Thermal properties of oxide

To evaluate the thermal stability of oxide glasses, various criteria have been used. Not only the simple parameters, as characteristic temperatures and values of activation energy and enthalpy changes, but also the combined criteria E/RTp and kf(T) have been taken into account. Three glasses with the composition of Li₂O·2SiO₂ (a), Li₂O_·2SiO₂·0.03TiO₂ (b) and Li₂O·2SiO₂·0.1TiO₂ (c) were prepared and the validity of the criteria was tested by applying them to these glasses. The results indicate that the sequence thermal stability of the studied glass system vs. crystallization depends not only on their composition but also on the used criteria. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal properties of oxide glasses

A criterion based on the length of induction period of crystallization was used to study the effect of added CoO and NiO oxides on the thermal stability of Li₂O·2SiO₂ glass system against crystallization. It was found out that the thermal stability of studied glasses against crystallization is Li₂O·2SiO₂ < Li₂O·2SiO₂·0.1CoO < Li₂O·2SiO₂·0.1NiO. The addition of CoO and NiO oxides to Li₂O·2SiO₂ glass system increases its thermal stability. These results coincide with the order determined by stability criteria based on the characteristic temperatures.     

Effect of nucleating agents on the crystallization of Li<Subscript>2</Subscript>O-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> system glass

The processes of nucleation of Li₂O-Al₂O₃-SiO₂ glasses with TiO₂ and TiO₂+ZrO₂ as nucleating agents were discussed. The DTA peak temperature and DTA peak height shown a strong dependence on the nucleation temperature in the glass with TiO₂, while in the glass with TiO₂+ZrO₂ this tendency was small. The optimum nucleation temperatures were 745 and 760°C for two glasses. It suggested that with TiO₂+ZrO₂ as nucleating agents, the crystallization had lower sensitivity for nucleation temperature, and the glass had higher nucleation efficiency than with TiO₂.     

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 stability of Li2O-SiO2-CaO-P2O5-F glass

Two glasses, the first one with the composition of Li₂O·2SiO₂ and the second one with the addition of CaO, P₂O₅ and CaF₂ in the stoichiometric ratio corresponding to fluoroapatite were prepared and their tendency to crystallize has been studied by non-isothermal DTA analysis. The values of kinetic parameters calculated using the isoconversional integral method have been used to determine the temperature dependencies of both the length of isothermal induction period and the length of overall isothermal crystallization for both glasses. The estimated dependencies indicate that the glass containing CaO, P₂O₅ and CaF₂ has a lower thermal stability.     

Synthesis of Zeolite ZSM-2 Using Zeolite NaX as Seeds

Introduction Oxygen and nitrogen have been produced tradition-ally by cryogenic distillation of air. Methods for the non-cryogenic separation based on selective adsorption have been developed and commercialized since the 1970s and have led to a cost-effective process for this important separation.1 Low-silica zeolites are important materials for producing oxygen by selective adsorption of nitrogen. In 19891990, a new generation of lith-ium-based adsorbents was developed.2,3 Highly lithium exc…     

The vitrification of alkali borate and alkali silicate melts

Glasses and crystals of compositions corresponding to the congruently melting compounds M₂O·2SiO₂ (M = Na. Rb, and Cs) and M₂O·4SiO₂ (M = K, Rb, and Cs) were studied by differential scanning calorimetry. The structure temperatures (T _f) and excess entropies at T _f of glasses were measured depending on the rate of cooling of the corresponding melts. The activation energies of glass formation (ΔE) and scale of cooperative motion in the transition region (ξ_a) were estimated. The totality of the data obtained were used to compare the thermodynamic (the ratio between the excess (with respect to the corresponding crystals) entropy of glass at T _f and the entropy of crystal melting), kinetic (fragility m = f(ΔE, T _f)), and microscopic (ξ_a) parameters of the vitrification of alkali silicate melts. The behaviors of alkali silicate and alkali borate melts were shown to be similar.     

