Hierarchical Structure of Porous Silicon Nitride Ceramics with Aligned Pore Channels Prepared by Ice‐Templating and Nitridation of Silicon Powder

A unique hierarchical porous structure of silicon nitride ceramic with 76.5% porosity is fabricated by combining an ice‐templating method and nitridation for a silicon powder. The porous silicon nitride ceramics were composed of a lamellar structure with aligned pore channels and ceramic walls filled with fibrous whiskers. This study is focused on the influences of freezing rate on the microstructures and properties of the silicon nitride ceramics. The properties were characterized by compressive strength and gas permeability, which were shown to vary with controlled microstructure. The compressive strength and the permeability reached up to 32.2 MPa and 0.035^?12 m², respectively.     

Hierarchically porous lanthanum zirconate foams with low thermal conductivity from particle-stabilized foams

Lanthanum zirconate (LZO) ceramic foams with hierarchical pore structure were fabricated by particle-stabilized foaming method for the first time, and the as-prepared ceramics have high porosity of 90.7%-94.9%, low thermal conductivity, and relatively high compressive strength. The LZO powder was synthesized by solid-state method. The porosity of the ceramic foams was tailored by suspensions with different solid loadings (20-40 wt%). The sample with porosity of 94.9% has thermal conductivity of 0.073 W/(m·K) and compressive strength of 1.19 MPa, which exhibits outstanding property of thermal insulation and mechanical performance, indicating that LZO ceramic foam is a promising thermal insulation material in high temperature applications.     

Fabrication of in-situ self-reinforced Si3N4 ceramic foams for high-temperature thermal insulation by protein foaming method

To meet demand for lightweight and high-strength ceramic foams, in-situ self-reinforced Si₃N₄ ceramic foams, with compressive strength of 13.2–45.9 MPa, were fabricated by protein foaming method combined with sintered reaction-bonded method. For comparison, ordinary protein foamed ceramics with irregular block microstructure were fabricated via reaction-bonded method, which had compressive strength of 3.6–20.5 MPa. Physical properties of these two types of samples were systematically compared. When open porosity was about 80%, both types of Si₃N₄ ceramic foams had excellent thermal insulation properties (<0.15 W m^?1 K^?1), while compressive strength of in-situ self-reinforced samples increased by more than 158% compared with ordinary samples. Under high-temperature oxidation conditions, microstructures of both types of samples were deformed with increase in oxidation temperature. Moreover, after oxidation temperature was increased to 1400 °C, oxidation weight gain decreased from 18.07% for ordinary samples to only 2.18% for self-reinforced samples. Thus, high-temperature oxidation resistance of Si₃N₄ ceramic foams was greatly improved.     

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.     

Tailoring the microstructure of high porosity Si3N4 foams by direct foaming with mixed surfactants

The mixed surfactants were successfully applied to fabricate the highly porous Si₃N₄ ceramic foams by the direct foaming method. The oppositely charged surfactants mixed in slurries could combined into catanionic surfactant by the electrostatic attraction and facilitate the formation of ultra-stable foams. The microstructure of the Si₃N₄ ceramic foams, including pore structure, mean pore size, pore size distribution and porosity were tailored by varying the mixing ratio of surfactant, mixed surfactants concentration and solid content of the initial slurries. Si₃N₄ ceramic foams with porosity of 92%-97%, mean pore size of 140-240 µm and compressive strength of 0.85-5.38 MPa were obtained by adjusting mixed surfactants between 0.1 and 0.4 wt% and solid content between 22 and 30 vol%. The compressive strength of Si₃N₄ ceramic foams in current work was much higher than most reported results.     

