Structural Transition Of Silicon Dioxide And Carbon Nanotubes Under High Pressure And High Temperature

Silica is a kind of mineral that exists widely in nature. It has manykinds of polymorphs and each polymorph can transit another underdifferent temperatures and different pressures, therefore, we can inferhow the rocks near the Earth's surface form by the study of the silicanear the Earth's surface. Coesite is a polymorph of silica existed in highpressure. Coesite is result of ultrahigh-pressure metamorphism. Thediscovery of coesite bearing rocks indicates that the rocks near theEarth's surface have been subducted by strong tectonic movement to thedepth of more than 90km by plate movements and experiencedultrahigh-pressure metamorphism there.In 1953, coesite was originally synthesized in the laboratory byCoes. In 1960, natural coesite was first found from Meteor aerolite craterby Chao and his colleagues. Later, coesite was found at severalultrahigh-pressure metamorphic belts in the world. As one of diagnosticultrahigh-pressure metamorphic minerals, coesite is generally used todetermine the peak metamorphic pressure. In order to explain theexistence of coesite near the Earth's surface, someone set forth thetheory of the ultrahigh-pressure metamorphic rocks exhumate to theEarth's surface. Ultrahigh-pressure metamorphism have aroused greatinterest of geologists because of the discovery of the coesite in natureand the synthesis of the coesite in the laboratory in order to explain hownatural coesite form. It is of great interest to study the conditions forcoesite formation under different types of stress state. In this paper, wereport new experimental results of quartz-coesite transition at differentpressures and different temperatures.As a kind of typical extreme condition, high-pressure Method hasbeen developed fast recent years on the aspects of big chamber and highpressure. The properties of high-pressure method are shortening thedistance between the atoms effectively, changing the states of electronsand affecting phrases changing courses. In the condition of high pressureand high temperature, the materials are given high energy. So the highpressure method is effective to synthesize new materials. High pressureand high temperature method can mimic the state in the Earth. That canenlighten and help us on comprehending many geologic phenomena.We have studied the synthetic regularity of coesite under highpressure and high temperature using different initial state nano-powderand amorphous mixture made from silicon dioxide nano-powder andcarbon nanotubes by mechanical ball milling(MBM)。We expect tosynthesize coesite under lower condition of pressure and temperature.The following is the main results of this thesis:1. Initial silicon dioxide nano-powder crystallized into cristobalitealpha and a little tridymite at 1000~1600℃, and did not crystallizewhen the sintering temperature is under 700℃. In theory, siliconshould crystallize into α-quartz at 700℃, tridymite at 1000℃ and1300℃, cristobalite only at 1600℃.2. All kinds of initial material crystallized into α-quartz under 2.0GPaand different temperature.3. Initial silicon dioxide nano-powder and the amorphous mixture(85:15) made from silicon dioxide nano-powder and carbonnanotubes by high-energy MBM for 100h both crystallized intocoesite under 3.9GPa and different temperature.4. The silicon dioxide nano-powder by high-energy MBM for 50h andthe amorphous mixture (85:15) made from silicon dioxidenano-powder and carbon nanotubes by high-energy MBM for 40hboth crystallized intoα-quartz and coesite under 3.9GPa and 320℃,and both crystallized into coesite under 3.9GPa and 800~1100℃.5. The silicon dioxide nano-powder by low-energy MBM for 50hcrystallized into α-quartz under 3.9GPa and 320℃ and crystallizedinto coesite under 3.9GPa and 800℃.6. The amorphous mixture (65:35) made from silicon dioxidenano-powder and carbon nanotubes by high-energy MBM for 40hcrystallized into coesite under 3.9GPa and 590~1100℃. But themixture by high-energy MBM for 100h crystallized intoα-quartz andcoesite under 3.9GPa and 590~1100℃ and the amount of coesitedecreased with temperature rising. The reason of causing thisphenomena is still explicit.The results above research show that the temperature crystallizinginto coesite in this paper is much lower than the one having beenreported.