以水熱法合成鎢摻雜二氧化釩奈米結構 於藍寶石基板;Hydrothermal synthesis of tungsten-doped vanadium dioxide nanostructures on sapphire

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Title:	以水熱法合成鎢摻雜二氧化釩奈米結構於藍寶石基板;Hydrothermal synthesis of tungsten-doped vanadium dioxide nanostructures on sapphire
Authors:	曾繼欣;ZENG, JI-SIN
Contributors:	材料科學與工程研究所
Keywords:	水熱法;二氧化釩;電學性質;hydrothermal method;vanadium dioxide;electrical properties
Date:	2020-08-19
Issue Date:	2020-09-02 15:44:41 (UTC+8)
Publisher:	國立中央大學
Abstract:	二氧化釩 (vanadium dioxide, VO2) ，是種具相變性質的過渡金屬氧化物，因此當發生金屬絕緣體相轉變 (metal-insulator transformation, MIT) 的過程中會從絕緣體―單斜晶相 (monoclinic) 轉換成金屬態―金紅石相 (rutile) 。此時物理性質也會發生突變，如電阻產生2~5個數量級突變，同時伴隨著磁性、光學性質上的改變，並且因為相轉換溫度 (約68 °C) 接近室溫，使其成為重要的功能材料研究對象。本研究目的在直接利用水熱法於藍寶石基板上沉積出摻雜鎢之二氧化釩結構，並進行後退火處理，探討其微觀結構和結晶相並嘗試優化相轉換溫度 (電阻突變溫度) 。實驗結果顯示在前驅物比例 (五氧化二釩:草酸) 為3:5時能使反應後得到最佳的基板覆蓋率且避免雜相V3O7生成；於水熱法180 °C、持溫4小時的低溫短時間條件下即可獲得純相二氧化釩B相的奈米結構。另外退火處理後之未摻雜二氧化釩結構相轉換溫度約68 °C、電阻變化量約3個數量級，與文獻上提出的相轉換溫度一致；而摻雜了2 at% 鎢的二氧化釩結構之相轉換溫度則下降至53 °C，電阻變化量約1個數量級，提高摻雜量時相轉換溫度並無明顯下降，且其X光繞射圖之M相特徵峰也無位移現象。不同的是以相同實驗條件下合成之摻鎢二氧化釩粉末其X光繞射圖的M相特徵峰位移量卻隨摻雜量上升而增加 (可證明鎢離子取代了部分的釩離子) 。由此推測摻雜後的二氧化釩更傾向於均質成核而非異質成核，因此只有少部分之結構可以沉積於藍寶石基板表面。 ;Vanadium dioxide (VO2) is a kind of transition metal oxide with phase change properties. Therefore, when metal-insulator transformation (MIT) occurs, the monoclinic VO2 will transform to the tetragonal rutile phase. At this time, the physical properties will also undergo mutations, such as 2 to 5 orders of magnitude mutations in resistance, accompanied by changes in magnetic and optical properties. Because the phase transition temperature (about 68 °C) of VO2 is close to room temperature, it has a great potential in the field of functional materials. The purpose of this research was to directly deposit W-doped vanadium dioxide on substrates by hydrothermal method, and performed post-annealing treatment. We explored its microstructure and crystalline phase, and tried to optimize the phase transition temperature (resistance mutation temperature) . The experimental results showed that when the precursor ratio (vanadium pentoxide: oxalic acid) was 3:5, the best substrate coverage could be obtained after the reaction and the formation of impurity V3O7 could be avoided. The pure vanadium dioxide B phase nanostructure could be successfully prepared through hydrothermal method at 180 °C for 4 hr. In addition, the Tc of the undoped VO2 structure was about 68 °C, and the resistance dropped by about 3 orders, which was roughly the same as the commonly known Tc of undoped VO2. When W content increased to 2 at%, the lowest Tc was 53 °C. The electrical resistance change was one order. The Tc did not decrease significantly when the doping amount was increased, and the M-phase characteristic peak of the X-ray diffraction pattern was also not shifted. However, the M phase characteristic peak displacement of the X-ray diffraction pattern of the tungsten-doped vanadium dioxide powder synthesized under the same experimental conditions increased with the increase of the doping amount. It was speculated that the doped vanadium dioxide tended to form homogeneous nucleation rather than heterogeneous nucleation, so only a small part of the structure could be deposited on the surface of the sapphire substrate.
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