Strength of pure alumina ceramics above 1 GPa

Up to now, commercially available alumina ceramics were claimed to have strength between 400 and 550 MPa. However, our study shows strength ~ 2 times higher for commercially available alumina than commonly believed. The average and characteristic strength, measured on 31 pure alumina ceramic discs by ball on three balls (B3B) test, were 1205 ± 93 MPa and 1257 MPa, respectively, with a Weibull modulus of m = 11.8. Tested specimens were in form of discs with a diameter of 5 mm and thickness 0.5 mm. The grain size distribution of the alumina is bimodal with an average grain size of ~ 850 nm measured at the surface. The fracture reveals a mixed transgranular / intergranular failure mode. To avoid incorporation of additional flaws, the discs were tested as sintered. The characteristic flexural strength measured in B3B was recalculated according to Weibull theory for standard 4-point bending bars of size 3 × 4 × 45 mm as bend 856 MPa. The measured strength of nearly 900 MPa shows the potential of strength for high purity alumina ceramics.     

Isobam assisted slurry optimization and gelcasting of transparent YAG ceramics

Gelcasting is a simple near-net shaping method to fabricate large-sized and/or complicated-structural ceramics. In this paper, a transparent yttrium alumina garnet (YAG) ceramic was successfully fabricated by gelcasting with a nontoxic, water soluble copolymer (isobutylene and maleic anhydride, Isobam) as both dispersant and gelling agent. The rheological behaviors of the slurries with different solids loading and Isobam contents were systematically investigated. The optimized slurry of 0.5 wt% Isobam and the solid loading of 68 wt% had the low viscosity and high stability, resulting in better homogeneity of the green body and better optical quality of transparent ceramics. A nearly pore-free structure of the sintered YAG ceramics with average grain size about 10.0 µm was obtained possessing an in-line transmittance of 75.7% at the wavelength of 1064 nm for a sample 2.5 mm thick.     

Fabrication of porous fibrous alumina ceramics by direct coagulation casting combined with 3D printing

A novel forming method for preparing porous alumina ceramics using alumina fibers as raw materials by direct coagulation casting (DCC) combined with 3D printing was proposed. Porous fibrous alumina ceramics were fabricated through temperature induced coagulation of aqueous-based DCC process using sodium tripolyphosphate (STPP) as dispersant and adding K₂SO₄ as removable sintering additives. The sacrificial coated sand molds was fabricated by 3D printing technology, followed by the infiltration of silica sol solution for the subsequent suspension casting. Stable alumina suspension of 40 vol% solid loading was obtained by adding 2.0 wt% STPP and 40 wt% K₂SO₄. The controlled coagulation of the suspension could be realized after heating at 90 °C for about 35 min. The ceramic sample sintered at 1450 °C for 2 h showed the highest compressive strength of 24.33 MPa with porosity of 57.38%. All samples sintered at 1300–1450 °C had uniform pore size distributions with average pore size of 7.2 µm, which indicated the good structure stability when sintered at high temperature.     

Characterization of porous alumina bodies fabricated by high-temperature evaporation of boric acid with sodium impurity

For steel-making refractories, porous alumina bodies fabricated by the evaporation of chemical impurities at high temperatures was studied. Boron hydroxide and sodium carbonate were added in different proportions as impurities into the starting aluminum hydroxide powder compacts, which were heated at various temperatures for 1 h to form porous alumina bodies. During heating, the borate compounds that reacted with sodium seems to be formed in the liquid phase, so that all alumina particles were able to grow into platelets. Although the sodium inside the compacts was completely evaporated by 1400 °C, the boron melt was maintained above 1400 °C. Hence, alumina platelets grew with a card-house structure, which preserved the high porosity of the alumina body. After heating at 1600 °C for 1 h, the remaining boron oxide was completely evaporated and sintering between the alumina particles began to decrease the porosity. The compressive strength and porosity of the alumina bodies obtained by heating at 1700 °C for 1 h were 0.8 MPa and 64%, respectively. It is expected that the resulting porous alumina bodies can be used as castable porous clinkers.     

