Researches On The Exfoliation Of Hexagonal Boron Nitride Nanosheets And Properties Of Their Composites

Two-dimensional（2D）materials show extraordinary potential in energy storage,catalysis and photoelectricity because of its spcial strctures and properties.Hexagonal boron nitride nanosheets（h-BNNs）possess high electrical insulation,thermal conductivity,breakdown strength and low dielectrical loss with a layered strcture that is similar to graphene.In this paper,triethanolamine（TEA）is used as the mono-solvent to exfoliate h-BNNs for the first time.We proposed a novel and facile way to prepare large-sized 2D materials continuously via a liquid-based exfoliation process without ultrasonic treatment.2,2,6,6-tetramethyl-piperidine 1-oxyl（TEMPO）-oxidized cellulose nanofibrils（TOCN）are a kind of natural polymer nanofibers with good renewability and biodegradation.TOCN films have compact layered strcture showing high light transmittance and flexibility,which is expected as the matrix to prepare composites.Then,the obtained h-BNNs are added into the TOCN to prepare the 1D/2D organic-inorganic layered nanocomposites.And,the cross-linked h-BNNs/TOCN（h-BNNs/CLTOCN）composites films are prepared via being soaked in CaCl₂ solution.The dielectric properties,thermal stability and optical performance of h-BNNs/TOCN and h-BNNs/CLTOCN films are studied.The conclusions of our research are summarized as follows:（1）The h-BN powder was added into a flask with TEA following with strring treatment,which exfoliated h-BN powder in a mono-solvent successfully.The adsorption effect between h-BN surface and the three dimensional network structure formed by hydrogen bonded TEA arised in the exfoliation system.Thus,the TEA acted as dispersant and stabilizer during the exfoliation process.The obtained h-BNNs were few-layers and the dispersions were of high stability after 9 months.（2）The h-BNNs was obtained via washing the dispersion by water to remove TEA.Then,h-BNNs/TOCN composites were prepared via liquid based method.The breakdown strength of h-BNNs/TOCN increased to 211 kV/mm with the content of12 vol.%h-BNNs.The dielectric loss decresed with the adding of h-BNNs,however,the dielectric constant and dielectric loss are of low stability varies with the frequency.In addition,the thermal stability should be improved.（3）Soaking the h-BNNs/TOCN in CaCl₂ solution,the carboxylate groups in TOCN could be cross-linked by Ca²⁺.The density of cross-linked h-BNNs/TOCN（h-BNNs/CLTOCN）increased in a large scale after the above process.The dielectric constant and dielectric loss of h-BNNs/CLTOCN were more stable than h-BNNs/TOCN varied with the frequency and the dielectric loss decreased apparently.The dielectric constant and dielectric loss of 2 vol.%h-BNNs/CLTOCN were 8.7 and0.02,respectively.When the constant of h-BNNs increased to 12 vol.%,the dielectric loss had fallen to 0.01,meanwhile,the dielectric constant decreased to 6.7.The properties displayed above indicates that the h-BNNs/CLTOCN composites wass promising to be applied in dielectric energy storage materials.In addition,the breakdown strength of 12 vol.%h-BNNs/CLTOCN reached 301 kV/mm,which is about 20%higher than 12 vol.%h-BNNs/TOCN and 500%higher than TOCN.Compared with h-BNNs/TOCN,the corresponding temperature of maximum decomposition rate of h-BNNs/CLTOCN were enhanced with 95⁹8℃.