Ammonium Paratungstate as Doping Source to Produce VO2 Controllable Phase Transition Powder

Vanadium oxides include V2O3, V3O5, V4O7, V5O9, V6O13, VO2 and V2O5, in which the atom ratio of oxygen to vanadium varies from 1.5 to 2.5. Among them, VO2 presents a reversible first-order metal-insulator transition (MIT) at a critical temperature (Tc). Intensive investigations have been made on the preparation of VO2 and its properties. It is generally believed that VO2 is a monoclinic structure (M), and presents semiconductive and relatively infrared transparent below Tc, whereas it transforms into tetragonal structure (R), and presents metallic and infrared reflection above Tc. These features make the VO2 suitable for the applications in intelligent energy windows coating, optical switching devices, optical data storage medium, electrodes for electrochromics, lithium batteries and supercapacitors, etc. Nevertheless, the high critical transition temperature of VO2 material (about 68 °C) limits its application.

VO2 doping with W (tungsten) is considered to be an efficacious way to overcome this disadvantage. a simple carbothermal reduction method has been presented by the simple direct mixture and calcination to prepare W-doped VO2 controllable phase transition powders, using ammonium paratungstate (APT) as the doping resource

The experimental procedures are as follows:

V1-xWxO2 (x = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) or W-doped VO2 controllable phase transition powders were synthesized by the carbothermal reduction method using the commercial V2O5 (1–10 μm, purify >99%) as the raw material, APT (purify >99%) as doping source, and C powders (<1 μm, purify >99%) as reducing agent, respectively. Firstly, amounts of APT, and C have been mixed by planetary milling with agate ball media for 6 h, in which the molar ratio of V2O5 to C for all samples was 2:1 and the molar ratios of APT were shown in Table 1. Secondly, the mixture were poured into a quartz crucible, and then the quartz crucibles have been transferred into the vacuum furnace with N2 protective atmosphere. The V1-xWxO2(x = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) solid solution powders have been obtained after annealing at 850 °C for 2 h.

A simple carbothermal reduction method has been presented by the simple direct mixture and calcination to synthesize W-doped VO2 controllable phase transition powders successfully using the commercial V2O5 as the raw material, APT as doping source, and C powders as reducing agent, respectively. Results showed that the prepared pure VO2 was a monoclinic structure and the W-doped VO2 was a tetragonal structure. The phase transition temperature of VO2 decreased gradually with increasing W-doping content. Tc of VO2 decreased gradually with increasing W-doping content by about 24 °C per W at%. Therefore, the carbothermal reduction method is a simple, high efficiency and low cost approach to synthesize VO2.

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