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A comparative study on the switching kinetics of W/VO2 powders and VO2 coatings and their implications for thermochromic glazing.

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Title: A comparative study on the switching kinetics of W/VO2 powders and VO2 coatings and their implications for thermochromic glazing.
Authors: Calvi, Lavinia1,2 (AUTHOR), Leufkens, Luc3,4 (AUTHOR), Yeung, Cindy P.K.3,4 (AUTHOR), Habets, Roberto3,4 (AUTHOR), Mann, Daniel3,4 (AUTHOR), Elen, Ken1,2 (AUTHOR), Hardy, An1,2 (AUTHOR), Van Bael, Marlies K.1,2 (AUTHOR), Buskens, Pascal1,3,4,5 (AUTHOR) pascal.buskens@tno.nl
Source: Solar Energy Materials & Solar Cells. Jun2021, Vol. 224, pN.PAG-N.PAG. 1p.
Subject Terms: Thermochromism, Reversible phase transitions, Powder coating, Metal-insulator transitions, Phase transitions, Critical temperature
Abstract: Monoclinic VO 2 (M) displays thermochromic properties based on its reversible metal-insulator transition. We studied the kinetics of the underlying structural phase transition (SPT) from monoclinic VO 2 (M) to rutile VO 2 (R) and vice versa both in powders and coatings, using isoconversional kinetic analysis based on datasets obtained through differential scanning calorimetry and UV–vis–NIR spectrophotometry. For VO 2 powders, prepared via solution-phase reduction of V 2 O 5 with oxalic acid and subsequent thermal anneal, we show that the activation energy | E a | of the SPT is temperature dependent, and decreases with increasing difference between the material's temperature and the critical switching temperature T 0. | E a | for both VO 2 (M) to VO 2 (R) and VO 2 (R) to VO 2 (M) is similar, and ranges between 138 and 563 kJ mol−1, depending on the temperature of the material. This indicates that similar defects play a key role in both SPTs. Upon doping with tungsten, T 0 was lowered from 66.93 °C (0 at. % W) to −28.38 °C (3.5 at. % W). | E a | , however, remained in the same range. Nonetheless, at W concentrations above 2 at. % a significant asymmetry was observed with higher | E a | for the switch from VO 2 (R) to VO 2 (M). The SPT of VO 2 (M) in coatings proceeded 4 times slower. This may result from the immobilization of the VO 2 domains on the substrate surface and within the coating network, making the SPT more difficult to progress. The findings of this study have important implications for the application of VO 2 (M) in energy efficient thermochromic glazing. Image 1 • Switching kinetics of VO 2 powders and coatings studied via isoconversional analysis. • Activation energy of the structural phase transition depends on the material's temperature. • Switching kinetics are symmetric for powders with ≤2 at. % W as dopant, and asymmetric for >2 at. %. • The switch in coatings proceeds approximately 4 times slower than in powders. • The switching kinetics impact the performance of these materials in thermochromic windows. [ABSTRACT FROM AUTHOR]
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Abstract:Monoclinic VO 2 (M) displays thermochromic properties based on its reversible metal-insulator transition. We studied the kinetics of the underlying structural phase transition (SPT) from monoclinic VO 2 (M) to rutile VO 2 (R) and vice versa both in powders and coatings, using isoconversional kinetic analysis based on datasets obtained through differential scanning calorimetry and UV–vis–NIR spectrophotometry. For VO 2 powders, prepared via solution-phase reduction of V 2 O 5 with oxalic acid and subsequent thermal anneal, we show that the activation energy | E a | of the SPT is temperature dependent, and decreases with increasing difference between the material's temperature and the critical switching temperature T 0. | E a | for both VO 2 (M) to VO 2 (R) and VO 2 (R) to VO 2 (M) is similar, and ranges between 138 and 563 kJ mol−1, depending on the temperature of the material. This indicates that similar defects play a key role in both SPTs. Upon doping with tungsten, T 0 was lowered from 66.93 °C (0 at. % W) to −28.38 °C (3.5 at. % W). | E a | , however, remained in the same range. Nonetheless, at W concentrations above 2 at. % a significant asymmetry was observed with higher | E a | for the switch from VO 2 (R) to VO 2 (M). The SPT of VO 2 (M) in coatings proceeded 4 times slower. This may result from the immobilization of the VO 2 domains on the substrate surface and within the coating network, making the SPT more difficult to progress. The findings of this study have important implications for the application of VO 2 (M) in energy efficient thermochromic glazing. Image 1 • Switching kinetics of VO 2 powders and coatings studied via isoconversional analysis. • Activation energy of the structural phase transition depends on the material's temperature. • Switching kinetics are symmetric for powders with ≤2 at. % W as dopant, and asymmetric for >2 at. %. • The switch in coatings proceeds approximately 4 times slower than in powders. • The switching kinetics impact the performance of these materials in thermochromic windows. [ABSTRACT FROM AUTHOR]
ISSN:09270248
DOI:10.1016/j.solmat.2021.110977