Porous ultrahigh temperature ceramics(UHTCs) are potential candidates as reusable thermal protection materials of transpiration cooling system in scramjet engine. However, low strength and low porosity are the main li...Porous ultrahigh temperature ceramics(UHTCs) are potential candidates as reusable thermal protection materials of transpiration cooling system in scramjet engine. However, low strength and low porosity are the main limitations of porous UHTCs. To overcome these problems, herein, a new and simple in-situ reaction/partial sintering process has been developed for preparing high strength and high porosity porous YB2C2. In this process, a simple gas-releasing in-situ reaction has been designed, and the formation and escape of gases can block the shrinkage during sintering process, which is favorable to increase the porosity of porous YB2C2. In order to demonstrate the advantages of the new method, porous YB2C2 ceramics have been fabricated from Y2O3, BN and graphite powders for the first time. The as-prepared porous YB2C2 ceramics possess high porosity of 57.17%–75.26% and high compressive strength of 9.32–34.78 MPa.The porosity, sintered density, radical shrinkage and compressive strength of porous YB2C2 ceramics can be controlled simply by changing the green density. Due to utilization of graphite as the carbon source, the porous YB2C2 ceramics show anisotropy in microstructure and mechanical behavior. These features render the porous YB2C2 ceramics promising as a thermal-insulating light-weight component for transpiration cooling system.展开更多
Layered compounds play pivotal roles as precursors for producing 2D materials through mechanical exfoliation(micro-mechanical cleavage) or chemical approaches. Therefore, searching for layered compounds with sharp ani...Layered compounds play pivotal roles as precursors for producing 2D materials through mechanical exfoliation(micro-mechanical cleavage) or chemical approaches. Therefore, searching for layered compounds with sharp anisotropic chemical bonding and properties becomes emergent. In this work, the stability, electronic structure, elastic properties, and lattice dynamics of YBCwere investigated. Strong anisotropy in elastic properties is revealed, i.e., high Young’s modulus in a-b plane but low Young’s modulus in c direction. The maximum to minimum Young’s modulus ratio is 2.41 and 2.45 for YBCwith P42/mmc and P4/mbm symmetry, respectively. The most likely systems for shear sliding or microdelaminating are(001)[100] and(001)[010]. The anisotropic elastic properties are underpinned by the anisotropic chemical bonding, i.e., strong bonding within the BCnets and weak bonding between Y atom layers and BCnets. YBCis electrically conductive and the contributions to the electrical conductivity are from delocalized Y 4deas well as Bpandpzelectrons. The layered crystal structure, sharp anisotropic mechanical properties, and metallic conductivity endorse YBCpromising as a precursor for new 2D BCnets.展开更多
基金supported by the National Natural Science Foundation of China under Grant Nos. U1435206 and 51672064by the Beijing Municipal Science & Technology Commission under Grant No. D161100002416001
文摘Porous ultrahigh temperature ceramics(UHTCs) are potential candidates as reusable thermal protection materials of transpiration cooling system in scramjet engine. However, low strength and low porosity are the main limitations of porous UHTCs. To overcome these problems, herein, a new and simple in-situ reaction/partial sintering process has been developed for preparing high strength and high porosity porous YB2C2. In this process, a simple gas-releasing in-situ reaction has been designed, and the formation and escape of gases can block the shrinkage during sintering process, which is favorable to increase the porosity of porous YB2C2. In order to demonstrate the advantages of the new method, porous YB2C2 ceramics have been fabricated from Y2O3, BN and graphite powders for the first time. The as-prepared porous YB2C2 ceramics possess high porosity of 57.17%–75.26% and high compressive strength of 9.32–34.78 MPa.The porosity, sintered density, radical shrinkage and compressive strength of porous YB2C2 ceramics can be controlled simply by changing the green density. Due to utilization of graphite as the carbon source, the porous YB2C2 ceramics show anisotropy in microstructure and mechanical behavior. These features render the porous YB2C2 ceramics promising as a thermal-insulating light-weight component for transpiration cooling system.
基金supported by the National Science Foundation of China under Grant No. U1435206 and No. 51672064the Beijing Municipal Science & Technology Commission under grant number Z151100003315012 and D161100002416001
文摘Layered compounds play pivotal roles as precursors for producing 2D materials through mechanical exfoliation(micro-mechanical cleavage) or chemical approaches. Therefore, searching for layered compounds with sharp anisotropic chemical bonding and properties becomes emergent. In this work, the stability, electronic structure, elastic properties, and lattice dynamics of YBCwere investigated. Strong anisotropy in elastic properties is revealed, i.e., high Young’s modulus in a-b plane but low Young’s modulus in c direction. The maximum to minimum Young’s modulus ratio is 2.41 and 2.45 for YBCwith P42/mmc and P4/mbm symmetry, respectively. The most likely systems for shear sliding or microdelaminating are(001)[100] and(001)[010]. The anisotropic elastic properties are underpinned by the anisotropic chemical bonding, i.e., strong bonding within the BCnets and weak bonding between Y atom layers and BCnets. YBCis electrically conductive and the contributions to the electrical conductivity are from delocalized Y 4deas well as Bpandpzelectrons. The layered crystal structure, sharp anisotropic mechanical properties, and metallic conductivity endorse YBCpromising as a precursor for new 2D BCnets.
