Synthesis of aluminum oxides from the products of the rapid thermal decomposition of hydrargillite in a centrifugal flash reactor: II. Structural and textural properties of aluminum hydroxide and oxide obtained from the product of the centrifugal thermal activation of hydrargillite (CTA product)

The basic properties of aluminum hydroxide compounds, namely, the flash product and the centrifugal thermal activation (CTA) product both before and after heat treatment at 400–1100°C are considered. The hydration conditions for obtaining 100% pseudoboehmite from the CTA product are determined. The structure, morphology, and texture of pseudoboehmite aluminum hydroxide synthesized from the CTA product and of the aluminum oxides obtained from this hydroxide in the temperature range 600–1100°C are described.     

Physicochemical properties of TiO<Subscript>2</Subscript> (anatase) prepared by the centrifugal thermal activation of hydrated titanium dioxide

The physicochemical properties of titania (anatase) prepared from hydrated titanium dioxide by centrifugal thermal activation (CTA) at 140–700°C were studied. It was found that the microstructure and the texture parameters of anatase prepared by the above method were considerably different from those of the samples prepared by the traditional thermal decomposition of titanium hydroxide. The conditions of centrifugal activation exerted a considerable effect on the structure and the texture parameters of the resulting anatase. The crystal structure of anatase prepared at a temperature lower than 650°C was imperfect, and it approached a regular structure only at a temperature of >650°C. At temperatures higher than 300°C, the samples of TiO₂ prepared using CTA were characterized by higher specific surface areas, fine pore structures, and comparable mesopore volumes, as compared with the samples prepared by commonly used synthetic methods.     

Optimized procedure for clay pillaring with aluminum species used in depollution

This study presents the improved method of smectite type clays pillaring, using aluminum salts. To achieve this goal, natural clay with a percentage of more than 95% montmorillonite was used. In order to analyze the pillaring process, a thermal procedure was used (thermal programmed desorption). The results of the pillaring process show that a controlled distance between the foliar band structures as long as 18·10^–10 m can be obtained. The clays modified by pillaring can be used as molecular sieves in microfiltration processes or as agents in residual water chemical depolluting processes. They also have zeolite properties which make them reusable.     

A new differential method for determining orders and rates of reactions and its application to solid state reactions. Part I. Some simple decompositions

The thermal dehydroxylation of natural Al-bearing geothite was investigated by IR spectroscopy. Venezuelan lateritic bauxites (which in addition to goethite contain kaolinite, gibbsite, ilmenite and quartz), as well as chemically isolated samples of Al-goethites, were heated to 300, 600 and 1000°C. The spectral features of the iron oxides formed during the thermal treatment depend on the heating temperature, showing that the first dehydroxylation product is Al-bearing protohematite which at temperatures above 600°C is recrystallized to Al-bearing hematite. Part of the aluminum which is occuled in this hematite originates from the gibbsite and to a smaller extent from the kaolinite.     

Effect of silica on the stability of the nanostructure and texture of fine-particle alumina

The effect of the modification of aluminum oxide with silicon oxide on the stability of fine-particle Γ- and δ-Al₂O₃ phases upon heat treatment in the wide temperature range of 550–1500°C was studied. It was found that the Γ- and δ-Al₂O₃ phases modified with silica are thermally stable up to higher temperatures than pure aluminum oxide. This is due to changes in the real structure of the modified samples, specifically, an increase in the concentration of extensive defects stabilized by hydroxyl groups bound to not only aluminum atoms but also silicon atoms. It is likely that Si-OH groups, which are thermally more stable than Al-OH groups, stabilize the microstructure of Γ- and δ-Al₂O₃ to higher temperatures, as compared with aluminum oxide containing no additives. Simultaneously, an increase in the thermal stability of the modified samples is accompanied by the retention of a high specific surface area and a developed pore structure at higher treatment temperatures.     

