Takeshi Tsuchida

Mechanochemical phenomena of gibbsite, bayerite and boehmite by grinding

Three hydrated aluminas, gibbsite, bayerite and boehmite, were ground in air for 0.25–20 h by a planetary ball mill. The effect of grinding on their texture, structure and thermal behaviors has been examined by means of XRD, TG, DTA, SEM, IR, particle size distribution and nitrogen gas adsorption. It was found that in the initial stages of grinding (within the first 1 h), the specific surface area, S, had increased in the gibbsite, decreased in the bayerite and had a maximum in boehmite, and then proceeded to achieve their respective equilibrium values. These behaviors were correlated with changes in the median size, d50, as a measure of the aggregate size of the particles. Prolonged grinding times, led to a decrease in the intensity of X-ray diffraction lines and an increase in the half-maximum line breadth. After 4 or 8 h grinding all the hydrated aluminas had been changed to the amorphous phase without mechanochemical dehydration. Moreover, the DTA endotherms attributable to the dehydration of the hydrated aluminas gradually diminished on grinding, and a set of broad endotherms appeared at about 150 or 200 °C for the amorphous phases. Whereas the transformation sequences of the unground hydrated aluminas to α-Al2O3 showed gibbsite → χ → κ → α, bayerite → η → θ → α and boehmite → γ → δ → θ → α, all the amorphous phases showed the amorphous hydrated alumina → amorphous alumina → η → α and the temperature of α-transformation had been lowered by more than 200°C.

Hydrothermal synthesis of submicrometer crystals of boehmite

Abstract The effect of solvents, mechanically ground gibbsites and amorphous hydrated alumina gels on the formation of well-crystallized and submicrometer-sized crystals of boehmite was studied. Nearly monodispersed boehmite particles of 0.2–0.3 μm with a parallelogram shape were hydrothermally synthesized from ball-milled gibbsite in water at 200°C for 6 h. The 100 nm-sized crystals of boehmite were prepared from amorphous hydrated alumina gels at 255°C for 24 h.

The effect of Cr3+ and Fe3+ ions on the transformation of different aluminum hydroxides to α-Al2O3

Abstract The α-transformation of pseudoboehmite, boehmite and bayerite containing Cr 3+ and Fe 3+ ions at concentrations of 0, 4.8, 9.1 and 16.7 mole%, which were prepared by coprecipitation, was investigated by means of TG, DTA, X-ray diffraction and diffuse reflectance spectra. It was found that the added metal ions influenced remarkably the temperature and sequence of the transformation to α-Al 2 O 3 . The effect depended on the state of existence of the ions, which varied with the kind of aluminum hydroxide. In pseudoboehmite and boehmite containing Cr 3+ ion, the α-transformation [via η (or γ) → δ → θ → α-Al 2 O 3 ] was inhibited by the stabilization of θ-Al 2 O 3 . The stabilization of θ-Al 2 O 3 was considered to be due to the formation of Cr 6+ ion, which resulted from the oxidation of part of the Cr 3+ ions incorporated in aluminum hydroxides during the dehydration in air. On the contrary, in bayerite containing Cr 3+ ion the α-transformation (via η → α) was accelerated: the added Cr 3+ ion was not incorporated in the bayerite structure and was crystallized separately as α-Cr 2 O 3 on heating. This α-Cr 2 O 3 particle in a nascent state appeared to act as an active nucleus to the crystallization of α-Al 2 O 3 . On the other hand, in all aluminum hydroxides containing Fe 3+ ion, the α-transformation [via η (or γ) → δ → α] was remarkably accelerated. Since the added Fe 3+ ion was not incorporated in all aluminum hydroxides and was crystallized as α-Fe 2 O 3 on heating, the same accelerating mechanism as in the bayerite containing Cr 3+ ion is assumed to operate. Moreover, the effect of α-Cr 2 O 3 , mixed mechanically with transition aluminas (η-, γ- and δ-Al 2 O 3 ) in different molar ratios on the α-transformation was also investigated for a comparison with the results obtained in aluminum hydroxides containing Cr 3+ ion prepared by coprecipitation.

The effect of grinding on the thermal decomposition of alumina monohydrates, α- and β-Al2O3.H2O

Abstract Well-crystallized alumina monohydrates, two boehmites and a diaspore, were ground in air for 0–24 h in a planetary ball mill. The effect of grinding on the texture, structure and thermal behaviour of dehydration and α-transformation was examined by means of XRD, TG, DTA, SEM and 27Al MAS-NMR. It was found that after 24 h of grinding, the boehmites changed entirely to an amorphous hydrated alumina, part of which transformed to alumina by an additional mechanochemical dehydration, while diaspore did not.

