【作者】 张广强；

【导师】 许大鹏；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2009， 博士

【摘要】 本文首次以含Si-OH形式结构水的非晶SiO₂粉体、α-方石英和α-鳞石英作为初始原料在高温高压下研究了柯石英的合成,并考察了通过机械球磨和碳纳米管（CNMTs）掺杂等手段改变原料的初始状态对柯石英合成条件的影响,还考察了柯石英在高温高压下的结构稳定性。实验结果表明,含Si-OH结构水的非晶SiO2粉体在0-4.2GPa、0-1200℃范围内的压致晶化产物α-石英和柯石英的P-T相边界斜率为负;首次发现Si-OH对柯石英的合成有显著的催化作用,在4.2GPa压力下得到了目前最低的柯石英合成温度190℃,并且把合成时间缩短到了秒量级。模拟折返过程的实验结果表明阶梯式降温降压到常温常压的时间为40h时,柯石英完全转化为α-石英,显示柯石英在漫长的折返过程中很难保存下来。α-方石英和α-鳞石英在高温高压的结构转变研究显示,α-鳞石英比α-方石英和α-石英更容易形成柯石英。此外,还以廉价的单质Si和石墨为原料,首次加入NH₄Cl作分散剂,使用改进振动方式的高能机械球磨机制备了分散性良好的粒径约为6nm的β-SiC粉体,而且制备β-SiC可以发出显著蓝移的中心位于382nm蓝紫光。更多还原

【Abstract】 The depth we can reach at present is 12 km by drilling technology, which is less than half of the average thickness of continental crust. It is obviously not possible to directly obtain the samples of the lower layers of the earth by this method. It is very meaningful to research the structures, states, physical and chemical properties of Earth’s interior minerals by high pressure experimental techniques, because all of them exist under high-pressure and high-temperature. The formation depth of the minerals can be inferred by the high-pressure experimental data.SiO₂ is an essential component of solid Earth, which mainly exists in the Earth’s Crust as quartz mineral and in the mantle as silicate minerals. It has many kinds of polymorphs, such as quartz, tridymite, cristobalite, coesite, stishovite etc. Each polymorph can transit other polymorphs under various temperatures and various pressures, and most of these polymorphs can exist as metastable phase at room temperature and atmospheric pressure. Therefore, a lot of important geological information was recorded by the phase transition of quartz minerals, and the polymorphs are very helpful to understand the processes of minerals groups on the Earth’s surface formationCoesite, as the first polymorph of SiO₂ under high pressure, was originally synthesized at laboratory by Coes, and after a few years, natural coesite was found in Meteor aerolite crater by Chao and his colleagues. The formation mechanism of coesite in the Earth’s crust has attracted much attention since it was found in the metasedimentary rocks of the Dora-Maira massif in the Western Alps by Chopin and in the high pressure eclogites of the Western Gneiss Region in Norway by Smith. Based on the synthesis conditions of coesite, the hypothesis of plate subduction-exhumation was put forward to explain the formation mechanism of coesite existing in the Earth’s crust, supposing that the coesite was formed in the process of a plate subducting into the upper mantle, and then exhuming back to the Earth’s crust. It is obvious that the synthesis condition of coesite is the basis to presume the formation depth of coesite in the Earth’s crust. Thus, the synthesis of coesite is of great interest to understand the natural occurrence of coesite, and to unravel the conditions of its formation and preservation in ultra-high pressure rocks. The change of synthesis conditions of coesite may have wide implications for the formation of coesite in subduction zone. However, a large number of studies have found that the synthesis conditions of coesite are very closely related to the initial states of materials.In previous experimental studies about the formation of natural coesite, only static high pressure was considered while the shear stress deriving from the collisions between the plates before and during the subduction process was not, which is not complete. During the process of mechanical ball milling, both a normal and shear stress can be produced by high speed collision of the steel balls. Comparing the collision between plates and mechanical ball milling, it is found that there is a common localization of pressure including normal and shear stress. Although the differences about the collision speed and space is very great, through their similarity, these two seemingly different and unrelated collision phenomena can be related to each other. In the present paper, a laboratory method of combining mechanical ball milling and static high pressure has been suggested for modeling the synthesis of coesite. Because the material structure, composition and stress state in the Earth’s interior is very non-uniform, it is not reasonable that the Earth is seen as a uniform static fluid system. Thereby, the