Harbin Institute of Technology AM:3000 ℃, ablation resistance!

Fiber reinforcement can generally improve the performance of the original material, especially for brittle materials, so fiber reinforced resin, metal, ceramic and carbon and other composite materials came into being. Carbon fiber reinforced carbon matrix (C/C) composite materials have been regarded as a major achievement in the history of materials since they were discovered in 1960. They have been widely used in aerospace, thermonuclear reactors, vehicle braking, thermophotoelectronics, testing, biomedicine and other fields. However, the preparation process of carbon-carbon composite materials is complicated and costly because of the long densification cycle, high preparation temperature and expensive petrochemical carbon materials. Therefore, there is an urgent need to develop a simple process for producing carbon/carbon composite materials using low-cost biomass instead of carbon sources, but this is still a challenge to date. Here,Harbin Institute of TechnologyProfessor Zhang Xinghong,Professor Zhou Yanchunreported.A use of sugar-derived carbon instead of petrochemical products as a dense matrix to prepareMethods for C/C Composites.sugar-derived carbon/Carbon (S-C/C) composite matrix with nano-graphite structure, high thermal stability, can be effectively combined with carbon fiber.The mechanical properties of S-C/C composites are comparable to those of composites prepared from petrochemical raw materials; more importantly,In10 MWm^-2The linear ablation rate is 0.03mm s after ablation at a temperature of 3000°C for 200 seconds under the heat flux^-1. This new typeS-C/C is promising for a wide range of applications from semiconductors to aerospace. Relevant results were published in the "Sugar-derived nanocrystalline graphite matrix C/C composites with excellent ablative resistance at 3000 C 《Advanced MaterialsThe first author isWenzheng Zhang，Mingyi Tan和 Daming ChenFor a common work.Figure1 shows a schematic diagram of the internal structure evolution when sugar-derived carbon is introduced into carbon fibers. First, the sugar is mixed with acrylamide and the crosslinking agent bisacrylamide to form an aqueous solution. After the aqueous solution permeates the carbon fiber, the acrylamide monomer undergoes gelation and crosslinking at 80°C to form a polymer network structure, thereby fixing the sugar in the gel network. Then, mesophase particles were formed after pre-carbonization at 200°C. Finally, after carbonization at 1200°C, the carbon in the saccharide material can be retained to form a sugar-based carbon (SC) matrix. Then, after a high temperature heat treatment at 3000 ° C., the SC matrix in the carbon fiber properties will be graphitized to form nano-graphite. Thus, the matrix of the S-C/C composite was sugar-based graphite (SG)(FIG. 1b). After graphitization, a uniform nanocrystalline graphite matrix structure is obtained, which is different from the uneven matrix carbon prepared by gas-phase hydrocarbon carbon source, the glassy carbon-like structure matrix carbon prepared by liquid-phase resin, and the large-size graphite matrix prepared by mesophase pitch.

Figure1: Schematic diagram of introducing SC into carbon fiber and forming SG matrix

Figure2a shows the HRTEM image of the SG matrix, from which it can be seen that the matrix is nanocrystalline graphite with a high degree of graphitization,graphite sheet spacingd(002) is 0.336 nm. Average crystal size between 10 nm and 20 nm. The author chose the figureThe microstructures in the different boxes in 2a were subjected to FTT transformation. Typical nano-graphite crystals have a variety of orientations in different regions, which is very obvious. Using high purity graphite as 100% sp2 reference sample, the sp2 content in SG was calculated by double window method.calculatedThe proportion of sp2 in SG and SC is 95.18 and 84.21 respectively. in addition,The peak energy loss of SG at 26.8 eV is higher than that of SC at 23.7 eV (Figure 2b). Compared with SC, the low loss EELS right shift of SG indicates that its graphitization degree has been improved. Here, the authors tested its thermal stability in air. It can also be seen from the heat flux curve that the highest exothermic oxidation peak of SG is 338°C higher than that of ISO-88. In addition, it can be clearly seen from the thermal stability data of different carbon materials that SG with a unique nanocrystalline structure has better thermal stability.The mechanism for better thermal stability is that nanocrystalline graphite may not be easily attacked by oxygen..

