Impedance spectroscopy of bismuth sodium titanate: Barium titanate ceramics with manganese doping

In this work we use Impedance Spectroscopy (IS) to study lead‐free ceramics of perovskite structure (Bi_1/2Na_1/2TiO₃)_0.925(BaTiO₃)_0.075 with Mn doping of 0, 0.2, 1.0, and 2.0 at.%. We compare our IS results with permittivity results, using dielectric to resistivity transformation equations which allow us to display results either as permittivity or as resistivity. We observe ionic conductivity dominating at lower temperatures, giving way to electronic conductivity at higher temperatures. The permittivity shows deviations from Curie‐Weiss behavior characteristic of relaxor ferroelectrics. The 0.2% Mn samples show the highest ionic resistivity, which will reduce heating for high voltage piezoelectric applications. They also have the highest Curie‐Weiss temperature and Burns temperature.     

Impedance spectroscopy on pH-sensors with lithium lanthanum titanate sensitive material

The ceramic Li_3xLa_2/3−x,_1/3−2xTiO₃ is used, in an all-solid-state configuration, as pH-sensor in aqueous buffer solution. This ceramic displays a pH sensitivity comparable to the one obtained with a commercial glass electrode and does not show any sensitivity to the redox potential of the solution. This is one of the remarkable properties of this ceramic pH-sensor. It is shown in this paper that the reproducibility and the behaviour of the pH-response depend on the morphology of the grain boundaries particularly on the size of the grains. A so-called ‘good’ pH-response is obtained with a ceramic showing big and homogeneous grains. Complex impedance spectroscopy reveals that such ceramic has a high grain boundary resistance. Furthermore, this electrochemical technique allows us to determine that the interface reaction involved in the pH detection has a time constant of the order of the second. Several assumptions such as ion exchange or acid-base reaction are proposed to explain the sensitivity of the ceramic material to the pH of the solution.     

Structural,microwave permittivity,and complex impedance studies of cation (Cr,Bi, Al,In) substituted SrNi-X hexagonal nano-sized ferrites

Sr₂Ni₂T_xFe_28-xO₄₆ (T = Cr³⁺, Bi³⁺, Al³⁺, In³⁺; x = 0.25) hexa-ferrites were fabricated via the sol-gel route. The phase temperature, which was obtained using thermo-gravimetric analysis (TGA), reveals that the X-type hexagonal phase of these materials is highly stable above 1200 °C. The phase formation was confirmed using X-ray diffraction (XRD) patterns. The variation in lattice strain shows the significant effect of substituted cations on the structure of the materials. The crystallite size of these materials was found in the range of 18–19 nm. The increase in dielectric constant, as well as with moderate loss, was observed in all substituted samples between 0.001 GHz and 1 GHz. The large values of dielectric constant were observed for Al³⁺ and In³⁺ substituted materials at higher frequency. The values of slope (n) reveal that all the materials exhibit frequency dependent ac conductivity. The Cole-Cole plots show the single semicircle for all samples. These semicircles of the Cole-Cole plots indicate that resistance inside the material is due to grains and grain boundaries. The both grain and grain boundaries resistance inside the material may cause large values of dielectric constant as well as the losses in the material. The large values of dielectric constant and impedance with moderate dielectric losses of these materials reflect their significant role in the fabrication of electronic devices as a radiation/microwave absorber or high frequency equipment components. Magnetic analysis shows the large values of magnetization and coercivity for Al³⁺ and In³⁺ substituted samples, which reflect that these materials might be beneficial for the absorption of electromagnetic radiation, formation of multilayer chip inductors and recording/storage purpose.     

