scholarly journals Rapid Synthesis and Chemical Durability of Gd2- x Nd x Zr2O7 (0.0 ≤ x ≤ 2.0) Sub-Micron Ceramics as Nuclear Waste Forms

2020 ◽  
Author(s):  
Kui Liu ◽  
Kuibao ZHANG ◽  
Ting DENG ◽  
Jianjun ZENG ◽  
Daimeng CHEN ◽  
...  
Keyword(s):  
Grain Size ◽  
Solvothermal Method ◽  
Pyrochlore Structure ◽  
Fluorite Structure ◽  
Nanocrystalline Powders ◽  
Release Rates ◽  
Chemical Furnace ◽  
Sample Density ◽  
Nuclear Waste Forms ◽  
Trivalent Actinide

Abstract In this work, Nd3+ was used as a surrogate and it was incorporated into Gd2Zr2O7 nanocrystalline ceramics to simulate the immobilization of trivalent actinide elements. A series of Gd2 − xNdxZr2O7 (0.0 ≤ x ≤ 2.0) nanocrystalline powders were fabricated by solvothermal method, and then Gd2 − xNdxZr2O7 sub-microncrystalline ceramics were prepared by sintering via self-propagating chemical furnace plus quick pressing (SCF/QP). All powders are in defective fluorite structure, and Nd doping hardly change the powder grain size. After analyzing the sintered ceramics, it can be found that the transition from defective fluorite structure to pyrochlore structure occurs when x ≥ 1.5. The sample density decreases with elevated Nd content, while the grain size gradually enlarges. Besides, the normalized release rates of Nd and Zr elements in the Nd2Zr2O7 waste form are kept in low values (below 10− 5 g•m− 2•d− 1), which exhibits its excellent aqueous stability.

Heavy Ion Irradiation of Zirconate Pyrochlores

2002 ◽  
Vol 713 ◽  
Author(s):  
J. Lian ◽  
L. M. Wang ◽  
J. Chen ◽  
R. C. Ewing ◽  
K. V. G. Kutty
Keyword(s):  
Ion Irradiation ◽  
Ion Beam ◽  
Amorphous State ◽  
Pyrochlore Structure ◽  
Fluorite Structure ◽  
Heavy Ion ◽  
Ionic Radii ◽  
Radiation Resistant ◽  
A Site ◽  
Nuclear Waste Forms

ABSTRACTZirconate pyrochlores, A2Zr2O7, are important potential nuclear waste forms for Puimmobilization. The binary Gd2(Ti2-xZrx)O7 has been shown to have increasing resistance to ionirradiation damage with the increasing Zr content, and Gd2Zr2O7 is radiation resistant to a 1 MeV Kr+ ion irradiation at 25 K to a dose of 5 dpa. In this study, a 1.5 MeV Xe+ irradiation was completed for zirconate pyrochlores A2Zr2O7 (A=La, Nd, Sm, Gd). The radiation resistance decreases with an increase of the ionic radius of A-site cation. La2Zr2O7 is the first zirconate pyrochlore to be amorphized by ion beam irradiation, and the critical amorphization temperature, Tc, is ∼310 K. The susceptibility of La2Zr2O7 to ion beam damage is related to its structure, which shows the largest deviation from the ideal fluorite structure. These results are also consistent with calculations of the cation antisite formation energy in the pyrochlore structure. The ion irradiation-induced pyrochlore-to-fluorite transformation occurred in all of the irradiated zirconate pyrochlore phases. Based on the results for Gd2Ti2-xZrxO7 and A2Zr2O7, the defect fluorite structures are stable when the ionic radii ratio rA/rB≤1.54; beyond this limit, the defect fluorite structure becomes increasingly unstable relative to the amorphous state.

Fabrication and Characterization of Ce-Doped Gd2Zr2O7 Ceramics as Simulated Waste Forms for Treating Pu

2015 ◽  
Vol 737 ◽  
pp. 639-641 ◽  
Author(s):  
Yan Kai Yang ◽  
Yi Xie ◽  
Xi Rui Lu
Keyword(s):  
Crystal Structure ◽  
Grain Size ◽  
Atmospheric Pressure ◽  
Pyrochlore Structure ◽  
Fluorite Structure ◽  
Waste Forms ◽  
Simulated Waste ◽  
Fabrication And Characterization ◽  
Structure And Microstructure

In order to investigate the disposal of plutonium by Gd2Zr2O7, Ce-doped Gd2Zr2O7 were fabricated by high temperature solid state reaction at 1500 °C under atmospheric pressure for 72 h. XRD and SEM studies were exploited to characterize the crystal structure and microstructure of the synthetic samples. The results revealed that Gd2Zr2O7 shows a single pyrochlore structure while Gd2Ce2O7 exhibit a defective fluorite structure. The microstructure of the samples was relatively dense, and the grain size was in the range of 3 - 6 μm.

