Electromechanical Properties of Pb(Yb1/2Nb1/2)O3-PbZrO3-PbTiO3 Ceramics

The electromechanical and electric-field-induced strain properties of x Pb(Yb_1/2Nb_1/2)O₃· y PbZrO₃·(1− x − y )PbTiO₃ ( x = 0.12, 0.25, 0.37; y = 0.10–0.40) ceramics have been studied systematically as a function of Pb(Yb_1/2Nb_1/2)O₃ (PYN) content and PbZrO₃/PbTiO₃ (PZ/PT) ratio. In addition, the effect of MnO₂ on the electromechanical properties of 0.12Pb(Yb_1/2Nb_1/2)O₃·0.40PbZrO₃·0.48PbTiO₃ was also investigated. The maximum transverse strain values of 1.6 × 10⁻³ for x = 0.12, 1.45 × 10⁻³ for x = 0.25, and 1.36 × 10⁻³ for x = 0.37 were obtained at the compositions which were regarded as the morphotropic phase boundary (MPB). The transverse strain was maximized at the MPB composition. The value of the maximum electromechanical coupling coefficient was 0.69 for y = 0.40 and x = 0.12 composition. In the 0.12Pb(Yb_1/2Nb_1/2)O₃·0.40PbZrO₃·0.48PbTiO₃ composition, the temperature of the maximum dielectric constant decreased and the grain size increased with an addition of MnO₂. The electromechanical coupling coefficient decreased while the mechanical quality factor rapidly increased with an addition of MnO₂. These resulted mainly from the acceptor effect of manganese ions that were produced by doping MnO₂ into the perovskite structure.     

Effect of MnO2 Addition on the Structure and Electrical Properties of Pb(Zn1/3Nb2/3)0.20(Zr0.50Ti0.50)0.80O3 Ceramics

Pb(Zn_1/3Nb_2/3)_0.20(Zr_0.50Ti_0.50)_0.80O₃ ceramics of pure perovskite structure were prepared by the two-stage method with the addition of 0–3.0 wt% MnO₂ and their piezoelectric properties were investigated systematically. The MnO₂ addition influences in a pronounced way both the crystal structure and the microstructure of the materials. The materials are transformed from the tetragonal to the rhombohedral structure, and the grain size is enhanced when manganese cations are added. The distortion of crystal structure for samples with MnO₂ addition can be explained by the Jahn–Teller effect. The values of electromechanical coupling factor ( k _p) and dielectric loss (tan δ) are optimized for 0.5-wt%-MnO₂-doped samples ( k _p= 0.60, tan δ= 0.2%) and the mechanical quality factor ( Q _m) is maximized for 1.0-wt%-MnO₂-doped samples ( Q _m= 1041), which suggests that oxygen vacancies formed by substituting Mn³⁺ and Mn²⁺ ions for B-site ions (e.g., Ti⁴⁺ and Zr⁴⁺ ions) in the perovskite structure partially inhibited polarization reversal in the ferroelectrics. The ceramics with 0.50–1.0 wt% MnO₂ addition show great promise as practical materials for piezoelectric applications.     

Perovskite Formation and Dielectric Characteristics of Pb(Zn1/3Ta2/3)O3 with PbTiO3 Substitution

Perovskite developments in the Pb(Zn_1/3Ta_2/3)O₃–PbTiO₃ system were explored. Formation yields and lattice parameters of the perovskite were determined from X-ray diffractometry results. Weak-field low-frequency dielectric properties of the system ceramics were investigated, followed by microstructure examination. Perovskite started to develop in Pb(Zn_1/3Ta_2/3)O₃ after the introduction of 30 mol% PbTiO₃, whereas complete stabilization was accomplished at 60% substitution. Dielectric relaxation behavior was not substantial across the entire composition range, whereas phase transition modes changed from diffuse to sharp with increased PbTiO₃ fraction.     

Phase Transition and Ferroelectric Characteristics of Pb[(Mg1/3Nb2/3)1-xTix]O3 Ceramics Modified with La(Mg2/3Nb1/3)O3

