共查询到20条相似文献,搜索用时 31 毫秒
1.
N. A. Inogamov V. V. Zhakhovskii S. I. Ashitkov Yu. V. Petrov M. B. Agranat S. I. Anisimov K. Nishihara V. E. Fortov 《Journal of Experimental and Theoretical Physics》2008,107(1):1-19
A femtosecond laser pulse with power density of 1013 to 1014 W/cm2 incident on a metal target causes ablation and ejection of the surface layer. The ejected laser plume has a complicated structure. At the leading front of the plume, there is a spall layer where the material is in a molten state. The spall layer is a remarkable part of the plume in that the liquid-phase density does not decrease with time elapsed. This paper reports theoretical and experimental studies of the formation, structure, and ejection of the laser plume. The results of molecular dynamics simulations and a theoretical survey of plume structure based on these results are presented. It is shown that the plume has no spall layer when the pulse fluence exceeds an evaporation threshold F ev. As the fluence increases from the ablation threshold F a to F ev, the spall-layer thickness for gold decreases from 100 nm to a few lattice constants. Experimental results support theoretical calculations. Microinterferometry combined with a pump-probe technique is used to obtain new quantitative data on spallation dynamics for gold. The ablation threshold is evaluated, the characteristic crater shape and depth are determined, and the evaporation threshold is estimated. 相似文献
2.
High-temperature field emission of Re, Pt, Ta, and W is studied by field-emission methods. Metal ions are found to evaporate mainly from the tops of thermal-field microprotrusions produced by high electric fields and temperatures on the emitter surface. For fi eld intensities of up to F=1–2 V/Å and temperatures of 1500–2000 K, the ion currents i are recorded from the entire emitter surface. They range from several tenths of nanoamperes to several nanoamperes. The activation energies of field evaporation determined from the Arrhenius plots logi=f(1/T) are found to be appreciably lower than those calculated within the charge exchange model for known parameters of the process and the metals evaporated. Reasons for such a difference in the activation energies and mechanisms of ion evaporation at high F and T are discussed. 相似文献
3.
M.B. Agranat S.I. Ashitkov N.A. Inogamov K. Nishihara V.E. Fortov 《Applied Surface Science》2007,253(15):6276-6282
Using microinterferometric method, a transition in laser plume from the regime with spallation to the regime without spallation is experimentally studied for the first time. The transition occurs when the fluence Finc of incident radiation exceeds a threshold of “evaporation” (Finc)ev. It has been shown previously that the spallation layer is formed at fluence above the ablation threshold (Finc)abl. Thus the spallation exists within the limits (Finc)abl<Finc<(Finc)ev. A laser beam has a maximum fluence (Finc)c on the axis of the beam. The threshold Fev separates two cases with qualitatively different morphology: (1) with unbroken shell covering the crater entirely if Fabl<Fc<Fev, and (2) with the shell having an aperture in the center (like the volcano muzzle) if Fc>Fev. 相似文献
4.
《Heat Recovery Systems and CHP》1995,15(6):591-599
The detailed thermodynamic cycle of the NH3NaSCN absorption refrigeration unit is presented, based on the thermodynamic properties of the working media. Correlations are developed, which express the coefficient of performance and the cooling capacity in terms of the required evaporation temperature, Tev, and the available ambient temperature, Tamb. A nomograph is also presented, which shows in a compact form the behaviour of the NH3NaSCN system and allows direct estimation of its main characteristics. It is concluded that if (Tamb − Tev) varies from 0 to 40°C, the theoretical coefficient of performance decreases linearly from 95 to 77%. For the same range of (Tamb − Tev) the theoretical cooling capacity varies from 1150 to 1300 kJ/kg NH3 if Tev varies from 0 to −15°C. Under the conditions examined, for Tamb − Tev > 23°C, the coefficient of performance of the NH3NaSCN system becomes higher than that of the NH3LiNO3 system. The observed increase reached 4% at Tamb − Tev = 40°C. 相似文献
5.
High-temperature field evaporation of tungsten and tantalum emitters in the temperature range from room temperature to 2500 K is studied using a static magnetic mass spectrometer equipped with a field source of ions. At room temperature, triply charged W3+ and Ta3+ ions alone are observed in the mass spectra. However, as the emitter temperature grows, the charge of the ions decreases. At T ≈ 1000 K, doubly charged W2+ and Ta2+ ions dominate in the spectra, and singly charged W+ and Ta+ ions appear in the temperature range 1900 < T < 2500 K. The evaporation rate of the singly charged ions is one to two orders of magnitude lower than the evaporation rate of the doubly charged particles. The energy parameters of field evaporation for differently charged tungsten ions are found. 相似文献
6.
