Through the Galerkin method the nonlinear ordinary differential equations （ODEs） in time are obtained from the nonlinear partial differential equations （PDEs） to describe the mo- tion of the coupled structure of a...Through the Galerkin method the nonlinear ordinary differential equations （ODEs） in time are obtained from the nonlinear partial differential equations （PDEs） to describe the mo- tion of the coupled structure of a suspended-cable-stayed beam. In the PDEs, the curvature of main cables and the deformation of cable stays are taken into account. The dynamics of the struc- ture is investigated based on the ODEs when the structure is subjected to a harmonic excitation in the presence of both high-frequency principle resonance and 1：2 internal resonance. It is found that there are typical jumps and saturation phenomena of the vibration amplitude in the struc- ture. And the structure may present quasi-periodic vibration or chaos, if the stiffness of the cable stays membrane and frequency of external excitation are disturbed.展开更多
Atomic-scale doping strategies and structure design play pivotal roles in tailoring the electronic structure and physicochemical property of electromagnetic wave absorption(EMWA)materials.However,the relationship betw...Atomic-scale doping strategies and structure design play pivotal roles in tailoring the electronic structure and physicochemical property of electromagnetic wave absorption(EMWA)materials.However,the relationship between configuration and electromagnetic(EM)loss mechanism has remained elusive.Herein,drawing inspiration from the DNA transcription process,we report the successful synthesis of novel in situ Mn/N co-doped helical carbon nanotubes with ultrabroad EMWA capability.Theoretical calculation and EM simulation confirm that the orbital coupling and spin polarization of the Mn–N4–C configuration,along with cross polarization generated by the helical structure,endow the helical converters with enhanced EM loss.As a result,HMC-8 demonstrates outstanding EMWA performance,achieving a minimum reflection loss of−63.13 dB at an ultralow thickness of 1.29 mm.Through precise tuning of the graphite domain size,HMC-7 achieves an effective absorption bandwidth(EAB)of 6.08 GHz at 2.02 mm thickness.Furthermore,constructing macroscale gradient metamaterials enables an ultrabroadband EAB of 12.16 GHz at a thickness of only 5.00 mm,with the maximum radar cross section reduction value reaching 36.4 dB m2.This innovative approach not only advances the understanding of metal–nonmetal co-doping but also realizes broadband EMWA,thus contributing to the development of EMWA mechanisms and applications.展开更多
Electrocatalytic reduction of CO_(2) converts intermittent renewable electricity into value-added liquid products with an enticing prospect,but its practical application is hampered due to the lack of high-performance...Electrocatalytic reduction of CO_(2) converts intermittent renewable electricity into value-added liquid products with an enticing prospect,but its practical application is hampered due to the lack of high-performance electrocatalysts.Herein,we elaborately design and develop strongly coupled nanosheets composed of Ag nanoparticles and Sn-SnO_(2) grains,designated as Ag/Sn-SnO_(2) nanosheets(NSs),which possess optimized electronic structure,high electrical conductivity,and more accessible sites.As a result,such a catalyst exhibits unprecedented catalytic performance toward CO_(2)-to-formate conversion with near-unity faradaic efficiency(≥90%),ultrahigh partial current density(2,000 mA cm^(−2)),and superior long-term stability(200 mA cm^(−2),200 h),surpassing the reported catalysts of CO_(2) electroreduction to formate.Additionally,in situ attenuated total reflection-infrared spectra combined with theoretical calculations revealed that electron-enriched Sn sites on Ag/Sn-SnO_(2)NSs not only promote the formation of*OCHO and alleviate the energy barriers of*OCHO to*HCOOH,but also impede the desorption of H*.Notably,the Ag/Sn-SnO_(2)NSs as the cathode in a membrane electrode assembly with porous solid electrolyte layer reactor can continuously produce~0.12 M pure HCOOH solution at 100 mA cm^(−2)over 200 h.This work may inspire further development of advanced electrocatalysts and innovative device systems for promoting practical application of producing liquid fuels from CO_(2).展开更多
