| High pressure physics is a subject to study the optics, electrics, magnetism, mechanics, microstructure, equation of state and phase transformation of materials under high pressure. High pressure research can bring discovery of the new structures and novel properties of materials, which have not appeared in the ambient pressure, As experimental evidence and theoretical base, the research is very significant for synthesizing new materials and improving properties of condensed matter. This dissertation includes three parts. First is the investigation of mechanical features of Bridgman anvil, especially, that with 20mm top diameter, this part belongs to high pressure technology. Second is the praperation of high-density nanocrystalline bulk senenium by rapid compression method, and last part is an attempt to make the bulk amorphous sulfur with structurally different content.(1) The features of Bridgman anvil in application of high pressureBy using the Bridgman anvil with 20mm top diameter, we research mechanical behavior of pyrophyllite gasket. Through changing the pressure, measuring the related size and analyzing the picture of recovered discs, the relationship between elastic module and pressure, and distribution of shear strength in the pyrophyllite gasket is studied in details. Results show that the elastic module of gasket rises with increasing pressure, and the shear strength of plastic zone distributes with a variation rule (i.e. the shear strength of plastic zone is risen but its increment rate is relatively reduced as increasing pressure).In addition, combining with the experimental results and theoretical analysis, it is confirmed that the shear strength within elastic zone can not be ignored, but a small quasi-hydrostatic pressure area exists in the middle of the gasket disk, which brings a practical evidence for the choice of sample size in high-pressure experiments. The investigation perfects the model of real pressure distribution in the disc gasket under high pressure.The pressure on the Bridgman anvils was calibrated by using the known phase transitions of bismuth. Compared with the measurement results on the different size and different type molds, it is made clear that the actual pressure increases on the center of Bridgman anvils and the increased rate also rises with the oil pressure increasing. The latter character is quite different from that of multi-anvil apparatus and belt apparatus.The method of larger pressure-jump was used to measure the change of temperature with pressure of selenium in adiabatic compression process. The results not only could induce the measurement of Wu-Jing parameters and Gruneisen parameters under high pressure, but also are helpful for estimation of the supercooling of selenium in next experiments.(2) Preparation of high-density nanocrystalline bulk selenium by rapid compressing of melt.Nanocrystalline (NC) materials have been attracted tremendous attention due to their unique physical and mechanical properties. Up to now there are many kinds of techniques synthesizing nanocrystalline bulk materials, however, few relatively with the high-pressure. In view of the relation of equivalence of pressure and temperature in thermodynamics, the viewpoint is proposed that changing the pressure rapidly has the same thermodynamic effect as changing the temperature abruptly at producing metastable structure bulk. Furthermore, in the rapid compressing process the whole sample, whether surface or interior is held in a synchronously thermal environment, where the thermal conduction is not working, and the bulk NC is beneficial to be prepared directly with large size.Four separate experiments have been conducted in this study, (a) Rapid compression to 2.8GPa for liquid selenium; (b) Rapid compression to 3.5GPa for liquid selenium; (c) Slow compression to 2.8GPa for liquid selenium; (d) Natural cooling at ambient pressure. Based on the XRD, SEM and TEM results of the recovered samples, it is clearly shown that homogenous nanostructures were formed only by the rapid compression processes, and the average crystal sizes were about 18.7 and 19.0nm in the samples recovered (a) and (b), respectively. The relative density of the nanocrystalline bulk is up to 98.17% of the theoretical value. It is suggested that rapid compression could induce pervasive nucleation and restrain grain growth during the solidification. Obtained bulk NC selenium is large with 16mm in diameter and 3.2mm in thickness. The research results show that the rapid compression of melt is an effective way for preparing high-density bulk nanocrystalline materials. The mechanism is related to fast supercooling, higher viscosity of the melt and lower diffusivity of atoms under high pressure.(3) Rapid compression induced solidification of structurally different content of amorphous sulfur.Since Mishima had found two kinds of amorphous ice, it becomes a new hot topic in the condensed matter physics that whether the same matter has different amorphous phases. Based on the related research we imagine that if the melt rapidly solidified at different temperatures by rapid compression, the different high-pressure phase would be "frozen" at different supercooling, and so the amorphous phase with different structure could be prepared. It has been reported that S8 rings of sulfur begin transition to the mixture of S8 rings and long polymer [S]n chains at 432K, indicated there are different structures in the melt sulfur at different temperature. Therefore, we present two comparative experiments to "freeze" the different structure by using rapid compressing melt sulfur at separated temperature above or under 432K. Recovered solid sulfur samples were analyzed by XRD, DSC, FT-IR and Raman spectrum. Characterization showed that all the recovered samples are mixture of S8 rings and long polymer [S]n chains, but their content is different each other. The experimental results supported the possibility to prepare different amorphous sulfur. More feasible scheme of experiment is further discussed in this part.
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