Temperature dependence of oxygen diffusion into clay‐doped PS films

Fluorescence technique was employed for the measurement of the diffusion coefficient of oxygen into polystyrene (PS) latex/modified Na‐activated bentonite (MNaLB) clay composite films. Three different MNaLB content (0, 5, and 20 wt%) composite films were prepared from PS/MNaLB mixtures by annealing them at 200°C, above the glass transition temperature of PS for 10 min. To determine the diffusivity of oxygen in PS/MNaLB composite films, diffusion measurements were performed over the temperature range from 25 to 70°C. Pyrene (P) was used as the fluorescent agent. The diffusion coefficients (D) of oxygen were determined by combining the fluorescence quenching method with Fickian transport model, and were found as a function of temperature for each MNaLB content film. The results showed that D values are strongly dependent on both temperature and clay content in composite film. It was also observed that D coefficients obey Arrhenius behavior, from where diffusion activation energies were measured. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Kinetic studies on sorption of n-hexane in vanadium substituted MFI type zeolites of various crystal morphologies

The MFI type materials isomorphously substituted with vanadium form crystals of two morphology types. Investigations of sorption kinetics for n-hexane indicated for both morphologies a non-typical increase in the value of corrected transport diffusion coefficient with the crystals dimensions. An increase in the D₀ values with the vanadium content of the crystals has also been found, although it is not so well expressed as that with the dimensions. The increase in the D₀ values is from 1.1 × 10⁻¹¹ to 1.1 × 10⁻¹⁰ m²/s and may be a consequence of an additional system of larger pores, which is not reflected in the adsorption isotherms due to common occurrence of these pores in all crystals. It is also possible that vanadium causes a superior structure ordering and a decrease in/weakening of diffusion barriers.     

DI-B experiment: planning, design and performance of an in situ diffusion experiment in the Opalinus Clay formation

The DI-B experiment is a long-term, natural-scale, in situ diffusion experiment, which is being performed in the Opalinus Clay formation at the Mont Terri Underground Rock Laboratory (URL), in Switzerland, employing nonradioactive tracers. One of the key aspects to be addressed for nuclear waste repository safety assessment purposes is the understanding of the transport mechanisms of the radionuclides contained in the radioactive waste. Consolidated clay formations display very low water hydraulic conductivities, so it is expected that the predominant transport process will be diffusion.The experimental set-up has been designed to withstand the site conditions and for monitoring and recording several physicochemical parameters (pH, conductivity, oxidation–reduction potential), as well as the pressures in the circuit and for the long-distance monitoring of the data acquisition system.The tracer selection has been made based on previous investigations carried out at CIEMAT, including a literature survey, laboratory sorption experiments and hydrogeochemical modeling for determining tracer stability under the physicochemical conditions to be expected in the site. The final selection includes ⁶Li, ⁸⁷Rb, D (as D₂O) and I (as I⁻). Hydrogeochemical modeling confirmed the stability of all the tracers selected. Batch sorption experiments showed that no sorption in the rock occurred in the case of ⁶Li, D and I (conservative tracers), whereas ⁸⁷Rb was 100% sorbed. However, ⁸⁷Rb was chosen because of its analogy with Cs, a relevant radionuclide commonly present in the nuclear spent fuel.Diffusion experiments have been carried out at laboratory scale with Opalinus Clay samples to provide diffusion parameters for modeling purposes. Effective diffusion coefficients, perpendicular and parallel to the bedding planes of the rock, respectively, were (1.68± 0.42)×10⁻¹¹ and (4.02± 0.30)×10⁻¹¹ m²/s for tritium, and (2.70± 0.27)×10⁻¹² and (1.38± 0.49)×10⁻¹¹ m²/s for iodide. Additional through-diffusion experiments (parallel to the bedding) were performed with the nonsorbing tracer ³⁶Cl⁻, in order to check the results obtained for iodide. The effective diffusion coefficient measured for chloride ions was (1.18± 0.27)×10⁻¹¹ m²/s, which is practically equal to the value obtained for iodide.Preliminary diffusion calculations have been carried out using two transport codes: GIMRT and CORE^2D, with conservative and nonconservative tracers, using effective diffusion coefficients (D_e) obtained experimentally in the laboratory (through-diffusion experiments) or selected from the literature. The diffusion profiles obtained from the calculations showed slight variations, which were consistent with the different modeling approaches employed. The predictive modeling results have been used to determine the initial tracer concentration that should be added to the circuit to assure well-defined profiles at the end of the experiment.This long-term in situ diffusion experiment will also provide useful data for the interpretation of previous diffusion experiments performed at the Mont Terri URL.     

