Characterization of packing structure of tape cast with non-spherical natural graphite particles

A three-dimensional microstructure of green and pressed tapes cast with graphite particles of non-spherical shape were examined quantitatively on the basis of the distribution of void sizes among the packed particles. The distributions measured over the cross-sections of the tape in three directions were expressed by the theoretical ones deduced for non-spherical particles. Particle shape was characterized by shape indices defined by Fourier analysis of particle outlines measured over the corresponding cross-sections of the green tape. The relationships were totally established between the limiting packing density, characterizing the void size distribution at the same section voidage, and the shape index of particle over the section. As a result, the normalized median void diameter as well as the limiting packing density of the pressed tape was found to increase with the particle shape index, corresponding to wider void size distribution. Therefore, based on developed correlations, the optimization of packing microstructure of the cast tape can be expected to result in high performance battery by using shape-modified graphite particles.     

Characterization of coarse particles in alumina powders by a wet sieving method

The wet-sieving method was applied to determine the content of coarse particles in commercial alumina powders by four independent research organizations participated in the Round-Robin Testing. The results showed that those powders clearly contain 700 to several thousands ppm of coarse particles with the size >25 μm, whereas they could not be detected with the conventional particle size analyzer because of their low concentration. SEM and polarized light microscope observations revealed that the coarse particles contained several types of aggregates which survived during the production processes of alumina by the Bayer process.     

Sol-gel alumina coating of wire mesh packing

Wire mesh packings have seen increasing applications to multiphase processes in recent years. Despite the high surface area, open structure and thermal and chemical resistance, wire mesh packings have a complex geometry which hinders some chemical applications, including changes of surface properties through application of a uniformly adhesive coating. In this work, the sol-gel deposition method of alumina coating ceramics was investigated for the first time on stainless steel wire mesh by using Dixon rings as example. The kinetics of deposition during the hydrolysis and polycondensation was followed for a range of initial composition of the coating such as the ratios of Al₂O₃ to water, acid content, polyethyleneimine binder content and the number of deposition cycles. Well-adhered alumina with a thickness up to 20 μm was successfully deposited. The molar ratios of acid to alumina and alumina to water of 0.25 and 0.01, respectively, 48 h of the aging time, 96 h of the mixing time and 2.1 g/L of polyethyleneimine binder formed a free of cracks coating of controlled thickness alumina on the Dixon rings.     

Drying mechanisms and stress development in aqueous alumina tape casting

Mass loss, shrinkage, Young’s modulus evolution and stress development of aqueous (alumina + latex) tape cast suspensions were observed during drying. Mass loss showed a constant drying rate period, followed by a falling rate period. Concurrently a linear shrinkage rate has been observed in the thickness direction, up to a drying point after which shrinkage abruptly stops. End of constant drying rate and end of shrinkage were not necessarily concomitant and depend on the latex/alumina ratio in the tape. A Young’s modulus value appears in the tape cast suspensions at the transition from liquid to solid like state. Then an increase of Young’s modulus is observed corresponding to latex film formation. The stresses generated by drying in the tape exhibited a first period of increase due to capillary pressure in the pores, a small relaxation immediately followed by a second stress increase due to latex film formation, and a stress plateau at the end of latex coalescence. Alumina powder granulometry and surface tension of the liquid had a preponderant influence on the first stress maximum whereas the properties of the latex and the drying conditions dominated the second stress maximum. By increasing latex proportion up to 25 wt.% on alumina basis, it was possible to make the first and the second stress rise concomitant.     

Inter-particle spacing in aqueous suspensions of nanopowders and its effects on particle packing,green body formation and fabrication of alumina

Despite the flexibility offered by a slurry-based processing of ceramics, fabrication of high-density products from nanopowders via conventional techniques is a challenge. The rheological behavior of nanopowder suspensions and the sintering processes of nanopowders have been thoroughly studied in the literature; however, the link between has commonly been overlooked. In this study, the packing behavior of alumina nanopowders and its relation to the rheological behavior of suspensions and the final sintered densities were investigated. For this reason, the availability of inter-particle spacing in aqueous nanopowder suspensions (IPS_aq) was discussed. Funk-Dinger's relation was modified by means of electric double layer and hydration layer formed around oxide powders in aqueous media. The effects of IPS_aq on the green body formation and alumina fabrication were explored in terms of densities and packing behavior. It was concluded that low viscosities or high green body densities do not necessarily lead to high-density end products. Yet, the availability of interparticle spacing in liquid media is vital to obtain efficient packing and high density in sintered products.     

