Наноразмерные дискретные покрытия оксида меди на кристаллах хлорида натрия, осажденные из паровой фазы в вакууме

Приведены результаты исследования морфологии дискретного наноразмерного медного покрытия на поверхности порошка хлорида натрия. Способом электронно-лучевого испарения и конденсации из паровой фазы в вакууме получено равномерное и однородное покрытие. Средний размер частиц меди согласно результатам лазерной корреляционной спектроскопии составил 27 нм. Представлены рекомендации относительно возможного практического применения.The results of investigation of morphology of discrete nanodimensional copper coating on the surface of sodium chloride powder are presented. Uniform and homogeneous coating was produced by the method of electron beam evaporation and condensation from vapor phase in vacuum. The obtained average size of copper particles in accordance with results of a laser correlation spectroscopy was 27 nm. Recommendations are given for a possible practical application

Crystal nucleation and cluster-growth kinetics in a model glass under shear

Crystal nucleation and growth processes induced by an externally applied shear strain in a model metallic glass are studied by means of nonequilibrium molecular dynamics simulations, in a range of temperatures. We observe that the nucleation-growth process takes place after a transient, induction regime. The critical cluster size and the lag-time associated with this induction period are determined from a mean first-passage time analysis. The laws that describe the cluster growth process are studied as a function of temperature and strain rate. A theoretical model for crystallization kinetics that includes the time dependence for nucleation and cluster growth is developed within the framework of the Kolmogorov-Johnson-Mehl-Avrami scenario and is compared with the molecular dynamics data. Scalings for the cluster growth laws and for the crystallization kinetics are also proposed and tested. The observed nucleation rates are found to display a nonmonotonic strain rate dependency

Salicylic Acid, an Ambimobile Molecule Exhibiting a High Ability to Accumulate in the Phloem

International audienceThe ability of exogenous salicylic acid (SA) to accumulate in castor bean (Ricinus communis) phloem was evaluated by HPLC and liquid scintillation spectrometry analyses of phloem sap collected from the severed apical part of seedlings. Time-course experiments indicated that SA was transported to the root system via the phloem and redistributed upward in small amounts via the xylem. This helps to explain the peculiarities of SA distribution within the plant in response to biotic stress and exogenous SA application. Phloem loading of SA at 1, 10, or 100 mM was dependent on the pH of the cotyledon incubating solution, and accumulation in the phloem sap was the highest (about 10-fold) at the most acidic pH values tested (pH 4.6 and 5.0). As in animal cells, SA uptake still occurred at pH values close to neutrality (i.e. when SA is only in its dissociated form according to the calculations made by ACD LogD suite software). The analog 3,5-dichlorosalicylic acid, which is predicted to be nonmobile according to the models of Bromilow and Kleier, also moved in the sieve tubes. These discrepancies and other data may give rise to the hypothesis of a possible involvement of a pH-dependent carrier system translocating aromatic monocarboxylic acids in addition to the ion-trap mechanism

Fine frequency shift of sigle vortex entrance and exit in superconducting loops

The heat capacity $C_{p}$ of an array of independent aluminum rings has been measured under an external magnetic field $\vec{H}$ using highly sensitive ac-calorimetry based on a silicon membrane sensor. Each superconducting vortex entrance induces a phase transition and a heat capacity jump and hence $C_{p}$ oscillates with $\vec{H}$. This oscillatory and non-stationary behaviour measured versus the magnetic field has been studied using the Wigner-Ville distribution (a time-frequency representation). It is found that the periodicity of the heat capacity oscillations varies significantly with the magnetic field; the evolution of the period also depends on the sweeping direction of the field. This can be attributed to a different behavior between expulsion and penetration of vortices into the rings. A variation of more than 15% of the periodicity of the heat capacity jumps is observed as the magnetic field is varied. A description of this phenomenon is given using an analytical solution of the Ginzburg-Landau equations of superconductivity

