Multifunctional aramid nanofibers reinforced RGO aerogels integrated with high-efciency microwave absorption, sound absorption and heat insulation performance

Although lightweight and three-dimensional(3 D) graphene aerogels are highly desirable for microwave absorption(MA) due to their high porosity,specific surface area,and 3 D conductive network,it still remains a large challenge to construct a multifunctional application framework to quickly adapt to the complex practical environment,making it to be efficiently applied in a variety of complex situation.Herein,multifunctional aramid nanofibers(ANFs) reinforced reduced graphene oxide aerogels(RGO@ANF) have been achieved by in-situ gel reaction,freeze-drying,and thermal annealing processes.The introduced ANFs in RGO aerogels can prevent the graphene sheets from over-stacking and enhance the connectivity of cell walls,thus leading to excellent compression resistance,MA,sound absorption,and thermal insulation performance.Under 70% strain,the maximum compressive stress of RGO@ANF aerogel reaches78.8 kPa,and reversible compressibility with reliable resistance to fatigue for 100 compressive cycles at20% strain.Further,the RGO@ANF aerogel exhibit a minimum reflection loss(RL_(min)) of-56.5 dB and a maximum effective absorption bandwidth(EAB) of 7.0 GHz at a thickness of 2.8 mm,basically covering the X and Ku bands.Moreover,the hybrid aerogel exhibited excellent sound absorption with an average absorption coefficient> 0.56 at 2-6 kHz and good thermal insulation performance with low thermal conductivity of about 49.18 mW m-1K-1.The integrated graphene aerogels with such multifunctional performances hold a great promise for applications such as MA,sound absorption,and heat insulation.

Analyses of non-aqueous reactive polymer insulation layer in high geothermal tunnel

The scenario of geothermal tunnel is commonly observed around the world,and increases with the new constructions in the long and deep tunnels,for example in China.Tunnel insulation is generally divided into active and passive insulation.In passive insulation,it is an effective way to set low thermal con-ductivity materials as the thermal insulation layer as the choice of insulation material mainly depends on the thermal conductivity.Polymer is a kind of material with good geothermal performance,but there are relatively few studies.In this context,the transient plane source(TPS)method was used to measure the thermal conductivity of the developed polymer.Then,the temperature field of the high geothermal tunnel insulated by the non-aqueous reactive polymer layer was simulated.With the parametric analysis results,the suggestions for the tunnel layers were proposed accordingly.It revealed that the thermal conductivity of polymer first increases and then decreases with temperature.There are two rising sec-tions(?40e10?C and 20e90?C),one flat section(10e20?C)and one descending section(>90?C).It is observed the thermal conductivity of polymer increases with increase of the density of insulation layer and the density,and the thermal conductivity decreases when exposed to high temperatures.The temperature of the surrounding rocks increases with increase of the thermal conductivity and the thickness of polymer.Finally,a more economical thickness(5 cm)was proposed.Based on the parametric study,a thermal insulation layer with thermal conductivity less than 0.045 W/(m K),thickness of 5 cm and a density less than 0.12 g/cm 3 is suggested for practice.

Hydrothermal Solidification of Diatomite and Its Heat Insulating Property

To meet the commercial requirements of inorganic heat insulators,the mixture of diatomite and Ca(OH)2 are evenly dispersed,mold-compacted,and then hydrothermally solidified due to the formation of tobermorite under an autoclaved process.Systematic investigations of the preparation conditions(including mix ratio,autoclaved factors,mold pressure,etc)were carried out to optimize the serving properties of such tobermorite-based products.As a result,a compressive strength of more than 30 MPa was realized for the specimen in high density(about 1.30(g·cm-3)).On the contrary,the specimen in light weight for example 0.63(g·cm-3)typically showed a thermal conductivity of around 0.12(W·m-1·K-1).The present work developed a feasible way to produce and to control the serving properties of diatomite-based heat insulators by a process of hydrothermal solidification,in which the optimized value of Ca/Si ratio was proposed to be 0.6~0.7,while the water content is 25% in weight,and hydrothermal reaction is performed at 180 ℃ for no more than 24 hours.

Recent Progress of Bionic Hierarchical Structure in the Field of Thermal Insulation Protection

Some living organisms with hierarchical structures in nature have received extensive attention in various fields.The hierarchical structure with multiple pores,a large number of solid-gas interfaces and tortuous conduction paths provide a new direction for the development of thermal insulation materials,making the living creatures under these extreme conditions become the bionic objects of scientific researchers.In this review,the research progress of bionic hierarchical structure in the field of heat insulation is highlighted.Polar bears,cocoons,penguin feathers and wool are typical examples of heat preservation hierarchy in nature to introduce their morphological characteristics.At the same time,the thermal insulation mechanism,fractal model and several preparation methods of bionic hierarchical structures are emphatically discussed.The application of hierarchical structures in various fields,especially in thermal insulation and infrared thermal stealth,is summarised.Finally,the hierarchical structure is prospected.

Ni Flower/MXene-Melamine Foam Derived 3D Magnetic/Conductive Networks for Ultra-Efficient Microwave Absorption and Infrared Stealth

The development of multifunctional and efficient electromagnetic wave absorbing materials is a challenging research hotspot.Here,the magnetized Ni flower/MXene hybrids are successfully assembled on the surface of melamine foam(MF)through electrostatic self-assembly and dip-coating adsorption process,realizing the integration of microwave absorption,infrared stealth,and flame retardant.Remarkably,the Ni/MXene-MF achieves a minimum reflection loss(RLmin)of−62.7 dB with a corresponding effective absorption bandwidth(EAB)of 6.24 GHz at 2 mm and an EAB of 6.88 GHz at 1.8 mm.Strong electromagnetic wave absorption is attributed to the three-dimensional magnetic/conductive networks,which provided excellent impedance matching,dielectric loss,magnetic loss,interface polarization,and multiple attenuations.In addition,the Ni/MXene-MF endows low density,excellent heat insulation,infrared stealth,and flame-retardant functions.This work provided a new development strategy for the design of multifunctional and efficient electromagnetic wave absorbing materials.

