Thermal analysis and thermochemistry of vitreous into crystalline state transition

Thermochemistry and structural mechanism of crystallization of MgO-Al₂O₃-SiO₂ glasses with TiO₂ as crystallization activator were studied. Thermal and HREM investigation proved that near the T _g temperature crystallization is going by rearrangement of glass structure elements and part of its components redistribution like at disorder — order phase transition in solid bodies. Nanocrystals of Mg-titanate and high quartz structure solid solution are formed then. Next enstatite and cordierite are crystallizing. Thermochemical and chemical bonds strength analysis indicate that during multistage crystallization of glasses, kind and order of crystal phase formation, is determined by the glass structure decomposition progress and its particular components release accompanying increase of temperature. It has been proved that molar heat capacity change (ΔC _p) accompanying the glass transition is the significant measure of degree of changes in the structure of glass preceding crystallization. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal behaviour of Fe-doped silicate–phosphate glasses

Thermal behaviour and structure of glasses from the SiO₂–P₂O₅–K₂O–MgO–CaO system modified by Fe₂O₃ addition were studied by DSC, XRD and FTIR methods. It has been found that the replacement of MgO and CaO modifiers by Fe₂O₃ in the structural network of silicate–phosphate glass results in decrease of the glass transition temperature (T _g) and heat capacity change (ΔC _p) accompanying the glass transformation. Simultaneously, the ability for crystallization, its course and the type of the forming phases depend on the relative proportions between iron and phosphorus as components forming the silicate–phosphate structure. The type of the crystal phases forming in the course of heating the considered glass has been found to be in agreement with the character of the domains occurring in this glass, confirmed by FTIR examinations.     

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.     

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.     

Thermal study of Mn-containing silicate–phosphate glasses

Silicate-phosphate glasses of SiO₂–P₂O₅–K₂O–MgO–CaO system containing manganese cations were investigated to obtain information about the influence of manganese ions on the thermal behavior of such glasses. Amorphous state of glasses and the course of phase transformation and crystallization taking place during their heating were investigated by DSC, XRD, and FTIR methods. It was shown that an increasing content of manganese replacing calcium and magnesium in the structure of analyzed glasses causes decrease of glass transition temperature (T _g) and heat capacity change (Δc _p) accompanying the glass transformation. Simultaneously, thermal stability of the glasses increased. Products of multistage crystallization of glasses containing up to 8 mol% of MnO₂ were: marokite (CaMn₂O₄), phosphate of Ca₉MgK(PO₄)₇ type, and diopside (CaMgSi₂O₆). Product of crystallization of glasses containing higher amount of manganese was braunite (Mn₇O₈SiO₄). This was accompanied by change of structure of magnesium calcium silicates from diopside-type structure to akermanite-type silicates (Ca₂MgSi₂O₇). The data interpretation was based on bonds and chemical interactions of the individual components forming the glass structure.     

The Detailed Mechanism of Oxidation of Ni-P Alloys

DTA in conjuction with X-ray diffraction analysis with a high-temperature camera and infrared spectroscopy was employed to determine the mechanism of oxidation of Ni-P alloys. Amorphous Ni-P powders were obtained from a nickel(II) sulphate bath as a nickel source and sodium dihydrophosphate(I) as a reducing agent. The crystallization product is composed of two phases: (f.c.c.) Ni and (b.c.t.) Ni₃P. The amorphous to crystalline transformation takes place in the temperature range 280–330°C. Ni₃P samples were heated from room temperature to 1050°C in air atmosphere at 5°C min⁻¹. It was found that the first stage of oxidation of Ni₃P goes through the intermediate phase of Ni₁₂P₅ formation to Ni₂P. Some exothermic reactions were observed. Heating runs were interrupted after each reaction for crystal structure determination by IR spectrometry. Infrared spectra are reported and it is shown that the structure units present in the amorphous products at about 700°C were the oxoanions PO₃ ⁻ and P₂O₇ ⁻. The final products of the oxidation process are NiO and Ni₃(PO₄)₂. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetics of the amorphous—anatase phase transformation in copper doped titanium oxide

Samples of TiO₂ doped with 2 and 5 mol% of Cu²⁺ were prepared by the sol-gel process. Titanium(IV) isopropoxide and copper(II) nitrate were used as precursors. The samples were prepared as monolithic shapes, dried at 80°C for 72 h and heat treated at various temperatures in the range 200–900°C for 2 h. The structural transformation and texture of the samples were investigated by X-ray powder diffraction (XRD) and nitrogen adsorption. Significant changes were observed during the crystallization process; on the one hand, the crystallization profiles show that crystallization occurs uniformly and is practically insensitive to the dopant concentration, but when the transformation at a given temperature is followed as a function of time, the rate of the amorphous-anatase transformation is larger for the sample containing 2 mol% Cu²⁺. Electron spin resonance (ESR) results show that in this sample there is no segregation of Cu²⁺ ions. The sample containing 2 mol% of Cu²⁺ was selected for the kinetic studies and the temperatures selected were 300, 325, 350, 375 and 400°C, which were taken from the amorphous to anatase crystallization profile. An activation energy of 137 ± 4 kJ/mol for the crystallization process was estimated from the kinetic data. These results showed that the effect of the open structure present in the TiO₂ amorphous phase provides the atomic mobility required for the crystallization. On the other hand, the differences in the crystallization rate due to the amount of Cu²⁺ were explained by the segregation of copper ions to the surface of the samples.     

