High Particle Volume Fractions Research Articles

Powder metallurgy of Al-based composites reinforced with Fe-based glassy particles: Effect of microstructural modification

ABSTRACTAl metal matrix composites reinforced with high volume fraction of Fe50.1Co35.1Nb7.7B4.3Si2.8 glassy particles were fabricated by mechanical milling followed by hot pressing. Elemental Al powders blended with 60 vol.% of glassy particles were mechanically milled for 1, 5, 10, 15, and 20 h, respectively. Selected samples were sintered by uniaxial hot pressing under Ar atmosphere. The changes in the microstructure along with their mechanical properties were investigated. Structural and microstructural characterization followed by microhardness and compression test results of the bulk composite material is reported. The use of high volume fraction of Fe-based glassy particles as reinforcement led to significant hardening of the Al matrix while leading to a remarkable combination of high strength and plasticity.

Micromechanical study on the effective elastic moduli of polymer-bonded explosives with imperfect interfaces

ABSTRACTThe particle/binder interfaces are wide spread since the particle volume fractions of polymer-bonded explosives (PBXs) are normally greater than 80%. Interfacial imperfection extensively exists and counts for the decline in effective properties. To investigate the impact of imperfect interfaces on the effective properties, a spring-interface model is introduced into the Mori–Tanaka method. Then the generalized self-consistent method is revised to predict the effective elastic moduli of PBXs. Results show that interfacial parameters have evident influences on the effective moduli. The effective modulus reduces as the interfacial imperfection aggravates, and this reduction is bigger in higher particle volume fraction. Further analyses indicate that, for PBXs, the shear moduli are independent of the interfacial normal compliance just as the bulk moduli independent of the tangential compliance. Since declines of the effective elastic moduli almost all result from the additional strain caused by imperfect interfaces, influences of the modified Eshelby tensors are negligible.

Predicting the Effective Elastic Properties of Polymer Bonded Explosives based on Micromechanical Methods

ABSTRACTDue to the extremely high particle volume fraction (greater than 85%) and damage feature of polymer bonded explosives (PBXs), conventional micromechanical methods lead to inaccurate estimates on their effective elastic properties. According to their manufacture characteristics, a multistep approach based on micromechanical methods is proposed. PBXs are treated as pseudo poly-crystal materials consisting of equivalent composite particles (explosive crystals with binder coating), rather than two-phase composites composed of explosive particles and binder matrix. Moduli of composite spheres are obtained by generalized self-consistent method first, and the self-consistent method is modified to calculate the effective moduli of PBX. Defects and particle size distribution are considered by Mori-Tanaka method. Results show that when the multistep approach is applied to PBX 9501, estimates are far more accurate than the conventional micromechanical results. The bulk modulus is 5.75% higher, and shear modulus is 5.78% lower than the experimental values. Further analyses discover that while particle volume fraction and the binder’s property have significant influences on the effective moduli of PBX, the moduli of particles present minor influences. Investigation of another particle size distribution indicates that the use of more fine particles will enhance the effective moduli of PBX.

Grain refinement of pure Al using Al2.5Cu0.5Ti particles with an L12 structure

In this study, an L12-modified Al2.5Cu0.5Ti intermetallic compound was chosen as a heterogeneous nucleation site to develop grain refiners for cast Al with a high volume fraction of heterogeneous nucleation site particles. Al2.5Cu0.5Ti intermetallic compound particles with an L12 structure were prepared by crushing the bulk material obtained by arc melting or directly by gas atomization. Al–10 vol% Al2.5Cu0.5Ti, Al–20 vol% Al2.5Cu0.5Ti, Al–30 vol% Al2.5Cu0.5Ti and Al–40 vol% Al2.5Cu0.5Ti refiners were fabricated by the spark plasma sintering method. The grain refining performance of the fabricated refiners for pure cast Al was investigated. It was found that fabricated Al–Al2.5Cu0.5Ti refiners were effective grain refiners for cast Al because Al2.5Cu0.5Ti particles with the L12 structure act as nuclei for Al grains in Al melts. Different refining performance characteristics appeared for refiners with different volume fraction of heterogeneous nucleation site particles, even though the total amount of particles in the melt was the same. Moreover, superior grain refining efficiency was obtained for the Al2.5Cu0.5Ti particles having a single phase than those with secondary phase. This is because the lattice matching between heterogeneous nucleation site particles and solidified Al is critical to obtain finer grain structures for Al casts.

