Diglyceryl acrylate as alternative additive dedicated to colloidal shaping of oxide materials – Synthesis,characterization and application in manufacturing of ZTA composites by gelcasting

The paper reports the synthesis of a new compound named diglyceryl acrylate (DGA) and its application in manufacturing of ZTA composites by gelcasting method. The compound can be used as a monomer able to polymerize in suspensions and also as a dispersing agent for fine-crystalline powders. The paper describes the synthesis of DGA, the properties of the monomer, the resulting polymer, ceramic suspensions and sintered bodies. The distribution of zirconia grains in alumina matrix was observed on SEM. Vickers hardness and fracture toughness of ZTA composites have been measured. The research revealed that with the use of DGA it is possible to obtain highly concentrated ceramic suspension (60 vol%) of low viscosity. The positive influence of the new monomer on the properties of ceramic suspensions and sintered bodies is explained on the basis of the analysis of spatial structures of all 8 regioisomers of DGA molecule.     

Elimination of Surface Spallation of Alumina Green Bodies Prepared by Acrylamide-Based Gelcasting via Poly(vinylpyrrolidone)

Surface spallation of gelcast alumina green bodies due to the exposure of the surface to air during gelation was successfully eliminated by adding a proper amount of poly(vinylpyrrolidone) (PVP) in aqueous acrylamide premix solution. The influences of PVP on the colloidal characteristics of alumina powder in aqueous solutions, the rheological properties of alumina slurries, the gelation process, and the properties and microstructures of the gelcast green bodies and the sintered samples were systematically investigated. Even though the addition of PVP to premix solution has slightly negative effects on the viscosity of alumina slurries, gelation and strength of green bodies, it has no obvious influence on the final structure and properties of alumina ceramics. Thus, PVP was considered an effective polymer to resolve the problem of the inhibition of gelation due to the presence of oxygen, which usually occurs in gelcasting during the formation of polymer network gel by in situ polymerization of monomers in air. The mechanism of PVP in eliminating the surface spallation problems of green bodies is also discussed.     

Gelcasting of alumina with a new monomer synthesized from glucose

Synthesis of a new low-toxic monomer obtained from glucose is reported and its application in gelcasting process is presented. The new monomer named 3-O-acrylic-d-glucose is an interesting alternative for the commercially available acrylic monomers. The presence of many hydroxyl groups in the monomer molecule allows to create hydrogen bonds in the polymer network. The resulting network does not need any external cross-linking agent to obtain a rigid ceramic part. The present paper describes the properties of ceramic slurries, green and sintered elements obtained from two alumina powders of different grain size (0.21 μm and 0.70 μm) with the use of the new monomer 3-O-acrylic-d-glucose and commercially available 2-hydroxyethyl acrylate.     

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。     

Gelcasting Performance of Alumina Aqueous Suspensions with Glycerol Monoacrylate: A New Low-Toxicity Acrylic Monomer

Gelcasting has been extensively studied and applied as a near-net-shaping technique for preparing complex ceramic parts. However, the use of acrylamide as a main component is an important drawback of this method as it is a hazardous, neurotoxic compound. In this context, a new low-toxicity monomer, glycerol monoacrylate, has been synthesized and successfully used for the gelcasting of ceramic suspensions. The presence of two hydroxyl groups in a monomer molecule allows the formation of a polymer network by the monomer polymerization and cooperative hydrogen bonding without any external crosslinking agent. The stability and rheological behavior of alumina gelcasting suspensions containing the new monomer has been studied by oscillatory measurements under the controlled deformation mode. The results were compared with those obtained for commercially available monomers, 2-hydroxyethyl acrylate and diethylene glycol diacrylate used as a crosslinking agent, in which case the effect of an initiator and activator was studied. The gelcasting performance for both the commercial and the newly synthesized monomers was studied and compared, showing that the former allows a faster gelation, whereas the latter leads to more uniform microstructures and higher sintered densities of up to 99% of TD.     

Gelcasting of alumina suspensions containing nanoparticles with glycerol monoacrylate

An acrylic monomer of low toxicity containing two hydroxyl groups has been synthesized and used for gelcasting in water. The results have been compared to those achieved with the use of a commercially available monomer (2-hydroxyethyl acrylate). Due to the chemical structure of the synthesized monomer, no addition of the crosslinking agent was necessary for gelation and similar results in terms of rheology of suspensions, density and microstructure of the bodies were obtained with respect to those obtained with the commercial monomer. However, higher time for gelation was observed.Two alumina powders with very different particle sizes were used in this study: a commercial submicron-sized powder (d₅₀ = 0.35 μm) and a nanometer-sized alumina obtained by freeze-drying from aluminum sulphate solutions. The rheological behavior of concentrated suspensions was studied in order to establish their stability and to analyse the effect of the different monomers used in the process. Once the suspensions were optimized, the influence of the size of the powder on the gelation process was studied. The sintered density of submicrometer-sized alumina was higher (99%) than that measured when the bimodal suspension was used due to the difficulty to obtain highly concentrated suspensions from nanometric powder.     

