Ti-Zr-Ni（-Pd）二十面体准晶的形成与储氘特性研究

【作者】 黄火根；

【导师】 武胜； 罗德礼；

【作者基本信息】 中国工程物理研究院 ， 核燃料循环与材料， 2011， 博士

【摘要】 Bergman型结构的Ti-Zr-Ni准晶具有储氢量高且吸氢快的优点,是一种极具潜力的储氢材料,有望在氢能或氘氚核聚变能领域得到广泛应用。然而,不同Ti-Zr-Ni准晶的储氢量的报道结果差异较大,且它们的放氢动力学特性较差,对其应用不利。Pd是一种重要的准晶合金组元,具有优良的吸放氢动力学特性。为此,本论文研究了Pd添加对Ti-Zr-Ni准晶合金的结构与氘化性能的影响。首先,利用吸铸方法制备合金,结合X射线衍射技术(XRD)、金相技术(OM)、透射电镜技术(TEM)、差示量热扫描计(DSC)等手段,研究Ti-Zr-Ni-Pd准晶的形成特征,探究Pd加入对合金的准晶形成能力、原子结构、电子结构与热力学稳定性的影响。其次,采用自制的气固反应系统、程序升温热脱附仪与X射线光电子谱仪(XPS)等技术,对照研究Ti-Zr-Ni准晶在Pd添加前后的氘化行为,并探讨Ti-Zr-Ni基准晶的储氢共性。得到结论如下：1)Ti40Zr40Ni20、Ti45Zr38Ni17、Ti41.5Zr41.5Ni17三种合金的准晶形成能力与制备过程中的冷却速率密切相关。合金熔体直接冷却将形成MgZn2结构C14 Laves相(简称C14相)。在吸铸条件下,主要形成二十面体准晶相(IQC),另有少量六角结构的α-Ti、体心立方β-(Ti, Zr)与C14等杂相析出。适量Pd替代Ti或Zr使前两个合金完全形成IQC相,其中Ti40Zr40Ni20合金中Ti与Zr的可替代量分别为2at.%～4at.%与2 at.%,而Ti45Zr38Ni17合金中Ti、Zr可替代量为4 at.%～6at.%与4at.%。Ti的可替代性优于Zr,主要因为前者与Pd原子尺寸更接近。2)Pd添加对Ti-Zr-Ni合金的原子密堆性、电子结构与热稳定性产生影响。由于Pd原子尺寸适中,促使合金的原子密堆性提高,趋向于得到更高配位数的拓扑密堆相,其电子结构偏离准晶相的稳定机制—Hume-Rothery规律。Pd替代Ti使准晶的热力学稳定性降低,初始相转变温度下降约300℃,这与Pd原子较小而易扩散有关。3)室温下,双相合金Ti40Zr40Ni20、Ti41.5Zr41.5Ni17、Ti45Zr38Ni17的饱和吸氘浓度分别达到11.6mmol/g、12.9mmol/g、12.7mmol/g。其中,Ti40Zr40Ni20的储氢能力超过文献报道值的2倍,而Ti41.5Zr41.5Ni17的储氢量接近最高文献值的65%。Ti36Zr40Ni20Pd4与Ti39Zr38Ni17Pd6纯准晶合金的饱和吸氘浓度约11.0mmol/g(～4.4wt.%),相当于氘金属比D/M=1.56,跟Bergman团簇中空位数目与金属原子数目的比值很接近。这反映出,理想的Bergman型Ti-Zr-Ni基准晶应该具有一致的储氘(或氢)能力,且接近11mmol/g(相当于2.2wt.%H2)。报道结果的差异,很大程度上源于准晶合金中各种结构缺陷的影响。4) Ti-Zr-Ni(-Pd)准晶吸氘后未观察到TiD2与ZrD2等产物析出。氘在Ti-Zr-Ni基准晶中绝大部分是固溶在四面体间隙位置。约4.4wt%的固溶氘,仅引起低于7%的准晶格膨胀,不会导致准晶相发生相转变,这反映出准晶在氢化过程中较强的结构稳定性。氘溶解引起的化学位移揭示氘占位更靠近Zr与Ti,这与它们之f间的化学亲和力较大密切相关。5) Ti-Zr-Ni基准晶吸氢快速而放氢困难的特性进一步得到证实。其吸氘(或氢)行为符合一级反应特征,吸氘快的原因是氘原子在准晶中的扩散激活能较低。Pd的添加可提高Ti-Zr-Ni准晶的吸氘动力学性能,使得Ti36Zr40Ni20Pd4准晶的室温吸氘速率常数达到0.03s-1,接近Ti40Zr40Ni20的两倍。放氘(或氢)困难应该与准晶格的收缩有关。6)初步结果显示,Ti-Zr-Ni-Pd准晶的氘离解坪台压与U、Ti相差不大,揭示出氘在Ti-Zr-Ni基准晶中的高稳定性。Ti36Zr40Ni20Pd4准晶的氕氘分离因子接近0.8,显示出负的氢同位素效应,这反映出氚相比于氕、氘在Ti-Zr-Ni-Pd准晶合金中更高的热力学稳定性。更多还原

