Dielectric Properties of Ceramics in Ba (Co1/3 Nb2/3)O3–Ba(Zn1/3Nb2/3)O3 Solid Solution

The dielectric properties of the Ba (Co_1/3 Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ system were determined. Ba (Co_1/3 Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ has a complex perovskite structure, a high dielectric constant, a low dielectric loss, and a low temperature coefficient of the resonant frequency. A solid-solution ceramic with 0.7Ba (Co_1/3 Nb_2/3)O₃·0.3 Ba(Zn_1/3Nb_2/3)O₃ has a dielectric constant of K=33.5, Q=11000 at 6.5 GHz, and a temperature coefficient of the resonant frequency of τ_f=0 ppm/°C. The temperature coefficient of resonant frequency can be varied by changing the composition. The Q values of the ceramics can be increased by annealing in a nitrogen atmosphere. These ceramics can be used for resonant elements and stabilized oscillators.     

Perovskite Phase Formation in the Relaxor System [Pb(Fe1/2Nb1/2)O3]1-x–[Pb(Zn1/3Nb2/3)O3]x

Phase formation and dielectric properties of the compositions in the system [Pb(Fe_1/2Nb_1/2)O₃]₁_ _x –[Pb(Zn_1/3Nb_2/3)O₃] _x were investigated as possible materials for multilayer ceramic capacitors. The formation of the phase with perovskite structure and dielectric properties of ceramics at room temperature in the entire composition range are presented. The undesirable pyrochlore phase can be suppressed up to x = 0.6 by adopting calcination of B-site oxides, followed by reaction with PbO. Compositions in the single-phase range can be sintered at less than 1000°C.     

Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics: I. Substitution of Ba3W2O9

A narrow region of Zn-vacancy-containing cubic perovskites was formed in the (1− x )Ba₃(ZnNb₂)O₉−( x )Ba₃W₂O₉ system up to 2 mol% substitution ( x =0.02). The introduction of cation vacancies enhanced the stability of the 1:2 B-site ordered form of the structure, Ba(Zn_{1− x} □ _x )_1/3(Nb_{1− x} W _x )_2/3O₃, which underwent an order–disorder transition at 1410°C, ∼35° higher than pure Ba(Zn_1/3Nb_2/3)O₃. The Zn vacancies also accelerated the kinetics of the ordering reaction, and samples with x =0.006 comprised large ordered domains with a high lattice distortion ( c/a =1.226) after a 12 h anneal at 1300°C. The tungstate-containing solid solutions can be sintered to a high density at 1390°C, and the resultant ordered ceramics exhibit some of the highest microwave dielectric Q factors ( Q × f =1 18 000 at 8 GHz) reported for a niobate-based perovskite.     

Adhesive-Bonded Ca(Mg1/3Nb2/3)O3/Ba(Zn1/3Nb2/3)O3 Layered Dielectric Resonators with Tunable Temperature Coefficient of Resonant Frequency

Ca(Mg_1/3Nb_2/3)O₃ and Ba(Zn_1/3Nb_2/3)O₃ ceramic cylinders with the same diameter were bonded by adhesive with low dielectric loss to yield the layered dielectric resonators, and the microwave dielectric characteristics were evaluated with TE_01δ mode. With increasing the Ba(Zn_1/3Nb_2/3)O₃ thickness fraction, the resonant frequency ( f ₀) decreased, while the effective dielectric constant (ɛ _{r ,eff}) and temperature coefficient of resonant frequency (τ _f ) increased. Good microwave dielectric characteristics were attained for the samples with the Ba(Zn_1/3Nb_2/3)O₃ thickness fraction of 0.5: ɛ _{r ,eff}=34.33, Q × f =57 930 GHz and τ _f =2.6 ppm/°C. Finite-element method was used to predict the microwave dielectric characteristics of the layered resonators and good agreements were attained between the experimental results and predicted ones. Also, both experiment and finite-element analysis indicated that the effects of the adhesive on f ₀, ɛ _{r ,eff}, and τ _f were slight, while that on Q × f value was significant.     

Electromechanical Properties of PMN–PZT Piezoelectric Single Crystals Near Morphotropic Phase Boundary Compositions

Pb(Mg_1/3Nb_2/3)O₃–PbZrO₃–PbTiO₃ (PMN–PZT) ferroelectric single crystals near morphotropic phase boundary compositions were fabricated by solid-state crystal growth. The Curie temperatures ( T _C) of the grown PMN–PZT crystals were found to be on the order of 210°C, with ferroelectric phase transition temperatures ( T _{R – T} ) in the range of 96°–165°C. The electromechanical coupling factors k ₃₃ and k ₃₂ were found to be >90% and >−87%, respectively. The coercive field E _C for all the compositions was on the order of 5 kV/cm, double the value of pure Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMNT) crystals. The temperature dependence of the piezoelectric and electromechanical properties and dc bias effect on the dielectric behavior were investigated. The temperature usage range under dc bias was found to be improved when compared with pure PMNT crystals with similar piezoelectric properties.     

