Finding the optimum balance between strength and toughness,as well as acquiring reliable thermal shock resistance and oxidation resistance,has always been the most concerned topic in the discussion of ultra-high tempe...Finding the optimum balance between strength and toughness,as well as acquiring reliable thermal shock resistance and oxidation resistance,has always been the most concerned topic in the discussion of ultra-high temperature ceramic composites.Herein,PyC modified 3D carbon fiber is used to reinforce ultra-high temperature ceramic(UHTC).The macroscopic block composite with large size is successfully fabricated through low temperature sintering at 1300℃without pressure.The prepared PyC modified 3D C_(f)/ZrC-SiC composites simultaneously possess excellent physical and chemical stability under the synergistic effect of PyC interface layer and low temperature 1/2 sintering without pressure.The fracture toughness is increased in magnitude to 13.05±1.72 MPa·m^(1/2)accompanied by reliable flexural strength of 251±27 MPa.After rapid thermal shock spanning from room temperature(RT)to 1200℃,there are no visible surface penetrating cracks,spalling,or structural fragmentation.The maximum critical temperature difference reaches 875℃,which is nearly three times higher than that of traditional monolithic ceramics.The haunting puzzle of intrinsic brittleness and low damage tolerance are resolved fundamentally.Under the protection of PyC interface layer,the carbon fibers around oxide layer and matrix remain structure intact after static oxidation at 1500℃for 30 min.The oxide layer has reliable physical and chemical stability and resists the erosion from fierce oxidizing atmosphere,ensuring the excellent oxidation resistance of the composites.In a sense,the present work provides promising universality in designability and achievement of 3D carbon fiber reinforced ceramic composites.展开更多
As for the air plasma sprayed ZrB_(2)-based coatings,B content change caused by inevitable oxidation is predictable but commonly ignored.Affected by air plasma flame,the B element loss and residual B_(2) O_(3) in the ...As for the air plasma sprayed ZrB_(2)-based coatings,B content change caused by inevitable oxidation is predictable but commonly ignored.Affected by air plasma flame,the B element loss and residual B_(2) O_(3) in the sprayed ZrB_(2) coating were observed.Moreover,how the B content change affects the microstructure,phase evolution,and ablation resistance(2.4 MW/m^(2),60 s)of ZrB_(2)-based coatings with different sec-ondary phases(SiC,MoSi_(2),and TaC)was investigated.The B element loss contributed to the increase in surface temperature and the decline in the sintering degree of the ZrO_(2) layer.The evaporation of residual B_(2)O_(3) caused damage to the coating structure in the form of pores,whose negative effect was enhanced and reduced by MoSi_(2) and TaC secondary phases,respectively.This work will provide some insight into thermally sprayed non-oxide ceramic coatings in the atmosphere.展开更多
Ultra-high temperature ceramics(UHTCs)exhibit a unique combination of excellent properties,including ultra-high melting point,excellent chemical stability,and good oxidation resistance,which make them promising candid...Ultra-high temperature ceramics(UHTCs)exhibit a unique combination of excellent properties,including ultra-high melting point,excellent chemical stability,and good oxidation resistance,which make them promising candidates for aerospace and nuclear applications.However,the degradation of hightemperature strength is one of the main limitations for their ultra-high temperature applications.Thus,searching for mechanisms that can help to develop high-performance UHTCs with good high-temperature mechanical properties is urgently needed.To achieve this goal,grain boundary segregation of a series of carbides,including conventional,medium entropy,and high entropy transition metal carbides,i.e.,Zr_(0.95)W_(0.05)C,TiZrHfC_(3),ZrHfNbTaC_(4),TiZrHfNbTaC_(5),were studied by atomistic simulations with a fitted Deep Potential(DP),and the effects of segregation on grain boundary strength were emphasized.For all the studied carbides,grain boundary segregations are realized,which are dominated by the atomic size effect.In addition,tensile simulations indicate that grain boundaries(GBs)will usually be strengthened due to segregation.Our simulation results reveal that grain boundary segregation may be a universal mechanism in enhancing the high-temperature strength of both conventional UHTCs and medium/high entropy UHTCs,since GBs play a key role in controlling the fracture of UHTCs at elevated temperatures.展开更多
