Porous Alumina Ceramics Research Articles

Preparation and properties of porous alumina with inter-locked platelets structure

Porous alumina ceramics with alumina platelets was prepared by vapor-solid reaction sintering of AlOF mesophase gas by the reaction of HF and Al2O3. The effect of heating treatment temperatures on porosity, the formation of inter-locked platelets structure and compressive strength of porous alumina ceramics was determined by Archimedes' method, XRD, SEM and compressive tests. The results indicated that after heating at temperatures from 1300 °C to 1600 °C, the porosity of alumina ceramics decreased from 61.6% to 48.4%. Increasing the heating treatment temperature was beneficial to form inter-locked structure between alumina platelets. The maximum compressive strength of porous ceramics with porosity of 48.4% can reach 29.8 MPa heated at 1600 °C; this strength was attributed to the strong bonding between the alumina platelets.

Fabrication, Characterization and Drainage Capacity of Single-Channel Porous Alumina Ceramic Membrane Tube.

The single-channel Al2O3-based porous ceramic membrane tubes (PCMT) were prepared with different grain size of Al2O3 powders by extrusion molding process, combing the traditional solid-phase sintering method. The effects of raw grain size and sintering temperature on the microstructure, phase structure, density, and porosity were investigated. The results revealed that with further increase in sintering temperature, the density of porous ceramics increases, while the porosity decreases, and the pore size decreases slightly. The pore size and porosity of porous ceramics increase with the increase in raw grain size, while the density decreases. Future, in order to study the water filtration of PCMT, the effect of porosity on the pressure distribution and flow velocity different cross-sectional areas with constant feed mass flow was analyzed using Fluent 19.0. It was found that an increase in the porosity from 30% to 45% with constant feed mass flow influenced transmembrane pressure, that varied from 216.06 kPa to 42.28 kPa, while the velocity change at the outlet was not obvious. Besides, it was observed that the surface pressure is almost constant along the radial direction of the pipe, and the velocity of water in the PCMT is increasing with the decreasing of distance to the outlet. It was also verified that the porosity being 39.64%, caused transmembrane pressure reaching to 77.83 kPa and maximum velocity of 2.301 m/s. These simulation and experimental results showed that the PCMT have good potential for water filtration.

Tunable Size of Hierarchically Porous Alumina Ceramics Based on DIW 3D Printing Supramolecular Gel.

A new three-dimensional (3D) printing gel is developed to construct hierarchically porous ceramics with adjustable millimeter-, micrometer-, and nanometer-scale size for application in thermal management. Not only does the gel based on supramolecular micelles exhibit excellent DIW 3D printability but also the supramolecular micelles act as templates that can precisely control the structure of micrometer-scale pores. The effect of millimeter- and μmicrometer-scale size on properties of porous ceramics is investigated in detail. The 3D-printed ceramic foam with millimeter-scale pores and smaller micrometer-scale pores shows better thermal insulation and lower compressive strength. For the thermal insulation, the local temperature of a chip exposed to contact heat is only 34.2 °C in the presence of a printed foam cap with a pore size of 41.5 μm, while the local temperature is 54.8 °C in the absence of the printed foam cap. The study provides a new method to construct hierarchically porous alumina ceramics with precisely tunable size, avoiding the issues of subtractive manufacturing and opening up new applications in portable devices or consumer electronics.

A selective methane gas sensor based on metal oxide semiconductor equipped with an on-chip microfilter

Gas sensors based on metal oxide semiconductor adduce a low cost and reliable solution for the leakage detection of methane, which is highly explosive. However, it is a fact that MOS gas sensors suffer from a heavy cross sensitive response to volatile organic compounds such as ethanol and acetic acid. With the help of filters, metal oxide semiconductor gas sensors could become quite selective to methane. Here, a novel on-chip microfilter was designed for metal oxide semiconductor gas sensors. The on-chip microfilter is fabricated from porous alumina ceramics loaded with platinum on the surface. The active filter was continuously heated by sensor chips. Whereas, it was kept intact with sensing films and reactive gas molecules have to diffuse through the porous ceramics before reaching the surface of metal oxide semiconductor. It was found that both selective response and sensitive response to methane were enhanced by the on-chip microfilter. This is similar to the reported active filter, which is prepared from catalytic films and deposited directly on the top of sensor layer. Additionally, it was indicated that the on-chip microfilter not only limited to tin dioxide but also valid for indium oxide based on sensors.