Jet impingement cooling and optimization study for a partly curved isothermal surface with CuO-water nanofluid

Numerical and optimization study of jet impingement cooling of a partly curved surface with CuO-water nanofluid was performed with Galerkin weighted residual finite element method and COBYLA (constrained optimization by linear approximation) optimization algorithm. Target surface was partly curved which has a semi-elliptic shape and kept at constant hot temperature. Simulations were performed for various values of Reynolds number and solid particle volume fraction. It was observed that effects of curved wall on the distribution of fluid flow and heat transfer characteristics are more pronounced for higher values of Reynolds number as compared to a flat wall configuration. Highest heat transfer is obtained with curved wall and significant differences are observed between the peak values of Nusselt number between a flat wall and curved wall case. The average Nusselt number is a linear increasing function of nanoparticle volume fraction and the trends in local and average heat transfer are similar for curved wall and flat wall configurations when nanoparticles are added. Average Nusselt number enhances by about 20% at the highest particle volume fraction as compared to water. A polynomial type correlation for the average Nusselt number was derived which depends on the Reynolds number and solid particle volume fraction for both configurations.

The Impotence of Non-Brownian Particles on the Gel Transition of Colloidal Suspensions.

The ability to predict transitions in the microstructure of mixed colloidal suspensions is of extreme interest and importance. The data presented here is specific to the case of battery electrode slurries whereby the carbon additive is reported to form strong colloidal gels. Using rheology, we have determined the effect of mixed particle systems on the critical gel transition . More specifically, we show that the introduction of a high volume fraction of large non-Brownian particles has little to no effect on . Although is unchanged, the larger particles do change the shape of the linear viscoelasticity and the nonlinear yielding behavior. There are interesting similarities to the nonlinear behavior of the colloidal gels with trends observed for colloidal glasses. A comparison of experimental data and the prediction from theory shows that the equation presented by Poon et al. is able to quantitatively predict the transition from a fluid state to a gel state.

Prominent role of a high volume fraction of Mg17Al12 particles on tensile behaviors of rolled Mg–Al–Zn alloys

Fabricating magnesium alloy sheets exhibiting superplasticity >500% by conventional rolling remains a challenge due to the difficulty in achieving fine-grained structure. Even more puzzling is the role of softened Mg17Al12 particles locating at grain boundaries, which play a promoting or retarding effect on superplasticity at elevated temperatures, has not been understood so far. Here, fine-grained (∼3.0 μm) Mg–9Al–1Zn (AZ91) alloy with a high volume fraction of spherical Mg17Al12 particles was prepared by conventional rolling. The rolled AZ91 alloy exhibits a high tensile strength of ∼370 MPa and ductility of ∼13% at room temperature. More importantly, high superplasticity of ∼563% and ∼735% is achieved at 250 and 300 °C, respectively. It is recognized that the deformation mechanisms of the Mg–Al–Zn alloys at elevated temperatures depend significantly on the amount of Mg17Al12 particles. This work highlights the prominent role of a high volume fraction of Mg17Al12 particles segregating along grain boundaries at elevated temperatures, which on one hand can effectively pin grain boundaries to retard grain coarsening, and on the other hand, are beneficial to relax the intergranular stress incompatibility, and hence significantly facilitate GBS behavior. The current study promotes significantly the industry application of high superplastic rolled AZ91 alloy sheets.

Analysis of single phase, discrete and mixture models, in predicting nanofluid transport

A numerical investigation of developing forced convective heat transfer and pressure drop of nanofluid flow inside a tube subject to a constant wall heat flux boundary condition is presented. The single-phase homogenous and two different two-phase models: Lagrangian-Eulerian model or (discrete phase model) and mixture model are utilized with both constant and temperature dependent properties to further investigate and clarify the differences and evaluate the assumption of the single-phase model. The obtained results were subjected to an intensive comparison with the available experimental data and numerical works in the literature. The influence of some important parameters such as, source and sink terms, injected particle mass flow rate, slip velocity, particle forces, Reynolds number, constant or temperature dependent properties and particle concentration on the heat transfer and flow characteristics of nanofluids were determined and discussed in detail. It was observed that the two phase Lagrangian-Eulerian model (DPM) overestimated the heat transfer coefficient values and the results from the mixture model displayed an unrealistic increase in heat transfer particularly for high particle volume fraction. The proposed single phase approach revealed a very good agreement with the experimental data and the maximum difference in the average heat transfer coefficient between the single-phase and DPM was found to be 5.9% considering variable properties. The results also revealed that increasing the injected particle mass flow rate does not have a significant effect on the heat transfer coefficient values and that the particles move with the same velocity of the fluid. Furthermore, the heat transfer coefficient increases as the particle volume fraction and Reynolds number increases, but it is accompanied by a higher pressure drop and wall shear stress values. DPM model provides a reasonable prediction for the thermal behavior of the nanofluids transport, the single-phase approach with temperature dependent viscosity and thermal conductivity is an accurate way to analyze the transport of nanofluids while requiring less CPU usage and memory for predicting the enhancement in nanofluids convective heat transfer.