The Kinetics of Ordering in Gadolinium Zirconate: an Unusual Oxygen Ion Conductor

1995 ◽  
Vol 398 ◽  
Author(s):  
Sossina M. Haile ◽  
Scott Meilicke
Keyword(s):  
Impedance Spectroscopy ◽  
Pyrochlore Structure ◽  
Fluorite Structure ◽  
Transformation Kinetics ◽  
X Ray Diffraction ◽  
Ion Conductor ◽  
Gadolinium Zirconate ◽  
X Ray ◽  
Oxygen Ion ◽  
Kinetics Of

ABSTRACTGadolinium zirconate, Gd2Zr2O7, undergoes an order-disorder transition at ∼1550°C, transforming from a defect fluorite structure (Fm3m) to a pyrochlore structure (Fd3m). Both cations and anions are ordered in the low-temperature, pyrochlore structure. In order to understand the interplay between anion and cation order parameters and ordering rates, the transformation kinetics of Gd2Zr2O7 have been examined via X-ray diffraction. Gadolinium zirconate is of particular interest because the oxygen ion conductivity of the ordered phase is significantly greatly than that of the disordered phase, in contrast to virtually every other known solid electrolyte. This difference in conductivity has provided a second technique for characterizing the transformation kinetics: in situ A.C. impedance spectroscopy. Results of the X-ray diffraction showed the growth of superstructure peak intensity to follow an apparent (time)½ dependence, rather than that expected from a nucleation and growth model. The impedance spectroscopy measurements, on the other hand, showed the conductivity to increase linearly with time. These results suggest the transition is second order in nature.

Mechanical Behaviour of Nanocrystalline Copper Related to Grain-boundary Structure

2002 ◽  
Vol 727 ◽  
Author(s):  
Y. Champion ◽  
P. Langlois ◽  
S. Guérin-Mailly ◽  
C. Langlois ◽  
M. J. Hÿtch
Keyword(s):  
Plastic Deformation ◽  
Grain Size ◽  
Structural Analysis ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Mechanical Behaviour ◽  
Tensile Testing ◽  
Metallic Nanostructures ◽  
Nanocrystalline Powders ◽  
Clear Understanding

AbstractUnderstanding the mechanical behaviour of metallic nanostructures is a key issue for their development. On the one hand, knowledge of the plastic behaviour at various temperatures is essential to control the synthesis, forming, and machining of such materials. Equally, a clear understanding of atomic and mesoscopic mechanisms, involving defects and their interactions, is essential for the control of ageing and functional properties. Regarding plastic deformation at room temperature, there is now evidence for unusual behaviour in nanostructured metals. In addition to high resistance and ductility, tensile testing reveals peculiar elasto-plastic deformation. Such behaviour was initially attributed to grain-boundary sliding. However, intergranular areas (including triple junctions) may possess special properties compared to their microcrystalline counterparts. For example, low activation energies have been measured for grain-boundary diffusion and it has been observed that grain-boundaries may act as dislocation sources and nucleation sites for deformation twinning.In this paper, we report on analysis on bulk copper nanostructures. Grain-boundaries are studied, by cross-correlating information from mechanical tensile testing and structural analysis, including X-ray diffraction (XRD) and transmission electron microscopy (TEM). Macroscopic bulk specimens (with grain size of about 80 nm) are prepared by powder metallurgy techniques, modified to fit to the special properties of nanocrystalline powders. Processing includes coldisostatic pressing, sintering and differential extrusion. The powders used (grain size of 40 nm) are synthesised by evaporation and cryo-condensation of a metallic vapour within liquid nitrogen. Results on mechanical testing and structural analysis will be reported. Emphasis will be placed on the structure of grain-boundaries (type of grain-boundary, grain-boundary thickness) studied by TEM and high resolution TEM image analysed using the geometric phase technique. The nanostructure was revealed to be consist in agglomerate of nano-size grains separated by low angle grain-boundaries. Agglomerates are themselves separerated by general high angle boundaries. These observations will then be related to the unusual mechanical true stress-true strain curves of the metallic nanostructures.

