With the development of automated driving vehicles, more and more vehicles will be fitted with more than one automotive radars, and the radar mutual interference will become very significant. Vehicle to everything (V2...With the development of automated driving vehicles, more and more vehicles will be fitted with more than one automotive radars, and the radar mutual interference will become very significant. Vehicle to everything (V2X) communication is a potential way for coordinating automotive radars and reduce the mutual interference. In this paper, we analyze the positional relation of the two radars that interfere with each other, and evaluate the mutual interference for different types of automotive radars based on Poisson point process (PPP). We also propose a centralized framework and the corresponding algorithm, which relies on V2X communication systems to allocate the spectrum resources for automotive radars to minimize the interference. The minimum spectrum resources required for zero-interference are analyzed for different cases. Simulation results validate the analysis and show that the proposed framework can achieve near-zero-interference with the minimum spectrum resources.展开更多
This paper examines various aspects of the design process and subsequent field test measurements of a large and complex substation grounding system. The study and measurements show that soil layering and lead interfer...This paper examines various aspects of the design process and subsequent field test measurements of a large and complex substation grounding system. The study and measurements show that soil layering and lead interference can have a significant impact on the appropriate test location that yields the exact substation ground impedance. Applying a specific percentage rule such as the 61.8% rule for uniform soils to obtain the true ground impedance may lead to unacceptable errors for large grounding systems. This poses significant problems when attempting to validate a design based on raw test data that are interpreted using approximate methods to evaluate substation ground impedance, and determine ground potential rise (GPR), touch and step voltages. Advanced measurement methodologies and modern software packages were used to obtain and effectively analyze fall of potential test data, compute fault current distribution, and evaluate touch and step voltages for this large substation. Fault current distribution between the grounding system and other metallic paths were computed to determine the portion of fault current discharged in the grounding system. The performance of the grounding system, including its GPR and touch and step voltages, has been accurately computed and measured, taking into account the impedance of the steel material used of the ground conductors and circulating currents within the substation grounding system.展开更多
Using liquid poly(methylvinyl)borosilazanes(PMVBSZ)as precursor,carbon fiber reinforced SiBCN matrix composites(C_f/SiBCN)were fabricated by a modified polymer infiltration and pyrolysis(PIP)process.With dicumyl perox...Using liquid poly(methylvinyl)borosilazanes(PMVBSZ)as precursor,carbon fiber reinforced SiBCN matrix composites(C_f/SiBCN)were fabricated by a modified polymer infiltration and pyrolysis(PIP)process.With dicumyl peroxide added as cross-linking agent,the PMVBSZ could be solidified at a low temperature of 120℃,leading to a high ceramic yield of~70%.The cross-linking mechanism and ceramization processes of the precursor were investigated in detail.Moreover,a modified infiltration technology was developed,which improved the efficiency and protected the precursor against moist air during PIP.Consequently,the obtained C_f/SiBCN composites had an oxygen content of around 1.22 wt%.Benefiting from the high ceramic yield and high efficiency of the modified PIP,C_f/SiBCN composites with an open porosity of~10%and uniform microstructure were obtained after only 7 cycles of PIP.The flexural strength and fracture toughness of the derived C_f/SiBCN composites were 371 MPa and 12.9 MPa·m^(1/2),respectively.This work provides a potential route for the fabrication of high performance C_f/SiBCN composites.展开更多
ZrB_2–ZrC–SiC ternary coatings on C/C composites are investigated by reactive melt infiltration of ZrSi_2 alloy into pre-coatings. Two different pre-coating structures, including porous B_4C–C and dense C/B, are de...ZrB_2–ZrC–SiC ternary coatings on C/C composites are investigated by reactive melt infiltration of ZrSi_2 alloy into pre-coatings. Two different pre-coating structures, including porous B_4C–C and dense C/B, are designed by slurry dip and chemical vapor deposition(CVD) process respectively. The coating prepared by reactive melt infiltration(RMI) into B_4C–C presents a flat and smooth surface with a three-layer cross-sectional structure, namely interior SiC transition layer, gradient ZrB_2–ZrC–SiC layer, and ZrB_2–ZrC exterior layer. In comparison, the coating prepared by RMI into C/B shows a more granular surface with a different three-layer cross-sectional structure, interior unreacted B–C pre-coating layer, middle SiC layer, and exterior ZrB_2–ZrC–ZrSi_2 layer. The forming mechanisms of the specific microstructures in two coatings are also investigated and discussed in detail.展开更多