Atmospheric control of gel-oxide transformation in sol–gel derived Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript> fibers

Via sol–gel processing metal–organic fibers were produced and dried up to 140 °C. For these gel fibers the influence of a treatment in different atmospheres was investigated for the temperature range of 200–850 °C. The atmospheres were nitrogen, water vapor, evaporated nitric and hydrochloric acid and evaporated hydrogen peroxide. In the presence of moisture and especially with acidic moisture fibers were transformed almost completely to their oxide composition (82 mol% Al₂O₃·18 mol% Y₂O₃). In these inorganic amorphous structures considerable differences were observed on several structural levels. On the atomic scale, the coordination of Al ions was investigated by ²⁷Al MAS NMR and skeletal density by He-pycnometry. Porosity in the nm scale was characterized by N₂-sorption. As a macroscopic effect of different treatment atmospheres, the longitudinal shrinkage was observed. For fibers treated at 500 °C the relative shrinkage varied by 100% (comparing water vapor and nitrogen atmosphere). No simple correlation between the release of organic constituents, the formation of porosity and the shrinkage could be found. These aspects were controlled by the rigidity of the inorganic network against atomic reconstitution. The kind of atmosphere was found to be an effective parameter to control various aspects of the xerogel structure.     

B<Subscript>12</Subscript>-induced formation of stishovite in sol–gel produced amorphous SiO<Subscript>2</Subscript> matrices

Composites containing vitamin B₁₂ (cyanocobalamin) dispersed in amorphous silica xerogel were studied structurally as a function of annealing temperature. Silica xerogel samples were prepared by the sol–gel method using an ethanol:H₂O:TEOS molar ratio of 4:11.6:1 and loaded with cyanocobalamin. We found that the structure of the cobalamin is unaltered, although decoordination of the benzimidazole nucleobase of B₁₂, whereas the amorphous quartz structure of the matrix is maintained under heat-treatment without low-cristobalite phase transformation, typically of this kind of materials. We found in our samples partial crystallization of the glass matrix in form of stishovite obtained at very lower pressure than those specified by the phase diagram, and temperatures about 400 °C due to the presence of vitamin B₁₂. The presence of stishovite is corroborated by the Rietveld refinement method.     

Calorimetric studies on 2TeO<Subscript>2</Subscript>–V<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

Glasses of the composition 2TeO₂–V₂O₅ were fabricated via the conventional melt-quenching technique. The amorphous and the glassy nature of the as-quenched samples were confirmed by X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC), respectively. The glass transition and crystallization parameters were evaluated under non-isothermal conditions using DSC. X-ray diffraction studies confirmed the presence of partially oriented crystallites in the heat-treated glasses. Kauzmann temperature (lower bound for the kinetically observed glass transition) was deduced from the heating rate dependent glass transition and crystallization temperatures.     

Influence of B<Subscript>2</Subscript>O<Subscript>3</Subscript> on the structure and crystallization of soil active glasses

Glasses of the SiO₂–P₂O₅–K₂O–MgO–CaO–B₂O₃ system acting as nutrients carriers in the soil environment were synthesised by the melt-quenching technique. Thermal properties were studied using DTA/DSC methods and the influence of B₂O₃ and P₂O₅ content on thermal stability and crystallization process of these glasses was examined. The structure of the glass network was characterized by FTIR, ³¹P, and ¹¹B MAS NMR. The chemical activity of the glasses in the 2 mass% citric acid solution was measured by the ICP-AES method. The analysis indicated that the formation of P–O–B units with chemically stable tetrahedral borate groups decreases the glass solubility in conditions simulating the soil environment.     

