A Vapor–Liquid–Solid Mechanism for Growing 3C‐SiC Single‐Domain Layers on 6H‐SiC(0001)

Growing device‐quality 3C‐SiC monocrystalline material is still an issue despite two decades of work dedicated to the subject. Using silicon as the substrate generates too many defects in the layers, owing to lattice mismatch, while it is very difficult to control the initial nucleation on an α‐SiC substrate so that 60° rotated domains are randomly formed. Herein, the elaboration of mono‐orientated 3C‐SiC layers on a 6H‐SiC(0001) on‐axis, Si face substrate using a vapor–liquid–solid mechanism is reported. This non‐conventional approach for growing monocrystalline layers involves feeding a Ge–Si melt by a propane flux at a temperature ranging from 1250 to 1550 °C. We show that, by using this technique, the 3C‐SiC material is almost always obtained on an hexagonal substrate, even if the crystal seed is oriented 8° off‐axis. Using on‐axis 6H‐SiC seeds and optimal growth conditions results in the reproducible deposition of single‐domain 3C‐SiC layers. A mechanism is proposed to clarify some aspects of this process.     

Remaining life assessment of welded pipes containing cracks

The presence of flaws, their size and location in the welded region affects the safe service life of pressure pipes operating at elevated temperature. A remaining life assessment was required to determine the probable remaining safe life of pressure pipes in a high-temperature steam distribution system in which crack-like defects had been identified in many welds. The crack-like defects indicated by NDE lacked the required information to fully identify their locations within the pipe wall. The effect of not knowing complete details of the location of the identified cracks was studied by considering a 360° circumferential crack with initial radial dimension a₀. The crack positions evaluated were: an internal surface crack, an external surface crack, and embedded cracks at various distances from the internal wall of the pipe. Crack growth was modeled using time-dependent fracture mechanics and a newly developed computer code.     

Cytochemical characterization and structural approach to Prosopis juliflora (Sw) D.C. seed gum extraction

Mesquite seed is a good gum source. In this work seed structure was characterized in order to help seed gum extraction. Seed gum was located between cotyledon and seed coat. Milling process yielded, basically, two fractions: endosperm and seed coat plus gum. This fact was supported by the cytochemical observation of a dense layer near the seed coat and a loose layer near the endosperm. Cells containing seed gum had thick cell walls and a proteic core. Seed gum extraction implications are discussed. Copyright © 2005 Society of Chemical Industry     

On the thermochemistry of the rare earth compounds with thep-block elements

A summary is given of the experimental work carried out to determine the heats of formation of the rare earth compounds with a number of elements of thep-block of the periodic table (in particular Al, In, Sn, Pb, As, Sb, and Bi). The experimental methods and the devices constructed to this end are briefly described and commented on. Some results recently obtained in the thermochemical investigation of binary (Pr-Al and Gd-Pb) and ternary (Ce-Ni-Al and La-Sb-Bi) alloys are presented.     

On the design of high-complexity cosine-modulated transmultiplexers based on the frequency-response masking approach

Two efficient techniques exploiting the frequency-response masking (FRM) approach are proposed in order to make it feasible to design prototype filters for highly selective nearly perfect-reconstruction cosine-modulated transmultiplexers and filter banks (CMTs and CMFBs) having a very large number of channels. In these design schemes, the number of unknowns is drastically reduced when compared with the corresponding techniques for designing direct-form prototype filters. Furthermore, in the proposed techniques, the main figures of merits, that is, the intersymbol interference and the interchannel interference for CMTs and the overall and aliasing distortions for CMFBs are taken into account in a controlled manner. In order to speed up the convergence of these two optimization techniques, simplifications for computing the resulting nonlinear constraints and the corresponding gradient vectors are proposed. They differ from each other in the sense that the first and second ones utilize the frequency-domain and time-domain constraints for controlling the figures of merit, respectively. Combining these two techniques results in numerically efficient algorithms for designing optimized CMTs (or CMFBs) with a reduced computational complexity (number of arithmetic operations per output sample), particularly when both branches of the FRM structure are required. Design examples are included illustrating the efficiency of the design methods and the high performance of the resulting CMT structures.     

A Cross-Layer Multi-Hop Simulator for IEEE 802.11e

In this work we simulate the ad hoc mode of IEEE 802.11e for routing optimisation. We simulate the behaviour of routing algorithms at the network layer by using a custom-made cross-layer network simulator developed by our team, which simultaneously considers the physical and Medium Access Control (MAC) layers. Although the simulator also supports the infrastructure mode, in this paper we focus on the ad hoc feature which was introduced by the authors. We opted for the simulator approach over the theoretical analysis, but we also present a mathematical model for IEEE 802.11 ad hoc networks. Some initial tests were performed by using a simple routing algorithm (to evaluate the behaviour of the system in terms of selection of the path between a source and a destination, and the correctness of the calculated metrics, which include end-to-end delay, packets lost, packets delivered), but more advanced cross-layer design solutions were also tested. When information from the physical and MAC layers is used as an input to the routing algorithm, improvements are achieved in the performance of the network. Several functions were compared and the algorithm that privileges shorter links accounting with the metric “collision rate” achieves the best results. When compared with a standard routing solution, this cross-layer approach allows to increase the number of packets delivered, while not significantly affecting the end-to-end delay of the packets.     

Design and validation of a novel multiplex real-time PCR assay for Vibrio pathogen detection

Three species--Vibrio cholerae, Vibrio parahaemolyticus, and Vibrio vulnificus--account for the majority of vibrio infections in humans. Rapid and accurate identification of Vibrio species has been problematic because phenotypic characteristics are variable within species. Additionally, biochemical identification and confirmation require 2 or more days to complete. Rapid and sensitive molecular techniques for the detection of vibrio pathogens would be useful for the surveillance and management of outbreaks. To facilitate the identification of human-pathogenic species, we designed and validated a highly sensitive, specific, and robust multiplex real-time PCR assay to identify V. cholerae, V. parahaemolyticus, and V. vulnificus using a four-dye configuration in a convenient lyophilized format. Multiple Vibrio strains were sequenced to verify candidate target TaqMan sites. Several individual assays within the multiplex contain multiple primers or probes to ensure detection of polymorphic variants. V. cholerae, V. parahaemolyticus, and V. vulnificus were detected either individually or in mixtures at ≤30 genomic copies. V. cholerae was specifically detected in the presence or absence of Vibrio mimicus. The Vibrio multiplex assay showed 100% specificity to all targets analyzed and no detection of nearest neighbor strains. Each assay exhibited 100% ± 10% efficiency. Multiplex real-time PCR can simplify pathogen detection and reduce costs per test since three species can be analyzed in a single reaction tube. Rapid and accurate detection of pathogenic vibrios in shellfish or seawater samples will improve the microbiological safety of seafood for consumers.     

Mechanical and dynamic mechanical properties of rice husk ash–filled natural rubber compounds

A laboratory‐sized two‐roll mill was used to incorporate rice husk ash into natural rubber (NR). A conventional vulcanization system was used for curing and cure studies were carried out on a Monsanto rheometer. Physical testing of the NR vulcanizates involved determining tensile and tear resistances and hardness. Swelling behavior of NR compounds and scanning electron microscopy were used to investigate the interaction between rice husk ash and natural rubber. Also, dynamical mechanical thermal analysis was used to assess filler–rubber interactions in terms of storage modulus (E′) and loss tangent (tan δ). For comparison purposes, two commercial fillers, precipitated silica (Zeosil‐175) and carbon black (N774), were also used. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2331–2346, 2002