Processing Impurities as Phase Assemblage Modifiers in Titanate Nuclear Waste Ceramics

The tolerance of titanate nuclear waste ceramics to fluctuations in the concentrations of processing contaminants was monitored using X-ray diffraction, electron microscopy, secondary ion mass spectrometry, and analysis of dissolution liquors. Several waste forms were fabricated, all of which contained idealized Purex waste simulant and, in addition, varying quantities of the common waste stream impurities P₂O₅, MgO, Fe₂O₃, Na₂O, and SiO₂. Incorporation of the oxides individually stabilized new phases including monazite (P₂O₅), pseudobrookite (MgO), loveringite (Fe₂O₃), freudenbergite (Na₂O), and pollucite (SiO₂)—only the latter phase deleteriously affected waste form performance by promoting cesium dissolution. However, when the processing contaminants were added simultaneously, a number of synergetic effects, particularly the stabilization of a soluble glassy phase, resulted in elemental losses which were an order of magnitude greater for some matrix and radwaste species. It was found that up to 25 wt% of the idealized Purex waste could be incorporated in the waste without diminution of its properties.     

Thermochemistry and Aqueous Durability of Ternary Glass Forming Ba-Titanosilicates: Fresnoite (Ba2TiSi2O8) and Ba-Titanite (BaTiSiO5)

Barium titanosilicates are possible oxide forms for the immobilization of short-lived fission products in radioactive waste. Ba₂TiSi₂O₈ (fresnoite) and BaTiSiO₅ (Ba-titanite) samples were prepared by a solid-state synthesis. The enthalpies of formation of Ba₂TiSi₂O₈ crystal and glass at 25°C and of BaTiSiO₅ glass were obtained from drop solution calorimetry in a molten lead borate (2PbO–B₂O₃) solvent at 701°C. The enthalpy of formation for fresnoite composition samples from constituent oxides was exothermic and became more exothermic with increasing crystallinity. Differential scanning calorimetry revealed that the crystallization rate of the fresnoite glasses increased with increasing devitrification. A modified Product Consistency Test-Procedure B (PCT-B) was used to collect solubility data on the fresnoite and titanate phases. The tests suggest that both glassy and crystalline fresnoite exhibit favorable aqueous stability and should be explored further as radioactive waste forms for long-term storage.     

Microstructures of Simulated Nuclear Waste

The microstructures of simulated tailored ceramic forms for the Idaho Chemical Processing Plant (ICPP) high-level waste were characterized by TEM. The ceramic forms were consolidated from simulated ICPP waste calcines with either silicayttria-based or silica–lithia-based additives by hot isostatic pressing. The hot isostatically pressed ceramic waste forms are composed of cubic CaF₂, monoclinic ZrO₂, stabilized cubic ZrO₂, tetragonal ZrSiO₄, and amorphous silicate phases which can be phase separated. The phaseseparated glass is a result of phase immiscibility in the soda aluminosilicate glass to the compositions of albite and mullite.     

Durability of Zirconium-Containing Ceramic Waste Forms in Water

The durability of three ceramic waste forms containing Zr as a constituent is examined at 90° and 150°C in HCl or deionized water. The three ceramic waste forms examined are: (A) 10-mol%-Y₂O₃-stabilized ZrO₂, (B) La₂Zr₂O₇ with a pyrochlore structure, and (C) CaZrO₃ with a perovskite structure. Each form contains an appropriate amount of Ce, Nd, and Sr to simulate the leaching behavior of actinides, lanthanides, and alkaline-earth fission products from highlevel nuclear waste. Two forms, (A) 10-mol%-Y₂O₃-stabilized ZrO₂ and (B) La₂Zr₂O₇ with a pyrochlore structure, show excellent durability in HCl and deionized water. For example, leach rates of all constituents in the La₂Zr₂O₇ waste form are less than about 10⁻⁴ g·m⁻²·d⁻¹ at 150°C for a leach time of 32 d in deionized water.     

Crystalline-Phase Formation in Hot Isostatic Pressing of Nuclear Waste Ceramics with High Zirconia Content

Ceramic forms with a significant glass phase have been prepared from Zr-rich simulated Idaho Chemical Processing Plant high-level nuclear waste by hot isostatic pressing of redox-controlled waste and chemical additive calcines. Crystalline-phase formation in the ceramics is a function of the waste loading, additives, and consolidation temperature. Actinide host phases are controlled by silica-zirconia crystal chemistry; the composition of the amorphous phase dominates the leaching behavior. Two phase assemblages—one consisting of CaF₂, ZrO₂, ZrSiO₄, and glass, the second of CaF₂, ZrO₂, YNaSiO₄, ZrSiO₄, and glass—showed leach resistance superior to melter-processed borosilicate glass forms for uranium and, at waste loadings >80 wt%, offer a factor of up to 3.2 times greater volume reduction. The presence of the designed glass phase in the waste forms makes them more flexible to variations in waste composition and lowers the required hot isostatic pressing temperature for consolidation as compared with polycrystalline waste ceramics, while maintaining the actinide leach resistance of the crystalline phases.     

