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William J. Weber - One of the best experts on this subject based on the ideXlab platform.
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Radiation tolerance of ceramics—insights from atomistic simulation of damage accumulation in pyrochlores
Energy and Environmental Science, 2010Co-Authors: Ram Devanathan, William J. Weber, Julian D. GaleAbstract:We have used molecular dynamics simulations to investigate the effects of radiation damage accumulation in two pyrochlore-structured ceramics, namely Gd2Ti2O7 and Gd2Zr2O7. It is well known from experiment that the titanate is susceptible to radiation-induced Amorphization, while the zirconate does not go amorphous under prolonged irradiation. Our simulations show that cation Frenkel pair accumulation eventually leads to Amorphization of Gd2Ti2O7, and both anion disorder and cation disorder occur during damage accumulation. Amorphization in Gd2Ti2O7 is accompanied by a density decrease of about 12.7% and a decrease of about 50% in the elastic modulus. In Gd2Zr2O7, Amorphization does not occur, because the residual damage introduced by radiation is not sufficiently energetic to destabilize the crystal structure and drive the material amorphous. Subtle differences in damage accumulation and annealing between the two pyrochlores lead to drastically different radiation response as the damage accumulates.
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Amorphization of nanocrystalline 3C-SiC irradiated with Si + ions
Journal of Materials Research, 2010Co-Authors: Weilin Jiang, Haiyan Wang, Ickchan Kim, Yanwen Zhang, William J. WeberAbstract:Irradiation induced Amorphization in nanocrystalline and single crystal 3C-SiC has been studied using 1 MeV Si+ ions under identical irradiation conditions at room temperature and 400 K. The disordering behavior has been characterized using in-situ ion channeling and ex-situ x-ray diffraction methods. The results show that, compared to single crystal 3C-SiC, full Amorphization of small 3C-SiC grains (~3.8 nm in size) at room temperature occurs at a slightly lower dose. Grain size decreases with increasing dose until a fully amorphized state is attained. The Amorphization dose increases at 400 K relative to room temperature. However, at 400 K, the dose for Amorphization for 2.0 nm grains is about a factor of 4 and 8 smaller than for 3.0 nm grains and bulk single crystal 3C-SiC, respectively. The behavior is attributed to the dominance of defect-stimulated interfacial Amorphization.
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Effect of ionization rates on dynamic recovery processes during electron-beam irradiation of 6H-SiC
Applied Physics Letters, 2007Co-Authors: In-tae Bae, William J. Weber, Manabu Ishimaru, Yoshihiko HirotsuAbstract:The authors have investigated the effects of 200 and 300keV electron-beam irradiations on Amorphization in 6H-SiC at 100 and 295K. Amorphization is induced by the accumulation of defects produced by direct atomic displacements. Dynamic recovery of these defects during irradiation, due to temperature increases and ionization effects, results in increases in the Amorphization dose. By comparing with previous data for 2MeV electrons and 1.5MeV Xe ions, the results demonstrate that ionization-enhanced recovery in 6H-SiC dramatically increases above an ionization rate threshold.
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atomistic simulation of Amorphization thermokinetics in lanthanum pyrozirconate
Applied Physics Letters, 2006Co-Authors: Jeanpaul Crocombette, Alain Chartier, William J. WeberAbstract:The kinetics of Amorphization in La2Zr2O7 pyrochlore is investigated using molecular dynamics simulations. Irradiation damage is simulated by continuous accumulation of cation Frenkel pairs at various temperatures. As observed experimentally, La2Zr2O7 first transitions to the fluorite structure, independent of the temperature, and Amorphization occurs at low temperatures. A model fit of the simulated dose-temperature curve reproduces experimental results in the literature, with a low temperature Amorphization dose D0=1.1 displacement per cation and an activation energy Eact=0.036eV. Present simulations indicate that point defect recombination can control the temperature dependence of Amorphization driven by point defect accumulation.
