摘要:

The vacuum requirements to obtain stable field emission cathodes are discussed. To keep the average work function of a tungsten emitter constant, the total pressure of all chemically active gases should not exceed 10−14Torr. To minimize ion bombardment of the emitter surface the helium pressure should be reduced by ion pumping even with tubes made of an alumino‐silicate glass. The relationship between the current density and the slope of a Fowler‐Nordheim plot has been calculated. Experimental evidence is given that variations in the work function can be compensated by changes in the field strength in such a way that stable field emitters are obtained. A qualitative explanation of such a mechanism is suggested.

ISSN:0021-8979    

DOI:10.1063/1.1728536

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The adsorption of oxygen on clean nickel (110) surfaces has been studied by low‐energy electron diffraction. Several successive structures are formed before the first monolayer is completed. These structures consist of both oxygen and nickel atoms, with each having more oxygen atoms and fewer nickel than the preceding. With increasing oxygen content the sticking probability becomes progressively lower, from about unity for the (1×2) structure to 0.01 for the final most oxygen‐rich structure. The temperature necessary for the degradation of any structure to that previously formed decreases from a value near the melting point of nickel for the (1×2) structure to about 250°C for the most oxygen‐rich structure. Normal cubic nickel oxide can be grown on top of the most stable of these oxygen‐nickel structures, but only at rather high temperatures and pressures. The oxide is oriented with a [100] direction normal to the (110) surface of the crystal, and with a [110] direction parallel to the [100] nickel direction in the surface plane.

ISSN:0021-8979    

DOI:10.1063/1.1728537

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The small signal power theorem for electron beams is reviewed in the light of recent controversies on the subject. The physical significance of the theorem is explained and its role as a mathematical identity is discussed. The assumptions and the boundary conditions of the linearized theory are carefully stated. The small signal power theorem is rederived in a simplified way. New examples and applications are treated.

ISSN:0021-8979    

DOI:10.1063/1.1728538

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The small signal power theorem for electron beams is transformed into a form which lends itself nicely to practical applications. This is achieved by the introduction of scalar and vector potentials and in four‐vector notations the theorem becomes particularly appealing. As an example of the application of the theorem, the one‐dimensional excitation of a cylindrical beam is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1728539

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Mercury‐doped germanium has been prepared by a modified zone leveling technique. Mercury as an impurity producesp‐type germanium with two energy levels. Hall measurements yield values of 0.087 and 0.23 eV for the lower and upper levels, respectively. Activation energies determined from the long wave‐length threshold of the photoconductive response are in good agreement with these values.

ISSN:0021-8979    

DOI:10.1063/1.1728540

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The modulus and internal friction of copolymers of tetrafluoroethylene and hexafluoropropylene (HFP) were measured with longitudinal waves at a frequency of 12 Mc between −80° and 260°C. The dielectric dissipation factor was measured at 0.1, 1.0, 10, and 100 kc between −180° and 200°C. Samples included copolymers with compositions ranging from pure polytetrafluoroethylene to pure polyhexafluoropropylene. A relaxation at −10°C (12 Mc) in polytetrafluoroethylene shifts to higher temperatures with increasing HFP content. Data suggest that 5 to 13 main‐chain carbon atoms are involved, a size consistent with assigning the relaxation to motions of a small point defect in the crystal lattice. Molecular models indicate that the effects of these defects are reduced by perfluoromethyl groups which enter the lattice as another kind of point defect. Dielectric and mechanical relaxation temperatures agree approximately and yield the same activation parameters. These parameters exhibit no significant variation with HFP content and follow a relation which applies to activated processes in inorganic solids. The relation also obtains for a relaxation at 197°C (12 Mc) in polytetrafluoroethylene. This relaxation shifts to lower temperatures and the activation enthalpy decreases with increasing HFP content. Dielectric data do not exhibit the relaxation, and heating polytetrafluoroethylene near the melting point does not introduce sufficient polar groups to alter this result. Another relaxation observed below −150°C in the dielectric but not in the ultrasonic data is independent of polymer density and HFP content.

