The Ti-Supported MnO2 electrode was modified by introducing SnO2 +RuO2 +MnO2 as an intermediate layer into the Ti/MnO2 interface. The anodic polarization curves were measured at various temperatures ranging from 30 to...The Ti-Supported MnO2 electrode was modified by introducing SnO2 +RuO2 +MnO2 as an intermediate layer into the Ti/MnO2 interface. The anodic polarization curves were measured at various temperatures ranging from 30 to 80 ℃ and the activation energy for the oxygen evolution reaction was evaluated. The experimental activation energy increased linearly with increasing the overpotential. The activation energy at the equilibrium potential was linearly correlated with the difference between the crystal field stabilization energies of Mn4+ at initial state and Mn4+ at transition state. The electrocatalysis characteristics of the anode were discussed by means of themechanism of the substitution reaction of the ligand(SN 1 and SN2) and molecular orbital theory.The results show that the anode has better electrocatalystic characteristics.展开更多
An iterative method is used to find the values of the Hamiltonian parameters for Yb3+ in a given low-symmetry crystalline site.Samples of Yb3+:RETaO4(RE = Gd,Y,and Sc) were prepared and their structures were determine...An iterative method is used to find the values of the Hamiltonian parameters for Yb3+ in a given low-symmetry crystalline site.Samples of Yb3+:RETaO4(RE = Gd,Y,and Sc) were prepared and their structures were determined.Based on the obtained structural data,their orbital-spin parameters and crystal field parameters were fitted by the superposition model(SM).Using the crystal field parameters obtained by the SM fitting as the initial parameters,the Hamiltonian parameters were fitted iteratively.The calculated and experimental energy levels for Yb3+:RETaO4 are consistent,and the maximal mean-root-square deviation is only 2.84 cm-1,indicating that the method is effective to determine the Hamiltonian parameters of Yb3+ in low-symmetry crystalline sites.展开更多
A series of M-Type barium hexaferrites with the general composition BaFe12-2xMoxZnxO19 were synthesized at 1100°C by a simple wet chemical mixture route. The properties of the prepared samples were examined by...A series of M-Type barium hexaferrites with the general composition BaFe12-2xMoxZnxO19 were synthesized at 1100°C by a simple wet chemical mixture route. The properties of the prepared samples were examined by X-ray diffraction, scanning electron microscopy, vibrating sample magnetometry, and Mössbauer spectroscopy. The diffraction patterns for all samples were found to agree well with the standard pattern of BaFe12O19 hexaferrite with no extraneous diffraction peaks. The products formed as well crystallized hexagonal platelet-like particles while the EDS measurements revealed the stoichiometric cationic ratios of the prepared samples. The spectral variations elucidated by Mössbauer spectroscopy were utilized to determine the different cation preferential site occupations as a function of x. Finally, the saturation magnetizations, magnetic anisotropies, and the anisotropy fields, determined from the magnetic measurements, showed consistency with the relative subspectral Mössbauer intensities and the single ion model for the anisotropy constant.展开更多
Excellent performances promoted by lattice oxygen have attracted wide attention for catalytic degradation of volatile organic compounds(VOCs).However,how to control the continuous regeneration of lattice oxygen from t...Excellent performances promoted by lattice oxygen have attracted wide attention for catalytic degradation of volatile organic compounds(VOCs).However,how to control the continuous regeneration of lattice oxygen from the support is seldom reported.In this study,we selected sepiolite supported manganese-cobalt oxides(Co_(x)Mn_(100-x)O_(y))as model catalysts by tuning Co/(Co+Mn)mass ratio(x=3%,10%,15%,and 20%)to enhance toluene degradation efficiency,owing to lattice oxygen regeneration by redox cycle existing at the interface and Mn species with high valence state,initiated by cobalt catalytic performance under the role of crystal field stability phase.The results of activity test show that the sepiolite-Co_(15)Mn_(85)O_(y)catalyst exhibit outperformances at 193℃with 10,000 h^(-1)GHSV.In addition,the catalyst existed at the bottom of the"volcano"curve correlated T_(50)or T_(90)with Co/(Co+Mn)weight ratio is sepiolite-Co_(15)Mn_(85)O_(y),conforming its outperformance.Further characterized by investigating active sites structural and electronic properties,the essential of superior catalytic activity is attributed to the grands of lattice oxygen continuous formation resulted from redox engineering based on the high atomic ratio of surface lattice oxygen with continuous refilled from the support and that of Mn^(4+)/Mn^(3+)cycle initiated by cobalt catalytic behaviors.All in all,redox engineering,not only promotes grands of active species reversible regeneration,but supplies an alternative catalyst design strategy towards the terrific efficiency-to-cost ratio performance.展开更多
