Electrical properties of (Bi2O3)0.75(RE2O3)0.25 ceramics (RE=Dy, Y, Ho, Er and Yb)

Five kinds of rare earth stabilized bismuth oxide ceramics, (Bi₂O₃)_0.75(RE₂O₃)_0.25 (RE=Dy, Y, Ho, Er and Yb), were synthesized by sintering a mixture of Bi₂O₃ and RE₂O₃ at 900–1100 °C and their electrical properties were investigated. The bulk density and the lattice constant linearly increased with an increase in the atomic weight of RE and the ionic radius of RE³⁺, respectively. The electrical conductivity at 300 °C slightly increased with the increasing ionic radius of RE³⁺, while at 500 and 700 °C, it was constant regardless of the ionic radius of RE³⁺. The migration activation energy and the association activation energy showed a maximum value and a minimum value at RE=Er, respectively.     

Electrical properties of niobium doped Bi4Ti3O12-SrBi4Ti4O15 intergrowth ferroelectrics

Nb-doped Bi₄Ti₃O₁₂-SrBi₄Ti₄O₁₅ intergrowth ceramics have been prepared by modified oxalate route. XRD phase analysis confirmed the formation of single phase compound. Nb-doping does not affect the basic crystal structure of the intergrowth. SEM micrographs showed that the grain size of the ceramics decreases with Nb-doping. The temperature dependence of dielectric constant and losses was investigated in the temperature range 30–800 °C and frequency range 1 kHz–1 MHz. With Nb-doping, the T_c of the ferroelectrics reduces and peak permittivity increases. Doping also introduces small relaxor behavior in the ferroelectrics. The dc conductivity of the ceramics decreases with doping. The remnant polarization (P_r) of the intergrowth ferroelectrics is increased with Nb doping.     

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

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

Electrochemical reforming of CH4−CO2 mixed gas using porous Gd-doped ceria electrolyte with Cu electrode

This paper reports on the composition and flow rate of outlet gas and current density during the reforming of CH₄ with CO₂ using three different electrochemical cells: cell A, with Ni−GDC (Gd-doped ceria: Ce_0.8Gd_0.2O_1.9) cathode/porous GDC electrolyte/Cu−GDC anode, cell B, with Cu−GDC cathode/ porous GDC electrolyte/Cu−GDC anode and cell C, with Ru−GDC cathode/ porous GDC electrolyte/ Cu−GDC anode. In the cathode, CO₂ reacts with supplied electrons to form CO fuel and O²⁻ ions (CO₂+2e⁻→CO+O²⁻). Too low affinity of Cu cathode to CO₂ in cell B reduced the reactivity of the CO₂ with electrons. The CO fuel, O²⁻ ions and CH₄ gas were transported to the anode through the porous GDC mixed conductor of O²⁻ ions and electrons. In the anode, CH₄ reacts with O²⁻ ions to produce CO and H₂ fuels (CH₄+O²⁻→2 H₂+CO+2e⁻). The reforming efficiency at 700−800 °C was lowest in cell B and highest in cell A. The Cu anode in cells A and C worked well to oxidize CH₄ with O²⁻ ions (2Cu+O²⁻→Cu₂O+2e⁻, Cu₂O+CH₄→2Cu+CO+2H₂). However, a blockage of the outlet gas occurred in all the cells at 700−800 °C. The gas flow is inhibited due to a reduction in pore size in the cermet cathode, as well as sintering and grain growth of Cu metal in the anode during the reforming.     

Fabrication and properties of Mn1.56Co0.96Ni0.48O4 free-standing ultrathin chips

Mn_1.56Co_0.96Ni_0.48O₄ (MCN) free-standing ultrathin chips are successfully fabricated by using screen printing. The structure, electrical and IR absorption properties have been investigated as a function of the sintering temperature. The X-ray diffraction, X-ray photoelectron spectroscopy and field emission scanning electron microscope analyses show remarkable improvements in crystallinity, stoichiometry and relative density for MCN chips. From the electrical experimental results, it is found that the resistivity of the chip samples sharply decreases from 2660 to 21.9 Ω cm. Such feature is attributed to the decrease in the grain boundary resistance and the increase in Mn³⁺/Mn⁴⁺ ratio with increasing sintering temperature. Furthermore, Fourier transform infrared spectra show that the absorption bands ν1 (around 1160 cm⁻¹) at 1100 °C almost disappears while the others still remain. It is ascribed to the increase in the density of samples. The intensities of symmetry bands ν2 (850 cm⁻¹) are also found to be consistent with an increase in ratio of Mn³⁺ to Mn⁴⁺ ions with increasing sintering temperature.     

