Phase separation and crystallization in glasses of the lithium silicate system xLi2O · (100 ? x)SiO2 (x = 23.4, 26.0, 33.5)

The phase separation in ultimately homogenized glasses of the lithium silicate system xLi₂O · (100 − x)SiO₂ (where x = 23.4, 26.0, and 33.5 mol % Li₂O) has been investigated. The glasses of these compositions have been homogenized using the previously established special temperature-time conditions, which provide the maximum dehydration and the removal of bubbles from the glass melt. The parameters of nucleation and growth of phase_separated inhomogeneities and homogeneous crystal nucleation have been determined. The absolute values of the stationary nucleation rates I _st of lithium disilicate crystals in the 23.4Li₂O · 76.6SiO₂ and 26Li₂O · 74SiO₂ glasses with the compositions lying in the metastable phase separation region have been compared with the corresponding rates I _st for the glass of the stoichiometric lithium disilicate composition. It has been established that the crystal growth rate have a tendency toward a monotonic increase with an increase in the temperature, whereas the dependences of the crystal growth rate on the time of low-temperature heat treatment exhibit an oscillatory behavior with a monotonic decrease in the absolute value of oscillations. The character of crystallization in glasses with the compositions lying in the phase separation region of the Li₂O-SiO₂ system is compared with that in the glass of the stoichiometric lithium disilicate composition. The inference has been made that the phase separation weakly affects the nucleation parameters of lithium disilicate and has a strong effect on the crystal growth.     

A study of the formation of Ln2 + xMe2 ? xO7 ? x/2 (Ln = Gd, Dy; Me = Zr, Hf) nanocrystals

It has been established that the process of producing the Ln_{2 + x} Me_{2 − x} O_{7 − x/2} (Ln = Gd, Dy; Me = Zr, Hf) nanocrystals by calcination of hydroxides, which, in turn, have been produced by coprecipitation of metal salts, includes several stages. At the beginning, the X-ray amorphous structure of the precursors remains unchanged during dehydration; during subsequent heating to 600–700°C, nanocrystals with a disordered fluorite structure begin to be formed. An increase in the temperature above 700°C leads to an increase in the size of crystallites (coherent scattering regions). This process is accompanied by changes occurring in their local structure. In the nanocrystalline powders of Cd₂Hf₂O₇ and Gd₂Zr₂O₇ synthesized at 1200°C (6 h), the pyrochlore-type superstructure with the lattice parameters doubled relative to fluorite has been revealed. It has also been found that, possibly, the Dy₂HfO₅ sample at 1600°C (3 h) has a modulated structure.     

On the local structure of Cd3+ centers in (BaGeO3)1 ? x ? y (Al2O3) x (0.45CaF2 · 0.55MgF2) y glasses

A correlation between the ¹⁵⁵Gd M?ssbauer spectra of the GdAlO₃ and Gd₃Al₅O₁₂ compounds and Gd³⁺-doped glasses in the (BaGeO₃)_{1 − x − y} (Al₂O₃) _x (0.45CaF₂ · 0.55MgF₂) _y system is revealed. The conclusion is drawn that, in the structure of the glasses under investigation, trivalent gadolinium atoms form structural units characteristic of mixed gadolinium and aluminum oxides. Original Russian Text ? S.A. Nemov, A.V. Marchenko, P.P. Seregin, 2008, published in Fizika i Khimiya Stekla.     