We report the observation of electric field induced random lasing in a dye doped liquid crystal system. This was achieved by using a liquid crystal host with negative dielectric anisotropy doped with laser dye PM 597 ...We report the observation of electric field induced random lasing in a dye doped liquid crystal system. This was achieved by using a liquid crystal host with negative dielectric anisotropy doped with laser dye PM 597 in a 75 μm cell with a homeotropic alignment layer. In the absence of an applied field, only amplified spontaneous emission was observed since the liquid crystal orientation was uniform. However, application of a field resulted in a fieldinduced planar-like configuration with local nonuniformity in liquid crystal orientation. This led to random lasing in the energized state(voltage greater than a transition threshold). The onset of lasing occurs by application of either a spatially homogenous or a spatially inhomogeneous electric field across the liquid crystal. The characteristics of the emission spectra as a function of different(i) dye concentration and(ii) applied voltage were investigated using nanosecond pulsed laser excitation at 532 nm. The effects of using an inhomogeneous field were compared to the use of a homogenous field and reported. It is shown that the spatial configuration can be used to alter the emission spectra of the system. The work is used to suggest a new configuration, referred to here as"reverse mode," for liquid crystal-based random lasers. This new configuration may provide additional avenues for their use in commercial devices.展开更多
We introduce a fast solver for the phase field crystal(PFC)and functionalized Cahn-Hilliard(FCH)equations with periodic boundary conditions on a rectangular domain that features the preconditioned Nesterov’s accelera...We introduce a fast solver for the phase field crystal(PFC)and functionalized Cahn-Hilliard(FCH)equations with periodic boundary conditions on a rectangular domain that features the preconditioned Nesterov’s accelerated gradient descent(PAGD)method.We discretize these problems with a Fourier collocation method in space,and employ various second-order schemes in time.We observe a significant speedup with this solver when compared to the preconditioned gradient descent(PGD)method.With the PAGD solver,fully implicit,second-order-in-time schemes are not only feasible to solve the PFC and FCH equations,but also do so more efficiently than some semi-implicit schemes in some cases where accuracy issues are taken into account.Benchmark computations of four different schemes for the PFC and FCH equations are conducted and the results indicate that,for the FCH experiments,the fully implicit schemes(midpoint rule and BDF2 equipped with the PAGD as a nonlinear time marching solver)perform better than their IMEX versions in terms of computational cost needed to achieve a certain precision.For the PFC,the results are not as conclusive as in the FCH experiments,which,we believe,is due to the fact that the nonlinearity in the PFC is milder nature compared to the FCH equation.We also discuss some practical matters in applying the PAGD.We introduce an averaged Newton preconditioner and a sweeping-friction strategy as heuristic ways to choose good preconditioner parameters.The sweeping-friction strategy exhibits almost as good a performance as the case of the best manually tuned parameters.展开更多
In this paper,we will develop a first order and a second order convex splitting,and a first order linear energy stable fully discrete local discontinuous Galerkin(LDG)methods for the modified phase field crystal(MPFC)...In this paper,we will develop a first order and a second order convex splitting,and a first order linear energy stable fully discrete local discontinuous Galerkin(LDG)methods for the modified phase field crystal(MPFC)equation.In which,the first order linear scheme is based on the invariant energy quadratization approach.The MPFC equation is a damped wave equation,and to preserve an energy stability,it is necessary to introduce a pseudo energy,which all increase the difficulty of constructing numerical methods comparing with the phase field crystal(PFC)equation.Due to the severe time step restriction of explicit timemarchingmethods,we introduce the first order and second order semi-implicit schemes,which are proved to be unconditionally energy stable.In order to improve the temporal accuracy,the semi-implicit spectral deferred correction(SDC)method combining with the first order convex splitting scheme is employed.Numerical simulations of the MPFC equation always need long time to reach steady state,and then adaptive time-stepping method is necessary and of paramount importance.The schemes at the implicit time level are linear or nonlinear and we solve them by multigrid solver.Numerical experiments of the accuracy and long time simulations are presented demonstrating the capability and efficiency of the proposed methods,and the effectiveness of the adaptive time-stepping strategy.展开更多
In this paper,we construct efficient schemes based on the scalar auxiliary variable block-centered finite difference method for the modified phase field crystal equation,which is a sixth-order nonlinear damped wave eq...In this paper,we construct efficient schemes based on the scalar auxiliary variable block-centered finite difference method for the modified phase field crystal equation,which is a sixth-order nonlinear damped wave equation.The schemes are linear,conserve mass and unconditionally dissipate a pseudo energy.We prove rigorously second-order error estimates in both time and space for the phase field variable in discrete norms.We also present some numerical experiments to verify our theoretical results and demonstrate the robustness and accuracy.展开更多
