Laser Heating of Boron Nitride and Graphite in a Diamond Anvil Cell
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show that temperatures above 5000 K could be reached. Numerous traces of melting in both hBN and cBN prove that the temperature achieved was above 3290 K (the melting temperature of hBN at a nitrogen pressure of 50 MPaS). At the periphery of the laser spot the sample was nearly unheated and remained unchanged. Thus the sample was subjected to a large temperature gradient. The BN samples heated in the nitrogen high-pressure medium did not change stoichiometry as determined from Raman spectra, X-ray diffiraction patterns, and electron energy loss spectroscopy (EELS). On the other hand, if we use argon, it incorporates into the BN samples after heating 4. The recovered samples were also studied by high resolution transmission electron microscopy (HRTEM). Importantly, the HP-HT phases remain in the recovered samples because of exceptionally high hysteresis of the transitions in carbon and BN, Fig. 1. 14
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Fig. 1. The phase diagram of BN. The pressure-temperature dashed lines show equilibrium phase boundaries according to Bundy and Wentorf 6 Dash-dot lines - according to Solorzenko . The thick solid lines indicate approximate boundaries for the direct cBN-hBN and hBN-cBN
transformations. The hatched area roughly
indicates the regions where the transformations are kinetically possible. Arrows show typical
experimental runs at constant pressure.
RESULTS Melting of cubic and hexagonal BN Melting of cBN is important for establishing the phase diagram and from the point of view of obtaining alloys with diamond and doping with electrically active impurities. In the previous studies only melting of hBN at pressures below 3 GPa was achieved 3' 5. We succeeded in melting both hBN and cBN by laser heating. Decomposition of BN before the melting was inhibited by use of nitrogen as a high-pressure medium. Among different melting criteria7 one of the most obvious and reliable is the change of the sample form. The starting sample is a platelet. After heating a characteristic drop appears at the hottest part, Fig. 2,3. Because the sample was in a temperature gradient one can observe all stages of the melting. Fig.4. demonstrates the process of melting which is spatially recorded over the plane of the sample. The melting starts at the surface with the appearance of roundings at defects and sharp edges, small drops appear at the surface, then the surface as whole is melted and finally the sample melts in the bulk.
Fig.2. Melting of cubic BN in nitrogen atmosphere at pressure 9.1 GPa. SEM image (a) the starting sample, (b) the same sample after laser heating of the right part, (c) boundary between melted (upper right side) and unmelted parts of the sample. 322
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Fig.3. A sample of cBN heated at 10.3 GPa in nitrogen atmosphere. (a) A drop of melted BN was formed at the right, heated part of the sample. A boundary between the drop (upper ri
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