Digital light processing of complex-shaped 3D-zircon (ZrSiO4) ceramic components from a photocurable polysiloxane/ZrO2 slurry

Explorations in the stereolithography fabricated polymer-derived ceramics are still far from maturity. Herein, a novel preceramic slurry, which consists of a photocurable epoxy-acrylic siloxane and particle-size gradated ZrO₂ fillers, was digital light processing shaped into 3D preceramic precursors. By taking advantage of the high reactivity of in-situ formed silica from the polysiloxane, 3D-zircon product can be synthesized through the sintering reaction between the polysiloxane and ZrO₂ particles in the precursor at a low temperature. During the sintering process, ZrSiO₄ phase starts to appear at the temperature of 1200 °C. A proper particle-size distribution of the ZrO₂ filler, 20 wt% of micropartciles and 4 wt% of nanoparticles, not only endowed the ceramic slurry with a low viscosity but also increase the purity of the zircon products. Besides, the addition of sintering aid NaF can promote the sintering reaction between the polysiloxane and ZrO₂ particles while increase the crystalize degree of the 3D-zircon products.     

Stereolithographical fabrication of dense Si3N4 ceramics by slurry optimization and pressure sintering

Photocurable gray-colored Si₃N₄ ceramic slurry with high solid loading, suitable viscosity and high curing depth is critical to fabricate dense ceramic parts with complex shape and high surface precision by stereolithography technology. In the present study, Si₃N₄ ceramic slurry with suitable viscosity, high solid loading (45 vol %) and curing depth of 50 μm was prepared successfully when surface modifier KH560 (1 wt%) and dispersant Darvan (1 wt%) were used. The slurry exhibits the shear thinning behavior. Based on the Beer-Lambert formula, D_p (the attenuation length) and E_c (the critical energy dose) of Si₃N₄ ceramic slurry with solid loading of 45 vol % were derived as 0.032 mm and 0.177 mJ/mm², respectively. Si₃N₄ ceramic green parts with complex shape and high surface precision were successfully fabricated by stereolithography technology. After optimizing the debinding and sintering process for green parts, dense Si₃N₄ ceramics with 3.28 g/cm³ sintering density were fabricated. The microhardness and fracture toughness of as-sintered Si₃N₄ ceramics are ~14.63 GPa and ~5.82 MPa m^1/2, respectively, which are comparable to those of the samples by traditional dry-pressed and pressureless sintering technology. These results show that ceramic stereolithography technology could be promising to fabricate high performance ceramics, especially for gray-colored monolithic Si₃N₄ ceramics.     

Preparation of photosensitive SiO2/SiC ceramic slurry with high solid content for stereolithography

Stereolithography is a popular three-dimensional (3D) printing technology, which is widely used for manufacturing ceramic components owing to its high efficiency and precision. However, it is a big challenge to prepare SiC ceramic slurry with high solid content for stereolithography due to the strong light absorption and high refractive index of dark SiC powders. Here, we propose a novel strategy to develop photosensitive SiO₂/SiC ceramic slurry with high solid content of 50–65 vol% by adding spherical silica with low light absorbance and applying a stacking flow model to improve the solid content of the slurry. The as-prepared slurry exhibits excellent stereolithography properties with a dynamic viscosity lower than 20 Pa s and curing thickness more than 120 μm. Therefore, it can be successfully applied for stereolithography-based additive manufacturing of SiC green bodies with large size (100 mm), sub-millimeter accuracy (0.2 mm), and complex structure. The stacking flow model also shows immense potential for the stereolithography of other dark-color ceramics with high solid content.     