ISSN:0021-8979    

DOI:10.1063/1.1728541

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Basic domain structures of epitaxially grown single‐crystal thin films of iron are investigated by the powder pattern technique. For films with (001) orientation, where two easy axes of magnetization lie within the plane of the film, a checkerboard pattern, i.e., a regular square domain pattern made up of 90° walls only, is expected to appear, since 90° walls should predominate over 180° walls in thin films. In the asevaporated condition, however, the film possesses a fairly complicated domain structure and the checkerboard pattern is obtained only after alternating‐field demagnetization. It is thus clearly indicated, in thin films, that there exists a lack of demagnetizing energy necessary for the formation of closure domains and that an external force is necessary to reach the demagnetized state with a closure domain structure.

ISSN:0021-8979    

DOI:10.1063/1.1728542

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The possibility of the rotation of a subgrain with respect to its neighbors as a natural process during recrystallization is analyzed thermodynamically and kinetically. It is found that it is energetically possible if the direction of rotation favors the elimination of low‐angle boundaries over that of high‐angle boundaries, the elimination of twist and asymmetric boundaries over that of the tilt and symmetric boundaries, and the elimination of the large‐area boundaries over that of the small‐area boundaries. Since the rotation direction has two degrees of freedom, there exists a relation between the two degrees of freedom such that directions whose two degrees of freedom fulfill such a relation provide no driving force for rotation. All other directions will supply free energy to rotate in one sense or the opposite. Kinetically one of the following two processes within one boundary can be rate‐controlling: the cooperative movement of dislocations in the boundary, and the cooperative diffusion of vacancies in the lattice. The rotation of a subgrain favors the elimination of one of the boundaries which contributes the largest fraction of driving force and also the largest fraction of resistance. This causes the subgrain to coalesce with the other subgrain separated by this boundary. The estimated time required for one such coalescence compares favorably with the observed rate of subgrain growth in Al.

ISSN:0021-8979    

DOI:10.1063/1.1728543

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

A paper with the above title appeared in this Journal in 1956 giving the charge distribution on, and capacitance of, an electrified right circular solid conducting cylinder for length‐to‐diameter ratios of 0.25 to 4. Much more accurate values, calculated for the range 0.125 to 8 on a digital computer, are tabulated in the present paper. Over this range the capacitance is given to 0.2% accuracy by the formulaC=[0.708+0.615(b/a)0.76]×10−10a farads,where 2bis the length andathe radius. A spherical harmonic expansion for the potential outside the circumscribed sphere is given.

ISSN:0021-8979    

DOI:10.1063/1.1728544

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

A study of three types of anomalous nickel‐iron magnetic films has been made: high coercive‐force films, which have unusually large hysteresigraph values of wall and rotational coercive forces; rotatable initial‐susceptibility films, for which the value of the initial susceptibility as found from low‐field hysteresigraph measurements is determined by previously‐applied high magnetic fields; and mottled films, whose high‐field hysteresis loops show low remanence and a high wall coercive force, and whose Bitter patterns have a spotted or mottled appearance. Anomalous films have been produced by a variety of techniques. In general, for a given technique, a mild treatment yielded high coercive‐force films, a moderate treatment gave rotatable initial‐susceptibility films, and a drastic treatment produced mottled films. The techniques used were evaporation at slow rates at high substrate temperatures, evaporation on aggregated metal deposits, electroplating on annealed gold films, and annealing normal films. According to a qualitative model postulated to explain these results, these production techniques cause the formation of small, scattered inhomogeneities which have high values of randomly oriented magnetic anisotropy associated with them. As the production technique becomes more drastic, the density and anisotropy values of these anisotropy centers increase, causing the gradual transition from high coercive‐force films through rotatable initial‐susceptibility films to mottled films. It is concluded that inhomogeneous strain in conjunction with magnetostriction is a likely cause of anisotropy centers. It is thought that ``rotatable anisotropy'' films discussed by previous workers are closely related to the films investigated in the present study.

ISSN:0021-8979    

DOI:10.1063/1.1728545

出版商:AIP    

年代:1962

数据来源: AIP