Porous Si3N4 ceramics prepared by aqueous gelcasting using low–toxicity DMAA system: Regulatable microstructure and properties by monomer content

In this study, the low–toxicity monomer N, N–dimethylacrylamide (DMAA), serving as both gelling agent and pore–forming agent, was adopted to fabricate porous Si₃N₄ ceramics with a regulatable microstructure and property by aqueous gelcasting. Results indicate that monomer content played an important role in regulating and optimizing the properties of sintered bodies. With increasing monomer content (5.94–30.69?wt%), both slurry viscosity (maximum 0.14?Pa?s at 95.40 s^?1) and green body strength (11.35–49.23?MPa) exhibited monotonic increasing trends, demonstrating superior mechanical properties to those obtained using the neurovirulent acrylamide (AM) gelling system. The increased monomer content not only improved porosity, but also promoted α→β–Si₃N₄ transformation as well as β–Si₃N₄ grain growth through enhancing the connectivity of interlocking pores and accelerating the vapor phase transport during liquid–phase sintering. These variations in phase composition and microstructure derived from the varied monomer content further resulted in monotonic changes in porosity (40.32–51.50%), mean pore size (0.27–0.38?μm), flexural strength (202.77–132.15?MPa), fracture toughness (2.93–2.32?MPa?m^1/2), dielectric constant (3.48–2.78) and loss (3.52–3.09?×?10^?3) at 10?GHz for sintered bodies, displaying an excellent comprehensive properties. This study suggests a promising prospect for DMAA in preparation of high–performance porous Si₃N₄ ceramics by aqueous gelcasting.     

Defect-free drying of large fine-particle zirconia compacts prepared by gelcasting method

In this work, drying of bodies prepared by gelcasting fine submicrometre-sized zirconia particles was studied and a drying process for defect-free bodies with large cross-sections was proposed. It was found that the cracking of large bodies could be prevented by reducing the monomer content and using appropriate non-volatile cosolvents. Glycerol and polyethylene glycols with different molecular weights were used as non-volatile cosolvents in aqueous ceramic suspensions. The complex effects of the individual cosolvents on the gelcasting process and, in particular, on the drying step were investigated and explained. The applicability of individual cosolvents for the gelcasting process were discussed and their optimal use was indicated.     

Preparation of porous SiC-Al2O3 ceramics via gelcasting utilising a shrinkable pore-forming agent and oxidised coarse-grained SiC

By utilising soaked millet as a shrinkable pore-forming agent, porous silicon carbide-alumina (SiC-Al₂O₃) ceramics were prepared via gelcasting. The fabrication of SiC-Al₂O₃ ceramics based on oxidised and unoxidised coarse-grained SiC was also studied. The water swelling, drying shrinkage, and low-temperature carbonisation of the millet were investigated. We found that the shrinkage of the soaked millet was greater than that of gel body during drying, which left large gaps that prevented shrinkage stresses from destroying the gel body. Low-temperature carbonisation of the millet should be performed slowly at 220–240?°C because its expansion rate increases to 45% at 250?°C, resulting in the cracking of samples. At a constant sintering temperature, the flexural strength of the SiC-Al₂O₃ ceramics prepared with SiC powders oxidised at 1000?°C was the highest, indicating that oxidised powders can successfully decrease the required sintering temperature and improve the flexural strength of composite ceramics. Based on our optimised process, porous SiC-Al₂O₃ ceramics were sintered at 1500?°C for 2?h. When their skeletons were fully developed, their pore sizes were in the range of 1.5–2?mm. Their porosity and flexural strength were 60.2–65.1% and 8.3–10.5?MPa, respectively.     

Processing of a natural hydroxyapatite powder: From powder optimization to porous bodies development

This paper deals with the development of macro-porous components made of a carbonated hydroxyapatite (HAp) nanopowder which was extracted from pig bones. Prior to sintering, the powder was treated at 700 °C for 1 h. During calcination, a partial carbonate decomposition occurred yielding CaO. In order to eliminate this by-product, the calcined HAp was washed in distilled water several times, checking the effect of washings by FT-IR spectroscopy. Then, the thermal stability of the as-calcined and washed powders treated in the range 800-1400 °C was investigated by XRD.After that, macro-porous materials made of washed HAp powder were prepared through a modified gelcasting process, using agar as a natural gelling agent and polyethylene spheres as the pore formers.     

