Browsing by Author "Zaluzec, NJ"
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- ItemHVEM-Tandem and EELS study of radiation damage in zirconolite(Cambridge University Press, 1997-08-10) Smith, KL; Zaluzec, NJ; Lumpkin, GRZirconolite (CaZrTi2O7) is the major host phase for actinides in Synroc, a promising waste form for the immobilization of high-level radioactive waste. The effect of radiation damage on the structure and durability of zirconolite are important to predictive modeling of zirconolite`s behavior in the repository environment and risk assessment. In this study, radiation damage effects in zirconolite were investigated by irradiating samples with 1.5 MeV Kr+ ions using the HVEM-Tandem at Argonne National Laboratory (ANL) and energy loss electron spectroscopy (EELS). The HVEM-Tandem consists of a modified AEI high voltage transmission electron microscope interfaced to a 2 MV tandem ion accelerator. EELS spectra were collected using a Philips 420 TEM, operated at 120 kV, fitted with a Gatan Model 607 Serial EELS. EELS data were recorded at resolutions of {approximately} 1.0 eV and at a dispersion of about {approximately} 0.25 eV. Selected area diffraction patterns (SADs) of individual grains of various zirconolites were monitored as a function of dose to establish the critical dose for amorphization (D{sub c}). The authors found that (1) Dc(zirconolite) is independent of the atomic weight of dopants in zirconolite and the mean atomic weight of the sample and that (2) the Bragg reflections in SAD patterns which persist to the highest doses are firstly those resulting from the fluorite sublattice and secondly the four (110)-type reflections which lie on the innermost of the two diffuse rings representative of amorphous zirconolite. © 1997 Microscopy Society of America 1997
- ItemIn situ radiation damage studies of Ca3Zr2FeAlSiO12 and Ca3Hf2FeAlSiO12(Australian Institute of Nuclear Science and Engineering (AINSE), 2009-11-25) Whittle, KR; Blackford, MG; Smith, KL; Lumpkin, GR; Zaluzec, NJGarnets, A3B2C3O12, are considered to be potential host phases for the immobilization of high-level nuclear waste as they can accommodate a number of elements of interest, including Zr, Ti and Fe. The naturally occurring garnet, kimzeyite, Ca3(ZrTi)2(SiAlFe)O12, can contain ∼30wt% Zr. An understanding of the radiation tolerance of these materials is crucial to their potential use in nuclear waste immobilization. In this study two synthetic analogues of kimzeyite of composition Ca3(ZrTi)2(SiAlFe)O12 and Ca3Hf2FeAlSiO12 were monitored in situ during irradiation with 1.0 MeV Kr ions using the intermediate voltage electron microscope-tandem user facility (IVEM) at Argonne National Laboratory. The structure of these materials was previously determined by neutron diffraction and 57Fe Mössbauer spectroscopy. Ca3(ZrTi)2(SiAlFe)O12 and Ca3Hf2FeAlSiO12 have very similar structural properties with cubic la3d symmetry, the only significant difference being the presence of Zr and Hf, respectively on the 6 coordinated B sites. © 2009 AINSE
- ItemIn-situ TEM studies of the radiation tolerance of ceramics(Australian Microscopy and Microanalysis Society, 2016-02-04) Aughterson, RD; Cairney, JM; Zaluzec, NJ; Lumpkin, GRFor the nuclear industry ion-irradiation has been used to simulate recoil damage from alpha decay, and exposure to neutrons in fission and fusion reactors [1]. The particular focus of this research is on the crystalline to amorphous transition facilitated by exposure to accelerated ions. The complex ceramic oxides chosen for this study, Ln2TiO5 (Ln = lanthanides and yttrium), have several uses within the nuclear industry. The compound Dy2TiO5 has been used within WWER type reactors due to its good resistance to irradiation induced swelling and structural failure [2]. Of particular interest are the cubic symmetry compounds with defect fluorite structure, which gives good radiation response. Previous studies show the series of Ln2TiO5 compounds may take on a variety of crystal symmetries depending on the lanthanide size and fabrication conditions used [3, 4]. The study of ion-irradiation response was carried out using the in-situ approach