Aqueous Gelcasting Research Articles

Low-toxic gelcasting of zircon ceramics

Zircon (ZrSiO4) ceramics are highly valued in various industries due to their exceptional physical and chemical properties. In the present work, zircon ceramics with high-density and desirable mechanical properties were successfully prepared using an aqueous gelcasting method, involving the use of a low-toxic and water-soluble monomer, N,N-dimethylacrylamide (DMAA), as the binder. A high solid loading and low viscosity zircon slurry was achieved by optimizing the pH and the dosage of the ammonium polyacrylate (PAA-NH3) dispersant. Zircon green bodies fabricated through the gelcasting method exhibited uniform microstructures and relatively high mechanical strength exceeding 11.0 MPa. The impact of solid loading on the density, decomposition, flexural strength, micro-hardness, and microstructure of the final ceramics were carefully studied. Notably, the ceramics, prepared by solidifying the slurry with a 45 vol% solid loading and sintering the green body at 1550 °C for 2 h, exhibited a relative density of 96.3 %, high phase purity, a uniform microstructure, and the highest flexural strength of 407.9 MPa. The results demonstrate that mechanically robust dense zircon ceramics can be effectively prepared using the low-toxic gelcasting method.

Direct-methane anode-supported solid oxide fuel cells fabricated by aqueous gel-casting

Direct methane Solid Oxide Fuel Cells (SOFCs) operated under catalytic partial oxidation (CPOX) conditions are investigated, focusing on the processing of the anode support and the anode deactivation caused by carbon deposition. Anode-supported SOFCs based on gadolinium-doped ceria (GDC) electrolyte, and NiO-GDC anode support were fabricated by the gel-casting method. Suitable aqueous slurries formulations of NiO–GDC were prepared, starting NiO-GDC nanocomposite powders, agarose as gelling agent and rice starch as pore former. Electrochemical and mechanical tests evidenced that the support of 550 ± 50 µm thickness and 10 wt% pore former is a good candidate for direct-methane SOFCs. The cells operating under stoichiometric conditions of CPOX reached a performance of 0.64 W·cm−2 at 650 ºC, a very close value to that measured under humidified hydrogen (0.71 W·cm−2). The best electrochemical stability of the cell is achieved at a CH4/O2 ratio of 2.5, showing no evidence of carbon deposition and reducing nickel re-oxidation significantly.

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.

One-step sintering for anti-fouling piezoelectric α-quartz and thin layer of alumina membrane

Lead-free piezoelectric ceramics are attracting significant attention in the construction of separation membranes because they offer a self-cleaning function and have no risk of lead leaching from the materials. In this study, the fabrication of cost-sensitive and self-cleaning ceramic composite membranes via co-sintering of a porous α-quartz green support and an alumina microfiltration layer is reported. The α-quartz green support prepared by aqueous gel-casting has benign wettability and flexural strength, which is beneficial for forming a thinner and homogeneous membrane layer by dip-coating. The effects of the co-sintering temperature and dip-coating parameters on the pore size, permeance, mechanical and chemical stabilities, and piezoelectric properties were further investigated. The optimal membrane had an average pore size of 190 nm by co-sintering at 1200 °C and a strongest ultrasound response of 15 mV at an alternating voltage (AV) of 100 V. Finite element analysis showed that the composite membrane generated a beneficial displacement and acoustic pressure (acpr) for cavitation effect. In-situ mitigation of fouling was demonstrated during 500 ppm oil-in-water (O/W) emulsion filtration, which resulted in a stationary flux (Jstat) of 190 L m−2 h−1·bar−1, increased by 65.7% at 100 V AV.

The hydrolysis process of γ-AlON powder and preparation of highly transparent ceramics with aqueous gel-casting

The untreated γ-AlON powders were proposed to prepare highly transparent γ-AlON ceramics by aqueous gel-casting technique and two-step sintering process. The related mechanism of hydrolysis of γ-AlON powders was discussed. The surface of the synthesized γ-AlON powders is covered with a layer of aluminum hydroxides compound. As the period of hydrolysis extends, the surface layer dissolved, and the boehmite and bayerite gradually formed, which is characterized by various techniques including XRD, FT-IR, XPS and TEM. The low viscosity slurry with high solids loading (53 vol%) was obtained by γ-AlON powders without any surface modification treatments as a result of the slow hydrolysis process. In addition, compared with the dry pressed sample, the sample (4.12 mm in thickness) prepared by aqueous gel-casting exhibits superior in-line transmittance of 81–83% in the visible region, which could be ascribed to the more homogenous and denser microstructure of the green body. Finally, the Weibull modulus and characteristic strength of γ-AlON transparent ceramic were determined to be 4.85 and 295 MPa, respectively.

