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.     

Gelcasting of through-pore hydroxyapatite ceramics

Although the near-net-shape forming ability of gelcasting has been well demonstrated, current attention mainly was focused on manufacturing compact ceramics. In this work we demonstrated that the combination of the gelcasting technique and the ceramic sintering process could prepare the near-net-shape through-pore hydroxyapatite (HAP) ceramics, in which the pore diameter at the range of < 2 μm could be adjusted by changing the content of HAP in green bodies and with the aid of alcohol immersion. Experiments demonstrated that the gels in green bodies and alcohol immersion played important roles in improving the strength of green bodies and decreasing the shrinkage and deformation of green bodies, and thus improving the through-pore and near-net-shape forming of HAP ceramics. Such method reported here possibly extends the gelcasting technique to manufacture other through-pore ceramics for precise filtration, protein separation, or high performance liquid chromatography.     

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。     

A Synergistic Low-Toxicity Gelcasting System by Using HEMA and PVP

A new low-toxicity gelcasting system with a 2-hydroxyethyl methacrylate (HEMA) monomer was applied for casting of alumina ceramics. Polyvinyl pyrrolidone (PVP) was adopted for modifying the homogeneity of the PHEMA (poly-HEMA) gel. The rheological properties of alumina suspension in the HEMA–PVP premix solution were studied. After preparation of a concentrated alumina suspension, homogenous alumina green body with a relatively high strength of about 19 MPa could be formed through the PVP-modified HEMA system. Dense complex-shaped ceramic parts can be successfully produced through the system. Besides, the surface exfoliation phenomenon that seems inherent to the acrylamide gelcasting system could also be eliminated by using the PVP-modified HEMA system. Analysis of the interaction of HEMA and PVP suggested that the improved microstructure and strength homogeneity, as well as the elimination of surface exfoliation in the new system, should be attributed to the intermolecular hydrogen bonding formed between PHEMA and PVP molecules.     

Alumina gelcasting by using HEMA system

A low-toxicity gel system based on the polymerization of low-toxicity 2-hydroxyethyl methacrylate (HEMA) was successfully developed. In order to obtain high solid loading ceramic slurry with low viscosities, a polyelectrolyte dispersant was selected. The results based on alumina suggested that the polyelectrolyte dispersant was more effective for the new HEMA system than tri-ammonium citrate (TAC), which was widely used for dispersing alumina powders. The green alumina bodies derived through the HEMA exhibited a mechanical strength as high as 18 MPa. SEM photos revealed that the green body also had a high homogeneity. Dense complex-shaped ceramic parts were produced through the new gelcasting system.     

先进陶瓷胶态成型新工艺的研究进展

总结了先进陶瓷胶态成型新工艺在清华大学的研究进展.简要介绍了几种高固相含量的浓悬浮体和高质量坯体的制备新技术,提出胶态成型新工艺固化机制的新见解.采用新工艺去除坯体中缺陷,实现高可靠性、复杂形状高性能陶瓷部件低成本的自动化、批量化制备,为先进陶瓷产业化铺平了道路.     

Producing Large Complex‐Shaped Ceramic Particle Stabilized Foams

Highly porous ceramic foams can be produced by combining particle stabilized foams and gelcasting concepts. Sulfonate‐type surfactants are selected to weakly hydrophobize the alumina surface and stabilize air bubbles in suspensions containing gelcasting additives, polyvinyl alcohol (PVA), and 2,5‐dimethoxy‐2,5‐dihydrofurane (DHF). The aim of this work was to prepare large complex‐shaped ceramic foam objects with homogeneous microstructure and high porosity. A key to avoiding drying cracks is to strengthen the wet green body via gelcasting. The influence of the amount of gelcasting additives on the mechanical strength of the ceramic foam green bodies is investigated as well as the effect of using cross‐linking agent versus the addition of just a binder. The presence of a cross‐linked polymeric network within the green body increases its mechanical strength and minimizes crack formation during drying.     