Direct evidence of oxygen evolution from Li1+ x (Ni1/3Mn1/3Co1/3)1? x O2 at high potentials

Li_1+x (Ni_1/3Mn_1/3Co_1/3)_1−x O₂ (NMC) oxides are among the most promising positive electrode materials for future lithium–ion batteries. A voltage “plateau” was observed on the first galvanostatic charging curve of NMC in the extended voltage region positive to 4.5 V vs. Li/Li⁺ for compounds with x > 0 (overlithiated compounds). Differences were observed in the cycling stability of the overlithiated and stoichiometric (x = 0) NMC oxides in this potential region. A differential plot of the charge vs. potential profile in the first cycle revealed that, for the overlithiated compounds, a large irreversible oxidative peak arises positive to 4.5 V vs. Li/Li⁺, while in the same potential region only a small peak due to the electrolyte oxidation is detected for the stoichiometric material. Differential Electrochemical Mass Spectrometry (DEMS) was used to investigate the high voltage region for both compounds and experimental evidence for oxygen evolution was provided for the overlithiated compounds at potentials positive to 4.5 V vs. Li/Li⁺. No oxygen evolution was detected for the stoichiometric compound.     

An XRD, FTIR and TPD Investigation of NO2 Surface Adsorption Sites of δ, γ Al2O3 and Barium Supported δ, γ Al2O3

In this paper an XRD, FTIR and TPD investigation of NO₂ surface adsorption sites of , Al₂O₃ and barium supported , Al₂O₃ is reported. Aim of this study is to bring additional light on the surface structures involved in NOx adsorption. Two samples of barium supported aluminas have been prepared and aged at 800 °C. These samples were characterised in comparison with the relative alumina support. The XRD characterisation of these samples shows the presence of barium carbonate and barium aluminate supported on alumina. The comparison of the FTIR spectra, before and after NO₂ adsorption, has revealed the formation, upon NO₂ contact, of a complex variety of nitrate and nitrite groups. The thermal desorption of nitrate and nitrite species has been simultaneously studied by means of FTIR spectroscopy and by TPD technique. By comparing the structural, adsorptive and spectroscopic results obtained on alumina and on barium supported alumina samples, a hypothesis on the basic sites active in NO₂ adsorption and of the possible decomposition paths induced by thermal heating are proposed.     

Solid solutions Na1 - xKxB3O5 (0.5 < x ≤ 1.0)

The phase relations in the NaB₃O₅-KB₃O₅ pseudobinary system are investigated using high-temperature X-ray powder diffraction, differential scanning calorimetry, and the annealing and quenching techniques. Solid solutions Na_{1 - x}K_xB₃O₅ (0.5 < x 1.0) based on the KB₃O₅ phase are found in the system at temperatures below 650–670°C. The replacement of potassium atoms by sodium atoms is accompanied by the contraction of the structure along the a and c axes. The parameter b (the direction of channels in the structure) is insensitive to the substitution of sodium atoms for potassium atoms. No isomorphic miscibility on the basis of NaB₃O₅ is revealed in the system. In the region from 0 to 50 mol % KB₃O₅, there exist two phases, namely, Na_{1 - x}K_xB₃O₅ (x 0.5) and -NaB₃O₅.Original Russian Text Copyright © 2004 by Fizika i Khimiya Stekla, Bubnova, Georgievskaya, Filatov, Ugolkov.     

New anode materials of Li1+xV1?xO2 (0?≤?x?≤?0.1) for secondary lithium batteries: correlation between structures and properties

Li_1+x V_1−x O₂ (0 ≤ x ≤ 0.1) compounds were studied as the anode materials for a lithium-ion battery. The crystal and electronic structures of the prepared materials were correlated with electrical conductivities and electrochemical properties. The electrochemical behaviors were significantly dependent on the composition of Li_1+x V_1−x O₂, and these were resulted from the perturbation of the local electronic structure arising from the increase in lithium contents in Li_1+x V_1−x O₂ rather than from the slight distortion in the crystal structure. The electrical conductivities of Li_1+x V_1−x O₂ increased with the increase in lithium contents in the compounds. Li_1.1V_0.9O₂ and Li_1.075V_0.925O₂ samples exhibit the first discharge capacities of 250 and 241 mAh g⁻¹ at 0.2 C-rate, respectively.     

