Microwave absorbing properties of La0.8Ba0.2MnO3 nano-particles

La0.8Ba0.2MnO3 nano-particles were synthesized by sol-gel process, and the crystal structure and morphology＇ were characterized by XRD and SEM, respectively. The complex permittivity and permeability were determined by microwave vector network analyzer in the frequency range of 2-18 GHz. The relationship between reflection coefficient and microwave frequency of La0.8Ba0.2 MnO3 was calculated based on measured data. The results show that the average diameter of La0.8Ba0.2MnO3 crystal powders is about 80 nm and the crystal structure is perovskite when being calcined at 800 ℃ for 2 h. The microwave absorbing peak is 13 dB at 6.7 GHz and the effective absorbing bandwidth above 10 dB reaches 1.8 GHz for the sample with the thickness of 2.6 mm. The microwave absorption can be attributed to both the dielectric loss and the magnetic loss from the loss tangents of the sample, but the former is greater than the latter.     

Synthesis of Nd0.7Sr0.3MnO3 ceramics by high-energy ball milling

Strontium doped perovskite-type manganite Nd_0.7Sr_0.3MnO₃ is synthesized by high-energy ball milling. Nd_0.7Sr_0.3MnO₃ single phase with perovskite structure is formed completely after milling 4 h and the perovskite phase decreases gradually and changes into amorphous state with increasing ball milling time up to 12 h. The interesting point in this work is that the Nd_0.7Sr_0.3MnO₃ perovskite can be formed again from the amorphous with subsequent 6 h ball milling. In addition, the re-obtained perovskite seems to be more stable than Nd_0.7Sr_0.3MnO₃ compound prepared using solid-state reaction method.     

Synthesis of YBa2Cu3Oy superconductors via attrition-milled intermediate oxide precursor containing BaCuO2.5

YBa₂Cu₃O_y (Y123) phase was synthesized from the oxide precursor containing BaCuO_2.5 in two distinct processes. in the first stage, a suitable amount of metallic Y, metallic Cu and barium nitrate (Ba(NO₃)₂) were mixed via attrition milling for 20 h. After 20 h of milling, the particle size of the powder was in the range of 0.05–0.2 μm. On subsequent heat treatment of the milled powder at 600°C for 5h with continuous argon flow, the intermediate precursor (IP) was synthesized. The IP contained BaCuO_2.5, Y₂O₃ and CuO and the overall composition of the IP was Y:Ba:Cu:O=1:2:3:7.4. The oxygen content of the IP was higher than that of the corresponding orthorhombic Y123 superconductor. In the second stage, the synthesized IP was converted to the superconducting Y123 phase with heat treatment at 900–1020°C in air without any additional low temperature oxygenation process. A DTA experiment at 20°C/min in air and a series of rapid heat, soak, and quench experiments showed that the BaCuO_2.5 constituent of the precursor decomposed at 838°C, which is higher than that of pure BaCuO_2.5 (760°C). The results of the TGA experiments suggested that the heating rate had a strong influence on the decomposition temperature of the BaCuO_2.5 of the IP. At a heating rate of>50°C/min, the BaCuO_2.5 did not decompose completely during heating to 1020°C and form orthorhombic YBa₂Cu₃O_7-x.     

Investigation of Co-Substituted Nanosized Mn2Y-Hexaferrites Synthesized by Sol-Gel Autocombustion Method

The structural and electrical properties of Co-substituted and nano-sized Y-type hexagonal ferrites have been investigated in the present work. The samples with chemical composition Ba₂Co _x Mn_2?x Fe₁₂O₂₂ (x = 0.0, 0.5, 1.0, 1.5, 2.0) were prepared by sol-gel autocombustion method. The powdered samples and pellets were sintered simultaneously at 1000 °C for 5 h and characterized by means of DTA/TGA, FTIR spectroscopy, x-ray diffraction (XRD), field emission gun scanning electron microscopy, and energy dispersive x-ray spectroscopy. The XRD analysis confirms that the investigated ferrites have single phased Y-type hexagonal structure without showing any impurity phase. Lattice constants (a and c), cell volume (V), crystallite size (D), and x-ray density (ρ _x ) have also been calculated from the XRD data. DC electrical resistivity is measured within the temperature range of 30-100 °C for each sample and is observed to increase with increasing Co-substitution. The dielectric constant (∈) has also been measured which is observed to decrease with Co-substitution. Thus, high electrical resistivity and low dielectric constant make these materials suitable for multi-layer chip inductors and also for RF components and circuits.     

