In this research, the amorphous/nanocrystalline ZrCu/Cu multilayered thin films with various conditions such as individual layer thickness, total layer thickness, and interface type have been successfully fabricated by the multi-gun sputtering processes. To investigate the mechanical properties and deformation behaviors of substrate-supported ZrCu/Cu multilayered thin films, these films deposited on the Cu or polyimide foils were prepared for tensile testing. Firstly, the tensile behaviors of the monolithic ZrCu thin film metallic glass and the ZrCu/Cu multilayered thin films deposited on the pure Cu foils are systematically examined. The extracted tensile modulus and strength of the 1-£gm-thick multilayered thin films are in good agreement with the theoretical iso-strain rule of mixture prediction. The extracted 2-£gm-thick multilayered film data are lower, but can be corrected back by considering the actual intact cross-sectional area during the tensile loading. Moreover, the current results reveal that the ZrCu/Cu multilayered coating exhibit much better tensile performance than the monolithic ZrCu coating. It indicates that the amorphous/nanocrystalline multilayered thin film structure can certainly enhance the mechanical properties of monolithic thin film metallic glasses under tension. Secondly, for the further investigation of tensile response, the polyimide-supported amorphous/nanocrystalline ZrCu/Cu multilayered thin films with various individual layer thicknesses from 10 to 100 nm were prepared. The relatively soft, smooth, and flexible polyimide foils as the substrates in this experiment can undergo sufficient deformation under In this research, the amorphous/nanocrystalline ZrCu/Cu multilayered thin films with various conditions such as individual layer thickness, total layer thickness, and interface type have been successfully fabricated by the multi-gun sputtering processes. To investigate the mechanical properties and deformation behaviors of substrate-supported ZrCu/Cu multilayered thin films, these films deposited on the Cu or polyimide foils were prepared for tensile testing. Firstly, the tensile behaviors of the monolithic ZrCu thin film metallic glass and the ZrCu/Cu multilayered thin films deposited on the pure Cu foils are systematically examined. The extracted tensile modulus and strength of the 1-£gm-thick multilayered thin films are in good agreement with the theoretical iso-strain rule of mixture prediction. The extracted 2-£gm-thick multilayered film data are lower, but can be corrected back by considering the actual intact cross-sectional area during the tensile loading. Moreover, the current results reveal that the ZrCu/Cu multilayered coating exhibit much better tensile performance than the monolithic ZrCu coating. It indicates that the amorphous/nanocrystalline multilayered thin film structure can certainly enhance the mechanical properties of monolithic thin film metallic glasses under tension. Secondly, for the further investigation of tensile response, the polyimide-supported amorphous/nanocrystalline ZrCu/Cu multilayered thin films with various individual layer thicknesses from 10 to 100 nm were prepared. The relatively soft, smooth, and flexible polyimide foils as the substrates in this experiment can undergo sufficient deformation under tension. The modulus and strength of the multilayered thin film are again demonstrated to be consistent with the theoretical iso-strain rule of mixture values. As the individual layer thickness decreases from 100 to 10 nm, the Young¡¦s moduli are only varied slightly. However, the maximum tensile stress exhibits a highest value for the 25 nm layer thickness. The higher crack spacing, or the lower crack density, of this 25 nm multilayer film leads to the highest strength. Thirdly, to avoid the stress and strain incompatibility owing to the mismatch of elastic modulus and strength levels from the connected amorphous/nanocrystalline layers, the Cu-supported amorphous/nanocrystalline ZrCu/Cu multilayered thin films with sharp and graded interfaces were successfully sputtered and examined by tensile testing. The extracted tensile properties of the multilayered films can be compared with the predicted values based on the two-phase and three-phase iso-strain rule of mixture model. The multilayered films with graded interfaces, each about 50 nm thick, consistently exhibit higher tensile strength and elongation. This can be rationalized by the reduced stress and strain incompatibility along the interfaces.

