Relaxors exhibit unique dielectric response properties such as diffuse phase transitions and strong dispersion of the dielectric constant.The origin of these properties is still not fully understood.Here,we review our...Relaxors exhibit unique dielectric response properties such as diffuse phase transitions and strong dispersion of the dielectric constant.The origin of these properties is still not fully understood.Here,we review our work on usingfirst-principles-based methods to elucidate the connections between composition,local structure and dynamics in Pb-based relaxors.展开更多
We use the first-principles-based molecular dynamic approach to simulate dipolar dynamics of BaZrO_(3)/BaTiO_(3)superlattice,and obtain its dielectric response.The dielectric response is decomposed into its compositio...We use the first-principles-based molecular dynamic approach to simulate dipolar dynamics of BaZrO_(3)/BaTiO_(3)superlattice,and obtain its dielectric response.The dielectric response is decomposed into its compositional,as well as the in-plane and out-of-plane parts,which are then discussed in the context of chemical ordering of Zr/Ti ions.We reveal that,while the in-plane dielectric response of BaZrO_(3)/BaTiO_(3)superlattice also shows dispersion over probing frequency,it shall not be categorized as relaxor.展开更多
To identify the parameters of the extended Debye model of XLPE cables,and therefore evaluate the insulation performance of the samples,the sparsity-promoting dynamicmode decomposition(SPDMD)methodwas introduced,aswell...To identify the parameters of the extended Debye model of XLPE cables,and therefore evaluate the insulation performance of the samples,the sparsity-promoting dynamicmode decomposition(SPDMD)methodwas introduced,aswell the basics and processes of its applicationwere explained.The amplitude vector based on polarization current was first calculated.Based on the non-zero elements of the vector,the number of branches and parameters including the coefficients and time constants of each branch of the extended Debye model were derived.Further research on parameter identification of XLPE cables at different aging stages based on the SPDMD method was carried out to verify the practicability of the method.Compared with the traditional differential method,the simulation and experiment indicated that the SPDMD method can effectively avoid problems such as the relaxation peak being unobvious,and possessing more accuracy during the parameter identification.And due to the polarization current being less affected by the measurement noise than the depolarization current,the SPDMD identification results based on the polarization current spectral line proved to be better at reflecting the response characteristics of the dielectric.In addition,the time domain polarization current test results can be converted into the frequency domain,and then used to obtain the dielectric loss factor spectrum of the insulation.The integral of the dielectric loss factor on a frequency domain can effectively evaluate the insulation condition of the XLPE cable.展开更多
Electromechanical and dielectric properties of PMN-PT ferroelectric ceramics are investigated.In particular,dielectric response studies focus on the investigation of the influence of the DC applied electric field on t...Electromechanical and dielectric properties of PMN-PT ferroelectric ceramics are investigated.In particular,dielectric response studies focus on the investigation of the influence of the DC applied electric field on the dielectric permittivity as a function of temperature and frequency.Results reveal an electric field driven dielectric anomaly in the dielectric permittivity curves,e(E),which in turn prevails in the whole ferroelectric phase region and continuously vanishes for temperatures near the paraelectric-ferroelectric phase transition temperature.A schematic model for the domains dynamics of the studied material is proposed taking into account the simultaneous contribution of both 90°and 180°domains walls.展开更多
The desirable characteristics of Ba_(6-3x)Nd_(8+2x)Ti_(18)O_(54)include high dielectric constant,low loss tangent,and high quality factor developed a new field for electronic applications.The microwave dielectric prop...The desirable characteristics of Ba_(6-3x)Nd_(8+2x)Ti_(18)O_(54)include high dielectric constant,low loss tangent,and high quality factor developed a new field for electronic applications.The microwave dielectric properties of Ba_(6-3x)Nd_(8+2x)Ti_(18)O_(54),with x?0:15 ceramics at different sintering temperatures(600–1300C)were investigated.The phenomenon of polarization produced by the applied electric field was studied.The dielectric properties with respect to frequency from 1MHz to 1.5 GHz were measured using Impedance Analyzer,and the results were compared and analyzed.The highest dielectric permittivity and lowest loss factor were defined among the samples.The complex dielectric modulus was evaluated from the measured parameters of dielectric measurement in the same frequency range,and used to differentiate the contribution of grain and grain boundary.展开更多
