Multiscale constitutive modeling of polymer materials

Materials are inherently multi-scale in nature consisting of distinct characteristics at various length scales from atoms to bulk material. There are no widely accepted predictive multi-scale modeling techniques that span from atomic level to bulk relating the effects of the structure at the nanometer (10-9 meter) on macro-scale properties. Traditional engineering deals with treating matter as continuous with no internal structure. In contrast to engineers, physicists have dealt with matter in its discrete structure at small length scales to understand fundamental behavior of materials. Multiscale modeling is of great scientific and technical importance as it can aid in designing novel materials that will enable us to tailor properties specific to an application like multi-functional materials. Polymer nanocomposite materials have the potential to provide significant increases in mechanical properties relative to current polymers used for structural applications. The nanoscale reinforcements have the potential to increase the effective interface between the reinforcement and the matrix by orders of magnitude for a given reinforcement volume fraction as relative to traditional micro- or macro-scale reinforcements. To facilitate the development of polymer nanocomposite materials, constitutive relationships must be established that predict the bulk mechanical properties of the materials as a function of the molecular structure. A computational hierarchical multiscale modeling technique is developed to study the bulk-level constitutive behavior of polymeric materials as a function of its molecular chemistry. Various parameters and modeling techniques from computational chemistry to continuum mechanics are utilized for the current modeling method. The cause and effect relationship of the parameters are studied to establish an efficient modeling framework. The proposed methodology is applied to three different polymers and validated using experimental data available in literature

Electronic properties of zig-zag carbon nanotubes : a first-principles study

Carbon nanotube (CNT) is a one dimensional (1-D) nanostructured material, which has been the focal point of research over the past decade for intriguing applications ranging from nanoelectronics to chemical and biological sensors. Using a first-principles gradient corrected density functional approach, we present a comprehensive study of the geometry and energy band gap in zig-zag semi-conducting (n,0) carbon nanotubes (CNT) to resolve some of the conflicting findings. Our calculations confirm that the single wall (n,0) CNTs fall into two distinct classes depending upon n mod 3 equal to 1 (smaller band gaps) or 2 (larger gaps). The effect of longitudinal strain on the band gap further confirms the existence of two distinct classes: for n mod 3 = 1 or 2, changing Eg by ~ ±110 meV for 1% strain in each case. We also present our findings for the origin of metallicity in multiwall CNTs

Sales Management Portal

Sales management and operation deals with set of business activities and processes that help a business organization run effectively and efficiently. It also means to coordinate sales operation that allows a business to consistently reach or even surpass its sales targets. Therefore as a sales manager, one would need to be able to evaluate and gain visibility into the current sales force and decide whether a particular objective would be met or not. In this project, we would be building such a tool that will help the sales manager, oversees the current state of business, in terms of managing different clients and the state of each order placed managed by sales personnel. The tool is intended to be an online portal in which the manager, who acts as an admin has the authority to approve/decline a sales request from the clients, keep track of their details, keep track of the production process of a previously accepted proposal, manage the sales force team

