Enhanced photocatalytic activity of TiO2 thin film coating on microstructured silicon substrate

Photocatalytic TiO2 thin film is prepared by sol–gel technique on microstructured silicon substrate produced by femtosecond laser cumulative irradiation. The photocatalytic activity is evaluated by the degradation of methylene blue (MB) solution under ultraviolet (UV) irradiation. For 6-ml MB solution with initial concentration of 3.0×10~-5 mol/L, the degradation rate caused by TiO2 thin film of 2-cm~2 area is higher than 70% after 10-h UV irradiation. Microstructured silicon substrate is found to enhance photocatalytic activity of the TiO2 thin film remarkably. The femtosecond laser microstructured silicon substrate is suitable to support TiO2 thin film photocatalysts.     

Influence of binder viscosity on the control of infrared emissivity in low emissivity coating

Low emissivity is the complex system and polymer binder is one of the most important factors that affect optical and mechanical properties of the coating. Low infrared emissivity coatings were prepared by using flake aluminum particles and three types of polymer resins as fillers and binders, respectively. The influence of polymer binder viscosity on pigment particles distribution, surface morphology and infrared emissivity of the coating was systematically investigated. The results indicate that infrared emissivity of the coating can be strongly affected by the resin viscosity at the same preparation condition, which induces different aluminum particles distribution and surface morphology of the coating. Low resin viscosity is helpful for aggregating pigments and reducing the top polymer layer thickness near the surface, thus the infrared emissivity is reduced. If the resin viscosity value is decreased by two orders of magnitude, the infrared emissivity values would be reduced as much as 0.2. Additionally, a theoretical model is proposed to account for this mechanism, which indicates that sedimentation, evaporation and diffusion play important roles in forming different aluminum particles distribution during the drying process of the coating.     

Broad band enhanced infrared light absorption of a femtosecond laser microstructured silicon

Absorptive properties of surface-structured silicon prepared by femtosecond laser pulses irradiating in SF₆ or N₂ are measured in a wide wavelength range of 0.3–16.0 μm. The SF₆-prepared surface-structured silicon shows enhanced light absorptance up to 80% or more in the entire measured wavelength range. The absorptance for N₂_prepared surface-structured silicon in the wavelength range of 9–14 µm is similar to that of a SF₆-prepared sample, although it decreases to about 30% in the wavelength range of 2–7 µm. Light absorption varies with the height and density of the spikes formed on silicon surfaces.     

Effects of annealing time on infrared emissivity of the Pt film grown on Ni alloy

Platinum films were sputter-deposited on polished nickel alloy substrates. The platinum thin films were applied to serve as low-emissivity layers to reflect thermal radiation. The platinum-coated samples were then heated in the air at 600 °C to explore the effects of annealing time on the emissivity of platinum films. The results show that the grain size of the Pt films increased with the increasing annealing time while their dc electrical resistivity decreased. Besides, the IR emissivitiy of the films gradually decreased with the increasing annealing time. Especially, when the annealing time reached 150 h, the average IR emissivity at the wavelength of 3-14 μm was only about 0.1. Moreover, the chemical analysis indicated that the Pt films on Ni-based alloy exhibit a good resistance against oxidation at 600 °C.     

飞秒激光微构造硅光电二极管的光电响应

在一定条件SF6气体氛围中,硅可在飞秒激光辐照区产生m量级的尖峰结构。针对不同尖峰高度的微构造硅,在不同温度下退火,采用电子蒸发的方法在正反面分别镀上铝电极,制备出了飞秒激光微构造光电二极管,并测试了其光电响应。实验结果表明：飞秒激光微构造光电二极管的响应随微构造硅光电二极管的尖峰高度和退火温度的不同而不同。尖峰高度为3～4 m的样品在973 K温度退火30 min后,响应度可达0.55 A/W。即使在1100 nm波长处,这种新型的硅光电二极管的响应仍可高达0.4 A/W。     

Characterization and application of cerium fluoride film in infrared antireflection coating

Cerium fluoride (CeF₃) thin films were evaporated to the germanium substrates at different substrate temperature from 100 °C to 250 °C. Structural and optical properties were characterized by X-ray diffraction (XRD), scan electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The morphology of samples deposited at different temperature can be closely related to preferred orientation. The infrared optical constants were obtained by fitting the transmission spectrum using Lorentz oscillator model. A simple example for fabrication of long-wave infrared broadband antireflection coating was also presented.     

Monte Carlo uncertainty simulation of surface emissivity at ambient temperature obtained by dual spectral infrared radiometry

The Monte Carlo statistical method was used to estimate the uncertainty of the emissivity, obtained by dual spectral infrared radiometry, of gray surfaces at ambient temperature. A large number of simulations were carried out, by selecting a wide range of values from parameters that directly affect the resulting emissivity: the emissivity of the target surface, the background temperature of the surrounding surfaces, the maximum error of the detectors, and the width and spectral location of the bands of the detectors.In summary, the level of uncertainty grows very quickly as the background temperature approximates the target surface. In terms of the spectral characteristics of the detectors, it was found that narrow bands, far apart from each other, and short wavelength bands provide the lowest uncertainty from all simulated emissivity values.     

