Influence of crystallite size on the oxidation kinetics of magnetite

The oxidation of magnetite yields the lacunar phase γ-Fe₂O₃, for sizes less than 5000 Å and the rhombohedral phase, α-Fe₂O₃, for sizes above 10 000 Å. For intermediate sizes, oxidation kinetics and X-ray analysis have confirmed that the γ-Fe₂O₃ phase forms at the beginning of the reaction, followed by phase α-Fe₂O₃ forming from γ-Fe₂O₃ and then directly from the still-unoxidized magnetite. Influence of size could be accounted for in terms of structure and stresses at the crystal lattice level.     

Microwave spectrum,centrifugal distortion,internal rotation and excited torsional states of isobutene

The microwave spectrum of isobutene has been recorded from 10 to 35 GHz. From the analysis of the ground and first two excited torsional state splittings, the following internal rotation parameters were calculated: V₃ = 2170 cal mol.^?1, V'₁₂ = ?210 cal. mol.^?1, I_α = 3.18amu Ų and angle (methyl-top to b-axis) 58.21°. Centrifugal distortion parameters were also obtained for the ground state.     

Time-resolved excitation spectra of xenon vapor in the 1500 Å region

Time-integrated and time-resolved excitation spectra, as well as fluorescence lifetimes, have been measured for xenon vapor as a funtion of pressure (for pure xenon as well as with different collision partners: krypton and helium), monitoring the 1700 Å second continuum afterglow. Three very different decay components have been observed: (a) A short component with lifetimes of the order of 2 ns, which is attributed to emission from the upper vibrational levels of the O⁺_u(¹Σ⁺_u) state of Xe*₂. (b) A long component with τ ≈ 60 ns, corresponding to emission from thermally relaxed vibrational levels of the 1_u,O^?_u(³Σ⁺_u) states of Xe*₂. No intervention of any Xe atomic excited species is invoked to explain the excitation and deactivation processes of Xe₂ molecules for these two components of the fluorescence. (c) A very long component with lifetimes in the 150–500 ns range, having a broad ? 1000 cm^?1 - excitation band, centered at ≈ 1470 Å and showing a striking screening and self-absorption effect as well as a strong effect when the partial pressure of a collision partner (Kr, He) is increased. The mechanism of this last excitation is not yet very well understood.     

RMN 13C d'α alcoxy-indenes et d'α alcoxy-styrenes : Topologie electronique du systeme Π et une reflexion sur la relation entre deplacement chimique et reactivite dans les ethers d'enol

¹³C NMR chemical shifts of ethylenic carbon atom C_β of α alkoxy-indenes and α-alkoxy-styrenes are affected by the conformation of the OR group with respect to the double bond and hence the efficiency of p- Π conjugation but, contrary to previous results, δC_α is not affected. The phenyl group exerts a diamagnetic Π- effect on C_α and C_β which is a function of its dihedral angle with the double bond. Although δC_β is especially sensitive to the Π-electron density, the chemical shifts of the ethylenic carbons are best understood by considering the total charge density. The σ and Π contributions to the total charge density are discussed using the preferred conformation of the molecules. This approach strongly suggests that the correlation between the gradual shift of δC_α to low frequency and the gradual shift of δC_β to high frequency (relative to TMS) as R changes from RCH₃ to RtC₄H₉ in certain series of enol-ethers is not due to pΠ conjugation variations. This method of interpreting ¹³C NMR shifts, as opposed to previous methods, is compatible with explanations of other physico-chemical properties of alkyl vinyl ethers. Although it is sometimes possible to correlate the gradual shifts of δC_β with (σ + Π) electron density variations in a homogeneous series, it seems impossible to predict the relation. In all cases, there is no evident correlation between gradual shifts of δC_β(orδC_α) and the reactivity of enol ethers because one cannot consider the role of the out-of-plane conformation of the alkoxy group (which increases from RCH₃ to RtC₄H₉); however, the out of plane conformation does not have an identical effect on δC_β and the reactivity.     

从传教士的活动看19世纪中叶至20世纪初法国对中国藏族地区的非军事侵略态度

文章以19世纪法国的世界殖民网络及法中关系为背景,以法国传教士的活动为线索,探讨了19世纪中叶至20世纪初法国对中国藏族地区所采取的非军事侵略态度,并对其采取这一态度的原因进行了分析。     

Crystal chemistry,magnetic, and electrical properties of the tetragonal plutonium oxide telluride Pu2O2Te

We describe the preparation and some physical properties of Pu₂O₂Te. The plutonium oxide telluride is isostructural with the corresponding rare-earth oxide tellurides which crystallize in the tetragonal system of La₂O₂Te-type. Magnetic susceptibility data from 4 K to room temperature are reported together with resistivity measurements. Pu₂O₂Te is found to be an antiferromagnet below 56 K and a semiconductor with an intrinsic energy gap of 0.65 eV. The magnetic behavior is interpreted in terms of superexchange coupling interactions via nonmetal p orbitals, i.e., in terms of 5f-p overlaps. This conclusion is supported by crystal chemistry considerations by comparison of cell volumes of Pu₂O₂Te and Nd₂O₂Te. In Pu₂O₂Te, the Pu crystal radius is found to be much lower than that of Nd in Nd₂O₂Te, suggesting some 5f electron “delocalization” leading to a crystal radius shrinkage. As for the hexagonal Pu₂O₂X compounds, with X = O, S, Se, the measured gap may be considered as the energy separation between the chalcogen np band and the 6d-7s conduction band, the occupied 5f states lying just below the np band with some 5f-np overlap.     

法国社会保障制度建设及其对我国的启示

法国作为典型的欧式福利国家,其社会保障水平在发达国家中处于较高的水平.经历了约两个世纪的漫长发展和二战后的不断完善后,这一制度对于缩小贫富差距和维护社会稳定起到了重要作用.但是随着社会不断的发展,法国的社会保障制度也暴露出自身的弊端.在分析这一制度建设取得的成就及其原因的基础上,进一步探讨其存在的问题对于我国社会保障制度的发展具有一定的启示作用.     

法国蔬菜生物多样性研究概况

比较详细地介绍了法国蔬菜生物多样性研究的历史、目标、对象、内容和技术路线，并简单介绍了它近期正在进行或将要进行的科研方向和内容．     

《出使英法义比四国日记》所昭示的桐城散文变革

出自古文家薛福成之手的《出使英法义比四国日记》,以欧洲社会为叙述对象,据事发论,称赞君主立宪制,提出挽救时弊、强盛中华的方略,以中华传统思想文化阐述西方政治、经济、科学观念,具有浓厚的改良主义色彩.取材、布局随意,在浅近文言中夹杂使用双音词、外来词,不自觉地离开了桐城"义法",成为古文蜕化、新文体孕育的开始.     

近代自然科学的兴起对西方自然观的影响——以启蒙运动中的法国为例

从16到18世纪末,天文学、生命科学和力学是近代自然科学成就最大的三个领域。根据近代自然科学所取得的巨大成就,法国的启蒙思想家们确立了一种比较彻底的唯物主义自然观,认为世界是物质的,而物质是运动着的,从而把唯物主义哲学推向了一个新的发展阶段。