Activation Energy Of Grains Research Articles

Effect of A/B ratio on the dielectric properties and insulation resistance of barium titanate sintered in a reducing atmosphere

The effect of the A/B ratio on the dielectric properties and insulation resistance of barium titanate (BaTiO3) sintered in a reducing atmosphere was investigated in this study. The increased demand for high capacitance, miniaturized, and reliable base metal electrode multilayer ceramic capacitors (BME MLCCs) has emphasized improving insulation properties. This study added Y2O3, MgO, Mn2O3, and SiO2 to tailor the barium titanate properties. We investigated how changes in the A/B ratio affect the microstructure, defect chemistry, and Y3+ ion occupation in BaTiO3, affecting dielectric properties and insulation resistance. It was observed that the activation energies for grain and grain boundary conductivity are dominated by oxygen vacancy and (VBa″−VO··) defect pair, respectively. The Ti3+ and oxygen vacancy concentrations were directly proportional to the room and high-temperature resistivities, respectively. The sample with an A/B ratio of 1.002 had the lowest oxygen vacancy and Ti3+ ratio, resulting in a higher resistance and lower thermally stimulated depolarization current (TSDC).

Characterization of structure and Li-ionic conductivity of La\(_{(2/3−x)}\)Li\(_{3x}\)TiO\(_{3}\) ceramics prepared by spark plasma sintering

In this work, La(2/3)-xLi3xTiO3 (LLTO) dense ceramic samples have been prepared by high-energy ball milling and spark plasma sintering (SPS) route. The crystal structures and microstructures of the samples were characterized by X-ray powder diffraction and FE-SEM, whereas their Li-ionic conductivity properties investigated by AC impedance spectroscopy. At 21 oC, the LLTO ceramic samples possessed the grain conductivity and grain boundary/total conductivity of σg = 8.3×10-4 S cm-1 and σgb = 2.3×10-5 S cm-1, respectively. In the investigated temperature range from 21 oC to 120 oC, the ion conduction is governed by thermally activated mechanism. The activation energies for grain and grain boundary conductivities are Eag = 0.26 eV and Eagb = 0.43 eV, respectively.

Effects of defects on electrical transport properties of anatase TiO<sub>2</sub> polycrystalline under high pressure: AC impedance measurement

The electrical transport properties of anatase TiO<sub>2</sub> polycrystalline have been systematically investigated by using high pressure <i>in-situ</i> impedance spectroscopy measurements. The anomalous behaviors of resistance, parameter factor and relaxation frequency of grain and grain boundary can be found at 6.4, 11.5 and 24.6 GPa. The results indicate that the first two discontinuous points (6.4 and 11.5 GPa) correspond to the phase transitions of TiO<sub>2</sub> from anatase to α-PbO<sub>2</sub> and then to baddeleyite, respectively. Above 24.6 GPa, TiO<sub>2</sub> completely transforms into the baddeleyite phase. Based on the change of grain resistance and grain boundary resistance under pressure, intrinsic defects play a crucial effect in the electrical transport properties of TiO<sub>2</sub> at high pressures. At 6.4 GPa, the occurrence of phase transition gives rise to the variation of defects’ role, from a deep energy level defect (as a recombination centre) changes into a shallow energy level defect (providing carriers for the conduction and valence bands). In addition, the position of defect in energy band changes with pressure increasing. The phase transition of TiO<sub>2</sub> at 6.4 GPa is the rearrangement of TiO<sub>6</sub> octahedron, while the other one at 11.5 GPa can be attributed to the migration of oxygen Schottky defects from inner to surface. Combining the packing factor and relaxation frequency, the electrical transport properties of TiO<sub>2</sub> under pressure are revealed, the packing factor and the relaxation frequency are closely related to the mobility and the carrier concentration, respectively. The activation energy of grain and grain boundary decrease with the pressure elevating, indicating that the transport of carriers in grain and grain boundary become easier under pressure, and the former is smoother than the latter owing to the activation energy of grain being smaller than that of grain boundary in the same pressure range. Moreover, the relaxation frequency ratio of TiO<sub>2</sub> grain and TiO<sub>2</sub> grain boundary decreases with pressure increasing, and the grain boundary effect under high pressure is not obvious.

