Structural,mechanical and electrical-contact properties of nanocrystalline-NbC/amorphous-C coatings deposited by magnetron sputtering

Niobium-carbide nanocomposite coatings with a carbon content varying from 43 to 64 at.% were deposited by dual DC magnetron sputtering. X-ray diffraction, x-ray photoelectron spectroscopy and electron microscopy showed that all coatings consisted of nanometer sized NbC grains embedded in a matrix of amorphous carbon. Mechanical properties and electrical resistivity showed a strong dependency on the amount of amorphous carbon (a-C) and NbC grain size in the coating. The highest hardness (23 GPa), elastic modulus (295 GPa) and the lowest resistivity (260 μΩ cm) were measured for the coating with about 15% of a-C phase. Contact resistance measurements using a crossed cylinder set-up showed lowest contact resistance for the coating containing 33% a-C (140 μΩ at a contact force of 100 N), which is comparable to a Ag reference (45 μΩ at a contact force of 100 N). Comparison with TiC-based nanocomposites studied under similar conditions showed that the NbC system has less tendency to form a-C and that lowest contact resistance is obtained at comparable amounts of a-C phase in both material systems (33% for NbC compared to 35% for TiC). With these good electrical contact properties, the NbC nanocomposites can be considered as a potential material for electrical contact applications.     

CHEMICAL ANALYSIS OF HIGH ASH BRAZILIAN COAL TAR. 3. HYDROCARBON CHARACTERIZATION

Abstract

In this paper we show the total analysis of the hydrocarbon fractions obtained from the chromatographic fractionation of a coal tar cut, previously described. All fractions were analyzed by Gas Chromatography coupled to Mass Spectrometry using a non-polar capillary column. A group-type distribution of aliphatic, olefins and monoaromatic hydrocarbons was found in the first three fractions obtained (I, II and III). In fractions IV and V polyaromatics with two or more condensed rings were founded. Many of these hydrocarbons are recognized to be carcinogenic and mutagenic, including pyrene, chrysene and others.     

Predominant enterobacteria on modified-atmosphere packaged meat and poultry

Enterobacteria on modified-atmosphere (MA) packaged meat (n = 54) and poultry (n = 32) products were enumerated, and 899 isolates were picked and ribotyped. For identification, 16S rRNA genes of representative strains were sequenced and analyzed. Altogether 54 (60%) of the samples contained enterobacteria >10⁴ CFU/g. In 34% of the poultry samples, enterobacteria counts were >10⁶ CFU/g suggesting that enterobacteria may contribute to spoilage of MA packaged poultry. The enterobacteria identified were predominantly Hafnia spp. (40%) and Serratia spp. (42%) with Hafnia alvei, Hafnia paralvei, Serratia fonticola, Serratia grimesii, Serratia liquefaciens, Serratia proteamaculans, and Serratia quinivorans being the species identified. In addition, 6% of the isolates were identified as Rahnella spp., 3% as Yersinia spp., and 1% as Buttiauxella spp. Percentage distributions of the predominant genera in different products showed that 89% of the Serratia spp. were from products packaged under a high-O₂ MA containing CO₂ (25–35%), whereas most (76%) isolates of Hafnia originated from anaerobically packaged red meat and poultry. These findings suggest that the gas mixture used for MA packaging influence the selection of enterobacteria growing on meat and poultry.     

Asymmetric Synthesis of 3‐Substituted Cyclohexylamine Derivatives from Prochiral Diketones via Three Biocatalytic Steps

Prochiral bicyclic diketones were transformed to a single diastereomer of 3‐substituted cyclohexylamine derivatives via three consecutive biocatalytic steps. The two chiral centres were set up by a C C hydrolase (6‐oxocamphor hydrolase) in the first step and by an ω‐transaminase in the last step. The esterification of the intermediate keto acid was catalysed by a lipase in the second step if possible. For two substrates the C C hydrolytic step as well as the esterification could be run simultaneously in a one‐pot cascade in an organic solvent. In one example, the reaction mixture of the first two steps could be directly subjected to bio‐amination in an organic solvent without the need to change the reaction medium. Depending on the choice of the ω‐transaminase employed and the substrate the cis‐ as well as the trans‐diastereomers could be obtained in optically pure forms.     

