摘要:

The inhomogeneous distribution of solute atoms in the presence of the strain field of dislocations causes additional phonon scattering in alloys. This nonuniformity results from the size difference between solute and solvent atoms. A previous theory considered the source of the scattering to be the mass difference between solute and solvent atoms. It is now shown that the size difference which causes the inhomogeneous distribution is, at the same time, an important additional source of scattering, which will always reinforce the anharmonic scattering of the strain field. The case of copper‐aluminum alloys is considered because of recent measurements made on this system. At higher temperatures (shorter phonon wavelengths) scattering by the individual solute atoms becomes relatively more important.

ISSN:0021-8979    

DOI:10.1063/1.1660194

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Faulting on {211} planes has been reported in filings of many bcc materials based on differences in the broadening of various peaks in the x‐ray powder pattern. These experiments have been repeated in this work using (a) recent techniques for correcting for various experimental errors and (b) new derivations of effects of faults on other planes. It is shown that the mean‐square strains⟨&egr;L2⟩are generally inversely proportional to the distanceLnormal to the reflection planes. As a result, it is possible to separate effective particle‐size (including faulting) and elastic strains from several first‐order reflections, rather than with multiple orders. With this approach, the results indicate that there are no faults produced by filing in Fe, &bgr; brass, and Mo&sngbnd;Re alloys. For Nb there are faults on {110}, and the concentration varies inversely with the interstitial content.

ISSN:0021-8979    

DOI:10.1063/1.1660195

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The ultrasonic harmonic generation technique employing fundamental and third harmonic wave amplitude measurements has been used to study the nonlinear elastic properties of SrTiO3single crystals. A complete set of nonlinearity parameters calculated from room temperature data support the values recently reported for the harmonic generation technique in which fundamental and second‐harmonic amplitudes were measured. The temperature variation of the third‐order elastic constantC111in the range 103°–300°K was also measured by third‐harmonic generation methods and was found to agree well with the corresponding second‐harmonic result. The experimental technique in most cases involved overlapped pulses in the pulse‐echo wave train, and the results were found to agree well with measurements for the first pulse in the normal pulse‐echo pattern.

ISSN:0021-8979    

DOI:10.1063/1.1660196

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The electrical breakdown of cesium vapor was measured in a guard‐ringed, variable parameter test vehicle, which had plane parallel rhenium electrodes. Experiments were conducted over a range of emitter temperatures from 1300° to 2000°K, with cesium pressures from 0.053 to 6.22 Torr and interrelectrode spacings from 0.001 to 0.07 cm. Because the breakdown occurred at relatively low voltages, the contact potential difference of the electrodes, as a function of temperature and cesium coverage, was taken into account. Paschen‐type curves exhibited minima in breakdown potential as a function of spacing at high pressures. At low pressures the necessary large spacings could not be achieved. The maximum breakdown voltage was 6 V at 0.053 Torr and the minimum breakdown voltage was 0.21 V at 6.22 Torr. At low cesium pressures the effect of the emitter temperature is pronounced. When the pressure is constant, the breakdown voltage shifts as a function of emitter temperature. At high pressures, the curves do not vary with emitter temperature at constant pressure, but shift in voltage as the pressure is changed. These experiments indicate that the Paschen behavior is not followed in the case of hot electrode discharges, for all ranges of temperature and pressure.

ISSN:0021-8979    

DOI:10.1063/1.1660197

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The interaction of laser‐blowoff metal plasmas with transverse magnetic fields in the 100‐kG range has been studied, the magnetic field being generated by a simple line current. Under these conditions it was found that the plasma crossed the field by means of an electrical polarization andE×Bdrift. An observed velocity reduction at fields greater than 10 kG has been attributed to an inhibiting effect of the magnetic field on the normal ambipolar energy transfer between electrons and ions in the expansion of laser‐blowoff plasmas.

ISSN:0021-8979    

DOI:10.1063/1.1660198

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Two‐dimensional small‐amplitude viscous, as well as nonviscous, wave motion is considered in a medium consisting of two superposed incompressible fluids of finite depths, separated at the interface by a membrane. The upper one has a free surface and the lower one is bounded below by a rigid bottom. This is rather a crude model of a general problem in biology. Here one is interested in studying similar motions in a medium consisting of several layers of fluid separated by membranes across which there is migration of particles. It is found that the membrane affects the dispersion relation but does not lead to the damping of the waves. The frequency in the viscous case decreases with viscosity. The modulus of decay of the amplitude is0{(&ngr;1&ngr;2)1/2/[&rgr;1(&ngr;1)1/2+&rgr;2(&ngr;2)1/2]},where &ngr;1, &ngr;2are the viscosities and &rgr;1, &rgr;2are the densities of the upper and the lower fluid, respectively.

