Porous YSZ Ceramics Reinforced by Different Kinds of Fibers

Porous YSZ ceramics reinforced by different fibers were prepared by gel‐casting with 15% solid content and pressureless sintering. The four kinds of fibers (mullite, aluminosilicate, Al₂O₃, and YSZ fibers) were added into the YSZ ceramics with the same 10% vol content. After sintered at 1500°C for 2 h, aluminosilicate and mullite fibers could not be found in the samples of porous YSZ ceramics, which showed they reacted with YSZ ceramics at high temperature, while YSZ and Al₂O₃ fibers still kept perfect after sintering. Furthermore, the influences of fiber content, sintering temperature, porosity of matrix materials on compressive strength and porosity of the porous YSZ ceramics were studied. The results showed that Al₂O₃ fiber showed more obvious reinforcing effect than YSZ fiber on porous YSZ ceramics. The fiber‐reinforcing effects depend on fiber content, sintering temperature, and porosity of matrix materials. The fiber addition can improve the shrinkage behavior of porous ceramics during sintering and strengthen the skeleton of porous ceramics.     

Porous fibrous ZrO2-mullite ceramics prepared via tert-butyl alcohol-based gel-casting

Mullite fiber was used to fabricate ZrO₂-mullite based porous ceramic via tert-butyl alcohol (TBA)-based gel-casting process using zirconite and bauxite as raw materials. Phase compositions, microstructure, pore size distribution, linear shrinkage, bulk density, apparent porosity, thermal conductivity, and compressive strength were analyzed to investigate influences of mullite fiber content and added Y₂O₃ on prepared porous ceramics. Results show that bird nest-like three-dimensional fibrous reticular skeleton structure was constructed with mullite fibers that evenly enwrapped rod-like mullite and ZrO₂ grains. Prepared porous fibrous ZrO₂-mullite ceramics had narrow pore size distribution that consisted of mullite and m-ZrO₂. With an increase in mullite fiber content, linear shrinkage and bulk density decreased, apparent porosity increased, and relatively good thermal conductivity was obtained. In addition, added Y₂O₃ reacted with Al₂O₃ and SiO₂ to form Y-Al-Si-O glass phase, which promoted sintering and densification of the ceramic, thus improving its compressive strength.     

Preparation and properties of mullite-bonded porous fibrous mullite ceramics by an epoxy resin gel-casting process

Porous fibrous mullite ceramics with a narrow range of pore size distribution have been successfully prepared utilizing a near net-shape epoxy resin gel-casting process by using mullite fibers, Al₂O₃ and SiC as raw materials. The effects of sintering temperatures, different amounts of fibers and Y₂O₃ additive on the phase compositions, linear shrinkage, apparent porosity, bulk density, microstructure, compressive strength and thermal conductivity were investigated. The results indicated that mullite-bonded among fibers were formed in the porous fibrous mullite ceramics with a bird nest pore structure. After determining the sintering temperatures and the amount of fibers, the tailored porous fibrous mullite ceramics had a low linear shrinkage (1.36–3.08%), a high apparent porosity (61.1–71.7%), a relatively high compressive strength (4.4–7.6 MPa), a low thermal conductivity (0.378–0.467 W/m K) and a narrow range of pore size distribution (around 5 µm). The excellent properties will enable the porous ceramics as a promising candidate for the applications of hot gas filters, thermal insulation materials at high temperatures.     

A new Al2O3 porous ceramic prepared by addition of hollow spheres

A method for making porous ceramic prepared by adding hollow spheres was developed, and the resulting porous ceramic was named as hollow spheres ceramic. Water soluble epoxy resin was used as a gel former in the gelcasting process of the Al₂O₃ hollow sphere and Al₂O₃ powder, the porous ceramic porosity varies from 22.3 to 60.1 %. The influence of amount of Al₂O₃ hollow sphere and sintering temperature on the microstructure, compressive strength and thermal conductivity were investigated. With an increasing amount of hollow sphere in the matrix, the porosity increases, which leads to decreased bulk density, compressive strength and thermal conductivity. The compressive strength of the porous ceramics has a power law relation with the porosity, and the calculated power law index is 4.5. The equations of the relationship between porosity and thermal conductivity of porous ceramics are proposed. The thermal conductivity of samples with 60.1 % porosity is as low as 2.1 W/m k at room temperature.     