Impedance spectroscopy of N‐substituted oligo‐oxyethylene polypyrrole films

The electrochemical properties of neutral (dedoped) and oxidized (doped) poly(1,11‐bis(1,1‐pyrrole)‐3,6,9‐trioxaundecane) (poly‐ I ) film electrodes were investigated using cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques. Poly‐ I was deposited on glassy carbon electrode (GCE) from acetonitrile solution containing 5.0 × 10^?3M 1,11‐bis(1,1‐pyrrole)‐3,6,9‐trioxaundecane ( I ) and 0.1M LiClO₄ supporting electrolyte. Doped poly‐ I exhibits a single semicircle in its complex‐capacitance plots, indicating a single dominant ion transport process, together with high capacitance values. These features make this polymer film a candidate for an energy storage material. Also, poly‐ I can be a candidate as a sensory material for the detection of Ag⁺ based on impedance parameters. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Impedance spectroscopy studies of dual membrane fuel cell

This paper reports impedance studies of a dual membrane fuel cell - an innovative design based on the idea for a junction between an oxygen ion conducting cathode/electrolyte part and proton conducting anode/electrolyte part through a mixed oxygen ion and proton conducting porous membrane. Thus oxygen, hydrogen and water are located in three independent chambers. This concept allows avoiding all the severe pitfalls connected to the presence of water at electrodes in both SOFC and PCFC. The performed measurements of the 3 compartments and the data analysis are improved by introducing some specialized approaches and techniques, which are discussed. The impedance contribution in the proof of the new concept is also presented.     

Grain growth in strontium titanate in electric fields: The impact of space-charge on the grain-boundary mobility

This study investigates grain growth in the perovskite oxide strontium titanate in an electric field. The seeded polycrystal technique was chosen as it provides a sensitive and controlled setup to evaluate the impact of different parameters on grain growth due to the well-defined driving force for grain growth. Current blocking electrodes were used to prevent Joule heating. The results show faster grain growth, and thus, higher grain-boundary mobility at the negative electrode. It is argued that the electric field causes point-defect redistribution, resulting in a higher oxygen vacancy concentration at the negative electrode. The local oxygen vacancy concentration is suggested to affect the space-charge potential at the grain boundaries. A thermodynamic treatment of the grain-boundary potential at a grain boundary without field shows that for a high oxygen vacancy concentration less space-charge and less accumulation of cationic defects to the boundary occurs. Therefore, at the negative electrode, a higher oxygen vacancy concentration results in less space-charge and less accumulation of cationic defects. The lower degree of defect accumulation requires less diffusion of segregated defects during grain-boundary migration, so that at the negative electrode faster grain growth is expected, as found in the experiments.     

Enhancement of dielectric properties and energy storage density of bismuth and lithium co-substituted strontium titanate ceramics

Polycrystalline Bismuth and Lithium Co-Substituted Strontium Titanate Sr _(1-x)(Bi,Li)xTiO₃, was prepared using the solid-state method with microwave assisted heating of initial materials. The effect of Bi³⁺ and Li⁺ concentration on the crystal structure, microstructure, and permittivity and energy storage properties of SrTiO₃ ceramics are investigated. The phase and structure have been confirmed by XRD along with Rietveld refinement studies. Morphological investigations have been carried out using FESEM. Frequency and temperature dependence of dielectric permittivity was investigated using impedance spectroscopy. The sample with x?=?0.02 has shown dielectric relaxation behavior. The activation energy of relaxation is found to be 1.2?eV and relaxation time equals 1.12 $\times$ 10^?7 sec. The room temperature P-E loop has been investigated, and the result confirms that there is no signature of the ferroelectric phase in all samples. The energy storage density was theoretically estimated in the present study using a P-E loop. The results showed an astonishing ten-time increase in energy storage density with 8% co-substitution. With increasing x, the grain size steadily decreased, and dielectric breakdown strength increased, yielding a higher energy storage density. The obtained results herald a promising future in the development of electrical capacitors for energy storage applications.     

