Nanocrystalline La0 Research Articles

Strain induced enhanced ferromagnetic behavior in inhomogeneous low doped La0.95Sr0.05MnO3+δ

We report an unusual high-temperature ferromagnetic transition in bulk single-phase nanocrystalline La0.95Sr0.05MnO3+δ, achieved through localized strain and inhomogeneous Sr-doping. Magnetization measurements show a well defined transition at 290 K and a broad one at ∼150 K. HRTEM imaging reveals the strain on the highly crystalline nanometer sized grains and Sr-doping gradients, while oxygen homogeneity at the grain interfaces is confirmed by EELS-spectra. The magnetic behavior, far from the expected bulk phase diagram, shows how local doping and strain can strongly tune the macroscopic properties of a bulk material.

Metal insulator transition of bulk and nanocrystalline La1−xCaxMnO3 perovskite manganite materials through in-situ ultrasonic measurements

Abstract Bulk and nanocrystalline La 1 − x Ca x MnO 3 ( x = 0.75, 0.80 and 0.85) perovskite manganite samples were prepared by using the solid-state reaction and sonochemical reactor methods, respectively. The X-ray diffraction studies confirmed that the prepared bulk and nanoperovskite manganite samples were crystalline in nature with an orthorhombic crystal structure. The microscopic studies revealed that the particle sizes of the bulk and nanoperovskite samples were 0.75–0.90 μm and 120–160 nm, respectively. In addition, ultrasonic velocities and attenuations of the bulk and nanosamples were measured by using the in-situ ultrasonic through transmission method. The temperature-dependent ultrasonic parameters showed an interesting anomaly in both bulk and nanoperovskite samples. The observed marked softening and hardening in sound velocities or attenuation were related to metal–insulator phase transition temperature. In nanosamples, the metal insulator transition took place at a lower temperature compared with the corresponding bulk samples. The observed results confirmed that nanoperovskite samples have a broad transition compared with the corresponding bulk samples. The observed broad peak at transition temperature in nanoperovskite samples was used to confirm the nanocrystalline nature of perovskites.

Synthesis and characterization of nanoparticulate La0.6Sr0.4CoO3−δ cathodes for thin-film solid oxide fuel cells

Nanocrystalline La0.6Sr0.4CoO3−δ (LSC) powder with an ultrafine microstructure is synthesized via salt-assisted spray pyrolysis and subsequently stabilized in water-based dispersions. Nanoparticulate cathode thin films of LSC and LSC–GDC (gadolinium doped ceria) nanocomposites (with 10–40 wt% of GDC) are prepared via single step spin coating on yttria stabilized zirconia (YSZ) substrates. In order to prevent the chemical reaction between the cathode and the electrolyte, a thin buffer layer of GDC is deposited using spin coating on the YSZ substrates. The electrochemical performance of the thin film cathodes is measured by impedance spectroscopy on symmetrical cells in the temperature range of 450–650 °C. LSC cathode thin films (250 nm thick) with 30 wt% GDC content exhibit the lowest area specific resistance (ASR) values of 0.32, 0.78 and 2.04 Ω cm2 in ambient air at 650, 600 and 550 °C, respectively.

Phase, morphology and core-level electron spectroscopy of nano-sized La0.65Sr0.35MnO3 powders prepared by solution combustion synthesis

Nanocrystalline La0.65Sr0.35MnO3 powders were synthesized by modified solution combustion route. The X-ray diffraction patterns of powders calcined at different temperatures were analyzed by Rietveld programme. The synthesized bulk polycrystalline sample were in 100% perovskite phase and showed an increase in particle size from 22.9nm to 55.9nm with the increase in the calcinations temperature from 650°C to 850°C. The grain boundary of sintered pellet observed under electron microscope was devoid of any precipitation related to MnO2, SrO and the La–Mn–Sr–O phases. The core level photoelectron spectra of lanthanum, strontium, manganese and oxygen were studied in detail. The average chemical composition obtained from the XPS measurement was found to be La0.62Sr0.38Mn1.11O3.44. Curie temperature for the 50nm powder was around 365K and saturation magnetization at 100K was 60emu/g.

