Rectangular Layouts and Contact Graphs

Contact graphs of isothetic rectangles unify many concepts from applications including VLSI and architectural design, computational geometry, and GIS. Minimizing the area of their corresponding {\em rectangular layouts} is a key problem. We study the area-optimization problem and show that it is NP-hard to find a minimum-area rectangular layout of a given contact graph. We present O(n)-time algorithms that construct $O(n^2)$-area rectangular layouts for general contact graphs and $O(n\log n)$-area rectangular layouts for trees. (For trees, this is an $O(\log n)$-approximation algorithm.) We also present an infinite family of graphs (rsp., trees) that require $\Omega(n^2)$ (rsp., $\Omega(n\log n)$) area. We derive these results by presenting a new characterization of graphs that admit rectangular layouts using the related concept of {\em rectangular duals}. A corollary to our results relates the class of graphs that admit rectangular layouts to {\em rectangle of influence drawings}.Comment: 28 pages, 13 figures, 55 references, 1 appendi

Heat conduction of single-walled carbon nanotube isotope-superlattice structures: A molecular dynamics study

Heat conduction of single-walled carbon nanotubes (SWNTs) isotope-superlattice is investigated by means of classical molecular dynamics simulations. Superlattice structures were formed by alternately connecting SWNTs with different masses. On varying the superlattice period, the critical value with minimum effective thermal conductivity was identified, where dominant physics switches from zone-folding effect to thermal boundary resistance of lattice interface. The crossover mechanism is explained with the energy density spectra where zone-folding effects can be clearly observed. The results suggest that the critical superlattice period thickness depends on the mean free path distribution of diffusive-ballistic phonons. The reduction of the thermal conductivity with superlattice structures beats that of the one-dimensional alloy structure, though the minimum thermal conductivity is still slightly higher than the value obtained by two-dimensional random mixing of isotopes.Comment: 7 Pages, 5 figures, accepted to Phys. Rev.

Radiation resistance of Ge, Ge0.93Si0.07, GaAs and Al0.08Ga0.92 as solar cells

Solar cells made of Ge, Ge(0.93)Si(0.07) alloys, GaAs and Al(0.08)Ga(0.92)As were irradiated in two experiments with 1-meV electrons at fluences as great as 1 x 10(exp 16) cm(exp-2). Several general trends have emerged. Low-band-gap Ge and Ge(0.93)Si(0.07) cells show substantial resistance to radiation-induced damage. The two experiments showed that degradation is less for Al(0.08)Ga(0.92)As cells than for similarly irradiated GaAs cells. Compared to homojunctions, cells with graded-band-gap emitters did not show the additional resistance to damage in the second experiment that had been seen in the first. The thickness of the emitter is a key parameter to limit the degradation in GaAs devices

Comparison of the effectiveness of sterilizing endodontic files by four different methods: An Invitro study

INTRODUCTION : Microorganisms induce a variety of infections and diseases in the human body and are largely ubiquitous in nature. Contamination directly or indirectly leads to transmission of infectious agents.6 The recent increase in the knowledge and the information on the transmission of the virus of the B hepatitis (HBV) and the virus of the immunodeficiency human being (HIV) has given new emphasis to the problem of the contamination crossed during the dental treatment. Infection control is a major issue in medicine and dentistry because of concern over communicable diseases transmitted in health care settings. The prevention of cross-contamination of infectious diseases among dental staff and patients is a major concern in a dental practice. In 1987, the Center of Disease Control (CDC) called the term “Universal Precaution” as being the set of procedures and measures that they aim at to protect the health and to provide security to the professionals of the area and, therefore, to the patients. These Universal Precautions understand the job of barriers of surfaces, sterilization of the instrument of clinical use, antisepsis, disinfection, cleanness and discarding of dismissable materials. AIMS AND OBJECTIVES : The purpose of this study is to compare and recommend the effective method of sterilizing endodontic files in dental practice. The aim of this study is to investigate the efficacy of four accepted methods of sterilizing endodontic instruments: (1) Autoclaving, (2) Carbon dioxide laser sterilization, (3) Chemical sterilization – Glutaraldehyde, (4) Glass-bead sterilization. MATERIALS AND METHODS : This study was done to investigate the efficacy of four accepted methods of sterilizing endodontic files. Part of the study was done at Dept. of Microbiology, Malar Hospital, Chennai and the CO2 laser system part of the study was done at KKR ENT Hospital, Chennai. The study was carried on 100 K-files, 21mm long and of size 25. Of these 20 files were taken as control group and the remaining 80 files were divided into 4 groups of 20 files in each group and they were tested for the efficacy of sterilization with different methods: autoclave, glass bead, glutaraldehyde and laser. RESULTS ; This study was done to determine the efficacy of sterilizing endodontic files by different methods – autoclave, glass bead, glutaraldehyde and laser. The endodontic files were sterilized after contamination with bacillus spores.The study showed that the endodontic files sterilized by autoclaving in an instrument box at 121°C for 15 minutes under 15 pounds pressure (Group A) showed total sterility. This method of sterilizing the endodontic files achieved complete sterilization. The files subjected to sterilization by glass bead sterilizer after wiping for 10 seconds with a 2 X 2 gauze soaked with surgical spirit and sterilized for 45 seconds at 240°C (Group B) showed presence of turbidity in 2 test tubes. Incomplete sterilization to the range of 10% was observed when the files were sterilized in glass bead sterilizer. The endodontic files sterilized by immersing in glutaraldehyde for 24hours (Group G) showed sterilization upto only 80%. This method showed contamination of 4 files after incubation. The files on sterilization by CO2 laser for 3 seconds per surface at 10 watts (Group L) showed 100% sterility. There was total sterility seen by this method of sterilization. The control group (Group C) where the files after contamination were not sterilized by any method showed growth in all the test tubes. Statistical analysis of the 4 sterilized groups showed a statistical significance (p ≤ 0.05). The statistical analysis of the sterilized groups with that of the control group showed that it is significant (p ≤ 0.05). On comparing the different groups among themselves, it was seen that there was no statistical significance among them. CONCLUSION : The following conclusions were drawn from the present study. 1. CO2 laser and autoclave were showing total sterility and are more efficient than glass bead and glutaraldehyde. 2. Autoclave can be used as a method for sterilization in clinical practice and in advanced clinics laser can be used a chair side method of sterilization. 3. Though laser is an effective method of sterilization, further studies have to determine the cutting efficiency and other mechanical properties of endodontic files after repeated exposure to CO2 laser