Effect and mechanism of Fe2O3 decomposition in the preparation of foaming ceramics from industrial solid waste

MgO–Al₂O₃–SiO₂ foam ceramics were prepared by direct sintering of asbestos tailings and coal fly ash by spontaneous bubble process. By X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermodynamic analysis, the effect of Fe₂O₃ decomposition on the preparation of foaming ceramics from industrial solid waste and the pore-forming process was studied. The results show that the foaming effect of foaming ceramics with higher raw iron content is better when the roasting temperature is higher. Increasing the content of iron is beneficial to the formation of cordierite, and increasing the roasting temperature is beneficial to the relative content of spinel in foaming ceramics. Combined with the change in the Fe valence state, content in porous ceramic samples, and thermodynamic analysis of the product, it is found that Fe₂O₃ decomposition is the fundamental cause of ceramic foaming at high temperature. Fe²⁺ produced after decomposition replaces Mg²⁺ into spinel and cordierite structures, and the O₂ produced by decomposition cannot be discharged in time to cause ceramic foaming. The research results can provide theoretical basis for the preparation of industrial solid waste foamed ceramics.     

Highly dense 0.3CaCeNbWO8-0.7LaMnO3 composite ceramics fabricated by cold sintering process

The cold sintering process (CSP) has been used for fabricating functional ceramics at a low sintering temperature. In this study, highly dense 0.3CaCeNbWO₈-0.7LaMnO₃ composite ceramics have been successfully fabricated by CSP. The phase structure, microstructure, and electrical properties of composite ceramics have been investigated. The composite ceramic is mainly composed of a tetragonal CaCeNbWO₈ phase with scheelite structure and an orthorhombic LaMnO₃ phase with perovskite structure. The relative density of composite ceramic is 94.5%, and is higher than that of single phase ceramic. The resistivity of composite ceramic exhibits negative temperature coefficient characteristics, with an aging coefficient less than 2%. Such a sintering methodology is of great significance, since it provides a feasible idea for preparing composite ceramics.     

The role of carbon in microstructure evolution of SiBCO ceramics

The composition of polymer derived ceramics could be readily tuned through controlling the structure and element content of the polymer precursors, and investigation on the effect of the element on microstructure evolution is important to the design of advanced ceramics. In this article, the effect of carbon content in SiBCO polymer precursors was systematically investigated. The polymer network and thermal stability of polymer precursors and the carbon content of pyrolyzed SiBCO ceramic could be readily tuned by controlling the DVB amount used. Carbon contributed to the formation of graphitic carbon in SiBC_xO ceramics and inhibited the growth of β–SiC and SiO₂ crystals at 1600 °C, but lead to an increase in the graphitic carbon phase at 1800 °C.     

Digital light processing fabrication of mullite component derived from preceramic precusor using photosensitive hydroxysiloxane as the matrix and alumina nanoparticles as the filler

By taking advantage of the low sintering temperatures of the preceramic polymers, stereolithography printed mullite components derived from preceramic polymer precursor containing alumina particles can be sintered at low temperatures. However, due to their high specific surface, nano alumina particles are difficult to be dispersed into the photocurable polysiloxane. Herein, to prepare mullite slurry, a photosensitive hydroxysiloxane was employed as the preceramic polymer matrix while γ-Al₂O₃ nanoparticle was added as the active filler. The introduction of photocurable hydroxysiloxane not only improved the homogeneity and rheological properties of mullite slurry but also shorted the ionic diffusion distance of Si-ion and Al-ion during the sintering process. Therefore, 3D mullite preceramic precursor stereolithography printed from hydroxysiloxane-Al₂O₃ slurry was endowed with a low sintering temperature around 1400 °C. During the sintering process of preceramic precursor, sintering aid AlF₃ can participate in the reaction and further promote the formulation of mullite grains.     

Preparation and thermal shock resistance of anorthite solar thermal energy storage ceramics from magnesium slag

Anorthite solar thermal energy storage ceramics were fabricated from magnesium slag solid waste by pressureless sintering. The effects of CaO/SiO₂ ratio and sintering temperature on the physical, chemical, and thermophysical properties of ceramics were explored. X-ray diffraction results demonstrated that thermal shock process contributed to the formation of anorthite, and increasing CaO/SiO₂ ratio promoted the transformation of anorthite (CAS₂) into melilite (C₂AS). Some micro-cracks were found according to SEM analysis, forming by the mismatch of thermal expansion coefficients among phases. The combined effects of the low thermal expansion coefficient of anorthite and micro-crack toughing endowed the ceramic with good thermal shock resistance. Optimum comprehensive performances were observed in the sample with a CaO/SiO₂ ratio of 0.58 sintered at 1160°C, of which the specific thermal storage capacity was 0.63 J·g^-1·°C^-1(room temperature). The bending strength increased by 0.22% after 30 thermal shock times (room temperature-800°C, wind cooling). Therefore, the anorthite ceramics exhibited great potential for solar thermal energy storage.     

