Shock tube measurements of branched alkane ignition times and OH concentration time histories

Ignition times and hydroxyl (OH) radical concentration time histories were measured behind reflected shock waves during the oxidation of three branched alkanes: iso‐butane (2‐methylpropane), iso‐pentane (2‐methylbutane), and iso‐octane (2,2,4‐trimethylpentane). Initial reflected shock conditions ranged from 1177 to 2009 K and 1.10 to 12.58 atm with dilute fuel/O₂/Ar mixtures varying in fuel concentration from 100 ppm to 1.25% and in equivalence ratio from 0.25 to 2. Ignition times were measured using endwall CH emission and OH concentrations were measured using narrow‐linewidth ring‐dye laser absorption of the R₁(5) line of the OH A‐X (0,0) band at 306.7 nm. The ignition times and OH concentration time histories were compared to modeled predictions of seven branched alkane oxidation mechanisms currently available in the literature and the implications of these comparisons are discussed. These data provide a unique database for the validation of detailed hydrocarbon oxidation mechanisms of propulsion related fuels. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 67–78 2004     

The effects of annealing in inert atmospheres and of oxygen concentration on chemiluminescence from polypropylene

It has been shown that heating polypropylene powder under a nitrogen atmosphere leads to the significant prolongation of the oxidation induction time measured by chemiluminescence in oxygen at 130 and 140 °C. While heating in nitrogen from 0 to 4 h at 140 °C leads to the linear increase of oxidation induction time, the maximum chemiluminescence intensity I_stat increases with the time of sample annealing until 2 h; then it starts to decay. The different and sometime unknown thermal history of the sample may thus explain the scatter of induction times of oxidation observed with different PPs whether they be pure or stabilised. Maximum chemiluminescence intensity plotted vs. concentration of oxygen in the surrounding atmosphere at 130 and 140 °C also increases linearly; however, this does not correspond with very small reduction of oxidation induction time. The four-parametric “master equations” used in our earlier papers were applied to fit the chemiluminescence runs both in oxygen and in nitrogen. The equation operates with the rate constants of hydroperoxide decomposition and oxidation spreading but at the same time, it takes into account the possible effect of oxidation products on decomposition of hydroperoxides.     

Elasticity of syndiotactic polypropylene: Insights from temperature and time dependence

The origin of the unusual and puzzling elasticity of drawn sPP samples was investigated. The mechanism responsible of the elasticity was studied for drawn samples characterized by a very simple structural organization, where there are no involvements of crystallographic modifications with different chain conformation. The elastic behavior of the drawn samples, valued through the hysteresis cycles, was determined at different temperatures. At room temperature the samples show remarkable elastic properties, whereas decreasing the temperature the elastic behavior becomes worse and worse, disappearing at temperatures lower than 0 °C. The elasticity also disappears in drawn samples after a long aging under tension. Thermomechanical and structural investigations, as well as shrinkage as a function of temperature and aging at room temperature of the fixed drawn samples, support the idea that the elastic behavior of sPP can be explained by the model of the plastic deformation of semicrystalline polymers. The interpretation is based on the presence of “tie” molecules axially connecting the crystals in the oriented samples, whose extension, chain conformation and/or crystallization determine the retractive stress of the oriented sample, as well as other mechanical properties. We show that many experiments on the drawn samples, either fixed or relaxed, are strictly connected to the morphology of the drawn sample, derived by the transition between the lamellar initial and the fibrillar final structure. By applying the model we can answer the questions derived from the experimental facts not yet well clarified, giving a new insight into the interesting elasticity of sPP.     

