Consolidation Behavior of High-Performance Ceramic Suspensions

Colloidal processing has been shown to produce low defect and uniform ceramic microstructures from submicrometer ceramic powders. These concepts were applied to colloidal pressing to determine critical design relationships for uniaxial consolidation of dense and uniform green bodies from colloidal suspensions. Carefully controlled constant rate of strain consolidation experiments were carried out using alumina in water. The compression index decreased from 0.143 for a poorly dispersed alumina system to 0.077 for a well-dispersed alumina suspension compression curve, indicating that the well-dispersed system is stiffer in consolidation. The compression curves showed that, as the degree of dispersion decreases, increased consolidation stresses are required to achieve a given particle packing density. The compression index increased with increasing strain rate for well-dispersed alumina suspensions. Permeability through the sample ranged from 3 × 10^–8 to 4 × 10⁻⁷ cm/s, decreasing with decreasing void ratio during consolidation. Well-dispersed samples gave lower permeabilities than did poorly dispersed samples over a given consolidation increment. Coefficients of consolidation were nonconstant over the experimental effective stress range, invalidating the general solution to the linear consolidation equation. An approximate incremental solution was applied which indicated rapid pressing cycles are possible by starting with a suspension having a high solids concentration. Application of this consolidation data to nonlinear consolidation models is recommended for more exact prediction of consolidation time.     

Coaxial Electrohydrodynamic Atomization of Ceramic Suspensions

In this work we demonstrate a novel method of preparing composite ceramic microstructures, using alumina–zirconia as an example. Suspensions of these two ceramics, containing a similar solids content, were simultaneously subjected to electrohydrodynamic atomization (EHDA) in the stable cone-jet mode and the resulting relics were analyzed by microscopical techniques before and after partial sintering at 1200°C. We show that the new method, coaxial EHDA (CEHDA), is capable of forming different annular microstructures, which are mainly controlled by the speed of the jetting.     

Mullite Suspensions for Reticulate Ceramic Preparation

The dispersing behavior of MgO-doped mullite suspension suitable for the production of reticulate ceramics by sponge impregnation was studied. Three deflocculants were tested and the optimum dispersing conditions for the mullite suspension were found. Electrokinetic studies of magnesia in water evidence that it undergoes pronounced solubilization that strongly affects the performance of the mullite suspension when added at the 1 wt% concentration, in terms of solid loading and aging effects. The optimum composition of the slurry was found, reticulate mullite samples were produced, and the green and sintered bodies were characterized.     

Curing of Highly Loaded Ceramic Suspensions in Acrylates

The objective of this study is to use differential scanning calorimetry (DSC) to characterize the curing kinetics of Al₂O₃ and hydroxyapatite suspensions in acrylates. The heat evolution of the acrylate premix and ceramic suspensions in dynamic and isothermal curing was measured. The calculated activation energies for the acrylate premix and the two ceramic suspensions were 133, 74, and 78 kJ/mol, indicating a catalytic effect of the ceramic fillers. The curing behavior of the two ceramic suspensions modeled with the autocatalytic polymerization model with satisfactory results.     

Ceramic Capillary Suspensions: Novel Processing Route for Macroporous Ceramic Materials

We introduce a novel method to produce macroporous ceramics by capillary suspensions. Adding a small amount (~1 vol%) of an immiscible secondary phase to a low concentration (~20 vol%) suspension can increase the yield stress by several orders of magnitude. This drastic change in flow behavior is induced by the creation of a sample‐spanning particle network in the suspension controlled by capillary forces. This strong network may persist even if the primary bulk phase is removed. Accordingly, capillary suspensions can be used as a precursor for manufacturing porous materials. Here, we focus on the specific features of this universal, low‐cost processing route for porous ceramics. An Al₂O₃ model system is used to demonstrate how to adjust porosity and pore size. With this system, we were able to achieve open porosities higher than 60% with an average pore size below 10 μm.     

Thermally Induced Wavy Hertzian Fracture

Reproducible "complexed" Hertzian cones were formed in glass as a result of thermally induced tensile stresses. These cones differ from normal cones by having wavy profile patterns, indicating two modes of fracture operating alternately. One mode is shallow and the other steep, together averaging a crack angle of 18.9 ± 1°. The thermal technique offers new test conditions for studying Hertzian fracture.     

