摘要

本研究是針對複合金屬氫化物 NaAlH4 在放氫過程之中間產物 Na3AlH6 之相變化加以探討，分析各種不同因素造成相變化之影響， 包括: 升溫速率、持溫溫度、持溫時間……等。為了探討以上所提到的變因，將使用高壓熱重分析儀(HPTGA)來進行量測並討論其結果。此外，將使用Ｘ光繞射分析儀(ex-situ X-ray diffraction analysis)和臨場同步輻射X光繞射儀(in-situ synchrotron X-ray diffraction)來觀察產物的結晶結構。本研究結果顯示，當在溫度220和240 °C之間，開始有 α- Na3AlH6 至 β- Na3AlH6 的相變化發生，相較於文獻所提到的溫度較低(250至260 °C之間)。根據熱重分析儀的結果，鋁氫化鈉在長恆溫時間下之條件，可看得到釋放的氫氣量隨著時間增加而增加，相變化也更促進。接著在不同升溫速率下的結果,ß- Na3AlH6 的出現範圍在較慢的升溫速率較廣而翻過來，在快的升溫速率，出現在很短的範圍內。
透過同步輻射X光繞射儀分析, 可發現鋁氫化鈉在220, 230, 240 °C的加熱過程當中可看得到 α- Na3AlH6 轉變成 ß- Na3AlH6 。表示 α- Na3AlH6 在220, 230, 240 °C的加熱過程當中已經正在進行vibrational 與rotational 狀態下，原子之間的bonding變弱，一旦降溫，原子之間的作用力回復正常，Lattice 的排列有些維持α- Na3AlH6但有些也變為 ß- Na3AlH6 甚至可能完全回復α- Na3AlH6 或直接分解為 NaH。故這也許 ß- Na3AlH3 一直以來都看不到,或出現於很弱的繞射訊號。

Abstract

Sodium tetrahydro aluminate is the most discovered in complex metal hydride family. The advantage in reversibility, and dehydrogenation amount exceed 5.5 wt% from that targeted by D.O.E for on-board light-duty vehicle utilization, makes it a popular candidate among all of complex metal hydride. In this study, as-received NaAlH4 was used and directly heated in thermogravimetric analysis (TGA) to see the quantity of Hydrogen that released from pure NaAlH4 isothermally, and how the results relate to the transformation of sodium tetrahydro aluminate into sodium hexahydro aluminate. In order to support the observation of this transformation, ex-situ XRD was used for structure and composition analysis of the reacted sodium tetrahydro aluminate in TGA. This study was also done by the help of in-situ synchrotron X-ray diffraction.
We found out that, the quantity of hydrogen gas generated from sodium tetrahydro aluminate is increased as the holding time and temperature increases. The result also showed the difference effect of the tetrahydro aluminate compound for its dehydrogenation amount by the variation of the heating rate. Where at lower heating rate, the quantity of gases that spill out are much more than the higher one.
Moreover, this study found out α- Na3AlH6 to ß- Na3AlH6 phase transformation taken place at temperature near the first dehydrogenation stage (210 °C), far lower than
the result from literature [31], and also, was taken place earlier (220 – 240 oC) than the one which was reported in literatures.
In the in-situ XRD result, the appearance of ß- Na3AlH6 could be found out at 220, 230, and 240 °C isothermally. However, the trace of this phase could only be detected in the middle of cooling process.

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