Porous Alumina Ceramics Research Articles

Residual strength of porous alumina ceramics and fractal characterization of their crack patterns after thermal shocks

Thermal shock behaviors of porous alumina samples with different porosities (4.6–16.0 %) and mean grain sizes (2.72–5.02 μm) were investigated under different quenching temperatures. The slit island method was used to analyze the structures of the pores. The effect of the pore structure on the thermal shock resistance of alumina ceramics was described quantitatively. The results revealed that the thermal shock resistance decreased when the fractal dimension of the pores increased. Alumina ceramic strips with a porosity and mean grain size of 4.6 % and 5.02 μm, respectively, were fabricated using raw powders with a mean particle size of ∼0.3 μm. These samples possessed the highest residual strength ratio (∼46.16 % at 600 °C) and critical temperature difference (∼262 °C). In addition, the fractal characterization of cracks on the surfaces of the alumina ceramics after thermal shock was carried out using the box-counting dimension method. In addition, we investigated the correlation between the mechanical properties and fractal dimensions of the surface cracks. The box-counting dimension increased with crack propagation and negatively correlated with the residual strength. Furthermore, the propagation rate of the surface cracks decreased as the temperature difference increased, resulting in a decrease in the growth rate of the fractal dimension.

Addition of AlO(OH) nanoparticles to assist stabilizing emulsion for digital light processing lightweight but robust porous alumina ceramics

It is proposed here that introducing AlO(OH) nanoparticles to build a wide range of particle size distribution to assist stabilizing emulsion and regulate its properties is an important prerequisite for using Pickering emulsion as digital light processing (DLP) feedstock. Combined with DLP, porous alumina ceramics with hierarchical porosity composed of macro-pores from the designed model (based on body-centered cubic lattice) and micro-pores formed by emulsion droplet, can be prepared by adjusting the microstructure of framework via changing AlO(OH) content, oil-to-water ratio and dispersant content. The AlO(OH) addition facilitates the further filling of particles at the phase interface of emulsion, thus increasing the viscosity and reducing the pore size. Porous alumina ceramics exhibit an average pore size distribution of 4.71 μm–6.06 μm, porosity of 71.92%–78.60 %, and relatively high compressive strength ranging from 5.02 MPa to 10.36 MPa, demonstrating a combination of high porosity and excellent mechanical properties.

Hierarchical porous ceramics constructed by interlocked alumina platelets with hollow glass spheres as pore formers

Hierarchical porous alumina ceramics were prepared through hydratable alumina gel-casting method. The matrices were constructed by interlocked alumina platelets. Hollow glass spheres as pore formers participated in the fluorine-catalyzed structure formation process. The samples with 20 wt% hollow spheres added (1000–1300 °C) had volume densities, apparent porosities, linear shrinkages and compression strengths of 0.59–0.90 g/cm3, 81.68 %∼74.20 %, 1.40 %∼14.07 % and 12.64–23.36 MPa, respectively. Changing the fraction of hollow spheres (10–30 wt%, 1100 °C) had effects on tailoring the compression strengths (3.13–13.40 MPa), without apparently affecting the densities (0.58–0.60 g/cm3)/porosities (79.70 %∼82.28 %)/shrinkages (1.47 %∼1.93 %). The pore size distributions demonstrated the interlocking pores built by alumina platelets, the pores that inherited from the gel network, and the large pores provided by hollow spheres. The reheated (1300 °C) samples presented good structure stability with shrinkages of 6.60 %∼18.57 %. The synthesized ceramics with both high porosities and satisfactory strengths could meet the requirements of high temperature gas/liquid filtration or catalyst supporting, etc.

Porous alumina‐based ceramics with very low thermal conductivity prepared by the starch consolidation route

AbstractAlumina‐based porous ceramics were fabricated via the starch consolidation casting route, using titanium oxide as a sintering additive. Alumina‐titania‐starch slurries with starch contents of 2.5, 5, 7.5, and 10 wt% relative to the alumina‐titania system were prepared. Titania was added to the slurries in proportions of 0, 1, 3, and 5 wt%, relative to alumina. The effects of starch and titania contents in the green bodies on the linear and volumetric shrinkage, as well as the total and open porosities of the sintered alumina product, were investigated. The effect of titania addition on mechanical, electrical, and thermal properties of the obtained porous alumina ceramics was studied. Microstructure and pores morphology were characterized by SEM. The alumina‐based ceramics obtained had external porosities that varied between 54 and 76 vol%. Thermal conductivities as low as 0.06–0.1 W/m.K were achieved and the volume electrical resistivity ranged from 3.6 to 600 GΩ·m, while compressive strengths up to 2.8 MPa for a titania ratio of 1 wt% were attained.

