Microwave-assisted Acid Dissolution of Sintered Advanced Ceramics for Inductively Coupled Plasma Atomic Emission Spectrometry

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Abstract
The microwave-assisted acid dissolution of sintered bodies of 28 structural and electronic advanced ceramic materials was systematically evaluated. These materials included zirconia-based ceramics, such as m-ZrO2 (a non-stabilized monoclinic zirconia), Ca-PSZ and Mg-PSZ (two partially stabilized zirconias), Y-FSZ (a fully stabilized zirconia) and Ce-TZP, Yb-TZP and Y-TZP/Ce (three tetragonal polycrystalline zirconias); alumina-based ceramics, such as Al2O3, mullite and spinel; ceria-based ceramics, such as CeO2–Gd2O3 (cubic ceria gadolinia); titania-based ceramics, such as TiO2; titanate-based ceramics, such as Al2TiO5, BaTiO3 and BIT (bismuth titanate); lead titanate-based ceramics, such as Ca-PT, La-PT, Nd-PT, Sm-PT and Gd-PT; lead zirconate titanate-based ceramics, such as PZT and PLZT; niobate-based ceramics, such as PMN (lead magnesium niobate); non-oxide-based ceramics, such as AlN, BN, Si3N4 and SiC; and oxide and non-oxide-based ceramics, such as β′-sialon (silicon aluminium oxynitride). Fifteen acids or mixtures of acids were tried, including HCl, HNO3, H2SO4, aqua regia, H2SO4–(NH4)2SO4 and mixtures of these acids with HF and with H2O2. A commercially available laboratory medium pressure microwave oven was used. Eleven optimized microwave methods were developed. These methods are simple (three stages maximum), fast (15–35 min digestion time) and mild (20–60% of the microwave oven power). By applying these microwave methods, it was possible to dissolve completely all the sintered advanced ceramics, except SiC and β′-sialon. These two non-oxide ceramics were the only samples that could not be dissolved by any of the acids or mixtures of acids tested. The microwave-assisted acid dissolution was compared for ICP-AES with conventional dissolution procedures, i.e., alkali fusion in a platinum crucible and in a graphite crucible and acid decomposition by conductive heating at elevated pressure (in a PTFE bomb). It was demonstrated that microwave-assisted dissolution presents many advantages over the other procedures. When compared with acid decomposition by conductive heating in a PTFE bomb, one of the most important advantages is the drastic shortening of the digestion time from hours to minutes. When compared with alkali fusions, one of the most important advantages is the use of smaller amounts of high-purity acids, which contain less impurities than the fluxes; because of this, matrix effects and contamination from the attack reagents are lower, and consequently there is an improvement in the analytical figures of merit of ICP-AES.

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