Properties of accretion shock waves in viscous flows with cooling effects

We study the properties of the shock waves for a viscous accretion flow having low angular momentum in presence of synchrotron cooling. We present all possible accretion solutions in terms of flow parameters. We identify the region of the parameter space for steady and oscillating shocks and show the effect of various energy dissipation processes on it. We discuss the role of the shock waves while explaining the observations from black hole candidates.Comment: 3 pages; 3 figures; prepared on the basis of the talk presented in the MG11 Meeting on General Relativity, Berlin, July 23-29, 200

Parameter space study of magnetohydrodynamic flows around magnetized compact objects

We solve the magnetohydrodynamic (MHD) equations governing axisymmetric flows around neutron stars and black holes and found all possible solution topologies for adiabatic accretion. We divide the parameter space spanned by the conserved energy and angular momentum of the flow in terms of the flow topologies. We also study the possibility of the formation of the MHD shock waves.Comment: 3 pages; 4 figures; prepared on the basis of the talk presented in the MG11 Meeting on General Relativity, Berlin, July 23-29, 200

Holes or Empty Pseudo-Triangles in Planar Point Sets

Let $E(k, \ell)$ denote the smallest integer such that any set of at least $E(k, \ell)$ points in the plane, no three on a line, contains either an empty convex polygon with $k$ vertices or an empty pseudo-triangle with $\ell$ vertices. The existence of $E(k, \ell)$ for positive integers $k, \ell\geq 3$, is the consequence of a result proved by Valtr [Discrete and Computational Geometry, Vol. 37, 565--576, 2007]. In this paper, following a series of new results about the existence of empty pseudo-triangles in point sets with triangular convex hulls, we determine the exact values of $E(k, 5)$ and $E(5, \ell)$, and prove bounds on $E(k, 6)$ and $E(6, \ell)$, for $k, \ell\geq 3$. By dropping the emptiness condition, we define another related quantity $F(k, \ell)$, which is the smallest integer such that any set of at least $F(k, \ell)$ points in the plane, no three on a line, contains a convex polygon with $k$ vertices or a pseudo-triangle with $\ell$ vertices. Extending a result of Bisztriczky and T\'oth [Discrete Geometry, Marcel Dekker, 49--58, 2003], we obtain the exact values of $F(k, 5)$ and $F(k, 6)$, and obtain non-trivial bounds on $F(k, 7)$.Comment: A minor error in the proof of Theorem 2 fixed. Typos corrected. 19 pages, 11 figure

GROWTH, FABRICATION AND CHARACTERIZATION OF Cu\u3csub\u3e2\u3c/sub\u3eZnSn(S\u3csub\u3ex\u3c/sub\u3eSe\u3csub\u3e1-x\u3c/sub\u3e)\u3csub\u3e4\u3c/sub\u3e PHOTOVOLTAIC ABSORBER AND THIN-FILM HETEROJUNCTION SOLAR CELLS

