Stability of SrFeO3 based materials in the presence of SiOx species in humid atmosphere at high temperatures and pressures

Strontium silicate was found on the surface of La_0.2Sr_0.8Fe_0.79Cr_0.2Mg_0.01O₃ exposed to hydrogen containing humid atmospheres at 1000 °C and 30 bars. Silica originated from the furnace tube material and was transported via the gas phase as a gaseous silica hydrate. Fe and Sr were initially preferentially expelled from the perovskite grain boundaries to give Sr₂SiO₄ at the surface, along with a secondary Fe-rich phase and a LaCrO₃-rich grain boundary region. Eventually, Fe and Sr were drawn from the grains, leaving a porous structure. This investigation highlights the importance of avoiding Si sources near Sr-rich membranes in humid atmospheres at high temperatures.     

Sintering behaviour of La1−xSrxFeO3−δ mixed conductors

The sintering properties of La_1−xSr_xFeO_3−δ (x = 0.1, 0.25) mixed conductors have been investigated with particular emphasis on the effect of secondary phases due to cation non-stoichiometry (±5 mol% La excess and deficiency). Secondary phases, located at grain boundaries in cation non-stoichiometric materials, increased the sintering temperature compared to single-phase materials. Extensive swelling in final stage of sintering was observed in all materials, which resulted in micro-porous materials. The swelling was most pronounced in the phase pure and two-phase materials due to La-deficiency, while refractory secondary phases in La-excess materials inhibited both sintering, grain growth and swelling. In La-deficient materials, formation of molten secondary phases resulted in rapid swelling due to viscous flow. The present findings demonstrated the importance of controlling sintering temperature and time, as well as careful control of the cation stoichiometry of La_1−xSr_xFeO_3−δ in order to achieve fully dense and homogenous La_1−xSr_xFeO_3−δ ceramics.     

La0.7Sr0.3Ga0.6Fe0.4O3-δ氧离子导体材料的制备与表征

以相应的金属氧化物和盐为原料,通过甘氨酸-硝酸盐法(GNP)合成出La0.7Sr0.3Ga06Fe04O3-δ(LSGF)粉末,经压制、烧结后,得到LSGF烧结体试样.采用X射线衍射仪(XRD)对所得前驱体及其煅烧后粉体的相组成进行分析;采用扫描电镜(SEM)及X射线冠电子能谱仪(EDS)对烧结体的微观组织和成分进行了观察,还对烧结体的致密度、电导率和热膨胀系数等进行了测试分析.实验结果表明,甘氨酸-硝酸盐法所制备的前驱体粉末在700℃煅烧5h后可获得具有单一钙钛矿结构的LSGF粉末,其晶粒尺寸在45 nm左右,所制备的LSGF烧结体在400～900℃范围的电导率在3.97～4.42 S·cm-1之间,热膨胀系数在9.2 ×10-6 ～11.12 ×10-6 K-1之间.     

Sintering and electrical conductivity in fast oxide ion conductors La2−xRxMo2−yWyO9 (R: Nd, Gd, Y)

Attrition and ball milling are used as mechanical means to reduce grain size of optimized fast oxide-ion conductors La_2−xR_xMo_2−yW_yO₉ (R: rare earths). Dilatometry is used to determine the optimal sintering conditions in order to obtain high density samples (greater than 96% of relative density) with help of scanning electron microscopy to characterize their microstructure. The optimal sintering temperatures are highly dependent on the chemical composition, and therefore identical annealing temperatures do not warrant similar relative densities. Complex impedance spectroscopy show that above the transition temperature of La₂Mo₂O₉ at 580 °C, the conductivity of all the studied compounds is lower than that of the parent compound, whereas just below the transition, in most cases the stabilization of the cubic phase increases conductivity. An interesting result is that tungsten substitution, which stabilizes La₂Mo₂O₉ against reduction, does not affect significantly the oxide ion conduction.     

