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

众所周知，二氧化钒VO2在67℃经历了半导体-金属转变，其对红外线的透过率也随之变化。因此，从节能的角度来看，利用该化合物的这种切换现象的智能窗吸引了更多的关注。然而，转变温度T_c=67℃太高，不能用于实际应用。此外，包括溅射技术在内的传统的VO2薄膜制备方法不适用于大面积窗口的制备。本研究的目的是通过一种廉价的镀膜方法获得室温附近的二氧化钒基薄膜。我们成功地开发了一种以聚钒酸为原料(包覆液)的新方法，该方法是由金属钒与过氧化氢反应合成的。通过在中等温度的还原氢气气氛中加热这种涂层薄膜，可以很容易地制备VO2薄膜。这种方法也有利于通过掺杂金属元素来控制T_c。例如，我们利用金属钒和钨的混合物与过氧化氢反应合成了类似的聚钨钒酸前驱体，成功地获得了钨掺杂的VO2薄膜V1-xWx02。在x=0.015处，薄膜在30℃左右发生了急剧的转变。我们证实了T_c每摩尔分数W降低了18C。我们还系统地研究了钨和钼双掺杂薄膜的掺杂效应。结果表明，各元素降低T_c的作用均符合Vegard类型规则。

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.