Conventional Sol-gel Process Research Articles

A novel necklace like structure assembled with MCM-41 and carbon nanotubes

Abstract We report the synthesis of MCM-41 particles which are pierced by carbon nanotubes forming a necklace like structure. We obtained these nanostructures using conventional sol–gel process for mesoporous MCM synthesis, incorporating highly disperse carbon nanotubes. This necklace like structure can be treated to eliminate the surfactant from mesoporous MCM particles obtaining thus a interconnected particles with high surface area (∼1000 m 2 g −1 ), and calcination treatment under air can also be used to obtain the SiO 2 structure free from carbon, maintaining the necklace structure. Some applications of these nanocomposites are suggested in this work.

A modified sol-gel process for multiferroic nanocomposite films

Multiferroic CoFe2O4–Pb(Zr,Ti)O3 (CFO-PZT) composite films with nanoscale mixture of the two phases were prepared by a modified sol-gel process, in which a mixed precursor solution of both CFO and PZT was used. X-ray diffraction and transmission electron microscopy examinations revealed the coexistence of perovskite PZT and spinel CFO that were mixed in nanoscale with mean grain sizes of 5–10nm. Magnetic properties of the CFO-PZT nanocomposite were examined, which were consistent with their microstructures. The magnetoelectric coupling between CFO and PZT was demonstrated by an external magnetic field induced electric polarization change. This modified sol-gel processing provides an alternative for multiferroic composite films, which is simpler and easier to control compared to the conventional layer-layer sol-gel process for multiferroic composite films.

Thermosoftening phenyl siloxane glasses prepared via sol concentration

Thermosoftening phenyl polysiloxane glasses have been prepared by concentration of the corresponding sol above the melting temperature of the resultant materials, which we call the ‘sol concentration method’ (SCM). Because the glass melt is directly obtained without a gelation process, a remarkable shortening of the process time and suppression of crack formation in the sol–gel transition region have been realized. That is the whole process time of the SCM (about 24 h) is much shorter than that of the conventional sol–gel process (about 10 days). Thermosoftening phenyl polysiloxane glasses with high transparency in the UV region were obtained by the SCM.

Preparation of lanthanum-doped TiO2 photocatalysts by coprecipitation

The lanthanum-doped TiO2 (La3+-TiO2) photocatalysts were prepared by coprecipitation and sol–gel methods. Rhodamine B was used as a model chemical in this work to evaluate the photocatalytic activity of the catalyst samples. The optimum catalyst samples were characterized by XRD, N2 adsorption–desorption measurement, SEM and electron probe microanalyses to find their differences in physical and chemical properties. The experimental results showed that the La3+-TiO2 catalysts prepared by coprecipitation exhibited obviously higher photocatalytic activities as compared with that prepared by the conventional sol–gel process. The optimum photocatalysts prepared by the coprecipitation and sol–gel process have similar adsorption equilibrium constants in Rhodamine B solution and particle size distribution in water medium although there are larger differences in their surface area, morphology and pore size distribution. The pores in the sol-gel prepared catalysts are in the range of mesopores (2–50 nm), whereas the pores in the coprecipitation prepared catalysts consist of bigger mesopores and macropores (>50 nm). The morphology of the primary particles and agglomerates of the La3+-TiO2 catalyst powders was affected by doping processes. The inhibition effect of lanthanum doping on the phase transformation is greater in the coprecipitation process than in the sol–gel process, which could be related with the different amount of Ti–O–La bonds in the precursors. This finding could be used for preparing the anatase La3+-TiO2 catalysts with more regular crystal structure through a higher heat treatment temperature. The optimum amount of lanthanum doping is ca. 1.0 wt.% and the surface atomic ratio of [O]/[Ti] is ca. 2.49 for 1.0 wt.% La3+-TiO2 catalysts prepared by the two processes. The obviously higher photocatalytic activity of the La3+-TiO2 samples prepared by the coprecipitation could be mainly attributed to their more regular anatase structure and more proper surface chemical state of Ti3+ species. The optimum preparation conditions are 1.0 wt.% doping amount of lanthanum ions, calcination temperature 800 °C and calcination time 2 h using the coprecipitation process. As compared with the sol-gel process, the coprecipitation process used relatively cheap inorganic raw materials and a simple process without organic solvents. Therefore, the coprecipitation method provides a potential alternative in realizing large scale production.