Synchronous deprotonation–protonation for mechanically robust chitin/aramid nanofibers conductive aerogel with excellent pressure sensing, thermal management, and electromagnetic interference shielding

Aerogels with regularly porous structure and uniformly distributed conductive networks have received extensive attention in wearable electronic sensors,electromagnetic shielding,and so on.However,the poor mechanical properties of the emerging nanofibers-based aerogels are limited in practical applications.In this work,we developed a synchronous deprotonation–protonation method in the KOH/dimethyl sulfoxide(DMSO)system at room temperature for achieving chitin cross-linked aramid nanofibers(CANFs)rather than chitin nanofibers(ChNFs)and aramid nanofibers(ANFs)separately by using chitin and aramid pulp as raw materials.After freeze-drying process,the cross-linked chitin/aramid nanofibers(CA)aerogel exhibited the synergetic properties of ChNF and ANF by the dual-nanofiber compensation strategy.The mechanical stress of CA aerogel was 170 kPa at 80%compressive strain,increased by 750%compared with pure ChNF aerogel.Similarly,the compressibility of CA aerogel was somewhat improved compared to ANF aerogel.The enhancement verified that the crosslinking reaction between ANF and ChNF during the synchronous deprotonation process was formed.Afterwards,the conductive aerogels with uniform porous structure(CA-M)were successfully obtained by vacuum impregnating CA aerogels in Ti_(3)C_(2)T_(x) MXene solution,displaying low thermal conductivity(0.01 W/(m·K)),high electromagnetic interference(EMI)shielding effectiveness(SE)(75 dB),flame retardant,and heat insulation.Meanwhile,the as-obtained CA-M aerogels were also applied as a pressure sensor with excellent compression cycle stability and superior human motion monitoring capabilities.As a result,the dual-nanofiber based conductive aerogels have great potentials in flexible/wearable electronics,EMI shielding,flame retardant,and heat insulation.

Preparation,with graphene,of novel biomimetic selfhealing microcapsules with high thermal stability and conductivity

This paper reports a comparative study of microcapsules with enhanced thermal stability and electrical conductivity inspired by the bionic thermal insulation of birds’feathers for self-healing aged asphalt.The work is based on an in situ polymerization with composite shell components of graphene and hexamethoxymethylmelamine resin.By using graphene,microcapsules with rough surfaces are achieved,improving the interface between microcapsules and asphalt.In addition,the microcapsules’initial thermal decomposition temperature is appropriately high,so that the stability of the microcapsule in the asphalt highway system is protected.The proportion of graphene in the microcapsule shell can regulate the microcapsule’s heat resistance because graphene modifies the shell’s structural makeup.Additionally,the microcapsules’electrical conductivity is relatively high.The self-healing capability of bitumen sharply increases,providing benefit to the effect of microcapsules on the properties of aged asphalt.

Multifunctional protective aerogel with superelasticity over−196 to 500℃

Protective materials that possess superelasticity and multifunctionality over a broad temperature range are urgently needed in various advanced applications.However,under harsh work conditions,the performance of current materials may largely deteriorate to lose protective functionality.Herein,we report a bidirectionally oriented multi-walled carbon nanotubes(MWCNTs)-reinforced chitosan carbon aerogel(CS-MWCNT)that possesses superelasticity,high electromagnetic interference shielding,thermal insulation,and infrared stealth at both low temperatures(such as liquid nitrogen)and high temperatures(such as alcohol flames).Highly oriented lamellar arch structures combined with an MWCNTs-reinforced carbon skeleton act as elastic segments to disperse the stress during compression and endow CS-MWCNT with the ability to recover to almost the original size after being compressed at−196-500℃.The lamellar structures make CS-MWCNT thermally insulating and infrared stealth with a low thermal conductivity of~0.03 W/(m·K).Furthermore,a high electromagnetic interference(EMI)shielding effect of 64 dB is realized via an absorption-dominant EMI shielding mechanism derived from the successive inherently conductive carbon lamella,and the EMI shielding performance is largely maintained after treatment under extreme conditions like low temperature,high temperature,as well as cyclic compression.This work provides a new strategy for the development of temperature-invariant multifunctional aerogels for harsh environment applications.

Multi-functional and multi-scenario applications for MXene aerogels with synergistically enhanced asymmetric modules

The development of multifunctional materials and synergistic applications of various functions are important conditions for integrated and miniaturized equipment.Here,we developed asymmetric MXene/aramid nanofibers/polyimides(AMAP)aerogels with different modules using an integrated molding process.Cleverly asymmetric modules(layered MXene/aramid nanofibers section and porous MXene/aramid nanofibers/polyimides section)interactions are beneficial for enhanced performances,resulting in low reflection electromagnetic interference(EMI)shielding(specific shielding effectiveness of 2483(dB·cm^(3))/g and a low R-value of 0.0138),high-efficiency infrared radiation(IR)stealth(ultra-low thermal conductivity of 0.045 W/(m·K)and IR emissivity of 0.32 at 3–5μm and 0.28 at 8–14μm),and excellent thermal management performances of insulated Joule heating.Furthermore,these multifunctional AMAP aerogels are suitable for various application scenarios such as personal and building protection against electromagnetic pollution and cold,as well as military equipment protection against infrared detection and EMI.