Biodegradable poly(l-lactic acid) nanofiber prepared by a carbon dioxide laser supersonic drawing

Biodegradable poly(l-lactic acid) (PLLA) nanofiber was prepared by a carbon dioxide (CO₂) laser supersonic drawing which was carried out by irradiating the laser on an as-spun fiber in a supersonic jet. The supersonic jet was generated by blowing off air into a vacuum chamber from a fiber supplying orifice. The flow velocity from the orifice can be estimated by applying Graham’s theorem from the pressure difference between the atmospheric pressure and the pressure of the vacuum chamber. The fastest flow velocity estimated was 396 m s⁻¹ when the chamber pressure was 6 kPa. The PLLA nanofiber having an average diameter of 0.132 μm was obtained when the supersonic drawing was carried out by irradiating the laser at 177 W cm⁻² on the as-spun fiber supplied at 0.1 m min⁻¹ in the vacuum chamber at 6 kPa. The obtained nanofiber had a draw ratio of about 323,000 and a degree of crystallinity of 45%, and its diameter uniformity was high. The CO₂ laser supersonic drawing was a new route for preparation of various nanofibers without using any solvent.     

Mean Gas Cluster Size Determination from Cluster Beam Cross-Section

This paper describes the simple experimental method of size determination of gas clusters in molecular beams formed from supersonic jets. Mean cluster size N is calculated from broadening of the transverse profile of beam intensity at a fixed distance behind the skimmer. The described method allows determining the mean sizes of the clusters of any pure gases. It does not require the building of some special models, or determination of empirical constants. Due to the high intensity of the supersonic beams, the measurements do not require any complex highly sensitive equipment. The effectiveness of the present method is validated by measurements in a cluster beams of test gases (easily condensable CO₂, Ar, and weakly condensable N₂) and the beam of C₂H₄ (ethylene), formed from a supersonic jet behind conical nozzles. The certainty of measured characteristics is confirmed by the results of numerical simulations. By using the described method the mean cluster sizes from 50 to 2000 molecules per cluster were determined. The correctness of the obtained cluster sizes of CO₂ and Ar is proved by comparison with results of other authors, obtained by other experimental methods, and estimations according to the empirical correlations using condensation scaling parameter Г^*.     

Mass spectrometric in-situ determination of NO2 in gas mixtures by resonance enhanced multiphoton ionisation

One- and two-colour photoionisation spectra for NO₂ have been investigated using a time of flight mass spectrometer as detector to find the most efficient REMPI process for analytical applications. Two different inlet systems have been employed: a pulsed supersonic jet expansion stage and a flow reactor. Selective and sensitive mass spectrometric determinations of free NO₂ have been possible even in the presence of high concentrations of organic nitrates, HNO₃ and other NO₂ precursors. Employing two-colour (1+1′+1) excitation using a concentration of HNO₃≤5·10¹⁴ molecules/cm³ a detection limit of 5·10¹¹ molecules/cm³ has been found for NO₂ whereas in the absence of HNO₃ a detection limit of 5·10¹⁰ molecules/cm³ is reported.     

High-resolution continuous-wave-diode laser cavity ring-down spectroscopy of the hydrogen fluoride dimer in a pulsed slit jet expansion: two components of the N=2 triad near 1.3 microm

