Bimodal Mixture Research Articles

Prediction of Diameter Distributions with Multimodal Models Using LiDAR Data in Subtropical Planted Forests

Tree diameter distributions are essential for the calculation of stem volume and biomass, as well as simulation of growth and yield and to understand timber assortments. Accurate and reliable prediction of tree diameter distributions is critical for optimizing forest structure compositions, scheduling silvicultural operations and promoting sustainable management. In this study, we investigated the potential of airborne Light Detection and Ranging (LiDAR) data for predicting tree diameter distributions using a bimodal finite mixture model (FMM) and a multimodal k-nearest neighbor (KNN) model (compared to the unimodal Weibull model (UWM)) over a subtropical planted forest in southern China. To do so, we first evaluated the capability of various LiDAR predictions (i.e., the bimodality coefficient (BC) and Lorenz-based indicators) to stratify forest structural types into unimodal and multimodal stands. Once the best LiDAR prediction for the differentiation was determined, the parameters of UWM (in non-specific and species-specific models) and FMM (in structure-specific models) were estimated by LiDAR-derived metrics and the tree diameter distributions of stands were generated by the estimated LiDAR parameters. When KNN was applied for constructing diameter distributions, optimal KNN strategies, including number of neighbors k, response configurations and imputation methods (i.e., Most Similar Neighbor (MSN) and Random Forest (RF)) for different species were heuristically determined. Finally, the predictive performance of estimated LiDAR the parameters of UWM, FMM and KNN for predicting diameter distributions were assessed. The results showed that LiDAR-predicted Lorenz-based indicators performed best for differentiation. Parameters of UWM and FMM were predicted well and the species-specific models had higher accuracies than the non-specific models. Overall, RF imputation from KNN with an optimal response set (i.e., DBH) were was stable than MSN imputation when k = 5 neighbors. In addition, the inclusion of bimodal FMM for differentiated all plots generally produced a more accurate result (Mean eR = 40.85, Mean eP = 0.20) than multimodal KNN (Mean eR = 52.19, Mean eP = 0.26), whereas the UWM produced the lowest performance (Mean eR = 52.31, Mean eP = 0.26). This study demonstrated the benefits of multimodal models with LiDAR for estimating diameter distributions for supporting forest inventory and sustainable forest management in subtropical planted forests.

Mechanical Properties of Liquid Phase Sintered SiC Materials by the Addition of Unimodal and Bimodal Particles

Both the mechanical properties and probability distribution of liquid phase sintered SiC (LPS-SiC) materials were investigated, based on a detailed analysis of their microstructures. In particular, the effect of the starting sizes of SiC particles on the characterization of LPS-SiC materials was evaluated. LPS-SiC materials were fabricated with a constant applied pressure, using a complex powder of different sized SiC particles. Two types of initial SiC powders used for the fabrication of LPS-SiC materials were a unimodal SiC single powder with the average particle size of 0.3 μm, and a bimodal SiC powder mixture with average particle sizes of 0.3 μm and 30 nm, respectively. A complex powder of Al2O3 and Y2O3 particles was also utilized as an additive material for the consolidation of the LPS-SiC materials. The characterization of the LPS-SiC materials was performed using microstructural observation, three-point bending test and micro Vickers hardness test. The LPS-SiC materials containing the unimodal SiC powder exhibited a good sintered density of about 3.11 g/cm3 and a low porosity of about 5.4%. The bimodal mixture of different SiC particles did not significantly affect the sintered density of LPS-SiC materials. However, LPS-SiC materials containing the bimodal SiC powder exhibited an average flexural strength about 630 MPa higher than that of unimodal SiC powder, accompanied by the creation of fine SiC grains in the range of about 0.2~1.4 μm. The LPS-SiC materials exhibited a similar hardness level of about 2200 Hv regardless of the starting SiC powders. The bimodal SiC powder also produced a higher shape parameter and variability in hardness in the Weibull statistical analysis. Key words: silicon carbide, liquid phase sintering, initial SiC particle size, grain size, weibull distribution, mechanical property

