Structural stability of ZnAl2O4 spinel irradiated by low energy particles

It is now well known that irradiation of metals and alloys can drive materials into complex configurations. Several examples, like the occurrence of order–disorder phase transitions driven by irradiation, are discussed by many authors. To understand the behavior of ceramics under irradiation, several spinels were irradiated. In this paper, experimental results on the irradiation of ZnAl₂O₄ by low-energy particles (4 MeV Au²⁺ ions) are presented. An order–disorder phase transition is observed. The unusual behavior of this spinel under irradiation is discussed within the framework of Martin’s theory of driven alloys under irradiation.     

Surface modification of MgAl2O4 and oxides with heavy ions of fission fragments energy

In this paper we report on results of surface modification in several candidate materials for inert matrix fuel hosts (MgAl₂O₄, MgO and Al₂O₃) induced by (0.5–5) MeV/amu Kr, Xe and Bi ion bombardment in the fluence range of 2 × 10¹⁰–10¹² ions/cm². The size and shape of nanoscale hillock-like defects, each of which was created by the impact of a single ion, have been studied by using atomic force microscopy (AFM) technique. It was found that mean hillock height on sapphire and spinel surfaces depends linearly on the incident electronic stopping power. The hillocks are highest in MgAl₂O₄, having a lower threshold for the lattice disorder in the bulk material via relaxation of electronic excitations. As a possible reason for the hillocks formation, the plastic deformation due to the defects created by the Coulomb explosion mechanism in the target subsurface layer is suggested.     

Real-time evolution of ion–surface interactions of MgAl2O3 and LiNbO3 detected by ion-induced photon spectroscopy

Ion-induced photon emission under 60 keV Cu⁻ implantation into the insulators of MgAl₂O₄ and LiNbO₃ was in situ measured over a wide wavelength range from 200 to 900 nm. The formation of the Mg and Li deficient layers for the respective insulator was detected based on the dose dependence of the Al, Mg and Li atomic line intensities. The Al I and Mg I line intensities from spinel exhibit different behaviors depending on irradiation temperatures: they gradually decrease while the (Al I)/(Mg I) ratio increases up to dose of 3×10¹⁷ ions/cm² at room temperature or exhibits a steady-state tendency at high temperature. Sharp decrease of Li I line intensity from LiNbO₃ under high dose rate bombardment indicates drastic changes of surface layer that may alter the phase stability and optical performance of insulators.     

Melting behaviour of oxide systems for heterogeneous transmutation of actinides. II. The system MgO–Al2O3–PuO2

Isothermal sections of the phase diagram of the system MgO–Al₂O₃–PuO₂ at various temperatures were calculated using sublattice models. The results show that below 2133 K no liquid occurs in the system. Above 2133 K liquid starts to form at the Al₂O₃–PuO₂ side. The phase diagram of the pseudo-binary system PuO₂–MgAl₂O₄ was also obtained from an isopleth T–x calculation.     

Radiation effects on MgAl2O4–yttria stabilized ZrO2 composite material irradiated with Ne ions at high temperatures

Spinel (MgAl₂O₄) and yttria stabilized ZrO₂ (YSZ) are candidates for fuel materials for use in nuclear reactors and the optical and insulating materials for fusion reactors. In our previous studies, the amorphization of spinel under 60 keV Xe ion irradiation at RT was observed. On the other hand, amorphization could not be confirmed in YSZ single crystals under the same irradiation conditions. In the present study, the damage evolution process of polycrystalline spinel–YSZ composite materials has been studied by in situ TEM observation during ion irradiation. The irradiation was performed with 30 keV Ne⁺ ions at a flux of 5 × 10¹³ ions cm⁻² s⁻¹ at 923 K and 1473 K, respectively. The observed results revealed a clear difference in morphology of damage depending on irradiation temperature and crystal grains. In the irradiation at 923 K, defect clusters and bubbles were formed homogeneously in YSZ grains. On the other hand, at 1473 K, only bubble formation was observed. The bubbles grew remarkably with increasing ion fluence in both grains. Even though the growth of the bubbles was observed in both grains, the average diameter of grown bubbles in spinel grains was larger than those in YSZ ones. The bubbles tended to form along the grain boundary at both temperatures.     

