烧绿石型氧化物的结构参数及性能参数的分子动力学模拟研究

发布时间：2018-03-28 01:34

本文选题：分子动力学　切入点：烧绿石　出处：《兰州大学》2017年博士论文

【摘要】：随着核能日新月异的发展,核废物的产生越来越多。如何安全有效地处理和处置核废物已成为核能发展中的首先要解决的关键问题之一。对高放废物进行固化,然后对固化体进行深层地质掩埋是目前最切实有效的处置方法。因此,寻找合适的固化基材成已为核能发展中亟待解决的问题之一。烧绿石相是自然界中天然存在的一种矿相,烧绿石型氧化物包含450多种合成化学成分,由于其抗辐照性能优越、结构稳定性好、机械性能强,且对核素包容率高、核素浸出率低,有望成为下一代高放射性核废物固化最理想的侯选基材之一,在近年来得到了科学界的广泛关注。本论文基于经典分子动力学方法,系统地研究了烧绿石的结构性能、力学性能和热学性能,并对烧绿石抗辐照性能进行了评估。烧绿石的分子动力学模拟采用的势函数模型为长程库仑势和短程Buckingham势。首先,基于短程Buckingham势近核部分不切实际的吸引力,采用普适的Ziegler-Biersack-Littmark(ZBL)排斥势对其进行修正,并运用指数函数进行了平滑连接。建立了适用于烧绿石体系尤其是动态性能模拟的势函数模型。据此模型模拟计算了包括晶格常数、弹性模量、热容量及热膨胀系数等烧绿石的静态性质,并给出了各个量的合理取值。结果表明,计算得到的各个静态性质参数与实验结果符合得很好。因此,本论文建立的势函数模型可较为准确地描述烧绿石体系中各离子间的相互作用,并期望应用于烧绿石低能离位碰撞及辐照损伤等动力学方面的研究。其次,研究了不同阳离子半径对烧绿石A2B2O7静态性能的影响,系统地计算分析了25种烧绿石的结构、弹性和热学性能。结果表明:烧绿石的晶格常数随A、B位阳离子半径的增大呈线性增长趋势,且B位阳离子比A位阳离子对晶格常数的影响略为显著;弹性模量随A、B位阳离子半径的增大呈现略微下降的趋势,B位阳离子半径对弹性模量的影响大于A位阳离子;热容量与热膨胀系数随A、B位阳离子半径的变化并不显著。然而,Er2Ti2O7的等压热容量(CP)出现异常,若将Er元素置于钛酸盐烧绿石的A位,可能会对其部分热学性能产生较大的影响。在烧绿石Gd2Ce2O7和Sm2Ce2O7中,热容量和热膨胀系数呈现急剧下降趋势,故烧绿石B位为Ce元素时,可能会对铈酸盐烧绿石中辐照损伤的退火效应有一定的影响。由于Ce和Pu在离子半径和价态变化等方面的相似性,通常采用无放射性的Ce代替有放射性的Pu进行高放废物的模拟固化研究。因此,若将Pu固化于烧绿石中的B位时,可能会对其热学性能产生较大影响,故固化基材的选择应将热学性能的改变作为重要考虑的因素。再次,运用分子动力学方法测定了在一系列烧绿石中低能离子与固体的相互作用及化学成分对其影响,得出了Gd2Zr2O7烧绿石中各类原子Gd、Zr、O48f和O8b沿80多个不等价晶向的离位阈能(Ed)轮廓图,共测定了320多个方向的Ed值。研究结果表明,Gd2Zr2O7中各类原子的Ed值均表现为各向异性,其中Zr原子的Ed值各向异性程度最高,而O8b的Ed值各向异性程度最低。基于测定的Ed最小值,本文推荐选取的Gd2Zr2O7中Gd、Zr和O的Ed值分别为56 eV,94 eV和25 eV。此外,本文研究了化学成分对10K温度下烧绿石中各类原子低能离位碰撞事例的影响,具体研究了阳离子半径对烧绿石A2B2O7(其中A位阳离子从Lu3+到La3+,B位阳离子从Ti4+到Ce4+)中各类原子沿[100]、[110]和[111]三个主要晶向离位阈能(Ed)的影响。研究结果表明,Ed的大小强烈依赖于原子类型、原子质量、入射晶向和晶格位置;低能离位碰撞后产生的缺陷包括:阳离子反位缺陷,阳离子弗仑克尔缺陷,阴离子弗仑克尔缺陷,各种不同的空位和填隙。然而,烧绿石中Ce的掺入导致了阴、阳离子离位阈能Ed的突变和一种不寻常的反位缺陷的形成,进而可能会影响其辐照效应。模拟得到的Ed值为多尺度的蒙特卡洛模拟提供了重要的输入参数。另外,这部分工作还揭示了材料的离位阈能Ed与其抗非晶化能力之间的关系:Ed的值越小,越容易发生有序-无序转变,临界非晶化剂量越高,临界非晶化温度越低,抗辐照能力越强。最后,对烧绿石的位移级联和辐照损伤进行了初步评估。采用分子动力学模拟研究了Gd2Zr2O7烧绿石中选取不同入射方向和不同初始能量的初级碰撞原子(PKA)对级联碰撞损伤的影响。结果表明,当PKA以一定能量沿不同方向入射时,位移级联所产生的辐照损伤程度差异很大,材料的抗辐照能力呈现出各向异性特性。选取不同初始动能的PKA沿特定方向入射时,级联碰撞所产生的损伤程度也各不相同:随PKA入射能量的增加,最终的产生缺陷数目增多,损伤半径增大,且缺陷重组率显著降低。通过对Gd2Zr2O7烧绿石中掺入不同比例Ce元素后辐照损伤的分子动力学模拟,结果表明随着掺Ce比例的增加,材料的抗辐照能力变化显著,且Ce元素掺入A位和B位对材料的抗辐照性能的影响不同。
[Abstract]:With the development of nuclear power nuclear waste generation change rapidly, more and more. How to effectively deal with the disposal of nuclear waste has become one of the first to solve the key problems in the development of nuclear energy. To solidify HLW, then solidified deep buried geological disposal is the most effective method. Therefore, looking for the appropriate curing substrate into one of the urgently needed for solving the problems in the development of nuclear energy. Pyrochlore phase is a kind of natural mineral found in nature, pyrochlore type oxide composition containing 450 kinds of chemical components, because of its superior anti irradiation properties, good structure stability, strong mechanical properties, and the high rate of radionuclide tolerance, radionuclide the leaching rate is low, is expected to become the most ideal for the next generation of high radioactive waste solidification candidate substrate, has been widespread concern