Piezoelectric porous α-quartz membrane by aqueous gel-casting with enhanced antifouling and mechanical properties

Porous α-quartz ceramics with improved mechanical strength were fabricated by using silica sol as a bonding agent in an aqueous gel-casting process. α-quartz dispersion was prepared with a solid load of 75 wt% and a viscosity of 1.66 Pa·s. α-Al2O3 particles were added to promote dispersion of the bonding agent. The optimum amounts of amorphous silica and α-Al2O3 were 10 % and 8 % of the mass of α-quartz powder, respectively. The thermal treatment of α-quartz green bodies was optimized for mechanical and permeation properties. Electrical fields up to 1 kV/mm were applied to the green bodies to accomplish α-quartz grains orientation and hence piezoelectric response. This caused an increase of piezo-acoustic response by 58 % at an excitation frequency of 200 kHz. In-situ mitigation of fouling was obtained by applying an alternating voltage of 60 V, during TiO2 suspension filtration, which resulted in an increase of the stationary flux by 23 %.

Preparation of porous Y2SiO5 ceramics with high porosity and extremely low thermal conductivity for radome applications

It is well known that aqueous gel-casting is challenging to prepare high-porosity ceramics due to the considerable drying shrinkage, cracking, and deformation of green bodies during drying caused by the high surface tension of water. Porous Y2SiO5 ceramics with high porosity were prepared by introducing carbon fibers as a support material in the drying process of aqueous gel-casting to reduce shrinkage during drying. Burning out the carbon fibers after drying does not negatively affect the properties of the porous ceramic. As prepared green bodies by aqueous gel-casting have low shrinkages of 8.69%–6.81% during drying processes and high compressive strength of 13.73 ± 1.55–10.66∼0.49 MPa. The higher compressive strength of the green body has a positive significance for processing porous ceramics into special-shaped structures. As prepared porous Y2SiO5 ceramics have high porosity of 73.94%–87.71%, lightweights of 1.16–0.55 g•cm3, extremely low thermal conductivities of 0.134 ± 0.006 to 0.051 ± 0.001 W•m−1•k−1, relatively low dielectric constants of 2.34–1.58, and tan δ are lower than 1.25 × 10−3. Porous Y2SiO5 ceramics with excellent dielectric properties and thermal insulation properties meet the requirements of thermal insulation and wave transmission integration of radome materials. Aqueous gel-casting also enriches the preparation methods of high-porosity Y2SiO5 ceramics.

Consolidation of complex-shape zircon compacts through agar gelation

Zirconium silicate (zircon, ZrSiO4) is a ceramic material with excellent thermal properties, such as low thermal conductivity and high thermal shock resistance. In this work, dense zircon complex shaped parts were obtained by aqueous gelcasting through agar gelation. The colloidal stability study was performed through zeta potential measurements as a function of pH and polyacrylic-based deflocculant concentration (PAA). The rheological study was carried out varying the solids content, the dispersant concentration, and the sonication mixing time, the best results being obtained for 0.1 wt.% PAA and 1 min sonication. The viscosity versus temperature curves were recorded on cooling for the zircon-agar mixtures showing a gelling temperature of approximately 35 °C. Better gelcasting performance was achieved with suspensions prepared at a solids loading of 40 vol.%. Dynamic sintering (25–1550 °C) and isothermal sintering tests (1550 °C and 1600 °C) were carried out, studying the shrinkage and densification of the material. Finally, the evolution of phases and the microstructure of the sintered parts were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Relative densities of 94.5% TD were reached at 1600 °C/2 h, but increased to 96% with 8 h holding, with a small grain size increase from 1.8 to 2.4 µm.