Ordering and quality factor in 0.95BaZn1/3Ta2/3O3–0.05SrGa1/2Ta1/2O3 production resonators

Ordering of the B-site cations in UMTS (universal mobile telecommunications systems) standard resonator pucks composed of perovskite structured, 0.95BaZn_1/3Ta_2/3O₃–0.05SrGa_1/2Ta_1/2O₃ (BZT–SGT) has been investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD) and powder neutron diffraction (PND). XRD patterns from samples sintered at 1550 °C/2 h but annealed and quenched at 50 °C intervals between 1400 and 1600 °C revealed that the order–disorder phase transition was at 1500 °C. In addition, a peak at 29.5° 2θ attributed to a Ba₈ZnTa₆O₂₄ phase was present due to ZnO loss. Electron diffraction patterns revealed that samples heat treated 1500 °C (including as sintered samples, 1525 °C/2h,) exhibited short-range 1:2 ordering along all <111> directions giving rise to an average short-range face centred cubic structure. Samples annealed and quenched from below 1500 °C showed 1:2 order. To avoid excessive ZnO loss, an annealing temperature was chosen at 1275 °C (for 24 and 168 h). Neutron diffraction data were best refined using two ordered BZT phases with slightly different lattice parameters. TEM revealed a microstructure in each case consisting of 1:2 small ordered domains in the centre of all grains but with every second grain exhibiting a concentric shell composed of an ordered single domain, containing elongated translational (APBs) but not orientational domains. The formation of the concentric ordered shell was attributed to grain boundary migration during grain growth. As-sintered samples gave unloaded quality factors (Q)=54,000 at 2 GHz which rose to 78,000 at 2 GHz after annealing for 24 h. No further improvement in Q was observed for longer annealing times.     

First Principles Modeling of Pd‐doped (La,Sr)(Co,Fe)O3 Complex Perovskites

(La,Sr)(Co,Fe)O₃ (LSCF) perovskites are well known promising materials for cathodes of solid oxide fuel cells. In order to reduce cathode operational temperature, doping on B‐sublattice with different metals was suggested. Indeed, as it was shown recently experimentally, doping with low Pd content increases oxygen vacancy concentration which is one of factors controlling oxygen transport in fuel cells. In this Communication, we modeled this material using first principles DFT calculations combined with supercell model. The charge density redistribution, density of states, and local lattice distortion around palladium ions are analyzed and reduction of the vacancy formation energy confirmed.     

Fabrication and microstructure of directionally solidified SrCe1 − xYxO3 − δ (x = 0.1, 0.2) high temperature proton conductors

SrCe_1 − xY_xO_3 − δ (x = 0.1, 0.2) high temperature proton conductors (HTPC) have been fabricated by directional solidification using a laser-heated float zone (LHFZ) method. The resulting microstructures have been studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The LHFZ method produces a cellular microstructure consisting of crystalline cells embedded in an amorphous matrix, showing a strong biaxial texture. EBSD studies show that all the crystalline cells have their <0 0 1> axis as described in the cubic Pm3m prototype unit cell parallel to the growth direction. The observed microstructural features are explained in terms of the fabrication process.     

Structural and electrical properties of lead free ceramic: Ba(La1/2Nb1/2)O3

Abstract

Abstract

Lead free perovskite Ba(La_1/2Nb_1/2)O₃ was prepared by conventional ceramic fabrication technique at 1375°C for 7?h in air atmosphere. The crystal symmetry, space group and unit cell dimensions were estimated using Rietveld analysis. X-ray diffraction analysis indicated the formation of a single phase monoclinic structure with space group P2/m. Energy dispersive X-ray analysis and scanning electron microscopy studies were carried to study the quality and purity of the compound. Permittivity data showed low temperature coefficient of capacitance (T _CC = 11%) up to 100°C. The circuit model fittings were carried out using the impedance data to find the correlation between the response of real system and idealised model electrical circuit. Complex impedance analyses suggested the dielectric relaxation to be of non-Debye type. The correlated barrier hopping model was employed to successfully explain the mechanism of charge transport in Ba(La_1/2Nb_1/2)O₃. The AC conductivity data were used to evaluate the density of states at Fermi level, minimum hopping length and apparent activation energy.     

