Detailed index profiles of ion implanted waveguides in KNbO3

Planar optical waveguides have been produced in KNbO₃, by He ion implantation at 2.0 MeV. The depth profiles for the two indices (n_a and n_c) have been determined using a dark mode reflectivity calculation. They show a pronounced anisotropic behaviour. For a dose of 2 × 10¹⁶ cm^-2 the nuclear damage peak has an index decrease ∼10% for n_a, and ∼7% for n_c, showing a sensitivity to nuclear damage at least three times that of LiNbO₃. The electronic damage region has ∼0.5% decrease for n_a, but an increase for n_c which is larger at shorter wavelengths. This anisotropic and dispersive behaviour is comparable with other implanted niobates, and a similar damage mechanism of radiation-enhanced diffusion is suggested.     

Phase transitions in R2 SnCl6-type crystals (R = NH3 C2H3, NH3C2H5 and N(CH3)4)

The thermal dilatation in (NH₃ ·CH₃) SnCl₆, (NH₃ · C₂H₅) SnCl₆ and [N(CH₃)] SnCl₆ was measured, and as the results it has turned out that (NH₃ ⁶·C₂H₅) SnCl₆ and [N(CH₃)₄]₂ SnCl₆ undergo the first order transitions at 128 K and 158 K, respectively. The low temperature phases of (NH · C₂H₅) SnCl₆ and [N(CH₃)₄]₂ SnC1₆ are found to be monoclinic and tetragonal, respectively, No phase transition was observed in (NH₃ ·CH₃)₂ SnCl₆ down to 77 K.     

Phase transition in Sol-Gel processed barium titanate ceramics

Ferroelectric barium titanate (BaTiO₃) ceramics have been prepared from barium acetate [Ba(CH₃COO)₂] and titanium isopropoxide [Ti(CH₃)₂CHO)₄] precursors by sol-gel technique The as-grown powder was found to be amorphous which crystallized to tetragonal phase after annealing at 700°C in air for one hour. The ceramics showed well saturated polarization-field (P-E) hysteresis loops at room temperature. The values of spontaneous polarization(P_s), remanent polarization (P_r) and coercive field (E_c) of the ceramics were found to be19.0, 12.66μC-cm^-2 and 30KVcm^-1 respectively. The room temperature dielectric constant (ε) and loss tangent (tanδ) of the ceramics were found to be 1135 and 0.012 respectively. The dielectric constant and tanδ showed anomaly peaks at 125°C showing ferroelectric to paraelectric phase transition. The polarization parameters also vanish at 125°C confirming the phase transition.     

Acoustic behavior of the incommensurate state of Rb2Zncl4 by brillouin scattering?

Temperature dependence of elastic stiffness constant C₁₁ was studied from 60 to -100°C by Brillouin scattering and refractive index measurements. It is found that C₁₁ decreases with decreasing temperature over the incommensurate phase and its temperature gradient becomes more or less flat at several degrees above the incommensurate-commensurate transition point T_c, where the dis-commensurate structure appears. This unusual temperature dependence of C₁₁ seems to be attributable to the incommensurately modulated structure of Rb₂ZnC1₄.     

PERMEATION THROUGH KAPTON® POLYIMIDE AT ELEVATED TEMPERATURES

The permeabilities of CH₄, CO₂, CH₃OH, H₂O, O₂, and CO through films of Kapton® polyimide were measured at temperatures of 50, 100, 150, 200, and 250°C and pressures below 1 atm. Apparent activation energies for the permeation of the pure components ranged from 31.6kJ mol^-1 for CH₄ to nearly 0 for H₂O under the conditions studied. The ideal permselectivity for methanol relative to methane decreased from over 100 to under 10 as the temperature was increased from 50 to 250°C.     

Structural and Optical Properties of Barium Strontium Titanate (Ba0.5Sr0.5TiO3) Thin Films

Ferroelectric Barium Strontium Titanate (Ba_0.5Sr_0.5TiO₃) or BST thin films on quartz substrates have been prepared by using a modified sol gel processing technique. The starting materials are Barium 2-ethylhexanoate Ba[CH₃(CH₂)₃CH(C₂H₅)CO₂]_2, Strontium 2-ethylhexanoate Sr[CH₃(CH₂)₃CH(C₂H₅)CO₂]₂ and Titanium(IV) isopropoxide [TiOCH(CH₃)₂]₄. The precursors except [TiOCH(CH₃)₂]₄ were synthesized in the laboratory. Transparent and crack-free films were fabricated on quartz substrates by spin coating. The as-fired films were found to be amorphous, which crystallized to cubic phase after annealing at 550°C in air for 1 hr. In this paper we report the structural and optical properties of BST films prepared by the modified sol-gel process.

