Mark G. Blackford

The durability of synroc

Abstract Polished specimens of well characterised Synroc (a polyphase titanate ceramic designed to immobilise high-level nuclear waste) containing 10 wt% simulated (PW-4b-D) waste were subjected to MCC-2 type dissolution tests at 150°C in doubly deionised water (surface area to volume ratio0.01 ± 0.0005 mm −1 ) for periods of up to 532 d. Scanning electron microscopy (SEM) of the polished surfaces and transverse sections confirmed that Synroc is highly durable and suffers little corrosion even after 532 d in doubly deionised water at 150°C (corrosion of the least resistant major phase, perovskite, was

Actinide and rare earth incorporation into zirconolite

Abstract Nd 3+ and Ce 3+ can substitute for about 65% of the Ca in monoclinic zirconolite, using AI as a charge compensator in a Ti site. Further substitution up to 85% of Nd 3+ and Ce 3+ produces an orthorhombic structure, while more than 85% substitution produces additional phases. Substitution of Ce 4+ in the Zr site appeared to be quite limited. Incorporation of U 4+ into the Ca and Zr sites in zirconolite gave results which were similar to those observed by others. Both trivalent and tetravalent Np and Pu can be substituted in the Ca and Zr sites, respectively, under oxygen partial pressures of 0.2−1 × 10 −5 atm, provided appropriate charge compensators are present. The implications of these results for formulating actinide-bearing zirconolite-rich ceramics are discussed briefly.

Determination of 25 elements in the complex oxide mineral zirconolite by analytical electron microscopy

Abstract In this paper we describe a technique for the determination of 25 elements in natural zirconolite using energy-dispersive analytical electron microscopy (AEM). The method presented here allows one to quantitatively investigate the chemistry of submicron-scale zones in complex oxide minerals. The effects of electron channeling, thickness variability and variations in detector resolution were minimized by using a controlled set of operating procedures and instrument parameters. To provide a high level of accurayy, kATI-factors were determined from standards for most of the 25 elements of interest, including all of the major elements. Each analytical spectrum is reduced to a set of raw peak counts (and errors) using a digital top-hat filter to suppress background followed by multiple least squares fitting of reference spectra. Counting times of 12–15 min per analysis were required to provide suitable counting statistics. Results are presented for zirconolite samples from the contact metamorphic aureole of the Bergell granodiorite intrusion, Switzerland-Italy. A comparison of 43 AEM analyses with 15 analyses obtained by wavelength-dispersive electron probe microanalysis (EPMA) shows that there is excellent agreement between the two data sets in the amounts of individual elements present, chemical trends and overall stoichiometry. An assessment of the combined data set shows that the major substitution mechanisms in the Bergell samples are coupled substitutions involving the M5,6- and M8-sites of the zirconolite structure: M8Ca2+ + M5,6Ti4+→M8REE3+ + M5,6(Al,Fe)3+ and M8Ca2+ + M5,6Ti4+→M8(Th,U)4+ + M5,6 (Mg,Fe)2+.

Preparation of boron doped porous titania networks containing gold nanoparticles with enhanced visible-light photocatalytic activity

The ability to decrease the electron/hole recombination rate, and decrease the band gap of titania to allow photoactivity on irradiation with visible light is attracting more and more attention. Here, boron doping of the titania, the deposition of gold nanoparticles, along with a meso-macroporous structure were obtained using a facile agarose gel templating process combined with sol-gel chemistry. The Au/B/TiO(2) nanocomposites were characterized using SEM, TEM, XRD, N(2) gas sorption, diffuse UV-vis, photoluminescence, and SIMS. The photocatalytic activity was assessed by degradation of an organic probe molecule (methylene blue) under visible light (λ > 420 nm). The resulting materials achieved photocatalytic activities up to 50% greater than the commercial Degussa P25 under visible light. The enhancement in photocatalytic activity was primarily attributed to the decrease in band gap as a result of the boron doping and its influence on the anatase to rutile phase formation: The doped materials were highly crystalline and an optimum anatase to rutile ratio (3:1) was obtained with 0.25 wt % boron in the sample calcined at 650 °C. In addition, the presence of the gold nanoparticles decreased recombination between the photoexcited electrons and holes, which further improved the photocatalytic activity.