Glycothermal Synthesis and Catalytic Properties of Nanosized Zn1?xCoxAl2O4 (x?=?0, 0.5, 1.0) Spinels in Phenol Methylation

Abstract  

Zn_1−x Co _x Al₂O₄ (x = 0, 0.5, 1.0) spinels were synthesised under microwave-assisted solvothermal conditions using 1,4-butanediol as reaction medium. The results of XRD and HRTEM have indicated nanocrystallinity of prepared materials (average crystallite size in the range 6–16 nm), and N₂ adsorption–desorption measurements have revealed high feature of their surface area and mesoporous nature. Acid–base properties of prepared materials were determined using TPD-NH₃ method and cyclohexanol test. All studied spinels that underwent examination were active in phenol methylation but Co-substituted zinc aluminate (Zn_0.5Co_0.5Al₂O₄) was found to be the most efficient in the selective formation of ortho-methylated products. It could be ascribed to its dominant basic character and the presence of weak and strong acid centres, and the highest surface area as well.     

Structural and dielectric behavior of yNi1 ? xCdxFe2O4 + (1 ? y)Ba0.8Sr0.2TiO3 magnetoelectric composites prepared through SHS route

The structural and dielectric properties of SHS-produced yNi_{1 − x} Cd _x Fe₂O₄ + (1 − y)Ba_0.8Sr_0.2TiO₃ (x = 0.2, 0.4, 0.6; y = 15, 30, 45%) magnetoelectric composites were characterized by XRD, SEM, and resistivity/dielectric measurements. SEM images reveal that SHS reaction can produce two pure phases simultaneously. The grown Cd-substituted nickel ferrite grains were well dispersed in a BST matrix. A decrease in resistivity with temperature shows the semiconducting nature of synthesized samples. The dielectric results demonstrated an attractive response of dielectric constant to frequency and temperature. The Curie temperature of about 480°C was observed in the Ni_0.4Cd_0.6Fe₂O₄ + Ba_0.8Sr_0.2TiO₃ composite.     

258.15 K下五元体系Li+,Na+,Mg2+//SO42-,Cl--H2O相平衡研究

中国含锂盐湖大部分位于青藏、新疆等干旱少雨、冬季寒冷且漫长的地区。为指导低温提锂工艺的开发和设计、利用冬季冷能进行目标离子的富集及明确含锂盐湖在低温下的析盐规律,采用等温溶解平衡法对258.15 K、二水氯化钠饱和条件下的交互五元体系Li⁺,Na⁺,Mg²⁺∥SO₄^2-,Cl^--H₂O相平衡关系进行研究并构建等温平衡相图。结果表明,相图中有4个共饱点、6个两盐结晶区(NaCl·2H₂O+Na₂SO₄·10H₂O、NaCl·2H₂O+MgCl₂·8H₂O、NaCl·2H₂O+MgSO₄·7H₂O、NaCl·2H₂O+Li₂SO₄·H₂...     

Na+,Mg2+//Cl-,SO42--H2O四元体系相平衡

针对海水淡化后浓海水综合利用问题,开展相平衡研究,提出硫酸钠水合物法增浓海水的方法。等温法测定了5℃时Na⁺, Mg²⁺//Cl^-, SO₄^2--H₂O四元体系及其子体系的相平衡溶解度数据,绘制并分析相图特征。结果表明：5℃时该体系存在2个等温共饱点和4个结晶区,十水硫酸钠结晶区形成面积较大,可用于硫酸钠水合物法增浓海水研究。相图分析和计算表明：该技术可使浓海水浓缩率高达82.78%,无水硫酸钠回收率99.6%。浓海水经精制处理后可作为制碱生产的原料,节省制碱生产过程中的化盐过程,为综合开发利用淡化后浓海水提供理论和数据基础。     