Effect of oxygen pressure on the oxidation kinetics of unstabilised polypropylene

The thermal oxidation of unstabilised polypropylene films at 80 °C and various oxygen pressures ranging from 0.02 MPa to 5 MPa has been studied by FTIR spectrophotometry (carbonyl growth). The induction time decreases and the maximum oxidation rate increases quasi-hyperbolically when the oxygen pressure increases. The asymptotic behaviour (corresponding to the regime of oxygen excess) is not reached at the highest pressure under study. A kinetic model derived from a classical mechanistic scheme but free of simplifying hypotheses, has been used to simulate the observed behaviour and to determine the elementary rate constants. It is shown that a good simulation of kinetic curves of carbonyl build-up in the whole pressure interval under study can be obtained with a set of physically reasonable rate constant values. The “inverse problem” cannot be, however, totally solved because certain constants are interdependent so that some rate constant values have to be arbitrarily chosen or taken from the literature.     

Effect of different carbons on ignition temperature and activation energy of black powder

By means of TG, DTG, and DTA, it was shown that the source of carbon can affect the decomposition and reaction characteristics of black powder compositions. Carbons with high volatile content lower the activation energy and reduce the ignition temperature of black powder. The removal of these volatiles causes an increase in ignition temperature and activation energy. It also showed that although sulfur has no effect on the ignition temperature, it does affect the preignition reaction. Sulfurless black powder also has larger activation energies than regular black powder. Compositions of black powder containing channel black have lower activation energies than those with charcoal.     

Effect of orientation on the free volume and oxygen transport of a polypropylene copolymer

The effect of uniaxial orientation on the free‐volume and oxygen‐transport properties of a propylene copolymer with 4.5 wt % ethylene was examined. The free‐volume hole size and hole density were measured with positron annihilation lifetime spectroscopy. Subsequently, the free‐volume characteristics were correlated with the oxygen‐transport properties. Orientation had only a small effect on the total amount of free volume: a small increase in the hole density was offset by a small decrease in the hole size. As a result, the oxygen solubility and amorphous‐phase density were unchanged by orientation. However, a pronounced decrease in the oxygen diffusivity when the draw ratio exceeded 6 indicated a change in the dynamic free volume. This was attributed to an increasing number of taut tie chains, which retarded oxygen diffusion. The reduced amorphous chain mobility was also manifest in the increased glass‐transition temperature, decreased bulk thermal expansivity, and decreased expansivity of free‐volume holes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1230–1243, 2005     

Ultra-weak photon emission from human hand: influence of temperature and oxygen concentration on emission

We have studied ultra-weak photon emission (UPE) from living organisms. We report here some features of the UPE from human hand by means of photon counting techniques. The intensity of the UPE depended on the position of human hand; nail>finger>palm. As the temperature declined, the intensity of the UPE from the palm decreased. Further, as oxygen concentration around the palm was lowered, the intensity of the UPE from the palm decreased. These results show the UPE from the palm partly contains emissions based on oxidation reaction on skin surface as a potential. When we used mineral oil between the photomultiplier tube and the palm, the intensity of the UPE increased twice as much, which indicates the UPE from the inside of the skin certainly exists. The fact may be explained by refractive index matching. As mentioned above, we considered the generation mechanism of photons emitted from the human hand.     

Effect of molecular oxygen on concentration autooscillations and autowaves in the Belousov-Zhabotinskii reaction

It has been established that molecular oxygen is responsible for the differences in the concentration regions for existence of autooscillations and autowaves in the Belousov-Zhabotinskii reaction when the reaction mixture is in contact with air. When gaseous oxygen is passed through the solution, it affects the oscillation parameters through two mechanisms: chemical interaction with malonic acid radicals, and removal of free bromine from the solution. The influence of stirring intensity on the autooscillation parameters in Belousov-Zhabotinskii reactions is related mainly to changes in the rate of access of molecular oxygen to the reaction mixture.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 6, pp. 661–666, November–December, 1988.     

