Elemental Analysis of Alloy Sample with Pulsed Micro-Discharge Optical Emission Spectrometry
WANG Xiao-hua, LI Wei-feng, GUO Zong-wei, HANG Wei*, HUANG Ben-li
Department of Chemistry and Ministry of Education Key Lab of Spectrochemical Analysis ＆ Instrumentation, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
Abstract：The pulsed micro-discharge ablation source was applied to the optical spectra analysis of the aluminum alloy at atmospheric pressure. This needle-plane micro-discharge device has the merits of low cost, easy operation and fast analysis. Pulsed discharge allows large instantaneous input power without melting the sample, which is vital to keep the stability of the discharge. In the duration of a few microseconds, high voltage of about -4 000 V was applied to the tungsten needle electrode, which quickly initiates a discharge channel between the electrode and electrode, and results in current of 20 A between the tungsten needle tip and the sample. Thus, the sample was ablated before ablated particles were excited. The deposition energy, whose value was approximately 8.5 mJ per discharge pulse, was transmitted between the two discharge electrodes by means of the electric current. The surface morphology of the ablation crater generated by this pulsed needle-plane discharge indicated that a local micro-plasma was formed in the electrode gap. In addition, the on-axial energy flux and current density were much higher than those of the off-axis region. In order to further investigate the ablation mechanisms and physical properties, the electrical characteristic of this plasma source was discussed. The time evolution process of the discharge plasma was studied through precise timing shooting technique. From the fast imaging results of the ICCD camera, it could be seen that the lifetime of the plasma source was comparable with the pulse width of the high-voltage discharge source, and what was more, the change of optical intensity of plasma source showed good agreement with the variation of discharge current. Combined with an optical spectrometer, the pulsed micro-discharge ablation source could well excite atomic lines of aluminum, magnesium, manganese, bronze, and more in the alloy sample. The spectral characteristics of the discharge plasma were studied further. The electron excitation temperature of the discharge plasma was determined as about 6 700 K and the electron number density was obtained in the order of 1017 cm-3 using the Boltzmann plot method and Stark broadening of line, respectively. Besides, it was verified that the discharge plasma generated in this experiment was in local thermal equilibrium state. A further attempt was made to demonstrate the ability of this technique in quantitative analysis. Our results showed that the pulse micro-discharge plasma employed as spectral analysis source was an effective method for the quantitative analysis of aluminum alloy sample.
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