Gelcasting Process Research Articles

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.

Magnetic properties of FeSiCr alloy powder coils made by gel casting process

A molding coil is molded directly onto enameled wire using soft magnetic materials. When the molding coil size becomes smaller and thinner, the granules consisting of magnetic powder and organic binder cannot evenly fill the coil center column during molding, thereby causing cracks or deterioration in the products. In this study, FeSiCr alloy powder coils were prepared using gel casting. The structures and magnetic properties of coils prepared by dry pressing and gel casting were compared. The experimental results showed that the low-viscosity slurry used in the gel casting process can significantly improve the powder filling uniformity in the coil center. Gel casting protects the enameled wire from being damaged and short-circuited. FeSiCr alloy powder coils prepared using the gel casting process have higher inductance and better DC-bias superposition characteristics than those prepared using dry pressing. However, if the slurry viscosity for gel casting is too high, the powder packing in the coil center column becomes worse, which leads to a lower inductance value. The binder content in gel casting is also lower than that in dry pressing, resulting in a higher initial permeability with the same or higher mechanical strength as that prepared by dry pressing due to the more homogeneous binder distribution. For small-sized and high-inductance FeSiCr alloy powder coils, the problems of uneven packing and cracks in the coil center column can be effectively solved using the gel casting process.

Preparation of porous MgAl2O4 ceramics by a novel pectin gel-casting process

Preparation of porous MgAl2O4 ceramics by a novel pectin gel-casting process

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%).

Biosilicate® Glass-Ceramic Foams From Refined Alkali Activation and Gel Casting

Biosilicate® glass-ceramics are among the most valid alternatives to 45S5 Bioglass. They combine a similar bioactivity and bioresorbability as the 45S5 with superior mechanical strength, owing to the crystallization of a Na–Ca silicate phase. This crystallization may be experienced upon viscous flow sintering of fine glass powders, thus configuring a sinter-crystallization process. As crystallization is seldom complete, sintering can also be applied to semicrystalline powders. The sintering/crystallization combination may be exploited for shaping highly porous bodies, to be used as scaffolds for bone tissue engineering, in the form of foams. The present study aims at exploring a gel-casting process, based on the room temperature foaming of powders suspended in a “weakly alkaline” (1 M NaOH) aqueous solution, followed by sintering at 1,000°C. The gelation of suspensions is attributed to the formation of hydrated compounds, later decomposed upon firing. Amorphous powders provided more intense gelation than semicrystalline ones, promoted a more homogeneous foaming, and stimulated a substantial crystallization upon firing. The homogeneity of foamed samples was assessed using micro-tomography and was further improved by casting foamed suspensions (“foam casting”) before setting.

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.

Combination of gel-casting and reactive melt infiltration for rapid fabrication of SiCw/SiC composites

Gel-casting combined with reactive melt infiltration (RMI) process was first proposed for the rapid preparation of SiC whisker-reinforced SiC ceramic matrix composites (SiCw/SiC). Gel-casting process can prepare isotropic whisker preforms with high volume fraction (≥40 vol.%) in near-net-size shape. RMI process can densify the whisker preform rapidly and achieve a high density. The influence of resin carbon (as the carbon source of SiC matrix) content on the microstructural evolution, phase composition and mechanical properties of SiCw/SiC composites was studied in detail. When the resin carbon content is 24 vol.%, SiCw/SiC can obtain good mechanical properties (218 MPa of flexural strength and 3.11 MPa·m1/2 of fracture toughness) with high density (2.9 g·cm−3) and low open porosity (~0.36 vol.%).

