Wide Range Of Volume Fractions Research Articles

Conduction paths of backbones for thermal enhancement of nanocomposite phase change materials

Thermal energy storage (TES) based on phase change materials (PCMs) has become a research hot spot due to its high energy storage density and maintained operating temperature during the phase change. However, as PCM has a poor thermal conductivity that can be as low as 0.2~0.5 W/m·K, the charging/discharging processes of PCM modules are significantly restrained, which severely affects the application of the TES technology in industrial sectors. This study concerns the improvement of the effective thermal conductivity of composite PCM formed by adding nanoparticles with high thermal conductivity into different PCMs. A theoretical model is established to reveal the intrinsic mechanism for the promotion of thermal conductivity of composite PCM consisting of nanoparticles. The results show that aggregation and interfacial thermal resistance are the main reasons for the change of the thermal conductivity. By forming effective conduction paths composed of backbones in the composite PCM, the average thermal conductivity can be improved significantly, which can be as high as 10~50 W/m·K with a wide range of volume fraction of the additives.

X-ray radiography of viscous resuspension

We use X-ray imaging to study viscous resuspension. In a Taylor-Couette geometry, we shear an initially settled layer of spherical glass particles immersed in a Newtonian fluid and measure the local volume fraction profiles. In this configuration, the steady-state profiles are simply related to the normal viscosity defined in the framework of the suspension balance model. These experiments allow us to examine this fundamental quantity over a wide range of volume fractions, in particular, in the semidilute regime where experimental data are sorely lacking. Our measurements strongly suggest that the particle stress is quadratic with respect to the volume fraction in the dilute limit. Strikingly, they also reveal a nonlinear dependence on the Shields number, in contrast with previous theoretical and experimental results. This likely points to shear-thinning particle stresses and to a non-Coulomb or velocity-weakening friction between the particles, as also evidenced from shear reversal experiments.

Colloidal dynamics and elasticity of dense wax particle suspensions over a wide range of volume fractions when tuning the softness by temperature

We report on the colloidal dynamics and rheology of liquid droplet emulsions and solid particle suspensions over a wide range of volume fractions. Diffusing wave spectroscopy and rheometry were utilized to study the micro/macroscale properties of 1-eicosene wax particles dispersed in water. By changing the temperature, the wax droplets change their properties from soft to hard or vice versa, which, depending on the density, may affect the system properties in a different way. Our measurements in the high volume fraction regime focus on the elasticity index and the plateau storage modulus, Gp′. The results show a more pronounced increase of the modulus for the wax particle suspensions around the random close packing volume fraction compared to the emulsion.

The role of clusters on heat transfer in sedimenting gas-solid flows

Abstract In this study, Eulerian-Lagrangian simulations of unbounded sedimenting gas-solid flows are conducted to quantify the effect of two-phase flow interactions on thermal transport. Cold particles are initially randomly distributed in a hot gas and settle under gravity for a wide range of volume fractions and Prandtl numbers. Two-way coupling spontaneously generates clusters of particles with a characteristic length scale much larger than the particle diameter, where fluctuations in particle concentration generate and sustain the carrier-phase turbulence. Simulations reveal that the gas phase is cooled relatively fast in the vicinity of clusters while hot spots persist in regions void of particles. This non-homogeneity was found to delay the overall heat transfer by as much as a factor of four compared to systems with a homogeneous particle distribution. To identify the mechanisms responsible for these variations, the spatially-averaged temperature equation is derived for statistically homogeneous (zero-dimensional) time-dependent particle-laden flows. It is found that the local fluid volume fraction-temperature covariance, referred to here as the fluid temperature seen by the particles, is the key term that accounts for the effect of clusters on heat transfer. Next, a framework for studying thermally fully-developed flows is presented, and statistics at steady state are reported. Consistent with previous work, local variations in the interphase heat transfer coefficient are found to vary by several orders of magnitude. Finally, a statistical modeling approach based on the concept of presumed-shape probability distribution functions is proposed.

