Facile fabrication of superhydrophobic polytetrafluoroethylene surface by cold pressing and sintering

A series of superhydrophobic polytetrafluoroethylene (PTFE) surfaces were prepared by a facile cold pressing and sintering method, and their microstructures and wetting behaviors could be artificially tailored by altering sintering temperature and using different masks. Specifically, the microstructures mainly depended on the sintering temperature, whereas the wetting behaviors, water contact angle (WCA) and sliding angle (SA), greatly hinged on both the sintering temperature and mask. Then a preferable superhydrophobic surface with WCA of 162 ± 2° and SA of 7° could be obtained when the sintering temperature was 360 °C and the 1000 grit abrasive paper was used as a mask. In addition, it was worth noting that the as-prepared surfaces exhibited excellent stability under UV illumination, which was the most key factor for them toward practical applications.     

Surface modification by vacuum annealing for field emission from heavily phosphorus-doped homoepitaxial (1 1 1) diamond

The relationship between field emission properties and C 1s core level shifts of heavily phosphorus-doped homoepitaxial (1 1 1) diamond is investigated as a function of annealing temperature in order to optimize surface carbon bonding configurations for device applications. A low field emission threshold voltage is observed from surfaces annealed at 800 °C for hydrogen-plasma treated surface, while a low field emission threshold voltage of wet-chemical oxidized surface is observed after annealing at 900 °C. The C 1s core level by X-ray photoelectron spectroscopy (XPS) showed a shoulder peak at 1 eV below the main peak over 800 and 900 °C annealing temperature for hydrogen-plasma treated and wet-chemical oxidized surfaces, respectively. When the shoulder peak intensity is less than 10% of the main peak intensity, lower threshold voltages are observed. This is due to the carbon-reconstruction which gives rise to a small positive electron affinity. By increasing annealing temperature, the shoulder peak ratios also increase, which indicates that a surface graphitization takes place. This leads to higher threshold voltages.     

Electronic properties of electrolyte/anodic alumina junction during porous anodizing

The growth of porous oxide films on aluminum (99.99% purity), formed in 4% phosphoric acid was studied as a function of the anodizing voltage (23-53 V) using a re-anodizing technique and transmission electron microscopy (TEM) study. The chemical dissolution behavior of freshly anodized and annealed at 200 °C porous alumina films was studied. The obtained results indicate that porous alumina has n-type semiconductive behavior during anodizing in 4% phosphoric acid. During anodising, up to 39 V in the barrier layer of porous films, one obtains an accumulation layer (the thickness does not exceed 1 nm) where the excess electrons have been injected into the solid producing a downward bending of the conductive and valence band towards the interface. The charge on the surface of anodic oxide is negative and decreases with growing anodizing voltage. At the anodizing voltage of about 39 V, the charge on the surface of anodic oxide equals to zero. Above 39 V, anodic alumina/electrolyte junction injects protons from the electrolyte. These immobile positive charges in the surface layer of oxide together with an ionic layer of hydroxyl ions concentrated near the interface create a field, which produces an upward bending of the bands.     

Preparation and properties of ZnS superhydrophobic surface with hierarchical structure

A novel ZnS hierarchical structure composed of nanorod arrays with branched nanosheets and nanowires grown on their upside walls, was synthesized over Au-coated silicon substrate via chemical vapor deposition technique. Contact angle and sliding angle of this hierarchical film with no surface modification were measured to be about 153.8° and 9.1° for 5 μl water droplets. Self-cleaning behavior and dynamic water-repelling performance were clearly demonstrated. In addition, electrowetting transition phenomenon from superhydrophobic to hydrophilic state happened when a critical bias ∼7.0 V was applied. Below this threshold voltage, the contact angle change is little. This work for the first time reports the creation of ZnS superhydrophobic surface and could enrich its research field as surface functional materials.     

Fabrication and characterization of a cotton candy like surface with superhydrophobicity

Superhydrophobic thin films were prepared on glass by air-brushing the in situ polymerization compositions of D₅/SiO₂. The wettability and morphology were investigated by contact angle measurement and scanning electron microscopy. The most superhydrophobic samples prepared had a static water contact angle of 157° for a 5 μl droplet and a sliding angle of ∼1° for 10 μl droplet. Thermal stability analysis showed that the surface maintained superhydrophobic at temperature up to 450 °C. Air trapping and capillary force on superhydrophobic behavior were evaluated.     

