Critical Amorphization Temperature Research Articles

Temperature dependence of irradiation-induced amorphization in a high-entropy titanate pyrochlore

AbstractThe temperature dependence of amorphization in a high-entropy pyrochlore, (Yb0.2Tm0.2Lu0.2Ho0.2Er0.2)2Ti2O7, under irradiation with 600 keV Xe ions has been studied using in situ transmission electron microscopy (TEM). The critical amorphization dose increases with temperature, and the critical temperature for amorphization is 800 K. At room temperature, the critical amorphization dose is larger than that previously determined for this pyrochlore under bulk-like 4 MeV Au ion irradiation but is similar to the critical doses determined in two other high-entropy titanate pyrochlores under 800 keV Kr ion irradiation using in situ TEM, which is consistent with reported behavior in simple rare-earth titanate pyrochlores. Graphical abstract

In-situ observation of irradiation-induced amorphization on fluorapatites doped with REEs (REE = La, Nd, Sm, Tb, Er, and Lu) of various ionic radii

The immobilization of nuclear waste containing various actinides requires the actinide host phases to maintain long-term stability under alpha decay damage. In this study, the irradiation resistance to amorphization of fluorapatite compounds with the formula of Ca9REE(PO4)5(SiO4)F2 (REE: rare earth element, REE = La, Nd, Sm, Tb, Er, and Lu) were investigated, where REEs3+ with decreased ionic radii were used to simulate actinides in immobilization wastes. In-situ 800 keV Kr2+ irradiation was performed with varying temperatures. The critical amorphization temperature, Tc, for these apatites were found to be 513.8, 503.7, 494.1, 493.7, 483.1 and 460.9 K, respectively. Tc shows a distinct decreasing trend with the decrease of ionic radii of doped REEs3+, indicating enhanced irradiation resistance. The selected REEs exhibited increasing electronegativity, which correlated with an increase in bonding ionic properties. This trend is unfavourable for the formation of a covalent network, thereby enhancing irradiation resistance. Besides, the decreased ionic radii of doped REEs3+ enhanced the migration and diffusion rates of smaller REE3+ after the collision cascade process. Additionally, the dopant REEs3+ showed a significant preference for CaⅡ sites. This preference decreased as the ionic radius of doped REE3+ and reduced the average ionic radius of CaⅠ sites. Consequently, the one-dimensional channel in apatite became larger, facilitating the rearrangement of F− ion. This study investigated the irradiation-induced amorphization of REE-silicon doped fluorapatites and the results highlighted the importance of composition of immobilization materials on irradiation performance.

The influence of helium-induced defects on the migration of strontium implanted into SiC above critical amorphization temperature

The presence of radiation-induced defects and the high temperature of implantation are breeding grounds for helium (He) to accumulate and form He-induced defects (bubbles, blisters, craters, and cavities) in silicon carbide (SiC). In this work, the influence of He-induced defects on the migration of strontium (Sr) implanted into SiC was investigated. Sr-ions of 360 keV were implanted into polycrystalline SiC to a fluence of 2 × 1016 Sr-ions/cm2 at 600°C (Sr-SiC). Some of the Sr-SiC samples were then co-implanted with He-ions of 21.5 keV to a fluence of 1 × 1017 He-ions/cm2 at 350°C (Sr + He-SiC). The Sr-SiC and Sr + He-SiC samples were annealed for 5 h at 1,000°C. The as-implanted and annealed samples were characterized by Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and Rutherford backscattered spectrometry (RBS). Implantation of Sr retained some defects in SiC, while co-implantation of He resulted in the formation of He-bubbles, blisters, and craters (exfoliated blisters). Blisters close to the critical height and size were the first to exfoliate after annealing. He-bubbles grew larger after annealing owing to the capture of more vacancies. In the co-implanted samples, Sr was located in three regions: the crystalline region (near the surface), the bubble region (where the projected range of Sr was located), and the damage region toward the bulk. Annealing the Sr + He-SiC caused the migration of Sr towards the bulk, while no migration was observed in the Sr-SiC samples. The migration was governed by “vacancy migration driven by strain fileds.”

