Index of content:
Volume 23, Issue 11, 01 November 1952
23(1952); http://dx.doi.org/10.1063/1.1702029View Description Hide Description
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.
23(1952); http://dx.doi.org/10.1063/1.1702030View Description Hide Description
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 interstitialcarbon 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.
23(1952); http://dx.doi.org/10.1063/1.1702031View Description Hide Description
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 (EV and EB ) 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, EV may be taken to be 90 kcal/mole. From the present experiments and the above value of EV , the activation energy for grain boundarydiffusion,EB , is calculated to be 75.4 kcal/mole.
23(1952); http://dx.doi.org/10.1063/1.1702032View Description Hide Description
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 compoundmanganese 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)max as high as 4.3×106 gauss‐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 magnetmaterial 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.
23(1952); http://dx.doi.org/10.1063/1.1702033View Description Hide Description
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.
23(1952); http://dx.doi.org/10.1063/1.1702034View Description Hide Description
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 10−3 mm 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.
23(1952); http://dx.doi.org/10.1063/1.1702035View Description Hide Description
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.
23(1952); http://dx.doi.org/10.1063/1.1702036View Description Hide Description
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, where T is the cathode temperature and g is the conductance of the electron stream.
23(1952); http://dx.doi.org/10.1063/1.1702037View Description Hide Description
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.
23(1952); http://dx.doi.org/10.1063/1.1702038View Description Hide Description
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 σ0 in the transient wave that impinges on a free surface is more than double the critical normal fracture stress σ c of the material. The number of scabs that will be formed will be equal to the first whole number smaller than the quotient σ0/σ c . The thickness of each scab is governed by the shape of the stress wave.
23(1952); http://dx.doi.org/10.1063/1.1702039View Description Hide Description
Numerical methods, such as those of Southwell, have made amenable to approximate analysis a large class of problems in the theory of elasticity. However, certain loadings must occasionally be considered which may give rise to theoretical stress singularities. In such cases an analytical study, at least in the immediate neighborhood of the singularities, would appear to be mandatory.
One such problem is that of determining the thermal stresses in a partially clamped elastic half‐plane. The solution given in this paper assumes self‐equilibrating tractions over the clamped portion of boundary so that the results apply approximately also to large finite areas.
23(1952); http://dx.doi.org/10.1063/1.1702040View Description Hide Description
A metallographic study was made of the diffusion zones present in copper‐α‐brass, copper‐nickel, and gold‐silver diffusion couples. Usual sandwich type couples and also vapor‐solid type couples in which the higher vapor pressure component was diffused either into or out of the couple through the vapor phase were investigated. Significant porosity was always generated by diffusion in the sandwich couples and also in the vapor‐solid couples when the higher vapor pressure component was diffused out of solid solution. Upon inward diffusion from the vapor porosity formed in the gold‐silver couples but not in the copper‐α‐brass or copper‐nickel couples.
It is concluded that porosity may form in any alloy system when the components diffuse at different rates and that it may cause appreciable error in the measurement of diffusion coefficients if ignored.
23(1952); http://dx.doi.org/10.1063/1.1702041View Description Hide Description
Radioactive tracer techniques have been employed to measure the rate of self‐diffusion in silver in polycrystalline and single crystalsolvents over a wide range of temperatures, and to investigate the effect of bombardment with 10‐Mev protons on this diffusion process. A least squares fit of the data from laboratory control experiments with both single crystal and polycrystallinesolvents in the temperature range 903°C to 640°C has yielded the diffusion constants Q=40,800±700 cal/mole and D 0=0.11±0.05 cm2/sec for the volume diffusion process. No effect of intense proton bombardment ranging from 0.5 to 5μa/cm2 has been detected on the volume self‐diffusion process in silver specimens which were simultaneously irradiated in the cyclotron target box and annealed at temperatures ranging from 852°C to 555°C. These results are, at least semiquantitatively, in accord with both the elastic collision model and the thermal spike model of radiation damage. Cyclotron techniques and apparatus are described in some detail.
23(1952); http://dx.doi.org/10.1063/1.1702042View Description Hide Description
The effect of the potential drop across the filament of a diode with directly heated emitter on the plate current of the diode is considered. Calculations are made for both space‐charge‐limited and retarding‐field conditions. Filament heating by direct current and by alternating current is considered. The results are given in convenient graphical form.
