Flame Hydrolysis Research Articles

Simultaneous UV direct writing of channel waveguides and Bragg gratings in germanium-doped planar silica

Channel waveguides incorporating photo-imprinted Bragg gratings were produced in germano-borosilicate glass layers fabricated by flame hydrolysis deposition, through single- and double-step exposure direct writing processes with UV laser irradiation. To our knowledge, this is the first time that UV exposure has been used to simultaneously realize both waveguiding and spectral selectivity functions.

Catalytic combustion of hydrocarbons over perovskites

The advantages and disadvantages of the catalysts so far employed or proposed for the low temperature catalytic combustion of hydrocarbons, in both static and mobile energy production devices, are discussed. Furthermore, a La0.9Ce0.1CoO3±δ perovskite has been prepared by a recently proposed new flame-hydrolysis (FH) method. This proved a high surface area, thermally highly resistant catalyst. The partial substitution of Ce for La in such a cobaltite led to a relatively low suprafacial activity, but to a high bulk oxygen mobility, leading to high intrafacial activity for the catalytic flameless combustion of methane. The best operating conditions have been also found, for supporting the so prepared active phase, by dip-coating of a cordieritic honeycomb support, after deposition of an alumina primer. A very active and durable catalyst was so obtained, useful for practical application in the environmentally friendly low temperature combustion of methane.

Preparation and characterization of SiO2–B2O3–P2O5 particles and films generated by flame hydrolysis deposition for planar light-wave circuits

Boron-phosphosilicate glass particles were deposited on Si wafer by flame hydrolysis deposition under controlled conditions. A few ten nanometer sized and spherical glass particles were synthesized in an oxy-hydrogen flame. The wafer surface temperature of 500 °C proved sufficient to form the Si–O–B framework while a higher temperature (approximately 730 °C) was required to produce Si–O–P formation. Gaussian bands at 3242 cm−1 provided evidence that the dissolved B2O3 concentration increased with the substrate temperature. In as-deposited porous films, a certain quantity of crystalline B2O3 remained, which was completely transformed into the noncrystalline phase after post deposition annealings. Film density measurements indicated that the B2O3 content in the doped silica films would be substantially lower than that in the precursor mixture.

Preparation of ultrafine silica powders

Numerous processes have been used for the preparation of ultrafine silica powders, which include the flame or plasma hydrolysis of silicon tetrachloride vapor at high temperatures [1, 2] and the hydrolytic polycondensation of organosilicates or silicon tetrachloride in liquid solvents [3, 4]. Silica powders produced by the above methods have unique properties such as fine particle size, large surface area and good dispersibility. However, their high production costs have limited the powders’ large-scale commercial applications [5]. Development of alternative cheaper routes to produce ultrafine silica powders with desirable characteristics is therefore of considerable interest. In this letter, we report the preparation of ultrafine silica powders by sol-gel process from water glass using ethyl acetate as a latent acid reagent. The interaction of ethyl acetate with water glass solution and the gelling characteristic of water glass in the presence of ethyl acetate were examined and the effect of calcination on the properties of the silica powders was studied. Water glass (technical grade) with SiO2 content of 26.4% and SiO2/Na2O mole ratio of 3.3 was used as a starting material, which was diluted with distilled water and vacuum filtered before use. Ethyl acetate (analytical grade) was added to water glass solution in a given mole ratio of CH3COOC2H5 to SiO2 under stirring at 25 ± 1◦C. After reaction for a period of time and appearance of silica gels, the resulting slurry was kept reacting for 40 min, and then acidified by HCl and stirred for another 1 h at 80 ◦C. It was filtered at the pump and thoroughly washed with distilled water to eliminate Cl− and Na+ ions. The wet gels obtained after filtration were stirred with n-butanol and distilled to remove water as azeotrope. Ultrafine silica powders can be obtained after azeotropic distillation and drying at 120 ◦C for 2 h. The calcination of the powders was carried out at 300 to 1200 ◦C for 2 h with an air atmosphere. The pH of the reaction system was monitored by using a pH meter (Model pHS2, Shanghai, China). The hydrolysis ratio of ethyl acetate was obtained by determining the content of alcohol generated in the process of gelling reaction via gas chromatography (Model BF3400, Beijing, China). The conditions were as follow: column, GDX-103, 2.6 mm × 2 m, 60–80 mesh; detector, TCD; carrier gas, He, volume rate, 45 mL/min; column temperature, 120 ◦C; inject and detection temperature, 210 ◦C. A series of sols and gels were prepared to examine the change in gelation time with the mole ratio of CH3COOC2H5 to SiO2. The gelation time of the reaction system was defined as the time when a visible solid phase was detected either in the form of a gel or precipitate. In this system gels were only formed at low ethyl acetate contents, whereas at higher contents phase separation of the solid and liquid phase occurred. This gelation time was a useful qualitative measure of the stability of the reaction system. The particle morphology and size of the silica sols and silica powders were observed by transmission electronic microscopy (TEM) (Model H-600, Hitachi, Tokyo, Japan). Specimens were made by dropping the silica sols and the prepared water suspension, including silica powders, onto carbon filmed copper grids. The phase structure of the silica powders was determined by X-ray diffractometry (Model D/MAX-III B, Rigaku Co., Tokyo, Japan) with Cu Kα radiation. The specific surface areas of the as-synthesized and calcined powders were measured via the Brunauer-Emmett-Teller (BET) method with nitrogen absorption using an automated volumetric analyzer (Model ASAP 2000, Micromeritics). In the solution of water glass, silica is present as monomeric, oligomeric and polymeric colloidal silicate ions. The colloidal species, with particles in the range 1 to 2 nm, are in equilibrium with monomeric and oligomeric species [6]. The behavior of silicates in solution is governed by the two following interdependent sets of equilibria [7]:

Electron-beam-induced densification of Ge-doped flame hydrolysis silica for waveguide fabrication

Experimentally we compare the densification induced by electron beam irradiation of Ge-doped silica, produced by flame hydrolysis deposition with the densification of thermally produced SiO2. By comparing these results to the predictions made by elasticity theory, we find good agreement for the thermal SiO2 by considering a single region of electron beam damage. For the Ge-doped flame-hydrolysis-deposited silica, we need to include in the model a second, shallow region, which densifies to a greater extent. X-ray photoelectron spectroscopy measurements suggest that the thickness of this additional region is comparable to a layer that was found to be depleted of Ge.

Catalytic flameless combustion of methane over perovskites prepared by flame–hydrolysis

A set of perovskite-type catalysts of general formula LaBO3±δ (B=Co, Mn, Fe) and a sample of La2NiO4±δ were prepared by means of a recently proposed innovative flame–hydrolysis procedure. The catalysts were characterised by nanometer-size particles (20–60nm), relatively high surface area (ca. 20m2/g), high thermal stability and high phase purity. Their high activity for the catalytic flameless combustion of methane confirmed the validity and versatility of the preparation method.Temperature-programmed -desorption and -reaction, coupled with mass spectrometric analysis, allowed to better understand some aspects of the catalytic behaviour shown by the present samples for the cited reaction. In particular, an interesting correlation between the availability of oxygen at various temperatures, as revealed by the so-called α and β oxygen desorption peaks, and reaction mechanism was found for the different B metals.

Asymmetric Mach-Zehnder germano-silicate channelwaveguide interferometersfor quantum cryptography systems

Asymmetric Mach-Zehnder channel waveguide interferometers possessing a 1.1 ns delay in one of the arms were fabricated using flame hydrolysis deposition of silica glass on silicon. Two modules were combined to form a single time-division interferometer for optical pulses. The composite interferometer exhibited a high fringe contrast ratio of 1:200 making it eminently suitable for use in compact, low error-rate quantum cryptography systems.

Comparison between the reactivity of commercial and synthetic TiO 2 photocatalysts

Titanium dioxide in powder form is largely used as a catalyst in the field of detoxification of aqueous effluents by photocatalysis. One of the most efficient commercial photocatalysts is TiO 2/P25 manufactured by flame hydrolysis. In this work, we compare the reactivity between this commercial TiO 2/P25 and a synthetic photocatalyst TiO 2/SG prepared in our laboratory by sol–gel method. The photocatalytic activities of TiO 2/P25 and TiO 2/FG were tested using a benzamide solution. We have noticed that the total organic carbon (TOC) evolution of the benzamide solution during the photodegradation was different between both photocatalysts (when the kinetic evolution of benzamide are similar). There is a slower decrease for TOC in the case of the TiO 2 synthetized by sol–gel method. The results of the adsorption and photodegradation experiments explain this difference between the two photocatalysts. As a matter of fact, it is known that the degradation rate depends on the capacity of the molecules to adsorb at the catalyst surface. So, we have pointed out that this adsorption capacity is lower in the case of TiO 2/SG. This explains the degradation rates found. And since the TOC variation depends on the intermediate degradation rate and not on the benzamide, it seems normal to note a lower evolution for the TOC in accordance with the irradiation times.

