Critical Amorphization Temperature Research Articles

Ion-beam and electron-beam irradiation of synthetic britholite

Britholite, ideally Ca 4− x REE 6+ x (SiO 4) 6O 2 (REE=rare earth elements), has the hexagonal structure of apatite, a candidate waste form for actinides. Two synthetic britholites: Ca 3.05Ce 2.38Fe 0.25Gd 5.37Si 4.88O 26 (N56) and Ca 3.78La 0.95Ce 1.45Zr 0.78Fe 0.14Nd 2.15Eu 0.50Si 6.02O 26 (N88) ( P63; Z=1) were irradiated with 1.0 MeV Kr 2+ and 1.5 MeV Xe + over the temperature range of 50–973 K. The process of ion irradiation-induced amorphization, including the effects of the target mass and the ion mass, and the recrystallization of amorphous domains due to ionizing irradiation were investigated. The critical amorphization temperature, T c was determined to be 910 K for N56 (1.0 MeV Kr 2+), 880 K for N88 (1.0 MeV Kr 2+) and 1010 K for N88 (1.5 MeV Xe +). The sequence of increasing T c correlates with the mass of the incident ion; whereas, the ratio of electronic to nuclear stopping power (ENSP) is inversely correlated with T c. Electron irradiations were conducted on previously amorphized britholite (N56) with an electron flux of 1.07 × 10 25 e −/m 2/s. The ionizing radiation resulted in recrystallization at the absorbed dose of 6.2 × 10 13 Gy. This result suggests that the ionizing radiation can induce recrystallization in silicate apatites, similar to that observed for phosphate apatite.

Effects of implantation temperature and ion flux on damage accumulation in Al-implanted 4H-SiC

The effects of implantation temperature and ion flux on damage accumulation on both the Si and C sublattices in 4H-SiC have been investigated under 1.1-MeV Al22+ irradiation at temperatures from 150 to 450 K. The rate of damage accumulation decreases dramatically, and the damage profile sharpens due to significant dynamic recovery at temperatures close to the critical temperature for amorphization. At 450 K, the relative disorder and the density of planar defects increase rapidly with the increasing ion flux, exhibiting saturation at high ion fluxes. Planar defects are generated through the agglomeration of excess Si and C interstitials during irradiation and post-irradiation annealing at 450 K. A volume expansion of ∼8% is estimated for the peak damage region.

Damage Evolution and Annealing of Au-Irradiated Samarium Titanate Pyrochlore

ABSTRACTDamage evolution and thermal recovery of 1 MeV Au2+ irradiated samarium titanate pyrochlore (Sm2Ti2O7) single crystals were studied by Rutherford backscattering spectroscopy and nuclear reaction analysis. The damage accumulation follows a nonlinear dependence on dose that is well described by a disorder accumulation model, which indicates a predominant role of defect-stimulated amorphization processes. The critical dose for amorphization at 170 and 300 K is ∼0.14 dpa, and a higher dose of ∼ 0.22 dpa is observed for irradiation at 700 K, which agrees with previous in-situ transmission electron microscopy (TEM) data for polycrystalline Sm2Ti2O7. Annealing in an 18O environment reveals a damage recovery stage at ∼ 850 K that coincides with a significant increase in 18O exchange due to oxygen vacancy mobility. This thermal recovery stage is also consistent with the critical temperature for amorphization measured by in-situ TEM in polycrystalline samples.

