Effects of protons and acceptor substitution on the electrical conductivity of La6WO12

Transport properties and non-stoichiometry of La_1−xCa_xW_1/6O₂ and La_1−yW_1/6O₂ (x=0, 0.005, 0.05; y=0.05, 0.1) have been characterized by means of impedance spectroscopy, the EMF-technique, H⁺/D⁺ isotope exchange, and thermogravimetry in the temperature range 300-1200 °C as a function of oxygen partial pressure and water vapor partial pressure. The materials exhibit mixed ionic and electronic conductivities; n- and p-type electronic conduction predominate at high temperatures under reducing and oxidizing conditions, respectively. Protons are the major ionic charge carrier under wet conditions and predominates the conductivity below ∼750 °C. The maximum in proton conductivity is observed for LaW_1/6O₂ with values reaching 3×10⁻³ S/cm at approximately 800 °C. The high proton conductivity for the undoped material is explained by assuming interaction between water vapor and intrinsic (anti-Frenkel) oxygen vacancies.     

Electronic conductivity of pure ceria

The electronic conductivity of pure ceria with two different impurity levels is examined by dc polarization technique based on the Hebb-Wagner ion blocking method. The impurity level for the ceria with 99.999% purity (5N-CeO₂) is about 1/100 of that with 99.9% purity (3N-CeO₂) as confirmed by the fluorescence intensity of impurities obtained by Raman spectroscopy. The electronic conductivity for the 5N-CeO₂ was measured at T = 973 K to 1173 K, and the results are essentially the same as those for the 3N-CeO₂. The electronic conductivity increases with decreasing of P(O₂) following slope values of − 1/4 to − 1/6. The − 1/4 dependent region becomes narrower for the 5N-CeO₂ than that for the 3N-CeO₂. For both types of ceria, the P(O₂) independent region appears in the same region of higher than 10^− 2 and 10^− 3 MPa at T = 1073 K and 973 K, respectively. Activation energies for the 5N-CeO₂ were 2.2 eV, 2.6 eV and 1.9 eV in P(O₂) dependent regions of − 1/6, − 1/4 and 0, respectively.     

Oxidation enthalpies for reduction of ceria surfaces

The thermodynamic properties of surface ceria were investigated through equilibrium isotherms determined by flow titration and coulometric titration measurements on high-surface-area ceria and ceria supported on La-modified alumina (LA). While the surface area of pure ceria was found to be unstable under redox conditions, the extent of reduction at 873 K and a P(O₂) of 1.6 × 10⁻²⁶ atm increased with surface area. Because ceria/LA samples were stable, equilibrium isotherms were determined between 873 and 973 K on a 30-wt% ceria sample. Oxidation enthalpies on ceria/LA were found to vary with the extent of reduction, ranging from −500 kJ/mol O₂ at low extents of reduction to near the bulk value of −760 kJ/mol O₂ at higher extents. To determine whether +3 dopants could affect the oxidation enthalpies for ceria, isotherms were measured for Sm⁺³-doped ceria (SDC) and Y⁺³-doped ceria. These dopants were found to remove the phase transition observed in pure ceria below 973 K but appeared to have minimal effect on the oxidation enthalpies. Implications of these results for catalytic applications of ceria are discussed.     

High sensitivity of magnetism of the “114” ferrimagnet CaBaCo4O7 to stoichiometry deviation

The effect of oxygen/cobalt off-stoichiometry upon magnetism in CaBaCo₄O₇ has been investigated. It is shown that the oxides CaBaCo₄O_7+δ and CaBaCo_4−xO_7−δ (0≤x≤0.20) synthesized below 1100 °C in air exhibit phase separation, where ferrimagnetic regions with T_C~56 K to 64 K coexist with regions of magnetic clusters. The latter are detected from ac-susceptibility measurements, which show various frequency dependent peaks at ~14–20 K, 37 K, and 45 K, depending on the stoichiometry. The origin of this phenomenon is attributed to the great sensitivity of the material to oxidation as the synthesis of temperature is lowered, leading to the introduction of additional Co³⁺ cations, with respect to the ideal formula CaBaCo₂²⁺Co₂³⁺O_7. This excess Co³⁺ tends to destroy the ferromagnetic zig-zag chains of the ferrimagnetic structure and creates various cobalt spin clusters, leading to the inherent phase separation in the samples.     

