Enhanced piezoelectricity in Na and Ce co-doped CaBi_(4)Ti_(4)O_(15) ceramics for high-temperature applications

The sodium(Na)and Ce co-doped calcium bismuth titanate(CBT;CaBi_(4)Ti_(4)O_(15))Aurivillius ceramics in a Ca_(1-x)(Na_(0.5)Ce_(0.5)),Bi_(4)Ti_(4)O_(15)(CNCBT;doping content(x)=0,0.03,0.05,0.08 and 0.12)system were synthesized by the conventional solid-state sintering method.All compositions show a single-phase orthorhombic(space group:A2jam)structure at room temperature.The shift of the Curie point(T_(c))towards lower temperatures(T)on doping results from the increased tolerance factor(t).The substitution-enhanced ferroelectric performance with large maximum polarization(P_(m))and facilitated domain switching is evidenced by the developed electrical polarization-electric field(P-E)and electrical current-electric field(I-E)hysteresis loops.The piezoelectric coefficient(d_(33)=20.5±0.1 pC/N)of the x=0.12 sample is about four times larger than that of pure CBT.The improved piezoelectric properties can be attributed to the high remanent polarization(P_(r))and relatively high dielectric permittivity(ε′).In addition,multi-sized(micron and sub-micron)domain structures were observed in the CNCBT ceramics by the piezoresponse force microscope(PFM).The multiple-sized ferroelectric domain structure with smaller domains is beneficial to the easy domain switching,enhanced ferroelectric performance,and improved piezoelectric properties of the CNCBT ceramics.The designed Aurivillius-phase ferroelectric ceramics with the T_(c) around 765℃and high piezoelectric coefficient(d_(33))are suitable for high-temperature piezoelectric applications.

Microwave characterization of two Ba_(0.6)Sr_(0.4)TiO_(3) dielectric thin films with out-of-plane and in-plane electrode structures

Ferroelectric(FE)thin films have recently attracted renewed interest in research due to their great potential for designing novel tunable electromagnetic devices such as large intelligent surfaces(LISs).However,the mechanism of how a polar structure in the FE thin films contributes to desired tunable performance,especially within the microwave frequency range,which is the most widely used frequency range of electromagnetics,has not been illustrated clearly.In this paper,we described several straightforward and cost-effective methods to fabricate and characterize Ba_(0.6)Sr_(0.4)TiO_(3)(BST)thin films at microwave frequencies.The prepared BST thin films here exhibit homogenous structures and great tunability(h)in a wide frequency and temperature range when the applied field is in the out-of-plane direction.The high tunability can be attributed to high concentration of polar nanoclusters.Their response to the applied direct current(DC)field was directly visualized using a novel non-destructive near-field scanning microwave microscopy(NSMM)technique.Our results have provided some intriguing insights into the application of the FE thin films for future programmable high-frequency devices and systems.

Magnetoelectric coupling at microwave frequencies observed in bismuth ferrite-based multiferroics at room temperature

1.Introduction,In the search for advanced multifunctional materials for next generation sensors and logic devices,magnetoelectric multifer-roics exhibiting simultaneously ferromagnetism and ferroelectric-ity within the same phase have received considerable attention in recent years[1-4].

High-entropy(Ca_(0.2)Sr_(0.2)Ba_(0.2)La_(0.2)Pb_(0.2))TiO_(3) perovskite ceramics with A-site short-range disorder for thermoelectric applications

Novel high-entropy perovskite-type(Ca_(0.2)Sr_(0.2)Ba_(0.2)La_(0.2)Pb_(0.2))TiO_(3)(CSBLP)ceramics with cubic structure of Pm-3 m space group were successful prepared by solid-state reaction method.Results of XRD,SEM-EDS,HRTEM confirmed a homogeneous distribution and equimolar arrangement of multicomponent cations on the A-site.The high-entropy CSBLP ceramics showed long-range structural order and short-range chemical disorder with widely dispersed nanoscale grains varying sizes of 4-6 nm in the microstructure.Because of increased configurational entropy,the CSBLP ceramic has an enhanced Seebeck coefficient(|S|=272μV/K at 1073 K)and low thermal conductivity(k=1.75 W/m·K at 1073 K)when annealed at1300℃.This work demonstrates the possibility of effectively reducing thermal conductivity and improving the performance of thermoelectric oxides through high-entropy composition design.

THE CONTRIBUTION OF ELECTRICAL CONDUCTIVITY,DIELECTRIC PERMITTIVITY AND DOMAIN SWITCHING IN FERROELECTRIC HYSTERESIS LOOPS

Triangular voltage waveform was employed to distinguish the contributions of dielectric permittivity,electric conductivity and domain switching in current-electricfield curves.At the same time,it is shown how those contributions can affect the shape of the electric displacement-electricfield loops(D-E loops).The effects of frequency,temperature and microstructure(point defects,grain size and texture)on the ferroelectric properties of several ferroelectric compositions is reported,including.BaTiO_(3);lead zirconate titanate(PZT);lead-free Na_(0.5)K_(0.5)NbO_(3);perovskite-like layer structured A_(2)B_(2)O_(7)with super high Curie point(T_(c));Aurivillius phase ferroelectric Bi_(3.15)Nd_(0.85)Ti_(3)O_(12);and multiferroic Bi_(0.89)La_(0.05)Tb_(0.06)FeO_(3).This systematic study provides an instructive outline in the measurement of ferroelectric properties and the analysis and interpretation of experimental data.

