Kinetics of Nonisothermal Crystallization of Bi2Sr2CaCu2Ox Glasses with Different Copper Valence States

The kinetics of the nonisothermal crystallization process in Bi₂Sr₂CaCu₂O_x glasses with different copper valance states, R(Cu⁺) = Cu⁺/(Cu⁺+ Cu²⁺) = 0.79–0.99, were examined by using differential scanning calorimetry. Four different kinetic equations were used for the data analyses. It was found that the values of activation energy for crystal growth, E _a, decreased with increasing Cu⁺ content. The value of E _a, which was estimated from the modified Ozawa equation, for the glass with R(Cu⁺) = 0.79 was 483 kJ·mol^-1 and for the glass with R(Cu⁺) = 0.99 was 353 kJ·mol^-1. In the former glass, the number of crystal nuclei was almost independent of the heating rate. But, in the latter glass, the number of crystal nuclei varied significantly with the heating rate. The crystallization mechanism in the glasses was closely related to the thermal stability and viscosity of glasses: the glass with R(Cu⁺) = 0.99 showing a high thermal stability and a low activation energy for viscous flow had a small value of E _a. The plot of In ( T _p²/α) against 1/ T _p, where T _p and α are crystallization peak temperature and heating rate, respectively, gave a reasonable value of E _a for the glass with R(Cu⁺) = 0.79 but was unsuitable for the evaluation of E _a for crystal growth of the glass with R(Cu⁺) = 0.99.     

High-Temperature Dynamic Mechanical Thermal Analysis of a Lithium Zinc Silicate Glass-Ceramic

High-temperature dynamic mechanical thermal analysis (DMTA) has been used to study a lithium zinc silicate glassceramic. The DMTA technique was capable of measuring the glass transition temperature of the original starting glass composition, as well as being able to follow crystallization and structural changes within the glass. The greater sensitivity of DMTA compared to differential thermal analysis, or differential scanning calorimetry, enabled the detection and measurement of the glass transition temperature of the residual glass phase in a largely crystalline glassceramic. An activation energy of 542 kJ·mol^-1 for this glass transition was obtained from a multifrequency experiment. A second transition was found in the crystallized glassceramics, and this is thought to be due to a crystal phase transition. Some evidence was also found supporting the view that in these glasses nucleation occurs by an amorphous phase separation mechanism.     

Kinetics of Enthalpy Relaxation at the Glass Transition in Ternary Tellurite Glasses

The kinetics of enthalpy relaxation (recovery) at the glass transition in x K₂O·(20− x )MgO·80TeO₂ glasses has been examined from heat capacity measurements using differential scanning calorimetry to clarify the features of the structural relaxation in ternary TeO₂-based glasses. Ternary glasses such as 10K₂O·10MgO·80TeO₂ show high thermal resistance against crystallization compared with binary glasses. The degree of fragility m estimated from the activation energy for viscous flow E _η and the glass transition temperature T _g is m = 55–62, indicating a fragile character in TeO₂-based glasses. Large heat capacity changes of 43.1–48.2 J·mol⁻¹·K⁻¹ are also observed at the glass transition. The activation energy for enthalpy relaxation Δ H is evaluated from the cooling rate dependence of the limiting fictive temperature, and values of Δ H = 897–1268 kJ·mol⁻¹ are obtained. Negative deviation from additivity in Δ H is also observed. Values of the Kovacs–Aklonis–Huchinson–Ramos (KAHR) parameter θ estimated from Δ H and T _g are 0.33–0.42 K⁻¹. It has been proposed that ternary glasses have more homogeneous and constrained glass structure compared with binary glasses.     

