A novel method for prediction of tensile strength of spherical particle‐filled polymer composites with strong adhesion

By analyzing the breakage of composites from a new perspective, the tensile strength of composites filled with spherical particles is studied in this article. A large number of experimental data are summed up, and a novel form of formula that can predict the tensile strength of composites is put forward. This form of formula focuses on the composites that possess strong adhesion strength between particles and polymer matrix. High particle volume fraction results in a phenomenon that the particles in composites have a tendency to aggregate. So, the influence of the particle aggregation is also studied in this article. A critical particle volume fraction is proposed to describe that at what value of particle volume fraction the clusters begin to form. And an aggregation factor is put forward to describe the extent that the clusters diminish the composite's tensile strength. A good agreement between the results of the prediction by this article and the experimental data in many references has been found, which validates the accuracy of the theory. POLYM. ENG. SCI., 57:137–143, 2017. © 2016 Society of Plastics Engineers     

Impact attrition of brittle structured particles at low velocities: rigorous use of a laboratory vibrational impact tester

Brittle particle impact attrition was measured over three orders of magnitude of impact velocity by use of both single- and multiple-impact testers. The multiple-impact tester was a resonant cantilever impactor with dynamic amplitude control and small-gap cavities, designed to ensure operation near the transition between the bouncing and resonant impact regimes. For this impactor, a novel technique using a trajectory simulation was developed to elucidate average impact velocities, effective particle restitution behavior, and average losses per impact from a set of nominal attrition rates (loss/time). This allowed direct comparison of data from the single-impact and multiple-impact testers.Results were obtained for three brittle, porous pharmaceutical particles with significantly-different, well-characterized internal structures. Results reveal a relatively unexplored mode of attrition that is distinguished by lack of gross fragmentation at low velocities, with a steep velocity dependence. This regime is expected for any brittle particle for which simple chipping is not seen—such as rounded solid particles and many agglomerates. At higher impact velocity, gross particle fragmentation is observed. The transition between these attrition regimes appears connected to particle structure, such as the size of attrition-resistant primary particles in an agglomerate or the point at which dominant flaws (that lead to fragmentation) are no longer critically active. The ranking of particles according to damage in high-velocity impacts was not predictive of damage in low-velocity impacts, because the particle attrition did not necessarily exhibit the same velocity-dependence in the two regimes. Such differences are critical for predicting performance in operations such as pneumatic conveying or fluidized beds, respectively.     

The effect of carbon source and molar ratio in Fe–Ti–C system on the microstructure and mechanical properties of in situ TiC/Fe composites

The size, distribution, and morphology of TiC particle in Fe–Ti–C system have a great influence on the mechanical properties of TiC/Fe composites. In this work, TiC/Fe composites were fabricated in the Fe–Ti–C system with different carbon source and molar ratio by combustion synthesis and hot-pressing method. Morphology and size of ceramic particles, as well as microstructure, interface bonding and mechanical properties of composites were compared. The results showed that the size of TiC particles decreased with increase of Fe content of Fe–Ti–C systems fabricated by the same carbon source, while the particles change from spherical shapes to cubic shapes which can reduce stress concentration between ceramic particles and matrix. Furthermore, TiC/Fe composites fabricated by 5Fe–Ti-carbon blacks (CBs) system exhibited superior yield strength (1523 MPa) compressive strength (2203 MPa) and microhardness (691.5 HV), caused by the high interface bonding strength and lamellar pearlite matrix which can commendably limit the dislocation slip. By comparison, TiC/Fe composites fabricated by 21Fe–Ti-carbon nanotubes (CNTs) system showed higher fracture strain (25.85%) on account of the ferrite matrix with favorable plastic. This work reveals the influence of carbon source and molar ratio of Fe–Ti–C system on TiC/Fe composites, which is helpful to further improve the properties of TiC/Fe composites.     

