Ultra-high temperature materials are desirable to withstand the severe aero-thermochemical environments of hypersonic flight,paving the groundworks for flight speeds exceeding Mach 5.Here,we present a novel ultra-high...Ultra-high temperature materials are desirable to withstand the severe aero-thermochemical environments of hypersonic flight,paving the groundworks for flight speeds exceeding Mach 5.Here,we present a novel ultra-high temperature composite with superior ablation resistances up to 3000℃for 900 s,utilizing a tailored ultra-high melting point HfC_(0.76)N_(0.24)matrix reinforced with short carbon fibers.The ablation-resistant capability of this composite is over 14 times greater than that of HfC at 3000℃.Furthermore,this research presents the first comprehensive investigation into the internal mechanisms governing thermal oxidation evolution of HfC_(0.76)N_(0.24)matrix through a combination of experimental results and theoretical simulations.The mechanistic details of these complex oxidation processes are elucidated in terms of chemical bonding and clusters evolutions,along with their relationship to cooperative oxygen atoms and molecules.Notably,nitrogen atoms do not directly generate gas and escape from the composites,rather,they interact with hafnium atoms to form Hf-C-N-O clusters with robust bonding for enhanced viscosity during ablation.These findings provide valuable insights into the transition from micro to macro scales,which will be the paradigm of inspiring and accelerating materials discovery in this field,as well as taking advantage of their full potential in the application of hypersonic aircraft and spacecraft vehicles.展开更多
Ultrahigh-temperature ceramics(UHTCs)are prominent candidates for use in thermal protection systems in the aerospace and nuclear industries.This study introduces a nitrogen-doped zirconium carbide that demonstrates re...Ultrahigh-temperature ceramics(UHTCs)are prominent candidates for use in thermal protection systems in the aerospace and nuclear industries.This study introduces a nitrogen-doped zirconium carbide that demonstrates remarkable ablation resistance,outperforming conventional carbide ceramics.The oxidation mechanisms of this material are elucidated through experimental and ab initio molecular dynamics simulations,representing the first analysis of such ultrahigh melting point ceramics from the perspective of structural development during the oxidation process.Transmission electron microscopy(TEM)analysis revealed the precipitation of nanocarbon and Zr–C–N–O phases at the interface between the oxidized and unoxidized regions following nitrogen doping.Nitrogen atoms preferentially combine with zirconium atoms at temperatures below the melting point of the oxide,forming robust Zr-C-N-O oxide network structures.These structures minimize oxide loss and maintain integrity during ablation,enhancing the material's performance in extreme environments.This study underscores nitrogen doping as a promising strategy to improve the ablation resistance of UHTCs,offering valuable insights for their application under demanding conditions.展开更多
Diffusion shaped film cooling holes with compound-angle diffuser structures exhibit superior cooling performance,which have gradually been applied in turbine blades of the advanced aeroengines.In our previous research...Diffusion shaped film cooling holes with compound-angle diffuser structures exhibit superior cooling performance,which have gradually been applied in turbine blades of the advanced aeroengines.In our previous research,the method of Servo Scanning three-dimensional Electrical Discharge Machining(SS-3D EDM)has been proven effective for high-precision machining of complex 3D cavities,offering notable advantages such as low tool cost,automatic compensation of electrode wear,and high machining flexibility.However,using tubular electrodes in SS-3D EDM,challenges persist under the conditions of the large layer depth.The lateral discharge phenomenon of tubular electrodes causes significant deformation at the electrode tip,increases the risk of lateral collisions,and complicates the accurate calculation of electrode wear length.To address these limitations,this research proposes a Trajectory Servo Scanning three-dimensional Electrical Discharge Machining(TSS-3D EDM)process.Axial servo motion of tubular electrode