Pre-exponential Constant Research Articles

From biomass to Energy: Investigating Chlorella pyrenoidosa’s potential for fuel and carbon materials

This study investigated the bioenergy potential of Chlorella pyrenoidosa (CP) for use as fuel and carbon material through chemical and thermal characterization. The thermo-kinetic characteristics of Chlorella pyrenoidosa were assessed using isoconversional, linear regression, and non-linear regression approaches. The physicochemical analysis revealed high carbon (53.1 %), volatile (69.35 %), and low moisture (2.19 %), ash content (3.42 %). The results indicated that the non-linear model fitting method was the most accurate with the approximated activation energy (Eα) and pre-exponential Arrhenius constant (Ln A) were 124.92 ± 2.74 kJ/mol and 23.38 ± 4.63 min−1, respectively. Additionally, the inclusion of sodium bicarbonate resulted in a significant increase in BET surface area. FTIR analysis revealed several functional groups beneficial for carbon material, while XRD analysis showed a broad peak correlated with an amorphous structure. This study highlighted the potential of Chlorella pyrenoidosa biomass for various applications, including carbon material and renewable fuel.

Silica polymerization and nanocolloid nucleation and growth kinetics in aqueous solutions

Amorphous silica precipitation from aqueous solutions at low temperatures (<100 °C) involves the initial nucleation and growth of silica polymers followed by aggregation, cementation and recrystallization to form opaline silica. In this study, silica polymerization was studied at temperature 23–80 °C, pH 2.48–9.65, and ionic strength 0.01–0.53 mol kg−1. Measured changes in the concentration of monomeric silica (SiO2(mono)) were fit assuming the polymerization reaction is fourth-order with respect to the difference between the concentration of silicic acid (Si(OH)4(aq)) and its concentration at equilibrium (Si(OH)4(aq)eq) with respect to amorphous silica:r=-k4AsmSi(OH)4(aq)-mSi(OH)4(aq)eq4where r is the overall rate of the reaction, As is the specific surface area of the formed polymers, calculated assuming particle-size-dependent solubility and a spherical polymer geometry, and k4 is the rate constant. The rate constant at zero ionic strength is given by:k0=Aexp-Ea/RTaOH-nwhere A is a pre-exponential constant (0.3822 mmol−3 cm−2 s−1), Ea is the activation energy (29.87 kJ mol−1), and n is equal to −0.80. The overall rate constant as a function of ionic strength (I) is given by:k4=k0Imwhere m is equal to 0.65. The rate constant increases by approximately six orders of magnitude from pH of 2 to 10 and one order of magnitude between temperature ∼20 and 80 °C. However, hydrolysis of silicic acid (Si(OH)4(aq)) at pH above ∼8 decreases the relative abundance of Si(OH)4(aq) while increasing the solubility of amorphous silica, thereby decreasing the rate of silica polymerization. The inferred reaction order is consistent with a reaction mechanism rate-limited by the formation of the cyclic tetramer across a wide range of pH. The time-dependence of the specific surface area during polymerization reflects the formation of critical nuclei towards the beginning of the polymerization process and Ostwald ripening of larger polymers towards the end of the polymerization process. The developed rate equation can predict rates of silica polymerization across a wide range of aqueous solution compositions.

Advanced isoconversional kinetic analysis: Insight in mechanisms and simulations. Successes and future

Various examples of application of advanced isoconversional kinetic analysis are discussed. The paper focuses on polymerization and crystallization mechanisms, protein denaturation and simulations in a model-free way under different temperature programs. It is shown how isoconversional kinetic analysis was developed in the recent years to gain meaningful kinetic information on changes in rate-limiting steps and was applied to multi-step processes. The complementary of the information obtained by estimating pre-exponential factors, rate constants and mathematical function that describes the reaction mechanism, in addition to the usual effective activation energy dependency, is highlighted. In addition, combination of model-free and model-based methods can be used for the elucidation of multi-step kinetics.

All-Nitrogen Energetic Material Cubic Gauche Polynitrogen: Plasma Synthesis and Thermal Performance.

