摘要:

Structure and morphology of polycrystalline diamond films prepared by chemical vapor deposition (CVD) have been studied using x‐ray texture analysis, angle‐resolved optical reflection, and scanning electron microscopy. The films under investigation exhibit a pronounced 110 fiber texture, i.e., a preferential alignment of {110} planes perpendicular to the growth direction. By thinning a 180‐&mgr;m‐thick CVD diamond film in an oxygen discharge the dependence of the degree of 110 texture on the film thickness has been investigated. It was found that the crystals formed at the beginning of the film growth are randomly oriented, and that a preferential orientation of {110} planes develops with increasing film thickness. Computer simulations show that this behavior can be explained by evolutionary selection, i.e., competing growth of differently oriented crystals, which implies that ⟨110⟩ is the direction of fastest growth. In addition, angle‐resolved optical reflection and scanning electron micrographs show that the surface of the diamond films consists of {111} faces. Possible explanations for these findings are discussed.

ISSN:0021-8979    

DOI:10.1063/1.346663

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The dislocation pile‐up avalanche model is used to explain the crystal size dependence for hot spot‐controlled initiation of chemical decomposition in cyclotrimethylenetrinitramine crystals subjected to drop‐weight impact testing. Deformation‐induced temperature rises, hot spot sizes, and lifetimes are related to previously reported values for direct thermal decomposition. A reasonable chemical reaction yield is estimated from available kinetic data.

ISSN:0021-8979    

DOI:10.1063/1.346664

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The transition character of paraelectric‐to‐ferroelectric phase transition and its relation to lattice soft‐mode behavior in the solid solutions of (Ba,Sr)TiO3, (Ba,Ca)TiO3, Ba(Ti,Zr)O3, and Ba(Ti,Sn)O3were investigated by the measurements of dielectric constants in the temperature range from −30 to 150 °C, and x‐ray line breadth of 100, 110, and 111 Bragg reflections at temperatures above the transition point. From the result of dielectric measurement, it was found that the specimens containing Ti‐site substituents have shown a diffuse phase transition character, whereas those having Ba‐site substituents have preserved a sharp phase transition. It was also noted from the change of line breath of Bragg reflections that for the solutions of Ba(Ti,Zr)O3and Ba(Ti,Sn)O3, the high dynamic strain at the ⟨1 0 0⟩ axis, induced from the soft‐mode of lattice vibration, become repressed when a large amount of solutes is contained, but for the solutions of (Ba,Sr)TiO3and (Ba,Ca)TiO3, it is recovered at temperatures near the transition point. Moreover, the dynamic strain at the ⟨1 1 1⟩ axis can be rapidly repressed by the addition of Ti‐site substituents. The different effects on the dynamic strains are attributed to the disparate distortions of oxygen octahedra in the perovskite lattice, induced by the solutes of different site substitution. It is also believed that the influence on the soft‐mode of lattice vibration has caused the different phase transition characters of the four BaTiO3solid solutions.

ISSN:0021-8979    

DOI:10.1063/1.346665

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Three numerical methods have been developed to model the equilibrium distribution of fluid phases in a multiphase saturated porous medium. The basic assumption made is that the distribution of phases is governed by the static interfacial free energy of the system, such that the equilibrium phase distribution corresponds to a minimum in the total interfacial free energy of the system. The example of determining the distribution of water vapor and liquid water in 2D numerical models of the pore space in a rock is considered. Starting with a numerical model of the pore space, the objective of each method is to obtain the minimum energy configuration of water vapor and liquid water in the pore space for some set level of water saturation. Two of the methods are simple and computationally fast methods that can produce fluid distributions close to, or matching, the equilibrium configuration. These methods can, however, produce metastable configurations due to the simplistic nature of the algorithms. The third method applied to this problem is a simulated annealing method. This method consistently produced the lowest possible energy configuration. It is concluded that simulated annealing can be successfully used to numerically model fluid distribution in multiphase saturated porous media.

