Anisotropic Shrinkage in Tape-Cast Alumina: Role of Processing Parameters and Particle Shape

Aqueous suspensions of alumina and other powders containing no visible morphological anisotropy were tape cast under different conditions to study the role of processing parameters and powder characteristics in the origin of in-plane sintering shrinkage anisotropy. High anisotropic shrinkage, with the typical shrinkage in the transverse direction exceeding that in the casting direction by more than 10%, was observed for tape-cast alumina cast at high solids loading and high shear rates. Tape casting without any binder also resulted in similar anisotropy. Lower solids loading with increased agglomeration resulted in a drastic reduction of shrinkage anisotropy. Under similar casting conditions, BaTiO₃ did not show any shrinkage anisotropy unlike alumina. Cubic zinc ferrite and titania powders also showed some consistent shrinkage anisotropy. A correlation was found between in-plane particle orientation and anisotropic shrinkage proving that powder characteristics, particularly particle shape, control anisotropic shrinkage even in apparently equiaxed powders.     

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.     

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.     

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.     

Particle Orientation During Tape Casting in the Fabrication of Grain-Oriented Bismuth Titanate

Slurries containing platelike Bi₄Ti₃O₁₂ particles have been tape cast to prepare green sheets with aligned particles. The slurries contain well-dispersed particles and show nearly Newtonian flow behavior. The effect of slurry composition and casting conditions on the particle orientation has been examined. The particle orientation in the green sheet is determined mainly by powder content; other parameters, such as binder content, casting speed, and blade opening, have little effect. The interaction between particles is a main cause for particle alingnment. The slurry with a large powder content is favorable for preparing dense grain-oriented ceramics.     

Tailoring particle alignment and grain orientation during tape casting and templated grain growth

The quality of crystallographic alignment in textured ceramics produced by tape casting and templated grain growth (TGG) has been little studied despite its demonstrated impact on magnetic, piezoelectric, and optical properties. Physical and crystallographic alignment of anisotropic template particles is shown to be directly linked to the casting rate, gap height, and casting viscosity during tape casting. These parameters are shown to affect the shape and magnitude of the shear rate profile under the doctor blade during casting which in turn causes a gradient in the torque acting on anisotropic particles. The magnitude of the torque, the time the slurry is exposed to torque during casting, and the ratio of casting height to template diameter are demonstrated to enable the particle alignment process to be tailored to produce well-aligned template particles. Crystallographic alignment of the textured ceramic was quantified by grain misalignment angle (full width at half maximum, FWHM) and degree of orientation (r) and is directly correlated with the degree of torque during casting. High-quality alignment (FWHM = 4.5°; r = 0.13) was demonstrated in the model TGG system consisting of submicrometer alumina and 5 vol% 11 μm diameter template platelet particles.     

Particle Shape and Size Effects on Anisotropic Shrinkage in Tape-Cast Ceramic Layers

The particle shape of a commercial low-temperature cofired ceramic (LTCC) composite powder was determined quantitatively in the as-received and milled state using a new particle image analyzer. All grades of the milled powder with average particle sizes of 3.0, 2.4, and 1.8 μm, respectively, exhibit a considerable stretched particle shape, because 40% of their particles have circularity values below 0.95. On the basis of the fast particle image analyzer, the influence of the raw materials on particle alignment during tape casting was investigated using "design of experiments" (DOE). In the cast LTCC green tapes, the degree of particle orientation was measured and correlated with the information from the particle shape analyses and with other material and process factors from the DOE. The results showed that the degree of particle alignment correlates significantly with the measured particle shape and size; more than 80% of the particles were oriented in the casting direction if their shape factor was below 0.5. The particle orientation causes shrinkage anisotropy. The use of a coarser LTCC powder with an average particle size d ₅₀ of 3.0 μm instead of 1.8 μm increased the sintering anisotropy factor of LTCC tapes and laminates significantly from 1.0% to 1.85% and from 3.6% to 7.6%, respectively. The use of more binder or less solvent led to higher shrinkage anisotropies too. The casting velocity showed only a minor effect on the degree of particle orientation and sintering anisotropy, which is due to the shorter shearing period in which particle rotation can take place.     

