Colloidal glasses

Colloidal suspensions are important model systems for the study of phase transitions. The glass transition, especially, can be followed more directly with colloidal systems and compared to theoretical predictions. At high volume fractions of the colloidal particles the density fluctuations are partially frozen in. This can be identified by the typical plateau values in the time correlation function. If one compares the experimental results with the mode coupling theory, a very good agreement can be obtained. Currently, some new experimental results concerning the dynamical heterogeneity in colloidal systems are under discussion and will certainly initiate new theoretical developments.     

Thermal behaviour and excess entropy of bioactive glasses and Zn-doped glasses

Bioactive glasses prepared in SiO₂–CaO–Na₂O and P₂O₅ system are used as biomaterials in orthopaedic and maxillofacial surgery. Zn presents high physiological interest. It enhances physiological effects of implanted biomaterials. In this work, the thermal characteristics (T _g, T _c and T _f) of pure bioactive glass elaborated with different amounts of CaO, Na₂O in pure glass and with different amounts of introduced Zn in glass (ranging from 0.1 to 10 in mass%), were studied. The excess entropy was calculated for different compounds. Glasses were prepared by the melting process. The thermal behaviour of obtained bioactive glasses was determined using differential thermal analysis. Therefore, the glass transition (T _g), the crystallization (T _c) and the melting temperatures (T _f) were revealed. Moreover, according to Dietzel formula, the thermal stability (TS) of the studied bioactive glasses has been calculated. The first results concerning the impact of different oxides, revealed a decrease of the TS, T _g, T _c and T _f when the SiO₂/CaO increases and revealed an increase of these thermal characteristics when the SiO₂/Na₂O and CaO/Na₂O ratios increase. Introducing Zn into the bioactive glasses induces a decrease of T _f and an increase of TS. Contrary to crystals, prepared glasses have entropy different to zero at T = 0 K and vary versus T _f. The excess entropy of pure glasses and Zn-doped glasses were calculated. The significant variations were registered.     

Lattice model of glasses

Glass-forming liquids have been extensively studied in recent decades, but there is still no theory that fully describes these systems, and the diversity of treatments is in itself a barrier to understanding. Here we introduce a new simple model that (possessing both liquid-crystal and glass transition) unifies different approaches, producing most of the phenomena associated with real glasses, without loss of the simplicity that theorists require. Within the model we calculate energy relaxation, nonexponential slowing phenomena, the Kauzmann temperature, and other classical signatures. Moreover, the model reproduces a subdiffusive exponent observed in experiments of dense systems. The simplicity of the model allows us to identify the microscopic origin of glassification, leaving open the possibility for theorists to make further progress.     

DTA of water-salt glasses

DTA investigations of glasses in the system Al₂(SO₄)₃-H₂O/55–65 mass% Al₂(SO₄)₃ revealed effects at the glass transformation (T _g), crystallization and melting temperature. During storage at ambient temperature,T _g increased, probably due to an annealing process; the increase was the least at the stability maximum for the glasses, at 61–63% Al₂(SO₄)₃.

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Zusammenfassung Gläser des Systems Al₂(SO₄)₃-H₂O mit 55–65 Masse-% Al₂(SO₄)₃ zeigen bei der DTA Effekte bei den Temperaturen von Glasumwandlung (T _g), Kristallisation und Schmelzen. Aufbewahren bei Raumtemperatur führt zum Ansteigen vonT _g, wahrscheinlich in Folge eines Temperprozesses; dieser Anstieg ist am geringsten bei der Zusammensetzung mit 61–63% Al₂(SO₄)₃, d. h. am Stabilitätsmaximum der Gläser.

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- 55–65 . % , _g, . _g, (61–63 .% ).

     

Na-Gd phosphate glasses

Summary The present paper deals with preparation, thermal properties and radioluminescence of Ce-doped Na-Gd phosphate glasses. Thanks to their high radioluminescence intensity, three times greater than that of BGO, these glasses are promising materials for the detection of neutrons, - and X-rays. The glasses with a Gd concentration up to 89 mol% were prepared by a rapid quenching technique in air. Their thermal properties were determined using DSC and temperatures of glass transition were measured in addition using TMA. Temperatures of glass transition, crystallization and melting depend on Gd concentration and they follow the liquidus line in a phase diagram of binary system in which two eutectics and a congruently melting compound exist. High glass-forming ability of this glass system was found evidenced. The glasses containing at least 30 mol% Gd were moisture resistant.     

Crystallization of silico-phosphate glasses

Silico-phosphate glasses of Ca_3/2PO₄-SiO₂ and NaCaPO₄-SiO₂ systems have been the topic of our studies. Microscopic and EDX investigations which have been carried out have shown that liquation occurs only in the case of glasses belonging to the NaCaPO₄-SiO₂ system. Additionally, it has been found that there are significant differences in the chemical compositions of the matrix and the inclusions. Based on the spectroscopic investigations it has been shown that the glasses of both series are characterized by complex domain composition and the structure of domains is close to that of the corresponding crystalline phase. Interpretation of the DTA results has been based on the knowledge of the texture and the structure of the materials studied. It has been found that liquation of the glasses is a multi-step process in which the matrix and the inclusions crystallize separately. Multi-step crystallization of the glasses belonging to the NaCaPO₄-SiO₂ system has been confirmed by the high temperature XRD investigations.     

