摘要:

Intergranular cracks occurred in shock‐loaded molybdenum for specimens of grain size 17 &mgr;m and larger; for smaller grain size specimens (<17 &mgr;m) there was almost no crack initiation except in very thin specimens with shock loading of high pressure and long pulse duration. Cracks were not nucleated by twin barriers, and grain‐boundary ledges appear to be responsible for initiating and propagating the cracks. The crack density was observed to decrease with increasing grain size for both constant peak pressure and pulse duration. In addition, crack density, as measured by the crack length intersecting the unit area of surface, generally increased with increasing shock pressure at constant pulse duration and with increasing pulse duration, at constant shock pressure.

ISSN:0021-8979    

DOI:10.1063/1.326049

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

We discuss apparent characteristic features of Rayleigh scattering of elastic waves from cracks. Interpreting these features, we suggest a procedure that in some experimental situations may be useful to distinguish generally shaped cracks from volume defects and to determine for elliptically shaped cracks the orientation, shape, and size.

ISSN:0021-8979    

DOI:10.1063/1.326050

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

When a diamond indentor with a spherical tip is pressed against a diamond flat, the high pressures generated can be calculated using Hertz’s equation. By use of Newton’s ring technique it is shown that the pressure with a thin film present between the diamonds is essentially the same pressure as calculated for diamond‐to‐diamond contact using Hertz’s equation.

ISSN:0021-8979    

DOI:10.1063/1.325995

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Pressures up to 120 GPa were generated when a diamond indentor of radius 10.0 &mgr;m was pressed against a very thin sample of Al2O3on a diamond flat. The thin film of Al2O3was prepared by sputtering of aluminum in an oxygen atmosphere. From the measurement of the electrical resistance of Al2O3as a function of pressure it was found that Al2O3remains an insulator at the highest pressure studied, namely, 120 GPa.

ISSN:0021-8979    

DOI:10.1063/1.325996

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Methods are developed to treat the nonlinear propagation of edge dislocations in atomic lattices. Static or low‐velocity calculations incorporate Eshelby’s analytic solution of the elastic equations. At higher, even transonic velocities, a novel fixed‐displacement boundary is used to allow steady‐state propagation of edge dislocations. These techniques are applied to crystals with the triangular‐lattice structure. The dependence of the Peierls‐Nabarro strain and propagation velocity on the range of the forces is studied. Propagation at higher strains and nonlinear effects, transonic dislocation velocities and climb, are also described.

ISSN:0021-8979    

DOI:10.1063/1.325997

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

If one interprets the ALR absorption lines of crystalline polymers in terms of oscillating elastic rods of length equal to the thicknessLof the crystalline lamellae, or the thicknessDof the crystalline core of the lamellae, one obtains for the elastic modulus of these rods values which are substantially higher than the axial elastic modulusEcof the crystal lattice. The effect is particularly conspicous with polymers which in the crystalline state exhibit a helical‐chain conformation. The explanation of this effect seems to be the elastic coupling of longitudinal oscillations of chain stems of subsequent lamellae by the intervening amorphous layers. The apparent increase of the ALR oscillation frequency by such coupling is particularly large if the elastic modulusEaof the amorphous layers is equal or even higher than that of the crystal lattice. The high value ofEaderived from such an analysis is a consequence of the very high ALR frequency (∼0.3 THz), which makes the amorphous layer react as a rigid glass.

ISSN:0021-8979    

DOI:10.1063/1.325998

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The thermal expansion coefficients of the orthorhombica,b, andcaxes in langbeinite‐type K2Cd2(SO4)3are &agr;=24(2) ×10−6, 12(1) ×10−6, and 19(2) ×10−6deg−1, &bgr;=5(1) ×10−8, 8(1) ×10−8, and 5(1) ×10−8deg−2, &ggr;=2(1) ×10−10, −6(1) ×10−10, and 6(1) ×10−10deg−3, respectively, between 287 and 425 K. Above the first‐order phase transition atTc=432 K, &agr;=19(1) ×10−6deg−1. The sulfate tetrahedra angular rotations are less than 5° between 298 and 417.5 K but are as much as 20° between 417.5 and 443.5 K. Over 70% of the rotations are estimated to take place within 2–3 K ofTc. The room‐temperature dielectric constants &egr;1, &egr;2, and &egr;3are about 8, 8, and 16, respectively, at 1 kHz: a jump of about 5% occurs atTc. Two spontaneous strains decrease markedly asTcis approached, but all three remain greater than 2×10−3. The ferroelastic displacements decrease by as much as 0.25 A˚ between 298 and 417.5 K to values ranging from 0.22 to 0.91 A˚. The absolute sense of the piezoelectric shear coefficientd14has been measured as positive, i.e., a tensile stress along [011] generates positive polarity on (100), negative on (1¯00). The magnitude ofd14is about 7.3(5) ×10−12C N−1, the electromechanical coupling factork14is 0.124(5), and the elastic compliances11is 1.72(2) ×10−11m2 N−1.

ISSN:0021-8979    

DOI:10.1063/1.325999

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A microstructural evaluation was made of experimental non‐Ohmic ZnO ceramics by optical, scanning, and transmission electron microscopy complemented with electron beam microprobe and x‐ray analyses. Four crystalline phases were identified: a major phase of ZnO grains containing Co and Mn in solid solution and three minor phases of bismuth silicate (12 Bi2O3⋅2SiO2), pyrochlore (Bi2[Zn4/3Sb2/3]O6) and spinel (Zn7Sb2O12) in the intergranular region. Transmission electron microscopy revealed that the ZnO grains are mostly in direct contact with each other and that the intergranular phase is located primarily in the multiple grain junctions. In those regions where ZnO grains are in contact with each other, the measured grain‐boundary width was ?25 A˚. It is suggested that the barrier to conduction in ZnO varistors resides in the vicinity of the grain boundary within the ZnO grains and not in the intergranular insulating layer.

ISSN:0021-8979    

DOI:10.1063/1.326000

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A strain effect has been found to explain the anomalous reduction in P diffusivity at surface concentrations greater than ∼4×1020cm−3. It is shown that at diffusion temperatures, misfit‐induced strain causes a reduction in the effective Si band gap up to ∼−130 meV at the solubility limit of P. This band‐gap narrowing results in reduced P diffusivity through a relative reduction of P+V=pairs in the surface region. This complex is the dominant species for P transport at high P concentrations. The diffusion of P in the tail region is dominated byV−vacancies liberated during P+V=pair dissociation events. Thus, the tail diffusivity will likewise be reduced by band‐gap narrowing in the surface region. These reductions in diffusivity can be by as much as a factor of 6 depending upon temperature and P doping.

ISSN:0021-8979    

DOI:10.1063/1.326001

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The crystal quality of CdTe layers grown on single‐crystalline CdS by chemical vapor deposition is evaluated. Different x‐ray techniques have been applied to investigate the crystal perfection and defects. Good CdTe layers grow in the sphalerite phase with strong twinning about the [111¯] growth direction. No mixture of cubic and hexagonal phases, polytypes, stacking faults, and fiber texture are observed. Imperfect layers contain polycrystalline areas.

ISSN:0021-8979    

DOI:10.1063/1.326002

出版商:AIP    

年代:1979

数据来源: AIP