Cation Interdiffusion in Polycrystalline Calcium and Strontium Titanate

A method has been developed to study bulk lattice interdiffusion between calcium and strontium titanate by fabrication of a diffusion couple using cosintering. The measured interdiffusion coefficients, ( C ), indicate that strontium impurity diffusion in calcium titanate occurs at a faster rate than calcium impurity diffusion in strontium titanate. These interdiffusion coefficients are composition independent when the concentration of the calcium cation exceeds that of the strontium cation; otherwise ( C ) is strongly composition dependent. Investigations into the effect of cation nonstoichiometry give results that are consistent with a defect incorporation reaction in which excess TiO₂, within the solid solubility limit, produces A-site cation vacancies as compensating defects. The interdiffusion coefficients increase with increasing concentrations of TiO₂, so it is concluded that interdiffusion of these alkaline-earth cations in their titanates occurs via a vacancy mechanism.     

Freudenbergite: A Possible Synroc Phase for Sodium-Bearing High-level Waste

Earlier studies have shown that the presence of excess sodium in high-level nuclear waste can give rise to increased leachability when the waste is incorporated in Synroc-type titanate ceramics. Freudenbergite (Na₂Al₂Ti₆O₁₆) can incorporate sodium and it exhibits satisfactory leaching resistance in water and reasonable resistance to radiation. From X-ray diffraction studies, freudenbergite is compatible with the other phases in Synroc. Thus, Synroc containing freudenbergite as an additional phase offers potential for immobilizing sodium accompanying high-level nuclear fuel reprocessing waste.     

X-ray Photoelectron Spectroscopy Study of High- and Low-Temperature Forms of Zirconium Titanate

X-ray photoelectron spectroscopy (XPS) spectra were measured for the high-temperature forms (quenched from 1500°C) of zirconium titanate and its solid solutions, the low-temperature forms (quenched from 1000°C), and cubic Ba(Zr_1/2Ti_1/2)O₃. Two distinct differences in the XPS spectra were noted between the high- temperature forms. The O 1 s spectrum of the high-temperature form consisted of two peaks of nearly equal intensities with a spacing of 2.2 eV, which was larger than the corresponding spacing of 1.6 eV, for Ba(Zr_1/2Ti_1/2)O₃, while the spectrum of the low-temperature forms was almost a single peak. The high-temperature form had an unassignable spectrum with a binding energy of 25 eV between the peaks of Zr 4 p and O 2 s . The unassignable spectrum could be a molecular orbital which had been hypothesized to form in these compounds.     

Formation of Monoclinic Hafnium Titanate Thin Films Via the Sol–Gel Method

Hafnium titanate films are generating increasing interest because of their potential application as high- k dielectrics materials for the semiconductor industry. We have investigated sol–gel processing as an alternative route to obtain hafnium titanate thin films. Hafnia-titania films of different compositions have been synthesized using HfCl₄ and TiCl₄ as precursors. The HfO₂–TiO₂ system composition with 50 mol% of TiO₂ and 50 mol% of HfO₂ has allowed the formation of a hafnium titanate film after annealing at 1000°C. The films exhibited a homogeneous nanocrystalline structure and a monoclinic hafnium titanate phase that has never been obtained before in thin films. The films resulted in the formation of homogeneously distributed nanocrystals with an average size of 50 nm. Different compositions, with higher or lower hafnia contents, produced anatase crystalline films after annealing at 1000°C.     

Electrical Properties of Lead Zirconate Titanate Thin Films With a ZrO2 Buffer Layer on an Electroless Ni-Coated Cu Foil

The effect of zirconia (ZrO₂) buffer layers on the phase development and electrical properties of lead zirconate titanate (PZT, 52/48) capacitors on an electroless Ni (P)-coated Cu foil was investigated. It was demonstrated that the buffer layer can be used to engineer the final properties. The incorporation of the ZrO₂ buffer layers retained acceptable capacitance densities (>350 nF/cm² for 50 nm thick ZrO₂), while significantly reducing leakage currents and improving reliability (<10⁻⁷ A/cm² after 1 h at 25 VDC for 100 nm thick ZrO₂), compared with PZT thin films directly on electroless Ni (P). The results are particularly important for embedded capacitor applications.     