Investigations of structure and transport in lithium and silver borophosphate glasses

Glasses in the system xLi₂O·(1−x)[0.5B₂O₃·0.5P₂O₅] and xAg₂O·(1−x)[0.5B₂O₃·0.5P₂O₅] have been prepared from melt quenching method. Glasses have been characterized for their densities, molar volumes, glass transition temperatures and heat capacities. Structural studies have been done using infrared and high resolution magic angle spinning nuclear magnetic resonance (HR MAS NMR) of ³¹P, ¹¹B and ⁷Li nuclei. Boron is present only in tetrahedral coordination except in Li₂O-rich glasses. Transport properties have been investigated over a wide range of frequency and temperature. Silver containing glasses are found to possess higher conductivities and lower barriers than lithium containing glasses. A structural model has been proposed in which pure B₂O₃–P₂O₅ compositions are assumed to be constituted of BPO₄ units and modification occurs selectively on the phosphate moiety. Tetrahedral boron units are thus expected to be retained in the glass structure.     

Kinetics of the pyrolysis and combustion of göynük oil shale

Thermal behaviour of the glass series (100–y)[0.5ZnO·0.1B₂O₃·0.4P₂O₅]·yTiO₂ (with y=0–39 mol% TiO₂) was investigated by DSC and TMA. The addition of TiO₂ results in a non-linear increase of glass transition temperature. The compositional dependences of thermal stability, evaluated by two criteria exhibit two maxima for the glasses doped with 10.7 and 35.9 mol% TiO₂. All the glasses crystallize on heating in the temperature range of 576–670°C. The crystallization mechanism was studied at the glasses with 19.4 and 35.9 mol% TiO₂ and the results showed that surface nucleation mechanism prevails in these glasses over the internal one.     

Devitrification behaviour of CaO−MgO(Y2O3)−SiO2 glasses

In this paper a thermoanalytical study of the kinetic parameters and mechanism of the devitrification process of CaO·SiO₂, 1.6CaO·0.4MgO·2SiO₂ and 1.4CaO·(0.6/3)Y₂O₃·2SiO₂ is reported. The experimental results suggest that, in the studied glasses, a surface nucleation process is operative; however, in finely powdered samples, that soften and efficiently sinter before devitrifying, surface nuclei behave as bulk nuclei. In this case lamellar crystalline structures are obtained.     

Thermal properties and stability of lithium titanophosphate glasses

DTA was used to study thermal properties and thermal stability of (50-x)Li₂O-xTiO₂-50P₂O₅ (x=0–10 mol%) and 45Li₂Ot-yTiO₂-(55-y)P₂O₅ (y=5–20 mol%) glasses. The addition of TiO₂ to lithium phosphate glasses results in a non-linear increase of glass transition temperature. All prepared glasses crystallize under heating within the temperature range of 400–540°C. The lowest tendency towards crystallization have the glasses with x=7.5 and y=10 mol% TiO₂. X-ray diffraction analysis showed that major compounds formed by annealing of the glasses were LiPO₃, Li₄ P₂O₇, TiP₂O₇ and NASICON-type LiTi₂(PO₄)₃. DTA results also indicated that the maximum of nucleation rate for 45Li₂O-5TiO₂-50P₂O₅ glass is close to the glass transition temperature.     

Thermochemistry of alkali-metal titanates

The molar enthalpies of reaction at 970 K of sodium and potassium titanates have been determined by high-temperature solution calorimetry for the reactions: mM₂TiO₃ + (n-m)TiO₂ = mM₂O·nTiO₂, where (m,n) = (1,1), (4,5), (1,2), (1,3) and (1,6) on M = Na, and (1,1), (2,3), (1,2), (1,4) and (1,6) on M = K. It has been indicated from the resultant data that 4Na₂O·5TiO₂, 2K₂O·3TiO₂ and K₂O·2TiO₂ might be stabilized at high temperatures by significant positive entropies . The enthalpies of formation at 970 K of these compounds have been derived from the combination of the present work with the literature data (ref.1,2). Melting points and degrees of hydrolysis of these compounds are reasonably correlated to their enthalpies of formation.     