V. N. Arustamov Kh. B. Ashurov Kh. Kh. Kadirov I. Kh. Khudaykulov 《Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques》2013,7(4):740-744
A criterion for the F-type mechanism of electron emission in a vacuum-arc cathode spot is developed. It is shown that the implementation of F-electron emission in the cathode spot necessitates the fulfillment of three conditions: first, generation of the optimal electric field E opt, second, implementation of atomic-ionic balance at which the E opt field is generated and, third, retention of the cathode temperature lower than the inversion temperature of Nottingham’s effect. The calculations show that the heat of the evaporation of cathode atoms which meets these requirements does not exceed λa ~ 1.1–1.2 eV. Taking the possibility of F-T-emission into account, the evaporation heat can be slightly higher than λa ~ 1.5–1.6 eV. However, in this case, it turns out to be fairly small. Note that the number of such metals is not very large. 相似文献
7.
H. Fukuyama 《Solid State Communications》1978,26(11):783-786
It is shown that within the Hartree-Fock approximation the electron gas in the quantum limit of a strong magnetic field has an instability as the temperature is lowered toward the charge density wave state with an wave-vector which has finite components in the directions not only parallel but also perpendicular to the field. The critical temperature, Tc, is estimated under the assumption of Tc??F?ωc, where ?F and ωc are the Fermi energy of the non-interacting system and the cyclotron frequency respectively. 相似文献
8.
《Surface science》1986,177(3):593-614
Ion energy distributions in low temperature field evaporation obtained by pulsed-laser time-of-flight atom-probe, in general, show a FWHM of the spatial zone of ion formation to be 0.3–0.5 Å; post field ionization is not responsible for the ion formation. However, when ions of two or more charge states coexist a low energy tail can be found for the higher charge state ions, similar to those found for gas ions in field ionization. This tail can extend as far as 10 Å above the surface. Ions in the tail can only be produced by post field ionization. Double peak structures are found in the energy distributions of some ion species such as Mo2+; the origin of which is not yet understood. At a constant rate of field evaporation, as the field is gradually reduced by continued field evaporation and the laser power density appropriately increased, the charge states shift to the lower ones. For Mo, however, Mo2+2 ions are formed before Mo+ ions can be detected. The narrowness of the energy distributions shows that they are stable. This finding has an important implication to the study of the stability or Coulomb explosion of multiple charged cluster ions, and also the theory of field evaporation. Under intense laser irradiation, higher charge state ions can reappear and a large fraction of ions may have an excess energy of a few hundred eV. These may be produced by multiphoton ionization and also by interaction with the over-heated electrons in laser-solid interactions. 相似文献
9.
We investigate the ferromagnetic phase transition in a heavily doped semiconductor or semimetal containing magnetic ions. The coupling between conduction electrons and magnetic ions is treated in the molecular field approximation. We show that the system undergoes a transition to an ordered state for arbitrarily small concentrations of the magnetic ions and the conduction electrons. In the limitJ/? F?1 we obtain the Ruderman-Kittel result for the Curie temperature, and forJ/? F?1 a generalization of Thompson’s strong coupling result. (J is the exchange coupling constant between conduction electrons and magnetic ions and? F is the kinetic Fermi energy at absolute zero.) The intermediate range is evaluated numerically. 相似文献
10.
The synthesized macrocycle L is the novel fluorescent receptor having the switching ability by the external stimuli as well as having the recognizing ability of various metal ions. In particular, this macrocycle L shows the possibility of the selectivity of metal ions even in the same charge ions of a different metal, and the values of association constant (M−1) of that for metal ions are consistent with the tendency of increasing charge number of metal ion. In addition, the values of quantum yield (ΦF) of metal complexes of macrocycle L were ranged from 0.021 to 0.111 enough to recognize the metal ions in macrocycle L. We know from the fluorescent pH titration of macrocycle L by acid/base that the change of fluorescence intersects at about pH=5. 相似文献
11.
A magnetic sector atom-probe has been employed to study slow field evaporation of most of the transition metals in vacuum, in neon and in a mixture of neon and hydrogen. Various metals were found to form metal-neon molecular ions. Slow field evaporation and the presence of hydrogen are favorable for their formation. All the experiments were done at 78 K. The metals that evaporate as nei'des abundantly are Ti, Zn, Zr, Nb, Mo, Pd and Ta, of which Ti, Nb and Pd produce neïde ions as much as 80 to 90%. There were also some neïdes with W, Re, Ir and Rh, definitely above the detection limit estimated to be 3% of the field evaporating metal ions. The role of hydrogen is thought to be two-fold: At the surface, hydrogen adsorption is assumed to cause a stronger metal-neon bond, while the electron shower from free space ionization of the auxiliary gas excites or ionizes the complex by electron impact to allow evaporation at a reduced field. 相似文献
12.
