Crystallization of glasses in the system SiO2-Li2O-TiO2-Al2O3, investigated in situ at high temperature by XRD and DTA methods

The processes of crystallization of fibres (diameter 10–15m) and coarse powders (grain size 500–1000m) with four compositions in the system SiO₂-Li₂O-TiO₂-Al₂O₃ were studied by conventional and in situ high-temperature XRD, DTA, SEM and optical microscopy. Activation energies of crystallization and morphological indices were deduced from the kinetic curves obtained by recording the high-temperature XRD peak intensity as a function of time. The glass-ceramic fibres drawn from compositions which exhibit glass-inglass phase separation show prevailing not-oriented bulk crystallization, whereas prevailing surface crystallization was found for single-phase glass fibres. Homogeneously-dispersed crystallization was obtained on heating fibres of these compositions. The partially cocrystallized glass fibres of eutectic composition between Li-metasilicate and-spodumene gave rise to a very fine and homogeneously-dispersed sub-microstructure.

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Zusammenfassung Die Prozesse der Kristallisation von Fasern (Durchmesser 10–15m) und groben Pulvern (Korngröße 500–1000m) von SiO₂-LiO₂-TiO₂-Al₂O₃ in vier verschiedenen Zusammensetzungen wurden mittels herkömmlicher und in situ XRD, DTA, SEM und optischer Mikroskopie untersucht. Aktivierungsenergien der Kristallisation und morphologische Indices wurden aus den durch Registrierung der Intensität der Hochtemperatur-XRT-Peaks in Abhängigkeit von der Zeit erhaltenen kinetischen Kurven ermittelt. Die aus Gemischen einer die Glas-in-Glas-Phasentrennung bedingenden Zusammensetzung gezogenen Glas-Keramik-Fasern zeigen vorwiegend eine nicht-orientierte Bulk-Kristallisation, während vorherrschend Oberflächenkristallisation bei Einphasen-Glasfasern festgestellt wurde. Eine homogen verteilte Kristallisation wurde beim Erhitzen der Fasern dieser Zusammensetzungen erhalten. Bei partiell ko-kristallisierten Glasfasern einer eutektischen Mischung von Li-Metasilikat und (-Spodumen liegt eine sehr feine und homogen-disperse Submikrostruktur vor.

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( 10–15 ) ( 500–1000 ) SiO₂-Li₂O-iO₂AL₂O₃ , , . , - . , --, ë, - . - . — - - - .

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This work was supported by the Italian C. N. R., Progetto Finalizzato Chimica Fine e Secondaria — Sottoprogetto Metodologie.

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We are indebted to Dr. B. Locardi of Stazione Sperimentale del Vetro, Murano (Venice) for furnishing the initial glasses and fibres.

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We are also indebted to the G. Giacomello Institute (C. N. R., Rome) for furnishing the computer programme for the unit cell parameter refinement. Assistance by M. Pistolato and F. Venuda in the experimental work is gratefully acknowledged.     

The application of the ion-interaction model to multicomponent 1-1 type electrolytes in mixed solvents

The ion-interaction model has been applied to published measurements for the multicomponent electrolytes HCl+MCl (M means Na or K) in mixed solvents (urea + water or methanol + water). The Pitzer parameters _HCl ⁽⁰⁾ , _HCl ⁽¹⁾ , C _HCl , ( _NaCl ⁽⁰⁾ + _NaH), _NaCl ⁽¹⁾ , _NaHCl in the HCl+NaCl + urea + water system were obtained by the method of least squares. A temperature and molality dependent equation for _HCl in this system was obtained. In the HCl+MCl + methanol + water system, the mixing parameter _HCl was obtained in terms of the Harned Rule. The relationship between Pitzer parameters and the composition of the mixed solvent was discussed.on leave from Department of Chemistry, Liaoning University Shenyang, China.     

