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21.
The influence of internal excitation on the reactions of O
2
+
+ CH4 and of CO
2
+
+ NO has been investigated using a slow flow drift tube. The rate coefficients for these reactions obtained as a function of relative kinetic energy in various buffer gases like He, Ne, Ar, and Kr showed higher values under conditions where the internal excitation of the reactant ions was enhanced. For both reactions the lowest reactivity at all kinetic energies was observed to occur in He, indicating that He is the least effective buffer for collisionally inducing internal excitation of molecular ions. 相似文献
22.
The tube model is applied for the treatment of stress-strain measurements on SBR networks. It is shown that this theory allows the separation of crosslink and constraint contributions to the stress-strain behavior and, also, a reliable determination of crosslink densities. The consideration of dynamical contributions is discussed and a special numerical method is developed. 相似文献
23.
《Electroanalysis》2006,18(18):1749-1756
Densely packed micro‐ and submicrometer electrode arrays of platinum and gold (the nominal number, N, of electrodes in each array varies between 225 and 3600) are fabricated by photolithographic technique and vapor deposition processes of metal films. The electrodes are conical‐shaped and only their apexes are exposed to the electrolytic solution. The electrode arrays are characterized electrochemically in Ru(NH3)6Cl3 aqueous solutions by using cyclic voltammetry at low scan rates, to establish the number of electrochemically active electrodes (Nac) in each array; the geometric characterization is performed by scanning electron microscopy. All the investigated arrays provide steady‐state voltammograms, indicating diffusionally independent behavior of each microelectrode. The number of microelectrodes that are active in the fabricated arrays depends on microelectrode density. In particular, for the arrays with N=3600 and N=225, the fraction of active sites is about 45% and 90%, respectively. The analytical performance of some of the Pt version of the arrays is tested in hydrogen peroxide solutions, allowing verifying that linear calibration plots over the concentration range (0.1–20 mM) are obtained. This dynamic range is larger than that typically recorded at smooth polycrystalline platinum electrodes (0.5–5 mM), and the better performance is attributed to both the higher aspect ratio of the cone geometry and the higher mass transport associated to each microelectrode of the array. Reproducibility (within 3.5%, r.s.d.) and long‐term stability (within 5%, r.s.d., after 8 h continuous use) of the electrode systems are satisfactory. A low detection limit, based on the signal to noise ratio equal to 3, of 0.05 mM is found, which is adequate for a rapid monitoring of H2O2 in real samples and industrial processes. 相似文献
24.
Isotachophoresis carried out in a 0.25 mm i.d. fused-silica capillary tube yielded high resolution, compared with that in a fluorinated ethylene-propylene polymer tube. The use of an ultraviolet-visible multichannel spectrophotometer with photodiode array as detector together with a cross flow cell (volume 0.01 μl) was investigated. The system was successfully applied to the analysis of cationic dyes such as neutral red, bismarck brown, and basic fuchsine. 相似文献
25.
Jijun Qiu Weidong Yu Xiangdong Gao Xiaomin Li 《Journal of Sol-Gel Science and Technology》2007,44(3):235-239
Ordered porous TiO2 films, including TiO2 nanotube arrays, are fabricated by a sol-gel dip-coating approach via ZnO nanorod templates obtained from aqueous solution
approach. The results indicate that the morphologies of ordered porous TiO2 films have been great affected by the sol-gel dip-coating cycle number. Open-ended TiO2 nanotube arrays can be obtained in optimum dip-coating cycle numbers. The TiO2 nanotubes with the inner diameter matching well with the diameters of ZnO nanorods, are well assembled and separate each
other. When the cycle number is less than this optimum value, no intact porous TiO2 film can be obtained. As the cycle number is larger than this optimum value, an ordered porous TiO2 film with many throughout holes is formed. The evolutive mechanism of ordered porous TiO2 films is proposed. 相似文献
26.
