Dielectric Properties of Pure and Gd‐Doped HfO2 Ceramics

The pure, 2 at.%, and 20 at.% Gd‐doped HfO₂ ceramics were prepared by the standard solid‐state reaction technique. Dielectric properties of these ceramics were investigated in the temperature range 300–1050 K and frequency range 20–5 × 10⁶ Hz. Our results revealed an intrinsic dielectric constant around 20 in the temperature below 450 K for all tested ceramics. Two oxygen‐vacancy‐related relaxations R1 and R2 were observed at temperatures higher than 450 K, which were identified to be a dipolar relaxation due to grain response and a Maxwell–Wagner relaxation due to grain‐boundary response, respectively. The dielectric properties of the pure and slightly doped (2 at.%,) samples are dominated by the grain‐boundary response, which results in a colossal dielectric behavior similar to that found in CaCu₃Ti₄O₁₂. The doping level of 20 at.% leads to the structural transformation from monoclinic phase to cubic phase. The dielectric properties of the heavily doped HfO₂ are dominated by the grain response without any colossal dielectric behavior.     

Structural and dielectric properties of CuO nanoparticles

The DC conduction and dielectric behaviour of copper oxide nanoparticles prepared by sol-gel method and sintered at 950 °C were studied in the temperature range of 200–526 K. The formation of single phase monoclinic CuO was confirmed by x-ray diffraction. Chemical composition of the CuO ceramic was investigated with X-ray photoelectron spectroscopy (XPS) technique. Although XRD analysis shows the formation of single phase CuO, XPS spectra revealed the presence of Cu³⁺ and Cu²⁺. Deviation from linearity ln (σ_DC) vs. 1/T plot at ~390 K was observed, which indicates that DC conduction in the CuO pellet is dominated by two different conduction mechanisms. The results obtained on AC conductivity indicate that AC conduction mechanism could be well explained by the multihopping model at low frequencies, while high frequency AC conductivity data can be described by small polaron tunnelling model. The dielectric relaxation mechanism in the CuO pellet was studied by impedance spectroscopy. It was found that while dielectric constant is an increasing function of temperature, it decreases with increasing frequency. The obtained impedance spectra indicated that the grain boundary effects and intergranular activities play a crucial role on the dielectric relaxation processes.     

Microstructure,Thermal Conductivity,and Electrical Properties of In Situ Pressureless Densified SiC–BN Composites

The microstructure, thermal conductivity, and electrical properties of pressureless densified SiC–BN composites prepared from in situ reaction of Si₃N₄, B₄C, and C were systematically investigated, to achieve outstanding performance as substrate materials in electronic devices. The increasing BN content (0.25–8 wt%) in the composites resulted in finer microstructure, higher electrical resistivity, and lower dielectric constant and loss, at the expense of only slight degradation of thermal conductivity. The subsequently annealed composites showed more homogeneous microstructures with less crystal defects, further enhanced thermal conductivities and electrical resistivities, and reduced dielectric constants and losses, compared with the unannealed ones. The enhanced insulating performance, the weakened interface polarization, and the reduced current conduction loss were explained by the gradual equalization of dissolved B and N contents in SiC crystals and the consequent impurity compensation effect. The schottky contact between graphite and p‐type SiC grains presumably played a critical role in the formation of grain‐boundary barriers. The annealed composites doped with 8 wt% BN exhibited considerably high electrical resistivity (4.11 × 10¹¹ Ω·cm) at 100 V/cm, low dielectric constant (16.50), and dielectric loss (0.127) at 1 MHz, good thermal conductivity [66.06 W·(m·K)^?1] and relatively high strength (343 MPa) at room temperature.     

Effect of Nd3+ and Cd2+ ions co-substitution on the dielectric and electron transport properties of spinel strontium nanoferrites

The work reports microemulsion synthesis of spinel nanoferrites having a nominal composition of Sr_1−xNd_xFe_2−yCd_yO₄ (0.0≤x≤0.1, 0.0≤y≤1.0). X-ray diffraction (XRD) patterns of powdered products annealed at 1073 K for three hours revealed the cubic Spinel structure with an average crystallite size in the range of 23–35 nm. The room temperature dc resistivity (ρ) has its maximum value for the composition Sr_0.92Nd_0.08Fe_1.2Cd_0.8O_4. However, for this composition the dielectric parameters and ac conductivity have their minimum values. The magnitudes of these parameters decrease with increasing frequency at low frequency region and then show almost independent frequency behavior at higher frequencies. The results are interpreted in terms of dielectric polarization with great detail. Electrochemical impedance spectroscopy results revealed that conduction process is predominantly governed by grain boundary volume. High resistivity and low dielectric constant of investigated material suggests that it may be employed in microwave devices.     