Hf-based carbides are highly desirable candidate materials for oxidizing environments above 2000℃.However,the static oxidation behavior at their potential service temperatures remains unclear.To fill this gap,the sta...Hf-based carbides are highly desirable candidate materials for oxidizing environments above 2000℃.However,the static oxidation behavior at their potential service temperatures remains unclear.To fill this gap,the static oxidation behavior of(Hf,Ti)C and the effect of Ti substitutions were investigated in air at 2500℃ under an oxygen partial pressure of 4.2 kPa.After oxidation for 2000 s,the thickness of the oxide layer on the surface of(Hf,Ti)C bulk ceramic is reduced by 62.29%compared with that on the HfC monocarbide surface.The dramatic improvement in oxidation resistance is attributed to the unique oxide layer structure consisting of various crystalline oxycarbides,HfO_(2),and carbon.The Ti-rich oxycarbide((Ti,Hf)C_(x)O_(y))dispersed within HfO_(2) formed the major structure of the oxide layer.A coherent boundary with lattice distortion existed at the HfO2/(Ti,Hf)C_(x)O_(y) interface along the(111)crystal plane direction,which served as an effective oxygen diffusion barrier.The Hfrich oxycarbide((Hf,Ti)CxOy)together with(Ti,Hf)C_(x)O_(y),HfO_(2),and precipitated carbon constituted a dense transition layer,ensuring favorable bonding between the oxide layer and the matrix.The Ti content affects the oxidation resistance of(Hf,Ti)C by determining the oxide layer's phase distribution and integrity.展开更多
Multiphase design is a promising approach to achieve superior ablation resistance of multicomponent ultra-high temperature ceramic,while understanding the ablation mechanism is the foundation.Here,through investigatin...Multiphase design is a promising approach to achieve superior ablation resistance of multicomponent ultra-high temperature ceramic,while understanding the ablation mechanism is the foundation.Here,through investigating a three-phase multicomponent ceramic consisting of Hf-rich carbide,Nb-rich carbide,and Zr-rich silicide phases,we report a newly discovered solid-state reaction process among multiphase multicomponent ceramic during ablation.It was found that this solid-state reaction occurred in the matrix/oxide scale interface region.In this process,metal cations are counter-diffused between the multicomponent phases,thereby resulting in their composition evolution,which allows the multicomponent phases to exist stably under a higher oxygen partial pressure,leading to the improvement of thermodynamic stability of three-phase multicomponent ceramic.Additionally,this solid-state reaction process appears synergistic with the preferential oxidation behavior among the oxide scale in enhancing the ablation performance.展开更多
Urban human thermal comfort(UHTC) is affected for interacting of weather condition and underlying surface framework of urban area. Urban underlying surface temperature value and Normalized Difference Vegetation Index(...Urban human thermal comfort(UHTC) is affected for interacting of weather condition and underlying surface framework of urban area. Urban underlying surface temperature value and Normalized Difference Vegetation Index(NDVI) were calculated using image interpreting and supervised classification technique by ERDAS IMAGE software using 1991 and 1999 Landsat TM images data. Reference to the relational standard of assessing human thermal comfort and other meteorology data of Hangzhou City in summer, air temperature and relative humidity variation of different land types of underlying surface were inversed. By choosing discomfort index as an indictor, the spatial distribution characteristic and the spatial variation degree of UHTC were estimated and mapped on a middle scale, that is, in six districts of Hangzhou. The main characteristics of UHTC spatial variation from 1991 to 1999 were revealed using a GIS-based calculation model. The variation mechanism were analyzed and discussed from the viewpoint of city planning, construction and environmental protection.展开更多