Factors Influencing Anisotropic Sintering Shrinkage in Tape-Cast Alumina: Effect of Processing Variables

The effect of the processing variables shear rate, solids loading, and sintering temperature on the anisotropy of sintering shrinkage of aqueous tape-cast alumina was studied. Higher shear rates and higher solids loading resulted in higher in-plane shrinkage anisotropy, whereas the shrinkage anisotropy in the thickness direction was higher for low solids loadings. The in-plane shrinkage anisotropy was found to be fairly constant above a certain critical shear rate (∼100 s⁻¹) independent of the solids loading. The shrinkage anisotropy through the thickness was higher than in-plane directions. A higher thickness direction sintering rate was observed and attributed to a greater number of interparticle necks in the thickness direction because of the platy nature of alumina particles and the greater thickness direction strains associated with binder removal. The binder did not significantly affect the in-plane sintering shrinkage but significantly affected the shrinkage in the thickness direction. It was suggested that emulsion binder particles occupy sites in between layers of particles in the thickness direction. The degree of anisotropic shrinkage was quantified using edge orientation polarograms and a direct correlation was obtained between the processing variables, shrinkage anisotropy, and the edge orientation index.     

Mechanical Properties of Tape-Cast Alumina-Glass Dental Composites

Alumina-glass dental composites were prepared by tape casting and sintering at 1120°C, followed by glass infiltration at 1100°C. Flexural strength and fracture toughness of the composites were investigated in terms of the influence of tape constituents, namely, alumina powder, binder, and plasticizer, on the mechanical properties. Both strength and toughness increased with increasing alumina fraction in tapes and decreased with increasing binder content in binder/plasticizer mixtures. These observations were consistent with the influence of the constituents on mean alumina particle distance in tapes, suggesting that the high strength of glass-infiltrated alumina composites is related to toughening by crack bowing. The strength and fracture toughness of the tape-cast composites, optimized for forming dental crowns, were 508 MPa and 3.1 MPa·m^1/2, respectively, obtained from biaxial tests. Shrinkage of the composites decreased with increasing thermocompression pressure, applied to the tapes prior to sintering, and heating rate to the sintering temperature.     

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.     

玻璃粉粒度对复合胶凝材料性能的影响

用激光粒度分析仪测定不同细度的玻璃粉粒度分布,以灰色关联方法( GIA)和扫描电子显微镜( SEM)研究了玻璃粉粒度对复合胶凝材料性能的影响。研究结果表明,玻璃粉中10μm以下的颗粒关联性均为正,说明这些颗粒对复合胶凝材料强度发展有积极贡献,其中以5~10μm的关联度为最大;而?45μm的颗粒呈负关联,其火山灰活性没有得到充分发挥;玻璃粉经适当物理磨细(35 min)后,能更有效地分散到复合胶凝材料的孔隙中,使复合胶凝材料的结构更加密实,从而提高了复合胶凝材料的抗压强度。     

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.     

Aqueous tape casting of silicon nitride

Slurries consisting of a low cost silicon nitride powder, sintering aids yttria and alumina, dispersants, binders, defoamers and water as a solvent were optimised for tape casting by electroacoustic and viscosity measurements and by casting experiments. The slurries exhibit shear-thinning behaviour due to the highly shear-thinning binder emulsion. Crack free tapes with a maximum thickness of approx. 250 μm and a binder content of 13–15 wt.% could be obtained. The green tapes with a high flexibility and green strength could be laminated easily by compression at room temperature. After sintering a dense microstructure developed. A significant shrinkage anisotropy parallel and perpendicular to the cast direction was observed.     

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.     

Cardanol as a dispersant plasticizer for an alumina/toluene tape casting slip

Cardanol, a naturally occurring C₁₅ alkyl chain substituted phenol obtained from cashew nut shell liquid (CNSL), is used as a dispersant and plasticizer for PMMA binder based alumina tape casting slips in toluene medium. The best dispersion of an alumina powder with BET surface area of 10.4 m²/g occurs at a cardanol concentration 2 wt.% of the powder at which the cardanol molecules form a monolayer on the particles with an end-on-adsorbed configuration through phenolic hydroxyl anchored on the surface. Cardanol added in excess of the dispersing agent acts as a plasticizer for PMMA, as revealed by significant decrease in the T_g of the polymer, reduction in the slurry viscosity and increase in the tape flexibility. Green tapes with good flexibility (failure strain, 10–20%) and strength (5.7–7 MPa) were obtained using a binder content 12 wt.% of alumina and employing a plasticizer to binder ratio in the range 0.66–0.82 by weight. The green tapes were thermally debinded with or without extraction of the plasticizer with methanol. The solvent extraction of the plasticizer prior to thermal debinding, however, did not show any effect on the density (97% TD) of the tapes sintered at 1500°C.     

