Porous SiO2 Layer Research Articles

Mass transport in binary TiO2:SiO2 and GeO2:SiO2 direct ink write glasses

AbstractThe mass transport mechanisms of Ti in TiO2:SiO2 and Ge in GeO2:SiO2 direct ink write, additively manufactured glasses were studied. Due to the low solubility of Ti in SiO2 and high melting point of TiO2 relative to SiO2, Ti transport was found to occur via solid state interdiffusion between adjoining SiO2 and TiO2 precursor particles. The diffusivity of titanium in SiO2 measured over typical sintering temperatures (1000–1300°C) using Rutherford backscattering spectrometry was . This provides an estimate of ∼30 nm for the diffusion length under typical sintering conditions (2 h at 1200°C). Although Ti and Ge have similar diffusivities in SiO2 glass at low concentrations, GeO2 was found to be much more mobile during the sintering of printed GeO2:SiO2 green bodies. This was evident in glasses with phase separated GeO2 regions over length scales of ∼10 µm and in experiments involving binary xerogel films in which GeO2 migrated over ∼10 µm through cracked, porous SiO2 layers. Large phase separated regions and long transport lengths in GeO2:SiO2 suggest that the transport of GeO2 occurs prior to the densification of the SiO2 matrix via an alternative mechanism such as capillary flow. These results inform important considerations in the design of index modifying inks for the direct ink write process, namely initial precursor phase, mutual solubility with the base SiO2 glass, and mass transport throughout the sintering process.

Stimulation and Enhancement of Near‐Band‐Edge Emission in Zinc Oxide by Distributed Bragg Reflectors

AbstractThe free exciton transition (near‐band‐edge emission, NBE) of ZnO at ≈388 nm can be strongly enhanced and even stimulated by an underlying photonic structure. 1D Photonic crystals, so‐called distributed Bragg reflectors, are utilized to suppress the deep‐level emission of ZnO (DLE, ≈500–530 nm). The reflector stacks are fabricated in a layer‐by‐layer procedure by wet‐chemical synthesis. They consist of low‐ε porous SiO2 layers and high‐ε TiO2 layers. Varying the thickness of the SiO2 layers allows tuning the optical bandgap in a wide range between ≈420 and 800 nm. A ZnO layer is deposited on top of the reflector stacks by sol–gel synthesis. The spontaneous photoluminescence (PL) emission of the ZnO film is modulated by the photonic structure. When the optical bandgap of the reflector is in resonance with the deep‐level emission of ZnO (DLE, ≈500–530 nm), then this defect‐related emission mode is suppressed. Strong NBE emission is observed even when the ZnO layer does not show any NBE emission (due to low crystallinity) in the absence of the photonic structure. With this cost‐efficient synthesis method, emitters for, e.g., luminescent gas sensors can be fabricated.

Long-term ablation behavior and mechanisms of 2D-Cf/ZrB2-SiC composites at temperatures up to 2400 °C

Long-term ablation behavior and mechanisms of 2D-Cf/ZrB2-SiC composites are studied by air plasma test at temperatures up to 2400 °C. After ablation at temperatures lower than 1800 °C for 60 s, a porous SiO2 layer forms on the sample surface, which turns to a continuous ZrO2-SiO2 glassy layer at higher temperatures (2000−2400 °C). As ablation temperature increases and ablation time extends, ZrO2 grains are precipitated from the ZrO2-SiO2 glass, further coarsening and sintered via a quasi-liquid phase assisted route, finally form a dense ceramic layer. Consequently, the 2D-Cf/ZrB2-SiC composites show a superior anti-ablation property even at 2400 °C for 300 s.

Structural combination of polar hollow microspheres and hierarchical N-doped carbon nanotubes for high-performance Li-S batteries.