Ca9Al(PO4)7 based sintered glass-ceramic for high dielectric breakdown strength

A novel Ca9Al(PO4)7 (CAP) based sintered glass-ceramic has been synthesized via the glass melting process and temperature controlled crystallization. The mechanisms of high dielectric breakdown strength (DBS) of the glass-ceramic were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy, crystal structure and high temperature complex impedance spectroscopy. CAP has a crystal structure similar to β-Ca3(PO4)2 (β-TCP) and a higher band gap (Egap), which makes it difficult for valence electrons to break away from binding and become free charges. At the same time, the isolated vacancy formed by Al replacing Ca becomes a capture site to prevent the transmission of free charge. Furthermore, CAP based glass-ceramic have a high activation energy of grain and grain boundary, which impedes the movement of free charges. All these endow this CAP based sintered glass-ceramic with a high DBS of 110.5 ± 5.3 kV/mm.

High frequency impedance measurements of sodium solid electrolytes

Solid-state sodium batteries are currently gaining enormous interest as a lower-cost and more environmentally friendly alternative to lithium batteries. They contain significantly less critical rare elements in both the electrolyte and the active material. However, to date, there is no efficient material combination of metallic anode, cathode, and solid electrolyte for room temperature applications that does not require an additional liquid electrolyte while maintaining high energy density. NaPSiO-based glass-ceramics show high ionic conductivity at room temperature, good corrosion resistance against ambient humidity and CO2, and stability against metallic sodium. However, the conductivity mechanisms of this promising class of materials are currently poorly understood. Herein, high frequency impedance measurements up to 108 Hz shed light on the contributions of grains and grain boundaries to the total impedance, including the distribution of relaxation time constants of the fully crystallized material. In addition, analysis of the temperature dependence allows separation of electrode contributions and determination of activation energies for grain and grain boundary conductivities. Our study provides the basis for fine-tuning the stoichiometry of NaPSiO-based glass-ceramics in terms of maximizing the conductive phase fraction to optimize the performance of future solid-state sodium batteries.

Investigation of the Densification Behavior of Alumina during Spark Plasma Sintering.

The article presents the results of the investigation of the mechanism of the densification behavior of alumina-based ceramics during spark plasma sintering. The role of the heating rates and additives were investigated. The first (initial) stage of sintering was investigated by the Young–Cutler model. The second (intermediate) stage of sintering was investigated as a process of plastic deformation of a porous body under external pressure. It was shown that, at the initial stage, the formation of necks between the particles is controlled by grain boundary diffusion (the activation energy is Qb ≈ 20 kTm). At this stage, accommodation of the shape of the alumina particles is also occurring (an increase in the packing density). The accommodation process facilitates the shrinkage of the powder, which is reflected in a decrease in the effective activation energy of shrinkage at low heating rates (10 °C/min) to Qb ≈ 17 kTm. At heating rates exceeding 10 °C/min, the intensity of the processes of accommodation of alumina particles turns out to be much slower than the existing diffusion processes of growth of necks between the alumina particles. It was shown that the grain boundary sliding mechanism that occurs in the second stage of sintering can play a decisive role under conditions of spark plasma sintering with a high heating rate. The found value of the activation energy at the second stage of sintering is also close to the activation energy of grain–boundary diffusion of alumina (Qb ≈ 20 kTm). The influences of the second phase particles of MgO, TiO2, and ZrO2 on densification behavior of alumina-based ceramics were investigated. Since at the first stage of sintering the densification relates with the formation of necks between the particles of alumina, the additives (0.5% vol) have no noticeable effect on this process. It was also shown that the second phase particles which are located at the grain boundaries of alumina are not involved in the slip process during the second sintering stage. Analysis shows that additives act only in the final (third) stage of spark plasma sintering of alumina.

Dielectric relaxations in NASICON-type Li1.25Zr1.75Al0.25(PO4)3 solid electrolyte

In this work, the conduction and relaxation behavior of lithium-ion conducting monoclinic Li[Formula: see text]Zr[Formula: see text]Al[Formula: see text](PO[Formula: see text] (LZAP) ceramic prepared via sol–gel method was investigated. Rietveld refinement of the X-ray diffraction data confirmed the structure of LZAP as monoclinic with space group [Formula: see text]21/[Formula: see text] at room temperature. The permittivity data were fitted using a modified Cole–Cole equation containing the complex conductivity terms to rationalize the giant dielectric constant and loss factor observed at lower frequencies. In addition, fittings revealed the presence of two thermally activated dielectric relaxations processes in LZAP ceramics, which were attributed to the lithium ions movement within the grain and grain boundary confines. The activation energies computed from the Arrhenius fitting of relaxation times associated with the grain and grain boundary regions were [Formula: see text]0.447 ± 0.009 eV and [Formula: see text]0.46 ± 0.01 eV, respectively. The activation energies for grain and grain boundary Li[Formula: see text] conduction were also found to have similar values ([Formula: see text]0.42 eV for grain, and 0.46 eV for grain boundary).