Effect of Wood Template on Magnetic Properties of Ni0.5Zn0.5Fe2-xCrxO4 Ceramics

2019 ◽  
Vol 956 ◽  
pp. 107-113
Author(s):  
Rui Li ◽  
Liang Cun Qian ◽  
Xue Feng Sun
Keyword(s):  
Grain Size ◽  
Saturation Magnetization ◽  
Spinel Structure ◽  
Single Phase ◽  
Sol Gel ◽  
Porous Ceramics ◽  
Nanocrystalline Powders ◽  
Test Results ◽  
Doping Amount ◽  
Wood Template

Porous Ni0.5Zn0.5Fe2-xCrxO4 (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) ceramics were prepared by a sol-gel method using basswood as the mold. At the same time, Ni0.5Zn0.5Fe2-xCrxO4 nanocrystalline powders were obtained by sintering the gel in oxygen at high temperature. The result of the XRD test showed that all samples presented the single-phase of the cubic spinel structure. With the increase of doping amount x, both porous ceramics and nanopowders decreased their grain size; At the same doping amount, the grain size of powder ferrites was about twice as large as that of porous ceramics. The magnetic test results showed that with the increase of doping amount x, the coercivity values increased monotonously and the saturation magnetization values decreased monotonously for all samples. Under the same doping content, the saturation magnetization values were lower but the coercivity values were higher as compared to the powder ferrites.

Ion-beam irradiation of Gd2Sn2O7 and Gd2Hf2O7 pyrochlore: Bond-type effect

2004 ◽  
Vol 19 (5) ◽  
pp. 1575-1580 ◽  
Author(s):  
Jie Lian ◽  
Rodney C. Ewing ◽  
L.M. Wang ◽  
K.B. Helean
Keyword(s):  
Room Temperature ◽  
Ion Beam ◽  
Pyrochlore Structure ◽  
Fluorite Structure ◽  
Structure Type ◽  
Radiation Response ◽  
Beam Irradiation ◽  
Ion Beam Irradiation ◽  
Bond Type ◽  
Wide Range

Ceramics with III-IV pyrochlore compositions, A3+2B4+2O7 (A = Y and rare earth elements; B = Ti, Zr, Sn, or Hf), show a wide range of responses to ion-beam irradiation. To evaluate the role of the B-site cations on the radiation stability ofthe pyrochlore structure-type, Gd2Sn2O7 and Gd2Hf2O7 have been irradiated by1 MeV Kr+. The results are discussed in terms of the ionic size and type ofbonding of Sn4+ and Hf4+ and compared to previous results for titanate andzirconate pyrochlores. Gd2Sn2O7 is sensitive to ion beam–induced amorphizationwith a critical amorphization dose of approximately 3.4 displacements per atom(dpa) (2.62 × 1015 ions/cm2) at room temperature and a critical amorphization temperature of approximately 350 K. Gd2Hf2O7 does not become amorphous at adose of approximately 4.54 displacement per [lattice] atom (3.13 × 1015 ions/cm2) at room temperature, but instead is transformed to a disordered fluorite structure upon ion-beam irradiation. Although the radius ratio of the A- to B-site cations provides a general indication of the type of radiation response of different pyrochlore compositions, the results for Gd2Sn2O7 emphasize the importance of bond type, particularly the covalency of the 〈Sn–O〉 bond in determining the radiation response.

Synthesis of WC-10wt.%Co Nanocrystalline Powders with Grain Growth Inhibitor by MTP

2007 ◽  
Vol 534-536 ◽  
pp. 1237-1240 ◽  
Author(s):  
Dong Kyu Park ◽  
Kwang Chul Jung ◽  
Jin Chun Kim ◽  
Sung Yeal Bae ◽  
In Sup Ahn
Keyword(s):  
Grain Size ◽  
Wear Resistance ◽  
Liquid Phase ◽  
Grain Growth ◽  
Growth Inhibitor ◽  
Liquid Phase Sintering ◽  
Nanocrystalline Powders ◽  
Grain Growth Inhibitor ◽  
Nano Crystalline ◽  
Carburizing Process

To improve the fracture strength and wear resistance of WC-Co cemented carbide, various technologies have been developed related producing the nano crystalline. There have been extensive studied conducted to retard grain growth during liquid phase sintering. However, when this inhibitor is added by simple mixing, a micro-pores forms during sintering In this study, WC-Co nanocrystalline powders with grain growth inhibitor in the site were prepared by MTP (Mechano-Thermic carburizing Process) to minimize this formation of micro pores and to retard grain growth effectively during sintering. In addition, then the phase and grain size of WC-Co nanocrystalline powders were evaluated according to the condition of MTP.