Residual stress originated from thermal expansion mismatch determines the mechanical properties of ceramic matrix composites(CMCs).Here,continuous SiC fiber reinforced SiC matrix(SiC_(f)/SiC)composites were fabricated...Residual stress originated from thermal expansion mismatch determines the mechanical properties of ceramic matrix composites(CMCs).Here,continuous SiC fiber reinforced SiC matrix(SiC_(f)/SiC)composites were fabricated by nano-infiltration and transient eutectic-phase(NITE)method,and the residual stress of the composites was investigated using high-temperature Raman spectrometer.With temperature increasing from room temperature to 1400℃,the residual stresses of the matrix and the fiber decrease from 1.29 to 0.62 GPa and from 0.84 to 0.55 GPa in compression respectively,while that of the interphase decreases from 0.16 to 0.10 GPa in tension.The variation of residual stress shows little effect on the tensile strength of the composites,while causes a slight decrease in the tensile strain.The suppression of fiber/matrix debonding and fiber pulling-out caused by the residual stress reduction in the interphase is responsible for the decreasing tensile strain.This work can open up new alternatives for residual stress analysis in CMCs.展开更多
Reactive melt infiltration(RMI)is an effective method for fabrication of highly dense carbon fiber reinforced ultra-high temperature ceramic matrix composites(Cf/UHTCs).In this work,C_(f)/SiC-ZrC-ZrB_(2)composites wer...Reactive melt infiltration(RMI)is an effective method for fabrication of highly dense carbon fiber reinforced ultra-high temperature ceramic matrix composites(Cf/UHTCs).In this work,C_(f)/SiC-ZrC-ZrB_(2)composites were fabricated by infiltrating ZrSi_(2)melt into porous C_(f)/B_(4)C-C preforms,where the physical and chemical reactions involved during the RMI process were identified and analyzed.Inhomogeneous infiltration between the inter-and intra-bundle pores was revealed,and was found to be strongly related to the pore structures of the C_(f)/B_(4)C-C preform.It is indicated that the inhomogeneous infiltration can be mitigated remarkably with increasing porosity and pore size of the preform.The effect of pore size on the RMI process was also investigated by a quantitative model,which agrees very well with the experiment results.It further indicates that the inhomogeneous infiltration can also be relieved at elevated RMI temperature.However,excessive infiltration at elevated temperature or more porous preform may cause serious erosion on interphase and fibers,leading to mechanical properties deterioration of the final composites.展开更多
In this work,the microstructure evolution,thermal expansion,thermal conductivity,and thermal shock resistance properties of the plasma-sprayed Xl-Gd_(2)SiO_(5),X2-Y_(2)SiO_(5),and X2-Er_(2)SiO_(5)coatings were evaluat...In this work,the microstructure evolution,thermal expansion,thermal conductivity,and thermal shock resistance properties of the plasma-sprayed Xl-Gd_(2)SiO_(5),X2-Y_(2)SiO_(5),and X2-Er_(2)SiO_(5)coatings were evaluated and compared by experimental measurement and theoretical exploration.Results showed that significant microstructure evolution such as crystallization of amorphous phase,grain growth,and defects reduction was observed in the RE_(2)SiO_(5)coatings after thermal aging at 1400℃.The Xl-Gd_(2)SiO_(5)coating exhibited higher CTE values than the X2-Y_(2)SiO_(5)and X2-Er_(2)SiO_(5)coatings,which was related to their crystal structure.The thermal conductivity of thermal-aged RE_(2)SiO_(5)coating was much higher than that of the as-sprayed RE_(2)SiO_(5)coating,and thermal conductivity was determined not only by crystal structure but also mainly by the microstructure of the coatings.The X2-Y_(2)SiO_(5)and X2-Er_(2)SiO_(5)coatings with lower thermal mismatch stre s ses presented much better thermal shock resistance than the X1-Gd_(2)SiO_(5)coating.展开更多
