Single-walled carbon nanotube formation with double laser vaporization technique

Single-walled carbon nanotubes (SWNTs) were prepared with double laser vaporization of a graphite target and a metal/alloy target inside an electric furnace at 1200 ^°C ambient temperature with 500 torr Ar gas atmosphere. Each target was vaporized simultaneously with a different Nd:YAG laser. Several kinds of metal/alloy target (Ni, Co, Fe, and permalloy) were tested in order to see the difference in the resulting SWNT yield and the diameter distribution of them. The Raman spectra of SWNT-containing soot prepared by use of this technique with permalloy/carbon system indicated that permalloy gives almost the same yield as compared with Ni/Co carbon composite rod with single laser vaporization technique, though the diameter distribution of them is slightly different. Also, time-resolved images of the plume by carbon and permalloy nanoparticles after laser vaporization were collected using a high-speed video camera. These images suggest that the hot plumes due to carbon and permalloy nanoparticles do not mix together so extensively, at least in a few hundred microseconds after laser vaporization. The effect of time delay between two laser pulses on the yield and the diameter distribution of SWNTs was also presented and discussed.     

Thermodynamic and kinetic considerations of nucleation and stabilization of acoustic cavitation bubbles in water

Qualitative explanation for a homogeneous nucleation of acoustic cavitation bubbles in the incompressible liquid water with simple phenomenological approach has been provided via the concept of the desorbtion of the dissolved gas and the vaporization of local liquid molecules. The liquid medium has been viewed as an ensemble of lattice structures. Validity of the lattice structure approach against the Brownian motion of molecules in the liquid state has been discussed. Criterion based on probability for nucleus formation has been defined for the vaporization of local liquid molecules. Energy need for the enthalpy of vaporization has been considered as an energy criterion for the formation of a vaporous nucleus. Sound energy, thermal energy of the liquid bulk (Joule-Thomson effect) and free energy of activation, which is associated with water molecules in the liquid state (Brownian motion) as per the modified Eyring's kinetic theory of liquid are considered as possible sources for the enthalpy of vaporization of water molecules forming a single unit lattice. The classical nucleation theory has then been considered for expressing further growth of the vaporous nucleus against the surface energy barrier. Effect of liquid property (temperature), and effect of an acoustic parameter (frequency) on an acoustic cavitation threshold pressure have been discussed. Kinetics of nucleation has been considered.     

Pulse sequences for uniform perfluorocarbon droplet vaporization and ultrasound imaging

Phase-change contrast agents (PCCAs) consist of liquid perfluorocarbon droplets that can be vaporized into gas-filled microbubbles by pulsed ultrasound waves at diagnostic pressures and frequencies. These activatable contrast agents provide benefits of longer circulating times and smaller sizes relative to conventional microbubble contrast agents. However, optimizing ultrasound-induced activation of these agents requires coordinated pulse sequences not found on current clinical systems, in order to both initiate droplet vaporization and image the resulting microbubble population. Specifically, the activation process must provide a spatially uniform distribution of microbubbles and needs to occur quickly enough to image the vaporized agents before they migrate out of the imaging field of view. The development and evaluation of protocols for PCCA-enhanced ultrasound imaging using a commercial array transducer are described. The developed pulse sequences consist of three states: (1) initial imaging at sub-activation pressures, (2) activating droplets within a selected region of interest, and (3) imaging the resulting microbubbles. Bubble clouds produced by the vaporization of decafluorobutane and octafluoropropane droplets were characterized as a function of focused pulse parameters and acoustic field location. Pulse sequences were designed to manipulate the geometries of discrete microbubble clouds using electronic steering, and cloud spacing was tailored to build a uniform vaporization field. The complete pulse sequence was demonstrated in the water bath and then in vivo in a rodent kidney. The resulting contrast provided a significant increase (>15 dB) in signal intensity.     

