Nanostructured La0 Research Articles

Relation between thickness, crystallite size and magnetoresistance of nanostructured La1- x Sr x Mn y O3±δ films for magnetic field sensors.

In the present study the advantageous pulsed-injection metal organic chemical vapour deposition (PI-MOCVD) technique was used for the growth of nanostructured La1−xSrxMnyO3±δ (LSMO) films on ceramic Al2O3 substrates. The compositional, structural and magnetoresistive properties of the nanostructured manganite were changed by variation of the processing conditions: precursor solution concentration, supply frequency and number of supply sources during the PI-MOCVD growth process. The results showed that the thick (≈400 nm) nanostructured LSMO films, grown using an additional supply source of precursor solution in an exponentially decreasing manner, exhibit the highest magnetoresistance and the lowest magnetoresistance anisotropy. The possibility to use these films for the development of magnetic field sensors operating at room temperature is discussed.

Electrochemical Behavior of a Nanostructured La1.25Gd0.25Mg0.5Ni7 Hydrogen Storage Material Modified with Magnetron Sputtered Nickel

A nanocrystalline La1.25Gd0.25Mg0.5Ni7 hydrogen storage material with a dominant A2B7 phase was obtained by mechanical alloying (MA) and heat-treatment (1123 K for 0.5 h). The concentration of the AB5 phase is 6.4 at.% and is marginally lower than that in the Gd-free parent La1.5Mg0.5Ni7 alloy. The powder fraction of 20–50 μm was encapsulated with thin (0.31 μm thick), amorphous nickel using magnetron sputtering. Electrochemical multicycling allowed the determination of the corrosion kinetic parameters, as well as the exchange current density of a H2O/H2 system and the effective diffusivity of hydrogen. Partial substitution of Gd for La in the La1.5Mg0.5Ni7 nanocrystalline parent alloy increases the electrode discharge capacity (up to 16%) but somewhat deteriorates its activation properties. Gd addition does not affect the corrosion degradation resistance, although the corrosion properties are weakened after particle encapsulation with the amorphous nickel coating. Finally, in spite of the Gd-modified La1.5Mg0.5Ni7 nanocrystalline alloy revealing a 10% lower exchange current density of the H2O/H2 system compared to the parent alloy, this property can be considerably improved (by >50%) by Ni particle encapsulation. The hydrogen diffusivity of the Gd-modified material is faster compared to the parent material but is slowed down by the Ni coating.

Hydrothermal Synthesis, Characterization, and Sintering Behavior of Core-Shell Particles: A Principle Study on Lanthanum Strontium Cobaltite Coated with Nanosized Gadolinium Doped Ceria

In this work, nanostructured (La0.6Sr0.4)0.99CoO3 (LSC)-Ce0.8Gd0.2O1.9 (CGO) core-shell particles were prepared by precipitating CGO nanoparticles on the surface of LSC particles under hydrothermal conditions. The as-prepared core-shell particles were sintered by spark plasma sintering (SPS) and conventional sintering, and the microstructure evolution and densification behavior were studied. Dense microstructures were reached using both sintering methods at relatively low temperatures. In the case of SPS, the core-shell architecture was partially maintained and nano-structured CGO grains were formed, while conventional sintering led to the formation of larger CGO grains. This work covers a detailed characterization of (a) the individual LSC-CGO core-shell particles and (b) the composites after densification.

Fabrication and characterization of novel powder reconstitution derived nanostructured spherical La2(Zr0.75Ce0.25)2O7 feedstock for plasma spraying

High-performance thermal spraying feedstock is of great significance for obtaining high quality plasma-sprayed thermal barrier coatings (TBCs). Rare-earth zirconates TBCs have attracted much more attention so far. In this paper, the nanostructured La2(Zr0.75Ce0.25)2O7 (LCZ) feedstock was fabricated by the nanopowder reconstitution method. The microstructures of the LCZ feedstock were characterized by transmission electron microscopy, X-ray diffraction, Raman spectroscopy and scanning electron microscopy. Moreover, the thermo-physical properties of the LCZ feedstock were also evaluated. The results indicate that the spherical LCZ feedstock with the median particle size (d50) of 37.40 μm consists of only pyrochlore phase. Compared with the nanostructured La2Zr2O7 (LZ) feedstock, the tap density and flowability of the LCZ feedstock are enhanced considerably. As for the LCZ coating, the thermal conductivity is much lower and the thermal expansion coefficient (TEC) is relatively higher than the LZ coating. Therefore, the nanostructured LCZ feedstock can be a promising advanced thermal spraying feedstock used for double ceramic layer TBCs in future.

