Aqueous Gelcasting Process Research Articles

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 %.

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.

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.

Fabrication of Al2O3-Bonded Fibrous Ceramics by An Aqueous Gel-Casting Process

The traditional gel-casting method mostly uses organic compounds, which are complex process, expensive and pollute the environment. In order to overcome the disadvantage of the traditional gel-casting method, an environment friendly aqueous gel-casting process is introduced in this paper. Al2O3-bonded fibrous ceramics with high porosity and low thermal conductivity have been prepared by a novel hydrated alumina gel-casting process using the ρ-Al2O3and mullite fibers or YSZ fibers as raw materials and the deionized water as hydration agent. A bird-nest-patterned three-dimensional reticular skeleton structure was established by the fibers to ensure a certain mechanical strength. The effects of the amount of mullite fibers andYSZ fibers on the properties of porous ceramic, such as phase composition, microstructure, sintering behavior and relative properties were investigated. Results showed that the as-prepared Al2O3-bonded fibrous mullite ceramics have high porosity of 66.9%∼78.6%, low thermal conductivity of 0.236∼0.479 W/m·K, and relatively high compressive strength of 0.80∼2.78 MPa. The as-fabricated fibrous YSZ ceramics had a high porosity (71.1%∼72.7%), a low thermal conductivity (0.209∼0.503 W/m·K) and a relatively high compressive strength (3.45∼4.24 MPa). Comparing to the as-fabricated fibrous mullite ceramics, the compressive strength of as-fabricated fibrous YSZ ceramics is promoted obviously.

Characterization of Alumina–Molybdenum Composites Prepared by Gel Casting Method

This work concerns the study of ceramic-metal composite materials processed by gel casting method. Due to limited possibilities of fabrication complex-shape elements obtained by classical methods of powder metallurgy (such as isostatic or uniaxial pressing) the colloidal processes are recently willingly applied in manufacturing of ceramic matrix composites. In the present work Al2O3–10 vol.% Mo composites were formed by aqueous gel casting process and sintered in argon atmosphere. Selected physical properties and phase composition have been described. The microstructure of the green and sintered samples was examined with a scanning electron microscope. Molybdenum particles were homogeneously distributed in the ceramic matrix. Moreover, Mo2C was formed during sintering process that had a positive contribution to enhancing the hardness of the composite. The composite after sintering was characterized by a density higher than 95% of theoretical density and high hardness. The performed studies have confirmed the possibility of application of gel casting method to produce Al2O3–Mo composite system.

Fibrous ZrO2-mullite porous ceramics fabricated by a hydratable alumina based aqueous gel-casting process

A near-net-shape ZrO2-mullite fibrous ceramic has been successfully fabricated by a novel aqueous gel-casting using the hydratable alumina (ρ-Al2O3), zircon and mullite fibers as the raw materials. The phase compositions, microstructure, sintering behaviors and properties of the as-prepared porous ceramic were observed and analyzed by XRD, FESEM with EDS, XRT and other testing methods. The effects of mullite fiber content and Y2O3 additive on the improvement of the comprehensive performance were investigated and discussed. The results showed that the hydrates from ρ-Al2O3 could react with zircon to form the binder to combine the mullite fibers together into a bird nest structure, and mullite strips and ZrO2 grains wrapped on the fibers decreased with increasing the contents of fibers. The as-prepared porous ceramic had the increase of apparent porosity and the decrease of shrinkage and bulk density as the amounts of fibers increased. And the low thermal conductivity (0.752–0.854 W(m·K)−1) and high compressive strength (10.4–14.4 MPa) played an important role on its wide applications. In addition, the introduction of Y2O3 additive could remarkably increase the compressive strength to 18.8 MPa by promoting the formation of YSZ and Y-Al-Si-O glass phase.

Fabrication and Characterization of Porous MgAl₂O₄ Ceramics via a Novel Aqueous Gel-Casting Process.

A novel and aqueous gel-casting process has been successfully developed to fabricate porous MgAl2O4 ceramics by using hydratable alumina and MgO powders as raw materials and deionized water as hydration agent. The effects of different amounts of deionized water on the hydration properties, apparent porosity, bulk density, microstructure, pore size distribution and compressive strength of the samples were investigated. The results indicated that the porosity and the microstructure of porous MgAl2O4 ceramics were governed by the amounts of deionized water added. The porous structure was formed by the liberation of physisorbed water and the decomposition of hydration products such as bayerite, brucite and boehmite. After determining the addition amounts of deionized water, the fabricated porous MgAl2O4 ceramics had a high apparent porosity (52.5–65.8%), a small average pore size structure (around 1–3 μm) and a relatively high compressive strength (12–28 MPa). The novel aqueous gel-casting process with easy access is expected to be a promising candidate for the preparation of Al2O3-based porous ceramics.

