Maxwell-Wagner Mechanism Research Articles

Synergistic insights to ion-doped hydroxyapatite: bridging XRD, electrical impedance, dielectric characteristics, and antibacterial properties

ABSTRACT In this work, ion-doped hydroxyapatite materials were synthesized using the sol-gel technique. XRD analysis confirmed the purity of the products, with no secondary peaks observed. Doping with manganese, lithium, potassium, and zinc ions resulted in a crystalline size of 47–48 nm. Fourier transform infrared spectroscopy validated the structural bands, while scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed nano-rod-like particles with sizes ranging from 30 to 80 nm in length and 13 to 40 nm in width. Impedance spectroscopy and dielectric relaxation studies showed electrical behaviour following the Maxwell-Wagner mechanism and non-Debye type relaxation. The materials exhibited strong antibacterial properties, making them promising for medical implant applications.

La3+ substitution impact on structural, magnetic, and electrical properties of CoFe2O4 synthesized via sucrose auto-combustion

Lanthanum (La)-substituted cobalt ferrites CoFe2-x La x O4 (x = 0.00–0.09) were successfully prepared using the sucrose sol-gel auto-combustion route. La-substitution effects on the structural, magnetic, and electrical/dielectric characteristics were investigated using different techniques. X-ray diffraction (XRD) results displayed a deviation from the single-phase structure at La-content ≥ 0.04 because of the formation of orthorhombic LaFeO3 at the boundaries of the grains. Both XRD and Fourier transform infrared (FT-IR) measurements showed that the crystal structure had not been significantly affected by La-substitution attributed to the crystallization out of this LaFeO3. Vibrating sample magnetometry (VSM) measurements revealed gradual decreases in the magnetization as the La-content increases, which could be referred to as the decrease in the super-exchange interaction in the octahedral sites because of the preferential substitution of large ionic radius La3+ ions in the octahedral positions. As the antiferromagnetic LaFeO3 formed, a severe reduction in the magnetization appeared. The increase in the magnetization at 0.09 was attributed to the cationic redistribution among sublattices. On the other hand, the coercivity data indicated the hard magnetic characteristics of all the samples. The electrical conductivity results showed the semiconducting character of all samples with obvious decreases with increasing La-content. According to Verwey’s hopping mechanism, these decreases were attributed to the preferential occupation of La3+ by octahedral positions. Dielectric results versus temperature indicated anomalies relaxations around 460 K with the successive addition of lanthanum attributed to the electrical inhomogeneity that occurred due to oxygen vacancy created or the Maxwell-Wagner mechanism.

Structural and electrical characteristics of manganese modified Bi0.5K0.5TiO3 ceramic

The Rietveld analysis of X-ray diffraction spectrum confirms the tetragonal (P4bm) symmetry of the manganese doped Bi0.5K0.5TiO3 (BKT) compound synthesized by the solid-state reaction route. The dielectric analysis of doping of 5% manganese (Mn) ions at the Ti-sites in the host BKT is analyzed on the basis of the Maxwell–Wagner mechanism in the material. The P-E loop indicates the ferroelectric nature of the compound. Correlated barrier hopping is found to be the dominant conduction mechanism in the compound. The leakage current confirms the negative temperature coefficient of resistance nature of the compound and the Ohmic electrode contact. The substitutions of smaller Mn ion with multiple valencies at the Ti site enhances the dielectric constant substantially, decreases the loss tangent, and lower the leakage current. These features are applicable for energy storage devices, memory devices, high-temperature sensors, and high-temperature capacitors.

