film was improved by applying the plasma etching. It is assumed that the shear stress caused by the mismatch in the strain of Al and PET leads to local delamination of the Al film. When the plasma etching was applied, the interfacial adhesion was considered to increase strong enough to prevent the delamination. Accordingly, the PET substrate retarded the strain concentration in Al film by constraining the deformation of Al film, leading to an increase in the failure strain of Al film. The failure strain of Al film with 50 μm-thick PET substrate was substantially lower than the Al film with 12, and 25 μm-thick PET. According to the shear lag theory, the shear stress at the interface increases with an increase in the load acting on the PET substrate. Thus, it is considered that the shear stress was high enough to cause the delamination of Al film even the interfacial adhesion increased with the plasma etching. The Al film deposited on 25μm-thick PET showed the maximum failure strain when the plasma etching time was 5 min. The result of the contact angle measurement indicated the effect of the chemical modification reaches constant when the plasma etching time was more than 5 min. On the other hand, the line surface profile indicated the plasma etching might form a periodic swell on the surface of the PET, and the swell appeared to become sharp as the plasma etching time increased. It is possible that the swell decreased the failure strain of the Al film by initiating the stress concentration during the tensile test. Due to the combination of the two effects, the failure strain of the Al film exhibited the maximum value when the plasma etching time was 5 min
Advancements in Optical Properties of Thermochromic VO2 Films through Experimental and Numerical Investigations
Get PDFThesis (Ph.D.)--University of Washington, 2016-03The Department of Energy reports that buildings consume more than 40% of primary energy in the U.S. and that this trend will continue for the foreseeable future. Furthermore, windows constitute a major path for energy losses from buildings and therefore also present a significant opportunity for efficiency improvement and waste reduction. With this in mind, the work in this dissertation is focused on improving the control of solar and thermal radiation through windows. These radiation spectra can be controlled independently because they peak at different wavelengths due to the much higher temperature (5500 °C) of the Sun compared to objects on Earth (25 °C). In this work, a thermochromic material is utilized to control solar irradiance and a low-emissive (low-E) material is used to control thermal radiation. Thermochromic materials possess optical properties that change in response to temperature and low-E coatings are reflective in the mid-infrared (thermal) region. VO2 is an exciting candidate for thermochromic coatings because its transmittance in the visible region is relatively constant, but its reflectance in the IR increases significantly with temperature. The main technical issues limiting VO2 are luminous transmittance (Tlum) that is too low and a transition temperature (τc) that is too high. For the low-E coating, (SnO2)x(In2O3)1-x (ITO glass) was chosen because it has both high luminous transmittance and low emissivity. In this dissertation it is shown that τc of VO2 can be lowered from 50 to 45 °C by reducing the grain size of the film from 70 to 31 nm. In the area of luminous transmittance, TiO2 is investigated as an anti-reflective coating which can be used to increase Tlum. Later in this work, it is demonstrated that the energy efficiency gained through VO2 can be further improved by combining it with a low-E coating. The multilayer design combines anti-reflection in the visible region, thermochromism the near-IR and low emissivity in the mid-IR for an outstanding energy-efficient coating across the solar and thermal spectra. This dissertation contributes knowledge that helps reduce the barriers which currently limit thermochromic materials from being utilized for energy efficiency in built environments
Strain Engineering of Monolayer MoS2 for Electrocatalysis Enhancement and Band Structure Modulation
No full textThesis (Ph.D.)--University of Washington, 2021In this dissertation, strain engineering is utilized to modulate the bandgap structure of monolayer MoS2 and enhance its catalytic performances. A general electron-beam lithography fabrication process is developed to fabricate a flexible electrochemical nanodevice, which is further combined with a biaxial beam bending method to apply biaxial tensile strain on the monolayer MoS2 samples. After introducing the tensile strain, the electronic states and the electrochemical capacitance of the monolayer MoS2 are characterized using two types of experiments. In the first experiment, the newly developed nanodevice is applied to measure the Density of States (DOS) of the MoS2. Measuring the intrinsic DOS and defect states of atomically thin layered materials are of profound importance for understanding their exotic physical properties and how to apply them in real life. Typically, the measurements of defect states require either ultra-high vacuum or low temperature. Here, we measured the defect states of two-dimensional (2D) materials in an ambient environment rooting on the electrochemical capacitance. The highest energy resolution of the electrochemical capacitance spectrum approaches 116 meV, close to the theoretical limit of thermal broadening at room temperature (3.5 kBT = 91 meV).