Enhancing Porous Alumina Ceramics for Bioapplications Through Targeted Surface Modification Techniques

Enhancing Porous Alumina Ceramics for Bioapplications Through Targeted Surface Modification Techniques

Machine learning assisted material development for lithography-based additive manufacturing of porous alumina ceramics

The transition of applying ceramic additive manufacturing (AM) from prototyping to mass production and from monolithic to multi-material (MM) components can be supported by continual development in materials and processes. Lithography-based ceramic manufacturing (LCM) can be used for MM printing of ceramics with high accuracy by introducing different approaches that enable discrete/smooth multidirectional material transitions. Adaptation of ceramic slurries plays important role for the successful printing and co-sintering. Especially for co-sintering, the shrinkage of the materials must be adapted so that no internal residual stresses occur. Machine learning (ML) offers promising opportunities for development of new materials and optimization of processes in AM by introducing relations between input features and output responses. In this article, ML algorithms were used for the prediction of shrinkage and porosity of alumina samples dependent on input features including material, printing, and thermal processing parameters so that defect-free porous/dense alumina combination can be achieved.

Starch consolidation casting of porous alumina and functional gradient porosity development

Starch consolidation casting (SCC) technique was successfully employed to produce both porous alumina and graded porous alumina ceramics. The solid content in the alumina suspension was maintained at 40 vol.%, with potato starch varying from 5 to 15%. Structures of the porous alumina (monolithic) samples obtained by SCC and uniaxial pressing were compared. In addition, the influence of the SCC consolidation temperature and the starch content were evaluated in the monolithic samples, while the consolidation temperature and the number of layers numbers were evaluated in the graded samples. The lower SCC consolidation temperature resulted in lower linear shrinkage and a slight increase in total porosity due to the increased pore size. The compressive strength values for the monolithic samples ranged from 60 to 200MPa, which can be considered high when compared to previous works. The graded samples exhibited porosity variations across layers and interfaces were free of cracks and imperfections. Linear shrinkage was the same for the adopted consolidation temperature and the porosity was slightly higher for the 3-layer samples. They achieved strength of 60MPa with fracture mode parallel to the applied load.

Hierarchical porous alumina ceramics as multi-functional support with excellent performance

Hierarchical porous nanostructure alumina ceramics (HP-Al2O3) were prepared through a novel approach involving polymer-assisted freeze-drying and subsequent calcination. The resulting material exhibits interconnected micro-meso-macro pores, distinguishing it from traditional Al2O3 nanoparticles (Al2O3-Nps) prepared without freeze-drying. By manipulating freeze-drying precursors and calcination temperatures, we tailor the morphology and structure of HP-Al2O3. Notably, the HP-Al2O3 demonstrates a remarkable 5.3-fold increase in adsorption efficiency for Congo red compared to Al2O3-Nps, attributed to its significantly larger specific surface area. The HP-Al2O3 is a versatile platform for building multi-functional composites, proved by immobilizing silver nanoparticles (AgNps) and graphitized carbonitride (g-C3N4) through chemical reduction and gas-solid reaction methods, respectively. When employed for the reduction of 4-nitrophenol and the degradation of Rhodamine B, Ag/HP-Al2O3 and g-C3N4/HP-Al2O3 display approximately 2.5 and 2.4 times higher reaction rate constants compared to Ag/Al2O3-Nps and g-C3N4/Al2O3-Nps. This enhanced catalytic performance is attributed to several key factors: the 3D porous framework of the HP-Al2O3 facilitates efficient catalyst immobilization, the microporous and mesoporous nature increases the active site and specific surface area, and the hierarchical porous structure enhances mass diffusion and light capture. Moreover, the self-supporting nature of the HP-Al2O3 simplifies separation and recovery from reaction solutions. With the integration of its hierarchical porous structure and commendable recovery properties, HP-Al2O3 emerges as a promising candidate for various functional applications, particularly in catalysis.