Communicated by Prof. E. C. Subbarao     

Degradation of PZT-4D hard piezoceramics under moderate heating

The changes produced in the dielectric permittivity, the transverse piezoelectric coefficient, the k₃₁ coupling factor, the s₁₁^E elastic compliance and the mechanical quality factor of PZT-4D hard piezoceramics by heating have been investigated. The ceramics were then repoled, and the reversible and irreversible components of the changes quantified. The results showed that depolarisation began at 150°C. A high level of poling was retained even after heating at 300°C (d₃₁=−83×10⁻¹² C N⁻¹ and k₃₁=0.225), only 20°C below the transition temperature. However, a significant irreversible degradation of the mechanical quality factor, Q_m, occurred at a temperature as low as 100°C. Experiments on thinned specimens showed that the degradation of Q_m took place in the Ag doped layer produced by the electrodes. Indentation surface cracks were also introduced into the ceramics to investigate the behaviour of cracks during the thermal treatments.     

Regeneration of S-poisoned Pd/Al2O3 catalysts for the combustion of methane

Regeneration of S-poisoned Pd/Al₂O₃ catalysts for the abatement of methane emissions from natural gas vehicles was addressed in this work.

Investigations were devoted to determine the temperature threshold allowing for catalyst reactivation under different CH₄ containing atmospheres. Under lean combustion conditions in the presence of excess O₂, partial regeneration took place only above 750 °C after decomposition of stable sulphate species adsorbed on the support. Short CH₄-reducing, O₂-free pulses led to partial catalyst reactivation already at 550 °C and to practically complete regeneration at 600 °C. Also in this case reactivation was associated with SO₂ release due to the decomposition of stable support sulphates likely promoted by CH₄ activation onto the reduced metallic Pd surface. Rich combustion pulses with CH₄/O₂ = 2 were equally effective to CH₄-reducing pulses in catalyst regeneration.

These results suggest that a regeneration strategy based on periodical natural gas pulses fed to the catalyst by a by-pass line might be efficient in limiting the effects of S-poisoning of palladium catalysts for the abatement of CH₄ emissions from natural gas engine.     

NO decomposition and reduction by CH4 over Sr/La2O3

Both NO decomposition and NO reduction by CH₄ over 4%Sr/La₂O₃ in the absence and presence of O₂ were examined between 773 and 973 K, and N₂O decomposition was also studied. The presence of CH₄ greatly increased the conversion of NO to N₂ and this activity was further enhanced by co-fed O₂. For example, at 773 K and 15 Torr NO the specific activities of NO decomposition, reduction by CH₄ in the absence of O₂, and reduction with 1% O₂ in the feed were 8.3·10⁻⁴, 4.6·10⁻³, and 1.3·10⁻² μmol N₂/s m², respectively. This oxygen-enhanced activity for NO reduction is attributed to the formation of methyl (and/or methylene) species on the oxide surface. NO decomposition on this catalyst occurred with an activation energy of 28 kcal/mol and the reaction order at 923 K with respect to NO was 1.1. The rate of N₂ formation by decomposition was inhibited by O₂ in the feed even though the reaction order in NO remained the same. The rate of NO reduction by CH₄ continuously increased with temperature to 973 K with no bend-over in either the absence or the presence of O₂ with equal activation energies of 26 kcal/mol. The addition of O₂ increased the reaction order in CH₄ at 923 K from 0.19 to 0.87, while it decreased the reaction order in NO from 0.73 to 0.55. The reaction order in O₂ was 0.26 up to 0.5% O₂ during which time the CH₄ concentration was not decreased significantly. N₂O decomposition occurs rapidly on this catalyst with a specific activity of 1.6·10⁻⁴ μmol N₂/s m² at 623 K and 1220 ppm N₂O and an activation energy of 24 kcal/mol. The addition of CH₄ inhibits this decomposition reaction. Finally, the use of either CO or H₂ as the reductant (no O₂) produced specific activities at 773 K that were almost 5 times greater than that with CH₄ and gave activation energies of 21–26 kcal/mol, thus demonstrating the potential of using CO/H₂ to reduce NO to N₂ over these REO catalysts.     