Fiber damage and uniform interphase preparation are the main challenges in conventional short fiber reinforced ceramic matrix composites.In this work,we develop a novel processing route in fabrication of short carbon ...Fiber damage and uniform interphase preparation are the main challenges in conventional short fiber reinforced ceramic matrix composites.In this work,we develop a novel processing route in fabrication of short carbon fiber reinforced ZrB_(2)-SiC composites(C_(sf)/ZrB_(2)-SiC)overcoming the above two issues.At first,C_(sf) preforms with oriented designation and uniform PyC/SiC interphase are fabricated via direct ink writing(DIW)of short carbon fiber paste followed by chemical vapor infiltration.After that,ZrB_(2) and SiC are introduced into the preforms by slurry impregnation and reactive melt infiltration,respectively.Microstructure evolution and optimization of the composites during fabrication are investigated in detail.The as-fabricated C_(sf)/ZrB_(2)-SiC composites have a bulk density of 2.47 g/cm^(3),with uniform weak interphase and without serious fiber damage.Consequently,non-brittle fracture occurs in the C_(sf)/ZrB_(2)-SiC composites with widespread toughening mechanisms such as crack deflection and bridging,interphase debonding,and fiber pull-out.This work provides a new opportunity to the material design and selection of short fiber reinforced composites.展开更多
This paper investigates the reliability problem of airborne free-space optical(FSO)communications,and a hybrid FSO/radio frequency(RF)communication system with parallel transmission is proposed,where the data stream i...This paper investigates the reliability problem of airborne free-space optical(FSO)communications,and a hybrid FSO/radio frequency(RF)communication system with parallel transmission is proposed,where the data stream is transmitted over both FSO and RF links simultaneously.Further,to combat channel fading,maximal ratio combining is utilized at the receiver for combining received signals from both links.The performances of the proposed system are analytically derived in terms of the outage probability and the average bite-error rate(BER).Numerical results show that the proposed hybrid FSO/RF system with parallel transmission outperforms a single airborne FSO or a single RF link,which provides technical guidance for designing reliable high-speed airborne communication systems.展开更多
基金support by China Information Communication Technologies Group Corporationsupported in part by Chinese Ministry of Education-China Mobile Communication Corporation Research Fund under Grant MCM20170101the European Union’s Horizon 2020 research and innovation programme under the Marie Skldowska-Curie Grant Agreement No.793345
文摘With the development of automated driving vehicles, more and more vehicles will be fitted with more than one automotive radars, and the radar mutual interference will become very significant. Vehicle to everything (V2X) communication is a potential way for coordinating automotive radars and reduce the mutual interference. In this paper, we analyze the positional relation of the two radars that interfere with each other, and evaluate the mutual interference for different types of automotive radars based on Poisson point process (PPP). We also propose a centralized framework and the corresponding algorithm, which relies on V2X communication systems to allocate the spectrum resources for automotive radars to minimize the interference. The minimum spectrum resources required for zero-interference are analyzed for different cases. Simulation results validate the analysis and show that the proposed framework can achieve near-zero-interference with the minimum spectrum resources.
文摘This paper examines various aspects of the design process and subsequent field test measurements of a large and complex substation grounding system. The study and measurements show that soil layering and lead interference can have a significant impact on the appropriate test location that yields the exact substation ground impedance. Applying a specific percentage rule such as the 61.8% rule for uniform soils to obtain the true ground impedance may lead to unacceptable errors for large grounding systems. This poses significant problems when attempting to validate a design based on raw test data that are interpreted using approximate methods to evaluate substation ground impedance, and determine ground potential rise (GPR), touch and step voltages. Advanced measurement methodologies and modern software packages were used to obtain and effectively analyze fall of potential test data, compute fault current distribution, and evaluate touch and step voltages for this large substation. Fault current distribution between the grounding system and other metallic paths were computed to determine the portion of fault current discharged in the grounding system. The performance of the grounding system, including its GPR and touch and step voltages, has been accurately computed and measured, taking into account the impedance of the steel material used of the ground conductors and circulating currents within the substation grounding system.
