【作者】 胡云；

【导师】 洪时明；

【作者基本信息】 西南交通大学 ， 材料学， 2010， 博士

【摘要】 高压物理学是研究物质在高压下的电学、光学、磁学、力学特性,以及高压下物质的微观结构、状态方程及相变等的学科。由于高压研究可以发现常压下物质所不具备的新结构、新性能、新现象及新规律,因此,它为新材料合成、制备及改性提供了重要的实验依据和理论基础。本论文共分为三个部分,第一部分为对Bridgman压砧加压性能的研究,属于高压技术研究；第二部分为快速增压法制备高密度纳米硒块体材料；第三部分为快速增压制备不同结构比例的大块非晶硫的探索,第二和第三部分属于高压下新材料的制备。分别摘要如下：(1) Bridgman对顶砧加压性能的研究在本实验室过去使用的端面直径为26mm的Bridgman对顶砧高压模具的基础上,进一步研究了直径为20mm的对顶砧模具的性能。并使用新的高压模具对叶腊石封垫的力学行为进行了一系列研究。通过改变施加压力研究了叶蜡石的剪切强度及弹性模量随压力的变化规律等力学行为,证实了封垫材料的弹性模量随压力增大而增大；得到了塑性区剪切强度随压力的变化规律(即在塑性区封垫材料的剪切强度随压力的增大而增大,且增大的趋势逐渐减小)。另外,将实验结果和理论分析相结合,证实了圆片的弹性区存在不可忽略的剪切强度,并进一步证明了圆片的中部存在一个准静水压区,为高压实验中样品腔尺寸的选择提供了更为科学的依据。这些实验结果完善了高压下圆片封垫内压力实际分布的模型。利用Bi丝已知的压致相变点对Bridgman压砧的压力进行了标定,结合上述测量结果的推算,得出了Bridgman式压砧中心实际压力随油压的升高而升高得更快的特性,这种特征不同于多压砧装置和年轮式模具。利用这套新的对顶砧高压模具配合快速增压压机测量了常温下绝热压缩过程中,硒的温度随压力的变化,从而为进一步测量硒的格林爱森参数和吴经参数打下基础,并为快速增压法制备纳米硒过冷度的估算提供了参考依据。(2)快速增压方法制备高密度纳米硒多晶块体材料纳米材料作为新型功能材料或结构材料的研究备受关注。制备纳米块体材料的方法已有多种,但与高压相关的并不多。我们考虑到在热力学上压力和温度的对等关系,提出对熔融液体快速改变压力导致凝固应与快速改变温度一样可以获得亚稳态结构的块体材料,同时因在这种过程中温度均匀,分子的凝聚行为将不受热传导率的限制,有利于一次性获得大块纳米晶体材料。本研究共设计了四组对比实验：(a)熔融硒快速增压到2.8GPa; (b)熔融硒快速增压到3.5GPa; (c)熔融硒慢速增压到2.8GPa；(d)熔融硒在常压下自然冷却。对四组实验所回收的固态硒分别进行SEM、TEM、XRD分析,发现(a)和(b)回收的样品均为纳米块体材料,平均晶粒尺寸分别为18.73nm和19.04nm, (a)回收样品的相对密度高达98.17%。其它两种实验获得的则是微米级多晶体。说明快速增压方法能够使熔体大量成核的同时又有效地抑制晶粒的生长,从而一次性获得均质的纳米晶体硒。这是首次通过快速增压法成功地制备出纳米多晶块体材料。获得的纳米晶硒块体材料的最大直径为16mm,厚度为3.2mm。研究结果显示,快速增压法是一种有效制备大块纳米晶材料的新途径。本文还对其形成机理进行了分析。(3)快速增压法制备不同结构比例的大块非晶硫的探索从Mishima发现冰有两种不同的非晶态以来,同种物质是否存在不同非晶相成为凝聚态物理中一个新的热点问题。相关研究结果启示我们：能否利用快速加压的方法使不同温度的熔体快速凝固,在不同的过冷度下“冻结”成不同的高压相,从而制备出不同结构的非晶相。前人报道熔融硫在432K时会由硫八环结构开始断裂转变为链状结构,因此我们设计了两组对比实验,旨在通过快速增压的方法“冻结”熔融硫在432K以上和以下的不同结构,从而制备出不同结构的非晶硫。回收样品经X射线衍射、差热分析、傅里叶红外光谱和拉曼光谱分析后,确定制备出来的非晶硫均为硫八环和链状硫的混合结构,只是它们的含量比例不同。我们对实验结果的原因进行了讨论分析,初步给出了制备不同结构非晶硫的可能性。更多还原

【Abstract】 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.更多还原