Shear-induced phase transition of nanocrystalline hexagonal boron nitride to wurtzitic structure at room temperature and lower pressure
Shear-induced phase transition of nanocrystalline hexagonal boron nitride to wurtzitic structure at room temperature and lower pressure
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DOI：
10.1073/pnas.1214976109
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发表时间：
2012-11-20
影响因子：
11.1
通讯作者：
Ma, Yanzhang
中科院分区：
文献类型：
--
作者：
Ji, Cheng;Levitas, Valery I.;Ma, Yanzhang
氮化硼（BN）、石墨、碳化硼（BC）和氮化硼碳（BCN）体系的无序结构被认为是这些体系中合成超硬相的重要前驱体材料。然而，这种材料的相变只能在极端的压力-温度条件下实现，这与工业应用无关。通过原位同步x射线衍射（XRD）测量，在室温6.7 GPa的压力下，对旋转金刚石顶槽（RDAC）施加大塑性剪切后，发现无序纳米晶六方BN (h)向超硬武锌矿BN (w)转变。然而，在静水压缩至52.8 GPa时，相同的hBN样品并没有转变为wBN，而可能经历了向高压无序相的可逆转变，并形成封闭的屈曲层。目前的相变压力是所有报道的室温下从hBN到wBN直接相变中最低的。通常，大的塑性应变会导致无序和非晶化；相反，在这里，高度无序的hBN转变为结晶的wBN。讨论了应变诱导相变的机理和相变压力低的原因。我们的研究结果表明，在塑料剪切作用下，从无序或非晶态前驱体合成超硬材料具有低压-室温的潜力。它们也为纳米晶材料和无序非晶材料在广泛剪切作用下的相变开辟了一条途径。
Disordered structures of boron nitride (BN), graphite, boron carbide (BC), and boron carbon nitride (BCN) systems are considered important precursor materials for synthesis of superhard phases in these systems. However, phase transformation of such materials can be achieved only at extreme pressure-temperature conditions, which is irrelevant to industrial applications. Here, the phase transition from disordered nanocrystalline hexagonal (h) BN to superhard wurtzitic (w)BN was found at room temperature under a pressure of 6.7 GPa after applying large plastic shear in a rotational diamond anvil cell (RDAC) monitored by in situ synchrotron X-ray diffraction (XRD) measurements. However, under hydrostatic compression to 52.8 GPa, the same hBN sample did not transform to wBN but probably underwent a reversible transformation to a high-pressure disordered phase with closed-packed buckled layers. The current phase-transition pressure is the lowest among all reported direct-phase transitions from hBN to wBN at room temperature. Usually, large plastic straining leads to disordering and amorphization; here, in contrast, highly disordered hBN transformed to crystalline wBN. The mechanisms of strain-induced phase transformation and the reasons for such a low transformation pressure are discussed. Our results demonstrate a potential of low pressure-room temperature synthesis of superhard materials under plastic shear from disordered or amorphous precursors. They also open a pathway of phase transformation of nanocrystalline materials and materials with disordered and amorphous structures under extensive shear.