Metal to insulator transition of VO2-based thin films prepared by a coating method and its application to smart windows

已知二氧化钒VO2在67C时发生半导体-金属跃迁，其红外线透过率也随之发生变化。因此，从节能的角度来看，利用这种化合物的开关现象的智能窗户吸引了很多关注。然而，转变温度Tc=67C对于实际应用来说太高了。此外，包括溅射技术在内的传统制备VO2薄膜的方法对于大面积窗口的生产是不实用的。本研究的目的是通过一种廉价的涂覆方法在室温下获得含Tc的二氧化氧基薄膜。我们成功地开发了一种以聚钒酸为原料（包衣液），用金属钒与过氧化氢反应合成包衣液的新方法。在还原H2气氛中，在中等温度下加热这种涂层膜，可以很容易地制备VO2薄膜。这种方法也有利于掺杂金属元素来控制Tc。例如，我们成功地获得了钨掺杂的VO2薄膜，V1-xWx02，使用同样合成的多钨钒酸前驱体，通过金属钒和钨的混合物与过氧化氢反应。x=0.015时的薄膜在约30℃时出现急剧转变。我们证实了Tc降低了18C / mol / %的w，并对钨钼双掺杂薄膜的掺杂效果进行了系统的研究。结果证实了Vegard型规则对各元素降低Tc的作用是成立的。

It is known that vanadium dioxide VO2 undergoes a semiconductor-metal transition at 67C and its transmittance of infrared ray changes concomitantly. Thus smart windows utilizing this switching phenomenon of the compound absorb much attention from an energy saving point of view. However, the transition temperature Tc=67C is too high for practical use. Moreover, conventional preparing methods of VO2 film including the sputtering technique are mot practical for producing large area window. The purpose of this study is to obtain VO2-based films with Tc near room temperature by an inexpensive coating method. We have succeeded in developing a new method using polyvanadic acid as a starting material (coating solution), which is synthesized by the reaction of metallic vanadium with hydrogen peroxide. A thin film of VO2 is easily prepared by heating such a coated film in a reducing H2 atmosphere at moderate temperature. This method is advantageous also for doping metal elements to control Tc. For example, we succeeded in obtaining tungsten doped VO2 films, V1-xWx02, using poly-tungsto-vanadic acid precursor synthesized similarly by reacting a mixture of metallic vanadium and tungsten with hydrogen peroxide. A film at x=0.015 showed a sharp transition at about 30C.We confirmed Tc is lowered by 18C per molar % of W.A systematic study of the doping effect was also carried out using double doped films with tungsten and molybdenum. As a result, the Vegard type rule was confirmed to hold for the effect of each element to reduce Tc.