Exploring the electrical behavior of iodine substituted CaCu3Ti4O12-xIx by impedance and modulus spectroscopy

This study explores the effect of iodine (an anion) substitution on the dielectric and resistive properties of CaCu3Ti4O12-xIx (x = 0, 0.005, 0.05, and 0.20) sintered at 1100 °C for 12 h with the help of impedance and modulus spectroscopy. The electrical measurements were done with varying temperatures (300–450 K) and frequency (20 Hz-1 MHz). The ionic distribution of different ions and iodine and their oxidation states were revealed by EDX and XPS analysis, supporting the synthesis of CCTO ceramics with iodine substitution. The complex plane plots (Z″ vs. Z′ and M″ vs. M′) and spectroscopic plots (Z″ vs. log f and M″ vs. log f) of various compositions at different temperatures suggest that the grains, grain boundaries, and sample-electrode interface contribute to their electrical response and can be represented by a combination of three R–C or R-CPE equivalent circuits. The depressed arcs in the Z″ vs. Z′ plot revealed multiple relaxations with a narrow distribution of time constants. The scaled master plots confirm that the resistive and polarization mechanisms are independent of temperature and are Maxwell-Wagner relaxations type. Broad relaxation peaks and mismatch in peak frequencies of Z″ and M″ vs. log f plots suggest that the relaxation mechanism is dominated by the short-range (localized) movement of charge carriers. The activation energies for grain and grain boundary relaxation processes are ∼0.15 eV and ∼0.2 eV, respectively. Increasing iodine doping from x = 0.005 to x = 0.05 leads to a large increase in the resistance of the specimen, which has been explained based on change of charge compensation for IO• defects from electronic defect e/ to ionic defects VCu// and/or VTi////. The dielectric constant is maximum for the composition, x = 0.2, whereas loss, tan δ is minimum for x = 0.05.

Grain and grain boundary behaviors and electrical transport properties of TiO<sub>2</sub> nanowires under high pressure

In this work, anatase Titanium dioxide (TiO<sub>2</sub>) nanowires are synthesized by the hydrothermal method, and its grain and grain boundary behaviors and electrical properties are investigated by alternating current (AC) impedance method under high pressure (up to 34.0 GPa). The relationship between the frequency dependence of impedance <i>Z''</i> and pressure indicate that the conduction mechanism of anatase phase TiO<sub>2</sub> nanowires in the test pressure range is electronic conductivity. It should be noted that the characteristic peaks of <i>Z''</i> move toward high frequency region with pressure increasing, demonstrating that the effect of grain interior on impedance becomes apparent. Additionally, the overall variation trends of grain and grain boundary resistance go downward with pressure increasing, and the descent rate of grain boundary is larger than those of grain before and after phase transition. However, in a range of phase transition (8.2–11.2 GPa, from anatase to baddeleyite phase), grain boundary resistance shows a discontinuously change (increases to 11.2 GPa and then decreases). Based on the different variation trends of grain and grain boundary resistance, it becomes obvious that the phase transition from anatase to baddeleyite phase first occurs at the surface of grain, and then extends to the interior of grain gradually. Also, as an intrinsic characteristic, the relaxation frequency is independent of the geometrical parameters. The pressure dependence of activation energy is obtained by fitting the pressure dependence of relaxation frequency. The activation energy of grain and grain boundary decrease with pressure increasing, implying that the contribution of pressure on the conductivity of sample is positive. Furthermore, the space charge potential for the whole test pressure range is positive, which is determined by the relationship between pressure and relaxation frequency. This fact illustrates that the anion defects are easily formed in the space charge region, and the oxygen defects are the main inducement for TiO<sub>2</sub> phase transformation.