Highly porous LAS-SiC ceramic with near-zero thermal expansion prepared via aqueous gel-casting combined with adding pore-forming agents

The main problem during preparing high porosity ceramics via aqueous gel-casting is the large drying shrinkage of green bodies. In this work, carbon fibers with high aspect ratio were introduced into aqueous gel-casting processes, which significantly inhibited the drying shrinkage of green bodies by overlapping fibers. At the same time, exhaustible carbon fibers sintered in air also act as sacrificial templates to further improve the porosity. Through the above method, porous LAS/SiC ceramics with high porosity and near-zero expansion were prepared. Drying shrinkage of the green bodies was reduced from 25.2% to 5.7% with the increase of fibers. The porosity of the obtained LAS/SiC ceramic is as high as 76.2%. When the fibers content is 20 vol% and LAS to SiC volume ratio is 6: 4, the composite has maximum value of compression and bending strength (18.2 and 14.0 MPa) and near-zero CTE (0.9 × 10−6 /K).

Effect of mullite phase formed in situ on pore structure and properties of high-purity mullite fibrous ceramics

Porous mullite ceramics are potential advanced thermal insulating materials. Pore structure and purity are the main factors that affect properties of these ceramics. In this study, high performance porous mullite ceramics were prepared via aqueous gel-casting using mullite fibers and kaolin as the raw materials and ρ-Al 2 O 3 as the gelling agent. Effects of addition of mullite fibers on the pore structure and properties were examined. The results indicated that mullite phase in situ formed by kaolin, and ρ-Al 2 O 3 ensured the purity of mullite samples and mullite fibers bonded together to form a nest-like structure, greatly improving the properties of ceramic samples. In particular, the apparent porosity of mullite samples reached 73.6%. In the presence of 75% of mullite fibers, the thermal conductivity was only 0.289 W/m K at room temperature. Moreover, the mullite samples possessed relatively high cold compressive strength in the range of 4.9–9.6 MPa. Therefore, porous mullite ceramics prepared via aqueous gel-casting could be used for wide applications in thermal insulation materials, attributing to the excellent properties such as high cold compressive strength and low thermal conductivity.

Fabrication and properties of highly transparent Li0.07Al2.76O3.64N0.36 ceramics by aqueous gelcasting and two-step preparation

Highly transparent Li0.07Al2.76O3.64N0.36 discs (φ 22 × 2.0 mm) and blocks (45 × 40 × 12 mm) were successfully prepared by aqueous gelcasting and two-step preparation. After introducing a small amount of lithium, the lower N1s binding energy and higher intensity of Al–N bonds on the surface of Li0.07Al2.76O3.64N0.36 powder cause Al–N breakage to be effectively avoided, which inhibits the hydrolysis of powder. Using MAM and MABM as monomers and gelling agents, a stable aqueous suspension with solid loading up to 55 vol% and low viscosity of 83 mPa s was prepared, and the rheological properties and gelating behaviour of the slurries were evaluated. Due to its composition and novel crystalline structure, the Li0.07Al2.76O3.64N0.36 component presented significantly different performances from γ-AlON. The Vickers hardness reached 15.44 ± 0.34 GPa at a load of 9.8 N, and the in-line transmittance reached ∼84.7% at 3.7 μm.

Structure and properties of Al2O3-bonded porous fibrous YSZ ceramics fabricated by aqueous gel-casting

To meet requirements for high porosity and high strength, novel aqueous gel-casting process has been successfully developed to fabricate Al2O3-bonded porous fibrous YSZ ceramics with ρ-Al2O3 and YSZ fibers as raw materials. Microstructure, phase composition, apparent porosity, bulk density, thermal conductivity, and compressive strength of fabricated porous ceramics were investigated, and effects of fiber content on properties were discussed. According to results, bird nest 3D mesh with interlaced YSZ fibers and Al2O3 binder was formed, ensuring the ability to obtain high performance, lightweight ceramics. An increase in the number of YSZ fibers led to more complex interlaced arrangement of fibers and denser network structure of porous ceramics at retaining their stability. Furthermore, their apparent porosity and bulk density increased, whereas thermal conductivity and compressive strength decreased with increasing the fiber content. In particular, comparatively high porosity (71.1–72.7%), low thermal conductivity (0.209–0.503 W/mK), and relatively high compressive strength (3.45–4.24 MPa) were obtained for as-prepared porous ceramics, making them promising for applications in filters, thermal insulation materials, and separation membranes.