The effect of different atmospheres on structural changes of titanate nanotubes during heating

Thermal stability of titanate nanotubes (Ti-NT) was studied in temperature range from room temperature up to 1000 °C in three different atmospheres—in air, vacuum and helium—and differences were observed. In air, vacuum and helium, the release of adsorbed and crystal water was detected in temperatures up to 200 °C. Transformation of the initial structure to anatase form of TiO₂ was found independently of the atmosphere used. But in air the transformation to anatase occurred at lower temperature. The difference between these atmospheres was approximately 100 °C. However, only in air, the transformation to sodium hexatitanate and rutile form of TiO₂ occurred at higher temperatures. In vacuum, only the anatase phase was detected up to 800 °C. In helium, the transformation to rutile was observed but not to sodium hexatitanate. The changes of the initial structure of Ti-NT during the elevation of temperature were studied by combination of in situ powder X-ray diffraction, differential scanning calorimetry, thermal gravimetric analysis and mass spectroscopy.     

Verflüchtigung von Spaltproduktjod aus Festem UCl4

It is possible to volatilize about 95% (at temperatures between 200°C and 600°C) of the iodine, which has been produced by irradiating solid UCl₄ with thermal neutrons. The delay times for iodine are decreasing from 2 hrs (at 200°C) to 7 sec (at 600°C). The release of the iodine is improved by using an inert carrier gas. When adding AgCl to the UCl₄ before irradiation is carried out, the release of iodine is suppressed at temperatures lower than 400°C. The adding of KI leads to a more rapid release of iodine at low temperatures.      

Structure and texture of oxyhydroxides formed by oxidation of nanodispersed aluminum with water

Aluminum oxyhydroxides were prepared by oxidation with water (55°C) of nanodispersed aluminum powders obtained by electric explosion of conductors in argon, nitrogen, carbon dioxide, and their mixtures. The structure and texture of aluminum oxyhydroxides were studied using methods of thermal desorption of nitrogen, derivatography, electron microscopy, and X-ray phase analysis.     

Synthesis and characterization of organosoluble polyetherimides with bulky triaryl imidazole moiety: Study of photophysical properties and kinetic of thermal degradation

Novel fluorescent polyetherimides containing bulky triaryl imidazole pendant and ether linkage in the main chains were prepared from a diamine, 2,2-phenoxy-bis[4-amino-2(4,5-diphenyl-2H-imidazole-2-yl)]propane, and commercially available dianhydrides. All the resulting polymers were amorphous with inherent viscosities ranged from 0.45 to 0.75 dl/g and were readily soluble in many organic solvents which could be solution-cast into transparent and tough films. They had useful levels of thermal stability associated with relatively high T _g (245?C254°C), 10% weight loss temperatures in excess of 500°C, and char yields at 600°C in air up to 62%. These polymers exhibited strong UV-vis absorption maxima in solution and in solid state. Photoluminescence efficiency of the polyetherimides was affected by their chemical structure with quantum yields in the range 10?C28%.     

Effect of multivalent cations,temperature, and aging on SOM thermal properties

Multivalent cations are suggested to influence the supramolecular structure of soil organic matter (SOM) via inter- and intra-molecular interactions with SOM functional groups. In this study, we tested the combined effect of cations, temperature treatment, and isothermal aging on SOM matrix properties. Samples from a peat and a mineral soil were either enriched with Na, Ca, and Al or desalinated in batch experiments. After treatment at 25, 40, 60, and 105 °C and after different periods of aging at 19 °C and 31 % relative humidity, we investigated the physicochemical matrix stability and the thermal stability against combustion. We hypothesized that multivalent cations stabilize the SOM matrix, that these structures disrupt at elevated temperatures, and that aging leads to an increase in matrix stability. The results show that cation-specific effects on matrix rigidity started to evolve in the peat only after 8 weeks of aging and were significantly lower than the temperature effects. Temperature treatment above 40 °C caused a non (or not immediately) reversible loss of water molecule bridges (WaMB) and above 60 °C a partly reversible melting process probably of semi-crystalline poly(methylene). Thermal stability increased with increasing cation valence and degree of protonation and was much less affected by temperature. Generally, Na-treated and control samples revealed lower thermal stability and lower increase in matrix rigidity with aging than those treated with Ca, Al, and H. We conclude that drying at elevated temperatures (>40 °C) may irreversibly change SOM structure via disruption of labile cross-links and melting of semi-crystalline domains.     