Ion-irradiation-induced amorphization ofLa2Zr2O7pyrochlore

The isometric pyrochlore structure, ${A}_{2}{B}_{2}{\mathrm{O}}_{7},$ is generally susceptible to radiation damage, but certain compositions are remarkably resistant to radiation damage. In the binary system ${\mathrm{Gd}}_{2}({\mathrm{Ti}}_{2\ensuremath{-}x}{\mathrm{Zr}}_{x}){\mathrm{O}}_{7},$ the radiation resistance increases dramatically with the substitution of Zr for Ti, until the pure end member ${\mathrm{Gd}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$ cannot be amorphized, even at doses as high as \ensuremath{\sim}100 dpa. Although zirconate pyrochlores are generally considered to be radiation resistant, we report results for the amorphization of a zirconate pyrochlore ${\mathrm{La}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$ by ion beam irradiation (\ensuremath{\sim}5.5 dpa at room temperature). The critical amorphization temperature ${T}_{c}$ is low, \ensuremath{\sim}310 K. The susceptibility to ion-beam-induced amorphization and structural disordering for zirconate pyrochlores is related to the structural deviation from the ideal fluorite structure, as reflected by the x parameter of the ${\mathrm{O}}_{48f}.$

Ion irradiation-induced amorphization and nano-crystal formation in garnets

The radiation susceptibility of garnet structure types (A 3B 2(XO 4) 3, I a3d, Z=8) has been examined by 1.0 MeV Kr 2+ irradiation with in situ transmission electron microscopy over the temperature range of 50–1070 K. The target garnets included five natural garnets: almandine, andradite, pyrope, grossular and spessartine, and five synthetic garnets incorporating various contents of actinides. The synthetic garnets were silicates (N-series) and ferrate–aluminates (G-series). The critical amorphization temperature ( T c), above which amorphization does not occur, were determined to be 1050 K for N77, 1130 K for N56, 1100 K for G3, 890 K for G4 and 1030 K for andradite. T c of the synthetic garnets increased with increasing mass density of the target and as the electronic-to-nuclear stopping power ratio decreased. During ion irradiation of the G3 garnet at a temperature of 1023 K near the T c, nano-crystals were produced in which the unit-cell parameter of the original garnet was halved (∼0.64 nm) as a result of a radiation-induced recrystallization process, without evidence for ordering of the cations.

Ion irradiation effects in natural garnets: Comparison with zircon

The behavior of garnet ( A 3 B 2( XO 4) 3; I a3d; Z=8 ) under ion-beam irradiation was investigated in order to compare its radiation susceptibility to another orthosilicate: zircon, ZrSiO 4. Five natural end-member compositions were examined by in situ transmission electron microscopy during irradiation with 1.0 MeV Kr 2+ over the temperature range of 50–1070 K. The critical amorphization temperature, above which amorphization does not occur, was 1030 K for andradite, but could not be determined for the other garnet composition because the T c was higher than the highest temperature of the experiment. Based on topologic criterion, the degree of structural freedom in garnet is ∼−2.25 and for zircon ∼−1.5. Based on topology the critical amorphization dose for garnet should be higher than that of zircon; however, the average amorphization dose of garnet (0.20 dpa) is lower than that of zircon (0.37 dpa) at room temperature. This may be the result of the assumed value for the displacement energies, E d, used in the calculation of dpa. Garnet did not decompose, while zircon decomposes to SiO 2+ZrO 2 during the ion irradiation at high temperature. This behavior may be related to the phase relations of garnet which melts congruently and zircon which decomposes to ZrO 2+SiO 2.

Heavy Ion Irradiation of Zirconate Pyrochlores

ABSTRACTZirconate pyrochlores, A2Zr2O7, are important potential nuclear waste forms for Puimmobilization. The binary Gd2(Ti2-xZrx)O7 has been shown to have increasing resistance to ionirradiation damage with the increasing Zr content, and Gd2Zr2O7 is radiation resistant to a 1 MeV Kr+ ion irradiation at 25 K to a dose of 5 dpa. In this study, a 1.5 MeV Xe+ irradiation was completed for zirconate pyrochlores A2Zr2O7 (A=La, Nd, Sm, Gd). The radiation resistance decreases with an increase of the ionic radius of A-site cation. La2Zr2O7 is the first zirconate pyrochlore to be amorphized by ion beam irradiation, and the critical amorphization temperature, Tc, is ∼310 K. The susceptibility of La2Zr2O7 to ion beam damage is related to its structure, which shows the largest deviation from the ideal fluorite structure. These results are also consistent with calculations of the cation antisite formation energy in the pyrochlore structure. The ion irradiation-induced pyrochlore-to-fluorite transformation occurred in all of the irradiated zirconate pyrochlore phases. Based on the results for Gd2Ti2-xZrxO7 and A2Zr2O7, the defect fluorite structures are stable when the ionic radii ratio rA/rB≤1.54; beyond this limit, the defect fluorite structure becomes increasingly unstable relative to the amorphous state.

