Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics: III. Effect of the Muffling Environment

The effect of different muffling environments on the structure and dielectric losses of Ba(Zn_1/3Nb_2/3)O₃ (BZN) microwave ceramics was investigated. The microwave dielectric losses of stoichiometric BZN pellets heated in ZnO-rich environments were severely degraded (e.g. Q × f ∼15 000 in ZnO powder) compared with samples muffled in their own powder ( Q × f ∼80 000). Structure analyses and gravimetric measurements confirmed that the ceramics muffled in ZnO powder or vapor absorb excess ZnO to form non-stoichiometric solid solutions with reduced cation order and Q . By using starting compositions in the (1− x )BZN−( x )BaNb_4/5O₃ binary ( x =0.04), the stoichiometry can be tailored to ensure that after the uptake of ZnO, the ceramics remain well ordered and are located in a high Q region of the system. For example, ZnO-vapor-protected (0.96)BZN−(0.04)BaNb_4/5O₃ reached a very high Q × f (∼1 05 000) after sintering at 1400°C for 5 h.     

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.     

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.     

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.     

Order-Disorder Phase Formation in the Complex Perovskite Compounds Ba(Ni1/3Nb2/3)O3 and Ba(Zn1/3Nb2/3)O3

The order-disorder phase formation of the complex perovskite compounds Ba(Ni_1/3Nb_2/3)O₃ (BNN) and Ba(Zn_1/3-Nb_2/3)O₃ (BZN) was investigated using X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The BNN and BZN samples were sintered over a temperature range of 1200° to 1500°C in air for 2 h. X-ray diffraction and transmission electron microscopy showed that these compounds exhibited a 1:2 ordering on the B-site within a narrow temperature range. When BNN and BZN were sintered above 1400° and 1350°C, respectively, a liquid phase formed in the grain boundary which was accompanied by disordering. The composition of the liquid phase resembled that of pyrochlore, with a small amount of nickel for BNN or zinc for BZN. The disordering with the formation of the liquid phase was attributed to the increase in defect concentration.     

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.     

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.     

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.     

Crystal Structure and Microwave dielectric Properties of Ba(Zn1/3Ta2/3)O3–(Sr,Ba)(Ga1/2Ta1/2)O3 Ceramics

The microwave dielectric properties and crystal structure of Ba(Zn_1/3Ta_2/3)O₃– (Sr,Ba)(Ga_1/2Ta_1/2)O₃ ceramics were investigated in the present study. The Q value of Ba(Zn_1/3Ta_2/3)O₃ was improved by adding 5 mol% Sr(Ga_1/2Ta_1/2)O₃. The maximum Q value of Q × f = 162000 GHz was obtained at 0.95Ba(Zn_1/3Ta_2/3)O₃. 0.05Sr(Ga_1/2Ta_1/2)O₃. For this composition, a lattice super structure caused by hexagonal ordering was observed. A further improvement in the Q value was attained when some Sr was replaced with Ba, and 0.95Ba(Zn_1/3Ta_2/3)O₃· 0.05(Sr_0.25Ba_0.75)(Ga_1/2Ta_1/2)O₃ exhibited a maximum Q value such that Q × f = 210000 GHz. Despite the increased Q value with the replacement of Sr by Ba, the c/a value, which indicates the degree of lattice distortion, remained constant near 3/2. The Q value thus improved without lattice distortion in the system Ba(Zn_1/3Ta_2/3)O₃-(Sr,Ba)(Ga_1/2Ta_1/2)O₃, whereas the improvement of Q value increased with lattice distortion in the solid solution system with Ba(Zn_1/3Ta_2/3)O₃ as an end member.     

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.     

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.     

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.     

Microstructure and Microwave Dielectric Properties of Ba(Zn1/3Ta2/3)O3 Ceramics with ZrO2 Addition

The effect of ZrO₂ on crystallographic order, microstructure, and microwave dielectric properties of Ba(Zn_1/3Ta_2/3)O₃ (BZT) ceramics was investigated. A small amount of ZrO₂ disturbed the 1:2 cation ordering. The average grain size of the BZT significantly increased with the addition of ZrO₂, which was attributed to liquid-phase formation. The relative density increased with the addition of a small amount of ZrO₂, but it decreased when the ZrO₂ content was increased. Variation of the dielectric constant with ZrO₂ addition ranged between 27 and 30, and the temperature coefficient of resonant frequency increased abruptly as the ZrO₂ amount exceeded 2.0 mol%. The Q value of the BZT significantly improved with the addition of ZrO₂, which could be explained by the increased relative density and grain size. The maximum Q × f value achieved in this investigation was ∼164 000 GHz for the BZT with 2.0 mol% ZrO₂ sintered at 1550°C for 10 h.     

Effect of Grain Size on Diffusion-Induced Grain-Boundary Migration in Ba(Zn1/3Nb2/3)O3 Ceramics

Diffusion-induced grain-boundary migration (DIGM) in Ba(Zn_1/3Nb_2/3)O₃ (BZN) ceramics was investigated with small (3.0 μm) and large (31. 4 μm) grain size specimens. The specimens were embedded in Nb₂O₅ or ZnO powders and then heat-treated at 1250° and 1310°C, respectively. The grain boundaries of the small grain size specimens were immobile, while those of the large grain size specimens migrated away from their centers of curvature. From the observed difference in migration behavior depending on grain size, the magnitude of the driving force for the DIGM was estimated.     

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.     

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.     

Microwave Dielectric Properties of Ti-Substituted Bi2 (Zn2/3Nb4/3)O7 Pyrochlores at Cryogenic Temperatures

Monoclinic pyrochlore ceramic Bi₂Zn_{2/3− x /3}Nb_{4/3−2 x /3}Ti _x O₇ (M–BZN) with x =0–0.4 is synthesized and the structure and microwave cryogenic properties are scrutinized. The dielectric constant (ɛ') and loss tangent (tanδ) of these ceramics are measured at a frequency of 3 GHz and temperature range of 15–300 K. With an increase in x value from 0 to 0.4, the dielectric constant and dielectric loss tangent of the investigated materials increase from 70 to 114 and 0.009 to 0.061, respectively. The Ti-substituted ceramics show an increase in dielectric constant with temperature, and the loss tangent shows a peak around 200 K. The peak in the dielectric loss tangent becomes more prominent with an increase of Ti content. The temperature where the dielectric loss tangent peak appears is found to be decreasing slightly with an increase of titanium doping. The observed dielectric characteristics of the titanium-doped M–BZN ceramics are attributed to the presence of the relaxation in these materials, originating from the disorder caused by the Ti⁴⁺ substitution.