Imines play a pivotal role as multifunctional intermediates in pharmaceutical and biological applications,and their selective synthesis under mild conditions has attracted increasing interest.In this study,a covalent-...Imines play a pivotal role as multifunctional intermediates in pharmaceutical and biological applications,and their selective synthesis under mild conditions has attracted increasing interest.In this study,a covalent-organic frameworks(COFs)modified with CeO_(2)quantum dots(QDs)were prepared through a simple chemical reaction using LZU1 COFs and Ce(NO_(3))_(3)·6H_(2)O serving as precursors.The optimized5CeO_(2)QDs@LZU1 decorated with about 3.9 wt%CeO_(2)QDs demonstrates excellent performance in the photocatalytic selective oxidative of amines in visible light conditions,achieving nearly 100%benzylamine conversion and over 99%imine selectivity within 6 h,which is significantly superior to that of pure LZU1 and CeO_(2).The remarkable enhancement in activity is mainly attributed to the fact that the interaction between CeO_(2)QDs and LZU1 COF in 5CeO_(2)QDs@LZU1 improves the visible light response and concurrently promotes the separation efficiency of photogenerated e-and h+pairs.Broad substrate scope also provides a prospect for the industrial synthesis of various imines.Our findings not only expand the application range of COFs by incorporating QDs but also lay the groundwork for their further rational design and optimization.展开更多
Some K_2NiF_4-type oxides(A_2BO_4) exhibited good catalytic activity for oxidative coupling of methane to form ethane and ethylene. The C_2 selectivity was well correlated with A-site ion when B-site was Ti^(4+) ion.
In this work,the influence of CO2 on the structural variation and catalytic performance of Na2WO4/Mn/Si O2 for oxidative coupling of methane to ethylene was investigated. The catalyst was prepared by impregnation meth...In this work,the influence of CO2 on the structural variation and catalytic performance of Na2WO4/Mn/Si O2 for oxidative coupling of methane to ethylene was investigated. The catalyst was prepared by impregnation method and characterized by XRD,Raman and XPS techniques. Appropriate amount of CO2 in the reactant gases enhanced the formation of surface tetrahedral Na2WO4 species and promoted the migration of O in MOx,Na,W from the catalyst bulk to surface,which were favorable for oxidative coupling of methane. When the molar ratio of CH4/O2/CO2 was 3/1/2,enriched surface tetrahedral Na2WO4 species and high surface concentration of O in MOx,Na,W were detected,and then high CH4 conversion of 33.1% and high C2H4 selectivity of 56.2% were obtained. With further increase of CO2 in the reagent gases,the content of active surface tetrahedral Na2WO4 species and surface concentration of O in MOx,Na,W decreased,while that of inactive species（Mn WO4 and Mn2O3） increased dramatically,leading to low CH4 conversion and low C2H4 selectivity. It could be speculated that Na2WO4 crystal was transformed into Mn WO4 crystal with excessive CO2 added under the reaction conditions. Pretreatment of Na2WO4/Mn/Si O2 catalyst by moderate amount of CO2 before OCM also promoted the formation of Na2WO4 species.展开更多
Thermodynamic models of calculating mass action concentrations for structural units or ion couples in RbCl-H2O binary and RbCl-RbNO3-H2O ternary strong electrolyte aqueous solutions were developed based on the ion and...Thermodynamic models of calculating mass action concentrations for structural units or ion couples in RbCl-H2O binary and RbCl-RbNO3-H2O ternary strong electrolyte aqueous solutions were developed based on the ion and molecule coexistence theory at 298.15 K.A transformation coefficient is needed to compare the calculated mass action concentration and the reported activity because they are obtained at different standard states and concentration units.The results show that the transformation coefficients between the calculated mass action concentrations and the reported activities of the same structural units or ion couples in RbCl-H2O binary and RbCl-RbNO3-H2O ternary strong electrolyte aqueous solutions change in a very narrow range.The transformed mass action concentrations of structural units or ion couples in RbCl-H2O binary system are in good agreement with the reported activities. The transformed mass action concentrations of RbCl and RbNO3 in RbCl-RbNO3-H2O ternary solution are also in good agreement with the reported activities,aRbCl and 3RbNOa,with different total ionic strengths as 0.01,0.05,0.1,0.5,1.0,1.5,2.0,3.0 and 3.5 mol/kg,respectively.All those results mean the developed thermodynamic model of strong electrolyte aqueous solutions can reflect structural characteristics of RbCl-H2O binary and RbCl-RbNO3-H2O ternary strong electrolyte aqueous solutions and the mass action concentration also strictly follows the mass action law.