Dielectric Properties of Bi_4Ti_3O_(12) Ceramics by Impedance Spectroscopic Method

Various lead-free ceramics have been investigated in search for new high-temperature dielectrics. In particular, Bi_4Ti_3O_（12） is a type of ferroelectric ceramics, which is supposed to replace leadcontaining ceramics for its outstanding dielectric properties in the near future. Ferroelectric ceramics of Bi_4Ti_3O_（12） made by conventional mixed oxide route have been studied by impedance spectroscopy in a wide range of temperature. X-ray diffraction patterns show that Bi_4Ti_3O_（12） ceramics are a single-phase of ferroelectric Bi-layered perovskite structure whether it is calcined at 800 ℃ or after sintering production. This study focused on the effect of the grain size on the electric properties of BIT ceramics. The BIT ceramics with different grain sizes were prepared at different sintering temperatures. Grain becomes coarser with the sintering temperature increasing by 50 ℃, relative permittivity and dielectric loss also change a lot. When sintered at 1 100 ℃, r values peak can reach 205.40 at a frequency of 100 k Hz, the minimum dielectric losses of four different frequencies make no difference, all close to 0.027. The values of Ea range from 0.52 to 0.68 e V. The dielectric properties of the sample sintered at 1 100 ℃ are relatively better than those of the other samples by analyzing the relationship of the grain, the internal stresses, the homogeneity and the dielectric properties. SEM can better explain the results of the dielectric spectrum at different sintering temperatures. The results show that Bi_4Ti_3O_（12） ceramics are a kind of dielectrics. Thus, Bi_4Ti_3O_（12） can be used in high-temperature capacitors and microwave ceramics.

Sensing Characteristics of Shear-Mode A1N Solidly Mounted Resonators with a Silicone Microfluidic System in Viscous Media

A1N solidly mounted resonators with silicone microfiuidic systems vibrating in shear mode are fabricated and characterized. The fabrication process is compatible with integrated circuits and the c-axis tilted A1N films are deposited, which allow in-liquid operation through excitation of the shear mode. The silicone microfluidic system is mounted on top of the sensor chip to transport the analyses and confine the flow to the active area. The properties of sensor operation in air, deionized water, ethanol, isopropanol, 80% glycol aqueous solution, glycol, and olive oil are characterized. The effects of different viscosities on the resonance frequency shift and Q-factor of the sensor have been discussed. The sensitivity and Q value in glycol of the sensor are 1.52 MHz cm2/μg and around 60, respectively. The results indicate the potential of a highly sensitive microfiuidic sensor system for the applications in viscous media.

Enhanced Microwave Absorption for High Filler Content Composite Molded from Polymer Coated Flaky Carbonyl Irons Modified by Silane Coupling Agents

A microwave absorbing sheet with a high complex permeability and a relatively low complex permittivity is obtained by molding of the densely coated flaky carbonyl iron particles(FCIPs)by styrenebutadiene-styrene block copolymer(SBS)in the assistance of coupling agent modification.Direct molding of the core-shell FCIPs without adding extra binder results in a large permeability due to the high filling ratio(55vol%)of absorbents.Importantly,the permittivity is well suppressed by the dense insulate polymer shell on the FCIPs,avoiding the severe impedance mismatch problem of the high filler content microwave absorbing materials.Investigations show that modifying the surface of FCIPs by proper amount of silane coupling agent is critical for the coating quality of the SBS shell,which is verified by resistivity and corrosion current density measurements,and can be interpreted by improved interfacial compatibility between the modified FCIPs and SBS.The obtained microwave absorbing sheet shows a minimum reflection loss of-38.74 dB at 1.57 GHz and has an effective absorption bandwidth from 1.1 to 2.3 GHz at a relatively small thickness of 2 mm.

Piezoelectric and ferroelectric properties of Na_(0.5)Bi_(4.5)Ti_(4)O_(15)-BaTiO_(3) composite ceramics with Mg doping

