學術論文

Radiation-induced segregation (RIS) is a non-equilibrium phenomenon that can significantly alter local composition and degrade material properties. Although extensive research has focused on RIS near grain boundaries, much less attention has been given to RIS near nanosized cavities, including voids and gas-filled bubbles, which are primary contributors to material swelling. Compared with grain boundaries, cavities exhibit unique characteristics as defect sinks, such as spherical geometry, potentially higher density, and shorter distances between sinks. Although conventional one-dimensional (1D) models can capture basic features of RIS near isolated cavities, our calculations showed that they failed to reproduce experimental RIS profiles near closely spaced bubbles, an arrangement commonly observed in irradiated materials due to heterogenous bubble nucleation. In this study, we combined two-dimensional (2D) simulations with atom probe tomography (APT) experiments to investigate RIS near helium bubbles in a Ni₅₀Fe₅₀ model alloy. The 2D simulations achieved good agreement with the experimental data. Moreover, we identified a non-linear coupling effect between neighboring bubbles: Ni segregation between two bubbles was higher than that from a single bubble, but lower than the linear sum of two isolated bubbles. These results demonstrate that the 2D RIS model is essential for simulating complex RIS behavior near cavities, thereby enabling more accurate predictions of microstructural evolution and property changes in materials under extensive irradiation.

Among GaN lateral power devices, with proper E-field management, the breakdown occurs in the device isolation region rather than the active region. In this paper, we investigate field-dependent carrier transport in the isolation structures. Isolation test structures with variations in buffer doping, un-intentionally doped (UID) GaN thickness, and implantation conditions were fabricated. Electrical characterization was performed over a wide range of voltage and temperature. With varying UID GaN thickness and buffer doping, the leakage current and breakdown characteristics remain the same for a specific implantation condition, indicating that the leakage conduction and breakdown are governed by the implanted GaN region. Analysis of the temperature-dependent I–V reveals that carrier transport in the implanted GaN can be well explained by hopping conduction, involving three distinctive regions of operation depending on the E-field. At low E-field, carrier transport is ohmic, consistent with variable range hopping, driven by thermal activation. At medium E-field, a field enhanced thermally activated hopping is observed, following σ ∼ σ(T)exp(E). At high E-field, activation-less hopping is visible, following σ ∼ exp(−1/E), which occurs above a critical E-field limited by localization length.

The radiation hardness of GaN-based devices is a critical metric for applications in extreme environments. This study investigates the structural changes in GaN and AlN induced by swift heavy ion (SHI) irradiation, characteristic of space radiation environments. A multilayered GaN/AlN structure is exposed to 950 MeV Au ions at fluences of 1 × 10¹² and 8 × 10¹² ions/cm². Subsequent post-irradiation characterization, including transmission electron microscopy and energy-dispersive x-ray spectroscopy, reveal no apparent amorphization across the entire sample. Notably, significant nanometer-sized cavities are observed in both GaN and AlN. The cavities in GaN exhibit an increase in number density and diameter with increasing SHI irradiation, with the average diameter progressing from 1.80 to 2.10 nm. In contrast, cavities in AlN appear considerably smaller. Molecular dynamics simulations, coupled with the inelastic thermal spike model, reproduce the presence of cavities in GaN and no cavities in the AlN structure. This difference is attributed to the faster heat dissipation and stronger bonding in AlN. Considering the overlapping of ion impacts at high fluences, simulations confirm the enlargement of cavity size in GaN. These findings contribute to a mechanistic understanding of the contrast in ion–matter interactions and induced microstructures between AlN and GaN under extreme ionizing radiation conditions. This disparity could potentially impact electronic performance through the formation of defect traps and interfacial strain fields.

The formation of helium bubbles and subsequent property degradation poses a significant challenge to tungsten as a plasma-facing material in future long-pulse plasma-burning fusion reactors. In this study, we investigated helium bubble formation in dispersion-strengthened tungsten doped with transition metal carbides, including TaC, ZrC, and TiC. Of the three dispersoids, TaC exhibited the highest resistance to helium bubble formation, possibly due to the low vacancy mobility in the Group VB metal carbide and oxide phases. Under identical irradiation conditions, large helium bubbles formed at grain boundaries in tungsten, while no bubbles were observed at the interfaces between the carbide dispersoid and tungsten matrix. Moreover, our results showed the interfaces could suppress helium bubble formation in the nearby tungsten matrix, suggesting that the interfaces are more effective in trapping helium as tiny clusters. Our research provided new insights into optimizing the microstructure of dispersion-strengthened tungsten alloys to enhance their performance.

The effect of different atomic substitutions at Mn sites on the magnetic and magnetocaloric properties in Ni₅₀Mn₃₅Co₂Sn₁₃ alloy has been studied in detail. The substitution of Ni or Co for Mn atoms might lower the Mn content at Sn sites, which would reduce the d-d hybridization between Ni 3d e_g states and the 3d states of excess Mn atoms at Sn sites, thus leading to the decrease of martensitic transformation temperature T_M in Ni₅₁Mn₃₄Co₂Sn₁₃ and Ni₅₀Mn₃₄Co₃Sn₁₃ alloys. On the other hand, the substitution of Sn for Mn atoms in Ni₅₀Mn₃₄Co₂Sn₁₄ would enhance the p-d covalent hybridization between the main group element (Sn) and the transition metal element (Mn or Ni) due to the increase of Sn content, thus also reducing the T_M by stabilizing the parent phase. Due to the reduction of T_M, a magnetostructural martensitic transition from FM austenite to weak-magnetic martensite is realized in Ni₅₁Mn₃₄Co₂Sn₁₃ and Ni₅₀Mn₃₄Co₂Sn₁₄, resulting in a large magnetocaloric effect around room temperature. For a low field change of 3 T, the maximum ΔS_M reaches as high as 30.9 J/kg K for Ni₅₀Mn₃₄Co₂Sn₁₄. A linear dependence of ΔS_M upon μ₀H has been found in Ni₅₀Mn₃₄Co₂Sn₁₄, and the origin of this linear relationship has been discussed by numerical analysis of Maxwell’s relation.