Novel rare gas interstitial fullerenes of C60 with Ar, Kr and Xe

Abstract In this paper we report the formation and characterisation of rare gas C 60 interstitial compounds of Ar, Kr and Xe. The materials were produced by hot isostatically pressing (HIP) the fullerene solid at temperatures between 200 and 550 °C and under rare gas pressures in the range 170–200 MPa. With this method, we have been able to make rare gas fullerene compounds with stoichiometries of Ar 1 C 60 , Kr 0.9 C 60 and Xe 0.66 C 60 . Thermal gravimetric analysis (TGA) showed that the HIPed materials contained rare gas after treatment, and gave a method for determining the stoichiometry. TGA also enabled the thermal stabilities of these materials with respect to rare gas loss to be investigated. The structure of the rare gas fullerenes was characterised by both X-ray and neutron powder diffraction. By Rietveld analysis of the diffraction data it has been shown that only the octahedral interstices of the fullerene face centred cubic (FCC) lattice were occupied by the rare gas, and the site occupancy of this site gave a stoichiometry agreeing within 5% of that obtained from TGA. The face centred to primitive cubic orientational ordering phase transition for these rare gas fullerenes was studied using neutron diffraction. The transition temperature was found to decrease as the size of the rare gas increases. This lowering is a result of the net weakening of the c 60 c 60 interaction potential, as the rare gas pushes the c 60 molecules slightly apart; a consequence of not only their size but also a result of their thermal motion (internal pressure) within the interstitial site. Differential scanning calorimetry (DSC) confirmed the transition temperatures obtained from neutron diffraction. In addition, transmission electron microscopy (TEM) and 13 C NMR studies have been performed on these materials and the results are discussed.

Partitioning of uranium and rare earth elements in synroc: effect of impurities, metal additive, and waste loading

Abstract AEM techniques employing digital filtering, least squares profile fitting, and experimental k-factor calibrations were used to investigate 16 Synroc samples containing simulated Purex (PW-4b-D) HLW at loadings of 10, 15, 19, and 23 wt%. A second group of Synroc samples with 10 wt% HLW also contained additional impurities of F, Na, MgO, P2O5, and Fe2O3. A third set of samples with 10 wt% HLW contained different metal additions of Al, Ni, and Ti, and a sample with no metal addition for comparison. In samples with low Na2O content, it was confirmed that element partitioning is mainly controlled by the ionic radius criterion, with smaller Y, Gd, and U ions having a preference for zirconolite and the larger Ce and Nd ions favoring perovskite. Average relative partitioning coefficients ( D Z P = metal oxide in zirconolite ÷ wt% metal oxide in perovskite ) of 8 samples with 10 wt% HLW and -∼0.5 wt% Na2O are 0.14 ± 0.01, 0.39 ± 0.03, 1.7 ± 0.2, 3.8 ± 1.0, and 2.2 ± 0.8 for Ce, Nd, Gd, Y, and U, respectively. Element partitioning is not strongly affected by additional impurities of F, MgO, P2O5, or Fe2O3; metal addition; or waste loading. Additions of up to ∼3.6 wt% Na2O lead to an increase in the amount of perovskite at the expense of zirconolite as well as a systematic shift in the partitioning of REEs and U from zirconolite into perovskite.

Heavy ion irradiation studies of columbite, brannerite, and pyrochlore structure types

Abstract Several natural and synthetic samples of pyrochlore, columbite, and brannerite were irradiated in situ in the high voltage electron microscope at Argonne National Laboratory using 1.5 MeV Kr+ ions at room temperature. The columbite samples were selected because they have the same stoichiometry as brannerite and they exhibit a range of cation order prior to irradiation. The results of this study demonstrate that all the samples pass through the crystalline–amorphous transformation at a relatively low ion dose. For the AB2O6 oxides, Dc ranges from 0.8 to 1.5×1014 ions cm−2, with little difference between the columbite and brannerite structure types. Within the group of columbite samples, there appears to be a systematic trend of increasing critical dose with increasing order parameter, although the critical doses are essentially within error of one another. Ordered columbites do not appear to revert to the subcell upon irradiation. The brannerite data set indicates a possible correlation between Dc and the mean atomic number of the A-site cation. The results for the two natural pyrochlores gave a slightly higher range of doses compared with the AB2O6 oxides. The pyrochlore data set indicates a possible correlation between Dc and the mean atomic number of the B-site cation. Electron diffraction results also suggest that the pyrochlore compositions irradiated in this work may not transform to the fluorite subcell until just before Dc is reached, if at all. For comparison, the samples investigated in this work have Dc values at room temperature that are generally on the low side of the range of values previously reported for synthetic zirconolite and pyrochlore samples (2–6×1014 ions cm −2 ) under similar irradiation conditions.

