Effect of the addition of silica sol on layered extrusion forming of Al2O3-based cores

Layered extrusion forming of ceramic cores with a nanoceramic suspension as a binder was conducted to explore a novel method to produce complex-shaped ceramic cores. Green bodies were prepared using Al₂O₃ particles as precursor materials and silica sol combined with aqueous polyvinyl alcohol solution as a binder. Increasing the silica sol content increased the viscosity of the slurry, enhanced the green bending strength, and decreased the green linear shrinkage. The green microstructure showed the nanosized silica particles were deposited on the surface of the Al₂O₃ particles and among the pores formed by Al₂O₃ particles irregular packing. In addition, increasing the silica sol content increased the bending strength, however, decreased linear shrinkage and open porosity of the sintered bodies. During sintering, the nanosized silica particles converted to the melting phase and reacted with Al₂O₃ and the microstructure of sintered bodies indicated the existence of sintering neck with silica sol addition.     

Effect of residual carbon on the phase transformation and microstructure evolution of alumina-mullite fibers prepared by sol-gel method

Sol-gel method, as one of the most effective methods, has been widely used in producing high-property ceramics. In this method, the pyrolysis of precursor significantly influences the following microstructure evolution and properties of ceramic materials. In this work, the effect of residual carbon, influenced by the pyrolysis atmosphere, on the phase transition and microstructure evolution of the alumina-mullite fibers was investigated. It was revealed that the phase transition path of the preheated fibers with 0.53 wt% residual carbon was amorphous phase → aluminosilicate → tetragonal mullite → orthorhombic mullite + θ-Al₂O₃. The phase transition path of the preheated fibers with 10.51 wt% residual carbon became amorphous phase → γ-Al₂O₃ → orthorhombic mullite + α-Al₂O₃ + θ-Al₂O₃. In addition, the residual carbon contributes to improve the densification of alumina-mullite fibers and refine the fiber grain size. The effect of residual carbon on the formation of mullite coarse grains was further discussed.     

ELECTROSTATIC SOL–SPRAY DEPOSITION OF NANOSTRUCTURED CERAMIC THIN FILMS

Electrospraying has been developed into an electrostatic spray deposition technique for the deposition of ceramic thin films. The cone-jet spraying mode appears to be the most preferable for this purpose, and the domain where the cone-jet mode exists was found to depend strongly on the nozzle design. A nozzle with a large diameter and a tilted outlet widens the windows for both the applied high DC voltage and the flow rates of a precursor liquid keeping the cone-jet mode intact. The results of three nozzle designs are compared, one of which has been selected for feeding two different precursor liquids simultaneously. With three relevant sols as precursor liquids, nanostructured thin films of ZnO, ZrO₂, and Al₂O₃ have been deposited. Their morphologies are dependent on the preparation of the precursor sols and the deposition temperature. Highly porous films were obtained by using a high deposition temperature and a sol prepared from a metal alkoxide or a metal acetate.     

Influence of yttrium oxide addition and sintering temperature on properties of alumina-based ceramic cores

Al₂O₃-based ceramic cores with a uniform microstructure were fabricated successfully by a traditional pressing forming method, in which Al₂O₃ powders were used as matrix and yttrium oxide as additive. The influences of yttrium oxide content and sintering temperature on properties of ceramic cores were studied carefully. Results indicated that a higher sintering temperature benefited the preparation of ceramic cores with excellent properties. As the temperature was above 1400°C, the reaction of Al₂O₃ and yttrium oxide occurred, leading to the formation of YAG phases. And, YAG was uniformly adhered on the surface of Al₂O₃ particles, exerting a good role in connecting Al₂O₃ particles. Based on XRD analyses, it was found that the increase in the sintering temperature could promote the formation of more YAG phases. When sintering temperature was adjusted to 1600°C, with the increase in the yttrium oxide content, their relative density developed a trend of decreasing first and then increasing, while the apparent porosity had an opposite change tendency. With the increase in the sintering temperature, the line shrinkage and bending strength of Al₂O₃-based ceramic cores both increased gradually. In our research, their bending strength reached to 53.5 MPa and apparent porosity was 33.9% when the ceramic cores were prepared with 9 wt% yttrium oxide at 1600°C.     