Electrochemical properties of electrospinning-fabricated layered perovskite used in cathode materials for a low temperature-operating solid oxide fuel cell

In this study, the microstructural and electrochemical properties of linear type nanofibers obtained by adding SmBa0.5Sr0.5Co2O5+d (SBSCO) layered perovskite oxide were investigated for use as cathode materials in a low temperature-operating solid oxide fuel cell.Linear type SBSCO fiber and Ce0.9Gd0.1O2−d coated SBSCO-fiber were fabricated using the electrospinning process. It was confirmed that the area specific resistance (0.75 Ω cm2) of the obtained SBSCO-fiber cathode was much lower than that (1.25 Ω cm2) of a powder-type SBSCO cathode at 550 °C. The result shows that the SBSCO fiber cathode exhibits lower polarization resistance in low temperature ranges than the powder SBSCO cathode materials do. A significantly lower activation energy (0.76 eV) was observed in fiber SBSCO cathode than in the nanostructured La0.8Sr0.2Co0.2Fe0.8O3−d ones which were used as control at 1.53 eV.

The Effect of Ni Doping on the Performance and Electronic Structure of LSCF Cathodes Used for IT-SOFCs

We investigated the effect of nickel doping on the electronic structure and performance of nanostructured La0.6Sr0.4Co0.2Fe0.8–0.03Ni0.03O3−δ prepared by the one-pot sol–gel method. The commercial undoped La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF0.8) was used as reference. Moreover, for comparison, Ni (3 mol %) was deposited by wetness impregnation over the La0.6Sr0.4Co0.2Fe0.8O3−δ. We show by in situ X-ray absorption spectroscopy at 900 °C under air flow that nickel enters the B perovskite site of the material and favors the stabilization of the cobalt oxidation state, as evidenced by the delay in the decrease of the average Co valence with respect to undoped samples. Our results are further supported by in situ X-ray Raman spectroscopy (XRS) that allowed us to monitor the temperature evolution of the O K-edge. XRS evidences that nickel-doped LSCF shows unmodified O2p-TM3d density of states, which proves that the Co oxidation state is preserved. Electrochemical impedance spectroscopy measurements were carried out o...

Nanostructured La–Sr–Mn–Co–O Films for Room-Temperature Pulsed Magnetic Field Sensors

The results of Co substitution for Mn in nanostructured La1- x SrxMn1- y Co y O3 films ( $x = 0.2$ ) grown on polycrystalline Al2O3 substrates by pulsed-injection metal-organic chemical vapor deposition technique, are presented. The Co content y in the films was changed as follows: 0; 0.023; 0.053; 0.078. The resistivity ( $\rho )$ dependences on temperature in zero magnetic field was investigated in the temperature range 5–300 K. It was found that the increase in Co content up to 0.078 results in the decrease of metal-insulator transition temperature and increase of the resistivity. The results of nanostructured films are compared with the epitaxial films grown on LaAlO3 substrate keeping the same technological conditions. The magnetoresistance (MR) and MR anisotropy (MRA) were investigated at room temperature (290 K) in permanent magnetic fields up to 2.35 T, and pulsed magnetic fields up to 20 T. It was found that the highest MR values are obtained for the intermediate Co content: $y = 0.053$ , while the MRA decreased with the increase of Co content. The results are discussed within the double-exchange interaction model for the perovskite oxides. The possibility to apply the manganite-cobaltite films for the development of room temperature B-scalar magnetic field sensors is considered.

Synthesis, structure and magnetic properties of nanostructured La1−xAxFe0.5Mn0.5O3 (A = Ca, Sr and Pb; x = 0 & 0.25) perovskites

The effect of hole doping on magnetic properties of LaFe0.5Mn0.5O3 have been investigated. All the ceramics samples La1−xAxFe0.5Mn0.5O3 (A=Ca, Sr and Pb; x=0 & 0.25) were synthesized at 500°C by sol-gel method and the particles size were found to be in nanodimension. The samples were characterized by X-ray and electron diffraction, HRTEM and both dc and ac-magnetization measurements. The X-ray and electron diffraction patterns were indexed by cubic Pm-3m space group. The particle size of the LaFe0.5Mn0.5O3 is ∼100nm, whereas the Pb-doped sample is ∼50nm and for Ca or Sr doped samples the size is ∼10–30nm. Both dc and ac-susceptibility measurements suggest that the effect of hole doping and A-site cationic radius in LaFe0.5Mn0.5O3 have no significant role on magnetic properties. However, the particle size plays an important role on magnetic property due to the development of surface ferromagnetic cluster at nanoscale. The competing interactions lead to magnetic phase separation where local ferromagnetic clusters coexist within the antiferromagentic matrix in all the samples.