Thermoelectric characterization of one- and two- dimensional materials

Thermoelectric materials are useful for a wide range of applications like waste heat removal, solid state cooling, and power generation in space missions etc. A material's thermoelectric figure of merit (zT), which determines its performance in the applications listed above, depends on its Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) as zT=S2σT/κ. Low dimensional materials like nanowires (1D) or atomically thin films (2D) are promising as they exhibit lower thermal conductivities or higher power factors (S2σ) compared to their bulk (3D) counterparts. The thermal and electrical properties of these low dimensional materials could be further tuned by modifying their microstructure to achieve higher zTs. To achieve such material tuning in a controlled fashion, it is necessary to understand the physical mechanisms that govern the relationships between a material's microstructure and its thermoelectric properties. Reliable experimental techniques and proper interpretation of the experimental results are essential to gain insights into the physical mechanisms of interest. This dissertation addresses the characterization of thermoelectric properties of one- and two-dimensional materials with the goal of studying the governing physical mechanisms. An experimental study on the Seebeck coefficient and electrical conductivity of atomically thin Molybdenum disulphide (MoS2), a two-dimensional semiconducting material, is presented. Seebeck coefficient and electrical conductivity are electronic transport properties. In MoS2 and atomically thin materials, the electron transport is heavily influenced by the localized states formed in their band gaps. By fitting the experimentally obtained temperature and gate voltage dependence of S and σ with a theoretical model, a determination of the nature of the localized states and the electron transport mechanism is made. For the one-dimensional materials, the focus is on the measurement of their thermal conductivity. Most of the advances in the figure of merit were achieved in the recent past by reducing the thermal conductivity. In this light, understanding the phonon transport in the low dimensional materials gains importance. A suspended bridge measurement platform is a very commonly used technique to measure thermal conductivity of one-dimensional materials. This technique is very useful for studying the underlying fundamental transport physics as it allows measurement on an individual 1D structure, as opposed to 3ω and TDTR methods which can only measure an assembly of 1D materials. Combining this measurement technique with precise microstructure characterization using transmission electron microscopy (TEM), the influence of the microstructure on thermal transport can be deducted. In a study done previously on silicon nanowires, different but similarly made 1D structures were used for microstructure characterization and thermal measurement. This mismatch introduces an uncertainty in the correlation between microstructure and the phonon transport. In this dissertation, a modification to the usual measurement platform is presented which allows TEM imaging and thermal measurement on the same 1D structure. Furthermore, refinements to the measurement principal that have been implemented in our lab to enable measurement on much finer 1D structures are discussed in this dissertation

Self-consistent calculations of strain-induced band gap changes in semiconducting (n, 0) carbon nanotubes

First-principles density-functional calculations of the electronic structure, energy band gaps (Eg), and strain-induced band gap changes in moderate-gap single-walled (n,0) carbon nanotubes (SWNTs) are presented. It is confirmed that (n,0) SWNTs fall into two classes depending upon n mod 3=1 or 2. Eg is always lower for “mod 1” than for “mod 2” SWNTs of similar diameter. For n\u3c10, strong curvature effects dominate Eg; from n=10 to 17, the Eg oscillations, amplified due to σ−π mixing, decrease and can be explained very well with a tight-binding model which includes trigonal warping. Under strain, the two families of semiconducting SWNTs are distinguished by equal and opposite energy shifts for these gaps. For (10,0) and (20,0) tubes, the potential surface and band gap changes are explored up to approximately ±6% strain or compression. For each strain value, full internal geometry relaxation is allowed. The calculated band gap changes are ±(115±10) meV per 1% strain, positive for the mod 1 and negative for the mod 2 family, about 10% larger than the tight-binding result of ±97 meV and twice as large as the shift predicted from a tight-binding model that includes internal sublattice relaxation

Carbon nanotubes for coherent spintronic devices

Carbon nanotubes bridge the molecular and crystalline quantum worlds, and their extraordinary electronic, mechanical and optical properties have attracted enormous attention from a broad scientific community. We review the basic principles of fabricating spin-electronic devices based on individual, electrically-gated carbon nanotubes, and present experimental efforts to understand their electronic and nuclear spin degrees of freedom, which in the future may enable quantum applications.Comment: 17 pages, 9 figures, submitted to Materials Toda

Storage and transport of energy in nanostructures

Nanostructures typically exhibit thermo-physical properties that are different from their bulk counterparts. The size dependence of thermo physical properties is attributed to changing energy and mass transport phenomenon with varying length scales. This size dependence can be profitably leveraged to build cheap and efficient energy storage and harvesting systems when the materials are highly abundant. In this thesis, we study two different materials which exhibit favorable properties at lower length scales. In the first case, we study the dependence of particle size on energy storage and Carbon dioxide absorption capability of Calcium oxide particles. We theoretically establish in this work that the CaO nanoparticles achieve higher and faster reaction conversions than the micrometer sized particles. We identify the parameters which contribute to the superior performance of CaO nanoparticles and thereby provide design recommendations to sustain the enhanced performance. In the second case, Silicon, another abundant material, in the form of a nanowire has been experimentally examined as a candidate material for thermoelectric applications to harvest waste heat. We designed and fabricated a device to gauge the thermoelectric figure of merit of nano-structured materials by simultaneous characterization of thermal, electrical and seebeck properties. Using the fabricated device, the silicon nanowires are shown to have a tenfold reduction in thermal conductivity from its bulk value thereby establishing silicon nanowires as a promising thermoelectric material