基于离散元方法的金属粉末铺粉动力学研究

增材制造技术在航空航天、医疗技术及运输和能源中的应用得到快速增长,其常用方式之一是基于粉末加工。该技术主要瓶颈之一往往与铺粉过程形成的粉层质量有关,颗粒动力学机制尚不清楚,且受铺粉装置的类型和颗粒的黏附性影响很大。本工作基于实验测量和表征的单个颗粒的真实物理和力学参数,采用离散元方法对增材制造常用气雾化金属粉末的铺粉过程进行了数值模拟分析,比较了刮刀和辊子两种铺粉装置中粉堆内颗粒速度和轨迹及最终铺粉层的质量,并分析了这些参数对颗粒黏附表面能的敏感程度。结果表明,相比于刮刀铺粉,在辊子铺粉过程中,由于辊子旋转运动的作用,粉堆内部存在多条拱形速度带和颗粒对流,且在形成铺粉层之前,颗粒需要经历爬坡上升和下坡滑落两个过程,运动轨迹更长。另外,两种铺粉装置中铺粉层颗粒总体积均随颗粒表面能的增加而降低,但与刮刀铺粉相比,辊子铺粉中铺粉层颗粒总体积小,且对颗粒表面能更加敏感。铺粉装置类型和颗粒黏附性对铺粉层质量的影响可以归因为铺粉层的形成机制,即颗粒从粉堆中进入铺粉间隙的难易程度及铺粉间隙中刮刀或辊子对颗粒的拖曳作用。     

Effect of Activated Carbon Bed on Binder Removal from Ceramic Injection Moldings

Weight losses during the binder removal from ceramic injection moldings that have been placed in beds of activated carbon powder and two types of alumina powder were compared. The effect of the specimen size and shape on debinding in a bed of activated carbon was investigated. Because of the structure of its particles, activated carbon powder allowed binder losses in ceramic moldings that consisted of submicrometer-sized particles to increase at a temperature of 130°C. The binder loss in a bed of activated carbon was dependent on the ratio of surface area to volume of the body that was extracted. Renewal of the sorption abilities of the bed of activated carbon after saturation with binder increased the rate of binder removal.     

Effect of coarse particles on the strength of alumina made by slip casting

The effect of coarse particles in alumina powder slurry on the microstructure and strength variation of the sintered alumina bodies made by slip casting was examined. A commercially available low-soda alumina powder was used as the raw material. Coarse agglomerated particles were added in the alumina powder slurry just before slip casting. Alumina ceramics were fabricated through slip casting process. The ceramic strength and fracture toughness were examined. Fracture origins and microstructure were observed with scanning electron microscopy (SEM). The internal structures of sintered bodies were examined by using mid-infrared microscope. As a result, direct observation technique using mid-infrared microscope enabled us to analyze a small amount of coarse agglomerated particles in sintered body. The strength variation of the sintered bodies was correlated with the size of coarse agglomerated particles detected by mid-infrared microscopy.     

石粉对自密实混凝土收缩性能的影响

为了研究石粉对自密实混凝土收缩性能的影响,设计了不同水胶比和石粉质量分数的试验,得到了水胶比为0.30、0.34、0.37,石粉质量分数为0%、10%、20%时自密实混凝土28 d龄期内自收缩和干燥收缩的变化规律。结果表明:自收缩随着水胶比的减小而增大,随着石粉质量分数的增加而增大;水胶比在0.30~0.37时,其对干燥收缩的影响不大;适量的石粉对干燥收缩几乎无影响,当石粉质量分数达到20%时,干燥收缩明显增大。在试验的基础上提出了考虑水胶比和石粉质量分数影响的自密实混凝土自收缩与干燥收缩预测模型。