Crystallization of Gels in the Apatite-Mullite System

Crystallization processes in gels of the apatite-mullite system were studied to obtain information for the synthesis of bioglass-ceramics and composite materials. SiO2-sol, Al(NO3)3·9H2O, Ca(NO3)2·4H2O, (NH4)3PO4·3H2O and CaF2 were used as precursors. CaF2 was added before and after gelation. Mixtures of mullite gel-glass and fluorapatite in the range 10 to 90 mol% were investigated for synthesis of composites. All the samples were heat treated at different temperatures in the range 950–1250°C and the structural changes were established using X-ray diffraction and IR-spectroscopy. When the gels were treated at 1050°C and at 1150°C, the main crystalline phases found were fluorapatite and mullite independent of the CaF2 content and the manner of its addition. At 1250°C the relative amounts of fluorapatite and mullite decrease and gehlenite appears. Composite materials containing fluorapatite and mullite as main crystalline phases can be obtained only when the content of mullite gel-glass in the initial mixture is more than 60 mol%.     

Phase Formation in Gels of the Apatite-Anorthite System

Phase formation in heat-treated composites containing anorthite gel and fluorapatite has been investigated. For the sol-gel synthesis TEOS, Al(NO3)3·9H2 O, Ca(NO3)2·4H2O, (NH4)3PO4·3H2O, NH4F and CaF2 were used as precursors. The gel mixtures were treated from 200 to 1250°C. It was found that the gels remain in an amorphous state up to 900°C. From X-ray diffraction structural analysis of anorthite gel-glasses heat treated at 600°C and 800°C it was found that a shift of the first maximum occurred from 1.85 to 1.77 Å, which could be interpreted as a transformation of the gel-glass structure with increasing temperature. During the heat treatment of the mixed gels from the apatite-anorthite system it was found that fluorapatite is present as a major crystal phase at 950°C. At 1250°C anorthite and gehlenite are the major crystalline phases.     

Preparation and Characterization of Forsterite (Mg2SiO4) Xerogels

Mg2SiO4 gels were prepared from alkoxide precursors, and the formation of the forsterite crystal phase was studied after heat treatments up to 1200°C. Prehydrolyzed TEOS in solution with 2-methoxyethanol was mixed with Mg(OEt)2, and the solution was hydrolyzed using excess water. The resultant gels were dried at 100°C to form xerogels which were subsequently powdered. These powders were characterized using thermal analysis (DTA and TGA), surface area analysis (BET), X-ray diffraction (XRD) and transmission electron microscopy (TEM).DTA and XRD indicated that forsterite crystallized at 770°C, and by 1000°C the powders were predominantly crystalline. BET gave powder surface areas between 400 and 550 m2 g–1. TEM revealed angular particles with sizes between 0.2 and 2 m. The low temperature of crystallization of forsterite indicates a high degree of intimate mixing between the precursor alkoxides, although XRD indicated some degree of inhomogeneity.     

Sol-Gel Synthesis and Microstructural Characterization of Silicon Oxycarbide Glass Sheets with High Fracture Strength and High Modulus

A synthesis route to silicon oxycarbide glass sheets (thickness 40 to 1000 m and area up to 20 × 35 cm2) has been developed for the first time starting from a methyl modified sol containing nano-particulate SiO2 and having a solid content of 70 wt%. The gel sheets obtained by casting and drying of this sol were sintered in N2 atmosphere at temperatures between 900–1650°C. Only by the incorporation of colloidal silica (0.10–0.35 mole per mole alkoxide) to the sol could crack-free, large area glass sheets be obtained. Fracture strength (three point bending) was found to attain a maximum (200–300 MPa) for the sheets sintered at 1000°C. Young's modulus attained a peak value between 120 and 130 GPa for the sheets sintered at 1200°C. HR-TEM studies showed an amorphous and homogeneous matrix up to a sintering temperature of 1200°C, whereas at 1450°C and 1650°C, crystallites of SiC and lamellar graphite were formed. It is concluded that addition of colloidal silica to the sol does not lead to inhomogeneities after sintering and therefore does not decrease the strength and elastic modulus.     