Kinetics of isotope exchange of dioxygen with a powdered high-temperature superconducting material Ba2YCu3O7?x

It has been found that the rate of isotope exchange in Ba₂YCu₃O_7–x–O₂ system is high. Its kinetics is exponential, first order with respect to dioxygen and the exchange is of the mixed first/third type.

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Ba₂YCu₃O_7–x–O₂ . -, -, - .

     

Thermal membrane potential across cation-exchange membranes for various halide solutions

Thermal membrane potential across cation-exchange membranes was measured for various halide solutions. Linear relationships between thermal membrane potential () and temperature difference (T) were observed, and the temperature coefficient of thermal membrane potential (/T) decreases with increase in the molality of the external solution as predicted by a theory reported. When counterions of membranes are hydrogen ions, the sign of the coefficients turns into minus near 0.01 mol/kg of the external solution for all membranes, although the coefficients are always positive for all the other forms. The dependence of the coefficients on the molality of the external solutions sometimes deviated slightly from the theory. This deviation is attributed to some change in water content of the membranes. A reproducible method of measuring water content of membranes was applied to discuss the change in the state of the membranes.     

Thermal properties of the clathrate compounds of binary cyanides

The stoichiometry of thermal decomposition of a particular group of clathrate compounds, of the type Cd(en)Pd(CN)₄·2G, with the same coordination component but different clathrate-enclosed components, was investigated. Irrespective of the experimental conditions, the liberation of the enclosed component G always proceeded in one step. Thermal investigation of the decomposition stoichiometry under identical experimental conditions showed that the temperature intervals in which the enclosed components are given off shift to lower temperatures in the order C₆H₅OH=> C₆H₆ > > C₄H₅N > C₄H₄S. Diffraction measurements indicated a considerable change in the structures of the original clathrate compounds after thermal release of the enclosed componentG.

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Zusammenfassung Die Stöchiometrie der thermischen Zersetzung einer bestimmten Gruppe von Klathratverbindungen vom Typ Cd(en)Pd(CN)₄·2G mit der gleichen Koordinationskomponente und verschiedenen Klathrat-Einschlußkomponenten wurde in dieser Studie untersucht. Ungeachtet der Versuchsbedingungen vollzieht sich die Freisetzung der eingeschlossenen KomponenteG stets in einer Stufe. Die unter gleichen Versuchsbedingungen durchgeführte thermische Untersuchung der Zersetzungsstöchiometrie zeigte, daß die Temperaturbereiche, in denen die eingeschlossenen Komponenten freigesetzt werden in der Reihenfolge C₆H₅OH > C₆H₆ > C₄H₅N > C₄H₄S in Richtung der niedrigeren Temperaturwerte verschoben werden. Die Diffraktionsmessungen weisen darauf hin, daß eine beträchtliche Änderung der Struktur der ursprünglichen Klathratverbindung nach thermischer Freisetzung der eingeschlossenen KomponenteG sichtbar wird.

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Résumé Dans cette étude on a établi la stoechiométrie de la décomposition thermique d'un groupe particulier de clathrates du type Cd(en)Pd(CN)₄·2G, avec le même composant de coordination et différents composants occlus. Indépendamment des conditions d'expériences, la libération du composant occlusG s'effectue toujours en une étape. L'étude thermique de la stoechiométrie de la décomposition effectuée dans les mêmes conditions d'expériences, a montré que les intervalles des températures dans lesquels les composants occlus sont libérés, se déplacent vers les températures plus faibles dans l'ordre suivant: C₆H₅OH > C₆H₆ > > C₄H₅N > C₄H₄S. Les mesures de diffraction indiquent qu'après la libération thermique du composant occlusG, une variation considérable de la structure initiale des clathrates originaux apparaît.

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Cd(en)Pd(CN)₄. 2G . , G . , , , ] ₆₅>₆₆>C₄H₆N>C₄H₄S. - , G.

