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.     

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.     

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.     

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.     

Radiation stability study on cerium loaded iron phosphate glasses by ion irradiation method

Journal of Radioanalytical and Nuclear Chemistry - To study radiation stability of iron phosphate glasses, cerium is used as surrogate of actinides. Cerium doped iron phosphate glasses have been...     

Crystallization kinetics of yttrium aluminate glasses

Journal of Thermal Analysis and Calorimetry - Yttrium aluminate glasses with eutectic AY-E and near-eutectic composition AY-NE were prepared in the form of glass microspheres. Their basic...     

Crystallization in glasses monitored by thermomechanical analysis

Thermomechanical analysis (TMA) can be used as a sensitive tool to follow crystallization behavior in non-crystalline materials. Newly developed method is based on slowing down of sample deformation caused by viscous flow above the glass transition due to macroscopic crystal growth. It is shown that a typical TMA sigmoidal curve reasonably well corresponds to direct measurement of crystal growth kinetics by means of optical microscopy. The method has been used to study crystallization kinetics in Ge₃₈S₆₂ glass. The TMA measurement is able to detect earlier stages of crystallization than obtained by differential scanning calorimetry measurement. The activation energy obtained from the shift of extrapolated end of TMA curve with heating rate (E = 263 ± 7 kJ mol^?1) is similar to the activation energy of ??-GeS₂ crystal growth in Ge₃₈S₆₂ glass (E _G = 247 ± 23 kJ mol^?1) obtained from direct optical microscopy measurements.     

Crystallization kinetic of fluoro- zirconate glasses by non-isothermal analysis

Fluoride glasses have been extensively studied due to their high transparency in the infrared wavelength. The crystallization kinetics of these systems has been studied using DTA and DSC techniques. Most of the experimental data is frequently investigated in terms of the Johnson-Mehl-Avrami (JMA) model in order to obtain kinetic parameters. In this work, DSC technique has been used to study the crystallization of fluorozirconate glass under non-isothermal conditions. It was found that JMA model was not fit to be applied directly to these systems, therefore, the method proposed by Málek has been applied and the Šesták-Berggren (SB) model seems to be adequate to describe the crystallization process. This revised version was published online in July 2006 with corrections to the Cover Date.     

Simultaneous determination of aluminium and iron in glasses,phosphate rocks and cement using first- and second-derivative spectrophotometry

First- and second-derivative spectrophoto-metric methods for the simultaneous determination of aluminium and iron in their mixtures are described. The methods are based on the colored complexes formed by aluminium and iron with hematoxylin in the presence of cetyltrimethylammonium bromide as a surfactant. The zero-crossing method has been utilized to measure the first- and second-derivative value of the derivative spectrum. Aluminium (0.05-1 microg ml(-1)) could be determined in the presence of iron (0.09-1.6 microg ml(-1)) and vice versa. The detection limits of aluminium and iron are 0.01 and 0.09 microg ml(-1), respectively in the first-derivative mode and 0.014 and 0.1 microg ml(-1) in the second-derivative mode. The proposed method has been applied to the simultaneous determination of aluminium and iron in glasses, phosphate rocks, cement and magnesite alloy.     

Simultaneous determination of aluminium and iron in glasses,phosphate rocks and cement using first- and second-derivative spectrophotometry

First- and second-derivative spectrophoto-metric methods for the simultaneous determination of aluminium and iron in their mixtures are described. The methods are based on the colored complexes formed by aluminium and iron with hematoxylin in the presence of cetyltrimethylammonium bromide as a surfactant. The zero-crossing method has been utilized to measure the first- and second-derivative value of the derivative spectrum. Aluminium (0.05–1 gml^-1) could be determined in the presence of iron (0.09–1.6 gml^-1) and vice versa. The detection limits of aluminium and iron are 0.01 and 0.09 gml^-1, respectively in the first-derivative mode and 0.014 and 0.1 gml^-1 in the second-derivative mode. The proposed method has been applied to the simultaneous determination of aluminium and iron in glasses, phosphate rocks, cement and magnesite alloy.     

Thermal conductivity of lithium iron silicate glasses

The thermal conductivity of some silicate glasses in the system Li₂O-SiO₂ (65.2 mol% SiO₂) modified by Fe₂O₃/SiO₂ substitution (up to 8 mol%) was determined at 30°C by the steady-state method.The conductivity values decrease progressively with increasing Fe₂O₃ content in the glass composition. The experimental thermal conductivities and those calculated from factors representing the contributions of individual oxides (on a wt.% basis) agree fairly well. The effect of the iron oxide content and the change in valency and coordination of iron ions on the conductivity, refractive index and density of the glasses are discussed.     

Shielding properties of lead and barium phosphate glasses

P₂O₅–PbO–BaO glass system has been investigated to see its potential use as radiation shielding material by studying the effect of the content of each of its composing oxides. The results show that this glass system is suitable as radiation shielding material and has some advantages comparing to the existing commonly used radiation protection glasses.     

