Pyrochlore glass-ceramics fabricated via both sintering and hot isostatic pressing for minor actinide immobilization

Pyrochlore glass-ceramics (GCs) have been investigated with samples fabricated via both sintering and hot isostatic pressing (HIPing) of a mixed oxide precursor. It has been demonstrated that sintering at 1200°C in air is necessary to obtain well-crystallized pyrochlore crystals in a sodium aluminoborosilicate glass through a one-step controlled cooling. The crystallization, structure, and microstructure of Eu₂Ti₂O₇ pyrochlore as the major phases in residual glass were confirmed with X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy, transmission electron microscopy, and Raman spectroscopy. The structures of major Eu₂Ti₂O₇ pyrochlore and minor [Eu_4.67O(SiO₄)₃] apatite in both sintered and HIPed samples were refined using synchrotron XRD data. While the processing atmosphere did not appear to affect the cell parameter of the main pyrochlore phase, very small volume expansion (~0.3%) was observed for the minor apatite phase in the HIPed sample. In addition, static leaching of the HIPed sample confirmed that pyrochlore GCs are chemically durable. Overall, pyrochlore GCs prepared via both sintering and HIPing with the Eu partitioning factor of ~23 between ceramics and the residual glass are suitable waste forms for minor actinides with processing chemicals.     

DFT+U study and in-situ TEM investigation of high-entropy titanate pyrochlore (Lu0.25Y0.25Eu0.25Gd0.25)2Ti2O7

High-entropy pyrochlore has become a promising immobilization matrix due to its ability to immobilize multiple nuclides especially for high-level radioactive waste. This work adopts first principle method to analyze the formation possibility of (Lu_0.25Y_0.25Eu_0.25Gd_0.25)₂Ti₂O₇ (HTP-1). By calculating the Gibbs free energy of possible chemical reactions, we predict that the driving force to synthesize HTP-1 is greater than the driving force to form single-component pyrochlore at temperatures above 1225 K. The characterization results show that the HTP-1 sample is successfully prepared by the solid-state reaction at the predicted temperature. To investigate the radiation resistance of HTP-1, the sample is irradiated by 800 keV Kr²⁺ ions, and the microstructure evolution is characterized by in-situ TEM. HTP-1 sample achieves complete amorphous at 0.26 dpa, indicating that its radiation resistance is between Eu₂Ti₂O₇ and Y₂Ti₂O₇, which is proved by the calculation results of x_O48f and antisite defect formation energy.     

Development of brannerite glass‐ceramics for the immobilization of actinide‐rich radioactive wastes

Brannerite‐based glass‐ceramics have been developed as potential waste forms for the immobilization of actinide‐rich radioactive wastes. For the first time, the formation of brannerite phases in glass has been demonstrated using uranium (U) and plutonium (Pu) with additions of gadolinium and hafnium as neutron absorbers. Both XRD and SEM‐EDS confirm that brannerite is the dominating phase with compositions close to Y_0.5U_0.5Ti₂O₆, Gd_0.2Pu_0.3U_0.5Ti₂O₆, and Gd_0.1Hf_0.1Pu_0.2U_0.6Ti₂O₆ internally crystallized in the glass. TEM SAED and Raman spectroscopy reveal the typical structure and vibration modes for brannerite. In addition, the presence of U⁵⁺ species as designed in the formulations has been confirmed by diffuse reflectance spectroscopy. More importantly, the U and Pu were partitioned exclusively in the ceramic phases with no detectable actinide in the glass.     

Structural and spectroscopic investigations on the crystallization of uranium brannerite phases in glass

Brannerite‐based glass‐ceramics with relatively high actinide waste loadings are potential waste forms for the immobilization of actinide‐rich radioactive wastes containing process chemicals. In this work, the crystallization of pentavalent uranium brannerite phases in glass with the incorporation of yttrium/cerium/europium has been investigated. The formation of brannerite phases in glass has been confirmed with X‐ray diffraction and Raman spectroscopy. The evolution of microstructures and the development of micro pores in brannerite phases have been revealed by scanning electron microscopy with the nominal formula, Y_0.51U_0.49Ti₂O₆, Ce_0.65U_0.35Ti₂O₆, and Eu_0.53U_0.47Ti₂O₆, being determined with energy‐dispersive spectroscopy. The presence of dominant U⁵⁺ species has been proven with both diffuse reflectance spectroscopy and X‐ray absorption near‐edge spectroscopy. In addition, the systematic Raman band shifts with the mean cation radius in the studied brannerite series have been observed and discussed.     