Boron Carbide Reinforced Alumina Composites

The mechanical properties of alumina have been successfully improved by adding isolated boron carbide particles of two different shapes. A K _Ic of 7.26 ± 0.20 MPa · m^1/2 for alumina—boron carbide whiskerlike composites and of 5.27 ± 0.12 MPa · m^1/2 alumina—boron carbide shardlike particle composites has been achieved. The fracture toughness of these composites is dependent on the volume fraction of the boron carbide particles as well as their size and shape. The flexural strength is also appreciably enhanced to a constant value with from 5 to 20 vol% boron carbide additions. The whiskerlike particles increase the flexural strength by 25% and the shardlike particles produce a 47% improvement.     

Fluidization of dried wastewater sludge

Experimental measurements were conducted of the minimum fluidization velocities and the entrainment (terminal) velocities for beds of dry, digested wastewater sludge particles. With the use of air, experiments were performed in an 80 mm i.d. glass column with carefully sieved, very narrow fractions of particles in a wide range of 0.65-5.3 mm. Physical examinations of the dried sludge particles revealed their irregular, mostly isometric shapes, their porous character, and their fibrous nature. New correlations, which cover the transitional flow region, have been developed to estimate the conditions of incipient fluidization and those of entrainment for such peculiar materials. Their explicit forms also enable the direct computation of the particle size corresponding to a chosen terminal velocity and the straightforward calculation of the terminal velocity of a given, irregularly shaped particle.     

Modelling titania formation at typical industrial process conditions: effect of surface shielding and surface energy on relevant growth mechanisms

This work investigates the effects of reduced accessible surface area of aggregate particles and of surface energy on relevant particle formation and growth mechanisms during titania formation from the vapour phase at industrial process conditions. Growth due to surface reaction and due to condensation is related to the fraction of the surface area that is exposed to the collision with single molecules. Surface shielding is found to hamper surface reaction and condensation once fractal aggregates start to form. It leads to significantly retarded precursor consumption and produces aggregate particles, which consist of more, but smaller primary particles. Surface energy data are varied within a range as proposed by available literature data. Moderate and high surface energy values result in a thermodynamic barrier to the formation of new particles and are shown to reduce the formation of seed particles by several orders of magnitude. This leads to the formation of aggregate particles which consist of a rather small number of primary particles and mainly grow by surface reaction. The primary contribution of condensation to growth of individual primary particles is shown to be very little. However, condensation should not be neglected as it has a strong impact on particle formation rates and hence on product characteristics such as the number of primary particles and primary particle size.     

Fragmentation of wood char in a laboratory scale fluidized bed combustor

Casuarina equisetifolia, a hard wood, and a popular energy crop in many tropical countries, was investigated experimentally for its char fragmentation in a laboratory scale atmospheric bubbling fluidized bed combustor. The effect of fuel shape and size on wood char fragmentation was studied. Wood particles of spherical, cylindrical (aspect ratio of 1), and cubical shapes of different sizes ranging from 10 to 25 mm were used in the experiments. Fragmentation of wood char was quantified in terms of various parameters, such as Number of Fragments (NF), Percentage of Fragmentation Events, Frequency of Fragmentation, Timing interval of Fragmentation, Size distribution of char and Fragmentation Index (FI). Also, qualitative observations on the evolution of char in terms of deformation, cracks and surface texture are discussed. It was observed that Casuarinaequisetifolia wood of sizes greater than 15 mm, of all shapes undergoes primary fragmentation during the devolatilization phase. Furthermore, chars fragment at the early stages (1st or 2nd quarter) of the char combustion phase, underscoring the significance of the phenomenon in fluidized bed combustion. For all the shapes of wood considered, there appears to be a cut-off size of the initial wood, below which its char certainly undergoes fragmentation. It was observed that the average char particle size at any instance during its combustion falls in a narrow range of 3.7–6.9 mm, 3–6.6 mm and 3–9.5 mm for spherical, cylindrical and cubical wood particles, respectively. Wood of initially cylindrical shape undergoes extensive fragmentation when compared with spherical and cubical shapes.     