基金financial supports from the National Key Research and Development Program of China (No. 2017YFB0703200) National Natural Science Foundation of China (No. 51702341)Chinese Academy of Sciences Innovative Funding (No. CXJJ-17-M169)
文摘Using liquid poly(methylvinyl)borosilazanes(PMVBSZ)as precursor,carbon fiber reinforced SiBCN matrix composites(C_f/SiBCN)were fabricated by a modified polymer infiltration and pyrolysis(PIP)process.With dicumyl peroxide added as cross-linking agent,the PMVBSZ could be solidified at a low temperature of 120℃,leading to a high ceramic yield of~70%.The cross-linking mechanism and ceramization processes of the precursor were investigated in detail.Moreover,a modified infiltration technology was developed,which improved the efficiency and protected the precursor against moist air during PIP.Consequently,the obtained C_f/SiBCN composites had an oxygen content of around 1.22 wt%.Benefiting from the high ceramic yield and high efficiency of the modified PIP,C_f/SiBCN composites with an open porosity of~10%and uniform microstructure were obtained after only 7 cycles of PIP.The flexural strength and fracture toughness of the derived C_f/SiBCN composites were 371 MPa and 12.9 MPa·m^(1/2),respectively.This work provides a potential route for the fabrication of high performance C_f/SiBCN composites.
基金financial support from the National Key Research and Development Program of China(No.2017YFB0703200)the research grant from Science and Technology Commission of Shanghai Municipality(No.16DZ2260600)
文摘ZrB_2–ZrC–SiC ternary coatings on C/C composites are investigated by reactive melt infiltration of ZrSi_2 alloy into pre-coatings. Two different pre-coating structures, including porous B_4C–C and dense C/B, are designed by slurry dip and chemical vapor deposition(CVD) process respectively. The coating prepared by reactive melt infiltration(RMI) into B_4C–C presents a flat and smooth surface with a three-layer cross-sectional structure, namely interior SiC transition layer, gradient ZrB_2–ZrC–SiC layer, and ZrB_2–ZrC exterior layer. In comparison, the coating prepared by RMI into C/B shows a more granular surface with a different three-layer cross-sectional structure, interior unreacted B–C pre-coating layer, middle SiC layer, and exterior ZrB_2–ZrC–ZrSi_2 layer. The forming mechanisms of the specific microstructures in two coatings are also investigated and discussed in detail.
基金financial support of the research grant from National Natural Science Foundation of China (No.51902328)the research grant from Science and Technology Commission of Shanghai Municipality (No. 19ZR1464700)the research grant from Key Program of the Chinese Academy of Sciences (No.ZDRW-CN2017-1)
文摘Residual stress originated from thermal expansion mismatch determines the mechanical properties of ceramic matrix composites(CMCs).Here,continuous SiC fiber reinforced SiC matrix(SiC_(f)/SiC)composites were fabricated by nano-infiltration and transient eutectic-phase(NITE)method,and the residual stress of the composites was investigated using high-temperature Raman spectrometer.With temperature increasing from room temperature to 1400℃,the residual stresses of the matrix and the fiber decrease from 1.29 to 0.62 GPa and from 0.84 to 0.55 GPa in compression respectively,while that of the interphase decreases from 0.16 to 0.10 GPa in tension.The variation of residual stress shows little effect on the tensile strength of the composites,while causes a slight decrease in the tensile strain.The suppression of fiber/matrix debonding and fiber pulling-out caused by the residual stress reduction in the interphase is responsible for the decreasing tensile strain.This work can open up new alternatives for residual stress analysis in CMCs.
基金The financial support from“The National Key Research and Development Program of China”(No.2017YFB0703200)“National Natural Science Foundation of China”(No.51702341)+1 种基金Chinese Academy of Sciences Innovative Funding(CXJJ-17-M169)“CAS Pioneer Hundred Talents Program”are greatly acknowledged.
