Non-isothermal Activation Energy Research Articles

Crystallization kinetics and growth mechanism of Pb(Zr0.52·Ti0.48)O3 nanopowders

In this study, Pb(Zr0.52� Ti0.48)O3 nanopowders were synthesized via sol-gel process. Particle morphology, crystalline phases and thermal behavior were characterized by scanning electron microscopy, X-ray diffraction and simultaneous thermal analyzer, respectively. The X-ray diffraction pattern showed perovskite phase clearly. The non-isothermal activation energy for the perovskite crys- tallization in Pb(Zr0.52� Ti0.48)O3 gel powders was 224.91 kJ mol -1 . Both growth morphology parameter (n) and crystallization mechanism index (m) are close to 3.0, indicating that the bulk nucleation is dominant in the perovskite PZT formation. To determine dielectric prop- erties, the calcined Pb(Zr0.52� Ti0.48)O3 nanopowders were pressed using uniaxial press. It was found that the Pb(Zr0.52� Ti0.48)O3 disks, by sintering at 1,200 C for 2 and 10 h, and at 1 kHz frequency, had 966 and 1,490 of the dielectric constant, respectively.

Isothermal and non-isothermal sublimation kinetics of zirconium tetrachloride (ZrCl4) for producing nuclear grade Zr

Sublimation of ZrCl4 is important for the production of nuclear grade metallic Zr in Kroll's process. The sublimation kinetics of ZrCl4 was investigated by thermogravimetric analysis under both isothermal and non-isothermal conditions. The sublimation rate of ZrCl4 increased with increasing temperature under isothermal conditions. ZrCl4 sublimation was confirmed to be a zero-order process under isothermal conditions, whereas it was first-order kinetics under non-isothermal conditions. The activation energy of ZrCl4 sublimation under isothermal conditions was 21.7 kJ mol−1. The activation energy for non-isothermal sublimation was 101.4 kJ mol−1 and 108.1 kJ mol−1 with the Kissinger method and Flynn–Wall–Ozawa method, respectively. These non-isothermal activation energies were very close to the heat of sublimation (103.3 kJ mol−1). Sublimation occurs by two elementary steps: surface reaction and desorption. Therefore, the overall activation energy of ZrCl4 sublimation is 104.8 (±3.4) kJ mol−1. The activation energy of the surface reaction and desorption steps are proposed to be 83.1 kJ mol−1 and 21.7 kJ mol−1, respectively.

Isothermal and non-isothermal crystallization behavior of PET nanocomposites

Polyethylene terephthalate (PET)/clay nanocomposites (PCNs) containing 1 wt% Cloisite 30B (C30B) were prepared via melt compounding. Modulated differential scanning calorimetry (MDSC) for isothermally crystallized samples revealed that the third endotherm at the highest temperature may be attributed to the recrystalization and melting of crystals, reorganized during heating. The first and second endotherms may be associated with melting of the secondary and primary crystals, respectively. The overall isothermal crystallization rate in PCNs was faster than in the neat resin. Growth kinetics revealed that the work required for chain folding and the equilibrium melting temperature in PCNs were somewhat higher than for neat PET. During isothermal crystallization, the steric hurdles introduced by clay layers lead to a reduction in the transport of the PET chains into crystallites. The effective non-isothermal activation energy for the PCNs was higher than for PET, possibly leading to less perfect crystals in the PCNs. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers

Thermal Stability and Thermal Decomposition Kinetics of 1-Butyl-3-methylimidazolium Dicyanamide

Thermal stability and thermal decomposition kinetics of 1-butyl-3-methylimidazolium dicyanamide ([bmin +][N(CN) 2 −]) were investigated using both isothermal and non-isothermal thermogravimetric analyses (TGA) under high pure nitrogen as carrier gas. The long-term thermogravimetric studies revealed that the highest temperature used should be 110 °C, at which [bmin +][N(CN) 2 −] lost less than 10% by mass in 10 hours. The non-isothermal activation energy values determined using Friedman and ASTM methods were (150±13) and (147±2) kJ·mol −1, respectively. Multivariate non-linear-regression methods showed that expanded Fn and CnB models were the best fit models with highest correlation coefficient of 0.9994, and the apparent activation energies were consistent with iso-conversional methods.

Effect of nanoclays on high and low temperature degradation of fluoroelastomers

High temperature degradation of a fluoroelastomer and its nanocomposites was carried out from room temperature to 700 °C using thermogravimetric analysis (TGA) in nitrogen and oxygen atmospheres. The presence of the unmodified nanoclay enhanced the onset of degradation in both the environments, because of polymer–filler interaction, exfoliation, uniform dispersion and high thermal stability of the layered silicates. In the derivative curve, there was a single T max, indicating one-stage degradation for all the samples. The non-isothermal activation energy of degradation was determined using the Kissinger and the Flynn–Wall–Ozawa methods. The nanocomposites showed higher activation energy than the neat elastomer. The activation energy of degradation, as observed by isothermal kinetics, was 165, 168 and 177 kJ mol −1 for the neat elastomer, modified and unmodified clay filled samples, respectively. Intrinsic viscosity, measured after low temperature ageing (125–175 °C) showed that the viscosity values were higher for the nanocomposites. The mechanism of degradation is discussed.

