Gel-casting Method Research Articles

The stability of aluminum oxynitride (AlON) powder in aqueous system and feasible gel-casting for highly-transparent ceramic

The stability of AlON powder in the aqueous system was studied during the gel-casting procedure of AlON ceramic. The pH value of AlON slurry, the phase assemblage and the morphology of AlON powder all suggested the excellent stability of untreated AlON powder even in hot water. Finally, highly-transparent AlON ceramic pellets (D = 110 mm and D = 22 mm) were fabricated by a feasible and easy aqueous gel-casting method. The polished AlON ceramics with 3.5 mm thickness possesses the highest inline transmittance above 86% at the wavelength of 2000 nm.

Effects of ZrB2 addition on texture development and properties of porous Si3N4-ZrB2 composites by magnetic field alignment

ABSTRACTTextured porous Si3N4-ZrB2 composites were prepared by the gel-casting method assisted by magnetic field alignment technology and subsequent pressureless sintering. These results showed that when a 6 T magnetic field was applied, the ZrB2 grains also formed a c-axis orientation in the textured green bodies, except for β-Si3N4 grains, which formed an a- or b-axis orientation. The increasing ZrB2 content inhibited the degree of texture of the Si3N4 and ZrB2 grains in the green bodies, respectively. After sintering at 1800 °C, new ZrN and BN phases without grain orientation characteristics formed by the reaction between Si3N4, ZrB2 and N2 and the ZrB2 phase disappeared completely. The sintering process promoted the degree of texture of Si3N4 as the matrix increases. Meanwhile, the variations in the degree of texture of Si3N4 with changes in the ZrB2 content were consistent with that in the green body. Additionally, when 10 wt% ZrB2 content was introduced, the apparent porosity reached its highest value and the bulk-density reached its lowest value, leading to the appearance of least strength. In textured porous composites, the strength was mainly affected by the densification process instead of the grain orientation. Also, no evident difference in strength existed between the two directions owing to the function of high porosity.

Investigation of the microstructure, cation distribution and optical properties of nanoscale NixMg1-xAl2O4 spinel pigments

Abstract Nanoscale NixMg1-xAl2O4 spinel pigments were synthesized by a citric acid precursor combined with the gel-casting method. The microstructure, cation distribution and optical properties as a function of calcining temperature and nickel content were investigated by the X-ray diffraction (XRD) Rietveld refinement, transmission/field emission scanning electron microscopy (TEM/FESEM), X-ray photoelectron spectroscopy (XPS), colour measurement and UV–vis–NIR spectrophotometry. Upon increasing the calcining temperature, both Ni2+ and Mg2+ hindered the migration of Al3+ to octahedral sites. When the Ni content increased, the cation site percentage of Ni2+ in the tetrahedral and octahedral sites varied slightly while that of Al3+ and Mg2+ change substantially. The cation exchange resulted in an increase in the inversion parameters and a decrease in the lattice parameters with increasing temperature or Ni content. Furthermore, Rietveld refinement also showed a shrinkage of the tetrahedra and an expansion of the distorted octahedra in the spinel structure. Short-range information based on optical spectra suggests that variation in the splitting energy of tetrahedra and octahedra caused the change in the spectral absorption. This study may deepen the understanding of the structural-optical property relationship of NixMg1-xAl2O4 spinel, which is vital to the further colour modification of ceramics and glazes.

High-strength macro-porous alumina ceramics with regularly arranged pores produced by gel-casting and sacrificial template methods

Macro-porous alumina ceramics were prepared by the gel-casting method using sacrificial polystyrene spheres as the template. The resulted porous alumina ceramics have regularly arranged pores and high cell wall densities, which confer high mechanical strength to the porous ceramics. The highest compressive strength for a porous alumina ceramic, with a relative density of 30%, in this work is 28 MPa, which is at least 74% higher than that of previously reported porous alumina ceramics with the same porosity. The results show that the cell wall density, window size and cell size can be adjusted by controlling the sintering temperature, solid loading percentage of the alumina slurry and the size of the polystyrene spheres. With the increase in cell wall density and decrease in cell size and window size, the compressive strength of the porous alumina increases. Thus, a novel porous structure with high porosity and high strength can be made by this flexible method which is also suitable for making complicated shapes and large sizes.