Effect of Deposition Temperature on the Microstructure and Performance of Vacuum Cold Sprayed Nano-Structured LSCF Cathodes for Solid Oxide Fuel Cells

Here, we investigate the feasibility of using vacuum cold spray process as a novel method for the cost effective fabrication of a nano-structured La0.4Sr0.6Co0.2Fe0.8O3 (LSCF) as a high-performance cathode for low-temperature solid oxide fuel cells (LT-SOFCs). Nano sized LSCF particles were used as feedstock. The effects of deposition temperature and particle size on the phase composition, microstructure, and electrochemical performance of the resulting cathodes were studied. The results indicate that 10~20 μm nano-structured porous LSCF cathodes can be successfully prepared by vacuum cold spray process. However, the deposition temperature shows significant influence on the microstructure. When the cathode was deposited at room temperature, there were many unbonded interfaces between the cathode and electrolyte. With the increase of deposition temperature, the interface bonding was improved. At a deposition temperature of 300 °C, it can be found that the cathode and electrolyte were bonded well. Besides, the results indicate that the cathode polarization resistance decreased significantly with the decrease of spray particle size. When particle size decreased from 150 nm to 50 nm, the cathode polarization resistance decreased from 11 Ωcm2 to 1.5 Ωcm 2 at 450 °C. Our results suggest that vacuum cold spray has potential to be a low-cost and flexible process suitable for the fabrication of nano-structured cathodes for LT-SOFCs.

Electrochemical Behavior of Nanostructured La0.8Sr0.2MnO3 as Cathodes for Solid Oxide Fuel Cells

In this work we outline the mechanisms contributing to the oxygen reduction reaction in nanostructured cathodes of La0.8Sr0.2MnO3 (LSM) for Solid Oxide Fuel Cells (SOFC). These cathodes, developed from LSM nanostructured tubes, can be used at lower temperatures compared to microstructured ones, and this is a crucial fact to avoid the degradation of the fuel cell components. This reduction of the operating temperatures stems mainly from two factors: i) the appearance of significant oxide ion diffusion through the cathode material in which the nanostructure plays a key role and ii) an optimized gas phase diffusion of oxygen through the porous structure of the cathode, which becomes negligible. A detailed analysis of our Electrochemical Impedance Spectroscopy supported by first principles calculations point towards an improved overall cathodic performance driven by a fast transport of oxide ions through the cathode surface. Figure 1

Formation of NiO/YSZ functional anode layers of solid oxide fuel cells by magnetron sputtering

The decrease in the polarization resistance of the anode of solid-oxide fuel cells (SOFCs) due to the formation of an additional NiO/(ZrO2 + 10 mol % Y2O3) (YSZ) functional layer was studied. NiO/YSZ films with different NiO contents were deposited by reactive magnetron sputtering of Ni and Zr–Y targets. The elemental and phase composition of the films was adjusted by regulating oxygen flow rate during the sputtering. The resulting films were studied by scanning electron microscopy and X-ray diffractometry. Comparative tests of planar SOFCs with a NiO/YSZ anode support, NiO/YSZ functional nanostructured anode layer, YSZ electrolyte, and La0.6Sr0.4Co0.2Fe0.8O3/Ce0.9Gd0.1O2 (LSCF/CGO) cathode were performed. It was shown that the formation of a NiO/YSZ functional nanostructured anode leads to a 15–25% increase in the maximum power density of fuel cells in the working temperature range 500–800°C. The NiO/YSZ nanostructured anode layers lead not only to a reduction of the polarization resistance of the anode, but also to the formation of denser electrolyte films during subsequent magnetron sputtering of electrolyte.

Effect of Deposition Temperature on the Microstructure and Performance of Vacuum Cold Sprayed Nano-Structured LSCF Cathodes for Solid Oxide Fuel Cells

Here, we investigate the feasibility of using vacuum cold spray process for the cost effective fabrication of a nano-structured La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) as a high-performance cathode for solid oxide fuel cells (SOFCs). The results indicate that the deposition temperature shows significant influence on the microstructure. When the cathode was deposited at room temperature, there were many unbonded interfaces between the electrolyte and cathode, and inside the cathode. At a deposition temperature of 300 °C, it can be found that the cathode and electrolyte were bonded well and the cathode showed the lowest polarization resistance (0.5 Ωcm2 at 600 °C). However, with the further increase of deposition temperature, a dense microstructure was obtained and resulted in a polarization resistance increase. Our results suggest that vacuum cold spray has potential to be a low-cost and flexible process suitable for the fabrication of nano-structured cathodes for SOFCs.

