To address a number of performance issues related to electron microscopy, the University of Wollongong has constructed a purpose built electron microscopy centre that was completed in July 2011, and purchased a comprehensive suite of specimen preparation equipment that is now operational for materials and life sciences. The centre will house up to 7 electron microscopes with two preparation laboratories, and has been designed to exceed the environmental specifications for the current generation of commercially available electron microscopes including aberration corrected S/TEMs. Currently two FEGSEm’s and a TEM have been relocated to the facility and are fully operational. Leica was selected as a major supplier for the specimen preparation equipment based on equipment specifi cations, product integration, demonstrated capability, ease of use, automation and applications support. These will be supplemented with an Leica M205A stereo, Leica DM2500 M and Leica DM6000 M optical microscopes for quality control during specimen preparation and their value as stand-alone research tools. These factors will facilitate production of high quality specimens, and the training of a large user-base typical of a university environment.
The Leica EM TXP, EM TIC020 and EM RES101 were purchased to prepare materials for SEM and TEM based studies. The Leica EM TXP was selected for its ability to prepare TEM specimens and perform target grinding. The ability to perform these functions while being viewed directly to assess quality and increase specimen throughput was seen as a major advantage. The Leica EM TIC020 was selected for its ability to prepare large, damage free areas of specimens for low voltage, high resolution SEM imaging, Electron Backscatter Diffraction (EBSD) and Energy Dispersive Spectroscopy (EDS). The Leica EM TXP and EM TIC020 instruments have already been utilized for the preparation of Magnesium diboride (MgB2) superconducting wires. MgB2 is a challenge to prepare because it is brittle and sensitive to water. Of particular interest are defects such as pores, cracks or impurities at the Mg and B grain boundaries after processing, which preparation artefacts can influence. Transverse and longitudinal orientations of MgB2 wires encapsulated in Niobium (Nb) metal and Inconel alloys, were prepared using the Leica EM TXP and EM TIC020 instruments. The specimens were glued to the small Leica EM TIC sample holders, then clamped for preparation on the Leica EM TXP using the procedure in Table 1.
Table 1: Leica EM TXP preparation of MgB2 superconducting wires.
|Tool insert||RPM||Force limit||Pump/mL||E-W speed/mms–1|
|Diamond disc cutter||15,000||–||18||0.025|
|Silicon carbide foil, 15 µm||1,000||8||8||0.2|
|Diamond lapping foil, 9 µm||2,000||8||8||0.2|
Ethanol was used as a lubricant and dispensed using the Leica EM TXP pump. The polishing procedure on the Leica EM TXP only used coarse lapping foils to provide a plane surface for final preparation with the Leica EM TIC020. A mechanical polishing procedure will be developed shortly to produce the best possible surface finish.
To minimize the effects of surface oxidation after preparation on the Leica EM TXP, the specimens were transferred directly to the Leica EM TIC020 and placed under vacuum. The specimens were ion milled with gun energies of 7 kV and gun currents of 2.6 mA. After polishing, the specimens were transferred directly to the SEM to minimize exposure to air. Figure 1a shows the transverse section with the Nb metal sheath encapsulating the MgB2 superconducting core. The ion polished surface and the interface between the dissimilar materials, are free of polishing artefacts. Figure 1b shows the MgB2 core in more detail, with pores present at some grain boundaries.
Figure 2a shows the longitudinal section of the superconducting wire where the elongated grains in the drawing direction are easily observed. Figure 2b shows the MgB2 core in more detail, where no physical defects were observed.
Figure 3 shows X-ray maps of a) Mg, b) B and c) O in the transverse section of the MgB2 superconducting wire. The x-ray maps show the elemental concentration as a function of the colour intensity in each respective map. Figure 3d shows all of the elemental maps superimposed. Here, it can be seen that O is present in low concentrations, mostly at the Mg grain boundaries where the superconducting properties can be affected.
The electron microscopy centre will also need to support growing areas in biology-based research projects needing TEM and SEM. A Leica EM UC7 ultramicrotome and EM FC7 cryo-attachment have been purchased to section soft polymers and biological materials to cater for the new research interests. Similarly, a Leica EM CPD030 critical point dryer will allow controlled drying of soft biological tissues and gels for SEM observation, and completes the suite of Leica instruments that will be installed in the new centre.