Internship folder for Josslyn Quansah for AEOP internship Summer 2024
Shock physics is mainly the investigation of effects of certain shock waves on different particles, highlighting the potential changes in structure, temperature, and viscosity. This can be used to model phase transitions, Geophyics/Planetary Sciences, molecular dynamic calculations, and the denotation of explosives.
Impedance matching uses pressure-particle velocity graphs and position-time graphs to find the pressure of an unknown sample through the shock velocity properties of a known material. This technique aids with shock mitigation strategies, determining the peak stress of explosives, and discovering vulnerabilities in detonation.
The SOP is used to track the amount of photons to determine the temperature of a dynamically compressed material; this complements the Visar technology in identifying shock breakout in a sample.
When preparing the sample, it is imperative that an antireflection coating is applied onto the surface to limit the photon back refelction at interface.
Depending on the use of either Omega-EP or Omege-60 one or two laser pulses focus on a specified diameter spot. By tweaking the intensity of the laser, pressure in the MgO sample will increase.
Measuring the X-Ray diffraction is collimated through a Ta pinhole, which brings into line the diffraction and records their transmission geometry on specific detectors. The detectors store the x-ray energy which can be converted into units of luminescence that are photosimulated through a calibrated scanner.
To predict the signal of crystal orientation, forward diffraction simulations are critical. The textural information modulates the intensity of powder direction in the azimuthal direction. The variation of this intensity can be calculated along the azimuth for both the B1 and B2 phase.
Firstly, Ramp Loading is used to allow samples to stay at the solid phase for longer as higher pressures are attained. To measure x-ray diffraction in this environment, certain criteria like specific x-ray sources and uniform pressure are required. The PXRDIP utilizes visar technology FUJI image plates, a diamond-sampld-diamond sandwich and more to capture powder x-ray diffraction. This diagnostic has enabled pressure history to be found which verifies the solidity and crystal structure of the sample.
To evaluate possible compostional space for large rocky exoplanets, ramp compression of iron is used to attain the essential values for density, pressure, and speed of sound along an isentropic compression path (that ramp compression allows). To accurelty determine the equation of state for the planetary materials at TPa pressures, experiments need to get as close to 2 TPa as possible, which ramp compression can come close to. In an experiment highlighted in this papers, the scientists were able to find the Lagrangian sound velocity and the pressure-density data needed to interperet the data of large rocky expoplanets. Multiple stpes were then used to transform this pressure-density data to a more isentropic path or "Vinet Fit" to more accuretely represent the data. During the experiment, other factors were discovered that could potentially affect the data being retreived from ramp compression. First, they found that the linear dependence of sound speed on density (Birch's Law) holds for Iron even under TPa pressures. This shows continuity and therfore does not affect any results from the sample. Secondly, they found that there was some variance in the sample heating during the ramp compression. It was then discovered that the sample heating was just a product of the energy from initial shock and the plastic work as a result of iron's strength under high pressures. This in turn means the variance is negligable. The results from the ramp compression will in turn provide an experiment based mass-radius relationship for a planet with a pure iron core (hypothetical) that can be useful to evaluate possible compositional space for large, rocky exoplanets.
Silicon is one of the lighter elements found at Earth's core, and is an element that holds the potential to be the most major light element due to its cosmochemics, low volatility, and continuous solid solution with Iron. DAC (Diamond Anvil Cell) experiments were attmepted to venture the P-T diagram for these Fe-Si alloys, but it was not as effective as Ramp Compression. This superior technique allows the compression path to mimic an isentropic one, and ensures the solid state is mantained even under very high pressures. This experiment with ramp-loading removes a need to rely on long extraplation of low-pressure data. In the expereiment featured in this paper, the PXRDIP was used in addition to the "diamond sandwich" (diamond-Fe/Si-diamond) to determine to the pressure history of the sample. At high pressure, a different amount of diffraction peaks were observed at due to different pressure, leading to variance in the data. At low pressure 3 diffraction peaks were visible, while at high pressure only 3 were. In addition, it was discovered that adding Si to Fe stabilized its B2 structure (respection the fcc and hcp phases). Incorporation of light elements into the core (not just the hypothetical solid iron core) can affect the number of properties of these celestial bodies. That includes mass radius relation, core size, and internal pressure. In conclusion of the experiment, it was clear that the crystal structure of Fe-Si alloys in planetary cores will depend on the amount of silicon content present.
