Chin Myung Whang

Effects of pH During the Base Catalyzed Reaction of Two-Step Acid/Base Catalyzed Process on the Microstructures and Physical Properties of Poly(dimethylsiloxane) Modified Silica Xerogels

Poly(dimethylsiloxane) (PDMS) modified silica xerogels were prepared by two-step acid/base catalyzed sol-gel process. By keeping the acid amount and hydrolysis period in acidic environment fixed, and adding different amounts of base to the sol afterwards, attempts had been made to study the effect of the different amounts of base catalyst on the micro-structure and physical properties of the prepared ORMOSILs. DTA, SEM, BET, FTIR were performed to characterize the derived specimens. The microstructure and physical properties are greatly influenced by the amount of base added to the sol. With increasing base content the crystallite size and porosity increase and the pore size distribution takes a broad spectrum. Whereas, the glass transition temperature seems to decrease with increasing of base catalyst amount. This is explained in terms of the change in the relative amounts of hydrolysis and condensation reactions due to the addition of different amount of base catalyst, which predominantly influences the condensation reactions. The results are reported in this communication along with possible explanations behind the observations.

Microstructures and mechanical properties of organically modified silicate prepared under various process conditions

Abstract Inorganic–organic hybrids or organically modified silicates (ORMOSILs), with tailorable optical and electrical properties depending on the inorganic and organic components chosen, processing conditions, etc., offer many new exciting properties that lead to different applications. In this study, tetraethoxysilane (TEOS) and poly(dimethylsiloxane) (PDMS) were chosen as the inorganic and organic components respectively, to prepare ORMOSIL matrix. Silanol-terminated PDMS can condense with oxide precursors to produce metal–O–Si bond and is known for its superior relaxation. The effects of H 2 O/TEOS ratio, PDMS molecular weight and different reaction schemes (i.e. reaction order) on the microstructures and mechanical properties of the SiO 2 –PDMS ORMOSILs have been studied. Attempts are made to explain the results in the light of presently available theories.

Effects of pH and Dye Concentration on the Optical and Structural Properties of Coumarin-4 Dye-Doped SiO2-PDMS Xerogels

Optical and structural properties of coumarin-4 dye-doped SiO2-PDMS xerogels synthesized by acid catalyzed (one-step) and acid-base catalyzed (two-step) sol-gel routes with varying pH (0.6 to 7) and dye content ( 5 × 10−4 to 5 × 10−2 mole) are reported. Spectroscopic methods such as photo-luminescence, FT-IR and FT-Raman were used for characterizations. The acid catalyzed xerogels prepared with pH < 2.5 exhibited two fluorescence peaks, I at ∼390 nm and II at ∼480 nm. The acid-base catalyzed xerogels synthesized with pH < 2.5 also exhibited two peaks, I at ∼400 nm and II at ∼475 nm. Peak II was not observed for the samples with pH ≥ 2.5. This phenomenon was attributed to the existence of pH dependent different forms of coumarin-4 molecule. The concentration-quenching phenomenon was observed for the acid catalyzed xerogels prepared with different dye concentration. TheFT-IR spectra indicated the existence of hydrogen bonds between the carbonyl groups of dye molecules and the silanol groups of gel matrix. The hydrogen bonding was the highest for the samples with the extremity pH, 0.6 and 7, resulting in the highest dye/gel matrix interactions, hence, the highest fluorescence peaks. The Raman studies indicated that the samples prepared with pH < 2.5 possessed relatively more number of 3-membered siloxane rings than 4-membered siloxane rings. The ring statistics was reversed for the samples with pH > 2.5. The number of silanol groups was higher for the samples with pH > 2.5.