Air permeability and tensile properties of novel micron-scale gradient porous plates fabricated by rolling and vacuum sintering

A new type of gradient porous plate (GPP) was successfully fabricated by rolling and vacuum sintering using powders and wire mesh porous plates (WMPPs). The air permeability and tensile strength tests were carried out and the fracture morphology was observed. The results showed that the GPPs with low porosity not only had micron-pore structure with the pore size below 10 μm, and air permeability reached 1.27 × 10−13 m2, but also hold tensile strength with 257.8 MPa. Particularly, its air permeability and tensile strength were significantly superior to nuclear graphite, stainless steel foam, and porous alumina ceramics, equivalent to porous titanium. This study provides an effective method for the fabrication of micron-scale metal GPPs, and is of great significance for expanding the application range (especially in the fields of micron-scale precision filtration and the porous throttles of the aerostatic bearing) of porous materials.

Microstructure and mechanical properties of cold sintered porous alumina ceramics

New innovative approach to fabricate porous alumina ceramics by cold sintering process (CSP) is presented using NaCl as pore forming agent. The effects of CSP and post-annealing temperature on the microstructure and mechanical strength were investigated. Al2O3–NaCl composite with bulk density of 2.92 g/cm3 was compacted firstly using CSP and then a porous structure was formed using post-annealing at 1200°C–1500°C for 30 min. Brazilian test method and Vickers hardness test were used to determine the indirect tensile strength and hardness of the porous alumina, respectively. Meanwhile, the phases and the microstructure were respectively examined using X-ray diffractometer and scanning electron microscope (SEM) complemented by the 3D image analysis with X-ray tomography (XRT). SEM structural and XRT image analysis of cold sintered composite showed a dense structure with NaCl precipitated between Al2O3 particles. The NaCl volatization from the composite was observed during the annealing and then complete porous Al2O3 structure was formed. The porosity decreased from 48 vol% to 28 vol% with the annealing temperature increased from 1200 °C to 1500 °C, while hardness and mechanical strength increased from 14.3 to 115.4 HV and 18.29–132.82 MPa respectively. The BET analysis also showed a complex pore structure of micropores, mesopores and macropores with broad pore size distribution.

Preparation and properties of porous alumina ceramics for ultra-precision aerostatic bearings

γ-alumina powder was mixed with α-alumina powder to fabricate porous ceramics. With an increase in the γ-alumina content, the porosity of the ceramics first increased and then decreased. The open porosity of the ceramics was the same as their total porosity when the γ-alumina content was equal to or greater than 20 wt%. The permeability and open porosity of the porous alumina had a power-function relationship. The addition of γ-alumina not only realised pore transfixion, but also effectively enhanced the compressive strength of the ceramics. The maximum pore size of the ceramics increased with an increase in the γ-alumina content, which contributed to the increase in the bearing capacity and stiffness. When the γ-alumina content was 50 wt%, the porous alumina exhibited an open porosity of 25% and a permeability of 3.2 × 10−15 m2, while the compressive strength and elastic modulus were 325 MPa and 145 GPa, respectively. The bearing stiffness of the film with a thickness of 7.5 μm reached 13.5 N/μm at the air supply pressure of 0.3 MPa. These ceramics showed potential for application in ultra-precision aerostatic bearings.

Porous Alumina Ceramics with Bifunctional Wettability for Oil/Water Separation

Porous Alumina Ceramics with Bifunctional Wettability for Oil/Water Separation

High-Entropy Perovskite Oxide Washcoated Porous Alumina Ceramic as a Superb Catalyst for Activating Peroxymonosulfate to Eliminate Refractory Organic Pollutants

High-Entropy Perovskite Oxide Washcoated Porous Alumina Ceramic as a Superb Catalyst for Activating Peroxymonosulfate to Eliminate Refractory Organic Pollutants

Preparation of self-lubricating porous alumina ceramics with PMMA /PAO6 microcapsules and their tribological properties

In this work, we showed that immersing polymethyl methacrylate (PMMA)/poly alpha olefins (PAO6) microcapsules into the porous matrix can improve the lubrication properties of porous alumina ceramics dramatically. PMMA/PAO6 microcapsules were synthesized by microemulsion polymerization, and the microcapsules were immersed in the porous alumina ceramic matrix by vacuum impregnation. The lubrication behaviors of the porous alumina ceramics with PMMA/PAO6 microcapsules have been investigated under different loads. As compared with the unprocessed porous alumina ceramics, the coefficient of friction (COF) of the porous ceramics impregnated with microcapsules could be reduced to 4% of that without microcapsules, and the wear rate could be reduced by two orders of magnitude. No obvious change of the COF was noticed for the matrices with different pore sizes. The good self-lubrication properties were achieved by releasing the PAO6 in the microcapsules during the friction process.

