摘要:

RETM5and RE2TM17(RE=rare earth, TM=transition metal) permanent magnets are discussed in this review. The scientific and technological considerations which have guided their development are described. Along with a discussion of the various research efforts reported in the literature, they help establish the framework within which most of the work can be integrated and understood. In regard to the RETM5alloys, the review limits itself to the technologically significant SmCo5materials; others are discussed minimally, and only to the extent that they contributed to overall magnet development efforts. Likewise, from among the fabrication technologies, the review is limited to those processes which have been demonstrated as capable of producing magnets in bulk useful form. Areas covered in this review include phase diagrams, crystal structures, magnetocrystalline anisotropy, coercivity mechanisms, fabrication technologies, processing‐microstructure‐property interrelationships, and thermal effects related to stability of magnet flux. Directions for future, additional work are discussed briefly as well.

ISSN:0021-8979    

DOI:10.1063/1.341084

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A pyroelectric photothermal spectroscopy has been developed for evaluating optical absorption spectra of thin films. The temperature change in a film illuminated with chopped monochromatic light is measured with transparent temperature sensors consisting of pyroelectric materials, and the output voltage is employed to calculate absorption spectra. In a practical system, the absorption coefficient down to 102cm−1can be determined for thin films 1 &mgr;m thick at desired temperatures.

ISSN:0021-8979    

DOI:10.1063/1.339875

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

An experimental investigation of the input impedance of a square loop antenna with a dielectric core is reported. The effects of the dielectric constant and the height of the core on the loop impedance are given. A comparison between the measured and theoretical results is included.

ISSN:0021-8979    

DOI:10.1063/1.339876

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The mechanism of polarization competition in laser diodes with a lateral quasi‐index‐guiding (QIG) structure is analyzed generally by way of the effective index approximation using a simplified QIG laser model. The influence of the relevant waveguide parameters on the polarization‐dependent threshold current of QIG laser diodes is investigated in detail by example of &lgr;=1.3‐&mgr;m ridge‐waveguide lasers. Thereby, it is found that for intermediate values of the effective index step, the TM mode exhibits a higher gain and lower threshold current, whereas for pure gain guiding or strong index guiding, the TE mode prevails. This behavior, which compares excellently to published experimental results, is proven as a basic feature of the two‐dimensional waveguiding mechanism in QIG devices. Accordingly, the effect of stress‐induced anisotropy of the optical gain has been found to be of minor importance as the origin for TM‐polarized QIG lasers made from lattice‐matched heterostructures. It is further demonstrated that, for certain device parameters, the QIG lasers with a small effective index step exhibit somewhat higher threshold currents than the purely gain‐guided devices of identical geometry.

ISSN:0021-8979    

DOI:10.1063/1.339877

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Novel structures for carrier‐induced electro‐optical phase modulation in crystalline silicon are examined. A new dual‐injection field‐effect transistor structure for guided‐wave light modulation at 1.3 &mgr;m is proposed and analyzed. It consists of an elongated cathode‐anode‐gate structure integrated in a rib waveguide. Dual‐gate and single‐gate control are considered. The overlap between the plasma charge density and the optical guided mode is computed. For a cathode‐anode voltage of 0.32 V, the effective refractive index of the waveguide mode changes by &Dgr;N=1×10−3when the gate voltage is altered by 12 V. Numerical estimates of the bias current, pinchoff voltage, interaction length, and modulator speed are given.

ISSN:0021-8979    

DOI:10.1063/1.339878

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The temporal behavior of excited state (3P2,3P1) of Kr atoms in a discharge‐pumped KrCl excimer laser has been studied using the hook method and the sidelight method with nanosecond time resolution. On the basis of these experimental results, the mechanism of the formation of KrCl* excimer is also discussed.

ISSN:0021-8979    

DOI:10.1063/1.339879

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The ultrasonic nondestructive characterization of heterogeneous media is difficult due to the inherent inhomogeneity of the material. Previous studies have shown that the amplitude information is not fully correlated to the medium characteristics. Our main interest is to evaluate the mean spacing between heterogeneities randomly distributed in the medium. The mean spacing between heterogeneities is related to the autocorrelation function for the echoes. However, this function in the case of highly heterogeneous media does not reflect the medium characteristics: the impulse response of the whole measurement set (especially the transducer) biases the obtained signature. Our studies are based upon the introduction of an original method giving a nonbiased signature of highly heterogeneous media derived from the spectrum of the rf backscattered signal. This signature immediately gives the mean heterogeneities spacing parameter.

ISSN:0021-8979    

DOI:10.1063/1.339880

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Simple form solutions are obtained for approximate two‐dimensional equations of motion and dominant conditions for free edges of energy‐trapped thickness vibrations of piezoelectric crystal plates. The resulting algebraic frequency equations are employed to calculate frequencies of fundamental and anharmonic overtones of thickness‐shear vibrations in infinite quartz strips of SC cut with a pair of free edges and in rectangular AT‐cut quartz plates with all four edges free. For circular quartz disks with free edge, frequencies of fundamental and harmonic overtones of thickness shear in AT cut, and frequencies of fundamental and anharmonic overtones of thickness shear in BT cut are computed. Presently predicted results are compared with either previously calculated results or existing experimental data. It is found that comparison is very close for all these cases.

ISSN:0021-8979    

DOI:10.1063/1.339881

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We study the backscattered signal from thin cylindrical shells irradiated by short ultrasonic pulses. Various materials (duraluminum, stainless steel, copper) and various thicknesses of the shells are considered. The shells are filled with air and immersed in water except in one experiment where the surrounding medium is ethanol. From the experimental results, we deduce the variations of the group velocity of the waves circumnavigating the shell. Two such waves are observed experimentally and their dispersion curves are compared with those of the waves propagating in plates immersed in the same fluid.

ISSN:0021-8979    

DOI:10.1063/1.339882

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

High‐speed elongating metal jets, such as are produced by shaped charges, are known to break up into a number of fragments after some stretching. Attempts to explain the lengths of the segments have usually related to Rayleigh’s work on the separation of a jet of water into droplets as a result of surface tension and on extensions of that work, although more recently research based on the ideas of plastic flow has had some success in showing the growth of instabilities of approximately correct wavelength. In this paper, the full equations of axially symmetric plastic flow are used, along with the von Mises condition, and a series solution sought to first order in a small parameterY/&rgr;W2, whereYis the yield strength of the metal, &rgr; its density, andWthe fall in velocity along the jet. The disturbances to velocity, shear stresses, and boundary oscillation arising from an initial small disturbance in the axial component of velocity have been found analytically. The results show that the amplitudes of the disturbances grow towards equality with the radius of the elongating jet for times and wavelengths compatible with observations of break up. Two possible criteria for break up are examined: namely, ‘‘necking’’ and the creation of voids in the jet when all principal stresses become tensile. There is radiographic evidence that both kinds of break up can occur. The results obtained here confirm the significance of these mechanisms, but are not able to distinguish sufficiently between them as the prime cause of break up in a given instance.

ISSN:0021-8979    

DOI:10.1063/1.339883

出版商:AIP    

年代:1988

数据来源: AIP