Nanocrystalline La0 Research Articles

Microstructure and H2S gas sensing properties of nanocrystalline perovskite-type La0.6Co0.4Mn0.5Ni0.5O3

Nanocrystalline La1−x Co x Mn1−y Ni y O3 (x = 0.2 and 0.4; y = 0.1, 0.3, and 0.5) thick films sensors prepared by sol–gel method were studied for their H2S gas sensitivity. The structural and morphological properties have been carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Average particle size estimated from XRD and TEM analyses was observed to be 30–35 nm. The gas response characteristics were found to depend on the dopants concentration and operating temperature. The maximum H2S gas response of pure LaMnO3 was found to be at 300 °C. In order to improve the gas response, material doped with transition metals Co and Ni on A- and B-site, respectively. The La0.6Co0.4Mn0.5Ni0.5O3 shows high response towards H2S gas at an operating temperature 250 °C. The Pd-doped La0.6Co0.4Mn0.5Ni0.5O3 sensor was found to be highly sensitive to H2S at an operating temperature 200 °C. The gas response, selectivity, response time and recovery time were studied and discussed.

Magnetism and magnetocaloric effect in nanocrystalline La0.67Ca0.33Mn0.9V0.1O3 synthesized by nebulized spray pyrolysis

Vanadium (V) substituted La0.67Ca0.33MnO3 was synthesized in the nanocrystalline form using nebulized spray pyrolysis. Highly uniform nanoparticles of average size in the range 30–40 nm as revealed by transmission electron microscopic studies were prepared. Rietveld refinement of XRD patterns shows that the sample adopts an orthorhombic structure with the Pnma space group. The systematic change in lattice parameters and magnetic phase transition (TC = 252 K) as compared with those of nanocrystalline La0.67Ca0.33MnO3 (TC = 260 K) indicate the substitutional effect of V. M–H curves in a certain temperature range reflect the superparamagnetic nature of the sample. The average particle size (∼28 nm) of the superparamagnetic clusters obtained from the Langevin fit is coincident with that of the nanocrystalline clusters revealed by the high-resolution transmission electron microscopy images. The Arrott plots reveal the first order character of magnetic phase transition of the sample. The peak magnetic entropy change (ΔSmax) of these nanopowders is estimated to be about 4.5 J kg−1 K−1 for a magnetic field of 5 T at 252 K. Compared with the bulk La0.67Ca0.33Mn0.9V0.1O3, the entropy change of the nanoparticles is found to be smaller, but the entropy change occurs over a broad temperature range, giving rise to a large relative cooling power.

Magnetism of nanocrystalline (La0.5Sr0.5)TiO3 doped with 3d cations

Nanocrystallites of nominal composition(La0.5Sr0.5)TiO3 12–15 nm in diameter exhibit a diamagnetic susceptibility that is greater by afactor of three than that of bulk ceramic material, due to a much-reduced Pauliparamagnetic contribution associated with oxidation (cation deficiency) of thematerial. Doping the nanocrystallites with 1.5 or 2.0% of every transition metalfrom V–Ni adds a Curie term to the susceptibility. Exchange coupling betweenthese paramagnetic ions is very weak. In the cases of Fe, Co and Ni there is anadditional hysteretic ferromagnetic magnetization, with a moment equivalentto a few tenths of a Bohr magneton per dopant atom, which is attributed to asecondary ferromagnetic impurity phase. Mössbauer analysis of samples prepared with 57Fe reveals the presence of some metallic iron. Metallic nickel is detected by x-ray diffraction,but no direct evidence of metallic cobalt was found in the ferromagnetic Co-dopedmaterial. The possibility of high temperature defect-related ferromagnetism in a metallicoxide is discussed.

Study of mechanochemically prepared nanocrystalline La0.8Sr0.2MnO3

The nanocrystalline La0.8Sr0.2MnO3 (LSM) is prepared by varying the revolutions per minute and milling time of planetary monomill during the mechanochemical method. The LSM forms in a relatively shorter milling time with an increase in the milling speed from 250 to 600 rpm. The structural phase transition from orthorhombic to rhombohedral phase in the LSM prepared by ball milling at the speed 250 rpm for 36 h is seen due to sintering it at 700 °C for 4 h. The crystallite size reduces with the increase in both the milling speed and the milling time individually or combined. The microhardness (HV) and sintered density increase with the reduction in the crystallite size. The temperature-activated transition temperature is suppressed by reducing the grain size in the nanometer range. The electrical dc conductivity increases with the reduction in the grain/crystallite size.

