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.     

Structural and Property Evolution of Aqueous-Based Lead Zirconate Titanate Tape-Cast Layers

We have developed aqueous suspensions of lead zirconate titanate (PZT) and an acrylic latex emulsion binder for tape casting. Rheological measurements were conducted to optimize their flow behavior at high solids volume fraction (φ_solids). Concentrated suspensions (φ_solids > 0.45) were tape cast onto a silicone-coated mylar carrier film, and the effect of cellulose (a viscosifier) and surfactant additions on wetting behavior was studied. Drying stress measurements were performed on tape-cast layers using a substrate deflection technique. The initial period of stress rise was analogous to that observed for binder-free ceramic films, whereas the stress decay was influenced by latex coalescence. Tape-cast layers exhibited a constant PZT volume fraction of 0.49 and uniform binder distribution independent of the PZT/latex volume ratio. The dielectric and piezoelectric properties of sintered multilayer samples were comparable to those obtained for bulk, isopressed samples. Our work provides guidelines for developing aqueous formulations for tape casting complex electroceramic oxide layers.     

碳化硅陶瓷水基浆料的流变性研究

主要研究了分散剂、pH、颗粒级配等因素对SiC水基浆料流变性能的影响,并分析了其原因。结果表明,各分散剂对SiC浆料粘度影响效果的大小顺序依次为:四甲基氢氧化铵(TMAH)>六偏磷酸钠(SHMP)>聚乙二醇(PEG)。TMAH、SHMP、PEG三种分散剂的最佳用量分别为0.8wt.%、0.6wt.%、1.0wt.%,最佳pH为13,最佳颗粒级配(F220/F1200,下同)为1.5,当固相体积分数超过50%后SiC浆料的粘度急剧增加。通过优化工艺参数,制备出了流动性较好,固相体积分数为50%、粘度为500mPa.s的SiC陶瓷浆料。     

Drying of Ceramic Layers with a Graded Pore Structure

Thin self-supporting ceramic TiO2-layers with a graded pore structure were prepared by using centrifugal deposition of powders and sols with different particle size distributions from mixed, diluted suspensions. During the evaporation drying step the layers have a strong tendency to warping and crack-formation because of the resulting difference in the capillary pressure in the upper and the bottom side pores. Four drying methods were investigated concerning their suitability for diminishing or eliminating capillary forces and for the production of planar, crack-free dried specimens. The drying techniques used in the experiments are briefly introduced. It should be emphasized that the most successful drying method for the layers mentioned above is a combination of microwave drying and subsequent critical point drying.     

Rheological, Structural, and Stress Evolution of Aqueous Al2O3:Latex Tape-Cast Layers

The rheological, structural, and stress evolution of aqueous alumina (Al₂O₃):latex tape-cast layers of varying composition were studied by shear rheology, direct visualization, and a controlled environment stress measurement device. Their low shear viscosity was nearly independent of the alumina:latex ratio for binary mixtures whose particle size ratio (λ=̄_alumina:̄_latex) approached unity, but varied over an order of magnitude for systems with particle size asymmetry. Direct visualization of these mixtures revealed that particle flocculation occurred as their total solids loading increased. Their structure was characterized at intermittent points during the drying process by imaging freeze-dried samples using scanning electron microscopy (SEM). Their corresponding stress histories exhibited three distinct regions: an initial period of stress rise, followed by a stress maximum, and, finally, a period of stress decay. Pure alumina layers exhibited a maximum stress of ∼1 MPa and a residual stress below 0.01 MPa. Pure latex films exhibited a maximum stress of ∼0.1 MPa and only a slight stress decay. The ceramic phase dominated the initial period of stress rise, while the latex phase strongly influenced the residual stress of composite layers cast from alumina:latex suspensions. Their maximum drying stress increased with decreasing Al₂O₃ particle size, whereas their residual stress increased with increasing latex T _g.     