Fabrication, mechanical properties and in vitro degradation behavior of newly developed Zn[sbnd]Ag alloys for degradable implant applications

Zn and Zn-based alloys have been recognized as highly promising biodegradable materials for orthopedic implants and cardiovascular stents, due to their proved biocompatibility and, more importantly, lower corrosion rates compared to Mg alloys. However, pure Zn has poor mechanical properties. In this study, Ag is used as a promising alloying element to improve the mechanical properties of the Zn matrix as well as its biocompatibility and antibacterial properties. Accordingly, we design three ZnAg alloys with Ag content ranging from 2.5 to 7.0wt% and investigate the influence of the Ag content on mechanical and corrosion behavior of the alloys. The alloys are developed by casting process and homogenized at 410°C for 6h and 12h, followed by hot extrusion at 250°C with extrusion ratio of 14:1. Degradation behavior is assessed by electrochemical and static immersion tests in Hank's modified solution. Microstructural analysis reveals that hot extrusion significantly reduces the grain size of the alloys. Zn-7.0%Ag alloy shows a reasonably equiaxed and considerably refined microstructure with mean grain size of 1.5μm. Tensile tests at room temperature suggest that increasing the Ag content steadily enhances the tensile strength, while it does not affect the tensile ductility significantly. Zn-7.0%Ag shows high yield strength and ultimate tensile strength of 236MPa and 287MPa, respectively, which is due to the grain refinement and high volume fraction of fine AgZn3 particles precipitating along the grain boundaries during the extrusion process. Among all these alloys, Zn-7.0%Ag displayed superplasticity over a wide range of strain rates (from 5×10−4s−1 to 1.0×10−2s−1) providing the possibility of exploiting forming processes at rapid rates and/or even at lower temperatures. In addition, extruded alloys exhibit slightly faster degradation rate than pure Zn. X-ray diffraction results show the presence of ZnO and Zn(OH)2 on the degraded surfaces. Moreover, scanning electron microscopy imaging reveals that micro-galvanic corrosion is more pronounced on the alloys with higher Ag content due to the higher volume fraction of AgZn3 particles.

Metal-matrix composite fabricated with gas tungsten arc melt injection and precoated with NiCrBSi alloy to increase the volume fraction of WC particles

AbstractThe volume fraction, dissolution, and segregation of WC particles in metal-matrix composites (MMCs) are critical to their wear resistance. Low carbon steel substrates were precoated with NiCrBSi coatings and processed with gas tungsten arc melt injection method to fabricate MMCs with high volume fraction of WC particles. The microstructures and wear resistance of the composites were investigated. The results showed that the volume fraction of WC particles increased with decreasing hopper height and was as high as 44% when hopper height was 100 mm. The dissolution of WC particles was minimal. The content of the alloying elements decreased from the top to the bottom of the matrix. More WC particles dissolved in the overlapping area, where Fe3W3C carbide blocks could be found. The wear loss of the MMCs after 40 min was 6.9 mg, which is 76 times less than that of the substrate after the 4 min test.

Hydrodynamics characterization of a choanoid fluidized bed bioreactor used in the bioartificial liver system: Fully resolved simulation with a fictitious domain method

Choanoid fluidized bed bioreactors (CFBBs) are newly developed core devices used in bioartificial liver-support systems to detoxify blood plasma of patients with microencapsulated liver cells. Direct numerical simulations (DNS) with a direct-forcing/fictitious domain (DF/FD) method were conducted to study the hydrodynamic performance of a CFBB. The effects of particle–fluid density ratio, particle number, and filter screens preventing particles flowing out of the reactor were investigated. Depending on density ratio, two flow patterns are evident: the circulation mode in which the suspension rises along one sidewall and descends along the other sidewall, and the non-circulation mode in which the whole suspension roughly flows upward. The circulation mode takes place under non-neutral-buoyancy where the particle sedimentation dominates, whereas the non-circulation mode occurs under pure or near-neutral buoyancy with particle–fluid density ratios of unity or near unity. With particle–fluid density ratio of 1.01, the bioartificial liver reactor performs optimally as the significant particle accumulation existing in the non-circulation mode and the large shear forces on particles in the circulation mode are avoided. At higher particle volume fractions, more particles accumulate at the filter screens and a secondary counter circulation to the primary flow is observed at the top of the bed. Modelled as porous media, the filter screens play a negative role on particle fluidization velocities; without screens, particles are fluidized faster because of the higher fluid velocities in the jet center region. This work extends the DF/FD-based DNS to a fluidized bed and accounts for effects from inclined side walls and porous media, providing some hydrodynamics insight that is important for CFBB design and operation optimization.

