Morphology-dependent photoluminescence property of red-emitting LnOCl:Eu (Ln=La and Gd)

Three different synthetic methods, the liquid phase process in HCl solution, the solvothermal reaction, and the surfactant-assisted solvothermal reaction, were explored to selectively control the particle shape and to enhance the luminescence intensity of the PbFCl-type red-emitting oxychloride phosphors LnOCl:Eu (Ln=La and Gd). The solvothermal pressure facilitated the low-temperature crystallization of the rod-shape particles for both Ln=La and Gd. It is noted that LaOCl:Eu nanorods show highly porous particle surface and quite low photoemission intensity. In contrast, the solvothermal synthesis could highly enhance the red-emission of GdOCl:Eu with no porous surface so as to be comparable to that of commercial Y₂O₃:Eu phosphor. An addition of surfactant material during solvothermal reaction yielded a rhomboidal-shape phosphor particles with no porous surface for both Ln=La and Gd. Interestingly, the elimination of surface porosity by using a surfactant significantly increased the emission intensity of LaOCl:Eu. It is proposed that the application of solvothermal technique for the synthesis of the PbFCl-type oxychloride phosphors is very effective to selectively control the particle shape and consequently to enhance the photoemission intensity if we use an appropriate surfactant material.     

Stereospecific polymerization of isobutyl vinyl ether. III. Polymerization with VCl3 • LiCl

The polymerization of isobutyl vinyl ether by vanadium trichloride in n-heptane was studied. VCl₃ ? LiCl was prepared by the reduction of VCl₄ with stoichiometric amounts of BuLi. This type of catalyst induces stereospecific polymerization of isobutyl vinyl ether without the action of trialkyl aluminum to an isotactic polymer when a rise in temperature during the polymerization was depressed by cooling. It is suggested that the cause of the stereospecific polymerization might be due to the catalyst structure in which LiCl coexists with VCl₃, namely, VCl₃ ? LiCl or VCl₂ ? 2LiCl as a solid solution in the crystalline lattice, since VCl₃ prepared by thermal decomposition of VCl₄ and a commercial VCl₃ did not produce the crystalline polymer and soluble catalysts such as VCl₄ in heptane and VCl₃ ? LiCl in ether solution did not yield the stereospecific polymer. It was found that some additives, such as tetrahydrofuran or ethylene glycol diphenyl ether, to the catalyst increased the stereospecific polymerization activity of the catalysts. Influence of the polymerization conditions such as temperature, time, monomer and catalyst concentrations, and the kind of solvent on the formed polymer was also examined.     

Cationoid living polymerisations

The cationoid living polymerisations have many of the characteristics of pseudo-cationic (φ-cat) polymerisations and none of those of cationic polymerisations. In contrast to the φ-cat polymerisations by protonic acids, which are transfer-dominated, those giving living polymerisations are propagated by esters which are part of a donor-acceptor complex involving a “third reagent”. There are three types: The first is exemplified by an ester which is activated by a metal halide, e.g. HMI (from monomer M + HI) + ZnBr₂, or oligoisobutyl acetate + BCl₃. The second type is exemplified by a polymer triflate complexed by a dialkyl sulphide. The reagents which change the reactivity of the ester are termed “modifiers”. The acceptors, e.g. ZnBr₂, which co-ordinate to a donor group on the polymer, are positive modifiers (or activators). The donors, such as R₂S, which coordinate onto an acceptor group at the end of the polymer, e.g. the acidic β-H, are negative modifiers (or moderators). The propagation by both types of modified ester probably involves a 6-centred cyclic transition state. The same applies to a third type of φ-cat propagation, in which a polymeric ester reacts with a D-A complex formed from M + I₂. The formation of unwanted non-living polymers of high DP and/or broad DPD, together with those resulting from living polymerisations, happens because the metal halides used in these reactions not only form the φ-cat propagating centres, but also react with ionogenic impurities to initiate concurrent, short-lived cationic polymerisations. These parasitic reactions can be suppressed by an excess of base, e.g. an ester or halide ions, which scavenge protons and carbenium ions. This phenomenon indicates that, whatever else is propagating the living polymerisations, it is unlikely to be carbenium ions. The kinetic equations for the living polymerisations, developed here, make it possible to calculate propagation rate-constants from rate and DP data; such calculations are shown for a variety of published results.     