The near-infrared overtone spectra of the prototypical hydrogen-bond hydrogen fluoride dimer (HF)2 contain rich information on hydrogen bond dynamics. We report a study of the N=2 triad involving excitations with two quanta of HF stretching in (HF)2 around 1.3 microm (7500-7800 cm(-1)) by means of continuous-wave-diode laser cavity ring-down spectroscopy in a pulsed supersonic slit jet expansion. The analysis of the rotationally resolved overtone spectra allows the study of vibrational mode-selective kinetics, such as hydrogen bond predissociation with lifetimes tauPD and tunneling rearrangement (switching) processes with periods tausw obtained from the tunneling splitting DeltaT in highly excited vibrational states. The Ka=1<--0 transition of the Nj=22 band of (HF)2 has been reinvestigated by us in a supersonic jet expansion; the much improved data obtained here are in excellent agreement with several previous experimental results. Our analysis provides subband-level positions and properties 0(Ka=1(A+))=7711.37956(66) cm(-1), DeltaT=0.0936(10) cm(-1), and tauPD=1.3-1.9x10(-9) s, depending on the level symmetry A+ and B+. We have also analyzed spectra of the Nj=21 band, which we have observed for the first time in a supersonic jet with rotational resolution. For the Ka=0<--0 transition of this band, we find the band center at 0(A+)=7550.3555(26) cm(-1) and a tunneling splitting of DeltaT=0.0150(37) cm(-1). This level involves mostly excitation of the H-bonded HF stretching with two quanta. The mode-selective tunneling switching is in agreement with a simple picture of inhibited tunneling. These experimental values are close to those calculated on the "SO(-3)" potential energy hypersurface of Klopper, Quack, and Suhm. The N=2 triad also exhibits a strongly mode-selective predissociation dynamics, with a predissociation lifetime tauPD=4.99(84)x10(-11) s in the Nj=21 level, which is more than 20 times shorter than that for the Nj=22 level.     

A molecular beam study of the evaporation of water from a liquid jet

A method to maintain a clean surface of a liquid in a high vacuum is described. Using a very thin and fast liquid jet it is not only possible to prevent freezing of the liquid but also to reduce the number of collisions between evaporating molecules to negligibly small values. Thus many of the standard, vacuum dependent, particle probing techniques for solid surfaces can be used for studies of rapidly vaporizing, high vapor pressure liquids. In a first molecular beam investigation we have used time-of-flight analysis to measure the velocity distribution of H₂O molecules vaporizing from thin jets of pure liquid water. The experiments were carried out for liquid jet diameters between 50 and 5 µm. In this range the expanding vapor is observed to undergo the transition to the collision-free molecular flow regime. From the measured velocity distributions the local surface temperature is determined to be less than 210 K. This appears to be the lowest temperature ever reported for supercooled liquid water.     

Nonequilibrium effects in a steady supersonic jet of a mixture of monatomic gases

On the basis of the moment equation system for parameters of a steady supersonic jet of a mixture of monatomic gases, the analysis of nonequilibrium effects as a velocity slip and temperature difference of components was carried out in hypersonic and spherically symmetric approximations. The limiting values of velocity slips and kinetic temperatures of mixture components depending on the source jet conditions and interaction potential were obtained.     

Possibilities for Energy Pumping in a Radio-Frequency Quadrupole by Shifted Supersonic Gas Jet. Part I: Accelerated and Excited Atom Transmission

Generation of an ion beam and its transmission into a mass analyzer is one of central problems in mass spectrometry. The use of a narrowly directed supersonic gas jet has a number of advantages in comparison with other sampling methods. The aim of this work was to confirm the declared earlier properties of the jet formed at the outlet of a cylindrical channel when the free path length of gaseous atoms at the beginning of the channel is comparable with the channel diameter. The paper describes the ability of such a supersonic jet to conserve an additional energy of jet gas atoms. A significant influence of the temperature of the gas flow on the yield of cyclohexane fragment ions was found, cyclohexane being an admixture in the noble gas jet passing through an electron ionization ion source. A possibility of obtaining a flow of metastable electronically excited atoms inside the jet is also shown. The results of the work confirm the availability of the supersonic gas jet for the design of a high efficiency ion source inside the radio-frequency quadrupole at the input of the mass analyzer.     

