刘江文, 邹长城, 王辉
, 欧阳柳章, 曾美琴, 朱敏
华南理工大学材料科学与工程学院 广东省先进储能材料重点实验室 广州 510640
LIU Jiangwen, ZOU Changcheng, WANG Hui, OUYANG Liuzhang, ZENG Meiqin, ZHU Min
中图分类号: TG139
文章编号: 1005-3093(2016)04-0248-07
通讯作者:
收稿日期: 2015-04-8
网络出版日期: 2016-04-25
版权声明: 2016 《材料研究学报》编辑部 《材料研究学报》编辑部
基金资助:
展开
摘要
设计并制备含有长周期堆垛有序结构(LPSO)的Mg94Cu4Y2储氢合金, 研究了合金在吸放氢过程中组织的转变机制以及吸放氢动力学性能。结果表明, Mg94Cu4Y2合金主要由Mg、Mg2Cu和高度固溶Cu、Y元素的含18R及14H型的LPSO组成。LPSO在首次吸氢过程中分解, 并原位生成均匀的(MgH2+MgCu2+YH3)纳米复合组织。在随后的脱氢和吸放氢循环中, 合金主要通过Mg/MgH2反应实现吸放氢。细小均匀分布的Mg2Cu和YH2对Mg/MgH2的催化作用, 使该合金表现出较优良的吸放氢动力学特性。
关键词：
Abstract
An alloy Mg94Cu4Y2 with a large quantity of long-period stacking ordered (LPSO) phases bearing Cu and Y was designed and prepared in this paper. The microstructural transformations and the hydrogen absorption/desorption properties of the alloy were characterized during hydrogenation and dehydrogenation processes. The cast Mg94Cu4Y2 alloy consists of phases such as Mg, Mg2Cu and LPSOs with 18R or 14H type. The LPSOs decomposed at the first hydrogenation, and in situ formed highly even dispersed nanocomposite (MgH2+MgCu2+YH3). The Mg/MgH2 was the main reaction during the subsequent dehydrogenation cycles. The alloy exhibits excellent hydrogen absorption and desorption kinetics because the nano-sized and even dispersed Mg2Cu and YH2 catalyzed effectively the Mg/MgH2 reactions.
Keywords：
镁基储氢合金有储氢密度高、资源丰富、价格低廉和环保等优点, 但是镁的氢化物MgH2的热力学稳定性较高, 脱氢动力学比较差。为了改进镁基合金的储氢性能, 很多学者做了大量的研究工作, 主要包括合金化、纳米化、催化以及形成复合组织等方法[1-3]。Reilly先研究了Mg-Cu储氢合金[4], 随后研究了Mg-Cu系储氢合金中Mg2Cu对Mg储氢性能的影响及其机理 [5-8]。用不同方法制备三元Mg-Cu-TM合金系的组织结构和储氢性能的研究工作, 也有少量报道。例如, 用快速凝固处理的Mg-Ni-Cu储氢合金[9, 10], 球磨法制备的Mg-Ni-Cu合金的储氢动力学研究[11]。
最近有关于含长周期堆垛有序结构(LPSO)的Mg-Ni-Y合金储氢性能的报道[12-14]。本文作者设计制备了含LPSO的MgNiY合金, 利用LPSO首次吸氢反应形成纳米复合结构, 使合金表现出优良的吸放氢动力学性能, 并阐明了其微观组织演变机制[12, 13]。对于Mg-Cu-Y合金, 调整成分和控制制备条件也可得到LPSO结构[15-19]。但是有关Mg-Cu-Y长周期结构合金的研究均未涉及其储氢性能, 更没有考察LPSO结构对合金储氢性能的影响及其机制。因此, 本文设计制备含Cu的Mg-Cu-Y合金, 研究Mg-Cu-Y合金中LPSO及其它相结构对合金储氢性能的影响, 以及合金吸放氢转变过程中的组织转变细节, 以期系统认识LPSO镁基储氢合金的储氢特性的微观机制。
自行设计Mg94Cu4Y2合金, 采用电磁感应真空熔炼结合铜模铸造的方法制备Mg94Cu4Y2。
