Compressive properties of pristine and SiC-Te-added MgB2 powders,green compacts and spark-plasma-sintered bulks

Pristine and (SiC+Te)-added MgB₂ powders, green and spark plasma sintered (SPS) compacts were investigated from the viewpoint of quasi-static and dynamic (Split-Hopkinson Pressure Bar, SHPB) compressive mechanical properties The amount of the additive (SiC+Te) was selected to be the optimum one for maximization of the superconducting functional parameters. Pristine and added MgB₂ show very similar compressive parameters (tan δ, fracture strength, Vickers hardness, others) and fragment size in the SHPB test. However, for the bulk SPSed samples the ratio of intergranular to transgranular fracturing changes, the first one being stronger in the added sample. This is reflected in the quasi-static K_IC that is higher for the added sample. Despite this result, sintered samples are brittle and have roughly similar fragmentation behavior as for brittle engineering ceramics. In the fragmentation process, the composite nature of our samples should be considered with a special focus on MgB₂ blocks (colonies) that show the major contribution to fracturing. The Glenn-Chudnovsky model of fracturing under dynamic load provides the closest values to our experimental fragment size data.     

The influence of fragmentation on the behavior of pyrite particles during pulverized coal combustion

Coal combustion is a complex series of reactions, dominated by transport mechanisms. A poorly understood aspect is the influence that fragmentation of excluded pyrite minerals has on the combustion thermal history and sulfur release. To explore this effect, fragmentation was incorporated into a mathematical model consisting of several stages, namely: heating up the particle, decomposition to pyrrhotite, fragmentation, and oxidization. A computational fluid dynamic (CFD) model was used to verify the heat and mass transfer predictions of the model. The model was then solved to simulate the particle heat and mass balance of a pyrite particle in a combustion environment.Fragmentation was found to increase particle temperature and sulfur release rate compared with a no fragmentation case, speeding up the particle temperature increase towards its melting point. Increased oxygen content in the bulk environment decreases the time required for a particle to reach its melting point and for complete sulfur release due to increased oxidation reaction rates. Model results also indicate that small pyrite particles need less time to reach both decomposition and melting temperatures; suggesting that smaller particles will spend more time in a molten state and react more completely.     

THE INFLUENCE OF SILANE COUPLING AGENT COMPOSITION ON THE SURFACE CHARACTERIZATION OF FIBER AND ON FIBER-MATRIX INTERFACIAL SHEAR STRENGTH

It is well known that the fiber-matrix interface in many composites has a profound influence on composite performance. The objective of this study is to understand the influence of composition and concentration of coupling agent on interface strength by coating E-glass fibers with solutions containing a mixture of hydrolyzed propyl trimethoxysilane (PTMS) and γ-aminopropyl trimethoxysilane (APS). The failure behavior and strength of the fiber-matrix interface were assessed by the single-fiber fragmentation test (SFFT), while the structure of silane coupling agent was studied in terms of its thickness by ellipsometry, its morphology by atomic force microscopy, its chemical composition by diffuse reflectance infrared Fourier transform (DRIFT), and its wettability by contact angle measurement. Deposition of 4.5 ×10 -3 mol/L solution of coupling agent in water resulted in a heterogeneous surface with irregular morphology. The SFFT results suggest that the amount of adhesion between the glass fiber and epoxy is dependent not only on the type of coupling agent but also on the composition of the coupling agent mixture. As the concentration of APS in the mixture increased, the extent of interfacial bonding between the fiber and matrix increased and the mode of failure changed. For the APS coated glass epoxy system, matrix cracks were formed perpendicular to the fiber axis in addition to a sheath of debonded interface region along the fiber axis.     

The fragmentation of coal particles during the coal combustion in a fluidized bed

This paper presents an experimental method for studying the fragmentation of coal particles during coal combustion in a fluidized bed and the quantitative fragmentation indexes of 10 typical Chinese coal ranks. The influences of a variety of factors such as the bed temperature, the size of coal particles, the coal rank and the fluidizing medium on the fragmentation index of coal particles are also studied. The research results show that the main reason for the fragmentation of coal particles is the primary fragmentation, and that the volatile matter can drastically influence the degree of fragmentation of coal particles.     

