Cellulose acetate based gelcasting process for Gd-containing ceramic bodies

A new route of gelcasting is applied to prepare Gd₂O₃ and Gd₂O₂S as well as Al₂O₃ ceramic parts, i.e. thin plates and fibres. Cellulose acetate is used as monomer. Instead of a water-based solvent system, ethylenediamine as solvent is employed; therefore, moisture- and air-sensitive ceramic powders can be processed, such as Gd₂O₂S. The presented work focuses on the characterisation of the slurry by rheological and chemical measurements, the polymerisation process and the first results of the fabrication of ceramic bodies. Both green and sintered parts were investigated using electron microscopy, X-ray diffractometry and physisorption.     

Microstructure and properties of aluminum nitride ceramics prepared by aqueous gelcasting method with nontoxic curdlan as gelling agent

Aluminum nitride ceramics were prepared by aqueous gelcasting method and pressureless sintering technique in N₂ atmosphere using Y₂O₃ as sintering additives with nontoxic curdlan as gel system. The solidification mechanism of curdlan was studied. The effects of curdlan content and solid content on the microstructure, relative density and flexural strength of green bodies were investigated. The influences of Y₂O₃ content and sintering soaking time on the microstructure and properties of sintered bodies were also studied. The results show that, as the temperature increases to 80 °C, the ceramic powders solidify through three-dimensional gel networks of curdlan during gelling process. The green bodies can be successfully fabricated through aqueous gelcasting method with modified powder as original materials. Suitable curdlan content and solid content contribute to fabricating green body with uniform microstructures and high flexural strength. The relative density and flexural strength of sintered bodies enhance as the Y₂O₃ content and soaking time increase. The flexural strength and thermal conductivity are about 107.5∼172.3 MPa and 75.2∼112.5 W/(m·K), respectively. The sintered body with 4 wt% Y₂O₃ soaking for 3 h exhibits the highest thermal conductivity because of appropriate relative density, uniform microstructure and reasonable intergranular phase distribution. The mechanical property and thermal conductivity of sintered bodies can be improved by optimizing the gelcasting process parameter, Y₂O₃ content, and soaking time. The nontoxic gelling system will have wide application for aqueous gelcasting ceramic with complex shape.     

Fabrication of textured ferroelectric ceramics by magnetic alignment via gelcasting

Grain-oriented ferroelectric ceramics have attracted more interest recently because they may provide near single crystal properties. In the present study, a novel process combining magnetic alignment and gelcasting was explored to prepare grain-oriented ferroelectric ceramics with different crystal structures. In a strong magnetic field, ceramic particles in slurry were aligned by the magnetic force and then locked in situ by polymerization via a gelcasting technique. This process was found effective for ferroelectric ceramics with a bismuth layer structure (Bi₄Ti₃O₁₂) and tungsten bronze structure (Sr_0.5Ba_0.5Nb₂O₆). The sintered samples show highly anisotropic structure and enhanced physical properties. However for perovskite structured ferroelectric ceramics (BaTiO₃), the green compact shows grain orientation, while after sintering the sample become random again.Thus for certain materials using the conventional ceramic processes, i.e., using conventional starting powders, gelcasting under strong magnetic fields (10 T) and pressure-less sintering, the preparation of dense grain-oriented ceramic materials is possible.     

Fabrication and microstructure of porous SiC ceramics with Al2O3 and CeO2 as sintering additives

In the present study, porous silicon carbide ceramics were prepared via spark plasma sintering at relatively low temperatures using Al₂O₃ and CeO₂ as sintering additives. Sacrificial template was selected as the pore forming mechanism, and gelcasting was used to fix the slurry in a short time. The evolution process of the microstructures during different steps was observed by SEM. The influence of the sintering temperature and sintering additives on the shrinkage and porosity of the samples was studied. The microstructures of different samples were characterized, and the mechanical properties were also evaluated.     

A novel method for preparation of interconnected pore-gradient ceramic foams by gelcasting

A novel approach was introduced to prepare pore-gradient Al₂O₃ ceramic foams with association of gelcasting process and polymer sphere template. This approach allows the design of pore connectivity and gradient height of ceramic foams by the appropriate selection of sphere sizes and numbers. The limitation of ceramic foams fabricated by polymeric sponge process on sponge carrier can be resolved by this approach. Epispastic polystyrene (EPS) spheres with different sizes were employed to array ordered templates. Influence of solid content and dispersant on the viscosity of Al₂O₃ slurry was studied. EPS spheres modified by oxygen plasma to increase the hydrophilicity of surfaces and influence of pre-removal of EPS template on the integrity of networks were investigated. Results showed that 55?vol.% Al₂O₃ slurries with 0.5?wt% dispersant kept good fluidity for casting and permeability in template. Water contact angle of EPS surface decreased from 95.2° to 20°. Some defects in green bodies such as edge-delamination and micropores disappeared after surface modification. The perfect structure and morphology of the ceramic foams were ascribed to the pre-removal of template by solvent. The hierarchical pore structure was fabricated with EPS spheres of 2.1, 2.6 and 3.2?mm diameters. Porosity and compressive strength of the pore-gradient Al₂O₃ ceramic foam were 68.5% and 3.06?MPa at 1,500?°C, respectively.     