在测试储氢性能前在氩气气氛手套箱中用锯条将铸态合金样品锉成约400 μm的粉末, 然后对粉末在QM-3SP2型行星式球磨机中进行简单球磨。球粉比为20∶1, 氩气气氛保护, 转速400 r/min, 时间120 min。
用AMC的气体反应控制器(Gas Reaction Controller)测试样品的吸放氢动力学, 样品质量控制在0.2-0.5 g。在动力学测试前对样品进行活化处理, 活化规范为380℃吸氢1 h , 真空放氢0.5 h, 活化两次。
用XRD、SEM和TEM分析样品的原始态和吸/放氢态的显微组织和相结构转变。XRD分析在Philip X-pert衍射仪上进行, Cu-Kα(λ=0.1.5406 nm), 管压U=40 kV, 管流I=40 mA, 扫描范围2θ=15°~85°。用带有能谱装置(Bruker 5010 EDS)的Zeiss Supra 40场发射扫描电子显微镜观察SEM显微组织。用JEM-2100透射电子显微镜进行TEM分析, 加速电压200 kV, 双倾试样台。用电解双喷结合离子减薄法制备块状试样的TEM样品。在手套箱中准备粉末TEM样品, 直接用碳微栅取样。
图1给出了Mg94Cu4Y2合金铸态时的SEM背散射电子照片, 明显可见三种衬度特征的区域。结合EDS能谱分析和后续的TEM(图2)及XRD分析(图3)可知, 图1中的黑色特征区域为块状纯Mg相, 灰色区域为固溶有Cu、Y元素的镁基LPSO结构。其中LPSO所占体积百分比约为50%, 在LPSO中混有少量呈白亮衬度特征成细针或细条块状相Mg2Cu, 与LPSO交替平行出现。
图1 铸态Mg94Cu4Y2合金的SEM背散射电子像
Fig.1 BSE micrograph showing the microstructure of the as cast Mg94Cu4Y2 alloy
图2 Mg94Cu4Y2合金精细结构的TEM照片
Fig.2 Fine structure of Mg94Cu4Y2 (a) typical BF TEM image showing the LPSO and adjacent Mg2Cu; (b) HRTEM micrograph of 14H-LPSO and inset showing corresponding SAEDP indexed zone axis
图3 Mg94Cu4Y2合金铸态和吸放氢前后的XRD图谱
Fig.3 XRD patterns of the alloy in the states of as cast, hydrogenated and dehydrogenated, respectively
图2给出了Mg94Cu4Y2合金铸态组织的TEM像和电子衍射分析。其中图2(a)对应于SEM照片中的LPSO和Mg2Cu组织特征, 可见粗条状LPSO与细片Mg2Cu交替生成。对LPSO部分进行电子衍射和HRTEM分析, 发现两类典型的长周期堆垛有序结构: 一种是14H型LPSO(图2(b)), 另一种是18R型LPSO(图2(c))。图2(d)给出了Mg相和相邻Mg2Cu相的形貌和电子衍射花样。
为了清晰了解该合金在吸放氢过程中的相变行为, 对Mg94Cu4Y2合金原始组织、吸氢态和吸氢后放氢态的材料进行了XRD相分析。图3给出了分别在铸态、300℃吸氢和300℃放氢后的XRD图谱。分析结果表明, 该合金铸态的主要相组成为Mg, LPSO和Mg2Cu相。合金完全氢化后的相组成为MgH2、MgCu2和YH3相, 有少部分Mg吸氢不完全, 且有少量未完全氢化的YH2。合金吸氢后再完全放氢, 其产物的相组成为Mg、Mg2Cu、YH2及少量残留的MgH2。根据上述结果, 结合文献[12, 13]对Mg-Ni-Y合金中LPSO吸放氢的研究和Mg-Cu 二元合金吸氢反应[4-6, 8], 可以推断合金的首次吸放氢反应可用式(1)-(3)表示, 即: Mg吸氢时形成MgH2; Mg2Cu与氢反应生成MgH2和MgCu2; 而LPSO中的Y元素与H的结合力很强, 在吸氢反应过程中先形成YH2, 充分吸氢后形成YH3, 同时造成LPSO分解, 其中的Mg吸氢形成MgH2, 而Cu和Mg结合形成Mg2Cu, Mg2Cu与氢反应生成MgH2和MgCu2。
而首次放氢反应过程可用式(4)-(6)表示: MgH2脱氢形成Mg, YH3相部分脱氢生成为YH2, 同时部分MgH2与MgCu2反应生成Mg2Cu和H2。
注意到LPSO在第一次吸氢后即发生了分解, 而且在随后的吸/放氢循环中不再生成。后续的吸/放氢反应可用式(7)-(9)表示。该合金中Cu、Y元素在后续吸放氢过程中形成Mg2Cu和YH2有催化作用[14]。
Mg94Cu4Y2合金在吸放氢过程中所涉及的化学反应式总结如下:
(1) 首次吸氢反应为:
