| The rapid development of aerospace and microelectronic technology has put forward an urgent demand for multifunctional high-performance ceramic materials,BN fibers show outstanding comprehensive performances covering mechanical,thermal,dielectric and so on by combining excellent physical&chemical properties of h-BN and unique fibrous structure,and meet the needs of many applications in various fields as multi-functional materials.To support the applications of the wave-transparent&thermal-insulated materials and the thermal management materials,we carried out a more facile way to fabricate BN micro-/nano-fiber based on C3N6H6?2H3BO3(M?2B)supramolecule.Specifically,it is focused to regulate the M?2B whisker's morphology,to study on conversion mechanism of the structural and morphology from M?2B to BN.And the fabrication of the three dimensional network of BN fibers(BNf-N)was conducted by combining the synthesis of M?2B whisker with freeze-casting method,followed by the systematical investigation on its preparation,structure,properties and applications.Firstly,the regulation on diameter and morphology of the M?2B whisker was studied,which was based on the crystallization process in synthesis of the supra-molecular whisker.The whisker's diameter was regulated from nanoscale(277±128 nm)to micron scale(3.273±2.011?m)by changing the ratio of C3N6H6 to H3BO3,as well as the concentration of the reactants.Considering large divergence of diameter distribution,the effect of cooling way on the M.2B growth and the whisker morphology was studied in detail.By virtue of the controlled cooling process,low divergence of M?2B whisker's diameter has been realized.Specifically,the whisker's diameter corresponding to the liquid nitrogen-assisted cooling method was 178±62 nm,while the diameter corresponding to the staged cooling process was 2.559±0.889?m.The evolution of M?2B whisker's profile during the ripening was explored.And the influence of M?2B crystal structure on whisker's profile was also analyzed via simulation.Based on the results,a mechanism on profile evolution has been proposed.The dynamical growth of M?2B gave birth to the primary profile of the nanobelt,and in the subsequent maturation,surface energy drove the M?2B nanobelt to cluster and merge into the micron whisker with polygon cross-section,and in final reached to the most thermodynamically stable form of the circular cross-section.Secondly,the evolution of molecular structure and morphology in the conversion process from M?2B to BNf was studied in detail.And the mechanism on structural evolution has been proposed.Firstly,H3BO3 molecular dehydration at 150-200 oC and produced g-B2O3 molten phase,which resulted in the self-healing of fiber morphology;secondly,the C-O-B bridge link between C3N6H6 molecule and B2O3 formed at 200-300oC;thirdly at about 500 oC,the triazine ring of C3N6H6 molecule decomposed into the N=C=O,-C?N,which were subsequently removed,meanwhile the nitridation of B2O3produced amorphous BN phase.Furthermore,the effects of high temperature treatment on the composition and performance of the BNf were investigated.Along with the temperature elevation,content of the O impurity decreased,accompanying with increasing degree of crystallization and the fiber density.The density of the BN fiber treated at 1400 oC reached 2.11 g?cm-3.Structure of the BN micro-/nano-fibers undergoing high temperature treatment was stable,and could tolerate long period corrosion of strong acid and alkali,could remained antioxidant under temperature above1000 oC.The dielectric constant of the BN fibers was in range of 3.7-5.0.Preparation of the BN micro-/nano-fiber based three-dimensional networks(BNf-N)were carried out,by virtue of the methods including one-step casting,two-step casting and impregnation-regrowth of M?2B whiskers.One-step casting method could synchronously realize synthesis of the M?2B whisker and construction of the fiber's network,and then turned into the BNf-N by heat treatment.It is advantageous to form integrity network with high-degree of fiber entanglement.Bulk density of the BNf-N varied within 0.029-0.041 g?cm-3 by adjusting the concentration of reactants.The latter two methods are aimed at increasing the density of the network structure,and density of the BNf-N could reach to 0.092-0.096 g?cm-3.The mechanical,thermal conduction,electromagnetic wave transparent and the adsorption performances of the BNf-N were systematically investigated.The compressive strength of the BNf-N was about 30.37-42.54 k Pa,with the elastic modulus in 0.48-1.14 MPa,it is enough to load-support in practical service.Owing to high porosity(97.90 vol%)of the network,thermal conductivity(TC)of the BNf-N was as low as 0.033-0.036 W?m-1?K-1,showing great thermal insulation.And degree of crystallization of the BN fiber had a dominant effect on the TC of the BNf-N,the BNf1700-N displayed high TC as 0.164 W?m-1?K-1.Due to low dielectric constant of the BN fiber and low fiber volume fraction of the network,the BNf-N exhibited a high electromagnetic wave transmission rate(>98.0%).Besides,the BNf-N showed excellent oil-water separation capacity due to ultra-hydrophobic and good lipophilicity of the BN fiber.The preparation and performance of the BN fiber reinforced epoxy matrix composites materials were initially investigated by using BNf-N as the fibers preform.As to BNf/epoxy composites,only 3.98 wt%BN fibers increased TC of the epoxy matrix composites by 23.8%,and the increase of crystallization and volume fraction of the BN fiber will enhance its thermal conductivity in further.In this paper,it was systematically investigated that included the synthesis of the M?2B whiskers,the ceramization of M?2B to BN micro-/nano-fibers,the construction of the three-dimensional networks and preparation and performance of BN micro-/nano-fiber reinforced composites.The formation mechanism on fibrous morphology of the M?2B was disclosed,as well as the structural evolution from M?2B to BNf.The density regulation of the BNf-N was realized,and function of the BN fiber in the composites is investigated.Our work provides theoretical support for ceramization of the supramoleculear materials and the guidance for the mass production and its application of the BN micro-/nano-fibers. |
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