Orientation dependence on piezoresponse of lead-free piezoelectric sodium bismuth titanate epitaxial thin films

Lead-free piezoelectric sodium bismuth titanate((Bi0.5Na0.5)TiO3,BNT)thin films were epitaxially grown onto(001)-,(110)-,and(111)-oriented Nb:SrTiO3(STO)single crystal substrates prepared by sol-gel processing.Highly oriented growth in(001),(110),and(111)BNT thin films was obtained in this work benefiting from the lattice match between the BNT film and the STO substrate.The different growth models in thin films with various orientations result in various surface morphologies dependent on the film orientation.The piezoresponse of the BNT thin films was represented exhibiting a strong orientation dependence that(110)>(001)>(111).This is contributed by the various domain switching contribution related to the crystal symmetry and polarization distribution in the three oriented thin films.

Synthesis and Piezoelectric Properties of Lead-free (Bi_(0.5)Na_ (0.5))_ 1-x(Ba_aSr_b)_xTiO_3 Ceramics

A new group of lead-free piezoelectric ceramics,（Bi0.5 Na0.5）1-x（BaaSrb）xTiO3（abbreviated as BNBST[100x-100a/100b],0〈x〈1,a＋b=1）,was synthesized.The ceramics were prepared by conventional ceramic sintering technique,and the ceramics with density of 95% of the theoretical one can be sintered without the atmosphere control during the sintering process.The results of the X-ray diffraction（XRD） data show that the ceramics possess a single perovskite phase.The measurements of dielectric and piezoelectric properties reveal that the ceramics provide relatively high piezoelectric charge constant d33 and high planar electromechanical coupling factor kp.For the BNBST6-95/5 ceramics,d33 is equal to 170pC/N,and kp is equal to 32.0%.The fabrication technique for these ceramics is conventional and stable.

Analyses of temperature-dependent interface states,series resistances,and AC electrical conductivities of Al/p Si and Al/Bi_4Ti_3O_(12)/p Si structures by using the admittance spectroscopy method

In this study, Al/p-Si and Al/Bi4Ti3O12/p-Si structures are fabricated and their interface states （Nss）, the values of series resistance （Rs）, and AC electrical conductivity （σac） are obtained each as a function of temperature using admit- tance spectroscopy method which includes capacitance-voltage （C-V） and conductance-voltage （G-V） measurements. In addition, the effect of interfacial Bi4Ti3012 （BTO） layer on the performance of the structure is investigated. The voltage- dependent profiles of Nss and Rs are obtained from the high-low frequency capacitance method and the Nicollian method, respectively. Experimental results show that Nss and Rs, as strong functions of temperature and applied bias voltage, each exhibit a peak, whose position shifts towards the reverse bias region, in the depletion region. Such a peak behavior is attributed to the particular distribution of Nss and the reordering and restructuring of Nss under the effect of temperature. The values of activation energy （Ea）, obtained from the slope of the Arrhenius plot, of both structures are obtained to be bias voltage-independent, and the Ea of the metal-ferroelectric-semiconductor （MFS） structure is found to be half that of the metal-semiconductor （MS） structure. Furthermore, other main electrical parameters, such as carrier concentration of acceptor atoms （NA）, built-in potential （Vbi）, Fermi energy （EF）, image force barrier lowering （△φb）, and barrier height （φb）, are extracted using reverse bias C 2-V characteristics as a function of temperature.

Ferroelectric and dielectric properties of La/Mn co-doped Bi_4Ti_3O_12 ceramics

La/Mn co-doped Bi4Ti3O12 ceramics, Bi3.25La0.75Ti3-xMnxO12 （x = 0.02, 0.04, 0.06, 0.08）, were prepared by the solid-state reaction method. The influence of manganese substitution for the titanium part in Bi3.25La0.75Ti3O12 on the sintering behaviour, microstructure, Raman spectra and electrical properties was investigated. The experimental results show that the phase composition of all samples with and without manganese doping, sintered at 1000 ℃, consists of a single phase with a bismuth-layered structure belonging to the crystalline phase Bi4Ti3O12. There is no evidence of any impurity phase, but a small change in crystallographic orientation is observed. The Curie temperature of Bi3.25La0.75Ti3-xMnxO12 ceramics is steadily shifted to lower temperature with increasing Mn-doping content. Moreover, the remnant polarisation （Pr） of Bi3.25La0.75Ti3- xMnxO12 samples increases with Mn-doping content, and the Bi3.25La0.75Ti2.92Mn0.08O12 sample exhibits the largest Pr of 16.6 μC/cm^2.