Transparent Superhydrophobic silica coatings on glass by sol-gel method

Wetting behavior of solid surfaces is a key concern in our daily life as well as in engineering and science. In the present study, we demonstrate a simple dip coating method for the preparation of Thermally stable, transparent superhydrophobic silica films on glass substrates at room temperature by sol-gel process. The coating alcosol was prepared by keeping the molar ratio of methyltriethoxysilane (MTES), trimethylmethoxysilane (TMMS), methanol (MeOH), water (H₂O) constant at 1:0.09:12.71:3.58, respectively with 13 M NH₄OH throughout the experiments and the films were prepared with different deposition time varied from 5 to 25 h. In order to improve the hydrophobicity of as deposited silica films, the films were derivatized with 10% trimethylchlorosilane (TMCS) as a silylating agent in hexane solvent for 24 h. Enhancement in wetting behavior was observed for surface derivatized silica films which showed a maximum static water contact angle (172°) and minimum sliding angle (2°) for 25 h of deposition time. The superhydrophobic silica films retained their superhydrophobicity up to a temperature of 550 °C. The silica films were characterized by field emission scanning electron microscopy (FE-SEM), surface profilometer, Fourier transform infrared (FT-IR) spectroscopy, thermo-gravimetric and differential thermal analysis (TG-DTA), percentage of optical transmission, water contact angle measurements. The imperviousness behavior of the films was tested with various acids.     

Improvement of corrosion resistance of NiTi sputtered thin films by anodization

Anodization of sputtered NiTi thin films has been studied in 1 M acetic acid at 23 °C for different voltages from 2 to 10 V. The morphology and cross-sectional structures of the untreated and anodized surfaces were investigated by field emission scanning electron microscopy (FE-SEM). The results show that increasing anodization voltage leads to film surface roughening and unevenness. It can be seen that the thickness of the anodized layer formed on the NiTi surface is in the nanometer range. The corrosion resistance of anodized thin films was studied by potentiodynamic scan (PDS) and impedance spectroscopy (EIS) techniques in Hank's solution at 310 K (37 °C). It was shown that the corrosion resistance of the anodized film surface improved with increasing voltage to 6 V. Anodization of austenitic sputtered NiTi thin films has also been studied, in the same anodizing conditions, at 4 V. Comparison of anodized sputtered NiTi thin films with anodized austenitic shape memory films illustrate that the former are more corrosion resistant than the latter after 1 h immersion in Hank's solution, which is attributed to the higher grain boundary density to quickly form a stable and protective passive ?lm.     

Preparation of superhydrophobic films on titanium as effective corrosion barriers

Stable superhydrophobic films were prepared on the electrochemical oxidized titania/titanium substrate by a simple immersion technique into a methanol solution of hydrolyzed 1H,1H,2H,2H-perfluorooctyltriethoxysilane [CF₃(CF₂)₅(CH₂)₂Si(OCH₂CH₃)₃, PTES] for 1 h at room temperature followed by a short annealing at 140 °C in air for 1 h. The surface morphologies and chemical composition of the film were characterized by means of water contact angle (CA), field emission scanning electron microscopy (FESEM), atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). The water contact angle on the surface of this film was measured to be as high as 160°. SEM images showed that the resulting surfaces exhibited special hierarchical structure. The special hierarchical structure along with the low surface energy leads to the high surface superhydrophobicity. The corrosion resistance ability and durance property of the superhydrophobic film in 3.5 wt.% NaCl solution was evaluated by the electrochemical impedance spectroscopy (EIS). The anticorrosion properties of the superhydrophobic film are compared to those of unmodified pure titanium and titania/titanium substrates. The results showed that the superhydrophobic film provides an effective corrosion resistant coating for the titanium metal even with immersion periods up to 90 d in the 3.5 wt.% NaCl solution, pointing to promising future applications.     

