2,692 research outputs found
Thermocapillary approaches to the deliberate patterning of polymers
No full textThe phenomenon of thermocapillarity, the response of fluids to thermal gradients due to thermal alteration of their surface tension, was first reported over a century ago. Since then, research has focused generally on either the fundamentals or mitigation of this effect during the processing of materials. Only in the past two decades has the deliberate use of thermocapillary forces for the patterning of polymers been actively pursued, either for the ordering of internal structure or the introduction of topographic features. This review seeks to highlight this work and further identify directions for further investigation. In particular, while thermocapillary forces are often inextricably bound to other mechanisms, there are emerging directions in the deliberate coupling of forces to improve the capabilities of each mechanism. Further, the applications of thermocapillary patterning to polymer-nanoparticle composites has recently provided another promising route to active architectures.Peer reviewed
Anti-reflection zinc oxide nanocones for higher efficiency thin-film silicon solar cells
No full textThesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2012.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student submitted PDF version of thesis.Includes bibliographical references (p. 77-80).Thin film silicon solar cells, which are commonly made from microcrystalline silicon ([mu]c-Si) or amorphous silicon (a-Si), have been considered inexpensive alternatives to thick polycrystalline silicon (polysilicon) solar cells. However, the low solar efficiency of these thin film cells has become a major problem, which prevents thin film silicon cells from being able to compete with other solar cells in the market. One source of inefficiency is the light reflection off the interface between the thin film cell's top Transparent Conducting Oxide (TCO) and the light absorbing silicon. In this work, we demonstrate the use of nanocone textured ZnO as the anti-reflection surface that mitigates this problem. The tapered structure of the nanocone forms a smooth transition of refractive index on the interface between the TCO (ZnO) and the silicon, effectively acting as a wideband Anti-Reflection coating (AR coating). Finite Difference Time Domain simulation is used to estimate the optimal ZnO nanocone parameter (periodicity and height) to be applied on a single junction microcrystalline silicon ([mu]c-Si) solar cell. Relative improvement over 25% in optical performance is achieved in the simulated structure when compared to state-of-the-art [mu]c-Si cell structure. Cheap and scalable colloidal lithography method is then developed to fabricate ZnO nanocone with the desired geometry. Since the ZnO texturing technique works by depositing ZnO on nanocone-textured glass substrate, the technique is potentially applicable to Transparent Conducting Oxides other than ZnO as well, making it a useful TCO texturing technique for solar cell applications.by Jonathan P. Mailoa.M.Eng
Focused Laser-Induced Marangoni Dewetting for Patterning Polymer Thin Films
No full textHighly-localized focused laser spike (FLaSk) heating of polymer thin films is a resist- and developer-free alternative to 2D laser direct write for creating patterns on the single micron or, by exploiting overlap effects, submicron scale. The massive temporal and spatial thermal gradients and resulting thermal Marangoni stresses generated by FLaSk are an effective means for the directed dewetting and patterning of such films. Here, the general applicability of this technique to glassy amorphous polymer thin film systems is investigated through systematic investigation of film thickness, glass transition temperature, and polymer mobility. The results reveal that the important parameters are the film thickness (coupled to the optical heating effects through anti-reflection coating effects) and the high-temperature polymer melt mobility, allowing for generation of single features with linewidths of down to ~1 μm. Further, the introduction of spatial mobility variations by using polymer brushes, bilayers, and microphase separated block copolymers leads to additional profile manipulation effects (i.e. spontaneous 2D pattern generation and flattened top profiles).Peer reviewe
Beyond the Shockley-Queisser limit : intermediate band and tandem solar cells leveraging silicon and CdTe technology
No full textThesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2016.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis. Page 156 blank.Includes bibliographical references (pages 141-153).The efficiencies of single-junction solar cells have been rapidly increasing and approaching their fundamental Shockley-Queisser efficiency limits. This is true for mature commercial technologies such as silicon and cadmium telluride. In order to enable solar cells with higher efficiency limits, new concepts need to be implemented which overcome the fundamental energy conversion mechanism limitations of single-junction solar cells. For this approach to be successful, it is advantageous to leverage existing manufacturing facilities and integrate these new solar cell architectures into commercially successful solar cell technologies such as silicon and cadmium telluride. In this thesis, two novel solar cell concepts are explored, categorized into three contributions. First, the application of intermediate band concept on silicon solar cells is explored by hyperdoping silicon, demonstrating room-temperature sub-band gap optoelectronic response from the material, and evaluating the feasibility of the intermediate band approach for improving silicon solar cell efficiency. Second, perovskite solar cells are integrated onto silicon solar cells to demonstrate mechanically-stacked perovskite/silicon tandem solar cell using low-cost silicon cell and monolithic perovskite/silicon tandem solar cell enabled by a silicon tunnel junction. Third, an analytic model is built to rapidly investigate the energy yield of different tandem solar cell architectures. When applied to cadmium telluride-based tandem solar cells, this model will help thin-film companies like First Solar narrow down the scope of future research and development programs on tandem solar cells.by Jonathan P. Mailoa.Ph. D
On the Douglas-Rachford splitting method and the proximal point algorithm for maximal monotone operators
No full textCover title. "This paper consists mainly of dissertation research results of the first author."--Cover.Includes bibliographical references (p. 31-34).Research supported in part by the Army Research Office. DAAL03-86-K-0171 Research supported in part by the National Science Foundation. ECS-8519058by Jonathan Eckstein, Dimitri P. Bertsekas
