Stimulated Brillouin scattering of CO2 radiation in compressed xenon

Stimulated Brillouin scattering (STBS) and phase conjugation of CO₂ laser radiation have been demonstrated experimentally for the first time in compressed xenon (59 atm at 21°C) located inside the low-Q cavity of this laser. The nonlinear medium was exposed to the action of counterpropagating focused multimode radiation beams. The difference between the frequencies of the longitudinal cavity modes was set at the frequency of the acoustic wave (v _s=32.2±0.3 MHz) excited as a result of STBS by 9.584 μm radiation. The duration of the radiation pulse τ _L was close to the acoustic phonon lifetime (τ _L<τ _ph≈3× 10⁻⁶s). The excitation of STBS was manifested experimentally as the locking of longitudinal modes, an increase in power and energy, and also an increase in the duration of the lasing pulse and a reduction in the divergence to the diffraction limit. Zh. éksp. Teor. Fiz. 116, 1941–1946 (December 1999)     

Structure of polymeric carbon dioxide CO2-V

The structure of polymeric carbon dioxide (CO2-V) has been solved using synchrotron x-ray powder diffraction, and its evolution followed from 8 to 65 GPa. We compare the experimental results obtained for a 100% CO2 sample and a 1 mol?% CO2/He sample. The latter allows us to produce the polymer in a pure form and study its compressibility under hydrostatic conditions. The high quality of the x-ray data enables us to solve the structure directly from experiments. The latter is isomorphic to the β-cristobalite phase of SiO2 with the space group I42d. Carbon and oxygen atoms are arranged in CO4 tetrahedral units linked by oxygen atoms at the corners. The bulk modulus determined under hydrostatic conditions, B0=136(10) GPa, is much smaller than previously reported. The comparison of our experimental findings with theoretical calculations performed in the present and previous studies shows that density functional theory very well describes polymeric CO2.     

Synchrotron small angle X-ray scattering study on the conformation of polystyrene in compressed CO2-toluene mixture

Synchrotron small angle X-ray scattering (SAXS) was performed to investigate the effect of dissolved CO2 in toluene on the conformation of polystyrene (PS) in the solution. It has been found that the second virial coefficient A2 and the radius of gyration Rg decrease with the increasing antisolvent CO2 pressure. The scattering intensity of PS chain followed l(h)∞h-αunder different antisolvent pressures (0, 0.6, 1.5, 2.4, 3.3, and 4.2 MPa), suggesting that the PS chain has self-similar structure behavior or a fractal structure in the presence of antisolvent CO2.All this reveals a large effect of antisolvent pressure or the solubility of CO2 in the solution on PS structure. The fractal dimensions increase with the increasing antisolvent pressure, indicating that the polymer chain changes from a swollen coil into a rather dense globule in the course of adding antisolvent CO2.     

单壁及双壁纳米碳管径向压缩特性的分子动力学研究

采用Tersoff-Brenner势与L-J势的分子动力学方法,研究了双石墨层作用下(13,0)(、22,0)锯齿形碳管以及它们组合的(13,0)/(22,0)双壁碳管的径向压缩力学特性.根据计算结果,讨论了三种碳管压缩过程中的构型、能量、压缩载荷等的变化及其差异.研究表明,压缩过程中,(22,0)碳管出现了明显的“塌陷”现象,“塌陷”时,能量及外载一度下降;(13,0)/(22,0)碳管的能量吸收能力及承受压缩载荷能力最大;(22,0)碳管的能量吸收能力与承受压缩载荷能力最差,且体积最容易压缩;当压缩应变小于12%时,(13,0)/(22,0)碳管的内管的构型、体积及其能量变化量均很小;内壁与外壁之间的Van der Waals能在整个压缩(13,0)/(22,0)碳管能量变化中仅占很小的份额.     

Semiclassical line mixing analysis in the first overtone band of CO compressed by N2

Infrared spectra of the 2 ← 0 absorption band of CO compressed by N₂ at various densities up to 456 Amagat and at temperatures of 296, 188 and 122 K have been analyzed. To interpret these spectra we have applied a semiclassical model with adiabatic corrections which accounts for the line-mixing effects. This model provides a satisfactory agreement with experimental bandshapes for all temperatures, especially for the lower N₂ densities. The discrepancy arising at higher densities may be due to imperfections of the calculated relaxation matrix and/or to the non-validity of the impact and binary collision approximations.     

Absorption of pulsed CO2 laser radiation by air and carbon dioxide

An optoacoustical method is used in experimental study of nonlinearity in absorption of pulsed CO₂ laser radiation by air with CO₂ and H₂O added and pure CO₂. The laser pulse consists of frequencies related to several rotational transitions, generated simultaneously. Nonlinearity in absorption was detected for laser radiation levels above 0.2 MW/cm².Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 49–51, October, 1982.     

