Ceramic Green Bodies Research Articles

Electrophoretic deposition of yttria-stabilised zirconia powder from aqueous suspensions for fabricating ceramics with channel-like pores

Abstract Electrophoretic deposition with simultaneous gas bubble formation by electrolysis can be used for producing ceramic green bodies, typically few millimetres in thickness, with unidirectionally aligned channel-like pores. The method is successfully applied to yttria-stabilised zirconia. Two types of aqueous suspension compositions are investigated. Suspensions with acetic acid additions are particularly suitable for forming green bodies with fine pore channels. Only small amounts of acetic acid, promoting the gas evolution, are needed for this purpose. Dissolution of yttria in the acidic range has to be considered, but the required low acid concentrations do not measurably affect the yttrium content of the deposits. Yttria dissolution can be minimised by a suspension composition containing an anionic polyelectrolyte and ammonia instead of acetic acid. The ammonia concentration influences the size of the tubular pores of the deposits formed under constant-voltage conditions. Using structured deposition electrodes, the regularity of the pore arrangement can be enhanced.

Analytic Model for the Diffusant Concentration in Ceramic Green Bodies During Diffusion-Controlled Thermal Binder Removal

The diffusion of the decomposition product of binder degradation has been modeled using the diffusion equation when a source term is present. An analytic solution to the governing nonlinear, unsteady-state partial differential equations has been obtained for a planar body with two simplifications. The first approximation utilizes the pseudo-steady-state approximation whereas the second approximates the diffusivity, as given by the free volume theory, in terms of a single temperature-dependent parameter. The analytic solution, which was compared to numerical solutions to establish the range of model validity, accurately describes the temporal and spatial distribution of the binder decomposition product during most of the heating cycle, especially when the concentration in the green body is the highest. The functional dependence of concentration within the body is established in terms of model parameters including the body size, degree of nonlinearity of the diffusivity, and the ratio of reaction rate to diffusivity.

Investigation of Glaze Layer Formation on Green Ceramic Bodies

Using a high-speed and high-resolution-camera system of droplets in glaze sprays and their impact on the surface of a green ceramic body based on silicate tiles have been characterized. The influence of the surface tension of the glaze slips on the droplet size distribution and their wetting behavior on contact with the green silicate body surface have been investigated. The unfired glaze layers have been studied by means of scanning electron microscopy and computer tomography. The impact angle of the droplets as well as the size of the droplets is of great importance for a high glaze surface quality.

Preparation and characterization of nanodispersions of yttria, yttrium aluminium garnet and lutetium aluminium garnet

Refractory and chemically inert nanoparticulate solids like yttria, yttrium aluminium garnet or lutetium aluminium garnet are notoriously difficult to disperse in aqueous solution, although such dispersions might prove useful for 2D- and 3D-printing, deposition of films or other shaping procedures for ceramic green bodies. This work reports on experiments to prepare such dispersions from nanopowders, using a range of selected carboxylic acids as ligands for electrostatic charging and stabilization of the various nanomaterials. The assessment of the system properties achieved combines ζ-potential and viscosity (flow curve) measurements in the resulting colloidal systems. Calculations of the molecular electrostatic potential of the ligands were used to correlate the molecular structure of the ligands and their capability to dock to the surface of the solid particles. The colloid chemical data received have then been interpreted on this background; citric acid and polyacrylic acid have been identified as the most attractive dispersants.

Producing Large Complex‐Shaped Ceramic Particle Stabilized Foams

Highly porous ceramic foams can be produced by combining particle stabilized foams and gelcasting concepts. Sulfonate‐type surfactants are selected to weakly hydrophobize the alumina surface and stabilize air bubbles in suspensions containing gelcasting additives, polyvinyl alcohol (PVA), and 2,5‐dimethoxy‐2,5‐dihydrofurane (DHF). The aim of this work was to prepare large complex‐shaped ceramic foam objects with homogeneous microstructure and high porosity. A key to avoiding drying cracks is to strengthen the wet green body via gelcasting. The influence of the amount of gelcasting additives on the mechanical strength of the ceramic foam green bodies is investigated as well as the effect of using cross‐linking agent versus the addition of just a binder. The presence of a cross‐linked polymeric network within the green body increases its mechanical strength and minimizes crack formation during drying.