Nonaqueous gel casting using multicomponent concentrated slurries through Michael additive reaction for fabricating silicon nitride dense ceramics

A novel gel casting system using nonaqueous, multicomponent, and concentrated slurries was designed for manufacturing Si3N4 dense ceramics with complex structures. The proposed process involves the dispersion of reagent particles in α-terpineol aided by the saturated adsorption of polyethyleneimine partially complexed with oleic acid (PEI-OA) as well as the addition of small amounts of multifunctional acrylates (MAs). The occurrence of the Michael additive reaction between the MAs and the remaining amines of PEI-OA fixed on the reagent particles resulted in a gradual increase in the viscosity of the slurry until its solidification. Using the Si3N4/α-terpineol slurry as a model system, the effects of processing parameters such as OA complex ratio, particle volume fraction, and MA structures on the solidification behaviors were systematically investigated. A slurry having higher particle concentration which was treated with PEI-OA comprised from lesser OA ratio found to effectively solidify within 2 h when mixed with MA having larger number of acryloyl group per molecule. The proposed gel casting process was then applied to the multicomponent Si3N4–Y2O3–Al2O3–TiO2–AlN/α-terpineol system. The gel casted complex structured green compacts were successfully densified up to 98% by liquid phase sintering at 1750 °C under 0.9 MPa N2 gas pressure.

The Influence of Si3N4w Volume Fraction on the Microstructure and Properties of Si3N4w/Si3N4 Composites via CVI Technique

Wave-transparent ceramic composites with excellent comprehensive performances are of particular significance for the development of space vehicles. In this study, Si3N4w was prepared by the gel-casting process and then combined with chemical vapor infiltration (CVI)-obtained Si3N4 to yield Si3N4w/Si3N4 composites with five Si3N4w volume fractions. The influence of the Si3N4w volume fraction on the microstructures, mechanical properties, and dielectric characteristics of the Si3N4w/Si3N4 composites were all studied. The results showed that Si3N4w/Si3N4 composites possess A sandwich structures. Higher Si3N4w volume fractions led to less incorporated amounts of the Si3N4 matrix at the same depth. The 25 vol% Si3N4w/Si3N4 composites integrated relatively higher Si3N4w volume fraction and load transfer ability of the matrix, leading to better flexural strengths reaching up to 41.37 MPa. The dielectric constants of the five prepared Si3N4w/Si3N4 composites ranged from 2.25 to 4.0. The dielectric losses were all lower than 0.005, demonstrating the good dielectric properties of the Si3N4w/Si3N4 composites.

Gel-Casting Prepared Porous Si3N4 Ceramics with Different Contents of Y2O3 and Al2O3 Additives

Porous Si3N4 ceramics were prepared via a controlled gel-casting process followed by gas pressure sintering with 8 wt.% Y2O3 and Al2O3 additives. The gel-casting parameters including slurry pH value, AM: MBAM ratio, degassing procedure, drying condition and decarbonize process were optimized, and the effects of different Y2O3: Al2O3 ratios on the microstructure and properties of the Si3N4 ceramics were investigated. With an optimized gel-casting process, a high solid loading of 46.5 vol.% was achieved, and as the Y2O3 content decreased from 6 to 1 wt.%, the porosity of Si3N4 ceramics declined from 36.3 to 27.5% while the β-Si3N4 phase content reduced from 38.9 to 15.0%. The bending strength and elastic modulus of the ceramics varied with different Y2O3: Al2O3 ratios and were influenced by the porosity as well as the tailored interlocking structure of elongated β-Si3N4 grains. The Si3N4 ceramics also exhibited low and stable dielectric constants.

Enhanced dielectric properties of highly dense Ba0.5Sr0.5TiO3 ceramics via non-toxic gelcasting

In this paper, a non-toxic water-soluble copolymer of isobutene and maleic anhydride (Isobam) was used for gelcasting of Ba0.5Sr0.5TiO3 (abbreviated as BST50) ceramics as both dispersant and gelling agent. The rheological properties and gelation behavior of BST50 suspensions have been investigated. BST50 suspension with 54 vol% solid loading and 0.1 wt% Isobam was optimized for the gelcasting process. Complex-shaped green bodies were prepared with homogeneous microstructure without large defects. BST50 samples sintered at 1500 °C were processed at a high relative density of 99.6%. The sample possessed a high dielectric tunability of 22% (30 kV/cm) and tanδ of 0.006 (1 kHz) which is decreased by 70% compared with that of dry-pressed samples. The present method has great potential in the preparation of other functional ceramics with complex shape and enhanced performance.