Solubility and mass transfer of H2, CH4, and their mixtures in vacuum gas oil: An experimental and modeling study

In this work, the solubility data and liquid-phase mass transfer coefficients of hydrogen (H₂), methane (CH₄) and their mixtures in vacuum gas oil (VGO) at temperatures (353.15-453.15 K) and pressures (1-7 MPa) were measured, which are necessary for catalytic cracking process simulation and design. The solubility of H₂ and CH₄ in VGO increases with the increase of pressure, but decreases with the increase of temperature. Henry's constants of H₂ and CH₄ follow the relation of ln H=-413.05/T + 5.27 and ln H=-990.67/T + 5.87, respectively. The molar fractions of H₂ and system pressures at different equilibrium time were measured to estimate the liquid-phase mass transfer coefficients. The results showed that with the increase of pressure, the liquid-phase mass transfer coefficients increase. Furthermore, the solubility of H₂ and CH₄ in VGO was predicted by the predictive COSMO-RS model, and the predicted values agree well with experimental data. In addition, the gas-liquid equilibrium (GLE) for H₂ + CH₄ + VGO system at different feeding gas ratios in volume fraction (i.e., H₂ 85% + CH₄ 15% and H₂ 90% + CH₄ 10%) was measured. The selectivity of H₂ to CH₄ predicted by the COSMO-RS model agrees well with experimental data. This work provides the basic thermodynamic and dynamic data for fuel oil catalytic cracking processes.     

Catalytic oxidation of methane over hexaaluminates and hexaaluminate-supported Pd catalysts

An aqueous (NH₄)₂CO₃ coprecipitation method, based on that of Groppi et al. [Appl. Catal. A 104 (1993) 101–108] was used to synthesize Sr_1−xLa_xMnAl₁₁O₁₉₋ hexaaluminates. These materials were first synthesized by alkoxide hydrolysis. This synthesis route requires special handling of the starting materials and is not likely to be commercially practical. The materials prepared by (NH₄)₂CO₃ coprecipitation have similar surface areas as those prepared by the alkoxide hydrolysis method. Their CH₄ oxidation activity, measured as the temperature needed for 10% conversion of methane, is higher than those prepared by alkoxide hydrolysis. The La-substantiated material, LaMnAl₁₁O₁₉₋, shows high surface area with 19.3 m²/g after calcination at 1400°C for 2 h. It is active for CH₄ oxidation with T_10% at 450°C using 1% CH₄ in air and 70 000 cm³/h g space velocity. The stability and activity of LaMnAl₁₁O₁₉₋ prepared by (NH₄)₂CO₃ coprecipitation method is a simple and important step forward for the application of CH₄ catalytic combustion for gas turbines.     

Catalytic combustion concept for gas turbines

Catalytic combustion for gas turbines was investigated, based on a partial catalytic combustion section followed by a homogeneous combustion zone. A pressurized test rig (<25 bar) was built to test the influence of various parameters on this concept using Pd and Pt catalysts.

The pressure influence on the apparent catalytic reaction rate was of the order 0.4, assuming that the reaction kinetics could be described by a power rate function which was of first order with respect to methane. Pd catalysts showed a pressure-dependent temperature for the transition of the active PdO to the much less active Pd. Combining Pd and Pt within one catalyst resulted in a considerably lower transition temperature.

Homogeneous combustion reactions set on from 650°C, depending on the methane concentration, pressure and flow. With inlet temperatures above 800°C the homogeneous combustion always started. At outlet temperatures below 1050°C high CO concentrations could be measured. At higher temperatures the CO, CH₄ and NO_x concentrations were lower than 5 ppm. During several experiments total conversion of CH₄ and CO was observed.     

Polymers of carbonic acid: 13. Polymerization of cyclotrimethylenecarbonate with tin tetrahalides

Polymerizations of cyclotrimethylenecarbonate (TMC) with SnCl₄, SnBr₄ were and SnI₄ were conducted in bulk. The reaction time, the temperature and the monomer/initiator (M/I) ratio were varied, and the influence of these parameters on yields and molecular weights was investigated. Yields above 90% were obtained with all these initiators, but the highest molecular weights (M_w ≈ 1.5 × 10⁵) resulted from SnI₄-initiated polymerizations. SnI₄ has the additional advantage that the polycarbonates were free of ether groups when prepared at moderate temperatures (e.g. 60°C). In contrast, SnCl₄ and SnBr₄ cause partial decarboxylation even at 60°C, and at higher temperatures up to 20 mol% of the carbonate groups may be replaced by ether linkages. Furthermore, mechanistic studies were conducted. SnCl₄ and SnBr₄ form solid 1:2 complexes with TMC at 20–25°C. Infra-red, ¹H and ¹³C nuclear magnetic resonance (n.m.r.) spectroscopies indicate complexation at the carbonyl oxygen. Kinetic measurements of polymerizations in solvents of different polarity suggest cationic polymerization mechanisms. In addition to CH₂---OH, CH₂Cl and CH₂Br end groups were detected by ¹H n.m.r. spectroscopy. The endgroup analyses of reaction mixtures and isolated poly(TMC) support the assumption of a cationic polymerization mechanism.     

