On the magnetic properties of Lithium Vanadium Bronzes LixV2O5

Magnetic susceptibility of V₂O₅ and of the α-, ?-, δ- and γ-Li_xV₂O₅ bronzes prepared either at high or at room temperature has been measured between liquid helium and ambient temperature. The results are representative of the localized character of the V⁴⁺ ion d-electron and, for high enough x-values, of the existence of antiferromagnetic interactions in a low dimensional system. The intrachain exchange integral J has been determined using the Bonner/Fischer model for Heisenberg chains with S = 1/2 spin. It is larger for the γ- than for the δ-phase. This result as well as the occurrence of long range ordering below T_N ≈ 130 K in the γ-phase may be ascribed to ordering of V⁴⁺ and V⁵⁺ ions in the former bronze and to the random distribution of V⁴⁺ ions in the latter one.     

Etude par diffraction X et microscopie electronique du système CaTiO3---Ca2Fe2O5

In order to check the extended defect model previously proposed for the nonstoichiometric perovskite A_nM_nO_3n−1 (n 2), the Ca₂Ti_2−2xFe_2xO_6−x solid solution has been studied by means of X-ray diffraction and electron microscope investigations. The results show the existence of a continuous evolution (0 x 1) with two different regions. Close to the CaTiO₃ composition (0 x 0.40) the oxygen vacancies seem to be randomly distributed and the symmetry remains pseudocubic. In the composition range 0.55 x 1, the electron microscope patterns show an ordering of the defects: the oxygen vacancies are ordered in the (0k0) planes in strings parallel to the [101] direction. It is concluded from these observations that perpendicular to the Oy direction isolated planes of tetrahedra arranged in rows separate planes of corner-linked octahedra. The n = 2; 2.5; 3, et 4 terms of the A_nM_nO_3n−1 series have been identified. C.S. planes characteristic of classical nonstoichiometric oxides with small cations do not appear: instead, tetrahedra planes succeed to the previous octahedra planes as the degree of nonstoichiometry increases.

Résumé

Afin de vérifier le modèle de non-stoechiométrie proposé dans un travail précédent pour les perovskites lacunaires A_nM_nO_3n−1 (n 2), la solution solide Ca₂Ti_2−2xFe_2xO_6−x a été, étudiée par diffraction X et microscopie électronique. Les résultats montrent l'existence d'une évolution continue (0 x 1) avec deux domaines différents. Pour les compositions proches de CaTiO₃ (0 x 0.40), les lacunes oxygénées semblent être distribuées statistiquement et la symétrie demeure pseudocubique. Dans le domaine de compositions 0.55 x 1, les images de microscopie électronique montrent un ordre des défauts: les lacunes d'oxygène sont ordonnées dans les plans (0k0) en rangées parallèles à la direction [101]. Ces observations ont permis de conclure que perpendiculairement à la direction Oy des plans isolés de files de tétraèdres séparent des plans d'octaèdres liés par leurs sommets. Les termes n = 2; 2.5; 3 et 4 des séries A_nM_nO_3n−1 ont été identifiés. Des plans de cisaillement rencontrés dans les oxydes non-stoechiométriques classiques de petits cations n'ont pas été mis en évidence: par contre, des plans de tétraèdres se substituent aux plans d'octaèdres initiaux lorsque le degré de non-stoechiométrie augmente.     

Preparation and characterization of Fully stoichiometric SrCoO3 by electrochemical oxidation

A nearly stoichiometric SrCoO₃ phase had been prepared up to now only using high oxygen pressure. A new method for preparing fully stoichiometric SrCoO₃ has been proposed using electrochemical oxidation. Brownmillerite-type SrCoO_2.50 is oxidized into a completely stoichiometric perovskite at a potential of 500 mV for 180 hours at room temperature in alkaline media (1 M KOH). The oxidized phase has a cubic unit cell (a=383.5 pm). It is metallic and, at temperatures below T=280 K, it shows ferromagnetic behavior. The magnetic moment at 0 K is 2.1 μ_B. The physical properties of SrCoO_3.00 have been explained in terms of partial occupancy of a σ*e band.     

