Organic Sol-Gel Method

Generally, the chemistry of an organic sol-gel method involves the hydrolysis and condensation of an organic precursor to form a sol, the aging process for the formation a vanadium polymer gel, and the postannealing process to obtain crystalline V02 coatings [26,36,37]. Various vanadium precursors have been used in the organic sol-gel process to synthesize V02 coatings, such as vanadium alkoxides [29], VO(acac) [26], and inorganic V205, etc. [24].

A series of novel vanadium dioxide nanostructures, including metastable monoclinic V02(B) nanoneedles and nanocorals, orthorhombic V02(0) nanoparticles, and monoclinic V02(M) nanofacets, were successfully prepared by Wang and Chen [41] by controlling the aging time and the postannealing conditions in the sol-gel process with VO(acac)2 as the precursor. Table 10.2 [41] exhibit the aging and post-heat treatment conditions to obtain the four V02 nanostructures.

Wu et al. [24] presented an organic sol-gel method to fabricate thermochromic V02 coatings on mica substrates by using low-cost V205 powders, isobutyl alcohol, and benzyl alcohol as the raw materials. This process was classified as an organic sol-gel process because of the generation of an organic vanadium precursor and

Table 10.2 The aging and post-heat treatment conditions to obtain various V02 nanostructures, including V02(B) nanocorals, nanoneedles, V02(0) nanoparticles, and V02(M) nanofacets

Aging treatment

Heat treatment

Products

#1 Aging for 24 h

550 C for 30 min + 600 C for 5 min in H2/N2

V02(B) nanocorals

#2 No aging

550 C for 30 min + 600 C for 5 min in H2/N2

V02(B) nanoneedles

#3 No aging

550 C for 30 min + 600 C for 5 min in vacuum

V02(0) nanoparticles

#4 No aging

#2 + 600°C for 4 h + air annealing thermal treatment in ambient atmosphere

V02(M) nanofacets

subsequent hydrolysis and condensation of the organic precursor to form an organic sol. The chemical reactions involved are listed as follows [24]:

1. Generation of an organic precursor

2. Hydrolysis reaction of the organic precursor solution

3. Condensation of the organic hydrolysis product

The effect of annealing temperature on the thermochromic properties of the as-deposited VO2 coatings was investigated in the annealing temperature range of 440ЭС-540°С with an interval of 20°C. The thermo-optical hysteresis curves with inset dTr/dT-T plots for the V02 films annealed between 440' C and 540°C are exhibited in Fig. 10.5a-f, respectively. It was found that the coatings annealed at 500°C exhibited not only the narrowest hysteresis width (Дтс), of 8°C, compared to those of the other coatings (~18°C) but also the highest IR modulation ДТШ, of 73%, and transmittance switching from 76% at 40°C to 3% at 90 C.

Yu et al. [26] studied the optical switching property of a V02 thin film fabricated using a VO[acac)2 precursor via a sol-gel method by varying the thermal aging time of the precursor. They found that the aging process significantly improved the coating ability with enhanced adhesivity. Moreover, the morphology and optical switching property of the deposited films were affected. Figure 10.6a-d shows the SEM images of the V02 coatings evolved under different aging conditions. It was observed that the aging process significantly affected the morphology of the coatings. Nonaged coatings were composed of sparse particles, with a low density. In contrast, as the aging time increased, coral-like grains appeared and compact coatings were formed. Furthermore, the effects of

Hysteresis loop of transmittance at 4.3 pm versus temperature with inset dTr/dT-T plots for the V0 films annealed between 440 C and 540 C

Figure 10.5 Hysteresis loop of transmittance at 4.3 pm versus temperature with inset dTr/dT-T plots for the V02 films annealed between 440 C and 540 C: (a) 440 C, (b) 460"C, (c) 480 C, (d) 500 C, (e) 5205C, and (f) 540 C. Cited from Ref. [24].

aging time on the optical switching properties of V02 coatings were investigated. As shown in Figs. 10.6e and 10.6f, the visible transmittance (7’[urn) at 253C decreased from ~90% to ~45% with the aging time increasing from 0 day to 7 days, while the IR switching efficiency (ДГгооопт) of the coatings aged for 7 days reached a maximum value of 51%.

SEM images of postannealed V0 coatings fabricated with aging time variations

Figure 10.6 SEM images of postannealed V02 coatings fabricated with aging time variations: (a) Nonaged, (b) 3 days, (c) 5 days, and (d) 7 days, (e) The optical transmittance spectra and (f) the MIT efficiency and visible transmittance of V02 coatings evolved with aging time variations. The MIT efficiency was calculated by the difference at high temperature and room temperature at 2000 nm. Visible transmittances were measured by room temperature value at 600 nm. Cited from Ref. [26].

 
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