Study of self-propagating high-temperature synthesis of aluminium nitride using a laser monitor

This study focused on the synthesis of aluminium nitride (AlN) by combusting aluminium nanopowder in air. To investigate the combustion of aluminium nanopowder, a copper bromide laser monitor was used. The optical system equipped with brightness amplification allowed the elimination of the background lighting effect and enabled the high time-resolved recording of the process. In particular, the laser monitor enabled us to detect changes in the morphology and optical properties of the surface of the aluminium nanopowder sample as well as to observe the propagation of the combustion waves in spite of the intense background lighting during combustion. The main time parameters of the combustion of aluminium nanopowder in air were determined. To improve and facilitate the processing of laser monitored high-speed video recordings, we proposed to analyse the time dependence of the intensity of the output signal of the laser monitor. The dependence was used to successfully detect the occurrences of all combustion waves and describe their dynamics. The time dependence also favourably represented the evolution of the reflection coefficient of the combustion products of aluminium nanopowder. This is the first time that this property of aluminium nanopowder has been investigated. The reflection coefficient evolution coupled with video recordings of the sample surface development during the combustion of nanopowder could be used to control the combustion process.     

Auto-combustion synthesis and electrochemical studies of La0.6Sr0.4Co0.2Fe0.8O3-δ – Ce0.8Sm0.1Gd0.1O1.90 nanocomposite cathode for intermediate temperature solid oxide fuel cells

In the present study, a nanocomposite cathode comprising Fe rich La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) based pervoskite semiconductor oxide and Sm-Gd co-doped ceria rich Ce_0.8Sm_0.1Gd_0.1O_1.90 (CSGO) in the ratio of 1:1 has been successfully synthesized by a simple glycine nitrate auto combustion method. The structural properties of the two phase nanocomposite were evaluated by X-ray diffraction and Raman spectroscopy. A detailed electrical properties of co-doped LSCF-CSGO nanocomposites have been studied with a comparison of LSCF added with 10?mol% and 20?mol% Gd singly doped ceria (LSCF-GDC10 and LSCF-GDC20) nanocomposites as a function of temperature in the range of 673–1073?K at air atmosphere by AC impedance spectroscopy. The total electrical conductivity of the co-doped LSCF-CSGO nanocomposites has been found to be 0.043?S?cm^?1 at 973?K which is higher than that of the LSCF composite containing singly doped compositions. The Sm co-doping in GDC phase has effectively helped to reduce the undesired electronic conduction produced in the doped ceria as the electron concentration of LSCF-CSGO was found to be ??2.62?×?10¹⁵ cm^?3 which was lower than the electron concentration of LSCF containing singly doped nanocomposite (LSCF-GDC20, ??2?×10¹⁶ cm^?3) estimated by Hall-Effect measurement. The activation energy of LSCF-CSGO nanocomposite has been found to be 0.05?eV for the oxygen reduction reaction by temperature dependent Arrhenius equation. The improved electrical properties in terms of high ionic conductivity and low activation energy have been achieved through the incorporation of Sm into GDC10 electrolyte phase in LSCF nanocomposite. The combustion synthesis method has also effectively helped to produce microstructure containing large grain size (~?6?µm) which is beneficial for enlarging triple phase boundary (TPB) area of cathodes utilized in solid oxide fuel cells (SOFC) operated at reduced/intermediate temperature (673–973?K).     

Combustion synthesis of hexagonal boron nitride nanoplates with high aspect ratio

High crystalline hexagonal boron nitride nanoplates with high aspect ratio of ~400 have been synthesized by combustion synthesis method through magnesiothermic reduction reaction between B₂O₃ and Mg in N₂ pressure. The synthesized hexagonal boron nitride nanoplates were about 50 nm in thickness and larger than 20 μm in lateral size. The six-fold symmetric spots electron diffraction pattern of transmission electron microscopy shows that the nanoplate is well crystallized. Hexagonal boron nitride nanoplates grow via an Oswald ripening process and have larger lateral size when it was prepared with larger magnesium particles. High temperature liquid magnesium provides an important environment for the growth and crystallization of boron nitride. This work provides an effective way to achieve low-cost and large-scale preparation of high-quality boron nitride nanoplates.     