Preparation and properties of porous SiC–Al2O3 ceramics using coal ash

In this paper, we first reported that porous SiC–Al₂O₃ ceramics were prepared from solid waste coal ash, activated carbon, and commercial SiC powder by a carbothermal reduction reaction (CRR) method under Ar atmosphere. The effects of addition amounts of SiC (0, 10, 15, and 20 wt%) on the postsintering properties of as-prepared porous SiC–Al₂O₃ ceramics, such as phase composition, microstructure, apparent porosity, bulk density, pore size distribution, compressive strength, thermal shock resistance, and thermal diffusivity have been investigated. It was found that the final products are β-SiC and α-Al₂O₃. Meanwhile, the SEM shows the pores distribute uniformly and the body gradually contacts closely in the porous SiC–Al₂O₃ ceramics. The properties of as-prepared porous SiC–Al₂O₃ ceramics were found to be remarkably improved by adding proper amounts of SiC (10, 15, and 20 wt%). However, further increasing the amount of SiC leads to a decrease in thermal shock resistance and mechanical properties. Porous SiC–Al₂O₃ ceramics doped with 10 wt% SiC and sintered at 1600°C for 5 hours with the median pore diameter of 4.24 μm, room-temperature compressive strength of 21.70 MPa, apparent porosity of 48%, and thermal diffusivity of 0.0194 cm²/s were successfully obtained.     

Synthesis and characterization of mullite-ZrO2 porous fibrous ceramic for highly efficient oil-water separation

This work aimed to proposing a new strategy for preparing the mullite-ZrO₂ porous fibrous ceramic used as alternative matrix material for oil-water separation by the aqueous gel-casting method. The properties of the fabricated porous fibrous ceramics in terms of microstructure, phase composition, apparent porosity, bulk density and compressive strength were investigated and the separation behavior was predicted by analyzing the structural changes. It is demonstrated that the phase composition of green bodies consisted of bayerite, boehmite, ZrSiO₄ and YSZ, and the sintered sample contained mullite, ZrO₂ and YSZ. As the YSZ fibers increased, the porosity of the fabricated porous ceramic increased with the maximum value of 70.65% due to the formation of more pores caused by YSZ fibers. Moreover, a significant increase in compressive strength (up to 9.52–21.86 MPa) was observed with the increase of YSZ fibers. Therefore, the fabricated porous ceramics could be appropriative for advanced applications of separation membranes for oil-water separation.     

Effect of alumina fiber content on pore structure and properties of porous ceramics

Porous Al₂O₃ ceramics with different contents of alumina fibers were prepared by gel-casting process. The effects of Al₂O₃ fiber content on pore size distribution, porosity, compressive strength, and load-displacement behavior of the ceramic materials were investigated. Initial results showed that with the increase of Al₂O₃ fiber content, the pore size and porosity of the material is increased, and the compressive strength is decreased. However, upon increasing the fiber content from 50 wt% to 67 wt%, the performance of the samples changed greatly. The compressive strength of the material increased, while the porosity remained unchanged, the pore size increased greatly, and the shape of the load displacement curve changed. It showed that when the fiber content increased from 50 wt% to 67 wt%, the loading body in the fiber-reinforced porous ceramics changed from particles to fibers.     

