【作者】 李倩；

【导师】 贺端威；

【作者基本信息】 四川大学 ， 原子与分子物理， 2023， 硕士

【摘要】 高压科学与技术一直与碳元素密切相关,碳是宇宙中非常丰富的元素,在生物圈中尤其如此。上世纪人们就已经发现金刚石和石墨都是碳的同素异形体,金刚石在正常环境条件下是一种热力学不稳定形式,但由于其高的内聚能和活化能,可以在石墨的热力学稳定区无限期存活。值得注意的是,金刚石具有最佳的机械、光学、电学等性能,不仅可以作为奢侈品,还被广泛地应用于机械加工、油气钻探、电子器件、医疗器械等领域,所以一直让人们为之着迷。在合成金刚石方面,科学家们付出了大量的努力,并且在高压研究方面也取得了许多技术进展。然而实验表明,在高温和各种压力下,金刚石也会转换为石墨,使其组分构成、晶体结构、晶界形态产生显著变化,这对于含金刚石材料的生产和应用是一个非常关键的问题。在实际的金刚石向石墨转变过程中,不仅需要满足运行的“热力学许可”,反应速率也非常重要,这通常受催化剂、缺陷结构、晶面类型、粒度、金刚石类型等的影响。而前人也做了大量的研究并总结出一定的规律,但理论和实验工作仍在继续,因为实现两者相变的原子机制尚未完全理解。众所周知,金刚石在高压压缩过程中通常处于非静水压状态,即在高压下金刚石颗粒间应力分布不均匀。而且通过高温高压法烧结聚晶金刚石时也会发现在颗粒之间未接触的低应力区会出现石墨化,导致高质量聚晶金刚石难以制备。此外在碳酸盐熔体存在的情况下,静水压力的增加也可以保护金刚石免受石墨化的影响,这可能是因为在静水压力下,金刚石更接近其稳定的温度-压力区间。总的来说,应力状态也是影响金刚石石墨化起始温度及石墨化速率的一个重要因素,但目前所有的解释都是基于实验的定性分析或猜测,并没有真实有效的定量数据作为支撑。本研究主要针对应力状态这一因素,定量研究金刚石在高温高压处理后的石墨化行为,根据实验结果再结合碳的相图,在实验条件允许的范围内,通过控制粒径分布来改善样品的应力状态,从而提高金刚石-石墨结构转变的温度点,减少多晶金刚石烧结体中石墨相的存在。本研究中的高温高压实验是在本实验室DS 6×14 MN和DS 6×8 MN两台铰链式六面顶压机上完成的,在高压(5 GPa和10 GPa)和高温(1000–2700℃)下,以纯金刚石粉末(对应于非静水压应力环境)和金刚石与氯化钠的混合粉末(对应于准静水压应力环境)作为前驱体处理,随后利用X射线衍射仪和扫描电子显微镜对回收样品中的石墨化进行初步测试与分析,探索应力状态对高温高压下金刚石石墨化行为的影响。为了对石墨进行定量分析,我们采用X射线衍射外标法,将纯金刚石粉末和石墨粉末以不同体积百分比混合后进行XRD测试,绘制关于石墨含量与石墨衍射峰面积的定标曲线。根据定标曲线和回收样品的X射线衍射谱定量分析金刚石的石墨化情况。最后将两种不同粒径(10μm和0.5μm)的金刚石混合粉末在10 GPa、1800℃条件下处理,与同样条件下单一粒径(10μm)的金刚石粉末烧结体进行对比分析,进一步验证应力状态对金刚石石墨化行为的影响。实验结果表明,在非静水压压缩下,由于金刚石晶粒间的应力分布不均匀,其石墨化更容易被触发,即起始石墨化温度相对较低。在5 GPa准静水压压缩下,石墨化起始温度约为1600℃,比非静水压压缩下的石墨化温度高约300℃。在10 GPa非静水压压缩下,1400℃开始观察到金刚石的石墨化,但是在准静水压压缩下即使升高温度至2300℃,也没有观察到石墨化现象。通过混合粒径来优化金刚石晶粒之间的应力分布,结果表明即使在非静水压环境下,也可以显著降低高温下金刚石的石墨化程度。以上的研究结果能帮助我们进一步理解金刚石-石墨相变机理与动力学过程,且可降低或者避免石墨化对于制备含金刚石材料的影响,为金刚石及其复合材料的生产与应用提供数据参考和指导。更多还原

