Polymorphic Phase Transition Temperature Research Articles

High Piezoelectric Performance of KNN-Based Ceramics over a Broad Temperature Range through Crystal Orientation and Multilayer Engineering.

Uninterrupted breakthroughs in the room temperature piezoelectric properties of KNN-based piezoceramics have been witnessed over the past two decades; however, poor temperature stability presents a major challenge for KNN-based piezoelectric ceramics in their effort to replace their lead-based counterparts. Herein, to enhance temperature stability in KNN-based ceramics while preserving the high piezoelectric response, multilayer composite ceramics were fabricated using textured thick films with distinct polymorphic phase transition temperatures. The results demonstrated that the composite ceramics exhibited both outstanding piezoelectric performance (d33~467 ± 16 pC/N; S~0.17% at 40 kV/cm) and excellent temperature stability with d33 and strain variations of 9.1% and 2.9%, respectively, over a broad temperature range of 25-180 °C. This superior piezoelectric temperature stability is attributed to the inter-inhibitive piezoelectric fluctuations between the component layers, the diffused phase transition, and the stable phase structure with a rising temperature, as well as the permanent contribution of crystal orientation to piezoelectric performance over the studied temperature range. This novel strategy, which addresses the piezoelectric and strain temperature sensitivity while maintaining high performance, is well-positioned to advance the commercial application of KNN-based lead-free piezoelectric ceramics.

Significantly enhanced piezoelectric temperature stability of KNN-based ceramics through multilayer textured thick films composite

The temperature instability of piezoelectric properties in KNN-based ceramics has been a major bottleneck impeding them toward practical applications. Herein, we prepared multilayer composite ceramics using two kinds of textured thick films with different polymorphic phase transition temperatures. The composite ceramics simultaneously exhibited outstanding piezoelectric properties (d33 = 400 ± 20 pC/N; S = 0.18 % at 40 kV/cm) and piezoelectric temperature stability (d33 and S variation within 5 % and 2 %, respectively over the temperature range of room temperature to 180 °C). The synergistic contribution of slightly fluctuating εr·Pr values, the robust phase and domain structures, and the complementary effect of piezoelectric fluctuation between the two composite layers led to the outstanding temperature stability of piezoelectric properties. Furthermore, the piezoelectric energy harvester produced using KNN-T1/T2 composite ceramics exhibits a large power density of 5.4 mW/cm3. These results prove great potential of KNN-T1/T2 ceramics in high temperature piezoelectric devices.

Simultaneous Enhancement of Piezoelectricity and Temperature Stability in KNN‐Based Lead‐Free Ceramics Via Layered Distribution of Dopants

AbstractThe past two decades have seen a great enhancement of piezoelectric coefficients (d33) to higher than 570 pC/N in (K, Na)NbO3 (KNN) piezoelectrics, but one notoriously unresolved issue is their severe temperature instability, obstructing them toward practical applications. The present work demonstrates a facile approach to overcome this problem by introducing a layered distribution of key dopants (Li and Sb) in a monolithic ceramic, featuring stepwise varied polymorphic phase transition (PPT) temperatures along the thickness direction. The dopant‐graded ceramic exhibits an outstanding d33 of 508 pC/N and a very large piezoelectric strain (Suni, of 0.18%). More importantly, an excellent temperature stability (d33 variation within 13% over the temperature range of 25–150 °C) is achieved, which is superior to that of most state‐of‐art KNN counterparts. These are attributed to the construction of spatially diffused PPT in combination with enhanced polarization, permittivity, and piezoresponse through interfacial effect, including the Maxwell–Wagner effect and intergranular stress by gradient doping. The results offer an alternative strategy for designing high‐performance piezoelectric materials with desirable temperature reliability.

Stress-modulated optimization of polymorphic phase transition in Li-doped (K,Na)NbO3

The effect of uniaxial compressive stress on the crystal structure of a 6 mol. % Li-doped (K,Na)NbO3 (LKNN6a) ceramic was investigated using in situ synchrotron X-ray diffraction, revealing the stress-induced relative change in monoclinic Pm and tetragonal P4mm phases. As such, stress-induced phase transformations, in addition to the lattice deformation and domain switching, are the contributing factors for the observed macroscopic mechanical behavior of LKNN6a. The in situ stress-dependent diffraction data also demonstrates a method to mechanically modulate the polymorphic phase transition temperature (TPPT) to a higher temperature, as observed by the temperature-dependent permittivity measurements under a constant bias stress. The external uniaxial compressive stress increases the stability of the lower symmetry monoclinic phase, shifting TPPT to a higher temperature by 60 °C for the maximum uniaxial compressive stress of 300 MPa in the studied composition. Importantly, the stress-induced stabilization of the room-temperature ferroelectric phase can be useful to optimize the phase transition region, as well as increase the temperature stability of lead-free KNN.

