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February 7th, 2012

VO2 foils eyed for mass production for thermochromic window applications

Published on February 7th, 2012 | By: pwray@ceramics.org

Photographs of sample films at room temperature. Credit, Gao et al.; RSC Energy Environ. Sci.

The notion of making functional and flexible ceramic foils is fascinating, but a little counterintuitive, isn’t it?

Thus, I am always intrigued when new techniques and applications are discovered. A while back I wrote about a group from ETH Zurich that mastered ultrathin, transparent and flexible foils using yttria stabilized polycrystalline tetragonal zirconia.

This weeks brings news of another transparent and flexible film based on vanadium dioxide developed by Chinese researchers at the Shanghai Institute of Ceramics, including ACerS member Yanfeng Gao. The new VO2 film is being eyed for use in thermochromic windows, which retain transparency in the visible range but dynamically regulate the passage of wavelengths that transfer solar heat and energy in the ultraviolet and infrared ranges.

Smart window researchers have had an interest in VO2 because they’d like to exploit a particular property: At 68°C (in the case of VO2 bulk single crystals), the material undergoes a reversible, thermally induced phase transition that shifts the optical properties in the near-infrared region from a low-temperature transparent state to a more reflective state.

In an email, Gao says materials based on VO2 nanoparticles, particularly VO2 foils that easily can be used with glass panels, are attractive for applications in construction and automotive industries. Heretofore, making such foils from solutions of nanoparticles has been tricky and unreliable because of the instability of the nanoparticles. But, Gao and his colleagues report in a paper in the Royal Society of Chemistry’s journal, Energy and Environmental Science, that they have figured out a new process that solves previous shortcomings and may be scalable to large-area mass production.

Gao, who works in SIC’s State Key Laboratory of High Performance Ceramics and Superfine Microstructure, says, “In this paper, we report a novel all-solution process that can be used to prepare transparent, stable and flexible VO2-based composite films. These films exhibit UV-shielding properties and an excellent temperature-responsive thermochromism in the near infrared region.”

The breakthrough? Gao says the answer came when the group coated the VO2 nanoparticles with a thin SiO2 shell. “The shell, Gao says, “significantly improved their anti-oxidation and anti-acid abilities.”

Essentially, the VO2 nanoparticles are given a SiO2 shell using tetraethyl orthosilicate (the thickness of the shell can be fine-tuned) and treated with a silane couple to increase dispersion. The last step is to cast the suspension on a PET substrate.

At 13.6% solar modulation efficiency, the researchers report in the paper that their VO2-based film is able to match the efficiency levels of other thermochromic films. This is also considerably higher than VO2 films produced by sputtering and other methods.

Gao says, “Traditional glass foils are usually based on thin notable metal layers for reflection of solar irradiation or organic dyes that can absorption solar heat. … The stability of these kinds of foils is still questionable. To our knowledge the current research reports on the first VO2 ceramic foils, and more importantly, the foils show excellent optical properties (visible transmittance and solar modulation ability, maybe the best in the world.).

As far as “smart” performance goes, Gao et al. report in the paper that they observed while testing a typical sample of the VO2 film, “… in a heating cycle from 35°C to 85°C, the transmittance at 1500 nm decreased from 57.7% (at 35°C) to 14.9% (at 81°C) gradually…  In a cooling cycle, the transmittance of film increased from 14.9% (at 75°C) to 57.7% (at 35°C).”

The promise of increased energy efficiency via thermochromic windows has drawn worldwide attention. Gao says it is a big concern for developing countries, such as China, which already has buildings occupying 52 billion square meters “waiting new techniques to improve their energy efficiency and to reduce greenhouse gas emissions. We are aiming to develop a new material along with a novel process that can be finally commercialized and used to for building glasses.”

Gao says scaling the group’s technique to large-area production is the next challenge, and says that a collaborative effort is worthwhile. “This method should be considered as a basis for mass production,” he says, “The method should combine with some techniques to efficiently fabricate and to improve performance-cost ratios. … We hope that colleagues working in related fields can join to consider innovations based on the current technology. As an important part of an eco-home, we hope that such kind of smart windows can be applied practically in the near future.”

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