The effect of particle size distribution on the microstructure and properties of Al2O3 ceramics formed by stereolithography

Stereolithography (SL) shows advantages for preparing alumina-based ceramics with complex structures. The effects of the particle size distribution, which strongly influence the sintering properties in ceramic SL, have not been systematically explored until now. Herein, the influence of the particle size distribution on SL-manufactured alumina ceramics was investigated, including bending strength at room temperature, post-sintering shrinkage, porosity, and microstructural morphology. Seven particle size distributions of alumina ceramics were studied (in μm/μm: 30/5, 20/3, 10/2, 5/2, 5/0.8, 3/0.5, and 2/0.3); a coarse:fine particle ratio of 6:4 was maintained. At the same sintering temperature, the degree of sintering was greater for finer particle sizes. The particle size distribution had a larger influence on flexural strength, porosity and shrinkage than sintering temperature when the particle size distribution difference reached 10-fold but was weaker for 10 μm/2 μm, 5 μm/2 μm and 5 μm/0.8 μm. The sintering shrinkage characteristics of cuboid samples with different particle sizes were studied. The use of coarse particles influenced the accuracy of small-scale samples. When the particle size was comparable to the sample width, such as 30 μm/5 μm and 5 mm, the width shrinkage was consistent with the height shrinkage. When the particle size was much smaller than the sample width, such as 2 μm/0.3 μm and 5 mm, the width shrinkage was consistent with the length shrinkage. The results of this study provide meaningful guidance for future research on applications of SL and precise control of alumina ceramics through particle gradation.     

红柱石粒度对氧化铝纤维增强红柱石基复合材料烧结性能的影响

以南非红柱石和多晶氧化铝纤维为原料,在纤维加入量(w)分别为5%、10%、15%和20%,烧成温度分别为1350℃和1500℃的条件下,研究了红柱石原料粒度为0.2～101.5μm和0.1～34.7μm时对传统无压烧结工艺制备的氧化铝纤维增强红柱石基复合材料烧结性能的影响。结果表明:随着红柱石粒度的减小,基体材料的莫来石化温度和烧结温度明显降低,但纤维团聚现象加剧;由于纤维与基体界面结合力较强,纤维的增强作用以纤维的脱粘和断裂为主;在材料烧结后,红柱石粒度的变化对其常温耐压强度影响不大。     

Evolution of the microstructure and mechanical properties of stereolithography formed alumina cores sintered in vacuum

Stereolithography (SL) was used to form alumina ceramic cores. The effect of sintering temperature on the microstructure and mechanical properties of the alumina ceramics are investigated, which were sintered in vacuum. The results indicate that, as the sintering temperature increased the particle size of alumina slightly increased, and the interlayer spacing first decreased and then increased. The open porosity of alumina ceramics significantly decreased as the sintering temperature in vacuum increased. The flexural strength and hardness increased as the sintering temperature increased. When sintered at 1150 °C, the flexural strength was found to be 33.7 MPa, the shrinkage was 2.3 %, 2.4 %, and 5.3 % in the X, Y, and Z directions, respectively, and the open porosity was 37.9 %. These results are similar to those found from sintering at 1280 °C in air.     

Fabrication of bioactive glass scaffolds by stereolithography: Influence of particle size and surfactant concentration

For decades, bioactive glass (BAG) has been utilized as a competent bone substitute owing to its intrinsic properties, such as outstanding biocompatibility and bioresorbability. Stereolithography (SL) is an additive manufacturing technology used to produce highly accurate three-dimensional BAG-based bone substitutes. However, the preparation of BAG-based SL resin is always a challenge, especially because of the inevitable sedimentation of BAG particles. In this study, BAG particles with different sizes were prepared by dry grinding (BAG_dry, greater particle size) and wet grinding (BAG_wet, finer particle size). Then, BAG_dry or BAG_wet SL resins with various amounts of surfactant were analyzed. The sedimentation rate for BAG powder increased with increasing particle size but decreased with increasing amounts of surfactant added to the resin. BAG_wet SL resins had a longer shelf life, so printing could still be finished after 14 days, whereas BAG_dry SL resins were no longer useable after 5 days. However, the BAG particle size did not affect the printing accuracy or scaffold strength. According to our results, BAG-based SL resin fabricated with BAG_wet (particle size <1.6 μm) and 5 wt% surfactant exhibited better resistance to sedimentation.     

