Oxidation of nickel and transport properties of nickel oxide

Oxidation kinetics of high purity nickel, as well as the nonstoichiometry and chemical diffusion in nickel oxide have been studied as a function of temperature (1373-1673 K) and oxygen pressure (10-10⁵ Pa) using microthermogravimetric techniques. In order to eliminate the possible participation of grain boundary diffusion in scale growth at lower temperatures, the oxidation rate measurements have always been started at the highest temperature (1673 K), when coarse-grained scale was formed, and the temperature and pressure dependence of the oxidation rate was determined by step-wise lowering the temperature of such pre-oxidized sample. Nonstoichiometry and the chemical diffusion coefficient in Ni_1−yO have also been determined on such coarse-grained oxide samples, obtained by complete oxidation of nickel at highest temperature (1673 K). It has been found, that under such conditions oxidation of nickel follows strictly the parabolic rate law, and the parabolic rate constant of this reaction is the following function of temperature and oxygen pressure: The results of nonstoichiometry measurements, in turn, may be described by the following relationship Finally, chemical diffusion coefficient in Ni_1−yO has been found to be independent on oxygen activity, indicating that the mobility of point defects in this oxide does not depend on their concentration, being the following function of temperature: It has been shown, that the parabolic rate constants of nickel oxidation, calculated from nonstoichiometry and chemical diffusion data are in excellent agreement with experimentally determined k_p values. All these results clearly indicate that the predominant defects in nonstoichiometric nickel oxide (Ni_1−yO) are double ionized cation vacancies and electron holes and the oxide scale on nickel growths by the outward volume diffusion of cations.     

A kinetic study of the thermal decomposition process of potassium metabisulfite: Estimation of distributed reactivity model

The thermal decomposition kinetics of potassium metabisulfite was studied by thermogravimetric (TG) and differential thermogravimetric (DTG) techniques using non-isothermal experiments. The apparent activation energy (E_a) is determined using the differential (Friedman) isoconversional method. The results of the Friedman's isoconversional analysis of the TG data suggests that the investigated decomposition process follows a single-step reaction and the observed apparent activation energy was determined as 122.4±2.1 kJ mol⁻¹. A kinetic rate equation was derived for the decomposition process of potassium metabisulfite with contracting area model, f(α)=2(1−α)^1/2, which is established using the Malek's kinetic procedure. The value of pre-exponential factor (A) is also evaluated and was found to be A=1.37×10¹² min⁻¹. By applying the Miura's procedure the distributed reactivity model (DRM) for investigated decomposition process was established. From the dependence α versus E_a, the experimental distribution curve of apparent activation energies, f(E_a), was estimated. By applying the non-linear least-squares analysis, it was found that the Gaussian distribution model (with distribution parameters E₀=121.3 kJ mol⁻¹ and σ=1.5 kJ mol⁻¹) represents the best reactivity model for describing the investigated process. Using the Miura's method, the A values were estimated at five different heating rates and the average A values are plotted against E_a. The linear relationship between the A and E_a values was established (compensation effect). Also, it was concluded that the E_a values calculated by the Friedman's method and estimated distribution curve, f(E_a), are correct even in the case when the investigated decomposition process occurs through the single-step reaction mechanism.     

Preparation and characterization of “green” hybrid clay-dye nanopigments

We obtained a low cost and abundant nanopigment material composed of Rhodamine B (Rh-B) organic dye compound and Unye bentonite (UB) clay from Turkey. The characterization of the nanopigment was investigated using scanning electron microscopy (SEM), particle size distribution, powder X-ray diffraction (PXRD), Fourier transformed infra-red spectroscopy (FT-IR) and thermal analysis techniques. According to the result of texture analyses, we showed that the particle size distribution (d: 0.5-mean distribution) of Rh-B/UB nanopigment material was around 100 nm diameter. It was also demonstrated that the samples had a particle size around nm diameter in SEM images. As seen in the PXRD and thermal analysis, there is a difference in basal spacing by 1.46° (2θ) and a higher mass loss by 7.80% in the temperature range 200–500 °C compared to the raw bentonite.     