Influence of Yb and Si on the fabrication of Yb:YAG transparent ceramics using spherical Y2O3 powders

Yb:YAG (yttrium aluminum garnet) transparent ceramics were fabricated by the solid-state method using monodispersed spherical Y₂O₃ powders as well as commercial Al₂O₃ and Yb₂O₃ powders. Pure YAG phase was obtained at low temperature due to homogeneous mixing of powders. Under the same sintering conditions, the Yb:YAG ceramics with different doping contents of Yb³⁺ had similar morphologies and densification rates. After being sintered at 1700 °C in vacuum, the ceramic samples had high transparencies. The Yb:YAG ceramics doped with 0.5 wt% SiO₂ formed Y–Si–O liquid phase and nonstoichiometric point defects that enhanced sintering. Compared with Nd doping, Yb doping hardly affected the YAG grain growth, sintering densification or optical transmittance, probably because Yb³⁺ easily entered the YAG lattice and had a high segregation coefficient.     

Low thermal conductivity in porous SiC–SiO2–Al2O3–TiO2 ceramics induced by multiphase thermal resistance

The introduction of multiple heterogeneous interfaces in a ceramic is an efficient way to increase its thermal resistance. Novel porous SiC–SiO₂–Al₂O₃–TiO₂ (SSAT) ceramics were fabricated to achieve multiple heterogeneous interfaces by sintering equal volumes of SiC, SiO₂, Al₂O₃, and TiO₂ compacted powders with polysiloxane as a bonding phase and carbon as a template at 600 °C in air. The porosity could be controlled between 66% and 74% by adjusting the amounts of polysiloxane and the carbon template. The lowest thermal conductivity (0.059 W/(m·K) at 74% porosity) obtained in this study is an order of magnitude lower than those (0.2–1.3 W/(m·K)) of porous monolithic SiC, SiO₂, Al₂O₃, and TiO₂ ceramics at an equivalent porosity. The typical specific compressive strength value of the porous SSAT ceramics at 74% porosity was 3.2 MPa cm³/g.     

In situ reactive synthesis and plasma-activated sintering of binderless WC ceramics

Binderless micrometre tungsten carbide ceramics (～1.1?μm) were in situ synthesised and densified by plasma-activated sintering (PAS) from mixed powders of tungsten trioxide and carbon black. The influence of sintering process and powders’ composition ratio on the phase composition, microstructure and mechanical properties of as-prepared samples was clarified in detail. The phase evolution was ascertained by X-ray diffraction to be WO₃→WO_2.72→WO₂→W₂C→WC, with the formation of CO and CO₂ gases. The sample with nearly single WC phase, dense structure and excellent mechanical properties was fabricated under the optimised process with the proper composition ratio. Owing to the micrograin size, the fracture toughness (8.88?MPa?m^1/2) was enhanced while high hardness (2159 HV10) was maintained, in the absence of any ceramic toughening phase.     

Digital light processing fabrication of porous cordierite ceramics from polysilxoane-based slurries

Herein, a pohotosentive polysiloxane (PSO)/talc/Al₂O₃ slurry was prepared for the digital light processing printing. Liquid photosensitive PSO was exploited as a triple-functioned material, acting both as the resin matrix, a high reactive Si source and a pore generator. Through adjusting the raw material components, polysiloxane can be sintered with talc and Al₂O₃ fillers after being pyrolyzed to 1200 ^oC while pores with variable diameters can be generated. Ternary component slurry prepared based on the above strategy possesses the characteristics of low viscosity, high reaction activity and good homogeneity. Sintering schedule of the printed thin-walled precursor was investigated to guarantee the morphology of cordierite product was consistent with the printed model. This work aims to provide a new strategy for DLP printing of MgO-Al₂O₃-SiO₂ ternary and other polymer derived ceramics.     