展开更多
The effect of temperature and hydrogen addition on undesired carbonaceous deposit formation during methane coupling was studied in DBD-plasma catalytic-wall reactors with Pd/Al2 O3, using electrical power to drive the...The effect of temperature and hydrogen addition on undesired carbonaceous deposit formation during methane coupling was studied in DBD-plasma catalytic-wall reactors with Pd/Al2 O3, using electrical power to drive the reaction.Experiments with thin catalyst layers allowed comparison of the performance of empty reactors and catalytic wall reactors without significantly influencing the plasma properties.The product distribution varies strongly in the temperature window between 25 and 200℃Minimal formation of deposits is found at an optimal temperature around 75℃ in the catalytic-wall reactors.The selectivity to deposits was c.a.10% with only 9 mg of catalyst loading instead of 45% in the blank reactor,while decreasing methane conversion only mildly.Co-feeding H2 to an empty reactor causes a similar decrease in selectivity to deposits,but in this case methane conversion also decreased significantly.Suppression of deposits formation in the catalytic-wall reactor at 75℃ is due to catalytic hydrogenation of mainly acetylene to ethylene.In the empty reactor,H2 co-feed decreases conversion but does not change the product distribution.The catalytic-wall reactors can be regenerated with H2-plasma at room temperature,which produces more added-value hydrocarbons.展开更多
A facile method for the synthesis of 2,3,3＇,4＇-biphenyltetracarboxylic dianhydride（a-BPDA） was reported,which comprises the steps of the dehalogenative coupling of dimethyl 4-chlorophthalate（4-DMCP） and dimethyl...A facile method for the synthesis of 2,3,3＇,4＇-biphenyltetracarboxylic dianhydride（a-BPDA） was reported,which comprises the steps of the dehalogenative coupling of dimethyl 4-chlorophthalate（4-DMCP） and dimethyl 3-chlorophthalate（3-DMCP） catalyzed by low-cost（Ph 3 P） 2 NiCl 2,the hydrolysis of tetra-ester and the dehydration of tetra-acid.In contrast to the conventional methods,this method has the advantage of low cost,convenient manipulation,available condition,high purity and good overall yield.Moreover,the single crystal structure of a-BPDA was analyzed by X-ray diffraction method.The X-ray data suggest that a-BPDA is a rigid,non-coplanar and non-linear structure.It contains three crystallographically independent molecules,in which the dihedral angles of the two linked phenyl rings are 44.75（4）°,46.37（3）° and 42.32（3）°,respectively.The title molecule is governed by a stronger intermolecular interaction in contrast to van der Waals interaction because of the special positions of anhydride groups.展开更多
The 0.8Pb(Zr0.48Ti0.52)O3 – 0.125Pb(Zn1/3Nb2/3)O3 – 0.075Pb(Mn1/3Nb2/3)O3 + x wt% Fe2O3 ceramics (PZT-PZN-PMnN), where x = 0 ÷?0.35, has been prepared by two-stage calcination method. The effect of Fe2O3 conten...The 0.8Pb(Zr0.48Ti0.52)O3 – 0.125Pb(Zn1/3Nb2/3)O3 – 0.075Pb(Mn1/3Nb2/3)O3 + x wt% Fe2O3 ceramics (PZT-PZN-PMnN), where x = 0 ÷?0.35, has been prepared by two-stage calcination method. The effect of Fe2O3 content on the crystal structure and electrical properties of ceramics has been investigated. The results of X-ray diffraction (XRD) show that all samples have pure perovskite phase with tetragonal structure, the c/a ratio increases with increasing Fe2O3 content. At x = 0.25, electrical properties of ceramics are best: the density (r) of 7.86 g/cm3, the electromechanical coupling factor (kp) of 0.64, the dielectric constant (εr) of 1400, the dielectric loss (tand) of 0.003, the mechanical quality factor (Qm) of 1450, the piezoelectric constant (d31) of 155 pC/N, and the remanent polarization (Pr) of 37 μC/cm2, which makes it as a promising material for high power piezoelectric devices.展开更多
