Mechanochemical activation of aluminum. 4. Kinetics of mechanochemical synthesis of aluminum carbide

The kinetics of mechanochemical synthesis of aluminum carbide Al₄C₃ from elements was studied with X-ray diffraction analysis, low-temperature argon adsorption, laser granulometry, chemical analysis, X-ray photoelectron spectroscopy, and electron scanning microscopy. The conversion was presented as a function of energy consumption (dose) upon the mechanical treatment of mixtures of aluminum and graphite powders with the composition Al-15 wt % C and Al-30 wt % C. A multistage mechanism of the mechanochemical reaction was revealed, and the following stages were separated and characterized: (i) independent grinding and mixing of reagents, (ii) formation of molecular-dense Al/C composites based on nanosized aluminum particles, (iii) chemical interaction of components with the formation of interatomic Al-C bonds, and (iv) crystallization of Al₄C₃ carbide. The formation of amorphous nuclei of aluminum carbide occurs on the contact surface of aluminum nanoparticles with carbon.     

Mechanochemical Activation of Aluminum: 3. Kinetics of Interaction between Aluminum and Water

Two regimes of oxidation by water are revealed for nanocrystalline aluminum prepared by the mechanical activation of its mixture with graphite and distributed in the matrix of amorphous carbon. At the temperatures 50°C < T < 90°C, nanosized aluminum particles interact with water under quasi-isothermal conditions. The main products are hydrogen and pseudoboehmite AlOOH; a low content of bayerite Al(OH)₃ is also formed. After the induction period, the kinetics of interaction can be satisfactorily described by the law of a diminishing sphere. The effective activation energy of the reaction is equal to 61 ± 10 kJ/mol and is identical for the samples of submicron aluminum prepared by different procedures. At temperatures above 90–95°C, the oxidation of mechanically activated aluminum by water is transformed into a thermally self-accelerated explosion process. Under these conditions, the oxidation of aluminum to α-Al₂O₃ is accompanied by an exothermal reaction between the metal and the carbon matrix during which aluminum carbide Al₄C₃ is formed.     

Study on kinetics of the pyrolysis process of aluminum sulfate

Abstract

Using high aluminum gangue as a raw material, aluminum sulfate 18 hydrate was made by the sulfuric acid leaching method under certain conditions. The product was characterized using X-ray diffraction (XRD) analysis of the phases, x-ray-fluorescence (XRF) analysis of the aluminum and sulfur content and thermogravimetric analysis of the crystallized water [Al₂(SO₄)₃·18H₂O]. Change characteristics of the crystal form and morphology during pyrolysis of octadecahydrate aluminum sulfate were studied by thermogravimetric Analysis (TGA), differential scanning calorimetry (DSC), differential thermogravimetry (DTG), XRD and scanning electron microscope (SEM). The theoretical basis for the preparation of metallurgical alumina from octadecahydrate aluminum sulfate was provided. According to the characteristics of the crystal structure change, the pyrolysis process of octadecahydrate aluminum sulfate can be separated into three stages. The first stage (dehydration stage 87–250?°C) had a weight loss rate of 40.5% and a loss of 15 water molecules; the weight loss rate of the second stage (dehydration stage 280–414?°C) was 8.1% with three water molecules lost; the weight loss rate of the third stage (decomposition stage 770–900?°C) was 36.1%, where three SO₃ molecules were lost. The pyrolysis products were mainly Al₂O₃. The activation energies of the three reaction stages were calculated using the Coats-Redfern method as 90.02?kJ/mol, 205.74?kJ/mol and 284.40?kJ/mol, respectively.     

Heat-Treatment-Induced Evolution of the Mesostructure of Finely Divided Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> Produced by the Sol–Gel Method

A method was developed for the low-temperature sol–gel synthesis of one of the most popular components of functional and structural materials—nanostructured yttrium aluminum garnet Y₃A_l5O₁₂—using precursors from the class of alkoxoacetylacetonates produced from the corresponding acetylacetonates. It was determined that increasing duration of heat treatment of yttrium-aluminum-containing xerogen in air to 6 h reduces the crystallization temperature of the Y₃A_l5O₁₂ phase from 920–930 to 750–800°C, which was confirmed by IR spectroscopy and X-ray powder diffraction analysis. The microstructure of nanocrystalline yttrium aluminum garnet obtained at 800°С was studied; it was found that the size of crystallites is 30–40 nm, the size of particles is 30–50 nm, and the size of pores is 20–30 nm. Small-angle neutron scattering demonstrated that, in the powders synthesized at 700–800°C, there is structural ordering of the short-range type, whereas in the nanocrystalline samples heat-treated at a higher temperature (850°С), there is no such ordering.     

