Microstructure,crystalline phase and electrical properties of Li_(0.06)(Na_(0.5)K_(0.5))_(0.94)Nb_(1-2x/5)Mg_xO_3 lead-free piezoelectric ceramics

MgO-modified Li0.06（Na0.5K0.5）0.94NbO3O3 （L6NKN） lead-free piezoelectric ceramics were synthesized by normal sintering at a rela- tively low temperature of 1000℃. The crystalline phase, microstructure, and electrical properties of the ceramics were investigated with a special emphasis on the influence of MgO content. The addition of MgO effectively improves the sintembility of the L6NKN ceramics. X-my diffr cfion analysis indicates that the morphotropic phase boundary （MPB） separating orthorhombic and tetragonal phases for the ceramics lies in the range of Mg doping content （x） from 0.3at% to 0.7at%. High electrical properties of the piezoelectric constant （d33=238 pC/N）, planar electromechanical coupling coefficient （kp=41.5%）, relative dielectric constant （εr=905）, and remanent polarization （Pr=38.3 μC/cm2） are obtained from the specimen with x=0.5at%, which suggests that the Li0.06（Na0.5K0.5）0.94Nb（1-2x/5）MgxO3 （x=0.5at%） ceramic is a promising lead-free piezoelectric material.

Effects of Na/K ratio on the phase structure and electrical properties of Na_xK_(1-x)NbO_3 lead-free piezoelectric ceramics

Lead-free piezoelectric NaxK1-xNbO3（x = 0.3-0.8）（NKN） ceramics were fabricated by normal sintering at 1060°C for 2 h.Microstructures and electrical properties of the ceramics were investigated with a special emphasis on the influence of Na content.The grain size of the produced dense ceramic was decreased by increasing Na content.A discontinuous change in the space distance was found at the composition close to Na0.7K0.3NbO3 ceramic, which indicates the presence of a transitional composition between two different orthorhombic phases, which is similar to the behavior of morphotropic phase boundary（MPB） in NaxK1-xNbO3 ceramics.Such MPB-like behavior contributes to the enhanced piezoelectric coefficient d33 of 122 pC/N, planar-mode electromechanical coupling coefficient kP of 28.6%, and dielectric constant εr of 703, respectively for the Na0.7K0.3NbO3 ceramic.Cubic temperature TC and the transitional temperature TO-T from orthorhombic to tetragonal phase are observed at around 420°C and 200°C, respectively.

Synergy of lead vacancies and morphotropic phase boundary to promote high piezoelectricity and temperature stability of PBZTN ceramics

The flourishing development of emerging electromechanical applications has stimulated an urgent demand for ferroelectric ceramics with high piezoelectric properties and broad temperature usage range.However,it remains a challenge to simultaneously obtain good piezoelectricity and reliable temperature stability in lead zirconate titanate(PZT)-based piezoelectric ceramics.To solve this issue,a synergetic strategy was proposed to introduce lead vacancies through niobium doping and construct morphotropic phase boundary(MPB).In this work,Pb_(0.905)Ba_(0.085)(V Pb″)_(0.01)[(Zr_(x)Ti_(1-x))_(0.98)Nb_(0.02)]O_(3)(PBZTN-x)material system was designed.Good comprehensive properties(d_(33)=864 pC/N,k_(p)=84%,T_(C)=201℃)and excellent temperature stability(less than 10%variation of electrical properties from 20℃ to 160℃)were obtained in PBZTN-0.540 ceramics.Good piezoelectricity can be attributed to high extrinsic contribution(domain wall motion)induced by Pb^(2+)vacancies and the existence of nano-domains emerged at MPB,while excellent temperature stability is mainly attributed to the minimized local stress in the lattice and the stable domain structure.

Effects of CeO_2 doping on the structure and properties of PSN-PZN-PMS-PZT piezoelectric ceramics

Quinary system piezoelectric ceramics PSN-PZN-PMS-PZT were prepared by using a two-step method. The effects of CeO2 doping on piezoelectric and dielectric properties of the system were investigated at morphotropic phase boundary （MPB）. The results reveal that the relative dielectric constant ε33^T｜ε0, the Curie temperature To, the piezoelectric constant d33, the mechanical quality factor Qm, and the electromechanical coupling coefficient Kp are changed with the increase of CeO2 content. On the other hand, the effects of CeO2 doping on the dielectric properties of PSN-PZN-PMS-PZT piezoelectric ceramics at high electric field are consistent with the change at weak electric field. The values of dielectric constant and dielectric loss are enhanced with the increasing of electric field.