Design and characterization of novel manganite perovskites Ba1-xBixTi1-xMnxO3 (0≤x≤0.2)

Polycrystalline manganite powders of Ba1-xBixTi1-xMnxO3 (x = 0, x = 0.1 and x = 0.2) were synthesized by the conventional solid-state reaction process. Their crystal structure, morphological, optical, dielectric and electrical properties were investigated. X-ray diffraction of the prepared samples was made at room temperature and confirmed the formation of a perovskite phase. Structural refinement, using the Rietveld method, revealed a tetragonal P4mm phase of pure BTO and a tetragonal P4/mmm phase with the presence of vacancies for both doped samples (x = 0.1 and x = 0.2). Scanning electron microscopy indicated that the perovskite samples had a grain size smaller than 1 μm. From UV–vis–NIR spectra, we found that the band gap reduces from 3.29 eV to 1.48 eV with the increase of Bi and Mn amounts, resulting in a shift of the absorption wavelength region toward the visible range. Dielectric analysis was conducted in a wide range of temperatures at different frequencies. Phase transitions were identified from thermal dielectric results, showing that the samples exhibited a non-relaxor behavior. The structural transformation from tetragonal to cubic structure corresponding to the transition from ferroelectric phase to paraelectric phase was observed in the dielectric properties investigation. The complex impedance spectroscopy indicated the presence of grain and grain boundary effects in the conduction mechanism. Electrical analysis showed that doping with Bi and Mn enhanced the DC conductivity. Furthermore, the DC conductivity temperature dependence confirmed that the studied samples present a semiconductor behavior. The activation energies of grain and grain boundaries depended on the amount of incorporated Bi and Mn. The activation energy of grain varied between 0.54 and 0.87 eV suggesting that the DC electrical conductivity is governed by ionized oxygen vacancies. The activation energy of grain boundaries varied between 0.85 and 0.58 eV.

Electro-mechanical and Polarization-Induced Antibacterial Response of 45S5 Bioglass-Sodium Potassium Niobate Piezoelectric Ceramic Composites.

Besides the excellent osteoconductivity and biocompatibility of 45S5 bioglass (BG), poor mechanical and electrical properties as well as susceptibility toward bacterial adhesion limit its widespread clinical applications. In this context, the present study investigates the effect of addition of piezoelectric sodium potassium niobate (Na0.5K0.5NbO3; NKN) on mechanical, dielectric, and antibacterial response of BG. BG-xNKN (x = 0, 10, 20, and 30 vol%) composites were synthesized at 800 °C for 30 min. The phase analyses using spectral techniques revealed the formation of the composite without any reaction between BG and piezoelectric ceramic NKN. The dielectric and electrical measurements were performed over a wide range of temperature (30-500 °C) and frequency (1 Hz-1 MHz) which suggests that space charge and dipolar polarizations are the dominant polarization mechanisms. The complex impedance analyses suggest that the average activation energies for grain and grain boundary resistances for BG-xNKN (x = 10, 20, and 30 vol%) composites are 0.59, 0.87, 0.94 and 0.76, 0.93, 1.06 eV, respectively. The issue of bacterial infection has been addressed by electrical polarization of the developed composite samples, at 20 kV for 30 min. Statistical analyses reveal that the viability of Gram-positive (S. aureus) and Gram-negative (E. coli) bacterial cells has been reduced significantly on positively and negatively charged BG-NKN composite samples, respectively. The qualitative analyses using the Kirby-Bauer test supports the above findings. Nitro blue tetrazolium and lipid peroxide assays were performed to understand the mechanism of such antibacterial response, which suggested that the combined effect of NKN addition and polarization significantly enhances the superoxide production, which kills the bacterial cells. Overall, incorporation of NKN in BG enhances the mechanical, electrical, and dielectric properties as well as improves the antibacterial response of polarized BG-xNKN composites.

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 Na0.5K0.5NbO3 (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.

