A series of (1-x)La0.6Dy0.1Sr0.3MnO3/0.5x(Sb2O3)(x=0.15) samples were prepared by the solid-state reaction method, and the influence of sintering temperature of the matrix on low-field magnetoresistance of (1-x)La0.6D...A series of (1-x)La0.6Dy0.1Sr0.3MnO3/0.5x(Sb2O3)(x=0.15) samples were prepared by the solid-state reaction method, and the influence of sintering temperature of the matrix on low-field magnetoresistance of (1-x)La0.6Dy0.1Sr0.3MnO3/0.5x (Sb2O3) was studied through the measurements of X-ray diffraction (XRD) patterns, scanning electron microscope (SEM) image, resistivity-temperature (ρ-T) curves, and magnetoresistance-temperature (MR-T) curves. The results indicate that for the samples with low sintering temperature of the matrix, lowfield magnetoresistance effect appears on the whole temperature range and can be explained by grain boundary effect; for the sample with high sintering temperature of the matrix, intrinsic magnetoresistance peak appears on the high-temperature range, low-field magnetore-sistance effect appears on low temperature range, and the magnetoresistance in the magnetic field of 0.2 T and on the comparatively large temperature range between 280 K and 225 K hardly changes with temperature and remains at 4.8%, which can be explained by the competition between the intrinsic magnetoresistance induced by double-exchange function inside grains and the tunneling magnetoresis-tance (TMR) induced by grain boundary effect. The temperature stability of magnetoresistance is beneficial to the practical applications of MR.展开更多
（TiCp＋ TiBw）/Ti-6Al-4V titanium matrix composites（PTMCs） have broad application prospects in the aviation and nuclear field. However, it is a typical difficult-to-cut material due to high hardness of the reinforc...（TiCp＋ TiBw）/Ti-6Al-4V titanium matrix composites（PTMCs） have broad application prospects in the aviation and nuclear field. However, it is a typical difficult-to-cut material due to high hardness of the reinforcements, high strength and low thermal conductivity of Ti-6Al-4V alloy matrix. Grinding experiments with vitrified CBN wheels were conducted to analyze comparatively the grinding performance of PTMCs and Ti-6Al-4V alloy. Grinding force and force ratios, specific grinding energy, grinding temperature, surface roughness, ground surface appearance were discussed. The results show that the normal grinding force and the force ratios of PTMCs are much larger than that of Ti-6Al-4V alloy. Low depth of cut and high workpiece speed are generally beneficial to achieve the precision ground surface for PTMCs. The hard reinforcements of PTMCs are mainly removed in the ductile mode during grinding. However, the removal phenomenon of the reinforcements due to brittle fracture still exists, which contributes to the lower specific grinding energy and grinding temperature of PTMCs than Ti-6Al-4V alloy.展开更多
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
For the first time, ultra-high temperature ceramic matrix composite bars were tested inside an arc-jet facility to investigate the impact of oxidation damage on strength retention. The composite bars, which were based...For the first time, ultra-high temperature ceramic matrix composite bars were tested inside an arc-jet facility to investigate the impact of oxidation damage on strength retention. The composite bars, which were based on a ZrB_(2)/SiC matrix reinforced with 45 vol% carbon fibers, were produced by slurry impregnation and sintering. The first batch was tested under 3-point (pt) bending, and two additional batches were exposed to plasma of dissociated air up to a temperature of 2200℃ for 2 min or 2.2 min and then subjected to 3-pt bending. More than 75% of the initial strength was retained even after repeated testing, demonstrating the durability and reusability of the material. Volatility diagrams were calculated to explain the oxide layering observed, while a numerical model was developed to correlate the experimental mechanical data with the elastic properties. The decrease of strength was attributed to a reduction of the cross section due to oxidation.展开更多
