Microwave-Assisted Acid Dissolution of Sintered Advanced Ceramics for Inductively Coupled Plasma Atomic Emission Spectrometry
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View Article Online / Journal Homepage / Table of Contents for this issue Microwave-assisted Acid Dissolution of Sintered Advanced Ceramics for Inductively Coupled Plasma Atomic Emission Spectrometry MARI´A T. LARREAa ,ISABELGO´ MEZ-PINILLAb AND JUAN C. FARIN˜ AS*c aCentro Nacional de Investigaciones Metalu´rgicas (CSIC), Avda. Gregorio del Amo 8, Ciudad Universitaria, 28040 Madrid, Spain bCentro de Espectrometrı´a Ato´mica de la Universidad Complutense de Madrid, Facultad de Geolo´gicas, Ciudad Universitaria, 28040 Madrid, Spain cInstituto de Cera´mica y V idrio (CSIC), 28500 Arganda del Rey, Madrid, Spain The microwave-assisted acid dissolution of sintered bodies of ties, high-temperature stability, high-temperature strength, 28 structural and electronic advanced ceramic materials was wear resistance, corrosion/oxidation resistance, chemical systematically evaluated. These materials included zirconia- inertness, or electrical, optical and/or magnetic properties.1 based ceramics, such as m-ZrO2 (a non-stabilized monoclinic These materials are used in structural applications (machine zirconia), Ca-PSZ and Mg-PSZ (two partially stabilized and cutting tools, wear components, heat exchangers, aero- zirconias), Y-FSZ (a fully stabilized zirconia) and Ce-TZP, space components, automotive engine components such as Yb-TZP and Y-TZP/Ce (three tetragonal polycrystalline turbocharge rotors and roller cam followers, power generation zirconias); alumina-based ceramics, such as Al2O3, mullite and components, biomedical implants, processing equipment used spinel; ceria-based ceramics, such as CeO2–Gd2O3 (cubic ceria for fabricating a variety of polymer, metal and ceramic parts) gadolinia); titania-based ceramics, such as TiO2; titanate- and in electronic applications (insulators, capacitors, actuators, based ceramics, such as Al2TiO5, BaTiO3 and BIT (bismuth transducers, varistors, electronic sensors and devices, oxygen titanate); lead titanate-based ceramics, such as Ca-PT, sensors, temperature sensors, electrooptical elements and mag- La-PT, Nd-PT, Sm-PT and Gd-PT; lead zirconate titanate- netic devices). In this way, the advanced ceramic materials can based ceramics, such as PZT and PLZT; niobate-based be classified into structural ceramics and electronic ceramics. ceramics, such as PMN (lead magnesium niobate); non-oxide- These ceramics include binary oxide-based materials, such as based ceramics, such as AlN, BN, Si3N4 and SiC; and oxide alumina (Al O ), zirconia (ZrO ), titania (TiO ) and ceria ∞ 2 3 2 2 and non-oxide-based ceramics, such as b -sialon (silicon (CeO2); ternary oxide-based materials, such as mullite aluminium oxynitride). Fifteen acids or mixtures of acids were (3Al2O3·2SiO2), spinel (MgO·Al2O3) and aluminium titanate tried, including HCl, HNO3,H2SO4, aqua regia, (Al2TiO5); non-oxide-based materials, such as boron nitride H2SO4–(NH4)2SO4 and mixtures of these acids with HF and (BN), aluminium nitride (AlN), silicon nitride (Si3N4) and with H2O2. A commercially available laboratory medium silicon carbide (SiC); perovskite-based materials, such as pressure microwave oven was used. Eleven optimized barium titanate (BaTiO3), alkaline earth or rare earth element- microwave methods were developed. These methods are simple modified lead titanate (e.g., Ca-PT or Gd-PT), lead zirconate (three stages maximum), fast (15–35 min digestion time) and titanate or PZT (PbZr1−xTixO3) and lanthanum-modified lead mild (20–60% of the microwave oven power). By applying zirconate titanate or PLZT (Pb1−yLayZr1−xTixO3); and oxide these microwave methods, it was possible to dissolve and non-oxide mixed compounds, such as silicon aluminium completely all the sintered advanced ceramics, except SiC and ∞ oxynitride or b -sialon (Si6−xAlxOxN8−x), a set of materials b∞-sialon. These two non-oxide ceramics were the only samples based on combinations of Si3N4 with Al2O3 and other oxides. that could not be dissolved by any of the acids or mixtures of The properties of these materials are greatly influenced by Published on 01 January 1997. Downloaded 11/4/2020 1:05:02 PM. acids tested. The microwave-assisted acid dissolution was the stoichiometric molar ratio of the macroconstituents,2 and compared for ICP-AES with conventional dissolution also by the presence of impurities,3 even at very low concen- procedures, i.e., alkali fusion in a platinum crucible and in a trations. Furthermore, small amounts of dopants,4 sintering graphite crucible and acid decomposition by conductive heating aids5 and other additives6 are added to these ceramics in order at elevated pressure (in a PTFE bomb). It was demonstrated to improve their processing and ultimate