Effect of De-Twinning on Tensile Strength of Nano-Twinned Cu Films.

Tensile tests were carried on the electroplated Cu films with various densities of twin grain boundary. With TEM images and a selected area diffraction pattern, nano-twinned structure can be observed and defined in the electroplated Cu films. The density of the nano-twin grain structure can be manipulated with the concentration of gelatin in the Cu-sulfate electrolyte solution. We found that the strength of the Cu films is highly related to the twin-boundary density. The Cu film with a greater twin-boundary density has a larger fracture strength than the Cu film with a lesser twin-boundary density. After tensile tests, necking phenomenon (about 20 µm) occurred in the fractured Cu films. Moreover, by focused ion beam (FIB) cross-sectional analysis, the de-twinning can be observed in the region where necking begins. Thus, we believe that the de-twinning of the nano-twinned structure initiates the plastic deformation of the nano-twinned Cu films. Furthermore, with the analysis of the TEM images on the nano-twinned structure in the necking region of the fractured Cu films, the de-twinning mechanism attributes to two processes: (1) the ledge formation by the engagement of the dislocations with the twin boundaries and (2) the collapse of the ledges with the opposite twin-boundaries. In conclusion, the plastic deformation of nano-twinned Cu films is governed by the de-twinning of the nano-twinned structure. Moreover, the fracture strength of the nano-twinned Cu films is proportional to the twin-boundaries density.

N-isopropylacrylamide gel dosimeter to evaluate clinical photon beam characteristics.

The introduction of beam intensity control concept in current radiotherapy techniques has increased treatment planning complexity. Thus, small-field dose measurement has become increasingly vital. Polymer gel dosimetry method is widely studied. It is the only dose measurement tool that provides 3D dose distribution. This study aims to use an N-isopropylacrylamide (NIPAM) gel dosimeter to conduct beam performance measurements of percentage depth dose (PDD), beam flatness, and symmetry for photon beams with field sizes of 3×3 and 4×4 cm(2). Computed tomography scans were used to readout the gel dosimeters. In the PDD measurement, the NIPAM gel dosimeter and Gafchromic™ EBT3 radiochromic film displayed high consistency in the region deeper than the build-up region. The gel dosimeter dose profile had 3% lower flatness and symmetry measurement at 5 cm depth for different fields compared with that of the Gafchromic™ EBT3 film. During gamma evaluation under 3%/3 mm dose difference/distance-to-agreement standard, the pass rates of the polymer gel dosimeter to the TPS and EBT3 film were both higher than 96%. Given that the gel is tissue equivalent, it did not exhibit the energy dependence problems of radiochromic films. Therefore, the practical use of NIPAM polymer gel dosimeters is enhanced in clinical dose verification.