On the synthesis and optical absorption studies of nano-size magnesium oxide powder

Magnesium oxide (MgO) nano-size powder is synthesized using magnesium nitrate hexahydrate and oxalic acid as precursors with ethanol as a solvent. The process involves gel formation, drying at 100 °C for 24 h to form magnesium oxalate dihydrate [α-MgC₂O₄·2H₂O] and its decomposition at 500, 600, 800, and 1000 °C for 2 h to yield MgO powder (average crystallite size ∼6.5-73.5 nm). The sol-gel products at various stages of synthesis are characterized for their thermal behaviour, phase, microstructure, optical absorption, and presence of hydroxyl and other groups like OCO, CO, C-C, etc. MgO powder is shown to possess an f.c.c. (NaCl-type) structure with lattice parameter increasing with decrease in crystallite size (t_av); typical value being ∼4.222(2) Å for t_av∼6.5 nm as against the bulk value of 4.211 Å. Infrared absorption has shown MgO to be highly reactive with water. Also, a variety of F- and M-defect centres found in MgO produce energy levels within the band gap (7.8 eV), which make it attractive for application in plasma displays for increasing secondary electron emission and reducing flickering effects. The possible application of the intermediate sol-gel products, viz., α-MgC₂O₄·2H₂O and anhydrous magnesium oxalate (MgC₂O₄) in understanding the plants and ESR dosimetry, respectively, has also been suggested.     

Influence of Ca2+ or Na+ extraframework cations on the thermal dehydration and related kinetic performance of LSX zeolite

Nitrogen adsorption performances of low-silica type X zeolites (LSX) containing Na⁺ or Ca²⁺ ions were studied and compared with Li⁺ ion, and their structural and thermal properties were investigated using various characterizations (XRD, TG-DTG, BET, XPS, SEM, TEM, and EDX with elemental mapping techniques). The kinetics of their thermal dehydration and decomposition was studied using thermogravimetry at three rates (5, 10, and 15 K/min) of linear increase of temperature under non-isothermal heating. Two model free procedures named, Kissinger, and Flynn–Wall–Ozawa (iso-conversional) and one model fitting method called Coats–Redfern based on single TG curves, as well as 41 mechanism functions were used. The kinetic parameters (apparent activation energy E, pre-exponential factor A and model) of the three phases for each sample obtained from the non-isothermal methods were then compared with the results from iso-conversional methods this showed that they strongly depend on the selection of appropriate mechanism function and the corresponding kinetic model from the perspective of crystal structure used. The results demonstrated that the E value obtained at low temperature was lower than that at high temperature, implying that the dehydration process of physisorbed water belongs to diffusion-based control, while decomposition of bonded water (chemisorbed) belongs to kinetic-based control at high temperature. These comparisons allow us to underline the strong effect of cations in association with water and their distribution in the micropores of LSX on the N₂ adsorption performance.     

Experimental study of thermal oxidation of nanoscale alloys of aluminium and zinc (nAlZn)

Aluminium-based alloys have wide applications but little is known about the thermal-chemical kinetics of nanoalloys. This work investigated the thermal oxidation of Zn and Al nanoalloys (nAlZn) with a BET equivalent diameter of 141 nm through the simultaneous TGA/DSC method. The thermal analysis was combined with elemental, morphology and crystalline structure analysis to elucidate the reaction mechanisms. It was found that the complete oxidation of nAlZn in air can be characterised by a three-stage process, including two endothermic and three exothermic reactions. With the help of ex-situ XRD, different reaction pathways were proposed for different stages, forming the end products of ZnO and ZnAl₂O₄. The reactivity comparison between Al and nAlZn suggested that different criteria should be used for different applications.     

The influence of aliovalent impurities on the oxidation kinetics of nickel at high temperatures

The influence of chromium and sodium on the nickel oxidation kinetics has been studied as a function of temperature (1373-1673 K) and oxygen activity (10−10⁵ Pa O₂), using microthermogravimetric techniques. It has been shown that the oxidation of Ni-Cr and Ni-Na alloys, like that of pure nickel, follows strictly the parabolic rate law being thus diffusion controlled. In agreement with the defect model of Ni_1−yO, it has been found that the oxidation rate of the Ni-Cr alloy is higher than that of pure nickel, the reaction rate is pressure independent and the activation energy of this process is lower. This implies that the concentration of double ionized cation vacancies in a Ni_1−yO-Cr₂O₃ solid solution is fixed on a constant level by trivalent chromium ions, substitutionally incorporated into the cation sublattice of this oxide. In the case of the Ni-Na alloy, on the other hand, the oxidation rate is lower than that of pure nickel, the activation energy is higher and the oxidation rate increases more rapidly with oxygen pressure. These results can again be explained in terms of the doping effect, by assuming that univalent sodium ions dissolve substitutionally in the cation sublattice of nickel oxide.     

