Effect of Externally Applied Plasticizer on Compaction Behavior of Spray-Dried Powders

The effects of an externally applied plasticizer on compaction behavior and green microstructure quality of spray-dried powders was investigated. The plasticizer was applied to the external surfaces of already spray-dried powders by spraying it on tumbling spray-dried granules. The apparent yield point of the spray-dried powder was reduced when the plasticizer was added. Microstructures of compacts made from these granules (with and without the externally applied plasticizer) were compared at different compaction pressures. Better knitting across granule interfaces and fewer defects were obtained for the granules with the externally applied plasticizer.     

Analysis of Powder Compaction Using a Compaction Rate Diagram

Compaction response diagrams are useful tools for characterizing the powder pressing operation. It is shown that taking the first derivative of the curve contained in a compaction response diagram yields a compaction rate diagram which reveals nuances in the pressing operation. These additional details can be quite important in the design of powder systems.     

Effect of Internal Lubricants on Defects in Compacts Made from Spray-Dried Powders

The role of internal lubricants in the closure of large intergranular pores during dry-pressing was investigated. Alumina was spray-dried with and without an internal lubricant to yield granules with similar characteristics other than lubricant effects. Green and sintered microstructures were evaluated at different compaction pressures. The defects were quantified by evaluating the fracture surface of ∼90% dense sintered compacts. The samples that contained an internal lubricant had higher green densities and fewer defects at comparable compaction pressures. The internal lubricant did not cause any significant reduction in green strength or increase in springback.     

Crystallization of Amorphous Components in Spray-Dried Powders

Spray-dried powders are typically produced as amorphous particles. Long storage of the particles tends to crystallize the powders, a reaction affected by moisture, time, and temperature. This work has examined partial crystallization from amorphous spray-dried powders by moisture sorption. Powders of citrus fiber with hibiscus extract, maltodextrin, coffee, tea, skim milk, and sucrose were produced with a laboratory-scale spray dryer. The powders were exposed to ambient temperature and various relative humidities, with weight measurements recorded over time. It has been found that, in different materials, the amorphous to crystalline state change is observed at varying rates depending on the relative humidities and molecular weights. This observation may be associated with all amorphous spray-dried materials.     

Crystallization of Amorphous Components in Spray-Dried Powders

Spray-dried powders are typically produced as amorphous particles. Long storage of the particles tends to crystallize the powders, a reaction affected by moisture, time, and temperature. This work has examined partial crystallization from amorphous spray-dried powders by moisture sorption. Powders of citrus fiber with hibiscus extract, maltodextrin, coffee, tea, skim milk, and sucrose were produced with a laboratory-scale spray dryer. The powders were exposed to ambient temperature and various relative humidities, with weight measurements recorded over time. It has been found that, in different materials, the amorphous to crystalline state change is observed at varying rates depending on the relative humidities and molecular weights. This observation may be associated with all amorphous spray-dried materials.     

Fracture Toughness of Spray-Dried Powder Compacts

The strengths and fracture toughness values were measured for alumina powder compacts containing two different binder systems. Diametral compression was used to measure both the tensile strength and the fracture toughness (through-thickness notch). This methodology was very useful in linking processing parameters, such as binder choice and compaction stress, to the quality of the green bodies. Observations of the compact structure before and after fracture showed that the binders segregated to the region between the spray-dried granules. The presence of the excess binder in this region was linked to both the failure mode and the creation of secondary cracks.     

Vacuum Drying for Ultrafine Alumina Powders

It is well known that the drying of liquid-borne powders will create agglomerates and the problem of agglomeration is particularly acute in the nanoscale range. To eliminate/mitigate the agglomeration problem, in this study, a vacuum drying technique was used for drying the colloid solution with θ-Al₂O₃ ultrafine particles. For comparison purposes, other drying methods including oven drying, microwave drying, and freeze drying were also applied for drying of the same kind of colloid solution. The results indicate that the redispersibility, which is closely related to the degree of agglomeration, of the dried powders obtained from vacuum drying is better than that obtained from freeze drying. More surprisingly, results showed that the dried powders obtained from the vacuum drying assisted by microwave heating has the redispersibility close to 100%.     

New Aqueous Injection Molding Process for Ceramic Powders

An aqueous injection molding process based on the gelling properties of certain naturally occurring polysaccharides, in particular agar, and its purified derivative, agarose, is described. The materials form nonviscous solutions at temperatures near 100°C which solidify to rigid gels upon cooling below the so-called gel point temperature, 37°C. Gels formed by these materials are strong, nominally 1500 g/cm² for agar and 2500 g/cm² for agarose at 3 wt% concentration. Molded parts can be dried and fired without use of absorbent powders or special debinding operations of any kind.     

