Sintering deformation caused by particle orientation in uniaxially and isostatically pressed alumina compacts

Effect of particle orientation on deformation during sintering is reported for model systems; one made with industrial grade low soda alumina, which has an elongated particle shape, and the other a special alumina with a spherical particle shape. To ensure the homogeneous packing density of particles, compacts were made by uniaxial pressing followed by cold isostatic pressing. The particle orientation was examined with a polarized light microscope and was found to be an important cause of sintering deformation. In a green body, for elongated shape of particles, the particle orientation occurred during uniaxial pressing, causing the anisotropic sintering shrinkage during sintering and thus the sintering deformation. No particle orientation nor shrinkage anisotropy was noted in the system made with the powder of spherical particle shape.     

Effect of particle shape and size on the shrinkage of stainless steel compacts

The effects of particle size and shape on the shrinkage of commercial 316 L stainless steel compacts are evaluated. Emphasis is placed on the effect of changes in surface area and the anisotropy in inter-particle contacts brought about by the irregular powders. The effects of fine particle addition on this anisotropy and surface area are presented and plained. An empirical relationship is derived which can be used to predict changes in the axial, radial and volume shrinkage when spherical particles are substituted into a commercial blend. The effect of spherical particle substitution on the shrinkage ratio is also discussed. It is shown how the combined effects of fine particle additions and substitutions of spherical particles for irregular particles can be used to produce a compact which exhibits a shrinkage ratio (radial to axial shrinkage) of unity.     

Preparation of SiO2 Glass from Model Powder Compacts: II, Sintering

The sintering behavior of powder compacts formed from spherical, nearly monosized SiO₂ particles was investigated. Highly ordered compacts sintered to high density and translucency at 1000°C. In contrast, less homogeneous samples prepared from flocculated suspensions remained highly porous after sintering under the same conditions. Densification kinetics were determined over the temperature range 900° to 1050°C for ordered compacts. The viscosity at each sintering temperature and the activation energy for viscous flow were determined using available sintering models. Sintering of ordered compacts is divided into several stages. Densification, mercury porosimetry, and electron microscopy results indicate that the first stage is dominated by the shrinkage of three-particle pore channels, whereas the second stage primarily involves the shrinkage of four-particle pore channels.     

Sintering of Glass Powders During Constant Rates of Heating

Analytical expressions for the initial sintering of glass powders were developed to describe shrinkage of powder compacts at a constant rate of heating. Data describing sintering of soda-lime glass spheres as measured by shrinkage of powder compacts at rates of heating from 0.46 to 2.91°C/min are consistent with the model analytical expressions. Compacts of glass spheres decreased in size exponentially with increasing temperature as predicted by the model.     

Isothermal Sintering of Spheroidized Cordierite-Type Glass Powders

Jagged (ball–milled) glass particles were spheroidized and pressed into compacts which were isothermally sintered in air. Both jagged- and spheroidized-particle compacts showed about the same 0.7 anisotropy of the ratio of axial to diametral shrinkage, but spheroidizing reduced the shrinkage rate. Thus, shrinkage anisotropy is not a simple particle shape effect; it may relate to differences in the axial and radial distributions of particle sizes present in these compacts.     

Effect of compaction on the sintering of borosilicate glass/alumina composites

The effect of initial compaction on the sintering of borosilicate glass matrix composites reinforced with 25 vol.% alumina (Al₂O₃) particles has been studied using powder compacts that were uniaxially pressed at 74, 200 and 370 MPa. The sintering behaviour of the samples heated in the temperature range 850–1150 °C was investigated by density measurement, axial and radial shrinkage measurement and microstructural observation. The density of the sintered composites increased continuously with temperature for compacts pressed at 74 MPa, while for compacts pressed at 200 and 370 MPa it reached the maximum value at 1050 °C and at higher temperatures it decreased slightly due to swelling. The results showed anisotropic shrinkage behaviour for all the samples, which exhibited an axial shrinkage higher than the radial shrinkage, and the anisotropic character increased with the initial compaction pressure.     

Sintering behaviour of composite materials borosilicate glass-ZrO2 fibre composite materials

The characteristics and properties of mixtures of a borosilicate glass and ZrO₂ fibres obtained from compacts isothermally sintered are presented. Three different particle sizes of glass powder and different sintering temperatures and times have been used to reveal the influence of the specific surface area and the glass matrix viscosity on sintering. The fundamental developed aspects are the sintering kinetics, the dense material characterisation and the study of the sintering mechanisms. The composite materials are suitable for sealing molten carbonate fuel cells (MCFC).     

