- Comparison of the thermal behaviour of α-AlF3 and aluminium fluoride hydrates
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The thermal decompositions of β-AlF3·3H2O, AlF3·3.5H2O, AlF3·1.5H2O, β-AlF3 and hydroxyfluorides of aluminium were investigated. The literature data were complemented with X-ray
- Menz,Zacharias,Kolditz
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- A simple aqueous phase synthesis of high surface area aluminum fluoride and its bulk and surface structure
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A simple novel synthesis route to aluminum fluoride, AlF3, from aqueous phase is reported. Al2O3 is dissolved in aqueous hydrofluoric acid, HF, and re-precipitated as AlF3 · 3H2O. Thermal decomposition results in thermally stable AlF3 with high specific surface areas between 120 and 60 m2/g depending on treatment temperatures (up to 450 °C). Bulk and surface structures of the resulting amorphous and crystalline materials were characterized by X-ray powder diffraction, infrared and solid state NMR spectroscopy (27Al, MAS), nitrogen physisorption and adsorption of paramagnetic probe molecules (vanadium complexes).
- Kleist, Wolfgang,Hae?ner, Carmen,Storcheva, Oksana,K?hler, Klaus
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- Etching AlAs with HF for epitaxial lift-off applications
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The epitaxial lift-off process allows the separation of a thin layer of III/V material from the substrate by selective etching of an intermediate AlAs layer with HF In a theory proposed for this process, it was assumed that for every mole of AIAs dissolved three moles of H2 gas are formed. In order to verify this assumption the reaction mechanism and stoichiometry were investigated in the present work. The solid, solution and gaseous reaction products of the etch process have been examined by a number of techniques, It was found that aluminum fluoride is formed, both in the solid form as well as in solution. Furthermore, instead of H2 arsine (AsH3) is formed in the etch process. Some oxygen-related arsenic compounds like AsO, AsOH, and AsO2 have also been detected with gas chromatography/mass spectroscopy. The presence of oxygen in the etching environment accelerates the etching process, while a total absence of oxygen resulted in the process coming to a premature halt. It is argued that, in the absence of oxygen, the etching surface is stabilized, possibly by the sparingly soluble A1F3 or by solid arsenic.
- Voncken,Schermer,Van Niftrik,Bauhuis,Mulder,Larsen,Peters,De Bruin,Klaassen,Kelly
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p. G347-G352
(2008/10/09)
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- Infrared Study of the Surface Properties of HTB-Type Al-, Cr-, Fe-Hydroxyfluorides
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The surface properties of iron, chromium, and aluminum fluorides in their hexagonal tungsten bronze (HTB) form have been investigated by infrared spectroscopy. The presence of hydroxyls is clearly observed. H/D exchange experiments with different deuterated probe molecules having various molecular sizes show that they are localized inside of the channels of the structure. The v(OH) bands in the infrared spectra of these materials are downward shifted compared to those of corresponding metal oxides, whereas the δ(OH) in-plane bending mode presents an unusual high wavenumber. These spectral features are compared to those observed on zeolites, for which hydroxyl environment and bridged conformation are similar. HTB compounds exhibit both strong Lewis and Br?nsted acid sites. The use of two basic probes with a different size (pyridine and ammonia) allows one to localize and to quantify these two kinds of acidity. Br?nsted acidity is related to the presence of hydroxy groups into the microporous channels and to chemisorbed HF, whereas Lewis acidity is due to defect sites both on the outer surface of crystallites and in the channels. The strength of acid sites is unambiguously found stronger than that reported for Al, Cr, and Fe oxides. This result is discussed taking into account the electronegativity of fluorine but also the bridged conformation of OH groups, as in the case of zeolites.
- Vimont, Alexandre,Lavalley, Jean-Claude,Francke, Lo?c,Demourgues, Alain,Tressaud, Alain,Daturi, Marco
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p. 3246 - 3255
(2007/10/03)
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