Exploration of phase changes at high pressures
Principle Supervisor: Dr Simon Bland
Most materials can assume multiple phases whilst in the solid state - for instance if iron is cooled from liquid at atmospheric pressure, the atoms will assemble in BCC, FCC and then BCC structures; whilst at higher pressures an HCP form can be produced. Often however the phase of materials at higher pressure remains relatively unexplored, as experiments designed to create these high pressures have introduced shock waves that have melted the material sample. In this PhD we will produce phase changes in materials through the use of various 'isentropic' or 'shockless' drive techniques, which compress a target to 100s of kbar pressure with a minimal change in temperature.
The principle aim of this PhD is to explore new, uncharted territory in phase space by combining pre-heating/cooling with controlled prescriptions of shock and ramp loading. The first part of the PhD will involve design, construction and testing of novel apparatus to heat or cool material samples in situ to a compression experiment. Heating many metals/metal alloys to ~1000K or cooling mixtures of gases to cryogenic temperatures will produce liquid initial conditions, which when compressed will form different solid allotropes. Once completed the apparatus will then be used in conjunction with the MAGPIE or MACH pulsed power facilities, to provide magnetically driven pressure waves, and with the new Imperial College gas gun, where ramp compressions will be accomplished using pulse-shaping impactors (e.g. graded-density flyers produced via rapid prototyping).
Experiments will initially use pure aluminium to 'calibrate' the techniques before moving onto materials including bismuth that incorporate a large number of solid-solid phase transitions. In addition to fielding an array of different diagnostics, such as VISAR, Het-V and conduction probes, the candidate will use 2D and 3D simulations to perfect the correct loading rate and uniformity for an experiment. The PhD will involve a large amount of cross over work with the Institute fo Shock Physics research partners at University College London, where work on diamond anvil cells produce static loadings of small scale samples of up to ~1Mbar; and may include the use of in-situ X-ray diffraction techniques developed in a separate project.