Etch Induction Time in CR-39 Detectors Etched in Na2CO3 Mixed NaOH Solution

Six different solutions of 6M NaOH, containing different amounts of Na2CO3 at 70℃ were used for the revelation of latent damage trails in CR-39 plastic track detectors. These detectors were earlier exposed to fission fragments from ^252 Cf source for 30 min in vacuum and were then etched in the respective solutions for different etching time intervals of 5-20 min starting from 5min up to 160min. The etch induction time in each detector was obtained by extrapolating the intersection of resulting curves of track lengths and track diameters with the time axis.     

Single- and multi-photon absorption of carbon dioxide laser lines by SO2 and CO2 molecules

Summary We have used 60 rovibrational lines of a Lumonics TEA CO₂ laser to record the spectra of coincidence of SO₂ IR absorption with CO₂ laser emission. We extended the same type of investigation to CO₂ molecules and observed an absorption at practically all the rovibrational lines of the laser. We have also recorded the increase of absorption with pressure of the studied molecule and with its temperature. Our work reveals very accurately the fine structure of these absorptions that shows a clear discreteness in the intensities of the lines. Some implication of this discreteness in multiphoton absorption is also presented, through data that indicate very strong vibrational coupling between ground and excited electronic states of SO₂. The authors of this paper have agreed to not receive the proofs for correction.     

Diffusion of CO2/CH4 confined in narrow carbon nanotube bundles

ABSTRACT

The diffusion of a CO₂/CH₄ mixture in carbon nanotube (CNT) bundles was studied using molecular simulations. The effect of diameter and temperature on the diffusion of the mixture was investigated. Our results show that the single-file diffusion occurs when CO₂ and CH₄ are confined in CNTs of diameter less than 1.0 nm. In CNTs of diameter larger than 1.0 nm, both molecules diffuse in the Fickian style. The transition from single-file to Fickian diffusion was demonstrated for both CO₂ and CH₄ molecules. A dual diffusion mechanism was observed in the studied (20, 0) CNT bundle, single-file diffusion of CO₂ in the interstitial sites of (20, 0) CNT bundle and Fickian diffusion of CO₂ and CH₄ in the pores. For CO₂, the interaction energies (CO₂–CO₂ and CO₂–CNT) are larger than that of CH₄ in all cases. But only a very small difference in the diffusion coefficient was observed between CO₂ and CH₄. Temperature has a negligible effect on the difference between diffusion coefficients of CO₂ and CH₄ in the studied CNT bundles. The adsorption, diffusion and permeation selectivities are discussed and compared, and the adsorption is demonstrated to be the rate limiting step for the separation of CO₂/CH₄ in CNT bundle membranes.     

The nature of the absorption bandshape density evolution for the first overtone of CO compressed by N2

Infrared spectra of the 2←0 absorption band of CO, compressed by N₂ at various densities up to 220 amagat at about 296 and 187 K have been analyzed. To interpret these spectra a line-by-line model has been applied, based on empirical corrections of the Lorentzian line-shape, which account roughly for the effects of line-mixing and the finite duration of collisions. At room temperature, this model provides a satisfactory agreement with experimental bandshapes, especially for the lower N₂ densities. At low temperature, the calculated absorption differs significantly from that observed around the band center, with a discrepancy increasing with the N₂ density. This difficulty is overcome by accounting for the binary—van der Waals complexes— CO…N₂ formation. The extent of these CO…N₂ complexes has been estimated from two models of bandshape fitting the experimental profiles and by a simple thermodynamic calculation.     

Symmetric and asymmetric vortex-antivortex molecules in a fourfold superconducting geometry

In submicron superconducting squares in a homogeneous magnetic field, Ginzburg-Landau theory may admit solutions of the vortex-antivortex type, conforming to the symmetry of the sample [L. F. Chibotaru, Nature (London) 408, 833 (2000)10.1038/35048521]. Here we show that these fascinating, but never experimentally observed states, can be enforced by artificial fourfold pinning, with their diagnostic features enhanced by orders of magnitude. The second-order nucleation of vortex-antivortex molecules can be driven by either temperature or an applied magnetic field, with stable asymmetric vortex-antivortex equilibria found on its path.     

Intensity,transition moment,and bandshapes for the second overtone of compressed CO

Infrared spectra of the CO second overtone absorption band have been studied for pressures up to 156 bar. The absolute intensity, obtained from direct measurements of the total band, is found to be (1.27 ± 0.04) × 10^-2cm^-2Am^-1 at 298 K. Using the vibrational moments for the ground state and the transitions 0–1 to 0–4, we have evaluated the coefficients M₀ to M₄ of the quartic dipole moment function for CO and the coefficients of the vibration-rotation interaction functions. The normalized 0–3 band contours of compressed CO were first generated by summing the pressure-broadened rotational lines with Lorentz profiles. A discrepancy appears between calculated and observed bandshapes and increases with density. An empirical model, which involves the sum of CO self-broadened lines with profile described as the product of a Lorentz function by a fitted exponential function, gives reasonable agreement with the experiments. The interference effects resulting from overlapping lines have been estimated within the impact approximation.     