Exhaust Gas Catalysts for Heavy-Duty Applications: Influence of the Pd Particle Size and Particle Size Distribution on the Combustion of Natural Gas and Biogas

In this study, an experimental investigation concerning exhaust gas catalysts for heavy-duty diesel engines fuelled by natural gas or biogas is presented. Miniature monoliths, 2.5 wt% Pd/Al₂O₃, have been prepared, characterised and tested. Various methods have been used in order to obtain different palladium particle sizes, including incipient wetness and microemulsion technique. Crystallite sizes between 2 and 40 nm were obtained. We observed that the metal particle size influences the activity for methane oxidation. Moreover, the homogeneity of the active material was found to affect the reaction rate.     

Lack of male teachers: a problem for students or teachers?

This article explores young people's perspectives on males and females as teachers, contrasting these with teachers' perceptions. It builds on 90 interviews of school students aged 13-14 and 60 follow-up interviews 4 years later. The first interviews were conducted in ethnographic context in two secondary schools in the mid-1990s in Helsinki, Finland. Whilst lack of male teachers is a recurrent theme in educational discussion, widely agreed among teachers, gender did not appear to be relevant when young persons talked about teachers. They appreciate teachers, irrespective of gender, who can teach and are friendly and relaxed, but who nevertheless keep order and make sure that students work. Male teachers who teach popular, non-academic subjects were often favoured by boys, but so were female teachers of academic subjects and increasingly as time went on. The interviews suggest that students do not need male teachers to act as 'male models'. They also suggest that male teachers should be sensitive in relationships with female students. When questioned explicitly, most of the interviewees did not regard lack of male teachers as a major problem     

磨浆机浓度的控制

精浆（或打浆）过程包括对纤维的压溃和切断作用。●压溃纤维使纸页强度更高,可以提高纸页的抗张强度、紧度与不透明度,而脱水速率则降低。●切断纤维可以提高纸页的透气度,降低纸页强度并使纸页变得更柔软,不透明度提高而脱水速率降低。使用磨浆机打浆,可以获得我们所希望的打浆结果。但应注意随着磨浆机磨盘的磨损,磨浆机的打浆作用会发生变化。由于这是关系到成纸质量的一个决定性因素,因此对磨浆机的控制非常重要。而浓度控制是其中的关键,它与流量控制一起构成了对磨浆机进行成功控制的基础。控制策略采用手动控制时,调节是根据纸页实…     

CVD-based tungsten carbide schottky contacts to 6H-SiC for very high-temperature operation

In this study, tungsten carbide, with its hardness, chemical inertness, thermal stability and low resistivity (25 μΩ cm)¹ is shown as a reliable contact material to n- and p-type 6H-SiC for very high temperature applications. WC films with thicknesses of 100–150 nm were deposited by chemical vapor deposition (CVD) from a WF₆/C₃H₈/H₂ mixture at 1173 K. A method to pattern CVD-tungsten carbide is suggested. TEM analysis of as deposited samples displayed a clear and unreacted interface. The electrical investigations of the p-type 6H-SiC Schottky contacts revealed a high rectification ratio and a low reverse current density (6.1 × 10⁻⁵ A cm⁻², −10 V) up to 773 K. On n-type, a low barrier (Φ_Bn=0.79 eV) at room temperature was observed. The low Φ_Bn value suggests WC to be promising as an ohmic contact material on highly doped n-type epi-layers. We will show a temperature dependence for the barrier height of tungsten carbide contacts that can be related to the simultaneous change in the energy bandgap, which should be considered when designing SiC devices intended for high temperature operation.