Physicochemical Treatments of Graphene Oxide to Improve Water Vapor/Gas Separation Performance of Supported Laminar Membranes: Sonication and H2O2 Oxidation.

We investigated the previously unexplored domain of water vapor/gas separation using graphene oxide (GO) membranes, expecting future applications, including gas dehumidifiers and superior humidity controllers. While the importance of manipulation of GO nanosheet size and surface chemistry in traditional water purification and gas separation has been acknowledged, their potential impact on water vapor/gas separation remained unexplored until now. We applied sonication and hydrogen peroxide treatments to GO water dispersions and systematically evaluated the size and surface chemistry of each GO nanosheet. Both treatments reduced the GO nanosheet size to shorten the diffusion length, which improved water permeance. In addition, hydrogen peroxide treatment improved the hydrophilicity of the nanosheet. Our novel findings demonstrate that optimization of GO nanosheet size and the increase in their hydrophilicity via hydrogen peroxide treatments for 5 h significantly enhance water permeance, leading to a remarkable water vapor permeance of 4.6 × 10-6 mol/(m2 s Pa) at 80 °C, a 3.1-fold improvement over original GO membranes, while maintaining a water vapor/nitrogen permeance ratio exceeding 10,000. These results not only provide important insights into the nature of water vapor/gas separation but also suggest innovative methods for optimizing the GO membrane structure.

Steam recovery from flue gas by organosilica membranes for simultaneous harvesting of water and energy.

Steam recovery from the spent gases from flues could be a key step in addressing the water shortage issue while additionally benefiting energy saving. Herein, we propose a system that uses organosilica membranes consisting of a developed layered structure to recover steam and latent heat from waste. Proof-of-concept testing is conducted in a running incinerator plant. The proposed system eliminates the need for a water supply while simultaneously recovering latent heat from the waste stream. First, the long-term stability of an organosilica membrane is confirmed over the course of six months on a laboratory-scale under a simulated waste stream. Second, steam recovery is demonstrated in a running waste incinerator plant (bench-scale), which confirms the steady operation of this steam recovery system with a steam recovery rate comparable to that recorded in the laboratory-scale test. Third, process simulation reveals that this system enables water-self-reliance with energy recovery that approximates 70% of waste combustion energy.

Permeation Properties of Water Vapor through Graphene Oxide/Polymer Substrate Composite Membranes.

Graphene oxide (GO) has attracted attention as an excellent membrane material for water treatment and desalination owing to its high mechanical strength, hydrophilicity, and permeability. In this study, composite membranes were prepared by coating GO on various polymeric porous substrates (polyethersulfone, cellulose ester, and polytetrafluoroethylene) using suction filtration and casting methods. The composite membranes were used for dehumidification, that is, water vapor separation in the gas phase. GO layers were successfully prepared via filtration rather than casting, irrespective of the type of polymeric substrate used. The dehumidification composite membranes with a GO layer thickness of less than 100 nm showed a water permeance greater than 1.0 × 10-6 mol/(m2 s Pa) and a H2O/N2 separation factor higher than 104 at 25 °C and 90-100% humidity. The GO composite membranes were fabricated in a reproducible manner and showed stable performance as a function of time. Furthermore, the membranes maintained high permeance and selectivity at 80°C, indicating that it is useful as a water vapor separation membrane.

A Brief Overview of the Microstructural Engineering of Inorganic-Organic Composite Membranes Derived from Organic Chelating Ligands.

This review presents a concise conceptual overview of membranes derived from organic chelating ligands as studied in several works. The authors' approach is from the viewpoint of the classification of membranes by matrix composition. The first part presents composite matrix membranes as a key class of membranes and makes a case for the importance of organic chelating ligands in the formation of inorganic-organic composites. Organic chelating ligands, categorized into network-modifying and network-forming types, are explored in detail in the second part. Four key structural elements, of which organic chelating ligands (as organic modifiers) are one and which also include siloxane networks, transition-metal oxide networks and the polymerization/crosslinking of organic modifiers, form the building blocks of organic chelating ligand-derived inorganic-organic composites. Three and four parts explore microstructural engineering in membranes derived from network-modifying and network-forming ligands, respectively. The final part reviews robust carbon-ceramic composite membranes as important derivatives of inorganic-organic hybrid polymers for selective gas separation under hydrothermal conditions when the proper organic chelating ligand and crosslinking conditions are chosen. This review can serve as inspiration for taking advantage of the wide range of possibilities presented by organic chelating ligands.