Fabrication and Characterization of PZT Thick Films for Sensing and Actuation

Lead Zirconate Titanate oxide (PZT) thick films with thicknesses of up to 10 μmwere developed using a modified sol-gel technique. Usually, the film thickness is less than1 μm by conventional sol-gel processing, while the electrical charge accumulation whichreveals the direct effect of piezoelectricity is proportional to the film thickness and thereforerestricted. Two approaches were adopted to conventional sol-gel processing – precursorconcentration modulation and rapid thermal annealing. A 10 μm thick film was successfullyfabricated by coating 16 times via this technique. The thickness of each coating layer wasabout 0.6 μm and the morphology of the film was dense with a crack-free area as large as 16mm2. In addition, the structure, surface morphology and physical properties werecharacterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomicforce microscopy (AFM) and electrical performance. The dielectric constant and hysteresisloops were measured as electric characteristics. This study investigates the actuation andsensing performance of the vibrating structures with the piezoelectric thick film. Theactuation tests demonstrated that a 4 mm x 4 mm x 6.5 μm PZT film drove a 40 mm x 7 mmx 0.5 mm silicon beam as an actuator. Additionally, it generated an electrical signal of 60mVpp as a sensor, while vibration was input by a shaker. The frequencies of the first twomodes of the beam were compared with the theoretical values obtained by Euler-Bernoullibeam theory. The linearity of the actuation and sensing tests were also examined.

Fabrication of (Ba,Sr)TiO<sub>3</sub> Thin Films by Hydrothermal Crystallization of Sol-Gel Precursor

Barium strontium titanate (BST) ferroelectric thin films have been successfully prepared on the single crystal silicon substrates using hydrothermal crystallization of sol-gel precursor, which combined the conventional sol-gel process and the hydrothermal method. The precursor film was deposited by the dip-coating method. The multiplayer BST gel films were hydrothermally treated in a Teflon vessel at low temperature of 160-180 °C for 1-3.5 h. The influences of the reaction conditions on the structure and crystallizing degree of BST thin films were discussed. We find that the temperature of as-dried gel films and post hydrothermal treatment are the critical parameters determining the quality of formation of BST thin films. Under optimized conditions, we can obtain well-developed perovskite BST films with smooth and pinhole-free surfaces. These results reveal that this technique is a promising low temperature process for the fabricating of thin films.

Photoluminescence characteristics of NASICON materials

Besides gas sensitivity, luminescence characteristics of NASICON (Na + super ion conductor) materials were found. In this paper, the photoluminescence (PL) properties of NASICON and doped-NASICON materials were investigated. NASICON was synthesized by conventional sol–gel process and doped with Dy 2O 3 by 1 wt.%, 3 wt.% and 5 wt.%, respectively. The ultraviolet light (325 nm, He–Cd laser) excited luminescent emissions of the resulted powders are recorded vs. wavelength in the range from 350 to 650 nm. The main peak of the pure NASICON is found at the wavelength of 474 nm (blue light), the transition energy is 2.616 eV. The luminescent intensity is increased after doping with Dy 2O 3.

Sol-gel based synthesis of complex oxide nanofibers

Electrospinning is a versatile and straight forward process for synthesizing one-dimensional (1D) nanostructures of diverse materials. Recently, a large variety of oxide ceramics have been synthesized in combination with conventional sol-gel processing. Here, the synthesis of BaTiO3 nanofibers via electrospinning is reported. The structural evolution from amorphous to crystalline is presented under various heat treatment conditions. Nanofibers with well-defined perovskite tetragonal phase were achieved with an average crystallite size of about 20 nm. Furthermore, single crystalline BaTiO3 nanofibers with 50 nm in diameter and lengths up to 1 μm were found, which is a novelty in electrospinning of ferroelectrics. XRD peak splitting confirmed the tetragonal perovskite structure, and this was fully supported by further evidence from selected area electron diffraction and Raman spectroscopy.

Dielectric and optical properties of BaTiO3 thin films prepared by low-temperature process

Barium titanate (BaTiO3) thin films have been prepared by low temperature processing on Pt/Ti/SiO2/Si substrates using sol-gel-hydrothermal (SGHT) technique, which combined the conventional sol-gel process and hydrothermal method. X-ray diffraction analysis showed that the barium titanate thin films are polycrystalline. As-reacted barium titanate films grown on Pt(111)/Ti/SiO2/Si(100) substrates had a dielectric constant (e) and loss tangent (tanδ) of 80 and 0.05 at 1 MHz, respectively. The optical constants including refractive index n, extinction coefficient k, and absorption coefficient α of the barium titanate thin films in the wavelength range of 2.5–12.6 μm were obtained by infrared spectroscopic ellipsometry.