Thermally induced gelation of alumina shaping-neutron scattering and rheological measurements

Thermally induced gelation forming based on methylcellulose is recently being explored as a simple and environmentally benign process. Alumina slurry containing 0.1 wt% methylcellulose is subjected to Quasi Elastic Neutron Scattering (QENS) and rheological measurements in gelation temperature regimes to evolve a possible mechanism of the forming process. A reduction in diffusivity of water in the slurry from 2.16 to 1.92 × 10-5 cm 2 ·s-1after exposure to 55°C is observed with QENS. This is found to be well correlated with a steep increase in viscosity from 1.2 Pa.s till 50°C to 50,000 Pa.s at 55°C. QENS studies revealed the diffusion of water occurs by jump diffusion with the jump lengths distributed randomly. Further, for the entire sample much longer residence time is found as compared to bulk water, which is due to hydrophilic interaction of water molecules with the methylcellulose in the slurry. Reduction in diffusivity of water along with the steep increase in viscosity could be understood as the strong, cross-linked polymer-solvent irreversible gel formation in presence of alumina which is responsible for the retention of a consolidated shape of the ceramic green body. Samples maintained the integrity while heat treatments achieving close to theoretical density values of3.98 g·cm-3 at 1550°C.

Fabrication of Integral Core/Shell Ceramic Casting Mould for Hollow Turbine Blade

The paper presents an integral core/shell fabrication of ceramic casting mould for hollow turbine blades by combining SL (Stereolithography) with gelcasting. This method could guarantee the positional accuracy between ceramic cores and shell, thus achieving the rapid fabrication of complex turbine blade castings. The paper focuses on the design of resin mould for gelcasting, the preparation of ceramic slurry, the pyrolysis of resin prototype and the sintering of ceramic green body. The feasibility and effectiveness of above mentioned method were verified through successful manufacturing of hollow turbine blades with double-walled structure.

Application of hydrophilic ionic liquid treatment to the morphological observations of hydrated porous ceramic green bodies

In this study, a simple and convenient method for observing the surface morphology of hydrated porous ceramic green bodies is proposed. The porous hyrdoxyapatite (HAp) green body was prepared by a gelcasting process and was dried in a humid chamber from 90 to 50% relative humidity at 25°C before subsequent treatment with a hydrophilic ionic liquid (IL). The surface morphology of the IL-treated porous HAp green body was observed using FE-SEM. The results showed that the pore morphology and microstructure of the HAp green body was readily observable without evidence of charging. The as-prepared sample showed pores approximately 300–600μm in diameter, which gradually contracted to approximately 200–400μm upon drying in the humid chamber. Following sintering at 1000°C, the pores had further contracted to approximately 100–300μm. The IL binds with the surrounding water to prevent the sample from drying in vacuum and acts as a conductive media, allowing the HAp ceramics to be observed in the electron microscope. In comparison to the micro-focused X-ray CT analysis, the fine pore structure (less than 100μm) could only be observed using FE-SEM when the porous body had also been subjected to the IL treatment.

Evolution of throttle-channel dual pores in YSZ ceramic monolith through in situ grown nano carbon wedges

Abstract Interconnectivity of the pore channels and their size distribution in a ceramic monolith influence the flow dynamics of permeating fluid. This work proposes an innovative pore-forming mechanism that begins with the growth of poly(p-phenylene terephthamide) (PPTA) in a ceramic green body of yttrium-doped zirconia (YSZ). The spatial confinement drove the formation of PPTA through an extrusion manner, resulting in submicron PPTA rods embedded in the matrix. Theses rods were carbonized under Ar between 800 and 1200 °C for 2 h before oxidizing atmosphere was introduced to facilitate their removal and sintering of YSZ. This dwelling temperature is crucial to the pore size distribution in the sintered ceramic due to the chemical and dimensional changes of the carbon wedges and YSZ grains. The sintered ceramic is characterized by interconnected throttle-to-channel pore structure, which manifested a marked improvement on fluid permeability and a capability of stretching viscous fluids passing through it.

Effect of Roasting Process on the Properties of Alumina-Based Ceramic Core

The roasting process of leachable alumina based ceramic core has been systematically studied in this paper, the results show that, the roasting process should be divided into two parts, i.e., dewaxing process and sintering process, and the plasticizer can be removed completely from the ceramic green body by the dewaxing process with little residue left behind. The sintering temperature and mineralizer are the decisive factors to ensure the strength of ceramic core. The apparent porosity of ceramic core will decrease obviously while the room temperature flexural strength will increase when the sintering temperature is rising. The room temperature flexural strength can exceed 14MPa when the ceramic core samples sintered at 1550°C. The ceramic core samples will present excellent comprehensive properties when the sintering temperature is 1550°C and the added quantity of SiO2 is 8~10%, i.e., apparent porosity is 44.7~46.9%, bulk density is 1.93~1.99 g/cm3, room temperature flexural strength is 10.65~11.93MPa.

