A magnetic pair-breaking effect in rare earth-doped manganites

The magnetic properties of La_0.60R_0.07Di_0.33MnO₃ ferromagnetic manganites (Di = Sr, Ba) are studied, where La is partly replaced by magnetic rare earths R. It is shown that (i) there is a ferromagnetic coupling between Mn and R spins, (ii) the Curie temperature is lowered compared to the parent La compound and (iii) its depression is correlated with the effective moment of the rare earth ion. This last relation is tentatively explained by a magnetic pair-breaking effect, where fluctuating R moments lower the double-exchange coupling between Mn atoms.     

Characterization of Ln0.5M0.5FeO3-δ (Ln = La, Nd, Sm; M = Ba, Sr) perovskites as SOFC cathodes

Ln_0.5M_0.5FeO_3-δ (Ln = La, Nd, Sm; M = Ba, Sr) perovskites have been synthesised by the glycine-nitrate route in order to obtain compounds with good morphological and electrical characteristics to be used as SOFC cathodes. A systematic study of the influence of the average ionic radius of the A cation (<r_A>), on the structural, morphological and electrical properties of these perovskites has been performed. The obtained materials have been characterised by ICP-AES analysis, X-ray powder and neutron powder diffraction, scanning electron microscopy and four-probe conductivity measurements. In this study <r_A> has been changed in the range 1.34 ≥ <r_A > ≥ 1.25 Å, while keeping constant the doping level, x = 0.5 (in A position) and A cation size disorder, σ² (r_A) = 0.0161 Å². Overall, the sample with the higher average ionic radius showed the highest values of conductivity. Chemical compatibility of the proposed cathodes with yttria-stabilised zirconia electrolyte (YSZ) has also been analysed. The data indicate that additional phase formations exist in all systems at temperatures above ~ 1000 °C.     

Effect of dimension on charge order domain in Ruddlesden-Popper manganites

The charge order (CO) domains of dimensionally controlled manganites Pr_1−xCa_xMnO₃ and Pr_1−xCa_1+xMnO₄ (x=0.5, 0.6 and 0.67), which have three-dimensional (3D) and two-dimensional (2D) Mn-O networks, respectively, have been studied by transmission electron microscopy (TEM). Although the electron diffraction data show similar dependences of the modulation wave vector on hole doping x, there are distinctive differences between the 3D and 2D systems in terms of the CO domain sizes. In the 2D system, the TEM images show that the domain size is almost constant irrespective of hole doping x. On the other hand, in the 3D system, the domain size of the incommensurate CO for x=0.6 is much smaller than those of the commensurate CO for x=0.5 and 0.67. Namely, in the 3D system, the CO states are strongly influenced by the incommensurability for the parent lattice. This difference indicates that the dimension of the Mn-O network plays a crucial role in the CO domain and suggests that the electron-lattice coupling of the 3D system is stronger than that of the 2D system.     

High-temperature electrical properties of magnesiowustite Mg1 − xFexO and spinel Fe3 − x − yMgxCryO4 ceramics

Phase relationships, thermal expansion and electrical properties of Mg_1 − xFe_xO (x = 0.1-0.45) cubic solid solutions and Fe_{3 − x − y}Mg_xCr_yO_4 ± δ (x = 0.7-0.95; y = 0 or 0.5) spinels were studied at 300-1770 K in the oxygen partial pressure range from 10 Pa to 21 kPa. Increasing iron content enlarges the spinel phase stability domain at reduced oxygen pressures and elevated temperatures. The total conductivity of the spinel ceramics is predominantly n-type electronic and is essentially p(O₂)-independent within the stability domain. The computer simulations using molecular dynamics technique confirmed that overall level of ion diffusion remains low even at high temperatures close to the melting point. Temperature dependencies of the total conductivity in air exhibit a complex behavior associated with changing the dominant defect-chemistry mechanism from prevailing formation of the interstitial cations above 1370-1470 K to the generation of cation vacancies at lower temperatures, and with kinetically frozen cation redistribution in spinel lattice below 700-800 K. The average thermal expansion coefficients of the spinel ceramics calculated from dilatometric data in air vary in the range (9.6-10.0) × 10^− 6 K^− 1 at 300-500 K and (13.2-16.1) × 10^− 6 K^− 1 at 1050-1370 K. Mg_1 − xFe_xO solid solutions undergo partial decomposition on heating under oxidizing and mildly reducing conditions, resulting in the segregation of spinel phase and conductivity decrease.     

