O. V. Ovchar

Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba(Co1/3Nb2/3)O3

Polycrystalline nonstoichiometric Ba(Co1/3Nb2/3)O3 (BCN) materials have been synthesized and investigated. Deviations from stoichiometry have been shown to lead to the formation of crystalline Ba6CoNb9O30 (barium deficiency) and Ba8CoNb6O24 (cobalt deficiency). The effect of phase composition on the microwave dielectric properties of BCN has been studied. The results demonstrate that the dielectric properties of BCN-based materials can be tuned by varying cobalt content. The materials obtained are high-Q microwave dielectrics with temperature-stable properties.

Effect of nonstoichiometry on the structure and microwave dielectric properties of cobalt metaniobate

The formation of cobalt metaniobate, CoNb2O6, with the columbite structure has been studied, and possible ways of optimizing solid-state synthesis conditions have been examined. Single-phase CoNb2−yO6−2.5y has been obtained in the composition range 0 ≤ y ≤ 0.05. The microstructure and microwave dielectric properties of Co1−xNb2O6−x materials have been investigated. The results indicate that the electrical Q of the nonstoichiometric materials drops sharply for x > 0 because of the presence of Nb2O5 as an impurity phase. For x < 0, Q gradually decreases with increasing Co content, which is caused by the rise in lattice strain and presence of Co4Nb2O9 impurity.

Synthesis and microwave dielectric properties of Zn1+x Nb2O6+x

We have studied the formation of zinc niobate, ZnNb2O6, with the columbite structure and the microstructure and microwave dielectric properties of Zn1+xNb2O6+x ceramics. The results demonstrate that, in the range 0.005 ≤ x ≤ 0.03, the excess zinc reduces the porosity of the material and increases its microwave quality factor Q. For x ≥ 0.03, the Q of the ceramics decreases because of the formation of an additional, zinc-enriched phase. Sintering in an oxygen atmosphere is shown to improve the dielectric properties of stoichiometric ZnNb2O6.

Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba 1 − x (Zn 1/2 W 1/2 )O 3 − x and Ba(Zn 1/2 − y W 1/2 )O 3 − y /2

We have synthesized Ba1 − x(Zn1/2W1/2)O3 − x and Ba(Zn1/2 − yW1/2)O3 − y/2 barium tungstates with different deviations from cation stoichiometry (x = 0.01–0.05, y = 0.01–0.05), determined the phase composition of ceramics fabricated from the tungstates, and investigated their electrical properties. Even slight deviations from cation stoichiometry in Ba(Zn1/2W1/2)O3 lead to the formation of the scheelite phase BaWO4, and its content increases with heat-treatment temperature. Barium or zinc deficiency in the systems studied improves the sintering behavior of Ba(Zn1/2W1/2)O3 and increases the degree of 1: 1 B-site cation ordering, which in turn ensures an increase in microwave quality factor, Q.

The Effect of Chemical Composition on the Structure and Dielectric Properties of the Columbites A2 + Nb2O6

This paper summarizes the synthesis and properties of the materials based on the columbites A 2+ 1+x Nb 2 O 6+x (A 2+ = Mg, Co, Zn) in the vicinity of their stoichiometric composition. The evolution of the phase composition, microstructure, and microwave dielectric properties of the studied materials have been considered with respect to different sintering atmospheres (air and oxygen). The correlation between the data obtained by electron microscopy, positron lifetime spectroscopy, and microwave characterization at the frequencies of 10 and 50-70 GHz has been presented and discussed. The A-site deficiency (x < 0) always leads to the formation of the secondary Nb-rich phase in the columbite matrix, which significantly deteriorates the microwave quality factor (Q) of the sintered materials. In contrast, the Nb deficiency improves the microstructure of the ceramics but initiates the formation of A 2+ -rich secondary phases. The competing effect of these factors results in the nonlinear variation Q(x). The resulting magnitudes (Qxf) attain values of 100,000-120,000 GHz depending on the A 2+ ion and sintering atmosphere.

Synthesis and properties of columbite-structure Mg1 − xNb2O6 − x

The formation of the columbite-structure magnesium niobate MgNb2O6 is a multistep process. Single-phase material can only be obtained through long-term high-temperature heat treatment. Deviations from stoichiometry have a significant effect on the microwave quality factor Q of the material: magnesium-deficient ceramics contain small amounts of Nb2O5 and have relatively low Q values, whereas an excess of magnesium leads to the formation of Mg4Nb2O9 (alpha-alumina structure) as an impurity phase, thereby drastically increasing the electrical Q.

Synthesis and dielectric properties of Sr0.6 − x Ba0.4Na2x Nb2O6 solid solutions

We have prepared Sr0.6 − xBa0.4Na2xNb2O6 (0 ≤ 2x ≤ 0.3) solid solutions with the tetragonal tungsten bronze (TTB) structure, investigated the sequence of phase changes involved, and determined the lattice parameters of the solid solutions. The electrical properties of polycrystalline samples have been studied in broad frequency and temperature ranges. The results indicate that an increase in sodium content, accompanied by a reduction in the concentration of vacancies on the A site of the TTB structure, reduces the rate of dielectric relaxation.

Synthesis and dielectric and nonlinear properties of BaTi1 − xZrxO3 ceramics

We have studied the formation of BaTi1 − xZrxO3 solid solutions with the perovskite structure. The phase composition, microstructure, and electrical properties of the BaTi1 − xZrxO3 materials have been investigated. The results demonstrate that their dielectric permittivity (ɛ) and loss tangent (tan δ) decrease with increasing zirconium content. At the same time, their nonlinearity coefficient (nR) remains large (nR = 30–50%) in relatively low fields (E = 30–50 kV/cm). The present data suggest that the materials studied here are potentially attractive for use in creating nonlinear devices of modern microelectronics.

Effect of Zn2TiO4 and ZnB2O4 additions on the microstructure and dielectric properties of AgNb1 − xTaxO3 solid solutions

We report the preparation and characterization of AgNb0.6Ta0.4O3 (ANT) based materials. The addition of Zn2TiO4 and ZnB2O4 influences the sintering temperature, phase composition, and microstructure of ANT ceramics. ANT doped with 1 wt % Zn2TiO4 or ZnB2O4 has high dielectric permittivity (400–470), low dielectric losses (tanδ ∼ 10−3), and a nonlinearity coefficient nR ≅ 3–9% (at a field strength E = 3 × 106 V/m).