The effects of 0–5 mol% addition of La(Mg_2/3Nb_1/3)O₃ (LMN) on the phase transition and ferroelectric behaviors of Pb[(Mg_1/3Nb_2/3)_1-xTi_x]O₃ (PMNT) ceramics with compositions near the morphotropic phase boundary (MPB) were studied. An evolution of structure from rhombohedral to tetragonal was found with increasing PbTiO₃ (PT) content across the MPB (at ∼32.5 mol% PT), and a coexistence of both rhombohedral and tetragonal phases was also found at the MPB. The dual-phase field extended toward the lower PT content side of the MPB, and, moreover, the rhombohedrality or tetragonality was reduced, especially for the compositions near the MPB, by the addition of La in PMNT. The ferroelectric transition was found to change from normal to diffuse as the La content increased and the compositions became more rhombohedral. In accordance with the structural evolution, the change of remanent polarization ( P _r) and coercive field ( E _c) also became gradually indistinct, and both P _r and E _c were reduced. For compositions near the MPB, both PMNT and La-modified PMNT had a similar electromechanical factor ( k _p) in a range around 0.55–0.60, but the mechanical quality factor ( Q _m) was significantly reduced for the La-modified PMNT. The piezoelectric coefficient ( d ₃₃), however, was largely improved with increasing La content in PMNT of compositions at MPB. A high value of d ₃₃∼ 815 pC/N was obtained for the 5-mol%-La-modified ceramics, but it was associated with a low value of Q _m.     

Phase Transition of Rare-Earth Aluminates (RE4Al2O9) and Rare-Earth Gallates (RE4Ga2O9)

Lattice parameters of RE₄Al₂O₉ (RE = Y, Sin, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) prepared at 1600–1800°C and those of RE₄Ga₂O₉ (RE = La, Pr, Nd, Sm, Eu, and Gd) prepared at 1400–1600°C were refined by Rietveld analysis for the X-ray powder diffraction patterns. The parameters increased linearly with the ionic radius of the trivalent rare-earth elements ( r _RE). High-temperature differential calorimetry and dilatometry revealed that both RE₄Al₂O, and RE₄Ga₂O, have reversible phase transitions with volume shrinkages of 0.5–0.7% on heating and thermal hystereses. The transition temperatures (7_tr) decreased from 1300°C (Yb) to 1044°C (Sm) for RE₄A1₂O₉, except for Y₄Al₂O₉ ( T_tr = 1377°C), and from 1417°C (Gd) to 1271°C (La) for RE₄Ga₂O, with increasing ionic radius of the rare-earth elements. These transition temperatures were plotted on a curve against the ionic radius ratio of Al³⁺ or Gd³⁺ and RE³⁺ ( r _A1Ga/r_RE) except for Y₄Al₂O₉.     

Elastic, Piezoelectric, and Dielectric Properties of 0.71Pb(Mg1/3Nb2/3)O3–0.29PbTiO3 Crystals Obtained by Solid-State Crystal Growth

Single crystals with the composition of 0.71Pb(Mg_1/3Nb_2/3)O₃–0.29PbTiO₃ (PMNT29) were grown using the solid-state crystal growth (SSCG) method. Compared with the conventional Bridgman grown crystals, the SSCG PMNT29 crystals were found to possess comparable piezoelectric, dielectric, and electromechanical properties ( d ₃₃∼1500 pC/N, ɛ^T ₃₃/ ɛ ₀∼5400 and k ₃₃∼90%), with the same Curie temperature ( T _C) and ferroelectric phase transformation temperature ( T _{R – T} ). The full set of elastic, piezoelectric, and dielectric material constants were determined by the resonance method for the SSCG grown PMNT29 crystals according to IEEE standards and compared with Bridgman grown crystals. The dielectric temperature behavior and the strain field characteristics were investigated, demonstrating excellent performance over the temperature range from room temperature to 95°C. Strain behavior at high electric fields was found to be different from Bridgman growth crystals and believed to be attributed to different domain structures.     

Structural and Electrical Properties of Er2O3-Doped Na1/2Bi1/2TiO3 Lead-Free Piezoceramics

Sodium bismuth titanates Na_1/2Bi_1/2TiO₃ (NBT) doped with 0–3 wt% Er₂O₃ were prepared by the conventional solid-state reaction method. The X-ray diffraction results revealed that the sintered Er-doped NBT ceramics exhibited a pure perovskite structure with Er³⁺ concentrations ranging from 0 to 1 wt%. At a low Er₂O₃ concentration, the Er-doped NBT ceramics showed enhanced electrical properties with dielectric constant ɛ₃₃^T/ɛ₀=636, a low dielectric dissipation factor (tan δ=3.3%), a low coercive field ( E _c=4.56 kV/mm), and a high piezoelectric constant ( d ₃₃=75 pC/N). The relationship between the composition and properties of Er-doped NBT ceramics has been discussed.     