On-line measurement of ultrafine aggregate surface area and volume distributions by electrical mobility analysis: I. Theoretical analysis

Electrical mobility analyzers are usually calibrated for spherical particles, and provide number, area and volume distributions for spherical particles. However, these instruments cannot be directly used to obtain the surface area and volume distributions for aggregates. Aggregates are important in technological applications, such as the manufacture of fine powdered materials, and in air pollution and atmospheric sciences. Thus, nanoparticle chain aggregates of low fractal dimension are another important limiting case, in addition to spheres; a method is described which makes it possible to relate aggregate surface area and volume distributions to the electrical mobility diameter. This is accomplished by equating the migration velocity of an aggregate to that of a sphere. Particles of equal migration velocities will trace similar paths in the mobility analyzer and have the same mobility diameter (neglecting the Brownian diffusive spread). By equating the migration velocities of a sphere and aggregate, the number and size of the primary particles composing the aggregate can be related to the diameter of a sphere with the same migration velocity.The calculation of aggregate surface areas and volumes requires two theoretical “modules”, one for the drag on the aggregates and the other for aggregate charging efficiency. Two modules selected from the literature were used. The results indicate that the surface area distributions of aggregates with random orientation are somewhat over-predicted when calculated directly from the mobility diameter. However, the volume distributions are greatly over-predicted, up to a factor of ten compared with values based on the mobility diameter. The affect of aggregate orientation on surface area estimates was also examined.     

A quantitative model of the motion of particles in the RSM/mintek on-stream particle size analyzer

The average radial velocities of particles in the regions adjacent to the inner and outer walls of the sizer helix have been calculated from measured particle concentrations using limited solutions to the continuity equations for particles and fluid. Reynolds number - drag coefficient curves based on the average velocities have been constructed and it was found that the correlation at the outer wall did not account satisfactorily for the effect of particle and fluid density. This was attributed to additional phenomena affecting particle motion that could not be taken into account in solving the continuity equations, so a separate curve had to be constructed for each particle density. The curves were used to predict the variation in separation size over a range of flow velocities and particle concentrations in the feed.     

管式炉中燃煤一次破碎特性的实验研究

燃煤在循环流化床锅炉中的破碎特性极大地改变了物料的粒度分布,对床内颗粒浓度、物料传热传质及煤颗粒的燃烧过程都有重要影响.由于循环流化床锅炉本身的复杂性及实验现象难于观察,在1台管式炉中研究了各种煤的一次破碎特性.实验发现,烟煤、贫煤、无烟煤和煤矸石的破碎形式并不相同.烟煤颗粒遵循环核分层破碎;贫煤、无烟煤既有表面破碎也有中心破碎,且少数颗粒因热爆性而迅速变为细小颗粒;煤矸石沿着颗粒层面发生破碎,破碎为一些碎片;此外,深入研究了颗粒粒径、炉膛温度和加热气氛对西山贫煤和阳泉无烟煤一次破碎特性的影响.     

Influence of macromixing on powder formation in a reactive coaxial jet

This paper deals with an experimental study and a modelling of the formation of particles in a reactive gaseous jet. Particles result from the gas-phase hydrolysis of tin tetrachloride in a turbulent coaxial jet. In presented experiments, the influence of velocities of gases and nozzle shape on particle size distributions has been studied. Experimental distributions have been evaluated by in situ light-scattering measurements. Theoretical particle size distributions resulting from a simple model are compared to experimental measurements. Results show that particles do not seem to grow only by agglomeration and mixing near the nozzle may strongly influence final particle size distributions. In fact, the development of mixing layers near the nozzle appears to have little effect on particle size distributions, so that mesomixing could be the major phenomenon.     

Hydrodynamic characteristics of a circulating fluidized bed

In a circulating fluidized bed (7.8 cm-ID x 260 cm-high), flow regime of coal-air system at room temperature has been determined. Bituminous coal particles used were either 0.73 mm or 1.03 mm in the mean diameter having density of 1400 Kg/m³. The transition velocities from bubbling to turbulent beds and the transport velocities between turbulent and fast beds have been determined. The resulting transition velocities between bubbling and turbulent beds were 103 cm/s for 0.73 mm and 130 cm/s for 1.03 mm coal particles, respectively. The transport velocities between turbulent and fast beds were 180 and 209 cm/s for 0.73 and 1.03 mm particles, respectively. In addition, chocking velocities were determined at different solid feeding rates. The resulting values were in the range of 2.55-2.65 m/s for 0.73 mm particle and of 2.77-2.84 m/s for 1.03 mm particle, respectively. The published literature data of the transition velocity between bubbling and turbulent bed have been correlated with particle properties.     

