Concept of thermodynamic capacity

The thermodynamic capacity of a species (C _i)in a homogeneous phase is defined as (∂n _i/∂μ_{i P, T, n j}wheren _iis the total number of moles ofi per unit quantity of the system irrespective of the actual system chemistry andμ _iis its chemical potential. Based on this definition, the thermodynamic capacity of oxygen in non-reactive and reactive gas mixtures and in binary and ternary liquid solutions has been computed. For reactive gas mixtures containing stable chemical species which do not undergo significant dissociation such as CO + CO₂, H₂ + H₂O and H₂ + CO₂, the capacity curves show a maximum at equimolar ratio and a minimum at higher oxygen potentials. If one of the chemical species partly dissociates as in the case of H₂S in H₂ + H₂S mixtures or SO₃ in SO₂ + SO₃ mixtures, capacity curves do not exhibit such maxima and minima, especially at high temperatures. It would be difficult to produce stable oxygen fugacities when the capacity has a low value, for example at compositions near the minimum. Oxygen capacities of non-ideal liquid solutions, Cu-O and Cu-O-Sn, and heterogeneous systems formed at saturation with the respective oxides are discussed.     

Intermediate gas phase precursors during plasma CVD of HMDSO

In plasma enhanced chemical vapor deposition (PECVD) from complex molecules like hexamethyldisiloxan (HMDSO) often not the molecules themselves but intermediate and reactive radicals or molecules are the precursors for film growth. Additionally, such PECVD processes are volume or mass flow limited under many process conditions. In these cases growth rate and film homogeneity is mainly dominated by the precursor content and its spatial distribution in the gas or plasma phase. Therefore the identification of such intermediate precursors is an important task to optimize a PECVD process and also helps us to understand the plasma chemical reactions during PECVD. A combined mass spectrometry and IR absorption study is used to identify intermediate gas phase precursors in HMDSO/O₂ PECVD remote plasmas. For this study a microwave plasma CVD system was used with HMDSO/O₂ ratios between 0.1 and 1 at typical operating pressures between 20 and 70 Pa. Three reactive intermediate species are proposed to act as a precursor for SiO_x film growth from HMDSO/O₂ plasmas. All three having a mass of 148 amu. The related reactive groups are the silanon (Si=O), silanol (Si-OH) and aldehyde (C=O) groups.     

A study of unsteady-state operating conditions of a supersonic inlet by the RANS/ILES method

Using the hybrid RANS/ILES method, a flow was investigated under unsteady-state conditions in a rectangular mixed-compression supersonic inlet at M_d = 2. The computations were made for Mach numbers of the incident flow M₀ = 1.8, 2, and 3. The geometry of two variants was investigated, viz., with the boundary layer bleed system and without it. Calculations were performed on the meshs containing (1.69–1.78) × 10⁶ cells. The flow rate through the supersonic inlet was varied within a wide range. Under most of the conditions investigated, the flow in the duct was unsteady as a consequence of separation of the boundary layer upon its interaction with the shock waves. For all geometric variants investigated and M₀, the throttle characteristics were constructed by the averaged flow parameters as well as the dependences of the flow rate through the boundary layer bleeding system and the static pressure pulsation level on the air inlet throttle ratio. Comparison of the computed results with the experimental data showed a good agreement in terms of both the averaged flow parameters and the pulsation characteristics.     

Unsteady triple-shock configurations and vortex contact structures initiated by the interaction of an energy source with a shock layer in gases

The effect of physical and chemical properties of the gaseous medium on the formation of triple Mach configurations and vortex contact structures and on the stagnation pressure and drag force dynamics has been studied for supersonic flows with external energy sources. For the ratio of specific heats that varies in a range of 1.1–1.4, a significant (up to 51.8%) difference has been obtained for the angles of triple-shock configurations in flows at Mach 4 past a cylindrically blunted plate. When studying the dynamics of the decreases in the stagnation pressure and drag force, it has been revealed that these effects are amplified and the vortex mechanism of drag reduction starts to prevail as the adiabatic index decreases.     

