Influence of Green Formulation and Pyrolyzable Particulates on the Porous Microstructure and Sintering Characteristics of Tape Cast Ceramics

Porous morphology and total porosity produced in sintered ceramic tapes was controlled by the amount and distribution of pyrolyzable graphite particles added to a colloidal suspension during a tape casting operation. A conceptual model of the green tape microstructure was used to explain the influence of graphite and tape formulation on sintering characteristics. The creation of a connected, open porous network in the sintered body was the result of graphite particle percolation within the green body. Additional voidage introduced by particle bridging was the source of excess porosity and also resulted in a bimodal pore size distribution. Sintering shrinkage was determined by the ceramic packing density, which was primarily determined by the tape formulation.     

Numerical investigation of particle shape and particle friction on limiting bulk friction in direct shear tests and comparison with experiments

In this study, the discrete element method (DEM) was used to investigate the influence of particle shape and interparticle friction on the bulk friction in a Jenike direct shear test. Spherical particle and non-spherical particles using two overlapping sphere giving particle aspect ratio of up to 2 and a full range of interparticle contact friction coefficient were studied numerically. These were compared with physical Jenike shear tests conducted on single glass beads and paired glass beads. To separate the influence of sample packing density from interparticle contact friction on the bulk shearing response, the same initial packing was used for each particle shape in the simulations. The interplay between contact friction and particle interlocking arising from geometric interaction between particles to produce the bulk granular friction in a direct shear test is explored and several key observations are reported. The results also show that particle interlocking has a greater effect than packing density on the bulk friction and for each particle shape; DEM can produce a good quantitative match of the limiting bulk friction as long as similar initial packing density is achieved.     

Effect of Solids Loading on Slip-Casting Performance of Silicon Carbide Slurries

The influence of solids loading and particle shape on the green microstructure of slip-cast bodies was investigated. Three commercial silicon carbide (SiC) powders (two coarse varieties with the same particle-size distribution (PSD) but different particle shapes and a finer powder) were used to prepare bimodal PSDs designed to maximize the packing density. Various surface-active agents (anionic, cationic, and non-ionic) were tested. Anionic dispersants were the most effective in dispersing aqueous SiC slurries. The effectiveness of dispersants was evaluated by sedimentation tests using very dilute slurries, by rheology, and by the packing density of slip-cast bodies prepared from suspensions loaded with 62.5 wt% solids, stabilized with a fixed amount of dispersant (0.25 wt%, relative to the solids). Then, the best dispersant was selected to study the effects of dispersant and solids concentrations on the degree of packing of bimodal suspensions that contained the sharper-edged coarse particles. It could be observed that the green density was dependent on both parameters, initially showing an increase to a maximum, followed by a decreasing trend. A high value of 74.5% of the theoretical density (TD) was obtained from suspensions that contained 70 wt% solids and 0.1 wt% dispersant. The substitution of the angular coarse particles by similarly sized but more spherical particles resulted in an additional increase in green density to >76 wt% TD. The results can be interpreted in terms of freedom of particles upon deposition on the cast layer, which enables particle rearrangement, and segregation phenomena.     

Effect of pore orientation on anisotropic shrinkage in tape-cast products

During tape casting, an anisotropic shrinkage can be observed, which is attributed to particle alignment during the casting process. The understanding of the relationship between green body microstructure and shrinkage anisotropy is of great importance for further miniaturization of multilayer ceramics. In the current study, four alumina powders with different particle shape (spherical, standard, plate-like and extreme plate-like) were used to cast green tapes. The sintering shrinkage behavior and the microstructure were analyzed. In particular, the pore orientation was determined quantitatively by using a modified linear intercept method. The relationship between pore alignment and anisotropic sintering shrinkage of cast green tapes is discussed in all three spatial directions. The shrinkage anisotropy could be correlated quantitatively with the pore anisotropy. Furthermore, this correlation was verified by mathematical modeling based on elongated particles and pores.     

Photomicrographic size distribution determination of non-spherical objects

Equations have been developed for calculating the size distribution of non-spherical objects by using photomicrographs prepared of sections or projections of samples. The equations contain the shape function characteristic of the shape of the objects in the sample which is also determined from the photomicrographs. The equations have been applied to the void size distribution determination in various sandstones and packed beds of beads, as well as to the particle size distribution determination of a sample of common salt. It has been found that the new equations resulted in plausible calculated size distributions in the case of highly anisotropic objects, whereas this was not the case when the sphere model was used as a basis for the calculations. In the case of the salt particles, however, the sphere model was almost equally adequate as the shape function method, because the shape function was, in this case, very nearly spherical.     