【Abstract】 Bergman-type Ti-Zr-Ni quasicrystals can absorb hydrogen rapidly in a large quantity, and have been regarded as a highly promising hydrogen storage material, which are unveiling their potential applications in fields of hydrogen energy or deuterium-tritium nuclear fusion energy. However, their potential applications have been restricted due to the reported large differences in hydrogen storage quantity in literature for various Ti-Zr-Ni quasicrystals and inferior hydrogen desorption kinetics. Palladium, as an important alloying element for quasicrystal formation, possesses superior hydrogen absorption/desorption kinetics. Therefore, this thesis aims to study the effect of Pd addition on the structures and deuterium storage/release properties of Ti-Zr-Ni quasicrystals.Firstly, the formation characteristics of Ti-Zr-Ni-Pd quasicrystals prepared by suction-casting method were investigated by using X-ray diffraction (XRD), Optical Microscopy (OM), Transition Electron Microscopy (TEM), Differential Scanning Calorimeter (DSC) apparatus. The effects of Pd addition on quasicrystal formation, atomic structure, electronic structure and thermodynamic stability were also evaluated. Secondly, the deuteration behaviors before/after Pd addition of Ti-Zr-Ni quasicrystalline alloys were studied by using home-made solid-gas reaction systems, Temperature Programmed Desorption (TPD) and X-ray Photoelectron Spectroscopy (XPS) techniques. Besides, the common features of hydrogen storage for Ti-Zr-Ni based quasicrystals were discussed.The results are concluded as follows:1) The quasicrystal forming abilities of Ti4oZr4oNi2o, Ti45Zr38Ni17 and Ti41.5Zr41.5Ni17 are close related to the cooling rate during preparation. Directly cooling the alloy melt led to the formation of a MgZn2-typed C14 Laves phase (abbr. C14), while suction casting it resulted in the formation of a main phase of icosahedral quasicrystal(IQC) together with the presence of a few hcp a-Ti, bcc (3-(Ti, Zr) and C14 phases. Moderate Pd substitution for Ti/Zr conduced to the formation of a single IQC phase in the Ti4oZr4oNi2o and Ti45Zr38Ni17 alloys. In the former alloy, the substitution quantity is 2 at.%-4 at.% for Ti and 2 at.% for Zr, while it is 4 at.%-6 at.% for Ti and 4at.% for Zr in the latter. Ti is more prone to be substituted than Zr, which should be owing to that Ti possesses the comparable atom size with that of Pd.2) Pd addition affects the close packness of atoms, electronic structure and thermal stability of Ti-Zr-Ni quasicrystals. Possessing a moderate atom size, Pd can increase the topological close packness of atoms, which makes the alloys tend to form a topologically densely-packed phase having higher coordination number and the deviation of the electronic structure from the Hume-Rothery rule for quasicrystal formation. The substitution of Pd for Ti caused Ti-Zr-Ni quasicrystals less stable and resulted in a decrease of 300℃for the initial phase transformation temperature, which could be attributed to easier diffusion of Pd with a smaller atom size.3) At room temperature, double-phase alloys Ti40Zr40Ni20, Ti41.5Zr41.5Ni17 and Ti45Zr38Ni17 can load deuterium up to 11.6 mmol/g,12.9 mmol/g and 12.7 mmol/g, respectively. The first value is 2 times larger than that in literature while the second is about 65% of the result reported in literature. For Ti36Zr4oNi2oPd4 and Ti39Zr38Ni17Pd6 containing a pure IQC phase, the saturated deuterium concentration is 11.0 mmol/g (-4.4wt%), which corresponds to the deuterium/metal ratio D/M of 1.56, close to the number ratio between the interstices and the metal atoms in Bergman-type cluster. Thus, it can be deduced that ideal Bergman-type Ti-Zr-Ni-based quasicrystals must possess almost the same deuterium (hydrogen)-absorbing ability～11.0mmol/g (corresponding to 2.2 wt.%). The difference in the amount of deuterium (or hydrogen) storage must lie in the variation in structure defects in the quasicrystal alloys.4) TiD2 and ZrD2 phases were not observed in the Ti-Zr-Ni(-Pd) quasicrystal after deuterium absorption. Most of the deuterium atoms are located in the tetragonal interstice. A dissolution of about 4.4 wt.% of deuterium caused the expansion of quasicrystal lattice of less than 7% without phase transformation, which reflects the structural stability of quasicrystal during hydrogenation. The chemical shift due to deuterium dissolution reveals that deuterium atoms are preferentially located near Zr and Ti, which is related to their strong chemical affinities.5) The characteristics of rapid absorption and difficult desorption for Ti-Zr-Ni based quasicrystals were confirmed in the study. The hydrogen absorption obeys the law of 1-stage reaction. Rapid absorption should be due to the low activation energy for deuterium diffusion in the quasicrystals. Pd addition improved the kinetic property of deuterium absorption for Ti-Zr-Ni quasicrystals, resulting in that the deuterium absorption velocity constant for Ti36Zr40Ni20Pd4 (0.03s-1) is almost two times of that for Ti40Zr40Ni20. The difficulty in deuterium desorption should be related to the shrink of the quasicrystal lattice.6) Preliminary results show similar deuterium-release equilibrium pressure between Ti-Zr-Ni-Pd quasicrystal and U/Ti, revealing the high thermo-stability of deuterium in the quasicrystals. In addition, due to the negative hydrogen isotope effect of Ti-Zr-Ni-Pd quasicrystals with H-D separate factor of about 0.8, higher thermodynamic stability of tritium in quasicrystals than that of hydrogen/deuterium can be expected.更多还原