Electrochemical time of flight method for determination of diffusion coefficients of glucose in solutions and gels

The diffusion coefficient of glucose in different media is an important parameter in life sciences, as well as in biotechnology and microbiology. In this work a simple, fast method is proposed that is based on the electrochemical time of flight principle. In most of the earlier time of flight experiments performed, a constant flight distance was applied. In the present work a scanning electrochemical microscope (SECM) was applied as a measuring tool. With use of the SECM, the flying distance could be changed with high precision, making measurements with several flight distances more accurate and reliable values could be obtained for solutions as well as for gels. The conventional voltammetric methods are not applicable for glucose detection. In our work electrocatalytic copper oxide coated copper microelectrodes and micro-sized amperometric enzyme sensors were used as detectors, while microdroplet-ejecting pneumatically driven micropipettes were used as a source. Figure Experimental set up for SCEM-TOF diffusion coefficient measurements Presented at the 9th International Symposium on Instrumental Analysis, Pécs, Hungary, 29 July-2 August 2008     

Development of medium pressure laser ionization,MPLI. Description of the MPLI ion source

A novel pulsed valve/ion source combination capable of time-resolved sampling from atmospheric pressure has been developed for use with laser ionization time of flight mass spectrometry. The source allows ionization extremely close to the nozzle of the pulsed valve, enabling ultra-sensitive detection of a number of compounds, e.g., NO, at mixing ratios <1 pptV. Furthermore, at analyte mixing ratios in the ppbV range, the temporal resolution of the system is in the sub-second regime, allowing time-resolved monitoring of highly dynamic and complex mixtures, e.g., human breath or reacting chemical mixtures in atmospheric smog chamber experiments. Rotational temperatures of approximately 50 K have been observed for analytes seeded in the supersonic jet expansion at a distance of 1 mm downstream of the nozzle orifice. The refinement of the original ion source has drastically reduced the impact of reflected laser light and the resultant electron impact signals previously observed. The general applicability of this technique is demonstrated here by coupling the source to commercially available as well as home-built time-of-flight mass spectrometers. Finally, we discuss the MPLI technique in view of the very recently introduced atmospheric pressure laser ionization (APLI) as well as the traditional jet-REMPI approach.     

Parameters controlling the generation and properties of plasma sprayed zirconia coatings

D.C. plasma jets temperature and velocity distributions as well as the arc root fluctuations at the anode were studied for Ar-H₂ (25 vol%) plasma forming gases. The parameters were the arc current up to 700 A, the total gas flow rate up to 100 slm, and the nozzle diameter which was varied from 6 to 10 mm. The trajectories of partially stabilized zirconia particles into the jet were studied by a 2D laser imaging technique and two fast (100 ns) two color pyrometers. The results have revealed the difficulty to inject small particles into the plasma flow since most were found to by-pass the jet rather than penetrate it. The results also show the broad trajectory distribution within the jet and the influence of the arc root fluctuations on the mean particle trajectory distribution within the jet. Beside the measurements of the particle surface temperature and velocity distributions in flight, the particle flattening and the cooling of the resulting splats were studied statistically for single particles all over the spray cone. Such studies have emphasized the drastic influence of the substrates or previously deposited layers temperature on the contact between them and the splats. At 200–300°C this contact is excellent (cooling rates of the order of 100 K/μs for 1 μm thick splats) and it results in a columnar growth within the splats and the layered splats of a bead (up to 500 layered splats). This growth can be observed through passes provided the bead surface temperature has not cooled too much (a few tens of K) before the next bead covers it. A/C values up to 60 MPa were achieved with PSZ coatings. The effect of impact velocity of the particles, of substrate preheating temperature, of relative movments torch to substrate, of substrate oxidation on A/C values and splat formation were also studied.     

