Ceramic aerogels are attractive materials due to their low density, large specific surface area, excellent fire resistance and low thermal conductivity, and are used in fields such as thermal/acoustic/electrical insulation, catalyst carriers, filters, and energy storage materials. Get applied. However, due to the inherent pearl necklace-like microstructure, it cannot form an effective continuous structure. The practical application of conventional ceramic aerogel is limited by its inherent brittleness. Although there have been studies using flexible one-dimensional fibers and ceramic sponges to solve the mechanical properties of ceramic aerogels. However, these aerogels will experience severe strength degradation and structural collapse under the action of external force or long-term exposure to high temperatures, which may cause catastrophic events. Therefore, ceramic fiber aerogels with sufficient mechanical strength (high strength and superelasticity) and excellent high temperature resistance are the key to reliable application of ceramic aerogels under extreme conditions.
To solve this problem, researchers Ding Bin and Researcher Si Yang of the Textile Technology Innovation Center of Donghua University in China have studied many materials, inspired by the extraordinary mechanical properties of wooden sponges with a sponge-like spring-like layered porous structure. Combining flexible ZrO2 -Al2O3 nanofibers with Al(H2PO4)3 (AHP) matrix, developed an anisotropic, layered structure and constant mechanical properties of ZrO2 -Al2O3 nanofiber aerogels (ZrAlNFAs). Due to the layered structure composed of many stacked multi-arch micro-scale structures arranged in parallel, ZrAlNFAs exhibits high compressive strength of up to 1100 kPa at 90% strain, and fatigue resistance of 1000 compressions at 60% strain Sex. In addition, the layered structure, high porosity (>98%) and full ceramic composition make ZrAlNFAs have temperature-constant compressibility, high fire resistance up to 1300°C, and thermal conductivity as low as 0.0322 W m–1 K–1 With high temperature insulation properties, they can be considered as promising alternatives to existing insulation materials in extreme environments. The result was published on "ACS NANO" under the title "Ultrastrong, Superelastic, and Lamellar Multiarch Structured ZrO2 –Al2O3 Nanofibrous Aerogels with High-Temperature Resistance over 1300 °C".
First, the flexible ZrO2 -Al2O3 nanofibers (ZrO2 -Al2O3 NFM) are prepared by electrospinning a mixture of precursor ZrO2 -Al2O3 sol/polyethylene oxide (PEO) solution. The nanofiber membrane was calcined in the air at 800°C for 1 hour to completely decompose the organic components and form a crystalline phase. After immersing the AHP in water, immerse ZrO2 -Al2O3 NFM for 30 minutes and stack them layer by layer. Subsequently, the layered assembly was freeze-dried into unbonded ZrAlNFAs. During the freezing process, the water in the AHP solution gradually solidifies into an ice crystal template, thereby forming a material with a nanofiber porous structure. Finally, the cross-linked ZrO2-Al2O3 NFM network was formed by calcining the unbound ZrAlNFAs in a muffle furnace at 800 °C for 1 h.
The obtained ZrAlNFAs has a porous layered multilayer structure, which is composed of a fluffy nanofiber layer, is connected to the nanofibers, and has a bonding structure of the nanofibers. It also has excellent heat insulation properties, which can prevent flowers from dehydration, fading or degradation.
ZrAlNFAs high strength, super elasticity and fatigue resistance
The 3D layered multilayer arch ZrAlNFAs exhibits excellent resilience when subjected to large compressive strains. The recoverable strain can even reach 90%, and after the pressure is released to the maximum stress of 1100 kPa, the sample recovers its original shape, which is much higher than the previously reported value of ceramic aerogel. In addition, after 1000 cycles of compression, the strength of ZrAlNFAs did not decrease significantly, and 70% of the original maximum stress was retained. The electron microscope results show that the shrinkage of the arched holes and the buckling of the nanofibers share the compressive strain on the ZrAlNFAs at a high level, so that it has strength bearing capacity, elastic recovery and good fatigue resistance, which also shows that the highly porous 3D Layered multilayer structures, flexible building blocks, and stable bonding structures are particularly important for this structure.
Temperature-constant superelasticity
The plastic deformation of ZrAlNFAs under 1000 times of compression at -120 and 500 °C is 12.5% and 22.1%, respectively, and after 1000 times of compression, it still maintains more than 70% of the initial maximum stress, similar to superelasticity at ambient temperature . The viscoelastic properties of ZrAlNFAs are almost constant over a wide temperature range from -120°C to 500°C. In addition, the damping ratio is only ~0.1, indicating the temperature-invariant stability of ZrAlNFAs.
ZrAlNFAs also exhibits long-term low/high temperature resistance and high compression fatigue resistance in liquid N2 (-196 °C) and 1100 °C. In addition, ZrAlNFAs still retain their elastic resilience properties when processed at 1300 °C and compressed simultaneously, which proves their high temperature resistance in a wide temperature range and their compressibility with temperature changes.
Fire resistance and heat insulation properties of ZrAlNFAs
The prepared ZrAlNFAs has high porosity and layered multi-arch pores, which may lead to low gas and solid heat transfer. In addition, the prepared ZrAlNFAs has resistance to high temperatures up to 1300°C. Burning ZrAlNFAs under the flame of a butane blowtorch, it was found that ZrAlNFAs remained non-combustible during the heating process, and maintained its original overall structure on both the macro and micro scales of the front and cross section. Traces of light ablation were also observed in the center of the front. Therefore, in addition to strong mechanical properties, the prepared ZrAlNFAs also have a combination of low thermal conductivity and high working temperature, indicating that they have broad application prospects in high temperature insulation.
Through the combination of electrospinning and AHP-bonded ZrO2-Al2O3 nanofiber membrane, the author developed a layered multilayer structure ZrAlNFAs with super mechanical strength and excellent elasticity as well as high temperature resistance. ZrAlNFAs has excellent mechanical compressibility at 90% strain due to its numerous multi-arched layered honeycomb structure and strong bonding structure, the ultimate stress is as high as 1100 kPa, and it has 1000 cycles at 60% strain. High compression fatigue resistance. In addition, the all-ceramic composition provides aerogels with superelasticity at constant temperature, high fire resistance up to 1300 °C, low thermal conductivity and high-temperature insulation performance. In short, their mechanical strength combined with their high strength, superelasticity and fatigue resistance as well as excellent high temperature resistance and thermal insulation properties make ZrAlNFAs a very promising thermal insulation material under extreme conditions.
Full text link: https://pubs.acs.org/doi/10.1021/acsnano.0c06423