Fluorine sites in glasses and transparent glass-ceramics of the system Na2O/K2O/Al2O3/SiO2/BaF2

The transparent glass-ceramics obtained in the silicate system Na₂O/K₂O/SiO₂/BaF₂ show homogeneously dispersed BaF₂ nano crystals with a narrow size distribution. The X-ray diffraction and the nuclear magnetic resonance spectroscopy were applied to glasses and the respective glass-ceramics in order to clarify the crystallization mechanism and the role of fluorine during crystallization. With an increasing annealing time, the concentration and also the number of crystals remain approximately constant. With an increasing annealing temperature, the crystalline fraction increases until a saturation limit is reached, while the number of crystals decreases and the size of the crystals increases. Fluoride in the glassy network occurs as Al-F-Ba, Al-F-Na and also as Ba-F structures. The latter are transformed into crystalline BaF₂ and fluoride is removed from the Al-F-Ba/Na bonds. However, some fluorine is still present in the glassy phase after the crystallization.     

Variable temperature 1H, 23Na,and 29Si MAS NMR studies on sodium silicate hydrates of composition Na2O · SiO2 · nH2O (n = 9, 6, 5): Local structure in crystals,melts, supercooled melts,and glasses

The local structure in crystals, melts, supercooled melts, and glasses of sodium silicate hydrates of composition Na₂O · SiO₂ · nH₂O (n = 9, 6, 5) is studied by variable temperature ¹H, ²³Na, and ²⁹Si MAS NMR spectroscopy. Detailed in situ investigations on the melting process of the crystalline materials reveal the importance of H₂O motion in the melting mechanism. Depending on the local coordination, crystallographically distinct Na sites show different behaviour during the melting process. Upon melting, the monomer silicate anions present in the crystalline hydrates undergo condensation reactions to oligomeric silicate anions. No recrystallization but glass formation occurs at low temperature if the melts were heated initially about 10 K above the melting point. In the glasses also oligomeric silicate anions are present with a preference for cyclotrimer species. In situ MAS NMR investigations and electric conductivity measurements of the melts, supercooled melts, and glasses suggest the distinction of three temperature ranges characterized by different local structure and dynamics of the sodium cations, water and silicate anions. These ranges comprise a glass and glass transition range A at low temperatures, an aggregation region B at intermediate temperatures, and a solution or electrolyte region C at high temperatures. In region B aggregation of sodium water complexes to hydrated polycation clusters is suggested, the dynamic behaviour of which is clearly different to that of the silicate anions, indicating that no long-lived contact ion pairs between sodium cations and silicate anions are formed.     

Nucleation and Crystal Formation in Lithium Disilicate‐Apatite Glass‐Ceramic from a Combined Use of X‐Ray Diffraction,Solid‐State NMR,and Microscopy

Glass‐ceramics are multi‐phase materials that are comprised of one amorphous phase and at least one crystalline phase. Their versatile performance and properties can be engineered by alterations of the three fundamental steps – formulation and production of the amorphous base glass, nucleation, and crystallization. Efforts have been made on syntheses of glass‐ceramics with different components, yet little is known about the details of nucleation and crystallization processes that are essential for tailoring glass‐ceramic properties. Herein, we investigate the nucleation and crystallization mechanisms of a multi‐component, that is SiO₂‐Al₂O₃‐CaO‐Li₂O‐K₂O‐P₂O₅‐F, glass‐ceramic system by a combined use of powder X‐ray diffraction (pXRD), solid‐state nuclear magnetic resonance (NMR), and electron microscopic (EM) techniques. The role of P₂O₅ in the nucleation and crystallization processes is particularly studied. We show that the formation of lithium silicate crystals being independent of the P₂O₅‐associated crystals, and the separation of P₂O₅ phases into individual growth domains of lithium orthophosphate and fluorapatite. We also observe the non‐uniform distribution of fluorapatite particles that explains the opalescence effect of this glass‐ceramic.     