is used to maintain the discharge gap of electrode bottom,and an innovative trajectory servo motion along the tangential orientation is introduced to stabilize the lateral discharge gap,enabling automatic compensation for tool wear at the rotating electrode tip.The effect of servo control parameters on machining depth accuracy is analyzed.Furthermore,a method for electrode wear length calculation is proposed based on the voltage signals of discharge gaps.An estimation method for the electrode wear coefficient is presented.Machining experiments on superalloys validate the effectiveness and capabilities of the TSS-3D EDM method by fabricating fan-shaped and conical diffusion shaped film cooling holes.The results show that the calculation error of tubular electrode wear length<5%,the dimensional error of hole profile dimensions as 4%–6%,the repeatability error<±4μm,and the material removal rate up to 0.664 mm^(3)/min using tubular electrodes with an outer diameter of 0.4 mm.展开更多
Agricultural land use(ALU)critically influences food production and water resource allocation.This study examines the dynamics of ALU in the North China Plain(NCP),a region characterized by intensive agri-culture and ...Agricultural land use(ALU)critically influences food production and water resource allocation.This study examines the dynamics of ALU in the North China Plain(NCP),a region characterized by intensive agri-culture and severe groundwater over-exploitation,focusing on the multidimensional drivers and their implications for water resource management.By employing an elaborate classification scheme based on satellite imagery and extensive first-hand field data,we identified significant shifts in crop patterns.From 2013 to 2017,there was a notable transition from double crops(primarily wheat-maize)to single crops(primarily maize),covering 4600 km^(2)and accounting for 42%of single crops in 2013.From 2017 to 2022,there was a shift from single crops to economic forests,encompassing 3600 km^(2)and 22%of eco-nomic forests in 2017,including orchards,timber trees,and shelter forest belts.These shifts resulted in an 11%decrease in grain acreage(6800 km^(2))but an 11%increase in crop water consumption(6.3 km^(3))during 2013-2022.Notably,water consumption by economic forests increased by 126%(9.4 km^(3))during this period.This study highlights the critical need to balance competing demands for food and water security,providing valuable insights applicable to other agriculturally intensive regions worldwide.展开更多
基金funding by the National Natural Science Foundation of China(52302128)the Foundation of State Key Laboratory of Science and Technology on Advanced Ceramic Fibers and Composites(No.6142907230303).
文摘Ultra-high temperature materials are desirable to withstand the severe aero-thermochemical environments of hypersonic flight,paving the groundworks for flight speeds exceeding Mach 5.Here,we present a novel ultra-high temperature composite with superior ablation resistances up to 3000℃for 900 s,utilizing a tailored ultra-high melting point HfC_(0.76)N_(0.24)matrix reinforced with short carbon fibers.The ablation-resistant capability of this composite is over 14 times greater than that of HfC at 3000℃.Furthermore,this research presents the first comprehensive investigation into the internal mechanisms governing thermal oxidation evolution of HfC_(0.76)N_(0.24)matrix through a combination of experimental results and theoretical simulations.The mechanistic details of these complex oxidation processes are elucidated in terms of chemical bonding and clusters evolutions,along with their relationship to cooperative oxygen atoms and molecules.Notably,nitrogen atoms do not directly generate gas and escape from the composites,rather,they interact with hafnium atoms to form Hf-C-N-O clusters with robust bonding for enhanced viscosity during ablation.These findings provide valuable insights into the transition from micro to macro scales,which will be the paradigm of inspiring and accelerating materials discovery in this field,as well as taking advantage of their full potential in the application of hypersonic aircraft and spacecraft vehicles.
基金funding by the National Natural Science Foundation of China(No.52302128)the Foundation of State Key Laboratory of Science and Technology on Advanced Ceramic Fibers and Composites(No.6142907230303).