Effects of Processing Conditions on Hydrothermal Synthesis of K_(0.5)Bi_(0.5)TiO_3 Nano-Powders

Potassium bismuth titanate nanosized powders were prepared by the hydrothermal method using Ti(C_4H_9O)_4 and Bi(NO_3)_3·5H_2O as raw materials in alkaline solution.The phase composition,particle size and morphology of the powders were studied by XRD and TEM analysis.The results showed that the powders were well crystallized and dispersed.The crystal phase of the powders was K_(0.5)Bi_(0.5)TiO_3 with the grain size of about 50 nm～100 nm.Hydrothermal temperature and alkaline concentrations had great effects on the phase composition and morphology of the resultant powders.Pure K_(0.5)Bi_(0.5)TiO_3 powders could be synthesized at 170℃～180℃with KOH concentration of 8 mol/L～12 mol/L.

Hysteresis loop behaviors of ferroelectric thin films: A Monte Carlo simulation study

The ferroelectric response of bismuth titanate Bi4Ti3O12 （BIT） thin film is studied through a Monte Carlo simulation of hysteresis loops. The ferroelectric system is described by using a Diffour Hamiltonian with three terms： the electric field applied in the z direction, the nearest dipole-dipole interaction in the transversal （x-y） direction, and the nearest dipole-dipole interaction in the direction perpendicular to the thin film （the z axis）. In the sample construction, we take into consideration the dipole orientations of the monoclinic and orthorhombic structures that can appear in BIT at low temperature in the ferroelectric state. The effects of temperature, stress, and the concentration of pinned dipole defects are assessed by using the hysteresis loops. The results indicate the changes in the hysteresis area with temperature and stress, and the asymmetric hysteresis loops exhibit evidence of the imprint failure mechanism with the emergence of pinned dipolar defects. The simulated shift in the hysteresis loops conforms to the experimental ferroelectric response.

Lanthanum-doped Bi_4Ti_3O_(12) ceramics prepared by high-pressure technique

We present an effective way in this paper to increase the density of lanthanum doped bismuth titanate ceramics, Bi4-xLaxTi3O12 （BLT）, thereby significantly improving the performance of the BLT ceramics. Dense BLT ceramicses, Bi4-xLaxTi3O12 （x = 0.25, 0.5, 0.75, 1.0）, are prepared by using nanocrystalline powders fabricated by a -gel method and high-pressure technique. The microstructures of the BLT ceramicses prepared separately by conventional-pressure and high-pressure techniques are investigated by using x-ray diffraction and transmission electron microscope. The influence of La-doping on the densification of bismuth titanate ceramics is investigated. The experimental results indicate that the phase compositions of all samples with various lanthanum dopings sintered at 900℃ possess layer- structure of Bi4Ti3O12. The green compacts are pressed under 2.5 GPa, 3.0 GPa, 3.5 GPa and 4.0 GPa, separately. It is found that the density of BLT ceramics is significantly increased due to the decreasing of porosity in the green compacts by high-pressure process.

