Haifeng Cheng

Infrared thermochromic properties of VO2 thin films prepared through aqueous sol-gel process

The stoichiometric vanadium(IV) oxide thin films were obtained by controlling the temperature, time and pressure of annealing. The thermochromic phase transition and the IR thermochromic property of 400 nm and 900 nm VO2 thin films in the 7.5 μm-14 μm region were discussed. The derived VO2 thin film samples were characterized by Raman, XRD, XPS, AFM, SEM, and DSC. The resistance and infrared emissivity of VO2 thin films under different temperature were measured, and the thermal images of films were obtained using infrared imager. The results show that the VO2 thin film annealed at 550 °C for 10 hours through aqueous sol-gel process is pure and uniform. The 900 nm VO2 thin film exhibits better IR thermochromic property than the 400 nm VO2 thin film. The resistance of 900 nm VO2 film can change by 4 orders of magnitude and the emissivity can change by 0.6 during the phase transition, suggesting the outstanding IR thermochromic property. The derived VO2 thin film can control its infrared radiation intensity and lower its apparent temperature actively when the real temperature increases, which may be applied in the field of energy saving, thermal control and camouflage.

Investigation of thermoelectric SiC ceramics for energy harvesting applications on supersonic vehicles leading–edges

Utilizing thermoelectric technology to aerodynamic heat harvesting on the leading-edge is worth noticing in the thermal protection systems. In this paper, a nose-tip model in a supersonic flow field is developed to predict the thermoelectric performance of SiC ceramics structures. The generation performance is numerically investigated in terms of the computational fluid dynamics and the thermal conduction theory. The output power and energy efficiency of the nose-tip model are obtained with Mach number varying from 2·5–4·5. The generated power reaches 1·708 W/m2at a temperature difference of 757 K at M = 4·5. With respect to the Thomson effect, the output power decreases rapidly. However, larger output power and energy efficiency would be obtained with the increase of Mach number, with or without considering the Thomson heat. Moreover, under the higher Mach numbers, larger range of output current value is available.

Facile and economical synthesis of an electrochromic copolymer for black based on electropolymerization of thiophene and 3,4-ethylenedioxythiophene

We report the facile and economical synthesis of an electrochromic copolymer for black based on electrochemical copolymerization of thiophene and 3, 4-ethylenedioxythiophene in boron trifluoride diethyl etherate. The resultant copolymer presents multicolor electrochromism with reversible color change between drab color and blue black. Furthermore, in the polar state the resultant copolymer shows strong and broad absorption in the whole visible region and then exhibits black color. The copolymer presents a transmittance variation of 25% at 522 nm, and corresponding response times for bleaching and coloration are 4.2 and 3.3 s, respectively. Good electrochemical stability can be achieved by the copolymer film, which retains 87% of its original electroactivity after 2000 cycles.

Thermophysical properties and stability of paraffin microemulsions as latent functionally thermal fluid

Paraffin microemulsions are multifunctional thermal fluid consisting of water as the continuous phase and paraffin as the dispersed phase. They can store or transfer a large amount of thermal energy by using the latent heat capacity of the paraffin during the phase transition as well as the sensible heat capacity of water. In this study, hexadecane O/W microemulsions were emulsified by an ionic surfactant SDS and a co-surfactant butanol. The phase behavior, effective stability, droplet size and thermophysical properties, such as latent heat and dynamic viscosity, were investigated experimentally. The results showed the prepared microemulsion with 30 wt.% hexadecane has expected high heat capacity about 93.4 J/g and low viscosity about 21 mpa·s. The average droplet size is 73 nm. The results indicate that they have potential applications in thermal energy storage and transfer.

Feasibility study on applying thermoelectric SiC ceramics for supersonic aerodynamic heat recovery

SiC ceramics are widely serving as ceramic composites matrix materials of high-velocity vehicles structures against extremely heating and oxidation for their sustainability at high temperature, and are also high temperature thermoelectric materials, which can transfer heat to electricity by temperature difference. Both of these advantages brought out an idea of generating electricity from aerodynamic heat by thermoelectric SiC structures on supersonic vehicles. A simple nose-tip thermoelectric module is set up, in this work, to predict the thermoelectric performance of SiC ceramics in a supersonic air flow environment (Mach number 3). The flow field parameters, temperature difference and temperature distributions of the module have been simulated by computational fluid dynamics methods. The thermoelectric performance and effect of Thomson heat were discussed. The maximum power output and energy efficiency reaches 0.0027 W and 0.0036 %, respectively, at 230 K temperature difference and a current of 0.122 A. The Thomson heat increases directly with the output current, and at a current above 0.15 A, over 50 % of the generated power has been turned back to thermal heat, resulting in the effective output power as well as energy efficiency decrease rapidly. The thermoelectric efficiency would be increased on higher-speed vehicles by enlarged temperature difference.