Phase and microstructure evolution of 0.2Zr1-xCexO2/Zr1-yCeySiO4 (0 ≤ x + y ≤ 1) ceramics designed to immobilize tetravalent actinides

https://doi.org/10.1016/j.jnucmat.2020.152318Get rights and content

Highlights

  • Ce-doped double-phase 0.2Zr1-xCexO2/Zr1-xCexSiO4 (0≤x ≤ 1) ceramics were fabricated.

  • Effect of Ce content on the phase and microstructure evolution was elucidated.

  • ZrSiO4 retained tetragonal, while ZrO2 transformed from monoclinic to cubic.

  • Lattice parameters increased with Ce-doping, revealing lattice immobilization of Ce.

  • Grain size and compactness increased with increasing Ce content.

Abstract

Zircon is important ceramic used widely due to its excellent properties, in particular for nuclear waste immobilization. However, it is difficult to obtain mono-phase zircon with high yield, usually result in double-phase ZrO2/ZrSiO4 ceramics. Herein, Ce-doped double-phase 0.2Zr1-xCexO2/Zr1-yCeySiO4 (0 ≤ x + y ≤ 1) ceramics were fabricated and their phase and microstructure evolution were studied. The results demonstrated that Ce content has great influence on the phase composition and microstructure. The ceramics with 0 ≤ x + y < 0.1 are tetragonal-ZrSiO4, monoclinic-ZrO2 and tetragonal-ZrO2 phases, with 0.1 ≤ x + y ≤ 0.3 are tetragonal-ZrSiO4, monoclinic-ZrO2, tetragonal-ZrO2 and cubic-ZrO2 phases, with 0.3 < x + y ≤ 0.6 are tetragonal-ZrSiO4, tetragonal-ZrO2 and cubic-ZrO2 phases, and with 0.6 < x + y ≤ 1 are cubic-ZrO2 and Ce2Si2O7 phases. With increasing Ce-doping content, the ZrSiO4 retained the tetragonal phase, while ZrO2 transformed from monoclinic to stable cubic phase. Furthermore, lattice parameters, grain size and compactness increased with increasing Ce content.

Introduction

Several ceramics such as phosphate [1,2], zirconolite [3], pyrochlore [4] and zircon [5] have been extensively studied due to their capacities to immobilize nuclear waste. Among these ceramics, Zircon (ZrSiO4) has been widely studied by scholars because of its excellent physical and chemical properties, and potential application in various fields [[6], [7], [8], [9]]. Particularly, in the field of high level radioactive waste treatment and disposal, zircon ceramic has been considered as host phase for the immobilization of weapons grade plutonium and other actinides due to its good chemical stability as well as the capacity to immobilize actinides in its lattice [5,10,11].

Up to now, considerable works have been devoted to the synthesis of zircon ceramic [[12], [13], [14], [15], [16], [17], [18]]. In particular, Pu-doped zircon based ceramics were prepared by Hancharet al. [19]. Uranothorite (Th1-xUxSiO4) has been synthesized in a complete series from thorite (ThSiO4) to coffinite (USiO4) [[20], [21], [22], [23]]. Zamoryanskaya et al. [24] synthesized the Ce-doped zircon ceramics. Recently, synthesis of pure CeSiO4 [25,26] and PuSiO4 [27] has been reported by Estevenon et al.. (Zr, Pu)SiO4 and (Hf, Pu)SiO4 zircon ceramics were prepared and studied by Burakov et al. [28]. Furthermore, Ferriss et al. [29] found that the Zr1-xPuxSiO4 and Zr1-xCexSiO4 solid solutions with high x values might be thermodynamically unstable compared to the respective oxide and silica mixtures based on Monte Carlo simulation. It was found that it is very difficult to synthesize mono-phase zircon ceramics with high yield. This may be due to any or all of the following reasons: (1) the slow diffusion kinetics, small formation free energy, and large activation energy for the reaction between ZrO2 and SiO2 exhibits [30,31], (2) the large change in the crystallographic environment of Zr between ZrO2 and ZrSiO4 structures, and the low solubility of SiO2 in ZrO2 [31], (3) the technology and synthesis method problems. Therefore, the double-phase ZrO2/ZrSiO4 ceramics were usually obtained in the preparation of mono-phase ZrSiO4 ceramics.

The evolution of phase and microstructure of mono-phase zircon ceramics has been widely investigated [[32], [33], [34], [35]]. Particularly, considerable works have been focused on the effect of element doping on the phase and microstructure of zircon ceramics. Ni-doped ZrSiO4 ceramics were prepared by the sol-gel method and their microstructural evolution was studied [36]. Tu et al. [37] studied the evolutions of phase and microstructure of Ce-doped zircon ceramics obtained from the sol-gel method. In our previous works [38,39], phase and microstructure evolution of Nd-doped, and Nd and Ce co-doped zircon ceramics designed to immobilize trivalent, and trivalent and tetravalent actinides have been studied. It was found that a small amount of the second phase ZrO2 was usually detected in zircon ceramics. The prepared ceramics exhibited double-phase ZrO2/ZrSiO4. The existence of ZrO2 phase may be not bad for nuclear waste immobilization. Zirconia ceramics have received considerable attention due to their ability to immobilize actinides and other oxides [40,41]. ZrO2 ceramic waste forms containing simulated plutonium was prepared by Gong et al. [42]. The results demonstrated that plutonium (simulated by Ce4+, U4+ or Th4+) and impurities (eg. Ca, Fe, Mg and Si) were incorporated into ZrO2. Previous works show that the approximate solubility limits of oxides in ZrO2 are: PuO2 22 mol% [43], UO2 85 mol% [44] and CeO2 18 mol% [45]. Therefore, ZrO2 ceramics have long been recognized as promising host matrices for nuclear waste immobilization. To the best of our knowledge, there have few studies on the actinides or simulated actinides immobilization behavior of double-phase ZrO2/ZrSiO4 ceramics. (Zr, Pu)SiO4 and (Zr, Pu)O2 double phase ceramics have been studied by Burakov et al. [46]. They found that the optimal Pu loading of zircon/zirconia ceramics is approximately 6 wt% Pu, and even radiation damaged zircon/zirconia ceramics retain high chemical resistance and mechanical durability. However, the information about the phase and microstructure evolutions of Ce-doped double-phase ZrO2/ZrSiO4 ceramics was unavailable. Therefore, it is of interest to study the simulated actinides immobilization behavior of the double-phase ZrO2/ZrSiO4 ceramics.