(2) 首次放氢反应为:
(3) 后续吸/放氢循环反应为:
在TEM镜筒中的高真空和电子束照射的双重作用下吸氢状态的样品各氢化物相极易脱氢, 分解为各脱氢产物。因此, 本文主要用TEM研究合金脱氢后的显微组织。 Mg94Cu4Y2合金经8次吸放氢循环后在脱氢状态下合金粉末的显微组织, 如图4所示。
图4 Mg94Cu4Y2合金放氢状态下的显微组织
Fig.4 TEM micrographs of dehydrogenated Mg94Cu4Y2, (a) Morphology; (b) Selected area electron diffraction patterns taken from (a); (c) TEM dark field image of Mg2Cu using reflection {022}; (d) HRTEM micrograph of Mg, YH2, Mg2Cu and MgCu2 particles
图4(a)中的TEM明场像表明, 样品的脱氢态各组成相晶粒很细小, 均小于100 nm。图4(b)中的选区电子衍射分析表明, 其相组成为MgH2、Mg2Cu和YH2, 与XRD的分析结果一致。图4(c)是对应于图4(a)左侧方框形区域的以Mg2Cu{002}衍射斑成像的TEM暗场像, 可见Mg2Cu呈小于100 nm的不规则形状晶粒。图4(d)给出了对应于图4(a)中左侧红色椭圆形区域的HRTEM像, 可见各相的晶粒都非常细小, 分别为相邻的小于20 nm的Mg, Mg2Cu, 以及未转变完成残留的小于5 nm的MgCu2。
图5给出了Mg94Cu4Y2合金在200℃、250℃、280℃、300℃和350℃下的吸氢动力学曲线。可以看出, 随着等温温度的提高样品的吸氢加快、吸氢量增加, 表现出较优异的吸氢动力学性能。试样在200℃保温60 min 后可吸氢约2%, 在280℃等温吸氢速度有明显提高, 60 min可达到5.5%吸氢量, 而在350℃等温吸氢60 min即可达到6%吸氢量。纯镁在温度低于300℃时吸氢速度不高, 为便于比较, 图中也给出了50 μm颗粒大小的纯镁350℃的吸氢动力学。可以看出, 二者在350℃的吸氢动力学性能相近, 但是纯镁350℃等温吸氢7 0min的吸氢量约6.2%, 而Mg94Cu4Y2合金的吸氢量可达6.6%。
图5 Mg94Cu4Y2合金在不同温度下的吸氢动力学曲线
Fig.5 Isothermal hydrogenation curves of the dehydrogenated sample under 2 MPa hydrogen pressure at 200, 250, 280, 300 and 350°C, respectively
图6给出了Mg94Cu4Y2合金吸氢后试样在不同温度下的等温放氢动力学曲线。图6表明, 普通MgH2在低于300℃时几乎不放氢。与纯镁相比, Mg94Cu4Y2合金的脱氢温度有所下降, 吸放氢动力学有很大的改善。随着温度的升高试样的脱氢速度提高, 合金在250℃等温180 min 后约有1%氢气释放, 放氢速度较低, 但是仍远比纯镁的高。300℃等温脱氢速度明显提高, 180 min放氢量约4.5%, 而350℃等温脱氢速度极高, 30 min左右基本上完全脱氢, 放氢量达6%左右, 远高于纯镁350℃时的脱氢性能。在部分温度下放氢曲线并不光滑, 可能与Mg2Cu(式 (5))的反应需要时间相关。
图6 Mg94Cu4Y2合金在不同温度下的放氢动力学曲线
Fig.6 Isothermal dehydrogenation curves of the hydrogenated sample at 250, 280, 300, and 350℃, respectively
改善镁基储氢合金动力学的主要方法有纳米化、合金化或利用催化等。在复合球磨法中可加入催化剂, 但是有以下问题: 1)由于分散性和均匀性等问题需要加入较大量的催化剂以提高催化效果[20-22], 结果是影响有效储氢量; 2)添加催化剂方法工艺较复杂[23-25]。多数合金元素在镁中的固溶度都比较低, 添加过多合金元素会超出固溶度而生成第二相, 也影响体系的储氢量。为解决上述问题, 本文设计合金成分时考虑了储氢量和合金化有效催化两个基本原则。本文的合金设计和前述研究结果表明, Mg94Cu4Y2合金除了Mg和Mg2Cu相, 还含有大量18R型和14H型LPSO相。合金中的Cu除形成Mg2Cu相, 其余Cu元素和几乎所有的Y元素都均匀分布在LPSO相中。Mg基LPSO的形成, 与过渡族元素和稀土元素的原子半径匹配、以及相互元素之间的混合负熵有关[17]。考虑到LPSO的特殊结构特点[18], 即TM与RE合金元素的有序化偏聚在密排面上, 而他们与H的作用力与Mg不同, 从而影响H原子扩散。更进一步的, 根据LPSO亚晶粒结构(LPSO中合金元素偏聚层)尺度范围影响氢性能, 可以设想LPSO中的TM与RE原子在Mg密排面界面的有序化偏聚层起到了“纳米或亚纳米化的界面效果”。