Effects of particle size and color on photocuring performance of Si3N4 ceramic slurry by stereolithography

Due to high absorbance of UV light and low solid loading, the stereolithography-based additive manufacturing of gray-colored and dense Si₃N₄ ceramic is of significant difficulty and challenge. The effects of geometric properties of ceramic powders on the curing performance were investigated by studying the absorption difference of the Si₃N₄ ceramic particles with different colors and particle sizes and ultraviolet light. The results show that the transmission of ultraviolet light and curing performance of the darker Si₃N₄ ceramic slurry are much poor. Under the same particle size, the Si₃N₄ ceramic slurry using lighter particles presents the smaller scattering coefficient. The scattering coefficient (～202) of the gray powder with ～0.8 μm average particle size is the smallest. Under the same color, the larger the particle size, the smaller the scattering coefficient. The smallest scattering coefficient of the white powder with ～2.0 μm average particle size is ～110.     

Numerical prediction of elastic properties for alumina green parts printed by stereolithography process

Stereolithography is a process based on the photopolymerization of a UV-reactive system consisting of ceramic particles dispersion in a curable resin. A key issue of this process is the control of the rigidity of green parts, which are strongly related to UV light exposure. This work is focused on the numerical prediction of green part stiffness according to stereolithography manufacturing parameters. A first macroscopic approach, based on the modelling of ceramic suspension polymerization, makes it possible to establish a relationship between the exposure and the Young's modulus. A second microscopic approach, using a periodic homogenization technique based on the strain energy, is applied to a 2D finite element model to evaluate the effective elastic properties. Numerical results show that macroscopic model is able to provide a Young’s modulus with a good level of accuracy. The modelling results from the microscopic model demonstrate an acceptable convergence with the experimental Young’s modulus.     

Effect of particle size distribution of raw powders on pore size distribution and bending strength of Al2O3 microfiltration membrane supports

To lower the sintering temperature of Al₂O₃ microfiltration membrane support, Al₂O₃ powders with particle size distribution of tri-modal are chosen. The results show that the function of fine Al₂O₃ grains depends on their agglomeration state: if fine Al₂O₃ grains distribute discretely, the bending strength of the support increases along with a slight increase in porosity; however, the aggregated fine grains are harmful to both bending strength and pore size distribution of the support. The bridging of medium Al₂O₃ grains between coarse grains contributes to increase the bending strength, but has less effect on porosity. The addition of medium (and/or fine) Al₂O₃ powder has less effect on the pore size distribution of the support if only coarse Al₂O₃ grain forms the support's framework, which suggests a new way to prepare the support with both high bending strength and high porosity at low temperature.     

Effect of synergism of solid loading and sintering temperature on microstructural evolution and mechanical properties of 60 vol% high solid loading ceramic core obtained through stereolithography 3D printing

Solid loading has a significant effect on the curing behavior of slurry and the microstructure and properties of the ceramic core. A high-solid loading slurry can effectively improve the sintering densification of ceramic particles and improve the interlayer bonding strength and mechanical properties at both 25 °C room and higher temperatures. Herein, based on the photopolymerization theory of ceramic slurry, the solid loading was increased from 45 to 60 vol% by adjusting the composition ratio of the resin ceramic powder. Additionally, the optimal sintering temperature of the 60 vol% solid loading ceramic core was 1200 °C. The synergistic effect of the solid loading and sintering temperature controls the sintering shrinkage of the sample within 3.2%; the porosity, high temperature, and room temperature flexural strength were approximately 30%, 24 MPa, and 10 MPa, respectively. The printing preparation of high-solid loading ceramic cores can be used to guide optimizing process parameters on an industrial scale.     