Effects of glycerol addition on the slurry dispersion and mechanical properties of alumina ceramics prepared by gel-casting process

The addition of glycerin reduced the dielectric constant of the aqueous alumina suspension, increased the zeta potential of the alumina powder, thickened the electric double layer, and improved the dispersibility of an alumina suspension for aqueous gel-casting. The glycerin-added suspension can effectively avoid the agglomeration, so that the green body had a more uniform microstructure and a higher relative density. The simultaneous addition of magnesium nitrate, EDTA and glycerol in aqueous-based alumina slurry can effectively promote the homogeneity of green microstructure and hence obtaining the sintered sample with relative sintered density of 99.7% and the mean grain size and distribution of 1.20 ± 0.44 μm. The (pressure-less sintered) sample Mg + EDTA + Gly exhibited superior Vickers hardness comparable to the samples prepared using HP, HIP, and SPS due to the small grain size (1.2 μm) and high sintered density (99.7%).

A novel route to fabricate Yb:YAG ceramic fiber and its optical performance

Transparent laser ceramics are the attractive gain medium for solid-state lasers. However, they have not been applied to the fiber lasers due to the difficulty of forming green fibers by traditional methods. In this study, a novel route combined aqueous gelcasting with capillary glass tube was designed to prepare Yb:YAG transparent ceramic fiber with a diameter of 1.0 mm and 43.0 mm in length for the first time. The slurry with high solid loading and low viscosity was smartly inhaled into the capillary glass tube to conquer the difficulty of casting. The green fibers without cracks and deformation had highly dense microstructure. The in-line transmittance of highly doped 15.0 at.% Yb:YAG transparent ceramic disc prepared from the same preparation conditions of Yb:YAG ceramic fiber was 80.0 % at 1064 nm. This study provides a novel route to prepare transparent ceramic fiber differing from extrusion processing, promoting the development of fiber laser.

Preparation of MgO–Y2O3 Composite Ceramic by Aqueous Gel Casting

MgO–Y2O3 composite ceramics were prepared by gel-casting technology. The surface of MgO powder is passivated with H3PO4 ethanol solution. The treated powder can be dispersed in water with a dispersant. The powder surface modification process was verified by pH, Zeta potentials, Fourier transmission infrared absorption spectroscopy (FT-IR), the absorbance of suspension and rheological properties. The obtained stable suspensions were gel cast, dried, and sintered at 1400 °C for 30 min under 35 MPa. As a comparison, the powder was compressed by die pressing and sintered at the same temperature and pressure. After sintering, the MgO–Y2O3 composite ceramic with a theoretical density of 97.29% can be prepared by aqueous gel casting process. The properties of MgO–Y2O3 composite ceramic fabricated by gel casting are better than die pressing.

A novel gelcasting induction method for YAG transparent ceramic

Yttrium aluminum garnet (YAG) transparent ceramics are an attractive material for solid-state laser, optical and high temperature structural applications because of their prominent advantages. However, it is difficult to achieve the high-quality ceramics by acrylamide (AM) aqueous gelcasting due to the inhomogeneous of gel body induced by traditional methods of “catalyst + initiator” or “initiator + heating”. In this work, a series of gel induction contrast experiments were designed to explore a novel gelcasting induction method. With different gelation process, the SEM of surfaces and fracture surfaces and as-fabricated ceramics quality, it was found the effectiveness of the novel gel induction method “vacuum defoaming + initiator”. The method had the ability to improve the uniformity of the gel body by regulating the form of energy injection and having enough time for the stirring of initiator. In addition, the decreasing residual organics would result in contributing to the subsequent debinding and ceramic quality improvement. Therefore, the “vacuum defoaming + initiator” was regarded as an effective induction method to eliminate defect and to achieve the in-line transmittance of as-fabricated YAG ceramic with excellent optical quality at 1064 nm reached 82.8%. This work greatly promotes the application of gelcasting in YAG transparent ceramics.

Fabrication of Sialon ceramic via non-aqueous gelcasting based on the HEMA gelling agent

In the traditional raw materials used for preparing Sialon ceramics, hydrolytic AlN, cannot easily be used for aqueous gelcasting. To overcome this problem, a new type of gelcasting technique was developed. By introducing 1-methyl-2-pyrrolidinone (NMP) as a solvent, Sialon ceramics were formed using a low-viscosity non-aqueous gel system based on the 2-hydroxyethyl methacrylate (HEMA) gelling agent in this work. Based on this gel system, the rheological properties of the slurry and the solidification and sintering properties of Sialon ceramics were studied. The results show that the best rheological properties of slurry can be obtained with the dispersant dosage of 6 vol%. As the HEMA content increases, the flexural strength of the green bodies increase. When the ratio of HEMA to TMPTA (trimethylolpropane triacrylate) is 10:1 and the HEMA content is 15 wt%, the flexural strength of the green body reaches a maximum of 13.07 MPa. β′-Sialon ceramic has the best comprehensive mechanical properties at the sintering temperature of 1700 °C. Its bulk density, Vickers hardness, fracture toughness and flexural strength were 3.207 g/cm3, 1484 HV, 7.11 MPa m1/2 and 351 MPa, respectively.

