Mixed conductivity of zircon-type Ce1−xAxVO4±δ (A=Ca, Sr)

Incorporation of alkaline-earth cations into the zircon-type lattice of Ce_1−xA_xVO_4+δ (A=Ca, Sr; x=0−0.2) was found to significantly increase the p-type electronic conductivity and to decrease the Seebeck coefficient, which becomes negative at x≥0.1. The oxygen ionic conductivity is essentially unaffected by doping. The ion transference numbers of Ce_1−xA_xVO_4+δ in air, determined by the faradaic efficiency measurements, are in the range from 2×10⁻⁴ to 6×10⁻³ at 973–1223 K, increasing when temperature increases or alkaline-earth cation content decreases. The results on the partial conductivities and Seebeck coefficient suggest the presence of hyperstoichiometric oxygen, responsible for ionic transport, in the lattice of doped cerium vanadates. The activation energies for the electron-hole and ionic conduction both decrease on doping and vary in the ranges 39–45 kJ/mol and 87–112 kJ/mol, respectively. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Ceria based mixed conductors with adjusted electronic conductivity in the bulk and/or along grain boundaries

Ceramic samples of Ce_1 ? xPr_xO_2 ? δ (CPO) and Ce_1 ? xGd_xO_2 ? δ (CGO) were obtained by different sintering schedules, including the use of cobalt as a sintering aid, added by mixing the precursor powders with cobalt nitrate solution; this allowed one to obtain different microstructural features and to change the transport properties, with emphasis on changes in grain boundary behaviour. Cobalt plays a double effect as sintering aid and also to induce important changes of grain boundary properties. Specific changes of grain boundary properties were ascribed by de-convolution of impedance spectra.Relatively high levels of mixed conductivity could be attained by adding a lanthanide species to yield ionic transport, whereas electronic conduction was promoted by the mixed valence character of PrO_x, combined with the additional contribution of Co-rich grain boundaries. These effects can be used to tune preferential electronic conductivity at bulk or grain boundary level. Oxygen permeability and a modified e.m.f. method were used to obtain the overall ionic transport number under oxidising conditions and its dependence on processing conditions. Additions of PrO_x induce bulk electronic conduction which assumes a greater role at lower temperatures. Further enhancement of electronic conductivity is attained by effects of Co-addition. Though Co-rich grain boundaries also yield significant levels of electronic conductivity in CGO, this contribution becomes minor at intermediate temperatures, due to differences between the activation energies for electronic and ionic conduction.     

Thermal stability and impedance spectroscopy studies of (Cu, Ni) substituted Bi4V2O11 ceramics

Polycrystalline samples of the oxygen ion conductor Bi₂V_0.9Cu_0.1−xNi_xO_5.35 with various contents of nickel (0 ≤ × ≤ 0.1) were investigated. The X-ray powder diffraction revealed the tetragonal structure of all compositions. DTA curves exhibit effects due to phase transition, one endothermic effect during heating and one exothermic one during cooling. The impedance of the ceramics with Pt electrodes was measured in the frequency range 10⁻¹–10⁷ Hz at constant temperatures between 350 and 920 K. The conductivity was determined by nonlinear least-squares analysis of the impedance spectra. Separation of the total resistance into grain interior and grain boundary components was feasible at temperatures below 580 K. The transition temperatures observed in DTA coincide with those observed in conductivity measurements. A phase transition, involving a reordering process of the oxygen ions is considered to be responsible for this phenomenon. The frequency dependent part of the intragrain conductivity was modeled by a constant phase admittance. The effective hopping rate was estimated by comparing the frequency dependent part and the dc limit of the intragrain conductivity. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997.     

High-temperature electrical transport in Al-doped calcium and strontium titanates

The electrical conductivity of perovskite-related oxides CaTi_1−xAl_xO_3−δ and SrTi_1−xAl_xO_3−δ (x=0−0.4) were investigated within the temperature range 900 to 1000 °C and the oxygen partial pressure range between 10⁻²⁰ and 0.21 atm using a dc four-point technique. The materials investigated show predominantly p-type electronic conductivity at high, n-type electronic conductivity at low, and ionic conductivity at intermediate oxygen partial pressures. The values of ionic conductivity in CaTi_1−xAl_xO_3−δ were found to be lower than those in CaTi_1−xFe_xO_3−δ. The effect of aluminium concentration on the high-temperature transport properties was examined. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Cu-Ce0.8Gd0.2O2−δ materials as SOFC electrolyte and anode

