Thermal degradation of hydroxypropyl trimethyl ammonium chloride chitosan–Cd complexes

Thermal degradation of hydroxypropyl trimethyl ammonium chloride chitosan–Cd complexes (HTCC–Cd) was investigated by thermogravimetric analysis. The results indicate that the degradation of HTCC–Cd in nitrogen atmosphere was two-step reaction. For the first step of degradation, the initial temperature of mass loss (T ₀), the final temperature of mass loss (T _f), and the temperature of maximum mass loss (T _p) increase linearly with the rising of heating rate (B). T _o = 1.241B + 220.3, T _p = 1.111B + 245.8, and T _f = 1.335B + 358.2. Using different methods, the kinetic parameters of the two steps were investigated. The results show that the activation energies of the first step of degradation obtained using Friedman and Flynn–Wall–Ozawa methods are 1.684 × 10⁵ and 1.646 × 10⁵ J mol^?1, and the corresponding activation energies for the second step are 1.165 × 10⁵ J mol^?1 and 1.373 × 10⁵ kJ mol^?1. The results obtained from Phadnis–Deshpande methods indicate that the two degradation processes are both nucleation and growth process, and follow A₄ mechanism with intergral form g(X) = [?ln(1 ? X)]⁴.     

A detailed study of isothermal crystallization of As2Se3 undercooled liquid

Isothermal crystallization of an As₂Se₃ undercooled melt was studied by differential scanning calorimetry and described using the classical theory of nucleation and crystal growth. The maximum rate of nucleation and crystal growth was observed to occur at approximately 235 and 350 °C, respectively. The activation energies of nucleation and crystal growth were determined to be ΔE _D = 311 kJ mol^?1 and ΔE* = 104 kJ mol^?1, respectively. The temperature dependencies of both the activation free energy of nucleation, ΔG*, and the critical diameter, r*, were also calculated.     

Temperature dependences of solid structure and properties of biodegradable poly(butylene succinate-co-terephthalate) (PBST) copolyester

In this paper, studies of the temperature dependence for spherulitic growth of PBST copolyester bearing 70 mol% butylene terephthalate units (named as PBST-70) ranged from 70 to 170 °C were first reported based on the Lauritzen–Hoffman secondary nucleation theory. The results showed that maximum spherulitic growth rate of PBST-70 was obtained under crystallization temperature of 90 °C, and more perfect spherulites were formed via increasing isothermal crystallization temperature by POM measurement. The classical regime I → II and regime II → III transitions occurred at the temperatures of 150 and 110 °C, respectively, using the empirical universal values of U* = 6300 J mol^?1 and T _∞ = T _g ? 30 K. Moreover, the effects of isothermal crystallization temperature on crystal lamellar thickness, thermal and tensile properties of PBST-70 were systematically investigated by small angle X-ray scattering, differential scanning calorimeter, and strength tester. The results indicated that the crystal lamellar thickness increased by increasing isothermal crystallization temperature. The endothermic peak shifted to higher temperature and the tensile properties of PBST-70 were enhanced under higher isothermal crystallization temperature.     

A Comparison Between Shaker and Bioreactor Performance Based on the Kinetic Parameters of Xanthan Gum Production

Xanthan gum production was studied using sugarcane broth as the raw material and batch fermentation by Xanthomonas campestris pv. campestris NRRL B-1459. The purpose of this study was to optimize the variables of sucrose, yeast extract, and ammonium nitrate concentrations and to determine the kinetic parameters of this bioreaction under optimized conditions. The effects of yeast extract and ammonium nitrate concentrations for a given sucrose concentration (12.1–37.8 g L^?1) were evaluated by central composite design to maximize the conversion efficiency. In a bioreactor, the maximum conversion efficiency was achieved using 27.0 g L^?1 sucrose, 2.7 g L^?1 yeast extract, and 0.9 g L^?1 NH₄NO₃. This point was assayed in a shaker and in a bioreactor to compare bioreaction parameters. These parameters were estimated by the unstructured kinetic model of Weiss and Ollis (Biotechnol Bioeng 22:859–873, 1980) to determinate the yields (Y _P/S), the maximum growth specific rate (μ _max), and the saturation cellular concentration (X*). The parameters of the model (μ _max, X*, m, λ, α, and β) were obtained by nonlinear regression. For production of xanthan gum in a shaker, the values of μ _max and Y _P/S obtained were 0.119 h^?1 and 0.34 g g^?1, respectively, while in a bioreactor, they were 0.411 h^?1 and 0.63 g g^?1, respectively.     