High surface area nano-sized La0.6Ca0.4MnO3 perovskite powder prepared by low temperature pyrolysis of a modified citrate gel

Abstract Single phase nanocrystalline La0.6Ca0.4MnO3 powder was synthesized by both the usual and a modified citrate gel precursor method, and the effects on the formation of homogeneous nano-sized powder with a perovskite structure were investigated. In the modified method, single phase La0.6Ca0.4MnO3 powder with an average particle size of 17.2 nm was obtained when the powder was pyrolyzed at 520°C for 2 h. Its specific surface area was 40.7 m2 g−1, about 4-fold larger than that of powder made by the usual citrate gel method.

Temperature Dependence of Phonon Modes in Nanocrystalline La0.67Ca0.33MnO3 as Observed by Infrared Spectroscopy

Low temperature infrared absorption measurements have been carried out on nanocrystalline La0.67Ca0.33MnO3 powder across the magnetic and the insulator-to-metal phase transitions. Interesting changes are observed in the temperature dependence of the mid-IR polaronic background and the stretching mode of the MnO, octahedron. Upon cooling below 250 K, the overall absorbance increases, but, unlike in the case of microcrystalline LCMO, the stretching mode is not completely screened even at 5 K. The temperature dependence of the stretching mode parameters points to a much reduced phonon-polaron coupling as compared to that in the bulk material.

Magnetocaloric properties of nanocrystalline La0.125Ca0.875MnO3

Some recent experimental studies show the invisibility of antiferromagnetic transition in the cases of manganites when their particle size is reduced to nanometer scale. In complete contrast to these cases, we have observed the signature of antiferromagnetic transition in the magnetocaloric properties of nanocrystalline La0.125Ca0.875MnO3 of average particle sizes 70 and 60 nm similar to its polycrystalline bulk form. The system exhibits inverse magnetocaloric effect in its polycrystalline and nanocrystalline forms. An extra ferromagnetic phase is stabilized at low temperature for the sample with particle size ∼60 nm.

Electronic properties of nanocrystalline LaNiO3 and La0.5Sr0.5CoO3 conductive films grown on silicon substrates determined by infrared to ultraviolet reflectance spectra

Electronic band structures of nanostructured LaNiO3 (LNO) and La0.5Sr0.5CoO3 (LSCO) films have been investigated by near-normal incident optical reflectance at room temperature. Dielectric constants of the conductive films in the photon energy range of 0.47–6.5 eV have been extracted with the Drude–Lorentz function. It is found that four interband electronic transitions can be uniquely assigned for the perovskite-type metallic oxides. Moreover, optical conductivity is approximately varied from 100 to 450 Ω−1 cm−1 and shows a different variation trend for the LNO and LSCO layers. The discrepancy could be ascribed to diverse electronic structure, grain size, and crystalline formation.

Specific physical properties of nanocrystalline (La0.65Sr0.35)0.8Mn1.2O3 ± Δ samples obtained by cold isostatic pressing

Single-phase powders of manganites (La0.65Sr0.35)0.8Mn1.2O3 ± Δ with average crystallite sizes of 30, 50, and 500 nm were produced by co-precipitation. The samples studied were obtained by cold isostatic pressing of powders at a pressure of 1 GPa without subsequent sintering. It is shown that the size of particles has a significant effect on the electromagnetic properties of the manganite samples. As the crystallite size decreases, the electrical resistance and coercive force increase and the tunneling magnetoresistance of the samples and the Curie temperature decrease.

Magnetic Properties of the La<sub>0.50</sub>Ba<sub>0.50</sub>MnO<sub>3</sub> Nanomanganites

Nanocrystalline La0.50Ba0.50MnO3 manganite was synthesized by an optimized sol-gel method. The initial sample was subjected to step-by-step heat treatment under air atmosphere. The ion stoichiometry, the morphology of crystallites of ceramics, and the magnetic properties were studied. It is established that the average crystallite size increases with increasing annealing temperature. All of the samples studied are characterized by a perovskite-like cubic structure, with the unit cell parameter a increasing continuously with the average crystallite size. The most significant lattice compression occurs in the sample with an average crystallite size of ~ 30 nm. The increase in the average crystallite size causes a nonmonotonic increase in the Curie temperature and in the spontaneous magnetic moment. The anomalous behavior of the magnetic properties of the La0.50Ba0.50MnO3 manganites obtained is explained by the competition between two size effects, namely, the frustration of the indirect exchange interactions Mn3+ – O – Mn4+ on the nanocrystallite surface and the crystal lattice compression due to the crystallite surface tension.

