Monolithic Silica Glass Research Articles

Cosolvent-free sol–gel synthesis of macroporous silica gels from tetramethoxysilane–tetraethoxysilane mixtures

Abstract Tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), and their mixtures were used for the cosolvent-free synthesis of macroporous silica gels as precursors for monolithic silica glasses. The liquid-state 29Si nuclear magnetic resonance (NMR) spectroscopy of precursor solutions indicated that cross-transesterification between TMOS and TEOS was completed in a few hours at 20 °C in the presence of an acid catalyst, whereas it was negligible when the catalyst was absent. In the precursor solutions prepared from TMOS, phase separation occurred after gelation, resulting in translucent gels. In contrast, in the solutions prepared from TEOS or a mixture of TMOS and TEOS at a TMOS mole fraction of 0.8, the phase separation can be induced before gelation, and opaque xerogels were easily obtained without fracture. The average size of macroscopic particles and macroporous structures were uniform over opaque xerogels prepared from TEOS. In contrast, in opaque xerogels prepared from the TMOS-rich mixture of TMOS and TEOS, the average particle size and macroscopic porosity inside them were notably smaller than those of the subsurface, probably because of a large exotherm upon gelation and the resulting temperature gradient in the gelling solutions. Such spatial morphology distribution made the sintering of the opaque gels into clear silica glasses difficult. Opaque gels prepared from TEOS and translucent gels prepared from solutions containing TMOS were transformed to clear silica glasses in high yields of ~99% by sintering in a helium atmosphere at 1050–1350 °C. Graphical abstract

Diffusionlike Drying of a Nanoporous Solid as Revealed by Magnetic Resonance Imaging

Drying plays a central role in various fabrication processes and applications of functional nanoporous materials, most prominently in relation to energy storage and conversion. During such processes, liquid coexists with air inside the sample, leading to transport as a result of concentration gradients of vapor and/or liquid. Experimentally, it is extremely challenging to unravel this transport phenomenology inside the hidden geometry of porous media. Here, we observe the drying of a model nanoporous material (monolithic mesoporous silica glass, Vycor) with magnetic resonance imaging. We show that, for various boundary conditions (air-flux intensities), no dry region develops, but the sample desaturates in depth. This desaturation is almost homogeneous throughout the sample for weak air flux, while saturation gradients can be observed for sufficiently strong air flux. We demonstrate that the transport of water is mainly ensured by liquid flow towards the free surface, resulting from a gradient of vapor pressure, associated with local saturation (via the desorption curve), leading to a gradient of liquid pressure (via the Kelvin law). Assuming otherwise standard hydrodynamic characteristics of the nanoconfined liquid, this results in a diffusionlike model, which appears to represent experimental data very well in terms of the spatial distribution of water over time inside the sample for various boundary conditions (air-flux intensities). Finally, we propose a predictive model of the detailed drying characteristics of a nanomaterial from knowledge of its pore size, permeability, and desorption curve. This provides an insight into the rational design of drying-based processes employing functional nanoporous materials and allows for a mechanistic understanding of drying phenomenologies in natural nanoporous media.

Cosolvent-free synthesis of macroporous silica gels and monolithic silica glasses from tetraalkoxysilane–water binary systems: comparison between tetramethoxysilane and tetraethoxysilane

Cosolvent-free synthesis of macroporous silica gels and monolithic silica glasses from tetraalkoxysilane–water binary systems: comparison between tetramethoxysilane and tetraethoxysilane

ABH damping of monolithic silica glass cantilever by structural and material modification using fs laser micromachining

AbstractCombining femtosecond laser exposure with chemical etching enables the fabrication of arbitrary 3D shapes in fused silica with micron precision. Here, we investigate the use of this process combined with additional tuning of the material properties for controlling the vibrational behavior of silica microcantilevers through an acoustic black hole (ABH) effect. This effect, caused by a tapered profile at the cantilever's end, enables the trapping of propagating acoustic waves. Such cantilevers with ABH design show a remarkable amplitude decrease in the resonance peak without adding any damping material as demonstrated experimentally. Finally, using Fourier‐transform infrared spectroscopy (FTIR), we observe residual effects of the laser exposure at the surface of the specimen that have noticeable influence on the dynamic behavior of the device.