Spatial variability of transport parameters in the Boom Clay

Several transport parameters (as hydraulic conductivity K, apparent diffusion coefficient D_p and diffusion accessible porosity η of HTO and iodide) have been intensively measured in the laboratory on high-quality cores taken at the Mol-1 borehole of the Mol site, in Belgium. The borehole was cored in 1997 from about 145 to 325 m depth, including the whole thickness of the Boom Formation, a Tertiary clay situated between 186 and 288 m depth (ground level), and part of the surrounding layers.The hydraulic conductivity measurements confirm the low permeability of the Boom Clay. An upper 90-m-thick clay layer within this formation can be considered as homogeneous with respect to the hydraulic conductivity. The vertical hydraulic conductivity K_v (i.e. K perpendicular to the bedding) is in the order of magnitude of 10⁻¹² m s⁻¹ and the average is 2.3×10⁻¹² m s⁻¹. This layer comprises from top to bottom the “Transition Zone”, the Putte Member, the Terhagen Member and the top of the Belsele-Waas Member of the Boom Formation. The 12 m at the base of the Formation, which corresponds to the lower part of the Belsele-Waas Member is characterised by larger K_v values (ranging between 10⁻¹¹ and 9×10⁻¹¹ m s⁻¹).The same thick clay layer can also be considered as homogeneous, regarding the values of the apparent diffusion coefficient and the diffusion accessible porosity η of tritiated water (HTO) and iodide. The average value of the diffusion accessible porosity is 0.37±0.03 for HTO and 0.16±0.02 for iodide. The apparent diffusion coefficient varies from 1.1×10⁻¹⁰ to 5.5×10⁻¹⁰ m² s⁻¹ for HTO and from 9.1×10⁻¹¹ to 5.2×10⁻¹⁰ m² s⁻¹ for iodide.     

Low-temperature thermionic emission from nitrogen-doped nanocrystalline diamond films on n-type Si grown by MPCVD

Temperature-dependent emission current–voltage measurements were carried out for nitrogen (N)-doped nanocrystalline diamond (NCD) films grown on n-type Si substrates by microwave plasma-assisted chemical vapor deposition (MP-CVD). Low threshold temperature (~ 260 °C) and low threshold electric field (~ 5 × 10^− 5 V/µm) were observed. Both the temperature dependence and the electric field dependence have shown that the obtained emission current was based on electron thermionic emission from N-doped NCD films. We have also studied the relation between nitrogen concentration and the saturation emission current. The saturation current obtained was as high as 1.4 mA at 5.6 × 10^− 3 V/µm at 670 °C when the nitrogen concentration was 2.4 × 10²⁰ cm^− 3. Low value of effective work function (1.99 eV) and relatively high value of Richardson constant (~ 70) were estimated by well fitting to Richardson–Dushman equation. The results of smaller φ and larger A′ suggest that N-doped NCD has great possibility of being a highly efficient thermionic emitter material.     

Enhanced performance of HFCVD nanocrystalline diamond self-mated tribosystems by plasma pretreatments on silicon nitride substrates

Nanocrystalline diamond (NCD) coatings were grown by the hot-filament chemical vapour deposition (HFCVD) method on hydrogen plasma pretreated silicon nitride (Si₃N₄) substrates. The friction and wear behaviour of self-mated NCD films, submitted to unlubricated sliding and high applied loads (up to 90 N), was assessed using an oscillating ball-on-flat configuration in ambient atmosphere. The reciprocating tests revealed an initially high friction coefficient peak, associated to the starting surface roughness of NCD coatings (R_q = 50 nm). Subsequently, a steady-state regime with low friction coefficient values (0.01–0.04) sets in, related to a smoother (R_q = 17 nm) tribologically modified surface. A polishing wear mechanism governing the material loss was responsible for mild wear coefficients (k  10^− 7 mm³ N^− 1 m^− 1). The hydrogen etching procedure notably increased the film adhesion with respect to untreated surfaces as demonstrated by the high threshold loads (60 N; 3.5 GPa) prior to film delamination.     