Transport properties of a new Li+ ion-conducting ormolyte: (SiO2-PEG)-LiCF3SO3

The transport properties of a new fast Li+ ion-conducting sol–gel-derived ormolyte, (SiO2–10 wt% PEG)–[Li/O] LiCF3SO3, with [Li/O] = 0–0.1 (mol/mol), is reported. The composition with [Li/O] = 0.04 exhibited the highest conductivity (σ25 °C = 1 × 10−4 S cm−1), with an enhancement of 103 over the host matrix (SiO2–10 wt% PEG xerogel), and has been found to be the ‘optimum conducting composition’. Direct determination of Li+ ion mobility (μ) and mobile ion concentration (n) revealed that the enhancement was due to the increase in both μ and n. Studies of the variation of σ, μ, and nversus temperature indicated that the system shows Arrhenius-type behavior. The activation energy and energies of migration and formation were evaluated from their respective Arrhenius plots. Measurement of the ion transference number (tion) confirmed that the ions are the sole charge carriers in the system. These results are discussed in the light of existing theories.

Photocatalytic Properties of the Transition Metal Doped TiO2 Powder Prepared by Sol-Gel Process

Transition metal-doped TiO2 powders as a photocatalyst were prepared by sol-gel process and Sb, Bi and Nb were introduced into them as dopants. The photocatalytic behaviors of the doped TiO2 powder were studied as a function of dopant, doping concentration and preparation conditions. X-ray diffraction, FT-Raman, B.E.T. and scanning electron microscopy were applied for structural and microstructural studies. Optical properties of the doped TiO2 powders were studied by UV-Visible Spectrometer and photocatalytic activity of the doped TiO2 was characterized in terms of the degradation of 1,4-dichlorobenzene. X-ray difraction analysis showed that doping with a transition metal ion suppresses anatase-to-rutile phase transition compared with the pure TiO2. The Sb and Nb-doped TiO2 powders did not exhibit any other diffraction peaks except those belonging to TiO2. On the other hand, a diffraction peak of Bi4Ti3O12 appears for 5 at.% Bi-doped samples. All of the doped TiO2 powders had higher specific surface area than undoped TiO2. Surface area increased with increasing dopant concentration depending on the dopant, from 33.9 m2/g to 55.4m2/g. The UV-visible absorption spectra of doped samples were red-shifted by 20~50nm according to the doping level. Also transition metal doped TiO2 powders exhibited better photocatalytic activity than the undoped TiO2. The increase in photoactivity is probably due to the increase in the interfacial electron transfer, red shifts, and better crystallinity.

Structural study of a sol–gel derived novel solid oxide fuel cell perovskite: (La1−xSrx)(Cr0.85Fe0.05Co0.05Ni0.05)O3−δ

The effect of composition on the structure of the new solid oxide fuel cell (SOFC) perovskite (La1−x Srx )(Cr0.85Fe0.05Co0.05Ni0.05)O3−δ is studied using x-ray diffractometry, pycnometry, and Fourier-transform infrared and Raman spectroscopy. Samples were synthesized using the Pechini method via doping the La site with Sr 2+ , to a composition with x = 0‐0.4 mole fraction, and at the Cr site with 0.05 mol of Co 2+ , Fe 2+ and Ni 2+ , and the structural results were compared with those for orthorhombic LaCrO3 .T he samples were orthorhombic perovskite of the Pnma space group. A second phase, monoclinic SrCrO4, was also observed, whose content increased linearly with increasing x. A large decrease in the lattice and hence volume was observed for the sample with x = 0.1, which was accompanied by a sharp decrease in the Cr−O bond length, tilt angle and crystallite size. The structural parameters were almost invariant for x = 0.1‐0.4, probably due to an increase in content of the high valence cations of smaller ionic radii, as suggested by an increase in the intensity of the defect band (∼515 cm −1 , IR). The octahedral CrO6 stretching, bending and tilting modes were found shifted accordingly. A mechanism employing a partial-charge model was proposed to explain the results. (Some figures in this article are in colour only in the electronic version)

Mechanical, electrical and micro-structural properties of La0.6Sr0.4Co0.2Fe0.8O3 perovskite-based ceramic foams

