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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 newly installed (Autumn 2014) Zeiss Sigma HD Analytical Field Emission Gun SEM (ASEM) is the flagship of the facility. It is a state-of-the-art ASEM designed to facilitate both element mapping and quantitative analysis of materials down to the ppm level. To this end, it is equipped with:

  • Two Oxford Instruments 150mm2 EDS detectors
  • An Oxford Instruments Wave WDS detector
  • An IXRF Xb micro-XRF

Image 1: New, state-of-the-art SEM facility.

In addition, it features a 5-element high-definition backscatter detector, chamber and in-lens secondary electron detectors and a cathodoluminescence detector.

The Sigma HD is designed to facilitate rapid element mapping at very high resolution (image framestore up to 12x9k pixels). Along with the dual large-area EDS detection setup, which can capture in excess of 1,000,000 counts per second, this enables unparalleled examination of distributions of major- and minor elements over large areas. In addition, using the micro-XRF source, trace element distributions can be imaged with a resolution of 10 micron. Since the ASEM is fully standardised, the element maps are quantitative, which allows for mapping of absolute element abundances. In addition, an advanced software package allows for automated phase identification and automatic detection and mapping of user-defined particles of interest.

Using integrated EDS (for major elements) and WDS (for minor elements), the ASEM can provide quantitative spot analyses with accuracy and precision rivalling that of an electron microprobe.

Combined, the element mapping and quantitative analysis capabilities of the ASEM provide a versatile tool for a quantitative understanding of element distributions in (geological) materials.

Image 2. Calcium image from igneous sample. (For a large version of this image, click here.)

Image 3. Element map from a layered sedimentary thin section. (For a larger version of this image, click here.)

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