Co-sintering of Cu-CGO cermet anodes on CGO (Ce_0.8Gd_0.2O_2−δ) electrolyte was conducted at low temperature (1000 °C) by introducing small amounts (2 mol.%) of CuO sintering aid to the electrolytic CGO. The Cu-CGO anodes with Cu contents from 20–50 vol.% were prepared by combustion synthesis followed by sintering and reduction. Symmetrical anode/electrolyte/anode assemblies of Cu-GCO/CGO/Cu-CGO were fabricated by manually depositing the anode combustion powder on a green substrate of the 2 mol% CuO-containing CGO, followed by co-pressing and co-sintering of the assembly at 1000 °C. The low-temperature sintered CGO is submicron with 95–99% relative density. CuO addition has no significant effect on either the total or ionic conductivity of the electrolyte, but p-type conduction in the temperature range, 900–1200 °C, is 25 times higher than that of undoped CGO. Oxygen-ion transference numbers of the Cu-containing CGO lie in the range 0.89–0.99, as determined by the modified e.m.f. technique under an oxygen/air potential gradient. The activation energy for ionic conduction, 83 kJmol⁻¹, is significantly lower than that for p-type electronic transport, 140 kJmol⁻¹. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Structural and electrical characterisation of strontium zirconate proton conductor co-sputter deposited coatings

SrZr_1−x Y _x O₃ coatings were co-sputtered from metallic Zr–Y (84–16 at.%) and Sr targets in the presence of a reactive argon–oxygen gas mixture. The structural and chemical features of the film have been assessed by X-ray diffraction and scanning electron microscopy. The electrical properties have been investigated for different substrates by Complex Impedance Spectroscopy as a function of crystalline state, temperature and atmosphere. The as-deposited coatings are amorphous and crystallise after annealing at 673 K for 2 h under air. The stabilisation of the perovskite structure is a function of the nominal composition. The films are dense and present a good adhesion on different substrates. Crystallisation and mechanical stresses are detected by alternating current (AC) impedance spectroscopy. Significant ionic conductivity in the 473–823 K temperature range is evidenced in air. Two different conduction regimes in the presence of steam are attributed to a modification of the charge carrier nature. In spite of low conductivity values (σ ~10⁻⁶ S.cm⁻¹ at 881 K), the activation energies are in agreement with that of Y-doped strontium zirconate ceramics (~0.7 eV in air).     

Ionic and electronic transport in perovskite-type La(Ga,M)O3−δ (M=Mg, Cr, Fe, Co, Ni, Nb)

Oxygen ion conduction in La_0.9Sr_0.1Ga_1−xM_xO_3−δ (M=Cr, Fe; x=0 – 0.20), LaGa_1−xM_xO_3−δ (M=Co, Ni; x=0.20 – 0.60), LaGa_1−x−yCo_xMg_yO_3−δ (x=0.35 – 0.60; y=0.10 – 0.25) and LaGa_0.85−xMg_0.15(Nb_0.33Mg_0.66)_xO_3−δ (x=0 – 0.20) is reported. At temperatures below 1200 K the ionic conductivity of La(Ga,M)O_3−δ (M=Co, Ni) increases with increasing oxygen nonstoichiometry, but is lower than for La(Ga,Mg)O_3−δ and (La,Sr)GaO_3−δ. Co-doping with Nb and Mg was found to result in decreasing ionic transport in La(Ga,Nb,Mg)O_3−δ due to blocking of oxygen sites by Nb⁵⁺. Small additions of Fe to the B-site of La_0.9Sr_0.1GaO_3−δ increase the ionic conductivity, whereas substitution of Cr for Ga has the opposite effect. Incorporation of transition metal cations into the Ga site leads to a higher p-type electronic conductivity in all studied perovskites. Paper presented at the 6th Euroconference on Solid Sate Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Electrical conductivity of Sr1−xTiO3−δ materialsmaterials

The high temperature conductivity of polycrystalline Sr_1−xTiO_3−δ samples in air was found to be lower than the conductivity of SrTiO₃ samples. However, the dependence of the electrical conductivity on the oxygen partial pressure showed that this trend can be reverted under reducing conditions. Both trends contradict the expected effects of A-site deficiency on the defect chemistry. Differences in average grain sizes give a plausible explanation for these findings. The dependence of the conductivity on the oxygen partial pressure suggests that p-type conductivity is dominant in air, for every sample, and one can thus assume that the number of grain boundaries plays a negative role on this contribution. Electrochemical permeability measurements confirmed that the ionic transport number of strontium titanate in air remains small. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Optimised NASICON ceramics for Na+ sensing

