Ceramic Suspensions Research Articles

Microfluidic rheology: A new approach to measure viscosity of ceramic suspensions at extremely high shear rates

This paper describes the operation principles of a micro-scale rheometry equipment based on the technology of microchannels on a chip and compares it with a conventional macro-scale rotational rheometer. Both techniques are used for the study of different solutions and suspensions of ceramic particles with different saccharides, which are subjected to a wide variety of shear rates from their preparation to their processing and injection in plasma spraying processes. The results have shown a clear difference between the rheological values obtained between both equipment, clearly influenced by the different measurement method. However, the microfluidic technique has higher accuracy to characterise fluids at high shear rates and low viscosities, and the apparent shear thickening observed in rotational rheometry for low viscosity samples due to wall slippage is not observed. In contrast, the fine microchannels of are easily clogged in concentrated suspensions.

A study of lead-free (K0.5N0.5)NbO3 piezoelectric ceramics processed by additive manufacturing

Lead-free (K[Formula: see text]N[Formula: see text]NbO3 piezoelectric ceramics have drawn wide attention due to its environment-friendly and excellent piezoelectric performance. Since the difficulty of post-processing of ceramics limit its further development and practical application, three-dimensional (3D) printing technology has been developed in this work. A homogeneous piezoelectric ceramic suspension with 40 vol% modified (K[Formula: see text]N[Formula: see text]NbO3 nanoparticles can be 3D printed well based on a stereolithography process. In addition, by the post heat treatment under different sintering temperature, a density value of 4.31[Formula: see text]g[Formula: see text]cm[Formula: see text] with the relative density of about 96% was obtained in the sample sintered at 1120[Formula: see text]C, which results in the larger dielectric constant and lower dielectric loss in this sample. These printed ceramics exhibit two successive phase transitions and reduce with increase of sintering temperature at measured temperature range.

Freeze cast porous membrane catalyst for hydrogen production via oxy-reforming

In this work, porous catalytic membrane with tailored microstructure has been developed for the production of hydrogen from the oxy-reforming process. Porous catalytic membranes have been produced by freeze-casting of ceria-based powders using camphene as solvent. The process parameters have been optimized in terms of type and amount of dispersant to obtain homogeneous and stable ceramic suspension suitable for the process, while the freezing temperature was chosen on the basis of the microstructure and porosity of the final samples. The obtained freeze cast sample produced showed very high levels of porosity (≈ 80%) and good gas permeability (1.0 × 10−11 m2). The ceria-based porous structure produced was finally evaluated as catalyst in high temperature oxy-reforming reaction for hydrogen production from methane showing high methane conversions (up to 90%) and hydrogen production. The use of a unique catalytic membrane other than pelleted catalysts allowed the obtaining of smooth thermal profiles without the evidence of endothermic or exothermic peaks.

Characterisation of mechanical and surface properties of novel biomimetic interpenetrating alumina-polycarbonate composite materials

ObjectiveTo determine the mechanical and surface characteristics of two novel biomimetic interpenetrating phase alumina-polycarbonate (Al2O3-PC) composite materials, comprising aligned honeycomb-like porous ceramic preforms infiltrated with polycarbonate polymer. MethodTwo composite materials were produced and characterised. Each comprised a porous structure with a ceramic-rich (polymer-poor) top layer, graduated through to a more porous ceramic-poor (polymer-rich) bottom layer. In addition, pure polycarbonate and dense alumina specimens were subjected to the same characterisation namely: density, compression, three-point bend, hardness, surface loss and surface roughness testing. Scanning electron microscopy and micro computerised tomography were employed for structural examination. ResultsThree-dimensional aligned honeycomb-like ceramic structures were produced and full interpenetration of the polymer phase was observed using MicroCT. Depending on the ceramic volume in the initial aqueous ceramic suspension, the density of the final interpenetrating composites ranged from 2.64 to 3.01g/cm3, compressive strength ranged from 192.43 to 274.91MPa, flexural strength from 105.54 to 148.47MPa, fracture toughness from 2.17 to 3.11MPa.m½, hardness from 0.82 to 1.52GPa, surface loss from 0.71 to 1.40μm and surface roughness, following tooth brushing, from 0.70 to 0.99μm. Composite specimens showed characteristic properties part way between enamel and polycarbonate. SignificanceThere was a correlation between the initial solid ceramic loading in the aqueous suspension, used to produce the porous ceramic scaffolds, and the subsequent characteristic properties of the composite materials. These novel composites show potential as aesthetic orthodontic bracket materials, as their properties fit part way between those of ceramic, enamel and polycarbonate.

