Effect of pore characteristics on permeability of sintered diatomite filter for microfiltration

Diatomite, because it is inherently porous and irregular, presents an interesting opportunity to investigate how the processing conditions of green bodies and the incorporation of spherical pores affect the final properties of a sintered ceramic filter. The water flux of a diatomite filter sintered at 1200°C was 6·3×10⁴ L m^?2 h^?1 bar^?1, which is higher than the water fluxes of typical ceramic filters such as spherical fly ash (1·6×10⁴ L m^?2 h^?1 bar^?1), γ-alumina (～1·0×10³ L m^?2 h^?1 bar^?1) and zirconia (1·6×10³ L m^?2 h^?1 bar^?1) filters. The results obtained in this study show that the pressure applied during the processing of green bodies and the incorporation of spherical pores directly affect pore characteristics and accordingly determine the permeability of the sintered diatomite filters.     

Fabrication of porous silica ceramics by gelation-freezing of diatomite slurry

Diatomite-derived porous silica ceramics with high porosities of up to 90% were fabricated using a gelation-freezing method, which resulted in unidirectional cellular or random microstructure with micrometer-sized cells. The ice crystals that were formed during freezing of a diatomite powder dispersed gel were removed by sublimation during vacuum drying, and the green bodies were sintered at 1150–1350 °C for 2 h in air. The thermal conductivity of the porous ceramics prepared with initial solid loadings of 5 and 10 vol% ranged from 0.09 to 0.16 W/(mK) at room temperature. The proposed method is therefore promising for the preparation of ceramic thermal insulators with very low thermal conductivity.     

Effect of humidity on the thermal conductivity of porous zirconia ceramics

This study focuses on the role of the water content on the effective thermal conductivity of porous ceramics placed in different conditions of relative humidity. Fully stabilized zirconia samples with variation in the capacity to take up water were prepared by varying the temperature of the thermal treatment. The pore volume fraction of the dried samples decreases from 56% down to 30%. Thermal conductivity measurements were made on samples placed in a chamber where the relative humidity was fixed between 3% and 99%. For all samples, the experimental values of the effective thermal conductivity increase significantly with the water content. Experimental results agree closely to analytical predictions based on the upper limit of the Hashin and Shtrikman expressions for calculating the thermal conductivity of the pores (constituted by air and water) and Landauer's effective medium expression for calculating the effective thermal conductivity of the material.     

Enhancing the thermal insulating properties of lanthanum zirconate porous ceramics via pore structure tailoring

The potentially useful role of lanthanum zirconate (La₂Zr₂O₇, LZO) porous bulk ceramics has been rarely explored thus far, much less the optimisation of its pore structure. In this study, LZO porous ceramics were successfully fabricated using a tert-butyl alcohol (TBA)-based gelcasting method, and the pore structures were tailored by varying the initial solid loading of the slurry. The as-prepared ceramics exhibited an interconnected pore structure with high porosity (67.9 %–84.2 %), low thermal conductivity (0.083–0.207 W/(m·K)), and relatively high compressive strength (1.56–7.89 MPa). The LZO porous ceramics with porosity of 84.2 % showed thermal conductivity as low as 0.083 W/(m·K) at room temperature and 0.141 W/(m·K) at 1200 °C, which is much lower than the counterparts fabricated from particle-stabilized foams owing to its unique pore structure with a smaller size, exhibiting better thermal insulating performance.     

3D printing boehmite gel foams into lightweight porous ceramics with hierarchical pore structure

We herein report a novel hierarchically porous ceramic foams derived from boehmite gel foams, which possess both high porosity and superior strength. The gel foams show excellent printability due to its predominant stability, high yield stress and storage modulus, which endows such foam material ideal ink for 3D printing lightweight and complex-shape materials via direct ink writing approach. The 3D printed ceramic foams possess programmable architecture assembled by porous filaments, uniform macro-pores with tunable size in the range of 4∼70 μm, as well as nanoscale pores in cell wall, after sintering at relatively low temperature of 1200–1300 °C. In this way, ceramic foams with high strength were achieved, attributed to the tiny grains, large amount of grain boundaries, uniform pores and hierarchical pore structure. Notably, the foams sintered below 1200 °C have significant advantage on specific surface area, which could reach up to 300-400 m²/g.     

