Yu Han Sun

Sol-gel broadband anti-reflective single-layer silica films with high laser damage threshold

A simple processing of preparing broadband anti-reflective single-layer silica films is presented in this article. By adding polyvinylpyrrolidone (PVP) into reactant mixture, PVP-containing SiO2 sol was obtained under base catalyzed hydrolysis and condensation of tetraethoxysilane. The spin-coating films and the dip-coating films were deposited on one side and two sides of quartz substrates. The anti-reflection band is 315 nm wide for dip-coating film and 559 nm wide for spin-coating film and the transmittance reached to 99.95 and 95.92% for dip-coating film and spin-coating film, respectively. By a Nd:YAG lasers the laser damage threshold of as-deposited films was measured at 1064 nm wavelength (1 ns pulse). It ranged from 24 to 33 J/cm2 with an average of 28.7 J/cm2. Compared to SiO2 sol without PVP, not only was the anti-reflection band broadened but the anti-reflection and laser damage threshold were retained.

Antireflective silica thin films with super water repellence via a solgel process

A solgel process was developed, through which silica films possessing both high antireflection and super water repellence were obtained. In this process, methyl-modified SiO2 sols synthesized by colloidal suspension of SiO2 particles and hexamethyldisilazane (HMDS) were used to deposit spinning-coating films on optical glass substrates. On the resultant films the contact angle for water increased with the increasing amount of HMDS in the reaction mixture. The biggest contact angle was 165 degrees, and the lowest reflectivity on one-sided film reached 0.03%. The antireflections were high all the while. One advantage of this process is that neither a roughened surface nor fluoroalkyltrialkoxylsilane (FAS) is needed to obtained super water repellence.

Durable solgel antireflective films with high laser-induced damage thresholds for inertial confinement fusion

We tested the use of two hydrophobic methyl-substituted silane precursors, methyltriethoxysilane and dimethyldiethoxysilane, to synthesize methyl-modified silica sols by a two-step method and a cohydrolysis method to produce durable antireflective films with high laser-induced-damage thresholds (LIDTs). Using small-angle x-ray scattering technology, we obtained details of the microstructure of clusters in sol and found various double fractal structural characteristics in the methyl-modified silica clusters; our findings were confirmed by transmission-electron micrographs. Through a ^29 Si magic-angle spin nuclear magnetic resonance study of the corresponding xerogels, we determined the double-fractal microstructure, which we then related to the LIDTs of AR films. The distribution configuration of methyls in clusters determined the double-fractal microstructure of clusters and then the LIDTs of AR films. The LIDTs of films produced by the cohydrolysis method (the highest was 38 J/cm² for 1-ns, 1064-nm laser action) were much higher than those from the two-step method because of the loose netlike clusters in the former configuration. During the 220-day aging, the transmittance of hydrophobic AR film decreased ∼0.2%. So it is practicable to prepare durable AR films with higher LIDTs than those of normal AR SiO2 films only by introducing hydrophobic methyls into a Si-O-Si matrix of clusters if an appropriate hydrophobic precursor is chosen.

A negative deviation from Porod's law in SAXS of organo-MSU-X

Small-angle X-ray scattering (SAXS) using synchrotron radiation as X-ray source has been employed to characterize the microscopic structure of organo-modified mesoporous molecular sieves (organo-MSU-X) prepared by a one-pot template-directed synthesis. It is shown that the SAXS profile is hardly constant with Poreds law showing a negative slope, i.e., negative deviation. This suggests that there is a diffuse interfacial layer located between the pores and the matrix. This suggests that the organic groups remain covalently linked to the matrix, as indicated by (SiCP)-Si-29 MAS NMR and FT-IR. The average thickness of the interfacial layer was found to be about 1 nm for each of the three samples with different kinds and the same amounts (20%) of organic groups. This kind of material has also been proved to possess both surface and mass fractal structures. The correction of the negative deviation from Poreds law is performed in order to obtain the pore structure of the amorphous porous silica materials

Superhydrophobic antireflective silica films: fractal surfaces and laser-induced damage thresholds

Several superhydrophobic antireflective silica films have been prepared by a solgel method that uses hexamethyl-disilizane (HMDS) as a modifier. In a high-power laser, laser-induced damage thresholds (LIDTs) of 23-30 J/cm2 were obtained at 1064-nm wavelength with 1-ns pulse duration. By atomic-force microscopy and optical microscopy, the fractal surfaces of films were studied, and multifractal spectra (MFSs) were calculated both before and after laser damage. The two-sided effect of HMDS on particle growth determined the surface fractal of a particle and the multifractal structure of a films surface. The bigger deltaalpha was, both before and after laser damage, the lower the LIDT was. The effect of methyl groups should be included in the determination of the MFS of the LIDT.