Frequency and Temperature‐Independent Electrical Transport Properties of 2BaO–0.5Na2O–2.5Nb2O5–4.5B2O3 Glass‐Ceramics

The temperature (300–973 K) and frequency (100 Hz–10 MHz) response of the dielectric and impedance characteristics of 2BaO‐0.5Na₂O–2.5Nb₂O₅–4.5B₂O₃ glasses and glass nanocrystal composites were studied. The dielectric constant of the glass was found to be almost independent of frequency (100 Hz–10 MHz) and temperature (300–600 K). The temperature coefficient of dielectric constant was 8 ± 3 ppm/K in the 300–600 K temperature range. The relaxation and conduction phenomena were rationalized using modulus formalism and universal AC conductivity exponential power law, respectively. The observed relaxation behavior was found to be thermally activated. The complex impedance data were fitted using the least square method. Dispersion of Barium Sodium Niobate (BNN) phase at nanoscale in a glass matrix resulted in the formation of space charge around crystal‐glass interface, leading to a high value of effective dielectric constant especially for the samples heat‐treated at higher temperatures. The fabricated glass nanocrystal composites exhibited P versus E hysteresis loops at room temperature and the remnant polarization (P_r) increased with the increase in crystallite size.     

Dielectric relaxation and electrical conductivity in Ca5Nb4TiO17 ceramics

The temperature dependence of dielectric properties and electrical conduction of Ca₅Nb₄TiO₁₇ ceramics were characterized in a broad temperature range. A dielectric anomaly with strong frequency dispersion was detected in the temperature range 700–1010 °C. This dielectric relaxation could be almost removed completely by annealing in an oxidizing atmosphere. Complex impedance analysis confirmed the electrical inhomogeneity of the ceramics with different contributions from the bulk and grain boundaries. This suggests that the main mechanism for the observed relaxation is the Maxwell–Wagner polarization. ac conductivity results revealed the variation of conduction mechanism with increasing temperatures from localized hopping to long-range motion of the doubly ionized oxygen vacancies.     

Impedance spectroscopy and conduction mechanism of magnetoelectric hexaferrite BaFe10.2Sc1.8O19

The dielectric relaxation and conduction properties of hexaferrite BaFe_10.2Sc_1.8O₁₉ (BFSO) ceramics have been investigated by impedance spectroscopy (1 Hz-2 GHz) at various temperatures (253-473 K). The frequency dependence of impedance and modulus spectra of BFSO shows that its dielectric responses are thermally activated. The scaling behaviors of impedance spectra indicate that the distribution of dielectric relaxation times in BFSO is temperature independent. The frequency-dependent conductivity spectra follow the universal-power-law at high temperatures but deviate slightly at low temperatures. An enormous increase in relaxation/conduction activation energies of BFSO above 413 K is also observed in both impedance and conductivity spectra. This indicates that at high temperatures, relaxation/conduction processes may be contributed mainly by the movable oxygen vacancies, whereas at low-temperature electron hopping dominates. The conductivity fitting results further suggest that electron/oxygen vacancy-related small polaron hopping should be the most probable conduction mechanism for BFSO.     

Effect of substitution of La3+ on structural and electrical properties of Sr(Fe0.5Nb0.5)O3 ceramic

In this work, we have mainly reported the effect of lanthanum substitution on structural, dielectric, impedance and transport properties of strontium iron niobate (i.e., Sr_1-xLa_x(Fe_0.5Nb_0.5)_1-x/4O₃ (x = 0, 0.05, 0.1, 0.15, 0.2)). The materials were synthesized using standard ceramic technology. The preliminary structural analysis was done by using the room temperature X-ray diffraction data. The samples of higher concentrations (x = 0.15 and x = 0.20) show the development of an additional phase (i.e., LaNbO₄ and Sr₃La₄O₉). Studies of frequency and temperature dependence of dielectric parameters exhibit an anomaly and relaxor behavior in the compounds. The electrical impedance and modulus analysis of frequency and temperature-dependent data show the contributions of grains and grain boundaries in the resistive and capacitive properties of the compounds. The study of transport properties of AC conductivity has provided the conduction and relaxation mechanism. The substitution of La³⁺ has significantly changed the dielectric constant, tangent loss, and transport properties of the material.     

