Nanocrystalline La0 Research Articles

A comparative study of magnetic training effect in bulk and nanocrystalline La0.46Ca0.54MnO3 compound

Transport, magneto-transport, and magnetic properties of La0.46Ca0.54MnO3 compounds having average grain size down to ∼15 nm have been studied. A magnetic training effect due to the external magnetic field cycling was distinctly observed in charge ordered antiferromagnetic bulk compound. Our present study indicates that the training effect was markedly modified along with the modification of the charge ordering due to the reduction of the grain size, and eventually both phenomenons disappeared in case of our lowest particle size sample (∼15 nm). Enhanced ferromagnetic correlation with the reduction of particle size plays the key role for the gradual diminishing of the training effect in the region of nanometer length scale.

Impact of a small amount of vacancy in both lanthanum and calcium on the physical properties of nanocrystalline La0.7Ca0.3MnO3 manganite

In this paper, we report the effect of both lanthanum and calcium (1%) on the structural and magnetic properties of nanocrystalline La0.7Ca0.3MnO3 manganite. Our powder specimens were synthesized using the sol–gel method with an annealing temperature of 850 °C. The X-ray diffraction patterns refined using Rietveld method confirms that our compounds are single phase and crystallize in the orthorhombic perovskite structure (Pbnm space group). The morphology of the samples, observed using a scanning electron microscope (SEM), reveals a spherical shape with an average grain size lower than 100 nm. Magnetization measurements versus temperature under low magnetic applied field (0.05T) show a paramagnetic–ferromagnetic transition for all compounds. The Curie temperature TC is found to increase with lacuna in both cases. In addition, the inverse of the susceptibility (1/χ) as a function of temperature indicates a deviation from the Curie Weiss lawn, signature of Griffiths phase occurrence. Experimental results for the critical β and γ exponents are typical of a behavior governed by the tricritical mean-field theory model.Using magnetization measurements as a function of magnetic applied field at several temperatures, we have deduced the magnetic entropy change, which undergoes an enlargement for both lacuna sites. The obtained results are compared to calculated ones based on the Landau theory and a good concordance is observed.

Ferromagnetic–paramagnetic transition temperature in bulk and nanostructured La0.7Sr x Ca0.3−x MnO3 (x = 0.10, 0.15, and 0.20) manganite materials

Series of bulk and nanostructured La0.7Sr x Ca0.3−x MnO3 (x = 0.10, 0.15, and 0.20) manganites were synthesized and characterized using different techniques. In the series, both the bulk and nanostructured La0.7Sr0.10Ca0.20MnO3 and La0.7Sr0.20Ca0.10MnO3 manganites have orthorhombic and rhombohedral structures, whereas La0.7Sr0.15Ca0.15MnO3 manganite has rhombohedral and orthorhombic structures, respectively. Online ultrasonic velocity and attenuation measurements were taken through an indigenously designed ultrasonic setup, and the analysis was done from 300 to 400 K during the aging of the samples to explore the structural/phase transitions. The bulk and nanocrystalline La0.7Sr x Ca0.3−x MnO3 perovskite samples show the particle size distribution in the range of 197–943 and 24–93 nm, respectively. The addition of Sr2+ alters the size of particles, which decrease in size. The observed anomaly in ultrasonic velocities, attenuations, and elastic moduli is correlated with the ferromagnetic–paramagnetic (FM–PM) transition temperature (T C) in both bulk and nanocrystalline perovskites. In addition, the shift in T C and the magnitude and width of observed anomaly are correlated with the value of x to study the behavior of T C.

Physical properties and ethanol sensing of perovskite La0.8Pb0.2Fe1−xMgxO3 compounds

The nanocrystalline La0.8Pb0.2Fe1−xMgxO3 (x=0.00, 0.05, 0.10 and 0.15) powders were prepared by sol–gel method using the citric acid route. The compounds were investigated with regard to their composition and microstructure, using XRD, Raman spectroscopy and FTIR methods. These compounds were found to exhibit an orthorhombic symmetry with the Pnma space group. The substitution of the Fe ion by the Mg one was found to increase the resistance of these compounds, where the resistance depended on both electrovalence compensation and oxygen vacancy compensation. The La0.8Pb0.2Fe1−xMgxO3 sensors showed p-type semiconducting properties within the temperature range of 100–400°C. The electric results revealed that x=0.15 sample presents high sensitivity and good selectivity to ethanol gas.

