本研究觀察q-到a-Al2O3相轉換過程中未完成相轉換的a-Al2O3還原成q-Al2O3的現象。q-到a-Al2O3的相轉換過程中會出現相轉換臨界晶徑（dcq～22 nm，dca～17 nm）及基礎晶徑（dp～50 nm）。後者係經由具臨界晶徑的a-Al2O3核晶聚合成長而得到的，相轉換在晶徑超過50 nm後才開始進入完成階段。以熱力學觀點來看，a-Al2O3晶粒在未完成相轉換（△Gr＞0）前，或晶徑介於20～50 nm之間都屬不穩定的狀態，應可還原為q-Al2O3。
本研究利用商用q-Al2O3粉為原料，先於1200℃持溫60秒後，q→a-Al2O3相變發生之a-生成量達30 wt%，再將樣品快速降溫至700℃、800℃、900℃及1000℃等溫度持溫，觀察在1200℃存在之a-及q- Al2O3晶粒的相變化現象。結果發現：粉末系統中不論是已進入相變過程但尚未完成相轉換的a-Al2O3或是離開1200℃後才進入相變之q-Al2O3所生成的a-晶粒，如屬未完成相轉換的a-Al2O3都會還原為q-Al2O3。
經過還原之q-晶粒可大至100 nm，並有雙晶結構（twin structure）出現。本研究亦同時指出還原為q-Al2O3之熱處理溫度需要在800℃以上，才可提供足夠的驅動力。

The phenomenon of a-Al2O3 crystallites of unfinished phase transformation would revert to q-Al2O3 was investigated in this study. The investigation was based on that the phase transformation of q- to a-Al2O3 is performed through nucleation and growth mechanism in which the critical size (dcq～22 nm, dca～17 nm) and primary size (dp =～50 nm) phenomena occur. And thus a-Al2O3 crystallites with crystallite sizes range from 20 to 50 nm or larger may as metastable and eventually can revert to q-phase.
Commercial q-Al2O3 powders pre-treated with ball milling were used as raw materials. The q-Al2O3 powder was calcined at 1200℃ for 60s, and the amount of a-formation induced by phase transformation was about 30 wt%. Then, the calcined samples were annealed at 700℃, 800℃, 900℃ and 1000℃ for various durations to check the reversion of a- to q-Al2O3, presumably the a-Al2O3 was metastable. It was found that there can be two kinds of metastable a-Al2O3 crystallites derived subsequently in the powder system: prior to and after annealed at 700-1000℃. No matter which one of them, if the phase transformation was not accomplished, it should revert to q-Al2O3.
It was found that some q-Al2O3 crystallites reverted from a-Al2O3 show sizes as large as 100 nm and have twin structure. It was also found that the thermal treatments at temperature ＞800℃ is required for providing sufficient driving force for performance of the reversion of a→q-Al2O3 in this study.

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