摘要:

A moments method is applied to the non‐self‐similar problem of the diffusion of radiation from a cavity, in the case that the transport of energy is purely radiative. The energy in the cavity is assumed to be introduced instantaneously, and the diffusion into the surrounding wall, particularly the motion of the radiation front, is followed for subsequent times. The moments method used has the characteristic that it gives, for the physical model assumed, exact results for the position of the front in both the short‐ and long‐time limits for all geometries.Both plane and spherical cavities are treated in detail. Series solutions which apply in the limits of short and long times are given. Equations describing the first‐order departure of spherical radiation flow from plane flow are derived, and the short‐time solution is given. In plane geometry all quantities of interest can be written in parametric form in terms of elementary functions, and numerical results are presented in this case. Spherical geometry yields a more complex parametric solution, which is also given.

ISSN:0021-8979    

DOI:10.1063/1.1709029

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A study has been made of the thermoelectric powers (S) and electrical resistivities (&rgr;) of CrFe alloys over the concentration range 0 to 48.2 at. % Fe and temperature (T) range from 25° to 1000°C. The plots ofSvsTand &rgr; vsTmove in an orderly progression from that of pure Cr to the curve for the Cr51.8Fe48.2alloy with no anomalies occurring. One might think thatSshould change sign as the alloy concentration passed through the region where Becket al.observed their ``spike'' in the electronic specific heat, but no such phenomenon was observed.

ISSN:0021-8979    

DOI:10.1063/1.1709030

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The full Navier‐Stokes equations are solved numerically in cylindrical coordinates in order to investigate the splash of a liquid drop onto a flat plate, into a shallow pool, or into a deep pool. Solution is accomplished with the Marker‐and‐Cell technique using a high‐speed computer. Results include data on pressures, velocities, oscillations, droplet rupture, and the effects of compressibility. They also show how the technique can be applied to a wide variety of other complicated fluid flow problems involving the transient behavior of a free surface.

ISSN:0021-8979    

DOI:10.1063/1.1709031

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A variety of optical absorption, emission and luminescence excitation spectra have been measured in an attempt to identify the centers involved in the thermoluminescence of commercial LiF:Mg. It is concluded that the principal trapping centers consist of a hole trapped near various groupings of Mg2+ions and vacancies. The optical absorption bands of these centers occur in the 3100–3800 Å region which contains several absorption bands corresponding to different geometries of the centers. It is suggested that the 2200‐Å band arises from Mg2+ion‐vacancy complexes which have captured two holes. During thermoluminescence, holes are transported from traps to emitting centers. The luminescent center appears to be theFcenter both in an isolated position and when adjacent to a complex involving Mg2+ions.

ISSN:0021-8979    

DOI:10.1063/1.1709032

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

An investigation has been made ofp‐njunction devices performing under conditions when the depletion layer was subjected to sudden generation of extreme carrier densities. Significant local distortion to the layer was identified, arising from the fact that the instantaneous high carrier density grossly changed the effective conductivity of the depletion layer. These findings are shown to be of particular significance when such junction devices are employed to analyze intense radiation phenomena.

ISSN:0021-8979    

DOI:10.1063/1.1709033

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The photoconductive response of several semiconducting diamonds was measured in the spectral range 0.8 to 4.7 &mgr; with crystal temperatures of 4.2° and 77°K. Photoconductive minima, most of which are not observed in the optical absorption spectrum of these diamonds, were recorded at 3.6, 3.5, 2.72, 2.65, 2.48, 2.41, 2.38, 2.35, 1.97, 1.87, 1.83, 1.78, 1.56, 1.44, 1.21, 1.05, 0.92, and 0.83 &mgr;, and minor features were recorded at 2.44, 2.25, 2.02, 1.33, and 1.13 &mgr;. Five activation energies, 0.21, 0.30, 0.37, 0.52, and 0.7 eV, for semiconducting diamonds have been observed. Good correlation is found between the photoconductive minima and the five activation energiesEAi(i=1,&cellip;, 5) through the termsEn=EAi+nEphwhereinEph= 0.165 eV, the energy of a Raman‐frequency phonon, andntakes integral values from 1 though 8. The minima may be explained as follows. Holes are excited to the valence band by the incident radiation. They lose energy mainly through the emission of Raman‐frequency phonons. When the energy relations are proper, holes are left in excited bound states at acceptor sites and do not contribute to the conduction process.

ISSN:0021-8979    

DOI:10.1063/1.1709034

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Quantitative measurements of the kinetics of nucleation in some simple substrate‐overgrowth systems have been made using improvedin situelectron microscopy techniques. The nucleation of bismuth and silver on evaporated carbon and the nucleation of bismuth on evaporated SiO substrates were studied as a function of substrate temperature and impinging flux. The results were analyzed in terms of the phenomeno‐logical theory of nucleation and in terms of Walton's atomistic model of condensation from the vapor phase. Both theoretical concepts led to specific conclusions concerning such nucleation parameters as the number of atoms in the critical nucleusn* and the free energy of desorption &Dgr;Gdes. The substrate temperature and impingement flux dependence of the maximum number of deposit particles that can be accommodated on the substrate surface were determined and tentatively interpreted on the basis of nucleation‐limited surface migration. Some nucleation induction time observations were made and were related to the clustering kinetics of adatoms, the growth process of individual nuclei, and the instrumental limitations of the detection method.

ISSN:0021-8979    

DOI:10.1063/1.1709035

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Isothermal changes in long‐range orderS, x‐ray line shifts due to stacking faults &Dgr;, and x‐ray particle size from Fourier analysis of line shapesDwere measured for filed samples of Ni3Al. Recovery temperatures between 200° and 270°C were investigated. The recovery of all three measured properties appeared to depend upon the mobility of vacancies in this temperature range. The evaporation of vacancies from faulted loops appeared to supply the vacancy flux for ordering and particle‐size changes. Fault shrinkage was found to cease quite abruptly between 215° and 200°C; at 200°C faulting initially increased, during which ordering did not occur, but after some 20 h the faulting began to anneal out and somewhat later ordering proceeded. An activation energy of 1.65 eV was found for the ordering reaction; this compares with a value of 1.5 eV forEmvin nickel.

ISSN:0021-8979    

DOI:10.1063/1.1709036

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Previous treatments of stress‐pulse‐activated piezoelectric and ferroelectric stress gauges and power generators neglect the secondary stresses produced by circuit‐enforced fields in not‐directly‐stressed parts of the transducer. It is pointed out that under short‐circuit conditions these secondary stresses generate strong effects in materials with high electromechanical coupling factors. To demonstrate this, a thin plate activated by a plane rectangular low‐amplitude stress front is considered. A rigorous theoretical analysis is sketched for this configuration. It is found that short‐circuit current pulses generated by the stress‐front transit are not rectangular, but vary exponentially in time. The exponent is proportional to the square of the electromechanical coupling factor and depends on the mechanical boundary conditions. Supporting experiments are reported.

ISSN:0021-8979    

DOI:10.1063/1.1709037

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Decay of the electron distribution injected into a polymeric solid at 2MeV was investigated by measuring the charge released during a triggered discharge. From the decay time constant &tgr;=&rgr;&egr;, resistivity &rgr; was calculated. For polymethylmethacrylate, the variation of &rgr; with temperature yielded an activation energy or trap depthW≈0.6 eV. Discharges resulting in Lichtenberg figures were also produced in polystyrene and polypropylene. Trap depths were estimated to beW≈0.6 eV for polystyrene andW≳0.4 eV for polypropylene.

ISSN:0021-8979    

DOI:10.1063/1.1709038

出版商:AIP    

年代:1967

数据来源: AIP