Processing and Electrical Properties in Lead-Based (Pb(Mg1/3Nb2/3)O3, Pb(Yb1/2Nb1/2)O3, PbTiO3) Systems

Lead-based ferroelectric (FE) ceramics exhibit superior electromechanical properties; therefore, there has been an increased focus on developing new lead-based FE materials with high Curie temperature ( T _c) and enhanced properties. The aim of this study was to investigate new compositions in the Pb(Mg_1/3Nb_2/3)O₃–Pb(Yb_1/2Nb_1/2)O₃–PbTiO₃ ( PMN–PYbN–PT) system to enhance the electromechanical properties while increasing the T _c and lowering the sintering temperature. The 0.575[0.5PMN–0.5PYbN]–0.425PT composition at PMN/PYbN (50/50) mole ratio were prepared by reactive sintering PMNT and PYbNT powder mixtures at 950°–1200°C for 4 h. PMNT and PYbNT powders were calcined via the columbite method. Samples were prepared by cold isostatic pressing at 80 MPa. Dense and fully perovskite 0.575[0.5PMN–0.5PYbN]–0.425PT ceramics were fabricated at 975°C for 4 h, and these samples displayed a remnant polarization ( P _r) of 32 μ C/cm², coercive field ( E _c) of 17 kV/cm, and a piezoelectric charge coefficient ( d ₃₃) of 475 pC/N. It is proposed that this ternary system can be tailored for various applications.     

Phase Transitional Behavior and Piezoelectric Properties of Lead-Free (Na0.5K0.5)NbO3–(Bi0.5K0.5)TiO3 Ceramics

(1− x )(Na_0.5K_0.5)NbO₃–(Bi_0.5K_0.5)TiO₃ solid solution ceramics were successfully fabricated, exhibiting a continuous phase transition with changing x at room temperature from orthorhombic, to tetragonal, to cubic, and finally to tetragonal symmetries. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal ferroelectric phases was found at 2–3 mol% (Bi_0.5K_0.5)TiO₃ (BKT), which brings about enhanced piezoelectric and electromechanical properties of piezoelectric constant d ₃₃=192 pC/N and planar electromechanical coupling coefficient k _p=45%. The MPB composition has a Curie temperature of 370°–380°C, comparable with that of the widely used PZT materials. These results demonstrate that this system is a promising lead-free piezoelectric candidate material.     

Microwave Dielectric Properties of CaTiO3-CaAl1/2Nb1/2O3 Ceramics Doped with Li3NbO4

The microwave dielectric properties of CaTi_{1− x} (Al_1/2Nb_1/2) _x O₃ solid solutions (0.3 ≤ x ≤ 0.7) have been investigated. The sintered samples had perovskite structures similar to CaTiO₃. The substitution of Ti⁴⁺ by Al³⁺/Nb⁵⁺ improved the quality factor Q of the sintered specimens. A small addition of Li₃NbO₄ (about 1 wt%) was found to be very effective for lowering sintering temperature of ceramics from 1450–1500° to 1300°C. The composition with x = 0.5 sintered at 1300°C for 5 h revealed excellent dielectric properties, namely, a dielectric constant (ɛ_r) of 48, a Q × f value of 32 100 GHz, and a temperature coefficient of the resonant frequency (τ_f) of −2 ppm/K. Li₃NbO₄ as a sintering additive had no harmful influence on τ_f of ceramics.     

Low-Temperature Sintering of K0.5Na0.5NbO3 Ceramics

The objective of this work was to lower the sintering temperature of K_0.5Na_0.5NbO₃ (KNN) without reducing its piezoelectric properties. The KNN was sintered using 0.5, 1, 2, and 4 mass% of (K, Na)-germanate. The influence of the novel sintering aid, based on alkaline germanate with a melting point near 700°C, on the sintering, density, and piezoelectric properties of KNN is presented. The alkaline-germanate-modified KNN ceramics reach up to 96% of theoretical density at sintering temperatures as low as 1000°C, which is approximately 100°C less than the sintering temperature of pure KNN. The relative dielectric permittivity (ɛ/ɛ₀) and losses (tanδ), measured at 10 kHz, the piezo d ₃₃ coefficient, the electromechanical coupling and mechanical quality factors ( k _p, k _t, Q _m) of KNN modified with 1 mass% of alkaline germanate are 397, 0.02, 120 pC/N, 0.40, 0.44, and 77, respectively. These values are comparable to the best values obtained for KNN ceramics sintered above 1100°C.     