Properties and preparation of thermoplastic polyurethane/silica hybrids using a modified sol–gel process

AbstractThermoplastic polyurethane elastomer/silica hybrids (TPU/SiO2) were prepared using a modified sol–gel process. This modified approach used tetrahydrofuran (THF) to extract polysilicic acid from aqueous sodium metasilicate solutions. A THF or aqueous solution containing sol particles was mixed with TPU solution to form TPU/silica hybrids. The Fourier transform infrared (FTIR) spectra and dynamical mechanical properties were recorded to depict the interfacial interaction. Storage modulus increased due to the reinforcement of the in situ silica. Tear strength decreased significantly with increasing in situ silica content. Cutting strength decreased slightly due to crack tip confinement. The modified process with extraction showed improved mechanical properties and better optical properties than the conventional sol–gel process without extraction. The fracture surface was revealed through SEM (scanning electron microscopy) to observe the degree of SiO2 dispersion, which in turn confirmed the optical and mechanical property results. POLYM. ENG. SCI., 47:77–86, 2007. © 2007 Society of Plastics Engineers

Fabricating sol–gel glass monoliths with controlled nanoporosity

Nanotopography is known to affect cell response, but the mechanisms are unknown. It is therefore important to be able to produce biomaterial surfaces with controllable nanopore sizes and morphologies from materials that can be used to make templates (scaffolds) for tissue regeneration. A rapid method of fabrication of sol–gel monoliths with controlled nanopore size is described in this paper. Pores of 2 nm in diameter were achieved, termed micropores by IUPAC convention. Conventional sol–gel processing yields pores an order of magnitude larger. Gelling time was also reduced from many hours to a few minutes, without using a gelling agent, and large crack-free monoliths were synthesized within 1 week.

Structure and dielectric properties of barium titanate thin films grown by sol-gel-hydrothermal process

Barium titanate (BT) thin films have been prepared on Pt∕Ti∕SiO2∕Si substrates through the use of a low temperature process. This technique combines the conventional sol-gel process and hydrothermal methods. X-ray diffraction analysis showed that the BT thin films had a pure perovskite structure. Au∕BT∕Pt capacitors have been fabricated and the dielectric properties have been discussed for SGHT-prepared BT films treated at 180 and 500°C, respectively. It was found that the BT films annealed at 500°C had much better ferroelectric properties. This improvement of ferroelectric properties through a heat treatment is probably associated with the change of density of hydroxyl groups in the films.

Supra-amphiphilic transparent mesoporous silica coating

Transparent mesoporous silica coatings were achieved by conventional sol-gel process. The obtained coatings display permanent supra- amphiphilicity, transparent appearance and good wetting property with very fast spread rate. Incorpo- ration of functional materials such as crystalline tita- nia nanoparticles into the coatings was also carried out without affecting the transparency and supra- amphiphilicity.

Gas-phase photocatalytic decontamination using polymer supported TiO2

An alternative method to the conventional sol–gel process is proposed to incorporate P-25 TiO2 (Degussa) onto polydimethylsiloxane (PDMS) and orthophthalic polyester (OP) polymers. These TiO2-incorporated films were characterized using SEM–EDS and showed a homogenous distribution of TiO2. The catalytic activity of the films was tested in aqueous phase under black light (365nm) illumination, using solutions containing dichloroacetic acid, salicylic acid and phenol. OP/TiO2 films presented the best performance, showing no loss of catalytic activity after reuse. Further tests were performed using this film in the form of fixed bed catalyst coated onto the internal walls of a plug-flow concentric gas-phase reactor (767mL) provided with a black light lamp (365nm) and an internal area of ∼900cm2. Ethanol vapor was used as the model compound at 0.05 or 0.02Lmin−1 mutually with humid synthetic air at 0.15 or 0.18Lmin−1, totaling a 0.2Lmin−1 flow rate. A continuous monitoring of the photoreactor outlet using GC–MS showed total mineralization of the model compound within 25min at a concentration of 126ppmv. The polymer substrate showed to be resistant to degradation and is a promising material for the decontamination of confined atmospheres.

Electrospinning: A Simple and Versatile Technique for Producing Ceramic Nanofibers and Nanotubes

Electrospinning is a remarkably simple method for generating nanofibers of polymers. When combined with conventional sol–gel processing, it provides a versatile technique for producing ceramic nanofibers with either a solid, porous, or hollow structure. This article presents a brief overview of recent progress in preparation of ceramic nanofibers by electrospinning, with a focus on an introduction to experimental procedures and analysis of several technical issues that are vital for a successful electrospinning experiment. We also highlight the unique capabilities of this technique in processing ceramic materials into nanostructures, and illustrate some potential applications of these nanostructures.