Thermoelectric and Mechanical Properties of Polycrystalline Yttrium-Containing GeTe Alloys

The introduction of yttrium (Y) into a germanium telluride (GeTe)-based solid solution is shown to make the material superplastic near polymorphic phase transition temperature TC. This effect makes it possible to increase the mechanical stability of GeTe-based alloys when they work in thermoelements in the thermal cycling mode near TC by the introduction of low Y additions (up to ≈2 at %).

Electrical properties of 0.98(K0.5Na0.5)NbO3-0.02AETiO3 piezoceramics doped with Li2CO3

Ceramics with chemical compositions of 0.98(K0.5Na0.5)NbO3-0.02AETiO3 doped with 1.0 wt.% Li2CO3 (NKN-AET-Li, AE = Mg, Ca, Sr and Ba) were prepared by a convention ceramic processing. The crystal structures, microstructure and electrical properties of all ceramics were investigated. The results show that all the samples possess a perovskite structure. It was found that the orthorhombic to tetragonal polymorphic phase transition (PPT) temperature (TO-T) shifted to near room temperature while maintaining a high Curie temperature Tc(≥400 °C) for NKN-CaT-Li and NKN-SrT-Li ceramics with high relative density (>96%). Piezoelectric values of NKN-CaT-Li and NKN-SrT-Li ceramics were found to be d33 = 197 pC/N and d33 = 243 pC/N; kp = 0.32 and kp = 0.34) near room temperature. The NKN-SrT-Li and NKN-CaT-Li ceramics showed good temperature stability of dielectric and piezoelectric properties within a temperature range of 50–200 °C.

Co-Doping Effect of BiGaO3 and (Bi,Na,K,Li)ZrO3 on Multi-Phase Structure and Piezoelectric Properties of (K,Na)NbO3 Lead-Free Ceramics

The phase boundary structure of (K,Na)NbO3 piezoelectric ceramic was modified by doping with Bi(Na,K,Li)ZrO3 and BiGaO3 through normal solid-state sintering. Rietveld refinements by X-ray diffraction revealed that the Bi(Na,K,Li)ZrO3/BiGaO3 co-doping in (K,Na)NbO3 led to a multi-phase structure at room-temperature, effectively moving the rhombohedral-orthorhombic (R-O) and orthorhombic-tetragonal (O-T) polymorphic phase transition temperatures close to the room temperature region. Increased levels of doping also generated a structural transition, i.e., triphasic R-O-T to diphasic R-T (T-rich) and finally to R-T (R-rich), contributing to shrinkage of the O phase as well as the increase of R phase fraction. A sensitive influence of the BiGaO3 doping (0.001 mole fraction level) on the structural properties such as the phase and microstructure was shown, resulting from the effect of the super-tetragonal structure of BiGaO3. The d33 property was strongly dependent on the phase and its volume fraction, in addition to the grain sizes. Eventually, enhanced and balanced properties of the piezoelectric coefficient and Curie temperature (d33 = 309 pC/N, TC = 343 °C) were obtained when the doped ceramic had a T-rich (86%) R-T structure.

(K0.5Na0.5)NbO3-Bi(Cu2/3Nb1/3)O3 Lead-free Ceramics: Phase Transition, Enhanced Dielectric and Piezoelectric Properties

(1 − x)(K0.5Na0.5)NbO3-xBi(Cu2/3Nb1/3)O3 [(1 − x)KNN-xBCN, 0 ≤ x ≤ 0.02] lead-free piezoelectric ceramics were prepared by a solid-state reaction method. The effects of BCN addition on the phase transition, microstructure and electrical properties of ceramics were studied. X-ray diffraction and Raman spectroscopy analysis confirmed that the BCN has diffused into KNN to form a solid solution. Both polymorphic phase transition, T O-T, and Curie temperature, T c, gradually decreased with increasing the BCN content. Moreover, the relative permittivity (e r) was increased greatly and the dielectric loss (tanδ) was almost decreased. Ferroelectric hysteresis loops (P–E) of samples showed that the remnant polarization (P r) was up to a maximum value with 26.52 μm/cm as x = 0.01. The piezoelectric properties of ceramics increased with increasing the x values. When x = 0.005 and 0.01, the ceramics exhibited high piezoelectric constant with 131 pC/N and 105 pC/N over a good piezoelectric stability under 350°C, respectively. These results indicate that BCN addition is an effective way to enhance the properties of KNN ceramics.