Bis(triethoxysilyl)ethane (BTESE)-Organosilica Membranes for H2O/DMF Separation in Reverse Osmosis (RO): Evaluation and Correlation of Subnanopores via Nanopermporometry (NPP), Modified Gas Translation (mGT) and RO Performance.

A 40 cm length Bis(triethoxysilyl)ethane (BTESE) membrane having different pore sizes was successfully prepared by changing the number of coating times for gas permeation (GP) and organic solvent reverse osmosis (OSRO) separation study. It was found that BTESE-6 membranes prepared through six-time coating consisted of small-sized pores in the range 0.56 to 0.64 nm estimated using modified Gas Translation (mGT) method and 0.59 to 0.67 nm estimated by nanopermporometry (NPP) method, respectively. These membranes demonstrated a high DMF rejection, RDMF > 95% with total flux, Jv total > 5 kg m-2 h-1 at operating condition feed pressure, Pf: 8 MPa; feed temperature, Tf : 50 °C; and feed flowrate, Qf : 30 mL/min; and they exhibited a high degree selectivity of He/SF6 in the range of ~ 260-3400 at a permeation temperature 200 °C. On the other hand, the larger pore sizes of the BTESE-4 membranes (pore size estimates > 0.76 nm to 1.02 nm) exhibited low DMF rejection and a low degree selectivity of He/SF6 around ~30% and 25, respectively, at the same operating condition as BTESE-6. Both GT and NPP methods can be considered as an indicator of the measurement membrane pore size. From this study, it was found that He and SF6 gases can be some of the potential predictors for water and DMF permeance. Furthermore, by comparing our OSRO membrane with other PV membranes for DMF/H2O separation, our BTESE-6 membranes still exhibited high flux in the range of 3-6 kg m-2 h-1 with a separation factor H2O/DMF in the range of 80-120.

The Effect of C/Si Ratio and Fluorine Doping on the Gas Permeation Properties of Pendant-Type and Bridged-Type Organosilica Membranes.

A series of pendant-type alkoxysilane structures with various carbon numbers (C1-C8) were used to fabricate sol-gel derived organosilica membranes to evaluate the effects of the C/Si ratio and fluorine doping. Initially, this investigation was focused on the effect that carbon-linking (pendant-type) units exert on a microporous structure and how this affects the gas-permeation properties of pendant-type organosilica membranes. Gas permeation results were compared with those of bridged-type organosilica membranes (C1-C8). Network pore size evaluation was conducted based on the selectivity of H2/N2 and the activation energy (Ep) of H2 permeation. Consequently, Ep (H2) was increased as the C/Si ratio increased from C1 to C8, which could have been due to the aggregation of pendant side chains that occupied the available micropore channel space and resulted in the reduced pore size. By comparison, these permeation results indicate that pendant-type organosilica membranes showed a somewhat loose network structure in comparison with bridged-type organosilica membranes by following the lower values of activation energies (Ep). Subsequently, we also evaluated the effect that fluorine doping (NH4F) exerts on pendant-type [methytriethoxysilane (MTES), propyltrimethoxysilane (PTMS)] and bridged-type [1,2-bis(triethoxysilyl)methane (BTESM) bis(triethoxysilyl)propane (BTESP)] organosilica structures with similar carbon numbers (C1 and C3). The gas-permeation properties of F-doped pendant network structures revealed values for pore size, H2/N2 selectivity, and Ep (H2) that were comparable to those of pristine organosilica membranes. This could be ascribed to the pendant side chains, which might have hindered the effectiveness of fluorine in pendant-type organosilica structures. The F-doped bridged-type organosilica (BTESM and BTESP) membranes, on the other hand, exhibited a looser network formation as the fluorine concentration increased.

Metal-Induced Aminosilica Rigidity Improves Highly Permeable Microporous Membranes via Different Types of Pendant Precursors.