Physical insights into the grain size effect on the electrical properties of nanocrystalline La2Zr2O7 pyrochlores

The role of grain size on the electrical properties of nanocrystalline La₂Zr₂O₇ (LZO) ceramics prepared by chemical co-precipitation is reported. Grain size from 10 to 70 nm was obtained. The XRD and Raman scattering showed a pyrochlore structure of La₂Zr₂O₇. Morphology and chemical analysis were done by SEM and XPS, respectively. Electrical conductivity studies using impedance spectroscopy resolved the conductivity borne to grain and grain boundary with a non-Debye nature of conduction. The activation energy (E_a) for electrical conduction through grain was almost the same for all the grain sizes and equal to 1.20 ± 0.03 eV. However, the E_a of the grain boundary had decreased from 1.17 eV to 0.78 eV with an increase in grain size, and the conduction is due to oxygen ion migration. The conduction mechanism was governed by the non-overlapping small polaron tunneling model. Additionally, ion-ion correlation in conduction is increased with increasing grain size.     

Enhanced magnetic and dielectric properties of Ti‐doped YFeO3 ceramics

Polycrystalline YFeO₃ (YFO) and YFe_1?(4/3)xTi_xO₃(YFTO) ceramics were prepared using the powder synthesized from the sol‐gel route. X‐ray diffraction analyses of the polycrystalline ceramics revealed the crystallization of the phase in orthorhombic crystal structure associated with the space group Pnma. The magnetization versus magnetic field hysteresis loops were obtained at room temperature for YFO and YFTO ceramics. The magnetic property changes from weak ferromagnetic in YFO to ferromagnetic in YFTO ceramics. The dielectric constant recorded at room temperature for YFTO ceramics was six times higher than that of YFO, whereas the dielectric loss gets reduced to 0.06 from 0.3 for YFO at 1 kHz. Impedance spectroscopy study carried out on YFO and YFTO ceramics confirmed the existence of non‐Debye‐type relaxation. Observed single semicircle in Z′ vs ?Z′′ plot established the incidence of intrinsic (bulk) effect and ruled out any grain boundary or electrode effects. The mechanism for the dielectric relaxation and electrical conduction process observed in YFO and YFTO ceramics was discussed by invoking electric modulus formalisms. Activation energy obtained by ac conductivity study suggested that the conduction process in YFO was linked up with the existence of the polaron and oxygen vacancies, whereas only oxygen vacancies contribute to the conduction process in YFTO ceramics.     

Study of structural,dielectric and electrical conduction behaviour of Gd substituted CaCu3Ti4O12 ceramics

Effect of Gd on microstructural, dielectric and electrical properties has been studied over wide temperature (300–500 K) and frequency range (100 Hz–1 MHz). Gd substitution in CCTO system results in decrease in the grain size and increase of Schottky potential barrier which causes lower value of dielectric constant. The dielectric constant remains nearly constant in temperature range 300–350 K. Doped samples show lower dielectric loss in middle frequency range (～10 kHz–1 MHz) at room temperature. The AC conductivity (σ_ac) obeys a power law, σ_ac = Afⁿ, where n is temperature dependent frequency exponent. The AC conductivity behaviour can be divided into three regions depending on conduction processes and the relevant charge transport mechanisms have been discussed.     

Transport properties of BaCe0.85Y0.15O3−δ at closed-cycle refrigerator temperature

The electrical conducting properties of both hydrated and dehydrated BaCe_0.85Y_0.15O_3?δ (barium cerate, BCY) were investigated at low temperature (473–203 K) by an AC impedance analyzer combined with a dielectric interface. For the BCY, the bulk and grain boundary conductivities were separated with the equivalent circuit model, and the bulk conductivity was approximately two orders of magnitude higher than the grain boundary conductivity. At very low temperature (203 K), a single semicircle was obtained in the impedance plot, whereas three distinct semicircles were plotted in modulus plot due to the three different resistance components in the system. The activation energy of bulk conductivity was 0.55 eV and 0.57 eV for the hydrated and dehydrated BCY samples, respectively.     

