A Mo-Si-C-N multi-layered anti-oxidation coating was in-situ fabricated by introducing nitrogen atmosphere during the fused sintering of Mo-Si slurry pre-layer on carbon/carbon composites. The phase composition and mi...A Mo-Si-C-N multi-layered anti-oxidation coating was in-situ fabricated by introducing nitrogen atmosphere during the fused sintering of Mo-Si slurry pre-layer on carbon/carbon composites. The phase composition and microstructure of the Mo-Si-C-N coating were characterized by X-ray diffractometry, optical microscopy, scanning electron microscopy with energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. The Mo-Si-C-N coating exhibited a three-layered structure. Besides the MoSi2/Si main-layer and the SiC bonding-layer, a surface layer of about 10 μm in thickness was synthesized on the coating surface. The surface layer mainly consisted of SiC nanowires and contained some Si3N4 and Si phases. SiC nanowires of 10 to 200 nm in diameter presented a terrace and distortion structure. Transmission electron microscopy indicated that the SiC nanowires grew along the preferred 〈111〉 direction. During oxidation test, SiC nanowires transmuted into SiO2 glass, which can play an important role in improving the oxidation resistance of C/C composites.展开更多
Charring composites are widely used in the thermal protection system(TPS) to consume the intense aerodynamic heating during vehicle reentry. The ablation and thermal responses for the charring composites can be studie...Charring composites are widely used in the thermal protection system(TPS) to consume the intense aerodynamic heating during vehicle reentry. The ablation and thermal responses for the charring composites can be studied by using a numerical ablation model, in which the surface ablation and volume ablation could be taken into account. The coupling interactions among temperature, gas motion and interior pressure producing the pyrolysis gas could make the computation more complicated. A multi-physics model is developed to simulate the thermal response coupled with volume ablation and surface ablation. After studying four typical ablation cases, the model is validated, and then the heat transfer mechanisms in ablation are investigated. It is found that the viscous dissipation energy by the motion of pyrolysis gas can be neglected in the simulation. Also, the flow of pyrolysis gas plays an important role in the temperature response, especially under high heat flux condition.展开更多
It is significant to compare the ablative and compressive mechanical behavior of different PICA-like materials in the engineering applications.The plasma wind tunnel ablation tests with high-entropy air and CO2 atmosp...It is significant to compare the ablative and compressive mechanical behavior of different PICA-like materials in the engineering applications.The plasma wind tunnel ablation tests with high-entropy air and CO2 atmospheres,and compressive experiments in the ambient and 150℃,were conducted for three kinds of PICA-like materials(CF/PR-Si,CBCF/PR-SiOC and NQF/PR-Si composites).The traditional carbon/phenolic(C/PR)braided composites were also used for comparison.PICA-like materials have the better thermal insulation than traditional C/PR composite,especially for CBCF/PR-SiOC composite.The ablation behavior of CF/PR-Si and CBCF/PR-SiOC PICA-like materials in the CO2 atmosphere can produce a large amount of SiO2 in the form of coatings,oxide layers and skeletons on the ablated surface,which are greatly different from that in the air atmosphere.The compressive behavior of PICA-like material is greatly depended on the fiber fabrics,and exhibits the large discrete characteristics.The longer fiber in the PICA-like materials plays the role in maintaining the material integrity,while it may increase the thermal conductivity.展开更多
To reveal the properties of ZrO2 at the atom and electron levels,the valence elec-tron structures of three ZrO2 phases were analyzed on the basis of the empirical electron theory of solids and molecules.The results sh...To reveal the properties of ZrO2 at the atom and electron levels,the valence elec-tron structures of three ZrO2 phases were analyzed on the basis of the empirical electron theory of solids and molecules.The results showed that the hybridization levels of Zr and O atoms in the m-ZrO2 were the same as those in the t-ZrO2,while those in the c-ZrO2 rose markedly.The electron numbers and bond energies on the strongest covalent bonds in the m-ZrO2 phase were the greatest,the values were 0.901106 and 157.5933 kJ/mol,respectively.Those in the t-ZrO2 phase took second place,which were 0.722182 and 123.9304 kJ/mol,and those in the c-ZrO2 phase were the smallest,which were 0.469323 and 79.0289 kJ/mol.According to the product of the bond energy on the strongest covalent bond and equivalent bond number(this value reflected the crystal cohesive energy),the order from the greatness to smallness was the c-ZrO2>t-ZrO2>m-ZrO2.This showed that the m-phase bonds were the tightest,their energy was the smallest,the crystal cohe-sive energy of the m-phase was the largest,and the m-phase existed most stably at room temperature.So it must need energy or higher temperature to take apart the stronger covalent bonds to form a new phase.展开更多