燃煤过程中颗粒物的形成机理研究进展

介绍了煤粉燃烧过程中颗粒物的形成机理，包括亚微米飞灰和残灰颗粒的主要形成途径．亚微米颗粒主要来自无机物的气化-凝结过程，在高温条件下无机矿物首先以氧化物、次氧化物或原子的形式气化，当温度降低时，无机蒸气通过均相成核、异相冷凝、凝并、团聚等过程形成细微颗粒．残灰由残留在焦炭颗粒中的矿物转化而成，焦炭破碎和表面灰的聚合是决定残灰最终粒径分布的主要过程，除此之外，对于含外来矿物较多的煤种，矿物破碎对残灰颗粒的形成也有十分重要的影响．最后对燃煤过程中颗粒物的形成机理研究提出了建议．     

An experimental study on the primary fragmentation and attrition of limestones in a fluidized bed

In this paper, an experimental study on the primary fragmentation and attrition of 5 limestones in a fluidized bed was conducted. The intensity of fragmentation and attrition were measured in the same apparatus but at different fluidizing velocities. It was found that the averaged size of the particles decreased by about 10-20% during the fragmentation process. The important factors for particle comminution include limestone types, heating rate, calcination condition and ambient CO₂ concentration. Fragmentation mainly occurred in the first a few minutes in the fluidized bed and it was more intense than that in the muffle furnace at the same temperature. The original size effect was ambiguous, depending on the limestone type. The comminution caused by attrition mainly occurred during calcination process rather than sulphation process. The sulphation process was fragmentation and attrition resisted. The attrition rate of sulphate was similar to that of lime in trend, decaying exponentially with time, but was one-magnitude-order smaller than that of lime. Present experimental results indicate that fragmentation mechanism of the limestone is dominated by CO₂ release instead of thermal stress.     

A population model of quartzite particles in a lab-scale fluidized bed due to the thermal fragmentation

A population model has been developed to simulate the size distribution of quartzite particles in a lab-scale fluidized bed. While quartzite particles as a kind of bed materials in the differential density circulating fluidized bed were loaded into the high temperature bed, the size distribution of quartzite particles would be changed due to the thermal fragmentation, which could significantly influence fluidization characteristics and heat transfer.With the purpose of comprehending the population of quartzite particles after thermal fragmentation, a fragmentation experiment of quartzite particles has been carried out in a lab-scale circulating fluidized bed. The results show that the fragmentation of particles mainly occurred on particles surface. Based on this experimental phenomenon, a mathematical population model was established to estimate the particle size distribution. The predicted value from the population model is close to the experimental value.     

Effects of particle shape and size on devolatilization of biomass particle

Experimental and theoretical investigations indicate particle shape and size influence biomass particle dynamics, including drying, heating rate, and reaction rate. Experimental samples include disc/flake-like, cylindrical/cylinder-like, and equant (nearly spherical) shapes of wood particles with similar particle masses and volumes but different surface areas. Small samples (320 μm) passed through a laboratory entrained-flow reactor in a nitrogen atmosphere and a maximum reactor wall temperature of 1600 K. Large samples were suspended in the center of a single-particle reactor. Experimental data indicate that equant particles react more slowly than the other shapes, with the difference becoming more significant as particle mass or aspect ratio increases and reaching a factor of two or more for particles with sizes over 10 mm. A one-dimensional, time-dependent particle model simulates the rapid pyrolysis process of particles with different shapes. The model characterizes particles in three basic shapes (sphere, cylinder, and flat plate). With the particle geometric information (particle aspect ratio, volume, and surface area) included, this model simulates the devolatilization process of biomass particles of any shape. Model simulations of the three shapes show satisfactory agreement with the experimental data. Model predictions show that both particle shape and size affect the product yield distribution. Near-spherical particles exhibit lower volatile and higher tar yields relative to aspherical particles with the same mass under similar conditions. Volatile yields decrease with increasing particle size for particles of all shapes. Assuming spherical or isothermal conditions for biomass particles leads to large errors at most biomass particle sizes of practical interest.     