Femtosecond rotational Raman coherence spectroscopy of cyclohexane in a pulsed supersonic jet

We combine the technique of femtosecond degenerate four-wave mixing (fs-DFWM) with a high repetition-rate pulsed supersonic jet source to obtain the rotational coherence spectrum (RCS) of cold cyclohexane (C(6)H(12)) with high signal/noise ratio. In the jet expansion, the near-parallel flow pattern combined with rapid translational cooling effectively eliminate dephasing collisions, giving near-constant RCS signal intensities over time delays up to 5 ns. The vibrational cooling in the jet eliminates the thermally populated vibrations that complicate the RCS coherences of cyclohexane at room temperature [Brügger, G.; et al. J. Phys. Chem. A 2011, 115, 9567]. The rotational cooling reduces the high-J rotational-state population, yielding the most accurate ground-state rotational constant to date, B(0) = 4305.859(9) MHz. Based on this B(0), a reanalysis of previous room-temperature gas-cell RCS measurements of cyclohexane gives improved vibration-rotation interaction constants for the ν(32), ν(6), ν(16), and ν(24) vibrational states. Combining the experimental B(0)(C(6)H(12)) with CCSD(T) calculations yields a very accurate semiexperimental equilibrium structure of the chair isomer of cyclohexane.     

Continued Study and Critical Evaluation of an Improved Impaction-Graphite Furnace System for the Determination of Metals in Aerosols1

Abstract

An evaluation of the parameters which effect the collection efficiency of metals in an aerosol via an improved impaction-graphite furnace atomic absorption spectrometric system showed that an optimum could be obtained at a jet diameter of 1.50 mm, and a jet exit -to-impaction surface distance of 3 mm. Low flow rates gave poor precision at low concentrations except when a large jet diameter was used. Preliminary studies by electron microscopy of particle size collected showed that the higher the flow rate and smaller the jet diameter, leads to a more dense concentration of particle sizes.

     

Reactions of hydrocarbons in a supersonic vacuum plasma jet

The plasma plume of a hydrogen plasma jet used for diamond synthesis is analyzed by a Pitot tube and by mass spectrometry. In the investigated pressure range of 2–10 mbar, supersonic gas velocities with Mach numbers of up to 2 were observed, which decreased with increasing pressure and increasing distance from the nozzle. The injection of the carbon-containing species either at the exit of the jet nozzle or simply into the background gas of the reaction chamber confirmed the importance of recirculation of background gas into the plasma plume. In the case of background injection the rise of the total carbon content in the plume with increasing distance from the nozzle is much slower than in the case of nozzle injection. The results of a numerical model of the hydrocarbon gas-phase reactions in the jet are presented. The model considers the entrainment of background gas into the plasma plume. Two domains along the jet axis can be distinguished. The first one in the vicinity of the nozzle is dominated by methyl radicals, the second one by atomic carbon. Increase of the hydrogen dissociation level results in the broadening of the atomic carbon domain and the rise of C₂ far from the nozzle. Background injection of CH₄ leads to lower total carbon content in the plume but has little effect on the species distribution along the jet axis.     

Condensation of Argon,Monosilane and Their Mixtures in a Pulse Free Jet

Pulse supersonic outflow of Ar, SiH₄ and Ar + SiH₄ gas mixture (where monosilane is a small admixture) was studied experimentally by the method of molecular beam mass-spectrometry. Using argon as an example we have shown that condensation processes at the quasi-stationary region of a pulse flow and within a stationary jet are similar. In the flows of pure gases clusters of argon and silane (hydrogenated silicon) and in the mixture argon – silane complexes were registered. The dependencies of the intensities of monomer and cluster ions on stagnation pressure were investigated. It was shown that in the mixture jet at low stagnation pressures the condensation process with the formation of monosilane clusters takes place and at high pressures mixed argon-silane complexes are formed. The parameters of flow transition into the regime of developed condensation were determined for pure gases and their mixture.     

Supersonic jet spectroscopy of naphthalene–fluorene bichromophoric cluster

We have investigated the supersonic jet spectroscopy and photophysics of 1-methylnaphthalene–fluorene (1MN–FL) cluster, and looked for indications of intramolecular electronic energy transfer (Intra-EET) from the FL (donor, D) to 1MN (acceptor, A). The clusters were identified by their time of flight (TOF) mass spectra. We observed clusters bands in the LIF spectrum near the region of 1MN origin. We have also observed cluster TOF–resonance enhanced multi-photon ionization (REMPI) spectrum near the electronic origin of the fluorene moiety. However, this spectrum was almost independent of the excitation wavelength, and was not observed in the LIF spectrum. This is probably due to fast Intra-EET resulting in lifetime broadening of the donor chromophore cluster spectrum, similar to that observed previously for the naphthalene–anthracene bichromophoric system.     