Crystalline structure and electrical properties of YCuxMn1–xO3 solid solutions

Solid solutions belonging to the Mn-rich region of the YCu_xMn_1–xO₃ system have been studied. The powders were prepared by solid-state reaction between the corresponding oxides. Sintered ceramics were obtained by firing at 1100–1325 °C. The incorporation of 30 at.% Cu to the yttrium manganite induces the formation of a perovskite-type phase, with orthorhombic symmetry. Increase of the Cu amount do not appreciably affects the orthorhombicity factor b/a, up to an amount of 50 at.% Cu. Above this Cu amount, a multiphase system has been observed, with the presence of unreacted-Y₂O₃, YMnO₃ and Y₂Cu₂O₅, along with a perovskite phase. DC electrical conductivity measurements have shown a semiconducting behaviour for all the solid solutions with perovskite-type structure. The room temperature conductivity increases with Cu until 33 at.% Cu, and then decreases. Thermally activated small polaron hopping mechanism, between Mn³⁺ and Mn⁴⁺ cations, controls the conductivity in these ceramics. Results are discussed as a function of the Mn³⁺/Mn⁴⁺ ratio for each composition.     

Improving the oxygen permeability of Ba0.5Sr0.5Co0.8Fe0.2O3−δ membranes by a surface-coating layer of GdBaCo2O5+δ

A GdBaCo₂O_5+δ layer was coated on the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membranes to enhance their oxygen permeability by employing the fast oxygen adsorption/desorption surface-exchange properties of the GdBaCo₂O_5+δ material. The oxygen flux of the coated and uncoated Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membranes was measured in the temperature range of 600–850 °C. The results reveal that the oxygen-permeation flux of the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membranes coated by a GdBaCo₂O_5+δ layer shows significant enhancement. The GdBaCo₂O_5+δ layer coated on the oxygen desorption side (He side) has much effect than that coated on the oxygen adsorption side (air side). At 850 °C, the oxygen flux with a single coating layer on the air side can rise 16%, while a single coating on the helium side will result into a rise of 23%.     

Effects of Na2CO3/K2CO3 on Chemical Looping Combustion Using Fe2O3/Al2O3 as Oxygen Carrier

The performance of the Fe₂O₃/Al₂O₃ oxygen carrier modified by the alkali metals, Na and K, was studied by thermogravimetric analysis and in a laboratory-scale fluidized-bed reactor. Different factors, such as the alkali metal content, the alkali metal type, and the interaction between Na and K, were taken into account. Both Na and K had strong catalytic effects on the conversion of Fe₂O₃. A high K content caused serious agglomeration. Increasing the Na content led to slagging because of the low-melting point solid solution. The possibility of agglomeration and slagging was reduced when Na and K were added together. The oxygen carrier with 1.5 wt % Na and 1.5 wt % K had better combustion efficiency and optimal thermal stability. Na and K could enhance the attrition resistance and prolong the lifetime of the oxygen carriers.     

Piezoelectric properties of Pb(Mn1/3Nb2/3)O3–PbZrO3–PbTiO3 ceramics with sintering aid of 2CaO–Fe2O3 compound

Since the electromechanical devices move towards enhanced power density, high mechanical quality factor (Q_m) and electromechanical coupling factor (k_p) are commonly needed for the high powered piezoelectric transformer with Q_m≥2000 and k_p=0.60. Although Pb(Mn_1/3Nb_2/3)O₃–PbZrO₃–PbTiO₃ (PMnN–PZ–PT) ceramic system has potential for piezoelectric transformer application, further improvements of Q_m and k_p are needed. Addition of 2CaO–Fe₂O₃ has been proved to have many beneficial effects on Pb(Zr,Ti)O₃ ceramics. Therefore, 2CaO–Fe₂O₃ is used as additive in order to improve the piezoelectric properties in this study. The piezoelectric properties, density and microstructures of 0.07Pb(Mn_1/3Nb_2/3)O₃–0.468PbZrO₃–0.462PbTiO₃ (PMnN–PZ–PT) piezoelectric ceramics with 2CaO–Fe₂O₃ additive sintered at 1100 and 1250 °C have been studied. When sintering temperature is 1250 °C, Q_m has the maximum 2150 with 0.3 wt.% 2CaO–Fe₂O₃ addition. The k_p more than 0.6 is observed for samples sintered at 1100 °C. The addition of 2CaO–Fe₂O₃ can significantly enhance the densification of PMnN–PZ–PT ceramics when the sintering temperature is 1250 °C. The grain growth occurred with the amount of 2CaO–Fe₂O₃ at both sintering temperatures.     