BiScO_(3)-BiFeO_(3)-PbTiO_(3)-BaTiO_(3) high-temperature piezoelectric ceramic and its application on high-temperature acoustic emission sensor

Piezoelectric ceramic based high-temperature acoustic emission(AE)sensor is required urgently in the structural health monitoring of high-temperature fields.In this research,a series of 0.45(BiSc_(x)O_(3)-BiFe_(1-x)O_(3))-0.48PbTiO_(3)-0.07BaTiO_(3)(BSc_(x)Fe_(1-x)-PT-BT,n(Sc)/n(Fe)=0.4/0.6-0.6/0.4)ceramics with both high Curie temperature and large piezoelectric constant were presented.The structure and electrical properties of BSc_(x)Fe_(1-x)-PT-BT ceramics as a function of n(Sc)/n(Fe)have been systematically investigated.All the ceramics possess a perovskite structure,and the phase approaches from the rhombohedral toward the tetragonal phase with the decrease of n(Sc)/n(Fe).The BSc_(0.5)Fe_(0.5)-PT-BT and BSc_(0.5)Fe_(0.5)-PT-BT piezoelectric ceramics exhibit good piezoelectricity(d_(33)=250-281 pC/N),high Curie temperature(T_(C)=430-450℃)and excellent temperature stability.These improvements are greatly attributed to the balance between rhombohedral and tetragonal phase near morphotropic phase boundary with dense microstructure of ceramics.AE sensor based BSc_(0.5)Fe_(0.5)-PT-BT piezoelectric ceramic was designed,prepared and tested.The high-temperature stability of AE sensor was characterized through pencil-lead breaking with in situ high-temperature test.The noise of AE sensor is less than 40 dB,and the acoustic signal is up to 90 dB at 200℃.As a result,AE sensors based on BSc_(x)Fe_(1-x)-PT-BT piezoelectric ceramics are expected to be applied into the structural health monitoring of high temperature fields.

GIANT ANHYSTERETIC RESPONSE OF FERROELECTRIC SOLID SOLUTIONS WITH MORPHOTROPIC BOUNDARIES: THE ROLE OF POLAR ANISOTROPY

Computer modeling and simulation for the Pb（Zr1-x Tix ）O3 （PZT） system reveal the role of polar anisotropy on the giant anhysteretic response and structural properties of morphotropic phase boundary （MPB） ferroelectrics. It is shown that a drastic reduction of the compositiondependent polar anisotropy near the MPB flattens energy surfaces and thus facilitates reversible polarization rotation. It is further shown that the polar anisotropy favors formation of polar domains, promotes phase decomposition and results in a two-phase multidomain state, which response to applied electric field is anhysteretic when the polar domain reorientation is only caused by polarization rotation other than polar domain wall movement. This is the case for the decomposing ferroelectrics under a poling electric field with the formation of a two-phase multidomain microstructure, wherein most domain walls are pinned at the two-phase boundaries. Indication of the microstructure dependence of the anhysteretic strain response opens new avenues to improve the piezoelectric properties of these materials through the microstructure engineering.

Dielectric properties of Pb (Zn_(1/3) Nb_(2/3)) O_3-PbZrO_3-PbTiO_3 solid solution near morphotropic phase boundary

The dielectric properties of Pb(Zn1/3Nb2/3)O3-PbZrO3-PbTiO3 (PZN-PZ-PT) system near the rhombohedral/tetragonal morphotropic phase boundary (MPB) are carefully studied in this paper.It is found that,for all samples,the curves around the temperatures of dielectric permittivity peak show the characteristics of diffuse phase transition.The change in PbZrO3/PbTiO3 ratio has much influence on the dielectric properties of the samples.The extent of diffuse phase transition increases with the increasing Zr/Ti ratio.The samples in rhombohedral region have typical diffuse phase transition in the temperature range measured.However,for the samples with tetragonal symmetry,a spontaneous normal ferroelectric-relaxor phase transition exists at temperature lower than that of permittivity peak.This normal ferroelectric-relaxor phase transition is confirmed by the experiment of thermally driven current.The analysis of TEM reveals that the samples in tetragonal region show a 90° macrodomain structure,while the samples in rhombohedral region have the configuration of microdomain structure.