Intensity,bandshapes and dipole correlation functions for the first overtone of compressed CO

Infrared spectra of the CO first overtone absorption band have been studied at room temperature for pressures up to 104.5 bar. The integrated intensities are found to be proportional to the gas density. The dipole moment autocorrelation function is used to assess a collisional model derived from the impact approximation and the Gordon M- and J-diffusion models developed classically, as well as the semi-classical M-diffusion model. The semi-classical M-model, which is better than the classical models, reproduces the experimental correlation functions well only for densities up to about 40 Am. From an analysis of the individual lines at low pressures, we conclude that their profiles are the product of a Lorentz function and of an empirical exponential function. The normalized band contours are calculated by summing the pressure-broadened rotational lines, assuming for these lines successively a Lorentz shape and two modified Lorentz shapes derived from the low-pressure line analysis. In the first case, a discrepancy between theoretical and experimental bandshapes become noticeable from about 20 Am and increases with density. The results are greatly improved by using two other models, one of which gives reasonable agreement with the experiments at all pressures. At high pressures, interference effects between the strongly overlapping lines are probably not negligible.     

The effect of internal CO2 conductance on leaf carbon isotope ratio

We estimated internal CO2 conductance (gi) for C3 plant species of different life forms (annual herbs, deciduous trees and evergreen trees) grown in a variety of environments to examine the effect of gi on their leaf carbon isotope ratio (delta13C). The purpose of this study was to test the validity of using delta13C as an index of photosynthetic water use efficiency (WUE). When comparing deciduous tree species grown in contrasting light environments, there was a strong positive relationship between delta13C and WUE. Similarly, delta13C was positively correlated to WUE when comparing the different species of evergreen trees. However, the difference in WUE between evergreen and deciduous tree species did not relate to that in leaf delta13C. In addition, WUE was similar between highland and lowland herbaceous plants, although the former had a much higher delta13C. The positive relationship between delta13C and WUE did not hold across different life forms and different altitudes when differences in gi did not relate to those in stomatal conductance, resulting in independence of chloroplast CO2 partial pressure from intercellular CO2 partial pressure.     

CO2活化温度对碳气凝胶结构及氢吸附性能影响

 以间苯二酚(R)和甲醛(F)为前驱体制备出RF碳气凝胶,并利用CO₂气体对其进行活化,通过热重分析(TG)、X射线衍射(XRD)、氢吸附等表征手段研究了活化过程中温度对碳气凝胶微结构及氢吸附性能的影响。结果表明：CO₂活化可使碳气凝胶比表面积大幅提高,且不会影响其微孔网络结构。随着活化温度的升高,碳气凝胶非晶性变得更加明显,失重率显著增加,微孔体积、比表面积及氢吸附量先增后减,有望通过改变CO₂活化温度增加碳气凝胶微孔数量来提高其氢吸附性能。     

Single-walled carbon nanotubes formation with a continuous CO2-laser: experiments and theory

It has been proved [1] that the use of a CO₂-laser system operating in continuous wave mode (cw) can be efficiently used for the production of carbon single-walled nanotubes (SWNTs). In this article we first describe in detail the variable experimental conditions (different ambient gases, static gas pressure, and gas flow) for SWNT formation and summarize the results of the characterization studies of the synthesized materials. Second, we analyze the influence of the different experimental conditions on the SWNTs formation process. We show that the heat transport, kinetic, and diffusion processes allow us to explain seemingly different formation conditions in a qualitative and semi-quantitative agreement with the experimental results. The presented self-consistent scenario for nanotube formation in a gas phase allowed us to propose future experiments on testing the mechanism of nanotube formation.     

A study of the gasification of carbon black with molten salt of Li2CO3 and K2CO3 for application in the external anode media of a direct carbon fuel cell

The characteristics of gasification reactions for carbon–carbonate mixtures were experimentally investigated at high temperatures up to 900 °C, considering the application of the mixtures to the external anode media of a direct carbon fuel cell. A thermo-gravimetric analysis (TGA) was conducted in either a nitrogen or carbon dioxide ambient environment for Li₂CO₃, K₂CO₃ and a mixture of these two substances with carbon black. Changes in the exit gas composition were also monitored during the heating process. It was shown that gasification in the mixture media occurs much more rapidly than carbonate decomposition at elevated temperatures, even for low concentrations of CO₂. It was also shown that the loading of carbonates to carbon significantly affects the global gasification reaction; it increased the reaction rate by an order of magnitude and decreased its activation energy. Based on the experimental observations, a simplified reaction model of gasification was suggested for the anode media of a DCFC, regarding carbonate-catalysed and metal-catalysed pathways of Boudouard reactions.