Sol-gel synthesis and properties of doped lanthanum chromite nanopowders

La_{1 ? x} Sr _x CrO₃ (0.2 ≤ x ≤ 0.3) powders have been prepared by a sol-gel process and dried in a microwave oven. Synthesis conditions were optimized using differential scanning calorimetry and mass spectrometry data. The phase composition of the synthesis products obtained at temperatures of 800, 850, 900, and 950°C was determined by X-ray diffraction. Scanning electron microscopy was used to determine the particle size of the powders and examine their morphology.     

Magnetic properties of nano-clusters lanthanum chromite powders doped with samarium and strontium ions synthesized via a novel combustion method

A novel approach to synthesize a single-phase orthorhombic perovskite lanthanum chromite LaCrO₃ clusters doped with Sm³⁺ and Sr²⁺ ions via gel combustion route was reported. The producing materials were synthesized using metal nitrates as oxidizers and triethanol amine (TEA), N-butyl amine (NBA) or ethylene diamine (EDA) as a fuel. The effect of the annealing temperature, type of organic fuel and the variation of the samarium and/or strontium substitution and its impact on crystal structure, crystallite size, microstructure and magnetic properties of the LaCrO₃ powders formed was systematically studied. The results revealed that a well crystalline single phase of pure LaCrO₃ can be achieved at annealing temperature from 800 to 1000 °C for 2 h. Moreover, each organic carrier materials exhibited a different degree of effectiveness in the synthesis of the mixed oxide powders. The crystal structure was influenced by doped Sm³⁺ and/or Sr²⁺ ions. The crystallite size of the produced powders was increased with the increase the annealing temperature, increasing the Sm³⁺ ion and the decrease of Sr²⁺ ion substitution. The microstructures of the produced powders were found to be nanoclusters octahedra-like shaped. The saturation magnetization of the LaCrO₃ powders increased continuously with an increase in the Sm³⁺ ion concentration and it decreased with an increase in the Sr²⁺ ion up to 0.3 at annealing temperature of 1000 °C for 2 h. The maximum saturation magnetization (0.279 emu/g) was achieved at the Sm³⁺ ion molar ratio 0.3 and annealing temperature 1000 °C. Moreover, wide coercivities can be obtained at different synthesis conditions (49.25 to 522  Oe).     

On the sintering kinetics of titania doped ceramic lanthanum chromite

The sintering behaviour of lanthanum chromite with the addition of 1 to 3 wt% TiO₂ was studied. Densification was examined as a function of sintering temperature and TiO₂ concentration. The results showed that pure LaCrO₃ could not be densified to >75% of the theoretical density, while the densities exceeding 90% of the theoretical were achieved with 3 wt% titania addition at 1600° C. Metallographic study reveals a normal grain growth following the rate equationD ²−D ₀ ² =kt. Activation energies both for densification and grain growth have been estimated to be ≈ (80±5) k cal/mole in case of 3 wt% TiO₂ addition.     

Anomaly diffuse and dielectric relaxation in strontium doped lanthanum molybdate

The dielectric properties of the La_2−xSr_xMo₂O_9−δ (x = 0-0.2) ceramics were investigated in the temperature range of 300-800 K. Dielectric measurement reveals that two dielectric anomalies, associated with the oxygen ion diffusion, exist in frequency spectrum with x = 0.5. The broad dielectric peaks in tan δ(ω) can be well fitted by a modified Cole-Cole approach. When x = 0.1, only one dielectric relaxation peak is observed, corresponding to different oxygen ion diffusion processes, as distinct from the only relaxation peak in the pure La₂Mo₂O₉. The relaxation parameters τ₀, the dielectric relaxation strength Δ, and the activation energy E_a were obtained. The result of this work shows that, the conductivity change caused by doping between the two phases is due to the combination of the dipolar effects and motion of ions.     

Hydrothermal processing and characterisation of doped lanthanum chromite for use in SOFCs

The perovskite powders Ca_0.3La_0.7CrO₃ and Sr_0.16La_0.84CrO₃ have been prepared using hydrothermal processing. The solid solutions were not formed directly in the autoclave, but the hydrothermally produced powders required calcination at a greatly reduced temperature to form the perovskite phase, reducing the tendency to produce hard agglomerates. Pellets with densities in excess of 95% TD were produced.     

Electrical conduction in titania-doped lanthanum chromite ceramics

The direct and alternating current electrical conductivities of lanthanum chromite ceramic doped with 1 to 3 wt% TiO₂ have been measured at temperatures from 20 to 200° C. The sample containing 3 wt% TiO₂ has been studied in detail because of all the samples it showed the highest sintered density and electrical conductivity. Alternating current conductivity, _ac, measurements taken over the frequency range 10 kHz to 1 MHz were found to follow a power law of the form _ac ⁿ , where is the frequency and 0.5<n<1, indicative of a hopping conduction mechanism. In addition, a dispersion in the dielectric constant was observed. The dispersion peak frequency has been found to be thermally activated with an activation energy of 0.423±0.002 eV. The results are successfully interpreted in terms of a model proposed by Pike, wherein a classical hopping of charge carriers (over a potential barrier) between the localized states is considered. The direct current conductivity activation energy predicted from this model is found to be consistent with the experimentally obtained value.     