Modelling study on the three-dimensional neutron depolarisation response of the evolving ferrite particle size distribution during the austenite–ferrite phase transformation in steels

The magnetic configuration of a ferromagnetic system with mono-disperse and poly-disperse distribution of magnetic particles with inter-particle interactions has been computed. The analysis is general in nature and applies to all systems containing magnetically interacting particles in a non-magnetic matrix, but has been applied to steel microstructures, consisting of a paramagnetic austenite phase and a ferromagnetic ferrite phase, as formed during the austenite-to-ferrite phase transformation in low-alloyed steels. The characteristics of the computational microstructures are linked to the correlation function and determinant of depolarisation matrix, which can be experimentally obtained in three-dimensional neutron depolarisation (3DND). By tuning the parameters in the model used to generate the microstructure, we studied the effect of the (magnetic) particle size distribution on the 3DND parameters. It is found that the magnetic particle size derived from 3DND data matches the microstructural grain size over a wide range of volume fractions and grain size distributions. A relationship between the correlation function and the relative width of the particle size distribution was proposed to accurately account for the width of the size distribution. This evaluation shows that 3DND experiments can provide unique in situ information on the austenite-to-ferrite phase transformation in steels.

A new approach to rapidly generate random periodic representative volume elements for microstructural assessment of high volume fraction composites

An algorithm based on event-driven molecular dynamics theory was developed to rapidly generate periodic representative volume elements (RVEs) with nonuniform distributions for both unidirectional fibre-reinforced and spherical particle-reinforced composites. Detailed statistical analyses were conducted for assessing the ability to generate RVEs with nonuniformly dispersed microstructures and either constant or random inclusion sizes for a wide range of volume fractions. The generated microstructures were directly compared with available microstructural optical images of a composite material, showing excellent statistical correlation and providing validation for the developed RVE generation approach. For further validation, finite element analysis was conducted using the generated RVEs in order to evaluate volume averaged elastic constants. The expected isotropic characteristics of the RVEs were correctly calculated, and excellent correlations with experimental data from the literature provided additional support for the algorithm accuracy. The versatile algorithm can rapidly generate RVEs with realistic reinforcement dispersions and high volume fractions up to 80%, which is advantageous compared to other algorithms. The proposed algorithm can be used as a design tool to accurately evaluate and tailor the mechanical properties of distinct composite material systems, and for their microstructural assessment including local damage predictions.

The Prediction of the Mechanical Properties for Dual-Phase High Strength Steel Grades Based on Microstructure Characteristics

The decrease of emissions from vehicle operation is connected mainly to the reduction of the car’s body weight. The high strength and good formability of the dual phase steel grades predetermine these to be used in the structural parts of the car’s body safety zones. The plastic properties of dual phase steel grades are determined by the ferrite matrix while the strength properties are improved by the volume and distribution of martensite. The aim of this paper is to describe the relationship between the mechanical properties and the parameters of structure and substructure. The heat treatment of low carbon steel X60, low alloyed steel S460MC, and dual phase steel DP600 allowed for them to reach states with a wide range of volume fractions of secondary phases and grain size. The mechanical properties were identified by a tensile test, volume fraction of secondary phases, and grain size were measured by image analysis. It was found that by increasing the annealing temperature, the volume fraction of the secondary phase increased, and the ferrite grains were refined. Regression analysis was used to find out the equations for predicting mechanical properties based on the volume fraction of the secondary phase and grain size, following the annealing temperature. The hardening mechanism of the dual phase steel grades for the states they reached was described by the relationship between the strain-hardening exponent and the density of dislocations. This allows for the designing of dual phase steel grades that are “tailored” to the needs of the automotive industry customers.

Fast, Robust Evaluation of the Equation of State of Suspensions of Charge-Stabilized Colloidal Spheres.