As one of the representatives of lead-free NBT ceramics,Na_(0.5)Bi_(4.5)Ti_(4)O_(15) has still attracted much attention due to its excellent dielectric properties and has become the focus of research.However,its piezoelectric properties are far from satisfactory.In order to improve the piezoelectric properties of Na_(0.5)Bi_(4.5)Ti_(4)O_(15),Na_(0.5)Bi_(4.5)Ti_(4-x)Mg_(x)O_(y)–BaTiO_(3)(NBTM–BT)composite ceramics were synthesized by a conventional mixed oxide route and sintered at 1040℃ through two-step method.We optimized the electrical properties of NBTM–BT by changing the stoichiometric ratio of Mg content and studied its microscopic mechanism.The piezoelectric coefficient(d33)is stable at about 20 pC/N.Moreover,the maximum remanent polarization(2Pr)of the ceramic is 3.08
C/cm^(2) with the coercive field of 18.01 kV/cm.The dielectric constant and dielectric loss for Na_(0.5)Bi_(4.5)Ti_(3.96)Mg_(0.04)–BT composite ceramic were found to be 486 and 0.17 at 10 kHz,respectively.The characteristic peaks of BT and Na_(0.5)Bi_(4.5)Ti_(4)O_(15) can be observed clearly from the X-ray diffraction analysis.SEM analysis showed that all samples were well crystallized.Consequently,the piezoelectric and ferroelectric properties of Na_(0.5)Bi_(4.5)Ti_(4)O_(15)–BT composite ceramic will be enhanced much by Mg doping,which means it has a wider range of applications in electronic devices such as piezoelectric devices.

Realizing simultaneously excellent energy storage and discharge properties in AgNbO_(3)based antiferroelectric ceramics via La^(3t)and Ta^(5+)co-substitution strategy

AgNbO_(3)based antiferroelectric(AFE)ceramics have large maximum polarization and low remanent polarization,and thus are important candidates for fabricating dielectric capacitors.However,their energy storage performances have been still large difference with those of lead-based AFEs because of their room-temperature ferrielectric(FIE)behavior.In this study,novel La^(3+)and Ta^(5+)co-substituted AgNbO_(3)ceramics are designed and developed.The introduction of La^(3+)and Ta^(5+)decreases the tolerance factor,reduces the polarizability of B-site cations and increases local structure heterogeneity of AgNbO_(3),which enhance AFE phase stability and refine polarization-electric field(PeE)loops.Besides,adding La^(3+)and Ta^(5+)into AgNbO_(3)ceramics causes the decrease of the grain sizes and the increase of the band gap,which contribute to increased Eb.As a consequence,a high recoverable energy density of 6.79 J/cm3 and large efficiency of 82.1%,which exceed those of many recently reported AgNbO_(3)based ceramics in terms of overall energy storage properties,are obtained in(Ag0.88La0.04)(Nb0.96Ta0.04)O_(3)ceramics.Furthermore,the discharge properties of the ceramic with discharge time of 16 ns and power density of 145.03 MW/cm3 outperform those of many lead-free dielectric ceramics.

The influence of Bi content on dielectric properties of Bi_(4-x)Ti_(3)O_(12-1.5x) ceramics

A kind of lead-free dielectric materials,such as the bismuth layered perovskite-type structure of Bi4-xTi3O12-1.5x(x=0.04,0.02,0,-0.02,-0.04),was prepared by the conventional solid-state method at 800C and sintered at 1100C.The variation of structure and electrical properties with different Bi concentration was studied.All the Bi4-xTi3O12-1.5x(x=0.04,0.02,0,-0.02,-0.04)samples exhibited a single structured phase.SEM could be a better approach to present the microstructure of Bi4-xTi3O12-1.5x(x=0.04,0.02,0,-0.02,-0.04)ceramics.It could be found that the grain size of Bi4.02Ti3O12.03 sintered at 1100C was smaller than that of others among the five samples through grain size mechanics.Through impedance spectra analysis,we knew,when the Bi content was fixed,that the dielectric constant and the loss values increased with the decrease of frequency.The Curie temperature of the five samples was about 670C.In particular,while at the frequency of 100 kHz,the lowest loss was 0.001 when Bi content was 3.98.The Bi4.02Ti3O12.03 ceramics with the minimum grain size had highest dielectric constant and the relatively low loss.Due to its high Curie temperature,high permittivity and low loss,the Bi_(4)Ti_(3)O_(12)(BIT)ceramics have a broad application prospect in high density memory,generator,sensor,ferroelectric tunnel junctions and so on.

Higher-order valley vortices enabled by synchronized rotation in a photonic crystal

Synchronized rotation of unit cells in a periodic structure provides a novel design perspective for manipulation of band topology.We then design a two-dimensional version of higher-order topological insulator(HOTI)by such rotation in a triangular photonic lattice with C_(3) symmetry.This HOTI supports the hallmark zero-dimensional corner states and,simultaneously,the one-dimensional edge states.We also find that our photonic corner states carry chiral orbital angular momenta locked by valleys,whose wave functions are featured by the phase vortex(singularity)positioned at the maximal Wyckoff points.Moreover,when excited by a fired source with various frequencies,the valley topological states of both one-dimensional edges and zero-dimensional corners emerge simultaneously.Extendable to higher or synthetic dimensions,our paper provides access to a chiral vortex platform for HOTI realizations in the terahertz photonic system.