Sodium impurities in ZnO-Bi2O3-Sb2O3 based varistors

Sodium impurities are frequently present in the raw oxides that constitute the complex formulation of ZnO-Bi₂O₃-Sb₂O₃ based varistors. But actually little is known about their effect on the microstructure and the electrical response of these materials. This is the main goal of the present contribution and according to the obtained results an excessive presence of this alkaline impurity can lead to a compositional change in the Bi-rich skeleton of the varistor microstructure. As a consequence the grain growth kinetics, the densification rate and the characteristic non-linear I-V response of these electroceramics are seriously affected.     

Effect of La substitution on the structural and electrical properties of BaBi4−xLaxTi4O15

Pure and lanthanum doped barium bismuth titanate BaBi_4−xLa_xTi₄O₁₅ (BBLT, x=0, 0.05, 0.15, 0.30) ceramics were prepared utilizing solid state method. The X-ray diffraction (XRD) data confirmed formation of single-phase Aurivillius compounds while SEM micrographs did not show evident grain size change of doped ceramics. Dielectric properties were investigated in 1.21 kHz to 1 MHz frequency range and in the temperature range of 20 to 727 °C. When Bi³⁺ is substituted with La³⁺, a significant disorder was induced and the material exhibited broadening of the phase transition. Impedance analysis confirmed the presence of two semicircular arcs in doped samples suggesting the existence of grain and grain-boundary conduction. The dc-conductivity and activation energies were evaluated for all compositions.     

Electrical conductivity relaxation in PVOH-LiClO4-Al2O3

We report on electrical conductivity relaxation measurements of solid polymer electrolytes (SPE) based on poly(vinyl alcohol) (PVOH) and LiClO₄ in which nanoporous Al₂O₃ particles with average pore diameter of 58 Å were dispersed. A power law frequency dependence of the real part of the electrical conductivity is observed as a function of temperature and composition. This behaviour is typical of systems in which correlated ionic motions in the SPE bulk material are responsible for ionic conductivity. This variation is well fitted to a Jonscher expression σ′(ω) = σ₀[1 + (ω/ω₀)^p] where σ₀ is the dc conductivity, ω₀ the characteristic angular frequency relaxation and p is the fractional exponent between 0 and 1. For a prototype membrane with composition 0.9PVOH − 0.1LiClO₄ + 7 wt.%Al₂O₃, it was found that the temperature dependence of σ₀ and ω₀, may be described by the VTF relationship, ? = ?₀ exp[−B/(T − T₀)], with approximately the same constant B and reference temperature T₀, indicating that ion mobility is coupled to the motions of the polymer chains. Moreover, p decreased with increasing temperature, from 0.68 at T = 319 K, to 0.4 at T = 437 K, indicating weaker correlation effects among mobile ions when the temperature is increased.     

Preparation of aluminium lanthanum oxyapatite tapes, La10AlSi5O26.5, by tape casting and reaction sintering

Lanthanum apatite is one of the most promising materials as an electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs) which operate close to 700 °C. Here, we report the preparation and optimization of dense La₁₀AlSi₅O_26.5 tapes obtained by the combination of tape casting technology and reaction sintering. Homogeneous concentrated mixed suspensions of La₂O₃, Al₂O₃ and SiO₂ were prepared in ethanol. The effect of the solid loading, the binder content, ultrasound and the initial particle size of the materials onto the characteristics of the tapes (cracking, composition, microstructure and electrical properties) has been studied. This study paves the way for further improvement in the processing of oxyapatite electrolyte for solid oxide fuel cells.     

Composite cathodes of La0.9Ca0.1Ni0.5Co0.5O3-Ce0.8Sm0.2O1.9 for solid oxide fuel cells

The mixed ionic and electronic conductors of La_0.9Ca_0.1Ni_0.5Co_0.5O₃-Ce_0.8Sm_0.2O_1.9 (LCNC-SDC) are investigated systematically for potential application as a cathode for solid oxide fuel cells based on a Ce_0.8Sm_0.2O_1.9 (SDC) electrolyte. The electrochemical impedance spectroscopy (EIS) measurements are performed in air over the temperature range of 600-850 °C to determine the cathode polarization resistance. The exchange current densities for oxygen reduction reaction (ORR), determined from the low-field cyclic voltammetry, high-field cyclic voltammetry, and EIS data are systematically investigated. The activation energies (E_a) for ORR determined from the slope of Arrhenius plots are in the range of 102.33-150.73 kJ mol⁻¹ for LCNC-SDC composite cathodes. The experimental results found that LCNC-SDC (70:30) composite cathode has a maximum exchange current density and a minimum polarization resistance of 0.30 Ω cm² for 850 °C among LCNC-SDC composite cathodes.     