A method based on in-tube solid-phase microextraction and capillary zone electrophoresis (CZE) was proposed for simultaneously determining four amphetamines (amphetamine, methamphetamine, 3,4-methylenedioxyamphetamine, and 3,4-methylenedioxymethamphetamine) in urine. A poly(methacrylic acid-ethylene glycol dimethacrylate) monolithic capillary column, which can provide sufficient extraction efficiency, was introduced for the extraction of amphetamines from urine samples. The hydrophobic main chains and acidic pendant groups of the monolithic column make it a superior material for extraction of basic analytes from aqueous matrix. After extraction, the samples were analyzed by CZE. The best separation was achieved using a buffer composed of 0.1 M disodium hydrogen phosphate (adjusted to pH 4.5 with 1 M hydrochloric acid) and 20% methanol v/v, with a temperature and voltage of 25 degrees C and 20 kV, respectively. By applying electrokinetic injection with field-amplified sample stacking, detection limits of 25-34 microg/L were achieved. Excellent method of reproducibility was found over a linear range of 0.1-5 mg/L. Determination of these analytes from abusers' urine sample was also demonstrated. 相似文献
27.
28.
Englert BC Scholz S Leech PJ Srinivasarao M Bunz UH 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(3):995-1000
We describe the synthesis of two cyclobutadiene(cyclopentadienyl)cobalt-containing poly(p-phenylene ethynylene)s (PPEs) and their use as precursors for stable ceramic surface coatings. Organometallic PPEs were shaped into hexagonally ordered assemblies by using the breath-figure method. Such breath figures can be washed away with an appropriate solvent. Upon pyrolysis at 500 degrees C under either nitrogen or air, the bubble arrays persist as ceramics and are insoluble in organic solvents or water. The formed pyrolyzed bubble arrays were analyzed by optical and scanning electron microscopy, as well as energy dispersive X-ray microanalysis (EDX). The composition of the ceramic materials is discussed based on EDX and IR data. 相似文献
29.
《Electroanalysis》2005,17(1):79-84
Fabrication, electrochemical characterization, and applications of low‐site density carbon nanotubes based nanoelectrode arrays (CNTs‐NEAs) are reported in this work. Spin‐coating of an epoxy resin provides a new way to create the electrode passivation layer effectively reducing electrode capacitance and current leakage. Cyclic voltammetry showed the sigmoidal shape curves with low capacitive current and scan‐rate‐independent limiting current. Square‐wave voltammetry showed well‐defined peak shapes in voltammograms of K3Fe(CN)6 and 4‐acetamidophenol (acetaminophen) and the peak currents to be proportioned to their concentrations, demonstrating the feasibility for voltammetric analysis of the CNTs‐NEAs. The CNTs‐NEAs were also used successfully for voltammetric detection of trace concentrations of lead(II) at ppb level at first‐time. The CNTs‐NEAs provide an excellent platform for ultra sensitive electrochemical sensors for chemical and biological sensing. 相似文献
30.
A simple theoretical model is described for deriving a 1-dimensional equation for the spreading of a tracer in a steady flow at the field scale. The originality of the model is to use a stochastic appoach not in the 3-dimensional space but in the 1-D space of the stream tubes. The simplicity of calculation comes from the local relationship between permeability and velocity in a 1-D flow. The spreading of a tracer front is due to local variations in the cross-sectional area of the stream tubes, which induces randomness in travel time. The derived transport equation is averaged in the main flow direction. It differs from the standard dispersion equation. The roles of time and space variables are exchanged. This result can be explained by using the statistical theory of Continuous Time Random Walk instead of a standard Random Walk. However, the two equations are very close, since their solutions have the same first and second moments. Dispersivity is found to be equal to the product of the correlation length by the variance of the logarithm of permeability, a result similar to Gelhar's macrodispersion.Nomenclature
A
total cross-section area of the sample
-
C
(resident) concentration of tracer
-
D,D
*
dispersion coefficient
-
F
flux of tracer
-
G
probability distribution function for permeability in the stream-tube segments
-
I
tracer intensity (mass crossing a surface per unit time)
-
K
permeability
-
L
length of the medium
-
M
number of stream tubes in the medium
-
N
number of segments along a stream tube
-
P
pressure
-
Q
total flow rate in the sample
-
a
length of an elementary stream-tube segment
-
g
probability distribution function for permeability in the space
-
i, j
indices, tube numbers
-
q
flow rate in each stream tube
-
s
variable cross-section area of a stream tube
-
t, t
time
-
u
front velocity
-
x
space variable in the flow direction
-
small local variation in time
-
,
t
longitudinal, transverse dispersivity
-
porosity of the porous medium
-
correlation length in the permeability field
-
viscosity of the fluid
-
time for filling an elementary stream tube segment
-
standard deviation of a stochastic variable
-
probability distribution of arrival times (Gaussian) 相似文献