Research of the Air Research and Development Command. LITERATURE CITED
( 1 ) Banexitz, J. J., Kenner, C. T., ~ \ & A L . CHEAT. 24, 1186 (1952). ( 2 ) Berkhout, H. IT., Goosens, S . , Chem. Weekblad 4 8 , 3 2 (1952). ( 3 ) Brnnisholz, G., Genton, RI., Plattner, E., Helv. Cham. Acta 36, 782 (1953). ( 4 ) Campbell, D. N., Kenner, C. T., A N ~ LCHEM. . 26, 560 (1954).
( 5 ) Cheng, K. L., Kurtz, T., Bray, R. H., Ibid., 24, 1640 (1952). ( 6 ) Diehl, H., Goetz, C. A., Hach, C. H., J . Am. Water W o r k s Assoc. 42, 40 (1950). (7) Flaschka, H., Fortschr. chem. Forsch. 3 , 2 5 3 (1955). (8) Flaschka, H., Huditz, F., Rader Rundschau 2, 181 (1952). ( 9 ) Gehrke, C. W.,Affs rung, H. E., Lee, Y. C., A N A L . (?HEM. 26, 1944 (1954). (10) Harvey, A. E., Komarmy, J. hI., Wyatt, G. RI., Ibid., 25, 498 (1953). (11) Holtz, A . H., Chem. Weekblad 47, 48 (1951).
Pribil, R., “Komplexometrie,” Chema 01, Prague, 1954. Reilley, . K,, Porterfield, W. W., h A L . CHEST. 28, 443 (1956). Schmid, R. W.,Reilley, C. S . , J . Am. Chem. SOC.78, 5513 (1956). Schwarzenbach, Gerald, “Die ‘komplexometrische Titration,” Ferdinand Enke TTerlag,Stuttgart, 1955. Schwarzenbach, G., Biedermann, W., Bangerter, F., Helv. C h i m Acta
8
29, 811 (1946).
RECEIVEDfor review August 29, 1956. Accepted October 30, 1956.
Detection of Polymorphic Phase Transformations by Continuous Measurement of Electrical Resistance PAUL D. GARN and STEWARD S. FLASCHEN Bell Telephone laboratories, Inc., Murray Hill, N. J.
b Because polymorphic phase transformations are in some cases not detectable by differential thermal analysis, the continuous measurement of electrical resistance was studied. The method i s useful as a supplement to differential thermal analysis or calorimetry for the detection of crystallographic phase transitions and the determination of melting points. The systems studied had specific conductances ranging from 3 X to lo-‘ ohm-’ cm.-’. The data obtained have been useful in determining phase diagrams. The method should be especially useful at room and lower temperatures, where differential thermal analysis i s not readily accomplished. In addition to its application to the detection of phase transformations by the measurement of discontinuous changes in resistivity, the method i s readily adaptable to the following of reactions involving a continuous resistivity change.
T
of the temperatures of polymorphic phase transformations is of considerable importance in several fields of research. The most convenient method in general use is differential thermal analysis, but this has not been successful in a few specific problems. The authors’ apparatus is not satisfactory for use below about 1 5 0 ” C. because of the high heat capacity and thermal lag of the furnace. Furthermore, the heat effect from some phase transformations is very large and the resulting endothermic or exothermic peak may be so broad that a thermal
effect from another transition as much as 30” to 40” C. higher in temperature is completely masked. I n the vicinity of 100” C., the driving off of adsorbed water may indicate an endothermal reaction. These deficiencies are not generally serious, but in some cases, the desired information is not obtainable by use of differential thermal analysis. Other supplementary methods of detection of phase transformations have been considered. The simplest method, considering either procedure or apparatus, is the continuous measurement of
HE DETERMINATIOX
268
ANALYTICAL CHEMISTRY
SP
electrical resistance of the sample as it is heated or cooled through the transition region. As a material undergoes a phase transformation, the thermal coefficient of electrical resistance changes because of a change in the electronic energy levels or, in some cases, because of a change in the mobility of ions within the lattice (or liquid, in the case of melting), This change of resistance may be detected by the use of a bridge circuit and appropriate recording equipment. Other investigators have measured
~E’ooi‘v 17r-777’
lOOK LOG
Figure 1. Bridge circuit used in measurement of continuous resistance
resistance (of loir resistance material) as a function of temperature or as a function of the prior heat treatment of the sample. Glaser, Moskowita, and Post (6, 8 ) used resistance measurement at room temperature in ascertaining the formation of borides and carbides. Glaser and Aloskowitz (’7) also measured the resistances of these materials at high temperatures. Andrew (1) has used resistance measurements a t room temperature in demonstrating the formation of carbides when a tungsten filament is heated in the presence of hydrocarbons. Shimizu ( I I ) , on the other hand, has used point-by-point measurement of electrical resistance a t elevated temperatures in his study of the thermal dehydration of clays. Colner and Zmeskal ( 3 ) have used a continuous recording of the potential drop across a portion of a stainless steel bar in their study of transformations in the steel. In this work, discontinuous changes in resistivity of materials ordinarily classed as insulators are used to detect phase transformations and to determine the temperatures of these transitions. Reisman, Triebn asser, and Holtaberg have concurrently used a similar method in their studies of the potassium niobate-potassium tantalate system (IO). Other means of detecting phase transformation-for example, dielectric constant measurements (2, ld)-may be used. Such techniques generally require apparatus not commonly used in chemical laboratories.