Ordered coalescence of nano-crystals in alkaline niobate ceramics with high remanent polarization

Lead-free alkali niobates Na_(0.5)K_(0.5)NbO_(3)(NKN)ceramics,with significantly enhanced ferroelectric remanent polarization(Pr),were prepared using Spark Plasma Sintering(SPS).Three types of boundaries were observed in the ceramics,being grain boundaries between faceted grains,domain boundaries that separate ferroelectric domains inside individual grains,and nanoscale sub-grain boundaries that reveal the nano-scale mosaicity of individual grains.Part of the sub-grain boundaries were from initial powder particles.The other sub-grain boundaries were built by ordered coalescence of nano-crystals during rapid SPS process.It was worthwhile to emphasize that the ordered coalescence of nano-crystals in bulk ceramics during sintering takes place and completes within minutes.These sub-grain features would disappear at higher temperature by long time sintering.Rapid Spark Plasma Sintering allowed us to capture this transient microstructure.The significantly enhanced ferroelectric Pr of NKN was attributed to nanoscale sub-boundaries,which stimulated the dynamics of ferroelectric domain formation and switching.

Enhanced energy storage performance under low electric field in Sm^(3+) doped AgNbO_(3) ceramics

Herein,Ag_(1-3x)Sm_(x)NbO_(3)(0≤x≤0.025)antiferroelectric ceramics were successfully synthesized by solid state methods.The effect of Sm 3þdoping on the structure,property and energy storage performance were studied.With the increasing Sm^(3+)concentrations,the average grain size decreased.Meanwhile,the stability of high temperature M phases(i.e.,the structure between T_(f) and T_(3))was expanded,which led to low loss for energy storage.Both of structure analysis and ferroelectric tests revealed the existence of weakly polar/AFE-like phase below T_(f).The Sm 3þdoping tended to suppress the ferroelectric behavior and expand the stability of antiferroelectricity.Consequently,a significantly enhanced energy storage performance(W_(rec)=3.8 J/cm^(3),η=73%)could be achieved in Ag_(0.97)Sm_(0.01)NbO_(3) ceramic,which was almost 1.5 times larger than that in non-doped AgNbO_(3)(W_(rec)=2.4 J/cm^(3),η=45%)under the similar applied field of 1705 kV/cm±.In particular,the performance of the ceramic showed great temperature stability with variation of±5%from 25℃ to 125℃.These results indicated that the Ag_(0.97)Sm_(0.01)NbO_(3) ceramic could be an ideal lead-free candidate used in the energy storage field.

Exploration about superior anti-counterfeiting ability of Sm^(3+) doped KSr_(2)Nb_(5)O_(15) photochromic ceramics:Origin and atomic-scale mechanism

Reversible luminescence modulation behavior upon the photochromic effect endows the photochromic ceramics with 3reat potential in anti-counterfeiting and data storage applications.Here,Sm^(3+)-doped KSr_(2)Nb_(5)O_(15) photochromic ceramics exhibit superior anti-counterfeiting ability:good covertness and considerable modulation ratio of luminescent emission intensity after photochromic reaction.The results show that the photochromism originated from oxygen and cation vacancies,which were directly identified by electron paramagnetic resonance and positron annihilation lifetime spectra.Unexpectedly,oxygen vacancies work more effectively than cation vacancies during photochromic reactions.Moreover,the extraordinary anti-counterfeiting ability was attributed to the high energy transfer rate,which was particularly caused by the short mean distance below 1 nm between the Sm^(3+) and vacancies.The work here has provided atomic-scale structural evidence and made a progress in understanding the photochromic origins and mechanism in color-center theory.

In-situ growth of carbon nanotubes on ZnO to enhance thermoelectric and mechanical properties

As a high-temperature thermoelectric(TE)material,ZnO offers advantages of non-toxicity,chemical stability,and oxidation resistance,and shows considerable promise as a true ready-to-use module under air conditions.However,poor electrical conductivity and high thermal conductivity severely hinder its application.Carbon nanotubes(CNTs)are often used as a reinforcing phase in composites,but it is difficult to achieve uniform dispersion of CNTs due to van der Waals forces.Herein,we developed an effective in-situ growth strategy of homogeneous CNTs on ZnO nanoparticles by exploiting the chemical vapor deposition(CVD)technology,in order to improve their electrical conductivity and mechanical properties,as well as reducing the thermal conductivity.Meanwhile,magnetic nickel(Ni)nanoparticles are introduced as catalysts for promoting the formation of CNTs,which can also enhance the electrical and thermal transportation of ZnO matrices.Notably,the electrical conductivity of ZnO is significantly boosted from 26 to 79 S·cm^(−1) due to the formation of dense and uniform conductive CNT networks.The lattice thermal conductivity(κ_(L))is obviously declined by the intensification of phonon scattering,resulting from the abundant grain boundaries and interfaces in ZnO-CNT composites.Importantly,the maximum dimensionless figure of merit(zT)of 0.04 at 800 K is obtained in 2.0%Ni-CNTs/ZnO,which is three times larger than that of CNTs/ZnO prepared by traditional ultrasonic method.In addition,the mechanical properties of composites including Vickers hardness(HV)and fracture toughness(K_(IC))are also reinforced.This work provides a valuable reference for dispersing nano-phases in TE materials to enhance both TE and mechanical properties.