Ion irradiation induced amorphization of precipitates in Zircaloy

Samples of Zircaloy-4 were ion irradiated at several doses and irradiation temperatures and studied using transmission electron microscopy to discern the amorphization and dissolution behavior of second-phase precipitates under irradiation. It is found that near the critical temperature for amorphization under neutron irradiation a similar amorphization morphology is obtained under proton irradiation as under neutron irradiation, that is, an amorphous layer starting at the precipitate-matrix interface and moving in with increasing dose. However, the rate of amorphous layer advancement is much slower than that seen under neutron irradiation, and saturates with dose so that it remains a partial precipitate amorphization, i.e., this precipitate amorphization mechanism does not lead to complete amorphization as it occurs with neutrons. The results indicate that the critical temperature for bulk amorphization is primarily dependent on the displacement cascade density achievable with the irradiating particle and secondarily with dose rate. Depletion of iron from the precipitates is also observed, although not directly linked to amorphization.

Radiation stability of nanostructured hydroxyapatite Ca10(PO4)6(OH)2 under ion irradiations

Radiation stability of nanostructured hydroxyapatite with different interfacial structure is studied. Hydroxyapatite nanoparticle (HAp-np) is fabricated by calcination in muffle furnace and densified nanocrystalline hydroxyapatite (HAp-nc) is synthesized by spark plasma sintering . X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterizations show the hexagonal structure for both HAp-np and HAp-nc. Irradiation responses of HAp-np and HAp-nc are studied via in situ TEM techniques. The samples are irradiated with 1 MeV Kr 2+ ions over the temperature range of 150 K to 523 K. In situ TEM observations reveal a higher critical amorphization dose of HAp-nc (0.075 dpa) compared to 0.05 dpa of HAp-np at room temperature, indicating enhanced amorphization resistance. The radiation tolerance shows a great dependence on temperature, and a higher critical dose can be observed at elevated temperatures. A lower critical amorphization temperature ( T c ) of HAp-nc (∼437 K) than HAp-np (∼545 K) indicates better radiation tolerance since the lower T c shows better defect annealing ability. This better radiation tolerance of HAp-nc is attributed to the higher sink strength and lower interface energy carried by densified grain boundaries as free-standing open surfaces of HAp-np can carry more isolated dangling bonds that results in a higher interface energy. This excess interface energy can lower the energy difference between crystalline phase and amorphous state, leading to HAp-np to be less radiation resistant. Radiation-induced amorphization is attributed to the accumulation of oxygen vacancies and the glass-like structure rearranged by displaced oxygen and phosphorus atoms.

Temperature dependence of irradiation-induced nanocrystallization in amorphous silicon carbide

Ion irradiation induced nucleation and growth of nanoparticles in amorphous SiC at elevated temperatures are investigated in this study. Both as-deposited amorphous and Kr ion irradiation-amorphized SiC films are used for the investigation. Similar behavior of particle precipitation in the two types of the films is observed. It is found that the threshold temperature for nucleation is between 550 and 700 K, which is considerably higher than the critical temperature for full amorphization in SiC irradiated with Kr ions. There is a temperature regime up to at least 900 K, where the nucleation and growth rates are only weakly dependent of the irradiation temperature. This temperature regime is potentially useful to tailor the size and density of nanocrystalline SiC precipitates by optimizing the ion irradiation conditions.

Phase stability of novel HfNbTaTiVZr refractory high entropy alloy under ion irradiation

HfNbTaTiVZr refractory high entropy alloy (RHEA) fabricated by arc melting shows the single-phase body-centered cubic (BCC) structure. Radiation-induced amorphization (RIA) is observed at 2 displacements per atom (dpa) at 298 K while the specimen remains its crystal structure at 423 K. The critical amorphization temperature (Tc) is likely within 298–423 K. RIA is mainly attributed to the accumulation of Frenkel pairs caused by elastic collisions under ion irradiation and the large atomic size mismatch in the studied RHEA facilitates the amorphization process. RIA of RHEA is for the first time reported and the finding challenges the current understanding of phase stability of RHEAs upon irradiation.

