A molecular dynamic study on the dissociation mechanism of SI methane hydrate in inorganic salt aqueous solutions

Gas hydrate is not only a potential energy resource, but also almost the biggest challenge in oil/gas flow assurance. Inorganic salts such as NaCl, KCl and CaCl₂ are widely used as the thermodynamic inhibitor to reduce the risk caused by hydrate formation. However, the inhibition mechanism is still unclear. Therefore, molecular dynamic (MD) simulation was performed to study the dissociation of structure I (SI) methane hydrate in existence of inorganic salt aqueous solution on a micro-scale. The simulation results showed that, the dissociation became stagnant due to the presence of liquid film formed by the decomposed water molecules, and more inorganic ions could shorten the stagnation-time. The diffusion coefficients of ions and water molecules were the largest in KCl system. The structures of ion/H₂O and H₂O/H₂O were the most compact in hydrate/NaCl system. The ionic ability to decompose hydrate cells followed the sequence of: Ca²⁺ > 2K⁺ > 2Cl⁻ > 2Na⁺.     

A micro shear stress sensor based on laterally aligned carbon nanotubes

This paper reports the development of a micro thermal shear stress sensor that utilizes multiwalled carbon nanotubes as the sensing element. The sensor was fabricated by laterally aligning randomly distributed nanotubes into a 360 μm long and 90 μm wide conductive trace between two triangular shaped micro electrodes through the use of a high frequency AC electric field. During operation, the aligned nanotubes are electrically heated to an elevated temperature and surface shear stress is measured indirectly by the amount of convective heat transfer from the heated nanotubes to the surrounding fluid flow.The nanotube alignment process was primarily controlled by three different phenomena: dielectrophoresis, joule heating, and Brownian motion. Numerical simulations, together with experimental verifications, indicated that a successful alignment could only be realized if: (1) the dielectrophoretic force was positive, (2) the electro-thermal force was also positive, and (3) the dielectrophoretic force was high enough to overcome Brownian motion. The aligned nanotube trace has a room-temperature resistance of 580 Ω, which corresponds to a conductivity of 2.7 × 10⁴ S/m. The absolute temperature coefficient of resistivity ranges from 0.01 to 0.04% °C⁻¹. This is about one order of magnitude smaller than the highly doped polysilicon sensing material used in the MEMS micro shear stress sensor. The shear stress sensitivity of the nanotube trace operated at a 3% overheat ratio is found to follow the theoretical sensor power ∝ (shear stress)^1/3 relationship, provided the shear stress level is higher than 0.34 mPa. This result confirms the feasibility of using aligned multi-walled carbon nanotubes as a thermal shear stress sensing material.     

Red emitting MTiO3 (M = Ca or Sr) phosphors doped with Eu3+ or Pr3+ with some cations as co-dopants

Ca (or Sr)TiO₃:Eu³⁺, M (Li⁺ or Na⁺ or K⁺) and CaTiO₃:Pr³⁺, M (Li⁺ or Na⁺ or Ag⁺ or K⁺ or Gd³⁺ or La³⁺) powders were prepared by combustion synthesis method and the samples were further heated to ～1000 °C to improve the crystallinity. The structure and morphology of materials were examined by X-ray diffraction (XRD) and a scanning electron microscopy (SEM). The morphologies of SrTiO₃:Eu³⁺, CaTiO₃:Eu³⁺ or CaTiO₃:Pr³⁺ powders co-doped with other metal ions were very similar. Small and coagulated particles of nearly cubical shapes with small size distribution having smooth and regular surface were formed. Photo-luminescence spectra of CaTiO₃:Pr³⁺ and co-doped either with Li⁺, Na⁺, K⁺, Ag⁺, La³⁺ or Gd³⁺ ions showed red emissions at 613 nm due to the ¹D₂ → ³H₄ transition of Pr³⁺. The variation of intensity of emission peak with different co-doping follows the order: K⁺ > Ag⁺ > Na⁺ > Li⁺ > La³⁺ > Gd³⁺. The characteristic emissions of CaTiO₃:Eu³⁺ lattices had strong emission at 614 and 620 nm for ⁵D₀ → ⁷F₂ with other weak transitions observed at 580, 592, 654, 705 nm for ⁵D₀ → ⁷F_n transitions where n = 0, 1, 3, 4 respectively in all host lattices. Photoluminescence intensity in SrTiO₃:Eu³⁺ is more than CaTiO₃:Eu³⁺ lattices. A remarkable increase of photoluminescence intensity (in ⁵D₀ → ⁷F₂ transition) was observed if co-doped with Li⁺ ions in CaTiO₃:Eu³⁺ and SrTiO3:Eu³⁺.     