Copper and rare earth-doped(RE = La, Gd, Nd) CuFe_(1.85)RE_(0.15)O_4 nano ferrites were prepared using the so nochemical method. The effective doping of rare-earth(La^(3+), Nd^(3+), Gd^(3+)) ions with copper nanoferri...Copper and rare earth-doped(RE = La, Gd, Nd) CuFe_(1.85)RE_(0.15)O_4 nano ferrites were prepared using the so nochemical method. The effective doping of rare-earth(La^(3+), Nd^(3+), Gd^(3+)) ions with copper nanoferrites was confirmed by X-ray diffraction. The tetrahedral and octahedral sites of the nano ferrites were identified through the Fourier transform infrared spectra. The doping of rare-earth elements enhances the optical bandgap energy of the nanoferrites that are observed through Ultraviolet-DRS spectra. The oxidation state of the elements Cu 2 p, La 3 d, Nd 3 d, Gd 3 d, Fe 2 p and O 1 s was analyzed. Scanning electron microscopy images indicate a spherical morphology with agglomeration to some elongate. The values of dielectric constant and conductivity decrease considerably due to doping rare-earth ions in copper nanoferrites. Low saturation magnetization and high coercivity values of rare earth-doped copper nanoferrites are observed from the typical hysteresis curves.展开更多
(1-x)(Bi_(0.5)Na_(0.5))TiO_(3)-xBaTiO_(3) lead-free ceramics have been obtained from the conventional solid-state reaction sintering method.The structural properties were investigated from X-ray diffraction and Raman ...(1-x)(Bi_(0.5)Na_(0.5))TiO_(3)-xBaTiO_(3) lead-free ceramics have been obtained from the conventional solid-state reaction sintering method.The structural properties were investigated from X-ray diffraction and Raman spectroscopy techniques.Results revealed well-crystallized ceramic samples with perovskite structure.Microstructural properties,obtained from scanning electron microscopy measurements,have shown high density with very low porosity level.The dielectric response,analyzed as a function of the temperature and several frequencies,showed very broad peaks with a strong frequency dependence of the temperature for the maximum dielectric permittivity for the modified system.Results were analyzed considering the influence of the BaTiO3 content on the studied physical properties.展开更多
A study of dielectric properties of composite films on the base of poly(vinylidene fluoride-trifluoroethylene)copolymer P(VDFTrFE)and ferroelectric ceramics of barium lead zirconate titanate(BPZT)solid solution is pre...A study of dielectric properties of composite films on the base of poly(vinylidene fluoride-trifluoroethylene)copolymer P(VDFTrFE)and ferroelectric ceramics of barium lead zirconate titanate(BPZT)solid solution is presented in this work.The composite films containing up to 50 vol.%of BPZT grains with size1m were prepared by the solvent cast method.Frequency dependences of real and imaginary components of the complex permittivity were determined.The concentration dependence of the dielectric constant was discussed.展开更多
Due to the temperature and frequency response of electromagnetic(EM)loss,how to realize the effective design of microwave absorption materials(MWAMs)at elevated temperature is highly desirable for practical applicatio...Due to the temperature and frequency response of electromagnetic(EM)loss,how to realize the effective design of microwave absorption materials(MWAMs)at elevated temperature is highly desirable for practical applications.Herein,transition metaldoped titanium nitride(M-TiN,M=Fe or Co)fibers were fabricated,the distortion of TiN lattice could cause the adjustable charge enrichment,which played a profound influence on the dielectric response and EM microwave absorption(EMWA)performances.Benefiting from the negative correlation between dielectric loss and temperature,more loss mechanism could be introduced,which would effectively enhance dielectric loss and EMWA performances at elevated temperature.The optimal EMWA performances of Fe-TiN fibers/polydimethylsiloxane(PDMS)composites were realized with a wide temperature range(298–423 K):the reflection loss(RL)could reach 99%(RL<−20 dB)at 12.2 GHz with 1.8 mm,when the filler content was only 15.0 wt.%.Compared with the undoped-TiN fibers/PDMS and Co-TiN fibers/PDMS composites,the excellent EMWA of Fe-TiN fibers/PDMS composite could be attributed to the reasonably synergistic polarization loss and conduction loss.Based on systematic analysis of the variable-temperature EM parameters and EMWA performances,the optimization of EMWA performances in wide temperature domain could be realized by introducing appropriate polarization loss and its compensating.Hopefully,this work provides a new strategy for regulating the dielectric response and designing effective MWAMs at elevated temperature.展开更多
