Fourier transform infrared spectroscopy studies on thermal decomposition of tetrakis-dimethyl-amido zirconium for chemical vapor deposition of ZrN

The decomposition behavior of tetrakis (dimethylamido) zirconium (TDMAZ) under various ambient gases was studied by using in-situ Fourier transform infrared spectroscopy (FTIR) aimed at understanding the gas phase reactions and also at selecting the appropriate process conditions for ZrN chemical vapor deposition (CVD). The infrared absorbance of the stretching vibration at 933.37 cm-1 was employed to monitor the degree of dissociation of the gaseous TDMAZ. In the case of argon and nitrogen atmospheres, TDMAZ starts to decompose at above 300 °C, while in a hydrogen atmosphere it starts to decompose at above 350 °C. To evaluate the effect of the decomposition behavior of the precursor on CVD ZrN, the ZrN films were grown at 150–375 °C under Ar, N₂, and H₂. A clear difference in transition temperature of controlling from surface reaction to gas phase mass transfer was observed: The ZrN growth rate decreased rapidly at above 300 °C under Ar or N₂ atmospheres, and increased continuously with increase of the deposition temperature under an H₂ atmosphere.     

Two-stage ignition of energetic materials with a liquid surface layer

Two-stage ignition regimes of energetic materials with a liquid surface layer were revealed by mathematical modeling of transient combustion processes. First, under the action of a radiant flux, the regime of forced gasification of a condensed phase with a degree of its surface depletion of 0.1–0.3 was observed. Gas-phase combustion occurs in the blow-off regime. As the radiant flux decreases, the gas flame approaches the surface and becomes determining, and the degree of condensed-phase depletion decreases.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 3, pp. 140–142, May–June, 1996.     

Experimental study on ignition and combustion characteristics of typical oils

The impact of radiant heat flux on ignition and combustion behavior of typical oils (diesel, lubricating oil, and aviation kerosene) was conducted in a cone calorimeter. A circular steel pan with a diameter of 10 cm was used to contain diesel, lubricating oil, and aviation kerosene without water sublayer. Using the standard oxygen consumption method, we obtained ignition time, heat release rate, mass loss rate, extinction coefficient, CO, and CO₂ yield, and average specific extinction area was calculated from the extinction coefficient. Janssens' method was adopted in this study to deal with ignition time and radiant heat flux under a 0.55 power rule. Results show that the fitting through Janssens' method is good for ignition time of diesel, lubricating oil, and aviation kerosene and radiant heat flux. Moreover, heat release rate, mass loss rate, and CO/CO₂ ratio appear to positively correlate with radiant heat flux, whereas average specific extinction area varies in a certain range. Copyright © 2013 John Wiley & Sons, Ltd.     

Modeling multi‐stage decomposition of cotton fabrics considering char oxidation in the presence of oxygen

A theoretical model, describing thermal degradation behavior of cotton fabrics that exposed to a constant radiant heat flux, is proposed in the paper. The model describes thermal and oxidative degradation of cotton fabric under the oxygen‐containing atmosphere and considers pseudo‐bi‐component separate‐stage kinetic process. Both exothermic and endothermic reactions are included in the decomposition process. At present, gas phase oxidation reactions are not included. Comparison with experimental results demonstrates that the predictions of the mass loss rate and temperature profile with these cotton fabrics are in agreement with the experiment. Effects of thermal radiation and ambient oxygen concentration on decomposition have also been investigated. The gas phase temperature is also predicted by the present numerical model. Results from numerical model will help contribute to a better understanding of the ignition mechanism of flame‐resistant cotton fabrics used for fire safety garments. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of nonreacting gases on the desorption of reaction-created CO from graphite

Temperature-programmed desorption (TPD) was used to determine whether inert gases influence the desorption step in the oxidation of carbon by CO₂. Surface oxides were formed on a graphite by reacting it at 1200 K with CO₂, and then quenching the reaction. The oxides then were removed by TPD to 1373 K, using He, Ar and Kr as carrier gases in separate desorptions. The oxides appeared as CO₂ and CO in the desorption carrier gases; some CO₂ was observed between 800 and 1050 K, but above 1050 K the product was almost entirely CO. The CO₂ desorption showed no effect from changing the carrier gas, but temperature of CO desorption peak and amount of CO desorbed by 1373 K both depended on which gas was used as a carrier. This shows that nonreactive gases affect the desorption step of the C-CO₂ reaction. Involved in the desorption step is movement of some surface oxide species to desorption sites; the nonreactive-gas influence occurs because the nonreactive gases affect this surface transport. In the presence of Ar and Kr, transport rates of surface oxides to desorption sites were higher than they were in the presence of He. Under reaction conditions this can result in greater CO desorption rates and faster overall reaction rates in the presence of Ar and Kr than occur in the presence of He.     