pLA ,LEADS
BRASS
PORCELAIN BUSHING
are welded together and formed into a loop so that the upper electrode may be pressed uniformly against the sample. The inset in the cell prevents the cell from short-circuiting in case the sample melts. The cell is set into a vertical platinum-wound furnace. MATERIALS
SWITCH LEAF
WASHER
4
‘SAMPLE
Figure 2. Sample holder for rneasurernent of continuous resistance
TO RECORDER
TO 6RlDGE 4
t :
PL
- lox
R h WIRE
C’SC
“Chemically pure” barium titanate was obtained from the Titanium Alloy Manufacturing Division, National Lead Co., Lot R-53395-116. The sample was ground to -270 mesh, pressed into a pellet a t 1000 pounds per square inch, and sintered 4 hours a t 13f5’ C. The barium, lanthanum, and strontium titanate samples w-ere prepared by mixing the required amount of Ianthanum oxalate, barium and strontium carbonates, and titanium dioxide, grinding the whole sample in a ball mill, and making into pellets as described for barium titanate. The potassium niobate and the potassium niobate-potassium tantalate materials were prepared by grinding together the proper quantities of potassium carbonate, niobium oxide, and tantalum oxide, and sintering. The samples were prepared by grinding to -270 mesh, pressing into pellets a t 1000 pounds per square inch, and sintering a t a temperature determined by the composition.
2,4’C./MIN. SC4-E
\
CV
I 2
T
APPARATUS
A direct current bridge was used for the detecting circuit, modified to adapt the circuit to the wide range of sample resistances. One fairly typical design is shown in Figure 1. The ratio switch was designed t o meet the requirements of the microvolt amplifier available, the maximum input impedance being 10,000 ohms. The potential range switch provides for bridge potentials of 1.5, 0.15, 0.015, and 0.0015 volt, using a single dry cell. The logarithmic potentiometer was used so that the sensitivity of adjustment would not vary with sample resistance. The signal across the ratio switch is fed into a microvolt amplifier and XI, XDrecorder. The amplifier is a Lee& & Northrup stabilized direct current microvolt amplifier, Type 9835-B. The recorder is a Leeds &. Sorthrup Speedomax XI, XZ,Type 69955, with a chart speed of 6 or 24 inches per hour. The Xz function is for temperature measurement. The XI function is zero-centered, as is the amplifier. The XI range is 5-0-5 mv. The signal from a thermocouple placed close to the sample is fed directly into the recorder. By operating in an “off balance” condition, samples with resistances above 1000 megohms could be used. The first sample holder used (Figure
Figure 3. Cell for continuous rneasurement of electrical resistance a t high temperatures
2) was made from a pair of switch leaves, a brass plate, and necessary insulators. The furnace is a Nichromewound vertical Hoskins furnace, FD101, 2 inches in inside diameter, 3.4 amperes. An oil bath consisting of a beaker nearly full of peanut oil is set inside the furnace. The oil is stirred by a motordriven stirrer.
The useful range of the method has been extended to higher temperature by the use of the sample holder shown in Figure 3, in which the upper electrode has been partially withdram-n. The sample is placed between tmo platinum disk-shaped electrodes. The upper electrode is weight-loaded by the cylindrical insulator to maintain good electrical contact. The electrodes are cut from 10-mil platinum sheet, to diameters of 12 and 9 mm. for the upper and lower electrodes, respectively. The lower lead wire may be welded to the electrode or, more conveniently, bent into a ring and the electrode pressed down against this ring. The thermocouple leads, 0.015 inch in diameter,
0 . 36 ’C,/ M I N.
