(Invited) Thermochromic Phase Transitions in VO2-Based Thin Films for Energy-Saving Applications

Materials that contribute to more sustainable technologies are in high demand, and the highest impact can be made in the structures that are most ubiquitous. Widely implemented, thermochromic ‘smart’ building windows have the potential to save energy by reducing the heating and cooling loads on HVAC systems.^1,2 Exploiting a crystalline phase transition in VO₂, these window coatings shift from infrared transparent on cold days when solar heating is desirable to infrared reflective as the ambient temperature increases. In pure VO₂, this transition occurs at 68°C, but the incorporation of 1-5% cation impurities can depress the transition temperature to Earth ambient temperatures.

In this talk we describe our results on infrared thermochromism in V_1-xM_xO₂thin films, for M = Ta, Nb, W, Hf, and others.

                To efficiently screen the vast compositional space of transitional metal impurities in VO₂, we employ a combinatorial materials technique for both material synthesis and characterization. Thin film libraries of continuously varying composition are prepared by pulsed laser deposition (PLD) from two or more unary metal oxide targets, e.g. V₂O₅ and Ta₂O₅. Taking advantage of the spatial non-uniformity of deposition rates from each target, we produce library samples of composition V_1-xM_xO₂where x can vary from <0.01 to 0.3 across a single 3” diameter wafer.

                A special high-throughput spectroscopic reflectance measurement apparatus has been constructed for screening these libraries for thermochromic transitions. A thermoelectric heater/chiller with a 3” diameter plate maintains library temperature at a setpoint between 5°C and 85°C, while near infrared (NIR) reflectance measurements are made at many library locations, and hence many unique compositions.

                In Figure 1, for each composition represented on the V_1-xTa_xO₂ sample library, the NIR reflectance switches from a low reflectance state to a high reflectance state, a consequence of the crystalline phase transition that occurs. The transition is reversible with a <10°C hysteresis, and the material returns to its low reflectance state as the material is cooled back to 20°C. As the Ta impurity content increases, the transition temperature decreases.

Figure 2 shows depression of transition temperatures in VO₂ due to the incorporation of Nb or Ta. Both are found to effect a temperature depression that is linear with concentration at -4 °C/atomic percent. Additionally, the higher valence state impurities, such as W, Nb, and Ta, depress the VO₂transition temperature while Hf incorporation has little to no effect.

‘Smart’ window coatings enabled by the metal-insulator phase transition in lightly-substituted VO₂have significant energy-saving potential in terms of decreased heating and cooling needs for commercial and residential buildings. Critically, the NIR transmission/reflection switch occurs due to ambient temperature, and so the technology is passive, requiring no external controls or power systems.

References

1. S. M. Babulanam, T. S. Eriksson, G. A. Niklasson, and C. G. Granqvist, Solar Energy Materials, 16, 347–363 (1987).

2. C. Lampert, "Chromogenic Switchable Glazing: Towards the Development of the Smart Window" Proceedings of Window Innovations Conference LBL-37766  (1995).