In this study, nickel-doped aminosilica membranes containing pendant groups were prepared with 3-aminopropyltriethoxysilane (APTES), trimethoxy[3-(methylamino)propyl]silane (MAPTS), 3 N,N-dimethyl aminopropyltrimethoxysilane (DAPTMS), N-[3-(trimethoxysilylpropyl]ethylene diamine (TMSPED), and 1-[3-(trimethoxysilyl)propyl] urea (TMSPU). Differences in the structures of terminal amine ligands significantly contributed to the formation of a coordinated structural assembly. Ultraviolet-visible spectroscopy (UV-vis), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and N2 adsorption isotherms revealed that short and rigid pendant amino groups successfully coordinated with nickel to produce subnanopores in the membranes, while an ion-exchange interaction was suggested for longer and sterically hindered aminosilica precursors. Moreover, the basicity of amine precursors affected the affinity of ligands for the development of a coordinated network. A pristine aminosilica membrane showed low levels of H2 permeance that range from 0.1 to 0.5 × 10-6 mol m-2 s-1 Pa-1 with a H2/N2 permeance ratio that ranges from 15 to 100. On the contrary, nickel coordination increased the H2 permeance to 0.1-3.0 × 10-6 mol m-2 s-1 Pa-1 with H2/N2 permeance ratios that range from 10 to 68, which indicates the formation of a microporous structure and enlargement of pore sizes. The strong level of coordination affinity between nickel ions and amine groups induced rearrangement of the flexible pendant chain into a more rigid structure.

Nanogradient Hydrophilic/Hydrophobic Organosilica Membranes Developed by Atmospheric-Pressure Plasma to Enhance Pervaporation Performance.

Organosilica membranes are a promising candidate for pervaporation dehydration owing to their tunable molecular sieving characteristics and excellent hydrothermal stability. Herein, we report a facile modification using an atmospheric-pressure water vapor plasma to enhance the pervaporation performance of organosilica membranes. The surface of methyl-terminated organosilica membranes was treated by water vapor plasma to develop an ultrathin separation active layer suitable for pervaporation dehydration. The surface hydrophilicity was increased by water vapor plasma due to oxidative decomposition of methyl groups to form silanol groups. The plasma-modified layer had a thickness of several nanometers and had a silica-like structure due to the condensation of silanol groups. The plasma-modified organosilica membranes exhibited an improved molecular sieving property owing to the formation of highly cross-linked siloxane networks with a pore size of approximately 0.4 nm. The membranes also exhibited an excellent permselectivity in the dehydration of alcohols due to the nanometer-thick separation active layer with controlled pore size and increased hydrophilicity. The plasma-modified membranes showed high H2O permeance exceeding 10-6 mol m-2 s-1 Pa-1 with permeance ratios for H2O/EtOH and H2O/IPA of 517-3050 and >10 000, respectively, in the dehydration of 90 wt % aqueous solutions at 50 °C, which is among the highest permselectivities for silica-based membranes. Furthermore, the plasma-modified membranes displayed highly efficient dehydration performance for a H2O/MeOH mixture. The H2O permeance and H2O/MeOH permeance ratio in the dehydration of a 90 wt % MeOH aqueous solution at 50 °C were (2.3-3.0) × 10-6 mol m-2 s-1 Pa-1 and 31-143, respectively, which exceeded the permeance-selectivity trade-off of conventional membranes including polymeric, silica-based, and zeolite membranes. The results indicate that the proposed plasma-assisted approach can enhance the pervaporation performance of organosilica membranes via the modification under atmospheric pressure and at room temperature.

Development of Highly Water-Permeable Robust PSQ-Based RO Membranes by Introducing Hydroxyethylurea-Based Hydrophilic Water Channels.

Copolymerization of bis[3-(triethoxysilyl)propyl]amine (BTESPA) and N-(2-hydroxyethyl)-N'-[3-(triethoxysilyl)propyl]urea (HETESPU) provided highly permeable robust reverse osmosis (RO) membranes that have an organically bridged polysilsesquioxane (PSQ) structure. The RO experiments with NaCl aqueous solution (2000 ppm) indicated that the introduction of hydroxyethylurea groups markedly improved the permeability of water (1.86 × 10-12 m3/m2sPa) to approximately 19 times higher than that of a membrane prepared via the BTESPA homopolymerization, with NaCl rejection remaining nearly unchanged (96%). This is the highest water permeability obtained so far for PSQ-based membranes that show higher than 90% NaCl rejection. The improvement of water permeability is likely due to aggregation through hydrogen bonding in the PSQ layer, which can be regarded as a hydrophilic water channel.

