Spatial and time resolved analysis of CN bands in the laser induced plasma from graphite

Analysis of the emission bands of the CN molecules in the plasma generated from a graphite target irradiated with 1-06/~m radiation pulses from a Q-switched Nd:YAG laser has been done. Depending on the position of the sampled volume of the plasma plume, the intensity distribution in the emission spectra is found to change drastically. The vibrational temperature and population distribution in the different vibrational levels have been studied as function of distance from the target for different time delays with respect to the incidence of the laser pulse. The translational temperature calculated from time of flight is found to be higher than the observed vibrational temperature for CN molecules and the reason for this is explained.

Time resolved study of CN band emission from plasma generated by laser irradiation of graphite

Time and space resolved studies of emission from CN molecules have been carried out in the plasma produced from graphite target by 1.06 urn pulses from a Q-switched Nd:YAG laser. Depending on the laser pulse energy, time of observation and position of the sampled volume of the plasma, the features of the emission spectrum are found to change drastically. The vibrational temperature and population distribution in the different vibrational levels have been studied as functions of distance, time, laser energy and ambient gas pressure. Evidence for nonlinear effects of the plasma medium such as self focusing which exhibits threshold-like behaviour are also obtained. Temperature and electron density of the plasma have been evaluated using the relative line intensities of successive ionization stages of carbon atom. These electron density measurements are verified by using Stark broadening method.

Deformation, Phase Transformation and Recrystallization in the Shear Bands Induced by High-Strain Rate Loading in Titanium and its Alloys

alpha-titanium and its alloys with a dual-phase structure (alpha+beta) were deformed dynamically under strain rate of about 10(4) s(-1). The formation and microstructural evolution of the localized shear bands were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results reveal that both the strain and strain rate should be considered simultaneously as the mechanical conditions for shear band formation, and twinning is an important mode of deformation. Both experimental and calculation show that the materials within the bands underwent a superhigh strain rate (9 x 10(5) s(-1)) deformation, which is two magnitudes of that of average strain rate required for shear band formation; the dislocations in the bands can be constricted and developed into cell structures; the phase transformation from alpha to alpha(2) within the bands was observed, and the transformation products (alpha(2)) had a certain crystallographic orientation relationship with their parent; the equiaxed grains with an average size of 10 mu m in diameter observed within the bands are proposed to be the results of recrystallization.

The evolution of shear bands of saturated soil

The development of the shear bands of saturated soil in coupling-rate- and pore-pressure-dependent simple shear has been discussed, using a simple model and a matching technique at the moving boundary of a shear band. Tt is shown that the development of shear bands are dominated by the coupling-rate and pore-pressure effect of the material. The strength of the soil acts as a destabilizer, whilst pore pressure diffusion makes the band expand. The theory is discussed and some computational solutions have been presented.

Behavior of Multiple Shear Bands in Zr-Based Bulk Metallic Glass

The deformation behavior of Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass was studied by in situ scanning electron microscopy (SEM) quasi-static uniaxial compression tests at room temperature. Multiple shear bands were observed with a large plasticity. Microscopic examination demonstrates that slipping, branching and intersecting of multiple shear bands are the main mechanisms for enhancing the plasticity of this metallic glass. Additionally, nano/micro-scale voids and cracks at the intersecting sites of shear bands and preferential etching of shear bands were observed as well. These observations demonstrated that the formation of shear bands in bulk metallic glasses is resulted mainly from local free volume coalescence.

Evolutions of Compaction Bands of Saturated Soils

The development of compaction bands in saturated soils，which is coupling—rate，inertial and pore—pressure—dependent，under axisymmetric loading was discussed，using a simple model and a matching technique at the moving boundary of a band．It is shown that the development of compaction bands is dominated by the coupling rate and pore—pressure effects of materia1．The soil strength makes the band shrinking，whilst pore pressure diffusion makes the band expand．Numerical simulations were carried out in this paper ·

Formation of shear bands in plane sheet

The formation of shear bands in plane sheet is studied, both analytically and experimentally, to enhance the fundamental understanding of this phenomenon and to develop a capability for predicting material failure. The evolution of voids is measured and its interaction with the process of shear banding is examined. The evolving dilatancy in plasticity is shown to have a vital role in analysing the shear-band type of bifurcation, and tremendously reduces the theoretical value of critical stresses. The analyses, referring to both localized and diffuse modes of bifurcation, fairly explain the corresponding observations obtained through testing a dual-phase steer sheet and provide a justification of the constitutive model used.