Effect of heat treatment on the migration behaviour of selenium implanted into polycrystalline SiC

This study reviews the migration behaviour of selenium in polycrystalline SiC, which acts as the main diffusion barrier in the coated fuel particles for Very High Temperature Reactors. Se ions of 200 keV were implanted into polycrystalline SiC wafers to a fluence of 1 × 1016 cm−2 at three temperatures, which were room temperature, 350 °C and 600 °C. The implanted samples were annealed at temperatures ranging from 1000 to 1500 °C in steps of 100 °C for 10 h. The migration of implanted Se was monitored by Rutherford backscattering spectrometry (RBS) while structural and morphological changes were monitored by Raman spectroscopy and scanning electron microscopy (SEM). Implantation of Se at room temperature amorphized the near surface region of the SiC substrates, while in samples implanted above the critical amorphization temperature the crystal structure was retained with some radiation damage. Annealing at 1000 °C resulted in the recrystallization of the amorphized SiC layer. In the case of room temperature implantation, the broadening of the implanted Se profile in RBS spectra was observed to occur after annealing at 1300 °C and became significant with an increase in annealing temperature. This broadening was accompanied by a peak shift towards the surface and loss of implanted Se. No broadening was observed in samples implanted above the critical amorphization temperature, but the peak shift towards the surface began after annealing at 1300 °C in samples implanted at 350 °C and 600 °C.

Radiation tolerant ceramics for nuclear waste immobilization: Structure and stability of cesium containing hollandite of the form (Ba,Cs)1.33(Zn,Ti)8O16 and (Ba,Cs)1.33(Ga,Ti)8O16

The radiation damage tolerance of nuclear waste forms is dependent on the material's resistance to defect formation and its ability to accommodate structural distortions that arise from defect creation. This study illustrates how the radiation tolerance of hollandite can be improved thorough compositional control of cesium stoichiometry. A hollandite series with the general form BaxCsyZnx+y/2Ti8-x-y/2O16 (0 < x < 1.33; 0 < y < 1.33) was exposed to heavy ion (Kr2+) irradiation at 27 °C, 100 °C, 200 °C and 300 °C followed by characterization with grazing incidence X-ray diffraction, transmission electron microscopy, and aqueous leaching tests. After exposure to 400 keV or 1 MeV Kr2+ irradiation, hollandite exhibited an onset of amorphization near 0.14 dpa and full amorphization ranging from 0.21 to 0.54 dpa depending on the cesium content. The radiation tolerance increased at elevated temperatures with a critical amorphization temperature between 200 °C and 300 °C. Elemental leaching decreased with increasing cesium content. Irradiated samples exhibited twice the fraction of cesium release compared to pristine samples. Experimental results also showed that cesium release from irradiated samples was at a minimum for the Ba0.33Cs1.00Zn0.83Ti7.17O16 sample.

Behavior of Host Materials with Surrogates of the Rare Earth–Actinide Fraction under Ion Irradiation

The behavior of rare earth titanate phases of monoclinic and orthorhombic symmetry under ion irradiation was studied. Three samples contain a set of rare earth elements (REE), and in one sample the high-level waste (HLW) is simulated by neodymium. The samples were prepared by sintering at 1400°С or cold crucible induction melting followed by melt crystallization. The samples were irradiated with 1-MeV Kr2+ ions at temperatures in the range 298–1023 K on a tandem installation consisting of an ion accelerator and a transmission electron microscope. The critical amorphization doses for the samples at 298 K were (1.5–2.5) × 1014 ion cm–2. These values correspond to those for monoclinic titanates Nd2Ti2O7 and La2Ti2O7, are close to those for REE–Ti pyrochlores and brannerite, and are lower by an order of magnitude than those for zirconolites at the same irradiation mode. The critical amorphization temperature of the phases studied (900 K) is comparable to that of Nd2Ti2O7 (920 K), but higher than that of La2Ti2O7 (840 K). Close radiation resistance of the REE2Ti2O7 and REE4Ti9O24 phases in which the rare earth elements are represented by a mixture of elements similar in composition to their set in spent nuclear fuel and liquid HLW from its reprocessing or solely by Nd confirms the assumption that Nd is a suitable surrogate for the whole rare earth–actinide fraction.