In this paper,based on the Lagrange Multiplier approach in time and the Fourierspectral scheme for space,we propose efficient numerical algorithms to solve the phase field crystal equation.The numerical schemes are u...In this paper,based on the Lagrange Multiplier approach in time and the Fourierspectral scheme for space,we propose efficient numerical algorithms to solve the phase field crystal equation.The numerical schemes are unconditionally energy stable based on the original energy and do not need the lower bound hypothesis of the nonlinear free energy potential.The unconditional energy stability of the three semi-discrete schemes is proven.Several numerical simulations in 2D and 3D are demonstrated to verify the accuracy and efficiency of our proposed schemes.展开更多
Crystal-field symmetry of lanthanide ions plays a critical role in suppressing quantum tunneling of magnetization(QTM)in single-molecule magnets(SMMs),but high-performance SMM design and modulation remain challenging ...Crystal-field symmetry of lanthanide ions plays a critical role in suppressing quantum tunneling of magnetization(QTM)in single-molecule magnets(SMMs),but high-performance SMM design and modulation remain challenging only in view of the geometric symmetry of the first coordination sphere.Herein,two bis(semicarbazone)/bis(thiosemicarbazone)dysprosium singleion magnets with pentagonal bipyramid geometry were reported,and bis(thiosemicarbazone)lanthanide complexes have never been reported to the best of our knowledge.They served as good archetypes to study the magneto-structural relationships based on the charge distribution.The complex with more ideal geometric symmetry displays fast zero-field QTM with negligible“effective barrier”,owing to the defective charge distribution.By modulating the transverse crystal field via the replacement of the O sites with the less charged and larger radius S atoms,it results in lower geometric but higher charge-distribution symmetry,giving rise to the significant suppression of QTM and the enhancement of reversal barrier up to ca.1,000 K.These results demonstrate that the charge-distribution symmetry can be chemically tailored by modification of the crystal field,which is essential for designing high-performance SMMs.展开更多
As the fourth-generation light source,solid-state lighting has developed rapidly in the past 30 years due to its advantages of high efficiency and environmental protection.It is widely used in various scenes such as a...As the fourth-generation light source,solid-state lighting has developed rapidly in the past 30 years due to its advantages of high efficiency and environmental protection.It is widely used in various scenes such as automobile headlights,projection displays,industrial production,and remote lighting.High-power,high-brightness white light-emitting diodes(LEDs)and laser diodes(LDs)technology put forward new requirements for the service stability of color conversion materials.Garnet phosphor ceramics have emerged with their unique advantages of withstanding high power excitation density and the flexibly tunable spectrum.In this article,the research progress of garnet based phosphor ceramics for high-power solid-state lighting was comprehensively reviewed.Firstly,the band gap and coordination environment regulations of luminescence centers of garnet phosphor were summarized.Secondly,the improvement of luminous efficacy via defects regulation was discussed.Thirdly,the relationship between the geometric design and the lighting performance was elucidated.Fourthly,the characterization methods of phosphor ceramics for laser lighting were introduced and illustrated.Finally,the development trend of garnet phos-phor ceramics in solid state lighting and display was prospected.展开更多
Electromagnetic wave(EMW)absorbing materials play a vital role in modern communication and information processing technologies to inhibit information leakage and prevent possible damages to environment and human bodie...Electromagnetic wave(EMW)absorbing materials play a vital role in modern communication and information processing technologies to inhibit information leakage and prevent possible damages to environment and human bodies.Currently,most of EMW absorbing materials are either composites of two or more phases or in the form of nanosheets,nanowires or nanofibers in order to enhance the EMW absorption performance through dielectric loss,magnetic loss and dielectric/magnetic loss coupling.However,the combination of complex shapes/multi phases and nanosizes may compound the difficulties of materials processing,composition and interfaces control as well as performance maintenance during service.Thus,searching for single phase materials with good stability and superior EMW absorbing properties is appealing.To achieve this goal,the EMW absorbing properties of transition metal carbides TMCs(TM=Ti,Zr,Hf,Nb and Ta)and high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C which belong to ultrahigh temperature ceramics,were investigated in this work.Due to the good electrical conductivity and splitting of d orbitals into lower energy t2glevel and higher energy eglevel in