根据本文研究结果, 可以设想如图7所示的Mg94Cu4Y2合金首次吸放氢和随后的吸放氢循环过程中组织转变机制。更加重要的是, LPSO这样的特殊初始结构使Mg94Cu4Y2合金中的LPSO在首次吸氢反应后形成(MgH2+MgCu2+YH3)纳米复合组织。这种纳米复合组织是通过反应原位生成, 其分布很均匀, 其机制可用如图7及式(3)所表达。 Mg-Cu-Y与Mg-Ni-Y合金中[12, 13]LPSO首次吸氢反应很相似, 不同之处是前者还存在Mg2Cu+H2
图7 Mg94Cu4Y2合金的吸放氢反应组织转变示意图
Fig.7 Sketch showing the microstructural mechanism of Mg94Cu4Y2 Alloy during the hydrogenation and dehydrogenation
LPSO首次吸氢分解形成以MgH2为主的纳米复合组织, 在随后的脱氢及吸放氢循环过程中不再形成LPSO, 而是按式(7)-(9)吸放氢反应机制进行。根据吸放氢过程中的组织结构分析, 该合金主要依靠Mg和MgH2之间的互相转换实现吸放氢。与纯镁相比, Mg94Cu4Y2合金的吸放氢动力学都有很大的改善。其原因也与Mg2Cu能促进分子H2在Mg表面的解离及对氢吸收的催化能力[26]有关, 特别是LPSO分解原位生成的Mg2Cu和在后续吸氢过程中形成的YH2对Mg/MgH2起到了有效的催化作用[4, 14]。
Mg94Cu4Y2合金主要由Mg、Mg2Cu和高度固溶Cu、Y元素的含18R及14H型LPSO的Mg基固溶体组成, 在首次氢化过程中LPSO分解后形成由MgH2+MgCu2+YH3组成的均匀的纳米复合结构, 该复合结构放氢后又形成由Mg+YH2+Mg2Cu的均匀复合纳米结构。在随后的吸放氢循环过程中, 这两类复合结构发生吸氢/脱氢转变。由于体系中均匀细小分布的YH2和Mg2Cu对Mg/MgH2的催化作用, 合金表现出优良的吸放氢动力学性能和较高的可逆储氢容量。
| [1] |
Thermodynamic tuning of Mg-based hydrogen storage alloys: Areview ,
Mg-based hydrides are one of the most promising hydrogen storage materials because of their relatively high storage capacity, abundance, and low cost. However, slow kinetics and stable thermodynamics hinder their practical application. In contrast to the substantial progress in the enhancement of the hydrogenation/dehydrogenation kinetics, thermodynamic tuning is still a great challenge for Mg-based alloys. At present, the main strategies to alter the thermodynamics of Mg/MgH2 are alloying, nanostructuring, and changing the reaction pathway. Using these approaches, thermodynamic tuning has been achieved to some extent, but it is still far from that required for practical application. In this article, we summarize the advantages and disadvantages of these strategies. Based on the current progress, finding reversible systems with high hydrogen capacity and effectively tailored reaction enthalpy offers a promising route for tuning the thermodynamics of Mg-based hydrogen storage alloys.