The size and strain effects on the Raman spectra of Ce1−xNdxO2−δ (0≤x≤0.25) nanopowders

Ultrafine Ce_1−xNd_xO_2−δ (x=0-0.25) powders were synthesized by self-propagating room temperature synthesis. Raman spectra were measured at room temperature in the 300-700 cm⁻¹ spectral range. The shift and asymmetric broadening of the Raman F_2g mode at about 454 cm⁻¹ in pure and doped ceria samples could be explained with combined size and inhomogenous strain effects. Increased concentration of O²⁻ vacancies with doping is followed by an appearance of new Raman feature at about 545 cm⁻¹.     

The sodium diffusion in aluminium-oxide

The transport of Na through the polycrystalline ceramic arc tube of high intensity discharge lamps has been investigated. This complex process consists of several steps: solution in the ceramics, diffusion through the ceramics, leaving the bulk phase, evaporation from the surface. Among the listed processes the kinetics of the diffusion was examined in the temperature range 400-1200 °C, separately from other disturbing effects. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) were used to determine the concentration depth profiles. The obtained results confirmed that the grain boundary diffusion plays an important role in the transport process of sodium through the ceramic wall. The bulk and the grain boundary diffusion coefficients and the temperature dependencies of these transport processes have been determined. The activation energy of Na bulk diffusion is 56.5 ± 6.7 kJ/mol at 900-1200 °C, respectively the activation energies of Na grain boundary diffusion amount to 97.5 ± 21.6 kJ/mol in the temperature range 700-1100 °C and 7.7 ± 4.0 × 10⁻² kJ/mol at 400-700 °C. The preexponential factor of the bulk diffusion was found to be D_o = 5.1 × 10⁻¹⁵ ± 9.5 × 10⁻¹⁷ cm²/s in the temperature range 900-1200°C, whereas the preexponential factors of grain boundary diffusion are D_o = 1.1 × 10⁻¹⁰ ± 1.1 × 10⁻¹¹ cm²/s at 700-1100 °C and D_o = 7.5 × 10^-15 ± 1.5 × 10⁻¹⁷ cm²/s at 400-700 °C.     

Optimal growth conditions for GdBa2Cu3O7 thin-film coated conductors characterized by polarized Raman scattering spectroscopy

High temperature superconducting GdBa₂Cu₃O₇ (GdBCO) thin films were grown by pulsed laser ablation. Textured MgO on metal substrates was used as a template for second generation wire applications. Growth conditions of GdBCO thin films were investigated for substrate temperature (T_s) and oxygen partial pressure (PO₂) during deposition. Superconducting critical currents of the films were obtained in the films grown at 790–810 °C of T_s and at 100–700 mTorr of PO₂. Scanning electron micrographs of the films revealed uniform and well-connected grains with some outgrown structures. X-ray θ–2θ scans of the films grown at 810 °C and 300–500 mTorr exhibited c-axis oriented texture. In-plane alignment and c-axis mosaic spread of the films were determined from X-ray Φ scans and rocking curves, respectively. Polarized Raman scattering spectroscopy was used to characterize optical phonon modes, oxygen content, cation disorder, and some possible second phases of the films. The Raman spectra of the films with large critical current density showed modes at 326–329 cm⁻¹, 444–447 cm⁻¹, 500–503 cm⁻¹ related to vibration of oxygen atoms. Origin of small peaks near 600 cm⁻¹ will be discussed as well. The information obtained from Raman scattering measurements will be useful for quality control of the conductors as well as optimization of the process conditions.     

Properties of doped ceria solid electrolytes in reducing atmospheres

In this work we present investigations of stability of rare earth-doped ceria electrolytes in reducing atmosphere. The effect of dopant type and dopant concentration on reducibility was studied on the basis of thermogravimetric and impedance spectroscopy measurements on materials grouped into two series: Ce_1 − xGd_xO_2 − x/2 (0 ≤ x ≤ 0.4) and Ce_0.85R_0.15O_1.925 (R = Y, Nd, Sm, Gd, and Dy). Relationship between an initial vacancy concentration introduced by the amount of dopant and the characteristic temperature of reduction was found. Much less pronounced dependence was observed for different dopants with the same concentration, which indicates that it is the dopant and vacancy concentration and not the dopant type, which is responsible for reducibility of ceria electrolytes. Impedance spectroscopy measurements allowed for calculation of changes of oxygen ions transport number during the reduction process.     