Lithium ion conducting PVA:PVdF polymer electrolytes doped with nano SiO2 and TiO2 filler

The effect of nano SiO₂ and TiO₂ fillers on the thermal, mechanical and electrochemical properties of PVA:PVdF:LiCF₃SO₃ have been investigated by three optimized systems of SPE (80PVA:20PVdF:15LiCF₃SO₃), CPE-I (SPE:8SiO₂) and CPE-II (SPE:4TiO₂). From the TGA curve least weight loss has been observed for CPE-II indicating high thermal stability compared to other systems. Stress–strain curve of the prepared samples confirm the enhancement of tensile strength in CPE-II compared to CPE-I and SPE. Conductivity studies show that addition of TiO₂ filler slightly enhances ionic conductivity 3.7×10⁻³ S cm⁻¹ compared to filler free system at 303 K. Dielectric plots have been analyzed and CPE-II possesses higher dielectric constant compared to CPE-I and filler free system. Temperature dependence of modulus plots has been studied for highest conductivity possessing sample. Wider electrochemical stability has been obtained for nano-composite polymer electrolytes. The results conclude that the prepared CPE-II shows the best performance and it will be well suited for lithium ion batteries.     

Modification of multi-walled carbon nanotubes for electric double-layer capacitors: Tube opening and surface functionalization

Multi-walled carbon nanotubes (MWCNTs) obtained opening the closed ends and using surface functionalization by means of a combination of partial oxidation in air and chemical modifications are characterized systematically in 0.3 M H₂SO₄ between 0 and 1.0 V, and these nanotubes were planned to be used as electrode materials in electric double-layer capacitors (EDLCs). Opening of MWCNTs, clearly observed by means of transmission electron microscopy (TEM), can be easily achieved by the partial oxidation in air through a seven-step temperature program identified by thermogravimetric/differential thermal analyses (TG/DTA). An increase in 175% specific capacitance is obtained for the MWCNTs, partially oxidized in air and chemically modified in H₂SO₄+HNO₃. The temperature-programmed desorption (TPD) data showed that evolutions of CO and CO₂ are, respectively, promoted by the application of partial oxidation in air and chemical modification in H₂SO₄+HNO₃. The above increase in specific capacitance for modified MWCNTs is attributed to an obvious increase in the BET surface area (double-layer capacitance) and the density of oxygen-containing surface functional groups (pseudocapacitance).     

Physical properties of the delafossite LaCuO2

High-quality LaCuO₂, elaborated by solid-state reaction in sealed tube, crystallizes in the delafossite structure. The thermal analysis under reducing atmosphere (H₂/N₂: 1/9) revealed a stoichiometric composition LaCuO_2.00. The oxide is a direct band-gap semiconductor with a forbidden band of 2.77 eV. The magnetic susceptibility follows a Curie-Weiss law from which a Cu²⁺ concentration of 1% has been determined. The oxygen insertion in the layered crystal lattice induces p-type conductivity. The electrical conduction occurs predominantly by small polaron hopping between mixed valences Cu^+/2+ with an activation energy of 0.28 eV and a hole mobility (μ_300 K=3.5×10⁻⁷ cm² V⁻¹ s⁻¹), thermally activated. Most holes are trapped in surface-polaron states upon gap excitation. The photoelectrochemical study, reported for the first time, confirms the p-type conduction. The flat band potential (V_fb=0.15 V_SCE) and the hole density (N_A=5.8×10¹⁷ cm⁻³) were determined, respectively, by extrapolating the curve C⁻² versus the potential to their intersection with C⁻²=0 and from the slope of the linear part in the Mott-Schottky plot. The valence band is made up of Cu-3d orbital, positioned at 4.9 eV below vacuum. An energy band diagram has been established predicting the possibility of the oxide to be used as hydrogen photocathode.     