Ultra-high temperature ceramic(UHTC) composites are widely used in high-temperature environments in aerospace applications. They experience extremely complex environmental conditions during service, including thermal,...Ultra-high temperature ceramic(UHTC) composites are widely used in high-temperature environments in aerospace applications. They experience extremely complex environmental conditions during service, including thermal, mechanical and chemical loading. Therefore, it is critical to evaluate the mechanical properties of UHTCs subject to an environment with elevated temperature, mechanical stress and oxygen. In this paper, an experimental investigation of the uniaxial tensile properties of a ZrB_2-SiC-graphite subject to an environment with a simultaneously elevated temperature, mechanical stress and oxygen is conducted based on a high-temperature mechanical testing system. To improve efficiency, an orthogonal experimental design is used. It is suggested that the temperature has the most important effect on the properties, and the oxidation time and stress have an almost equal effect. Finally, the fracture morphology is characterized using scanning electron microscopy(SEM), and the mechanism is investigated. It was concluded that the main fracture mode involved graphite flakes pulling out of the matrix and crystalline fracture, which indicates the presence of a weak interface in the composites.展开更多
To reveal the properties of stabilizers in ZrO2 on nanoscopic levels,the valence electron structures of four stable ZrO2 phases and c-ZrO2 were analyzed on the basis of the empirical electron theory of solids and mole...To reveal the properties of stabilizers in ZrO2 on nanoscopic levels,the valence electron structures of four stable ZrO2 phases and c-ZrO2 were analyzed on the basis of the empirical electron theory of solids and molecules.The results showed that the hybridization levels of Zr atoms in c-ZrO2 doped with Ca and Mg dropped from B17 to B13,the hybridization levels of Zr atoms in c-ZrO2 doped with Y and Ce dropped from B17 to B15,and that the four stabilizing atoms all made the hybridization levels of O atoms drop from level 4 to level 2.The numbers of covalent electrons in the strongest covalent bond in the descending order are c-ZrO2>Zr0.82Ce0.18O2>Zr0.82Y0.18O1.91>Zr0.82Mg0.18O1.82>Zr0.82Ca0.18O1.82.The bond energies of the strongest covalent bond and the melting points of the solid solutions in the descending order are Zr0.82Ce0.18O2>c-ZrO2>Zr0.82Y0.18O1.91>Zr0.82Mg0.18O1.82>Zr0.82Ca0.18O1.82.The percent-ages of the total number of covalent electrons in the descending order are c-ZrO2>Zr0.82Y0.18O1.91>Zr0.82Ce0.18O2>Zr0.82Mg0.18O1.82>Zr0.82Ca0.18O1.82.From the above analysis,it can be concluded that the stabilizing degrees of the four stabilizers in the descending order are CaO>MgO>Y2O3>CeO2.展开更多
Materials chemomechanics is an exciting and fast growing field where mechanics meets chemistry, and has drawn increasing attention in recent years. It is concerned with the phenomena and mechanisms of coupled chemical...Materials chemomechanics is an exciting and fast growing field where mechanics meets chemistry, and has drawn increasing attention in recent years. It is concerned with the phenomena and mechanisms of coupled chemical and mechanical interactions. Chemomechanical coupling phenomena exist in many research areas, ranging from the development of advanced batteries, biomechanical engineering, hydrogen embrittlement, and high temperature oxidation. In fact, a typical chemomechanical problem usually involves the processes of diffusion, chemical reaction, deformation and stress generation. Such a strong coupling between chemistry and mechanics can give rise to the complex spatial-temporal evolution of composition, microstructure and morphology in materials.展开更多
The Zr-rich(Zr0.8Ti0.2)B2 and the Ti-rich(Ti0.8Zr0.2)B2 solid solutions are formed when TiB2 and ZrB2 are hot-pressed.To forecast the properties of the two solid solutions,their valence electron structure was analyzed...The Zr-rich(Zr0.8Ti0.2)B2 and the Ti-rich(Ti0.8Zr0.2)B2 solid solutions are formed when TiB2 and ZrB2 are hot-pressed.To forecast the properties of the two solid solutions,their valence electron structure was analyzed based on the empirical electron theory(EET)of solids and molecules.We used three different models,the average atom model,the average cell model and the real cell model,and compared with the calculation results from the three models.In the real cell model,the lattice constants of the solid solu-tions were supposed to be changed or unchanged.The results showed that different models could only result in slight change in the hybridization levels of the metal atoms in the two solid solutions and little difference between the calculation values.However,they can not change the variant trend of the va-lence electron structure nor the properties of the solid solutions.Thus,the three models and the methods are appropriate and the calculation results are reasonable and consistent.展开更多
