Foam-gelcasting preparation,microstructure and thermal insulation performance of porous diatomite ceramics with hierarchical pore structures

Hierarchically pore-structured porous diatomite ceramics containing 82.9∼84.5% porosity were successfully prepared for the first time via foam-gelcasting using diatomite powder as the main raw material. Sizes of mesopores derived from the raw material and macropores formed mainly from foaming were 0.02∼0.1 μm and 109.7∼130.5 μm, respectively. The effect of sintering temperature, additive content and solid loading of slurry on pore size and distribution, and mechanical and thermal properties of as-prepared porous ceramics were investigated. Compressive strength of as-prepared porous ceramics increased with sintering temperature, and the one containing 82.9% porosity showed the highest compressive strength of 2.1 ± 0.14 MPa. In addition, the one containing 84.5% porosity and having compressive strength of 1.1 ± 0.07 MPa showed the lowest thermal conductivity of 0.097 ± 0.001 W/(m·K) at a test temperature of 200 ̊C, suggesting that as-prepared porous ceramics could be potentially used as good thermal insulation materials.     

PZT ceramics fabricated based on stereolithography for an ultrasound transducer array application

The PZT ceramics with different weight content from 78% to 89% were printed using the stereolithography method. The piezoelectric properties as well as the microstructure of the ceramics were investigated in detail. X-ray patterns and Raman spectra demonstrate that the steady PZT tetragonal phase has already formed in these sintered ceramics. Besides, the piezoelectric constant and dielectric constant were 212–345 pC/N and 760–1390, which were sligthly lower than that of the dry pressing disk. Furthermore, a two-dimensional ultrasound transducer array (8?×?8) was designed and developed to evaluate the properties of the 3D-printed PZT ceramics.     

Fabrication of porous silica ceramics by gelation-freezing of diatomite slurry

Diatomite-derived porous silica ceramics with high porosities of up to 90% were fabricated using a gelation-freezing method, which resulted in unidirectional cellular or random microstructure with micrometer-sized cells. The ice crystals that were formed during freezing of a diatomite powder dispersed gel were removed by sublimation during vacuum drying, and the green bodies were sintered at 1150–1350 °C for 2 h in air. The thermal conductivity of the porous ceramics prepared with initial solid loadings of 5 and 10 vol% ranged from 0.09 to 0.16 W/(mK) at room temperature. The proposed method is therefore promising for the preparation of ceramic thermal insulators with very low thermal conductivity.     

Effect of synergism of solid loading and sintering temperature on microstructural evolution and mechanical properties of 60 vol% high solid loading ceramic core obtained through stereolithography 3D printing

Solid loading has a significant effect on the curing behavior of slurry and the microstructure and properties of the ceramic core. A high-solid loading slurry can effectively improve the sintering densification of ceramic particles and improve the interlayer bonding strength and mechanical properties at both 25 °C room and higher temperatures. Herein, based on the photopolymerization theory of ceramic slurry, the solid loading was increased from 45 to 60 vol% by adjusting the composition ratio of the resin ceramic powder. Additionally, the optimal sintering temperature of the 60 vol% solid loading ceramic core was 1200 °C. The synergistic effect of the solid loading and sintering temperature controls the sintering shrinkage of the sample within 3.2%; the porosity, high temperature, and room temperature flexural strength were approximately 30%, 24 MPa, and 10 MPa, respectively. The printing preparation of high-solid loading ceramic cores can be used to guide optimizing process parameters on an industrial scale.     

Effect of SiC powder on the properties of SiC slurry for stereolithography

Silicon carbide (SiC) is a kind of structural ceramics with excellent properties and it is widely used in industrial fields. Stereolithography (SL) is a potential additive manufacturing technique to fabricate fine complex SiC components, the resin-based SiC slurry with superior rheological and photo-polymerization properties is important for SL. In this paper, we investigated the influence of SiC powder on the properties of the SiC slurries for SL. The physical characteristics of SiC powder such as particle size, size distribution and appearance were tested and observed, and their influence on the dispersion, sedimentation and photo-polymerization property of the SiC slurry were investigated and discussed in detail based on their correlative theory, we finally prepared SiC slurry with superior rheological and photo-polymerization properties, and fabricated the fine complex SiC green body with low defects, high accuracy and high bending strength successfully. The SiC slurry with the solid content of 40 vol% was fabricated by the SiC powder with the median diameter D₅₀ ≈ 10.0 μm and a narrow particle size distribution, it is Bingham fluid with good fluidity and the viscosity of it is 464.40 mPa s under the shear rate of 51.08 s^?1, the cured SiC parts with Z – axis dimension change of 0.75% was finally fabricated, the three points bending strength of it is 50.18 MPa. Our research work provides some fundamental understanding of the SL technique for fabricating fine complex SiC components, explored a suitable way to fabricate high quality SiC green parts through SL, and offers some valuable references for preparing SiC slurry with superior rheological and photo-polymerization properties.     

