Preparation And Thermally Conductive Mechanism Study Of Electrical Insulation Hexagonal Boron Nitride/Cellulose Nanofibrils Composite

Electronics continuously develop in the direction of miniaturization,integration and high-power.Considerable heat will be accumulated if the heat could not be discharged timely,which will affect the operation quality and lifetime of electronics,or even cause an explosion and endanger people’s safety.At present,thermally conductive plastic,rubber,and resin are usually used as electrical insulation materials in electronics.In view of the outstanding issues of above materials such as poor thermal conductivity,easy aging and non-degradation,cellulose nanofiber（CNF）with good film-forming property,chemical and thermal stability performance,environmentally friendly and biodegradable properties was selected as substrate,hexagonal boron nitride（h-BN）with high thermal conductivity and excellent electrical insulation property was chosen as functional filler,luxuriant thermally conductive pathways were supposed to be constructed in composite by using 3D template method,aiming to reduce the phonon scattering and improve the phonon transmission efficiency,further achieving the aim of enhancing the thermal conductivity of the composite.In addition,the modification methods,thermal stability,thermal conductivity and electrical insulation performance of h-BN were systematically studied.The main results were concluded as follows:1.The hydroxyl group content of h-BN was increased after NaOH alkalization,which was beneficial to reduce the phonon scattering and improve the phonon transfer efficiency,the thermal conductivity of h-BN was increased from 13.993 W/m K to 16.651 W/m K,showing an increasement of 19.0%.The thickness of h-BN was effectively reduced by using liquid phase ultrasonic treatment,and the specific surface area was improved.The thermal conductivity of BNNS was high up to 17.884 W/m K,indicating an increasement of 21.8%.Sodium lignosulfonate（LS）played an effective dispersive role in h-BN,the phonons scattering and transfer efficiency could be affected within h-BN due to the steric hindrance caused by the three-dimensional supramolecular structure of lignin,leading to the decrease of thermal conductivity.The volume resistivity of h-BN obtained by different modification methods was in the range of3～4×10¹⁴Ω?cm,maintaining excellent electrical insulation performance.2.The thermal conductivity of BN-OH/CNF composite fabricated by blending method was0.678 W/m K at the BN-OH loading of 23.0 wt%,while this feature of BN-OH/CNF composite fabricated by 3D template method was as high as 1.488 W/m K at the BN-OH/CNF ratio of 3:1,and the volume resistivity was 3.832×10¹⁴Ω?cm.Indicating that the effective contact between BN-OH was impeded by CNF in blending method,the phonon scattering was thus increased,and led to a lower thermal transfer efficiency.Meanwhile,luxuriant thermally conductive pathways were formed in BN-OH/CNF composite by using 3D template method,exhibiting better thermal conductive characteristic.The tensile strength,T_onset and T_max of BN-OH/CNF composite with23.0 wt%BN-OH were 39.0 MPa,218 ^oC and 292 ^oC,respectively,showing good mechanical performance and thermal stability.3.Lignin nanoparticle（LNP）with abundant phenolic hydroxyl group had hydroquinone conjugated structure.Effective links between BN-OH particles were realized through borax cross-linking,thus the thermal conductive pathways were formed,which increased the phonon transmission channels and strengthened the thermal conductivity of the composite.The results showed that the thermal conductivity of the composite prepared by 3D template method was high up to 2.084 W/m K at the BN-LNP content of 40 wt%,which was 46.3%higher than that of the BN-OH/CNF composites under the same BN-OH loading.The volume resistivity and tensile strength of BN-OH/CNF composite were 3.688×10¹⁴Ω?cm and 49.2 MPa,respectively,exhibiting good electrical insulation and mechanical performances.4.The binding force between h-BN and CNF would sharply decrease at a high h-BN loading on account of the inherent property as an inorganic mineral particle.PVA was commonly used as polymer reinforcing agent,an appropriate amount of PVA could enhance the binding force between the BN-LNP and CNF,which was beneficial to improve the mechanical strength of the composite.When the amount of PVA gradually increased to 40 wt%,the thermal conductivity of the composite increased from 2.049 W/m K to 2.242 W/m K,and the tensile strength increased from 49.1 MPa to 73.2 MPa.Indicating that PVA and LNP had similar functions,i.e.,more links were generated between the BN-OH particles,and enriched hydroxyl groups on the PVA surface enhanced the binding force with CNF,which could further improve the thermal conductivity and mechanical properties of the composite.In conclusion,the high-temperature alkalization treatment of h-BN effectively increased its surface activity.The BN-OH/CNF composite prepared by 3D template method realized the construction of thermally conductive pathways between BN-OH particles,LNP effectively improved the heat transfer between BN-OH particles through the effective link by cross-linking reaction of borax,which was the supplementary thermally conductive channels,greatly reducing the phonon scattering and improving the phonon transfer efficiency.Moreover,not only the thermal conductivity of BN-OH/CNF composites was improved,but the mechanical properties and thermal stability of the composite were simultaneously enhanced by using LNP and PVA,which endowed the resulting composite utilization potentiality in miniaturized electronic devices.Therefore,the present study provided the theoretical basis and method reference for the development of green,thermally conductive and electrical insulative materials.