Meanwhile, the absolute energy positions of the DOS can be obtained by integrating a reference electrode in our electrochemical system. With this approach, the DOS and defect states in monolayer MoS2 are mapped out by measuring its electrochemical capacitance. We can also monitor the DOS evolution during the electrochemical reactions. Our preliminary result on the mercury ions adsorption shows that the Mo defects in monolayer MoS2 dominate the chemical adsorption procedure. This work paves the way towards a new platform for measuring the intrinsic DOS and defect states of 2D materials under an ambient environment.
Following the electrochemical capacitance measurements, the hydrogen evolution reaction property was determined using the monolayer MoS2 as the catalyst. The catalytic property is highly dependent on the number of their active sites and the turnover frequency (TOF) of catalysts. While increasing the number of active sites eventually approaches a saturation point, theoretically raising the TOF can improve the catalysis performance linearly. On the other side, the TOF has a strong correlation with the DOS of materials. Thus, it is critical to develop a strategy for establishing a relationship between the electronic structure and TOF. The purpose of this endeavor was to use strain engineering to uncover this missing puzzle piece. Tensile strain contributes to spin-orbital coupling in monolayer MoS2, resulting in an increased electronic state near the conduction band edge. As demonstrated experimentally, this unusual electronic structure in strained monolayer MoS2 causes an unanticipated three-zone catalysis process and an ultrahigh TOF of 27.75 s-1 in the hydrogen evolution reaction, which is superior to platinum. This study establishes a connection between the strain condition, electronic states, and catalytic property
Laser Surface Treatment for Targeted Strengthening of Metallic Multilayer Thin Films
No full textThesis (Ph.D.)--University of Washington, 2024In this dissertation, pulsed laser surface treatment is applied to two multilayer thin film material systems, Ti/Ni and Cu/Al, to locally strengthen the surface layers of a multilayer film while preserving the original structure and properties of the base layers. Picosecond laser surface treatment was applied to Ti/Ni to investigate the effect of laser pulse energy, individual layer thickness, and total film thickness on the surface strengthening. Intermetallic phase precipitation was shown to be a dominant mechanism for laser-induced strengthening of the Ti/Ni multilayer. Therefore, Cu/Al, a material with a plethora of potential intermetallic phases, was treated with nano- and picosecond pulse lasers to generate intermetallic phases with the goal of confined surface strengthening. The laser surface treatment method was then applied as an array to optimize the laser treatment parameters for scaling to industrial applications.Picosecond pulsed laser treatment of Ti/Ni showed that for increasing laser pulse energy, the multilayer cross-section initially consists of a clear multilayer structure, followed by the onset of an intermixed layer at the film surface that grows in thickness as energy increases until the film is fully intermixed. Intermixing was not sufficient to cause surface strengthening, instead there was an increase in hardness after intermetallic phases were generated at higher pulse energy. The effect of individual layer thickness was compared between 20 nm and 50 nm layers, and the results showed that the formation of an intermixed layer and the onset of intermetallic phase precipitation required less energy for larger layers. Incorporating the effect of a 500 nm and 1 µm total film thickness shows more energy is required to generate the intermixed layer and reach the critical energy to form intermetallic phases for thicker Ti/Ni films.
Initial testing of the Cu/Al multilayer system revealed a reflectivity of over 90% when treated with light at 1064 nm, necessitating the addition of a 25 nm Ti capping layer to improve absorption of the incident energy. Nanosecond and picosecond pulsed laser treatments were applied to the Cu/Al + Ti multilayer films to generate intermetallic phases and measure the corresponding increase in hardness. Surface treatment with the nanosecond laser resulted in a smooth, microcracked surface with no intermixed layer but a dramatic increase in hardness to a maximum of over 10 GPa for nanoindentation at shallow contact depth that decreases to the as-deposited hardness of around 4 GPa with increasing contact depth. Picosecond laser treatment results in the formation of a highly textured surface and an intermixed layer with clear intermetallic phase generation, however the resulting strengthening is generally limited to 5-7 GPa over a range of shallow contact depths.
Picosecond laser treatment was applied in a hexagonal array pattern to extend the treatment from a single location test to an area treatment. Arrays were created with 0% shot overlap as well as 50% shot overlap in x and y directions. Both single-pulse arrays and multiple pulse arrays were generated, with an emphasis on developing single-pulse arrays that show clear intermetallic phase generation and a corresponding increase in hardness to avoid complications associated with multi-pulse testing such as thermal accumulation
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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