Magnetotransport study of nanocrystalline and polycrystalline manganites La0.8Sr0.2MnO3 in high magnetic fields

Comparative study of magnetotransport and magnetic properties in strong magnetic fields up to 47 T is reported for nanocrystalline and polycrystalline mixed valence manganites La0.8Sr0.2MnO3. The low temperature resistivity enhancement observed only in nanocrystalline manganite is ascribed to the electrostatic blockade of charge carriers between the fine grains/crystallites. Small polaron model is found to describe the high temperature conductivity both in poly- and nanocrystalline manganites. The quantitative analysis supplies the low temperature blocking energy barrier and polaron activation energy. An evolution of resistivity with temperature and magnetic field reveals the intrinsic and extrinsic contributions to large negative magnetoresistance effect. The magnetoresistance is found to be correlated with a squared magnetization.

Synthesis and characterization of La0.6Sr0.4Fe0.8Cu0.2O3−δ oxide as cathode for Intermediate Temperature Solid Oxide Fuel Cells

Nanocrystalline La0.6Sr0.4Fe0.8Cu0.2O3−δ (LSFCu) material was synthetized by combustion method using EDTA as fuel/chelating agent and NH4NO3 as combustion promoter. Structural characterization using thermodiffraction data allowed to determine a reversible phase transition at 425°C from a low temperature R-3c phase to a high temperature Pm-3m phase and to calculate the thermal expansion coefficient (TEC) of both phases. Important characteristics for cathode application as electronic conductivity and chemical compatibility with Ce0.9Gd0.1O2−δ (CGO) electrolyte were evaluated. LSFCu presented a p-type conductor behavior with maximum conductivity of 135Scm−1 at 275°C and showed a good stability with CGO electrolyte at high temperatures. This work confirmed that as prepared LSFCu has excellent microstructural characteristics and an electrical conductivity between 100 and 60Scm−1 in the 500–700°C range which is sufficiently high to work as intermediate temperature Solid Oxide Fuel Cells (IT-SOFCs) cathode. However a change in the thermal expansion coefficient consistent with a small oxygen loss process may affect the electrode-electrolyte interface during fabrication and operation of a SOFC.

Small polaron conduction in lead modified lanthanum ferrite ceramics

In the present work we have illustrated the physics of the electrical characteristics of nanocrystalline La1−xPbxFeO3, (0⩽x⩽0.2) powder prepared using auto-combustion synthesis. The effect of lead doping on the dielectric, impedance and ac conductivity characteristics of lanthanum ferrite has systematically been investigated. The synthesized powders were phase pure and crystallized into centro-symmetric Pnma space group. As compared to pure LaFeO3 ceramics (dielectric constant∼14,000), the dielectric constant is grossly increased (∼30,000) in Pb doped LaFeO3. The temperature dependence of dielectric constant of 10.0 at.% Pb doped LaFeO3 exhibits dielectric maxima similar to that observed in ferroelectric ceramics with non-centrosymmetric point group. For La0.8Pb0.2FeO3 ceramics, the frequency dependence of the dielectric constant and loss tangent at various temperatures (300–450K) exhibit typical colossal dielectric constant (CDC) like behavior. From the impedance spectroscopy we have estimated the grain and grain boundary resistance and capacitance of Pb doped LaFeO3 that follow a small polaron hopping conduction model. Long range movement of the charge carriers govern the CDC behavior.

Mössbauer spectroscopy studies of phase evolution in SrFe12O19/La0.5Ca0.5MnO3 composites

A composite system composed of SrFe12O19 (SFO) and La0.5Ca0.5MnO3 (LCMO) was prepared by a two-stage sol–gel/hydrothermal synthesis technique. Starting conditions were varied to obtain products with 1:1 and 1:2M ratios of SFO:LCMO. The morphology and composition of the composites were characterized by SEM and room-temperature Mössbauer spectroscopy, revealing the growth of nanocrystalline La0.5Ca0.5MnO3 particles (d⩽150nm) on the surface of SrFe12O19 platelets. Mössbauer spectra not only confirm the presence of well-formed M-type hexagonal ferrite in the composite materials, but also demonstrate the formation of a new, intermediate phase La0.5Ca0.5Mn(Fe)O3 at the interface between SFO and LCMO. The signature of the intermediate Fe-containing phase increases with the starting molar content of La0.5Ca0.5MnO3, as a greater fraction of the SrFe12O19 surface is covered by La0.5Ca0.5MnO3, increasing the total interfacial area. The trends in coercivity in the series of samples between 10K and 300K are reported and discussed in terms of sample morphology.