Gelcasting of alumina based ceramic cores containing yttria for single crystal and directional solidification blades

Abstract

Ceramic cores play an essential role in investment casting. In this paper, gelcasting process has been successfully employed to fabricate alumina based ceramic cores containing yttria for single crystal and directional solidification blades. Based on an investigation of the formability of different ceramic slurries, material compositions of ceramic cores are determined by experiments. A proper sintering process is developed to get low sintered shrinkage, high apparent porosity and high room temperature flexural strength. The high temperature properties of ceramic cores are improved by dipping in water based yttria sol and resintering. The test results show that comprehensive properties of alumina based ceramic cores containing yttria fabricated by gelcasting are better than those of AC-1 ceramic cores made by the Beijing Institute of Aeronautical Materials, China, and that the ceramic cores can be applied to single crystal and directional solidification blade casting.     

Fractions design of irregular particles in suspensions for the fabrication of multiscale ceramic components by gelcasting

A new approach for the preparation of suspensions with a high solid loading and low viscosity by using irregular particles was proposed. These suspensions were prepared for the fabrication of multiscale ceramic components by gelcasting. Based on the Funk-Dinger function and fractal theory, the closest packing theory was applied to optimize the volume fractions of different particles. The maximum solid loading of slurries prepared for gelcasting was 62 vol%, and the viscosity at a shear rate of 100 s^?1 was only 0.29 Pa s. By as-prepared suspensions, a decimeter-scale ceramic part with submillimeter features was fabricated successfully by gelcasting, which verify the feasibility of the proposed method.     

Studies of drying and sintering characteristics of gelcast BaTiO3-based ceramic parts

Drying green gelcast parts is an essential step in the gelcasting manufacturing process. In this work, the liquid desiccant method was used for drying of BaTiO₃-based semiconducting ceramic gelcast parts. The results show that the loading level of ceramic powders and the liquid desiccant concentration significantly effect the drying process and the sintering characteristics of the ceramic parts. Lowering the loading level of ceramic powders and increasing the concentration of the liquid desiccant, non-uniform and differential drying in various regions due to great solvent gradient, induces structural and residual stresses which cause defects, such as cracking, bending and other malformations, which make the articles useless during the drying process and sintering procedure. However, when the solid loading of green gelcast parts is increased to more than 45 vol.%, the stresses developed during drying can be greatly reduced, and a higher concentration of the liquid desiccant can be used without inducing defects in the drying process and defect free ceramic with a smooth surface can be obtained. Moreover, the effects of loading level of ceramic powders and thickness of parts on the density of ceramic parts were studied. Higher solid content in the gel, and lower thickness of parts, increase the density of ceramics.     

Laser-quality Tm:(Lu0.8Sc0.2)2O3 mixed sesquioxide ceramics shaped by gelcasting of well-dispersed nanopowders

Tm³⁺-doped mixed sesquioxide transparent ceramics are attractive candidates for the generation of robust ~2.1 μm lasers. In this paper, laser-quality Tm:(Lu_0.8Sc_0.2)₂O₃ mixed sesquioxide ceramics were shaped for the first time by gelcasting of well-dispersed nanopowders, which were obtained using a modified coprecipitation method. The dispersibility of starting nanopowders was largely improved using alcohol-water solvent. The rheological properties of slurries were optimized for gelcasting. We also investigated the densification behavior of the gel-casted green compacts. In contrast to the dry-pressing route, it was found that gelcasting could yield more homogeneous and transparent ceramics. The optical in-line transmittance of the ceramic rod 12 mm in length was as high as 80.3% at 2090 nm. Upon pumping the ceramic rod by 796 nm diode laser, a 1.88 W CW laser at 2090 nm was acquired with a slope efficiency of 24.6% (with respect to the input pump power).     