A model investigation of the influence of particle shape on portland cement hydration

The NIST Virtual Cement and Concrete Testing Laboratory (VCCTL) is used to simulate the influence of particle shape on the hydration kinetics and setting of portland cement. Building on previous work in reconstructing particle shapes from real cements, real-shape particles are used to produce three-dimensional digitized cement paste microstructures, and the hydration of these microstructures is tracked using VCCTL. The degree of hydration and percolation of solids is monitored and compared to experimental data at several water-cement ratios. The simulations predict that shapes of particles influence cement hydration in two ways: the additional surface / volume ratio relative to spherical particles results in greater rates of hydration, and the anisometry in shape influences the degree of hydration at which the particles and hydration products percolate to form a stiff three-dimensional network.     

The phase behaviors of cylindrical diblock copolymers and rigid nanorods' mixtures

Mixtures of cylindrical forming diblock copolymers (DBCPs) and mobile nanorods (NRs) are systematically investigated via dissipative particle dynamics (DPD) simulations. Final morphology of such composites depends not only on the characteristics of the copolymers, but also on the physical or chemical features of NRs, such as NR number, length, and surface adsorption (neutral, A and B attractive). A consideration of enthalpic and entropic interactions is necessary when physically or chemically distinct NRs are introduced into the copolymer/nanorod composites. For the short NRs, the phase behavior is similar to that of spherical nanoparticles (NPs). For the long NRs, the self-assembly of NRs can influence both the orientation and morphology of diblock/nanorod mixtures. If more NRs are incorporated, under stronger confinement from the host phase separated domains, the long NRs will aggregate and self-assemble into a certain spatial organization, inducing the morphological transitions of the composites from one phase to another. This behavior is not encountered for a similar system doped with spherical particles, emphasizing the role of particle shape in the interaction between doping particles and the host phase.     

Entrainment behaviour of high-density Geldart A powders with different shapes

Atomised and milled Ferrosilicon with average particle diameters of 38 and 50 µm respectively were fluidised with air at ambient conditions. The entrainment rate of the more spherical atomised particles was on average six times that of the irregularly shaped milled particles over the range of superficial velocities investigated. In an attempt to decouple the effect of particle size from shape, the bed was divided into theoretically isolated bins based on the distributions of particle sizes. This indicated that the atomised particles had a higher entrainment rate for particles smaller than approximately 25 µm whereas the opposite was true for particles greater than this size. None of the entrainment correlations investigated was able to predict the switch in entrainment behaviour as a function of particle sphericity and diameter. Furthermore, the traditional critical particle diameter associated with cohesive Geldart A particles was not observed for either of the two particle shapes. It is therefore concluded that neither the hydrodynamic nor Van der Waals forces acting on the particles can adequately explain the entrainment rate behaviour of differently shaped high-density Geldart A particles.     

Segregation by size difference in a conical fluidized bed of pharmaceutical granulate

Axial and radial segregation studies of a dry placebo pharmaceutical granulate exhibiting a continuous, bimodal particle size distribution have been carried out in a bench-scale conical fluidized bed. Experiments were conducted at superficial gas velocities of 0.5, 0.75, and 1.0 m/s based on the cone inlet diameter and static bed heights of 0.12 and 0.17 m above the distributor. Axial profiles of the mixing index show that granule greater than 800 μm segregate to the bottom of the bed. Radial segregation data shows that the large granule tends to accumulate at the centre bottom and becomes better mixed as the gas velocity is increased. Static bed height showed no influence on radial or axial segregation in this study. It is theorized that observed segregation patterns are because the local velocities in the dilute central core region of the conical bed are less than the terminal velocities of the largest particles in the particle size distribution.