Investigation of the corrosion behaviour of a NiCrMo alloy in nuclear process heat helium by means of X-ray diffraction and electron probe microanalysis

Samples of the nickel base alloy Nimonic 86 were exposed to PNP-500 type helium for 1000 h at 650, 750, 850, 900 and 950°C. X-ray diffraction showed 5 additional phases, formed at the sample surfaces during the heat treatment. They were shown to be: (1) a spinel Cr₂MnO₄ (at T ≤ 850°C) containing increasing amounts of Al and Ti with increasing temperature, (2) a sesquioxide Cr₂O₃ (at T ≥ 900°C) containing substantial amounts of Mn for T ≤ 850°C, (3) an ≠₁ oxide Ni₃Mo₃SO (mainly at 750°C) containing some Fe, (4) an ≠₁ carbide (Ni, Cr, Si)₃Mo₃C (at T ≤ 750°C), (5) Cr₂₃C₆ (at T ≤ 850°C) containing some Mo.     

Interfacial microstructure and shear strength of reactive air brazed oxygen transport membrane ceramic–metal alloy joints

To fabricate a multi-layered structure for maximizing oxygen production, oxygen transport membrane (OTM) ceramics need to be joined or sealed hermetically metal supports for interfacing with the peripheral components of the system. Therefore, in this study, Ag–10 wt% CuO was evaluated as an effective filler material for the reactive air brazing of dense Ce_0.9Gd_0.1O_2–δ–La_0.7Sr_0.3MnO_3±δ (GDC–LSM) OTM ceramics. Thermal decomposition in air and wetting behavior of the braze filler was performed. Reactive air brazing was performed at 1050 °C for 30 min in air to join GDC–LSM with four different commercially available high temperature-resistant metal alloys, such as Crofer 22 APU, Inconel 600, Fecralloy, and AISI 310S. The microstructure and elemental distribution of the ceramic-ceramic and ceramic-metal interfaces were examined from polished cross-sections. The mechanical shear strength at room temperature for the as-brazed and isothermally aged (800 °C for 24 h) joints of all the samples was compared. The results showed that the strength of the ceramic-ceramic joints was decreased marginally by aging; however, in the case of metal-ceramic joints, different decreases in strengths were observed according to the metal alloy used, which was explained based on the formation of different oxide layers at the interfaces.     

A new symmetric solid-oxide fuel cell with La0.8Sr0.2Sc0.2Mn0.8O3-δ perovskite oxide as both the anode and cathode

A novel perovskite-type La_0.8Sr_0.2Sc_0.2Mn_0.8O₃ (LSSM) oxide was synthesized and evaluated as the electrode material of a symmetric solid-oxide fuel cell. Characterization was done by electrical conductivity, crystal structure stability, redox stability, catalytic activity for methane oxidation and oxygen electro-reduction. LSSM shows greater electrical conductivity than the typical La_0.8Sr_0.2Cr_0.5Mn_0.5O₃ (LSCM) perovskite oxide under both anode and cathode operating conditions. It also shows excellent chemical and structural stability due to the backbone effect of Sc³⁺ for the perovskite lattice structure. A symmetric electrolyte-supported cell with 0.3 mm thick scandium-stabilized zirconia electrolyte and LSSM as cathode and anode shows peak power densities of 310 and 130 mW cm² at 900 °C, respectively, when operating on wet H₂ and wet CH₄. Stable performance is demonstrated.     

Optical properties of buffer and superconducting layers of the YBa2Cu3O7 ?δ/La1.75Ba0.25CuO4/La0.8Ba0.2MnO3 heterostructure for second-generation superconducting cables on an Ni5% W tape

An epitaxial YBa₂Cu₃O_{7 − δ}/La_1.75Ba_0.25CuO₄/La_0.8Ba_0.2MnO₃ heterostructure has been obtained on a biaxially textured tape of an Ni5% W alloy. It is shown that when films of a buffer layer of La_0.8Ba_0.2MnO₃ applied onto a tape of an Ni alloy in a reducing atmosphere are annealed in an oxygen atmosphere it is the compensation of oxygen vacancies that plays the dominant role in the formation of optical and magnetooptical properties of the layer, rather than stresses at the film-substrate interface. The shape of the reflection spectrum of the buffer layer after annealing corresponds to that of an La_0.8Ba_0.2MnO₃ single crystal, which indicates that the oxygen content is close to the stoichiometric one. The temperature dependences of the transverse Kerr effect and coefficient of light reflection of the films at a fixed wavelength can be used for the evaluation of T _c and the temperature of the optical response of the metal-insulator transition of the buffer layer on the basis of lanthanum manganite. Original Russian Text ? O.Yu. Gorbenko, A.R. Kaul’, Yu.P. Sukhorukov, E.A. Gan’shina, S.V. Samoylenkov, N.N. Loshkareva, A.V. Telegin, O.V. Melnikov, O. Stadel, 2007, published in Fizika Metallov i Metallovedenie, 2007, Vol. 104, No. 6, pp. 578–585.     

