Orthorhombic-tetragonal Phase Transition Temperature Research Articles

Phase transition, dielectric and piezoelectric properties of K0.5Na0.5NbO3–CaTi0.9Zr0.1O3 lead-free ceramics

Lead-free (1 − x)K0.5Na0.5NbO3–xCaTi0.9Zr0.1O3 + 0.75 mol%MnO2 piezoelectric ceramics have been prepared by an ordinary sintering technique and their phase transition, dielectric and piezoelectric properties have been studied. The results of X-ray diffraction show that CaTi0.9Zr0.1O3 diffuse into K0.5Na0.5NbO3 lattices to form a solid solution with a perovskite structure. After the addition of CaTi0.9Zr0.1O3, both the cubic–tetragonal and tetragonal–orthorhombic phase transition temperatures decrease, and a relaxor behavior is induced. Coexistence of the orthorhombic and tetragonal phases is formed in the ceramics with 0.03 < x < 0.07 at room temperature. Owing to the higher number of possible polarization states resulting from the coexistence of the two phases, the piezoelectric properties of the ceramics are enhanced significantly. The ceramic with x = 0.05 exhibits the following optimum properties: d33 = 203 pC/N, kp = 45.0%, and TC = 342 °C.

Influence of CuO Additive on Physical Properties of BaTiO&lt;sub&gt;3&lt;/sub&gt; Ceramics

Various ceramics were prepared by the conventional solid-state reaction technique according to the compositional formula of (1-x) BaTiO3+xCuO withx= 0.00 ~ 0.05. The influence of CuO additive on microstructure, crystalline structure and dielectric and piezoelectric properties were investigated. It has been found that CuO acts like a sintering aid in the CuO-modified BaTiO3ceramics to promote the ceramic densification and suppress the abnormal grain growth during sintering process, and decreases the orthorhombic-tetragonal phase transition temperature. The CuO-modified BaTiO3ceramics show desirable microstructures with high density, small average grain size and uniform grain size distribution. For the one withx= 0.01, it has the good piezoelectric properties ofd33= 300 pC/N,kp= 0.49,k33= 0.62 andQm= 425 at room temperature, and exhibits aε'peak at -5°C and ad33maximum of 353 pC/N at 0°C, respectively.

Effects of poling over the orthorhombic-tetragonal phase transition temperature in compositionally homogeneous (K,Na)NbO3-based ceramics

Li-, Ta-, and Mn-modified (K,Na)NbO3 ceramics with various compositional homogeneity have been prepared by conventional and precursor methods. The homogeneous ceramic has demonstrated a sharper peak in temperature dependent piezoelectric response. The dielectric and piezoelectric properties of the homogeneous ceramics have been characterized at the experimental subcoercive electric fields near the temperature of the orthorhombic–tetragonal phase transition with respect to poling in both phases. Poling in the tetragonal phase is shown to enhance the low-signal dielectric and piezoelectric properties in the orthorhombic phase.

Phase transition and electrical properties of LiNbO3-modified K0.49Na0.51NbO3 lead-free piezoceramics

Lead-free (1-x)K0.49Na0.51NbO3-xLiNbO3 (KNN-LN, x = 0 ~ 0.08) piezoelectric ceramics were prepared by the conventional solid-state sintering method. The effects of LiNbO3 doping amount x on the phase transition behavior and the electrical properties of KNN-LN ceramics were investigated. By increasing LiNbO3 doping amount x, the orthorhombic-tetragonal polymorphic phase transition (PPT) temperature (T o–t) of KNN-LN ceramics shifted downwards, however, the Curie temperature (T c) slightly moved upwards. The room temperature phase structure thus changed from orthorhombic to tetragonal across the compositions with 0.05 ≤ x ≤ 0.06, named as PPT region. The composition with x = 0.06 in the tetragonal side of PPT region exhibited optimized electrical properties of d 33 = 246pC/N, k p = 41.6%, e r = 679, tgδ = 0.028, and Q m = 52. In addition to its very high T c = 467 °C, this ceramic can be an excellent candidate for replacing the lead-based piezoceramics in high temperature applications.