AC and DC conductivity study of ceramic compound NaGd(WO4)2 using impedance spectroscopy

In this works, the NaGd(WO4)2 compound was obtained by the conventional solid-state reaction method at 923 K, has been investigated by X-ray diffraction, vibrational and impedance spectroscopy. The X-ray diffraction’s results indicate that the sample possess single crystalline phase in the tetragonal system with space group I41/a. The unit cell parameters are: a = 5.246(6) A, c = 11.380(1) A, and V = 313.257(8) A3. Thus, the impedance spectroscopy measurements were performed in the (209Hz–5 MHz) frequency and the (613–711 K) temperature ranges. Besides, the ac conductivity contribution is interpreted using the universal Jonscher’s power law and is thermally activated with an activation energy of grain, grain boundary and total were 0.91(4), 0.79(3) and 0.97(5) eV respectively. The mechanism of conduction is due to the displacement of Na+ ion that can be interpreted by the model NSPT (non-overlapping small polaron tunneling).

Effect of sintering temperature on structural and dielectric properties of Bi and Li co-substituted barium titanate ceramic

Bi and Li co-substituted barium titanate, Ba0.98(Bi,Li)0.02TiO3 ceramic samples were sintered at different temperatures using conventional solid state sintering technique. X-Ray Diffraction patterns confirm tetragonal phase in all the sintered samples. Microstructure analysis using Scanning Electron Microscopy (SEM) reveals increasing grain sizes with an increase in sintering temperature. Dielectric spectroscopy performed in the range of 40Hz to 2MHz at room temperature shows that the dielectric constant increases with increasing sintering temperature, reaching a maximum of value of 1200 at 40Hz where the dielectric loss observed was less than 0.02 for samples sintered at 1300°C. Temperature dependant impedance spectroscopy data in the range of 30–300°C was used to measure AC conductivity. The activation energy of grains was deduced through Arrhenius plots. Loss tangent at different frequencies for 1300°C sintered samples was less than 0.1 over the entire temperature range. The high dielectric constant with a low dielectric loss at elevated temperatures make Ba0.98(Bi,Li)0.02TiO3 samples suitable for Multi-Layer Ceramic Capacitors (MLCC)s used in high-temperature applications.

Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics

SiO2-added barium strontium titanate ceramics Ba0.4Sr0.6TiO3-xwt%SiO2 (x=0, 0.5, 1, 3, BSTSx) were prepared via a traditional solid state reaction method. The effect of SiO2 additive on the microstructure, dielectric response and energy storage properties was investigated. The results confirmed that with the increase of SiO2 additive, diffuse phase transition arises and the dielectric constant decreases. An equivalent circuit model and Arrhenius law were used to calculate the activation energy of grain and grain boundary, which indicated that the dielectric relaxation at high temperature was caused by oxygen vacancy. While appropriate SiO2 additive led to improve the breakdown strength, further increase of SiO2 deteriorated the energy storage because of the low densification. Finally, optimized energy storage performance was obtained for BSTS0.5 ceramics: dielectric constant of 1002, dielectric loss of 0.45%, energy density of 0.86J/cm3 and energy storage efficiency of 79% at 134kV/cm.

Investigation on the dielectric properties of Mg-doped (Ba0.95Ca0.05)(Ti0.85Zr0.15)O3 ceramics

Mg-doped Ba0.95Ca0.05Ti0.85Zr0.15O3 ceramics were prepared through a solid-state reaction method and their dielectric relaxation behavior was investigated. All of compositions exhibit perovskite strucutre with acceptor-Mg in the B-sites. The grain size was reduced and homogenized by Mg doping. The Tc and the magnitude of diffusion change regularly in the studied compositional ranges. The relaxor diffuseness from ferroelectric phase transition reached maximum at x=0.01. The impedance study showed that the conductance activation energy of grain (Ea-g) is higher than that of grain-boundary (Ea-gb). And Ea-gb varies with Mg concentrations, which is more stable than Ea-g. These results were explained by oxygen vacancies generated and gathered as cluster rotate around the MgTi″ in the grains.

Impedance spectroscopy and scaling behaviors of Sr3Co2Fe24O41 hexaferrite

The impedance spectroscopy of Z-type hexaferrite Sr3Co2Fe24O41 (SCFO) has been investigated as a function of temperature from 303 to 503 K. The frequency dependent impedance (Z″) and modulus (M″) spectra show that for the air annealed SCFO, the electrical responses of SCFO are thermal activated and there is a distribution of relaxation times. The scaling behaviors of Z″ and M″ spectra further suggest that the distribution of relaxation times is temperature independent. The Cole-Cole plots in impedance formalism show that the electrical response of SCFO originates from both the grain and the grain-boundaries. The activation energies for grain and grain boundary are 0.66 eV and 0.67 eV, respectively. The frequency dependent conductivity (σ′) spectra follow the universal power law. The fitting results of σ′ spectra show that the small polaron hopping is the most probable conduction mechanism for SCFO. Moreover, the scaling behavior of σ′ spectra further confirms that the distribution of local electrical response times is temperature independent. The air plus O2 annealed SCFO shows thermally activated electrical responses and scaling behaviors as well. The above results show that although the electrical responses of SCFO are temperature dependent, the relaxation mechanism in SCFO is temperature independent.

