摘要:

In this paper the end effect loss in a dc magnetohydrodynamic generator with rectangular cross section is considered. The case for nonconducting walls is examined, and a simple expression for the losses in terms of the maximum power output is obtained. The end effect loss is compared to viscous and turbulent flow losses and it is shown to be the predominant loss over a wide range of operating conditions.

ISSN:0021-8979    

DOI:10.1063/1.1736100

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

A rolling texture transition from the (110) [1¯12] common silver type to the (123) [4¯1¯2] copper type was produced in high‐purity silver by changing the temperature of deformation. The texture determined at 0°C for a strip rolled at 0°C was a simple (110) [1¯12]. Rolling at 200°C produced a texture of the (123) [4¯1¯2] type, plus a cube texture component due to partial recrystallization. Our results also suggest that the relatively high oxygen content in common silver is probably not responsible for the formation of the (110) [1¯12] texture. The change in texture from one type to the other in high‐purity silver can be obtained by merely varying the rolling temperatures.

ISSN:0021-8979    

DOI:10.1063/1.1736101

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

It is shown that the stability limit for pressurized horizontal floating zones is related to the eigenvalues of a membrane of the same shape. In particular, a rectangle of lengthaand widthbis stable if&pgr;2(4/a2+1/b2)≥&rgr;g/&ggr;,and a circle of radiusRis stable if(&bgr;11/R)2≥&rgr;g/&ggr;,where &rgr; is the density,gis the acceleration of gravity, &ggr; is the surface tension constant, and &bgr;11=3.8317 is the first root of the Bessel functionJ1(x). This latter case checks with experiments on melted circular zones of tin.

ISSN:0021-8979    

DOI:10.1063/1.1736102

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

A theoretical analysis of the efficiency of a thermionic converter is made in terms of the following parameters:Va, the potential difference between the top of the potential barrier in the interelectrode space and the Fermi level of the anode;VL, the potential drop across a load impedance in series with the converter; andVl, the potential drop in the necessary electrical connection to the cathode. The analysis is carried out by developing an expression for the efficiency of the converter and then maximizing this expression with respect toVLandVl. This method yields optimum values of load impedance, cathode lead geometry, and cathode work function in terms ofVa, cathode temperature, cathode emission constant (usually denoted byA), and effective emissivity of the cathode. A hypothetical example is worked out numerically and the results show that (1) a low value ofVais required for high efficiency, and (2) relatively low values of cathode work function are required for maximum efficiency at ordinary cathode temperatures.

ISSN:0021-8979    

DOI:10.1063/1.1736103

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Creep in pore‐free polycrystalline aluminum oxide has been studied as a function of temperature, grain size, and strain rate. The behavior may be described by&sgr;&agr;d2&egr;˙, wheredis the mean grain diameter. Deformation is not controlled by basal slip but by a diffusional creep process as described by Nabarro and Herring, [F. R. N. Nabarro,Conference on Strength of Solids(The Physical Society, London, 1948), p. 75; C. Herring, J. Appl. Phys.21, 437 (1950)].

ISSN:0021-8979    

DOI:10.1063/1.1736104

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The stacking fault probability has been measured by x‐ray means on a number of alloys of gold, silver, and copper with varying zinc content. By using these probabilities, the relative magnitudes of the stacking fault energies have been deduced.

ISSN:0021-8979    

DOI:10.1063/1.1736105

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

A sensitive vacuum microbalance was used to determine the sputtering yields for the argon‐silicon system over an energy range of 34–800 ev. Preliminary data for the ion bombardment of silicon with CO2was also obtained. Variation of the discharge voltage provided information on the relative sputtering effectiveness of Ar+and Ar2+ions. Extrapolation of the low‐energy data indicates a probable threshold energy of 15–20 ev. The sputtering data indicate that the target surfaces were clean and reproducible. The experimental method is shown to be well suited for sputtering studies.

ISSN:0021-8979    

DOI:10.1063/1.1736106

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Photomicrographs of pore and grain boundary structures in sintered powder compacts are presented to provide the basis for qualitative description of the important phases of the course of densification. From this guide, appropriate grain shapes and pore shapes and locations are selected for the formulation of diffusion sintering models. The principle models presented are for bulk diffusion transport with the grain boundaries as vacancy sinks when the pore phase is continuous and coincident with three grain edges, and also when the pore phase is discontinuous and located at four‐grain corners. These models predict that the rate of density change is constant when the diffusion coefficient and grain size are constant. The need for simultaneous isothermal densification and grain growth data is indicated. The explicit change in densification rate with discontinuous grain growth is predicted in terms of pore spacing and grain size.

ISSN:0021-8979    

DOI:10.1063/1.1736107

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

During sintering in alumina powder compacts, the density has been found to increase linearly with the logarithm of time, and the grain size increases with the one‐third power of time. Incorporation of the time dependence of grain size increase into latestage bulk diffusion sintering models (from Part I) [R. L. Coble, J. Appl. Phys.32, 787 (1961)] leads to corrected models by which a semilogarithmic behavior is predicted. The presence of density gradients in normally fabricated pellets makes impossible the deduction of whether theoretical density will be achieved from the early stages of thecourseof densification. Diffusion coefficients calculated from the intermediate and later stages of sintering bear order‐of‐magnitude agreement with those calculated from the initial‐stage sintering measurements in alumina. All diffusion coefficients from sintering data are higher than Kingery's measured diffusion coefficients for oxygen. It is hypothesized that the sintering process must then be controlled by bulk diffusion of aluminum ions while the oxygen transport takes place along the grain boundaries. In controlling the sinterability of alumina to theoretical density, it appears that magnesia does not ``inhibit'' discontinuous grain growth, but instead increases the sintering rate such that discontinuous growth nuclei do not have time to form.

ISSN:0021-8979    

DOI:10.1063/1.1736108

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

An approximate solution is found of a boundary‐value problem arising from the continuity equation in an inhomogeneous semiconductor, leading to rotational current vectors. Results are used to predict the effect of carrier‐concentration gradients on magnetoresistance. The predicted weak‐field effects are especially significant in degenerate semiconductors andn‐type III–V intermetallics where the ``intrinsic'' magnetoresistance is small. In strong fields, even small gradients in carrier concentration can completely alter the field dependence of the magnetoresistance. Experimental results indicate that transverse currents, which do not occur in the simple case discussed, do appear in general, and further perturb the magnetoresistance. The influence of inhomogeneous magnetic fields is discussed briefly.

ISSN:0021-8979    

DOI:10.1063/1.1736109

出版商:AIP    

年代:1961

数据来源: AIP