Dielectric relaxations in fine-grained SrTiO3 ceramics with Cu and Nb co-doping

SrTi_1?3x(Cu_xNb_2x)O₃ (x?=?0.05, 0.1, 0.15 and 0.2) ceramics were synthesized using a solid-state reaction method at 1400?°C in air. Their structures, valence states, conduction mechanisms and dielectric properties were investigated in detail. Fine grains and vibration modes of the samples related to the doping effects were observed. A distorted pseudo-cubic structure was confirmed by XRD and Raman spectroscopy. All ceramics exhibited a colossal dielectric constant and dielectric relaxations. The dielectric constant in the low- and high-frequency ranges for x?=?0.05 and 0.1 was ascribed to the contribution of the grain boundary and grain, respectively, and the electrode polarization was significant at x?=?0.15 and 0.2. The dielectric relaxation peaks obeyed the T^?1/4 law at low temperature for all samples, confirming the polaron relaxation process. The electric modulus analysis confirmed that low-temperature dielectric relaxation was related to the grain response, and the electric conduction exhibited the same behavior with the dielectric relaxation. The variable-range-hopping conduction indicated a highly distorted structure and the localization of carriers in SrTi_1?3x(Cu_xNb_2x)O₃, which was consistent with the XRD and Raman results. The mixed-valence structure of Cu was identified by XPS, and the polaron hopping between the mixed-valence Cu ions was supposed to be responsible for the dielectric relaxation and electric conduction.     

Influence of Na doping on the magnetic properties of LaFeO3 powders and dielectric properties of LaFeO3 ceramics prepared by citric sol-gel method

The emergence of ferromagnetism in perovskite oxide LaFeO₃ nanoparticles and colossal dielectric response in ceramics has inspired researchers to study the effect of various dopants on the magnetic and dielectric properties of LaFeO₃ powders and ceramics. However, the influence of alkali element Na doping has not been studied yet, and the origin of such ferromagnetic behavior is still ambiguous. The primary objective of the present work is to elucidate the effect of Na doping on the magnetic properties of La_1−xNa_xFeO₃ (x=0, 0.1 and 0.2) powders and dielectric properties of corresponding ceramics prepared by citric sol-gel method. FE-SEM results showed that the introduction of Na dopant actually resulted in the formation of nonstoichiometric La_1−xFeO₃ and x/2 Na₂O. Compared to the canted AFM behavior for the pure powder, ferromagnetic behavior with enhanced magnetization of 2.11 emu/g at 10 kOe could be obviously observed at room temperature for the powder with x=0.2. XPS measurement suggested nonstoichiometric Fe/La ratio which leads to the distortion of lattice structure and enlarged canting angle between the two AFM coupled Fe sublattice should be responsible for the enhancement of magnetization in the Na-doped samples. Meanwhile, the introduction of Na dopant lowered the growth temperature of grains of the parent LFO and resulted in larger average grain size, which in turn leaded to great enhancement of ε′ into the order of 10⁵ at 100 Hz at the cost of high tanδ for the Na-doped ceramics.     

Giant dielectric response and mixed-valent structure in the layered-ordered double-perovskite ceramics

Origin of giant dielectric response in the layered-ordered double-perovskite La₂CuSnO₆ ceramics was investigated in the present work. Apart from the main layered-ordered double-perovskite La₂CuSnO₆ phase, minor La₂Sn₂O₇ secondary phase was detected. There were two dielectric relaxations in the curve of temperature dependence of dielectric properties of La₂CuSnO₆ ceramics. Both of dielectric relaxations were thermal activated process. The low temperature relaxation should be attributed to the mixed-valent structure (Cu²⁺/Cu³⁺) since the activation energy was similar to that of La₂CuTiO₆ ceramics. While the high temperature relaxation was closely related to the thermal activated hopping process of electrical conduction. After annealing the sample in N₂ and O₂ atmosphere, the trend of dielectric behaviors and ac conductivities were the same at high temperature. This result confirmed that the high temperature relaxation was closely related to the electrical conduction.     

