Showing papers by "Hamid Garmestani published in 2014"

An Experimental Insight into the Structural and Electronic Characteristics of Strontium-Doped Titanium Dioxide Nanotube Arrays

Abstract: The possibility of in situ doping during electrochemical anodization of titania nanotube arrays is demonstrated and the mechanism and variations in structural and electronic characteristics of the nanotube arrays as after doping is systematically explored. In the presence of strontium as the dopant, bulk analysis shows strontium mainly incorporated into the lattice of TiO2. Surface analysis, however, reveals phase segregation of SrO in the TiO2 matrix at high Sr doping levels. The near edge X-ray absorption fine structure (NEXAFS) spectroscopy analysis reveals that Sr2+ doping only alters the Ti and O ions interaction in the TiO2 lattice on the surface with no effect on their individual charge states. An in-depth understanding of the dopant incorporation mechanism and distribution into TiO2 nanotube arrays is achieved using high resolution transmission electron microscopy (HRTEM) and the high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with the electron energy loss spectroscopy (EELS) measurements on the surface and bulk of the nanotubes. Upon their use to photoelectrochemically split water, the Sr-doped TiO2 nanotube film shows incident photon conversion efficiencies (IPCE) as high as 65%. The enhanced light activity in conjunction with the ordered one-dimensional morphology makes the fabricated films promising candidates for water photoelectrolysis.

Three‐Dimensional Reconstruction and Microstructure Modeling of Porosity‐Graded Cathode Using Focused Ion Beam and Homogenization Techniques

Abstract: In this study, microstructure of a porosity-graded lanthanum strontium manganite (LSM) cathode of solid oxide fuel cells (SOFCs) has been characterized using focused ion beam (FIB) and scanning electron microscopy (SEM) combined with image processing. Two-point correlation functions of the two-dimensional (2D) images taken along the direction of porosity gradient are used to reconstruct a three-dimensional (3D) microstructure. The effective elastic modulus of the two-phase porosity-graded cathode is predicted using strong contrast (SC) and composite inclusion (CI) homogenization techniques. The effectiveness of the two methods in predicting the effective elastic properties of the porosity-graded LSM cathode is investigated in comparison with the results obtained from the finite element model (FEM).

High Shear Deformation to Produce High Strength and Energy Absorption in Mg Alloys

Abstract: Magnesium alloys have the potential to reduce the mass of transportation systems however to fully realize the benefits it must be usable in more applications including those that require higher strength and ductility. It has been known that fine grain size in Mg alloys leads to high strength and ductility. However, the challenge is how to achieve this optimal microstructure in a cost effective way. This work has shown that by using optimized high shear deformation and second phase particles of Mg2Si and MgxZnZry the energy absorption of the extrusions can exceed that of AA6061. The extrusion process under development described in this presentation appears to be scalable and cost effective. In addition to process development a novel modeling approach to understand the roles of strain and state-of-strain on particle fracture and grain size control has been developed.

An optimum approximation of n-point correlation functions of random heterogeneous material systems.

Abstract: An approximate solution for n-point correlation functions is developed in this study. In the approximate solution, weight functions are used to connect subsets of (n-1)-point correlation functions to estimate the full set of n-point correlation functions. In previous related studies, simple weight functions were introduced for the approximation of three and four-point correlation functions. In this work, the general framework of the weight functions is extended and derived to achieve optimum accuracy for approximate n-point correlation functions. Such approximation can be utilized to construct global n-point correlation functions for a system when there exist limited information about these functions in a subset of space. To verify its accuracy, the new formulation is used to approximate numerically three-point correlation functions from the set of two-point functions directly evaluated from a virtually generated isotropic heterogeneous microstructure representing a particulate composite system. Similarly, three-point functions are approximated for an anisotropic glass fiber/epoxy composite system and compared to their corresponding reference values calculated from an experimental dataset acquired by computational tomography. Results from both virtual and experimental studies confirm the accuracy of the new approximation. The new formulation can be utilized to attain a more accurate approximation to global n-point correlation functions for heterogeneous material systems with a hierarchy of length scales.

