Influence of particle size on the breaking of aluminum particle shells

Rupturing the alumina shell(shell-breaking)is a prerequisite for releasing energy from aluminum powder.Thermal stress overload in a high-temperature environment is an important factor in the rupture of the alumina shell.COMSOL Multiphysics was used to simulate and analyze the shell-breaking response of micron-scale aluminum particles with different particle sizes at 650℃in vacuum.The simulation results show that the thermal stability time and shell-breaking response time of 10μm–100μm aluminum particles are 0.15μs–11.44μs and 0.08μs–3.94μs,respectively.They also reveal the direct causes of shell breaking for aluminum particles with different particle sizes.When the particle size is less than 80μm,the shell-breaking response is a direct result of compressive stress overload.When the particle size is between80μm and 100μm,the shell-breaking response is a direct result of tensile stress overload.This article provides useful guidance for research into the energy release of aluminum powder.

Detailed modeling of aluminum particle combustion——From single particles to cloud combustion in Bunsen flames

A numerical model for aluminum cloud combustion which includes the effects of interphase heat transfer,phase change,heterogeneous surface reactions,homogeneous combustion,oxide cap growth and radiation within the Euler–Lagrange framework is proposed.The model is validated in single particle configurations with varying particle diameters.The combustion process of a single aluminum particle is analyzed in detail and the particle consumption rates as well as the heat release rates due to the various physical/chemical sub-models are presented.The combustion time of single aluminum particles predicted by the model are in very good agreement with empirical correlations for particles with diameters larger than 10μm.The prediction error for smaller particles is noticeably reduced when using a heat transfer model that is capable of capturing the transition regime between continuum mechanics and molecular dynamics.The predictive capabilities of the proposed model framework are further evaluated by simulating the aluminum/air Bunsen flames of Mc Gill University for the first time.Results show that the predicted temperature distribution of the flame is consistent with the experimental data and the double-front structure of the Bunsen flame is reproduced well.The burning rates of aluminum in both single particle and particle cloud configurations are calculated and compared with empirical correlations.Results show that the burning rates obtained from the present model are more reasonable,while the correlations,when embedded in the Euler–Lagrange context,tend to underestimate the burning rate in the combustion stage,particularly for the considered fuel-rich flames.

Electrical and Optical Properties of Nano Aluminum Film/Particle Structure

The electrical and optical effects of particles on the nano aluminum film deposited by thermal evaporation was investigated. From the characterization results of scanning electron microscope（SEM）, the accumulation in tens of nanometers had been observed. The current-voltage（I-V） curve of the sample indicates its nonlinear electrical characters expecting the corresponding nonlinear optical properties. By the theoretical calculation, nonlinear conduction of the carrier transportation may result from the barrier-well-barrier structure, where negative resistance and Coulomb blockade effect appears. The simulation results are approximately matched with the experimental results. By testing the fluorescence emission spectrum of the sample, peaks were found to be located at 420 and 440 nm. In addition, the full width at half maximum（FWHM） had been obviously broadened by means of adding 2, 5-diphenyloxazole（DPO）. Therefore, discrete energy levels could be estimated inside those particles.

Studies on aluminum powder combustion in detonation environment

The combustion mechanism of aluminum particles in a detonation environment characterized by high temperature(in unit 10^(3)K),high pressure(in unit GPa),and high-speed motion(in units km/s)was studied,and a combustion model of the aluminum particles in detonation environment was established.Based on this model,a combustion control equation for aluminum particles in detonation environment was obtained.It can be seen from the control equation that the burning time of aluminum particle is mainly affected by the particle size,system temperature,and diffusion coefficient.The calculation result shows that a higher system temperature,larger diffusion coefficient,and smaller particle size lead to a faster burn rate and shorter burning time for aluminum particles.After considering the particle size distribution characteristics of aluminum powder,the application of the combustion control equation was extended from single aluminum particles to nonuniform aluminum powder,and the calculated time corresponding to the peak burn rate of aluminum powder was in good agreement with the experimental electrical conductivity results.This equation can quantitatively describe the combustion behavior of aluminum powder in different detonation environments and provides technical means for quantitative calculation of the aluminum powder combustion process in detonation environment.

Microstructure of in situ Al_3Ti/6351Al composites fabricated with electromagnetic stirring and fluxes

The 6351 wrought aluminum alloy and K2TiF6-CaF2-LiCl components were selected as raw materials to fabricate in situ Al3Ti particulate reinforced aluminum alloy at 720℃via direct melt reaction method with electromagnetic stirring(EMS).CaF2 and LiCl acted as fluxes to lower the reaction temperature of the system.It is shown that the electromagnetic stirring and fluxes accelerate the emulsion process of K2TiF6.Optical microscopy,scanning electron microscopy,transmission electron microscopy and energy dispersive spectrum were utilized to analyze the microstructure and components of composites.Compared to composites fabricated without EMS and fluxes,the sizes of endogenetic Al3Ti are refined from 10-15μm to 2-4μm,which are often accompanied with silicon element.The morphology of Al3Ti or Al3TiSi0.22 exhibits triangle,quadrilateral and other clumpy patterns. Because of the Ca elements from CaF2,the sizes of Mg2Si decrease from 8-10μm to 1-2μm due to the formation of Ca2Si.

Investigation on Reaction Interface between the Aluminum and K2ZrF6 by Freezing the Molten Salt Reaction

A reaction interface between the aluminum and K_2ZrF_6 during molten salt reaction process was frozen by quenching the mold in water, and the interface structure was analyzed to determine the formation process of Al_3Zr. Results show that a clear conical interface existed between the K_2ZrF_6 and aluminum. A zirconium accumulation layer with the thickness of about 2–3 lm was formed at the aluminum side of the interface. Many initially formed Al_3Zr particles（with the size of 0.4–16 lm） distributed in this layer, most of which located at the interface. The morphology of Al_3Zr particles is closely related with their size. For the size of 0.4–1 lm, the Al_3Zr appeared as globular and ellipsoid shapes. When it grew to the size of 1–2 and 2–16 lm, it exhibited the rule cube shape, and rule cuboids shape, respectively.