Current thin-film solar cell technologies based on CuInxGa1-xSe2 (CIGS) and CdTe photo-absorber materials use rare and expensive elements, such as In, Te, Ga, and toxic Cd which severely limit the mass production and deployment of these solar cells. Thus, a major research effort is focused toward the development of new photovoltaic (PV) absorber materials comprising of earth-abundant, low-cost, and environmentally benign constituent elements that can support terawatt (TW)-scale PV generation in the near future and be economically sustainable. Cu-based I2-II-IV-VI4 quaternary kesterite compound Cu2ZnSn(SxSe1-x)4 (CZTSSe) have recently emerged as a potential photo-absorber material for thin-film solar cells. All constituent elements in CZTSSe are abundant in earth’s crust, are much cheaper and possess no acute toxicity. CZTSSe is an intrinsically p-type material with a large optical absorption coefficient (α\u3e104 cm-1) and exhibits a tunable direct optical bandgap in the range of 1.0 eV ≤ Eg ≤ 1.5 eV corresponding to chalcogen ratios of 0 ≤ x ≤ 1. The theoretical Shockley-Queisser efficiency limit for a single junction CZTSSe solar cell is estimated to be ~32% – similar to that of CIGS solar cells. All these merits make CZTSSe an ideal photo-absorber material for thin-film solar cells. In this dissertation, a comprehensive investigation is undertaken on the growth and characterization of Cu2ZnSn(SxSe1-x)4 photovoltaic absorber material followed by thin-film solar cell fabrication and cell characterization. CZTSSe films were fabricated by a vacuum-based two-step process of ZnS/Cu/Sn stacked precursor layer deposition on bi-layer molybdenum (Mo)-coated soda-lime glass (SLG) substrates via thermal evaporation and successive annealing of the precursor stacks under a mixed sulfur and selenium vapor at 550°C. The heterojunction was formed by deposition of n-CdS layer on top of p-CZTSSe absorber film via a low-cost chemical bath deposition (CBD) technique. The structural, compositional and morphological characterization of the CZTSSe films were carried out by Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The best solar cell was obtained with a Cu-poor and slightly Zn-rich absorber composition corresponding to elemental ratios of Cu/(Zn+Sn) = 0.773, Zn/Sn = 1.13 and S/(S+Se) = x = 0.58. Thickness of the CZTSSe film was measured to be 1.3-1.4 μm with an estimated bandgap of ~1.3 eV. The photovoltaic performance of the fabricated cells were evaluated under simulated AM1.5G (100 mW/cm2) solar radiation. The champion cell exhibited an open-circuit voltage (VOC) of 506 mV, a short-circuit current density (JSC) of 22.92 mA/cm2, and a fill-factor (FF) of 35% resulting in a total area efficiency (η) of 4.06% without any antireflection coating. Performance of the fabricated solar cells were found to be limited by high series resistance (RS), low shunt resistance (RSh), and poor fill factor (FF). The sources of high series resistance were attributed to the small multi-grain microstructure of the polycrystalline CZTSSe film, presence of micro air-voids, and a Mo(SSe)x interfacial layer at the Mo back contact. AFM studies revealed micro-pores on the film surface which act as low resistance shunt paths and are attributed to the source of low shunt resistance. Impedance spectroscopy were performed on the solar cells to model and extract the equivalent AC circuit parameters. Fitting of the experimental results showed the presence of a blocking barrier at the back contact and a recombination center resembling a constant phase element (CPE). Temperature dependent illuminated current-voltage (J-V) studies indicated a major recombination phenomena occurring at the heterojunction interface corresponding to an activation energy of 1.12 eV. Further investigation of the electronic defect levels in the fabricated solar cells have been carried out by current-mode deep level transient spectroscopy (I-DLTS). Two dominant deep acceptor defects at Ev+0.12 eV, and Ev+0.32 eV have been observed and were identified as the CuZn(-/0) and CuSn(2-/-) antisites respectively

Development of a Low-cost, Portable, and Programmable Solar Module to Facilitate Hands-on Experiments and Improve Student Learning

Solar energy has become one of the major renewable energy sources to meet a significant portion of the global energy demand in the near future. The photovoltaic (PV) industry is growing at a fast pace and is predicted to reach tera-watt scale power production capacity by 2050. Thus, creating highly qualified engineers by providing effective hands-on education is of primary importance. This paper discusses on the design, fabrication, and implementation of a low-cost, portable, and programmable solar module to improve the student learning outcomes of a solar power and renewable energy course by facilitating hands-on experiments. Solar cells within the smart module can be electronically interconnected in various configurations and I-V characteristics of the resulting module or any individual cell can be measured using integrated microcontroller and measurement circuits interfaced by a LabVIEW-based interactive data acquisition software. The developed plug-and-play smart solar module is an affordable and effective teaching tool that can be used for in-class demonstrations or to perform hands-on experiments. An experiment-based project was included into the course syllabus where students perform solar cell I-V characterization and extract the cell parameters by analyzing the experimental data. Student perception on the learning outcome and the impact of the project were measured based on student feedback. All of the students found that the experiment-based project helped them to reinforce the theoretical knowledge and a majority of the students believe that the learning outcomes of the course would be incomplete without it

Three Brane Action and The Correspondence Between N=4 Yang Mills Theory and Anti De Sitter Space

Recently, a relation between N=4 Super Yang Mills in 3+1 dimensions and supergravity in an $AdS_5$ background has been proposed. In this paper we explore the idea that the correspondence between operators in the Yang Mills theory and modes of the supergravity theory can be obtained by using the D3 brane action. Specifically, we consider two form gauge fields for this purpose. The supergravity analysis predicts that the operator which corresponds to this mode has dimension six. We show that this is indeed the leading operator in the three brane Dirac-Born-Infeld and Wess-Zumino action which couples to this mode. It is important in the analysis that the brane action is expanded around the anti de-Sitter background. Also, the Wess-Zumino term plays a crucial role in cancelling a lower dimension operator which appears in the the Dirac-Born-Infeld action.Comment: 12 pages, LaTex, no figures; error in the form of the final dimension six operator corrected, some references and comments added, main conclusions unchange