丙酮团簇的多光子电离解离与结构计算

用355nm激光多光子电离解离飞行时间质谱观测到在超声分子束中形成的最多为12个分子的团簇离子及其碎片.用密度泛函方法对n=2～5的丙酮团簇结构进行计算,给出了优化构型及其基态能量.结果表明,两个丙酮分子组成团簇时稳定结构为近似垂直构型.3～5个丙酮分子组成团簇时以环状结构最稳定.     

Comparison of Laminar and Turbulent Thermal Plasma Jet Characteristics—A Modeling Study

Modeling results are presented to compare the characteristics of laminar and turbulent argon thermal plasma jets issuing into ambient air. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of ambient air into the laminar and turbulent argon plasma jects, respectively. It is shown that since only the molecular diffusion mechanism is involved in the laminar plasma jet, the mass flow rate of ambient air entrained into the laminar plasma jet is comparatively small and less dependent on the jet inlet velocity. On the other hand, since turbulent transport mechanism is dominant in the turbulent plasma jet, the entrainment rate of ambient air into the turbulent plasma jet is about one order of magnitude larger and almost directly proportional to the jet inlet velocity. As a result, the characteristics of laminar plasma jets are quite different from those of turbulent plasma jets. The length of the high-temperature region of the laminar plasma jet is much longer and increases notably with increasing jet inlet velocity or inlet temperature, while the length of the high-temperature region of the turbulent plasma jet is short and less influenced by the jet inlet velocity or inlet temperature. The predicted results are reasonably consistent with available experimental observation by using a DC arc plasma torch at arc currents 80–250 A and argon flow rates (1.8–7.0)×10⁻⁴ kg/s.     

Two-dimensional model for thermal plasma chemical vapor deposition

A formulation of a global mathematical two-dimensional model for Thermal Plasma Chemical Vapor Deposition (TPCVD) is reported. Both gas-phase and surface chemical kinetics as well as ordinary and thermal diffusion are incorporated. Flow is assumed to be steady, laminar and swirlless at this stage. The results include velocity, pressure, density, temperature and chemical species distributions in the reactor, and the heat flux and the film growth characteristics at the substrate.The model has been applied to a low pressure diamond TPCVD. Two basic cases have been investigated: (1) supersonic jet regime, and (2) high speed subsonic jet regime. The results for both cases are presented and compared. In both cases, the hydrocarbon species needed for the diamond formation are assumed to be premixed in the plasma jet.The main conclusions are: (1) The low pressure high speed jets are very narrow and slow down only at the substrate through a bow shock, (2) the faster the jet, the bigger the total deposited amount of diamond but also the higher the heat flux and diamond growth rate nonuniformities.     

Shape analysis of the velocity distribution in supersonic Ar beams: Comparison between experiment and theory

The shape parameters of the velocity distribution for beam densityF(v) = C($\text{v}\text{α}$)² exp[?($\text{v}\text{?}\text{u}\text{α}$)²]

from a supersonic expansion of Ar are determined both experimentally via time-of-light(TOF) analysis and computationally by solving the Boltzmann equation for a radial flow field via the method of moments. TOF spectra are recorded by means of the detection of metastable Ar atoms. This technique eliminates velocity discrimination in the detection process. Significance test and a sensitivity analysis for the experimental and theoretical results are included. The agreement of measured and calculated shape parameters is very good. Small-angle scattering of particles, travelling with small relative velocity along the same stream line, is still effective beyond the transition region to free molecular flow and continues to modify the distribution function. A primitive model that correlates the shape parameters c₃ and c₄ with the terminal speed ratio and describes their variation with the distance from the nozzlee is developed.