Determination of interface layer thickness of a pseudo two-phase system by extension of the Debye equation

The Debye equation with slit-smeared small angle x-ray scattering (SAXS) data is extended from an ideal two-phase system to a pseudo two-phase system with the presence of the interface layer, and a simple accurate solution is proposed to determine the average thickness of the interface layer in porous materials. This method is tested by experimental SAXS data, which were measured at 25 °C, of organo-modified mesoporous silica prepared by condensation of tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) using non-ionic neutral surfactant as template under neutral condition.

Design of reflective tantala optical coatings using sol-gel chemistry with ethanoic acid catalyst and chelator

Abstract Sol-gel processing of Ta(OC2H5)5 to produce tantala highly reflective (HR) coating has been systematically studied in the absence of inorganic anions using an ethanoic acid catalyst in part using IR. In the presence of ethanoic acid, both hydrolysis and chelation co-exist, depending on the preparative parameters; most of the precursor was found to be converted into an amorphous chemically modified tantala sol and finally a gel, the crystallization of which did not start below 700 K. A ternary phase diagram was used to outline the regions of stability of the system. Two-layer and multi-layer tantala thin HR solid films (0.3 μ thick with a refractive index of 1.7) can be prepared by spin-coating of such sols.

Low pressure polymer precursor for synthesis of diamond at low temperature

Abstracts are not published in this journal

Sol-gel chemistry of tantala HR coatings: Structure and laser-damage resistance

Sol-gel processing of Ta(OC2H5)5 to produce high reflective and high refractive index (HR) tantala coatings has been followed by infrared spectroscopy (FTIR), where in the presence of ethanoic acid, both chelation and hydrolysis reactions co-exist, depending on the preparative parameters. However, most of the precursor was found to convert into an amorphous tantala sol and finally a gel. Transmission electron microscopy (TEM) and light-scattering showed that the primary sol particles (whose size was about 5 nm) aggregated into clusters up to 60–80 nm via different linkages: ring-like, cross-linked and colloidal structures. Thermal analysis-X-ray diffraction (TGA-XRD) indicated that the gel did not crystallise below 700 K. Tantala films (0.3 μm thick; refractive index of 1.7) were prepared by spin-coating of these sols and had properties closely related to the sol structure.

Determination of SiO2 colloid core size by SAXS

A colloid is a suspension in which the dispersed phase (i.e. sol particle or colloidal particle) is so small (in nanoscale) that gravitational forces are negligible and interactions are dominated by short-range forces, such as van der Waals attraction and surface charges [1]. According to the physical chemistry theory of the colloidal system [2], a colloidal particle is composed of a colloidal core and an interface layer that wraps about the colloid core. The average size of the colloidal cores, denoted as D, is an important parameter of a colloid system, which can help one to understand the structure and the aggregating mechanism of the colloid. To the best of our knowledge, the determination of D remains unreported. In this short paper we try to determine D of SiO2 colloid cores prepared by base-catalyzed hydrolysis and condensation of alkoxides in alcohol by small angle X-ray scattering (SAXS). The analysis was directly based on slit-smeared SAXS data. Four samples of SiO2 colloids were examined. The samples were prepared by co-condensation of tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) using NH4OH as catalyst. TEOS and MTES were named precursors. The non-ionic C8H17(C6H4)(EO)10H (denoted Tx-10) was used as surfactant. The molar ratio of precursor: H2O : C2H5OH : NH4OH was 1 : 2 : 40 : 0.0005. TEOS and ethanol were mixed at ambient condition (25 ◦C) for 30 min to form solution (I); certain amounts of NH4OH, H2O and Tx-10 were mixed with the same volume of ethanol to make solution (II), which was then added into solution (I) to produce a homogeneous and transparent solution. After 30 min stirring, the solution was kept at ambient condition (25 ◦C or 50 ◦C) for some time. So-produced samples are listed in Table I. SAXS experiment was performed using synchrotron radiation as X-ray source with a long-slit collimation system at Beijing Synchrotron Radiation Facility. Incident X-ray wavelength λ was 0.154 nm. The scattered X-ray intensities were recorded using imagery plate technology. The data were corrected for absorption and the background scattering of the sample. According to SAXS theory [3], the scattering of an ideal two-phase system having sharply defined phase