The main goal of this work was to gain knowledge on the substitution of the simulated tetravalent actinide (Ce) for Zr in ZrO2/ZrSiO4 and the effect of Ce content on the phase and microstructure evolution of 0.2Zr1-xCexO2/Zr1-yCeySiO4 (0 ≤ x + y ≤ 1) ceramics. Here, cerium (Ce) was employed as the surrogates for plutonium (Pu) due to the similar ionic radius (Pu 0.96 Å and Ce 0.97 Å) and similar outer shell electron distribution (Pu 5f66d07s2 and Ce 4f15d16s2) [47]. However, the use of Ce as Pu surrogate for zircon-based ceramics should be considered as limited [48]. Because in CeO2-SiO2 system the cerium (IV) reduction to cerium (III) could be observed over 1400 °C even under air atmosphere [49]. (Zr, Ce)O2 solid solutions can be synthesized by solid state reaction method [50]. Furthermore, it was found that the increase in Ce content promote the phase transformation from monoclinic to cubic. Estevenon et al. [25] reported that CeSiO4 could not be obtained starting from Ce (IV) reactants. Pure CeSiO4 can be prepared through a hydrothermal treatment at 150 °C using a starting mixture of 1 mol L−1 Ce(III) nitrate and Na2SiO3 solutions and by adjusting the initial pH to 8. In addition, CeSiO4 crystallized in the zircon structure (I41/amd space group) with the following unit cell parameters: a = 6.9603(1) Å, c = 6.1946(2) Å, i.e. V = 300.11(2) Å3. A series of Ce-doped double-phase ZrO2/ZrSiO4 ceramics with the general formula of 0.2Zr1-xCexO2/Zr1-yCeySiO4 (0 ≤ x + y ≤ 1) were prepared by solid state reaction method. In order to understand the Ce immobilization behavior of double-phase ZrO2/ZrSiO4 ceramics, the powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were employed to analyze the obtained ceramics. The effect of Ce content on the phase and microstructure evolution of Ce-doped double-phase ZrO2/ZrSiO4 ceramics was investigated in detail.

Section snippets

General formula design

The double-phase 0.2ZrO2/ZrSiO4 ceramics were obtained with the ZrO2/SiO2 molar ratio of 1.2:1 (excess of 20 mol% ZrO2). According to the theory of isomorphism, we speculate the Zr sites in the double-phase 0.2ZrO2/ZrSiO4 structure can be substituted by Ce. Considering the ions charge balance and the Ce is in the tetravalent state, the chemical general formula 0.2Zr1-xCexO2/Zr1-yCeySiO4 (0 ≤ x + y ≤ 1) was used. However, if Ce(IV) was reduced to Ce(III) at high temperature, the theoretical

Phase evolutions

The effect of Ce content (x) on the phase of the Ce-doped 0.2ZrO2/ZrSiO4 ceramics sintered at 1550 °C for 72 h in air was investigated in detail by analyzing their XRD patterns. Fig. 1 shows the XRD patterns of 0.2Zr1-xCexO2/Zr1-yCeySiO4 (0 ≤ x + y ≤ 1) compositions. The detailed phase compositions of 0.2Zr1-xCexO2/Zr1-yCeySiO4 (0 ≤ x + y ≤ 1) ceramics are listed in Table 2. It can be clearly observed that the Ce content has great influence on the phase compositions of the obtained ceramics. In

Conclusions

In summary, a series of 0.2Zr1-xCexO2/Zr1-yCeySiO4 (0 ≤ x + y ≤ 1) ceramics have been successfully obtained by doping Ce into 0.2ZrO2/ZrSiO4 via solid state reaction method. The phase and microstructure evolution of Ce-doped zirconia/zircon ceramics have been elucidated. The results demonstrated that Ce has been successfully immobilized into the ZrO2/ZrSiO4 lattice as designed. It was found that the Ce content has great influence on the phase composition and microstructure of the obtained

CRediT authorship contribution statement

Yi Ding: Investigation, Funding acquisition, Writing - original draft. Zhengdi Jiang: Investigation, Writing - review & editing. Tianheng Xiong: Data curation, Investigation. Zimei Bai: Investigation. Dandan Zhao: Writing - review & editing. Hui Dan: Investigation, Methodology. Tao Duan: Conceptualization, Methodology, Supervision, Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the National Defense Basic Scientific Research Program (No. JCKY2019404D001), the application basic research project of Science and Technology Department of Sichuan Province (No. 20YYJC3547) and the doctor research Foundation of Southwest University of Science and Technology (No. 13zx7136).

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