Influence of particles packing in granules on the particles orientation in compacts

Particles orientation during compaction was studied in alumina granules of different packing structures and deformation properties. These granules were classified mainly in two types: loose granules prepared with flocculated slurries and dense granules prepared with dispersed slurries. Particles orientations in the granules and in the compacts were examined quantitatively with the cross-polarized light microscopy. A large difference was noted in the packing structures of granules and compacts. Orientation of particles was detected only in the surface vicinity of the dense granules. These dense granules show only a slight change in particle orientation locally and its initial structures were mostly preserved even after compaction. As a result, the green compacts containing these granules also show very low net particles orientation. In contrast, loose granules show no orientated particles. However, a major rearrangement of the particles was noted during compaction, resulting in a high net particle orientation in the compacts. The particles orientation in the green compacts affected the anisotropy in the sintering shrinkage significantly; high anisotropy was observed in compacts of high particle orientation fabricated from the loose granules.     

Aqueous tape casting of alumina using natural rubber latex binder

Eco-friendly and sustainable tape casting of alumina powder suspensions using concentrated natural rubber (poly isoprene) latex binder has been studied. The high negative zeta potential values of the aqueous alumina slurry (−53 to −72 mV) and rubber latex (−67 to −84 mV) at pH in the range of 9–11.5 enables their co-dispersion to produce tape casting slurries of solids (alumina + rubber) concentration >60 vol% with adequate flow characteristics. Drying of the slurry tape-cast on Mylar substrate is achieved within 15 min at 70 °C due to its high solid concentration. The green tapes containing 14.2 to 18.1 wt% of rubber shows tensile strength and strain at failure in the ranges of 1.85 to 1.61 and 41–254%, respectively. The flexible green tapes turn rigid by annealing at 200 °C due to the self-cross-linking of rubber chains induced by the Lewis acid sites of alumina. Thickness reduction to the extent of 20% by rolling of the green tape before annealing improves the green microstructure which results in an enhancement in sintered density from 93 to 98% of the theoretical value. However, the additional rolling and annealing steps consume extra time and energy compared to the tape casting processes using other reported binders.     

Simulation of random packing of polydisperse particles

An optimization based model that simulates random packings of polydisperse particles to mimic porous media systems is presented in this paper. The model simulates systems with properties that map back to physically measured quantities. The model is composed of three main procedures: detachment of an existing sphere from a target sphere by moving its center to a new position, movement of an existing sphere to touch the target sphere, and addition of a new sphere to touch the target sphere. Each procedure is performed to satisfy a constrained nonlinear optimization formulation. Input parameters include radius and coordination numbers density functions. A monodisperse and polydisperse packings were simulated by incorporating physically based properties of glass bead packings obtained from 3D computed tomography images. In each simulated system, the packing density obtained was very close to the packing density of the physical system. The packings were tested for isotropy, homogeneity and randomness and then compared to experimental microtomography images. Findings indicate that the packings generated were isotropic, homogenous, random, and in a good agreement with the experimental data.     

Synthesis and mechanical properties of porous alumina from anisotropic alumina particles

A porous alumina body was synthesized from anisotropic alumina particles (platelets). The uniaxial pressure in fabricating the green compact body had an influence on the relative density of the alumina body after heating. When green compacts, which had been uniaxially pressed at 1 and 3 MPa, were heated at 1400 °C for 1 h, the relative densities of the resulting alumina bodies were 25.0% and 35.5%, respectively. The compressive strength of compacts that were uniaxially pressed at 1 and 3 MPa were 0.8 and 4.3 MPa, respectively. In an attempt to increase the compressive strength of these porous alumina bodies, aluminum nitrate and magnesium nitrate solution treatments were performed, followed by reheating to 1400 °C for 1 h. When a 0.5 mol/l aluminum nitrate solution was used, the compressive strength of the porous alumina body uniaxially pressed at 1 MPa changed from 0.8 MPa (without solution treatment) to 1.5 MPa. Furthermore, when 0.1 mol/l magnesium nitrate solution was used, the compressive strength of the porous alumina increased to 1.7 MPa. Thus, solution treatment of the porous alumina body had a strong positive effect on its mechanical strength.     

Phase-inversion tape casting and synchrotron-radiation computed tomography analysis of porous alumina

A variant of tape casting based on the phase inversion phenomenon was adopted for fabrication of porous ceramic wafer. A slurry was prepared by dispersing alumina powder in an N-methyl-2-pyrrolidone (NMP) solution of the polymers polyethersulfone (PES) and polyvinylpyrrolidone (PVP). The slurry was cast using a doctor blade, and immersed in water to solidify the polymer solution via phase inversion. The green tape was dried and sintered at 1500 °C. The as-prepared ceramic wafer was characterized using synchrotron-radiation computed tomography (SR-CT). It was revealed that the ceramic wafer contained typical finger-like macrovoids, and the porosity resulting from these macrovoids was ~30%. The overall porosity of the wafer was 59%, as derived from the density data measured by Archimedes method in mercury. It is concluded that the phase inversion tape casting is a simple and effective method for preparation of porous ceramics.     