【Abstract】 High-pressure science and technology have always been closely related to carbon,which is a very rich element in the universe,especially in the biosphere.In the last century,it has been discovered that diamond and graphite are allotropes of carbon.Diamond is a thermodynamic unstable form under normal environmental conditions,but it can survive indefinitely in the thermodynamic stable region of graphite due to its high cohesive energy and activation energy.It is worth noting that diamond have the best mechanical,optical,electrical and other properties,not only as luxury goods,but also widely used in machining,oil and gas drilling,electronic equipments,medical equipments,and other fields,so people have always been fascinated by them.Scientists have made a lot of efforts in the synthesis of diamond,and there are many technological advances in high-pressure research.However,experiments have shown that diamond can be converted into graphite at high temperatures and various pressures,resulting in significant changes in its composition,crystal structure,and grain boundary morphology,which is a crucial issue for the production and application of diamond containing materials.In the actual process of diamond to graphite transformation,it is not only necessary to meet the "thermodynamic permission" of operation,but also the reaction rate is very important,which is usually affected by catalyst,defect structure,crystal plane type,particle size,diamond type and so on.While previous researchers have also done a lot of research and summarized certain laws,theoretical and experimental work is still continuing because the atomic mechanism that enables phase transitions between the two is not yet fully understood.As is well known,diamond is usually in a state of non-hydrostatic during high-pressure compression,which means that the stress distribution between diamond particles is uneven under high pressure.Moreover,it is also found that graphitization occurs in the low-pressure region where the particles are not in contact with each other when sintering polycrystalline diamond by the high-temperature and high-pressure method,which makes it difficult to prepare high-quality polycrystalline diamond.In addition,the increase of hydrostatic pressure can also protect diamond from the influence of graphitization in the presence of carbonate melts,which may be due to the fact that diamond is closer to its stable temperature-pressure range under hydrostatic pressure.In general,the state of stress is also an important factor affecting the initial temperature of graphitization and rate of diamond graphitization.However,all current explanations are based on experimental qualitative analysis or speculation,and there is no real and effective quantitative data to support them.This study is mainly aimed at the factor of stress state,quantitatively studying the graphitization behavior of diamond after the treatment of high temperature and high pressure.Based on the experimental results and the phase diagram of carbon,the stress state of the sample is improved by controlling the particle size distribution within the allowable range of experimental conditions,and the temperature point of diamond-graphite structure transition is increased,thereby the presence of graphite phase in polycrystalline diamond sintered bodies is reduced.The high-temperature and high-pressure experiments in this study were conducted on DS 6 × 14 MN and DS 6 × 8 MN hinge-type cubic presses in our laboratory.Pure diamond powder(corresponding to non-hydrostatic stress environment)and mixed powder of diamond and sodium chloride(corresponding to quasi-hydrostatic stress environment)were used as precursors under high pressure(5GPa and 10 GPa)and high temperature(1000 – 2700 ℃).Subsequently,the graphitization in the recovered samples was preliminarily tested and analyzed by Xray diffractometer and scanning electron microscope to explore the effect of stress state on graphitization behavior of diamond under high pressure and high temperature.In order to quantitatively analyze graphite,we used an X-ray diffraction external standard method.We mixed pure diamond powder and graphite powder with different volume percentages and conducted XRD testing to draw the calibration curves of graphite content and graphite peak’ area.The graphitization of diamond was quantitatively analyzed based on the calibration curve and the X-ray diffraction spectrum of the recovered sample.Finally,mixed grain-size diamond powder(10 μm and 0.5 μm)was treated at 10 GPa and 1800 ℃,and compared with the sintered body of single grainsize(10 μm)under the same conditions to further verify the influence of stress state on graphitization behavior of diamond.The experimental results show that the graphitization of diamond under nonhydrostatic compression is much more easily triggered,in other words,the initial temperature of graphitization is relatively low,due to the uneven stress distribution within diamond grains.Under the quasi-hydrostatic compression of 5 GPa,the initial graphitization temperature is about 1600 ℃,which is about 300 ℃ higher than that under non-hydrostatic compression.Under the non-hydrostatic compression of 10 GPa,the graphitization of diamond could be observed at 1400 ℃,but no graphitization was observed even at 2300 ℃ under quasi-hydrostatic compression.The stress distribution among diamond grains is optimized by mixing particle sizes.The results show that the graphitization of diamond at high temperature can be significantly reduced even under non-hydrostatic environment.The above research results can help us further understand the mechanism and kinetic process of diamond-graphite phase transition,reduce or avoid the influence of graphitization on the preparation of diamondcontaining materials,and provide data reference and guidance for the production and application of diamond and its composite materials.更多还原