MgTiO3 Modified (Na0.53K0.41Li0.06)Nb0.91Sb0.09O3 Piezoelectric Ceramics with Improved Thermal Stability

MgTiO3-modified (Na0.53K0.41Li0.06)Nb0.91Sb0.09O3 ceramics were synthesized using solid-state reaction method. The addition of MgTiO3 makes the structure of the ceramics changes from the coexistence of orthorhombic and tetragonal phases to a single tetragonal phase. The orthorhombic-tetragonal phase transition temperature T O-T decreases to below room temperature, while the Curie temperature increases with the addition of MgTiO3. As a result, the tetragonal phase region is expanded, and good thermal stability of electrical properties is obtained in MgTiO3 modified ceramics. The 0.5mol% MgTiO3 modified ceramics exhibit outstanding properties of d 33∼258pC/N, ∼1431, k p∼0.435, and P r∼18.1 μC/cm2, with improved thermal stability.

Effect of Li on the microstructure and electrical properties of (K 0.17Na 0.83)NbO 3 lead-free piezoceramics

Lead-free (K 0.17Na 0.83−xLi x)NbO 3 piezoceramics were synthesized by conventional solid state sintering and the effect of Li on the microstructure and electrical properties was investigated. The experimental results show that 4 mol% Li doping facilitates the grain growth and increases the density of the ceramics. The measurement of dielectric properties reveals that the orthorhombic-tetragonal ( T O-T) and tetragonal-cubic ( T C) phase transition temperatures change linearly with increasing Li content and the sample with x = 0.06 exhibits the highest T C of 429 °C and lowest T O-T of 181 °C. Because of the wide apart of T O-T from room temperature, no polymorphic phase structures were obtained and the piezoelectric strain coefficient d 33 and piezoelectric coupling constant k p varied between low values of 57, 61 pC/N and 0.18, 0.24, respectively with less than 6 mol% Li doping.

Discharge Current Measurement and Depolarization Property of Lead-Free Niobate Piezoceramics

Lead-free Na0.5K0.5NbO3 (NKN) piezoelectric ceramics was synthesized by an ordinal solid state reaction method. The ceramics was electrically poled under 3 kV/mm at 150 oC for 30 min. The discharge current-flow was measured for the poled ceramics at the heating rate of 1 oC/min from room temperature up to 500 oC. The two sharp peaks were observed at 209 and 417 oC. These temperatures were corresponded to orthorhombic-tetragonal (TO-T) and tetragonal-cubic (TC) phase transition temperatures, respectively. The sharp peaks were mainly originated from the decrease of the spontaneous polarization. Additionally, the several large broad patterns were also observed at 280, 363 and 375 oC, According to the electromechanical coupling factor kp, which is the indicative parameter of the polarization, mainly decreased at TO-T and TC. These broad peaks were not associated with the spontaneous polarization. The peaks seem to have been originated from the discharge current of the oxygen ion migration on elevating temperature inside the poled NKN ceramics.

Co 2O 3 doped (Na 0.65K 0.35)NbO 3 piezoceramics

Lead-free (Na 0.65K 0.35)NbO 3 + x wt.% Co 2O 3 (KNN– xCo) piezoceramics were synthesized by conventional ceramic processing and the effects of low Co 2O 3 concentration on the microstructure and electrical properties were investigated. The experimental results show that the orthorhombic–tetragonal phase transition temperature ( T O–T) decreases slightly whereas the tetragonal–cubic phase transition temperature ( T C) and crystal structure keep unchanged with the increasing of Co 2O 3 content. The doping of Co 2O 3 facilitates the grain growth and improves the density and piezoelectric properties of the ceramics. The sample of x = 0.2 exhibits good piezoelectric properties of piezoelectric coefficient d 33 = 127pC/N and electromechanical coupling coefficient k p = 35.1% with density of ρ = 4.31/cm 3. These results strongly suggest that the composition of (Na 0.65K 0.35)NbO 3 is another promising lead-free candidate for investigation besides (Na 0.5K 0.5)NbO 3 system.