Pressure-induced phase transition in nanocrystalline La0.9Mn0.8Fe0.2O3.15 perovskite studied by Mössbauer spectroscopy

We report here on a pressure-induced phase transition in nanocrystalline La-deficient perovskite oxide, synthesised through the sol–gel technique, using Mössbauer spectroscopy, electrical resistivity and X-ray diffraction measurements at room temperature. At ambient pressure, the high-spin Fe3+ ion is distributed in two different environments. Below 0.6 GPa, a sudden dip in the resistivity curve and a single-environment Fe3+ doublet in the Mössbauer pattern are indicative of lattice compaction. A high-spin to low-spin Fe3+ transition at 2.1 GPa, and an orthorhombic to monoclinic structural transformation at 4.9 GPa, as revealed through Mössbauer patterns, are also associated with sudden dips in the electrical resistivity. X-ray diffraction patterns support the results.

Synthesis and structure of nanocrystalline La0.50Ba0.50MnO3

Nanocrystalline manganite La0.50Ba0.50MnO3 was synthesized by the soft-chemical method. For this purpose, the sol-gel method was modified and improved. A trihydric saturated alcohol, e.g., glycerol, was suggested as a new organic matrix. The crystal structure of the composite was studied by X-ray powder diffraction at room temperature. A La0.50Ba0.50MnO3 powder annealed in air at T = 500°C is characterized by a perovskite-like cubic structure with the unit-cell parameter a = 3.869 A. The chemical composition of the sample was studied by electron-probe X-ray microanalysis. The La: Ba: Mn cation ratio in the material was 1: 1: 2. The surface topography was examined with a scanning electron microscope. The crystallite size was ∼30 nm. The dependence of the crystal structure and the surface topology on the annealing temperature was studied. The high-temperature treatment in air resulted in the growth of larger, micrometer-size, crystallites.

Synthesis, microstructure and catalytic property of nanocrystalline La1−xCexMnO3

Synthesis, microstructure and catalytic property of nanocrystalline La_1−<i>x</i>Ce_<i>x</i>MnO₃

Study of phase transition on nanocrystalline (La, Sr)(Mn, Fe)O system using high-pressure Mössbauer spectroscopy and high-pressure X-ray diffraction

Pressure-induced structural changes on nano-crystalline La0.8Sr0.2Mn0.8Fe0.2O3 were studied using high-pressure Mossbauer spectroscopy and high-pressure X-ray diffraction. Mossbauer measurements up to 10 GPa showed first order transition at 0.52 GPa indicating transformation of Fe4 + to high spin Fe3 + , followed by another subtle transition at 3.7 GPa due to the convergence of two different configurations of Fe into one. High-pressure X-ray diffraction measurements carried up to 4.3 GPa showed similar results at 0.6 GPa as well as 3.6 GPa. Attempts were made to explain the changes at 0.6 GPa by reorientation of grain/grain boundaries due to uniaxial stress generated on the application of pressure. Similarly variation at 3.6 GPa can be explained by orthorhombic to monoclinic transition.

Structural and chemical properties of nanocrystalline La0.5Sr0.5CoO3−δ layers on yttria-stabilized zirconia analyzed by transmission electron microscopy

Nanocrystalline La1−xSrxCoO3−δ (LSC) thin films with a nominal Sr content x = 0.5 were deposited on 3.5 mol% yttria-stabilized zirconia (YSZ) substrates by a low-temperature sol–gel process followed by a rapid thermal annealing procedure at temperatures up to 900 °C. The structural and chemical stability of the as-prepared nanocrystalline LSC and demixing effects within the thin film or at the LSC/YSZ interface were studied after long-time exposure at temperatures between 700 °C and 1,000 °C. The grain size and surface topography were analyzed by scanning electron microscopy. Transmission electron microscopy combined with selected-area electron diffraction, energy-dispersive X-ray spectrometry, and electron-spectroscopic imaging was applied for the investigation of the microstructure and the analysis of the local chemical composition and element distribution on the nanoscale. Chemical potential calculations, which were performed to assess the decomposition of LSC/YSZ as a function of temperature, show good agreement with the experimental results.

H2S sensing characteristics of La0.7Pb0.3Fe0.4Ni0.6O3 based nanocrystalline thick film gas sensor

In this paper, the structural and gas sensing properties of nanocrystalline La0.7Pb0.3Fe0.4Ni0.6O3 was prepared by a sol–gel citrate method. The structural characteristics of the material were studied by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD pattern shows a nanocrystalline solid solution of La0.7Pb0.3Fe0.4Ni0.6O3 with a perovskite phase. The gas sensing properties were studied for different reducing gases such as H2S, ethanol, CO and LPG. The La0.7Pb0.3Fe0.4Ni0.6O3 powder showed large response to H2S gas at an operating temperature 250 °C. Further improvement in response was achieved by the addition of 0.5 wt% Pd to La0.7Pb0.3Fe0.4Ni0.6O3. The sensor showed a good response to 150 ppm H2S gas at an operating temperature 200 °C.