Increasing demand is appearing for the fast, robust prediction of the equation of state of colloidal suspensions, notably with a view to using it as input data to calculate transport coefficients in complex flow solvers. This is also of interest in rheological studies, industrial screening tests of new formulations, and the real-time interpretation of osmotic compression experiments, for example. For charge-stabilized spherical particles, the osmotic pressure can be computed with standard liquid theories. However, this calculation can sometimes be lengthy and/or unstable under some physicochemical conditions, a drawback that precludes its use in multiscale flow simulators. As a simple, fast, and robust replacement, the literature reports estimations of the osmotic pressure that have been built by adding the Carnahan-Starling and the cell model pressures (CSCM model). The first contribution is intended to account for colloid-colloid contacts, and the second, for electrostatic effects. This approximation has not yet been thoroughly tested. In this work, the CSCM is evaluated by comparison with data from experiments on silica particles, Monte Carlo simulations, and solutions of the accurate Rogers-Young integral equation scheme with a hard-sphere Yukawa potential obtained from the extrapolated point-charge renormalization method for a wide range of volume fractions, surface charge densities, and interaction ranges. We find that the CSCM is indeed perfectly adequate in the electrostatically concentrated regime, where it can be used from vanishingly small to high surface charge because there is error cancellation between the Carnahan-Starling and cell model contributions at intermediate charge. The CSCM is thus a nice extension of the cell model to liquid-like dense suspensions, which should find application in the domains mentioned above. However, it fails for dilute suspensions with strong electrostatics. In this case, we show that, and explain why, perturbation methods and the rescaled mean spherical approximation are good alternatives in terms of precision, ease of implementation, computational cost, and robustness.

Piezoelectric sensitivity and hydrostatic response of novel lead-free 2–0–2 composites with two single-crystal components

This paper reports the piezoelectric performance and important related hydrostatic parameters of 2–0–2 composites based on lead-free ferroelectric and piezoelectric single crystals with 4mm symmetry. We demonstrate that ferroelectric domain-engineered alkali niobate-tantalate based single crystals provide large values of the piezoelectric coefficients g3j∗ and related parameters when used in a 2–0–2 composite system with a relatively wide range of volume fractions of single-crystal components. An ‘aspect-ratio effect’ as a result of the presence of inclusions of a piezoelectric Li2B4O7 single crystal in a polymer medium is studied for the first time for a case where the elastic compliance s12 in the single crystal/polymer layer of the composite passes through zero. It is observed that changes in the aspect ratio and volume fraction of the Li2B4O7 inclusions influence the hydrostatic piezoelectric coefficient gh∗, squared figure of merit dh∗gh∗ and electromechanical coupling factor kh∗ of the composite, and large values of gh∗∼102 mV·m/N, dh∗gh∗∼ (10−11–10−10) Pa−1 and kh∗≈ 0.6–0.7 are achieved. A link between max kh∗ and a change in sgns12 is first described for the 2–2-type composite, and a comparison of the hydrostatic parameters of the novel and related composites is made. The present results show the potential of lead-free 2–0–2 composites that are suitable for piezoelectric sensor, energy-harvesting and hydroacoustic applications.

Parametric multiphysics finite-volume theory for periodic composites with thermo-electro-elastic phases

The zeroth-order multiphysics finite-volume micromechanics has been proposed to model coupled thermo-electro-mechanical behaviors of unidirectional composites embedded with piezoelectric phases. Parametric mapping is implemented within the multiphysics finite-volume theory’s framework, facilitating modeling of multiphase piezoelectric materials with complex microstructures with relatively coarse unit cell discretization. The resulting theory admits piezoelectric materials with complete anisotropy and arbitrary poling direction and enables rapid generation of the entire set of coupled thermo-mechanical, piezoelectric properties, figures of merits, as well as the local fluctuations of fields within the composite microstructures with greater fidelity than its predecessor. The proposed method is verified extensively by comparison with the finite-element homogenization technique, which produces an excellent agreement in a wide range of volume fractions but offers much better stability and efficiency. The contr...

Structurally Stable Attractive Nanoscale Emulsions with Dipole-Dipole Interaction-Driven Interdrop Percolation.