Shape distinction of particulate materials by laser diffraction pattern analysis

A pattern of a diffraction image depends on the particle shape, while the size of the pattern depends on the sectional area of the particle. In this work, the method to extract differences from the diffraction patterns due to different shapes of non-spherical particles was studied conceptually. In this respect, a radial segment (wedge) photo-detector was assumed as a detector. Diffraction patterns and intensity patterns detected by the radial segment detector were calculated for many kinds of two-dimensional shapes, corresponding to the projections of particles, as a circle, ellipses, triangles, quadrangles, other anonymous shapes, also shapes extracted from real phytoplanktons. From these detected light intensity patterns, we extracted (or define) two indexes: “circular index” and “peak number.” It was shown that various shapes can be distinguished by means of two-dimensional mapping with these parameters. In addition, an applicability of a concentric detector was examined to estimate the particle size when the particle is non-spherical but is a single particle in the measurement. As a result, it was found that the circle equivalent diameter determined with usual scheme agreed well with the sectional area equivalent diameter of the original particle even in any cases of non-spherical samples. From these results, it was shown that the particle size and shape in wide range can be distinguished from the three-dimensional mapping with “circular index”, “peak number” and “particle size”.     

Axial dispersion coefficients in packed beds at low reynolds numbers

Peclet numbers describing axial dispersion in gas flow through packed beds of spheres were obtained using a two measurement point, pulse technique in a test section four inches inside diameter and 5.14 feet high. Three packing sizes were investigated, corresponding to tube to particle diameter ratios of 6.4, 17, and 66 In the experiments the contribution of the velocity profile to axial spreading was reduced by using thermal conductivity detectors which responded to dispersion only in the central part of the bed cross-section. In this region of a packed bed the velocity profile is relatively flat. The results point to a particle diameter effect which is more pronounced than has been previously reported. This is in accord with the diffusive mechanism of axial dispersion in a packed bed provided dispersion caused by the velocity profile does not affect the measured pulse response. In the absence of velocity profile effects, the spreading of residence times in void cells is caused primarily by the shedding of the decelerated boundary layers on the downstream side of the particles. At low velocities however, molecular diffusion predominates. Implicit in this discussion is the hypothesis that the uniformity of shape and size of packing particles has an important bearing on the manner in which the Peclet number approaches its limiting value as the gas velocity is increased.     

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.     

Tape casting of nanocrystalline ceria gadolinia powder

A ceramic ceria gadolinia solid solution membrane for solid oxide fuel cells was fabricated by tape casting using a nanopowder of 37 nm average particle size. A novel combination of solvent and dispersant was used to disperse the nanoparticles. The polymer was added in a dilute stage to guarantee a homogeneous distribution. After casting a remarkable densification of the cast tape suspension from a solid loading of 20 up to 42 vol.% was observed during drying. The green tape was sintered to >92% theoretical density and was dense towards perfusion. The resulting grain size in the sintered specimen still was <200 nm.     

Influence of coke structure on the properties of the carbon-graphite artefact

The properties of the graphite artefact are influenced by many properties of the coke filler which are nebulously referred to as ‘coke structure’ or ‘coke quality’. Unfortunately, no one quality of a coke dominates the properties of the final graphite due to the inter-property interactions which take place during processing through to graphitization. Therefore, the influence and interactions of the following coke properties on the quality of the final graphite will be discussed: macro- (or aspect ratio) and microstructure; density, pore size and shape as a function of particle size; real density; particle strength; the intrinsic particle expansion and the puffing character (or sulphur level) of the coke. In addition, some discussion concerning the effect of feedstock type and the conversion of the feedstocks to form green coke followed by processing to calcined coke will be presented.     

On the effective density of non-spherical particles as derived from combined measurements of aerodynamic and mobility equivalent size

Effective densities derived from combined mobility and aerodynamic sizing provide a valuable tool for the characterization of non-spherical particles. Different effective densities have been introduced depending on the primary measurement parameters (mass, mobility and/or aerodynamic size) and the flow regime (transition, free-molecular). Here we explore the relationship between these effective densities, their physical interpretation and their dependence on particle shape, density and various equivalent diameters. We also provide an overview over the wide range of practical implications of the effective density concept with a particular focus on the characterization of particles with irregular or even unknown shape using commercially available instruments such as DMA, SMPS, FMPS, ELPI, APS, TEOM and multi-stage impactors. Finally, we identify new perspectives for particle characterization by extending the effective density concept into the free-molecular regime and by suggesting a triple-instrument approach for on-line determination of both particle density and shape as well as the dynamic shape factor for different flow regimes.     