Dielectric and electric relaxations induced by the complex defect clusters in (Yb+Nb) co‐doped rutile TiO2 ceramics

The effect of the Yb+Nb substitution for Ti on the microstructure, crystal structures, and dielectric properties of (Yb_1/2Nb_1/2)_xTi_1?xO₂ (0.01≤x≤0.1) ceramics is investigated in this study. The results reveal that the solid solubility limit of the (Yb_1/2Nb_1/2)_xTi_1?xO₂ ceramics is x=0.07, and the average grain sizes considerably decrease from 12 μm to 6 μm with x increasing from 0.01 to 0.1. Three types of dielectric relaxations are observed at temperature ranges of 10‐30 K, 80‐180 K, and 260‐300 K, caused by the electron‐pinned defect dipoles, polaron hopping, and interfacial polarizations, respectively. The conduction mechanism changes from nearest‐neighbor‐hopping to polaron hopping mechanism, which is confirmed by ac conductivity measurements. The present work indentifies the correlation between the colossal permittivity and polaron hopping process in the titled compound.     

Impedance and AC Conductivity of GdCr1−xCoxO3 (x = 0, 0.33, 0.5, 0.67 and 1) Perovskites

Perovskite series GdCr_1?xCo_xO₃ (x = 0, 0.33, 0.5, 0.67 and 1) was obtained using a solution combustion method. The powder XRD was used for identification and structural characterization of the obtained perovskites. All compounds crystallize within the space group Pnma. The morphology of samples was studied using SEM. The impedance and AC conductivity of GdCr_1?xCo_xO₃ were studied using impedance spectroscopy in a frequency range from 10 Hz to 10 MHz and in temperature interval 297–337 K. Changes in electric modulus and DC conductivity, with increasing of the value of x in the structures, were observed. The AC conductivity obeyed the universal power law, σ(ω) = σ(0) + Aωⁿ and revealed semiconductor behavior. The calculated activation energies of existing processes varied with the cobalt content and applied frequency. The impedance spectra showed non‐Debye behavior with a distribution of relaxation times for relaxation and conductive processes. The conduction mechanism for pure orthochromite and orthocobaltite was defined and two types of conduction were observed in the investigated temperature range for the complex perovskites. In order to explain the results, an equivalent circuit with fitted values of circuit components was proposed.     

The Lowered Dielectric Loss and Grain‐Boundary Effects in La‐doped Y2/3Cu3Ti4O12 Ceramics

The effects of La concentration on the electrical conductivity and electric modulus of Y_2/3?xLa_xCu₃Ti₄O₁₂ ceramics (0.00 ≦ x ≦ 0.20) were investigated in detail. Proper amount of La substitution in Y_2/3?xLa_xCu₃Ti₄O₁₂ ceramics made the dielectric loss decreased. When x = 0.10, Y_2/3?0.10La_0.10Cu₃Ti₄O₁₂ ceramics exhibited the highest grain‐boundary resistance (0.893 MΩ) and the lowest dielectric loss (about 0.025 at 1 kHz), meanwhile the samples exhibited a relatively high dielectric constant above 6000 over a wide frequency range from 40 Hz to 1 MHz. The decreased dielectric loss was attributed to the enhanced grain‐boundary resistance. With the increase in La concentration, the dielectric relaxation behaviors correlated with the grain‐boundary effects were significantly enhanced. By La doping, the activation energies for the conduction in grain boundaries were slightly depressed, and the activation energies for the relaxation process in grain boundaries were slightly changed. Based on the activation values, it can be concluded that the doubly ionized oxygen vacancies had substantial contribution to the conduction and relaxation behaviors in grain boundaries.     

