Granular Microstructure Research Articles

Axially moving materials with granular microstructure

The mechanics of axially moving media is significant because of their broad engineering applications. In many engineering applications, it is beneficial to understand the dynamical material response from a microstructural viewpoint. Here we focus upon wave propagation in axially moving materials with granular microstructure. To this end, the granular micromechanics approach is utilized since the resulting continuum model is known to predict wave dispersion. To consider axially moving materials, this approach is enhanced to account for the axial velocity using an Eulerian description of the system accounting for convective terms in the material derivatives and utilizing variational approach. The dispersive behavior of axially moving 1D materials are then derived and compared with the dispersive behavior of non-moving materials. In the absence of microstructure, the axially moving material model simplifies to published literature and shows non-dispersive non-symmetric forward and backward waves. In the case of axially moving materials with granular microstructure, the model predicts dispersive non-symmetric waves. In this case, there are two acoustic and two optical wave branches. Axial velocity leads to narrowing and widening in the frequency band gaps in the forward and backward waves, respectively. Negative group velocity is also observed in certain wavenumber ranges. Clearly, the stopbands created by the axial velocity and the non-symmetric dispersive behavior studied here should be considered in engineering designs for vibration control when the axially moving material possesses granular microstructure. The results presented here can also be used to help obtain parameters needed for axially moving granular metamaterials to be designed for particular applications.

Thermal analysis of large granular assemblies using a hierarchical approach coupling the macro-scale finite element method and micro-scale discrete element method through artificial neural networks

A hierarchical approach for modelling the thermal response of large-scale granular assemblies by coupling the micro-scale particle-level thermal interactions with the macro-scale continuum system is proposed. The coupling is done by using a machine learning tool that is trained to replicate the effect of discrete particle nature on the macro-scale system using finite elements. A trained Artificial Neural Network (ANN) tool that can estimate the effective local thermal conductivity for each finite element considering the influence of the presence of stagnant gas in the interstitial voids, gas pressure and the granular microstructure is used. This way of hierarchical coupling using ANN eliminates the need to perform thermal discrete element simulations for each finite element at every increment by directly predicting the effective local conductivity. The proposed hierarchical approach is applied to a breeder blanket of fusion reactor that consists of more than 15 million particles to demonstrate the efficacy of the method. The influence of the drop in gas pressure across the breeder unit and the heat generation on the temperature distribution of the full-scale breeder unit is analysed numerically.

Elastic wave propagation in dry granular media: Effects of probing characteristics and stress history

Elastic wave propagation provides a noninvasive way to examine polydisperse, frictional granular materials. The discrete element method (DEM) allows for a micromechanical interpretation of the acoustic response. Using experimentally measured granular microstructures as input, after straining them to various cyclic, oedometric compression states, we numerically perform both static and dynamic probing to deduce elastic moduli/wave velocities from small-strain modulus degradation and time/frequency-domain signals.Static probing allows to understand the path dependency of the elastic moduli, i.e., the stress response to small strain increments, as well as their degradation behavior, for medium-to-large probing strains. Complementarily, dynamic probing provides insights into the acoustic properties (dispersion relations) at different wavelengths, for various probing characteristics. The wave velocities extracted from the space-time evolution of particle motion are sensitive to travel distance and input waveform, but converge to the same long-wavelength velocities far away from the source, where short wavelengths are dispersed and attenuated. Analyzing the same space-time data in the frequency domain leads to the same results, much less dependent on the probing characteristics than in the time domain, and much faster than static probing.Concerning the dependence on the stress history, as expected, the moduli (and wave velocities) increase under initial oedometric compression. First unloading reveals a significant plastic, irreversible decrease of the moduli, whereas reloading along the oedometric path leads to a reduced degradation. The elastic regime, i.e., the probing strain at which the moduli begin to degrade, gradually decays, as the change of the deviatoric to mean stress ratio increases towards its maximum where plasticity/irreversibility is strongest. Interestingly, the moduli from static probing show that the degradation curves immediately before and after a load reversal are almost identical, suggesting that the elastoplastic behavior is symmetric around the turning point for tiny strains, as also confirmed by identical vibrational densities of states at reversal. Remarkably, the moduli, their degradation behavior and the shapes of the dispersion relations (normalized by their large wavelength limits), are all very similar during unloading and reloading, whereas they reflect a strongly different material behavior under initial loading.