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.     

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.     

Colloidal processing: enabling complex shaped ceramics with unique multiscale structures

Colloidal processing of fine ceramic powders enables the production of complex shaped ceramics with unique micro and macro structures which are not possible to produce via conventional dry processing routes. Because of this enhanced structural control and shaping capabilities, colloidal processing has been exploited to produce ceramic components with ever increasing complexity and functionalities. In this review, we revisit some of the research efforts on this topic to highlight its relevance and growing importance for the advanced manufacturing of functional ceramics. Selected examples of colloidal systems with increasing level of complexity are discussed to showcase the wide range of structures that can be generated through wet processing approaches. The historical development and background knowledge pertaining to colloids and surface interactions is first briefly reviewed. The major colloidal shape forming and additive manufacturing processes that utilize colloidal pastes and inks are then reviewed, highlighting the control of suspension rheology needed in these techniques. Next, methodologies that combine suspended particles with a pore‐forming phase are discussed as a means to produce porous ceramic materials. Further control over the interactions between anisotropic particles and their alignment in suspensions can be gained via externally applied fields (such as magnetic) to produce texturally aligned green bodies. This leads to bioinspired ceramics that can programmably morph into complex shaped objects upon sintering. Hierarchical porous structures with high mechanical efficiency are also shown as an example of the multiscale designs that can be generated through advanced colloidal processing. As drying of ceramic bodies is an inevitable consequence of wet colloidal processing, the current understanding of this critical processing step is reviewed. Finally, the gaps in knowledge in these fields are discussed to provide our perspective on where the field may support advances in ceramics in the future.     

Monomer systems for the gelcasting of foams

The association of the gelcasting process with the aeration of ceramic suspensions allowed the development of a novel category of porous ceramics with unprecedented mechanical properties. One of the critical points in the processing of porous ceramics by this route involves the setting of the foams, which is based on a gelling reaction by the in situ polymerization of organic monomers dissolved in the liquid phase. A variety of monomeric systems that are known to be suitable gelling agents for setting ceramic suspensions into dense forms were investigated in this work, with a view to the production of ceramic foams. These systems, namely ammonium acrylate, N-hydroxymethylacrylamide, methacrylic acid, methacrylamide and methylene-bysacrylamide, were studied considering the main requirements to produce ceramic foams, which include short setting time and high wet green strength. The effects of monomer type on powder dispersion, reaction kinetics, and green strength of wet and dried gelled bodies were investigated. The results revealed that the chemical characteristics of each system can affect the dispersion and rheological properties of suspensions. Polymerization kinetics and wet and dry mechanical strength varied markedly, depending on the type of monomer used.     

Study on Gelcasting of Fused Silica Glass Using Glutinous Rice Flour as Binder

Gelcasting is a colloidal processing method for fabricating high-strength and complex shape ceramic green bodies. However, industry has been reluctant to use the gelcasting technique because the most commonly used gel, acrylamide (AM), is a neurotoxin. Here, we report an attempt at the gelcasting of fused silica glass using a natural and nontoxic gel, glutinous rice flour (GRF) as binder. The GRF-based aqueous system was found to behave excellently in the gelcasting process. Flexural strength of fused silica green bodies solidified with only 3 wt% GRF is up to 11.87 MPa. Bulk density and flexural strength of fused silica glass sintered at 1275 °C are 1.75 g/cm³ and 47.02 MPa, respectively.     

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.     

Gelcasting of Alumina

Gelcasting is a novel method for molding ceramic powder based on a synthesis of concepts derived from traditional ceramics and polymer chemistry. Gelcasting of alumina is described in this paper. The process is based on the in situ polymerization of acrylamide monomers as the setting mechanism for forming the green body. It has the following features: slurries with high solids loading and low viscosity (1.8 Pa·s at 62 vol% solids), dried bodies containing less than 4 wt% binder, and the ability to fabricate complex-shaped bodies.     