Performance and stability of niobium-substituted Ba0.5Sr0.5Co0.8Fe0.2O3 − δ membranes

The phase stability, thermal expansion, electrical conductivity, and oxygen permeation of perovskite-type oxides Ba_0.5Sr_0.5(Co_0.8Fe_0.2)_1 − xNb_xO_3 − δ (x = 0 − 0.2) have been investigated. Room-temperature X-ray diffraction of as-prepared powders indicates that in the investigated compositional range solid solutions are formed. Long-term annealing experiments both in flowing air and nitrogen, at 750 °C, demonstrate that the phase instability observed in parent Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ (BSCF) is suppressed already at the minimum substitution of 5 mol% of niobium for (Co, Fe). Both electrical conductivity and thermal expansion are found to decrease with increasing niobium concentration, which behaviors can be explained by defect chemical considerations, taking into account charge compensation mechanisms by doping BSCF with Nb⁵⁺ donor cations. The oxygen permeation flux of 10 mol% Nb-substituted BSCF, in the range 800-900 °C, is reduced by 10% relative to that found for parent BSCF. Switching from helium to a CO₂-containing purge gas results in a severe reduction or cessation of the oxygen flux. Options are discussed to avoid undesired formation of surface carbonates.     

First-principles calculations of elastic and thermo-physical properties of Al, Mg and rare earth lanthanide elements

The elastic constants of the Al, Mg and rare earth (RE) lanthanide elements have been calculated at T=0 K by using the projector augmented-wave (PAW) method within the generalized gradient approximation (GGA). The bulk moduli, shear moduli, Young's moduli and Poisson's ratio of poly-crystalline solid are estimated from the calculated elastic constants of single crystal. Based on the quasi-harmonic Debye model, the Debye temperature, heat capacity, Grüneisen parameter and linear thermal expansion coefficient are also estimated. The present calculated results are in reasonable agreement with the available experimental data and other theoretical results. The present calculation of elastic constants for Ce also indicates that the PAW potential (named “Ce_3”), for which one f electron is kept frozen in the core and hence fix the valency of Ce to three (Ce_3) does not yield good results for the elastic constants.     

Electrical characterization of strontium tartrate single crystals

The a.c. and d.c. conductivity of SrC₄H₄O₆·3H₂O are measured and are found to lie between usual conductivities of semiconductor and insulator. Temperature dependence of d.c. conductivity shows intrinsic conduction, which is confirmed by the slope of versus data. Due to application of thermal energy, noticeable conductivity peaks imply liberation of water molecules during dehydration and the formation of strontium oxalate. The conductivity plot has a nature similar to the intrinsic-to-extrinsic transition found in normal semiconductors. There occurs Efros hopping conduction in our samples.     

Growth of rare earth silicides on silicon

Dysprosium metal layers deposited on (100)Si substrates were annealed in situ at 300 °C for different time intervals in order to produce DySi_2−x layers at several stages of growth. Electron microscopy work showed that silicon reacts with Dy metal through the simultaneous growth of an amorphous interlayer and a polycrystalline phase to form DySi_2−x. Compressive stresses due to volumetric changes during the reaction deform the produced silicide layer and induce roughness at the surface and interface.     

Enhancing the phase stability and ionic conductivity of scandia stabilized zirconia by rare earth co-doping

Effect of co-doping Yb, Gd and Ce in scandia stabilized zirconia (SSZ) on the phase stability, high temperature aging behavior and ionic conductivity was studied. Both binary (10 mol% SSZ) and the ternary (co-doped) compositions were found to be in single cubic phase in the as-processed condition. However, the binary composition exhibited the rhombohedral ‘β’ phase after sintering whereas the ternary compositions remained in the single cubic phase. The sintered pellets were aged at 900 °C for 500 h in air to study the phase stability at high temperature. Transmission electron microscopy revealed that the aged samples of Yb and Gd co-doped compositions contain small amount of the tetragonal phase which resulted in considerable degradation in conductivity (more than 20%). The Ce co-doped sample, on the other hand, was in single cubic phase after aging and this ensured that conductivity reduction was minimal in this composition. The co-doped samples however, showed higher conductivity before and after aging compared to the binary composition. The rhombohedral ‘β’ phase was absent in all the co-doped ternary compositions even after high temperature aging.     

Synthesis and characterization of apatite-type La9.67Si6-xAlxO26.5-x/2 electrolyte materials and compatible cathode materials

Apatite silicates have recently been reported as promising electrolyte materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this work, a series of apatite-type compounds La_9.67Si_6-xAl_xO_26.5-x/2 (LSAO) with x = 0-2 are synthesized by the sol-gel process at calcining temperature of 800-900 °C. Thermal expansion coefficient, relative density and electrical conductivity of these samples with different Al doped contents are investigated. A symmetrical cell, which is composed of La_9.67Si₅AlO₂₆ electrolyte and (La_0.74Bi_0.10Sr_0.16)MnO_3+δ (LBSM) cathode, is fabricated and electrochemically characterized. LBSM cathode shows a good electrochemical performance, which proves LBSM to be a promising candidate cathode for LSAO-based electrolyte.     