Piezoelectric Properties of(Na0.5K0.5-xLix)NbO3 Ceramics

Lead-free piezoelectric ceramics （Na0.5K0.5-xLix）NbO3 （x=0.057-0.066） were synthesized by an ordinary sin-tering technique. Substituting Li for K can lead to structural distortion, which improves the Curie temperature （To） greatly. By adding appropriate LiNbO3 content, piezoelectric constant d33 values reach 202-212 pC/N. Electromechanical coefficients of the planar mode reach 44.4%-46.8%. The dielectric loss is below 2.6%, which is much lower than reported （about 50%）. The To of （Na0.5K0.5-xLix）NbO3 （x=0.057-0.066） is in the range of 490-510℃, at least 70℃ higher than that of pure （Na0.5K0.5）NbO3 ceramics. The results show that （Na0.5K0.5-xLix）NbO3 ceramic is a kind of good lead-free high-temperature piezoelectric material.

Structural changes concurrent with ferromagnetic transition

Ferromagnetic transition has generally been considered to involve only an ordering of magnetic moment with no change in the host crystal structure or symmetry, as evidenced by a wealth of crystal structure data from conventional X-ray diffractometry （XRD）. However, the existence of magnetostriction in all known ferromagnetic systems indicates that the magnetic moment is coupled to the crystal lattice; hence there is a possibility that magnetic ordering may cause a change in crystal structure. With the development of high-resolution synchrotron XRD, more and more magnetic transitions have been found to be accompanied by simultaneous structural changes. In this article, we review our recent progress in understand- ing the structural change at a ferromagnetic transition, including synchrotron XRD evidence of structural changes at the ferromagnetic transition, a phenomenological theory of crystal structure changes accompanying ferromagnetic transitions, new insight into magnetic morphotropic phase boundaries （MPB） and so on. Two intriguing implications of non-centric symmetry in the ferromagnetic phase and the first-order nature of ferromagnetic transition are also discussed here. In short, this review is intended to give a self-consistent and logical account of structural change occurring simultaneously with a ferromagnetic transition, which may provide new insight for developing highly magneto-responsive materials.

Piezoelectricity in K_(1-x)Na_xNbO_3: First-principles calculation

The piezoelectric properties of K1-xNaxNbO3 are studied by using first-principles calculations within virtual crystal approximation. To understand the critical factors for the high piezoelectric response in K1-xNaxNbO3, the total energy, piezoelectric coefficient, elastic property, density of state, Born effective charge, and energy barrier on polarization rotation paths are systematically investigated. The morphotropic phase boundary in K1-xNaxNbO3 is predicted to occur at x = 0.521, which is in good agreement with the available experimental data. At the morphotropic phase boundary, the longitudinal piezoelectric coefficient d33 of orthorhombic K0.5Na0.5NbO3 reaches a maximum value. The rotated maximum of d＊33 is found to be along the 50° direction away from the spontaneous polarization （close to the [001] direction）. The moderate bulk and shear modulus are conducive to improving the piezoelectric response. By analyzing the energy barrier on polarization rotation paths, it is found that the polarization rotation of orthorhombic K0.5Na0.5NbO3 becomes easier compared with orthorhombic KNbO3, which proves that the high piezoelectric response is attributed to the flattening of the free energy at compositions close to the morphotropic phase boundary.

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.

Dielectric and piezoelectric perfomance of lead-free ceramics of boron sodium gadolinate niobate at morphotropic phase boundary

New lead-free piezoceramic nanocomposites of Boron Sodium Gadolinium Niobate(BNGN),with general formula(1-x)B_(0.5)Na_(0.5)GdO3xB_(0.5)Na_(0.5)NbO_(3),exhibiting a Morphotropic Phase Boundary(MPB),have been synthesized following hydrothermal method followed by solid state sintering.The occurrence of MPB at the composition with x=0.55,at which rhombohedral and monoclinic phases are found to coexist,has been confirmed using powder XRD.This accounts for the occurrence of large remnant polarization when the sintered ceramic pellets are subjected to electric poling at 2KV/mm.Uniform microstructure of various compositions is confirmed by SEM imaging.Dielectric and piezoelectric properties of the samples are found to be comparable to those of commercial grade PZT.At the MPB,the d_(33)coefficient is found to be 556 pC/N,which is close to that of commercial grade PZT,which makes BNGN a promising material to substitute lead containing PZT in the near future.

ertical morphotropic phase boundary in lead-free piezoelectric ceramics(K,Na,Li)NbO_(3)-BaZrO_(3)-(La,Na)TiO_(3) system

A morphotropic phase boundary(MPB)with temperature-independent behavior,the so-called vertical MPB was investigated in lead-free(K,Na,Li)NbO_(3)–BaZrO_(3)–(La,Na)TiO_(3)ternary ceramic system.The specimens were synthesized by a conventional solid-state reaction method,and their crystal structures as well as their MPB were determined from X-ray diffraction patterns measured from room temperature to 300℃.The vertical MPB composition was determined to be 0.9025(K_(0:45)Na0:5Li_(0:05))NbO_(3)–0.09BaZrO_(3)–0.0075(La,Na)TiO_(3)and the Curie temperature was found to be about 195℃.It was successfully confirmed that ceramic samples of this system could be sintered in a reducing atmosphere.For lead-free piezoceramic applications of multilayer actuators using Ni inner electrodes,the results obtained in this work have important practical implications.