Amorphization and recrystallization of the ABO 3 oxides

Single crystals of the ABO 3 phases CaTiO 3, SrTiO 3, BaTiO 3, LiNbO 3, KNbO 3, LiTaO 3, and KTaO 3 were irradiated by 800 keV Kr +, Xe +, or Ne + ions over the temperature range from 20 to 1100 K. The critical amorphization temperature, T c, above which radiation-induced amorphization does not occur varied from approximately ∼450 K for the titanate compositions to more than 850 K for the tantalates. While the absolute ranking of increasing critical amorphization temperatures could not be explained by any simple physical parameter associated with the ABO 3 oxides, within each chemical group defined by the B-site cation (i.e., within the titanates, niobates, and tantalates), T c tends to increase with increasing mass of the A-site cation. T c was lower for the Ne + irradiations as compared to Kr +, but it was approximately the same for the irradiations with Kr + or Xe +. Thermal recrystallization experiments were performed on the ion-beam-amorphized thin sections in situ in the transmission electron microscope (TEM). In the high vacuum environment of the microscope, the titanates recrystallized epitaxially from the thick areas of the TEM specimens at temperatures of 800–850 K. The niobates and tantalates did not recrystallize epitaxially, but instead, new crystals nucleated and grew in the amorphous region in the temperature range 825–925 K. These new crystallites apparently retain some `memory' of the original crystal orientation prior to ion-beam amorphization.

Crystalline-to-amorphous phase transformation in ion-irradiated GaAs

The crystalline-to-amorphous phase transformation in GaAs irradiated with B, C, O, Si, Ar, or Ge ions has been investigated by transmission electron microscopy and Rutherford backscattering with ion channeling. The critical relationships between ion flux and substrate temperature which define the threshold conditions for amorphization in GaAs by irradiation with various ions have been measured. The amorphous phase forms over a wide range of irradiation conditions by a collapselike nucleation process when a critical free energy is exceeded. The threshold conditions for nucleation are shown to be thermally activated, with a single activation energy of $0.9\ifmmode\pm\else\textpm\fi{}0.1\mathrm{eV},$ independent of ion species, energy, or fluence. However, despite the independence of the measured activation energy on ion species, the critical temperature for amorphization does not scale linearly with the rate of energy deposition or displacement density, indicating that details of the collision cascade which influence defect quenching, trapping, clustering and annihilation processes determine the pathway to amorphization. We speculate that this results from the difficulty of nucleating the amorphous phase at temperatures where defects are highly mobile, and that nucleation occurs locally at stable defect complexes which are formed more readily in the dense collision cascades created by heavier ions.

Radiation damage and nanocrystal formation in uranium–niobium titanates

Two uranium–niobium titanates, U 2.25Nb 1.90Ti 0.32O 9.8 and Nb 2.75U 1.20Ti 0.36O 10, formed during the synthesis of brannnerite (UTi 2O 6), a minor phase in titanate-based ceramics investigated for plutonium immobilization. These uranium titanates were subjected to 800 keV Kr 2+ irradiation from 30 to 973 K. The critical amorphization dose of the U-rich and Nb-rich titanates at room temperature were 4.72×10 17 and 5×10 17 ions/ m 2 , respectively. At elevated temperature, the critical amorphization dose increases due to dynamic thermal annealing. The critical amorphization temperature for both Nb-rich and U-rich titanates is ∼933 K under a 800 keV Kr 2+ irradiation. Above the critical amorphization temperature, nanocrystals with an average size of ∼15 nm were observed. The formation of nanocrystals is due to epitaxial recrystallization. At higher temperatures, an ion irradiation-induced nucleation-growth mechanism also contributes to the formation of nanocrystals.

