Stanislav Kurajica

Crystallization kinetics of mullite from single-phase gel determined by isothermal differential scanning calorimetry

Abstract Transformation kinetics of single-phase gel with mullite composition was studied by isothermal differential calorimetry (DSC) in temperature range from 937 to 959°C. Single exotherm was observed for annealing temperatures below 947°C, and two overlapped exothermic peaks were seen above this temperature. According to XRD analysis, mullite was the only phase crystallized either under non-isothermal or isothermal heat treatment. Johnson-Mehl-Avrami (JMA) equation for nucleation and growth could not describe mullite crystallization adequately, even below 947°C. Using bimodal JMA-type model, that proposes mullite crystallization in two steps, the fitting was remarkably good in the whole temperature range. Obtained kinetics data do not allow one to characterize the gel, either as typical single phase one (nucleation-controlled process with two rate constants and small apparent activation energies), or as hybrid gel (mullite formation via spinel and high apparent activation energies). The rate constants were an order of magnitude smaller than is proposed for single phase gel. The apparent activation energies, however (Ea1 = 1053 ± 51 kJ/mol, and Ea2 = 1028 ± 22 kJ/mmol, were in great discrepancy to those already cited for single phase gels, but they were in very good agreement with data evaluated for diphasic and hybrid gels. Mullite a-axis length and effective fraction of mullite that is formed in the first and second step of the process provided an insight in the mechanism of mullite crystallization. It is assumed that not the nucleation and crystallization limitations, rather the phase separation is the controlling process in mullite formation from single phase gel under applied experimental conditions.

Inverse spinel structure of Co-doped gahnite

Abstract Powder ZnAl2O4 (gahnite) samples doped with 0-100 at% Co were obtained by a sol-gel technique. X-ray powder diffraction was used to characterize the samples. Gahnite samples are cubic with the normal spinel structure, space group Fd3̄m. Cobalt doping caused a nonuniform increase of unit-cell parameter. The structure of the gahnite samples was refined by the Rietveld method. The location of Co2+ was determined by EPR spectroscopy. Cobalt doping of gahnite induces the inverse spinel structure at only 4 at% Co, and the inversion parameter increases with Co2+ doping level. Metal-oxide distances in the (Al,Co)O6 octahedra dominantly influence the unit-cell parameter of Co-doped gahnite

Resolution of overlapping peaks and the determination of kinetic parameters for the crystallization of multicomponent system from DTA or DSC curves: I. Non-isothermal kinetics

Abstract A new computer program has been created to separate overlapping exothermic and endothermic DTA (DSC) effects, obtained under non-isothermal conditions, occurring due to simultaneous nucleation and growth or more growth processes, and also to calculate the kinetic parameters of the processes represented by each peak. A new graphical method for the initialization of the required parameters and the refinement of the initial values by the least squares method is presented. The procedure for the determination of the kinetic parameters from DTA data is demonstrated on a model system and on a kinetic analysis of mullite formation from diphasic gel.

Cobalt incorporation in mullite

Abstract Mullite samples doped with cobalt were derived from diphasic gels with constant atomic ratio (Al + Co)/Si = 3:1, where 0, 1, 2, and 3 at% of aluminum was replaced by cobalt. X-ray powder diffraction showed that the samples contained mullite phase and some amount of α-Al2O3 (for pure and doped samples) and CoAl2O4 (for doped samples). Cobalt doping caused an increase in unit-cell parameters of the mullite phase. Transmission electron microscopy and energy dispersive X-ray spectroscopy were used for sample microanalysis and determination of the chemical composition of the Co-doped mullite phase. The Rietveld method was performed for quantitative phase analysis of the samples and for structure refinement of the mullite phase in the samples. It was found that a small amount of Co2+, 0.36 wt%, substituted for Al3+ in the AlO6 octahedra of the mullite structure. Simultaneously, the same amount of tetrahedral Al3+ ions was likely substituted by Si4+ in the (Al,Si)O4 tetrahedra for the purpose of charge compensation. The remaining cobalt reacted with alumina forming CoAl2O4, and dissolved in the glassy phase. The proposed formula for Co-doped mullite is CoyAl4+2x-2ySi2-2x+yO10-x

A new approach to solid-state reactions kinetics analysis: the application of assisting functions to basic equations for isothermal conditions

Abstract Functions describing simple models for solid-state reaction kinetics under isothermal conditions were transformed in order to obtain new functions with improved properties suitable for kinetic analysis. The transformations were performed trough multiplying well known functions as Johnson–Mehl–Avrami or Jander’s with assisting functions. In such manner, new functions, Φ , were obtained enabling calculation of kinetic parameters in a simple way. The relationships between the values of the extremes of Φ functions and various kinetic parameters have been established. The calculation of kinetic parameters by application of new equations have been demonstrated on six model systems. Very good agreement between preset and calculated parameters for each model system has been observed. Using the same approach, the rate-determining step, i.e. the reaction model, and proper kinetic equation could also be determined.