Flux crystal growth of Ba2[(UO2)2Ti2O8]: Structures,spectroscopies and implications

We report a flux crystal growth of Ba₂[(UO₂)₂Ti₂O₈] and subsequent structural and spectroscopic studies using multiple techniques. The layered crystal structure, built up with sheets of edge-sharing dimeric uranyl pentagonal bipyramids and dimeric TiO₅ square pyramids with interlayer Ba(II) ions, was revealed by synchrotron single crystal x-ray diffraction and confirmed with electron diffraction using a transmission electron microscope. The presence of only hexavalent uranium was confirmed by both diffuse reflectance and x-ray absorption near-edge structure spectroscopies, consistent with the bond valence sum calculations. Its vibrational modes were revealed by Raman spectroscopy. In addition, its implications as a potential hexavalent uranium waste form for the immobilization of uranium-rich radioactive wastes were discussed.     

Preparation and luminescence properties of Eu2O3 doped glass-ceramics containing NaY(MoO4)2

Eu₂O₃ doped transparent glass-ceramics containing NaY(MoO₄)₂ crystalline phase were prepared via melting-crystallization. The optimum heat treatment condition (660℃/3h) was determined by DSC, XRD, SEM and transmittance curves. The transmittance of glass-ceramic can reach 80 % in the visible region. The emission spectra of Eu₂O₃ doped glass-ceramics consist of Eu³⁺ ions characteristic emission peaks at 591nm (⁵D₀→⁷F₁) and 614nm (⁵D₀→⁷F₂). The optimal doping concentration of Eu₂O₃ in the glass-ceramics is 0.9 mol%, and fluorescence lifetime is 1.37042ms. The change of the ratio of red emission intensity to orange emission intensity leads to the shift of chromaticity coordinates from orange to red region, and the chromaticity coordinate (0.6337, 0.3635) of 0.9 mol% Eu₂O₃ doped glass-ceramic is closest to the standard red light coordinate. The results show that this kind of glass-ceramic is expected to be good red emission material.     

Hot pressing preparation of Y2Ti2O7-based glass-ceramics as potential waste forms for actinide immobilization

A series of Y₂Ti₂O₇ glass-ceramic which is the main crystal phase is pyrochlore for actinide immobilization were synthesized by a new method combining oxide pre-crystallization and hot pressing. Y₂Ti₂O₇ was evenly distributed in the glass medium, increasing the temperature promoted the rapid growth of Y₂Ti₂O₇ grains, and increasing the pressure was conducive to maintaining the uniformity of Y₂Ti₂O₇ grains. Driven by a pressure, the glass component filled the gap well between the Y₂Ti₂O₇ grains, and the glass-ceramic obtained a compactness close to the theoretical density, so that it had an excellent microstructure and served the immobilization of actinide better.     

Identification and quantification of reaction phases at Si3N4–Ti interfaces by using analytical transmission electron microscopy techniques

Silicon nitride (Si₃N₄) and titanium (Ti) were successfully bonded by using a capacitor discharge joining method. The resulting sample interfaces were characterized by scanning electron microscopy (SEM) and analytical transmission electron microscopy (TEM) techniques. SEM and chemical analyses by energy dispersive X-ray spectrometry (EDX) and wave length X-ray spectrometry (WDX) showed that if there is a reaction layer it is very small. Sample preparation from metal–ceramic joints for TEM by using conventional techniques is difficult. To overcome this problem, samples were prepared by using a focus ion beam (FIB) and investigated by TEM techniques. Analytical TEM techniques such as electron energy loss spectroscopy (EELS) revealed that Si₃N₄ interacted with Ti and reaction phases were formed at the interface. These phases are approximately 50 nm thick Ti₃N₂ layer at the interface next to Si₃N₄ followed by continuous Ti₆Si₃N phase as a matrix containing Ti₃N particles.     

Synthesis and Unique Photoluminescence Properties of Eu2Ti2O7 and Eu2TiO5

The europium titania materials, pyrochlore Eu₂Ti₂O₇ and orthorhombic Eu₂TiO₅, were synthesized from a mixture of Eu₂O₃ and TiO₂ using the solid‐state reaction method. The structural and optical properties of these titania materials were investigated using X‐ray diffraction analysis, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and diffuse reflectance spectroscopy. Temperature dependence of the PL intensity was measured between T = 20 and 450 K and analyzed on the basis of various theoretical models. A remarkable increase in the PL intensity with increasing T was observed in these titania materials at higher temperatures, above ~140 K, and well explained by a trap/reservoir model. Interestingly, a dramatic decrease in the electric‐dipole emission component relative to the magnetic‐dipole one was observed in Eu₂Ti₂O₇ above T ~ 140 K. The schematic energy‐level diagram for Eu³⁺ in the Eu₂Ti₂O₇ host was proposed for the sake of a better understanding of the PL and PLE processes in this type of phosphorescent material.     

