摘要:

Simple expressions are obtained, in certain cases, for the temperature increase in a heat‐producing body when the normal heat flow from the body is blocked by defective cooling on portions of the surface of the body. Defective areas in the shape of long strips and round spots are considered.

ISSN:0021-8979    

DOI:10.1063/1.1702029

出版商:AIP    

年代:1952

数据来源: AIP

摘要:

The mechanism for self‐diffusion in single crystals of graphite has been examined theoretically. Calculations for vacancy and direct interchange mechanisms are based on atomic interactions within the graphite hexagonal layers. These interactions are obtained from the known systematic change of carbon‐carbon bond strength with interatomic distance. In order to be able to calculate the energy of formation of an interstitial carbon atom, a potential function has been devised to account for interaction between the planes. This potential function consists of two terms, a van der Waals' attraction and an exponential type repulsion. The adjustable constants have been evaluated from known physical data. The interplanar cohesive energy of graphite is calculated to be 4.36 kcal/mole.Comparison of the total activation energies for self‐diffusion via vacancies, direct interchange, and interstitial atoms indicates that direct interchange is the preferred mechanism. The activation energy for self‐diffusion by means of direct interchange is estimated to be 90 kcal/mole. The present theoretical treatment is not applicable to diffusion along grain boundaries or pores.

ISSN:0021-8979    

DOI:10.1063/1.1702030

出版商:AIP    

年代:1952

数据来源: AIP

摘要:

Self‐diffusion in graphite was measured over the temperature range from 1835°C to 2370°C by observing the penetration of C‐14 tracer initially applied to one end of a graphite rod.The experimental data were found to be in agreement with a diffusion mechanism consisting of concurrent volume and grain boundary processes. Activation energies for both processes (EVandEB) could not be uniquely obtained from the data, however. An independent determination of either is required in order to evaluate the other. On the basis of recent theoretical work,EVmay be taken to be 90 kcal/mole. From the present experiments and the above value ofEV, the activation energy for grain boundary diffusion,EB, is calculated to be 75.4 kcal/mole.

ISSN:0021-8979    

DOI:10.1063/1.1702031

出版商:AIP    

年代:1952

数据来源: AIP

摘要:

Permanent magnets in present use consist chiefly of alloys containing cobalt or nickel, or both. Because these elements are currently scarce, possible substitute materials were examined. The intermetallic compound manganese bismuthide was chosen for this investigation because it has the highest recorded magnetic crystal anisotropy. The new permanent magnet was prepared by hot pressing finely pulverized MnBi in the presence of a strong magnetic field to facilitate particle alignment. Some of these compacts showed a maximum energy product (BH)maxas high as 4.3×106gauss‐oersteds, a coercive force (Hc) of 3400 oersteds and a residual flux density (Br) of 4300 gauss. This preliminary work shows that aside from its possibilities as a substitute for magnets requiring cobalt, this new magnet material has a value which exceeds present materials for special applications where a high coercive force is desirable, e.g., where one cannot use a magnet that is long compared to the air gap required. There is also theoretical possibility of further improvement in maximum energy product.

ISSN:0021-8979    

DOI:10.1063/1.1702032

出版商:AIP    

年代:1952

数据来源: AIP

摘要:

The directions of particle motion associated with acoustic waves have been computed and are displayed graphically, for each of the three waves, as a function of propagation direction. The numerical example used is cubic nickel and the maximum polarization angle found is 11½ degrees.

ISSN:0021-8979    

DOI:10.1063/1.1702033

出版商:AIP    

年代:1952

数据来源: AIP

摘要:

An investigation has been made of the use of beta‐emissive radioisotopes for generating high voltages. Voltage was obtained by supporting a beta‐emitter on an insulated electrode in a low‐pressure gas and allowing it to charge itself by virtue of its electron emission. This method represents a direct conversion of nuclear energy into electrical energy. The apparatus is described. Data on insulation breakdown, the effect of gas pressure, and the electrode material are given. The maximum voltage was obtained at a pressure of 10−3mm of Hg. The electrical characteristics of such a generator are discussed. Using an emitter containing 0.25 curie of strontium 90, a voltage of 365 kilovolts was obtained.

ISSN:0021-8979    

DOI:10.1063/1.1702034

出版商:AIP    

年代:1952

数据来源: AIP

摘要:

Fluorescence x‐rays may be more useful analytically in metallurgical microradiography than the line emission from a tube target because of the greater homogeneity of the former radiation. Although the intensity of fluorescence is very low, exposure times are not prohibitive for some applications. In the present report, the method and apparatus are described and illustrative examples are shown.

ISSN:0021-8979    

DOI:10.1063/1.1702035

出版商:AIP    

年代:1952

数据来源: AIP

摘要:

The noise power spectrum of a diode with a temperature limited current and also for a diode with a retarding field is shown to be the sum of two parts. One, the ``pure'' shot effect, is the spectrum which would obtain if the electrons were all emitted with zero velocity, that is, if the cathode were at zero temperature (and still maintained the same emitting properties). The second is the thermal contribution, namely, 4kTg, whereTis the cathode temperature andgis the conductance of the electron stream.

ISSN:0021-8979    

DOI:10.1063/1.1702036

出版商:AIP    

年代:1952

数据来源: AIP

摘要:

The electric potential produced by positive and negative point current sources located in a homogeneous conducting sphere, a problem of interest in the field of electrocardiography, is obtained for arbitrary source locations and separations. Certain special cases of the general solution that are of particular value in both experimental and theoretical electrocardiographic research are also presented.

ISSN:0021-8979    

DOI:10.1063/1.1702037

出版商:AIP    

年代:1952

数据来源: AIP

摘要:

Further quantitative data bearing on the scabbing of metals under explosive attack are presented in this paper. In particular, the mechanics of the generation of multiple scabs is established. Experimental data have been obtained that show that multiple scabbing will occur whenever the maximum stress &sgr;0in the transient wave that impinges on a free surface is more than double the critical normal fracture stress &sgr;cof the material. The number of scabs that will be formed will be equal to the first whole number smaller than the quotient &sgr;0/&sgr;c. The thickness of each scab is governed by the shape of the stress wave.

ISSN:0021-8979    

DOI:10.1063/1.1702038

出版商:AIP    

年代:1952

数据来源: AIP