Crystal microstructure, infrared absorption, and microwave electromagnetic properties of （La1-xDyx）2/3Sr1/3MnO3

The manganite perovskite polycrystal samples of (La_1−x Dy _x )_2/3Sr_1/3MnO₃ (x = 0, 0.1, 0.2, 0.35, and 0.5) doped with Dy were prepared by solid state reaction in atmosphere to measure their X-ray diffraction (XRD) patterns, scanning electric microscope (SEM) images, infrared absorption spectra, and microwave electromagnetic properties. The displacement of the XRD peaks of the samples was found, and the 2ϑ increases from 0.05° to 0.5°. The grains of undoped La_2/3Sr_1/3MnO₃ not only have the greatest size, but also the most regular shape. The size of the grains decreases as the Dy doping content increases from 0 to 0.5. The infrared absorption spectra of all samples were measured at room temperature. An absorption peak corresponding to the stretching vibration mode of Mn-O bonds appears within the range of 591–629 cm⁻¹. The absorption peak shifts from a higher frequency to a lower one with the decrease of the average ionic radius of A-site. The frequency dependence of microwave-absorbing properties, imaginary components of the complex magnetic permeability μ″ and dielectric permeability ε″ for all samples was measured at room temperature from 8 to 13 GHz. The results show that the loss of microwave absorption can be attributed to both the magnetic and electric losses. The increase of Dy content not only enhances the microwave absorption but also causes the displacement of the absorption peaks.     

Synthesis of titanium carbide by thermo-chemical methods with TiH2 and carbon black powders

A new synthesizing method for producing submicron TiC powders was studied by using TiH₂ and carbon black powders. It is well known that hydrogen absorption transforms titanium from metal (h.c.p.) to brittle hydride (f.c.c.) powders by ball-milling. This research focused on obtaining submicron-sized TiC powders from the ball-milled mixture of TiH₂ and carbon black by thermal treatment. The hydrogen, carbon, iron, and oxygen composition changes in the mixed powders were analyzed. Thereafter, a differential thermal analysis (DTA) test was performed to observe change of phase with ball-milling time. The TiC powders were obtained by heat treating the powders milled for 5h at various temperatures (600–1200°C). The phase microstructure was investigated via DTA, X-ray diffraction (XRD), and scanning electron microscope (SEM). The mixture milled for 2h had an f.c.t. structure containing 66.73 at. %H transformed to f.c.c. by milling for 4h. After 5h of ball-milling, submicron-sized particles of 273 nm were obtained. At the isothermal heat-treating temperature of 500°C, the Ti single phase was formed completely, and the TiC phase of lattice parameter 0.310 nm was completely formed over the temperature of 1000°C.     

不同Ce掺杂比例对La1-xCexMnO3薄膜输运性质及机理研究

本文中研究了电子型掺杂钙钛矿薄膜La_1-xCe_xMnO₃的输运性质和外场作用下的输运机理。研究表明,La_1-xCe_xMnO₃薄膜呈现出典型的金属-绝缘体转变,且与Ce的掺杂浓度相关。电阻温度曲线表明,在低温时,电子-电子散射和磁畴对电子的散射是电阻形成的主要原因,而在高温下小极化子的跳跃机制起主要贡献。通过激光照射样品表面,发现光场诱导金属绝缘体转变温度向着低温区间偏移,该现象产生的原因在于La_1-xCe_xMnO₃薄膜内部铁磁相与顺磁相的共存,此外,高能量的激光对样品的电阻变化影响更明显。进一步研究表明,Ce的掺杂浓度将会通过金属-绝缘相变对La_1-xCe_xMnO₃的磁电阻效应产生显著的调制作用。本文将为新型能量转换器件的开发与应用提供新的研究思路。     

Direct production of porous cathode material (La1−x Sr x MnO3) using a reactive DC thermal plasma spray system

The cathode material (La_0.8Sr_0.2MnO₃) in solid oxide fuel cells (SOFCs) was plasma sprayed on mild steel in a reactive DC thermal plasma spray process. This high-speed process of depositing thin films for the components of SOFCs was examined experimentally. The results showed that a coating layer of La_0.8Sr_0.2MnO₃ with a particular porosity could be obtained directly using both prereacted La_0.8Sr_0.2MnO₃ and mixed raw materials (La₂O₃, SrCO₃, and MnO or MnCO₃) as feed materials with or without a pore former. The heat treatment of the plasma coating material at 1073 K (800 °C) for 3 h significantly enhanced the desired crystallization of La_0.8Sr_0.2MnO₃ in the coated material.     