in the field of Science in recent years. This thesis based on classical molecular dynamics For the systematic study of the structure and properties of pyrochlore, mechanical properties and thermal properties of pyrochlore anti irradiation properties were evaluated. The molecular dynamics simulation of pyrochlore potential function model used for long-range and short-range Coulomb potential Buckingham potential. First of all, attractive short-range Buckingham potential near the nuclear part based on unrealistic. By using the universal Ziegler-Biersack-Littmark (ZBL) for correcting the repulsive potential, and with the exponential function of smooth connection is established. Pyrochlores system especially the potential function model of dynamic performance simulation. The simulation model including the lattice constant, the calculated elastic modulus, heat capacity and thermal expansion coefficient of static properties of pyrochlore, and gives the reasonable value of each quantity. The results show that the static properties of the calculated parameters are in good agreement with experimental results. Therefore, this paper is to establish the Potential function model can accurately describe the interaction between ion burning green system, and expect to study collision and irradiation damage dynamics of bits used in Pyrochlores from low energy. Secondly, the effects of different cationic radius on the static performance of the A2B2O7 pyrochlore, systematically calculated and analyzed 25 kinds of Pyrochlores the structure, elastic and thermal properties. The results show that the pyrochlore lattice constants A, B cations increases linearly with the increase of the radius, and B cations than A cations on the lattice constant effect slightly significant; elastic modulus with A increased slightly decreased B cation radius. Effect of B cation radius on the elastic modulus is greater than A cations; heat capacity and thermal expansion coefficient with the change of B A, a cation radius is not significant. However, isobaric heat capacity of Er2Ti2O7 (CP) is abnormal if Er In the titanate pyrochlore A, may have a greater impact on some of its thermal properties. The pyrochlore Gd2Ce2O7 and Sm2Ce2O7, thermal capacity and thermal expansion coefficient have shown a sharp decline, so the pyrochlore B for Ce elements, may be to cerate Pyrochlores in irradiation annealing effect the injury has certain effect. Due to the similarity of Ce and Pu in the ionic radius and valence variation etc., usually by non radioactive Ce instead of radioactive Pu simulation study on curing of high level radioactive waste. Therefore, if the Pu in the curing of pyrochlore B in position, may have a significant impact the thermal performance, so the choice of substrate should be cured thermal properties change as an important factor. Thirdly, effects of ion solid interactions and chemical composition of its in a series of Pyrochlores were determined by molecular dynamics method Gd2Zr2O7, the pyrochlore in various atomic Gd, Zr, O48f and O8b along the more than 80 inequivalent orientations from a