Flyweight, Superelastic, Electrically Conductive, and Flame-Retardant 3D Multi-Nanolayer Graphene/Ceramic Metamaterial

Highly Flexible and Superelastic Graphene Nanofibrous Aerogels for Intelligent Sign Language

Highly flexible and superelastic aerogels at large deformation have become urgent mechanical demands in practical uses, but both properties are usually exclusive. Here a trans-scale porosity design is proposed in graphene nanofibrous aerogels (GNFAs) to break the trade-off between high flexibility and superelasticity. The resulting GNFAs can completely recover after 1000 fatigue cycles at 60% folding strain, and notably maintain excellent structural integrity after 10000 cycles at 90% compressive strain, outperforming most of the reported aerogels. The mechanical robustness is demonstrated to be derived from the trans-scale porous structure, which is composed of hyperbolic micropores and porous nanofibers to enable the large elastic deformation capability. It is further revealed that flexible and superelastic GNFAs exhibit high sensitivity and ultrastability as an electrical sensors to detect tension and flexion deformation. As proof, The GNFA sensor is implemented onto a human finger and achieves the intelligent recognition of sign language with high accuracy by multi-layer artificial neural network. This study proposes a highly flexible and elastic graphene aerogel for wearable human-machine interfaces in sensor technology.

Lightweight, flame retardant Janus carboxymethyl cellulose aerogel with fire-warning properties for smart sensor

Lightweight, flame retardant biomass aerogels combining with multi-functionalities are promising for thermal insulation, noise absorption and smart sensors. However, high flammability hinders the application of these aerogels in extreme condition. Herein, lightweight, flame retardant aerogel with fire-warning properties fabricated from resource-abundant graphite and green carboxymethyl cellulose (CMC) is reported. During sonicating expandable graphite (EG) in CMC solution, CMC not only fabricates the downsizing process via hydrogen bonding effect but also forms stable dispersions. Then biomass aerogel is fabricated by freeze-drying strategy and enhanced by metal ionic cross-linking method. This aerogel demonstrates Janus properties for electrical conductivity and thermal conductivity. Due to the synergistic flame retardant effect of graphite nanocomposite and metal ions with a barrier effect and catalytic carbonization capacity, the flame retardancy of these aerogels are enhanced with fire-warning properties. Furthermore, these aerogels are used for monitoring physical deformations as smart sensors, which provides inspiration and a sustainable solution for developing low-cost biomass aerogel with multifunction.

Multiscale Interpenetrated/Interconnected Network Design Confers All-Carbon Aerogels with Unprecedented Thermomechanical Properties for Thermal Insulation under Extreme Environments

With ultralight weight, low thermal conductivity, and extraordinary high-temperature resistance, carbon aerogels hold tremendous potential against severe thermal threats encountered by hypersonic vehicles during the in-orbit operation and re-entry process. However, current 3D aerogels are plagued by irreconcilable contradictions between adiabatic and mechanical performance due to monotonicity of the building blocks or uncontrollable assembly behavior. Herein, a spatially confined assembly strategy of multiscale low-dimensional nanocarbons is reported to decouple the stress and heat transfer. The nanofiber framework, a basis for transferring the loading strain, is covered by a continuous thin-film-like layer formed by the aggregation of nanoparticles, which in combination serve as the fundamental structural units for generating an elastic behavior while yielding compartments in aerogels to suppress the gaseous fluid thermal diffusion within distinct partitions. The resulting all-carbon aerogels with a hierarchical cellular structure and quasi-closed cell walls achieve the best thermomechanical and insulation trade-off, exhibiting flyweight density (24 mg cm-3 ), temperature-constant compressibility (-196-1600 °C), and a low thermal conductivity of 0.04 829 W m-1 K-1 at 300 °C. This strategy provides a remarkable thermal protection material in hostile environments for future aerospace exploration.

Large-scale assembly of isotropic nanofiber aerogels based on columnar-equiaxed crystal transition

Ice-templating technology holds great potential to construct industrial porous materials from nanometers to the macroscopic scale for tailoring thermal, electronic, or acoustic transport. Herein, we describe a general ice-templating technology through freezing the material on a rotating cryogenic drum surface, crushing it, and then re-casting the nanofiber slurry. Through decoupling the ice nucleation and growth processes, we achieved the columnar-equiaxed crystal transition in the freezing procedure. The highly random stacking and integrating of equiaxed ice crystals can organize nanofibers into thousands of repeating microscale units with a tortuous channel topology. Owing to the spatially well-defined isotropic structure, the obtained Al2O3·SiO2 nanofiber aerogels exhibit ultralow thermal conductivity, superelasticity, good damage tolerance, and fatigue resistance. These features, together with their natural stability up to 1200 °C, make them highly robust for thermal insulation under extreme thermomechanical environments. Cascading thermal runaway propagation in a high-capacity lithium-ion battery module consisting of LiNi0.8Co0.1Mn0.1O2 cathode, with ultrahigh thermal shock power of 215 kW, can be completely prevented by a thin nanofiber aerogel layer. These findings not only establish a general production route for nanomaterial assemblies that is conventionally challenging, but also demonstrate a high-energy-density battery module configuration with a high safety standard that is critical for practical applications.