High‐Performance Small‐Amount Fe2O3‐Doped (K,Na)NbO3‐Based Lead‐Free Piezoceramics with Irregular Phase Evolution

[(K0.43Na0.57)0.94Li0.06][(Nb0.94Sb0.06)0.95Ta0.05]O3 + x mol% Fe2O3 (KNLNST + x Fe, x = 0~0.60) lead‐free piezoelectric ceramics were prepared by conventional solid‐state reaction processing. The effects of small‐amount Fe2O3 doping on the microstructure and electrical properties of the KNLNST ceramics were systematically investigated. With increasing Fe3+ content, the orthorhombic‐tetragonal polymorphic phase transition temperature (TO‐T) of KNLNST + x Fe ceramics presented an obvious “V” type variation trend, and TO‐T was successfully shifted to near room temperature without changing TC (TC = 315°C) via doping Fe2O3 around 0.25 mol%. Electrical properties were significantly enhanced due to the coexistence of both orthorhombic and tetragonal ferroelectric phases at room temperature. The ceramics doped with 0.20 mol% Fe2O3 possessed optimal piezoelectric and dielectric properties of d33 = 306 pC/N, kp = 47.0%, = 1483 and tan δ = 0.023. It was revealed that the strong internal stress in the KNLNST + x Fe ceramics with higher Fe3+ contents (x = 0.40, 0.60) stabilized the orthorhombic phase, leading to the irregular “V” type rather than the usually observed monotonic phase transition with composition change in the ceramics.

Electromechanical properties of (Ba,Sr)(Zr,Ti)O3 ceramics

In this work, we report dielectric, piezoelectric and ferroelectric properties of perovskite Ba0.95Sr0.05ZrxTi(1−x)O3 (BSZT, x=0.05, 0.10, 0.15, 0.20) ceramics synthesized by using a solid-state reaction method. It is demonstrated that when the Zr concentration was 5at% and the sintering temperature was 1400°C, the BSZT ceramic gives the best overall electromechanical performance, including a large remnant polarization of ~ 12.1µC/cm2, large dielectric constants (εrpeak≈12,600, εrRT≈3500) and an excellent d33 value of ~355pC/N. Curie temperature of this ceramic is about ~100°C. The enhanced electromechanical properties of the BSZT ceramics as compared with Ba0.95Sr0.05TiO3 (BST) can be explained by optimization of the tolerance factor and shift of ferroelectric polymorphic phase transition temperatures (TPPT).

Structure, dielectric and piezoelectric properties of (1-x)K0.48Na0.48Li0.04NbO3-xBa0.85Ca0.15Zr0.1Ti0.9O3 ceramics

Lead free piezoelectric ceramics (1-x)KNLN-xBCZT (0 ≤ x ≤ 0.015) are prepared using ultrasonic irradiation assisted solid state reaction process. The coexistence of orthorhombic and tetragonal phases is identified in the range of 0.008 ≤ x ≤ 0.013. The polymorphic orthorhombic-tetragonal phase transition temperature shifts towards room temperature with the increase of the BCZT concentration. Improved polarization and enhanced piezoelectric properties are obtained in 0.99KNLN-0.01BCZT ceramic at room temperature, which is attributed to the shift of their orthorhombic-tetragonal phase transition towards room temperature as well as the high density and high Pr value. These results indicate that 0.99KNLN-0.01BCZT ceramic is a promising lead-free piezoelectric material.

Roles of Li and Ta in Pb-free piezoelectric (Na,K)NbO3 ceramics

Piezoelectric coefficient (d33) of (Na,K)NbO3 (NKN) is enhanced not only at its morphotropic phase boundary (MPB) composition but also enhanced at its polymorphic phase transition (PPT) temperature between orthorhombic and tetragonal phases (TO-T). Thus, for NKN-based ceramics, even higher d33 could be obtained if both MPB and PPT are simultaneously optimized. This temperature as well as composition dependence of piezoelectric properties of NKN-based ceramics requires a systematic approach that differentiates factors for MPB and PPT. In this paper, the roles of Li and Ta known to affect d33 and TO-T were identified in relation with lattice parameters.