Electroactive alumina particles embedded in an acrylic elastomer

Composites of alumina particles embedded in a polar acrylic rubber polymer matrix are being investigated as potential electroactive polymer actuators. The measured FTIR spectra, XRD patterns, and SEM micrographs suggest that the alumina particles with an average diameter of 9.873 ± 0.034 μm having the rhombohedral form of the corundum phase are intercalated homogeneously within the acrylic rubber matrix. At an Al₂O₃ volume fraction of 0.144, the electrical conductivity increases from 10⁻⁹ to 10⁻⁸ (Ω m)⁻¹ at 500 Hz. The storage modulus without applied electrical field, G′_o, increases from 9,533 to 105,540 Pa, an order of—a single—magnitude increase, as the particle volume fraction is varied from 0 to 0.144. The increase in the matrix rigidity of the hybrid organic‐inorganic composites are because of the stress transfer from matrix to the reinforcement particles, the partial substitution of a soft matrix with a stiffer filler, and the segmental immobilization caused by the interaction between the matrix polymer chains and the filler surface. Under an applied electric field, induced dipole moments and particle‐particle interaction are generated, leading to an increase in the matrix rigidity. Under an applied electrical field, G′_{2 kV/mm} increases from 9,775 to 139,080 within the same volume fraction range, a difference of more than 30% is observed because of presence of an electrical field. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

A packing algorithm for particles of arbitrary shapes

Particle packing is a subject of both academic and industrial importance, and a number of packing algorithms have been proposed and widely used. However, most of these packing algorithms can only deal with spheres and a few regular shapes. If applied to arbitrarily shaped particles, they would have difficulties in at least one of three aspects. First, arbitrary shapes are notoriously difficult to model mathematically. Secondly, efficient algorithms for collision and overlap detection of arbitrary shapes are difficult to derive and more difficult to implement. Thirdly, the packing program would be very computationally expensive for routine and practical use. This paper describes a new, digital approach to particle packing, which can avoid many of the difficulties suffered by conventional methods. The key innovation is digitisation of both particle shapes and the packing space. Thus, a particle is now just a coherent collection of pixels or voxels, regardless of its shape, moving on a square lattice, onto which the packing space is mapped. Using the digital approach, it is easy to pack particles of any shapes and sizes into a container of any geometry, generally requiring no more than an ordinary PC. Although, as yet, the packing algorithm does not involve physical forces explicitly, it can simulate some physical phenomena such as size segregation. The ability to pack particles in their real shapes, rather than approximated as spheres, opens up many new industrial and academic opportunities, some of which are discussed. Examples of packing density predictions of particles subject to various effects, including vibration and rotation, are given.     

Thermomechanical and thermodilatometric analysis of green alumina porcelain

A modulated force thermomechanical analysis (mf-TMA) and thermodilatometric analysis (TDA) of the green ceramic mixture of kaolin (27 wt.%), Al₂O₃ (50 wt.%) and feldspar (23 wt.%) up to 1000 °C is presented. The mf-TMA reflects changes during heating the green ceramics with higher sensitivity than TDA. Discrepancies between mf-TMA and TDA revealed that the elastic behavior of the green porcelain samples is determined most importantly by processes on the crystal boundaries (escaping of the water molecules at the low temperatures up to 150 °C and solid state sintering at the temperatures above 450 °C). Processes in the crystal interiors (e.g. dehydroxylation) have a lesser function. Thermodilatometric results depend more on the processes which take place inside the crystals than on the processes on the crystal surfaces.     

Nacre-like alumina composites reinforced by zirconia particles

Nacre-like alumina is a class of bio-inspired ceramic composite manufactured by field-assisted sintering of green bodies made primarily of alumina platelets with an anisotropic microstructure. Here we investigate the addition of zirconia particles to enhance the mechanical properties of the composite. The resulting structure is a nacre-like anisotropic structure which features deflection and reinforcement during crack propagation. Monoclinic zirconia has no impact on the mechanical properties of the composite while tetragonal zirconia improves its fracture resistance properties. Both types of zirconia seem to slow down grain growth during sintering. The addition of zirconia stabilised in the tetragonal phase is thus a good option to obtain a composite with a fine microstructure and higher mechanical properties than a standard nacre-like alumina, with a flexural strength of 626 ± 39 MPa and a crack initiation toughness of 6.1 ± 0.6 MPa.m^0.5.     