Adsorption of Pb(II) from aqueous solution to MX-80 bentonite: Effect of pH, ionic strength, foreign ions and temperature

The adsorption of Pb(II) from aqueous solution to MX-80 bentonite was studied using batch technique under ambient conditions. Removal percent (%) and distribution coefficient (K_d) were determined as a function of shaking time, pH, ionic strength and temperature. The results showed that the adsorption behavior of Pb(II) on bentonite was strongly dependent on pH and ionic strength. The presence of complementary cations depressed the adsorption of Pb(II) on bentonite in the order of Li⁺ ≈ Na⁺ > K⁺ at pH 2–5. The adsorption isotherms were simulated by the Langmuir, Freundlich, and Dubinin–Radushkevich (D–R) models very well. The thermodynamic parameters (ΔH⁰, ΔS⁰, and ΔG⁰) for the adsorption of Pb(II) were determined at three different temperatures of 291 K, 308 K and 328 K. The adsorption reaction was exothermic and the process of adsorption was favored at low temperature. The results suggest that bentonite is suitable as a sorbent material for the recovery and adsorption of Pb(II) from aqueous solution.     

Electrochemical behaviour of uranium (IV) in DMF at vitreous carbon

The electrochemical behaviour of UCl₄ (0.01 mol L⁻¹ up to 0.05 mol L⁻¹) in 0.1 mol L⁻¹ TBAPF₆/DMF solution at vitreous carbon was studied, at room temperature, by cyclic voltammetry and potentiostatic techniques. The electrolytic solutions were analyzed by UV spectroscopy (UV), and the electrodeposited films were characterized by Rutherford Backscattering Spectroscopy (RBS) and X-ray diffraction (XRD). The cyclic voltammetric results, at low UCl₄ concentrations (0.01 mol L⁻¹), point that the reduction of U(IV) to U(0) occurs in two steps involving mainly U(IV) and U(III) species. The first electron transfer reaction is quasi-reversible and the second irreversible. The diffusion coefficient of U(IV) in DMF and the charge rate constant were determined to be 4.78 × 10⁻⁷ cm² s⁻¹ and 1.93 × 10⁻³ cm s⁻¹ (at 0.02 V s⁻¹), respectively.RBS data obtained from samples prepared at constant potential (−3.10 V) during 3 h at room temperature, indicated the presence of uranium particles deposited all over the vitreous carbon surface with aggregates in some places, confirming that the second reduction step corresponds to uranium electrodeposition. No crystallographic ordering could be seen by XRD, pointing to an amorphous character of the uranium films.     

Fluorescence quenching method for monitoring oxygen diffusion into PS/CNT composite films

Oxygen permeabilities of nanocomposite films consisting of multi wall carbon nanotubes (MWNT) and polystyrene (PS) were determined to investigate the oxygen diffusion depending on MWNT and temperature. A method which is based on quenching of an excited phosphorescent by oxygen was applied for the measurements. The composite films were prepared from mixtures of (MWNT) and surfactant-free pyrene (P)-labeled (PS) latexes of various compositions at room temperature. These films were then annealed at 170 °C which is well above the glass transition (T_g) temperature of polystyrene, for 10 min. Diffusion experiments were performed for eight films with different MWNT content (0, 1.5, 3, 5, 10, 15, 25 and 40 wt%) to evaluate the effect of MWNT content on oxygen diffusion. Diffusion coefficients were found to increase from 1.1 × 10⁻¹² to 41 × 10⁻¹² cm² s⁻¹ with increasing MWNT content. On the other hand, to examine the effect of temperature on oxygen diffusion, diffusion measurements were performed over a temperature range of 24–70 °C for three different MWNT contents (3, 15, and 40 wt%) within the films. The results indicated that the values of the diffusion coefficient D are strongly dependent on both temperature and MWNT content in the film. It was also observed that the diffusion coefficients obey Arrhenius behavior, from which diffusion energies were determined, which increased with increase of MWNT content and temperature.     