|
| [2] |
Metal hydride materials for solid hydrogen storage: a review ,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">Hydrogen is an ideal energy carrier which is considered for future transport, such as automotive applications. In this context storage of hydrogen is one of the key challenges in developing hydrogen economy. The relatively advanced storage methods such as high-pressure gas or liquid cannot fulfill future storage goals. Chemical or physically combined storage of hydrogen in other materials has potential advantages over other storage methods. Intensive research has been done on metal hydrides recently for improvement of hydrogenation properties. The present review reports recent developments of metal hydrides on properties including hydrogen-storage capacity, kinetics, cyclic behavior, toxicity, pressure and thermal response. A group of Mg-based hydrides stand as promising candidate for competitive hydrogen storage with reversible hydrogen capacity up to 7.6 wt% for on-board applications. Efforts have been devoted to these materials to decrease their desorption temperature, enhance the kinetics and cycle life. The kinetics has been improved by adding an appropriate catalyst into the system and as well as by ball-milling that introduces defects with improved surface properties. The studies reported promising results, such as improved kinetics and lower decomposition temperatures, however, the state-of-the-art materials are still far from meeting the aimed target for their transport applications. Therefore, further research work is needed to achieve the goal by improving development on hydrogenation, thermal and cyclic behavior of metal hydrides.</p>
|
| [3] |
Recent progress in hydrogen storage ,
<p id="">The ever-increasing demand for energy coupled with dwindling fossil fuel resources make the establishment of a clean and sustainable energy system a compelling need. Hydrogen-based energy systems offer potential solutions. Although, in the long-term, the ultimate technological challenge is large-scale hydrogen production from renewable sources, the pressing issue is how to store hydrogen efficiently on board hydrogen fuel-cell vehicles<sup><a href="#bib1" id="bbib1" class="intra_ref"> [1]</a> and <a href="#bib2" id="bbib2" class="intra_ref">[2]</a></sup>.</p>
|
| [4] |
Reaction of hydrogen with alloys of magnesium and copper ,
ABSTRACT
|
| [5] |
Mg-based hydrogen storage materials: Surface segregation in Mg2Cu and related catalytic effects , |
| [6] |
Hydrogen storage properties of Mg/Cu and Mg/Pd laminate composites and metallographic structure ,
The Mg-based laminate composites, Mg/Cu and Mg/Pd, were prepared by repetitive-rolling, which is considered to suit for mass production. Mg/Cu laminate composites (Mg/Cu=2) absorb and desorb hydrogen reversibly at 473K, and the laminate composites have a better reaction kinetics than melting-casting alloys. TEM observations revealed that the as-rolled Mg-based laminate composite had the sub-micrometer-ordered layered structure with dense dislocations and vacancies. After initial activation and dehydrogenation process, the samples have kept the sub-micrometer-ordered laminate structure with dense dislocations and vacancies. The nano-structure of Mg-based laminate composites leads to lower hydrogen desorption temperature and better kinetics, which would contribute to achieve high capacity hydrogen storage materials. In Mg/Pd laminate composites (Mg/Pd=6), Mg 6 Pd is formed during initial activation process. This Mg 6 Pd also can store hydrogen reversibly through the disproportionation and recombination process.
|
| [7] |
Formation and hydrogen storage properties of in situ prepared Mg-Cu alloy nanoparticles by arc discharge ,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">Mg–Cu alloy nanoparticles were <em>in situ</em> prepared by a physical vapor condensation method (arc discharge) in a mixture of argon and hydrogen. Four crystalline phases, Mg, Mg<sub>2</sub>Cu, MgCu<sub>2</sub> and MgO, were formed simultaneously during the arc-discharge evaporation. Detailed experiments revealed that nanostructured hydrogen-active phases of Mg<sub>2</sub>Cu and Mg exhibit enhanced hydrogen absorption kinetics possibly due to the small grain size and surface defects. The maximal hydrogen storage contents of Mg–Cu alloy nanoparticles can reach 2.05 ± 0.10 wt% at 623 K.</p>