NASICON dense ceramics were obtained from solid state reaction between SiO₂, Na₃PO₄·12H₂O and two different types of zirconia: monoclinic ZrO₂ and the yttria-doped tetragonal phase (ZrO₂)_0.97(Y₂O₃)_0.03. Higher temperatures were needed to obtain dense samples of the yttrium free composition (1265 °C). The electrical conductivity, at room temperature, of the yttria-doped samples sintered at 1230 °C (0.20 S/m) is significantly higher than the value obtained with the material prepared from pure ZrO₂. The impedance spectra show that the differences in conductivity are predominantly due to the higher grain boundary resistance of the undoped ceramics, probably due to formation of monoclinic zirconia and glassy phases along the grain boundary. Further improvement of the electrical conductivity could be achieved after optimization of the grain size and density of grain boundaries. A maximum conductivity value of about 0.27 S/m at room temperature was obtained with the yttria-doped samples sintered at 1220 °C for 40 h. Yttria-doped and undoped ceramics were tested as Na⁺ potentiometric sensors. The detection limit of the yttria-doped sample (10⁻⁴ mol/l) was one order of magnitude lower than the obtained with the undoped material. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16 – 22, 2001.     

Heat conductivity and the Lorentz number of the Sm1−x GdxS “black” phase

Measurement of the heat conductivity and electrical resistivity of two Sm_1−x Gd_xS compositions with x=0.1 and 0.14 is reported within the 80–300 K interval. An analysis of experimental data on the electronic component of heat conductivity permits a conclusion that the d subband of “heavy” carriers in the conduction band of these materials lies above the s “light”-carrier subband. Fiz. Tverd. Tela (St. Petersburg) 41, 26–29 (January 1999)     

Electrical and structural properties of poly (ethylene oxide)/silver triflate polymer electrolyte system dispersed with MgO nanofillers

Solvent-free films of poly (ethylene oxide)–silver triflate (PEO–AgCF₃SO₃)/MgO-based nanocomposite polymer electrolytes (PEO)₅₀AgCF₃SO₃–x wt.% MgO (x = 1, 3, 5, 7, and 10) obtained using solution casting technique were found to exhibit an appreciably good complexation of MgO nanofiller within the polymer electrolyte system and non-Debye type of relaxation as revealed by Fourier transform infrared and complex impedance analyses. Optimized filler (5 wt.% MgO) when incorporated into the polymer electrolyte resulted in a maximum electrical conductivity of 2 × 10⁻⁶ S cm⁻¹ in conjunction with a silver ionic transference number (t _Ag+) of 0.23 at room temperature (298 K). Detailed structural, thermal, and surface morphological investigation indicated a slight reduction in the degree of crystallinity owing to the addition of MgO nanofiller.     

P-Type electronic conduction in CeO2- and LaGaO3-based solid electrolytes

Modifications of the e.m.f. and faradaic efficiency techniques, taking into account electrode polarization in the measuring cells, in combination with the use of electrodes having sufficiently high polarization resistances enable a precise determination of minor electronic contributions to the conductivity of solid electrolytes. These methods were used to determine the p-type conductivity of compositions based on La(Sr)Ga(Mg)O_3-δ (LSGM) and Ce(Gd)O_2-δ (CGO) at 900–1270 K. The oxygen ion transference numbers of these materials under oxygen/air gradient vary in the range 0.999–0.970, increasing with decreasing temperature. Substitution of 2 % gadolinium in Ce_0.80Gd_0.20O_2-δ with praseodymium was found to increase the electron-hole conduction by 2.5 – 4 times. At temperatures above 700 K, both the partial oxygen ionic and p-type electronic conductivities of LaGaO₃-based phases are higher than those in CGO. The electron-hole transport in LSGM tends to increase with the magnesium concentration, while the activation energy is essentially independent of composition. Electronic conduction in CGO and LSGM electrolytes was also found to be influenced by the ceramic microstructure. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Mixed conductive electrode materials for sensors and SOFC