Study of the velocity distribution in an intense pulsed hydrogen beam

We present the results of measurements of the velocity distributions of particles in a pulsed hydrogen beam obtained from a dissociator with a radio frequency discharge (duration 1.0 ms, repetition rate 1 Hz). It is shown that the hydrogen inside the dissociator is heated up to ～2800 K, so the thermal dissociation of hydrogen molecules is essential. In order to cool the atoms, the gas was let through a pyrex channel 5 mm in diameter. The cooling channel walls being at room temperature and the channel having a length of 50 mm, we have obtained a supersonic beam of hydrogen atoms with a Mach number M_| = 2.7±0.25. When the channel walls were cooled by the flowing liquid nitrogen and the channel was 70 mm long we obtained a beam of cooled atoms with a Mach number M_| = 4.14±0.35. The velocity distribution of atoms depends on the power of the rf discharge inside the dissociator and on the gas consumption per pulse, and varies during the discharge pulse. For a temperature of the cooling channel walls T_ch = 77 K, a gas consumption N = 3.3×10¹⁷ molecules per pulse and a discharge power of 0.23 kW cm^?3, we have obtained an atomic beam with intensity I(0) = (2.8±0.8)×10²⁰ atoms sr^?1 s^?1 and a most probable velocity ν_MP = (1.97±0.07)×10⁵ cm s^?1.     

Effect of DOP-based compounds on fire retardancy, thermal stability, and mechanical properties of DGEBA cured with 4,4′-DDS

The structure-property-relationships, thermal stability and flame retardancy of a DGEBA-DDS system containing various organo-phosphorus compounds as flame retardants is investigated. Three non-reactive (DOP-ethyl, DOP-ethylhexyl and DOP-cyanur) and one reactive (DOP-glycidyl) phosphorous compounds are added separately to the epoxy resin and the mixtures are cured with 4,4′-DDS in a substoichiometric ratio. The addition of such DOPO-compounds leads to improved flame retardancy at low phosphorus contents of about 2 wt.% (about 20 wt.% of additive) without significantly affecting other important properties such as fracture toughness (K_lc) and glass transition temperature (T_g) of the matrix. Neither the type nor the amount of additive affects the fracture toughness of cured epoxies up to additive concentrations of between 18 and 24 wt.%. Furthermore, the loss in glass transition temperature of the cured resin can be correlated with the amount and chemical reactivity of the organo-phosphorus additive. The reactive DOP-glycidyl and the non-reactive DOP-cyanur additive are observed to maintain the highest glass transition temperature of the epoxy system mainly due to a higher extent of the cross-linking reaction. The results presented in this study highlight the potential of optimising the flame retardancy and the resulting physical and mechanical properties of epoxy systems for liquid composite moulding applications by varying the chemical structure of the organo-phosphorus compounds.     

Simulation of hypersonic flows on unstructured grids

The paper describes a solver for the compressible inviscid flow equations which is based on a flux vector splitting strategy able to deal with chemical reaction effects. The methodology here adopted is based on a modification of the Flux Vector Splitting technique due to van Leer.¹¹ The scheme operates on completely unstructured grids and has been coupled with an adaptive remeshing procedure to compute high speed flows. Solutions for two-dimensional problems for non-reactive and reactive air in thermodynamic equilibrium are presented.     

The formation of vortex shocks in a 3D subsonic flow behind a weak shock wave emerging from a channel

The structure of a 3D subsonic flow behind a diffracted shock wave was studied by experimental and numerical methods for the incident shock wave Mach numbers M ₀ close to unity. It is established that vortex shocks appear in the flow behind the diffracted shock wave even when M ₀ decreases to 1.04, which is much lower than the threshold Mach number obtained analytically for a 2D automodel case. The time interval from the outflow start to the local supersonic zone formation, as well as the experimentally measured time of appearance of the first vortex shock, increase with decreasing M ₀.     