Radiation-induced amorphization of Langasite La3Ga5SiO14

Single crystals of Langasite La3Ga5SiO14 (LGS) were irradiated by 1 MeV Kr2+ ions at temperature range from 298 to 898 K in order to simulate the damage effect of neutron radiation on Langasite, a candidate sensor material proposed as high temperature and pressure sensors in nuclear reactors. The microstructure evolution of LGS as functions of irradiation dose and temperature was followed by in-situ TEM observation through electron diffraction pattern. LGS is found to be sensitive to ion beam irradiation-induced amorphization from displacive heavy ions with a low critical dose of ∼0.5 ± 0.2 dpa (neutron fluence of (1.6 ± 0.6 ) × 1019 neutrons/cm2) at room temperature. The critical amorphization temperature, Tc, is determined to be 910 ± 10 K. Under simultaneous ionizing electron (300 keV, 45 nA) and displacive heavy ion irradiations (1-MeV Kr2+ and flux of 6.25 × 1011 ions/cm2·s), LGS displayed greater stability of crystal structure against amorphization, possibly due to the electron radiation-induced recovery of displacive damage by heavy ions.

Insights into the radiation behavior of Ln2TiO5 (Ln = La-Y) from defect energetics

First-principles calculations have been performed to study the structural properties and defect energetics of orthorhombic Ln2TiO5 (Ln=La, Pr, Nd, Sm, Gd, Dy and Y). The lattice parameters decrease with increasing lanthanide cation radius. The mean 〈Ln1-O〉 and 〈Ln2-O〉 bond lengths increase systematically by respectively 1.16% and 1.19% from Dy2TiO5 to La2TiO5. Meanwhile, mean 〈Ti-O〉 bond length increases slightly with increasing Ln cation radius from Y2TiO5 to La2TiO5. The calculated results of point defect formation energies suggest that Ln2-Ti antisite defect is the most energetically favorable defect type for Ln2TiO5. The trend of Ln2-Ti antisite defect formation energy of Ln2TiO5 is precisely consistent with the radiation resistance, as demonstrates by the critical amorphization temperature, which indicates that the formation of Ln2-Ti antisite defect may has a significant influence on the radiation behavior of Ln2TiO5.

Curium-doped stannate pyrochlore: Durability under radiation and leaching in water

The radiation resistance of the phase (Gd,Cm)2Sn2O7 with a pyrochlore-type structure containing 3.0 wt % 244Cm was studied. It was established that amorphization occurs at a dose of 1019 α-decay/g (1.52 displacements per atom), which is 2–5 times higher than that needed for amorphization of titanate and titanate–zirconate pyrochlore phases with a similar structure. The heating of the amorphous ceramics restores the structure of the pyrochlore. The restoration process begins in the temperature interval of 600–700°C. This allows us to estimate the critical amorphization temperature as 650°C. On the 14th day, the rate of Cm leaching from the initial sample in water at 90°C is 10–1; Gd, 10–2; and Sn, 10–3 g/(m2 day). After amorphization the leaching rate increases by an order of magnitude (Cm) and two orders of magnitude (Gd), but it does not change for Sn. Compared to the zirconate and titanate–zirconate phases, stannate pyrochlore is markedly less resistant in water and cannot be regarded as a matrix for the immobilization of REE-actinide fraction wastes.

Temperature dependence of the radiation tolerance of nanocrystalline pyrochlores A2Ti2O7 (A = Gd, Ho and Lu)

