- 學位論文
以TiVFe合金為基礎的四元至七元儲氫高熵合金之設計與研究
Design and Study on the Hydrogen-Storage 4- to 7-Element High-Entropy Alloys Based on Composition of TiVFe
摘要
本研究為首度系統性的，以TiVFe為基礎合金，分別以不同順序，加入Zr、Co、Mn、Cr、Ni等金屬元素，進行四元至七/八元等莫耳儲氫合金的設計、熔煉鑄造、活化與吸氫動力學曲線測試。鑄造態合金，在本實驗設備許可條件下，進行40 atm、400℃的兩小時活化。在吸氫動力學曲線測試中，凡可以吸放氫者，才進行室溫及80℃的PCT吸放氫曲線測試。在第四元分別加入Zr、Cr、與Mn，成為TiVFeZr、TiVFeCr、TiVFeMn等三種四元合金後，僅發現，加入Zr的TiVFeZr鑄造態合金，有明顯的吸放氫效應。TiVFeCr與TiVFeMn四元鑄造態合金則沒有明顯的吸放氫。任選一種沒有吸氫的TiVFeCr，依次加入Co、Mn、Zr、Ni等元素，成為五至八元等莫耳鑄造態合金，亦僅發現合金加至第七元的Zr時，才有吸放氫現象。但到八元鑄造態合金，又成為不吸放氫者。顯示，Zr在此系列鑄造態合金，對合金的吸放氫，有關鍵性的作用。上面所提的，可吸放氫TiVFeZr鑄造態合金以及TiVFeMnZrCrCo鑄造態合金，分別編碼為4E及7E鑄造態合金。為便於區分，以下凡有編碼者，皆為明顯可吸放氫者。4E分別加入Co、Mn，成為5E-1及5E-2鑄造態合金。5E-1分別加入Cr、Mn，成為6E-1及6E-2合金。5E-2分別加入Co、Cr、Ni，成為6E-2、6E-2及6E-4鑄造態合金。6E-1加入Mn，又成為7E鑄造態合金。從以上變換元素的添加順序過程中，復可見Zr在吸放氫的關鍵性作用。經過室溫的PCT吸放氫曲線測試，得到4E、5E-1、5E-2、6E-1、6E-2、6E-3、6E-4及7E鑄造態合金，室溫最大儲氫量H/M比，分別為1.12、1.21、1.07、0.80、0.79、1.18、1.01、及0.84。利用以上各元素與氫的平均結合焓的計算，可得以上合金與氫的平均結合焓，分別為-87.6、-69.9、-70.7、-56.8、-60.7 、-64.6、-65.0、及-62.3 kJ/mol。大致顯示，在本研究合金中，與氫的平均結合焓越負者，室溫最大儲氫量H/M之比值越大。進一步計算，合金中各元素的兩兩原子半徑差絕對值的平均，作為合金晶格的應變能大小的量測。大致可解釋，6E-3合金與氫的平均結合焓，雖然僅-64.6 kJ/mol，但其應變能最小，故有大的室溫最大儲氫量H/M比。最後計算以上各合金，在室溫的混合熵（-TΔS），分別為-3.5、-4.0、 -4.0、-4.5、-4.5、-4.5、-4.5、及-4.9 kJ/mol，顯示混合熵對儲氫量的影響不大。80℃的PCT吸放氫顯示較室溫吸氫量為低，但有較小的遲滯效應。有關各合金的吸放氫平台，在本研究中，亦均一一加以討論。針對鑄造態合金的SEM金相、EDS成份、以及XRD繞射圖的鑑定，顯示本研究各合金的吸放氫能力，與吸氫相的種類及數量有關。大致含3至5個相，吸放氫相與以無序及有序BCC及Laves相有關。除7E鑄造態合金外，其他鑄造態合金，吸氫前後普遍顯示有，如A →（吸氫）AHx →（放氫）A’的相變態，亦即，吸放氫後，相不可逆，存在氫誘發相變態反應。本研究大部份合金顯示，吸氫動力學為氫擴散機制控制過程。本研究合金吸氫前後，經過機械合金後，均極易非晶質化。非晶質化後，吸放氫不明顯。鑄造態與吸氫後6E-1合金，非晶質化後，再經過800℃熱處理後的XRD圖是相同的。但對7E合金則不相同。顯示本研究合金系統的複雜性。6E-1與7E兩非晶質合金經熱處理後，吸氫亦不明顯。
並列摘要
This is the first systemized study on the design, melting, casting, activation and kinetics of hydrogen absorption of 4- to 7/8-multicomponent equal-mole hydrogen-storage alloys that are based on the composition of TiVFe with the addition of Zr, Co, Mn, Cr, and Ni in various combinations. As-cast alloys were activated under the available experiment condition in laboratory, i.e., 40 atm and 400℃ for 2 h. All that can significantly absorb hydrogen in kinetic experiment were carried out for PCT isotherms at room temperature (RT) and 80℃. The fourth component element, which is one of Zr, Cr and Mn, that was added in TiVFe to become TiVFeZr, TiVFeCr, and TiVFeMn alloys, only TiVFeZr can absorb hydrogen significantly. In comparison, quaternary TiVFeCr and TiVFeMn alloys cannot absorb hydrogen. On further adding the fifth to eighth element in sequence of Co, Mn, Zr, and Ni in TiVFeCr to become quinternary to octonary equal-mole alloys, only the as-cast 7-multicomponent Zr-containing alloy can absorb hydrogen. However, the as-cast octonary equal-mole alloy cannot. This obviously shows that the key function of Zr in this series of as-cast alloy on the hydrogen absorption. The aforementioned as-cast TiVFeZr and TiVFeMn-ZrCrCo alloys that can absorb hydrogen are designated as 4E and 7E alloys, respectively. In order to differentiate the absorbable from the unabsorbable, all those can absorb hydrogen in this study are designated with notation as nE-m, where n