where the test materials were exposed to 1 MeV Kr2+ ions and monitored for their transition from crystalline to amorphous state. This was carried out using the intermediate voltage electron microscope (IVEM)-Tandem facility at Argonne National Laboratory. The critical dose of irradiating ions, Dc, required to render the Ln2TiO5 completely amorphous was determined by monitoring selected area electron diffraction patterns for loss of diffraction spots (Bragg maxima) and replacement with diffuse rings. Within the Ln2TiO5 series of compounds studied a linear relationship appears between the radii of the lanthanide and the radiation response. This follows a similar trend found in related pyrochlore compounds. Possible explanations for this trend are investigated. For the nuclear industry ion-irradiation has been used to simulate recoil damage from alpha decay, and exposure to neutrons in fission and fusion reactors [1]. The particular focus of this research is on the crystalline to amorphous transition facilitated by exposure to accelerated ions. The complex ceramic oxides chosen for this study, Ln2TiO5 (Ln = lanthanides and yttrium), have several uses within the nuclear industry. The compound Dy2TiO5 has been used within WWER type reactors due to its good resistance to irradiation induced swelling and structural failure [2]. Of particular interest are the cubic symmetry compounds with defect fluorite structure, which gives good radiation response. Previous studies show the series of Ln2TiO5 compounds may take on a variety of crystal symmetries depending on the lanthanide size and fabrication conditions used [3, 4]. The study of ion-irradiation response was carried out using the in-situ approach where the test materials were exposed to 1 MeV Kr2+ ions and monitored for their transition from crystalline to amorphous state. This was carried out using the intermediate voltage electron microscope (IVEM)-Tandem facility at Argonne National Laboratory. The critical dose of irradiating ions, Dc, required to render the Ln2TiO5 completely amorphous was determined by monitoring selected area electron diffraction patterns for loss of diffraction spots (Bragg maxima) and replacement with diffuse rings. Within the Ln2TiO5 series of compounds studied a linear relationship appears between the radii of the lanthanide and the radiation response. This follows a similar trend found in related pyrochlore compounds. Possible explanations for this trend are investigated.
- ItemInvestigating radiation damage in ceramics: the role of in situ microscopy(Cambridge University Press, 2010-08-01) Smith, KL; Lumpkin, GR; Whittle, KR; Blackford, MG; Zaluzec, NJRadiation damage effects in ceramics are of interest in both current (e.g. nuclear waste forms) and next generation (Gen IV) nuclear technologies (e.g. transmutation targets, inert matrix fuels, waste forms etc.). The mechanisms of radiation damage production and recovery in ceramics often vary dependent on the damage source, e.g., energetic neutrons in reactor systems versus alpha recoil damage in nuclear waste forms. Furthermore, the kinetics of damage recovery are complicated by multiply activated processes and in certain cases, longer-term process may modify the structural state left by irradiation in the short term. Here, we review a selection of data collected by the current authors and associates, which highlight the roles played by in situ microscopy in unraveling the mechanisms of radiation damage in ceramics. In situ irradiation of a variety of ceramics (e.g. pyrochlores, perovskites, polymorphs of TiO2 and analogues etc.) have been carried out using the IVEM-Tandem Facility at Argonne National Laboratory [1-7]. The IVEM-Tandem Facility is comprised of an intermediate voltage TEM (IVEM), a Hitachi H-9000NAR, interfaced to two ion accelerators (at 30° to vertical). Specimens were generally prepared by crushing and suspending small fragments on holey carbon coated Cu grids. Samples were most often, irradiated using 1 MeV Kr ions at temperatures from 50 to 900K. Figure 1 shows (a) a bright field (BF) image and (b) selected area diffraction pattern (SADP) of a grain of a typical specimen (in this case TiO2) before