Preperation Of Insotropic Polycrystalline Nanoscale Graphite With Sugar Derived Carbon And Its Application In Carbon/Carbon Composites

Isotropic nanocrystalline graphite exactly caters to the insatiable demand for light structure and high performance,which has been a popular candidate in the aerospace field.In recent years,small-molecule biomass material represented by saccharide have been favored by researchers due to its characteristic of abundance in nature,low-cost,easy purification process,and high freedom of carbonized production,which has become the carbon source for preparation of high-performance carbonaceous materials.For isotropic nanocrystalline graphite,the critical points of preparation mainly concentrate on graphitization of carbonaceous structure,construction of isotropic structure and acquisition of nanocrystalline densified bulk.One important judgement for graphitization is that carbonaceous material transformed from condensed phase must experience a plastically liquid phase at low temperature to generate an aromatized planar molecule structure of stacking lamella.Although the saccharide possesses crystal structure and fixed melting point,the carbonization is mainly completed through polycondensation of molecular and formation of macromolecular carbon chains,which makes it difficult for graphite crystalline to recrystallize.The construction of isotropic structure is mainly embodied in the granulation of carbonaceous,so that the graphite crystallites can only recrystallize inside the particle instead of self-assembling with graphite crystallites in adjacent particles,which realizes the anisotropy on micro level and isotropy on macro level.The acquisition of nanocrystalline dense bulks mainly depends on the molecular size of precursor and control of carbonization.The most challenging issues for obtaining saccharide carbon are large amount of carbon loss and uncontrollable foaming,hindering the densification of carbonaceous.Therefore,current researches are mostly limited within the preparation of low-dimensional and porous carbonaceous materials such as carbon quantum dots,hydrothermal carbon spheres and graphene foams.It is still in blank when it comes to the preparation of densified macrostructure by extracting carbon in theoretical content from saccharides.Here,we report a strategy for introducing the acrylamide hydrogel system into high concentration glucose solution and dividing it into nano capsules.The molecular sieve effect of acrylamide film gradually removes the free water and hydron from glucose hydrogel.The carbonaceous materials could be completely extracted from glucose(carbon yield > 96%)with the generation of isotropic granulation structure.The carbonized acrylamide film belongs to high-quality graphite nucleation,which promotes the stacking lamellar structure of glucose-derived polymers generated by aromatization gradually graphitizing.Since the matrix composition of carbon fiber reinforced carbon matrix composites is highly designable,a large number of researchers have designed the matrix of carbon fiber reinforced carbon matrix composites in various aspects.Due to the single chemical composition of carbon fiber reinforced carbon matrix composites,their macro properties will be greatly changed due to the differences in microstructure and fiber-matrix interface properties,and the mechanical properties and thermal conductivity of composites with different matrix texture types will also be affected [20].The structure type of carbon matrix is the main factor that determines the ablative and denudative resistance of carbon fiber reinforced carbon matrix composites.Increasing the degree of graphitization can reduce the sensitivity of carbon oxidation reaction of the matrix,and reducing the grain size can resist the large area of spalling caused by heat flow erosion.However,the carbon fiber reinforced carbon matrix composites prepared by using hydrocarbon as carbon source have some problems,such as the matrix carbon is difficult to graphitize(resin)or form large grain graphite(asphalt)after graphitization,and the anisotropic carbon matrix(PyC)with uneven texture,which leads to the lack of dimensional and ablative resistance of carbon fiber reinforced carbon matrix composites in the environment of high dynamic pressure and high heat flow.At present,it is difficult to design the texture of carbon fiber reinforced carbon matrix composites prepared from hydrocarbon carbon sources,and it is difficult to change the texture of carbon fiber reinforced carbon matrix composites through process design.At the same time,there is a problem of interface incompatibility between carbon fiber reinforced carbon matrix composites prepared by liquid hydrocarbon carbon sources,which usually requires adding an interface layer on the fiber surface by means of vapor deposition or adding surface nano modification,resulting in interface defects between different carbon source matrices.This is also a major factor limiting the high temperature ablative and denudation resistance of carbon fiber reinforced carbon matrix composites.Therefore,it is very important to reduce the defects of carbon matrix and control the texture effectively to improve the ablative and denudation resistance of carbon fiber reinforced carbon matrix composites in the ablative environment.Here,nanometer polycrystalline stone ink is introduced into carbon fiber reinforced carbon matrix composites as a matrix,which perfectly solves the problems of difficult graphitization of carbon fiber reinforced carbon matrix composites matrix or too large graphite grains and uneven texture.We used non-hydrocarbon small molecule carbohydrate as carbon source,used acrylamide gel network to fix the carbon in the carbohydrate in the form of small cells,and macrointroduced into the carbon fiber precast.After in situ carbonization,after high temperature heat treatment,we could obtain the uniform texture of sugar derived nano polycrystalline ink matrix.After repeated filling,high density carbon fiber reinforced sugar derived nano polycrystalline ink-matrix composites can be obtained.At the same time,the sugar-derived nanocrystalline graphite matrix can be well combined with carbon fiber,which solves the problem of interface mismatch during the preparation of carbon fiber reinforced carbon matrix composites.During the ablation experiment,the surface response temperature of the material exceeds2000 ℃,and the line ablation rate is less than 0.02 mm/s,showing excellent ablative resistance and denudation resistance.