On the Contrast in Visual Cryptography Schemes

A visual cryptography scheme is a method to encode a secret image SI into shadow images called shares such that certain qualified subsets of shares enable the ``visual' recovery of the secret image. The ``visual' recovery consists of xeroxing the shares onto transparencies, and then stacking them. The shares of a qualified set will reveal the secret image without any cryptographic computation. In this paper we analyze the contrast of the reconstructed image in k out of n visual cryptography schemes. (In such a scheme any k shares will reveal the image, but no set of k-1 shares gives any information about the image.) In the case of 2 out of n threshold schemes we give a complete characterization of schemes having optimal contrast and minimum pixel expansion in terms of certain balanced incomplete block designs. In the case of k out of n threshold schemes with we obtain upper and lower bounds on the optimal contrast. Received 27 September 1996 and revised 13 February 1998     

Single-wall carbon nanotubes prepared with different kinds of Ni-Co catalysts: Raman and optical spectrum analysis

The addition of acetates and nitrates for the synthesis of single-wall carbon nanotubes via laser ablation was explored. Targets containing nominal amounts of acetates or nitrates in addition to Ni and Co catalysts were compared to a standard target containing only Ni and Co at temperatures ranging from 1000 to 1200 °C. The as produced web-like soot was characterized by transmission electron microscopy, Raman spectroscopy and optical absorption spectroscopy. All samples showed a linear increase in SWCNT mean diameter with temperature; however, the rate of mean diameter change with temperature differed for the various targets, more so at lower temperatures. The addition of nitrates improved the SWCNT relative yield over all the temperatures used, whereas the inclusion of acetates improved the relative yield only at lower temperatures. The above results were discussed and analyzed according to the porous structure of the targets resulting from the decomposition of the acetates and nitrates and consequential thermal diffusion changes.     

Spontaneous and piezoelectric polarization effects on the outputcharacteristics of AlGaN/GaN heterojunction modulation doped FETs

We report on the calculation of electrical characteristics of AlGaN/GaN heterojunction field effect transistors (HFETs). The model is based on the self-consistent solution of the Schrodinger and Poisson equations coupled to a quasi-2D model for the current flow. Both single and double heterojunction devices are analyzed for [0001] or [000-1] growth directions. The onset of a parasitic p-channel for particular growth directions and alloy concentrations is also shown     

Breakdown quenching in high electron mobility transistor by usingbody contact

In this paper, the effect of a body contact (BC) to quench breakdown effects and increase the breakdown voltage in high-electron mobility transistors (HEMTs) is theoretically investigated. The body contact is formed by a highly p-type doped substrate connected to an ohmic back contact. By means of a two-dimensional (2-D) self consistent Monte Carlo simulator we show that the BC prevents holes generated by impact ionization (II) from accumulating in the channel and in the buffer, thus inhibiting the parasitic bipolar effect (PBE). This improves the breakdown behavior and extends the range of the usable drain voltages     

Liquid-crystallization induced reactions. Microstructure and morphology of copolyesters synthesized by transesterification of PET with some LCP monomers

The transesterification of poly(ethylene terephthalate) (PET) with a mixture of sebacic acid (S), 4,4′-diacetoxybiphenyl (B) and 4-acetoxybenzoic acid (H), carried out under conditions expectedly favoring the formation of a p(ET-SBH) random copolyester, produces biphasic materials with an isotropic matrix and a highly fibrous, liquid-crystalline dispersed phase. Spectroscopic, calorimetric, microscopic and diffractometric characterization of the fractions separated by solvent extraction has shown that the two phases consist of practically random copolyesters having different average composition. Interestingly, the degree of aromaticity of the matrix is even lower than that of PET, whereas that of the minor phase is appreciably higher than that calculated for the SBH copolyester that would be produced from the monomer mixture in the absence of PET. This unexpected result is interpreted on the basis of an enthalpy-driven progressive diffusion of aromatic-rich material toward the mesophase which segregates at an early stage of the polycondensation within the isotropic mixture of low molar mass oligomers initially produced by the PET acidolysis. Thus, an increasing differentiation, rather than an equilibration, of the composition of the two phases takes place. It is noteworthy that, despite the strong compositional difference, the two phases of these products show fairly good compatibility and interfacial adhesion.     

Pyrolysis of glass-hardened polyamides used in the automotive industry

Thermogravimetry and on-line flash pyrolysis–gas chromatography–mass spectrometry were used for studying thermal decomposition processes in filled polyamides deriving from used cars. The thermal decomposition of each polymer begins at about 350 °C and proceeds with a weight loss of 100% for the non-glass fibres, under nitrogen atmosphere. The analysis of the pyrolyzate compounds shows that from polyamide 66, the most abundant product volatile at degradation temperatures is cyclopentanone, while from polyamide 6 there is a considerable yield of ε-caprolactam. This revised version was published online in November 2006 with corrections to the Cover Date.     

Local surface interpolation with Bézier patches

A surface interpolation method for meshes of cubic curves is described. A mesh of cubic curve is constructed between the given vertices. This mesh is filled with Bézier patches, so that the surface is represented as a union of geometrically continuous bicubic quadrilateral and/or quartic triangular Bézier patches. The method is local and uses Farin's [Farin '83] conditions of G¹ continuity between patches. The procedure for finding the needed control points of the Bézier patches is simple and efficient.