A novelty technique,namely,pre-oxidation,has been proposed to improve the strength and thermal shock behavior of ZrB2-SiC-graphite ceramic composites,which is a promising candidate for ultra-high temperature ceramics ...A novelty technique,namely,pre-oxidation,has been proposed to improve the strength and thermal shock behavior of ZrB2-SiC-graphite ceramic composites,which is a promising candidate for ultra-high temperature ceramics (UHTCs) in aerospace engineering. The composite is pre-oxidized at temperatures of 900 ℃ and 1100 ℃ for 1 h and 3 h in air. A theoretical model considering surface heat transfer has been given and analyzed. A water-quenching technique is adopted at different temperatures from 300 ℃ to 600 ℃ in order to investigate the thermal shock behavior of pre-oxidized composites. The critical thermal shock temperature improves more than 40% after pre-oxidizing at 1100 ℃ for 3 h. However,the composites strength is not improved as a result of the insensitivity to surface defects.展开更多
ZrB_2-SiC based ultra-high temperature ceramic（UHTC） struts were firstly proposed and fabricated with the potential application in the combustor of scramjets for fuel injection and flame-holding for their machinabil...ZrB_2-SiC based ultra-high temperature ceramic（UHTC） struts were firstly proposed and fabricated with the potential application in the combustor of scramjets for fuel injection and flame-holding for their machinability and excellent oxidation/ablation resistance in the extreme harsh environment. The struts were machined with electrospark wire-electrode cutting techniques to form UHTC into the desired shape, and with laser drilling to drill tiny holes providing the channels for fuel injection. The integrated thermal-structural characteristic of the struts was evaluated in high-temperature combustion environment by the propane-oxygen free jet facility, subject to the heat flux of 1.5 MW/m^2 lasting for 300 seconds, and the struts maintained integrity during and after the first experiment. The experiments were repeated for verifying the reusability of the struts. Fracture occurred during the second repeated experiment with the crack propagating through the hole. Finite element analysis（FEA） was carried out to study the thermal stress distribution in the UHTC strut. The simulation results show a high thermal stress concentration occurs at the hole which is the crack initiation position. The phenomenon is in good agreement with the experimental results. The study shows that the thermal stress concentration is a practical key issue in the applications of the reusable UHTC strut for fuel injection structure in scramjets.展开更多
基金supported by Key Program of National Natural Science Foundation of China(No.52032003)National Natural Science Foundation of China(Nos.51872059 and 51772061)+2 种基金Science Foundation of the National Key Laboratory of Science and Technology on Advanced Composites in Special Environments(No.6142905202112)China Postdoctoral Science Foundation(No.2021M690817)Heilongjiang Provincial Postdoctoral Science Foundation(No.LBH-Z20144).
文摘Finding the optimum balance between strength and toughness,as well as acquiring reliable thermal shock resistance and oxidation resistance,has always been the most concerned topic in the discussion of ultra-high temperature ceramic composites.Herein,PyC modified 3D carbon fiber is used to reinforce ultra-high temperature ceramic(UHTC).The macroscopic block composite with large size is successfully fabricated through low temperature sintering at 1300℃without pressure.The prepared PyC modified 3D C_(f)/ZrC-SiC composites simultaneously possess excellent physical and chemical stability under the synergistic effect of PyC interface layer and low temperature 1/2 sintering without pressure.The fracture toughness is increased in magnitude to 13.05±1.72 MPa·m^(1/2)accompanied by reliable flexural strength of 251±27 MPa.After rapid thermal shock spanning from room temperature(RT)to 1200℃,there are no visible surface penetrating cracks,spalling,or structural fragmentation.The maximum critical temperature difference reaches 875℃,which is nearly three times higher than that of traditional monolithic ceramics.The haunting puzzle of intrinsic brittleness and low damage tolerance are resolved fundamentally.Under the protection of PyC interface layer,the carbon fibers around oxide layer and matrix remain structure intact after static oxidation at 1500℃for 30 min.The oxide layer has reliable physical and chemical stability and resists the erosion from fierce oxidizing atmosphere,ensuring the excellent oxidation resistance of the composites.In a sense,the present work provides promising universality in designability and achievement of 3D carbon fiber reinforced ceramic composites.