Wetting of MgO and α-Al2O3 by molten Mg: a model of droplet with high vapor pressure

The wetting of MgO and α-Al₂O₃ polycrystalline plates by molten Mg in purified argon was studied between 973 and 1273?K using an improved sessile drop method. The MgO/Mg system is basically non-reactive while Al₂O₃/Mg is reactive. The common features in the non-reactive and reactive systems are the high tendency for oxidation and the high evaporation rate of Mg. It is shown that wetting kinetics in both systems is governed by the evaporation and pinning phenomenon leading to contact angle versus time curves passing through a minimum: the apparent contact angle first decreases and then increases. The intrinsic contact angle should be estimated using the initial value (advancing contact angle) and the maximum value just before the disappearance of the droplet caused by Mg evaporation (receding contact angle).     

A Similarity Criterion for Supersonic Flow Past a Cylinder with a Frontal High-Porosity Cellular Insert

We have experimentally and numerically studied the influence of the ratio of the diameter of a cylinder with a frontal gas-permeable porous insert made of nickel sponge to the average pore diameter in the insert on the aerodynamic drag of this model body in supersonic airflow (M_∞ = 4.85, 7, and 21). The analytical dependence of the normalized drag coefficient on a parameter involving the Mach number and the ratio of cylinder radius to average pore radius in the insert is obtained. It is suggested to use this parameter as a similarity criterion in the problem of supersonic airflow past a cylinder with a frontal high-porosity cellular insert.     

A scheme for solving strongly coupled chemical reaction equations appearing in the removal of SO2 and NOx from flue gases

The removal of NO_x from mixtures of NO-NO₂-N₂ and NO-NO₂-O₂-H₂O is discussed theoretically in this study, and the removal of ₂SO and xNO is further discussed when a gas system of NO_x-N₂-O₂-H₂O contains CO₂ and SO₂. The involved chemical reaction rate equations in the process of SO₂/NO_x removal are solved numerically using Treanor's method, in which a scheme separating chemical reactions into fast and slow groups has been proposed for improving the numerical stability. Numerical results show that the contribution of ion reactions to xNO removal is negligible, and that high temperature is not beneficial for the NO oxidation. However, high concentration of O₂ is conducive to the NO oxidation. Addition of water facilitates the NO_x removal, and increasing water vapor concentration enhances the NO_x removal efficiency; inclusion of CO₂ and SO₂ into the system favors the NO removal.     

Chemical-reaction dependence of plasma parameter in reactive silane plasma

Electron temperature in a silane glow-discharge plasma, being an important plasma parameter for determining photo-induced instability in the resulting hydrogenated amorphous silicon (a-Si:H), has been studied under various film-preparation conditions. We have used an optical-emission-intensity ratio of Si¹ to SiH¹ (I_Si¹/I_SiH¹) which corresponds to the high-energy-tail slope of the electron-energy-distribution function in the plasma as a measure of electron temperature in a reactive silane glow-discharge plasma. We have found quite differently from the conventional non-reactive glow-discharge plasma such as hydrogen plasma that the electron temperature in the silane plasma is strongly modified by the substrate temperature (gas temperature) especially under high silane-gas partial-pressure condition. This anomalous behavior of the electron temperature in the silane plasma has been explained by means of gas-phase-polymerization reaction and electron-attachment process to the polymers in the plasma. The electron temperature has been remarkably reduced when a hydrogen-dilution method and a cathode-heating method are used which are considered to control polymer-formation reactions in the silane plasma together with utilization of conventional electron-temperature-controlling methods such as a very high plasma-excitation frequency and an application of magnetic field for electron-confinement. As a consequence of the reduction of electron temperature in the silane plasma, highly stabilized a-Si:H has been successfully obtained even under high growth rate conditions of 1.5 nm s⁻¹.     