|
| [8] |
Hydriding and dehydriding kinetics of Mg in a Mg/Mg2Cu eutectic alloy: Pressure sweep method , |
| [9] |
Hydrogenation and dehydrogenation behaviours of nanocrystalline Mg20Ni10-xCux (x=0-4) alloys prepared by melt spinning ,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">In order to improve the hydriding and dehydriding kinetics of the Mg<sub>2</sub>Ni-type alloys, Ni in the alloy was partially substituted by element Cu, and the nanocrystalline Mg<sub>2</sub>Ni-type Mg<sub>20</sub>Ni<sub>10−x</sub>Cu<sub>x</sub> (<em>x</em> = 0, 1, 2, 3, 4) alloys were synthesized by melt-spinning technique. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus. The results show that the substitution of Cu for Ni does not change the major phase Mg<sub>2</sub>Ni. The hydrogen absorption capacity of the alloys first increases and then decreases with rising Cu content, but the hydrogen desorption capacity of the alloys grows with increasing Cu content. The melt spinning significantly improves the hydrogenation and dehydrogenation capacity and kinetics of the alloys.</p>
|
| [10] |
Hydrogen-storage properties of Mg-23.5Ni- (0 and 5) Cu prepared by melt spinning and crystallization heat treatment ,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">Mg–23.5 wt% Ni and Mg–23.5 wt% Ni–5 wt% Cu alloys for hydrogen storage were prepared by melt spinning and crystallization heat treatment. The alloys were ground by a planetary ball mill for 2 h in order to obtain a fine powder. The activated Mg–23.5Ni and Mg–23.5Ni–5Cu alloys absorbed 4.34 and 4.84 wt% H, respectively, at 573 K under 12 bar H<sub>2</sub> for 60 min. The activated Mg–23.5Ni and Mg–23.5Ni–5Cu alloys desorbed 4.27 and 4.81 wt% H, respectively, at 573 K under 1.0 bar H<sub>2</sub> for 30 min. The hydriding rates of the alloys are quite high, even at 473 K, while the dehydriding rates of the samples at 473 K are nearly zero.</p>
|
| [11] |
Mg-Ni-Cu mixtures for hydrogen storage: A kinetic study ,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">Mg-based ternary mixtures (40 ≤ Mg wt% ≤ 80) containing increasing amount (up to 30 wt %) of Ni and Cu were prepared by ball milling (BM) under Ar for 16 h and subsequently activated at high temperature (623 K) by charging/discharging cycles at H<sub>2</sub> pressure of 50 bar/0.7 bar. The work aims to analyze the influence of the mixtures' composition on the storage properties (H<sub>2</sub> intake and sorption kinetics) and to describe the role played by an <em>ad-hoc</em> activation in reaching these same properties. The storage capacity of the mixtures decreases by decreasing the Mg starting content, the H<sub>2</sub> active phases being “free Mg” and the “bonded Mg” intermetallic compounds Mg<sub>2</sub>Ni and Mg<sub>2</sub>Cu. After full activation (3 charging/discharging runs), “free Mg” hydrogenates 10 times quicker than the “bonded Mg phases”, while the discharging of both “free” and “bonded” Mg hydrides takes place simultaneously with similar kinetics. The best kinetic performance is shown by the samples with Mg = 60 wt% and 70 wt% and the highest Ni content (30% and 20% respectively), with sorption rates up to 7 times higher than those of the pure Mg/MgH<sub>2</sub> system.</p>
|
| [12] |
Superior hydrogen storage kinetics of Mg12YNi alloy with a long-period stacking ordered phase ,
The hydrogen absorption/desorption kinetics of the Mg12YNi alloy with an 18R-type long-period stacking ordered phase was investigated. It was found that the hydrogen-induced decomposition of the 18R phase with a grain size of 200-300 nm occurs at 573 K, leading to the formation of YH2 and YH3 particles with an average size of 10 nm. In the subsequent hydrogen absorption/desorption process, the fine YH2 and/or YH3 particles act as catalysts that enhance remarkably the hydrogen storage kinetics. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
|
| [13] |
Facilitating de/hydrogenation by long-period stacking ordered structure in Mg based alloys ,