Determination of critical cooling rates in metallic glass forming alloy libraries through laser spike annealing
No full textThe glass forming ability (GFA) of metallic glasses (MGs) is quantified by the critical cooling rate (RC). Despite its key role in MG research, experimental challenges have limited measured RC to a minute fraction of known glass formers. We present a combinatorial approach to directly measure RC for large compositional ranges. This is realized through the use of compositionally-graded alloy libraries, which were photo-thermally heated by scanning laser spike annealing of an absorbing layer, then melted and cooled at various rates. Coupled with X-ray diffraction mapping, GFA is determined from direct RC measurements. We exemplify this technique for the Au-Cu-Si system, where we identify Au56Cu27Si17 as the alloy with the highest GFA. In general, this method enables measurements of RC over large compositional areas, which is powerful for materials discovery and, when correlating with chemistry and other properties, for a deeper understanding of MG formation.Peer reviewe
Multiscale Patterning of a Metallic Glass using Sacrificial Imprint Lithography
No full textBulk metallic glasses have been advanced as a means to achieve durable multiscale, nanotextured surfaces with desirable properties dictated by topography for a multitude of applications. One barrier to this achievement is the lack of a bridging technique between macroscale thermoplastic forming and nanoimprint lithography, which arises from the difficulty and cost of generating controlled nanostructures on complex geometries using conventional top-down approaches. This difficulty is compounded by the necessary destruction of any resulting reentrant structures during rigid demolding. We have developed a generalized method to overcome this limitation by sacrificial template imprinting using zinc oxide nanostructures. It is established that such structures can be grown inexpensively and quickly with tunable morphologies on a wide variety of substrates out of solution, which we exploit to generate the nanoscale portion of the multiscale pattern through this bottom-up approach. In this way, we achieve metallic structures that simultaneously demonstrate features from the macroscale down to the nanoscale, requiring only the top-down fabrication of macro/microstructured molds. Upon detachment of the formed part from the multiscale molds, the zinc oxide remains embedded in the surface and can be removed by etching in mild conditions to both regenerate the mold and render the surface of the bulk metallic glass nanoporous. The ability to pattern metallic surfaces in a single step on length scales from centimeters down to nanometers is a critical step toward fabricating devices with complex shapes that rely on multiscale topography for their intended functions, such as for biomedical and electrochemical applications.Peer reviewe
Thermocapillary Multidewetting of Thin Films
No full textThermocapillary dewetting of liquids and molten films has recently emerged as a viable alternative to conventional microprocessing methods. As this thermal gradient-induced mechanism is universal, it can be applied to any material. This work explores the sequential dewetting of materials with varying melting points, including polymers and metals, to create aligned morphologies. The variation in melting point allows for the dewetting of single layers at a time or mobility-limited simultaneous dewetting. As a result, a variety of multimaterial structures can be produced with built-in alignment, such as arrays of concentric circles, lines with periodic segmentation, or islands on holes. This approach employs photothermal methods to induce the necessary thermal gradient, manipulating several variables in order to influence the consequent structures. Adjusting laser power and light intensity allows for the control of temperature for selective dewetting of films; altering beam size and exposure time affects the extent of dewetting in terms of diameter size; overlap effects and simultaneous dewetting can result in complex architectures. This controlled writing of patterns also presents a technique to create both masks at low temperatures for conductive multilayers as well as templates for electrospray deposition.Peer reviewe
‘Decolonisation’ in China, 1949-1959
No full textIn this chapter Jonathan Howlett adopts perspectives and models from wider literatures on decolonisation to explore the Chinese Communist Party’s elimination of the British semi-colonial presence from China after the revolution of 1949 and to place it within its global context. He focuses in particular on the CCP’s attempts to address the economic, cultural and human legacies of semi-colonialism within a comparative context. In so doing, the author seeks to complicate our understanding of the Sino-British relationship by exploring one of its most dramatic phases and to further illuminate this neglected period in Chinese history
Sensitivity Analysis of Optical Metrics for Spectral Splitting Photovoltaic Systems: A Case Study
No full textSpectral splitting of sunlight to increase photovoltaic (PV) efficiency beyond the Shockley-Queisser limit has gained interest in recent years. Sensitivity analysis can be a useful tool for system designers to determine how much deviation from ideal conditions can be tolerated for different optical parameters. Understanding the origin of these sensitivities can offer insight into materials and device design. We employ 2-D TCAD simulations to analyze the sensitivity of system performance to two optical parameters: spectral fidelity (the fraction of photons directed to the intended material) and the spatial uniformity of illumination intensity. We analyze a system using crystalline silicon (Si) and cuprous oxide (Cu[subscript 2]O) as absorbers. We find that the spectral fidelity of the light directed to the Si cell has to be greater than 90% for the system to outperform a high-efficiency single-junction Si device. Varying the fidelity of the light directed to the Cu[subscript 2]O cell from 55% to 90% changes system efficiency by less than 10% relative. In some cases, increasing the fidelity of this light reduces system efficiency. We find no significant impact of spatial variation on length scales from 600 μm to 4.8 mm in devices with emitter sheet resistance less than 500 Ω/□.United States. Department of Defense. Assistant Secretary of Defense for Research & Engineering (Contract FA8721-05-C-0002)National Science Foundation (U.S.) (Contract ECCS-1102050)National Science Foundation (U.S.) (Singapore-MIT Alliance for Research and Technology (SMART))American Society for Engineering Education. National Defense Science and Engineering Graduate FellowshipNational Science Foundation (U.S.). Graduate Research Fellowship Progra
- …