LIF-based imaging of preferential evaporation of a multi-component gasoline surrogate in a direct-injection engine 总被引:1,自引:0,他引:1
In direct-injection engines, fuel components of different volatility can segregate in the transient evaporating spray. The resulting spatial mixture inhomogeneities may impact ignition and combustion. The technique presented here images the effect of preferential evaporation of a multi-component gasoline surrogate fuel in an optically-accessible direct-injection engine motored on nitrogen. It is based on laser-induced fluorescence (LIF) of two aromatic tracers with different volatilities, 1,4-difluorobenzene (representing light to medium components) and 1-methylnaphthalene (heavy component) added to a base-fuel of n-pentane, iso-octane, and n-undecane. LIF from the two tracers is spectrally separated and detected on two cameras, with channel crosstalk corrected in post-processing. Consistent with previous measurements in a high-pressure vessel, the light components are preferentially found downstream, towards the front of the evaporated fuel jet. Throughout large regions of the field of view, about 20% surplus of 1-methylnaphthalene is found, and throughout smaller ones about 40% of 1,4-difluorobenzene. To better assess the impact of the (unknown) local temperature on the measurement accuracy, two-color thermometry based on LIF of anisole (methoxybenzene) is performed in separate experiments. In the relevant range of crank-angles the local temperature is found to be 25 K lower in regions of high fuel concentration than in the rest of the charge, implying a systematic temperature-induced error in the fuel-tracer ratio of 0.11. 相似文献
13.
Thin films of CaF2 co-doped with low concentrations of Eu and Sm ions were grown by pulsed laser deposition (PLD) using a KrF (λ=248 nm) as the ablation source. To the best of our knowledge, the work presented here is the first report of rare-earth-doped CaF2 films grown by PLD with this source. Combined laser excitation-emission spectroscopy was used to map out electronic transitions of Eu3+ with 7F0→5D1 excitation and the 5D0→7F1 emission. At the low concentrations used here the crystal field center of cubic symmetry is dominant in the films that are same for laser targets. However, charge compensated centers are present in the bulk crystal precursor. The removal of the charge compensated centers in the films and the target is likely caused by the target preparation where high pressure and temperature were applied. 相似文献
14.
H. Maletta K. R. P. M. Rao I. Nowik 《Zeitschrift für Physik A Hadrons and Nuclei》1972,249(3):189-194
It is demonstrated that the localized magnetic moments of Fe in Mo which undergo spin compensation at low temperatures vias—d exchange interaction (Kondo-effect) also exhibit, at temperaturesT>T k , relaxation phenomena in the Mössbauer spectra due to the same exchange interaction. Utilizing the Korringa relation, the product of the exchange parameterJ and the density of statesp(? F is estimated to be ¦Jp(? F )¦=0.008. At low temperatures, however, deviations from this relation have been observed. The relaxation rate becomes temperature independent and decreases with increasing external magnetic field. 相似文献
15.
V. P. D’yakonov V. P. Pashchenko É. E. Zubov V. I. Mikhailov Yu. Bukhantsev I. M. Fita V. A. Turchenko N. A. Doroshenko A. Szevczik R. Zuberek G. Szymczak 《Physics of the Solid State》2003,45(5):914-921
The magnetic and transport properties of La1?x Mn1+x O3 manganites with excess manganese are studied. It is shown that magnetic and charge ordering heavily depends on the superstoichiometric manganese content, magnetic field, and pressure. The magnetoresistive effect (MRE) is enhanced as the manganese concentration increases. In addition to the paramagnet-ferromagnet transition, the temperature dependences of the magnetization exhibit anomalies at low temperatures in samples with x=0.1–0.4. The magnetization decreases at T<45 K in fields H<0.2 kOe and increases as H changes from 0.2 to 10 kOe. An analysis shows that the features observed at low temperatures are most probably related to the transition from the ferromagnetic state to the canted spin structure in clusters of mixed-valence manganese ions. The temperature dependences of the magnetization and resistivity remain unchanged as the pressure increases. It is demonstrated that the Curie and metal-dielectric transition temperatures shift to higher values as the manganese concentration increases under pressure. The temperature of the MRE peak increases under pressure, while the MRE decreases. 相似文献
16.