Effects of fine grains and sintering additives on stereolithography additive manufactured Al2O3 ceramic

Al₂O₃ ceramics are fabricated by stereolithography based additive manufacturing in present reports. To improve the densification and performance of Al₂O₃ ceramic, the introduction of fine grains or sintering additives has been studied by traditional fabrication techniques. However, no research has focused on the effects of adding fine grains and sintering additives on the stereolithography additive manufactured Al₂O₃ ceramic. In this study, both fine grains and sintering additives were added firstly, and then the effects of fine grains and sintering additives on the relative density, microstructure, mechanical properties, and physical properties of the stereolithography additive manufactured Al₂O₃ ceramics were investigated. Finally, defect-free Al₂O₃ ceramic lattice structures with high precise and high compressive strength were manufactured.     

Effect of wet screening on particle size distribution and coal properties

Wet screening is one of the methods used to remove fine material from the coal feed to gasification. Sasol Synfuels in South Africa undertook an investigation to quantify fine coal generation in the coal supply to gasification. Coal samples were wet screened in the laboratory and results compared to the normal dry screening procedure. It was found that the fines (−0.5 mm) increased almost five times when the coal was wet screened compared to dry screening. This study was subsequently initiated by Sasol Technology R&D to establish the mechanism of fine coal generation during wet screening, as well as the effect of wet screening on particle size distribution (PSD) and chemical properties of coal. Changes in the PSD and chemical properties of coal from individual coal sources used at Sasol Synfuels were compared. Composite coal samples with a predetermined PSD of all individual coal sources used at Sasol Synfuels were screened under wet and dry conditions. The PSD was again determined after screening, as well as the mineral composition (by X-ray diffraction) of the fines. Results indicated that wet screening caused clay minerals to be removed from the coal structure leading to an increase in the fines. This removal of minerals weakened the coal structure causing further size degradation of coarser fractions.     

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.     

Enhanced mechanical properties of 3D printed alumina ceramics by using sintering aids

Stereolithography based 3D printing provides an efficient pathway to fabricate alumina ceramics, and the exploration on the mechanical properties of 3D printed alumina ceramics is crucial to the development of 3D printing ceramic technology. However, alumina ceramics are difficult to sinter due to their high melting point. In this work, alumina ceramics were prepared via stereolithography based 3D printing technology, and the improvement in the mechanical properties was investigated based on the content, the type and the particle size of sintering aids (TiO₂, CaCO₃, and MgO). The flexural strength of the sintered ceramics increased greatly (from 139.2 MPa to 216.7 MPa) with the increase in TiO₂ content (from 0.5 wt% to 1.5 wt%), while significant anisotropy in mechanical properties (216.7 MPa in X-Z plane and 121.0 MPa in X–Y plane) was observed for the ceramics with the addition of 1.5 wt TiO₂. The shrinkage and flexural strength of the ceramics decreased with the increase in CaCO₃ content due to the formation of elongated grains, which led to the formation of large-sized residual pores in the ceramics. The addition of MgO help decrease the anisotropic differences in shrinkage and flexural strength of the sintered ceramics due to the formation of regularly shaped grains. This work provides guidance on the adjustment in flexural strength, shrinkage, and anisotropic behavior of 3D printed alumina ceramics, and provides new methods for the fabrication of 3D printed alumina ceramics with superior mechanical properties.     

Role of fabrication route and sintering on wear and mechanical properties of liquid-phase-sintered alumina

Liquid-phase-sintered Al₂O₃ (LPS) fabricated by slip casting, tape casting, isopressing, uniaxial pressing, piston and auger extrusion showed substantial differences in wear due to differences in morphology as observed in image analyses of SEM micrographs. The abrasive wear was low in the case of uniaxial pressing and high in the case of tape casting in the ‘dry sand and rubber wheel’ test. The wear surface of the tape cast specimen exhibited extensive microcracking possibly due to orientation of Al₂O₃ platelet (major face) parallel to the abraded surface whereas some degree of perpendicular orientation in extruded surface resulted in lower wear loss. In wet-milling wear test, the isopressed balls of a 95–97 wt% LPS derived from reactive powder (<1 μm) showed 25% lower wear loss than that of the extruded balls of a 91–94 wt% LPS derived from coarse powder (70–100 μm). Sintering at a lower temperature with longer duration and batch milling of the composition in between 12 and 16 h resulted in low wear loss. Flexural strength also improved by longer sintering time but did not show any improvement by increasing milling time. However, the variation in flexural strength was minimized by isopressing the extruded specimen. A high indentation fracture toughness at 49.03 N test load was associated with (i) large elongated reinforcement grains in a fine-grained microstructure with overall elongated morphology and (ii) with an intergranular fracture.     