固体氧化物燃料电池阴极材料La1.2Sr0.8Co0.8Ni0.2O4+δ的研究

以聚丙烯酸、柠檬酸和尿素三种复合配体作为络合剂,采用溶胶凝胶法制备成La1.2Sr0.8Co0.8Ni0.2O4+δ前驱物,经800℃煅烧获得阴极粉体。利用丝网印刷法将LSCN和GDC制备成复合阴极。利用XRD、TEM、ED、SEM、压汞仪和电化学工作等分别对LSCN粉体的物相、结构和电极的形貌、孔结构和电性能等进行了表征。研究结果表明,800℃煅烧获得了颗粒尺寸约为80nm、正交晶型结构的La1.2Sr0.8Co0.8Ni0.2O4+δ阴极粉体,单电池以含水蒸气的H2为燃料在750℃得到了稳定的电性能。     

A2B2O7(A=Ln3+,B=Zr4+,Hf4+Ti4+Ce4+)型透明陶瓷研究进展

A₂B₂O₇型化合物具有高熔点、高密度和高折射率等特点,在照明材料、航空材料、医疗成像、磁光材料等领域具有潜在应用价值。随着陶瓷制备工艺的进步,透明陶瓷的光学、力学和热学等性能不断提升使A₂B₂O₇型透明陶瓷受到越来越多研究者的关注,本文介绍了一些A₂B₂O₇型透明陶瓷的制备工艺、性能参数和其潜在的应用价值。     

Li2SrSiO4:Eu2+,Tb3+中Eu2+和Tb3+的发光特性和能量传递

用高温固相法在N2/H2=95/5(v/v)还原气氛下合成了Li2SrSiO4:Eu2+,Tb3+荧光粉发光材料,通过荧光光谱研究其发光特性,并从理论上探讨了Eu2+与Tb3+之间的能量转移类型。结果表明:该发光材料主发射峰值550nm,与Eu2+在4f7-4f65d1产生跃迁有关;通过掺杂,共存于Li2SrSiO4基质中的Tb3+通过电多级相互作用将能量传递给Eu2+;在500~650nm范围内对Eu2+具有很强的敏化作用,使其在主发射峰550nm的发射强度显著增强;当名义化学组成为Li2Sr0.995SiO4:0.005Eu2+,0.010Tb3+时,发光强度为最佳。     

Electrochemical behaviour of FexCo3?xO4 with (x?=?0, 1, 2 and 3) oxides thin film electrodes in alkaline medium

Spinel-type Fe _x Co_3−x O₄ thin films with x = 0, 1, 2 and 3 were prepared, on stainless steel supports, using the thermal decomposition method at 400 °C. The X-ray diffraction patterns show the presence of a spinel-type structure with a low crystallinity. The electrochemical behaviour was investigated in 1 M KOH, using open-circuit-potential measurements and cyclic voltammetry. The studies allowed to observe the redox reactions occurring at the Fe _x Co_3−x O₄ (x = 1 and 2) oxide surface, namely Fe₃O₄/Fe(OH)₂ or Fe₃O₄/Fe₂O₃, Co₃O₄/CoOOH, Co(OH)₂/CoOOH and CoO₂/CoOOH by comparing the experimental data with those obtained for the Co₃O₄ and Fe₃O₄ films as well as with those referred to in the literature. The results show that iron ions play the major role in the solid-state surface redox transitions in the negative potential range, whereas the cobalt ions are the key species in the positive potential range. However, the contribution of each component, although small, has to be considered in both potential regions.     

Ionic conductivity of (As2Se3)1 ? x(AgHal)x (Hal = I, Br) nanocomposites

Nanocomposites based on chalcogenide glasses have been synthesized. A differential thermal analysis of (As₂Se₃)_{1 − x} (AgI) _x and (As₂Se₃)_{1 − x} (AgBr)x (0 ≤ x ≤ 0.5) samples has been performed. The size of nanofragments that undergo elementary structural transformations has been evaluated. The data obtained are in agreement with the evaluated sizes of X-ray coherent scattering regions. The electrical properties of the glasses under consideration have been studied using impedance spectroscopy in the temperature range 293–393 K. It has been demonstrated that the ionic component of the electrical conductivity dominates in glasses with a high content of silver halide.     