Numerical Simulation of Anisotropic Shrinkage in a 2D Compact of Elongated Particles

Microstructural evolution during simple solid-state sintering of two-dimensional compacts of elongated particles packed in different arrangements was simulated using a kinetic, Monte Carlo model. The model used simulates curvature-driven grain growth, pore migration by surface diffusion, vacancy formation, diffusion along grain boundaries, and annihilation. Only the shape of the particles was anisotropic; all other extensive thermodynamic and kinetic properties such as surface energies and diffusivities were isotropic. We verified our model by simulating sintering in the analytically tractable cases of simple-packed and close-packed, elongated particles and comparing the shrinkage rate anisotropies with those predicted analytically. Once our model was verified, we used it to simulate sintering in a powder compact of aligned, elongated particles of arbitrary size and shape to gain an understanding of differential shrinkage. Anisotropic shrinkage occurred in all compacts with aligned, elongated particles. However, the direction of higher shrinkage was in some cases along the direction of elongation and in other cases in the perpendicular direction, depending on the details of the powder compact. In compacts of simple-packed, mono-sized, elongated particles, shrinkage was higher in the direction of elongation. In compacts of close-packed, mono-sized, elongated particles and of elongated particles with a size and shape distribution, the shrinkage was lower in the direction of elongation. The results of these simulations are analyzed, and the implication of these results is discussed.     

Diffusion Sintering: II, Initial Sintering Kinetics of Alumina

The initial sintering kinetics of alumina have been studied by measuring the isothermal shrinkage of compacts of several alumina powders in air. The shrinkage of these compacts can best be described by a grain-boundary vacancy diffusion model for the temperature range 1200° to 1600°C. The behavior of the compacts is consistent with the model after an initial shrinkage has occurred. The magnitude of this initial shrinkage is constant for identical specimens and is independent of the sintering temperature.     

Diffusion Sintering: I, Initial Stage Sintering Models and Their Application to Shrinkage of Powder Compacts

Consideration is given to several geometrical models that contribute to shrinkage. Various shapes of particles, vacancy sinks, and diffusion paths are described as they affect sintering shrinkage. These simplified models are extended to compacts of nonuniform particles so that much of the kinetics of sintering of a substance can be determined by measuring shrinkage rates of powder compacts. A nonideal compact may sinter as though it had once been an ideal compact after a specific amount of shrinkage has occurred. This shrinkage is characteristic of the particular compact and its origin and is independent of temperature.     

Three-Dimensional Model for Calculating Duration of Viscous-Flow Sintering

A method for estimating the duration of isothermal sintering of a powdered body and its final porosity and shrinkage is considered. The method, based on a three-dimensional model in the form of a cubic packing of spherical particles, uses sintering physics equations, the physicochemical properties of materials, and particle sizes. The model allows for describing not only the initial state (coalescence of particles) and the final state (compression of individual pores), but also the intermediate consolidation stage according to the general mass transfer mechanism. The sintering parameters for a sample of container glass powder are calculated using the viscous flow mechanism. The absolute discrepancy between the estimated results and experimental data is equal to 2.0% in porosity and 1.5% in shrinkage. __________ Translated from Steklo i Keramika, No. 5, pp. 19 – 24, May, 2005.     

Model for Sintering Devitrifying Glass Particles with Embedded Rigid Fibers

We extend the Clusters model to account for the presence of rigid inclusions and use it to analyze the experimental sintering kinetics of composites of 60SiO₂·24B₂O₃·16Na₂O glass particles and zirconia fibers. We followed the densification kinetics of such composites as a function of the particle size, volume fraction of fibers, fiber to pore size ratio, temperature, and time of thermal treatment. The parameters of the extended Clusters model are the glass particle size distribution and shape factor, the fiber volume fraction and radii, the glass viscosity and surface tension, the number of nucleating sites per unit surface, and the crystal growth rate in the parent glass. Hydrostatic tensions caused by the fibers were also included in the calculations. The modified Clusters model with only one adjustable parameter, which is largely dominated by viscosity but also includes particle shape, allowed us to account for the effect of surface crystallization and fiber content as inhibitors of densification and successfully describe the sintering kinetics of the studied composites.     

Behavior of Pores in the Sintering of Acicular Fe2O3 Powder

The sintering of acicular Fe₂O₃ powder was studied by isothermal dilatometry, hysteresis Hg porosimetry, and SEM of powder compacts using a resin-impregnating technique, and compared with an ordinary equiaxed powder. It is shown that the pores in the acicular powder compacts remain connected until a later stage of sintering than those in the spherical powder, and that the observed rapid initial densification, along with preferential shrinkage in the parallel-to-pressing direction, can be interpreted in terms of particle rearrangement.     

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.     

Initial Sintering of Rutile

The initial shrinkage of powder compacts of rutile was measured in air at 700° to 1130°C. The shrinkage behavior agrees well with a model based on grain-boundary diffusion. The apparent activation energy for the shrinkage rate is 76.9 ± 2.5 kcal/mole. Changes in ambient atmosphere (O₂, N₂, vacuum) had no effect on the initial sintering kinetics.     