Effect of manganese addition on sintering behavior and mechanical properties of alumina toughened zirconia

The effect of MnO₂ additives on the sintering behavior and mechanical properties of alumina-toughened zirconia (ATZ, with 10 vol% alumina) composites was investigated by incorporating different amounts of MnO₂ (0, 0.5, 1.0, and 1.5 wt%) and sintering at various temperatures ranging from 1300 to 1450 °C. The addition of MnO₂ up to 1.0 wt% improved the sintered density, hardness, flexural strength, and fracture toughness of the composite. However, the addition of 1.5 wt% MnO₂ degraded the relative density, hardness, and flexural strength of the composite due to the transformation of the ZrO₂ phase from tetragonal to monoclinic and grain coarsening. Optimal results were obtained with 1.0 wt% MnO₂ and sintering at 1450 °C, which improved the mechanical properties (hardness: 13.5 GPa, flexural strength: 1.2 GPa, fracture toughness: 8.5 MPa m^1/2) and lowered the sintering temperature compared to the conventional sintering temperature of ATZ composites (1550 °C). Thus, the ATZ composite doped with MnO₂ is a promising material for structural engineering ceramics owing to its improved mechanical properties and lower sintering temperature.     

Sintering behaviour and properties of zirconia ceramics prepared by pressureless sintering

Improvements in the sintering process and powder quality can lead to wider application of zirconia in ceramics. In this study, the effects of different temperatures on the stability, relative content of the tetragonal phase, and composition of Al₂O₃–ZrO₂ ceramic powders were explored using pressureless-assisted sintering. The crystallinity of the sintered Al₂O₃–ZrO₂ samples was significantly improved. The content of the tetragonal-phase ZrO₂ in sintered ceramic powders was 52.07%, 52.46%, 56.16%, 63.99%, and 64.90%, respectively, which was significantly higher than those of the raw materials. The average particle size of the sintered samples decreased from 1.07 μm to 0.17 μm with an increase in temperature, indicating that the ceramic powder particles were refined. The sample that was subjected to pressureless-assisted sintering at 1200 °C and held for 1 h exhibited the best stability and more uniform particle distribution compared to other samples. The particle size distribution data were closer to the standard line, satisfying the requirements of the normal distribution law. The results revealed that a high temperature was more favourable to the solid solution, and the formation of an Al₂O₃–ZrO₂ solid solution can diminish the influence of the volume expansion of ceramic powders on the sample properties during sintering. Therefore, the addition of the sintering aid Al₂O₃ significantly promotes the densification of the powders, and the pressureless sintering technique reduces the sintering temperature of the solid solution, thus imparting a crystalline structure and excellent mechanical properties to the material.     

高固含量多分散粒径分布乳液的研究2.二元种子法制备二元分散粒径分布乳液

通过在聚合过程中加入第２种子乳液,制备出了具有二元分散粒径分布的乳液,研究了第２种子乳液量对乳液粒径及粒径分布的影响。实验结果表明,通过改变第２种子乳液的量可以调节乳液搂闰径及粒径分布。当大粒子质量分数为８０％时,乳液粘度最小。通过粒径分布的多分散化,可显著降低乳液的最低成膜温度。     

Refractories containing fused and sintered alumina aggregates: Investigations on processing,particle size distribution and particle morphology

