Controllable Preparation And Properties Of Suger Carbon Biomass Conversion Porous Carbon-based Materials

The surface and the inner wall of ramjet aircraft will face extreme environmental conditions such as ultra-high temperature during the service,it is necessary to implement effective thermal protection measures for the internal components of aircraft,therefore,the high efficiency heat insulation materials are urgently needed.Porous carbon material is a three-dimensional porous material composed of carbon materials with different pore structures.It shows extensive application potential in thermal management and electromagnetic shielding due to the advantages of light weight,high porosity,high temperature resistance and heat insulation.In recent years,the preparation of porous carbon materials using biomass materials as precursors has triggered a research boom.However,there are some problems in the research of biomass-based porous carbon,such as uncontrollable preparation of micro and macro structure,single function and poor structural strength.This paper aims to achieve the controllable preparation of macroscopical and microscopic structures of porous carbon materials based on the sugar-carbon conversion technology.By regulating the types of carbon precursors（small-molecule sucrose,macromolecule chitosan as well as sucrose and chitosan collaboration）and fabrication methods,designed and prepared biomass sugar carbon conversion porous carbon-based materials with different densities and macro properties.The collaborative optimization of thermal insulation performance and mechanical strength was realized to meet the needs of different application scenarios.The influence of the changes in the micro structure on the material performance was investigated,the structural characteristics of biomass-based porous carbon materials were systematically studied.It has laid a solid theoretical foundation for its practical application in related fields and exhibits important practical application value.The main research contents of this paper are as follows:The small molecule sucrose was used as the carbon source,three-dimensional macroscopic precursor was first constructed through the formless physical mechanical stirring foaming method combined with gel injection mold technology,and large-scale multifunctional sucrose-based carbon foam was prepared by high-temperature heat treatment.Inspired by honey stored in honeycombs,the epoxy hydrogel with three-dimensional spatial network structure was used as a structure guiding agent to fix sucrose molecules in it,which solved the uncontrollable foaming problem of sucrose during the heating process.Benefiting from this,the carbon yield of carbon foam was effectively improved to 44.7%.The microporous structure of carbon foam can be controlled by adjusting the sucrose content;therefore,the controllable preparation of carbon foam can be achieved.The sucrose-based carbon foam shows the characteristics of light weight and high strength.When the density is 0.179 g/cm³,the compressive strength of the sample is up to 4.71 MPa,specific compression strength is up to 26.31 MPa·cm³·g^-1.Duo to the hierarchical porous structure from micro to nano scale,the sucrose-based carbon foam can effectively block the heat flow.The room temperature thermal conductivity is as low as 0.092 W/m·K and the high temperature thermal conductivity（1200°C）is only 0.269W/m·K as a density of 0.091 g/cm³.It shows good thermal stability and flame ablation resistance.The sucrose-based carbon foam also has good oil adsorption and separation capacity,and the maximum adsorption capacity can reach 10 g·g^-1.In order to obtain more lightweight biomass-based porous carbon to further improve its heat insulation performance,an ultralightweight chitosan/MXene composite carbon aerogel with multifunctional properties was designed and prepared from chitosan and MXene nano sheets through a direct freeze-drying method combined with high temperature heat treatment.The composite carbon aerogel shows good structural bearing capacity due to the uniform pore structure,and the specific compressive strength can reach 3.18 MPa·cm³·g^-1.MXene nanosheets are stacked on the pore wall of the composite carbon aerogel,building a hierarchical porous structure from micrometer to nanometer,which contributes to the excellent thermal insulation performance.When the density is0.0119 g/cm³,the room temperature thermal conductivity of the sample is as low as0.0259 W/m·K,similar to the thermal conductivity of air,and the high temperature thermal conductivity（800°C）is only 0.0627 W/m·K.When the heat treatment temperature is 800°C,the content of MXene nanosheet is 0.1322 vol%,the average electromagnetic shielding efficiency of the composite carbon aerogel in X band is as high as 61.4 d B,and the specific electromagnetic shielding efficiency is 5155.46 d B·cm³·g^-1.Good structural strength and hydrophobic characteristic provide guarantee for the long-term use of composite carbon aerogels.There was little decrease in the thermal conductivity of the sample and the electromagnetic shielding effectiveness was still as high as 40.5 d B after being stored in a humid and hot environment（50°C,45–65%humidity）for 30 days.Finally,in order to achieve collaborative optimization of mechanical strength and thermal insulation properties of porous carbon materials,a composite carbon foam with pure biomass carbon source was fabricated using small molecule sucrose as the main carbon source,and chitosan as the auxiliary carbon source and structure directing agent through the guidance of ice template and combined with high-temperature carbonization treatment.The small molecules of sucrose can be fixed in the three-dimensional spatial network structure of chitosan hydrogel,which inhibits the uncontrolled foaming of sucrose.The introduction of sucrose significantly increases the residual carbon content of composite carbon foams.CNTs,as a skeleton support,effectively reduces the volume shrinkage of samples,and creates a large number of nano pores in situ by overlapping on the pore wall.Benefiting from this,the composite carbon foam shows the characteristics of light weight,high strength as well as excellent heat insulation performance.When the carbonization temperature is 1200°C and the density of composite carbon foam is 0.1226g/cm³,the compressive strength is 2.18 MPa,and the specific compressive strength reaches 17.79 MPa·cm³·g^-1.When the density of the sample is only 0.0773 g/cm³,the thermal conductivity at room temperature is as low as 0.0648 W/m·K,and the thermal conductivity at high temperature（900°C）is only 0.1644 W/m·K.At the same time,it exhibits good thermal stability and flame ablation resistance.The rich hierarchical porous structure endows the composite carbon foam with good oil adsorption and separation performance.The adsorption capacity for kerosene reaches 12.56 g·g^-1,and remains 11.35g·g^-1 after 20 adsorption-desorption cycles.