We propose a simple strategy to effectively improve the hydrogenation and dehydrogenation kinetics of Mg based hydrogen storage alloys. We designed and prepared an Mg91.9Ni4.3.Y-3.8 alloy consisting of a large quantity of long-period stacking ordered (LPSO) phases. A type of highly dispersed multiphase nanostructure, which can markedly promote the de/hydrogenation kinetics, has been obtained utilizing the decomposition of LPSO phases at first a few of hydrogenation reactions. The fine structures of LPSO phases and the microstructural evolutions of the alloy during hydrogenation and dehydrogenation reactions were in detail characterized by means of transmission electron microscopy (TEM). The LPSO phases transformed to MgH2, Mg2NiH4, and YH3 after the first hydrogenation. The highly dispersed nanostructure at macro and micro (nano) scale range remains even after several de/hydrogenation cycles. The alloy shows excellent hydrogen storage properties and its reversible hydrogen absorption/desorption capacities are about 5.8 wt% at 300 degrees C. Particularly, the alloy exhibits very fast dehydrogenation kinetics. The dehydrogenated sample can release approximately 5 wt% hydrogen at 300 degrees C within 200 s and 5.5 wt% within 600 s. We elucidate the structural mechanism of the alloy with outstanding hydrogen storage performance. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
|
| [14] |
Microstructure and hydrogen storage properties of melt-spun Mg-Cu-Ni-Y alloys ,
Microstructoral and hydrogen storage properties of three nanocrystalline melt-spun Mg-base alloys (Mg(90)Cu(2.5)Ni(2.5)Y(5), Mg(85)Cu(5)Ni(5)Y(5) and Mg(80)Cu(5)Ni(5)Y(10)) have been investigated in view of their application as reversible hydrogen storage materials. The activation procedure and the hydrogen sorption kinetics of these alloys were studied by thermogravimetry at different temperatures in the range from 100 C to 380 degrees C. It has been found that these alloys can reach reversible gravimetric hydrogen storage densities of up to 4.8 wt.%-H(2). Even at a low temperature of 100 degrees C, the hydrogenation kinetics of the investigated alloys is rather high in the range of 1.5 wt.%-H(2) per hour. In the hydrogenated state, these alloys consist of MgH(2), high temperature Mg(2)NiH(4), Mg(2)NiH(0.3), YH(2), YH(3) as well as MgCu(2). The presence of MgCu(2) indicates the reaction of Mg(2)Cu with hydrogen. After repeated hydrogenation/dehydrogenation the preservation of a nanocrystalline grain structure has been confirmed by scanning electron microscopy, energy-filtered and conventional transmission electron microscopy. Additionally, the distribution of hydrogen in the hydrogenated sample was mapped by means of electron energy loss spectroscopy. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
|
| [15] |
Study on Mg-TM-Ln type nanostructured materials ,Mg-TM-Ln型镁基纳米结构材料研究 ,
讨论了Mg-Cu-Y、Mg-Ni-Y和Mg-Zn-Y等三种重要的Mg-TM-Ln型的多组元Mg基纳米结构材料的成分构成、制备过程、微观结构和力学性能以及它们的形成机制.其中,制备方法涉及非晶部分晶化法、机械合金化/粉末冶金法(MA/PM法)和快速凝固(原子雾化)/粉末冶金法(RS/PM法).结果表明,在Mg-Cu-Y系合金中,分布于非晶相基体之上的纳米晶体相使非晶合金的断裂应力增加.而不同成分的旋淬Mg-Cu-Y非晶条带弯曲断裂韧性的不同,很可能是由于存在于它们之中的纳米晶颗粒的性质有别而造成;在Mg-Ni-Y系合金中,Y部分地置换Ni将严重地影响其晶化行为,使得Mg-Ni-Y三元非晶合金中的纳米晶体相颗粒变得更加细小,而合金晶化行为的改变则导致了其杨氏模量的明显变化;在所有的金属合金中,RS/PM Mg97Zn1Y2合金的比强度是最高的.而在所有的Mg基合金中,RS/PM Mg-Zn-Y合金具有最佳的综合性能.
|
| [16] |
Study on the long period ordered (LPO) phasein the Mg-TM (TM=Cu, Ni, Zn)-Y systems ,
Mg-TM(TM=Cu, Ni, Zn)-Y体系长周期有序(LPO)相的研究 ,
|
| [17] |
Precipitates with peculiar morphology consisting of a disk-shaped amorphous core sandwiched between 14H-typed long period stacking order crystals in a melt-quenched Mg98Cu1Y1alloy ,
The microstructure of a melt-quenched Mg 98 Cu 1 Y 1 alloy has been studied by high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). We have found Cu- and Y-rich precipitates, which are uniformly dispersed in Mg-matrix grains. The precipitates are aligned parallel to the c-plane of the Mg-matrix crystal, and have the peculiar morphology consisting of a disk-shaped amorphous core sandwiched between 14H-typed long period stacking order (LPSO) crystals. A relatively stable supper-cooled liquid phase in an Mg-Cu-Y alloy system, and the formation and growth of the LPSO crystals from the amorphous phase are responsible for the peculiar morphology of the precipitates.