Multifunctional Hierarchical Metamaterial for Thermal Insulation and Electromagnetic Interference Shielding at Elevated Temperatures

The custom design of lightweight cellular materials is widely concerned due to effectively improved mechanical properties and functional applications. However, the strength attenuation and brittleness behavior hinder honeycomb structure design for the ceramic monolith. Herein, the ceramic matrix composite metamaterial (CCM) with a negative Poisson's ratio and high specific strength, exhibiting superelasticity, stability, and high compressive strength, is customized by combining centripetal freeze-casting and hierarchical structures. CCM maintains a negative Poisson's ratio response under compression with the lowest value reaching -0.16, and the relationship between CCM's specific modulus and density is E ∼ ρ^1.3, which indicates the mechanical metamaterial characteristic of high specific strength. In addition to the extraordinary mechanical performance endowed by hierarchical structures, the CCM exhibits excellent thermal insulation and electromagnetic interference shielding properties, in which the thermal conductivity is 30.62 mW·m^-1·K^-1 and the electromagnetic interference (EMI) shielding efficiency (SE) reaches 40 dB at room temperature. The specific EMI shielding efficiency divided by thickness (SSE/t) of CCM can reach 9416 dB·cm²·g^-1 at 700 °C due to its stability at elevated temperatures, which is 100 times higher than that of traditional ceramic matrix composites. Moreover, the designed hierarchical structure and metamaterial properties provide a potential scheme to implement cellular materials with collaborative optimization in structure and function.

Ultralight elastic Al2O3 nanorod-graphene aerogel for pressure sensing and thermal superinsulation

Novel nanorod aerogels have gained tremendous attention owing to their unique structure. However, the intrinsic brittleness of ceramics still severely limits their further functionalization and application. Here, based on the self-assembly between one-dimensional (1D) Al₂O₃ nanorods and two-dimensional (2D) graphene sheets, lamellar binary Al₂O₃ nanorod-graphene aerogels (ANGAs) were prepared by the bidirectional freeze-drying technique. Thanks to the synergistic effect of rigid Al₂O₃ nanorods and high specific extinction coefficient elastic graphene, the ANGAs not only exhibit robust structure and variable resistance under pressure, but also possess superior thermal insulation properties compared to pure Al₂O₃ nanorod aerogels. Therefore, a series of fascinating features such as ultra-low density (3.13-8.26 mg cm^-3), enhanced compressive strength (6 times higher than graphene aerogel), excellent pressure sensing durability (500 cycles at 40% strain) and ultra-low thermal conductivity (0.0196 W m^-1 K^-1 at 25 °C and 0.0702 W m^-1 K^-1 at 1000 °C) are integrated in ANGAs. The present work provides fresh insight into the fabrication of ultralight thermal superinsulating aerogels and the functionalization of ceramic aerogels.

Highly Deformable High-Strength SiO2 Aerogel Designed with an Alternating Structure of Hard Cores and Flexible Chains for Thermal Insulation

Thermally insulating aerogels can now be prepared from ceramics, polymers, carbon, and metals and composites between them. However, it is still a great challenge to make aerogels with high strength and excellent deformability. We propose a design concept of hard cores and flexible chains that alternately construct the aerogel skeleton structure. The approach gives the designed SiO₂ aerogel excellent compressive (fracture strain 83.32%), tensile. and shear deformabilities, corresponding to maximum strengths of 22.15, 1.18, and 1.45 MPa, respectively. Also, the SiO₂ aerogel can stably perform 100 load-unload cycles at a 70% large compression strain, demonstrating an excellent resilient compressibility. In addition, the low density of 0.226 g/cm³, the high porosity of 88.7%, and the average pore size of 45.36 nm effectively inhibit heat conduction and heat convection, giving the SiO₂ aerogel outstanding thermal insulation properties [0.02845 W/(m·K) at 25 °C and 0.04895 W/(m·K) at 300 °C], and the large number of hydrophobic groups itself also gives it excellent hydrophobicity and hydrophobic stability (hydrophobic angle of 158.4° and saturated mass moisture absorption rate of about 0.327%). The successful practice of this concept has provided different insights into the preparation of high-strength aerogels with high deformability.

Direct synthesis of highly stretchable ceramic nanofibrous aerogels via 3D reaction electrospinning

Ceramic aerogels are attractive for many applications due to their ultralow density, high porosity, and multifunctionality but are limited by the typical trade-off relationship between mechanical properties and thermal stability when used in extreme environments. In this work, we design and synthesize ceramic nanofibrous aerogels with three-dimensional (3D) interwoven crimped-nanofibre structures that endow the aerogels with superior mechanical performances and high thermal stability. These ceramic aerogels are synthesized by a direct and facile route, 3D reaction electrospinning. They display robust structural stability with structure-derived mechanical ultra-stretchability up to 100% tensile strain and superior restoring capacity up to 40% tensile strain, 95% bending strain and 60% compressive strain, high thermal stability from −196 to 1400 °C, repeatable stretchability at working temperatures up to 1300 °C, and a low thermal conductivity of 0.0228 W m⁻¹ K⁻¹ in air. This work would enable the innovative design of high-performance ceramic aerogels for various applications.

Ceramic aerogels are generally brittle and often tend to structurally collapse under large external tensile strain. Here the authors synthesize large-scale stretchable ceramic aerogels with interwoven crimped nanofibers by combining electrohydrodynamic method and 3D reaction electrospinning.