Porous silicon prepared from monolithic porous silica glass using a two‐step magnesiothermic reduction

AbstractMagnesiothermic reduction to convert silicon dioxide into silicon is difficult to control because the reduction often proceeds through the formation of magnesium silicide. To improve the yield of silicon, we have applied a two‐step magnesiothermic reduction to produce porous silicon from a monolithic porous silica glass substrate. At the first step, the porous silica glass substrate with pore size of approximately 40 nm was reacted with magnesium at 525°C for 6 hours, in which magnesium silicide was formed in the several tens of micrometers‐deep surface layer, while the inside of the substrate remained unreacted. Subsequently, in the second step, the product was heat‐treated at 650°C for 72 hours to produce silicon. In this step, the obtained magnesium silicide reacted with the unreacted silica glass and the intermediate products, which consist of silicon in the intermediate oxidation states generated in the first step of the reaction. The obtained silicon retained the characteristic porous structure of the initial porous silica glass. The sample also retained the monolithic shape of the initial substrate, even after being divided into several pieces. In addition, we found that periodical layered structures were generated in the course of the reduction procedure. We successfully obtained a monolithic porous silica glass sample, the surface region of which consists of porous silicon.

Capillarity-Driven Oil Flow in Nanopores: Darcy Scale Analysis of Lucas–Washburn Imbibition Dynamics

We present gravimetrical and optical imaging experiments on the capillarity-driven imbibition of silicone oils in monolithic silica glasses traversed by 3D networks of pores (mesoporous Vycor glass with 6.5 nm or 10 nm pore diameters). As evidenced by a robust square root of time Lucas–Washburn (L–W) filling kinetics, the capillary rise is governed by a balance of capillarity and viscous drag forces in the absence of inertia and gravitational effects over the entire experimental times studied, ranging from a few seconds up to 10 days. A video on the infiltration process corroborates a collective pore filling as well as pronounced imbibition front broadening resulting from the capillarity and permeability disorder, typical of Vycor glasses. The transport process is analyzed within a Darcy scale description, considering a generalized prefactor of the L–W law, termed Lucas–Washburn–Darcy imbibition ability. It assumes a Hagen–Poiseuille velocity profile in the pores and depends on the porosity, the mean pore diameter, the tortuosity and the velocity slip length and thus on the effective hydraulic pore diameter. For both matrices a reduced imbibition speed and thus reduced imbibition ability, compared to the one assuming the nominal pore diameter, bulk fluidity and bulk capillarity, can be quantitatively traced to an immobile, pore wall adsorbed boundary layer of 1.4 nm thickness. Presumably, it consists of a monolayer of water molecules adsorbed on the hydrophilic pore walls covered by a monolayer of flat-laying silicone oil molecules. Our study highlights the importance of immobile nanoscopic boundary layers on the flow in tight oil reservoirs as well as the validity of the Darcy scale description for transport in mesoporous media.

Luminescent sintered silica glass prepared by adsorbing Pr ions into mesoporous SiO2–PVA nanocomposite

A monolithic silica glass doped with Pr3+ was prepared by immersing a mesoporous SiO2–polyvinly alcohol (PVA) nanocomposite in a Pr nitrate solution and then sintering it at 1100℃ in air. The sintered glass exhibited pink emission when excited by ultraviolet irradiation. The photoluminescence intensity of the glass depended on the total concentration of Pr ions, which was adjusted by changing the concentration of the Pr nitrate solution and the immersion time. The glass with the highest photoluminescence intensity was prepared by using a 1- or 10-mM Pr solution at an immersion time of 60 min. To reveal how the immersion conditions affected the photoluminescence intensity, we analyzed and discussed the adsorption kinetics of the Pr ions onto the surface of the mesoporous nanocomposite. At immersion times below 60 min, the Pr ions diffused into pores of the nanocomposite without aggregating. To prepare Pr3+-doped silica glass with high photoluminescence, both the concentration of the Pr solution and the immersion time must be controlled.