Influence of electrolyte temperature on properties and infrared emissivity of MAO ceramic coating on 6061 aluminum alloy

6061 aluminum alloy was treated by MAO at various temperatures of the alkali silicate electrolyte using pulsed bipolar current mode for ten minutes. The surface microstructures and properties were studied using SEM, EDX, and XRD. The infrared emissivities of the MAO ceramic coatings were measured at the 70 °C using FTIR spectrometer. The electrolyte temperature strongly affected all the surface properties. The MAO alumina ceramics prepared in cold electrolytes have volcano-like and accumulated particles microstructures, while those prepared in hot electrolytes were: rougher, thinner and contained grainy spherical hollow bulgy microstructures with more pore density and more sillimanite and cristobalite phases which enhanced the IR emissivity. Also, the increment of sillimanite and cristobalite phases moved the apparent peaks toward longer wavelengths, and broadened the opaque region of the IR spectra. As a result, the increment of electrolyte temperature from 12.3 °C to 90.5 °C increased the average of LWIR emissivity from 80.4% to 94.4%, respectively, for the MAO ceramic coatings.     

Investigation of the effects of Ni-based alloy K465 on the normal spectral emissivity during oxidation

The normal spectral emissivity of Ni-based alloy K465 during oxidation is experimentally measured at 810, 914 and 998 °C for 12 h in air over the wavelength from 1.3 to 2.4 μm. The combined standard uncertainty of the normal spectral emissivity is less than 3%. The oscillations of the emissivity and the effects of oxidation temperature, heating time and wavelength on the emissivity are investigated. The oscillations of the emissivity are formed by the interference effect between the radiation from the surfaces of the substrate and the oxidation film. The oscillation extremums of the emissivity shift towards larger wavelengths as the oxidation process proceeds. The results show that the normal spectral emissivity increases as the temperature increases at the initial time. The normal spectral emissivity decreases as wavelength increases except for the occurrence of the oscillations of the emissivity. The normal spectral emissivity increases rapidly at the initial heating time, and the change of emissivity becomes slow when the oxidation tends to be saturated gradually. Besides, the emissivity fitting models versus heating time and wavelength are established, which fit the experimental results very well. The emissivity relative errors of the fitting models are less than 4%.     

Apparent directional spectral emissivity determination of semitransparent materials

An inverse estimation method and corresponding measurement system are developed to measure the apparent spectral directional emissivities of semitransparent materials. The normal spectral emissivity and transmissivity serve as input for the inverse analysis. Consequently, the refractive index and absorption coefficient of the semitransparent material could be retrieved by using the pseudo source adding method as the forward method and the stochastic particle swarm optimization algorithm as the inverse method. Finally, the arbitrary apparent spectral directional emissivity of semitransparent material is estimated by using the pseudo source adding method given the retrieval refractive index and absorption coefficient. The present system has the advantage of a simple experimental structure, high accuracy, and excellent capability to measure the emissivity in an arbitrary direction. Furthermore, the apparent spectral directional emissivity of sapphire at 773 K is measured by using this system in a spectral range of 3 μm–12 μm and a viewing range of 0?–90?. The present method paves the way for a new directional spectral emissivity measurement strategy.     

Laser pulse transient method for measuring the normal spectral emissivity of samples with arbitrary surface quality

A laser pulse transient method for measuring normal spectral emissivity is described. In this method, a laser pulse (λ=1064 nm) irradiates the top surface of a flat specimen. A two-dimensional temperature response of the bottom surface is measured with a calibrated thermographic camera. By solving an axisymmetric boundary value heat conduction problem, the normal spectral emissivity at 1064 nm is determined by using an iterative nonlinear least-squares estimation procedure. The method can be applied to arbitrary sample surface quality. The method is tested on a nickel specimen and used to determine the normal spectral emissivity of AISI 304 stainless steel. The expanded combined uncertainty of the method has been estimated to be 18%.     

Experimental and theoretical studies of the evolution of normal spectral emissivity during successive steps of tungsten oxidation

We studied the influence of oxidation on the normal spectral emissivity of tungsten in the wavelength range 1–10 μm under well-controlled conditions. The optical measurements have been performed under an oxygen pressure of 1.3 Pa and in the temperature range 800–1000 K, until the oxide was thick enough to be opaque to the radiation concerned. The experimental curves of emissivity versus exposure show an induction time followed by a maximum before a steady-state value is reached. In conjunction with a growth model, supported by X-ray diffraction and electron microscopy studies, we propose an optical model of heterogeneous media which correctly accounts for the observed variations of emissivity during the whole reaction. By inserting the optical constants of the various media into this model, we show that it is possible to characterize, by emissivity the morphology of the oxide on the surface during the reaction, until the oxide is thick enough to be opaque.     

