The parameterized feedback vertex (arc) set problem is to find whether there are k vertices (arcs) in a given graph whose removal makes the graph acyclic. The parameterized complexity of this problem in general directed graphs is a long standing open problem. We investigate the problems on tournaments, a well studied class of directed graphs. We consider both weighted and unweighted versions. 相似文献
A novel estimate for the line-to-ground capacitance that accurately predicts the pull-in instability parameters for narrow electrostatically actuated microbeams is proposed. Parameters in the proposed formula are obtained by least square fitting data from a fully converged numerical solution with the method of moments. For a narrow microbeam, it is shown that the new formula significantly improves upon classical formulas that neglect fringing field effects due to the finite thickness of the microbeam cross-section 相似文献
The growth of CIGS thin films on soda-lime glass substrates at different substrate temperatures by dual ion beam sputtering system in a single-step route from a single quaternary sputtering target with the composition of Cu (In0.70 Ga0.30) Se2 was reported. The effects of the substrate temperature on structural, optical, morphological and electrical properties of CIGS films were investigated. Stoichiometry of one such film was investigated by X-ray photoelectron spectroscopy. All CIGS films had demonstrated a strong (112) orientation located at 2θ ~26.70o, which indicated the chalcopyrite structure of films. The value of full-width at half-maximum of (112) peak was reduced from 0.58° to 0.19° and crystallite size was enlarged from 14.98 to 43.05 nm as growth temperature was increased from 100 to 400 °C. However, atomic force microscope results showed a smooth and uniform surface at lower growth temperature and the surface roughness was observed to increase with increasing growth temperature. Hall measurements exhibited the minimum film resistivity of 0.09 Ω cm with a hole concentration of 2.42 × 1018 cm?3 and mobility of 28.60 cm2 V?1 s?1 for CIGS film grown at 100 °C. Film absorption coefficient was found to enhance nominally from 1 × 105 to 2.3 × 105 cm?1 with increasing growth temperature from 100 to 400 °C. 相似文献
A highly practical, one‐step, facile synthesis of aromatic, heteroaromatic, allylic and aliphatic nitriles from primary alcohols catalyzed by ferric nitrate [Fe(NO3)3⋅9H2O] in the presence of TEMPO, aqueous ammonia and air at room temperature is described.
The discovery and understanding of gecko ‘frictional-adhesion’ adhering and climbing mechanism has allowed researchers to mimic and create gecko-inspired adhesives. A few experimental and theoretical approaches have been taken to understand the effect of surface roughness on synthetic adhesive performance, and the implications of stick–slip friction during shearing. This work extends previous studies by using a modified surface forces apparatus to quantitatively measure and model frictional forces between arrays of polydimethylsiloxane gecko footpad-mimetic tilted microflaps against smooth and rough glass surfaces. Constant attachments and detachments occur between the surfaces during shearing, as described by an avalanche model. These detachments ultimately result in failure of the adhesion interface and have been characterized in this study. Stick–slip friction disappears with increasing velocity when the flaps are sheared against a smooth silica surface; however, stick–slip was always present at all velocities and loads tested when shearing the flaps against rough glass surfaces. These results demonstrate the significance of pre-load, shearing velocity, shearing distances, commensurability and shearing direction of gecko-mimetic adhesives and provide us a simple model for analysing and/or designing such systems. 相似文献
Enteric copolymers of acrylic acid and methyl methacrylate (2.5:7.5 and 2:8) were prepared using tetrahydrofuran as solvent
and AIBN as free radical initiator for colon targeting. FTIR and 1H NMR spectra of the copolymers showed absence of vinyl bond/protons present in the monomers suggesting successful polymerization.
Flurbiprofen sodium microspheres (M1 and M2) made with the copolymers, by oil/oil solvent evaporation, were spherical, anionic
(zeta potential –57.8 and –53.7 mV) and contained 5.47 and 5.89% drug. FTIR spectrum of microspheres showed peaks for aromatic
C = C stretching and substituted benzene ring, indicating entrapment of flurbiprofen. PXRD revealed crystalline structure
of flurbiprofen while copolymer and microspheres were amorphous. DSC thermograms showed a sharp melting endotherm of flurbiprofen
sodium at 129.26°C against broad endotherms of copolymers and microspheres. The microspheres released 43 and 36% drug at pH
6.8 in 2 h and 99 and 96% at pH 7.4 in next 3–4 h.The microspheres did not adhere on gastric-mucosa at pH 1.2 but showed mucoadhesion
time of 18 min and 9 min on intestinal mucosa at pH 6.8. Thus, the microspheres on oral administration, would release the
drug in colon, suggesting the potential of the hemocompatible copolymers for pH dependent colon targeted drug delivery system. 相似文献
We have studied the defect formation energies of the various native (vacancies, interstitials, and antisites) and Au defects
in Hg1−xCdxTe using density functional-based total energy calculations with ultrasoft pseudo-potentials. These studies are important
for infrared (IR) detection technology where the device performance can be severely degraded because of defects. To calculate
formation energies, we modeled the neutral and charged defects using supercells containing 64 atoms. From the formation energies,
we have determined the defect concentrations as a function of stoichiometry and temperature. We find the prevalent neutral
defects to be Au at the Hg site (AuHg), Hg vacancies (VHg), and Te antisites (TeHg). We have also explicitly studied charged defects and have found Te
Hg2+
, Au
Hg1−
, V
Hg1−
, V
Hg2−
, and V
Te2+
to have low formation energies. We have identified AuHg to be the prevalent Au defect, having concentrations several orders of magnitude greater than the other Au defects. We find
that the charge state of VHg is primarily (1−) or (2−) depending on the electronic chemical potential. 相似文献
Summary We presume that plane strain state of deformation prevails when the interior of a long gun barrell or a cylindrical pressure vessel is dynamically loaded. The viscoplastic material of the body is taken to exhibit strain-rate hardening and thermal softening. Two thin ellipsoidal voids located symmetrically on the horizontal axis and near the center of the cylinder wall act as nuclei for the initiation of shear bands. We note that deformations of the cylinder are nonhomogenous even in the absence of the voids. It is therefore interesting to investigate when the bands initiate from the void tips and the interaction, if any, among them.It is found that shear bands initiate first at void tips closer to the center of the cylinder. These bands propagate faster to the inner surface of the cylinder as compared to those initiating from the other void tips which propagate towards the outer bounding surface of the cylinder. Whereas contours of constant maximum principal logarithmic strain originating from the outer void tips spread out laterally in both directions as they propagate into the cylinder, those originating from the inner void tips spread out in only one lateral direction as they propagate into the body. 相似文献