New Experimental Results on the Phase Diagram of Boron Nitride (original) (raw)
Related papers
Chemistry of Materials, 1993
Graphitic BCzN has been compressed with Co metal at a pressure of 5.5 GPa and temperatures of 1400-1600 OC. The principal resulting products were crystals (average dimension 3 pm) with cubiclike facets. The powder X-ray diffraction pattern revealed two kinds of cubic phase, in approximately equal amounts, which were identified as diamond and cBN on the basis of their lattice parameters. Microelemental analyses on individual crystal fragments by K-edge electron energy-loss spectroscopy confirmed this disproportionating crystallization scheme: half of the grains were composed of carbon-only signals of which gave fine-structure characteristic of sp3 bonding and the other half gave spectra characteristic of sp3 boron and nitrogen. The crystallization of cBN as well as diamond in the catalytic solvent of pure Co metal is observed here for the first time and is of relevance to the mechanism of the accepted catalytic action of cobalt on the hexagonal/cubic transformation.
Europhysics Letters (epl), 2004
One of the challenges in characterization of strain-induced transformations is to create uniform pressure. In this letter, conditions for nearly homogeneous pressure distribution are predicted and achieved experimentally. Compared to hydrostatic loading, plastic shear generally reduces the transformation pressure significantly. We observed, however, an unexpected phenomenon: the transformation of hexagonal to superhard wurtzitic BN under pressure and shear initiated at a pressure comparable to that in hydrostatic compression (∼ 10 GPa). In situ X-ray diffraction revealed that plastic shear increases the disorder, while hydrostatic compression does not. This increase neutralizes the transition pressure reduction caused by shear. For the same disorder, shear reduced the transformation pressure significantly, and caused a complete, irreversible transformation.
One of the challenges in characterization of strain-induced transformations is to create uniform pressure. In this letter, conditions for nearly homogeneous pressure distribution are predicted and achieved experimentally. Compared to hydrostatic loading, plastic shear generally reduces the transformation pressure significantly. We observed, however, an unexpected phenomenon: the transformation of hexagonal to superhard wurtzitic BN under pressure and shear initiated at a pressure comparable to that in hydrostatic compression (∼ 10 GPa). In situ X-ray diffraction revealed that plastic shear increases the disorder, while hydrostatic compression does not. This increase neutralizes the transition pressure reduction caused by shear. For the same disorder, shear reduced the transformation pressure significantly, and caused a complete, irreversible transformation.
Epitaxy of cubic boron nitride on (001)-oriented diamond
Nature Materials, 2003
C ubic boron nitride (c-BN),although offering a number of highly attractive properties comparable to diamond, like hardness, chemical inertness and a large electronic bandgap, up to now has not found the attention it deserves. This mostly has to do with preparational problems, with easy chemical routes not available and, instead, the necessity to apply ion-bombardment-assisted methods. Hence, most of the c-BN samples prepared as thin films have been nanocrystalline, making the prospect of using this material for high-temperature electronic applications an illusion. Although heteroepitaxial nucleation of c-BN on diamond substrates has been demonstrated using the high-pressure-hightemperature technique 1,2 , none of the low-pressure methods ever succeeded in the epitaxial growth of c-BN on any substrate.Here,we demonstrate that heteroepitaxial c-BN films can be prepared at 900°C on highly (001)-oriented diamond films, formed by chemical vapour deposition, using ion-beam-assisted deposition as a low-pressure technique. The orientation relationship was found to be c-BN(001)[100]||diamond(001)[100]. High-resolution transmission electron microscopy additionally proved that epitaxy can be achieved without an intermediate hexagonal BN layer that is commonly observed 3 on various substrates. Beside their potential use as protective coatings for cutting tools and optical instruments, cubic boron nitride (c-BN) films have been considered as an ideal material for electronic devices applicable at high temperatures, because it is a wide-bandgap semiconductor and, unlike diamond, it can be doped both n-and p-type. During the past few years, thin c-BN films have been deposited by a variety of experimental approaches 4-9 .However,most of these films were composed of very small grains (some nanometres in size) resulting in a high density of defects and grain boundaries. A significant improvement in film quality could be
The crystallographic texture of graphite-like and diamond-like boron nitride bulk materials
2001
X-ray powder diffraction and quantitative texture analysis have been employed to study the crystallographic relation of parent and product phases in direct transformations of graphite-like BN polymorphs into diamond-like BN phases at a pressure of 7.7 GPa and temperatures up to 2600 K. It has been found that the transformation of both CVD-produced bulk slightly textured turbostratic BN (tBN) and highly textured graphite-like hexagonal BN (hBN) results in a preferred orientation of the crystallites of the new cubic BN (cBN) due to the (111) cBN (001) tBN, hBN epitaxial relation. Phase transformation of a CVD-produced bulk highly textured rhombohedral BN (rBN) results in the formation of highly textured wurtzitic BN (wBN) according to the wBN (001) rBN crystallographic relation. C 2001 Kluwer Academic Publishers
Features of a cBN-to-graphite-like BN phase transformation under pressure
Diamond and Related Materials, 2000
Phase transformations of cubic boron nitride to graphite-like forms cBNª gBN have been studied within the 1670᎐2970 K temperature range and at high pressures, which suppresses the dissociation decomposition of the compound, by using a recessed-type high pressure apparatus with a toroid-type lock. To clarify the degree of structure imperfection effect on the type and mechanism of transformation, various types of bulk cBN initial materials were used. A partial or complete cBNª gBN transformation has been observed in the studied temperature range. It has been found that in a pure recrystallized material with a low level of the structural imperfection, a two-stage transformation occurs to form an intermediate metastable rhombohedral Ž. Ž. phase rBN. Transformation into the hexagonal modification of graphite-like boron nitride hBN is realized only in material with a highly imperfect structure. From the analysis of the crystallographic correspondence between cBN, rBN and hBN lattices, and the differences in structure between the initial and forming materials, the possible structural mechanisms of the cBNª gBN transformation were proposed.