Fabrication of SiC bodies by optimized gel-casting method

In this research, acrylate gel maker monomers Mathacrylamide (MAM) and Methylenebisacrylamide (MBAM) were used through a gel casting process to fabricate SiC bodies. Investigation of rheological behavior of suspensions showed that optimum amount of surfactant (Tetramethylammonium hydroxide) was equaled with 0.7 wt% of ceramic powders and ceramic powders could be added up to 37 vol% of solvent without any problems in casting. The effects of monomers content, monomers ratio and also solid loading on gelation time, bending strength of samples were investigated. Results showed that increasing the monomers content (MAM + MBAM) and ceramic solid loading led to the reduction of gelation time. Also alteration of these parameters resulted in variation of samples bending strength from ~6 MPa to 19 MPa, the best strength was obtained in the sample containing 35 wt% monomers with the MAM/MBAM ratio of 3 and 35 vol% SiC. Finally the samples containing different amounts of SiC were casted according to the pre-determined optimum parameters and after being dried they were sintered at 2150 °C for 1 h. According to the results, the highest density, Young's modulus, hardness and indentation fracture resistance were obtained in sample containing 37 vol% SiC and were equaled with 96.89%, 499.24 GPa, 27.51 GPa and 6.3 MPa·m1/2, Respectively. Microstructural studies showed that cleavage fracture planes, crack deflection and branching were the most important mechanisms on toughness enhancement.

Processing of Macroporous Alumina Ceramics Using Pre-Expanded Polymer Microspheres as Sacrificial Template

Shaped porous ceramics have proven to be the most adapted materials for several industrial applications, both at low and high temperatures. Recent research has been focused on developing shaping techniques, allowing for a better control over the total porosity and the pores characteristics. In this study, macroporous alumina foams were fabricated by gel-casting using pre-expanded polymeric microspheres with average sizes of 40 μm, 20 μm, and 12 μm as sacrificial templates. The gel-casting method, as well as the drying, debinding, and presintering conditions were investigated and optimized to process mechanically strong and highly porous alumina scaffolds. Furthermore, a reliable model relating the amount of pre-expanded polymeric microspheres and the total porosity of the presintered foams was developed and validated by mercury intrusion porosimetry measurements. The electron microscopy investigation of the presintered foams revealed that the size distribution and the shape of the pores could be tailored by controlling the particle size distribution and the shape of the wet pre-expanded microspheres. Highly uniform and mechanically stable alumina foams with bimodal porosity ranging from 65.7 to 80.2 vol. % were processed, achieving compressive strengths from 3.3 MPa to 43.6 MPa. Given the relatively open pore structure, the pore size distribution, the presintered mechanical strength, and the high porosity achieved, the produced alumina foams could potentially be used as support structures for separation, catalytic, and filtration applications.

Fabrication of Porous Al₂O₃ Ceramics with Submicron-Sized Pores Using a Water-Based Gelcasting Method.

The gelcasting method is usually employed to fabricate relatively dense ceramics. In this work, however, porous Al2O3 ceramics with submicron-sized pores were fabricated using the water-based gelcasting method by keeping the Al2O3 content at low levels. By controlling the water content in the ceramic slurries and the sintering temperature of the green samples, the volume fractions and the size characteristics of the pores in the porous Al2O3 can be readily obtained. For the porous Al2O3 ceramics prepared with 30 vol.% Al2O3 content in the slurries, their open porosities were from 38.3% to 47.2%, while their median pore sizes varied from 299.8 nm to 371.9 nm. When there was more Al2O3 content in the slurries (40 vol.% Al2O3), the porous Al2O3 ceramics had open porosities from 37.0% to 46.5%, and median pore sizes from 355.4 nm to 363.1 nm. It was found that a higher sintering temperature and Al2O3 content in the slurries increased the mechanical strength of the porous Al2O3 ceramics.