A potentially enhanced radiation resistance of nanocrystalline materials, as a consequence of the high density of interfaces and surfaces, has attracted much attention both to understand the fundamental role of these defect sinks and to develop them for high-radiation environments. Here, irradiation response of nanocrystalline A2Ti2O7 (A = Gd, Ho and Lu) pyrochlore powders with grain sizes of 20–30 nm was investigated by 1-MeV Kr2+ ion bombardment. In situ transmission electron microscopy (TEM) revealed that the critical amorphization fluence for each nanocrystalline compound at room temperature was greater than that for their coarse-grained counterparts, indicating an enhanced amorphization resistance. The effect of temperature on the irradiation response of one of these compounds, nanocrystalline Lu2Ti2O7, was further examined by performing ion irradiation at an elevated temperature range of 480–600 K. The critical amorphization temperature (Tc) was found to be noticeably higher in nanocrystalline Lu2Ti2O7 (610 K) than its coarse-grained counterpart (480 K), revealing that nanocrystalline Lu2Ti2O7 is less resistant to amorphization compared to its coarse-grained phase under high temperatures. We interpret these results with the aid of atomistic simulations. Molecular statics calculations find that cation antisite defects are less energetically costly to form near surfaces than in the bulk, suggesting that the nanocrystalline form of these materials is generally less susceptible to amorphization than coarse-grained counterparts at low temperatures where defect kinetics are negligible. In contrast, at high temperatures, the annealing efficiency of antisite defects by cation interstitials is significantly reduced due to the sink properties of the surfaces in the nanocrystalline pyrochlore, which contributes to the observed higher amorphization temperature in the nano-grained phase than in coarse-grained counterpart. Together, these results provide new insight into the behavior of nanocrystalline materials under irradiation.

Influence of radiation damage on the thermal properties of silicon carbide implanted with heavy noble gas ions

Diffusion of heavy noble gas atoms in irradiation damaged single crystalline silicon carbide and the thermal etching of it is investigated at temperatures of 1300°C and 1400°C. For this purpose 360keV krypton and xenon ions were implanted in commercial 6H-SiC wafers at 600°C, which is far above the critical amorphization temperature of the target material. Width broadening of the implantation profiles and the retention of krypton and xenon during isothermal annealing was determined by RBS-analysis, whilst damage profiles were simultaneously obtained by α-particle channelling. No diffusion and no loss of the implanted species is detected in the implanted samples after isothermal annealing for 40h at 1400°C. However, thermal etching of the target material is observed at both annealing temperatures and leads at 1400°C to a significant shift of the implantation profile towards the surface due to sublimation. RBS analysis shows that this occurs mainly during the initial stage of isothermal annealing, while surface loss during prolonged annealing is minimal. The resulting topographical modification of the surface during annealing was studied by scanning electron and atomic force microscopy. It indicates that the observed phenomenon is due to a relatively strong dependence of thermal etching on the defect density in the surface region, while the evolving surface roughness seems not to play a decisive role.

Irradiation-induced disordering and amorphization of Al3Ti-based intermetallic compounds

An in situ ion-irradiation study, simultaneously examined using transmission electron microscopy, was performed to investigate irradiation-induced disordering and amorphization of Al3Ti-based intermetallic compounds. Thin foil samples of two crystalline structures: D022-structured Al3Ti and L12-structured (Al,Cr)3Ti were irradiated using 1.0 MeV Kr ions at a temperature range from 40 K to 573 K to doses up to 4.06 × 1015 ions/cm2. The results showed that both the compounds underwent an order-disorder transformation under irradiation, where both Al3Ti and (Al,Cr)3Ti ordered structures were fully transformed to the disordered face-centered cubic (FCC) structure except at the highest irradiation temperature of 573 K. A slightly higher irradiation dose was required for order-disorder transformation in case of Al3Ti as compared to (Al,Cr)3Ti at a given temperature. However, their amorphization resistances were different: while the disordered FCC (Al,Cr)3Ti amorphized at the irradiation dose of 6.25 × 1014 ions/cm2 (0.92 dpa) at 40 K and 100 K, the Al3Ti compound with the same disordered FCC structure maintained crystallinity up to 4.06 × 1015 ions/cm2 (5.62 dpa) at 40 K. The critical temperature for amorphization of (Al,Cr)3Ti under Kr ion irradiation is likely between 100 K and room temperature and the critical temperature for disordering between room temperature and 573 K.