Hybrid PAC-submerged membrane system for trace organics removal: I. Adsorption kinetics study of PAC in a bubbled solution

Part I of this study presents a theoretical method combined with experiments to determine the adsorption kinetics of powdered activated carbon (PAC) in the hybrid PAC-submerged membrane (SM) system with air bubbling for trace organics removal. The homogeneous surface diffusion model (HSDM) was applied to describe the kinetics of the adsorbate uptake. The differences between the model solutions and the corresponding experimental results were minimized by means of Levenberg–Marquardt algorithm so that two kinetic parameters D_S and k_f involved in HSDM were obtained simultaneously. The D_S was found to be 1.14 × 10⁻¹⁶ m²/s and the k_f value was correlated with the bubbling rate (Q_b) and carbon dosage (C_c), which are required in the modeling of the hybrid PAC-SM system presented in Part II of this study. The k_f was enhanced from 1.18 × 10⁻⁴ to 4.18 × 10⁻⁴ m/s when the bubbling intensity increased in the intermittent bubbling tests, suggesting that from energy consumption point of view, the high intensity intermittent bubbling is more efficient in improving the liquid film mass transfer than continuous bubbling with the same net bubbling rate.     

Electrochemical studies on the binding of a carcinogenic anthraquinone dye, Purpurin (C.I. 58 205) with DNA

The interaction of the anthraquininoid dye, Purpurin (C.I. 58 205) with fish-sperm DNA was electrochemically investigated in 0.02 mol L⁻¹ pH 7.0 Britton–Robinson (B–R) buffer. After interaction with DNA, the irreversible oxidation peak of the dye decreased and underwent a positive shift of potential, suggesting intercalation between the dye and DNA, this being consistent with fluorescence quenching results. The binding constant (K) and binding site size (s) of the two species were 9.6 × 10⁶ L mol⁻¹ and 1.7 respectively, as determined using voltammetric titration, indicating that the dye was strongly bound to the DNA.     

Photochemical production of nitric oxide from a nitrosyl phthalocyanine ruthenium complex by irradiation with light in the phototherapeutic window

The photochemical behavior of [Ru(NO)(NO)₂pc] (pc = phthalocyanine) is reported in this paper. In addition to ligand localized absorption bands (λ < 300 nm), the electronic spectrum of this complex in dichloromethane solution was dominated by an intense absorption at 640 nm characterized as Q-bands. Irradiation of [Ru(NO)(NO)₂pc] at 366 and 660 nm led to the production of nitric oxide (NO) as detected by a NO-sensor. NO production by light irradiation at high energy involved excitation of d_π–π^* transition, while a photoinduced electron transfer occurred at long wavelength irradiation. The NO quantum yields varied from 1.4 × 10⁻³ to 2.3 × 10⁻² mol einstein⁻¹, depending on oxygen concentration.     

An efficient light-harvesting ruthenium dye for solar cell application

A novel, heteroleptic ruthenium dye comprising a vinyl group between the carboxylate and bipyridine segments as well as extended π-conjugation of the ancillary ligand, employing alkyl-bithiophene, was synthesized. The dye displayed a remarkably high absorption coefficient of 2.51 × 10⁴ M⁻¹ cm⁻¹ (at 562 nm) for its metal-to-ligand charge transfer band. The photo-to-current conversion efficiency of the corresponding dye-sensitized solar cell was 9.12% under AM 1.5 (100 mW/cm²) irradiation. Furthermore, owing to both the very strong metal-to-ligand charge transfer band and the large number of dye molecules adsorbed on the TiO₂ electrode, the conversion efficiency of the dye-sensitized cell was >7.5% at a light intensity ≤198 mW cm⁻².     