possibility of the existence of high pressure micro-region in the Earth’s interior must be considered.Water, as a common mineralizer in nature, exist widely in the Earth’s interior and plays a very important role on the formation of various minerals and especially on the phase transitions of SiO₂. The effect of water is not neglected in the research of the synthesis coesite under high pressure and high temperature. The effect of free water on the synthesis of coesite has been studied by Kameyama et al, and they found that the presence of free water had remarkable effects on the kinetics of crystallization of coesite. Besides free water, there is a number of structural water existing as Si-OH in many minerals. However, the effect of Si–OH on the synthesis of coesite is little known.In the present paper, we have investigated the crystallization of amorphous SiO₂ powder with silicon-hydroxy group （Si-OH） in the range of 0-4.2 GPa and 150-1200℃. The obtained samples were detected by X-ray diffraction, TEM, TG-DTA, Raman and FT-IR spectroscopy. The results show that the crystallization products are coesite andα-quartz, and the P-T boundary of the synthesizedα-quartz and coesite has been successfully obtained. The slope of our P-T boundary is negative, which is opposite to the thermodynamic equilibrium phase boundary of coesite andα-quartz, and we give the theoretical explanations about this problem. Through investigating crystallization process under high pressure and high temperature, we have found that the pressure-induced crystallization of amorphous SiO₂ with Si-OH can complete rapidly. In this crystallization course, it is quickly crystallized into coesite orα-quartz in 1 min.Because the conditions located at between the equilibrium phase boundary are the thermodynamic stable conditions of coesite, the syntheticα-quartz from it will slowly transforms to coesite under these conditions, and the lower the temperature, the lower the transformation velocity ofα-quartz to coesite.Via normal-pressure high-temperature sintering and mechanical ball milling, the amorphous SiO₂ powders with the same grain sizes were prepared, but they contained the different quantity and different form of water. Comparing the crystallization results of these powders under high pressure and high temperature, the catalytic action of silicon-hydroxy groups was found for the first time. Using amorphous SiO₂ containing 5.6 wt% silicon-hydroxy groups as initial material, single-phase coesite can be synthesized under 4.2 GPa and 190℃. This synthesis temperature is the lowest synthesis temperature of coesite so far, and the synthesis time can be shortened to the order of seconds.Based on the hypothesis of plate subduction-exhumation, natural coesite was formed in the process of a plate subducting into the upper mantle, and then exhuming back to the Earth’s crust. It is an important problem whether coesite can be preserved in the course of exhuming back to the Earth’s crust. We have designed the experiment for simulating the exhuming back of coesite, and find that coesite will transform toα-quartz if the time of exhuming back to Earth’s crust is more than 40h. It is obvious that coesite can not exhume back from upper mantle in so short time, so there are certain problems to explain the formation of coesite using the hypothesis of plate subduction-exhumation.Via normal-pressure high-temperature sintering and mechanical ball milling, the amorphous SiO₂ powders with the same grain sizes were prepared, but they contained the different quantity and different form of water. Comparing the crystallization results of these powders under high pressure and high temperature, the catalytic action of silicon-hydroxy groups was found for the first time. Using amorphous SiO₂ containing 5.6 wt% silicon-hydroxy groups as initial material, single-phase coesite can be synthesized under 4.2 GPa and 190℃. This synthesis temperature is the lowest synthesis temperature of coesite so far, and the synthesis time can be shortened to the order of seconds.Based on the hypothesis of plate subduction-exhumation, natural coesite was formed in the process of a plate subducting into the upper mantle, and then exhuming back to the Earth’s crust. It is an important problem whether coesite can be preserved in the course of exhuming back to the