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.

Suspension Temperature as a Rheological Control Parameter in Magnetic Separation

It is noted that the significant (but usually shadow) magnetic separation parameters of liquid-dispersion media (suspensions, including ceramic suspensions, for separating the disperse phase of ferro-particle impurities) are the dynamic viscosity and the density of the medium. The temperature dependences of the slip viscosity and the dispersion component of the slip (water) are compared; variants of their description in the working temperature intervals by the corresponding exponential and logarithmic functions which could be valid in other media also, including media used for other, nonindustrial, purposes, are presented. The form of such a function for slip is used in the base expressions for the efficiency and limiting rate of its filtration magnetic separation—with a convenient (in practice) validation of the slip temperature in these expressions.

Accurate and uniform fabrication of Li2TiO3 pebbles with high properties based on Stereolithography technology

Li2TiO3 is a vital candidate breeder to solve tritium self-consistency of fusion reaction. In this study, Digital Light Processing (DLP) based on Stereolithography technology was used to fabricate Li2TiO3 pebbles for the first time. Ceramic suspensions with different solid loadings were prepared by mixing modified Li2TiO3 powder with UV-curable premixture. The size error of Li2TiO3 green pebbles was corrected by adjusting the deformation factor of equatorial radius, and the Li2TiO3 green pebbles were successfully fabricated with an accurate size. After sintering processing, the Li2TiO3 pebbles with a uniform diameter of 1.5 mm and better sphericity of 1.01 were yielded. The sintering behavior of Li2TiO3 pebbles were investigated. Results showed that the Li2TiO3 pebbles formed with 35 vol% solid loading, and sintered at 1050 ℃ had a uniform pore distribution, and also had optimal properties, such as high relative density of 93.6 %TD, and prominent crush load of 92.3 N.

Cement free castable. Part 6. Comparative assessment of natural (clay) and artificial ceramic binders (HCBS)

A comparative assessment of artificial (based on highly concentrated ceramic binders suspensions - HCBS) and natural (based on clays and kaolins) ceramic binders of similar composition has been carried out. Analysis of their properties and characteristics shows both their significant similarity and significant difference. As with HCBS, the binding (strength) properties of clays and kaolins are determined by the content of colloidal particles and nanoparticles in them. In contrast to HCBS, clays and kaolins are characterized by a significant content of chemically bound water, a high water requirement of molding systems, and a sharp, different (by an order of magnitude and higher) shrinkage during drying. If binders based on clays are considered from the position of HCBS classification, then according to their chemical composition they can be classified as acid-amphoteric (like materials based on chamotte). However, in terms of chemically bound water, volumetric concentration of the suspension, they are closer to HCBS of amphoteric and basic compositions. The addition of refractory clays and kaolins is very effective in the production of ceramic concrete.

Multi-material additive manufacturing of functionally graded carbide ceramics via active, in-line mixing

Advanced ceramics are required in many applications including armor, engine components, and wear parts for abrasive, corrosive, and high temperature environments. Heterogeneous structuring in these materials has the potential to unlock extrinsic mechanisms that improve damage tolerance, of vital importance for structural functionality. However, traditional ceramic powder processing and forming techniques limit the design space to simple geometries with chemically homogenous microstructures. Thus, additive manufacturing by direct ink writing (DIW) was applied to fabricate multi-phase carbide specimens with tailored composition and mesostructure. A custom DIW system was developed to allow simultaneous extrusion and mixing of multiple inks, comprised of ceramic particulate suspensions, through a single nozzle. Boron carbide (B4C) and silicon carbide (SiC) were chosen for this study due to their excellent mechanical properties. Aqueous B4C and SiC inks were loaded to 47.5 vol% ceramic content and showed yield-pseudoplastic behavior. The carbide inks were characterized and modified to exhibit similar rheological behavior (yield stress and viscosity), and were used to produce B4C–SiC parts with either discrete or continuous composition variation. Specimens were hot pressed at 35 MPa and 1950 °C, yielding near full density with hardness (Knoop) values of 20–23 GPa. Tailored heterogeneity, achieved via active in-line mixing, is revealed through microstructural characterization. Cracking observed in the specimen with discretely varied composition is the result of thermally-induced residual stress, and is elucidated through analytical calculations.