A heat transfer model for three-fluid separated heat pipe exchanger was analyzed,and the temperature transfer matrix for general three-fluid separated heat exchanger working in parallel-flow or counter-flow mode was o...A heat transfer model for three-fluid separated heat pipe exchanger was analyzed,and the temperature transfer matrix for general three-fluid separated heat exchanger working in parallel-flow or counter-flow mode was obtained.It was found that the forms of temperature transfer matrix are similar for heat pipe rows with equal or different heat transfer surface area.Furthermore,by using the temperature transfer matrix of the heat pipe exchanger,the relationship between heat transfer effectiveness θ 1,θ 2 and M,NTU,U,Δt i were derived for the exchanger operating in parallel-flow or counter-flow mode,and a simple special example was adopted to demonstrate the correctness of these relationships.展开更多
Data centers,as the infrastructure of all information services,cost tremendous amount of energy.Reducing the hot spot temperature in the data center room is benefit to prevent overheating of devices,and to increase co...Data centers,as the infrastructure of all information services,cost tremendous amount of energy.Reducing the hot spot temperature in the data center room is benefit to prevent overheating of devices,and to increase cooling system efficiency.In this paper,we study the problem of optimal power distribution among racks for minimal hot spot temperature.The temperature rise matrix(TRM)model is used for the purpose of fast estimation of the thermal environment.The accuracy of the model is evaluated by conducting numerical simulations of computational fluid dynamics(CFD).Using the TRM model,optimal distributing of heating power is converted into a linear programming problem,which can be solved by highly efficient algorithms,such as Simplex.Furthermore,with realistic constraints including rack idle power and power upper limit,an iteration method is proposed to calculate the optimal power distribution along with the optimal on/off states of the racks.Obtained solutions are discussed and validated by comparing with CFD simulations.Results show that the TRM model is acceptable in evaluating temperature rises in the forced-convection-dominated scenarios,and the proposed method is able to obtain optimal power distributions under various levels of total power demand.展开更多
基金supported by the National Natural Foundation of China (No. 19934003) the Natural Science Research Key Program of Anhui Educational Committee (No. KJ2011A259)+3 种基金the Opening Program of Cultivating Base of Anhui Key Laboratory of Spintronics and Nanomaterials (Nos. 2010YKF04 2011YKF05)the Professors’and Doctors’Research Startup Foundation of Suzhou University (Nos. 2011jb01 2011jb02)
文摘A series of (1-x)La0.6Dy0.1Sr0.3MnO3/0.5x(Sb2O3)(x=0.15) samples were prepared by the solid-state reaction method, and the influence of sintering temperature of the matrix on low-field magnetoresistance of (1-x)La0.6Dy0.1Sr0.3MnO3/0.5x (Sb2O3) was studied through the measurements of X-ray diffraction (XRD) patterns, scanning electron microscope (SEM) image, resistivity-temperature (ρ-T) curves, and magnetoresistance-temperature (MR-T) curves. The results indicate that for the samples with low sintering temperature of the matrix, lowfield magnetoresistance effect appears on the whole temperature range and can be explained by grain boundary effect; for the sample with high sintering temperature of the matrix, intrinsic magnetoresistance peak appears on the high-temperature range, low-field magnetore-sistance effect appears on low temperature range, and the magnetoresistance in the magnetic field of 0.2 T and on the comparatively large temperature range between 280 K and 225 K hardly changes with temperature and remains at 4.8%, which can be explained by the competition between the intrinsic magnetoresistance induced by double-exchange function inside grains and the tunneling magnetoresis-tance (TMR) induced by grain boundary effect. The temperature stability of magnetoresistance is beneficial to the practical applications of MR.