Scanning electron microscopy of the surfaces of ion implanted SiC

This paper gives a brief review of radiation damage caused by particle (ions and neutrons) bombardment in SiC at different temperatures, and its annealing, with an expanded discussion on the effects occurring on the surface. The surface effects were observed using SEM (scanning electron microscopy) with an in-lens detector and EBSD (electron backscatter diffraction). Two substrates were used, viz. single crystalline 6H-SiC wafers and polycrystalline SiC, where the majority of the crystallites were 3C-SiC. The surface modification of the SiC samples by 360keV ion bombardment was studied at temperatures below (i.e. room temperature), just at (i.e. 350°C), or above (i.e. 600°C) the critical temperature for amorphization of SiC. For bombardment at a temperature at about the critical temperature an extra step, viz. post-bombardment annealing, was needed to ascertain the microstructure of bombarded layer. Another aspect investigated was the effect of annealing of samples with an ion bombardment-induced amorphous layer on a 6H-SiC substrate. SEM could detect that this layer started to crystalize at 900°C. The resulting topography exhibited a dependence on the ion species. EBSD showed that the crystallites forming in the amorphized layer were 3C-SiC and not 6H-SiC as the substrate. The investigations also pointed out the behaviour of the epitaxial regrowth of the amorphous layer from the 6H-SiC interface.

Pressure and temperature phase diagram of Gd2Ti2O7 under irradiation

The pressure and temperature phase diagram of Gd2Ti2O7 under irradiation are calculated by means of molecular dynamics calculations. The critical temperature for amorphization obeys a linear law with pressure. Gd2Ti2O7 under irradiation transits towards the fluorite above this temperature and amorphizes below. The configuration of the Ti interstitial reveals to be the key of the amorphizability of Gd2Ti2O7. Its stability depends upon disorder and pressure. Low pressure promotes the stabilization of Ti linked-polyhedra that drive the system to the amorphous state under irradiation. Conversely, high pressure activates its destabilization to interstitials that recombine with vacancies, driving the system to the fluorite structure under irradiation.

Ion Beam Irradiation-Induced Amorphization of Nano-Sized KxLnyTa2O7-v Tantalate Pyrochlore

Nano-sized (~10-15 nm) tantalate pyrochlores KxLnyTa2O7-v (Ln = Gd, Y, and Lu) were irradiated with 1 MeV Kr2+ beams at different temperatures and their radiation response behaviors were studied by in-situ TEM observations. All of these nano-sized KxLnyTa2O7-v pyrochlores are sensitive to radiation-induced amorphization with low critical doses (~0.12 dpa) at room temperature and high critical amorphization temperatures above 1160 K. The K+ plays a key role in determining the radiation response of tantalate pyrochlores, in which the K+-rich KLuTa2O7 displays greater amorphization susceptibility than K0.8GdTa2O6.9 and K0.8YTa2O6.9 with lower K+ occupancy at the A-site. The reduced amorphization tolerance of the composition with a greater K+ content is consistent with the prominently larger K+/Ta5+ cationic radius ratio, which may result in more structural deviation from the parent fluorite structure and less capability to accommodate radiation induced defects. An empirical correlation between critical amorphization temperature and ionic size was derived, generally describing the dominant effect of ionic sizes in controlling radiation response of a wide range of pyrochlore compounds as potential nuclear waste forms. The results of the tantalate pyrochlore in this work highlight that nanostructured pyrochlores are not intrinsically radiation tolerant and their responses are highly compositional dependent.

Microstructure and Cs Behavior of Ba-Doped Aluminosilicate Pollucite Irradiated with F+ Ions

Radionuclide 137Cs is one of the major fission products that dominate heat generation in spent fuels over the first 300 years. A durable waste form for 137Cs that decays to 137Ba is needed to minimize its environmental impact. Aluminosilicate pollucite CsAlSi2O6 is selected as a model waste form to study the decay-induced structural effects. Whereas Ba-containing precipitates are not present in charge-balanced Cs0.9Ba0.05AlSi2O6, they are found in Cs0.9Ba0.1AlSi2O6 and identified as monoclinic Ba2Si3O8. Pollucite is susceptible to electron-irradiation-induced amorphization. The threshold density of electronic energy deposition for amorphization was determined to be ∼235 keV/nm3. Pollucite can be readily amorphized under F+ ion irradiation at 673 K. A significant amount of Cs diffusion and release from the amorphized pollucite occurs during the irradiation. However, cesium is immobile in the crystalline structure under He+ ion irradiation at room temperature. The critical temperature for amorphization is not higher than 873 K under F+ ion irradiation. If kept at or above 873 K all the time, the pollucite structure is unlikely to be amorphized; Cs diffusion and release are improbable. A general discussion regarding pollucite as a potential waste form is provided in this report.