H o t Stage for Microscopic Observations between Room Temperature and MARJORIE J. VOLD
AND
350' C.
TODD M. DOSCHER
Department of Chemistry, University of Southern California, Lo, Angeles, Calif.
THIS
note demribea a small furnace found useful in studying phase relations with the aid of microscopic observation between room temperature and about 350" C. The design problem is that of maintaining a temperature gradient of up to 100' C. per mm. in one direction (so that the sample can be a t a high temperature with the microscope objective still a t room temperature) but with substantially zero gradient in the plane perpendicular thereto, over an area of 2 sq. cm. Additional requirements are a means for rapid and accurate measurement of the temperature and a fairly low lag in varying the temperature.
space into which is inserted the heating element composed of a 1-foot length of No. 26 Nicrome resistance wire wound as a spiral The heating element is wrapped around the brass ring fairly tightly and is separated from it by two stri s of mica in order to prevent the development of hot spots. &he brass ring with winding, is then embedded in position with shredded a&estos The leads to the heating element are brought out through in the block, and secured to the binding posts 8~ shown. Two '/l@-inch lass disks, separated by an air space, 1/8rinct asbestos spacer, Et into the well created by the brass ring, leaving approximately inch from the top of the ring to the top glass disk. These glass disks serve to insulate the heated air chamber of the furnace from the condenser system of the microscope, as well aa prevent too great a heat loss from the center of the main hot stage chamber. This is an effective technique, since the lower chamber is heated by the brass ring almost as effectively as is the upper chamber. The disks may be chipped from microscope slides-held under water while chipping t o prevent cracking-or a oircular glass cutter may be used. The microscope objective is protected from the furnace by a Pyrex disk 0.125 inch thick, cut from a well-annealed plate. The latter condition is important if the furnace is to be used with a polarizing microscope, in order to prevent depolarization by the disk. The 0.125-inch disk is used rather than another dummy air space, a~ formed on the underside of the main hot stage chamber, because of its greater convenience when changing specimens This disk is sufficiently thick to prevent large heat losses from the center of the heated air chamber-heat conduction from its periphery, where it makes contact with the asbestos insulation, probably accounts for much of the heat loss to the environment-but does not conduct so well as to serve as a source of heat for the air I -IfI I c chamber. This was confirmed by using a pair of reversed thermocouples-one junction sealed to the glass disk and the other Figure 1. Cross Section of Microscope H o t Stage positioned in the center of the air space-and determining that the temperature of the underside of the disk was an average of C. Heating element C. Asbestos disk 0.5" to 1.0' C. lower than the temperature of the air chambrr. D . Capillary on wire coil H. Pyrex dirk
. drilled
g- t
E.
K. Glass disks
Brass ring
F. Mica strip
Commercially available heating stages are generally designed for biological applications and are not serviceable much above 60" C. Those few which can be heated to high temperatures are generally made of metal and therefore have a fairly high heat capacity which remlts in enormous temperature lag. The exterior of these stages becomes warm, with resultant inconvenience in manipulation. Metallic parts intended to distribute the heat uniformly usually undergo oxidation and deterioration with short use, and temperature inhomogeneity develops far greater than the usually claimed 1 above 200" C. Custom-made hot stages for specific applications have been described (1-4, 6, 11). Wallace and Willard (10) devised a fairly simple, sturdy, and generally applicable hot stage not unlike the present design, which nevertheless suffered from several defects. The heated chamber was 2.5 cm. (1inch) in depth and made of Alundum wound with Kichrome wire; this construction led to the development of severe vertical temperature inhomogeneity within the furnace proper. For use with a 1OX objective the specimen had to be raised so close to the top of the stage that high temperatures were not attained readily, nor without the development of large horizontal temperature gradients. O
Figure 2.
Section through Control Area of H o t Stage
A . Magnrria block B . Asbestos paper C. Heating element D. Capillary E. Brass ring
The present furnace (Figures 1 and 2) is constructed of a lightweight porous f i e brick (Johns-Manville magnesia brick). The bottom section consists of a block, 2.5 inches square and 0.75 inch high, from which the cfntral portion has been drilled, leaving the terraces, AA' and BB . A thin brass ring, 0.75 inch in diameter and 6/16 inch high, fits into a slot, thus creating an annular
F. Mica strip C. Thermocouple coil H. Binding port
J . Thermocouple leads
A l/&nch hole is drilled through the block and the brass ring to permit the entrance of a thermocouple for measuring the temperature within the ring. A thermocouple is preferred to a thermometer because of its much smaller heat capacity and i t 9 ability to measure a much wider range of temperature. 154
ANALYTICAL EDITION
February, 1946
C o per- constantan thermocouple wire, k o . 30 B. & S. gage, has been used successfully. Since the e.m.f. is 0.04 to 0.05 mv. per O C., a Leeds &
Northrup potentiometer and galvaoqmeter, with a sensitivity of 0.015 miaroampere per mm., is satisfactor for measurements good to 0.3" C. two or three loops of the thermocouple wire are left within the air space, conduction of heat along the wire is low enough so that standard calibration tables may he used. Otherwise, the calibration may run 2 to 3' C.lower rhan the reference tables at 300" C.