     

IR and thermal studies on lithium oxomolybdenum(VI) oxalate

A new molybdenum(VI) complex, Li₂[Mo₂O₆(C₂O₄)] · 2 H₂O (LMO), was prepared and characterized by chemical analysis and IR spectral studies. Its thermal decomposition was studied by using TG and DTA techniques. LMO loses its two moles of water between 75 and 170° to give the anhydrous product, which decomposes in three stages between 240 and 380°. The first two stages occur in the temperature ranges 240–280° and 280–305°, to give intermediates with the tentative compositions Li₆[Mo₆O₁₉(C₂O₄)₂] and Li₆[Mo₆O₂₀(C₂O₄)], respectively. In the third stage, which extends up to 380°, Li₆[Mo₆O₂₀(C₂O₄)] decomposes to give the end-product, Li₂Mo₂O₇.

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Zusammenfassung Ein neuer Molybdän(VI)-Komplex der Formel Li₂[Mo₂O₆(C₂O₄)] · 2 H₂O (LMO) wurde dargestellt und durch chemische Analyse und IR-spektroskopisch charakterisiert. Die thermische Zersetzung dieses Komplexes wurde mittels TG und DTA untersucht. LMO verliert die zwei Wassermoleküle zwischen 75 und 170° unter Bildung des wasserfreien Produktes, das zwischen 240 und 380° in drei Stufen zersetzt wird. Die in den Temperaturbereich von 240–280° und 280–305° verlaufenden ersten zwei Reaktionsschritte ergeben Intermediäre der tentativen Zusammensetzung Li₆[Mo₆O₁₉(C₂O₄)₂] bzw. Li₆[Mo₆O₂₀(C₂O₄)]. In dem sich bis 380° erstreckenden dritten Reaktionsschritt wird Li₆[Mo₆O₂₀(C₂O₄)] unter Bildung des Endproduktes Li₂Mo₂O₇ zersetzt.

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Li₂[MO₂O₆(C₂O₄] · 2 ₂ . . - 70–170° , , 240–380°. 240–280° 280–305° - Li₆[Mo₆O₁₉(C₂O₄)₂] Li₆[Mo₆O₂₀(C₂O₄)]. - 380° Li₂Mo₂O₇.

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The authors are grateful to Prof. S. N. Tandon, Head of the Chemistry Department, for providing the research facilities.     

The preparation, characterization, and thermal behaviour of some lithium aluminum oxides: Li3AlO3 and Li5AlO4

The purpose of this research was to study the best conditions for the synthesis of the double oxides Li₅AlO₄ and Li₃AlO₃ in the solid state starting from the simple oxides, and to determine their heats of formation.Li₅AlO₄ was obtained from Li₂O₂ or Li₂O and -Al₂O₃ in a Li/Al molar ratio of 51, and was characterized by X-ray methods. Lithium orthoaluminate, Li₃AlO₃, was obtained from Li₂O₂ and -Al₂O₃ in a molar ratio 31. The postulated formula, Li₃AlO₃, was confirmed by chemical analysis.The temperature ranges in which the compounds are stable were established by the DTA method, and were found to be very limited for Li₃AlO₃ (400–430°) but greater for Li₅AlO₄ (440 — more than 600°).The heats of formation of Li₅AlO₄ and Li₃A10₃, also determined by means of the DTA method, were found to be –552.3 ± 0.8 kcal/mole and –416.8 ± 2 kcal/mole, respectively.

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Zusammenfassung Es wurden die Bildungsverhältnisse der doppelten Oxide Li₅AlO₄ und Li₃AlO₃ in fester Phase ausgehend von den einfachen Oxyden geprüft und ihre Bildungswärmen bestimmt. Li₅AlO₄ wurde aus Li₂O₂ oder Li₂O und -Al₂O₃ beim Molverhältnis von Li/Al 5 1 erhalten und röntgenographisch identifiziert. Das Orthoaluminat Li₃AlO₃ erhielt man beim Molverhältnis 3 1 von Li₂O₂ und -Al₂O₃. Die Zusammensetzung von Li₃AlO₃ wurde durch chemische Analyse nachgewiesen. Die DTA-Prüfung zeigte, daß Li₃AlO₃ nur im sehr kleinen Temperaturgebiet (400–430°), Li₅AlO₄ hingegen im weiteren Bereich zwischen 440–600° stabil ist. Die Bildungswärmen betrugen für Li₅AlO₄ –552.3±0.8, für Li₃AlO₃ –416.8±2 kcal/Mol.