Surface texturing of polytetrafluoroethylene by hot embossing

In this study, hot embossing by reusable Ni mold with features in the form of rectangular diffraction gratings of 4 μm period was successfully employed for surface texturing of polytetrafluoroethylene (PTFE) film above the glass transition temperature of PTFE amorphous phase with the aim to enhance surface hydrophobicity. Imprint pressure was set to 0.5 MPa and it was at least tenfold lower than reported by other authors using cold stamping. Embossed gratings were clearly seen on the surface of all imprinted samples even after the annealing at 140 °C and aging for 1 month at room temperature. The best results were achieved when imprint temperature was 150 °C. Measurements of the water contact angle on imprinted PTFE surfaces have showed that increase of the average contact angle for the current test setup was 8°. Using imprint stamp with the more favorable features may lead to somewhat higher hydrophobicity.     

Characterisation of the anodic layers formed on 2024 aluminium alloy, in tetraborate electrolyte containing molybdate ions

Anodic layer growth on 2024 aluminium alloy at 70 °C, under 40 V, during 60 min, in 50 g L⁻¹ di-sodium tetraborate solution containing di-sodium molybdate from 0.1 to 0.5 M (pH 10) is examined. Anodising behaviours strongly depend on additive concentration. Development of anodic films is favoured with weak molybdate additions (<0.3-0.4 M). The film thicknesses increase and the porosity of anodic layers decreases. Molybdenum (+VI), detected by X-ray photoelectron spectroscopy (XPS) analysis, is present in the anodic films and the Mo incorporation, studied by energy dispersive spectroscopy (EDS) analysis, increases with molybdate concentration. However, for high molybdate concentrations (>0.4 M), anodising behaviour becomes complex with the formation of a blue molybdenum oxide at the cathode. The growth of aluminium oxide is hindered. As the anodic layers are thinner, the Mo(+VI) incorporation significantly decreases. These two configurations implicate different corrosion performances in 5% sodium chloride solution at 35 °C. As the alkaline anodic layer formed with 0.3 M molybdate species is the thickest and the Mo incorporation is the more pronounced, its corrosion resistance is the highest. The effect of morphology and composition of anodic films on pitting corrosion is also discussed.     

Fabrication of superhydrophobic surfaces on zinc substrates and their application as effective corrosion barriers

Stable superhydrophobic surfaces have been effectively fabricated on the zinc substrates through one-step platinum replacement deposition process without the further modification or any other post processing procedures. The effect of reaction temperatures on the surface morphology and wettability was studied by using SEM and water contact angle (CA) analysis. Under room temperature, the composite structure formed on the zinc substrate was consisted of microscale hexagonal cavities, densely packed nanoparticles layer and micro/nanoscale structures like the flowers. The structure has exhibited great surface roughness and porosity contributing to the superhydrophobicity where the contact angle could reach an ultra high value of around 170°. Under reaction temperature of 80 °C, the composite structure, on the other hand, was hierarchical structure containing lots of nanoscale flowers and some large bushes and showed certain surface roughness (maximum CA value of about 150°). In addition, an optimal superhydrophobic platinum surface was able to provide an effective anticorrosive coating to the zinc substrate when it was immersed into an aqueous solution of sodium chloride (3% NaCl) for up to 20 days. The corrosion process was monitored through electrochemical means and the results are compared with those of unprotected zinc plates.     

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).     