Preparation of ZTA ceramic by aqueous gelcasting with a low-toxic monomer DMAA

The present work reports the development of aqueous gelcasting of ZTA ceramic with a low-toxicity monomer DMAA. The rheological properties and the gelation behaviors of the slurries for gelcasting were investigated. It was proved that the time available for casting the slurry (idle time) can be controlled by the amounts of initiator. The ZTA green bodies exhibited a mechanical strength as high as 21 MPa. After sintered at 1600 °C for 2 h, the highest bending strength and fracture toughness of the sintered ZTA samples were as high as 643.3 ± 75 MPa and 6.3 ± 0.3 MPa m^1/2, respectively. SEM photographs revealed that the green bodies and sintered part had a uniform microstructure. The volume fraction of tetragonal phase zirconia was as high as 90%. Dense ZTA green bodies and ceramic parts with complex shaped were produced through the new gelcasting system.     

Acrylic Triblock Copolymer Design for Thermoreversible Gelcasting of Ceramics: Rheological and Green Body Properties

Thermoreversible gelcasting (TRG) is an attractive net-shape powder-based processing technique which relies on the temperature-driven gelation of a polymer solution. This study uses the TRG of alumina to investigate the implications of triblock copolymer design (block length, endblock fraction, and midblock chemistry) on rheological and green body properties. The liquid-to-solid transition and relaxation time in the gel state are controlled by the polymer's endblock length while the total polymer length controls the viscosity at high temperature. Although triblock design and concentration do not affect the green body porosity or sintered density, they do have significant effects on green body behavior. Triblocks with a high fraction of rubbery midblock behave as elastomers and confer significant toughness to the green bodies. In contrast, those with glassy midblocks increase the strength of the body but also behave in a brittle manner. Green body strength increases with increasing triblock concentration and is well described by a model for the strength of ceramic bodies with the binder localized at the particle necks.     

Microstructure and properties of aluminum nitride ceramics prepared by aqueous gelcasting method with nontoxic curdlan as gelling agent

Aluminum nitride ceramics were prepared by aqueous gelcasting method and pressureless sintering technique in N₂ atmosphere using Y₂O₃ as sintering additives with nontoxic curdlan as gel system. The solidification mechanism of curdlan was studied. The effects of curdlan content and solid content on the microstructure, relative density and flexural strength of green bodies were investigated. The influences of Y₂O₃ content and sintering soaking time on the microstructure and properties of sintered bodies were also studied. The results show that, as the temperature increases to 80 °C, the ceramic powders solidify through three-dimensional gel networks of curdlan during gelling process. The green bodies can be successfully fabricated through aqueous gelcasting method with modified powder as original materials. Suitable curdlan content and solid content contribute to fabricating green body with uniform microstructures and high flexural strength. The relative density and flexural strength of sintered bodies enhance as the Y₂O₃ content and soaking time increase. The flexural strength and thermal conductivity are about 107.5∼172.3 MPa and 75.2∼112.5 W/(m·K), respectively. The sintered body with 4 wt% Y₂O₃ soaking for 3 h exhibits the highest thermal conductivity because of appropriate relative density, uniform microstructure and reasonable intergranular phase distribution. The mechanical property and thermal conductivity of sintered bodies can be improved by optimizing the gelcasting process parameter, Y₂O₃ content, and soaking time. The nontoxic gelling system will have wide application for aqueous gelcasting ceramic with complex shape.     

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.     

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.     

Magnesia induced coagulation of aqueous PZT powder suspensions for direct coagulation casting

Coagulation characteristics of poly(acrylate) dispersed PZT powder suspensions by MgO coagulating agent have been studied. The PZT powder suspensions undergoes coagulation at MgO concentrations much lower than the equivalent amount to react with the dispersant indicating a major shift in the coagulation mechanism from the corresponding alumina powder suspensions. Unlike in alumina powder suspensions, the Mg²⁺ ions generated from the MgO reacts with the ammonium poly(acrylate) adsorbed on particle surface that result in cross-linking of PZT particles by Mg²⁺ through the ammonium poly(acrylate) molecules. The particle bridging induces faster coagulation of the slurry cast in a mould as required for an ideal DCC process. The compressive strength and stability against oscillatory stresses of the wet-coagulated bodies increased with increase in number of cross-links between the particles. The PZT green bodies prepared by the DCC process sintered to near theoretical density and the MgO added as coagulating agent (～0.1 wt%) had only minor influence on its piezoelectric characteristics.     

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.     

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.     