Failure and fragmentation of ceramic target with varying geometric configuration under ballistic impact

The failure and fragmentation of monolithic bare alumina 99.5% ceramic target and energy dissipation of steel 4340 projectile have been studied in a series of ballistic experiments carried out, with the incidence velocities in a range, 122–290 m/s. The velocity drop and energy dissipation increased with incidence velocity for 10 mm thick target with damage zone extended upon the whole area of rear face at higher velocities. The ballistic results obtained with the 10 mm thick target have been compared with the ballistic performance of the 5 mm thick target used in a previous study to explore the effects of target thickness on the failure mechanism. A model for the residual velocity of projectile after perforation of the single layered ceramic target has been developed based on the Lambert Jonas model by using the experimental data available for 5 mm and 10 mm thick alumina 99.5% target against 10.9 mm projectile. The residual velocities and damage patterns were reproduced with a reasonable amount of accuracy by a three-dimensional finite element model developed on commercial ABAQUS/CAE. The effect of obliquity and projectile diameter to target thickness ratio (D/T) on ballistic performance has been determined by the numerical simulation model with impact velocity in a range of 300–500 m/s. A spatial variation of ejected fragments velocity at different time steps was plotted to develop a velocity profile for the ceramic fragments coming out of the target. A semi-empirical model has been proposed for residual velocity after perforation of a monolithic ceramic target, relating to the incidence velocity and projectile diameter to target thickness ratio. The monolithic ceramic targets have been investigated for a comparative assessment of energy dissipation by the ceramic layer to eventually design an efficient front layer of a ceramic based composite armour in future studies.     

Hypervelocity impact modeling with different equations of state

This study focuses on the simulation of hypervelocity impact problems with different equations of state (EOSs). We used a high-order multi-material Godunov method in Eulerian form and applied an “exact” multi-material Riemann solver to find more accurate individual phase advection fluxes regardless of the aggregate state. We investigated effects of melting in strong shock waves, evaporation in rarefaction waves and spallation. We found that a careful treatment of the spall formation mechanism is of greatest importance in obtaining accurate numerical results. Three types of wide-range EOSs are discussed—caloric EOS in unified analytical form for condensed and rarefied matter; thermodynamically complete stable multi-phase EOS in tabular form which does not allow for metastable states; and thermodynamically complete metastable EOS in tabular form which permits metastable gas, liquid or solid phases.     

尖山铁矿溜井管理中块度配矿研究

通过对尖山铁矿粉矿堵井，泥石流跑溜和各系统下料喷堵矿等现象的研究与分析，在品位配矿的基础上，提出了块度配矿的生产管理措施。经过两年的实践，取得了较好的效果。     

Particle size reduction of anthracite coals during devolatilization in a thermobalance reactor

The effects of devolatilization temperature (750-900 °C), coal size (2-12 mm) and coal properties (carbon content, Hardgrove index (HGI), pore volume) of anthracite coals on the primary fragmentation and particle size reduction during devolatilization have been determined in a thermobalance reactor. The fragmentation index increases with increasing devolatilization temperature and particle size. The fragmentation index is also influenced by coal properties, such as carbon content, HGI, pore volume, etc. Thus, the reduction ratio of particle size before and after devolatilization increases with increasing devolatilization temperature and particle size.     

Multi-scale simulation of needle-shaped particle breakage under uniaxial compaction

The breakage of needle-shaped particles within a random packed bed subjected to uni-directional compaction has been simulated using the discrete element method (DEM). Elongated particles with a chosen aspect ratio have been created by linking individual spherical discrete elements by rigid bonds, characterized by a given ultimate bending strength. A randomly packed bed of these elongated particles has been formed and gradually compressed between two infinite parallel solid planes. The particle size distribution as function of the compaction ratio has been studied in dependence on the individual particle strength, the initial particle length, and their distribution. The simulations have shown that the fragmentation generally follows the sequential halving kinetics and that the formation of fines is most profound in systems with a distribution of particle strengths, both within and between individual particles.