Zinc substitution effect on the structural,spectroscopic and electrical properties of nanocrystalline MnFe2O4 spinel ferrite

This paper reports the structural, morphological, spectroscopic, dielectric, ac conductivity, and impedance properties of nanocrystalline Mn_1-xZn_xFe₂O₄. The nanocrystalline Mn–Zn ferrites were synthesized using a solvent-free combustion reaction method. The structural analysis using X-ray diffraction (XRD) pattern reveals the single-phase of all the samples and the Rietveld refined XRD patterns confirmed the cubic-spinel structure. The calculated crystallite size values increase from 8.5 nm to 19.6 nm with the Zn concentration. The surface morphological analysis using field emission scanning electron microscopy and the transmission electron microscopy confirms the nano size of the prepared ferrites. X-ray photoelectron spectroscopy was used to study the ionic state of the atoms present in the samples. Further, the high-resolution Mn 2p, Zn 2p, Fe 2p, and O 1s spectra of Mn_1-xZn_xFe₂O₄ does not result in the appearance of new peaks with Zn content, indicating that the Zn substitution does not change the ionic state of Mn, Zn, Fe, and O present in nanocrystalline Mn_1-xZn_xFe₂O₄. The investigated electrical properties show that the dielectric constant, tan δ and ac conductivity gradually decrease with increasing Zn substitution and the sample Mn_0·2Zn_0·8Fe₂O₄ has the lowest value of conductivity at 303 K. The ac conductivity measured at different temperatures shows the semiconducting nature of the ferrites. The impedance spectra analysis shows that the contribution of grain boundary is higher compared with the grain to the resistance. The obtained results suggest that the Zn substituted manganese ferrite nanoparticles can act as a promising candidate for high-frequency electronic devices applications.     

Developing a Combustion Model of a Solid Propellant Containing Capsules of Liquid Oxidizer

A combustion model of a solid propellant containing liquid oxidizer capsules was developed. The model is based on the combustion cycle of a unit cell, an oxidizer droplet surrounded by the adjusting amount of binder. Three combustion stages are considered: (1) binder decomposition parallel to oxidizer heating up to boiling temperature, (2) simultaneous binder and oxidizer gasification, and (3) gasification of the remaining condensed phase species. Simple global kinetics are assumed for the gas phase reactions and binder decomposition abiding Arrhenius law, and boiling process for the oxidizer abiding Clausius‐Clapeyron curves. The obtained results show similar trends as do other relevant models and experimental results, especially for the effect of droplet size and propellant composition on the burn rate. The predicted effect of initial temperature on the burn rate is less significant than for common solid propellants. The results indicate that for small droplets the oxidizer will heat up to boiling temperature even before it is revealed on the burning surface, due to heat conduction through the surrounding binder.     

Recent advances in high temperature catalytic combustion

Catalytic combustion of methane has been investigated for the application to gas turbines. As the combustion is operated at high temperatures and high space velocity, heterogeneous reaction and surface-initiated gas phase reaction proceed concurrently. Thermal resistance to maintain large surface area is, therefore, requested to attain high combustion efficiency above 1000°C. Hexaaluminate compounds were effective in maintaining large surface area. On the other hand, palladium catalysts were generally employed for the combustion of methane below 1000°C. The prototype catalyst combustors were successfully tested with their high combustion efficiency and low NO_x emission by using Pd based- and/or hexaaluminate catalysts.     

Computational and Experimental Investigation of an Industrial Biomass Furnace

An industrial biomass boiler of 200 kW thermal power is analyzed by computational and experimental methods. Gas composition and temperature within the furnace and in the downstream heat exchanger are measured. Combustion, flow and heat transfer processes within the furnace, exhaust tract as well as the forced convection on the water side of the heat exchanger are computationally investigated by the finite volume method-based computational fluid dynamics procedures. The predictions are compared with the experiments. A satisfactory overall agreement between the predicted and measured temperatures is observed with an average deviation about 5 %. For oxygen mole fractions a fair agreement is found that shows deviations within the range of 10–20 %. For carbon monoxide, calculations exhibit a stronger underprediction.     

典型结构一体化加力燃烧室性能数值仿真研究

为了简化发动机加力燃烧室的结构和减少零件数量以达到减轻发动机重量的目的;减少气流在加力燃烧室中在非加力状态下的流体损失,提高发动机的燃烧效率和工作稳定性,在额定工况下对A、B、C、D四种特定的一体化加力燃烧室的结构设计的气动性能进行了数值仿真研究,以寻求找到一个合理的一体化结构参数。计算结果表明,该新型一体化加力燃烧室均有稳定的流场结构,并在支板下方的凹腔区形成低速回流区,可以用来组织火焰和稳定燃烧,在四种方案中,B方案的总压恢复能力较强,流体损失小,流场特性最佳,燃烧性能最好。     

发射药点火燃烧测温技术探索研究

采用六通道光学高温计,对两种发射药的点火燃烧温度进行试验,获得了这两种发射药在不同压力点的点火燃烧温度,并对所获得的测温结果进行了分析,为进一步开展发射药的点火燃烧温度研究打下基础。     

网孔结构和可压缩材料抑制爆炸波的研究评述

对用网孔或可压缩材料抑制爆炸波的研究现状进行了评述 ,介绍了两种抑制爆炸波强度的理论和研究结果。指出了目前研究存在的问题 ,并对要解决的关键问题提出了见解