Effect of monomers content in enhancing solid-state densification of silicon carbide ceramics by aqueous gelcasting and pressureless sintering

The possibility of obtaining solid-state sintered silicon carbide (SiC) through aqueous gelcasting using commercial SiC powders was demonstrated. Green bodies were prepared from thixotropic SiC slurries in aqueous medium with optimized pH and solid-loading. The monomer system in gelcasting provides strength to the green bodies through formation of a gel network by polymerization and the carbon from polymeric gel enhances the densification of SiC, thereby avoiding addition of carbon externally to the gelcasting batches. Maximum bulk density of 3.16 g/cm³ (98.4% of relative density) was achieved in gelcast SiC on sintering at 2150 °C in argon atmosphere. The effect of carbon on SiC densification is evinced from the changes in microstructure of sintered SiC with increase in carbon content. The density and microstructure of gelcast and sintered SiC was comparable to that obtained from dry pressing and sintering of additive mixed SiC powders.     

Preparation,mechanical and thermal properties of Si3N4 ceramics by gelcasting using low–toxic DMAA gelling system and gas pressure sintering

In this work, Si₃N₄ ceramics were fabricated through an aqueous gelcasting method using a low–toxic monomer called N, N–dimethylacrylamide (DMAA) followed by gas pressure sintering at 1850 °C for 2 h under 6 MPa N₂ atmosphere. The effect of solid loading on performance of slurries, green and sintered bodies was investigated systematically. The results show that the slurries with a solid loading as high as 50 vol% (viscosity 0.17 Pa.s at 100 s^–1) were achieved. With the increase of solid loading (30–50 vol%), the green bodies exhibited a monotonically decreased, however high enough in general, flexural strength of 16.50–11.52 MPa, which was comparable to that of widely–used neurovirulent acrylamide (AM) gelling system. In regard to the sintered bodies, increasing solid loading significantly promoted sintering and improved mechanical properties and thermal conductivity as a result of the increased density, bimodal distribution structure, as well as suitable interfacial bonding strength. The best performance parameters of Si₃N₄ ceramics, bulk density of 3.25 g/cm³, apparent porosity of 0.67%, flexural strength of 898.92 MPa, fracture toughness of 6.42 MPa m^1/2, Vickers hardness of 2.81 GPa, and thermal conductivity of 34.69 W m^–1 K^–1, were obtained at 50 vol% solid loading. This work renders low–toxic DMAA gelling system promising prospect in preparation of high–performance Si₃N₄ ceramics by gelcasting.     

New route for processing of multilayer Al2O3-Co3O4 materials through gelcasting

Multilayer dense ceramic materials composed of Al₂O₃ and Al₂O₃-Co₃O₄ layers have been obtained by gelcasting. The key stage in the process was the optimization of the polymerization idle time in order to ensure strong adhesion between layers without cracks and delamination in a green state and after sintering. The significant advantage of this method is occurence of strong connections between consituent layers due to the slight migration of the slurry to the gelled bottom part of the sample, what is not obseved in techniques based on lamination processes. The multilayer samples were composed of two Al₂O₃ layers and two Al₂O₃-Co₃O₄ layers arranged alternately. The rheological characterization of the slurries was done. The properties of the sintered multilayer bodies were examined in comparison to the single-layer alumina samples. Observations in SEM and ligth miscroscope were performed. The presence of the transition layer in the sintered bodies was observed.     

Mechanical and electrical properties of lithium stabilized sodium beta alumina solid electrolyte shaping by non-aqueous gelcasting

A novel non-aqueous gelcasting system based on the polymerization of 2-hydroxyethyl methacrylate (HEMA) with low toxicity is applied to the fabrication of lithium stabilized beta alumina solid electrolyte (Li-BASE). Triethanolamine and polyethylene glycol (MW. 1000) are used as dispersant and plasticizer, respectively. Effects of dispersant concentration, solids loading and plasticizer concentration on the rheological behavior of the suspensions are investigated and optimized. Cracking and warpage phenomena in the gelcast green body during drying process are discussed. Green body gelcast from the optimized suspension shows homogeneous microstructure with a relative density of 59.4 % and a bending strength of 17 MPa. Properties, including the relative density, phase composition, microstructure, bending strength and ionic conductivity, of Li-BASEs sintered from green body formed by gelcasting and cold isostatic press are evaluated. Results show that gelcasting forming technique performs well in preparing BASE ceramics with high Weibull modulus and ionic conductivity.     

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.     

Postcasting Contraction: Improving the Density of Gelcast Nanoparticle Green Bodies with Heated Liquid Desiccants

To achieve high‐density green bodies, a novel route without needing high solids loading suspensions, that is postcontraction process has been developed. In this process, the wet green body is immersed in heated liquid desiccants (polyethylene glycol [PEG] aqueous solution), which is prepared by gelcasting with thermoresponsive gel system. The density of green bodies is improved through releasing water quickly and shrinking rapidly. It is attributed to the cooperative action of the interior aggregation forces stemming from the shrinkage of network‐structured polymer and the exterior osmotic pressure from PEG aqueous solution. Alumina nanoparticle green bodies with different solids loadings were prepared by gelcasting with PNIPAM Poly(N‐isopropylacrylamide) gel system. The postcontraction process was almost completed in 30 min in liquid at 50°C, and meanwhile the bodies were almost dried. The relative densities of all green bodies and sintered parts are in accordance with each other, around 43% and 94%, respectively.