Bounds for the Roots of Lacunary Polynomials

A polynomial P(X)  = X^d + a_d − 1X^d − 1 + ⋯ is called lacunary when a_d − 1 =  0. We give bounds for the roots of such polynomials with complex coefficients. These bounds are much smaller than for general polynomials.     

Development of an optical fibre reflectance sensor for copper (II) detection based on immobilised salicylic acid

Immobilized salicylic acid onto XAD-2 (styrene–divinylbenzene cross-linked copolymer) has been attempted in this study as a reagent phase for the development of an optical fibre copper (II) sensor. The measurements were carried out at a given wavelength of 690.27 nm since it yielded the largest divergence different in reflectance spectra before and after reaction with the analyte element. The optimum response was obtained at pH 5.0. The linear dynamic range of Cu(II) was found within the concentration range of 1.0–2.0 mmol L⁻¹ with its LOD of 0.5 mmol L⁻¹. The sensor response from different probes (n = 9) gave an R.S.D. of 8.4% at 0.55 mmol L⁻¹ Cu(II). The effect of interfered ions at 1:1 molar ratio of Cu(II):foreign ion was also studied in this work.     

Simultaneous determination of epinephrine and ascorbic acid at the electrochemical sensor of triazole SAM modified gold electrode

The electrochemical sensor of triazole (TA) self-assembled monolayer (SAM) modified gold electrode (TA SAM/Au) was fabricated. The electrochemical behaviors of epinephrine (EP) at TA SAM/Au have been studied. The TA SAM/Au shows an excellent electrocatalytic activity for the oxidation of EP and accelerates electron transfer rate. The diffusion coefficient is 1.135 × 10⁻⁶ cm² s⁻¹. Under the optimum experiment conditions (i.e. 0.1 mol L⁻¹, pH 4.4, sodium borate buffer, accumulation time: 180 s, accumulation potential: 0.6 V, scan rate: 0.1 Vs⁻¹), the cathodic peak current of EP versus its concentration has a good linear relation in the ranges of 1.0 × 10⁻⁷ to 1.0 × 10⁻⁵ mol L⁻¹ and 1.0 × 10⁻⁵ to 6.0 × 10⁻⁴ mol L⁻¹ by square wave adsorptive stripping voltammetry (SWASV), with the correlation coefficient of 0.9985 and 0.9996, respectively. Detection limit is down to 1.0 × 10⁻⁸ mol L⁻¹. The TA SAM/Au can be used for the determination of EP in practical injection. Meantime, the oxidative peak potentials of EP and ascorbic acid (AA) are well separated about 200 ± 10 mV at TA SAM/Au, the oxidation peak current increases approximately linearly with increasing concentration of both EP and AA in the concentration range of 2.0 × 10⁻⁵ to 1.6 × 10⁻⁴ mol L⁻¹. It can be used for simultaneous determination of EP and AA.     

