X: X–ray source
C: Collimator
S: Sample (limited rotation)
F: Scattering Foil
DA: Absorption Detector,
DR: Refraction Detector
Sc: Sample Scanner
In XRF, the target material is irradiated with X-rays, which results in the release of a photon of characteristic energy. Each element has its own set of characteristic emissions and can be used for qualitative and semi-quantitative elemental analysis in a wide range of materials, above aluminium in the periodic table. Minimal sample preparation is required. A variant, total reflection X-ray fluorescence uses X-rays impinging on the surface of a sample at glancing incidence such that total reflection occurs. This excites photon emission from atoms in only the topmost layers of the material. Whilst this technique is very sensitive it does require very flat samples.
This is an analytical technique that measures the energies of photoelectrons emitted from atoms of a sample when it is irradiated with soft (or low energy) X-rays. XPS is surface-sensitive and is frequently used for quantitative elemental analysis of fracture surfaces, to determine the effect of surface preparation on surface chemistry and for monitoring chemical changes in adhesive samples. The technique, which is capable of detecting all elements with the exceptions of hydrogen and helium, can provide information on chemical structure (e.g. oxidation states) and element distribution present on the surface of any solid material. The technique is surface-sensitive with a maximum operational depth of less than 10 nanometres with a spatial resolution of less than 10 mm. XPS can be used to determine the effect of elevated temperature and surface preparation on surface chemistry and is used to examine the cause of adhesion problems. The technique can be used in conjunction with inert gas ion sputtering to determine the variation in chemical composition with depth (NB. Many polymeric material samples are sensitive to ion beam damage).
X-ray topography is characterized by the spatially resolved detection of scattering of a sample. It combines both the advantages of radiographic imaging and the analytical information of X-ray scattering like wide and small angle diffraction, refraction and total reflection. Scanning techniques under Small and Wide Angle scattering conditions permit the topographic characterisation of any crystalline or amorphous solid or liquid. Wide Angle X-ray Scattering (WAXS) (diffraction) is elastic scattering (no energy shift) and is sensitive to the atomic and molecular structure below the scale of nanometers. Small Angle X-ray Scattering (SAXS) is suitable for studying colloids, polymers and biological materials. Particle dimensions smaller than 50 nm can be determined. In combination, the two techniques can be used to provide information on molecular orientation. The high quantum efficiency of scintillation counters permits the intensity measurement at reasonable speed.
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X: X–ray source C: Collimator S: Sample (limited rotation) F: Scattering Foil DA: Absorption Detector, DR: Refraction Detector Sc: Sample Scanner |
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X-ray diffraction topography (courtesy of BAM)
X-Ray Refraction-Topography developed at BAM utilises the optical effect of refraction at interfaces, which for X-rays happens at small scattering angles of a few minutes of arc. Using a Kratky camera, a very narrow X-ray beam traverses a sample, which is scanned in the surface plane and scattering intensities are taken at all positions. The technique provides nearly linear contrast by inner surfaces and interfaces. 2-D images are generated with a possible spatial resolution of about 10 µm x 300 µm). Usually the strong intensity of the X-ray refraction signal enables scanning of samples within relatively short time. Between 0.1 and 5 seconds per sample position are required for 2 % signal to noise ratio depending on the sample absorption and the inner surface concentration.
It is possible using X-Ray Refraction to generate inclination topography projections of bonded joints manufactured from non-metallic materials (e.g. fibre-reinforced polymer composites) to identify regions of poor bonding. The technique can also be used to determine crack density in polymers after chemical ageing (i.e. crazing) and for studying fibre debonding in fibre-reinforced laminates. For some practical applications, like evaluating the flow pattern of injection moulding parts, it is often sufficient, to image solely the changes in orientation by texture topography. The rotation slit is fixed at an inclination angle of maximum slope of the rotation profile. While the sample is scanned any changes in texture and preferred orientation result in intensity changes.
A limitation of Refraction-Topography is the averaging of the interface signals over the thickness of a sample. But a transversal section image can be achieved by a combination of Refraction line scans and computer tomography techniques. At zero scattering-angle, multiple linear scans of the sample are repeated at different inclination angles about an axis perpendicular to the incident beam. The absorption signals can be reconstructed according to the rules for “parallel beam filtered back projection.”
Lap-joint scanning arrangement (courtesy of BAM)
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Mean inner surface signal of 12 linear scans (courtesy of BAM) (-1.2 to 1.2 degrees incident angle relative to the joint surfaces)
Next: Auger Electron Spectroscopy (AES)
