Conjugated polymers are very attractive materials for the scientists and industry due to low cost of the organic compounds, their lightweight, easy large-area processing from solution at low temperature and mechanical flexibility. Moreover, these materials are multifunctional and advanced technologies require both simultaneous n- and p-type conductance, i.e. ambipolarity. However, there are some hindrances which do not allow the wide spreading of this new generation of semiconductors into the market, first of all, due to their instability to ambient conditions. Moreover, determination of the tunable parameters which are responsible for high efficiency and controlled crystal packing ordering of the devices is rather complicated. A lot of efforts are done in order to improve the performance of the organic electronics as well as to shed light on the relation between the chemical structure and their intrinsic properties. Additionally, the governing factors which define the conductive properties of these materials are still under debate and this remains a great challenge for the researchers. One way to gain insight into the characteristics of polymeric materials is to begin exploring the polymers from their small constitutive units and then step-by-step to construct and characterize every compound up to macromolecular level. In this work, the semiconducting block copolymers, as promising candidates for application in organic transistors, are investigated starting from their small donor and acceptor blocks up to monomers and macromolecules, using computational methods running on different time and length scales. It is found out that the charge transport depends on the symmetry of molecules and the hopping mobilities can be predicted from isolated stacks of dimers, which are defined by minimum energy, without knowledge of the actual crystal structure. Interestingly, the polymers moieties prefer to build up mixed stacks and the flanks form segregated columns if there are no present defects in the samples. At each step of the investigation the results are compared with available experimental data.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / by Kord Sterling Smith. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / by Alexander Y.C. Cheng. / Ph.D.

Thesis. 1976. Sc.D.--Massachusetts Institute of Technology. Dept. of Nuclear Engineering. / Microfiche copy available in Archives and Science. / Bibliography: leaves 221-224. / Sc.D.

Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Nuclear Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / Ph.D.

Thesis (Nucl.E.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by John A. Bernard, Jr. / Nucl.E.

The effect of ultrasonic vibration on the anodic oxidation of silver foil in KOH was studied. An increase in charging capacity of approximately 20% was found to be the result of cavitation erosion. Silver wire was exposed to tension and torsion during oxidation but no change in charge acceptance was caused by these stresses. The oxide growth rate on silver foil electrodes was compared to rate equations that have been proposed for other metals. Uhlig's equations for the growth of semiconductor oxides was found to describe most of the data. The data did not fit other rate equations. Determination of the location of radioactive Ag110 in the oxide layer indicated that silver oxide grows by direct transport of the silver ion through the oxide, if a uniform oxide thickness is assumed. However, the dissolution-precipitation-model of oxide growth describes the data better and allows for the non-uniform oxide thickness which is characteristic of silver.

The use of aerial platforms such as unmanned aerial vehicles (UAVs), popularly known as drones, has emerged as a promising solution for providing reliable and cost-effective wireless communications. In particular, UAVs can be quickly and efficiently deployed to support cellular networks and enhance their quality-of-service (QoS) by establishing line-of-sight communication links. With their inherent attributes such as mobility, flexibility, and adaptive altitude, UAVs admit several key potential applications in wireless systems. Remarkably, despite these inherent advantages of UAVbased communications, little work has analyzed the performance tradeoffs associated with using UAVs as aerial wireless platforms. The key goal of this dissertation is to develop the analytical foundations for deployment, performance analysis, and optimization of UAV-enabled wireless networks. This dissertation makes a number of fundamental contributions to various areas of UAV communications that include: 1) Efficient deployment of UAVs, 2) Performance evaluation and optimization, and 3) Design of new flying, three-dimensional (3D) wireless systems. For deployment, using tools from optimization theory, holistic frameworks are developed for the optimal 3D placement of UAV base stations in uplink and downlink scenarios. The results show that the proposed deployment approaches significantly improve the downlink coverage for ground users, and enable ultra-reliable and energy-efficient uplink communications in Internet of Things (IoT) applications. For performance optimization, a novel framework is developed for maximizing the performance of a UAV-based wireless system, in terms of data service, under UAVs’ flight time constraints. To this end, using the mathematical framework of optimal transport theory, the optimal cell associations, that lead to a maximum data service to ground users within the limited UAVs’ hover duration, are analytically derived. The results shed light on the tradeoff between hover time and quality-of-service in UAV-based wireless networks. For performance evaluation, this dissertation provides a comprehensive analysis on the performance of a UAV-based communication system in coexistence with a terrestrial network. In particular, a tractable analytical framework is proposed for analyzing the coverage and rate performance of a network with a UAV base station and deviceto-device (D2D) users. The results reveal the fundamental tradeoffs in such a UAV-D2D network that allow adopting appropriate system design parameters. Then, this dissertation sheds light on the design of three new drone-enabled wireless systems. First, a novel framework for effective use of cache-enabled UAVs in wireless networks is developed. The results demonstrate how the users’ quality of experience substantially improves by exploiting UAVs’ mobility and user-centric information. Second, a new framework is proposed for deploying and operating a drone-based antenna array system that delivers wireless service to ground users within a minimum time. The results show significant enhancement in QoS, spectral and energy efficiency while levering the proposed drone antenna array system. Finally, to effectively incorporate various use cases of drones ranging from aerial users to base stations, the new concept of a fully-fledged 3D cellular network is introduced. For this new type of 3D wireless network, a unified framework for deployment, network planning, and performance optimization is developed that yields a maximum coverage and minimum latency in the network. In a nutshell, the analytical foundations and frameworks presented in this dissertation provide key guidelines for effective design and operation of UAV-based wireless communication systems. / Ph. D.

The abstract kinetic equation Tψ’=-Aψ is studied with partial range boundary conditions in two geometries, in the half space x≥0 and on a finite interval [0, r]. T and A are abstract self-adjoint operators in a complex Hilbert space. In the case of the half space problem it is assumed that T is a (possibly) unbounded injection and A is a positive compact perturbation of the identity satisfying a regularity condition, while in the case of slab geometry T is a bounded injection and A is a bounded Fredholm operator with a finite dimensional negative part. Existence and uniqueness theory is developed for both models. Results are illustrated on relevant physical examples. / Ph. D.

It is shown that multigroup transport equations with nondiagonal cross section matrices arise when the modal approximation is applied to energy dependent transport equations. This work is a study of such equations for the case that the cross section matrix is nondiagonalizable. For the special case of a two-group problem with a noninvertible scattering matrix, the problem is solved completely via the Wiener-Hopf method. For more general problems, generalized Chandrasekhar H equations are derived. A numerical method for their solution is proposed. Also, the exit distribution is written in terms of the H functions. / Ph. D. / incomplete_metadata