Crystallisation of zirconolite from an alkoxide precursor

Abstract Crystallisation of zirconolite (CaZrTi2O7) from a stoichiometric alkoxide precursor was studied by X-ray diffraction, differential thermal analysis and high resolution electron microscopy. The X-ray amorphous oxide formed by drying mixed ethanolic solutions crystallised to a disordered fluorite structure on heating to ~ 700 °C. Zirconolite with a pseudo-trigonal structure formed at ~ 900 °C, and at higher temperatures gradually transformed to fully ordered monoclinic zirconolite. Activation energies for the amorphous to fluorite and fluorite to pseudo-trigonal zirconolite transformations were obtained from the DTA measurements by the Ozawa method, but were not consistent with the rates of transformation observed by X-ray diffraction in isothermal measurements. The pseudo-trigonal zirconolite was found by high-resolution electron microscopy to be highly disordered monoclinic zirconolite.

Dissolution of synthetic brannerite in acidic and alkaline fluids

Abstract The dissolution of synthetic brannerite in aqueous media at 40 and 90 °C under atmospheric redox conditions has been studied. At 40 °C, the presence of phthalate as a buffer component in the pH range of 2–6 has little effect on uranium release from brannerite. Bicarbonate increases uranium release and enhances the dissolution of brannerite. Compared to UO 2 , brannerite is more resistant to dissolution in bicarbonate solutions. In under-saturated conditions at 90 °C, the dissolution of brannerite is incongruent (preferential release of uranium) at pH 2 and nearly congruent at pH 11. TEM examinations reveal a polymorph of TiO 2 (pH 2 specimen) and a fibrous Ti-rich material (pH 11 specimen) as secondary phases. XPS analyses indicate the existence of U(V) and U(VI) species on the surfaces of specimens both before and after leaching, and U(VI) was the dominant component on the specimen leached in the pH 11 solution.

Solvothermal synthesis and photocatalytic application of porous Au/TiO2 nanocomposites

Porous titania networks embedded with gold nanoparticles have been synthesized and tested in photocatalytic applications. A solvothermal method was adopted for the in situ reduction of the gold precursor to gold nanoparticles within a preformed TiO2 matrix synthesized by combining a templating technique and sol–gel chemistry. The titania materials were crystalline with anatase crystals of 13 ± 2 nm in diameter; XRD peaks of metallic gold were observed when the gold content was over 1 wt% (gold crystal diameters of 18–25 nm for 1–8 wt% Au). The variation in gold particle size for different gold content was observed by transmission electron microscopy, which also confirmed highly porous structures of the final Au/TiO2 composites. The actual gold content generally increased with the Au content used during synthesis; was enhanced when the pH of the gold precursor solution increased from 2 to 10; and remained relatively constant with changes in the solvothermal temperature (120–180 °C). UV-Visible diffuse reflectance spectra of the Au/TiO2 showed strong absorption at ∼550 nm due to the successful addition of gold nanoparticles to the titania. The number of surface hydroxyl groups was influenced by solvothermal treatment temperature, synthesis pH and gold content. The addition of gold nanoparticles to the TiO2 significantly suppressed recombination of the photon-induced electron–hole pairs. The photocatalytic efficiency of the composites was assessed by monitoring the photocatalytic degradation of methylene blue in aqueous solution; maximum efficiency was more than double that of the pure TiO2 control when using 2.0 to 4.0 wt% gold solutions at pH 7 and optimized solvothermal conditions of 180 °C for 14 h. The photocatalytic efficiency was significantly altered with the addition of gold and was greatly influenced by the anatase crystal size and relative XRD peak intensity, the gold content, and the capability of the material to decrease photon-induced electron–hole recombination.