Effect of Au film and absorption groups on minority carrier life of porous silicon

In this paper, the samples were prepared with or without metal assistance in different time. The effect of Au film and absorption groups on minority carrier life of porous silicon was studied by using μ-PCD measurements, and the result was further proved by FTIR spectra and SEM images. The results were showed that the existed hydrogen-related groups on the surface of porous silicon would decrease the minority carrier life and the grid Au film also can decrease the minority carrier life. The minority carrier life may be controlled by the more effective factor, metal elements or absorption groups.     

High-temperature spectral emissivity of SiC in the IR range

A three-layer optical model is proposed for the IR radiator based on SiC with the help of which the SiC emissivity is calculated at a temperature 1650 K and the bands in the 9–25-μm region are interpreted.     

Low infrared emissivity of polyurethane/Cu composite coatings

Polyurethane/Cu composite coatings with low infrared emissivity near to 0.10 at the wavelength of 8-14 μm were prepared by a simple and convenient process. The influences of the content of Cu powder, surface roughness, coating thickness and temperature on infrared emissivity of the coatings were systematically investigated. The results indicated that the emissivity decreases significantly with increasing content of Cu powder and coating thickness. The coatings with smooth surface exhibit lower emissivity values than those with rough coatings. Moreover, we found the relationship between the emissivity of coatings and temperature presents a “U” type, and the emissivity reaches to the minimum at about 380 K. The mechanisms of low emissivity were proposed by optical theories, which are found to be in a good agreement with the experimental results.     

Resistive switching in Au/TiO2/Pt thin film structures on silicon

The conditions and mechanisms of preliminary treatment in strong electric fields (forming) and subsequent resistive switching in Au/TiO₂/Pt thin-film structures on silicon were investigated. The thin TiO₂ films in these structures were prepared by different methods, namely, vacuum evaporation of metallic titanium followed by thermal oxidation in air and radio-frequency cathode sputtering of titanium dioxide from a powder target. The current-voltage and voltage-capacitance characteristics of the structures, as well as the dependences of their conductivity on the time of exposure to a dc voltage of different polarities and on the temperature were measured. The data obtained permitted the conclusion that the physical mechanism underlying the forming process consists in a sharp increase in the density of surface states in TiO₂ films due to the electric breakdown of the Schottky barrier at the contact with the platinum electrode, whereas the resistive switching of the structures is governed by the variation in the population of surface states in the TiO₂ band gap and/or in the defect concentration in the barrier region of the structures acted upon by voltage pulses of different polarities.     

Microstructure and infrared emissivity property of coating containing TiO2 formed on titanium alloy by microarc oxidation

Ceramic coatings containing TiO₂ were formed on Ti6Al2Zr1Mo1V alloy surface by microarc oxidation (MAO) method. The microstructure, phase and chemical composition of the coatings were analyzed by SEM, XRD and EDS techniques. The coating mainly consists of rutile TiO₂ and a small amount of anatase TiO₂. The infrared emissivity values of coated and uncoated titanium samples when exposed to 700 °C were tested. It was found that the coating exhibits a higher infrared emissivity value (about 0.9) in the wavelength range of 8–14 μm than that of the uncoated titanium alloy, although which shows a slight increase from 0.1 to 0.3 with increasing exposure time at 700 °C. The relatively high infrared emissivity value of the MAO coating is possibly attributed to the photon emission from the as formed TiO₂ phase.     

Visible and near-infrared responsivity of femtosecond-laser microstructured silicon photodiodes

We investigated the current-voltage characteristics and responsivity of photodiodes fabricated with silicon that was microstructured by use of femtosecond-laser pulses in a sulfur-containing atmosphere. The photodiodes that we fabricated have a broad spectral response ranging from the visible to the near infrared (400-1600 nm). The responsivity depends on substrate doping, microstructuring fluence, and annealing temperature. We obtained room-temperature responsivities as high as 100 A/W at 1064 nm, 2 orders of magnitude higher than for standard silicon photodiodes. For wavelengths below the bandgap we obtained responsivities as high as 50 mA/W at 1330 nm and 35 mA/W at 1550 nm.     

Size effects on the spectral thermal emissivity of alkali halides

The far-i.r. vibrational modes of LiF slabs and KCl microcrystals are determined by observation of the spectral emission. For the measurement of the emissivity a special sample compartment and a high-temperature black-body radiation standard have been developed for the spectral region 30–120μm.The observed resonance absorptions of LiF slabs on metallic substrates can be assigned to definite virtual modes. The spectra of the slabs agree well with theoretical calculations if the frequency dependence of the damping in the dielectric function is taken into account.Three different types of KCl microcrystals are investigated: cubes, octahedra and irregular crystals. The emissivity spectra of the three shapes show a dominant peak in the Reststrahlenband. This peak is nearly independent of the crystal type and of the covering density. In addition a further distinct resonance occurs near the TO frequency. The intensity of this mode depends on the covering density of the particles on the sample area. An explanation of this phenomenon is proposed. Besides these two marked peaks a secondary structure in the Reststrahlenband is observed. This structure is explained to some extent by a theory recently published by Fuchs.