Preparation of porous Al2TiO5-Mullite ceramic by starch consolidation casting and its corrosion resistance characterization

Stabilized Al₂TiO₅ (AT)-mullite (M) porous ceramics were fabricated by starch consolidation casting using corn starch as curing agent and their microstructure, mechanical properties, pore size distribution and corrosion resistance were examined. Results showed that AT-M porous ceramic with the flexural strength of 11.5 MPa, apparent porosity of about 54.7% and pore size distribution in the range of 1–15 µm could be obtained with 10 wt% corn starch addition. Corrosion resistance results showed mass losses in hot H₂SO₄ solution and NaOH solution for 10 h to decreased from 1.03% to 0.36% and 4.39–2% when the calcination temperature increased from 1400 °C to 1450 °C, which proved these AT-M porous ceramics to possess an excellent corrosion resistance in acidic condition when calcined at 1450 °C.     

A novel method for the preparation of porous zirconia ceramics with multimodal pore size distribution

This work presents a new route of processing porous ceramic materials by vacuum induction melting of metallic ternary alloys from the system Ni–Zr–Y. Following internal oxidation results in an interpenetrating network of nickel and oxide phase consisting of yttria and zirconia. After dissolution of the metallic nickel matrix the oxide phase remains as a stable porous ceramic material with a multimodal pore size distribution. The porosity, pore size distribution and specific surface area can be controlled by alloy composition, solidification conditions and oxidation parameters.     

Improved mechanical strength and thermal resistance of porous SiC ceramics with gradient pore sizes

Thermal insulation applications of porous SiC ceramics require low thermal conductivity and high mechanical strength. However, low thermal conductivity and high mechanical strength possess a trade-off relationship, because improving the mechanical strength requires decreasing the porosity, which increases the thermal conductivity. In this study, we established a new strategy for improving both the mechanical strengths and thermal resistances of porous SiC ceramics with micron-sized pores by applying a double-layer coating with successively decreasing pore sizes (submicron- and nano-sized pores). This resulted in a unique gradient pore structure. The double-layer coating increased the flexural strengths and decreased the thermal conductivities of the porous SiC ceramics by 24–70 % and 29–49 % depending on the porosity (48–62 %), improving both their mechanical strengths and thermal resistances. This strategy may be applicable to other porous ceramics for thermal insulation applications.     

Processing and properties of silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity

Silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity were prepared by sintering nano-SiC powder-carbon black template compacts at 600–1200 °C for 2 h in air. The microstructure of the silica-bonded porous nano-SiC ceramics consisted of SiC core/silica shell particles, a silica bonding phase, and hierarchical (meso/macro) pores. The porosity and thermal conductivity of the silica-bonded porous nano-SiC ceramics can be controlled in the ranges of 8.5–70.2 % and 0.057–2.575 Wm⁻¹ K⁻¹, respectively, by adjusting both, the sintering temperature and template content. Silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity (0.057 Wm⁻¹ K⁻¹) were developed at a very low processing temperature (600 °C). The typical porosity, average pore size, compressive strength, and specific compressive strength of the porous nano-SiC ceramics were ∼70 %, 50 nm, 2.5 MPa, and 2.7 MPa·cm³/g, respectively. The silica-bonded porous nano-SiC ceramics were thermally stable up to 1000 °C in both air and argon atmospheres.     

Preparation of porous diatomite ceramics by an alkali treatment near room temperature

Porous diatomite ceramics were prepared by alkali treatment of the raw powder without any sintering procedure. The alkali treatment was carried out by dissolution of the surface by KOH solution to form solid crosslinking among the powder particles near room temperature. The effect of the amount and concentration of KOH solution on the properties of the porous diatomite ceramics was investigated. Variation of the treatment factors was found to be effective for controlling properties such as porosity, strength and thermal conductivity. Mixing a larger amount of high-concentration KOH solution with the raw powder could promote dissolution and the formation of thicker necks between particles by solid crosslinking. The resultant porous diatomite ceramics showed comparable compressive strength and thermal conductivity to those of sintered materials. The proposed method is therefore a useful tool for the preparation of high-performance porous ceramics near room temperature.     

In-situ growth of mullite whiskers and their effect on the microstructure and properties of porous mullite ceramics with an open/closed pore structure

Porous mullite ceramics with an open/closed pore structure were prepared by protein foaming method combined with fly ash hollow spheres. Both the open porosity and total porosity of samples were enhanced by increasing the hollow sphere content. Mullite whiskers with a diameter of 0.2–4 μm were grown in-situ in the porous mullite ceramics with an AlF₃ catalyst, conforming to a vapor-solid growth mechanism. The pore structure of the porous mullite ceramics was significantly affected by the mullite whiskers which increased the open porosity and total porosity. Moreover, the median pore size was reduced from 65.05 μm to 36.92 μm after the introduction of mullite whiskers. The flexural strength and the thermal conductivity of the samples decreased with increasing total porosity. The porosity dependence of the thermal conductivity was well described by the universal model, providing a reference for the prediction of thermal conductivity of porous ceramics with open/closed pores.     

