Temperature Of Maximum Nucleation Rate Research Articles

The best diffusivity proxy for crystal nucleation in stoichiometric oxide glasses

Due to the difficulty in measuring the mobility of the “structural units” of supercooled liquids and glasses during crystal nucleation, the effective diffusivity (D) has been frequently estimated via viscosity (Dη) or nucleation time-lags (Dτ). However, it has been recently reported that, below the glass transition temperature (Tg), the experimental steady-state nucleation rates (Ist) can reach much higher values than those predicted by the Classical Nucleation Theory (CNT) when assuming D ≈ Dη or D ≈ Dτ. Hence, another possibility would be inferring D from experimental crystal growth velocity data (D ≈ DU), which is a natural approach since nucleation and growth are likely controlled by the same mass transfer process through the crystal/liquid interface. Therefore, the approximations D ≈ Dη and D ≈ DU were tested in this work using lithium disilicate glass as a model system. To avoid the influence of compositional changes, we measured the nucleation rates, growth velocity, and viscosity using samples of the same glass batch. With this strategy, we found that the very long experimental nucleation times implemented below Tg shifted the experimental temperature of maximum nucleation rate to temperatures lower than those previously reported for this glass former. Most important is that the CNT gives a much better description of the experimental Ist(T) data in the whole analyzed temperature range if D is approximated by DU instead of Dη, indicating that crystal growth is indeed an adequate proxy for nucleation diffusivity.

Microstructure and mechanical properties of a nucleant-free basaltic glass-ceramic

ABSTRACTThe crystallisation kinetics of magnetite from basalt glass was studied using glass samples prepared by melting basaltic rocks from the Serra Geral Formation, without the addition of the nucleating agent, using differential scanning calorimetry under isothermal conditions. The temperature of maximum nucleation rate for magnetite was determined as 650 C. Upon further heating to 860 C, a second phase crystallises, which was identified by X-ray diffraction as a pyroxene (augite, Ca(Mg,Fe)SiO). The best mechanical properties were exhibited by a glass-ceramic nucleated at 650 C for 0.5 h and further heat-treated at 860 C for 1 h. The basalt glass-ceramic exhibited a fine grain and homogeneous microstructure, with high hardness and reasonable resistant to NaOH attack.

Study of the crystallization of leucite in feldspar glass matrix

Feldspathic glass–ceramics reinforced with leucite are usually used in dental prosthesis. This study focused on leucite crystallization kinetics due to its importance to the end product of a dental crown processing. Leucite grains were nucleated and grown from feldspathic glass frit powders with particle size smaller than 45 μm. The nucleation and crystallization kinetics of leucite crystals in the glass matrix was investigated under isothermal and non-isothermal conditions through differential thermal analysis. The samples were also characterized by X-ray diffraction and scanning electron microscopy. The temperature of maximum nucleation rate was determined from the DTA curves of samples heat treated at different temperatures. The activation energy (E) of leucite crystallization was determined by the Kissinger method and the Avrami parameter (n) indicated that surface crystallization is the dominant mechanism in the glass.

Nucleation and growth behavior of glasses in the TeO 2–Bi 2O 3–ZnO glass system

The kinetics of crystallization of glasses in the (90 − x)TeO 2–10Bi 2O 3–xZnO system with x = 15, 17.5, 20 and 25 have been studied using differential thermal analysis (DTA), hot stage XRD and optical microscopy. Thermal properties, activation energy for crystallization, Johnson–Mehl–Avrami exponent, nucleation and growth regimes and rates were determined and shown to depend on glass composition. These variations were related to the domain of glass formation in the TeO 2–Bi 2O 3–ZnO ternary glass system: the glasses with x = 15 and 25 are at the limit of the glass formation domain while the glasses with x = 17.5 and 20 are in the middle of the glass-forming region. From the Johnson–Mehl–Avrami parameter, which increases slightly from 1.7 to 2.2 when x increases from 15 to 25, and from optical micrographs, the predominant crystallization in the investigated glasses is expected to be 2 dimensional bulk crystallization governed by diffusion. Using XRD, we found that Bi 2O 3 crystals are the first crystals to form in the glasses when heat treated at their respective temperatures of maximum nucleation. When heat treated at a higher temperature the Bi 2O 3 crystals were found to transform into Bi 2Te 4O 11. We observed an increase in the crystal size when the glasses were heat treated at their respective temperatures of maximum nucleation revealing a probable overlap between the nucleation and growth regimes in all the glasses, the largest overlap being for the glass with x = 15. Thus, this glass is not an appropriate material candidate for applications which require controlled nucleation and growth as a large distribution of crystal sizes can be obtained in the glass when heat treated at its temperature of maximum nucleation rate. The glass with x = 25 is suspected to be also an inappropriate candidate for this kind of applications as it has a higher tendency to fully crystallize in a small temperature range than the other glasses as evidenced by its high N q and I n.