Effect of Curing Protocol on Mechanical Behavior of Green Ceramic Bodies Fabricated in Ceramic Microstereolithography

The present work is based on investigation of mechanical behavior of green ceramic parts fabricated in scanning based ceramic microstereolithography (CM SL). This study is essential as many times green ceramic bodies fabricated by microstereolithography (MSL) can be directly incorporated in microelectromechanical systems (MEMS). These green parts are fabricated from ceramic suspensions obtained by dispersing ceramic particles in photocurable (meth)acrylate resins separately. As fabricated parts in CMSL are essentially polymer-ceramic composites whose strength mainly depends on cross link density. Nanoindentation and glass transition temperature derived from DSC were used to compare cross link density of green ceramic bodies fabricated from (meth)acrylate suspensions. Further, double bond conversion, which is indicative of cross link density, was verified by FTIR. These results suggested that there is significant influence of curing protocol on mechanical behavior of green ceramic parts

Preparation of γ-Al2O3/α-Al2O3 ceramic foams as catalyst carriers via the replica technique

This work describes an effective method for the preparation of open-cell ceramic foams for their further use as catalyst supports. The polyurethane sponge replica technique was applied using a ceramic suspension based on a mixture of α-alumina, magnesia and titania and polyvinyl alcohol solution as a liquid component. The polyurethane sponge was etched with NaOH and covered with colloidal silica to obtain better adhesion of the slurry to the walls of the polymeric material onto it. The surface area of the ceramic carrier was increased by adding a layer of γ-alumina. Deposition of an active catalytic phase (Pt) was done by impregnation. Properties of the carriers and the final catalyst were investigated by a number of physico-chemical methods such as TEM, SEM, XRD and computer tomography. Hydrogenation of ethyl benzoylformate was performed to elucidate the catalytic properties of foam catalysts illustrating their applicability.

Microstructure and gas permeation performance of porous silicon nitride ceramics with unidirectionally aligned channels

AbstractPorous Si3N4 ceramics with unidirectionally aligned channels were prepared via freezing ceramics suspension with distinct solid contents under different freezing temperatures. The samples obtained using lower solid content in ceramic suspension at higher freezing temperature exhibit larger Darcian and non‐Darcian constants due to their higher open porosity, larger pore size and lower tortuosity. Moreover the investigation on individual contributions of viscous energy losses and inertial energy losses on the total pressure drop during permeation process indicates that with decreasing the solid content or the freezing temperature the viscous energy losses increase but the inertial energy losses decrease for samples owing to the differences in their pore structures. It is worth mentioning that porous Si3N4 ceramics with unidirectionally aligned channels exhibit larger Darcian and non‐Darcian constants than those with similar pore size distributions and open porosity owing to their lower tortuosity, thus rendering them appropriate for filters and membrane supports.

CeO2 thin film supported on TiO2 porous ceramics

The work demonstrates an easy and simple method for obtaining a TiO2 porous ceramic coated with a CeO2 thin film (COTF) on the surface. Replica technique used a 30 PPI polyurethane sponge and TiO2 ceramic suspension. Dip coating technique was used to coat the material surface and cerium polymeric resin synthesis was obtained by polymeric precursors method. Thermal analysis indicated a thermostable material from 500 °C. X-ray diffraction analysis of calcined CeO2, at 600 to 900 °C, resulted in monophasic and crystalline with a fluorite type cubic structure. Scanning electron microscopy made it possible to monitor COTF formation and growth from islands shape to a more homogenous form over 10 depositions.