Gel-casting for manufacturing porous alumina ceramics with complex shapes for transpiration cooling

Gel-casting for manufacturing porous alumina ceramics with complex shapes for transpiration cooling

Zero sintering-induced shrinkage of porous oxide ceramics

Porous oxide ceramics are widely used in extreme working conditions owing to their excellent resistance to high temperatures and corrosion. However, sintering is an inevitable process applied to ceramics, from the green body to the final product. The highly complex structures exacerbate the shrinkage-induced irregular deformation and crack formation in the sintering process. A pioneering approach is developed in this study to achieve zero shrinkage for porous alumina ceramics during multistep sintering, using a combination of active fillers - ZrAl3 and Al75Si25. The response surface method is used to optimize the material compositions and sintering process, to achieve shrinkages of less than 0.05% for the entire process. The sintering expansion mechanism is investigated by analyzing the pyrolysis and microstructures of samples at different temperatures. The combination of ZrAl3 and Al75Si25 can attain the continuous expansion of the matrix in a wide temperature range of 600–1400 °C. Furthermore, typical alumina components are fabricated and used to verify the effectiveness of the proposed approach. Owing to shrinkage suppression, the profile deviation of the samples is less than 0.1 mm, and the proportion of microcracks is reduced by 97.8%. The suggested approach shows potential applications in near-net, low-defect fabrication of complex fine ceramic components.

Fabrication and mechanical properties of triply period minimal surface porous alumina ceramics based on Digital Light Processing 3D printing technology

Fabrication and mechanical properties of triply period minimal surface porous alumina ceramics based on Digital Light Processing 3D printing technology

Fabrication of porous aluminum ceramics beyond device resolution via stereolithography 3D printing

This paper presents a new method for fabricating high-resolution porous ceramics using the 3D printing technology. Stereolithography 3D printing was used to create porous alumina parts with polyamide 12 (PA12) powder, alumina ceramic particles, and acrylic resin as the sacrificial porogen, aggregate, and mixture, respectively. After subsequent de-binding and sol impregnation treatments, a porous alumina part with a particle size of 15 μm was obtained after sintering. The use of aluminum sol enhanced the mechanical properties of porous alumina ceramics, thus enhancing their strength. A discrete element method model was used to simulate and explain the results, thereby providing insights into the optimization of the method design. Accordingly, this study demonstrated the potential for using the stereolithography 3D printing technology to fabricate high-resolution porous ceramics with tailored properties for various applications.

Porous Alumina-Bentonite Ceramics: Effects of Fillers and Molding Technique

In porous ceramics processing, the green body shaping technique largely determines the control of the final porous structure and material properties. The study is aimed at finding affordable approaches for the shaping of two different narrow-fraction fillers: F240 electro-corundum and hollow alumina microspheres. The results revealed the influence of accessible shaping techniques (semi-dry pressing, direct casting, and slip casting) on the structural and mechanical properties of porous alumina ceramics. The starting materials were characterized by XRD, SEM, EDX, and BET. The manufactured ceramics were studied in terms of microstructure, density, porosity, and flexural strength. Free stacking of the fillers’ particles during the direct and slip casting resulted in a higher porosity of ceramics compared with that of semi-dry pressing, while reducing its mechanical strength. Direct casting appeared preferable for ceramics with hollow microspheres because it maintained the integrity of the filler particles and preserved their inherent porosity in the ceramics. The optimal parameters for porous ceramics processing were determined as follows: pressing at 30 MPa and sintering at 1280–1320 °C with a bentonite content of 15 wt.%. In this case, the average density and open porosity of F240 samples reached 2.22 g cm−3 and 40.4%, while samples containing hollow microspheres reached 2.20 g cm−3 and 36.7%, respectively.