Flow-injection determination of iron(III) in soil by biamperometry using two independent redox couples

A flow-injection biamperometric method for the determination of iron(III) has been described. The detector consists of two chambers separated by a salt bridge, and one platinum wire working electrode is embedded in each chamber, respectively. When iron(III) solution and hydrogen peroxide solution simultaneously flow through two chambers, the reduction of iron(III) at one platinum electrode is associated with the oxidation of hydrogen peroxide at the other platinum electrode, forming such a system as similar to a reversible couple one. The biamperometric system can perform the determination of iron(III) without any external potential difference. The linear relationship is obtained from 1.0 × 10⁻⁶ to 1.0 × 10⁻⁴ mol l⁻¹ with a detection limit of 6.0 × 10⁻⁷ mol l⁻¹. The proposed method exhibits the satisfactory reproducibility with a relative standard derivation (R.S.D.) of 1.4% for 17 successive determinations of 2.0 × 10⁻⁵ mol l⁻¹ iron(III) and is applied to the determination of iron(III) in soil.     

Development of a mercury optical sensor based on immobilization of 4-(2-pyridylazo)-resorcinol on a triacetylcellulose membrane

A new optical sensor for mercury(II) ions is developed based on immobilization of 4-(2-pyridylazo)-resorcinol (PAR) on a triacetylcellulose membrane. Chemical binding of Hg²⁺ ions in solution with a PAR immobilized on the triacetylcellulose surface could be monitored spectrophotometrically at 525 nm. The optode shows excellent response over a wide concentration range of 5–3360 μM Hg(II) with a limit of detection of 1.5 μM Hg(II). The influence of factors responsible for the improved sensitivity of the sensor were studied and identified. The response time of the optode was 20 min for a stable solution, and was 15 min for a stirrer solution. The influence of potential interfering ions on the determination of 5 × 10⁻⁵ M Hg(II) was studied. The sensor was applied for determination of Hg(II) in water samples.     

Design,simulation and validation of a novel uncooled infrared focal plane array

This paper describes a novel single-layer bi-material cantilever microstructure without silicon (Si) substrate for focal plane array (FPA) application in uncooled optomechanical infrared imaging system (UOIIS). The UOIIS, responding to the radiate infrared (IR) source with spectral range from 8 to 14 μm, may receive an IR image through visible optical readout method. The temperature distribution of the IR source could be obtained by measuring the thermal–mechanical rotation angle distribution of every pixel in the cantilever array, which is consisted of two materials with mismatching thermal expansion coefficients. In order to obtain a high detection to the IR object, gold (Au) film is coated alternately on silicon nitride (SiN_x) film in the flection beams of the cantilevers. And a thermal–mechanical model for such cantilever microstructure is proposed. The thermal and thermal–mechanical coupling field characteristics of the cantilever array structure are optimized through numerical analysis method and simulated by using the finite element simulation method. The thermal–mechanical rotation angle simulated and thermal–mechanical sensitivity tested in the experiment are 2.459 × 10⁻³ and 3.322 × 10⁻⁴ rad/K, respectively, generally in good agreement with what the thermal–mechanical model and numerical analysis forecast, which offers an effective reference for FPA structure parameters design in UOIIS.     

A distributed model of water balance in the Motueka catchment,New Zealand

A distributed water balance model is used to simulate the soil moisture regime of the Motueka catchment. The model is a major simplification of the Distributed Hydrology–Vegetation–Soil Model (DHVSM) with modifications suitable for the study area. The model was applied at 25-m resolution with a 1-day time-step for 10 years. The simulated hydrograph showed good correspondence with the observed hydrograph and there was good agreement of simulated and measured mean annual discharges (57.3 m³ s⁻¹ as compared with 58.7 m³ s⁻¹). Five different land cover scenarios were used to predict the effects of vegetation change on the hydrological regime: (1) current land cover; (2) prehistoric land cover; (3) maximum pine planting; (4) pine trees on easy slopes; and (5) pine trees on steep slopes. The pine scenarios all reduced the mean annual flow by about 2 m³ s⁻¹, while the prehistoric scenario reduced the mean annual flow by about 6 m³ s⁻¹. The pine scenarios (3, 4, and 5) reduced the 7-day 5-year low flow from 7.4 m³ s⁻¹ to between 6.5 m³ s⁻¹ and 6.8 m³ s⁻¹, respectively; and the prehistoric scenario reduced the 7-day 5-year low flow to 5.3 m³ s⁻¹.     

