Liquid-solid extrusion directly following vacuum infiltration(LSEVI)is an infiltration-extrusion integrated forming technique,and transverse weld between upper residual magnesium alloy and magnesium matrix composites ...Liquid-solid extrusion directly following vacuum infiltration(LSEVI)is an infiltration-extrusion integrated forming technique,and transverse weld between upper residual magnesium alloy and magnesium matrix composites is a common internal defect,which can severely reduce the yield of composite products.To improve current understanding on the mechanism of transverse welding phenomenon,a thermo-mechanical numerical model of LSEVI for magnesium matrix composites was developed.The formation of transverse weld during extrusion was visualized using finite element simulation method,and the formation mechanism was discussed from the aspect of velocity field using a point tracking technique.The simulation results were verified by the experimental results in term of weld shape.展开更多
The acoustic bottom backscattering strength was measured at the frequency range of 6–24 kHz on a typical sandy bottom in the South Yellow Sea by using omnidirectional sources and omnidirectional receiving hydrophones...The acoustic bottom backscattering strength was measured at the frequency range of 6–24 kHz on a typical sandy bottom in the South Yellow Sea by using omnidirectional sources and omnidirectional receiving hydrophones. In the experiment, by avoiding disturbances due to scattering off the sea surface and satisfying the far-field condition, we obtained values of acoustic bottom backscattering strength ranging from –41.1 to –24.4 dB within a grazing angle range of 18°–80°. In the effective range of grazing angles, the acoustic scattering strength generally increases with an increase in the grazing angles, but trends of the variation were distinct in different ranges of frequency, which reflect different scattering mechanisms. The frequency dependence of bottom backscattering strength is generally characterized by a positive correlation in the entire frequency range of 6–24 kHz at the grazing angles of 20°, 40° and 60° with the linear regression slopes of 0.222 9 dB/kHz, 0.513 0 dB/kHz and 0.174 6 dB/kHz, respectively. At the largest grazing angle of 80°, the acoustic backscattering strength exhibits no evident frequency dependence.展开更多
Sea-surface acoustic backscattering measurements at moderate to high frequencies were performed in the shallow water of the south Yellow Sea, using omnidirectional spherical sources and omnidirectional hydrophones. Se...Sea-surface acoustic backscattering measurements at moderate to high frequencies were performed in the shallow water of the south Yellow Sea, using omnidirectional spherical sources and omnidirectional hydrophones. Sea-surface backscattering data for frequencies in the 6–25 k Hz range and wind speeds of(3.0±0.5)and(4.5±1.0) m/s were obtained from two adjacent experimental sites, respectively. Computation of sea-surface backscattering strength using bistatic transducer is described. Finally, we calculated sea-surface backscattering strengths at grazing angles in the range of 16°–85°. We find that the measured backscattering strengths agree reasonably well with those predicted by using second order small-roughness perturbation approximation method with "PM" roughness spectrum for all frequencies at grazing angles ranged from 40° to 80°. The backscattering strengths varied slightly at grazing angles of 16°–40°, and were much stronger than roughness scattering. It is speculated that scattering from bubbles dominates the backscattering strengths at high wind speeds and small grazing angles. At the same frequencies and moderate to high grazing angles, the results show that the backscattering strengths at a wind speed of(4.5±1.0) m/s were approximately 5 d B higher than those at a wind speed of(3.0±0.5) m/s. However, the discrepancies of backscattering strength at low grazing angles were more than 10 d B. Furthermore the backscattering strengths exhibited no significant frequency dependence at 3 m/s wind speed. At a wind speed of 4.5 m/s, the scattering strengths increased at low grazing angles but decreased at high grazing angles with increasing grazing angle.展开更多