Investigation of thermoelectric SiC ceramics for energy harvesting applications on supersonic vehicles leading–edges

Utilizing thermoelectric technology to aerodynamic heat harvesting on the leading-edge is worth noticing in the thermal protection systems. In this paper, a nose-tip model in a supersonic flow field is developed to predict the thermoelectric performance of SiC ceramics structures. The generation performance is numerically investigated in terms of the computational fluid dynamics and the thermal conduction theory. The output power and energy efficiency of the nose-tip model are obtained with Mach number varying from 2·5–4·5. The generated power reaches 1·708 W/m²at a temperature difference of 757 K at M = 4·5. With respect to the Thomson effect, the output power decreases rapidly. However, larger output power and energy efficiency would be obtained with the increase of Mach number, with or without considering the Thomson heat. Moreover, under the higher Mach numbers, larger range of output current value is available.     

Initial phases of the collapse of an empty cavity in a liquid

This paper relates to the collapse of an empty spherical cavity in a liquid. The analysis is mainly theoretical, the object being to study the effects of liquid compressibility on the initial phases of the motion of the cavity. A method, similar to the Rayleigh-Janzen theory of subsonic aerodynamics, is described whereby the motion of the cavity can be systematically computed as it collapses from an initial radius R₀ down to a stage when the collapse Mach number approaches unity. Instead of the usual differential equation, a parameter $\text{R}\text{R}\text{?}\text{.R}{}^2$ describes the cavity motion. Results obtained are in excellent agreement with those obtained from numerical computations. Distributions of fluid properties behind the cavity are obtained at different stages of the collapse. These results, apart from demonstrating the effects of liquid compressibility, also indicate the existence of high-intensity pressure fields in the surrounding flow field even at these subsonic speeds. A general feature of the present theory is that it offers a method by means of which the subsonic phase of the collapse can be linked with the supersonic phase.     

Optical breakdown in supersonic air jet

The characteristics of a free supersonic air jet with a diameter of 10 mm and Mach numbers within 1.7–3.7 have been studied in the presence of energy supply via pulsed optical discharge generated by a periodic-pulsed mechanically-Q-switched CO₂ laser. The coefficient of laser radiation absorption by optical discharge plasma in supersonic air jet has been studied and it is established that this parameter can reach 60%. It is demonstrated for the first time that a threshold density of air corresponding to sharp growth in the absorption efficiency amounts to 1.8–2 kg/m³ and is independent of the Mach number in airflow. Distributions of the gasdynamic parameters (including dynamic pressure and temperature) in supersonic air jet with energy supply via optical discharge have been determined.     

Optical and chemical characterization of thin TiNx films deposited by DC-magnetron sputtering

Thin titanium nitride (TiN_x) films were deposited on silicon substrates by means of a reactive DC-magnetron plasma. Layers were synthesized under various conditions of discharge power and nitrogen flows in two operation modes of the magnetron (the so-called “balanced” and “unbalanced” modes). The optical constants of the TiN_x films were investigated by spectroscopic ellipsometry (SE). X-ray photoelectron spectroscopy (XPS) was used to determine the relative atomic concentration and chemical states of the TiN_x films. The density and thickness of the films have been investigated by means of grazing incidence X-ray reflectometry (GIXR). The results of the layer analyses were combined with plasma investigations carried out by means of energy resolved mass spectrometry (ERMS) under the same conditions. It is shown that the magnetron mode has a clear influence on the titanium deposition rate and the incorporation of nitrogen into the layers.     

Parameters of electrode discharges in supersonic air flows

Results are given of experimental investigations of basic microscopic parameters of plasma of a longitudinal-and-transverse dc discharge in a supersonic wind tunnel with a backward-facing step. The results of measurements of electric field E, of temperatures of gas T _g and of excitation of electron levels T _e , and of reduced electric field E/N are compared with the measured or previously found parameters of transverse and longitudinal discharges in supersonic flows and in stationary air in a wide range of pressures and currents.     

