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.     

Fabrication of cordierite filter by in-situ solidification for high temperature dust collection

Cordierite ceramic filters have been shown a potential candidate for a high temperature dust collection, owing to its very low thermal expansion coefficient characteristic as an attribute of excellent thermal shock resistance. It is preferable that a design of porous cordierite filter should have low pressure drop with high permeability while keeping the structural integrity. This study describes a simple and direct approach for fabricating a porous cordierite filter with honeycomb-type structure by introducing various non-through holes into mechanically foamed slurry, which is subsequently in-situ solidified by polymerization reaction. The gelcasting cordierite-based slurry was mechanically foamed in nitrogen-filled chamber. The resulting foamed slurry was cast directly into circular molds equipped with a cover having different number of pins. The gelled-bodies were carefully dried at 25 °C with a controlled humidity, followed by sintering under optimized condition. The dust collection performance, pressure drop, and gas permeability were evaluated as a function of the number of non-through holes in porous cordierite. Based on the results of the tests, the holey porous cordierite filter offered a better filter design for dust collection application.     

泡沫陶瓷的研制

泡沫陶瓷是一种新型的功能陶瓷材料。它具有独特的结构和性能，在工业中有着广泛的应用前景。泡沫陶瓷具有密度小、透气性高、耐高温、抗化学腐蚀等特性。本研究对用颗粒强化的氧化铝骨架合成泡沫陶瓷进行了分析。这种材料比其它多孔陶瓷材料具有更好的热化学性质。这种材料可以用有机海绵浸浆获得，然后烧去海绵，留下多孔陶瓷网。这种方法的优点是它包含了过程参数和陶瓷结构，同时合成物的烧结情况及其它条件的影响在文中也有阐述。     

A novel method based on ultrastable foam and improved gelcasting for fabricating porous mullite ceramics with thermal insulation–mechanical property trade-off

Foam instability and long drying cycle limits the widespread use of foaming method. In this paper, a kind of porous mullite ceramic with thermal insulation–mechanical property trade-off were fabricated via novel ultrastable foam and improved gelcasting procedure. The solidification process and stability of foam slurry, as well as the thermal, mechanical property and pore structure of the porous mullite ceramics were investigated. The results showed that porous mullite ceramics with different bulk densities could be prepared via varying volume of foam which was stable enough to be maintained in slurry for a long time. The accelerated gelation rate as well as the gelation degree resulted in the improved gelcasting method led to a shortened period of drying and demould. The obtained pores, which were small, smooth, and unimodal distributed in size in porous mullite ceramics, contributed to achieving the trade-off between thermal insulation and mechanical property.     

凝胶注模制备多孔陶瓷的研究进展

凝胶注模是将传统的陶瓷粉体成型与有机物原位聚合相结合的一种新型成型工艺,为制备高性能、复杂结构的多孔陶瓷提供了一条新途径.在阐述凝胶注模工艺原理的基础上,综述了凝胶注模结合不同成孔方法制备多孔陶瓷的成孔机制以及最新研究进展,并展望了凝胶注模制备多孔陶瓷的发展趋势.     

Development of ethylene glycol-based gelcasting for the preparation of highly porous SiC ceramics

Aqueous gelcasting is inappropriate for the preparation of highly porous ceramics, due to the large drying shrinkage of green bodies caused by the high surface tension of water. To solve this problem, non-aqueous gelcasting using organic solvents with much lower surface tension was developed. However, for most organic solvents, the precipitation polymerization of gels led to the low strength of green bodies, which was inconvenient for the fabrication of large size workpieces. In this work, a novel ethylene glycol-based gelcasting was developed to prepare highly porous SiC ceramics. Ethylene glycol induced the solution polymerization of gels and increased the strength of green bodies effectively. In addition, the high flexibility of the ethylene glycol-based gels could release the inner stress in the drying process. Highly porous SiC ceramics with large size were successfully prepared by the optimized gelcasting method.     

Preparation and properties of porous ceramics from nickel slag by aerogel gelcasting

To further resource industrial solid waste, porous ceramics with high porosity were prepared by a gelcasting method using nickel slag and kaolin as raw materials and hydrophilic nontoxic SiO₂ aerogel as a gelling agent. The effects of nickel slag content, dispersant and solid content on the properties and microstructure of porous ceramics were investigated in detail in terms of density, compressive strength, porosity, phase composition and micromorphology. The results confirmed that a certain amount of nickel slag can effectively improve the porosity of porous ceramics, while the addition of dispersant can promote the flow of the slurry, enhanced the denseness of the raw billet and significantly improved the compressive strength. However, its excessive use had a negative effect on the ceramic density and porosity. At the same time, the solid content played a key role in the performance of porous ceramics prepared by gelcasting, and too much solid content was also not conducive to the generation of pores. When the nickel slag content was 55%, the amount of dispersant was 2%, and the solid content was 60 vol%, the porous ceramic had a better overall performance, the density of the porous ceramic was 510 kg/m³, the compressive strength was 1.3 MPa, and the porosity reached 80.1%. The major crystalline phases of porous ceramics prepared by nickel slag were cordierite and anorthite.     

