| Ln-M共掺杂钙钛锆石的固溶行为研究 (Ln=La, Nd, Gd, Ho, Yb; M=Al, Fe, Nb) | |
| Alternative Title | The solubility behavior of Ln-M co-doped zirconolite (Ln=La, Nd, Gd, Ho, Yb; M=Al, Fe, Nb) |
| 吉诗银 | |
| Subtype | 博士 |
| Thesis Advisor | 李玉红 ; 王志光 |
| 2020-06-04 | |
| Degree Grantor | 兰州大学 |
| Place of Conferral | 兰州 |
| Degree Name | 理学博士 |
| Degree Discipline | 粒子物理与原子核物理 |
| Keyword | 钙钛锆石 高放射性废物 陶瓷固化基材 固溶机制 晶相转变 |
| Abstract | 伴随核能的高速发展和广泛应用，核废物尤其高放射性废物（HLW）的长期、安全、有效处理与处置已成为一个亟待解决的难题。HLW最终处置方案的国际共识是深层地质处置。深层地质处置流程中最关键的是HLW固化。钙钛锆石（zirconolite，CaZrTi2O7）作为性能优异的HLW陶瓷固化基材之一，具备三种阳离子晶格，因此具有灵活的固溶模式，研究清楚HLW在钙钛锆石中的固溶行为对未来实际应用具有重要意义。本论文系统性的研究了过渡金属M (Fe3+, Al3+, Nb5+)作为电荷补偿取代Ti4+时，Ln3+（模拟三价锕系元素）在zirconolite的固溶率、晶相演变和固溶行为。主要研究内容为：(1) Ln-Fe共掺杂Ca1-xLnxZrTi2-xFexO7（Ln=La, Nd, Gd, Ho, Yb0.1&lex&le1.0）体系，Ln-Fe在zirconolite的固溶率和晶相转变上的协同效应(2) 成体系的研究Al3+和Fe3+ 同时作为电荷补偿时，Ln3+在Ca1-xLnxZrTi2-x (Al, Fe)xO7（Ln=La, Nd, Gd, Ho, Yb0.1&lex&le0.9）型zirconolite中的溶解度、晶相转变和固溶行为(3) CaZr1-xNdxTi2-xNbxO7（x=0.05-1.0）固溶体中高价态Nb5+作为电荷补偿时，固溶体的晶相演变和阳离子占位机制。 首次成体系的研究了电荷补偿离子Fe3+ 和Ln3+ 共掺Ca1-xLnxZrTi2-xFexO7（Ln=La, Nd, Gd, Ho, Yb0.1&lex&le1.0）在晶相转变和固溶率上的协同效应。研究发现，在0.1&lex&le0.7的Ca1-xLaxZrTi2-xFexO7中zirconolite与perovskite共存，当x=0.8-1.0时perovskite、zirconia和pseudobrookite三种晶相共存。Zirconolite-2M和zirconolite-3O存在Ca1-xLnxZrTi2-xFexO7（Ln= Nd, Gd）中，且zirconolite-2M到zirconolite-3O的相变分别发生在x=0.6和0.9时。Ho-Fe在zirconolite-2M单相中的溶解度超过90 %。Ca1-xYbxZrTi2-xFexO7型zirconolite-2M单相陶瓷只能在0.1&lex&le0.6范围内形成。Ca1-xNdxZrTi2-xFexO7陶瓷中zirconolite-2M和3O的晶格常数都随掺杂剂含量增加而增加。Ca1-xGdxZrTi2-x FexO7中，zirconolite-2M的晶格常数a随x的增加而增加，晶格常数b和c没有明显的变化。Ho-Fe共掺杂zirconolite-2M的晶格常数a随掺杂剂含量增加而增加，晶格常数b和c则呈现出相反的变化趋势。而在x=0.1-0.6的Ca1-xYbxZrTi2-xFexO7型zirconolite-2M晶格常数b和c随x增加而减小，晶格常数a却没有明显变化。Ca1-xLnxZrTi2-xFexO7（Ln= Nd, Gd, Ho, Yb）固溶体中zirconolite-2M的晶格常数展现出不同的变化趋势，说明Ln3+和Fe3+在zirconolite中的置换机制有所差异。 此外，我们创新性的采用Al3+和Fe3+离子同时取代zirconolite的Ti4+作为电荷补偿，合理的设计了一系列Ca1&minusxLnxZrTi2&minusx(Al, Fe)xO7 (Ln=La, Nd, Gd, Ho, Yb) 实验，通过同步辐射X射线技术研究掺杂剂的溶解度、晶相演变和阳离子的局域配位环境。Zirconolite单相不能在Ca1&minusxLaxZrTi2&minusx(Al, Fe)xO7中形成，且perovskite的含量逐渐增加并最终成为主相。Zirconolite-2M和zirconolite-3O在Ca1&minusxNdxZrTi2&minusx(Al, Fe)xO7和Ca1&minusxGdxZrTi2&minusx(Al, Fe)xO7中观察到，且2M到3O的相变分别在x=0.7和0.9时发现。Ho和Yb在zirconolite-2M单相Ca1&minusxLnxZrTi2&minusx(Al, Fe)xO7 (Ln=Ho, Yb)中的固溶率分别能达到90 % 和70 %。Ca1&minusxLnxZrTi2&minusx(Al, Fe)xO7 (Ln=Nd, Gd, Ho, Yb)固溶体中zirconolite-2M的晶格常数变化与掺杂的阳离子半径和置换机制密切相关。XANES揭示在zirconolite-2M型Ca1&minusxHoxZr Ti2&minusx(Al, Fe)xO7中Fe3+同时占据TiO5和TiO6晶格位置，TiO5与TiO6的比值随掺杂剂增加而减小Zr4+的配位环境为ZrO7和微量ZrO8配位。 Zirconolite和pyrochlore作为潜在的HLW陶瓷固化基材，同属于缺陷萤石的衍生结构，在一定条件下能互相转化。我们选取高电荷态的Nb5+取代Ti4+作为电荷补偿，制备了一系列CaZr1-xNdxTi2-xNbxO7 (x=0.05-1.0) 陶瓷固溶体样品。研究发现，微量的perovskite杂相一直存在于CaZr1-xNdxTi2-xNbxO7 (x=0.05-1.0) 陶瓷中。样品的主相随x的增加发生zirconolite-2M到zirconolite-4M到pyrochlore的晶相转变，相变分别在x=0.25和0.3时观察到，在x=0.25-04样品中zirconolite-2M和zirconolite-4M共存。近似单相的pyrochlore在x&ge0.5的样品中观察到。但是，在x=1.0时出现了一个新的cubic-215晶相，它属于缺陷萤石或部分无序的pyrochlore结构。此外，zirconolite-2M、zirconolite-3T和perovskite共存于x=0.2样品中。SXRD数据的Rietveld精修发现，Nd3+和Nb5+分别进入zirconolite-2M的Zr和Ti1（TiO6）晶格点位，在pyrochlore (A2B2O7) 结构中Nd3+、Ca2+和Zr4+占据A晶格，Nb5+、Ti4+和Zr4+占据B晶格。 |