Dielectric Properties of (1−x)CaTiO3–xCa(Zn1/3Nb2/3)O3 Ceramic System at Microwave Frequency

The microwave dielectric properties of the (1− x )CaTiO₃– x Ca(Zn_1/3Nb_2/3)O₃ ceramic system have been investigated. The ceramic samples sintered at 1300°–1450°C for 4 h in air exhibit orthorhombic pervoskite and form a complete solid solution for different x value. When the x value increased from 0.2 to 0.8, the permittivity ɛ_r decreased from 115 to 42, the unloaded quality factor Q × f increased from 5030 to 13 030 GHz, and the temperature coefficient τ_f decreased from 336 to −28 ppm/°C. When x =0.7, the best combination of dielectric properties, a near zero temperature coefficient of resonant frequency of τ_f∼−6 ppm/°C, Q × f ∼10 860 GHz and ɛ_r∼51 is obtained.     

Dielectric and Electrostrictive Properties of Ferroelectric Relaxors in the System Pb(Mg1/3Nb2/3)O3:PbTiO3: Ba(Zn1/3Nb2/3)O3

Ceramic dielectrics which have been fabricated in the Pb(Mg_1/3 Nb_2/3)O₃:PbTiO₃:Ba(Zn_1/3Nb_2/3)O₃ composition system are shown to exhibit two distinct dielectric maxima, both of which show the characteristic loss spectra of ferroelectrics with diffuse phase transitions. The height of the individual maxima can be controlled by the Zn:Mg ratio in the starting material and, in suitably chosen compositions, a wide range of almost temperature-independent high dielectric permittivity is possible. These dielectrics show strong electrostrictive deformations under high electric fields but the electrostrictive strain is much less temperature-sensitive than in other relaxors.     

Dielectric and Ferroelectric Properties of Ceramics in the Pb(Zn 1/3Nb2/3)O3-BaTiO3-PbTiO3 System

The use of Pb(Zn_1/3Nb_2/3)O₃ ceramics is restricted by the formation of a pyrochlore phase detrimental to both dielectric and piezoelectric properties. Recently it has been shown that a 6 mol% addition of BaTiO₃ to PZN suppresses the formation of pyrochlore phase. Phase relations and dielectric properties of ceramics in the PZN-BT-PT system are reported here. Compositions with the perovskite structure, having high dielectric constant and low temperature coefficient of capacitance, have been identified.     

Microstructures and Microwave Dielectric Characteristics of Ca(Zn1/3Nb2/3)O3 Complex Perovskite Ceramics

Ca(Zn_1/3Nb_2/3)O₃ microwave dielectric ceramics were prepared using a solid-state reaction process, and their microwave dielectric properties were evaluated as functions of sintering and postdensification annealing conditions. The relationship between microwave dielectric properties and processing was interpreted through the variation of microstructures. The dielectric constant showed slight variation with sintering and annealing conditions, but the Q × f value increased at first and then decreased with increased sintering temperature, and annealing in oxygen indicated significant improvement in Q × f , especially for the specimens sintered at higher temperatures. The good microwave dielectric properties were obtained in the ceramics sintered at 1225°C in air for 3 h and annealed at 1100°C in oxygen for 8 h: ɛ= 34.1, Q × f = 15 890 GHz, τ_f=−48 ppm/°C.     

Growth of Pb((Zn1/3Nb2/3)0.91Ti0.09)O3 Single Crystals Using an Allomeric Seed Crystal and Their Electrical Properties

Single crystals of Pb((Zn_1/3Nb_2/3)_0.91Ti_0.09)O₃ (PZNT 91/9), 28 mm in diameter and 30 mm in length, have been successfully grown using a modified Bridgman technique with an allomeric seed crystal. X-ray fluorescence analysis (XRFA) measurement confirms that the effect of segregation is not serious. The segregation coefficient k for PbTiO₃ content during crystal growth is 0.99, which causes some fluctuation in the composition along the growth direction. The fluctuation of composition and the complicated domain structure cause a variation of electric properties. Dielectric measurement indicates that PZNT 91/9 crystals exhibit an almost normal ferroelectric phase transition at ∼183°C from the tetragonal phase to the cubic phase. In addition, a weak frequency-dependent ferroelectric-ferroelectric phase transition is observed at ∼85°C, which is attributed to partial conversion of the rhombohedral phase to a tetragonal phase. The dielectric thermal hysteresis behavior and the existence of polarization above the Curie temperature verify that the phase transitions at ∼85° and 183°C are first order with a slight diffuse character and first order, respectively. It is demonstrated that the effects of segregation can be decreased and the homogeneity of the obtained PZNT 91/9 single crystals can be improved by optimizing growth parameters.     