Preparation of Si3N4 ceramics with high strength and high reliability via a processing strategy

In this study, the preparation of Si₃N₄ ceramics with high mechanical reliability is investigated. The influences of several processing steps on the bending strength and the Weibull modulus are reported including: (i) coating of the Si₃N₄ powder with its sintering aids, (ii) oxidation of the coated powder, (iii) cold isostatic pressing, (iv) gelcasting of the green bodies and (v) gas pressure sintering. It was found that all the aforementioned steps contribute to improvements of strength and reliability of Si₃N₄ ceramics. Via an optimised processing strategy, Si₃N₄ ceramics with a bending strength and a Weibull modulus as high as 944.7±29.5 MPa and 33.9, respectively, could be prepared. Additionally, it was also found that surface modifications, i.e. coating and oxidation of Si₃N₄ powder, increased the rheological properties of the powder suspension in aqueous media, which is favourable for in situ colloidal forming such as gelcasting.     

Gelcasting of ceramic suspension in acrylamide/polyethylene glycol systems

A new polyethylene glycol/acrylamide-based gelcasting system has been developed. The new system performs at least as well as, and in some cases better than, the original acrylamide-based system. The development of this system is for the sake of eliminating the surface-exfoliation phenomenon of green bodies gelcast in air. This study concentrates attention on dispersion, rheological and gelation behavior in the new system and flexural strength of green body.     

Aqueous Gelcasting of Fused Silica Using Colloidal Silica Binder

An aqueous‐based gelcasting of fused silica ceramics by using colloidal silica binder was developed. Fused silica slurries having different volume percentage of solid loading from 63 to 74 vol% in colloidal silica were made and the rheological properties were evaluated. It was found that the slurry with 73 vol% of solid loading with viscosity 0.70 Pa.s is suitable for this gelcasting system. The influence of solid loading on physical and mechanical properties of gelcast green and sintered bodies has been studied. The fabricated green body by using colloidal silica binder exhibited a flexural strength of 9 MPa and 88% of theoretical density with 2.2% of drying shrinkage while the sintered sample exhibited flexural strength of 60 MPa and 95% of theoretical density with 4.3% of sintering shrinkage. It was observed that, the nano silica particles from the colloidal silica binder is filling the interstitial positions in the consolidated fused silica green body and enhancing physical and mechanical properties.     

Fabrication and properties of porous Si3N4 ceramics by aqueous gelcasting using low-toxic DMAA gelling agent

A low-toxic and water-soluble monomer N, N-dimethylacrylamide (DMAA) was employed as a gelling agent in the gelcasting of porous Si₃N₄ ceramics. The process conditions and composition for slurry preparation (with a solid loading of 36?vol%), the consolidation and sintering of green bodies were investigated and optimized. The effects of various factors such as zeta potential, pH value of the premix solution, dispersant dosage and ball milling time on the rheological properties of the slurries were investigated. The results suggest that the best rheological properties (66.5 mPa.s at a shear rate of 96.3?s^?1) of the slurries were obtained when pH value ranged between 9 and 11, dispersant dosage reached 1?wt%, and ball milling time was 6?h. All the as-prepared green bodies showed a homogeneous microstructure and high flexural strength ≥ 26?MPa with a maximum up to 46.3?MPa when the ratio of DMAA to MBAM, initiator dosage, polymerization temperature and time were 14, 1?wt%, 70?°C and 90?min, respectively. The sintered bodies had a homogeneous microstructure, excellent and regulatable properties, a flexural strength of 216.3–327.3?MPa, and a porosity of 39.6–29.1% by varying the sintering temperature from 1710?°C to 1810?°C and the holding time from 1?h to 3?h. The superior comprehensive effect makes DMAA a promising candidate for an environmentally friendly gelling agent in gelcasting of porous Si₃N₄ ceramics.     

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.