Mechanical, electrical and micro-structural properties of new electronic conducting ceramic foams are reported. Ceramic foams are prepared using the slurry of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) by the polymeric sponge method, which is followed by spray coating for increasing the number of coatings?sinterings on polyurethane foams of 30, 45 and 60?ppi (pores per linear inch). An increase in the number of coatings?sinterings and ppi improved the compressive strength, density and electrical conductivity by decreasing the porosity to ~76%, as also observed by the SEM study. The three-times coated?sintered ceramic foams (60?ppi) exhibited optimum values of compressive strength of ~1.79?MPa and relative density of ~0.24 at 25??C and electrical conductivity of ~22?S?cm?1 at 600??C with an activation energy of ~0.22?eV indicating its suitability as a solid oxide fuel cell current collector. The experimental results are discussed in terms of the Gibson and Ashby theoretical model.

Effects of Ultrasonic Irradiation on Physical Properties of Silica/PEG Hybrids

The effect of ultrasonic radiation is reported for silica-poly(ethylene glycol) system prepared without the solvent using sol-gel processing by varying various parameters such as ultrasonic irradiation time, PEG content and HCl/TEOS molar ratio. The property of sonogel is compared with classic gel which has been prepared with ethanol as a solvent by traditional sol-gel processing. SEM, BET, DTA-TGA, density and Vickers hardness measurements are carried out for analyzing the samples. The gelation time is found strongly dependent on radiation time, PEG content and pH value, and has been discussed on the basis of existing theories. The SiO₂-10 &20 wt% PEG sonogel exhibited superior optical, physical and gel properties as compared to the classic gel, hence, found suitable for device applications. The ultrasonic radiation increased the density and surface area, and also reduced the pore size which is well supported by the shift in the peak of DTA curve. The DTA thermogram was found similar to that of pure silica gel.

Apatite Formation on PDMS-Modified SiO2-CaO-P2O5 Hybrids Prepared with Different P2O5 Content by Sol-Gel Method

Bioactive ORMOSILS (organically modified silicate), PDMS-CaO-SiO2-P2O5 with five different P2O5 content (0, 0.01, 0.03, 0.06, 0.09 mol%) have successfully been synthesized by sol-gel process. The hybrids have been prepared with polydimethylsiloxane (PDMS), tetraethoxysilane (TEOS), calcium nitrate tetrahydrate [Ca(NO3)2 4H2O] and triethyl phosphate (TEP) as starting materials and subsequently soaked into the simulated body fluid (SBF) for different period of time and the bioactivity of hybrids was determined by examining the apatite formation on the surface of the specimen by FT-IR, Thin-Film X-ray Diffraction, and Scanning Electron Microscopy (SEM). All of the prepared samples with different P2O5 content showed in vitro bioactivity. It was observed that the increase in P2O5 content up to 0.03 mole % increases the apatite formation compared to P2O5- free hybrids. However, further increase in P2O5 concentration slows down the formation of the apatite layer most probably due to the decrease of pH of SBF by dissolution of a large amount of phosphate ions.

Nd3+-Doped TiO2 Nanoparticles Prepared by Sol-Hydrothermal Process

Neodymium ion doped TiO 2 nanoparticles were prepared by sol-hydrothermal process using titanium isopropoxide as the TiO 2 precursor. The prepared neodymium ion doped TiO 2 nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The photocatalytic degradation of 1,4-dichlorobenzene (DCB) was conducted in the suspension of Nd 3+ doped TiO 2 nanoparticles. The photocatalytic behaviors and microstructure of the prepared Nd 3+ -TiO 2 nanopowder were investigated as the function of dopant content. The effect of Nd 3+ content of the doped TiO 2 nanoparticles on the DCB photodegradation reaction rate was investigated and the first order reaction rate constant (k = 0.153min -1 ) for the doped nanoparticles was found to be significantly larger than that for the undoped TiO 2 nanoparticles. The result showed the optimum value of the Nd 3+ content to be 2 mole %.