The present study investigated pressed high-purity alumina refractories containing either white fused or tabular (sintered) alumina aggregates under comparable conditions. Using factorial experiments especially the effects of the pressing pressure, the particle size distribution model and the particle morphology were evaluated. White fused alumina exhibited a higher refractoriness under load as well as a lower total compression and creep rate in creep in compression experiments. However, tabular alumina had a higher cold crushing strength and Young's modulus before and after thermal shock. Yet, no significant effect regarding the relative loss of the Young's modulus due to thermal shock was determined. Generally, a higher pressing pressure reduced the apparent porosity and increased the cold crushing strength, the Young's modulus and the refractoriness under load. The batches according to a recently suggested modified Andreasen particle size distribution model contained a considerably higher amount of the coarsest particle fraction, while the medium particle size fractions were reduced. Surprisingly, for both alumina raw materials the modified Andreasen model resulted in a virtually identical apparent density and a slightly lower apparent porosity compared to the conventional Andreasen model. Furthermore, the thermomechanical properties were essentially unaffected, while the cold crushing strength and the Young's modulus were somewhat lower. For both raw materials the addition of blocky coarse grain fractions yielded a lower apparent porosity and higher apparent density compared to angular grains due to improved particle packing. Remarkably, the creep in compression and the creep rate were reduced as well. Consequently, the modified Andreasen model together with a designed particle morphology might allow the fabrication of shaped alumina products with a much higher content of coarse grained particles resulting in at least similar or even improved physical, mechanical and thermomechanical properties irrespective of the used alumina raw material.     

Phase-field simulation of platelike grain growth during sintering of alumina

The toughness of alumina can be improved by utilizing the in situ formation of platelike anisotropic grains during sintering, that is, abnormal grain growth (AGG). Computer simulations of AGG may be effective to realize the conditions for obtaining the desired self-composite microstructure. In the first part of this study, sintering experiments of high-purity alumina powders were conducted to confirm the effects of powder size distribution as well as the amounts of additives. In the second part, a phase-field method for simulating the platelike grain growth was proposed. The large platelike grains were reproduced when the critical driving force of coarsening was set up. The incubation time of AGG was also observed in the case of the narrow size distribution. Although the morphology of the platelike grains did not exactly agree with the experimental observations, a possibility of the present method as a computational tool for simulating platelike AGG was verified.     

Effects of binary particle size distribution on minimum pick-up velocity in pneumatic conveying

Minimum pick-up velocities (U_pu) for entrainments of particle mixtures having binary particle size distributions (PSD) are measured in a horizontal pneumatic-conveying line using the weight-loss method. Geldart's groups A, B, and C glass beads having diameters of 400, 170, 40, and 5 μm are used. Variations in U_pu as a function of particle mass fraction (m) are examined. The capability of empirical correlations of monodisperse U_pu in predicting U_pu of binary mixtures is investigated. For group B particle mixtures (i.e. 400 & 170 μm), the particles are entrained separately resulting in linear U_pu variations with m, which is accurately predicted by the monodisperse U_pu correlation. For mixtures involving group A and B particles (i.e. 170 & 40, 400 & 40 μm), the two particles are collectively entrained resulting in U_pu that vary non-linearly with m and that cannot be predicted by the correlation. For mixtures involving group B and C particles (i.e. 400 & 5, 170 & 5 μm), U_pu are comparable to that of the monodisperse group B particles, therefore they are accurately predicted by the correlation. The significant impacts of binary PSD on U_pu found presently indicates that PSD effects on particle entrainment process warrants further investigations.     

An improved estimation of size distribution from particle profile measurements

Several methods are available to measure particle size. The majority of them, such as sieving, are off-stream techniques where samples must first be separated from the main stream for analysis.Therefore, the search for on-line particle size analysis systems has provided the impetus for the introduction of image-based particle size analysers to the mineral industry in the past three decades. Generally, the estimation of particle size distribution on the basis of image analysis depends on measuring a single parameter of particle profile. For example the equivalent area diameter (d_A) or mean Feret's diameter (d_F) distributions, then transforming this data to the equivalent size distribution. However, due to the irregularity of particles being analysed, it is believed that this kind of analysis may increase the error in estimation of particle size distribution since profiles of irregular particles carry more information than can be represented by a single parameter.In this paper, a proposed technique which measures two parameters, equivalent area diameter (d_A) and mean Feret's diameter (d_F), for each particle profile has been developed. The accuracy of the technique has then been investigated in the laboratory by successfully estimating (unfolding) the size distribution, where size refers to sieve size, of three samples of different particle shapes with known size distribution.