文摘Ultrahigh-temperature ceramics(UHTCs)are prominent candidates for use in thermal protection systems in the aerospace and nuclear industries.This study introduces a nitrogen-doped zirconium carbide that demonstrates remarkable ablation resistance,outperforming conventional carbide ceramics.The oxidation mechanisms of this material are elucidated through experimental and ab initio molecular dynamics simulations,representing the first analysis of such ultrahigh melting point ceramics from the perspective of structural development during the oxidation process.Transmission electron microscopy(TEM)analysis revealed the precipitation of nanocarbon and Zr–C–N–O phases at the interface between the oxidized and unoxidized regions following nitrogen doping.Nitrogen atoms preferentially combine with zirconium atoms at temperatures below the melting point of the oxide,forming robust Zr-C-N-O oxide network structures.These structures minimize oxide loss and maintain integrity during ablation,enhancing the material's performance in extreme environments.This study underscores nitrogen doping as a promising strategy to improve the ablation resistance of UHTCs,offering valuable insights for their application under demanding conditions.
基金co-supported by the Tsinghua University Initiative Scientific Research Program,China(No.20244186005)the Science Center for Gas Turbine Project,China(No.P2022-A-IV-002-003)the National Natural Science Foundation of China(No.92060108)。
文摘Diffusion shaped film cooling holes with compound-angle diffuser structures exhibit superior cooling performance,which have gradually been applied in turbine blades of the advanced aeroengines.In our previous research,the method of Servo Scanning three-dimensional Electrical Discharge Machining(SS-3D EDM)has been proven effective for high-precision machining of complex 3D cavities,offering notable advantages such as low tool cost,automatic compensation of electrode wear,and high machining flexibility.However,using tubular electrodes in SS-3D EDM,challenges persist under the conditions of the large layer depth.The lateral discharge phenomenon of tubular electrodes causes significant deformation at the electrode tip,increases the risk of lateral collisions,and complicates the accurate calculation of electrode wear length.To address these limitations,this research proposes a Trajectory Servo Scanning three-dimensional Electrical Discharge Machining(TSS-3D EDM)process.Axial servo motion of tubular electrode is used to maintain the discharge gap of electrode bottom,and an innovative trajectory servo motion along the tangential orientation is introduced to stabilize the lateral discharge gap,enabling automatic compensation for tool wear at the rotating electrode tip.The effect of servo control parameters on machining depth accuracy is analyzed.Furthermore,a method for electrode wear length calculation is proposed based on the voltage signals of discharge gaps.An estimation method for the electrode wear coefficient is presented.Machining experiments on superalloys validate the effectiveness and capabilities of the TSS-3D EDM method by fabricating fan-shaped and conical diffusion shaped film cooling holes.The results show that the calculation error of tubular electrode wear length<5%,the dimensional error of hole profile dimensions as 4%–6%,the repeatability error<±4μm,and the material removal rate up to 0.664 mm^(3)/min using tubular electrodes with an outer diameter of 0.4 mm.
基金supported by the National Key Research and Development Program of China(2021YFB3900604)National Natural Science Foundation of China(52079065,52325901,and 42471399)World Bank China Programmatic Research on Water.
文摘Agricultural land use(ALU)critically influences food production and water resource allocation.This study examines the dynamics of ALU in the North China Plain(NCP),a region characterized by intensive agri-culture and severe groundwater over-exploitation,focusing on the multidimensional drivers and their implications for water resource management.By employing an elaborate classification scheme based on satellite imagery and extensive first-hand field data,we identified significant shifts in crop patterns.From 2013 to 2017,there was a notable transition from double crops(primarily wheat-maize)to single crops(primarily maize),covering 4600 km^(2)and accounting for 42%of single crops in 2013.From 2017 to 2022,there was a shift from single crops to economic forests,encompassing 3600 km^(2)and 22%of eco-nomic forests in 2017,including orchards,timber trees,and shelter forest belts.These shifts resulted in an 11%decrease in grain acreage(6800 km^(2))but an 11%increase in crop water consumption(6.3 km^(3))during 2013-2022.Notably,water consumption by economic forests increased by 126%(9.4 km^(3))during this period.This study highlights the critical need to balance competing demands for food and water security,providing valuable insights applicable to other agriculturally intensive regions worldwide.