Synthesis,characterization and ceramization of a novel vinyl-rich liquid precursor for Si(O)C ceramic

Polymethylsilane (PMS) was partially modified with 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane ([CH₃(CH₂åCH)SiO]₄, D4Vi) via conventional hydrosilylation. The as-synthesized vinyl-rich liquid precursor (V-PMS) was characterized by the viscosity test, gel-permeation chromatography, Fourier-transform infrared spectroscopy, nuclear magnetic resonances spectroscopy. The results indicate that the obtained precursor is well soluble in common solvents and exhibits a controllable viscosity of 326.9–714.6 mPa s at room temperature. The thermal properties of V-PMS were investigated by differential scanning calorimetry and thermogravimetric analysis. The V-PMS can be cured readily at 150 °C in inert atomosphere. The ceramic yield of V-PMS reaches 81% at 1200 °C, 38% higher than that of PMS. The final pyrolytic residue is hard, dense monolithic up to 1400 °C under Ar atmospheres. The controllable viscosity, excellent thermal curability and high ceramic yield enable the liquid precursor a promising material to shape various Si(O)C ceramic materials for high-temperature application.     

Gelcasting of alumina foams using agarose solutions

Aqueous gelcasting of dense or cellular ceramics by using biopolymers as gel-formers, instead of monomers, is a promising technology mainly in terms of environmental aspects. The main difficulty of using biopolymer solutions in processing of cellular ceramics by foaming method is their high viscosity, which prevents the foaming capacity of the ceramic suspension. In this work, the procedure for preparing concentrated agarose solutions (4 wt.%) by dissolving under overpressure conditions was evaluated for the gelcasting of alumina foams, and the rheological behaviour of alumina suspensions containing agarose was studied. The viscosity of the gelling solution obtained under overpressure conditions was lower than that prepared by simply heating at 90 °C, thus providing high foaming capacity of the alumina suspensions and consequently manufacturing of highly porous ceramics (86–90%). The microstructure of alumina foams was typically composed of approximately spherical cells interconnected by circular windows. The use of different agarose concentrations in alumina suspensions effected the rheological conditions, which resulted in changes in the pore and window sizes of the resulting ceramics. Depending on agarose concentration (0.50–1.0 wt.% on a dry solids basis) in the starting (35 vol.%) alumina slurry, the mean pore size ranged from 529 to 375 μm, while the mean window size varied from 113 to 77 μm.     

Rheological examination of C60 in low density solutions

Fullerene (C₆₀) in solutions of decahydronaphthalene (decalin) and a petroleum solvent viscous standard (PSVS) were studied to understand the rheological properties of fullerene-laden solutions. Although fullerene solubility limits have been published for a variety of solvents, little has been reported on the effect that fullerenes have on flow properties of fluids. In this study, the solvents were studied up to the point of saturation, whereby measurements of solubility, density, viscosity and elasticity were conducted varying the concentration level of C₆₀. Rheological measurements based on molecular interactions and on distortion of the flow were studied. No significant elastic contribution from the fullerenes resulted for the solutions below saturation. A pseudoplastic behavior with a lubrication effect imparted by the C₆₀ molecules was observed in the decalin solutions at concentrations below the saturation level. The PSVS solutions remain Newtonian for all C₆₀ concentrations while leading to an increase in viscosity.     

Evaluation of microwave processed glass–ceramic coating on nimonic superalloy substrate

MgO–Al₂O₃–TiO₂ based glass–ceramic coatings were formed on nimonic superalloy substrates by microwave and conventional heat treatment processes. The resultant glass–ceramic coatings were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), image analysis, nanohardness and Young's modulus evaluation by depth sensitive indentation (DSI) technique. Nanohardness and Young's modulus values of the microwave heat treated glass–ceramic coatings were improved in comparison to those of the conventionally treated glass–ceramic coatings due to presence of finer sized crystallites in the microwave processed coatings. Slight enhancement in the nanohardness and Young's modulus values with soaking time for the microwave processed coatings were explained in terms of the microstructural refinement and the reinforcement of the parent glass matrix.     

Mechanical properties of graphene platelet-reinforced alumina ceramic composites

Alumina (Al₂O₃) ceramic composites reinforced with graphene platelets (GPLs) were prepared using Spark Plasma Sintering. The effects of GPLs on the microstructure and mechanical properties of the Al₂O₃ based ceramic composites were investigated. The results show that GPLs are well dispersed in the ceramic matrix. However, overlapping of GPLs and porosity within ceramics are observed. The flexural strength and fracture toughness of the GPL-reinforced Al₂O₃ ceramic composites are significantly higher than that of monolithic Al₂O₃ samples. A 30.75% increase in flexural strength and a 27.20% increase in fracture toughness for the Al₂O₃ceramic composites have been achieved by adding GPLs. The toughening mechanisms, such as pull-out and crack deflection induced by GPLs are observed and discussed.     