in situ irradiation and (c) the SADP of the same grain after a series of irradiations. Multiple measurements were taken and averaged to ascertain the critical fluence (of particular ions) for amorphisation, Fc, of a given sample at various temperatures. Fc data were then were used to establish the critical temperature, Tc, of the particular sample, where Tc is the temperature at which the recovery process is at least equal to the damage rate (that is the temperature above which a sample can not be amorphised). IVEM-Tandem data on a range of pyrochlores with different compositions have been compiled and used to develop an empirical model that can predict the Tc values for many pyrochlores within +/- 80°C [2, 6]. IVEM-Tandem results for specific solid solution series of pyrochlores are informative and will be discussed more fully in the conference presentation. LaxSr1-3x/2TiO3 perovskites show a non-linear relationship for Tc with x [3]. In combination with other data, in situ TEM suggests that cubic perovskites in the 0
- ItemIon beam irradiation of ABO4 compounds with the fergusonite, monazite, scheelite, and zircon structures(https://doi.org/10.1111/jace.17288, 2020-06-04) de los Reyes, M; Aughterson, RD; Gregg, DJ; Middleburgh, SC; Zaluzec, NJ; Huai, P; Ren, CL; Lumpkin, GRThe effects of irradiation on CaWO4, SrWO4, BaWO4, YVO4, LaVO4, YNbO4, and LaNbO4 were investigated on thin crystals using 1.0 MeV Kr ions at 50‐1000 K. All of the ABO4 compounds can be amorphized with calculated damage cross sections (σa = 1/Fc0) in the range of ~0.30‐1.09 × 10‐14 cm2 ion−1 at zero Kelvin. The analysis of fluence‐temperature data returned critical temperatures for amorphization (Tc) of 311 ± 1, 358 ± 90, 325 ± 19, 415 ± 17, 541 ± 6, 636 ± 26, and 1012 ± 1 K, respectively, for the compounds listed above. Compared with previous in situ irradiation of ABO4 orthophosphate samples using 0.8 MeV Kr ions, the Tc values of LaVO4 and YVO4 are higher than those of LaPO4 and YPO4 by 82 K and 124 K, respectively. The Tc values of the three scheelite structures, CaWO4, SrWO4, and BaWO4, indicate that they are the most radiation tolerant compounds under these conditions. The A‐B cation anti‐site energies, EfAB, determined by DFT range from 2.48 to 10.58 eV and are highly correlated with the A‐B cation ionic radius ratio, rA/rB, but are not correlated with Tc across the different structure types, suggesting that the formation and migration energies of Frenkel defects play a more important role in damage recovery in these compounds. We also discuss the role of cation and anion charge/iconicity as determined by DFT. ABO4 compounds with the zircon structure and B = P or V have a distinct advantage over those with B = Si as the damaged regions do not appear to be significantly affected by polymerization of (PO4)3− or (VO4)3− groups which might stabilize the amorphous fraction and ultimately lead to phase separation as observed in zircon (ZrSiO4). © 1999-2020 John Wiley & Sons, Inc.
- ItemIon irradiation of ternary pyrochlore oxides(American Chemical Society, 2009-07-14) Lumpkin, GR; Smith, KL; Blackford, MG; Whittle, KR; Harvey, EJ; Redfern, SAT; Zaluzec, NJPolycrystalline synthetic samples of Y2Ti2−xSnxO7 with x = 0.4, 0.8, 1.2, and 1.6, together with Nd2Zr2O7, Nd2Zr1.2Ti0.8O7, and La1.6Y0.4Hf2O7, were irradiated in situ in the intermediate voltage electron microscope (IVEM)-Tandem Facility at Argonne National Laboratory using 1.0 MeV Kr ions at temperatures of 50 to 650 K. Determination of the critical amorphization fluence (Fc) as a function of temperature has revealed a dramatic increase in radiation tolerance with increasing Sn content on the pyrochlore B site. Nonlinear least-squares analysis of the fluence-temperature curves gave critical temperatures (Tc) of 666 ± 4, 335 ± 12, and 251 ± 51 K for the Y2Ti2−xSnxO7 samples with x = 0.4, 0.8, and 1.2, respectively. The sample with x = 1.6 appears to disorder to a defect fluorite structure at a fluence below 1.25 × 1015 ions cm−2 and remains crystalline to 5 × 1015 ions cm−2 at 50 K. Additionally, the critical fluence-temperature response curves were determined for Nd2Zr1.2Ti0.8O7 and La1.6Y0.4Hf2O7, and