High-pressure / High-temperature Synthesis and Characterization of Boron-doped Diamond
Zeitschrift für Naturforschung B, 2006
Bulk samples (with volumes up to ∼ 7.5 mm 3 ) of boron-doped diamonds (BDD) were synthesized by means of direct reaction between boron carbide and graphite in a multianvil apparatus at high pressures and high temperatures (HPHT). X-ray diffraction data revealed the presence in BDD of a very small amount of a highly boron-enriched phase (B 50 C 2 ) and traces of the B 13 C 2 used as an initial material. The absence of B 50 C 2 in the product of treatment of pure B 13 C 2 under the same HPHT conditions suggests that boron-rich carbides exsolute from diamond on quenching leading to boron depletion of the diamond matrix. These observations imply that boron solubility in diamond increases at high pressure and high temperature. This result may have important implications for the understanding of the mechanism of boron incorporation into diamond at HPHT synthesis and for the interpretation of the data on superconductivity of polycrystalline BDD.
Plastic shear significantly reduces the phase transformation ͑PT͒ pressure when compared to hydrostatic conditions. Here, a paradoxical result was obtained: PT of graphitelike hexagonal boron nitride ͑hBN͒ to superhard wurtzitic boron nitride under pressure and shear started at about the same pressure ͑ϳ10 GPa͒ as under hydrostatic conditions. In situ x-ray diffraction measurement and modeling of the turbostratic stacking fault concentration ͑degree of disorder͒ and PT in hBN were performed. Under hydrostatic pressure, changes in the disorder were negligible. Under a complex compression and shear loading program, a strain-induced disorder was observed and quantitatively characterized. It is found that the strain-induced disorder suppresses PT which compensates the promotion effect of plastic shear. The existence of transformation-induced plasticity ͑TRIP͒ was also proved during strain-induced PT. The degree of disorder is proposed to be used as a physical measure of plastic straining. This allows us to quantitatively separate the conventional plasticity and transformation-induced plasticity. Surprisingly, it is found that TRIP exceeds the conventional plasticity by a factor of 20. The cascade structural changes were revealed, defined as the reoccurrence of interacting processes including PTs, disordering, conventional plasticity, and TRIP. In comparison with hydrostatic loading, for the same degree of disorder, plastic shear indeed reduces the PT pressure ͑by a factor of 3-4͒ while causing a complete irreversible PT. The analytical results based on coupled strain-controlled kinetic equations for disorder and PT confirm our conclusions.
The Journal of Chemical Physics, 2006
Plastic shear significantly reduces the phase transformation ͑PT͒ pressure when compared to hydrostatic conditions. Here, a paradoxical result was obtained: PT of graphitelike hexagonal boron nitride ͑hBN͒ to superhard wurtzitic boron nitride under pressure and shear started at about the same pressure ͑ϳ10 GPa͒ as under hydrostatic conditions. In situ x-ray diffraction measurement and modeling of the turbostratic stacking fault concentration ͑degree of disorder͒ and PT in hBN were performed. Under hydrostatic pressure, changes in the disorder were negligible. Under a complex compression and shear loading program, a strain-induced disorder was observed and quantitatively characterized. It is found that the strain-induced disorder suppresses PT which compensates the promotion effect of plastic shear. The existence of transformation-induced plasticity ͑TRIP͒ was also proved during strain-induced PT. The degree of disorder is proposed to be used as a physical measure of plastic straining. This allows us to quantitatively separate the conventional plasticity and transformation-induced plasticity. Surprisingly, it is found that TRIP exceeds the conventional plasticity by a factor of 20. The cascade structural changes were revealed, defined as the reoccurrence of interacting processes including PTs, disordering, conventional plasticity, and TRIP. In comparison with hydrostatic loading, for the same degree of disorder, plastic shear indeed reduces the PT pressure ͑by a factor of 3-4͒ while causing a complete irreversible PT. The analytical results based on coupled strain-controlled kinetic equations for disorder and PT confirm our conclusions.