Thermal studies of ZnO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript>–TeO<Subscript>2</Subscript> glasses

The effect of TeO₂ additions on the thermal behaviour of zinc borophosphate glasses were studied in the compositional series (100 − x)[0.5ZnO–0.1B₂O₃–0.4P₂O₅]–xTeO₂ by differential scanning calorimetry, thermodilatometry and heating microscopy thermal analysis. The addition of TeO₂ to the starting borophosphate glass resulted in a linear increase of glass transition temperature and dilatometric softening temperature, whereas the thermal expansion coefficient decreased. Most of glasses crystallize under heating within the temperature range of 440–640 °C. The crystallization temperature steeply decreases with increasing TeO₂ content. The lowest tendency towards crystallization was observed for the glasses containing 50 and 60 mol% TeO₂. X-ray diffraction analysis showed that major compounds formed by annealing of the glasses were Zn₂P₂O₇, BPO₄ and α-TeO₂. Annealing of the powdered 50ZnO–10B₂O₃–40P₂O₅ glass leads at first to the formation of an unknown crystalline phase, which is gradually transformed to Zn₂P₂O₇ and BPO₄ during subsequent heating.     

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.     

Structure and Magnetic Properties of Nickel–Zinc Ferrite Nanoparticles Prepared by Glass Crystallization Method

The magnetic and microstructure properties of Fe₂O₃–0.4NiO–0.6ZnO–B₂O₃ glass system, which was subjected to heat treatment in order to induce a magnetic crystalline phase (Ni_0.4Zn_0.6-Fe₂O₄ crystals) within the glass matrix, were investigated. DSC measurement was performed to reveal the crystallization temperature of the prepared glass sample. The obtained samples, produced by heat treatment at 765°C for various times (1, 1.5, 2, and 3 h), were characterized by X-ray diffraction, IR spectra, transmission electron microscopy, and vibrating sample magnetometer. The results indicated the formation of spinel Ni–Zn ferrite in the glass matrix. Particles of the ferrite with sizes ranging from 28 to 120 nm depending on the sintering time were observed. The coercivity values for different heat-treatment samples were found to be in the range from 15.2 to 100 Oe. The combination of zinc content and sintering times leads to samples with saturation magnetization ranging from 12.25 to 17.82 emu/g.     

Structure and Magnetic Properties of Nickel–Zinc Ferrite Nanoparticles Prepared by Glass Crystallization Method

Summary. The magnetic and microstructure properties of Fe₂O₃–0.4NiO–0.6ZnO–B₂O₃ glass system, which was subjected to heat treatment in order to induce a magnetic crystalline phase (Ni_0.4Zn_0.6-Fe₂O₄ crystals) within the glass matrix, were investigated. DSC measurement was performed to reveal the crystallization temperature of the prepared glass sample. The obtained samples, produced by heat treatment at 765°C for various times (1, 1.5, 2, and 3 h), were characterized by X-ray diffraction, IR spectra, transmission electron microscopy, and vibrating sample magnetometer. The results indicated the formation of spinel Ni–Zn ferrite in the glass matrix. Particles of the ferrite with sizes ranging from 28 to 120 nm depending on the sintering time were observed. The coercivity values for different heat-treatment samples were found to be in the range from 15.2 to 100 Oe. The combination of zinc content and sintering times leads to samples with saturation magnetization ranging from 12.25 to 17.82 emu/g.     

Correlation between structural and optical properties of TiO<Subscript>2</Subscript>:ZnO thin films prepared by sol–gel method

We have studied structural and optical properties of thin films of TiO₂, doped with 5% ZnO and deposited on glass substrate (by the sol–gel method). Dip-coated thin films have been examined at different annealing temperatures (350–450 °C) and for various layer thicknesses (89–289 nm). Refractive index, porosity and energy band gap were calculated from the measured transmittance spectrum. The values of the index of refraction are in the range of 1.97–2.44, the porosity is in the range of 0.07–0.46 and the energy band gap is in the range of 3.32–3.43. The coefficient of transmission varies from 50 to 90%. In the case of the powder of TiO₂, doped with 5% ZnO, and aged for 3 months in ambient temperature, we have noticed the formation of the anatase phase (tetragonal structure with 20.23 nm grains). However, the undoped TiO₂ exhibits an amorphous phase. After heat treatments of thin films, titanium oxide starts to crystallize at the annealing temperature 350 °C. The obtained structures are anatase and brookite. The calculated grain size, depending on the annealing temperature and the layer thickness, is in the range of 8.61–29.48 nm.     