Low-cost Free-standing ferroelectric polymer films with high polarization produced via pressing-and-folding

Ferroelectric polymer poly(vinylidene fluoride)(PVDF)shows excellent electro-activity and is promising for flexible electronic devices.However,the processing of PVDF into the favourable ferroelectric structure(β-phase)presents difficulties,while its copolymer with trifluoroethylene(PVDF-TrFE)can directly crystallize into β-phase,but shows limited thermal stability and high-cost processing.As a result,an easily implementable method,pressing-and-folding(P&F),was used to produce highly compatible blended films of PVDF and PVDF-TrFE without using any hazardous solvent or complex polymer processing equipment.Hot-pressed PVDF(molecular weight:530 kg/mol)and PVDF-TrFE(molar ratio:51/49)films were firstly stacked before undergoing P&F treatment.Compared to extrusion-blended films before and after P&F,the P&F stacked films showed isotropic crystalline structure of b-phase,as confirmed using X-ray diffraction and infrared spectroscopy.The ferroelectric remnant polarization of the P&F stacked films is 0.068 C/m^(2),surpassing pure PVDF-TrFE(0.062 C/m^(2))and the simulated value of remnant polarization of pure PVDF(~0.065 C/m^(2)).The above findings promise to provide inspirations for new processing strategy on PVDF-based functional polymers.

Temperature-dependent deformation in silver-particle-covered copper nanowires by molecular dynamics simulation

Cu nanowires covered by Ag particles is studied for potential applications in the next-generation microelectronics.To date,the deformation mechanism in the Cu-Ag core-particle is not clear.Here,molecular dynamics simulation is used to describe the Cu-Ag core-particle system.The results show that the equilibrium structure of Ag particles is reconstructed,when the particle≤1.0 nm.At low temperature(1 K)indicate that three different deformation processes take part in the core-particle structure,depending on the size of Ag particles.When the particle diameter≤2.0 nm,the prevailing deformation mechanism is the emission of dislocations from the Cu surface.For the particle diameters ranging from 3.0 to 6.0 nm,the emission of misfit dislocations from the Ag-Cu interface is the dominant deformation mechanism.If the Ag particle≥6.0 nm,the deformation mechanism can be characterized by the slip band,consisting of the dislocations and amorphous atoms.For elevated temperatures(2-400 K),the mechanical properties of the Ag-Cu core-shell system are nearly independent of temperature,whereas the structure with particles larger than 2.0 nm showed a strong dependence of its mechanical properties on temperature.Based on the results,the diameter-temperature plastic deformation map is proposed.

Effects of zinc substitution on the dielectric properties of Ca_(5)Nb_(4)TiO_(17) microwave ceramics

Ca_(5)Nb_(4)TiO_(17) ceramics with 0≤x≤0.4 were prepared through a solid-state reaction method.Effects of zinc substitution on sintering behavior and microwave dielectric properties of Ca_(5)Nb_(4)TiO_(17) ceramics were investigated.The sintering temperature was significantly lowered from 1480℃for pure Ca_(5)Nb_(4)TiO_(17) to 1260℃ for x=0.4.The microwave dielectric properties are strongly correlated with the composition.It is worth noting that the temperature cofficient of resonant frequency(τf)displays a tendency toward positive value,ranging from-126.4 ppm/℃ to-8.6 ppm/℃.A temperature stable microwave ceramic with dielectric constant of 52 and Q×f value of 9937GHz is achieved at x=0.4 and is a potential candidate for application as cores in dielectrically loaded antennas.

Study on properties of tantalum-doped La_(2)Ti_(2)O_(7) ferroelectric ceramics

A series of ceramics with a general formula,La_(2)Ti_(2-x)Ta_(x)O_(7),in which x=0.05;0.1;0.2;and 0.3,were prepared by Spark Plasma Sintering(SPS).Effects of tantalum substitution for titanium on the structure,dielectric,and piezoelectric properties were studied.Results revealed that the structure changed gradually from 4-layer to 3-layer due to the higher valence of Ta.The solid solution limit of tantalum in La_(2)Ti_(2)O_(7)lattice was in the proximity of x=0.2.The ferroelectric Curie temperature(Tc)decreased with increasing tantalum doping content.dc resistivity reached a maximum value at x=0.2 with a value of~1.0×10^(8)Ωcm at 600℃.The influence of texture on the piezoelectric properties of La_(2)Ti_(2-x)Ta_(x)O_(7)ceramics was also investigated.A maximum d_(33)value~2:1 pC/N was obtained at x=0.2.