Computer simulation of super-magnetoelastic behavior near critical region of magnetic materials based on phase-field method

Since the discovery of ferromagnetic morphotropic phase boundary(MPB)in 2010,the connotation and extension of MPB have been becoming more and more abundant.Over the last dozen years,much experimental work has been done to design magnetostrictive materials based on the MPB principle.However,due to the difficulty in direct experimental observations and the complexity of theoretical treatments,the insight into the microstructure property relationships and underlying mechanisms near the ferromagnetic MPB has not been fully revealed.Here,we have reviewed our recent computer simulation work about the super-magnetoelastic behavior near the critical region of several typical materials.Phase-field modeling and simulation are employed to explore the domain configuration and engineering in single crystals as well as the grain size effect in polycrystals.Besides,a general nano-embryonic mechanism for superelasticity is also introduced.Finally,some future perspectives and challenges are presented to stimulate a deeper consideration of the research paradigm between multiscale modeling and material development.

Superior piezoelectric performance with high operating temperature in bismuth ferrite-based ternary ceramics

Due to the thermal depolarization effect,adequate piezoelectric performance with high operating temperature is regarded to be challenging to accomplish concurrently in piezoceramics for applications in specific piezoelectric devices.In this work,we synthesized(0.8−x)BiFeO_(3)-x PbTi_(3)-0.2Ba(Zr_(0.25)Ti_(0.75))O_(3)(abbreviated as BFO-x PT-BZT)ternary solid solutions with 0.15≤x≤0.30 by conventional solid-state reaction method.The MPB composition with a coexisting state of rhombohedral-tetragonal phases exhibits enhanced electromechanical properties,including Curie temperature of 380℃,large-signal equivalent piezoelectric coefficient d^(∗)_(33)of 395 pm V^(-1),small-signal piezoelectric coefficient d_(33)of 302 pC N^(-1),and electromechanical coupling factor k_(p)of 50.2%,which is comparable to commercial PZT-5A ceramics,indicating potential in high-temperature applications.Furthermore,in-situ X-ray diffraction(XRD)and piezoelectric force microscopic(PFM)techniques demonstrate that multiphase coexistence and complex nanodomains promote piezoelectric response via synergism.The x=0.24 composition exhibits the highest in-situ d_(33)of 577 pC N^(-1)and good temperature stability in 30−280℃,indicating that BZT-modified BFO-PT ceramics are promising candidates for high-temperature piezoelectric devices.

Ultralow operating voltage for energy conversion performance in Hf_(1-x)Zr_(x)O_(2) thin films

Emerging ferroelectric and antiferroelectric HfO_(2)-based thin films are attractive candidates for energy conversion and storage applications. In this work, the polar phase transformation between tetragonal and orthorhombic phases associated with ferroelectric or antiferroelectric behaviors is utilized to manipulate the electrocaloric cooling and energy storage performances in Zr-doped, woken up HfO_(2) ultrathin films. A giant electrocaloric temperature change of up to 11.85 K in Hf_(0.5)Zr_(0.5)O_(2) with the morphotropic phase boundary (MPB) state and a high energy storage density of 39.34 J/cm^(3) in the tetragonal phase-dominant Hf0.25Zr0.75O2 system are obtained. More interestingly, contrary to overdoping and excessive electric fields, an appropriate Zr concentration of 0.5 and an applicable driving field of 1.91 MV/cm are desired for the electrocaloric effect, resulting in an ultralow operating voltage as low as 1.3 V in this 6.8 nm thick Hf_(0.5)Zr_(0.5)O_(2) film. These findings illustrate that the structural design strategy is a visible method for achieving optimal energy-related behaviors and highlight the great possibilities for building future energy-related devices.