Modelling biodiesel–diesel spray combustion using multicomponent vaporization coupled with detailed fuel chemistry and soot models

A multicomponent vaporization model is integrated with detailed fuel chemistry and soot models for simulating biodiesel–diesel spray combustion. Biodiesel, a fuel mixture comprised of fatty-acid methyl esters, is an attractive alternative to diesel fuel for use in compression-ignition engines. Accurately modelling of the spray, vaporization, and combustion of the fuel mixture is critical to predicting engine performance using biodiesel. In this study, a discrete-component vaporization model was developed to simulate the vaporization of biodiesel drops. The model can predict differences in the vaporization rates of different fuel components. The model was validated by use of experimental data of the measured biodiesel drop size history and spray penetration data obtained from a constant-volume chamber. Gas phase chemical reactions were simulated using a detailed reaction mechanism that also includes PAH reactions leading to the production of soot precursors. A phenomenological multi-step soot model was utilized to predict soot emissions from biodiesel–diesel combustion. The soot model considered various steps of soot formation and destruction, such as soot inception, surface growth, coagulation, and PAH condensation, as well as oxidation by oxygen and hydroxyl-containing molecules. The overall numerical model was validated with experimental data on flame structure and soot distributions obtained from a constant-volume chamber. The model was also applied to predict combustion, soot and NOx emissions from a diesel engine using different biodiesel–diesel blends. The engine simulation results were further analysed to determine the soot emissions characteristics by use of biodiesel–diesel fuels.     

Double-spray counterflow diffusion flame model

In this spray model we consider two gaseous streams approaching each other from opposite directions in a counterflow. The two opposed streams each carry a distribution of liquid droplets. The sprays vaporize, and the vaporized fuel and oxidizer gases diffuse and convect toward a chemical reaction region near the stagnation plane, at which the reactants burn. A set of steady-state ordinary differential equations is derived to describe the temperature of the gas flow and the mass fractions of each reactant. We solve the differential equations in three consequent cases, each more complicated than the previous one: (i) fast vaporization and fast chemistry; (ii) finite-rate vaporization and fast chemistry; and (iii) finite-rate vaporization and finite-rate chemistry. Comparisons are made of our model results to previous fuel-spray-only and purely gaseous counterflow diffusion flame models. The parametric dependences of vaporization-zone movement, flame movement, temperature rise and degree of reactant leakage through the flame are examined. In addition, the strain rate dependence of these quantities is examined up to and including extinction.     

The Possible Coupling of LNG Regasification Process with the TSA Method of Oxygen Separation from Atmospheric Air

Liquefied Natural Gas (LNG) must be vaporized before it is used in the combustion process. In most regasification terminals, energy that was previously expended to liquefy natural gas is dissipated in the environment. The paper proposes the use of the thermal effect of LNG regasification for the atmospheric air separation as a possible solution to the LNG exergy recovery problem. The presented idea is based on the coupling of the LNG regasification unit with an oxygen generator based on the Temperature Swing Adsorption (TSA) process. Theoretical analysis has revealed that it is thermodynamically justified to use the LNG enthalpy of vaporization for cooling of the TSA adsorption bed for increasing its adsorptive capacity. It has been shown that 1 kg of LNG carries enough exergy for separating up to approximately 100 g of oxygen using the TSA method. Although the paper suggests using the enthalpy of LNG vaporization for atmospheric air separation, similar processes for other gas mixture separations using the TSA method can be applied.     

Correlation between drop vaporization and self-ignition

A parametric analysis of numerical solutions to problems of vaporization and self-ignition of liquid hydrocarbon drops was performed, and a new criterion determining the conditions of drop self-ignition was suggested. According to this criterion, self-ignition at a given reduced distance from the drop begins when the required reduced gas temperature and equivalence ratio are reached. A new model of heating and vaporization of drops in dense gas suspensions was suggested. The model was verified in multidimensional calculations of self-ignition and combustion of drop clouds. Calculations showed that the model correctly described the phenomenology of local formation and anisotropic propagation of self-ignition waves in suspensions of drops in gases.     