Effect of mullite phase formed in situ on pore structure and properties of high-purity mullite fibrous ceramics

Porous mullite ceramics are potential advanced thermal insulating materials. Pore structure and purity are the main factors that affect properties of these ceramics. In this study, high performance porous mullite ceramics were prepared via aqueous gel-casting using mullite fibers and kaolin as the raw materials and ρ-Al₂O₃ as the gelling agent. Effects of addition of mullite fibers on the pore structure and properties were examined. The results indicated that mullite phase in situ formed by kaolin, and ρ-Al₂O₃ ensured the purity of mullite samples and mullite fibers bonded together to form a nest-like structure, greatly improving the properties of ceramic samples. In particular, the apparent porosity of mullite samples reached 73.6%. In the presence of 75% of mullite fibers, the thermal conductivity was only 0.289 W/m K at room temperature. Moreover, the mullite samples possessed relatively high cold compressive strength in the range of 4.9–9.6 MPa. Therefore, porous mullite ceramics prepared via aqueous gel-casting could be used for wide applications in thermal insulation materials, attributing to the excellent properties such as high cold compressive strength and low thermal conductivity.     

Novel Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> aerogel/porous zirconia composite with ultra-low thermal conductivity

Highly porous zirconia fibers networks with a quasi-layered microstructure were successfully fabricated using vacuum squeeze moulding. The effects of inorganic binder content on the microstructure, room-temperature thermal and mechanical properties of fibrous porous zirconia ceramics were systematically investigated. Al₂O₃–SiO₂ aerogel was impregnated into fibrous porous ceramics, and the microstructures, thermal and mechanical properties of Al₂O₃–SiO₂ aerogel/porous zirconia composites were also studied. Results show that the Al₂O₃–SiO₂ aerogel/porous zirconia composites exhibited higher compressive strength (i.e., 1.22 MPa in the z direction) and lower thermal conductivity [i.e., 0.049 W/(m/K)]. This method provides an efficient way to prepare high-temperature thermal insulation materials.     

Porous ceramics based on high-thermal-stability Al2O3–ZrO2 nanofibers for thermal insulation and sound absorption applications

Al₂O₃ fibers are promising candidates for porous ceramics, but the sudden growth of grains in the fibers above 1200 °C will limit their applications for high temperature. Herein, we reported the successful fabrication of the Al₂O₃–ZrO₂ nanofibers by electrospinning and the nanofiber-based porous ceramics by a combination of gel-casting, freeze-drying and high-temperature sintering. Results show that the addition of Zr could greatly improve the thermal stability (up to 1400 °C) of the Al₂O₃-based nanofibers, owing to the inhibition of the sudden growth of the grains in the fibers at high temperature. The Al₂O₃–ZrO₂ nanofiber-based porous ceramics after sintering at 1100–1400 °C possessed a multi-level pore structure and exhibited high thermal stability, ultra-high porosity (97.79–98.04%), ultra-low density (0.075–0.091 g/cm³) and thermal conductivity (0.0474–0.0554 W/mK), and excellent sound absorption performance with the average sound absorption coefficient of 0.598–0.770. These porous ceramics are expected to be employed in the fields of high-temperature thermal insulation and sound absorption.     

Low thermal conductivity in porous SiC–SiO2–Al2O3–TiO2 ceramics induced by multiphase thermal resistance

The introduction of multiple heterogeneous interfaces in a ceramic is an efficient way to increase its thermal resistance. Novel porous SiC–SiO₂–Al₂O₃–TiO₂ (SSAT) ceramics were fabricated to achieve multiple heterogeneous interfaces by sintering equal volumes of SiC, SiO₂, Al₂O₃, and TiO₂ compacted powders with polysiloxane as a bonding phase and carbon as a template at 600 °C in air. The porosity could be controlled between 66% and 74% by adjusting the amounts of polysiloxane and the carbon template. The lowest thermal conductivity (0.059 W/(m·K) at 74% porosity) obtained in this study is an order of magnitude lower than those (0.2–1.3 W/(m·K)) of porous monolithic SiC, SiO₂, Al₂O₃, and TiO₂ ceramics at an equivalent porosity. The typical specific compressive strength value of the porous SSAT ceramics at 74% porosity was 3.2 MPa cm³/g.     

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.     