| Other Abstract | With the rapid development and wide application of nuclear power, it is becoming an urgent problem for the long-term, safe and effective management and disposal of nuclear waste, especially high-level radioactive waste (HLW). Deep geological disposal of HLW is the best solution as the international consensus. The key of deep geological disposal is the immobilization of HLW. Zirconolite (CaZrTi2O7), as one potential ceramic matrix, have three cation sublattice for the immobilization of actinides. Therefore, it is important to clarify the substitution mechanism of actinides in zirconolite for the future practical application. In this thesis, we systematically study the phase evolutions, solid-solution behavior and solubility of Ln3+ (as the simulation of actinides) in zirconolite, when the transition metal M (Fe3+, Al3+, Nb5+) replaces Ti4+ as charge compensation. The major contents are as follows: (1) Ln-Fe co-doped zirconolite-type Ca1-xLnxZrTi2-xFexO7 (Ln=La, Nd, Gd, Ho, Yb0.1&lex&le1.0) system, study the synergistic effect of Ln3+ and Fe3+ on the phase transformations and solubility of Ln3+ in zirconolite. (2) Systematic study the phase transformations, substitution mechanism and solubility of Ln3+ in zirconolite-type Ca1-xLnxZrTi2-x(Al, Fe)xO7 (Ln=La, Nd, Gd, Ho, Yb0.1&lex&le0.9), here equivalent Al3+, Fe3+ replace Ti4+ simultaneously as charge compensators. (3) Study the effect of high valent Nb5+ and Nd3+ co-doped on the dopants solubility, phase transformations and structural information in CaZr1-xNdxTi2-xNbxO7 (x=0.05-1.0) solid-solutions. Firstly, we systematically investigated the synergistic effects of Ln3+ (as the surrogates for actinides) and Fe3+ co-doped Ca1-xLnxZrTi2-xFexO7 (Ln=La, Nd, Gd, Ho, Yb0.1&lex&le1.0) on phase evolution and solubility of Ln3+ in zirconolite. Results show that zirconolite (2M or 3O) with perovskite co-exist in the Ca1-xLaxZrTi2-xFexO7 (x=0.1-0.7) ceramics while perovskite, zirconia and pseudobrookite co-exist in the range of 0.8&lex&le1.0. Both of zirconolite-2M and zirconolite-3O were found in the Ca1-xLnxZrTi2-xFexO7 (Ln=Nd, Gd) ceramics and their phase transformations were observed at x=0.6 and 0.9, respectively. Ho-Fe co-doped Ca1-xHoxZrTi2-xFexO7 to form zirconolite-2M single-phase can achieve to 0.9 f.u. (formula unit). In Ca1-xYbxZrTi2-xFexO7 ceramics, single phase zirconolite-2M was only observed in range of 0.1&lex&le0.6. In Ca1-xNdxZrTi2-xFexO7 ceramics, the lattice parameters of both zirconolite-2M and zirconolite-3O gradually increase with x increasing. The lattice parameter a increase with increasing of x while slightly change of b and c were found in zirconolite-2M phase of Ca1-xGdxZrTi2-xFexO7 ceramics. In zirconolite-2M type Ca1-xHoxZrTi2-xFexO7 ceramics, lattice parameter a increase with x increasing, reverse trend on lattice parameter b and c. Both lattice parameter b and c show a decreasing trend in single zirconolite-2M type Ca1-xYbxZrTi2-xFexO7 (0.1&lex&le0.6) solid-solutions while slight change on lattice parameter a. The different lattice parameters variation