Thermal Stability and Humidity Resistance of ScTaO4 Modified (K0.5Na0.5)NbO3 Ceramics

Lead-free (Na0.5K0.5)NbO3-xmol% ScTaO4 (x = 0–1.5) ceramics are prepared using the conventional solid-state reaction method and their properties are investigated in detail. The results indicate that the piezoelectric properties and density are improved by the introduction of ScTaO4. Due to the high orthorhombic-tetragonal phase transition temperature TO-T (around 200° C), stable piezoelectric properties against temperature are obtained. In a wide temperature range of 15–160° C, kp of the (Na0.5K0.5)NbO3–0.5 mol% ScTaO4 ceramic remains almost unchanged and d31 increases slightly from 59 pC/N to 71 pC/N. The deliquescent problem is effectively solved by the addition of ScTaO4. The piezoelectric properties of ScTaO4 modified (Na0.5K0.5)NbO3 ceramics show no obvious reduction and dielectric loss increases slightly after 120 h of immersion. From the analysis, it is suggested that the density is an important factor that improves the humidity resistance of the specimens.

Lead-free (K0.5Na0.5)0.95(LiSb)0.05Nb0.95O3-BaTiO3 piezoceramics

Lead-free (1-x)(K0.5Na0.5)0.95(LiSb)0.05Nb0.95O3-xBaTiO3 (abbreviated as (1-x)KNNLS-xBT) piezoceramics were synthesized by conventional solid state sintering and the effect of BaTiO3 on the microstructure, dielectric and piezoelectric properties was investigated. It was found that both orthorhombic-tetragonal (T O-T) and tetragonal-cubic (T C) phase transition temperatures decreased obviously with increasing BaTiO3 content. Although proper amount of BaTiO3 facilitated the sintering of (1-x)KNNLS-xBT ceramics, the addition of BaTiO3 affected the relaxor behavior slightly and it was not beneficial to improve piezoelectric strain coefficient d 33, remnant polarization P r and piezoelectric coupling constant k p.

Dielectric and piezoelectric properties of MnO2-doped K0.5Na0.5Nb0.92Sb0.08O3 lead-free ceramics

Lead-free MnO2-doped K0.5Na0.5Nb0.92Sb0.08O3 ceramics have been fabricated by a conventional ceramic technique and their dielectric and piezoelectric properties have been studied. Our results show that a small amount of MnO2 (0.5–1.0 mol%) is enough to improve the densification of the ceramics and decrease the sintering temperature of the ceramics. The co-effects of MnO2 doping and Sb-substitution lead to significant improvements in the ferroelectric and piezoelectric properties. The K0.5Na0.5Nb0.92Sb0.08O3 ceramic with 0.5 mol%MnO2 doping possesses optimum propeties: d33 = 187 pC/N, kP = 47.2%, er = 980, tanδ = 2.71% and Tc = 287 °C. Due to high tetragonal-orthorhombic phase transition temperature (TO-T ~ 150 °C), the K0.5Na0.5Nb0.92Sb0.08O3 ceramic with 0.5 mol%MnO2 doping exhibits a good thermal stability of piezoelectric properties.

Universality in phase diagram of (K,Na)NbO3–MTiO3 solid solutions

Detailed phase diagrams are necessary for improving the temperature stability of piezoelectric properties of (Na,K)NbO3-based solid solutions. We report the phase diagrams of the (1−x)Na0.5K0.5NbO3–xMTiO3 (M=Pb, Ba, Sr, Ca, and Bi0.5Li0.5) solid solutions over a broad temperature range of 10 K≤T≤∼770 K. It is found that the phase diagrams show several universal characteristics: (a) Curie temperature TC-T decreases sharply at low x range; (b) the relation between TC-T and x deviates from Vegard’s law; and (c) the tetragonal-orthorhombic phase transition temperature TT-O and the orthorhombic-rhombohedral phase transition temperature TO-R are almost independent of M. We suggest that valence mismatch is the main reason for the rapid decrease in TC-T and the deviation of TC-T from Vegard’s law. It is also found that TC-T is mainly affected by the A-site cations, whereas TT-O and TO-R are mainly affected by the B-site cations.

Properties of (Na 0.53K 0.4Li 0.07)Nb 1− x% Sb x% O 3 ceramics with high lithium and antimony content

(Na 0.53K 0.4Li 0.07)Nb 1− x% Sb x% O 3 (NKLNS x) piezoelectric ceramics with high content of lithium and antimony have been prepared by the solid-state reaction technique. The effect of Sb content on the crystallographic structure, phase transitions, piezoelectric and ferroelectric properties of NKLNS x ceramics is studied. The X-ray diffraction results suggest that the structure of the ceramics changes from pure tetragonal phase to the coexistence of orthorhombic and tetragonal phases with the increasing x, and that a little amount of second phase, LiSbO 3, is formed. The increase of Sb content induces a decrease of the Curie temperature T C and an increase of the orthorhombic–tetragonal phase transition temperature T O-T. The two compositions, NKLNS8.8 and NKLNS9.1, with T O-T closest to room temperature, possess optimum properties, with d 33 ∼ 290 pC/N, ɛ r ∼ 1340, and P r ∼ 24.2 μC/cm 2. These properties are superior to those of previously reported (K, Na, Li)(Nb, Sb)O 3 systems with low content of lithium and antimony. The thermal stability investigations show that the change of k p is minor for the ceramics that do not undergo a phase transition in the examined temperature range.