Electrocatalytic and Enhanced Photocatalytic Applications of Sodium Niobate Nanoparticles Developed by Citrate Precursor Route

Development of cost effective and efficient electrocatalysts is crucial to generate H2 as an alternative source of energy. However, expensive noble metal based electrocatalysts show best electrocatalytic performances which acts as main bottle-neck for commercial application. Therefore, non-precious electrocatalysts have become important for hydrogen and oxygen evolution reactions. Herein, we report the synthesis of high surface area (35 m2/g) sodium niobate nanoparticles by citrate precursor method. These nanoparticles were characterized by different techniques like X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. Electrocatalytic properties of cost-effective sodium niobate nanoparticles were investigated for HER and OER in 0.5 M KOH electrolyte using Ag/AgCl as reference electrode. The sodium niobate electrode showed significant current density for both OER (≈2.7 mA/cm2) and HER (≈0.7 mA/cm2) with onset potential of 0.9 V for OER and 0.6 V for HER. As-prepared sodium niobate nanoparticles show enhanced photocatalytic property (86% removal) towards the degradation of rose Bengal dye. Dielectric behaviour at different sintering temperatures was explained by Koop’s theory and Maxwell-Wagner mechanism. The dielectric constants of 41 and 38.5 and the dielectric losses of 0.04 and 0.025 were observed for the samples sintered at 500 °C and 700 °C, respectively at 500 kHz. Conductivity of the samples was understood by using power law fit.

Structural and electrical properties of Gd-doped BiFeO3:BaTiO3 (3:2) multiferroic ceramic materials

Gadolinium doped BiFeO3:BaTiO3 (3:2) polycrystalline multiferroic ceramics have been prepared by high-temperature solid state reaction technique. X-ray diffraction (XRD) analysis at room temperature of the prepared materials confirmed the formation of the compounds with rhombohedral crystal structure. The average particle size of as prepared samples have been found in the range of 35 nm to 55 nm for different doping concentrations. The average grain size of as prepared samples are less than 100 nm which is confirmed from SEM study. The SEM of annealed compounds showed the uniform distribution of grains and the formation of dense ceramic with average grain size in the order of 4 µm. Dielectric studies of the materials reveals that the dielectric constant ( $${\varepsilon _r}$$ ) and tangent loss (tan δ) decreases with doping concentrations at room temperature. The variation of $${\varepsilon _r}$$ and tan δ with temperature was explained on the basis of Maxwell–Wagner mechanism. The values of grain resistance ( $${R_b}$$ ) and grain capacitance (Cb) were obtained from Nyquist plots for the different doping concentrations at 300 °C. The activation energy ( $${E_a}$$ ) was calculated from the curve of frequency dependent ac conductivity ( $${\sigma _{ac}}$$ ) within the range 0.19 eV to 0.45 eV. The remnant polarization of the samples (0.53 µC/cm2) was measured from polarization versus electric field (P–E) hysteresis curves. The ferromagnetic behaviour of the Gd-doped BiFeO3:BaTiO3 (3:2) sample has been studied by SQUID for the lowest doping concentration. The value of remnant magnetization was found 0.0235 emu/g at room temperature.

Structural and dielectric properties of multiferroic composite system Ba0.5Sr0.5TiO3- Ni0.4Co0.2Zn0.4Fe2O4

ABSTRACTIn this paper, structural and dielectric properties of multiferroic composite system (1-x) Ba0.5Sr0.5TiO3 (BST) – (x) Ni0.4Co0.2Zn0.4Fe2O4 (NCZF) x = 0.1, 0.2, 0.3, 0.4 molar ratios have been studied. X-ray diffraction analysis of composite samples confirmed the presence of both the phases. Density measurements result indicate that composite samples although possess lower density in comparison to both the pure phases but density of the composite samples increases with NCZF content. Dielectric constant and loss of the composite samples decreases with increase in frequency due to Maxwell-Wagner relaxation mechanism. Dielectric constant of the composite samples firstly increases with increase in NCZF content followed by a decrease.

Structural, Magnetic and Electrical Properties of Barium Titanate and Magnesium Ferrite Composites

Structural, magnetic and electrical characteristics of multiferroics (1-x)BaTiO3-xMgFe2O4 composites with weight fractions of x = 0.3, 0.5 and 0.7 are reported. MgFe2O4 powders were prepared using sol-gel auto combustion technique. It was combined with commercial BaTiO3 to form composites by using wet milling solid state reaction technique. Formation of tetragonal perovskite for the ferroelectric BaTiO3 and cubic spinel for the ferrimagnetic MgFe2O4 phases, were identified from the XRD pattern. The average grain size for each composite was about 0.5 μm. The M-H loop showed soft ferrimagnetic properties due to the presence of MgFe2O4 in the composites. The increment of the MgFe2O4 weight fraction increased the saturation magnetization and slightly changed the coercive field. The complex impedance plot can be represented by a parallel R and C circuit. Composite sample with x = 0.5 has the highest resistance with lowest capacitance and dielectric constant value at room temperature. The dielectric constant showed a very strong dispersion at low frequencies, due to the Maxwell-Wagner mechanism and a slight dispersion at higher frequencies. Based on the results obtained, all of the composite samples exhibited high dielectric constant and tangent loss at the low frequency range.