TMC6octahedral arrangement,TMCs(TM=Ti,Zr,Hf,Nb and Ta)exhibit good EMW absorbing properties.Especially,Hf C and Ta C exhibit superior EMW absorbing properties.The minimum reflection loss(RLmin)value of Hf C is-55.8 d B at 6.0 GHz with the thickness of 3.8 mm and the effective absorption bandwidth(E_(AB))is 6.0 GHz from 12.0 to 18.0 GHz at thickness of 1.9 mm;the RL_(minvalue)of Ta C reaches-41.1 d B at 16.2 GHz with a thickness of 2.0 mm and the EABis 6.1 GHz with a thickness of 2.2 mm.Intriguingly,the electromagnetic parameters,i.e.,complex permittivity and permeability are tunable by forming single phase solid solution or high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C.The R_(Lminvalue)of high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C is-38.5 d B at 9.5 GHz with the thickness of 1.9 mm,and the EABis 2.3 GHz(from 11.3 to 13.6 GHz)at thickness of 1.5 mm.The significance of this work is that it opens a new window to design single phase high performance EMW absorbing materials by dielectric/magnetic loss coupling through tuning the conductivity and crystal field splitting energy of d orbitals of transition metals in carbides,nitrides and possibly borides.展开更多
Controlling molecular magnetic anisotropy via structural engineering is delicate and fascinating,especially for single-molecule magnets(SMMs).Herein a family of dysprosium single-ion magnets(SIMs)sitting in pentagonal...Controlling molecular magnetic anisotropy via structural engineering is delicate and fascinating,especially for single-molecule magnets(SMMs).Herein a family of dysprosium single-ion magnets(SIMs)sitting in pentagonal bipyramid geometry have been synthesized with the variable-size terminal ligands and counter anions,through which the subtle coordination geometry of Dy(Ⅲ)can be finely tuned based on the size effect.The effective energy barrier(Ueff)successfully increases from 439 to 632 K and the magnetic hysteresis temperature(under a 200 Oe/s sweep rate)raises from 11 to 24 K.Based on the crystal-field theory,a semi-quantitative magneto-structural correlation deduced experimentally for the first time is revealed that the Ueff is linearly proportional to the structural-related value S20 corresponding to the axial coordination bond lengths and the bond angles.Through the evaluation of the remanent magnetization from hysteresis,quantum tunneling of magnetization(QTM)is found to exhibit negative correlation with the structural-related value Stun corresponding to the axial coordination bond angles.展开更多
Rare earth phosphates have been used extensively in luminescent phosphors.Hexagonal Ce1-xGdxPO4 with crystal field manipulation was successfully synthesized using a hydrothermal method.The photoluminescence emission i...Rare earth phosphates have been used extensively in luminescent phosphors.Hexagonal Ce1-xGdxPO4 with crystal field manipulation was successfully synthesized using a hydrothermal method.The photoluminescence emission intensity of hexagonal CePO4 was obviously enhanced by the crystal structure manipulation with gadolinium ions codoping.Compared to pure CePO4,the intensity photoluminescence was enhanced about 15 folds with x=0.05.The effect of gadolinium doping was systematically investigated by X-ray diffraction,transmission electron microscopy,X-ray photoelectron spectroscopy and photoluminescence spectroscopy.Crystal field asymmetry can be effectively tuned by different amount of Gd^3+codoping,and the crystal field asymmetry is conductive to electron population of high energy level of Ce^3+ions.This material has potential applications in optics,electronics fields,and so on.Meanwhile,the method can be extended to another kind of high performance photoluminescence materials preparation.展开更多
Mg_(3)Sb_(2) as a Zintl compound is a promising thermoelectric material with the intrinsically low lattice thermal conductivity and excellent n-type electrical properties,but its p-type electrical transport properties...Mg_(3)Sb_(2) as a Zintl compound is a promising thermoelectric material with the intrinsically low lattice thermal conductivity and excellent n-type electrical properties,but its p-type electrical transport properties are poor.Here,the thermoelectric performance of Mg_(3)Sb_(2) under the effect of biaxial strain is investigated by using first-principles method and Boltzmann transport theory.The application of biaxial strain enables tuning the band structure of Mg_(3)Sb_(2) in such a way that the band degeneracy of both the conduction band and valence band increases.As the biaxial strain increases,the Seebeck coefficient of ptype Mg_(3)Sb_(2) has a remarkable increase,leading to a significant improvement in power factor.This is mainly ascribed to the achievement of valence band orbital degeneracy.Meanwhile,the lattice thermal conductivity exhibits very slight biaxial strain dependence within the strain range considered in this work,which increases from 1.28 to 1.62 W m^(-1) K^(-1) at 300 K.Finally,the highest ZT of p-type Mg_(3)Sb_(2) at 700 K can be up to 2.6 along the in-plane direction under-2.5%biaxial strain,which is almost three times that of the unstrained counterpart.The realization of high thermoelectric performance of p-type Mg_(3)Sb_(2) will promote its practical applications as thermoelectric generators.展开更多
基金Supported by the National Natural Science Foundation of China.
