From Current Literature

On Thin Transparent Metallic Sheets

The well-known phenomenon of the passage of light through thin sheets of metal has been studied on metallic layers deposited electrolytically or obtained by sputtering onto some transparent supporting substance (glass, mica, etc.). Attempts to obtain free thin sheets without a supporting layer, for which purpose the metal was deposited on some other metallic plate that was then dissolved, led to the production of free sheets, in the best case, down to \(0.01\,\mu\); but already at a thickness of \(0.5\,\mu\) these sheets exhibited considerable porosity and inhomogeneity. Last year K. Müller1 developed a method

Sector aperture: \(i = 57^\circ\ 136^\circ 33.4\)  265  253  \(\mu\mu\)

Fig. 1.

for obtaining free metallic sheets, completely homogeneous in structure, with a thickness 10 times smaller than had hitherto been possible.

For this purpose he, as before, deposited the metal electrolytically on a plate of another metal (copper) having a small thickness (\(0.01\,\mathrm{mm}\)); over the thin layer of metal he then deposited a second—covering—layer of copper, protecting the thin interlayer obtained from any external mechanical influences. By removing the two outer copper layers, the author succeeded in obtaining gold and nickel sheets 6 cm in diameter and \(0.04\,\mu\) thick, and 2 cm in diameter and \(0.02\,\mu\) thick and, with still smaller dimensions, only \(0.01\,\mu\) thick. In addition, he obtained thin sheets (\(0.04\,\mu\)) of silver, platinum, and iron. It is interesting to note that a gold sheet supported by a metallic ring is very sensitive to movements in the air, whereas nickel sheets display considerable elasticity: at a thickness of \(0.04\,\mu\)

and, with a diameter of 6 cm, such a sheet permits a deflection of 1 mm. Moreover, these sheets can withstand considerable pressure; used as screens covering windows of 0.6 mm, nickel sheets 0.25 μ thick can withstand a pressure of 250 mm of mercury.

The thickness of a sheet (about 0.04 μ) was determined by weighing, using the known specific gravity, and, for smaller thicknesses, from the data obtained—by the amount of electricity passed. Taking the atomic constants given by Hölle, Bolin, and Vegard, one may assume that the thinnest of the gold and nickel sheets obtained (0.01 μ) contain 30 atomic layers.

Fig. 1 shows a photograph obtained in this way with different amounts of light entering the spectrograph and with various thin sheets placed in the path of the light. It turned out that the gold sheet is most transparent for green rays. Photometry of the light passing through the gold showed that the maximum transmission (at a thickness of 0.01 μ, corresponding to \(\lambda = 546\,\mu\mu\)) shifts, as the thickness of the sheet decreases, toward longer waves, becoming less sharp. At the same time, as the thickness of the gold sheet decreases, its reflecting power also shifts from the yellow rays toward the red end of the spectrum.

Fig. 2. Microphotographic images of a scale divided into 0.01 mm: a) with a nickel sheet \(2.5 \cdot 10^{-6}\) cm thick between the objective and the scale; b) the same without the sheet.

Fig. 2 gives a microphotograph demonstrating the complete homogeneity of a thin nickel sheet: on the right is a photograph of a 0.01 mm scale, on the left the same scale photographed through a nickel sheet of 0.025 μ. The transparency of nickel sheets increases when they are heated in an oxygen atmosphere, but after heating in hydrogen the sheet again assumes its usual properties. Owing to their large specific surface and the associated good cooling, thin sheets can withstand currents of considerable strength. Thus, for example, a nickel sheet of 0.04 μ can withstand a load of 0.3 ampere per 1 cm of width. The thin metallic sheets obtained by Müller can evidently serve as material for many varied investigations in almost all areas of physics, both as objects of study and as construction material.

P. Belikov.

1. C. Müller. Sitzungsber. d. preuss. Akad. 25, 1925. ↩