The enhancing effect of a cation-π interaction on the cooperativity of halogen bonds: A computational study

In this work, we investigate the effect of a cation-π interaction on the cooperativity of X⋯N halogen bonds in PhX⋯NCX⋯NH₃ complexes, where Ph = phenyl and X = Cl, Br, I. Molecular geometries and interaction energies of the resulting complexes are studied at the MP2/aug-cc-pVDZ(-PP) computational level. The mechanism of the cooperativity between halogen bonds is analyzed using parameters derived from the noncovalent index, quantum theory of atoms in molecules and natural bond orbital methodologies. It is found that the divalent cations (Be²⁺, Mg²⁺) have a larger influence on the cooperativity of halogen bonds than monovalent ones (Li⁺, Na⁺). The formation of a cation-π interaction leads to strengthening of the halogen bonds, hence increases their cooperativity.     

CaMoO4:RE3+,Yb3+,M+ phosphor with controlled morphology and color tunable upconversion

CaMoO₄:RE³⁺,Yb³⁺ (RE = Er, Ho, Tm) phosphors were successfully synthesized by a facile hydrothermal method. XRD patterns confirmed tetragonal structure under different RE³⁺ and M⁺ ions doping conditions. Particles shapes and sizes were confirmed by SEM and TEM analyses. Particles shape and size were well tuned by control of solution pH; spherical balls consisting of nano-grains at low pH of ∼2, rice grain shapes at moderate pH of ∼6, and thin flakes at higher pH of ∼12, were observed. Fine tunability of upconversion (UC) emission color was achieved by doping multiple RE³⁺ ions within a single CaMoO₄ host. Blue, green and orange upconverted emission were observed by doping Tm³⁺, Er³⁺ and Ho³⁺ in the CaMoO₄, respectively. Further, the emission colors were well tuned by the combination of Tm, Er and Ho ions and their concentrations. CaMoO₄:Tm³⁺,Ho³⁺,Yb³⁺ exhibited perfect white emission with well tunability from cool white to warm white colors. Substitution of part of Ca ions by M⁺ (M = Li, Na, K, Rb) ions affected the crystal field symmetry around RE³⁺ ions and hence changed the transition probabilities between their f–f transition levels, consequently intensified the UC intensities. The blue (Tm³⁺), green (Er³⁺), and orange (Ho³⁺) upconversion intensities of CaMoO₄:RE³⁺,Yb³⁺,0.10 K⁺ phosphors increased by 60, 50 and 40 folds compared to the unsubstituted analogues, respectively. The K substituted CaMoO₄:RE³⁺,Yb³⁺,K⁺ phosphors exhibited intense UC emissions visible by naked eye even pumped by less than 1 mW laser power and can have potential application in displays and variety of other applications.     

Ion–π interaction in impacting the nonlinear optical properties of ion–buckybowl complexes

Ion–buckybowl complexes have received considerable attention in modern chemical research due to its fundamental and practical importance. Herein, we performed density functional theory (DFT) to calculate the geometical structure, binding interactions, dipole moments and the first hyperpolarizabilities (β_tot) of ion–buckybowl complexes (ions are Cl⁻ and Na⁺, buckybowls are quadrannulene, corannulene and sumanene). It is found that the stabilities of ion–buckybowl compounds primarily originate from the interaction energy, which was proved by a new isomerization energy decomposition analysis approach. Plots of reduced density gradient mirror the ion–π weak interaction has been formed between the ions and buckybowls. Significantly, the buckybowl subunits cannot effectively impact the nonlinear optical (NLO), but the kind of ion has marked influence on the second-order NLO responses. The β_tot values of Cl⁻–buckybowl complexes are all larger as compared to that of Na⁺–buckybowl complexes, which is attributed to the large charge-transfer (CT) from Cl⁻ to buckybowl. Our present work will be beneficial for further theoretical and experimental studies on the NLO properties of ion–buckybowl compounds.     

