Dissertation
Shock-induced graphite to diamond transformation: role of graphite crystal structure and microstructure
Doctor of Philosophy (PhD), Washington State University
01/2020
Handle:
https://hdl.handle.net/2376/111551
Abstract
Plane shock wave experiments were performed on graphite samples to examine the effects of initial crystal structure and microstructure on the shock-induced graphite to diamond phase transformation. Three graphite types, with a range of microstructures, were studied: as-deposited pyrolytic graphite (PG), ZYH-grade highly oriented pyrolytic graphite (HOPG), and ZYB-grade HOPG. Three types of plate impact experiments were used to quantify the transformation processes and the high pressure phases achieved for each graphite type.
Laser interferometry was used to measure shock speeds and transmitted wave profiles during shock compression. Transformation stresses, peak states, and transformation timescales were determined for each graphite type from these data. Wave transmission experiments showed that the ZYB and ZYH-grade HOPG have comparable wave structures above the ~22 GPa transformation stress. In PG, the phase transition occurs at ~46 GPa, more than double the HOPG transition stress. For all graphite types, regardless of microstructure, rapid transformations were observed and the peak states were consistent with the diamond response.
Real-time in situ X-ray diffraction (XRD) measurements were used to probe the crystal structures during shock compression below and above the phase transformation stress for each graphite type. Ambient XRD data showed significant differences between HOPG and PG: HOPG crystallites had a hexagonal graphite structure, whereas PG crystallites had a more disordered turbostratic carbon structure. Below their respective phase transformation stresses, XRD measurements on each graphite type showed compression mainly along the crystallite c-axis. Above the phase transformation stresses, XRD measurements on both HOPG types showed the unambiguous transformation to hexagonal diamond (HD); in contrast, XRD measurements on PG showed the formation of nanocrystalline cubic diamond (CD).
Front-surface impact experiments were used to measure the sound speeds in the peak states for each graphite type and to establish the longitudinal moduli of the high pressure phases in the shocked state. The longitudinal moduli of the HD phases formed from HOPG were significantly larger than ambient elastic CD values showing that HD is stiffer than ambient CD. The longitudinal moduli of the CD phases formed from PG were consistent with or slightly lower than ambient CD values.
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Details
- Title
- Shock-induced graphite to diamond transformation: role of graphite crystal structure and microstructure
- Creators
- Travis Jay Volz
- Contributors
- Yogendra M Gupta (Advisor)Matthew D McCluskey (Committee Member)James A Hawreliak (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Physics and Astronomy, Department of
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Number of pages
- 357
- Identifiers
- 99900581704101842
- Language
- English
- Resource Type
- Dissertation