Synthesis,IR spectrum,thermal property and crystal structure of cyano-bridged heteronuclear polymeric complex, [Cd(teta)Ni(μ-CN)<Subscript>2</Subscript>(CN)<Subscript>2</Subscript>] · 2H<Subscript>2</Subscript>O

The cyano-bridged heteronuclear polymeric complex, [Cd(teta)Ni(μ-CN)₂(CN)₂] · 2H₂O (1), (teta = triethylenetetraamine) was synthesized and characterized by FT-IR spectroscopy, thermal analysis and single crystal X-ray diffraction techniques. It crystallizes in the orthorhombic system, space group Pccn. The asymmetric unit also contains two uncoordinated water molecules. The coordination geometry around the Cd(II) centre is a highly distorted octahedral. In the crystal structure, intramolecular N–H···O and intermolecular N–H···O, O–H···O and O–H···N hydrogen bonds, beside the cyano-bridged chains made up of tetracyanonickelate ions coordinated to Cd(II) ions, where the Ni(II) ion is coordinated by four cyanoligands in a square-planar arrangement, link the molecules into polymeric networks parallel to (001) plane, where the hydrogen bonded water molecules occupy the cavities between the layers. The FT-IR spectrum was reported in the 4,000–400 cm⁻¹ region. Vibration assignments were given for all the observed bands and the spectral feature also supported the structure of the polymeric complex. The decomposition reaction takes place in the temperature range 20–1,000 °C in the static air atmosphere.     

The effect of Sb content on glass-forming ability,the thermal stability,and crystallization of Ge–Se chalcogenide glass

The glass formation and devitrification of intermediate alloys in the Sb–Ge–Se system were studied by differential scanning calorimetry. A comparison of various simple quantitative methods to assess the level of stability of the glassy materials in the bove mentioned system is presented. All of these methods are based on characteristic temperatures, such as the glass transition temperature, T _g, the onset temperature of crystallization, T _in , the temperature corresponding to the maximum crystallization rate, T _p, or the melting temperature, T _m . In this case, k _gl may be more suitable for estimating the glass thermal stability in above composition range than ΩT. In this work the parameter K _r (T) is added to the stability criteria. The thermal stability of some ternary compounds of the Sb–Ge–Se type has been evaluated experimentally and correlated with the activation energies of crystallization by this kinetic criterion and compared with those evaluated by other criteria. All the results of criteria and kinetic parameter K _r (T) confirm that the thermal stability decrease with increasing Sb content in the glassy system. The crystallization results are analyzed and both the activation energy of crystallization process and the crystallization mechanism are characterized. Finally, identification of the crystalline phases was made by recording the X-ray diffraction pattern of the transformed material. This pattern shows the existence of microcrystallites of two phases, the first is germanium Selenide GeSe₂ and the second is Sb₂Se₃ in amorphous matrix for annealed of Sb_2.5Ge_22.5Se₇₅ and Sb₁₀Ge₁₅Se₇₅ glass.     

Preparation of γ-Alumina Membranes From Sulphuric Electrolyte Anodic Alumina and Its Transition to α-Alumina

Gamma-alumina membrane was prepared from anodic (amorphous) alumina (AA) obtained in a sulphuric acid electrolyte. The transformation scheme, i.e., the crystallization to form metastable alumina polymorphs and the final transition to α-Al₂O₃ with heating was studied by TG-DTA and X-ray diffraction (XRD) using fixed time (FT) method. When heating at a constant rate, the crystallization occurred at 900°C or higher and the final formation of α-Al₂O₃ occurred at 1250°C or higher, which temperatures were higher than the case of using anodic (amorphous) alumina prepared from oxalic acid electrolyte. Relative content of S of the products was obtained by transmission electron microscope (TEM)-energy dispersive spectroscopy (EDS). The proposed thermal change of anodic alumina membrane prepared from sulphuric acid is as follows: 1. At temperatures lower than ca 910°C: Formation of a quasi-crystalline phase or a polycrystalline phase (γ-, δ- and θ-Al₂O₃); 2. 910–960°C: Progressive crystallization by the migration of S toward the surface within the amorphous or the quasi-crystalline phase, forming S-rich region near the surface; 3. 960°C: Change of membrane morphology and the quasi-crystalline phase due to the rapid discharge of gaseous SO₂; 4. 960–1240°C: Crystallization of γ-Al₂O₃ accompanying δ-Al₂O₃; and 5. 1240°C: Transition from γ-Al₂O₃ (+tr. δ-Al₂O₃) into the stable α-Al₂O₃. The amorphization which occurs by the exothermic and the subsequent endothermic reaction suggests the incorporation of SO₃ groups in the quasi-crystalline structure. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mechanisms of Incipient Chemical Reaction between Ca(OH)2and SiO2under Moderate Mechanical Stressing

The structure of mechanically induced precursor in an amorphous state between Ca(OH)₂and SiO₂was determined by solid state MAS and CP/MAS²⁹SiNMR spectroscopies, and the changes in the behavior of crystallization by linear heating were studied by X-ray diffractometry. A large broad Q₀peak due to the short-range order of hydrated calcium silicates appeared in the²⁹SiNMR spectra of mechanically induced precursors. The formation of the new ordering is confined in the near-surface region of SiO₂particles, as confirmed by comparing MAS and CP/MASNMR spectra. When the amorphous precursor obtained above was subsequently heated, a rapid solid state reaction led to single phasedα-CaSiO₃as the final product.