A nanocomposite of poly(N‐vinylcarbazole) with nanodimensional alumina

A nanocomposite of poly(N‐vinylcarbazole) (PNVC) and Al₂O₃ was prepared by precipitation of a preformed PNVC in a tetrahydrofuran solution onto an aqueous suspension of nanodimensional Al₂O₃. Prolonged extraction of a PNVC–Al₂O₃ composite by benzene failed to extract the loaded PNVC from the Al₂O₃, as shown by Fourier transform infrared studies. Scanning electron microscopy analyses revealed distinct morphological features of the composite, and transmission electron microscopy analyses confirmed that the particle sizes were in the range of 120–240 nm. Thermogravimetric analyses demonstrated the enhanced stability of the nanocomposite relative to the base polymer. Direct current conductivity of the PNVC–Al₂O₃ composites was found to be about 0.14 × 10^?6 S/cm. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2233–2237, 2003     

High-entropy La(Fe0.2Co0.2Ni0.2Cr0.2Mn0.2)O3 ceramic exhibiting high emissivity and low thermal conductivity

A novel, high-entropy, perovskite-structured, solid solution La(Fe_0.2Co_0.2Ni_0.2Cr_0.2Mn_0.2)O₃ ceramic was successfully synthesized via high-temperature solid-state reaction. The crystal structure, microstructure, infrared emissivity, and thermophysical properties were investigated. The experimental results indicated that La(Fe_0.2Co_0.2Ni_0.2Cr_0.2Mn_0.2)O₃ exhibited an infrared emissivity as high as .92 in the near-infrared region of .76–2.50 μm. The thermal conductivity was 1.38–1.72 W m⁻¹ K⁻¹ in the temperature range of 25–1200°C.     

添加剂对铝酸钠溶液种分产物质量的影响

研究了由脂肪酸类表面活性剂与脂肪烷烃按一定比例复配而成的复合型添加剂对铝酸钠溶液二段法分解过程及最终产品氧化铝性能的影响及其作用机理。结果表明,当添加量为60×10-6g·L-1时,添加剂能够有效强化铝酸钠种分过程、改善分解产物质量,使溶液分解率提高大于2％;氢氧化铝颗粒小于45 μm的质量分数降低约7％;氧化铝颗粒小于45 μm的质量分数降低约6.7％。晶体形貌表明,添加剂的加入有效促进了氢氧化铝颗粒的附聚、长大过程,颗粒多呈球状,结晶比较完善。     

Fe掺杂对Pt-Sn-K/γ-Al_2O_3异丁烷脱氢催化剂性能的影响

采用共沉淀法制备不同Fe含量的Fe(x)-γ-Al_2O_3复合氧化物载体,并采用真空浸渍法制备了Pt-Sn-K/Fe(x)-γ-Al_2O_3催化剂。对制备的催化剂进行XRD、N_2物理吸附-脱附和NH_3-TPD表征,研究Fe的掺杂对Pt-Sn-K/Fe(x)-γ-Al_2O_3催化剂的结构及其催化异丁烷脱氢反应性能的影响。结果表明,Fe的引入可以改变催化剂的反应活性和产物选择性,当Fe_2O_3掺杂质量分数为4%时,催化剂具有最高的异丁烯收率,50 h的平均收率达到42.9%。     

Effects of Mg/Fe substitution on perovskite stabilization and dielectric properties of Pb(Zn1/2W1/2)O3

The effects of compositional modification by substituting Mg or Fe for Zn on the perovskite formation, crystallographic aspects, and dielectric properties of Pb(Zn_1/2W_1/2)O₃ were investigated. Microstructural evolutions in the sintered ceramics were also examined. Stabilization of the perovskite structure was accomplished by a much smaller concentration of Mg substitution. Lattice parameters of the perovskite decreased with increasing substituent fractions in general. By contrast, the permittivity values increased significantly with increasing Mg/Fe substitution.     

