Acid Solution Processed VO2-Based Composite Films with Enhanced Thermochromic Properties for Smart Windows

As a typical thermochromic material, VO2 coatings can be applied to smart windows by modulating the transmission of near infrared (NIR) light via phase transition. However, the inherent undesirable luminous transmittance (Tlum) and solar modulation efficiency (ΔTsol) of pure VO2 impede its practical application. In order to solve this problem, the porous VO2 based composite film was prepared by magnetron sputtering and subsequent acid solution process with Zn2V2O7 particles used as a sacrificial template to create pores, which showed excellent Tlum (72.1%) and enhanced ΔTsol (10.7%) compared with pure VO2 film. It was demonstrated that the porous structure of the film caused by acid solution process could improve the Tlum obviously and the isolated VO2 nanoparticles presented strong localized surface plasmon resonance (LSPR) effects to enhance the ΔTsol. Therefore, this method will provide a facile way to prepare VO2 based films with excellent thermochromic performance and thus promote the application of the VO2 based films in smart windows.

Single-nanoantenna driven nanoscale control of the VO2 insulator to metal transition

The ultrafast concentration of electromagnetic energy in nanoscale volumes is one of the key features of optical nanoantennas illuminated at their surface plasmon resonances. Here, we drive the insulator to metal phase transition in vanadium dioxide (VO2) using a laser-induced pumping effect obtained by positioning a single gold nanoantenna in proximity to a VO2 thermochromic material. We explore how the geometry of the single nanoantenna affects the size and permittivity of the nanometer-scale VO2 regions featuring phase transition under different pumping conditions. The results reveal that a higher VO2 phase transition effect is obtained for pumping of the longitudinal or transversal localized surface plasmon depending on the antenna length. This characterization is of paramount importance since the single nanoantennas are the building blocks of many plasmonic nanosystems. Finally, we demonstrate the picosecond dynamics of the VO2 phase transition characterizing this system, useful for the realization of fast nano-switches. Our work shows that it is possible to miniaturize the hybrid plasmonic-VO2 system down to the single-antenna level, still maintaining a controllable behavior, fast picosecond dynamics, and the features characterizing its optical and thermal response.

Optical Forces on an Oscillating Dipole Near VO2 Phase Transition

We investigate optical forces on oscillating dipoles close to a phase change vanadium dioxide (VO2) film, which exhibits a metal-insulator transition around 340 K and low thermal hysteresis. This configuration emulates the interaction between an illuminated nanosphere and an interface and we employ a classical description to capture its important aspects. We consider both electric and magnetic dipoles for two different configurations, namely with the dipole moments parallel and perpendicular to the VO2 film. By using Bruggeman theory to describe the effective optical response of the material, we show that the thermal hysteresis present in the VO2 transition clearly shows up in the behavior of optical forces. In the near-field regime, the force on both dipoles can change from attractive to repulsive just by heating (or cooling) the film for a selected frequency range. We also verified that the optical forces are comparable to the Casimir-Polder force in a similar system, revealing the possibility of modulating or even changing the sign of the resultant force on an illuminated nano-object due to the presence of a thermochromic material. We hope that this work contributes to set the grounds for alternative approaches to control light-matter interactions using phase-change materials.

Microencapsulation of Thermochromic Material by Silicon Oxide Nanoparticles

This study is mainly focused on the fabrication of SiO2 as an inorganic shell material encapsulated an organic thermochromic (TC) core material comprises of either the (i) three-component as-synthesized blue dyes [BDTCM@SiO2] or (ii) off-the-shelf (commercial) black dyes [CDTCM@SiO2]. Both the SiO2 encapsulated thermochromic systems have successfully demonstrated the color transition at around 31 °C. For the three-component thermochromic microcapsules, we have used the crystal violet lactone (CVL) as a leuco dye, bisphenol-A (BPA) as a color developer, and 1-tetradecanol (TD) as a solvent. Different ratios of the thermochromic dye and the metal oxide were prepared to examine the effect of the core@shell ratio on the microstructural and thermal properties of the encapsulated microcapsules. The mean particles sizes of the BDTCM@SiO2 are below 100 nm, whereas, the CDTCM@SiO2 samples exhibited the mean particle sizes varied in a range of 100-1000 nm. The endothermic phase transition due to melting and in general, the thermal stabilities of these SiO2 encapsulated TCMs have been explored for the purpose of deploying these systems for thermal energy savings or storage applications.