Evolution of steel surface composition with heating in vacuum and in air

X-ray photoelectron spectroscopy (XPS) has been used to investigate the changes in surface composition of three steels as they have undergone heating. The steels were mild steel, and two austenitic stainless steels, commonly designated 304 and 316 stainless steels. XPS measurements were made on the untreated samples, and then following heating for 30 min in vacuo and in a 1 × 10⁻⁶ Torr partial pressure of air, at temperatures between 100 °C and 600 °C.Mild steel behaves differently to the two stainless steels under the heating conditions. In mild steel the iron content of the surface increased, with oxygen and carbon decreasing, as a function of increasing temperature. The chemical state of the iron also changed from oxide at low temperatures, to metallic at temperatures above 450 °C.In both stainless steels the amount of iron present in the surface decreased with increasing temperature. The decrease in iron at the surface was accompanied by an increase in the amount of chromium at the steel surface. At temperatures above 450 °C the iron in both 304 and 316 stainless steels showed significant contributions from metallic iron, whilst the chromium present was in an oxide state. In 316 stainless steel heated to 600 °C there was some metallic chromium present in the surface layer.The surfaces heated in air showed the least variation in composition, with the major change being the loss of carbon from the surfaces following heating above 300 °C. There was also a minor increase in the concentration of chromium present on both the stainless steels heated under these conditions. There was also little change in the oxidation state of the iron and chromium present on the surface of these steels. There was some evidence of the thickening of the surface oxides as seen by the loss of the lower binding energy signal in the iron or chromium core level scans.The surfaces heated in vacuum showed a similar trend in the concentration of carbon on the surfaces, however the overall concentration of oxygen decreased throughout the heating of these steels. There were also significant changes in the oxidation state of the iron and chromium on these surfaces with significant amounts or iron and chromium present in the metallic form following heating up to 600 °C.It appears that the carbon contamination on the surfaces plays an important role in the fate of the surface oxide layer for all of the steels heated in a vacuum environment.     

Anti-icing performance of superhydrophobic surfaces

This article studies the anti-ice performance of several micro/nano-rough hydrophobic coatings with different surface chemistry and topography. The coatings were prepared by spin-coating or dip coating and used organosilane, fluoropolymer or silicone rubber as a top layer. Artificially created glaze ice, similar to the naturally accreted one, was deposited on the nanostructured surfaces by spraying supercooled water microdroplets (average size ∼80 μm) in a wind tunnel at subzero temperature (−10 °C). The ice adhesion strength was evaluated by spinning the samples in a centrifuge at constantly increasing speed until ice delamination occurred. The results show that the anti-icing properties of the tested materials deteriorate, as their surface asperities seem to be gradually broken during icing/de-icing cycles. Therefore, the durability of anti-icing properties appears to be an important point for further research. It is also shown that the anti-icing efficiency of the tested superhydrophobic surfaces is significantly lower in a humid atmosphere, as water condensation both on top and between surface asperities takes place, leading to high values of ice adhesion strength. This implies that superhydrophobic surfaces may not always be ice-phobic in the presence of humidity, which can limit their wide use as anti-icing materials.     

Effect of calcination temperature on the morphology and surface properties of TiO2 nanotube arrays

Well-ordered TiO₂ nanotube arrays were prepared by electrochemical anodization of titanium in aqueous electrolyte solution of H₃PO₄ + NH₄F at a constant voltage of 20 V for 3 h, followed by calcined at various temperatures. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and Photoluminescence (PL) were used to characterize the samples. The results showed that the as-prepared nanotube arrays before being calcined were amorphous and could transform to anatase phase at a heat treatment temperature higher than 400 °C. As the calcination temperatures increased, crystallization of anatase phase enhanced and rutile phase appeared at 600 °C. However, further increasing the calcination temperature would cause the collapse of nanotube arrays. PL intensity of the nanotube arrays annealed at 500 °C was the lowest, which was probably ascribed to better crystallization together with fewer surface defects of the nanotube arrays.     

Silane overpressure post-implant annealing of Al dopants in SiC: Cold wall CVD apparatus

We report on the successful post-implantation annealing of 4H-SiC samples that were implanted at elevated temperature with high-dose Al implants. Anneals at temperatures up to 1700 °C were conducted in a silane ambient at a process pressure of 150 Torr using a cold wall CVD reactor. A series of 30 min anneals were performed in 3% silane premixed in 97% UHP argon (Ar), which was further diluted in a 6 slm Ar carrier gas. The surface morphology of the samples was studied via plan-view secondary electron microscopy (SEM) and atomic force microscopy (AFM). The resulting surface morphology showed no evidence of step bunching or any other surface degradation. We also report the electrical characterization of Al⁺ implanted p⁺/n 4H-SiC diodes realized on samples that were annealed at 1600 °C. The current-voltage characteristics of the p⁺/n diodes and the resistivity of the implanted layer were measured at room temperature. Most of the diodes had a turn-on voltage of 1.75 V, with ideality factors of up to 1.2 and very low reverse leakage current at −100 V, corresponding to an average reverse leakage current density of (9.7 ± 0.4) × 10⁻⁹ A/cm². The breakdown voltage was near the theoretical value for an epitaxial 4H-SiC layer. However, the resistivity value of the implanted Al⁺ layer was 11 Ω cm, which was an order magnitude higher than the expected value.     

