摘要:

A systematic study of impurity segregation in spherulitic crystallization is described. The experiments deal principally with high polymers, in which the role of ``impurities'' is fulfilled by stereoirregular molecules or by molecules of low molecular weight. It is shown that these species are rejected preferentially by growing crystals and that their diffusion plays a vital part in governing over‐all morphology. In particular, openness of texture is related to the concentration of impurity present; and coarseness of texture, which is a measure of the ``diameters'' of crystalline fibers between which impurities become concentrated during crystallization, is determined by &dgr;=D/G, whereDis the diffusion coefficient in the melt andGis the radial growth rate of the spherulite. Results provide substantial support for a theory of spherulitic crystallization proposed by the authors.

ISSN:0021-8979    

DOI:10.1063/1.1713606

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The influence of impurity segregation on the kinetics of spherulitic crystallization is examined. Radial growth rates over a range of temperatures are reported for a number of polymers containing various proportions of impurity species of widely varying molecular weight. Variations of growth rate with molecular weight indicate that transport processes within regions extending some distance from solid‐liquid interfaces play a major part in controlling growth kinetics. In the unusual case of a polymer containing impurities of very low molecular weight, extended radial diffusion of impurities may even give rise to parabolic growth [(radius) ∝ (time)½] in place of linear growth as normally found. Conventional interpretations fail to account satisfactorily for dependence on molecular weight, and are clearly inadequate in that they deal with transport processes only as ``jump'' processesatthese interfaces. Complications that transport processes within the body of the melt introduce into the problem of formulating an absolute theory of growth kinetics are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1713607

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Theoretical consideration is given to the equation governing the shapes of molten zones when they are not necessarily surfaces of revolution. This equation is seen to reduce to equations previously given for both vertical and horizontal molten zones when the shape is a surface of revolution. Solution of the equation governing the shape of the vertical molten zone when it is a surface of revolution gives corrected values for the maximum stable zone lengths for large rod radii. Detail consideration is given to the influence of a surface film on vertical zone stability.

ISSN:0021-8979    

DOI:10.1063/1.1713608

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

For recent experimental results on thin superconductive films, the maximum average energy density as a macroscopic criterion of superconductivity is in better agreement with observed behavior than are any of the earlier criteria—maximum magnetic field, maximum current density, or maximum energy density. The earlier criteria are briefly reviewed, and characteristic formulas and critical curves are applied to an example for comparison with the proposed criterion. To determine the full significance of the proposed criterion, further experimental and theoretical work is needed.

ISSN:0021-8979    

DOI:10.1063/1.1713609

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The recent increase of activity in the field of slow‐electron diffraction has resulted in a need for agreement on conventions of terminology in the description of a diperiodicsurface structurerelative to a triperiodicsubstrate. Vector description of the surface structurenetin terms of the substrate surface net is suggested. Fractional Miller indices result when the surface structuremeshis larger than the substrate surface mesh. The surface structure may be either that of a deposit or that of theselvedgeof the substrate.With the relaxation of one of the diffraction conditions in the diperiodic structure the reciprocal lattice becomes a set of parallel rods intersecting the Ewald sphere continuously with change of wavelength of the incident radiation or change of orientation of the crystal.The paper discusses the applicability of the seventeen two‐dimensional space groups and the eighty diperiodic groups in three dimensions to these diperiodic structures. A comparison is made between the different effects of surface symmetry on etch pits and epitaxial deposits on the cleavage face of muscovite.

ISSN:0021-8979    

DOI:10.1063/1.1713610

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Single crystals of zinc tungstate were grown by the Cˇzochralski method from melts prepared by various methods. Of the three chemical etches evaluated for purposes of revealing dislocations in this material, solutions of potassium or sodium hydroxides were most satisfactory. The (100) and (010) planes were shown to be slip planes. Both of these planes are parallel to the growth direction of crystals grown for maser applications. Annealing experiments indicated appreciable mobility of the dislocations at about 800°C. Etch pit counts revealed typical dislocation densities of 104−105pits/cm2in areas near the center, and around 106pits/cm2at outer regions of the crystal. Gaseous inclusions were frequently surrounded in their immediate vicinity by dislocations, and regions with a high density of these inclusions also had a high dislocation density. Some miscellaneous crystallographic information is also presented to enable crystals to be accurately orientated by simple optical methods.

ISSN:0021-8979    

DOI:10.1063/1.1713611

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The thin amorphous oxide formed in air on electropolished aluminum replicates the surface of the metal, resulting in identical topographies at the metal‐oxide and oxide‐oxygen interfaces. The observed configurations are attributed to surface features developed in the metal during electropolishing; these features vary widely depending upon electropolishing conditions. The formation of crystalline oxide at 600°C occurs at the metal‐amorphous oxide interface even in the presence of thick amorphous layers formed by anodic oxidation. Evidence is given that cracks develop in the amorphous film on heating and that subsequent thermal oxidation proceeds through these defects.

ISSN:0021-8979    

DOI:10.1063/1.1713612

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Solutions are presented to the problems which usually prevent accurate x‐ray determinations of preferred orientation. The relationship between observed and corrected intensities and the angle dependent absorption factors are treated, including the case of displaced and distorted line profiles caused by low absorption in thick specimens. The absorption factors are derived experimentally from measurements of the scattered (Compton) background radiation without recourse to calculations or tables. Two simple methods, either by varying the wavelength &lgr; or the ordernof the x‐ray reflection, span the region of the pole figure in which data are unreliable or unobtainable if only one wavelength or order is used. Random samples are not required for normalizing the data to ``times random'' units. The intensity for a random sample can be calculated from the corrected data itself and the equationIhkl=[[double integral operator]Ihkl(&agr;,&bgr;) sin&agr;d&agr;d&bgr;]/2&pgr;where (&agr;,&bgr;) are the coordinates of a point on the usual polar net. It is suggested that textures can generally be represented by Gaussian or superposition of Gaussian orientation distributions.

ISSN:0021-8979    

DOI:10.1063/1.1713613

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

A fundamental study of the ductile‐brittle behavior of the transition‐metal monocarbides has been initiated through an investigation of the plastic deformation of single crystals of TiC, ZrC, and NbC. A vacuum furnace was used with an Instron testing machine for compression measurements in the temperature range 800° to 1600°C. The specimens were Linde single‐crystal boules, cleaved into bars approximately 1×1×4 mm. The compressive stress was applied longitudinally in a ⟨100⟩ direction in the NaCl structure. The three carbides studied behaved similarly, but NbC was several times harder than TiC and ZrC. The critical resolved shear stress &tgr; decreases exponentially with increasing temperature, increases approximately linearly with increasing carbon content, is little affected by surface condition, and increases as the logarithm of the strain rate. No yield point was observed. The effective stress exponent depends on the strain rate, ranging from 1 to 10. These results are discussed in relation to recent theories of the stress‐strain relation.

ISSN:0021-8979    

DOI:10.1063/1.1713614

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Diffusion of chromium in pure nickel has been studied by radioactive tracer and residual activity technique. The diffusion coefficient (in units of cm2/sec) in the range 600°−900°C is given byDCr/Ni=0.03 exp(−40 800/RT) and in the range 350°−600°C is given byDCr/Ni=5.45×10−9exp(−13 700/RT).The bimodal diffusion behavior has been explained on the basis of solid solubility of chromium in nickel, which falls very rapidly from 1000°C downwards. The entropy of activation for high‐temperature diffusion falls very well on the entropy vs activation energy curve for lattice diffusion in nickel.

ISSN:0021-8979    

DOI:10.1063/1.1713615

出版商:AIP    

年代:1964

数据来源: AIP