María T. Larrea

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

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.

Matrix effect of aluminium, calcium and magnesium in axially viewing inductively coupled plasma atomic emission spectrometry

The matrix effect due to Al, Ca and Mg in axial view mode ICP-AES was investigated over analyte lines with total excitation energy from 1.62 to 16.51 eV. A global qualitative explanation of the action of the matrix is proposed. The energy is directly transferred between matrix and analyte specimens (atoms or ions) during inelastic collisions, which are particularly relevant in the 1.62–8 eV excitation energy range, where the direct action with argon specimens is less probable to occur. Concrete applications of this global qualitative explanation are given, taking into account the resonance energy effect and the spin conservation rule. The characteristic matrix effect of Al and Ca for lines excited by charge transference mechanism was observed. In the presence of Mg, other possible matrix–analyte interactions may reduce the efficiency of this mechanism. The particular behavior of the matrix effect for lines in the 10.5–11.5 eV energy range can be considered as experimental evidence of the Penning ionization–excitation mechanism, which probably actuates along other matrix–analyte interactions.

Improvement in the ion exchange chromatographic separation of rare earth elements in geological materials for their determination by inductively coupled plasma atomic emission spectrometry

Ion exchange chromatographic separation of the rare earth elements (REEs) in geological materials for their determination by inductively coupled plasma atomic emission spectrometry has been improved. The time taken for the procedure, the consumption of reagents and the external contamination of the samples have been reduced. The emission of corrosive vapours has been totally eliminated. The efficiency of the chromatographic separation by elution with two concentrations of hydrochloric acid has been confirmed at higher flow rates in a high pressure chromatographic column. A prototype has been designed to combine the use of a pressure operated chromatographic column with the simultaneous evaporation of the eluates and the recovery of the acid. The column was charged with the solution resulting from the decomposition of the sample and the matrix elements were eluted with 2 mol l–1 HCl and the REEs with 6 mol l–1 HCl. The REE fraction from the column was concentrated by distillation. The whole process (separation and concentration) was performed in 1 h 15 min. The saving in time reached a factor of 30. The recoveries of the REEs during the ion exchange procedure ranged from 96 to 105%. The accuracy and precision have been evaluated by analysing seven geological reference materials: NIM-G (Granite), NIM-L (Lujavrite), NIM-N (Norite), SY-2 (Syenite), SY-3 (Syenite), MRG-1 (Gabbro), and JB-1a (Basalt). Differences between obtained and certified values were not statistically significant for α= 0.05 (t-test). The precision varied from 1.8 to 18.2%.

Determination of rare earth and concomitant elements in magnesium alloys by inductively coupled plasma optical emission spectrometry

An Inductively Coupled Plasma Optical Emission Spectrometry method for simultaneous determination of Al, Ca, Cu, Fe, In, Mn, Ni, Si, Sr, Y, Zn, Zr and rare earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) in magnesium alloys, including the new rare earth elements-alloyed magnesium, has been developed. Robust conditions have been established as nebulizer argon flow rate of 0.5mLmin(-1) and RF incident power of 1500W, in which matrix effects were significantly reduced around 10%. Three acid digestion procedures were performed at 110°C in closed PFA vessels heated in an oven, in closed TFM vessels heated in a microwave furnace, and in open polypropylene tubes with reflux caps heated in a graphite block. The three digestion procedures are suitable to put into solution the magnesium alloys samples. From the most sensitive lines, one analytical line with lack or low spectral interferences has been selected for each element. Mg, Rh and Sc have been studied as internal standards. Among them, Rh was selected as the best one by using Rh I 343.488nm and Rh II 249.078nm lines as a function of the analytical lines. The trueness and precision have been established by using the Certified Reference Material BCS 316, as well as by means of recovery studies. Quantification limits were between 0.1 and 9mgkg(-1) for Lu and Pr, respectively, in a 2gL(-1) magnesium matrix solution. The method developed has been applied to the commercial alloys AM60, AZ80, ZK30, AJ62, WE54 and AE44.

Influence of strain rate on the microstructure of a thermo-mechanically processed medium carbon microalloyed steel

The microstructure of a medium carbon microalloyed steel has been studied in the as-cast condition as well as after high temperature torsion deformation. The range of deformation parameters of the tests covered the conditions used in the forming process of a real part. The influence of thermomechanical treatment conditions (true strain, true strain rate and temperature) on the final austenite grain size has been studied. The evolution of the austenite grain size is analyzed under conditions of static and dynamic recrystallization. It was found that the strain rate can be as important as the strain to influence the final microstructure and therefore the mechanical properties of the final product.