The near-Surface Region of Cubic Boron Nitride Single Crystal from the Li ₃ N-hBN System

Abstract

Cubic boron nitride single crystals are synthesized with lithium nitride as a catalyst under high pressure and high temperature. The main phases in the near-surface region, which around the single crystal are determined as a mixture of hexagonal boron nitride (hBN), cubic boron nitride (cBN) and lithium boron nitride (Li3BN2). High resolution transmission electron microscopy examinations show that there exist lots of nanometer-sized cubic boron nitride nuclei in this region. The interface phase structures of cubic boron nitride crystal and its near-surface region are investigated by means of transmission electron microscopy. The growth mechanism of cubic boron nitride crystal is analyzed briefly. It is supposed that Li3BN2 impels the direct conversion of hBN to cBN as a real catalyst, and cBN is homogeneously nucleated in the molten state under high pressure and high temperature.

New Insights into the Mechanism for Growing Cubic Boron Nitride Crystals under High Pressure

“…This indicates that the nucleation of cBN is related to such-recrystallized hBN. A similar phenomenon has been previously observed. ,, …”

“…A similar phenomenon has been previously observed. 18,42,43 Considering the success of the seed method in growing large crystals of diamond, 48−51 this work tested the feasibility for preparing cBN crystals, as shown in Figure 3a. The aspurchased cBN crystals with sizes of 0.9−1.2 mm are used as seeds, and they are in an octahedral shape with well-defined facets of (111) planes and display a dark-yellow color (Figure 3b), due to defects.…”

“…An intermediate pressure value of ∼4.5 GPa is observed for Li 3 N. According to these P – T diagrams, the lower limit of formation temperature for Li 3 N, Mg 3 N 2 , and Ca 3 N 2 is in the 1300–1350 °C range, slightly higher than that of Sr 3 N 2 (i.e., 1200 °C). Such variations in the formation pressure and temperature are likely because the different intermediate ternaries of M–B–N (M = Li, Mg, Ca, and Sr) are produced, ,,,,,− given by normalL normali 3 normalN + h B N = normalL normali 3 normalB normalN 2 M 3 N 2 + 2 h B N = M 3 B 2 N 4 ( M = normalM normalg , normalC normala , and normalS normalr ) …”

“…These facts indicate that cBN may also form in a similar dissolution−precipitation process to that of diamond, 39−41 rather than a solid-state reaction. 42,43 However, experimental evidence that supports such a picture for the formation of cBN is still sparse, calling for more experimental efforts into the associated matter.…”

“…According to a number of recent experiments, intermediate phases are produced with possible compositions of Li 3 BN 2 and M 3 B 2 N 4 , which are necessary ingredients for the formation of cBN. ,,, cBN starts to form in the vicinity of melting of those intermediate phases, as previously observed by means of in situ high- P – T synchrotron X-ray diffraction (XRD) experiments. , In this regard, cBN crystals are likely nucleated from a eutectic melt of Li 3 BN 2 (BN) n or M 3 B 2 N 4 (BN) n , by which the B–N dimers are transported for the continuous growth of crystals. These facts indicate that cBN may also form in a similar dissolution–precipitation process to that of diamond, − rather than a solid-state reaction. , However, experimental evidence that supports such a picture for the formation of cBN is still sparse, calling for more experimental efforts into the associated matter.…”