This study introduces an extremely stable attractive nanoscale emulsion fluid, in which the amphiphilic block copolymer, poly(ethylene oxide)-block-poly(ϵ-caprolactone) (PEO-b-PCL), is tightly packed with lecithin, thereby forming a mechanically robust thin-film at the oil-water interface. The molecular association of PEO-b-PCL with lecithin is critical for formation of a tighter and denser molecular assembly at the interface, which is systematically confirmed by T2 relaxation and DSC analyses. Moreover, suspension rheology studies also reflect the interdroplet attractions over a wide volume fraction range of the dispersed oil phase; this results in a percolated network of stable drops that exhibit no signs of coalescence or phase separation. This unique rheological behavior is attributed to the dipolar interaction between the phosphorylcholine groups of lecithin and the methoxy end groups of PEO-b-PCL. Finally, the nanoemulsion system significantly enhances transdermal delivery efficiency due to its favorable attraction to the skin, as well as high diffusivity of the nanoscale emulsion drops.

Real-space imaging of translational and rotational dynamics of hard spheres from the fluid to the crystal.

Using real-space imaging of single particles, we investigate the interplay between translational and rotational motion of tracer particles in suspensions of colloidal particles over a wide range of volume fractions from dilute fluid to densely packed crystal. To this end, we introduce a new type of spherical colloidal tracer particles containing two differently labelled fluorescent cores. The tracer particles can be combined with host particles enclosing a single fluorescent core and chemical and physical properties identical to the tracers. This leads to a system of spherical colloidal particles, in which spatio-temporal trajectories of rotation and translation of individual particles can be recorded simultaneously with full 360° resolution of rotational dynamics. Our analysis shows that translation and rotation of colloidal particles are uncorrelated and decoupled for all volume fractions irrespective of the phase of the particle system.

Asymmetric Double Tetragonal Domain Packing from ABC Triblock Terpolymer Blends with Chain Length Difference

A specific phase structure was observed for binary blends of poly(isoprene)-block-poly(styrene)-block-poly(2-vinylpyridine) (ISP) triblock terpolymers with asymmetric chain lengths of two end-blocks. Tetragonal-packed cylinders were obtained from various binary blends on a wide range of volume fractions, although the sizes of I and P cylinders were highly asymmetric. Those structures have never been found for monodisperse ABC triblock terpolymers, and the three specific features have been confirmed: (1) I cylinders were metamorphosed into rod domains, and their interfaces have nonconstant mean curvatures; (2) the cross-sectional area ratio of I/P domain is qualitatively changed with the volume fraction of each component; and (3) spherical and cylindrical domains of P component coexist. The molecular design adopted in the present work, that is, I and P blocks in two parent terpolymers have both fairly large chain length difference, must lead to these new morphologies.

Strong Ligand–Block Copolymer Interactions for Incorporation of Relatively Large Nanoparticles in Ordered Composites

For well-ordered block copolymer (BCP) nanocomposites containing nanoparticles (NPs), the ratio of NP core diameter (dcore) and BCP domain width (L) has been generally limited with dcore/L < 0.3 when NP/BCP interactions are relatively neutral or weak. Here, we systematically investigate the spatial distribution of gold nanoparticles (NP) ranging in size up to 0.8 times that of the target domain width in symmetric polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) using a hydrogen-bonding-mediated (H-bonding) assembly. NPs with core diameters (dcore) 2, 5, 9, and 15 nm are coated with ligands bearing phenol groups as the hydrogen-donating sites for H-bonding including 2-(11-mercaptoundecyl)hydroquinone and thiol-terminated poly(4-hydroxystyrene). These hydroxylated NPs are selectively incorporated into the P2VP block (domain width L = 7–37 nm) over a wide range of volume fractions (ϕ = 3–26 vol %). Particle distributions are biased toward the center of in the P2VP domains when dcore/L is greater than 0.3 and ...