Influence of CoO Nanoparticles on Properties of Barium Zirconium Titanate Ceramics

Composites of Ba(Zr0.07Ti0.93)O3 ceramic and CoO nanoparticles (at 1.0 vol.% to 3.0 vol.%) have been fabricated to investigate the effects of the CoO nanoparticles on the properties of the composites. X-ray diffraction data revealed that the modified samples contained Ba(Zr0.07Ti0.93)O3 and CoO phases. Addition of CoO nanoparticles improved the magnetic behavior and resulted in slight changes in ferroelectric properties. The composites showed a magnetoelectric effect in which the negative value of the magnetocapacitance increased with increasing CoO concentration. Examination of the dielectric spectra showed that the two phase-transition temperatures as observed for unmodified Ba(Zr0.07Ti0.93)O3 merged into a single phase-transition temperature for the composite samples. The composite samples also showed broad relative permittivity versus temperature (er–T) curves with frequency dispersion. This dielectric behavior can be explained in terms of the Maxwell–Wagner mechanism. In addition, the Vickers hardness (Hv) value of the samples increased with increasing CoO content.

Impact of Cu2O doping on high dielectric properties of CuO ceramics

Single-phase CuO ceramic samples were prepared with the starting nano powders of CuO + xCu2O (x = 0, 1, 3, 7% in mole ratio) via solid state reaction method, and characterized by X-ray diffraction, Raman scattering and scanning electron microscopy. For all the samples, the temperature dependences of dielectric constants and losses were measured at the frequencies of 102, 103, 104, 105 and 106 Hz, respectively. With increasing doping content of Cu2O, a strong correlation is demonstrated at given temperature and frequency between the measured dielectric constants and the unit-cell volumes of CuO. The strong correlation is argued in terms of the change in densities of CuO defects (e.g. Cu/O vacancies and/or interstitial Cu impurities) due to Cu2O doping, which is supported by the formation energies of CuO defects and the corresponding unit-cell volume from first-principles calculations. The high dielectric constant (∼103–105) of CuO ceramic is therefore attributed to the reduction in resistance due to Cu/O defects in the grain by Maxwell-Wagner mechanism.

Study of the anisotropy of the dielectric response of Na1/2Bi1/2TiO3 relaxor ferroelectric

The dielectric response, conductivity, and domain structure of (Na1/2Bi1/2)TiO3 single crystals are studied in the temperature range of 290–750 K for the [100], [110], and [111] crystallographic directions. It is shown that the region of optical isotropization is observed in polarized light in the temperature range of 570–620 K. In this case, the birefringence (Δn) decreases and disappears (together with the image of the domain structure) for the [100] directions. The region of optical isotropization in the [111] directions is characterized by the disappearance of the image of the domain structure and by the existence of individual regions with partial quenching. The domain structure in the [110] directions remains distinguished against the background of a significant decrease in Δn in the indicated temperature range. The region of isotropization is also manifested in the temperature dependence of the imaginary part of the dielectric response and is determined by the isotropic character of the conductivity in the range of 570–620 K. The bulk conductivity has a thermally activated character with activation energies E a = 50−60 meV at T 620 K. The low-frequency dispersion of the dielectric response is determined by the Maxwell–Wagner mechanism and is due to an increase in the ionic conductivity at temperatures above 620 K. The anisotropy of the susceptibility holds in the entire studied ranges of frequencies (25 Hz–1 MHz) and temperatures.