Digital Light Processing 3D-Printed Ceramic Metamaterials for Electromagnetic Wave Absorption

Combining 3D printing with precursor-derived ceramic for fabricating electromagnetic (EM) wave-absorbing metamaterials has attracted great attention. This study presents a novel ultraviolet-curable polysiloxane precursor for digital light processing (DLP) 3D printing to fabricate ceramic parts with complex geometry, no cracks and linear shrinkage. Guiding with the principles of impedance matching, attenuation, and effective-medium theory, we design a cross-helix-array metamaterial model based on the complex permittivity constant of precursor-derived ceramics. The corresponding ceramic metamaterials can be successfully prepared by DLP printing and subsequent pyrolysis process, achieving a low reflection coefficient and a wide effective absorption bandwidth in the X-band even under high temperature. This is a general method that can be extended to other bands, which can be realized by merely adjusting the unit structure of metamaterials. This strategy provides a novel and effective avenue to achieve "target-design-fabricating" ceramic metamaterials, and it exposes the downstream applications of highly efficient and broad EM wave-absorbing materials and structures with great potential applications.

Structured Ultra-Flyweight Aerogels by Interfacial Complexation: Self-Assembly Enabling Multiscale Designs

The rapid co-assembly of graphene oxide (GO) nanosheets and a surfactant at the oil/water (O/W) interface is harnessed to develop a new class of soft materials comprising continuous, multilayer, interpenetrated, and tubular structures. The process uses a microfluidic approach that enables interfacial complexation of two-phase systems, herein, termed as "liquid streaming" (LS). LS is demonstrated as a general method to design multifunctional soft materials of specific hierarchical order and morphology, conveniently controlled by the nature of the oil phase and extrusion's injection pressure, print-head speed, and nozzle diameter. The as-obtained LS systems can be readily converted into ultra-flyweight aerogels displaying worm-like morphologies with multiscale porosities (micro- and macro-scaled). The presence of reduced GO nanosheets in such large surface area systems renders materials with outstanding mechanical compressibility and tailorable electrical activity. This platform for engineering soft materials and solid constructs opens up new horizons toward advanced functionality and tunability, as demonstrated here for ultralight printed conductive circuits and electromagnetic interference shields.

Fabrication, Structure, Performance, and Application of Graphene-Based Composite Aerogel

Graphene-based composite aerogel (GCA) refers to a solid porous substance formed by graphene or its derivatives, graphene oxide (GO) and reduced graphene oxide (rGO), with inorganic materials and polymers. Because GCA has super-high adsorption, separation, electrical properties, and sensitivity, it has great potential for application in super-strong adsorption and separation materials, long-life fast-charging batteries, and flexible sensing materials. GCA has become a research hotspot, and many research papers and achievements have emerged in recent years. Therefore, the fabrication, structure, performance, and application prospects of GCA are summarized and discussed in this review. Meanwhile, the existing problems and development trends of GCA are also introduced so that more will know about it and be interested in researching it.

Magnetically Aligned Ultrafine Cobalt Embedded 3D Porous Carbon Metamaterial by One-Step Ultrafast Laser Direct Writing

Spatial manipulation of nanoparticles (NPs) in a controlled manner is critical for the fabrication of 3D hybrid materials with unique functions. However, traditional fabrication methods such as electron-beam lithography and stereolithography are usually costly and time-consuming, precluding their production on a large scale. Herein, for the first time the ultrafast laser direct writing is combined with external magnetic field (MF) to massively produce graphene-coated ultrafine cobalt nanoparticles supported on 3D porous carbon using metal-organic framework crystals as precursors (5 × 5 cm2 with 10 s). The MF-confined picosecond laser scribing not only reduces the metal ions rapidly but also aligns the NPs in ultrafine and evenly distributed order (from 7.82 ± 2.37 to 3.80 ± 0.84 nm). ≈400% increment of N-Q species within N compositionis also found as the result of the special MF-induced laser plasma plume. (). The importance of MF is further exmined by electrochemical water-splitting tests. Significant overpotential improvements of 90 and 150 mV for oxygen evolution reaction and hydrogen evolution reaction are observed, respectively, owing to the MF-induced alignment of the NPs and controlled elemental compositions. This work provides a general bottom-up approach for the synthesis of metamaterials with high outputs yet a simple setup.

Elastic Recovery Properties of Ultralight Carbon Nanotube/Carboxymethyl Cellulose Composites

Ultralight materials exhibit superelastic behavior depending on the selection, blending, and carbonization of the materials. Recently, ultimate low-density materials of 5 mg/cm³ or less have attracted attention for applications such as sensors, electrodes, and absorbing materials. In this study, we fabricated an ultralight material composed of single-walled carbon nanotubes (CNT) and sodium carboxymethyl cellulose (CMC), and we investigated the effect of density, composition, and weight average molecular weight of CMC on elastic recovery properties of ultralight CNT/CMC composites. Our results showed that the elastic recovery properties can be improved by reducing the density of the composite, lowering the mass ratio of CNTs, and using CMC with small molecular weight.