In Situ Rheological Investigation of the Coagulation in Aqueous Alumina Suspensions

Electrostatically stabilized alumina suspensions can be destabilized by the enzyme-catalyzed decomposition of urea (direct coagulation casting). Depending on the conditions, this reaction can shift the pH of a suspension to the buffer pH of the reaction products or increase the ionic strength at the buffer pH. The coagulation for both mechanisms was investigated using in situ rheological measurements. Using a vane tool in oscillation mode, the measuring conditions were optimized to find a reasonable method for time-dependent measurements. Constant parameters (stress or strain) proved to be unsuitable, because the linear viscoelastic region shifted considerably during the coagulation. Furthermore, the gel structure produced on coagulation via increase of ionic strength (Δ I ) was very sensitive to the oscillation. Therefore, for long-time experiments, a short continuous measurement with a low strain was followed by amplitude sweeps with increased intervals to determine the linear values of G ' and G ". In this way, the increase of the moduli G ' and G " could be followed for longer times, and it was possible to demonstrate two results. First, the final G ' of the network was about 10 times higher for Δ I -coagulated material than for suspensions coagulated via pH shift (ΔpH). Second, particle rearrangement processes took place in Δ I -coagulated networks even after the chemical changes were finished, whereas ΔpH-coagulated samples were "frozen-in" when approaching the isoelectric point and showed no further physical changes afterward.     

Reversible Cluster Aggregation and Growth Model for Graphene Suspensions

We present a reversible cluster aggregation model for 2‐D macromolecules represented by line segments in 2‐D; and, we use it to describe the aggregation process of functionalized graphene particles in an aqueous SDS surfactant solution. The model produces clusters with similar sizes and structures as a function of SDS concentration in agreement with experiments and predicts the existence of a critical surfactant concentration (C_crit) beyond which thermodynamically stable graphene suspensions form. Around C_crit, particles form dense clusters rapidly and sediment. At C ? C_crit, a contiguous ramified network of graphene gel forms which also densifies, but at a slower rate, and sediments with time. The deaggregation–reaggregation mechanism of our model captures the restructuring of the large aggregates towards a graphite‐like structure for the low SDS concentrations. © 2017 American Institute of Chemical Engineers AIChE J, 63: 5462–5473, 2017     

聚氯乙烯的离子型热可逆交联研究

通过二胺的烷基化反应，制备了不同链长的二叔胺，利用二叔胺与聚氯乙烯(PVC)链上的活性氯形成季铵盐离子键的反应，制备了具有离子型热可逆交联特性的PVC。在研究中发现，对于硬聚氯乙烯(PVC-U)，适当份数的交联剂能够提高材料的杨氏模量、屈服强度、断裂强度和断裂伸长率；应用于软聚氯乙烯(PVC-S)时，断裂强度、断裂伸长率均得到提高，而永久变形量明显降低，并且交联时间的延长对力学性能影响显著。     

Novel Thermally Conductive Thermoplastic/Ceramic Composite Foams

Multifunctional materials that are lightweight and thermally conductive but electrically insulating are important for modern electronics, computer, and telecommunication technologies. Here, a novel foam structure of a polymer/matrix composite filled with ceramic platelets with improved thermal conductivity is reported. Such improvement is caused by the stress‐induced alignment of thermally conductive fillers in the cell wall of the plastic foam. The foam structure is very promising for use as a lightweight electronic packaging material owing to its light weight, thermal conduction ability, electrical insulation, and good processability.

     

Liquid Junction Potentials and Transference Numbers in Montmorillonite Suspensions

The EMF's of concentration cells with and without transference were determined. The cells consist of two compartments each containing an equal concentration of K-montmorillonite with a different KCl concentration at the two sides of the liquid junction. The EMF's were determined with Ag/AgCl electrodes and a K-sensitive glass electrode. The liquid junction potential and the transference number of the ionic species in the suspensions were calculated from the data of the EMF. The liquid junction potential (being zero for KCl solution), and the transference number of K⁺, were found to increase with the increase in the clay concentration. This indicates that pH measurements in clay suspensions involve a liquid junction at the KCl bridge which cannot be disregarded.     

Retention of Liquid by Solids in Suspensions and Sediments

Abstract

The effects of associated immobile liquid on the settling behavior of particles in solid-liquid suspensions are investigated. The effects are studied with respect to such sedimentation parameters as suspension porosity ?, final sediment volume fraction u_s, internal void fraction of the flow unit u_i, Steinours parameter A, and McKay's packing factory which corrects for the effective volume and density of the flow units in a sedimenting suspension. It is shown that the degree of liquid association with the settling particles is a function of their physicochemical characteristics. Suspensions containing clays and carbonates, owing to their ionic nature and loose packing behavior, absorb large amounts of liquids and result in larger settled sediments of a compressible nature. On the other hand, systems like glass beads-liquid suspensions absorb little associated liquid and virtually result in noncompressible settled sediments due to their rigid spherical shapes and close packing. The addition of hydrocolloids (such as natural gums) provides ionic stability to suspensions by giving them low cmpressibility and low values of u_s, u_i, A, and p.     