Mechanical properties of calcium- and strontium-substituted lanthanum chromite

Mechanical properties of acceptor (calcium and strontium)-substituted lanthanum chromites are reported as a function of composition, temperature and environment. The strength dependence on temperature for these perovskite conductors was found to depend on the acceptor type, with the calcium-substituted chromites showing a significant reduction in strength with increasing temperature, while the strength of strontium-substituted chromites was essentially invariant with temperature. The decrease in strength observed upon annealing in highly reducing environments was correlated to changes in lattice structure, stoichiometry and fracture morphology. A significant observation was the decrease in the cohesive strength of the grains relative to grain boundaries, beyond a critical oxygen vacancy concentration in the chromites. The structural changes in the chromite lattice upon reduction also resulted in decreased fracture toughness. © 1998 Chapman & Hall     

On some transport properties of strontium-doped lanthanum chromite ceramics

D.C. Electrical conductivity, Hall mobility and magnetic susceptibility measurements on La_1?x Sr _x CrO₃ (0?x?0.25) perovskite ceramic system, and their temperature dependence, have been carried out to understand the nature of the transport mechanism in them. The electrical conductivity and Hall mobility displayed thermally activated temperature dependence with activation energies that varied from 0.13 to 0.23 ev. The variation of the d.c. conductivity of the polycrystalline La_1?x Sr _x CrO₃ with strontium content (X) has been found to be strongly affected by the changes in microstructure. Magnetic susceptibility data indicate that the electronic transport is due to the presence of Cr⁴⁺ ions in the lattice, and that the localized level of hopping is associated with the chromium 3d band.     

Solubility of lanthanum in strontium titanate in oxygen-rich atmospheres

Perovskite (ABO₃)-type lanthanum substituted strontium titanate ceramics (lanthanum content x), which had been sintered in pure oxygen at 1400 °C, were investigated from x = 0 up to x = 0.6 by light optical and scanning electron microscopic means in conjunction with X-ray analysis, X-ray diffraction, and pyconometry in order to determine the mechanism being responsible for the compensation of the electronic excess charge resulting from the "donor" lanthanum. A pure strontium vacancy compensation mechanism was observed for lanthanum contents up to x = 0.3. Above x = 0.4 titanium vacancies occur additionally but their concentration remains negligible compared to the predominating strontium vacancies. No indication of a solubility limit of lanthanum at x = 0.4, as stated in former works was observed. At x = 0.5 and 0.6 the lattice structure was found to be slightly distorted, tetragonally and orthorhombically, respectively. The lattice parameter obeys Vegard’s law up to the end member La_2/3□_1/3TiO₃ (□: vacant site). These results were completely confirmed by pycnometry data. This revised version was published online in November 2006 with corrections to the Cover Date.     

Sintering behavior and dielectric properties of Ce doped strontium barium niobate ceramics with silica sintering additive

Sr_0.5Ba_0.5Nb₂O₆ ceramics with and without CeO₂ dopant were prepared by a partial co-precipitation method and a liquid phase sintering process. The cooperative effects of Ce doping and silica sintering additive on the sintering behaviors and the dielectric properties of Sr_0.5Ba_0.5Nb₂O₆ ceramics was investigated. It was observed that the lattice parameters of a-axis and c-axis of Sr_0.5Ba_0.5Nb₂O₆ ceramics decrease with the increase of Ce dopant, namely contraction of crystal cell volume occurs. Amorphous silica used for sintering additive can effectively restrain abnormal grain growth and prevent the rise of sintering activation energies caused by Ce doping, but Ce doping has more effect on the average size of the grains and the dielectric properties than the silica sintering additive when the Ce dopant and the silica sintering additive are introduced. Both the Curie temperatures and the maximum of dielectric constant at Tc decrease as the Ce³⁺ concentration increases.     

Fabrication of Sr- and Co-doped lanthanum chromite interconnectors for SOFC

Many studies have been performed dealing with the processing conditions of electrodes and electrolytes in solid oxide fuel cells (SOFCs). However, the processing of the interconnector material has received less attention. Lanthanum chromite (LaCrO₃) is probably the most studied material as SOFCs interconnector. This paper deals with the rheology and casting behaviour of lanthanum chromite based materials to produce interconnectors for SOFCs. A powder with the composition La_0.80Sr_0.20Cr_0.92Co_0.08O₃ was obtained by combustion synthesis. Aqueous suspensions were prepared to solids loading ranging from 8 to 17.5 vol.%, using ammonium polyacrylate (PAA) as dispersant and tetramethylammonium hydroxide (TMAH) to assure a basic pH and providing stabilization. The influence of the additives concentrations and suspension ball milling time were studied. Suspensions prepared with 24 h ball milling, with 3 wt.% and 1 wt.% of PAA and TMAH, respectively, yielded the best conditions for successful slip casting. Sintering of the green discs was performed in air at 1600 °C for 4 h leading to relatively dense materials.     