Electrochemical Behavior of Nanostructured La0.8Sr0.2MnO3 as Cathodes for Solid Oxide Fuel Cells

La0.8Sr0.2MnO3 (LSM) is one of the most commonly used cathodes in Solid Oxide Fuel Cells (SOFC). In spite of the fact that nanostructured cathodes are expected to display improved performance, the high operating temperature (~ 1000°C) of LSM-based SOFCs hinders their stability. In the present work, we have developed nanostructured cathodes prepared from LSM nanotubes of enhanced performance, allowing its use at lower temperatures (~ 800°C). We observed that our cathodes have qualitative improvements compared with microstructured materials: firstly, the diffusion in the gas phase is optimized to a negligible level and secondly, evidence of ionic conduction is found, which is extremely rare in LSM cathodes. We propose that this important change in the electrochemical properties is due to the nanostructuration of the cathode and deserves further attention, including the exploration of other materials.

Improved polarization behaviour of nanostructured La0.65Sr0.3MnO3 cathode with engineered morphology

We report a simple synthesis of La0.65Sr0.3MnO3 nanorods (LSM-R) through hydrothermal reaction followed by calcination at high temperature (700–850 °C). Thermogravimetric analysis and XRD study reveals that 850 °C is adequate for phase pure LSM-R formation. The microstructure of the powder has been clinically studied using FESEM and TEM. It is observed that the intermediate hydrothermal treatment plays key role in formation of such nanorods. Different bulk properties of LSM-R like sinteractivity, CTE, density, electrical conductivity etc. have been comprehensively studied. A maximum electrical conductivity of around 250 S/cm at 800 °C is obtained when LSM-R specimen is sintered at 1200 °C/2 h. The cathodic polarization of such LSM-R is measured using impedance analysis. It is observed that polarization value initially decreases attains minimum and then starts to increase with increase in cathode sintering temperature and dwelling time. A minimum cathodic polarization value of 0.32 Ω/cm2 at 800 °C is obtained at an optimized cathode sintering condition of 1000 °C/2 h.

Development of nanostructured La0.8Sr0.2MnO3−δ-Er0.4Bi1.6O3 cathodes via an infiltration process with different polymeric agents for intermediate temperature solid oxide fuel cells

Dual-phase composite cathodes consisting of La0.8Sr0.2MnO3−δ (LSM) nanoparticles on the Er0.4Bi1.6O3 (ESB) scaffold have been synthesized via an infiltration technique using different polymeric agents for intermediate temperature solid oxide fuel cell (IT-SOFC) applications. It was found that the LSM infiltration with Triton-X yielded well-distributed LSM nano-catalysts (<50 nm) on the ESB surface, while the use of glycine resulted in the highly agglomerated ‘layer-like’ structure. The nanostructured LSM-ESB fabricated with Triton-X exhibited low electrode resistance of 0.23 Ω-cm2 at 650 °C, which is ∼60% lower than that of the conventional LSM-ESB (0.56 Ω-cm2). Electrochemical impedance analysis revealed that this performance enhancement is primarily attributed to the enhanced charge transfer process of oxygen reduction reactions with homogeneously increased reaction sites in the characteristic nanostructure.

Electrochemical Characteristics of Nickel and Ceria Infiltrated Nano-Structured La0.2Sr0.8TiO3 Anodes for Solid Oxide Fuel Cells By Infiltration

A La-doped SrTiO3 (LST) is an alternative anode material of solid oxide fuel cells (SOFC). The LST has a high electronic conductivity and chemically stable under a reduced atmosphere. In addition, it has excellent carbon deposition resistance and high tolerance of sulfur in hydrocarbon fuels. However, the LST suffers from a low catalytic activity toward the oxygen reduction reaction. Thus, it needs to improve the catalyst activity of the LST anode. In this study, nickel and gadolia-doped ceria (GDC) infiltrated nano-structured LST anodes were proposed in order to improve the catalytic activity of the LST-based oxide anode. Nano-sized Ni and GDC were homogeneously dispersed in a LST backbone by infiltration. Ni/GDC infiltrated LST showed higher catalytic activity than Ni and GDC infiltrated ones, which means that the synergistic effect can be expected in the Ni/GDC infiltrated LST. It seems that the polarization resistance (Rp) depended on the infiltrated Ni and GDC content and their dispersion state or microstructure. We analyzed the correlation between the microstructure of the infiltrated anode and electrochemical characteristics.