Structural and magnetic properties of CoFe2O4 nanopowders,prepared using a modified Pechini method

Cobalt ferrite nanopowders were synthesized by means of the sol-gel method, using citric acid as a chelating agent, and various alcohols as gelling agent: ethanol (ET), ethylene glycol (EG), polyvinyl alcohol (PVA), 1,3 propanediol (PD) and a mixture of PVA and EG. The simultaneous TG/DTA analysis revealed different thermal behaviours of the synthesized gels, depending on the gelling agent. The powders obtained at 500 °C and annealed at 700 °C and 1000 °C contain a single CoFe₂O₄ phase. Scanning electron microscopy (SEM) revealed the influence of the gelling agent on the morphology of cobalt ferrite particles. The coercivity and the saturation magnetization of the powders obtained at 500 °C showed a strong dependence on the crystallite size, determined by the nature of the gelling agent.     

Preparation of alumina foams by the thermo-foaming of powder dispersions in molten sucrose

The alumina powder disperses in molten sucrose due to the hydrophilic interaction between the particle surface and sucrose hydroxyls. The thermo-foaming of the dispersions is due to the bubbles created by the water vapour produced by the OH condensation at 150 °C which are stabilized by the alumina particles adsorbed on the gas–liquid interface as well as the increase in viscosity. The foaming time, the foam setting time and the foam volume depend on the alumina powder to sucrose weight ratio. The alumina foams have interconnected cellular microstructure and the cells are having a near spherical morphology. The porosity (97.84–93.29 vol.%.) decreased and the average cell size (0.54–1.2 mm) increased with the increase in alumina powder to sucrose weight ratio (0.4–1.4). The alumina foams with density in the range of 0.239–0.267 g/cc showed compressive strength in the range of 1.02–1.47 MPa.     

Temperature induced gelation of non–aqueous alumina suspension using oleic acid as dispersant

A novel temperature induced gelation method for alumina suspension using oleic acid as dispersant is reported. Non–aqueous suspension with high solid loading and low viscosity is prepared using normal octane as solvent. Influence of oleic acid on the dispersion of suspension was investigated. There was a well disperse alumina suspension with 1.3 wt% oleic acid. Influence of gelation temperature on the coagulation process and properties of green body was investigated. The sufficiently high viscosity to coagulate the suspension was achieved at −20 °C. The gelation temperature was controlled between the melting point of dispersant and solvent. The gelation mechanism is proposed that alumina suspension is destabilized by dispersant separating out from the solvent and removing from the alumina particles surface. The alumina green body with wet compressive strength of 1.07 MPa can be demolded without deformation by treating 53 vol% alumina suspension at −20 °C for 12 h. After being sintered at 1550 °C for 3 h, dense alumina ceramics with relative density of 98.62% and flexural strength of 371±25 MPa have been obtained by this method.     

Effect of polymer concentration on porosity and pore size characteristics of alumina membrane substrates prepared by gelcasting

The effect of urea–formaldehyde (UF) polymer concentration on porosity and average pore size of alumina membrane substrates prepared by gelcasting has been studied. The soluble UF oligomers formed in the initial stages of polymerization act as steric stabilizer for alumina particles in the suspension. The porosity and average pore size of the substrate samples decreased with both the decrease of amount of polymer in the gelcast body and the increase of sintering temperature. Membrane substrates obtained by sintering of gelcast bodies containing UF polymer concentrations from 24.3 to 15.6 wt% at temperatures from 1250 to 1450 °C showed porosity and average pore size of 62.5–27 vol% and 0.43–0.20 μm, respectively. The membrane substrates prepared by the gelcasting method had narrow pore size distribution.     

Influence of pressure on filtration of aqueous alumina suspensions

The dispersibility of colloidal alumina particles (median size 310 nm) was related to the surface potential, the solid concentration in a suspension and the pressure applied to the particles. The consolidation behavior of colloidal alumina particles with an isoelectric point pH 8.7 was examined using a developed pressure filtration apparatus at 1–10 MPa of applied pressure. The height of 7 or 20 vol% alumina suspensions at pH 3.0, 7.8 and 9.0 as a function of filtration time was fitted by a filtration model developed for a flocculated suspension rather than a traditional filtration model for a dispersed suspension. An increased pressure, a decrease of particle concentration and a porous microstructure of colloidal cake reduced the consolidation time of alumina suspension. The wet alumina compacts were significantly compressed during filtration but relaxed after the release of the applied pressure. However, the packing density of alumina compact after calcination at 700 °C was almost independent of the filtration pressure and controlled by the structure of network of alumina particles in a solution.     