Controllable preparation of porous ZrB2–SiC ceramics via using KCl space holders

In this work, a novel and facile technique based on using KCl as space holders, along with partial sintering (at 1900 °C for 30 min), was explored to prepare porous ZrB₂–SiC ceramics with controllable pore structure, tunable compressive strength and thermal conductivity. The as-prepared porous ZrB₂–SiC samples possess high porosity of 45–67%, low average pore size of 3–7 μm, high compressive strength of 32–106 MPa, and low room temperature thermal conductivity of 13–34 W m⁻¹ K⁻¹. The porosity, pore structure, compressive strength and thermal conductivity of porous ZrB₂–SiC ceramics can be tuned simply by changing KCl content and its particle size. The effect of porosity and pore structure on the thermal conductivity of as-prepared porous ZrB₂–SiC ceramics was examined and found to be consistent with the classical model for porous materials. The poring mechanism of porous ZrB₂–SiC samples via adding pore-forming agent combined with partial sintering was also preliminary illustrated.     

A novel way to fabricate highly porous fibrous YSZ ceramics with improved thermal and mechanical properties

Inspired by nest structure, highly porous fibrous yttria-stabilized zirconia (YSZ) ceramics were fabricated through tert-butyl alcohol (TBA)-based gel-casting process and pressureless sintering by using YSZ fibers as raw material and adding K₂SO₄ as removable sintering aid. Different sintering temperature and soaking time were investigated to achieve optimal thermal and mechanical properties. The results show that all specimens consist of crystallized t-YSZ phase. Fibers interconnect with good interfacial bonding on junctions. Under higher sintering temperature, porosity drops gradually while compressive strength increases significantly. With prolonged soaking time, there is no obvious change in porosity and compressive strength increases gradually. All specimens have uniformly distributed pores with average size of 30.2 μm and show good structural stability at high temperature. Ultra-low thermal conductivity is achieved and ductile fracture mode with high elongation makes it more applicable in high-temperature thermal insulating applications.     

Fabrication and properties of mullite fiber matrix porous ceramics by a TBA-based gel-casting process

A bird nest-like structure was designed by using the mullite fiber as the matrix and SiO₂ as the high temperature binder. This special material was successfully prepared by a TBA-based gel-casting process. The randomly arranged fiber laps bonded by SiO₂ binder was the most important structure characteristic of this porous material. The effect of sintering temperature on the properties, i.e. porosity, bulk density, linear shrinkage, compressive strength, thermal conductivity and the microstructure was studied. The composite exhibited significant pseudoductility. The fracture mechanism of this composite under compression was discussed. The results indicated that the sintering temperature ranging from 1500 to 1600 °C was suitable for yielding mullite fiber matrix porous ceramics which had a low thermal conductivity (0.19–0.22 W/m K), a relatively high compressive strength (3–13 MPa) and a high resilience (66–70%) for applications in the thermal insulators and high-temperature elastic seal field.     

Process induced alignment of carbon nanotube decreases longitudinal thermal conductivity of Al2O3 based porous composites

Alumina (Al₂O₃) based porous composites, reinforced with zirconia (ZrO₂), 3 and 8 mol% Y₂O₃ stabilized ZrO₂ (YSZ) and 4 wt% carbon nanotube (CNT) are processed via spark plasma sintering. The normalized linear shrinkage during sintering process of Al₂O₃-based composite shows minimum value (19.2–20.4%) for CNT reinforced composites at the temperature between 1650 °C and 575 °C. Further, the combined effect of porosity, phase-content and its crystallite size in sintered Al₂O₃-based porous composite have elicited lowest thermal conductivity of 1.2 Wm⁻¹K⁻¹ (Al₂O₃-8YSZ composite) at 900 °C. Despite high thermal conductivity of CNT (∼3000 Wm⁻¹K⁻¹), only a marginal thermal conductivity increase (∼1.4 times) to 7.3–13.4 Wm⁻¹K⁻¹ was observed for CNT reinforced composite along the longitudinal direction at 25 °C. The conventional models overestimated the thermal conductivity of CNT reinforced composites by up to ∼6.7 times, which include the crystallite size, porosity, and interfacial thermal resistance of Al₂O₃, YSZ and, CNT. But, incorporation of a new process induced CNT-alignment factor, the estimated thermal conductivity (of <6.6 Wm⁻¹K⁻¹) closely matched with the experimental values. Moreover, the high thermal conductivity (<76.1 Wm⁻¹K⁻¹) of the CNT reinforced porous composites along transverse direction confirms the process induced alignment of CNT in the spark plasma sintered composites.     