Numerous theoretical calculations have demonstrated that polynitrogen with an extending polymeric network is an ultrahigh-energy all-nitrogen material. Typical samples, such as cubic gauche polynitrogen (cg-N), have been synthesized, but the thermal performance of polynitrogen has not been unambiguously determined. Herein, macroscopic samples of polynitrogen were synthesized utilizing a coated substrate, and their thermal decomposition behavior was investigated. Polynitrogen with carbon nanotubes was produced using a plasma-enhanced chemical vapor deposition method and characterized using infrared, Raman, X-ray diffraction X-ray photoelectron spectroscopy and transmission electron microscope. The results showed that the structure of the deposited polynitrogen was consistent with that of cg-N and the amount of deposition product obtained with coated substrates increased significantly. Differential scanning calorimetry (DSC) at various heating rates and TG-DSC-FTIR-MS analyses were performed. The thermal decomposition temperature of cg-N was determined to be 429 °C. The apparent activation energy (Ea) of cg-N calculated by the Kissinger and Ozawa equations was 84.7 kJ/mol and 91.9 kJ/mol, respectively, with a pre-exponential constant (lnAk) of 12.8 min-1. In this study, cg-N was demonstrated to be an all-nitrogen material with good thermal stability and application potential to high-energy-density materials.

Gas Permeability through Polyimides: Unraveling the Influence of Free Volume, Intersegmental Distance and Glass Transition Temperature.

The relationships between gas permeability and free volume fraction, intersegmental distance, and glass transition temperature, are investigated. They are analyzed for He, CO2, O2, CH4, and N2 gases and for five similar polyimides with a wide range of permeabilities, from very low to extremely high ones. It has been established here that there is an exponential relationship between permeability and the free volume fraction, and between permeability and the most probable intersegmental distance as measured by WAXS; in both cases, with an exponential coefficient that depends on the kinetic gas diameter as a quadratic polynomial and with a preexponential positive constant. Moreover, it has been proven that the intersegmental distance increases linearly with the free volume fraction. Finally, it has been established that the free volume fraction increases with the glass transition temperature for the polymers tested, and that they depend on each other in an approximate linear way.

Pb diffusion in magnetite: Dating magnetite crystallization and the timing of remanent magnetization in banded iron formation

The ferrimagnetic mineral magnetite (Fe3O4) is abundant in banded iron formation (BIFs), and has the potential to provide UPb or PbPb age information on these rocks because it incorporates small amounts of U during growth. Combined with age measurements, paleomagnetic studies of BIF magnetites may also yield insight into the history of Earth's magnetic field and its relationship to early evolution of Earth's interior and atmosphere.Reliable magnetite ages utilizing Pb isotopes require knowledge of Pb diffusion in the magnetite structure. For this reason, we undertook an experimental investigation of Pb diffusion in magnetite by diffusing Pb2+ ions into pre-polished slabs of natural magnetite oriented parallel to {001} or {111}. A mixture of PbSO4 and Fe2O3 was used as a surface powder source to supply Pb2+ diffusant at the sample surface and at the same time buffer the oxygen fugacity of the system at magnetite-hematite (MH)—a typical fO2 for banded iron formations (BIFs) due to the common presence of both iron oxides (and where Pb2+ is stable relative to other Pb valence states). Diffusion experiments spanned temperatures of 500–675 °C and durations of 75 to 2035 h. Following each experiment, in-diffused Pb was depth-profiled using Rutherford backscattering spectroscopy (RBS) and Pb diffusivities were calculated from the profiles using an infinite half-space diffusion model. The following diffusion law for Pb2+ in magnetite is based upon 12 independent diffusivity measurements:DPb (m2·s−1) = (9 × 10−17 m2·s−1) exp.(−98,000 J·mol−1)/RT)where the uncertainties in the pre-exponential constant and activation energy are ±6% and ± 15%, respectively.Pb diffusion in magnetite over the temperature range of our study is orders of magnitude slower than projected for other divalent cations based on down-temperature extrapolation of previously measured diffusion laws (e.g., for Mn2+, Fe2+, Co2+, and Ni2+). This finding is encouraging in terms of the potential suitability of magnetite for UPb age determinations of BIFs and other magnetite-bearing rocks. Indeed, classical Dodson closure temperatures well above 500 °C are not unrealistic in cases where magnetite crystals having large diffusion domains (e.g., >100 μm in radius) are cooled relatively rapidly (e.g., at 100 °C/MYr). This is of particular significance for paleomagnetic studies, since the Curie temperature of magnetite is 580 °C and therefore the age of magnetization in magnetite-bearing rocks may be directly dated. However, slow cooling of magnetites having small diffusion domains can lead to Pb loss at temperatures of 200 °C or lower. Pb mobilization is evaluated for various time-temperature scenarios that involve both heating and cooling as well as “closed-loop” time-temperature paths. We conclude that UPb or PbPb age determinations of BIF magnetites are potentially reliable, but isotopic results should be assessed in concert with knowledge of the thermal history of the host rock and the effective grain size of the magnetites.