The study of thermal decomposition kinetics of zinc oxide formation from zinc oxalate dihydrate

This study is devoted to the thermal decomposition of ZnC₂O₄·2H₂O, which was synthesized by solid-state reaction using C₂H₂O₄·2H₂O and Zn(CH₃COO)₂·2H₂O as raw materials. The initial samples and the final solid thermal decomposition products were characterized by Fourier transform infrared and X-ray diffraction. The particle size of the products was observed by transmission electron microscopy. The thermal decomposition behavior was investigated by thermogravimetry, derivative thermogravimetric and differential thermal analysis. Experimental results show that the thermal decomposition reaction includes two stages: dehydration and decomposition, with nanostructured ZnO as the final solid product. The Ozawa integral method along with Coats–Redfern integral method was used to determine the kinetic model and kinetic parameters of the second thermal decomposition stage of ZnC₂O₄·2H₂O. After calculation and comparison, the decomposition conforms to the nucleation and growth model and the physical interpretation is summarized. The activation energy and the kinetic mechanism function are determined to be 119.7 kJ mol^?1 and G(α) = ?ln(1 – α)^1/2, respectively.     

Isothermal crystallization kinetics of poly(ethylene terephthalate)s of different molecular weights

The isothermal crystallization kinetics and morphology of poly(ethylene terephthalate) (PET) polymers of different molecular weights have been studied by means of differential scanning calorimetry and transmission microscopy (TM). The kinetic parameters of Avrami exponent n, the rate constant k, half time t _1/2, rate at 50 % crystallinity, τ _1/2 for crystallization of different PETs were evaluated from double logarithmic plots of log {?ln[1 ? X(t)]} versus log t, where X(t) is extent of crystallinity at a given crystallization temperature. The crystallization rate of polymers with high molecular weight found to be lower than that of polymers with low molecular weight, at the same crystallization temperature. It was found that the nucleation mechanism and growth dimension of polymers with low molecular weight are different from those of polymers with high molecular weight. The results of TM and isothermal crystallization kinetics showed a consistent trend for the crystallization of all PET polymers studied, comprising a primary stage and a secondary stage. The activation energy in the PET polymers of low molecular weight was found to be lower than that of polymers with high molecular weight.     

Experimental and computational study of the thermal decomposition of 3-buten-1-ol in m-xylene solution

An experimental study of the thermal decomposition of a β-hydroxy alkene, 3-buten-1-ol, in m-xylene solution, has been carried out at three different temperatures: 553.15, 573.15, and 593.15 K. The temperature dependence of the rate constants for the decomposition of this compound in the corresponding Arrhenius equation is given by ln k (s^?1) = (27.34 ± 1.24)–(19,328 ± 712) (kJ mol^?1) T ^?1. A computational study has been performed at the MP2/6-31+G(d) level of theory to calculate the rate constants and the activation parameters by the classical transition state theory. The Arrhenius equation obtained theoretically, ln k (s^?1) = (28.252 ± 0.025)–(19,738.0 ± 14.4) (kJ mol^?1) T ^?1, agrees very satisfactorily with the experimental one. The bonding characteristics of reactant, transition state, and products have been investigated by the natural bond orbital analysis which provides the natural atomic charges and the Wiberg bond indices used to follow the progress of the reaction. The enthalpy of the reaction has been calculated using experimental values taken from literature and theoretic calculations. The agreement between both values is satisfactory.     

Growth,structural, crystallisation,thermal decomposition and dielectric behaviour of melaminium bis(hydrogen oxalate) single crystal