For single-walled carbon nanotubes(SWCNTs)with a length of 1-50μm,the surface plasmon-polariton(SPP)resonance is within the terahertz frequency range;therefore,SWCNT lattices can be used to design frequency-selective...For single-walled carbon nanotubes(SWCNTs)with a length of 1-50μm,the surface plasmon-polariton(SPP)resonance is within the terahertz frequency range;therefore,SWCNT lattices can be used to design frequency-selective surface(FSS).A numerical model of electromagnetic wave diffraction on a two-dimensional periodic SWCNT lattice can be described by an integro-differential equation of the second-order with respect to the surface current along SWCNT.The equation can be solved by the Bubnov-Galerkin method.Frequency dependence of reflecting and transmitting electromagnetic waves for FSSs near the SPP resonance is studied numerically.It is shown that the resonances are within the lowerfrequency part of the terahertz range.We also estimate the relaxa-tion frequency of an individual SWCNT and demonstrate the applic-ability of the Kubo formula for graphene conductivity to array of strips similar in size to SWCNTs under consideration.展开更多
Ca_(3)Ti_(2)O_(7) with Ruddlesden-Popper structure exhibits the largest polarization among the known hybrid improper ferroelectrics.However,the high Curie temperature impedes the thorough study of phase transition thr...Ca_(3)Ti_(2)O_(7) with Ruddlesden-Popper structure exhibits the largest polarization among the known hybrid improper ferroelectrics.However,the high Curie temperature impedes the thorough study of phase transition through dielectric characterization.According to the previous theoretical design rule,the Curie temperature can be suppressed by increasing the tolerance factor.So,in the present work,high-quality Ca_(3-x)LaxTi_(2-x)Al_(x)O_(7)(x=0.0,0.1,0.2,0.3)ceramics with increased tolerance factors were successfully prepared.The amplitude of oxygen octahedral tilt mode indeed decreases with increasing tolerance factors,leading to a degeneration of ferroelectric polarization.However,the unexpected rigid rotation mode causes the composition-invariable coercive fields.The Curie temperatures decrease linearly with increasing x and tolerance factors.The variable-temperature dielectric constant confirms first-order improper ferroelectric transitions in Ca_(3)Ti_(2)O_(7)-based ceramics.The results of variable temperature Xray diffraction reveal the coexistence of two-phases below Curie temperature.The present work confidently confirms the first-order improper ferroelectric transition in Ca_(3)Ti_(2)O_(7)-based ceramics by combining results of variable-temperature dielectric response and in-situ X-ray powder diffraction.展开更多
Processing of materials in the form of ceramics normally involves several steps including calcination at a relatively low temperature for synthesis of the end-product powder and sintering at a high temperature for den...Processing of materials in the form of ceramics normally involves several steps including calcination at a relatively low temperature for synthesis of the end-product powder and sintering at a high temperature for densification.The work we have been developing introduces a novel approach enabling synthesis plus sintering of materials in a single running experiment by using electric fields,ending with dense ceramics that display grains noticeably finer than in conventional processing.This new paradigm is fully illustrated with experiments conducted on amorphous CaCu3Ti4O12 precursor powder,shown to experience,on heating,crystallization through intermediate phases,followed by chemical reaction leading to synthesis of the end-product powder,plus densification depending on field adjustment.The processing time and furnace temperature are considerably reduced,demonstrating that enhanced synthesis and sintering rates applied under field input.Similar results found in Bi2/3Cu3Ti4O12are also shown.The different factors that may contribute to this unique scenario,including Joule heating,defect generation,and reduction of free energy for nuclei formation promoted by the applied field,are briefly discussed.Overall,the findings we bring here are exclusive as they show an exploitable way that allows rapid processing of materials with good control over particle and grain coarsening.展开更多
BiFeGaO3-BaTiO3(BFG-BT)based ceramics with a large piezoelectric coefficient are potential high performance lead-free piezoelectric compounds.In this work,textured and random BFG-BT ceramics were realized by the solid...BiFeGaO3-BaTiO3(BFG-BT)based ceramics with a large piezoelectric coefficient are potential high performance lead-free piezoelectric compounds.In this work,textured and random BFG-BT ceramics were realized by the solid state reaction method with and without BaTiO3(BT)templates.Textured ceramics were obtained by a reactive templated grain growth(RTGG)method leading to a high-temperature electromechanical strain of S=0.27%at 40 kV/cm and to an effective piezoelectric coefficient(d33*)up to 685 pm/V at 180℃.The easy movement of oriented domains enhanced the electromechanical strain under an applied electric field in textured sample(Lotgering factor f=66.3%).Structural investigations reveal that the proportion and degree of distortion of BFG-BT rhombohedral phase(R3c)reached its maximum in textured ceramics,resulting in large ferrodistortive displacements under electric fields.In addition,the dense nanodomains with low domain wall energies,inferred from the high-resolution transmission electron microscope(HR-TEM)observations,contribute to the extra displacement of the textured sample under an applied electric field.In textured ceramics,the remnant polarization was stable(about 17μC/cm2)from room temperature to 180℃,contributing to the stable ferroelectric property at high temperatures.Through the introduction of BT templates,high-density nanodomains were formed and the Burns temperature was enhanced in textured ceramics.The electromechanical strain,polarization and dielectric behavior were correlated to the textured or random forms of the BFG-BT based ceramics.展开更多