Grain boundary segregation for enhancing the thermal stability of alumina-mullite diphasic fibers by La2O3 addition

Improved thermal stability of fibers is increasingly demanded for high-temperature applications. In this study, alumina-mullite diphasic fibers with 0–5 wt % La₂O₃ addition were synthesized via the sol-gel method. The precipitation of LaAl₁₁O₁₈ occurred when the content of added La₂O₃ exceeded a critical range. Subsequently, the effects of 1 wt % La₂O₃ on alumina-mullite diphasic fibers were systematically discussed in terms of pyrolysis removal, phase transition pathways, microstructure and thermal stability. The results indicated that alumina-mullite diphasic fibers consisted of γ, θ-Al₂O₃ and mullite phases. The spherical γ, θ-Al₂O₃ grains, ranging in size from 20 to 100 nm, were dispersed within the mullite matrix with an irregular shape. Additionally, the La element was segregated into the γ, θ-Al₂O₃ grain boundaries during the crystallization process. This resulted in stabilized γ, θ-Al₂O₃ and mullite grains and an improved strength retention rate of 84 % after thermal exposure at 1200 ℃ for 5 h.     

Effect of rheological properties of AlOOH sol on the preparation of Al2O3 nanofiltration membrane by sol-gel method

In order to directly prepare an High Flux Al₂O₃ nanofiltration membrane on an Al₂O₃ support with an average pore size of 4 μm, AlOOH sol was prepared with aluminum isopropoxide as the precursor, The effect of rheology on the dip-coating of AlOOH sol and the effect of viscoelasticity on the heat treatment of AlOOH gel film to prepare defect-free Al₂O₃ nanofiltration membrane were studied. The results indicate that AlOOH sol will increase its viscosity with the increase of the standing time. When the viscosity increases to a certain extent, the colloidal particles will gradually transform into gels, and change from Bingham fluid to Herschel-Bulkley pseudoplastic fluid. The thickness of the AlOOH gel film is related to the viscosity of the AlOOH sol. The flow viscosity of AlOOH sol should be about 0.0025～0.005 Pa·s, while the thickness of the AlOOH gel film after dip-coating is about 6.5～12 μm. The storage modulus and loss modulus of AlOOH gel film increase with the increase of aging time. Only when the storage modulus of the AlOOH gel is greater than the saturated vapor pressure of the solvent under normal pressure (0.1 MPa), it will not crack due to the evaporation of the sol during the heat treatment process. After the storage modulus exceeds 0.1 MPa, the surface of the heat-treated Al₂O₃ ceramic membrane is smooth and crack-free, the rejection rate for crystal violet dye is 99.8%, and its average pore size is 2.75 nm, that has the capability of nanofiltration. Due to the lack of intermediate layer, the pure water flux of the Al₂O₃ nanofiltration membrane is as high as 201.7 l.m^-2bar^-1h^-1, and the steady-state filtration flux is 48.7 l.m^-2bar^-1h^-1 when filtering 20 mg/l crystal violet solution. By controlling the rheological properties of AlOOH sol, a high flux Al₂O₃ nanofiltration membrane was prepared.     

The controllable microstructure of porous Al2O3 ceramics prepared via a novel freeze casting route

In traditional aqueous slurry freezing casting processing, the growth method of ice crystals is hard to control, resulting in the uncontrollable pore's morphologies of the porous ceramics. In the experimental, the pure Al₂O₃ sol was used to substitute water as a medium for preparing ceramic slurry. With Al₂O₃ sol addition, it becomes easy to control the microstructure and pore's morphologies of the porous Al₂O₃ ceramics via adjusting of the solid loading, composition of the ceramic slurries, as well as the cooling methods. The SEM micrographs showed that the sol-contained ceramic slurry combined with freeze casting processing can easily prepare the porous Al₂O₃ ceramics with different pore sizes and different morphologies. The porous Al₂O₃ ceramics prepared from 70 wt.% to 90 wt.% solid loading sol-contained Al₂O₃ slurries and sintered at 1500 °C for 2 h have open porosities from 81.7% to 64.6%.     