CHARACTERIZATION OF SHEAR BANDS IN TWO BULK METALLIC GLASSES WITH DIFFERENT INHERENT PLASTICITY

Shear banding characterization of Zr64.13Cu15.75Ni10.12Al10 and Zr65Cu15Ni10Al10 bulk metallic glasses (BMGs) with significant difference in inherent plasticity and quite similar chemical composition was studied by depth sensitive macroindentaion tests with conical indenter. Well-developed shear band pattern can be found for both BMGs after indentation. Distinct difference in the shear band spacing, scale of plastic deformation region and the shear band branching in the two BMGs account for the different plasticity.

Characterization Of Shear Bands In Two Bulk Metallic Glasses With Different Inherent Plasticity

Shear banding characterization of Zr64.13Cu15.75Ni10.12Al10 and Zr65Cu15Ni10Al10 bulk metallic glasses (BMGs) with significant difference in inherent plasticity and quite similar chemical composition was studied by depth sensitive macroindentaion tests with conical indenter. Well-developed shear band pattern can be found for both BMGs after indentation. Distinct difference in the shear band spacing, scale of plastic deformation region and the shear band branching in the two BMGs account for the different plasticity.

Improved fluorescence from Tm3+/Er3+/Ce3+ triply doped bismuth-silicate glasses for S+C-bands amplifiers

Fluorescence of Tm3+/Er3+ codoped bismuth-silica (BS) glasses and the sensitization of Ce3+ are investigated. It shows that Ce3+ codoping with Tm3+/Er3+ in BS glasses results in a quenching of Tm3+ ion emission from F-3(4) to the H-3(6) level. Consequently, the 1.47 mu m emission occurs after the population inversion between the H-3(4) and F-3(4) levels. Furthermore, the codoped glasses show the broad emission spectra over the whole S and C bands with full-width at half-maximum (FWHM) up to about 119nm, as it combines 1.55 mu m emission band of Er3+ with 1.47 mu m emission band of Tm3+ under 800nm excitation.

Correlations between electronic energy bands spin splitting and magnetic profile symmetry of magnetic superlattice

We have theoretically investigated the energy band structures of two typical magnetic superlattices formed by perpendicular or parallel magnetization ferromagnetic stripes periodically deposited on a two-dimensional electron gas (2DEG), where the magnetic profile in the perpendicular magnetization is of inversion anti-symmetry, but of inversion symmetry in parallel magnetization, respectively. We have shown that the energy bands of perpendicular magnetization display the spin-splitting and transverse wave-vector symmetry, while the energy bands of the parallel magnetization exhibit spin degeneration and transverse wave-vector asymmetry. These distinguishing spin-dependent and transverse wave-vector asymmetry features are essential for future spintronics devices applications. (c) 2008 Elsevier B.V. All rights reserved.

A full spd tight-binding treatment for electronic bands of graphitic tubes

A tight-binding (TB) treatment with the inclusion of d orbitals is applied to the electronic structures of graphitic tubes. The results show that the high angular moment bases in TB scheme are necessary to account the severe curvature effect in ultra-thin single wall carbon nanotubes, especially for properly reproducing the band edge overlap behavior in (5, 0) tube, predicted by the existing ab initio calculations. In the large diameter limit, the participation of two synnmetry-allowed d bases provides a natural replication to the recent measured electronic dispersions of valence band of graphene when the strong anisotropy due to the two-dimensional planar hexagonal sheet structure is dealt with properly. In addition, the detailed relation between the two sets of quantum numbers of screw symmetry and that of zone folding is formulated in appendix. (C) 2008 Elsevier Ltd. All rights reserved.