本文選題：分子動(dòng)力學(xué)　切入點(diǎn)：燒綠石　出處：《蘭州大學(xué)》2017年博士論文

【摘要】：隨著核能日新月異的發(fā)展,核廢物的產(chǎn)生越來越多。如何安全有效地處理和處置核廢物已成為核能發(fā)展中的首先要解決的關(guān)鍵問題之一。對(duì)高放廢物進(jìn)行固化,然后對(duì)固化體進(jìn)行深層地質(zhì)掩埋是目前最切實(shí)有效的處置方法。因此,尋找合適的固化基材成已為核能發(fā)展中亟待解決的問題之一。燒綠石相是自然界中天然存在的一種礦相,燒綠石型氧化物包含450多種合成化學(xué)成分,由于其抗輻照性能優(yōu)越、結(jié)構(gòu)穩(wěn)定性好、機(jī)械性能強(qiáng),且對(duì)核素包容率高、核素浸出率低,有望成為下一代高放射性核廢物固化最理想的侯選基材之一,在近年來得到了科學(xué)界的廣泛關(guān)注。本論文基于經(jīng)典分子動(dòng)力學(xué)方法,系統(tǒng)地研究了燒綠石的結(jié)構(gòu)性能、力學(xué)性能和熱學(xué)性能,并對(duì)燒綠石抗輻照性能進(jìn)行了評(píng)估。燒綠石的分子動(dòng)力學(xué)模擬采用的勢(shì)函數(shù)模型為長程庫侖勢(shì)和短程Buckingham勢(shì)。首先,基于短程Buckingham勢(shì)近核部分不切實(shí)際的吸引力,采用普適的Ziegler-Biersack-Littmark(ZBL)排斥勢(shì)對(duì)其進(jìn)行修正,并運(yùn)用指數(shù)函數(shù)進(jìn)行了平滑連接。建立了適用于燒綠石體系尤其是動(dòng)態(tài)性能模擬的勢(shì)函數(shù)模型。據(jù)此模型模擬計(jì)算了包括晶格常數(shù)、彈性模量、熱容量及熱膨脹系數(shù)等燒綠石的靜態(tài)性質(zhì),并給出了各個(gè)量的合理取值。結(jié)果表明,計(jì)算得到的各個(gè)靜態(tài)性質(zhì)參數(shù)與實(shí)驗(yàn)結(jié)果符合得很好。因此,本論文建立的勢(shì)函數(shù)模型可較為準(zhǔn)確地描述燒綠石體系中各離子間的相互作用,并期望應(yīng)用于燒綠石低能離位碰撞及輻照損傷等動(dòng)力學(xué)方面的研究。其次,研究了不同陽離子半徑對(duì)燒綠石A2B2O7靜態(tài)性能的影響,系統(tǒng)地計(jì)算分析了25種燒綠石的結(jié)構(gòu)、彈性和熱學(xué)性能。結(jié)果表明:燒綠石的晶格常數(shù)隨A、B位陽離子半徑的增大呈線性增長趨勢(shì),且B位陽離子比A位陽離子對(duì)晶格常數(shù)的影響略為顯著;彈性模量隨A、B位陽離子半徑的增大呈現(xiàn)略微下降的趨勢(shì),B位陽離子半徑對(duì)彈性模量的影響大于A位陽離子;熱容量與熱膨脹系數(shù)隨A、B位陽離子半徑的變化并不顯著。然而,Er2Ti2O7的等壓熱容量(CP)出現(xiàn)異常,若將Er元素置于鈦酸鹽燒綠石的A位,可能會(huì)對(duì)其部分熱學(xué)性能產(chǎn)生較大的影響。在燒綠石Gd2Ce2O7和Sm2Ce2O7中,熱容量和熱膨脹系數(shù)呈現(xiàn)急劇下降趨勢(shì),故燒綠石B位為Ce元素時(shí),可能會(huì)對(duì)鈰酸鹽燒綠石中輻照損傷的退火效應(yīng)有一定的影響。由于Ce和Pu在離子半徑和價(jià)態(tài)變化等方面的相似性,通常采用無放射性的Ce代替有放射性的Pu進(jìn)行高放廢物的模擬固化研究。