Controlled crystallization and ionic conductivity of a nanostructured LiAlGePO 4 glass–ceramic

A Li 1.5[Al 0.5Ge 1.5(PO 4) 3] glass composition was subjected to several crystallization treatments to obtain glass–ceramics with controlled microstructures. The glass transition ( T g), crystallization onset ( T x ) and melting ( T m) temperatures of the parent glass were characterized by differential scanning calorimetry (DSC). The glass has a reduced glass transition temperature T gr = T g/ T m = 0.57 indicating the possibility of internal nucleation. This assumption was corroborated by the similar DSC crystallization peaks from monolithic and powder samples. The temperature of the maximum nucleation rate was estimated by DSC. Different microstructures were produced by double heat treatments, in which crystal nucleation was processed at the estimated temperature of maximum nucleation rate for different lengths of time. Crystals were subsequently grown at an intermediate temperature between T g and T x . Single phase glass–ceramics with Nasicon structures and grain sizes ranging from 220 nm to 8 μm were then synthesized and the influence of the microstructure on the electrical conductivity was analysed. The results showed that the larger the average grain size, the higher the electrical conductivity. Controlled glass crystallization allowed for the synthesis of glass–ceramics with fine microstructures and higher electrical conductivity than those of ceramics with the same composition obtained by the classical sintering route and reported in literature.

Thermal behaviour and properties of Na2O-TiO2-P2O5 glasses

Differential scanning calorimetry (DSC) and thermomechanical analysis (TMA) were used to study the thermal behaviour of (50-x)Na2O-xTiO2-50P2O5 and 45Na2O-yTiO2-(55-y)P2O5 glasses. The addition of TiO2 to the starting glasses (x=0 and y=5 mol% TiO2) resulted in a nonlinear increase of glass transition temperature and dilatation softening temperature, whereas the thermal expansion coefficient decreased. All prepared glasses crystallize under heating within the temperature range of 300–610°C. The contribution of the surface crystallization mechanism over the internal one increases with increasing TiO2 content. With increasing TiO2 content the temperature of maximum nucleation rate is also gradually shifted from a value close to the glass transition temperature towards the crystallization temperature. X-ray diffraction measurements showed that the major compounds formed by glass crystallization were NaPO3, TiP2O7 and NaTi2(PO4)3. The chemical durability of the glasses without titanium oxide is very poor, but with the replacement of Na2O or P2O5 by TiO2, it increases sharply.

Ionic blocking effect in partially crystallized lithium disilicate

A lithium disilicate glass was heat treated at 454°C, the temperature of maximum nucleation rate, up to 25days. At this temperature the growth rate is negligible, and therefore these thermal treatments lead to a glassy matrix in which ellipsoidal crystals of about the same size were randomly dispersed. Crystallized volume fraction (α) of up to 20% was reached. Electrical conductivity of glassy, partially crystallized, and 100% crystallized lithium disilicate was then measured by impedance spectroscopy. The crystallized surface layer was eliminated by polishing in order to solely analyze the effect of bulk crystals. The complex plane plots of impedance data showed one or two semicircles depending on (α). The high frequency semicircle represents the vitreous matrix while the low frequency one was ascribed to the blocking effect imposed to the lithium ions by the existing crystals in the partially crystallized samples. This hypothesis was confirmed by the analysis of the relaxation frequency related to each one of the semicircles and by comparison to the relaxation frequency of lithium disilicate glass and crystal. Results show that the blocking effect produces a second semicircle in the impedance diagram starting from a crystallized volume fraction of 15%.

Re-examination of effects of nucleation temperature and time on glass crystallization

The effects of nucleation temperature and time on the kinetics of non-isothermal glass crystallization have been re-examined to demonstrate the limitations of some approximate solutions used to extract kinetic parameters from differential thermal analysis (DTA) experiments. Those features were analyzed by numerical solutions of equations describing the dependence of fraction crystallized on the rates of nucleation and growth, and the corresponding transient time, reported for lithium disilicate. It was shown that the temperature of maximum nucleation rate varies on changing the nucleation time. Some guidelines were established to assist the selection of suitable conditions to perform crystallization studies by DTA, and to extract the values of activation energy and dimensionality of growth from the dependences of crystallization peak temperature on heating rate, and nucleation time. The main limitations of these methods were identified and discussed.