A study of Eu-doped La2Zr2O7 nanocrystals prepared by salt-assistant combustion synthesis

Nanocrystalline (La1−xEux)2Zr2O7 series solid solutions were prepared by a simple and controllable salt-assisted combustion process. The samples were characterized by XRD, TEM and EDS. The results showed the partial substituted products were single-phase solid solutions. The effect of thermal treatment temperature on the crystal size of nanocrystals and the activation energy of crystal growth during calcinations were studied. And the activation energy of (La1−xEux)2Zr2O7 series with x=0.05, 0.1, 0.25, 0.5, and 0.75 was 25.4, 27.8, 28.0, 32.4, and 30.6kJ/mol, respectively. TEM images revealed that the products were composed of well-dispersed cubic-like particles with the range of size at 20–50nm. EDS results furtherly confirmed that the composition of the obtained compound was in agreement with the stiochiometric formular. The emission spectra of (La1−xEux)2Zr2O7 nanocrystals at 385nm upon excitation showed that there were some intense and prevailing emission peaks located at 590–650nm.

Synthesis of nano-crystalline La1–xSrxCoO3–δ perovskite oxides by EDTA–citrate complexing process and its catalytic activity for soot oxidation

In the present study, La1–xSrxCoO3–δ (x=0.2, 0.3, 0.4, 0.5) perovskite nano-particles have been successfully prepared by EDTA–citrate complexing process and the catalytic performance was examined for the soot oxidation activity. The physio-chemical properties of these materials were characterized by means of X-ray diffraction (XRD), thermogravimetry–differential thermal (TG–DTA) analysis, BET surface area and X-ray photoelectron spectroscopy (XPS) analysis, scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) analysis. TG–DTA analysis suggested that, in the temperature range of 350–500°C, the pyrolysis of metal/nitrate/citrate/EDTA complexes takes place and the single phase La1–xSrxCoO3–δ perovskite oxides can be formed at 900°C and the phase formation has been confirmed by XRD analysis. The surface composition of the samples derived from XPS analysis indicated an impoverishment of lanthanum content at the surface with the increase of strontium content. For La0.6Sr0.4CoO3–δ sample, Sr-enriched perovskite surface has been observed along with the more number of surface oxides (M(Sr, La, Co)O) resulting in higher catalytic activity for soot oxidation than the others.

Structure, magnetic and electrical transport properties of the perovskites La0.67−xEuxSr0.33MnO3

In this paper we report the structural, magnetic and electrical properties of a series of nanocrystalline La0.67−xEuxSr0.33MnO3 (0≤x≤0.3) materials which were prepared by the solid-state reaction method in air. The X-ray powder diffraction has shown that all our synthesized samples are a single phase and have crystallized in the hexagonal symmetry with R3¯C space group. The scanning electron microscopy has shown smooth and densified structures, clean and pure images. Electric and magnetic measurements show that all our samples had exhibited a ferromagnetic to paramagnetic transition and a metallic to semiconductor one when temperature increases. The substitution of La3+ by Eu3+ leads to a continuously simultaneous decrease of the Curie temperature TC (from 350K for x=0.0–258K for x=0.3) and the metal-semi-conductor transition temperature Tp (from 310K for x=0.0–224K for x=0.3). The electrical resistivity data were analyzed using different theoretical models and it has been concluded that at low temperatures (ferromagnetic metallic region) the resistivity may originate from grain/domain boundary, electron–electron scattering and two-magnon scattering effects. While in the paramagnetic insulating regime, the variation of resistivity with temperature may be explained by adiabatic small polaron Hopping mechanism and variable-range hopping mechanisms. The values of activation energies were found decreasing, while the density of states at the Fermi-level, N (EF) was increasing with decreasing 〈rA〉. A suitable explanation for the observed behavior is given.

Critical behaviour and magnetocaloric effect of nano crystalline La0.67Ca0.33Mn1−xFexO3 (x = 0.05, 0.2) synthesized by nebulized spray pyrolysis