Near‐Net‐Shaped Porous Ceramics for Potential Sound Absorption Applications at High Temperatures

We present an interesting processing route for obtaining alumina/mullite‐based ceramics with controlled porosity and airflow resistance leading to promising microstructures for application as sound absorbers. The use of ceramic materials aims for potential applications where high temperatures or corrosive atmospheres are predominant, e.g., in combustion chambers of gas turbines. For the production of the porous ceramics we combined freeze gelation and sacrificial templating processes to produce near‐net‐shaped parts with low shrinkage (<3%) based on environmental‐friendly and low cost conditions. The obtained microstructure presents a bimodal pore size distribution, with small pores derived from the freeze gelation process (~30 μm) connecting large pores (2–5 mm diameter) originated from the expanded polystyrene template particles. These connections, called “windows” in this study, show a significant impact on the sound absorption properties, allowing the pressure diffusion effect to take place, resulting in a significant improvement of the sound absorption coefficient. By varying the template particle content and the slurry solid content, it is possible to control the sound absorption behavior at different frequencies of the open‐celled ceramics. These ceramics feature a high open porosity, from 77% to 82%, combined with sufficient compressive strength ranging from 0.27 to 0.68 MPa and sound absorption coefficients of 0.30–0.99, representing a highly promising combination of properties for noise control and reduction at corrosive environments and high temperatures.     

New consolidation process inspired from making steamed bread to prepare Si3N4 foams by protein foaming method

A new consolidation process had been developed for preparing Si₃N₄ ceramic foams by using protein foaming method, which was inspired from the preparation of steamed bread. The main advantage of this consolidation process was no crack development during foamed slurry consolidation process. By using this new consolidation, Si₃N₄ ceramic foams with open porosities of 79.6–87.3% and compressive strength of 2.5–22 MPa were prepared. Protein addition and solid content on mechanical properties and pore structures of the as-prepared ceramic foams were investigated. Results indicated that the open porosity decreases with protein addition and solid content while compressive strength increased with solid content. With the increase of solid content, pores of the ceramic foams became regular in shape and uniform in size while both size and number of windows on the walls decreased.     

Effects of heating rate on the structure and properties of SiOC ceramic foams derived from silicone resin

Silicon oxycarbide ceramic foams were fabricated in a single step manufacturing process using in situ foaming of SiOC powders loaded silicone resin. The effects of heating rate on the porosity, compressive strength and microstructure of the ceramic foams were investigated. The porosity (total and open) increased firstly and then decreased with increasing heating rate. It was possible to control the total and open porosity of ceramic foams within a range of 81.9–88.2% and 62.4–72.5% respectively, by adjusting the heating rate from 0.25 °C/min to 3 °C/min while keeping the silicone resin content at 90 vol%. However, the compressive strength decreased with increasing the heating rate progressively, and the average compressive strength of the foams was in the range of 1.0–2.3 MPa. Micrographs indicated that the ceramic foams which cross-linked at a heating rate less than 1 °C/min had a well-defined open-cell and regular pore structure.     

Highly Porous Zirconia Ceramic Foams with Low Thermal Conductivity from Particle‐Stabilized Foams

Zirconia ceramic foams with ultra‐high porosity of 96%–98% have been fabricated using sodium dodecyl sulfate (SDS) as the particle stabilizer of zirconia particles for the first time. The wet foams stabilized by zirconia particles are ultra‐stable due to partially hydrophobic zirconia particles modified by SDS. Zirconia foams exhibit close cells with thin cell wall and small grain size. Increasing SDS concentration favors the foamability of the suspension, and further increases the porosity of ceramic foams. Zirconia ceramic foams with porosity of 98.1% have compressive strength of 0.26 ± 0.05 MPa. Decreasing solid loading leads to the porosity of ceramic foams. The compressive strength could be improved significantly by increasing the sintering temperature. Zirconia ceramic foams with porosity of 97.9% has low thermal conductivity of 0.027 ± 0.004 W·(m·K)^?1, which could be used as thermal insulation and refractory material.     

Fibrous porous mullite ceramics modified by mullite whiskers for thermal insulation and sound absorption

In order to meet the demand for thermal insulation and sound absorption, fibrous porous mullite ceramics (FPMC) with high porosity and an interconnected pore structure were prepared, followed by a pore structure modification with in situ grown mullite whiskers on the three-dimensional framework of the FPMC. The resultant hierarchical material exhibited superior sound absorption performance in the low-to-medium frequency to most reported sound-absorbing materials, as well as a sufficient compressive strength of 1.26 MPa with low thermal conductivity of 0.117 W·m^?1·K^?1. Moreover, the effects of solid content and mullite whiskers on the microstructure and physical properties of the material were analyzed. The increase of solid content led to increased compressive strength and thermal conductivity and decreased frequency corresponding to the first sound absorption peak. The thermal conductivity and compressive strength of the material increased as the mullite whiskers grew, while the median pore size decreased.     