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.     

Enhanced thermal stability of poly(vinyl alcohol) in presence of melanin

A novel method was developed to enhance the thermal stability of PVA by using natural and synthetic melanins from oxidation of dopamine. Thermogravimetric (TG) curves indicated that the synthetic melanin changed the thermal degradation behaviors of PVA and largely improved the decomposed temperature by 80–110 °C in nitrogen when incorporation of synthetic melanin with low content (0.5–2 mass%). The thermal degradation kinetics suggested the activation energies of PVA/synthetic melanin blends were much higher than these of pure PVA. Isothermal TG curves conformed that the PVA/synthetic melanin blends exhibited more thermal stability than pure PVA. Moreover, the chemical structure changes of macromolecular after degradation were characterized by using fourier transform infrared and the results suggested that elimination reaction on the first degradation step did not took place for the PVA/synthetic melanin blends at 270 °C.     

Time‐Resolved High and Low Temperature Phase Transitions of the Nanocrystalline Cubic Phase in the Y2O3‐ZrO2 and Fe2O3‐ZrO2 System

The nanocrystalline cubic Phase of zirconia was found to be thermally stabilized by the addition of 2.56 to 17.65 mol % Y₂O₃ (5.0 to 30.0 mol % Y, 95.0 to 70.0 mol % Zr cation content). The cubic phase of yttria stabilized zirconia was prepared by thermal decomposition of the hydroxides at 400°C for 1 hr. 2.56 mol % Y₂O₃‐ZrO₂ was stable up to 800°C in an argon atmosphere. The samples with 4.17 to 17.65 mol % Y₂O₃ were stable to 1200°C and higher. All samples at temperatures between 1450°C to 1700°C were cubic except the sample with 2.56 mol % Y₂O₃ which was tetragonal. The crystallite sizes observed for the cubic phase ranged from 50 to 150 Å at temperatures below 900°C and varied from 600 to 800 nm between 1450°C and 1700°C. Control of furnace atmosphere is the main factor for obtaining the cubic phase of Y‐SZ at higher temperature. Nanocrystalline cubic Fe‐SZ (Iron Stabilized Zirconia) with crystallite sizes from 70 to 137 Å was also prepared at 400°C. It transformed isothermally at temperatures above 800°C to the tetragonal Fe‐SZ and ultimately to the monoclinic phase at 900°C. The addition of up to 30 mol % Fe(III) thermally stabilized the cubic phase above 800°C in argon. Higher mol % resulted in a separation of Fe₂O₃. The nanocrystalline cubic Fe‐SZ containing a minimum 20 mol % Fe (III) was found to have the greatest thermal stability. The particle size was a primary factor in determining cubic or tetragonal formation. The oxidation state of Fe in zirconia remained Fe³⁺. Fe‐SZ lattice parameters and rate of particle growth were observed to decrease with higher iron content. The thermal stability of Fe‐SZ is comparable with that of Ca‐SZ, Mg‐SZ and Mn‐SZ prepared by this method.     

Synthesis and characterization of thermally stable,high‐modulus polyimides containing benzimidazole moieties

A series of novel benzimidazole‐containing aromatic polyimides were prepared from synthesized 5,4′‐diamino‐2‐phenyl benzimidazole (DAPBI), and commercial dianhydrides by the conventional two‐step polymerization. The obtained films were amorphous and could afford flexible, transparent, and tough films with excellent thermal and mechanical properties. They showed high levels of tension strength of up to 234 MPa, modulus of up to 5.6 GPa without any stretching. According to thermal stability measurements, the glass‐transition temperatures of the polymers were observed between 329 and 425 °C. The 5% weight‐loss temperatures of most polyimides were above 600 °C in nitrogen. Excellent properties of these polyimides were proved to be attributed to the rigid‐rod structure and hydrogen bond of intermacromolecular. SAXS and SEM results showed self‐molecular orientation caused the formation of rod‐like extended conformations. It was demonstrated that high degree of supramolecular order led to the increase of thermal stability and mechanical properties of the polyimide films. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2024–2031, 2009     