Substitutional charge disorder as in PbMg_(1/3)Nb_(2/3)O_(3),structural cation vacancies as in Sr_(x)Ba_(1-x)Nb_(2)O_(6) and isovalent substitution of off-centered cations as in BaTi_(1-x)Sn^(x)O_(3) and BaTi1-xZrxO_(...Substitutional charge disorder as in PbMg_(1/3)Nb_(2/3)O_(3),structural cation vacancies as in Sr_(x)Ba_(1-x)Nb_(2)O_(6) and isovalent substitution of off-centered cations as in BaTi_(1-x)Sn^(x)O_(3) and BaTi1-xZrxO_(3) give rise to quenched electric random-fields(RFs),which we proposed to be at the origin of the peculiar behavior of relaxor ferroelectrics 20 years ago.These are,e.g.a strong frequency dispersion of the dielectric response and an apparent lack of macroscopic symmetry breaking in the low temperature phase.Both are related to mesoscopic RF-driven phase transitions,which give rise to irregularly shaped quasi-stable polar nanoregions below the characteristic temperature T^(*),but above the global transition temperature Tc.Their co-existence with the paraelectric parent phase can be modeled by time-dependentfield equations under the control of quenched RFs and stress-free strain(in the case of order parameter dimension n2).Transitions into global polar order at Tc may occur in uniaxial relaxors as observed on the uniaxial relaxor ferroelectric Sr0.8Ba_(0.2)Nb_(2)O_(6) and come close to RF Ising model criticality.Re-entrant relaxor transitions as observed in solid solutions of Ba_(2)Pr_(0.6)Nd_(0.4)(FeNb_(4))O_(15) are proposed to evidence the coexistence of distinct normal and relaxor ferroelectric phases within the framework of percolation theory.Keywords.展开更多
The relaxor behavior of PLZT ferroelectric ceramics has been analyzed in a wide frequency and temperature ranges,below and above the temperature for the formation of the so-called polar nano-regions(PNRs).An approxima...The relaxor behavior of PLZT ferroelectric ceramics has been analyzed in a wide frequency and temperature ranges,below and above the temperature for the formation of the so-called polar nano-regions(PNRs).An approximation to the dynamical behavior of the PNRs has been discussed using Cole-Cole’s relaxation model and Jonscher’s Universal Relaxation Law.The analysis considers both the dipolar contribution and those ones associated with DC and AC electric conductivities,this latter not being previously reported in the literature for relaxor materials.The effectiveness of the developed model has been verified from the agreement between the experimental data and the theoretical calculations.This study also offers an indirect method to predict the DC component of the electrical conductivity.展开更多
In the present work,the crystal structure,ferroelectric and dielectric properties of the dense single-phase Li_(2)CaTa_(2)O_(7) ceramics with A-site ordered double-layer Ruddlesden-Popper structures have been investig...In the present work,the crystal structure,ferroelectric and dielectric properties of the dense single-phase Li_(2)CaTa_(2)O_(7) ceramics with A-site ordered double-layer Ruddlesden-Popper structures have been investigated by the experiments and first-principles calculations.A polar Pna21 phase was determined by the Rietveld refinement against the X-ray diffraction pattern at room temperature,and it was confirmed by its lowest calculated energy and rigid phonon modes.The ferroelectricity was found by observing the ferroelectric hysteresis loop at room temperature,and its remanent polarization was similar to the value calculated from the Berry phase and Born effective charge approaches.The ferroelectricity is a nonconventional proper one since its polarization is induced from the displacement of oxygen anions instead of tantalum cations based on the individual atomic polarization.A first-order transition from an antiferroelectric to paraelectric phase around 900 K was found by the DSC and variable-temperature dielectric measurements.From the present work,the room-temperature ferroelectricity is experimental confirmed in the present ceramics,inspiring the search for new ferroelectricity in the ceramics with A-site ordered Ruddlesden-Popper structures.展开更多
基金This work was supported by the O±ce of Naval Research,under Grant No.N00014-11-1-0578by the NSF under grant DMR11-20901.Computational support was provided by a Challenge Grant from the HPCMO of the U.S.Department of Defense.
文摘Relaxors exhibit unique dielectric response properties such as diffuse phase transitions and strong dispersion of the dielectric constant.The origin of these properties is still not fully understood.Here,we review our work on usingfirst-principles-based methods to elucidate the connections between composition,local structure and dynamics in Pb-based relaxors.