Damage accumulation and recovery in gold-ion-irradiated barium titanate

Single-crystal barium titanate (BaTiO 3) wafers were irradiated 60° off the surface normal at 170 and 300 K using 1.0 MeV Au 2+ ions over a fluence range from 0.03 to 0.19 ions/nm 2 . Disorder on both the Ba and Ti sublattices has been studied in situ using Rutherford backscattering spectrometry along the 〈1 1 0〉 axial direction. At these irradiation temperatures, the temperature dependence of disordering is small. The dose for amorphization under these conditions is on the order of 0.5 dpa, which is 50% of that required to amorphize SrTiO 3 under similar conditions. At low damage levels, recovery of disorder is observed at room temperature, suggesting at least one lower temperature recovery stage. For more highly damaged states, two distinct recovery stages have been identified between 420 and 570 K and between 720 and 870 K. The recovery stage between 420 and 570 K is associated with the critical temperature for full amorphization (∼550 K) in BaTiO 3. The higher temperature recovery stage is most likely associated with epitaxial recrystallization.

Irradiation-induced amorphization: Effects of temperature, ion mass, cascade size, and dose rate

An empirical model based on cascade ``quenching'' and epitaxial recrystallization has been developed to describe the accumulation of the amorphous fraction during ion beam irradiation experiments. The model is based on the assumption that the amorphous fraction that remains after the formation of a cascade is related to a crystallization efficiency parameter A. For low values of A, as would be expected at low temperatures, for heavy-ion irradiations, or for materials that are good glass formers, the accumulation of the amorphous fraction as a function of dose is an exponential function. For high values of A, as would be expected at elevated temperatures, for light-ion irradiations, or for materials that are poor glass formers, the accumulation of the amorphous fraction as a function of dose is a sigmoidal function. Amorphization dose varies as a function of temperature and is reflected by the temperature-dependent crystallization efficiency. The effects of ion mass and energy on critical amorphization dose and temperature are discussed in terms of the cascade size. The dose-rate effect on the critical temperature of amorphization is derived considering the thermal annealing of the damaged material.

Micro-Raman and ion channeling study of crystal damage in Si induced by focused Co ion beam implantation

The lattice damage of silicon produced by ion implantation at extremely high current density of 0.8 A/cm2 (2.5×1018 cm-2 s-1) was investigated. In a focused ion beam system, implantation was carried out with 70 keV Co ions, fluences of 1.2×1016 cm-2 and 6.7×1015 cm-2 into Si (111) at room temperature and elevated temperatures between 355 °C and 400 °C. Radiation damage measurements were performed by Rutherford backscattering/channeling spectroscopy and micro-Raman analysis. The radiation damage was studied as a function of pixel dwell-time and implantation temperature. The critical temperature for amorphization increases with current density. Although the fluence of the focused ion implantation was constant, crystalline layers were obtained for short and amorphous layers for long pixel dwell-times. The critical dwell-time of crystalline/amorphous transition increases with implantation temperature. From the results a typical time for defect annealing of 10-5 s at 400 °C and an activation energy of (2.5±0.6) eV were deduced.

