Catalyst Design by NH4OH Treatment of USY Zeolite

Hierarchical zeolites are a class of superior catalysts which couples the intrinsic zeolitic properties to enhanced accessibility and intracrystalline mass transport to and from the active sites. The design of hierarchical USY (Ultra‐Stable Y) catalysts is achieved using a sustainable postsynthetic room temperature treatment with mildly alkaline NH₄OH (0.02 m ) solutions. Starting from a commercial dealuminated USY zeolite (Si/Al = 47), a hierarchical material is obtained by selective and tuneable creation of interconnected and accessible small mesopores (2–6 nm). In addition, the treatment immediately yields the NH₄⁺ form without the need for additional ion exchange. After NH₄OH modification, the crystal morphology is retained, whereas the microporosity and relative crystallinity are decreased. The gradual formation of dense amorphous phases throughout the crystal without significant framework atom leaching rationalizes the very high material yields (>90%). The superior catalytic performance of the developed hierarchical zeolites is demonstrated in the acid‐catalyzed isomerization of α‐pinene and the metal‐catalyzed conjugation of safflower oil. Significant improvements in activity and selectivity are attained, as well as a lowered susceptibility to deactivation. The catalytic performance is intimately related to the introduced mesopores, hence enhanced mass transport capacity, and the retained intrinsic zeolitic properties.     

Multi-level analysis of an interaction network between individuals in a mailing-list

It is well known now that most real-world complex networks have some properties which make them very different from random networks. In the case of interactions between authors of messages in a mailing-list, however, a multi-level structure may be responsible for some of these properties. We propose here a rigorous but simple formalism to investigate this question, and we apply it to an archive of the Debian user mailing-list. This leads to the identification of some properties which may indeed be explained this way, and of some properties which need deeper analysis.     

Modification of emission wavelength in organic random lasers based on photonic glass

Control of the emission wavelength of a random laser (RL) system over a 7-nm waveband is demonstrated using a green-emitting π-conjugated polymer infiltrated into a photonic glass formed by nano/micro-size monodisperse silica spheres. The use of a solution-based conjugated polymer enables the complete filling of the voids within the photonic glass without suffering from quenching and the gain can therefore be maximized. The emission wavelength of these structures is set by a combination of the material system spectral gain and of the transport mean free path, the latter being controlled by the mean diameter of the spheres in the nano-scale range. Transport mean free paths of photons in the RL’s active region are calculated using Mie scattering theory and corroborated with coherent backscattering measurements. Further wavelength modification is also possible by changing the pump spot size and the pump fluence.     

Location and calculation-free node-scheduling schemes in large wireless sensor networks

In wireless sensor networks that consist of a large number of low-power, short-lived, unreliable sensors, one of the main design challenges is to obtain long system lifetime without sacrificing system original performance (sensing coverage and sensing reliability). To solve this problem, one of the potential approaches is to identify redundant nodes at the sensing interface and then assign them an off-duty operation mode that has lower energy consumption than the normal on-duty mode. In our previous work [J. Wireless Commun. Mobile Comput. 3 (2003) 271; Processing of ACM Wireless Sensor Network and Application Workshop 2002, September 2002], we proposed a node-scheduling scheme, which can provide a 100% sensing coverage preservation capability. This, however, requires each node to be aware of its own and its neighbors’ location information. Also, in that scheme, each node has to do accurate geometrical calculation to determine whether to take an off-duty status. In this paper, we propose and study several alternative node-scheduling schemes, which cannot completely preserve the original system coverage, but are nonetheless light-weighted and flexible compared with the previous one. Our simulation results compare these schemes with the previous one and demonstrate their effectiveness.     