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.     

Preparation of calcium hexaluminate porous ceramics by gel-casting method with polymethyl methacrylate as pore-forming agent

Calcium hexaluminate (CA₆) porous ceramics were prepared by gel-casting method, with α-Al₂O₃ and CaCO₃ as raw materials and polymethyl methacrylate (PMMA) microspheres as pore-forming agent. The effects of the amount of pore-forming agent PMMA microspheres on the phase composition, bulk density, apparent porosity, flexural strength, microstructure, thermal shock stability and thermal conductivity of CA₆ porous ceramics were systematically studied. The pores of CA₆ porous ceramics are mainly formed by the burning loss of PMMA microspheres and the decomposition of organic matter. Adding an appropriate amount of PMMA microspheres as pore-forming agent has a positive effect on the thermal shock stability of CA₆ porous ceramics. When the amount of pore-forming agent is 15 wt%, the volume density of CA₆ porous ceramics is 1.33 g/cm³, the porosity is 63%, the flexural strength is 13.9 MPa, the thermal shock times can reach 9 times, and the thermal conductivity is 0.293 W/(m·K), which can meet the application in refractory, ceramics or high temperature cement industries.     

High gas permeability and directional channels of mullite porous ceramics prepared by pectin-based freeze-drying method

New gel system for preparing mullite porous ceramics by gel-casting freeze-drying was proposed, using pectin as gel source and alumina and silica as raw materials. Directional channels were formed due to sublimation of water during freeze-drying and decomposition of pectin during high temperature sintering to prepare porous mullite ceramic membranes. Effects of solid content on the properties of mullite ceramics in terms of phase composition, microstructure, apparent porosity, bulk density, pore size distribution, compressive strength, thermal conductivity, pressure drop, and gas permeability were investigated. It was found that prepared porous mullite possessed high apparent porosity (56.04%–75.34%), low bulk density (.77–1.37 g/cm³), uniform pore size distribution, relatively high compressive strength (.61–3.03 MPa), low thermal conductivity (.224–.329 W/(m·K)), high gas permeability coefficient (1.11 × 10⁻¹⁰–4.73 × 10⁻¹¹ m²), and gas permeance (2.18 × 10⁻²–9.32 × 10⁻³ mol⋅m⁻²⋅s⁻¹⋅Pa⁻¹). These properties make prepared lightweight mullite ceramic membranes promising for application in high temperature flue gas filtration. Proposed gel system is expected to provide a new route to prepare porous ceramics with high porosity and directional channels.     

Seed assisted in-situ synthesis of porous anorthite/mullite whisker ceramics by foam-freeze casting

Porous anorthite/mullite whisker ceramics with both high strength and low thermal conductivity have been successfully prepared by combining seed-assisted in situ synthesis and foam-freeze casting techniques. The addition of mullite seed was conducive to a reduction in the sintering shrinkage, pore size, and anorthite grain size. This increased the high aspect ratio of mullite whiskers, which enhanced the strength and diminished the thermal conductivity. Mullite whiskers overlapped to form a stable three-dimensional network structure similar to the bird's nest, which was also beneficial to heighten the mechanical properties of the prepared porous ceramics. Through this method, the prepared materials had a high apparent porosity of 87.7–90.2%, a low bulk density of 0.29–0.36 g/cm³, a high compressive strength of 0.65–3.31 MPa, and low thermal conductivity of 0.067–0.112 W/m·K. The results indicated that the method described here can fabricate porous ceramics with excellent properties for further thermal insulating applications.