Ion-Irradiation-Induced Amorphization Of Cadmium Niobate Pyrochlore

ABSTRACTIrradiation-induced amorphization of Cd2Nb2O7 pyrochlore was investigated by means of in-situ temperature-dependent ion-irradiation experiments in a transmission electron microscope, combined with ex-situ ion-implantation (at ambient temperature) and RBS/channeling analysis. The in-situ experiments were performed using Ne or Xe ions with energies of 280 and 1200 keV, respectively. For the bulk implantation experiments, the incident ion energies were 70 keV (Ne+) and 320 keV (Xe2+). The critical amorphization temperature for Cd2Nb2O7 is ∼480 K (280 keV Ne+) or ∼620 K (1200 keV Xe2+). The dose for in-situ amorphization at room temperature is 0.22 dpa for Xe2+, but is 0.65 dpa for Ne+ irradiation. Both types of experiments suggest a cascade overlap mechanism of amorphization. The results were analyzed in light of available models for the crystalline-to-amorphous transformation and were compared to previous ionirradiation experiments on other pyrochlore compositions.

Radiation stability of gadolinium zirconate: A waste form for plutonium disposition

Zirconate and titanate pyrochlores were subjected to 1 MeV of Kr+irradiation. Pyrochlores in the Gd2(ZrxTi1-x)2O7system (x= 0, 0.25, 0.5, 0.75, 1) showed a systematic change in the susceptibility to radiation-induced amorphization with increasing Zr content. Gd2Ti2O7amorphized at relatively low dose (0.2 displacement per atom at room temperature), and the critical temperature for amorphization was 1100 K. With increasing zirconium content, the pyrochlores became increasingly radiation resistant, as demonstrated by the increasing dose and decreasing critical temperature for amorphization. Pyrochlores highly-enriched in Zr (Gd2Zr2O7, Gd2Zr1.8Mg0.2O6.8, Gd1.9Sr0.1Zr1.9Mg0.1O6.85, and Gd1.9Sr0.1Zr1.8Mg0.2O6.75) could not be amorphized, even at temperature as low as 25 K.

Comparison of Ion‐Beam Irradiation Effects in X2YO4 Compounds

The effects of 1.5 MeV Kr‐ion irradiation on seven X2YO4 phases with the olivine (A2VIBIVO4), spinel (AIVB2VIO4), and phenakite structures have been investigated using in situ and high‐resolution transmission electron microscopy (HRTEM) over a wide temperature range (20‐873 K). At low temperatures (<200 K), the olivine and phenakite are susceptible to radiation‐induced amorphization with a critical amorphization dose of 0.2‐0.5 displacement per atom (dpa). The critical amorphization dose increases with increasing irradiation temperature at varying rates for the various phases, resulting in a distinct critical amorphization temperature for each phase. For the Mg‐Fe series of olivine, the susceptibility to amorphization at higher temperatures (room temperature or above) increases with increasing Fe content. Although the spinel phases are, in general, much more resistant to amorphization, a high‐pressure metastable spinel phase, gamma‐SiFe2O4, is easily amorphized at doses <0.2 dpa at temperatures below 723 K. This phase decomposes after irradiation at 873 K. At 20 K, complete amorphization of the FeCr2O4 spinel (chromite) is achieved at ~4 dpa, but no evidence of amorphization is observed in MgAl2O4 spinel after 5.4 dpa. The distinct differences in the relative susceptibility of these phases to amorphization are discussed in terms of the structural and chemical controls on the amorphization process.

Neutron irradiation induced amorphization of silicon carbide

This paper provides the properties of bulk stoichiometric silicon carbide which has been amorphized under neutron irradiation. Both high purity single crystal hcp and high purity, highly faulted (cubic) chemically vapor deposited (CVD) SiC were irradiated at approximately 60°C to a total fast neutron fluence of 2.6 × 10 25 n/m 2. Amorphization was seen in both materials as evidenced by TEM, electron diffraction and X-ray diffraction techniques. Physical properties for the amorphized single crystal material are reported including large changes in density (−10.8%), elastic modulus as measured using a nanoindentation technique (−45%), hardness as measured by nanoindentation (−45%), and standard Vickers hardness (−24%). Similar property changes are observed for the amorphized CVD SiC. Using measured thermal conductivity data for the CVD SiC sample, the critical temperature for amorphization at this neutron dose and flux, above which amorphization is not possible, is estimated to be greater than ∼125°C.