Achieving significantly enhanced piezoelectricity in aurivillius ceramics by improving initial polarization and dielectric breakdown strength

In this work, the neglected role of the dielectric breakdown strength (BDS) in piezoelectricity of bismuth layer-structured ferroelectrics (BLSFs) is revealed, enhanced initial polarization and BDS work together to improve the piezoelectricity of CaBi₄Ti₄O₁₅ (CBT) ceramics via Li/Bi co-substitution. The experimental work, first-principles calculations and finite element simulation are carried out to investigate the effect of Li/Bi co-substitution in depth. The stronger spontaneous polarization (P_s) from the lattice distortion and the more favorable domain switching from the improved grain growth both contribute to an enhanced initial polarization. Besides, the changed grain-scale microstructure improves the hardness and inhibits the local discharge, the higher resistivity related to strong defect dipoles suppresses the heat generation during the poling process, all resulting in an increase of BDS by about 100%, and allowing more domains to align. This work demonstrates an effective strategy to develop BLSFs with enhanced piezoelectricity for high temperature piezoelectric applications.     

Compositional‐driven multiferroic properties in samarium substituted LaFeO3‐PbTiO3 solid solutions

Samarium‐substituted LaFeO₃‐PbTiO₃ solid solutions Pb_1?xLa_x/2Sm_x/2Ti_1?xFe_xO₃ were synthesized within the solid solubility limit 0.10≤x≤0.30. The structural analysis reveals the phase transition from tetragonal to cubic with the variation in composition. Refined lattice and structural parameters vary gradually with composition, and the structural data correlate very well with the microstructural, Raman, and dielectric data. Magnetic ordering in the solid solutions continuously increases with the increase in Fe content. Electric polarization hysteresis loops reveal enhancement in remnant polarization with addition of La/Sm.     

Tailoring the chemical heterogeneity of Mn-modified 0.75BiFeO3-0.25BaTiO3 ceramics for piezoelectric sensor applications

The Mn-modified 0.75BiFeO₃-0.25BaTiO₃ (75BFBTMn) piezoelectric ceramic possesses a high depolarization temperature of 500 °C and a large piezoelectric coefficient of 110 pC/N, showing the potential for high temperature piezoelectric sensors. However, 75BFBTMn ceramic usually suffers dielectric degradation and abrupt drop of piezoelectric coefficient in the range of 300 °C to 500 °C. Combined the high-energy synchrotron X-ray diffraction analysis with Backscatter-SEM results, it is demonstrated that the electrical thermal instability is owing to the existence of chemical inhomogeneity. The Air-annealing treatment is able to decrease the volume fraction of pseudo-cubic phase and the lattice distortion, removes the chemical inhomogeneity in the grain and free Bi₂O₃ at grain boundary, and then eliminates dielectric anomalies and piezoelectric degradation with temperature. These results indicate that air-annealing is a simple but effective method to eliminate the chemical inhomogeneity in 75BFBTMn ceramics, thereby improving the property thermal stability for high temperature piezoelectric sensor applications.     

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.     

Structural and dielectric properties,and nonlinear electrical response of the CaCu3-xZnxTi4O12 ceramics: Experimental and computational studies

CaCu_3-xZn_xTi₄O₁₂ ceramics (x = 0, 0.05, 0.10) were successfully prepared by a conventional solid-state reaction method. Their structural and dielectric properties, and nonlinear electrical response were systematically inspected. The X-ray diffraction results indicated that single-phase CaCu₃Ti₄O₁₂ (JCPDS no. 75–2188) was obtained in all sintered ceramics. Changes in the lattice parameter are well-matched with the computational result, indicating an occupation of Zn²⁺ doping ions at Cu²⁺ sites. The overall tendency shows that the average grain size decreases when x increases. Due to a decrease in overall grain size, the dielectric permittivity of CaCu_3-xZn_xTi₄O₁₂ decreases expressively. Despite a decrease in the dielectric permittivity, it remains at a high level in the doped ceramics (~3,406–11,441). Besides retention in high dielectric permittivity, the dielectric loss tangent of x = 0.05 and 0.10 (~0.074–0.076) is lower than that of x = 0 (~0.227). A reduction in the dielectric loss tangent in the CaCu_3-xZn_xTi₄O₁₂ ceramics is closely associated with the enhanced grain boundary response. Increases in grain boundary resistance, breakdown electric field, and conduction activation energy of grain boundary as a result of Zn²⁺ substitution are shown to play a crucial role in improved grain boundary response. Furthermore, the XPS analysis shows the existence of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺, indicating charge compensation due to the loss of oxygen lattice. Based on all results of this work, enhanced dielectric properties of the Zn-doped CCTO can be explained using the internal barrier layer capacitor model.     