trends of zirconolite in Ca1-xLnxZrTi2-xFexO7 (Ln=Nd, Gd, Ho, Yb) may be attributed to the different substitution mechanisms of Ln3+ and Fe3+ in zirconolite-2M. In addition，we innovatively employed both Al3+ and Fe3+ to replace Ti4+ of zirconolite as charge compensation, and rationally designed a series Ca1&minusxLnxZrTi2&minusx(Al, Fe)xO7 (Ln=La, Nd, Gd, Ho, Yb) experiments. And utilized synchrotron X-ray methods to unravel the dopants solubility, phase transformations and coordination environment of cations. Zirconolite single-phase cannot form and the fraction of perovskite increase with x increasing in Ca1&minusxLaxZrTi2&minusx(Al, Fe)xO7 solid-solutions. In the Ca1&minusxLnxZrTi2&minusx (Al, Fe)xO7 (Ln=Nd, Gd) ceramics, both zirconolite-2M and zirconolite-3O were observed. Phase transformation of zirconolite-2M to zirconolite-3O occurs at x=0.7 and 0.9 for Ca1&minusxNdxZrTi2&minusx(Al, Fe)xO7 and Ca1&minusxGdxZrTi2&minusx(Al, Fe)xO7, respectively. The solubility of Ho in Ca1&minusxHoxZrTi2&minusx(Al, Fe)xO7 to form single-phase zirconolite-2M can reach to 0.9 f.u.. In the Ca1&minusxYbxZrTi2&minusx(Al, Fe)xO7 ceramics, single zirconolite-2M can only be observed at x&le0.7, zirconolite-2M (and/or 3T) with trace zirconia was observed when x=0.9. XANES revealed that Fe3+ ions incorporate into both TiO5 and TiO6 of Ca1&minusxHoxZrTi2&minusx(Al, Fe)xO7, and the ratio of TiO5 to TiO6 decreases with the increase of dopants, the coordination environment of Zr4+ is ZrO7 with trace ZrO8. Zirconolite and pyrochlore, as potential HLW ceramic matrices, belong to anion-deficient fluorite-derived superstructures, and could transform each other under specific conditions. we use a high valent Nb5+ substituted Ti4+ as charge balance, a series of CaZr1-xNdxTi2-xNbxO7 (x=0.05-1.0) solid solutions were synthesized. Trace perovskite was observed in all the CaZr1-xNdxTi2-xNbxO7 (x=0.05-1.0) ceramics. The evolutions of zirconolite-2M to zirconolite-4M to pyrochlore were found at x=0.25 and 0.3, respectively. And co-existence of zirconolite-2M and zirconolite-4M in x=0.25-0.4 samples. Approximately single-phase pyrochlore appeared when x&ge0.5. However, a new cubic-215 crystalline phase which belong to partially disordered pyrochlore or defective fluorite structure appeared in the sample with x=1.0. Zirconolite-2M, zirconolite-3T and perovskite co-existed in the sample with x=0.2. The Rietveld refinement base on SXRD determined that Nd3+ and Nb5+ incorporated into Zr site and Ti1 (TiO6) site of zirconolite-2M, respectively. In pyrochlore(A2B2O7) structure, Nd3+, Ca2+ and Zr4+ occupy the A sublattice while Nb5+ and Ti4+incorporate B sublattice. |
| Pages | 119 |
| URL | 查看原文 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | https://ir.lzu.edu.cn/handle/262010/465255 |
| Collection | 核科学与技术学院 |
| Affiliation | 核科学与技术学院 |
| First Author Affilication | School of Nuclear Science and Technology |
| Recommended Citation GB/T 7714 | 吉诗银. Ln-M共掺杂钙钛锆石的固溶行为研究 (Ln=La, Nd, Gd, Ho, Yb; M=Al, Fe, Nb)[D]. 兰州. 兰州大学,2020. |
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