جستجوی مقالات مرتبط با کلیدواژه « dielectric loss » در نشریات گروه « مواد و متالورژی »

Zinc oxide (ZnO) ceramic is of interest due to its unique properties such as semiconductor, photocatalytic, biocompatibility and high dielectric constant. This research investigates the effect of annealing temperature on the dielectric properties, microstructure, and electrical resistance of ZnO ceramics. ZnO samples, after sintering at 1250°C, were annealed at temperatures of 850, 950, and 1050°C for 4 hours. X-ray diffraction analysis results showed that annealing reduces microstrain in the samples and increases the size of crystallite. No significant differences were observed in the size and morphology of the grains in the annealed and non-annealed samples. Annealing resulted in a significant decrease in the dielectric constant and dielectric loss of ZnO ceramics. The sample annealed at 950°C had the lowest dielectric constant and dielectric loss. Annealing at 950°C reduced the dielectric constant of ZnO ceramic from 111,000 to 35,000 and the dielectric loss from 22 to 6, measured at 1 kHz. The reduction in electrical losses may be attributed to the decrease in microstrain and the increase in crystallite size. The conductivity of annealed samples was lower than ZnO ceramic. Annealing caused an increase in the electrical resistance of the grain and grain boundary of ZnO ceramics. A significant increase in the electrical resistance of the grain boundary may indicate a decrease in the concentration of oxygen vacancies, which leads to a decrease in the dielectric constant of ZnO ceramics.

Bismuth titanate ceramic has been attracted as a lead-free ferroelectric due to its high Curie temperature in high temperature applications as well as high frequency applications. In this study, the effect of niobium oxide as an additive on the microstructure and dielectric properties of bismuth titanate ceramic was investigated. For this purpose, niobium oxide was added to bismuth titanate, synthesized by the solid state method, in the amounts of 0.3, 0.6 and 1.2 mole %. The results showed that niobium oxide has a significant effect on the microstructure and dielectric properties of bismuth titanate ceramic. The addition of niobium oxide improved the density and the densification of bismuth titanate ceramics. For a sample containing mole % of niobium oxide, the highest density was obtained at 98% of the theoretical density. The amount of 0.6 mole % of niobium oxide is reported as the optimal value. In this sample, the highest dielectric constant (420) and the least amount of dielectric loss (0.04) were obtained. Also, the dependence of the dielectric properties of this sample on the frequency was less than bismuth titanate. Improvements in the dielectric properties of bismuth titanate can be attributed to grain growth and higher density by the addition of niobium oxide additive. The results of this study show that the grain size of bismuth titanate ceramic will have a significant effect on dielectric constant and dielectric loss.

In the present work, effect of WO3 dopant on the sintering behavior, microstructure evolution, and microwave dielectric loss of Ba(Co1/3Nb2/3)O3 ceramics were systematically investigated. (1-x) Ba(Co1/3Nb2/3)O3 – (x) WO3 compounds, where x=0, 0.002, 0.004, 0.008, and 0.02, were prepared by the conventional solid state synthesis route followed by sintering at 1300-1450ºC for 10h at air atmosphere. Solid solution limit of WO3 oxide in the Ba(Co1/3Nb2/3)O3 compound and formation of any secondary phase were determined by X-ray diffraction (XRD) technique. In addition, the obtained XRD patterns were simulated by Rietveld refinement and degree of 1:2 cation ordering was calculated based on the refinement results. Scanning electron microscopy (SEM) was employed to study microstructural development of the ceramic samples and to directly identify secondary phase formation and their morphology. XRD results demonstrated that WO3 could solve into Ba(Co1/3Nb2/3)O3 structure for x<0.02, while detailed investigation by SEM directly indicated that even for Ba(Co1/3Nb2/3)O3 – 0.002 WO3 (x=0.002) composition additional phases were precipitated during high-temperature sintering. According to the XRD results, it was found that BaWO4, and Ba9CoNb14O45 compounds were formed as secondary phases. On the other hand, Rietveld refinement simulation showed that addition of WO3 into Ba(Co1/3Nb2/3)O3 results in a significant decline in the 1:2 cation ordering degree, where it was deceased from 95% to 59% when x was increased from x=0 to x=0.02. Quality factor, Q, (inverse of dielectric loss, 1/tanδ) of the prepared ceramics were measured at the microwave frequency range and it was found that incorporation of WO3 noticeably lowered the quality factor of Ba(Co1/3Nb2/3)O3 materials, where Q×f (f denotes resonance frequency) was measured to be 61,000 GHz for x=0 composition, whereas, measurements did not show any resonant peaks for x=0.02 ceramics, which means the ceramics suffer from a huge microwave dielectric loss.

In this report، various compositions were selected based on wollastonite in CaO-SiO2-MgO system. XRD analysis revealed that wollastonite was the main crystalline phase in the sintered glass-ceramics. Sinterability and crystallization of glass ceramics were analyzed at various temperatures. 830 oC، was selected as the optimum sinterability temperature. Finally dielectric properties of glass-ceramic samples at 830 oC، in 12-8 GHz frequency were reported. Dielectric constant and dielectric loss of glass ceramics are 6. 7-7. 5 and 0. 0028-0. 0034 respectively.

The ever increasing demand for the low loss، low constant dielectrics (for high speed signal transmission) in microwave industry constantly accelerates the search for new microwave dielectrics. Also glass-ceramics have been shown dielectric properties، generally superior to metals and organic polymers. In this report، sintrability and microwave dielectric properties of CaO-SiO2-MgO glass ceramics has been investigated. The results of sinterability demonstrated that this composition is sutable for LTCC technology with low sintering temperature (<1000 oC) 800-860 oC. The glass ceramic prepared at 860 oC exhibited dielectric constant of 6. 13 and dielectric loss of 0. 0037 in 8-12 GHz.