Preparation and Performance of Ni-YSZ/YSZ Co-Ceramic Using Multiple Gel-Tape Casting

Use starch, corn meal and activated carbon as pore-making agent, Ni-YSZ/YSZ composite ceramic green body was prepared by multiple gel-tape casting with acryloyl ammonium-polyvinyl alcohol. The specimen was sintered at 1450°C for 4h, then reduced in hydrogen atmosphere at 800°C for 4h, Ni-YSZ/YSZ composite ceramic disk was obtained, the porosity, thermal expansion coefficient, electrical properties and microstructures of the specimen was tested. The results showed that the porosity of Ni-YSZ/YSZ composite ceramic disk reaches about 30% when use starch as pore-making agent, expansion coefficient is proportional to porosity, both of them reduced with the temperature increased. The YSZ is continued and compact with network structure, Ni particulates are well distributed in it, suitable for intermediate temperature solid oxide fuel cell anode materials. Image analysis of SEM micrograph showed that the structure is loose, porous, uniform and compact in Ni/YSZ/YSZ multi-layer composite ceramics, which meet the requirement of intermediate temperature SOFC.

Dielectric characteristic of nanocrystalline Na0.5K0.5NbO3 ceramic green body

Dielectric spectroscopy was applied to porous nanocrystalline Na0.5K0.5NbO3 (NKN) ceramic green body, wherein influences of percolation effect and water adsorption at pore surface of the ceramic green body on dielectric response were examined over wide temperature (150 to 450 K) and frequency (100 Hz to 1 MHz) ranges. Dielectric permittivity of the ceramic green body is about 2–3 orders of magnitude higher than that of pure NKN powder or NKN ceramic. Furthermore, the high dielectric permittivity and high humidity sensitivity of the ceramic green body can appear again with aging a period of time in air. The data from this investigation make potential applications for NKN as a giant dielectric material or a humidity sensing material.

Rapid sintering of crack-free zirconia ceramics by pressure-less spark plasma sintering

Heating of a ceramic green body is a key step in sintering. We have created inside a spark plasma sintering apparatus pressure-less sintering conditions that allow homogeneous and extremely rapid heating. Dense and crack-free zirconia ceramic was sintered at heating rates of up to 500 °C min -1 and dwell times of 2 min. This extremely fast sintering process is accompanied by extremely rapid grain growth, indicating a non classical sintering mechanism. No grain size gradients were observed inside the sintered zirconia ceramics.

Sintering Procedure of Calcium Hexaluminate Ceramics

Thermal expansion behavior of ceramic green bodies could be used to determine appropriate sintering procedure for ceramic materials. In this paper, green bodies containing alumina and calcium hydroxide was prepared by slip casting and their thermal expansion behavior was measured. Thermal expansion curve indicated that calcium aluminate (CaO•Al2O3, CA) and calcium dialuminate (CaO•2Al2O3, CA2) are yielded by reaction sintering as intermediate phase compounds in green samples at temperature about 1160°C and 1240°C-1410°C, respectively; and calcium hexaluminate (CaO•6Al2O3, CA6) is yielded as target product at 1450°C-1560°C. Controlled sintering procedure was determined according to thermal expansion curve of green samples. Analysis result to properties of sintered samples indicated that thermal expansion behavior revealed to be a very simple and powerful tool to determine reaction sintering procedure of calcium hexaluminate ceramics.

New laser machining technology of Al2O3 ceramic with complex shape

A new method was developed for the fabrication of complex-shaped Al2O3 ceramic parts by combining laser machining and gelcasting technique. The unwanted ceramic powders parts were selectively removed by laser machining specified by a computer program, and the gelcast Al2O3 green bodies were machined to a designed shape by a CO2 laser at a wavelength of 10.6μm. The influences of solid loading, laser output power, scanning speed and nitrogen purge on the machining of green Al2O3 ceramic bodies were studied. The experimental parameters were optimized, the green Al2O3 bodies with solid loading of 40vol% or below were easier to be machined, while the green bodies with solid loading of 45vol% or above were hard to be further machined due to the surface sintering. Better machining quality and deeper machining depth could be obtained when the laser power is 30W. The green Al2O3 bodies with complex shape were obtained by the laser machining.

Effect of Glass Fiber Modification on Tensile Strength and Shrinkage of Ceramic Mould

Aiming to improve the crack-resistance of the mould of ceramic mould casting, glass fibers whose average diameter was approximately 16 µm, length 1mm were dispersed, and the effect of the glass fibers on the tensile strength and shrinkage rate of the ceramic mould was investigated in this study. In the ceramic green body, quartz sand was selected as refractory, silica sol was binder. The results show that, with increasing glass fiber, the tensile strength of the ceramic mould increased linearly from 0.175 MPa to 0.221 MPa at the same sintering temperature, and the shrinkage rate fell from 1.37% to 1.33% at room temperature. With the sintering temperature rising, the tensile strength increased and the shrinkage decreased by the glass fiber dispersion, but their variation rule were not changed. The investigation proposed a new method to improve the crack-resistance of the ceramic mould, i.e., glass fiber dispersion into the ceramic mould.