Ultrahigh-strength alumina ceramic foams via gelation of foamed boehmite sol

Ceramic foams with significant porosity and robust mechanical properties have received extensive attentions. However, it is still difficult to achieve excellent compressive strength at high porosity levels. In this work, a gelation of foamed boehmite sol method is proposed to settle this issue. The technological parameters during preparation process are systematically investigated. As-prepared alumina ceramic foams possess unprecedentedly high compressive strength of 34.1-89.1 MPa at high porosity levels of 66.0%-87.2%, which is attributed to the present of hierarchical pore structure, small grain size and pore size. This work demonstrates a facile and novel method for the fabrication of high-performance alumina ceramic foams toward practical applications.     

Three-dimensional (3D) flexible nanofibrous network knitting on hierarchical porous architecture

Hierarchical structured porous ceramics have attracted tremendous research interests because of their numerous excellent properties including robust mechanical strength and large surface area. In this work, silicon carbide (SiC)-based porous ceramics with three levels of pore hierarchy are fabricated from silicon particle-stabilized foams and a subsequent one-step calcination after they were embedded with coke. Three-dimensional (3D) flexible nanofibrous network is adhered and wrapped on cell walls of porous ceramics, which is readily fine-tuned and tailored by the temperature to provide optimized pore structure. The resultant SiC-based porous ceramics present a density of 1.03 g/cm³ at a porosity of 72% with a large quantity of hierarchical micro- and macropores. This hierarchical structure leads to robust compressive strength (23.52 MPa) and large surface area (64.32 m²/g). The fabrication method is straightforward and sought-after, providing a facile technical route for advanced hierarchical porous ceramics used in filtration and catalysis fields.     

Preparation and microstructure evolution of novel ultra-low thermal conductivity calcium silicate-based ceramic foams

In this study, municipal solid waste incineration fly ash (MSWI FA) was used as a new raw material for the ceramics industry and a novel ultra-low thermal conductivity calcium silicate-based foams (CSFs) was prepared by the direct foaming method. The effects of the addition of foam and borax on the sintering behavior and microstructural evolution of the CSFs were investigated. With the optimal amount of foam, the CSFs had an apparent porosity of 63.43%–67.49%, bulk density of 0.75–0.84 g/cm³, compressive strength of 1.83–3.21 MPa, and room-temperature thermal conductivity of 0.213–0.235 W/(m·K). Notably, the whisker morphology, pore structure, and sintering behavior of the samples can be controlled by changing the amount of borax. The prepared ceramic foams can be applied in the fields of thermal insulation, filtration, and catalyst carriers.     

Fabrication of novel BPO4 ceramic foams using the combination of the direct foaming method and freeze-drying techniques

In this study, ammonium phosphate monobasic and boric acid were used as the primary starting materials to produce BPO₄ powder by solid-state reaction. Using BPO₄ powders as the main raw material, BPO₄ ceramic foams were prepared for the first time using the direct foaming method and freeze-drying techniques. The effects of the additive content and solid loading on the slurry's rheological behavior were investigated, and the microstructures and properties of the as-prepared BPO₄ ceramic foams were examined. The results reveal that the porosity of the BPO₄ ceramic foams synthesized at 1223 K ranged from 84.2% to 90.4%, the compressive strength ranged from .12 MPa to .72 MPa, and the thermal conductivity ranged from .32 W/(m·K) to .74 W/(m·K) (298 K). The findings of this study have great significance for the development of new thermal insulation ceramic materials.     