Infrared Microscopy for Examination of Structure in Spray-Dried Granules and Compacts

Novel infrared microscopy has been developed to improve the liquid-immersion method, which is a technique that has been developed by the authors to study the packing structures of powder particles in ceramic green bodies. This paper demonstrates the high potential of the novel technique by presenting clear structures of Si₃N₄ powder granules and their compacts.     

Physical and Functional Properties of Blackberry Freeze- and Spray-Dried Powders

The aim of the present work was to develop two products from blackberry juice by freeze and spray drying with potential use as food colorants or healthy ingredients. A characterization of the physical and functional properties of the powdered juices was done. Maltodextrin or a mixture of trehalose and maltodextrin were assessed as carrier matrices. Freeze-dried, maltodextrin-containing powders presented the best retention of bioactive compounds and antiradical activity; however, they showed a narrow relative humidity range for storage in the glassy state. Spray-dried powders showed better physical properties, bearing higher glass transition temperature and lower molecular mobility than freeze-dried formulations.     

A Simple Direct Casting Route to Ceramic Foams

A simple direct foaming and casting process using ovalbumin-based aqueous slurries for fabricating ceramic and metal foams is demonstrated. Foaming of aqueous ceramic slurries and the foam microstructure were seen to be a strong function of slurry rheology. Setting of foams with ceramic solids loading above 20 vol% was achieved by addition of acid, which also prevented binder migration. Acid addition resulted in excessive shrinkage, causing cracking of foams with ceramic loading below 20 vol%. Addition of sucrose to the slurries suppressed shrinkage leading to defect-free foams with porosity exceeding 95%. Overall porosity and foam microstructure could be controlled through ceramic solids loading, ovalbumin–water ratio, foaming time and sucrose amount, and sintering temperature. The ceramic foams fabricated by the process were strong enough to be green machined to different shapes.     

Egg White as an Environmentally Friendly Low-Cost Binder for Gelcasting of Ceramics

Slurries with different solids loading (34–50 vol%) of alumina were made with egg white (ovalbumin)–water premix. The ovalbumin content in the premix was decreased with increase in solids loading to lower slurry viscosity. The slurries were cast into bulk shapes and heated to 80°C, resulting in denaturation of ovalbumin forming a gel. The gelled samples were dried under controlled humidity and sintered in air to densities 94%–97% of theoretical. Alumina slurries formed with egg white (no water) were used to form tapes as thin as 60 μm in the sintered state.     

Formation of Lead Zirconate Titanate Powders by Spray Pyrolysis

Twin-fluid atomization spray pyrolysis (SP) has been investigated for the production of lead zirconate titanate (PZT) powders, using aqueous solutions of lead acetate and zirconium and titanium alkoxide precursor reagents. The particle size distribution of the PZT powder showed a d ₅₀ value of 0.3 μm, but with a small fraction of relatively large particles, several micrometers in size. Most particles were spherical but many of the largest particles, in the size range ca. 1–5 μm, were irregular. It was demonstrated that the morphology of the final PZT powder was controlled by decomposition processes occurring during the initial drying stages, at ≤200°C. A pyrochlore or fluorite-type intermediate crystalline phase was present in the final powders, but when the maximum reactor temperature was raised, and/or when the levels of excess lead in the starting solutions were increased, the proportion of the desired perovskite phase increased. However, at the highest process temperatures studied, ∼900°C, small crystallites of another phase formed on the surface of the PZT particles; these were probably lead oxide carbonate particles. Overall, a starting solution composition containing around 5 mol% excess Pb, and a maximum reactor temperature of 800°C, were selected as offering the most suitable conditions for producing PZT (52/48) powder, with minimal secondary phases(s). Preliminary densification studies showed that the powders could be sintered at 1150°–1200°C to give pellets of 95%–96% theoretical density.     