Synthesis and characterization of monodispersed BaSO4/Y2O3:Eu3+ core–shell submicrospheres

Spherical BaSO₄ particles have been coated with Y₂O₃:Eu³⁺ phosphor layers (BaSO₄/Y₂O₃:Eu³⁺) by the wet chemical method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dipersive spectroscopy (EDS), photoluminescence spectra were utilized to characterize the BaSO₄/Y₂O₃:Eu³⁺ core–shell-structured phosphor particles. The obtained core–shell phosphors consist of well dispersed submicron spherical particles with narrow size distribution. XRD result shows that no reaction occurred between the BaSO₄ cores and the Y₂O₃:Eu³⁺ shells even after annealing at 1400 °C. TEM and EDS results show that BaSO₄ particles are well coated with the shell of Y₂O₃:Eu³⁺. The BaSO₄/Y₂O₃:Eu³⁺ core–shell particles show a red emission corresponding to ⁵D₀?⁷F₂ of Eu³⁺ under the excitation of ultraviolet.     

Microstructure evolution and bonding strength of the Al2O3/Al2O3 interface brazed via Ni-Ti intermetallic phases

In this study, Al₂O₃ workpieces were vacuum brazed by using Ni-45Ti binary alloy. The interfacial microstructure evolution of the joints obtained at different brazing temperatures was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The bonding strength of the joints was measured by shear testing. The results indicated that Ni₂Ti₄O and AlNi₂Ti were the main reaction products in the joint areas. Moreover, the Ti₂Ni intermetallic compound formed in the brazing seam. The typical layer structure of the brazed joints was Al₂O₃/AlNi₂Ti/Ni₂Ti₄O/Ti₂Ni + NiTi/Ni₂Ti₄O/AlNi₂Ti/Al₂O₃. With the brazing temperature increasing, the thickness of the Ni₂Ti₄O reaction layer adjacent to the Al₂O₃ substrate increased significantly, while the AlNi₂Ti phase had a tendency to dissolve with the brazing temperature increasing. The mechanism for the microstructure evolution was also discussed. The maximum shear strength of 125.63±4.87 MPa of the joints was obtained when brazed at 1350 °C for 30min. The fracture occurred hardly in the interface between Al₂O₃ and Ni-45Ti filler alloy.     

Phase transition of Eu2Ti2O7 under high pressure and a new ferroelectric phase with perovskite‐like layered structure

Ferroelectrics with perovskite‐like layered (PL) structure are well‐known for their high T_c and the application prospect of high‐temperature‐piezoelectric sensing. In this study, the PL‐structure Eu₂Ti₂O₇ was prepared by 1‐step high‐pressure sintering, which show the pyrochlore structure of Eu₂Ti₂O₇ would change into PL structure at 11 GPa, 1300°C. The PL‐structure Eu₂Ti₂O₇ is metastable, which will change back to pyrochlore structure at about 900°C in the air. The PL‐structure Eu₂Ti₂O₇ was confirmed as a high‐temperature ferroelectric material for the first time. The ferroelectric domain switching was directly observed using piezoelectric force microscope. The piezoelectric constant of the PL Eu₂Ti₂O₇ ceramic was measured as 0.7‐0.9 pC/N and its thermal depoling temperature (T_d) was determined as 800°C, which is associated with the PL‐pyrochlore transition.     

Synthesis,structure, and luminescence of Eu3+-activated La4Ti3O12 nanoparticles with layered perovskite structure

La_4-4xEu_4xTi₃O₁₂ (x = 0.01-0.15) nanoparticles were synthesized via the polymerizable complex method. The X-ray powder diffraction (XRD) measurements and Rietveld refinements verified that all the samples crystallized in a pure trigonal perovskite structure. La₄Ti₃O₁₂ has band energy of 3.88 eV, which can be greatly narrowed by Eu³⁺-doping. This benefits the red-shift of the optical absorption from UV- to near-UV wavelength region. La₄Ti₃O₁₂ has the self-activated luminescence at room temperature with a broadband centered at 520 nm. La_4-4xEu_4xTi₃O₁₂ (x = 0.01-0.15) phosphors present red-emitting luminescence with the strongest ⁵D₀→⁷F₂ transition at 613 nm. The energy transfer was discussed on the doping concentration. Importantly, the phosphors present dominant absorption in near-UV region (395 nm). An experimental red-emitting lamp was fabricated using a 395 nm LED-chip in combination with the single phosphor of La_4-4xEu_4xTi₃O₁₂ (x = 0.1). The results indicate that La_4-4xEu_4xTi₃O₁₂ could be a promising candidate used in solid-state lighting and display under near UV-light.     

