Preparation,thermal and phase evolution and functional properties of non-stoichiometric strontium-doped lanthanum manganite perovskite ceramics

Powders of (La_0.85Sr_0.15)_0.98MnO_3-δ (LSM85) and (La_0.80Sr_0.20)_0.98MnO_3-δ (LSM80) perovskites have been synthesized and characterized in detail, and then sintered to evaluate their electrical and magnetic properties at low temperatures and at near room temperature. Microstructural observations/analyses after the dilatometric essays show that the perovskites have compositions that deviate from the nominal ones. Furthermore, magnetic characterization of both samples reveals a ferromagnetic behavior with a Curie temperature above 300 K for LSM80 and of 260 K for LSM85. In addition, LSM85 is insulator from 15 up to 300 K, whereas LSM80 is metallic up to 160 K and insulator up to 300 K. This significant discrepancy of behaviour is attributable to structural and compositional differences between the two perovskites.     

Microwave sintering study of strontium-doped lanthanum manganite in a single-mode microwave with electric and magnetic field at 2.45 GHz

The objective of this work is to study the changes in the physical and mechanical properties of strontium-doped lanthanum manganite (LSM) material and LSM-YSZ (ZrO₂ doped with 8 mol% yttria) composite, obtained by colloidal processing and sintered by 2.45 GHz microwave sintering at 1200 and 1300 °C using two different single-mode cavities. One circular cavity with TE₁₁₁ mode that has maximum in the electric field (E-field) and one rectangular cavity with TE₁₀₂ mode that has maximum in the magnetic field (H-field). As compared to conventional sintering at 1300 and 1400 °C, the microwave-heated samples exhibited a denser structure for shorter sintering times. LSM-based materials showed higher heating behavior in H-field, which translates into higher energy absorption. This fact can be attributed to an electromagnetic pressure induced by the combined effect of current loops subjected to H-field. Therefore, the interaction between the material and the electromagnetic waves depends on the dominant field of them.     

Preparation of nano-crystalline strontium-doped lanthanum manganate (LSM) powder and porous film by aerosol flame deposition

To improve the performance of the cathode, many groups have studied the relationship between the cathode microstructure and the electrochemical performance. In this study, a porous La_0.8Sr_0.2MnO_3±δ (LSM) cathode film with primary particles of under 10 nm diameters was prepared by aerosol flame deposition (AFD). The AFD technique was applied to synthesize the spherical particles and dense LSM thin film. The cathode performance was improved, and the polarization resistance was decreased by extending the active triple phase boundary. The electrochemical characteristics were investigated in the temperature range of 600–900 °C and oxygen partial pressure range of 0.1–1.0 atm. For oxygen reduction reaction on the LSM cathode, it was turned out that both oxygen atom diffusion and oxygen ion transfer from the three phase boundary to the yttria-stabilized zirconia electrolyte lattice were the rate-determining steps with comparable contributions. The polarization resistance of the prepared LSM film decreased from 206 to 1.7 Ω cm² with increasing temperature from about 600 to 900 °C and the activation energy was 1.48 eV.     

Role of spin-glass phase for magnetoresistance enhancement in nickel substituted lanthanum calcium manganite

Effect of Ni substitution in lanthanum calcium manganite (LCMO) has been investigated for change in magnetoresistance (MR). Scanning electron microscopy images revealed decrease in grain size from 3.72 µm to 0.55 µm by Ni substitution. Maximum increase in MR has been found 28% at low temperature (100 K) for x=0.10, Ni substitution at Mn site. Metal insulator transition temperature has been decreased from 253.2 K for x=0.0–90 K for x=0.10. Above x=0.10, Ni substitution no metal-insulator transition temperature appeared due to the presence of porosity in the samples. Ni substitution lowered the magnetic transition temperature from 255 K for x=0.0–125 K for x=0.25. Lowering of irreversible temperature (T_irr) from 250 K for x=0.0–135.4 K for x=0.20 has been obtained by zero field cooled (ZFC) and field cooled (FC) measurements confirm reduction of ferromagnetic clusters and spin-glass phase like behavior due to Ni presence. The spin-glass phase presence allows spin-polarized tunneling even at low magnetic field, which ultimately results in enhancement of MR at low temperature. Core level X-ray photoelectron spectroscopy measurements confirm Ni²⁺ charge state of Ni ions and increase in Mn⁴⁺/Mn³⁺ ratio with increasing Ni content. Increase in resistivity and weakening of ferromagnetism with Ni substitution at Mn site has been observed due to the reduction in grain size and dilution of double exchange interaction.     