氢氧化镁粉体的干燥动力学研究

采用以六水氯化镁为原料,氢氧化钠和氨水为混合沉淀剂的直接沉淀法,制备得到平均粒径约为2μm的氢氧化镁粉体。通过不同干燥介质温度和不同湿物料层厚度的干燥动力学实验,获得氢氧化镁粉体的干燥曲线和干燥速率曲线。采用Weibull函数模型,对实验数据进行拟合分析,得到氢氧化镁粉体的干燥动力学参数和干燥动力学方程,探讨干燥动力学参数的影响因素。研究结果表明,可以用Weibull函数模型来描述氢氧化镁粉体的干燥过程,尺度参数与干燥介质温度及物料层厚度有关,随着干燥介质温度的提高,或物料层厚度的降低,干燥速率增大,尺度参数减小,但形状参数变化不大。     

Ink-Jet Printing of Binders for Ceramic Components

Layered manufacturing methods for fabricating ceramic components can involve selective deposition of binder using ink-jet printing. Selection of a proper binder plays a critical role in fabricating parts with good surface finish, dimensional accuracy, and high resolution. Several polymeric solution-phase binders were investigated in terms of their physical properties, printing performance, and binder-powder bed interaction. It was observed that the molecular weight should be <15 000 for the binder to be penetrated into dense powder compacts. Binder infiltration kinetics and printed line width were also significantly influenced by powder-bed characteristics, such as surface roughness and pore size, as well as the physical properties of the binder, such as viscosity and surface tension.     

Materials research & measurement needs for ceramics additive manufacturing

We report on a recent workshop dedicated to additive manufacturing (AM) of ceramics that was held at the National Institute of Standards and Technology (NIST) in November 2019. This two-day all-invited meeting brought together experts from industry, government agencies and academia to review the state of the field and identify the most pressing applied materials research and metrology issues which, if addressed, could accelerate the incorporation of AM methods into commercial ceramic manufacturing. Besides the AM technologies, the discussions included consideration of the necessary post-processing steps. We highlight some of the successes and challenges for the adoption of ceramics AM on an industrial scale, as viewed by the workshop participants. We also propose actions for the ceramic community to facilitate the wider commercialization of these fabrication methods.     

Determination of the Minimum Time for Binder Removal and Optimum Geometry for Three-Dimensional Porous Green Bodies

A model is developed to optimize two aspects of the thermal removal of binder from green ceramic components. The model, which accounts for flow in porous media arising from the thermal decomposition of binder in three-dimensional bodies with anisotropic permeability, describes the pressure within the body as a function of position, time, and temperature during the heating cycle. The model is used with variational calculus to predict the heating profile that minimizes the cycle time for the thermal removal of binder. The model is also used to determine which body geometry maximizes the buildup of pressure in parallelepipeds, a common shape of multilayer ceramic capacitors.     

可再分散聚合物粉末研究进展

介绍了可再分散聚合物粉末在国内外的应用、制备及作用机理研究现状,说明其存在耐水性能较差及价格偏高等问题,今后应进一步加强耐水性能及作用机理等方面的研究。     

A process control algorithm for reaction-diffusion minimum time heating cycles for binder removal from green bodies

Minimum time heating cycles have been simulated for binder removal from ceramic green bodies where the product of binder decomposition exits the green body via diffusion. The model consists of the reaction-diffusion partial differential equation, ordinary differential equations describing the reaction kinetics and a heating function, and algebraic equations, one of which imposes a constraint on concentration or pressure to avoid failure of the green body. Two solution approaches were compared: an earlier approximate method based on the pseudo-steady state assumption combined with a variational calculus algorithm and a new approach based on the finite element method combined with a process control algorithm. The agreement between the two solution strategies reinforces the validity of the pseudo-steady state approximation and the utility of the process control methodology. The latter, which was also applied to problems in which no approximate solution was obtainable, is thus a general method for obtaining minimum time heating cycles.     

Forming and Sintering of in Situ Alumina Composite with Hydraulic Inorganic Binder

Two types of barium aluminate binders were prepared by heat treatment of barium aluminate precursors, synthesized by using solution processes, at low temperatures ( T < 500°C). One was barium monoaluminate (BaAl₂O₄), and the other was barium aluminate binder (BAH binder) composed of amorphous phase, barium aluminate monohydrate (BaAl₂O₄·H₂O), and BaAl₂O₄. The setting time of the BAH binder was controlled by adjusting the heat-treatment temperature of the BAH binder precursor. The addition of the synthesized BaAl₂O₄ powders to Al₂O₃ powders improved the bending strength of Al₂O₃ matrix green bodies. The synthesized BaAl₂O₄ powders led to the in situ forming of barium hexaaluminate (BaO· x Al₂O₃, x = 6.9: BA6) platelets in the matrix by reacting with Al₂O₃ during sintering. The formed BA6 platelets inhibited the grain growth of the matrix Al₂O₃ grains.