Critical Amorphization Dose Research Articles

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

The 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

Investigation of the damage behavior in SiC without any additives irradiated with Si ions by GIXRD, Raman and TEM

The SPS-prepared SiC without any additives was irradiated with 15.9 MeV Si ions at room temperature and 500 °C. The damage behavior was investigated by grazing incident X-ray diffraction (GIXRD) spectroscopy, Raman spectroscopy and transmission electron microscopy (TEM). GIXRD and Raman spectra show that the disorder of SiC increases and the lattice swelling occurs with the increase of irradiation at room temperature. High temperature irradiation was associated with the reduction of swelling and the recovery of disorder. TEM results indicate that at room temperature, with increasing dose, the material transitions from slight amorphization to complete amorphization. The amorphization dose is estimated to be around 1.2 dpa. The combined effects of stacking faults, high-energy ions accompanied by higher electronic energy loss inducing displacement cascades, and cascade-induced annealing delay damage accumulation, leading to an increased critical dose for amorphization. Moreover, the transient thermal spikes due to higher electronic energy loss, leading to defect annealing and the formation of Frank loops at lower irradiation temperatures.

Atomic-scale observation and prediction of gallium ion implantation induced amorphization on natural diamond surface

Gallium ion implantation is an important solution to modify the physical properties of diamonds, in which the critical condition of gallium ion implantation-induced amorphization of diamond has a crucial influence on the modification effect. Therefore, this work attempts to establish an analytical prediction model of the critical implantation dose for diamond amorphization considering the acceleration voltage, implantation angle and crystal orientation. The prediction accuracy of the model is validated with the electron backscatter diffraction pattern technology. The results show that the model has a satisfactory prediction accuracy when the acceleration voltage is higher. Moreover, the atomic-scale observation of gallium ion implantation-induced amorphization of natural diamonds is realized on a spherical aberration projection electron microscope. The results presented in this work are significant to modify the natural diamond tool for service life improvement with focused ion beams in the future as well as to guide doping diamonds to manufacture n-type semiconductor diamonds.

Enhanced irradiation-resistance of polymer-derived SiC by incorporation of multiwalled carbon nanotubes

In this work, multiwalled carbon nanotubes were introduced into polycarbosilane to fabricate carbon nanotubes reinforced nano-SiC composites (MWCNTs/ PCS-nano-SiC). Radiation effects on MWCNTs/ PCS-nano-SiC were studied by irradiation with 2 MeV Au2+ ions at room temperature and doses ranging from 2 × 1014 to 8 × 1014 ions/cm2. The sample pyrolyzed at 1400 °C and containing 3 wt% carbon nanotubes exhibited excellent overall performances. The multiple graphite layers in the MWCNTs provided "absorption traps" for defects, improving the efficiency of defect recombination. Tightly combined MWCNTs/ PDC interface is required to enable the two phases to protect each other during irradiation. The critical amorphization dose was increased upon structural optimization, and the hardness degradation was significantly reduced. The rise of Young's modulus at a high damage dose was discovered because of the "interface-driven shrinkage" of nano-SiC. The present study provides insight into SiCf/ SiC design for an advanced nuclear system.

Mechanical property and irradiation resistance of high entropy pyrochlore (Sm0.2Eu0.2Gd0.2Y0.2Lu0.2)2Ti2O7

High entropy ceramics have recently attracted a lot of attention due to their superior physical properties. In this work, a novel high entropy pyrochlore (Lu0.2Y0.2Gd0.2Eu0.2Sm0.2)2Ti2O7 (HTP-1) was designed and prepared. Sample was characterized by X-ray diffraction, Raman spectroscopy and transmission electron microscopy coupled with an energy dispersive spectrometry. The results show that single-phase HTP-1 is successfully synthesized and exhibits severe lattice distortion. The Vickers hardness of HTP-1 measured by the microhardness tester is ∼ 922.28 Hv, which is significantly higher than the result calculated with the mixture rule, demonstrating that high entropy pyrochlore has high mechanical hardness. In-situ TEM was performed on HTP-1 irradiated with 800 keV Kr2+ at room temperature to investigate irradiation resistance of sample. The critical amorphization dose of HTP-1 is 0.27 dpa, which indicates the irradiation resistance of HTP-1 is between Sm2Ti2O7 and Y2Ti2O7.