Light-induced levitation of ultralight carbon aerogels via temperature control

We demonstrate that ultralight carbon aerogels with skeletal densities lesser than the air density can levitate in air, based on Archimedes' principle, when heated with light. Porous materials, such as aerogels, facilitate the fabrication of materials with density less than that of air. However, their apparent density increases because of the air inside the materials, and therefore, they cannot levitate in air under normal conditions. Ultralight carbon aerogels, fabricated using carbon nanotubes, have excellent light absorption properties and can be quickly heated by a lamp owing to their small heat capacity. In this study, an ultralight carbon aerogel was heated with a halogen lamp and levitated in air by expanding the air inside as well as selectively reducing its density. We also show that the levitation of the ultralight carbon aerogel can be easily controlled by turning the lamp on and off. These findings are expected to be useful for various applications of aerogels, such as in communication and transportation through the sky.

Applications of Ceramic/Graphene Composites and Hybrids

Research activity on ceramic/graphene composites and hybrids has increased dramatically in the last decade. In this review, we provide an overview of recent contributions involving ceramics, graphene, and graphene-related materials (GRM, i.e., graphene oxide, reduced graphene oxide, and graphene nanoplatelets) with a primary focus on applications. We have adopted a broad scope of the term ceramics, therefore including some applications of GRM with certain metal oxides and cement-based matrices in the review. Applications of ceramic/graphene hybrids and composites cover many different areas, in particular, energy production and storage (batteries, supercapacitors, solar and fuel cells), energy harvesting, sensors and biosensors, electromagnetic interference shielding, biomaterials, thermal management (heat dissipation and heat conduction functions), engineering components, catalysts, etc. A section on ceramic/GRM composites processed by additive manufacturing methods is included due to their industrial potential and waste reduction capability. All these applications of ceramic/graphene composites and hybrids are listed and mentioned in the present review, ending with the authors' outlook of those that seem most promising, based on the research efforts carried out in this field.

Recent Progress in Graphene/Polymer Nanocomposites

Nanocomposites, multiphase solid materials with at least one nanoscaled component, have been attracting ever-increasing attention because of their unique properties. Graphene is an ideal filler for high-performance multifunctional nanocomposites in light of its superior mechanical, electrical, thermal, and optical properties. However, the 2D nature of graphene usually gives rise to highly anisotropic features, which brings new opportunities to tailor nanocomposites by making full use of its excellent in-plane properties. Here, recent progress on graphene/polymer nanocomposites is summarized with emphasis on strengthening/toughening, electrical conduction, thermal transportation, and photothermal energy conversion. The influence of the graphene configuration, including layer number, defects, and lateral size, on its intrinsic properties and the properties of graphene/polymer nanocomposites is systematically analyzed. Meanwhile, the role of the interfacial interaction between graphene and polymer in affecting the properties of nanocomposites is also explored. The correlation between the graphene distribution in the matrix and the properties of the nanocomposite is discussed in detail. The key challenges and possible solutions are also addressed. This review may provide a constructive guidance for preparing high-performance graphene/polymer nanocomposite in the future.

Polymer-Derived SiOC Integrated with a Graphene Aerogel As a Highly Stable Li-Ion Battery Anode

Amorphous polymer-derived silicon oxycarbide (SiOC) is an attractive candidate for Li-ion battery anodes, as an alternative to graphite, which is limited to a theoretical capacity of 372 mAh/g. However, SiOC tends to exhibit poor transport properties and cycling performance as a result of sparsely distributed carbon clusters and inefficient active sites. To overcome these limitations, we designed and fabricated a layered graphene/SiOC heterostructure by solvent-assisted infiltration of a polymeric precursor into a modified three-dimensional (3D) graphene aerogel skeleton. The use of a high-melting-point solvent facilitated the precursor's freeze drying, which following pyrolysis yielded SiOC as a layer supported on the surface of nitrogen-doped reduced graphene oxide aerogels. The fabrication method employed here modifies the composition and microstructure of the SiOC phase. Among the studied materials, the highest levels of performance were obtained for a sample of moderate SiOC content, in which the graphene network constituted 19.8 wt % of the system. In these materials, a stable reversible charge capacity of 751 mAh/g was achieved at low charge rates. At high charge rates of 1480 mA/g, the capacity retention was ∼95% (352 mAh/g) after 1000 consecutive cycles. At all rates, Coulombic efficiencies >99% were maintained following the first cycle. Performance across all indicators was majorly improved in the graphene aerogel/SiOC nanocomposites, compared with unsupported SiOC. The performance was attributed to mechanisms across multiple length scales. The presence of oxygen-rich SiO_4-xC_x tetrahedral units and a continuous free-carbon network within the SiOC provides sites for reversible lithiation, while high ionic and electronic transport is provided by the layered graphene/SiOC heterostructure.

Effect of nonlocality in spatially uniform anisotropic metamaterials

In this study, we investigate an effect of spatial dispersion in anisotropic metamaterials of regular periodic geometry. We indicate conditions under which a local and nonlocal approach are convergent, as well as the areas of particularly strong nonlocality. Our analysis also reveals that new resonance transitions altering the topology of an iso-frequency surface arise in the presence of spatial dispersion. For the first time, we demonstrate that nonlocality can serve as a new mechanism for tailoring effective dispersion of an anisotropic metamaterial, which opens new venues for novel applications requiring strong direction discrimination of the incident radiation.

