Nanostructured Ceramics of Potassium Sodium Bismuth Titanate: Hydrothermal Synthesis and Piezoelectric Response at Morphotropic Phase Boundary

Potassium Sodium Bismuth Titanate (KNBT) ceramics, with the general formula (1 - x)K0.5Bi0.5TiO3 -xNa0.5Bi0.5TiO3, have been synthesized following hydrothermal route, starting with solid solutions of pure perovskite nanoceramics of KBT and NBT in desired stoichiometric weight ratios, followed by sintering between 850°C and 1000°C for few hours. Pure KNBT nanoceramics with perovskite structure, having mean particle size around 30 nm, could be obtained. Morphology of the samples is found to depend strongly on composition. A change of composition results in a phase change, as evident from X-ray structure analysis. This phase change is a result of rhombohedral to tetragonal morphotropic phase boundary (MPB) in the sample with x around 0.80. Composition dependent occurrence of MPB leads to formation of needle like structures with micrometer length scales. These are typical of tetragonal lamellar structures, suggesting partial induction of tetragonal polar order from rhombohedral structure at MPB. Dielectric and piezoelectric properties, such as dielectric constant and loss, piezoelectric coefficients and figures of merit, exhibit threshold maxima in their values at the composition corresponding to MPB. These values reported for a lead-free piezoceramic, synthesized by a comparatively simple hydrothermal route, are highly promising, and comparable to well-known PZT.

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.

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.

Flexible highly-effective energy harvester via crystallographic and computational control of nanointerfacial morphotropic piezoelectric thin film

Controlling the properties of piezoelectric thin films is a key aspect for designing highly efficient flexible electromechanical devices. In this stud）~ the crystallographic phenomena of PbZr1-xTixO3 （PZT） thin films caused by distinguished interfacial effects are deeply investigated by overlooking views, including not only an experimental demonstration but also ab initio modeling. The polymorphic phase balance and crystallinity, as well as the crystal orientation of PZT thin films at the morphotropic phase boundary （MPB）, can be stably modulated using interfacial crystal structures. Here, interactions with MgO stabilize the PZT crystallographic system well and induce the texturing influences, while the PZT film remains quasi-stable on a conventional A1203 wafer. On the basis of this fundamental understanding, a high-output flexible energy harvester is developed using the controlled-PZT system, which shows significantly higher performance than the unmodified PZT generator. The voltage, current, and power densities are improved by 556%, 503%, and 822%, respectively, in comparison with the previous flexional single-crystalline piezoelectric device. Finally, the improved flexible generator is applied to harvest tiny vibrational energy from a real traffic system, and it is used to operate a commercial electronic unit. These results clearly indicate that atomic-scale designs can produce significant impacts on macroscopic applications.

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.

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.

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.

Effects caused by antiferroelectric nanodomains in PZT-based coarse-grained ceramics with compositions from the morphotropic boundary region

Presented results demonstrate importance of taking into account such a phenomenon as the solid solution decomposition at the boundaries separating coexisting phases in lead zirconate-titanate-based solid solutions with compositions belonging to the morphotropic boundary region of the"temperature–composition"phase diagram.It is shown that in the local decomposition of solid solutions in the vicinity of the boundaries separating the tetragonal and rhombohedral phases in lead zirconate-titanate-based solid solutions lead to the changes of the solid solution's chemical composition and to the formation of segregates.It is also shown that the proper thermoelectric treatment of samples containing these segregates can give substantially higher values of piezoelectric parameters in the lead zirconate-titanate-based compounds.

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.

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.

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.

Investigation on the Microstructure and Electrical Properties of the Compositionally Modified PZT Ceramics Prepared by Mixed-Oxide Method

The structural and electrical properties of Pb[ZrxTi0.95-x(Mo1/3In2/3)0.05]O3 piezoelectric ceramics system with the composition near the morphotropic phase boundary were investigated as a function of the Zr/Ti ratio. Studies were performed on the samples prepared by the conventional method of thermal synthesis of mixed oxides. The materials structure was investigated by X-ray diffractometry to demonstrate the co-existence of the tetragonal and rhombohedral phases. In the present system, the MPB, in which the tetragonal and rhombohedral phases coexist, is in a composition range of 0.47 ≤ x ≤ 0.50. The lattice constants of the a and c axes for the samples were calculated from the XRD patterns. Microstructure of the sintered ceramics was observed by scanning electron microscopy (SEM) of free surfaces specimens. The relative permittivity, dielectric dissipation, piezoelectric coefficient and electromechanical coupling factor reach at maximum value x = 0.49 (εr = 7300.345 (at the Curie temperature), tanδ = 0.002050, d31 = 94.965 PC/N and kp = 0.513 and a Curie temperature of 430°C). These properties are very promising for applications in ultrasonic motors.

Effects of Thermal Conditions in the Phase Formation of Undoped and Doped Pb(Zr_1-xTi_x)O₃Solid Solutions

The purpose of this research is to study the effect of thermal conditions such as temperature ramp rate and isothermal times in the phase formation of Pb(Zr1-xTix)O3 solid solutions with composition near the morphotropic phase boundary (MPB) by using the conventional ceramic method. The perovskite phase formation and morphology of undoped Pb(Zr0.52Ti0.48)O3 (abbreviated PZT) and doped new material Pb0.98Gd0.02[(Zr0.52Ti0.48)0.98(Mg1/3Nb2/3)0.01(Ni1/3Sb2/3)0.01]O3 (abbreviated PZT-PGMNNS) specimens calcined between 700°C and 900°C have been examined by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier Transform Infrared (FTIR) while the thermal evolution of the initial precursor was followed by TG-DTA. So the results of these studies have been discussed.

Enhanced Ferroelectric and Converse Piezoelectric Properties of Dense Lead-Free Na_0.4K_0.1Bi_0.5TiO₃Ceramics for Actuator Applications

In this work, lead-free Na0.4Bi0.5 TiO3-K0.1Bi0.5TiO3 (NKBT) piezoelectric ceramics were prepared by the solid-state reaction method and their structural, ferroelectric and dielectric properties were systematically studied. NKBT sintered at 1150℃ for 4 h exhibited highly dense (96% - 98% of the theoretical density) and uniform grains of size 1.1 μm. The coexistence of tetragonal (P4bm) and rhombohedral (R3c) phases due to the presence of morphotrophic phase boundary for the chosen composition has been confirmed by the Rietveld refinement studies. Enhanced ferroelectric properties such as remnant polarization (Pr) and coercive field (EC) are found to be 37 μC/cm2 and 30 kV/cm, respectively. The optimized synthesis procedure for NKBT ceramics resulted in enhanced strain (%) and converse piezoelectric coefficient (d33*) of 0.3 % and 554 pm/V which is attributed to smaller grain size and switching of non 180o domains. NKBT ceramics with such excellent piezoelectric properties can be considered as a promising candidate for actuator applications.

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.

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.