Scanning transmission electron microscopy sample preparation

Substrate Characterization. Test coupons and panels of 7075-T6 aluminum, an alloy used extensively for aircraft structures, were degreased In a commercial alkaline cleaning solution and rinsed In distilled, deionized water. The samples were then subjected to either a standard Forest Products Laboratories (FPL) treatment ( 0 or to a sulfuric acid anodization (SAA) process (10% H2SO4, v/v 15V 20 min), two methods used for surface preparation of aircraft structural components. The metal surfaces were examined by scanning transmission electron microscopy (STEM) In the SEM mode and by X-ray photoelectron spectroscopy (XPS). 

Scanning transmission electron microscopy (STEM), coupled with EDX, has been used to determine metal particle sizes. The specimens for STEM are prepared by dispersing the sample ultrasonicaUy in methanol and placing one drop of the suspension onto a Formvar film supported on a copper grid. 

Samples for transmission electron microscopy were prepared in the following manner. Films were grown with different current rates up to various thicknesses on platinum working electrodes. The film thickness was controlled by the period of current flow. The films were transferred onto carbon coated electron microscope grids by stripping with formvar and subsequently removing the formvar with methylene chloride. As-synthesized films were directly used for scanning electron microscopy. 

Particle Induced X-Ray Emission (PKE) is similar to EDS analysis except that it uses high-energy particles instead of electrons to create the vacancies that lead to X-ray emission. It offers orders of magnitude better detection limits for trace elements than does EDS. Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) require extensive sample preparation. When coupled with another technique (EDS for example), they can be useful in determining the nature of surface films or in illuminating the microstructural causes of corrosion. Environmental Scanning Electron Microscopes (ESEM) that are equipped with EDS spectrometers allow SEM-EDS type data to be obtained from samples in the hydrated state. 

A titania-coated silica, as prepared and calcined, was used as support to prepare two An catalysts. The catalysts were characterized by several techniques. X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and tested in the CO Preferential Oxidation (CO PROX) reaction. The dispersion of Ti onto the silica was veiy homogeneous. On the other hand, the incorporation of Au was limited to less than 1 % and the particles size varied in the range 3-10 ran in both samples. Reaction studies were carri out on a fix bed reactor and on a Temporal Analysis of Products (TAP) reactor. The results reveal the participation of the surface -OH groups on the mechanism of the selective CO oxidation. 

The consolidated titanate waste pellets are similar in appearance to their glass counterparts, i.e., both are dense, black and apparently homogeneous. Microscopic analyses, however, reveal important differences between these two waste forms. While little definitive work has been done with glassy waste forms, it is apparent that several readily soluble oxide particulates of various nuclides are simply encapsulated in the glass matrix. The titanate waste form has undergone extensive analyses which includes optical microscopy, x-ray, scanning electron microscopy, microprobe, and transmission electron microscopy (l ) The samples of titanate examined were prepared by pressure sintering and consisted of material from a fully loaded titanate column. Zeolite and silicon additions were also present in the samples. 

The oldest microscopy technique for materials analysis was optical microscopy. Even to this day, for feature sizes above 1 pm, this is one of the most popular tools. For smaller features, electron microscopy techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are the tools of choice. A third family of microscopy includes scanning probe tools such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM). In these relatively recent techniques, sample preparation concerns are of minor importance compared to other problems, such as vibration isolation and processing of atomically sharp probes. Therefore, the latter techniques are not discussed here. This chapter is aimed at introducing the user to general specimen preparation steps involved in optical and electron microscopy [3 7], which to date are the most common... 

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