Modified electrical properties of chemical solution deposited epitaxial BiFeO3 thin films by Mn substitution

The effect of Mn substitution on microstructure and electrical properties of epitaxial BiFeO₃ (BFO) thin films grown by an all-solution approach was investigated. Raman analysis reveals that the Mn atoms substitution at Fe sites can result in Jahn-Teller distortion and thus lead to the weakness of long-range ferroelectric order. In addition, the break-down characteristics of BFO thin films are improved with the increase of Mn atoms content, although the leakage current is gradually increased. Meanwhile, the grain size, the dielectric constant and loss are also increased with the increase of Mn content. The P-E hysteresis loops and PUND results demonstrate that the intrinsic ferroelectric polarization is effectively improved with Mn atoms substitution as the grain size increased and Mn atoms play a role of nucleation sites. However, the ferroelectric properties are deteriorated with the excess substituted Mn content due to the higher leakage current.     

High dielectric constant of NiFe2O4–LaFeO3 nanocomposite: Interfacial conduction and dielectric loss

Materials exhibiting colossal dielectric constant are the most sought-after materials due to their variety of applications in various electronics industries. NiFe₂O₄ and LaFeO₃ belonging to the spinel and perovskite structures, respectively, were coupled into a nanocomposite by adapting a one-pot sol-gel synthesis. The ratio of NiFe₂O₄:LaFeO₃ was varied and the synthesized materials were studied for their dielectric behaviors. Interestingly, among the samples studied, the nanocomposite with the ratio of 1:2 of NiFe₂O₄–LaFeO₃ exhibited a high dielectric constant value of 10390 at a frequency of 1 kHz with a several-fold increase in conductivity. The high conductivity resulted in a high dielectric loss. The origin of such a high dielectric constant and loss have been attributed to the Maxwell-Wagner type space charge polarization arising from the microstructure that consists of large and continuous grain boundaries, and the conducting phase at the interface, respectively.     

Structural,dielectric and magnetic behavior of BST modified rare earth ortho-ferrite LaFeO3

This communication reports the effect of rare earth ortho-ferrite LaFeO₃ on the structural, micro-structural, dielectric, electrical, conduction and magnetic behaviors of BST i.e. [(LaBa_0.5Sr_0.5)_0.5(FeTi)_0.5O₃]. After successful preparation of sample in desired conditions by the conventional solid state route, its single phase formation in orthorhombic symmetry have been analyzed and confirmed by X-ray diffraction technique. The compound shows improved optical, dielectric and magnetic behavior. The dielectric and electrical characteristics of the sample has been studied in different conditions of temperature (25-500 °C) and frequency (1-1000 kHz). A thermally activated transport characteristic evidences the semiconductor nature of the sample. Room temperature magnetic hysteresis confirms the presence of sizeable ferromagnetism in the studied compound. Based on other interesting results, this material could be considered favorable candidate for electronic device applications.     

Effect of Ba2+ substitution on the structural and electrical properties of SrBi4Ti4O15 ceramic

We studied the effect of Barium doping on the structural and electrical properties of SrBi₄Ti₄O₁₅ (SBBT) ferroelectric ceramic. The samples were synthesized by the conventional solid-state reaction method. X-ray diffraction (XRD) and Raman scattering techniques have been employed to characterize the structural property. Scanning electron microscope images showed plate like grain morphology with random orientation of platelets. Lower Curie temperature and enhanced dielectric constant at the transition temperature were clearly observed with increasing the concentration of Ba²⁺ ion. Detailed studies on the effect of barium on electrical behavior of the SBBT systems have been carried out by the non destructive complex impedance spectroscopy (CIS technique). The Nyquist plots suggest that the grain and grain boundary were responsible in the conduction mechanism of the materials. Ferroelectric measurements revealed that Ba²⁺ doping leads to reduction in the remnant polarization. The piezoelectric coefficients (d₃₃) of the ceramics were enhanced with Ba²⁺ doping.     