Formation of gels and gel-glasses in the VO2(V2O5)-SiO2 systems

Blue-coloured gels have been prepared in the VO₂-SiO₂ system up to 80 mol% VO₂ by sol-gel technology using TEOS and aqueous solutions of VOSO₄·5H₂O. It is established by means of VIS and ESR spectra that at low temperatures VO²⁺ complexes are formed. An oxidation of V⁴⁺ has taken place with increasing temperature, and V₂O₅ and cristobalite have been separated. Silica gel glasses stable up to 800°C have been obtained from gels containing 1–3 mol% VO₂.     

Synthesis and Characterization of PZT Fibers via Sol-Gel

A fiber forming PZT gel containing 58.5 wt% PZT was synthesized by using zirconium-n-propylate, titanium-iso-propylate, lead acetate and butoxyethanol. Unseeded PZT gels and gels containing 0.5 wt% PZT perovskite seeds (Ø = 200–300 nm) could be extruded through a monofilament nozzle (Ø = 100 m) at pressures between 50 and 150 bar, whereas PZT gels, containing 1 and 2 wt% PZT particles, were pressed through the nozzle at higher pressures (200–300 bar). The microstructure of unseeded and seeded (0.5, 1, 2 wt% PZT) PZT fibers was characterized by SEM. Unseeded fibers had three different shells at 450°C: an external dense shell (approx. 200 nm thick), a middle shell consisting of a porous structure (1.5m thick) and the center of the fiber, characterized by a matrix containing globular particles. At 700°C, a 200–250 nm thick and dense external shell and a porous fiber interior were be observed. 2 wt% of PZT seeds was necessary to densify the fiber completely. The seeds were located in the center of each PZT perovskite rosette.     

Effect of ZnO on the performance of Ag/SiO2 catalyst for the vapor-phase synthesis of 3-methylindole

The vapor-phase synthesis of 3-methylindole over Ag/SiO2 doped with ZnO was investigated. The catalysts were characterized by XRD, H2- TPR, NH3-TPD and TG techniques. The results indicated that ZnO promoter greatly enhanced the initial activity of the catalyst but disfavored its stability. H2-TPR and XRD results showed that the reduction peak of Ag2O shifted to higher temperature and the intensity of silver diffraction peaks was much weaker after the addition of ZnO promoter to Ag/SiO2. This indicated that there existed the interaction between Ag2O and SiO2-ZnO which promoted the silver particles dispersing on the support and inhibited the sintering of silver during the reaction. NH3-TPD and TG results revealed that the acid amounts of the catalyst and coking increased after adding ZnO to Ag/SiO2, which resulted in the deactivation of Ag/SiO2-ZnO catalyst rapidly.     

Structural Characterization of Silver Doped Silica Prepared by Two Different Wet Chemical Methods

Two sets of silver doped silica samples were prepared, one using the traditional sol-gel method and the other from colloidal silica. In samples prepared by the first method, the addition of Ag promotes crystallization of the SiO₂ matrix when annealed at temperatures below those marked by the phase diagram. Before crystallization of the silica glass into the -cristobalite phase occurs, the silver diffuses throughout the amorphous network to form silver colloidal particles at annealing temperatures that depend on the silver concentration. In samples obtained from the colloidal silica, larger Ag particles are formed at lower annealing temperatures. Further annealing at higher temperatures crystallizes the glass into the already mentioned phase. The reason of having larger Ag particles at lower temperatures in the latter set of samples is probably because they have a more open structure, produced by a wider distribution in the Si—O—Si bridging angle.     

Size effect in the oxidation of platinum nanoparticles on graphite with nitrogen dioxide: An XPS and STM study

The interaction of NO₂ with model catalysts prepared by platinum evaporation onto the surface of highly oriented pyrolytic graphite has been investigated at room temperature and a pressure of 3 × 10^?6 Torr by X-ray photoelectron spectroscopy and scanning tunneling microscopy. In the catalyst containing only small (<2.5 nm) platinum particles, these particles oxidize to PtO and PtO₂. The action of NO₂ on the graphite support and on the graphite-supported Pt catalyst causes graphite oxidation. The oxygen concentration in the model catalyst is higher than on the support. This is supposed to be due to the spillover of oxygen atoms from platinum particles to graphite.     

