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.     

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.     

Probing of 1:2 Ordering in Ba(Ni1/3Nb2/3)O3 and Ba(Zn1/3Nb2/3)O3 Ceramics by XRD and Raman Spectroscopy

The 1:2 ordering in Ba(Ni_1/3Nb_2/3)O ceramics sintered at 1350-1500°C has been investigated by using XRD and Raman spectroscopy. Both of the techniques show that the degree of the 1:2 ordering decreases as the sintering temperature increases. However, XRD discerns the 1:2 ordering only for the samples sintered at 1350-1400°C, whereas Raman spectroscopy discerns the 1:2 ordering for all the samples. Similar results have been obtained for Ba(Zn_1/3Nb_2/3)O₃ ceramics, where only the temperature range is slightly different. It is demonstrated that Raman spectroscopy can be a useful tool for probing of the 1:2 ordering in the A(B'^II_1/3B"^V_2/3)O₃-type complex perovskite compounds.     

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.     

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.     

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.     

Effects of Sintering Atmosphere on Densification Behavior and Piezoelectric Properties of Pb(Ni1/3Nb2/3)O3–PbZrO3 Ceramics

Densification of polycrystalline Pb(Ni_1/3Nb_2/3)O₃–PbTiO₃–PbZrO₃ (PNN–PT–PZ) specimens was enhanced as the partial pressure of O₂ in the sintering atmosphere was increased. This observation was attributed to the increase in the internal pressure of a closed pore due to the thermal decomposition of PbO at a low partial pressure of O₂. The relative dielectric permittivity (ε_r), d ₃₃, k _p, and grain size of sintered specimens were also increased as the partial pressure of O₂ in the sintering atmosphere was increased. The observed dependence of piezoelectric properties on the partial pressure of O₂ was discussed in terms of the enhanced formation of the A-site vacancy ( V "_Pb) or the suppression of the B-site defect ( V ¨_O) as the oxygen potential increased.     

Effects of Stacking Scheme on Microwave Dielectric Characteristics of Ca(Mg1/3Nb2/3)O3–Ba(Zn1/3Nb2/3)O3 Layered Dielectric Resonators

Ca(Mg_1/3Nb_2/3)O₃ (CMN) and Ba(Zn_1/3Nb_2/3)O₃ (BZN) ceramic disks were stacked with three stacking schemes, designated as CMN/BZN, CMN/BZN/CMN, and BZN/CMN/BZN, to yield layered dielectric resonators, and the microwave dielectric characteristics were evaluated with the TE_01δ mode. Both experiments and finite element analysis showed that the microwave dielectric characteristics of the layered resonator were determined not only by the volume fraction of BZN but also by the stacking scheme. For each stacking scheme, a good combination of microwave dielectric characteristics with an effective dielectric constant of 34.33–34.52, a Q × f value of 58 800–62 080 GHz, and a near-zero temperature coefficient of resonant frequency could be achieved by adjusting the volume fraction of BZN. The effects of the stacking scheme on the microwave dielectric characteristics of the temperature-stable layered resonator were discussed by combining finite element analysis and dielectric composite models.     

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.     

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.     

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.     

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.     

Effect of Excess PbO on the Sintering Characteristics and Dielectric Properties of Pb(Mg1/3Nb2/3)O3–PbTiO3-Based Ceramics

Additions of excess PbO to the perovskite Pb[(Mg_1/3Nb_2/3)_0.92Ti_0.08]O₃ solid solution enhanced the formation of a liquid phase at 840°C, which served as a densification aid for the ceramics. The liquid phase allowed elimination of pores and promoted grain growth during sintering. With additions of 1 to 2 wt% excess PbO, densities in excess of 97% of theoretical were obtained at a sintering temperature of 950°C. The peak dielectric constants of the resulting ceramics were over 18 000 at 30°C and dissipation factors less than 1%. Additions of PbO in excess of 2 wt% resulted in inferior dielectric properties due mainly to the dilution of the ferroelectric phase.     

Structure and Dielectric Properties of the Ba(Mg1/3Nb2/3)O3-La(Mg2/3Nb1/3)O3 System

A complete range of perovskite solid solutions can be formed in the (1 − x )Ba(Mg_1/3Nb_2/3)O₃- x La(Mg_2/3Nb_1/3)O₃ (BMN-LMN) pseudobinary system. While pure BMN adopts a 1:2 cation ordered structure, 1:1 ordered phases are stabilized for 0.05 ≤ x ≤ 1.0. Dark-field TEM images indicate that the La-doped solid solutions are comprised of large 1:1 ordered domains and no evidence was found for a phase-separated structure. This observation coupled with the systematic variations in the intensities of the supercell reflections supports a charge-balanced "random-site" model for the 1:1 ordering. The substitution of La also induces a transformation from a negative to positive temperature coefficient of capacitance in the region 0.25 ≤ x ≤ 0.5.     