Post-irradiation examination of uranium-doped rock-like oxide fuels

Post-irradiation examinations of rock-like oxide fuels were performed in JAERI to evaluate irradiation behavior and geochemical stability. Five kinds of fuels were prepared using 20% enriched U instead of Pu; a single-phase fuel of an yttria-stabilized zirconia containing UO₂ (U-YSZ), two particle-dispersed type fuels of U-YSZ particles + MgAl₂O₄/Al₂O₃ powder, two homogeneously blended type fuels of U-YSZ powder + MgAl₂O₄/Al₂O₃ powder. The fuels were irradiated in JRR-3 for about 100 days and estimated irradiation conditions were as follows; linear power was 15 kW m⁻¹, thermal neutron fluence was 7 x 10²⁴ m⁻² and fuel temperatures at the surface were 740–1130 K. From the results of non-destructive examinations, the stainless steel cladding surfaces were partially discolored by oxidation and no remarkable deformation of the pins was observed. Significant pellet fragmentation was not observed in spite of the crack formation as observed in irradiated LWR UO₂ fuels. Nonvolatile FPs were observed only in pellets but volatile Cs moved partly to the plenum. From these examinations, no significant difference in macroscopic irradiation behavior was distinguished among 5 fuels.     

Heavy ion irradiation effects in the rare-earth sesquioxide Dy2O3

Polycrystalline pellets of the rare-earth sesquioxide Dy₂O₃ with cubic C-type rare-earth structure were irradiated with 300 keV Kr²⁺ ions at fluences up to 5 × 10²⁰ Kr/m² at cryogenic temperature. Irradiation-induced microstructural evolution is characterized using grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM). In previous work, we found a phase transformation from a cubic, C-type to a monoclinic, B-type (C2/m) rare-earth structure in Dy₂O₃ during Kr²⁺ ion irradiation at a fluence of less than 1 × 10²⁰ Kr/m². In this study, we find that the crystal structure of the top and middle regions of the implanted layer transform to a hexagonal, H-type (P6₃/mmc) rare-earth structure when the irradiation fluence is increased to 5 × 10²⁰ Kr/m²; the bottom of the implanted layer, on the other hand, remains in a monoclinic phase. The irradiation dose dependence of the C-to-B-to-H phase transformation observed in Dy₂O₃ appears to be closely related to the temperature and pressure dependence of the phases observed in the phase diagram. These transformations are also accompanied by a decrease in molecular volume (or density increase) of approximately 9% and 8%, respectively, which is an unusual radiation damage behavior.     

Carbon deposition from a γ-irradiated CO2/CO/CH4/C2H6 gas mixture on the manganese oxides MnO, Mn3O4 and Mn2O3

The composition of oxides formed on steel surfaces within power reactors may influence heat transfer efficiency. Previous studies have indicated that carbon is deposited on spinal-type oxides containing manganese, iron, cobalt, nickel and chromium. In this investigation, characterised manganese oxides have been subjected to γ-irradiation under conditions similar to those experienced in reactors in an effort to understand the catalytic processes involved in deposit initiation and growth. Mn₃O₄ and Mn₂O₃, under the conditions present in the γ-cell, were reduced to MnO during the time of exposure. Relative carbon deposition rates were observed to follow the trend MnO>Mn₃O₄≈Mn₂O₃.     

镧系元素模拟衰变子体对An_2Zr_2O_7固化体结构及其γ辐照性能的影响

锆基烧绿石An2Zr2O7以优异的抗辐照性能和化学稳定性成为高放废物中锕系核素的理想固化基材,高放废物固化体在长期贮存过程中不断衰变产生衰变子体,必将影响固化体的结构和性能。本文以镧系核素Nd模拟锕系核素Pu、Am,La模拟其衰变子体U、Np,通过溶胶凝胶方法合成了(LaxNd1-x)2Zr2O7模拟固化体。样品经高能γ辐照,辐照剂量为233.78kGy。利用X射线衍射、Raman振动光谱和结构精修方法对辐照前后的系列样品进行了分析。结果表明:(LaxNd1-x)2Zr2O7系列固化体均为单一的烧绿石结构相;固化体的晶格常数随La的增加呈线性增加,晶体结构趋于有序化,意味着衰变子体有助于固化体趋向于更加有序的烧绿石结构。γ辐照和结构精修结果表明,随着子体的增加,An—O48f键长增大,离子键结合力减小,在辐照情况下晶格易发生无序化,抗辐照能力减弱。     

