Electron microbeam facility
Our microscope can be used to image objects such as minerals and microfossils at a magnification far exceeding the capabilities of an optical microscope.
The Electron Microbeam facility in the School of Earth and Ocean Sciences houses two Scanning Electron Microscopes and an X-ray Diffractometer. The Scanning Electron Microscope (SEM) is used for characterization, imaging and analysis of sub-micron features in materials. The X-ray Diffractometer is used for identifying and characterizing minerals, either alone or in complex mixtures.
Our facilities include a state-of-the-art Zeiss Sigma HD Field Emission Gun Analytical SEM which is used for high-resolution imaging and X-ray element mapping as well as quantitative analysis of major, minor and trace elements. In addition our FEI XL30 Field Emission GunEnvironmental SEM is used for high-resolution imaging and semi-quantitative X-ray element analysis of samples. Carbon- and gold-coating facilities are available for non-conducting samples. The Philips PW1710 Automated Powder Diffractometer is used for identifying and characterizing minerals, either alone or in complex mixtures.
Our instruments are available to users from all colleges within Cardiff University, as well as other academic institutions and industry partners. Those interested in using the facilities should contact Duncan Muir (MuirD1@cardiff.ac.uk).
Our on-site analytical capacity includes:
Our state-of-the-art Zeiss Sigma HD Field Emission Gun Analytical SEM (ASEM) is the flagship of our facility. The Schottky Thermal Field Emission Gun provides a stable beam that can be operated between 0.2 and 30 kV and 4 pA to 100 nA. As well as being used for high-resolution imaging, it is set up for rapid X-ray element mapping and quantitative analysis of elements.
The ASEM is equipped with:
- an in-lens and Everhart-Thornley secondary electron detector
- a backscatter electron detector
- two Oxford Instruments 150 mm2 energy dispersive X-ray spectrometers (EDS)
- an Oxford Instruments Wave wavelength dispersive X-ray spectrometer (WDS)
- an iXRF micro-XRF source
- an Oxford Instruments Aztec electron back-scatter diffraction (EBSD) detector
- a Centaurus cathodoluminescence detector
How it helps
A combination of EDS (for major and minor elements), WDS (for high-precision analysis of major and minor elements) and micro-XRF (for trace elements) means complete characterization of elements heavier than carbon can be achieved with accuracy and precision rivalling an electron microprobe. The dual 150 mm2 EDS detectors, which can capture in excess of 1 000 000 counts per second, enable rapid, high-resolution 2D mapping of distributions of major and minor elements over relatively large areas. Since the ASEM is fully standardized, the element maps are quantitative, thus absolute element abundances can be mapped. In addition, using the micro-XRF source with an accelerating voltage of 50 kV, trace element distributions of elements heavier than Ti22 can be imaged with a resolution of 15 µm. EBSD provides information on crystal orientations with sub-micron resolution and can be combined with EDS chemical data for textural and microstructural studies. Image referencing software allows easy navigation around the sample using images from a flatbed scanner or digital camera.
This is a high resolution SEM allowing magnifications up to 500,000x on conventional coated/conducting samples but it also has the capability for high resolution SEM of uncoated and even hydrated "wet" samples.
Our FEI XL30 Field Emission Gun Environmental SEM (ESEM) is fitted with a secondary electron (SE) and back-scatter electron (BSE) detector along with an Oxford Instruments Inca Energy (EDS) X-ray analysis system. Resolution is better than 10 nm using the SE detector, and the BSE detector allows imaging of mean atomic mass variations. The EDS is used for standardless semi-quantitative point analyses and X-ray mapping of elements heavier than carbon. Variable pressure controls mean uncoated samples can be imaged along with gold- or carbon-coated samples. A gaseous secondary electron detector (GSE) combined with a Peltier cooling stage enables operation in ‘wet’ ESEM mode. The stage is adjustable in x-y-z dimensions and can be tilted allowing irregularly shaped samples to be accommodated.
XRD is an important tool for determining the intercrystalline structures of minerals and chemical compounds. The method is non-destructive and mineral species can be identified (‘fingerprinted’) in bulk samples even if only in very small particles. Mineralogical make-up of sediments and sedimentary rocks can also be estimated (for species with concentrations in excess of about 5%). An Environmental Chamber is available for running air-sensitive samples.
Our laboratory contains a Philips Automated Powder Diffractometer, model PW1710. Solid and powdered samples can be analysed and matched against a database of 70 000 recorded phases, thereby identifying and quantifying unknown phases. Each pure mineral or compound has a specific X-ray diffraction pattern and these are matched against the unknowns. The diffractometer gives compound or mineral names, as opposed to a list of elements as in other analytical methods.