基金the discussion with Drs A.A.Bokov,J.Hlinka,Z.-G.Ye,J.Petzelt,and L.Bellaiche.This work is financially supported by the National Natural Science Foundation of China(NSFC),Grant Nos.51390472,11574246National Basic Research Program of China,Grant No.2015CB654903+1 种基金Z.J.also acknowledges support from China Scholarship Council(CSC No.201506280055)the 111 Project(B14040)。
文摘We use the first-principles-based molecular dynamic approach to simulate dipolar dynamics of BaZrO_(3)/BaTiO_(3)superlattice,and obtain its dielectric response.The dielectric response is decomposed into its compositional,as well as the in-plane and out-of-plane parts,which are then discussed in the context of chemical ordering of Zr/Ti ions.We reveal that,while the in-plane dielectric response of BaZrO_(3)/BaTiO_(3)superlattice also shows dispersion over probing frequency,it shall not be categorized as relaxor.
基金supported by the Science and Technology Project of Guizhou Power Grid Co.,Ltd. (No.GZKJXM20210405).
文摘To identify the parameters of the extended Debye model of XLPE cables,and therefore evaluate the insulation performance of the samples,the sparsity-promoting dynamicmode decomposition(SPDMD)methodwas introduced,aswell the basics and processes of its applicationwere explained.The amplitude vector based on polarization current was first calculated.Based on the non-zero elements of the vector,the number of branches and parameters including the coefficients and time constants of each branch of the extended Debye model were derived.Further research on parameter identification of XLPE cables at different aging stages based on the SPDMD method was carried out to verify the practicability of the method.Compared with the traditional differential method,the simulation and experiment indicated that the SPDMD method can effectively avoid problems such as the relaxation peak being unobvious,and possessing more accuracy during the parameter identification.And due to the polarization current being less affected by the measurement noise than the depolarization current,the SPDMD identification results based on the polarization current spectral line proved to be better at reflecting the response characteristics of the dielectric.In addition,the time domain polarization current test results can be converted into the frequency domain,and then used to obtain the dielectric loss factor spectrum of the insulation.The integral of the dielectric loss factor on a frequency domain can effectively evaluate the insulation condition of the XLPE cable.
基金The authors would like to thank the National Council of Scientific and Technological Development(CNPq)grant 303447/2019-2,Minas Gerais Research Foundation(FAPEMIG)grants PPM-00661-16 and APQ-02875-18 and Coordenacao de Aperfeicoamento de Pessoal de Nível Superior—Brasil(CAPES)—Finance Code 001 Brazilian agencies for the financial supportThe financial support of the PGC2018-099158-B-I00 project(Spanish Government)is also gratefully acknowledged.
文摘Electromechanical and dielectric properties of PMN-PT ferroelectric ceramics are investigated.In particular,dielectric response studies focus on the investigation of the influence of the DC applied electric field on the dielectric permittivity as a function of temperature and frequency.Results reveal an electric field driven dielectric anomaly in the dielectric permittivity curves,e(E),which in turn prevails in the whole ferroelectric phase region and continuously vanishes for temperatures near the paraelectric-ferroelectric phase transition temperature.A schematic model for the domains dynamics of the studied material is proposed taking into account the simultaneous contribution of both 90°and 180°domains walls.
基金This research was financially supported from the Research University Grant Scheme(RUGS)Project No.:05-02-12-2180RUUniversiti Putra Malaysia(UPM).The authors also acknowledged the Department of Physics,Faculty of Science,UPM and Institute of Advanced Technology(ITMA),UPM.
文摘The desirable characteristics of Ba_(6-3x)Nd_(8+2x)Ti_(18)O_(54)include high dielectric constant,low loss tangent,and high quality factor developed a new field for electronic applications.The microwave dielectric properties of Ba_(6-3x)Nd_(8+2x)Ti_(18)O_(54),with x?0:15 ceramics at different sintering temperatures(600–1300C)were investigated.The phenomenon of polarization produced by the applied electric field was studied.The dielectric properties with respect to frequency from 1MHz to 1.5 GHz were measured using Impedance Analyzer,and the results were compared and analyzed.The highest dielectric permittivity and lowest loss factor were defined among the samples.The complex dielectric modulus was evaluated from the measured parameters of dielectric measurement in the same frequency range,and used to differentiate the contribution of grain and grain boundary.