Freeze-Casted Porous Lamellar-Structured Alumina Ceramics and Polymer-Infiltrated Composites: Synergistic Enhancement of Structural Properties

Freeze-Casted Porous Lamellar-Structured Alumina Ceramics and Polymer-Infiltrated Composites: Synergistic Enhancement of Structural Properties

Influence of Fabrication Method and Surface Modification of Alumina Ceramic on the Microstructure and Mechanical Properties of Ceramic-Elastomer Interpenetrating Phase Composites (IPCs).

The paper presents experimental results of the work conducted to improve the adhesion between alumina ceramics and urea-urethane elastomer in the interpenetrating phase composites (IPCs), in which these two phases are interpenetrating three-dimensionally and topologically throughout the microstructure. Measurements of the contact angle, surface roughness, and shear tests were used to evaluate the effectivity and select the quantity of a silane coupling agent and the ceramic fabrication method. The tests were conducted using samples of dense alumina ceramic obtained by three- or four-step methods. In the four-step process, hot isostatic pressing (HIP) was applied additionally. As a result of the coupling agent coat and HIP application, the ceramic substrate wettability by the elastomer was improved. The water contact angle was reduced from 80 to 60%. In the next step, porous ceramic preforms were fabricated using HIP sintering and a solution of silane coupling agent treated their surface. The composites were produced using vacuum-pressure infiltration of porous alumina ceramics by urea-urethane elastomer in liquid form. The influence of the coupling agent application on the microstructure and mechanical properties of the composites was estimated. The microstructure of the composites was identified using SEM microscopy and X-ray tomography. As a result of using the coupling agent, residual porosity decreased from 7 to 2%, and compressive strength, as well as stress at a plateau, increased by more than 20%, from 25 to 33 MPa and from 15 to 24 MPa, respectively, for the composites fabricated by infiltration ceramic preforms with 40% of porosity.

Reusing Waste Coffee Grounds in the Preparation of Porous Alumina Ceramics

Porous ceramics can be used in various industrial applications, such as thermal insulation, orthopedic implants, high-temperature filtration, lightweight structural components, and catalyst supports, etc., and can be obtained using various methods. In this study, the sacrificial fugitive method was used to prepare a porous alumina ceramic. The appropriate amount of sacrificial fugitive was combined with raw ceramic powder as a pore-forming agent, and was then evaporated or burned out either before or during the sintering process to create the desired pores. Various materials can be used as pore-forming agents; in this work, eco-friendly waste coffee grounds (WCG) were utilized. First, alumina ceramic green bodies were prepared via slip casting of 60 wt. % alumina suspensions with five different amounts of WCG (0 wt. %, 1 wt. %, 5 wt. %, 10 wt. % and 15 wt. %) and the dispersant Dolapix (0.2 wt. %), and using PVA (0.5 wt. %) as a binder for all solutions. The effect of the various amounts of WCG on the alumina ceramic green bodies, and subsequently on the obtained sintered ceramics, was tracked and validated through different analyses. Suspension viscosity was determined through a rotational viscometer. Simultaneous differential thermal and thermogravimetric (DTA/TGA) analyses were used to observe the thermal decomposition of WCG and to determine the sintering regime. After sintering, the density, porosity, and shrinkage of the samples were examined and calculated. In addition, the phase composition and crystallite size of all sintered samples were determined by powder X-ray diffraction (PXRD) analysis, as well as their morphology and composition using Scanning Electron Microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). The results show that density decreased from 3.743 to 2.172 g/cm3 and porosity increased from 6.12% to 45.52%, both with the increasing amount of WCG (from 0 wt. % to 15 wt. %).

Carbon Fibers and Graphite as Pore-Forming Agents for the Obtention of Porous Alumina: Correlating Physical and Fractal Characteristics