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Amorphization of ceramic materials by ion beam irradiation
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 1998Co-Authors: L. M. Wang, S. X. Wang, Rodney C. Ewing, Weiliang Gong, William J. WeberAbstract:Abstract Ion-beam-induced Amorphization of a wide variety of ceramic materials has been investigated using in situ TEM with 1.5 MeV Kr+ or Xe+ ions at temperatures between 20 and 1000 K. Except for a few ‘Amorphization resistant’ materials which usually have simple crystal structures, most ceramic materials under study amorphized after a fraction of a dpa (displacement per atom) at cryogenic temperatures. In general, critical Amorphization dose increases with the irradiation temperature at a rate determined by the kinetics of the Amorphization and crystallization processes. Based on a cascade quenching model and an analysis on the structural resistance to recrystallization, a semiempirical parameter which can easily be calculated from both structural and chemical parameters of a material, has been developed to predict the susceptibility of ceramics to Amorphization. The calculated results for over ten phases in the Al2O3–MgO–SiO2 system agree quite well with the experimental data. The results for phases in the Al2O3–MgO–SiO2 system have also suggested a parallel in the kinetics between ion-beam-induced Amorphization and glass formation. The critical Amorphization temperature, above which irradiation-induced Amorphization cannot be completed, is found to be closely related to the glass transition temperature. The ratio between glass transition and melting temperatures can also be used to predict the susceptibility of a ceramic material to Amorphization, equivalent to the Debye temperature criterion.
Rodney C. Ewing - One of the best experts on this subject based on the ideXlab platform.
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size dependence of radiation induced Amorphization and recrystallization of synthetic nanostructured cepo4 monazite
Acta Materialia, 2013Co-Authors: Rodney C. Ewing, Yiqiang Shen, Xiang Sun, Zhili Dong, Jie LianAbstract:Abstract Monazite, CePO 4 , is considered an important phosphate-type structure for the incorporation and disposal of actinides. Nanocrystalline monazite with particle sizes ranging from 20 nm to greater than 100 nm was synthesized. The displacive and ionizing effects of radiation were investigated, separately and simultaneously, for 1 MeV Kr 2+ and 200 keV electron irradiations. In situ transmission electron microscopy observations indicated that CePO 4 nanoparticles can be readily amorphized by 1 MeV Kr 2+ 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 (20 nm) are less resistant to Amorphization compared with larger-sized particles (40 nm). 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 200 keV electron-beam irradiation, CePO 4 previously amorphized by 1 MeV Kr 2+ experienced an ionizing-radiation-induced recrystallization. A greater recrystallization rate was observed for the smaller-sized particles. Under simultaneous electron and displacive ion irradiations, CePO 4 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 CePO 4 implies that an optimized size regime may exist in which nanostructured materials are more tolerant of radiation-induced Amorphization.
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radiation and thermal effects on porous and layer structured materials as getters of radionuclides
Current Opinion in Solid State & Materials Science, 2004Co-Authors: Lumin Wang, J Chen, Rodney C. EwingAbstract:Abstract The long term radiation and thermal effects on porous and layer structured materials that may function as getters for radionuclides have been evaluated using accelerated laboratory experiments including energetic electron, ion or neutron irradiation, as well as high-temperature thermal annealing. The materials studied include: zeolites, layered silicates (mica and smectite clays), open framework structured apatite and crystalline silicotitanate (CST) which is an important synthetic ion-exchange material for the chemical separation of high-level liquid radioactive wastes. In situ transmission electron microscopy during irradiation by energetic electrons and ions has shown that all the studied materials are susceptible to irradiation-induced Amorphization. Amorphization can be induced by ionization and/or direct displacement processes. Amorphization may be preceded or accompanied with dehydration, layer spacing reduction and gas bubble formation. In the case of zeolites, CST and some layer silicates, radiation effects are significantly enhanced at higher temperatures. In fact, thermal annealing at high temperatures alone can cause complete Amorphization of zeolites. Our experiments have shown that Amorphization or even partial Amorphization will cause a dramatic reduction (up to 95%) in ion-exchange and sorption/desorption capacities of zeolite for radionuclides, such as Cs and Sr. Because the near-field or chemical processing materials (e.g., zeolites or CST) will receive a substantial radiation dose after they have incorporated radionuclides, our results suggest that radiation effects may, in some cases, retard the release rate of sorbed or ion-exchanged radionuclides.