The role of Co in the BaTiO3–Na0.5Bi0.5TiO3 based X9R ceramics

Ceramics in BaTiO3–Na0.5Bi0.5TiO3–Nb2O5–Co3O4 (BT–NBT–Nb–Co) system were fabricated to satisfy the EIA-X9R specification (−55~200°C, ΔC/C≤±15%) of multilayer ceramic capacitors. The role of Co in this system was investigated. The core–shell structure was found to exist in the system, as confirmed by transmission electron microscopy (TEM), accounting for the high dielectric temperature stability. The Co can regulate the reaction between Nb and BT–NBT to adjust the core–shell structure, where the optimum dielectric properties were achieved at Nb/Co=3, with dielectric constant being 930 at room temperature and capacitance variation being less than 15% in the temperature range from −55°C to 200°C. The impedance spectroscopy was fitted by 3RC equivalent circuit, corresponding to the grain, grain boundary and ceramic/electrode interface region for the Co modified BT–NBT system. The conduction activation energy of grain was found to decrease with increasing Co concentration, where the conduction mechanism of grain and grain boundary are both sensitive to the Co concentration.

Structural Properties and Ionic Conductivity of Nanocrystalline Zr5Ti7O24 Ceramic Synthesized by Autoignited Combustion Technique

Nanoparticles of Zr5Ti7O24 ceramic have been prepared by the autoignited combustion technique. The phase purity of the ceramic was examined by x-ray diffraction method, yielding lattice parameters of a = 14.3 A, b = 5.2 A, and c = 5.1 A. A narrow size distribution of the nanoparticles was determined by transmission electron microscopy with a peak at 21.55 nm. Coexistence of Zr/TiO6 octahedra and Zr/TiO4 tetrahedra was observed in the Raman spectrum. The optical bandgap of the ceramic was found to be 2.36 eV. The sample was pressed into a circular pellet and sintered at 1490°C, obtaining 94.7% of theoretical density. The microstructure of the sintered pellet was analyzed by scanning electron microscopy, and the elemental composition was confirmed by energy-dispersive spectrometry. Impedance spectroscopic analysis of the sample showed that ions are the main source of conduction. Two semicircles in the Cole–Cole plot provide evidence of grain and grain boundary contributions to the conductivity. The total ionic conductivity of the sample was 5.57 × 10−1 S/m at 700°C. The activation energies of grains and grain boundaries above 500°C were 1.08 eV and 0.72 eV, respectively. Nanocrystalline Zr5Ti7O24 ceramic is a promising material for use as an electrolyte in solid oxide fuel cells.

The origin of giant dielectric relaxation and electrical responses of grains and grain boundaries of W-doped CaCu3Ti4O12 ceramics

The origin of giant dielectric relaxation behavior and related electrical properties of grains and grain boundaries (GBs) of W6+-doped CaCu3Ti4O12 ceramics were studied using admittance and impedance spectroscopy analyses based on the brick–work layer model. Substitution of 1.0 at. % W6+ caused a slight decrease in GB capacitance, leading to a small decrease in the low-frequency dielectric constant. Surprisingly, W6+ doping ions have remarkable effects on the macroscopic dielectric relaxation and electrical properties of grains. X-ray photoelectron spectroscopy analysis suggested that the large enhancements of grain resistance and conduction activation energy of grains for the W6+-doped CaCu3Ti4O12 ceramic are caused by reductions in concentrations of Cu3+ and Ti3+ ions. Considering variation of dielectric properties together with changes in electrical properties of the W6+-doped CaCu3Ti4O12 ceramic, correlation between giant dielectric properties and electrical responses of grains and GBs can be described well by the internal barrier layer capacitor model. This model can ascribe mechanisms related to giant dielectric response and relaxation behavior in CaCu3Ti4O12 ceramics.