Three-Dimensional Graphene Hybrid SiO2 Hierarchical Dual-Network Aerogel with Low Thermal Conductivity and High Elasticity

We describe lightweight three-dimensional (3D) graphene hybrid SiO2 aerogels (GSAs) with hierarchically robust interconnected networks fabricated via an in situ deposition procedure after a hydrothermal assembling strategy with graphene oxide sheets. The nano-/micron-thick SiO2 coating conformably grew over porous graphene templates with two constituents (e.g., graphene and SiO2) and formed chemically bonded interfaces. In addition, it significantly refined the primary graphene pores by hundreds of microns into smaller porous patterns. Studies of its mechanical properties verified that the graphene interframework made the ceramic composites elastic, while SiO2 deposition enhanced the strength required it to resist deformation. The higher SiO2 contents resulted in lower elasticity but larger strength because of the apparent nanosize effect of SiO2 ceramic thickness; GSAs with a density of 82.3–250.3 mg/cm3 (corresponding to SiO2 sol with concentration ranging from 5 to 20 wt %) could reach a good balance of strength and elasticity. Benefiting from hierarchical micronetworks consisting of semiclosed or closed pores, GSAs offer excellent thermal-insulation performance, with thermal conductivity as low as 0.026 W/(m·K). GSAs offer improved fire-resistant capacity rather than that of pure carbon-based aerogels via the synergic protection of SiO2 ceramic accretion. This highlights the promising applications of GSAs as lightweight thermal-shielding candidates for industrial equipment, civil architectures, and defense transportation vehicles.

Precise control of versatile microstructure and properties of graphene aerogel via freezing manipulation

A deep understanding of the shaping technique is urgently required to precisely tailor the pore structure of a graphene aerogel (GA) in order to fit versatile application backgrounds. In the present study, the microstructure and properties of GA were regulated by freeze-casting using an ice crystal template frozen from -10 °C to -196 °C. The phase field simulation method was applied to probe the microstructural evolution of the graphene-H2O system during freezing. Both the experimental and simulation results suggested that the undercooling degree was fundamental to the nucleation and growth of ice crystals and dominated the derived morphology of GA. The pore size of GA was largely regulated from 240 to 6 μm via decreasing the freezing temperature from -10 °C to -196 °C but with a constant density of 8.3 mg cm-3. Rapid freeze casting endowed GA with a refined pore structure and therefore better thermal, electrical, and compressive properties, whereas the GA frozen slowly had superior absorption properties owing to the continuous and tube-like graphene lamellae. The GA frozen at -196 °C exhibited the highest Young's modulus of 327 kPa with similar densities to those reported in the literature. These findings demonstrate the diverse potential applications of GA with regulated pore morphologies and also contribute to cryogenic-induced phase separation methods.

3D Printing of Ultralight Biomimetic Hierarchical Graphene Materials with Exceptional Stiffness and Resilience

Biological materials with hierarchical architectures (e.g., a macroscopic hollow structure and a microscopic cellular structure) offer unique inspiration for designing and manufacturing advanced biomimetic materials with outstanding mechanical performance and low density. Most conventional biomimetic materials only benefit from bioinspired architecture at a single length scale (e.g., microscopic material structure), which largely limits the mechanical performance of the resulting materials. There exists great potential to maxime the mechanical performance of biomimetic materials by leveraging a bioinspired hierarchical structure. An ink-based three-dimensional (3D) printing strategy to manufacture an ultralight biomimetic hierarchical graphene material (BHGMs) with exceptionally high stiffness and resilience is demonstrated. By simultaneously engineering 3D-printed macroscopic hollow structures and constructing an ice-crystal-induced cellular microstructure, BHGMs can achieve ultrahigh elasticity and stability at compressive strains up to 95%. Multiscale finite element analyses indicate that the hierarchical structures of BHGMs effectively reduce the macroscopic strain and transform the microscopic compressive deformation into the rotation and bending of the interconnected graphene flakes. This 3D printing strategy demonstrates the great potential that exists for the assembly of other functional materials into hierarchical cellular structures for various applications where high stiffness and resilience at low density are simultaneously required.

Structurally Controlled Cellular Architectures for High-Performance Ultra-Lightweight Materials

The design and synthesis of cellular structured materials are of both scientific and technological importance since they can impart remarkably improved material properties such as low density, high mechanical strength, and adjustable surface functionality compared to their bulk counterparts. Although reducing the density of porous structures would generally result in reductions in mechanical properties, this challenge can be addressed by introducing a structural hierarchy and using mechanically reinforced constituent materials. Thus, precise control over several design factors in structuring, including the type of constituent, symmetry of architectures, and dimension of the unit cells, is extremely important for maximizing the targeted performance. The feasibility of lightweight materials for advanced applications is broadly explored due to recent advances in synthetic approaches for different types of cellular architectures. Here, an overview of the development of lightweight cellular materials according to the structural interconnectivity and randomness of the internal pores is provided. Starting from a fundamental study on how material density is associated with mechanical performance, the resulting structural and mechanical properties of cellular materials are investigated for potential applications such as energy/mass absorption and electrical and thermal management. Finally, current challenges and perspectives on high-performance ultra-lightweight materials potentially implementable by well-controlled cellular architectures are discussed.