Amorphization of Zr 3Fe under electron irradiation

The intermetallic compound Zr 3Fe has been made amorphous by 0.9 MeV electron irradiation. By performing this irradiation in situ, it was possible to conduct a systematic study of the influence of temperature, dose rate, electron energy and specimen orientation on the amorphization process. The critical temperature and the critical dose for amorphization were determined, and shown to depend on dose rate. By varying the electron energy, we determined the displacement energies for the Zr and Fe atoms in Zr 3Fe, and showed that, at low electron energy, the amorphization rate is dependent on specimen orientation. We analyze these results in terms of a model based on amorphization occurring at a damage level where the modified free energy of the irradiated crystal exceeds the free energy of the amorphous phase. This model is shown to predict the amorphization kinetics, i.e. the critical temperature and critical dose for amorphization. We also compare amorphization induced by electron and ion irradiation.

Heavy-ion irradiation effects in theABO4orthosilicates: Decomposition, amorphization, and recrystallization

The effects of displacive, heavy-ion irradiation on the properties of synthetic single-crystals of ${\mathrm{ZrSiO}}_{4}$ (zircon), ${\mathrm{HfSiO}}_{4}$ (hafnon), tetragonal ${\mathrm{ThSiO}}_{4}$ (thorite), and monoclinic ${\mathrm{ThSiO}}_{4}$ (huttonite) have been investigated. All four $\mathrm{AB}{\mathrm{O}}_{4}$-type orthosilicate materials became amorphous in a process that exhibited a two-stage dependence on temperature when irradiated by 800 keV ${\mathrm{Kr}}^{+}$ or ${\mathrm{Xe}}^{+}$ ions in the temperature range of 20 to 1100 K. The critical amorphization temperature above which amorphization did not occur increased in the following order: huttonite, zircon, hafnon, thorite. At temperatures below 500 K, the tetragonal and monoclinic polymorphs of ${\mathrm{ThSiO}}_{4}$ required approximately the same ion fluence for amorphization. Monoclinic ${\mathrm{ThSiO}}_{4}$ is, therefore, not ``more resistant'' to radiation damage than the tetragonal-symmetry form, but instead, its amorphous phase recrystallizes at a lower temperature. A model that accounts for the observed two-stage behavior was developed and used to calculate the recrystallization activation energies for both stages. When irradiated with heavy ions above a certain characteristic temperature, an unexpected decomposition was observed in which all four $\mathrm{AB}{\mathrm{O}}_{4}$ orthosilicates phase separated into the crystalline oxides: ${\mathrm{ZrO}}_{2},$ ${\mathrm{HfO}}_{2},$ or ${\mathrm{ThO}}_{2}$ plus amorphous ${\mathrm{SiO}}_{2}.$

Dwell-time dependence of the defect accumulation in focused ion beam synthesis of CoSi 2

Cobalt disilicide microstructures were formed by 70 keV Co 2+ focused ion beam implantation into Si(1 1 1) at substrate temperatures of about 400°C and a subsequent two step annealing (600°C, 60 min and 1000°C, 30 min in N 2). It was found that the CoSi 2 layer quality strongly depends on the pixel dwell time and the implantation temperature. Only for properly chosen parameters continuous CoSi 2 layers could be obtained. Scanning electron microscopy and Rutherford backscattering/channelling investigations were carried out combined with a special preparation technique for structures which are smaller than the analysing beam. The quality of the CoSi 2 layers which is correlated to the damage was investigated as a function of dwell-time (1–250 μs) and target temperature (355–415°C). The results show that the irradiation damage increases with the dwell-time. The Si top layer was amorphized for longer dwell-times although the substrate temperature was always above the critical temperature for amorphization of about 270°C according to the model of Morehead and Crowder. For the high current density of a focused ion beam (1–10 A/cm 2) the damage creation rate is higher than the rate of dynamic annealing.