Impedance and modulus spectroscopy characterization of lead free barium titanate ferroelectric ceramics

In this work, dense BaTiO₃ ceramics were successfully prepared by the solid state reaction process. X-ray diffraction technique showed single-phase polycrystalline sample with perovskite structure. Dielectric behavior and the impedance relaxation were investigated in a wide range of temperature (room temperature (RT) – 40 °C) and frequency (1 kHz–1 MHz). A broad dielectric constant peak was observed over a wide temperature range around the phase transition temperature. The complex impedance plot exhibited two impedance semicircles identified over the frequency range of 1 kHz–1 MHz, which is explained by the grain and grain boundary effects. The presence of non-Debye type of relaxation has been confirmed by the complex impedance analysis. The centers of the impedance semicircles lie below the real axis, which indicates that the impedance response is a Cole–Cole type relaxation.     

Impedance spectroscopy and magneto-dielectric analysis of La3+ substituted Co2Z hexaferrite

Single phase Ba₃Co₂Fe₂₄O₄₁ (Co₂Z) and Ba_2.8La_0.2Co₂Fe₂₄O₄₁ (La-Z) were prepared by solid state ceramic method. The dielectric, impedance and electric modulus were systematically investigated as a function of temperature from 50 °C–250 °C and in the frequency range of 1 Hz–1 MHz. Frequency dependent imaginary impedance and electric modulus implies that La³⁺ substitution significantly affect hopping and relaxation mechanism of charge carriers. Cole-Cole plots (Z" vs Z') has been fitted with suitable equivalent circuit for both Co₂Z & La-Z. The obtained grain and grain boundary resistances confirmed that charge carriers experienced lower resistance in La-Z. Arrhenius plots of relaxation time (τ) vs 1/T showed parallel conduction mechanism in system. Complex electric modulus spectrum distinguished the capacitive contribution of grain, grain boundary and electric effect. A very high magnetodielectric effect at low frequencies has been observed for La-Z.     

Structural,optical, and Raman studies of Gd doped sodium bismuth titanate

The effects of gadolinium (Gd) on lead free sodium bismuth titanate (Na_0.5Bi_0.5TiO₃, NBT) ceramics are investigated. X-ray diffraction (XRD) studies indicate that perovskite phase (rhombohedral, R3c) is formed for all Gd doped NBT (Gd_x:NBT) compositions (x?=?0, 0.02, 0.04, 0.06, 0.08). XRD peak shifts to the higher angles for all compositions except for x?=?0.08. Amphoteric nature of Gd in NBT sites are observed i.e., it occupies Bi-site up to x?=?0.06 and then distributes to Ti-site. Octahedral distortion (c/a) increases in the range 0.02?≤?x?≤?0.06 and then decreases. Raman spectra suggest that the introduction of Gd⁺³ ion induces structural changes without disturbing the long range order. The material can be readily excited using UV (360?nm) and shows emission peaks at ~592?nm and 687?nm. Optical property evaluation indicates that the lowest band gap (E_g = 2.78?eV) is observed at x?=?0.08. When x?>?0.04, the photoluminescence (PL) intensity decreases indicating the onset of concentration quenching. The critical energy distance (found to be 14?Å) and Dexter's theory based analysis indicate that concentration quenching is attributable to multipole-multipole (specifically dipole-dipole) interactions in the system. Commission International de Eclairage (CIE) chromatic color coordinates are reported for all doped systems; the observed patterns mirror PL analysis results. For instance, PL intensity shrinks beyond 4?at%; this corresponds to a regression in the CIE trajectory with respect to concentration.     

Structural and Raman spectroscopic studies of poled lead-free piezoelectric sodium bismuth titanate ceramics

Lead-free piezoelectric sodium bismuth titanate (NBT) ceramics are synthesized by a solid state reaction method. Poled NBT ceramics possess rhombohedral R3c structure. Vibrational phonon frequencies associated to Bi–O, Na–O, TiO₆ octahedra and oxygen vibrations are observed. Experimentally unreported lowest frequency region peak centered at 111 cm⁻¹ is assigned to E(TO1) mode related to Bi–O vibrations. Appearance of lowest frequency mode may be due to well-polished surfaces with reduced Rayleigh scattering background and well-defined ordering of Bi and Na ions in the lattice.     