Dynamics of single-wall carbon nanotube synthesis by laser vaporization

The key spatial and temporal scales for single-wall carbon nanotube (SWNT) synthesis by laser vaporization at high temperatures are investigated with laser-induced luminescence imaging and spectroscopy. Graphite/(Ni, Co) targets are ablated under typical synthesis conditions with a Nd:YAG laser at 1000 °C in a 2-in. quartz tube reactor in flowing 500-Torr Ar. The plume of ejected material is followed for several seconds after ablation using combined imaging and spectroscopy of Co atoms, C₂ and C₃ molecules, and clusters. The ablation plume expands in stages during the first 200 7s after ablation and displays a self-focusing behavior. Interaction of the plume with the background gas forms a vortex ring which segregates and confines the vaporized material within a ~1-cm³ volume for several seconds. Using time-resolved spectroscopy and spectroscopic imaging, the time for conversion of atomic and molecular species to clusters was measured for both carbon (200 7s) and cobalt (2 ms) at 1000 °C. This rapid conversion of carbon to nanoparticles, combined with transmission electron microscopy analysis of the collected deposits, indicate that nanotube growth occurs over several seconds in a plume of mixed nanoparticles. By adjusting the time spent by the plume within the high-temperature zone using these in situ diagnostics, single-walled nanotubes of controlled (~100 nm) length were grown and the first estimate of a growth rate on single laser shots (0.2 7m/s) was obtained.     

Influence of the accommodation coefficient on the water drop evaporation process in a radiation field

The process of water drop evaporation in a field of intense laser radiation is examined on the basis of the hydro-thermodynamics equations under the assumption of quasistationarity in the conditions in the surrounding medium and in the radius. The influence of the accommodation coefficient on heating of the drops on the magnitude of the surface jump in water vapor density and on the position of the upper bound of the convective evaporation region is analyzed. It is shown that the surface jump in the vapor does not alter the rate of convective evaporation of the drop in practice, but can result in substantially different time dependences of the radius of a diffusely evaporating drop as compared with those found without it. The solution obtained is compared with the solution of the problem of evaporation in a Stefan approximation.     

Estimation of electromagnetic energy accumulation time in a water drop exposed to infrared laser radiation

The time of accumulation of electromagnetic energy in a water drop exposed to infrared laser radiation is estimated numerically taking into account the energy of interaction of water molecules and optical parameters of the water drop. The calculated value of electromagnetic energy accumulation time is in good agreement with the experimental value.     

On acoustic radiation from a nonlinearly vibrating charged drop

An expression for the time-varying shape of an incompressible liquid drop immersed in a compressible dielectric medium is derived to the second-order approximation in drop vibration amplitude. It is shown that the acoustic radiation spectrum of the drop has a monopole component, which makes a considerable contribution to the integral radiation intensity. Its appearance is associated with the time variation of the zeroth-mode vibration amplitude, showing up in the second order of smallness.     

计算氢同位素水分子汽化焓的比较法

本文首次提出计算氢同位素水分子汽化焓的比较法,得到0～374℃时H2O,D2O,DTO和T2O的汽化焓,并提出用D2从含氚重水中分离出DT气的适宜温度.     

用电加热方法测量液氮的汽化潜热

对测量液氮汽化潜热实验进行改进,引入计算机数据采集技术,记录液氮的汽化过程,通过浸入液氮中的通电电阻释放热量,来改善汽化程度,这样既可以长时间地观察液氮的汽化过程,又能较准确的测量汽氮汽化潜热.     

Contribution of vaporization and boiling to thermal-spike sputtering by ions or laser pulses

Here we consider what, in our terminology, we designate as normal vaporization, normal boiling, and phase explosion. In the case of vaporization, one is dealing with the emission of particles (atoms or molecules) from the extreme outer surface of either a solid or liquid for any temperature exceeding 0 K. In the case of boiling, one is (at least ideally) dealing with heterogeneously nucleated bubbles which diffuse to the outer surface of a liquid or solid and then escape, the latter being possible for temperatures equal to or exceeding the boiling temperature (T(b)). In the case of phase explosion one is dealing with the consequences of what happens when a liquid approaches the thermodynamic critical temperature (T(tc) or T(c)), and massive homogeneous nucleation takes place. Although these three mechanisms have been reviewed in reasonable detail in recent work, we will here present evidence, apparently not previously considered, that boiling, whether the distance scale is atomically small (5-15 nm, as for laser-pulse impact on a metal in the absence of thermal diffusion) or much larger, has a prohibitive kinetic obstacle because it requires bubble diffusion if the bubbles are formed other than at the outer surface. That is to say, boiling will never be a significant process whether with ion or laser-pulse impact. This leaves vaporization and phase explosion as the only possible thermal-spike processes capable of expelling material from an ion- or laser-pulse bombarded surface in a significant quantity. But even with vaporization it can be shown that a kinetic obstacle, although not as severe as for boiling, will enter. The final result is that only phase explosion will normally be relevant for sufficiently short time scales.     