Mechanisms and kinetics of polymerization of thermoplastic polyimides. II. Study of “bridged” dianhydride/aromatic amine systems

The aim of the present work was to identify and follow the main and side reactions involved in the ring dehydration of amic acid prepared from “bridged” dianhydrides whose central substituent is an electron acceptor or donor, and an aromatic diamine. Several isomeric structures may appear as a result of the opening reactivity and selectivity of anhydride groups towards the aromatic amine. Reaction mechanisms and kinetics were thus studied in solvent phase with HPLC and ¹³C-NMR and in solid molten phase by FTIR and solid ¹³C-NMR. The experimental conditions (liquid and solid) and the structure of the products (type of central substituent) affecting the mechanisms and kinetics of the reactions were noted. © 1993 John Wiley & Sons, Inc.     

Insulator–metal transition in Nd0.8Na0.2Mn(1−x)CoxO3 perovskites

The electric and magnetic properties of the perovskites Nd_0.8Na_0.2Mn_(1−x)Co_xO₃ (0x0.2) prepared by the usual ceramic procedure were investigated. The insulator-to-metal-like (IM) transition, closely related to a ferromagnetic arrangement, was revealed for the composition of x=0.04 and a similar tendency was detected for x=0. The insulating behavior persists down to low temperatures for higher contents of cobalt ions in spite of the transition to the bulk ferromagnetism. The properties are interpreted in terms of the steric distortion, tilting of the Mn(Co)O₆ octahedra and the double-exchange interactions of the type Mn³⁺–O²⁻–Mn⁴⁺and Mn^3.5+δ–O²⁻–Co²⁺, respectively. Presence of antiferromagnetic domains in the ferromagnetic matrix for the most of cobalt-substituted samples is supposed.     

Chemical depth profiling and 3D reconstruction of III–V heterostructures selectively grown on non‐planar Si substrates by MOCVD

The chemical characterization of novel 3D architectures with nanometre‐scale dimensions is extremely challenging. The chemical composition of InGaAs/AlAs quantum wells selectively grown in SiO₂ trenches, 100–300 nm wide, is studied. Combining high lateral resolution 3D ToF‐SIMS analysis and Auger measurements, the chemical composition of individual trenches was obtained confirming the uniformity of these III–V heterostructures. These results correlate well with an average approach using SIMS depth profiling. The effects of ion beam orientation on the surface topography of confined structures were highlighted. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Acetylene-terminated resins: Mechanism of thermal polymerization and properties of cured polymers

The polymerization kinetics and mechanisms of model compounds (mono- and difunctional) characteristic of prepolymers of ATR (acetylene-terminated resins) have been studied by HPLC, FTIR, UV, NMR (liquid and solid) and HPLC in the temperature range of 150–220°C. Using monofunctional compounds, we were able to show that the thermal polymerization of acetylene functions is not a simple reaction. Initially, the majority of compounds formed have a low polymerization degree, followed by the formation of species with higher and progressive polymerization degrees. Spectroscopic techniques such as NMR and FTIR lead to the unambiguous determination of reaction conversion up to 95% in the case of bifunctional prepolymers. The understanding of network structures, on the other hand, is more delicate in polyaromatic systems, since the chains formed are very similar to the basic skeleton of the prepolymer. The kinetic curves obtained isothermally between 150–220°C have shown that while kinetic monitoring of the disappearance of the first 90% of ethynyl functions poses no particular problems, there is a considerable uncertainty for the last 10%, regardless of the technique chosen. We have demonstrated that me thermal properties are dependent on the molecular weight and the structure of the polymer.     

中温固体氧化物燃料电池阴极材料LaBiMn2O6的制备与性能研究

采用固相法合成了中温固体氧化物燃料电池(IT-SOFCs)阴极材料LaBiMn₂O₆,并利用X射线衍射(XRD)和电化学阻抗谱(EIS)进行表征.结果表明该材料与电解质Ce_0.7Bi_0.3O_1.85(CBO)在1 000 ℃烧结12 h不发生反应.交流阻抗和直流极化测试结果发现,阴极极化电阻随测试温度的增加而逐渐减小,700 ℃空气中的极化电阻为0.71 Ω·cm²;氧分压测试结果显示,在600~700 ℃范围内,电极反应的速率控制步骤为电极上发生的电荷转移反应.电极过电位为85 mV时,700 ℃的阴极电流密度达到 216 mA·cm^-2 ,表明LaBiMn₂O₆是一种潜在的中温固体氧化物燃料电池(IT-SOFCs)阴极材料.