Chemical Expansion Due to Hydration of Proton‐Conducting Perovskite Oxide Ceramics

The crystal structures of proton‐conducting BaZr1−xYxO3−x/2 (BZY05–BZY20) and BaCe0.8Y0.2O2.9 (BCY20) during hydration/dehydration has been studied by in situ high‐temperature X‐ray diffraction and thermal analysis. A contraction/expansion of the crystal lattice associated with dehydration/hydration was observed for all materials at elevated temperatures and the polymorphic phase transition temperatures of BaCe0.8Y0.2O2.9 were depressed by lowering the vapor pressure of water. A thermodynamic formalism is introduced to describe the chemical expansion associated with the hydration of oxygen vacancies in acceptor‐doped oxides. A conventional point defect model was applied to describe the lattice strain associated with the hydration. The chemical expansion is discussed with respect to the available volumetric data on the hydration of proton‐conducting oxide materials and its likely impact on ceramic fuel cells/hydrogen separation membranes utilizing a proton‐conducting electrolyte.

Effect of Na excess on the dielectric and piezoelectric properties of (Na0.53 K0.47 )(Nb0.55 Ta0.45 )O3 ceramics

Na-excess (Na0.53+xK0.47)(Nb0.55Ta0.45)O3 ceramics (x = 0.0, 0.005, 0.01, 0.015, 0.02, and 0.03) were prepared through the solid-state reaction method. The effects of Na excess and Ta substitution on dielectric behaviors and piezoelectric properties were investigated. Crystallized single phases were confirmed using X-ray diffraction. In particular, the orthorhombic–tetragonal polymorphic phase-transition temperature (TO–T) was observed near room temperature, and the piezoelectric properties were enhanced, even for small Na excesses and Ta substitution. The optimized piezoelectric coefficient d33 and planar electromechanical coupling coefficient kp were estimated to be 320–340 pC/N and 0.36, respectively.

Dielectric, Piezoelectric, Pyroelectric Properties and Polymorphic Phase Transition of 0.95 (K<sub>X</sub>Na<sub>1-X</sub>) NbO<sub>3</sub>-0.05LiSbO<sub>3</sub> Lead-Free Ferroelectric Ceramics

0.95 (KXNa1-X) NbO3-0.05 LiSbO3(KNN-LS-X) (X=0.4-0.5) lead-free ceramics were prepared by conventional solid method. The effect of K/Na ratio on the dielectric, piezoelectric, pyroelectric and polymorphic phase transition was studied. The results show that the electrical properties strongly depend on the K/Na ratio. The KNN-LS-X(X=0.45) ceramics exhibit enhanced properties (εr=891.267,d33=222pC/N,Kp=0.43517,Qm=64.72,p=15×10-4C/m2K). Enhanced electrical properties of the KNN-LS-X (X=0.45) ceramics could be attributed to the effect of K/Na ratio modifying the polymorphic orthorhombic-tetragonal phase transition temperature to room temperature, which could make KNN-LS-X (X=0.45) ceramics use as a new ferroelectric sensor.

Dielectric and piezoelectric properties of SrZrO3-modified (K0.45Na0.51Li0.04)(Nb0.90Ta0.04Sb0.06)O3 lead-free piezoceramics

Lead-free (1–x)(K0.45Na0.51Li0.04)(Nb0.90Ta0.04Sb0.06)O3−xSrZrO3 ceramics were fabricated by a conventional ceramic technique, and the effects of SrZrO3 content on crystal phases and electrical properties of the ceramics were investigated. No secondary phase was identified by XRD analysis, indicating that SrZrO3 has completely diffused into the lattice and formed the solid solutions. The Curie temperature TC kept almost unchanged while the polymorphic phase transition temperature TO–T shifted slightly toward lower temperature with the increasing SrZrO3 content. Anomalous broad dielectric relaxation was observed in the εr–T curves near TC when x≥0.006. The properties of the ceramics were improved by adding a small amount of SrZrO3. The optimum electrical performances of d33=210pC/N, kp=0.318, kt=0.310 and ε33T/ε0=677 were obtained at x=0.003.

Structure and electrical properties of [(K0.49Na0.51)1−xLix](Nb0.90Ta0.04Sb0.06)O3 lead-free piezoceramics

Lead-free [(K0.49Na0.51)1−xLix](Nb0.90Ta0.04Sb0.06)O3 (KNLNTS) ceramics were prepared by a conventional ceramic processing method, and the effects of Li content on the crystal phases and electrical properties of the ceramics were investigated. A secondary phase was identified by XRD analysis when x=0.08, indicating that the maximum solid solubility of lithium in KNLNTS solid solutions is lower than 8 mol%. The Curie temperature TC increased while the polymorphic phase transition (PPT) temperature TO–T decreased with the increasing Li content. The coexistence of orthorhombic and tetragonal phases was observed near room temperature when x=0.04. The optimum electrical performance, i.e. piezoelectric coefficient d33=215 pC/N, electromechanical coupling coefficient kp=0.313, kt=0.306, relative permittivity ε33T/ε0=805 and loss tangent tan δ=0.045, was obtained for x=0.04.