基金supported by the Foundation of Heilongjiang Province (No. LC04C16)the Foundation of Aerospace Technology (No. HTJSZC-0506) the Post-Doctor Foundation of Heilongjiang Province, China
文摘A Mo-Si-C-N multi-layered anti-oxidation coating was in-situ fabricated by introducing nitrogen atmosphere during the fused sintering of Mo-Si slurry pre-layer on carbon/carbon composites. The phase composition and microstructure of the Mo-Si-C-N coating were characterized by X-ray diffractometry, optical microscopy, scanning electron microscopy with energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. The Mo-Si-C-N coating exhibited a three-layered structure. Besides the MoSi2/Si main-layer and the SiC bonding-layer, a surface layer of about 10 μm in thickness was synthesized on the coating surface. The surface layer mainly consisted of SiC nanowires and contained some Si3N4 and Si phases. SiC nanowires of 10 to 200 nm in diameter presented a terrace and distortion structure. Transmission electron microscopy indicated that the SiC nanowires grew along the preferred 〈111〉 direction. During oxidation test, SiC nanowires transmuted into SiO2 glass, which can play an important role in improving the oxidation resistance of C/C composites.
基金supported by the National Natural Science Foundation of China(Grant Nos.11672089&11732002)the Natural Science Foundation of Heilongjiang Province,China(Grant No.A2017003)the Fundamental Research Funds for the Central Universities(Grant No.HIT.NSRIF.2017017)
文摘Charring composites are widely used in the thermal protection system(TPS) to consume the intense aerodynamic heating during vehicle reentry. The ablation and thermal responses for the charring composites can be studied by using a numerical ablation model, in which the surface ablation and volume ablation could be taken into account. The coupling interactions among temperature, gas motion and interior pressure producing the pyrolysis gas could make the computation more complicated. A multi-physics model is developed to simulate the thermal response coupled with volume ablation and surface ablation. After studying four typical ablation cases, the model is validated, and then the heat transfer mechanisms in ablation are investigated. It is found that the viscous dissipation energy by the motion of pyrolysis gas can be neglected in the simulation. Also, the flow of pyrolysis gas plays an important role in the temperature response, especially under high heat flux condition.
基金supported by the National Natural Science Foundation of China(Grant Nos.11672089,11732002&11902333)。
文摘It is significant to compare the ablative and compressive mechanical behavior of different PICA-like materials in the engineering applications.The plasma wind tunnel ablation tests with high-entropy air and CO2 atmospheres,and compressive experiments in the ambient and 150℃,were conducted for three kinds of PICA-like materials(CF/PR-Si,CBCF/PR-SiOC and NQF/PR-Si composites).The traditional carbon/phenolic(C/PR)braided composites were also used for comparison.PICA-like materials have the better thermal insulation than traditional C/PR composite,especially for CBCF/PR-SiOC composite.The ablation behavior of CF/PR-Si and CBCF/PR-SiOC PICA-like materials in the CO2 atmosphere can produce a large amount of SiO2 in the form of coatings,oxide layers and skeletons on the ablated surface,which are greatly different from that in the air atmosphere.The compressive behavior of PICA-like material is greatly depended on the fiber fabrics,and exhibits the large discrete characteristics.The longer fiber in the PICA-like materials plays the role in maintaining the material integrity,while it may increase the thermal conductivity.
基金the Major Project of the National Natural Science Foundation of China(Grant No.90505015)
文摘To reveal the properties of ZrO2 at the atom and electron levels,the valence elec-tron structures of three ZrO2 phases were analyzed on the basis of the empirical electron theory of solids and molecules.The results showed that the hybridization levels of Zr and O atoms in the m-ZrO2 were the same as those in the t-ZrO2,while those in the c-ZrO2 rose markedly.The electron numbers and bond energies on the strongest covalent bonds in the m-ZrO2 phase were the greatest,the values were 0.901106 and 157.5933 kJ/mol,respectively.Those in the t-ZrO2 phase took second place,which were 0.722182 and 123.9304 kJ/mol,and those in the c-ZrO2 phase were the smallest,which were 0.469323 and 79.0289 kJ/mol.According to the product of the bond energy on the strongest covalent bond and equivalent bond number(this value reflected the crystal cohesive energy),the order from the greatness to smallness was the c-ZrO2>t-ZrO2>m-ZrO2.This showed that the m-phase bonds were the tightest,their energy was the smallest,the crystal cohe-sive energy of the m-phase was the largest,and the m-phase existed most stably at room temperature.So it must need energy or higher temperature to take apart the stronger covalent bonds to form a new phase.
