Nanoparticles Of La0 Research Articles

Influence of particle size and agglomeration in solid oxide fuel cell cathodes using manganite nanoparticles

In this work we studied the influence of particle size and agglomeration in the performance of solid oxide fuel cell cathodes made with nanoparticles of La0.8Sr0.2MnO3. We followed two synthesis routes based on the Liquid Mix method. In both procedures we introduced additional reagents in order to separated the manganite particles. We evaluated cathodic performance by Electrochemical Impedance Spectroscopy in symmetrical (CATHODE/ELECTROLYTE/CATHODE) cells. Particle size was tuned by the temperature used for cathode sintering. Our results show that deagglomeration of the particles, serves to improve the cathodes performance. However, the dependence of the performance with the size of the particles is not clear, as different trends were obtained for each synthesis route. As a common feature, the cathodes with the lowest area specific resistance are the ones sintered at the largest temperature. This result indicates that an additional factor related with the quality of the cathode/electrolyte sintering, is superimposed with the influence of particle size, however further work is needed to clarify this issue. The enhancement obtained by deagglomeration suggest that the use of this kind of methods deserved to be considered to develop high performance electrodes for solid oxide fuel cells.

Structural and magnetic analysis of La0.67Ca0.33MnO3 nanoparticles thermally treated: Acoustic detection of the magnetocaloric effect

Nanoparticles of La0.67Ca0.33MnO3 were synthesized via the sol-gel method, thermally treated and characterized using X-ray diffraction, magnetization, electron spin resonance and magnetoacoustic experiments. The formation of the desired perovskite structure was verified and the average size of the nanoparticles was also determined. An increase of the particle size by rising the treatment temperature was observed. The Curie temperature and the isothermal entropy variation of the samples were obtained from the magnetization data. The isothermal entropy change, produced under the application of an external magnetic field, which expresses the magnetocaloric effect, became significantly larger for the samples treated at higher temperatures. These results are in good agreement with those obtained by magnetoacoustics, based on the direct and contactless measurement of the temperature change, validating the ability of the technique to study the magnetocaloric effect in reduced mass and nanoparticles samples.

Dynamic Magnetic Properties of La0.8Ag0.2MnO3 Nanoparticles

In thisresearch,nanoparticlesof La0.8Ag0.2MnO3 with mean particle sizes d of 16 and 22 nm have been prepared by sol-gel method. Dynamic magnetic properties of the samples have been carried out by AC magnetic susceptibility and electron spin resonance techniques. The structural properties of the samples have been characterized by X-ray diffraction using Rietveld refinement and transmission electron microscopy. The analysis of the AC magnetic susceptibility by phenomenological models reveals the interacting superparamagnetic behaviors in La0.8Ag0.2MnO3 nanoparticles. From the Vogel-Fulcher model, the values of 1.3 × 104 and 2.8 × 104 erg/cm3 are obtained for the effective magnetic anisotropy constant for d= 16 and 21 nm, respectively. Electron spin resonance signals of the samples are well described by double Lorentzian line shapes which suggest the presence of ferromagnetic clusters in the paramagnetic phase and possible phase separation at room temperature.

Elaboration of Low Field and Room Temperature Magnetoresistive Sensor of La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3 </sub>Nanoparticles Synthesized by Sol-Gel Method

Low field and room temperature operation range are two critical requirements for magnetic sensors. In this paper, nanoparticles of La0.7Sr0.3MnO3 (LSMO) were elaborated to fulfill these requirements. La0.7Sr0.3MnO3 nanoparticles were synthesized using sol-gel method. The physical properties of LSMO were examined by X-Ray diffraction and Scanning Electron Microscope. The ferromagnetic nature of LSMO samples was confirmed by magnetization measurements. Nanoparticles were pressed in shape of pellets and sintered at different temperatures of 900°C, 1000°C, 1100°C and 1200°C. Magnetoresistive measurements were taken at room temperature with magnetic field (μH) in range ±110 mT. The experimental results show maximum magnetoresistive ratio and sensitivity of 1.3% and 0.023%/mT respectively for sample sintered at 1000°C. Other sensor characteristics namely hysteresis error, response time and noise measurement were also given.

Fabrication of Nanoperovskite Type Oxides and Its Efficient Usage on Removal of Trypan Blue Dye

A1-xAxBO3 belongs to the perovskite oxide, ABO3, structure. In this study, nanoparticles of La0.5Ca0.5FeO3 were fabricated by the sol–gel citrate technique. The family of perovskite-type oxides could be considered as an adsorbent material for the removal of dyes. This study has investigated the efficiency of La0.5Ca0.5FeO3, as an adsorbent for the removal of diazo dye Trypan Blue (TB), from an aqueous solution. The adsorption studies were carried out at different pH values, various adsorbent dosages and contact times in a batch experiment. The kinetic studies indicate that the removal process obeys a pseudo-first-order kinetic equation. Also, the isotherm evaluations reveal that the adsorption of TB by the nanoparticles follows the Langmuir model. This nanoparticle material, with good specific affinity towards dye molecules, is a promising adsorbent for dye removal from natural water.