Effects of SrTiO3 on dielectric and piezoelectric properties of K0.48Na0.48Li0.04Nb0.96Ta0.04O3-based piezoceramics

► Sodium potassium niobate based piezoceramics modified with SrTiO 3 (ST) were prepared. ► Crystal structure, microstructure and dielectric properties of ceramics were investigated. ► Addition of ST more than 3 mol% changed ferroelectric behavior from normal to relaxor. ► Coexistence of two structures in ceramic with 1 mol% ST enhanced piezoelectric constant. In this study, (100 − x ) K 0.48 Na 0.48 Li 0.04 Nb 0.96 Ta 0.04 O 3 − x SrTiO 3 (0 ≤ x ≤ 10) ceramics were fabricated via normal sintering of synthesized powder by using solid state reaction. All ceramics revealed pure perovskite structure, indicating formation of solid solution between KNNLT and ST up to 10%. With increasing x , the crystal structure of ceramics changed from orthorhombic to tetragonal and finally pseudocubic symmetry when x = 10. Ceramic containing 1% ST had orthorhombic and tetragonal symmetries, simultaneously. Investigation of the variation of dielectric constant of ceramics versus temperature revealed that for ceramic with x = 1, polymorphic phase transition (PPT) temperature between orthorhombic and tetragonal is less than room temperature. Thus coexistence of two different structures in this ceramic is due to vicinity of its composition to morphotropic phase boundary (MPB). As a result, the maximum piezoelectric constant was measured for this ceramic. Ceramics containing 5 and 7.5% ST tend to appear relaxor ferroelectric behavior which is because of chemical inhomogeneities in both A- and B-sites of the ABO 3 perovskite structure.

Effects of NaSbO3 on phase structure and electrical properties of K0.5Na0.5NbO3–LiTaO3–NaSbO3 piezoelectric ceramics

The (0.96–x)K0.5Na0.5NbO3–0.04LiTaO3–xNaSbO3 (abbreviated as KNN–LT–xNS, x = 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08) lead-free piezoelectric ceramics were fabricated by conventional ceramic technique. The crystal structure, dielectric, ferroelectric and piezoelectric properties of the ceramics were investigated. The Curie temperature (TC) and the polymorphic phase transition temperature (TO−T) of the ceramics decreased gradually with the increase of NaSbO3. In addition, a coexistence of orthorhombic and tetragonal phases in the ceramics was identified in the composition range of 0.05 ≤ x ≤ 0.08. The ceramic with a composition of x = 0.06, which was close to the orthorhombic side of the polymorphic phase transition (PPT) region, exhibited excellent electrical properties with piezoelectric coefficient d33 = 233 pC/N, planar electromechanical coupling coefficient kp = 0.328, remnant polarization Pr = 14.7 μC/cm2, coercive field Ec = 11.7 kV/cm, relative permittivity \( \varepsilon_{33}^{\text{T}} /\varepsilon_{0} \) = 1,033, and loss tangent tan δ = 0.063. The ceramics had relatively low Qm value in the range of 10–37.

Phase transition behavior and temperature-stable piezoelectric properties of new quaternary (K, Na)NbO3 based ceramics

(K, Na)NbO3-based lead free materials have been found to exhibit good piezoelectric properties due to the orthorhombic–tetragonal polymorphic phase transition (PPT) temperature compositionally shifted downward to near room temperature. However, this transition correspondingly results in a strong temperature dependence of the dielectric and piezoelectric properties. In this work, new quaternary (1−x) (K0.4425Na0.52Li0.0375)(Nb0.8925Sb0.07Ta0.0375)O3 (KNLNST)–xSrTiO3 (ST) lead-free piezoelectric ceramics were fabricated by a conventional ceramic technique and their structure and piezoelectric properties were also studied. The results of X-ray diffraction reveal that SrTiO3 diffuses into the KNLNST lattices to form a new solid solution with a perovskite structure. After the addition of SrTiO3, tetragonal–orthorhombic phase transition shifts to lower temperatures. The good piezoelectric properties of 0.995 KNLNST–0.005 ST material were found to be d33∼295pC/N, kp∼42%, and εr∼1902, with greatly improved temperature stability over the temperature range of 0–100°C, demonstrating practical potential for actuator and ultrasonic transducer applications.