Kinetic phenomena in non-crystalline materials studied by thermal analysis

The structural relaxation and viscosity behavior of Ge₃₈S₆₂ glass has been studied by thermomechanical analysis. The relaxation response to any thermal history is well described by the Tool-Naraynaswamy-Moynihan model. The apparent activation energy of structural relaxation is very close to the activation energy of viscous flow (Eη=478±12 kJ mol^-1). However, the activation energy of crystal growth obtained by optical microscopy is about one half of this value. Similar result has been obtained from isothermal DSC measurement (Ea=220±20 kJ mol^-1). The kinetic analysis of these data reveals interface controlled crystal growth with zero nucleation rate. This revised version was published online in July 2006 with corrections to the Cover Date.     

The kinetics of Al-Si spinel phase crystallization from calcined kaolin

The crystallization of Al-Si spinel from medium ordered kaolin with high content of kaolinite was investigated using the differential thermal analysis (DTA). The apparent activation energy of the process was evaluated from the dependence of exothermic peak of crystallization on heating rate. Within the applied interval of heating rate (1-40 K min⁻¹) the temperature of peak maximum increases from initial value of 1220.5 K in about 54.2 K. The apparent activation energy of the process 856±2 kJ mol⁻¹was calculated using the Kissinger equation. The growth morphology of Al-Si spinel crystal was evaluated from the Avrami parameter. The average value of morphology parameter determined within the observed interval of heating rate is 3.08±0.03. This value indicates that crystallization mechanism of Al-Si spinel phase proceeds by bulk nucleation of the new phase with constant number of nuclei and that the three-dimensional growth of crystals is controlled by the reaction rate on the phases interface.     

Nucleation and crystal growth in Na2O · 2 CaO · 3 SiO2 glass: a DTA study

The non-isothermal devitrification of Na₂O · 2 CaO · 3 SiO₂ glass has been studied by differential thermal analysis in order to evaluate, from DTA curves, the temperature of maximum nucleation rate, T_m, and the activation energy values, E_c, for crystal growth.The temperature, T_m=580°C, is very close to the glass transition temperature, T_g=570°C, and the value of E_c=78 Kcal mole^?1 for the surface crystal growth is nearly the same as the value E_c=89 kcal mole^?1 for the bulk crystal growth; both are consistent with the activation energy for viscous flow. It is also pointed out that the nucleation rate—temperature curve and the crystallization rate—temperature curve are partially overlapped.     

Crystallization Kinetics of Glassy As2Se3

Isothermal crystallization of bulk As₂Se₃ glass was studied in temperature range 270-360°C. Johnson-Mehl-Avrami (JMA) equation describes the crystallization process in the whole temperature range. By means of analysis of JMA equation the temperature dependence of kinetic exponent n was found, its value changes from 3.8 to 1.9 with increasing temperature. The relationship between the value of n and crystal morphology was briefly discussed. Furthermore the value of apparent activation energy E was determined as well as melting enthalpy. Temperature dependence of crystal growth rate was also determined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Investigation of the Crystallization Kinetics of Zn(Met)(AcO)_2·H_2O in Mixed Solution of Water and Acetone by Microcalorimetry

Introduction The complexes of zinc salts with -amino acid as additive have a wide application in medicine, foodstuff and cosmetics.1-3 The synthesis methods of the com-plexes of zinc salts with -amino acid have been re-viewed.4 The solubility of Zn(AcO)2-Met-H2O system at 298.15 K has ever been investigated by the semi-micro-phase equilibrium method.5 The phase diagram is a simple one, in which the solid complex of Zn(AcO)2 and Met can not be obtained at 298.15 K. Zn(Met)2+ in solution w…     

Crystallization in glasses monitored by thermomechanical analysis

Thermomechanical analysis (TMA) can be used as a sensitive tool to follow crystallization behavior in non-crystalline materials. Newly developed method is based on slowing down of sample deformation caused by viscous flow above the glass transition due to macroscopic crystal growth. It is shown that a typical TMA sigmoidal curve reasonably well corresponds to direct measurement of crystal growth kinetics by means of optical microscopy. The method has been used to study crystallization kinetics in Ge₃₈S₆₂ glass. The TMA measurement is able to detect earlier stages of crystallization than obtained by differential scanning calorimetry measurement. The activation energy obtained from the shift of extrapolated end of TMA curve with heating rate (E = 263 ± 7 kJ mol^?1) is similar to the activation energy of ??-GeS₂ crystal growth in Ge₃₈S₆₂ glass (E _G = 247 ± 23 kJ mol^?1) obtained from direct optical microscopy measurements.     

