Thermal Properties and Surface Modification of Pd-Based Nanocrystalline Alloys

• Main Authors: Kuan-Wen Wang, 王冠文
• Other Authors: Tsong-Pyng Perng
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/15611736820247753457

博士 === 國立清華大學 === 材料科學工程學系 === 94 === Nanostructured materials are very attractive because of their large surface area and surface energy. By controlling the surface area or surface energy, the properties of nanomaterials can be manipulated. A common surface modification practice is to cover the nanoparticle surface with a layer of surfactant. The surfactant may prevent the particles from agglomeration and grain growth during sintering, and may also increase dispersity as the surface energy changes. Thus, the surface energy and properties of nanomaterials can be changed by this technology. In this study, thermal properties and surface modification of the Pd-based nanocrystalline alloys were studied. The thermal properties of nanocrystalline alloys include the desorption of surfactants, sintering, surface segregation, and selective oxidation. How the surface modification affects these properties of the nanocrystalline alloys is the main topic of this research. In the first section, desorption of surfactants, stearic acid (SA) and polyethylene glycol (PEG), from nanocrystalline Pd was studied by thermal analysis. The desorption temperatures measured in different systems were compared. The residual surfactant influences the sintering behavior of the nanocrystalline pellet. In section two, temperature programmed desorption (TPD) was used to identify the desorption temperatures of SA or PEG on the PdxNi1-x nanocrystalline alloys. The desorption temperature could be correlated to the surface energy of the alloys and the surfactants affected the sintering behavior of the alloys. In parts three to six, Pd70Ag30 alloy nanocrystalline was studied in detail. The preparation process of the nanoparticles was reviewed in part three. The nanoparticles with different homogeneities were prepared and surface segregation of these alloys was studied. There is the least change in surface composition in the most homogeneous Pd70Ag30 alloy nanocrystalline prepared by the strongest reducing agent, NaBH4. In sections four, the surface segregation of Pd70Ag30 nanoparticles with the same homogeneity but different surface modifications (SA or PEG) were studied. Surface modification did affect the surface segregation and sintering of the nanoparticles. Different surfactants behaved differently. In sections five and six, the selective oxidation of Pd70Ag30 alloy nanocrystalline in air was examined. The selective oxidation of Pd competes with surface segregation of Ag at lower temperatures (<300oC). At higher temperatures, intensive oxidation takes place and becomes more dominant. Surface modification (SA or PEG) can change the selective oxidation property at lower temperatures. As for the suppression of sintering behavior, SA can retard the grain growth of the materials more effectively. The desorption temperature of SA from materials is higher than that of PEG.