Synthesis and characterization of cobalt-free Ba0.5Sr0.5Fe0.8Cu0.2O3−δ perovskite oxide cathode nanofibers

The cobalt-free perovskite-oxide, Ba_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ (BSFC) is a very important cathode material for intermediate-temperature proton-conducting solid oxide fuel cells. Ba_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ nanofibers were synthesized for the first time by a sol-gel electrospinning. Process wherein a combination of polyvinylpyrrolidone and acetic acid was used as the spinning aid and barium, strontium, iron and copper nitrates were used as precursors for the synthesis of BSFC nanofibers. X-ray diffraction studies on products prepared at different calcination temperatures revealed a cubic perovskite structure at 900 °C. The temperature of calcination has a direct effect on the crystallization and surface morphology of the nanofibers. High porosity, and surface area, in addition to an electrical conductivity of 69.54 S cm⁻¹ at 600 °C demonstrate the capability of BSFC nanofibers to serve as effective cathode materials for intermediate-temperature solid oxide fuel cells.     

Preparation and properties of K2NiF4-type perovskite oxides La2NiO4 catalysts for steam reforming of ethanol

The performance of La₂NiO₄ perovskite catalysts, prepared using a citric acid complexation method, for the steam reforming of ethanol was studied. The catalysts were characterized by X-ray diffractometry (XRD), specific surface area measurements (BET), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The experimental results show that the calcination temperature and the amount of citric acid (CA) have a significant influence on the characteristics of the catalysts and their catalytic activity. Among the catalysts tested, the La₂NiO₄ catalyst calcined at 700 °C with n(La):n(Ni):n(CA) of 2:1:3 exhibits the best activity and excellent stability as well as very low coke formation.     

Transport phenomena in the La<Subscript>0.72</Subscript>Ba<Subscript>0.28</Subscript>MnO<Subscript>3</Subscript> single crystal

Results of an experimental study of the temperature dependences of the magnetization M, electrical resistivity, magnetoresistance, thermo-and magnetothermo-emf, and Hall effect of the La_0.72Ba_0.28MnO₃ single crystal are presented. An analysis of the temperature dependences of kinetic properties shows that, at low temperatures, electrons are principal charge carriers in La_0.72Ba_0.28MnO₃ and the metallic conduction takes place. As the temperature increases to T ≈ 145 K, the sign of the ordinary Hall coefficient reverses; this indicates the change in the type of the majority charge carriers. Within a certain temperature range which lies substantially below the Curie temperature (T _C ), a metal-semiconductor transition occurs. Near the Curie temperature and within the paramagnetic range, the manganite under study is a semiconductor; the conduction is mainly effected by holes activated to the mobility edge. The critical behavior of the resistance and magnetoresistance is discussed.     

A Study on Effect of Reactive and Rare Earth Element Additions on the Oxidation Behavior of Mo–Si–B System

Mo–9Si–8B–1Ti, Mo–9Si–8B–1.8Ti, Mo–9Si–8B–0.2La and Mo–9Si–8B–0.4La₂O₃ (at.%) alloys were prepared using mechanical alloying followed by hot isostatic pressing and field assisted sintering. XRD, SEM and EBSD analysis confirmed the formation of Mo solid solution, A15 and T2 phases in the alloys. Isothermal oxidation behavior of the specimens was studied in the temperature range from 750 to 1,300 °C for up to 100 h. Both the Ti and La containing alloys showed superior oxidation behavior compared to unalloyed Mo–Si–B at 900 °C at the initial periods of oxidation. Ti-added alloys suffered higher rate of weight loss at higher temperatures (1,000–1,300 °C) due to the formation of non-protective low viscosity SiO₂-TiO₂-B₂O₃ scale. La-alloyed Mo–Si–B showed superior oxidation resistance at intermediate temperatures (900 °C) as well as at higher temperatures. Enrichment of La at the oxide/alloy interface was found to be the reason for improved oxidation behavior of La-alloyed Mo–Si–B. Amongst the four materials studied, the La₂O₃ containing alloy showed the best oxidation resistance at 900 °C.     