Network tailoring of organosilica membranes via aluminum doping to improve the humid-gas separation performance.

Organosilica membranes have recently attracted much attention due to excellent hydrothermal stability which enables their use in the presence of water. In particular, during humid-gas separations at moderate-to-high temperatures, these membranes have shown excellent water permeance and moderate water selectivity, which has been a breakthrough in separation performance. In the present work, we found that aluminum doping into the bis(triethoxysilyl)ethane (BTESE)-derived organosilica structure further improves water selectivity (H2O/N2, H2O/H2) while maintaining a level of water permeance that reaches as high as several 10-6 mol (m-2 s-1 Pa-1). Single-gas permeation and nitrogen adsorption experiments have revealed that aluminum doping promotes densification of the pore structure and improves molecular sieving. In addition, water adsorption and desorption experiments have revealed that aluminum doping enhances water adsorption onto the pore walls, which blocks permeation by other gasses and significantly improves water permeation selectivity during the separation of humid gases. Our results provide a strategy for the fabrication of a membrane that provides both a high level of water permeance and enhanced water selectivity.

Hydrothermal Stability of Hydrogen-Selective Carbon-Ceramic Membranes Derived from Polybenzoxazine-Modified Silica-Zirconia.

This work investigated the long-term hydrothermal performance of composite carbon-SiO2-ZrO2 membranes. A carbon-SiO2-ZrO2 composite was formed from the inert pyrolysis of SiO2-ZrO2-polybenzoxazine resin. The carbon-SiO2-ZrO2 composites prepared at 550 and 750 °C had different surface and microstructural properties. A carbon-SiO2-ZrO2 membrane fabricated at 750 °C exhibited H2 selectivity over CO2, N2, and CH4 of 27, 139, and 1026, respectively, that were higher than those of a membrane fabricated at 550 °C (5, 12, and 11, respectively). In addition to maintaining high H2 permeance and selectivity, the carbon-SiO2-ZrO2 membrane fabricated at 750 °C also showed better stability under hydrothermal conditions at steam partial pressures of 90 (30 mol%) and 150 kPa (50 mol%) compared with the membrane fabricated at 500 °C. This was attributed to the complete pyrolytic and ceramic transformation of the microstructure after pyrolysis at 750 °C. This work thus demonstrates the promise of carbon-SiO2-ZrO2 membranes for H2 separation under severe hydrothermal conditions.

Microporous Nickel-Coordinated Aminosilica Membranes for Improved Pervaporation Performance of Methanol/Toluene Separation.

The nickel-doped bis [3-(trimethoxysilyl) propyl] amine (BTPA) derived membrane has a microporous coordinated network that has high potential to be an ideal separation barrier for methanol-toluene azeotropic mixtures via the pervaporation process. Ni-BTPA membranes were modified by employing a nickel dopant over amine groups in mole ratios (mol/mol) that ranged from 0.125 to 0.50. The incorporation of different amounts of nickel dopant into flexible amine-rich organosilica precursors of BTPA increased the rigidity and resulted in a porous structure with a large specific surface area (increased from 2.36 up to 282 m2 g-1) and a high pore volume (from 0.024 up to 0.184 cm3 g-1). Methanol-toluene separation performance by the nickel-doped BTPA (Ni-BTPA) membranes was increased with increases in the nickel concentration. Ni-BTPA 0.50 showed separation performance that was superior to other types of membranes, along with a high-level of flux at 2.8 kg m-2 h-1 and a separation factor higher than 900 in a 10 wt % methanol feed solution at 50 °C. These results suggest that the balance between the microporosity induced by amine-nickel coordination and an excessive amount of nickel-ion facilitates high levels of flux and separation of methanol.

Multiple Amine-Contained POSS-Functionalized Organosilica Membranes for Gas Separation.