Preparation,crystallization,microstructure and dielectric properties of lead bismuth titanate borosilicate glass ceramics

Various bulk and transparent glasses were prepared by rapid melt quenching technique in the glass system 55[(Pb_(x)Bi_(1-x))TiO_(3)]-44[2Si_(O)2B_(2)O_(3)]-La2O3(x=0-0.7).The X-ray diffraction(XRD)studies of the glass samples confirmed the amorphous nature.The differential thermal analyses(DTA)were carried out from room temperature to 900℃with a heating rate of 10℃/min.The DTA patterns of the samples showed one or more exothermic sharp peaks shifting towards lower temperature side with increasing concentration of bismuth oxide(BiO).On the basis of DTA results,the solid solution of bismuth titanum oxide(Bi_(2)Ti_(2)O_(7))/lead bismuth titanium oxide(Pb3Bi4Ti6O21)was precipitated in borosilicate glassy matrix as a major phase.The glasses were subjected to 4 h and 8 h heat treatment schedules to convert into glass ceramics.XRD analysis of these glass ceramic samples showed that the major crystalline phase of the entire glass ceramic samples with 0≤x≤0.5 is found to have cubic crystal structure,while it is tetragonal for glass ceramic sample with x=0.7.The scanning electron microscopy(SEM)micrographs indicated the uniform distribution of Bi_(2)Ti_(2)O_(7)and Pb3Bi4Ti6O21 crystallites in the glassy matrix.

Structure and Electric Properties of (1-x)(Bi_(1/2)Na_(1/2)) TiO_3-xBaTiO_3 Systems

The structural, dielectric and piezoelectric properties of （1-x）（Bi1/2Na1/2） TiO3-xBaTiO3 ceramics were investigated for the compositional range, x=0.02, 0.04, 0.06, 0.08, 0.10. The samples were synthesized by a conventional solid-state reaction technique. All compositions show a single perovskite structure, and X-ray powder diffraction patterns can be indexed using a rhombohedral structure. Lattice constants and lattice distortion increase while the amount of BaTiO3 increases. The X-ray diffraction results show the morphotropic phase boundary （MPB） of （1-x）（Bi1/2Na12） TiO3-xBaTiO3 exists in near x=0.06-0.08. Temperature dependence of dielectric constant eT33/ε0 measurement reveals that all compositions experience one structural phase and two ferroelectric phases transition below 400℃： rhombohedral （or rhombohedral plus tetragonal） ferroelectric phase ←→ tetragonal antiferroelectric phase ←→ tetragonal paraelectric phase. Relaxor behaviors exist in the course of ferroelectric to antiferroelectric phase transition. Dielectric and piezoelectric properties are enhanced in the MPB range for （ 1-x）（Bi1/2Na1/2）TiO3-xBaTiO3.

Oxygen vacancy migration and its lattice structural origin in A-site non-stoichiometric bismuth sodium titanate perovskites

Off-stoichiometry of perovskite structural Bi0.5Na0.5TiO3(BNT)ferroelectrics can give rise to considerable oxide-ion conductivity.The inherent structural characteristics are urgent to be resolved due to its particular sensitivity of the conduction mechanism to the nominal composition and synthesis process.Herein,a thorough study of the temperature-dependent neutron,X-ray diffraction and Raman spectrum is carried out on a series of equivalently substituted A-site deficient non-stoichiometric and pristine BNT.Phase transition and defect association are systemically investigated in these dominated rhombohedral phases at room temperature,associated with well saturated ferroelectric states.Significant structural evolution identified by Rietveld refinements and the origin of the electrical performance are clarified at elevated temperatures,focusing on the subtle distortions of ionic displacements,oxygen octahedral tilts and local chemical environments for oxygen vacancies.The ion migration ability mediated by oxygen vacancies that are not energetically favorable in BNT mainly depends on the external substitutional disorder,and is strongly affected by the dopant concentration.Together with the lone pair substitution concept,superior oxide ionic conductivity is achieved,and an alternative strategy is provided in designing BNT based oxide ion conductors.

Investigation of Ceramic Based Composites by Using 2D Graphene Filler

The ceramic composites of sodium bismuth titanate with reduce graphene oxide NBT/rGO of different compositions were fabricated by solid state sintering method and characterized. In this work, the graphene oxide (GO) and reduce graphene oxide (rGO) was successfully synthesized by Hummer’s modified method which is confirmed by FTIR and XRD results. The reduce graphene oxide used as 2D filler in piezoelectric creamic material. The crystalline structure of NBT/rGO composite was confirmed by X-ray diffraction with rhombohedral symmetry. The dispersion of rGO in the ceramic can be detect by the optical microcopy images. The electrical conductivity of sodium bismuth titanate shows increase at higher values of frequency and conductivity nanocomposites of different wt% were start decreases up to certain value of frequency. The broadening of peaks in frequency explicit plots of electrical conductivity with the help of LCR Meter (Impedance Capacitance and Resistance). The crystalline size of reduced graphene oxide and NBT is calculated by Scherrer’s formula of XRD peaks.