Laser sintering effects on the physical properties and structure of 0.94(K0.5Na0.5)NbO3-0.06LiTaO3 lead-free piezoelectric ceramics

Taking laser as the heat source for ceramic sintering, 0.94K0.5Na0.5NbO3-0.06LiTaO3 ceramics with unique physical properties were prepared. Fine ferroelectric properties with remnant polarization of 18 μC/cm2 and coercive field of 18.8 kV/cm were obtained. The piezoelectric properties and density of the ceramics were d33∼120 pC/N, kp∼36.6%, TC∼420 °C, and ρ∼4.125 g/cm3. The shift of the two phase transition temperatures (tetragonal-orthorhombic phase transition temperature TO-T shifting to 100 °C from 190 °C while Curie temperature Tc shifting to 420 from 394 °C) in laser-sintered ceramics reflects large structure distortion of phase transition and Li+ substitution of high temperature phase. Orientation degree of the texture structure based on anisometric grains of submicron diameter in the ceramics was 23%. The texture structure corresponding to the fine ferroelectric properties was resulted from the mass transform in liquid phase and strictly heating directivity of laser irradiation. O2 discharge through coaxial nozzle takes some actions like O2 annealing and effectively suppression on the volatilization of Na2O during laser sintering. Lager substitution of Li+ leads to the presence of small amount of K3Li2Nb5O15 phase damaging in d33. By analysis of the sintering effects on the properties and structure evolution, laser sintering was shown as a promising method in preparation of functional ceramics with excellent and applicable physical properties, compared to a traditional furnace sintering method.

Tantalum influence on physical properties of (K 0.5Na 0.5)(Nb 1− xTa x)O 3 ceramics

Lead-free (K 0.5Na 0.5)(Nb 1− x Ta x )O 3 ceramics with x = 0.00–0.30 were prepared by the solid-state reaction technique. The effects of Ta on microstructure, crystallographic structure, phase transition and piezoelectric properties have been investigated. It has been shown that the substitution of Ta decreases Curie temperature T C and orthorhombic–tetragonal phase transition temperature T O–T, while increasing the rhombohedral–orthorhombic phase transition temperature T R–O. In addition, piezoelectric activity is enhanced with the increase of Ta content. The ceramics with x = 0.30 have the high value of piezoelectric coefficient d 33 = 205 pC/N. Moreover, k p shows little temperature dependence between −75° C and 75 °C, and d 33 exhibits very good thermal stability over the range from −196 °C to 75 °C in the aging test.

Microstructure and electrical properties of (Li, Ag, Ta, Sb)-modified (K0.50Na0.50)NbO3 lead-free ceramics with good temperature stability

0.98[(K0.4725Na0.4725Li0.055)NbO3]–0.02Ag(Ta1−xSbx)O3 lead-free piezoelectric ceramics were prepared by the conventional mixed-oxide method. The effects of the Sb content on the microstructure and the electrical properties of the ceramics were systematically investigated. The experimental results show that Sb strongly affects the microstructure and the electrical properties of the ceramics and that the partial substitution of Sb slightly decreases the Curie temperature (Tc) and shifts the orthorhombic–tetragonal phase transition temperature (TO–T) to below room temperature. The thermal-depoling behaviour, age characteristics and temperature stability of the ceramics with x = 0.40 were also studied. The ceramics with x = 0.40 exhibit optimum properties (d33 = 250 pC N−1, kp = 44%, Tc = 444 °C, εr ∼ 1186, tan δ ∼ 2.7%, Pr ∼ 32.0 µC cm−2, Ec ∼ 14.8 kV cm−1), good thermal-depoling behaviour and age characteristics and improved temperature stability. These results show that the ceramic with x = 0.40 is a promising candidate material for high-temperature lead-free piezoelectric ceramics.