Devitrification Kinetics and Mechanism of K2O–CaO–SrO–BaO–B2O3–SiO2 Glass-Ceramic

The devitrification kinetics and mechanism of a low-dielectric, low-temperature, cofirable K₂O–CaO–SrO–BaO–B₂O₃–SiO₂ glass-ceramic have been investigated. Crystalline phases including cristobalite (SiO₂) and pseudowollastonite ((Ca,Ba,Sr) SiO₃) are formed during firing. Activation energy analysis shows that the nucleation of the crystalline phases is controlled by phase separation of the glass. The crystallization kinetics of both cristobalite and pseudowollastonite obey Avrami-like behavior, and the results show an apparent activation energy close to that of the diffusion of alkaline and alkali ions in the glass, suggesting that diffusion is rate limiting. The above conclusion is further supported by analysis of measured growth rates.     

Crystallization of (Na2O–MgO)–CaO–Al2O3–SiO2 Glassy Systems Formulated from Waste Products

Aluminosilicate and silicate glass-ceramics were obtained from controlled devitrification of CaO–Al₂O₃–SiO₂ glassy systems starting from Spanish and Italian coal fly ash or Italian municipal incinerator slag mixed with other byproducts, such as glass cullet and dolomite. The nucleation mechanism and the crystallization kinetics were investigated by thermal, diffractometric, and microstructural measurements. Moreover, the experimentally observed devitrification and the identification of the crystalline phases formed were compared with the indications derived from Ginsberg, Raschin-Tschetveritkov, and Lebedeva diagrams used for petrological glass-ceramics. All the glasses showed a good crystallization tendency with the formation of dendritic pyroxene and acicular wollastonite together with feldspar and iron spinels starting from the surface. The activation energy values for crystallization ranging from 472 to 832 kJ ·mol⁻¹ were found to be close to those typical for aluminosilicate glasses; moreover, the possibility to vitrify and devitrify up to 100 wt% of slag and up to 40–50 wt% of ash mixed with glass cullet and dolomite makes the vitrification treatment a suitable disposal procedure.     

Internal Friction, Dielectric Loss, and Ionic Conductivity of Tetragonal ZrO2-3% Y2O3 (Y-TZP)

The defect structure in 3 mol% Y-TZP was studied by correlated internal friction, dielectric loss, and ionic conductivity experiments. A prominent mechanical and dielectric loss peak occurs in the temperature range between 380 and 550 K that depends on the frequency of measurement. The relaxation parameters were determined as H_m = 90 ± 3 kJ·mol⁻¹, τ_∞= (1.0_+1.5^−0.6) × 10₋₁₄ s for the mechanical relaxation and H_d = 84 ± 3 kJ·mol⁻¹, τ_∞= (1.6_+1.7^−0.9) × 10^–13 s for the dielectric relaxation. The ionic conductivity below 790 K is controlled by an activation enthalpy of H _σ= 89 ± 3 kJ·mol⁻¹; at higher temperatures H _σ= 60 ± 3 kJ·mol^–1. An atomistic model is presented which assumes that oxygen vacancies are trapped by yttrium ions forming anisotropic complexes which—by reorientation—cause anelastic and dielectric relaxation. At higher temperatures (>790 K) these complexes are dissociated, which leads to the reduced activation enthalpy for ionic conductivity.     

Secondary Grain Growth of Li2O-A12O3-SiO2-TiO2 Glass-Ceramics

The kinetics of secondary grain growth in a Ti0₂-nucleated β-spodumene solid-solution glass-ceramic was studied. The thermal stability of the grains was excellent. Grain growth followed the cube-root-of-time law. The activation energy of the grain boundary migration was 55 ± 10 kcal/mol. Grain growth inhibition due to Ti0₂ precipitates and the residual glassy phase was closely examined. The excellent thermal stability of the grains is due to grain growth inhibition by the residual glassy phase, not by rutile precipitates. It is suggested that the diffusion of A²⁺, and probably the simultaneous diffusion of Li⁺, through the residual glass is the rate-limiting process for the grain boundary migration.     