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Résumé On a recherché les meilleures conditions pour réaliser la synthèse à l'état solide des oxydes doubles Li₅AlO₃ et Li₃AlO₃ en partant des oxydes individuels et pour déterminer leur chaleur de formation. Li₅AlO₄ a été obtenu en partant de Li₂O₂ (ou Li₂O) et de -Al₂O₃ dans le rapport molaire Li/Al=5/1; il a été caractérisé par étude aux rayons X. L'orthoaluminate de lithium, Li₃AlO₃, a été obtenu en partant de Li₂O₂ et de -Al₂O₃ mélangés dans le rapport 3/1. L'analyse chimique a confirmé la formule présumée Li₃AlO₃. L'ATD a permis de déterminer le domaine de stabilité thermique de ces composés: très restreint pour Li₃AlO₃ (400–430°), plus grand pour Li₅AlO₄ (440-plus de 600°), ainsi que leurs chaleurs de formation: –416.8±2 kcal/mole et –552.3±0.8 kcal/mole, respectivement.

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— Li₅AlO₄ Li₃AlO₃ . Li₅AlO₄ Li₂O₂ Li₂O -l₂₃ Li/Al 51, . , Li₃AlO₃, Li₂O₂ -l₂₃ 31. . , , , Li₃AlO₃ (400—430 °?) Li₅AlO₄ (440- 600 °?). Li₅AlO₄ Li₃AlO₃ 552.3 ± 0.8 416.8 ± 2 /, .

     

Interdependence between the dispersion of a solid decomposition product and its formation rate

In an analysis of the possible mechanism and kinetics of a thermal decomposition reaction with the formation of a solid product, the following features were considered: the collective rearrangement character of the transformation; the formation of a product with a different non-equilibrium defectiveness and free energy; the free energy relationship in the series of processes leading to products with different dispersions; the formation of intermediate structures; and the spinodal character of their decomposition. Relationships are presented between the rate of solid product formation, the process temperature, and the surface area and size of the particles.

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Zusammenfassung In einer Analyse des möglichen Mechanismus und der Kinetik einer unter Bildung eines festen Produktes verlaufenden thermischen Zersetzungsreaktion werden folgende Besonderheiten erörtert: der kollektive Umordnungscharakter der Umwandlung, die Bildung eines Produktes mit unterschiedlicher Nichtgleichgewichts-Gitterstörung und unterschiedlicher freier Energie, die Beziehung der freien Energie in zu Produkten unterschiedlicher DispersitÄt führenden Prozessen, die Bildung intermediÄrer Strukturen und der spinodalen Charakter ihrer Zersetzung. Beziehungen zwischen der Bildungsgeschwindigkeit des festen Produktes, der Temperatur des Prozesses und der OberflÄchengrö\e und Grö\e der Partikel werden angegeben.

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: ; ; , ; . , , .

     

Structural Study of Methyl Acrylate in a Nematic Liquid Crystal by 1H NMR with 13C Satellites

The molecular structure and conformation of methyl acrylate have been determined by analyzing the ¹H NMR spectrum with ¹³C satellites of the molecules dissolved in the liquid crystal ZLI 1132. In the structural analysis, direct coupling constants have been corrected for molecular vibrations and converted to those in the r structure. The correlation between reorientational motion and internal rotation has been taken into account by using the mean external potential described in terms of bond interaction tensors. The potential for the internal rotation about the bond has been represented as V() = (V ₁/2) (1 – cos ) + (V ₂/2)(1 – cos 2), where is zero for the s-cis configuration, and the determined values of V ₁ and V ₂ are 2.7(1) and 8.4(13) kJ mol^–1, respectively. The relative abundance of the s-cis conformer, 76(1)%, agrees with a gas-phase value, 67(11)%, determined by electron diffraction within experimental errors.     