掺杂RE3+对CaAl2O4:Eu2+,Nd3+发光性能的影响

以燃烧法合成了CaAl2O4∶Eu2+,Nd3+,RE3+紫色长余辉发光材料。实验结果表明,掺杂辅助激活剂Pr3+和Ce3+对CaAl2O4∶Eu2+,Nd3+磷光体发光性能有明显影响。掺杂Pr3+的CaAl2O4∶Eu2+,Nd3+样品的发射峰蓝移;掺杂Ce3+的CaAl2O4∶Eu2+,Nd3+样品的发射峰红移。Pr3+或Ce3+掺杂,可以提高CaAl2O4∶Eu2+,Nd3+磷光体的初始亮度,Pr3+或Ce3+在其中起到增加陷阱密度,提高发光亮度的作用。     

SmBaCo2O5+δ-Ce0.8Sm0.2O1.9复合阴极的热膨胀及电化学性能

采用甘氨酸-硝酸盐法(GNP)合成SmBaCo2O5+δ(SBCO)阴极材料和Ce0.8Sm0.2O1.9(SDC)电解质材料,制备不同比例的SBCO-SDC复合阴极,考察SDC含量对复合阴极的热膨胀、电导率和电化学性能的影响。结果表明,SBCO与SDC在1100℃混合煅烧未发生明显的化学反应,两者之间具有良好的化学相容性。SDC的加入可有效改善复合阴极的热膨胀性能,随着SDC含量的增加,SBCO-SDC复合阴极的热膨胀系数(TEC)逐渐减小,同时其电导率也逐渐下降。此外,SDC的加入导致SBCO-SDC复合阴极界面电阻(ASR)增加。当SDC含量为20%时,750℃测试的ASR为0.145Ω.cm2,500～800℃范围内电导率大于100 S.cm-1,满足IT-SOFC阴极材料的要求。     

Insights into the Reactivity of La1?xSrxMnO3 (x?=?0?÷?0.7) in High Temperature N2O Decomposition

Abstract  

In this paper a wide range of La_1−x Sr _x MnO₃ (x = 0–0.7) perovskites was synthesized by Pechini route, characterized by XRD (including high temperature measurements), XPS, differential dissolution phase analysis, TPR H₂, oxygen exchange and tested in N₂O decomposition at 900 °C. At low degree of Sr substitution for La (x ≤ 0.3), high catalytic activity was found for perovskites with hexagonal structure (x = 0.1–0.2) and can be related to fast oxygen mobility caused by the lattice disordering during polymorphic phase transition from the hexagonal to cubic structure. For multiphase samples (x > 0.3) increase of activity and oxygen mobility can be attributed to the formation of the layer-structured perovskite–LaSrMnO₄ on the surface.     

Scanning Tunneling Microscopy Study of H6+xP2Mo18?xVxO62 (x?=?0–3) Wells–Dawson Heteropolyacid Catalysts: Correlation of NDR Peak Voltage with Reduction Potential and Oxidation Catalysis

Abstract  

Scanning tunneling microscopy (STM) and tunneling spectroscopy studies were carried out to examine the redox properties of vanadium-containing H_6+x P₂Mo_18−x V _x O₆₂ (x = 0, 1, 2, 3) Wells–Dawson heteropolyacid (HPA) catalysts. The HPAs formed two-dimensional well-ordered monolayer arrays on a graphite surface and exhibited a distinctive current–voltage behavior called negative differential resistance (NDR). The NDR peak voltages of H_6+x P₂Mo_18−x V _x O₆₂ HPAs were correlated with reduction potentials determined by temperature-programmed reduction and with catalytic activity for oxidative dehydrogenation of isobutyraldehyde to methacrolein. The NDR peak voltage of H_6+x P₂Mo_18−x V _x O₆₂ appeared at less negative voltage with increasing reduction potential and oxidation catalysis.