Room temperature magnetization measurements of some substituted rare earth iron garnets

The saturation magnetization (M _s) and the initial magnetic susceptibility (χ _in) of substituted yttrium iron garnet (YIG) Ho_3−x Y _x Fe₅O₁₂ where (x=0,0.3,1.5), pure terbium iron garnet Tb₃Fe₅O₁₂, and substituted aluminum iron garnet (AlIG) Ho₃Fe_5−x Al _x O₁₂ where (x=0.05,0.7) at room temperature are reported. It has been observed that M _s and χ _in decrease in a linear manner as Ho³⁺ replaces Y³⁺ in yttrium iron garnet. For substituted (AlIG), a drastic decrease in both M _s and χ _in as Al³⁺ ions replace Fe³⁺ ions. The rate of decrease of M _s and χ _in with the increase of Al³⁺ for substituted (AlIG) is greater than that for substituted (YIG). That is attributed to the decrease in the superexchange interaction (self and mutual iron interactions) with Al substitution.     

Control of luminescence and conductivity of delafossite-type CuYO2 by substitution of rare earth cation (Eu, Tb) and/or Ca cation for Y cation

Delafossite-type oxides of CuTb_yY_1−yO₂, CuEu_yY_1−yO₂, CuCa_xTb_yY_1−x−yO₂ and CuCa_xEu_yY_1−x−yO₂ have been prepared by solid state reactions. The lattice-parameter dependence on the composition implies substitution of the Tb³⁺, Eu³⁺ and Ca²⁺ cations for the Y³⁺ site. Noticeable sharp emission lines due to the f-f transitions (⁵D₄→⁷F_J, J=3-6) of Tb³⁺ or due to the f-f transitions (⁵D₀→⁷F_J, J=0-4) of Eu³⁺ are observed at room temperature. Electrical conductivities of CuCa_xTb_yY_1−x−yO₂ and CuCa_xEu_yY_1−x−yO₂ are larger than those of CuTb_yY_1−yO₂ and CuEu_yY_1−yO₂, indicating the increase of the hole concentration caused by the substitution of Ca²⁺ for the Y³⁺ site. These results indicate the controllability of the luminescence and conductivity in CuCa_xTb_yY_1−x−yO₂ and CuCa_xEu_yY_1−x−yO₂ delafossite-type oxides by simultaneous substitution of the rare earth Tb³⁺ or Eu³⁺ cation and the Ca²⁺ cation for the Y³⁺ site.     

Thermal conductivity and magnetic properties of the B substituted Ca3Co4O9

In this study, we reported the effects of the boron (B) substitution into the Ca site in the Ca₃Co₄O₉ system on the electrical, thermal and magnetic properties between 300 K and 5 K. The results indicated that the B-substitution into the system caused an increase of resistivity due to the decrease on carrier concentration. Thermal conductivity decreased for the x = 0.5 B-substituted sample and then increased with increasing the B-content. Analysis on the thermal conductivity of samples showed that the phonon–phonon interaction term is the dominant component in the total thermal conductivity for all the samples. It was found that the point defect contribution to the thermal conductivity increased by increasing the B-content. The temperature dependence of magnetic susceptibility showed a paramagnetic behavior at room temperature and ferrimagnetic behavior below 20 K for unsubstituted sample. But, the magnetization decreased in the B-substituted samples. The substitution of B into the Ca site destroyed the interlayer coupling, which resulted in the decrease of the ferromagnetic behavior. The susceptibility data was fitted using Curie–Weiss law with temperature independent term and the μ_eff values were calculated to be 1.42 μ_B and 3.89 μ_B for unsubstituted sample and the highest B-substitution, respectively.     

Influence of the electrical field applied during thermal cycling on the conductivity of LLDPE/CNT composites

The effect of the electrical field on the conductivity of linear low-density polyethylene/carbon nanotubes/ (LLDPE/CNT) composites during temperature cycling has been investigated. Under applied voltage a positive resistivity temperature coefficient was observed during heating already at low (2–3 wt%) CNT content, followed by a large negative temperature coefficient during cooling whose value depends on the applied voltage. The resistivity values after thermal cycling were markedly lower, while they slightly increased in the absence of an electrical field. The effects of thermal cycling on structural and physical properties of the composites have been evaluated by X-ray diffraction and differential scanning calorimetry.     

Magnetic and transport properties of some light rare earth tungstates

Measurements relating to molar magnetic susceptibility, dc electrical conductivity and thermoelectric power of Nd, Sm, Eu and Gd are reported. The ac electrical conductivity at a few temperature ranges is also given. It is found that it follows the Curie-Weiss law behaviour and this has been attributed to the crystal field effect. The experimental value of Bohr magneton for the magnetic ions has been found to be in good agreement with theory. Thermoelectric power is negative in the measured temperature range suggesting these materials to bep-type semi-conductors and holes as the dominant charge carriers. The results are explained using band theory.     

