The search for alternative methods of synthesizing cubic boron nitride (cBN), one of
the hardest known materials, at low thermo-baric conditions has stimulated considerable
research interest due to its great potential for numerous practical industrial applications.
The practical applications are motivated by the material’s amazing combination of extraordinarily superior properties.
The cBN phase is presently being synthesized from
graphite-like boron nitride modifications at high thermo-baric conditions in the presence
of catalytic solvents or by ion–beam assisted (chemical and physical) deposition methods.
However, the potential and performance of cBN have not been fully realized largely due
to central problems arising from the aforementioned synthesis methods.
The work reported in this dissertation is inspired by the extensive theoretical investigation of the influence of defects in a ecting the transformation of the hexagonal boron nitride (hBN) phase to the cBN phase that was carried out by Mosuang and Lowther (Phys
Rev B 66, 014112 (2002)). From their investigation, using an ab-initio local density approach, for the B, C, N, and O simple defects in hBN, they concluded that the defects
introduced into hBN could facilitate a low activation–energy hexagonal-to-cubic boron
nitride phase transformation, under less extreme conditions.
We use ion implantation as a technique of choice for introducing ‘controlled’ defects
into the hot–pressed polycrystalline 99.9% hBN powder samples. The reasons are that the
technique is non–equilibrium (not influenced by dilusion laws) and controllable, that is
the species of ions, their energy and number introduced per unit area can be changed and
monitored easily. We investigate the structural modifications of hBN by ion implantation.
Emphasis is given to the possibilities of influencing a low activation–energy hBN-to-cBN
phase transformation. The characterization of the structural modifications induced to the
hBN samples by implanting with He+ ions of energies ranging between 200 keV and 1.2
MeV, at fluences of up to 1.0 1017 ionscm2, was accomplished by correlating results
from X-Ray Di raction (XRD), micro-Raman (-Raman) spectroscopy measurements,
and two-dimensional X-Y Raman (2D-Raman) mapping measurements. The surface to
pography of the samples was investigated using Scanning Electron Microscopy (SEM).
E orts to use Surface Brillouin Scattering (SBS) were hampered by the transparency of
the samples to the laser light as well as the large degree of surface roughness. All the
implantations were carried out at room temperature under high vacuum.
2D-Raman mapping and -Raman spectroscopy measurements done before and after
He+ ion irradiation show that an induced hBN-to-cBN phase transformation is possible:
nanocrystals of cBN have been observed to have nucleated as a consequence of ion implantation,the extent of which is dictated by the fluences of implantation.
The deviationof the measured spectra from the Raman spectra of single crystal cBN is expected, has been observed before and been attributed to phonon confinement e ects. Also observed are phase transformations from the pre-existing hBN modification to: (a) the amorphous boron nitride (aBN), (b) the rhombohedral boron nitride (rBN) modifications, (c) crystalline and amorphous boron clusters, which are a result of the agglomeration of elementary boron during and immediately after ion implantation.
These transformations were observed at high energies. Unfortunately, the XRD measurements carried out could not complement the Raman spectroscopy outcomes probably because the respective amounts of the transformed materials were well below the detection limit of the instrument used in the former case.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/5581 |
| Date | 29 August 2008 |
| Creators | Machaka, Ronald |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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