and m are number of components and different kind of alloys with same number of component, respectively. Thus adding Co and Mn to 4E becomes 5E-1and 5E-2, respectively. In turn, adding Cr and Mn to 5E-1 becomes 6E-1 and 6E-2, while adding Co, Cr, and Ni to 5E-2 becomes 6E-2, 6E-3, and 6E-4, respectively. At last, alloy formation by adding Mn in 6E-1 results in 7E alloy. Again, from the change in element adding sequence one can easily see the key function of Zr on hydrogen absorption. After the RT-PCT experiment, one can obtain the maximal RT H-capacities (H/M) for 4E, 5E-1, 5E-2, 6E-1, 6E-2, 6E-3, 6E-4, and 7E as-cast alloys are 1.12, 1.21, 1.07, 0.80, 0.79, 1.18, 1.01, and 0.84, respectively. By the calculation of averaging the formation enthalpies between hydrogen and each component element in each alloy, one can obtain enthalpies of 4E, 5E-1, 5E-2, 6E-1, 6E-2, 6E-3, 6E-4, and 7E are -87.6, -69.9, -70.7, -56.8, -60.7, -64.6, -65.0, and -62.3 kJ/mol, respectively. It shows roughly that the more negative the average formation enthalpies, the more the H/M ratio for alloys. On further calculation in averaging the absolute values of difference between two atomic radii in alloys, which manifests the degree of mismatch strain in lattice, one can explain why the low enthalpy of -64.6 kJ/mol for 6E-3 can have high H/M since it has the lowest mismatch strain among alloys studied. Finally we calculate the mixing configuration entropy energies at RT, and obtain -3.5, -4.0, -4.0, -4.5, -4.5, -4.5, -4.5, and -4.9 kJ/mol for 4E, 5E-1, 5E-2, 6E-1, 6E-2, 6E-3, 6E-4, and 7E, respectively. The influence of entropy on the H/M ratio is relatively small. The hydrogen absorption and desoption capacities for alloys in PCT isotherms at 80℃ are lower than those counterparts at RT. The plateaus in PCT isotherms for the alloys are also discussed in this study. SEM micrographs, EDS composition, and XRD patterns for alloys were also carried out in this experiment. It shows that the hydrogen absorption and desoption capacities for alloys studied in this experiment have something to do with the kinds and amount of the hydrogen absorbing and desorbing phases. All alloys in this study contain 3 to 5 phases such as disordered and ordered BCC and Laves phases that can absorb and desorb hydrogen. Except as-cast 7E alloy, all other 7 alloys generally occur hydrogen-induced phase transformation reactions, such as A → (H absorbing) AHx →(H desorbing) A’, during the hydrogen absorbing and desorbing processes. That is, there are irreversible phases after absorbing and desorbing hydrogen. Most alloys are diffusion-controlled in H absorbing kinetic study. Mechanically alloying the alloys of whatever H absorbing or not gives easily amorphous powders. XRD patterns for 800℃-heat treating the amorphized powders from both as-cast and as-H absorbing states of 6E-1alloy are the same, while those for 7E alloy at different states are not. This means that there must be complexity in this alloy series. The amorphized 6E-1 and 7E alloys followed by heat treatment also show little H absorption.