In-situ diffraction of laser shock heating

Date: 10-17 January 2011
Beamline: I11 at Diamond
Team on site: Steve Fearn, Morgan Jones, Rudi Winter (Aber)
Alistair Lennie, Julia Parker, Stephen Thompson,
Chiu Tang (I11)
Support: Jonathan Potter (Diamond, I11)
Julien Marchal, Brian Wills (Diamond, detectors)
Richard Fearn (Diamond, software)
Instrument: X-ray energy: 15keV
Detector: Mythen PSD
Sample env.: Sample enclosure with pyrometer, webcam
and laser and x-ray windows
mounted at the centre of the diffractometer.
Sample tilted 10° relative to incoming beam.
Laser: Synrad 125W CO2 (10.6μm)
Samples: Various refractory ceramics and natural rock
specimens

The objective of this experiment was to record x-ray diffractograms with high time resolution to study shock-induced strain in a number of refractory ceramics. The strain was induced by firing an infrared laser at the sample while recording x-ray diffractograms a variable distance (up to 8mm) away from the impact point.

Generally, shock waves travel at the speed of sound in a homogeneous material. However, in a granular material, the contact points between the grains play an important role: Where two grains impinge on each other, energy is dissipated and the contact points act effectively as a secondary source of strain waves.

X-ray diffraction is sensitive to strain because it picks up the change of the lattice constants of a strained material. This results in a shift of the Bragg peaks of the sample; compressive strain produces a shift to larger angles. In practice, there will be un-strained and strained material within the x-ray beam spot as the shock waves travels through it, resulting in a peak broadening (from the superposition of both components) rather than an actual shift. The effect is very small, and will take both the time and angular resolution of the instrument to the limit.

Fig.: Refractory sample during laser pulse.
Fig.: part of the team on the beamline.
Photo: Alistair Lennie

The picture on the left shows Steve, Morgan and Rudi (left to right) on the beamline. The CO2 laser is the long black instrument sitting on the stepper motor controlled sample environment table, behind the laptop in the foreground. The sample cell and detector are out of shot on the left.

The small picture shows a sample (a slice of alumina-zirconia silicate refractory, approx. 12mm×12mm and 800μm thick) mounted in the purpose-built sample chamber. The picture was taken by a webcam inside the sample chamber. The green spot near the centre of the sample is the x-ray beam spot; it is visible because of fluorescence from one of the components of the sample. The darker spot to its left is the laser impact point.

Dynamic pump-probe experiments

Fig.: Pulse diagram showing the timing pattern of a single exposure.

Timings are critical in an experiment such as this. To achieve data acquisition synchronised with the laser pulses, the rising flank of the laser pulse was used to trigger the beamline's data acquisition software, which in turn had a configurable trigger delay. The whole sequence was repeated many times per sample, with a low enough rate to ensure the sample wasn't heating up.

Individual data acquisitions were kept as short as possible to maximise the sensitivity to the shock wave travelling through the beam spot. It was confirmed that a 1ms exposure is sufficient to capture the strongest Bragg peaks (blue symbols in the diagram below - channels with fewer than three counts are omitted for clarity), and that adding up many 1ms frames reproduces a diffractogram taken with a longer acquisition time faithfully.

Fig.: A single 1ms exposure and a composite diffractogram.

The red line in the graph is a sum of 1242 1ms frames (reduced to scale). Such composite diffractograms have excellent statistics and can be used to analyse peak shifts and broadenings.

We have obtained 135000 individual frames during this beamtime. A data analysis procedure is now being devised that will allow us to select frames which contain a particular Bragg peak and to trace its features as a function of distance from the laser impact site and timing.