INSTRUMENTATION by Ralph H. Müller
Microscope for Ultrahigh Temperatures Developed T N T H I S SEVENTEENTH YEAK of com-
•*• men ting on instrumental advances and developments it has been our continuing good fortune to have advice, suggestions, and comments from experts the world over. Some have even been kind enough to point out errors we have made, but so politely that they were apparently unaware of our deftness in such matters. Microscope for Ultrahigh Te mperatu res
In our search for matters new and interesting we found one of the finest examples of modern instrument design right down the corridor, some 50 feet from our office and laboratories. It's one thing to turn out a sleek, streamlined machine or instrument and quite another to develop an instrument which represents a half-century jump over prevailing practices and techniques. D. M. Olson, of the University of California's Los Alamos Scientific Laboratory, one of the country's out-
standing design engineers, has developed a microscope for ultrahigh temperatures. This is a reflecting microscope, whose objective is a concave ellipsoidal mirror. With it, specimens can be examined and photographed at 30 to 600 X, in vacuum or controlled atmosphere, at any temperature from ambient to 2500° C. (4500° F.). Although designed primarily for metallurgical research, it is obviously of wide interest to investigators wishing to examine materials in controlled environment, at very high temperatures, either statically or dynamically. Existing high temperature microscopes have severe limitations, lacking either sufficient resolution or the ability to examine specimens at very high temperatures without damage to their objective lenses. Olson's approach is a return to a very old microscope system—that based on reflection rather than refraction. Since a reflecting objective is inherently and completely achromatic, it can be manufactured without optical
complications in the large diameter required to combine high resolution with great working distance. I n this microscope, the objective is a concave elliptical mirror, whose working distance is 4 inches and numerical aperture 0.47. Specimens as large as 1 / 2 inch in diameter can be heated to temperatures above 2500° C. by radiation from a resistance heater around them and viewed by the mirror. Furnace and microscope are enclosed in a sealed vessel in which the environment can be a vacuum as small as a fraction of a micron or an inert atmosphere u p to about 4000 p.s.i. A general view of the high temperature microscope is shown in Figure 1 with eyepiece and/or camera at the left. The furnace which contains the specimen and initial portion of the optical system is seen at the right foreground. A more detailed schematic (Figure 2) shows the furnace. Heating is provided by a partially split tantalum tube which surrounds the specimen and its supporting structure.
Figure 2 A. Quartz Window B. Reflecting Objective C. Aperture Plate D. Specimen E. Heater Tube F. Radiation Shields G. Electrode Η. Χ Motion I. Focus J. Quartz Window
Figure 1 .
High temperature microscope.
General view
VOL 34, NO. 2, FEBRUARY 1962
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INSTRUMENTATION
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Heat loss is reduced by tantalum shielding consisting of 15 wrapped layers of 5-mil tantalum sheet, coaxial with the heater tube which it sur rounds. Top and bottom shields are stacks of 10 tantalum washers each 10 mils thick. Heating current to the furnace is through water-cooled copper conduc tors. Power is supplied at 0 to 20 volts by a 15-kv.-amp. step-down transformer, with a variable voltage transformer in the 220-volt primary circuit for temperature control. About 400 amperes at 9.5 volts will maintain the furnace at 2500° C , and excess power is available for operation at still higher temperatures, in which case, however, the tantalum heater and inner radiation shields must be replaced by tungsten. The specimen stage, within the heater tube, is supported by a vertical post assembled from tungsten rods and a perforated tube intended to provide rigid specimen support with high re sistance to heat flow. As shown in Figure 2, the support post can be raised or lowered by means of a cam permitting V 2 -inch travel. This motion is controlled by the drum marked focus. The support post is mounted in a plate moving in two hor izontal dovetail slides and can be moved to the extent of 1/2 inch along the χ or y axis by two screw microme ters outside the furnace shell. This is seen more clearly in Figure 1. The furnace assembly, specimen support, and positioning mechanisms and microscope objective arc all con tained in a double-walled, water-cooled vessel of stainless steel. The arrangement is seen more clearly in Figure 3, in which the upper assem bly, containing the reflecting objective and quartz window, is swung aside.