Quantum chemical calculations on the catalytic dehydration of 2-propanol

The charge on atoms and the bond strengths in CH₃CHOHCH₃, CH₃ CH₃, CH₃CHOHCH ₂ ^– , CH₃ CH ₂ ^– and CH₃ CH₃...O(H)CH(CH₃)₂ have been calculated by the CNDO/2 method. The results are in agreement with published experimental observations on catalytic dehydration. A modified mechanism of anti-elimination on solid catalysts is proposed.

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CH₃CHOHCH₃, CH₃ CH₃, CH₃CHOHCH ₂ ^– , CH₃ CH ₂ ^– CH₃ CH₃...O(H)CH(CH₃)₂, CNDO/2. . .

     

Generalized quantum mechanical two-centre problems. IV. On the accuracy of simple LCAO approximations for H 2 + states and the Eckart criterion

If x denotes an exact solution of the quantum mechanical two centre Coulomb problem, we optimize a normalized LCAO approximation by making the overlap S = (x¦) a maximum. In this context we study how a weight factor (r _a r _b )^–1 in the definition of the inner product changes the approximation and the expectation value of electronic energy. Finally we compare the lower bound given by the Eckart criterion with the exact overlap. Results are reported for H ₂ ⁺ states 1s_g and 2p_u.Dedicated to Professor Hermann Hartmann on occasion of his 70th birthday on May 4th, 1984     

ESR studies of photoinduced centers on weakly reduced anatase

The paramagnetic centers of reduced anatase (g₁=1.999, g₂=1.984, g₃=1.979) and (g₁=2.046, g₂=2.008), assigned to the stabilization of photoinduced holes near anion vacancies, have been studied. For the reduced oxide a new type of O₂ photoadsorption at h >1 eV due to the generation of only electron surface centers has been discovered.

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, : (g₁=1,999, g₂=1,984, g₃=1,979) (g₁=2,046, g₂=2,008), . O₂ h>1 , .

     

Silane-silanol condensation catalyzed by organotin compounds

The rate of condensation of H–SiR₂–(OSiR₂)_n–H (n=2, 3 or 4 and R=CH₃) and HO-SiRR–(OSiRR)₃–OH (R=C₆H₅) in xylene at 100 °C, catalyzed by dibutyltin diacetate, dilaurate and di-(2-ethyl)hexanoate, has been studied by infrared spectroscopy. The kinetic law is first order in each reactant and the reaction is accompanied by autocondensation of the silanol. The mecha nism involves complexation of the silanol with the catalyst, followed by the attack of a silane or silanol molecule.

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H–SiR₂–(OSiR₂)_n–H ( n=2,3 4 R=CH₃) HO–SiRR–(OSiRR)₃–OH ( R=C₆H₅) 100° C, , -(2-) , . . .

     

Influence of sodium salt addition on the morphological and catalytic properties (ethane hydrogenolysis) of aluminasupported cobalt

NaNO₃ addition to Co/Al₂O₃ results in an increase in the reducibility, in a smaller dispersity of the metal and in a decrease in the ethane hydrogenolysis rate due to a decorative effect.

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NaNO₃ Co/Al₂O₃ ,

     

Electronic state of the active site of A V?P?O/SiO2 catalyst in oxidation of chlorine-containing hydrocarbons

V⁴⁺ ions in slightly associated states are supposed to play the role of V–P–O/SiO₂ active sites. V⁴⁺ ions are reduced to V³⁺ during deactivation of the V–P–O/SiO₂ catalyst.

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V–P–O/SiO₂ V⁴⁺ . V–P–O/SiO₂ V⁴⁺ V³⁺.