Effects of fine grains and sintering additives on stereolithography additive manufactured Al2O3 ceramic

Al₂O₃ ceramics are fabricated by stereolithography based additive manufacturing in present reports. To improve the densification and performance of Al₂O₃ ceramic, the introduction of fine grains or sintering additives has been studied by traditional fabrication techniques. However, no research has focused on the effects of adding fine grains and sintering additives on the stereolithography additive manufactured Al₂O₃ ceramic. In this study, both fine grains and sintering additives were added firstly, and then the effects of fine grains and sintering additives on the relative density, microstructure, mechanical properties, and physical properties of the stereolithography additive manufactured Al₂O₃ ceramics were investigated. Finally, defect-free Al₂O₃ ceramic lattice structures with high precise and high compressive strength were manufactured.     

Fabrication of transparent MgAl2O4 ceramics by gelcasting and cold isostatic pressing

Highly transparent MgAl₂O₄ ceramics have been fabricated by aqueous gelcasting combined with cold isostatic pressing (CIP), pressureless sintering and hot isostatic pressing (HIP) from high purity spinel nanopowders. The gelling system used AM and MABM as monomer and gelling agent. The influences of dispersant and PH on the rheological behavior of the MgAl₂O₄ slurries were investigated. The spinel slurry with low solids loading (25 vol%) and low viscosity (0.15 Pa s) was obtained by using 6 wt% Duramax-3005 (D-3005) as dispersant. After CIP, the green body had a relative density of 48% with a narrow pore size distribution. The influence of sintering temperature on densification and microstructure was studied, choosing 1500 °C as the sintering temperature. After HIP (1650 °C/177 MPa/5 h), transparent MgAl₂O₄ ceramic with the thickness of 3 mm was obtained, whose in-line transmittance was 86.4% at 1064 nm and 79.8% at 400 nm, respectively. The ceramic exhibited a dense microstructure with the average grain size of 23 μm. The Vickers hardness and flexure strength of the sample reached 13.6 GPa and 214 MPa, respectively.     

Synthesis and characterization of high-density B2O3-added forsterite ceramics

Boria (B₂O₃)-added forsterite (Mg₂SiO₄) ceramics were synthesized and their properties were characterized. The addition of B₂O₃ was aimed to produce high density forsterite ceramics at a low sintering temperature. The raw materials were purified silica sand and commercial magnesia powders. Fosterite powder was produced from a solid reaction between the raw powders at 1100 °C prior to addition of B₂O₃, uniaxial pressing and sintering at 1200 °C. The amount of added B₂O₃ varied between 0%, 4%, and 8% by weight. Elemental analysis was performed by X-ray fluorescence (XRF) spectroscopy on the purified silica powder, whereas phase analyses were obtained from X-ray diffraction (XRD) data. Characterization of the ceramics included diameter shrinkage, density-porosity, thermal expansion, Vickers hardness, and dielectric constant. The results showed that the silica powder contained 98.7 at% Si with minor impurities, including 0.5 at% Ti, but the only identified crystalline phase was quartz. Further phase analysis of the ceramics showed that the addition of B₂O₃ reduced the amount of formed forsterite and increased the amount of cristobalite, proto- and clino-enstatite (MgSiO₃) as well as suanite (Mg₂B₂O₅). The highest forsterite content was found in B₂O₃-free ceramics, approximately 88.1 wt%. Moreover, the addition of B₂O₃ also reduced the diameter of the sample by more than 21%, resulting in a very dense ceramic with an apparent porosity of only 0.3%. The Vickers hardness significantly increased from 0.3 GPa for the B₂O₃-free ceramic to 10.9 GPa for the 4% B₂O₃ sample. The dielectric constant of the B₂O₃-added forsterite ceramics was improved by approximately 2–6 times that of the B₂O₃-free ceramic, which was primarily attributed to the loss of porosity in the samples.     

