摘要:

A discussion is given of the processes by which atoms are displaced from their lattice sites by incident fast electrons. In addition to direct displacement by the incident electron, expressions are derived for the fraction of atoms displaced by fast moving atoms and by fast secondary electrons, both of which are generated initially by the incident electron. A discussion of thermal spikes is given. A thermal spike process is discussed that might lead to the generation of defects with x irradiation in materials that exhibit a threshold energy for a displacement by a charged particle.

ISSN:0021-8979    

DOI:10.1063/1.1735653

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

X‐ray examinations of brittle fractures in polycrystalline chromium, produced at temperatures between 193°K and 623°K, have been made. Estimates of the amount of deformation present at the fracture surfaces showed that very large plastic strains were associated with brittle cleavage fracture at the higher temperatures, such that the length of a self‐sustaining crack calculated according to the modified Griffith crack formula of Hall, was more than an order greater than the mean grain diameter of the material. Plastic strains of this order would be expected to result in ductile behavior. It is suggested that brittle failure was caused by the presence of a fine dispersion of precipitate particles, which delayed the action of cross slip in sealing off the early stages of cracking. The spacing of the precipitate particles was estimated to be between 6000 and 7000 A.

ISSN:0021-8979    

DOI:10.1063/1.1735654

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

Experimental observations are presented of dislocation multiplication, of the defect structure left behind by a moving dislocation, and of cross‐glide of individual dislocations in LiF crystals. New dislocation loops form at many different sites in the wake of a moving dislocation. These loops have the same Burgers vector as the parent dislocation but do not, in general, lie on the same atomic plane. The rate of formation of new loops depends upon the magnitude of the applied stress. Such creation of new loops leads eventually to the formation of a wide glide band.A moving screw dislocation trails many line defects behind it that lie parallel to its direction of motion. The existence and nature of these trails and the observed dislocation multiplication can be explained in terms of a mechanism which involves the formation, by cross‐glide, of jogs on a screw dislocation. This cross‐glide multiplication mechanism was originally proposed by Orowan and by Koehler. It is demonstrated that cross glide occurs easily in LiF, so that this mechanism is plausible. Some interesting complications arise when jogs are formed that are longer than several atomic spacings but less than several hundred. The defect trails exert a dragging of the screw dislocations that is not negligible compared to the yield stress of a crystal.

ISSN:0021-8979    

DOI:10.1063/1.1735655

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

The mobilities of positive ions in H2and D2have been measured, using a pulsed Townsend technique. For hydrogen, the mobility &mgr;0corrected to 0°C was 11.8 (cm2/v×sec) atE/p0=26(v/sec×mm Hg), increasing to a maximum of 15.3 atE/p0=48, then decreasing to 11.6 atE/p0=150. For deuterium, the mobility was about 0.75 the value for hydrogen throughout the range ofE/p0. The experiment was performed with uranium‐purified gas in an ultra‐high vacuum system. Ions were not identified as atomic, diatomic, or triatomic; there was no conclusive evidence of more than one ion at any value ofE/p0. The H2mobility data are significantly lower than those attributed to Mitchell in the range 20<E/p0<40, and lower than the value ≈12.5 for zero‐field mobility generally reported.

ISSN:0021-8979    

DOI:10.1063/1.1735656

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

The five independent elastic constants for yttrium single crystals have been determined by the pulse‐echo technique over the temperature range, 4.2–400°K. The experimental values extrapolated to 0°K are:C11=8.34±0.02,C33=8.01±0.02,C44=2.690±0.006,C12=2.91±0.03, andC13=1.9±0.4 in units of 1011d/cm2. The degree of elastic anisotropy is low throughout the entire temperature range. The curves illustrating the temperature dependence of the elastic constants show several inflections, and the curve forC11crosses that forC33near 390°K. The behavior is somewhat unusual, and is probably explicable on the basis that the degree of band overlap in yttrium is quite sensitive to interatomic spacing. Such a sensitivity has previously been postulated to explain the temperature dependence of the Hall coefficient.

ISSN:0021-8979    

DOI:10.1063/1.1735657

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

The optical absorption by free carriers in a semiconductor containing a dispersed colloidal phase is treated, taking into account the dipolar diffusion of minority carriers about the particles.

ISSN:0021-8979    

DOI:10.1063/1.1735658

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

A correlation is reported between certain optical extinction bands in sapphire, quartz, the silver and alkali halides, and the calculated plasma resonance of electrons in colloidal particles which are either known or expected to be present. This correlation suggests not only that pile neutrons produce lithium particles in lithium fluoride, but aluminum particles in sapphire, and silicon particles in quartz.

ISSN:0021-8979    

DOI:10.1063/1.1735659

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

The solutions of the equations of space charge flow of electron beams for radial flow between concentric spherical electrodes are formulated in such a manner as to permit the introduction of arbitrary initial conditions. This allows one to solve a variety of physical problems with the aid of a table of Airy functions. A particular case, which exhibits a potential minimum between electrodes, is presented as an illustrative example.

ISSN:0021-8979    

DOI:10.1063/1.1735660

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

The attenuation of the plane shock wave produced in a solid by a flying plate of the same material is treated neglecting dissipative processes and effects of material rigidity. Explicit formulas for the position of the shock front and the shape of the pulse as functions of time are obtained by application of Friedrich's method. A numerical example for an aluminum target and projectile is presented to illustrate some of the features of the calculation, and an experiment is proposed to test the theory. The experiment should also allow a reasonably precise measurement of sound velocity immediately behind the shock front to be obtained. This possibility applies equally well to the case of target and projectile of different materials.

ISSN:0021-8979    

DOI:10.1063/1.1735661

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

The domain dynamics of polarization reversal in metal electroded BaTiO3single crystals have not been adequately described in the literature. The present paper contains quantitative measurements of the velocity of the sidewise motion of 180° domain‐walls in BaTiO3crystals, most of which were electroded with 200‐A thick films of Au. The wall velocity has been measured as a function of several variables including the applied electric field, the crystal thickness and the impurity content added to the crystal growth melts.The wall velocity as a function of the fieldEis described byv∞exp (−&dgr;/E) where &dgr; andv∞are constants over several decades of velocity. The quantities &dgr; andv∞are the activation field and extrapolated wall velocity forE=∞, respectively. This field dependence of the wall velocity is the same as that reported previously for liquid electroded specimens. The thickness dependence of the wall velocity, which is through &dgr;, is very pronounced and similar to that observed in measurements of other variables under different experimental conditions. The present data are explained in terms of a surface layer estimated to be of the order of 100‐A thick. It is suggested that high fields within the surface layer may give rise to electron field emission and the electroluminescence which is observed during polarization reversal. Effects on the wall motion which persist for times of the order of minutes are ascribed to dielectric relaxation phenomena. The observed dependence of the wall motion on the impurity content of the cyrstal, and several other parameters, may result from changes in the characteristics of the surface layer.

ISSN:0021-8979    

DOI:10.1063/1.1735663

出版商:AIP    

年代:1960

数据来源: AIP