Papers by Keyword: Dielectric Response

Abstract: M-type hexagonal ferrites have wide range of applications in magnetic recording media, microwave devices, micro electrochemical systems, high frequency devices, magneto-optical devices and many more. In present research, M-type strontium hexagonal ferrites doped with ‘magnesium’ having chemical composition (SrMg_xFe_12-xO₁₉) for x= 0.00, 0.05, 00.10, 0.15, 0.20, were synthesized to investigate the influence of rare earth metal doping on the structure and dielectric properties via sol-gel auto combustion technique. Molecular absorption/transmission, structural properties and dielectric response were investigated by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) and Dielectric measurements of ‘Mg’ doped strontium hexaferrites. X-ray diffraction analysis verified the magneto-plumbite structure. Crystal sizes were found in varying order for different concentrations of ‘Mg’ ranging from 12.357 to 15.375 nm. The FT-IR spectra exhibited higher frequency band (500–515.84 cm^-1) indicating tetrahedral site’s vibrations of metallic cations and lower frequency band (385.35–375.16 cm^-1) exhibiting octahedral sites due to metallic oxygen bond that confirmed the hexagonal structure. The resonance peaks were observed in dielectric constant, loss, tangent loss, AC conductivity, electric modulus and quality factor versus frequency graphs. The dielectric properties were found to be enhanced gradually by increasing concentration of magnesium. The best Q-factor was found for magnesium concentration (x=0.20). The dielectric parameters specify that these ferrite nanoparticles are good applicants for the higher frequency implementations.

Abstract: The present work investigate the microwave absorption properties of reduced graphene oxide (RGO)-Silicon carbide (SiC)-Linear low-density polyethylene (LLDPE) composites prepared in different concentration of fillers(10, 20, 30, 40 wt. %) with LLDPE matrix. Synthesis of RGO is confirmed from XRD analysis and SiC is used as received from supplier. Complex permittivity of the composites is measured using Nicolson Ross method showing an increasing trend with increasing filler concentrations with maximum and for 40 wt. % composite sample. Based on transmission line theory and using measured value of complex permittivity, conductor backed single and double layer absorber is designed by thickness optimization. The calculated reflection loss (RLc) value of ~-71 dB at 11.23 GHz is observed for 40 wt. % composite sample of 7 mm thickness with -10 dB absorption bandwidth of 1.48 GHz and -20 dB bandwidth of 0.64 GHz.

Abstract: The CaTiO₃ samples were prepared by high-energy ball milling process followed by sintering process from 1040 to 1200°C. X-ray diffraction (XRD), microstructural analysis, and dielectric properties over a wide range of frequency varying from 0.01 Hz to 1 GHz at room temperature were investigated. The formation of a single phase CaTiO₃ with orthorhombic structure was achieved at 1120°C and above. From a morphological point of view, sintering temperature promoted grain growth. Dielectric properties in the frequency range 0.01 Hz - 1 MHz revealed a relaxation-type process. Interfacial phenomena were the possible physical mechanisms that gave rise to these relaxation-type plots. Extending the frequencies above ~1 MHz yielded a frequency-independent characteristic of dielectric constant (ε'). These turned out to the relatively small dielectric loss (tan δ) values. The origin of the dielectric responses in the frequency range 1 MHz - 1 GHz was attributed to the domination of dipolar polarization. The grain size effect in sintered CaTiO₃ samples was prominent, notably in dielectric responses above ~1 MHz. Increase in sintering temperature remarkably led to an enhancement in dielectric constant values and reduction in dielectric loss values. Therefore, a significant correlation existed between microstructural features and dielectric properties.

Abstract: In this work, soft combustion technique was used to study the effect of lanthanum (La) doping on the properties of Bi_0.5Na_0.5TiO₃ (BNT). The amount of La dopant investigated for (Bi_0.5Na_0.5)_(1-1.5x)La_xTiO₃ (BNLT) pellets were x = 0, 0.01, 0.02, 0.05 and 0.10 (0, 1, 2, 5 and 10 mol% La). The addition of La dopant into the BNT structure caused lattice distortion and altered the crystal symmetry of the BNT from hexagonal to tetragonal symmetry for BNLT. BNT and BNLT pellets were sintered at 1100°C for 3 hours and pure perovskite phase BNT was obtained with the addition of La up to 5 mol%. A secondary phase of Bi₂(Ti₂O₇) appeared when the pellet was doped with 10 mol% of La dopant. Increasing of La dopant in the pellet resulted in the increasing of dielectric constant and decreasing of dielectric loss. An optimum property was obtained by 5 mol% BNLT with dielectric constant of 753 and dielectric loss of 0.0377. Normal 0 false false false EN-US X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;}

Abstract: The polycrystalline samples of Ba4CaRTi3Nb7O30 (R = Eu, Dy), members of tungsten-bronze family, were prepared by high-temperature solid state reaction method and studied for their dielectric and electrical properties. X-ray diffraction (XRD) analysis reveals the formation of single-phase compounds having orthorhombic crystal structure at room temperature. Microstructural analysis by scanning electron microscope (SEM) shows that the compounds have well defined grains, which are distributed uniformly throughout the sample. Detailed dielectric properties of the compounds as a function of frequency and temperature show that the compounds undergo non-relaxor kind of ferroelectric-paraelectric phase transition of diffuse nature. Ferroelectric, piezoelectric and pyroelectric studies of the compounds have been discussed in this paper. The temperature dependence of dc conductivity of the compounds have been investigated. The conductivity study over a wide temperature range suggests that the compounds have negative temperature coefficient of resistance (NTCR) behaviour.