Microstructure and mechanical properties of in-situ grown mullite toughened 3Y-TZP zirconia ceramics fabricated by gelcasting

In-situ grown mullite toughened zirconia ceramics (mullite-zirconia ceramics) with excellent mechanical properties for potential applications in dental materials were fabricated by gelcasting combined with pressureless sintering. The effect of sintering temperature on the microstructure and mechanical properties of mullite-zirconia ceramics was investigated. The results indicated that the columnar mullite produced by reaction was evenly distributed in the zirconia matrix and the content and size of that increased with the increase of sintering temperature. Mullite-zirconia ceramics sintered at 1500 °C had the optimum content and size of the columnar mullite phase, generating the excellent mechanical properties (the bend strength of 890.4 MPa, the fracture toughness of 10.2 MPa.m^1/2, the Vickers hardness of 13.2 GPa and the highest densification). On the other hand, zirconia particles were evenly distributed inside the columnar mullite, which improved the mechanical properties of columnar mullite because of pinning effect. All of this clearly confirmed that zirconia grains strengthened columnar mullite, and thus the columnar mullite was more effective in enhancing the zirconia-based ceramics. Simultaneously, the residual alumina after reaction was distributed evenly in the form of particle, which improved the mechanical properties of the sample because of pinning effect. Overall, the synergistic effect of zirconia phase transformation toughening with mullite and alumina secondary toughening improved the mechanical properties of zirconia ceramics.     

Preparation and characterization of porous mullite ceramics via foam-gelcasting

Porous mullite ceramics were prepared via foam-gelcasting using industrial grade mullite powder as the main raw materials, Isobam-104 as the dispersing and gelling agent, sodium carboxymethyl cellulose as the foam stabilizing agent, and triethanolamine lauryl sulfate as the foaming agent. The effects of processing parameters such as type and amount of additive, solid loading level and gelling temperature on rheological properties and gelling behaviors of the slurries were investigated. The green samples after drying at 100 °C for 24 h were fired at 1600 °C for 2 h, and the microstructures and properties of the resultant porous ceramic samples were characterized. Based on the results, the effects of foaming agent on the porosity level, pore structure and size and mechanical properties of the as-prepared porous mullite ceramics were examined. Porosity levels and pore sizes of the as-prepared samples increased with increasing the foaming agent content up to 1.0%, above which both porosity levels and pore sizes did not change. The compressive strength and flexural strength of the as-prepared sample with porosity of 76% and average pore size of 313 μm remained as high as 15.3±0.3 MPa and 3.7±0.2 MPa, respectively, and permeability increased exponentially with increasing the porosity.     

Preparation of zirconium carbide foam by direct foaming method

Ultra light, highly porous, closed-cell structured ZrC foam can be produced in two steps. First, pre-ceramic foam is prepared by direct foaming of zirconia sol and phenolic resin. In the next step, the foamed green body is converted into ZrC foam after carbothermal reduction at 1600 °C under argon atmosphere. The obtained ZrC foam has porosity of 85% and possesses uniform cells with an average size of about 40 μm. The foam also displays excellent thermal stability up to 2400 °C. Its compressive strength and thermal conductivity at room temperature are 0.4 MPa and 0.94 W/(m K), respectively.     

Processing of low-density alumina foam

A novel method for synthesizing low-density alumina foam has been developed. The alumina foam with 98.5% porosity was synthesized by an unconventional route from an aqueous aluminum nitrate–sucrose solution. The resin formed by heating this solution underwent foaming and set into solid green foam, which was sintered at 1873 K. The thermogram of the green foam showed mass loss in four stages. The foam exhibited interconnected porous network with window size in the range 103–226 and 167–311 μm for foams sintered at 1223 and 1873 K, respectively. The alumina foam sintered at 1223 K exhibited gamma phase and that sintered at 1873 K exhibited alpha phase.     