Titanate Ceramics for the Stabilization of Partially Reprocessed Nuclear Fuel Elements

A titanate ceramic designed to immobilize high-level waste generated by Amine reprocessing of heavy-water reactor fuel and fabricated by uniaxial hot-pressing was characterized. X-ray diffraction and selected-area electron diffraction were used to identify a six-phase assemblage consisting of betafite, a hollandite structure type, perovskite, uraninite, hibonite, and Magnéli phases. Secondary electron imaging of polished surfaces revealed many microvoids consistent with a measured density of 5.25 g · cm⁻² (∼90% of theoretical density). The waste form was chemically heterogeneous at the hundreds of micrometers scale, as backscattered electron imaging and energy-dispersive X-ray spectroscopy indicated that regions rich in either uranium or titanium oxides were common. Grain sizes ranged from 0.1 to 0.5 μm. All crystals were faceted with the exception of anhedral uraninite grains. Dissolution experiments conducted at 90°C in distilled water gave uranium and titanium loss rates which were solubility limited (10⁻¹ to 10⁻⁴ g · m⁻²· d⁻¹), while cesium dissolution, which was not contrained by solubility limits, was more rapid (23.3 g · m⁻²· d⁻¹). Hydrothermal testing at 150°C in distilled water resulted in precipitation of uranium(VI) titanate (UTiO₅) and brookite (TiO₂).     

Solubility of Hydrogen Defects in Doped and Undoped BaTiO3

Thermal desorption studies and electrical conductivity measurements have been used to investigate the solubility of hydrogen defects in barium titanate ceramics in oxidizing atmospheres. Hydrogen is found to dissolve in the perovskite lattice of undoped and acceptor-doped BaTiO₃ by the formation of hydroxide ions on regular oxygen ion sites. The hydrogen defects act as mobile donors and may substitute the oxygen vacancies in compensating fixed acceptors. From the dependence of the solubility on the water vapor pressure and the dopant concentration, the incorporation/desorption equilibrium reaction is deduced. A defect model is derived which consistently extends previously proposed models. The influence of Ba₂TiO₄ and Ba₆Ti₁₇O₄₀ second phases on the solubility of hydrogen defects is discussed.     

HIPed Tailored Hollandite Waste Forms for the Immobilization of Radioactive Cs and Sr

Hot isostatically pressed tailored hollandite waste forms were used to demonstrate the immobilization of Cs and Sr are separable from spent nuclear fuel, as well as Ba and Rb. Four hollandite formulations were investigated, two samples with ∼12 wt% waste loading (on an oxide basis) and two with ∼18 wt% waste loading. Two of the samples were Al-substituted and the other two contained Mg. The hollandite in the Al-substituted samples contained all the waste cations, as designed, but this was not the case in the Mg-substituted samples. The hollandite in the Mg-substituted samples did not contain all the waste cations, with ∼50% of the Sr forming SrTiO₃ as a secondary phase. This resulted in waste forms that were not as durable, with respect to Cs, as their Al counterparts. The formation of SrTiO₃ had little effect on the Sr release rates and was not detrimental to the Mg-substituted hollandite waste form. For the Al-substituted samples, the MCC-1 normalized release rates were <0.06 g·(m²·day)⁻¹ at 0–28 days for all elements, while the Cs release rates remained at 2.0 g·(m²·day)⁻¹ at 0–28 days for the Mg-substituted samples.     

Peculiar Dielectric Behaviors of (Na1/2Bi1/2)0.87Pb0.13TiO3 Thin Films

PbTiO₃-doped sodium bismuth titanate (Na_1/2Bi_1/2)_{1− x} Pb _x TiO₃ of perovskite structure is one of the best-known piezoelectrics/ferroelectrics. However, it has not been properly investigated in any thin-film forms. In this study, the dielectric properties of (Na_1/2Bi_1/2)_0.87Pb_0.13TiO₃ thin films synthesized via a sol–gel route were investigated. They exhibit a strong frequency dispersion of the dielectric permittivity at relatively high frequencies, which is shifted to lower frequencies with increasing temperature. The electrical behavior can be fitted using Jonscher's universal law for dielectric relaxation. The peculiar dielectric behaviors observed can be ascribed to the coexistence of two different dielectric phases in the films, which is believed to be associated with the growth of the local Pb²⁺TiO₃ nanoclusters upon substitution of Pb²⁺ for Na⁺/Bi³⁺ in the (Na_1/2Bi_1/2)_{1− x} Pb _x TiO₃ films.     

Solid-State Synthesis of Ultrafine BaTiO3 Powders from Nanocrystalline BaCO3 and TiO2

Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO₂ (70 nm) with BaCO₃ of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO₃ in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO₃ already leads to formation of the single-phase BaTiO₃ after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d ₅₀=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO₃ and TiO₂ represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders.     

