Preparation And Performance Of Flame-retardant Organic/Inorganic Composite Aerogels With Sensitive Fire-warning Capability

Organic aerogels show promising prospects in architectural decoration,transportation,aerospace,and so on,due to their ultralow density and excellent thermal insulation.However,most organic aerogels are mainly composed of carbon and hydrogen,which are easy to be ignited in case of fire,thus posing serious threat to people’s lives and property.To address this issue,flame retardants are incorporated to improve the fire safety of the organic aerogels.Nevertheless,the risk of flashover and flame spread still exists at an early stage before the fire is out of control,especially at high temperature,which cannot meet the increasing fire safety requirements.Herein,chitosan-based and polyimide-based composite aerogels with fire-warning capability were fabricated by incorporating montmorillonite,carbon nanotubes,and titanium carbide（MXene）nanosheets into organic aerogels,which effectively improved the flame retardancy of aerogels and addressed the response delay of the traditional smoke fire-warning system,thereby wining more time for evacuation and rescue.It provides novel ideas for the preparation and application of intelligent fire-safe materials in the era of the Internet of everything,which indicates important scientific significance and huge application value.The main content and results of this study are mainly as follows:（1）Flame retardant CCS/MMT/A-CNT（CCA）aerogels with ultrasensitive fire-warning were prepared via freeze-drying using carboxymethyl chitosan（CCS）,montmorillonite（MMT）and amino-functionalized carbon nanotube（A-CNT）as raw materials,and glutaraldehyde（GA）as the cross-linker.When being burned,the CCA aerogel presented an ultrasensitive fire-warning response as short as 0.25 s.Compared with the CCS aerogel,the peak heat release rate（PHRR）,total heat release（THR）,CO release and total smoke production（TSP）of CCA aerogels were significantly decreased.The limiting oxygen index（LOI）values of CCA aerogels were higher than 27%,and they self-extinguished quickly when the fire was removed.In addition,the possible fire-warning mechanism was revealed as follows:at high temperature,A-CNT fast removed amino groups and was reduced to CNT with excellent electrical conductivity;combined with the inherent semiconducting behavior,A-CNT exhibited sensitive thermal-induced electrical resistance change.Besides,CCS underwent fast charring reactions and bonded the CNT together,which led to the formation of conductive pathways and the sharp decrease of the electrical resistance of CCA aerogels.When being burned,CCS crosslinked and formed char quickly with the assistance of the solid acid catalysis effect of MMT to bond the MMT and A-CNT more tightly,and formed more compact and continuous three-dimensional network carbon layer.Meanwhile,owing to the excellent barrier effect of MMT and the hierarchical porous structure of the aerogel,the transfer of combustible gases and heat was effectively blocked,thus significantly improving the flame retardancy of CCA aerogels.Moreover,CCA aerogels exhibited good mechanical resilience,excellent anti-fatigue and piezoresistive sensing properties,showing promising application in human movements monitoring.（2）Polyamide acid（PAA）was firstly synthesized by 4,4’-diaminodiphenyl ether,pyromellitic anhydride and trimethylamine using N,N-dimethylacetamide as solvent.Subsequently,multifunctional polyimide@MXene（PI@MXene）aerogels with fire-warning,temperature sensing and piezoresistive sensing were successfully prepared via freeze-drying,thermal imidization and dip-coating with MXene aqueous solution.It was found that PI@MXene aerogels possessed excellent mechanical performance.It could recover after being compressed at 80%strain,and the maximum stress of PI@MXene aerogels at 30%strain achieved 209 k Pa,69%higher than that of polyimide（PI）aerogel.Besides,a linear function relationship between the maximum thermoelectric voltage and the temperature difference of PI@MXene aerogels from 50℃to 250℃was observed.When the PI2@MXene-1 aerogel was exposed to an alcohol lamp for the first time,the fire-warning system was triggered in 4.67 s.Besides,the trigger time was only 3.66 s at the fifth cycle of the fire-warning test.The incorporation of MXene not only endowed PI@MXene aerogels with sensitive temperature sensing and fire-warning,but also improved the flame retardancy and thermal stability of the aerogel.When PI2 and PI2@MXene-1 aerogels were burned for60 s and then removed,the PI2 aerogel self-extinguished at 1.5 s while the PI2@MXene-1aerogel self-extinguished within 1 s.This was due to the combination of the carbonization of PI,the lamella barrier of MXene and the titanium dioxide（Ti O₂）formed by partial oxidation of MXene to form a compact protective char layer,blocking the transmission of oxygen,combustible gas and heat,which improved the flame retardancy of the aerogels.（3）Silver selenide nanowire（Ag₂Se NW）was synthesized by selenium dioxide（Se O₂）and silver nitrate（Ag NO₃）,and used as thermoelectric synergistic reinforcement materials of MXene.Subsequently,PI@（Ag₂Se NW/MXene）（PAM）composite aerogels with fire-warning and electrothermal functions were prepared by one-step dip coating.It was found that the maximum thermoelectric voltages of the PAM aerogels at the same temperature difference were higher than those of the PI@MXene aerogels.When the mass ratio of Ag₂Se to MXene was 1:1,the PA₁M₁ aerogel presented the best thermoelectric efficiency.When encountering flame,the PA₁M₁ aerogel triggered the fire-warning system at 4.03 s.Even at the 20th cycle of the fire-warning test,the fire-warning trigger time was still less than 6 s.In addition,the LOI values of PAM aerogels were higher than 48%,showing exceptional flame retardancy.Moreover,the temperature of the maximum thermal weight loss rate and the mass residual rate of the PA₁M₁ aerogel were higher than those of the PI@MXene aerogel,and its heat release rate was significantly reduced.This was mainly because Ag₂Se NW acted as skeleton to strengthen the char layer,thus improving its barrier effect.Besides,the dip-coating of polydimethylsiloxane（PDMS）endowed the PAM aerogels with excellent hydrophobicity and self-cleaning performance,which further expanded the applications of aerogels.