Studies on glass transition of lithium-iron phosphate glasses

Amorphous analogs of lithium-iron phosphates (LFP) were prepared by standard press-quenching method and their thermal stabilities as well as structural properties were studied for the first time. Glass transition temperature T _g determined by DTA method was observed at the temperature range 492–523°C, depending on the glass composition. The maxima of crystallization peaks were observed in the 555–579°C range. In products obtained after heating up to 700°C the XRD patterns have revealed the presence of: LiFePO₄ (triphylite), FePO₄ (heterosite), α-FePO₄ (quartz like structure) and Li₃Fe₂(PO₄)₃ (Nasicon like structure) phases.     

Crystallization and second harmonic generation in potassium-sodium niobiosilicate glasses

Transparent glasses having molar composition (23−x)K₂O·xNa₂O·27Nb₂O₅·50SiO₂ (x=0, 5, 10, 15 and 23) have been synthesized by the melt-quenching technique and their devitrification behaviour has been investigated by DTA and XRD. Depending on the composition, the glasses showed a glass transition temperature in the range 660-680 °C and devitrified in several steps. XRD measurements showed that the replacement of K₂O by Na₂O strongly affects the crystallization behaviour. Particularly, in the glasses with only potassium or low sodium content the first devitrification step is related to the crystallization of an unidentified phase, while in the glass containing only sodium, NaNbO₃ crystallizes. For an intermediate sodium content (x=10 and 15) a potassium sodium niobate crystalline phase, belonging to the tungsten-bronze family, is formed by bulk nucleation. This system looks promising to produce active nanostructured glasses as the tungsten-bronze type crystals have ferroelectric, electro-optical and non-linear optical properties. Preliminary measurements evidenced SHG activity in the crystallized glasses containing this phase.     

Molybdenum phosphate glasses modified by Cu2O

Glasses in the system Cu₂O-P₂O₅-MoO₃ were prepared. The transformation temperature of glasses,T _g, ranges from 122 to 436°C and depends strongly on their composition.T _g values are mainly controlled by the oxygen-packing density. The structure of glasses was studied by infrared spectroscopy. The presence of various phosphate and molybdate units, from discrete monomeric to those with three-dimensional networks, has been observed. The electrical conductivity of prepared glasses depends mainly on Cu₂O content and varies from 10^–14–1cm^–1, for the glass free of Cu₂O, to 10^–6–1cm^–1, for glasses with compositions near of the binary Cu₂O-MoO₃ composition.This work was supported by grant No 1/731/94 of the Slovak Grant Agency for Science. The paper was presented on TERMANAL'94, XIII. International Conference on Thermal Analysis, October 1994, High Tatras, Slovakia.     

Thermochemistry of phosphate glasses for immobilization of dangerous waste

Vitrification is currently considered to be an effective method for immobilization of radioactive waste. It is based on the enclosing of harmful elements in the structure of the glass. This work presents the results of studies on the thermal properties of glasses from P₂O₅–Al₂O₃–Na₂O and P₂O₅–Al₂O₃–Fe₂O₃–Na₂O systems for rendering nuclear waste in the form of salts such as sulfates, halides, and phosphates with high sodium content. These substances are not accepted by borosilicate glass, commonly used up to now for nuclear waste immobilization. Formation of sinters of glass-waste mixtures was selected as the method for immobilization, and the thermal chemistry of this process was studied. CaCl₂ was used as the model chloride waste substance. The process of immobilization consists of its sintering with Na, Al, Fe-phosphate glasses containing more than 50 wt% P₂O₅ as the amorphous matrix. Thermal analysis showed that all glasses exhibit an ability for crystallization, with that the intensiveness of this process is determined by the chemical composition of these glasses. The addition of Fe₂O₃ to the glass intensified crystallization process. Leaching of components of sinters tests established that glass containing Fe₂O₃ in its composition most effectively binds waste in comparison to Al₂O₃ containing phosphate glass. The test results allow for the statement that the waste substance in the form of chloride salts such as CaCl₂ is stable bound in the glass–crystalline sinters, which ensures its effective immobilization.     

Crystallization study of Na-Gd phosphate glass using non-isothermal DTA

Na-Gd phosphate glasses doped with Ce³⁺ are intensively studied due to their high intensity radioluminescence. Crystallization kinetics of glass with nominal composition of NaGd(PO₃)₄ was investigated using non-isothermal DTA at heating rates between 10 and 115 K min⁻¹ and evaluated by the Kissinger and Ozawa peak methods. The activation energy for crystallization was determined for heating rates lower than 72 Kmin⁻¹ as 789.91 and 802.77 kJ mol⁻¹ by using the Kissinger and Ozawa methods, respectively. Formation of nuclei, their dimensions and movement of the crystallization front were observed using isothermal optical thermomicroscopy.