properties. Because that microwave-assisted dissolution presents many advantages of this, the determination of these elements is vital to manufac- over the other procedures. When compared with acid turing control, property improvement, failure prevention and decomposition by conductive heating in a PTFE bomb, one of quality assurance. the most important advantages is the drastic shortening of the Inductively coupled plasma atomic emission spectrometry digestion time from hours to minutes. When compared with (ICP-AES) is nowadays the most widely used technique for alkali fusions, one of the most important advantages is the use the analytical characterization of advanced ceramics. It has of smaller amounts of high-purity acids, which contain less been applied to the chemical analysis of a wide variety of impurities than the fluxes; because of this, matrix eVects and 7–10 11–14 9,10,12 samples: Al2O3, ZrO2-based materials, AlN, contamination from the attack reagents are lower, and 9,10,15–17 9,10,18–20 13,21 Si3N4, SiC, perovskite-based materials, consequently there is an improvement in the analytical figures 13,22,23 13 13,24 13 BaTiO3, Gd-PT, PZT, PLZT, etc. For the chemi- of merit of ICP-AES. cal analysis of these refractory and chemically resistant mate- Keywords: Microwave-assisted dissolution; structural sintered rials, complete dissolution of the samples with extreme chemical advanced ceramics; electronic sintered advanced ceramics; treatments must be performed. This has been realized by two inductively coupled plasma atomic emission spectrometry general procedures: acid decomposition and alkali fusion. Acid decomposition procedures include: (i) conductive heating at + 25 atmospheric pressure with H2SO4 HF in a platinum dish, + + 14 + The terms advanced, engineering or technical ceramics refer HCl H2SO4 HF in a platinum crucible, H2SO4 11–13,24,26 13,21 to ceramic materials which exhibit superior mechanical proper- (NH4)2SO4 in a glass beaker or a platinum dish Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1323–1332) 1323 View Article Online + 7 8 and H3PO4 H2SO4 in a quartz beaker, a PTFE beaker or lead to the aforementioned systematic errors. In addition, the a platinum crucible;27 and (ii) conductive heating at elevated high reagent blank of the fluxes and the large dilution factor 8,12,13,21–23,25,28–31 32 28 pressure with HCl, HNO3, H2SO4, of the sample solution can lead to a decrease in the sensitivity. + 9,10 + 9,10,32,33 + 28 44,45 HCl HNO3, HCl H2SO4, H2SO4 HClO4, The microwave-assisted heating technique is a promising + 28 28,34 + 35,36 H2SO4 H3PO4, HF, HCl HF, alternative for sample preparation owing to the advantages + 9,10,17,37,38 + 13,21,24,28 + 28 HNO3 HF, H2SO4 HF, HClO4 HF, over the conventional acid decomposition and alkali fusion + 28 + + 26 H3PO4 HF, HCl HNO3 HF, procedures: strict control of the heating power and of the time + + 9,10,19,20,33,39,40 + 32 HNO3 H2SO4 HF and HF H2O2 in a the heating is applied, more amenable to automation, ability PTFE bomb. Alkali fusion procedures include: (i) fusion in a to use small volumes of reagents, lower reagent blank, complete 23,41 platinum crucible with Na2CO3, digestion without the use of HClO4, extremely short digestion + 12,13,21 + 15 Na2CO3 Na2B4O7, Na2CO3 Na2SO4, times, lower risk of contamination from the decomposition + 42 + 25 + 7 Na2B4O7 KNO3, CaCO3 H3BO3, Li2CO3 H3BO3 vessel and from the laboratory environment, no losses of + + 43 and Na2B4O7 H3BO3 LiOH; and (ii) fusion in a graphite volatile elements, etc. 8 13,21,23,24 crucible with LiBO2 and Li2B4O7. For comparative Despite the great interest that this technique presents, only purpose, Table 1 summarizes the literature methods using acid two papers32,46 devoted to their application to ceramic mate- decomposition by conductive heating at elevated pressure in a rials could be found. Table 2 summarizes the microwave- PTFE bomb for the dissolution of advanced ceramic powders. assisted dissolution methods used in these papers. Tata´r et al.46 However, these dissolution procedures, which are a major studied the influence of various physico-chemical properties source of errors and diYculties, present several serious disad- (crystalline phase, particle size and specific surface area) of Y vantages. Acid decomposition procedures are time consuming, Al2O3 samples on e ciency of microwave-assisted digestion + ° often require the use of potentially hazardous acids and can with H2SO4 (1 1) at 170 C as a function of time. lead to systematic errors such as contamination, loss of volatile Matusiewicz32 compared a commercial microwave-assisted elements and incomplete digestions. Alkali fusion procedures, digestion system with the conventional PTFE bomb decompo- Y although e cient and relatively rapid, are very laborious, sition for the dissolution of Al2O3, AlN, BN and Si3N4 samples.