Figure 4. Relation between ternperature and resistance of barium titanate
The sodium bromide \vas a commercial reagent grade material. Synthetic quartz was prepared in this laboratory. The sources of the raw materials for the preparation of barium, lanthanum, and strontium titanate and potassium niobate-tantalate were: BaC03, c . P . , Fisher Scientific Co. SrC03, c.P.,Fisher Scientific Co. Lan(CzOa)a,c.P.,Fisher Scientific Co. TiOp, c . P . , Sational Lead Co. KVCO,. c.P.. Merck SjbpO6: 99+%, Amend Talos, 99+%, Amend VOL. 29, NO. 2, FEBRUARY 1957
269
PROCEDURE
The sample was pressed into a cylindrical pellet about 10 mm. in diameter and 0.5 to 3 mm. thick, using a WatsonStillman press at 1000 pounds per square inch. Brass washers were pressed on the ends of the pellet to obtain a large contact area and this assembly n-as inserted between the switch leaves. Spring tension then holds the sample in place. The sample holder was then set onto the furnace so that the sample was immersed in the oil. The potential across the bridge, the resistance ratio, and the variable resistance were adjusted to give an initial signal of low magnitude, so that the recorder pen was near the center of the chart, and the heating cycle was begun. The heating was controlled manually by use of a variable autotransformer. After the heating cycle record is obtained, a similar cooling record may be obtained if desired.
talate sample. The experimental value by the continuous resistance method was 145" C. A series of barium titanate-strontium titanate samples was tested by the resistance measurement technique. The transition temperature and composition are related by the equation T
=
110.6
- 3.11 c
where T is the temperature in degrees centigrade and c is the composition in mole per cent of strontium titanate. The probable errors in the constants are 1 0 . 9 2 and k0.024, respectively. The deviations from the best line are within the batch formulation error. No other type of measurement has been found nearly so satisfactory for determining the relation between transition temperature and composition of barium titanate.
RESULTS
A demonstration of the definition obtainable by this method is shonm in Figure 4, which gives curves for heating rates of about 0.4' and 2.4" C. per minute. The transition temperature of this barium titanate is 125.0' k 0.5" C. This transition is not readily detected by differential thermal analysis. Actual resistances were not determined in any case, but the actual resistances of the specimens described are in the order of 10 to 10,000 megohms. A measure of the resistance was taken and continuously recorded. The specific conductances of the materials which have been studied range from 3 X 10-4 to ohm-' cm.-' (9). The method has also been used in the construction of phase diagrams. The high temperature phase diagram of the potassium niobate-potassium tantalate system has been given (6). The low temperature portion of this diagram has been under investigation. A portion of the data was obtainable by differential thermal analysis, but because of equipment limitations, the entire phase diagram could not be constructed from differential thermal analysis data; for this reason, the continuous resistance method is being used. The data obtained by the two methods agree well. Differential thermal analysis gives a transition point of 209" C. for potassium niobate and 182" C. for 95 mole % niobate-5 mole % ' tantalate. Resistance measurement data give values of 210" and 181" C. for these transitions. Potassium niobate undergoes another transition a t 417" C. The transition temperature varies ITith added tantalate. Extrapolation of the data for 0, 5, and 10 mole yo potassium niobate yielded a value of 150" C. for a 607, potassium niobate40% potassium tan270
ANALYTICAL CHEMISTRY
220°C.
700'C.
A
55OoC.
'7..\
60OoC.
7 3 8'
T
Figure 6. indications A.
Phase transition
Quartz
6. Sodium bromide
Figure 6 shows the results obtained for the a 4 transition of quartz, and for the melting of sodium bromide, using the high temperature cell described above. The quartz transition is a change from one hexagonal structure to another, involving very slight structural changes and very little activation energy (13). This transition is detectable only as a change in slope except a t high heating rates. The sodium bromide results are predictable. As the melting point is approached, the resistance decreases more rapidly than would be expected of an ionic crystal because of premelting. Subsequent melting of the sample results in a very rapid drop in resistance. The temperature at which the decrease occurs confirms the melting point (740" C.) found by differential thermal analysis ( 4 ) . CONCLUSIONS
Figure 5. tions
Phase transition indica-
A. KNbo.90 Tao.~oOa(ceramic) 6. KNb03 (single crystal)
The method yields, in general, only a change in slope for the mixed systems which have been studied, but for pure materials. such as barium titanate or potassium niobate, the crystalline phase transitions are extremely sharp. Figure 5 s h o w the kinds of indication that appear t o be characteristic of the mixed systems and pure crystalline materials. The bariuni titanate sample showed an anomalous behavior (Figure 4), in that the resistance increased sharply a t the transition point.