Ionic conductivity in Bi2Sn2O7 ceramics

Impedance spectroscopy measurements, in the temperature range from room temperature to 600 K, were performed in order to investigate the dielectric and ionic properties of Bi₂Sn₂O₇ ceramics. The results show that the conductivity in this pyrochlore is associated with the hopping of ions. An activation energy of 1.26 eV was observed and the dielectric constant exhibits a strong contribution from ionic conduction.     

Crystallization behavior of Bi1.25Y1.75Fe5O12 prepared by coprecipitation process

A non-isothermal study of the crystallization kinetic of coprecipitation of Bi_1.25Y_1.75Fe₅O₁₂ was carried out by differential scanning calorimetry (DSC). The Avrami exponent n suggesting the dimensionality of crystal growth was determined using the Ozawa equation. From non-isothermal DSC data presented values in range of 775–1023 kJ/mol and 3.28–2.10 for the activation energy of crystallization and the Avrami exponent, respectively. These Avrami exponent values were consistent with surface and internal crystallizations growth simultaneously.     

Synergistic effect in structural and supercapacitor performance of well dispersed CoFe2O4/Co3O4 nano-hetrostructures

Herein, we report a facile homogeneous urea – assisted hydrothermal approach for the design of CoFe₂O₄/Co₃O₄ nano hetrostructure. A variation in Co concentration leads to smartly designed composite material namely CFC-11 and CFC-12 where CFC-12 appreciates the benefits of both CoFe₂O₄ and Co₃O₄ nanoparticles. The physico – chemical properties of as developed materials were investigated by X-ray diffraction (XRD), field emission electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron microscopy (XPS) and Raman spectroscopy. The specific surface area and pore size distribution was determined by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halendo (BJH) respectively. Magnetic measurements via. vibrating sample magnetometer (VSM) demonstrate that saturation magnetization decreases with the incorporation of Co₃O₄ antiferromagnetic nanoparticles. To explore the utility of as designed nano-hetrostructures as supercapacitor electrodes, we employed cyclic voltammetry (CV) and electrochemical impedence spectroscopy (EIS) measurements. A high specific capacitance of 761.1?F?g^?1 at 10?mV?s^?1, admirable cyclic durability of 92.2% and a low resistance value obtained from impedence measurements was observed for CFC-12. The favorable performance demonstrates the synergistic effect of CoFe₂O₄ and Co₃O₄ nanoparticles and thus promise an excellent material for energy storage devices.     

Fabrication and properties of highly transparent Tm3Al5O12 (TmAG) ceramics

Highly transparent Tm₃Al₅O₁₂ (TmAG) ceramics were fabricated by solid-state reaction and vacuum sintering. Densification, microstructure evolution, mechanical, thermal, and optical properties of the TmAG ceramics were investigated. Fully dense TmAG ceramic with average grain size of 15 μm was obtained by sintering at 1780 °C for 20 h. The in-line transmittance was 80.5% at 2000 nm. The absorption coefficients at 682 nm and 785 nm were 8.03 cm⁻¹ and 8.33 cm⁻¹, respectively. The Vickers hardness, the Young modulus, the bending strength, and the fracture toughness values were 15.14 GPa, 343 GPa, 230 MPa, and 2.35 MPa m^1/2, respectively. The thermal conductivity at room temperature was 3.3 W/m K and the average linear thermal expansion coefficient from 20 °C to 1000 °C was 8.915 × 10⁻⁶ K.     

Synthesis of nano-sized Yb3Al5O12 powders by the urea co-precipitation method

Nano-sized polycrystalline ytterbium aluminum garnet (YbAG, Yb₃Al₅O₁₂) powders were successfully synthesized by a simple urea co-precipitation method. The thermal behavior of the YbAG precursor was investigated. The calcined amorphous YbAG precursor was directly converted to 20–30 nm monophase YbAG at as low as temperature 900 °C, without any other intermediate phases. The nano-sized YbAG powders calcined at 900 °C distributed evenly with a slight aggregation, and the specific surface area of the powders reached 29.7 m² g⁻¹. The absorption spectrum of the YbAG powders was measured, and there were two strong absorption peaks centered at 931 and 965 nm. The results did not correspond to the absorption peaks of Yb:YAG crystal exactly, as the lattice parameters were different between YbAG and YAG.     