Homogeneous nucleation versus glass transition temperature of silicate glasses

This paper provides experimental and theoretical evidence for a correlation between the maximum internal nucleation rate, I max= I( T max) [where T max is the temperature of maximum nucleation rate] and the reduced glass transition temperature, T gr, for 51 glass-forming liquids. In addition, it demonstrates an analogous correlation between T max, the time-lag of nucleation at T max and the reduced glass transition temperature. An explanation is given for these remarkable trends.

Isothermal and non-isothermal crystallization kinetics of zinc-aluminosilicate glasses

Isothermal and non-isothermal differential thermal analysis (DTA), and X-ray diffraction (XRD) were used to study the nucleation and crystallization behavior of multicomponent Li 2O–ZnO–Al 2O 3–B 2O 3–SiO 2 glasses with ZrO 2 and TiO 2 as nucleating agents. The temperature of maximum nucleation rate for investigated glasses was found to be 1028 K. The crystallization of two phases (mullite s.s. and gahnite) occurs in a narrow temperature and time interval, respectively, therefore, the DTA curves are characterized by the superposition of exothermic peaks. Recently derived mathematical models have been applied for resolving the overlapping. The models enabled establishing the kinetic parameters for crystal growth of individual phases. The activation energies for crystal growth were found to be in the range of 459–471 and 368–386 kJ mol −1 for mullite and gahnite, respectively. Avrami exponent n reveals three-dimensional growth of both phases.

Transparent surface crystallized glasses with optical non-linear LaBGeO 5 crystals

Transparent surface crystallized glasses consisting of ferroelectric stillwellite-type LaBGeO 5 crystalline phase and showing second harmonic generation were successfully prepared. The crystallized glasses were fabricated through two-step heat-treatment in air (e.g. 670 ° C, 3 h+750 ° C, 5 h) for 25La 2O 3·25B 2O 3·50GeO 2 glass. The temperature of maximum nucleation rate was determined to be 670°C from differential thermal analyses, which is consistent with the glass transition temperature. The LaBGeO 5 crystalline phase was confirmed to be formed at the surface region of about 60 μm and the grains were more oriented in the interior. The second harmonic intensity from the surface was in the order of 1/100 in comparison with that of quartz α-SiO 2.

Thermal Analysis of Nucleation and Growth of Crystalline Phases in Vitrified Industrial Wastes

Devitrification behavior of vitrified industrial waste (electric arc furnace dust and foundry sand) was examined by differential thermal analysis (DTA), X‐ray diffractometry, and scanning electron microscopy (SEM). Crystalline phases identified were primarily solid solutions of spinel and augite. The temperature of maximum nucleation rate for spinel was determined by DTA to be 635°C. Variations in the quench temperature resulted in changes of both grain size and volume fraction of an equixed spinel. The evolution of morphological changes upon heat treatment was examined by SEM. TiO2 additions were ineffective as spinel nucleating agents, because additions of >5 wt% resulted in decreased spinel formation and the crystallization of titanium‐iron and titanium‐divalent‐metal oxides. Controlling process and heat‐treatment temperatures to optimize the amount of the spinel and to achieve a high‐grade abrasive product also are discussed.

The non-isothermal devitrification of Li 2TiGe 3O 9 glass

The non-isothermal devitrification of a lithium titanium germanate glass of stoichiometry Li 2TiGe 3O 9 has been studied by differential thermal analysis. The temperature of maximum nucleation rate and the value of activation energy for crystal growth were evaluated from DTA curves. The influence on the crystallization process of the specific surface area of the samples and of the heat treatments of nucleation is emphasized. The investigated glass exhibited the same molar ratio of GeO 6/GeO 4 groups and the same T g as lithium tetragermanate glass.

Nucleation and crystallization kinetics of CaO-Al2O3-2SiO2 in powdered anorthite glass

The nucleation and crystallization kinetics of CaO-Al2O3-2SiO2 crystals in powdered anorthite glass with particle size <44 μm in which CaO-Al2O3-2SiO2 crystals were found to crystallize in the heating process of the glass, were studied by nonisothermal measurements using differential thermal analysis (DTA). The temperature of maximum nucleation rate was determined from the DTA curves of samples heat treated at different temperatures. The activation energy and kinetic parameters were simultaneously calculated from the DTA data using previously reported kinetic models. The crystallization process of a sample heat treated at the temperature of the maximum nucleation rate was fitted to kinetic equations with an Avrami constant, n≅2 and a dimensionality of crystal growth, m≅2, indicating that the constant number of nuclei of CaO-Al2O3-2SiO2 precipitated in a glass matrix grew two-dimensionally with an activation energy taken as an average of the values calculated by the Kissinger and also the Augis and Bennett method of 679±4 kJ mol−1.