Evaporation of Charged Drops

A model of evaporation of a multiply charged liquid drop is developed. The model self-consistently takes into account the main factors influencing the charged drop evaporation, including effects of the drop surface curvature and charge on the saturated vapor pressure, repeated fragmentation of drops during evaporation, and the capability of drops having a unit charge and a certain stabilization radius not to evaporate even in an unsaturated vapor medium. Analytical dependences are derived that can be used to calculate an integral lifetime of a charged drop with allowance for its fragmentation into smaller drops. Our estimates demonstrate that the evaporation time of charged drops is much smaller than that of uncharged drops.     

A mass spectrometric study of the vaporization of cerium tribromide under Knudsen and Langmuir conditions

The molecular and ionic vaporization of cerium tribromide from the effusion cell (Knudsen conditions) and the open surface of a CeBr₃ single crystal (Langmuir conditions) were studied by high-temperature mass spectrometry. The CeBr₃ and Ce₂Br₆ molecules and the CeBr₄⁻ and Ce₂Br₇⁻ negative ions were observed as vapor constituents. The partial pressures of the molecules in saturated vapor and the ratio between the vaporization coefficients of monomers and dimers under free vaporization conditions were determined. The enthalpies and the activation energies of sublimation in the form of monomers and dimers were measured. Ion–molecular equilibria with the participation of negative ions were studied. The enthalpies of formation of molecules and ions in the gas phase were obtained.     

表面单分子膜的垂悬液滴方法研究

提出一种扩展或收缩气/液界面单分子层的分子密度，用荧光成像技术检测单分子层的扩展或收缩效果的新方法——垂悬液滴分析方法-此方法通过改变液滴体积，对液滴界面上的表面活性剂分子实施扩展和收缩，具有Langmuir-Blodgett(LB槽)的功能-对表面活性剂荧光分子，用液滴的激光感生荧光图像，可以实时测量液滴界面的相对分子密度变化；用偏振荧光分析技术，可获取荧光偶极矩在液滴界面的相对取向-对罗丹明(B)表面活性剂分子ODRB的实验结果表明：1) 表面单分子层分子密度在压缩过程中遵循σ/σ_{0关键词：}     

Determination of temperature and concentration of a vapor–gas mixture in a wake of water droplets moving through combustion products

Characteristic temperatures and concentrations of a vapor–gas mixture in a wake of water droplets moving through combustion products (initial temperature 1170 K) were determined using the Ansys Fluent mathematical modeling package. We investigated two variants of motion: motion of two droplets (with sizes from 1 mm to 3 mm), consecutive and parallel, and motion of five staggered droplets. The influence of the relative position of droplets and also of distances between them (varied from 0.01 mm to 5 mm) on temperatures and concentrations of water vapor was established. The distances determine the relation between the evaporation areas and the total volume occupied by a droplet aggregate in the gas medium. The results of modeling for conditions that take into account vaporization on the droplet surface at average constant values of evaporation rate and also with consideration of the change in the latter, depending on the droplet temperature field, are compared. We determined conditions under which the modeling results are comparable for the assumption of a constant vaporization rate and with regard to the dependence of the latter on temperature. The earlier hypothesis on formation of a buffer vapor layer (“thermal protection”) around a droplet, which decreases the thermal flow from the external gas medium, was validated.     

Polymerization of butadiene on nanoparticles’ surfaces and formation of metal/polymer nanocomposites

In this paper, we demonstrate that laser vaporization of metals in the presence of a small concentration of butadiene vapor leads to the polymerization of butadiene and incorporation of the metal nanoparticles within the polymer matrix. The metal nanocomposites are characterized by electron microscopy, X-ray diffraction and EDX. The results from high pressure mass spectrometry indicate that multiple additions of butadiene molecules on the metal cations Fe⁺, Ni⁺ and Pt⁺, generated by laser vaporization, take place at room temperature thus providing an efficient means of initiating further polymerization reactions. The Pt⁺ reactions show extensive fragmentations and elimination steps generating hydrocarbon ions. The laser vaporization/polymerization method provides the ability to encapsulate several different metals or metal oxides which undoubtedly will play a significant role in tuning the various properties of the polymer composites.