Modulation of phase boundary and domain structures to engineer strain properties in BNT-based ferroelectrics

Bismuth sodium titanate(BNT)ceramics exhibit outstanding strain responses but are unfavorable for application in high-sensitivity displacement actuators due to the large negative strain resulting from irreversible changes in their phase transition and domain structure.Here,(1−x)Bi_(0.50)Na_(0.41)K_(0.09)TiO_(3)-xNaNbO_(3)(BNKT−xNN)solid solutions were prepared to improve the strain properties through the strategy of modulating the phase boundary and domain structures.The introduction of sodium niobate could effectively regulate the relative content of the tetragonal(P4bm)and rhombohedral(R3c)phases in the phase boundary region.The ferroelectric-to-relaxor phase transition(TF−R)was reduced,and the ergodic relaxor(ER)state was nurtured at room temperature.Excellent zero-negative strain properties of S=0.41%and d33*=742 pm/V were achieved from the reversible transition between the ER and ferroelectric states under an applied electric field(x=0.04).Additionally,understanding the domain states via piezoelectric force microscopy(PFM)and firstorder reversal curve(FORC)revealed that the superior strain responses originated from the reversible inter-transformation of substable macrodomains and polar nanoregions(PNRs)in the phase boundary.This study provides new insight into the interplay between the evolution of phase boundaries and domain structures and the strain properties of BNT-based ceramics.

Dielectric properties of bismuth-containing pyrochlores:A comparative analysis

The comparative analysis of the dielectric properties of bismuth-containing pyrochlores with different manifestation of atomic order/disorder was carried out.We examined the dielectric properties(including behavior in electric fields)of two pyrochlore compounds:BZN(presumably a composition close to Bi_(1.5)Zn_(0.5)Nb_(1.5)O_(6.5))ceramics with chemical disorder in both A and B cation sublattices and Bi_(2)Ti_(2)O_(7)single crystal with fully chemical ordered structure.The fundamental differences between the dielectric properties of the BZN ceramics and Bi_(2)Ti_(2)O_(7)single crystal were shown.In particular,in the dielectric relaxation behavior(which cannot be described via Arrhenius law in the Bi_(2)Ti_(2)O_(7))or in the influence of the electric fields on the dielectric permittivity(split-ting of the field-cooled and zero-field-cooled behaviors was observed for Bi_(2)Ti_(2)O_(7)below estimated freezing temperature).The results of this study highlights the special role of Bi_(2)Ti_(2)O_(7)as a candidate material for studying aspects of geometric frustration related with pyrochlore structure in non-magnetic medium and specifies the future directions of research.

Construction of charge transfer chain in Bi_(12)TiO_(20)-Bi_(4)Ti_(3)O_(12)/α-Bi_(2)O_(3)composites to accelerate photogenerated charge separation