Fabrication and photoluminescence of monolithic silica glass doped with alumina nanoparticles using SiO<sub>2</sub>-PVA nanocomposite

Fabrication and photoluminescence of monolithic silica glass doped with alumina nanoparticles using SiO<sub>2</sub>-PVA nanocomposite

ChemInform Abstract: Cosolvent‐Free Sol—Gel Synthesis and Optical Characterization of Silica Glasses Containing LaF3 and (La,Er)F3 Nanocrystals

AbstractReview: 49 refs.

Preparation and photoluminescence of monolithic silica glass doped with Tb3+ ions using SiO2–PVA nanocomposite

The monolithic silica glass doped with Tb3+ ions was fabricated using the SiO2–PVA nanocomposite as the glass precursor. In order to dope Tb3+ ions in the monolithic silica glass, the mesoporous SiO2–PVA nanocomposite was immersed in the Tb3+ ions contained solution and subsequently sintered at 1100°C in air. Consequently the monolithic transparent silica glass was obtained, exhibiting green fluorescence attributed to 5D4→7F5 main transitions under UV excitation. The Tb concentration in the sintered glass could be controlled by immersion time of the nanocomposite in the solution.

Highly transparent, bright green, sol–gel-derived monolithic silica-(Tb,Ce)PO4 glass-ceramic phosphors

A cosolvent-free sol–gel method yields monolithic silica glasses containing (Tb,Ce)PO4 nanocrystals, free from Rayleigh scattering, and exhibiting ultraviolet-induced bright green photoluminescence.

Cosolvent-free sol–gel synthesis of rare-earth and aluminum codoped monolithic silica glasses

Cosolvent-free sol–gel synthesis of rare-earth and aluminum codoped monolithic silica glasses

Imbibition in mesoporous silica: rheological concepts and experiments on water and a liquidcrystal

We present, along with some fundamental concepts regarding imbibition of liquidsin porous hosts, an experimental, gravimetric study on the capillarity-driveninvasion dynamics of water and of the rod-like liquid crystal octyloxycyanobiphenyl(8OCB) in networks of pores a few nanometers across in monolithic silica glass(Vycor). We observe, in agreement with theoretical predictions, square root oftime invasion dynamics and a sticky velocity boundary condition for both liquidsinvestigated. Temperature-dependent spontaneous imbibition experiments on8OCB reveal the existence of a paranematic phase due to the molecular alignmentinduced by the pore walls even at temperatures well beyond the clearing point.The ever present velocity gradient in the pores is likely to further enhance thisordering phenomenon and prevent any layering in molecular stacks, eventuallyresulting in a suppression of the smectic phase in favor of the nematic phase.

Preparation of SiO2-PVA nanocomposite and monolithic transparent silica glass by sintering

We present a procedure for fabricating transparent silica glass that involves the sintering of green bodies prepared from an inorganic–organic nanocomposite. The nanocomposite was prepared from fumed silica and poly(vinyl alcohol) (PVA). We investigated the SiO2–PVA nanocomposite by Fourier transform infrared spectrometry (FT-IR) measurement, transmission electron microscopy and electron energy-loss spectroscopy, and the results revealed homogeneously dispersed SiO2 nanoparticles and PVA. We examined the relationship between the pH and ζ potential of the SiO2 suspension and the formability of the SiO2–PVA nanocomposite. Formability of the SiO2–PVA nanocomposite was dependent on the pH of the SiO2 suspension, and a monolithic SiO2–PVA nanocomposite without cracks was obtained using a SiO2 suspension at around the isoelectric point (pI). By sintering the SiO2–PVA nanocomposite in air at 1100°C, monolithic transparent silica glass was obtained with no cracks. The glass is highly transparent from the ultraviolet to the visible region.