Stealth materials with high dependability at elevated temperatures and outstanding mechanical properties are urgently needed for practical applications.As one-dimensional ultrahigh temperature ceramic(UHTC)materials,z...Stealth materials with high dependability at elevated temperatures and outstanding mechanical properties are urgently needed for practical applications.As one-dimensional ultrahigh temperature ceramic(UHTC)materials,zirconium carbide whiskers(ZrCw)have attracted a great deal of attention due to their desirable mechanical and ablation resistance performance in high-temperature environments.We have successfully synthesized ZrCw using a carbothermal reduction technique without the introduction of metal catalytic in this paper.ZrCw shows a typically prismatic structure with the diameter of 1e2 mm and the aspect ratio of up to 250.The growth of ZrCw is controlled by a solid-liquid-solid(SLS)and vaporsolid(VS)compound mechanism in conjunction with the auxiliary action of mesophase Na3ZrF7.The ZrCw/paraffin hybrids achieve the minimum reflection loss(RL(min))of25.77 dB at 13.28 GHz under the thickness of 1.25 mm,and reach an effective absorption bandwidth(EAB)of 3.04 GHz(14.96 e18.00 GHz)with a thickness of only 1.0 mm.This work presents a promising approach for large-scale producing high-purity whiskers,and verifies that ZrCw has extensive application prospects in the field of stealth materials.展开更多
7075 aluminum billets were fabricated by micro droplet deposition manufacturing technique, and the influence of interracial bonding between metal droplets on the tensile properties was studied. Three sets of samples w...7075 aluminum billets were fabricated by micro droplet deposition manufacturing technique, and the influence of interracial bonding between metal droplets on the tensile properties was studied. Three sets of samples were manufactured under different temperature conditions, and their mechanical properties were compared. The results show that the temperature of the metal droplets and substrate significantly affect the tensile strength of the sample. Moreover, with proper temperature setting, the 7075 aluminum billets manufactured by micro metal droplet deposition could achieve very good mechanical properties with a tensile strength of 373 MPa and an elongation of 9.95%, which are very similar to those of an ex truded sample. Moreover, a metallurgical bonding diagram based on numerical calculations of interfacial temperature was established to predict the interfacial bonding state. In addition, the fracture morphologies of these specimens were observed. It is indicated that there was a significant transformation of failure mechanism with the improvement of metallurgical bonding, which agreed well with the numerical results.展开更多
As an alternative to short fibers,non-woven fabrics(NWFs)were made using different types of long fibers to optimize the performance of paper-based friction materials and their technology.In this investigation,the fill...As an alternative to short fibers,non-woven fabrics(NWFs)were made using different types of long fibers to optimize the performance of paper-based friction materials and their technology.In this investigation,the fillers and resin were impregnated into these NWFs to prepare three kinds of wet friction material.The tribological,mechanical,and thermal properties of the new wet friction material were studied.The results indicate that the dynamic friction coefficient of the new friction material is approximately 0.12 and the static friction coefficient is approximately 0.15;the better wear rate is 0.81334×10^(-14)m^(3)·(N·m)^(-1).In addition,the temperature for 10%mass loss yielded 100°C enhancement and the tensile strength was improved by 200%,compared to previously reported values.Most importantly,the advantages include a simple preparation flow,low cost,and resource conservation.This is a promising approach for the future development of paper-based friction materials.展开更多
Carbon fiber reinforced AZ91 D matrix composites with pyrolytic(PyC) coating deposited on fiber surface(PyC-Cf/AZ91 D composites) have been fabricated by Liquid-solid extrusion following vacuum pressure infiltrati...Carbon fiber reinforced AZ91 D matrix composites with pyrolytic(PyC) coating deposited on fiber surface(PyC-Cf/AZ91 D composites) have been fabricated by Liquid-solid extrusion following vacuum pressure infiltration technique(LSEVI). Interfacial microstructure and failure behavior of the composites were investigated. Instead of interfacial reaction products, block-shaped interfacial precipitates Mg(17) Al(12) were detected at the interface, which indicates that interfacial reaction was restrained by LSEVI and PyC coating. Nano-MgO was detected at the interface. Interfacial failure behavior of the PyC-Cf/AZ91 D composites,which was the failure between PyC coating and AZ91 D alloy due to the mismatch of thermal expansion and relatively poor bonding, was proposed. Fracture surface of the PyC-Cf/AZ91 D composites was characterized by fibers pulling-out tests. PyC coating served not only as protection to the fibers, but also an adjustment of the interface of the composites.展开更多