Scaling of shear-induced diffusion and clustering in a blood-like suspension

The transport of cells and substances in dense suspensions like blood heavily depends on the microstructure and the dynamics arising from their interactions with red blood cells (RBCs). Computer simulations are used to probe into the detailed transport-related characteristics of a blood-like suspension, for a wide range of volume fractions and shear rates. The shear-induced diffusion of RBCs does not follow the established linear scaling with shear rate for higher volume fractions. The properties directly related to RBC deformability —stretching and flow orientation— are not sufficient to explain this departure according to the model of Breedveld, pointing to the dominance of collective effects in the suspension. A cluster size analysis confirms that collective effects dominate high volume fractions, as the mean cluster size is above 2 and the number of “free RBCs” is significantly decreased in denser suspensions. The mean duration of RBC contacts in clusters is increased in the high volume fraction and shear rate cases, showing that these clusters live longer.

Diblock copolymers of polystyrene‐b‐poly(1,3‐cyclohexadiene) exhibiting unique three‐phase microdomain morphologies

ABSTRACTThe synthesis and molecular characterization of a series of conformationally asymmetric polystyrene‐block‐poly(1,3‐cyclohexadiene) (PS‐b‐PCHD) diblock copolymers (PCHD: ∼90% 1,4 and ∼10% 1,2), by sequential anionic copolymerization high vacuum techniques, is reported. A wide range of volume fractions (0.27 ≤ ϕPS ≤ 0.91) was studied by transmission electron microscopy and small‐angle X‐ray scattering in order to explore in detail the microphase separation behavior of these flexible/semiflexible diblock copolymers. Unusual morphologies, consisting of PCHD core(PCHD‐1,4)–shell(PCHD‐1,2) cylinders in PS matrix and three‐phase (PS, PCHD‐1,4, PCHD‐1,2) four‐layer lamellae, were observed suggesting that the chain stiffness of the PCHD block and the strong dependence of the interaction parameter χ on the PCHD microstructures are important factors for the formation of this unusual microphase separation behavior in PS‐b‐PCHD diblock copolymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1564–1572

On continuum modelling of dense inelastic granular flows of relevance for high shear granulation

This article investigates a number of possible formulations of a continuum description for modelling dense inelastic granular flows. The revised Enskog theory (RET) for expressing the granular temperature and formulation of transport coefficients has been used within the continuum framework. The framework assumes particles as inelastic spheres and can describe a granular system at a wide range of volume fractions. The transport coefficients depend on the volume fraction through a modified expression for the radial distribution function. The proposed radial distribution function is based on previous studies on the behaviour of the shear viscosity in which an earlier divergence of the latter compared to the other transport coefficients has been demonstrated for sheared dense granular systems. Our results show that the newly developed radial distribution function maintains the ability of RET to predict the occurrence of instabilities in a homogeneous cooling granular gas. The introduced function also improves predictions for the velocity and volume fraction profiles in a Couette cell dense shear flow. Thus the proposed expression shows promising features in terms of improving predictions for volume fractions relevant in high shear granulators. We have also observed that a different expression may be needed for the densest regions.

On the effect of incremental forming on alpha phase precipitation and mechanical behavior of beta-Ti-10V-2Fe-3Al

A combination of good ductility and fatigue resistance makes β-titanium alloys interesting for many current and potential future applications. The mechanical behavior is primarily determined by microstructural parameters like (beta phase) grain size, morphology and volume fraction of primary / secondary α-phase precipitates, and this allows changing and optimizing their mechanical properties across a wide range. In this study, we investigate the possibility to modify the microstructure of the high-strength beta titanium alloy Ti-10V-2Fe-3Al, with a special focus on shape and volume fraction of primary α-phase. In addition to the conventional strategy for precipitation of primary α, a special thermo-mechanical processing is performed; this processing route combines the conventional heat treatment with incremental forming during the primary α-phase annealing. After incremental forming, considerable variations in terms of microstructure and mechanical properties can be obtained for different thermo-mechanical processing routes. The microstructures of the deformed samples are characterized by globular as well as lamellar (bimodal) α precipitates, whereas conventional annealing only results in the formation of lamellar precipitates. Because of the smaller size, and the lower amount, of α-phase after incremental forming, tensile strength is not as high as after the conventional strategy. However, high amounts of grain boundary α and lamellar αp-phase in the undeformed samples lead to a significantly lower ductility in comparison to the matrix with bimodal structures obtained by thermo-mechanical processing. These results illustrate the potential of incremental forming during the annealing to modify the microstructure of the beta titanium Ti-10V-2Fe-3Al in a wide range of volume fractions and morphologies of the primary α phase, which in turn leads to considerably changes, and improved, mechanical properties.