Synthesis of hierarchical porous silicon oxycarbide ceramics from preceramic polymer and wood biomass composites

Hierarchical porous SiOC ceramics were successfully prepared using a polysiloxane preceramic polymer mixed with wood biomass by annealing at different temperatures under Ar atmosphere. These SiOC ceramics display a trimodal pore size distribution in the micro-, meso- (micropores + mesopores, 1.7–14 nm) and macro-size scale (1–15 μm). The mesopores and micropores mainly originate from the formation of large amounts of SiC crystals and nanowires, graphite-like microcrystallites, and nm-scale pores of ray parenchyma cells and pits of the wood biomass. The SiOC sample prepared at a higher temperature processes the specific surface area up to 180.5 m²/g. The specific surface area, pore volume and average pore width of the samples can be controlled by adjusting the pyrolysis temperature.     

Fabrication and rheological behavior of tape-casting slurry for ultra-thin multilayer transparent ceramics

Disk transparent ceramics with multilayer composite structure serves well as laser gain medium because of its excellent thermal and optical properties. For such a type of material, the preparation of corresponding outstanding slurry is the first yet most essential step, and it may determine the quality of both subsequent green tape and the sintered ceramics. In this work, the tape-casting slurry with typically pseudoplastic behavior was carefully modulated in the mixture solvent of anhydrous ethanol and ethyl acetate, and its rheological property was systematically investigated using fine-controlled rotary rheometer. The obtained result reveals 4wt.% fish oil to be the best dispersed agent. With the obtained slurry for tape-casting followed by lamination and warm isostatic-pressing, the multilayer ultrathin TCs was successfully fabricated via high-temperature vacuum sintering technology. The obtained full-dense microstructure with high optical quality may endow the studied ceramic with great potential for highly efficient solid-state laser gain medium.     

Fabrication and characterization of porous mullite ceramics with ultra-low shrinkage and high porosity via sol-gel and solid state reaction methods

Porous mullite ceramics with ultra-low shrinkage and high porosity were prepared by solid state reaction between MoO₃ and mullite precursor powders which were synthesized from tetraethylorthosilicate and aluminium nitrate nonahydrate via sol-gel methods. The synthetic process of mullite precursor powder and effects of MoO₃ amount on the phase composition, microstructure, physical properties such as firing shrinkage, open porosity, bending strength, water absorption and bulk density of porous mullite ceramics were investigated. The results indicated that the addition of MoO₃ not only lowered the mullite forming temperature from 985.4 to 853.3 °C, but also restrained densification behavior of samples due to the formation of mullite and Al₂O₃–MoO₃ solid solution, besides, MoO₃ also improves the formability, open porosity and bending strength of samples. The optimal amounts of MoO₃ is 8 wt%, and the resultant samples exhibit outstanding properties, including a low shrinkage rate of 1.86 ± 0.07%, an open porosity of 61.91 ± 0.16% and a bending strength of 9.35 ± 1.11 MPa.     

Evolution of the microstructure and mechanical properties of stereolithography formed alumina cores sintered in vacuum

Stereolithography (SL) was used to form alumina ceramic cores. The effect of sintering temperature on the microstructure and mechanical properties of the alumina ceramics are investigated, which were sintered in vacuum. The results indicate that, as the sintering temperature increased the particle size of alumina slightly increased, and the interlayer spacing first decreased and then increased. The open porosity of alumina ceramics significantly decreased as the sintering temperature in vacuum increased. The flexural strength and hardness increased as the sintering temperature increased. When sintered at 1150 °C, the flexural strength was found to be 33.7 MPa, the shrinkage was 2.3 %, 2.4 %, and 5.3 % in the X, Y, and Z directions, respectively, and the open porosity was 37.9 %. These results are similar to those found from sintering at 1280 °C in air.     