Magnetic properties of CNX whiskers

Carbon–nitrogen whiskers have been prepared by pyrolysis of 1,2-diaminopropane at 950 °C or of allylamine at 900 °C followed by quenching. They are scrolls of carbon film typically 250 nm thick and up to 1 mm long with about five layers in a structure like a “cigare russe” or “brandy snap”, about 50 μm in diameter. Approximately 8 wt% of nitrogen is incorporated into the carbon films, which are practically amorphous, exhibiting a broad diffraction peak at d = 0.34 nm. The whiskers are on the border of metallic conductivity with a resistivity of about 10⁻⁶ Ωm, and they may show either a positive or a negative temperature coefficient of resistance. The pyrolysis produces either whiskers, soot or both. Magnetization measurements of the whiskers made from 1,2-diaminopropane reveal a large diamagnetic susceptibility of χ = −170 × 10⁻⁹ m³  kg⁻¹ and a small ferromagnetic component of unknown origin with σ_S of up to 0.2 A m² kg⁻¹, whereas the soot shows a purely diamagnetic signal, with χ ≈ −40 × 10⁻⁹ m³ kg⁻¹.     

Study of LiNi0.5Mn1.5O4 synthesized via a chloride-ammonia co-precipitation method: Electrochemical performance, diffusion coefficient and capacity loss mechanism

LiNi_0.5Mn_1.5O₄ powders are prepared via a new co-precipitation method. In this method, chloride salts are used as precursors and ammonia as a precipitator. The impurity of chlorine can be removed via a thermal decomposition of NH₄Cl in the subsequent calcination. X-ray diffraction pattern reveals that the final product is a pure spinel phase of LiNi_0.5Mn_1.5O₄. Scanning electron microscopy shows that the powders have an octahedron shape with a particle size of about 2 μm. Electrochemical test shows that the LiNi_0.5Mn_1.5O₄ powders exhibit an excellent cycling performance and after 300 cycles, the capacity retention is 83%. The lithium diffusion coefficient is measured to be 5.94 × 10⁻¹¹ cm² s⁻¹ at 4.1 V, 4.35 × 10⁻¹⁰ cm² s⁻¹ at 4.75 V and 7.0 × 10⁻¹⁰ cm² s⁻¹ at 4.86 V. The mechanism of capacity loss is also explored. After 300 cycles, the cell parameter ‘a’ decreases by 0.54% for the quenched sample (LiNi_0.5Mn_1.5O_4−δ) and by 0.42% for the annealed sample (LiNi_0.5Mn_1.5O₄). Besides, it is the first time to identify experimentally that the Ni and Mn ions dissolved in the electrolyte can be further deposited on the surface of anode.     

Immobilization of horseradish peroxidase on self-assembled (3-mercaptopropyl)trimethoxysilane film: Characterization, direct electrochemistry, redox thermodynamics and biosensing

Highly organized (3-mercaptopropyl)trimethoxysilane (3-MPT) films have been prepared via self-assembled coupled with sol–gel linking technology. Horseradish peroxidase (HRP) is successfully immobilized onto the densely packed three-dimensional (3D) 3-MPT network and the direct electrochemistry of HRP is achieved without any electron mediators or promoters. Redox thermodynamics of HRP on the 3-MPT films, which is obtained from the temperature dependence of the reduction potential, suggests that the positive shift of redox potentials of HRP at the interface of 3-MPT originates from the solvent reorganization effects and conformational change of the polypeptide chain of HRP. Based on the direct electrochemistry and electrocatalytic ability of HRP, a sensitive third-generation amperometric H₂O₂ biosensor is developed with two linear dependence ranges of 5.0 × 10⁻⁷ to 1.0 × 10⁻⁴ and 1.0 × 10⁻⁴ to 2.0 × 10⁻² mol L⁻¹.     

The role of pressure on thin amorphous carbon films deposited using microwave surface wave plasma CVD

We report the effects of gas composition pressure (GCP) on the optical, structural and electrical properties of thin amorphous carbon (a-C) films grown on p-type silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition (MW SWP CVD). The films, deposited at various GCPs ranging from 50 to 110 Pa, were studied by UV/VIS/NIR spectroscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and current–voltage characteristics. The optical band gap of the a-C film was tailored to a relatively high range, 2.3–2.6 eV by manipulating GCPs from 50 to 110 Pa. Also, spin density strongly depended on the band gap of the a-C films. Raman spectra showed qualitative structured changes due to sp³/sp² carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness < 0.5 nm). The photovoltaic measurements under light illumination (AM 1.5, 100 mW/cm²) show that short-circuit current density, open-circuit voltage, fill factor and photo-conversion efficiency of the film deposited at 50 Pa were 6.4 μA/cm², 126 mV, 0.164 and 1.4 × 10^− 4% respectively.     