Radiative transfer simulation of dust-like aerosols: Uncertainties from particle shape and refractive index

The composition, particle shape, number concentration, size distribution, and spatial and temporal distributions of dust aerosols cause significant uncertainties in relevant radiative transfer simulations. The spherical particle approximation has been generally recognized to introduce errors in radiative transfer calculations involving dust aerosols. Although previous studies have attempted to quantify the effect of non-spherical particles, no consensus has been reached as to the significance of the dust aerosols non-spherical effect on flux calculations. For this study, we utilize a newly developed ultra-violet-to-far-infrared spectral database of the single-scattering properties of tri-axial ellipsoidal, mineral dust-like aerosols to study the non-spherical effect on radiative forcing. The radiance and flux differences between the spherical and ellipsoidal models are obtained for various refractive indices and particle size distributions. The errors originating from using the spherical model and the uncertainties in the refractive indices are quantified at both the top and bottom of the atmosphere. The dust non-spherical effect on the net flux and heating rate profile is obtained over the entire range of the solar spectrum. The particle shape effect is found to be related to the dust optical depth and the surface albedo and can be an important uncertainty source in radiative transfer simulation. The particle shape effect is largest over water surfaces and can cause up to a 30% difference in dust forcing at the top of the atmosphere.     

3D-customized thin-walled ceramic mouldings produced by deep drawing of green tapes

Thin-walled 3D-customized ceramic components can be used, e.g. for inert and thermal resistant housings as well as special surface structured kiln furniture. For manufacturing such components, deep drawing of ceramic green tapes has been used. The technology has been almost not applied for processing of ceramic green tapes. A new approach was developed to realize a homogenous ceramic particle packing and hence uniform green density within a deep drawn green tape. After debinding and sintering, defect-free model structures with a dense microstructure (density > 99%) were achieved. Within this study, the deep drawing of ceramic green tapes by using a new approach was investigated. The developed approach brings several beneficial properties together, e.g. a reproducible form deviation and more importantly a homogeneous particle distribution allowing homogeneous and dense structures. With this results, the transformation of non-complex and cheap ceramic green tapes to 3D-customized and near-net-shape thin-walled mouldings becomes possible.     

Shrinkage of Tape Cast Products During Binder Burnout

During sintering of tape cast products, anisotropic shrinkage occurs, which can be attributed to an anisotropic green tape structure concerning particle and pore orientation. Little is known about the shrinkage during binder burnout (BBO) and its relation to the microstructure of green tapes including the binder–plasticizer phase. Therefore, the article determines the shrinkage behavior of green tapes derived from alumina powders with different particle shape during binder burnout and prefiring in all spatial directions. The shrinkage after prefiring relative to the green and the debindered states is also discussed. The interrelation between shrinkage behavior and microstructure is investigated in dependence on different process parameters and specifically on the thermal behavior of the binder–plasticizer phase in the green tapes. It is shown that the subtraction of the BBO shrinkage from the total shrinkage results in completely different data for the sintering shrinkage anisotropy in z direction.     

Highly filled thermoplastic composites. II: Effects of particle size distribution on some properties

The effect of irregularly shaped glass particle size and size distribution on the packing density and flexural mechanical properties of highly-filled composites with a rubbery thermoplastic matrix was studied. Increasing the particle's median size and size distribution width significantly increases the packing density of the composites. Compression molding causes the glass particles to fracture at a decreasing level with an increasing distribution width. Particle median size, rather than size distribution, affects the mechanical properties; The flexural modulus and strength increase and the ultimate deflection in flexure decreases with a decreasing median size. A “glass network” is formed in the compression molded composites because of the mechanical interlocking of particles. The nature of this continuous glass phase predominates the composites mechanical behavior. The particle's size and shape determine the nature of the glass network and, thus, have a dominating effect on the mechanical properties. The latter are significantly affected by the particle's surface properties. A specific silane treatment of the glass particles acts to reduce the particle/particle friction, resulting in a higher packing density. The treatment also acts as a cohesive liquid to increase the strength of the glass network, and to increase the particle/polymer adhesion, increasing the composites' strength and ductility.     