Effect of solid loading on properties of Cr2O3 refractories by aqueous gelcasting

Dense chromic oxide refractories were prepared by aqueous gelcasting process. Effect of solid loading on rheological property of slurries, properties of green bodies and sintered bodies were investigated. AN-2000 was selected as the dispersant for obtaining slurries with high solid loading and low viscosity. The solid loading of suspension that can meet the requirement for casting was as high as 52 vol%. However, it was found that the highest relative density of the green body and the sintered body separately reached 66·17% and 85·38% when the solid loading came to 50 vol%, while the flexural strength of them were 13·60 MPa and 62·71 Mpa. Once the solid loading increased to 52 vol%, the relative density and mechanical properties of the green body decreased noticeably, which caused by the increase of the slurry viscosity. Besides, SEM images proved that green bodies and sintered bodies had the homogeneous microstructure when the solid loading was 48-50 vol%. As the solid loading was 52 vol%, hard agglomerations and pores formed in the green body, which had a significant influence on the properties of the sintered body.

Preparation of Al2O3–Ni Cermet Composites by Aqueous Gelcasting

The main objective of producing ceramic-metal composites is to improve the fracture toughness of the brittle ceramic matrix. Classical methods of powder metallurgy (such as isostatic or uniaxial pressing) are not capable of fabricating complex-shape elements. For this reason, colloidal processes have been applied recently in manufacturing of ceramic matrix composites. In the present work, Al2O3–3 vol.% Ni composites were prepared by aqueous gelcasting process and sintering in reducing atmosphere. The commercially available monomers 2-hydroxyethyl acrylate and N,N′-methylenebisacrylamide as cross-linking agents were used. The activator was N,N,N′,N′,-tetramethylenediamine (TEMED) and ammonium persulfate was added as an initiator of the polymerization process. Composite slurries were obtained from alumina powders (grain size 0.21 μm) and 3% (in respect to the total solid volume) of two nickel powders of different grain size (~10 and ~0.4 μm). The paper describes the rheological properties of the obtained composite slurries. The properties of green and sintered elements have also been presented. It was found that the presence of nickel particles had a significant influence on the slurry polymerization process. The green and sintered relative density was in every case higher than 57 and 97%, respectively.

Fabrication of integral ceramic mold for investment casting of hollow turbine blade based on stereolithography

PurposeThe purpose of this paper is to develop a novel process of integral ceramic molds for investment casting of hollow turbine blades.Design/methodology/approachAt first, a resin pattern of a hollow turbine blade prototype is fabricated by stereolithography (SL). And then aqueous gelcasting process is utilized to fill the resin pattern with ceramic slurry of low viscosity and low shrinkage, through in situ polymerization of the slurry a ceramic mold is formed. At last, the ceramic mold for investment casting of hollow turbine blade is obtained by vacuum drying, pyrolyzing and sintering.FindingsAn integral ceramic mold is successfully fabricated by combining SL and gelcasting process, cores and shell are connected with each other and thus high relative position accuracy is guaranteed. Properties of integral ceramic mold at room temperature and high temperature satisfy the requirements of directional casting for complex‐shaped thin‐walled blades.Research limitations/implicationsBecause the integral ceramic mold is a close body, it is very difficult to directly measure its inner dimensions and the relative position accuracy of cores and shell, and the further research is needed.Originality/valueThis method enhanced the versatility of using SL prototype in the fabrication of integral ceramic mold for investment castings. Although this paper took a hollow turbine blade as an example, this method is also capable of fabricating integral ceramic molds for other complex investment castings.

Near‐Net Shape β‐Si4Al2O2N6 Parts by Hydrolysis Induced Aqueous Gelcasting Process

In this paper, a new net‐shaping process, an hydrolysis‐induced aqueous gelcasting (GC) (GCHAS) has been reported for consolidation of β‐Si4Al2O2N6 ceramics from aqueous slurries containing 48–50 vol%α‐Si3N4, α‐Al2O3, AlN, and Y2O3 powders mixture. Dense ceramics of same composition were also consolidated by aqueous GC and hydrolysis assisted solidification routes. Among three techniques used, the GCHAS process was found to be superior for fabricating defect‐free thin wall β‐Si4Al2O2N6 crucibles and tubes. Before use, the as purchased AlN powder was passivated against hydrolysis. The sintered β‐Si4Al2O2N6 ceramics exhibited comparable properties with those reported for similar materials in the literature.