Modified active electrode materials based upon rare earth manganites were developed for different solid electrolyte electrochemical cells. The preparation, structure, thermal expansion, the state of oxygen on the surface, the electronic and ionic conductivity of the perovskites Ln_1−xCa(Sr)_xMn_1−y(Co, Ni)_yO_3−δ with various compositions and electrode kinetics on the manganite electrode/solid electrolyte interfaces were investigated. The value of the bulk conductivity was larger than 150 S/cm (at 1100 K) and increased significantly with increasing contents of Ni or Co. The thermal expansion coefficients of rare earth manganites were close to those of ZrO₂ based solid electrolytes. The expansion coefficients of Co or Ni subsituted lanthanum manganites increase with Co or Ni substitution and are over 12•10⁻⁶K⁻¹. The ionic conductivities were determined using encapsulated zirconia microelectrodes based on a Hebb-Wagner analysis of the currentvoltage curves. The relatively high oxide ion conductivity of 10⁻⁵ S/cm at 900...1000 K was found by Ni or Co doped manganites. Studies of the electrode kinetics using complex impedance spectroscopy show that Co and Ni doped manganites have advantages if used as electrodes as compared with these for noble metals. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11–18 Sept. 1994     

Impedance spectroscopy study of BNKLT polycrystalline ceramic

Complex impedance analysis of perovskite structured polycrystalline, [Bi_0.5(Na_1−x−y K _x Li _y )_0.5]TiO₃, at x=0.2, y=0.1 ceramic was synthesized by a mixed oxide method. The formation of single-phase material was confirmed by X-ray studies, and it was found to be rhombohedral structure at room temperature. Under scanning electron microscope, grains separated by well-defined boundaries are visible, which is in good agreement with impedance analysis. The BNKLT ceramic shows excellent piezoelectric properties and the optimum properties measured are: d ₃₃=251 pC/N, g ₃₃=24×10⁻³ mV/N, k _p =30.5% and k _t =28.1%. A complex impedance spectroscopy (CIS) study has been carried out to investigate the electrical properties. Impedance and modulus plots helped to separate the grain and grain boundary to the overall polarization or electrical behavior. CIS analysis suggests the presence of temperature-dependent relaxation process in the material. A possible hopping mechanism for electrical transport processes in the studied material is evident from the modulus analysis. The modulus mechanism indicates the non-Debye type of conductivity relaxation in the materials, which is supported by impedance data. The activation energies have been calculated from impedance (E _τ =0.58 eV) and electric modulus (E _τ =0.40 eV) studies, which suggests that the conduction is ionic in nature. The variation in width of the curves, M"/M"_maxM'/M'_{\max} and Z"/Z"_maxZ'/Z'_{\max} at FWHM, allows to conform that the relaxation process involved is of non-Debye type.     

Oxygen ion transport numbers: assessment of combined measurement methods

Several modifications of the faradaic efficiency and electromagnetic field (EMF) methods, taking electrode polarisation resistance into account, were considered based on the analysis of ion transport numbers and p-type electronic conductivity of ceramics at 973–1,223 K. In air, the activation energies for p-type electronic and oxygen ionic transport are 115 ± 9 and 71 ± 5 kJ/mol, respectively. The oxygen ion transference numbers vary in the range 0.992–0.999, increasing when oxygen pressure or temperature decreases. The apparent electronic contribution to the total conductivity, estimated from the classical faradaic efficiency and EMF techniques was considerably higher than true transference numbers due to a non-negligible role of interfacial exchange processes. The modified measurement routes give reliable and similar results when p(O₂) values at the electrodes are high enough, whilst decreasing the oxygen pressure leads to a systematic error for all techniques associated with measurements of concentration cell EMF. This effect, presumably due to diffusion polarisation, increases with decreasing temperature. The most reliable results in the studied p(O₂) range were provided by the modified faradaic efficiency method.     

Optical properties and electronic structure of YNi<Subscript>5 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> intermetallic compounds

The optical properties of hexagonal intermetallic compounds YNi_{5 − x} Cu _x (x = 0, 1, 2) have been investigated by ellipsometry in the spectral range of 0.22–15 μm. It is shown that the replacement of nickel atoms by copper atoms leads to local changes in the optical-conductivity spectra. A new absorption band is found at 3.5–4.5 eV; its intensity depends on the copper content. The plasma and relaxation frequencies of conduction electrons are determined. The electronic structure and interband optical conductivity of these compounds are calculated within the electron density functional theory using the pseudopotential method. The main parameters of the band structure and the total and partial densities of electronic states are determined. Qualitative agreement is obtained between the experimental and theoretical frequency dependences of the optical conductivity.     