Glass-ceramics from fly ash with added Li2O

Abstract The effects of the addition of Li2O to fly ash on the devitrification behaviour of the derived glass are investigated with the aid of differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The phases crystallizing first for the as-quenched glass and the glass sample previously nucleated are different. Solid solutions of lithium-magnesium silicate (Li(2 + 2x)Mg(1 − x)SiO4) and a small quantity of β-eucryptite (LiAlSiO4) solid solution are formed from the as-quenched glass, while only solid solutions of lithium-magnesium silicate are formed from the sample previously nucleated. In both cases, these crystalline phases are then converted, at higher temperatures, into a large amount of β-eucryptite solid solution together with small quantities of diopside crystals (CaMgSi2O6). Kinetic parameters for nucleation and crystal growth were estimated from the DTA curves. The temperature of maximum nucleation rate was 620°C and the activation energy for crystal growth E = 316 kJ mol−1. The crystal morphology was investigated by SEM, and the crystal shape was consistent with the morphological index n calculated by DTA. The glass-ceramic obtained from a previously nucleated glass showed a fine-grained texture.

Nucleation and crystal growth in a fly ash derived glass

The devitrification behaviour of a fly ash derived glass, examined by differential thermal analysis (DTA), X-ray diffraction and scanning electron microscopy (SEM), is reported and discussed. The crystallized phases were identified as mullite (3Al2O3·2SiO2) and anorthite (CaO·Al2O3·2SiO2). Kinetic parameters for nucleation and crystal growth were estimated from the DTA curves. The temperature of maximum nucleation rate was 790°C and the activation energy for crystal growth E=370 kJ mol−1. The crystal morphology was investigated by SEM and the crystal shape found to be consistent with the morphological index n calculated by DTA. The glass-ceramic obtained from a previously nucleated glass showed a fine-grained texture.

Influence of Dopants on Nucleation and Growth of High‐Quartz Solid Solution in Lithium Aluminosilicate Glass

The kinetic parameters of nucleation and crystal growth of high‐quartz solid solution in multicomponent lithium aluminosilicate glasses doped with various transition‐metalions were studied by nonisothermal DTA. The crystallization of glasses nucleated at different temperatures was carried out, and plots of the DTA peak versus the nucleation temperatures were used to determine the maximum nucleation rate temperature. Peak temperature data of nucleated samples at varying heating rates (5–20 K/min) were used to determine the activation energy for crystallization via the JMA equation. The temperature of maximum nucleation rate depends greatly on the doped transition‐ metal ions present. The activation energy for crystallization obtained for undoped glass or glasses doped with Fe2O3 is of the same order as that already published, and the Avrami exponent is consistent with predominantly three‐dimensional crystal growth. The much higher activation energy values for glasses doped with CoO could be a consequence of two crystallization processes proceeding simultaneously.

A study of the effects of CeO 2 on the crystallization of CaO/P 2O 5 glass

In previous studies, the effects of rare earth oxides on the physical properties of CaO/P 2O 5 glass-ceramics have been investigated. It was found that the addition of rare earth oxides elevated the physical properties of CaO/P 2O 5 glass-ceramics. CeO 2 played a key role as the nucleation agent. The main objective of the experiment is to examine the effect of the addition of CeO 2 on the crystallization of CaO/P 2O 5 glasses. Addition of CeO 2 appears to produce a higher temperature of maximum nucleation rate (615°C) than that of CaO/P 2O 5 glasses (580°C). The activation energy of crystal growth and morphology of crystal growth are also presented. It is found that the surface crystallization occurs in the presence of CeO 2.

Crystal nucleation and growth in an LiNaSiO 3 glass

The non-isothermal devitrification of LiNaSiO 3 glass was studied using differential thermal analysis and X-ray diffraction. A crystallisation mechanism in two steps was found. The temperature of maximum nucleation rate and the kinetic parameters of crystal growth were evaluated from the DTA curves.

Nucleation in glass-forming systems. A DTA study.

A simple method recently proposed (ref. 1) for evaluating the temperature of maximum nucleation rate is applied to several different glass-forming systems. The results well agree with those obtained by means of traditional methods.