we obtained Tc values of 685 ± 53 K and 473 ± 52 K, respectively, for these pyrochlores. Nd2Zr2O7 did not become amorphous after a fluence of 2.5 × 1015 ions cm−2 at 50 K, but there is evidence that it may amorphize at a higher fluence, with an estimated Tc of 135 K. The observed Tc results are discussed with respect to the predicted Tc values based upon a previously published empirical model (Lumpkin, G. R.; Pruneda, M.; Rios, S.; Smith, K. L.; Trachenko, K.; Whittle, K. R.; Zaluzec, N. J. J. Solid State Chem. 2007, 180, 1512). In the Y2Ti2−xSnxO7 pyrochlores, Tc appears to be linear with respect to composition, and is linear with respect to rA/rB and x(48f) for all samples investigated herein. © 2009, American Chemical Society
- ItemIon irradiation of ternary pyrochlores(Materials Research Society, 2008-12-01) Whittle, KR; Smith, KL; Blackford, MG; Redfern, SAT; Harvey, EJ; Zaluzec, NJ; Lumpkin, GRSynthetic pyrochlore samples Y2Ti2-xSnxO7 (x=0.4, 0.8, 1.2, 1.6), Nd2Zr2O7, Nd2Zr1.2Ti0.8O7, and La1.6Y0.4Hf2O7, were irradiated in-situ using the IVEM-TANDEM microscope facility at the Argonne National Laboratory. The critical temperatures for amorphisation have revealed a dramatic increase in tolerance with increasing Sn content for the Y2Ti2-xSnxO7 series. This change has also found to be linear with increasing Sn content. Nd2Zr1.2Ti0.8O7 and La1.6Y0.4Hf2O7 were both found to amorphise, while Nd2Zr2O7 was found to be stable to high doses (2.5x10^15 ions cm-2). The observed results are presented with respect to previously published results for irradiation stability predictions and structural disorder. © Materials Research Society 2009
- ItemIon irradiation of the TiO2 polymorphs and cassiterite(Mineralogical Society of America, 2010-01) Lumpkin, GR; Blackford, MG; Smith, KL; Whittle, KR; Zaluzec, NJ; Ryan, EA; Baldo, PThin crystals of rutile, brookite, anatase, and cassiterite were irradiated in situ in the transmission electron microscope using 1.0 MeV Kr ions at 50–300 K. Synthetic rutile and natural cassiterite, with 0.1–0.2 wt% impurities, remain crystalline up to a fluence of 5 x 1015 ions cm–2 without evidence for amorphization at 50 K. Natural brookite and anatase, with 0.3–0.5 wt% impurities, become amorphous at fluences of 8.1 x 1014 and 2.3 x 1014 ions cm–2, respectively. We have also studied two natural rutile samples containing ~1.7 and 1.2 wt% impurities. These samples became amorphous at 9.2 x 1014 and 8.6 x 1014 ions cm–2 at 50 K, respectively. Further analyses of the fluence-temperature data for natural brookite, rutile, and anatase give critical amorphization temperatures of 168 ± 11, 209 ± 8, and 242 ± 6 K, respectively. Results are briefly discussed with respect to several criteria for radiation resistance, including aspects of the structure, bonding, and energetics of defect formation and migration. © 2010, Mineralogical Society of America
- ItemIon-beam irradiation of lanthanum compounds in the systems La2O3–Al2O3 and La2O3–TiO2(Elsevier, 2010-10) Whittle, KR; Lumpkin, GR; Blackford, MG; Aughterson, RD; Smith, KL; Zaluzec, NJThin crystals of La2O3, LaAlO3, La2/3TiO3, La2TiO5, and La2Ti2O7 have been irradiated in situ using 1 MeV Kr2+ ions at the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), Argonne National Laboratory (ANL). We observed that La2O3 remained crystalline to a fluence greater than 3.1×1016 ions cm−2 at a temperature of 50 K. The four binary oxide compounds in the two systems were observed through the crystalline–amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (Tc) of 647 K for LaAlO3, 840 K for La2Ti2O7, 865 K for La2/3TiO3, and 1027 K for La2TiO5. The Tc values observed in this study, together with previous data for Al2O3 and TiO2, are discussed with reference to the melting points for the La2O3–Al2O3 and La2O3–TiO2 systems and the different local environments within the four crystal structures. Results suggest that there is an observable inverse correlation between Tc and melting temperature (Tm) in the two systems. More complex relationships exist between Tc and crystal structure, with the stoichiometric perovskite LaAlO3 being the most resistant to amorphisation. © 2010, Elsevier Ltd.