Defect structure and oxide ion transport in Sr- and Cr-doped LaCoO3 − δ

The results of oxygen nonstoichiometry, δ, measured by means of coulometric technique as a function of oxygen partial pressure, p_o2, in temperature range 1223 ≤ T, K ≤ 1323 are presented for the perovskite-type doped with chromium solely LaCo_0.7Cr_0.3O_3 − δ and simultaneously doped both with strontium and chromium La_0.7Sr_0.3Co_0.7Cr_0.3O_3 − δ cobaltites. The limit stability of the latter was found to exceed that of undoped cobaltite LaCoO_3 − δ on six orders of magnitude of p_o2 at a given temperature. The modeling of the defect structure of these perovskites was carried out and its adequate model was found. Chemical and self-diffusion coefficients of oxygen vacancies and oxygen ionic conductivity and ionic transport numbers were measured for the first time for La_0.7Sr_0.3Co_0.7Cr_0.3O_3 − δ as a function of oxygen partial pressure p_o2and temperature in the ranges − 4 ≤ log(p_o2, atm) ≤ 0 and 1223 ≤ T, K ≤ 1323, respectively. The additional substitution of Sr for La in LaCo_0.7Cr_0.3O_3 − δ was shown to lead to noticeable increase of ionic conductivity and oxygen chemical diffusion coefficient at given values of oxygen partial pressure and temperature as compared to lanthanum cobaltite doped with chromium solely. Self-diffusion coefficient of oxygen vacancies and their mobility in La_0.7Sr_0.3Co_0.7Cr_0.3O_3 − δ were found to be dependent on oxygen partial pressure and nonstoichiometry unlike undoped and doped with chromium lanthanum cobaltites.     

Low temperature sintering of sub-stoichiometric Ni-Cu-Zn ferrites: Shrinkage, microstructure and permeability

We have studied sub-stoichiometric Ni-Cu-Zn ferrites with iron deficiency (i.e., <50mol% Fe₂O₃) of composition Ni_0.20Cu_0.20Zn_0.60+zFe_2−zO_4−(z/2) with 0≤z≤0.06. The temperature of maximum shrinkage rate is shifted from T=1000 °C for z=0 towards lower temperatures down to T=900 °C for a sub-stoichiometric ferrite with z=0.02. Dense samples are obtained after firing at 900 °C for z>0 only. Sub-stoichiometric compositions (z>0) do not form single-phase spinel ferrites after sintering at 900 °C, but rather represent mixtures of CuO and a stoichiometric ferrite with slightly modified composition. The formation of small amounts of CuO at grain boundaries is demonstrated by XRD and SEM. The permeability is increased from μ=80 for stoichiometric ferrites (z=0) to μ=660 for z=0.02. The formation of CuO during sintering of sub-stoichiometric ferrites supports densification and is a prerequisite for low temperature firing of multilayer inductors. Addition of 1 wt% Bi₂O₃ as liquid phase sintering aid is required to provide sufficient densification of the stoichiometric ferrite (z=0) at 900 °C. Addition of 0.37 wt% Bi₂O₃ to a sub-stoichiometric ferrite (z=0.02) results in dense samples after firing at 900 °C; however, the microstructure formation is dominated by heterogeneous grain growth.     

Oxygen deficiency, vacancy clustering and ionic transport in (La,Sr)CoO3 − δ

The equilibrium p(O₂)-T-δ diagrams of perovskite-type La_1 − xSr_xCoO_3 − δ (x = 0.3-0.7), collected at 873-1223 K in the oxygen partial pressure range 10^− 5-1 atm by coulometric titration and thermogravimetric analysis, were analyzed in order to appraise the effects of the point-defect interactions. The nonstoichiometry variations were adequately described combining the rigid-band approach for delocalized holes and the pair-cluster formation reaction involving oxygen vacancies and Co²⁺ cations, whilst coulombic repulsion between the positively charged vacancies can be neglected. The resultant relationships between the oxygen chemical potential and mobile vacancy concentration were used for numerical regression analysis of the steady-state oxygen permeation through dense La_1 − xSr_xCoO_3 − δ membranes, affected by the surface exchange kinetics when Sr²⁺ content is higher than 40-50%. The calculated ionic conductivity is strongly influenced by the defect association processes, and decreases with decreasing concentration of the mobile vacancies as clustering starts to prevail on reduction. The Mössbauer spectroscopy studies of La_1 − xSr_xCoO_3 − δ, doped with 1 mol% ⁵⁷Fe isotope and moderately reduced at p(O₂) ≈ 10⁻⁵ atm, show no long-range vacancy ordering at x ≤ 0.5.     