Theoretical investigation of the interaction between aromatic sulfur compounds and [BMIM]+[FeCl4]− ionic liquid in desulfurization: A novel charge transfer mechanism

In this work, interaction nature between a group of aromatic sulfur compounds and [BMIM]⁺[FeCl₄]⁻ have been investigated by density functional theory (DFT). A coordination structure is found to be critical to the mechanism of extractive desulfurization. Interaction energy and extractive selectivity follow the order: thiophene (TH) < dibenzothiophene (DBT) ≈ benzothiophene (BT). Alkylation of TH or BT (e.g. 3-methylthiophene, and 3-methylbenzothiophene) leads to a stronger interaction with ionic liquid, but steric hindrance effects of some alkylic derivatives (e.g. 2,7-dimethylbenzothiophene) lead to a weaker interaction with ionic liquid. The mechanism of extractive desulfurization is attributed to the charge transfer effect. During extractive desulfurization, electrons on aromatic sulfur compounds transfer into the Lewis part of ionic liquid, namely, [FeCl₄]⁻. Furthermore, it is better to consider the Lewis acidity of Fe-containing ionic liquid by the whole unit (such as [FeCl₄]⁻ and aromatic sulfur compounds (X)) rather than only Fe or S atom.     

Influences of substituting Na+ ions in Eu2+, Mn2+ doped Ba9Y2Si6O24 phosphors

Near-ultraviolet (NUV)-excitable phosphors composed of Ba_8.9-(1/2)pNa_pEu_0.1Y₂Si₆O₂₄ (p = 0–0.8) and Ba_8.7-qNa_0.4Eu_0.1Mn_qY₂Si₆O₂₄ (q = 0–0.4) were prepared via a solid-state reaction in a reducing atmosphere. The X-ray diffraction patterns of the obtained phosphors were examined to index the peak positions. After Na⁺ substitution for Ba²⁺ in the Ba_8.9Eu_0.1Y₂Si₆O₂₄ host lattice, the clear shift from green to yellow emission was observed. The Gaussian components of Ba_8.9-(1/2)pNa_pEu_0.1Y₂Si₆O₂₄ (p = 0, 0.4, and 0.8) phosphors were exploited by using the three different Eu²⁺ ion sites in the host lattice. The dependence of the luminescent intensity of the Mn²⁺ co-doped (q = 0–0.4) host lattices on the fixed Na⁺ and Eu²⁺ contents was also investigated. Co-doping Na⁺ with Mn²⁺ and Eu²⁺ emitters in the host structure enabled efficient energy transfer from Eu²⁺ to Mn²⁺. The mechanism underlying this energy transfer was also discussed. The Commission Internationale de I’Eclairage (CIE) coordinates near the yellow and red regions of the obtained phosphors were observed. Each of Ba_8.9Eu_0.1Y₂Si₆O₂₄, Ba_8.7Na_0.4Eu_0.1Y₂Si₆O₂₄, and Ba_8.6Na_0.4Eu_0.1Mn_0.1Y₂Si₆O₂₄ phosphors with a 405 nm LED chip was fabricated. The color rendering index (CRI, R_a) at correlated color temperature (CCT) with the CIE coordinates was exhibited for the LEDs. The thermal quenching and activation energy for the Ba_8.7Na_0.4Eu_0.1Y₂Si₆O₂₄ and Ba_8.6Na_0.4Eu_0.1Mn_0.1Y₂Si₆O₂₄ phosphors were measured.     