Fabrication, microstructures and properties of SiCf/SiC composites prepared with two kinds of SiC fibers as reinforcements

Three-dimensional (3D) SiC/SiCf composites were fabricated by a polymer impregnation and pyrolysis (PIP) method using 3D four directional braided preforms from two kinds of SiC fibers as the reinforcements and liquid polycarbosilane as the SiC precursor. Both kinds of SiC fibers were prepared from the same precursor and process but with different heat treatment temperatures. The one prepared at a higher temperature (No.1) had a higher crystallinity than the another one (No.2). A thin pyrolytic carbon (PyC) interlayer was coated on the preforms by a methane CVD method before impregnation. Their microstructures and mechanical properties were investigated by TEM, SEM, Raman spectroscopy and mechanical tests. Results showed that both had a polycrystalline structure with grain sizes of 8-15 nm. Fiber pullout was seen from the fracture cross-section of the composites after a bending test, indicating pseudo-ductile fracture behavior. The composite using No.1 SiC fibers had an average flexural strength, elastic modulus and fracture toughness of 955.0±42.8 MPa, 110.3±1.7 GPa and 28.5±2.8 MPa·m1/2, respectively, which are superior to those of the material formed using No.2 SiC fibers. This was ascribed to the high modulus and excellent thermal resistance of the stoichiometric No.1 SiC fibers. The PyC layer adhering to No.1 SiC fibers was ordered and smooth while that deposited on No.2 SiC fibers had a loose and granular microstructure, which was attributed to the different surface chemistries of the two types of SiC fibers.

Deformation, Annealing, Melt‐Rock Interaction, and Seismic Properties of an Old Domain of the Equatorial Atlantic Lithospheric Mantle

AbstractWe analyzed a set of mantle xenoliths from the Fernando de Noronha archipelago to constrain the roles of deformation, cooling, annealing, and melt percolation in the evolution of the mantle lithosphere of the equatorial Atlantic. The peridotites are dominantly lherzolites with coarse granular or porphyroclastic microstructures. Equilibrium temperatures range between 850 and 1,000 °C. Olivine crystal preferred orientations (CPO) have mainly orthorhombic patterns characterized by clear [100] and [010] point maxima or a fiber‐[100] tendency; these patterns imply deformation by dislocation creep with dominant activation of the [100](010) slip system. Olivine fiber‐[010] patterns are less common; they probably result from recrystallization or melt‐rock interaction. Annealing after deformation partially recovered the microstructures and reduced the olivine CPO strength. Pyroxene CPOs often have weak consistency with olivine CPO, implying post‐deformation refertilization. Chemical compositions are systematically more fertile than those from abyssal peridotites from the Mid‐Atlantic Ridge. These compositions together with the microstructures suggest two stages of melt‐rock interaction. An early refertilization of the base of the lithosphere due to continuous percolation of small melt fractions and later, more important, but local chemical changes (both refertilization and dunitization) as well as recrystallization due to reactive melt percolation likely associated with the Cenozoic volcanism. By comparing the calculated seismic properties with existing seismological data we argue that the moderate seismic anisotropy displayed by the Fernando de Noronha mantle xenoliths likely records past horizontal asthenospheric flow parallel to the spreading direction frozen in the lithospheric mantle as the plate cooled.

Photocatalytic Degradation of Azithromycin by Nanostructured TiO₂ Film: Kinetics, Degradation Products, and Toxicity.