Aqueous Gelcasting Process for β‐Si4Al2O2N6 Ceramics

Dense β‐Si4Al2O2N6 ceramics were obtained from α‐Si3N4, α‐Al2O3, AlN, and Y2O3 upon sintering (for 4 h at 1675°C) green bodies consolidated by an aqueous gelcasting and a conventional dry‐powder processing route. The as‐purchased AlN powder was treated with H3PO4 and Al(H2PO4)3 and dispersed in aqueous‐organic premix solution along with α‐Si3N4, α‐Al2O3, and Y2O3 powder with the help of a commercial amino alcohol‐based cationic (Dolapix A88) dispersing agent to obtain suspensions with 45–50 vol% solids loading for gelcasting purpose. The gelcat β‐Si4Al2O2N6 exhibited superior hardness (1423±6 Hv), fracture toughness (3.95±0.3 MPa·m1/2), and coefficient of thermal expansion (3.798 × 10−6/°C between 30° and 1000°C) in comparison with the ceramic consolidated by conventional dry‐powder pressing route, which exhibited only 1317±5 Hv, 3.30±0.2 MPa·m1/2 and 3.532 × 10−6/°C between 30° and 700°C.

Preparation of γ-LiAlO 2 green bodies through the gel-casting process

An aqueous gel-casting process using methacrylamide (MAM) as monomer and N, N′-methylenebisacrylamide (MBAM) as crosslinker was developed for γ-LiAlO 2 ceramics. The precursor γ-LiAlO 2 powder with mean particle size of 3.4 μm was synthesized via solid-state reaction process. The zeta potential of the γ-LiAlO 2 powders in deionized water and viscosity of the slurries with different solid loadings were measured. It was found that the solid loading of the γ-LiAlO 2 slurry that met the requirement of gel-casting could reach 56 vol.%. The effects of pH values and the kind of initiator on the copolymerization of MAM/MBAM were investigated. The green bodies with the binder content of 5.3 wt% exhibited a compressive strength of 12.6 MPa and a yield point as high as 6.4 MPa. SEM results revealed homogeneous and compact morphology of the green bodies. In addition, the compressive strength of the sintered γ-LiAlO 2 bodies were studied simultaneously.

An Aqueous Gelcasting Process for Sintered Silicon Carbide Ceramics

An aqueous gelcasting process for the preparation of dense as well as porous‐sintered SiC ceramics has been described in this paper. A commercial silicon carbide powder coated with phenolic resin was used in this investigation. For the purpose of comparison, a pure SiC powder was also studied. ς potential and viscosity studies revealed that the pure SiC powder requires an electro‐steric stabilization, whereas the phenolic resin‐coated powder requires an electrostatic stabilization in order to produce their corresponding aqueous slurries with high solids content. Thermogravimetry and differential thermal analysis techniques have been used to study the decomposition behavior of phenolic resin. Aqueous slurries containing 25–50 vol% SiC powder were gelcast and sintered at 2150°C for 1 h. The sinterability of gelcast SiC samples was found to be highly influenced by the SiO2 formed on the surface of SiC during aqueous processing, as confirmed by the Fourier transform infrared spectroscopy study. The results obtained from various characterization techniques suggest that in order to make dense SiC parts with >3.13 g/mL bulk density (a theoretical density of 97.5%) by an aqueous gelcasting process, the starting phenolic resin (∼5%)‐coated SiC powder should possess a median particle size of <11.0 μm, surface area of >3.2 m2/g, a compact (green) density of >1.67 g/mL, and a B content of >0.5%. Further, by using polyethylene granules and organic foaming agents, sintered SiC foam with a porosity of >80%, a compressive strength of >16 MPa and a coefficient of thermal expansion of 4.574 × 10−6/°C between 30° and 700°C can be prepared by an aqueous gelcasting process, followed by sintering at 2150°C for 1 h.

Microstructure and texture evolution in gel-cast α-alumina/alumina platelet ceramic composites

Highly textured alumina ceramics have been fabricated by an alginate-based, aqueous gel-casting process. It has been shown that the fine-grained α-alumina matrix can be transformed into a single-phase and highly c-axis textured alumina by gel-casting with the addition of a small amount of alumina platelets, with the strongest texture derived from 9.1 vol.% of the seed platelets in the gelled precursor tape.

Gelcasting of alumina from acidic aqueous medium using acrylic acid

An aqueous gelcasting process for alumina using an acrylic acid monomer has been investigated. Concentrated aqueous alumina slurries with pH 3 were set in moulds at 80°C during 10–15 min by polymerisation of the monomer contained in it using ammonium persulphate as free-radical initiator. The green density maximum (57% TD, corrected for organic content) was obtained at ∼6 wt% binder content, at which the green strength (by diametrical compression test) measured was 11.5 MPa. The green bodies with binder content ⩾4 wt% were amenable to conventional machining operations such as lathing, milling and drilling. The total linear shrinkage of 55 vol% A16SG alumina slurry was 1.4% during drying and 15–17% during binder removal and sintering to 97% TD.