Porous YSZ ceramics by water-based gelcasting

Gelcasting, as a novel method to form ceramic bodies, has been successfully developed to fabricate porous YSZ ceramics with an open porosity of 33.1–50.3%, mean pore size of 0.66–0.98 μm and the nitrogen permeability of 215–438 m³/m².bar.h. In order to further illustrate the features of this water-based gelcasting process to prepare porous ceramics, the same YSZ powders were blended with the same additives, and then cold pressed and sintered at the same conditions employed for gelcasting process. Compared with the cold pressed samples, the gelcast bodies exhibit higher open porosity, lower closed porosity, relatively larger pore size and thus higher gas permeability. Therefore, the developed gelcasting process is a very effective method to fabricate porous ceramics for filters or supports.     

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.     

Preparation and characteristics of highly porous BN-Si3N4 composite ceramics by combustion synthesis

Nitride based ceramics are considered as a kind of promising material for structural and functional integration due to their robust structure, extreme environmental resistance and electromagnetically transparency. It is still challenging to prepare nitride based ceramics with homogeneous and controllable microstructure because of their low self-diffusion coefficient and difficulty in sintering. Here, we developed a gelcasting-SHS process by combining gelcasting forming and self-propagating high temperature synthesis (SHS) for the preparation of porous BN-Si₃N₄ composite ceramics. First, carbon residue problem in the gelcasting -SHS process was studied. Based on the result, porous BN-Si₃N₄ composite ceramics with high porosity (69.42 ~ 86.48%), high strength (21.7 ~ 81.0 MPa) and low dielectric constants (1.42 ~ 2.87) were synthesized. In addition, the thermal shock resistance of porous BN-Si₃N₄ composite ceramics until 1000 ℃ was evaluated.     

Enhancing the thermal insulating properties of lanthanum zirconate porous ceramics via pore structure tailoring

The potentially useful role of lanthanum zirconate (La₂Zr₂O₇, LZO) porous bulk ceramics has been rarely explored thus far, much less the optimisation of its pore structure. In this study, LZO porous ceramics were successfully fabricated using a tert-butyl alcohol (TBA)-based gelcasting method, and the pore structures were tailored by varying the initial solid loading of the slurry. The as-prepared ceramics exhibited an interconnected pore structure with high porosity (67.9 %–84.2 %), low thermal conductivity (0.083–0.207 W/(m·K)), and relatively high compressive strength (1.56–7.89 MPa). The LZO porous ceramics with porosity of 84.2 % showed thermal conductivity as low as 0.083 W/(m·K) at room temperature and 0.141 W/(m·K) at 1200 °C, which is much lower than the counterparts fabricated from particle-stabilized foams owing to its unique pore structure with a smaller size, exhibiting better thermal insulating performance.     

Gelcasting of Alumina Using Epoxy Resin as a Gelling Agent

A water-soluble epoxy resin and hardener system for gelcasting of ceramics was developed. Polymerization of the system is a nucleophilic addition reaction, and not affected by oxygen. Therefore, this gelcasting system can be carried out in an air atmosphere. The ceramic slurries were prepared by mixing the aqueous premix solution of epoxy resin with alumina powder and dispersant. For cross-linking, a polyamine hardener was added. Both the rheological properties of the slurries and the gelling behaviors of the aqueous premix solutions were evaluated. The slurry with a solid loading of 80 wt% is optimized for gelcasting. The flexural strength of the sintered ceramics is 335±47 MPa with 98% of theoretical density.     

Porous ceramics with near-zero shrinkage and low thermal conductivity from hazardous secondary aluminum dross

Herein, a simple, versatile, and low-cost approach has been proposed to realize the green utilization of secondary aluminum dross, the hazardous solid waste, namely directly sintering dry-pressed green bodies from secondary aluminum dross to fabricate porous ceramics according to high-temperature foaming process spontaneously without adding spare foaming agents. Aluminum nitride (AlN) in secondary aluminum dross was employed to realize high-temperature foaming due to its oxidation, which makes traditional AlN and salts removal process needless. Moreover, near-zero shrinkage or even expansion during sintering of porous ceramics have occurred because in-situ foaming process together with the oxidation of Al particles well offset the sintering shrinkage. After sintering at 1400°C for 2 h, porous ceramics composed of α-Al₂O₃ and spinel phases with open porosity of 37.91%, sintering expansion rate of 1.13%, flexural strength of 45.67 MPa, and thermal conductivity of 0.97 W/(m·K) have been prepared. Cenospheres as pore-forming agents have been added to further improve the porosity, and alumina-based porous ceramics with open porosity of 28.39%–43.20% and flexural strength of 15.80–52.48 MPa have been obtained. This effective solution for recycling secondary aluminum dross could supply high-performance porous ceramics, which is expected to be applied in the fields of light-weight structural components and thermal insulations.     