Production of HfB<Subscript>2</Subscript>–SiC (10–65 vol % SiC) Ultra-High-Temperature Ceramics by Hot Pressing of HfB<Subscript>2</Subscript>–(SiO<Subscript>2</Subscript>–C) Composite Powder Synthesized by the Sol–Gel Method

The formation of HfB₂–SiC (10–65 vol % SiC) ultra-high-temperature ceramics by hot pressing of HfB₂–(SiO₂–C) composite powder synthesized by the sol–gel method was studied. By the example of HfB₂–30 vol % SiC ceramic, it was shown that the synthesis of nanocrystalline silicon carbide is completed at temperatures of as low as ≥1700°C (crystallite size 35–39 nm). The production of the composite materials with various contents of fine silicon carbide at 1800°C demonstrated that the samples of the composition HfB₂–SiC (20–30 vol % SiC) are characterized by the formation of SiC crystallites of the minimum sizes (36–38 nm), by the highest density (89%), and by higher oxidation resistance during heating in an air flow to 1400°C.     

Effect of surfactants on the structure and texture characteristics of aluminum oxide

The effect of surfactants (polyvinyl alcohol and cetyltrimethylammonium bromide), which were introduced at the aluminum hydroxide synthesis stage, on the structure and texture characteristics of aluminum oxide was studied by a set of physicochemical techniques. The introduction of the above surfactants did not cause considerable changes in the thermal transformations of aluminum hydroxides, but it affected the genesis of the formed carbon. An analysis of the diffuse reflectance spectra and electron micrographs indicated that the aluminum oxide obtained in the presence of polyvinyl alcohol and calcined at 300°C was covered with polyene-type coke. An increase in the treatment temperature to 550°C led to the formation of condensed aromatic coke; in this case, the specific surface area of the sample increased from 125 to 500 m²/g. The samples calcined at 550°C were γ-Al₂O₃ with a unit cell parameter of 7.933 Å and a crystallite size of no more than 30–40 Å. The pore size distribution was bimodal, with maximums at 35–65 and 380–415 Å, regardless of treatment temperature.     

Synthesis of Highly Dispersed Zirconium Diboride for Fabrication of Special-Purpose Ceramic

Reduction of zirconium dioxide with boron carbide and nanofibrous carbon in argon yielded a highly dispersed powder of zirconium diboride. Characteristics of zirconium diboride powders were examined by various analytical methods. The material obtained is represented by a single phase, zirconium diboride. Powder particles are for the most part aggregated. The average size of particles and aggregates is 10.9–12.9 μm with a wide size distribution. The specific surface area of the samples is 1.8–3.6 m² g^–1. The oxidation of zirconium diboride begins at a temperature of 640°C The optimal synthesis parameters were determined: ZrO₂: B₄C: C molar ratio of 2: 1: 3 (in accordance with stoichiometry), process temperature 1600–1700°C, synthesis duration 20 min.     

Synthesis and characterization of a novel AlQ3‐containing polymer

In this article, the synthesis of a tris(8‐hydroxyquinoline)aluminum (AlQ₃)‐containing poly(arylene ether) (4) is reported. The presence of AlQ₃ pendants in polymer 4 is confirmed by NMR, ultraviolet–visible, photoluminescence, and gel permeation chromatography analyses. This is the first report of the attachment of AlQ₃ complexes as side chains to a polymer. Polymer 4 has a glass‐transition temperature of 217.8 °C and is thermally stable with a 5% weight‐loss temperature greater than 500 °C under nitrogen, as determined by differential scanning calorimetry and thermogravimetric analyses, respectively. Polymer 4 is quite soluble in common organic solvents, such as tetrahydrofuran, N,N‐dimethylacetamide, and CHCl₃. A composite that is 80 wt % polymer 4 and 20 wt % AlQ₃ forms a transparent and tough film when cast from its chloroform solution. The application of this AlQ₃‐containing polymer in light‐emitting diodes is under investigation. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2887–2892, 2000     

