Abstract:The possibilities of different phase transitions to cBN with Li 3 N as catalyst at high temperature and high pressure (1600-2200 K, 4.8-6.0 GPa) are analyzed, in the framework of the second law of thermodynamics. The Gibbs free energy (∆G) of three reactions which may happen in the Li 3 N-BN system: hBN + Li 3 N→Li 3 BN 2 , hBN→cBN, and Li 3 BN 2 →cBN + Li 3 N, is calculated, with the influence of high temperature and high pressure on volume included. We show that ∆G of hBN + Li 3 N→Li 3 BN 2 and hBN→cBN are between −35~−10 KJ·mol , respectively. However, ∆G of Li 3 BN 2 →cBN + Li 3 N can be positive or negative. The area formed by the positive data is a V-shaped area, which covers the most part of the cBN growing V-shaped area. It confirms that Li 3 BN 2 is stable in the P-T area of cBN synthesis, and cBN is probably transformed directly from hBN. Analysis suggests that Li 3 BN 2 promotes the transition from hBN to cBN.
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.
Photocatalytic degradation plays a crucial role in wastewater treatment, and the key to achieving high efficiency is to develop photocatalytic systems that possess excellent light absorption, carrier separation efficiency, and surface-active sites. Among various photocatalytic systems, S-type heterojunctions have shown remarkable potential for efficient degradation. This work delves into construction of S-type heterojunctions of ternary indium metal sulfide and bismuth ferrite nanofibers with introduction of sulfur vacancy defects and morphology modifications to enhance the photocatalytic degradation performance. Through the impregnation method, BiFeO3/Zn2S4 heterojunction materials were synthesized and optimized30% BiFeO3/Zn2S4 heterojunction exhibited superior photocatalytic performance with higher sulfur vacancy concentration than Zn2S4. The in-situ XPS results demonstrate that the electrons between Zn2S4 and BFO are transferred via S-Scheme, and after modification, Zn2S4 has a more favorable surface morphology for electron transport, and its flower-like structure interacts with the nanofibers of BFO, which has a further enhancement of the reaction efficiency for degrading pollutants. This exceptional material demonstrated a remarkable 99% degradation of Evans blue within 45 min and a significant 68% degradation of ciprofloxacin within 90 min. This work provides a feasible idea for developing photocatalysts to deal with the problem of polluted water resources under practical conditions.
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