Bismuth-containing fluoride glasses

The example of bismuth-containing glasses based on InF₃ and ZrF₄ is used to consider the effect of bismuth on glass formation, properties, and structure and the appearance of broadband luminescence at low wavelengths in the IR spectra of fluorozirconate glasses.     

Crystallization of iron phosphate glasses

Differential thermal analysis (DTA), thermogravimetric analysis (TG), X-ray diffraction and Mössbauer spectroscopy were employed in the investigation of crystalline products of FeO_x-P₂O₅ glasses generated by various heat treatments. In glasses with a high value of =Fe²⁺/(Fe²⁺ + Fe³⁺), absorption of oxygen occurs in a broad temperature range identified by TG. Depending on the value of , two exotherms appear in the DTA curves, the low-temperature one corresponding to crystallization of the Fe₃(PO₄)₂ type regions, and the high-temperature one being related to various phases with dominating FePO₄. Each exotherm has its own transformation region, identical in absolute value. The Mössbauer spectra of glasses which underwent thermal treatment at higher temperatures exhibit some indication of phases of the types Fe₃(PO₄)₂ · xH₂O and FePO₄ · xH₂O.

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Zusammenfassung Differentialthermoanalyse (DTA), thermogravimetrische Analyse (TG), Röntgendiffraktion und Mössbauerspektroskopie wurden bei der Untersuchung (von kristalliner Produkte) durch verschiedene thermische Behandlungen hergestellten FeO_x-P₂O₅ Gläsern eingesetzt. In den Glasarten mit einem hohen Wert von =Fe²⁺/(Fe²⁺ + Fe³⁺) erfolgt die Sauerstoffabsorption in einem durch TG [nachgewiesenen breiten Temperaturbereich. In Abhängigkeit von dem -Wert erscheinen zwei Exothermen in den DTA-Kurven, von denen der bei niedriger Temperatur die Kristallisation des Fe₃(PO₄)₂ entspricht, und jener bei höheren Temperaturen in verschiedenen Phasen dem FePO₄. Jede Exotherme hat ihr eigenes Umwandlungsgebiet, das in absolutem Wert identisch ist. Die Mössbauer-Spektren der Gläser welche einer Wärmebehandlung bei höheren Temperaturen unterzogen worden sind, weisen einige Indikationsphasen der Typen Fe₃(PO₄)₂ · xH₂O und FePO₄ · xH₂O auf.

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, , FeO_x-P₂O₅, . , =Fe²⁺/(Fe²⁺+Fe³⁺) . , - : Fe₃(PO₄)₂, – FePO₄. , . , , Fe₃(PO₄)₂ · ₂ FePO₄ · ₂.

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The authors thank Mrs. Simonyiová for assistance in the experimental part of these investigations, and Dr. Horváth and Dr. Vondrovic for performing the X-ray and TG analyses.     

Thermal stability of fluorochloroindate glasses

New fluorindate glass compositions have been investigated in order to improve optical transmission as well as thermal properties. Chloride inclusion extends transmission of a fluoride matrix to longer wavelength in infrared region. In the present work thermal parameters of an IZnBS composition, based on InF₃, ZnF₂, BaF₂ and SrF₂, with various amounts of alkaline chlorides were investigated by differential scanning calorimetry. The chloride presence decreased all characteristic temperatures and increased both thermal stability and glass forming ability up to 10% of MCl content, where M=Li, Na, K and Rb. The presence of NaCl promoted glass phase separation. For samples containing same concentration of NaCl, this effect is accentuated for increasing the contents of SrF₂. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photoluminescence of terbium-containing sol-gel glasses

Glasses of the following composition: T glass SiO₂, TL glass SiO₂/ligand, T-Tb glass SiO₂/terbium complex were obtained by the method of acid catalyzed hydrolytic homo- and cocondensation of tetraethoxysilane (the source of SiO₂) with 3-(3-triethoxysilylpropyl)pentane-2,4-dione (the ligand) and terbium(III) tris[3-3-triethoxysilylpropyl)pentane-2,4-dionate] (terbium complex). The study of the fluorescence spectra showed that the cationic luminescence is achieved by the excitation of terbium-containing glasses at the absorption band of the ligand (λ_exc 275–280 nm). Energy transfer from the matrix (SiO₂) to the ligand and cation with the following emission of Tb³⁺ is possible on exciting the T-Tb glass by the light with the wavelength 360 nm. However, this method of generating terbium cation emission seems to be less efficient.     