Abstract: It is important to reveal the relationship between the characteristic quantity of return voltage and polarization characteristics of dielectric in theory. In recent years, some researchers have proposed some equivalent circuit models to study the relationship between the equivalent circuit and the characteristic quantity of return voltage. However, seldom shows how the mathematical model of equivalent circuit parameters estimated is established and these parameters are estimated. The initial slope of return voltage is of great significance to identify the parameters of equivalent circuit based on linear dielectric response theory. Therefore, this paper establishes a mathematical model with the data of initial slope of return voltage for estimating its parameter, and then, a hybrid genetic algorithm is used for solving the problem. The result shows that the model established is feasible and effective.

Abstract: Multiferroic bismuth manganites (BiMnO₃) possess both ferromagnetic and ferroelectric properties. Their electrical properties can be controlled by doping for useful applications. In this work single-phase cobalt doped bismuth manganite nanoparticles having general formula BiMn_1-xCo_xO₃ (x=0, 0.2, 0.4, 0.6) were synthesized by co-precipitation method. Structural properties like lattice parameters and crystallite size of samples were determined by the data obtained by X-rays diffraction. The dielectric constant (ε) and dielectric loss tangent (tanδ) of samples were investigated as a function of frequency from 20Hz-3MHz using ac measurement data. For all the compositions dielectric constant was decreased with increasing frequency, however it increased with the increase in cobalt content. However cobalt addition causes a decrease in loss tangent as compared to pure BiMnO₃ composition. The origin of this behavior is discussed in terms of Maxwell-Wagner and Koops model. Substitution of Mn with Co, in BiMnO₃-based compounds is supposed to cause better properties in terms of tangent loss.

Abstract: The compound Dy₂Si₂O₇ exists in two polymorphs, the low temperature triclinic phase (type B) and a high temperature orthorhombic phase (type E).The dc and ac electrical conductivities of E-Dy₂Si₂O₇ are measured in the temperature range 290-510 K and frequency range 1 kHz to 1 MHz . The dc electrical transport data are analyzed according to Motts variable-range hopping model. The disorder parameter (T_o) and density of states at fermi level are obtained. The ac conductivity σ_ac (ω) is obtained through the dielectric parameters. The ac conductivity can be expressed as σ_ac (ω) =B ω ^s , where s is slope and it decreases with increase in temperature. The conduction mechanism in the compound is discussed in low and high temperature regions in the light of theoretical models.

Abstract: The mixed-oxide system Ba (Ti_1-xZr_x)O₃ (BTZ) with the simple perovskite structure was reported to exhibit the remarkable dielectric response around x = 0.08. In order to understand the origin of the response, we have investigated the crystallographic features of ferroelectric states in BTZ with 0 ≤ x ≤ 0.15 mainly by transmission electron microscopy. According to the constructed phase diagram of BTZ, when the Zr content increases from x = 0 around 300 K, the ferroelectric tetragonal state in BaTiO₃ is changed into the ferroelectric orthorhombic state around x = 0.03, and then into the ferroelectric monoclinic state around x = 0.06. That is, the monoclinic state with a polarization vector in the {110}_PC planes is present in BTZ with 0.07 ≤ x ≤ 0.15, where the subscript PC denotes the paraelectric cubic state. The notable feature of the monoclinic state is that it exhibits the herringbone-type domain structure consisting of nanometer-sized ferroelectric domains. It is thus understood that the remarkable dielectric response in BTZ should be associated with the presence of nanometer-sized domains with the monoclinic symmetry.

Abstract: A high dielectric constant nano-composite was fabricated using polyurethane (PU) as matrix and poly (p-chloromethyl styrene) (PCMS) grafted with copper phthalocyanine oligomer (CuPc, a planar multiring semiconductor with dielectric constant>10⁵) (name the grafting product as PCMS-g-CuPc) as filler. According to the TEM-observed morphologies, PCMS-g-CuPc balls (~80 nm) distribute uniformly in PU matrix, while in PCMS-g-CuPc balls the PCMS acts as “matrix” which contains dispersed CuPc particles with a average size of ca. 25nm [1/20 of that of CuPc in simple blend of PU and CuPc (PU/CuPc)]. At 100Hz, the dielectric constant of the nanocomposite reaches 345, almost 7 times higher than that of PU/CuPc. The enhanced dielectric response in the nano-composite can be explained by the remarkably strengthened interface exchange coupling effect as well as the Maxwell-Wagner-Sillars polarization mechanism due to the significantly reduced CuPc particle size in the nano-composite.