Effect of sintering temperature on the structural, dielectric and ferroelectric properties of tungsten substituted SBT ceramics

Structural, dielectric and ferroelectric properties of tungsten (W) substituted SrBi₂(Ta_1−xW_x)₂O₉ (SBTW) [x=0.0, 0.025, 0.05, 0.075, 0.1 and 0.2] have been studied as a function of sintering temperature (1100-1250 °C). X-ray diffraction patterns confirm the single-phase layered perovskite structure formation up to x=0.05 at all sintering temperatures. The present study reveals an optimum sintering temperature of 1200 °C for the best properties of SBTW samples. Maximum T_c of ∼390 °C is observed for x=0.20 sample sintered at 1200 °C. Peak-dielectric constant (ε_r) increases from ∼270 to ∼700 on increasing x from 0.0 to 0.20 at 1200 °C sintering temperature. DC conductivity of the SBTW samples is nearly two to three orders lower than that of the pristine sample. Remnant polarization (P_r) increases with the W content up to x≤0.075. A maximum 2P_r (∼25 μC/cm²) is obtained with x=0.075 sample sintered at 1200 °C. The observed behavior is explained in terms of improved microstructural features, contribution from the oxygen and cationic vacancies in SBTW. Such tungsten substituted samples sintered at 1200 °C exhibiting enhanced dielectric and ferroelectric properties should be useful for memory applications.     

Oxygen nonstoichiometry, defect structure and electrical properties of LaFe0.7Ni0.3O3 − δ

Oxygen nonstoichiometry (δ), total conductivity (σ) and thermoelectric power (S) of the LaFe_0.7Ni_0.3O_3 − δ sample have been studied as functions of temperature and oxygen partial pressure. Based on the results of the direct reduction of the sample in hydrogen flow at 1100 °C the absolute oxygen content (3 − δ) has been found to vary from 2.999 to 2.974 in the range of 1273-1373 K and 10^− 3-0.21 atm. The point defect equilibrium models have been proposed and fitted to the set of experimental data in the form of log p(O₂) = f(δ)_T dependences. The values of standard thermodynamic quantities of defect formation reactions have been assessed. The joint analysis of oxygen nonstoichiometry, total conductivity and thermoelectric power has been performed using a small-polaron approach. The values of partial conductivity, partial thermopower and mobilities of electronic charge carriers have been calculated. The p-type semiconducting behavior of LaFe_0.7Ni_0.3O_3 − δ has been explained by the higher mobility values of electron holes than those of electrons in the whole range of thermodynamic parameters studied.     

Oxygen relaxation and phase transition in GdBaCo2O5 + δ oxide

Electrical conductivity, internal friction techniques and dilatometer have been used to investigate the oxygen relaxation, phase transition and thermal expansion behavior of GdBaCo₂O_5 + δ. The main electronic charge carriers in GdBaCo₂O_5 + δ are electronic holes, which could be assigned to the formation of Co⁴⁺. The oxygen exchange kinetics intensely depends on oxygen partial pressure and is also closely related to temperature. Both electrical conductivity and internal friction give rise to an abnormal at about 75 °C, which are related to the insulator-metal transition occurring in GdBaCo₂O_5 + δ. One large relaxation internal friction peak, due to the motion of oxygen within Gd-O plane, is also found in the oxide. The average thermal expansion coefficient (TEC) of GdBaCo₂O_5 + δ is about 21.4 × 10⁻⁶ K⁻¹ between 500 °C and 900 °C.     

The effect of duration of diffusion on Ag diffusion coefficients in YBa2Cu3O7

The Ag diffusion in superconducting YBa₂Cu₃O₇ (YBaCuO) ceramic has been studied over the duration of the diffusion range 5-24 h in the temperature range 700-850 °C by the energy-dispersive X-ray fluorescence (EDXRF) technique. For the excitation of silver atoms, an annular Am-241 radioisotope source (50 mCi) emitting 59.543 keV photons was used. The temperature dependences of silver diffusion coefficients in grains (D₁) and over the grain boundaries in the range 700-850 °C (D₂) are described by the relations D₁=1.4×10⁻² exp[−(1.18±0.10)/kT] and D₂=3.1×10⁻⁴ exp[−(0.87±0.10)/kT].     

Electrical properties of KTiOPO4 single crystal in the temperature range from −100 °C to 100 °C

The electrical property of a KTiOPO₄ single crystal was studied by means of a dielectric spectroscopy method in the temperature range from −100 to 100 °C. Dielectric dispersion began at a temperature, T_S=−80 °C. It is believed that this dielectric dispersion is related to the ionic hopping conduction, which arises mainly from the jumping of K⁺ ions. The activation energy concerned with hopping conduction is E_a∼0.20 eV above T_S. T_S=−80 °C can be the minimum temperature for the hopping K⁺ ion.     

