多孔Ni₃Al的制备工艺与催化行为研究

【作者】 何晓宇；

【导师】 黄伯云； 刘咏；

【作者基本信息】 中南大学 ， 材料学， 2008， 硕士

【摘要】 微管反应器作为结构紧凑且高效，使用酒精或碳氢化合物作为原料的氢气生产系统引起了研究人员的高度重视，比起传统反应器来，这种反应器有高的表面积一体积比，高的热导率和高的质量迁移。金属问化合物Ni₃Al是一种性能良好的高温结构材料，比不锈钢优异的耐热性能，有着十分广阔的应用前景。本论文，开展了Ni₃Al多孔材料研究。通过粉末冶金反应烧结方法，改变造孔剂的含量，调整烧结温度和时间，优化了Ni₃Al多孔材料的制备工艺。在Ni、Al元素粉末中添加NaCl造孔剂，改变造孔剂的含量，研究了其对材料密度和孔隙度的影响，并对Ni-Al反应机理进行了研究，推导出烧结样品密度随NaCl添加量变化的预测公式。通过XRD、SEM、HRTEM等现代材料分析技术分析了材料的相组成和显微结构。获得以下主要结论：1．Ni₃Al的多孔结构主要是依赖于在高温时，作为造孔剂的NaCl在真空状态下从材料中挥发后而形成。通过NaCl颗粒的大小可控制材料中孔隙的大小，所得孔隙形状与NaCl颗粒的形状一致。Ni₃Al多孔材料的孔隙度随NaCl添加量的增加而提高，即材料密度随NaCl添加量的提高而降低，基本呈线性关系。当NaCl的添加量为50％（wt．％）时，得到孔隙分布均匀的多孔Ni₃Al材料。2．从理论上，造孔剂的含量对多孔Ni₃Al孔隙度的影响，可由下述方程描述1/（1-θ）=1/（ρ₀）·[K·ρ_m1+(e^ε1-1)ρ₀₁/ρ（01）ρ_m1e^ε1 +ρ_m2+(e^ε2-1)ρ₀₂/ρ（02）ρ_m2e^ε2·x/（1-x）]。在实验中，且在特定的工艺条件下，当采用工艺条件为压制压力为600MPa，以10℃／min的升温速率，先在600℃保温3h，然后在801℃保温1h，1000℃保温1h，可获得多孔Ni₃Al材料，采用最小二乘法得到多孔Ni₃Al的孔隙度θ与NaCl的添加量x之间的关系表达式为3．采用甲醇作为碳源，多孔Ni₃Al作为催化剂载体，可催化分解制备出碳纳米管。当反应温度为600℃时，能使催化剂具有较高的活性同时催化裂解甲醇的速率也较快，产生的碳纳米管的数量较多。随催化时间的增加，纳米碳管的产量增加，当催化时间达到24h，碳纳米管的产量达到最大值。经高分辨透射电镜（HRTEM）分析表明生成的碳纳米管为多壁碳纳米管，管壁约为35层。4．多孔Ni₃Al材料中生长碳纳米管的可能的生长机理为：在甲醇催化分解的过程中，纳米的Ni颗粒的形成是由于表面的选择氧化作用，在Ni₃Al表面生成纳米的Ni颗粒，或由于反应不完全而残留下的纳米的Ni颗粒。甲醇在600℃时能分解出游离的碳原子，游离的碳原子以纳米的Ni颗粒为基体，生长出碳纳米管，碳纳米管在多孔Ni₃Al材料孔隙内部生长，在多孔Ni₃Al的孔隙内部生成大量的碳纳米管。更多还原

【Abstract】 In recent years, microchanneled reactors have received much attention as compact and efficient hydrogen production systems utilizing alcohol or hydrocarbon, due to their high surface to volume ratio and high rates of heat and mass transfer compared with conventional reactors.Ni₃Al intermetallic compound is one kind of new materials not only with high heat-resistance but also with good formability and mechanical strength. In this respect, intermetallic compound Ni₃Al, an excellent high-temperature structural material, has considerable promise. In this paper, the investigation focuses on the preparation of porous materials. Reaction Sintering of Powder Metallurgy method is adopted. The kind and the amount of pore forming agent are changed, as well as the sintering temperature and time. Adding different pore forming agents into the Ni, Al element powder, made an optimized choice. The effects of the amount of pore forming agent and the sintering temperature in a definite period on the density and porosity of the materials are studied. A formula that is used to calculate the density of the sintered sample is set up. The phases and microstructure of intermetallic compound porous material are investigated by means of X-ray diffraction （XRD）, scanning electron microscopy （SEM）, and high resolution transmission electron microscopy （HRTEM）. The results are listed as follows:1. The form of structure of porous Ni₃Al mainly depend on the volatilization of NaCl from porous Ni₃Al materials at high temperature.NaCl is adopted as pore-from material to prepare porous Ni₃Al materials, the size of pore could be controlled by size of NaCl particle directly, and the shape of pore correspondence with the shape of NaCl particle. Addition amount of NaCl has an important effect on this material porosity and porous structure; this specimen porosity rise as addition amount of NaCl increases but density decline and the relationship of them nearly a line.2.In theory, as preparing porosity Ni₃Al by addition pore-forming material, 1/（1-θ）=1/（ρ₀）·[K·ρ_m1+(e^ε1-1)ρ₀₁/ρ（01）ρ_m1e^ε1 +ρ_m2+(e^ε2-1)ρ₀₂/ρ（02）ρ_m2e^ε2·x/（1-x）]isobtained. In experiment, while the rate of temperature rise is 10℃/min,under the sintering process is 600℃for 3h, 801℃for 1h, 1000℃for lh and the pressing pressure is 600MPa,at this process, the relationship between porosity of Ni₃Al and amount of NaClis1/（1-θ）=1.2608 + 2.9235 x/（1-x） .3. Carbon nanotube is prepared using porous Ni₃Al as catalyst. The result indicates: while reaction temperature is 600℃, the catalytic both have the high activity and more fast catalyst speed than at others temperature. Meanwhile, the production of carbon nanotube is also large. As the catalytic time increase, the production of carbon nanotube increase. While the catalytic time reaches 24 h, the production of carbon nanotube is the most and not increased. HRTEM analyse indicate that carbon nanotube is muti-wall carbon nanotube and the number of wall is about 35.4. The mechanical of muti-wall carbon nanotube growing in the porous Ni₃Al is that: while the catalytic decomposition of methanol happens, due to the election of oxidation on the surface of Ni₃Al, nano Ni particle is formed on the surface Ni₃Al. Carbon atom which from the decomposition of methanol become carbon nanotube. As a result, carbon nanotubes are growing in inward of hole of porous Ni₃Al material; meanwhile, much of carbon nanotubes are growing in the hole of porous Ni₃Al.更多还原