Composite Glass-Ceramics in the Systems MgO-SiO2, MgO-Al2O3-SiO2 and Fluorapatite Obtained by Sol-Gel Technology

The crystallization processes in the synthesis of composite glass-ceramics in the systems fluorapatite (FA)-MgO·SiO₂ gel-glass (GG) and FA-MgO·Al₂O₃·SiO₂ (GG) have been studied. The composites were prepared by mixing of MgO·SiO₂ and MgO·Al₂O₃·2SiO₂ gel-glasses with FA (from 20 mol% to 80 mol%). The obtained samples were thermally treated at 950°C, 1050°C, 1150°C and 1250°C for 2 hours and the structural changes were investigated, applying XRD-analysis, IR-spectroscopy, Raman spectroscopy and SEM. It has been proved that depending on the chemical composition of the gel-glass and the temperature of treatment, composite glass-ceramics may be prepared consisting only FA, FA and a silicate phase or FA, other phosphate phases and a silicate phase. Because of the absence of calcium or aluminum ions in the composition of the GG, FA is easily decomposed and phases as 7CaO·2MgO·3P₂O₅ and MgO·2P₂O₅ are formed.     

Structure and Electron Spin Resonance of Annealed Sol-Gel Glasses Containing Ag

In this work, SiO₂ samples with silver, prepared using the sol-gel method, were analyzed after being thermally treated in air in the range of 100 to 800°C. The sol-gel starting solutions were prepared by mixing tetra-ethyl-orthosilicate (TEOS), water and ethanol. Samples with 4 different H₂O/TEOS molar ratios (3.3, 5, 7.5 and 11.7 respectively) and with different nominal Ag concentrations were prepared (1, 2 and 4%vol. of Ag). It was found that upon annealing, different silver spices were formed, such as Ag₂ ⁺, Ag⁺, Ag°, and metallic silver aggregates. The identification of these spices was carried out by means of X-ray diffraction, Electron Paramagnetic Resonance (EPR), optical emission and optical absorption. It was also found that the specific type of silver spices observed depends on the structure of the SiO₂ matrix and on the annealing temperatures. It was found that samples prepared from precursor solutions with a low H₂O/TEOS ratio have a more open structure, and therefore silver diffuses faster and forms agglomerates at lower temperatures. Samples prepared from solutions with larger H₂O/TEOS ratios have a more dense structure, which allows the formation of atomic or molecular spices in addition to silver particles. A systematic study of this system was carried out using EPR on samples prepared from solutions having different H₂O/TEOS molar ratios, various Ag concentrations and subjected to different thermal treatments.     

Structural Development of Single Phase (Type I) Mullite Gels

Single phase (type I) mullite gels were prepared by sol-gel techniques starting from alkoxides (Al-butylate, tetraethylorthosilicate) and alkoxides plus nitrates (tetraethylorthosilicate, Al(NO₃)₃·9H₂O). After drying at 150°C the aluminosilicate gels are non-crystalline and remain so up to 900°C. Above 900°C the gels transform into Al₂O₃-rich mullite plus a coexisting SiO₂ phase. Structural studies on temperature-dependent dehydroxylation and condensation of the gels were carried out by large angle X-ray scattering, by infrared spectroscopy and by²⁹ Si NMR spectroscopy. Heat-treatment (<150°C) of dried gels first causes removal of the H₂O and organic residuals weakly bound at the open pore surfaces of the gels while the stronger, structurally bound OH groups are not affected. At temperatures <600°C OH groups are released and recombine to molecular H₂O. If the temperature does not exceed 800°C the newly formed H₂O is trapped in closed nanopores of the gel-network. Corresponding electron microscopical investigations reveal agglomerates of 10 nm sized primary particles virtually unaffected by the heat-treatment below 900°C. NMR investigation provided a new structural model on type and distribution of coordination polyhedra in aluminium silicate gel networks. Unlike Si, which according to ²⁹Si NMR is always 4-fold coordinated with O, ²⁷Al NMR spectroscopy revealed that Al cordination is more complex and is influenced by thermal treatment. Al occurs six-fold (octahedrally) and four-fold (tetrahedrally) coordinated. A third ²⁷Al NMR signal which has been attributed to five-fold-coordinated Al in the literature increased in intensity with the heat-treatment. A comparison of NMR data of the gels with those of mullite suggests that tetrahedra triclusters (3 tetrahedra having one oxygen atom in common) occur as major structural units in aluminium silicate gels rather than five-fold-coordinated Al. Triclusters of tetrahedra may compensate the excess negative charge in the network caused by Si⁴⁺ Al³⁺ substitution. The charge compensation model is supported by aluminosilicate gels doped with network modifiers (e.g., Na⁺). Since equimolar addition of Na⁺ compensates Si⁴⁺ Al³⁺ substitution the formation of triclusters is no longer required which actually can be deduced from²⁷ Al NMR studies.     