Synthesis and electrical properties of strontium-doped lanthanum ferrite with perovskite-type structure

In this study, we prepared strontium-doped lanthanum ferrites with the perovskite-type structure for application as solid oxide fuel cell (SOFC) cathodes. We used the Pechini method to prepare strontium-doped lanthanum ferrites with the strontium:lanthanum molar ratios of 20:80 and 40:60. The resulting doped materials were characterized using various analytical tools. The calcination process was conducted at 450 °C because above this temperature, the stabilization of mass loss occurred and no phase transformation was observed. The X-ray diffraction results confirmed the mixing of the powder phases after the calcination process and the presence of a single powder phase in the air-sintered samples. The high-resolution transmission electron microscopy results revealed the presence of agglomerated nanoparticles smaller than 20 nm in size in the samples. The electrochemical impedance spectroscopy results revealed that the sample with 20% strontium exhibited a conductivity of 3.9 × 10⁻³ S cm⁻¹ at 95 °C and activation energy of 0.37 eV. In contrast, the sample with 40% strontium exhibited a conductivity of 3.5 × 10⁻² S cm⁻¹ and activation energy of 0.29 eV. These results suggest that with an increase in the strontium content, the conductivity of the samples increased, where as the activation energy of the conduction process decreased. Therefore, the ferrites synthesized in this work are potential catalysts for SOFC cathodes.     

Catalytic wet oxidation of phenol over lanthanum strontium manganite

The catalytic performances of lanthanum strontium manganite (La_0.8Sr_0.2)Mn_0.98O₃, in catalytic wet oxidation (CWO) of a phenol solution under milder conditions of temperature (398–498 K) and pressure (P_o2=4 bar), in a batch reactor, have been investigated. Aim of this study is the evaluation of the effect of temperature, catalyst loading, phenol concentration and stirrer speed on phenol conversion. Experimental data obtained from the different test conditions are best-fitted to evaluate the effective reaction order and apparent activation energy.     

Studies on slip casting behavior of lanthanum strontium manganite

Dispersion conditions for slip (slurry) formulation of a powder mixture of lanthanum strontium manganite (La_0.84Sr_0.16MnO₃ - LSM) and carbon (pore former) in water was studied through detailed zeta-potential and rheological measurements. The zeta potential variation with pH for the aqueous suspensions of only LSM or carbon exhibited a maximum value in alkaline medium (−40 mV to −50 mV at a pH of 10-11), establishing the pH window for their co-dispersion for slurry formulation. A study of the viscosity variation with shear rate for the slurries with varying solid content (in the range of 45-65 wt.%) exhibited pseudo-plastic flow behavior, indicating presence of flocculates in them. The yield stress values obtained from the Casson equation reduced with decreasing solid content, indicating reduction in the flocculate strength. The slip with solid content of 50 wt.% exhibited optimum flow characteristics to form long tubes with uniform wall thickness (wall thickness 2-4 mm and length of 150-200 mm). The tubular specimens formed after controlled carbon burn out and sintering at 1400 °C for 1 h possessed about 35% open porosity. The porosity remained the same upon further sintering at 1400 °C for 8 h.     

Characterization of porous lanthanum strontium manganite (LSM) and development of yttria stabilized zirconia (YSZ) coating

Cylindrical shapes of lanthanum strontium manganite (LSM) with varying amounts of pore former have been fabricated by the extrusion technique. Density of the sintered samples was found to decrease with the addition of pore former. Pore size distribution in a sintered sample with 37% porosity was within the range of 2–20 μm. Electrical conductivity of LSM samples with 20% and 32–37% porosity was found to be ∼92 S/cm and ∼80 S/cm, respectively. The thermal expansion coefficient of the LSM samples was found to be independent of porosity in the 20–37% porosity range. Sintered LSM tube was coated with YSZ by dip coating. Cross-sectional view of the coated sample showed formation of a dense layer of YSZ on the porous cathode structure.     