Single crystals of melaminium bis (hydrogen oxalate) (MOX) single crystals have been grown from aqueous solution by slow solvent evaporation method at room temperature. X-ray powder diffraction analysis confirms that MOX crystallises in monoclinic system with space group C2/c. The calculated lattice parameters are a = 20.075 ± 0.123 Å b = 8.477 ± 0.045 Å, c = 6.983 ± 0.015 Å, α = 90°, β = 102.6 ± 0.33°, γ = 90° and V = 1,159.73 (Å)³. Thermogravimetric analysis at three different heating rates 10, 15 and 20 °C min^?1 has been done to study the thermal decomposition behaviour of the crystal. Non-isothermal studies on MOX reveal that the decomposition occurs in two stages. Kinetic parameters [effective activation energy (E _a), pre-exponential factor (ln A)] of each stage were calculated by model-free method: Kissinger, Kim–Park and Flynn–Wall method and the results are discussed. A significant variation in effective activation energy (E _a) with conversion progress (α) indicates that the process is kinetically complex. The linear relationship between the ln A and E _a was established (compensation effect). DTA analyses were conducted at different heating rates and the activation energy was determined graphically from Kissinger and Ozawa equation. The average effective activation energy is calculated as 276 kJ mol^?1 for the crystallization peak. The Avrami exponent for the crystallization peak temperature determined by Augis and Bennett method is found to be 1.95. This result indicates that the surface crystallization dominates overall crystallization. Dielectric study has also been done, and it is found that both dielectric constant and dielectric loss decreases with increase in frequency and is almost a constant at high frequency region.     

Effect of cooling rate and Al content on solidification characteristics of AZ magnesium alloys using cooling curve thermal analysis

The solidification behavior of AZ Magnesium alloys in various cooling conditions was investigated using a computer-aided cooling curve thermal analysis method. In each case, the cooling curve and its first and second derivative curves have been plotted using accurate thermal analysis equipment and solidification characteristics were recognized from these curves. The cooling rates used in the present study range from 0.22 to 8.13 °C s^?1. The results of thermal analysis show that the solidification parameters of AZ alloys such as nucleation temperature (T _N,α), nucleation undercooling (?T _N,α), recalescence undercooling (?T _R,α), range of solidification temperature (?T _S) and total solidification time (t _f) are influenced by variation of cooling rate. Also, the effect of Al content on these parameters was studied. Microstructural evaluation was carried out to determine the correlation between the cooling rate and secondary dendrite arm spacing.     

Thermoluminescence and linear thermal expansion of MgAl2O4

The co-efficient of linear thermal expansion (α _av) and dose dependence of thermoluminescence (TL) in MgAl₂O₄(s) were measured. The change in length per unit length was recorded as a function of temperature between room temperature to 1,273 K at a heating rate of 8 K min^?1, in flowing argon atmosphere. The average of three measurements was quoted as the α _av for MgAl₂O₄(s). The linear thermal expansion was measured to an accuracy of ±3 %. The dose dependence of the TL was found to be super linear in the dose range of 0–10 kGy with a k value of 0.503 indicating that the MgAl₂O₄(s) ceramic is ideally suited for the dose estimation of self-irradiated inert matrix fuel in a once through fuel cycle for actinide burning.     

Crystal growth kinetics in GeS2 amorphous thin films

The crystal growth kinetics of germanium disulfide in undercooled melts has been studied by optical microscopy under isothermal conditions. The linear growth kinetics of GeS₂ has been observed in the temperature range 672 ≤ T ≤ 711 K in thin film samples. The activation energy of crystal growth assuming Arrhenius behavior has been determined as E _G = 166 ± 8 kJ mol^?1 for thin film samples. From the dependence of reduced growth rate on undercooling, the interface driven 2-D surface nucleated model was estimated.     

Apparent transfer coefficient for ORR at polycrystalline platinum under convection conditions: a potential modulation study

The oxygen reduction reaction has been studied in the thin-layer flow through cell at polycrystalline platinum in 0.5 M H₂SO₄ at different flow rates. The apparent transfer coefficient (α′) and the corresponding Tafel slope have been calculated using the previously introduced ac voltammetry method [1, 2]. In addition to the correction of the high electrolyte resistance in the thin-layer cell, the contribution from other adsorption processes presented by the adsorption resistance has been also subtracted. The effect of increasing convection on the obtained α′ and Tafel slopes has been examined and it was found that the apparent cathodic transfer coefficient is 0.5 (corresponding to a Tafel slope of ?120 mV dec^?1) in all cases independent of the flow rate or frequency, suggesting that the first electron transfer to oxygen is the rate-determining step. The difference between Tafel slopes obtained here, i.e., by ac voltammetry method, and that obtained from usual Tafel slopes is due to the potential dependence of oxygen adsorbates, which in the method used here is—at least in part—separated from the potential dependence of the rate constant.     