Dispersion,rheology and aqueous tape casting of alumina–zirconia composites

Abstract

Alumina and zirconia were dispersed individually in aqueous media using Darvan C as the dispersant and at optimised pH condition. Based on sedimentation, rheology, yield stress, electrodeposition and zeta potential measurements, 2 wt-% of the dispersant and a pH of 10·5 were found to be the optimum condition for the codispersion of alumina and zirconia. Aqueous tape casting slurries with a solid loading of 32 wt-% were prepared under the optimised conditions of dispersion. Alumina–zirconia (50 : 50) composite tapes of 40 μm thickness and 56% green density were obtained.     

Influence of tape cast parameters on texture formation in alumina by templated grain growth

Templated grain growth (TGG) is known as one of the methods to introduce texture in ceramic materials. In order to obtain a highly textured ceramic material with TGG, it is crucial that the seeding particles are well aligned in the green product. Using the shear forces of the tape cast process makes it possible to align seeding particles with shape anisotropy. The influence of different tape cast process parameters on the formation of texture in alumina ceramics using platelet alumina seeding particles has been investigated and quantified. Experiments have been carried out in which the tape cast speed, the deairing time and the gap between doctor blade and carrier film were varied. Further the effect of the seeding particle concentration and the solids loading of the tape cast suspension are investigated. The results are presented in terms of pole figures and lotgering factors.     

Preparation of aluminum nitride green sheets by aqueous tape casting

Aluminum nitride green sheets were prepared by aqueous tape casting. The characteristics of a treated AlN were studied in aqueous ball-milling media. The oxygen content picked up with the increase of ball-milling time. It was noted that the oxygen content of AlN powder with the dispersant DP270 was lower than that of AlN powder without the dispersant DP270. The isoelectric points of the treated AlN with and without DP270 were, respectively, at pH 3.35 and pH 3.90. The dispersant DP270 not only efficiently dispersed AlN powder in water to form a stable suspension, but also formed a coat onto AlN surface to limit hydrolysis of the AlN powder. The tape casting slurry exhibited a typical shear-thinning behavior. Aqueous AlN green tape had a smooth surface and a narrow pore size distribution. Its relative density was 52.6%. No other crystalline phase was detected by XRD except for AlN and sintering aid yttria in AlN green sheet.     

Characterization and modeling of microstructural stresses in alumina

Brittle failure is often influenced by difficult to measure and variable microstructure‐scale stresses. Recent advances in photoluminescence spectroscopy (PLS), including improved confocal laser measurement and rapid spectroscopic data collection have established the potential to map stresses with microscale spatial resolution (< 2 μm). Advanced PLS was successfully used to investigate both residual and externally applied stresses in polycrystalline alumina at the microstructure scale. The measured average stresses matched those estimated from beam theory to within one standard deviation, validating the technique. Modeling the residual stresses within the microstructure produced qualitative agreement in comparison with the experimentally measured results. Microstructure scale modeling is primed to take advantage of advanced PLS to enable its refinement and validation, eventually enabling microstructure modeling to become a predictive tool for brittle materials.     

I: Dispersibility of robust alumina particles in non-aqueous solution

This article presents the preparation of well dispersed alumina slurries containing relatively large alumina particles that can withstand accelerated weather conditions. Besides using conventional dispersants such as phosphate esters and menhaden fish oil, dispersants widely used in the surface coatings industries such as Disperbyk 110 and Triton X 100 have also been employed. However, sedimentation tests, sediment density, viscosity and gloss measurements indicate the failure of the anionic dispersants as well as menhaden fish oil to disperse the alumina particles in non-aqueous medium. Instead the binder polyvinyl butyral itself acts as the best dispersant. The well dispersed alumina slurry is stabilized in the presence of a commercially available rheology modifier, Bentone SD2 for a period of 144 h at 60 °C. The rheological behavior of the aged sample was studied under various conditions. Although a rise in viscosity of the suspension was observed when the slurry was exposed to accelerated weather conditions, a lower shear thinning index and higher gloss values indicate a better dispersion state with aging.     

Random loose packing of small particles with liquid cohesion

Random packing of wet grains is numerically investigated using a multiscale discrete element method. The cohesion between wet grains is evaluated by the Weber number, with which the macroscopic and microscopic properties of wet packing are compared with those of dry systems. The effect of capillary force on wet packing partially resembles that of van der Waals force on the dry adhesive packing, because both packing fraction (ϕ) and coordination number (Z) of wet packing decrease with increasing surface tension, following a Boltzmann-like exponential decay that leads to a metastable state with ϕ ≈ 0.37 and Z ≈ 3.8. The decay processes of wet grains, however, are much faster than that of dry ones due to the additional liquid-phase viscous dissipation. Moreover, in the cases of strong cohesion, the relationship between Z and ϕ can be well interpreted by the Edwards' ensemble theory, while those with weak cohesion follow an exponential formula instead. © 2018 American Institute of Chemical Engineers AIChE J, 65: 500–511, 2019