Codoping effects on thermochromic VO2 coatings

• Main Author: Cheng, Eg Ming
• Other Authors: Long Yi
• Format: Final Year Project
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Materials::Nanostructured materials; DRNTU::Engineering::Materials::Microelectronics and semiconductor materials::Thin films; DRNTU::Engineering::Materials::Functional materials
• Online Access: http://hdl.handle.net/10356/55859
• Institution: Nanyang Technological University

Green technology has much gained attention in recent years with increasing effects of global warming and diminishing fossil fuels in the earth. Hence, energy saving technology has been greatly concerned in the market, especially smart window coatings for office building usages. Thermochromism of VO2 has become ideal candidate for application while its phase transition temperature (τC) is the closest to room temperature. Many studies showed that single doping of element can help to improve the thermochromic properties of VO2. Some research groups have also studied the codoping effect in thermochromic properties of VO2. However, most of the study’s results unable to reach or barely reached the commercial requirements in smart window application. Thus, the aim of this project is using other doping elements to investigate the codoping effect on thermochromic properties of smart window coating materials. The chosen codoping elements are Tungsten (W) and Magnesium (Mg). Vanadium (V) oxide (V2O5), W and Mg powder are the source materials to be used in the project. The W and Mg powders were firstly dissolved in hydrogen peroxide (H2O2); the solution was then stirred at 70°C. After the powders were fully dissolved, oxalic acid (H2C2O4) powder was added into solution and waited for reaction until stabilized. PVP was added into a concentrated and blue color stable precursor for promoting adhesiveness of precursor. The precursor was dip-coated onto a fused silica substrate followed by heat treatment in tube furnace. A series of characterization and testing were executed. Phases in thin films were determined by X-ray diffraction (XRD). Field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and ultraviolet-visible light-near infrared (UV-Vis-NIR) spectroscopy were applied to find out the surface morphology, film roughness, and thermochromic properties of thin films respectively. Experiment results showed that while codoping level was increased from 0 to 3.5 at% W (with fixed 3.8 at% Mg), dense-packed nano-particles of VO2 were able to achieve; phase transition temperature (τC) was reduced from 75°C to 37.5°C and hysteresis loop was in a range of 5°C to Codoping Effects on Thermochromic VO2 Coatings ii 15°C from 0.5 at% to 3.5 at%. On the other hand, luminous transmittance (Tlum) was varying at different codoping level; solar modulating ability (ΔTsol) was decreased linearly. In conclusion, parameter constraint needs to be firmly applied and carefully control of codoping level. Codoping W and Mg had improved the thermochromism of VO2 and meet part of the commercial requirements.