Non-debye nature in thermal relaxation and thermal properties of lithium borate glasses studied by modulated DSC

Complex heat capacity, C _p ^*=C _p ^'–iC _p ^', of lithium borate glasses xLi₂O·(1–x)B₂O₃ (molar fraction x=0.00–0.30) has been investigated by Modulated DSC. We have analyzed the shape of C _p ^* by the Cole-Cole plot, performed fitting by the Havriliak-Negami equation, and then determined the parameters related to the non-Debye nature of thermal relaxation. Moreover, the concentration dependence of the thermal properties has been investigated. Glass transition temperatures become higher with the increase of molar fraction of Li₂O and shows the board peak around x=0.26. Temperature ranges of glass transitions become narrower with the increase of Li₂O concentration.     

Structural characterisation and thermo-physical properties of glasses in the Li<Subscript>2</Subscript>O–SiO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–K<Subscript>2</Subscript>O system

This article aims to shed some light on the structure and thermo-physical properties of lithium disilicate glasses in the system Li₂O–SiO₂–Al₂O₃–K₂O. A glass with nominal composition 23Li₂O–77SiO₂ (mol%) (labelled as L₂₃S₇₇) and glasses containing Al₂O₃ and K₂O with SiO₂/Li₂O molar ratios (3.13–4.88) were produced by conventional melt-quenching technique in bulk and frit forms. The glass-ceramics (GCs) were obtained from nucleation and crystallisation of monolithic bulk glasses as well as via sintering and crystallisation of glass powder compacts. The structure of glasses as investigated by magic angle spinning-nuclear magnetic resonance (MAS-NMR) depict the role of Al₂O₃ as glass network former with four-fold coordination, i.e., Al(IV) species while silicon exists predominantly as a mixture of Q ³ and Q ⁴ (Si) structural units. The qualitative as well as quantitative crystalline phase evolution in glasses was followed by differential thermal analysis (DTA), X-ray diffraction (XRD) adjoined with Rietveld-reference intensity ratio (R.I.R.) method, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The possible correlation amongst structural features of glasses, phase composition and thermo-physical properties of GCs has been discussed.     

All-solid-state batteries with Li2O-Li2S-P2S5 glass electrolytes synthesized by two-step mechanical milling

Sulfide solid electrolytes, which show high ion conductivity, are anticipated for use as electrolyte materials for all-solid-state batteries. One drawback of sulfide solid electrolytes is their low chemical stability in air. They are hydrolyzed by moisture and generate H₂S gas. Substituting oxygen atoms for sulfur atoms in sulfide solid electrolytes is effective for suppression of H₂S gas generation in air. Especially, the xLi₂O·(75-x)Li₂S·25P₂S₅ (mol%) glasses hardly generated H₂S gas in air. However, substituting oxygen atoms for sulfur atoms caused a decrease in conductivity. The x?=?7 glass showed high chemical stability in air and maintained high conductivity of 2.5?×?10^?4 S cm^?1 at room temperature. Performance of cells using the 7Li₂O·68Li₂S·25P₂S₅ and the 75Li₂S·25P₂S₅ glasses as solid electrolytes were compared. All-solid-state C/LiCoO₂ cell using the 7Li₂O·68Li₂S·25P₂S₅ glass produced performance as good as that obtained using the 75Li₂S·25P₂S₅ glass. Capacity retention and change of interfacial resistance of the former cell were superior to those of the latter cell after storage at 4.0 V and 60 °C. The diffusion of oxygen element into the 7Li₂O·68Li₂S·25P₂S₅ glass was less than that into the 75Li₂S·25P₂S₅ glass after storage at the voltage of 4.0 V at 60 °C. Improvement of the stability of sulfide solid electrolytes to moisture was related to cell performance as well as an increase in conductivity.     