The stacking structure of Nb_(2)CSe_(2),a newly synthesized layered metal carbo-selenide,was elucidated by scanning transmission electron microscopy.Nb,CSe2 features Se-Nb-C-Nb-Se quintuple atomic layers.These layers ...The stacking structure of Nb_(2)CSe_(2),a newly synthesized layered metal carbo-selenide,was elucidated by scanning transmission electron microscopy.Nb,CSe2 features Se-Nb-C-Nb-Se quintuple atomic layers.These layers are stacked in Bernal mode.In this mode,Nb,CSe2 crystallizes in a trigonal symmetry(space group P3m1,No.164),with lattice parameters of a=3.33 A and c=18.20 A.Electronic structure calculations indicate that the metal carbo-selenide has Fermi energy crossing the bands where it touches to give a zero gap,indicating that it is an electronic conductor.As evidenced experimentally,the electrical conductivity is as high as 6.6×10^(5) S·m^(-1),outperforming the counterparts in the MXene family.Owing to the layered structure,the bonding in Nb_(2)CSe_(2) with an ionic formula of(Nb^(1.48+)),(C^(1.74-))(Se^(0.61-))_(2) is highly anisotropic,with metallic-covalent-ionic bonding in intralayers and weak bonding between interlayers.The layered nature is further evidenced by elastic properties,interlayer energy,and friction coefficient determination.These characteristics indicate that Nb_(2)CSe_(2) is an analog of molybdenum disulfide(MoS_(2)),which is a typical binary van der Waals(vdW)solid.Moreover,vibrational properties are reported,which may offer an optical identification standardfor new ternary vdW solids in spectroscopic studies,including Raman scatteringand infrared absorption.展开更多
基金supported by the National Natural Science Foundation of China(Nos.10672121 and 11072125)
文摘Through the Galerkin method the nonlinear ordinary differential equations （ODEs） in time are obtained from the nonlinear partial differential equations （PDEs） to describe the mo- tion of the coupled structure of a suspended-cable-stayed beam. In the PDEs, the curvature of main cables and the deformation of cable stays are taken into account. The dynamics of the struc- ture is investigated based on the ODEs when the structure is subjected to a harmonic excitation in the presence of both high-frequency principle resonance and 1：2 internal resonance. It is found that there are typical jumps and saturation phenomena of the vibration amplitude in the struc- ture. And the structure may present quasi-periodic vibration or chaos, if the stiffness of the cable stays membrane and frequency of external excitation are disturbed.
基金supported by the National Natural Science Foundation of China(22265021)the Aeronautical Science Foundation of China(2020Z056056003)Jiangxi Provincial Natural Science Foundation(20232BAB212004).
文摘Atomic-scale doping strategies and structure design play pivotal roles in tailoring the electronic structure and physicochemical property of electromagnetic wave absorption(EMWA)materials.However,the relationship between configuration and electromagnetic(EM)loss mechanism has remained elusive.Herein,drawing inspiration from the DNA transcription process,we report the successful synthesis of novel in situ Mn/N co-doped helical carbon nanotubes with ultrabroad EMWA capability.Theoretical calculation and EM simulation confirm that the orbital coupling and spin polarization of the Mn–N4–C configuration,along with cross polarization generated by the helical structure,endow the helical converters with enhanced EM loss.As a result,HMC-8 demonstrates outstanding EMWA performance,achieving a minimum reflection loss of−63.13 dB at an ultralow thickness of 1.29 mm.Through precise tuning of the graphite domain size,HMC-7 achieves an effective absorption bandwidth(EAB)of 6.08 GHz at 2.02 mm thickness.Furthermore,constructing macroscale gradient metamaterials enables an ultrabroadband EAB of 12.16 GHz at a thickness of only 5.00 mm,with the maximum radar cross section reduction value reaching 36.4 dB m2.This innovative approach not only advances the understanding of metal–nonmetal co-doping but also realizes broadband EMWA,thus contributing to the development of EMWA mechanisms and applications.