Electrochemomechanical Behavior of Polypyrrole-Coated Nanofiber Scaffolds in Cell Culture Medium

Glucose-gelatin nanofiber scaffolds were made conductive and electroactive by chemical (conductive fiber scaffolds, CFS) and additionally electrochemical polypyrrole deposition (doped with triflouromethanesulfonate CF3SO3-, CFS-PPyTF). Both materials were investigated in their linear actuation properties in cell culture medium (CCM), as they could be potential electro-mechanically activated cell growth substrates. Independent of the deposition conditions, both materials showed relatively stable cation-driven actuation in CCM, based on the flux of mainly Na+ ions from CCM. The surprising result was attributed to re-doping by sulfate anions in CCM, as also indicated by energy-dispersive X-ray (EDX) spectroscopy results. Overall, the electrochemically coated material outperformed the one with just chemical coating in conductivity, charge density and actuation response.

Large-area superelastic graphene aerogels based on a room-temperature reduction self-assembly strategy for sensing and particulate matter (PM2.5 and PM10) capture

Graphene aerogels are emerging low density and superelasticity macroscopic porous materials with various applications. However, it still remains a challenge to develop a versatile strategy under ambient conditions for fabricating large-area, high-performance graphene aerogels, which is crucial for their practical applications. Here, we report a novel room-temperature reduction self-assembly (RTRS) strategy to fabricate large-area graphene aerogels under ambient conditions. The strategy is based on using unique hydrazine hydrates as reducing agents to generate stable microbubbles beneficial for the formation of macroporous graphene hydrogels. Interestingly, the resultant hydrogel followed by a simple pre-freeze treatment can be naturally dried into graphene aerogels without noticeable volume shrinkage or structure cracking. Benefiting from the mild conditions, a large-area graphene aerogel with a diameter of up to 27 cm was prepared as an example. The as-formed aerogels exhibit a stable honeycomb-like coarse-pores structure, a low density of 3.6 mg cm-3 and superelasticity (rapidly recoverable from 95% compression) which are suitable for pressure/strain sensors. Moreover, the aerogel exhibits superior particulate matter adsorption efficiency (PM2.5: 93.7%, PM10: 96.2%) and good recycling ability. Importantly, the preparation process is cost-effective and easily scalable without the need for any special drying techniques and heating processes, which provides an ideal platform for mass production of graphene aerogels toward practical applications.

Graphene Oxide-Enabled Synthesis of Metal Oxide Origamis for Soft Robotics

Origami structures have been widely applied in various technologies especially in the fields of soft robotics. Metal oxides (MOs) have recently emerged as unconventional backbone materials for constructing complex origamis with distinct functionalities. However, the MO origami structures reported in the literature were rigid and not deformable, thus limiting their applications to soft robotics. Herein, we reported a graphene oxide (GO)-enabled templating synthesis to produce complex MO origami structures from their paper origami templates with high structural replication. The MO origami structures were next stabilized with elastomer, and the MO-elastomer origamis were able to be adapted into multiple actuation systems (including magnetic fields, shape-memory alloys, and pneumatics) for the fabrication of MO origami robots. Compared with conventional paper origami robots, the MO robots were lightweight, mechanically compliant, fire-retardant, magnetic responsive, and power efficient. We further demonstrated the legendary phoenix-fire-reborn concept in the soft robotics fields: a paper origami robot sacrificed itself in a fire scene and transformed itself into a downsized Al₂O₃ robot; the Al₂O₃ robot was able to crawl through a narrow tunnel where the original paper robot was unfit. These MO reconfigurable origamis provide an expanded material library for building soft robotics, and the functionalities of MO robots can be systematically engineered via the intercalation of various metal ions during the GO-enabled synthesis.

Resilient Si3N4 Nanobelt Aerogel as Fire-Resistant and Electromagnetic Wave-Transparent Thermal Insulator

With the prevailing energy challenges and the rapid development of aerospace engineering, high-performance thermal insulators with various functions are attracting more and more attention. Ceramic aerogels are promising candidates for thermal insulators to be applied in harsh environments because of their low thermal conductivity and simultaneously excellent thermal and chemical stabilities. In general, the effective properties of this class of materials depend on both their microstructures and the intrinsic properties of their building blocks. Herein, to enrich the family and broaden the application fields of this class of materials, we prepared ultralight α-Si₃N₄ nanobelt aerogels (NBAs) with tunable densities ranging from 1.8 to 9.6 mg cm^-3. The α-Si₃N₄ NBA realized resilient compressibility (with a recoverable strain of 40-80%), fire resistance (1200 °C butane blow torch), thermal insulation (0.029 W m^-1 K^-1), and electronic wave transparency (a dielectric constant of 1-1.04 and a dielectric loss of 0.001-0.004) in one material, which makes it a promising candidate for mechanical energy dissipative, fire-resistant, and electronic wave-transparent thermal insulator to be applied in extreme conditions. The successful preparation of such resilient and multifunctional α-Si₃N₄ NBAs will open up a new world for the development and widespread applications of ceramic aerogels.