- ItemM(n+1)AXn phases are they tolerant/resistant to damage(Australian Institute of Physics, 2011-02-03) Whittle, KR; Riley, DP; Blackford, MG; Aughterson, RD; Moricca, SA; Lumpkin, GR; Zaluzec, NJTernary carbide materials have been proposed as having applications within the future nuclear technologies, both fusion (ITER/DEMO) and fission (Gen IV). These new designs require a material to have the ability to tolerate radiation damage to high levels, with a high level of predictability. As part of such a process two systems, specifically Ti3AlC2 and Ti3SiC2 have been studied to determine their radiation tolerance, using in-situ ion beam irradiation with 1 MeV Xe ions, coupled with transmission electron microscopy. Irradiations have shown that both systems show little amorphisation at 300K up to doses of at least 6.25 x 1015 ions cm-2 (~28-30 dpa). However, there is a subtle difference between Ti3AlC2 and Ti3SiC2, with Ti3SiC2 showing more evidence for damage. Further irradiations using 500 KeV Xe to fluences equivalent to 100 dpa have also been undertaken, with crystalline material visible and evidence of recrystallisation. Explanations and possible mechanisms for recovery from damage are presented, along with implications for future potential uses.
- ItemA model for electron scattering in irradiated pyrochlore-fluorite systems(Cambridge University Press, 2009-07-26) Lumpkin, GR; Blackford, MG; Smith, KL; Whittle, KR; Zaluzec, NJCertain minerals, materials for energy, and high technology devices may undergo irradiation, either internally through alpha decay, or externally by neutrons or ions [1]. Many of these materials become amorphous when irradiated beyond a critical dose level. Electron diffraction patterns are routinely used to study the structure of these materials, including the observation of order-disorder phenomena during irradiation [2]. Here we show how the intensities of Bragg beams change from a dynamical to a kinematic-like state during the accumulation of amorphous domains in the material. The transition from the crystalline to the amorphous state can be considered as a two-phase problem, with fc + fa = 1, where fc and fa are the fractions of the crystalline and amorphous phases, both having the same composition. For fc = 1 the perfect crystal scatters into the nodes of the reciprocal lattice according to the many-beam dynamical equations and for fa = 1 the amorphous phase scatters according to the Debye equation. Figure 1 shows the strong Bragg-Bragg interactions that occur in the perfect crystal according to relationships like (hkl)1 + (hkl)2 = (hkl)3. However, once there are sufficient amorphous domains in the material, the diffuse scattering from these domains will begin to attenuate the double diffraction relationship between Bragg beams via relationships like (hkl)1 + (am)1 = (am)2 and (am)1 + (hkl)1 = (am)2. Multiple diffuse scattering, e.g., (am)1 + (am)2 = (am)3, will eventually come into play as shown in Figure 1. Thus, the relative intensities of the Bragg beams change dramatically with increasing radiation dose as fa increases. The above model also suggests that the intensity scattered from the amorphous phase will be dependent upon the orientation of the specimen and that Bragg-diffuse interactions may be observed. We tested this model by irradiating TEM samples of Gd2Ti2O7 in the IVEM-Tandem Facility at Argonne National Laboratory using 1.0 MeV Kr ions. The top row of Figure 2 reveals a systematic change in the Bragg beam intensities of the crystalline fraction from a dynamical pattern toward a kinematic-like pattern for the [110] zone axis with increasing dose. The weaker superlattice beams are lost in the amorphous background or in the enhanced small angle scattering. As shown previously [3], kinematically strong beams always the last to disappear during irradiation. Additional dynamical scattering effects are shown in the bottom row of Figure 2, in which the specimen is tilted to a 〈111〉 systematic row. These SAED patterns exhibit stronger scattering at lower dose levels from the amorphous fraction due to the removal of numerous Bragg beams from the Ewald sphere upon tilting, together with multiple Bragg-diffuse scattering along the systematic row. Certain superlattice beams (e.g., 333) can be observed to high dose levels, indicating that the crystalline domains retain the ordered pyrochlore structure. The model provides a general description of changes in Bragg beam intensities during the crystalline-amorphous transformation, but it is especially relevant to pyrochlore-fluorite systems in which the superlattice beams are very weak in the kinematic-like state. This has major implications for the interpretation of order-disorder phenomena for different damage mechanisms. References [1] Ewing, R.C., Weber, W.J., Lian, J., J. Appl. Phys. 95, 5949-5972 (2004). [2] Lian, J., Chen, J., Wang, L.M., Ewing, R.C., Farmer, J.M., Boatner, L.A., Helean, K.B., Phys. Rev. B 68, 134107 (2003). [3] Smith, K.L., Zaluzec, N.J., Lumpkin, G.R., In situ studies of ion irradiated zirconolite, pyrochlore, and perovskite. J. Nucl. Mater. 250, 36-52 (1997). © 2009 Microscopy Society of America