Experimental Investigation on Laser Ignition and Combustion Characteristics of NEPE Propellant

The ignition and combustion property of solid propellant is the main content in internal ballistic research, which has a great significance for propulsion application and combustion mechanism. In this study, the detailed gas‐phase reaction mechanism of Nitrate Ester Plasticized Polyether Propellant (NEPE) was developed. It is helpful to understand the intricate processes of solid‐propellant combustion. The factors which may have influences on ignition delay time and temperature distribution of propellant surface was analyzed by laser ignition experiment. Using high‐speed camera and an infrared thermometer, the ignition and combustion process and the surface temperature distribution of NEPE propellant under laser irradiation were measured. Laser heat flux, ambient pressure and initial temperature of NEPE propellant have an influence on the ignition delay time and the surface temperature. Results show that the ignition delay time decreases with the increase of laser heat flux, ambient pressure and initial temperature of NEPE propellant. At the same time, with the increase of laser heat flux, the influences of ambient pressure and initial temperature on the ignition delay time decrease. Besides, laser irradiation, ambient pressure and initial temperature have significant influences on the surface temperature distribution of the propellant.     

Evaluating ignition data using the flux time product

A protocol based on the flux time product (FTP)¹ is used to analyze ignition data obtained from the Cone Calorimeter under an impressed flux in the range 20–70 KWm^?2 for different orientations and modes of ignition for conditioned cellulosic materials. The mean, maximum and minimum ignition times are depicted graphically by orientation and mode of ignition. Flux time products, FTP indices, critical irradiances and estimates of the convective heat loss associated with a change in specimen orientation are derived using the mean time-to-ignition data. It is demonstrated that consideration of the thermal thickness of a specimen may not be necessary when the proposed FTP methodology is utilized to determine valid correlations between the time-to-ignition and the incident radiant flux.     

聚甲基丙烯酸甲酯材料强制点燃研究

使用锥形量热仪研究了聚合物材料PMMA(Poly(methyl methacrylate))在不同热通量下的强制点燃,得到其相应的点燃时间和临界表面温度。同时考虑到强制点燃过程中气、固相的能量和质量传递及化学动力学过程,给出了一种描述聚合物材料强制点燃的气相和固相数学模型,并对实验结果进行了计算,同时与文献数据进行了比较,其计算结果与实验值吻合较好。     

A theoretical basis for flammability properties

Theoretical formulations are presented for the fire growth processes under external radiant heating. They included ignition, burning and energy release rate, and flame spread. The behaviour of these processes with external heating is described along with the critical conditions that limit them. These include the critical heat fluxes for ignition, flame spread and burning rate. It is shown how these processes and their critical conditions depend on a limited number of properties measurable by a number of standard test methods. The properties include heat of combustion, the heat of gasification, ignition temperature and the thermal properties of the material. Alternatively, the properties could be related to parameters easily found from data; namely: (1) the critical heat flux (CHF) for ignition; (2) the slope of the energy release rate with externally imposed flux, defined as heat release parameter (HRP); and (3) the ignition parameter, defined as thermal response parameter (TRP). It is further shown that the flame heat flux differences between small laminar flame ignition sources and larger turbulent flames can affect flame spread due to heat flux and ignition length factors. Finally, it is found that the critical energy release rates theoretically needed for ignition, sustained burning, and turbulent upward flame spread are roughly 13, 52, and 100 kW/m², respectively, and independent of material properties. Copyright © 2005 John Wiley & Sons, Ltd.     

Effects of NTO Oxidizer Temperature and Pressure on Hypergolic Ignition Delay and Life Time of UDMH Organic Gel Droplet