Polythermal solubility of light fullerenes in valeric and caproic acids in the temperature range 20–80°C

The polythermal solubility of fullerene C₆₀ and a fullerene mixture (60 wt % C₆₀ + 39 wt % C₇₀ + 1 wt % higher fullerenes C _n , n = 76, 78, 84, 90...) in valeric and caproic acids was studied in the temperature range 20–80°C. The solubility diagrams are presented and characterized.     

Low-temperature mechanochemical synthesis of nanosized silicon carbide

The methods of X-ray diffraction analysis, scanning electron microscopy, synchronous thermal analysis, and adsorption are used to study the mechanochemical synthesis of silicon carbide through the reaction Si + C → β-SiC. The reaction is found to take place in several stages. At the first stage, i.e., at activation doses below approximately 5 kJ/g, the powders of the components are independently ground to increase the specific surface area of the mixture to 145 m²/g, graphite is amorphized, and the sizes of the coherent-scattering regions of silicon drastically diminish. At the second stage (doses of 5–15 kJ/g), dense Si/C aggregates are formed and two fractions (coarse and fine) with different particle sizes arise in silicon crystallites. As the activation dose is enhanced, the amount of the fine fraction rises, while the sizes of coherent-scattering regions decrease to 2–3 nm. When samples are heated at 800°C, the fine fraction of silicon interacts with carbon to yield silicon carbide with crystallite sizes of 3–4 nm, whereas the coarse fraction of silicon recrystallizes. At the third stage, i.e., at doses of higher than 15 kJ/g, the mechanochemical synthesis of SiC occurs through the following scheme: fine fraction Si + C → amorphous SiC → crystallization of SiC.     

The heat of wetting of aluminum hydroxyoxide obtained by the thermal activation of hydrargillite

Changes in the volume and surface properties of thermally activated hydrargillite, so-called centrifugal thermal activation (CTA) product (empirical formula Al₂O₃ · 0.85H₂O), during its calcining in air with gradually increasing temperature from 90 to 1100°C were studied. At each stage of calcining, weight loss, phase composition, texture characteristics, and the heat of wetting with water at 25°C were determined. Measurements of the heat of wetting showed that the energy and, therefore, chemical state of the surface changed during thermal treatment. The data obtained were used to calculate the heats of adsorption q _ads of water vapor by CTA samples with different water contents; the range of q _ads variations was 50–250 kJ/mol. The values obtained are compared with literature data.     

Thermodynamic properties of aluminum monooxycarbide Al2OC

It was shown that the published data on the thermodynamic properties of aluminum monooxycarbide Al₂OC (ed) are not consistent with the phase diagram of the Al₂O₃-Al₄C₃ system. A thermodynamic modeling of the equilibrium state of the Al₂O₃-Al₄C₃ system made it possible to obtain new estimates of the standard entropy and enthalpy of formation of aluminum monooxycarbide: S°(298.15 K, cd. Al₂OC) = 45.3 J/(K mol) and Δ_f H°(298.15 K, cd, Al₂OC) = ?625.6 kJ/mol.     

Study on thermal decomposition processes of polysiloxane polymers—From polymer to nanosized silicon carbide

The objective of this study is to determine experimentally the usefulness of different polysiloxanes as precursors for bulk ceramic products. Such resins are an alternative for currently commercially used polycarbosilanes. Four types of polysiloxanes were used. The polymers differed in C/Si molar ratio. Thermogravimetric measurements of polymers were made to determine curing and heat treatment conditions. Ceramic yield (Y_c) after heat treatment over the temperature range from 20 to 1700 °C was determined. Structure, microstructure and phase composition of ceramic products obtained from the polymers were investigated. It was found that during thermal decomposition of polymers in the temperature range from 20 to 1000 °C amorphous inorganic Si–C–O ceramics were formed. When the temperature exceeded 1500 °C nanosized 3C and 2H types of silicon carbide crystallized from the resin precursors with C/Si molar ratio higher than 1. On the contrary, heat treatment of polymer with C/Si molar ratio close to 1 did not lead to the formation of nanocrystalline silicon carbide.     