Oxidative reforming of methane to syngas over a NiO-CaO catalyst

Methane was successfully converted to synthesis gas by oxidative reforming over a reduced NiO-CaO catalyst first prepared by Choudhary et al. [1]. In order to compare this work with previously reported results, the effect of space velocity and temperature was investigated. Additionally, the lifetime of the catalyst was tested with both a dry and a wet feed, as well as the influence of CH₄/O₂ ratio on catalyst performance. In accordance with the results of Choudhary on this and other catalysts, the syngas yield was found to deviate from the expected equilibrium values. This may be attributed to the catalyst temperature being higher than the measured bulk gas temperature. Lifetime tests ( 500°C for 50 hours and 700°C for 20 hours) showed that the catalyst slowly lost activity and selectivity. In contrast, the inclusion of low steam feed rates resulted in stable operation (no deactivation and constant bed temperature). Decreasing the CH₄/O₂ ratio resulted in improved CH₄ conversion while, contrary to the expected equilibrium trend, CO selectivity remained constant. When space velocity was increased from 200 000 to 500 000 h⁻¹, CH₄ conversion and CO selectivity remained unchanged, but both values decreased when space velocity was again lowered to 200 000 h⁻¹.     

Mechanistic evidence of the partial oxidation of methane to formaldehdye over silica based oxide catalysts by temperature programmed reaction studies

The catalytic behaviour of SiO₂ supported MoO₂ and V₂O₅ catalysts in the partial oxidation of methane to formaldehyde with O₂ (MPO) in the range 400–800°C has been investigated by temperature programmed reaction (TPR) tests. Both the sequence of the onset temperature of product formation and the product distribution patterns signal that MPO on silica based oxide catalysts occurs mainly via a consecutive reaction path: CH₄ → HCHO → CO → CO₂. At T >/ 700°C a parallel surface assisted gas-phase reaction pathway leads to the formation of minor amounts of C₂ products both on SiO₂ and MoO₃/SiO₂ catalysts. The redox properties of MoO₃/SiO₂ and V₂O₅SiO₂ catalysts have been systematically evaluated by H₂ and CH₄ temperature programmed reduction (H₂-TPR, CH₄-TPR) measurements. H₂-TPR results do not account for the reactivity scale of oxide catalysts in the MPO. CH₄-TPR measurements indicate that the enhancement in the specific activity of the silica is controlled by the capability of MoO₃ and V₂O₅ promoters in providing ‘active’ lattice oxygen species.     

NO reduction by CH4 in the presence of O2 over La2O3 supported on Al2O3

Dispersing La₂O₃ on δ- or γ-Al₂O₃ significantly enhances the rate of NO reduction by CH₄ in 1% O₂, compared to unsupported La₂O₃. Typically, no bend-over in activity occurs between 500° and 700°C, and the rate at 700°C is 60% higher than that with a Co/ZSM-5 catalyst. The final activity was dependent upon the La₂O₃ precursor used, the pretreatment, and the La₂O₃ loading. The most active family of catalysts consisted of La₂O₃ on γ-Al₂O₃ prepared with lanthanum acetate and calcined at 750°C for 10 h. A maximum in rate (mol/s/g) and specific activity (mol/s/m²) occurred between the addition of one and two theoretical monolayers of La₂O₃ on the γ-Al₂O₃ surface. The best catalyst, 40% La₂O₃/γ-Al₂O₃, had a turnover frequency at 700°C of 0.05 s⁻¹, based on NO chemisorption at 25°C, which was 15 times higher than that for Co/ZSM-5. These La₂O₃/Al₂O₃ catalysts exhibited stable activity under high conversion conditions as well as high CH₄ selectivity (CH₄ + NO vs. CH₄ + O₂). The addition of Sr to a 20% La₂O₃/γ-Al₂O₃ sample increased activity, and a maximum rate enhancement of 45% was obtained at a SrO loading of 5%. In contrast, addition of SO⁼₄ to the latter Sr-promoted La₂O₃/Al₂O₃ catalyst decreased activity although sulfate increased the activity of Sr-promoted La₂O₃. Dispersing La₂O₃ on SiO₂ produced catalysts with extremely low specific activities, and rates were even lower than with pure La₂O₃. This is presumably due to water sensitivity and silicate formation. The La₂O₃/Al₂O₃ catalysts are anticipated to show sufficient hydrothermal stability to allow their use in certain high-temperature applications.     