Kondo-like electronic transport and ferromagnetic cluster-glass behavior in La0.7Sr0.3MnO3nanostructures

We have studied the electronic transport and magnetic properties of a nanocrystalline La0.7Sr0.3MnO3system. We observed low temperature Kondo effect with magnetic cluster behavior.

Development of nanosized lanthanum strontium aluminum manganite as electrodes for potentiometric oxygen sensor

Nanocrystalline La0.8Sr0.2Al0.9Mn0.1O3 (LSAM) was synthesized by a microwave-assisted citrate method, and characterized by electron microscopy and X-ray diffraction. Electrical behavior of LSAM was investigated by impedance spectroscopy and activation energy of conduction was obtained. Joining of sintered bodies of LSAM and yttria-stabilized tetragonal zirconia polycrystals (YTZP), an extensively studied oxygen ion conducting electrolyte, was examined by isostatic hot pressing methods. Characteristics of the joining region were evaluated with microprobe Raman spectroscopy, and products formed at the interface, primarily strontium zirconate, was confirmed by examination of high temperature chemical reaction between LSAM and YTZP powders. The electrical properties of the LSAM were exploited for development of a high temperature oxygen sensor in which LSAM functioned as the electrode and YTZP as electrolyte.

Nanocrystalline La1−xSrxCo1−yFeyO3 perovskites fabricated by the micro-emulsion route for high frequency response devices fabrications

Nanocrystalline La1−xSrxCo1−yFeyO3 (x=0.00–0.60) perovskites were fabricated by a cheap economic route (i.e. micro-emulsion method) and characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). TGA analysis showed ~35% weight loss. The crystallite size determined by XRD and SEM ranged from 30 to 80nm and ~30 to 50nm, respectively. The dielectric behavior was evaluated in the range of 1.0×106Hz to 3.0×109Hz at 298K, the dielectric parameters resulting appreciably enhanced by co-doping with Sr and Fe. The maximum dielectric parameters (ε′=103.35, ε″=58.92 and tanδ=0.57) were observed for La0.4Sr0.6Co0.4Fe0.6O3 at 15×106Hz. Results suggest the potential use of these nanocrystalline perovskites in GHz-operated microwave devices.

Effect of Mn‐Site for Al Substitution on Structural, Electrical and Magnetic Properties in La0.67Sr0.33Mn1−xAlxO3 Thin Films by Sol‐Gel Method

Nanocrystalline La0.67Sr0.33Mn1−xAlxO3 (x = 0.00, 0.05, 0.10, 0.15, 0.20, and 0.25) thin films have been prepared on quartz substrates by sol‐gel method. The structural and morphology studies were investigated via X‐ray diffraction (XRD) and field emission scanning electron microscope (FESEM). XRD graph patterns show rhombohedral distorted perovskite structures. FESEM images show that the average grain size decreased as the concentration of x increased. Electrical property was investigated using four‐point probe technique. Resistivity results show that metal‐insulator transition (MIT) temperatures (Tp) decreased when the concentration of x increased. Tp shifted to lower temperature when the concentration of x increased. The data was analyzed based on theoretical models, where the ferromagnetic resistivity is followed with the equation ρ = ρo + ρ2T2, where ρo is due to the significance of grain boundary effects and a second‐term ~ρ2T2 appears that might be applied to the electrons scattering. In the high temperature regime (T > Tp), the resistivity data can be well described by small polaron hopping (SPH) and variable range hopping (VRH) mechanisms. Magnetic property was investigated using a vibration sample magnetometer. All samples that were obtained showed hysteresis curve with the highest value of magnetization for sample x = 0.10.