文摘The Ti-Supported MnO2 electrode was modified by introducing SnO2 +RuO2 +MnO2 as an intermediate layer into the Ti/MnO2 interface. The anodic polarization curves were measured at various temperatures ranging from 30 to 80 ℃ and the activation energy for the oxygen evolution reaction was evaluated. The experimental activation energy increased linearly with increasing the overpotential. The activation energy at the equilibrium potential was linearly correlated with the difference between the crystal field stabilization energies of Mn4+ at initial state and Mn4+ at transition state. The electrocatalysis characteristics of the anode were discussed by means of themechanism of the substitution reaction of the ligand(SN 1 and SN2) and molecular orbital theory.The results show that the anode has better electrocatalystic characteristics.
基金supported by the National Natural Science Foundation of China (Grant Nos. 90922003,51172236,and 50872135)the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. YYYJ-1002)
文摘An iterative method is used to find the values of the Hamiltonian parameters for Yb3+ in a given low-symmetry crystalline site.Samples of Yb3+:RETaO4(RE = Gd,Y,and Sc) were prepared and their structures were determined.Based on the obtained structural data,their orbital-spin parameters and crystal field parameters were fitted by the superposition model(SM).Using the crystal field parameters obtained by the SM fitting as the initial parameters,the Hamiltonian parameters were fitted iteratively.The calculated and experimental energy levels for Yb3+:RETaO4 are consistent,and the maximal mean-root-square deviation is only 2.84 cm-1,indicating that the method is effective to determine the Hamiltonian parameters of Yb3+ in low-symmetry crystalline sites.
基金supported by a generous grant from the Deanship of Scientific research at the University of Jordan under contract number 1404
文摘A series of M-Type barium hexaferrites with the general composition BaFe12-2xMoxZnxO19 were synthesized at 1100°C by a simple wet chemical mixture route. The properties of the prepared samples were examined by X-ray diffraction, scanning electron microscopy, vibrating sample magnetometry, and Mössbauer spectroscopy. The diffraction patterns for all samples were found to agree well with the standard pattern of BaFe12O19 hexaferrite with no extraneous diffraction peaks. The products formed as well crystallized hexagonal platelet-like particles while the EDS measurements revealed the stoichiometric cationic ratios of the prepared samples. The spectral variations elucidated by Mössbauer spectroscopy were utilized to determine the different cation preferential site occupations as a function of x. Finally, the saturation magnetizations, magnetic anisotropies, and the anisotropy fields, determined from the magnetic measurements, showed consistency with the relative subspectral Mössbauer intensities and the single ion model for the anisotropy constant.
基金Supported by the National Natural Science Foundation of China(21707023)Provincial Key Research and Development Plan of Hunan Province(2018SK2034)New Faculty Start-Up Funding from Xiangtan University(18QDZ16)。
文摘Excellent performances promoted by lattice oxygen have attracted wide attention for catalytic degradation of volatile organic compounds(VOCs).However,how to control the continuous regeneration of lattice oxygen from the support is seldom reported.In this study,we selected sepiolite supported manganese-cobalt oxides(Co_(x)Mn_(100-x)O_(y))as model catalysts by tuning Co/(Co+Mn)mass ratio(x=3%,10%,15%,and 20%)to enhance toluene degradation efficiency,owing to lattice oxygen regeneration by redox cycle existing at the interface and Mn species with high valence state,initiated by cobalt catalytic performance under the role of crystal field stability phase.The results of activity test show that the sepiolite-Co_(15)Mn_(85)O_(y)catalyst exhibit outperformances at 193℃with 10,000 h^(-1)GHSV.In addition,the catalyst existed at the bottom of the"volcano"curve correlated T_(50)or T_(90)with Co/(Co+Mn)weight ratio is sepiolite-Co_(15)Mn_(85)O_(y),conforming its outperformance.Further characterized by investigating active sites structural and electronic properties,the essential of superior catalytic activity is attributed to the grands of lattice oxygen continuous formation resulted from redox engineering based on the high atomic ratio of surface lattice oxygen with continuous refilled from the support and that of Mn^(4+)/Mn^(3+)cycle initiated by cobalt catalytic behaviors.All in all,redox engineering,not only promotes grands of active species reversible regeneration,but supplies an alternative catalyst design strategy towards the terrific efficiency-to-cost ratio performance.
文摘We report the observation of electric field induced random lasing in a dye doped liquid crystal system. This was achieved by using a liquid crystal host with negative dielectric anisotropy doped with laser dye PM 597 in a 75 μm cell with a homeotropic alignment layer. In the absence of an applied field, only amplified spontaneous emission was observed since the liquid crystal orientation was uniform. However, application of a field resulted in a fieldinduced planar-like configuration with local nonuniformity in liquid crystal orientation. This led to random lasing in the energized state(voltage greater than a transition threshold). The onset of lasing occurs by application of either a spatially homogenous or a spatially inhomogeneous electric field across the liquid crystal. The characteristics of the emission spectra as a function of different(i) dye concentration and(ii) applied voltage were investigated using nanosecond pulsed laser excitation at 532 nm. The effects of using an inhomogeneous field were compared to the use of a homogenous field and reported. It is shown that the spatial configuration can be used to alter the emission spectra of the system. The work is used to suggest a new configuration, referred to here as"reverse mode," for liquid crystal-based random lasers. This new configuration may provide additional avenues for their use in commercial devices.
