Double layer SiO2/Al2O3 high emissivity coatings on stainless steel substrates using simple spray deposition system

High emissivity coatings are widely used in spacecraft and industrial furnaces, which have attracted a great attention recently due to energy saving applications. In this study, a simple spray coating method was used to produce double layer high emissivity coatings on stainless steel (SS) substrate by sol-gel process at room temperature. Initially silica (SiO2) sol, prepared using sol-gel process, was deposited on sandblasted SS substrate with required thickness, followed by deposition of aluminium oxide (Al2O3) layer. The gradual increase in the thickness of Al2O3 layer resulted in increase in the emittance. The optimized double layer Al2O3 (23 μm)/SiO2 (9.13 μm) coating on SS substrate exhibited high emittance (ε = 0.92- 0.94) and low absorptance (α = 0.30-0.34). The high emissivity coating, when exposed to a temperature of 1000 ºC in air, showed nearly the same value of emissivity. The SiO2/Al2O3 coating thus provides a simple and cost effective method for the preparation of high emissivity coatings

High performance single layer nano-porous antireflection coatings on glass by sol-gel process for solar energy applications

Antireflection coatings have received a great importance due to their ability to enhance the efficiency of the solar cells and solar selective coatings by minimizing the reflections of the incident light from the front surface. In this study, a silica (SiO2) sol, prepared using sol–gel process, was deposited on cleaned glass substrates by dip coating method and subjected to heat treatment at 400 °C. The thickness and porosity of the coating were optimized to achieve high transmittance. The thickness was optimized by varying the lifting speed of the substrate from the sol. The porosity was induced in the coating by using polymeric additives and through heat treatments. The optimized single layer SiO2 coating on cleaned glass substrate exhibited a maximum transmittance of 97.5% at λ=500 nm wavelength. The hybrid sol was found to give reproducible coatings up to a period of 30 days when stored at 16 °C. The present process provides a simple and cost effective method for the preparation of antireflection coatings, which have huge potential to enhance the efficiency of solar cells, receiver tubes and other solar devices

Dual-scale rough multifunctional superhydrophobic ITO coatings prepared by air annealing of sputtered indium tin alloy thin films

A novel method to fabricate multifunctional indium tin oxide (ITO) coatings is discussed. Superhydrophobic ITO coatings are fabricated by radio frequency balanced magnetron sputter deposition of indium–tin alloy on glass substrates followed by complete oxidation of the samples in air. The chemical nature and structure of the coatings are verified by X-ray diffraction, X-ray photoelectron spectroscopy and micro-Raman spectroscopy. Field emission scanning electron microscopic studies of the coatings display rod-like and blob-like microstructures, together with fractal-like nanostructures infused on top. Microscale roughness of the ITO coatings is measured by three-dimensional profilometry and is found to be in the range of 0.1–3 μm. Thus the presence of micro- and nano- sized structures result in dual-scale roughness. The variation in the contact angle with the deposition time is studied using a contact angle goniometer. High water contact angles (>160°) and low contact angle hysteresis (5°) are obtained at an optimum microscale roughness. The ITO coatings also exhibit other functional properties, such as low sheet resistance and semi-transparent behaviour in the visible region. The loss in the transparency of the ITO coatings is attributed to the presence of higher scale of roughness. The photoluminescence measurements show large photoemission in the visible region. It is expected that further improvements in the multifunctional properties of transparent conducting oxides will open new frontiers in designing novel materials with exotic properties

Design of Ag-Si3N4 nanocermet coatings for photothermal conversion applications

For efficient photothermal conversion in solar selective coatings, high absorptance (α) in the visible region (VIS) and low thermal emittance (ε) in the infrared (IR) region at high operating temperature are the most desirable requirements. For this, nanocermet coatings are suitable candidate materials owing to their high photothermal conversion efficiency. Here we report the preparation of Ag-Si3N4 nanocermet spectrally selective coatings using magnetron sputtering technique from the sputtering of Ag and Si3N4 targets. The field emission scanning electron microscopy of the coatings confirmed the presence of Ag nanoparticles with an average diameter of 15-25 nm dispersed uniformly in the Si3N4 matrix. High absorptance (α = 0.933) in the VIS region and low emittance (ε = 0.15 on Cu substrate) in the IR region have been achieved for the optimized nanocermet coatings. The absorptance spectra of the coatings exhibited a strong resonance peak due to the surface plasmon resonance of the Ag nanoparticles, which shifted to higher wavelength with an increase in the size of Ag nanoparticles. Thermal stability tests of these coatings revealed their stability up to 300ºC in vacuum for 2 hrs

Consequence of oxidant to monomer ratio on optical and structural properties of Polypyrrole thin film deposited by oxidation polymerization technique

This paper reports the effect of oxidant to monomer (O/M) ratio on optical and structural properties of Polypyrrole (PPy) thin film deposited by chemical oxidation polymerization technique. Noticeable changes have observed in the properties of PPy thin films with O/M ratio. Cauliflower structure have been observed in FE-SEM images, wherein grain size is observed to decrease with increase in O/M ratio. AFM results are in good agreement with FE-SEM results. From FTIR spectra it is found that, PPy is in highly oxidized form at low O/M ratio but oxidation decreased with increase in O/M ratio. Also C-C stretching vibrations of PPy ring is decreased whereas C=C stretching is increased with ratio. Absorption peak around 450 nm corresponds to π-π∗ transition and around 800 nm for polarons and bipolarons. The intensity of such peaks confirms the conductivity of PPy, which is observed maximum at low O/M ratio and found to decrease with increase in ratio. Optical band gap (BG) is found to increase from 2.07 eV to 2.11 eV with increase in the O/M ratio

Optically transparent, superhydrophobic methyltrimethoxysilane based silica coatings without silylating reagent

The superhydrophobic surfaces have drawn lot of interest, in both academic and industries because of optically transparent, adherent and self-cleaning behavior. Surface chemical composition and morphology plays an important role in determining the superhydrophobic nature of coating surface. Such concert of non-wettability can be achieved, using surface modifying reagents or co-precursor method in sol–gel process. Attempts have been made to increase the hydrophobicity and optical transparency of methyltrimethoxysilane (MTMS) based silica coatings using polymethylmethacrylate (PMMA) instead of formal routes like surface modification using silylating reagents. The optically transparent, superhydrophobic uniform coatings were obtained by simple dip coating method. The molar ratio of MTMS:MeOH:H2O was kept constant at 1:5.63:1.58, respectively with 0.5 M NH4F as a catalyst and the weight percent of PMMA varied from 1 to 8. The hydrophobicity of silica coatings was analyzed by FTIR and contact angle measurements. These substrates exhibited 91% optical transmittance as compared to glass and water drop contact angle as high as 171 ± 1◦. The effect of humidity on hydrophobic nature of coating has been studied by exposing these films at relative humidity of 90% at constant temperature of 30 ◦C for a period of 45 days. The micro-structural studies carried out by transmission electron microscopy (TEM)