Low Loss Dielectrics in the Ca(B'1/2Nb1/2)O3 [B'=Lanthanides, Y] System

The Ca(B'_1/2Nb_1/2)O₃ [B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, and In] complex perovskites have been prepared by conventional solid-state ceramic route. The structure and microstructure of the ceramics have been characterized by X-ray diffraction and scanning electron microscopy methods. The ceramics have dielectric constant (ɛ_r) in the range 23–32, normalized Q -factor ( Q _u× f ) 11 000–38 000 GHz and temperature coefficient of resonant frequency (τ_f) −43–5.2 ppm/°C. The microwave dielectric properties of Ca(B'_1/2Nb_1/2)O₃ ceramics are found to depend on the ionic radii of B'-site elements and tolerance factor ( t ). The substitution of Ba²⁺ and Sr²⁺ for Ca²⁺ resulted a phase transition in Ca(B'_1/2Nb_1/2)O₃ ceramics. The (Ca_0.05Ba_0.95) (Y_1/2Nb_1/2)O₃ has τ_f close to zero (1.2 ppm/°C) with ɛ_r=35 and Q _u× f =48 500 GHz and is proposed as a useful material for base station applications. Dielectric properties of the Ca(B'_1/2Nb_1/2)O₃ ceramics were tailored by the addition of TiO₂ and CaTiO₃.     

Low-Temperature Observation of Relaxor Ferroelectric Domains in Lead Zinc Niobate

Domain reorientation in single crystals of lead zinc niobatelead titanate solid solutions was examined, because the reorientation contributes to the electrically controlled change of shape and change of response in piezoelectric transducers and actuators. An optical microscope technique was used to explore the buildup of macropolar domains from micropolar regions in relaxor compositions. Poorly defined "ambiguous" spindlelike domains changed to distinct lamellar domains as PbTiO₃ content was increased. The domain walls in Pb(Zn_1/3Nb_2/3)O₃-rich samples moved with a wavelike motion. The motion ceased or "froze-in" below –130° and –30°C for field-biased and nonbiased samples, respectively. The domains were observed at various temperatures from 300° to –185°C and electric fields up to ±10kV/cm.     

Dielectric Behavior of the Relaxor Pb(Mg1/3Nb2/3)O3—PbTiO3 Solid-Solution System in the Microwave Region

Dielectric behavior of the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃—PbTiO₃ solid-solution system was studied from—50° to 200°C in the 100 to 12 × 10⁹ Hz frequency region, and a broad dielectric relaxation was measured for compositions throughout the system. The relative microwave permittivity of the composition 0.9Pb(Mg_1/3Nb_2/3)O₃·0.1 PbTiO₃ decreased by 1 order of magnitude from the 1-MHz value of 11800, and similar decreases were observed for other compositions in the system. Dielectric loss (tan δ) values ranged from 0.5 to 1.0 at microwave frequency. The temperature of the broad dielectric constant maximum shifts toward higher values with increased frequency.     

Cation Ordering Transformations in the Ba(Zn1/3Nb2/3)O3-La(Zn2/3Nb1/3)O3 System

Single-phase perovskites were formed in the (1−_x)Ba(Zn_1/3Nb_2/3)O₃-( _x )La(Zn_2/3Nb_1/3)O₃ system for compositions with 0.0≤ x ≤0.6. Although the stability of the trigonal "1:2" ordered structure of the Ba(Zn_1/3Nb_2/3)O₃ end member is very limited (0.0≤ x ≤0.05), low levels of lanthanum induce a transformation to a cubic, "1:1" ordered structure that has a broad range of homogeneity (0.05≤ x ≤0.6). Samples with x > 0.6 were comprised of La₃NbO₇, ZnO, and a perovskite with x = 0.6. The cubic 1:1 phases were fully ordered and no evidence was found for a compositionally segregated microstructure. These observations could not be reconciled in terms of a "space-charge" model; rather, they supported a charge-balanced, "random-site" structure for the 1:1 cation-ordered Ba(β_1/2'β_1/2")O₃ phases.     

High Piezoelectric and Dielectric Properties of La-Doped 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb(ZrxTi1−x)O3 Ceramics Near Morphotropic Phase Boundary

La-doped 0.3Pb(Zn_1/3Nb_2/3)O₃–0.7Pb(Zr _x Ti_{1− x} )O₃ ( x =0.5–0.53) piezoelectric ceramics with pure perovskite phase were synthesized by a two-step hot-pressing route. The piezoelectric properties of various compositions near the morphotropic phase boundary (MPB) were systematically investigated. Not only was the exact MPB of this system determined via X-ray diffractometry analysis, but also the peak of piezoelectric properties was found near the MPB. The optimum piezoelectric properties of this series were observed in the specimen with Zr/Ti=51/49. The piezoelectric coefficient ( d ₃₃) and electromechanical coupling factor ( k _p) were 845 pC/N and 0.70, respectively, which have not been reported in this system so far. Large permittivity (ɛ_r=4088) and permittivity maximum (ɛ_m=29 500) were also obtained for the poled specimens. The temperatures ( T _max) of the permittivity maxima ranged from 206° to 213°C with various Zr/Ti ratios.     