Synthesis of BaCu(B2O5) Ceramics and their Effect on the Sintering Temperature and Microwave Dielectric Properties of Ba(Zn1/3Nb2/3)O3 Ceramics

BaCu(B₂O₅) ceramics were synthesized and their microwave dielectric properties were investigated. BaCu(B₂O₅) phase was formed at 700°C and melted above 850°C. The BaCu(B₂O₅) ceramic sintered at 810°C had a dielectric constant (ɛ_r) of 7.4, a quality factor ( Q × f ) of 50 000 GHz and a temperature coefficient of resonance frequency (τ_f) of −32 ppm/°C. As the BaCu(B₂O₅) ceramic had a low melting temperature and good microwave dielectric properties, it can be used as a low-temperature sintering aid for microwave dielectric materials for low temperature co-fired ceramic application. When BaCu(B₂O₅) was added to the Ba(Zn_1/3Nb_2/3)O₃ (BZN) ceramic, BZN ceramics were well sintered even at 850°C. BaCu(B₂O₅) existed as a liquid phase during the sintering and assisted the densification of the BZN ceramic. Good microwave dielectric properties of Q × f =16 000 GHz, ɛ_r=35, and τ_f=22.1 ppm/°C were obtained for the BZN+6.0 mol% BaCu(B₂O₅) ceramic sintered at 875°C for 2 h.     

Microstructural Inhomogeneity in Sintered Pb(Mg1/3Nb2/3)O3-PbTiO3 Based Dielectrics

The sintering behavior and microstructural development of dielectric ceramics based on Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ solid solutions are greatly affected by the formation of a liquid phase at ∼ 1290°C. Prolonged sintering at and above this temperature gives rise to an excessive PbO loss and the resultant variation in composition leads to an inhomogeneous microstructure. The inhomogeneity is characterized by the formation of a dense, localized region containing a PbO-rich liquid near the surface with a porous interior region in the bulk of the sample.     

Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics. IV. Tuning τf and the Part Size Dependence of Q×f

High Q ceramics of Ba₃W₂O₉ (BW)-substituted Ba(Zn_1/3Nb_2/3) O₃ (BZN) were prepared with a zero τ_f through the partial substitution of Zn by Ni and Co. The small concentrations of B-site vacancies introduced by the substitution of BW accelerated the kinetics and stability of the cation ordering and lowered the sintering temperature. Dense, zero τ_f, ordered solid solutions such as 0.99Ba(Zn_0.3Co_0.7)_1/3Nb_2/3O₃–0.01BW with ɛ_r=34.4 and Q × f =82 000 at ∼8 GHz could be obtained after sintering at 1380°C for 5 h and annealing at 1300°C for 24 h. Partially ordered ceramics in the Zn/Co and Zn/Ni solid solutions show a large gradient in the ordering throughout the pellets, which produces a resonant frequency dependence of their Q × f value. The ordering gradient is associated with the increased constraints on the growth of the 1:2 ordered structure within the interior of larger and thicker pellets and can be minimized by extended annealing.     

Dielectric Characteristics of Bi- and Ti-Substituted Pb(Mg1/3Nb2/3)O3

Pb[Mg_1/3Nb_2/3]O₃ was gradually substituted by Bi[Mg_2/3Nb_1/3]O₃ (BiMN) up to 30 mol%, with an overall modification by a constant fraction of PbTiO₃ (10 mol%). Monophasic perovskite powders could be prepared via the B-site precursor route. Ceramic samples of the system showed a typical relaxor behavior of frequency-dependent dielectric dispersion. Values of the maximum dielectric constant decreased substantially with increasing BiMN concentration, whereas corresponding temperatures changed only moderately.     

Tetragonal Tungsten Bronze Niobate, K0.2Sr0.4NbO3: A New Material for Capacitors with Flat Dielectric Curves

A new ceramic for capacitors, which exhibits flat dielectric curves with relative permittivity values (ɛ) as high as 3000 (Y7R or X7R class), has been synthesized. Use of a niobate, K_0.2Sr_0.4NbO₃ (KSN), with a tetragonal tungsten bronze structure (TTB), as a major phase for the fabrication of such ceramics is shown for the first time. Nevertheless, the addition of perovskites as minor agents—PbMg_1/3Nb_2/3O₃ (PMN)—is necessary to obtain stable temperature. Moreover, the use of lithium salts as sintering agents has been shown to be important for the fabrication of these ceramics.