Multiferroic properties of Indian natural ilmenite

In this communication, the main results and analysis of extensive studies of electric and magnetic characteristics (relative dielectric constant, tangent loss, electric polarization, electric transport, impedance, magnetic polarization and magneto-electric coupling coefficient) of Indian natural ilmenite (NI) have been presented. Preliminary structural analysis was studied by Rietveld refinement of room temperature XRD data, which suggests the rhombohedral crystal system of NI. Maxwell–Wagner mechanism was used to explain the nature of the frequency dependence of the relative dielectric constant. The impedance analysis reveals that below 270 °C, only the bulk contributes, whereas at higher temperature, both grain boundary and the bulk contribute to the resistive characteristics of the material. The magnitude of the depression angles of the semicircles in the Nyquist plot has been estimated. The correlated barrier hopping model has been used to explain the frequency dependence of ac conductivity of the material. The activation energy of the compound has been estimated using the temperature dependence of dc conductivity plot. The obtained polarization hysteresis loops manifest improper ferroelectric behavior of NI. The existence M–H hysteresis loop supports anti-ferromagnetism in the studied material. The magneto-electric voltage coupling coefficient is found to be 0.7 mV/cm Oe. Hence, other than dielectric constant, electric polarization, magnetization and magneto-electric studies support the existence of multiferroic properties in NI.

Dielectric properties of Sr1-Ba Fe0.5Nb0.5O3; (x= 0.0, 0.1 and 0.2) ceramics prepared by the molten salt technique and their electrode effects

Abstract In the present work, strontium barium iron niobate (Sr 1- x Ba x Fe 0.5 Nb 0.5 O 3 ; SBFN) ( x = 0.0, 0.1 and 0.2) powders were synthesized via the molten salt technique . Pure phase perovskite powders were obtained at a relatively low calcination temperature of 800 °C which was 400 °C lower than for the solid-state reaction technique (conventional technique). SBFN ceramics were fabricated from the calcined powders and their properties were investigated. XRD data of the ceramics was consistent with an orthorhombic symmetry. All ceramics showed very high dielectric constants where the x = 0.1 ceramic showed an extremely high dielectric constant ( e r ~ 1.98 × 10 5 at 298 °C and 1 kHz, for an Ag electrode) when compared to SFN ceramics prepared by the conventional solid-state reaction technique. The dielectric behavior of the ceramics could be explained in terms of the Maxwell-Wagner mechanism as the ceramics exhibited heterogeneous conduction where an electrode response gave a strong effect in the low frequency and high temperature regions. In addition, the type of electrode (sample-electrode interface) also affected the dielectric constants of the ceramics.

Structural, electrical and magnetic characteristics of improper multiferroic: GdFeO3

Studies of dielectric, impedance, conductivity, magnetic and magneto-electric (ME) properties of GdFeO3 ceramics fabricated by chemical method are reported here. The synthesized powder is phase-pure and crystallizes in the orthorhombic crystal structure. Below 50 °C, the impedance has only grain contribution, while at higher temperatures, it has both grain and grain boundary contributions. Based on the depression angle of the Nyquist plot, the inhomogeneity of the sample is estimated. The capacitance data reveal that at low temperatures, the sample behaves as a leaky capacitor while at higher temperatures the sample shows the effect of the diffusion of thermally excited charge carriers across a barrier. In the low-frequency domain, the dielectric characteristics were explained on the basis of the Maxwell–Wagner mechanism, while in the high-frequency range those were correlated to the grain effect. The frequency dependent characteristic of the tangent loss is explained as a combined contribution from the Debye-like relaxation and dc conductivity related mechanism at higher temperatures. The temperature dependence of the dielectric characteristic and data are found to fit with two Gaussian peaks centered at 148 °C and 169 °C. While the first peak is explained on the basis of the Maxwell–Wagner mechanism, the second has its origin in magnetic reordering and the shifting of Gd3+ ions along the c-axis. The magnetic reordering also results in a sharp decrease of conductivity between 169 °C and 243 °C. The frequency dependent ac conductivity is explained on the basis of the correlated barrier hopping model and the quantum mechanical hopping model for the different frequency domain. The existence of P–E and M–H loops support its improper ferroelectric behavior and canted anti-ferromagnetism respectively. The ME coefficient of the sample is found to be 1.78 mV cm−1 Oe−1.