Effects of particle size distribution and sintering temperature on properties of alumina mold material prepared by stereolithography

Alumina mold materials prepared by stereolithography usually have considerable sintering shrinkage, and their properties related to casting have been rarely studied. In this study, alumina molds materials were prepared by stereolithography, and the effects of particle size distribution and sintering temperature on the properties of the materials were investigated. Results show that the viscosity of the slurries decreases as the fraction of fine powder increases, and the particle size distribution affects the curing behaviors slightly. Sintering shrinkage increases as the fraction of fine powder or the sintering temperature increases. Although lower sintering shrinkage can be achieved by sintering at 1350 °C or 1450 °C, the mold materials sintered at lower temperatures would continue to shrink under the service temperature of 1550 °C, and thus 1550 °C is determined as the optimal sintering temperature. As the fraction of fine powder increases, the creep resistance first increases and then decreases, and specimens prepared with 0.1 fraction of fine powder exhibit the best creep resistance with the droop distance of 4.44 ± 0.45 mm. Specimens prepared with 0.1 fraction of fine powder and sintered at 1550 °C exhibit linear shrinkage of 6.36% along the X/Y direction and 11.39% along the Z direction, and have a flexural strength of 78.15 ± 3.50 MPa and porosity of 30.12 ± 0.08%. The resulting material possesses relatively low sintering shrinkage, proper mechanical strength, porosity and high-temperature properties that meet the requirements for casting purposes.     

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.     

Fabrication of transparent neodymium-doped yttrium aluminum garnet ceramics by high solid loading suspensions

Dense neodymium-doped yttrium aluminum garnet (Nd:YAG) transparent ceramic was obtained by slip casting and solid-state reaction. The colloidal behavior of the aqueous suspensions of neodymia, yttria, and alumina mixed powders using Dispex A as dispersant was investigated. The variation in zeta potential due to pH alteration was studied. The isoelectric point (IEP) was at pH 4.5 and 4 for the specimens with and without Dispex A, respectively. The optimal dispersion conditions were achieved for the suspensions at pH 9.6 with 0.4 wt% Dispex A. The green body prepared by slip casting was vacuum sintered from 1200 °C to 1750 °C. The grain size of the sintered body increased, and the pore size decreased with increasing sintering temperature. Pore-free Nd:YAG transparent ceramic with a grain size of 5–10 μm was obtained by sintering at 1750 °C for 10 h. The in-line transmittance of the annealed specimen reached 80.8% at 1064 nm.     

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.     

Digital light processing stereolithography of zirconia ceramics: Slurry elaboration and orientation-reliant mechanical properties

Digital Light Processing (DLP) is a promising technique for the preparation of ceramic parts with complex shapes and high accuracy. In this study, 3 mol% yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) UV-curable slurries were prepared and printed via DLP. Two different solid loadings (40.5 and 43.6 vol%, respectively) and printing directions were investigated to assess the influence of these parameters on physical and mechanical properties of the sintered parts. Zirconia samples were sintered at 1550 °C for 1 h, achieving a very high relative density (99.2%TD), regardless of solid loading and printing direction. FE-SEM micrographs shown a homogeneous and defect-free cross section with an average grains size of 0.56 ± 0.19 µm. Finally, mechanical properties were influenced by printing direction and zirconia vol%. Indeed, the composition with the higher solid loading (i.e. 43.6 vol%) had the highest three-point flexural strength (751 ± 83 MPa) when tested perpendicular to the printing plane.     

Effects of particle size and color on photocuring performance of Si3N4 ceramic slurry by stereolithography

Due to high absorbance of UV light and low solid loading, the stereolithography-based additive manufacturing of gray-colored and dense Si₃N₄ ceramic is of significant difficulty and challenge. The effects of geometric properties of ceramic powders on the curing performance were investigated by studying the absorption difference of the Si₃N₄ ceramic particles with different colors and particle sizes and ultraviolet light. The results show that the transmission of ultraviolet light and curing performance of the darker Si₃N₄ ceramic slurry are much poor. Under the same particle size, the Si₃N₄ ceramic slurry using lighter particles presents the smaller scattering coefficient. The scattering coefficient (～202) of the gray powder with ～0.8 μm average particle size is the smallest. Under the same color, the larger the particle size, the smaller the scattering coefficient. The smallest scattering coefficient of the white powder with ～2.0 μm average particle size is ～110.     