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The microscope objective is a con cave, quartz-base mirror, coated with rhodium for high reflectance. A thin layer of silicon monoxide over the rhodium provides for abrasion resist ance and easy cleaning. Its concave reflecting surface is a segment of an ellipsoid of revolution with major and minor axes of 62 and 21.9 inches, re spectively. The mirror segment (4 inches in diameter with a 2-inch di ameter central opening) is symmetrical about the major axis of the ellipsoid, which coincides with that of the fur nace axis. The specimen to be examined is raised or lowered until its upper sur face is at the first focal point of the mirror, 4 inches below the vertex of the ellipsoid. Light radiated or re flected from the specimen surface is collected by the mirror and directed toward the quartz exit window in the bottom of the furnace. The various components, constituting obstructions (Figure 3), were so designed and ar ranged to cause minimum obstruction to the reflected beam, so that some 84% of the light reaches the exit win dow. Since the second focus of the ellipsoidal objective is 10 feet from its vertex, a folded optical system of pre cisely oriented front surface plane mir rors brings a real image of the speci men surface to view with economy of space under the instrument table. The image at this point is magnified by 30 diameters. By means of a R.amsden eyepiece or projection lens a total mag nification of 600X can be attained. As seen in Figure 2, the quartz window at the top of the furnace permits the temperature of the specimen to be measured with an optical pyrometer. In addition, a lamp housing mounted over the window (Figure 1) provides intense illumination of nonluminous
The Cahn GRAM Electrobalance is designed for weighing small samples, from one gram down as far as you can go.
See the n e w RG Recording Version at the Pittsburgh Symposium Booth 460 WRITE FOR FULL DETAILS:
instrument company Figure 3. Details of furnace assembly, specimen support, positioning mechanisms, and microscope objective
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144 A
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ANALYTICAL CHEMISTRY χ
INSTRUMENTATION
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Figure 4. Photomicrograph of delta ferrite iron
Recording
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Another fine instrument in the Cary,tradition of highest quality is the new Model 15 Recording Spectrophotometer. Significant design advancements contribute to its outstanding, versatile performance. Instrument operating limits, 1750-8000 A, extend precision usefulness overa broader range. Reduced beam size (1.0x0.3 cm) assures maximum reliability with minimum samples. Coupled scan and chart drive affords extreme operating simplicity with single variable speed control. APPLIED PHYSICS
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INSTRUMENTS Raman/UV/IR
Recording Spectrophotometers
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specimens or supplements the radiation from hot surfaces. More extensive details of Olson's microscope are to be found in TJ. S. Patent 2,969,712. A forthcoming paper in Metals Progress by D. M. Olson, Β. Β. Brixner, and M. C. Smith will deal with the operation of the microscope, its research possibilities, and unusual examples of microscopy at ultrahigh temperatures. Brixner is the inventor of the 1,000,000-frames-per-second camera [J. Opt. Soc. Am. 45, 876 (1955)] which we had the privilege of describing some years ago. Smith is the noted metallurgist and author of "Principles of Physical Metallurgy" and "Alloy Series in Physical Metal lurgy" (Harper and Bros., 1956). One striking example of an applica tion of this microscope is shown in Fig ure 4. This is one of the first photo micrographs ever made of delta ferrite iron, a material long known to exist but never seen before. It is easy to become enthusiastic about the achievements of one's good friends and colleagues. That we are not alone in this attitude is attested by the fact that at the October Metals Congress and Exposition in Detroit, Olson was awarded a medal for show ing the microstructure of niobium at 3270° F.—recognition for "results by unconventional techniques." In our "ho-hum" corner we still hear a few bleats from the incompetent who say that these things are gadgets and are not nearly as important as the scientific results which they reveal. This can well be a generation which will peer through this microscope, finding what it had long suspected but had never seen, and view with awe and as tonishment, some things it never dreamed of.
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