Influence of grain size on transition temperature of thermochromic VO2

Abstract

A thermochromic low-emittance coating: Calculations for nanocomposites of In2O3:Sn and VO2 Appl. Phys. Lett. 99, 131907 (2011); 10.1063/1.3641869 Mg doping of thermochromic VO 2 films enhances the optical transmittance and decreases the metal-insulator transition temperature Vanadium(IV) oxide (VO 2 ) is a unique material that undergoes a reversible phase transformation around 68 C. The material could potentially be used as an energy-efficient coating for windows since its reflectance in the infrared (IR) increases significantly more than in the visible region. Currently, VO 2 is limited by a transition temperature ðs c Þ that is too high, luminous transmittance that is too low or both. In this study, a transition temperature of 45 C is achieved for a reactively sputtered, undoped film by restricting grain size to approximately 30 nm. It is concluded that a higher density of grain boundaries (smaller grain size) provides a greater number of nucleating defects which in turn reduces s c . Similarly, a higher density of grain boundaries may reduce the hysteresis width (difference between transition temperatures in heating and cooling). Also in this study, a new set of optical performance metrics is proposed in which the solar spectrum is divided into the ultraviolet (UV), visible and near infrared (NIR) regions. This approach is more closely aligned with the goals of limiting UV, allowing luminous and modulating NIR transmission. Using these metrics, the optical properties of the low-s c sample were: 2% UV transmittance, 47% luminous transmittance, and 23% NIR modulation (decrease from 43 to 33%). This study demonstrates that the grain size of VO 2 should be viewed as an important parameter for controlling the transition temperature of the material. V C 2015 AIP Publishing LLC. [http://dx.

Synthesis of vanadium oxide films with controlled morphologies: Impact on the metal-insulator transition behaviour

“…Several authors have reported results correlating the film thickness, grain size and crystallinity with the MIT and provided rather conflicting views relating film morphology and MIT behaviour as each case has only one or limited types of morphologies being studied. Therefore, it is important to properly understand this property with respect to various microstructures and their influence on the grain boundary density and crystallinity.…”

Interdependence of Structure, Morphology, and Phase Transitions in CVD Grown VO2 and V2O3 Nanostructures

“…Recent reports have shown that the phase transition temperature in VO2 materials can be influenced by grain size and film morphology, as well as by electronic doping and strain induced phenomena. [12][13][14][15] Also V2O3…”

Influence of Na diffusion on thermochromism of vanadium oxide films and suppression through mixed-alkali effect

“…This characterization elucidates that the average (between cooling and heating) c was approximately 55 • C for the samples in this study. In the literature it has been reported that c for bulk monocrystalline VO 2 is 67 • C to 68 • C, but it is also known that factors such as grain size, residual stress, percent crystallinity and impurities can affect the transition temperature substantially [9,17,33]. In the present study, the relatively low c is attributed to the nanocrystalline structure.…”

“…Zhang et al reactively DC sputtered VO 2 films with round to polygonal grains on the order of 40 nm to 100 nm [32]. Miller and Wang (the authors of the current study) reactively DC sputtered VO 2 films with needle-like grains that coarsened from 30 nm to 70 nm as annealing time increased from 30 min to 75 min [33]. Gurvitch et al RF sputtered pure V films and then oxidized them in O 2 .…”