Use of metastable, dissociated and charged gas species in synthesis: a low pressure analogue of the high pressure technique

Author: Jayadevan K.P.   Kumar N.V.   Mallya R.M.   Jacob K.T.  

Publisher: Springer Publishing Company

ISSN: 0022-2461

Source: Journal of Materials Science, Vol.35, Iss.10, 2000-05, pp. : 2429-2434

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Abstract

Oxidation of silver using microwave-induced oxygen plasma and oxygen-ozone gas mixture was studied as a function of temperature and partial pressure. The oxide Ag_2O was formed at temperatures well above its normal decomposition temperature in oxygen plasma at a pressure of 5 Pa. The higher oxide AgO_1−x was formed in O_2+O_3 gas mixtures at lower temperatures. The oxygen chemical potentials for the oxidation of Ag to Ag_2O, Ag_2O to AgO_1−x and AgO to Ag_2O_3 were evaluated from thermodynamic data and compared with the experimental results to obtain information on the chemical potential of oxygen in microwave plasma and gases containing ozone. The oxygen potential of the gas phase in microwave plasma operating at a pressure of 5 Pa was found to be in excess of 36 kJ/mol at 750 K. This is equivalent to a pressure of diatomic oxygen gas greater than 3 × 10^7 Pa. In the O_2+O_3 mixture at ambient pressure containing 5 mole percent O_3, the oxygen potential is ∼112 kJ/mol at 465 K. The equivalent pressure of diatomic oxygen is ∼4 × 10^17 Pa. Thus, metastable species such as O_3 or charged species such as O^− present in plasma can be used as a powerful reagent for the syntheses of metastable oxides. Similar techniques can be used for other metastable inorganic solids such as nitrides for functional applications.