基金supported by the National Natural Science Foundation of China(Grant Nos.11472092,11672088,11502058)the National Basic Research Program of China(Grant No.2015CB655200)
文摘Ultra-high temperature ceramic(UHTC) composites are widely used in high-temperature environments in aerospace applications. They experience extremely complex environmental conditions during service, including thermal, mechanical and chemical loading. Therefore, it is critical to evaluate the mechanical properties of UHTCs subject to an environment with elevated temperature, mechanical stress and oxygen. In this paper, an experimental investigation of the uniaxial tensile properties of a ZrB_2-SiC-graphite subject to an environment with a simultaneously elevated temperature, mechanical stress and oxygen is conducted based on a high-temperature mechanical testing system. To improve efficiency, an orthogonal experimental design is used. It is suggested that the temperature has the most important effect on the properties, and the oxidation time and stress have an almost equal effect. Finally, the fracture morphology is characterized using scanning electron microscopy(SEM), and the mechanism is investigated. It was concluded that the main fracture mode involved graphite flakes pulling out of the matrix and crystalline fracture, which indicates the presence of a weak interface in the composites.
基金the Major Project of the National Natural Science Foundation of China(Grant No.90505015)
文摘To reveal the properties of stabilizers in ZrO2 on nanoscopic levels,the valence electron structures of four stable ZrO2 phases and c-ZrO2 were analyzed on the basis of the empirical electron theory of solids and molecules.The results showed that the hybridization levels of Zr atoms in c-ZrO2 doped with Ca and Mg dropped from B17 to B13,the hybridization levels of Zr atoms in c-ZrO2 doped with Y and Ce dropped from B17 to B15,and that the four stabilizing atoms all made the hybridization levels of O atoms drop from level 4 to level 2.The numbers of covalent electrons in the strongest covalent bond in the descending order are c-ZrO2>Zr0.82Ce0.18O2>Zr0.82Y0.18O1.91>Zr0.82Mg0.18O1.82>Zr0.82Ca0.18O1.82.The bond energies of the strongest covalent bond and the melting points of the solid solutions in the descending order are Zr0.82Ce0.18O2>c-ZrO2>Zr0.82Y0.18O1.91>Zr0.82Mg0.18O1.82>Zr0.82Ca0.18O1.82.The percent-ages of the total number of covalent electrons in the descending order are c-ZrO2>Zr0.82Y0.18O1.91>Zr0.82Ce0.18O2>Zr0.82Mg0.18O1.82>Zr0.82Ca0.18O1.82.From the above analysis,it can be concluded that the stabilizing degrees of the four stabilizers in the descending order are CaO>MgO>Y2O3>CeO2.
文摘Materials chemomechanics is an exciting and fast growing field where mechanics meets chemistry, and has drawn increasing attention in recent years. It is concerned with the phenomena and mechanisms of coupled chemical and mechanical interactions. Chemomechanical coupling phenomena exist in many research areas, ranging from the development of advanced batteries, biomechanical engineering, hydrogen embrittlement, and high temperature oxidation. In fact, a typical chemomechanical problem usually involves the processes of diffusion, chemical reaction, deformation and stress generation. Such a strong coupling between chemistry and mechanics can give rise to the complex spatial-temporal evolution of composition, microstructure and morphology in materials.
基金Supported by the National Natural Science Foundation of China(Grant No.90505015)
文摘The Zr-rich(Zr0.8Ti0.2)B2 and the Ti-rich(Ti0.8Zr0.2)B2 solid solutions are formed when TiB2 and ZrB2 are hot-pressed.To forecast the properties of the two solid solutions,their valence electron structure was analyzed based on the empirical electron theory(EET)of solids and molecules.We used three different models,the average atom model,the average cell model and the real cell model,and compared with the calculation results from the three models.In the real cell model,the lattice constants of the solid solu-tions were supposed to be changed or unchanged.The results showed that different models could only result in slight change in the hybridization levels of the metal atoms in the two solid solutions and little difference between the calculation values.However,they can not change the variant trend of the va-lence electron structure nor the properties of the solid solutions.Thus,the three models and the methods are appropriate and the calculation results are reasonable and consistent.