The structure, critical exponents and magnetocaloric effect (MCE) of Nebulized Spray Pyrolysis (NSP) synthesized nano crystalline La0.67Ca0.33Mn1−xFexO3 (x = 0.05, 0.2) were investigated. The Reitveld refinement of XRD patterns show that the samples adopt an orthorhombic structure with Pnma space group. TEM inspection reveals that the average particle size is about 15 nm and 42 nm for NSP synthesized LCMFe0.05 and LCMFe0.2 samples respectively. The temperature and field dependent magnetization studies reveal the superparamagnetic state of La0.67Ca0.33Mn0.95Fe0.05O3 and spin-glass-like state of La0.67Ca0.33Mn0.8Fe0.2O3. The critical behaviour at the transition region studied using modified Arrott plot provides a second order nature of phase transition for both samples. The magnetocaloric studies show the maximum value of magnetic entropy change (ΔSmax) is in the range 2.3 J kg−1 K−1 at 158 K for LCMFe0.05 and 0.3 J kg−1 K at 92 K for LCMFe0.2 respectively at 5 T field. The field dependence of the magnetic entropy changes are also analysed, which show a power law dependence (ΔSM ∞Hn, n = 0.72 (2)) at transition temperature, TC = 162 K for LCMFe0.05 and n = 1.11(3) at 92 K for LCMFe0.2.

Thermoelectric and magnetic properties of nanocrystalline La0.7Sr0.3CoO3

This work reports on the effect of grain size on the electrical, thermal, and magnetic properties of La0.7Sr0.3CoO3 samples obtained by solid-state reaction combined with ball milling. Electrical characterization made for samples with grain sizes ranging from 26 nm to 0.57 μm shows a size-induced metal-to-insulator transition for the sample with smallest grain size. As a consequence of the grain size reduction, there is a change of the thermopower sign and the thermal conductivity displays extremely low values. Interestingly, the thermoelectric figure of merit displays an enhancement as a consequence of the grain size reduction. The Curie temperature was found to be constant whereas the saturation magnetization decreases as grain size decreases. Thereby electrical and magnetic properties of nanocrystalline samples are interpreted in terms of a core-shell structure in which grain boundaries display an insulating behavior while the core region of each grain exhibits the bulk properties.

Size effect on the structural, magnetic, and magnetotransport properties of electron doped manganite La0.15Ca0.85MnO3

Nanocrystalline La0.15Ca0.85MnO3 samples of various grain sizes ranging from ∼17 to 42 nm have been prepared by sol-gel technique. Phase purity and composition were verified by room temperature x-ray diffraction and SEM-EDAX analysis. The bulk La0.15Ca0.85MnO3 is known to order antiferromagnetically around 170 K and to undergo a simultaneous crystal structural transition. DC magnetization measurements on 17 nm size La0.15Ca0.85MnO3 show a peak at ∼130 K (TN) in zero-field-cooled (ZFC) state. Field-cooled magnetization bifurcates from ZFC data around 200 K hinting a weak ferromagnetic component near room temperature due to surface moments of the nanoparticle sample. Low temperature powder neutron diffraction experiments reveal that the incomplete structural transition from room temperature orthorhombic to low temperature orthorhombic-monoclinic state also occurs in the nanoparticle sample as in the bulk. Magnetization in the ordered state decreases as particle size increases, thus indicating the reduction of the competing ferromagnetic surface moments.

Structural Characterization of Nanocrystalline La<sub>1-x</sub>Sr<sub>x</sub>CrO<sub>3</sub> Thick Films for H<sub>2</sub>S Gas Sensors

The nanocrystalline of La1-xSrxCrO3 (x = 0.0, 0.1, 0.2, 0.3 & 0.4) were prepared by sol-gel method and their crystal structures & morphology were characterized by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). XRD patterns indicate that the average particle size of the nanocrystalline La0.7Sr0.3CrO3in the range of 30 - 35 nm. The gas sensing properties were studied towards reducing gases like Ammonia gas (NH3), liquefied petroleum gas (LPG), hydrogen sulphide (H2S) and H2 gas and it is observed that undoped LaCrO3 shows response to H2S gas at relatively high operating temperature 300°C. The La1-xSrxCrO3 based sensor with x = 0.3 shows better sensitivity towards H2S gas at an operating temperature 210°C. The effect of Sr doping on sensitivity, response time and recovery time of the sensor in the presence of H2S and other reducing gases were studied and discussed.