混合金属络合物羟基镍铝交联蒙托土的合成

用取代法和共聚法合成了一系列羟基镍铝交联蒙托土。(27)~Al-NMR,UV谱表明两种合成方法都可以得到具有Kessin结构的羟基镍铝柱。取代法合成的交联蒙托土具有均一的(001)面衍射(d_(001)约1.80nm),而共聚法的样品有两种羟基镍铝柱支撑着层间,分别在1.80nm和1.55nm附近产生复合(001)面衍射。用高温老化处理可改善交联蒙托土的热稳定性。经过处理的样品在700℃焙烧后仍可保持稳定,与不经处理的样品相比,热稳定性提高200℃以上。     

Crystallization study of SrSnO3:Fe

Alkaline earth stannates have recently become important materials in ceramic technology due to its application as humidity sensor. In this work, alkaline earth stannates doped with Fe³⁺ were synthesized by the polymeric precursor method, with calcination at 300 °C/7 h and between 400 and 1100 °C/4 h. The powder precursors were characterized by TG/DTA after partial elimination of carbon. Characterization after the second calcination step was done by X-ray diffraction, infrared spectroscopy, and UV?Cvis spectroscopy. Results confirmed the formation of the SrSnO₃:Fe with orthorhombic perovskite structure, besides SrCO₃ as secondary phase. Crystallization occurred at 600 °C, being much lower than the crystallization temperature of perovskites synthesized by solid state reaction. The analysis of TG curves indicated that the phase crystallization was preceded by two thermal decomposition steps. Carbonate elimination occurred at two different temperatures, around 800 °C and above 1000 °C.     

Surface Modification of Silicagels. Evaluation of Thermally - Treated Commercial Silicagels

Abstract

The chromatographic properties and the structure and texture of thermally treated silicagels have been studied. Commercial silicagels (Partisil and Lichrosorb) were thermally modified at 300, 400, 500, 600, 700, 800 and 900°C. The evolution of specific surface, pore volume and distribution of mean pore diameter as a function of temperature was investigated. It is shown that the modification at 600°C is the most interesting one: reducing the hydroxyl concentration of a silicagel by a controlled thermal treatment, all the surface hydroxyl groups could be involved in a chemical modification.     

Specifics of the thermal transformations of the wurtzite phase of boron nitride

The kinetics of the transformation of the BN wurtzite phase to the graphite modification was studied at normal pressure and 600–970°C. At these temperatures and certain thermal treatment durations, along with the formation of the graphite phase, the reverse transition from g-BN to w-BN occurs, a behavior indicative of a higher thermodynamics stability of the wurtzite phase.     

Alumina cryogels with superior thermal stability for catalyst supports

Alumina cryogels were synthesized from a colloidal boehmite sol through a sol–gel processing and subsequent freeze drying, and thermal stability was examined by comparison to that of the corresponding xerogel, precipitate and commercial alumina. N₂ adsorption, X-ray powder diffraction and transmittance electron micrography observations revealed that the stability was higher for the cryogel than for others in particular at temperatures above 1,000 °C. The higher stability was ascribed to the fine and uniform primary particles with fibrous shapes formed by the sol–gel technique and furthermore to the suppression of aggregation of the primary particles owing to the subsequent freeze drying. It was also found that aluminum sec-butoxide employed as a precursor for the preparation of boehmite sol was preferable compared to aluminum iso-propoxide.     

Thermal conversions of polymers with macroheterocycles in the chain

Thermal conversion of polyhexazocyclanes and compound modeling the fragments of polymers were explored within 300–600°C under vacuum (10^?5 torr) and in water vapor. At temperatures below 450°C, the hydrolytic degradation plays a key role in the thermal degradation of polyhexazocyclanes. At a higher temperature, the homolytic processes prevail, water, strongly retained by the macroheterocycle, is most important in the hydrolysis process. It was shown that the increase in the thermohydrolytic stability of polymers with the system of condensed cycles in the macroheterocycles and that of polyhexazocyclanes modified by polymer analogous conversion at the secondary amino groups of the isoindole cycle was caused by the absence of bound water.