Electrospun microfibers with embedded leuco dye-based thermochromic material for textile applications

Electrospinning is an inexpensive and versatile technique for fabricating micro- and nano- scaled fibers. There have been limited attempts to employ it for the fabrication of thermochromic (TC) fibers, and the fabrication of a three-component (dye, developer, and solvent) TC material has required the use of a more complicated coaxial electrospinning technique. Herein, a simple and novel method for creating thermochromic fibers by electrospinning single strands of poly (methyl methacrylate) (PMMA) with embedded thermochromic powder of a polymer encapsulated three-component system was employed. Unlike past leuco dye-based thermochromic fibers, an unmodified syringe tip can be used for the spinning process and only one flow rate needs to be determined. A solution of solvent (either N-dimethylformamide or chloroform), PMMA, and a commercially available black thermochromic powder was prepared and spun using a custom-made electrospinning apparatus. The spun fibers exhibited a clear color transition from grey to white and had average diameters of 2.53 µm and 1.96 µm for chloroform and N-dimethylformamide based fibers, respectively. The fibers were characterized by scanning electron and optical microscopy to determine their morphology, Fourier transform infrared spectroscopy to determine their chemical composition, and differential scanning calorimetry and thermogravimetric analysis to characterize their thermal properties.

Reversible Thermochromic Ink based on Crystal Violet Lactone/Boric Acid/Hexadecyl Alcohol for Anti-Counterfeiting Printing

The purpose of the study is to prepare reversible thermochromic materials with good discoloration properties, which are used as fillers to prepare a new type of reversible thermochromic ink. Reversible thermochromic materials were prepared by solid phase method with crystal violet lactone as chromogenic agent, boric acid as chromogenic agent and hexadecanol as cosolvent, and reversible thermochromic ink was prepared by using thermochromic material as filler. The scanning electron microscope images of reversible thermochromic materials showed that the prepared materials were spherical and had the advantages of non-adhesion. The printability of reversible thermochromic ink was discussed by changing the parameters including content of thermochromic material, printing pressure, ink sequence and using printability tester. The test of color change performance of thermochromic ink showed that the color change temperature range was 47.5–51.1 ◦ C, the color change time was 46 s, the recolor time was 29 s, and the stability was excellent. The reversible thermochromic ink has a certain application value in anti-counterfeiting printing.

Bio-inspired Artificial Helical Chromatophore for Stealth Mode

<p>Stealth technology has been very usefully applied in the military fields and is now becoming more prominent as a strategic technology. In nature, the firefly squid can protect itself from enemies using camouflage as a stealth mode. On the other hand, it is able to send fluorescent signals to attract prey by switching into a bright mode. Despite the development of many existing biomimetic materials, there are significant constraints related to their color-changeable velocity and mobility. Herein, we have developed a bio-inspired artificial thermochromic material system, which can reversibly switch between stealth and bright modes and thus provide a means to adapting to one’s environment analogous to the strategy applied by firefly squids. Through vertical contraction, a helically coiled yarn artificial muscle, selectively coated by Rhodamine B and TiO₂, can switch between fluorescent and stealth modes with a maximum speed of 0.31 cm/s. Upon external thermal impulse, artificial thermochromic muscle can spin up to 309° and achieve a negative strain of 84.6%. In addition, this research demonstrates thermochromic effects even in underwater aqueous conditions, showing applicability toward underwater robotics. With the cost-effectiveness of the demonstrated system, the developed artificial thermochromic muscles can be implemented into a variety of applications, such as colorimetric sensors and aqueous color-changeable soft robotics.</p>

Bio-inspired Artificial Helical Chromatophore for Stealth Mode

<p>Stealth technology has been very usefully applied in the military fields and is now becoming more prominent as a strategic technology. In nature, the firefly squid can protect itself from enemies using camouflage as a stealth mode. On the other hand, it is able to send fluorescent signals to attract prey by switching into a bright mode. Despite the development of many existing biomimetic materials, there are significant constraints related to their color-changeable velocity and mobility. Herein, we have developed a bio-inspired artificial thermochromic material system, which can reversibly switch between stealth and bright modes and thus provide a means to adapting to one’s environment analogous to the strategy applied by firefly squids. Through vertical contraction, a helically coiled yarn artificial muscle, selectively coated by Rhodamine B and TiO₂, can switch between fluorescent and stealth modes with a maximum speed of 0.31 cm/s. Upon external thermal impulse, artificial thermochromic muscle can spin up to 309° and achieve a negative strain of 84.6%. In addition, this research demonstrates thermochromic effects even in underwater aqueous conditions, showing applicability toward underwater robotics. With the cost-effectiveness of the demonstrated system, the developed artificial thermochromic muscles can be implemented into a variety of applications, such as colorimetric sensors and aqueous color-changeable soft robotics.</p>