Kinetics of Dissolution and Crystallization in a β-Spodumene Glass-Ceramic

The kinetics of partial melting in a β-spodumene glass-ceramic was measured by holding fully crystallized material above the solidus but below the liquidus temperature. The kinetics was apparently limited by the rate of diffusion of silica in the crystalline phase. The measured lattice diffusivity of silica was 8×10⁻¹⁰ exp(-251 kJ mol⁻¹ /RT ) m² s⁻¹ with a possible error of ±60 kJ mol⁻¹ in the activation energy. The partially molten samples were heat-treated below the solidus to obtain the kinetics of crystallization, which was found to be interface-reaction-controlled.     

Crystallization Kinetics and Properties of Nonstoichiometric Cordierite-Based Thick-Film Dielectrics

Dielectric thick films based on a nonstoichiometric cordierite (2.4MgO·2Al₂O₃·5SiO₂, containing 3 wt% B₂O₃, 3 wt% P₂O₅, and 3 wt% PbO) were investigated, in regard to their microstructure, crystallization kinetics, and properties. A stable glass-ceramic thick-film microstructure that was formed on a 96% alumina substrate was observed after firing at a temperature of 915°C for 30 min in a nitrogen atmosphere. No µ-cordierite was observed in the X-ray diffraction (XRD) patterns of the thick film. The crystallization kinetics were studied via quantitative XRD analysis using the Avrami equation, and the rate constant increased as the temperature increased. The decreasing tendency of the Avrami parameter, relative to temperature, suggested a change in growth directionality during crystallization. The activation energy for crystallization of the thick film was determined to be ~83 kcal/mol (~350 kJ/mol). The coefficient of thermal expansion (CTE) and the dielectric constant of the glass phase were evaluated using the bulk-sample data. For the case of a 3-wt%-PbO sample fired at 950°C for 30 min in a nitrogen atmosphere, the remaining glass was estimated, using the parallel mixing rule, to have a dielectric constant of 15.3 at 1 MHz. The dielectric constant of the remaining glass was dependent on the PbO content and the heat-treatment temperature. The estimated CTE of the remaining glass for the 3-wt%-PbO sample was 19 × 10^-6/°C.     

Non-Isothermal Crystallization Kinetics of a Blast Furnace Slag Glass

The capability of the blast furnace slag to be vitrified was investigated by means of differential thermal analysis (DTA), scanning electron microscopy, and X-ray diffraction (XRD) in samples prepared by melting without the addition of any other ingredients. Differential thermal analysis revealed the presence of two major exothermic peaks. XRD indicates the presence of more than one crystalline phase: gehlenite Ca₂Al₂SiO₇, BaAl₂Si₂O₈, and pyroxene Ca(Mg,Al)(Si,Al)₂O₆. The activation energy of crystallization was estimated on the basis of DTA carried out on particle size 312–500 μm at different heating rates. Analysis of non-isothermal DTA data yielded values of 457.5 KJ/mol and 2.21 for the activation energy of crystallization and the Avrami exponent, respectively. A value for the activation energy corresponding to structural relaxation at temperatures around the glass transition has also been determined for this glass.     

Li2O─SiO2─Al2O3─MeIIO Glass-Ceramic Systems for Tile Glaze Applications

In order to verify the possibility of using glass-ceramic materials as tile coatings, the devitrification processes of three industrial formulations belonging to the Li₂O─Al₂O₃─SiO₂ glass-ceramic system were investigated by differential thermal analysis, X-ray diffractometry, scanning electron microscopy, and IR spectroscopy. Compositional variations were made by addition of large amounts of MgO or CaO or PbO (ZnO) oxides as well as through smaller additions of other oxides. In these systems the surface crystallization contributes appreciably to the bulk crystallization mechanism. All the systems investigated show a high tendency toward crystallization even at very high heating rates, developing a very close network of interlocked crystals of synthetic β-spodumene-silica solid solutions (LiAlSi₄O₁₀). The results of this research are expected to establish the conditions under which these glass-ceramic systems can be practically used as tile glazes.     

