摘要:

Velocity interferometry diagnostics are used to monitor transmitted compressive shock and release waves in yttria (Y2O3)‐stabilized tetragonal and cubic zirconia. In tetragonal zirconia compressive yielding due to lattice slip or lattice instability is observed at 13–17 GPa and again at 30–31 GPa. This represents an additional fine structure to the dynamic compression of zirconia that has not previously been observed. A correlation with initial sample density is seen for the lower‐amplitude yield. A single yield over the range of the data occurs at 5–6 GPa for cubic zirconia. Approximately 4%–6% porosity in the cubic zirconia accounts for initial yield somewhat lower than earlier studies. From unloading wave profile measurements, initial release wave velocities at the Hugoniot state and stress derivatives of these data are determined. Release moduli data increase monotonically over a Hugoniot stress range of 11–45 GPa. A stress derivative of this data ofK’l= 1.85 is obtained. Stress derivatives determined directly from the initial release slope of individual profiles are significantly higher and suggest deformation or phase transformation kinetics on release. Release waves from Hugoniot states of 11–13 GPa suggest formation of a rarefaction shock and may imply a dynamic stress‐induced transformation not indicated by the shock profile. A tensile spall strength of 1.6 GPa was determined for the tetragonal zirconia. This high value may be related to the large fracture toughness of this transformation‐toughened ceramic.

ISSN:0021-8979    

DOI:10.1063/1.350631

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Diamond coatings were prepared on silicon substrates for fracture studies. Two thicknesses of coatings were evaluated: 6 and 12 &mgr;m. The diamond films increased the strength of the silicon for the same size fracture initiating crack and thus caused an apparent toughness increase from 1.2 MPam1/2for the uncoated silicon to about 1.6 MPam1/2for the 12 &mgr;m coated silicon. Fractography showed that the indentation impression on the coated surface was altered by the coating, but this did not alter the formation of the radial crack beneath the surface so that indentation fracture mechanics can be used for thin coatings with thicknesses below 12 &mgr;m. Fractography also showed that the coatings separated from the substrate under and near the indentation site, but was still intact away from the indentation. Most of the fracture in the diamond coating was transgranular indicating good intergranular adhesion.  

ISSN:0021-8979    

DOI:10.1063/1.350632

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Shock‐wave profiles of highly ordered pyrolytic graphite shocked normal to the basal plane of the graphite crystal structure have been measured. For graphite with sufficient orientational order a martensitic transformation to a diamond‐like phase is observed with a transition onset pressure 19.6±0.7 GPa, the stability limit of the graphite structure under shock compression. The minimum overpressure required for the transformation is not more than 6 GPa and the two‐wave structure of the transition is overdriven to a single wave above 40 GPa.

ISSN:0021-8979    

DOI:10.1063/1.350633

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The microstructure of lead titanate zirconate and PbTiO3‐based ceramics is investigated with thermal waves. The photoacoustic data obtained by a gas‐microphone method show the presence of thermal‐wave diffraction within the samples. The comparison with scanning electron micrographs allows the influence of the grain size and the volume of the glassy phase on the photoacoustic signal to be determined. The influence of the thermal parameters is also discussed. Analysis of the photoacoustic behavior of the investigated ceramic samples is made using the Opsal–Rosencwaig theory.

ISSN:0021-8979    

DOI:10.1063/1.350634

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

An investigation on the effects of the disorder of superlattice periods on x‐ray‐diffraction patterns is reported. Three kinds of disorder (random fluctuations, systematic deviations, and local deviations) have been considered. The results of the simulations show that their effects are quite different. The systematic deviation widens the superlattice peaks and shifts the positions of the low‐angle superlattice peaks. The effect of the systematic deviation on the reduction of peak intensity is more serious than that of random fluctuation. The local deviation makes the superlattice peak weaker and wider or splits it into double peaks. These three kinds of disorder can be distinguished.

ISSN:0021-8979    

DOI:10.1063/1.350635

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The elastic properties of sputter‐deposited Ag/Ni superlattices have been studied by means of the Brillouin light scattering technique. Measurements have been taken on both free‐standing and glass‐supported superlattices, the former deposited at 100 K, the latter at 300 K. The two effective elastic constantsc11andc44have been determined on specimens 5 &mgr;m thick for different superlattice periods;c44decreases by almost 40% as the bilayer periodicity decreases from 19 to 1.3 nm, whilec11shows no appreciable variation. Measurements performed on supported specimens, 0.5 &mgr;m thick, enabled us to get information also on the constantc33, which exhibits a monotonic increase with period. The elastic anomalies found can be related to the appreciable lattice distortion observed by x‐ray diffraction experiments. Both Brillouin scattering and x‐ray analysis have shown that the interface quality improves appreciably when the temperature of deposition is lowered from 300 to 100 K.