Rethinking the modeling approach for asphaltene precipitation using the PC-SAFT Equation of State

The Perturbed Chain version of the Statistical Associating Fluid Theory Equation of State (PC-SAFT EoS) has been successfully applied to the modeling of the phase behavior of petroleum systems at reservoir conditions and the prediction of asphaltene precipitation. Although the predictive capabilities of this method and its wide range of applications are much better than other modeling techniques based on conventional cubic equations of state, there are certain features that are still missing, such as the polydisperse nature of asphaltenes. Furthermore, the simulation parameters are usually tuned to Asphaltene Onset Pressure (AOP) data –which are usually obtained by depressurizing live oil samples− or titration experiments of dead oil samples at ambient conditions. These techniques are subject to significant uncertainties caused by slow kinetics of asphaltene aggregation and the limited sensitivity of the instruments used. The focus of this work is on the experimental study and modeling of asphaltene precipitation from n-alkane - diluted crude oil mixtures considering the polydispersity of asphaltenes. To investigate the effect of asphaltene polydispersity, a crude oil sample is studied by sequential precipitation with different normal alkanes. A recently developed experimental technique called the “Indirect Method” is used for the detection and quantification of asphaltene precipitation. Based on our previous research using the indirect method, it was found that both the onset and the amount of asphaltene precipitation change with the aging time for a wide range of volume fractions of the added n-alkane. However, the data showed that the amount of precipitated asphaltene at 90 volume percent of a given precipitant is nearly time invariant. This experimental data point is not controlled by the kinetics of asphaltene aggregation and, for this reason, is used in this study to adjust the simulation parameters. The experimental data are modeled using the Perturbed Chain form of the Statistical Associating Fluid Theory. An enhanced characterization method is used to minimize the number of adjustable parameters. In this work, a distribution function is proposed to represent the asphaltene molecular weight distribution for different asphaltene sub-fractions. It has been found that the inclusion of four asphaltene sub-fractions to represent the polydispersity of asphaltenes is necessary to match the amount of asphaltene precipitated. The modeling results show a reasonable agreement for the precipitated amount at 90 volume percent of n-pentane, n-hexane, n-heptane and n-octane. The PC-SAFT EoS predicts an onset point that requires less addition of n-alkane than the corresponding experimental point after one day of aging time. This result suggests that a thermodynamic limit exists for the onset of asphaltene precipitation and that it corresponds to experiments that require very long aging times, as it has been previously reported by Maqbool et al. [1]. By fitting the simulation parameters to the amount of precipitated asphaltenes (and not the detected onset points) at 90 volume percent of added asphaltene precipitant, we can successfully remove the variability of the data with respect to the aging time. This tuning procedure also enables a more accurate prediction of the amount of precipitated asphaltenes at reservoir conditions, which is a more important result for the modeling of asphaltene deposition than the Asphaltene Onset Point itself. The integration of this novel characterization procedure as part of the Asphaltene Deposition Tool (ADEPT) [2–4] developed at Rice University is a topic of ongoing research.

Crystallization Kinetics of Concurrent Liquid–Metastable and Metastable–Stable Transitions, and Ostwald’s Step Rule

Experimental measurements of colloidal crystallization in a wide range of volume fractions of charged particles were performed to investigate the liquid-metastable-stable transition process. To fit the obtained experimental data, we developed a theoretical model to formulate the kinetics of the concurrent liquid-metastable and metastable-stable transitions. This model is well-supported by our observations. We found that when the ratio of the metastable-stable transition rate to the liquid-metastable rate is very large, the metastable state can become undetectable, although it still exists, offering a possible explanation for very few exceptions to Ostwald's step rule.