Stopped-flow kinetic analysis of the oxidation of semicarbazide by hexachloroiridate(IV)

A kinetic analysis of the oxidation of semicarbazide (SEM) by the single-electron oxidant [IrCl₆]^2? has been carried out by stopped-flow spectrometric techniques. The reaction proved to be first order each in [IrCl₆ ^2?] and [SEM]_tot, leading to overall second-order kinetics. The variation in the observed second-order rate constant k′ with pH was explored over the pH range of 0–7.11. Spectrophotometric titration revealed a stoichiometry of Δ[IrCl₆ ^2?]/Δ[SEM]_tot = 4:1 for the redox reaction. On the basis of the rate law, the redox stoichiometry, and the rapid scan spectra, a reaction mechanism is proposed which involves parallel attacks of [IrCl₆]^2? on both H₂NCONHNH₃ ⁺ and H₂NCONHNH₂ as rate-determining steps, followed by several rapid reactions. The rate expression, derived from the reaction mechanism, was utilized to simulate the k′–pH profile yielding a virtually perfect fit and indicating that the reaction path involving H₂NCONHNH₃ ⁺ does not make a significant contribution to the overall rate. The reaction between [IrCl₆]^2? and H₂NCONHNH₂ was further studied as a function of both temperature and ionic strength. From the temperature dependence, activation parameters were obtained as: ?H ₂ ^?  = 34.9 ± 1.5 kJ mol^?1 and ?S ₂ ^?  = ?78 ± 5 J K^?1 mol^?1. The observed ionic strength dependence suggests that the rate-determining step is between [IrCl₆]^2? and a neutral species of SEM. Hence, both the temperature and ionic strength dependency studies are in good agreement with the proposed reaction mechanism, in which the rate-determining step involves an outer sphere electron transfer.     

Synthesis,thermal decomposition and dielectric behavior of bis(thiourea)silver(I)nitrate

New semi-organic bis(thiourea)silver(I)nitrate (TuAgN) single crystals have been grown from slow evaporation solution growth technique. Single crystal X-ray diffraction study reveals that the crystal belongs to orthorhombic system with the non-centrosymmetric space group C2221 and the calculated cell parameters are a = 33.3455 (6) Å, b = 45.2957 (7) Å, c = 20.3209 (5) Å, α = β = γ = 90°, and V = 30692.8 (10) Å ³. The thermal stability and decomposition behavior of TuAgN compound have been studied by thermogravimetric analysis at three different heating rates 5, 10, and 15 °C min^?1. The effective activation energy (E _a) and pre-exponential factor (ln A) of thermal decomposition of thiourea from TuAgN compound at three different heating rates are estimated by model free methods: Arrhenius, Flynn–Wall, Kissinger, and Kim–Park. The calculated effective activation energies were found to vary with the fraction (α) reacted. The compensation effect between the (ln A) and (E _a) has also been studied. Dielectric properties of TuAgN crystal have been studied in a wide range of frequencies and temperatures. AC conductivity has also been carried out.     

Effect of in impurity on crystallization kinetics of (Se.7Te.3)100−xInx system

The effect of In impurity on the crystallization kinetics and the changes taking place in the structure of (Se₇Te₃) have been studied by DTA measurements at different heating rates (α=5 deg·min^?1, 10 deg·min^?1, 15 deg·min^?1 and 20 deg·min^?1). From the heating rate dependence of the values ofT _g,T _c andT _p, the glass transition activation energy (E _t) and the crystallization activation energy (E _c) have been obtained for different compositions of (Se₇Te₃)_100?xIn_x (0≤×≤20). The variation of viscosity as a function of temperature has been evaluated using Vogel-Tamman-Fulcher equation. The crystallization data are analysed using Kissinger's and Matusita's approach for nonisothermic crystallization. It has been found that for samples containing In=0, 10, 15, 20 at%, three dimensional nucleation is predominant whereas for samples containing In=5 at%, two dimensional nucleation is the dominant mechanism. The compositional dependence ofT _g and crystallization kinetics are discussed in terms of the modification of the structure of the Se?Te system.     