Effect of forming process on the integrity of pore-gradient Al2O3 ceramic foams by gelcasting

Pore-gradient Al₂O₃ foams were produced by gelcasting using the epispastic polystyrene (EPS) sphere template. This approach allows the design of porous ceramics with degree of pore connectivity and height of gradient layers via appropriate selection of the sizes and numbers of spheres. The fabrication processing of open-cell porous ceramics limited by polymeric sponge template, sharp cracks at the strut edges and closed pores can be resolved by this approach. To achieve the optimal manufacturing conditions of maintaining integrity of the network, the effects of solid loads, height of the slurry and the pre-removal of the polymeric foam template on the struts of the ceramic foams were studied. The results revealed that 55 vol.% Al₂O₃ slurries with 0.5 wt.% ammonium polyacrylate kept good fluidity for casting and avoided the inner inordinate shrinkage. Different shrinkage behavior of the top and bottom of the sample was effectively reduced due to approximately same water vapor diffusion areas on the top and bottom. The integrity of dendritic solidification structure maintained perfectly through template pre-removed in dichloromethane compared with direct heating.     

氧化硅凝胶注模成形技术研究

研究了分散剂的种类、用量、料浆pH值、球磨时间、有机单体含量等因素对氧化硅料浆性能的影响,制备出固相体积分数达42vol%,流变性能够满足凝胶注模工艺要求的二氧化硅料浆,并成功制备了复杂形状的二氧化硅坯体。对凝胶注模成形二氧化硅坯体低温烧结技术进行了研究,提出了素烧、浸渍工艺,有效地降低了二氧化硅坯体的烧成温度。实验表明:二氧化硅凝胶注模成形坯体在1200℃素烧,冷却后浸渍硅溶液,可在1300℃烧结。     

废旧泡沫塑料的回收利用

将废旧泡沫磺化制备聚苯乙烯磺酸及其钠盐,目标产品是具有良好絮凝性能的助凝剂。以产品和聚合硫酸铁对陶瓷废水进行絮凝净化试验,取得了良好效果,并研究了最佳实验条件。结果表明,使用两者可以提高絮凝效果,用聚苯乙烯磺酸钠作为助凝剂的处理成本低和方法简单,值得推广应用。这对废旧泡沫的回收利用有明显的效果,减少对环境的危害。     

粉煤灰发泡陶瓷的制备及性能研究

面对日益匮乏的陶瓷原料,利用固体废弃物来制备发泡陶瓷已是当今趋势。以粉煤灰为主要原料,研究铬渣掺量、碎玻璃掺量和粉磨工艺对粉煤灰发泡陶瓷的影响。结果表明,掺入适量的铬渣可改善粉煤灰发泡陶瓷的性能,小掺量的碎玻璃对粉煤灰发泡陶瓷的性能影响较小。当原料配比为m(粉煤灰)∶m(铬渣)∶m(长石)∶m(碎玻璃)=60∶10∶20∶10时,采用湿法粉磨3 h,可以制得平均孔径为0.64 mm,体积密度为368.54 kg/m³,抗压强度为8.11 MPa的发泡陶瓷。     

注凝成型(gelcasting)工艺及其新发展

简要概括了注凝成型的工艺过程及其特点,着重介绍了低毒性凝胶系统的选择、注凝成型技术的扩大应用2个方面.分析了注凝成型现阶段存在的问题,并指出了新的发展方向.     

陶瓷胶态注射成型中压力的影响

陶瓷胶态注射成型是一种结合了凝胶注模成型和注射成型优点的新成型工艺。不同于凝胶注模成型中通过温度诱发反应,在胶态注射成型中是利用温度与压力共同作用引发陶瓷浆料固化。采用自行研制的凝胶点测试装置,研究了不同温度条件下压力对氧化铝浆料固化过程的影响,发现提高压力能促进浆料固化,不仅能缩短其聚合诱导期,而且能加速固化过程。此外,探讨了胶态成型的作用机制,为胶态注射成型的自动化设计提供了相关工艺参数。     

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.