The possibility of using C20 fullerene and graphene as semiconductor segments for detection,and destruction of cyanogen-chloride chemical agent

Detection of hazardous chemical species by changing the electrical conductivity of a semiconductor matter is a proposed and applied way for decreasing their subsequent unpleasant effects. Recently, many examples of using inorganic or organic materials, polymeric, and also nano-sized species as sensors were reported in which, in some cases, those matters were strongly affective and suitable.In this project, we have made an assessment on whether the graphene segment or C₂₀ fullerene, able to sense the existence of cyanogen chloride NCCl? In order to gain trustable results, the possible reaction pathways along with the adsorption kinetics were investigated. Moreover, the electronic density of states DOS showed that C₂₀ fullerene senses the existence of cyanogen chloride agent with a clearer signal (ΔE_g = 0.0110 eV) compared to the graphene segment (ΔE_g = 0.0001 eV). Also the adsorption energy calculations showed that cyanogen chloride could be adsorbed by the fullerene in a multi-step process (E_ads1 = −0.852 kcal mol⁻¹; E_ads2 = −0.446 kcal mol⁻¹; E_ads3 = −2.330 kcal mol⁻¹).     

Electrochemical determination of ascorbic acid in fruits on a vanadium oxide polypropylene carbonate modified electrode

A novel vanadium oxide polypropylene carbonate modified glassy carbon electrode was developed and used for the measurement of ascorbic acid (AA). The electrode was prepared by casting a mixture of vanadium tri(isopropoxide) oxide (VO(OC₃H₇)₃) and poly(propylene carbonate) (PPC) onto the surface of a glassy carbon electrode. The electrochemical behavior of the VO(OC₃H₇)₃–PPC film modified glassy carbon electrode was investigated by cyclic voltammetry and amperometry. This modified electrode exhibited electrocatalytic response to the oxidation of ascorbic acid. Compared with a bare glassy carbon electrode, the modified electrode exhibits a 220 mV shift of the oxidation potential of ascorbic acid in the cathodic direction and a marked enhancement of the current response. The response current revealed a good linear relationship with the concentration of ascorbic acid in the range of 4 × 10⁻⁸ and 1 × 10⁻⁴ mol L⁻¹ and the detection limit of 1.5 × 10⁻⁸ mol L⁻¹ (S/N = 3) in the pH 8.06 Britton–Robinson solution. Quantitative recovery of the ascorbic acid in synthetic samples has been obtained and the interferences from different species have been studied. The method has been successfully applied to the determination of ascorbic acid in fruits. The concentrations of ascorbic acid measured by this method are in good agreement with the literature value. It is much promising for the modified films to be used as an electrochemical sensor for the detection of ascorbic acid.     

Perchlorate-selective polymeric membrane electrode based on a cobaloxime as a suitable carrier

A cobaloxime ([chlorobis(dimethylglyoximeato)(triphenylphosphine)] cobalt (III), [Co(dmgH)₂pph₃Cl]) incorporated in a plasticized poly(vinyl chloride) membrane was used to develop a perchlorate-selective electrode. The influence of membrane composition on the electrode response was studied. The electrode exhibits a Nernstian response over the perchlorate concentration range 1.0 × 10⁻⁶ to 1 × 10⁻¹ mol l⁻¹ with a slope of −56.8 ± 0.7 mV per decade of concentration, a detection limit of 8.3 × 10⁻⁷, a wide working pH range (3–10) and a fast response time (<15 s). The electrode shows excellent selectivity towards perchlorate with respect to many common anions. The electrode was used to determine perchlorate in water and human urine.     

Generalized Strong Pseudoprime Tests and Applications

We describe probabilistic primality tests applicable to integers whose prime factors are all congruent to 1 mod r where r is a positive integer;r =  2 is the Miller–Rabin test. We show that if ν rounds of our test do not find n ≠  =  (r +  1)²composite, then n is prime with probability of error less than (2 r) ^− ν. Applications are given, first to provide a probabilistic primality test applicable to all integers, and second, to give a test for values of cyclotomic polynomials.