THE ANALYST'S COLUMN L.T.HALLETT,Science Editor
F WE spoke about a transmission elec ] tron microscope, many people might ask whether we were talking about the familiar electron microscope found in most well equipped laboratories, and the answer would be yes. There are, how ever, emission electron microscopes, al though we rarely hear about them because they have not been commercially available and are therefore built by individual inves tigators for special studies. I t is a fact, however, that the very first electron mi croscopes were the emission type, but their development did not keep pace with that of the transmission microscopes because of the wider field of application and the higher resolving power of the latter. The Philips Research Laboratories, Eindhoven, Netherlands, has been pioneering in build ing a commercial emission instrument through the efforts of Rathenau (formerly with Philips but now a professor at the University of Amsterdam) and his assist ant, Baas. Burgers and Ploos van Amstel in 19.35 published details of their emission micro scope and showed how it could be applied to metallurgical problems. More elabo rate microscopes with higher resolution were built later by others, and research studies at the present time are being car ried out with such instruments at t h e Sorbonne, at Ohio State University, and at the Bell Telephone Research Labora tories. The Philips Co., as a result of its own studies begun in 1948 and those of others which are published, has built three experimental instruments which it hopes will be the forerunners of a number of others after their usefulness has been demonstrated. One of these instruments is located in the Naval Research Lab in Washington, D. C.; one instrument is located at the North American Philips Co., Mt. Vernon, Ν. Υ. ; and one remains in the Philips Laboratory in The Nether lands. On November 21 and 22 the North American Philips Co. organized a very informal symposium to acquaint inter ested scientists with the possibilities of emission microscope uses and also to demonstrate the instrument. R. D. Heidenreich of the Bell Telephone Labora tories discussed the application of the instrument which he had built. While much of his talk was devoted to the appli cation of ferrous metals, he is really using the instrument to stud}' vacuum tube problems of interest to the Bell Tele phone Co. LaVerne Birks, Naval Research Laboratory, described his brief experience with the Philips instrument, reporting on some preliminary studies on the operation of the equipment using metal alloys. Baas described the prin ciples, operation, and application of the Philips instrument. The emission instrument superficially is like a conventional transmission instruVOLUME
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ment in that it has a microscope column, vacuum system, and power supply. In fact, t h e projector assembly, including a standard 35-mm. film camera and a 4inch viewing screen, is identical with the corresponding part of the new Philips 75-kv. transmission electron microscope. The magnification can be varied 150 to 800 diameters. While t h e theoretical resolution is 100 Α., practically 1.000 A. is obtained, owing to unavoidable roughness of the sample surface. In order to obtain a visible image of the polished surface of a specimen, it must be a conductor and must be heated and acti vated by evaporating onto the surface a thin deposit of another metal such as barium or cesium. Because t h e absorp tion of the activator depends on the exposed lattice planes of the metallic specimen, grains of identical composition but different orientation can be distin guished by differences in brightness. On a heterogeneous alioy the emission of the various composing phases will generally be different. The electron optical investigation of a cesium-activated surface is feasible be tween 400° and 740° C. Barium is used between 850° and 1250° C. Restrictions in the metals and alloys which can be investigated are determined by whether the specimen reacts with the activator or the formation of an oxide film, impene trable to electrons. I t can be seen that traces of gaseous impurities in the evac uated system can cause trouble if they react with the metal surface. Reactive metals such as Al, Si, Ti, and Mg are therefore almost impossible to study, while less reactive ones like Fe, Co, Ni, Μη, Cr, and Cu can be readily studied. Hydrocarbons have been found to be excellent t o remove the oxide layers; wafer and oxygen must be absent. Using barium the instruments can be used for 5 to 6 specimens before cleaning, while with cesium it should be cleaned after each specimen. I t can be gathered from the above considerations that severe limitations attend the use of the approach. I t would seem best suited to the study of transformation in heat treat ment of alloys and not for "cold" speci mens, which can more easily be handled with etching and observation with the light microscope, or by x-ray diffraction techniques. In fact, Heidenreich said, in his opinion, a combination of such tech niques was the best approach to such metal lurgical studies. In the study of phase transformations a motion picture tech nique yields the most information because the changes are relatively rapid. Baas showed some films which he had taken, and they showed very vividly and clearly the phase changes which took place. The speakers were candid in their dis cussion and emphasized the limitations as well as the virtues of this new approach to the study of alloys. Birks made the comment that much work must be done before the usefulness of this approach could be assessed. He noted that like microscopy in general, there appeared to be considerable art involved. One prob lem is the fact that when manganese containing steels are subjected to vacuum and heat, the manganese might boil out. Heidenreich said he hoped to extend the range of the instrument by varying the wave length which might make it possible to bring into view grains at a characteristic wave length. Certainly metallurgists should give this approach careful consideration in the future. The price of the Philips model is around $20,000, and it is hoped t h a t if sufficient interest is shown, 25 instruments can be scheduled for the near future. VOLUME
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