ISSN:0021-8979    

DOI:10.1063/1.1660199

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

An interpretation of initiation pressure hysteresis phenomena for fixed‐point pressure calibration is presented according to the theory of nucleation in condensed systems and is substantiated for the bismuth I–II point by hydrostatic pressure experiments with a manganin pressure gauge. Kinetic initiation and completion of the bismuth I–II transformation were observed at constant pressure and temperature, and several different isobaric initiation pressures were measured at the same temperature. An asymmetrical relationship of the equilibrium pressure between the initiation pressures of the forward and reverse transformations was measured, and it was demonstrated that the high‐pressure side of the strain hysteresis of initiation is within 0.17 kbar of the equilibrium pressure. The microstructure and thickness of the sample influenced the initiation pressures and the region of indifference, but the center of the region appeared reproducible. A region of indifference of 0.03 kbar was measured. The origin of the real strain hysteresis of initiation is identified for polymorphic transformations and initiation pressures are discussed in terms of the strain energy and chemical kinetics of nucleation. It is shown that other interpretations of hysteresis phenomena are not satisfactory. It is reasoned that the equilibrium pressure rather than a higher pressure value is a more accurate calibration pressure for the initiation of the bismuth I–II transformation in solid‐media apparatus.

ISSN:0021-8979    

DOI:10.1063/1.1660132

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Experimental data for the iodine photodissociation laser are presented over the following ranges of operating conditions: CF3I pressure, 0.4–500 Torr; argon pressures, 5–200 Torr; and flash energy, 225–1764 J. Peak powers of 400 W and optimal gains of 45 dB/m were observed. The experimental results for CF3I pressures less than 200 Torr and for the first 25 &mgr;sec of lasing action are within 10&percent; of the predictions from a four‐level kinetic model of the laser. These results indicate the utility of kinetic models for describing pumping, radiative, and collisional processes.

ISSN:0021-8979    

DOI:10.1063/1.1660133

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The pumping mechanism of a high‐current pulsed CO2laser has been investigated. It was found that there is a time delay of the laser pulse behind the current pulse. From the dependence of the time delay on the plasma parameters, such as electron density, electron temperature, gas temperature, gas pressure, and emission of oxygen atomic line (7771 Å), it can be shown that during the current pulse, the CO2is totally dissociated into CO and O. The subsequent recombinations of CO and O into CO2are responsible for the pumping of the upper level of the CO2laser (10.6 and 9.4 &mgr;). The time delay between the current pulse and laser pulse is thought to be due to the time required for CO and O to recombine to reach the threshold population for lasing for the particular optical cavity. The threshold upper level population is obtained by knowing theQvalue of the optical cavity, wavelength of the laser line, and linewidth of the radiation line. By equating the amount of CO2formed during the delay time to the threshold upper laser population, the reaction rate of CO+O→CO2is thus obtained. The agreement between the rate obtained and previously determined supports the proposed pumping mechanism.

ISSN:0021-8979    

DOI:10.1063/1.1660134

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The stimulated emission of CdS platelets excited by light pulses from a N2laser is investigated at 2° and 77°K, as a function of the level of excitation and for differently prepared crystals. Pure and perfect crystals show peaks due to exciton‐exciton interactions (P), and due to the recombination of a free exciton with simultaneous emission of one or two LO phonons [(Ex‐LO) and (Ex‐2L)]. Platelets grown in an atmosphere of excess S or Cd show up to five additional stimulated emission lines, located between P and Ex‐LO. Imperfect crystals show a broad emission band extending from P to beyond Ex‐LO. At low excitation levels only P appears in the stimulated emission spectra. With increasing level of excitation the intermediate impurity lines (JJ) in Cd‐ and S‐rich crystals also show stimulated emission and the maximum emission intensity shifts from the P peak sequentially through all JJ lines always toward the next peak at lower wavelengths. At still higher excitation levels, Ex‐LO, the next peak at lower wavelengths, gains maximum intensity and in one observation even Ex‐2LO grew to become the dominant stimulated emission line. In imperfect crystals the maximum of the broad emission band shifts from a position near P to Ex‐LO with increasing level of excitation. Further, the P line is found to split in a magnetic fieldH⊥C. The intensity ratio of the upper to the lower P branch changes with the excitation intensity, such that with lower excitation intensity the higher energetic branch dominates the emission while at higher excitation intensity the lower energetic branch has higher emission intensity than the other branch. A reasonable explanation of the dependence on excitation intensity is given in terms of electron‐assisted transitions which increase the absorption in the tail region of the absorption edge, and thus spoil the necessary gain for stimulated emission.

ISSN:0021-8979    

DOI:10.1063/1.1660135

出版商:AIP    

年代:1971

数据来源: AIP