Effect of starch addition on microstructure and properties of highly porous alumina ceramics

Porous alumina ceramics with ultra-high porosity were prepared through combining the gel-casting process with the pore-forming agent technique. Porosity and pore size distribution of the sintered bulks were evaluated with and without adding starch, respectively. In particular, the influences of starch addition on the properties, including thermal conductivity and compressive strength were studied. It was found that the incorporation of starch increased the nominal solid loading in the suspension and subsequently promoted the particle packing efficiency. The porosity is raised with increasing starch content from 0 to 30 vol%, which brings the decrease in thermal conductivity, whereas the compressive strength isn't seriously degraded. The further higher starch addition (40 vol%), however, would deteriorate the performance of the alumina porous ceramics. It is believed that the appropriate starch amount (lower than 30 vol%), working as a pore-forming agent, suppresses the driving force of densification without affecting the connections of neighboring grains while excessive starch amount would lead to the collapse of the porous structure.     

Mechanism of enhancement in magnetoresistance properties of manganite perovskite ceramics by current annealing

Studies on spintronics have provided solid evidence that the grain boundaries (GBs) in polycrystalline manganite can produce a strong extrinsic magnetoresistance (MR). This type of MR, called Low-field MR (LFMR), is larger than the intrinsic MR and can be triggered over a wide range of temperature. However, the existence of more GBs would bring about the weakening of magnetism and decrease the magnitude of MR simultaneously. Here we show that during annealing the application of electric-current to a representative ferromagnetic manganite perovskite, polycrystalline La_2/3Sr_1/3MnO₃ (LSMO), can produce more GBs and improve low-field magnetization, which leads to enhanced MR effect and field-response sensitivity as compared to the traditional-annealed sample. By using static micromagnetic models combined with the theories of spin-polarized intergrain tunneling and charge carrier hopping across domain wall, the observed enhancement of magnetoresistive response in current-annealed LSMO can be well explained.     

Chromium removal using a porous carbon felt cathode

Removal of hexavalent chromium by an electrochemical process using a carbon felt electrode in a flow-through electrochemical cell was studied. Optimum operating conditions, such as pH and current density were investigated. It was found that at pH 3.5, current density 400 A m^–2 and flow velocity of 1.11 cm s^–1, the total chromium and Cr(VI) concentrations can be reduced to less than 5 mg l^–1 within 230 min operation for 5 l of solution with an initial concentration of 50 mg l^–1. It was found that the solution pH is a critical factor for the chromium removal process. To achieve high chromium removal efficiency, pH and current density must be carefully controlled.     

Explosion in a highly porous medium

Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 25, No. 3, pp. 89–96, May–June, 1989.     

Reduced graphene oxide supported highly porous V2O5 spheres as a high-power cathode material for lithium ion batteries

Reduced graphene oxide (rGO) supported highly porous polycrystalline V(2)O(5) spheres (V(2)O(5)/rGO) were prepared by using a solvothermal approach followed by an annealing process. Initially, reduced vanadium oxide (rVO) nanoparticles with sizes in the range of 10-50 nm were formed through heterogeneous nucleation on rGO sheets during the solvothermal process. These rVO nanoparticles were oxidized to V(2)O(5) after the annealing process in air at 350 °C and assembled into polycrystalline porous spheres with sizes of 200-800 nm. The weight ratio between the rGO and V(2)O(5) is tunable by changing the weight ratio of the precursors, which in turn affects the morphology of V(2)O(5)/rGO composites. The V(2)O(5)/rGO composites display superior cathode performances with highly reversible specific capacities, good cycling stabilities and excellent rate capabilities (e.g. 102 mA h g(-1) at 19 C).     

Effect of pre-oxidation on the microstructure,mechanical and dielectric properties of highly porous silicon nitride ceramics

Highly porous Si₃N₄ ceramics have been fabricated via freeze casting and sintering. The as-sintered samples were pre-oxidized at 1200–1400 °C for 15 min. The effect of pre-oxidation temperature on the microstructure, flexural strength, and dielectric properties of porous Si₃N₄ ceramics were investigated. As the pre-oxidation temperature increased from 1200 °C to 1400 °C, firstly, the flexural strength of the pre-oxidized specimens remained almost constant at 1200 °C, and then decreased to 14.2 MPa at 1300 °C, but finally increased to 25.6 MPa at 1400 °C, while the dielectric constant decreased gradually over the frequencies ranging from 8.2 GHz to 12.4 GHz. This simple process allows porous Si₃N₄ ceramics to have ultra-low dielectric constant and moderate strength, which will be feasible in broadband radome applications at high temperatures.     