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.     

Impedance spectroscopy studies of the dissolution of ferrous- and zinc-based materials in aqueous timber preservatives

The relative degradation rates of metallic materials in aqueous, copper-based timber preservatives have been investigated using electrochemical impedance spectroscopy. The metals examined were AS/NZS 1595:1998, grade CA1 mild steel, AISI 316 stainless steel (UNS S31600) and AS 1397, grade Z275 hot-dipped galvanised coil-coated sheet. The electrolytes consisted of commercially sourced copper–chrome–arsenate, copper azole and alkaline copper quaternary electrolytes diluted to a concentration of 0.1 mol dm⁻³ copper. The electrochemical impedance response of each system has been modelled and the resulting relative rates of corrosion have been shown to broadly correspond to general trends taken from previous weight loss and direct current studies incorporating similar electrolytes. The hot-dipped galvanised steel corroded in an active manner at rates that were two to four orders of magnitude greater than that of the ferrous-based materials; the latter appeared to passivate in all instances.     

Microstructural evolution,non‐Ohmic properties,and giant dielectric response in CaCu3Ti4−xGexO12 ceramics

The microstructural evolution, non‐Ohmic properties, and giant dielectric properties of CaCu₃Ti_4?xGe_xO₁₂ ceramics (x=0‐0.10) are systematically investigated. The Rietveld refinement confirms the existence of a pure CaCu₃Ti₄O₁₂ phase in all samples. Significantly enlarged grain sizes of CaCu₃Ti_4?xGe_xO₁₂ ceramics are associated with the liquid phase sintering mechanism. Enhanced dielectric permittivity from 6.90×10⁴ to 1.08×10⁵ can be achieved by increasing Ge⁴⁺ dopant from x=0‐0.10, whereas the loss tangent is remarkably reduced by a factor of ≈10. Non‐Ohmic properties are enhanced by Ge⁴⁺ doping ions. Using impedance and admittance spectroscopies, the underlying mechanisms for the dielectric and nonlinear properties are well described. The improved nonlinear properties and reduced loss tangent are attributed to the enhanced resistance and conduction activation energy of the grain boundaries. The largely enhanced permittivity is closely associated with the enlarged grain sizes and the increase in the Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺ ratios, which are calculated from the X‐ray absorption near‐edge structure.     

Non‐Ohmic Properties and Electrical Responses of Grains and Grain Boundaries of Na1/2Y1/2Cu3Ti4O12 Ceramics

The dielectric and non‐Ohmic properties of Na_1/2Y_1/2Cu₃Ti₄O₁₂ ceramics sintered under various conditions to obtain different microstructures were investigated. Microstructure analysis confirmed the presence of Na, Y, Cu, Ti, and O and these elements were well dispersed in the microstructure. Na_1/2Y_1/2Cu₃Ti₄O₁₂ ceramics exhibited non‐Ohmic characteristics with large nonlinear coefficients of about 5.7–6.6 irrespectively of sintering conditions. The breakdown electric field of fine‐grained ceramic with the mean grain size of ≈1.7 μm (≈5600 V/cm) was much larger than those of the course‐grained ceramics with grain sizes of ≈9.5–10.4 μm (≈1850–2180 V/cm). Through optimization of sintering conditions, a low loss tangent of about 0.03 and very high dielectric permittivities of 18 000–23 000 with good temperature stability were successfully accomplished. The electrical responses of the grains and grain boundaries can, respectively, be well described using admittance and impedance spectroscopy analyses based on the brickwork layer model. A possible mechanism for the origin of semiconducting grains is discussed. The colossal dielectric response was reasonably described as closely correlated with the electrically heterogeneous microstructure by means of strong interfacial polarization at the insulating grain‐boundary layers. The non‐Ohmic properties of Na_1/2Y_1/2Cu₃Ti₄O₁₂ ceramics were primarily related to their microstructure, i.e., grain size and volume fraction of grain boundaries.     