23(1952); http://dx.doi.org/10.1063/1.1702043View Description Hide Description
The hafnium‐hydrogen system at room temperature consists of three phases. The lattice parameters of the hafnium metal used in this investigation and of the three phases are as follows: hafnium,a 0=3.200A, c 0=5.061A, and c=1.58; deformed cubic phase, a 0=4.702±0.012A, c 0=4.678±0.012A, and c=0.995; face‐centered cubic phase, a 0=4.708±0.002A; and face‐centered tetragonal phase, a 0=4.882±0.002A, c 0=4.384±0.002A, and c=0.898. The deformed cubic phase extends up to HfH1.53 and converts into the face‐centered cubic phase between HfH1.53 and HfH1.70. The face‐centered cubic phase transforms into the tetragonal phase between HfH1.80 and HfH1.87. The tetragonal phase extends up to HfH1.98.
23(1952); http://dx.doi.org/10.1063/1.1702044View Description Hide Description
Thermionic emitters consisting of barium oxide dispersed throughout a body of porous tungsten can be successfully made if suitable compounds of barium are employed as the source of the oxide. Compounds such as the carbonate, which can oxidize tungsten and react to form Ba3WO6:,are unsuitable. However, if by various means, BaO is dispersed in tungsten,reaction occurs at operating temperature of the cathode to slowly generate free barium: 6BaO+W→Ba3WO6+3Ba, which diffuses to the surface and lowers the work function.
Cathodes consisting of approximately 5 percent of BaO in W are capable of supplying continuous emission in excess of 100 amp/cm2, give equivalent dc and pulsed emission, and are highly resistant to damage by arcing, temporary poor vacuum, ion bombardment, and high temperatures.
Emission current of approximately 8 amp/cm2 was obtained at an operating temperature of 1000°C from a typical cathode of this type. The life of this cathode, at 1100°C, was in excess of 650 hours. Constants of the Richardson equation for a relatively inactive cathode were φ=1.56 ev, A=0.6 amp/cm2/deg2.
23(1952); http://dx.doi.org/10.1063/1.1702045View Description Hide Description
The creep rate of copper under deuteron bombardment has been investigated using the 16‐Mev external beam of the University of Pittsburgh cyclotron. Measurements were made of the second stage creep rate of a copper wire under deuteron bombardment at a temperature of 260°C and a loading of 10,000 psi. Within the precision of the experiment ±20 percent, the creep rate during and after bombardments of 10 to 20 hours duration was unchanged from the creep rate preceding bombardment. Theoretical considerations of the type discussed by Slater, predict that with the available deuteron flux of 1012 particles per cm2 per sec there should be no appreciable change in creep rate. The available deuteron flux cannot displace enough atoms from their normal lattice sites to produce a dislocation involving a hundred atoms or more; further, the steady‐state density of displaced atoms is not enough to impede the motion of those dislocations present in the crystals. While regions of increased local temperature are produced during bombardment, these also should not affect the creep rate. Thus, apparently the considerations of solid‐state theory are confirmed and creep rates should not be expected to increase for all metals under bombardment.
23(1952); http://dx.doi.org/10.1063/1.1702046View Description Hide Description
Graphs of the theoretical pressure distribution resulting from diffraction of a weak shock by a rigid right‐angled corner are presented and compared with graphs of measured pressure distributions. The agreement is found to be satisfactory. A graph of the electric field resulting from diffraction of an electromagnetic pulse by a perfectly conducting right‐angled corner is also presented.
23(1952); http://dx.doi.org/10.1063/1.1702047View Description Hide Description
At time t=0 a unit sphere containing a perfect gas at uniformly high pressure is allowed to expand suddenly into a homogeneous atmosphere. Solutions for short times later are sought by analytic (i.e., not numerical) methods. Viscosity and heat conduction are neglected. The particle velocity,sound speed, and entropy are developed in powers of y, which is proportional to the time (more precisely, the distance moved by the head of the rarefaction wave in time t), with coefficients depending on a slope coordinate q=(1/2N)[(2N−1) +(1−x)/y], where x is the radial coordinate, N=(½)(γ+1)/(γ−1), and γ is the ratio of specific heats. The zero‐order coefficients are the plane shock‐tube solution. First‐order corrections are derived for the various regions. Boundary conditions are approximated for small y at the surfaces of discontinuity, and the method for matching the solutions in the different regions is outlined. This matching process is carried out for the expansion of a diatomic gas into diatomic air.
23(1952); http://dx.doi.org/10.1063/1.1702048View Description Hide Description