Grain Characteristics of Nanocrystalline ZrO2 Powders

2007 ◽  
Vol 336-338 ◽  
pp. 2558-2561 ◽  
Author(s):  
Hui Yan Yin ◽  
Min Fang Han
Keyword(s):  
Grain Size ◽  
Nanocrystalline Powders ◽  
Crystalline Domain ◽  
Grain Sizes ◽  
Grain Distribution ◽  
Agglomeration Coefficient

The grain characteristics of four kinds of ZrO2 nanocrystalline powders were manifested and discussed in this paper. The grain size of sample 1 tested by XRD, TEM and BET is consistent. The agglomeration coefficient of Sample 1 is higher, about 62.5, and the span of grain distribution is 14.58. The grain sizes of Sample 2 and 3 tested by XRD, TEM and BET are different. These differences identify the crystalline domain including 2~3 unite, which make the departure spheres in TEM. There are more soft agglomerates in Sample 2 with the agglomeration coefficient of 25.71 and the span of grain distribution being 18.94 than that in Sample 3 with the agglomeration coefficient of 6.89 and the span of grain distribution being 8.91. The grain sizes of Sample 4 tested by XRD, TEM and BET are obviously different. That identifies the large hard agglomerates.

Investigations of discrepancies between laboratory studies of the flow of ice: density, sample shape and size, and grain-size

1994 ◽  
Vol 19 ◽  
pp. 146-154 ◽  
Author(s):  
T. H. Jacka
Keyword(s):  
Grain Size ◽  
Creep Rate ◽  
Aspect Ratio ◽  
Sample Size ◽  
Minimum Creep Rate ◽  
Test Sample ◽  
Sample Length ◽  
Fine Grained ◽  
Sample Shape ◽  
Sample Density

Laboratory results are presented concerning ice creep at minimum creep rate (at ~1% strain) for fine-grained, initially isotropic, polycrystalline samples. The effect on the creep rate of ice density, sample shape (aspect ratio) and size, grain-size and ratio of grain-size to sample size is examined. Provided sample density is above ~0.83 Mg m−3 (i.e. the close-off density), there is no effect of density on ice-creep rate. Results provide no evidence of a creep rate dependence on test sample length for cylindrical samples. Sample diameter, however, does affect creep rate. Over the range of sample diameters studied (16.2 to 90 mm) creep rate decreases monotonically by a factor of ~4. This effect is independent of sample aspect ratio. Experiments examining size effects in simple shear indicate no dependence of minimum flow rate on shape or size in this stress configuration. Two grain-sizes were represented within the samples tested for the effect of sample size. As expected from earlier work, no grain-size effect on minimum creep rate is evident. In addition, there was no evidence of an effect on creep rate of the ratio of grain-size to sample size.

TEM Study of Short-Range-Order in Zirconolite Induced by High Temperature Ion Irradiation

1999 ◽  
Vol 5 (S2) ◽  
pp. 756-757
Author(s):  
S. X. Wang ◽  
L. M. Wang ◽  
R. C. Ewing
Keyword(s):  
Oxygen Vacancies ◽  
Ion Irradiation ◽  
Pyrochlore Structure ◽  
Fluorite Structure ◽  
Waste Immobilization ◽  
Temperature Irradiation ◽  
High Level Nuclear Waste ◽  
Nuclear Waste Immobilization ◽  
Shortrange Order ◽  
High Level

Zirconolite (CaZrTi207) is an important phase proposed for high level nuclear waste immobilization. Zirconolite was irradiated by 1 MeV Kr+ at various temperatures. At room temperature, zirconolite became amorphous after a dose of 7x1014 ions/cm2.1 Amorphization dose increased with temperature due to thermal annealing. The critical temperature, above which amorphization does not occur, was estimated to be 654 K. During the low temperature irradiation (<654 K), concurrent with amorphization, zirconolite transformed from a monoclinic structure to the cubic pyrochlore structure and then to the fluorite substructure. The structural change is due to the disordering between cations and between oxygen and oxygen vacancies.After an irradiation at 673 K to a dose of 3.6x1015 ions/cm, the zirconolite samples remained crystalline. The diffraction pattern consists of strong maxima from the fluorite structure and diffuse maxima surrounding the Bragg positions of the pyrochlore superlattice (FIG. 1). Diffuse scattering patterns have been reported in other phases, and were generally attributed to the shortrange- order (SRO) domains.

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