基金NSF grants DMS-1720213,DMS-1719854,and DMS-2012634NSF grants DMS-1720213 and DMS-2111228.The work of S.M.Wise was partially supported by DMS-1719854 and DMS-2012634.
文摘We introduce a fast solver for the phase field crystal(PFC)and functionalized Cahn-Hilliard(FCH)equations with periodic boundary conditions on a rectangular domain that features the preconditioned Nesterov’s accelerated gradient descent(PAGD)method.We discretize these problems with a Fourier collocation method in space,and employ various second-order schemes in time.We observe a significant speedup with this solver when compared to the preconditioned gradient descent(PGD)method.With the PAGD solver,fully implicit,second-order-in-time schemes are not only feasible to solve the PFC and FCH equations,but also do so more efficiently than some semi-implicit schemes in some cases where accuracy issues are taken into account.Benchmark computations of four different schemes for the PFC and FCH equations are conducted and the results indicate that,for the FCH experiments,the fully implicit schemes(midpoint rule and BDF2 equipped with the PAGD as a nonlinear time marching solver)perform better than their IMEX versions in terms of computational cost needed to achieve a certain precision.For the PFC,the results are not as conclusive as in the FCH experiments,which,we believe,is due to the fact that the nonlinearity in the PFC is milder nature compared to the FCH equation.We also discuss some practical matters in applying the PAGD.We introduce an averaged Newton preconditioner and a sweeping-friction strategy as heuristic ways to choose good preconditioner parameters.The sweeping-friction strategy exhibits almost as good a performance as the case of the best manually tuned parameters.
基金Research of R.Guo is supported by NSFC grant No.11601490Research of Y.Xu is supported by NSFC grant No.11371342,11626253,91630207.
文摘In this paper,we will develop a first order and a second order convex splitting,and a first order linear energy stable fully discrete local discontinuous Galerkin(LDG)methods for the modified phase field crystal(MPFC)equation.In which,the first order linear scheme is based on the invariant energy quadratization approach.The MPFC equation is a damped wave equation,and to preserve an energy stability,it is necessary to introduce a pseudo energy,which all increase the difficulty of constructing numerical methods comparing with the phase field crystal(PFC)equation.Due to the severe time step restriction of explicit timemarchingmethods,we introduce the first order and second order semi-implicit schemes,which are proved to be unconditionally energy stable.In order to improve the temporal accuracy,the semi-implicit spectral deferred correction(SDC)method combining with the first order convex splitting scheme is employed.Numerical simulations of the MPFC equation always need long time to reach steady state,and then adaptive time-stepping method is necessary and of paramount importance.The schemes at the implicit time level are linear or nonlinear and we solve them by multigrid solver.Numerical experiments of the accuracy and long time simulations are presented demonstrating the capability and efficiency of the proposed methods,and the effectiveness of the adaptive time-stepping strategy.
基金supported by National Natural Science Foundation of China(Grant Nos.11901489 and 11971407)supported by National Science Foundation of USA(Grant No.DMS-1720442)。
文摘In this paper,we construct efficient schemes based on the scalar auxiliary variable block-centered finite difference method for the modified phase field crystal equation,which is a sixth-order nonlinear damped wave equation.The schemes are linear,conserve mass and unconditionally dissipate a pseudo energy.We prove rigorously second-order error estimates in both time and space for the phase field variable in discrete norms.We also present some numerical experiments to verify our theoretical results and demonstrate the robustness and accuracy.
基金The work of Q.Zhuang is supported by the National Natural Science Foundation of China(No.11771083)The research of S.Zhai is supported in part by the Natural Science Foundation of China(No.11701196)+3 种基金the Natural Science Foundation of Fujian Province(No.2020J01074)The work of Z.Weng is supported in part by the Natural Science Foundation of China(No.11701197)Supported by the Fundamental Research Funds for the Central Universities(No.ZQN-702)the Key Laboratory of Intelligent Computing and Information Processing of Ministry of Education(Xiangtan University)(No.2020ICIP03).
文摘In this paper,based on the Lagrange Multiplier approach in time and the Fourierspectral scheme for space,we propose efficient numerical algorithms to solve the phase field crystal equation.The numerical schemes are unconditionally energy stable based on the original energy and do not need the lower bound hypothesis of the nonlinear free energy potential.The unconditional energy stability of the three semi-discrete schemes is proven.Several numerical simulations in 2D and 3D are demonstrated to verify the accuracy and efficiency of our proposed schemes.