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 m1/2, 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.

Nanosized copper powders prepared by gel-casting method and their application in lubricating oil

Abstract Nanosized copper powders were prepared by a gel-casting method using copper nitrate, acrylamide (AM) and N, N′-methylenebisacrylamide (MBAM) as the main raw materials. The as-prepared copper powders were characterized by X-ray diffractometry and scanning electron microscopy, and then added into a 48# industrial white oil. Dispersion and wear properties of the compounded lubricating oil were tested. The results show that the copper powders prepared are of high purity, fine dispersibility with mean particle size of about 60 nm and with a narrow particle size distribution. The nanosized copper powders can be well dispersed in the lubricating oil. The addition of the copper powders obviously improves the anti-wear properties of the lubricating oil owing to their good self-repairing performance. Compared with 48# industrial white lubricating oil, the friction coefficient of GCr15 steel with the compounded oil containing 0.6% copper powders reduces by 0.07 and nearly no wear chippings are found in the scratches of the friction counter parts.

Photocatalytic degradation of methylene blue using TiO2 supported in ceramic material

The present work evaluates the use of ceramic material, produced with the aid of industrial waste through the Gel-Casting method, as a support for catalysts in the photocatalytic degradation of Methylene Blue. The analysis was carried out in a reactor containing UVC lamps through batch processing. The results show that the reaction of photocatalytic degradation of the dye presents pseudo-first order kinetics, with values of degradation between 60 and 70 %. The study evaluated the deactivation of the catalysts synthesized and supported in the ceramic material. In this case, it was observed that the larger the mass of deposited catalyst on the ceramic support, the longer the lifespan of the sample, since a larger mass takes longer to deactivate.

Preparation, microstructure and properties of AlN/CBC composites

Gelcasting method with nontoxic biopolymer as gel former can be used to prepare ceramics with complex shape. AlN ceramic bonded carbon (AlN/CBC) composites with light weight were fabricated by gelcasting and pressureless sintering method using nontoxic agarose as gel former with Y2O3+CaF2 as sintering additive. The effect of sintering additive content and sintering temperature on the microstructure, flexural strength and thermal conductivity of composites was investigated. The results reveal that, as the temperature decreases, the raw powders solidify into three-dimensional net structures through abundant hydrogen bonds in the agarose. High sintering temperature facilitates densification and improves mechanical and thermal properties of the composites. Increasing sintering additive content can also tune the density and raise flexural strength. The AlN/CBC composite containing 7wt% sintering additive content, which was sintered at 2000°C, exhibits the highest thermal conductivity. Relative density, C content and microstructure uniformity have effects on properties of AlN/CBC composite.

A novel highly porous ceramic foam with efficient thermal insulation and high temperature resistance properties fabricated by gel-casting process

The design of super thermal insulation and high-temperature resistant materials for high temperature furnaces is crucial due to the energy crisis and the huge wasting. Although it is told that numerous studies have been reported about various of thermal insulation materials prepared by different methods, the applications of yttria-stabilized zirconia (YSZ) ceramic foams fabricated through tert-butyl alcohol (TBA)-based gel-casting process in bulk thermal isolators were barely to seen. In this paper, highly porous yttria-stabilized zirconia (YSZ) ceramic foams were fabricated by a novel gel-casting method using tert-butyl alcohol (TBA) as solvent and pore-forming agent. Different raw material ratio, sintering temperature and soaking time were all investigated to achieve optimal thermal insulation and mechanical properties. We can conclude that porosity drops gradually while compressive strength increases significantly with the rising temperature from 1000-1500°C. With prolonged soaking time, there is no obvious change in porosity but compressive strength increases gradually. All specimens have uniformly distributed pores with average size of 0.5-2μm and show good structural stability at high temperature. The final obtained ceramic foams displayed an outstanding ultra-low thermal conductivity property with only 200.6 °C in cold surface while the hot side was 1000 °C (hold 60 min to keep thermal balance before testing) at the thickness of 10 mm.