Structural, dielectric and impedance characteristics of CoTiO3

The present work reports the synthesis and characterization of the ilmenite-type rhombohedral structured CoTiO3 ceramic. The polycrystalline powder of CoTiO3 was prepared by the mixed-oxide technique. X-ray structural analysis of the compound confirmed the formation of a single-phase compound. The study of microstructure by scanning electron microscopy shows that the compound has well defined grains which are distributed uniformly throughout the surface. It shows that the grain size lies in the range of 8–10 μm. The Fourier transform infrared spectroscopy (FTIR) studies confirm the presence of TiO, TiOTi, TiOO, CoO bond in the studied compound. The frequency dependence of dielectric constant was explained on the basis of Maxwell–Wagner mechanism and Koop's phenomenological theory. The frequency-temperature dependence of impedance analysis shows that the bulk effect dominates up to 250 °C, and the appearance of second semicircular arc above 280 °C indicates the presence of grain boundary effect in the sample. The depressed semicircles in Nyquist plot with depression angles clearly indicate the distribution of relaxation times in the ceramic samples. It also shows that the material has negative temperature co-efficient of resistance similar to that of semiconductors. Similar behavior has also been observed in the study of IV characteristics of the material. Through the study of dc conductivity of CoTiO3 and using the relation; ln σdc α Ea/KBT, activation energy of the compound was calculated. The frequency dependence of ac conductivity is explained on the basis of Correlated barrier Hopping Model.

The structural, electrical and magnetoelectric properties of soft-chemically-synthesized SmFeO3 ceramics

The structural, electrical and magnetoelectric properties of SmFeO3 ceramic samples, synthesized using a soft-chemical method, were studied. A structural analysis of the material was carried out by the Rietveld refinement of room temperature x-ray diffraction data. The temperature dependence of the dielectric peaks was analyzed by fitting them with two Gaussian peaks corresponding to two phase transitions—one being electric, and the other being magnetic in nature. The depression angle of the semicircles in a Nyquist plot representing the grain and grain boundary contributions in the sample was estimated. The grain boundary effect, appearing at temperatures above 75 °C, is explained using the Maxwell–Wagner mechanism. The impedance study reveals a semi-conducting grain with an insulating grain boundary, leading to the formation of surface and internal barrier layer capacitors and resulting in a very high dielectric constant. The effect of dc conductivity on the loss tangent at low frequencies and high temperature has been analyzed. The frequency dependence of ac conductivity in the two different regions can be explained on the basis of correlated barrier hopping and quantum mechanical tunneling models. The material is found to exhibit canted antiferromagnetism and improper ferroelectric characteristics. The value of the magnetoelectric voltage-coupling coefficient (α) of a SmFeO3 ceramic is found to be 2.2 mV cm−1 Oe−1.

Anomalous behavior of the dielectric response of quantum paraelectric CaTiO3 with iron impurities

We report on the studies of lattice dynamics and crystal structure of natural perovskite with the general formula CaTiO3:0.01Fe by dielectric spectroscopy and X-ray diffraction in a wide temperature range. In contrast to the “chemically pure” synthetic perovskite, the lattice dynamics of the Fe-containing natural sample exhibits a structural instability in the vicinity of 240 K. Our studies of the behavior of magnetic moment of CaTiO3:0.01Fe demonstrate that in the temperature region from 4.3 to 300 K this material is paramagnetic. No changes in the crystal symmetry have been observed in the structural studies, which can be explained by the small degree of lattice distortions, as has been observed previously for isotopically substituted SrTiO318. Conductivity measurements and analysis of the dielectric response dispersion of CaTiO3:0.01Fe have shown that the dispersion is due to the Maxwell–Wagner mechanism and can be attributed to oxygen vacancies present in the mineral.