Effects of pore structure on thermal conductivity and strength of alumina porous ceramics using carbon black as pore-forming agent

In the present work, carbon black (CB) works as a pore-forming agent in the preparation of alumina porous ceramics. The pore structures (i.e. mean pore size, pore size distribution and various pores size proportions) were characterized by means of Micro-image Analysis and Process System (MIAPS) software and mercury intrusion porosimetry. Then their correlation and thermal conductivity as well as strength were determined using grey relation theory. The results showed that the porosity and mean pore size increased against the amount of CB, whereas the thermal conductivity, cold crushing strength and cold modulus of rupture reduced. The <2 μm pores were helpful for enhancing the strength and decreasing the thermal conductivity whereas the >14 μm pores had the opposite effects.     

High-strength thermal insulating porous mullite fiber-based ceramics

How to improve the strength of fibrous porous ceramics dramatically under the premise of no sacrificing its low density and thermal conductivity has remained a challenge in the high-temperature thermal insulation field. In this paper, a new kind of high-strength mullite fiber-based ceramics composed of interlocked porous mullite fibers was prepared by nanoemulsion electrospinning and dry pressing method. Results show that as to the porous ceramics with the same density (～ 0.8 g/cm³), the three-dimensional skeleton structure composed of porous mullite fibers was much denser than that composed of solid mullite fibers. Therefore, porous mullite fiber-based ceramics exhibited a higher compressive strength (5.53 MPa) than that of solid mullite fiber-based ceramics (3.21 MPa). In addition, porous mullite fiber-based ceramics exhibited a superior high-temperature heat insulation property because the porous structure in fibers could reduce the radiant heat conduction. This work provides new insight into the development of high-temperature thermal insulators.     

Effect of pore structure on gas permeability constants of porous alumina

Porous alumina was fabricated using different particle size, sintering temperature, and particle size and content of poly (methyl-methacrylate) (PMMA) as pore former. The Forchheimer equation was used to investigate the relationship between porosity and average pore size, and obtain the permeability constants k₁ and k₂ (the viscous effect and the inertial effect, respectively). Compared to Darcy's law, the Forchheimer equation established a more realistic and reliable relationship between fluid pressure and fluid velocity. k₁ and k₂ were found to be more sensitive to the average pore size than to the porosity of alumina. Moreover, reliable relationships were confirmed between the average pore size and k₁, k₂, and their ratio (k₁/k₂).     

Effect of the addition of polyvinylpyrrolidone as a pore-former on microstructure and mechanical strength of porous alumina ceramics

The effect of the solvent on the properties of porous alumina ceramics was studied when polyvinylpyrrolidone (PVP) was used as an organic pore-former. In particular, porous alumina ceramics were produced by dry-pressing of mixed PVP–alumina powder; the mixing of PVP and alumina powder was achieved via ball milling using water or acetone as solvent, or dry ball milling. Due to the different solubility of PVP in water and acetone, porous alumina ceramics with different pore structures and mechanical properties were obtained. Because of its cylindrical pores being aligned to some extent, the sample prepared using acetone as solvent exhibited the highest bending strength (140.2 MPa) and Young's modulus (57.4 GPa), which were 1.6 times and 3.4 times higher compared to that prepared without PVP. Moreover, the addition of PVP via wet ball milling led to more uniform dispersion of PVP in alumina, hence limiting the grain growth during sintering process and increasing the grain bonding.     

Tailoring the pore structure and property of porous biphasic calcium phosphate ceramics by NaCl additive

This study investigated the effects of NaCl additive on the phase composition, pore structure and mechanical property of porous biphasic calcium phosphate (BCP) ceramics, which were prepared by freeze-casting. The results indicated that the addition of NaCl promoted transformation of β-tricalcium phosphate to hydroxyapatite in the BCP ceramics; the OH group in HA phase of BCP ceramic was partially replaced by chloride ion. As the mass fraction of NaCl in the slurry increased from 0 to 3%, the porosity of obtained porous BCP ceramics decreased from 77.76% to 60.22%; the average width of dendritic pores increased from 74.37 µm to 111.27 µm; the compressive strength achieved threefold increase. As the amount of NaCl additive reached 4.5%, the porosity, pore width, and compressive strength of the porous BCP ceramics were comparable with those modified by 3% NaCl. NaCl is regarded as an effective additive to tailor the pore structure and property of freeze-cast porous ceramics.     

