摘要:

Results are presented for the temperature dependence of orientation birefringence, &Dgr;n(T), from −195° to 150°C for the following crosslinked polymers: polydimethyl siloxane, polyethylene, ethylene/propylene copolymers (84/16, 75/25, 48/52 mole ratio), and ethylene/vinyl acetate copolymers (96/4, 82/18). On cooling from the rubbery state under a small constant load, the birefringence magnitude shows an abrupt increase when crystallization takes place, reaching a maximum value (&Dgr;nc) when crystallization is complete; &Dgr;ncis determined principally by the degree of orientation in the rubbery state. In the rigid semicrystalline state, belowTg, the magnitude of orientation birefringence decreases with decreasing temperature. The transition temperature for the ethylene/propylene copolymers is lowered by increasing the propylene content; the nature of the transition changes progressively from a crystallization in polyethylene to a glass transition in the copolymer having 52 mole % propylene. The observed difference in optical parameters for polyethylene and its copolymers suggests a difference in physical structure which strongly affects the optical properties. We believe that the structure of the copolymers is microcrystalline, contrasting to the spherulitic crystallinity of polyethylene (and polydimethyl siloxane).

ISSN:0021-8979    

DOI:10.1063/1.1709299

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The shear strength of grossly deformed copper, silver, and gold has been measured in an opposed anvil shear apparatus at pressures up to 150 kbar in the temperature range from 27°–900°C. The shear data agree with independent strength measurements at low pressures, but differ significantly from high‐pressure shear strength measurements by other investigators. The data also fit a simple empirical formula relating the temperature and pressure dependence of the shear strength.

ISSN:0021-8979    

DOI:10.1063/1.1709300

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Several interesting features concerning the reflection of obliquely incident electromagnetic waves from a dielectric half‐space moving at relativistic velocities are discussed. Two cases of the movement are considered: (a) The dielectric medium moves parallel to the interface. (b) The dielectric medium moves perpendicular to the interface. Both polarizations of the incident wave are considered: (a) The electric field of the incident wave lies in plane of incidence. (b) The electric field of the incident wave lies in a plane which is normal to the plane of incidence. The variation of the Brewster angle as a function of the velocity of the medium is examined in particular. Also considered in detail are the energy relations of this moving boundary problem.

ISSN:0021-8979    

DOI:10.1063/1.1709301

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

It is shown that silicon doped with B, Al, Ga, P, As, or Sb is a fast, sensitive, and efficient detector of 10.6‐&mgr; radiation from a CO2laser. Experimental results are presented for several moderately compensated specimens. An investigation was made at temperatures from 23°–40°K of the electrical characteristics, time constant, noise, and 10.6‐&mgr; photoresponse. The results are interpreted in terms of idealized, non‐Ohmic impurity‐photoconductor models. Detectivities within a factor of three of ideal background‐limited performance were attained, and response times in the nanosecond region were found.

ISSN:0021-8979    

DOI:10.1063/1.1709302

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Single crystals of cobalt‐doped thallium manganese fluoride (TlMn1−xCoxF3) have been studied by antiferromagnetic resonance for cobalt concentrations of 0≤x≤0.001. In this range the antiferromagnet remains cubic at 4.2°K, with its crystalline anisotropy varying linearly with cobalt concentration fromK1/M= −5.1 Oe for pure TlMnF3to +8.2 Oe, passing through zero forx=0.0004. Other magnetic properties, such as Ne´el temperature and exchange field, are unaffected by the small cobalt concentrations. Multiple non‐equivalent equilibrium spin‐orientation states, which have been observed in spin‐flopped resonance in the ⟨100⟩ and ⟨110⟩ crystal planes whenK1/Mis greater than zero, are described.

ISSN:0021-8979    

DOI:10.1063/1.1709303

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Single crystals of Bi containing ordered MnBi filaments (∼4% by volume) have been grown by a zone‐melting technique in the presence of a longitudinal magnetic field. Metallographic and x‐ray examinations showed that the long axis of the MnBi needles (∼4 &mgr; in diameter and ∼200 &mgr; long) was aligned along the preferred growth direction, which is perpendicular to the trigonal axis, of the single crystalline Bi matrix. The long axis of the needles is thecaxis of MnBi, the direction of easy magnetization. These needles rendered the eutectic Bi‐MnBi crystal ferromagnetic with a strong magnetic anisotropy, as revealed by magnetic moment measurements. The dependences on temperature and magnetic field of its electrical and thermal properties were essentially the same as those of pure Bi, which is a nonmagnetic semimetal. Thus, a controlled growth technique has been used here to achieve an ordered structural arrangement that combines the properties of semimetals and ferromagnetics into a single material.