The Properties of 0.5BZT - 0.5BCT Piezoelectric Ceramic from Gel Casting Process by Using Ethylene Glycol Diglycidyl Ether (EGDGE) Epoxy Resin as a Gelling Agent

This research is concerned with properties of 0.5BZT-0.5BCT through a water based gel casting process by using Ethylene Glycol Diglycidyl Ether (EGDGE) epoxy resin as a gelling agent. A solution of an ammonium salt of an acrylic polymer in water (NH4PAA) was used as an additive in order to make the dispersion of the powders easier and provide better stability of the slurry and dispersant. The results were found that viscosities of the 0.5BZT-0.5BCT with EGDGE epoxy resin was minimised by the addition of 1.6 wt% of dispersant based on the dried powder weight. The highest green strengths of around 25 MPa were achieved at 30 wt% EGDGE resin content and 44 vol% solids loading. The gel casting samples were studied at sintering temperatures between 1300-1500 °C. The highest values of piezoelectric and dielectric properties corresponded to 0.5BZT-0.5BCT gel casting samples fabricated with a gel casting slurry incorporating 30 wt% EGDGE resin content, 44 vol% solids loading and sintered at 1500 °C, with d33 and εr values of 310 pC/N and 2515, 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.

Parameterized design and fabrication of porous bone scaffolds for the repair of cranial defects

In bone tissue engineering, the structure of a scaffold is very important for cell growth and bone regeneration. It is better to make the scaffold resemble the native cancellous bone because natural cancellous bone can promote scaffold revascularization, which then accelerates cell proliferation. This study presents a parameterized design and fabrication method for cranial scaffold construction. A native human cranial sample was first scanned using micro computed tomography (CT), followed by 3D reconstruction, after which the internal structure of the bone trabecula was created. Based on an extracted negative bone trabecula model, the design components of “cavity”, “connecting pipe” and “spatial framework” were proposed to describe the scaffold model. Then, by using the parameterized component model and an assembly and deformation algorithm, the bionic scaffold was designed. Its porous distribution, connection, porosity and area size were easily controlled. Finally, a biomaterial scaffold case was fabricated using a gelcasting process, and cell culture testing was performed in vitro to verify the scaffold's biocompatibility. The results show that the scaffold can promote cell growth and that cells accumulate in the form of a mass within three days.

A Taguchi approach to the influence of infiltration parameters on microstructure and properties of W–ZrC composites prepared by the displacive compensation of porosity (DCP) method

In the present work, tungsten carbide (WC) preforms were fabricated by gel-casting process using nontoxic Na-alginate and then reactively infiltrated by molten Zr2Cu to produce W-ZrC composite via displacive compensation of porosity process. An orthogonal array design (OAD) was employed as a statistical experimental method for the fabrication of W-ZrC composites. This paper deals with the analysis of the infiltration parameters such as temperature and time on the microstructures and properties of W-ZrC composites based on Taguchi's experimental design methods. The results of ANOVA showed that infiltration temperature as the most consequential factor affecting the amount of ZrC phase, flexural strength and Vickers hardness. The optimized sample had a flexural strength of 460 ± 4 MPa, Vickers hardness of 9.5 ± 0.4 GPa, relative density of 95.21 ± 1.07 and deflection at the fracture point of 0.19 ± 0.04 mm.