ИССЛЕДОВАНИЕ ИСКУССТВЕННЫХ КЕРАМИЧЕСКИХ ВЯЖУЩИХ НА ОСНОВЕ СИЛИКАТНЫХ МАТЕРИАЛОВ ДЛЯ ПРОИЗВОДСТВА КЕРАМИКИ

this article discusses the possibility of using artificial ceramic binders obtained by the technology of highly concentrated binder suspensions in the production of ceramic materials and products. The results of complex studies of suspensions of artificial ceramic binders based on feldspar and quartz sand, as well as the results of tests of the samples obtained on their basis are presented. A definite advantage of entering the suspensions in the molding compositions of the slurries used in the manufacture of certain varieties of pottery is proved. The solution of the problem of import substitution of raw materials components, caused mainly by the difficult political situation in the country, as well as the lack of its own raw material base of good quality, in connection with which the paper proposes the use of technology of highly concentrated binder suspensions, which allows to optimize the quality of domestic raw materials. The use of technology of highly concentrated binder suspensions, allows not only to expand the assortment and range of products produced by this technology, but also to improve the quality and economic feasibility of individual varieties of ceramics. The emphasis on the introduction of technology of highly concentrated binding suspensions in certain areas of ceramics production is explained by the formation of a completely different structure of materials due to the increased content of nanoparticles in the composition of artificial ceramic binding suspensions, which contributes to a significant increase in mechanical strength while optimizing other basic technological quality indicators for ceramic materials and products. The possibility of reducing the firing temperature by 50-100°C, however, this issue requires additional research.

Cellular SiC/Iron Alloy Composite

Cellular SiC/iron alloy composite with a spatial structure of mutually intersecting skeletons created with a porous ceramic preform has not been obtained before, despite promising spectrum of potential uses. We tested the possibility of obtaining such material using a SiC material with an oxynitride bonding and grey cast iron. Porous ceramic preforms were made by pouring the gelling ceramic suspension over a foamed polymer base which was next fired. The obtained samples of materials were subjected to macroscopic and microscopic observations as well as investigations into the chemical composition in microareas. It was found that the minimum width of a channel in the preform, which in the case of pressureless infiltration enables molten cast iron penetration, ranges from 0.10 to 0.17 mm. It was also found that the ceramic material applied was characterized by good metal wettability. Were the channels are wide enough for the metal penetration we observed that the ceramics/metal contact area always has a transition zone in which mixing of both components takes place.

COMPLEX DEFLOCCULENT BASED ON SODA, LIQUID GLASS AND OXYETHYLIDENEDIPHOSPHONIC ACID IN CERAMIC CASTING TECHNOLOGY

Here are discussed some results to use oxyethylidenediphosphonic acid (OEDPA) and various additives as a deflocculant composition for controlling the rheological properties of a ceramic suspension. According to the results of determining thixotropy, it was found that a complex thinner based on soda and OEDPA is the most effective. It is desirable to replace a portion of the soda with liquid glass. By the full factorial experiment method, it was established the optimal composition of the components of the complex thinner. The proposed thinner is highly effective in a wide range of OEDPA, soda, and liquid glass concentrations. The resulting suspensions have stable, high structural–mechanical properties. A mathematical model describing the effect of the composition of the thinner on the viscosity and thixotropy of the suspension is presented. The effect of the diluent on the mechanical properties of the molded dried and fired samples was determined. The addition of OEDPA to the thinner significantly lowers the rheological parameters and the rheological properties of the suspensions approach the rheological properties of Newtonian systems. It was found that OEDPA lowers the mass buildup rate, which can be increased by lowering the moisture content in the suspension, which makes it possible to increase the post-casting and -drying density of the samples. The introduction of this additive leads to an increase in the hydrate shell around the clayey particle and an increase in the stability of the suspension, but the increase in the ζ - potential in comparison with the production additive is very small. The mechanism of action of a complex additive is proposed, including ion-exchange, complexation and chemisorption. An increase in the strength and density of dry and calcinated clay samples as well as a decrease in the shrinkage and porosity are observed. The complex additive makes it possible to optimize the technology at the stages of casting slip and to decrease the number of parts rejected during molding, drying and firing.