Double-negative-index ceramic aerogels for thermal superinsulation

Ceramic aerogels are attractive for thermal insulation but plagued by poor mechanical stability and degradation under thermal shock. In this study, we designed and synthesized hyperbolic architectured ceramic aerogels with nanolayered double-pane walls with a negative Poisson’s ratio (−0.25) and a negative linear thermal expansion coefficient (−1.8 × 10−6 per °C). Our aerogels display robust mechanical and thermal stability and feature ultralow densities down to ~0.1 milligram per cubic centimeter, superelasticity up to 95%, and near-zero strength loss after sharp thermal shocks (275°C per second) or intense thermal stress at 1400°C, as well as ultralow thermal conductivity in vacuum [~2.4 milliwatts per meter-kelvin (mW/m·K)] and in air (~20 mW/m·K). This robust material system is ideal for thermal superinsulation under extreme conditions, such as those encountered by spacecraft.

Controllable Crimpness of Animal Hairs via Water-Stimulated Shape Fixation for Regulation of Thermal Insulation

Animals living in extremely cold plateau areas have shown amazing ability to maintain their bodies warmth, a benefit of their hair's unique structures and crimps. Investigation of hair crimps using a water-stimulated shape fixation effect would control the hair's crimpness with a specific wetting-drying process thereafter, in order to achieve the regulation of hair thermal insulation. The mechanism of hair's temporary shape fixation was revealed through FTIR and XRD characterizations for switching on and off the hydrogen bonds between macromolecules via penetration into and removal of aqueous molecules. The thermal insulation of hairs was regulated by managing the hair temporary crimps, that is, through managing the multiple reflectance of infrared light by hair hierarchical crimps from hair root to head.

Electrospun Janus-like pellicle displays coinstantaneous tri-function of aeolotropic conduction, magnetism and luminescence

A new Janus-like pellicle with top-bottom structure, functionalized by conductive aeolotropism, magnetism and luminescence (defined as a CML Janus-like pellicle), is conceived and constructed via electrospinning by combining microcosmic with macroscopic partitions. [PANI/PMMA]//[Eu(BA)₃phen/PMMA] and [Fe₃O₄/PMMA]//[Tb(BA)₃phen/PMMA] Janus-like microribbons are selected as building units to construct a conductive aeolotropism-luminescence layer (CL layer) and magnetism-luminescence layer (ML layer), and the two layers are combined to form a CML Janus-like pellicle. Macroscopic partition is achieved by designing the Janus-like structure of the pellicle, while Janus-like microribbons are used for the microcosmic partition by separating rare earth luminescent compounds from dark-colored magnetic Fe₃O₄ NPs and conductive PANI. The CML Janus-like pellicle has stronger luminescence compared to the contrast samples. The magnetism of the CML Janus-like pellicle can be adjusted by changing the doping amount of Fe₃O₄ NPs. The CML Janus-like pellicle can achieve a strong and variable conductive aeolotropism via changing the doping amount of PANI and the highest conductive aeolotropism ratio can reach ca. 10⁸ times when the PANI content is 70%. Microcosmic and macroscopic partitions are simultaneously integrated into the CML Janus-like pellicle, which results in almost no detrimental mutual influences between the two layers, and the overall performances of the CML Janus-like pellicle are greatly improved.

Janus-like pellicle with conductive aeolotropism, magnetism and luminescence is designed and constructed via electrospinning by combining microcosmic with macroscopic partitions.

Scalable Fabrication of Thermally Insulating Mechanically Resilient Hierarchically Porous Polymer Foams

The requirement of energy efficiency demands materials with superior thermal insulation properties. Inorganic aerogels are excellent thermal insulators, but are difficult to produce on a large-scale, are mechanically brittle, and their structural properties depend strongly on their density. Here, we report the scalable generation of low-density, hierarchically porous, polypropylene foams using industrial-scale foam-processing equipment, with thermal conductivity lower than that of commercially available high-performance thermal insulators such as superinsulating Styrofoam. The reduction in thermal conductivity is attributed to the restriction of air flow caused by the porous nanostructure in the cell walls of the foam. In contrast to inorganic aerogels, the mechanical properties of the foams are less sensitive to density, suggesting efficient load transfer through the skeletal structure. The scalable fabrication of hierarchically porous polymer foams opens up new perspectives for the scalable design and development of novel superinsulating materials.

Three-dimensional graphene-based polymer nanocomposites: preparation, properties and applications

Motivated by the unique structure and outstanding properties of graphene, three-dimensional (3D) graphene-based polymer nanocomposites (3D-GPNCs) are considered as new generation materials for various multi-functional applications. This review presents an overview of the preparation, properties and applications of 3D-GPNCs. Three main approaches for fabricating 3D-GPNCs, namely 3D graphene based template, polymer particle/foam template, and organic molecule cross-linked graphene, are introduced. A thorough investigation and comparison of the mechanical, electrical and thermal properties of 3D-GPNCs are performed and discussed to understand their structure-property relationship. Various potential applications of 3D-GPNCs, including energy storage and conversion, electromagnetic interference shielding, oil/water separation, and sensors, are reviewed. Finally, the current challenges and outlook of these emerging 3D-GPNC materials are also discussed.