Evaluation of bulk compression using a discrete element procedure calibrated with data from triaxial compression experiments on single particles

Confined compression of bimodal mixtures of nearly monodisperse spherical cellulose acetate (CA) particles (diameters 1.5 and 2.0 mm) was studied numerically with the discrete element method (DEM) and experimentally using a materials tester equipped with suitable tablet tooling. An extended truncated sphere contact model was used in the simulations, enabling them to be carried out to high relative densities (approaching and sometimes exceeding unity). In order to calibrate this model, the contact pressure development was extracted from prior experimental investigations on single 2.0-mm large CA particles. Results from the simulations were evaluated with the Kawakita and Heckel compression equations and compared to the corresponding data obtained from bulk compression experiments. Generally, a high degree of similarity between experiments and simulations was observed, showing the usefulness of combining confined single particle compression experiments with a suitable numerical model when predicting the performance of powder compression to high relative densities.

Investigating shock processes in bimodal powder compaction through modelling and experiment at the mesoscale

Impact-driven compaction is a proposed mechanism for the lithification of primordial bimodal granular mixtures from which many meteorites derive. We present a numerical-experimental mesoscale study that investigates the fundamental processes in shock compaction of this heterogeneous matter, using analog materials. Experiments were performed at the European Synchrotron Radiation Facility generating real-time, in-situ, X-ray radiographs of the shock’s passage in representative granular systems. Mesoscale simulations were performed using a shock physics code and set-ups that were geometrically identical to the experiments. We considered two scenarios: pure matrix, and matrix with a single chondrule. Good agreement was found between experiments and models in terms of shock position and post-shock compaction in the pure powder setup. When considering a single grain embedded in matrix we observed a spatial porosity anisotropy in its vicinity; the compaction was greater in the region immediately shockward of the grain, and less in its lee. We introduced the porosity vector, C, which points in the direction of lowest compaction across a chondrule. This direction-dependent observation may present a new way to decode the magnitude, and direction, of a single shock wave experienced by a meteorite in the past

The hiding-exposure effect revisited: A method to calculate the mobility of bimodal sediment mixtures

Predicting seabed mobility is hampered by the limited accuracy of sediment transport models when the bed is composed of mixed sediments. The hiding-exposure (HE) effect modifies the threshold of motion of individual grain classes in sediment mixtures and its strength is dependent on the grain size distribution. However, an appropriate method of predicting this effect for bimodal sediment mixtures remains to be developed. The prototypical example of a bimodal mixture is that consisting of a well-sorted sand and gravel for the fine and coarse fractions respectively. Through a comprehensive series of laboratory experiments, the HE effect has been quantified for a full range of sand-gravel mixtures from pure sand to pure gravel, the choice of which has been underpinned by an integrated study of offshore geophysical and sedimentological data found in coastal and shelf seas. In the sand–gravel mixtures used in the present study the critical shear stress needed to mobilise the sand and gravel fractions increased by up to 75% and decreased by up to 64%, respectively, compared to that needed to mobilise well-sorted sediment of similar size. The HE effect was found to be dependent on the percentage of gravel (coarse mode) present in the bimodal mixture, whereby the effect for the mixture is the weighted sum of the HE effect for the fine and coarse modes.

Thermal stability and microstructure evolution of ultra-fine grained Al-Mg alloy

.The Al-Mg alloys are widely used in the automotive and marine industry due to their excellent properties such as corrosion resistance, good weldability and intermediate mechanical properties along with their low price. Magnesium has a high solubility in the aluminium ~17.35 wt. %. Its addition increases the work-hardening ability, thus an essential work hardening can be expected in those alloys. Refining the grain size of the aluminium-magnesium alloys to the ultra-fine (<1 μm) and even nanometric scale using severe plastic deformation processes become the most effective strategy to increase their mechanical strength, whereas the post plastic deformation treatments are strategies to increase ductility. Experiments were conducted to evaluate the mechanical behaviour of an AlMg5 alloy after ECAP processing and subjected to the post-ECAP annealing treatment. The initial grain size was ~270 μm and was reduced to ~20 μm by ECAP with post-ECAP annealing at 350 °C for 30 min. The annealing treatment led to some sort of bimodal grain mixture with the larger grains embedded in the original UFG structure that provide an excellent combination of strength and ductility.