Structure of Na2O–MgO–CaO–SiO2 glasses by combined Raman spectroscopy and thermodynamic modeling approach

The present contribution deals with the Raman spectra and structure of Na₂O–MgO–CaO–SiO₂ glasses. Six glasses with the trisilicate overall composition 15Na₂O·xMgO·(10–x)CaO·75SiO₂ (x = 0, 2, 4, 6, 8, 10) were studied. The structure of studied glasses was described by the thermodynamic model of Shakhmatkin and Vedishcheva. From the 27 components with the stoichiometry given by the composition of stable crystalline phases, only eight were found in significant abundance in the studied glasses—namely: SiO₂, 2MgO·SiO₂ (M2S), MgO·SiO₂ (MS), Na₂O·3CaO·6SiO₂ (NC3S6), Na₂O·CaO·5SiO₂ (NCS5), Na₂O·MgO·4SiO₂ (NMS4), Na₂O·SiO₂ (NS), and Na₂O·2SiO₂ (NS2). The correlation analysis points out that the strong positive correlations between the equilibrium molar amounts of: {M2S–MS–SiO₂}, {NC3S6–NCS5}, and {NMS4–NS–NS2}. From the components of significant abundance, only the content of MS and NC3S6 change significantly within the studied compositional series. These two components were identified with the result of the principal component analysis of Raman spectra that indicated the presence of two independent spectral components. Using the method of Malfait the partial Raman spectra of MS and NC3S6 components were found. The obtained results very well reproduce the experimental Raman spectra and confirmed in such way the thermodynamic model.     

Kinetic analysis of the thermal stability of lithium silicates (Li4SiO4 and Li2SiO3)

The kinetics describing the thermal decomposition of Li₄SiO₄ and Li₂SiO₃ have been analysed. While Li₄SiO₄ decomposed on Li₂SiO₃ by lithium sublimation, Li₂SiO₃ was highly stable at the temperatures studied. Li₄SiO₄ began to decompose between 900 and 1000 °C. However, at 1100 °C or higher temperatures, Li₄SiO₄ melted, and the kinetic data of its decomposition varied. The activation energy of both processes was estimated according to the Arrhenius kinetic theory. The energy values obtained were −408 and −250 kJ mol⁻¹ for the solid and liquid phases, respectively. At the same time, the Li₄SiO₄ decomposition process was described mathematically as a function of a diffusion-controlled reaction into a spherical system. The activation energy for this process was estimated to be −331 kJ mol⁻¹. On the other hand, Li₂SiO₃ was not decomposed at high temperatures, but it presented a very high preferential orientation after the heat treatments.     

Study of the kinetic aspects of formation of calcium monosilicate in the model system CaSO<Subscript>4</Subscript>·2H<Subscript>2</Subscript>O-Na<Subscript>2</Subscript>O·SiO<Subscript>2</Subscript>

The possibility and kinetic aspects of formation of X-ray amorphous calcium monosilicate in the model system CaSO₄·2H₂O-Na₂O·SiO₂ at room temperature were studied.     

The Non-Isothermal Devitrification of Potassium Germanate Glasses

The devitrification behaviour of the glasses K₂O·xGeO₂ with x=4, 7 or 8 was examined by means of differential thermal analysis (DTA), the Fourier transformation infrared (FTIR) transmittance spectra and X-ray diffraction (XRD). The glass transition temperatures were related to the molar ratio GeO₄/GeO₆. For the glass with x=4, metastable K₄Ge₉O₂₀> crystals are initially formed and then converted at higher temperatures into stable K₂Ge₄O₉ crystals. The glasses with x=7 or 8 both devitrify into K₂Ge₇O₁₅> crystals. The effects of the specific surface area of the samples on the devitrification mechanisms were established. Bulk nucleation predominates in the glass with x=4, while the glasses with x=7 or 8 crystallize from the surface. The activation energies for crystal growth were evaluated from the DTA curves. This revised version was published online in July 2006 with corrections to the Cover Date.