186 research outputs found
Carrier transport mechanism of AlGaN/GaN Schottky barrier diodes with various Al mole fractions
No full textThe evolution of the dominant carrier transport mechanism of AlxGa1-xN/GaN Schottky barrier diodes with increasing Al mole fraction x (x = 0.2, 0.3, 0.4, and 0.5) is investigated. The Schottky barrier height (SBH) linearly increases with the work function of Schottky metal with the slope factor of similar to 0.5 irrespective of x. The SBH, the ideality factor, and the reverse leakage current, however, show an increasing deviation from the predicted values given by the thermionic-emission theory as x increases. The XPS analysis reveals enhanced predominant incorporation of oxygen donors and an increase in the energy band bending at the surface of AlGaN with increasing x, indicating that the carrier transport by tunnelling through the thin, heavily doped Schottky barrier becomes dominant. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1187sciescopu
Heat transfer through a condensate droplet on hydrophobic and nanostructured superhydrophobic surfaces
No full textUnderstanding the fundamental mechanisms governing vapor condensation on non-wetting surfaces is crucial to a wide range of energy and water applications. In this thesis, we reconcile classical droplet growth modeling barriers by utilizing two-dimensional axisymmetric numerical simulations to study individual droplet heat transfer on non-wetting surfaces (90° < θ_a < 170°). Incorporation of an appropriate convective boundary condition at the liquid vapor interface reveals that the majority of heat transfer occurs at the three phase contact line, where the local heat flux can be up to 4 orders of magnitude higher than at the droplet top. Droplet distribution theory is incorporated to show that previous modeling approaches under predict the overall heat transfer by as much as 300% for dropwise and jumping-droplet condensation. To verify our simulation results, we study condensed water droplet growth using optical and ESEM microscopy on bi-philic samples consisting of hydrophobic and nanostructured superhydrophobic regions, showing excellent agreement with the simulations for both constant base area and constant contact angle growth regimes. Our results demonstrate the importance of resolving local heat transfer effects for the fundamental understanding and high fidelity modeling of phase change heat transfer on non-wetting surfaces.Submission published under a 24 month embargo labeled 'U of I Access', the embargo will last until 2018-05-01The student, Shreyas Chavan, accepted the attached license on 2016-04-25 at 17:05.The student, Shreyas Chavan, submitted this Thesis for approval on 2016-04-25 at 17:16.This Thesis was approved for publication on 2016-04-27 at 09:11.DSpace SAF Submission Ingestion Package generated from Vireo submission #9489 on 2016-07-07 at 13:50:52Made available in DSpace on 2016-07-07T20:35:15Z (GMT). No. of bitstreams: 2
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Previous issue date: 2016-04-27Embargo set by: Seth Robbins for item 93182
Lift date: 2018-07-07T20:35:34Z
Reason: Author requested U of Illinois access only (OA after 2yrs) in Vireo ETD systemU of I Only Restriction Lifted for Item 93182 on 2018-07-08T09:15:36Z
Essential spherical isometries
No full textA theorem of Fillmore, Stampfli and Williams asserts that a bounded linear Hilbert space operator is an essential isometry if and only if it is a compact perturbation of either an isometry or a coisometry with finite-dimensional kernel. In this note, we discuss the spherical analog of this result. It turns out that the spherical analog of this result does not hold verbatim, and this failuremay be attributed to the fact that in dimension d>2, there exist spherical isometries with finite-dimensional joint cokernel, which are not essential spher-ical unitaries. We also discuss some strictly higher-dimensional obstructions in representing an essential spherical isometry as a compact perturbation of aspherical isometry
Phase change phenomena on water repelling and biphilic surfaces
No full textThis Dissertation was approved for publication on 2019-04-10 at 09:54.Water-repelling surfaces have been studied for many decades. Hydrophobic and superhydrophobic surfaces are beneficial in phase change heat transfer applications, specifically during condensation because of the enhanced heat transfer and during freezing because of the anti-freezing properties. The current study is focused on enhanced phase change phenomena on superhydrophobic and biphilic surfaces. Hydrophobic surfaces that enable dropwise condensation exhibit 5-10X higher heat transfer. Coalescence induced droplet jumping on superhydrophobic surfaces further increases the heat transfer by 30%. Here, biphilic surfaces consisting of hydrophilic spots on a superhydrophobic background are studied for enhanced condensation. Water droplets nucleating at the hydrophilic spots grow to sizes defined by the biphilic geometry, followed by coalescence and departure. A high fidelity model that captures departure dynamics during droplet jumping on biphilic surfaces and predict the overall condensation heat transfer has been developed. By controlling the spatial geometry and length scale of the hydrophilic spots, enhanced (10X) jumping-droplet condensation heat transfer is obtained.