Effect of pH of colloidal suspension on crystallization and activation energy of deep levels in SnO2 thin films obtained via sol-gel

Colloidal suspensions of tin dioxide (SnO₂) are prepared by the sol-gel method from suspensions with distinct pHs. The particles in solution are embedded by an electrical layer. Decreasing the pH contributes to the destruction of this layer, leading to a high degree of aggregation among particles (clusters) due to the generation of cross-linked bonds (Sn-O-Sn) between them. The aggregation affects the electrical properties of films deposited by dip-coating from these solutions, due to the higher packing produced by acid pH. The X-ray diffractograms of films indicate higher crystallinity for lower pH. The Arrhenius plot leads to activation energies of the deepest level, which was between 67 and 140 meV, for the films prepared from suspensions with pH 6-11. Lower pH films also presented higher electrical conductivity. Obtained activation energies may be related to different types of defects, which could be associated with oxygen vacancies with distinct neighborhoods, influenced by the pH and potential barriers between grains, due to distinct packing caused by cross-linked bonds. TGA/DTA results indicate an easier crystallization process for lower pH, which ends at lower temperature, in good agreement with X-ray diffraction data.     

Fluorescence properties of riboflavin-functionalized mesoporous silica SBA-15 and riboflavin solutions in presence of different metal and organic cations

Riboflavin was covalently linked to mesoporous SBA-15 silica surface via grafting technique. Then fluorescence properties of the system obtained were analyzed in the presence of several metal and organic cations. Both quenching and strengthening of fluorescence as well as significant changes in the maximum fluorescence wavelength were observed. The results were compared with absorption and fluorescence data obtained for riboflavin water solutions.     

Thermal behavior and nucleation kinetics of 1,5-dimethyl-2-nitroimino-1, 3, 5-triazinane crystal

Nitroguanidine derivatives have increasingly gained attention because of their high insecticidal activities and wide spectrum. In this paper, nitroguanidine derivative 1,5-dimethyl-2-nitroimino-1, 3, 5-triazinane was synthesized, and its crystal structure was determined by X-ray technique. The thermal behaviors of 1, 5-dimethyl-2-nitroimino-1, 3, 5-triazinane in a nitrogen atmosphere were also studied under non-isothermal conditions by thermogravimetry (TG) and differential scanning calorimetry (DSC) techniques. The TG and DSC studies showed that the sample started to melt at 408.1 K with high melting enthalpy of 121.3 J/g and was stable up to at least 423.2 K, which indicated that the sample could be effectively utilized for various devices below 423.2 K. The melting entropy of 1,5-dimethyl-2-nitroimino-1, 3, 5-triazinane calculated from melting point and melting enthalpy using Eq. (1) was 51.476 J mol⁻¹ K⁻¹. In addition, the nucleation parameters of 1,5-dimethyl-2-nitroimino-1, 3, 5-triazinane in ethanol, such as the radius of critical nucleus and the Gibbs free energy barrier, had also been investigated based on classical nucleation theory.     

Electrochemical performance of LiFePO4/C synthesized by solid state reaction using different lithium and iron sources

LiFePO₄/C cathode materials were prepared from different lithium and iron sources, using glucose as the carbon source and the reducing agent, via a solid state reaction. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), galvanostatic charge-discharge test and cyclic voltammetry (CV). The results showed that the LiFePO₄/C is olivine-type phase, and composed of relatively large particles of about 400 nm and some nano-sized particles, which favor the electronic conductivity. The LiFePO₄/C cathode material synthesized from Li₂CO₃ and Fe₂O₃ had the smallest particles and the highest uniformity. It delivered the capacity of 145.8 mA h/g at 0.2 C, and had good reversibility and high capacity retention. The precursor of LiFePO₄/C was characterized by thermogravimetry (TG) to discuss the crystallization formation mechanism of LiFePO₄.     