A new polyhedral oligomeric silsesquioxane (POSS) designed with eight -(CH2)3-NH-(CH2)2-NH2 groups (PNEN) at its apexes was used as nanocomposite uploading into 1,2-bis(triethoxysilyl)ethane (BTESE)-derived organosilica to prepare mixed matrix membranes (MMMs) for gas separation. The mixtures of BTESE-PNEN were uniform with particle size of around 31 nm, which is larger than that of pure BTESE sols. The characterization of thermogravimetric (TG) and gas permeance indicates good thermal stability. A similar amine-contained material of 3-aminopropyltriethoxysilane (APTES) was doped into BTESE to prepare hybrid membranes through a copolymerized strategy as comparison. The pore size of the BTESE-PNEN membrane evaluated through a modified gas-translation model was larger than that of the BTESE-APTES hybrid membrane at the same concentration of additions, which resulted in different separation performance. The low values of Ep(CO2)-Ep(N2) and Ep(N2) for the BTESE-PNEN membrane at a low concentration of PNEN were close to those of copolymerized BTESE-APTES-related hybrid membranes, which illustrates a potential CO2 separation performance by using a mixed matrix membrane strategy with multiple amine POSS as particles.

Recent Progress in a Membrane-Based Technique for Propylene/Propane Separation.

The similar physico-chemical properties of propylene and propane molecules have made the separation process of propylene/propane challenging. Membrane separation techniques show substantial prospects in propylene/propane separation due to their low energy consumption and investment costs, and they have been proposed to replace or to be combined with the conventional cryogenic distillation process. Over the past decade, organosilica membranes have attracted considerable attention due to their significant features, such as their good molecular sieving properties and high hydrothermal stability. In the present review, holistic insight is provided to summarize the recent progress in propylene/propane separation using polymeric, inorganic, and hybrid membranes, and a particular inspection of organosilica membranes is conducted. The importance of the pore subnano-environment of organosilica membranes is highlighted, and future directions and perspectives for propylene/propane separation are also provided.

TiO2 Coatings Via Atmospheric-Pressure Plasma-Enhanced Chemical Vapor Deposition for Enhancing the UV-Resistant Properties of Transparent Plastics.

Herein, TiO2 coatings were deposited on photodegradable polymers for protection from UV irradiation using the atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) technique. Polymethylmethacrylate (PMMA) and polycarbonate (PC) substrates were coated with titanium tetraisopropoxide as the precursor in an open-air atmospheric-pressure nonequilibrium argon plasma jet. The AP-PECVD-derived TiO2 coatings exhibited good adhesion to PMMA and PC. The TiO2 coatings could shield more than 99% of UV light in the wavelength range of 200-300 nm, without affecting the transmittance of visible light. UV irradiation tests on polymer films demonstrated that the degradation rates of PMMA and PC were significantly reduced by one-tenth after they were coated with TiO2 films.

Ultrafast Synthesis of Silica-Based Molecular Sieve Membranes in Dielectric Barrier Discharge at Low Temperature and Atmospheric Pressure.

Microporous silica membranes have shown promise as potential candidates for energy-efficient chemical separation. Herein, we report the ultrafast synthesis of silica membranes, on the order of minutes, in atmospheric-pressure, low-temperature plasma. Direct deposition in the discharge region of atmospheric-pressure plasma enables the immediate formation of a thin silica layer on a porous substrate. The plasma-deposited layer had a thickness of ∼13 nm and was confined to the immediate surface of the substrate. With an increase in deposition temperature, we observed an increase in the inorganic nature of the plasma-deposited layer and simultaneous improvement in the membrane performance. Consequently, the resulting membranes exhibited outstanding permeance for small-sized gas molecules, such as H2 (>10-6 mol m-2 s-1 Pa-1), with a high H2/SF6 permeance ratio of ∼6300, providing a nonthermal alternative for the fabrication of silica-based membranes.

SiC mesoporous membranes for sulfuric acid decomposition at high temperatures in the iodine-sulfur process.

Inorganic microporous materials have shown promise for the fabrication of membranes with chemical stability and resistance to high temperatures. Silicon-carbide (SiC) has been widely studied due to its outstanding mechanical stability under high temperatures and its resistance to corrosion and oxidation. This study is the first to prepare mesoporous SiC membranes for use in sulphuric acid decomposition to achieve thermochemical water splitting in the iodine-sulfur process. Single-gas permeation was carried out to confirm the stability of this mesoporous membrane under exposure to steam and H2SO4 vapor. Benefiting from the excellent chemical stability of the α-Al2O3 membrane support and the SiC particle layer, the SiC membrane exhibited stable gas permeance without significant degradation under H2SO4 vapor treatment at 600 °C. Additionally, with extraction, the membrane reactor exhibited an increased conversion from 25 to 41% for H2SO4 decomposition at 600 °C. The high performance combined with outstanding stability under acidic conditions suggests the developed SiC membrane is a promising candidate for H2SO4 decomposition in a catalytic membrane reactor.

