Ambient pressure drying (APD) of silica aerogels has emerged as an attractive method adapting to large-scale production. Spring-back is a unique phenomenon during APD of silica aerogels with volume expansion after its shrinkage under capillary force. We attribute the intense spring-back at elevated drying temperatures to a dense structure formed on the surface and the formation of positive internal pressure. Furthermore, an APD-assisted foaming method with an in situ introduction of NH4HCO3was proposed. NH4HCO3decomposing at drying temperatures hastened the emergence of positive pressure, thereby increasing the expansion volume. Compared to the previous method, the porosity of silica aerogel increased from 82.2% to 92.6%, and mesopore volume from 1.79 cm3g−1to 4.54 cm3g−1. By adjusting the amount of the silicon source, silica aerogels prepared by the APD-assisted foaming method generated higher volume expansion and lower thermal conductivity. After calcination to remove undecomposed ammonium salts, the hydrophobic silica aerogel with a density of 0.112 g cm−3reached a mesopore volume of 5.07 cm3g−1and a thermal conductivity of 18.9 mW m−1·K−1. This strategy not only improves the thermal insulation properties, but also offers a significant advancement in tailoring silica aerogels with specific porosity and mesopore volume for various applications.
Bin Liu,Jie Fan,Hongqiang Nian,Yiran Li,Huimin Xiang,Yanchun Zhou
Sintering and phase transitions of materials in current large-area thermal insulation systems are critical problems for hypersonic vehicles. Searching for thermal insulation materials with good structure stability, thermal stability, and superior insulation performance is urgent for the development of these vehicles. Herein, we reported a novel porous BaZrO 3 ceramic fabricated by a particle-stabilized foam method. By controlling the sintering temperature, the porosity (92.96%–94.62%), thermal conductivity (0.088–0.193 W/(m·K)), and strength (0.30–1.31 MPa) of the porous ceramics are tunable. Most importantly, these porous BaZrO 3 ceramics exhibit excellent sintering resistance. After being heat treated at 1300°C for 6 h, the shrinkage of porous BaZrO 3 ceramics is nearly 0. The shrinkage of the samples heat-treated even at 1600°C is still lower than 3%, demonstrating outstanding high-temperature structure stability. Our results demonstrate that porous BaZrO 3 ceramics are promising candidates for high-temperature thermal insulation applications.
Sheet-like lithium-rich cathode materials Li 1.2 Mn 0.54 Ni 0.16 Co 0.10 O 2 (LMNCO) assembled by primary grains of 100–500 nm in sizes were facilely prepared by a glucose-urea bubbling and post-annealing process. The post-annealing temperature was found to play an important role on the grain size , crystallinity , cation ordering and electrochemical performance of the cathode materials. The sample post-annealed at 900 °C showed larger grains, higher crystallinity, better layered structure, higher lithium storage capacity and better rate performance than the one annealed at 800 °C. An initial discharge capacity of above 300 mA h g −1 was obtained at 0.1 C (20 mA g −1 ) for the sample annealed at 900 °C, and at 1, 2, 5 and 10 C, discharge papacies of 223, 192, 157 and 128 mA h g −1 were achieved respectively. The material synthesizing process proposed in this study may be extended to prepare a wide range of sheet-like functional oxides that may be potentially applied in energy storage or other fields.
Waste foundry sand (WFS) is a solid waste by-product generated during the sand casting process in foundries. Promoting resourceful recycling of WFS is of significance for environmental and efficient resource utilization. In this study, WFS was used as the raw material to fabricate ceramic foams using the particle-stabilized foam method. The results showed that the ball-milled fine WFS particles in the slurry were able to adhere to air bubbles, leading to the formation of ceramic foams with closed pores after drying and sintering. The liquid phase generated during the sintering process from melted WFS contributed to the development of dense pore walls. The porosity of the ceramic foams first decreased from 30.93% to 22.83%, and then increased to 40.22% with the rise in slurry pH from 2 to 5. Moreover, the porosity gradually decreased from 49% to 3% as the sintering temperature increased from 1000°C to 1300°C. The variations in the pore structure significantly influenced their properties. The WFS-based ceramic foams were produced with 0.68‒1.32 g/cm 3 volume density, 32.4‒210.3 MPa compressive strength, and 0.18‒0.87 W/(m K) thermal conductivity. This study facilitated the reuse of WFS, enhancing cost-effective, efficient production of high-performance ceramic materials for building insulation.
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