Rational Design and Porosity of Porous Alumina Ceramic Membrane for Air Bearing

Air bearing has been widely applied in ultra-precision machine tools, aerospace and other fields. The restrictor of the porous material is the key component in air bearings, but its performance is limited by the machining accuracy. A combination of optimization design and material modification of the porous alumina ceramic membrane is proposed to improve performance within an air bearing. Porous alumina ceramics were prepared by adding a pore-forming agent and performing solid-phase sintering at 1600 °C for 3 h, using 95-Al2O3 as raw material and polystyrene microspheres with different particle sizes as the pore-forming agent. With 20 wt.% of PS50, the optimum porous alumina ceramic membranes achieved a density of 3.2 g/cm3, a porosity of 11.8% and a bending strength of 150.4 MPa. Then, the sintered samples were processed into restrictors with a diameter of 40 mm and a thickness of 5 mm. After the restrictors were bonded to aluminum shells for the air bearing, both experimental and simulation work was carried out to verify the designed air bearing. Simulation results showed that the load capacity increased from 94 N to 523 N when the porosity increased from 5% to 25% at a fixed gas supply pressure of 0.5 MPa and a fixed gas film thickness of 25 μm. When the gas film thickness and porosity were fixed at 100 μm and 11.8%, respectively, the load capacity increased from 8.6 N to 40.8 N with the gas supply pressure having been increased from 0.1 MPa to 0.5 MPa. Both experimental and simulation results successfully demonstrated the stability and effectiveness of the proposed method. The porosity is an important factor for improving the performance of an air bearing, and it can be optimized to enhance the bearing’s stability and load capacity.

A novel catalytic-type gas sensor based on alumina ceramic substrates loaded with catalysts and printed electrodes

l The porous alumina ceramics act as the supports of catalysts and the substrates of Pt heating electrodes, simultaneously. l The preparation of platinum electrodes in batch enable sensors with a high uniformity. l The high sensitivity and stability of prototype H 2 sensors was achieved. l Heat transfer analysis by FEM modeling was carried out to verify the sensing performance of H 2 sensors. Catalytic combustion gas sensors have been widely used for leakage detection of combustible gases. Therefore, the catalytic sensor with low preparation cost and high repeatability in a mass production has great importance. In this view, a new structure of high-performance catalytic sensor was proposed. Catalytic-type gas sensors based on screen-printing Platinum (Pt) heating electrode onto a porous alumina substrate was fabricated. This design enables porous alumina ceramics to act as the support of catalyst and substrate of heating electrode, simultaneously. The screen-printing technique obviously enhances the uniformity of the sensor devices in batch. To demonstrate the feasibility of this design, a prototype hydrogen sensor was prepared with a relatively high sensitivity around 30 mV/Vol% at 245°C and an excellent long-term stability. Additionally, sensing performances of hydrogen devices were also verified by finite element method (FEM) modeling. The simulation results suggested that the response time and response value were related to the thickness of substrate and the operating temperature of sensors. The facile and large-scale fabrication method of the sensor device in this work is expected to have potential utility for industrial detection and for personalized applications. .

In Situ Investigation of Strain Localization in Sintered, Porous Segmented Alumina.

Evaporation of paraffin and ultra-high-molecular-weight polyethylene admixed with alumina powder for the slip casting and sintering process allowed the obtainment of segmented porous alumina ceramics with 50% total porosity, whose deformation behavior we studied. Structurally, these ceramic materials were composed of large and small pores, and a system of discontinuities subdividing the samples into segments. Using digital image correlation (DIC), strain distribution maps were obtained that allowed the observation of strain localization zones, where primary cracks propagated along the interblock discontinuities. Two stages were revealed to be responsible for different mechanisms that provided the sample with damage tolerance under compression loading: the first stage was crack propagation along the block boundaries, which was followed by the second stage of microcracking and fragmentation, consisting of filling of the free spaces with fragments, compaction band generation, and stabilization of the crack. Both stages comprise a cycle that is repeated again and again until the full volume of the sample is occupied by the compaction bands.