Photogenerated charge separation and transfer is one of the bottleneck steps in photocatalysis,and efficient charge separation strategies are strongly desired.Here,mimicking the electron transport chain in natural photosynthesis,we report the design and fabrication of a charge transfer chain using bismuth-based semiconductor as a proof-of-concept.In view of the thermodynamic energy band positions and structural similarity based on the density functional theory(DFT)analysis,heterostructured combination ofα-Bi_(2)O_(3),perovskite-like Bi_(4)Ti_(3)O_(12),and sillenite Bi12TiO20 was designed for fabrication of charge transfer chain.By tuning the molar ratio of Bi and Ti precursors,the Bi_(4)Ti_(3)O_(12)and Bi12TiO20 particles were formed on the surface ofα-Bi_(2)O_(3)by an insitu transformation process,giving rise to Bi_(12)TiO_(20)-Bi_(4)Ti_(3)O_(12)/α-Bi_(2)O_(3)composites with charge transfer chain.We propose that the effective charge transfer is accomplished amongα-Bi_(2)O_(3),Bi12TiO20,and Bi_(4)Ti_(3)O_(12),which significantly improves the photogenerated charge separation and transfer,as indicated by photoluminescene,time-resolved photoluminescene,and electrochemical impedance spectra results.As expected,the Bi_(12)TiO_(20)-Bi_(4)Ti_(3)O_(12)/α-Bi_(2)O_(3)shows the superior photocatalytic activity for the degradation of environmental pollutants with high concentration.Even for the refractory pollutants like 4-chlorophenol,the optimal Bi_(12)TiO_(20)-Bi_(4)Ti_(3)O_(12)/α-Bi_(2)O_(3)composite shows 28 times higher than that ofα-Bi_(2)O_(3)for photocatalytic degradation,verifying the superiority of photogenerated charge transfer chain in photocatalysis.This work demonstrates the feasibility of the charge transfer chain strategy to boost the photogenerated charge separation,which is of great significance for designing energy and environmental-related materials in heterogonous photocatalysis.

Cooperation of oxygen vacancies and 2D ultrathin structure promoting CO2 photoreduction performance of Bi4Ti3O12

Reduction of CO2to solar fuels by artificial photosynthesis technology has attracted considerable attention. However, insufficient separation of charge carriers and weak CO2adsorption hamper the photocatalytic CO2 reduction activity. Herein, we tackle these challenges by introducing oxygen vacancies (OVs) on the two-dimensional Bi4Ti3O12ultrathin nanosheets via a combined hydrothermal and postreduction process. Selective photodeposition experiment of Pt over Bi4Ti3O12discloses that the ultrathin structure shortens the migration distance of photo-induced electrons from bulk to the surface, benefiting the fast participation in the CO2reduction reaction. The introduction of OVs on ultrathin Bi4Ti3O12 nanosheets leads to enormous amelioration on surface state and electronic structure, thereby resulting in enhanced CO2adsorption, photoabsorption and charge separation efficiency. The photocatalytic experiments uncover that ultrathin Bi4Ti3O12nanosheets with OVs reveal a largely enhanced CO2photoreduction activity for producing CO with a rate of 11.7 lmol g-1h-1in the gas–solid system, 3.2 times higher than that of bulk Bi4Ti3O12. This work not only yields efficient ultrathin photocatalysts with OVs, but also furthers our understanding on enhancing CO2reduction via cooperative tactics.

Phase evolution in(1x)(Na_(0.5)Bi_(0.5))TiO_(3)-xSrTiO_(3) solid solutions:A study focusing on dielectric and ferroelectric characteristics

In the present work,the nature of phase evolution of(1x)(Na_(0.5)Bi_(0.5))TiO_(3)-xSrTiO_(3)(NBT-xST)solid solutions with x of 0e0.6 is revealed by characterizing the dielectric and ferroelectric properties.Two unique dielectric anomalies associated with high-temperature nanoregions(PNRs)in the ergodic relaxor(ER)state and low-temperature PNRs in the nonergodic relaxor(NR)state are identified.Characteristic temperatures,including TB,TRT*,Tm,Td and TT*,are determined in fresh and poled states on the basis of the characteristics of the evolution of these two dielectric anomalies.The whole evolution of the transition from the NR state to the ER state is reflected by the temperature-dependent polarization versus electric field(P-E)hysteresis loops,i.e.,from the square loops,via the double-like loops,to the slim loops.The characteristic temperatures,including TP-N,TN-R and TR-dis,are determined by the characteristics of the evolution of P-E loops.Accordingly,a phase diagram of NBT-xST was constructed according to these characteristic temperatures.Most importantly,the relationship between polarization responses and heterogeneous polar phase coexistence has been established and a schematic diagram is given.This work will help to understand the phase evolution and its impact on the macroscopic properties of NBT and the associated NBT-based solid solutions.