The molecular susceptibility and paramagnetic shift of Rb2CoCl4 single crystals grown using the slow evaporation method were measured, and from these experimental results we obtained the transferred hyperfine interaction due to the transfer of spin density from Co2+ ions to Rb+ ions. The transferred hyperfine field was obtained for the ferroelectric, incommensurate, and normal phases. In the case of Rb(I), the transferred hyperfine interaction decreases with increasing temperature in the incommensurate phase, and increases with increasing temperature in the normal phase. The value of Hhf in the incommensurate and normal phases increases abruptly with increasing temperature in the case of Rb(II). These results indicate that the effects due to the transfer of spin density from Co2+ ions to the Rb(I) and Rb(II) ions are large above Ti. In particular, the effect due to the transfer of spin density to Rb(II) ions in the normal phase is very large; the variations with temperature of the transferred hyperfine interactions of the Rb(I) and Rb(II) nuclei are more or less continuous in Tc1 and Ti, and are not affected by the ferroelectric-incommensurate-normal phase transitions. 相似文献
17.
133Cs (I=7/2) nuclear magnetic resonance in a Cs2CuCl4 single crystal grown by using the slow evaporation method was measured in its three mutually perpendicular crystal planes. The 133Cs resonances of two different groups with two crystallographically inequivalent cesium nuclei, Cs(I) and Cs(II), in the unit cell were recorded. The transferred hyperfine fields for Cs(I) and Cs(II) calculated from the paramagnetic shift and the molecular susceptibility measurements could be expressed by the linear equation Hhf=AT+B. The angular dependence of the 133Cs nuclear magnetic resonance spectra showed that the Cs(I) and the Cs(II) nuclei had different values for the quadrupole coupling constant. The electric field gradient tensors of Cs(I) and Cs(II) were symmetric, and the orientations of their principal axes did not coincide. The Cs(I) ion surrounded by 11 chlorine ions had a small quadrupole parameter, a smaller charge distribution, and a small value for the transferred hyperfine field. However, the Cs(II) ion surrounded by nine chlorine ions had a larger quadrupole parameter, a larger charge distribution, and a larger value for the transferred hyperfine field. 相似文献
18.
A. V. Filippov M. N. Vasil’ev A. V. Gavrikov A. F. Pal’ O. F. Petrov A. N. Starostin V. E. Fortov 《JETP Letters》2007,86(1):14-19
Charging of dust particles in a plasma with the two-temperature energy distribution of electrons has been studied. It has been shown that the dust-particle potential divided by the electron temperature decreases with increasing electron temperature in the plasma with cold ions. Owing to this behavior, the potential of the dustparticle surface increases with the electron temperature more slowly than the linear function and is lower than the electron temperature (divided by the elementary charge) for T e > 5.5 eV in hydrogen and for T e > 240 eV in argon. The fraction of fast electrons at which these electrons begin to contribute to the charge of dust particles has been determined. It has been shown that the charge of micron particles can reach 106 elementary charges. The effect of the cold and thermal field emission on the charge of dust particles has been analyzed. The possibility of obtaining ultrahigh charges (to 107 elementary charges on dust particles with a radius of 50–100 μm irradiated by a 25-keV 1-mA electron beam has been demonstrated. 相似文献
19.
A. N. Pavlenko A. M. Matsekh N. I. Pecherkin R. Kneer V. V. Lel A. S. Surtaev 《Thermophysics and Aeromechanics》2006,13(1):85-96
Experimental data on heat transfer with intense evaporation in the falling films of liquid nitrogen were analysed. According
to data generalization, heat transfer at evaporation becomes more intense under the precrisis modes at high heat fluxes for
two studied boundary conditions on the heat-releasing surface: T
w ≈ const and q
w ≈ const. The relative contributions of conductive and convective components of heat transfer for different heat fluxes were
estimated due to statistical treatment of the wave characteristics carried out by the capacitance probes for measurement of
the local liquid film thickness. It was found out that heat transfer intensification is mainly caused by a drastic decrease
in thermal resistance of the local zones with intensely evaporating residual layer between large waves. At that, the convective
component of heat transfer related to wave perturbations on a free surface of a liquid film decreases significantly with a
rise of heat fluxes. New data on pulsations of the local temperature of the heat-releasing surface were obtained at different
points along the flow with the modes of “dry spot” formation.
The work was financially supported by the Russian Foundation for Basic Research (grant No. 03-02-04027-NNIO-a) and DFG (Deutsche
Forschungsgemeinschaft, project Nos. Re-463-37-1 and SFB-540). 相似文献
20.
Y. Ono 《Zeitschrift für Physik B Condensed Matter》1979,35(2):133-140
The influence of a weak magnetic field on the Anderson transition is investigated by extending Götze's recent zero field theory. Whereas the diagonal components of the conductivity tensor are not essentially effected and the Hall conductivity tends to zero linearly when the Fermi energyε F approaches the mobility edgeε c from above, the Hall coefficient goes to zero discontinuously whenε F=ε c at zero temperature. At finite temperatures, however, no such discontinuity appears. 相似文献