Porous Alumina Ceramics with Multimodal Pore Size Distributions.

Pore networks with multimodal pore size distributions combining advantages from isotropic and anisotropic shaped pores of different sizes are highly attractive to optimize the physical properties of porous ceramics. Multimodal porous Al2O3 ceramics were manufactured using pyrolyzed cellulose fibers (l = 150 µm, d = 8 µm) and two types of isotropic phenolic resin spheres (d = 30 and 300 µm) as sacrificial templates. The sacrificial templates were homogeneously distributed in the Al2O3 matrix, compacted by uniaxial pressing and extracted by a burnout and sintering process up to 1700 °C in air. The amount of sacrificial templates was varied up to a volume content of 67 Vol% to form pore networks with porosities of 0–60 Vol%. The mechanical and thermal properties were measured by 4-point-bending and laser flash analysis (LFA) resulting in bending strengths of 173 MPa to 14 MPa and heat conductivities of 22.5 Wm−1K−1 to 4.6 Wm−1K−1. Based on µCT-measurements, the representative volume-of-interest (VOI) of the samples digital twin was determined for further analysis. The interconnectivity, tortuosity, permeability, the local and global stress distribution as well as strut and cell size distribution were evaluated on the digital twin’s VOI. Based on the experimental and simulation results, the samples pore network can be tailored by changing the fiber to sphere ratio and the overall sacrificial template volume. The presence pore formers significantly influenced the mechanical and thermal properties, resulting in higher strengths for samples containing fibrous templates and lower heat conductivities for samples containing spherical templates.

Composition-engineered LaCoO3-based monolithic catalysts for easily operational and robust peroxymonosulfate activation

Herein, we report for the first time the construction of perovskite oxide catalysts on monolithic supports for peroxymonosulfate (PMS) activation to eliminate organic pollutants in wastewater. Specifically, the commercially available porous alumina ceramic (PAC) was utilized as monolithic support, on which Ca − Ti co-modified LaCoO3 perovskites (L1−xCxC1−yTyO3) were successfully loaded through a simple, controllable, and reagent-saving “dripping − calcination” process. The resulting monolithic catalysts (L1−xCxC1−yTyO3@PAC) featured large sizes (cm-scale), highly open porous architectures, and compositionally favorable active components, by which easy catalyst recycling, rapid mass transfer of guest species, and high yet stable catalytic performance were harvested in PMS activation for degradation of the typical antibiotic pollutant metronidazole (MNZ). By optimizing the introduction amount of Ca and Ti, the resulting L0.8C0.2C0.4T0.6O3@PAC showed a high turnover frequency (TOF) value of 12.6 min−1 for MNZ with a low cobalt leaching amount of 0.203 mg L−1, which were significantly superior to the unmodified catalyst LCO3@PAC (TOF: 7.9 min−1; cobalt leaching: 1.71 mg L−1). Roles of Ca and Ti modifiers in the LaCoO3-based monolithic catalyst, degradation pathway of MNZ, and SO4−•/1O2 dominated catalytic mechanism were investigated and elucidated. Our work provides a promising avenue for rational design of low-cost, easily operational, and robust monolithic catalysts toward PMS activation for large-scale environmental remediation.

High-strength porous alumina ceramics prepared from stable wet foams

Porous ceramics have been widely used in heat insulation, filtration, and as a catalyst carrier. Ceramics with high porosity and high strength are desired; however, this high porosity commonly results in low strength materials. In this study, porous alumina with high porosity and high strength was prepared by a popular direct foaming method based on particle-stabilized wet foam that used ammonium polyacrylate (PAA) and dodecyl trimethyl ammonium chloride (DTAC) as the dispersant and hydrophobic modifier, respectively. The effects of the dispersant and surfactant contents on the rheological properties of alumina slurries, stability of wet foams, and microstructure and mechanical properties of sintered ceramics were investigated. The microstructure of porous ceramics was regulated using wet foams to achieve high strength. For a given PAA content, the wet foams exhibited increasing stability with increasing DTAC content. The most stable wet foam was successfully obtained with 0.40 wt% PAA and 0.02 wt% DTAC. The corresponding porous alumina ceramics had a porosity of 82%, an average grain size of 0.7 µm, and a compressive strength of 39 MPa. However, for a given DTAC content, the wet foams had decreasing stability with increasing PAA content. A possible mechanism to explain these results is analyzed.