Synthesis and Evaluation Catalytic Efficiency of Perovskite-Type Oxide Nanopowders in Removal of Bromocresol Purple from Aqueous Solution

A1-xAxBO3 belongs to the perovskite oxides of the ABO3 structure. In this study, nanoparticles of La0.5Ca0.5FeO3 were fabricated by sol-gel citrate technique. A series of common analytical techniques were used to characterize the crystallinity, morphology, specific surface area, and grain size of the nanopowders. These properties were characterised by means of XRD, SEM, EDX and FTIR. The calculated particles size using the Scherrer's formula was about 20 nm. Moreover, the family of perovskite-type oxides could be considered as an adsorbent/catalyst material for the removal of dyes. This study has also investigated the efficiency of La0.5Ca0.5FeO3, as an adsorbent for removal of dye (Bromocresol Purple (BP)), from an aqueous solution. The adsorption studies were carried out at different pH values, various adsorbent dosages and contact time in a batch experiments. The kinetic studies indicate that the removal process obeys the Pseudo-first-order kinetic equation. Also, the isotherm evaluations reveal that the adsorption of BP by the nanoparticles follows the Langmuir model. In addition, this nanoparticle with good specific affinity towards dye molecules was a promising adsorbent for dye removal from natural water.

Structural and magnetic characterization of La0.8Sr0.2MnO3 nanoparticles prepared via a facile microwave-assisted method

Nanoparticles of La0.8Sr0.2MnO3 (LSMO) with different particle sizes are synthesized by a very fast, inexpensive, reproducible, and environmentally friendly method: the microwave irradiation of the corresponding mixture of nitrates. The structural and magnetic properties of the samples are investigated by the X-Ray diffraction (XRD), Fourier transform infra-red (FT-IR) spectroscopy, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and magnetic (DC magnetization and AC susceptibility) measurements. The XRD study coupled with the Rietveld refinement show that all samples crystallize in a rhombohedral structure with the space group of R−3C. The FT-IR spectroscopy and FE-SEM images indicate formation of the perovskite structure of LSMO. The DC magnetization measurements confirm the decrease in the particle size effects on the magnetic properties, e.g. reduction in the ferromagnetic (FM) moment and increase in the surface spin disorder. Magnetic dynamics of the samples studied by AC magnetic susceptibility shows that the magnetic behavior of the nanometer-sized samples is well-described by the Vogel-Fulcher and critical slowing down laws. Strong interaction between magnetic nanoparticles of LSMO was detected by fitting the experimental data with the mentioned models.

Structural and Magnetic Properties of Microwave Assisted Sol-Gel Synthesized La0.9Sr0.1MnO3 Manganite Nanoparticles

The structural and magnetic properties of nanoparticles of La0.9Sr0.1MnO3 (LSMO) were studied using powder X-ray diffraction (XRD), transmission electron microscope (TEM), and magnetic measurements. The XRD refinement result indicates that samples crystallize in the rhombohedral structure with R-3C space group. The dc magnetization measurements revealed that samples exhibit no hysteretic behavior at room temperature, symptomatic of the superparamagnetic (SPM) behavior. The results of ac magnetic susceptibility measurements show that the susceptibility data are not in accordance with the Néel-Brown model for SPM relaxation but fit well with conventional critical slowing down model which indicates that the dipole-dipole interactions are strong enough to cause superspin-glass-like phase in LSMO samples.

Tunable magnetic and magnetocaloric properties of La0.6Sr0.4MnO3 nanoparticles

Nanoparticles of La0.6Sr0.4MnO3 with different particle sizes are synthesized by the nitrate-complex auto-ignition method. The structural and magnetic properties of the samples are investigated by X-Ray diffraction (XRD), Fourier transform infra-red (FT-IR) spectroscopy, transmission electron microscopy (TEM), and DC magnetization measurements. The XRD study coupled with the Rietveld refinement shows that all samples crystallize in a rhombohedral structure with the space group of R-3 C. The FT-IR spectroscopy and TEM images indicate formation of the perovskite structure with the average sizes of 20, 40, and 100 nm for the samples sintered at 700, 800, and 1100 °C, respectively. The DC magnetization measurements confirm tuning of the magnetic properties due to the particle size effects, e.g., reduction in the ferromagnetic moment and increase in the surface spin disorder by decreasing the particle size. The magnetocaloric effect (MCE) study based on isothermal magnetization vs. filed measurements in all samples reveals a relatively large MCE around the Curie temperature of the samples. The peak around the Curie temperature gradually broadens with reduction of the particle size. The data obtained show that although variations in the magnetic entropy and adiabatic temperature decrease by lowering the particle size, variation in the relative cooling power values are the same for all samples. These results make this material a proper candidate in the magnetic refrigerator application above room temperature at moderate fields.

