Multilevel Assembly Scheme For The Controllable Fabrication And Property Modulation Of Boron Nitride Aerogel

Aerogel is a kind of porous material featuring a nanoscale skeleton.It could serve as macroscopic materials while maintaining nanoscale characteristics,and therefore holds great prospects in a variety of applications.The performance of aerogel is influenced by two factors-composition and structure,whose contribution to the properties is not based on a mere superposition.Multidimensional features could make a certain aerogel display some unique behaviors.Accordingly,it is important to develop elegant ways for microstructure design and property modulation.Nonetheless,due to the limitations of current strategies in aerogel field,there are several obstacles while fabricating many types of aerogels.The current situation requires the expanding of idea library in this field.This work focuses on the fabrication of boron nitride aerogel,and meanwhile explores the principles for property modulation.The main points are as follows.(1)Based on the in-depth analyses of the upsides and downsides of existing preparation routes in aerogel field,this thesis,in the first step,established the concept of multi-level assembly by fully drawing the lessons from supramolecular chemistry.The overall idea of this scheme can be summarized as follows:in the initial stage,assembling raw molecules into supramolecular architectures with diverse structures by harnessing the assembly process;then boron nitride aerogels converted from them could possess diverse skeletons.This scheme not only allows to tailor the microstructure but also gives rise to interconnected networks that form in-situ.Subsequently,a series of attempts were made to verify the feasibility and flexibility of this concept from different aspects.(2)This work presents a building-block-based design strategy.Changing the type and molar ratio of building blocks(i.e.,manipulating the "actor" of assembly)could make the as-formed supramolecular gels exhibit different assembly patterns,and the final boron nitride aerogels converted from them could possess various superstructures.The as-prepared boron nitride aerogels in this part displayed similarly low thermal conductivity,while their water affinity exhibited noticeable discrepancy as the microstructure varied.(3)This work demonstrates a solvent-based design strategy.Adjusting the solvent proportion during the formation of supramolecular gels(i.e.,manipulating the"environment" of assembly)could also change the assembly pattern,and accordingly,the resultant boron nitride aerogels could exhibit different hierarchy.The change in structural hierarchy had little effect on the thermal conductivity of boron nitride aerogel,but shifted the wettability from hydrophobicity to hydrophilicity and occasioned several-fold difference in ion adsorption performance.(4)The above approaches,changing either the "actor" or "environment" of assembly,could be attributed to the thermodynamic-based strategy.This work then shifts to the kinetic aspect and displays a temperature-mediated assembly scheme for the design of hierarchical boron nitride nanoribbon networks.Setting various cooling rate during the formation of supramolecular assemblies could render the final boron nitride nanobelts varied in their width and also affect the stacking pattern of inner crystals within individual nanoribbons.(5)The aforementioned attempts center around the assembly process,while this research proceeds to explore how the conversion process could be harnessed to tune the properties of boron nitride aerogel.With all parameters during assembly stage fixed,a mere change in atmosphere during high temperature treatment could influence the properties of boron nitride aerogel,be it phase composition,microstructure,pore structure,wetting behavior and mechanical performance.(6)In the extending section,this work synthesized other materials,such as boron-carbon-nitrogen-oxygen systems and graphite carbon nitride with engineerable microstructure,by changing the initial building molecules.In addition,this research explored how the ion adsorption performance of boron nitride aerogel could be further improved,and used the optimized aerogel adsorbent in adsorption desulfurization practices.These explorations,from different aspects,verify that the design principle of multilevel assembly scheme is feasible and extendable in terms of materials fabrication and properties modulation.This work develops an effective scheme for the controllable preparation and property modulation of boron nitride aerogel,which helps to extend the tool library in aerogel field and also provides inspirations for the design of other materials.