Packing of Cylindrical Particles with a Length Distribution

The packing of cylindrical particles with log-normal and modified power-law length distributions has been experimentally studied. The results indicate that the packing density is heavily dependent on the parameters in the two distributions. However, this dependence cannot be predicted by the direct analogy to that for the packing of spherical particles. It is postulated that the packing of nonspherical particles be governed by two factors: the shape effect and the size effect, which respectively correspond to the unmixing and mixing states of a particle mixture and are quantified from the specific volumes of the two states. Analysis of the results suggests that the shape effect is dominant for the packing of cylindrical particles with a wide length distribution.     

Determination of non-spherical particle size distribution from chord length measurements. Part 1: Theoretical analysis

In this paper, extensive theoretical studies are described on two important issues in translating a chord length distribution (CLD) measured by FBRM instrument into its particle size distribution (PSD) including PSD-CLD and CLD-PSD translation models for general non-spherical particles. Analytical solutions to calculate the PSD-CLD models for spherical and ellipsoidal particles are developed. For non-spherical particles, a numerical method is given to calculate the PSD-CLD model. The iterative non-negative least squares (NNLS) method is proposed in the CLD-PSD model, because of its many advantages converting measured CLD into its PSD, such as insensitivity to measurement noise and particle shape. The effectiveness of the proposed methods is validated by extensive simulations.     

Understanding centrifugal casting in the manufacture of functionally graded materials

Centrifugal casting is a shaping technique intended for ceramic tubular structure manufacture, where the particle size and density are exploited to produce asymmetrical membranes and functionally graded materials. However, some well-established insights about particle segregation are debatable and remain unclear. For instance, small particles do not necessarily stay in the inner region, and the bigger particles do not accumulate in the outer. Herein several manufacturing parameters were studied through Taguchi’s robust design, using discrete element method-based simulations to generate the data. Alumina powder was used as model material and water as the liquid phase, to assess the green cast formation time, the cast thickness, the roundness, the changes in relative density, and particle size distribution along the cast’s cross-section. The mean particle diameter and the rotation speed are the most influential parameters for the casting time. The volume fraction of solids on the precursor slurry is decisive regarding the cast structural properties, and particle segregation is negligible, except for size differences above one order of magnitude. When a fraction of denser nickel powder was added, density segregation was also observed, but the size differences can overshadow its effect. In addition, alumina and nickel particles were cast in a lab-scale centrifuge and experimentally validated the segregation of particles. The centrifugal casting method was successfully applied for producing the Al₂O₃-Ni composites with a gradient distribution of the Ni phase.     

Thermophoretic force measurements of aggregates of micro-spheres

Direct measurements of the thermophoretic force on spherical and non-spherical particles have been made using an electrodynamic balance mounted in a vacuum chamber. The non-spherical particles consisted of aggregates of polystyrene latex spheres. By varying the system pressure the force was measured over a wide range of Knudsen numbers ranging from the near-continuum regime to the free-molecule regime. The gases were monoatomic helium and diatomic nitrogen. The data were analyzed and normalized to determine the thermophoretic shape factor that relates the force on a non-spherical particle to that on a sphere of the same volume. The shape factors and the thermophoretic forces are compared with available theories. The measured thermophoretic forces are shown to agree with theories for the Knudsen regime and the free-molecule regime, and the thermophoretic shape factors are shown to be consistent with theories based on the limiting cases of large and small Knudsen numbers.     

Influence of particle size distribution on rheology and particle packing of silica-based suspensions

The effect of particle size, particle size distribution and milling time on the rheological behaviour and particle packing of silica suspensions was investigated using slurries containing total solids loading of 46 vol.%. Three silica powders with different average particle sizes (2.2, 6.5 and 19 μm), derived from dry milling of sand, and a colloidal fumed silica powder with 0.07 μm were used. Different proportions of colloidal fumed silica powder were added to each of the coarser silica powders and the mixtures were ball-milled for different time periods. The influence of these factors and of the particle size ratio on the rheological behaviour of the suspensions and densities of green slip cast bodies was studied.The results show that the flow properties of slips are strongly influenced by the particle size distribution. The viscosity of suspensions increases with the addition of fine particles, imposing some practical limitations in terms of volume fraction of fines that can be added. On the other hand, increasing the size ratio enhanced the shear thinning character of the suspensions, while decreasing the size ratio led to an accentuation of the shear thickening behaviour. For all mixed suspensions, green densities increased with increasing milling time, due to size reduction of silica powders and a more efficient deagglomeration of fumed silica. Increasing amounts of fumed silica led to a first increase of particle packing up to a maximum, followed by a decreasing trend for further additions. Good relationships could be observed between rheological results and packing densities.