Compositional changes during electron microprobe analysis of alkali silicate glasses

Local changes of compositions of alkali silicate glasses with time of electron bombardment during electron microprobe analysis are observed. The concentration of an element in the probed region varies with time in different ways depending upon the glass composition, the intensity of electron beam and the composition and thickness of conducting coating.The results are summerized as follows:(1) Generally, the X-ray intensities of the alkali metals decrease with time in the low-alkali glasses. On the other hand, the X-ray intensities of the alkali metals increase after a short period of decrease in the high-alkali glasses.(2) The increase of the X-ray intensities of the alkali metals is more rapid for higher beam loadings. Moreover, high beam loadings cause the increase of the X-ray intensities of the alkali metals in the low-alkali glasses.(3) For the same beam loading, the X-ray intensities change relatively quickly in Na2O-SiO2 glass, somewhat slowly in K2O-SiO2 glass, and even more slowly in Cs2O-SiO2 glass.(4) The X-ray intensities of the alkali metals of crystallized glasses decrease with time irrespective of their compositions.(5) In the absence of conducting coating, X-ray intensities of the alkali metals always increase with time.(6) In alkali-free glasses, X-ray intensity of each element is practically constant with time.The results suggest that the X-ray intensity changes result predominantly from the migration of the alkali ions. The decrease of the X-ray intensities of the alkali metals is consistent with the model proposed by Lineweaver. The increase of the X-ray intensities of the alkali metals, which is inconsistent with the Lineweaver mechanism, is explained as follows. Vaporization of the glass, which is more severe in the high-alkali glasses and for higher beam loadings, leads to a damage of the conducting coating in the region under the beam. A negative charge builds up on and near the surface and the positive alkali ions are attracted to the surface to neutralize the charge.