2D carbon fiber reinforced AZ91 D matrix composites(2D-C_f/AZ91 D composites) were fabricated by liquid–solid extrusion and vacuum pressure infiltration technique(LSEVI). In order to modify the interface between ...2D carbon fiber reinforced AZ91 D matrix composites(2D-C_f/AZ91 D composites) were fabricated by liquid–solid extrusion and vacuum pressure infiltration technique(LSEVI). In order to modify the interface between fibers and matrix and protect the fiber, pyrolytic carbon(Py C) coating was deposited on the surface of T700 carbon fiber by chemical vapor deposition(CVD). Microstructure observation of the composites revealed that the composites were well fabricated by LSEVI. The segregation of aluminum at fiber surface led to the formation of Mg_(17)Al_(12) precipitates at the interface. The aluminum improved the infiltration of the alloy and Py C coating protected the fibers effectively. The ultimate tensile strength of 2D-C_f/AZ91 D composites was about 400 MPa. The fracture process of 2D-C_f/AZ91 D composites was transverse fiber interface cracking–matrix transferring load–longitudinal fibers bearing load–longitudinal fibers breaking.展开更多
The effect of hydrogen on the growth mechanism of pyrocarbon has attracted much attention. The influence of hydrogen on the dissociation from CH4to C2H2on pristine graphene,N-doped graphene and vacancy graphene have b...The effect of hydrogen on the growth mechanism of pyrocarbon has attracted much attention. The influence of hydrogen on the dissociation from CH4to C2H2on pristine graphene,N-doped graphene and vacancy graphene have been investigated by using density functional theory.There are two kinds of heterogeneous reaction pathways when the hydrogen is involved, i.e., dehydrogenation reactions and H-abstraction reactions. The transition state calculations were performed to acquire the reaction pathways on each substrate after obtaining the most stable adsorption configurations of the reactants and products. The results indicate that the adsorptions of reactants are not affected by hydrogen. The dehydrogenation reactions are more favored on vacancy graphene, and the H-abstraction reactions are more favored on the pristine graphene and N-doped graphene. The dehydrogenation reactions on the vacancy graphene are most favored among all these reactions in favor of the deposition of pyrocarbon. The dehydrogenation reactions on these three substrates are affected by hydrogen.展开更多
基金The authors would like to gratefully acknowledge the financial support of National Natural Science Foundation of China(Grant No.51305345)Natural Science Basic Research Plan in Shaanxi Province of China(Grant No.2014JQ6228).
文摘Liquid-solid extrusion directly following vacuum infiltration(LSEVI)is an infiltration-extrusion integrated forming technique,and transverse weld between upper residual magnesium alloy and magnesium matrix composites is a common internal defect,which can severely reduce the yield of composite products.To improve current understanding on the mechanism of transverse welding phenomenon,a thermo-mechanical numerical model of LSEVI for magnesium matrix composites was developed.The formation of transverse weld during extrusion was visualized using finite element simulation method,and the formation mechanism was discussed from the aspect of velocity field using a point tracking technique.The simulation results were verified by the experimental results in term of weld shape.
基金The Opening Fund of Pilot National Laboratory for Marine Science and Technology(Qingdao)under contract No.QNLM2016ORP0209the National Natural Science Foundation of China under contract Nos 41330965,41676055 and 41527809the Taishan Scholar Project Funding under contract No.tspd20161007
文摘The acoustic bottom backscattering strength was measured at the frequency range of 6–24 kHz on a typical sandy bottom in the South Yellow Sea by using omnidirectional sources and omnidirectional receiving hydrophones. In the experiment, by avoiding disturbances due to scattering off the sea surface and satisfying the far-field condition, we obtained values of acoustic bottom backscattering strength ranging from –41.1 to –24.4 dB within a grazing angle range of 18°–80°. In the effective range of grazing angles, the acoustic scattering strength generally increases with an increase in the grazing angles, but trends of the variation were distinct in different ranges of frequency, which reflect different scattering mechanisms. The frequency dependence of bottom backscattering strength is generally characterized by a positive correlation in the entire frequency range of 6–24 kHz at the grazing angles of 20°, 40° and 60° with the linear regression slopes of 0.222 9 dB/kHz, 0.513 0 dB/kHz and 0.174 6 dB/kHz, respectively. At the largest grazing angle of 80°, the acoustic backscattering strength exhibits no evident frequency dependence.
基金The National Natural Science Foundation of China under contract Nos 41330965 and 41527809the Opening Fund of Qingdao National Laboratory for Marine Science and Technology under contract No. QNLM2016ORP0209the Taishan Scholar Project Funding under contract No. tspd20161007.