Molecular Dynamics Simulation Study of Solid Vibration Permeation in Microporous Amorphous Silica Network Voids.

Microporous silica membranes have silica polymer network voids smaller than 3 Å where only small gas molecules such as helium (2.6 Å) and hydrogen (2.89 Å) can be transported. These silica membranes are highly expected to be available for H2 separation. In order to examine gas permeation mechanisms in the silica polymer network voids, factors such as membrane porous structures, gas diffusivity, and gas permeability were studied via membrane permeation molecular dynamics simulation. The thermal motions of silica membrane constituent atoms were examined according to classic harmonic oscillation potential using a suitable amorphous silica structure and non-equilibrium molecular dynamics (NEMD) simulations of gas permeation. The dynamic model successfully simulated the gas permeation characteristics in an amorphous silica membrane with a suitable Hooke's potential parameter. The introduction of the oscillative thermal motion of the membrane atoms enhanced gas diffusivity. Helium and hydrogen diffusivity and permeability were analyzed using gas translation (GT) and solid vibration (SV) models. The diffusion distance of gas molecules between adsorption sites was around 5.5-7 Å. The solid-type vibration frequencies of gas molecules in the site were on the order of 1013 and were reasonably smaller for heavier helium than for hydrogen. Both the GT and SV models could explain the temperature dependency of helium and hydrogen gas diffusivities, but the SV model provided a more realistic geometrical representation of the silica membrane. The SV model also successfully explained gas permeability in an actual silica membrane as well as the virtual amorphous silica membrane.

Tailoring Ultramicroporosity To Maximize CO2 Transport within Pyrimidine-Bridged Organosilica Membranes.

Amine-functionalized organosilica membranes have attracted an increasing amount of attention because of significant potential for the capture of postcombustion CO2. The appealing separation performance of these membranes, however, is generally obtained via compromises to gas permeance. In the present study, a novel, ultramicroporosity-tailored composite (organo)silica membrane with high flux was synthesized via sol-gel cocondensation of a pyrimidine-bridged organoalkoxysilane precursor 4,6-bis(3-(triethoxysilyl)-1-propoxy)-1,3-pyrimidine (BTPP) with a second intrinsically rigid network precursor (1,2-bis(triethoxysilyl)ethane or tetraethylorthosilicate). The surface chemistry, ultramicroporosity, and chain-packing state of the initial BTPP-derived membranes can be carefully tuned, which has been verified via Fourier transform infrared spectroscopy, water-contact angle measurement, X-ray diffraction, and positron annihilation lifetime spectroscopy. The composite (organo)silica xerogel specimens presented a slightly improved ultramicroporosity with noticeable increases in gas adsorption (CO2 and N2). However, a surprising increase in CO2 permeance (>2000 GPU), with moderate CO2/N2 selectivity (∼20), was observed in the resultant composite (organo)silica membranes. Furthermore, gas permeance of the composite membranes far surpassed the values based on Maxwell predictions, indicating a possible molecular-scale dispersion of the composite networks. This novel, porosity-tailored, high-flux membrane holds great potential for use in industrial postcombustion CO2 capture.

UV-Protective TiO2 Thin Films with High Transparency in Visible Light Region Fabricated via Atmospheric-Pressure Plasma-Enhanced Chemical Vapor Deposition.

This article focuses on control of film thickness and roughness to improve the ultraviolet (UV)-protective performance of TiO2 films prepared by atmospheric-pressure plasma-enhanced chemical vapor deposition using titanium(IV) isopropoxide (TTIP) as the precursor and argon as the plasma working gas. The relationship between the film morphology and UV-protective performance suggested that a decrease in roughness is the key factor to achieve performance improvement. The effects of substrate temperature and precursor concentration were investigated, and the results showed that an increase in both substrate temperature and precursor concentration reduced the roughness and improved the transparency to visible light without reducing the ability to block UV light. Finally, a TiO2 film with greater than 99% UV light blockage and greater than 95% transmittance of visible light was obtained.