Can we trust TEM images of silicate glasses?
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Can we trust TEM images of silicate glasses? Nan Jiang Department of Physics and Astronomy Arizona State University Tempe, AZ 85287-1504, USA ABSTRACT Electron irradiation-induced modifications in two glasses, K2O – SiO2 and Au doped Na2O – B2O3 – SiO2, were observed in electron microscope. The products of modifications were “nanoparticle” like contrasts in transmission electron microscopy (TEM) images, which can be easily confused with real nano-particles and phase separation. The driving force for the modifications in the glasses is the tendency of elimination of non-bridging oxygen (NBO) through the removal of cations. The phase separation into cation rich and poor region is their nature under electron irradiation. Therefore, it is absolutely essential to record in situ frames when the TEM images are used to provide microstructure information of glasses. Additionally, charging effects in glasses have also been discussed. INTRODUCTION In the past decades, TEM played an important role in studying microstructure of glasses, such as phase separation and precipitation. The spatial resolution was usually required from submicron to tenth of nanometer. Recently, the spatial resolution becomes more and more crucial when the observed objects scale down to nanometer or even to sub-nanometer, such as the precipitation of nano-particles in glasses. In order to measure physical and chemical properties of individual nano-particles, TEM is usually the first choice of experimental method, which can provide both configuration and composition information at atomic scale. For example, TEM images give straightforward microstructure information of sizes of nano-particle and their distribution. On the other hand, glasses are very susceptible to electron irradiation. The modifications of glasses by electron irradiation include phase decomposition, formation of gas bubbles and crystallization. Most previous studies on the irradiation effects in glasses were aimed at understanding the immobilization of high-level nuclear reaction waste [1]. However, little attention has been paid to the accuracy of information obtained from TEM images. In this article, we will give two examples of TEM images of two glass systems. In both cases, electron irradiation significantly modifies the glass microstructures, and the modifications can be easily confused with the nanometer particles and (nanometer) phase separation. Through the discussion of the fundamental of beam – glass interaction, we emphasize that it is necessary to set up in situ observations as routine procedure to the glass systems. EXPERIMENTAL DETAILS Two glass systems were used in this study: K2O – SiO2 and Au doped 10Na2O – 20B2O3 – 70SiO2 (in mole%). The later was annealed at 600ºC in order to precipitate Au nano-particles. (The samples were supplied by J. Qiu of Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science.) All TEM specimens were prepared by grinding the glass into a
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powder in acetone and mounting suspended pieces on a holy-carbon-film
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