Electrical and magnetic properties of double perovskite: Y2CoMnO6

In this communication, the structural, micro-structural, dielectric, electrical, magnetic, and leakage-current characteristics of a double perovskite (Y₂CoMnO₆) ceramic material have been reported. The material was synthesized via a high-temperature mixed-oxide route. The compound crystallizes in a monoclinic structure which is confirmed from preliminary X-ray structural study. The morphological study by using scanning electron micrograph reveals the almost homogeneous distribution of grains throughout the surface of the sample. The nature of frequency-dependence of dielectric constant has been described by the Maxwell-Wagner model. The occurrence of a dielectric anomaly in the temperature dependence of dielectric permittivity study demonstrates the ferroelectric-paraelectric phase transition in the material. From the Nyquist plots, we found the existence of both grain and grain boundary effects. The frequency dependence of conductivity was studied by the Jonscher’s Power law, and the conduction phenomenon obeys the large overlapping polaron tunneling model. By using the Arrhenius equation, the activation energy has been calculated which is nearly equal to the energy required for the hoping of the electron. Both impedance and conductivity analysis demonstrate that the sample exhibits negative temperature coefficient of resistance (NTCR) properties indicating the semiconducting type of material at high temperatures. The anti-ferromagnetic character of the material is observed from the nature of magnetic hysteresis loop. The leakage current analysis suggests that the conduction process in the material follows the space charge limited conduction phenomenon. Such material will be helpful for modern electronic devices and spintronic applications.     

Structural,dielectric properties and electrical conduction behaviour of Dy substituted CaCu3Ti4O12 ceramics

Dy substituted CCTO ceramics were synthesized using solid state reaction method. Effect of Dy on structural, microstructural, dielectric and electrical properties has been studied over a wide temperature (300–500 K) and frequency range (100 Hz–1 MHz). Rietveld refinement, carried out on the samples, confirmed single phase formation and indicated overall decrease in lattice constant. Microstructure showed bimodal distribution of grains in CCTO with bigger grains surrounded by smaller grains. Dy substitution reduced grain size. Dy substitution in CCTO reduces the dielectric constant which may be attributed to increase of the Schottky potential barrier. The dielectric constant remains nearly constant in temperature range 300–400 K. The AC conductivity obeys a power law, σ_ac=A fⁿ, where n is the temperature dependent frequency exponent. The AC conductivity behaviour can be divided into three regions, over entire temperature range, depending on conduction processes. The relevant charge transport mechanisms have been discussed.     

Influence of Grain and Grain‐Boundary Resistances on Dielectric Properties of KNbO3 Under Small DC Bias Field

Lead‐free piezoelectric potassium niobate (KNbO₃) system was synthesized by conventional solid‐state ceramic route. Rietveld analysis of X‐ray diffraction data of this system revealed that the sample crystallized in pure orthorhombic perovskite phase at room temperature. SEM micrograph of this system depicted presence of grains having diffuse brick structure with an average grain size of 500 nm. Dielectric properties of KNbO₃ ceramic were investigated under different DC bias voltage in a broad frequency (from 20 Hz to 1 MHz) and temperature (from 200°C to 500°C) ranges in its three crystalline phases. The dielectric constant was found to increase with increasing bias field in all three phases. The loss tangent of this system was found to increase first, and then it becomes constant with increasing bias field. These properties have been explained in terms of variation of grain and grain‐boundary resistances with bias field.     

Differentiated Electric Behaviors of La2/3Cu3Ti4O12 Ceramics Prepared by Different Methods

Microstructure and electric behaviors of La_2/3Cu₃Ti₄O₁₂ (LCTO) ceramics prepared by the sol‐gel method (SG) and solid‐state method (SS) have been systemically investigated. The results indicated that LCTO‐SG ceramics sintered at 1105°C for 15 h showed larger grain size, higher density, and especially higher dielectric constant up to about 0.9–1.6 × 10⁴ at 10²~10⁵ Hz compared to LCTO‐SS ceramics. The higher dielectric constant of the LCTO‐SG ceramics might be due to the stronger internal barrier layer capacitor (IBLC) effect. More notably, compared with LCTO‐SS ceramics, two kinds of dielectric anomalies, one conduction activation energy value and same activation energies for the conduction and relaxation process in LCTO‐SS ceramics, the LCTO‐SG ceramics showed three kinds of dielectric anomalies, two values of conduction activation energy, and decrease in conduction activation energy with increasing temperature. The activation energies for the conduction and relaxation process in LCTO‐SG ceramics showed great difference below about 210°C, suggesting that the mechanism of electrical conduction and dielectric relaxation seem to be different in LCTO‐SG ceramics. These remarkable differences in electric behaviors of LCTO ceramics prepared by sol‐gel and solid‐state methods were firstly found and analyzed.