Singlet oxygen evolution from layered transition metal oxide cathode materials and its implications for lithium-ion batteries

For achieving higher energy density lithium-ion batteries, the improvement of cathode active materials is crucial. The most promising cathode materials are nickel-rich layered oxides LiNi_xCo_yMn_zO₂ (NCM) and over lithiated NCM (often called HE-NCM). Unfortunately, the full capacity of NCM cannot be utilized due to its limited cycle-life at high state-of-charge (SOC), while HE-NCM requires high voltages. By operando emission spectroscopy, we show for the first time that highly reactive singlet oxygen is released when charging NCM and HE-NCM to an SOC beyond ≈80%. In addition, on-line mass-spectrometry reveals the evolution of CO and CO₂ once singlet oxygen is detected, providing significant evidence for the reaction between singlet oxygen and electrolyte to be a chemical reaction. It is controlled by the SOC rather than by potential, as would be the case for a purely electrochemical electrolyte oxidation. Singlet oxygen formation therefore imposes a severe challenge to the development of high-energy batteries based on layered oxide cathodes, shifting the focus of research from electrochemically stable 5?V-electrolytes to chemical stability toward singlet oxygen.     

Author index

Films of TiC, TiN and their composite were prepared on molybdenum by a reactive sputtering method with CH₄ and N₂ as the reactive gases and argon as the sputtering gas and applying bias potentials to the substrate material.The films were characterized by X-ray photoelectron spectroscopy and Auger electron spectroscopy. The quantitative chemical composition of the TiC and TiN coatings was determined as a function of the partial pressures of CH₄ (P_CH4) and N₂ (P_N2) during the reactive sputtering. For the TiC coating the most suitable P_CH4 range which gives the stoichiometric composition (carbon-to-titanium ratio, 0.8–1.0) without impurities was found to be (2–5) × 10^?4 Torr (substrate temperature, 300 °C; bias potential, ? 300 V). For the TiN coating the structure and composition of the films prepared by reactive sputtering were observed to depend greatly on the condition of applying the bias potential. The suitable P_N2 range which gives golden films of the stoichiometric composition was higher than 1 × 10^?4 Torr (substrate temperature, 200–300 °C; bias potential from ?75 to ?200 V).On the basis of these experimental studies of TiC and TiN coatings successive coatings of TiC and TiN were deposited onto a molybdenum substrate to achieve higher thermal stability and better adhesion to the substrate. The successive coating method is a promising technique for use in fusion reactors.     

Numerical modelling of fluids mixing,heat transfer and non‐equilibrium redox chemical reactions in fluid‐saturated porous rocks

Non‐equilibrium redox chemical reactions of high orders are ubiquitous in fluid‐saturated porous rocks within the crust of the Earth. The numerical modelling of such high‐order chemical reactions becomes a challenging problem because these chemical reactions are not only produced strong non‐linear source/sink terms for reactive transport equations, but also often coupled with the fluids mixing, heat transfer and reactive mass transport processes. In order to solve this problem effectively and efficiently, it is desirable to reduce the total number of reactive transport equations with strong non‐linear source/sink terms to a minimum in a computational model. For this purpose, the concept of the chemical reaction rate invariant is used to develop a numerical procedure for dealing with fluids mixing, heat transfer and non‐equilibrium redox chemical reactions in fluid‐saturated porous rocks. Using the proposed concept and numerical procedure, only one reactive transport equation, which is used to describe the distribution of the chemical product and has a strong non‐linear source/sink term, needs to be solved for each of the non‐equilibrium redox chemical reactions. The original reactive transport equations of the chemical reactants with strong non‐linear source/sink terms are turned into the conventional mass transport equations of the chemical reaction rate invariants without any non‐linear source/sink terms. A testing example, for some aspects of which the analytical solutions are available, is used to validate the proposed numerical procedure. The related numerical solutions have demonstrated that (1) the proposed numerical procedure is useful and applicable for dealing with the coupled problem between fluids mixing, heat transfer and non‐equilibrium redox chemical reactions of high orders in fluid‐saturated porous rocks; (2) the interaction between the solute diffusion, solute advection and chemical kinetics is an important mechanism to control distribution patterns of chemical products in an ore‐forming process; and (3) if the pore‐fluid pressure gradient is lithostatic, it is difficult for the chemical equilibrium to be attained within permeable cracks and geological faults within the crust of the Earth. Copyright © 2005 John Wiley & Sons, Ltd.