Development of diagnostics to study the mechanisms and kinetics of shock induced phase transformations
Supervisor: Dr Bill Proud
Optical pyrometry is used by AWE to obtain partial release interface temperature measurements of shocked metals in an attempt to constrain EOS physics models. To achieve these interface temperature measurements, transpararent window anvils are used to prevent the full release of pressure at the surface of the material interest, thus allowing optical detectors enough time to record meaningful radiance measurements. Two of the most common window materials used in this technique is sapphire and lithium fluoride. The current material of choice at AWE is LiF. Dynamic emissivity measurement capability is also being developed to enable more accurate temperature calculations to be made from radiance data and also to determine phase change for the shocked metal samples.
There is a strong desire to gain a fuller understanding of the mechanisms and kinetics of shock induced phase transformations for condensed materials leading to a comprehensive EOS model and P-V-T relation for materials of interest. It is intended that this research PhD will focus on developing diagnostics that are capable of studying dynamic temperature and phase and to employ them on gas gun physics experiments. The aim is to gain a better understanding of phase kinetics, the influence impedance matched transparent anvils have on temperature and emissivity measurements, the disparity between interface and bulk temperature measurements and to produce data to aid a constrained EOS for shocked materials.
The PhD studentship will initially look at developing diagnostics to study the shock induced properties of potassium chloride (KCI). This material has been extensively studied for its mechanical and, to a lesser extent, electrical properties at high strain rates. The dynamic B1 to B2 phase transition of this ionic crystal structure from rock salt (NaCl) into cesium chloride (CsCl) has been observed for shock loading at around 2GPa. Other observations resulting from this transformation include strong absorption of light and induced polarisation. These types of observation have also been seen in other transparent dielectrics such as sapphire and lithium flouride. However, for these materials, such observations are seen only when much higher dynamic pressures are applied. They are also accompanied by unwanted shock generated luminescence. By developing diagnostics to study these phenomena in depth, the student will gain an understanding of the mechanisms and kinetics of this type of phase transformation and will look to apply this understanding in the development of temperature and emissivity diagnostics. Investigation into how these high strain rate associated features employing anvil windows will be conducted. It is hoped that advantage can be taken of the induced opacity in KCI by measuring the radiance of opaque shock front. Potassium chloride has the advantage over Al2O2 and LiF that the pressure needed to induce opacity does not appear to induce shock luminescence. The colour temperature of the thermally radiating opaque shock front from strongly shocked sapphire and LiF is mixed with, and therefore masked by, luminescent radiation from hotspots with substantially higher temperature readings than the expected bulk temperature. Thus by studying KCI, it is envisaged that experiments can be designed to investigate the disparity between interface and bulk temperature measurements.
Initial experiments will take place at Cranfield University using KCI, followed by experiments at the Institute of Shock Physics laboratory facilities where the study of LiF, sapphire and bismuth is intended.