Porous alumina ceramics with different porosities were prepared via atmospheric pressure sintering using a sacrificial template method with alumina powder as the raw material and carbon fiber (CF) and graphite as pore-forming agents. The effects of the contents and ratios of the pore-forming agents and the aspect ratios of CF on the microstructure, mechanical properties, pore size, and pore-size distribution of the porous alumina samples were investigated. In addition, the surface fractal dimension (Ds) of porous alumina samples with different pore-forming agents was evaluated based on the mercury intrusion porosimetry data. The pore-size distribution of the prepared porous alumina samples showed single, double, or multiple peaks. The pore structure of the samples maintained the fibrous shape of the original CF and the flake morphology of graphite with a uniform pore-size distribution, but the pore structure and morphology were different. With the increase in the content of the pore-forming agent, the porosity of the samples gradually increased to a maximum of 63.2%, and the flexural strength decreased to a minimum of 12.36 MPa. The pore structure of the porous alumina samples showed obvious fractal characteristics. Ds was closely related to the pore structure parameters of the samples when the content of the pore-forming agent was 70 vol.%. It decreased with an increase in the sample porosity, most probable pore size and median pore size, but increased with an increase in the sample flexural strength.

UV curing-assisted 3D plotting of ceramic feedstock containing thermo-regulated phase-separable, photocurable vehicle for dual-scale porosity structure

We propose a novel approach for manufacturing dual-scale porosity alumina structures by UV curing-assisted 3D plotting of a specially formulated alumina feedstock using a thermo-regulated phase separable, photocurable camphene/triethylene glycol dimethacrylate (TEGDMA) vehicle. In particular, 3D plotting process was conducted at - 5 °C, and thus an alumina suspension prepared using liquid camphene/TEGDMA at room temperature could undergo phase separation, resulting in camphene crystals surrounded by walls comprised of liquid photopolymer enclosing alumina particles. To enhance the shape retention ability of extruded filaments, polystyrene (PS) polymer was used as the tackifier . The phase-separated feedrod could be extruded favorably through a nozzle and rapidly photopolymerized by UV light during the 3D plotting process. Thr ee-dimensionally interconnected macropores were tightly constructed, which were separated by microporous alumina filaments, where micropores were created by the removal of camphene crystals via freeze-dying. The macroporosity of porous alumina ceramics was controlled by adjusting the distance between deposited filaments, while their microporosity was kept constant, leading to tightly tailored overall porosity and mechanical properties.

Rice Husk as a Pore-Forming Agent: Impact of Particle Size on the Porosity and Diametral Tensile Strength of Porous Alumina Ceramics

This study describes the porosity and particle size effects of rice husk pore former on the diametral tensile strength of porous alumina (Al2O3) ceramics. Porous Al2O3 ceramics with high porosity and sufficient diametral tensile strength were successfully prepared by the pore-forming agent method using rice husk (RH) as the pore former according to the sample formulation Al2O3-xRHy (where ‘x’ denotes the particle size range in µm and ‘y’ denotes the percent weight content (wt%) of RH). The thermogravimetric analysis (TGA) and X-ray diffractometer (XRD) results revealed that silica was retained as rice husk ash in the developed porous Al2O3 after the decomposition of the starting rice husk pore former. Microstructures of the as-prepared porous Al2O3 ceramics having different RH additions exhibited hierarchical pore structures with increased particle size of the pore-forming agent. Porosity increased with larger particle size range of rice husk where the Al2O3-63RH5 demonstrated the least porosity (44.2 vol%), while the highest porosity (70.9 vol%) was demonstrated by the Al2O3-125–250RH20. The diametral tensile strength of the RH-shaped porous alumina ceramics declined from 16.97 to 0.65 MPa with increased particle size of the rice husk.

Fabrication of porous Al2O3 ceramics using carbon black as a pore forming agent by spark plasma sintering

Fabrication of ceramic materials containing high amount of porosity with controlled size and structure is a critical factor for various applications. In this study, porous alumina ceramics were prepared by addition of 5–30 vol% carbon black (CB) powders with different specific surface area (SSA) into the alumina matrix as a pore forming agent and application of partial sintering via spark plasma sintering (SPS) technique. After removal of CB particles from the ceramic structures, physical, mechanical, and thermal properties of the porous alumina ceramics were investigated. The open porosity level was detected between 38.48 and 59.81% depending on the type and amount of CB removed. The compressive strength of the sintered samples was measured as ∼150 MPa, which is ∼3 times higher than that of the conventionally sintered samples thanks to the strong neck formation between grains. The large pores formed by removal of CB agglomerates in case of using CB powders with SSA of 107 and 254 m2/g resulted in a change in thermal conduction mechanism from lattice vibration to thermal convection.