Properties and Antimicrobial Activity of Nanosilver Deposited Cotton Fabric Coated with γ-Methacryloxypropyl Trimethoxysilane

The simple and industrially adjustable process of silver nanoparticles deposition on cotton fabric followed by coating with γ-methacryloxypropyl trimethoxysilane (MPS), as well as the properties of obtained nanocomposite has been investigated. Silver nanoparticles were prepared through the reduction of silver nitrate with sodium borohydrate and deposited on cellulose fabric. In order to stabilize obtained nanostructured material and improve its properties, the fabric was treated by immersion in cross linkable MPS. The morphology of the finished fabrics was characterized by Scanning Electron Microscopy (SEM). It has been determined that process does not impair nanocharacter of silver particles and yield uniform distribution throughout the fabric surface. The concentration of Ag in the fabric samples was determined by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). The average deposited amount of nanoparticles was found to be 63 ppm and 48 ppm for samples without and those with MPS, respectively. The character of obtained polysiloxane coating was examined using Fourier Transform Infrared (FTIR) spectroscopy. The presence of SiOSi bonds on the cellulose surface pointed out to self-condensation between silanol groups. Differential Thermal and Thermo-Gravimetric Analysis (DTA/TGA) revealed that MPS deposited on fabric provided modification of thermal properties. The change of the surface properties after the modification was ascertained by contact angle measurements. The antimicrobial activity of the antibacterial finish on the cotton fabric, expressed as bacterial reduction efficiency, has been determined. Bacterial reduction of at least 88.5% against both, Staphylococcus aureus and Escherichia coli, has been obtained.

A comparative study of toluene oxidation on different metal oxides

This work reports the results of experimental and theoretical investigation of toluene oxidation on different metal oxide based catalysts (manganese oxide, MnOx, mixed manganese-iron oxide, MnFe, perovskite-type manganese oxide, LaMnO3 and commercial Pt-Al2O3 catalyst). Particular attention was devoted to single and mixed manganese based oxides and ceria based materials as alternatives to conventionally used noble metal containing catalysts. Toluene oxidation was performed under steady-state conditions in an integral fixed bed reactor operating over a wider range of reaction temperatures and at various space times. The influence of reaction variables on the rate of toluene oxidation was examined using the simple first-order kinetic model and the one-dimensional (1D) pseudo-homogeneous model to describe the reaction system. The proposed model was verified comparing the theoretical predictions with the experimental laboratory results. The results of catalytic tests indicated that the mixed manganese-iron oxide (MnFe) exhibited remarkable catalytic activity for the toluene oxidation, almost comparable with the activity of the commercial Pt-Al2O3. The reaction temperature T50 corresponding to 50% of the toluene conversion was observed at 419 K for the MnFe oxide and at 405 K for the Pt-Al2O3. A very good agreement of experimental data with the proposed 1D model was obtained. Based on the shape of the light-off curve and the values of the apparent activation energies, which decreased from 120.36 kJ/mol to 16.88 kJ/mol with reaction temperature increase, it was concluded that the reaction rate was probably limited by the mass transfer, no matter the relatively small catalyst particle size fraction employed in this study (315 400μm).

A structural investigation of tris(ethyl acetoacetate)aluminium (III)

Aluminium tris(ethyl acetoacetate), Al(C6H9O3)3 has been prepared and characterized by means of 1H and 13C nuclear magnetic resonance spectroscopy, scanning electron microscopy and X-ray powder diffraction (XRPD). It was determined that two stereoisomeric complexes can be distinguished in the solution: meridional and facial. The crystal structure of Al(C6H9O3)3 has been determined from XRPD data. The compound crystallizes in the monoclinic space group P21/n.

High Surface Area Ceria Nanoparticles via Hydrothermal Synthesis Experiment Design

Hydrothermal synthesis of CeO2 was optimized on two reactant concentrations and synthesis temperature and duration, in order to achieve material having the greatest specific surface area (SSA). Taguchi method of experimental design was employed in evaluation of the relative importance of synthesis parameters. CeO2 nanoparticles were characterized using X-ray diffraction, nitrogen adsorption-desorption isotherms, and scanning electron microscopy. Optimum conditions for obtaining particles with greater SSA were calculated according to Taguchi?s model ?the-higher-the-better.? Synthesis temperature was found to be the only parameter significant for enabling nanoparticles with greater SSA. Mesoporous nanocrystalline ceria with SSA as great as 226?m2?g?1 was achieved, which is unprecedented for the hydrothermally synthesized ceria. The reason for this achievement was found in temperature dependence of the diffusion coefficient which, when low, favors nucleation yielding with fine particles, while when high it favors crystal growth and formation of one-dimensional structures. The occurrence of 1D-structure in sample exhibiting the smallest SSA was confirmed. Very fine crystallites with crystallite size as low as 5.9?nm have been obtained being roughly inverse proportional to SSA. Selected samples were tested as catalyst for soot oxidation. Catalyst morphology turned out to be decisive factor for catalytic activity.

Sol-Gel Derived Mullite-Gahnite Composite

Mullite-gahnite composites with different phase-proportions were prepared using sol-gel process. Crystallization path was determined using differential thermal analysis (DTA). X-ray powder diffraction (XRD) was used to study the crystal phases development. The course of the thermal reactions is dominated by the intermediate formation of two spinel phases. The former phase was attributed to gahnite, while the latter to Al-Si spinel. Zn loading decreases amounts of mullite and α-alumina, while increases gahnite and amorphous phase. The observed microstructure of sintered bodies is characterized by fine gahnite particles distributed among larger mullite grains, which is highly favourable for ceramics with high mechanical requirements.