基金co-supported by the National Natural Science Foundation of China (Nos. 51235004, 51375235)the Fundamental Research Funds for the Central Universities (No. NE2014103) of ChinaPriority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) of China
文摘（TiCp＋ TiBw）/Ti-6Al-4V titanium matrix composites（PTMCs） have broad application prospects in the aviation and nuclear field. However, it is a typical difficult-to-cut material due to high hardness of the reinforcements, high strength and low thermal conductivity of Ti-6Al-4V alloy matrix. Grinding experiments with vitrified CBN wheels were conducted to analyze comparatively the grinding performance of PTMCs and Ti-6Al-4V alloy. Grinding force and force ratios, specific grinding energy, grinding temperature, surface roughness, ground surface appearance were discussed. The results show that the normal grinding force and the force ratios of PTMCs are much larger than that of Ti-6Al-4V alloy. Low depth of cut and high workpiece speed are generally beneficial to achieve the precision ground surface for PTMCs. The hard reinforcements of PTMCs are mainly removed in the ductile mode during grinding. However, the removal phenomenon of the reinforcements due to brittle fracture still exists, which contributes to the lower specific grinding energy and grinding temperature of PTMCs than Ti-6Al-4V alloy.
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supported by the Italian Space Agency(ASI)as part of the project AM3aC2A:Multi-scale approach for modelling CMC and UHTCMC materials.This work was also supported by project ECOSISTER(National Recovery and Resilience Plan(NRRP)Mission 04 Component 2 Investment 1.5-NextGenerationEU,Call for tender n.3277 dated 30/12/2021,Award Number:0001052 dated 23/06/2022)。
文摘For the first time, ultra-high temperature ceramic matrix composite bars were tested inside an arc-jet facility to investigate the impact of oxidation damage on strength retention. The composite bars, which were based on a ZrB_(2)/SiC matrix reinforced with 45 vol% carbon fibers, were produced by slurry impregnation and sintering. The first batch was tested under 3-point (pt) bending, and two additional batches were exposed to plasma of dissociated air up to a temperature of 2200℃ for 2 min or 2.2 min and then subjected to 3-pt bending. More than 75% of the initial strength was retained even after repeated testing, demonstrating the durability and reusability of the material. Volatility diagrams were calculated to explain the oxide layering observed, while a numerical model was developed to correlate the experimental mechanical data with the elastic properties. The decrease of strength was attributed to a reduction of the cross section due to oxidation.
文摘A heat transfer model for three-fluid separated heat pipe exchanger was analyzed,and the temperature transfer matrix for general three-fluid separated heat exchanger working in parallel-flow or counter-flow mode was obtained.It was found that the forms of temperature transfer matrix are similar for heat pipe rows with equal or different heat transfer surface area.Furthermore,by using the temperature transfer matrix of the heat pipe exchanger,the relationship between heat transfer effectiveness θ 1,θ 2 and M,NTU,U,Δt i were derived for the exchanger operating in parallel-flow or counter-flow mode,and a simple special example was adopted to demonstrate the correctness of these relationships.
基金supported by the Project of Shanghai Municipal Science and Technology Commission (No.22DZ2291100)the National Natural Science Foundation of China (No.51976062)the Opening Project of the Key Laboratory of Heat Transfer Enhancement and Energy Conservation of Education Ministry (South China University of Technology,No.202000105).
文摘Data centers,as the infrastructure of all information services,cost tremendous amount of energy.Reducing the hot spot temperature in the data center room is benefit to prevent overheating of devices,and to increase cooling system efficiency.In this paper,we study the problem of optimal power distribution among racks for minimal hot spot temperature.The temperature rise matrix(TRM)model is used for the purpose of fast estimation of the thermal environment.The accuracy of the model is evaluated by conducting numerical simulations of computational fluid dynamics(CFD).Using the TRM model,optimal distributing of heating power is converted into a linear programming problem,which can be solved by highly efficient algorithms,such as Simplex.Furthermore,with realistic constraints including rack idle power and power upper limit,an iteration method is proposed to calculate the optimal power distribution along with the optimal on/off states of the racks.Obtained solutions are discussed and validated by comparing with CFD simulations.Results show that the TRM model is acceptable in evaluating temperature rises in the forced-convection-dominated scenarios,and the proposed method is able to obtain optimal power distributions under various levels of total power demand.