织构PNN-PZT陶瓷的光固化成型制备及其电学性能研究

为高效制备织构压电陶瓷,以球状Pb(Ni_1/3Nb_2/3)O₃-PbZrO₃-PbTiO₃ (PNN-PZT) 为基体粉体,片状BaTiO₃ （BT）为模板粉体,采用光固化成型技术代替传统流延技术制备织构压电陶瓷。研究了粉体形貌对打印浆料流动性的影响、浆料的光敏参数以及不同BT含量织构陶瓷的晶体结构和电学性能。结果表明,球状粉体浆料具有低黏度的特性,能够有效提高打印浆料的固含量,最大固含量可达86%（质量）。此时陶瓷浆料的临界曝光量与透射深度分别为127.5 mJ/cm²和21.1 μm。打印后的PNN-PZT-BT陶瓷沿[00l]_c方向生长,BT模板粉体含量从1%增长到5%,陶瓷的织构度由42%增到92%。当BT含量为3%时,样品具有最高的压电常数d₃₃=1047 pC/N。与传统的流延法相比,SLA技术的工艺优势在于制备周期短,稳定性高,能够有效降低织构陶瓷的制备难度。     

织构PNN-PZT陶瓷的光固化成型制备及其电学性能研究

为高效制备织构压电陶瓷,以球状Pb(Ni_1/3Nb_2/3)O₃-PbZrO₃-PbTiO₃ (PNN-PZT) 为基体粉体,片状BaTiO₃ （BT）为模板粉体,采用光固化成型技术代替传统流延技术制备织构压电陶瓷。研究了粉体形貌对打印浆料流动性的影响、浆料的光敏参数以及不同BT含量织构陶瓷的晶体结构和电学性能。结果表明,球状粉体浆料具有低黏度的特性,能够有效提高打印浆料的固含量,最大固含量可达86%（质量）。此时陶瓷浆料的临界曝光量与透射深度分别为127.5 mJ/cm²和21.1 μm。打印后的PNN-PZT-BT陶瓷沿[00l]_c方向生长,BT模板粉体含量从1%增长到5%,陶瓷的织构度由42%增到92%。当BT含量为3%时,样品具有最高的压电常数d₃₃=1047 pC/N。与传统的流延法相比,SLA技术的工艺优势在于制备周期短,稳定性高,能够有效降低织构陶瓷的制备难度。     

Fabrication of piezoelectric nano-ceramics via stereolithography of low viscous and non-aqueous suspensions

A high-performance piezoelectric nano-ceramic was fabricated through stereolithography of low viscosity and high solid loading ceramic/polymer composite suspensions. Through the proper fitting calculation of experimental data, the maximum theoretical solid loading, rheological and curing behaviors of the suspension system were evaluated and lucubrated. The suspensions with a 40 vol% solid loading of the BaTiO₃ nanoparticles displayed shear thinning behavior and a relatively low viscosity of 232 mPa·s at 46.5 s⁻¹ shear rate. After post-process, the 3D printed ceramic specimens showed a nanometer-level grain size with a density of 5.69 g/cm³, which corresponds to about 95% of the theoretical density. The printed ceramics exhibit a piezoelectric constant of 163 pC/N and relative permittivity of 2762 respectively. The results achieved in this research indicate that the stereolithography process is a promising 3D printing technology to fabricate piezoelectric materials with complex geometries and exquisite features for the applications of ceramic components.     

Fabrication of SiC ceramic architectures using stereolithography combined with precursor infiltration and pyrolysis

Stereolithography based additive manufacturing provides a new route to produce ceramic architectures with complex geometries. In this study, 3D structured SiC ceramic architectures were fabricated by stereolithography based additive manufacturing combined with precursor infiltration and pyrolysis (PIP). Firstly, photosensitive SiC slurry was prepared. Then, stereolithography was conducted to fabricate complex-shaped green SiC parts. Polymer burn-out was subsequently performed, and porous SiC preforms were produced. After that, precursor infiltration and pyrolysis was used to improve the density and strength. Finally, 3D-structured SiC ceramic architectures with high accuracy and quality were obtained. It is believed that this study can give some fundamental understanding for the additive manufacturing of SiC ceramic structures.     

Properties regulation of SiC ceramics prepared via stereolithography combined with reactive melt infiltration techniques

Stereolithography(SLA) combined with reactive melt infiltration (RMI) is an effective way to fabricate silicon carbide(SiC) ceramic components with complex shapes and high precision. The purpose of this paper is to increase the content of SiC in the sintered body and improve the properties of SiC ceramics prepared by SLA/RMI technologies by the impregnation of the precursor of carbon source after debinding. The effects of the concentration of phenolic resin solution on the strength of sintered body were studied. The results show that this method can reduce the coefficient of thermal expansion and improve the thermal conductivity of the final body. At the same time, when the concentration of phenolic resin solution is 40 wt%, the final body obtained the best comprehensive properties. The value of bulk density, flexural strength and elastic modulus were 2.89 g/cm³, 244.17 ± 5.13 MPa and 402.39 GPa, respectively. This strategy provides a promising prospect for the preparation of space optical mirrors with complex shapes and high strength by the SLA/RMI method.