Synthesis and characterization of nanocrystalline NiO-GDC via sodium alginate-mediated ionic sol-gel method

In this study, nanocrystalline nickel oxide gadolinium-doped ceria (NiO-GDC) powder was synthesized in-situ using Na-Alginate as the template via ionic sol-gel technique. The effects of calcination time and temperature on the particle size and the physiochemical properties of nanocrystalline NiO-GDC are presented in this paper. Using this method, gel beads were formed by contacting sodium alginate solution as the gelling template and metal (gadolinium/cerium/Ni) nitrates as the precursor. The obtained nanocrystallites were characterized using Field Emission Scanning Electron Microscopy, powder X-ray diffraction, energy dispersive X-ray spectroscopy, thermo gravimetric analysis, nitrogen adsorption/desorption analysis, and Fourier transform infrared spectroscopy. It was observed that the increasing calcination temperature had affected both the particle size and the surface area of the NiO-GDC, whereas the increasing calcination time had only impacted the size of the particles. The smallest mesoporous nanocrystalline NiO-GDC powder (12.1225 ± 0.005 m²/g surface area), composed of cubic GDC (5.18 nm crystallite size) and cubic NiO (7.99 nm crystallite size) were synthesized at a calcination temperature of 500 °C for 2 h. This study hopes to inspire more researches on the ionic-gelation method for synthesizing other metal nanostructures as well as other reaction parameters.     

Properties evaluation and fabrication of green clay reformulated from water sludge

Water sludge from Sungai Dua Penang fresh water processing plant has been successfully converted into functional pottery clay with improved physicochemical behavior and properties. Water sludge was generally made of kaolinite mineral that consisted of silica and alumina. At 7 h of milling duration, water sludge demonstrated a narrow particle size distribution at the size range of 107–150 µm. Water sludge owned a specific surface area of 27 m²/g with 8.8 nm (diameter) pore size and 0.05 cm³/g pore volume. Plasticity of clay body increased when clay formulation involved fine particles, e.g. water sludge or bentonite, which promote water adsorption ability. Fine particles with large surface area and better compaction also explained the enhanced hardness of pottery clay. The incorporation of clay minerals such as bentonite and sodium silicate into the formulation has facilitated metals immobilization within the clay body. Final clay product has a terra cotta color and performed a uniform shrinkage without obvious fracture. The fabrication of pottery wares from water sludge with minimized metals leakage has not only higher reutilize value but also a cost effective green method for handling waste and environmental issues.     

Processing,properties and microstructure of SiC foam derived from epoxy-modified polycarbosilane

SiC foams having controlled porosity were fabricated using epoxy modified polycarbosilane (EMPCS). The EMPCS was synthesized by refluxing adequate amount of epoxy and polycarbosilane (PCS) in THF solution at 150 °C. The EMPCS having epoxy content of 0%, 10% and 20% by weight were termed as PCS, 10EMPCS and 20EMPCS respectively. Thermal foaming of the EMPCS was carried out at 1000 °C under inert atmosphere followed by ceramization at 1200, 1400 and 1600 °C under vacuum. The cell size of the ceramized SiC foam was found to be varying between 100 and 700 µm. The ceramized SiC foams were characterized for their density, porosity and compressive strength. Total porosity was found to be 81.8 ± 3.9, 87 ± 4.1 and 90.6 ± 4.6% for the PCS, 10EMPCS and 20EMPCS based SiC foams while their bulk densities were found to be 0.6 ± 0.03, 0.4 ± 0.02 and 0.3 ± 0.01 g/cc respectively. Compressive strength was found to be the highest for the SiC foams ceramized at 1600 °C for all the types of EMPCS. The compressive strength of the 10EMPCS is found to be 2.2 ± 0.2 MPa, 2.5 ± 0.2 MPa and 3.8 ± 0.3 MPa for the foams pyrolyzed at 1200 °C, 1400 °C and 1600 °C respectively while the strength was 1.9 ± 0.1 MPa, 2.1 ± 0.2 MPa and 2.9 ± 0.2 MPa for the 20EMPCS based SiC. The 20EMPCS based SiC foam of thickness 10 mm was exposed to oxy-acetylene flame for 120 s, back face temperature was found to be around 300 °C. Microstructure and phase analysis was carried out to understand the effect of epoxy content and ceramization temperature on physical, mechanical and thermal properties of different types of the SiC foams.     

