李 程

English

博士学位論文の要旨及び審査結果の要旨 (Summary of Thesis(DR))othe

李, 程

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Porous Materials by Vapor Phase Dealloying著者 李 程号 61学位授与機関 Tohoku University学位授与番号 工博第5289号URL http://hdl.handle.net/10097/00122220  り ちぇん 氏 名   李 程 授   与 学   位  博士（工学） 学位授与年月日 平成２８年９月２６日 学位授与の根拠法規 学位規則第４条第１項 研究科，専攻の名称 東北大学大学院工学研究科（博士課程）知能デバイス材料学専攻 学 位 論 文 題 目          Porous Materials by Vapor Phase Dealloying（気相脱合金化法に よる多孔質材料）  主査　東北大学教授 陳  明偉 　東北大学教授 後藤　孝東北大学教授　加藤 秀実 論文内容要旨 Porous material is defined as material containing pores or voids. The pore structure endues the porous material many unique properties different from bulk state, such as low density, large specific surface area and high active site density, etc. The length scale of pore size distributes from sub-nano meter to over-macro meter based on different fabrication methods and the skeleton or matrix of the materials can be both organic or inorganic materials. This kind of high range modification brings large potential of applications such as structure materials, filters, energy storage materials and catalysts.    Fig. 1 roughly summarizes the widely used fabrication methods for porous materials with different length scale. Foaming process and sintering process are commercialized fabricating methods for macro scale porous materials such as metal foam and porous ceramic, while templating method and dealloying method are developed recently for meeting the request for the highly ordered nano scale porous materials. Compared with templating method, dealloying method is much easier to operate as fabricating nanoporous materials via selectively dissolving one component from the mother alloy and self organization of reminding component. The prospect of actual industrial application of this method is limited by two demerits. One is the unavoidable usage of chemical solutions like strong acids or bases that will cause environment problems. The other is the high cost for recycling the dissolved component, which is over 60% of the precursor alloy, because of the valence state transformation by chemical reaction. 指  導  教  員 東北大学教授　陳　明偉論 文 審 査 委 員? 139 ?Fig. 1. A hierarchy of porous materials, with average pore size increasing from left to right. Blue represents conductors and yellow is insulators. Applications for porous materials with distribution of length scales are shown in green.   In this thesis, vapor phase dealloying method is developed based on an old technique called sublimation purifying that is used for recycling or purifying validity metal such as Mg, Zn and some rare earth elements. By taking advantage of the saturated vapor pressure difference between two components, selective evaporation of the volatile component takes place with designed temperature and pressure. Since the precursors for the vapor phase dealloying do not contain primary pore structure before dealloying, and the pore structure comes from the self organization of left atoms as same as dealloying process, the mechanism of pore formation is also suppose to depend on the surface diffusion of left atom. Moreover, parting limit and polarization phenomena have been involved in the process as well.  Zinc has been chosen as the sacrificed component based on its high saturated vapor pressure as well as the low cost. The zinc rich precursor alloy was set in the middle of a furnace combined with nitrogen and hydrogen cylinders, and rotation pump as shown in Fig. 2. Ar gas (250 SCCM) mixed with H2 gas (10 SCCM) flow was used for preventing samples from oxidation. The pressure during the dealloying was 100 Pa. ? 140 ? Fig. 2. Schematic illustration of vapor phase dealloying setup.  In chapter 4, zinc has been separately evaporated from AgZn3 ribbon samples. However, no pore structure has been observed even when the zinc concentration decreased to 3 at.%.   In chapter 5, porous nickel with pore sizes from 390 nm to 570 nm was fabricated via vapor phase dealloying. During annealing, the Ni2Zn11 changed to NiZn first, and then became FCC nickel. The coarsening exponent of nickel is about 4.38, which is very close to the kinetic parameter, ~4, indicating that the diffusion rate of nickel is close to the evaporating rate of zinc. The activation energy for forming the ligament of nickel is about 0.62 eV, which is close to the nickel surface self diffusion energy in high vacuum. One step growth graphene and nitrogen doped graphene on the porous nickel were succeed. Compared with the process of growing grapheme on nanoporous nickel, which has been reported by our group earlier, the nickel substrate was replaced by the undealloyed Ni-Zn ribbon. During heating process, dezincification happened so that Ni-Zn ribbon changed to the porous zinc before reaching the grapheme growth temperature. With higher heating rate the pore size was decreased as well as the quality of the graphene. When testing the hydrogen evolution reaction (HER) catalytic performance, the nickel substrate need to be removed by the hydrochloric acid because the signal of nitrogen doped grapheme would be covered by it. The ligament size of free standing nitrogen doped grapheme was almost consist with the size of nickel substrate. The free standing nitrogen doped grapheme with heating rate of 40 K/min has the smallest onset overpotential and 50 K/min sample has the smallest Tafel slope due to high density of defects introduced by the shrinkage after nickel removing as the low graphene quality.   In chapter 6, nanoporous cobalt with pore size from 270 nm to 400 nm was fabricated via vapor phase dealloying. By dealloying the Co5Zn21 precursors in vacuum, zinc separately evaporated from ? 141 ?the matrix and the remaining cobalt formed a bicontinuous structure. The coarsening exponent of cobalt is about 3.14, which is smaller than the kinetic parameter, ~4, implying surface relaxation kinetics of roughened metal surface in solutions. Moreover, the activation energy for forming the ligament of cobalt is about 81.7 kJ/mol, which is smaller than the cobalt self diffusion energy in bulk cobalt. The relationships of hardness with dealloying temperature have opposite tendency to both specific surface area and porosity. The sample annealed at 923 K for 60 min exhibited both the maximal specific surface area (6.96 m2g-1) and the minimal micro-indentation hardness (43 MPa).   In chapter 7, the dealloying pressure was further decreased by utilize of extra turbomolecular pump so as to lower the critical dealloying temperature. Minimum ligament size of porous cobalt was reduced to less than 20 nm (Fig. 3) while the limitation of the minimum ligament size of nickel was larger than 300 nm. According to the ab initio calculation, the morphology evaluation during the dealloying process is highly related with the change of surface energy. Stable pore structure is preferred when the surface energy decreases.   Fig. 3. SEM micrograph of Co5Zn21 compressed powder dealloyed at 523 K for 480 min.    Generally, vapor phase dealloying method was developed and porous materials can be fabricated by this method. Additionally, no chemical solvent involving, and metallic state of the dealloyed zinc province the vapor phase dealloying method the potential of actual industrial production process.   ? 142 ?? 143 ?

Porous Materials by Vapor Phase Dealloying

https://tohoku.repo.nii.ac.jp/?action=repository_action_common_download&item_id=124807&item_no=1&attribute_id=18&file_no=1

気相脱合金化法による多孔質材料

Abstract

Similar works

Full text

Available Versions

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