Radiation induced amorphization of carbides in additively manufactured and conventional ferritic-martensitic steels: In-situ experiments on extraction replicas

In-situ irradiations using 1 MeV Kr2+ ions in a transmission electron microscope were performed on extraction replica samples containing M23C6 carbides and MX carbonitrides from two ferritic-martensitic (FM) steels: (i) 9%Cr-1%W-TaV based Eurofer97 and (ii) 9%Cr-1%Mo-VNb (all in wt.%) based additively manufactured (AM) Grade91 steel. The irradiations were performed between 100 and 773 K up to a maximum dose of ∼2.4 displacements per atom (dpa). The M23C6 carbides are highly susceptible to radiation induced amorphization (RIA), while the MX type nanoprecipitates are highly amorphization resistant across the entire irradiation temperature range. Between 100 and 423 K, RIA of M23C6 carbides occurs very rapidly with critical amorphization doses ranging between ∼0.35 and 0.9 dpa, increasing to higher doses at 573 K. Complete amorphization of the M23C6 carbides up to doses of ∼2.4 dpa is not possible at 773 K under the present irradiation conditions. The critical amorphization dose of M23C6 carbides increases nearly exponentially with irradiation temperature. The critical temperature for the crystalline-to-amorphous phase transformation (Tc→a) of M23C6 carbides irradiated as replica samples, i.e. without the surrounding metal matrix, was estimated to be ∼812 K. Comparing the present results with neutron irradiation data on bulk samples reveals a decrease in the critical amorphization dose for M23C6 and an increase of Tc→a, highlighting the effect of dose rate on the amorphization behavior that is qualitatively consistent with literature on other non-metals. Changes in the minor chemistry of M23C6, such as presence or absence of W, V, Nb, Mo, seems to have little effect on the amorphization behavior. In-situ irradiations on extraction replica samples provide a novel pathway to explore radiation tolerance of nanoprecipitates in nuclear structural materials

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.

Synthesis of novel hybrid carbon nanomaterials inside silicon carbide nanotubes by ion irradiation

A novel hybrid carbon nanomaterial was synthesized by ion irradiation of a C–SiC coaxial nanotube. The hybrid consisted of one-dimensionally stacked graphene nanodisks with diameters less than 50 nm and cylindrical multiwalled carbon nanotubes inside an amorphous SiC tubular layer. A sudden emergence of new continuous graphitic layers in the microstructure were observed by in situ transmission electron microscopy following ion irradiation, where these layers were perpendicular to the nanotube's length direction. The SiC crystals in the C–SiC coaxial nanotube became amorphous, also due to the ion irradiation, although the critical amorphization dose was higher than that for bulk SiC. Most remarkably, the carbon layer remained crystalline, even after an irradiation dose higher than 20 dpa (displacement per atom). Such results show that these carbon layers possess better resistance against amorphization when subjected to ion irradiation than the SiC layers in the C–SiC coaxial nanotube. The lattice plane spacing of the carbon layer increases up to the point when irradiation damage lead to the complete amorphization of SiC crystals, after which it starts to decrease. This demonstrates that the carbon layer experiences high compression stress during ion irradiation. Thus, the ion irradiation of C–SiC coaxial nanotubes gives rise to a novel development of the microstructure and can be considered as one of the new synthetic processes for making novel carbon nanomaterials.

Electron beam damage of perfluorosulfonic acid studied by soft X-ray spectromicroscopy