基金supported by the National Key Research and Development Program of China(2018YFA0306001)the National Natural Science Foundation of China(22073115,22105230,22131011,21821003)the Pearl River Talent Plan of Guangdong(2017BT01C161)
文摘Crystal-field symmetry of lanthanide ions plays a critical role in suppressing quantum tunneling of magnetization(QTM)in single-molecule magnets(SMMs),but high-performance SMM design and modulation remain challenging only in view of the geometric symmetry of the first coordination sphere.Herein,two bis(semicarbazone)/bis(thiosemicarbazone)dysprosium singleion magnets with pentagonal bipyramid geometry were reported,and bis(thiosemicarbazone)lanthanide complexes have never been reported to the best of our knowledge.They served as good archetypes to study the magneto-structural relationships based on the charge distribution.The complex with more ideal geometric symmetry displays fast zero-field QTM with negligible“effective barrier”,owing to the defective charge distribution.By modulating the transverse crystal field via the replacement of the O sites with the less charged and larger radius S atoms,it results in lower geometric but higher charge-distribution symmetry,giving rise to the significant suppression of QTM and the enhancement of reversal barrier up to ca.1,000 K.These results demonstrate that the charge-distribution symmetry can be chemically tailored by modification of the crystal field,which is essential for designing high-performance SMMs.
基金This work was financially supported from the National Key Re-search and Development Program of China(No.2021YFB3501700)the National Natural Science Foundation of China(Nos.52202135,61975070,51902143 and 61971207)+7 种基金the Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD)the Key Research and Development Project of Jiangsu Province(Nos.BE2021040 and BE2019033)the Natural Science Foundation of Jiangsu Province(Nos.BK20191467 and BK20221226)the Postgrad-uate Research&Practice Innovation Program of Jiangsu Province(No.KYCX21_2568)the International S&T Cooperation Program of Jiangsu Province(Nos.BZ2019063,BZ2020045 and BZ2020030)the Natural Science Foundation of the Jiangsu Higher Education In-stitutes of China(Nos.19KJB430018 and 20KJA430003)the Special Project for Technology Innovation of Xuzhou City(Nos.KC19250,KC20201,KC20244 and KC21379)Open Project of State Key Laboratory of Advanced Materials and Electronic Components(No.FHR-JS-202011017).The authors would like to show great appreci-ation to Prof.Ole Bjarlin Jensen from the Technical University of Denmark for his long-term support and guidance on diode laser-related knowledge and technique.
文摘As the fourth-generation light source,solid-state lighting has developed rapidly in the past 30 years due to its advantages of high efficiency and environmental protection.It is widely used in various scenes such as automobile headlights,projection displays,industrial production,and remote lighting.High-power,high-brightness white light-emitting diodes(LEDs)and laser diodes(LDs)technology put forward new requirements for the service stability of color conversion materials.Garnet phosphor ceramics have emerged with their unique advantages of withstanding high power excitation density and the flexibly tunable spectrum.In this article,the research progress of garnet based phosphor ceramics for high-power solid-state lighting was comprehensively reviewed.Firstly,the band gap and coordination environment regulations of luminescence centers of garnet phosphor were summarized.Secondly,the improvement of luminous efficacy via defects regulation was discussed.Thirdly,the relationship between the geometric design and the lighting performance was elucidated.Fourthly,the characterization methods of phosphor ceramics for laser lighting were introduced and illustrated.Finally,the development trend of garnet phos-phor ceramics in solid state lighting and display was prospected.
基金the National Natural Science Foundation of China under grant No.51972089,No.51672064 and No.U1435206。
文摘Electromagnetic wave(EMW)absorbing materials play a vital role in modern communication and information processing technologies to inhibit information leakage and prevent possible damages to environment and human bodies.Currently,most of EMW absorbing materials are either composites of two or more phases or in the form of nanosheets,nanowires or nanofibers in order to enhance the EMW absorption performance through dielectric loss,magnetic loss and dielectric/magnetic loss coupling.However,the combination of complex shapes/multi phases and nanosizes may compound the difficulties of materials processing,composition and interfaces control as well as performance maintenance during service.Thus,searching for single phase materials with good stability and superior EMW absorbing properties is appealing.To achieve this goal,the EMW absorbing properties of transition metal carbides TMCs(TM=Ti,Zr,Hf,Nb and Ta)and high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C which belong to ultrahigh temperature ceramics,were investigated in this work.Due to the good electrical conductivity and splitting of d orbitals into lower energy t2glevel and higher energy eglevel in TMC6octahedral arrangement,TMCs(TM=Ti,Zr,Hf,Nb and Ta)exhibit good EMW absorbing properties.Especially,Hf C and Ta C exhibit superior EMW absorbing properties.The minimum reflection loss(RLmin)value of Hf C is-55.8 d B at 6.0 GHz with the thickness of 3.8 mm and the effective absorption bandwidth(E_(AB))is 6.0 GHz from 12.0 to 18.0 GHz at thickness of 1.9 mm;the RL_(minvalue)of Ta C reaches-41.1 d B at 16.2 GHz with a thickness of 2.0 mm and the EABis 6.1 GHz with a thickness of 2.2 mm.Intriguingly,the electromagnetic parameters,i.e.,complex permittivity and permeability are tunable by forming single phase solid solution or high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C.The R_(Lminvalue)of high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C is-38.5 d B at 9.5 GHz with the thickness of 1.9 mm,and the EABis 2.3 GHz(from 11.3 to 13.6 GHz)at thickness of 1.5 mm.The significance of this work is that it opens a new window to design single phase high performance EMW absorbing materials by dielectric/magnetic loss coupling through tuning the conductivity and crystal field splitting energy of d orbitals of transition metals in carbides,nitrides and possibly borides.