Fabrication of ZrB2-SiC ceramic composites by optimized gel-casting method

The ZrB2-SiC ceramics with homogenous microstructures were successfully fabricated by the optimized gel-casting method and pressureless sintering. The effects of the processing parameters, including the monomer content, cross-linker/monomer ratios, initiator addition, catalyst concentration and polymerization temperature, on the final gel properties were systematically investigated. The rheological behavior and stability of ZrB2-SiC suspensions were evaluated, and the microstructures and mechanical properties of sintered ceramics were also analyzed. The homogenous gel networks containing low monomer content (4wt%) and without catalysts had been successfully obtained, which could be mainly attributed to the homogeneous distribution of crosslinking points and temperature-induced gelation. The crack-free and complex-shaped ZrB2-SiC ceramic composites were achieved by the optimized gel-casting, which could reach the highest relative density of 97.2% and the flexural strength of 402 ± 57MPa, respectively. This study provides an optimized gel-casting process for fabricating ZrB2-SiC ceramics with excellent properties by low colloidal additives and without catalysts.

Simultaneous enhancement of elasticity and strength of Al2O3-based ceramics body from cellulose nanocrystals via gel-casting process

A green gel-casting method was developed by the combination of rod-like cellulose nanocrystals (CNC) and low toxicity monomer N, N-dimethylacrylamide (DMAA), which was proved to be a promising substitute of traditional neurotoxin monomer acrylamide (AM). The hydrophilic nature and homogeneous dispersion of CNC in aqueous suspension ensured the essential compatibility with the hydrosoluble polymerization system, and therefore provided remarkable mechanical enhancement of green body. The bending strength of the green body was highly increased by 68% with the introduction of 0.9 wt‰ CNC. Meanwhile, the fabricated green body exhibited significant improvement in flexibility and elasticity, with the unique bendable and recovery performances after drying treatment at room temperature for 1h. The computer stimulation by the COMSOL Multiphysics confirmed the special mechanical enhancement effect induced by the presence of highly-crystalline and rigid CNC.

Effect of Dispersant Amount to Fabrication of 3-D Porous Cordierite through Gelcasting Method

In this research, cordierite was produced using oxide based raw materials (MgO, SiO2 and Al2O3) through glass route method. A mixture of raw material was gone through melting at 1550 °C followed by water quenching. Glass powder produced were then milled for 5 hour to get fine powder. Cordierite powder produced was then used to prepare 3-D porous cordierite ceramic. In this research, gelcasting method was choosen. The dispersant amount added in the cordierite slurry was varied (2.5-5.5g). The cordierite pellet were then characterized using x-ray difraction. Results shows high purity (99%) cordierite was obtained. The porosity and compressive strength were tested. The porosity show when increasing the dispersant amount, the porosity will decrease. For compressive strength increase when the dispersant amount increase.

Porous mullite thermal insulators from coal gangue fabricated by a starch-based foam gel-casting method

Porous mullite materials with high porosity of 71.8–88.2% were prepared by foaming of coal gangue/Al2O3/starch composite slurry and subsequent pore structure stabilization using starch consolidation and sintering. Coal gangue was recycled to prepare porous mullite foams. Starch was used as gellant, replacing commonly used poisonous chemical and expensive animal protein additives in foaming. Sintered mullite foams exhibited tri-modal pore structure, including large-sized pores (50–300 μm) replicating gas bubbles, moderate-sized pores (~10 μm) embedded in pore walls, and small-sized pores (<5 μm) appearing among mullite crystals. The compressive strength of mullite foams was between 0.21 and 8.7 MPa with low thermal conductivity between 0.1056 and 0.3848 W/m K, indicating that porous mullite foams are candidate materials for applications in thermal insulation.