Oxygen vacancies as a link between the grain growth and grain boundary conductivity anomalies in titanium-rich strontium titanate

Titanium-rich (Sr/Ti?=?0.995) strontium titanate (ST) ceramics, air-sintered in a temperature range of 1400–1625?°C, were reported to possess anomalies in the grain growth and analogous anomalies in the grain boundary (GB) conductivity activation energy. However, these two interface-related phenomena, occurring at GBs, could not be associated with each other using a simple “brick-layer” model. In this work we revise the topic and advocate that the deviation from the model comes from the oxygen vacancies localized at GBs of the rapidly-cooled ST ceramics. To verify this, we annealed the ceramics in oxygen and performed their systematic and comparative analysis using impedance spectroscopy. A levelling-off in the GB conductivity activation energy, which increases for ≤1.24?eV, and a four-fold decrease in the GB permittivity are observed after annealing. Thus, we confirm a key role of oxygen vacancies in relation between the grain growth and GB conductivity anomalies of as-sintered Ti-rich ST ceramics.     

Impedance spectroscopy study of anodic growth of thick zirconium oxide films in H2SO4, Na2SO4 and NaOH solutions

Anodic zirconium oxide films were grown potentiodynamically at a constant sweep rate up to the breakdown potential on rod electrodes made of 99.8% metallic zirconium. Different media of different pH were tested, namely 0.5 M H₂SO₄ (pH 0.3), 0.1 M Na₂SO₄ (pH 9) and 0.1 M NaOH (pH 13). By electrochemical impedance spectroscopy and scanning electron microscopy the oxide film thickness was monitored during the voltage scan. The behaviour was found to be different in the presence and absence of sulphate anions. In the presence of SO₄²⁻, the films were dense but breakdown occurred at 300–340 nm. In NaOH, two relaxations appeared above 50 V and were ascribed to a bi-layered coating structure and the maximum layer thickness was 720 nm before breakdown.     

The influence of processing on the microstructure and the microwave properties of Co-F-codoped barium strontium titanate thick-films

The influence of processing on the microstructure and the dielectric properties of Co-F-codoped Ba_0.6Sr_0.4TiO₃ (BST) thick-films has been investigated. BST powders with different particle sizes were prepared and applied on alumina substrates by screen-printing. The resulting thick-films were sintered at different holding times and characterized with respect to their microstructure and microwave properties. The microstructure of the thick-films shows a clear dependency on sintering time and initial particle size. In addition to grain growth, the formation of a secondary phase is observed at the interface between substrate and BST with increasing sintering time. The dielectric characterization at microwave frequencies shows an increase of tunability with larger grain size while the dielectric loss is even lowered. This shows the strong influence of the microstructure on the material properties and the possibility of tailoring the material through specific processing.     

Microstructural and high-temperature impedance spectroscopy study of Ba6MNb9O30 (M=Ga,Sc, In) relaxor dielectric ceramics with tetragonal tungsten bronze structure

This work reports on the microstructural and high-temperature impedance spectroscopy study of a family of dielectric ceramics Ba₆MNb₉O₃₀ (M=Ga, Sc, In) of tetragonal tungsten bronze (TTB) structure with relaxor properties. For Ba₆GaNb₉O₃₀ and Ba₆InNb₉O₃₀ pellets, the SEM images have revealed good, dense internal microstructures, with well-bonded grains and only discrete porosity; in contrast Ba₆ScNb₉O₃₀ pellets had a poorer microstructure, with many small and poorly-bonded grains gathered in agglomerates, resulting in significant continuous porosity and poorly defined grain boundary regions. The electroactive regions were characterised by the bulk and grain boundaries capacitances and resistances, while their contribution to the electrical conduction process was estimated by determining activation energies from the temperature (Arrhenius) dependence of both electric conductivities and time constants. For Ga and In analogues the electronic conductivity are dominated by the bulk response, while for Sc analogue, the poorly defined grain boundaries give a bulk-like response, mixing with the main bulk contribution.