Micro machining of pre-sintered ceramic green body

Ceramic materials are widely used in industrial products because of their excellent properties such as resistance to erosion, high temperature and wear. Those properties pose difficulty in processing ceramic materials to desired shapes and dimensions, especially in micro machining of ceramics using micro tools because of the low mechanical strength of tiny tools and the extreme hardness of ceramics. The machining of ceramic green body may be a solution to the problem. However, the low strength of ceramic green body results in low machining accuracy such as damage at the edges of micro features. In this paper, the pre-sintering of ceramic materials was used to enhance the material strength, and hence to improve the machining accuracy. Microstructures of pre-sintered ceramic green bodies at different temperatures were observed and analyzed. The mechanical strength of pre-sintered ceramics was measured. It was confirmed that the mechanical strength of ceramics increased with an increase of pre-sintering temperature. There was no obvious shrinkage after pre-sintering at below 850 °C. The edge damage of machined micro features was reduced significantly. The tool wear increased with the increase of pre-sintering temperature. Finally, a 3D micro feature was machined and the fully sintering shrinkage was compensated accurately.

Rheology of aqueous mullite–starch suspensions

One of the forming methods developed for the manufacture of porous materials by direct consolidation, in which a ceramic suspension consolidates into non-porous molds (e.g. metal molds) by thermogelation of an organic agent, uses starch as both consolidator/binder of the ceramic suspension and pore former at high temperature. Changes in the rheological behavior of the aqueous suspensions are produced by starch gelatinization thermal process. This process as well as the presence of both the ceramic particles and added processing additives, influences the kinetics of green ceramic body formation and its microstructural features. In this work, the thermogelling behavior of mullite aqueous suspensions (40 vol.%; 0.45 wt.% of a polyacrylic polyelectrolyte as dispersant) containing 10 vol.% of different native starches (potato, cassava, and corn) was studied by dynamic rheology in order to determine the experimental conditions that must be used for forming mullite green bodies by thermal consolidation. Viscoelastic properties ( G′ and G″) as a function of temperature (30–95 °C) and deformation (0.1–625.0% at 40 °C) were determined by temperature sweep tests and dynamic strain sweep tests, respectively. From these tests, and considering previous results of the rheological behavior of starch suspensions, we analyzed the influence of ceramic particles on the starch gelatinization process and the strength of the developed gels. On the other hand, shear flow properties of aqueous mullite–starch suspensions were also analyzed to obtain information on the rheological behavior of the suspensions at room temperature.

Analysis of the elastic properties of an interpenetrating AlSi12–Al 2O 3 composite using ultrasound phase spectroscopy

An interpenetrating composite fabricated by squeeze-casting a eutectic aluminium–silicon alloy into a porous alumina preform is studied in this work. The preform was fabricated by pyrolysis of cellulose fibres used as pore forming agent, pressing of the green ceramic body and subsequent sintering of alumina particles. The resulting preform had both micropores within the ceramic walls and macropores between those walls, which were infiltrated by the liquid metal. Composites with alumina contents varied in the range of 18–65 vol.% were studied. Three longitudinal and three shear elastic constants of the composites were determined using ultrasound phase spectroscopy on rectangular parallelepiped samples. Complete stiffness matrix of one sample was determined by modifying the sample geometry by cutting at the corners of the sample and subsequent ultrasonic measurements. All composites exhibit a moderately anisotropic behavior, which can be attributed to a non-random pore orientation distribution caused by uni-axial pressing of the preforms prior to sintering. The experimental results are compared with several theoretical micromechanical models.

Recent developments in gelcasting of ceramics

Gelcasting is a well-established colloidal processing method with a short forming time, high yields, high green capacity and low-cost machining, and has been used to prepare high-quality and complex-shaped dense/porous ceramic parts. In this article, we reviewed recent developments in gelcasting technology for ceramic preparation. For environmental pollution reduction during ceramic preparation by gelcasting, the development of low-toxic and nontoxic gelcasting systems is discussed. The occurrence and control of inner stress and surface-exfoliation in ceramic green bodies prepared by gelcasting are analyzed, and then some methods to control and eliminate the inner stress and surface-exfoliation in ceramic gelcast green bodies, especially for colloidal injection molding of ceramics (CIMC) are proposed. Finally, the applications of gelcasting for the fabrication of porous ceramics and complex-shaped ceramics (e.g., microbeads, rutile capacitor, thin-wall rutile tube, refractory nozzle) are summarized.