     

Augmentation of the Generalised n×n Eigenvalue Equation to a Generalised (n+1)×(n+1) Eigenvalue Equation

Generalised n×n eigenvalue equation B| _i = _i S ^b | _i (i=1,...,n) where B and S ^b are n×n Hermitian matrices while S ^b is in addition positive definite is considered. This equation is augmented to a generalised (n+1)(n+1) eigenvalue equation H| _k = _k S| _k (k=1,...,n+1) where Hermitian matrices H and S represent matrices B and S ^b , respectively, augmented by one additional row and one additional column. It is shown how the eigenvalues _k and the eigenvectors | _k of the augmented eigenvalue equation can be expressed in terms of the eigenvalues _i and the eigenvectors | _i of the original eigenvalue equation. Operation count to obtain by this method all augmented eigenvalues and eigenvectors is of the order O(n ²). Unless matrices involved are of some special kind such as sparse matrices or alike, this operation count is one order of magnitude smaller than operation count required by other presently known methods. In many practical cases operation count to obtain a single selected eigenvalue and/or eigenvector by this method is of the order O(n). In the case of the generalised eigenvalue equation, all other methods usually require again O(n ³) operations, even if only a single eigenvalue and/or eigenvector is required. Thus in many cases of interest operation count to obtain a selected eigenvalue and/or eigenvector by this method is two orders of magnitude smaller than operation count required by other methods.     

Kinetics and selectivity of the oxidation of alkanes in Cl2?H2O and Cl2?Hg2+?H2O solutions

Relative oxidation rate constants of several normal, iso-, cyclo-and methylcycloalkanes have been measured in Cl₂–H₂O and Cl₂–Hg²⁺–H₂O solutions at 343K. The selectivity of C–H bond dissociation in both systems is the same. HOCl is assumed to be the active species for the two systems.

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Cl₂–H₂O Cl₂–Hg²⁺–H₂O 343 , -,- ; C–H . , HOCl.

     

The consistency of repeated H2?O2 titration cycles on supported Pt catalysts

H₂ and O₂ uptakes at 296 K on a Pt/Al₂O₃ catalyst remained constant during repeated H₂–O₂ titration cycles conducted in a high vacuum adsorption system. Previously reported variations in these uptakes are attributed to adsorption of contaminants during desorption conditions. Also, it was found that nearly 40% of the H₂ adsorbed could be removed by degassing at 296 K for 1 hr.

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H₂ O₂ 296 Pt/Al/₂O₃ H₂–O₂, . . , 40% H₂ 1 296 .

     

Activity coefficients for ternary systems: VI. The system HCl + MgCl2 + H2O at different temperatures; application of Pitzer's equations

Electromotive-force measurements have been made on HCl–MgCl₂–H₂O mixtures at 5, 15, 25, 35 and 45°C at eleven different ionic strengths from 0.1–5.0 mol-kg ^–1 . The results are interpreted in terms of the simple Harned's equations, as well as the more complicated Pitzer ion-component treatment of multicomponent electrolyte mixtures. Activity coefficients for HCl in the salt mixtures obey Harned's rule up to and including I=5.0. For the salt in the acid mixtures, Harned's rule holds true up to and including I=0.5. The contribution of higher-order electrostatic terms (E and E') in the Pitzer equations is important for accurate evaluations of unlike cation-cation interactions (_H,Mg), and cation-anion-cation interactions (_H,Mg,Cl). The values of^S_H,Mg and _H,Mg,Cl (determined with E and E'), _H,Mg and _H,Mg,Cl (determined without E and E'), as well as the trace activity coefficients of HCl, _tr ^A , in solutions of MgCl₂ (where ionic strength fraction of the salt,y _B = 1) at all the experimental temperatures and ionic strengths, are reported. Results of this study are compared with those for similar systems. At I=0.1 and 25°C, the results of the Brönsted-Guggenheim specific interaction theory are discussed briefly.