Mitigation of thermal and fatigue behavior in K0.5Na0.5NbO3-based lead free piezoceramics

K(0.5)Na(0.5)NbO(3)-(KNN) based lead free materials have been found to exhibit good piezoelectric properties (d(33) approximately 250 pCN) due to the orthorhombic-tetragonal polymorphic phase transition (PPT) temperature compositionally shifted downward to near room temperature. However, associated with the PPT are issues of temperature and domain instability, making them impractical for applications. In this work, CaTiO(3) (CT) was used to effectively shift the PPT downward in effort to mitigate these issues. As expected, CT modified KNN based materials exhibited nearly temperature independent properties (-50 approximately 200 degrees C) and fatigue-free behavior, together with its relatively high d(33) value of approximately 200 pCN, make the CT modified KNN based materials excellent candidates for lead free actuators and transducers.

BiFeO3-doped (Na0.5K0.5)NbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1−x)(Na0.5K0.5)NbO3-xBiFeO3 (x=0∼0.07) were synthesized by the solid-state reaction. Differential scanning calorimetry (DSC) measurements revealed that an increase in the amount of BiFeO3 dopant resulted in a decrease in the orthorhombic-tetragonal and tetragonal-cubic phase transition temperature of the material. One percent BiFeO3 additive suppressed grain growth, which not only benefits the sintering of ceramics but also enhances the piezoelectric and ferroelectric properties, where d33=145pC/N, kp=0.31, Qm=80, Pr=11.3 μC cm−2 and Ec=16.5 kV cm−1. As xBF>0.01, both piezoelectric and ferroelectric properties decreased rapidly with an increasing amount of dopant.

Piezoelectric properties and hardening behavior of K5.4Cu1.3Ta10O29-doped K0.5Na0.5NbO3 ceramics

Lead-free piezoelectric ceramics K0.5Na0.5NbO3+xmol% K5.4Cu1.3Ta10O29 have been prepared by a conventional ceramic fabrication technique. All the ceramics possess a perovskite structure with orthorhombic symmetry. Our results reveal that the addition of K5.4Cu1.3Ta10O29 is effective in improving the densification of the ceramics. Besides, after the addition of K5.4Cu1.3Ta10O29, the Curie temperature and the tetragonal-orthorhombic phase transition temperature decrease and the P-E loop becomes constricted, in particular, for the ceramic with x=0.75. Based on the symmetry-conforming principle of point defects, it is suggested that defect dipoles are formed by the acceptor dopant ions Cu2+ and O2− vacancies along the polarization direction. As a result of the low migration rate of defects, the defect dipoles remain in the original orientation during the P-E loop measurement and thus provide restoring forces to reverse the switched polarizations. Similarly, the defect dipoles do not response along with the polarization in the normal piezoelectric activities and thus provide “pinning” to the deformed polarization, making the ceramics become “hardened.” For the ceramic with x=0.75, the mechanical quality factor Qm becomes maximum at a value of 1530, while the other piezoelectric properties remain reasonably high: piezoelectric coefficient d33=90pC∕N, planar and thickness mode electromechanical coupling factors kP=41 and kt=46%.

Perovskite lithium and bismuth modified potassium-sodium niobium lead-free ceramics for high temperature applications

[ ( K ( 1 − x ) ∕ 2 Na ( 1 − x ) ∕ 2 Li x ) 1 − 3 y Bi y ] Nb O 3 (abbreviated as KNLBN) ceramics were prepared by conventional solid-state sintering. The addition of Li+ and Bi3+ makes the orthorhombic-tetragonal phase transition temperature (TO-T) decrease from 200°C for pure (K0.5Na0.5)NbO3 ceramics to −10°C for KNLBN ceramics (x=0.06, y=0.005), while maintaining high Curie temperature (TC=455°C) and piezoelectric properties (d33=185pC∕N, kp=0.43, kt=0.45, ε33T∕ε0=1020, and tanδ=0.023) at room temperature. In addition, good temperature stability of electrical properties is obtained in KNLBN ceramics (x=0.06, y=0.005) owing to the decrease of TO-T. These results indicate that KNLBN (x=0.06, y=0.005) ceramic is a promising lead-free piezoelectric candidate material for high temperature applications.