The Sandberg Geomaterials Department has its own in-house Hitachi SU 3500 scanning electron microscope (SEM)1 fitted with a state-of-the-art Oxford Instruments energy dispersive X-Ray microanalysis system and X-Max X-ray detector capable of light element detection. The equipment is supervised by staff with over 30 years of experience in SEM and associated petrographic/analytical techniques. Sandberg is UKAS accredited for the SEM analysis of concrete and crushed rock fill.
Hitachi Scanning Electron Microscope (SEM) fitted with a state-of-the-art Oxford Instruments INCA energy dispersive X-Ray microanalysis system.
SEM Capabilities
The Sandberg SEM is able to carry out very high-resolution imaging but also has the additional ability to examine samples at low vacuum, enabling easily damaged materials or fragile hydrated samples, such as clays or polymers, to be examined. High-resolution, low-vacuum imaging in secondary electron, as well as backscattered electron mode, is possible using the Hitachi UVD secondary electron detector.
The microscope has a maximum resolution of 7nm (0.000007mm) and can operate at magnifications of up to x100,000, enabling the morphology of cement hydrates in concrete and clays to be revealed in great detail. The microscope also has a large sample chamber, allowing samples of up to 70mm in diameter to be examined.
Typical X-ray spectrum from a GGBS particle in concrete. Spot chemical analysis measuring about 0.01mm in diameter.
The SEM is fitted with a backscattered electron detector that can be used to provide images of samples that show variations in chemical composition. Materials containing elements with a high atomic mass, such as iron, appear brighter in electron images than materials containing elements with predominantly low atomic masses, such as magnesium or carbon.
Chemical analysis / X-ray Microanalysis
The Oxford Instruments INCA analysis system, coupled with an X-Max 50 dry X-ray detector, allows for the simultaneous analysis of multiple elements and can provide a cost-effective solution to establishing general information about the chemical composition. A typical analysis would commence with the identification of the elements present, which are then analysed simultaneously and expressed as either elements or oxides.
The ability to undertake chemical analyses is indispensable for most types of SEM-based analysis. With the Sandberg SEM, it is possible to make spot chemical analysis of particles as small as 1 µm (0.001mm) in diameter. Much larger areas of up to 4mm x 4mm can also be analysed. X-ray mapping can be used to show the spatial distribution of elements.
Working at low accelerating voltages, it is possible to obtain chemical compositions of very thin surface coatings that are less than 0.001mm thick. This ability is vital for staining investigations.
Image Analysis and X-phase Mapping
The SEM has the ability to carry out image analysis based either on contrast/ brightness or on chemical or mineral phase composition. This technique has many applications and can be useful for quantifying proportions of morphological features such as microscopic voids/pores, for examining the distributions of elements such as chloride around a crack in concrete or for investigating the distribution of specific compounds on a crack surface such as the image shown below:
Footnote
- Due to the high demand for our instrumental analysis services and consultancy, a second SEM has been procured: a Hitachi Flex SEM 1000 Mk II Advanced Compact scanning electron microscope complete with a comprehensive Oxford Instruments AZtec energy dispersive X-ray microanalysis facility. The AZtec package installed on the new SEM includes advanced software options such as AZtec Feature and Auto Phase Map, which are particularly suited to quantifying deleterious minerals such as pyrrhotite and pyrite in concrete blocks subjected to testing in accordance with IS 465.
The new scanning electron microscope represents a valuable addition to Sandberg’s existing SU3500 SEM and significantly increases the capacity for material investigations requiring SEM analysis ↩︎