The 90° brillouin scattering in β-BiNbO4 single crystal

The acoustic modes propagated in the bc plane of triclinic β-BiNbO₄ single crystal are investigated by the Brillouin scattering method at room temperature. The Brillouin spectra corresponding to the longitudinal acoustic mode are analyzed to evaluate the elastic constants. Obtained elastic constants are c33 = 16 ± 2, c22=27 ± 3 and c23+2c44=25 ± 3 in the unit of 10¹¹ dyne· mid;cm^-2.     

Diamond and graphite crystallization from C–O–H fluids under high pressure and high temperature conditions

Crystallization of diamond was studied in the CO₂–C, CO₂–H₂O–C, H₂O–C, and CH₄–H₂–C systems at 5.7 GPa and 1200–1420°C. Thermodynamic calculations show generation of CO₂, CO₂–H₂O, H₂O and CH₄–H₂ fluids in experiments with graphite and silver oxalate (Ag₂C₂O₄), oxalic acid dihydrate (H₂C₂O₄·2H₂O), water (H₂O), and anthracene (C₁₄H₁₀), respectively. Diamond nucleation and growth has been found in the CO₂–C, CO₂–H₂O–C, and H₂O–C systems at 1300–1420°C. At a temperature as low as 1200°C for 136 h there was spontaneous crystallization of diamond in the CO₂–H₂O–C system. For the CH₄–H₂–C system, at 1300–1420°C no diamond synthesis has been established, only insignificant growth on seeds was observed. Diamond octahedra form from the C–O–H fluids at all temperature ranges under investigation. Diamond formation from the fluids at 5.7 GPa and 1200–1420°C was accompanied with the active recrystallization of metastable graphite.     

Computations of the Mie scattering coefficient corrected for the forward scattering

Computations of the actual Mie scattering coefficient (K_a) have been performed by taking into account the acceptance angle (₀) of the optical receiving system. Results show that in problems concerning the determination of the aerosol particle parameters through scattering methods, the total scattering coefficient (K_r) computed by taking ₀ = 0 has to be substituted by K_a. Tables of K_a have been arranged for the following values of the particle refractive index (m), size parameter (x) and ₀: M = 1·33, X = 0·1 (0·1) 200 and m = 1·55, X = 0·1 (0·1) 100, for ₀ = 0·1° (0·1°) 1·0°.     

Synthesis of ordered small pore mesoporous silicates with tailorable pore structures and sizes by polyoxyethylene alkyl amine surfactant

Ordered mesoporous silicates with tailorable pore structures and small pore sizes have been synthesized by using polyoxyethylene alkyl amine surfactant PN-430 [CH₃(CH₂)₁₇N(EO)_x(EO)_y, x + y = 5] as a structure-directing agent under acidic condition. Two-dimensional (2-D) hexagonal (p6mm) mesoporous silicates have been prepared via an evaporation-induced self-assembly (EISA) process. The N₂ sorption isotherms show that the product has a small uniform pore size distribution of 1.8 nm by BJH model, a BET surface area of 730 m²/g and a pore volume of 0.36 cm³/g. 3-D cubic (Pm-3n) mesoporous silicate with small uniform pore size (1.76 nm) can also be prepared at high concentration of PN-430 by EISA method in tetrahydrofuran solvent. The solvothermal post-treatment by n-hexane at 70 °C for 3 d to the above material results in the phase transition of the mesostructure from Pm-3n to P6₃/mmc based on XRD and TEM analyses. In comparison, by using nonionic oligometric alkyl-ethylene oxide surfactant such as Brij 78 (C₁₈H₃₇EO₂₀) or Triton X-100 (CH₃C(CH₃)₂CH₂C(CH₃)₂C₆H₄ EO₁₀) as co-templates, high-quality hexagonal (p6mm) small pore mesoporous silicates have also been prepared in ethanol media. Our results show that the blend templates composed of PN-430 and a small amount of nonionic surfactant can increase the efficiency of organic and inorganic hybrid species assembly, improve the quality of the structural regularity, and decrease the pore size to about 1.65 nm.