Ferroelectric phase transition and electrical conduction mechanisms in high Curie-temperature PMN-PHT piezoelectric ceramics

Ferroelectric phase transition characteristic and electrical conduction mechanism of the high Curie-point (T_C) 0.15Pb(Mg_1/3Nb_2/3)O₃−0.4PbHfO₃−0.45PbTiO₃ (PMN-PHT) piezoelectric ceramics were studied by the temperature dependent Raman spectra and electrical properties. Sole first-order ferroelectric phase transition is demonstrated by the thermal hysteresis behavior of the temperature dependent dielectric constant and the dramatic drop of the derivative of inverse dielectric constant ξ= d(1/ε_r)/dT around T_C in the PMN-PHT ceramics. The temperature dependent Raman spectroscopy not only provides further evidence for the ferroelectric to paraelectric phase transition appearing around T_C in the PMN-PHT ceramics, but also reveals the successive phase symmetry changes of the polar nanoregions (PNRs), in which apparent anomalies appear in the Raman peaks' wavenumber, wavenumber distance, intensity, intensity ratio, and line width of some selected Raman modes upon heating. Typical sole cole-cole circle is obtained for the PMN-PHT ceramics in the temperature range of 440–560 °C, based on which the activation energy (E_a) of the electrical conduction is calculated being ~1.2 eV. Such low value of E_a indicates that the oxygen vacancies formed in the PHT-PMN ceramics induced by the evaporation of Pb during the sintering process dominate the high-temperature extrinsic electrical conduction.     

Structural,magnetic, vibrational and impedance properties of Pr and Ti codoped BiFeO3 multiferroic ceramics

Single-phase (Bi_1−xPr_x)(Fe_1−xTi_x)O₃ ceramics (x=0.03, 0.06, and 0.10 as BPFT-3, BPFT-6 and BPFT-10, respectively) were synthesized by conventional solid state reaction method. The effect of varying Pr and Ti codoping concentration on the structural, magnetic, dielectric and optical properties of the BPFT ceramics have been investigated. X-ray diffraction indicated pure rhombohedral phase formation for BPFT-3 and BPFT-6 ceramics, however, a structural phase transition from a rhombohedral to an orthorhombic phase has been observed for BPFT-10 ceramic. The maximum remnant magnetization of 0.1824 emu/g has been observed in BPFT-6. With increasing codoping concentration the room temperature dielectric measurements showed enhancement in dielectric properties with reduced dielectric loss. UV–vis diffuse reflectance spectra demonstrated the strong absorption of light in the visible region for a band gap variation 2.31–2.34 eV. Infrared spectroscopy indicated the shifting of Bi/Pr–O and Fe/Ti–O bonds vibrations and change in Fe/Ti–O bond lengths. Decrease in the conductivity on increasing Pr and Ti concentration in BFO is attributed to an enhancement in the barrier properties leading to suppression of lattice conduction path arising due to lattice distortion as confirmed from impedance analysis.     

Enhanced electrical transport properties of Cr-substituted Mg2–Y-type hexaferrites for power applications

The impact of Cr³⁺ substitution on the electrical and dielectric properties of Y-type hexaferrites with composition Ba₂Mg₂Cr_xFe_12-xO₂₂ (x = 0.0, 0.5, 1.0, 1.5, and 2.0) was synthesized by the sol-gel auto combustion technique. The lattice parameters 'a' and 'c' slightly increase with the substitution content of Cr³⁺, and some other physical parameters, including porosity, microstrain, dislocation density, specific surface area, and stacking fault coefficient, were determined from XRD data. FTIR spectra confirm the formation of iron oxide base material by the appearance of three signature bands at precisely 429, 474, and 499 cm^-1 due to Fe–O vibrations at octahedral and tetrahedral sites. Substituting Cr³⁺ at the octahedral site enhanced the room temperature DC electrical resistivity from 6.89 × 10⁹ to 2.27 × 10¹⁰ Ω-cm. Temperature-dependent DC electrical resistivity exhibits semiconducting behavior. There is a direct relation between resistivity and activation energy. The dielectric behavior of Cr³⁺ substituted samples in the frequency range of 1 MHz–6 GHz was understood based on the conduction mechanism through the hopping of electrons between Fe³⁺ and Fe²⁺ ions and the Maxwell-Wagner Model. In impedance spectroscopy studied using Cole-Cole plots, most dielectric response arises from the contribution of the grain boundary effect. With the substitution of Cr^3+, dielectric losses decreased. A very high Q-value around 1 GHz was observed, suggesting that these hexaferrites are efficient materials for many high-frequency applications, such as multi-layer chip inductors (MLCI), dielectric resonators, and power applications.     