Foam-gelcasting preparation,microstructure and thermal insulation performance of porous diatomite ceramics with hierarchical pore structures

Hierarchically pore-structured porous diatomite ceramics containing 82.9∼84.5% porosity were successfully prepared for the first time via foam-gelcasting using diatomite powder as the main raw material. Sizes of mesopores derived from the raw material and macropores formed mainly from foaming were 0.02∼0.1 μm and 109.7∼130.5 μm, respectively. The effect of sintering temperature, additive content and solid loading of slurry on pore size and distribution, and mechanical and thermal properties of as-prepared porous ceramics were investigated. Compressive strength of as-prepared porous ceramics increased with sintering temperature, and the one containing 82.9% porosity showed the highest compressive strength of 2.1 ± 0.14 MPa. In addition, the one containing 84.5% porosity and having compressive strength of 1.1 ± 0.07 MPa showed the lowest thermal conductivity of 0.097 ± 0.001 W/(m·K) at a test temperature of 200 ̊C, suggesting that as-prepared porous ceramics could be potentially used as good thermal insulation materials.     

Investigating the effect of SPRM on mechanical strength and thermal conductivity of highly porous alumina ceramics

For recycling waste refractory materials in metallurgical industry, porous alumina ceramics were prepared via pore forming agent method from α-Al₂O₃ powder and slide plate renewable material. Effects of slide plate renewable material (SPRM) on densification, mechanical strength, thermal conductivity, phase composition and microstructure of the porous alumina ceramics were investigated. The results showed that SPRM effectively affected physical and thermal properties of the porous ceramics. With the increase of SPRM, apparent porosity of the ceramic materials firstly increased and then decreased, which brought an opposite change for the bulk density and thermal conductivity values, whereas the bending strength didn’t decrease obviously. The optimum sample A2 with 50 wt% SPRM introducing sintered at 1500 °C obtained the best properties. The water absorption, apparent porosity, bulk density, bending strength and thermal conductivity of the sample were 31.7%, 62.8%, 1.71 g/cm³, 47.1 ± 3.7 MPa and 1.73 W/m K, respectively. XRD analysis indicated that a small quantity of silicon carbide and graphite in SPRM have been oxidized to SiO₂ during the firing process, resulting in rising the porous microstructures. SEM micrographs illustrated that rod-like mullite grains combined with plate-like corundum grains to endow the samples with high bending strength. This study was intended to confirm the preparation of porous alumina ceramics with high porosity, good mechanical properties and low thermal conductivity by using SPRM as pore forming additive.     

Fabrication of wood-like porous silicon carbide ceramics without templates

The porous silicon carbide ceramics with wood-like structure have been fabricated via high temperature recrystallization process by mimicking the formation mechanism of the cellular structure of woods. Silicon carbide decomposes to produce the gas mixture of Si, Si₂C and SiC₂ at high temperature, and silicon gas plays a role of a transport medium for carbon and silicon carbide. The directional flow of gas mixture in the porous green body induces the surface ablation, rearrangement and recrystallization of silicon carbide grains, which leads to the formation of the aligned columnar fibrous silicon carbide crystals and tubular pores in the axial direction. The orientation degree of silicon carbide crystals and pores in the axial direction strongly depends on the temperature and furnace pressure such as it increases with increasing temperature while it decreases with increasing furnace pressure.     

Preparation and properties of porous ceramics from nickel slag by aerogel gelcasting

To further resource industrial solid waste, porous ceramics with high porosity were prepared by a gelcasting method using nickel slag and kaolin as raw materials and hydrophilic nontoxic SiO₂ aerogel as a gelling agent. The effects of nickel slag content, dispersant and solid content on the properties and microstructure of porous ceramics were investigated in detail in terms of density, compressive strength, porosity, phase composition and micromorphology. The results confirmed that a certain amount of nickel slag can effectively improve the porosity of porous ceramics, while the addition of dispersant can promote the flow of the slurry, enhanced the denseness of the raw billet and significantly improved the compressive strength. However, its excessive use had a negative effect on the ceramic density and porosity. At the same time, the solid content played a key role in the performance of porous ceramics prepared by gelcasting, and too much solid content was also not conducive to the generation of pores. When the nickel slag content was 55%, the amount of dispersant was 2%, and the solid content was 60 vol%, the porous ceramic had a better overall performance, the density of the porous ceramic was 510 kg/m³, the compressive strength was 1.3 MPa, and the porosity reached 80.1%. The major crystalline phases of porous ceramics prepared by nickel slag were cordierite and anorthite.