Organic gel propellants are promising candidates for a variety of rocket motor and scramjet applications, since they are intrinsically safe and provide high performance. It is well known that organic gel fuel droplets exhibit distinct combustion characteristics compared with conventional liquid fuel droplets, and furthermore an understanding of the ignition delay and lifetime of these droplets is critical to the improvement of combustor design. In this work, investigations of the combustion of unsymmetrical dimethylhydrazine (UDMH) organic gel droplets in different nitrogen tetroxide (NTO) oxidizing atmospheres were conducted using two sets of experimental apparatus. The combustion characteristics under different conditions of temperature and pressure were compared and analyzed based on the flame shapes observed during experimentation. From these trials, an unsteady combustion model was developed and used for the numerical simulation of spray‐sized UDMH organic gel droplet combustion in an NTO atmosphere. The hypergolic ignition and burning characteristics of the organic gel droplets under conditions simulating either engine startup or steady state combustion were compared, and changes in ignition delay and droplet lifetime with ambient temperature and pressure were analyzed. The experimental and numerical results show that the UDMH organic gel droplets exhibit periodic swell‐burst behavior following the formation of an elastic film at the droplet surface. Each droplet burst results in fuel vapor ejection and flame distortion, the intensity of which declines with increasing ambient pressure. However, the swell‐burst period is extended with increasing ambient pressure, which results in potential flameout. Under conditions of low temperature and pressure similar to those at engine startup, the ignition delay and lifetime of spray‐sized gel droplets decrease with increasing temperature or pressure, although there is a sharp increase in droplet lifetime when the ambient pressure reaches a critical value associated with flameout. The ignition delay was found to be a rate‐limited phenomenon linked to the droplet heating rate. The proportion of ignition delay and droplet lifetime due to droplet heating‐up decreased with increasing temperature or decreasing pressure. Conversely, at high temperatures and pressures simulating the engine’s steady state operating conditions, the droplets were observed to flameout after several swell‐burst periods and both ignition delay and lifetime decreased monotonically with increasing temperature or pressure. The ignition delay time was determined to be rate‐limited by gas phase chemical reactions and contributed very little to the overall droplet lifetime compared with the engine startup condition.     

Effect of dissolved gases on subcooled flow boiling heat transfer

The influence of various dissolved gases (He, N₂, Ar, CO₂, C₃H₈) on subcooled boiling heat transfer was investigated for flow of water and of heptane in an annulus with a heated core. Flow velocity, liquid bulk temperature, system pressure, gas partial pressure and heat flux were all varied over a wide range.In comparing the measured heat transfer coefficients with those for subcooled boiling of the corresponding degassed liquids, it was found that the coefficients were always increased owing to the desorption of the dissolved gases. The extent of the increase depended on the solubility of the given gas in the given liquid and could be as much as several hundred per cent. In addition, the solid surface temperature required for the inception of bubble formation was reduced considerably, in some cases far below the saturation temperature of the pure liquid.Attempts were made to extend the prediction of incipient boiling temperatures to cases where gases are dissolved in the liquid.     

Gas sorption in poly(butylene terephthalate). II. Influence of crystallinity and molecular orientation

Sorption measurements of CO₂, Ne, and Ar in poly(butylene terephthalate) (PBTP) films were studied by the gravimetric method with a recording microbalance at 298 K in the pressure range of 1–22 bar. The semicrystalline samples were annealed and oriented at 373 K. The sorption isotherms for CO₂ in PBTP films in the glassy state can be well described by the dual-sorption theory. The nonlinear sorption behavior of Ne and Ar can be satisfactorily analyzed using the sorption model developed for noble gases in PBTP. For the undrawn annealed films, it has been found that the increment of crystallinity leads to the reduction of the equilibrium gas concentration. For the oriented films, the gas concentration rises with increasing draw ratio. It appears that the sorption behavior for all tested gases in the oriented PBTP films does not depend on the changes of crystallinity and crystalline morphology under extension. The difference of the critical pressure p^* indicates the change in the size of the frozen microvoids existing in the noncrystalline phase, which was altered by annealing and drawing. © 1993 John Wiley & Sons, Inc.     

Experimental study of peat ignition upon exposure to radiant energy

Peat ignition upon exposure to radiant energy was studied experimentally for various kinds typical of bogs of Tomsk Region (Russia) and bogs near the city of Edinburgh (UK). The exposure time and energy density required for ignition of various kinds of peat; the characteristic burning surface temperature; and the range of radiant flux in which the combustion mode changed from flaming to smoldering were determined.     

Fabrication of porous magnesium spinel with directional pores by unidirectional solidification

A porous magnesium spinel (MgAl₂O₄) with directional pores was fabricated by unidirectional solidification in pressurized argon and hydrogen mixed gases. Two different kinds of pores with large directional and small facet shape were formed in the solidified samples. The former pores were dominant in the porous structure. A small amount of free corundum phase was formed in the solidified porous spinel as a secondary phase by vaporization of MgO component during the solidification process. With increasing total gas pressure, the pore size of the solidified samples decreased while no change in the porosity. The porosity and pore size of the samples increased with increasing hydrogen partial pressure. The porosities of the samples fabricated under 10%H₂–90%Ar and 1%H₂–99%Ar mixed gases were 30 and 10%, respectively, and that under Ar atmosphere was very low, non-porous.     