Investigation on PVDF-HFP microporous membranes prepared by TIPS process and their application as polymer electrolytes for lithium ion batteries

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) microporous membranes were prepared via thermally induced phase separation (TIPS) process. Then they were immersed in a liquid electrolyte to form polymer electrolytes. The effects of polymer content in casting solution on the morphology, crystallinity, and porosity of the membranes were studied by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and a mercury porosimeter, respectively. Ionic conductivity, lithium-ion transference number, and electrochemical stability window of corresponding polymer electrolytes were characterized by AC impedance spectroscopy, DC polarization/AC impedance combination method, and linear sweep voltammetry, respectively. The results showed that spherulites and “net-shaped” structure coexisted for the membranes. Polymer content had no effect on crystal structure of the membranes. The maximum transference number was 0.55. The temperature dependence of ionic conductivity followed the Vogel–Tammann–Fulcher (VTF) relation. The maximum ionic conductivity was 2.93 × 10⁻³ Scm⁻¹ at 20 °C. Electrochemical stability window was stable up to 4.7 V (vs. Li⁺/Li).     

Synthesis and ionic conduction of apatite-type materials

Apatite is a mineral with general formula M₁₀(XO₄)₆Z₂, where M are metallic elements such as Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ln³⁺ etc.; X=P, V, S, Si, Ge, Re, Cr etc; Z=F⁻, Cl⁻, I⁻, OH⁻, O²⁻, S²⁻ etc. Some materials with apatite structure (S.G. P6₃/m) exhibit quite high cationic (Li⁺, H⁺ etc.) and/or anionic (F⁻, Cl⁻ etc.) conduction. Recently, it was reported that some rare earth silicates, e.g., La₁₀(SiO₄)₆O₃, exhibit quite high oxide-ion conductivity. In this paper, we discuss chemical composition, structure, synthetic procedure and ionic conduction of apatite-type materials. Recent improvements are briefly reviewed. High ionic conductivity has been observed for both cation deficient, oxygen stoichiometric La_9.33(SiO₄)₆O₂ and cation stoichiometric, oxygen excess La₁₀(SiO₄)₆O₃ compositions. Grain boundary conductivity is usually low, which tends to dominate the impedance response. The resistance, particularly the grain boundary resistance is also found to depend on pO₂. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Electrical characterization of highly Titania doped YSZ

The defect fluorite region of the ternary system ZrO₂-Y₂O₃-TiO₂ encompasses compositions which offer both, good electronic and oxygen ion conductivity which enable good catalytic activity for the direct oxidation of methane in a solid oxide fuel cell (SOFC). The electrical properties of compositions Y_xTi_yZr_1−(x+y)O_2−x/2 (with x=0.15, 0.2, 0.25 and y=0.15, 0.18) were characterised in order to find the composition with highest ionic and electronic conductivity. High titanium dopant concentrations (Y) of 15 and 18 atom%, near the solubility limit of Ti⁴⁺ in the fluorite structure, have been introduced to achieve a high electronic conductivity at low oxygen partial pressure. The yttrium content x has been varied between 15 and 25 atom% to find the fluorite composition with the highest ionic conductivity for each titanium level. In the pO₂-range from 0.21 to 10⁻¹³ atm the conductivity is predominantly ionic and constant over that range. The maximum ionic conductivity is 0.01 Scm⁻¹ for the compositions, which contain 15 atom% yttrium. Substantial electronic conductivity is introduced into the system at low oxygen pressures below 10⁻¹³ atm via reduction of Ti⁴⁺ ions to Ti³⁺. The maximum electronic conductivity of 0.2 Scm⁻¹ at 930 °C has been measured for a sample with 18 atom% titanium. The slope of all log(σ) vs. log(pO₂) plots follows a pO ₂ ^−1/4 -dependence. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Assessment of the electronic conductivity of core-shell, Fe-doped LSGM ceramics by impedance spectroscopy

The structure, microstructure and low-temperature electrical properties of core-shell-type mixed conductors based on lanthanum gallate with Fe-doped grain boundaries are analyzed in depth. Electron probe microanalysis revealed that the iron concentration in the grain-boundary regions (shell) is below 1 at.% and their thickness is no more than 1.5 μm. The low-temperature (< 400 °C) electronic conductivity is enhanced by up to 2-3 orders of magnitude with respect to the corresponding undoped ceramics, as revealed by the analysis of impedance spectra combined with microstructural information. The electronic transport numbers lie in the range between 0.35 and 0.1 at 275 to 400 °C, decreasing at higher temperatures, where the influence of grain boundaries on the overall transport properties vanishes and the ionic conductivity increases.