An Investigation on Magnetic Interacting La<sub>0.6</sub>Sr<sub>0.4</sub>MnO<sub>3</sub> Nanoparticles

Nanoparticles of La0.6Sr0.4MnO3 (LSMO) with an equivalent average particle size of ~ 20nm were synthesized by nitrate-complex auto-ignition method. The structural and magnetic properties were investigated systematically. The X-Ray diffraction study coupled with the Rietveld refinement of XRD data showed that sample crystallize in a rhombohedral structure with space group of R-3C. The stretching and bending modes in Fourier transform infrared (FTIR) spectroscopy is observed which indicates the formation of the perovskite structure of LSMO. The nature of DC magnetization versus field M(H) plot for sample resembles qualitatively with ferromagnetic (FM) systems. The AC-susceptibility data obtained at different frequencies confirmed the presence of a frequencydependent freezing temperature around room temperature. Based on the interparticle interaction strength and phenomenological models (Neel-Brown, Vogel-Fulcher and Critical slowing down) the magnetic dynamic behavior of nanoparticles was characterized. By fitting the experimental data with mentioned models the possibility existence of superparamagnetic/spin glass states were considered. The obtained values from fitting performance showed that there is a strong interaction between magnetic nanoparticles of LSMO.

A Comparative Study on Electrochemical Behavior of (La,Sr)FeO<sub>3</sub> and (La,Sr)FeO<sub>3</sub>-(La,Sr)MnO<sub>3</sub> Anodes for Solid Oxide Electrolysis Cells

La0.8Sr0.2FeO3 electrodes modified with nanoparticles of La0.8Sr0.2MnO3 (LSF/LSM) were prepared by infiltration method. LSM solution had good wettability on LSF surfaces and good electrochemical compatibility with LSF. The impedance of half-cells incorporating the electrodes was also examined. The deconvolution of the impedance spectrum indicated that the peak positions are changed after the LSF infiltration. The LSF/LSM electrode exhibited smaller polarization resistance than pure LSF electrode. Finally, a transform from asymmetry to symmetry appeared in the polarization curves of LSF/LSM electrode with the increase of temperature.

XAFS investigation of the role of orientational disorder in the stabilization of the ferromagnetic metallic phase in nanoparticles of La0.5Ca0.5MnO3

The inclusion of the contribution of Jahn–Teller distortion of MnO6 units, in addition to double-exchange, has been largely successful in explaining the magneto-transport behavior of manganites. However, our recent experiments on La0.5Ca0.5MnO3 demonstrated the limitation of these factors in explaining the radical difference between the magneto-transport properties of bulk and nanocrystalline forms. While bulk La0.5Ca0.5MnO3 exhibits insulator character (4–300 K) and an anti-ferromagnetic–ferromagnetic transition at 200 K, the nanocrystalline form stabilizes in a metallic ferromagnetic phase (4–300 K). This is counter-intuitive since large Jahn–Teller distortion, which promotes anti-ferromagnetism or insulator character, exists in the nanocrystals too (as indicated by x-ray diffraction results). In this work, we resolve this paradox by considering the role of structural disorder. Employing x-ray absorption spectroscopy, we establish that the disorder in inter-octahedral coupling is enhanced by 57% in the nanocrystals, as the octahedral units are randomly oriented with respect to each other. This orientational disorder promotes metallic ferromagnetism by destroying the stringent orbital ordering that is needed for anti-ferromagnetism and the co-operative nature of the orbital order.

Nanoparticles of La0.8Ca0.2Fe0.8Ni0.2O3−δ perovskite for solid oxide fuel cell application

Polycrystalline samples of La0.8Ca0.2Fe0.8Ni0.2O3−δ (LCFN) with perovskite type structure have been prepared by combustion, freeze drying, citrate-gel process and liquid mix method. The analysis of X-ray powder diffraction indicated that the samples were single phase and crystallized in an orthorhombic (space group, Pnma no. 62) structure. Transmission electron microscopy (TEM) analysis on the synthesized powder at 600 °C by liquid mix method showed clusters of 150 nm formed by nanoparticles of 20 nm. Electrochemical performance of LCFN cathodes, which are used for intermediate temperature solid oxide fuel cells, were investigated. The polarization resistance was studied using two different electrolytes: Y-doped zirconia (YSZ) and Sm-doped ceria (SDC). The dc four-probe measurement exhibits a total electrical conductivity, over 100 S cm−1 at T ≥ 600 °C, pointing out that strontium can be substituted for the cheaper calcium cation without destroying the electrochemical properties. Experimental results indicate that nanoparticles have more advantages in terms of smaller particle size and better electrochemical performance.

