摘要:

We measured quasisimultaneously the energy distributions of positive ions at the powered rf and grounded electrode of a parallel plate 13.56 MHz discharge using an energy selective mass spectrometer. The resulting ion energy distributions reflect the discharge potential conditions expected from a capacitive plasma sheath model. By means of an externally supplied dc bias of the powered electrode we are able to influence the potential structure and to control ion energy and ion flux independently. The ratio between mean ion energy and mean sheath thickness reflects the effect of collisions on the ion energy distributions and enables estimates of sheath thickness and bulk plasma parameters to be made which are compared with values obtained by Langmuir probe measurements. We are able to demonstrate that changes in sheath potential also affect, via secondary electrons, the ionization regime in the discharge and this can be utilized to control the species composition in the discharge.©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364331

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A simple model is proposed to derive the Gibbs potential change associated with the formation of secondary nuclei in the crystallization of systems consisting of isotactic polypropylene nucleated with small amounts of indigo. A basic assumption is that the average basal surface tension of critical nuclei lowers as an effect of chain–nucleant interaction. The actual fold length of individual stem pairs may be lower than critical due to chain–nucleant encounters. Two limiting cases are considered, namely, when the fold lengths of adjacent stem pairs are independent of one another (random fold), and when they are not (persistence). Corresponding expressions of the nucleation free enthalpy are used to analyze available data of crystallization kinetics. Both scenarios describe the physical situation well, most likely because under the actual experimental conditions critical nuclei consist, on average, of one stem pair. The random fold scheme, however, is more informative as it provides (at least in principle) a method for determining both the basal and lateral contributions to surface tension through an analysis of the crystallization kinetics. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364332

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

BaTiO3powder has been synthesized by heat treating (BaTi) citrate polyester resins at 500 °C for 8 h. X-ray powder diffractometry (XRD) and Raman spectroscopy data suggest the presence of a high-temperature hexagonal phase at room temperature. However, high-resolution transmission electron microscopy indicates that the peaks that were ascribed to the above phase in the Raman and XRD spectra may originate from stacking faults in the particles. Our results disprove the existence of the high-temperature hexagonalBaTiO3at room temperature.©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364333

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A simple analytical model for ion-solid interactions, suitable for low energy beam depositions, is reported. An approximation for the nuclear stopping power is used to obtain the analytic solution for the deposited energy in the solid. The ratio of the deposited energy in the bulk to the energy deposited in the surface yields a ceiling for the beam energy above which more defects are generated in the bulk resulting in defective films. The numerical evaluations agree with the existing results in the literature. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364334

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The electro-optical behavior of polymer stabilized cholesteric texture cells has been investigated for three different polymers. The switching process was studied with respect to the electric field dependence of the diffuse reflectivity, diffuse transmittance, and the dynamics of the reorientation process. For certain polymer concentrations, a two-stage reorientation process was observed. This behavior is consistent with the cholesteric liquid crystal being divided between two distinct environments. In the first, the liquid crystal is strongly dominated by the polymer network, while in the second a bulklike behavior, comparable to the unstabilized cholesteric material, is observed. Scanning electron micrographs of the polymer networks further support this model. Measurements of the diffuse scattering indicate that the polymer influenced regions contribute largely to the observed back scattering, whereas the bulklike material contributes primarily to forward scattering. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364335

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Melting is encountered in almost all laser materials processing. This article deals with a one-dimensional heat conduction problem to investigate the melting rate during laser materials processing. The problem is solved approximately to obtain a correlation among melt depth, power density, and laser irradiation time. Based on this correlation, the dynamics of melting, a relationship between the melt depth and power density and an average melting velocity are expressed by simple analytic formulas. These expressions are further simplified for high power densities (I⩾109W/m2). The times to reach the melting and boiling temperatures at the surface of the workpiece are also calculated. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364336

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The rapid laser driven shock compression of solids is modeled using a hydrodynamic computer code. By postprocessing output from these simulations and using a new, efficient x-ray diffraction algorithm we calculate time-dependent x-ray diffraction images which can be directly compared with experimental x-ray diffraction data. The measurement of shock compression of crystalline solids by x-ray diffraction is illustrated with three examples, one a square wave, and two triangular waves. The development of these waves and the associated x-ray diffraction patterns are followed as the waves propagate through the crystalline solid and interact with a free surface. Simulated x-ray streak camera data created with this hydrodynamic-x-ray diffraction model are presented and are compared to experimental data. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364337

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A new method has been applied to dynamically compact ceramic powders in the cylindrical configuration. In this method, a converging oblique detonation is used instead of the sliding detonation used in the standard method. The oblique detonation is generated by a configuration using two explosive layers. X-ray flash photographs have been made that show the detonation and shock fronts in both the standard and new configuration. In the present article, the shock wave and particle velocities in theB4Cpowder have been calculated using the shock and detonation angles obtained from the photographs in combination with the measured detonation velocity. In the two-layer configuration, the pressure applied to the powder was increased by a factor of 3.5 compared to the one-layer configuration, in agreement with calculations. The working principle of the two-layer configuration is discussed and compared with a computer simulation of the process. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364338

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The use of Raman spectroscopy for strain characterization of materials often requires scattering configurations in which the optical phonon wavevector does not coincide with any of the strain-modified phonon eigenvectors. It is shown that the optical phonon frequencies in strained zincblende crystals exhibit angular dispersion similar to that of optical phonons in unstrained biaxial and uniaxial crystals. The entire formalism is developed on the basis of the relative magnitude of the longitudinal-transverse frequency splittings and the strain-induced frequency splittings. The shifts produced at grazing incidence are above the usual spectroscopic limits of accuracy and can influence the interpretation of data in terms of strain relaxation or defects. The model is applied to diamond- as well as zincblende-type structures. Specific examples are worked out in detail for scattering geometries under grazing incidence in [001] and [111] strained heterojunctions. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364339

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Combining hardness indentation tests and micro-Raman spectroscopy it is shown that metallic Si-II is produced near the interface of a diamond indenter and silicon to a depth of about 0.5 &mgr;m, where the highest stresses (hydrostatic and deviatoric) exist. At fast unloading rates Si-II transforms to the amorphous state, whereas a mixture of the r8 high pressure polymorph Si-XII and the bc8 phase Si-III forms upon a slow load release. The region of Si-III+Si-XII is surrounded by the wurtzite structured Si-IV, where the stresses during the indentation had not been high enough to cause the transition to the metallic state. Thus, because of shear deformation a direct transformation to Si-IV takes place. Outside the phase-transformed regions the classical aspects of indentation-induced deformation by dislocation glide, twinning and crack formation are observed. Annealing of the high pressure phases leads to the formation of Si-IV at moderate temperatures and to the reversal to the original diamond structure (Si-I) at temperatures above 500 °C. Using the laser beam of the Raman spectrometer to anneal the samples the phase transitions could be monitored directly. The formation of silicon polymorphs other than amorphous or metallic structures during hardness indentation is, to the best of our knowledge, reported here for the first time. The results compare well with the polymorphism in Si that is known from diamond anvil cell experiments. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364340

出版商:AIP    

年代:1997

数据来源: AIP