基金the National Science Fund for Distinguished Young Scholars(Grant No.52125103)the National Natural Science Foundation of China(Grant Nos.52301232,52071041,12074048,and 12147102)China Postdoctoral Science Foundation(Grant No.2022M720552).
文摘Electrocatalytic reduction of CO_(2) converts intermittent renewable electricity into value-added liquid products with an enticing prospect,but its practical application is hampered due to the lack of high-performance electrocatalysts.Herein,we elaborately design and develop strongly coupled nanosheets composed of Ag nanoparticles and Sn-SnO_(2) grains,designated as Ag/Sn-SnO_(2) nanosheets(NSs),which possess optimized electronic structure,high electrical conductivity,and more accessible sites.As a result,such a catalyst exhibits unprecedented catalytic performance toward CO_(2)-to-formate conversion with near-unity faradaic efficiency(≥90%),ultrahigh partial current density(2,000 mA cm^(−2)),and superior long-term stability(200 mA cm^(−2),200 h),surpassing the reported catalysts of CO_(2) electroreduction to formate.Additionally,in situ attenuated total reflection-infrared spectra combined with theoretical calculations revealed that electron-enriched Sn sites on Ag/Sn-SnO_(2)NSs not only promote the formation of*OCHO and alleviate the energy barriers of*OCHO to*HCOOH,but also impede the desorption of H*.Notably,the Ag/Sn-SnO_(2)NSs as the cathode in a membrane electrode assembly with porous solid electrolyte layer reactor can continuously produce~0.12 M pure HCOOH solution at 100 mA cm^(−2)over 200 h.This work may inspire further development of advanced electrocatalysts and innovative device systems for promoting practical application of producing liquid fuels from CO_(2).
基金Project supported by the National Natural Science Foundation of China(22176054,22306060)China Postdoctoral Science Foundation(2022M721134)the Fundamental Research Funds for the Central Universities(2023MS060)。
文摘Imines play a pivotal role as multifunctional intermediates in pharmaceutical and biological applications,and their selective synthesis under mild conditions has attracted increasing interest.In this study,a covalent-organic frameworks(COFs)modified with CeO_(2)quantum dots(QDs)were prepared through a simple chemical reaction using LZU1 COFs and Ce(NO_(3))_(3)·6H_(2)O serving as precursors.The optimized5CeO_(2)QDs@LZU1 decorated with about 3.9 wt%CeO_(2)QDs demonstrates excellent performance in the photocatalytic selective oxidative of amines in visible light conditions,achieving nearly 100%benzylamine conversion and over 99%imine selectivity within 6 h,which is significantly superior to that of pure LZU1 and CeO_(2).The remarkable enhancement in activity is mainly attributed to the fact that the interaction between CeO_(2)QDs and LZU1 COF in 5CeO_(2)QDs@LZU1 improves the visible light response and concurrently promotes the separation efficiency of photogenerated e-and h+pairs.Broad substrate scope also provides a prospect for the industrial synthesis of various imines.Our findings not only expand the application range of COFs by incorporating QDs but also lay the groundwork for their further rational design and optimization.
基金Subject supported by the National Natural Science Foundation of China
文摘Some K_2NiF_4-type oxides(A_2BO_4) exhibited good catalytic activity for oxidative coupling of methane to form ethane and ethylene. The C_2 selectivity was well correlated with A-site ion when B-site was Ti^(4+) ion.