Direct crystallization of silicate–phosphate glasses of NaCaPO4–SiO2 system

The subject of the study was silicate–phosphate glasses of NaCaPO₄–SiO₂ system which are precursors of glass–crystalline materials. Glass–crystalline materials of NaCaPO₄–SiO₂ system obtained via crystallization of glasses belong to a group of the so-called bioactive materials. In order to obtain glass–crystalline materials with pre-established parameters, it is necessary to conduct crystallization of glasses at specific conditions. In order to design direct crystallization process properly, it is necessary to know the structure and microstructure of the glassy precursor. Microscopic investigation showed that liquation takes place in all the studied glasses. Based on DSC examinations, it has been found out that crystallization of the glasses of NaCaPO₄–SiO₂ system is a multistep process. The presence of several clearly separated exothermic peaks in DSC curves of investigated glasses makes it possible to crystallize only the separated phase with the matrix remaining amorphous or vice versa. Conducted detailed X-ray and spectroscopic studies of the materials obtained by heating in a gradient furnace (in the temperature specified on the basis of DSC) showed that separated phase and matrix crystallizes separately. Therefore, bioactive glass–crystalline materials can be obtained due to the existence of the phase separation phenomenon and pre-established sizes of the crystalline phase.     

The effects of La2O3 on the early stages of crystallization for MgO-Al2O3-SiO2-TiO2-La2O3 glass-ceramics

The early stages of crystallization for MgO-Al₂O₃-SiO₂-TiO₂-La₂O₃ glasses with different La₂O₃ concentrations were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The glass transition temperature (Tg) of the glass decreases at first and then increases again with increasing La₂O₃ concentration. This indicates that the structure of the glass becomes weaker at first and then stronger again. Lanthanum acts in glasses as network modifier and will usually decrease the network connectivity of the glass structure. Nevertheless, if the La₂O₃ concentration is high enough, the oxygen and other ions start to agglomerate around La, resulting in a more closely packed structure. Heat-treatment of the sample with x = 0.1 at 770–810 °C results in the precipitation of a droplet phase with higher mean atomic weight embedded in a matrix with lower mean atomic weight. The initial crystalline phase magnesium aluminum titanate (MAT) precipitates from the droplet phase. Nevertheless, for the sample with x = 0.4, dendrite-like structure could be observed after heat-treatment of the glass at 810 °C. Furthermore, the crystalline phase first precipitated is the lanthanum containing perrierite, which could be attributed to the rearrangement of the glass structure as an effect of La³⁺ incorporation.     

Interaction of Aluminum Metaphosphates in the Setting of Potassium Silicate Solutions in Terms of the Crystalline Phase Composition.

Aluminum phosphates are known as inorganic hardening agents for the setting of alkali silicate solutions, but only few studies have been published on the setting mechanism of potassium water glass. The solution behavior of two aluminum metaphosphates in alkaline environments were investigated photometrically determining the dissolved aluminum content. The crystalline phase composition of the hardened potassium silicate systems was determined by X-ray diffraction. New insights into the setting mechanism were obtained concerning the structure of the aluminum metaphosphate and the SiO₂/K₂O ratio of three different potassium silicate solutions. With increasing pH value aluminum tetrametaphosphate reacts rapidly and forms crystalline potassium tetrametaphosphate dihydrate by an ion-exchange-reaction. In parallel, a depolymerization of the cyclic metaphosphate structure occurs leading to potassium dihydrogen phosphate as final fragmentation product. With aluminum hexametaphosphate no ion-exchange reaction product was observed. Only potassium dihydrogen phosphate could be found in higher quantities compared to the reaction with aluminum tetrametaphosphate.     

Glass forming ability and thermal stability of oxyfluoride glasses

New oxyfluoride glasses based on the system ZrO₂-Pr₂O₃-ZrF₄-BaF₂ were obtained, and the glass forming ability of this system was evaluated. The effects of glass composition on both thermal stability and the crystalline phases formed upon heat treatment were determined by the DSC and XRD methods, respectively. The composition with higher thermal stability and better glass forming ability contained 2 mol% of oxides.     

Annealing Behavior of Silica Gel Powders Modified with Silver Crystalline Aggregates

The annealing behaviour of silica powders added with silver, prepared by the sol-gel method, was studied using X-ray diffraction. Partial crystallization of amorphous SiO₂ samples as low as 600°C has been observed. For that, silver needed to be added to the precursor solution in such a way that it formed aggregates. Silica xerogel samples were prepared using a molar ratio ethanol/H₂O/TEOS of 4:11.6:1 and loaded with silver in three different ways: in the form of silver nitrate, silver chloride, or chemically synthesised silver fine particles. The microstructure of the silica xerogel powders was studied as a function of annealing temperature. Attention was paid to the evolution of the glass matrix as well as the silver aggregates in the SiO₂ matrix. Partial crystallization of the glass matrix was achieved at temperatures much lower than those specified by the phase diagram, independently of preparation method of the silver aggregates.     