Raman Study of Isothermal Devitrificatn Kinetics of NaPO3 Glass

A laser Raman spectroscopic method was developed by which the increase in crystallinity of a devitrifying glass can be monitored continuously and isothermally at the devitrification temperature, without sample quenching, in a molecularly specific manner that yields high-quality kinetic data for the glass-crystal transformation. Applied to the study of vitreous (NaPO₃)_n( gl ), the method yielded continuous data which were fitted, by a nonlinear least-squares method, to an equation of the form α( t ) = 1- exp (- k_Nt^N ) with sufficient precision to permit optimization with respect to both N and k_N. The values of N and k_N vary with glass thermal history, i.e. with variations in time of heating at T_g for otherwise identical samples, although the material does not phase separate. The activation energy from any plot of In ( k_N )^{1/ N} vs (1/ T ) does not vary, but the offset of the curves indicates different preexponential terms in the Arrhenius expression. The results are analyzed in terms of current theories.     

Electrical Conductivity of Zirconia Stabilized with Scandia and Yttria

Electrical conductivity of zirconia stabilized with scandia and yttria (Sc₂O₃+Y₂O₃= 8 mol%) has been measured by the complex impedance method in the temperature range 573 to 1173 K. With increasing Sc₂O₃ concentration, electrical conductivity increases at temperatures above 640 K, but it decreases below this temperature. Electrical conductivity in the electrolytes examined is a result of two processes: an activation energy of 59 to 79 kJ·mol⁻¹ predominant at high temperatures and an activation energy of 109 to 125 kJ·mol⁻¹ predominant at low temperatures.     

Mechanism and Kinetics of Reactions in the System Ni-Ti-O

Reactions at T =600° to 1250°C between nickel oxide and titanium dioxide, metallic nickel and titanium dioxide, and the oxidation of Ni-TiO₂ cermet were studied. The formation of compouuds was observed by temperature-programmed reduction in hydrogen flow. At T > 600°C, NiTiO₃ forms. During the initial stages of the reaction, a solid solution of TiO₂ in NiO is formed. The reaction is influenced by anatase-rutile transformation. The apparent activation energy of the formation of NiTiO₃ is considerably lower during the reaction in the mixture of metallic nickel with titanium dioxide (218 kJ°mol^-1 for powder and 255 kJ.mo1^-1 for cermet oxidation) than it is for the reaction between NiO and rutile (385 kJ.mol^-1).     

Synthesis and Sintering of Cerium(II) Monosulfide

Cerium monosulfide (CeS) powder was synthesized by the reduction of Ce₂S₃ powder with metallic Ce, which was obtained from ceria (CeO₂) powder using carbon disulfide (CS₂) gas. To obtain the maximum amount of CeS from a mixture of Ce₂S₃ and Ce, an excess amount of metallic Ce, a stoichiometric composition, was necessary in the synthesis at 1273 K for 10.8 ks. The preliminary sintering experiments also were performed using a synthetic CeS powder containing a small amount of Ce, Ce₂O₂S, and β-Ce₂S₃ as impurities. It was found that the oxygen content in the sintered compact decreases gradually as the sintering temperature increases, because of the removal of the impurities due to the evaporation of the volatile CeO. Single-phase CeS was formed by sintering at 2173 K. To evaluate the activation energy for densification of single-phase CeS, a CeS powder was prepared by milling an initial sintered compact and was used as an ingredient for hot-press experiments. Densification data during hot-press sintering were analyzed using a kinetic equation, showing that boundary diffusion is a rate-limiting process. The results suggest that this boundary diffusion model can explain well the densification data, with an apparent activation energy of 479 kJ·mol^-1.     

