摘要:

The possibility of using an ultrasonic oblique‐incidence technique for improved sensitivity in interfacial weakness detection in adhesive bonds was considered. The analysis of the sensitivity of transverse waves to interface imperfections on an interface between two semi‐infinite half‐spaces with bond rigidities of various finite values was carried out. Excellent sensitivity is obtained for discriminating bond weaknesses from a perfect or welded situation to a poor or smooth situation compared to longitudinal wave impingement and reflection. The optimal angle of incidence for the inspection of an aluminum‐to‐aluminum adhesively bonded structure in an immersion technique is presented. Additionally, a signal feature in frequency domain for adhesive and cohesive weakness distinction is proposed.

ISSN:0021-8979    

DOI:10.1063/1.340294

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Fluctuations in plasma potential bring about an apparent broadening in the ion energy distribution when it is derived from theI‐Vcharacteristic of a retarding grid energy analyzer. This effect may have detrimental consequences for the prospects of applying energy analyzers to measure diffusion of the background distribution when electrostatic turbulence is present. The effect is demonstrated by inducing spatially homogeneous oscillations of the plasma potential in a beam‐plasma system, applying a directional ion energy analyzer for beam detection. A theoretical analysis is presented, and quantitative agreement with the experimental results is obtained.

ISSN:0021-8979    

DOI:10.1063/1.341128

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We have obtained atomic‐resolution images of NbSe2single crystals in air. Constant‐height images clearly show the expected atomic structure, and can distinguish the two inequivalent halves of the unit cell. Constant‐current images show an anomalously high atomic corrugation, associated with elastic deformation of the sample. Surface contamination probably plays an important role in transmitting the tip‐sample forces. A larger‐scale apparent buckling of the surface with a period of several times the atomic spacing is sometimes observed.

ISSN:0021-8979    

DOI:10.1063/1.340295

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Single crystals of an Al‐Mn superlattice have been grown by molecular‐beam epitaxy. For thicknesses less than the critical thicknessdc&bartil;30 A˚, Mn atoms form the fcc lattice in the [111] direction of an Al surface grown on sapphire‐C substrate. The modulated structure of the Mn surface has been observedinsituby reflection high‐energy electron diffraction. On the other hand, for thicknesses more thandc, Mn grows in the [110] direction of the bcc lattice (&agr; phase). After annealing a compression of the lattice has been observed by x‐ray diffraction. The annealing has produced well‐oriented alloys with fairly large domain sizes.

ISSN:0021-8979    

DOI:10.1063/1.340296

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The interaction between iron and defects produced by electron irradiation in iron‐doped silicon has been observed by using deep‐level transient spectroscopy. A new electron trap level due to the iron‐vacancy‐oxygen complex [Fe+(V⋅O)] was detected atEc−0.36 eV. The role of theAcenter (V⋅O) in the formation of the complex was confirmed by the observation that during low‐temperature annealing (<150 °C) the increases in the concentration of the [Fe+(V⋅O)] complex was accompanied by an equal and opposite change in the concentration of theAcenter. Besides the interaction with theAcenter, iron was found to influence the annealing behavior of the divacancy andEcenter.

ISSN:0021-8979    

DOI:10.1063/1.340297

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Elastoplastic properties of Al2O3single crystal (14–38 GPa) and polycrystal (6–33 GPa) under shock compression were studied by the measurements of Hugoniot compression curves and stress histories. The Hugoniot‐elastic limit (HEL) stresses were determined to be 14.4–17.3 GPa for single crystals and 6.7–9.6 GPa for polycrystals with 2.7%–3.2% porosity. In the plastic region, the single‐crystal Hugoniot stresses, at a given density, exceed the isothermal x‐ray static data and its extrapolation by 6.5–8 GPa. And, the polycrystalline Hugoniot data exceeds, at a given density, the single‐crystal Hugoniot data by about 1 GPa. These Hugoniot offsets are not caused by temperature increase, but rather by the shear strength and the remaining porosity. The stress histories of a single crystal show very similar shapes to that of a perfect elastoplastic material. The apparent yield stresses of single crystal at the HEL and in the plastic region are estimated to be 9.5–12 GPa and 4–6 GPa, from the HEL data and the quasirarefaction shock in the stress histories, respectively. As a result, it is concluded that both Al2O3single crystal and polycrystal behave as quasi‐elastoplastic material with some loss of shear strength, unlike many other brittle materials (elastoisotropic material with large catastrophic loss) such as Si, SiO2, etc. Furthermore, the present results suggest that the packing state and bonding state of crystal should be also ranked among the factors deciding the heterogeneous shock yielding of brittle materials, in addition to the thermophysical properties, which are related to the shear banding phenomenon or the other yielding mechanism.