Magnetic properties of nanocrystalline La0.52Sr0.28Mn1.2O3

Phase-pure La0.52Sr0.28Mn1.2O3 manganite nanopowders with average crystallite sizes of 30, 60, and 200 nm have been synthesized using coprecipitation and multiple cold isostatic pressing at 1 GPa. The crystallite size is shown to have a significant effect on the electrical and magnetic properties of the nanopowders: with decreasing particle size, their resistivity rises by several orders of magnitude, their Curie temperature decreases significantly, and the peak in their magnetic susceptibility broadens. The electrical and magnetic properties of powder compacts are compared to those of ceramic samples. The powder compacts show conventional magnetic hysteresis behavior, whereas the ceramics produced by sintering the compacts at 1270 K have an anomalous hysteresis. A mechanism is proposed that accounts for the anomalous hysteresis behavior.

Microstructure Characterization and Electrical Conductivity Measurement of La1−xCaxCrO3 (x = 0.25, 0.4, 0.5) Prepared by Aspartic Acid‐Assisted Solution Combustion

Nanocrystalline La1−xCaxCrO3 (x = 0.25, 0.40, 0.50) powders were synthesized by gel‐combustion process using aspartic acid as a fuel. Phase identification of the samples was carried out by powder X‐ray diffraction. Pore distribution and microstructure of sintered pellets were investigated by small angle neutron scattering (SANS) and scanning electron microscopy (SEM). SANS and relative density measurement results indicate better sinterability for products with lower Ca2+ concentration. Four‐probe direct current electrical conductivity of LaCrO3 with different Ca2+ concentrations was measured in the temperature range 350°C–1000°C in air as well as in hydrogen atmosphere. Linear behavior of Arrhenius plots of electrical conductivities in both air and hydrogen atmosphere indicates charge carrier transport is due to thermally activated hopping conduction of small hole‐polarons. It was observed that the electrical conductivity decreased with increasing substitution of Ca2+ concentration instead of increasing small polaron concentration as expected. This trend in electrical conductivity is attributed to the neutralization of some small polarons due to formation of oxygen vacancies with increase in Ca2+ concentration.

INFLUENCE OF GRAIN SIZE ON MAGNETOCALORIC EFFECT IN THE NANOCRYSTALLINELa0.8Te0.2MnO3

The nanocrystalline La0.8Te0.2MnO3samples are prepared by sol-gel method and show rhombohedral crystal structure with R3c space group at room temperature. The calculated crystallite sizes are ~55 nm, 40 nm and 25 nm for calcined at 700°C, 800°C and 900°C temperatures. The SEM images show the grain size increases as the calcination temperature increases and the values are in good agreement with that obtained from X-ray diffraction analysis. The samples undergo paramagnetic to ferromagnetic transition and follow Curie–Weiss law in the paramagnetic region. The maximum entropy change are ~3.2 J kg-1K-1, 3 J kg-1K-1and 2 J kg-1K-1for a field change of 20 kOe for 55 nm, 40 nm and 25 nm samples respectively. In the framework of Landau theory of phase transition, the experimentally observed magnetic entropy change and theoretical predicted model fits well for all the nanoparticles.

Suppression of Charge and Antiferromagnetic Ordering in La0.5Ca0.5MnO3 Nanoparticles

The intermediate band-width manganite La0.5Ca0.5MnO3 has a CE type charge, orbital and antiferromagnetic order below 195 K owing to strong electronic correlation and commensurability effect. We have critically investigated in details all the possible mechanisms behind the destabilization of charge/orbital ordering in La0.5Ca0.5MnO3 with the reduction of grain size in the nanometric regime as evidenced from our various experimental results. We have tried to found out a universal scenario to explain such behavior in nanometric manganites in general. This study shows that the room temperature Pnma perovskite structure, correlated with the lattice distortion, Mn-O distances and octahedral tilts, is practically unaffected with the reduction of particle size. However, in these nanocrystalline La0.5Ca0.5MnO3, the stable charge ordered state with antiferromagnetic ordering becomes significantly suppressed and FM ordering spontaneously appears below T(c) = 260 K as compared to its bulk counterpart. From strong experimental evidences on nano and bulk manganites, we revisit this issue and also discuss the absence of correlation between size reduction and pressure effects on manganites. We argue from a phenomenological analysis that the above observations could be best attributed to enhanced surface disorder in the nanosized half-doped manganite grains which is likely to be general in all nano size charge ordered manganites.