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Amorphization of ceramic materials by ion beam irradiation
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 1998Co-Authors: L. M. Wang, S. X. Wang, Rodney C. Ewing, Weiliang Gong, William J. WeberAbstract:Abstract Ion-beam-induced Amorphization of a wide variety of ceramic materials has been investigated using in situ TEM with 1.5 MeV Kr+ or Xe+ ions at temperatures between 20 and 1000 K. Except for a few ‘Amorphization resistant’ materials which usually have simple crystal structures, most ceramic materials under study amorphized after a fraction of a dpa (displacement per atom) at cryogenic temperatures. In general, critical Amorphization dose increases with the irradiation temperature at a rate determined by the kinetics of the Amorphization and crystallization processes. Based on a cascade quenching model and an analysis on the structural resistance to recrystallization, a semiempirical parameter which can easily be calculated from both structural and chemical parameters of a material, has been developed to predict the susceptibility of ceramics to Amorphization. The calculated results for over ten phases in the Al2O3–MgO–SiO2 system agree quite well with the experimental data. The results for phases in the Al2O3–MgO–SiO2 system have also suggested a parallel in the kinetics between ion-beam-induced Amorphization and glass formation. The critical Amorphization temperature, above which irradiation-induced Amorphization cannot be completed, is found to be closely related to the glass transition temperature. The ratio between glass transition and melting temperatures can also be used to predict the susceptibility of a ceramic material to Amorphization, equivalent to the Debye temperature criterion.
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Electron Irradiation of Zeolites
MRS Proceedings, 1998Co-Authors: S. X. Wang, L. M. Wang, Rodney C. EwingAbstract:Three different zeolites (analcime, natrolite, and zeolite-Y) were irradiated with 200 keV and 400 keV electrons. All zeolites amorphized under a relatively low electron fluence. The transformation from the crystalline-to-amorphous state was continuous and homogeneous. The electron fluences for Amorphization of the three zeolites at room temperature were: 7.0× 10 19 e/cm 2 (analcime), 1.8×10 20 e/cm 2 (natrolite), and 3.4×10 20 e/cm 2 (zeolite-Y). The different susceptibilities to Amorphization are attributed to the different channel sizes in the structures which are the pathways for the release of water molecules and Na + . Natrolite formed bubbles under electron irradiation, even before complete Amorphization. Analcime formed bubbles after Amorphization. Zeolite-Y did not form bubbles under irradiation. The differences in bubble formation are attributed to the different channel sizes of the three zeolites. The Amorphization dose was also measured at different temperatures. An inverse temperature dependence of Amorphization dose was observed for all three zeolites: electron dose for Amorphization decreased with increasing temperature. This unique temperature effect is attributed to the fact that zeolites are thermally unstable. A semi-empirical model was derived to describe the temperature effect of Amorphization in these zeolites.
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Electron-irradiation- and ion-beam-induced Amorphization of coesite
Physical review. B Condensed matter, 1996Co-Authors: Weiliang Gong, Rodney C. Ewing, Lumin Wang, Jin ZhangAbstract:Electron-irradiation-induced Amorphization in coesite was observed {ital in} {ital situ} as a function of temperature (15-750 K) at incident electron energy of 1.0 MeV by TEM. Amorphization induced by ion-beam irradiation (1.5 MeV Kr{sup +}) in coesite was studied {ital in} {ital situ} as a function of temperature (15-875 K) using the high-voltage electron microscope (HVEM) Tandem Facility at Argonne National Laboratory. Electron-irradiation-induced Amorphization in coesite was also observed at 200 keV and 300 K. Previously, effect of temperature on Amorphization was considered to only result in annealing, ie recovery of damaged region. This study shows that temperature has an enhancing effect on Amorphization and may even play a dominant role in radiation-induced Amorphization of some crystalline materials. The effect of temperature in enhancing Amorphization was first theoretically and experimentally recognized in electron- and ion-beam-irradiation-induced Amorphization of coesite. Coesite has a melting temperature, {ital T}{sub {ital m}} (875 K), below its glass transition temperature, {ital T}{sub {ital g}} (1480 K). A thermodynamic analysis has been made to model the critical Amorphization dose-temperature dependencies of electron- and ion-beam irradiations. We propose that the thermodynamic contributions to Amorphization include the free-energy increase due to defect accumulation caused by irradiation and chemical disordering,more » {Delta}{ital G}{sub def} and {Delta}{ital G}{sub dis}, and a thermal contribution, {Delta}{ital G}{sub therm}. The thermodynamic condition for Amorphization is generalized by the expression {Delta}{ital G}{sub total}={Delta}{ital G}{sub therm}+{Delta}{ital G}{sub def}+{Delta}{ital G}{sub dis}{ge}{Delta}{ital G}{sub am }, where {Delta}{ital G}{sub total>} is the total free-energy increase for the irradiated crystal, and {Delta}{ital G}{sub am} is the free-energy level required for solid-state Amorphization. (Abstract Truncated)« less