Direct ink writing of aluminium oxynitride (AlON) transparent ceramics from water-based slurries

In this study, transparent AlON ceramics were fabricated via the direct ink writing (DIW) method from the water-based ceramic slurry. The solids loading of the ceramic slurry was optimised by changing the dispersant content, and the printability and water content were then adjusted by adding hydroxyethyl cellulose (HEC). The structure of the green body was complete and no impurity phases were detected. The effects of sintering temperature and dwell time on the bulk density, phase evolution, microstructure, in-line transmittance, and mechanical properties of the ceramics were studied systematically. High optical and mechanical properties of 10 × 10 × 0.9 mm³ single-phase AlON ceramic tiles were obtained by sintering at 1960 °C for 10 h in a nitrogen atmosphere: in-line transmittance of 81.90% at a wavelength of 780 nm, fracture toughness of 1.74 MPa·m^1/2 (2.94 N), and Vickers hardness of 18.56 GPa (2.94 N). This study provides a novel method for synthesising AlON transparent ceramics from water-based ceramic slurries.     

多孔Al2O3-ZrO2陶瓷的微观结构

以Al₂O₃、ZrO₂陶瓷粉体为溶质,以莰烯为溶剂,以Texaphor963作为添加剂,制备出低粘度高稳定性的陶瓷浆料,采用冷冻注模工艺制备出具有较高强度的陶瓷坯体,采用无压烧结工艺,得到了多孔Al₂O₃-ZrO₂陶瓷制品,并对其微观结构进行了研究。     

Fabrication and properties of porous anorthite/mullite ceramics with both high porosity and high strength

Porous anorthite/mullite ceramics with both high porosity and high strength have been successfully fabricated by foam-gelcasting and pressureless sintering technology, using α-Al₂O₃, SiO₂, and CaCO₃ as starting materials and MnO₂ as sintering aids. The porous mullite ceramics prepared in this study had 83.3% porosity and 0.3 W/m·K thermal conductivity, exhibited compressive strength value as high as 6.1 MPa. The samples fabricated with mullite content of 30 mol% possessed 79.4% porosity and 5.9 MPa compressive strength showed thermal conductivity as low as 0.19 W/m·K. With the addition of MnO₂, the properties of the prepared materials varied slightly when mullite content changed in a large scale. The results showed that the addition of MnO₂ promoted the reaction, affected sintering and grain growth, and contributed to high strength and low-thermal conductivity.     

Transient liquid phase diffusion process for porous mullite ceramics with excellent mechanical properties

Porous mullite ceramics were fabricated by the transient liquid phase diffusion process, using quartz and fly-ash floating bead (FABA) particles and corundum fines as starting materials. The effects of sintering temperatures on the evolution of phase composition and microstructure, linear shrinkage, porosity and compressive strength of ceramics were investigated. It is found that a large amount of quartz and FABA particles can be transformed into SiO₂-rich liquid phase during the sintering process, and the liquid phase is transient in the Al₂O₃-SiO₂ system, which can accelerate the mullitization rate and promote the growth of mullite grains. A large number of closed pores in the mullite ceramics are formed due to the transient liquid phase diffusion at elevated temperatures. The porous mullite ceramics with high closed porosity (about 30%) and excellent compressive strength (maximum 105?MPa) have been obtained after fried at 1700?°C.     

Fabrication and properties of SiC porous ceramics using a polyurethane preparation process

SiC porous ceramics can be prepared by introducing the polyurethane preparation method into the production process of ceramic biscuits, followed by sintering at 1300?°C for 2?h under N₂ flux after the cross-linking of polycarbosilane at 220?°C for 4?h in air. The microstructures, mechanical properties and infiltrations of the SiC porous ceramics are investigated in detail. The best dispersal effect comes from the SiC slurry with xylene as the solvent and a mixture of Silok®7096 (1?wt%) and Anjeka®6041 (4?wt%) as the dispersant. The compressive strength of SiC porous ceramics with high porosity (69.53%) reaches 16.9?MPa. The heat treatment can increase infiltration, the rate of which (4.296?×?10^?7 mm²) after the heat treatment at 750?°C in air is approximately two times faster than that before the heat treatment. The SiC porous ceramics fabricated in this study will have potential application in active thermal protection systems.