Burning Rates of Nanoaluminum–Water Solid Propellants at Various Pressures

The burning rates of solid propellants comprising different nanoaluminum powders and distilled water were examined at pressurized conditions up to . Several aluminum powders were tested using a strand burner technique: uncoated nanopowder (ALEX), stearic-acid coated nanopowder (L-ALEX), Viton-coated nanopowder (V-ALEX) with two different particle sizes, and bimodal mixture of V-ALEX and micrometer-size powders. Propellants containing ALEX, L-ALEX, and bimodal compositions followed the power law relation throughout the pressure range, with pressure exponents of , 0.5, and 0.28, respectively. Propellants containing V-ALEX powder demonstrated two different regimes: at pressures lower than 1.8 MPa, the burning rates were constant with respect to pressure; whereas for higher pressures, the burning rates followed the power law relation with and for powder diameters of 60 and 100 nm, respectively. The combustion efficiency, defined as the fraction of aluminum that oxidized during combustion, showed high values (90–95%) for all tested compositions.

Metal injection moulding of a 17-4 PH stainless steel: a comparative study of mechanical properties

While the metal injection moulding (MIM) of the precipitation-hardenable 17-4 PH stainless steel has found wide-ranging applications in a number of industries and popularity as a workhorse alloy, data on the as-sintered properties ‘that can be expected’ is limited and scattered in the literature. A quantitative comparison of the mechanical properties of as-sintered 17-4 PH materials prepared via MIM is currently not available in the literature. -5 μm, -15 μ m, and −45 μ m starting 17-4 PH stainless steel powder materials and their bimodal mixtures were used to formulate with feedstocks investigated in this work. This paper compares the as-sintered mechanical properties resulting from the measured tensile tests against those properties reported in the literature, those specified in the ASTM and MPIF standard minimum specification for MIM materials, those properties specified in the material suppliers’ datasheets and the mechanical properties of 17-4 PH wrought material – for completeness. While the as-sintered properties of the CSIR 17-4 PH material are generally lower than expected, the comparison shows that the measured properties are within the industry standard specifications. Findings from this study are a testament that a range of mechanical properties is feasible from the 17-4 PH alloy.

Building area extraction from the high spatial resolution remote sensing imagery

An approach to building area extraction from high-resolution remote sensing imagery is proposed based on the local gradient orientation density function (LGODF) and the bimodal density function (BDF). Firstly, the LGODF is calculated by moving the window of 35 × 35 based on the gradient magnitude and the gradient direction of each pixel in the image. Then, the BDF is obtained by multiplying the movable bimodal Gaussian mixture function and LGODF in each window. Finally, peaks with difference of 90° are searched for in the BDF, and the central point of the corresponding windows are determined as building pixels. For the validity of the proposed method, seven representative sub-images from PLEIADES images covering Shenzhen China are selected. Experimental results reveal that the precision achieve 94.08% and the recall up to 96.70%.

Investigation of LBM-processed bimodal powder mixtures of the nickel base alloy HX and WC–Co

Laser beam melting (LBM) is an additive manufacturing technology (AM), which enables the production of individual, complex metal parts. The development of AM-specific materials is a necessary step to exploit the full technological potential and should be a useful contribution for the ongoing process of implementing, establishing, and realizing AM processes in industrial fields of application. Theoretically, the LBM process offers the opportunity to process any weldable metal powder feedstock. Since LBM parameters affect the solidification conditions directly, this process is predestinated for future material developments. In the present paper, first research results to process the conventional LBM powder (Hastelloy X; HX) that was blended with a fine WC–Co powder are presented. Due to its high temperature and corrosion resistance, HX is widely used in industrial AM applications. WC–Co was added to improve the wear and mechanical properties. In this study, the blended, bimodal powder feedstock was analyzed, and the influence of the key process parameters on the porosity, morphology, and grain structure was investigated. The aim was to obtain the first basic information concerning the behavior of such powder blends during LBM processing and the resulting material properties. In summary, processing powder blends is possible. Since an increased porosity was determined for a high WC–Co content, the process parameters need to be further improved. In addition, it was determined that the energy input during the powder melting process directly affects the distribution of WC–Co particles within the HX matrix. High-energy inputs lead to a degradation of the WC particles. Low-energy inputs foster a consistent embedding of WC–Co particles within the HX matrix.

Erosion Behavior of Sand-silt Mixtures: the Role of Silt Content

ABSTRACT Yao, P.; Hu, Z.; Su, M.; Chen, Y.P., and Ou S.Y., 2018. Erosion behavior of sand-silt mixtures: the role of silt content. In: Shim, J.S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1171–1175. Coconut Creek (Florida), ISSN 0749-0208. It is well known that critical clay content is key factor controlling overall cohesion of the mixtures and increasing erosion threshold. Besides cohesive clay fraction, existing studies show that non-cohesive silt can enhance bed stability of a sandy bed as well. Many studies were conducted with an artificial bimodal mixture with distinct peaks of sand and silt fraction to understand the role of silt particles. However, it is unclear to what extent a bimodal mixture represents natural sand-silt mixtures. In this study, we conducted series of annular flume experiments on sediment samples collected from typical silt dominated tidal flat...

Compressibility and tablet forming ability of bimodal granule mixtures: Experiments and DEM simulations

Compressibility and tablet forming ability (compactibility) of bimodal mixtures of differently sized granules formed from microcrystalline cellulose were studied experimentally and numerically with the discrete element method (DEM). Compression data was analysed using the Kawakita equation. A multi-body contact law that accounts for contact dependence resulting from plastic incompressibility/geometric hardening was used in the DEM simulations. The experimental Kawakita a and 1/b parameters both depended non-monotonically on composition (weight fraction of large particles). For the a parameter, this dependence was explained by variations in the porosity of the initial granule beds; for the 1/b parameter, other factors were found to be of importance as well. The numerical results generally compared favourably with the experiments, demonstrating the usefulness of the DEM at high relative densities, provided that a suitable multi-particle contact model is used. For all mixtures, the tensile strength of the formed tablets increased with increasing applied pressure. The tensile strength generally decreased with increasing fraction of large particle, and this decrease was more rapid for large differences in particle size. A possible interpretation of these findings was proposed, in terms of differences in lateral support of small particles in the vicinity of large particles.

Corrosion Behavior of WC-Co Coatings with VC Additions Applied by Thermal Sprayed HVOF

The addition of nanostructured VC particles in coatings WC-Co improves the mechanical properties of cermet processed by the thermal spray technique HVOF. These coatings are used to protect components from wear, high temperature and corrosion. In the other hand these coatings are an alternative to replace chrome plating in aerospace industry. The objective was study the corrosion resistance of bimodal coatings WC-Co-VC applied by thermal spray HVOF with different concentration of VC on AISI 304. Bimodal mixtures were obtained from commercial micro and nanostructured powders. Then it was deposited on AISI 304 using a HVOF hybrid gun and robotized arm. The electrochemical characterization was performed by polarization resistance and potentiodynamic polarization curves. It was determined that the anodic polarization curves the coatings present a tendency to passivation; however, they were more pronounced in sodium hydroxide. Addition of nanostructured VC particles to the cermet WC-12Co didn’t affect its corrosion resistance.

Experimental observations on sorting patterns of heterogeneous sediment mixtures in low constrained flows

Field and laboratory investigations indicate that gravel bed rivers with bimodal grain size distribution and low lateral confinement, such as in the case of braided or multi-channel rivers, can present simultaneously active channel variations, both in the planimetric and altimetric directions, together with planimetric and vertical sorting. Such aspects were reproduced in new flume experiments considering three flow confinements with characteristic aspect ratios from about 80 to 5. Three long runs of about 60 hours were carried out under constant feeding rate conditions, with a bimodal mixture of natural sediments, a fixed flume slope of 3.18 %, and width imposed by lateral walls from 0.50 m to 0.12 m. We present here the results obtained with the first run, with a width of 0.5 m. We observed fluctuations of bed surface granulometric composition, bed slope, and outlet solid discharge; each of which with its own periodicity. In particular, the rearrangement of the bed surface texture was rapid. Cyclic bed states were observed: a stage of coarsening and aggradation corresponding to low values of outlet solid discharge; a stage of fining and degradation concomitant to high values of transport rate. The other two on-going runs (narrower configurations) aim at investigate the effect of lateral confinement on the morphodynamics of the system.

Interface engineering in ferromagnetic high-thermal conductivity iron-diamond/metal composites for electric conversion applications

The objective of this work is to investigate whether any combination of metal and magnetic particles may fit the specifications of electric conversion applications, which require, among other properties, sufficiently high magnetic permeability and thermal conductivity and a low (adjustable) thermal expansion coefficient. After having explored a wide variety of combinations, guided by both chemical and physical considerations, it was decided to investigate composites fabricated by gas pressure infiltration of Ag or Ag3wt%Si alloys into compacts of bimodal mixtures of diamond (high thermal conductivity) and iron particles (high magnetic permeability). Three average particle sizes of each component were used to fabricate the composites, namely, diamond particles of 230, 285 and 295 μm and iron particles of 30, 42 and 398 μm. In addition the volume fraction varied in the ranges 0.1–0.59 (diamond) and 0.12–0.43 (iron). In order to avoid alloying with the infiltrating metal and iron-diamond reaction, iron particles were coated with amorphous carbon. The results indicate that only composites containing a volume fraction of carbon-coated iron particles higher than 0.4 showed properties (a thermal conductivity higher than 200 W/mK and a relative magnetic permeability above 0.3) within the range valid for electric conversion applications. Composites containing non-coated iron particles reached in almost all cases very low values of both properties.

Preparation of Porous Stainless Steel Hollow-Fibers through Multi-Modal Particle Size Sintering towards Pore Engineering.

The sintering of metal powders is an efficient and versatile technique to fabricate porous metal elements such as filters, diffusers, and membranes. Neck formation between particles is, however, critical to tune the porosity and optimize mass transfer in order to minimize the densification process. In this work, macro-porous stainless steel (SS) hollow-fibers (HFs) were fabricated by the extrusion and sintering of a dope comprised, for the first time, of a bimodal mixture of SS powders. The SS particles of different sizes and shapes were mixed to increase the neck formation between the particles and control the densification process of the structure during sintering. The sintered HFs from particles of two different sizes were shown to be more mechanically stable at lower sintering temperature due to the increased neck area of the small particles sintered to the large ones. In addition, the sintered HFs made from particles of 10 and 44 μm showed a smaller average pore size (<1 μm) as compared to the micron-size pores of sintered HFs made from particles of 10 μm only and those of 10 and 20 μm. The novel HFs could be used in a range of applications, from filtration modules to electrochemical membrane reactors.