- ItemOn the characterisation of order-disorder in ion-irradiated pyrochlore compounds by electron scattering methods(Materials Research Society, 2008-12-01) Lumpkin, GR; Whittle, KR; Blackford, MG; Smith, KL; Zaluzec, NJSelected area electron diffraction patterns are routinely used to determine the effects of radiation damage in nuclear materials. Using zone axis orientations, the intensities of Bragg beams change from a dynamical to kinematic-like state due to the presence of amorphous domains in the material. Such changes in beam intensities, together with the increased diffuse scattering from the increasing amorphous fraction, present a major obstacle to the determination of cation or anion disorder in the crystalline fraction. © Materials Research Society 2009
- ItemPyrochlore-defect fluorite phase transitions and stability in the Y2Sn2-xZrxO7 system(Committee of Asia-Pacific Societies of Microscopy, 2012-02-07) de los Reyes, M; Whittle, KR; Elliman, RG; Zaluzec, NJ; Ashbrook, SE; Mitchell, MR; Lumpkin, GRThe Y2Sn2-xZrxO7 pyrochlore series undergoes a phase transformation from a cubic pyrochlore structure type (Fd3m) to defect fluorite (Fm3m) actuated by an increase in Zr content, coupled with thermal annealing above 1500 °C. X-ray diffraction analysis reveals the onset of a pyrochlore to defect fluorite transition at Y2Sn0.8Zr1.2O7 with the loss of long range ordering. This is confirmed further by selected area electron diffraction (SAED) illustrating shorter range ordering in the defect fluorite phase incommensurate with unit cell size. This transformation however, occurs at a much higher Zr content than that predicted by classical radius ratio models. The diffuse scattering features observed in electron diffraction patterns of defect fluorite phases indicate some form of shorter range ordering involving compositional-displacive structural modulation. The behaviour of these materials during irradiation will be discussed and linked with the observed structural parameters (diffuse scattering, unit cell size).
- ItemRadiation damage and the effects of disorder(Australian Institute of Nuclear Science and Engineering (AINSE), 2009-11-25) Whittle, KR; Blackford, MG; Lumpkin, GR; Zaluzec, NJRadiation damage and its effects on a material is an important component in the prediction of the long term stability of waste form materials. As part of the ongoing goal of increasing the accuracy of long-term predictions of radiation damage, two types of material, based on proposed materials with a waste form application have been irradiated. Results have shown that Y2TiO5 (Y2.67Ti1.33O6.57), and Yb2TiO5 (Yb2.67Ti1.33O6.67), both disordered pyrochlore-based materials, behave significantly different to the fully occupied pyrochlore equivalent. For example the critical temperature, the temperature above which materials remain crystalline during irradiation, is found to decrease substantially, ∼150K from the ordered equivalents, e.g. Y2Ti2O7. A second material based on LaTiO5 has been found to behave differently to both La2/3TiO3 and La2Ti2O7, with a change in Tc of ∼200K. Both systems are discussed with reference to both crystalline and chemical effects, highlighting both using experimental, both x-ray and electron based, and simulation analysis. © 2009 AINSE
- ItemRadiation damage in materials - effects of disorder(Materials Research Society, 2009-05-24) Whittle, KR; Blackford, MG; Lumpkin, GR; Smith, KL; Zaluzec, NJRadiation damage and the effect on physical and chemical properties is an important component in the prediction of the long-term stability of waste form materials. As part of the ongoing goal of increasing the accuracy of long-term predictions of radiation damage, two types of material, based on proposed materials with a waste form application have been irradiated. Results have shown that Y2TiO5 (Y2.67Ti1.33O6.67), and Yb2TiO5 (Yb2.67Ti1.33O6.67), both of which are non-stoichiometric, disordered pyrochlore-based compounds, behave significantly different to the stoichiometric, ordered pyrochlore equivalent. For example the critical temperature, the temperature above which materials remain crystalline during irradiation, is found to decrease from the ordered equivalents, e.g. Y2Ti2O7. A second material based on La2TiO5 has been found to behave differently to both LaSUB>2/3TiO3 and La2Ti2O7, with a change in Tc of ~ 200 K. © Materials Research Society 2009
- ItemRadiation tolerance and disorder - can they be linked?(Materials Research Society, 2009-12-02) Whittle, KR; Blackford, MG; Smith, KL; Lumpkin, GR; Zaluzec, NJThe future expansion of nuclear power provides materials challenges that are not easily overcome, for example the safe immobilisation of nuclear waste is an important component in any future expansion of nuclear power. The use of ceramic-based materials, as opposed to borosilicate glasses, is now being investigated widely. The ability of ceramics to be tailored to a specific waste stream is now understood and obtainable quickly and with minimal cost. A second component that limits the expansion of fission-based technologies is the development of materials that are not only tolerant of radiation damage, but are also capable of retaining mechanical strength at high temperatures. One concern for any material however, is the effect of radiation damage, primarily from alpha-decay damage, which in many systems can transform the material from crystalline to amorphous. The effects of such radiation damage on both the structural and chemical properties can range from trivial to critical, for example volume expansion and are the primary focus of much research. As part of a long-term research programme the effects on radiation tolerance of a range of ordered and disordered materials are discused.
