Doctor of Philosophy
Chemical and Biochemical Engineering
Heating and cooling buildings consumes a tremendous amount of energy worldwide, often >50% for Northern climates such as Canada. A major part of this energy is lost through windows. The use of thermochromic windows provides a new intriguing option, in which the window automatically regulates the amount of solar transmission in response to environmental temperature changes. Of the potential thermochromic materials, vanadium dioxide (VO2) is known to display a fully reversible semiconductor-metal transition (SMT). VO2 responds thermochromically to environmental temperature changes by changing its crystal structure as a function of temperature. VO2 is transparent to infrared radiation (IR) when its temperature is below a so-called phase transition temperature (Tc), but is IR light reflective above Tc, while retaining visible light transmittance. Such an intrinsic property makes VO2 an attractive material for designing a new generation of "smart" thermochromic coatings to efficiently utilize solar energy. The main goal of this dissertation focuses on developing innovative VO2 based particles and their integration into polymeric smart coatings using an economical and green chemistry approach, while examining the assemblies optical and thermal properties for potential application in smart windows.
Water as a “green” solvent was examined in Ch. 2 using ammonium metavanadate as a low toxicity VO2 source, which was reduced by aspartic acid, for the novel synthesis of monoclinic vanadium dioxide [VO2(M)]. In this chapter, various experimental parameters including annealing temperature, annealing time, anti-oxidation time, reagent concentration and W(tungsten) doping concentration were examined. This study examined finding the optimum conditions for synthesising an oxidation stable, W-doped, monoclinic vanadium oxide exhibiting a low Tc with small hysteresis. Meanwhile, the luminous transmittance and solar modulation ability of VO2 based films was found to be enhanced simultaneously, an excellent result for potential commercial usage. The molar ratio of ammonium metavanadate and aspartic acid was varied systematically as well as the annealing temperature and reaction time. An aspartic acid/NH4VO3 molar ratio of “0.7/1” and “0.6/1”, calcined at 800 °C for 2 h was found to deliver the purest VO2(M) phase. Optimized VO2(M) was then doped with tungsten (W) with nominal concentrations up to 4 at. %, with the annealing conditions optimized. The lowest transition temperature of 53.6 °C was found for the 3% W-nominal doping concentration using an 800 °C annealing temperature for 2 h. This material was shown to be stable against oxidation and showed an integrated luminous transparency (Tlum) of 68% at 22 °C and a solar modulation efficiency (ΔTsol) of 20.7% between 22 °C and 80 °C in a polyvinylpyrrolidone (PVP) coating. Additionally, a pure VO2(M) phase was achieved at lower annealing temperatures (i.e. 450 °C) and shorter times with the addition of W or PVP, which was not possible with the undoped sample. In general, these coatings showed better optical properties compared to most films reported in the literature, although their VO2 transition temperature was still relatively high, not near room temperature. Window coatings for thermochromic glass at higher switching temperatures are also of industrial interest for solar thermal applications, making these results useful for the scientific community.
To make small particles with low Tc, a hydrothermal method was applied. Ch. 3 investigated reducing ammonium metavanadate (NH4VO3) with hydrazine in water using a hydrothermal method, followed by a relatively brief time heat treatment (calcination). A high yield of VO2(M) crystals was obtained using this synthesis methodology. The reaction parameters (concentration, temperature and time) were examined to optimize the synthesis conditions of VO2 (M) using a parametric approach. Then, the influences of the calcination parameters including temperature and ramping rate were examined on the size, morphology, phase purity and phase transition temperature of Mo and W-doped VO2 (M). Various nominal Mo and W contents from 0 to 4 at. % were investigated. The results showed that flower-like and stick-shaped morphologies of W-doped VO2 and Mo-doped VO2 particles were obtained, respectively. It was found that tungsten doping was more effective for reducing the phase transition temperature of VO2 (68 °C) to an ambient temperature (23 °C) compared to molybdenum doping (54.4 °C). Furthermore, a high transition reduction efficiency of 23 K/at. % was obtained for the W-doped VO2 crystals. The thermochromic properties of VO2/PVP coatings on glass were investigated with the un-doped coatings exhibited high infrared modulation (up to 35% at 2000 nm), and simultaneously high visible light transmittance (> 62%). For the 1at. % W-doped coatings, high infrared modulation was obtained (up to 28% at 1500 nm) with high visible transmittance (> 75%). This could open an economical route to large-scale VO2 (M) particles synthesis to produce low cost smart windows.
To further improve the chemical stability and thermochromic performance of VO2, Ch. 4 examines a novel approach for the rapid synthesis of VO2/SiO2 composite structure. In this study, ammonium metavanadate was reduced with maleic acid using water as the green solvent, with VO2/SiO2 obtained at various (Si/V) molar ratios. The effect of SiO2 addition on the size, dispersion, chemical stability and thermochromic performance of VO2 was investigated. This study reveals that the SiO2 significantly improved the anti-oxidation stability of VO2 in air at room temperature for testing times up to seven months. Further, the infrared (IR) contrast transmittance displayed a VO2/SiO2 composite film which demonstrated an excellent IR switching quality (20%) that was 4X greater than the plain VO2 film (5%), while maintaining high visible transmittance (70%) at the wavelength of 680 nm. Moreover, using a reflux approach at low temperature reaction, the phase transition temperature (Tc) of 2 at. % W-doped VO2 was reduced considerably to 23 °C, when tungstic acid (as the W-doping source) was pre-dissolved in hydrogen peroxide (H2O2) prior to use.
To examine a low temperature approach to making VO2(M), Ch. 5 examines a novel synthetic approach at room temperature (RT) using hydrolysis for preparation of tungsten W-doped VO2 particles. The hydrolysis of vanadyl acetylacetonate [VO(acac)2] was achieved at RT by adding HCl as a catalyst, converting the green aqueous solution into a blue one, indicating the formation of VO2(M). In addition to examining different drying methods, various experimental parameters including concentration, reaction time, annealing time and W doping concentration were investigated. The W-doping system is different from the high temperature hydrothermal process usually reported in the literature. The W-doped VO2 NPs were found to exhibit a low semiconductor-metal phase transition temperature (SMT). It was discovered that small NPs of vanadium (V) oxide, present in the formed product, could enhance the dispersion of VO2(M) (IV) particles in a polyvinylpyrrolidone (PVP) matrix. By implementing this dispersion strategy, a VO2(M)/PVP coated glass with an excellent infrared (IR) switching efficiency (40%) and good visible transmittance (35%) was accomplished. In addition to PVP, hydrophobic poly(methyl methacrylate) (PMMA) and hydrophilic poly(4-vinylpyridine) (P4VP) were inspected as matrix polymers to achieve weather resistant films and films with good dispersion of VO2 particles, respectively. Additionally, the effect of variable film thickness with controlled ratios of VO2/PMMA and VO2(M)/P4VP on the optical performance of the films was investigated. VO2(M)/P4VP based-film demonstrated an excellent IR switching ability (47%) with great visible transmittance (41%) compared to (30%) and (29%) of VO2/PMMA, respectively. The better performance is ascribed to the uniform dispersion of particles in P4VP. Importantly, the synthetic method is a green chemistry approach and scalable, which can potentially enable a large scale production of VO2.
Zomaya, Dicho, "Synthesis of Chemically Stable VO2 Particles Using Green Solvents and their Application as Smart Coatings" (2019). Electronic Thesis and Dissertation Repository. 6089.