Enthalpie der Polymerisation von 7-Oxabicyclo[2.2.1]-heptan sowie exo- und endo-2-Methyl-7-oxabicyclo-[2.2.1]-heptan

Ohne Zusammenfassung     

Die Korrektur der Sekund?rfluorescenz in der R?ntgenspektrometrie

Zusammenfassung Die unmittelbar aus der Theorie abgeleitete Gl. (3) enthält keine explizite Korrektur der Sekundärfluorescenz, wie sie mit Gl. (4) in [3] vorgeschlagen wurde. Es war daher von Interesse, die Wirksamkeit der Gl. (4) im Vergleich mit Gl. (3) zu überprüfen, wobei der durchschnittliche Analysenfehler unbekannter Proben als Prüfkriterium diente.Die Anwendung auf theoretisch berechnete und praktisch gemessene Fe-Cr-Ni-Proben und auf Gemische von Metalloxiden ergab eine sehr geringe Überlegenheit von Gl. (4) über Gl. (3). Da erstere jedoch rechentechnisch komplexer aufgebaut ist, erscheint Gl. (3) als für die Praxis geeigneter.

---

The correction of secondary fluorescence in X-ray spectrometry

Summary The Eq. (3) was directly derived from the theory and does not contain the correction of secondary fluorescence proposed by the authors of [3] with their semi-empirical Eq. (4) Therefore, it was of interest to test the efficiency of Eq. (4) in comparison with Eq. (3), the average analysis error serving as test criterion.The application to theoretically calculated and empirically measured Fe-Cr-Ni-systems and to mixtures of metal oxides resulted in a very small superiority of Eq. (4) to Eq. (3). Since Eq. (4) is mathematically more complicated than Eq. (3) the latter seems to be more suitable for practice.

     

Developments in the theory of cationic polymerization-VII [1a,b]: Theoretical attempts at improving initiators for cationic polymerization of alkenes

The initiation of the cationic polymerisation of alkenes is examined in detail by means of simple thermodynamic concepts. From a consideration of the kinetic requirements it is shown that the ideal initiator will yield a stable, singly charged anion and a cation with a high reactivity towards the monomer by simple, well defined reactions. It must also be adequately soluble in the solvent of choice and for the experimental method to be used. The calculations are applied to carbocation salts as initiators and a method of predicting their relative solubilities is described. From established and predicted data for a variety of carbocation salts the position of their ion:molecule equilibria and their reactivity towards alkenes are examined by means of Born-Haber cycles. This treatment established the relative stabilities of a number of anions and the reason for dityl, but not trityl salts initiating the polymerisation of isobutene.The outcome of our treatment is a general method for selecting theoretically promising new initiators.     

Tl-based superconducting films prepared by aerosol spray deposition and thallinated in an open system

Superconducting Tl-based films were prepared on a LaAlO₃ single crystal substrate. Spray pyrolysis of Ba, Ca and Cu nitrate solutions was used for deposition of the precursor films. They were subsequently ex-situ thallinated in flowing oxygen (open system). While the superconducting Tl-2212 phase formed at an annealing temperature of 880°C, thallination at 900°C led to the formation of a Tl-2223 /Tl-2212 mixture. The amount of Tl-2223 increased with prolonged thallination, whereas the Tl-2212 phase progressively disappeared. Films prepared in such a manner were c-oriented and contained only low amounts of non-superconducting impurities. The resulting samples were characterized by XRD and SEM and their T _C values were determined by resistive four-point measurements. They showed critical temperatures in the range of T _ON = 125–135 K, T _C0 = 91–93 K. Differences between the composition and properties of the obtained films and those thallinated in closed systems under 50 kPa of oxygen are discussed in this paper. Results show that the Tl-2212 → Tl-2223 transformation proceeds at a slower rate under flowing oxygen than in a closed system. Presented at 5-th International Conference Solid State Surfaces and Interfaces, November 19–24, 2006, Smolenice Castle, Slovakia.