Metal-particle ignition and oxide-layer instability

The use of metals as high-energy fuel additives is generally compromised by the appearance of a strongly protective oxide layer that covers the fuel surface. The previous work concentrated on the elimination of the oxide layer by a global, symmetry-conserving attack, using an admixture of aggressive chemical constituents in the ambient atmosphere, a strong flux of radiation, or strongly shearing gas flows designed to intensely strain the surface layer. This paper shows that symmetry breaking leads to a different approach to ignition. For a liquid oxide layer, destabilization can be obtained via the Marangoni effect associated with longitudinal surface stress, thus breaking the translational symmetry in longitudinal directions. The thermodynamic state of the layer is described in a thin-film model, which leads to a creeping-flow approximation. Ignition by way of the Marangoni effect is then shown to result from spreading of punctures and ruptures in the oxide layer, which is a prerequisite for layer thinning or even complete layer removal. Boron-particle ignition is selected to illustrate the theory, because the well-known difficulties with boron ignitability have greatly impaired the use of boron fuel in propulsion devices. It is shown that, for ambient temperatures below 1634 K, the oxide surface layer can be destabilized by way of punctures and ruptures, owing to the peculiar property of the boron oxide, namely, positive Marangoni numbers. Previous models of boron-particle ignition insisted on conservation of symmetry and expressly excluded the appearance of punctures and ruptures. Because of this constraint, a critical ambient temperature of 1900 K for boron ignition was obtained, so that the new value of 1634 K opens up a novel approach to ignition. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 2, pp. 39–52, March–April, 2006.     

Radiative ignition of thermoplastic and charforming materials

Small samples of poly(methyl methacrylate), red oak and possibly low-density polyethylene ignite under the influence of thermal radiation with a propensity that increases with the area irradiated, consistent with ignition requiring that the lower flammability limit of pyrolysis gases be reached. The time to self-extinction of small samples of charring, burning materials (low-density polyethylene, red oak and possibly polycarbonate) in the absence of external radiant flux increases with burning area, suggesting that the increased absorptivity of char increases the importance of radiative feedback.     

Initiation of a Charge of a Gel-Like Reactive Substance by an Electric Explosion. Part 1. Thermal Processes

Ignition of a reactive substance by a high-temperature discharge cavity is analyzed by an asymptotic method. The effect of heat transfer into the lateral surface of the charge and decrease in the intensity of the radiant flux from the cavity on the moment and critical conditions of ignition is studied. Calculation results are compared with available experimental data.     

Effect of Radiant Heat Transfer on the Minimum Spark–Ignition Energy of Gas Suspensions

A mathematical model of spark ignition of a gas suspension is constructed on the basis of a two–temperature thermal diffusion model of gas–suspension combustion and the radiant heat transfer is modeled in a diffusion approximation. The dependences of the minimum ignition energy on the parameters which describe the disperse phase and the domain of the disperse–phase parameters in which the radiant heat transfer affects greatly the minimum spark–ignition energy is determined by solving the problem numerically. The analytical formula that was obtained for determination of the critical spark–ignition energy of the gas suspension and that takes into account the radiant heat transfer in the gas suspension gives values different from numerical results by not more than 30% in a broad range of determining parameters of the problem. The theoretically obtained values of the minimum spark–ignition energy of a gas suspension of coal dust agree satisfactorily with experimental data.     

Thermal analysis of polymer ignition

Gas phase criteria for the onset of flaming combustion of solids in fires are used to locate a critical temperature T_cr in a nonisothermal analysis (TA) experiment that corresponds to the surface temperature of the solid at ignition in a fire test, T_ign. This critical TA temperature occurs at low conversion of solid to gaseous fuel so it is independent of the heating rate in the test or the thermal decomposition reaction model. However, T_cr depends on the thermal properties of the polymer and the conditions of the fire test in which the gas phase criteria were measured. Nonisothermal analysis data in nitrogen and air were obtained for 20 polymers by thermogravimetric analysis and microscale combustion calorimetry. The critical temperatures T_crs obtained from TA experiments compared favorably with analytic results for a simple polymer ignition model and finite element simulations and were in qualitative agreement with ignition temperatures measured in standardized fire tests.     

Role of the gas phase in the transition of condensed material to combustion on ignition by a radiation flux

A model of ignition and the transition to combustion in condensed material under the action of a pulse of radiant flux is outlined. Chemical reactions on both sides of the phase interface are taken into account. It is shown that taking account of the thermophysical and chemical properties of the phases has a significant effect on the limits of stable ignition. The dynamics of the basic characteristics of the process is studied with pulsed heating of gasifying condensed material.Translated from Fizika Goreniya i Vzryva, Vol. 27, No. 4, pp. 7–12, July–August, 1991.