ISSN:0021-8979    

DOI:10.1063/1.346666

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The permeation of oxygen through Ag normally occurs by a sequence of steps which include the initial dissociative adsorption of molecular oxygen at the upstream surface, the dissolution of the atoms into the bulk, and the subsequent migration of the atoms between octahedral sites of the lattice until they arrive at the vacuum interface downstream. The dissociative adsorption step, however, proceeds slowly, as indicated by the low sticking coefficient of O2on Ag(10−6–10−3). The application of a dc field in 0.5 Torr of O2(E/n∼10−14V cm2) on the upstream side of a Ag membrane generated gas phase atomic oxygen that substantially enhanced the transport. The transport flux was observed to increase from a value of 4.4×1013cm−2 s−1to a glow discharge value of 2.83×1014cm−2s−1at a membrane temperature of 650 °C. This suggests that the dissociative adsorption step limits the supply of oxygen atoms to the upstream side of the membrane. When the upstream O2was replaced by an equal pressure of CO2, only a small permeation signal was observed, but the application of the glow discharge substantially increased the transport flux from 3.25×1012cm−2s−1to 1.74×1014cm−2s−1. This method of separating O2from a CO2environment may be a possible mechanism for providing a supply of oxygen for astronauts in a manned mission to Mars.

ISSN:0021-8979    

DOI:10.1063/1.346734

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In order to elucidate the diffusion mechanism of hydrogen in post‐plasma‐hydrogenation of amorphous silicon (a‐Si) film prepared by chemical vapor deposition (CVD), the change in the hydrogen depth profiles with plasma exposure time and with successive hydrogenation of hydrogen isotopes were measured by secondary ion mass spectrometry and infrared absorption. The post‐hydrogenation process of the CVDa‐Si film is explained by a model composed of fast diffusion (small activation energy) of atomic hydrogen through weakly bound sites such as interstitials, its capture by reactive sites such as weak SiSi bonds and dangling bonds, and an exchange between weakly bound and bonded hydrogens.

ISSN:0021-8979    

DOI:10.1063/1.346735

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A dynamic approach is presented for determining a set of equivalent elastic, piezoelectric, and permittivity constants for a superlattice consisting of a periodic multilayer of anisotropic materials. The objective is to predict the acoustic properties of such a periodic structure in the long‐wavelength (low‐frequency) limit. It is shown that the acoustic wave propagation properties for propagation in any plane defined by an arbitrary direction and the superlattice axis are contained in a 6×6 system matrix requiring only a subset of at most 29 material constants (from a total of 46) for its construction. The computation of an equivalent system matrix for a superlattice is demonstrated and the manner of obtaining the equivalent density, six permittivities, 21 stiffness, and 18 piezoelectric constants is given.

ISSN:0021-8979    

DOI:10.1063/1.346736

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Heavily Zn‐doped epitaxial layers have been grown on In‐doped GaAs substrates by organometallic vapor phase epitaxy in order to investigate the strain relaxation mechanism and hence the nucleation and propagation of misfit dislocations. Samples doped with Zn higher than 3×1020cm−3still show a perfectly strained state beyond the coherent length obtained for undoped GaAs epilayers. Hence, epilayers having a higher Zn concentration seem to have a higher activation barrier for nucleation and propagation of misfit dislocations. Furthermore, for the Zn‐doped/undoped/substrate(In‐doped) double‐epilayer structure, the elastic strain in the heavily Zn‐doped layer is found to be unrelaxed, regardless of the increase in the thickness of the Zn‐doped upper layer. This result suggests dislocation generation at the growing surface and glide along the {111} slip planes into the heterointerface.

ISSN:0021-8979    

DOI:10.1063/1.346737

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Diamondlikea‐C:H films have been deposited by decomposition of C2H2. By the combination of Brillouin scattering and the ultralow load indentation technique, the following mechanical properties of the films were studied: the microhardnessH, the Young’s modulusE, the shear modulus &mgr;, and the Poisson’s ratio &ngr;. From the measured data of mass density, hydrogen concentration, and the ratio ofsp3/sp2bonds the filling factors of these films were calculated. The results show that the hardness, Young’s modulus, and shear modulus of these films depend strongly on the volume concentration of voids and are directly proportional to the filling factor.

ISSN:0021-8979    

DOI:10.1063/1.346738

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Epilayers of Cd1−xMnxTe grown on CdTe substrates by liquid‐phase epitaxy (LPE) are characterized by x‐ray, modulated (piezo‐ or photo‐) reflectivity, and Raman spectroscopy. The epilayers are found to be of excellent structural quality, having sharp excitonic signatures in the reflectivity spectra and highly uniform Mn concentrations. Intentional introduction of In donors during LPE growth of Cd1−xMnxTe has been verified by spin‐flip Raman signals which show large Raman shifts with a Brillouin function behavior characteristic of donors in diluted magnetic semiconductors. During the LPE growth we also encountered the formation of large‐area, free‐standing platelets of Cd1−xMnxTe. They have been similarly characterized with x‐rays, as well as modulation and Raman spectroscopy.

ISSN:0021-8979    

DOI:10.1063/1.346739

出版商:AIP    

年代:1990

数据来源: AIP