Manufacture of Fine-Pored Ceramics by the Gelcasting Method

The fine-pored materials represent a wide range of applications and searches are being continued to develop methods of their manufacturing. In the article, based on measurements on fine-grained powders of Al2O3, TiO2, and SiO2, it has been demonstrated that gelcasting can be relatively simple method of obtaining of nanoporous materials with high values of both specific surface area and open porosity. The powders were dispersed in silica sol, and the gelling initiator was NH4Cl. The usefulness of experiment design theory for developing of fine-pored materials with high porosity and specific surface area was also shown.

Hierarchically bimodal mesoporous silica fibers as building units for silica monolith with trimodal porous architecture

This work reports the controllable preparation of a unique kind of hierarchically mesoporous silica (HMS) fibers with bimodal porosities based on a simple TEOS/P123/HCl(aq.) templating system via the partitioned cooperative self-assembly (PCSA) process. Experimental results show that the formation of hierarchical mesostructures, especially the 2nd mode porous structure, depends on the interval time pertaining to the PCSA process. Synthetic conditions, including the interval time, temperature and stirring, are all found to be important in the evolution of fiber particle morphology. Moreover, such HMS fibers can be used as building units to prepare mesoporous silica monoliths with hierarchical trimodal pore systems via a simple gel-casting method, with additional (the 3rd modal) macropores originating from the packing of micron-sized HMS fibers. Such materials might further expand the application of both HMS fibers and HMS monoliths in various fields.

Synthesis and properties of porous alumina ceramics with inter-locked plate-like structure through the tert-butyl alcohol-based gel-casting method

Porous alumina ceramics with inter-locked plate-like structure were successfully fabricated by the tert-butyl alcohol-based gel-casting method. The effects of Al/Si mole ratio on the structure and properties of the obtained products were investigated. The formation process of the inter-locked structure was analyzed. The products fabricated with the one-step method, in which the growth of plate-like alumina and the formation of the inter-locked structure were combined together, exhibited a high porosity of 76.21%, a compressive strength of 0.525 MPa and a low thermal conductivity of 0.495 W/mK, while the products fabricated by the two-step method after sintered at 1500 °C using the grinded powders after 700 °C heat-treatment as the starting materials possessed a relatively high compressive strength of 4.80 MPa. The obtained porous alumina ceramics were promising flexible insulant materials for the application in high-temperature field.

Preparation of a B4C hollow microsphere through gel-casting for an inertial confinement fusion (ICF) target

B4C hollow microspheres have been proposed to be a promising target pellet for inertial confinement fusion (ICF). However, the preparation of a B4C hollow microsphere is challenging and has been rarely reported. In this study, we developed a gel-casting method followed by a calcining process to fabricate a core-shell structured B4C microsphere on a molybdenum core. The microstructure, stoichiometry, surface roughness, and wall thickness of the microsphere was characterized by Scanning Electron Microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray photography. The results indicated that a core-shell structured microsphere with uniform microstructure and high strength were obtained by gel-casting, accompanied by calcining at 1400°C. The diameter of the molybdenum core was 2mm, and the wall thickness was approximately 100–180µm. It should be noted that the main phase of core-shell structured microsphere was B4C, with a small amount of residual carbon and B2O3. The core-shell structured B4C microsphere was subsequently processed by laser drilling and corrosion to obtain the B4C hollow microsphere. It was indicated that the B4C hollow microsphere has respectable corrosion resistance. Atomic Force Microscope (AFM) analysis demonstrates that the B4C hollow microsphere exhibits outer and inner surface roughness of 197nm rms and 19.3nm rms based on a 5µm square scan, respectively. This study provides a new method for preparing a high quality B4C hollow microsphere, which have potential application as a target pellet for ICF.