|
| [18] |
Elevated temperature Mg97Y2Cu1 alloy with long period ordered structure , |
| [19] |
Development of long-period ordered structures during crystallisation of amorphous Mg80Cu10Y10 and Mg83Ni9Y8 , |
| [20] |
Strom-Olsen, Nanocrystalline magnesium for hydrogen storage , |
| [21] |
Thermal stability of nanocrystalline magnesium for hydrogen storage ,
Magnesium hydride is considered to be one of the most interesting alternatives for the reversible storage of hydrogen. It is abundant, inexpensive, easy to handle, environmentally benign and exhibits a high hydrogen storage capacity of up to 7.665wt.%. Furthermore, nanocrystalline Mg powder prepared by high energy ball milling and the addition of suitable catalysts shows very fast absorption and desorption kinetics. The thermal stability of the nanocrystalline microstructure as well as the respective sorption kinetics of ball-milled MgHwith or without 0.565mol% NbOas catalyst have been investigated after cycling and annealing at the technically relevant temperatures between 300 and 40065C. While kinetics for pure MgHslows down substantially already after a few cycles at 30065C, MgHwith NbOcatalyst still shows fast sorption kinetics after annealing up to 37065C. At higher temperatures, the kinetics for the catalyzed material also breaks down, which is attributed to a deterioration of the catalyst. Continuous coarsening of the microstructure during annealing leads to an increased fraction of the storage capacity that can only be recharged at a slower rate. This is discussed in terms of retarded growth conditions for the MgHphase.
|
| [22] |
Effect of Nb2O5 on MgH2 properties during mechanical milling ,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">Recently, it was shown that hydrogen absorption–desorption kinetics in magnesium were improved by milling magnesium hydride (MgH<sub>2</sub>) with transition metal oxides. Herein, we investigate the role of the most effective of these oxides, Nb<sub>2</sub>O<sub>5</sub> when added in larger volume fraction. The effect of Nb<sub>2</sub>O<sub>5</sub> on magnesium crystalline structure, particle size and (ab)desorption properties has been characterised. Moreover, we report that pure MgH<sub>2</sub> can also show fast hydrogen sorption kinetics after a long milling time. The effects of Nb<sub>2</sub>O<sub>5</sub> on MgH<sub>2</sub> sorption properties are rationalised in a new approach considering Nb<sub>2</sub>O<sub>5</sub> as a dispersing agent, which helps reduce MgH<sub>2</sub> particle size during milling.</p>
|
| [23] |
Hydrogen storage properties of nanosized MgH2-0.1TiH2prepared by ultrahigh-energy-high-pressure milling , |
| [24] |
Remarkable enhancement in dehydrogenation of MgH2 by a nano-coating of multi-valence Ti-based catalysts ,
ATi-based multi- valence catalyst was coated on the surface of ball milled Mg powders (similar to 1 mu m in diameter), aiming to decrease the desorption temperature and increase the kinetics of hydrogen release from MgH2 by its catalytic effect on thermodynamics. The catalysis coating was prepared by the chemical reaction between Mg powders and TiCl3 in THF solution, which is similar to 10 nm in thickness and contains multiple valences in the form of Ti (0), TiH2 (+2), TiCl3 (+3) and TiO2 (+4). It is believed that the easier electron transfer among these different Ti valences plays a key role in enhancing the hydrogen recombination for the formation of a hydrogen molecule (e.g. H- + H- -> 2e H-2). This recombination is generally regarded as the key barrier for hydrogen desorption of MgH2. Experimentally, temperatureprogrammed desorption (TPD) and isothermal dehydrogenation analysis demonstrate that the MgH2 coated Ti based system (denoted as Mg-Ti) has excellent dehydrogenation properties, which can start to release H-2 at about 175 degrees C and release 5 wt% H-2 within 15 min at 250 degrees C. The dehydrogenation reaction entropy (DS) of the system is changed from 130.5 J K 1 mol(-1) H-2 to 136.1 J K-1 mol(-1) H-2, which reduces the Tplateau to 279 degrees C at an equilibrium pressure of 1 bar. A new mechanism has been proposed that multiple valence Ti sites act as the intermediate for electron transfers between Mg2+ and H-, which makes the recombination of H-2 on Ti (in forms of compounds) surfaces much easier.
|
| [25] |
Hydrogen storage properties of the Mg-Ti-H system prepared by high-energy-high-pressure reactive milling , |
| [26] |
Surface analysis of Mg2Ni-Mg, Mg2Ni and Mg2Cu , |
/
| 〈 |
|
〉 |