Syntheses, characterization and electrical property of a new silver diphosphonate with zeolite-like framework and three-dimensional silver interactions: [Ag4(O3PCH2CH2PO3)]

A new silver organodiphosphonate, [Ag₄(O₃PCH₂CH₂PO₃)] (1), has been synthesized and characterized by X-ray diffraction, IR, TGA-DSC, electricity measurement and element analysis. 1 crystallizes in the monoclinic space group P2(1)/n (a=6.0115(16) Å, b=8.630(2) Å, c=8.462(2) Å, β=97.693(4)°, Z=2, R₁=0.0604, wR₂=0.1450). 1 contains one-dimensional channels and a three-dimensional Ag?Ag interacted substructural net. TGA and XRD indicate little weight loss up to 300 °C and little structure change after heated at 170 °C for 2 h, respectively. The grain interior conductivity of 1 increases continuously from 50 to 170 °C. Results of EHT calculations show that under thermal or optical excitation the conductivity of 1 is mainly due to transfer of π antibonding electrons of -PO₃ group through O atom to Ag 5s orbital, which also leads to enhancement of Ag?Ag interactions and promotes formation of Ag?Ag substructural net.     

Preparation and Characterization of Sol-Gel Derived Y2O3 Thin Films

Sol-gel derived Y2O3 thin films have been prepared on platinum coated silicon wafers and fired to temperatures ranging from 400°C to 750°C. Multiple coats were used to obtain films up to 0.5 m thick with an intermediate firing of 400°C between coatings. Top Pt electrodes were sputtered to form monolithic capacitors. These films exhibited a dielectric constant of 18 and a leakage current of 10–11–10–7 A/cm2, making them attractive candidates for high dielectric constant dielectric films in high density DRAMs.     

超细Ag颗粒对葡萄糖氧化酶生物传感器响应灵敏度的增强效应

生物传感器在临床医学、环境和食品工业等方面都有重要的用途［１］,且由于其具有体积小、精度高、灵敏度好和便于现场测定等优点,已成为当前研究的热点课题之一．其中研究得较多的是氧化酶电流型生物传感器,特别是介体型,由于其响应快,灵敏而倍受重视．现已报道的可...     

Formation and Crystallization Behavior of Gels in the Na2O-SiO2-TiO2-ZrO2 System

SiO₂-TiO₂-ZrO₂ and 5Na₂O·95(SiO₂ + TiO₂ + ZrO₂) gels were synthesized and role of Na₂O in gel formation and crystallization behavior of gels were studied. From Si(OC₂H₅)₄, Ti(iso-OC₃H₇)₄, Zr(n-OC₃H₇)₄ and NaOCH₃ solutions in EtOH without H₂O, transparent and opaque gels were obtained. Opaque bulk gels, rich in TiO₂ or ZrO₂ composition in Na₂O containing SiO₂-TiO₂-ZrO₂ system, contain agglomerated spherical particles of diameter small <10 m, in contrast with opaque gels having large particles <30 m in alkali-free SiO₂-TiO₂-ZrO₂ system. Crystallization temperature (Tc) was measured by DTA on dried gels. Compared with the alkali-free SiO₂-TiO₂-ZrO₂ gels, 5 mol% Na₂O containing gels gave lower Tc in SiO₂ rich compositions and higher in TiO₂ rich or ZrO₂ rich compositions.     