Advanced indium-tin oxide ceramics for sputtering targets

Indium-tin oxide (ITO) ceramic sputtering targets are widely used in formation of electrically conductive transparent thin films for electrodes in flat panel displays, solar cells, antistatic films and others, and which are commonly produced by a conventional dc magnetron sputtering process. The ceramic targets should be of high purity with a uniform microcrystalline structure and should possess high density and high electrical conductivity. In the present work, the challenges of the ceramic composition (e.g. the ratio of In₂O₃ and SnO₂) and manufacturing are considered; they include the use of high quality starting materials, particularly In₂O₃ powders with respect to purity, morphology and sinterability, manufacturing routes and sintering process. Positive experience in the development and manufacturing of ITO ceramic planar sputtering targets using in-house prepared In₂O₃ powders is reported. ITO ceramic tiles with areas up to 1500–1700 cm² and densities of 99+% of TD are manufactured. Physical properties of the ITO ceramics and sputtered films have been studied.     

A new Al2O3 porous ceramic prepared by addition of hollow spheres

A method for making porous ceramic prepared by adding hollow spheres was developed, and the resulting porous ceramic was named as hollow spheres ceramic. Water soluble epoxy resin was used as a gel former in the gelcasting process of the Al₂O₃ hollow sphere and Al₂O₃ powder, the porous ceramic porosity varies from 22.3 to 60.1 %. The influence of amount of Al₂O₃ hollow sphere and sintering temperature on the microstructure, compressive strength and thermal conductivity were investigated. With an increasing amount of hollow sphere in the matrix, the porosity increases, which leads to decreased bulk density, compressive strength and thermal conductivity. The compressive strength of the porous ceramics has a power law relation with the porosity, and the calculated power law index is 4.5. The equations of the relationship between porosity and thermal conductivity of porous ceramics are proposed. The thermal conductivity of samples with 60.1 % porosity is as low as 2.1 W/m k at room temperature.     

α-Al2O3悬浮体的流变性及凝胶注模成型工艺的研究

陶瓷凝胶注模型工艺是一种新颖的成型工艺,具有很好的应用前景。它将高分子化学单体聚合的思路引入到陶瓷的成型工艺中,可制备高强度、高均匀性的陶瓷坯体。     

Powder morphology modification and sinterability improvement of Ce0.8Sm0.2O1.9 derived from solution combustion process

Ce_0.8Sm_0.2O_1.9 powders were synthesized by solution combustion process using a relatively low fuel (urea) concentration. The morphology of the powders was studied in relation to the synthetic process. The sinterability and microstructure of the ceramic specimens were investigated with respect to the powder morphology. The as-combusted powder showed sphere-like fine particles constituted by an aggregation of Ce_0.8Sm_0.2O_1.9 nanocrystallites. A ball-milling process modified the morphology of the powder and improved the sinterability of the ceramic specimens. The fuel concentration of the combustion synthetic system played an important role in determining the morphology and the sintering reactivity of the resulting powders. The microstructure evolution of the ceramic specimens with sintering temperature was found to be closely related with the powder morphology. Ce_0.8Sm_0.2O_1.9 ceramics with high relative densities (95.8-98.0%) and fine grains (0.2-0.3 μm) were prepared from the ball-milled powder at relatively low sintering temperatures of 1100-1200 °C.     

Preparation of boron carbide–aluminum composites by non-aqueous gelcasting

Gelcasting is an attractive forming process to fabricate ceramic parts with near-net-shape. In the present work, non-aqueous gelcasting of boron carbide (B₄C)–aluminum (Al) composites was studied. A stable B₄C–Al slurry with solids loading up to 55 vol.% for gelcasting was prepared. The slurry was solidified in situ to green body with the mean value of relative density of 64% and flexural strength of 21 MPa. The SEM images showed that powders in green body compact closely by the connection of polymer networks. B₄C–Al samples were also obtained by the process of gelcasting and sintering at 1300 °C for 1 h in 0.1 MPa Ar atmosphere. The average bulk density of sintered body was 2.05 g cm⁻³.     

New route for processing of multilayer Al2O3-Co3O4 materials through gelcasting

Multilayer dense ceramic materials composed of Al₂O₃ and Al₂O₃-Co₃O₄ layers have been obtained by gelcasting. The key stage in the process was the optimization of the polymerization idle time in order to ensure strong adhesion between layers without cracks and delamination in a green state and after sintering. The significant advantage of this method is occurence of strong connections between consituent layers due to the slight migration of the slurry to the gelled bottom part of the sample, what is not obseved in techniques based on lamination processes. The multilayer samples were composed of two Al₂O₃ layers and two Al₂O₃-Co₃O₄ layers arranged alternately. The rheological characterization of the slurries was done. The properties of the sintered multilayer bodies were examined in comparison to the single-layer alumina samples. Observations in SEM and ligth miscroscope were performed. The presence of the transition layer in the sintered bodies was observed.