Mesoporous (ZrO2-TiO2)/Al2O3 ternary carriers as hydrodesulfurization catalysts support

Ternary systems at various compositions were synthesized by coprecipitating Zr and Ti (to get a ZrO₂-TiO₂ 40-60 mol%) chlorides in aqueous basic media (provided by urea thermal decomposition) over an alumina substrate. Materials characterization included N₂ physisorption, X-ray diffraction, thermal analysis, high-resolution electron microscopy and Raman and UV-vis spectroscopies. High interaction among components was clearly evidenced by various techniques. Textural properties of ternary oxides could be tuned depending on composition of formulations. Mixed oxides with 10 or 20 mol% of ZrO₂-TiO₂ (at 40-60% mol, in turn) had the most suitable combination of textural properties (surface area, average pore diameter and pore volume) for the intended application (support of catalyst for hydrodesulfurization de oil-derived middle distillates). The suitability of those ternary supports was demonstrated in the dibenzothiophene hydrodesulfurization where the corresponding supported MoS₂ catalysts (at 2.8 atom Mo nm⁻²) were much more active (on a per mass of catalyst basis) than when impregnated over either alumina or zirconia-titania oxides.     

Morphology and properties of poly(methyl methacrylate)/clay nanocomposites by in-situ solution polymerization

Poly(methyl metacrylate)/montmorillonite (PMMA)/(MMT) nanocomposites were prepared by in-situ solution polymerization of methyl methacrylate monomer in the presence of the organic modified MMT-clay. After the organic modification by ionic exchanging with amine salts, the organoclay becomes more hydrophobic and compatible then pristine clay with methyl methacrylate monomer. The modified clays are characterized by wide angle X-ray diffraction (WAXRD). The powdered X-ray diffraction and transmission electron microscopy (TEM) techniques were employed to study the morphology of the PMMA/clay nanocomposites which indicate that the modified clays are dispersed in PMMA matrix to form both exfoliated and intercalated PMMA/modified clay nanocomposites. The thermo-mechanical properties were measured by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC). Gas permeability analyzer (GPA) shows the excellent gas barrier property of the PMMA nanocomposites which is in good agreement with the morphology. The optical property was measured by UV-vis spectroscopy which shows that these materials have good optical clarity, and UV resistance.     

Kinetic study of isothermal crystallization process of Gd2Ti2O7 precursor's powder prepared through the Pechini synthetic approach

Crystallization process of Gd₂Ti₂O₇ precursor's powder prepared by Pechini-type polymerized complex route has been studied under isothermal experimental conditions in an air atmosphere. It was found that the crystallization proceeds through two-parameter Šesták–Berggren (SB) autocatalytic model, in the operating temperature range of 550 °C≤T≤750 °C. Based on the behavior of SB parameters (M, N), it was found that in the lower operating temperature range, the crystallites with relatively low compactness exist, which probably disclosed low dimensionality of crystal growth from numerous nucleation sites, where the amorphous solid is produced. In the higher operating temperature region (above 750 °C), it was established that a morphological well-defined and high-dimensional particles of the formed pyrochlore phase can be expected. It was found that at T=850 °C, there is a change in the rate-determining reaction step, from autocatalytic into the contracting volume mechanism.     

Preparation and structural characterization of zwitterionic surfactant intercalated into NiZn-layered hydroxide salts

Three zwitterionic surfactants, dodecyl dimethyl carboxylbetaine (DCB), dodecyl dimethyl sulfobetaine (DSB) and N-dodecyl-β-aminoprpionate (DAP), intercalated into NiZn-layered hydroxide salts (NZL-DCB, NZL-DSB and NZL-DAP) were synthesized by the coprecipitation method. The effect of surfactant content, pH, temperature and time of hydrothermal treatment on preparation was investigated and discussed. The NZL-DCB, NZL-DSB and NZL-DAP were characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetry analysis and differential thermal analysis (TGA/DTA). The results showed that basal spacings of NZL-DCB, NZL-DSB and NZL-DAP were around 3.45, 3.68 and 3.94 nm, respectively. DCB, DSB and DAP probably form an overlapped bilayer in the gallery. TGA/DTA data indicated that NZL-DCB, NZL-DSB and NZL-DAP displayed three loss weight stages: loss of adsorbed and structural water, dehydroxylation of matrix and decomposition of nitrate ions, decomposition and combustion of surfactants. Furthermore, chemical analysis data, BET surface area and scanning electron microscopic (SEM) were also measured and analyzed.