Photocatalytic Effects of Rutile Phase TiO2 Ultrafine Powder with High Specific Surface Area Obtained by a Homogeneous Precipitation Process at Low Temperatures

The photocatalytic characteristics of nanostructured TiO2 ultrafine powder with rutile phase produced using the homogeneous precipitation process at low temperatures (HPPLT) were compared with those of commercial P-25 TiO2 powder by flame hydrolysis. The TiO2 powder by the HPPLT showed much higher photoactivity in the removal rate, showing lower pH values in the solution than the P-25 powder when eliminating metal ions such as Pb and Cu from the aqueous metal-EDTA solutions. This can be inferred as the more rapid photo-oxidation or -reduction of metal ions from the aqueous solution, together with relatively higher efficiencies in the use of an electron-hole pair formed on the surface of the TiO2 particles under UV light irradiation. Also, in the view of the TiO2 particle morphology, compared to the well-dispersed spherical P-25 particles, the agglomerated TiO2 secondary particles by the HPPLT consist of acicular typed primary particles with a thickness in the range of 3–7 nm and the primary particles radialize in all directions, which would be more effective to photocatalytic reactions without the large electron-hole recombination on the surface of the TiO2 particle under UV light irradiation. It can be, therefore, thought that the higher photoactivity of the rutile TiO2 powder by the HPPLT in the aqueous solutions results mainly from having a larger surface area by the acicular shaped primary particles with very thin thickness and radialization in all directions.

Surface chemistry of SiO 2 and TiO 2–SiO 2 glasses as determined by titration of soot particles

Surface charge is a critical issue for various types of glass processing including surface finishing (polishing) and cleaning. Unlike monomeric silicic acid with a p K a of 9.8, glass surfaces can have variable p K a values as determined by extent of Si–O–Si bonding, composition, and structure. The purpose of this study was to determine the surface charge, point of zero charge, and p K as for high purity fused SiO 2 and TiO 2–SiO 2 (∼7 wt% TiO 2) glasses. To achieve necessary surface areas for titration, colloidal soot particles were used in this study rather than glass monolith. Soot particles have the same inherent physical and chemical properties as the product glasses that are made by the same chemical vapor flame hydrolysis deposition process. Titration experiments performed in 10 −1–10 −5 M NaCl solutions revealed dissociation constants (i.e., intrinsic p K a1 and p K a2 values) of 0.0±0.2 and 7.0±0.1, respectively for the fused SiO 2 particles, and 0.0±0.1 and 5.0±0.2 , respectively for the TiO 2–SiO 2 particles. Points of zero charge for each material were calculated as 3.5±0.1 and 2.5±0.1 for the fused SiO 2 and TiO 2–SiO 2 particles, respectively. The role of TiO 2 in lowering point of zero charge and p K a2 values is hypothesized to be the result of four-fold coordination as compared to the six-fold coordinated TiO 2 used in surface chemistry studies reviewed by this paper.

Birefringence free planar optical waveguide made by flame hydrolysis deposition (FHD) through tailoring of the overcladding

Stresses developing in a planar waveguide resulting from the different thermal expansion coefficients of the substrate and the three glass layers (buffer, core and cladding) were analyzed using a finite element method. It can be shown that mainly the thermal expansion of the overcladding determines the birefringence in the finished waveguide. Based on that result, recipes for an overcladding made with the flame-hydrolysis-deposit ion (FHD)-process were devised. We demonstrate the absence of birefringence in a commercial waveguide layer overclad with this glass. The high doping levels required for the cladding to have a thermal expansion coefficient sufficient for this raises concerns about the moisture sensitivity of such a glass. We examined the depth dependent composition of the glass using WD-ESCA (wavelength dispersive electron microprobe) and show, that at the surface a layer depleted of dopants is formed during the high temperature sintering process, This layer can serve as a protective coating to isolate the underlying, higher doped layer from the effects of moisture. Analysis of the stresses shows that this does not effect the birefringence behavior of the waveguide.