EFFECT OF PORE COMPOSITION ON THERMAL SHOCK RESISTANCE OF POROUS CERAMICS

Porous alumina ceramics with different pore composition (intrinsic pores and extrinsic pores) were prepared by gel injection molding process combined with pore forming technology and effects of intergranular pores and extrinsic pores on thermal shock fracture resistance and thermal shock damage resistance were studied. The results show that, when the solid phase content or total porosity is the same, the porous ceramic material with extrinsic pores has higher damage tolerance, thermal shock critical temperature difference (ΔTc) and residual strength retention rate. A ratio parameter (KIC/E) for evaluating the thermal shock stability of materials is deduction. The larger the ratio is, the better the thermal shock resistance of materials.

Tailored pore structures and mechanical properties of porous alumina ceramics prepared with corn cob pore‐forming agent

AbstractIn this work, the effects of porosity and different particle sizes of pore‐forming agent on the mechanical properties of porous alumina ceramics have been reported. Different grades of porous alumina ceramics were developed using corn cob (CC) of different weight contents (5, 10, 15, and 20 wt%) and particle sizes (<63 µm, 63‐125 µm and 125‐250 µm) as the pore‐forming agent. Experimental results showed that total porosity and pore cavity size of the porous alumina ceramics increased with rising addition of CC pore former. Total porosity increased with increasing particle size of CC with the Al2O3‐<63CC5 sample exhibiting the lowest total porosity of 41.3 vol% while the highest total porosity of 68.1 vol% was exhibited by the Al2O3‐125‐250CC20. The particle size effect of CC on the mechanical properties revealed that diametral tensile strength and hardness of the porous alumina ceramics deteriorated with increasing particle size of CC pore former. The Al2O3‐<63CC5 sample exhibited the highest diametral tensile strength and hardness of 25.1 MPa and 768.2 HV, respectively, while Al2O3‐125‐250CC20 exhibited the lowest values of 1.1 MPa and 35.9 HV. Overall, porous alumina ceramics with the smallest pore sizes under each particle size category exhibited superior mechanical properties in their respective categories.

Thermal Conductivity Optimization of Porous Alumina Ceramics via Taguchi Model

A comparative analysis of the thermal conductivity for porous alumina using Taguchi method has been reported in the current research. Porous alumina is one of the most critical ceramics amongst those that are widely used in the thermal insulator industry; this is because of their physical properties. Thus, the investigation of these properties is highly desirable. Test variables were performed for the thermal conductivity studies-weight per cent of a pore-forming agent (yeast), sintering temperature, and soaking time. Through implementing the experimental design using the Taguchi method for thermal conductivity of porous alumina was statistically analyzed. The Signal-to-noise ratio and variance analysis investigated the influence of different parameters on the porous media's thermal conductivity. The result of research determines that the addition of the pore-forming agent obtained a higher thermal insulator. Based on the optimum conditions obtained from the Taguchi method factor was 20wt.% weight of yeast cell , sintering temperature at 1200 C , and the holding time 1.5 h. that give higher value of the S/N ratio.

Low thermal expansion coefficient and high thermal conductivity epoxy/Al2O3/T-ZnOw composites with dual-scale interpenetrating network structure

Substrates with high thermal conductivity (TC), outstanding mechanical strength, and low coefficient of thermal expansion (CTE) and dielectric constant are ideal materials for electronic packaging. The preparation of ceramic/polymer composites is a feasible method to meet the above requirements. Herein, a kind of reticulated porous alumina ceramics (RPCs) was introduced into epoxy resin (EP) doped with tetrapod-like zinc oxide whiskers (T-ZnOw) to produce EP/Al2O3/T-ZnOw composites with continuous interpenetrating network structure. The addition of 11.7 vol% RPCs and 6 vol% T-ZnOw caused an increase of TC to 1.968 W·m−1·K−1. Compared to pure EP, the CTE of EP/11.7 vol% Al2O3/6 vol% T-ZnOw composite was reduced by an order of magnitude and the flexural modulus increased 5.7 times. The novel substrate materials, fabricated via the polymer replica and vacuum impregnation methods, featured attractive properties for potential use in electronic packaging applications with low fillers loading.