Polymeric aluminum porphyrin: Controllable synthesis of ultra-low molecular weight CO2-based polyols

Carbon dioxide-based polyols with ultra-low molecular weight (ULMW, M_n < 1000 g/mol) are emergent polyurethane precursors with economic and environmental benefits. However, the lack of effective proton-tolerant catalytic systems limits the development of this field. In this work, the polymeric aluminum porphyrin catalyst (PAPC) system was applied to the copolymerization of CO₂ and propylene oxide, where sebacic acid, bisphenol A, poly(ethylene glycol), and water were used as chain transfer agents to achieve the controlled synthesis of CO₂-polyols. The molecular weight of the resulting CO₂-polyols could be facilely regulated in the range of 400–930 g/mol at low catalyst loadings, fully demonstrating its catalytic advantages of high activity, high product selectivity, and excellent proton tolerance of PAPC. Meanwhile, the catalytic efficiency of PAPC could reach up to 2.1–5.2 kg/g under organic CTA conditions, even reaching 1.9 kg/g using water as the CTA. The cPC content could be controlled within 1.0 wt% under the optimized conditions, indicating the excellent controllability of the PAPC system. ULMW CO₂-polyols combines the advantages of low viscosity (∼3000 mPa s at 25 °C), low glass transition temperature (∼−73 °C), and high carbonate unit content (∼40%), which is important for the development of high-performance polyurethanes.     

Kinetics of aluminum extraction from aluminum ash by leaching with sulfuric acid

Abstract

To explore the high value of aluminum ash development and utilization, factors (granularity of aluminum ash, leaching temperature, leaching time, sulfuric acid concentration, and liquid solid ratio) affecting the leaching rate of aluminum were evaluated. The kinetics of the leaching process was also investigated. The results indicate that the leaching rate of aluminum in aluminum ash can reach 88.9% under the following conditions: leaching temperature, 100?°C; leaching time, 120?min; sulfuric acid concentration, 2.5?mol/L; and liquid–solid ratio (mass ratio), 1.3:1. Studies on the leaching dynamics of aluminum in aluminum ash showed that leaching was divided into two stages. At the initial stage (i.e. at the leaching rate x?≤?0.3), the external diffusion resistance and the diffusion resistance of the solid product layer are negligible, and leaching is controlled by chemical reaction at the interface. The apparent activation energy was 5733.25?J/mol. At the leaching rate x?=?0.3–0.9, the leaching rate was controlled by the non-steady diffusion of the liquid film in the porous solid by the fluid reactant H₂SO₄. The apparent activation energy was 25390.87?J/mol.     

Hydrolysis of aluminum sulfates in the presence of ammonium sulfite

Hydrolysis of aluminum sulfate and potash alum in the presence of an ammonium sulfite additive in the temperature range T _boil-200°C was studied. A possible reaction mechanism was suggested. The composition and structure of the precipitates formed were determined and certain differences in their morphology were noted. Recommendations for practical implementation of the method of hydrolytic recovery of aluminum from solutions of its sulfate salts were given.     

Impact of a Supersonic Dissociated Air Flow on the Surface of HfB<Subscript>2</Subscript>–30 vol % SiC UHTC Produced by the Sol–Gel Method

A new method to produce ultra-high-temperature ceramic composites under rather mild conditions (1700°C, 30 MPa, treatment time 15 min) was applied to synthesize a relatively dense (ρ_rel = 84.5%) HfB₂–30 vol % SiC material containing nanocrystalline silicon carbide (average crystallite size ～37 nm). The elemental and phase compositions, microstructure, and some mechanical properties of this material and also its thermal behavior in an air flow within the temperature range 20–1400°C were investigated. Using a high-frequency induction plasmatron, a study was made of the effect of a supersonic dissociated air flow on the surface of the produced ultra-high-temperature ceramic composite shaped as a flat-end cylindrical sample installed into a copper water-cooled holder. On 40-min exposure of the sample to the supersonic dissociated air flow, the sample did not fail, and the weight loss was 0.04%. Although the heat flux was high, the temperature on the surface did not exceed 1400–1590°C, which could be due to the heat transfer from the sample to the water-cooled model. The thickness of the oxidized layer under these conditions was 10–20 μm; no SiC-depleted region formed. Specific features of the microstructure of the oxidized surface layer of the sample were noted.     