Modeling the rheological properties of highly nano-filled polymers

Organic and inorganic materials are usually added to polymers in order to achieve some benefits such as reducing the product cost, as well as achieving higher modulus and strength. Addition of these materials would change polymers’ behavior. Adding nano-materials to polymers on the other hand is a new challenge in the field of polymer composites where previous studies were unable to achieve good correlation with nano-composites at higher particle volume fractions. In this research, Yamamoto network theory is developed to investigate the behavior of highly nano-filled systems. For this purpose, five different types of sub-chain and two types of junctions are considered and the effect of particle size, concentration, and the model parameters in association with the behavior of the junctions are studied. Moreover, some experiments are performed on polystyrene filled with nano-silica at different particle size and concentration values in frequency mod in the linear region. At last, we compared the results of our final model with the experiments in order to evaluate its accuracy, which confirmed a very good agreement.

Fabrication, microstructure and mechanical properties of Mg matrix composites reinforced by high volume fraction of sphere TC4 particles

Abstract A novel liquid settling method was investigated and applied to fabricate TC4 spherical particle reinforced AZ91 alloy matrix composites. This method was called liquid state settling technique in which TC4 particles would settle down under the force of gravity. High volume fraction (50%) particle reinforced AZ91 composites could be easily obtained via this novel method. This is difficult to achieve for other traditional liquid fabrication methods. In addition, there was a good dispersion of TC4 particles in the AZ91 matrix and no clusters were found, which indicate that this method was feasible. Interfacial reaction occurred and the reaction product was confirmed to be Al2Ti. Three kinds of pre-dispersion technologies were used before the settling process and different interfacial microstructures were found. Theoretical calculation and experimental results both indicated that the interfacial product which was embedded in the matrix strengthened the composites and improved the tensile strength.

Feasibility of a small-scale production system approach for palm sugar sweetened dark chocolate

Palm sugar is highly produced and patronised in the south-eastern part of Asia, e.g. Indonesia which is also the world’s third largest cocoa producer. Recently, interest in palm sugar sweetened chocolate and better approaches for production methods that can be easily applied in developing countries are rising. This work investigated the influence of palm sugar on the quality attributes of dark chocolate in terms of fineness, rheological behaviour and aroma profile compared to that of sucrose. Furthermore, a small-scale processing approach using the combination of Stephan mixer and ball mill compared to the conventional method was investigated. The results showed that palm sugar sweetened chocolate exhibited a higher viscosity, a higher particle volume fraction and a higher degree of particle agglomeration due to its relatively high moisture and presence of glucose and fructose. Furthermore, chocolates sweetened with palm sugar displayed a distinctive aroma profile with the abundant presence of 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one and pyrazine-based compounds. Applying the alternative processing method leaded to the production of chocolate with rather high degree of agglomeration and viscosity as compared to chocolate produced by means of conventional processing. Moreover, the alternative processing method resulted in the presence of myrcene, β-trans-ocimene and isoamyl acetate which were not observed in the conventionally produced chocolates. The alternative processing method, however, seems to have potential for small-scale production of dark chocolate sweetened with palm sugar.

Eulerian-Eulerian Simulation of Particle-Liquid Slurry Flow in Horizontal Pipe

In this study, a computational fluid dynamics (CFD) simulation which adopts the inhomogeneous Eulerian-Eulerian two-fluid model in ANSYS CFX-15 was used to examine the influence of particle size (90 μm to 270 μm) and in situ particle volume fraction (10% to 40%) on the radial distribution of particle concentration and velocity and frictional pressure loss. The robustness of various turbulence models such as thek-epsilon(k-ε),k-omega(k-ω), SSG Reynolds stress, shear stress transport, and eddy viscosity transport was tested in predicting experimental data of particle concentration profiles. Thek-epsilon model closely matched the experimental data better than the other turbulence models. Results showed a decrease in frictional pressure loss as particle size increased at constant particle volume fraction. Furthermore, for a constant particle volume fraction, the radial distribution of particle concentration increased with increasing particle size, where high concentration of particles occurred at the bottom of the pipe. Particles of size 90 μm were nearly buoyant especially for high particle volume fraction of 40%. The CFD study shows that knowledge of the variation of these parameters with pipe position is very crucial if the understanding of pipeline wear, particle attrition, or agglomeration is to be advanced.