Recycling of extracted titanium slag and gold tailings for preparation of self-glazed ceramic foams

Self-glazed ceramic foams were successfully synthesized via powder sintering method, using extracted titanium slag (ETS) and gold tailings (GT) as raw materials without adding any sintering aids and foaming agents. Influence of ETS addition and sintering temperature on crystal phase evolution, physical–mechanical properties, and micro-morphology of ceramic foams was systematically studied. Results indicated that products sintered at 1180 °C with 30 wt% ETS and 70 wt% GT showed the best performance, i.e., bulk density of 1.66 g cm^?3, flexural strength of 20.4 MPa, water absorption of 0.14%, open porosity of 0.23%, and glaze Vickers hardness of 6.5 GPa. Moreover, it was observed that there existed strong correlation between bulk density and bending strength. Self-glazed ceramic foams developed in this study are expected to be used as building envelope materials and provide new ideas for effective reuse of other similar solid wastes.     

Highly porous YSZ ceramic foams using hollow spheres with holes in their shell for high-performance thermal insulation

Yttria-stabilized zirconia (YSZ) porous ceramic foams were fabricated using YSZ microspheres with holes on the surface to determine their properties as insulation materials. Highly porous YSZ ceramics with bimodal pore structures, such as internal pores in single hollow spheres and external pores between the spheres, were successfully prepared using YSZ spheres as raw materials. Additionally, holes were added to the shells to reduce continuous thermal pathways and significantly enhance the insulation properties. Furthermore, by adding holes on the surface of the sphere, the porous foams using a hollow sphere exhibit a maximized porosity of 80.69%, remarkably enhanced their insulation properties with low thermal conductivity (0.10 W/m-K), and have sufficient compressive strength to protect the green body (5.7 MPa). The mechanical strength of the YSZ porous foam was maintained owing to the uniform arrangement of the supports.     

Main characteristics of foams and a study of the production process of zirconia foam ceramics

Conclusions Some combined studies of the syneresis, resistance, and the rheological properties of two-phase foams used to obtain foam ceramics have been carried out. A series of characteristics have been proposed for the evaluation of the properties of the three-phase mineralized foam ceramics. The connection between the technological and rheological properties of mineralized zircon foam and the connection between the properties of the foams and the foam ceramics obtained from them have been shown.A zircon foam ceramic with a total porosity of 78–90% and an ultimate compressive strength of 2–17 MPa has been prepared.It has been shown that it is possible to obtain a no-shrink (on molding) zircon foam ceramic.Translated from Ogneupory, No. 2, pp. 53–57, February, 1980.     

Recycling of silicon kerf waste for preparation of porous SiCw/SiC membrane supports by in situ synthesis

Recycling has enormous economic benefits and practical significance under the context of gradually increasing solid wastes. In order to recycle and reuse the silicon kerf waste, in this work, porous SiC_w/SiC ceramics were successfully prepared by in situ synthesis from silicon kerf waste and fired at 1400-1500°C for 4 hours. The results showed that these porous ceramics, reinforced by the interlocking whisker, revealed high apparent porosity (48.02%-51.76%) and cold compressive strength (5.68-9.54 MPa). Furthermore, the practicable pore size (2.09-2.53 μm) and decent durability showed the potential of these porous ceramics as membrane supports. This work verified the possibility of the SiC-based ceramics prepared from the silicon kerf waste.     

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.     

The sound absorption performance of the highly porous silica ceramics prepared using freeze casting method

Highly porous silica ceramics with unidirectional pores were prepared using the freeze casting method. By adjusting the solid content and freezing temperature, the porosity of the ceramics was tailored in the range of 78.20%-84.59% and pore size in the range of 8.4-71.4 μm, respectively. Sound absorption properties of porous silica ceramics was studied and the effect of structural factors was systematically investigated. The results showed that higher porosity and smaller pores of the porous ceramics favored the sound absorption in the entire sound wave frequency. By backing the sample with small pore size porous ceramics, the sound absorption property was enhanced, particularly in the low and medium frequency range, thus the sound absorption peak shifted towards lower frequency. The presence of air gap in the back would also favor sound absorption in low and medium frequency range. The as-fabricated porous silica ceramics owed excellent sound absorption properties due to their unidirectional pores and low flow resistances.