基金Project supported by the Science and Engineering Research Board(SERB)New Delhi,India(SR/FTP/PS-068/2014)
文摘Copper and rare earth-doped(RE = La, Gd, Nd) CuFe_(1.85)RE_(0.15)O_4 nano ferrites were prepared using the so nochemical method. The effective doping of rare-earth(La^(3+), Nd^(3+), Gd^(3+)) ions with copper nanoferrites was confirmed by X-ray diffraction. The tetrahedral and octahedral sites of the nano ferrites were identified through the Fourier transform infrared spectra. The doping of rare-earth elements enhances the optical bandgap energy of the nanoferrites that are observed through Ultraviolet-DRS spectra. The oxidation state of the elements Cu 2 p, La 3 d, Nd 3 d, Gd 3 d, Fe 2 p and O 1 s was analyzed. Scanning electron microscopy images indicate a spherical morphology with agglomeration to some elongate. The values of dielectric constant and conductivity decrease considerably due to doping rare-earth ions in copper nanoferrites. Low saturation magnetization and high coercivity values of rare earth-doped copper nanoferrites are observed from the typical hysteresis curves.
基金The authors thank the National Council of Scientific and Technological Development(CNPq)grants 401072/2014-2 and 303447/2019-2,Minas Gerais Research Foundation(FAPEMIG)grants PPM-00661-16 and APQ-02875-18 and Coordenacao de Aperfeicoamento de Pessoal de Nível Superior-Brasil(CAPES)-Finance Code 001 Brazilian agencies for the financial support.
文摘(1-x)(Bi_(0.5)Na_(0.5))TiO_(3)-xBaTiO_(3) lead-free ceramics have been obtained from the conventional solid-state reaction sintering method.The structural properties were investigated from X-ray diffraction and Raman spectroscopy techniques.Results revealed well-crystallized ceramic samples with perovskite structure.Microstructural properties,obtained from scanning electron microscopy measurements,have shown high density with very low porosity level.The dielectric response,analyzed as a function of the temperature and several frequencies,showed very broad peaks with a strong frequency dependence of the temperature for the maximum dielectric permittivity for the modified system.Results were analyzed considering the influence of the BaTiO3 content on the studied physical properties.
基金supported by the Russian Scientific Foundation(Grant N 15-19-00138).
文摘A study of dielectric properties of composite films on the base of poly(vinylidene fluoride-trifluoroethylene)copolymer P(VDFTrFE)and ferroelectric ceramics of barium lead zirconate titanate(BPZT)solid solution is presented in this work.The composite films containing up to 50 vol.%of BPZT grains with size1m were prepared by the solvent cast method.Frequency dependences of real and imaginary components of the complex permittivity were determined.The concentration dependence of the dielectric constant was discussed.
基金support of the National Nature Science Foundation of China(No.U1704253)the Key Scientific Research Project Plan in Universities of Henan Province(No.23A430037)the Science and Technology Planning Project of Henan Province(No.212102210474).
文摘Due to the temperature and frequency response of electromagnetic(EM)loss,how to realize the effective design of microwave absorption materials(MWAMs)at elevated temperature is highly desirable for practical applications.Herein,transition metaldoped titanium nitride(M-TiN,M=Fe or Co)fibers were fabricated,the distortion of TiN lattice could cause the adjustable charge enrichment,which played a profound influence on the dielectric response and EM microwave absorption(EMWA)performances.Benefiting from the negative correlation between dielectric loss and temperature,more loss mechanism could be introduced,which would effectively enhance dielectric loss and EMWA performances at elevated temperature.The optimal EMWA performances of Fe-TiN fibers/polydimethylsiloxane(PDMS)composites were realized with a wide temperature range(298–423 K):the reflection loss(RL)could reach 99%(RL<−20 dB)at 12.2 GHz with 1.8 mm,when the filler content was only 15.0 wt.%.Compared with the undoped-TiN fibers/PDMS and Co-TiN fibers/PDMS composites,the excellent EMWA of Fe-TiN fibers/PDMS composite could be attributed to the reasonably synergistic polarization loss and conduction loss.Based on systematic analysis of the variable-temperature EM parameters and EMWA performances,the optimization of EMWA performances in wide temperature domain could be realized by introducing appropriate polarization loss and its compensating.Hopefully,this work provides a new strategy for regulating the dielectric response and designing effective MWAMs at elevated temperature.
