جستجوی مقالات مرتبط با کلیدواژه « dielectric constant » 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.

In recent years, research into materials with high dielectric constants, including doped titanium oxide, has increased because of the potential for modern microelectronics applications and high-density energy storage. The aim of this study was to investigate the effects of zinc oxide as an acceptor additive on the dielectric properties and microstructure of titanium oxide ceramics. The amount of 0.5, 1 and 2 wt% of the additive was added to the titanium oxide and the samples were prepared by powder pressing method and then sintered at 1420 ºC for 5 hours. The properties of the samples including density, phases, dielectric constant, dielectric loss and microstructure were investigated. The results showed that zinc oxide improved the density of titanium oxide. The results of X-ray diffraction test showed that there was no secondary phase and impurities in the samples containing the additive, which could indicate the replacement of the additive in the crystalline lattice of titanium oxide and the formation of a solid solution. The addition of zinc oxide resulted in the decreasing of dielectric constant and dielectric loss of titanium oxide. Zinc oxide additive caused grain growth and increase of grain size of titanium oxide ceramic. Microstructures with very large grain sizes and the possibility of creating oxygen vacancy with using zinc oxide additives can lead to the reduction in the dielectric constant of the titanium oxide ceramic.

A composite radome is a durable structure to protect telecommunication radar antennas against mechanical and environmental factors. Due to its placement in front of the antenna and the possibility of affecting the received and transmitted waves, in addition to the mechanical properties, the electromagnetic properties of this structure are very important. In this study, the mechanical properties (tensile strength in the direction of fibers) and the electromagnetic properties (dielectric constant at X-band frequency) for a GFRP composite radome has been studied and optimized. For this purpose, two types of glass fibers E and D series, and two types of epoxy resin and polyester resin have been used. In addition, three common methods of composite fabrication including hand layup, vacuum bag, and vacuum infusion have been used. Also, by using the Taguchi test design method and signal-to-noise analysis, the optimal composition was obtained for each of the properties, which for tensile strength of the combination of E series glass fibers and epoxy resin and vacuum bag fabrication method, the highest tensile strength was 320.9 MPa and for the dielectric constant of D series glass fibers and epoxy resin and vacuum bag fabrication method with the lowest value of 2.85 were determined. After determining the optimal composition for each of the properties, the optimum state was investigated by considering all the mechanical and electromagnetic properties factors together by using the multi-response optimization method (gray relation). Finally, a combination of D-series glass fibers, epoxy resin, and vacuum bag fabrication method was proposed.

Zinc oxide (ZnO) is widely used in various fields such as electronic devices, converters, varistors, catalysts, photoelectric devices, and piezoelectric application due to its unique properties. In the present study, the effect of titanium oxide (TiO2) additive on the properties of ZnO ceramic was studied. 0.5, 1 and 2 wt% of TiO2 were added to ZnO and various properties of ZnO ceramic including density, phase behavior, dielectric properties and microstructure were investigated. The obtained samples had a high relative density and about 98% of the theoretical density. The results of X-ray diffraction (XRD) showed that TiO2 additive leads to the formation of a secondary phase of Zn2TiO4 in ZnO ceramic and with increasing the amount of TiO2, the amount of impurity phase increases and the peaks move to lower angles. TiO2 additive considerably reduced the dielectric constant and the dielectric loss of ZnO ceramic. With addition of 2 wt% TiO2, the dielectric constant of ZnO was decreased from 48200 to 1400 and the dielectric loss of ZnO was decreased from 20 to 5.4, measured at 100 kHz. The results of microstructure study showed that TiO2 additive will have a significant effect on the microstructure of ZnO ceramic, so that with increasing the amount of TiO2 additive, grain growth decreases. The reduction in the constant dielectric of ZnO ceramic can be attributed to the smaller grain size, the existence of the impurity phase and the defect of the cation vacancy in the presence of TiO2 additive.

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 this research, cadmium sulfide (CdS) and cadmium sulfide doped with 3 cc manganese (3cc Mn-CdS) nanostructures have been prepared by ultrasound-assisted method and obtained products have been used for preparation of PVC: CdS and PVP: 3 cc Mn-CdS nanocomposites as an interfacial layer of the metal-polymer-semiconductor (MPS) Schottky structures. The structural, morphological, purity and optical properties of prepared nanostructures have been investigated by XRD, SEM, EDX and UV-Vis analyzes. The XRD of CdS sample confirmed the formation of the cadmium sulfide with cubic phase and its average nanocrystallite size obtained 6 nm. EDX analysis of both samples confirmed the pure phase of the prepared nanostructures. The energy gap of the CdS and 3 cc Mn-CdS nanostructures was calculated through the energy gap diagram 4.2 eV and 3.6 eV, respectively, that these values are bigger than from its bulk value (2.5 eV) due to the quantum confinement effect.Dielectric parameters such as ɛ′, ɛ″ and tan δ, of Al /PVP: CdS/ p-Si (MPS1) and Al /PVP: 3 cc Mn-CdS / p-Si (MPS2) Schottky structures are calculated and compared using C/G-f measurements in the frequency range of 100 Hz –1 MHz.The results showed that the dielectric parameters are strong function frequency. Also, doping of cadmium sulfide nanostructures with a very small amount of manganese source leads to a decrease in the dielectric constant, conductivity, and increase in the series resistance and loss tangent of the MPS2 compared to MPS1 Schottky structure.

