【作者】 朱玲玲；

【导师】 崔得良；

【作者基本信息】 山东大学 ， 材料物理与化学， 2010， 博士

【摘要】 通过借鉴溶剂方法合成纳米氮化硼的经验,我们首次利用溶剂对氮化硼的粒径和结晶质量进行了调控,并进一步将恒压溶剂热方法应用于纳米氮化硼的可控制备,初步探讨了合成tBN过程中的关键影响因素和机理。在实现可控制备氮化硼纳米颗粒的基础上,系统研究了在液态环境中纳米氮化硼的相变过程,探索了这个过程中的基本规律及影响因素。另一方面,我们首次研究了水热热压过程中纳米氮化硼的相变规律,为以工业品hBN纳米晶为原料低成本制备wBN和cBN奠定了基础。在前期优化实验条件的基础上,采用溶剂热方法,以BCl3和Li3N为原料合成了氮化硼纳米颗粒。利用苯和吡啶性质的差异,在250℃制备了具有不同粒径和结晶质量的两种aBN；进一步地,我们首次将恒压溶剂热方法应用于BCl3和Li3N的反应,在相对独立地调控温度和压力条件下,高产率制备了tBN(最高产率为63%)。系统研究了实验过程中各项参数对制备tBN的影响,初步提出了恒压溶剂热条件下tBN的生成机理。为了实现温和条件下cBN纳米颗粒的制备,我们系统研究了氮化硼纳米颗粒在液态环境中的相变规律及其影响因素。结果表明,氮化硼纳米颗粒在液态环境中相变为wBN和cBN时所需的温度和压力与固态环境下相比明显降低,相变过程在温和条件下就可以完成。另外,通过优选溶剂和降低原料的颗粒尺寸及结晶度可以进一步降低相变温度和压力。以SM-Ⅰ(主要物相为aBN,颗粒粒度不均匀)为原料时,氮化硼纳米颗粒在吡啶、苯胺和氯苯中热压后得到的样品中wBN和cBN的相对含量之和的最大值分别为34.7%、37.1%和45%,表明吡啶、苯胺和氯苯对氮化硼sp2→sp3相变的促进能力依次增强。此外,纳米氮化硼的起始状态对wBN和cBN的生成至关重要,在三种原料SM-Ⅰ、SM-Ⅱ和SM-Ⅲ中,原料SM-Ⅱ(主相为aBN,粒度均匀、结晶质量较差)中的纳米颗粒更容易发生向wBN和cBN的转变,在260℃和150 MPa下热压15 h后,wBN和cBN的相对含量之和达到91.3%；相比之下,原料SM-Ⅲ(主相为tBN,结晶质量较高)中的tBN向wBN和cBN的转变最为困难,样品中wBN和cBN的相对含量之和最大值仅为13.5%。为了探索更有利于wBN和cBN生成的途径,降低液态相变方法制备cBN的成本,我们首次以工业品hBN纳米晶(平均粒径～50 nm)为原料,研究了水热热压过程中hBN纳米晶与H2O的反应性质,以及该过程中BN样品中的相变规律。结果表明,由于颗粒尺寸减小,hBN纳米晶与H2O在温和条件下即可发生反应,而且反应程度随温度、压力及反应时间的变化发生显著变化。此外,在水热热压过程中存在hBN→tBN→aBN的相变：hBN纳米晶在270℃以上热压时转变为tBN,并随着热压温度升高到310℃进一步转变成aBN。与此同时,在aBN样品中出现了少量cBN晶核。为了进一步提高样品中cBN的相对含量,我们用LiCl/NaF饱和水溶液取代水,结果发现卤离子的加入可促进sp2态BN向sp3态BN转变。在温和条件下实现纯cBN纳米颗粒的低成本制备具有很大的实际应用价值,为此我们提出了新的液态相变方法和水热热压相变方法,并利用它们开展了纳米氮化硼相变规律的初步探索。尽管这些方法还远未达到完善的程度,但是它们为立方氮化硼等超硬材料的制备提供了一种温和、经济且更为可控的途径。更多还原

【Abstract】 On the basis of analyzing the previous results of synthesizing BN nanoparticles by solvothermal method, we have prepared BN nanoparticles with specific particle size and crystallinity by using different solvents. Besides, a new constant-pressure solvothermal method was applied to the synthesis of tBN, and the key factors in the synthesis procedure were investigated. Furthermore, the phase transformation of BN nanoparticles when being hot-pressed in solvents was systematically studied, and the key affecting factors have been explored. On the other hand, the phase transformation of hBN induced by hydrothermal hot-pressing process was firstly studied. The detailed results are as following:By optimizing the preparation parameters, two kinds of aBN with different particle size and crystallinity were synthesized at 250℃by solvothermal method, using BCl3 and Li3N as the starting materials. After that, a constant-pressure solvothermal method was firstly used in the reaction of BCl3 and Li3N. By using this route, tBN was synthesized in a controllable way, and the yield was as high as 63%. Based on the experimental results, a preliminary model for explaining the reaction mechanism was proposed.In order to synthesize pure cBN nanoparticles under mild conditions, the phase transformation of boron nitride nanoparticles in solvents was investigated, and the key factors were comprehensively explored. The results indicate that the required temperature and pressure for the formation of wBN and cBN were greatly decreased in solvents, and the phase transformation completed at rather moderate conditions. Besides, the temperature and pressure for BN transformation could be further decreased by optimizing solvent and decreasing the particle size and crystallinity of BN nanoparticles. When sample SM-II was used as the starting material, the content of wBN and cBN in the sample prepared by hot-pressing in pyridine, aniline and chlorobenzene 34.7%,37.1% and 45%, respectively. This result reveals that the induction effects on the transformation from sp2 to sp3 BN was as follows: chlorobenzene> aniline> pyridine. Additionally, the initial state of starting material also played a key role on the formation of wBN and cBN. Among the three starting materials SM-Ⅰ, SM-Ⅱand SM-Ⅲ, BN in SM-Ⅱtransformed into wBN and cBN under the modest conditions, and the content of wBN and cBN in the sample obtained by hot-pressing SM-II at 260℃and 150 MPa was as high as 91.3%. In comparison, the transformation from tBN to wBN and cBN in SM-III was rather difficult, and the maximum content of wBN and cBN was only 13.5%.On the purpose of synthesizing wBN and cBN at more moderate conditions, the reaction between hBN nanocrystals and H2O, as well as the phase transformation induced by hydrothermal hot-pressing process, were firstly studied by using commercial hBN nanocrystals as the starting material. The results indicate that hBN reacted with H2O and ammonium borate formed under very mild condition. Due to the decrease of the particle size, this reaction was greatly accelerated with the increase of temperature and pressure, as well as the prolonging of hot-pressing duration. Furthermore, a phase transformation of hBN→tBN→aBN occurred during the hydrothermal hot-pressing of hBN nanocrystals, which was transformed into tBN at the temperature above 270℃, and then transformed into aBN as the temperature further increased to 310℃. Meanwhile, a small amount of cBN nucleus was observed in the BN sample. In order to increase the content of cBN, LiCl/NaF aqueous solution was used as the liquid medium, which resulting in the formation of relative large amount of sp3-bonded BN.With the purpose of synthesizing pure cBN using commercial hBN nanocrystals under mild conditions, we developed a new constant-pressure solvothermal synthesis method and a hydrothermal hot-pressing method, the phase transformation of BN nanoparticles under moderate conditions was also explored. Although these methods are still far from being perfect, at least they provided us the possibility of synthesizing cBN in a moderate and controllable way.更多还原