Application of titanium-containing sorbents for treating liquid radioactive waste with the subsequent conservation of radionuclides in Synroc-type titanate ceramics

New nanocrystalline titanium-containing sorbents, namely, layered hydrazinium titanate LHT-9, (N₂H₅)_0.5Ti_1.87O₄ and synthetic ivanyukite-Na Na₂K[Ti₄(OH)O₃(SiO₄)₃] · 7H₂O, possess high sorption capacity towards cations of different valences and, due to their high chemical stability in acidic and alkaline media, provide the purification of liquid radioactive waste of any composition. The calcination of the product of the radionuclide sorption on LHT-9 and ivanyukite at 1000–1200°C yields mineral-like Synroc-type titanate ceramics applicable for long-term immobilization of sorbed radionuclides with a significant (by 17–250 times) decrease in the amount of radioactive waste.     

Influence of Eu doping on the structural,electrical and optical behavior of Barium Zirconium Titanate ceramic

The influence of Eu³⁺ doping on the structural, dielectric and optical behavior of Barium Zirconium Titanate (BZT) with general formula Ba_1?xEu_2x/3Zr_0.05Ti_0.95O₃ (x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05) has been investigated in the present study. The X-ray diffraction (XRD) data show a phase transition from orthorhombic to tetragonal symmetry due to the incorporation of Eu³⁺ ions in BaZr_0.05Ti_0.95O₃ matrix. A secondary phase of Eu₂Ti₂O₇ is observed for the composition with x ≥ 0.03. The Raman spectroscopic study confirms the structural change as observed in XRD, while the Fourier transformation infra red (FT-IR) spectra reveal that Eu³⁺ doping in BZT creates vacancies in the system. The temperature dependent dielectric study shows that the transition temperature and maximum dielectric constant decrease with increase in Eu³⁺content. It is observed that the dielectric diffuseness increases up to x≤0.02 followed by a decrease at higher concentrations of Eu³⁺. The optical behavior of prepared samples is studied through UV–visible spectroscopy, and it is found that the optical band-gap value increases with Eu³⁺ concentration up to 2% and then decreases for higher concentrations (x > 2%).     

Investigation of Ti2AlC formation mechanism through carbon and TiAl diffusional reaction

Ti₂AlC formation mechanism with carbon and TiAl alloy as reactants at 1000–1200 °C have been investigated through Raman spectrometer and transmission electron microscope (TEM). Reaction products TiC_x and Ti₂AlC are confirmed. Full width at half height (FWHH) of Ti₂AlC Raman peak is used to characterize crystallinity, which indicates better perfection of Ti₂AlC grains near TiAl area and with higher reaction temperature. Variations of TiC_x microstructure, especially stacking faults(SF) and nanotwins, are observed by TEM along reaction layer. Ti₂AlC lattice uniformity of different reaction temperature is also confirmed by TEM, which is consistent with Raman result. TiC_x is believed first formed through reaction between TiAl and C. With help of carbon vacancy, Al-doped TiC_x SF and nanotwin can be formed under hot pressing, which becomes the basal frame of Ti₂AlC.     

Controllable synthesis of bifunctional material Ca2Ti2O6:Eu3+ and its comparative study on luminescence and photocatalytic properties with CaTiO3:Eu3+