Fabrication of sol–gel 70GeO2–30SiO2 thick films from TEOG and DEOS and investigation of the 5 eV band

Crack-free germanosilicate films, 3.6 µm-thick and containing up to 70 mol% germanium dioxide, which are inaccessible through the conventional sol–gel process, were fabricated using tetraethyl orthogermanate (TEOG) and diethylorthosilicate (DEOS) as precursors for germania and silica, respectively. The studies using viscosity, TEM and SEM revealed that DEOS contributed largely to stabilizing the GeO2–SiO2 sol and suppressing crack formation in thick films. XRD study showed that the films remained amorphous after being sintered at 600 °C in air for 60 min and annealed at 550 °C under a flowing H2/N2 atmosphere for 120 min. An intense absorption band at around 241 nm was distinctly observed in the films. The intensity of this absorption band was found to be effectively bleached by UV illumination. Weak photoluminescence emission bands which originated from the neutral oxygen di-vacancy were detected near 375 and 275 nm. Therefore, the 5 eV absorption band observed in this work was mainly caused by the neutral oxygen monovacancy. A saturated absorptivity change of the UV-bleachable band after prolonged illumination was found to be 389 cm−1 for 70GeO2–30SiO2 films.

Anhydrous Sol-Gel Synthesis of Titania-Doped Siloxane Polymer for Integrated Optics

Sol-gel synthesis of organic-inorganic hybrid materials for planar waveguides and devices has received growing interest due to its low-cost processing and good suitability for doping. Titania is an important optical dopant, but homogeneous incorporation of titania in silica is difficult to be achieved by the conventional sol-gel process (aqueous system) because of the significant difference between the hydrolysis rates of the precursors. In this paper, we report an anhydrous sol-gel process for synthesising titania-doped siloxane polymers. The process consists of a hydrolysis of 3-methacryloxypropyltrimethoxysilane (MPS) with boric acid under anhydrous conditions, and a condensation with dimethyldimethoxysilane (DMDMS), diphenyldimethoxysilane (DPhDMS) and titanium ethoxide (TET). Optical characterisations for the produced titania-doped polymer were performed, and results showed that TET doping is useful for reducing the OH concentration of the synthesised polymer and is also effective for improving the optical quality of spin coatings. DMDMS and DPhDMS are favourable in reducing the birefringence and in increasing the thermostability of the material, and the methacryl groups of MPS are UV-polymerizable, which is useful for low cost fabrication of waveguides by photolithographic process. The results of ellipsometry scanning measurements show that titania is homogeneously incorporated in the hybrid matrix, suggesting that the anhydrous sol-gel process is useful for preparation of UV-sensitive titania-doped siloxane polymers for optical applications.

Evolution in Microstructural Properties of Cetyltrimethylammonium Bromide‐Templated Mesoporous Tin Oxide upon Thermal Crystallization

Mesoporous crystalline SnO2 was synthesized using a supermolecule‐templating process, in which cetyltrimethylammonium bromide (CTAB) was introduced into a homogenously peptized tin oxyhydroxide sol, followed by calcination in air. The templated powders were calcined either with or without being subjected to a hexamethyldisilazane (HMDS) pretreatment, and the microstructures were characterized as a function of calcination temperature up to 800°C. Organized mesostructure was initially obtained, and grain growth on calcination was the primary factor that destabilized the mesostructure. Compared with the conventional sol–gel process, the templating process promotes the formation of mesoporosity, enlarges pore volume, and drastically slows grain growth. The HMDS treatment further inhibits grain growth above 400°C. The combination of these processes facilitates formation of fully crystallized mesoporous SnO2 materials with very large surface areas. Fully crystallized mesoporous SnO2 that has a surface area of 220 m2/g has been thus obtained by calcination up to 500°C.

Preparation of UO2 Kernel for HTR-10 Fuel Element

A 10 MW high temperature gas-cooled reactor (HTR-10) was constructed in Institute of Nuclear Energy Technology (INET) of Tsinghua University, China. HTR-10 reached its first criticality successfully in December, 2000 and realized its full power operation in the beginning of 2003. Fabrication technology for HTR-10 spherical fuel element has been established and developed in INET through a lot of R&D activities during the past 20 years. A process known as total gelation process of uranium (TGU) has been developed to prepare UO2 kernel for HTR-10 fuel element in INET. The TGU process combined advantages of both external gelation and internal gelation method of conventional sol-gel process. The sol prescription consists of not only urea, polyvinylalcohol (PVA), tetrahydrofurfuyl alcohol (THFA), but also hexamethylene tetra-amine (HMTA) that is always employed in internal gelation process. The TGU process and facilities are described in this paper. Fuel kernel production for HTR-10 fuel element using this TGU process has completed up to April, 2002. This paper also reported the quality inspection data of kernel production.

티타늄 뭉치 화합물의 합성과 분자 구조

Since Vecht and Quddus have reported the preparation of alkaline earth sulfides by using the reaction between alkaline earth alkoxides and H 2 S in 1976, a thio sol-gel process has been reported for the synthesis of metal sulfides using alkoxides or thiolates. The thio sol-gel process in which metal alkoxides act as the metal source uses H 2 S for a thiolysis reaction similar to H 2 O for the hydrolysis reaction in the conventional sol-gel process as follows: