Stacked Voids Research Articles

Material extrusion additive manufacturing of WC-9Co cemented carbide

Additive manufacturing (AM) cemented carbide is primarily challenged by critical issues of difficult-to-eliminate pores, cracks, deleterious phases, and non-uniform microstructure, which result in poor mechanical properties. This study utilizes material extrusion (MEX) AM technology to prepare a WC-9Co cemented carbide. The results show that green body printing and solvent debinding process significantly affect its stacked voids, interlayer bonding defects, and debound cracks. These green body defects are eliminated through liquid phase flow and WC particles flow-rearrangement during sintering, resulting in Co-rich regions or Co pools, abnormal growth of WC grains, and residual pores. A green body free of visible defects is prepared using a feedstock with a powder loading of 54 vol%, a printing temperature of 150 ℃, a microfilament overlap ratio of 30 %, and a printing-layer thickness of 0.1 mm. Followed by a two-step solvent debinding and continuous thermal debinding-vacuum-pressure sintering, a nearly fully dense WC-9Co cemented carbide is fabricated, with a uniform microstructure of two phases and free of pores, cracks, and Co pools. Its Vickers hardness, transverse rupture strength, and fracture toughness reach 1525±3 HV30, 3492±45 MPa, and 20.4±0.5 MPa·m1/2, respectively, which are significantly better than those of similar reports and could be comparable to similar cemented carbides prepared by powder metallurgy. This study could provide a new technological path for developing high-performance cemented carbide complex-structured components.

Structural Analysis of Sucrose-Derived Hard Carbon and Correlation with the Electrochemical Properties for Lithium, Sodium, and Potassium Insertion

Hard carbon possesses the ability to store Li, Na, and K ions between stacked sp2 carbon layers and voids (micropores). We have explored hard carbon as a candidate for negative electrode materials ...

Study the influence of the air void topology on porous asphalt clogging

This paper investigates the effect of air void topology on the clogging properties of asphalt mixtures with porosity in the range 13.6%–31.8%. The air void properties of the asphalt mixtures were quantified by means of computed tomography of the asphalt. In addition, virtual pore networks were generated using the Intersected Stacked Air voids (ISA) method and then, 3D printed using transparent resin. The porous asphalt and the transparent resin with realistic pores were clogged by letting water and dust pass through the pores during hydraulic conductivity tests. The clogging ratio was measured by quantifying the reduction in hydraulic conductivity after each clogging cycle. The volume of pores clogged was quantified by means of thresholded images of the transparent resin with realistic pores. Furthermore, the clogging ratio of each test specimen was compared against its air void topologies, such as average pore diameter, tortuosity, Euler number, and macroporosity. It was found that the main factor influencing the clogging ratio of asphalt mixture was the air void diameter and the reason for this, is because particles of similar size to the pores have a high chance to block them. Tortuosity, Euler number, and macroporosity, do not have a significant influence on the clogging ratio of asphalt mixture. Finally, it was concluded that to build mixtures that do not clog, the size distribution of the clogging particles must be analysed prior to building the road and the maximum aggregate size and gradation carefully selected to produce pores which are bigger than the clogging particles.

Virtual porous materials to predict the air void topology and hydraulic conductivity of asphalt roads

This paper investigates the effects of air void topology on hydraulic conductivity in asphalt mixtures with porosity in the range 14%–31%. Virtual asphalt pore networks were generated using the Intersected Stacked Air voids (ISA) method, with its parameters being automatically adjusted by the means of a differential evolution optimisation algorithm, and then 3D printed using transparent resin. Permeability tests were conducted on the resin samples to understand the effects of pore topology on hydraulic conductivity. Moreover, the pore networks generated virtually were compared to real asphalt pore networks captured via X-ray Computed Tomography (CT) scans. The optimised ISA method was able to generate realistic 3D pore networks corresponding to those seen in asphalt mixtures in term of visual, topological, statistical and air void shape properties. It was found that, in the range of porous asphalt materials investigated in this research, the high dispersion in hydraulic conductivity at constant air void content is a function of the average air void diameter. Finally, the relationship between average void diameter and the maximum aggregate size and gradation in porous asphalt materials was investigated.

Visualizing, analyzing, and managing voids in the material extrusion process

A problem with the planning solutions for the additive manufacturing material extrusion process is a lack of optimization strategies to improve upon the standard raster and contour tool paths. Bead deposition tool paths can cause unwanted voids, which in turn creates a set of potential failure points within the finished product. This paper aims to identify, minimize, and manage void regions during the tool path generation. The goal is to minimize voids in each layer and to prevent stacked void regions, i.e., avoid creating an internal chimney. Material extrusion processes, with a wide selection of nozzle sizes (0.4 to 21 mm), are considered suitable candidates for this solution. The mathematical model is established based on the component geometry and the available build options for a given machine-material configuration. A C++ program has been developed to select a set of standard (available) tool path parameters to determine the optimal output process variables (bead width, raster angle, and the overlap percentage). Case studies are presented to show the merits of this approach.

Void Profile from Planck Lensing Potential Map

Abstract We use the lensing potential map from Planck CMB lensing reconstruction analysis and the “Public Cosmic Void Catalog” to measure the stacked void lensing potential. We have made an attempt to fit the HSW void profile parameters from the stacked lensing potential. In this profile, four parameters are needed to describe the shape of voids with different characteristic radii R V . However, we have found that after reducing the background noise by subtracting the average background, there is a residue lensing power left in the data. The inclusion of the environment shifting parameter, , is necessary to get a better fit to the data with the residue lensing power. We divide the voids into two redshift bins: cmass1 ( ) and cmass2 ( ). Our best-fit parameters are , , , , for the cmass1 sample with 123 voids and , , , , for the cmass2 sample with 393 voids at 68% C.L. The addition of the environment shifting parameter is consistent with the conjecture that the Sloan Digital Sky Survey voids reside in an underdense region.

Cosmic Voids in the SDSS DR12 BOSS Galaxy Sample: the Alcock–Paczyński test

Abstract We apply the Alcock–Paczyński (AP) test to stacked voids identified using the final data release (DR12) of the Baryon Oscillation Spectroscopic Survey (BOSS). We also use 1000 mock galaxy catalogs that match the geometry, density, and clustering properties of the BOSS sample in order to characterize the statistical uncertainties of our measurements and take into account systematic errors such as redshift space distortions. For both BOSS data and mock catalogs, we use the ZOBOV algorithm to identify voids, we stack together all voids with effective radii of in the redshift range of 0.43–0.7, and we accurately measure the shape of the stacked voids. Our tests with the mock catalogs show that we measure the stacked void ellipticity with a statistical precision of 2.6%. The stacked voids in redshift space are slightly squashed along the line of sight, consistent with previous studies. We repeat this measurement of stacked void shape in the BOSS data, assuming several values of within the flat model, and we compare this to the mock catalogs in redshift space to perform the AP test. We obtain a constraint of at the 68% confidence level from the AP test. We discuss the sources of statistical and systematic noise that affect the constraining power of this method. In particular, we find that the measured ellipticity of stacked voids changes more weakly with cosmology than the standard AP prediction, leading to significantly weaker constraints. We discuss how constraints will improve in future surveys with larger volumes and densities.

Real-space density profile reconstruction of stacked voids

We present a non-parametric, model-independent method to reconstruct the spherical density profiles of void stacks in real space, without redshift-space distortions. Our method uses the expected spherical symmetry of stacked voids to build the shape of the spherical density profile of cosmic voids in real space without any assumption about the cosmological model. We test the reconstruction algorithm with both a toy model and a full dark matter simulation. We present the result for the simulation: the reconstruction of the spherical density profile for a simulated stacked void in real space. We also present a first application of the algorithm to reconstruct real cosmic void stacks density profiles in real space from the Sloan Digital Sky Survey (Sutter et al. 2012b). We discuss capabilities of the algorithm and possible future improvements. Reconstructed density profiles from real voids open the way to the study of the spherically averaged dynamical structure of voids.

A measurement of the Alcock–Paczyński effect using cosmic voids in the SDSS

We perform an Alcock-Paczynski test using stacked cosmic voids identified in the SDSS Data Release 7 main sample and Data Release 10 LOWZ and CMASS samples. We find ~1,500 voids out to redshift $0.6$ using a heavily modified and extended version of the watershed algorithm ZOBOV, which we call VIDE (Void IDentification and Examination). To assess the impact of peculiar velocities we use the mock void catalogs presented in Sutter et al. (2013). We find a constant uniform flattening of 14% along the line of sight when peculiar velocities are included. This flattening appears universal for all void sizes at all redshifts and for all tracer densities. We also use these mocks to identify an optimal stacking strategy. After correcting for systematic effects we find that our Alcock-Paczynski measurement leads to a preference of our best-fit value of $\Omega_{\rm M}\sim 0.15$ over $\Omega_{\rm M} = 1.0$ by a likelihood ratio of 10. Likewise, we find a factor of $4.5$ preference of the likelihood ratio for a $\Lambda$CDM $\Omega_{\rm M} = 0.3$ model and a null measurement. Taken together, we find substantial evidence for the Alcock-Paczynski signal in our sample of cosmic voids. Our assessment using realistic mocks suggests that measurements with future SDSS releases and other surveys will provide tighter cosmological parameter constraints. The void-finding algorithm and catalogs used in this work will be made publicly available at http://www.cosmicvoids.net.

A FIRST APPLICATION OF THE ALCOCK-PACZYNSKI TEST TO STACKED COSMIC VOIDS

We report on the first application of the Alcock-Paczynski test to stacked voids in spectroscopic galaxy redshift surveys.We use voids from the Sutter et al. (2012) void catalog, which was derived from the Sloan Digital Sky Survey Data Release 7 main sample and luminous red galaxy catalogs. The construction of that void catalog removes potential shape measurement bias by using a modified version of the ZOBOV algorithm and by removing voids near survey boundaries and masks. We apply the shape-fitting procedure presented in Lavaux & Wandelt (2012) to ten void stacks out to redshift z=0.36. Combining these measurements, we determine the mean cosmologically induced "stretch" of voids in three redshift bins, with 1-sigma errors of 5-15%. The mean stretch is consistent with unity, providing no indication of a distortion induced by peculiar velocities. While the statistical errors are too large to detect the Alcock-Paczynski effect over our limited redshift range, this proof-of-concept analysis defines procedures that can be applied to larger spectroscopic galaxy surveys at higher redshifts to constrain dark energy using the expected statistical isotropy of structures that are minimally affected by uncertainties in galaxy velocity bias.

PRECISION COSMOGRAPHY WITH STACKED VOIDS

We present a purely geometrical method for probing the expansion history of the Universe from the observation of the shape of stacked voids in spectroscopic redshift surveys. Our method is an Alcock-Paczynski (AP) test based on the average sphericity of voids posited on the local isotropy of the Universe. It works by comparing the temporal extent of cosmic voids along the line of sight with their angular, spatial extent. We describe the algorithm that we use to detect and stack voids in redshift shells on the light cone and test it on mock light cones produced from N-body simulations. We establish a robust statistical model for estimating the average stretching of voids in redshift space and quantify the contamination by peculiar velocities. Finally, assuming that the void statistics that we derive from N-body simulations is preserved when considering galaxy surveys, we assess the capability of this approach to constrain dark energy parameters. We report this assessment in terms of the figure of merit (FoM) of the dark energy task force and in particular of the proposed EUCLID mission which is particularly suited for this technique since it is a spectroscopic survey. The FoM due to stacked voids from the EUCLID wide survey may double that of all other dark energy probes derived from EUCLID data alone (combined with Planck priors). In particular, voids seem to outperform Baryon Acoustic Oscillations by an order of magnitude. This result is consistent with simple estimates based on mode-counting. The AP test based on stacked voids may be a significant addition to the portfolio of major dark energy probes and its potentialities must be studied in detail.