Kinetics of ultrafine coating of SiO2 nanoparticles, suspended in non-thermal CH4/C2H6 plasma

Capacitively coupled RF discharges in methane and ethane (1000–3000 Pa, 5–15 W/cm³) were used for the in-flight coating of SiO₂ nanopowders (NP) with an a-C:H layer [A. Kouprine, F. Gitzhofer, M. Boulos, A. Fridman, Polymer-like C:H thin film coating of nanopowders in capacitively coupled RF discharge, Plasma Chemistry and Plasma Processing 24 (2) (2004) 189–215]. In this present work a model of the chemical kinetics of this plasma processing has been developed, based on the GRI-Mech 3.0 mechanism [G.P. Smith, D.M. Golden, M. Frenklach, Gas Research Institute, Detailed chemical reaction mechanism. Available from: ], and calculations are performed, using the Chemkin-II code. The model includes the formation of two solid phases: an amorphous C:H coating, deposited on the suspended NP, and soot, incepted in the gas phase. Non-equilibrium plasma effects are taken into account by the rates of radical species’ production, using the reference data on the “G-value”. The CH₃ and H species appear to have key roles in surface activation and the nanofilm growth. The results demonstrate the competitive character of the formation of the two solid phases: the C:H coating is dominant under T_gas < 1400–1600 K, depending on the conditions, and soot commencing to dominate at higher temperatures. This division corresponds to the passage from non-thermal plasma conditions, with the dominance of single-valent radicals and solid phase growth on the substrate only, to the plasma thermalisation with the formation of two-valent radicals and v. inception of the solid phase. The results are validated experimentally by the observation of soot formation as a function of the gas temperature, by the mass spectrograph data on the gas composition after plasma reforming, and by the reference data on coating growth rates and induction times for soot formation.     

STUDIES ON PHOSPHONIUM YLIDS‐VIII. WITTIG REACTION WITH N,N′-3,5-CYCLO-HEXADIENE-1,2-DIYLIDENEBIS[BENZAMIDE]

Abstract

The reaction of methylenetriphenylphosphoranes (1) with N,N′-3,5-cyclohexadiene-1,2-diylidenebis [benzamide] (2) afforded new ylid-phosphoranes of type 3. The mechanism that accounts for formation of adducts 3 is discussed. Structural reasoning for compounds 3 was based on compatible analytical, chemical and spectroscopic results.     

Inspiration from Old Dyes: Tris(stilbene) Compounds as Potent Gram‐Positive Antibacterial Agents

Herein we describe the preparation and structure‐activity relationship studies on range of stilbene based compounds and their antibacterial activity. Two related compounds, each bearing carboxylic acid moieties, exhibit good activity against several bacterial strains, including methicillin‐resistant Staphylococcus aureus MRSA (ATCC 33592 and NCTC 10442). Compound 10 was most active against Moraxella catarrhalis with minimum inhibitory concentrations (MICs) of 0.12–0.25 μg mL^?1 and against Staphylococcus spp. with MICs ranging from 2–4 μg mL^?1. The derivative 17 showed increased activity with MICs of 0.06–0.25 μg mL^?1 against M. catarrhalis and 0.12–1 against Staphylococcus spp. This level of activity is similar to that reported for S. aureus for antibiotics, such as vancomycin, with MICs of ≤2.0 μg mL^?1 and clindamycin with MICs of ≤0.5 μg mL^?1. As an indicator of toxicity, 17 was tested for its ability to lyse sheep erythrocytes, and showed low haemolytic activity. Such results highlight the value of tris(stilbene) compounds as antibacterial agents providing suitable properties for further development.     