Transport Properties of Nanoparticles of Complex Oxides: Likely Presence of Coulomb Blockade at Low Temperature

In this paper we report transport properties at low temperatures (3 K < or = T < or = 300 K) in nanostructured functional oxides. Electrical resistivity in the nanoparticles of La0.5CoO3 (LSCO), a ferromagnet, show a gradual change-over from a completely metallic state (in the bulk sample) to a completely insulating state (in the sample with the smallest particle size of approximately 35 nm), while still remaining a ferromagnet, albeit with a lower Tc. In the LSCO nanoparticles (diameter <60 nm) there is a change-over in the temperature dependence of the resistivity at the lowest temperature (T <10 K) which we could identify as arising from the Coulomb blockade in the conducting grains which are separated by insulating grain boundaries that allow transport by tunneling. However, nanoparticles of the nonmagnetic material, LaNiO3 (LNO) remain metallic (similar to the bulk sample) throughout the temperature range studied with the resistivity reaching a temperature independent value at the lowest temperature measured. We suggest that the essential difference between the electrical transport in these nanostructured materials arises due to the physical nature of the grain boundaries which is insulating in LSCO due to absence of long range spin order that is needed for the metallic state.

Size induced arrest of the room temperature crystallographic structure in nanoparticles of La0.5Ca0.5MnO3

We have shown, using high resolution x-ray diffraction studies of nanoparticles, that size reduction can lead to an arrest of the high temperature phase in perovskite materials, which prevents growth of the low temperature phase that needs a specific crystal structure for its stabilization. This has been shown in the context of ∼15nm diameter nanocrystals of La0.5Ca0.5MnO3, in which the orthorhombic distortion at room temperature freezes in and, as a consequence, the charge order cannot set in and the ferromagnetic state that sets in at higher temperature is stabilized at low temperature.

Fe-doped La0.5Sr0.5TiO3−δ nanoparticles: A diluted magnetic oxide system

We report room-temperature ferromagnetism in ∼11.4–14.6nm nanoparticles of La0.5Sr0.5Ti0.985Fe0.015O3−δ. Nanoparticles of undoped and Fe-doped samples were synthesized by a polymerized complex method and were characterized using x-ray diffraction, transmission electron microscopy, and vibrating-sample magnetometer. The undoped samples exhibit a diamagnetic behavior, whereas the Fe-doped samples are ferromagnetic having the specific magnetizations of 0.041–0.101emu∕g at 10kOe. Our results indicate that the ferromagnetic property of La0.5Sr0.5Ti0.985Fe0.015O3−δ system is intrinsic and is not a result of any secondary magnetic phase or cluster formation. The finding of room-temperature ferromagnetism in this system would stimulate further interest in the area of diluted magnetic oxides.

Effect of Size Reduction on Charge Ordering in La0.5Ca0.5MnO3

In this paper we report size-induced collapse of charge ordering in nanoparticles of La0.5Ca0.5MnO3 (LCMO). This happens when the particle size approximately 100 nm or below. The investigation shows that a tuning of size can tune the nature of ground state in manganites. The particles were synthesized by sol-gel route (polyol based) and were characterized by X-ray-diffraction (XRD) and Transmission Electron Microscope (TEM). The collapse of charge ordering stabilizes the ferromagnetic phase with metallic character. The resulting phase appears to be electronically phase segregated. We base our conclusions on detailed magnetization and resistivity measurements as well as the evolution of the structure on size reduction. The observed data have been explained as arising due to a combined effect of both phases. Size reduction reduces the orthorhombic distortion and enhances the Double exchange that stabilizes the ferromagnetic phase and increases the band width. It is likely that this enhanced band-width leads to weakening of the charge-ordering and its eventual collapse.

Pushing nanoparticles of La0.7Sr0.3MnO3

Pushing nanoparticles of La0.7Sr0.3MnO3 (LSMO) on a native SiO2 surface using atomic force microscopy (AFM) in the tapping mode is presented. The pushing is accompanied by a repulsive tip–sample interaction between the AFM tip and the LSMO nanoparticles and the physisorption of the LSMO on the SiO2 surface. The AFM images show scratch artifacts on the surface, indicating that artificial scratches are strongly related to the pushing of the LSMO nanoparticles. A possible approach to pushing nanoparticles is proposed.