Synthesis of Bi2O3 and Bi4(SiO4)3 Thin Films by the Sol-Gel Method

Bi2O3 thin films were prepared by dipping silica slides in ethanolic solutions of tris(2,2-6,6-tetramethylheptane-3, 5-dionato)bismuth(III) [Bi(dpm)3] [1] and heating in air at temperatures 500°C. Bi4(SiO4)3 homogeneous thin films were obtained from the reaction of the bismuth oxide coating with the silica glass substrate at temperatures higher than 700°C. For heat treatments at temperatures between 600°C and 700°C, Bi2SiO5 coatings were obtained. The composition and microstructure evolution of the films were determined by Secondary Ion-Mass Spectrometry (SIMS), X-Ray Photoelectron Spectroscopy (XPS) and Glancing Angle X-Ray Diffraction (GA-XRD). The synthesis procedure was reproducible and allowed the control of the Bi2O3 phase composition. Moreover, the thin film annealing parameters were correlated with the formation of bismuth silicates, among which Bi4(SiO4)3 (BSO) is very appealing for the production of fast light-output scintillators [2].     

Preparation of TiO2 coating films containing Pd fine particles by sol-gel method

TiO₂ coating films of 0.3–0.4 m in thickness that contain fine Pd particles have been prepared by sol-gel method using Ti(OC₃H ₇ ⁱ )₄ and PdCl₂ as starting materials in an attempt to obtain optical materials that show surface plasma resonance in the visible range. A temperature higher than 900°C was required for formation of Pd metal particles when the heat-treatment was conducted in air. Heat-treatment of pre-heated amorphous films in air at 800°C resulted in precipitation of PdO particles, the size of which could be varied by changing the time of heat-treatment, and subsequent heat-treatment in H₂/Ar gas converted the PdO particles into Pd metal particles. Heat-treatment of pre-heated amorphous films in H₂/Ar gas also resulted in precipitation of Pd metal particles. The size of the Pd metal particles precipitated in the films was 6 to 90 nm, depending on the conditions of heat-treatment. The resultant TiO₂ films containing Pd metal particles were brownish grey in color and showed optical absorption in the visible range over 400 nm, which is attributed to surface plasma resonance of Pd metal particles.     

Atomic-Scale Structure of Water-Based Zirconia Xerogels by X-Ray Diffraction

The structural evolution of zirconia thin films and gel powders has been evaluated by X-ray diffraction. Maxima (r ₁ and r ₂) of the experimental radial distribution function RDF and the bond angles were determined and correlated with TGA (thermogravimetric analysis), DTA (differential thermal analysis) and MS (mass spectrometry). The results indicate that the topological short-range structure (<5 Å) of amorphous zirconia thin films, independent of drying temperature, resembles that of crystalline tetragonal ZrO₂. In contrast, amorphous zirconia powder gels dried at temperatures below 120°C show atomic arrangements similar to that of tetragonal ZrO₂. The structure of these gels annealed at temperatures between 165–340°C resembles a distorted tetragonal ZrO₂, monoclinic-like structure. Zirconia powders and films contain crystalline tetragonal ZrO₂ at 400°C.     

ESR-spectrum of silver atoms stabilized in gamma-irradiated aluminium silicate modified by nickel

ESR technique was used to study the effect of conditions chosen to prepare samples to obtain and stabilize silver atoms forming under the action of⁶⁰Co -irradiation at 77 K in aluminium silicate modified by nickel and containing silver ions. The use of¹⁰⁹Ag isotope made it possible to detect two types of silver atoms Ag_I and Ag_II in -irradiated aluminium silicate modified by nickel and containing silver ions introduced by various techniques. The parameters of the ESR-spectrum of the silver particles under study have been determined and their thermal stability has been investigated.