Earth’s crust. We have designed the experiment for simulating the exhuming back of coesite, and find that coesite will transform toα-quartz if the time of exhuming back to Earth’s crust is more than 40 h. It is obvious that coesite can not exhume back from upper mantle in so short time, so there are certain problems to explain the formation of coesite using the hypothesis of plate subduction-exhumation.There is a small quantity of fine particles coesite in the samples synthesized from the mixture of amorphous SiO₂ and MWCNTs under 2.0 GPa and 300℃for 30 min, and the reason why the coesite can be synthesized under such low pressure is“the principle of small-area role”under high pressure. We suggest it is possible to form a small quantity of fine particles coesite in the upper crust （about 10 km）by taking into account shear stress, seismic shock wave, and“small-area role”.In the present paper, theα-cristobalite andα-tridymite have been producted fromα-quartz, and the structural transformation ofα-cristobalite andα-tridymite to coesite has been first investigated. The experimental results show thatα-cristobalite can be transformed to coesite under 3.5 GPa and 1200℃for 30 min, and be transformed to coesite at 900℃when the pressure was increased to 3.5 GPa. It is interesting to note that, theα-cristobalite milled for a specified period of time is easier to form coesite under high pressure and high temperature. For example, under 3.5 GPa and 600℃theα-cristobalite milled for 2.5 h can be transformed to single-phase coesite. Theα-tridymite material, are mainly transformed toα-quartz at 300-1200℃when the pressure is lower than 3.0 GPa, are transformed to coesite at 900℃under the pressure is increased to 3.5 GPa, but are transformed to single-phase coesite at 600℃under the pressure is increased to 3.9 GPa. Through comparing the structural transformation regulation ofα-quartz,α-cristobalite andα-tridymite under high pressure and high temperature, we found thatα-tridymite is easiest to be transformed to coesite under high pressure.The reasons behind this phenomenon may be thatα-tridymite is similar to coesite in structure and the density ofα-tridymite is lowest Another research system is the preparation of the silicon carbide nanopowders. Nanostructured silicon carbide （SiC） is a very promising material in many application areas which include composite materials, microelectronics, optoelectronics, and biomedical engineering. Due to its high thermal stability, high hardness and strength, as well as intrinsic resistance to oxidation and corrosion, SiC nanoparticle is regarded as an attractive candidate for advanced ceramics applications. In recent years, the luminescent properties of SiC nanomaterials have received increasing attention because of the enhanced emission intensity and the blue-shifted photoluminescence with decreasing the grain size of SiC. Therefore, SiC nanoparticle is considered as a good candidate as blue and ultraviolet light emitter in displays. Moreover, SiC nanoparticle is potentially useful in bio-labeling and medicine. Up to now, various synthesis methods have been proposed for preparation of high purity ultrafine SiC powders. Although these synthesis methods can be used for preparing ultrafine SiC particles, they are not suitable for mass production because of high costs and low yields. Therefore, it is necessary to find a feasible, low-cost, and mass production method for fabricating ultrafine SiC powders. By using improved ball mill and the introduction of small amount of NH4Cl into the starting mixture of Si and graphite, high-quality SiC nanoparticle has been high-efficiently synthesized.The synthesis products were characterized by XRD, FT-IR, TEM and SAED. The results show the grain size of the synthesizedβ-SiC nanoparticle gradually decreased with increasing the milling time from 20 to 60 h, but the grain size basically keep unchanged above 60 h. The dispersion ofβ-SiC nanoparticle is the best when the milling time is 40 h. Under 325 nm excitation, a stable and intensive broad emission peak at 387 nm has been observed in the PL spectrum of the synthetic nanoparticles, and this emission shows an obvious blueshift of bandgap. Therefore,β-SiC nanoparticles synthesized by HMBM have great potential for the preparation of advanced ceramic and light-emitting devices.更多还原