Thermal expansion of doped lanthanum gallates

Thermal expansion of several compositions of Sr and Mg-doped LaGaO₃ including an A-site deficient composition (La_0.9Sr_0.1)_0.98(Ga_0.8Mg_0.2)O_2.821 were measured in the temperature range from 298 to 1273 K. The effect of doping on thermal expansion was studied by varying the composition at one site of the perovskite structure (either A or B), while keeping the composition at the other site invariant. Thermal expansion varied nonlinearly with temperature and exhibited an inflexion between 550 and 620 K, probably related to the change in crystal structure from orthorhombic to rhombohedral. The dependence of average thermal expansion coefficient (α _av) on the dopant concentration on either A or B site of the perovskite structure was found to be linear, when the composition at the other site was kept constant. Mg doping on the B-site had a greater effect on the average thermal expansion coefficient than Sr doping on the A-site. Cation deficiency at the A-site decreases thermal expansion when compositions at both sites are held constant.     

Silica-coated lanthanum–strontium manganites for hyperthermia treatments

La_0.76Sr_0.24MnO_3 + δ particles, prepared by performing a traditional, solid-state method of synthesis, were coated by uniform layers of silica via initiating hydrolysis and condensation of TEOS in aqueous–alcoholic alkali environment. The eventually obtained samples exhibited Curie temperature at ∼40 °C, and comprised core-shell particles of ∼250 nm in diameter. By varying stoichiometric ratio of cations within manganite cores of the particles, Curie point of the resulting material can be varied too, thus opening a way for the simple design of biocompatible, temperature-self-regulating particles for application in hyperthermia treatments, with Curie point thereof adjusted to a destined biological context of application.     

Sintering kinetic studies in nonstoichiometric strontium titanate ceramics

The effect of nonstoichiometry on the densification of SrTiO₃ ceramics with Sr/Ti ratios from 0.997 to 1.02 was systematically addressed. The kinetics of densification was studied by dilatometric analysis. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for crystallographic and microstructure characterization. Ti excess enhanced matter transport during sintering whereas Sr excess decreased it. The shrinkage rate and average grain size increased with the decrease of Sr/Ti ratio. Close values of the activation energy for the initial densification and the near constant onset temperature for densification suggest that identical transport mechanisms control the densification of all the compositions. Small excesses of TiO₂ and SrO were mostly incorporated into the perovskite lattice inducing alterations in the defect chemistry of the material and the mass transport during sintering is controlled by Sr vacancies. Very small stoichiometric variations have a strong influence on the sintering kinetics and resulting microstructure of ST ceramics.     

Key issues and challenges in device level fabrication of MEMS acoustic sensors using piezo thin films doped with strontium and lanthanum

The development of next-generation piezoelectric micro-electro-mechanical systems (piezo-MEMS) demands reliable and reproducible processes to fulfil the ever-growing requirements. Research on doped PZT thin films with improved ferroelectric and piezoelectric thin films is on the rise; however, most of them mainly discuss the fabrication process flows and results. This paper attempts to present an overview of some of the key issues and challenges encountered in the device level fabrication of piezo-MEMS acoustic sensors using PZT thin films doped with Strontium and Lanthanum for underwater applications. A recipe on the technical challenges would have saved more time and cost for the researchers as several unsuccessful fabrications and damages were encountered. This paper discusses the issues starting from fabrication of sputtering target to deposition, annealing, patterning and isolation of device layers, electrical contacts, packaging as per the application requirements, etc. in detail.

     

A cylindrical model of an absolutely black body of lanthanum chromite

A cylindrical model of an absolutely black body of lanthanum chromite for reproducing temperatures in the 800–1500°C range is described. Data on the emissive power of lanthanum chromite are given and the construction and metrological characteristics of a model of an absolutely black body are described.     

Effect of lanthanum replacement by strontium in lanthanum nickelate crystals synthetized using gelatin as organic precursor

La_1-xSr_xNiO₃ (0 ≤ x ≤ 0.8) nanopowders were successfully synthesized using gelatin as a polymerization agent. Powders were calcined for 4 h at 700 and 900 °C and characterized using thermal analysis, X-Ray diffraction and scanning electron microscopy. Perovskite phase formation was achieved at 700 °C, with an increase in crystallinity and secundary phases forming in powders calcined at 900 °C. The structure of the lanthanum nickelate was confirmed as rhombohedral and transition to tetragonal structure occurred by partially substituting lanthanum with strontium.