Formation Of γ-Al2O3 Research Articles

Mullite formation from non-crystalline precursors

X-ray diffraction, differential thermal analysis, and29Si and27Al MAS NMR spectroscopy were used to characterize the formation of mullite from non-crystalline precursors. The precursors were produced by the sol-gel route from organic starting compounds (SGM), and by coprecipitation of inorganic starting materials (CM). There is NMR evidence that both types of mullite precursor consist of Si-O and Al-O tetrahedra, and of Al-O octahedra. Furthermore, fivefold-coordinated Al-O polyhedra, or alternatively strongly distorted Al-O tetrahedra, do occur in the precursors. SGM and CM precursors transform to mullite in multi-step reactions with intermediate formation ofγ-Al2O3, and a non-crystalline nearly pure SiO2 phase. Theγ-Al2O3 compound has a highly distorted spinel structure, and may contain some minor amount of Si. The diffusion-controlled decomposition of the precursors toγ-Al2O3 + SiO2 at low temperature (⩽1000°C) suggests that the precursors are diphasic, consisting of nanometre-sized, long-range-disordered Al2O3- and SiO2-rich domains. The reaction ofγ-Al2O3 + SiO2 above about 1000°C initially produces Al2O3-rich mullite. This is explained by a possible nucleation of mullite on the surfaces ofγ-Al2O3 crystallites. The gradual transformation of the unstable, low-temperature (⩾1000 to ⩽1400°C) Al2O3-rich mullites to the stable high-temperature (⩾ 1400°C) 3/2-type mullites (3Al2O3 · 2SiO2) is essentially controlled by the annealing temperatures, whereas annealing times play a minor role.

Structural Changes Induced on Mullite Precursors by Thermal Treatment: A 27Al MAS‐NMR Investigation

NMR study of mullite precursors has shown that local arrangement of Al in samples synthesized by spray pyrolysis differs considerably from the one adopted by samples obtained by polymeric or colloidal routes. Aluminum is tetra‐ and pentahedrally coordinated in the first type of samples but is tetra‐ and octahedrally coordinated in the second ones. Segregation of SiO2 and Al2O3 is directly produced in colloidal preparation; however, this phenomenon occurs only in polymeric gels when they are heated between 980° and 1100°C. In polymeric samples, thermal treatment at ∼980°C produces the formation of γ‐Al2O3. A similar treatment in spray‐pyrolized powders gives directly 3:2 mullite. From these results, exothermic and expansive effects detected at ∼980°C were ascribed to changes in coordination of Al produced during the atomic rearrangement that accompanies formation of these two phases (γ‐Al2O3 or mullite). Above 1200°C, incorporation of Si in the Al‐rich phase induces the formation of 3:2 mullite in polymeric and colloidal samples.

Topotaxial phase transition induced by ion bombardment in α-Al2O3

Abstract The topotaxial phase transition, which is induced by gold-ion bombardment, from α-Al2O3, to γ-Al2O3 has been observed. The formation of γ-Al2O3 has been confirmed by transmission electron microscopy and X-ray diffraction. Between the γ-Al2O3 phase and original matrix structure of α-Al2O3 there is a topotaxial relation of (11l)γ//(0001)α and [110]γ//[0001]α, but the matrix is amorphous. We discuss the crystal structure of the γ-Al2O3 phase and a mechanism for the topotaxial phase transition with the amorphous matrix.

プラズマスプレー・ロール拡散接合法により製造した炭素繊維強化アルミニウム複合材料の高温安定性

Applicability of the combination process of plasma spray and roll diffusion bonding method to the fabrication of CF/Al composites has been investigated. In the present studies, oxidation states of CF/Al composites fabricated by this process (open system) and the stability of CF/Al composites during heating (873 K), which is one of the important properties expected for CF/Al composites, are investigated.The results are as follows:(1) In the case of CF/Al composites fabricated by this process, formation of γ-Al2O3 on fiber surface occurs.(2) The γ-Al2O3 formed on fiber surface maintains the tensile strength of CF/Al composite during heating at 873 K by inhibiting harmful reaction between fiber and matrix.The formation of γ-Al2O3 is industrially important as a method of fiber coating without additional process and cost.

Formation of metastable phases in flame- and plasma-prepared alumina

The formation of metastable phases in plasma- and flame-prepared alumina particles is examined in terms of the classical nucleation theory, rate of transformation of metastable to stable forms, and the thermal history of the particles during solidification. It is suggested that homogeneous nucleation of the solidification of liquid droplets at considerable undercooling results in the formation ofγ-Al2O3 rather thanα-Al2O3 because of its lower critical free energy for nucleation. The phase finally observed depends upon the thermal history of the particles during evolution of the heat of fusion and upon the kinetics of the transformation of the nucleating phase to the stable phase. This means that the cooling rate of the particles is relatively unimportant and under the conditions existing in flames and plasmas, metastable alumina will be formed on solidification. The metastable form will be retained on cooling particles less than approximately 10 μm diameter, but particles larger than this may transform toα-Al2O3 during the solidification exotherm

Thermodynamic functions by e.m.f. measurements on solid galvanic cells in non-stationary conditions: system Al + Al2O3

E.m.f. measurements were carried out on the cell,[graphic ommitted] after a cathodic reduction of the aluminum surface-oxide layer, in order to bring the aluminum activity at the interface equal to that of the pure phase. After the polarization current was cut off, the decay e.m.f. against time curves were recorded and were found to be step-shaped. The first plateau corresponds to the formation of a more oxidized two-phase system at the anode interface. The system is rapidly destroyed by iron diffusing from the bulk which restores the original system and the e.m.f. rapidly decrease; then the slope becomes very slight and can be accounted by the decrease of the aluminum activity at the cathode interface. The difference of these two plateaus corresponds to the predicted variation of the Gibbs function in the transition from Fe3O4+Fe2O3 to Fe-wustite. By estrapolating the last portion of the curve at the time at which the current is switched off, the standard molar free energy of formation of γ-Al2O3 is calculated.