文摘Sea-surface acoustic backscattering measurements at moderate to high frequencies were performed in the shallow water of the south Yellow Sea, using omnidirectional spherical sources and omnidirectional hydrophones. Sea-surface backscattering data for frequencies in the 6–25 k Hz range and wind speeds of(3.0±0.5)and(4.5±1.0) m/s were obtained from two adjacent experimental sites, respectively. Computation of sea-surface backscattering strength using bistatic transducer is described. Finally, we calculated sea-surface backscattering strengths at grazing angles in the range of 16°–85°. We find that the measured backscattering strengths agree reasonably well with those predicted by using second order small-roughness perturbation approximation method with "PM" roughness spectrum for all frequencies at grazing angles ranged from 40° to 80°. The backscattering strengths varied slightly at grazing angles of 16°–40°, and were much stronger than roughness scattering. It is speculated that scattering from bubbles dominates the backscattering strengths at high wind speeds and small grazing angles. At the same frequencies and moderate to high grazing angles, the results show that the backscattering strengths at a wind speed of(4.5±1.0) m/s were approximately 5 d B higher than those at a wind speed of(3.0±0.5) m/s. However, the discrepancies of backscattering strength at low grazing angles were more than 10 d B. Furthermore the backscattering strengths exhibited no significant frequency dependence at 3 m/s wind speed. At a wind speed of 4.5 m/s, the scattering strengths increased at low grazing angles but decreased at high grazing angles with increasing grazing angle.
基金supported by the National Natural Science Foundation of China(Grant Nos.51872234,51872232 and 52231004)the Joint Funds of the National Natural Science Foundation of China(Grant No.U21B2067)the Key R&D Program of Shaanxi Province(Grant Nos.2019ZDLGY04-02 and 2021ZDLGY14-04)and Natural Science Basic Research Plan in Shaanxi(2022JC-25).
文摘Stealth materials with high dependability at elevated temperatures and outstanding mechanical properties are urgently needed for practical applications.As one-dimensional ultrahigh temperature ceramic(UHTC)materials,zirconium carbide whiskers(ZrCw)have attracted a great deal of attention due to their desirable mechanical and ablation resistance performance in high-temperature environments.We have successfully synthesized ZrCw using a carbothermal reduction technique without the introduction of metal catalytic in this paper.ZrCw shows a typically prismatic structure with the diameter of 1e2 mm and the aspect ratio of up to 250.The growth of ZrCw is controlled by a solid-liquid-solid(SLS)and vaporsolid(VS)compound mechanism in conjunction with the auxiliary action of mesophase Na3ZrF7.The ZrCw/paraffin hybrids achieve the minimum reflection loss(RL(min))of25.77 dB at 13.28 GHz under the thickness of 1.25 mm,and reach an effective absorption bandwidth(EAB)of 3.04 GHz(14.96 e18.00 GHz)with a thickness of only 1.0 mm.This work presents a promising approach for large-scale producing high-purity whiskers,and verifies that ZrCw has extensive application prospects in the field of stealth materials.
基金the National Natural Science Foundation of China(No.51521061)the Defense Industrial Technology Development Program(No.A1120133026)+2 种基金the Doctoral Fund of Ministry of Education of China(No.20126102110022)the“111”Project of China(No.B08040)the Research Fund of the State Key Laboratory of Solidification Processing(NWPU)(No.85-TZ-2013)
文摘7075 aluminum billets were fabricated by micro droplet deposition manufacturing technique, and the influence of interracial bonding between metal droplets on the tensile properties was studied. Three sets of samples were manufactured under different temperature conditions, and their mechanical properties were compared. The results show that the temperature of the metal droplets and substrate significantly affect the tensile strength of the sample. Moreover, with proper temperature setting, the 7075 aluminum billets manufactured by micro metal droplet deposition could achieve very good mechanical properties with a tensile strength of 373 MPa and an elongation of 9.95%, which are very similar to those of an ex truded sample. Moreover, a metallurgical bonding diagram based on numerical calculations of interfacial temperature was established to predict the interfacial bonding state. In addition, the fracture morphologies of these specimens were observed. It is indicated that there was a significant transformation of failure mechanism with the improvement of metallurgical bonding, which agreed well with the numerical results.