Pure pyrochlore Ca₂Ti₂O₆, perovskite CaTiO₃, and their mixed crystalline phases with different proportions were controllably synthesized via a solvothermal method, followed by a subsequent calcination process. RIR (reference intensity ratio) data of Ca₂Ti₂O₆ were first obtained by X-ray diffraction (XRD), which can be used to quantitatively analyze the phase composition. When Eu³⁺ is doped into these calcium titanium oxides, they can be used as luminescent and photocatalytic materials. The structure, luminescence, and photocatalytic properties of pure pyrochlore Ca₂Ti₂O₆:Eu³⁺ and perovskite CaTiO₃:Eu³⁺ were comparatively studied in detail. The relative intensities of the excitation peaks and the emission peaks in Ca₂Ti₂O₆:Eu³⁺ and CaTiO₃:Eu³⁺ are different, which is attributed to the different symmetries of Eu³⁺ inhabiting the two kinds of lattices. In addition, although the luminescence intensity of CaTiO₃:3%Eu³⁺ is higher than that of Ca₂Ti₂O₆:3%Eu³⁺ under excitation at 394 nm, the luminescence intensity of Ca₂Ti₂O₆:3%Eu³⁺ is superior to that of CaTiO₃:3%Eu³⁺ under excitation at 464 nm and 533 nm. Photocatalytic experiments show that Ca₂Ti₂O₆:3%Eu³⁺ has better photocatalytic performance than CaTiO₃:3%Eu³⁺, which is mainly due to its smaller crystallite size, higher specific surface area and pyrochlore structure. In addition, biphase (Ca₂Ti₂O₆–CaTiO₃):3%Eu³⁺ has the best photocatalytic activity compared with the single phase Ca₂Ti₂O₆:3%Eu³⁺ and CaTiO₃:3%Eu³⁺, owing to the presence of heterojunctions that significantly reduced the band gap. It is anticipated that the discovery of this bifunctional Ca₂Ti₂O₆:Eu³⁺ would expand the application of rare earth-doped calcium titanium oxide materials.     

Direct and template‐free synthesis of one‐dimensional nanostructures of sodium trititanate and sodium hexatitanate

One‐dimensional (1D), pure phases of sodium trititanate and sodium hexatitanate (Na₂Ti₃O₇ and Na₂Ti₆O₁₃ ) nanowires (Na‐TNW) were synthesized via a direct hydrothermal approach. The effect of the variation in growth parameters (NaOH concentration and reaction duration) on the morphology and structure of the Na‐TNW has been investigated using bulk (RAMAN and XRD) and surface (TEM, SEM and N₂ sorptiometry) techniques. By varying the NaOH concentration, the particle size decreases without variation in the obtained trititanate structure. Also by varying the growth duration, the nanowires were elongated with reduced aggregation accompanied by structural conversion from trititanate to hexatitanate. Therefore, this study provides a full concept for the direct preparation of Na‐TNW with controlled characteristics.     

Interfacial reactions between Ti and Y2O3/Ca4Ti3O10 composites

In titanium casting, the interfacial reactions between selected Y₂O₃/Ca₄Ti₃O₁₀ composites in contact with titanium at 1600°C for 30 minutes in an argon environment were characterized using X-ray diffraction, scanning electron microscopy, and analytical transmission electron microscopy. Before contact with titanium, reaction phases of the Y₂O₃/Ca₄Ti₃O₁₀ composites formed through hot pressing at 1500°C or annealing at 1600°C. These phases were identified as Ca₄Ti₃O₁₀, Y₂O₃, and minor amounts of CaO. The amount of Ca₄Ti₃O₁₀ in the composites gradually decreased as the amount of Y₂O₃ increased. When the Y₂O₃/Ca₄Ti₃O₁₀ composites were in contact with titanium, a reaction layer of α-Ti and Y₂O₃ was formed near the interface of the composite side. Large amounts of calcium and oxygen were expelled toward the composite side from Ca₄Ti₃O₁₀ near the interface to form CaO, which precipitated in the Ca₄Ti₃O₁₀ and Y₂O₃ phases. Furthermore, no reaction phases were detected in titanium. Ca₄Ti₃O₁₀ and Y₂O₃ can function as diffusion barrier layers to effectively suppress the diffusion of oxygen into titanium.     

Current advances on titanate glass-ceramic composite materials as waste forms for actinide immobilization: A technical review

As the emerging versatile waste forms for immobilizing actinide-rich radioactive wastes, glass-ceramic composite materials based on some durable ceramic phases are being developed. They have apparent advantages over the conventional borosilicate glasses and multi- or single- phase ceramics as they essentially combine the chemical and processing flexibilities of glasses to accommodate processing impurities and excellent chemical durability of ceramic phases to host actinides. More recently, some new advances have been made on scientific and technological aspects including new glass-ceramic systems; improved understanding of ceramic phase evolution in glass; actinide validation studies and simplified processing techniques. This review is intended to cover the current advances on the development of glass-ceramic composite waste forms focusing on titanate ceramic phases (zirconolite, pyrochlore and brannerite) for immobilizing various actinide-rich radioactive wastes arising from the nuclear fuel cycle.