A two-step chemical scheme for auto-igniting and propagating kerosene flames at reheat conditions

A methodology to build reduced schemes able to capture the main flame features and combustion mode in a sequential burner is proposed. This is here applied to derive a two-step scheme for kerosene flames at reheat conditions typical of aircraft engines for Large Eddy Simulations applications. It comprises 6 species (kerosene, O2, N2, CO2, CO, H2O) and two reactions where only two kinetic constants are adjusted: the pre-exponential constant of the reactions are changed as a function of the progress variable and equivalence ratio in order to match both auto-ignition times and premixed flame speeds in rich conditions. Vitiation levels are taken into account through the equivalence ratio ϕpf characterizing the oxidizer stream composition in the reheat combustor. The novel two-step scheme is validated against semi-detailed chemistry for 1D laminar kerosene/vitiated air flames with decreasing inlet velocity (thereby decreasing reactant residence time) changing from auto-igniting flames at high inlet velocities to propagating flames flashing back at low inlet velocities. The scheme is proved to capture auto-igniting as well as propagating flames found in afterburner conditions at very low computational cost and with a precision comparable to complex chemical schemes.

Determination of diffusion coefficient of C2H6 and CO2 in hydrocarbon solvents by molecular dynamics simulation

In this paper, the diffusion coefficients of C2H6 and CO2 in various hydrocarbon solvents have been determined using molecular dynamics simulation at different temperatures and atmospheric pressure. Effect of force field type and number of cell molecules on MD simulation results were investigated. The optimum number of molecules in amorphous cell was obtained and it was found that COMPASS force field showed the best result in comparison with the other investigated force fields forces (Universal, Dreiding, Cvff, and Pcff). In addition, the diffusion coefficients of ethane and CO2 were found to comply with the Stokes–Einstein equation. These coefficients were calculated from the slope of mean square displacement (MSD) curves. Temperature changes also showed an increasing effect on the diffusion coefficients and the activation energies (Ea) and pre-exponential constants (D0) for C2H6 diffusion in heptane and hexane solvents were obtained using Arrhenius theory. The simulated results indicated a good agreement with the experimental data for C2H6 in heptane, hexane, octane, dodecane, and hexadecane solvents and CO2 in hexadecane solvent.

Development of Kinetic Parameters for Nitric Acid Leaching of Phlogopite and the Characterization of Solid Products

AbstractSouth Africa is a net importer of fertilizer products, importing all of its potassium, as well as 60–70% of its nitrogen requirements. Thus, domestic prices are impacted significantly by international prices, shipping costs, and exchange rates. Producing these fertilizers locally would be far more economical. Phlogopite, a rich source of potassium, is discarded in large quantities during mining operations; the objective of the present study, therefore, was to determine the acid-leaching characteristics and behavior of phlogopite as a means of releasing potassium. Phlogopite samples were leached with nitric acid (source of nitrogen for fertilizers) at various concentrations, temperatures, and reaction times. The feed phlogopite and leached residue samples corresponding to conversions of 14% (LP1), 44% (LP2), and 100% (LP3) were collected and analyzed using X-ray fluorescence spectroscopy (XRF), X-ray diffractometry (XRD), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), Brunauer–Emmett–Teller surface area and porosity analysis (BET), thermogravimetric analysis (TGA), and field emission gun-scanning electron microscopy (FEG-SEM). The feed phlogopite was highly crystalline. The absence of defects in the lattice meant that the motion of H+ atoms penetrating into the lattice was restricted, suggesting internal diffusion-controlled leaching. Furthermore, results obtained from the various analytical techniques corroborated each other in terms of the release of cations during leaching. All leaching experiments were conducted batchwise, in a closed system. The gravimetric data from the experiments were used to identify a suitable model which predicts accurately the leaching behavior. The reaction was found to be internal diffusion-controlled, and the D1 model, which represents one-dimensional diffusion through a flat plate, predicts the leaching behavior most accurately. The observed activation energies (Ea) and pre-exponential constants (k0) varied with initial nitric acid concentration ([H+]0).

