Inorganic Sol-Gel Method

V205 is the most commonly used inorganic precursor to prepare sol-gel V02 coatings because of its low cost and easy availability [19, 33]. In fact, the V205 sol is very stable and easy to form, simply by quenching the melted V205 in water. In the last decades, studies have focused on the effects of the procedure parameters on the quality and switching properties of the final V02 coatings, especially the postannealing treatment to reduce V2Os into V02 [24, 35,38].

In 1996, Yin et al. [30, 40] first reported the synthesis of V02 coatings by an inorganic sol-gel process by quenching melted V2Os into water to form a V2Os colloidal gel. The process included four steps: quenching, coating, drying, and vacuum annealing treatment. The effect of the drying process on the coating blistering was investigated. According to the results, the heating rate was the most important factor affecting the surface blistering. When the heating rate was less than 0.5°C min-1, no blistering was observed on the coating surface. Moreover, the effects of film thickness, quenching temperature, and annealing time on the resistivity

XRD patterns of sol-gel V2O5 films annealed in a temperature range of

Figure 10.1 XRD patterns of sol-gel V2O5 films annealed in a temperature range of (a) 500 C-600 C for 20 min and (b) 400 C-450 C for a different amount of time by rapid thermal annealing (RTA). Cited from Ref. [38].

switching property were studied as well. It was found that lower quenching temperatures were more favorable for better resistivity and switching properties and the magnitude of the resistivity change increased with increasing film thickness and annealing time as well.

In 2002, Yuan et al. [38] prepared orientated V205 thin films on a Si02/Si substrate using a V205 sol to study the valence reduction process from V205 to V02 thin films. The initial V205 films were obtained by spin coating. Then, the films were treated by various rapid thermal annealing (RTA) conditions in the temperature range of 400°C-600°C and their X-ray diffraction (XRD) patterns were collected to study the valence reduction process. From the XRD results (Figs. 10.1a and 10.1b), it was demonstrated that the reduction process of V2Os to V02 followed the sequence V2Os ->■ V3O7 -> V4O9 -*■ V60i3 -*■ V02, that is, from the phase V„02n+i to V02 [38].

Liu et al. [32, 39] prepared thermochromic V02 coatings by postannealing treatment of V2Os coatings on a polished fused quartz substrate. A thermodynamic calculation was conducted to direct experiments. The calculation results showed that V2Os decomposed into V02 at 550°C under an atmospheric pressure of less than

0.06 Pa. Therefore, they annealed the initial V2Os coatings at a pressure of 0.05 Pa at 450°C, 500°C, 550°C, and 600°C for 6 h

Hysteresis curves of the resistance and IR emissivity of the V0coatings on quartz with thickness of

Figure 10.2 Hysteresis curves of the resistance and IR emissivity of the V02 coatings on quartz with thickness of (a), (c) 400 nm and (b), (d) 900 nm annealing at 550"C for 10 h under air pressure of 0.05 Pa. Cited from Ref. [39] and reproduced with permission from Springer

and for annealing times ranging from 2 h to 14 h at 550°C. V205 and V307 were still observed for the samples annealed at 450 C and 500°C for 6 h. The V02 phase appeared at 550°C, with the absence of V205. Pure polycrystalline V02 coatings formed at 600 C for 6 h. Considering the high-temperature-induced coarse surface at 600°C, the V02 coatings were synthesized at 550°C for various time. Pure V02 coatings were obtained by annealing the sample at 550 C for 10 h. Furthermore, the effect of film thickness on resistance switching and IR chromic properties was investigated. V02 coatings with thicknesses of 400 nm and 900 nm on fused quartz substrates were prepared by annealing at 550' C for 10 h under an air pressure of 0.05 Pa. As shown in Figs. 10.2c and 10.2d, the resistance of the 400 nm coatings dropped by ~3 orders of magnitude at 68°C and the half-maximum width of the hysteresis cycle was about 7°C, while the V02 coatings of 900 nm exhibited a resistance change of over 4 orders of magnitude at 68 C and a half-maximum width of only

SEM images of annealed V0s coatings

Figure 10.3 SEM images of annealed V20s coatings (~150 nm in thickness) at (a) 300°C, (b) 500 C, and (с) 700°C. AFM images of V02 films annealed at 500' C with thicknesses of (d) 30 nm, (e) 150 nm, and (f) 320 nm. Hysteresis loops of the IR (Л = 25 pm) transmittance against temperature for V02 films with thicknesses of (g) 30 nm, (h) 150 nm, and (i) 320 nm. The insets show the derivative of the temperature dependence of transmittance (dTr/dT). Cited from Ref. [34].