Drying of Granular Ceramic Films: II, Drying Stress and Saturation Uniformity

Films composed of ceramic particles were observed during drying. The films were prepared from 20 vol% aqueous dispersions of α-alumina and α-quartz and were free of any organic binder. Conditions for uniform film saturation during drying were established by consideration of a liquid transport model and by direct observation of the drying films. Drying stresses were measured in situ by a substrate deflection method based on an optical interference technique. Simultaneous stress and weight measurements were used to determine stress as a function of saturation. The maximum stress occurred near 100% saturation and was approximately 2 and 1.1 MPa for films produced from 0.35-and 0.68-μm particles, respectively. The maximum stress decreased from 2 to 0.9 MPa for films produced from the 0.35-μm particles when 0.005 wt% surfactant was added to the slurry. The surfactant decreased the liquid surface tension from 72 to 32 dyn/cm. These observations are direct evidence of the effects of capillary tension on the state of stress in a ceramic body. Mechanical properties of the green ceramic films were estimated by use of a linear elastic fracture model. Knowledge of the critical cracking thickness and maximum stress in the film was used to estimate the fracture resistance of the granular film. The fracture resistance values are approximately 0.02 and 0.007 MPa·m^1/2 for films produced from alumina and silica, respectively. The difference in mechanical behavior of the silica and alumina films is similar to that expected by the difference in Hamaker constants between the two materials.     

Analysis of Effectiveness and Stress Development during Convective and Microwave Drying

The aim of the article is to study the effectiveness of convective and microwave drying with respect to drying time and stress generation in clay-like dried bodies. The theoretical analysis of stresses was confined to the constant drying rate period because clay shrinks and the stresses are generated mainly in this period. The theoretical prediction of stress development was validated using an acoustic emission method by monitoring the micro- and macrocracks formation caused by the stresses. Apart from the stress analysis, the combined convective-microwave drying was examined experimentally during whole process in order to show that the volumetric heat supply due to microwave heating enhances convective drying and, apart from this, such a combined drying process develops less stress in dried material. The theoretical and experimental studies were performed on a kaolin sample in the form of a thin plate.     

Analysis of Effectiveness and Stress Development during Convective and Microwave Drying

The aim of the article is to study the effectiveness of convective and microwave drying with respect to drying time and stress generation in clay-like dried bodies. The theoretical analysis of stresses was confined to the constant drying rate period because clay shrinks and the stresses are generated mainly in this period. The theoretical prediction of stress development was validated using an acoustic emission method by monitoring the micro- and macrocracks formation caused by the stresses. Apart from the stress analysis, the combined convective-microwave drying was examined experimentally during whole process in order to show that the volumetric heat supply due to microwave heating enhances convective drying and, apart from this, such a combined drying process develops less stress in dried material. The theoretical and experimental studies were performed on a kaolin sample in the form of a thin plate.     

Prediction and Explanation of Aging Shrinkage in Tape-Cast Ceramic Green Sheets

A simple test to predict linear shrinkage of tape-cast green sheets as a function of time was developed. Shrinkage was found to correlate inversely with the amount of organic phase bound to ceramic particle surfaces. The test was developed for alumina green tapes with acrylic binder. The correlation was found to be independent of the medium used for the two slurry systems evaluated, water and toluene/ethanol. Treatments developed to dimensionally stabilize green sheets reduced shrinkage after 35 days of aging to less than 0.15%. Stabilization treatments involved heating and/or polymer swelling which facilitated the binding of organic phase to alumina particles, causing the tape to be in a more relaxed state.     

Modeling of Multiphase Transport during Drying of Honeycomb Ceramic Substrates

Multiphase transport model to simulate drying of honeycomb ceramic substrates in a conventional (hot air) drier is developed. Heat and moisture transport in the honeycomb walls as well as channels is modeled. The model predictions are validated against experiments done for drying of cylinder-shaped substrates by comparing histories and axial profiles of moisture loss and point temperature histories at various locations. Drying experiments are performed at two different values of air temperature, 103°C and 137°C, at a relative humidity value of 5%. Sensitivity analysis reveals that the drying process is controlled by heat and water vapor transport. External heat transfer is the dominant resistance mechanism for energy transport, whereas internal convection and binary diffusion dominate the resistance to vapor transport.     

Atomic Force Microscopy Study of Ceramic Powder Compacts during Drying

Atomic force microscopy (AFM) was used to image the surface of ceramic powder compacts that contained polymer and solvent during drying. Topographical information was continuously collected as a function of drying time. Surface changes were directly observed during imaging. Surface crystals appeared on damp zirconia sheets after drying, subsequently contributing to secondary flaws in the form of holes in fired ceramic products. In cases where few surface crystals were observed in the green stage, fine grain size and high strength were observed after firing. The fired material was viewed via AFM and shown to be directly related to the drying process that was observed earlier.     