In this paper, nanostructured TiO2 film was prepared by the by sol-gel process and dip-coating technique with titanium tetraisopropoxide as a precursor. After heat treatment at 550 °C, the deposited film was characterized by means of micro-Raman spectroscopy and atomic force microscopy (AFM). It was found that the TiO2 film consisted of only the TiO2 anatase phase and showed a granular microstructure. Photocatalytic degradation of azithromycin by using sol-gel nanostructured TiO2 film was studied to define the most effective degradation process for potential use in wastewater treatment. Different factors were evaluated during photocatalysis, such as pH (3, 7, and 10), water matrix (ultrapure water and synthetic municipal waste water effluent), influence of another pharmaceutically active compound (sulfamethoxazole, one of the most often detected pharmaceutic compounds in waste waters), and radiation sources (low pressure ultraviolet (UV) mercury lamps with a UV-A and UV-C range; a light-emitting diode (LED) lamp with a radiation peak at 365 nm). The most effective degradation process was achieved with the UV-C irradiation source in matrices at pH 10. The water matrix had little effect on the photocatalytic degradation rates of azithromycin. The presence of sulfamethoxazole in the water matrix decreased the degradation rate of azithromycin, however, only in matrices with a pH level adjusted to 10. During the experiments, five azithromycin degradation products were identified and none of them showed toxic properties, suggesting effective removal of azithromycin. LED 365 nm as the irradiation source was not as effective as the UV-C lamp. Nevertheless, considering the cost, energy efficiency, and environmental aspects of the irradiation source, the LED lamp could be a “real-life” alternative.

In situ alloying of elemental Al-Cu12 feedstock using selective laser melting

ABSTRACTThis investigation developed selective laser melting (SLM) processing parameters for the in situ fabrication of an Al-Cu12 alloy from pure elemental blends of aluminium and copper powders. Use of elevated pre-heat temperatures (400°C) created a coarser dendritic cell microstructure consisting of supersaturated Al-rich with a uniform Al2Cu phase granular microstructure compared to non-pre-heated samples. Al-Cu12 in situ samples achieved maximum tensile strength values comparable to that of sand cast pre-alloyed Al-Cu12. Processing at elevated pre-heat temperatures created components with higher ultimate tensile strength and ductility compared to standard room temperature-built samples due to it assisting a more complete melting of Al and Cu particles. Additionally pre-heating enabled an artificial age hardening, producing an equilibrium α + θ microstructure. The creation of an alloy in situ through the use of elemental powder blends represents a low-cost and flexible methodology for exploration of new SLM material compositions and potential candidate materials for semi-solid processing using SLM.

Micro-scale investigation of the role of finer grains in the behavior of bidisperse granular materials

This paper presents a numerical study of the effect of fine content on the mechanical behavior of bidisperse granular materials using the discrete element method. Triaxial compression tests are performed on different samples with fine contents varied from 0 to 40%. It was found that, starting from 20%, fine content has a visible effect on the shear strength. The optimal fine content is about 30%, at which the shear strength is the best. An investigation into the granular micro-structure showed that the fine particles, on one hand, come into contact with coarse particles, but on the other hand, separate the latter ones as fine content increases beyond 20%. Thus, the part of the shear stress carried by the coarse–fine contacts increases, while the part carried by the coarse–coarse contacts decreases. For fine content ≤ 30%, the coarse–coarse contacts primarily carry the shear stress. Above this optimal fine content, the fine–coarse contacts overtake the coarse–coarse ones. The fine–fine contacts have little contribution to supporting the shear stress. For the studied range of fine content, the coarse particles primarily carry the shear stress, leaving the fine particles under relatively low stresses. Moreover, the matrix composed of fine particles is greatly softened by the shear loading. A classification of binary mixtures depending on their micro-structure was also proposed.

Micromagnetic simulator for complex granular systems based on Voronoi tessellation

Abstract A micromagnetic code based on Voronoi tessellation and the non-uniform fast Fourier transform (NUFFT) method is presented. The code is capable of efficiently and accurately simulating magnetization dynamics in large and structurally complex granular systems, such as multilayer granular media used for perpendicular magnetic recording, bit patterned media, granular nanowires, and read heads. In these systems the granular microstructure and distributions in grain and interface properties play an important role in device performance. The presented Voronoi simulator allows comprehensive studies to be performed as it accounts for the detailed granular microstructure and distributions that characterize true systems. Simulation time is greatly reduced by a non-uniform fast Fourier transform algorithm and implementation on graphics processing units (GPUs). Simulations of conventional magnetic recording, heat-assisted magnetization reversal, domain wall dynamics in granular nanowires, and particulate tape recording are presented.