Investigation of the Crystallization Kinetics of Aqua Methioninezinc (II) Sulfate by Microcalorimetry

Summary.  The crystal growth process of aqua methioninezinc (II) sulfate (Zn(Met)SO₄·H₂O) from water and acetone was investigated using a Calvet microcalorimeter. The heat produced and the rate of heat production during the crystal growth process at 293.15, 295.15, 298.15, and 300.15 K were measured. On the basis of these results the rate constant, the Eyring parameters (the apparent activation enthalpy, the activation entropy, and the activation free energies), and the Arrhenius parameters (the activation energy, the pre-exponential factor) of the crystal growth process have been obtained. The results have shown that this crystal growth process accords with the Burton-Cabrera-Frank dislocation theory. Corresponding author. E-mail: weizhang@nwu.edu.cn Received August 16, 2002; accepted (revised) December 3, 2002 Published online May 15, 2003     

Synthesis and characterization of Na2O–CaO–SiO2 glass–ceramic

The Na₂O–CaO–SiO₂ ternary glass–ceramic with the composition of 49 mass% Na₂O, 20 mass% CaO, and 31 mass% SiO₂ was prepared by the conventional method. The ternary glass–ceramic was characterized using X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy techniques. The Na₂CaSiO₄ phase, having the cubic crystal system, with the crystallite size of 25.14 nm and lattice parameter of 0.7506 nm was determined from the XRD pattern. The activation energy of the glass–ceramic calculated from the DTA curves was found to be 162.02 kJ mol^?1. The Avrami exponent was found to be ~2 indicating a one-dimensional growth process. The mass loss percent from ambient temperature to 1,173 K is less than 1 %. The density was calculated to be 2,723 kg m^?3. The fine-grained microstructure with the particle sizes less than 1 μm was confirmed by the scanning electron microscope micrograph.     

Indomethacin: The Interplay between Structural Relaxation,Viscous Flow and Crystal Growth

Non-isothermal differential scanning calorimetry (DSC) was used to study the influences of particle size (d_aver) and heating rate (q⁺) on the structural relaxation, crystal growth and decomposition kinetics of amorphous indomethacin. The structural relaxation and decomposition processes exhibited d_aver-independent kinetics, with the q⁺ dependences based on the apparent activation energies of 342 and 106 kJ·mol⁻¹, respectively. The DSC-measured crystal growth kinetics played a dominant role in the nucleation throughout the total macroscopic amorphous-to-crystalline transformation: the change from the zero-order to the autocatalytic mechanism with increasing q⁺, the significant alteration of kinetics, with the storage below the glass transition temperature, and the accelerated crystallization due to mechanically induced defects. Whereas slow q⁺ led to the formation of the thermodynamically stable γ polymorph, fast q⁺ produced a significant amount of the metastable α polymorph. Mutual correlations between the macroscopic and microscopic crystal growth processes, and between the viscous flow and structural relaxation motions, were discussed based on the values of the corresponding activation energies. Notably, this approach helped us to distinguish between particular crystal growth modes in the case of the powdered indomethacin materials. Ediger’s decoupling parameter was used to quantify the relationship between the viscosity and crystal growth. The link between the cooperativity of structural domains, parameters of the Tool-Narayanaswamy-Moynihan relaxation model and microscopic crystal growth was proposed.     

Crystallite growth of rice husk ash silica

The effect of temperature of firing and time of annealing on crystallite size of silica obtained by burning boiled rice husk was studied by the X-ray broadening technique. The results showed that nuclei of disordered cristobalite were present in ash silica, and crystallite growth was governed by two processes, namely nucleation and growth taking place simultaneously with a rate varying with the temperature of treatment. The nucleation process manifested itself in the low temperature range of 800–900°C, while growth was more pronounced in the temperature range 1000–1100°C, occurring with different activation energies of 81.3 and 52.4 cal mole^?1, respectively. Both processes occurred with the same intensity at 964°C. A growth process and crystal perfection followed with an activation energy of 36.8 cal mole^?1.     

Non-isothermal crystallization kinetics of a Si–Ca–P–Mg bioactive glass

In this work, the crystallization process of a SiO₂–3CaO·P₂O₅–MgO glass was studied by non-isothermal measurements using differential thermal analysis carried out at various heating rates. X-ray diffraction at room and high temperature was used to identify and follow the evolution of crystalline phases with temperature. The activation energy associated with glass transition, (E _g), the activation energy for the crystallization of the primary crystalline phase (E _c), and the Avrami exponent (n) were determined under non-isothermal conditions using different equations, namely from Kissinger, Matusita & Sakka, and Osawa. A complex crystallization process was observed with associated activation energies reflecting the change of behavior during in situ crystal precipitation. It was found that the crystallization process was affected by the fraction of crystallization, (x), giving rise to decreasing activation energy values, E _c(x), with the increase of x. Values ranging from about 580 kJ mol^?1 for the lower crystallized volume fraction to about 480 kJ mol^?1 for volume fractions higher than 80 % were found. The Avrami exponents, calculated for the crystallization process at a constant heating rate of 10 °C min^?1, increased with the crystallized fraction, from 1.6 to 2, indicating that the number of nucleant sites is temperature dependent and that crystals grow as near needle-like structures.     

