Obtenção de novos materiais pelo processo sol-gel : oxidos e fosfatos de ferro

AUTOR(ES)
DATA DE PUBLICAÇÃO

1990

RESUMO

Thermal and hydrotermal transformations of non-crystalline iron (III) hydroxoacetate (HAF), hydroxophosphate (HPF) and metaphosphate (MPF) were followed using these methods: iron (II) and total-iron determinations; infrared spectrophotometry; surface area and density measurements; Mössbauer, (VIS-UV) photoacoustic and electron spin resonance spectroscopies;scanning electron microscopy. HAF and HPF were prepared by hydrolysis of iron (III) nitrate with ammonia, in the presence of acetate and phosphate, respectively. MPF was precipitated by admixture of aqueous iron (III) nitrate and sodium metaphosphate. These three substances are non-crystalline (by X-ray diffraction) and have the following chemical compositions: i) HAF: FeO1.26-1.30 (CH3COO)0.40-0.70.(0.96-1.45)H2O; ii)HPF: FeO1.39 (HPO4)0.11.0.75H2O, FeO1.30 (HPO4)0.20.H2O, FeO (HPO4)0.50.1.6H2O, FeO0.79 (HPO4)0.71.0.98H2O; iii)MPF: Fe2P4O13.3.81H2O; Hydrothermal treatments (in aqueous dispersions at 120°C, 48 hours under air or N2 in closed ampoules) were done on HAF and on mixtures of HAF and magnetite. The results show that: i) phosphate and citrate ions in concentrations equal to or greater than 5.10 moles.l inhibit crystallization and stabilize the magnetic fraction of the solid mixture; ii) acetate ions are desorbed; both phosphate and citrate are sorbed; iii) HAF crystallises to hematite, in the absence of phosphate and citrate. Heating of HAF (270°C, 1 hour under N2) leads to partial loss of acetate and formation of magnetite. More complete transformations are observed at higher temperatures. Heating of HPF to 770°C yelds various crystalline phases: FePO4.2H2O, FePO4, Fe4(P2O7)3 and a-Fe2O3, depending on phosphorus content of the sample. At 10.2 % phosphorus content, HPF resists crystallization up to 590°C under air; at 14 % phosphorus there is partial convertion to FePO4, at this temperature. Heating of MPF gives the following results: i) coexistence of amorphous and crystalline phases, under many conditions; formation of three different crystalline phases, under various conditions; ii) formation of foam at 620°C and a Fe(II)-Fe(III) glass at 1200°C. Salts and hidroxosalts of iron (III), prepared by sol-gel and precipitation techniques are thus suitable starting materials in the preparation of pigments, glasses and ceramics. Control of amorphous/crystalline transformations can be adequately done by using suitable co-anions, both in thermal and hydrotermal processes.

ASSUNTO(S)

oxido de ferro processos quimicos compostos organometalicos

ACESSO AO ARTIGO

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