In this work, Mn0.5Zn0.5Fe2O4 ferrite powders were prepared through glycine-nitrate combustion method at different molar ratios of glycine to nitrate. X-ray diffraction patterns showed that samples crystallized successfully in a spinel-type structure in all reactions. Magnetic properties were measured using a vibrating sample magnetometer. The saturation magnetization values of the as-synthesized samples were found to be in the range of 59.5-63.3 emu/g. Dielectric and electromagnetic properties were examined by a LCR meter. The sample synthesized at the lowest G/N ratio showed the lowest magnetic loss and the highest permeability. The complex impedance analysis illustrated only one semicircle indicating the predominant effect of grain boundary property of the material.

In this work, lithium ferrite and bismuth titanate have been synthesized to fabricate a multiferroic composite by microwave-induced combustion method. Multiferroic composites with the formula (where x=0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) were prepared. X-ray diffraction patterns showed that perovskite structure of bismuth titanate and spinel structure of lithium ferrite have been formed successfully in the as-synthesized nanostructured powders. Dielectric properties were examined by a LCR meter. Multiferroic composites containing the higher content of Li ferrite (70 wt % and 90 wt% Li ferrite), in comparison to the pure bismuth titanate and Li ferrite, indicate higher values of dielectric constant. In bismuth titanate and the composite with 10 wt% Li ferrite, dielectric constant remains constant as the frequency changes; in contrast, this character decreases significantly with frequency for Li ferrites and the other composites in such a way in Li ferrite, dielectric constant decreases from 8500 at 100 Hz to 180 at 1 MHz. Some dielectric peaks are observed in the dielectric constant vs. temperature plots which more or less shift to the higher temperatures as the bismuth titanate increases. For instance, the maximum dielectric constant of 2250 is observed at 25 ºC for Li ferrite; nevertheless, the maximum value of 5990 is found at 400 ºC for sample containing 70 wt% bismuth titanate.

In this study, Lithium ferrite has been produced by sol-gel method and influence of heat treatment at 1000˚C on its structure and properties has been investigated. Based on X-ray diffraction (XRD) data, it was identified that Li ferrite with cubic crystal structure and P4332 space group is formed successfully before and after heat treatment process. The microstructure of produced powder has been investigated by field emission scanning electron microscope (FESEM). The secondary particles with an average particle size of about 75 nm are observed into initial plate-like clusters. These particles grow to about 10 times larger after heat treatment. Magnetic properties measured by vibrating sample magnetometer (VSM) indicates that the value obtained for saturation magnetization is 53.54 emu/g. After heat treatment, saturation magnetization didn’t change. Magnetic permeability as the most important electromagnetic properties measured in all frequencies showed a significant improvement owing to heat treatment. This improvement is also observed in dielectric constant. Evaluation of magnetic permeability and dielectric constant variation with temperature shows that magnetic and dielectric Curie temperatures are in the range of 830-900˚K and 590-610˚K, respectively.

In this research, the effect of nanometer niobium oxide additions on electrical properties of barium titanate was investigated. 0.1, 0.2, 0.3 and 0.6 wt.% niobium oxide was added to barium titanate using planetary mill and ethanol media. The dried powder mixtures were spark plasma sintered at 1100, 1200 and 1300 °C and the characteristics of sintered samples were evaluated by XRD, SEM and LCR meter. XRD results showed no decomposition of initial phases and formation of new phase by adding niobium oxide while the microscopic investigation revealed the particle size reduction of barium titanate by increasing niobium oxide. Sample containing 0.3 %wt. niobium oxide sintered at 1300 °C had the maximum density of 99.5 and dielectric constant of 6300. On the other hand, sample containing 0.1 %wt. niobium oxide sintered at 1100 °C obtained the lowest density of 95.5 which showed optimal PTCR properties rather than other samples because of the sharp change in slope of the resistivity at Curie temperature.

Antenna protection against external factors (e. g. wind، rain، and objects colliding) is done by radome. In addition to acceptable mechanical properties، radome should satisfy suitable electrical properties such as dielectric constant and loss tangent to prevent negative effect on antenna performance. In order to build radome to be utilizing at high temperatures (over 1000 °C)، for flying objects with hypersonic speed (5 to 10 Mach)، engineering ceramics are used. Porosity affects the electrical and mechanical properties of articles. It simultaneously reduces the dielectric constant and flexural strength. In this study porous ceramic articles of beta-Sialon were fabricated by ball milling beta-Sialon powder، monomers and water for 10 hours and under 20 MPa uniaxial pressure. Average porosity size of this sample is 200 nm with 30% of its porosities are below 100 nm. Dielectric constant and loss tangent of this sample with 20% porosity in 8-12 GHz is 5. 3 and 0. 04، respectively.

Recently، due microelectronic technology developments، it is important to improve the electronic properties ceramic dielectrics، for some applications e. g. capacitors. Accordingly، one of these advanced materials that has attracted much attention in recent research is NiO-based ceramics which shows an extraordinarily a range of high dielectric constant. In the present work، for the first time Li0. 05Ni0. 95O ceramic dielectrics were prepared by mechanical alloying. The structure and microstructure of the milled powders were characterized by X-Ray diffraction (XRD) and Scanning Electron Microscopy (SEM) at different milling times. The quantitative analyses of the XRD results were done by the Rietveld refinement. According to the results، by milling operation، nanocrystallization and particles refinement were developed in which the crystallite and particles size of the powders reached to values of about 20 nm to 200 nm، after 24h of milling. In order to improve the dielectric properties، the 24h milled Li0. 05Ni0. 95O powders were mixed with iron oxide (Fe2O3) and after sintering process، the morphology of the Li0. 05Fe0. 02Ni0. 93O sample studied by SEM. It revealed that the grains sizes remained in the submicron range. By measuring the dielectric properties of the Li0. 05Fe0. 02Ni0. 93O sample using LCR meter at 120 Hz-200 kHz frequency range، high dielectric constant (about 5000) and relatively low loss (about 0. 9) were obtained at 20 kHz which remained constant up to 200 kHz.

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.