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.     

Some connections between viscoelastic properties of PVC and plasticized PVC and molecular kinetics

A coupling model that has been shown in the past to be capable of relating macroscopically measured relaxation parameters to molecular ones has been presented. In this article the coupling model is applied to the analysis of stress relaxation data collected by Cama and Sternstein on PVC and plasticized PVC. The Kohlrausch-Williams-Watts form, exp — (t/τ*)^1?n, using n = 0.77 is found to be capable of describing the stress relaxation master curve at temperatures below the glass transition, T_g. From the temperature-independent apparent activation energy found by Cama and Sternstein, the primitive activation energy of the α-relaxation was calculated to be 7.5 kcal/mol, which is a reasonable value for the energy barrier to internal rotational isomerism in PVC. Support for this value is found from the data on two plasticized PVCs with different T_gs and apparent activation energies. By applying the coupling model in a similar manner, the primitive activation energies were found to be 8.5 kcal/mol for PVC plasticized with 6 pph dioctylphthalate and 7.7 kcal/mol for PVC plasticized with 6 pph tricresyl phosphate. Within experimental uncertainties, the three primitive activation energies can be considered to be the same. This finding is consistent with the physical basis for primitive activation energy and its identification with the internal rotation barrier, which should be independent of the type and amount of plasticizer in the system. Analysis of Cama and Sternstein's data on the effect of repeated stress aging on stress relaxation of quenched samples of PVC and plasticized PVC show that the coupling constant n increases systematically with each successive stress-aging cycle until it approaches the value for slow-cooled samples. These results are consistent with the notion that stress-aging changes the structural state of the glass in ways similar to physical aging.     

A study of enthalpic relaxation of a liquid crystal

A liquid crystal, BL038, which was observed not to crystallize, has a glass transition at 215 K and a nematic to isotropic transition at 380 K. Samples aged below the glass transition at various temperatures T _a, exhibited an endotherm at the transition which developed with extent of ageing time, t _a. We attribute this endotherm to the relaxation of the glass towards the equilibrium liquid. The progress of the relaxation process was measured using differential scanning calorimetry. On subsequent reheating, the aged glass showed an apparent shift in the glass transition to higher temperatures. The endotherm was used to define the extent of enthalpic relaxation and the maximum value observed was found to increase initially then decrease, with the extent of undercooling from the glass transition temperature, Δ T, passing through a maximum for a Δ T = 15 K. From the temperature dependence of the relaxation times, an apparent activation enthalpy for the relaxation process of 85 ± 10 kJ mol^-1 was determined. The small value of the activation enthalpy compared with that found in the ageing of polymers reflects differences in the molecular species involved in relaxation processes.     

Crystallization mechanism and microstructure evolution of Li2O–Al2O3–SiO2 glass-ceramics with Ta2O5 as nucleating agent

Li₂O–Al₂O₃–SiO₂ glass-ceramics were prepared with Ta₂O₅ as nucleating agent, the crystallization mechanism and microstructure evolution were investigated by DTA, XRD, and SEM technologies. With increasing amount of Ta₂O₅ from 2 to 6 mol%, the crystallization activation energy decreased from 297.73 to 218.66 kJ mol^?1, while the crystallization index increased from 1.76 to 3.39. In addition, the cluster of dendritic crystals and lamellar structure obtained in T-2 glass-ceramics indicated a typical two-dimensional crystallization mechanism, and the formation of spherical β-quartz solid solution in T-4 specimens, with average size of 50–70 nm, was mainly due to bulk crystallization mechanism. It was considered that Ta₂O₅ promoted the nucleation and crystallization of LAS glass by precipitating the crystalline precursor phase of Ta₂O₅, which acted as nuclei for the subsequent crystal growth. Eventually, the diffusion and crystallization process, microstructure morphology, as well as the secondary grain growth were also investigated.     

