摘要:

Data for the diffusion of ethane in polyethylene are presented together with the results of optical microscopy and of large‐ and small‐angle x‐ray diffraction for the same samples. The orientation of the lamella boundaries with respect to the concentration gradient of the diffusing molecules is important in determining the diffusion rate. In unoriented, as‐crystallized polymers, the diffusion constant varies inversely with the lamella thickness. The results support the concept of lamella boundary diffusion. Lamella perfection is also important. In both oriented and unoriented polymers, the solubility constant apparently varies inversely with the lamella thickness, suggesting absorption in the lamella boundaries.

ISSN:0021-8979    

DOI:10.1063/1.1713829

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

A unique fracture of electrostrictive origin has been observed in ceramic barium titanate subjected to damped high‐voltage oscillations. Such a nonexplosive method of producing impulse loading with an internal variable time marker makes this an attractive material in which to study fracture propagation.

ISSN:0021-8979    

DOI:10.1063/1.1713830

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The redistribution of solute during phase transformations where solute partitioning occurs at the two‐phase interface has been mathematically treated for (i) the special case of equal diffusion rate in both the liquid and solid and (ii) diffusion only in the liquid under the additional force of a constant electric field. This one‐dimensional analysis shows that (i) diffusion in the solid has a small effect on the grown‐in solute distribution fork0<1 but an appreciable effect fork0>1 and (ii) the presence of an electric field can significantly alter the grown‐in solute distribution. Further, one can deduce from the latter analysis a new experimental method for determining the effective ionic mobility of the solute species in the liquid phase.

ISSN:0021-8979    

DOI:10.1063/1.1713831

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The technique of double evaporation of CdS and S was investigated. A significant increase of resistivity to 107&OHgr;·cm, and the onset of space‐charge‐limited current were observed in films made by this technique. It is hypothesized that the improvement noted compared with films from a single source evaporation of CdS is due to the reduction in number of sulfur vacancies.

ISSN:0021-8979    

DOI:10.1063/1.1713832

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The entropy factor and effective jump frequency for the thermally activated unpinning of dislocations are computed by use of the statistical mechanics treatment of absolute rate theory. Three problems are considered in detail: The first is a model problem of three mass points, the second a dislocation line of length 2lpinned by an impurity at its center, and third a dislocation line pinned uniformly over its length. The unique feature about the theoretical treatment of the diffusion of dislocations in crystals, as contrasted with the diffusion of point defects, is that good quantitative approximations for the frequencies of the system in both the ground state and the activated state can be obtained from the vibrating string model. The approximations of this model are good for the lowest frequencies, which turn out to be the important ones for the processes considered. The entropy factor is very large for long loop lengths. For a typical case of a binding energy of110eV, the frequency factor for a dislocation pinned at its center is the order of 5×1010cps, independent of the loop length or of the attack frequency. For a continuously pinned dislocation, the effective frequency is greater than the Debye frequency.

ISSN:0021-8979    

DOI:10.1063/1.1713833

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Measurements of the scattering by crystals of NaCl, KCl, and KBr, containing ≈10−6divalent impurity concentration, at &lgr;4358 Å are reported for the temperature range 200°–1000°K. The results for equilibrium heat treatments are entirely reversible, but not for quench treatments. The equilibrium results are consistent with the predictions of the theory of charged dislocations and scattering theory for defect clouds associated with the dislocations. Formation energies for anion and cation vacancies are obtained, viz.: NaCl,g+=0.6,g−=1.4; KCl,g+=0.65,g−=1.75; KBr,g+=0.80,g−=1.40, energies in eV. Precipitation of impurities at dislocations occurs atT=273°, 213°, 223°K for NaCl, KCl, and KBr, respectively.

ISSN:0021-8979    

DOI:10.1063/1.1713834

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

&tgr;0, the stress necessary to produce a plastic strain of 2×10−6, has been measured with the use of electrical resistance strain gauges in neutron‐irradiated copper single crystals as a function of neutron dose (∼1016to 1020neutrons/cm2) and testing temperature (77° and 300°K). &tgr;0increases from about 0.3 kg/mm2for ∼1016neutrons/cm2to over 5 kg/mm2for ∼1020neutrons/cm2; in annealed copper crystals, &tgr;0is ∼0.02 kg/mm2(previously determined by other researchers). At all neutron doses, &tgr;0is about 75% of the macroscopic flow stress. &tgr;0is observed to increase with the cube root of the neutron dose; the macroscopic flow stress shows the same dose dependence.The results are explained on the basis of dislocation generation and obstacle cutting mechanisms controlling the yielding. These mechanisms are thought to be simultaneously operative.

ISSN:0021-8979    

DOI:10.1063/1.1713835

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Elongated damage structures resulting from fission fragments traveling nearly parallel to the surface have been observed in (100) cleavage faces of bulk LiF crystals. These tracks have lengths up to 2.3 &mgr; and are about 100 Å wide. The direction in which the fission fragment has entered the LiF crystal can be determined from the characteristics of tracks which cross typical fracture steps in the crystal surface. When the fragment enters the surface it registers a surface disturbance in the form of a trough defined by two ridges. Deeper in the crystal the fragment produces a single ridge and finally ceases to register a surface disturbance when it has penetrated more than about 30 Å into the crystal. Track length distributions have been determined from a large number of tracks for two specimens. These show good agreement with a theoretical distribution based on straightforward geometrical considerations. Details of the theoretical development are outlined in an appendix. There are, however, a disproportionately large number of tracks visible in <100> crystallographic directions. This would be explained if fragments registered surface disturbances from a greater depth in these particular directions.

ISSN:0021-8979    

DOI:10.1063/1.1713836

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The generation, due to glide motion of dislocations, of the second harmonic of an ultrasonic stress or strain wave propagating in a crystal is studied by using the string model of dislocations in the presence of a static bias stress. The calculated results agree semiquantitatively with the experiment by Hikata, Chick, and Elbaum. The dependence of the amplitude of the second harmonic on bias stress, when this stress is sufficient to cause dislocation unpinning, can be accounted for by the dislocation damping effect that is associated with the anomalous velocity change due to unpinning.

ISSN:0021-8979    

DOI:10.1063/1.1713837

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

An analysis of the flow of polymer melts as well as of melt fracture at high shear rates has been carried out. On the basis of the flow properties of polyethylene, polypropylene, polystyrene, polymethylmetacrylate melts, and silicone rubbers measured over a wide range of shear rates from 10−3up to 103sec−1, the following conclusions have been drawn: Under low rates of shear the behavior of polymer melts is of Newtonian nature, whereas at high rates the character of flow depends on the value of the dimensionless criterion&kgr;=&tgr;/&eegr;N&ggr;˙=&kgr;3&tgr;·R/&eegr;N·V, where &tgr; is shear stress&ggr;˙is shear rate, &eegr;Nis Newtonian viscosity,Ris characteristic dimension, andVis mean velocity of flow. The fracture of polymer melts occurs above a critical constant value of the number &kgr;: &kgr;crit=0.025. Available flow data, analysed by the method of reduced variables, show that most previously suggested criteria are essentially equivalent. This treatment eliminates their restricted validity and technological applicability and brings the description of fracture close to molecular theories of non‐Newtonian flow.

ISSN:0021-8979    

DOI:10.1063/1.1713838

出版商:AIP    

年代:1964

数据来源: AIP