Ferroelectric Properties of Pb(Zn1/3Nb2/3)O3–PbTiO3–RNbO3(R=Na, K) Ceramics

We investigate the ferroelectric properties of Pb(Zn_1/3Nb_2/3)O₃–PbTiO₃(PZN–PT)-based ceramics, which are stabilized by adding a small amount of NaNbO₃ (NN) and KNbO₃ (KN). As the content of alkali niobate increased, the ferroelectric properties of Pb(Zn_1/3Nb_2/3)O₃–PbTiO₃–RNbO₃ (PZN–PT–RN; R=Na, K) became softer, which was more pronounced in PZN–PT–KN. The difference in the piezoelectric properties between PZN–PT–KN and PZN–PT–NN was explained by the cation size effect. Because the ionic size of Na is smaller than that of K, the Na ion can retain the ferroelectricity of the solid solution more effectively. The field-induced strain of 85PZN–5PT–10NN under 10 kV/cm was as high as 0.1%. Also, the addition of NN increased the tunability of dielectric constant significantly. At a composition of 85PZN–5PT–20NN, the tunability was 90% and no hysteresis was observed. In contrast to RN, the increase in the content of PT caused the transition from relaxor to normal ferroelectrics, which were accompanied by the structural change from the rhombohedral to tetragonal phase.     

Far-Infrared Reflection Spectra of Dielectric Ceramics for Microwave Applications

Far-infrared reflection spectra of dielectric ceramics, BaSm₂Ti₅O₁₄, BaTi₄O₉, and some pcrovskites such as Ba(Zn_1/3Nb_2/3)O₃, have been measured at room temperature using a Fourier transform infrared spectrometer in order to investigate the effect of the crystal structure on the dielectric properties. As for perovskites, Sr(Zn_1/3Nb_2/3)O₃ and Sr(Mg_1/3Nb_2/3)O₃, in which B site ions are ordered, were also measured. Reflectance data were analyzed by means of a factorized form of dielectric functions instead of the classical dispersion theory, and all of the spectra were well fitted. The values of dielectric constants and tan δ calculated from the reflectance data were in good agreement with resonant cavity measurements at 5 GHz. Furthermore, results of this study have shown that the main contribution to the microwave dielectric properties is caused by low-frequency optically active modes located at 50 to about 300 cm⁻¹, and for perovskite structures it is suggested that the ordering of B site ions is significant in obtaining low dielectric losses.     

Synthesis of Pb(Mg1/3Nb2/3)O3 in Excess Lead Oxide by Mechanical Activation

Twenty hours of mechanical activation of mixed oxides at room temperature led to the formation of Pb(Mg_1/3Nb_2/3)O₃ (PMN) in excess PbO. The crystallinity of the activation-derived perovskite PMN phase was further established when the activated PMN–PbO phase mixture was subjected to calcination at 800°C. Pyrochlores, such as Pb₃Nb₄O₁₃ and Pb₂Nb₂O₇, were not observed as transitional phases on mechanical activation and subsequent calcination, although 50% excess PbO was deliberately added. The perovskite PMN phase was recovered by washing off excess PbO using acetic acid solution at room temperature. It was sintered to a relative density of 98.9% of theoretical at 1200°C for 1 h and the sintered PMN exhibited a dielectric constant of ∼14 000 at 100 Hz and a Curie temperature of −11°C.     

Influence of Cation Order on the Electric Field-Induced Phase Transition in Pb(Mg1/3Nb2/3)O3-Based Relaxor Ferroelectrics

The effect of cation ordering on an electric field-induced relaxor to normal ferroelectric phase transition in Pb(Mg_1/3Nb_2/3)O₃ (PMN)-based ceramics was investigated. Both A-site La doping and B-site Sc doping were found to enhance the chemical ordering in these relaxor ceramics. However, the enhanced chemical orderings showed different impacts on the dielectric and ferroelectric properties in these perovskite materials. The 5% La doping was observed to shift the dielectric maximum temperature ( T _max) to a significantly lower temperature and suppress the electric field-induced transition to a ferroelectric phase. In contrast, the 5% and 10% Sc doping showed little effect on T _max but strengthened the ferroelectric coupling. The difference is discussed on the basis of cation size and charge imbalance. An electric field-temperature phase diagram is also proposed for the 0.90PMN–0.10Pb(Sc_1/2Nb_1/2)O₃ based on its history dependence of the electric field-induced phase transition.