Zn- and Cu-substituted Co2Y hexagonal ferrites: Sintering behavior and permeability

Y-type polycrystalline hexagonal ferrites Ba₂Co_2−x−yZn_xCu_yFe₁₂O₂₂ with 0≤x≤2 and 0≤y≤0.8 were prepared by the mixed-oxide route. Single phase Y-type ferrite powders were obtained after calcinations at 1000 °C. Samples sintered at 1200 °C show a permeability that increases with the substitution of Zn for Co and display maximum permeability of μ′=35 at 1 MHz for x=1.6 and y=0.4. A resonance frequency f_r=500 MHz is observed for Zn-rich ferrites with y=0 and 0.4. The saturation magnetization increases with substitution of Zn for Co. Addition of Bi₂O₃ shifts the temperature of maximum shrinkage down to T≤950 °C. Moreover, an increase of the Cu-concentration further lowers the sintering temperature to T≤900 °C, enabling co-firing of the ferrites with Ag metallization for multilayer technologies. However, low-temperature firing reduces the permeability to μ′=10 and the resonance frequency is shifted to 1 GHz. Thus substituted hexagonal Y-type ferrites can be used as soft magnetic materials for multilayer inductors for high frequency applications.     

Antiferroelectric phase transition in pyrochlore-like (Dy1− xCax)2Ti2O7 − δ (x = 0, 0.1) high temperature conductors

Low-temperature ordering transitions in polycrystalline high temperature conductors (Dy_1 − xCa_x)₂Ti₂O_7 − δ (x = 0, 0.1) prepared using co-precipitation, mechanical activation and solid-state reactions at 1400 or 1600 °C have been studied by impedance spectroscopy at low frequencies and thermal mechanical analysis (TMA). The dielectric permittivity and loss tangent of the ceramics obtained have been measured as a function of temperature at low frequencies (0.5−500 Hz). The results provide evidence for the relaxation of point defects, most likely oxygen vacancies, at 500−600 °C and an antiferroelectric low-temperature phase transition of the second order, associated with re-arrangement process in the oxygen sublattice of pyrochlore structure. The temperature of the antiferroelectric transition is 700 to 800 °C, depending on the synthesis procedure and ceramic composition. Calcium doping of Dy₂Ti₂O₇ leads to the formation of additional oxygen vacancies and, in the case of the samples prepared via co-precipitation, increases the peaks in permittivity due to the relaxation process and ordering transition by three or six times, respectively.     

High-throughput characterization of BixY3−xFe5O12 combinatorial thin films by magneto-optical imaging technique

Bi_xY_3−xFe₅O₁₂ thin films have been grown on GGG (Gd₃Ga₅O₁₂) (1 1 1) substrates by the combinatorial composition-spread techniques under substrate temperature (T_sub) ranging from 410 to 700 °C and O₂ pressure of 200 mTorr. In order to study the effect of substrates on the deposition of Bi_xY_3−xFe₅O₁₂ thin films, garnet substrates annealed at 1300 °C for 3 h were also used. Magneto-optical properties were characterized by our home-designed magneto-optical imaging system. From the maps of Faraday rotation angle θ_F, it was evident that the Faraday effect appears only when T_sub = 430-630 °C. θ_F reaches to the maximum value (∼6°/μm, λ = 632 nm) at 500 °C, and is proportional to the Bi contents. XRD and EPMA analyses showed that Bi ions are easier to substitute for Y sites and better crystallinity is obtained for annealed substrates than for commercial ones.     

Nd-laser irradiations effects and AFM-investigations on structural and surface features of optimally lutetium thorium-co-doped-2212-BSCCO

Lutetium-thorium optimally co-doped sample with general formula Bi_2−x−yLu_xTh_ySr₂Ca₁Cu₂O₈ where x = y = 0.15 mol was selected to be the target for three different doses of Nd-laser beam irradiations the 1st 15 W/cm² for 60 min, 2nd 30 W/cm² for 120 min and 3rd 45 W/cm² for 240 min, respectively. Nd-Laser irradiations doses were monitoring as function of micro-structure, superconducting and structural features of Lu-Th-optimally co-doped 2212-BSCCO. AFM and SE-microscopy were applied to investigate changes occurred in the surface's layers after each Nd-laser irradiation dose. Both of bulk superconductivity and surface resistivity were measured to establish the promotion occurred on the superconducting features due to Nd-laser irradiation doses.