Zinc(II) complex of terpyridine-crown macrocycle: A new motif in fluorescence sensing of zwitterionic amino acids

A new fluorescence macrocyclic receptor based on the Zn(II) complex of a C₂-terpyridine and a crown ether has been developed for molecular recognition of zwitterionic amino acids in water/DMF solution with remarkable selectivity towards l-aspartate (K = 4.5 × 10⁴ M⁻¹) and l-cysteine (K = 2.5 × 10⁴ M⁻¹).     

Thermal stability of Mesoporous SBA-15 and Sn-SBA-15 Molecular Sieves: An in situ HTXRD study

Highly ordered SBA-15 and Sn-SBA-15 mesoporous molecular sieves with Si/Sn = 80, 60, 40 and 10 have been prepared through direct synthesis route under milder acidic conditions, which were used for the in situ high temperature X-ray diffraction (HTXRD) studies in the temperature range 298–1573 K for the first time in the literature. SBA-15 is found to be thermally stable up to 1473 K and appearance of α-cristobalite was observed at 1573 K also supported by the thermogravimetric (TG) data. A strong negative thermal expansion was observed on heating from 298 to 1573 K (α_a = −4.3 × 10⁻⁶ K⁻¹). Sn containing samples (Si/Sn = 80 and 60) showed a positive thermal expansion (6.75 × 10⁻⁶ K⁻¹ and 9.04 × 10⁻⁶ K⁻¹, respectively). On the other hand, the samples with Si/Sn = 40 and 10 showed a strong negative thermal expansion (−4.12 × 10⁻⁶ K⁻¹ and −7.56 × 10⁻⁶ K⁻¹) similar to SBA-15. The linear thermal expansion coefficient varied in the order: Si/Sn = 60 > 80 > 40 > 0 > 10. Sn⁴⁺ ions exhibit both tetrahedral and octahedral coordination depending upon the location of these ions either on the walls of the silica (Si/Sn = 80 and 60) or in the corona region of the structure (Si/Sn = 40 and 10), respectively. The thermal decomposition of the samples (TG data) is correlated to the thermal expansion behavior (HTXRD data). The decomposition behavior of template ions located within the pores is strongly influenced by the presence of Sn in the framework and a ‘soft’ interaction probably exists between the template ions and the Sn sites.     

Application of silica gel as an effective modifier for the voltammetric determination of dopamine in the presence of ascorbic acid and uric acid

Amorphous silica gel modified carbon paste electrode (CPE) offers substantial improvements in voltammetric sensitivity and selectivity towards determination of dopamine (DA). Cyclic voltammetry of Fe(CN)₆^3−/4− as a negatively charged probe revealed that the surface of the silica gel modified carbon paste electrode had a high density of negative charge at pH 8.0. Therefore, the modified electrode adsorbed DA (pK_a = 8.9) and enhanced its voltammetric response while repulsed ascorbic acid (AA) (pK_a = 4.2) and uric acid (UA) (pK_a = 5.4) and inhibited their interfering effects. The influence of various experimental parameters including percent of silica gel in the CPE, pH of solution, and accumulation time and potentials, on the voltammetric response of DA was investigated. At the optimum conditions, the analytical curve was linear for dopamine concentrations from 2.0 × 10⁻⁷ to 1.0 × 10⁻⁶ mol L⁻¹ and 2.0 × 10⁻⁶ to 1.5 × 10⁻⁴ mol L⁻¹ with a detection limit (3σ) of 4.8 × 10⁻⁸ mol L⁻¹. The prepared electrode was used for determination of DA spiked into DA injection and human serum samples, and very good recovery results were obtained over a wide concentration range of DA.