The influence of Zn2+ ions on the microstructure, electrical and gas sensing properties of La0.8Pb0.2FeO3 perovskite

The nanocrystalline La0.8Pb0.2Fe1−xZnxO3 (x=0, 0.05, 0.10 and 0.20) powders with orthorhombic perovskite structure were synthesized by a novel self-combustion method using polyvinyl alcohol as colloidal medium. The samples were investigated with regard to their composition and microstructure, using XRD and SEM-EDX methods, their electric properties and gas-sensing properties (resistivity response) to ethanol, acetone and hydrogen. The crystalline lattice constants, the porosity, specific surface area and electric resistivity increase with the increasing amount of zinc ions, which substitute the iron in La0.8Pb0.2FeO3-based perovskite (x=0). The studied samples show p-type semi-conducting properties for all the studied gases within the temperature range of 100–380°C. For acetone at the concentration of 400ppm in the case of the sample with x=0, the response value is 140; it increases with the increasing quantity of Zn2+ ions up to 560 for x=0.1, and significantly decreases (down to 45) for x=0.2. One can state that the sensitivity of La0.8Pb0.2Fe1−xZnxO3 samples is strongly affected by the amount of Zn2+ ions. The spectacular increase of the response to acetone (560) for the sample with x=0.1 is remarkable.

Influence of substrate temperature on structural, morphological and electrical properties of La0.6Sr0.4Co0.2Fe0.8O3 thin films for IT-SOFCs

The crack free, homogeneous, nanocrystalline La0.6Sr0.4Co0.2Fe0.8O3 thin films have been deposited on Ce0.9Gd0.1O1.95 electrolyte by the spray pyrolysis technique. The films with typically 1–2μm thickness, get crystallized on annealing at 800°C. In the present work, the effect of substrate temperature on microstructure, crystallization and conductivity has been systematically studied. The deposition of film at 300°C shows higher in plane electrical conductivity of 26.6Scm−1 with activation energy, Ea=0.23eV. The higher conductivity attributes to uniform, thin and dense microstructure of thin films. No interfacial reaction products between La0.6Sr0.4Co0.2Fe0.8O3 and Ce0.9Gd0.1O1.95 were detected upon annealing at 800°C/4h. The uniformity and porosity distribution as a function of deposition temperature clarified from Scanning Electron micrograph.

Nanocrystalline La0.84Sr0.16MnO3 and NiO-YSZ by Combustion of Metal Nitrate-Citric Acid/Glycine Gel – Phase Evolution and Powder Characteristics

Phase evolution behaviour of lanthanum strontium manganite (LSM) and nickel oxide-yttria stabilized zirconia (NiO-YSZ) composite from the respective oven dried fuel – metal nitrate gels (citrate-metal nitrate for LSM and glycine-metal nitrate for NiO-YSZ) was studied using TG-DTA, XRD and FTIR. The TG-DTA revealed sharp exotherms around 200° and 221°C attributed to the setting in of combustion of gels of LSM and NiO-YSZ respectively. The XRD results indicated formation of nanocrystalline LSM and NiO-YSZ phases above 200° and 221°C respectively. The FTIR spectra of LSM and NiO-YSZ gels heated above 200° and 221°C exhibited characteristic absorption bands corresponding to metal-oxygen vibrations in LSM and NiO-YSZ indicating formation of the respective compounds. However, chemical and phase pure LSM and NiO formed above 700°C indicate the minimum calcination temperature to be used in the processing. The role of calcination treatment on powder characteristics in the temperature range of 700° to 1350°C was studied. With increasing calcination temperature of the as-formed powders, the characteristics such as crystallite size, median particle size (D50), green density of the compacts increased while specific surface area decreased. Typical sintering studies at 1400° and 1450°C revealed that sinterability and per cent linear shrinkage decreased with increasing calcination temperature. However, the required amount of porosity for SOFC application could be obtained with incorporation of a pore former in case of LSM and varying the sintering time in case of NiO-YSZ composite.