Observation of oxygen contamination at ZnSe/GaAs interfaces using SIMS

ZnSe epilayers were grown on GaAs (1 0 0) substrates using MBE. The native contamination (oxide and carbon) was removed in situ from the substrate surfaces by conventional thermal cleaning and by exposure to atomic hydrogen. A maximum substrate temperature of 600 °C was required for the thermal cleaning process, while a substrate temperature of 450 °C was sufficient to clean the substrate using hydrogen. ZnSe epilayers were also grown on As capped GaAs epilayers, which were decapped at a maximum temperature of 350 °C. SIMS profiles showed the existence of oxygen at the interface for all of the substrate preparation methods. The oxygen surface coverage at the interface was found to be 0.03% for the atomic hydrogen cleaned substrate and 0.7% for the thermally cleaned substrate.     

Fabrication of superhydrophobic silica-based surfaces with high transmittance by using tetraethoxysilane precursor and different polymeric species

The preparation of superhydrophobic silica-based surfaces via the sol-gel process through the addition of different polymeric species into the precursor solution was done in this study. The surface roughness of the films was obtained by removing the organic polymer at a high temperature, and then the hydrophobic groups were bonded onto the films with a monolayer by chemical reaction with hexamethyldisilazane (HMDS). The characteristic properties of the as-prepared films were analyzed by contact angle measurements, scanning electron microscopy (SEM), atomic force microscopy (AFM), nitrogen adsorption/desorption, and UV-vis scanning spectrophotometer. The experimental results revealed that the superhydrophobic thin films with high transmittance could easily be prepared using polypropylene (PPG), polyethylene (PEG), and poly(vinylpyrrolidone) (PVP). Surface roughness and pore size were enhanced using PPG polymeric species. The distribution of pore size was from the microporous to the mesoporous and marcoporous regions. In addition, the contact angles of the rough surfaces prepared at 500 °C without modification of HMDS were smaller than 5° but larger than 156° after modified by HMDS.     

Fabrication of superhydrophobic surfaces on zinc substrates

Stable superhydrophobic surfaces were fabricated on the zinc substrates through simple silver replacement deposition process with the modification of octadecyl mercaptan. The effects of reaction conditions on the surface morphology and wettability of the prepared surfaces were carefully studied. The results show that the fabrication of a best superhydrophobic surface depends largely on the moderate reactant concentration. When the concentration of AgNO₃ solution was 2 mmol/L, the zinc substrate was covered by a dendritic outline structure. Aggregated silver nanoparticles were formed on the substrate in accordance with some certain laws, exhibiting great surface roughness. The typical hierarchical micro-nanostructures, flower-like structures and porous structures also could be found from the SEM images. The maximal water contact angle (CA) value of about 161 ± 2°, and the minimal sliding angle (SA) of about 2° were obtained under the same reaction condition.     

Effects of annealing on morphological,structural and electrical properties of thermally evaporated WO3 thin films

Tungsten trioxide (WO₃) thin films were prepared by thermal evaporation method onto quartz substrates at room temperature. Effect of annealing temperature (from 200 to 800 °C) to morphology, crystallographic structure and electrical properties were investigated. In order to investigate the temperature dependant resistivity properties of the films dark current–voltage measurements were done at the temperatures of 30, 60, 90, 120 and 150 °C. From the AFM pictures it is seen that the increasing annealing temperature causes an increase in grain sizes. At elevated temperatures the grains combine to each other and thus form continuous and homogenous surfaces. From the XRD patterns it was seen that the as-prepared and annealed films at 200, 300, 310 and 320 °C were amorphous. On the other hand at 330 °C and higher temperatures the films were found as in crystallized structures (monoclinic phase). From the current–voltage measurements it was seen that the contacts areohmic and the current increased with increasing temperatures. From the calculated values it was seen that the produced films shows good semiconducting nature.