基金supported by the National Key R&D Program of China(Grant No.2017YFB0308303)Shaanxi Province Technology Innovation Guide Special Project(Grant No.2017CGZH-RGGJ-01)+1 种基金Shaanxi Provincial Key Research(Grant No.2018ZDCXL-GY09-05)the Analytical&Testing Center of Northwestern Polytechnical University,and the seed Foundation of Innovation and Creation for Graduate Students in Northwestern Polytechnical University(ZZ2019082)。
文摘As an alternative to short fibers,non-woven fabrics(NWFs)were made using different types of long fibers to optimize the performance of paper-based friction materials and their technology.In this investigation,the fillers and resin were impregnated into these NWFs to prepare three kinds of wet friction material.The tribological,mechanical,and thermal properties of the new wet friction material were studied.The results indicate that the dynamic friction coefficient of the new friction material is approximately 0.12 and the static friction coefficient is approximately 0.15;the better wear rate is 0.81334×10^(-14)m^(3)·(N·m)^(-1).In addition,the temperature for 10%mass loss yielded 100°C enhancement and the tensile strength was improved by 200%,compared to previously reported values.Most importantly,the advantages include a simple preparation flow,low cost,and resource conservation.This is a promising approach for the future development of paper-based friction materials.
基金supported by the National Natural Science Foundation of China (Nos. 51472203, 51575447, 51521061)
文摘Carbon fiber reinforced AZ91 D matrix composites with pyrolytic(PyC) coating deposited on fiber surface(PyC-Cf/AZ91 D composites) have been fabricated by Liquid-solid extrusion following vacuum pressure infiltration technique(LSEVI). Interfacial microstructure and failure behavior of the composites were investigated. Instead of interfacial reaction products, block-shaped interfacial precipitates Mg(17) Al(12) were detected at the interface, which indicates that interfacial reaction was restrained by LSEVI and PyC coating. Nano-MgO was detected at the interface. Interfacial failure behavior of the PyC-Cf/AZ91 D composites,which was the failure between PyC coating and AZ91 D alloy due to the mismatch of thermal expansion and relatively poor bonding, was proposed. Fracture surface of the PyC-Cf/AZ91 D composites was characterized by fibers pulling-out tests. PyC coating served not only as protection to the fibers, but also an adjustment of the interface of the composites.
基金supported by the National Nature Science Foundation of China (Nos. 51472203, 51521061, 51575447 and 51432008)
文摘2D carbon fiber reinforced AZ91 D matrix composites(2D-C_f/AZ91 D composites) were fabricated by liquid–solid extrusion and vacuum pressure infiltration technique(LSEVI). In order to modify the interface between fibers and matrix and protect the fiber, pyrolytic carbon(Py C) coating was deposited on the surface of T700 carbon fiber by chemical vapor deposition(CVD). Microstructure observation of the composites revealed that the composites were well fabricated by LSEVI. The segregation of aluminum at fiber surface led to the formation of Mg_(17)Al_(12) precipitates at the interface. The aluminum improved the infiltration of the alloy and Py C coating protected the fibers effectively. The ultimate tensile strength of 2D-C_f/AZ91 D composites was about 400 MPa. The fracture process of 2D-C_f/AZ91 D composites was transverse fiber interface cracking–matrix transferring load–longitudinal fibers bearing load–longitudinal fibers breaking.
基金supported by the National Natural Science Foundation of China (Nos. 51821091, 51872234 and 51972271)
文摘The effect of hydrogen on the growth mechanism of pyrocarbon has attracted much attention. The influence of hydrogen on the dissociation from CH4to C2H2on pristine graphene,N-doped graphene and vacancy graphene have been investigated by using density functional theory.There are two kinds of heterogeneous reaction pathways when the hydrogen is involved, i.e., dehydrogenation reactions and H-abstraction reactions. The transition state calculations were performed to acquire the reaction pathways on each substrate after obtaining the most stable adsorption configurations of the reactants and products. The results indicate that the adsorptions of reactants are not affected by hydrogen. The dehydrogenation reactions are more favored on vacancy graphene, and the H-abstraction reactions are more favored on the pristine graphene and N-doped graphene. The dehydrogenation reactions on the vacancy graphene are most favored among all these reactions in favor of the deposition of pyrocarbon. The dehydrogenation reactions on these three substrates are affected by hydrogen.