Crystallization of niobium germanosilicate glasses

Niobium germanosilicate glasses are potential candidates for the fabrication of transparent glass ceramics with interesting non-linear optical properties. A series of glasses in the (Ge,Si)O₂-Nb₂O₅-K₂O system were prepared by melting and casting and their characteristic temperatures were determined by differential thermal analysis. Progressive replacement of GeO₂ by SiO₂ improved the thermal stability of the glasses. Depending on the composition and the crystallization heat-treatment, different nanocrystalline phases—KNbSi₂O₇, K₃Nb₃Si₂O₁₃ and K_3.8Nb₅Ge₃O_20.4 could be obtained. The identification and characterization of these phases were performed by X-ray diffraction and Raman spectroscopy.The 40 GeO₂-10 SiO₂-25 Nb₂O₅-25 K₂O (mol%) composition presented the higher ability for volume crystallization and its nucleation temperature was determined by the Marotta's method. An activation energy for crystal growth of ∼529 kJ/mol and a nucleation rate of 9.7×10¹⁸ m⁻³ s⁻¹ was obtained, for this composition. Transparent glass ceramics with a crystalline volume fraction of ∼57% were obtained after a 2 h heat-treatment at the nucleation temperature, with crystallite sizes of ∼20 nm as determined by transmission electron microscopy.     

Crystallization kinetics of metallic glasses

In this study, the temperature-heating rate diagram of the main crystallization process of two metallic glasses, Fe₇₄Ni_3.5Mo₃B₁₆Si_3.5 and Fe₄₁Ni₃₈Mo₃B_18, was obtained from one experimental differential scanning calorimetry (DSC) scan and the knowledge of their activation energy as determined by an isoconversional method. A good concordance was observed between the diagram curves obtained by calculation (isoconversional approach) and the experimental data, which verifies the reliability of the method and the validity of the kinetic approach in these alloys.     

Thermal properties of oxide glasses

A criterion based on the length of induction period of crystallization was used to evaluate the thermal stability of M₂O·SiO₂ (M?=?Li, Na) glasses against crystallization. It was founded out that the stability of studied glasses against crystallization is Li₂O·SiO₂?<?Na₂O·SiO₂. The results coincide with the order determined by stability criteria based on temperatures and the values of activation energy. A criterion based on the length of induction period enables to discriminate among the thermal stabilities of the silicate glass systems.     

Thermal properties of oxide glasses

Glasses with the composition Li₂O·2SiO₂·nTiO₂ and Li₂O·2SiO₂·nZrO₂, where n=0, 0.03, 0.062, 0.1, were prepared and the onset and peak temperatures have been determined by DTA. From these characteristic temperatures, the kinetic parameters describing the nucleation and crystal growth have been obtained by isoconversional methods. The kinetic parameters have been used for the calculation of nucleation and crystal growth times for individual glasses so determining the order of glass stability at reheating. The stability of glasses increases with the content of TiO₂ or ZrO₂ where the increase is higher for ZrO₂. Within the concentration range under study, the increase of both times with the metal oxide concentration is quadratic. It has been discussed that the crystallization kinetics does not obey the Arrhenius law and, therefore, when using the evaluation methods based on this law, the results should not be extrapolated outside the temperature range of the measurements.     

Structural recovery of polymer glasses

The structural recovery of polystyrene glasses subjected to thermal cycles in the glass transition region is studied by differential scanning calorimetry and by dilatometry. Three different types of behaviour are observed in respect of the peak in the specific heat capacity or thermal expansion coefficient on heating, and are found to be in full agreement with the predictions of the KAHR model. First, for well-stabilized glasses, the dependence of the (main) peak temperature on heating rate and annealing time yields consistent values for the structure parameter x by means of the peak-shift method. Second, for poorly-stabilized glasses, the (upper) peak temperatures shift quite differently, but again consistently with theory. Third, for glasses intermediate between poorly-and well-stabilized conditions, both main and upper peaks can be observed simultaneously in the same endotherm.     

New silver tellurite glasses

The glass forming ranges in the TeO₂-AgO_0.5 and TeO₂-AgO_0.5-AgI systems have been assessed for two cooling rates; results based on heat capacity and electrical conductivity measurements as well as derived from infrared spectroscopy are presented. The ternary glasses are excellent conductors due to the presence of Ag⁺ ions mainly supplied by AgI. The decrease in C _P with increasing AgI-content suggests the presence of AgI microdomains which are finely distributed and do not contribute to the configurational heat capacity change. Fast motion of Ag⁺ ions (low activation energy) occurs in the glassy matrix which shows slow structural relaxation (high activation barrier).This research is performed within the frame of a P.I.R.M.A.T. action of the CNRS.     

Dielectric studies of As-Se glasses

Dielectric constants and loss tangents of As-Se glasses have been measured between 300 K and the respective glass transition temperatures and between 1 kHz and 20 kHz. The variation of dielectric constants has been interpreted in terms of both heteropolarity of bonds and average bond energies employing a chemically ordered network model. Various contributions to total molar polarizations have been estimated. Rapid rise of loss tangent in the vicinity of glass transitions has been interpreted in terms of rapid increase; of d.c. conductivity. Communication No. 108 from Solid State and Structural Chemistry Unit.