Surface Controlled Growth of Thin-Film Strontium Titanate Nanotube Arrays on Silicon

Abstract: We demonstrate the vertical self-organized growth of thin-film SrTiO3 (STO) nanotube arrays (NTAs) on SiO2 substrate. The surface morphology and crystal orientation of the grains at the exterior wall of the backbone TiO2 nanotube arrays were found to play an important role in the growth rate as well as the final morphology of the STO NTAs. A formation mechanism is proposed that involves nucleation of SrTiO3 nanocubes through a semitopochemical route followed by a self-assembly process resulting from the Ostwald-ripening-assisted oriented attachment of SrTiO3 nanocubes. It was shown that under appropriate reaction kinetics the nanotube architecture of the overall template can be maintained to form STO NTAs. The application of this novel platform enables a controlled and efficient mass fabrication of STO NTAs on widely used inexpensive silicon substrates, which can potentially lead to full integration with electronics in the near future.

Analysis of Micro-Texture and Grain Size Distributions in Machined Aluminum Alloy 7075

Abstract: the effects of turning on the gradients of micro-texture and grain size of Al alloy 7075 processed by turning is studied. Analytical models based on Boussinesq equation and stress/force transformation, predict that the equivalent von-Mises stress penetrate almost 1 mm below the machined surface with a maximum of 1400 MPa. The induced temperature below the surface is and reaches up to 240°C. As a result of deformation below the machined surface, the grains below the surface show a preferred grain shape orientation different from the grains in the central region. The normalized population of (121) plane increases at the machined surface compared to the central part of the sample. Orientation distribution functions reveal that at the central part of the sample, the material is mostly randomly oriented and the closest texture component is the recrystallized texture. On the other hand, the strongest texture attribute at the machined surface is β-fiber torsion. The average grain size shows a 10% decreases moving away from center while below the machined surface there is a 50% increase in the average grain size. This can be due to the grain growth caused by higher temperatures right below the machining tool. The reported results provide a toolset to determine the final properties of the material after machining in addition to providing means to more accurately describing the machining mechanics.

Characterization of the Effects of Active Filler-Metal Alloys in Joining Ceramic-to-Ceramic and Ceramic-to-Metal Materials

Abstract: This study investigated actively brazing Alumina-to-Alumina with Ag-Cu-Ti as the filler metal system and Alumina-to-Copper with Cu-Ti-Co as the filler-metal system. The research was conducted on four samples, two of which were alumina brazed to alumina (Samples 1 & 2), and the other two were alumina brazed to copper (Samples 3 & 4). The filler metal composition for each sample was as follows: Sample 1 consisted of Cu-96%, and Ti-4%; Sample 2 consisted of Ag-70%, Cu-26%, and Ti-4%; Sample 3 consisted of Cu-85%, Ti-10%, and Co-5%; and Sample 4 consisted of Cu-55%, Ti-40%, and Co-5%. The phase transformations between the filler and base metal of each brazed joint were studied using EDS, SEM, optical microscopy, and X-ray diffraction.

Biomimetic engineering of a fully bio-based system in nanomedicine

Abstract: Design of materials and devices by mimicking the fascinating systems found in nature, have enabled scientists to discover new techniques to treat diseases from diagnosis to therapeutic care. With the recent advances in nanomedicine, the length-scale of this design has further extended down into a nano-sized array. Inspired by the two natural assemblies found in nature; cellulose and collagen; we have designed a new class of green functional material with nano-sized arrangement. The fabricated material composed of collagen hydrogel reinforced by cellulose nanowhiskers in order to effectively enhance the rigidity of collagen and to better mimic the morphology and profile features existed in biological tissues. The biocompatibility of the hydrogel nanocomposite was also investigated by the invasion and proliferation of human bone marrow derived mesenchymal stem cells around the materials at 8 day of culture. We believe that our biomimetically-engineered platform in this study could increase the biomedical applications of fully bio-based systems such as scaffolding in tissue engineering.