- ItemRadiation tolerance of A2Ti2O7 compounds at the cubic-monoclinic boundary(Australian Nuclear Association, 2006-10-15) Lumpkin, GR; Harvey, EJ; Smith, KL; Blackford, MG; Zaluzec, NJCeramic waste forms provide attractive alternatives to the direct disposal of spent fuel or the immobilisation of high-level radioactive waste in borosilicate glass. They are particularly suited for the disposal of actinide wastes (e.g., from partitioning strategies, or for excess Pu from defence purposes) and furthermore they exhibit very low dissolution rates in aqueous fluids, making them attractive candidates for certain repository scenarios (e.g., deep disposal). For general background information on these materials, including studies of the crystal chemistry, aqueous durability, and the behaviour of natural analogues in geological systems, readers are referred to references [1-4]. Over the design lifetime of ceramic waste forms, the actinide elements will undergo alpha decay, resulting in damage to the crystalline structure primarily due to alpha recoil collision cascades. In certain materials, this will lead to a crystalline-amorphous transformation accompanied by volume expansion and reduced chemical durability. The performance in aqueous fluids may be compromised by cracking, increased surface area, and decreased thermodynamic stability of the amorphous phase. Consequently, the radiation damage effects have been of particular interest in ceramic waste forms. Detailed reviews of radiation damage effects can be found in references [5-7]. Some aspects of the alpha decay process have been simulated by irradiation with heavy ions under controlled experimental conditions. In this study, we conducted in situ ion irradiation experiments using the IVEM-Tandem Facility at Argonne National Laboratory to determine the radiation response of Gd2Ti2O7 pyrochlore and two monoclinic, layered perovskite-type phases, Nd2Ti2O7 and La2Ti2O7. For each compound, the critical amorphization dose Dc was determined as a function of temperature and used to establish Tc , the critical temperature, above which the compound remains crystalline. Together with previous data for the A2Ti2O7 compounds, our results show a clear reversal in the trend of Tc versus the cation-anion radius ratio rM/rX. Our experimental results are discussed in the general context of the potential factors that control the susceptibility of a given compound to amorphisation, which include aspects of the structure, bonding, and disorder energy. For the A2Ti2O7 compounds we also show that the critical temperature correlates with the electronic structure (e.g., 4f occupancy) of lanthanide cations. This appears to be a unique result in the world of ion irradiation studies, but the story is complicated by the phase transition from pyrochlore to the layered perovskite structure in this system. Our ultimate goal here is to illustrate the need for a detailed understanding of the physical processes of radiation damage and the potential for predictive modelling of waste form performance.