基金Russian Foundation for Basic Research(RFBR),project number 21-52-15034。
文摘For single-walled carbon nanotubes(SWCNTs)with a length of 1-50μm,the surface plasmon-polariton(SPP)resonance is within the terahertz frequency range;therefore,SWCNT lattices can be used to design frequency-selective surface(FSS).A numerical model of electromagnetic wave diffraction on a two-dimensional periodic SWCNT lattice can be described by an integro-differential equation of the second-order with respect to the surface current along SWCNT.The equation can be solved by the Bubnov-Galerkin method.Frequency dependence of reflecting and transmitting electromagnetic waves for FSSs near the SPP resonance is studied numerically.It is shown that the resonances are within the lowerfrequency part of the terahertz range.We also estimate the relaxa-tion frequency of an individual SWCNT and demonstrate the applic-ability of the Kubo formula for graphene conductivity to array of strips similar in size to SWCNTs under consideration.
基金financially supported by the National Natural Science Foundation of China under Grant Nos.51772266 and 51790493the National Key R&D Program of China under Grant No.2016YFA0300101.
文摘Ca_(3)Ti_(2)O_(7) with Ruddlesden-Popper structure exhibits the largest polarization among the known hybrid improper ferroelectrics.However,the high Curie temperature impedes the thorough study of phase transition through dielectric characterization.According to the previous theoretical design rule,the Curie temperature can be suppressed by increasing the tolerance factor.So,in the present work,high-quality Ca_(3-x)LaxTi_(2-x)Al_(x)O_(7)(x=0.0,0.1,0.2,0.3)ceramics with increased tolerance factors were successfully prepared.The amplitude of oxygen octahedral tilt mode indeed decreases with increasing tolerance factors,leading to a degeneration of ferroelectric polarization.However,the unexpected rigid rotation mode causes the composition-invariable coercive fields.The Curie temperatures decrease linearly with increasing x and tolerance factors.The variable-temperature dielectric constant confirms first-order improper ferroelectric transitions in Ca_(3)Ti_(2)O_(7)-based ceramics.The results of variable temperature Xray diffraction reveal the coexistence of two-phases below Curie temperature.The present work confidently confirms the first-order improper ferroelectric transition in Ca_(3)Ti_(2)O_(7)-based ceramics by combining results of variable-temperature dielectric response and in-situ X-ray powder diffraction.
基金partly supported by the Coordenacao de Aperfeicoamento de Pessoal de Nível Superior-Brasil(CAPES)-Finance Code 001,through Grant Nos.BEX 3276/14-7 and BEX 9291/13-0
文摘Processing of materials in the form of ceramics normally involves several steps including calcination at a relatively low temperature for synthesis of the end-product powder and sintering at a high temperature for densification.The work we have been developing introduces a novel approach enabling synthesis plus sintering of materials in a single running experiment by using electric fields,ending with dense ceramics that display grains noticeably finer than in conventional processing.This new paradigm is fully illustrated with experiments conducted on amorphous CaCu3Ti4O12 precursor powder,shown to experience,on heating,crystallization through intermediate phases,followed by chemical reaction leading to synthesis of the end-product powder,plus densification depending on field adjustment.The processing time and furnace temperature are considerably reduced,demonstrating that enhanced synthesis and sintering rates applied under field input.Similar results found in Bi2/3Cu3Ti4O12are also shown.The different factors that may contribute to this unique scenario,including Joule heating,defect generation,and reduction of free energy for nuclei formation promoted by the applied field,are briefly discussed.Overall,the findings we bring here are exclusive as they show an exploitable way that allows rapid processing of materials with good control over particle and grain coarsening.
基金financially supported by the National Key R&D Program of China(Nos.2016YFB0402701 and 2016YFA0201103)the National Basic Research Program of China(No.2015CB654605)+1 种基金the National Natural Science Foundation of China(Nos.51831010 and 51672293)the Instrument Developing Project of Chinese Academy of Sciences(No.ZDKYYQ20180004)。
文摘BiFeGaO3-BaTiO3(BFG-BT)based ceramics with a large piezoelectric coefficient are potential high performance lead-free piezoelectric compounds.In this work,textured and random BFG-BT ceramics were realized by the solid state reaction method with and without BaTiO3(BT)templates.Textured ceramics were obtained by a reactive templated grain growth(RTGG)method leading to a high-temperature electromechanical strain of S=0.27%at 40 kV/cm and to an effective piezoelectric coefficient(d33*)up to 685 pm/V at 180℃.The easy movement of oriented domains enhanced the electromechanical strain under an applied electric field in textured sample(Lotgering factor f=66.3%).Structural investigations reveal that the proportion and degree of distortion of BFG-BT rhombohedral phase(R3c)reached its maximum in textured ceramics,resulting in large ferrodistortive displacements under electric fields.In addition,the dense nanodomains with low domain wall energies,inferred from the high-resolution transmission electron microscope(HR-TEM)observations,contribute to the extra displacement of the textured sample under an applied electric field.In textured ceramics,the remnant polarization was stable(about 17μC/cm2)from room temperature to 180℃,contributing to the stable ferroelectric property at high temperatures.Through the introduction of BT templates,high-density nanodomains were formed and the Burns temperature was enhanced in textured ceramics.The electromechanical strain,polarization and dielectric behavior were correlated to the textured or random forms of the BFG-BT based ceramics.
