Abstract
The effects of post-annealing on the crystal structure, microstructure, and microwave dielectric properties for Ba(Co0.7Zn0.3)1/3Nb2/3O3 ceramics were investigated. The as-prepared materials were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The microwave dielectric properties are measured at 6 GHz using a network analyzer. Ba5Nb4O15 and/or Ba8(Co,Zn)1Nb6O24 secondary phases were found on the surface according to sintering conditions due to volatilization of some Zn and Co elements. The experimental results show that the beneficial effect of the annealing steps to improved the microwave dielectric properties. Excellent microwave dielectric properties were achieved for the coarse-grained microstructures by a higher sintering temperature and with a shorter holding time followed by annealing steps at lower temperatures with a longer holding time. This improvement can be attributed to 1:2 cation ordering within the crystal, which is taking place during annealing process. The Ba(Co0.7Zn0.3)1/3Nb2/3O3 ceramic could be used successfully for realization of dielectric microwave resonators, since it has a high quality factor Qf value of 123,700 GHz, a high dielectric constant εr value of 34.5 and a temperature coefficient of the resonant frequency τf of 0 ppm/°C.
Highlights
Compounds with perovskite structure with a general formula ABO3 have been of continuous importance for many years [1]
We study the effect of this additional annealing step on the structure, microstructure, and microwave dielectric properties of Ba(Co0.7 Zn0.3 )1/3 Nb2/3 O3 (BCZN) ceramics and compare the results with those obtained without this annealing
At a depth of 90 μm below the surface (Figure 2b), all reflections can be attributed to the cubic structure of Ba(Co0.7 Zn0.3 )1/3 Nb2/3 O3, whereas no peaks attributable to Ba8 (Co,Zn)1 Nb6 O24 were structure of Ba(Co0.7Zn0.3)1/3Nb2/3O3, whereas no peaks attributable to Ba8(Co,Zn)1Nb6O24 were observed. These results clearly show that the Ba8 (Co,Zn)1 Nb6 O24 phase is only formed on the surface observed. These results clearly show that the Ba8(Co,Zn)1Nb6O24 phase is only formed on the surface of of the BCZN ceramics and that it represents a secondary phase
Summary
Compounds with perovskite structure with a general formula ABO3 have been of continuous importance for many years [1]. Applications as dielectric resonators require materials with a high quality factor at the resonance frequency (Q f ), a high dielectric constant (εr ) and a near-zero temperature coefficient of the resonance frequency (τ f ) [2,5] In this context, Ba(Co0.7 Zn0.3 )1/3 Nb2/3 O3 (BCZN) ranks among the best oxide materials with regard to applications such as dielectric resonators in the range of microwave frequencies due to its high Q f value (~56,000–85,000 GHz), its high εr (~34.5) and its small τ f (~0) [6,7]. As a continuation of our previous work, the present paper describes improvements of the microwave dielectric proprieties of BCZN ceramics achieved by a supplementary annealing process, which is added to the synthesis procedure described previously. We study the effect of this additional annealing step on the structure, microstructure, and microwave dielectric properties of BCZN ceramics and compare the results with those obtained without this annealing. The relationship between the heat treatment and the BCZN characteristics will be discussed in detail
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