threshold (Ed) profile, a total of more than 320 directions were measured Ed values. The results show that the Gd2Zr2O7 of atomic Ed were anisotropic, which Zr atom the Ed value of the highest degree of anisotropy, and the O8b Ed value of the degree of anisotropy is lowest. The minimum value of determination of Ed based on Gd2Zr2O7 is recommended in this paper selected Gd, Zr and O Ed were 56 eV, 94 eV and 25 eV. in addition, the effects of chemical composition on the burning effect of various atoms from a low energy in the case of collision pyrochlore 10K temperature, specific study of the cationic radius on the pyrochlore A2B2O7 (including A cations from Lu3+ to La3+, B cations from Ti4+ to Ce4+) in all kinds of atoms along the [100], [110] and [111] three main crystal to displace threshold energy (Ed). The results show that the influence of Ed, the size of the strong Strongly depends on the atomic types, atomic mass incident, crystal orientation and lattice position; including defects from a low-energy collision: cation antisite defects, cationic Ephron Kerr f Kerr anion defects, defects, different kinds of vacancy and interstitial. However, pyrochlore in the incorporation of Ce led to the formation of Yin. From the cation antisite defects a threshold Ed mutation and an unusual, and may affect the radiation effect. The simulated value of Ed Monte Carlo simulation for multi scale provided important input parameters. In addition, this work also reveals a threshold from the relationship between Ed and its anti amorphization the ability of the material: the value of Ed is smaller, more prone to the order disorder transition, the critical amorphization dose increases the critical amorphization temperature is lower, the ability of anti radiation stronger. Finally, the displacement cascade Pyrochlores and conducted a preliminary evaluation of radiation damage Estimated. Using the molecular dynamics simulation of primary atomic collisions from different incident direction and different initial energy of Gd2Zr2O7 Pyrochlores in (PKA) on the effect of cascade collision damage. The results showed that when PKA with certain energy along the different direction of incidence, the difference degree of radiation damage caused by large displacement cascade, the anti radiation ability of materials exhibit anisotropic properties. Different initial kinetic energy of PKA along the specific direction of incidence, the extent of damage produced by the collision cascade is different: with the increase of the incident energy of PKA, the final number of defects, defects and damage radius increased, the recombination rate was significantly reduced. The Gd2Zr2O7 burn the molecular dynamics simulation of radiation damage the different proportion of Ce doped pyrochlore after the results show that with increasing the ratio of Ce, anti irradiation ability changes were observed, and Ce doped A and B on The effects of the material on the radiation resistance are different.

【学位授予单位】：兰州大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TB303

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