Phase transition and IR properties of tungsten-doped vanadium dioxide nanopowders

doi:10.1016/j.jallcom.2009.01.092

Journal of Alloys and Compounds

Volume 480, Issue 2, 8 July 2009, Pages 537-540

https://doi.org/10.1016/j.jallcom.2009.01.092 Get rights and content

Abstract

Thermochromic vanadium dioxide exhibits a semiconducting to metallic phase transition at T_t = 341 K, involving strong variations in electrical, magnetic, optical transmittance. Tungsten-doped vanadium dioxide nanopowders were synthesized by thermolysis with slight improvement and active white powdery tungstic acid (WPTA) used as a substitutional dopant. The results show that the phase transition temperature of the doped VO₂ powders were decreased to 298.6 K, which is very close to the room temperature. Tungsten-doped has enhanced the IR properties of VO₂ nanopowders because the contrast of its IR transmission below and above the room temperature is up to 92% in our experiment.

Introduction

Vanadium oxides compounds (V₂O₃, V₂O₅, V₆O₁₃, etc.) present a first order phase transition [1] and exhibit a semiconducting to metallic phase transition at their transition temperatures, involving strong variations in electrical, magnetic, optical transmittance in the infrared region. Among these oxides, VO₂ has attracted much attention and been extensively studied [1], [2], [3], because its transition temperature, T_t, is close to room temperature (T_t < 341 K) [4]. And it can be considered as a very good candidate for a lot of switching applications: thermal sensors, smart IR optical windows [5], [6], [7], smart IR emissive coatings [8], and switching electrical resistances [9].

The VO₂ material, for example, exhibits infrared transmission with a monoclinic structure at T < T_t, where T is the ambient temperature. It becomes, however, infrared reflection and has a tetragonal rutile structure at T > T_t. According to described above, it is required for a smart window. In winter, it allows infrared solar transmittance and keeps the indoor warm; in summer, the smart window VO₂ coating blocks infrared solar transmittance and makes the indoor cool. In addition, a “smart window” used in international safeguard satellites could help protect sensitive optical surveillance systems from accidental damage or even sabotage [10]. If we could make use of VO₂ for the smart windows and in automobiles, electricity consumption can be lowered by 30%, as well as other fuels conserved [11], [12], [13], because about 50% of the total solar energy is distributed to the infrared spectral range.

To make VO₂ effective as an intelligent window material, it is desirable to lower the transition temperature from 341 K to near room temperature for practical applications [11], [12], [13], [14], [15]. Doping studies have shown that the transition temperature can be altered by the incorporation of metal ions into the VO₂ lattice [16], [17]. It has been found that the most effective metal ion is tungsten [18], because it produces reversely large T_t shifts for small dopant concentrations [19].

In this paper, we have slightly improved the method of thermolysis to prepare tungsten-doped vanadium dioxide nanopowders using white powdery tungstic acid (WPTA) as the dopant. The process can be easily applied to commercial production lines due to its short cycle period and simple operation, and obtained a better result to reduce transition temperature. We have reduced the phase transition temperature to 298.6 K, which is very close to room temperature. And at 293 K and 303 K, the contrast of the semi-metal states of the transmittance(ΔT) is measured being 92%. Such doped VO₂ nanopowders have a potential to be used as smart windows materials.

Section snippets

Experimental

Undoped and tungsten-doped vanadium dioxide were prepared by the reference [20]. We slightly improved the method. Hydrazine hydrogen chloride was added with less amount (1.00 g) and without solution when prepare vanadyl chloride (VOCl₂) solution.15 g ammonium bicarbonate (NH₄HCO₃) was dropped into the VOCl₂ solution with stirring (as the VO²⁺ is stable in the acidity condition) when prepare the precursor of nanopowders.

X-ray diffraction analyses

The XRD patterns of W-doped VO₂ particles with various W contents are presented in Fig. 1. It can be seen from Fig. 1that all the W-doped VO₂ are well crystallized and consistent with JCPDS 43-1051, even the W atomic percent is up to 7.21%. But for powders with dopant level higher than 8.02%, extra reflections are seen in the X-ray diffraction patterns which do not correspond to any vanadium oxide phase. This is most likely because some of the tungsten go towards the formation of tungsten

Conclusions

Well-crystallized and narrow size distribution nanopowders of vanadium dioxide and tungsten-doped vanadium dioxide were successfully synthesized by thermolysis method with white powdery tungstic acid used as a substitutional dopant. The micro-structure was studied in detail by XRD and TEM. A crunode at 8.02% W doping for W-doped VO₂ particles can be observed from XRD with W contents up to 10.03%. According to the curves of DSC, the transition temperature of the W-doped powders is reduced to

Acknowledgments

This work is supported by Shanghai Board of Education Fund under contract/grant number CL 200810.We thank Professor Lehan Wei for the supply of a constant current source.

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Phase transition and IR properties of tungsten-doped vanadium dioxide nanopowders

Abstract

Introduction

Section snippets

Experimental

X-ray diffraction analyses

Conclusions

Acknowledgments

Thin Solid Films

Sol. Energy

Thin Solid Films

J. Solid State Chem.

Thin Solid Films

Sol. Energy Mater. Sol. Cells

Thin Solid Films

Mater. Res. Bull.

J. Solid State Chem.

Thin Solid Films

Opt. Mater.

Mater. Sci. Eng. B

J. Chem. Phys.

J. Am. Chem. Soc.