Li Weiping

Erosion of a Polyimide Material Exposed to Simulated Atomic Oxygen Environment

Abstract Oxygen plasma source generated by thermal cathode filament discharge has been used to study the erosion process of polyimide (PI) materials in atomic oxygen (AO) environment, and their mass loss, surface morphology and surface chemical compositions have been examined by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) after exposure to incremental AO flux. The data indicate that the physical adsorption of AO at the samples surface results in the increase of oxygen concentration when polyimide is exposed to AO flux. Then selective chemical reactions of groups of polyimide materials with AO yield volatile organic compounds, sample mass loss is on linear increase and carpet-like surface morphology forms. In the initial exposure to AO, the reaction occurs mainly between AO and carbon in specific location of aromatic ring, then the reaction rate of C=O groups gradually increases. After AO exposure, the oxygen concentration increases while nitrogen and carbon concentration decreases. Reaction rate of groups containing nitrogen is slower compared with carbon and oxygen.

Fabrication of Nanostructured Electroforming Copper Layer by Means of an Ultrasonic-assisted Mechanical Treatment

Abstract Electroformed copper layer with nanostructure is obtained using a subsequent mechanical treatment under the conditions of ultrasonic vibration according to the demand of high performance material in aeronautics. The microstructure of the electroformed copper layer is observed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The tensile strength is evaluated with a tensile tester. It is found that bulk crystal of electroformed coppers surface layer is changed to nanocrystals (about 10 nm in size) after the ultrasonic-assisted mechanical treatment (UMT) but the whole monocrystalline structure still remains. The tensile strength exhibited by the new copper layer is two times better than the regular electroformed copper layer, while the fracture strain remains constant. In addition, the strengthening mechanism of UMT process is proved to be dislocation strengthening mechanism.

Effect of Cathodic Hydrogen Evolution on the Coercivity and Thermal Stability of Sintered NdFeB Magnets

Abstract The effect of cathodic hydrogen evolution on coercivity and thermal stability was investigated in sintered NdFeB magnets. The magnetic properties, phase structure and morphology were studied by SQUID-VSM, DSC, XRD, TEM and SEM. After cathodic hydrogen evolution, the Hcj decreases from 1034.8 kA·m−1 to 963.16 kA·m−1. The temperature coefficient α declines from −0.253%·°C−1 to −0.3229%·°C−1, and the β declines from −0.7518%·°C−1 to −0.7738%·°C−1. A mechanism was proposed to explain the results. In the process of cathodic hydrogen evolution, some of the generated hydrogen atoms react with the Nd2Fe14B phase and Nd-rich phase, forming Nd2Fe14BHx and NdHx, respectively. The formation of NdHx causes a volume expansion, which results in intergranular cracks and stress. X-ray diffraction and morphology characterization confirms the presence of these defects. These defects would significantly promote the nucleation of reverse magnetic domains, and further decline the coercivity and thermal stability of magnets.