Crystallization of nano calcium fluoride in CaF2–Al2O3–SiO2 system

The effects of alumina and CaF₂ content on the crystallization behavior, fluorine loss, phase separation mechanism and optical properties of oxyfluoride glass ceramics were investigated. Three series of glasses in which their SiO₂/Al₂O₃ ratio was different such as 1.8, 2.18 and 2.5 were examined. Results showed that the adopted ratios played key role significantly in the mechanism of phase separation of the glass. It modified from spinodal decomposition to nucleation and growth by decreasing the mentioned ratio.UV spectroscopy showed that owing to the small size of precipitated CaF₂ crystals, i.e. 20 nm, in the glass with a silicon oxide to alumina ratio of 2.18 and initial CaF₂ amount of 35 mol%, it remained transparent after heat treatment at 740 °C.     

Influence of modifiers and glass-forming ions on the crystallization of glasses of the NaCaPO4–SiO2 system

Silicate?Cphosphate glasses of the XYPO₄?CSiO₂ and XYPO₄?CSiO₂?CAlPO₄ (where X?=?Na⁺ and/or K⁺ and Y?=?Ca²⁺ and/or Mg²⁺) systems have been the subject of the presented investigations. Bioactive glasses from these systems are the base for obtaining glass-crystalline biomaterials through a direct crystallization. However, growth of crystalline phases very adversely affects the bioactivity of the glasses. Uncontrolled growth of crystalline phases can be reduced by means of a glass phase separation phenomenon in the silicate?Cphosphate glasses because boundaries of inclusion-matrix phase may be a barrier limiting the growth of crystalline phases. Microscopic and EDX investigations which have been carried out have shown that glass phase separation occurs in glasses belonging to XYPO₄?CSiO₂ and XYPO₄?CSiO₂?CAlPO₄ systems. Introduction of aluminum ions into the glass structure leads to a rapid homogenization of its texture. Based on DSC examinations it has been found out that crystallization of the glasses of XYPO₄?CSiO₂ systems is a multistep process. The presence of several (the number depends on the type of modifiers and glass-forming ions) clearly separated exothermic peaks in DSC curves of investigated glasses makes it possible to crystallize only the inclusions with the matrix remaining amorphous or vice versa. It has been shown that, crystallization of glasses of XYPO₄?CSiO₂?CAlPO₄ system is single-stage process, which is the consequence of the homogenizing effect of aluminum ions on their texture.     

Effect of iron sulfides on the physicochemical processes occurring during solidification of synthetic calcium aluminosilicate monolith

Phase formation processes occurring during solidification of two samples of silicate materials obtained by filtration combustion of a mixture of fly ash, limestone, and sand were studied. During the solidification of the material produced in the combustion mode in which impurity pyrite FeS₂ completely decomposed by the interaction with atmospheric CO₂ and water, dicalcium silicate, melilite, and glass phase gradually transformed into a mixture of serpentine-type binder (Ca, Fe, Mg)₃[(Si, Al)₂O₅](OH)₄ · nH₂O and calcium carbonates. The narrowing of the combustion wave front allows one to preserve pyrite particles encapsulated in the glass phase. In this case, the mechanism of solidification of the combustion product changes: the expose to sulfuric acid released by pyrite oxidation of the silicate components of the material results in the formation of gypsum and hydrosilicates filling the pore space. This enables one to reach high strength characteristics of the material without prepressing. The presence of a lower-pH medium in the pore space inhibits ettringite crystallization, thus preventing mechanical destruction.     

Crystallization of MgFe2O4 from a glass in the system K2O/B2O3/MgO/P2O5/Fe2O3

Spherical magnetic Mg-Fe-O nanoparticles were successfully prepared by the crystallization of glass in the system K₂O/B₂O₃/MgO/P₂O₅/Fe₂O₃. The magnetic glass ceramics were prepared by melting the raw materials using the conventional melt quenching technique followed by a thermal treatment at temperatures in the range 560–700 °C for a time ranging from 2 to 8 h. The studies of the X-ray diffraction, electron microscopy and FTIR spectra confirmed the precipitation of finely dispersed spherical (Mg, Fe) based spinel nanoparticles with a minor quantity of hematite (α-Fe₂O₃) in the glass matrix. The average size of the magnetic nano crystals increases slightly with temperature and time from 9 to 15 nm as determined by the line broadening from the XRD patterns. XRD studies show that annealing the glass samples for long periods of time at temperature ≥604 °C results in an increase of the precipitated hematite concentration, dissolution of the spinel phase and the formation of magnesium di-borate phase (Mg₂B₂O₅). For electron microscopy, the particles were extracted by two methods; (i) replica extraction technique and (ii) dissolution of the glass matrix by diluted acetic acid. An agglomeration of the nano crystals to larger particles (25–35 nm) was observed.     