Molecular dynamics modelling of radiation damage in normal, partly inverse and inverse spinels

The radiation response of perfect crystals of MgAl₂O₄, partially inverted MgGa₂O₄ and fully inverse MgIn₂O₄ were investigated using molecular dynamics. Dynamical cascades were initiated in these spinels over a range of trajectories with energies of 400 eV and 2 keV for the primary knock-on event. Collision cascades were set up on each of the cation and anion sublattices and were monitored up to 10 ps. Simulations in the normal MgAl₂O₄ spinel for the 2 keV energy regime resulted in similar defect structures as obtained at the post-threshold 400 eV energies, with little clustering occurring. The predominant defect configurations were split interstitials and cation antisites. For the inverse spinels, a much wider variety of lattice imperfections was observed. More defects were also produced due to the formation of interstitial–vacancy cation chains and oxygen crowdions.     

Low-temperature diffusion in inert-gas bombarded Al2O3

Previous work on diffusion in inert-gas bombarded Al₂O₃ has revealed the presence of four diffusion processes, of which two take place well below the temperatures for self-diffussion, one agrees with self-diffusion, and one occurs at temperatures well above those for self-diffusion. The present work serves to explore in greater detail the two low-temperature processes. It is shown that the first, which is found in -Al₂O₃, Al(OH)₃, and γ-Al₂O₃beginning at about 100° C, is consistent with a range of ΔH's of 28 to 50 kcal/mole. The mechanism of the process is hinted at by the fact that it overlaps the temperatures both for Al(OH)₃decomposition and for point-defect motion in -Al₂O₃; the correlation with point defects is believed, however, to be the more significant. The second process, which is found only in -Al₂O₃ beginning at 500–650° C, implies an essentially single ΔH lying between 69 and 79 kcal/mole. It was suggested previously by Matzke and Whitton on the basis of electron diffraction that the process could be attributed to the amorphous-crystalline transition of -Al₂O₃. Further aspects of low-temperature diffusion in Al₂O₃ were revealed by comparing autoradiographs of specimens of -A₂O₃which were bombarded to various doses and then either heated to 850° C or immersed in unheated 12N NaOH. Thus regions exposed to a high dose and which would be expected to be amorphous, had an increased sticking factor, a greater tendency to lose gas during heating, and an enhanced chemical reactivity.     

Post irradiation examination of uranium inclusions in inert matrices.

The inert matrix materials CeO₂, MgO, Y₂O₃, MgAl₂O₄ and Y₃Al₅O₁₂ were selected as candidates for inert matrices for the EFTTRA²-T3 neutron irradiation experiment. Most targets contain 20% enriched ²³⁵U fissile inclusions with an average size of roughly 150 μm. The volume fraction of the fissile phase is either 2.5 vol% UO₂ or 19.6 vol% of Y_5.78UO_x in the inert matrices. The samples were irradiated for 198.9 full power days in the High Flux Reactor in Petten. The calculated burn-up is between 17.3 and 19.5% FIMA. The temperature of the cladding was kept at 600 ± 25 K. A dimensional change of at least +5 % is measured after neutron irradiation for Y₃Al₅O₁₂ and MgAl₂O₄ with macro dispersions of UO₂; the other targets with a macro dispersion of UO₂ show a volume change of less than + 1 vol%. The fractional release of the fission gas Xe is more than 40% for the MgAl₂O₄ and Y₃Al₅O₁₂ matrices with a macro dispersion of UO₂, the other targets show a fractional release of Xe of less than 15%. Cracks are observed in MgO and MgAl₂O₄ targets which is possibly related to the stress caused by swelling of the UO₂ inclusions.     