Effect of graphite particles as additive on the curing behaviour of β-tricalcium phosphate suspensions and scaffold fabrication by digital light processing

Reducing the viscosity of high solid-loading ceramic suspensions and controlling the resolution of ceramic green parts produced by digital light processing (DLP) 3D printing are two important concerns in the ceramic additive manufacturing industry. The presence of ceramic particles within a photopolymerizable system leads to light scattering that reduces the resolution of the ceramic green scaffolds. This study introduced a graphite additive to solve these problems and focused on the effects of graphite concentration on the viscosity, curing behaviour and scaffold fabrication of β-TCP ceramic suspensions. As a result, it was found that an appropriate addition of graphite reduced the viscosity of the ceramic suspensions, and the light scattering decreased with the increase of graphite concentration. Both the cure depth (Cd) and excess width (Cex) also decreased with the increase of graphite concentration. But, graphite has a larger effect on the width curing than depth curing.

Thermal behaviour of vitreous ceramic coatings obtained by electrophoretic deposition for furnace components

In this study, three different vitreous ceramic coatings have been designed to improve radiation heat transfer and thereby increase the thermal efficiency of fired heaters or furnaces working at high temperatures. The vitreous ceramic coatings were produced through Electrophoretic Deposition technique (EPD) of ceramic suspensions. These ceramic formulations were designed based on components which increase emissivity, such as SiO2 and a Black dye (based on chromium, copper and iron oxides), added in 25 wt%. These coatings showed emissivity values around 0.89 at room temperature and around 0.82 at 550 °C in the middle infrared (MIR) spectral range, with slight differences between them. The SiO2 and Black dye additions provide an important protective effect on the coatings’ thermal stability as it was proved by the absorbance level at long times, higher than 85% in the near infrared (NIR) spectral range. These results were also supported by microstructural characterisation, substrate-coatings adhesion strength and thermal stability tests.

Novel freeze-casting device with high precision thermoelectric temperature control for dynamic freezing conditions.

A novel freeze-casting device utilizing a thermoelectric element for high precision temperature control allowing for dynamic freezing conditions of freeze-cast materials is presented. Freeze-casting is a processing route for producing materials of anisotropic porosity in the form of aligned and well-defined microchannels. In freeze-casting, particulates of a material are suspended in a fluid and a thermal gradient is applied across for directional freezing. Controlling the thermal gradient across the suspension amounts to controlling the kinetics and freezing direction in the suspension and thus the resulting structural features and dimensions of the microchannels. The performance of the device presented here was evaluated by directional freezing of both water and aqueous ceramic suspension samples using both linear and exponential freezing profiles. The freezing front was successfully tracked by continuously measuring the temperature gradient along the sample using thermocouples directly mounted on the freeze-casting mold. The current minimum operational temperature of the freeze-caster is ∼220 K, with freezing front velocities in the range of ∼5 μm/s to 30 μm/s for sample lengths of 5 mm-25 mm.

Фазовый состав, структура и некоторые свойства материалов на основе ВКВС боксита композиционного состава в системе Al2O3 – SiO2 – SiC

The effect of temperature and duration of heat treatment of samples based on a highly concentrated ceramic binder suspension (HCBS) of a composite composition (bauxite + 11% VDKS), as well as with an additional content of 15% silicon carbide on their phase composition, structure, and some properties is studied. Long-term heat treatment (60 and 120 hours) in the temperature range 1300 - 1400 ° C is carried out in a tunnel kiln for firing dinas. In the process of heat treatment, the process of formation of secondary mullite, the oxidation of SiC followed by mullite formation, proceeds. The influence of the heat treatment duration on the phase composition, as well as the strength, apparent density, growth, SIC oxidation state, and bending strength of samples containing 15% SiC are characterized. It is found that the minimum porosity value (2.7 %) is observed when the heat treatment duration is 8 hours. This drop in porosity and a slight decrease in density is due to a certain degree of mullitization or SiC oxidation, but also to the formation of a significant volume of closed porosity. With increasing the duration of the firing process to 60 and 120 hours, the porosity indicators increase significantly, and the density decreases sharply.