Formation Of Amorphous Alumina Research Articles

Calcination-induced changes in structure, morphology, and porosity of allophane

Abstract Allophane with an Al/Si molar ratio of 1.6 was hydrothermally synthesized, followed by calcination at defined temperatures up to 1300 °C. The heated products were then characterized using a combination of techniques including X-ray diffraction, Fourier transform infrared spectroscopy, thermal analyses, nuclear magnetic resonance, transmission electron microscopy, and N2 physisorption. In allophane, the imogolite local structure (ImoLS) had a relatively low thermal stability, and the adsorbed water was removed at a low temperature. Five major steps were outlined for the thermal evolution process of allophane. (i) At 200 °C or lower, the loss of adsorbed water and partial dehydroxylation of Si OH accompanying with the formation of Si O Si resulted in the loss of ImoLS. (ii) As the temperature rose, further dehydroxylation of Si OH as well as disassociation of Al OH occurred, leading to decreases in the specific surface area and porosity due to the continued agglomeration of hollow spherules, whereas the spherical morphology of allophane was largely retained. (iii) At approximately 500 to 900 °C, the disconnection of AlO octahedra and SiO tetrahedra caused increased disintegration of the allophane structure and formation of amorphous alumina and silica. (iv) At approximately 1000 °C, nanosized mullite was crystallized by the reaction between the yielded amorphous alumina and silica. (v) Finally, further growth of the already-formed mullite crystals and formation of cristobalite occurred, with the consumption of excess silica.

Microstructure and mechanical properties of plasma sprayed Al2O3-YSZ composite coatings

Alumina-yttria stabilized zirconia (Al2O3-YSZ) composite coatings are recognized as a potential alternative material for thermal barrier coatings. In this paper, Al2O3-YSZ composite coatings were deposited using atmosphere plasma spraying. The effects of alumina addition and annealing treatment on the microstructure and mechanical properties of the plasma sprayed Al2O3-YSZ composite coatings were investigated by XRD, SEM and an instrumented indentation testing system. Owing to the existence of YSZ in the composite coatings, the formation of amorphous alumina was suppressed. The quenching stress is high enough to trigger microcracking, these microcracks will further propagate or close under the post deposition thermal mismatch stress. Young's modulus of the coatings keeps an upward tendency from 50.56 GPa to 55.93 GPa and hardness increase from 5.36 GPa to 5.93 GPa with alumina contents varying from 10 wt% to 55 wt%. After an annealing treatment, Young's modulus and hardness increase compared with the as-sprayed ones. Fracture toughness of Al2O3-YSZ composite coatings is higher than the YSZ one, and it increases firstly and then decreases with alumina addition varying from 10 wt% to 55 wt%. The fracture toughness of coatings is related to the amount of heterogeneous specific interface between Al2O3 and YSZ splats and the high microcracks density in the composite coatings.

Mechanochemical synthesis of alumina nanoparticles: Formation mechanism and phase transformation

Abstract Nanosized alumina powders were synthesized by mechanochemical treatment of stoichiometric mixture of anhydrous AlCl3 and CaO. X-ray powder diffraction (XRD), differential thermal and thermogravimetric analysis (DSC-TG), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to understand the formation mechanism and the exact phase transformation sequence in this system. Milling of the initial raw materials resulted in the formation of crystalline CaO and partially amorphous AlCl3, with no sign of chemical reaction between the constituents. Heating the as-milled powder led to the formation of amorphous aluminum hydroxide (Al(OH)3) and calcium chloride (CaCl2). Based on the results obtained in this study, AlCl3 hydrolyses in the heating stage and CaO adsorbs the produced HCl forming CaClOH and CaCl2 crystalline phases. Heat treatment of the amorphous Al(OH)3 resulted in the formation of amorphous alumina (Al2O3). Amorphous alumina transformed into η-, κ-, and α‐Al2O3 when calcined at higher temperatures. Based on Rietveld refinement, it was concluded that η-Al2O3 and κ-Al2O3 phases are stable up to an average critical crystallite size of around 13 nm and 39 nm respectively.

Controlled rate thermal decomposition of synthetic bayerite under vacuum

Influences of the controlled decomposition rate and controlled residual pressure on the thermal decomposition process of synthetic bayerite were investigated by applying a method of controlled rate evolved gas detection (CREGD). Comparing with the conventional process under linearly increasing temperature, the thermal decomposition process of the synthetic bayerite at a controlled decomposition rate (∼10 −2 mg min −1) under controlled residual pressure (∼10 −3 Pa) was characterized by the lowered reaction temperature profile and small changes in the specific surface area of the sample during the course of reaction. Formation of an amorphous alumina as the decomposition product is another important result of the process control by the present CREGD. The amorphous alumina crystallizes to η-Al 2O 3 at around 1100 K. It is worth noting that the characteristics of the reaction process under controlled rate conditions and the formation of amorphous alumina as the decomposition product are very similar to those of mechanochemically activated bayerite under conventional linear heating.

Thermal transformations of aluminium nitrate hydrate

By means of the thermogravimetric (TG) method, and differential thermal analysis (DTA) with simultaneous chemical analysis of the gaseous reaction products, X-ray diffraction phase analysis, and infrared spectroscopy, stages of decomposition of Al(NO 3) 3·9H 2O were studied. It has been found that the thermal decomposition of Al(NO 3) 3·9H 2O proceeds in a few stages. First stage is connected with partial dehydration of the compound without decomposition of the nitrate groups. The successive stages comprise the hydrolytic processes, which are connected with further dehydration and loss of the nitrate groups. The process proceeds with the formation of amorphous alumina.