Coexistence of Immiscible Liquid Phases in the LaPO<Subscript>4</Subscript>–SiO<Subscript>2</Subscript>–NaF–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> System

It was determined that the system LaPO₄–SiO₂–NaF–Nb₂O₅ within the temperature range 850–1200°C has regions of immiscibility of liquid phases (silicate and phosphate–salt melts). The coexisting melts have contrast chemical and phase compositions and structural-textural features, because of which the methods for extracting rare-earth elements and niobium from these melts differ. The silicate melts form glass, whereas the phosphate–salt melts have high crystallization ability. The mutual solubility of the liquid phases does not exceed 5%. The components of the system are contrastively distributed between the silicate and phosphate–salt melts. A fraction of 95–97% of niobium is extracted into the silicate melt, and 93–95% of La and P is extracted into the phosphate–salt melt.     

Study of crystallization process of soda lead silicate glasses by thermal and spectroscopic methods

The crystallization process of some glasses in the ternary Na₂O–SiO₂–PbO system with good chemical stability that can be used for waste inertization was studied using X-ray diffraction (XRD), infrared spectroscopy (FT-IR), differential thermal analysis (DTA) and scanning electron microscopy. The parent glasses were characterized by XRD and FT-IR, and their vitreous state was determined. DTA measurements evidenced glass transition (T _g) and crystallization temperatures (T _c). The thermal treatments were conducted at vitreous transition temperature (400 °C) and at highest effect of crystallization (650 °C). XRD evidenced the lead and sodium silicate crystalline phases in samples treated at 650 °C for 12 h. Micrometer crystallites dispersed in the glass matrices have affected the transparence of glasses and made them opaque after treatment at 650 °C. The influence of oxide quantities in compositions on the crystallization tendency was revealed. A PbO higher content than that of SiO₂ as well as lower Na₂O content decreased the tendency of crystallization.     

Pulse annealing electron paramagnetic resonance with probing transition ions

The analysis of the sequence of electron paramagnetic resonance (EPR) spectra of trace amounts of substitutional probing paramagnetic ions incorporated in (nano)crystalline samples submitted to isothermal and isochronal pulse annealing treatments can offer a wealth of information on the thermally induced compositional and structural changes of the host material. The potential of this new thermal analysis method is illustrated here with results of such investigations on the thermal decomposition of crystalline zinc hydroxide (Zn(OH)₂) and anhydrous zinc carbonate basic (Zn₅(CO₃)₂(OH)₆) precursors containing trace amounts of substitutional Mn²⁺ probing ions into nanostructured zinc oxide-ZnO. The quantitative analysis of the sequence of isochronal pulse annealing EPR spectra could provide, besides the thermal decomposition curves of the two precursors, additional information about the structure of the resulting nanostructured ZnO, some of it hard to get by standard structural diffraction techniques. The analysis of both isochronal and isothermal pulse annealing EPR data was further used to investigate the crystallization mechanism of the initially formed nanostructured disordered ZnO and to quantitatively describe the further growth of the resulting ZnO nanocrystals with the increasing annealing temperature and duration.     

Thermal and structural studies of nanocrystallization of oxyfluoride glasses

The influence of fluorine content on the structure and crystallization of oxyfluoride glasses from the Na₂O–Al₂O₃–SiO₂–LaF₃ system was studied by DTA/DSC, XRD, FTIR and SEM methods. It has been found that the increase in the fluorine content in the structure of oxyfluoride glasses causes the increase of the flexibility of their structure, which inhibits the process of crystallization of the silicate- aluminium matrix. Simultaneously the ability of the glass for LaF₃ crystallization, which shows a multistage character, is increasing. Analysis of the local atomic interactions in the structure of glasses has been used to explain the course of the crystallization.     