5°C. Accordingly, better IR chromic property (Figs. 10.2e and 10.2f) was observed on the thicker V02 coatings with a higher IR emittance variability of 0.6 and a narrower hysteresis half-maximum width of 3°C than on 400 nm V02 coatings (0.5 in IR emittance variability and a hysteresis half-maximum width of 4°C) [39].

Shi et al. [34] designed a series of experiments to investigate the effect of annealing temperature, annealing time, and film thickness on the grain size and morphology of the final V02 coatings. Dip coating was used to prepare the initial V2Os coatings on a Si substrate, and postannealing treatment was conducted in a N2 atmosphere. Figure 10.3a-c shows the SEM images of the coatings annealed at 300°C, 500°C, and 700°C, respectively. Extremely small grains were observed on the coatings annealed at 300°C, where only slight crystallization occurred. As the annealing temperature was increased to 500°C, a homogeneously distributed coating with the grain size of 120 nm formed. With a further increase in the annealing temperature, to 700°C, a film assembled by agglomerated nanoparticles with the grain size of 30 nm was prepared, which was explained by the recrystallization of the films at high temperatures. Subsequently, V02 films were prepared at 500 C for different annealing times (20 min, 40 min, 60 min, and 90 min). No obvious effect of the annealing time was observed on the grain size, morphology, and phase structure of the films, but annealing for a longer length of time proved to be favorable for the formation of a more stoichiometric V02 phase. The effect of thickness on surface morphology was observed. AFM images of 30 nm, 150 nm, and 320 nm thick films in Fig. 10.3d-f indicate that the grain size becomes smaller as the film thickness increases, along with the evolution of a compact and homogenous film. Moreover, the IR thermochromic properties of the V02 films of different thicknesses were compared, as shown in Fig. 10.3g-i. It was found that as the film thickness increased, the ability of the films to modulate the IR transmittance gradually improved and the transition temperature dropped.

Li et al. [35] further investigated the stability of V02 thermochromic coatings prepared by the inorganic sol-gel method on exposure to air. The as-prepared V02 thermochromic coatings were treated by annealing in air at different temperatures and for different lengths of time, and the morphologies and IR switching properties of the resulting coatings were compared. Figure 10.4a- g shows the SEM images of as-prepared V02 coatings and coatings annealed at 200 C, 250°C, 300°C, 350°C, 370°C, and 400°C in air for 20 min, respectively. The grain growth was observed for the V02 coatings obtained at 300°C. And the grainy structure became more obvious, with a reduction in the mesoporous structure. When the annealing temperature was over 300°C, the grain size continued to increase and the crystals varied from grainy to a bar-like structure,

SEM images of (a) as-prepared V0 coatings and coatings after annealing at

Figure 10.4 SEM images of (a) as-prepared V02 coatings and coatings after annealing at (b) 200 C, (c) 250=C, (d) 300 C, (e) 350 C, (f) 370 C, and (g) 400 °C in air for 20 min. (h) The air annealing temperature and (i) annealing time (350' C) evolution of the increased tc and the variation in the 1R switching efficiency. Cited from Ref. [35].

resulting in more compact coatings. Notably, a new phase appeared when the annealing temperature was above 300°C, indicating the oxidation of V02 into V205 in air. The response to these changes in morphology and phase structure, the transition temperature (rc), and the IR switching efficiency varied. It was exhibited (Fig. 10.4h,i) that Tt increased and the IR switching efficiency decreased as the annealing temperature rose. Moreover, similar results were obtained with increasing annealing time at 350’C. These results indicate that V02 thermochromic coatings are quite stable in air below 200°C and further increase in temperature induces the oxidation of V02, along with a deterioration in the IR thermochromic properties.

 
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