Dielectric and impedance spectroscopy of (Ba, Sm)(Ti, Fe)O3 system in the low-medium frequency range

The (Ba, Sm)(Ti, Fe)O3 system, combination of non-ferroelectric samarium ortho-ferrite and ferroelectric barium titanate, was prepared by using a standard high-temperature solid-state reaction technique. Structural analysis of the material using room temperature XRD data confirmed the formation of the single-phase compound. Detailed structural analysis of the system using Rietveld refinement method exhibits the tetragonal structure with polar space group P4mm. Room temperature micrograph of the sample recorded by a scanning electron microscope shows uniform distribution of grains of various dimension and with less voids confirming the formation of high density sample. Studies of dielectric and impedance spectroscopy characteristics of the sample were carried out in the low-frequency (100–1300 mHz) and the high-frequency region (100 Hz–1 MHz) at different temperatures (25–300 °C). The frequency dependence of dielectric constant of both low and high frequency plots was explained on the basis of Maxwell–Wagner mechanism, Shockley Read statistics and Koops phenomenological theories. Ferroelectric phase transition appears in both the low- and high-frequency regions. Nyquist plots show the contributions of grain, grain boundary and electrode effect. The frequency dependence of conduction mechanisms were explained on the basis of correlated barrier hopping model and overlapping large polaron tunneling models. The P–E hysteresis loop confirms the ferroelectricity in the material.

Modulus spectroscopy of CaCu3Ti4O12 ceramics: clues to the internal barrier layer capacitance mechanism

To date, all existing literature on the so-called ‘high permittivity’ perovskite oxide CaCu3Ti4O12 (CCTO) in the form of ceramics, single crystals and thin films show the grains (bulk) to exhibit semiconductivity with room temperature, RT, resistivity of ∼10–100 Ω cm. Here we show that CCTO grains can be highly resistive with RT resistivity >1 GΩ cm when CCTO ceramics are processed at lower temperature (700 °C). With increasing processing temperature, the semiconducting CCTO phase commonly reported in the literature emerges from grain cores and grows at the expense of the insulating phase. For sintering temperatures of ∼1000–1100 °C, the grains are dominated by the semiconducting phase and the insulating phase exists only as a thin layer grain shell/grain boundary region. This electrical microstructure results in the formation of the so-called Internal Barrier Layer Capacitance (IBLC) or Maxwell–Wagner mechanism that produces the commonly reported high effective permittivity at radio frequencies in dense ceramics. The relationship between Cu loss at elevated processing temperatures and the transformation of the grain resistivity from an insulating to semiconducting state with increasing processing temperature is also discussed.

Dielectric property and relaxation mechanism of CaCu3Ti4O12 ceramic

In this paper, the dielectric property of CaCu3Ti4O12 ceramic is measured by Novocontrol wide band dielectric spectrometer in a temperature range of -100-100 ℃ and frequency range of 0.1 Hz-10 MHz, and the corresponding dielectric relaxation mechanism is discussed. Firstly, on the basis of quantitative analysis of macroscopic shell-core structure, the possibility of colossal dielectric constant (CDC) originating from the surface insulated layer effect is rejected. Secondly, after the analysis of the nature of classical Maxwell-Wagner sandwich polarization and its activation energy, classical Maxwell-Wagner mechanism is also abandoned. Finally, a new model of trapped electron relaxation at the boundary of Schottky barrier is proposed. The new mechanism correctly reflects the essential connection between intrinsic point defects, conductivity and dielectric constant of CaCu3Ti4O12material.

Gate stack dielectric degradation of rare-earth oxides grown on high mobility Ge substrates

We report on the reliability characteristics and their analysis, of rare-earth oxides (REOs) dielectric degradation, when used as interfacial buffer layers together with HfO2 high-k films (REOs/HfO2) on high mobility Ge substrates. Metal-oxide-semiconductor (MOS) devices with these stacks, show dissimilar charge trapping phenomena under varying levels of constant-voltage-stress (CVS) conditions, influencing the measured densities of the interface (Nit) and border (NBT) traps. In the present study, we report on C-Vg hysteresis curves related to both Nit and NBT. We propose a new model based on the Maxwell-Wagner mechanism, and this model explains the current decay transient observed under CVS bias from low to higher fields of MOS gate stack devices grown on Ge substrates. The proposed model is unlike to those used for other MOS devices. Finally, CVS measurements for very long times at moderate fields reveal an initial current decay due to relaxation, followed by charge trapping and generation of stress-induced leakage which eventually lead to hard breakdown.