Unravelling the nature of ferromagnetic-paramagnetic phase transition and its bearing on colossal magnetoresistance in nanocrystalline La1−xCaxMnO3 (0.3 ≤ x ≤ 0.4)

The nature of the ferromagnetic (FM)-paramagnetic (PM) phase transition at changes from first order to second order when the average crystallite size, d, falls below 100 nm in optimally hole-doped nanocrystalline La1−xCaxMnO3. In zero field, the systems with d < 100 behave as a three-dimensional (3D) uniaxial dipolar ferromagnet in the asymptotic critical region but in finite fields, the asymptotic critical behavior changes over to that of a 3D Ising ferromagnet. How effectively polaron correlations couple magnetic degrees of freedom to the lattice decides the order of the FM-PM transition and the magnitude of colossal magnetoresistance (CMR). d alters the electron-lattice coupling and thereby enables the tuning of CMR over an unusually wide temperature range.

Thermopower and electrical resistivity of La1−xSrxMnO3 (x=0.2, 0.3): Effect of nanostructure on small polaron transport

Abstract In this work we report on thermopower S(T) and electrical resistivity ρ(T) measurements performed on La1−xSrxMnO3 (x = 0.2, 0.3) ceramic samples, obtained by cold compaction and sintering of mechanically alloyed nanocrystalline powders. The samples studied in this work display grain sizes between d ∼ 70 nm and O(2 μm). The effects induced by the nanostructure on the thermoelectric and electrical properties in the paramagnetic state have been studied and discussed in terms of current theories about small polaron electrical transport in manganites. The observed changes in the conductivity and thermopower activation energies can be interpreted as a consequence of enhanced carrier localization and higher polaron formation energy WH for nanocrystalline La1−xSrxMnO3.

Investigation on electrical transport, CO sensing characteristics and mechanism for nanocrystalline La1−xCaxFeO3 sensors

The appropriate doping of Ca in nanocrystalline LaFeO3 not only reduces the electrical resistance, but also enhances the sensing response. A minimum of resistance for nanocrystalline La1−xCaxFeO3 (x=0–0.35) occurs at about x=0.3 with a value of Fe4+/Fe3+≈1. The electrical conduction of La1−xCaxFeO3 (x=0–0.35) can be well described by the mechanism of small polaron hopping. Results of X-ray photoelectron spectroscopy (XPS) show that the proportion of adsorbed oxygen Oads enhances monotonously with Ca doping from x=0–0.35. However, there exists an optimal Ca content (about x=0.2) for obtaining highest response to 200ppm CO among La1−xCaxFeO3-based sensors (0≤x≤0.35). Such results indicate that some parts of adsorbed oxygen species do not effectively release their electrons to the surface of La1−xCaxFeO3 at higher Ca dopants. The gas sensing response to CO for La1−xCaxFeO3 sensors depends not only upon the amount of adsorbed oxygen on grain surfaces of sensors, but also upon the desorption capabilities of adsorbed oxygen species through reacting with CO.

Nanocrystalline La0·8K0·2Fe0·5Mn0·45CO0·05O3 catalysts for removal of soot from diesel engine exhausts

A citrate gel process and a citrate gel assisted dip coating method have been used to prepare the nanocrystalline La0·8K0·2Fe0·5Mn0·45CO0·05O3 (LKFMCO) powders and an LKFMCO catalyst. The catalyst was characterised by an X-ray diffraction (XRD), a scanning electron microscope (SEM), and a surface area analyzer (BET). Results showed that the LKFMCO catalyst presented a rather irregular porous structure, in which pores were distributed from about 0·5 to 1·5 μm. Following up on the bench test, soot reductions in diesel engine exhausts were investigated with the LKFMCO catalyst. It is shown that CO2 and CO concentration reduced under the treatment of the LKFMCO catalyst. The values of CO concentration reached to their highest level at different temperature. The LKFMCO catalyst exhibits a high soot catalytic activity.

Synthesis of La0.8Sr0.2FeO3 perovskites nanocrystals by Pechini sol–gel method

Pechini sol–gel method has been employed for the preparation of nanocrystalline La0.8Sr0.2FeO3 perovskite. The synthesis procedure was optimized by varying the calcination temperature. The results revealed the decomposition of the resin precursor and confirmed the formation of the perovskite phase starting at 600°C with spherical morphology (<50nm) and subsequently increased crystal size at higher temperatures, in all cases with a structure consisting of agglomerates. The presence of pores formed by the combustion of the resin precursor was appreciable in perovskites obtained at 600 and 850°C, whereas sintering was observed at 1000°C.