Microfabrication of High-Aspect Ratio KNN Lead-Free Piezoceramic Pillar Arrays by Aqueous Gelcasting

The present paper reported a novel approach for the fabrication of a high-aspect ratio (K, Na)NbO3 (KNN) piezoelectric micropillar array via epoxy gelcasting, which involves the in situ consolidation of aqueous KNN suspensions with added hydantoin epoxy resin on a polydimethylsiloxane (PDMS) soft micromold. KNN suspensions with solid loadings of up to 45.0 vol.% have rheological behavior, which was suitable for the gelcasting process. The uniform green KNN bodies derived from the optimized suspension of 42.0 vol.% solid loading and 15.0 wt.% resin had exceptionally high mechanical strength (9.14 MPa), which was responsible for the integrity of the piezoceramic micropattern structure. The square-shaped KNN piezoelectric pillar array with lateral dimensions of up to 5 μm and an aspect ratio of up to five was successfully fabricated.

Ti2AlC bulk ceramics produced by gelcasting and Al-rich pressureless sintering

Ti2AlC bulk ceramic was produced via aqueous gelcasting followed by Al-rich pressureless sintering. The processing parameters were optimized and two pressureless sintering methods of Al-rich and C-rich were compared and analyzed.The results showed that stable Ti2AlC slurry and homogeneous green body can be obtained at the pH value, zeta potential, dispersant content and solid content of 10, 65.2 mV, 1.6 wt%, and 48 vol%, respectively. Impurities of 0.6 ppm O2 and 0.2 ppm N2 containing in the flowing argon promoted the thermal dissociation of the gelcasted bulk Ti2AlC, forming whiskers of Al2OC and cloud-like floccule of AlN and simultaneously resulting in the delamination of a duplex layer, an outer ring layer of Ti3AlC2 and a loose and porous cracking layer of Ti2AlC–Ti3AlC2. The synergistic effect of the porous reaction layer and the unclosed channels formed after debinding resulted in a severe decrease in the density and mechanical properties of the C-rich pressureless sintered samples. In contrast, the Al-rich pressureless sintering system can provide a strong and effective protection for the sintering of the gelcasted Ti2AlC green body. The density, hardness, flexural strength and fracture toughness of AlRC sample reached 3.98 g/cm3, 4.29 GPa, 341 MPa, 4.83 MPa m1/2, respectively.

Viscoelastic behaviors and drying kinetics of different aqueous gelcasting systems for large Nd: YAG laser ceramics rods

AbstractLarge and nondeforming Nd: YAG ceramic prepared by wet forming is of great importance as gain medium to obtain high‐power solid‐state lasers. However, it is difficult to achieve high‐quality laser ceramics due to insufficiency of the in‐depth understanding of transformation mechanism of gels viscoelasticity and effective control means during drying process. In this work, the rheological behaviors, viscoelastic characteristics, and mechanical strengths in classical acrylamide (AM) and novel Isobam (PIBM) gelcastings were systematically compared to explore the suitable route for the large‐sized 2% Nd: YAG transparent ceramics with high aspect ratio (>10). AM system exhibited a higher complex viscosity (1.82 × 105 Pa s), a shorter gel time (92.9 seconds), and a higher flexural strength (about 24.46 MPa) than PIBM system, and especially its ability to quickly gel was beneficial to the homogeneity of green body. In addition, the order of drying rates of wet gels in four drying media was observed as follows: 55℃ hot air> ethanol> solid desiccant> PEG‐11000 and the moisture diffusion coefficients were calculated and simulated to offer the deep consideration of drying kinetics. The “ethanol + 55℃ hot air” was regarded as an effective composite drying method to eliminate defect and to achieve φ8 mm × 160 mm Nd: YAG ceramic with the in‐line transmittance of 83% @1064 nm. Therefore, not only the cognition of gel process, but also the defects control strategy is proposed. More importantly, this work greatly promotes the application of wet forming and laser ceramics in high‐power lasers.