Flexural strength evaluations and fractography analyses of slip cast mesoporous submicron alumina

Mesoporous submicron α?Al₂O₃ green compacts were fabricated using slip casting of ultrafine alumina powders. The pre-sintered samples were sintered in air atmosphere at 1300 °C, 1350 °C, 1400 °C, and 1500 °C to obtain a variety of grain morphologies namely submicron equiaxed and micro rod structures. The resulting grain diameters lie between ～ 270 nm and ～ 1590 nm and total porosity fraction between 0.05% and 13%. The room temperature flexural strength (σ) evaluations and fractography analyses of sintered alumina samples were performed. It was observed that the total porosity fraction dictates the flexural strength as compared to grain diameter. Further, it was found that the flexural strength exhibited a decreasing trend for an increase in the total porosity fraction, and proved to be a better effective parameter than open porosity fraction. The fractography analyses suggest that samples sintered at 1300 °C and 1350 °C predominantly underwent intergranular fracture, while those sintered at 1400 °C and 1500 °C underwent a mixture of intergranular and transgranular fracture.     

Synthesis of fly ash based geopolymer mortar considering different concentrations and combinations of alkaline activator solution

Geopolymerisation is a process that can transform alumina and silica rich waste materials into valuable binding materials, having excellent mechanical properties. The present experimental study shed a light on the variation in compressive strength of fly ash based geopolymer mortar by varying the molarity of sodium hydroxide as 12 M, 14 M, 16 M and accompanying by sodium silicate (Na₂SIO₃) in 2:1 (Na₂SIO₃/ NaOH) with same molarities. All the geopolymer mixes were oven cured at 80 °C for 24 h and after that kept at room temperature up to the time of testing. The compressive strength was checked subsequently at the ages of 3, 7, 14 and 28 days. The experimental results reveal that the addition of sodium silicate enhances the strength development in geopolymer mortar. The ultimate compressive strength of 40.42 MPa was obtained by incorporating sodium silicate along with 16 M concentrated sodium hydroxide. Furthermore, increasing trend of the compressive strength was found with increasing molar concentration of sodium hydroxide and curing period.     

Preparation and properties of porous alumina ceramics with oriented cylindrical pores produced by an extrusion method

Porous alumina ceramics with unidirectionally-oriented pores were prepared by extrusion. Carbon fibers of 14 μm diameter and 600 μm length to be used as the pore-forming agent were kneaded with alumina, binder and dispersing agent. The resulting paste was extruded, dried at 110 °C, degreased at 1000 °C and fired at 1600 °C for 2 h. SEM showed a microstructure of dispersed highly oriented pores in a dense alumina matrix. The pore area in the cross section was 25.3% with about 1700 pores/mm². The pore size distribution of the fired body measured by Hg porosimetry showed a sharp peak corresponding to the diameter of the burnt-out carbon fibers. The resulting porous alumina ceramics with 38% total porosity showed a fracture strength of 171 MPa and a Young's modulus of 132 GPa. This strength is significantly higher than the reported value for other porous alumina ceramics even though the present pore size is much larger.     

Microporous alumina substrate with porosity >70% by gelcasting

Microporous alumina membrane substrate in tubular and planar configurations have been prepared by gelcasting of alumina powder slurry using high amount of urea–formaldehyde as gelling agent followed by humidity controlled drying, binder removal and sintering of the gelled bodies. Porosity of the substrate samples sintered at 1350 °C was more than 70% as measured by mercury porosimeter. More than 51% porosity could be retained even after sintering of the samples at 1450 °C. Average pores size of the membrane substrate samples sintered at temperature in the range from 1250 to 1550 °C varied between 0.42 and 0.56 with a maximum at 1350 °C. More uniform pores were observed in sample sintered at 1450 °C. Urea-formaldehyde polymer present in the gelcast body acts as template for micropores.