Effect of sample layer thickness and temperature rise time on the pyrolysis temperature of cellulose

The temperature profiles throughout a rapidly heated sample layer were calculated as functions of time and sample thickness using the one-dimensional heat equation. The rates of decomposition throughout the layer were then calculated using the Arrhenius equation. The decomposition temperature, T_1/e, for a layer is defined as the temperature at which a fraction of 37% of the material is not yet decomposed. The theoretical results are used to predict T_1/e in cellulose for different Curie-point temperatures and heating rates. In general, T_1/e increases with increasing heating rates. In vacuum Curie-point pyrolysis (e.g., pyrolysis-mass spectrometry) of thin samples (thickness < 0.04 mm), T_1/e is almost uniform throughout the sample. When loss of heat occurs due to an ambient gas (e.g., pyrolysis-gas chromatography) the temperature rise time increases so the decomposition proceeds at a lower temperature; this is most pronounced at the outside of a very thick layer.     

Effect of radiation absorption modes on ignition time of translucent polymers subjected to time-dependent heat flux

Majority of previous solid ignition models, including numerical and analytical ones, considered only surface absorption of incident heat flux for simplification. However, the influence of in-depth absorption on pyrolysis and subsequent ignition cannot be ignored for infrared translucent polymers. This work addresses this problem and focuses on time-dependent heat flux to establish an analytical model for ignition behaviors prediction by means of theoretical analysis. Ignition temperature was utilized as the ignition criterion, and both surface and in-depth absorption scenarios were considered. Thermally thick polymethyl methacrylate and polyamide 6 were selected as reference materials to verify the reliability and applicability of the proposed model by comparing the analysis results with experimental data as well as numerical simulations. A method for determining the approximation parameters of the theoretical analysis was presented to derive the relationship between ignition time and the coefficients in heat flux expressions. The results show that the higher surface temperature owing to surface absorption accelerates the pyrolysis rate and results in a shorter ignition time, while in-depth absorption affects the ignition time inversely. The effect of surface heat loss was also evaluated quantitatively through both analytical and numerical models. The uncertainty of the proposed model is mainly caused by the selection of the approximation parameters. Nevertheless, it provides an alternative approach to estimate the ignition time of translucent polymers besides numerical simulation.

     

Effect of WO3 particle size on the type and concentration of surface oxygen

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Effect of concentration and temperature on surface tension of sodium hyaluronate saline solutions

The effect of concentration and temperature on the surface tension of sodium hyaluronate (NaHA) saline solutions was investigated using the technique of the shape of pendant drops. The decay rate of the surface tension with the increase of NaHA concentration was well-described by the empirical Hua-Rosen equation. Adsorption at the air-liquid interface was estimated using the Gibbs equation. The temperature dependence of a dilute solution and a semidilute entangled solution was numerically fitted with a second-order polynomial equation. The surface behavior of the NaHA saline solutions was interpreted in terms of their known viscoelastic properties.     

Effect of temperature and dopant concentration on the conductivity of samaria-doped ceria electrolyte

The conductivity, σ, of a samaria-doped ceria electrolyte is studied as a function of temperature and dopant concentration, x, which was from 5 to 30 mol%. It is shown that a maximum in σ versus x corresponds to a minimum in activation energy. It is found that the conductivity is completely due to oxygen vacancy conduction. The conductivity increases with increasing samaria doping and reaches a maximum for (CeO₂)_0.8(SmO_1.5)_0.2, which has a conductivity of 5.6×10^–1 S/cm at 800 °C. A curvature at T=T _c, the critical temperature, has been observed in the Arrhenius plot. This phenomenon may be explained by a model which proposed that, below T _c, nucleation of mobile oxygen vacancies into ordered clusters occurs, and, above T _c, all oxygen vacancies appear to be mobile without interaction with dopant cation. In addition, the composition dependences of both the critical temperature and the trapping energy are consistent with that of the activation energy. Electronic Publication     

Melting temperature and enthalpy of isotactic polypropylene

The fusion process, melting temperature and heat of fusion of carefully crystallized fractions of isotactic polypropylene (molecular weights from 5·4 × 10⁴ to 2·2 × 10⁵) have been investigated. The equilibrium melting temperature, obtained by the extrapolation method in experiments involving very low degrees of crystallinity, corresponds to 208°. The fusion enthalpy was found to be 1·386 cal/mole. This low value and the high depression of Tm^o suggest a relatively high interfacial energy; a value of 285 erg/cm² is estimated for σ_ec.     