Electrospinning preparation and microstructure characterization of homogeneous diphasic mullite ceramic nanofibers

In this work, diphasic mullite (3Al₂O₃·2SiO₂) nanofibers with good homogeneity were prepared by electrospinning method. Aluminum nitrate (AN) and aluminum isopropoxide (AIP) were used as alumina sources, commercial colloidal silica as silica source, and polyvinyl alcohol (PVA) as polymer additive. Precursor nanofibers with continuous and uniform structures were acquired at mass ratio of PVA to precursor sol from 0.06 to 0.09. γ-Al₂O₃ phase was obtained at 878 ^°C and mullite phase formed at 1322 ^°C upon heating of the precursor under air atmosphere. Calcined samples suggested mullite as dominant phase at 1300 ^°C and amorphous SiO₂ could even exist at 1400 ^°C. As-prepared nanofibers possessed continuous structures with subequal average diameters at 900–1200 °C. However, such morphological characteristics were lost at temperatures above 1300 ^°C due to rapid growth of crystal grains. Al and Si elements were uniformly distributed in fibers and mixed at nanoscale, confirming homogeneity and diphasic features of nanofibers.     

Synthesis and characterization of sol–gel derived continuous spinning alumina based fibers with silica nano-powders

Silica nano-powders were used as the Si source to substitute acidic silica sol in the SiO₂–Al₂O₃ sol mixture for benefiting the sintering of the fibers at 1250 °C. With the increase of nano-silica, grain diameter and porosity of the fibers decreased firstly and increased subsequently, a minimum value of 35.86 nm and 0.86% was exhibited at the nano-silica content of 20%. The solid content, linear growth model and homogeneity of the precursor sol were not affected by the presence of nano-silica, although the continuous spinning length became low. NMR Analysis of ²⁷Al indicated that polymerization degree of the sol was enhanced by nano-silica. The nano-particle contributed to the reduced intermolecular distance of gel, so the appropriate content existed for resulting the reduced grain size and compact structure of the fibers.     

Influence of chrome-bearing sols vacuum impregnation on the properties of magnesia-chrome refractory

Two kinds of nanosized chrome-bearing sols, i.e. Cr₂O₃ precursor sol and MgCr₂O₄ spinel precursor sol, were obtained by homogeneous precipitation. Properties of the sol vacuum impregnated magnesia-chrome refractory, such as bulk density, cold crushing strength, pore distribution, and chemical composition, etc., are superior to those of the un-impregnated sample. SEM micrographs show a different microstructure of the impregnated sample as compared to the un-impregnated one. The influence of vacuum impregnation on copper slag corrosion resistance of magnesia-chrome refractories has also been evaluated. The results show that both sols could improve the magnesia-chrome bricks corrosion resistance in impregnation.     

Microstructure Development of PZT Ceramics by Doping with Small Amounts of Al2O3, SiO2, and Fe2O3

Lead zirconate titanate (PZT) ceramics are used in a wide range of applications as sensors and actuators. Typically, they are formulated by the mixed oxide route, using several mixing and milling steps. Due to wear, these processes introduce impurities into the ceramic mass, which on their part can strongly influence densification behaviour and final properties of the PZT. In this study, the effect of such impurities, such as iron oxide, alumina, and silica, on the sintering behaviour and microstructure development of PZT ceramic is evaluated. A commercial Nb‐doped PZT powder was used and doped by adding Al₂O₃, SiO₂, and Fe₂O₃ with an amount of up to 0.001 mol%. Bulk samples were prepared and sintered in air. The mass loss, density, and grain size were correlated by regression analysis using the doping elements and levels, respectively. Due to the complex interactions between the oxides and the ceramic properties, the experiments were performed with the design‐of‐experiment method (DoE). The results showed a significant influence of these low amounts of doping levels on the microstructure development. Moreover, it was shown that doping after calcination affects the microstructure in a similar way to doping before calcination. Thus, a possibility to compensate concentration variations in the calcined ceramic mass is demonstrated, to homogenize the chemical composition and the final microstructure of the sintered PZT ceramic in the manufacturing processes.     

Temperature-induced changes in morphology and microstructure of electrospun mullite nanofibers fabricated from a hybrid mullite sol

Mullite nanofibers with small diameter and high surface area are an ideal candidate as the reinforcements in composite materials, and have promising applications in the fields of catalysis, filtration, thermal storage and so forth. In this work, electrospun mullite nanofibers were successfully synthesized using a hybrid mullite sol. The morphology and microstructure of fibers calcined at different temperatures were investigated. The morphology of fibers synthesized at 900 °C is porous with coarse surface, and after crystallization it becomes compact with smooth surface. The densities of fibers increase with the increasing temperatures. At 1200 °C the surface of fibers becomes coarse again, as a result of the grain growth of mullite. The crystallization path of fibers was revealed that the Al-rich mullite (4Al₂O₃·SiO₂) together with amorphous silica formed at 1000 °C, changed into mullite with higher silica contents as temperature further increased, and finally transformed into a stable 3Al₂O₃·2SiO₂ phase at 1200 °C. During this crystallization process, the flow of amorphous silica phase and the formation of mullite crystal structure benefit the densification of fibers, leading to the resultant fibers with fine and compact microstructure. The present findings can provide a guideline for the preparation of the promising high-mechanical mullite nanofibers and the synthesized nanofibers display great potential as reinforcements in structural ceramic composites.     