Nanocrystalline materials have been under extensive study in the past two decades. The reduction in grain size induces many abnormal behaviors in the properties of nanocrystalline materials, that have been investigated systematically and quantitatively. As one of the most fundamental relations in materials science, the structure-property relation should still apply on materials of nano-scale grain sizes. The characterization of grain boundaries (GBs) and related entities remains a big obstacle to understanding the structure-property relation in nanocrystalline materials. It is challenging experimentally to determine the topological properties of polycrystalline materials due to the complex and disordered grain boundary network presented in the nanocrystalline materials. The constantly improving computing power enables us to study the structure-property relation in nanocrystalline materials via Monte Carlo and molecular dynamic simulations. In this study, we will first propose a geometrical construction method based on inverse Monte Carlo simulation to generate digital microstructures with desired topological properties such as grain size, interface area, triple junction length as well as their statistical distributions. The influences on the grain shapes by different topological properties are studied. Two empirical geometrical laws are examined including the Lewis rule and Aboav-Weaire law. Secondly, defect free nanocrystalline Copper (nc-Cu) samples are generated by filling atoms into the Voronoi structure and then relaxed by molecular dynamics simulations. Atoms in the relaxed nc-Cu samples are then characterized into grain atoms, GB interface atoms, GB triple junction atoms and vertex atoms using a newly proposed method. Atoms in each GB entity can also be identified. Next, the topological properties of nc-Cu samples before and after relaxation are calculated and compared, indicating that there exists a physical limit in the number of atoms to form a stable grain boundary interface and triple junction in nanocrystalline materials. In addition, we are able to obtain the statistical averages of geometrical and thermal properties of atoms across each GB interfaces, the so-called GB profiles, and study the grain size, misorientation and temperature effects on the microstructures in nanocrystalline materials. Finally, nc-Cu samples with different topological properties are deformed under simple shear using MD simulation in an attempt to study the structure-property relation in nanocrystalline materials.

The composition-dependent Curie temperature and bias-dependant dielectric permittivity of Barium Strontium Titanate (BST) makes it very attractive for tunable application in the RF/Microwave regime. In this research work, the performance of BST varactors fabricated on the conventional Pt/Ti/SiO2/Si bottom electrode stack were compared with those fabricated using chemical vapor deposited Nanocrystalline Diamond (NCD) as the diffusion barrier layer instead of SiO2. The varactors fabricated on NCD films displayed much better symmetry in capacitance-voltage behavior and better overall quality factors than varactors fabricated on SiO2. The improvement in performance can be attributed to existence of stable interfaces in the devices fabricated on NCD which reduced the bottom electrode losses at high frequencies. The SiO2 based BST varactors on the other hand displayed better reliability and breakdown fields. The main purpose of this research work is to develop a robust Metal Insulator Metal (MIM) structure to achieve better all round performance of BST varactors. In the second part of this research work, the prospect of developing diamond based layered Surface Acoustic Wave (SAW) devices using Ba0.8Sr0.2TiO3 as the piezoelectric layer is investigated. Structural characterization of BST thin films deposited on Si/NCD/Pt and Si/SiO2/Ti/Pt stack were performed using X-Ray Diffraction (XRD) and Atomic Force Microscopy (AFM). Cross-sectional studies on the two stacks were performed using Scanning Electron Microscopy (SEM). X-Ray Mapping (XRM) was then done to ascertain the quality of the interfaces and to check for interdiffusion between layers. MIM structures in the Coplanar Waveguide (CPW) configuration were fabricated using conventional lithography and etching techniques for high frequency measurements. The performance of the fabricated varactors was characterized from 100 MHz to 1 GHz. For the SAW application, structural characterization of Ba0.8Sr0.2TiO3 on Chemical Vapor Deposited (CVD) diamond was done and the deposition procedure was optimized to obtain thick BST films. SAW bandpass filters and resonators were designed wherein the device geometry was varied over a wide range in order to characterize the variation in device performance with geometry. Finally interdigital capacitor structures were fabricated and used for conducting Curie temperature measurements on the deposited BST films in order to determine the operation range of the deposited BST films.