EXAFS study of rubidium-doped single-wall carbon nanotube bundles

International audienceThe local structure around the rubidium ions inserted in single-wall carbon nanotube bundles (Rb-doped SWCNT) is studied by Rb K-edge extended x-ray-absorption fine structure (EXAFS). The dependence of the local order around the rubidium ions is investigated as a function of the time of doping (i.e., as a function of the stoichiometry of the sample). The first coordination shell of the rubidium ions, related to the distance between rubidium and the first nearest-neighboring carbon atoms, has a clear time doping dependence. Comparison between ab initio simulations of the EXAFS spectra and experimental data questions the interstitial site (between three tubes) as the preferential insertion site in SWCNT bundles. The results indicate that the rubidium ions are mainly located inside the tubes and around the bundles. The results are in good agreement with combined x-ray and neutron diffraction experiments performed on the same samples

A systematic study of supersonic jet noise

December 1971Includes bibliographical references (leaves 39-40)The purpose of this work is to study the acoustic fields associated with two different nozzle configurations; a rectangular and a circular. Both nozzles are designed with the same exit Mach number and have an identical momentum and energy flux. By presenting a comparison of the two nozzles, it proposed to establish and identify the dominant noise generating mechanisms. A basic difference in shape changes the relative importance of different noise mechanisms. The other main aim of this study is to establish scaling laws of supersonic jet noise. A shock tube is a very versatile apparatus for such an analysis. By first changing the driver, driven pressure and molecular weights, a wide range of stagnation pressures and temperatures could be achieved. The case with which these conditions are simulated is, however, traded off with the short test time, of the order of milliseconds. A short test time allows the use of a heat sink nozzle and eliminates the use of an anechoic chamber.So far tests have been made in the range of 1000-5000°R, for different levels of expansion and an exit Mach number of 2.7. In comparing the two nozzles, it is found that the rectangular nozzle is indeed quieter than the circular nozzle. The rectangular nozzle is more effective under overexpanded conditions, and a factor of 1.6 in acoustic efficiency at low temperature (1200°R) and about 3 at high temperature is related to a rapid deceleration of the jet through a system of strong shocks. The low acoustic efficiency of the overexpanded rectangular jet is related to a rapid deceleration of the jet through a system of strong shocks. At high temperature, this effect is not observed because an important density ratio exists across the shear layer which becomes very unstable due to the Taylor instability.For both the circular and rectangular nozzle, the effect of temperature showed an increase in the directivity angle at high temperature which is correlated to an increase in eddy convective velocity, rather than refraction due to density gradients, which seems to play a secondary role. The low temperature overexpanded jet showed a difference of about 2.6 db in the OPWL between the two nozzles. However, at this condition, for the rectangular nozzle, a difference of 8 db between the maximum and minimum noise direction is observed. Hence, a suitable orientation of the nozzle could cause a considerable reduction in the noise level. The rectangular nozzle seems to be very effective under overexpanded conditions. The scaling laws, which are in the preliminary stages, were developed for the change in the OPWL as a function of stagnation pressure. For the circular nozzle, additional scaling was done for temperature and acoustic efficiency.These scaling laws need to be verified for additional temperatures. Also, further work should be initiated in the potential use of the rectangular nozzle as a noise suppressor and as a model for better comprehension of noise generating mechanisms.Office of Noise Abatement, Dept. of Transportation. DOT-TSC-14

Assessment of Nitrogen Nutrition Status of Grasses under Water Deficit and Recovery

Grasslands are rarely irrigated. They are therefore systematically submitted to more or less severe water deficits: as well as mineral deficiencies, water scarcity often also results in a reduction of nitrogen (N) status. Although identified some time ago, qualitatively, the interaction with N still remains difficult to take into account in quantitative analyses of crop physiology under water deficits. This paper illustrates how the nitrogen (N) status of the crop changes under water deficits. A N nutrition index (INN) was defined as the ratio of the actual N concentration of forage with the theoretical N concentration under optimal conditions, the latter only depending on the above ground biomass. The objective of the paper is to describe the effect of water deficits on INN, using a new assay recently proposed by Faruggia et al. ( 2004)