Criticality in failure under compression: Acoustic emission study of coal and charcoal with different microstructures.

A systematic study of acoustic emission avalanches in coal and charcoal samples under slow uniaxial compression is presented. The samples exhibit a range of organic composition in terms of chemical elements as well as different degrees of heterogeneity in the microstructure. The experimental analysis focuses on the energies E of the individual acoustic emission events as well as on the time correlations between successive events. The studied samples can be classified into three groups. The more homogeneous samples (group I) with pores in the micro and nanoscales, with signatures of hardening effects in the stress-strain curves, exhibit the cleanest critical power-law behavior for the energy distributions g(E)dE∼E^{-ε}dE with a critical exponent ε=1.4. The more heterogeneous samples with voids, macropores, and granular microstructures (group III), show signatures of weakening effects and a larger effective exponent close to the value ε=1.66, but in some cases truncated by exponential damping factors. The rest of the samples (group II) exhibit a mixed crossover behavior still compatible with an effective exponent ε=1.4 but clearly truncated by exponential factors. These results suggest the existence of two possible universality classes in the failure of porous materials under compression: one for homogeneous samples and another for highly heterogeneous samples. Concerning time correlations between avalanches, all samples exhibit very similar waiting time distributions although some differences for the Omori aftershock distributions cannot be discarded.

Microfibers of microporous carbon obtained from the pyrolysis of electrospun β-cyclodextrin/pyromellitic dianhydride nanosponges

Novel activated carbon materials are being constantly developed and applied in many research and industrial applications. The choice of the precursor represents a key step, to achieve satisfying resulting features in terms of surface development, structural features, carbon contents, and amount of ash. In this work, a novel carbon product has been obtained starting from an electrospun β-cyclodextrin/pyromellitic dianhydride linked nanosponge. After the pyrolysis, the polymeric mat resulted in carbon material still displaying fibrous morphology, characterized by a diameters distribution of 1.3 ± 0.5 μm. A peculiar granular microstructure was observed via FESEM, a 692 m2/g surface area and a 5–12 Å pore size distribution, with pore volume of 0.14 cm3/g, resulted via BET characterization, while −32.8 ± 0.7 mV resulted after ζ potential analysis. These results confirmed β-cyclodextrin/PMDA-linked electrospun nanosponges as a suitable precursor for a novel carbon material preparation.

Comparisons of weathered lateritic, granitic and volcanic soils: Compressibility and shear strength

Lateritic soil is rich in oxides of iron and aluminum (sesquioxide). The effects of these oxides on its mechanical behaviour are not well-understood. In this study, the compression and shear behaviour of a lateritic (LAT) soil was tested and compared to those of two other weathered granitic and volcanic soils (CDG and CDV) for the first time within the critical state framework. The mineralogy, chemical compositions and microstructure of these three soils were also measured by x-ray diffraction (XRD), x-ray fluorescence (XRF) and scanning electron microscopy (SEM) respectively. It is found that only LAT contains goethite, while other minerals are similar in all the soils. The content of iron and aluminum oxides in LAT is about 40% higher than that in CDV and CDG. LAT is found to be less compressible than CDV and CDG by 18% and 36% respectively, even though the former has a higher clay content than the latter two soils. The lowest compressibility of LAT is mainly attributed to its high content of sesquioxide, which induces the formation of more aggregates. The LAT has more intra-aggregate pores and hence less inter-aggregate pores than CDV and CDG. A smaller volume of inter-aggregate pores of LAT results in a lower compressibility. It is found that the critical state friction angles of LAT, CDV and CDG are 42°, 33° and 38°, respectively. The highest friction angle of LAT is likely attributed to the content of large particles and presence of goethite. Due to the high sesquioxide contents of LAT, many clay fines also form large aggregates leading to a more granular microstructure. Consequently, the number of particle contacts is increased and therefore enhancing particle interlocking. Goethite mineral found in LAT could enhance inter-aggregate interlocking through its contiguous singly coordinated OH groups.