Shear Thinning of Noncolloidal Suspensions.

Shear thinning-a reduction in suspension viscosity with increasing shear rates-is understood to arise in colloidal systems from a decrease in the relative contribution of entropic forces. The shear-thinning phenomenon has also been often reported in experiments with noncolloidal systems at high volume fractions. However its origin is an open theoretical question and the behavior is difficult to reproduce in numerical simulations where shear thickening is typically observed instead. In this letter we propose a non-Newtonian model of interparticle lubrication forces to explain shear thinning in noncolloidal suspensions. We show that hidden shear-thinning effects of the suspending medium, which occur at shear rates orders of magnitude larger than the range investigated experimentally, lead to significant shear thinning of the overall suspension at much smaller shear rates. At high particle volume fractions the local shear rates experienced by the fluid situated in the narrow gaps between particles are much larger than the averaged shear rate of the whole suspension. This allows the suspending medium to probe its high-shear non-Newtonian regime and it means that the matrix fluid rheology must be considered over a wide range of shear rates.

Not just fractal surfaces, but surface fractal aggregates: Derivation of the expression for the structure factor and its applications

Densely packed surface fractal aggregates form in systems with high local volume fractions of particles with very short diffusion lengths, which effectively means that particles have little space to move. However, there are no prior mathematical models, which would describe scattering from such surface fractal aggregates and which would allow the subdivision between inter- and intraparticle interferences of such aggregates. Here, we show that by including a form factor function of the primary particles building the aggregate, a finite size of the surface fractal interfacial sub-surfaces can be derived from a structure factor term. This formalism allows us to define both a finite specific surface area for fractal aggregates and the fraction of particle interfacial sub-surfaces at the perimeter of an aggregate. The derived surface fractal model is validated by comparing it with an ab initio approach that involves the generation of a "brick-in-a-wall" von Koch type contour fractals. Moreover, we show that this approach explains observed scattering intensities from in situ experiments that followed gypsum (CaSO4 ⋅ 2H2O) precipitation from highly supersaturated solutions. Our model of densely packed "brick-in-a-wall" surface fractal aggregates may well be the key precursor step in the formation of several types of mosaic- and meso-crystals.

An advanced 3D modeling method for concrete-like particle-reinforced composites with high volume fraction of randomly distributed particles

An advanced numerical method is proposed to generate more realistic 3D meso-scale models for concrete-like particle-reinforced composites with high volume fraction of particles. In traditional methods, it usually consumes an excessive amount of computational time, and even worse, it cannot be realized when the volume fraction exceeds the computational limitation. A novel idea is presented in this paper in which free fall acceleration is applied to simulate the falling process of randomly distributed particles. Firstly, certain numbers of convex particles are generated based on a grading curve, and then the particles are randomly placed into a higher space than the required specimen size in height. The time for the intersection detection is significantly saved as the intersection possibility among particles in a larger space is decreased. Secondly, the surfaces of the generated particles are meshed with shell elements, and then the particles fall freely into a certain container. Finally, the matrix and particles are meshed with solid elements for further numerical analysis. Comparing with other modeling methods, this method can establish models with higher particle volume fraction. Besides, the minimum gap between adjacent particles can be flexibly controlled by changing the thickness of the shell elements. So numerical models with different volume fractions can be obtained easily. Moreover, computational cost and time can be effectively reduced. Based on this method, some numerical simulations have been carried out, and the results of numerical simulations in different loading cases all have good consistency with experimental data, which implies the accuracy of the method and the isotropy of the phase arrangements. This method is of great importance in performance analysis, structural optimization, and material design for particle-reinforced composites.

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., 2016. © 2016 Society of Plastics Engineers

Effective thermal properties of an aluminum matrix composite with coated diamond inhomogeneities

The paper focuses on the calculation of the effective thermal properties of aluminum matrix composites containing diamond particles. We propose a model explaining non-monotonic behavior of the overall thermal conductivity (TC) and monotonic behavior of the thermal expansion coefficient (CTE) at high volume fraction of diamond particles. The model also accounts for the effect of particle coatings that increases the interfacial bonding between the diamond particles and metal matrix. Effect of the coating is modeled using differential method of replacing an inhomogeneous inclusion by an equivalent homogeneous one. The effective properties are calculated using Maxwell homogenization scheme and compared with experimental data. We also establish and verify experimentally cross-property connection between thermal conductivity and thermal expansion coefficient for Al\\diamond composites.