In Situ Whisker-Reinforced AlPO4-Modified β-Eucryptite Glass-Ceramic: I, Morphology and Crystallization Kinetics

A whisker-reinforced glass-ceramic composite in the Li₂O-Al₂O₃-SiO₂-P₂O₅ system has been fabricated by a single-stage process that simultaneously forms the glass-ceramic material and whiskers of TiO₂ in situ . The method utilizes typical glass-ceramic processing techniques and requires precise addition of a TiO₂ nucleation agent during the glass-melting operation, followed by controlled nucleation and crystallization. The maximum nucleation temperature, 740°C, was determined by differential thermal analysis (DTA), and the result was confirmed by scanning electron microscopy (SEM) of the microstructures formed by nucleation/isothermal crystallization heat treatments. The apparent activation energy for crystallization of the material was determined to be 285 ± 3 kj/mol. The average aspect ratio of the TiO₂ whiskers depends on temperature and time during crystallization. The X-ray diffraction patterns of the in situ composite show that eucryptite(ss) and rutile exist as two different phases; no additional phases were observed. Elemental X-ray mapping by electron microprobe indicates that these highly crystallized composites consist of modified β-eucryptite glass-ceramic matrix and acicular grains of TiO₂.     

Sintering, Crystallization, and Properties of B2O3/P2O5-Doped Li2O·Al2O3·4SiO2 Glass-Ceramics

Sintering, crystallization, microstructure, and thermal expansion of Li₂O·Al₂O₃·4SiO₂ glass-ceramics doped with B₂O₃, P₂O₅, or (B₂O₃+ P₂O₅) have been investigated. On heating the glass powder compacts, the glassy phase first crystallized into high-quartz s.s., which transformed into β-spodumene after the crystallization process was essentially complete. The effects of dopants on the crystallization of glass to high-quartz s.s. and the subsequent transformation of high-quartz s.s. to β-spodumene were discussed. The major densification occurred only in the early stage of sintering time due to the rapid crystallization. All dopants were found to promote the densification of the glass powders. The effect of doping on the densification can fairly well be explained by the crystallization tendency. All samples heated to 950°C exhibited a negative coefficient of thermal expansion ranging from about −4.7 × 10^-6 to −0.1 × 10^-6 K^-1. Codoping of B₂O₃ and P₂O₅ resulted in the highest densification and an extremely low coefficient of thermal expansion.     

Effect of Al2O3 Additions on the Thermal Expansion Behavior of a Li2O-ZnO-SiO2 Glass-Ceramic

The effect of Al₂O₃ substitution on the thermal expansion behavior of a Li₂O-ZnO-SiO₂ glass-ceramic was investigated using differential thermal analysis, X-ray diffractometry, and dilatometry. The coefficient of thermal expansion of the original Al₂O₃-free glass-ceramic measured between 20° and 900°C decreased from 12.4 × 10^-6°C^-1 to 6.3 × 10^-6°C^-1 with Al₂O₃ substitution for ZnO up to 11 wt%. The results were correlated to the changes in the phase assemblage with Al₂O₃ addition.     

Directionally Crystallized Sodium Lead Silicate Glass-Ceramic

The iron-nucleated devitrification of a sodium lead silicate glass was studied; it produces a glass-ceramic consisting of sodium metasilicate crystals in a lead silicate glass matrix. The nucleating agent is not incorporated in the crystal phase, Anisotropic devitrification of the glass in a thermal gradient produces an aligned microstructure with needles of sodium metasilicate oriented with the c axis parallel to the direction of crystallization.     

Study of Devitrification of Lithium Glass

An etching technique has been found which allows the devitrification phenomena in glass to be followed. Using this technique a study was made on 30 mole % Li₂O-SiO₂ glass at temperatures of 520°, 560°, and 600°C. By studying the average crystal size versus time at a given temperature, the growth rate was determined. It was found to be 7.5 × 10⁻⁴, 8 × 10⁻³, and 3 × 10⁻² cm. per hour for the three temperatures. The nucleation of crystals appeared to be linear with time at each temperature. An activation energy of 49 kcal. per mole was found for the growth process. Crystals were identified as Li₂Si₂O₅.