Ferroelectric domains and phase transition of sol-gel processed epitaxial Sm-doped BiFeO_(3)(001)thin films

BiFeO_(3),a room-temperature multiferroic material,has recently been increasingly applied as a potential lead-free piezoelectric material due to its large piezoelectricity achieved by doping.In this work,12%Smdoped BiFeO_(3)epitaxial thin films were fabricated on Nb-doped SrTiO_(3)(001)single crystal substrates via sol-gel method.The epitaxy was verified by reciprocal space mapping(RSM)and transmission electron microscope(TEM).The TEM results indicated the coexistence of R3c and Pbam phases in the film.The domains and piezoelectric properties from room temperature to 200℃were characterized by piezoresponse force microscopy(PFM).Domains became active from 110℃to 170℃,and domain configurations changed obviously.A partially fading piezoresponse indicated the emergence of antiferroelectric Pbam.The in-situ domain analysis suggested that the phase transition was accompanied by domain wall motion.Switching spectroscopy PFM(SS-PFM)was further conducted to investigate the piezoresponse during the phase transition.Anomalous responses were found in both ON and OFF states at 170℃,and the film exhibits typical antiferroelectric behavior at 200℃,implying that the completion of phase transition and structure turned to the Pbam phase.This work revealed the origin of the high piezoresponse of Sm-doped BiFeO_(3)thin films at the morphotropic phase boundary(MPB).

Phase diagram for Bi-site La-doped BiFeO_(3)—BaTiO_(3)lead-free piezoelectric ceramics

Three different series of lead-free ceramics,i.e.,(1-y)Bi1.03(1_x)LaxFeO_(3)-yBaTiO_(3)(y=0.27,x=0.00-0.12),(y=0.30,x=0.00-0.10),and(y=0.33,x=0.00-0.08)are prepared via a conventional solid-state reaction with water quenching.From X-ray diffraction and electrical property measurements,two morphotropic phase boundaries(MPBs)are discovered in all three ceramic systems.The first MPB(MPB-Ⅰ)appeared between rhombohedral and tetragonal phases,whereas the second MPB(MPB-Ⅱ)appeared between tetragonal and cubic-like phases.The highest direct piezoelectric coefficients(d_(33)=201,274,and 268 pC/N)are mainly attributed to the typical MPB-I of the rhombohedral and tetragonal phases.However,the highest converse piezoelectric coefficients(d_(33*)=490,500,and 570 pm/V with Curie temperature>330℃)are obtained for compositions near to the MPB-II.A significant enhancement in the dielectric constant at low temperature is associated with the local structural heterogeneity by La^(3+)doping,which serves as an origin for a high piezoelectric strain response.Based on the crystal structure as well as on the dielectric,ferroelectric,and piezoelectric properties,a phase diagram is constructed for La-doped BiFeO3-BaTiO3 ceramics.This phase diagram reveals the relationship between piezoelectric performance and crystal structure.

Processing, phase transitions, and dielectric properties of BSPT ceramics

Solid solutions on the base of composition from morphotropic phase boundary(1-x)BiScO_(3)–xPbTiO_(3) with x=0.64,0.645,0.65 were prepared from nitrates solutions.Variations in phase content,structure parameters,microstructure,dielectric and piezoelectric properties of dense ceramic samples were studied.The first-order phase transitions were observed at temperatures near 700 K.The influence of the processing conditions on the morphology,temperature of phase transitions,and dielectric parameters was observed.In some ceramics effect of dielectric relaxation was observed at high temperatures.This effect was explained by the contribution from ionic transport due to the oxygen vacancies creation during high temperature sintering.

Investigation of lead-free BiFeO_(3)–BaTiO_(3)piezoelectric ceramics through precise composition control

BiFeO_(3)-BaTiO_(3)is a promising lead-free piezoelectric ceramic,exhibiting high Curie temperature and superior electrochemical characteristics.In this work,e_(1-x)TBiFeO_(3)-xBaTiO_(3)(BF-xBT,x=0.26,0.28,0.30,0.32,0.34,0.36)ceramics were fabricated using the conventional solid-state reaction method through precise composition control.Multiple characterization techniques,including X-ray powder diffraction(XRD),scanning electron microscope(SEM),and electrical property testing systems,were applied to systematically examine the crystallographic structure,microstructure,as well as the dielectric,ferroelectric and piezoelectric properties of the BF-xBT ceramics.The XRD results confirm that all compositions exhibit a typical perovskite structure,transitioning from a single rhombohedral phase to a rhombohedral-cubic phase mixture as the BT content increases.SEM shows apparent core-shell microstructures in the ceramics.Notably,the results demonstrated that the BF-0.30BT ceramic exhibits the maximum piezoelectric constant(d_(33))-217 pC/N,while the BF-0.34BT ceramic displays the maximum converse piezoelectric constant(d_(33)^(*))-323 pm/V,which highlights the suitability of BF-BT ceramics for high-performance piezoelectric applications.