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Scanning Electron Microscopy/X-ray Diffraction Facility

The Scanning Electron Microscope is used to image objects such as minerals and microfossils at a magnification far exceeding the capabilities of an optical microscope. As well as imaging, the X-rays emitted from the specimen can be used for elemental analysis. The X-ray Diffractometer is used for identifying and characterising minerals, either alone or in a complex mixture. The Facility comprises:

  • An Analytical SEM (ASEM)
  • An Environmental SEM (ESEM)
  • An X-ray Diffraction Laboratory

Analytical SEM

The Analytical SEM is based on a Cambridge Instruments (Zeiss NTS) S360 with a secondary electron detector (SE) for surface imaging, a 4-quadrant back-scatter electron detector (4QBSD) for density imaging, and a cathodoluminescence detector. The analytical instrumentation is comprised of an Oxford Instruments INCA ENERGY (EDX) x-ray analysis system and INCA WAVE (WDX) wavelength dispersive x-ray spectrometer. Image recording is via a Mitsubishi Thermal Imager or via the OI INCA system to hard drive, USB flash drive or CD writer.

With high resolution imaging under optimum conditions with a suitable sample, a resolution of the order of 5nm is achievable. This allows images to be collected at a magnification of X 100K or more, in practice however magnifications up to X 20K are usually sufficient. Back scattered electron-imaging produces a density image particularly of polished mineral surfaces and is used extensively for searching for small (< 1micron) dense minerals for example gold or platinum group minerals. Other applications include elemental zoning within minerals and multi-phase imaging.

Energy dispersive X-ray analysis of virtually all minerals and alloys is possible providing the surface is suitably polished. Materials routinely analysed include sulphides, sulphosalts, oxides, platinum group minerals, carbonates, silicates and glass. Detection limits for EDX analysis are variable but typically 0.02-0.05%. The software also allows for multi-element x-ray mapping and line scanning.

Wavelength dispersive X-ray analysis using a single channel crystal spectrometer allows analysis of elements from Boron up, again with a variable detection limit but typically in the region of 0.001-0.005% in the element range calcium-gold. Single element mapping and line scanning is possible.

 The Analytical Scanning Electron Microscope.

 

Environmental SEM

The Environmental SEM was installed in the department in July 2001 following a successful bid in the 2000 round of JIF (Joint Infrastructure Fund) with additional funding provided by HEFCW (Higher Education Funding Council for Wales). 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.

The microscope is a Veeco FEI (Philips) XL30 ESEM (Environmental Scanning Electron Microscope) FEG (Field Emission Gun). It is fitted with a secondary electron detector (SE), a back scatter electron detector (BSE) and a gaseous secondary electron detector (GSE). It also has an Oxford Instruments INCA ENERGY (EDX) x-ray analysis system. There is a Peltier cooling stage for use in "wet" ESEM mode. Image recording is via a SONY video graphics printer or digital by processing image frames in a 16 bit framestore computer for output to hard drive, USB flash drive or CD writer. The Windows graphical user interface allows quick and easy operation by mouse and keyboard shortcuts but there is also a manual user interface.

The ESEM FEG has 3 main modes of operation. X-ray analysis is possible in all modes:

 

  • High resolution conventional SEM in high vacuum mode using SE or BSE with a resolution of 2nm at 30KV and magnifications of 75x to 500,000x.
  • Low vacuum mode (often known as environmental mode) using GSE or BSE at pressures of 0-1.5 Torr using gas (nitrogen or water vapour) in the microscope chamber for charge neutralisation. This is suitable for any solid, dry specimen and removes the need for coating specimens.
  • ESEM or "wet" mode using GSE at pressures of 0-10 Torr and temperatures of ±20° C from ambient (when the Peltier cooling stage is fitted) using water vapour in the microscope chamber. Adjusting the pressure of the water vapour and the temperature of the cooling stage to saturation conditions (100% RH) allows a wet sample to remain hydrated whilst inside the chamber. This is particularly useful for biological specimens.

The Environmental Scanning Electron Microscope.

 

X-ray Diffraction Laboratory

The X-ray Diffraction Laboratory contains a Philips Automated Powder Diffractometer, model PW1710. The diffractometer gives compound or mineral names, as opposed to a list of elements as in other analytical methods. Solid and powdered samples can be analysed and matched against a database of 70,000 recorded phases, thereby identifying and quantifying the unknown phases that are contained within a sample. Each pure mineral or compound has a specific X-ray diffraction pattern and it is these that are matched against the unknowns. This method is a non-destructive analytical technique.

For this technique, samples must be solid. The sample is powdered and packed into an aluminium holder. It is then placed in the Goniometer and bombarded with X-rays generated from a copper or cobalt tube. The diffracted rays are collected by a detector and the information relayed to a computer where, using the Bragg equation, it is converted to d-values of specific intensities. This information can then be shown graphically in the form of a diffraction pattern or 'diffractogram'. The diffractograms from the unknown sample can then be matched against a database using the PC-Identify software. For mixed phase samples, a percentage of each phase present can be calculated from the areas of the major peaks for each of the phases present.  This would be classed as semi-quantitative analysis.

For smaller amounts of powdered sample, they can be mixed with a little solvent and pipetted onto glass slides, they can then be inserted into the Goniometer.

An Environmental Chamber is available for running air-sensitive samples.

The XRD is a widely used research tool. The intercrystalline structures of new minerals and chemical compounds can be determined. Mineral species can be identified ("fingrprinted") 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%).

 The X-Ray Powder Diffractometer.

 

SEM Contact Information

Peter Fisher
Email: FisherPC@cf.ac.uk
Telephone: +44(0)29 208 75059

Lindsey Axe
Email: Axe@cf.ac.uk
Telephone: +44(0)29 208 74310

XRD Contact Information

Anthony Oldroyd
Email: Oldroyd@cf.ac.uk
Telephone: +44(0)29 208 75801 /  75059