Small Metallic Grains Research Articles

Optical properties of granular magnesium films

We studied the optical properties of magnesium cermets formed by co- sputtering magnesium metal and MgF 2. An absorption peak was observed at about 2500 Å, in good agreement with the predictions of the Maxwell-Garnett theory. The weakness and large width of the absorption is indicative of very small metal grains.

Effect of charging energy on superconductivity in granular metal films

Previous model calculations of the superconducting properties of aggregates of superconducting grains coupled by Josephson tunnel barriers did not take into account the charging energy associated with small metal grains. It is pointed out that in granular superconductors, in which the grain size is 20--100 A, the charging energy can be orders of magnitude larger than the Josephson coupling energy and superconducting coupling between the grains is quenched unless the grains are in intimate electrical contact.

High Tc's in Ultra-Thin Films

Measurements are reported which show that the maximum Tc that has been reached in quenched ``soft-metal'' films is correlated with the maximum Tc computed from the analysis of McMillan,1 assuming an optimum value for the electron-phonon coupling. The enhanced electron-phonon coupling may be attributed to the lowering of the phonon frequencies, which would be caused by the small metallic grains and greatly increased surface area expected in these porous quenched films. Molecular dynamics calculations by Dickey and Paskin2 indicate that the changes in the phonon frequencies in small particles are large enough to account for the changes in Tc. Data are presented which show that the region with the greatest increase in Tc occurs within a few monolayers of the metal-vacuum, metal-insulator, or metal-metal surfaces.3 If similar, near monolayer structures of, high Tc, transition metal alloys can be made, temperatures above 25°K should be possible.

Light from Reflection Nebulae. I. Dielectric Versus Metallic Particles

view Abstract Citations (19) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Light from Reflection Nebulae. I. Dielectric Versus Metallic Particles Greenberg, J. Mayo ; Roark, Terry P. Abstract Calculations have been performed for a wide variety of models of reflection nebulae and for a range of nebular grain optical characteristics. Several variations in the theory have been considered, based on pure single scattering and single scattering modified by internal nebular extinction. All the calculations use the scattering by spheres as given by Mie theory. A criterion based on the B - V color difference between the star and the nebula is developed which may be used to exclude either classical dielectric grains (radius 0.3 ) or small (radius 0 Os metallic, graphite, or dielectric grains. For those cases in which the illuminating star is at or near the front surface of the nebula the small metallic and dielectric grains always produce a significantly bluer color of the nebula than do the classical dielectric grains. Furthermore, on the basis of a homogeneous plane-parallel slab geometry for the nebula, the degree of variability of the nebular color is much greater for the classical dielectric grains than for the metallic grains. Publication: The Astrophysical Journal Pub Date: March 1967 DOI: 10.1086/149084 Bibcode: 1967ApJ...147..917G full text sources ADS | Related Materials (2) Part 2: 1970ApJ...161..947G Part 3: 1970ApJ...161..961H

Recent Falls of Meteorites in France and Italy

THE French Academy of Sciences has recently received several important and interesting accounts of the fall of two or three meteoric masses in France and Italy. On the 23rd of July, about half-past five on a still afternoon, with a perfectly clear sky and a bright sun, a violent report, followed by a rumbling noise, was heard in the commune of Lancé, canton of St. Arnaud (Loir-et-Cher). On the following day it was ascertained that the noise had been heard over a wide area of country, and had caused much uneasiness; and a letter arrived from a landowner of Lile-Bouchard, announcing that he had seen a “fiery lance”shooting across the sky in a direction from S.W. to N.E. with great swiftness. Whilst on its way its point appeared to split, giving rise to two meteors, which continued their way parallel to each other for some distance. Another observer south of Tours had also seen them, and described them as having the shape of a bottle, and being of an orange colour. M. De Tastes, who communicated the first account to the Academy, on proceeding to St. Arnaud, was fortunate enough to learn that one of these meteorites had been seen to fall near Lancé, and he was also successful in finding it. Its weight was 47 kilogrammes (about 103 lbs.), and it had penetrated to a depth of 1.40 metres (about 5 ft. 9 in.). On being removed, it broke into three pieces. Of the second meteorite nothing was heard for some time, but it was ultimately found at a place called Pont-Loisel, about 12 kilometres (71/2 miles) to the south-west of the place where the other had fallen; and an account of it is given to the Academy by M. Daubrée. It is of exactly the same mineralogical character as the one first found, thus showing it originally belonged to it, but its weight is only 250 grammes, and it had only penetrated to the depth of about half a metre. On ascertaining the course of the meteorite, it was found that this, the smallest portion, had fallen first, and that the larger one had continued its course for some distance farther. In this respect it resembled the meteorite which fell on March 14, 1864, near Orgueil (Tarn-et-Garonne), in which the smallest portion, weighing about 15 grammes, first fell, and then the heavier one, weighing 40 grammes. M. Daubrée has recently analysed the meteorite, and his results are somewhat remarkable. The largest piece is of an unequal spheroidal shape, with a rounded surface; it is covered all over with a crust, probably caused by the incandescence and superficial fusion. In appearance the fracture is black, and almost basaltic looking, showing a globular structure and numerous small spheroidal grains. Here and there small metallic grains are to be seen, yellow in colour, like iron bisulphide, these and other metallic-looking grains showing much better when the surface is polished. Its specific gravity was 3.80. Treated with water, a very small quantity of chloride of sodium dissolved out, and M. Daubreée remarks that this is not the first time that this salt has been found in meteorites; and he brings forward evidence to show that it could not have been derived from the soil in which the meteorite was buried, but that it must have formed part of it when it fell. No traces of any salts of potash, nor of any sulphates or hyposulphates could be found. Dissolved in nitric acid, a silicate was found, which was proved to consist chiefly of magnesium and protoxide of iron, and there was an undissolved residue, part of which was colourless, the remainder dark black. By means of spectrum analysis, copper was thought to be recognised; but calcium, barium, and strontium were shown to be absent. No carbon was found; but, as usual, cobalt and nickel accompanied the iron. The following is the complete analysis:—