Study On The Preparation And Properties Of Nanoporous Ag-based Alloys By Dealloying

Nanoporous metallic materials refer to the metallic materials with nano-scaled pores. Due to the nano-scaled pores and the three-dimensional bicontinuous ligament/channel structure, nanoporous metallic materials have asignificantly highsurfacetovolumeratio, and specific physical, chemical, optical properties. Nanoporous metallic materials show great potential in many fields, such as catalysis, sensors, actuators, fuel cells, and so forth. Nanoporous metallic materials can be fabricated by the process of dealloying, which refers to the selective dissolution of one or more active components out of an alloy. The preparation of nanoporous metallic materials by dealloying has attracted great attention in recent years. Systematicinvestigation on dealloying process is of great importance for the preparation of functional nanoporous materials.In thisthesis, rapidly solidified Mg-Ag based precursor alloys were dealloyed in acid solutions, and the formation of Ag-based nanoporous materials has been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The influence of alloy composition, dealloying solution and elemental doping on the dealloying process and the formation of nanoporous structure was systemically studied. The hydrogen sensing properties and the electrochemical catalytic activity towardselectro-oxidationof ethanol were characterized. The influence of vacancies in precursor alloys on the dealloying process was also investigated. The results show that the rapidly solidified Mg77Ag23precursor alloy ribbons were mainly composed of Mg54Ag17, while the rapidly solidified Mg65Ag35and Mg50Ag50precursor alloy ribbons both comprised a single MgAg phase. The Mg50Ag50precursor alloy ribbons can not be dealloyed in HCl solutions, while the Mg77Ag23and Mg65Ag35precursor alloy ribbons can be fully dealloyed and the dealloying resultsin the formation of typical three dimensional bicontinuous nanoporous Ag. The dealloying resistance and the ligament size in nanoporous Ag increase with the increasing Ag content in Mg-Ag precursor alloys, indicating that the precursor alloy composition has a significant influence on the dealloying process and the resultantnanoporous structure. The dealloying behavior of the Mg77Ag23precursor alloy in different acid solutions was investigated. The results show that the Mg77Ag23precursor alloy can be fully dealloyed in different acid solutions and the nanoporous Ag with different microstructural characters can be successfully fabricated. Crack-free nanoporous Ag ribbons with good mechanical integrity and small ligament/channel sizes can be obtained by dealloying Mg-Ag precursor alloys in suitable acid solutions. The Mg-Ag-Pd precursor alloys can be fabricated by doping of Pd into Mg-Ag alloys. The doping of Pd has a significant influence on the dealloying and results in the formation of ultrafine nanoporous Ag-Pd bimetallic material. The nanoporous microstructure was composed of channels and islands, which are also nanoporous, with ultrafine ligament/channel structure. Nanoporous Ag-Pd bimetallic material shows excellent hydrogen sensing properties and high catalytic activity towardselectro-oxidationof ethanol. Our present findings provide maximum flexibility in tailoring microstructure, fabrication of crack-free nanoporous metallic materials with refined ligament/channel structure and excellent properties. Moreover, the MgAg-phase-composed MgxAg100-x (x=65,62,58,54,50) precursor alloys exhibit different dealloying behaviors in HCl solutions, indicating the vacancy content in precursor alloys has a significant influence on dealloying process.