Coagulation of Concentrated Suspensions of Ultrafine Alumina Powders by pH Shift

Highly concentrated suspensions of ultrafine alumina powders are prepared using 2-phosphonobutane-1,2,4-tricarboxylic acid as dispersant. The coagulation of these suspensions is carried out at around 278 K by adding a blend of acetic anhydride plus ethylene glycol. Coagulation takes place through a pH shift because of a time-delayed hydration of the acetic anhydride. The coagulation kinetics are studied by viscosity measurements with respect to the temperature and composition of the coagulant blend. Finally, the method is used to produce complex 3D parts of large dimensions for bio-applications by direct coagulation casting and the fired bodies are characterized.     

碳酸钙悬浮液混凝处理后的流变性

应用LVDV-Ⅲ+型可编程流变仪测定了微米级碳酸钙悬浮液混凝处理后的流变特性,考察因素包括悬浮液固相质量浓度、pH值、混凝剂种类及添加量、搅拌速度.结果表明,混聚后碳酸钙悬浮液在测定范围内流变曲线符合Herschel-Bulkley模型.表观粘度随颗粒浓度的增大而增大,其中屈服应力随体系浓度的增大而增大,刚性系数随体系...     

Hierarchically Structured Materials by Anodic Coagulation Casting of Fibrinogenic Alumina Suspensions

Anodic coagulation casting of fibrinogenic ceramic suspensions is a novel processing technology, which is based on the electrically induced transformation of the water soluble fibrinogen into the insoluble fibrin. Contrary to the direct coagulation casting (DCC) technology, green formation does not depend on a pH‐shift and as the fibrin coagulate forms on an anode, it can be combined with the electrophoretic deposition (EPD) technology. In this study, the conversion of fibrinogen into fibrin is activated via electron transfer processes at an electrode material and is combined with the green formation of alumina by embedding the ceramic particles in the protein matrix. The focus of this work was to establish a technology to shape thin hierarchically structured ceramic films and thick porous materials with a distinct pore structure. Film thickness and porosity were controlled by the applied voltage and the processing‐time. The range of the established green bodies included two‐dimensional and simple three‐dimensional shapes including multilayered deposition and fiber coatings. Overall the process of anodic coagulation casting can be reported to be successful for all established ceramic shapes except multilayers, where delamination was observed. The deposited alumina ceramics were characterized using light microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and synchrotron micro computed tomography (μCT), while the coagulation mechanism was studied using high‐performance liquid chromatography (HPLC).     

Thermally Induced Delamination of Symmetrically Graded Multilayers

A fracture mechanics model is used for calculating the steady-state energy release rate of symmetrically graded ceramic multilayers. Results from the present study and previous results on asymmetric multilayers are generalized to simple design rules for avoidance of delamination of graded multilayers: (A) For asymmetric multilayers, delamination can be suppressed by inserting a few additional interlayers. (B) For symmetric multilayers, multilaying is not efficient; instead materials should be selected which have a sufficiently low thermal expansion mismatch. The later can be judged by making bilayers of identical thickness as the desired symmetric multilayers. If the bilayer delaminates it is unlikely that symmetric multilayers can be processed, and other materials (having a smaller thermal expansion coefficient mismatch) should be tested.     

Fundamentals of Preparing Suspensions of Silicon and Related Ceramic Powders

A dispensability model previously developed for silicon powder is extended to theoretically predict the behavior of silicon nitride and silicon carbide powders, and their nonstoichiometric analogs, in a variety of potential dispersing media.     

Process Mechanism for Vacuum-Assisted Microfluidic Lithography with Ceramic Colloidal Suspensions

The infiltration kinetics and the drying mechanism of suspensions involved in vacuum-assisted microfluidic lithography (μFL) have been investigated for the fabrication of complex micropatterned ceramic structures. Infiltration lengths of alumina suspensions with various solid loadings into microchannels were analyzed as a function of channel widths ranging from 10 to 100 μm. The use of well-dispersed ethanol-based suspensions with lower viscosities and wider channels allowed for easier and longer infiltration due to lower fluidic resistance in the channels. In contrast to the micromolding in capillaries (MIMIC) method, vacuum-assisted μFL has a distinct drying mechanism in which there is a critical level of solid loading of the suspension with respect to the volume of the microchannel for the fabrication of a defect-free pattern structure.