Studies on the activation energy from the ac conductivity measurements of rubber ferrite composites containing manganese zinc ferrite

Manganese zinc ferrites (MZF) have resistivities between 0.01 and 10 Ω m. Making composite materials of ferrites with either natural rubber or plastics will modify the electrical properties of ferrites. The moldability and flexibility of these composites find wide use in industrial and other scientific applications. Mixed ferrites belonging to the series Mn_(1−x)Zn_xFe₂O₄ were synthesized for different ‘x’ values in steps of 0.2, and incorporated in natural rubber matrix (RFC). From the dielectric measurements of the ceramic manganese zinc ferrite and rubber ferrite composites, ac conductivity and activation energy were evaluated. A program was developed with the aid of the LabVIEW package to automate the measurements. The ac conductivity of RFC was then correlated with that of the magnetic filler and matrix by a mixture equation which helps to tailor properties of these composites.     

Thermal, electrical, and electrochemical properties of Lanthanum-doped Ba0.5Sr0.5 Co0.8Fe0.2O3-δ

Crystal structure, thermogravimetry (TG), thermal expansion coefficient (TEC), electrical conductivity and AC impedance of (Ba_0.5Sr_0.5)_1-xLa_xCo_0.8Fe_0.2O_3-δ (BSLCF; 0.05?x?0.20) were studied in relation to their potential use as intermediate temperature solid oxide fuel cell (IT-SOFC) cathode. A single cubic pervoskite was observed by X-ray diffraction (XRD). The TEC of BSLCF was increasing slightly with the increasing content of La, and all the compounds showed abnormal expansion at high temperature. Proved by the TG result, it was associated with the loss of lattice oxygen. The electrical conductivity, which is the main defect of Ba_0.5Sr_0.5 Co_0.8Fe_0.2O_3-δ (BSCF), was improved by La doping, e.g., the compound of x=0.20 demonstrated a conductivity of σ=376 S cm⁻¹ at 392 °C. The increase of electrical conductivity resulted from the increased concentration of charge carrier induced by La doping. In addition, the AC impedance revealed the better electrochemical performance of BSLCF. For example, at 500 °C, the sample with composition x=0.15 yielded the resistance values of 2.12 Ω cm², which was only 46% of BSCF.     

Effect of nickel substitution on defect chemistry, electrical properties, and dimensional stability of calcium-doped yttrium chromite

The effect of nickel substitution on defect chemistry, electrical properties, and dimensional stability of calcium-doped yttrium chromite was studied for use as an interconnect material in high temperature solid oxide fuel cells (SOFCs). The compositions of Y_0.8Ca_0.2Cr_1 − xNi_xO_3 ± δ (x = 0-0.15), prepared using the glycine nitrate process, showed single phase orthorhombic perovskite structures over a wide range of oxygen partial pressures (4.6 × 10^− 20 atm ≤ pO₂ ≤ 0.21 atm at 900 °C). X-ray diffraction (XRD) analysis indicated that most of the nickel ions replacing chromium ions are divalent and act as acceptor dopants, leading to a substantial increase in conductivity. In particular, the conductivity at 900 °C in air increased from 10 S/cm to 34 S/cm with 15% nickel substitution, and an increase in charge carrier density was confirmed by Seebeck measurements, which validated the predominant Ni²⁺ oxidation state. A point defect model was derived, and the relationship between electrical conductivity and oxygen partial pressure was successfully fitted into the proposed model. The defect modeling results indicated that nickel substitution improves the stability of calcium-doped yttrium chromite toward reduction and suppresses the oxygen vacancy formation, which results in significantly increased electrical conductivity in reducing environment. The electrical conductivity of Y_0.8Ca_0.2Cr_0.85Ni_0.15O_3 ± δ at 900 °C in reducing atmosphere (pO₂ = 10^− 17 atm) was 5.8 S/cm, which was more than an order of magnitude higher than that of Y_0.8Ca_0.2CrO_3 ± δ (0.2 S/cm). Improved stability in reducing atmosphere was further confirmed by dilatometry measurements showing reduced isothermal “chemical” expansion, and the isothermal expansion in reducing atmosphere (pO₂ = 10^− 17 atm) at 900 °C decreased from 0.07% for Y_0.8Ca_0.2CrO_3 ± δ to 0.03% for Y_0.8Ca_0.2Cr_0.85Ni_0.15O_3 ± δ. Based on these results, enhanced electrical performance and mechanical integrity is expected with nickel substitution on calcium-doped yttrium chromite in SOFC operating conditions.