Thermo-chromic nanoparticles/polymer composite films for smart glass and energy efficiency in building
Assoc. Prof. Dr. Jatuphorn Wootthikanokkhan and Onruthai Srirodpai
Nanotec-KMUTT Center of Excellence on Hybrid Nanomaterials for Alternative Energy
School of Energy, Environment and Materials
King Mongkut’s University of Technology Thonburi
Glass has been widely used in modern buildings in various forms. These include windows, skylight, facades, roof, etc. This is due to the fact that the uses of glass in buildings and houses allow natural light entering the inner spaces, creating pleasant, comfortable, and relaxing atmospheres. However, the above benefits are obtained at the expense of energy efficiency in building. It has been reported that 25% to 35% of energy wasted in buildings is due to inefficient windows. [1,2] This is due to the fact that spectrum of electromagnetic radiation from the Sun striking the Earth’s atmosphere comprised of several regions including ultraviolet (UV), visible and infrared (IR). Particularly, the radiation in the infrared region which accounts for 49% of the heating of Earth.
To reduce the energy consumption in building, spectrum of the solar radiation, penetrating the windows need to be controlled. This can be achieved by using a kind of “smart windows” capable of regulating the solar energy in response to an external stimulus such as light (photo-chromic glass), heat (thermo-chromic glass), or electricity (electro-chromic glass). Particularly, the use of thermos-chromic glass is of interesting due to its several advantages. These include the fact that an installation of the thermos-chromic glass is similar to that of the traditional windows. There is no power supply needed and the production process is less expensive than the electrochromic analogue.
There are several kind of thermos-chromic materials. These include V2O3, V2O5, VO2, V6O13, and Ti2O3.  Especially, monoclinic VO2 (M) is considered to be an ideal thermosensitive materials. This was due to the fact that its transition temperature (Tc) is about 68 °C, which is the lowest as compared to other materials. Above the transition temperature, crystal structure of the metal oxide changes from monoclinic to tetragonal. In other words, morphology of the material changes from semi-conductor phase to metallic phase. This was accompanied by a reversible changes in various properties such as electrical conductivity, specific heat, magnetic properties, optical transmittance and reflectance in the infrared range. Besides, the transition temperature of VO2 can be further reduced by doping it with some metals such as molybdenum (Mo), niobium (Nb), chromium (Cr), and tungsten (W). Especially, work by Batista et al.,  demonstrated that W is the most effective doping agent for VO2. In this case, Tc of VO2 decreased by 7 °C/ % atomic of the dopant, without any detrimental effect on thermos-chromic properties of the material.
Synthesis and fabrication of VO2 film
Generally, VO2 thin films coated on glass substrate can be prepared by two main strategies which are the vacuum techniques (such as sputtering  chemical vapor deposition ) and the solution based techniques (such as sol-gel method  and polymer assist deposition ). Noteworthy, each techniques are capable of producing VO2 with some unique advantages and limitations. Specifically, VO2 film obtained from the vacuum techniques are of high uniformity but the fabrication process is rather complex and require expensive equipment. On the other hand, the liquid phase technique are relatively simple and less expensive. However, uniformity and properties of the prepared film have yet to be further improved . Besides, scaling up of the process to prepare VO2 for commercial uses is still challenging. In this regard, it is of interest to develop an alternative technique in which film uniformity, chromic properties and cost effectiveness of the VO2 film can be compromised. One possible strategies is that by preparing the VO2 in a form of nano-powder, followed by compounding it with a transparent polymeric binder.
Research work on the development of VO2 based thermo-chromic glass at HyNAE
In general, VO2 powder can be prepared by several methods such as pyrolysis , hydrothermal , and reduction-hydrolysis . At HyNAE lab, we found that a hydrothermal technique is the most suitable and effective technique for preparing the VO2 particles. The optimum conditioned for preparing tungsten doped VO2 (M) particles, using V2O5 as a raw material, were also studied and optimized. For example, by doping with 0.5 wt% of tungsten, Tc of the synthesized VO2 reduced from 68 °C to 44 °C.
Figure 1. A Diagram illustrating the processes for synthesis of VO2
Figure 2 XRD patterns of the prepared VO2 (left) and DSC thermograms of VO2 and doped VO2 (right).
Attempts were also made to apply the synthesized VO2 particles onto glass substrates by various methods. These including mixing it with some polymers before fabricating via solution casting, electrospinning, and extrusion. Particularly, by compounding the VO2 powder with ethylene vinyl acetate copolymer (EVA) followed by extruding the composite film via an extrusion process, it was possible to use the EVA/VO2 nanocomposite film as a binder for fabricating laminated glass. This is an aspect of our ongoing work.
Figure 3. A prototype of VO2/EVA composite coated glass (left) and a normal glass (right)
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