Nanocrystalline (NC) metals with averaged grain size smaller than 100 nm have shown promising mechanical properties such as higher hardness and toughness than conventional coarse-grained metals. Unlike conventional metals in which the deformation is controlled by dislocation activities, the microstructural evolution in NC metals is mainly dominated by grain rotation and stress-driven grain boundary motion (SDGBM) due to the high density of grain boundaries (GBs). SDGBM is thus among the most studied modes of microstructural evolution in NC materials with particular interests on their fundamental atomistic mechanisms. In the first part of this thesis, molecular dynamics simulations were used to investigate the influences of Triple Junctions (TJs) on SDGBM of symmetric tilt GBs in copper by considering a honeycomb NC model. TJs exhibited asymmetric pinning effects to the GB migration and the constraints by the TJs and neighboring grains led to remarkable non-linear GB motion in directions both parallel and normal to the applied shear. Based on these findings, a generalized model for SDGBM in NC Cu was proposed. In the second part, the interaction of SDGBM with crack, voids and precipitates was investigated. It was found that depending on the GB structure, material type and temperature, there is a competition between different atomistic mechanisms such as crack healing, recrystallization and GB decohesion. It is hoped that the findings of this work could clarify the micro-mechanisms of various experimental phenomena such as grain refinement in metals during severe plastic deformation, which can be used to design optimized route of making stabilized bulk NC metals. / February 2016

This research work presents the first comprehensive study on nanocrystalline materials produced in bulk quantities using a novel continuous electrodeposition process. A series of nanocrystalline Ni and Ni-Fe alloy continuous foils were produced and an intensive investigation into their structure and various properties was carried out. High-resolution transmission electron microscopy (HR-TEM) revealed the presence of local strain at high and low angle, and twin boundaries. The cause for these local strains was explained based on the interpretation of non-equilibrium grain boundary structures that result when conditions of compatibility are not satisfied. HR-TEM also revealed the presence of twin faults of the growth type, or “growth faults”, which increased in density with the addition of Fe. This observation was found to be consistent with a corresponding increase in the growth fault probabilities determined quantitatively using X-ray diffraction (XRD) pattern analysis. Hardness and Young’s modulus were measured by nanoindentation. Hardness followed the regular Hall-Petch behaviour down to a grain size of 20 nm after which an inverse trend was observed. Young’s modulus was slightly reduced at grain sizes less than 20 nm and found to be affected by texture. Microstrain based on XRD line broadening was measured for these materials and found to increase primarily with a decrease in grain size or an increase in intercrystal defect density (i.e., grain boundaries and triple junctions). This microstrain is associated with the local strains observed at grain boundaries in the HR-TEM image analysis. A contribution to microstrain from the presence of growth faults in the nanocrystalline Ni-Fe alloys was also noted. The macrostresses for these materials were determined from strain measurements using a two-dimensional XRD technique. At grain sizes less than 20 nm, there was a sharp increase in compressive macrostresses which was also owed to the corresponding increase in intercrystal defects or interfaces in the solid.

This research work presents the first comprehensive study on nanocrystalline materials produced in bulk quantities using a novel continuous electrodeposition process. A series of nanocrystalline Ni and Ni-Fe alloy continuous foils were produced and an intensive investigation into their structure and various properties was carried out. High-resolution transmission electron microscopy (HR-TEM) revealed the presence of local strain at high and low angle, and twin boundaries. The cause for these local strains was explained based on the interpretation of non-equilibrium grain boundary structures that result when conditions of compatibility are not satisfied. HR-TEM also revealed the presence of twin faults of the growth type, or “growth faults”, which increased in density with the addition of Fe. This observation was found to be consistent with a corresponding increase in the growth fault probabilities determined quantitatively using X-ray diffraction (XRD) pattern analysis. Hardness and Young’s modulus were measured by nanoindentation. Hardness followed the regular Hall-Petch behaviour down to a grain size of 20 nm after which an inverse trend was observed. Young’s modulus was slightly reduced at grain sizes less than 20 nm and found to be affected by texture. Microstrain based on XRD line broadening was measured for these materials and found to increase primarily with a decrease in grain size or an increase in intercrystal defect density (i.e., grain boundaries and triple junctions). This microstrain is associated with the local strains observed at grain boundaries in the HR-TEM image analysis. A contribution to microstrain from the presence of growth faults in the nanocrystalline Ni-Fe alloys was also noted. The macrostresses for these materials were determined from strain measurements using a two-dimensional XRD technique. At grain sizes less than 20 nm, there was a sharp increase in compressive macrostresses which was also owed to the corresponding increase in intercrystal defects or interfaces in the solid.