基金supported by the National Key Research and Development Program of China(2018YFA0306001)the National Natural Sciences foundation of China(21620102002,21822508,21821003)the Pearl River Talent Plan of Guangdong(2017BT01C161)。
文摘Controlling molecular magnetic anisotropy via structural engineering is delicate and fascinating,especially for single-molecule magnets(SMMs).Herein a family of dysprosium single-ion magnets(SIMs)sitting in pentagonal bipyramid geometry have been synthesized with the variable-size terminal ligands and counter anions,through which the subtle coordination geometry of Dy(Ⅲ)can be finely tuned based on the size effect.The effective energy barrier(Ueff)successfully increases from 439 to 632 K and the magnetic hysteresis temperature(under a 200 Oe/s sweep rate)raises from 11 to 24 K.Based on the crystal-field theory,a semi-quantitative magneto-structural correlation deduced experimentally for the first time is revealed that the Ueff is linearly proportional to the structural-related value S20 corresponding to the axial coordination bond lengths and the bond angles.Through the evaluation of the remanent magnetization from hysteresis,quantum tunneling of magnetization(QTM)is found to exhibit negative correlation with the structural-related value Stun corresponding to the axial coordination bond angles.
基金supported by the Natural Science Foundation of Fujian Province(No.2017J01688)。
文摘Rare earth phosphates have been used extensively in luminescent phosphors.Hexagonal Ce1-xGdxPO4 with crystal field manipulation was successfully synthesized using a hydrothermal method.The photoluminescence emission intensity of hexagonal CePO4 was obviously enhanced by the crystal structure manipulation with gadolinium ions codoping.Compared to pure CePO4,the intensity photoluminescence was enhanced about 15 folds with x=0.05.The effect of gadolinium doping was systematically investigated by X-ray diffraction,transmission electron microscopy,X-ray photoelectron spectroscopy and photoluminescence spectroscopy.Crystal field asymmetry can be effectively tuned by different amount of Gd^3+codoping,and the crystal field asymmetry is conductive to electron population of high energy level of Ce^3+ions.This material has potential applications in optics,electronics fields,and so on.Meanwhile,the method can be extended to another kind of high performance photoluminescence materials preparation.
基金This work was supported by the National Natural Science Foundation of China under Grant Nos.11775163,12175166,12175079the National Key R&D Program of China(2019YFA0210003).
文摘Mg_(3)Sb_(2) as a Zintl compound is a promising thermoelectric material with the intrinsically low lattice thermal conductivity and excellent n-type electrical properties,but its p-type electrical transport properties are poor.Here,the thermoelectric performance of Mg_(3)Sb_(2) under the effect of biaxial strain is investigated by using first-principles method and Boltzmann transport theory.The application of biaxial strain enables tuning the band structure of Mg_(3)Sb_(2) in such a way that the band degeneracy of both the conduction band and valence band increases.As the biaxial strain increases,the Seebeck coefficient of ptype Mg_(3)Sb_(2) has a remarkable increase,leading to a significant improvement in power factor.This is mainly ascribed to the achievement of valence band orbital degeneracy.Meanwhile,the lattice thermal conductivity exhibits very slight biaxial strain dependence within the strain range considered in this work,which increases from 1.28 to 1.62 W m^(-1) K^(-1) at 300 K.Finally,the highest ZT of p-type Mg_(3)Sb_(2) at 700 K can be up to 2.6 along the in-plane direction under-2.5%biaxial strain,which is almost three times that of the unstrained counterpart.The realization of high thermoelectric performance of p-type Mg_(3)Sb_(2) will promote its practical applications as thermoelectric generators.
