Electron energy loss spectroscopy of Na in Na, Na 2 O, and silicate glasses
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S. Zhou and J. Qiu Department of Materials Science, Zhejiang University, Hongzhou, People’s Republic of China (Received 24 January 2008; accepted 2 June 2008)
Measuring Na in silicate glasses can be difficult in transmission electron microscopy due to modifications induced by electron irradiation. The modifications involve not only the loss of Na from the illuminated region, but also the formation of Na and Na2O. This work compares the electron energy loss spectroscopy (EELS) of plasmon and Na L23 edge in Na2O–SiO2 glass with those in Na and Na2O. The interpretations of the fine structures in Na L23 edge were also given with the aid of full multiplescattering calculations. It demonstrates that the formation of metallic Na can be easily identified by its bulk plasmon at about 5.8 eV, and the formation of Na2O can be better seen by Na L23-edge fine structure.
I. INTRODUCTION
Sodium plays an important structural role in glasses, glass ceramics, and minerals. In the soda-lime silicate glass, the most common form of glass produced ever, for example, Na serves as a flux to lower the glass melting temperature, and thus the ease of glass formation is increased. From the structure point of view, introducing Na2O to silica can breakdown the SiO2 network and form nonbridging oxygens. This results in an increase in thermal expansion coefficient and a decrease in viscosity at any specific temperature.1 It is believed that Na ions locate close to the negatively charged nonbridging oxygens to maintain local electroneutrality in the structure. Because of the lack of techniques capable of assessing glass structure at high spatial resolution, there has been an argument about the distribution of Na: whether Na ions distribute entirely randomly or there is a tendency to form alkali-rich regions (for a review see Ref. 1). Recently, it has been demonstrated that electron energy loss spectroscopy (EELS), combining with (scanning) transmission electron microscope [(S)TEM], has potential ability to probe glass structure and composition at nanometer scale.2–4 However, Na in the silicate glasses is notoriously unstable under the high-energy and highintensity electron probe, because of the high mobility of the Na+ ions under the electric field.5 Therefore, characterizing Na in glasses by electron probe is difficult.6 Pre-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0296 J. Mater. Res., Vol. 23, No. 9, Sep 2008
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viously, we discovered that Na+ ions in the silicate glasses undergo the transitions Na+ → Na0 → Na+ near the surfaces shortly after being exposed to electron beam in TEM.7 The initial Na+ ions are in the glass network as modifiers, while the final Na+ ions are in the form of Na 2 O. Therefore, characterizing the Na-containing glasses as long as Na is concerned must be done under very low electron beam intensity and requires a very short exposure. Under these conditions, Na K-edge EELS and characteristic x-ray (i.e., energy di
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