Using XRD and velocity interferometry to determine material strength in shock-compressed Diamond - 07/05/2024
Diamond has high strength, thermal conductivity, mechanical impedence, and broad optical transparency. These are attributes that make Diamand and ideal window in shock and ramp compression experiments. However, the behavior of diamond beyond the elasitc limit is unknown - suprisingly. To figure out what this behavior would entail, XRD is the only suitable technique to infer flow stress/ strength. This is because it can directly measure the strain anisotropy of Diamond while other methods use more indirect and time-consuming tactics. HEL is the applies stress where failure begins. It is relevant that below the HEL there is no permenant deformation to the material, but above it there is material failure, and subsequently stress dissipation. The objective for the experiment in this paper is demonstrating a technique that illustrates the material strength of daimond under laser shock compression up to 300 GPa. In the experiment they used an XFEL (x-ray free electron laser) on each IP (image plate) shot, and used the VISAR technology to measure shock timing and free surface velocity of each shot. The CSPAD's recorded the XRD (x-ray diffraction) from the different planes in the diamond. Results from this experiment show that there is imporvement now in determing the strength and EOS measurements of diamond when XRD and VISAR are combined. All in all, diamond was shown to keep a GPa of 20 in terms of strength for 150-300 GPa of pressure. However, the accuracy of this measurement can be improved through increasing probed volume or reducing grain size in the sample material.
In the experiment described in this article, the objective was to characterize the yield strength of diamond my meausring how single crystal and polycrystalline diamonds react under 100-1000 GPa of pressure. It was hypothesized that the strength of the diamond can influence shock wave response due to significant strength displayed during ramp wave loading. This would suggest that diamond is more resistant than previously thought. During the experiment, it was found that 6 ns pulses resulted in good shock steadiness, but a limited amount of peak stress available for data review. 1 to 4 ns pulses resulted in higher peak stress, but very little shock steadiness as shock decay was visible. Depending on the data that is being sought, it is wise to find which time period is sufficent. AR coatings as described in the reading are relevant in ensuring shock veloicty can be adequately determined. When there is a poor coating, shock velocity is determined by transit time of shock across quartz. When studying the results of the experiment, two wave structures were observed in low stress tests. The first was was transparent, and the second was visible showing the difference between elastic and inelastic waves. The shock velocity was determined from both waves to accurately update the data. After gathering the particle veloicty, stress, and density of these waves in addition to impedence matching, and hugoniot approximations a conclusion was formed. Single crystal and polycrystalline diamond are comparable under high strain-rate dynamic loading.
The response of shock compressed diamond is widely assumed to be hydrodynamic under multimegabar stresses. The lateral stress is assumed to be equal to the longitudinal stress. This has been the accepted "status quo" in high stress shock wave literature. However, it was discovered that this assumpiton is invalid when the deviatoric stress can not be ignored. Deviatoric stress is the logitudinal stress minus the mean pressure. At higher pressures above Mbar, it has been found that the hydrodynamic approximation can be used because the deviatoric stress is negligble. However, this is only reasonable for metals and other types of solid at high pressures. For stronger more stiff materials, it remains an unresolved scientific issue. With diamond as the target of the experiement, it brings in several questions regarding these discoveries. At what pressures does the hydrodynamic approximation stay vaild for diamond? At multi-bar pressures, how would the crystal anisotropy manifest in shocked diamond? If the hydrostatic assumption is unvalid, how can we understand/model the inelastic respons of diamond after shock? In the experiment to answer these question, they got synthetic diamond samples that used either LiF or SiO2 as a window. AR coating were used on both, and the VISAR measured velocity of reflective surfaces in the sample/shock wave transit times. (Three different orientations of diamond were used). The conclusions of this experiment was that the two wave structure from the [110] and [111] diamond and the strong orientation dependence of the elastic waves speeds and amplitude shows that the hydrodynamic approximation is not valid for diamond (single crystal) below high enough pressures to melt.