Scanning transmission X-ray microscopy (STXM) was used to study chemical changes to perfluorosulfonic acid (PFSA) spun cast thin films as a function of dose imparted by exposure of a 200 kV electron beam in a Transmission Electron Microscope (TEM). The relationship between electron beam fluence and absorbed dose was calibrated using a modified version of a protocol based on the positive to negative lithography transition in PMMA [Leontowich et al, J. Synchrotron Rad. 19 (2012) 976]. STXM was used to characterize and quantify the chemical changes caused by electron irradiation of PFSA under several different conditions. The critical dose for CF2-CF2 amorphization was used to explore the effects of the sample environment on electron beam damage. Use of a silicon nitride substrate was found to increase the CF2-CF2 amorphization critical dose by ∼x2 from that for free-standing PFSA films. Freestanding PFSA and PMMA films were damaged by 200 kV electrons at ∼100 K and then the damage was measured by STXM at 300 K (RT). The lithography cross-over dose for PMMA was found to be ∼2x higher when the PMMA thin film was electron irradiated at 120 K rather than at 300 K. The critical dose for CF2-CF2 amorphization in PFSA irradiated at 120 K followed by warming and delayed measurement by STXM at 300 K was found to be ∼2x larger than at 300 K. To place these results in the context of the use of electron microscopy to study PFSA ionomer in fuel cell systems, an exposure of 300 e−/nm2 at 300 K (which corresponds to an absorbed dose of ∼20 MGy) amorphizes ∼10% of the CF2-CF2 bonds in PFSA. At this dose level, the spatial resolution for TEM imaging of PFSA is limited to 3.5 nm by radiation damage, if one is using a direct electron detector with DQE = 1. This work recommends caution about 2D and 3D morphological information of PFSA materials based on TEM studies which use fluences higher than 300 e−/nm2.

In-situ TEM study of radiation-induced amorphization and recrystallization of hydroxyapatite

The radiation response of hydroxyapatite, a nuclear waste form candidate, was investigated using 1 MeV Kr2+ and 200 keV electron irradiations combined with in-situ TEM observation. The results showed a remarkable difference in the behavior of nano-crystalline hydroxyapatite under the two different radiation conditions. A crystalline-to-amorphous transformation occurred under the 1 MeV Kr2+ irradiation due to the accumulation of displacement damage, with a critical amorphization dose (Dc) of 2 × 1014 ions/cm2 (0.05 dpa) at room temperature. The Dc increased dramatically with temperature due to effective dynamic annealing of radiation-induced defects, such that no amorphization occurred above a critical temperature (Tc) of 545 K. In contrast, under 200 keV electron irradiation, which mainly dissipates its energy through inelastic ionization processes, the pre-amorphized hydroxyapatite exhibited rapid recrystallization, likely due to the bond breaking and reforming at the amorphous-crystalline interface assisted by the ionization. The electron-irradiation induced recrystallization in amorphous hydroxyapatite was extremely sensitive, with a critical electron dose several orders of magnitude lower than most previously reported ceramics. Under concurrent ion and electron irradiations, the materials retained crystalline even at the critical dose of 0.05 dpa, and remained so until the dose reached 0.15 dpa, lending further proof to the enhanced defect recovery by ionizing electrons. The radiation data of hydroxyapatite highlights the material's sensitivity to radiation conditions and its excellent dynamic defect annealing ability.

In-situ TEM study of the ion irradiation behavior of U3Si2 and U3Si5

U3Si2 and U3Si5 are two important uranium silicide phases currently under extensive investigation as potential fuel forms or components for light water reactors (LWRs) to enhance accident tolerance. In this paper, their irradiation behaviors are studied by ion beam irradiations with various ion mass and energies, and their microstructure evolution is investigated by in-situ transmission electron microscopy (TEM). U3Si2 can easily be amorphized by ion beam irradiations (by 1 MeV Ar2+ or Kr2+) at room temperature with the critical amorphization dose less than 1 dpa. The critical amorphization temperatures of U3Si2 irradiated by 1 MeV Kr2+ and 1 MeV Ar2+ ion are determined as 580 ± 10 K and 540 ± 5 K, respectively. In contrast, U3Si5 remains crystalline up to 8 dpa at room temperature and is stable against ion irradiation-induced amorphization up to ∼50 dpa by either 1 MeV Kr2+ or 150 KeV Kr+ at 623 K. These results provide valuable experimental data to guide future irradiation experiments, support the relevant post irradiation examination, and serve as the experimental basis for the validation of advanced fuel performance models.