Flame hydrolysis deposition of glass on silicon for the integration of optical and microfluidic devices

Flame hydrolysis deposition (FHD) of glasses has previously found applications in the telecommunications industry. This paper shows how the technology can be used to deposit silica with different refractive indices and thereby produce low-loss planar waveguides for use in analytical applications. We also show that the glasses can be patterned using a new reactive ion etch and sealed using a modification of anodic bonding, such that the resulting microstructures can be readily incorporated within a lithographically defined "chip", integrating both optical and fluidic circuitry on the same device. In the example described in this paper, waveguides, analytical microtiter chambers and fluidic capillary channels, with the necessary high aspect ratio features (and with depths up to 40 microm) were all produced in glass, using the appropriate deposition and etching technologies. The performance of the chip was assessed in the framework of a low-volume fluorescence assay, using waveguides to address miniaturized microtiter chambers with volumes of 230 and 570 pL. Devices featuring different optical detection configurations, including both in-line and orthogonal waveguide geometries, were fabricated. In the optimal configuration, the experimental detection limit was determined as ca. 20 pM (equivalent to 10 zmol) of a cyanine fluorophore, Cy5. The applicability of the device as a biochip platform was further illustrated by analytical measurements on fluorescently labeled oligodeoxynucleotides.

Measurements of Temperature and OH Radical Distributions in Flame Hydrolysis Deposition Process

The effects of SiCladdition on flame structures have been studied in flame hydrolysis deposition (FHD) processes using Coherent anti-Stokes Raman spectroscopy (CARS) and planar laser induced fluorescence (PLIF) to measure temperatures and OH concentrations, respectively. The results demonstrate that even a small amount of SiCl addition can change thermal and chemical structures of H/O diffusion flames. When SiCl is added to a flame temperature decreases in non-reacting zone due to the increases in both specific heat and density of the gas mixture, while flame temperature increase in particle formation zone due to the heat release through hydrolysis and oxidation reactions of SiCl. It is also found that OH concentration decreases dramatically in particle formation zone where temperatures increase. This can be attributed to consumption of oxidative species and generation of HCl during silica formation.

Effect of annealing on Ge-doped SiO2 thin films

Thermal annealing effects on optical and structural properties of Ge-doped SiO2 thin films prepared by the chemical vapor deposition and flame hydrolysis deposition methods were investigated. The thin film prepared by the former method showed inhomogeneous Ge distribution, and Ge oxygen-deficient centers were observed. When it was thermally annealed at temperatures higher than 800 °C, the Ge distribution became uniform. The concentration of oxygen deficient centers was found to decrease with the thermal annealing in an O2 atmosphere, while it increased with the thermal annealing at 1000 °C in N2. This suggests that improvement of the film quality can be achieved by thermal annealing. On the other hand, neither inhomogeneity of Ge distribution nor the appearance of oxygen deficient centers was observed in the film prepared by the latter method, and its film quality was scarcely affected by the thermal annealing.

Inductively coupled plasma etching of Ge-doped boron-phosphosilicate glass for planar lightwave circuit devices

Boron-phosphosilicate glass films were deposited on silicon wafers using H 2/O 2 flame hydrolysis deposition. GeO 2 was doped up to 12 wt% for core layer. Boron and phosphorus contents were varied up to 24 and 2.4 wt%, respectively. Doping level was controlled by changing the flow rate of gaseous chemicals. The layers were patterned and then etched using inductively coupled plasma to form optical waveguides. Cr and CF 4 were used as the etch mask and the reactive gas, respectively. The effect of germanium on the etch rate is small. The etch rate of the 24 wt% boron-doped layer was larger by about 10%, compared to the 12 wt% boron-doped layer. The etch rate, selectivity and plasma-induced surface damage were affected by chamber pressure, gas flow rate, capacitively coupled power and inductively coupled power. DC bias voltage between electrode and plasma in the process chamber was the parameter which has the largest effect in controlling the etch results. DC bias voltage could be effectively controlled by capacitively coupled power and inductively coupled power.

Fabrication of integrated microanalytical chambers and channels for biological assays using flame hydrolysis deposition glass

The aim of this study was to develop and optimise a process to etch deep, high aspect ratio microstructures in glasses. To this end, we examined the use of flame hydrolysis deposition (FHD) to form integrated lab-on-a-chip structures involving silica-on-silicon. The FHD process allows buried optical waveguide structures to be produced, which are isolated from the surface using a glass overlayer, or cladding. This arrangement allows sensing regions, such as microanalytical chambers and capillary channels for microfluidics, to be defined by etching through the upper cladding. In this paper we also demonstrate a simple fluorescence assay measurement, to illustrate the potential application of this technology.