Oxidation behavior of alloy HK40 in H2?H2O atmosphere

In the present paper, the isothermal oxidation behavior of alloy HK40 in H₂? H₂O atmosphere in the temperature range 850–950 °C was systematically investigated by means of TGA, XRD, SEM, and EDS. The results demonstrated that the oxidation of alloy HK40 under low oxygen partial pressure obeyed a cubic instead of parabolic rate law. The values of the oxidation rate constant k at 850, 900, and 950 °C were 0.0079, 0.01886 and 0.04031 mg³/(cm⁶ h) at 850, 900, and 950 °C, respectively. The scales were composed of MnCr₂O₄ and Cr₂O₃. MnCr₂O₄ mainly existed in the form of blades in the outer part of the scale and its amount gradually increased with increasing temperature. In addition, there were silica particles along the scale/metal interface and indications were found that they contribute to the improvement of scale spallation resistance at high temperature as found by scratch tests.     

Preparation and properties of Ce0.8Ca0.2O1.8 anode material by glycine-nitrate process

Ce0.8Ca0.2O1.8（CDC82） anode material was prepared by glycine-nitrate process（GNP）. Thermogravimetric（TG） analysis and differential scanning calorimetric（DSC） methods were adopted to characterize the reaction process of CDC82 material. X-ray diffractometry（XRD）, scanning electron microcopy（SEM）, direct current four probe （four-probe DC） and temperature process reduce（TPR） techniques were adopted to characterize the properties of CDC82 material. After the precursor was sintered at 750℃ for 4 h, CDC82 material with pure-fluorite structure and nanometer size was obtained. The total conductivity of CDC82 changes little with temperature in air at 50-850℃, and the maximum value is 0.04 S/cm at 750 ℃. The total conductivity wholly becomes larger when the atmosphere changes from air to hydrogen, which greatly increases with increasing temperature and reaches the maximum value of 1.09 S/cm at 850 ℃. Some impurities such as CeMg and La203 exist after the mixture of CDC82 anode and La1-xSrxGa1-yMgyO3-δ （LSGM） electrolyte material is sintered at 1 200 ℃ for 15 h. The CDC82 material as anode material has excellent catalytic property for hydrogen and methane.     

Temperature dependent surface morphology and lithium diffusion kinetics of LiCoO<Subscript>2</Subscript> cathode

In an attempt to understand the effect of synthesis temperature upon surface morphology and lithium diffusion kinetics of LiCoO₂, the compound was synthesized at four different temperatures, viz., 600, 700, 800 and 900 °C using a novel gelatin-assisted combustion method. LiCoO₂ synthesized at 800 °C is found to be a mixture of rhombohedral and cubic LiCoO₂ and a temperature of 900 °C leads to the formation of cubic LiCo₂O₄ compound, thus favoring lower temperatures such as 600 and 700 °C to prepare phase pure rhombohedral LiCoO₂. Cyclic voltametry and impedance spectral studies evidence that LiCoO₂ synthesized at 600 °C exhibits better electrochemical cycling behavior and considerably reduced internal resistance upon cycling, which are substantiated further from the higher lithium diffusion coefficient value. The study demonstrates the possibility and superiority of synthesizing electrochemically active LiCoO₂ with preferred surface morphology and better lithium diffusion kinetics at a relatively lower temperature of 600 °C, using a gelatin-assisted combustion method.     

Combustion Synthesis in a Mechanically Activated Mg-C-TiO2-H3BO3 System

TiC-TiB₂ nanocomposite was fabricated by self-propagating high-temperature synthesis (SHS) of mechanically milled powders. H₃BO₃, TiO₂, Mg, and C as starting materials were milled for 0.5, 1, and 3?h then pressed to form pellets. Green compacts were placed in a tube furnace which had been preheated to three different temperatures of 900, 1000, and 1100?°C with argon atmosphere, for the synthesis. The prepared samples were studied by XRD, SEM, and TEM analytical technique. TiC and TiB₂ were not formed during milling process. According to the XRD patterns of synthesized samples, ball milling for 0.5?h is the optimum time for mechanical activation of the mixed powders. Further investigation indicated that 900?°C is the best temperature for the synthesis of this mixture. By using Williamson-Hall method, the average crystallite sizes of TiB₂ and TiC were calculated 40.7 and 75.6?nm, respectively, which were confirmed by TEM images.