Piezoelectric Properties and Phase Relations of Pb(Mg1/3Nb2/3)O3-PbTiO3−PbZrO3 Ceramics with Barium or Strontium Substitutions

When a small amount of Ba or Sr is substituted for Pb in Pb(Mg_1/3 Nb_2/3)O₃-PbTiO₃-Pb2rO 3 , the morphotropic boundary and the compositions which show the highest planar coupling coefficient and dielectric constant shift slightly toward the decreasing PbTiO₃ content. The tetragonality of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ and Pb(Mg_1/2 Nb_2/3)-O₃-PbTiO₃-PbZrO₃ ceramics decreased with increasing Ba or Sr content. The lattice parameter (α axis) in the rhombohedral or pseudocubic phase increased with the increase of Ba but decreased with the increase of Sr substitution. Although the Curie temperature was lowered with the increase of Ba or Sr, the dielectric constants of the ceramics were increased. The dielectric and piezoelectric properties of the ternary compositions near the morphotropic boundary were improved through selection of sub-stituent and base composition. A planar coupling coefficient of 0.66 and a low Young's modulus were obtained with substitution of 5 mole % Ba. A dielectric constant greater than 3500 and a planar coupling of 0.63 can be obtained by substituting 5 mole % Sr.     

Crystal Growth and Electrical Properties of Pb((Zn1/3Nb2/3)0.91Ti0.09)O3 Single Crystals Produced by Solution Bridgman Method

Piezoelectric Pb((Zn_1/3Nb_2/3)_0.91Ti_0.09)O₃ (PZNT 91/9) single crystals 40 mm in diameter were successfully grown from solution by the Bridgman method with a PbO flux. The crystals were grown in a platinum crucible heated to 1130°C. Growth rate was 0.35 mm/h. The obtained crystals were ~40 mm in diameter 20 mm in length and were a rust-brown color. The Curie temperature, T _C, ranged from 175° to 185°C, and the dielectric constant before poling at room temperature was 2000-8900 within a wafer. After electrical poling, specimens had electromechanical coupling coefficients in rectangular bar mode, k _33´, of 79%-88%, which were larger than for PZT ceramics ( k _33´ < 70%). These PZNT 91/9 single crystals grown by the Bridgman process satisfy the requirements for array-type transducers used in echocardiographic equipment. Results confirm that the Bridgman method is useful for mass-producing large crystals of PZNT 91/9.     

Effect of Lead Content on the Structure and Electrical Properties of Pb((Zn1/3Nb2/3)0.5(Zr0.47Ti0.53)0.5)O3 Ceramics

The effects of lead content on the structure and electrical properties of Pb((Zn_1/3Nb_2/3)_0.5(Zr_0.47Ti_0.53)_0.5)O₃ ceramics were investigated. Specimens with various lead concentrations were prepared by the conventional oxide-mixing method. When the lead concentration was slightly less than the stoichiometric amount, a large amount of pyrochlore phase was formed along with the perovskite phase. On the other hand, excessive amounts of lead led to the formation of PbO on the surface of the specimen. These second phases were seriously detrimental to electromechanical properties. The highest piezoelectric properties were observed when an excess of 1 mol% lead was added. By optimizing the specimen composition, excellent piezoelectric and dielectric properties ( k _p= 0.7, d ₃₃= 490 pC/N, and ɛ_m= 15000) were obtained.     

Effect of Rare-Earth Additives on Electromechanical Properties of Modified Lead Titanate Ceramics

The influence of rare-earth additives, such as La, Nd, Sm, and Gd, and poling conditions on the electromechanical properties of (Pb_{1−3 x /2}Ln _x )(Ti_0.98Mn_0.02)O₃ compositions, x = 0.04–0.12, were investigated. The type and amount of additive were found to affect the lattice anisotropy, dielectric constant, and electromechanical properties. A large electromechanical anisotropy ( k _t / k _p ) could be obtained in 10 mol% Sm-modified and 8 mol% Gd-modified lead titanate ceramics, and seemed to correlate to a low Poisson's ratio.     

Phase Structure and Electrical Properties of (K0.48Na0.52)(Nb0.95Ta0.05)O3-LiSbO3 Lead-Free Piezoelectric Ceramics

(1− x )(K_0.48Na_0.52)(Nb_0.95Ta_0.05)O₃– x LiSbO₃ [(1− x )KNNT− x LS] lead-free piezoelectric ceramics were prepared by the conventional solid-state sintering method. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal phases was identified in the composition range of 0.03< x <0.05. The ceramics near the MPB exhibit a strong compositional dependence and enhanced electrical properties. The (1− x )KNNT– x LS ( x =0.04) ceramics exhibit good electrical properties ( d ₃₃=250 pC/N, k _p=45.1%, k _t =46.3%, T _c=348°C, T _{o − t} =74°C, P _r=25.9 μC/cm², E _c=10.7 kV/cm, ɛ_r∼1352, tan δ∼3%). These results show that (1− x )KNNT– x LS ceramic is a promising lead-free piezoelectric material.     