Hierarchical structured materials constructed with conductive carbon materials have been extensively studied as S host materials for Li-S batteries. However, their outwardly developed hierarchical structures, which do not contain structures or materials to inhibit polysulfide dissolution, lead to the dissipation of dissolved polysulfides and poor dispersion properties during the slurry-making process, which results in non-uniformity in the cathodes. Herein, an assembly of polar materials (hollow structured SiO2 microspheres) and electrically conductive hierarchical N-doped bamboo-like carbon nanotubes (b-NCNTs) is designed as an efficient S host material for minimizing the dissolution of polysulfides during Li-S battery operations. Highly aligned and packed b-NCNTs are grown in hollow structured SiO2 microspheres. The SiO2 layer coated on the surface of the hollow CoFe2O4 microspheres plays a key role in the synthesis of easily dispersible hierarchical b-NCNTs microspheres (b-NCNTs@SiO2). The S-loaded b-NCNTs@SiO2 electrodes show better cycling stability than S-loaded b-NCNTs electrodes. The polysulfide trapping of the polar SiO2 layer and the well-developed b-NCNTs minimize the dissolution of polysulfides during cycling. In addition, the introduction of electronegative N atoms into the b-NCNTs lattice enhances their polysulfide trapping ability. The S-loaded b-NCNTs@SiO2 electrodes exhibit stable discharge capacities of >771 mA h g-1 over 195 cycles at a current density of 0.5 C and a high reversible capacity of 486 mA h g-1 even at a high current density of 5.0 C.

Thermo-optical properties of porous silica thin films produced by sol-gel method

In this work, the spectral dispersion of optical constants of porous silica thin films have been presented. To determine the optical properties of porous SiO2 films, the ellipsometer Woolam M − 2000 working in the spectral range 190–1700 nm was applied. The thermo-optical coefficients of porous SiO2 films and thermal thickness dependence in temperature range 25–300 °C have been determined. The porous SiO2 layers exhibited low refractive indices. The thermo-optical coefficients of presented samples were negative due to the porous structure of SiO2 films. Moreover, it has been stated that thinness has been independent of temperature. After returning to starting conditions, the refractive indices were closer to starting values which confirmed partial reversibility of the heating and cooling process of porous. The thermal properties of porous silica films have been discussed.

Resistive switching of CuO nanofibers embedded into hollow channels of SiO2 layer

In this work, we have formed CuO memristive array embedded into porous SiO2 layer. Such an approach would create synaptic system on silicon substrate using standard integrated electronics operations. It also provides control over the geometric parameters of artificial synapses, which determines their synaptic weights. The results of synaptic behavior have shown that the value of synaptic weights and currents are dependent on the thickness of the CuO filled SiO2 porous layer.

Water Sorption in Electron-Beam Evaporated SiO2 on QCM Crystals and Its Influence on Polymer Thin Film Hydration Measurements

Spectroscopic ellipsometry (SE) and quartz crystal microbalance (QCM) measurements are two critical characterization techniques routinely employed for hydration studies of polymer thin films. Water uptake by thin polymer films is an important area of study to investigate antifouling surfaces, to probe the swelling of thin water-containing ionomer films, and to conduct fundamental studies of polymer brush hydration and swelling. SiO2-coated QCM crystals, employed as substrates in many of these hydration studies, show porosity in the thin electron-beam (e-beam) evaporated SiO2 layer. The water sorption into this porous SiO2 layer requires correction of the optical and mass characterization of the hydrated polymer due to changes in the SiO2 layer as it sorbs water. This correction is especially important when experiments on SiO2-coated QCM crystals are compared to measurements on Si wafers with dense native SiO2 layers. Water adsorption filling void space during hydration in ∼200-260 nm thick SiO2 layers deposited on a QCM crystal resulted in increased refractive index of the layer during water uptake experiments. The increased refractive index led to artificially higher polymer swelling in the optical modeling of the hydration experiments. The SiO2-coated QCM crystals showed between 6 and 8% void as measured by QCM and SE, accounting for 60%-85% of the measured polymer swelling in the low humidity regime (<20% RH) and 25%-40% of the polymer swelling in the high humidity regime (>70% RH) from optical modeling for 105 and 47 nm thick sulfonated polymer films. Correcting the refractive index of the SiO2 layer for its water content resulted in polymer swelling that successfully resembled swelling measured on a silicon wafer with nonporous native oxide.