A series of experiments was carried out to compare the structural and electronic properties of intrinsic nanocrystalline silicon (nc-Si:H) thin films deposited via continuous wave (cw) and pulsed (p)-PECVD at 150°C substrate temperature. Working at this temperature allows for the easy transfer of film recipes from glass to plastic substrates in the future. During the p-PECVD process the pulsing frequency was varied from 0.2 to 50 kHz at 50% duty cycle. Approximately 15% drop in the deposition rate was observed for the samples fabricated in p-PECVD compared to cw-PECVD. The optimum crystallinity and photo (σph) and dark conductivity (σD) were observed at 5 kHz pulsing frequency, with ~10% rise in crystallinity and about twofold rise in the σph and σD compared to cw-PECVD. However, for both the cw and p-PECVD nc-Si:H films, the observed σph and σD were one to two orders and three orders of magnitude higher respectively than those reported in literature. The average activation energy (EA) of 0.16 ∓ 0.01 eV for nc-Si:H films deposited using p-PECVD confirmed the presence of impurities, which led to the observation of the unusually high conductivity values. It was considered that the films were contaminated by the impurity atoms after they were exposed to air. Following the thin film characterization procedure, the optimized nc-Si:H film recipes, from cw and p-PECVD, were used to fabricate the absorber layer of thin film solar cells. The cells were then characterized for J-V and External Quantum Efficiency (EQE) parameters. The cell active layer fabricated from p-PECVD demonstrated higher power conversion efficiency (η) and a maximum EQE of 1.7 ∓ 0.06 % and 54.3% respectively, compared to 1.00 ∓ 0.04 % and 48.6% respectively for cw-PECVD. However, the observed η and EQE of both the cells were lower than a reported nc-Si:H cell fabricated via p-PECVD with similar absorber layer thickness. This was due to the poor Short-circuit Current Density (Jsc), Open-circuit Voltage (Voc), and Fill Factor (FF) of the cw and p-PECVD cells respectively, compared to the reported cell. The low Jsc resulted from the poor photocarrier collection at longer and shorter wavelengths and high series resistance (Rseries). On the other hand, the low Voc stemmed from the low shunt resistance (Rsh). It was inferred that the decrease in the Rsh occurred due to the inadequate electrical isolation of the individual cells and the contact between the n – layer and the front TCO contact at the edge of the p-i-n deposition area. Additionally, the net effect of the high Rseries and the low Rsh led to a decrease in the FF of the cells.

