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1. An in situ study on Kr ion–irradiated crystalline Cu/amorphous-CuNb nanolaminates

   https://doi.org/10.1557/jmr.2019.24  

   Fan, Zhe ; Fan, Cuncai ; Li, Jin ; Shang, Zhongxia ; Xue, Sichuang ; Kirk, Marquis A. ; Li, Meimei ; Wang, Haiyan ; Zhang, Xinghang ( July 2019 , Journal of Materials Research)

   Nanocrystalline and nanolaminated materials show enhanced radiation tolerance compared with their coarse-grained counterparts, since grain boundaries and layer interfaces act as effective defect sinks. Although the effects of layer interface and layer thickness on radiation tolerance of crystalline nanolaminates have been systematically studied, radiation response of crystalline/amorphous nanolaminates is rarely investigated. In this study, we show that irradiation can lead to formation of nanocrystals and nanotwins in amorphous CuNb layers in Cu/amorphous-CuNb nanolaminates. Substantial element segregation is observed in amorphous CuNb layers after irradiation. In Cu layers, both stationary and migrating grain boundaries effectively interact with defects. Furthermore, there ismore »a clear size effect on irradiation-induced crystallization and grain coarsening. In situ studies also show that crystalline/amorphous interfaces can effectively absorb defects without drastic microstructural change, and defect absorption by grain boundary and crystalline/amorphous interface is compared and discussed. Our results show that tailoring layer thickness can enhance radiation tolerance of crystalline/amorphous nanolaminates and can provide insights for constructing crystalline/amorphous nanolaminates under radiation environment.« less
2. Investigating the dislocation reactions on Σ3{111} twin boundary during deformation twin nucleation process in an ultrafine-grained high-manganese steel

   https://doi.org/10.1038/s41598-021-98875-z  

   Hung, Chang-Yu ; Shimokawa, Tomotsugu ; Bai, Yu ; Tsuji, Nobuhiro ; Murayama, Mitsuhiro ( September 2021 , Scientific Reports)

   Some of ultrafine-grained (UFG) metals including UFG twinning induced plasticity (TWIP) steels have been found to overcome the paradox of strength and ductility in metals benefiting from their unique deformation modes. Here, this study provides insights into the atomistic process of deformation twin nucleation at Σ3{111} twin boundaries, the dominant type of grain boundary in this UFG high manganese TWIP steel. In response to the applied tensile stresses, grain boundary sliding takes place which changes the structure of coherent Σ3{111} twin boundary from atomistically smooth to partly defective. High resolution transmission electron microscopy demonstrates that the formation of disconnectionmore »on Σ3{111} twin boundaries is associated with the motion of Shockley partial dislocations on the boundaries. The twin boundary disconnections act as preferential nucleation sites for deformation twin that is a characteristic difference from the coarse-grained counterpart, and is likely correlated with the lethargy of grain interior dislocation activities, frequently seen in UFG metals. The deformation twin nucleation behavior will be discussed based on in-situ TEM deformation experiments and nanoscale strain distribution analyses results.

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3. Photovoltaic Response of Thin-Film CdTe Solar Cells under Accelerated Neutron Radiation in a TRIGA Reactor

   Powell, Kaden ; Lund, Matthew ; Wang, Meng-Jen ; Qian, Yang ; Magginetti, David ; Reese, Matthew ; Cazalas, Edward ; Sjoden, Glenn ; Yoon, Heayoung ( June 2020 , Electronic Materials Symposium)