工艺参数对Mo/Cu粉末凝胶注模成形的影响

本文采用甲基丙烯酸羟乙酯（HEMA）-1,6-己二醇二丙烯酸酯非水基凝胶注模体系制备了浓Mo/Cu粉末浆料。研究了分散剂用量、单体含量和固相体积分数对浆料流变行为的影响。并研究了单体含量、单体/交联剂比例、引发剂用量、温度等工艺参数对固化行为和坯体抗弯强度的影响。结果表明,固相体积分数对浆料就变行为的影响最大,其次是引发剂用量和单体含量。随着单体含量的增加和单体/交联剂比例的减小,坯体抗弯强度增加;引发剂用量对坯体抗弯强度的影响较小。根据上述结果,Mo/Cu粉末非水基凝胶注模的合理工艺参数如下：HEMA含量为25 vol.% ~ 30 vol.%, 单体/交联剂比例为10：1 ～ 15：1,引发剂用量为1.5 vol.% ~ 2.5 vol.%,固化温度在60℃与80℃之间。     

Gelcasting of alumina-chitosan beads

Several papers have reported the advantageous combination of chitosan and ceramic particles for such applications as biomimetic scaffolds, membranes, pollution remediation and gelcasting complex shapes. This work presents a novel gelcasting consolidation mechanism, based on the effects of pH changes on chitosan solubility and zeta potential of alumina particles. Unlike other chitosan-based gelcasting methods, it employs a small content of organic material (lower than 3 wt%) and does not require crosslinking agents (such as glutaraldehyde). With this new method alumina beads with 0.5-1 mm diameter could be produced, whose porosity and specific surface area could be tuned for various applications.     

Gelcasting of Al2O3–W composites: Broadband dielectric spectroscopy and rheological studies of tungsten influence on polymerisation kinetics

Gelcasting is a method which allows for the production of high-quality ceramic-metal composites. In this work, the problem of local polymerisation in gelcasting of alumina-tungsten composite and metal influence on activation energy of glycerol monoacrylate was investigated. The broadband dielectric spectroscopy method was used for the first time to study the kinetics of gelation in ceramic processing. Polymerisation reaction rates constant of pure monomer, and monomer with ceramic and the metal filler, within the temperature range from 313 K to 333 K were determined. The obtained results were compared with rheological measurements. The calculated activation energy of polymerisation for monomer with 0.5 vol% of tungsten is 66 kJ/mol in comparison to a pure monomer which is 290 kJ/mol. This indicates the catalytic influence of metal on the polymerisation. Therefore, in the alumina suspension with tungsten polymerisation begins near the surface of the metal particles resulting in polymer structure inhomogeneity and lower mechanical properties of green bodies.     

水溶性高分子PVP对陶瓷凝胶注模成型坯体表面起皮的抑制作用与机理

在凝胶注模成型的丙烯酰胺单体溶液中加入适量的聚乙烯吡咯烷酮(ＰＶＰ),可有效地克服在空气气氛下单体聚合形成凝胶网络的氧阻聚问题,从而消除了坯体表面起皮现象,达到精确控制坯体尺寸的目的．本文研究了加入ＰＶＰ对Ａｌ_２Ｏ_３粉料的分散性、陶瓷浆料的静/动态流变特性、生坯的强度以及显微结构的影响;并通过红外光谱初步分析了ＰＶＰ消除陶瓷坯体表面起皮现象的作用机制,即利用ＰＶＰ增稠作用和ＰＶＰ分子间的氢键作用在表面处起到粘结粉体的作用．     

Rapid fabrication of alumina-based ceramic cores for gas turbine blades by stereolithography and gelcasting

Injection moulding is accepted as one of the most important methods for shaping complex ceramic cores, which are used to form intricate internal cooling passages of gas turbine blades. But the relatively long lead time and high costs involved in the fabrication of hard tooling render it uneconomical for new products development and low-volume production. In the study, a rapid prototyping process is developed to fabricate complex-shaped alumina-based ceramic core by combining stereolithography (SL) with gelcasting. SL is utilized to fabricate an integral sacrificial resin mold, and gelcasting is utilized to form a wet ceramic core green body through polymerization of aqueous ceramic slurry. The freeze-drying process is adopted to treat the wet green body surrounded by the resin mold, the drying shrinkage is decreased, and the generation of crack can be prevented. The sintering shrinkage of ceramic core is controlled by adding magnesium oxide power and developing a novel sintering process. After the resin mold is burnt out, the complex-shaped alumina-based ceramic core is obtained.     

Short-period segmented-in-series solid oxide fuel cells on flattened tube supports

Segmented-in-series solid oxide fuel cells (SIS-SOFCs) were prepared on flattened-tube partially stabilized zirconia supports. The distinguishing characteristic of these cells was the short repeat period, 2.4 mm, and small active cell length, 1.3 mm, compared to ≈10 mm in previous SIS-SOFCs. The support tubes, formed by gelcasting, were bisque fired and then screen printing was used to sequentially deposit Ni-YSZ anodes, YSZ electrolytes, and Pt-YSZ composite interconnects. After high-temperature co-firing, LSM–YSZ and LSM cathode layers were screen printed and fired. Each flattened tube side had 12–16 individual cells. For testing, the open tube ends were sealed and humidified hydrogen flowed inside of the tubes; air was flowed over the outside of the tubes. Maximum total power at 800 °C was ≈8 W and maximum power density was ≈0.7 W cm⁻², calculated using cell active area. Good stability was observed during a ≈650 h steady-state test. Excellent stability was also observed over ≈20 redox cycles.