文摘Reactive melt infiltration(RMI)is an effective method for fabrication of highly dense carbon fiber reinforced ultra-high temperature ceramic matrix composites(Cf/UHTCs).In this work,C_(f)/SiC-ZrC-ZrB_(2)composites were fabricated by infiltrating ZrSi_(2)melt into porous C_(f)/B_(4)C-C preforms,where the physical and chemical reactions involved during the RMI process were identified and analyzed.Inhomogeneous infiltration between the inter-and intra-bundle pores was revealed,and was found to be strongly related to the pore structures of the C_(f)/B_(4)C-C preform.It is indicated that the inhomogeneous infiltration can be mitigated remarkably with increasing porosity and pore size of the preform.The effect of pore size on the RMI process was also investigated by a quantitative model,which agrees very well with the experiment results.It further indicates that the inhomogeneous infiltration can also be relieved at elevated RMI temperature.However,excessive infiltration at elevated temperature or more porous preform may cause serious erosion on interphase and fibers,leading to mechanical properties deterioration of the final composites.
基金financially supported by the National Key R&D Program of China(No.2018YFB0704400)the National Science and Technology Major Project(No.2017-VI-0020-0092)+2 种基金the Shanghai Technical Platform for Testing on Inorganic Materials(No.19DZ2290700)the Shanghai Sailing Program(No.19YF1453900)the Natural Science Foundation of Shanghai(No.20ZR1465700)。
文摘In this work,the microstructure evolution,thermal expansion,thermal conductivity,and thermal shock resistance properties of the plasma-sprayed Xl-Gd_(2)SiO_(5),X2-Y_(2)SiO_(5),and X2-Er_(2)SiO_(5)coatings were evaluated and compared by experimental measurement and theoretical exploration.Results showed that significant microstructure evolution such as crystallization of amorphous phase,grain growth,and defects reduction was observed in the RE_(2)SiO_(5)coatings after thermal aging at 1400℃.The Xl-Gd_(2)SiO_(5)coating exhibited higher CTE values than the X2-Y_(2)SiO_(5)and X2-Er_(2)SiO_(5)coatings,which was related to their crystal structure.The thermal conductivity of thermal-aged RE_(2)SiO_(5)coating was much higher than that of the as-sprayed RE_(2)SiO_(5)coating,and thermal conductivity was determined not only by crystal structure but also mainly by the microstructure of the coatings.The X2-Y_(2)SiO_(5)and X2-Er_(2)SiO_(5)coatings with lower thermal mismatch stre s ses presented much better thermal shock resistance than the X1-Gd_(2)SiO_(5)coating.
基金support from the Key Research Program of Frontier Sciences,CAS(No.QYZDY-SSW-JSC031)the projects supported by State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,Wuhan University of Technology(No.2021-KF-5)State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,Donghua University(No.KF2116)are greatly acknowledged.
文摘Fiber damage and uniform interphase preparation are the main challenges in conventional short fiber reinforced ceramic matrix composites.In this work,we develop a novel processing route in fabrication of short carbon fiber reinforced ZrB_(2)-SiC composites(C_(sf)/ZrB_(2)-SiC)overcoming the above two issues.At first,C_(sf) preforms with oriented designation and uniform PyC/SiC interphase are fabricated via direct ink writing(DIW)of short carbon fiber paste followed by chemical vapor infiltration.After that,ZrB_(2) and SiC are introduced into the preforms by slurry impregnation and reactive melt infiltration,respectively.Microstructure evolution and optimization of the composites during fabrication are investigated in detail.The as-fabricated C_(sf)/ZrB_(2)-SiC composites have a bulk density of 2.47 g/cm^(3),with uniform weak interphase and without serious fiber damage.Consequently,non-brittle fracture occurs in the C_(sf)/ZrB_(2)-SiC composites with widespread toughening mechanisms such as crack deflection and bridging,interphase debonding,and fiber pull-out.This work provides a new opportunity to the material design and selection of short fiber reinforced composites.
文摘This paper investigates the reliability problem of airborne free-space optical(FSO)communications,and a hybrid FSO/radio frequency(RF)communication system with parallel transmission is proposed,where the data stream is transmitted over both FSO and RF links simultaneously.Further,to combat channel fading,maximal ratio combining is utilized at the receiver for combining received signals from both links.The performances of the proposed system are analytically derived in terms of the outage probability and the average bite-error rate(BER).Numerical results show that the proposed hybrid FSO/RF system with parallel transmission outperforms a single airborne FSO or a single RF link,which provides technical guidance for designing reliable high-speed airborne communication systems.