Synthesis and on-line ultrasonic characterisation of bulk and nanocrystalline La0.68Sr0.32MnO3 perovskite manganite

La0.68Sr0.32MnO3 perovskite manganite samples were prepared using sonochemical reactor and solid state reaction technique. The ultrasonic velocity, attenuation and elastic moduli of samples were measured using ultrasonic through transmission method, at a fundamental frequency of 5MHz over a wide range of temperatures. The temperature dependence of the ultrasonic parameters shows an interesting anomaly in all the compositions. The observed dramatic softening and hardening in sound velocities or attenuation is related to phase transitions. The linear magnetostriction effect is more dominant in the perovskite than volume magnetostriction effect which is evident from the observed anomalous in both longitudinal and shear velocities and attenuation. Further, a decrease in grain size in the sintered sample leads to a shift in the ferromagnetic transition temperature (TC) from 375 to 370K.

Collapse of charge ordering and enhancement of magnetocaloric effect in nanocrystalline La0.35Pr0.275Ca0.375MnO3

We demonstrate the possibility of enhancing both the magnetocaloric effect (MCE) and refrigerant capacity (RC) in nanostructured mixed phase manganites. A comparative study of the magnetic and magnetocaloric properties of La0.35Pr0.275Ca0.375MnO3 in single crystalline and nanocrystalline forms is presented. While the conventional trend is reduction of magnetization and MCE with nanostructuring, we show that the opposite is true in the case of mixed phase manganites. The charge-ordered state is largely suppressed and ferromagnetic order is established in the nanocrystalline sample with an average particle size of 50 nm. Consequently, a strong enhancement of MCE and RC and a strong reduction of thermal and field hysteresis losses are achieved in the nanocrystalline sample. This finding opens up a way of exploring magnetic refrigerant materials at the nanometer scale for active magnetic refrigerators.

Influence of polymerizing agent on structure and spectroscopic properties of nano‐crystalline La0.8Sr0.2MnO3 powders

AbstractStrontium‐doped lanthanum manganite (La1‐xSrxMnO3 or LSM) is the material most widely used on solid oxide fuel cells cathode. In this work, nano‐scale La0.8Sr0.2MnO3 powders have been synthesized by polymeric precursor‐based methods using different polymerizing agents with the aim of evaluating the influence of this change in the final powder. The powders calcined at 700 °C for 2 h have been characterized by X‐ray diffraction (XRD), scanning electron microscopy (FE‐SEM) and Fourier transform infrared spectroscopy (FTIR) in order to investigate the quality of both synthesis routes. It is shown that the crystal structure and morphology of the particles are significantly dependent on the preparation conditions. Single phase and La0.8Sr0.2MnO3 nano‐crystalline particles less than 30 nm were obtained using ethylene glycol as polymerizing agent. FTIR results have been indicated that only the powder obtained using gelatin presented small traces of hydroxyl groups on its surface. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Acetone gas-sensing performance of Sr-doped nanostructured LaFeO 3 semiconductor prepared by citrate sol–gel route

Nanocrystalline La 1− x Sr x FeO 3 ( x = 0, 0.1, 0.2, 0.3 and 0.4) was prepared by sol–gel citrate method. The structural and microstructural characterization has been carried out with the help of X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. XRD studies confirm the formation of LaFeO 3 with perovskite phase. Further studies by Fourier transform infrared spectroscopy were also conducted. DC electrical properties of the prepared nanoparticles were studied by D.C. conductivity measurements. The gas-sensing performance of the as-prepared La 1− x Sr x FeO 3 nanoparticles was investigated towards different reducing gases like acetone (CH 3COCH 3), ethanol (C 2H 5OH), hydrogen sulfide (H 2S), hydrogen (H 2) and liquefied petroleum gas (LPG). The nanocrystalline La 0.7Sr 0.3FeO 3 exhibited good sensing performance towards acetone gas with rapid response and high sensitivity at 275 °C as compared with LaFeO 3.

Enhanced low field magnetoresistance in nanocrystalline La0.7Sr0.3MnO3 synthesized on MgO nanowires

We report on the structure and transport properties of nanocrystalline manganite La0.7Sr0.3MnO3 (LSMO) synthesized on nanowires-engineered MgO substrates by pulsed laser deposition, which is compared with reference samples deposited directly on flat MgO substrates. Such LSMO/MgO nanocomposites show enhanced low field magnetoresistance, especially at low temperature, due to the dominant spin-polarized intergrain tunneling. This work suggests that growing on nanoengineered substrates is a viable route to achieve nanostructured materials with desired crystalline structure and physical properties.