Fabrication of all-solid-state battery using Li5La3Ta2O12 ceramic electrolyte

The chemical and electrochemical properties of Li₅La₃Ta₂O₁₂ (LLTa) solid electrolyte were extensively investigated to determine its compatibility with an all-solid-state battery. A well-sintered LLTa pellet with a garnet-like structure was obtained after sintering at 1200 °C for 24 h. Li ion conductivity of the LLTa pellet was estimated to be 1.3×10⁻⁴ S cm⁻¹. The LLTa pellet was stable when in contact with lithium metal. This indicates that Li metal anode, which is the best anode material, can be applied with the LLTa system. A full cell composed of LiCoO₂/LLTa/Li configuration was constructed, and its electrochemical properties were tested. In the resulting cyclic voltammogram, a clear redox couple of LiCoO₂ was observed, implying that the all-solid-state battery with the Li metal anode was successfully operated at room temperature. The redox peaks of the battery were still observed even after one year of storage in an Ar-filled glove-box. It can be concluded that the LLTa electrolyte is a promising candidate for the all-solid-state battery because of its relatively high Li ion conductivity and good stability when in contact with Li metal anode and LiCoO₂ cathode.     

Electrical properties of Li1.3Ge1.4Ti0.3Al0.3(PO4)3 superionic ceramics

Preparation and electrical characterization of NASICON-type compound, Li_1.3Ge_1.4Ti_0.3Al_0.3(PO₄)₃, are described. The solid solution is obtained with Ge⁴⁺ → Ti⁴⁺ and Ge⁴⁺ → Al³⁺ substitutions in LiGe₂(PO₄)₃. The powder has been fabricated by a solid state reaction and the structural characteristics of it have been studied by X-ray. Ceramic samples have been sintered by varying the sintering duration from 1 to 3 h. Samples were studied by complex impedance spectroscopy in the frequency range 1 MHz-1.2 GHz and temperature range 300-600 K. Two regions of relaxation dispersion were found. The dispersions were related to the fast Li⁺ ion transport in the grains and grain boundaries. Variation of the sintering duration has no considerable effect on electrical properties of the ceramics.     

Microstructure, electrical properties, and aging behavior of ZnO-Pr6O11-CoO-Cr2O3-Y2O3-Er2O3 varistor ceramics

The microstructure, electrical properties, and aging behavior of the ZnO-Pr₆O₁₁-CoO-Cr₂O₃-Y₂O₃-Er₂O₃ varistor ceramics were investigated for different contents of Er₂O₃. The microstructure consisted of ZnO grain and an intergranular layer (Pr, Y, and Er-rich phases) as a secondary phase. The increase of Er₂O₃ content decreased the average grain size and increased the sintered density. As the Er₂O₃ content increased, the breakdown field increased from 4206 V/cm to 5857 V/cm and the nonlinear coefficient increased from 32.6 to 48.6. The varistor ceramics added with 1.0 mol% Er₂O₃ exhibited excellent stability by exhibiting −0.2% in the variation rate of the breakdown field and −2.7% in the variation rate of the nonlinear coefficient for aging stress of 0.95 E_1 mA/150 °C/24 h.     

Influence of porosity on the electrical properties of La9.33(SiO4)6O2 oxyapatite

Oxyapatites are very promising materials in terms of ionic conductivity. They can be considered as a potential electrolyte for fuel cells as SOFC. The influence of porosity on the electrical properties of La_9.33(SiO₄)₆O₂ oxyapatite is reported here. Hot pressed pellets with various densification ratios have been characterized by the complex impedance method. The high frequency response associated with the bulk contribution is much more affected by the porosity than the grain boundaries contribution: as a consequence, the electrical behaviour of the samples has been considered in assimilating the porous ceramics to composite materials made of apatite with various amounts of air inclusions. Thus, the porosity dependence of conductivity, activation energy and permittivity are reported here. A percolation threshold has been highlighted for porosity values greater than 30%, involving great lowering of the electrical performances.     

Microwave assisted sintering of CaCu3Ti4O12

CaCu₃Ti₄O₁₂ electroceramic was prepared by a microwave assisted solid-state reaction technique from CaCO₃, CuO and TiO₂ powders. Processing involved the preparation of raw material, mixing and milling, calcination, pellet forming and sintering processes. Conventional furnace and microwave assisted sintering processes were employed in order to improve phase structures, morphology and dielectric properties of CaCu₃Ti₄O₁₂ ceramics. Surface and fracture FESEM analysis showed that the microwave assisted sintered CaCu₃Ti₄O₁₂ produced better densification and more uniform grain size compared to the conventional sintered sample.