ISSN:0021-8979    

DOI:10.1063/1.340298

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

It is shown through numerical simulations that the gap between performed fragments closes during explosive launch, trapping the detonation products until radial expansion of the fragments is sufficient to separate the fragments. Circumferential strains from the numerical calculations are in good agreement with the plastic strains from recovered fragments. Additionally, considerable insights into the dynamics of the explosive launch process are obtained by comparing spall failure and nucleation and growth of ductile voids in the recovered fragments against the numerical simulations.

ISSN:0021-8979    

DOI:10.1063/1.341150

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The response of a transverse piezoresistance gauge under plane shock wave loading is different than that of a longitudinal piezoresistance gauge. For a given combination of gauge and embedding matrix the response of the transverse gauge can be higher than that of the longitudinal gauge in a certain range of shock stresses, and lower at a different stress range. This ‘‘crossover phenomenon’’ defines a certain stress level (&sgr;*), for which the response of the two gauges is equal. In this paper we analyze this crossover of the gauge’s responses analytically. The analysis accounts for the crossover stress in terms of the yield strength of gauge material and the deviatoric stresses in the embedding matrix. We show that when the yield strength of the gauge is higher than that of the matrix, there is no crossover of the gauge’s response. Experimental results are presented to support the analysis.

ISSN:0021-8979    

DOI:10.1063/1.340246

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Photoinduced transient spectroscopy (PITS) and photoluminescence spectroscopy measurements were carried out on samples of liquid‐encapsulated Czochralski‐grown semi‐insulating GaAs which have been diffused with copper at 550 °C. The PITS curves, taken at different depths from the surface, confirm that the copper‐related acceptor level at 0.51 eV is due to a complex, which we assign to CuGaVAs. The PITS results show that an acceptor level at 0.51 eV in GaAs, previously assigned to iron impurity, can also be caused by copper. Photoluminescence studies show that the emission peak at 1.36 eV, commonly attributed to CuGa, also varies with depth. This variation is explained by the predominance of copper in the form Cu+inear the copper‐covered surface. We have shown, from both PITS and photoluminesence results, the significance of the capping effect of a copper film during annealing, and this effect must be taken into account in the analysis of copper diffusion data in GaAs.

ISSN:0021-8979    

DOI:10.1063/1.341148

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Fast real‐time ellipsometry is used to studyinsitu, as a function of the substrate, the growth of the first tens of monolayers of hydrogenated amorphous silicon (a‐Si:H) deposited by a rf glow discharge of SiH4. The high sensitivity of this technique is illustrated and the early stage of the growth is found to strongly depend upon the nature of the substrate. A nucleation mechanism followed by incomplete coalescence is observed on metal and hydrogenated amorphous germanium (a‐Ge:H) substrates. On the contrary, fused silica (SiO2) and tin dioxide (SnO2) are superficially reduced:  this reduction creates at the interface a mixed layer ofa‐Si:H and silicon oxide on the silica substrate, and produces elemental tin at the surface of the SnO2substrate. In this last case, tin is found to diffuse in the furthera‐Si:H growing film. On crystalline silicon (c‐Si), thea‐Si:H growth shows incomplete coalescence followed by homogeneous growth, probably together with the reduction of the nativec‐Si oxide layer.

ISSN:0021-8979    

DOI:10.1063/1.340247

出版商:AIP    

年代:1988

数据来源: AIP