Effects of Particle-Size Distribution in Initial-Stage Sintering

Diffusion models for the initial-stage sintering of spherical particles were developed for distributions of particle sizes. A linear array of particles with different sizes was treated quantitatively; number, rather than weight, distributions are considered directly to permit summation of particle-pair interactions. Evaluation of a single particle size which will characterize the equivalent shrinkage of the distributed array of random particles yields a size much smaller than that which would designate the division at 50 wt% finer conventionally used. Also, the effective size is smaller for the lattice-diffusion model than for the grain-boundary-diffusion-controlled model. The results show that the rates for binary mixtures are intermediate between the behaviors of the end-member sizes, in accordance with previous experimental findings. A square array of equal spheres with contacting interstitial spheres was also analyzed. Coherent shrinkage requires a retardation of shrinkage along the diagonal contacts (due to tensile loads). Procedures for evaluating the stress transfer were generated for this and similar particle packings. The calculated shrinkage rate is again intermediate between that predicted from previous models and the end-member sizes.     

Influence of erodent particle types on solid particle erosion of polyphenylene sulphide composite under low particle speed

The influence of erodent particle types on solid particle erosion of randomly oriented short glass fiber and mineral particle reinforced polyphenylene sulphide (PPS) was investigated. The solid particle erosion studies were carried out using low speed solid particle erosion test rig with 150 to 212‐μm brown fused aluminum oxide (Al₂O₃), 150 to 200‐μm silica sand and 150 to 250‐μm glass bead. Glass bead eroding particles appear spherical in shape whereas aluminum oxide and silica sand eroding particles have sharp and angular edges. The erosion tests were conducted at six different contact angles of 15, 30, 45, 60, 75, and 90°, respectively. The results showed a strong dependence of the eroding particle types on the erosive wear behavior of PPS composite. The peak erosion rate occurred at 45° contact angle for silica sand eroding particles while the peak erosion rate occurred at 30° contact angle for aluminum oxide and glass bead particles. The morphologies of eroded surfaces were characterized by the scanning electron microscopy. In case of aluminum oxide and silica sand, the erosive wear mechanism occurs firstly by the erosion of matrix, followed by the fracture of un‐supported fibers and their detachment; however, the erosive wear mechanism occurs different for glass bead particles. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Sintering of glass matrix composites with small rigid inclusions

We investigated the effect of dispersed crystalline particle volume content Φ on sintering of glass matrix composites (GMC) for low-temperature co-fired ceramics (LTCC) applications. Such composites typically consist of alumo-borosilicate glass and α-Al₂O₃ powders of similar average particle size (D₅₀ ≈ 3 μm). Sintering shrinkage was observed by dilatometry and heating microscopy and was backed up by glass viscosity measurements. Microstructure analysis revealed that α-Al₂O₃ particles do neither show significant dissolution into the liquid phase nor detectable crystallization throughout LTCC firing schedules. Therefore, in this study α-Al₂O₃ particles were treated as small rigid inclusions. It was found that Φ lowers the shrinkage rate of GMC. While the lowering is small for small Φ and at the early stage of densification it progressively increases during sintering, and final shrinkage shifts up to 170 K to higher temperatures for Φ = 0.45. The behaviour observed could be explained assuming that sintering is controlled by the effective viscosity, which progressively increases non-linearly during densification due to the gradually wetting of the surface area of corundum particles. We could demonstrate that Al₂O₃ cluster can cause residual pores and reduce the attainable shrinkage. The reduction of attainable shrinkage is found to depend on Φ³, reaching about 8% at Φ = 0.45.     

生物玻璃的烧结与析晶

利用玻璃的烧结收缩,结合差热分析和X射线衍射分析,研究了CaO-SjO_2-P_2O_5系统生物玻璃的烧结和析晶动力学。确定它的烧结机制主要是粘性流动烧结。A玻璃和Ⅰ-2玻璃的烧结活化能分别为540kJ/mol和476kJ/mol。析晶相是磷灰石和硅灰石。A玻璃和Ⅰ-2玻璃的析晶表观活化能分别约为480kJ/mol和390kJ/mol。通过分析烧结温度、Na_2O含量对烧结和析晶的影响,选择了合适的烧结条件,控制烧结和析晶程度,获得了既有足够的自身强度,又有一定的结合强度的多孔玻璃陶瓷。     

Limitations on the sintering of graded particle systems

Improvement of compact density is commonly achieved by blending coarse and fine particles, but these compacts will not densify without the presence of a significant amount of liquid phase. It was proposed that two step sintering (TSS) could be applied to sinter the fine particle matrix, potentially accommodating the presence of inclusions of large particles. This hypothesis was false. Compacts were prepared with similar green density but with different ratios of coarse, medium, and fine particles and then subjected to TSS. The results indicated that constrained sintering limits densification on both ends of the particle packing spectrum: A fine particle matrix containing large particles fails to densify because the matrix cannot shrink around the inclusion; the densification of fine particle pockets in a skeletal network composed of large particles does not allow sufficient shrinkage in the pockets of small particles.