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VO 2在67 ℃时发生超导-金属转变，其红外线透射率随之变化。因此，从节能的角度来看，利用化合物的这种切换现象的智能窗吸引了很多关注。然而，转变温度Tc= 67 ℃对于实际应用来说太高。此外，传统的制备VO_2薄膜的方法，包括溅射法，对于大面积窗口的制作是不实用的。本研究的目的是通过一种廉价的镀膜方法获得Tc接近室温的VO 2基薄膜。我们成功地开发了一种新的方法，使用多钒酸作为起始原料（涂层溶液），它是通过金属钒与过氧化氢反应合成的。通过在中等温度下在还原性H2气氛中加热这种涂覆的膜，容易地制备VO 2的薄膜。该方法对于掺杂金属元素以控制Tc也是有利的。例如，我们成功地获得了钨掺杂的VO 2膜，V1-xWxO 2，使用类似地通过使金属钒和钨的混合物与过氧化氢反应合成的多钨钒酸前体。x=0.015的薄膜在30 ℃左右出现了一个急剧的转变，我们证实了每摩尔%的W使Tc降低了18个C。我们还用钨和钼的双掺杂薄膜系统地研究了掺杂效应。其结果是，Vegard型规则被证实为成立的效果，每个元素，以减少Tc。

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.