A meshless method for solving the EEG forward problem

We present a numerical method to solve the quasistatic Maxwell equations and compute the electroencephalography (EEG) forward problem solution. More generally, we develop a computationally efficient method to obtain the electric potential distribution generated by a source of electric activity inside a three-dimensional body of arbitrary shape and layers of different electric conductivities. The method needs only a set of nodes on the surface and inside the head, but not a mesh connecting the nodes. This represents an advantage over traditional methods like boundary elements or finite elements since the generation of the mesh is typically computationally intensive. The performance of the proposed method is compared with the boundary element method (BEM) by numerically solving some EEG forward problems examples. For a large number of nodes and the same precision, our method has lower computational load than BEM due to a faster convergence rate and to the sparsity of the linear system to be solved.     

Complex permittivity characterization of benzocyclobutene for terahertz applications

High-quality polymers such as the benzocyclobutene polymer (BCB) provide interesting dielectric feature for terahertz applications. Already used in silicon integrated circuit technologies, this material could become one of the most promising candidates for the realization of future THz waveguides and interconnections on a silicon substrate but also after active devices process on the top of any other technology (GaAs, InP, GaN…). A frequency-dependent complex permittivity of spin-coated thick layers of this low-k dielectric is obtained from transmittance spectra measured with Fourier transform spectroscopy in the frequency range of 0.5-5.4 THz. The dielectric constant and the loss tangent are discussed according to curing conditions of the photosensitive resin used. A low loss tangent value of 7 − 9 × 10^-3 at 1 THz is obtained with polymerisation in oxygen-free atmosphere. An incomplete curing and a high dose UV exposure have a weak impact on losses. These results associated with the high compatibility of this polymer with silicon and metals make BCB layers well suited for the design of microelectronic THz devices and circuits.     

Laser-Empowered Random Metasurfaces for White Light Printed Image Multiplexing

Printed image multiplexing based on the design of metasurfaces has attracted much interest in the past decade. Optical switching between different images displayed directly on the metasurface is performed by altering the parameters of the incident light such as polarization, wavelength, or incidence angle. When using white light, only two-image multiplexing is implemented with polarization switching. Such metasurfaces are made of nanostructures perfectly controlled individually, which provide high-resolution pixels but small images and involve long fabrication processes. Here, it is demonstrated that laser processing of nanocomposites offers a versatile low-cost, high-speed method with large area processing capabilities for controlling the statistical properties of random metasurfaces, allowing up to three-image multiplexing under white light illumination. By independently controlling absorption and interference effects, colors in reflection and transmission can be varied independently yielding two-image multiplexing under white light. Using anisotropy of plasmonic nanoparticles, a third image can be multiplexed and revealed through polarization changes. The design strategy, the fundamental properties, and the versatility of implementation of these laser-empowered random metasurfaces are discussed. The technique, applied on flexible substrate, can find applications in information encryption or functional switchable optical devices, and offers many advantages for visual security and anticounterfeiting.     

Size effects on the DC characteristics and low frequency noise of double polysilicon NPN bipolar transistors

DC and low frequency noise measurements have been carried out for double polysilicon NPN bipolar transistors. Our experiments have highlighted unexpected geometrical dependencies for the base saturation current density and low frequency noise. A model, taking into account a variation of the interfacial oxide thickness at the periphery of the emitter has been proposed and it has been successfully applied to the experimental results.     

Recovering missing slices of the discrete fourier transform using ghosts

The discrete Fourier transform (DFT) underpins the solution to many inverse problems commonly possessing missing or unmeasured frequency information. This incomplete coverage of the Fourier space always produces systematic artifacts called Ghosts. In this paper, a fast and exact method for deconvolving cyclic artifacts caused by missing slices of the DFT using redundant image regions is presented. The slices discussed here originate from the exact partitioning of the Discrete Fourier Transform (DFT) space, under the projective Discrete Radon Transform, called the discrete Fourier slice theorem. The method has a computational complexity of O(nlog(2)n) (for an n=N×N image) and is constructed from a new cyclic theory of Ghosts. This theory is also shown to unify several aspects of work done on Ghosts over the past three decades. This paper concludes with an application to fast, exact, non-iterative image reconstruction from a highly asymmetric set of rational angle projections that give rise to sets of sparse slices within the DFT.