Numerical analysis of point-sharp indentation-load relaxation simulated using the finite-element method to characterize the power-law creep deformation of a visco-elastoplastic solid

The modified indentation-load relaxation (ILR), i.e. indentation-stress relaxation (ISR), of point-sharp indentation on a visco-elastoplastic solid (VEPS) with the power-law creep deformation (PLCD) simulated using the finite-element method differs obviously from that on a linear VEPS because of (1) remarkable dwell time-dependent piling-up around an indentation at constant indentation depth, (2) nonexponential ISR, and (3) YE -dependent ISR rate, where Y and E are the yield stress and Young’s modulus of the indented solid, respectively. Based on the numerical analysis of the characteristic ISR, a stepwise procedure is proposed to estimate the pre-exponential creep constant and stress exponent of PLCD from an ILR test.

Study on Static Strain Aging Kinetics of High-Carbon Steel Wires and Its Impact on High-Strength Steel Cords

Under quasi-static loading, an irregular failure mode of high-strength thin carbon steel cords were observed after low-temperature thermal aging. Character and kinetics of damage in such wire ropes highly depend on the plastic elongation of the steel wires, which is significantly modified by the strain aging effect. In this paper, the static strain aging effect on heavily drawn high-carbon steel wires and their cords is experimentally studied in the 80–200 °C temperature range. The kinetics of the aging process is studied in detail. Experimental data are fit by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) kinetic model. The temperature dependence of the static strain aging process is given by means of the Arrhenius equation. The associated JMAK exponents, the apparent activation energy and the pre-exponential constant are determined. Quantitative analysis of the affected strength and strain parameters is given, and based on this, the macroscopic failure mechanism is fundamentally explained.

Evaluation of thermal hazards based on thermokinetic parameters of 2,4-dinitroanisole

ABSTRACT As a high-energy insensitive explosive which is expected to replace 2,4,6-trinitrotoluene (TNT), it is necessary to evaluate the thermal risk of 2,4-dinitroanisole (DNAN). In this article, the thermal performance of DNAN was tested using differential scanning calorimeter (DSC) and accelerating rate calorimeter (ARC). According to the test results, the decomposition activation energy (E), the pre-exponential constant (A), activation entropy (ΔS ≠), activation enthalpy (ΔH ≠), activation Gibbs free energy (ΔG ≠) at T P0 of the reaction and the critical temperature of thermal explosion (T b) were obtained. The kinetic model under adiabatic conditions was established. The thermal hazard assessment of DNAN was carried out according to the thermodynamic parameters, and the assessment results are of great significance to the application of DNAN.

Investigation on thermal degradation kinetics and mechanisms of chicken manure, lignite, and their blends by TGA.

In this study, thermogravimetric analysis (TGA) was performed under the air environment for four different heating rates (10, 20, 30, and 40 °C min-1) in order to find out thermal degradation and mechanisms of the chicken manure, a Turkish lignite, and their blends (25 lignite + 75 manure, 50 lignite + 50 manure, and 75 lignite + 25 manure). To calculate thermal kinetics and responsible solid-state mechanisms of the samples, the Flynn-Wall-Ozawa and Coats-Redfern methods were applied. Significant differences between Turkish lignite and chicken manure samples were observed in terms of thermal kinetics and mechanisms. D1 and D4 mechanisms were found to be the responsible mechanisms for the main oxidation region of the lignite and chicken manure/blends, respectively. A similar decreasing trend for the calculated activation energies and pre-exponential constants was observed with increasing biomass content in the manure blends from 25 to 75% by both Flynn-Wall-Ozawa and Coats-Redfern methods. Furthermore, biomass content has an effect on the mechanisms of chicken manure blends during the combustion. D3 was found to be the responsible solid-state mechanism for the third regions (pre-combustion of the manure) of the chicken manure samples. However, D1 and D2 mechanisms were found to be responsible mechanisms for the blends.