Polymer-Like C:H Thin Film Coating of Nanopowders in Capacitively Coupled RF Discharge

Nanopowders of amorphous SiO₂, with typical particle sizes of 30–80 nm, were treated under the non-equilibrium plasma conditions, created by a capacitively coupled (CC) RF discharge formed in pure methane or ethane. The plasma gas flow rate varied between 0.02 and 0.06 slpm, with reactor pressures between 1000 and 5000 Pa, and applied RF power inputs between 700 and 1500 W. The plasma properties were monitored through measurements of the rotational temperature, as derived from the C₂ 5160 Å Swan band and N₂ second positive 3577 Å band, and the atomic hydrogen excitation temperature, from the H_β H_γ and H_δ lines during the powder treatment process. The compositions of the gases that passed through the plasma were analyzed by mass spectrometry. In spite of the evidence for the presence of C_nH_2n+2 and C_nH_2n (n=1–3) species and acetylene in the discharge, the homogeneous formation of soot was not observed. At the same time, the introduced nanoparticles were observed to act as centers for the inception and growth of C:H thin coatings in the form of a polymer-like hydrocarbon layers, of thickness between <5 and 30 nm. The results of TEM, IR spectroscopy, thermo-gravimetric and precision calorimetric analysis performed on the treated powders provide evidence to the formation of an amorphous, high density C:H matrix on the particles' surfaces.     

Volumetric emission of argon plasmas in the presence of vapors of Fe,Si, and Al

The net volumetric emission was calculated for argon plasmas at atmospheric pressure in the presence of metal vapors for different elements, over the temperature range from 3000 to 30,000 K. The computations are based on the escape factor model, using a semi-empirical method for the determination of line profiles and line broadening effects. Results for iron, .silicon, and aluminum show an important influence of the presence of even the smallest concentrations of the metal vapors on the net emission coefficient of the plasma. The effect is strongest for iron, followed by aluminum and .silicon. Special attention is given to self-absorption effects which are most important in the first millimeter o% the optical path of the emitted radiation. The effect is incorporated into the calculation procedure of the net emission coefficient and can be used as a volumetric energy sink as long as the absorption length is shorter than the radius of the control volume used in the computation scheme.     

Thermal Properties Evolution of PCB FR4 Epoxy Composites for Mechatronic During Very Long Ageing

Very long ageing times, up to 8000 h at 110 °C, were achieved on printed circuit boards PCB FR4 Laminate commonly used in automotive design. This composite is fabricated from glass fiber embedded in a flame retardant epoxy resin. Ageing is performed in an oven under air atmosphere at a temperature lower than the glass transition temperature, thus physical ageing must take place. Thermogravimetric analysis indicates that during heating, the flame retardant interacts first with O₂ atmosphere for temperature higher than 180 °C. Fourier transform infrared spectroscopic analysis shows that chemical degradation of the samples due to oxidation occurs after ageing at 110 °C for time longer than 940 h. From thermally modulated differential scanning calorimetry analysis, two different modifications of the epoxy resin are evidenced: one fraction for which oxidation modified drastically the molecular structure and one fraction which is not yet oxidized. These two fractions had different molecular mobilities leading to different molecular relaxation processes at the glass transition.     

Diffusion phenomena of a cold gas in a thermal plasma stream

The present study involves both experimental investigation and mathematical modeling of the diffusion process of a cold gas injected into a main plasma stream. The cold gas (nitrogen or helium) was injected axially through a water cooled tube located along the centerline of an induction plasma torch. The 2-D distribution of the temperature, velocity and concentration profiles in the plasma flow were measured using enthalpy probe techniques. The results are compared with the predictions of a 2-D, LTE, turbulent mathematical model. The effects of the nature (composition) of the injected gas and its mass flow rate are investigated. The enthalpy probe measurements and the predictions of the model are in good agreement. The effective (turbulent and molecular) transport properties are estimated from a comparison of the measured and calculated profiles of the temperature, velocity and concentration fields. This study sheds light on the basic diffusion mechanisms involved in a widely used configuration of induction plasma reactors, i.e. in which the material to be treated is injected axially into the plasma, through a central water cooled tube.