Effect of temperature gradient on the erosion behavior of SiC coating for carbon/carbon composites in a combustion environment

SiC coating with a thickness of 50–70µm was prepared on the surface of C/C composites by in-situ reaction method. The SiC coated C/C composites were then tested in a wind tunnel where a temperature gradient from 200 to 1600°C could be obtained to investigate their erosion behavior. The results of wind tunnel test indicated that the service life of C/C composites was prolonged from 0.5 to 44h after applying the SiC coating. After the wind tunnel test, three typical oxidation morphologies, including glassy SiO2 layer, porous SiO2 layer and clusters of honeycomb-like SiO2 grains, were found on the SiC coated C/C composites. With the decrease of oxidation temperature, the amount of glassy SiO2 declined and the thermal stress increased, which induced the cracking followed by the degradation of the SiC coating.

Development of cost efficient solar receiver tube with a novel tandem absorber system

New single layer, double layer and triple layer absorber tandems were designed and developed for high performance Organic Ranking Cycle (ORC) based solar thermal applications. A Cr-Mn-Fe composite oxide based absorber layer is devolved by controlled chemical oxidation and designed to have a nanoporous structure. It is covered with double antireflective layers a ZrO2-SiO2 nanocomposite layer developed by the sol-gel route and a porous SiO2 layer obtained from SiO2 nanoparticle suspension. Double and triple layer tandem absorbers were developed by a combination of chemical, sol-gel and nanoparticle suspension methods. By varying process parameters like duration, temperature and withdrawal speed, different combinations of Cr-Mn-Fe oxides/ZrO2-SiO2 double layer tandem and Cr-Mn-Fe oxides/ZrO2-SiO2/SiO2 triple layer tandem composite absorber layers with varying refractive indices were developed on an economical variety of SS substrate. The optimized thickness of the absorber layer along with the antireflective layers shown a high absorptance (α=94.9) and low warm emittance (ε=0.122 at 300°C). More specifically, this novel double layer tandem coating has excellent optical properties along with high corrosion resistance (withstands>200h in salt spray test). It has good thermal stability in an open air atmosphere and is well suited for low and medium temperature solar thermal applications.

Optofluidic Modulation of Self-Associated Nanostructural Units Forming Planar Bragg Microcavities.

Bragg microcavities (BMs) formed by the successive stacking of nanocolumnar porous SiO2 and TiO2 layers with slanted, zigzag, chiral, and vertical configurations are prepared by physical vapor deposition at oblique angles while azimuthally varying the substrate orientation during the multilayer growth. The slanted and zigzag BMs act as wavelength-selective optical retarders when they are illuminated with linearly polarized light, while no polarization dependence is observed for the chiral and vertical cavities. This distinct optical behavior is attributed to a self-nanostructuration mechanism involving a fence-bundling association of nanocolumns as observed by focused ion beam scanning electron microscopy in the slanted and zigzag microcavities. The outstanding retarder response of the optically active BMs can be effectively modulated by dynamic infiltration of nano- and mesopores with liquids of different refraction indices acting as a switch of the polarization behavior. The unprecedented polarization and tunable optofluidic properties of these nanostructured photonic systems have been successfully simulated with a simple model that assumes a certain birefringence for the individual stacked layers and accounts for the light interference phenomena developed in the BMs. The possibilities of this type of self-arranged nanostructured and optically active BMs for liquid sensing and monitoring applications are discussed.

Miniature all-silica fiber-optic sensor for simultaneous measurement of relative humidity and temperature.

This Letter presents a miniature fiber-optic sensor created at the tip of an optical fiber suitable for simultaneous measurement of relative humidity and temperature. The proposed sensor is based on two cascaded Fabry-Perot interferometers, the first configured as a relative humidity sensing element made from silica micro-wire coated with thin porous SiO2 layer, and the second as a temperature sensing element made from a segment of a standard single-mode fiber. The sensor has linear characteristics for both measurement parameters and a sensitivity of 0.48 deg/%RH and 3.7 deg/°C.

Recycling of iron and silicon from drinking water treatment sludge for synthesis of magnetic iron oxide@SiO₂ composites.