Systematic synthesis of (Gd1−xLax)2O2SO4:Tb3+ and (Gd1−xLax)2O2S:Tb3+ nanophosphors for remarkably enhanced luminescence

Coprecipitation with rare-earth nitrate, ammonium sulfate, and ammonium hydroxide produced hydroxide-type amorphous precursors incorporating sulfate and carbonate anions, from which [(Gd_1−xLa_x)_0.99Tb_0.01]₂O₂SO₄ and [(Gd_1−xLa_x)_0.99Tb_0.01]₂O₂S (x = 0, 0.15, 0.3, 0.5, 0.65, 0.8, and 1) were obtained as two series of nanophosphors by calcination at 950°C in air and hydrogen, respectively. The detailed characterization by X-ray diffractometer, scanning electron microscopy/transmission electron microscopy, Brunauer–Emmett–Teller, and particle sizing confirmed that solid solutions were directly formed and that the products have small crystallite size, unimodal size distribution, and high specific surface area, revealing the advantages of the synthesis method. Photoluminescence study revealed that La³⁺ admixture may significantly improve the 545 nm main emission of Tb³⁺ for both the phosphor series. Furthermore, the 545 nm main emission of [(Gd_1−xLa_x)_0.99Tb_0.01]₂O₂SO₄ was identified to have an excellent thermal stability, which retained over 90% of its room-temperature intensity at 150°C (no quenching for Gd₂O₂SO₄:Tb³⁺). The two series of phosphors were comparatively studied for their excitation and luminescence performances, as a function of temperature and La³⁺ content, and the results were rationalized by considering bandgap, crystal structure, UV absorption, and the character of chemical bonds.     

Effects of alumina incorporation in coprecipitated NiO–Al2O3 catalysts

The effect of Al₂O₃ levels on the properties of NiO in coprecipitated NiO–Al₂O₃ samples were investigated, using samples with up to 60.7 wt.% Al₂O₃ that had been calcined in the range 300–700°C. Characterization techniques included BET surface area of fresh and reduced catalysts, X-ray diffraction analysis of structure and crystallite size, magnetic susceptibility measurements, oxidizing power, and reducibility in H₂. Only NiO was detected in samples with up to 4.1 wt.% Al₂O₃ for all sample calcination temperatures. Surface areas were similar for all fresh samples but decreased rapidly after calcination at high temperatures. The surface area loss was less for the higher Al₂O₃-containing samples. Nickel oxide crystallite sizes increased at higher calcination temperatures, but remained approximately the same for each Al₂O₃ level.

The NiO was nonstoichiometric (NiO_1+x), with x decreasing at higher calcination temperatures and increasing with small amounts of added Al₂O₃ through a maximum at about 3 wt.% Al₂O₃. However, this did not correlate well with microstrain in the NiO crystallites nor with reducibility, which decreased with Al₂O₃ addition. At higher levels of Al₂O₃ (13.6 wt.% and above), surface areas increased with higher Al₂O₃ loadings, but NiO crystallite sizes remained approximately the same, independent of both Al₂O₃ content and calcination temperature. X-ray diffraction patterns were very diffuse, and it was not possible to rule out the presence of pseudo-spinel combinations of NiO and Al₂O₃. Reducibility was more difficult than with low Al₂O₃ levels, and nonstoichiometry was low and independent of Al₂O₃ content.

Reducibilities of all samples calcined at 300°C correlated well with the final BET surface area of the reduced samples, indicating that more dispersed NiO crystallites are more difficult to reduce, a conclusion that supports a model for reduction proposed previously.