Electric surface properties of fractal aggregates—rotational and translational electrokinetics

Fractal aggregates from γ-Al 2O 3 obtained by flame hydrolysis with different hydrodynamic radius are studied by electric light scattering (rotational) and electrophoresis (translational) electrokinetics. The nonequilibrium, electric surface properties of the aggregates characterized by their electric polarizability are studied. The electric polarizability depends linearly on the aggregates dimensions and decreases with indifferent electrolyte concentration increase tending to a constant value. The aggregate structure (the existence of “inner” counterions in the pore space) could be supposed as responsible for an “internal” electric polarizability, relaxing in the kHz region. The electrophoretic mobilities do not depend on the κa-parameter ( κ −1=thickness of diffuse double layer, a=radius of initial spherical γ-Al 2O 3 particles).

Electron spin resonance and thermoluminescence in powder form of clear fused quartz: effects of grinding

Clear fused quartz (CFQ) tubes were powdered either manually by using a mortar and pestle (for coarse production) or mechanically, using a micronising mill (for fine production). A high and multisignal electron spin resonance (ESR) background was found in the fine powder even after annealing it at 900°C for 20 min. In the case of the coarse powder, the signal (ESR background) varied inversely with particle size and was quite high for particle sizes lower than 38 μm. In a subsidiary experiment, using fine SiO 2 powder (99.8% pure, with the particle size of ∼0.007 μm), manufactured by using flame hydrolysis, only a weak background signal was found. The 60Co gamma-ray irradiated powders (∼22 Gy) were subjected to ESR analysis or thermoluminescence (TL) readout. The ESR intensity of the coarse powder varied directly with particle size. Thus, the intensity for a particle size of 20–38 μm was very low and almost the same as the unirradiated intensity. In TL readout the results were the opposite: the TL intensity of the coarse powder varied inversely with the particle size down to 38 μm, after which it decreased with decreasing particle size of the material. The fine powder, produced by grinding the CFQ tubes, was insensitive to gamma-rays (at least at doses of up to 50 Gy); but for the flame hydrolysis SiO 2 the situation was the opposite. The minimum detectable dose (MDD) for the CFQ in powder form using ESR was ∼2 Gy, which is ∼2 times higher than that for the bulk form, while the MDD for the powder using TL was ∼20 μGy, which is ∼2 times lower than that for the bulk form of the material.

Characterization of borophosphosilicate glass soot fabricated by flame hydrolysis deposition for silica-on-silicon device applications

The use of flame hydrolysis deposition (FHD) to fabricate porous silica glass soot in the B2O3-P2O5-SiO2 glass system (BPSG) is described for silica-on-silicon device applications. The deposition conditions with a Si substrate temperature (∼200 °C) and a flame temperature (1300–1500 °C) are appropriate to synthesize the SiO2 and P2O5-SiO2 non-crystalline glass soot. However, further investigations for the B2O3-P2O5-SiO2 glass soot are needed to obtain complete amorphous phases. The densification process of porous silica glass soot in the three systems of SiO2, P2O5-SiO2 and B2O3-P2O5-SiO2 is also described to estimate the onset of sintering temperature. The OH absorption measurements are performed to try to identify incorporation of hydroxyl contaminants in the systems of P2O5-SiO2 and B2O3-P2O5-SiO2.

Deep, vertical etching of flame hydrolysis deposited hi-silica glass films for optoelectronic and bioelectronic applications

We report on the use of CHF3, C2F6, and SF6 as etch gases for deep reactive ion etch processing of germano-boro-silicate glass films prepared by flame hydrolysis deposition (FHD). The glass film under study had a composition of 83 wt % SiO2, 12 wt % GeO2, and 5 wt % B2O3. The scope of the study was to identify the benefits and drawbacks of each gas for fabrication of deep structures (>10 μm) and to develop an etch process in each gas system. The etch rate, etch profile, and surface roughness of the FHD glass films were investigated for each gas. Etch rates and surface roughness were measured using a surface profiler and etch profiles were assessed using a scanning electron microscope. It was found that SF6 had the highest FHD glass etch rate and nichrome mask selectivity (>100:1) however, it had the lowest photoresist mask selectivity (<1:2) and highest lateral erosion. CHF3 had the lowest FHD glass etch rate but high selectivity over both nichrome (>80:1) and photoresist (>10:1) and the etch profile was found to be smooth and vertical. C2F6 had a similar etch profile to that of CHF3, but the selectivity over both mask materials was lower than in CHF3. Fused silica was used as a reference material where it was found the percentage drop in etch rate in C2F6, SF6, and CHF3 was −12%, −15%, and −37%, respectively. From the results presented here CHF3 proved to be the most versatile etch process as either photoresist or nichrome masks could be used to attain depths of 20 μm, or more.