Pyrolysis kinetic modelling of abundant plastic waste (PET) and in-situ emission monitoring

BackgroundRecycling the ever-increasing plastic waste has become an urgent global concern. One of the most convenient methods for plastic recycling is pyrolysis, owing to its environmentally friendly nature and its intrinsic properties. Understanding the pyrolysis process and the degradation mechanism is crucial for scale-up and reactor design. Therefore, we studied kinetic modelling of the pyrolysis process for one of the most common plastics, polyethylene terephthalate (PET). The focus was to better understand and predict PET pyrolysis when transitioning to a low carbon economy and adhering to environmental and governmental legislation. This work aims at presenting for the first time, the kinetic triplet (activation energy, pre-exponential constant, and reaction rate) for PET pyrolysis using the differential iso-conversional method. This is coupled with the in-situ online tracking of the gaseous emissions using mass spectrometry.ResultsThe differential iso-conversional method showed activation energy (Ea) values of 165–195 kJ mol−1, R2 = 0.99659. While the ASTM-E698 method showed 165.6 kJ mol−1 and integral methods such as Flynn-–Wall and Ozawa (FWO) (166–180 kJ mol−1). The in-situ Mass Spectrometry results showed the gaseous pyrolysis emissions, which are C1 hydrocarbons and H–O-C=O along with C2 hydrocarbons, C5–C6 hydrocarbons, acetaldehyde, the fragment of O–CH=CH2, hydrogen, and water.ConclusionsFrom the obtained results herein, thermal predictions (isothermal, non-isothermal and step-based heating) were determined based on the kinetic parameters. They can be used at numerous scale with a high level of accuracy compared with the literature.

Manganese-doped ceria nanoparticles grain growth kinetics

Grain growth kinetics of nanosized ceria, pure and doped with 10, 20 and 30% of manganese, were investigated. Hydrothermally produced nanoparticles were isothermally annealed at temperatures in a range between 500 and 700 ​°C for different annealing times. The average nanoparticle crystallite sizes were determined using X-ray diffraction data via the Scherrer equation and compared to grain size gained by using transmission electron microscopy. On the basis of obtained data, the kinetics of ceria nanoparticles grain growth was studied using an isothermal and a non-isothermal model. It was shown that the presence of manganese inhibits ceria grain growth by reducing the grain growth rate. Activation energy for ceria grain growth, between 226 and 229 ​kJ ​mol−1 for pure and 114 and 130 ​kJ ​mol−1 for doped samples were obtained. Although doped samples show lesser grain growth activation energies, they also have lesser pre-exponential constant and thus increased thermal stability.

The kinetics of thermal decomposition and hot-stage microscopy of selected energetic cocrystals

ABSTRACT With the increased interest in cocrystals, there is a need to quantify the decomposition characteristics of these new materials. The thermal decomposition of four energetic cocrystals composed of 4-amino-3,5-dinitropyrazole (ADNP)/diaminofurazan (DAF), 2,4,6-trinitrotoluene (TNT)/2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL20), 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX)/CL20, and 1-methyl-3,5-dinitro-1,2,4-triazole (MDNT)/CL20 was studied using simultaneous differential scanning calorimetry (DSC), thermogravimetry analysis (TGA), and hot-stage microscopy. The kinetic parameters of their thermal decomposition reaction were determined using both the Kissinger and Ozawa kinetic analyses methods. Each cocrystal’s peak exothermic temperature (decomposition temperature), activation energy, and pre-exponential constant are reported and compared to the corresponding stoichiometric physical mixture. The results showed that cocrystallization clearly affects the thermal decomposition and reaction kinetics of these materials, and importantly can make the material more, or less, susceptible to decomposition.