阳离子有序相Ce2Zr2O7.97的石墨还原制备及其结构表征

采用石墨还原法探索了CeO₂-ZrO₂二元体系在不同还原温度、还原时间条件下的物相组成及结构,借助X射线衍射、红外振动光谱等手段对还原样品进行了物相表征及结构分析。实验结果表明：通过石墨还原能在较低温度（T_red＝1050 ℃）、较短时间（t＝24 h）内合成纯相Ce₂Zr₂O_7.97,较传统先还原（95%Ar+5%H₂,T_red≥1400℃,t＞48 h）后氧化（O₂或空气气氛）工艺更简单经济;Ce₂Zr₂O_7.97相能在1050～1300 ℃、24～72 h还原条件下稳定存在并保留至室温而不发生相分解,其机制可能归因于石墨所形成的弱还原环境;Ce₂Zr₂O_7.97相拥有与烧绿石Ce₂Zr₂O₇相近的有序阳离子亚晶胞结构,但由于其氧空位（O3、O4、O11）被大部分氧离子所填充,导致该相结构对称性降低。本文提出的石墨还原法制备富氧相Ce₂Zr₂O_7.97有望成为一种较传统制备工艺更简单经济的新方法,而Ce₂Zr₂O_7.97相能在还原气氛中稳定存在,可为An₂Zr₂O_7+x固化体的存放环境提供借鉴和参考。     

Thermodynamic studies on Cs4U5O17(s) and Cs2U2O7(s) by emf and calorimetric measurements

The oxygen potentials over the phase field: Cs₄U₅O₁₇(s)+Cs₂U₂O₇(s)+Cs₂U₄O₁₂(s) was determined by measuring the emf values between 1048 and 1206 K using a solid oxide electrolyte galvanic cell. The oxygen potential existing over the phase field for a given temperature can be represented by: Δμ(O₂) (kJ/mol) (±0.5)=−272.0+0.207T (K). The differential thermal analysis showed that Cs₄U₅O₁₇(s) is stable in air up to 1273 K. The molar Gibbs energy formation of Cs₄U₅O₁₇(s) was calculated from the above oxygen potentials and can be given by, Δ_fG⁰ (kJ/mol)±6=−7729+1.681T (K). The enthalpy measurements on Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) were carried out from 368.3 to 905 K and 430 to 852 K respectively, using a high temperature Calvet calorimeter. The enthalpy increments, (H⁰_T−H⁰₂₉₈), in J/mol for Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) can be represented by, H⁰_T−H⁰_298.15 (Cs₄U₅O₁₇) kJ/mol±0.9=−188.221+0.518T (K)+0.433×10⁻³T² (K)−2.052×10⁻⁵T³ (K) (368 to 905 K) and H⁰_T−H⁰_298.15 (Cs₂U₂O₇) kJ/mol±0.5=−164.210+0.390T (K)+0.104×10⁻⁴T² (K)+0.140×10⁵(1/T (K)) (411 to 860 K). The thermal properties of Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) were derived from the experimental values. The enthalpy of formation of (Cs₄U₅O₁₇, s) at 298.15 K was calculated by the second law method and is: Δ_fH⁰_298.15=−7645.0±4.2 kJ/mol.     

掺杂Cr2O3对UO2芯块烧结行为的影响

提高燃料燃耗的一个有效手段是通过增大UO₂晶粒尺寸来减少元件内部气体压力,在大晶粒UO₂芯块中,裂变气体到达晶界表面的距离增加,因而裂变气体的释放速率降低,元件内部气体压力的增高缓慢。本文研究了添加Cr₂O₃对UO₂晶粒尺寸的影响。对纯UO₂、添加0.5% Cr₂O₃及5% Cr₂O₃ 3种配方的芯块进行了试验,在5%H₂Ar保护下,以10 ℃/min和5 ℃/min的升温速率升温至1 700 ℃,然后烧结2 h或4 h,对比纯UO₂芯块与添加Cr₂O₃的芯块发现,添加Cr₂O₃可有效增大晶粒尺寸;较长的烧结时间可促进晶粒长大;较低的升温速率也使晶粒长大。烧结过程产生液相烧结,液相浸润晶粒边界,促进晶粒长大。     