Evidence for an impact‐induced magnetic fabric in Allende, and exogenous alternatives to the core dynamo theory for Allende magnetization

AbstractWe conducted a paleomagnetic study of the matrix of Allende CV3 chondritic meteorite, isolating the matrix's primary remanent magnetization, measuring its magnetic fabric and estimating the ancient magnetic field intensity. A strong planar magnetic fabric was identified; the remanent magnetization of the matrix was aligned within this plane, suggesting a mechanism relating the magnetic fabric and remanence. The intensity of the matrix's remanent magnetization was found to be consistent and low (~6 μT). The primary magnetic mineral was found to be pyrrhotite. Given the thermal history of Allende, we conclude that the remanent magnetization was formed during or after an impact event. Recent mesoscale impact modeling, where chondrules and matrix are resolved, has shown that low‐velocity collisions can generate significant matrix temperatures, as pore‐space compaction attenuates shock energy and dramatically increases the amount of heating. Nonporous chondrules are unaffected, and act as heat‐sinks, so matrix temperature excursions are brief. We extend this work to model Allende, and show that a 1 km/s planar impact generates bulk porosity, matrix porosity, and fabric in our target that match the observed values. Bimodal mixtures of a highly porous matrix and nominally zero‐porosity chondrules make chondrites uniquely capable of recording transient or unstable fields. Targets that have uniform porosity, e.g., terrestrial impact craters, will not record transient or unstable fields. Rather than a core dynamo, it is therefore possible that the origin of the magnetic field in Allende was the impact itself, or a nebula field recorded during transient impact heating.

Effect of Particle Size Distribution on Powder Packing and Sintering in Binder Jetting Additive Manufacturing of Metals

The binder jetting additive manufacturing (AM) process provides an economical and scalable means of fabricating complex parts from a wide variety of materials. While it is often used to fabricate metal parts, it is typically challenging to fabricate full density parts without large degree of sintering shrinkage. This can be attributed to the inherently low green density and the constraint on powder particle size imposed by challenges in recoating fine powders. To address this issue, the authors explored the use of bimodal powder mixtures in the context of binder jetting of copper. A variety of bimodal powder mixtures of various particle diameters and mixing ratios were printed and sintered to study the impact of bimodal mixtures on the parts' density and shrinkage. It was discovered that, compared to parts printed with monosized fine powders, the use of bimodal powder mixtures improves the powder's packing density (8.2%) and flowability (10.5%), and increases the sintered density (4.0%) while also reducing the sintering shrinkage (6.4%).

Porosity of bimodal sediment mixture with particle filling

The porosity of a bimodal sediment mixture is affected by the filling of fine particles in the voids of coarse particles when the particle size range is wide. The classical ideal packing models tend to overestimate the filling, and, thus, underestimate the porosity of the mixture. In this study, an existing random filling model is improved by considering a three-dimensional packing configuration, and a new model is developed by considering how many fine particles are required and how many are available to cover the surface of coarse particles in the sediment mixture. The developed models are validated using measured data and compared with existing models. The new model can reproduce well the variation trend of the mixture porosity as the fraction of fine particles varies.

Analysis of the Influence of Starting Materials and Processing Conditions on the Properties of W/Cu Alloys.

In this work, a study of the influence of the starting materials and the processing time used to develop W/Cu alloys is carried out. Regarding powder metallurgy as a promising fabrication route, the difficulties in producing W/Cu alloys motivated us to investigate the influential factors on the final properties of the most industrially demanding alloys: 85-W/15-Cu, 80-W/20-Cu, and 75-W/25-Cu alloys. Two different tungsten powders with large variation among their particle size—fine (Wf) and coarse (Wc) powders—were used for the preparation of W/Cu alloys. Three weight ratios of fine and coarse (Wf:Wc) tungsten particles were analyzed. These powders were labelled as “tungsten bimodal powders”. The powder blends were consolidated by rapid sinter pressing (RSP) at 900 °C and 150 MPa, and were thus sintered and compacted simultaneously. The elemental powders and W/Cu alloys were studied by optical microscopy (OM) and scanning electron microscopy (SEM). Thermal conductivity, hardness, and densification were measured. Results showed that the synthesis of W/Cu using bimodal tungsten powders significantly affects the final alloy properties. The higher the tungsten content, the more noticeable the effect of the bimodal powder. The best bimodal W powder was the blend with 10 wt % of fine tungsten particles (10-Wf:90-Wc). These specimens present good values of densification and hardness, and higher values of thermal conductivity than other bimodal mixtures.