Structure and Optical Properties of Doped SiO2Mesoporous Glasses

Monolithic mesoporous silica glasses were synthesized. The presence of Cu2+and Fe3+cations during the synthesis of sol-gel precursors leads to different morphologies and pore sizes. The materials are characterized via IR and Raman scattering spectra to detect surface groups and -Si-O-Si- rings (i.e., 3–6 Si atoms) and morphology is examined through electron microscopy. N2sorption isotherms reveal details of the mesoporous structure of the materials, which are endowed with significantly large surface areas and pore volumes. Vapor percolation occurs in these samples because of a void arrangement consisting of pore bulges delimited by narrower necks. The optical characterization shows the luminescence spectrum and thermoluminescent behavior subjected to successive exposures of beta particles.

Amine-buffered Phase Separating Tetraethoxysilane–Water Binary Mixture: A Simple Precursor of Sol–Gel Derived Monolithic Silica Gels and Glasses

Abstract Amines are used as pH control agents to form macroporous silica gels utilizing macroscopic phase separation of acid-catalyzed tetraethoxysilane–water binary mixture. The wide pKa variety of amines from basic to acidic regions is suitable to buffer the solution at neutral pH range, at which the phase separation becomes prominent. This method allows monolithic silica gels and glasses to be prepared reproducibly from alkoxide-based solutions with minimal use of pH control agents.

Spontaneous Imbibition Dynamics of ann-Alkane in Nanopores: Evidence of Meniscus Freezing and Monolayer Sticking

Capillary filling dynamics of liquid n-tetracosane (n-C24H50) in a network of cylindrical pores with 7 and 10 nm mean diameter in monolithic silica glass (Vycor) exhibit an abrupt temperature-slope change at Ts = 54 degrees C, approximately 4 degrees C above bulk and approximately 16 degrees C, 8 degrees C, respectively, above pore freezing. It can be traced to a sudden inversion of the surface tension's T slope, and thus to a decrease in surface entropy at the advancing pore menisci, characteristic of the formation of a single solid monolayer of rectified molecules, known as surface freezing from macroscopic, quiescent tetracosane melts. The imbibition speeds, that are the squared prefactors of the observed square-root-of-time Lucas-Washburn invasion kinetics, indicate a conserved bulk fluidity and capillarity of the nanopore-confined liquid, if we assume a flat lying, sticky hydrocarbon backbone monolayer at the silica walls.

Sol-gel synthesis of monolithic silica gels and glasses from phase-separating tetraethoxysilane–water binary system

A facile synthetic route for monolithic silica gels and glasses from a phase-separating binary mixture of tetraalkoxysilane and water, free from alcohols, organic additives, or other third components, has been developed.

Influence of temperature on the physical properties of TEOS silica xerogels

In the present paper, the experimental results on the influence of temperature on the physical properties such as bulk density, percentage of porosity, percentage of volume shrinkage and weight loss of silica xerogels, are reported. The xerogels have been prepared by hydrolysis and polycondensation of tetraethoxysilane (TEOS) using a new catalyst (citric acid) followed by drying at ambient temperatures (<90°C) and atmospheric pressure. The lowest density silica xerogels obtained were of the order of 0.22×10 3 kg m −3. The xerogels were then heated in air at different temperatures ranging from room temperature (25°C) to 1200°C with a controlled heating rate of 1.5°C min −1. It has been found that highly transparent (≈90 %), monolithic, pure and densified (2.2×10 3 kg m −3) silica glasses were obtained at a temperature as low as 980°C when compared to conventional melting of silica glasses (≈2000°C). The experimental results have been discussed by taking into account the processing conditions and related chemical changes. Besides the porosity and density measurements, the gels and glasses were also characterised by infrared (IR) spectroscopy and scanning electron microscopy (SEM).

Preparation and characterisation of monolithic Pr-doped silica glasses by a sol–gel method

Pr–Al co-doped silica glasses have been prepared using Pr(NO 3) 3 6H 2O, Al(NO 3) 3 9H 2O and a sol of fumed silica and HNO 3 acidified Si(OC 2H 5) 4 as starting materials. The glasses were characterised by density, transmission electron microscope (TEM), EDAX, refractive index, linear shrinkage, absorption and fluorescence spectra. The glasses were also examined in relation to total –OH content with Pr-concentration. Aluminium co-doping modifies the optical properties of the Pr-doped glasses.