Synthesis and characterization of La0.8Sr0.2Co0.5Fe0.5O3±δ nanopowders by microwave assisted sol–gel route

Nano-crystalline La0.8Sr0.2Co0.5Fe0.5O3±δ powder has been successfully synthesized by microwave assisted sol–gel (MWSG) method. The decomposition and crystallization behavior of the gel-precursor was studied by Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) analysis. From the result of FT-IR and X-ray diffraction patterns, it is found that a perovskite La0.8Sr0.2Co0.5Fe0.5O3±δ was formed by irradiating the precursor at 700 W for 3 min, but the well-crystalline perovskite La0.8Sr0.2Co0.5Fe0.5O3±δ was obtained at 700 W for 35 min. Morphological and specific area analysis of the powder were done by transmission electron microscopy (TEM), scanning electron microscope (SEM) and Brunauer–Emmett–Teller (BET). The surface areas measured was 38.9 m2/g and the grain size was ∼23 nm. Electrochemical properties of pure LSCF cathode on YSZ electrolyte at intermediate temperatures were investigated by using AC impedance analyzer, which shows a low area specific resistance (0.077 Ω cm2 at 1073 K and 0.672 Ω cm2 at 953 K). Moreover, the synthesis period of 20 h usually observed for conventional heating mode is reduced to a few minutes. Thus, the MWSG method is proved to be a novel, extremely facile, time-saving and energy-efficient route to synthesize LSCF powders.

Synthesis of single phase Ce doped nanocrystalline LaMnO3

Abstract It has been reported that the electron (Ce) doped LaMnO3 compounds are multi-phased when prepared through solid state reaction and precipitation method. However, the laser ablated thin films of the above show single phase nature. Here, we report the synthesis of single phase nanocrystalline La0.85Ce0.15MnO3 and La0.8Ce0.2MnO3 powders through citrate-complex method. In this method, the metal ions are homogeneously dispersed in amorphous organic (citrate) complex and are allowed to react while calcination. Thus, the reacted metal ions give single phase compounds. The measured average particle size from transmission electron micrograph is found to be 30 nm for La0.8Ce0.2MnO3 sample. Temperature dependence of magnetization of both the samples show ferro- to paramagnetic transition and zero field cooled and field cooled magnetization curves suggest cluster glass behaviour. Temperature dependence of resistance of the La0.8Ce0.2MnO3 sample exhibits a metal to insulator transition at 110 K and shows no impurity peak around 250 K, which is generally observed in multi-phase samples. Magnetoresistance decreases with increasing temperature in ferromagnetic regime, as observed in typical hole doped nanocrystalline manganites.

Tunable room temperature low-field spin polarized tunneling magnetoresistance of La0.7Sr0.3MnO3 nanoparticles

The authors have studied low-field spin polarized tunneling magnetoresistance (LFMR) at room temperature of a series of nanocrystalline La0.67Sr0.33MnO3 samples having average crystallite size (ϕ) from 14to150nm. Interestingly with decrease in ϕ, LFMR enhances up to a critical ϕ of 28nm, beyond which LFMR starts decreasing, indicating a pronounced crossover of LFMR value (LFMRmax∼1.67%) with decrease in ϕ. Magnetization versus magnetic field study at room temperature reveals that with decrease in ϕ LFMR increases untill the nanoparticles remain magnetically multidomain (MD), but as soon as it falls in the single domain (SD) regime, LFMR immediately starts diminishing. Their study explores the fact that for MD nanoparticles room temperature LFMR is decided only by surface spin susceptibility (χb), whereas for SD nanoparticles LFMR arises as a result of a sensitive balance between χb and thermal effect.

Low-field magnetoresistance in nanocrystalline La0.7Sr0.3MnO3 films

Nanocrystalline La0.7Sr0.3MnO3 films with thickness t=10-60nm were grown on LaAlO3(100) substrates by radio-frequency magnetron sputtering. Their electrical resistivity and low-field magnetoresistance (MR) were measured. Metal-insulator transitions occur above 275K for films with t=20-60nm, but the electron localization prevails in the 10nm thick film. Furthermore, only the 10nm thick film has an MR that depends on the inverse of temperature, consistent with the model of spin-polarized tunneling. This relationship may reflect a critical aspect of the structure of grain∕grain-boundaries. Accordingly, the tunneling MR in this film is 27% at 75K.