正、反向共沉淀法对Pr_2Zr_2O_7纳米粒子表观活化能的影响

以氨水作为沉淀剂,采用正、反向共沉淀法制备Pr2Zr2O7纳米粒子。利用XRD、SEM、TEM、TG-DTA等测试手段表征了样品物相及形貌;研究其制备过程中合成动力学和晶粒生长动力学,采用Doyle-Ozawa法和Kissinger法分别计算正、反向沉淀粒子在主要反应阶段的表观活化能。结果表明:反向沉淀的滴定速率为2 mL·min-1、母盐溶液初始浓度0.05 mol·L-1、反应体系温度273 K、pH值11、煅烧温度为1 173 K,保温2 h的条件下获得的样品形貌近球形、无团聚现象、一次粒径约60 nm。Pr2Zr2O7前驱体的分解过程分为3个阶段,正、反向粒子各阶段平均表观活化能分别为:71.2、97.8、183.2 kJ·mol-1和45.37、84.34、152.16kJ·mol-1;晶粒生长活化能分别为19.02和11.95 kJ·mol-1,后者比前者的晶粒生长活化能降低了7.07 kJ·mol-1;反向共沉淀制备工艺优于正向共沉淀法。     

正、反向共沉淀法对Pr2Zr2O7纳米粒子表观活化能的影响

以氨水作为沉淀剂,采用正、反向共沉淀法制备Pr₂Zr₂O₇纳米粒子。利用XRD、SEM、TEM、TG-DTA等测试手段表征了样品物相及形貌;研究其制备过程中合成动力学和晶粒生长动力学,采用Doyle-Ozawa法和Kissinger法分别计算正、反向沉淀粒子在主要反应阶段的表观活化能。结果表明：反向沉淀的滴定速率为2mL·min^-1、母盐溶液初始浓度0.05mol·L^-1、反应体系温度273K、pH值11、煅烧温度为1173K,保温2h的条件下获得的样品形貌近球形、无团聚现象、一次粒径约60nm。Pr₂Zr₂O₇前驱体的分解过程分为3个阶段,正、反向粒子各阶段平均表观活化能分别为：71.2、197.8、183.2kJ·mol^-1和45.37、84.34、152.16kJ·mol^-1;晶粒生长活化能分别为19.02和11.95kJ·mol^-1,后者比前者的晶粒生长活化能降低了7.07kJ·mol^-1;反向共沉淀制备工艺优于正向共沉淀法。     

Calorimetric study on Ge<Subscript>23</Subscript>Se<Subscript>67</Subscript>Sb<Subscript>10</Subscript>–0.5CsCl glass

The mechanical behavior of Ge₂₃Se₆₇Sb₁₀ glass can be improved by adding CsCl facilitating the nano-crystaline formation. Understanding the crystallization mechanism of chalcogenide glass can help in directing the subsequent annealing processing and tuning the microstructure and physical properties. In this work, 99.5Ge₂₃Se₆₇Sb₁₀–0.5CsCl glass was prepared and its transformation kinetics was investigated under non-isothermal conditions with heating rate up to 400 K min^?1. Using Vogel–Fulcher–Tammann equation, the ideal glass transition temperature was determined as T _0g = 434.1 K. Using the classical JMA theory, the average activation energy and average growth exponent were determined as 135.0 and 2.4 kJ mol^?1, while using the model considering impingements (MCI), the two parameters were determined as 120.9 and 3.2 kJ mol^?1, respectively. Compared to JMA theory, the MCI model can fit the transition curves better, and it shows that the growth mode of the present glass is between two-dimension and three-dimension. By comparing with the result of Ge₂₃Se₆₇Sb₁₀ glass, it is found that addition of CsCl can reduce the growth dimension and activation energy during crystallization.     

Non-isothermal crystallization of K<Subscript>2</Subscript>O·TiO<Subscript>2</Subscript>·3GeO<Subscript>2</Subscript> glass

The crystallization of K₂O·TiO₂·3GeO₂ glass under non-isothermal condition was studied. In powdered glass with particle sizes less than 0.15 mm, surface crystallization was dominant and an activation energy of crystal growth of E _a,s=327±50 kJ mol⁻¹ was calculated. In the size range 0.15 to 0.45 mm, both surface and volume crystallization occurred. For particle sizes >0.45 mm, volume crystallization dominated with spherulitic morphology of the crystals growth and E _a,v=359±64 kJ mol⁻¹ was calculated.     

Avrami exponent of crystallization in tellurite glasses

Nucleation process and crystal growth for three samples of the (20-x)Li₂O–80TeO₂–xWO₃ glass system were studied using X-ray diffraction and differential scanning calorimetry techniques. X-ray diffraction data confirmed the amorphous characteristic of the as-quenched samples and indicated the growth of crystalline phases formed due to the thermal treatment for annealed samples. These results reveal the presence of three distinct γ-TeO₂, α-TeO₂ and α-Li₂Te₂O₅ crystalline phases in the TL sample, and two distinct α-TeO₂ and γ-TeO₂ crystalline phases in the TLW5 and TLW10 samples. The activation energy and the Avrami exponent were determined from DSC measurements. The activation energy values X-ray diffraction data of the TLW10 glass sample suggest that γ-TeO₂ phase occur before the α-TeO₂. The results obtained for the Avrami exponent point to that the nucleation process is volumetric and that the crystal growth is two or three-dimensional.