Temperature dependent near-UV molar absorptivities of aliphatic aldehydes and ketones in aqueous solution

Temperature dependent molar absorptivities are reported for acetone, 2-butanone, 2-pentanone, 3-pentanone, acetaldehyde, propionaldehyde, and n-butyraldehyde in aqueous solution. Molar absorptivities are given at eight temperatures in the range 6.5–69.5°C for wavelengths greater than 200 nm, a spectral resolution of 2.0 nm, and a spacing of 2.5 nm. For both ketones and aldehydes a shift to shorter wavelengths of approximately 10 nm is observed in the aqueous phase absorption spectrum relative to that found in the gas phase. For the ketones, there is an increase in the total intensity of the spectrum of approximately 5% over the range of temperatures studied. For the aldehydes a much larger change in the intensity of the absorption spectrum is observed, due to the temperature dependence of the hydration reaction RCHO + H₂O ⇄ RCH(OH)₂; K_hyd = [RCH(OH)₂/[RCHO]. The change in the spectral intensity with temperature is used to determine thermodynamic parameters for the hydration reaction, giving the following results (at 25°C): acetaldehyde, K_hyd = 1.13 ± 0.06, ΔH = −19.7±0.6kJ/mol, ΔS= −65.0±2.5J/mol-K; propionaldehyde, K_hyd=1.02±0.06, ΔH=-20.8±0.8kJ/mol, ΔS=-69.6±3.1J/mol-K; n-butyraldehyde, K_hyd=0.50±0.05, ΔH=-27.0±2.2kJ/mol, ΔS= −96.5± 8.2 J/mol-K. The implications of these results for aqueous phase atmospheric chemistry are discussed.     

Effects of vacuum annealing on the particle size and phase composition of nanocrystalline tungsten carbide powders

The effects of the vacuum annealing temperature (400–1400°C) on the phase and chemical composition, particle size, and microstresses of the nanocrystalline powders of tungsten carbide WC with 20–60 nm particles were studied by X-ray diffraction and electron microscopy. Vacuum annealing of WC nano-powders at T _ann ≤ 1400°C was accompanied by decarbonization, resulting from the interaction of carbon with the oxygen impurity. Changes in the chemical composition of the nanocrystalline powder of tungsten carbide led to changes in its phase composition. The annealing was accompanied by growth of powder particles due to the aggregation of nanoparticles and by a decrease of microstresses.     

Sensor Performance of Nanostructured TiO<Subscript>2</Subscript>–SnO<Subscript>2</Subscript> Films Prepared by an Aqueous Sol–Gel Process

Nanostructured TiO₂–SnO₂ thin films and powders were prepared by a facile aqueous particulate sol–gel route. The prepared sols showed a narrow particle size distribution with hydrodynamic diameter in the range 17.2–19.3 nm. Moreover, the sols were stable over 5 months, since the constant zeta potential was measured during this period. The effect of Sn:Ti molar ratio was studied on the crystallisation behaviour of the products. X-ray diffraction analysis revealed that the powders were crystallised at the low temperature of 400 °C containing anatase-TiO₂, rutile-TiO₂ and cassiterite-SnO₂ phases, depending on annealing temperature and Sn:Ti molar ratio. Furthermore, it was found that SnO₂ retarded the anatase to rutile transformation up to 800 °C. The activation energy of crystallite growth was calculated in the range 0.96–6.87 kJ/mol. Transmission electron microscope image showed that one of the smallest crystallite sizes was obtained for TiO₂–SnO₂ binary mixed oxide, being 3 nm at 600 °C. Field emission scanning electron microscope analysis revealed that the deposited thin films had nanostructured morphology with the average grain size in the range 20–40 nm at 600 °C. Thin films produced under optimized conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO gas at low operating temperature of 200 °C, resulting in increased thermal stability of sensing films as well as a decrease in their power consumption.