Effect of Alkali Elements Content on the Structure and Electrical Properties of (K0.48Na0.48Li0.04)(Nb0.90Ta0.04Sb0.06)O3 Lead-Free Piezoelectric Ceramics

In this communication, we report on the effects of alkaline elements (K, Na, and Li) content on the electrical properties of (K_0.48Na_0.48Li_0.04)(Nb_0.90Sb_0.06Ta_0.04)O₃ ceramics. Because of the high volatility, the alkaline elements evaporate during the high-temperature sintering process, generating vacancy defects in the ceramics. As a result, the ferroelectric and piezoelectric properties of the ceramics become weakened. Our results reveal that the volatilization loss can be effectively compensated by a 1 mol% excess of alkaline elements. The remanent polarization P _r, piezoelectric coefficient d ₃₃, and electromechanical coupling coefficient k _p of the compensated ceramics are improved by 15%, 16%, and 20%, respectively, having a value of 30 μC/cm², 310 pC/N, and 0.6.     

Single-Calcination Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 and Pb(Mg1/3Nb2/3)O3 Ceramics Using a Coating Method

A coating approach for synthesizing 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃ (0.9PMN–0.1PT) and PMN using a single calcination step was demonstrated. The pyrochlore phase was prevented by coating Mg(OH)₂ on Nb₂O₅ particles. Coating of Mg(OH)₂ on Nb₂O₅ was done by precipitating Mg(OH)₂ in an aqueous Nb₂O₅ suspension at pH 10. The coating was confirmed using optical micrographs and zeta-potential measurements. A single calcination treatment of the Mg(OH)₂-coated Nb₂O₅ particles mixed with appropriate amounts of PbO and PbTiO₃ powders at 900°C for 2 h produced pyrochlore-free perovskite 0.9PMN–0.1PT and PMN powders. The elimination of the pyrochlore phase was attributed to the separation of PbO and Nb₂O₅ by the Mg(OH)₂ coating. The Mg(OH)₂ coating on the Nb₂O₅ improved the mixing of Mg(OH)₂ and Nb₂O₅ and decreased the temperature for complete columbite conversion to ∼850°C. The pyrochlore-free perovskite 0.9PMN–0.1PT powders were sintered to 97% density at 1150°C. The sintered 0.9PMN–0.1PT ceramics exhibited a dielectric constant maximum of ∼24 660 at 45°C at a frequency of 1 kHz.     

Microwave Dielectric Properties of La5CrTi3O15 and La4MCrTi3O15 (M=Pr, Nd, and Sm) Ceramics

The La₅CrTi₃O₁₅ and La₄MCrTi₃O₁₅ (M=Pr, Nd, and Sm) microwave dielectric ceramics were prepared by the conventional solid-state ceramic route. The structure and microstructure of the ceramics were studied by X-ray diffraction and scanning electron microscopy methods. The dielectric properties of the ceramics were measured in the microwave frequency region using a network analyzer by the resonance method. The ceramics show a dielectric constant (ɛ_r) in the range of 37 to 39.5, a quality factor ( Q _u× f _o) 17,300 to 34,000 GHz, and a temperature coefficient of resonant frequency (τ_f) in the range from −22 to −38 ppm/°C.     

Phase Relations in the Garnet Region of the System Y2O3–Fe2O3–FeO–Al2O3

Liquidus equilibrium relations for the air isobaric section of the system Y₂O₃–Fe₂O₃–FeO–Al₂O₃ are presented. A Complete solid-solution series is found between yttrium iron garnet and yttrium aluminum garnet as well as extensive solid solutions in the spinel, hematite, orthoferrite, and corundum phases. Minimum melting temperatures are raised progressively with the addition of alumina from 1469°C in the system Y–Fe–O to a quaternary isobaric peritectic at 1547°C and composition Y _0.22 Fe _1.08 Al _0.70 O _2.83* Liquidus temperatures increase rapidly with alumina substitutions beyond this point. The thermal stability of the garnet phase is increased with alumina substitution to the extent that above composition Y _0.75 Fe _0.65 Al _0.60 O ₃ garnet melts directly to oxide liquid without the intrusion of the orthoferrite phase. Garnet solid solutions between Y _0.75 Fe _1.25 O ₃ and Y _0.75 Fe _0.32- Al _0.93 O ₃ can be crystallized from oxide liquids at minimum temperatures ranging from 1469° to 1547°C, respectively. During equilibrium crystallization of the garnet phase, large changes in composition occur through reaction with the liquid. Unless care is taken to minimize temperature fluctuations and unless growth proceeds very slowly, the crystals may show extensive compositional variation from core to exterior.