More attention has been paid to the deterioration of water bodies polluted by drinking water treatment sludge (DWTS) in recent years. It is important to develop methods to effectively treat DWTS by avoiding secondary pollution. We report herein a novel investigation for recovery of Si and Fe from DWTS, which are used for the synthesis of two iron oxide@SiO2 composites for adsorption of reactive red X-3B (RRX-3B) and NaNO2. The results show that Fe(3+) (acid-leaching) and Si(4+) (basic-leaching) can be successfully recovered from roasted DWTS. Whether to dissolve Fe(OH)3 precipitation is the key point for obtaining Fe3O4 or γ-Fe2O3 particles using the solvothermal method. The magnetic characteristics of Fe3O4@SiO2 (390.0 m(2) g(-1)) or Fe2O3@SiO2 (220.9 m(2) g(-1)) are slightly influenced by the coated porous SiO2 layer. Peaks of Fe-O stretching vibration (580 cm(-1)) and asymmetric Si-O-Si stretching vibrations (1080 cm(-1)) of Fe3O4@SiO2 indicate the successful coating of a thin silica layer (20-150 nm). The adsorption capacity of RRX-3B and NaNO2 by Fe3O4@SiO2 is better than that of Fe2O3@SiO2, and both composites can be recycled through an external magnetic field. This method is an efficient and environmentally friendly method for recycling DWTS.

Ablation behavior of ZrB2–SiC protective coating for carbon/carbon composites

Ablation behavior of ZrB2–SiC protective coating for carbon/carbon composites during oxyacetylene flame test at 2500°C was investigated by analyzing the microstructure differentiation caused by the increasing intensity of ablation from the border to the center of the surface. After ablation, a continuous SiO2 scale, a porous SiO2 layer inlaid with fine ZrO2 nuclei, and a continuous ZrO2 scale respectively emerged in the border region, the transitional region, and the center region. In order to investigate the ablation microstructure in the initial stage, the sub-layer microstructure was characterized and found to be mainly formed by coral-like structures of ZrO2, which showed huge difference with the continuous structure of ZrO2 on the surface layer. A kinetic model concerning the thickness change induced by volatilization and oxidation during ablation was built to explain the different growth mechanisms of the continuous ZrO2 scale and the coral-like ZrO2 structure.

Formation of structures with noncatalytic CdTe nanowires

The processes of the noncatalytic synthesis of structures with CdTe nanowires by magnetron sputtering deposition are studied. It is shown that the deposition of magnetron sputtered CdTe onto substrates covered by a porous SiO2 layer can result in CdTe nanowires formation. The porosity of SiO2 layers with thicknesses from 2 to 15 nm fabricated by magnetron sputtering deposition is estimated.

Oxidation behavior of Nb–Si–B alloys with the NbSi2 coating layer formed by a pack cementation technique

The oxidation behavior of Nb–Si–B alloys with NbSi2 coating layers was investigated at 1100°C under ambient atmosphere. Continuous NbSi2 layers with a thickness of 40–60μm were coated by depositing Si on the alloy surfaces at 900°C using a pack cementation technique. A sharp interface was observed between the NbSi2 layer and the pure Nb, while a (Nb5Si3+T2) two-phase region formed between the NbSi2 layer and the alloy with the T2 phase composition. During oxidation, a glassy SiO2 layer formed significantly increasing the oxidation resistance of the coated alloys. However, evaporation of B occurred forming a porous SiO2 layer during the oxidation of the alloys with high B content. The oxidation behavior of the coated pure Nb and T2 phase alloys is discussed based on a microstructural analysis of the oxidized samples.

Design and Fabrication of Luminescent Antireflective Sol-Gel Coatings with a Bi-Layer Structure

Transparent coatings consisting of a Y2O3:Eu3+,Bi3+ layer (bottom) and a porous SiO2 layer (top) with high and low refractive index, respectively, were fabricated on quartz glass substrates by a sol–gel dip-coating method. The thickness of both the layers and the refractive index of the SiO2 layer were designed aiming at the realization of luminescent antireflective (AR) coatings. The target thickness of 140 nm for the Y2O3:Eu3+,Bi3+ layer was obtained by controlling the withdrawal speed in dip-coating. The target refractive index of 1.30 for the porous SiO2 layer was achieved by controlling the composition of coating solutions, which were composed of tetraethylorthosilicate, an aqueous ammonia solution, and ethanol, as well as the withdrawal speed. Bi-layer structured coatings with the bottom Y2O3:Eu3+,Bi3+ layer and the top SiO2 layer could reduce Fresnel reflection considerably, showing an excellent AR effect. Moreover, photoluminescence intensity from the Y2O3:Eu3+,Bi3+ layer was increased by stacking the porous SiO2 layer. This phenomenon was explained with the increased absorption of excitation light by decreasing Fresnel reflection at the surface (excitation enhancement) and the reduced total internal reflection of emission light by easing the difference in the refractive index at the interface (emission enhancement).