ISSN:0021-8979    

DOI:10.1063/1.1709304

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Landau damping of longitudinal electron oscillations in a boundless, Maxwellian, collisionless magneto‐plasma is studied in an approximation which neglects the coupling to transverse oscillations. The longitudinal component of the electric field is explicitly developed as a superposition of exponentially damped sinusoidal oscillations for an initial perturbation sinusoidal in space and Maxwellian in velocity. A formula is derived for the minimum time for Landau damping, i.e., the time which must elapse before only the lowest frequency oscillation is significant.An analysis is presented for the strong magnetic field case in which &ohgr;c≫2&pgr; |sin&thgr;| (&lgr;/&lgr;D)&ohgr;p, where &ohgr;cis the cyclotron frequency, &thgr; is the angle between the propagation vector and the external magnetic, &lgr;Dis the Debye length, &lgr; is the wavelength, and &ohgr;pis the plasma frequency. For this case the Laundau zeros move continuously along rays from their location in the absence of a magnetic field when the angle&thgr; is zero to the origin as this angle approaches 90 deg. With this same variation of angle the minimum time increases from the null field value to infinity. These results are independent of the cyclotron frequency, provided that the above inequality is satisfied. For no external magnetic field the minimum time is found to increase with increasing wavelength and decreasing thermal spread of the initial perturbation when the spread is less than that of the unperturbed distribution.

ISSN:0021-8979    

DOI:10.1063/1.1709305

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The interdependent problems of determining the current density field and the electron temperature and number density profiles in nonequilibriumJ×Bdevices are formulated realistically, and solved numerically. The two‐dimensional formulation includes the important effects of thermal and concentration diffusion, thermal and velocity boundary layers, and finite reaction rates on the electrical behavior of crossed‐field devices, and allows each effect to be studied separately. As a result, this study predicts and interprets the asymmetry of the current distribution that has often been reported in experimental studies. Computations in potassium‐seeded nitrogen plasmas have shown that streamwise nonuniformities can be very pronounced both in the core and in the electrode boundary regions of high‐current density devices. In the limiting case of instantaneous reaction rates (ionization equilibrium at the electron temperature) and instantaneous energy relaxation, current lines in the core display a striking increase in slope throughout the narrow region between the insulator segments, where they become almost perpendicular to the flow direction and much denser than in the remaining part of the core. Under these conditions, there is no evidence of current ``shorting'' through the boundary layer, although theTedistribution is such that high‐luminosity regions appear over the electrodes, particularly over the cathode. The general and flexible methods developed in this study allow realistic evaluation of suggested designs under various operating conditions.

ISSN:0021-8979    

DOI:10.1063/1.1709306

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A similarity principle is introduced for multipacting discharges in geometrically similar electrode configurations. For phase‐similar breakdowns, it is shown that the rf and dc voltages are proportional to (fl)2, the electron velocity is proportional tofl, and the rf and dc magnetic fields are proportional tof, wherefis the frequency of the rf voltage andlis the characteristic length for similar configurations. If the electron kinetic energy is similar as well,flmust also be invariant. These similarity relations are applied to previous work in parallel plates, and it is shown that the results of the different multipacting cases can be unified under the similarity principle. A physical explanation is also given for the success of the constantvf/viassumption, whereviandvfare the respective emission and impact electron velocities. When applied to geometries other than parallel plates, the similarity principle yields important scaling information and allows prediction of the breakdown behavior in these geometries, where analytical solutions are otherwise not available.

ISSN:0021-8979    

DOI:10.1063/1.1709307

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A high‐pressure optical cell, modeled after the Drickamer cell I, has been developed for absorption and luminescence in the 0–40 kbar and in the 77°–383°K ranges. The windows consist of a NaCl‐sapphire combination in order to retain transparency down to 77°K. The window transmission varies as a function of temperature and/or pressure between tolerable limits. The largest contribution to nonreproducible errors in intensity measurements arises from the sample—salt interfaces. The pressure scale has been approximately determined at low temperatures by means of sudden changes in light transmission during the KCl phase transformation around 20 kbar. The liquid—solid phase transition in heptane is also studied.

ISSN:0021-8979    

DOI:10.1063/1.1709308

出版商:AIP    

年代:1967

数据来源: AIP