Properties of silica sol bonded corundum‐spinel castables for steel ladles

In order to overcome the shortcoming of the calcium aluminate cement (CAC) bonded castables, we prepared corundum‐spinel castables using silica sol as binder and tabular corundum, sintered magnesia‐alumina spinel, and reactive alumina as raw materials in this study. The effect of spinel grain size and solid content of silica sol on the flow value, sintering, mechanical strength and microstructure of the specimens treated at varying temperature of 400, 1000, 1500, and 1650°C for 5 hours in an air atmosphere were studied by SEM and EDS analyses. The results indicate that silica sol is suitable as a binder for corundum‐spinel systems. And silica sol with solid content of 25% bonded samples containing ≤90 μm spinel perform quite better than the others. At the same time, silica sol bonded samples had high strength in medium temperature. This is because that the closer proximity of silica sol and alumina powder and the high activity of nanometer SiO₂ in silica sol are beneficial for the reaction of SiO₂ and Al₂O₃ to generate mullite needed for reinforcement of castables matrix.     

Preparation of Porous Al2O3-Ceramics by Biotemplating of Wood

A new approach for preparing of microcellular ceramic materials is the reproduction of wood morphologies by biotemplating, where the structural features of the native wood are maintained in the ceramic product. Biomorphic Al₂O₃-ceramics were manufactured via the sol-gel route by repeated infiltration of low viscous alumina sols into wood preforms and subsequently sintered in air at 1550°C. The microstructure and phase formation during processing were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), density and porosimetry measurements. In contrast to conventional processed, highly-porous Al₂O₃-ceramics, the microstructure of the biomorphic Al₂O₃-ceramics is characterized by an uniaxial pore morphology with pore diameters in the micrometer range, depending on the initially used wood template.     

Silica sol modified phenolic resin and its effect on mechanical properties of Al2O3-SiC-C bricks

Phenolic resin(PF) is commonly used as a binder in Al₂O₃-SiC-C bricks; however, it reduces the strength and accelerates the damage after carbonisation under temperatures of 300–800 °C. To improve the medium-temperature mechanical properties of Al₂O₃-SiC-C bricks, the mechanism of silica sol modify PF was analyzed, and silica sol modified phenolic resin (PFS) was introduced into Al₂O₃-SiC-C bricks to explore the effect of PFS on the mechanical properties of Al₂O₃-SiC-C bricks. The results showed that the silanol groups (Si-OH) in the silica sol can react with the hydroxyl groups (-OH) in the PF to form a Si-O-C bond, which makes PFS have a higher residual carbon rate and greater thermal stability. Overall, the introduction of PFS enhances the mechanical properties of the Al₂O₃-SiC-C bricks. When the content of silica sol in PFS is 10 wt%, the cold modulus of rupture and compressive strengths increased by 62.6% and 22.1%, respectively.     

Combined effects of SiO2 ratio and purity on physical properties and microstructure of in situ alumina-mullite ceramic

The in situ formation of mullite (Al₆Si₄O₁₃) is a complex process based on solid-state reactions strongly affected by the characteristics of Al₂O₃ and SiO₂ sources. This study investigated the combined effects of variable SiO₂/Al₂O₃ ratios and the presence of low-melting-point impurities on the physical properties and microstructure of in situ alumina-mullite ceramics. Two grades of synthetic amorphous silica, known as microsilica, of similar physical properties and significant differences in the content of alkali-based impurities (0.8 and 3.6 wt%), were combined with thin calcined alumina particles in different proportions (from silica-free up to stoichiometric mullite), pressed and sintered. The samples were tested for total porosity, flexural strength, and pore size distribution, and their crystalline phases and microstructures formed were investigated. Small amounts of both microsilica grades (up to 8.9 wt%) hindered the densification of the alumina-mullite matrix and favored grain growth events, increasing porosity and reducing strength. For samples containing higher microsilica loads (16.4–28.2 wt%), the impurities content significantly affected the amount of liquid phases formed. Such impurities altered the ratio and shape of the pores and the total amount of mullite after sintering. Therefore, different microstructures and levels of flexural strength and total porosity were observed.     