Ultralight, Recoverable, and High-Temperature-Resistant SiC Nanowire Aerogel

Ultralight ceramic aerogels with the property combination of recoverable compressibility and excellent high-temperature stability are attractive for use in harsh environments. However, conventional ceramic aerogels are usually constructed by oxide ceramic nanoparticles, and their practical applications have always been limited by the brittle nature of ceramics and volume shrinkage at high temperature. Silicon carbide (SiC) nanowire offers the integrated properties of elasticity and flexibility of one-dimensional (1D) nanomaterials and superior high-temperature thermal and chemical stability of SiC ceramics, which makes it a promising building block for compressible ceramic nanowire aerogels (NWAs). Here, we report the fabrication and properties of a highly porous three-dimensional (3D) SiC NWA assembled by a large number of interweaving 3C-SiC nanowires of 20-50 nm diameter and tens to hundreds of micrometers in length. The SiC NWA possesses ultralow density (∼5 mg cm^-3), excellent mechanical properties of large recoverable compression strain (>70%) and fatigue resistance, refractory property, oxidation and high-temperature resistance, and thermal insulating property (0.026 W m^-1 K^-1 at room temperature in N₂). When used as absorbents, the SiC NWAs exhibit an adsorption selectivity of low-viscosity organic solvents with high absorption capacity (130-237 g g^-1). The successful fabrication of such an attractive material may provide promising perspectives to the design and fabrication of other compressible and multifunctional ceramic NWAs.

Three-Dimensional Printing Hollow Polymer Template-Mediated Graphene Lattices with Tailorable Architectures and Multifunctional Properties

It is a significant challenge to concurrently achieve scalable fabrication of graphene aerogels with three-dimensional (3D) tailorable architectures (e.g., lattice structure) and controllable manipulation of microstructures on the multiscale. Herein, we highlight 3D graphene lattices (GLs) with complex engineering architectures that were delicately designed and manufactured via 3D stereolithography printed hollow polymer template-mediated hydrothermal process coupled with freeze-drying strategies. The resulting GLs with overhang beams and columns show a 3D geometric configuration with hollow-carved features at the macroscale, while the construction elements of graphene cellular on the microscale exhibit a well-ordered and honeycomb-like microstructure with high porosity. These GLs demonstrate multifunctional properties with robust structure, high electrical conductivity, low thermal conductivity, and superior absorption capacitance of organic solvents. Moreover, the GLs were utilized as a subtle sensor for the fast detection of chemical agents. Aforementioned superior properties of GLs confirm that the combination of 3D tailorable manipulation and self-organization design of structures on the multiscale is an effective strategy for the scalable fabrication of advanced multifunctional graphene monoliths, suggesting their promising applications as chemical detection sensors, environmental remediation absorbers, conductive electrodes, and engineering metamaterials.

Efficient Flame Detection and Early Warning Sensors on Combustible Materials Using Hierarchical Graphene Oxide/Silicone Coatings

Design and development of smart sensors for rapid flame detection in postcombustion and early fire warning in precombustion situations are critically needed to improve the fire safety of combustible materials in many applications. Herein, we describe the fabrication of hierarchical coatings created by assembling a multilayered graphene oxide (GO)/silicone structure onto different combustible substrate materials. The resulting coatings exhibit distinct temperature-responsive electrical resistance change as efficient early warning sensors for detecting abnormal high environmental temperature, thus enabling fire prevention below the ignition temperature of combustible materials. After encountering a flame attack, we demonstrate extremely rapid flame detection response in 2-3 s and excellent flame self-extinguishing retardancy for the multilayered GO/silicone structure that can be synergistically transformed to a multiscale graphene/nanosilica protection layer. The hierarchical coatings developed are promising for fire prevention and protection applications in various critical fire risk and related perilous circumstances.

Thermally Stable and Electrically Conductive, Vertically Aligned Carbon Nanotube/Silicon Infiltrated Composite Structures for High-Temperature Electrodes

Traditional ceramic-based, high-temperature electrode materials (e.g., lanthanum chromate) are severely limited due to their conditional electrical conductivity and poor stability under harsh circumstances. Advanced composite structures based on vertically aligned carbon nanotubes (VACNTs) and high-temperature ceramics are expected to address this grand challenge, in which ceramic serves as a shielding layer protecting the VACNTs from the oxidation and erosive environment, while the VACNTs work as a conductor. However, it is still a great challenge to fabricate VACNT/ceramic composite structures due to the limited diffusion of ceramics inside the VACNT arrays. In this work, we report on the controllable fabrication of infiltrated (and noninfiltrated) VACNT/silicon composite structures via thermal chemical vapor deposition (CVD) [and laser-assisted CVD]. In laser-assisted CVD, low-crystalline silicon (Si) was quickly deposited at the VACNT subsurfaces/surfaces followed by the formation of high-crystalline Si layers, thus resulting in noninfiltrated composite structures. Unlike laser-assisted CVD, thermal CVD activated the precursors inside and outside the VACNTs simultaneously, which realized uniform infiltrated VACNT/Si composite structures. The growth mechanisms for infiltrated and noninfiltrated VACNT/ceramic composites, which we attributed to the different temperature distributions and gas diffusion mechanism in VACNTs, were investigated. More importantly, the as-farbicated composite structures exhibited excellent multifunctional properties, such as excellent antioxidative ability (up to 1100 °C), high thermal stability (up to 1400 °C), good high velocity hot gas erosion resistance, and good electrical conductivity (∼8.95 Sm^-1 at 823 K). The work presented here brings a simple, new approach to the fabrication of advanced composite structures for hot electrode applications.