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 Stability of Spark‐Plasma‐Sintered Lead Vanadate Iodoapatite

Spark‐plasma‐sintered lead vanadate iodoapatite Pb9.85(VO4)6I1.7, a promising nuclear waste form for the immobilization of I‐129, was irradiated with energetic ions, electrons, and gamma rays, to investigate its radiation stability. In situ TEM observation of the 1 MeV Kr2+ irradiation shows that lead vanadate iodoapatite generally exhibits higher tolerance against ion irradiation‐induced amorphization than lead vanadate fluorapatite, and the spark plasma sintering can further enhance its radiation stability attributed to the enhanced crystallinity, reduced defect concentration, and denser microstructure. The critical amorphization dose and critical temperature for the SPS‐densified iodoapatite at 700°C are determined to be 0.25 dpa at room temperature and 230°C, respectively. No significant phase transformation or microstructural damage occurred under energetic electron and gamma irradiations. Raman spectra of gamma‐ray‐irradiated iodoapatite indicate improved V–O bond order at 500 kGy dose. Generally, the spark‐plasma‐sintered iodoapatite exhibits excellent radiation stability for nuclear waste form applications. The significantly enhanced radiation stability of the SPS‐densified iodoapatite suggests that SPS holds great promise for fabricating iodoapatite waste form with minimum iodine loss and optimized radiation tolerance for effective management of highly volatile I‐129.

Structural and Optical Properties of GaS Single Crystals Irradiated by Hydrogen

Using Raman light scattering and Rutherford backscattering, we studied the structural disorder of layered GaS crystals before and after hydrogen (H 2 + ) implantation with energy of 140 keV. Initially, the elemental components of GaS were distributed uniformly in depth, and this distribution remained stable up to a dose of 5 × 1015 at./cm2. Doses up to 1 × 1015 at./cm2 increased the photoresponse (from 0.66 to 5.3 times) over a wide wavelength range from 490 nm to 900 nm. Additionally, the irradiated samples displayed new photoresponse peaks with maximums at λ = 668 nm and λ = 739 nm, corresponding to new energy levels of 0.59 eV and 0.77 eV, respectively. However, further dose increase up to 5 × 1015 at./cm2 dramatically reduced the photoresponse due to structural disorder (amorphization). The experimental value of the critical dose for initial amorphization was greater than 1 × 1015 at./cm2, which agrees with the calculated value. Raman scattering confirmed the photoresponse results.

Characterization of the ion-amorphization process and thermal annealing effects on third generation SiC fibers and 6H-SiC

The objective of the present work is to study the irradiation effects on third generation SiC fibers which fulfill the minimum requisites for nuclear applications, i.e. Hi-Nicalon type S, hereafter HNS, and Tyranno SA3, hereafter TSA3. With this purpose, these fibers have been ion-irradiated with 4 MeV Au ions at room temperature and increasing fluences. Irradiation effects have been characterized in terms of micro-Raman Spectroscopy and Transmission Electron Microscopy and compared to the response of the as-irradiated model material, i.e. 6H-SiC single crystals. It is reported that ion-irradiation induces amorphization in SiC fibers. Ion-amorphization kinetics between these fibers and 6H-SiC single crystals are similar despite their different microstructures and polytypes with a critical amorphization dose of ∼3 × 1014 cm−2 (∼0.6 dpa) at room temperature. Also, thermally annealing-induced cracking is studied via in situ Environmental Scanning Electron Microscopy. The temperatures at which the first cracks appear as well as the crack density growth rate increase with increasing heating rates. The activation energy of the cracking process yields 1.05 eV in agreement with recrystallization activation energies of ion-amorphized samples.

Amorphization Driven by Defect-Induced Mechanical Instability

Using ab initio molecular dynamics simulations, we perform a comparative study of the defect accumulation process in silicon carbide (SiC) and zirconium carbide (ZrC). Interestingly, we find that the fcc Si sublattice in SiC spontaneously and gradually collapses following the continuous introduction of C Frenkel pairs (FPs). Above a critical amorphization dose of ~0.33 displacements per atom (dpa), the pair correlation function exhibits no long-range order. In contrast, the fcc Zr sublattice in ZrC remains structurally stable against C sublattice displacements up to the highest dose of 1.0 dpa considered. Consequently, ZrC cannot be amorphized by the accumulation of C FPs. We propose defect-induced mechanical instability as the key mechanism driving the amorphization of SiC under electron irradiation.