Effect of solidification path on the microstructure of Al2O3–Y2O3–ZrO2 ternary oxide eutectic ceramic system

The solidification path of the Al₂O₃–Y₂O₃–ZrO₂ ternary oxide eutectic composite ceramic is determined by a high temperature DTA and laser floating zone (LFZ) directional solidification method to investigate the effect of solidification path on the microstructure of the ternary oxide. The DTA and microstructure analyses show that the YAG or Al₂O₃ tends to form as primary phase under the unconstrained solidification conditions, and then the system enters ternary eutectic solidification during cooling from 1950 °C at rate of 20 °C/min. The as-solidified composite ceramic shows a divorced irregular eutectic structure consisting of Al₂O₃, YAG and ZrO₂ phases with a random distribution. The primary phases are however completely restrained at the directional solidification conditions with high temperature gradient, and the ternary composite by LFZ presents well coupled eutectic growth with ultra-fine microstructure and directional array. Furthermore, the eutectic transformation and growth mechanism of the composite ceramic under different solidification conditions are discussed.     

Strong and tough laminated ceramics prepared from ceramic foams

Here, we present a novel strategy to prepare laminated ceramics by combining the ceramic foams and hot-pressing sintering. Al₂O₃ and ZrO₂ ceramic foams prepared by the particle-stabilized foaming method was cut into thin slices and then directly laminated and hot-pressing sintered. Al₂O₃/ZrO₂ laminated ceramics with various structures were prepared. Compared with the slices prepared by conventional process, ceramic foams can easily regulate the thickness of laminate to resemble the nacre-like structure. In addition, the grain in the ceramic foams have lower activity and shrinkage rate, thereby weakening the residual tensile internal stress caused by grain coarsening and differences in coefficient of thermal expansion. The effects of layer number and thickness ratio on residual stress and the structure-activity relationship between mechanical properties and microstructure were investigated. The fracture toughness, flexural strength, and work of fracture of the optimal Al₂O₃/ZrO₂ laminated ceramics are 8.2 ± 1.3 MPa·m^1/2, 356 ± 59 MPa, and 216 J·m^?2, respectively.     

Studies on the Crystallization Behavior of Al2O3-HfO2 Ceramic Fibers Prepared by Melt-spinning of Polymer Precursors

In this paper, Al₂O₃-HfO₂ ceramic fibers with different element distributions were prepared by melt-spinning of polymer precursors. The crystallization behavior of the two fibers was investigated and compared. From in-situ XRD, the critical crystal size and initial crystal size of the crystal structure transition from γ-Al₂O₃ to α-Al₂O₃ were calculated to be 14 nm and 40 nm, respectively. The activation energy calculated by the non-isothermal crystallization kinetic accounted for the difference in the crystal structure transition process. As a result of the lower crystallization activation energy, the γ-Al₂O₃ reached the critical size earlier in the Al₂O₃-HfO₂ ceramic fibers with a relatively uneven distribution of elements, and the fibers exhibited a lower crystal structure transition temperature of α-Al₂O₃. While the Al₂O₃-HfO₂ fibers with uniform distribution of elements showed better crystal size stability at high temperatures.     

Preparation of barium hexa-aluminate ceramic fibres and refractories

Abstract

Barium hexa-aluminate refractories, ceramic fibres, and ribbon can be obtained by sol–gel processing from precursor solutions containing a barium salt and an aluminium salt, the solutions having the stoichiometric BaO . 6Al₂O₃ composition. Barium chloride is a suitable barium salt and the preferred aluminium salts are the chlorohydrates Al₂ (OH)_n Cl_{6- n} , with n being 1 or 2, preferably 1. The precursor solutions form filaments and ribbons, and also gels which can be used as binders in refractory grain mixes. The gel filaments and ribbons convert to barium hexa-aluminate ceramic fibres and ribbons on sintering, preferably by rapid heating to at least 1400°C. Previous work on the sintering of gels of stoichiometric BaO . 6Al₂O₃ composition is summarised and compared with the behaviour of oxide stoichiometric BaO . 6Al₂O₃ powder mixes of alumina and barium carbonate when sintered. The effect of chromium (III) addition is also summarised