   Cadmium telluride (CdTe) solar cells are a promising photovoltaic (PV) technology for producing power in space owing to their high-efficiency (> 22.1 %), potential for specific power, and cost-effective manufacturing processes. In contrast to traditional space PVs, the high-Z (atomic number) CdTe absorbers can be intrinsically robust under extreme space radiation, offering long-term stability. Despite these advantages, the performance assessment of CdTe solar cells under high-energy particle irradiation (e.g., photons, neutrons, charged particles) is limited in the literature, and their stability is not comprehensively studied. In this work, we present the PV response of n-CdS / p-CdTe PVs under acceleratedmore »neutron irradiation. We measure PV properties of the devices at different neutron/photon doses. The equivalent dose deposited in the CdTe samples is simulated with deterministic and Monte Carlo radiation transport methods. Thin-film CdTe solar cells were synthesized on a fluorine-doped tin oxide (FTO) coated glass substrate (≈ 4 cm × 4 cm). CdS:O (≈ 100 nm) was reactively RF sputtered in an oxygen/argon ambient followed by a close-spaced sublimation deposition of CdTe (≈ 3.5 μm) in an oxygen/helium ambient. The sample was exposed to a 10 min vapor CdCl2 in oxygen/helium ambient at 430˚C. The samples were exposed to a wet CuCl2 solution prior to anneal 200ºC. A gold back-contact was formed on CdTe via thermal evaporation. The final sample contains 16 CdTe devices. For neutron irradiation, we cleaved the CdTe substrate into four samples and exposed two samples to ≈ 90 kW reactor power neutron radiation for 5.5 hours and 8.2 hours, respectively, in our TRIGA (Training, Research, Isotopes, General Atomics) reactor. We observed a noticeable color change of the glass substrates to brown after the neutron/gamma reactor exposure. Presumably, the injected high-energy neutrons caused the breaking of chemical bonds and the displacement of atoms in the glass substrates, creating point defects and color centers. The I-V characteristics showed noticeable deterioration with over 8 hour radiations. Specifically, the saturation current of the control devices was ≈ 25 nA increasing to 1 μA and 10 μA for the 5.5-hour and 8.2-hour radiated samples, respectively. The turn-on voltage of the control devices (≈ 0.85 V) decreased with the irradiated sample (≈ 0.75 V for 5.5-hour and ≈ 0.5 V for 8.2-hour exposures), implying noticeable radiation damage occurred at the heterojunction. The higher values of the ideality factor for irradiated devices (n > 2.2) compared to that of the control devices (n ≈ 1.3) also support the deterioration of the p-n junction. We observed the notable decrease in shunt resistance (RSH) and the increase in series resistance (Rs) with the neutron dose. It is possible that Cu ions introduced during the CuCl2 treatment may migrate into CdTe grain boundaries (GBs). The presence of Cu ions at GBs can create additional leakage paths for photocarrier transport, deteriorating the overall PV performance. We estimated the radiation dose of CdTe in comparison to Si (conventional PV) using a UUTR model (e.g., MCNP6 2D UTR Reactor simulations). In this model, we simulated Si and CdTe at the center point of the triangular fuel lattice and used an “unperturbed flux” tally in the water. Our simulations yielded a dose rate of 6916 Gy/s of neutrons and 16 Gy/s of photons for CdTe, and 1 Gy/s of neutrons and 21 Gy/s of photons for Si (doses +/- <1%). The large dose rate of neutrons in CdTe is mainly attributed to the large thermal neutron absorption cross-section of 113Cd. Based on this estimation, we calculate that the exposure of our CdTe PVs is equivalent to several million years in LEO (Low-Earth Orbit), or about 10,000 years for Si in LEO. Currently, we are working on a low-dose neutron/photon radiation on CdTe PVs and their light I-Vs and microstructural characterizations to gain better understanding on the degradation of CdTe PVs.« less
4. Atomistic simulations of superplasticity and amorphization of nanocrystalline anatase TiO2

   https://doi.org/DOI:10.1016/j.eml.2018.05.009  

   X. Zhang, H.J. Gao ( July 2018 , Extreme mechanics letters)

   As an important type of functional material, nanocrystalline TiO2 with anatase phase has been used for solar energy conversion and photocatalysis. However, there have been only a few limited studies on the mechanical behaviors of nanocrystalline anatase. We performed a series of large-scale atomistic simulations to investigate the deformation of nanocrystalline anatase with mean grain sizes varying from 2 nm to 6 nm and amorphous TiO2 under uniaxial tension and compression at room temperature. The simulation results showed that for uniaxial tension, the fracture strains of simulated samples increase as the mean grain size decreases, and a superplastic deformation occurredmore »in the nanocrystalline sample with a grain size of 2 nm. Such superplasticity of nanocrystalline anatase is attributed to the dominance of grain boundary sliding and nanoscale cavitation during deformation. The simulation results also showed that during uniaxial compression, the amorphization induced by high local compressive stress is the controlling plastic deformation mechanism, resulting in a good compressibility of nanocrystalline TiO2. During both tension and compression, nanocrystalline TiO2 exhibited good deformability, which is attributed to the fact that the grain boundaries with high volume fractions and disordered structures accommodated large plastic strains. Our present study provides a fundamental understanding of the plastic deformation of nanocrystalline anatase TiO2, as well as a route for enhancing the tensile and compressive deformability of nanostructured ceramics.« less
5. Thermal transport in monocrystalline and polycrystalline lithium cobalt oxide

   https://doi.org/10.1039/C9CP01585J  

   He, Jinlong ; Zhang, Lin ; Liu, Ling ( June 2019 , Physical Chemistry Chemical Physics)

   Efficient heat dissipation in batteries is important for thermal management against thermal runaway and chemical instability at elevated temperatures. Nevertheless, thermal transport processes in battery materials have not been well understood especially considering their complicated microstructures. In this study, lattice thermal transport in lithium cobalt oxide (LiCoO 2 ), a popular cathode material for lithium ion batteries, is investigated via molecular dynamics-based approaches and thermal resistance models. A LiCoO 2 single-crystal is shown to have thermal conductivities in the order of 100 W m −1 K −1 with strong anisotropy, temperature dependence, and size effects. By comparison, polycrystalline LiCoO 2more »is more isotropic with much lower thermal conductivities. This difference is caused by random grain orientations, the thermal resistance of grain boundaries, and size-dependent intra-grain thermal conductivities that are unique to polycrystals. The grain boundary thermal conductance is calculated to be in the range of 7.16–25.21 GW m −2 K −1 . The size effects of the intra-grain thermal conductivities are described by two empirical equations. Considering all of these effects, two thermal resistance models are developed to predict the thermal conductivity of polycrystalline LiCoO 2 . The two models predict a consistent thermal conductivity–grain size relationship that agrees well with molecular dynamics simulation results. The insights revealed by this study may facilitate future efforts on battery materials design for improved thermal management.« less