基金supported by the National Key R&D Program of China(No.2021YFA0715800)the Science Center for Gas Turbine Project(No.P2021-A-IV-003-001)+1 种基金the National Nat-ural Science Foundation of China(No.52125203)the Innova-tion Foundation for Doctor Dissertation of Northwestern Polytech-nical University(No.CX2021007)。
文摘As for the air plasma sprayed ZrB_(2)-based coatings,B content change caused by inevitable oxidation is predictable but commonly ignored.Affected by air plasma flame,the B element loss and residual B_(2) O_(3) in the sprayed ZrB_(2) coating were observed.Moreover,how the B content change affects the microstructure,phase evolution,and ablation resistance(2.4 MW/m^(2),60 s)of ZrB_(2)-based coatings with different sec-ondary phases(SiC,MoSi_(2),and TaC)was investigated.The B element loss contributed to the increase in surface temperature and the decline in the sintering degree of the ZrO_(2) layer.The evaporation of residual B_(2)O_(3) caused damage to the coating structure in the form of pores,whose negative effect was enhanced and reduced by MoSi_(2) and TaC secondary phases,respectively.This work will provide some insight into thermally sprayed non-oxide ceramic coatings in the atmosphere.
基金supported by the National Natural Science Foundation of China(No.51672064)。
文摘Ultra-high temperature ceramics(UHTCs)exhibit a unique combination of excellent properties,including ultra-high melting point,excellent chemical stability,and good oxidation resistance,which make them promising candidates for aerospace and nuclear applications.However,the degradation of hightemperature strength is one of the main limitations for their ultra-high temperature applications.Thus,searching for mechanisms that can help to develop high-performance UHTCs with good high-temperature mechanical properties is urgently needed.To achieve this goal,grain boundary segregation of a series of carbides,including conventional,medium entropy,and high entropy transition metal carbides,i.e.,Zr_(0.95)W_(0.05)C,TiZrHfC_(3),ZrHfNbTaC_(4),TiZrHfNbTaC_(5),were studied by atomistic simulations with a fitted Deep Potential(DP),and the effects of segregation on grain boundary strength were emphasized.For all the studied carbides,grain boundary segregations are realized,which are dominated by the atomic size effect.In addition,tensile simulations indicate that grain boundaries(GBs)will usually be strengthened due to segregation.Our simulation results reveal that grain boundary segregation may be a universal mechanism in enhancing the high-temperature strength of both conventional UHTCs and medium/high entropy UHTCs,since GBs play a key role in controlling the fracture of UHTCs at elevated temperatures.
基金This work was supported by the National Natural Science Foundation of China grant numbers[52072410].
文摘Hf-based carbides are highly desirable candidate materials for oxidizing environments above 2000℃.However,the static oxidation behavior at their potential service temperatures remains unclear.To fill this gap,the static oxidation behavior of(Hf,Ti)C and the effect of Ti substitutions were investigated in air at 2500℃ under an oxygen partial pressure of 4.2 kPa.After oxidation for 2000 s,the thickness of the oxide layer on the surface of(Hf,Ti)C bulk ceramic is reduced by 62.29%compared with that on the HfC monocarbide surface.The dramatic improvement in oxidation resistance is attributed to the unique oxide layer structure consisting of various crystalline oxycarbides,HfO_(2),and carbon.The Ti-rich oxycarbide((Ti,Hf)C_(x)O_(y))dispersed within HfO_(2) formed the major structure of the oxide layer.A coherent boundary with lattice distortion existed at the HfO2/(Ti,Hf)C_(x)O_(y) interface along the(111)crystal plane direction,which served as an effective oxygen diffusion barrier.The Hfrich oxycarbide((Hf,Ti)CxOy)together with(Ti,Hf)C_(x)O_(y),HfO_(2),and precipitated carbon constituted a dense transition layer,ensuring favorable bonding between the oxide layer and the matrix.The Ti content affects the oxidation resistance of(Hf,Ti)C by determining the oxide layer's phase distribution and integrity.
基金supported by the National Natural Science Foundation of China(52072410 and 51602349).