In-situ synthesis and properties of porous cordierite ceramics with adjustable pore structure

Using high purity oxide (MgO, Al2O3 and SiO2) as raw materials, porous cordierite ceramics with a single phase and adjustable pore structure have been in-situ synthesized by foam gel-casting process. The effects of sintering temperature and foaming agent concentration on the phase composition, microstructure and properties were systematically investigated. The results indicated that pure cordierite could be obtained at sintering temperatures of 1430 °C and 1450 °C, while the sample sintered at 1450 °C showed better comprehensive performance. By adjusting the foaming agent concentration in the range of 2–32 g/L, the pore size of the materials could be controlled between 45.3 μm and 151.3 μm with the total porosity of 85.4–87.8%. When the sintering temperature was 1450 °C and the foaming agent concentration was 8 g/L, the prepared samples showed excellent properties and the near net size sintering was achieved. The sintering linear shrinkage was about 0.26%, the total porosity was 86.6 ± 0.2%, the bulk density was 0.35 ± 0.01 g/cm3, the compressive strength was 2.63 ± 0.33 MPa, the thermal conductivity was 0.217 ± 0.026 W/(m·K), the thermal expansion coefficient was 1.70 × 10−6/oC, and the permeability of deionized water was 3.62 × 10−11 m2.

Green gelcasting of CaCu3Ti4O12 ceramics

CaCu3Ti4O12 (CCTO) ceramics with desirable dielectric properties were prepared by a green gelcasting forming method combined with pressureless sintering. A non-toxic natural macromolecule, glutinous rice flour (GRF) was used as the gelling agent for the gelcasting process. The significant parameter, namely, the dosage of gelling agent was carefully studied so as to optimize the gelcasting process. The results showed that the dosage of GRF had evident effect on both gelcasting process and the final dielectric properties of CCTO ceramics. CCTO ceramics from the green body solidified with 3.5 wt% GRF showed relatively optimal dielectric properties, namely a dielectric constant up to 1.60 × 104 and a loss of only 0.063 (at 1 kHz test frequency). CCTO ceramics fabricated by this green gelcasting method have superior dielectric properties compared with that of the ceramics obtained by the dry pressing method.

The Effect of TiO2 Addition on the Electrical Properties of Ce0.8Sm0.2O2−δ Electrolyte Material for SOFCs

Sm and Ti co-doped ceria-based mixed ion-electronic conductor with the compositions of Sm0.2Ce0.8−xTixO2−δ (x = 0–0.15) are prepared by a gel-casting process. The effects of addition of Ti on the phase compositions, the sintering behavior, and the electrochemical properties of sample are investigated. The article tentatively discusses the relation of conductivity with Po2 by defect principles. Results show that the single-phase fluorite structure can be formed only at 500 °C, the addition of certain quantity TiO2 can promote the sintering densification with a uniform grain growth. When the sintering temperature is 1350 °C, the sample with 10% addition of Ti has the highest relative density, it reaches 98.94% of the theoretical density, which is much higher than the one prepared by the solid state method. The conductivity of samples decreases while the conductive activation energy increases by adding TiO2. Besides, it should not be neglected that the conductivity of samples in H2 is much higher than in air, which attributes to the reduction of Ti and Ce ions in reducing atmosphere. Overall, above test and analysis results suggest that TiO2 could be used only as an effective sintering aid but cannot as co-dopant on the optimization of electrical properties for ceria-based electrolytes.

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.

Complex-shaped high speed steel with high mechanical performance fabricated by gelcasting sintering

Abstract Hot isostatic pressing is the most common method to prepare powder metallurgy high speed steel. However, due to the limitation of the steel capsule, it is still a challenge to directly produce complex-shaped high speed steel. Therefore, a gelcasting sintering process is proposed in this study. Complex-shaped high speed steel parts were prepared. First, fine powders (10 μm) were employed to prepare a gelcasting slurry. Then, the slurry was poured into a silicone mould followed by debonding and pressureless sintering. Nearly full densification was achieved in the gelcasting sintered samples. Homogeneous microstructure was observed with fine carbides (1 – 2 μm) evenly distributed in the matrix. Compared to traditional samples prepared by hot isostatic pressing, the bend strength increased from 2 800 MPa to 3 800 MPa. Additionally, the oxygen content of the sintered samples was lower than 100 ppm.