Magnetocaloric nanostructured La0.7Ca0.3−xBaxMnO3 (x < 0.3) ceramics produced by combining polyol process and Spark Plasma Sintering

Ferromagnetic La0.7Ca0.3−xBaxMnO3 (x < 0.3) ceramics were produced by an original pathway, combining soft chemistry and Spark Plasma Sintering (SPS), within a few minutes at a temperature of 800 °C (instead of several hours at 1300 °C with the conventional ceramic route). Their structural, chemical, magnetic and magnetocaloric properties were investigated by combined X-ray diffraction (XRD), Mn K-edge X-ray absorption near-edge structure (XANES) and routine magnetometry and compared to those of their conventionally solid-solid made counterparts. All the samples are single phase. A slight unit cell dilatation is observed in the produced ceramics when the Ba atomic content increases, due to the progressive replacement of Ca2+ cations by larger Ba2+ ones. In every case, the volume cell size is higher in the polyol-SPS samples than in their conventionally made counterparts. The qualitative and quantitative analysis of the XANES spectra allow to attribute this unit cell size change to a manganese electronic state modification in relation with the experimental material processing conditions: polyol-SPS samples exhibit higher Mn3+/Mn4+ atomic ratio. Moreover, the microstructure of these samples is highly dense and ultrafine grained with a compactness of about 94% and an average size grain of less than 150 nm. Interestingly, the para-to ferromagnetic transition in these ceramics is found to be significantly broadened at Curie temperatures, TC, which are smaller than those measured on their bulk counterparts. As a consequence, the thermal variation of the magnetic entropy change is broader in these ceramics compared to in the conventionally made ones, making them particularly valuable for magnetocaloric applications.

Influence of preparation methods on the structure and catalytic performance of nanostructured La0.7Ba0.3Co0.3Ni0.7O3 for CO oxidation

Nano-perovskite oxides La0.7Ba0.3Co0.3Ni0.7O3 were synthesized by the co-precipitation and polymerized complex route based on the Pechini process methods (denoted as LBCN-Cop and LBCN-P, respectively) and employed in the removal of carbon monoxide (CO) from (N2 + CO + O2) gas mixture through oxidation. In this paper, the effect of the preparation method on the catalytic activity of nanocatalysts is discussed. The catalysts were characterized by XRD, FTIR, BET, TPR, SEM and EDX techniques. The specific surface area of the prepared catalysts varied from 24.55 (LBCN-Cop) to 34.14 (LBCN-P) m2 g−1. SEM investigations revealed that the LBCN-P nanoparticles were uniform spheres in shape with diameters ranging from 50 to 60 nm, whereas more agglomeration of particles was found for the LBCN-Cop catalyst. The results of catalytic activity measurements showed the superiority of the catalyst sample prepared by the Pechini method for CO oxidation at low temperatures compared with that prepared by the co-precipitation method.

Fast Resistance Relaxation in Nanostructured La–Ca–Mn–O Films in Pulsed Magnetic Fields

The results of fast resistance relaxation of nanostructured La1− x Ca x MnO3 (LCMO) films with different compositions $x$ (0.218, 0.285, 0.296, 0.41) grown by the metal–organic chemical vapor deposition technique are presented. The fast ( $\sim 200\mu \text{s}$ ) resistance relaxation process observed after the magnetic field pulse was switched off was studied in pulsed fields up to 14 T in the temperature range of 80–170 K. It was found that the remnant resistivity of the fast relaxation process is proportional to the magnetoresistance of the films up to 10 T. The smallest remnant relaxation values were found for the films with highest insulator–metal transition temperature $T_{m}$ . It was shown that the fast process could be analyzed using the Kolmogorov–Avrami–Fatuzzo model, taking into account the reorientation of magnetic domains into their equilibrium state. It was concluded that LCMO films with $x=0.218$ should be favored for the development of fast pulsed magnetic field sensors operating at cryogenic temperatures.

Structure and microstructure dependent transport and magnetic properties of sol–gel grown nanostructured La0.6Nd0.1Sr0.3MnO3 manganites: Role of oxygen

In the present communication, we report the results of the studies on structural and microstructural properties of sol–gel grown nanostructured La0.6Nd0.1Sr0.3MnO3 (LNSMO) manganites sintered and annealed at different temperatures (viz., 700, 800 and 900°C). It is shown that resistivity, magnetoresistance (MR) and magnetic behavior are strongly dependent on structure and microstructure of the LNSMO samples. Suppression in resistivity, decrease in MR and enhancement in metal to insulator transition temperature (TP) have been discussed in detail on the basis of grain morphology and role of oxygen. Magnetism of LNSMO samples has been discussed on the basis of variation in ferromagnetic to paramagnetic transition temperature (TC), irreversibility, freezing temperature, thermal hysteresis, M–H loop area, coercive field and remnant magnetization with process temperature and annealing environment.