In terms of freezing and frost formation, understanding the mechanisms of frost formation is essential to a variety of Heating, Ventilating, Air Conditioning and Refrigeration (HVAC&R) applications. When water vapor in the ambient condenses on a chilled substrate in the form of liquid water and then freezes, it is known as condensation frosting. The dominant mechanism governing the spread of condensation frosting is inter-droplet ice bridge frost wave propagation. When a subcooled condensate water droplet freezes on a hydrophobic or superhydrophobic surface, neighboring droplets still in the liquid phase begin to evaporate. The evaporated water molecules deposit on the frozen droplet and initiate the growth of ice bridges directed toward the water droplets being depleted. Neighboring liquid droplets freeze as soon as the ice bridge connects. In this study, the significance of individual droplet freezing on frost wave propagation is studied. 10X slower frost wave propagation speeds on superhydrophobic surfaces are observed. Furthermore, at larger length scales, during bulk freezing of water, it has been shown that superhydrophobic surfaces offer no delay in freezing.
Although frosting delay has been shown with superhydrophobic surfaces, complete elimination of frosting has not been achieved. Given enough time, frosting will initiate and spread to cover the entire surface. In the HVAC&R sectors, the most common approach to remove frost from a surface (defrost) is to reverse the system cycle direction and heat the working fluid. However, water retention on the heat exchanger surface during defrosting decreases the long term heat transfer performance. In this study, the defrosting behavior of superhydrophobic and biphilic surfaces comprising of spatially distinct superhydrophobic and hydrophilic domains is used to accelerate defrosting. During defrosting, biphilic surfaces are shown to exhibit enhanced surface cleaning with no water retention. Furthermore, an ultra-efficient method to defrost a surface covered with ice/frost by focusing energy at the substrate-ice interface is studied. To remove ice/frost efficiently, only the interfacial layer adhering the ice/frost to the solid surface is melted by using a localized ‘pulse’ of heat, allowing gravity or gas shear in conjunction with the ultra-thin lubricating melt water layer to remove the ice/frost. A high fidelity numerical model is developed to simulate pulse defrosting. This work not only provides a fundamental understanding of phase change processes on superhydrophobic and biphilic surfaces, but also elucidates its applications for a plethora of energy industries.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2021-05-01The student, Shreyas Chavan, accepted the attached license on 2019-04-09 at 16:13.The student, Shreyas Chavan, submitted this Dissertation for approval on 2019-04-09 at 16:16.DSpace SAF Submission Ingestion Package generated from Vireo submission #13540 on 2019-08-22 at 16:20:55Made available in DSpace on 2019-08-23T20:44:41Z (GMT). No. of bitstreams: 2
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Previous issue date: 2019-04-10Embargo set by: Seth Robbins for item 112291
Lift date: 2021-08-23T20:44:50Z
Reason: Author requested closed access (OA after 2yrs) in Vireo ETD systemEmbargo set by: Seth Robbins for item 112291
Lift date: 2021-08-23T20:46:41Z
Reason: Author requested closed access (OA after 2yrs) in Vireo ETD systemEmbargo set by: Seth Robbins for item 112291
Lift date: 2021-08-23T20:47:38Z
Reason: Author requested closed access (OA after 2yrs) in Vireo ETD systemEmbargo set by: Seth Robbins for item 112291
Lift date: 2021-08-23T20:48:32Z
Reason: Author requested closed access (OA after 2yrs) in Vireo ETD systemLimited Restriction Lifted for Item 112291 on 2021-08-24T09:15:24Z
ANALYSIS OF PUBLICATION PROFILE OF ACADEMIC LIBRARIANS IN MAHARASHTRA
No full textResearch has always been regarded as the most important intellectual activity in the higher education system; therefore this research article aims to provide a comprehensive picture of research productivity in symposia, seminars, conferences, journals, book chapters, research projects and patents carried out by various academic librarians during 2004 to 2013. A questionnaire was designed for data collection and the same was sent to the academic librarians by e-mail/in print. The results indicate that there were total 1325 research productivity in symposia, seminars, conferences, journals, book chapters, research projects and patents during the year 2004 to 2013. Highest 405 (30.57%) research papers published in conferences. Maximum 28.98% research publications appeared during the year 2013. Dr. S.P. Chavan was the most prolific author he has published 85 (6.41%) research papers. Librarian preferred English language to write research papers. Researchers communicated their research through variety of communication channels. The librarian mostly preferred to journals for communication channel. Highest 57 (74.03%) librarians have used single authorship pattern for their publications. Mostly 68 (88.31%) librarians have attending the workshops/symposia/seminars/conferences to acquire new skills, latest technology, update knowledge
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