Structural and impedance analysis of Bi0.5Na0.5Ti0.80Mn0.20O3 ceramics

The Bi_0.5Na_0.5Ti_0.80Mn_0.20O₃ ceramic was synthesized by a conventional solid-state reaction technique. Rietveld refinement of X-ray diffraction data confirms the rhombohedral crystal structure of the compound with R3c space group. The optical band gap energy of the compound is found to be 1.93 eV. The substitution of 20% Mn ions at the Ti sites results in the improved dielectric characteristics and a shift in the ferroelectric to paraelectric electric phase transition peak from 330 °C to 370 °C in the material. The frequency dispersion of dielectric constant and its footprint in the Nyquist and Cole-Cole plots have been analyzed. The analysis of complex impedance and modulus spectroscopy confirms the non-Debye type of relaxation mechanisms in the material with contributions from both the grain and grain boundary to the electrical properties. The frequency dependence of AC conductivity data exhibits overlapping large polaron tunneling conduction mechanism in the compound.     

Dielectric and electrical response of hydroxyapatite – Na0.5K0.5NbO3 bioceramic composite

In recent years, the development of electrically active materials for orthopedic implant applications attracted attention owing to the fact that inherent electricity of bone mediates it's various metabolic processes. In this perspective, the present work investigates the effect of incorporation of varying amounts of piezoelectric biocompatible Na_0.5K_0.5NbO₃ (NKN) on dielectric and electrical properties of hydroxyapatite (HA) over a wide range of temperature (30–500?°C) and frequency (1?Hz?–?1?MHz). The composites HA-x NKN (x?=?10–30?wt%) were synthesized by solid state ceramic method and optimally sintered at 1075?°C for 2?h. X-Ray Diffraction (XRD) and Fourier Transform Infrared (FTIR) patterns confirmed the formation of phase pure HA and NKN in the composite system. The dielectric response of the samples has been compared with that of the existing theoretical models. The dielectric measurement suggests that the space charge as well as dipolar polarization is the dominant polarization mechanisms. The complex plane impedance and modulus spectroscopic analyses were performed to reveal the conduction mechanism. The activation energies for grain and grain boundary resistance for HA- (10–30?wt%) NKN were 1.03, 1.464, 1.28 and 1.34, 1.56, 1.30?eV, respectively. These results suggest that ionic conduction is the dominant conduction mechanism. Hydroxyl ions and oxygen vacancies are observed to be responsible for the conduction in HA-xNKN composite system. Overall, HA- x NKN composite system can be suggested as a potential material for electro-active orthopedic implant application.     

Dielectric behavior dependence on temperature and Cr-doping contents of Aurivillius Bi5Ti3FeO15 ceramics

The dielectric behavior of Cr-doped Bi₅Ti₃FeO₁₅ (BTFCO) ceramics was systemically studied by temperature-dependent dielectric/impedance spectroscopy from 200 K to 400 K. Two dielectric relaxation processes were found in grain interiors of the BTFCO ceramics. Both of them showed similar activation energy. This usually brings a great challenge to discern the different mechanisms of the conductivity and/or dielectric responses in Fe–Cr-based materials; nevertheless the origins of the two relaxations were unambiguously determined by adjusting the Cr doping contents. One relaxation at high temperature region was proposed to be associated with the localized electron hopping between Fe³⁺ and Fe²⁺, whereas another one at lower temperatures was assigned to the localized hole transfer between Cr³⁺ and Cr⁶⁺ inside the grains. Moreover, the transition temperature between the two relaxations showed a nearly linear reducing trend with the Cr-doping content.     

Anisotropic Dielectric and Electrical Properties of Hot-Forged SrBi4Ti4O15 Ceramics

The effect of grain orientation on the dielectric and electrical properties of SrBi₄Ti₄O₁₅ ceramics are studied by impedance and modulus spectroscopy. The degree of orientation calculated from the X-ray diffraction pattern is found to be 94.2% along the c-axis of the crystal structure. The ratio of permittivity along the perpendicular to parallel direction is found to be ∼2.5 at the Curie temperature. The nonsuperimposition of the normalized Z″ and M″ versus frequency plot revealed that the conduction is localized and deviate from ideal Debye-like behavior. The conduction mechanism has been explained on the basis of jump relaxation model.