Effect of working temperature on the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites

In the present study, the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites (hybrid composites) in the working temperature range from 23 °C to 90 °C was studied by experimental and constitutive methods. Monotonic and cycle loading-unloading single-lap-shear tests were employed to determine the interfacial properties of hybrid composites. The experimental results show that both interfacial shear strength and shear stiffness decrease with increasing working temperature. A regression function was adopted to evaluate the decaying degree of interfacial properties with increasing working temperature. The shear stress-displacement relationship under monotonic loading exhibits nonlinear behavior after an initial elastic region. The envelope lines of shear stress-displacement of hybrid composites under cyclic loading indicate that the nonlinearity in the curve is caused by the plastic deformation of polypropylene in the interphase region. A constitutive model was built to describe the nonlinear shear stress-displacement relation of hybrid composites at different working temperatures. A full suite of temperature-dependent plastic parameters in the model was obtained from cyclic loading-unloading tensile tests. The predicted shear stress–displacement curves agreed well with experimental results from different working temperatures. In addition, the failure mode of hybrid composites varied with working temperature.     

Effect of carrier concentration on the facilitated oxygen transport in a polymer membrane

Facilitated transport of oxygen in a solid Polymer membrane containing cobaltporphyrin (CoP) which carries oxygen specifically and reversibly, leads to permselectivity of oxygen against nitrogen in the membrane. The increase in concentration of the CoP carrier is expected to enhance the oxygen transport. The membranes of poly(octylmethacrylate-co-vinylimidazole) containing 0.8 ～ 10 wt% CoP were prepared, and the effects of the CoP-concentration on the transport and the diffusion constants of oxygen are studied. Although the induction period before the steady state of oxygen permeation was prolonged with the CoP concentration in the polymer membrane, the glass transition temperature (T_g) of the membrane was increased and the diffusion constants of oxygen were decreased with the CoP concentration to yield unexpected reduction of the oxygen permeation in the highly CoP loaded membrane.     

Dependence of the kinetics of gas phase methane oxidation at high pressures on the concentration of oxygen and on temperature

A decrease of the rate of high pressure oxidation of very rich methane-oxygen mixtures with increasing oxygen concentration was found. The results were confirmed by kinetic simulation of the process. A very distinct temperature dependence of the rate of oxygen conversion can be explained on the basis of a critical change in the reaction mechanism.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 53–56, January, 1996.     

Dual optical sensor for oxygen and temperature based on the combination of time domain and frequency domain techniques

In measuring specific conditions in the real world, there are many situations where both the oxygen concentration and the temperature have to be determined simultaneously. Here we describe a dual optical sensor for oxygen and temperature that can be adapted for different applications. The measurement principle of this sensor is based on the luminescence decay times of the oxygen-sensitive ruthenium complex tris-4,7-diphenyl-1,10-phenanthroline ruthenium(III) [Rudpp] and the temperature-sensitive europium complex tris(dibenzoylmethane) mono(5-amino-1,10-phenanthroline)europium(III) [Eudatp]. The excitation and emission spectra of the two luminophores overlap significantly and cannot be discriminated in the conventional way using band pass filters or other optical components. However, by applying both the frequency and time domain techniques, we can separate the signals from the individual decay time of the complexes. The europium complex is entrapped in a poly(methyl methacrylate) (PMMA) layer and the ruthenium complex is physically adsorbed on silica gel and incorporated in a silicone layer. The two layers are attached to each other by a double sided silicone based tape. The europium sensing film was found to be temperature-sensitive between 10 and 70 °C and the ruthenium oxygen-sensitive layer can reliably measure between 0 and 21% oxygen.