Evolution of Carbon during Burnout and Sintering of Tape-Cast Aluminum Nitride

The surface nature and the composition of AlN powder, as-received and exposed to binder burnout, were studied using XPS and TEM. The surface of as-received AlN powder was covered by a thin layer of aluminum oxynitride and oxide mixture. A small portion of residual carbon from binder burnout was bound to oxygen atoms on the AlN powder surface, and the majority of the carbon was amorphous graphitoid carbon which covered the AlN powder surface uniformly. AlN samples were made using tape casting and pressureless sintering. Surface-carbon-to-oxygen ratio of AlN powder after binder burnout was evaluated using XPS. The surface C/O atomic ratios were observed to correlate with the sintering behavior, the composition of the second phase, the second phases distribution, and grain-boundary composition, as well as thermal conductivity of AlN samples.     

陶瓷粉体常用的干燥方法

详细叙述了陶瓷粉体采用的热风干燥、红外线干燥、喷雾干燥、冰冻干燥等干燥方法,分析了各种干燥方法的优缺点。在实际应用中,可根据不同的陶瓷粉体选择相适应的干燥方法,以便能获得理想的干燥效果。     

陶瓷干燥设备的发展历程

本文主要介绍了陶瓷干燥设备的发展历程。     

Effect of Particle Shape on Anisotropic Packing and Shrinkage Behavior of Tape-Cast Glass–Ceramic Composites

In this study, the influence of particle shape anisometry and particle alignment in tape-cast green sheets on the shrinkage behavior of low-temperature co-fired ceramics (LTCCs) was investigated quantitatively. A new method for the characterization of particle shape with the use of a particle image analyzer is presented, and its application to real material systems demonstrated. A commercial LTCC system and three developed composite powders with different average particle sizes were analyzed. After tape casting, particle alignment in the green sheets was analyzed using image analysis of SEM micrographs of cross sections. The investigations showed that the degree of particle alignment correlates significantly with the particle shape and size of the materials. A further increase in particle orientation was seen after the lamination process. Additionally, the powder packing of both single layers and laminates was analyzed by mercury porosity. The anisotropic shrinkage behavior during the sintering process was determined by means of optical dilatometry. The data obtained on the particle morphology, particle orientation in the tapes, and their effects on the shrinkage anisotropy will be discussed.     

陶瓷工业干燥技术和设备

本文分析了陶瓷干燥过程的基本原理、干燥的种类、各瓷种所用干燥器分类以及微波干燥技术的先进性。     

微弧氧化陶瓷膜的制备及形貌研究

在硅酸钠、钨酸钠、硼酸、偏铝酸钠和乙二胺四乙酸二钠的复合电解液中,利用微弧氧化方法,在ZL102铸造铝合金上制备了陶瓷膜层。结果表明：脉冲电源获得的氧化膜表面由熔化凝固后的氧化物颗粒组成,颗粒呈不规则球形,周围有明显的熔化痕迹,表面粗糙凹凸不平且不均匀。通过对陶瓷膜层能谱分析,膜层中出现了Al、Si、Na、O、W元素。     

SiC浆料的流变性能研究

为制备结构均匀的多孔SiC陶瓷的料浆,本文比较了不同质量分数(40%~60%)的SiC浆料的剪切应力和黏度随剪切速率的变化.结果表明:浆料的等电点在3.6左右,Zeta电位值随着pH值的增加而减小,碱性环境下分散效果更好;用Casson方程拟合浆料的剪切应力,相关系数在0.98以上,随着固相含量的增加,拟合的极限粘度和屈服应力增加;当剪切速率达到100 s-1时,固相含量为40%~55%浆料的粘度都在1 Pa·s以下,固相含量为60% 的浆料粘度大概是2.1 Pa·s.     

国外高温陶瓷辊棒组成与性能的研究

本文对国外一种高温陶瓷辊棒的矿物－化学组成显微结构与其物化性能之间的关系进行了研究。结果表明,这是一种在莫来石基质中镶嵌着许多粗颗粒刚玉骨料的高温结构陶瓷材料制品。