基金support from the Ministry of Science and Technology (Nos.2012BAC20B10)the National Natural Science Foundation of China (Nos. 21321061 and 20976109)
文摘In this work,the influence of CO2 on the structural variation and catalytic performance of Na2WO4/Mn/Si O2 for oxidative coupling of methane to ethylene was investigated. The catalyst was prepared by impregnation method and characterized by XRD,Raman and XPS techniques. Appropriate amount of CO2 in the reactant gases enhanced the formation of surface tetrahedral Na2WO4 species and promoted the migration of O in MOx,Na,W from the catalyst bulk to surface,which were favorable for oxidative coupling of methane. When the molar ratio of CH4/O2/CO2 was 3/1/2,enriched surface tetrahedral Na2WO4 species and high surface concentration of O in MOx,Na,W were detected,and then high CH4 conversion of 33.1% and high C2H4 selectivity of 56.2% were obtained. With further increase of CO2 in the reagent gases,the content of active surface tetrahedral Na2WO4 species and surface concentration of O in MOx,Na,W decreased,while that of inactive species（Mn WO4 and Mn2O3） increased dramatically,leading to low CH4 conversion and low C2H4 selectivity. It could be speculated that Na2WO4 crystal was transformed into Mn WO4 crystal with excessive CO2 added under the reaction conditions. Pretreatment of Na2WO4/Mn/Si O2 catalyst by moderate amount of CO2 before OCM also promoted the formation of Na2WO4 species.
基金Project supported by Publication Foundation of National Science and Technology Academic Books of China
文摘Thermodynamic models of calculating mass action concentrations for structural units or ion couples in RbCl-H2O binary and RbCl-RbNO3-H2O ternary strong electrolyte aqueous solutions were developed based on the ion and molecule coexistence theory at 298.15 K.A transformation coefficient is needed to compare the calculated mass action concentration and the reported activity because they are obtained at different standard states and concentration units.The results show that the transformation coefficients between the calculated mass action concentrations and the reported activities of the same structural units or ion couples in RbCl-H2O binary and RbCl-RbNO3-H2O ternary strong electrolyte aqueous solutions change in a very narrow range.The transformed mass action concentrations of structural units or ion couples in RbCl-H2O binary system are in good agreement with the reported activities. The transformed mass action concentrations of RbCl and RbNO3 in RbCl-RbNO3-H2O ternary solution are also in good agreement with the reported activities,aRbCl and 3RbNOa,with different total ionic strengths as 0.01,0.05,0.1,0.5,1.0,1.5,2.0,3.0 and 3.5 mol/kg,respectively.All those results mean the developed thermodynamic model of strong electrolyte aqueous solutions can reflect structural characteristics of RbCl-H2O binary and RbCl-RbNO3-H2O ternary strong electrolyte aqueous solutions and the mass action concentration also strictly follows the mass action law.
基金supported by the Innovation Fund for Chemistry of NWO,grant number 731.014.303by SASOL。
文摘The effect of temperature and hydrogen addition on undesired carbonaceous deposit formation during methane coupling was studied in DBD-plasma catalytic-wall reactors with Pd/Al2 O3, using electrical power to drive the reaction.Experiments with thin catalyst layers allowed comparison of the performance of empty reactors and catalytic wall reactors without significantly influencing the plasma properties.The product distribution varies strongly in the temperature window between 25 and 200℃Minimal formation of deposits is found at an optimal temperature around 75℃ in the catalytic-wall reactors.The selectivity to deposits was c.a.10% with only 9 mg of catalyst loading instead of 45% in the blank reactor,while decreasing methane conversion only mildly.Co-feeding H2 to an empty reactor causes a similar decrease in selectivity to deposits,but in this case methane conversion also decreased significantly.Suppression of deposits formation in the catalytic-wall reactor at 75℃ is due to catalytic hydrogenation of mainly acetylene to ethylene.In the empty reactor,H2 co-feed decreases conversion but does not change the product distribution.The catalytic-wall reactors can be regenerated with H2-plasma at room temperature,which produces more added-value hydrocarbons.