Cellulose is one of the most abundant biopolymers found in nature. Cellulose based derivatives have a number of advantages including recyclability, reproducibility, biocompatibility, biodegradability, cost effectiveness and availability in a wide variety of forms. Due to the benefits of cellulose based systems, this research study was aimed at developing novel cellulosic nanoparticles with potential pharmaceutical and personal care applications. Two different cellulosic systems were evaluated, each with its own benefits and proposed applications. The first project involves the synthesis and characterization of polyampholyte nanoparticles composed of chitosan and carboxymethyl cellulose (CMC), a cellulosic ether. EDC carbodiimide chemistry and inverse microemulsion technique was used to produce crosslinked nanoparticles. Chitosan and carboxymethyl cellulose provide amine and carboxylic acid functionality to the nanoparticles thereby making them pH responsive. Chitosan and carboxymethyl cellulose also make the nanoparticles biodegradable and biocompatible, making them suitable candidates for pharmaceutical applications. The synthesis was then extended to chitosan and modified methyl cellulose microgel system. The prime reason for using methyl cellulose was to introduce thermo-responsive characteristics to the microgel system. Methyl cellulose was modified by carboxymethylation to introduce carboxylic acid functionality, and the chitosan-modified methyl cellulose microgel system was found to be pH as well as temperature responsive. Several techniques were used to characterize the two microgel systems, for e.g. potentiometric and conductometric titrations, dynamic light scattering and zeta potential measurements. FTIR along with potentiometric and conductometric titration was used to confirm the carboxymethylation of methyl cellulose. For both systems, polyampholytic behaviour was observed in a pH range of 4-9. The microgels showed swelling at low and high pH values and deswelling at isoelectric point (IEP). Zeta potential values confirmed the presence of positive charges on the microgel at low pH, negative charges at high pH and neutral charge at the IEP. For chitosan-modified methyl cellulose microgel system, temperature dependent behaviour was observed with dynamic light scattering. The second research project involved the study of binding interaction between nanocrystalline cellulose (NCC) and an oppositely charged surfactant tetradecyl trimethyl ammonium bromide (TTAB). NCC is a crystalline form of cellulose obtained from natural sources like wood, cotton or animal sources. These rodlike nanocrystals prepared by acid hydrolysis of native cellulose possess negatively charged surface. The interaction between negatively charged NCC and cationic TTAB surfactant was examined and it was observed that in the presence of TTAB, aqueous suspensions of NCC became unstable and phase separated. A study of this kind is imperative since NCC suspensions are proposed to be used in personal care applications (such as shampoos and conditioners) which also consist of surfactant formulations. Therefore, NCC suspensions would not be useful for applications that employ an oppositely charged surfactant. In order to prevent destabilization, poly (ethylene glycol) methacrylate (PEGMA) chains were grafted on the NCC surface to prevent the phase separation in presence of a cationic surfactant. Grafting was carried out using the free radical approach. The NCC-TTAB polymer surfactant interactions were studied via isothermal titration calorimetry (ITC), surface tensiometry, conductivity measurements, phase separation and zeta potential measurements. The major forces involve in these systems are electrostatic and hydrophobic interactions. ITC and surface tension results confirmed two kinds of interactions: (i) electrostatically driven NCC-TTAB complexes formed in the bulk and at the interface and (ii) hydrophobically driven TTAB micellization on the NCC rods. Conductivity and surface tension results confirmed that the critical micelle concentration of TTAB (CMCTTAB) shifted to higher values in the presence of NCC. Phase separation measurements allowed us to identify the formation of large aggregates or hydrophobic flocs depending on the TTAB concentration. Formation of NCC-TTAB complexes in aqueous solutions was confirmed by a charge reversal from negative to positive charge on the NCC rods. The effect of electrolyte in shielding the negative charges on the NCC was observed from ITC, surface tensiometry and phase separation experiments. Several mechanisms have been proposed to explain the above results. Grafting of PEGMA on the NCC surface was confirmed using FTIR and ITC experiments. In phase separation experiments NCC-g-PEGMA samples showed greater stability in the presence of TTAB compared to unmodified NCC. By comparing ITC and phase separation results, an optimum grafting ratio (PEGMA : NCC) for steric stabilization was also proposed.

No description available.

The main objective of this work was to study the applicability of Nano porous/Nanocrystalline TiO2 films for serving the growing demand for fast accurate and low cost air quality analysis techniques. The sensitivity of response to variations in microstructure pointed the way to obtain controlled, reproducible and regular microstructures with critical dimensions proportionate with the Debye- length of the oxide. This work aimed to study the sensor properties of thin films of Nanocrystalline Titania and Titania nanotubes at room temperature. Nano porous/Nanocrystalline TiO2 films were fabricated through evaporation driven convection/sol-gel method by controlling the process conditions. We also fabricated ordered TiO2 nanotube arrays by carefully controlling the anodization conditions. TiO2 nanotube arrays were fabricated through potentiostatic anodization of a Ti metal sheet in various electrolyte mediums. The electrolyte mediums consisted of mixtures of acids ranging from HF, HCl, HNO3, H2SO4, H3PO4 and CH3COOH and polar organic solvents like C2H6OS(DMSO) and C2H4(OH)2(Ethylene Glycol) . Well ordered films consisting of TiO2 nanotube arrays (25 μm to 250 μm in length) were obtained. Contacts were deposited on to the annealed samples. Capacitive gas sensors were fabricated and their response to ammonia gas was evaluated.