Enhanced oxygen vacancy aggregation and colloid production in Al2O3

Optical luminescence and absorption measurements on Al₂O₃ (sapphire) electron irradiated at temperatures between 200 and 270°C with and without an applied electric field have been used to identify an oxygen vacancy aggregation process leading to the formation of aluminium colloids. This process, which is only observed when an electric field is applied during irradiation, occurs within the volume and is considered as a possible precursor for RIED. The results help to clarify the observed similarity between RIED and colloid production, and help to provide an explanation for the observation of gamma alumina in sapphire which has suffered RIED.     

Argon irradiation damage at a Cu/Al2O3 interface for different alumina structures

The evolution of damages at a Cu/Al₂O₃ device interface after Ar⁺ irradiation, depending on alumina structure, and the effect of surface roughness on sputtering have been studied. A polycrystalline Cu/Al₂O₃ bilayer and polycrystalline Cu on amorphous alumina were irradiated with 400 keV Ar⁺ ion beam at doses ranging from 5 × 10¹⁶ to 10¹⁷ Ar⁺/cm² at room temperature. The copper layer thicknesses were between 100 and 200 nm. RBS analysis was used to characterize the interface modification and to deduce the sputtering yield of copper. The SEM technique was used to control the surface topography. A RBS computer simulation program was used to reproduce experimental spectra and to follow the concentration profile evolutions of different elements before and after ion irradiation. A modified TRIM calculation program which takes into account the sputtering yield evolution as well as the concentration variation versus dose gives a satisfactory reproduction of the experimental argon distribution. The surface roughness effect on sputtering and the alumina structure influence at the interface on mixing mechanisms are discussed.     

Er2Ti2O7烧绿石基玻璃陶瓷固化体的制备工艺研究

为获得含单一钛酸盐烧绿石的玻璃陶瓷固化体,本文以Er₂Ti₂O₇作为钛酸盐烧绿石代表,在传统的烧结法制备工艺基础上,通过热雾喷解和晶核掺入的技术改进,分别研究了烧结温度、烧结时间、玻璃与陶瓷晶核质量配比等参数对Er₂Ti₂O₇基玻璃陶瓷固化体物相及结构的影响规律。XRD结果表明：在不同的烧结温度、烧结时间和质量配比下,均能获得含单一Er₂Ti₂O₇烧绿石的玻璃陶瓷固化体,Er₂Ti₂O₇前驱体在玻璃中的相稳定性好,未发生相分解,玻璃组分亦未受陶瓷相的影响而析出第二相;玻璃组分对陶瓷颗粒存在张应力作用,玻璃含量越高,Er₂Ti₂O₇的晶胞常数越大;提高烧结温度和烧结时间均有利于增强固化体中Er₂Ti₂O₇的结构有序性,但烧结温度效果强于烧结时间。SEM结果表明：Er₂Ti₂O₇在玻璃基体上呈四方形生长,与玻璃界面清晰,相容性好。固化体的物性测试结果表明,玻璃组分占比将直接影响固化体的表观孔隙率和致密度。根据以上结果可知,新工艺制备的样品具有相纯度高、烧绿石相与玻璃相容性好、两相比例可调等优点,较好地解决了钛酸盐烧绿石在玻璃基体中易发生相分解的问题。     

基于铌酸银的耐高温放射性碘吸附剂的制备及性能

通过机械混合法制备了一种基于铌酸银(AgNbO_3)的耐高温放射性碘吸附剂(AgNbO_3/Al_2O_3)。和常规载银吸附剂(Ag/Al_2O_3)相比,AgNbO_3/Al_2O_3吸附剂的吸附性能更为稳定;特别是在650℃以上时,其对放射性碘的去污因子远高于常规载银吸附剂。表征测试结果表明,该吸附剂结构稳定性良好,可耐受较长时间的高温。热重测试和高温脱附试验等结果表明,碘化银在AgNbO_3/Al_2O_3吸附剂表面稳定性的提高是该吸附剂在高温时吸附性能更佳的主要原因,其将来有望用于核事故中的应急处置。