基金Supported by the Ministry of Education Project Combining the Industry and Teaching with Research of Guangdong Province,China(No.2011B090400062)
文摘A facile method for the synthesis of 2,3,3＇,4＇-biphenyltetracarboxylic dianhydride（a-BPDA） was reported,which comprises the steps of the dehalogenative coupling of dimethyl 4-chlorophthalate（4-DMCP） and dimethyl 3-chlorophthalate（3-DMCP） catalyzed by low-cost（Ph 3 P） 2 NiCl 2,the hydrolysis of tetra-ester and the dehydration of tetra-acid.In contrast to the conventional methods,this method has the advantage of low cost,convenient manipulation,available condition,high purity and good overall yield.Moreover,the single crystal structure of a-BPDA was analyzed by X-ray diffraction method.The X-ray data suggest that a-BPDA is a rigid,non-coplanar and non-linear structure.It contains three crystallographically independent molecules,in which the dihedral angles of the two linked phenyl rings are 44.75（4）°,46.37（3）° and 42.32（3）°,respectively.The title molecule is governed by a stronger intermolecular interaction in contrast to van der Waals interaction because of the special positions of anhydride groups.
文摘The 0.8Pb(Zr0.48Ti0.52)O3 – 0.125Pb(Zn1/3Nb2/3)O3 – 0.075Pb(Mn1/3Nb2/3)O3 + x wt% Fe2O3 ceramics (PZT-PZN-PMnN), where x = 0 ÷?0.35, has been prepared by two-stage calcination method. The effect of Fe2O3 content on the crystal structure and electrical properties of ceramics has been investigated. The results of X-ray diffraction (XRD) show that all samples have pure perovskite phase with tetragonal structure, the c/a ratio increases with increasing Fe2O3 content. At x = 0.25, electrical properties of ceramics are best: the density (r) of 7.86 g/cm3, the electromechanical coupling factor (kp) of 0.64, the dielectric constant (εr) of 1400, the dielectric loss (tand) of 0.003, the mechanical quality factor (Qm) of 1450, the piezoelectric constant (d31) of 155 pC/N, and the remanent polarization (Pr) of 37 μC/cm2, which makes it as a promising material for high power piezoelectric devices.
基金supported by the Shenyang National Laboratory for Materials Science,Institute of Metal Research,Chinese Academy of Sciences,Basic and Applied Basic Research Program of Guangdong Province(No.2021B0301030003)Ji Hua Laboratory(No.X210141TL210)。
文摘The stacking structure of Nb_(2)CSe_(2),a newly synthesized layered metal carbo-selenide,was elucidated by scanning transmission electron microscopy.Nb,CSe2 features Se-Nb-C-Nb-Se quintuple atomic layers.These layers are stacked in Bernal mode.In this mode,Nb,CSe2 crystallizes in a trigonal symmetry(space group P3m1,No.164),with lattice parameters of a=3.33 A and c=18.20 A.Electronic structure calculations indicate that the metal carbo-selenide has Fermi energy crossing the bands where it touches to give a zero gap,indicating that it is an electronic conductor.As evidenced experimentally,the electrical conductivity is as high as 6.6×10^(5) S·m^(-1),outperforming the counterparts in the MXene family.Owing to the layered structure,the bonding in Nb_(2)CSe_(2) with an ionic formula of(Nb^(1.48+)),(C^(1.74-))(Se^(0.61-))_(2) is highly anisotropic,with metallic-covalent-ionic bonding in intralayers and weak bonding between interlayers.The layered nature is further evidenced by elastic properties,interlayer energy,and friction coefficient determination.These characteristics indicate that Nb_(2)CSe_(2) is an analog of molybdenum disulfide(MoS_(2)),which is a typical binary van der Waals(vdW)solid.Moreover,vibrational properties are reported,which may offer an optical identification standardfor new ternary vdW solids in spectroscopic studies,including Raman scatteringand infrared absorption.