- ItemRadiation tolerance of A2Ti2O7 materials - a question of bonding?(Cambridge University Press/Springer Nature, 2007) Whittle, KL; Lumpkin, GR; Smith, KL; Blackford, MG; Harvey, EJ; Zaluzec, NJThe resistance of Ln2Ti2O7 (Ln=lanthanide) compounds to radiation damage is an important topic in the understanding and development of new materials by which radioactive nuclear waste can safely be immobilised. A model has been developed, from previously published density functional theory and molecular orbital theory simulations of the band structure for Ln2Ti2O7 materials. This model provides a chemical interpretation of radiation stability. © 2007 Materials Research Society
- ItemRadiation tolerance of M(n+1)AX(n) phases, Ti3AlC2 and Ti3SiC2(Elsevier, 2010-08-01) Whittle, KR; Blackford, MG; Aughterson, RD; Moricca, SA; Lumpkin, GR; Riley, DP; Zaluzec, NJDuring investigations of novel material types with uses in future nuclear technologies (ITER/DEMO and GenIV fission reactors), ternary carbides with compositions Ti3AlC2 and Ti3SiC2 have been irradiated with high Xe fluences, 6.25 × 1015 ions cm−2 (25–30 dpa), using the IVEM-TANDEM facility at Argonne National Laboratory. Both compositions show high tolerance to damage, and give indications that they are likely to remain crystalline to much higher fluences. There is a visible difference in tolerance between Ti3AlC2 and Ti3SiC2 that can be related to the changes in bonding within each material. These initial findings provide evidence for a novel class of materials (+200 compounds) with high radiation resistance, while, significantly, both of these materials are composed of low-Z elements and hence exhibit no long-term activation. © 2010, Elsevier Ltd.
- ItemThe response of complex ceramic oxides exposed to ion-irradiation, compared using two TEM characterisation techniques; bulk, ex-situ, and thin crystal, in-situ(Australian Microscopy and Microanalysis Society, 2015-02-12) Aughterson, RD; Cairney, JM; Ridgway, MC; Zaluzec, NJ; Lumpkin, GRThe response of materials exposed to high energy particles has been an active area of research for several decades, due both to potentially improved and detrimental effects on the material properties. For the nuclear industry ion-irradiation has been used to simulate recoil damage from alpha decay, and exposure to neutrons in fission and fusion reactors [1]. Whilst there are many changes to the host material caused via the impact of accelerated ions the particular focus of this research is on the crystalline to amorphous transition. The amorphisation of the host material can lead to detrimental effects on its properties such as swelling, embrittlement, and micro-cracking leading to eventual structural failure. The complex ceramic oxides chosen for this study, Ln2TiO5 (Ln = lanthanides and yttrium), have several uses within the nuclear industry. The compound Dy2TiO5 has been used within Russian WWER type reactors for two decades due to its good resistance to irradiation induced swelling and structural failure [2]. Of particular interest are the cubic symmetry compounds with defect fluorite structure, which gives good radiation response. Previous studies have indicated that the series ofLn2TiO5 compounds may take on a variety of crystal symmetries depending on the lanthanide size and fabrication conditions used [3, 4].Previous to any ion-irradiation exposure the materials of interest were tested for homogeneity of stoichiometry and crystallography. Characterisation was carried out via backscattered electron imaging (Z contrast) to highlight any variations in elemental composition. This was followed up with multiple spot analyses using energy dispersive x-ray spectroscopy to confirm the homogeneous nature of the material plus the stoichiometry. X-ray diffraction was used to determine the long range symmetry of the test materials plus confirm the single structure nature. Any materials found to have more than one crystal structure type or greater than 5% secondary phase were rejected for further ion-irradiation based experiments. The preliminary study of ion-irradiation response was carried out using the in-situ approach where the test materials were exposed to 1 MeV Kr2+ ions and monitored for their transition from crystalline to amorphous state. The in-situ ion-irradiation was carried out using the intermediate voltage electron microscope (IVEM)-Tandem facility at Argonne National Laboratory. The critical dose of irradiating ions, Dc, required to render the Ln2TiO5 completely amorphous was determined by monitoring selected area electron diffraction patterns for loss of diffraction spots (Bragg maxima) and replacement with diffuse rings (refer to Figure 1). Further bulk Se+ ion-irradiation was carried out at the Australian National University using the TANDEM, heavy ion accelerator. The damage penetration depth was characterised ex-situ using cross-sectional TEM. The cross-sectional damage depth profile of the bulk sample was compared with simulation, SRIM (Stopping Range of Ions in Matter), based calculations and a critical dose of amorphisation value attained. By using these two TEM characterisation approaches the thin crystal in-situ results can be compared with the more “realistic” bulk approach. ©2015 Australian Microscopy and Microanalysis Society