因此,若將Pu固化于燒綠石中的B位時(shí),可能會(huì)對(duì)其熱學(xué)性能產(chǎn)生較大影響,故固化基材的選擇應(yīng)將熱學(xué)性能的改變作為重要考慮的因素。再次,運(yùn)用分子動(dòng)力學(xué)方法測(cè)定了在一系列燒綠石中低能離子與固體的相互作用及化學(xué)成分對(duì)其影響,得出了Gd2Zr2O7燒綠石中各類原子Gd、Zr、O48f和O8b沿80多個(gè)不等價(jià)晶向的離位閾能(Ed)輪廓圖,共測(cè)定了320多個(gè)方向的Ed值。研究結(jié)果表明,Gd2Zr2O7中各類原子的Ed值均表現(xiàn)為各向異性,其中Zr原子的Ed值各向異性程度最高,而O8b的Ed值各向異性程度最低�；跍y(cè)定的Ed最小值,本文推薦選取的Gd2Zr2O7中Gd、Zr和O的Ed值分別為56 eV,94 eV和25 eV。此外,本文研究了化學(xué)成分對(duì)10K溫度下燒綠石中各類原子低能離位碰撞事例的影響,具體研究了陽離子半徑對(duì)燒綠石A2B2O7(其中A位陽離子從Lu3+到La3+,B位陽離子從Ti4+到Ce4+)中各類原子沿[100]、[110]和[111]三個(gè)主要晶向離位閾能(Ed)的影響。研究結(jié)果表明,Ed的大小強(qiáng)烈依賴于原子類型、原子質(zhì)量、入射晶向和晶格位置;低能離位碰撞后產(chǎn)生的缺陷包括:陽離子反位缺陷,陽離子弗侖克爾缺陷,陰離子弗侖克爾缺陷,各種不同的空位和填隙。然而,燒綠石中Ce的摻入導(dǎo)致了陰、陽離子離位閾能Ed的突變和一種不尋常的反位缺陷的形成,進(jìn)而可能會(huì)影響其輻照效應(yīng)。模擬得到的Ed值為多尺度的蒙特卡洛模擬提供了重要的輸入?yún)?shù)。另外,這部分工作還揭示了材料的離位閾能Ed與其抗非晶化能力之間的關(guān)系:Ed的值越小,越容易發(fā)生有序-無序轉(zhuǎn)變,臨界非晶化劑量越高,臨界非晶化溫度越低,抗輻照能力越強(qiáng)。最后,對(duì)燒綠石的位移級(jí)聯(lián)和輻照損傷進(jìn)行了初步評(píng)估。采用分子動(dòng)力學(xué)模擬研究了Gd2Zr2O7燒綠石中選取不同入射方向和不同初始能量的初級(jí)碰撞原子(PKA)對(duì)級(jí)聯(lián)碰撞損傷的影響。結(jié)果表明,當(dāng)PKA以一定能量沿不同方向入射時(shí),位移級(jí)聯(lián)所產(chǎn)生的輻照損傷程度差異很大,材料的抗輻照能力呈現(xiàn)出各向異性特性。選取不同初始動(dòng)能的PKA沿特定方向入射時(shí),級(jí)聯(lián)碰撞所產(chǎn)生的損傷程度也各不相同:隨PKA入射能量的增加,最終的產(chǎn)生缺陷數(shù)目增多,損傷半徑增大,且缺陷重組率顯著降低。通過對(duì)Gd2Zr2O7燒綠石中摻入不同比例Ce元素后輻照損傷的分子動(dòng)力學(xué)模擬,結(jié)果表明隨著摻Ce比例的增加,材料的抗輻照能力變化顯著,且Ce元素?fù)饺階位和B位對(duì)材料的抗輻照性能的影響不同。
[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.

【學(xué)位授予單位】：蘭州大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2017
【分類號(hào)】：TB303

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