Synthesis of Al3BC in air from mechanically activated Al/B/C powder mixtures

Abstract A ternary aluminum borocarbide, Al 3 BC was prepared for the first time by self-propagating high-temperature synthesis (SHS) induced by mechanical activation of Al/B/C powder mixtures in air. The effect of mixing molar ratio of Al/B/C and grinding time on the formation of Al 3 BC was investigated. On the other hand, Al 3 BC was also formed by mechanical activation and subsequent annealing of Al/B/C=3/1/1 powder mixture. The lattice constants of Al 3 BC obtained in two methods were compared.

Synthesis of ZrC and ZrN in air from mechanically activated Zr-C powder mixtures

Abstract Mixtures of zirconium and carbon powders in various atomic fractions (5–75 at.% of C) were ground in a planetary ball mill. When the mixtures thus activated mechanically were transferred into a graphite crucible and exposed to air, they ignited spontaneously and the self-propagating high-temperature synthesis (SHS) took place in two successive steps. Zirconium carbide and zirconium nitride were formed, depending on the initial C concentration. The almost stoichiometric ZrC was formed in air from the ZrC powder mixtures with C = 50–60 at.%. From the measurement of the lattice constant and X-ray photoelectron spectroscopy (XPS) spectra, the formation of solid solutions between ZrC and ZrN was also estimated. A possible reaction mechanism for the formation of the zirconium carbide and nitride is discussed.

Thermoanalytical study on the chlorination of hydrated aluminas and transition aluminas

Abstract The chlorination processes of four hydrated aluminas (bayerite, gibbsite, pseudoboehmite, boehmite) and four transition aluminas (η-,γ-,δ-,θ-Al2O3) were studied in the presence of active carbon by means of a gas-flow DTA apparatus. In the case of hydrated alumina systems three exothermic peaks appeared at about 230, 460 and 500°C or above, which corresponded to the formation of hydrogen chloride, white needle-like crystals and the chlorination of hydrated aluminas, respectively. On the other hand, in transition alumina systems, only one exothermic peak due to the chlorination of transition alumina appeared at 580–670°C. The relationship between the chlorination behavior and structure of transition aluminas was discussed.

Hydrothermal synthesis and characterization of diaspore, β-Al2O3•H2O

An acicular single crystal of diaspore was synthesized under the conditions of 1 N NaOH, 450 ° C and 1200 kg cm−2 and characterized by means of X-ray diffraction, scanning electron microscopy, electron diffraction and infrared spectroscopy. The reactivity of three kinds of natural ores containing diaspore which were used as a seed crystal for synthesis of diaspore was also compared.

Thermoanalytical study on the reaction of the CuO-Al2O3(η, γ and α) systems

Abstract The reaction processes proceeding in the CuO-Al 2 O 3 (η, γ and α) systems were investigated by means of TG, DTA and high-temperature X-ray diffraction. In the CuO—η-Al 2 O 3 and CuO—γ-Al 2 O 3 systems, the formation of 80% CuAl 2 O 4 (reaction 1) occurred on heating to 1000°C. The resulting CuAl 2 O 4 decomposed to CuAlO 2 , Al 2 O 3 and O 2 (reaction 2) in the temperature range 1130–1230°C. The incongruent melting of CuAlO 2 (reaction 3) occurred at about 1250°C. On the other hand, the CuO—α-Al 2 O 3 system did not show the formation of CuAl 2 O 4 on heating to 1000°C, and CuO decomposed to Cu 2 O (reaction 4) at 1045°C and then the resulting Cu 2 O reacted with α-Al 2 O 3 to form CuAlO 2 (reaction 5). The difference in reaction process between the CuO—η-Al 2 O 3 , CuO—γ-Al 2 O 3 and CuO—α-Al 2 O 3 systems can be attributed to the reactivity of η-, γ- and α-Al 2 O 3 in CuAl 2 O 4 formation. CuO + Al 2 O 3 = CuAl 2 O 4 (1) 4 CuAl 2 O 4 = 4 CuAlO 2 + 2 Al 2 O 3 + O 2 (2) 2 CuAlO 2 = Al 2 O 3 + liquid (3) 4 CuO = 2 Cu 2 O + O 2 (4) Cu 2 O + Al 2 O 3 = 2 CuAlO 2 (5)

Kinetics of the dehydration of boehmites prepared under different hydrothermal conditions

Abstract The kinetics of the dehydration of five boehmites, which were prepared under different hydrothermal conditions (300°C, 85 atm, 20 h — 150°C, 4.5 atm, 5 h), were studied by means of isothermal TG at an air pressure of 150 mm Hg and at a constant flow rate of nitrogen (30 ml min −1 ) containing water vapor of partial pressures between 10 −4 and 23.8 mm Hg. It was found that the dehydration temperature of boehmites was lowered as the preparation conditions became more mild. Moreover, the rate-controlling step of the dehydration of boehmites varied with their preparation conditions. The water vapor pressure led to decreases in the rate of dehydration.