FigureNanostructure and thermal stability of 2SG matrix and ISO-88

InIn C/C composites, there is an interfacial zone between the fiber and the matrix. In the preparation of S-C/C composite material, it is necessary to repeat the process of mixing aqueous solution impregnation-gelation-pre-carbonization-carbonization-graphitization for many times, so that more than 90% of the pores of S-C/C composite material are filled. After densification, the surface of the S-C/C composite is very flat (Figure 3a), and the internal fiber bundles are filled with SG. The interfacial region between the SG matrix and the carbon fibers was microstructurally analyzed by FIB (Figure 3b). As can be seen from the bright field image, the microstructure difference between carbon fiber and SG matrix is very obvious. The area of Figure 3d-1 shows carbon fibers, which are typically amorphous carbon, and the area of Figure 3d-2 shows the amorphous interfacial region near the fibers. The area of Figure 3d-3 shows the microstructure of the nanocrystalline graphite matrix and the region of the amorphous interface away from the fiber where the amorphous diffraction pattern gradually disappears. The regions shown in FIG. 3d-4 exhibit good crystalline morphology. As can be seen from the above microstructural evolution of the entire interface,InThere is an amorphous transition layer between the SG matrix and the carbon fiber.Figure3: Microstructure and interface morphology of S-C/C composites

When used as a hot structural component in a spacecraft,C/C composites will encounter severe high temperature ablation and heat flux erosion environment, which can be simulated by arc wind tunnel device. Figure 4a shows the temperature profile and macroscopic morphology of the C/C composite after 200 seconds of ablation at the arc wind tunnel stagnation point.The final line ablation rate is0.03 mm s^-1The ablation performance is better than that of similar materials reported in the literature.. DueS-C/C has a high thermal conductivity, and the surface response temperature of the material gradually increases with time. Figure 4d shows that the sample surface is covered by a layer of air during the first 30 seconds, which can effectively protect the surface material from oxidation attack. The reason for this phenomenon is that the pyrolysis gases in the carbon material can react with the oxygen in the boundary layer, thereby slowing the ablation of the material surface. After 30 seconds of heat flux, the top of the sample gradually warms up and emits light until about 90 seconds, when the surface reaction temperature gradually exceeds 1000°C. The sample gradually recedes under the action of heat flux, but the overall shape is still cylindrical. After 120 seconds, the temperature is gradually increased to above 2500°C and the sample emits light. After approximately 160 seconds, the temperature rose sharply to 3000°C and this maximum temperature was maintained until the end of the test. It is worth mentioning that the S-C/C remained cylindrical throughout the ablation process, and the ablation process was very uniform. Figure 4f compares macroscopic photographs of the sample before and after ablation. It can be seen from the photos after ablation that the surface of the sample remains flat, and there is no collapse or large area peeling. Analysis of the microstructure of the sample after ablation shows that the surface is very flat (Figure 4d). After testing in a high heat flux environment, it is confirmed that the S-C/C composite has excellent resistance to ablation and erosion, and does not appear. Spalling and obvious shape change.Figure4: S-C/C arc wind tunnel ablation performance test results

The mechanical properties of S-C/C are bending strength up to 187.24 MPa, tensile strength up to 231.16 MPa, and compressive strength up to 113.65 MPa(Fig.5a), comparable to similar materials reported in the literature. In terms of thermophysical properties, the thermal conductivity of the S-C/C prepared in this study is as high as 117.6W m in the xy direction at room temperature.^-1K^-1,The thermal conductivity in the z direction exceeds 60 W m-1 K^-1. Figure5C compares the ablation resistance of C/C composites prepared by different carbon sources. The ablation rate of C/C composites prepared from hydrocarbon carbon sources is greater than 0.05mm s under high temperature or high heat flux conditions^-1, much higherS-C/C composite. The above results clearly show thatCarbon prepared by substituting carbohydrates for non-renewable carbon sources of hydrocarbons/Carbon composites also have excellent ablative resistance.Figure5: Mechanical and Thermophysical Properties of S-C/C Composites

In summary, the author proposes a new method for preparing carbon fiber reinforced carbon matrix composites, that is, forming nano-crystalline graphite matrix in carbon fiber, which is made of sugar-derived carbon source. This is a milestone in the development of carbon fiber reinforced carbon matrix composites.The S-C/C matrix has excellent thermal stability, and the initial oxidation temperature is higher than that of other types of carbon. Unlike the process of preparing carbon/carbon composites from petrochemical raw materials, S-C/C composites do not require the deposition of a PyC interphase layer. It is intriguing that the interfacial transition zone can be formed between the SG matrix and the carbon fiber, thus forming a strong bond between the matrix, which ensures the excellent thermal and mechanical properties of the S-C/C composite. By 10 MPa m in an arc-shaped wind tunnel environment.^-2The ablation resistance of the material was verified by an ablation test of 200 seconds.After evaluation, the surface response temperature of the material exceeds3000°C with an ablation rate of 0.03mm s^-1, showing excellent ablation resistance.TheSG adds new members to the carbon/carbon composite family, provides new ideas for researchers and industrial production of carbon-based composites, and is expected to make carbon-based composites be applied in more fields.

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