A Stochastic Model for Crystal-amorphous Transition in Low Temperature Molecular Beam Epitaxial Si(111)

Molecular beam epitaxial Si (111) grown below a certain temperature result in amorphous structure due to the limited surface mobility of atoms in finding correct epitaxial sites. In spite of many experimental and theoretical studies, the mechanism of crystal‐amorphous transition and its dynamics related to the growth conditions are not well understood. In this article, we present a theoretical model based on the formation of stacking fault like defects as a precursor to the amorphous transition of the layer. The model is simulated based on a stochastic model approach and the results are compared to that of experiments for temperatures in the range of 500–900 K and growth rate in the range of 0.1–3.0 Å/s. The agreement between our results and experimental observations is excellent

Graded-bandgap AlGaAs solar cells for AlGaAs/Ge cascade cells

Some p/n graded-bandgap Al(x)Ga(1-x)As solar cells were fabricated and show AMO conversion efficiencies in excess of 15 percent without antireflection (AR) coatings. The emitters of these cells are graded between 0.008 is less than or equal to x is less than or equal to 0.02 during growth of 0.25 to 0.30 micron thick layers. The keys to achieving this performance were careful selection of organometallic sources and scrubbing oxygen and water vapor from the AsH3 source. Source selection and growth were optimized using time-resolved photoluminescence. Preliminary radiation-resistance measurements show AlGaAs cells degraded less than GaAs cells at high 1 MeV electron fluences, and AlGaAs cells grown on GaAs and Ge substrates degrade comparably

Enhanced strength and temperature dependence of mechanical properties of Li at small length scales and its implications for Li metal anodes

Most next-generation Li-ion battery chemistries require a functioning lithium metal (Li) anode. However, its application in secondary batteries has been inhibited because of uncontrollable dendrite growth during cycling. Mechanical suppression of dendrite growth through solid polymer electrolytes (SPE) or through robust separators has shown the most potential for alleviating this problem. Studies of the mechanical behavior of Li at any length scale and temperature are virtually non-existent because of its extreme reactivity, which renders sample preparation, transfer, microstructure characterization and mechanical testing prohibitively challenging. We conduct nano-mechanical experiments in an in-situ Scanning Electron Microscope and show that micron-sized Li attains extremely high strengths of 105 MPa at room temperature and of 35MPa at 90$^\circ$C. We demonstrate that single crystalline Li exhibits a power-law size-effect at the micron- and sub-micron length scales, with the strengthening exponent of -0.68 at room temperature and of -1.00 at 90$^\circ$C. We also report the elastic and shear moduli as a function of crystallographic orientation gleaned from experiments and first principles calculations, which show a high level of anisotropy up to the melting point, where the elastic and shear moduli vary by a factor of ~4 between the stiffest and most compliant orientations. The emergence of such high strengths in small-scale Li and sensitivity of this metal's stiffness to crystallographic orientation help explain why the existing methods of dendrite suppression have been mainly unsuccessful and have significant implications for practical design of future-generation batteries.Comment: 43 pages, 4 figures, Supporting Informatio