A. V. Shalimova - One of the best experts on this subject based on the ideXlab platform.
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Are the abilities of crystalline alloys to Amorphization upon melt quenching and severe plastic deformation identical or different
Materials Letters, 2016Co-Authors: R.v. Sundeev, A. M. Glezer, A. V. ShalimovaAbstract:Abstract The Amorphization behavior of the titanium-nickelide- and zirconium-based crystalline alloys, which differ in the ability to Amorphization upon melt quenching, have been studied upon severe plastic deformation. It was found that the ability to deformation-induced Amorphization upon severe plastic deformation of crystalline multicomponent alloys is determined by the cumulative ability to deformation-induced Amorphization of crystalline phases contained in the alloy. It is shown that the parameters characterizing the ability to the formation of amorphous state upon melt quenching and upon high-pressure torsion are substantially different and are controlled by different physical parameters.
Kevin J. Hemker - One of the best experts on this subject based on the ideXlab platform.
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Observed Mitigation of Local Amorphization in Boron-Rich Boron Carbide
SSRN Electronic Journal, 2019Co-Authors: Ankur Chauhan, Mark C Schaefer, Richard A Haber, Kevin J. HemkerAbstract:Boron carbide is extremely hard but has been shown to undergo stress-induced Amorphization when subjected to large nonhydrostatic stresses. This localized Amorphization has been associated with the sudden loss of shear strength and poor ballistic performance. Recent quantum mechanics predictions suggest that boron-enrichment may be used to mitigate Amorphization in boron carbide. As a means to test this hypothesis, stoichiometric boron carbide (nominally B4C) and a novel composition of B-rich boron carbide (nominally B6.3C) were investigated. Nanoindentation followed by Raman spectroscopy revealed an obvious reduction in the Raman peaks associated with Amorphization in the B-rich material. Transmission electron microscopy observations of the region below the nanoindents facilitated direct observation of Amorphization, confirmed the Raman result that Amorphization is reduced in the B-rich specimens, and provided additional insight into deformation mechanisms. It is concluded that boron-rich alloys offer one path to mitigating local Amorphization in boron carbide.
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Experimental observations of Amorphization in stoichiometric and boron-rich boron carbide
Acta Materialia, 2019Co-Authors: Ankur Chauhan, Mark C Schaefer, Richard A Haber, Kevin J. HemkerAbstract:Abstract Boron carbide is extremely hard but has been shown to undergo stress-induced Amorphization when subjected to large nonhydrostatic stresses. This localized Amorphization has been associated with the sudden loss of its shear strength and poor ballistic performance. Recent quantum mechanics predictions suggest that boron-enrichment may be used to mitigate Amorphization in boron carbide. As a means to test this hypothesis, stoichiometric boron carbide (nominally B4C) and a novel composition of B-rich boron carbide (nominally B6.3C) were investigated. Nanoindentation followed by Raman spectroscopy revealed an obvious reduction in the Raman peaks associated with Amorphization in the B-rich material. Transmission electron microscopy observations of the region below the nanoindents facilitated direct observation of Amorphization, confirmed the Raman finding that Amorphization is reduced in the B-rich specimens, and provided additional insight into deformation mechanisms. It is surmised that boron-rich alloys offer a path to reducing local Amorphization in boron carbide.
R.v. Sundeev - One of the best experts on this subject based on the ideXlab platform.
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Are the abilities of crystalline alloys to Amorphization upon melt quenching and severe plastic deformation identical or different
Materials Letters, 2016Co-Authors: R.v. Sundeev, A. M. Glezer, A. V. ShalimovaAbstract:Abstract The Amorphization behavior of the titanium-nickelide- and zirconium-based crystalline alloys, which differ in the ability to Amorphization upon melt quenching, have been studied upon severe plastic deformation. It was found that the ability to deformation-induced Amorphization upon severe plastic deformation of crystalline multicomponent alloys is determined by the cumulative ability to deformation-induced Amorphization of crystalline phases contained in the alloy. It is shown that the parameters characterizing the ability to the formation of amorphous state upon melt quenching and upon high-pressure torsion are substantially different and are controlled by different physical parameters.