Kinetics and Energetics of Solute Segregation in Granular Block Copolymer Microstructures

The segregation kinetics of deuterated polystyrene (d-PS) within the grain boundary regions of a lamellar poly(styrene-b-isoprene) copolymer is analyzed using a combination of electron imaging and ...

Micromorphic and micropolar continua: A comparative study based on homogenisation of particle simulations

AbstractExtended continuum‐mechanical theories, such as the micromorphic, the micropolar or the microstrain continuum, are suitable formulations when additional characteristic microstructural deformation modes should be included into macroscopic modelling techniques. This is especially the case when the microscopic view of the considered material reveals a granular or particulate structure. Then, by means of a particle‐centre‐based homogenisation technique, it is possible to link the microstructural information that can be gained, for example, from Discrete‐Element (DE) simulations, to the extended deformation and stress states of the above mentioned extended continua. In this context the present contribution focuses on two main aspects: at first, different granular microstructures, in particular bonded and unbonded granular systems, are investigated using a discrete‐element formulation with the emphasis on the evolution of strain localisation phenomena. Secondly, the obtained results of these initial‐boundary‐value problems are homogenised towards micropolar and micromorphic stress measures. Depending on the microstructural characteristics, the activation and evolution of the extended stress states are compared and evaluated with special emphasis on the size of the chosen homogenisation domain.

Storage of aerobic granular sludge embedded in agar and its reactivation by real wastewater.

Aerobic granular sludge (AGS) was preserved using an agar embedding method to maintain its stability. No obvious damage was imposed on the granular appearance during 30 days of cold and dry storage, but the granular microstructure had an uneven surface with a large number of holes. The results were consistent with the extinction of microbial communities and the monitored consumption of extracellular polymeric substances, in which granular specific oxygen utilization rate and mixed liquor volatile suspended solids/mixed liquor suspended solids ratio, respectively, decreased by 72.4% and 62.5% during storage. A mass conversation calculation indicated that the loss of granular mass was 1.6393 g. An offensive odour was smelled during storage, and the results indicated that a material transformation and mitigation were involved between AGS and the gas phase. Although the granular structure was destroyed to a certain extent, no obvious damage was imposed on the granular skeleton during storage. After it was aerated again after a feeding with real wastewater, the residual skeleton served as a carrier for the rapid proliferation of microorganisms, and good granular properties were obtained after 11 days of reactivation.

Investigation on the fabrication of dicing blades with different sintering methods for machining hard-brittle material wafers

Hard-brittle materials applied in various micro-electromechanical systems should be sliced into miniature and microparts often with complex microstructures. However, it is difficult for bladed dicing to machine those materials without damage because of their hard and brittle properties. This article presents innovative design and fabrication of dicing blades and the associated dicing process optimization, particularly by focusing on the blade wheel fabrication through sintering methods including traditional hot press sintering, vacuum sintering and spark plasma sintering. Four key parameters, namely, the radial wear of the dicing blade, the current of relay, the number of chips larger than 50 μm and the largest chip size, are employed to assess the cutting performance of dicing blades fabricated. The analysis and experimental results indicate that the spark plasma sintering method achieves the lowest number of chips and the smallest chip size due to its rapid spark between metal particles during the sintering process. The spark plasma sintering method can also produce a granular microstructure with sufficient porosities, which results in uniform bonding strength of diamond abrasive grits within the dicing blade. For the dicing process parameters, the back cutting depth has the most obvious influence on the dicing blade tool life, cutting current and slicing damages, while the feed rate is ranked the next. The least influencing parameter is the dicing spindle rotational speed.