Role of oxygen pressure on the stability of lanthanum strontium manganite–yttria stabilized zirconia composite

We have investigated the structural and chemical stability of La_0.8Sr_0.2MnO₃ (LSM)–8 mol.% yttria stabilized zirconia (YSZ) composite. LSM and YSZ powders were mixed and sintered at 1400 °C for 10 h in controlled atmosphere (PO₂ = 0.21 to 10^?6 atm). The unit cell volume of LSM increases during exposure to reduced oxygen partial pressure while it remains unchanged for YSZ. During reduction in the oxygen partial pressure from 0.21 atm to 10^?6 atm, the solubility of manganese in YSZ increases from ～10 at.% to ～15 at.%. Lower oxygen partial pressure also results in the grain growth and formation of La₂Zr₂O₇ and MnO_x (Mn₃O₄) compounds lowering the stability of the LSM–YSZ composite. On subsequent sintering in 0.21 atm PO₂, the La₂Zr₂O₇ and MnO_x compounds tend to disappear indicating the reversibility of the interaction. The reversibility of LSM–YSZ reaction has been independently confirmed using La₂Zr₂O₇ and MnO_x.     

Numerical investigation of mass transfer enhancement through a porous body affected by electric field

The use of electrohydrodynamic (EHD) actuator has emerged recently as an effective mean to enhance mass transfer. However, the effect of nonthermal EHD-induced mass transfer in such porous body has remained unclear. Hence, in this paper, the mass transfer enhancement with EHD technique in a porous body is numerically investigated using the finite volume method. The flow patterns and the moisture content have been studied for different Reynolds numbers and the applied voltages. The numerical results show that the EHD flow field amplifies the mass transfer by a factor of 20.5 for Re?=?200 and 2.12 for the Re?=?4,000 and V?=?28?kV in comparison to the case without the electric field.     

Production of highly porous fractal coatings by the plasma-explosion method

Experiments on deceleration of a dense metal plasma obtained under explosive loading of porous specimens have shown that a highly porous coating is formed on the target surface. The coating has a vortex structure; finely dispersed fractal filaments are located on the surface of the vortices. The sprayed surface consists of the recrystallized material of the porous specimen with small admixtures of the target material. In the case of deceleration of a multicomponent plasma, the coating has an analogous structure and consists of the specimen components and target material, which are uniformly distributed over the surface.Institute of Geosphere Dynamics, Russian Academy of Sciences, Moscow 117334. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 2, pp. 110–113, March–April, 1995.     

Processing of highly porous bioglass monoliths by hydrothermal hot pressing

Porous bioglass monoliths have been processed by hydrothermal hot pressing (HyHP) from sol-gel and melt-derived bioglass powders of composition (in mol %): SiO₂–CaO–P₂O₅ (55.0-40.0-5.0) and SiO₂–CaO–Na₂O–P₂O₅ (47.2-26.4-23.8-2.6), respectively. An open porosity of >70% ever reached in 3D structures is reported for monoliths issued from sol-gel powders. Dissolution studies were performed in simulated body fluid (SBF) for 1–30 days. The monoliths were analysed using X-Ray Diffraction (XRD), Fourier Transform Infra-Red (FTIR) spectroscopy and Scanning Electron Microscopy (SEM) to observe the formation of an apatite-like layer and elemental concentration of SBF was evaluated using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). A higher kinetics in the development of apatite layer was observed for sol-gel derived monoliths. This result is explained by the high surface areas of the nanosized sol-gel powders and the possibility of HyHP to create large porosity (mesoporous monoliths) and retain large surface areas. HyHP is also effective in processing 3D-bioglass structures with porosity gradient by co-sintering powders of different size.     

Modelling the porous cathode of a SOFC: oxygen reduction mechanism effect

A composite electrode comprising a porous mixture of solid electrolyte (typically yttria stabilized zirconia, YSZ) and electronically conducting, electrocatalytic material (typically strontium doped lanthanum manganite, LSM) is generally used to improve the cathodic performance of the solid oxide fuel cell (SOFC). The advantage of the composite electrode is that the reaction zone is spread from the electrode/electrolyte interface into the electrode, effectively resulting in a functionally diffuse interface where the charge transfer reaction occurs. The present study proposes a one-dimensional dc and ac model that takes into account mass and charge conservation, transport of species and reaction kinetics. It considers the porous electrode to be a homogeneous medium characterized by a number of parameters, and in particular ionic conductivity and the diffusion coefficient. The influence of kinetic and transport parameters as well as that of the microstructure on the shape of both polarization curves and impedance diagrams is discussed.