Size dependence of radiation-induced amorphization and recrystallization of synthetic nanostructured CePO4 monazite

Monazite, CePO4, is considered an important phosphate-type structure for the incorporation and disposal of actinides. Nanocrystalline monazite with particle sizes ranging from 20nm to greater than 100nm was synthesized. The displacive and ionizing effects of radiation were investigated, separately and simultaneously, for 1MeV Kr2+ and 200keV electron irradiations. In situ transmission electron microscopy observations indicated that CePO4 nanoparticles can be readily amorphized by 1MeV Kr2+ irradiation. Nanostructured monazite displays greater critical amorphization doses and lower critical temperatures than that of bulk natural monazite, suggesting enhanced amorphization tolerance. A strong size dependence on radiation-induced amorphization was observed in which the smaller-sized particles (20nm) are less resistant to amorphization compared with larger-sized particles (40nm). The excess surface energy of nanostructured materials, as suggested by the larger surface area upon the reduction of particle size, may alter the energy difference between amorphous and crystalline phases, thus affecting the radiation stability. With 200keV electron-beam irradiation, CePO4 previously amorphized by 1MeV Kr2+ experienced an ionizing-radiation-induced recrystallization. A greater recrystallization rate was observed for the smaller-sized particles. Under simultaneous electron and displacive ion irradiations, CePO4 displayed greater tolerance against amorphization, probably as a result of radiation-induced recovery of displacive damage by the ionizing radiation. The strong size dependence of displacive radiation-induced amorphization and ionizing-radiation-enhanced recrystallization processes for nanostructured CePO4 implies that an optimized size regime may exist in which nanostructured materials are more tolerant of radiation-induced amorphization.

Chemistry and radiation effects of davidite

Davidite (A1−xM21O38) samples from five different geological localities contain approximately 0.2 to 9.5 wt% UO2 (0.02 to 0.65 atoms per formula unit) and <0.1 to 1.3 wt% ThO2 (<0.01 to 0.09 atoms per formula unit). Maximum amounts of other notable cations include 3.7 wt% V2O3, 4.1 wt% Cr2O3, 2.5 wt% Y2O3, 5.6 wt% La2O3, 6.0 wt% Ce2O3, 4.0 wt% MnO, 2.4 wt% ZnO, 2.7 wt% SrO, and 4.9 wt% PbO. As a result of the variation in age and Th-U content, the calculated α decay dose ranges from ~0.2 to 44 × 1016 α/mg (~0.06 to 14.5 dpa). For samples with ages of 275–295 Ma, the critical dose for amorphization based on electron diffraction is ~0.8 × 1016 α/mg. Natural davidite is commonly altered to rutile, ilmenite, titanite, and other minor phases.

Peculiarities of radiation-induced defect formation in nanocrystals embedded in a solid matrix

The modification of basic radiation-physics effects (elastic defect formation and amorphization) in nanoclusters placed in a solid matrix compared to the same processes in single crystals is considered theoretically. We introduce a diffusion model of defect formation and study the influence of the nanocluster-matrix interface on the displacement energy. Based on the percolation treatment, we show a significant change in the critical nanocluster amorphization dose compared to the case of a crystal. The influence of various types of irradiation on the amorphization processes is considered. Judgments about the variety of radiation-physics effects in nanoobjects are expressed.

Response of strontium titanate to ion and electron irradiation

The response of strontium titanate (SrTiO 3) to ion and electron irradiation is studied at room temperature. For an accurate energy to depth conversion and a better determination of ion-induced disorder profile from Rutherford backscattering spectrometry measurement, a detailed iterative procedure is described and applied to ion channeling spectra to determine the dechanneling yield and the disorder profiles for the Sr and Ti sublattices. The result shows a large underestimation in disorder depth, ∼40% at the damage peak, which indicates a large overestimation of the electronic stopping power for 1.0 MeV Au ions in SrTiO 3 predicted by the SRIM (Stopping and Range of Ions in Matter) code. Overestimation of heavy ion stopping power may lead to an overestimation of the critical dose for amorphization. The current study also demonstrates possible ionization effects in SrTiO 3 under ion and electron irradiation. Pre-amorphized SrTiO 3 exhibits strong ionization-induced epitaxial recovery at the amorphous/crystalline interface under electron irradiation.