New Phase Transitions in Perovskite Oxides: BiFeO3, SrSnO3, and Pb(Fe2/3W1/3)1/2Ti1/2O3

This paper reports on two new aspects of oxide perovskites: the first part is devoted to new phase transitions, especially at high temperatures; multiferroics such as BiFeO₃ and Pb–Fe–W–titanate are emphasized but nonmagnetic materials such as SrSnO₃ are included. The work summarized on bismuth ferrite emphasizes its metal–insulator transition near 1200 K (atmospheric pressure) and 47 GPa (room temperature); that on SrSnO₃ emphasizes order–disorder phase transitions; and that on lead–iron tungstate–titanate exhibits a classic second-order ferroelectric phase transition, of which rather few are known in the literature. The second part of the paper presents a discussion of constant phase elements for oxide perovskite ceramics; this is a modern way of characterizing their dielectric relaxation, particularly near phase transition temperatures.     

Preparation of Hollow BaTiO3 and Anatase Spheres by the Layer-by-Layer Colloidal Templating Method

Hollow BaTiO₃ and anatase spheres were prepared from multilayered colloidal titanate particles. An inorganic precursor, titanium (IV) bis(ammonium lactate) dihydroxide (TALH) (chemical formula: [CH₃CH(O–)CO₂–NH₄]₂Ti(OH)₂) was used. First, a layer-by-layer (LBL) colloid-templating method was employed using TALH to generate monodispersed hollow titanate spheres. These spheres were then treated in a Ba(OH)₂ solution or distilled water under hydrothermal conditions to transform them into hollow BaTiO₃ or anatase spheres, respectively.     

Synthesis of Zirconium Titanate-Based Materials by Colloidal Filtration and Reaction Sintering

Zirconium titanate exhibits crystallographic anisotropy in thermal expansion, which makes it a suitable candidate for low thermal expansion materials. In this work, zirconium titanate has been synthesized by reaction sintering the green bodies, which have been obtained by colloidal filtration of concentrated suspensions of yttria-tetragonal zirconia polycrystals (Y–TZP) and titania. Powders were mixed in a 50/50 mol% ratio (ZT50) to obtain pure zirconium titanate. Rheological characterization of the suspensions has allowed the establishment of optimum green processing conditions. Sintering has been performed at 1400°C for 2 h, and the obtained materials have been characterized by X-ray diffraction, and scanning electron microscopy with energy-dispersive X-ray microanalysis. The ZT50 material has Zr₅Ti₇O₂₄ as the major phase, although Y₂ ((Zr_0.3Ti_0.7)₂O₇) and unreacted Y–TZP can still be detected.     

Development of Dysprosium Titanate Based Ceramics

Dysprosium titanate (Dy₂TiO₅) based ceramic is one of the control rod materials for thermal reactors. This ceramic has been prepared by conventional mixing and calcination technique. The formation of Dy₂TiO₅ phase from the Dy₂O₃–TiO₂ starting mixture has been found to be a two-step process. In the first step, the formation of dysprosium dititanate (Dy₂Ti₂O₇) phase takes place at 700°C. This Dy₂Ti₂O₇ reacts at around 1100°C with remaining Dy₂O₃ present in the mixture to form Dy₂TiO₅. The Dy₂TiO₅ phase has three polymorphs (orthorhombic, hexagonal, and fluorite). The fluorite phase is reported to have lowest in-service irradiation swelling characteristics when used as control rod elements in thermal reactors. In the present investigation, the fluorite polymorph of Dy₂TiO₅ has been stabilized by doping with molybdenum trioxide.     

Hydrothermal Soft Chemical Synthesis and Particle Morphology Control of BaTiO3 in Surfactant Solutions

Barium titanate (BaTiO₃) particles with book-like and spherical morphology were prepared by using a hydrothermal soft chemical process in the presence of a cationic surfactant. A layered titanate of H_1.07Ti_1.73O₄ with a lepidocrocite-like structure and plate-like particle morphology was used as the precursor. The layered titanate was hydrothermally treated in a Ba(OH)₂–(HTMA-OH) ( n -hexadecyltrimethylammonium hydroxide) solution or a Ba(OH)₂–(HTMA-Br) ( n -hexadecyltrimethylammonium bromide) solution in a temperature range of 80°–250°C to prepare BaTiO₃. The intercalation reaction of HTMA⁺ with the layered titanate promotes the structural transformation reaction from the layered titanates to BaTiO₃, while it inhibits the structural transformation reaction to anatase under the hydrothermal conditions. The particle morphology of BaTiO₃ prepared by this method dramatically changes with changing reaction conditions. HTMA⁺ plays an important role in changing particle morphology in the hydrothermal soft chemical process.