ISSN:0021-8979    

DOI:10.1063/1.350636

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Radio‐frequency sputtered barium titanium silicate (BST), Ba2Si2TiO8, thin films were grown on crystalline Si(100) substrates at substrate temperatures ranging from 750 to 955 °C and were characterized using x‐ray diffraction, optical microscopy, and scanning electron microscopy. The result of x‐ray diffraction analysis indicates that the BST films deposited at an optimum substrate temperature of 845 °C were stronglyc‐axis oriented. The corresponding film growth rate in the direction normal to the film surface and lateral grain growth rate were 1.95 nm/min and 0.77 &mgr;m/min, respectively, at the initial stage of deposition. The former decreased with sputtering time and the latter increased with grain size. The fast lateral grain growth rate indicates a strong interaction between the overgrown BST film and the Si substrate. The increase in lateral grain growth rate suggests a surface diffusion controlled nucleation and growth mechanism in the initial stage of the deposition, and a coalescence mechanism dominating in the later stage. The activation energy for lateral grain growth was 359±30 kJ/mol for 0.01 &mgr;m size grains, and decreased to 148±20 kJ/mol for 1 &mgr;m size grains, which is in good agreement with the proposed growth model.

ISSN:0021-8979    

DOI:10.1063/1.350637

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A series of investigations are presented which address various aspects of the growth, by molecular beam epitaxy, ofn‐type (Si doped)on‐axisGaAs/GaAs(111)B.In situcharacterization by reflection high‐energy electron diffraction has identified four surface phases on the static (zero growth rate) surface, and three reconstructions which occur, depending upon the substrate temperature, during growth. Then‐type doping properties of GaAs/GaAs(111)B epilayers have been compared withn‐GaAs/GaAs(100) structures. Hall effect and low‐temperature photoluminescence measurements have demonstrated that it is possible to dope GaAs/GaAs(111)B with Si in the 6×1014to 1018cm−3range. A variable growth temperature study is also presented which examines the surface structural, electrical, optical, and surface morphological properties ofn‐GaAs/GaAs(111)B grown in the 400 to 650 °C temperature range. The onset of electrical conduction, and optically active material, was found to be directly related to changes in the dynamic surface structure. The variable growth temperature study also revealed a temperature regime within which it was possible to significantly improve the surface morphology of on‐axis GaAs/GaAs(111)B structures whilst retaining good electrical and optical properties.

ISSN:0021-8979    

DOI:10.1063/1.350638

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Gas‐source molecular‐beam epitaxy (MBE) has been used to grow SiGe alloys with Si2H6and GeH4as sources on (100) Si substrates. Single‐crystalline epilayers with Ge composition as high as 33% have been produced at 610 °C, the lowest temperature hitherto used for gas‐source SiGe MBE. Growth parameters, growth modes, and the structural characteristics have been studied by a variety ofinsituandexsitutechniques. Double‐crystal x‐ray diffraction data for the alloys have been obtained for the first time in thin mismatched layers.

ISSN:0021-8979    

DOI:10.1063/1.350639

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The growth steps were observed on both {100} and {111} planes of diamonds prepared by a combustion‐flame deposition method using C2H2‐O2. The etch pits on {100} planes of diamonds were also observed with a scanning electron microscope. The orientations of both growth steps and etch pits on {100} planes were parallel to ⟨110⟩, whereas the step directions on {111} planes were [1¯1¯2]‐oriented and the lateral growth rate was the slowest in the [1¯1¯2] direction. The former is described with the hypothesis of so‐called (2×1) surface reconstruction, and the latter is successfully explained by a step reconstruction model. The occurrence of the surface reconstruction would indicate that not only atomic hydrogen but also OH radicals and atomic oxygen, which adequately exist in the diamond growing region in a flame, prevent the surface from being passivated or adsorbed by impinging atoms and/or radicals under the growing conditions.

ISSN:0021-8979    

DOI:10.1063/1.350640

出版商:AIP    

年代:1992

数据来源: AIP