Influence of microstructure and crystalline phases on impedance spectra of sodium conducting glass ceramics produced from glass powder

Crystallization of highly ionic conductive N5 (Na₅YSi₄O₁₂) phase from melted Na_3+3x-1Y_1-xP_ySi_3-yO₉ parent glass provides an attractive pathway for cost-effective manufacturing of Na-ion conducting thin electrolyte substrates. The temperature-dependent crystallization of parent glass results in several crystalline phases in the microstructure (N3 (Na₃YSi₂O₇), N5 and N8 (Na8.1Y Si6O18) phases) as well as in rest glass phase with temperature dependent viscosity. The electrical properties of dense parent glass and of compositions densified and crystallized at 700 °C, 800 °C, 900 °C, 1000 °C, and 1100 °C are investigated by impedance spectroscopy and linked to their microstructure and crystalline phase content determined by Rietveld refinement. The parent glass has high isolation resistance and predominantly electrons as charge carriers. For sintering at ≥ 900 °C, sufficient N5 phase content is formed to exceed the percolation limit and form ion-conducting pathways. At the same time, the highest content of crystalline phase and the lowest grain boundary resistance are observed. Further increase of the sintering temperature leads to a decrease of the grain resistance and an increase of grain boundary resistance. The grain boundary resistance increases remarkably for samples sintered at 1100 °C due to softening of the residual glass phase and wetting of the grain boundaries. The conductivity of fully crystallized N5 phase (grain conductivity) is calculated from thorough impedance spectra analysis using its volume content estimated from Rietveld analysis, density measurements and assuming reasonable tortuosity to 2.8 10⁻³ S cm⁻¹ at room temperature. The excellent conductivity and easy processing demonstrate the great potential for the use of this phase in the preparation of solid-state sodium electrolytes.

     

Glass transition behavior and crystallization kinetics of Cu0.3(SSe20)0.7 chalcogenide glass

The glass transition behavior and crystallization kinetics of Cu_0.3(SSe₂₀)_0.7 chalcogenide glass were investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD). Two crystalline phases (SSe₂₀ and Cu₂Se) were identified after annealing the glass at 773 K for 24 h. The activation energy of the glass transition (E_g), the activation energy of crystallization (E_c), the Avrami exponent (n) and the dimensionality of growth (m) were determined. Results indicate that this glass crystallizes by a two-stage bulk crystallization process upon heating. The first transformation, in which SSe₂₀ precipitates from the amorphous matrix with a three-dimensional crystal growth. The second transformation, in which the residual amorphous phase transforms into Cu₂Se compound with a two-dimensional crystal growth.     

Characterization of the electronic structure and thermal stability of HfO2/SiO2/Si gate dielectric stack

X‐ray photoelectron spectroscopy was used to investigate thermal stability of HfO₂ on SiO₂/Si substrate prepared by atomic layer deposition, followed by annealing at different temperature. Hf silicate and Hf silicide are formed at the interface of HfO₂ and SiO₂ during deposition. The Hf silicide disappears, while the amount of the Hf silicate is intensified after post‐deposition annealing treatment at 400 °C. Phase separation of the Hf silicate layer occurs when the annealing temperature is over 400 °C, resulting in the Hf silicate decomposition into Si and Hf oxides. Moreover, crystallization at high temperature leads to grain boundaries formation, which deteriorates the gate leakage current, as observed by the electrical measurements. The similar annealing temperature dependence of both internal electric field and the amount of Hf silicate implies that the Hf silicate plays a key role in building up the internal electric field, which is attributed to generation of oxygen vacancies (V_o) in the Hf silicate layer. Copyright © 2017 John Wiley & Sons, Ltd.     

Rigid amorphous fraction in poly(ethylene terephthalate) determined by dilatometry

Volumetric thermal analysis of semicrystalline poly(ethylene terephthalate), PET, with different content of crystalline phase was carried out using mercury-in-glass dilatometry. The effect of crystals on the thermal properties of amorphous phase (glass transition temperature, T _g, thermal expansion coefficients, α) were determined. At cold-crystallization (106°C, up to 4 h), crystalline content of 2.4–25.3 vol.% was achieved. Increasing content of crystalline phase broadens the glass transition region and increases T _g. The change of thermal expansion coefficient during glass transition is lower than that predicted by the two-phase model, which indicates the presence of a third fraction — rigid amorphous fraction (RAF), whose content steadily increases during crystallization. However, its relative portion (specific RAF) is significantly reduced. Further significant decrease in specific RAF appears after annealing at a higher temperature.