文摘Multiphase design is a promising approach to achieve superior ablation resistance of multicomponent ultra-high temperature ceramic,while understanding the ablation mechanism is the foundation.Here,through investigating a three-phase multicomponent ceramic consisting of Hf-rich carbide,Nb-rich carbide,and Zr-rich silicide phases,we report a newly discovered solid-state reaction process among multiphase multicomponent ceramic during ablation.It was found that this solid-state reaction occurred in the matrix/oxide scale interface region.In this process,metal cations are counter-diffused between the multicomponent phases,thereby resulting in their composition evolution,which allows the multicomponent phases to exist stably under a higher oxygen partial pressure,leading to the improvement of thermodynamic stability of three-phase multicomponent ceramic.Additionally,this solid-state reaction process appears synergistic with the preferential oxidation behavior among the oxide scale in enhancing the ablation performance.
文摘Urban human thermal comfort(UHTC) is affected for interacting of weather condition and underlying surface framework of urban area. Urban underlying surface temperature value and Normalized Difference Vegetation Index(NDVI) were calculated using image interpreting and supervised classification technique by ERDAS IMAGE software using 1991 and 1999 Landsat TM images data. Reference to the relational standard of assessing human thermal comfort and other meteorology data of Hangzhou City in summer, air temperature and relative humidity variation of different land types of underlying surface were inversed. By choosing discomfort index as an indictor, the spatial distribution characteristic and the spatial variation degree of UHTC were estimated and mapped on a middle scale, that is, in six districts of Hangzhou. The main characteristics of UHTC spatial variation from 1991 to 1999 were revealed using a GIS-based calculation model. The variation mechanism were analyzed and discussed from the viewpoint of city planning, construction and environmental protection.
基金Sponsored by the National Natural Science Foundation of China ( Grant No.10572044,90505015)the Program for New Century Excellent Talents in University (Grant No.NCET-05-0346)
文摘A novelty technique,namely,pre-oxidation,has been proposed to improve the strength and thermal shock behavior of ZrB2-SiC-graphite ceramic composites,which is a promising candidate for ultra-high temperature ceramics (UHTCs) in aerospace engineering. The composite is pre-oxidized at temperatures of 900 ℃ and 1100 ℃ for 1 h and 3 h in air. A theoretical model considering surface heat transfer has been given and analyzed. A water-quenching technique is adopted at different temperatures from 300 ℃ to 600 ℃ in order to investigate the thermal shock behavior of pre-oxidized composites. The critical thermal shock temperature improves more than 40% after pre-oxidizing at 1100 ℃ for 3 h. However,the composites strength is not improved as a result of the insensitivity to surface defects.
基金Funded by the Major Research Plan of the National Natural Science Foundation of China(No.91216302)the Major State Basic Research Development Program of China(973 Program)(No.2015CB655200)the National Natural Science Foundation of China(Nos.11672088,11472092,and 11502058)
文摘ZrB_2-SiC based ultra-high temperature ceramic（UHTC） struts were firstly proposed and fabricated with the potential application in the combustor of scramjets for fuel injection and flame-holding for their machinability and excellent oxidation/ablation resistance in the extreme harsh environment. The struts were machined with electrospark wire-electrode cutting techniques to form UHTC into the desired shape, and with laser drilling to drill tiny holes providing the channels for fuel injection. The integrated thermal-structural characteristic of the struts was evaluated in high-temperature combustion environment by the propane-oxygen free jet facility, subject to the heat flux of 1.5 MW/m^2 lasting for 300 seconds, and the struts maintained integrity during and after the first experiment. The experiments were repeated for verifying the reusability of the struts. Fracture occurred during the second repeated experiment with the crack propagating through the hole. Finite element analysis（FEA） was carried out to study the thermal stress distribution in the UHTC strut. The simulation results show a high thermal stress concentration occurs at the hole which is the crack initiation position. The phenomenon is in good agreement with the experimental results. The study shows that the thermal stress concentration is a practical key issue in the applications of the reusable UHTC strut for fuel injection structure in scramjets.