Influence of Y and Mn alternately doped order on the microstructures and dielectric properties of Ba0.6Sr0.4TiO3 films

ABSTRACTThe influences of Y and Mn alternately doped order on the microstructures and dielectric properties of the Ba0.6Sr0.4TiO3 (BST) films were reported in this paper. The Y and Mn alternately doped BST films were designed as YBST/MnBST/… and MnBST/YBST…multilayer films orderly expressed as (Y/Mn)M and (Mn/Y)M for short, and prepared on Pt/Ti/SiO2/Si wafers by an improved sol-gel method, where Y and Mn represent yttrium doped BST layer and manganese doped BST layer, and M is cycle unit, respectively. The microstructures of the alternately doped BST films were observed by SEM and the capacitance-voltage curves at 100 kHz or 1 MHz were measured by a HP4284A LCR meter and the dielectric properties in the range of 1GHz were measured by an E4991A impedance analyzer. Compared to Y or Mn doped multilayer BST film, (Y/Mn)M and (Mn/Y)M show higher dielectric tunability and lower dielectric loss with higher dielectric constant. Moreover, (Y/Mn)M show better dielectric properties than (Mn/Y)M because (Y/Mn)M show granular microstructures independent of M, while the (Mn/Y)M show granular microstructures when M is 2 and add to a surface layer of columnar microstructures on the granular microstructures when M is 4. The related mechanisms were obtained in terms of the XRD phase structures, the cross-sectional SEM microstructures and the AFM morphologies.

Thin films of Ag–Au nanoparticles dispersed in TiO2: influence of composition and microstructure on the LSPR and SERS responses

Thin films containing monometallic (Ag,Au) and bimetallic (Ag–Au) noble nanoparticles were dispersed in TiO2, using reactive magnetron sputtering and post-deposition thermal annealing. The influence of metal concentration and thermal annealing in the (micro)structural evolution of the films was studied, and its correlation with the localized surface plasmon resonance (LSPR) and surface enhanced Raman spectroscopy (SERS) behaviours was evaluated. The Ag/TiO2 films presented columnar to granular microstructures, developing Ag clusters at the surface for higher annealing temperatures. In some cases, the films presented dendrite-type fractal geometry, which led to an almost flat broadband optical response. The Au/TiO2 system revealed denser microstructures, with Au nanoparticles dispersed in the matrix, whose size increased with annealing temperature. This microstructure led to the appearance of LSPR bands, although some Au segregation to the surface hindered this effect for higher concentrations. The structural results of the Ag–Au/TiO2 system suggested the formation of bimetallic Ag–Au nanoparticles, which presence was supported by the appearance of a single narrow LSPR band. In addition, the Raman spectra of Rhodamine-6G demonstrated the viability of these systems for SERS applications, with some indication that the Ag/TiO2 system might be preferential, contrasting to the notorious behaviour of the bimetallic system in terms of LSPR response.

Effect of Morphological Differences on the Cold Formability of an Isothermally Heat-Treated Advanced High-Strength Steel

Steel sheets of Fe-0.2C-2Mn-0.2Si-0.03Ti-0.003B (m%) for the automotive industry were isothermally heat-treated, comprising austenitizing and subsequent isothermal annealing at temperatures between 300°C and 500°C. As a consequence, microstructures ranging from granular bainite over lower bainite to auto-tempered and untempered martensite were obtained. In tensile, hole expansion and bending tests, the performances in different forming conditions were compared and the changes of microstructure and texture were studied by complementary electron backscatter diffraction (EBSD) analyses. Samples with granular bainitic microstructures exhibited high total elongations but lower hole expansion ratios; in subsequent EBSD and texture analyses, evidence for inhomogeneous deformation was found. In contrast, the lath-like bainitic/martensitic microstructure showed higher strength and lower elongation to fracture. This results in a reduced bendability, but also in a high tolerance against damage induced by the shearing of edges, and, thus, allows homogeneous deformation to higher strains in the hole expansion test.

Nanoscale thermal transport aspects of heat-assisted magnetic recording devices and materials

Abstract