Chitosan Hydrogel‐Capped Porous SiO2 as a pH Responsive Nano‐Valve for Triggered Release of Insulin

AbstractA pH responsive, chitosan‐based hydrogel film is used to cap the pores of a porous SiO2 layer. The porous SiO2 layer is prepared by thermal oxidation of an electrochemically etched Si wafer, and the hydrogel film is prepared by reaction of chitosan with glycidoxypropyltrimethoxysilane (GPTMS). Optical reflectivity spectroscopy and scanning electron microscopy (SEM) confirm that the bio‐polymer only partially infiltrates the porous SiO2 film, generating a double layer structure. The optical reflectivity spectrum displays Fabry–Pérot interference fringes characteristic of a double layer, which is characterized using reflective interferometric Fourier transform spectroscopy (RIFTS). Monitoring the position of the RIFTS peak corresponding to the hydrogel layer allows direct, real‐time observation of the reversible volume phase transition of the hydrogel upon cycling of pH in the range 6.0–7.4. The swelling ratio and response time are controlled by the relative amount of GPTMS in the hydrogel. The pH‐dependent volume phase transition can be used to release insulin trapped in the porous SiO2 layer underneath the hydrogel film. At pH 7.4, the gel in the top layer effectively blocks insulin release, while at pH 6.0 insulin penetrates the swollen hydrogel layer, resulting in a steady release into solution.

A simple route to coat mesoporous SiO2 layer on carbon nanotubes

Herein we present a simple method to coat a porous SiO2 layer on carbon nanotubes (CNTs) with the aid of the cationic surfactant cetyltrimethyl ammonium bromide (CTAB). The coating process was studied systematically, and a possible coating mechanism was proposed. Temperature and the ratio of CTAB/CNTs/H2O were found to play key role in the coating process. This method can be applied to both multiwalled carbon nanotubes (MWNTs) and single walled carbon nanotubes (SWNTs). The individualized nature of the CNTs (both MWNTs and SWNTs) was maintained during the coating process. Furthermore, Raman spectroscopy showed that this method is nondestructive to the electronic structure of CNTs. The CNT/porous SiO2 core/shell structure will serve as a platform for further surface functionalization of CNTs.

In situDetection of Adsorbates at Silica/Solution Interfaces by Fourier Transform Infrared Attenuated Total Reflection Spectroscopy Using a Silica-Coated Internal Reflection Element

A silica-coated ZnSe internal reflection element (IRE) was used to acquire attenuated total reflection (ATR) Fourier transform infrared (FT-IR) spectra of ethyl acetate at an n-heptane/silica surface. A thin silica film was deposited onto the IRE by withdrawing the substrate from a suspension of fumed silica; upon drying, a durable, porous SiO2 layer is formed on the IRE that is stable in contact with aprotic solvents. The layer thickness is targeted to be comparable to the depth of penetration of infrared radiation beyond the IRE/film interface. These films were used to investigate the adsorption of ethyl acetate onto silica from n-heptane. A multidimensional least-squares fit of the spectroscopic data reveals multiple surface-site energies on the silica surface, as reflected in both the solution concentration dependence of ethyl acetate accumulation on the surface and the frequencies of vibrational modes that shift upon adsorption.

Erbium Doped SiO2 Layers Formed on the Surface of Silicon by Spark Processing

We present structural and spectroscopic analyses of luminescent erbium-doped porous SiO2 layers on silicon formed using a spark processing technique. Scanning electron microscopy reveals a surface of irregular holes covered by a SiO2 layer. Concomitant energy-dispersive X-ray mapping experiments show that the erbium concentration in the porous layer can be controlled by varying the molarity of the erbium solution deposited on the substrate prior to spark processing. Both visible and near-infrared photoluminescence spectroscopy, under conditions of varying temperature and excitation power, have been used to study the nature of the erbium centers formed in the porous layer. Self-quenching of Er3+ photoluminescence at 1.54 μm occurs at the highest concentrations of erbium employed.