Structure and Piezoelectric Properties of Lead-Free Na_(0.5)Bi_(0.5)TiO_3 Nanofibers Synthesized by Electrospinning

Lead-free Na0.5Bi0.5TiO 3（NBT） nanofibers with the perovskite structure were prepared by the electrospinning method.The nanofibers were about 200-300 nm in diameter and up to several hundred microns in length.The crystal structures and morphologies of the nanofibers were characterized by X-ray diffraction（XRD）,Raman spectroscopy,scanning electron microscopy（SEM）,and transmission electron microscopy（TEM）.The effective piezoelectric property of individual NBT nanofiber was examined by piezoresponse force microscopy（PFM）.The NBT nanofibers crystallized in pure perovskite phase after annealing above 700℃in air and comprised a great number of fine particles with size of 60-80 nm.In addition,the electromechanical energy conversion models for NBT nanofibers were built and demonstrated high voltage output as high as several millivolts.Such a result qualifies NBT nanofibers as a promising candidate for leadfree electromechanical conversion devices.

Effect of rare earth on physical properties of Na_(0.5)Bi_(0.5)TiO_(3)system:A density functional theory investigation

Na_(0.5)(Bi_(3/4)RE_(1/4))_(0.5)TiO_(3)(RENBT,RE=Nd,Gd,Dy,and Ho)compounds were investigated in the framework of first-principles calculations using the full potential linearized augmented plane wave(FP-LAPW)method based on the spin-polarized density functional theory implemented in the WIEN2k code.Combined charge density distribution and Ti K-edge X-ray absorption spectra reveal that the RENBT compositions with high polarization values are accompanied by a higher TiO_(6)distortion,DyNBT,and NdNBT compounds.The effect of the rare-earth elements on the polarization is confirmed experimentally with the collection of the hysteresis loops.The investigation of the electronic properties of the compounds highlights the emergence of a magnetization owing to the 4f orbital effect of the rare-earth elements.Besides,the investigation of the chemical ordering shows a short-range chemical ordering for the pure composition and an increased A-site disorder for dysprosium doped NBT system.The increased disorder may speak for increased relaxor properties in the RE doped compositions.

A REVIEW ON LEAD-FREE PIEZOELECTRIC CERAMICS STUDIES IN CHINA

There are a large number of research publications on the hot topic of environmental friendly leadfree piezoelectric materials worldwide in the last decade.The number of researchers and institutions involved from China is much larger than other countries or regions.The publications by Chinese researchers cover a broad spectrum on the preparations,structures,properties and applications of lead-free piezoelectric ceramics.This has motivated us to come out with a review on recent advances in development of lead-free piezoelectric ceramics in China.The emphases are especially on the preparation and electric properties of barium titanate-based materials,bismuth sodium titanate and related materials,alkaline niobate and related materials,bismuth layerstructured materials,as well as texture engineering of ceramics and some of their single crystals.Hopefully,this could give further impetus to the researchers to continue their e®orts in this promising area and also draw the attentions from legislature,research o±ce,industrial and publics.

Progress and perspective of high strain NBT-based lead-free piezoceramics and multilayer actuators

In this review,the evolution of high strain Na_(0.5)Bi_(0.5)TiO_(3)-based lead-free piezoceramics and their multilayer actuators has been explored.First,in terms of Na_(0.5)Bi_(0.5)TiO_(3)-based ceramic materials,the origin of high strain,the typical chemical modification methods of obtaining large strain and extrinsic factors affecting the large strain are discussed.Then it briefly summarizes the problems existing in Na_(0.5)Bi_(0.5)TiO_(3)-based ceramics for multilayer actuator applications.Strategies to optimize strain performance by means of microstructure control and phase structure design are also discussed.Thereafter,in terms of multilayer actuator,we describe its characteristics,applications and preparation process systematically,as well as the recent development of Na_(0.5)Bi_(0.5)TiO_(3)-based multilayer actuator.At last,perspectives on directions of following work and promising fields for the applications of the materials and their devices are presented.