Oxygen ion conductivity in ceria-based electrolytes co-doped with samarium and gadolinium

In a systematic study, two compositional series of ceria-based oxides, both co-doped with Sm and Gd, were synthesised using a low temperature method and evaluated as oxygen ion-conducting electrolytes for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs). Series one, Ce1-2xSmxGdxO2-x, had equal concentrations of Sm and Gd but varying total dopant concentration. Series two, Ce0.825SmxGd0.175-xO1.9125, had a fixed total dopant concentration but the Sm:Gd concentration ratio was varied. The materials were characterised using scanning and transmission electron microscopy, inductively coupled plasma mass spectrometry and X-ray diffraction. Impedance spectra were recorded on dense pellets of these materials. From these, total, bulk and grain boundary conductivities and capacitances along with activation energies, pre-exponential constants and enthalpies of ion migration and defect association were obtained. These gave a detailed insight into the fundamental conduction processes in the materials. Ce0.825Sm0.0875Gd0.0875O1.9125 had the highest total ionic conductivity at temperatures of 550 °C and above and also demonstrated an enhanced conductivity with respect to its singly-doped parent compounds, Ce0.825Sm0.175O1.9125 and Ce0.825Gd0.175O1.9125, at 400 °C and above. This compares favourably with previously-reported values and has promising implications for the development of IT-SOFCs.

Static recrystallization kinetics of ferrite in cold-deformed medium carbon steel

Static recrystallization (SRX) kinetics of a cold rolled commercial medium carbon steel (AISI 4130) at subcritical temperatures was studied for proposing a comprehensive Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. The value of the recrystallization activation energy (QSRX) was near the activation energy for lattice diffusion in α-iron (251 kJ mol−1). Therefore, the atomic mechanism for SRX of ferrite in this material was considered to be the self-diffusion in α-iron. It was revealed for the first time that the pre-exponential constant (K0) in the equation of the overall rate constant (K) depend exponentially on the reduction in thickness (r). The Avrami exponent (n) of ∼1 was determined, which was interpreted in terms of the nucleation of recrystallization on grain boundaries. Accordingly, a JMAK equation with incorporation of the effects of annealing temperature and reduction in thickness was proposed, which can be used for grain refinement of this material for industrial applications.

Kinetic study of the effect of the heating rate on the waste tyre pyrolysis to maximise limonene production

The formation of isoprene and dl-limonene during waste tyre pyrolysis was investigated in terms of the effect of the heating rate (up to 100°C/min). Ion current signals were used to track during pyrolysis the evolution of the predominant ions of isoprene (isoprene 67) and dl-limonene (limonene 93), by using a thermogravimetric analyser coupled with mass spectrometry (TGA/MS). The combined model-free and model-based kinetics were used to estimate the activation energy (Ea) for isoprene and dl-limonene formation at 131 and 115kJ/mole, respectively, based on the Kissinger method. Reaction order (n) values were estimated at 1.2 and 1.1 for isoprene and dl-limonene, respectively. Better model fit (R2=0.998) of the experimental data to the Arrhenius equation for isoprene and dl-limonene, respectively, was observed when the Kissinger method was used compare to Friedman method. Although the Ea values for isoprene and dl-limonene were not significantly different, the combined three kinetic parameters (Ea, pre-exponential constant (A), and n) may be significantly different. Therefore, for dl-limonene formation selectivity over isoprene, the differences in the three kinetic parameters values for each compound model and heating rate on the reaction progress was significant. The reaction progress at peak isoprene and dl-limonene formation rate increased from 0.42 to 0.45 and more significantly from 0.35 to 0.44, respectively as the heating rate was increased from 15 to 100°C, confirming that the preferred strategy to maximise dl-limonene production is rapid heating to the moderate final pyrolysis temperature.

Numerical and analytical solutions for determination of interdiffusion coefficients in superalloys during homogenization

The solution of the Fick's second law of diffusion in conjunction with the concept of residual segregation index was used for studying the interdiffusion of Ti in the matrix of as-cast Ni-based superalloys during homogenization at elevated temperatures. It was found that the consideration of two-dimensional diffusion model necessitates the numerical solution with the resulting non-unique and inconsistent diffusion parameters. Conversely, the one-dimensional diffusion model with the analytical solution resulted in the reasonable activation energy and pre-exponential constants. The outcome of this research is important for studying the precipitation/dissolution phenomena during processing.