The Oxidation of Iron(II) with Oxygen in NaCl Brines

The oxidation of nanomolar levels of iron(II) with oxygen has been studied in NaCl solutions as a function of temperature (0 to 50?°C), ionic strength (0.7 to 5.6 mol?kg^?1), pH (6 to 8) and concentration of added NaHCO₃ (0 to 10 mmol?kg^?1). The results have been fitted to the overall rate equation: $$\mathrm{d}\mbox{[Fe(II)]}/\mathrm{d}t=-k_{\mathrm{app}}\mbox{[Fe(II)]}[\mbox{O}_{2}]$$ The values of k _app have been examined in terms of the Fe(II) complexes with OH^? and CO ₃ ^2? . The overall rate constants are given by: $$k_{\mathrm{app}}=\alpha_{\mathrm{Fe}2+}k_{\mathrm{Fe}}+\alpha_{\mathrm{Fe(OH)}+}k_{\mathrm{Fe(OH)}+}+\alpha_{\mathrm{Fe(OH)}2}k_{\mathrm{Fe(OH)}2}+\alpha_{\mathrm{Fe(CO3)}2}k_{\mathrm{Fe(CO3)}2}$$ where α _i is the molar fraction and k _i is the rate constant of species i. The individual rate constants for the species of Fe(II) interacting with OH^? and CO ₃ ^2? have been fitted by equations of the form: $$\begin{array}{l}\ln k_{\mathrm{Fe}2+}=21.0+0.4I^{0.5}-5562/T\\[6pt]\ln k_{\mathrm{FeOH}}=17.1+1.5I^{0.5}-2608/T\\[6pt]\ln k_{\mathrm{Fe(OH)}2}=-6.3-0.6I^{0.5}+6211/T\\[6pt]\ln k_{\mathrm{Fe(CO3)}2}=31.4+5.6I^{0.5}-6698/T\end{array}$$ These individual rate constants can be used to estimate the rates of oxidation of Fe(II) over a large range of temperatures (0 to 50?°C) in NaCl brines (I=0 to 6 mol?kg^?1) with different levels of OH^? and CO ₃ ^2? .     

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.     

Study on thermal decomposition and oxidation kinetics of cation exchange resins using non-isothermal TG analysis

Kinetics of two successive thermal decomposition reaction steps of cationic ion exchange resins and oxidation of the first thermal decomposition residue were investigated using a non-isothermal thermogravimetric analysis. Reaction mechanisms and kinetic parameters for three different reaction steps, which were identified from a FTIR gas analysis, were established from an analysis of TG analysis data using an isoconversional method and a master-plot method. Primary thermal dissociation of SO₃H⁺ from divinylbenzene copolymer was well described by an Avrami–Erofeev type reaction (n = 2, g(α) = [?ln(1 ? α)]^1/2]), and its activation energy was determined to be 46.8 ± 2.8 kJ mol^?1. Thermal decomposition of remaining polymeric materials at temperatures above 400 °C was described by one-dimensional diffusion (g(α) = α ²), and its activation energy was determined to be 49.1 ± 3.1 kJ mol^?1. The oxidation of remaining polymeric materials after thermal dissociation of SO₃H⁺ was described by a phase boundary reaction (contracting volume, g(α) = 1?(1 ? α)^1/3). The activation energy and the order of oxygen power dependency were determined to be 101.3 ± 13.4 and 1.05 ± 0.17 kJ mol^?1, respectively.     

A study of non-isothermal kinetics of limestone decomposition in air (O2/N2) and oxy-fuel (O2/CO2) atmospheres

The non-isothermal experiments of limestone decomposition at multi-heating rates in O₂/N₂ and O₂/CO₂ atmospheres were studied using thermogravimetry. The limestone decomposition kinetic model function, kinetic parameters of apparent activation energy (E), and pre-exponential factor (A) were evaluated by Bagchi and Malek method. The results shown that in 20 % O₂/80 % N₂ atmosphere, the limestone decomposed slowly following the contracting sphere volume model controlled by boundary reaction (spherical symmetry) in two stages, and the E increased by about 50 kJ mol^?1 in the second decomposition stage. But in 20 % O₂/80 % CO₂ atmosphere, the presence of high-concentration CO₂ significantly inhibited the limestone decomposition, and made the decomposition process occur at high temperature with a rapid rate; the decomposition kinetics was divided into three stages, the first stage was an accelerated decomposition process following the Mampel Power law model with the exponential law equation, the second stage followed the nth order chemical reaction model as an α–t deceleration process, and the third stage belonged to the random nucleation and nuclei growth model with the Avrami–Erofeev equation. And with the heating rate increasing, the reaction order n showed a slight rise tendency. The E was about 1,245 kJ mol^?1 in 20 % O₂/80 % CO₂ atmosphere, but was only about 175 kJ mol^?1 in 20 % O₂/80 % N₂ atmosphere. The E and A increased markedly in the O₂/CO₂ atmosphere.