文摘Substitutional charge disorder as in PbMg_(1/3)Nb_(2/3)O_(3),structural cation vacancies as in Sr_(x)Ba_(1-x)Nb_(2)O_(6) and isovalent substitution of off-centered cations as in BaTi_(1-x)Sn^(x)O_(3) and BaTi1-xZrxO_(3) give rise to quenched electric random-fields(RFs),which we proposed to be at the origin of the peculiar behavior of relaxor ferroelectrics 20 years ago.These are,e.g.a strong frequency dispersion of the dielectric response and an apparent lack of macroscopic symmetry breaking in the low temperature phase.Both are related to mesoscopic RF-driven phase transitions,which give rise to irregularly shaped quasi-stable polar nanoregions below the characteristic temperature T^(*),but above the global transition temperature Tc.Their co-existence with the paraelectric parent phase can be modeled by time-dependentfield equations under the control of quenched RFs and stress-free strain(in the case of order parameter dimension n2).Transitions into global polar order at Tc may occur in uniaxial relaxors as observed on the uniaxial relaxor ferroelectric Sr0.8Ba_(0.2)Nb_(2)O_(6) and come close to RF Ising model criticality.Re-entrant relaxor transitions as observed in solid solutions of Ba_(2)Pr_(0.6)Nd_(0.4)(FeNb_(4))O_(15) are proposed to evidence the coexistence of distinct normal and relaxor ferroelectric phases within the framework of percolation theory.Keywords.
基金The authors would like to acknowledge the Third World Academy of Sciences(RG/PHYS/LA Nos.99-050,02-225,and 05-043),the Abdus Salam International Centre for Theoretical Physics(ICTP),Trieste,Italy,for financially supporting the Latin-American Network of Ferroelectric Materials(NT-02)and the National Council of Scientific and Technological Development(CNPq)grant 303447/2019-2Minas Gerais Research Foundation(FAPEMIG)grants PPM-00661-16 and APQ-02875-18Coordenacao de Aperfeicoamento de Pessoal de Nível Superior-Brasil(CAPES)-Finance Code 001,and Sao Paulo Research Foundation(FAPESP)grants 06/60013-5 and 2018/24352-7 Brazilian agencies for the financial support.
文摘The relaxor behavior of PLZT ferroelectric ceramics has been analyzed in a wide frequency and temperature ranges,below and above the temperature for the formation of the so-called polar nano-regions(PNRs).An approximation to the dynamical behavior of the PNRs has been discussed using Cole-Cole’s relaxation model and Jonscher’s Universal Relaxation Law.The analysis considers both the dipolar contribution and those ones associated with DC and AC electric conductivities,this latter not being previously reported in the literature for relaxor materials.The effectiveness of the developed model has been verified from the agreement between the experimental data and the theoretical calculations.This study also offers an indirect method to predict the DC component of the electrical conductivity.
基金financially supported by the National Natural Science Foundation of China under Grant Nos.51772266,51790493 and 51961145105the Natural Science Foundation of Zhejiang Province under Grand No.LY20E020012+1 种基金the National Key R&D Program of China under Grant No.2016YFA0300101the Scientific Research Fund of Zhejiang Provincial Education Department under Grant No.Y201941595.
文摘In the present work,the crystal structure,ferroelectric and dielectric properties of the dense single-phase Li_(2)CaTa_(2)O_(7) ceramics with A-site ordered double-layer Ruddlesden-Popper structures have been investigated by the experiments and first-principles calculations.A polar Pna21 phase was determined by the Rietveld refinement against the X-ray diffraction pattern at room temperature,and it was confirmed by its lowest calculated energy and rigid phonon modes.The ferroelectricity was found by observing the ferroelectric hysteresis loop at room temperature,and its remanent polarization was similar to the value calculated from the Berry phase and Born effective charge approaches.The ferroelectricity is a nonconventional proper one since its polarization is induced from the displacement of oxygen anions instead of tantalum cations based on the individual atomic polarization.A first-order transition from an antiferroelectric to paraelectric phase around 900 K was found by the DSC and variable-temperature dielectric measurements.From the present work,the room-temperature ferroelectricity is experimental confirmed in the present ceramics,inspiring the search for new ferroelectricity in the ceramics with A-site ordered Ruddlesden-Popper structures.