Mullitization kinetics from silica- and alumina-rich wastes

Mullite was produced from a mixture of raw wastes with high contents of silica and aluminium hydroxides. The first were wastes from slate rocks and had muscovite, chlorite and quartz as their main crystalline phases. The second were aluminium sludges resulting from the physico-chemical treatment of the wastewaters generated by aluminium surface treatment industries. As-received raw materials were weighed to produce the 2:1 alumina:silica stoichiometric mixture, corresponding to an excess of alumina relatively to the ideal 3:2 mullite composition. The non-isothermal activation energies for primary (431 kJ/mol) and secondary mullite crystallization (454 kJ/mol) were determined by differential dilatometry. Sintered products were composites of mullite and α-alumina dispersed in a glassy phase and presented flexural strengths higher than 100 MPa after sintering temperatures at 1285 °C.

Phase transformation and growth of mullite in kaolin ceramics

The phase transformation and growth of mullite (3Al2O3·2SiO2) in kaolin ceramics have been investigated using X-ray diffractometer, transmission electron microscope, select area electron diffractometer, energy dispersion spectrometer and differential thermal analysis. The mullite which was transformed from kaolin appears at 1050 °C by XRD and tallies with DTA. The initial mullite crystal showed a plate-like morphology. The Al2O3 content in mullite crystal increased from 49.57 to 71.37 wt.% but the lattice parameters of a, b and c axes decreased from 8.085, 8.106 and 3.215 Å to 7.882, 7.974 and 2.946 Å, respectively, with the grain width increasing from 20 to 70 nm when the kaolin was sintered at 1300 °C for 30 min. The nonisothermal activation energy of mullite crystallization in kaolin ceramics was 1182.3 kJ mol−1. The growth morphology parameters n and m were both about 2.0, indicating that the bulk nucleation was dominant in mullitization and the crystal growth was controlled by diffusion. Seemingly, this study has been attempted to provide an integrative presentation of the thermal–structural characterization together with detailed kinetic and mechanistic interpretations.

Nucleation and Crystallization Kinetics of Hydrated Amorphous Lactose Above the Glass Transition Temperature

The crystallization kinetics of amorphous lactose in the presence and absence of seed crystals were investigated at 57.5% relative humidity. Isothermal crystallization studies were conducted gravimetrically in an automated vacuum moisture balance at several temperatures between 18 and 32°C. The crystallization rate constants were then determined from Johnson–Mehl–Avrami (JMA) treatment and isothermal activation energies were obtained from Arrhenius plots. Based on microscopic observations, a reaction order of 3 was used for JMA analysis. The nonisothermal activation energies were determined by differential scanning calorimetry using Kissinger's analysis. Isothermal activation energies for amorphous lactose with and without seed crystals were 89.5 (±5.6) kJ/mol and 186.5 (±17.6) kJ/mol, respectively. Nonisothermal activation energies with and without seed crystals were 71 (±7.5) kJ/mol and 80.9 (±8.9) kJ/mol, respectively. The similarity of the isothermal and nonisothermal activation energies for the sample with seeds suggested that crystallization was occurring by growth from a fixed number of preexisting nuclei. Markedly different isothermal and nonisothermal activation energies in the absence of seeds suggested a site-saturated nucleation mechanism, and therefore allowed calculation of an activation energy for nucleation of 317 kJ/mol.

Thermal and chemical events in the decomposition course of manganese compounds

Thermogravimetry and differential thermal analysis, infrared spectroscopy (solid phase and gas phase), and X-ray diffraction were used to characterize the pathways of the decomposition of the hydrated acetate, oxalate and nitrate of manganese in air. The non-isothermal activation energy ( ΔE, kJ mol −1) was determined for the thermal processes monitored throughout the decomposition course. The results showed that Mn( CH 3 COO) 2·4 H 2 O dehydrates in three steps up to 130 °C, and then decomposes to a mixture of manganese oxides Mn 3O 4 (major) and Mn 2O 3 (minor) through the intermediates Mn(OH)CH 3COO and/or MnOCH 3COO and MnCO 3. MnC 2 O 4·2 H 2 O dehydrates in one step at 150 °C, and then decomposes via MnCO 3 into Mn 3O 4 (hausmannite structure) at 350 °C. Mn( NO 3) 2·4 H 2 O undergoes stepwise dehydration up to 175 °C, and decomposes above 200 °C via an unstable oxynitrate intermediate yielding MnO 2; this decomposes at about 550 °C to the α-Mn 2O 3 phase.

Determination of kinetic parameters for the reoxidation of direct reduced iron under rising temperature conditions

This paper presents the main kinetic parameters for the reoxidation of direct reduced iron (DRI) samples in air under non-isothermal conditions and compares the data with isothermal kinetic parameters for a typical DRI sample. It is found that the non-isothermal activation energy (Δ E) values are less than the isothermal Δ E values. For coal based DRI, the non-isothermal Δ E values lie in the range 20–35 kJ mol −1. For gas based DRI the value is much higher, about 55kJ mol −1. For the reoxidation of DRI, the kinetic compensation effect (KCE) has been found to be valid.

The use of a method for evaluating the non-isothermal activation energy by modelling a non-isothermal change with kinetic parameters dependent on the d

A method for determining the non-isothermal activation energy, elucidated previously by the present authors, was used to evaluate the change in this kinetic parameter with the degree of conversion.

Kinetics of sintering of irregular particles of glass : Part II. Non-isothermal sintering

Part I of this series discussed isothermal sintering of a commercial window glass powder. In this paper results of a non-isothermal study are reported. It is shown that the isothermal kinetic equation can be used for analysis of non-isothermal data. The non-isothermal activation energy E value is significantly different from the isothermal E value.