155
flat capillaries about 12 to 15 mm. long, supported on a wire coil of the thermocouple wire, which in turn rests on the surface of the uppermost bottom glass cover of the furnace (see diagrams). To test the accuracy of the hot stage, melting points were determined on the following:
6 Figure 3. Temperature Difference between Center of H o t Stage and Other Parts
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1 division 1/10 inch. With this design, the air space within Determined et 250' C. the brass ring can be maintained a t temperatures up to 350" C. with the entire exterior of the block only faintly warm to the touch. Moreover, the heat capacity is sufficiently low so that a small change in heat input is reflected in a rapid adjustment of the temperature to its new value. The maximum rate of temperature rise compat ihle with maintenance of uniform temperature within the air
Temperature, C. Observed Reported
Substance Tin Urea a
231.7 * 0.3 132.7 1 0 . 3
231.9" 132.6
National Bureau of Standards sample
I n addition, phase changes in the system sodium nitrate-silvei nitrate were determined to see if the points a t the liquidus and solidus curves could be determined with accuracy. These result* are: Mole % ' Sodium Nitrate
Phase Change Melting Melting Freezing Melting
0.00
29.9
100.00
Temperature, C. Observed Reported (61 208.8 f 0 . 3 217.6 0.3 235.2 * 0.3 308.1 f 0 . 3 f
208.6 217.6 235.0 308
In some cases, where the sample is spread ovei more or less circular area, the temperature homogeneity may be insufficient for the most exacting work. To provide for such cases the authors have developed a modification of the furnace. The necessary changes in construction are indicated in Figure 4, and the temperature distribution obtained is shown in Figure 5. ~t
H
A
Figure 4.
Cross Section
of Modified H o t Stage
A . Microslide on wire coil B ; Microscope stage C. Fan shalt
$pace is 1.5' to 2.0" C. per minute. The temperature gradient under such circumstances is illustrated in Figure 3, which was determined by using two reversed thermocouple junctions and a galvanometer sensitive to 0.1 a C. The temperature rise is controlled by means of a variac and a L2-ohm resistance, which is interposed in series between the furnace and the variac. The mean spontaneous rate of cooling with the heating element turned off is 20" C. per minute between 300" and 100"C., and 10O C. below 100" C. The minimum distance from the microscope objective to the center of the heated space is approximately 4.2 mm., which with the usual design of microscope makes feasible the use of a 16-mm. (lox) objective, so that a magnification of. the order of 100power can be readily obtained. For intermittent observations the 0.125-inch Pyrex disk may be replaced by a '/winch disk, permitting the use of a 20X objective. However, under such circumstances the heating rate should be below 0.7" to 1.0" C. per minute and the temperature should not exceed 200" to 250" C. in order to maintain the temperature homogeneity as indicated above. The sample has to be mounted on a slide, cover slip, or other mount that can be contained entirely within the heated space. The field of view is adjusted entirely by moving the whole furnace about on the microscope stage. With a rotating stage, the furnace can be fastened to it, and moved with the adjustable spacers. The authors have been studying soap systems which must be protected from the air during heating and from vapor loss (7, 8, 9). It has proved convenient to contain such samples in sealed,
I n this modification the bottom section is 1 inch high. The brass ring is '/16,inch high and is wrapped with two turns of coiled Nichrome wire instead of one, allowing for a heated space slightly greater than 0.5 inch in depth. Homogeneity is secured by the use of the stirrer blade, which is cut out from a piece of sheet copper, Q/le by S//S inch. Eight blades are cut out as shown, and bent so as to circulate the air acrosa the air space. T h e Pyrex disk should be cut to make a fairly tight fit in order t o prevent leakage. The shaft, F~~~~~5 m T~~~~~~~ made of l / d n c h Bessemer steel rod, ture ~~~~~~b~~~~~ with is silver-soldered t o the blade. The Modified H~~stage block and the brass ring are slotted to receive the shaft, so that the blade I division 1/11 Inch. Determined at 250' C. clears the top disk by at least '/a2 and preferably '/lo inch. The shaft is driven by an air motor (Aero-Mix, Precision Scientific Co.) which is clamped by a brass block directly to the microscope stage, so that it is moved integrally with it. The specimen is again supported on a coil of the thermocouple wire, which is stiff enough to be suspended as shown.
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ACKNOWLEDGMENT
The authors wish to express their appreciation to R. D. Vold for his interest during the development of this instrument. LITERATURE CITED
(1) (2) (3) (4) (5) (6) (7)
Clevenger, IND. ENG.CHEM.,16, 854 (1924). Cottrell, J.Am. Chem. Soc., 34, 1328 (1912). Cram, Ibid., 34, 954 (1912). Ertel and Lange, 2.anorg. Chem., 171, 168 (1928). Hiasink, Z . physik. Chem., 32, 542 (1900). Oberhoffer, 2.Elektrochem., 15, 634 (1909). Vold, M. J., Macomber, M . , and Vold, R. D., J. A m . Chem. SOC., 63 1 6 8 f\ -l-R 41~ ----I. --I
(8) Vold, R. D., J . Phys. Chem., 43, 1212 (1939). (9) Vold, R. D., and Vold, M. J., J . Am. Chem. SOC.,61, 808 (1939). (10) Wallace and Willard, J . Chem. Education, 8, 706 (1931). ( l l j ,Walton, IND.ENQ.CHEM.,ANAL.ED., 1, 106 (1929).