The use of a continuous measurement of electrical resistance compared to point-by-point measurement is analogous to the use of differential thermal analysis compared to calorimetry. In each case, one obtains a measure of a dependent variable which in itself is not of immediate interest in order to determine the value of the independent variable (temperature) a t a-hich an event, in this case a phase transformation, occurs. The experimental technique is simpler and less time-consuming than point-by-point measurement of electrical resistance and should thus prove, as is differential thermal analysis, a useful and widely applicable supplementary laboratory technique. LITERATURE CITED
Andrews, M. R., J . Phys. Chem. 27, 270-83 (1923). Budnikov, P. P., Barro, V. M., Mchledlishvili-Petros an, 0. P.,
Soobsheniya Akad. %auk Ghtin S. S. R. 14, 27-31 (1953). Colner, W. H., Zmeskal, O., Trans.
A m . SOC. Metals 44, 1158-68 (1952). (4) Flaschen, S. S., Garn, P. D., unpublished measurements. ( 5 ) Garn, P. D., Flaschen, S. S., ANAL. CHEM.29, 271 (1957). (6) Glaser, F. W.,J . Metals 4 , 391-6 (1952). (7) Glaser, F. IT., Moskowitz, D., Powder M e t . Bull. 6 , 178-85 (1953). (8) Glaser, F. W.,hloskowitx, D., Post,
B., J . A p p l . Phys. 24, 731-3 (1953). (9) "International Critical Tables of Numerical Data, Physics, Chemistry and Technology," vol. VI, pp. 148, 341, McGraw-Hill, New York, 1929. (10) Reisman, A,, Triebwasser, S., Holtsberg, F., J. Am. Chem. SOC.77, 4228-30 (1955). . (11) Shimizu, S., Tohoku Imp. Uniu. Sci. Repts. 22, 633-61 (1933).
(12) Yon Hippel, A., Breckenridge, It. G . Chesley, F. G., Tisza, I,aszlo, Ind. Eng. Chem. 38, 1097-109 (1946). (13) \Veils, A. F.,,,"Strurtural Inorganic Chemistry, p. 463, Oxford Cniversity Press, London, 1949. RECEIVEDfor review June 30, 1956. Accepted October, 18, 1956. Division of Analytical Chem~stry, 127th >feetin& ACS, Cincinnati, Ohio, March-April 1955.
Analytical Applications of a Differential Thermal Analysis Apparatus PAUL D. GARN and STEWARD
S. FLASCHEN
Bell Telephone laboratories, Inc., Murray Hill, N.I.
b An apparatus for differential thermal analysis is described, which is useful as a tool in the study of inorganic materials. In it have been incorporated as many advantages as possible of systems previously set up. Thermograms of magnesium carbonates and talcs indicate transition and decomposition temperatures. A phase diagram of the potassium niobate-potassium tantalate system was determined from differential thermal analysis data. Thermograms of potassium maleate were prepared from different starting materials. Identification of the magnesium carbonates and talcs according to the source aids in setting up firing schedules in the production of ceramics. The phase diagram of the potassium niobate-potassium tantalate system is of interest in the study of ceramics.
D
thermal analysis has been used for many years for detecting phase transitions, principally in minerals. Excellent reviews of differential thermal analysis apparatus and techniques have been prepared by Grim (6) and by Smothers, Chiang, and Wilson (16). I n addition to its normal use in determining temperatures of phase transformations, differential thermal analysis is useful as a control tool or as a routine tool for comparing similar but not identical materials. As a control tool i t may be used t o distinguish raw materials quickly and easily in those cases in n-hich the treatment of the material must be modified if slight changes in the material are encountered. As a comparison tool, differential thermal analysis may be used in some rases to test materials that IFFERENTIAL
yield anomalous results by other tests. Determination of transition temperature of samples with systematically varied compositions yields the data necessary to establish the phase diagram of the system. DESIGN OF APPARATUS
The apparatus used in this work is unique only in that a n attempt has been made to incorporate as many advantages as possible of systems previously set up. The high temperature furnace (Figure 1) was adapted from the design used by Coffeen (4).
It has a platinum heating element wound on refractory alumina tubing. A thin cylindrical platinum shield is placed inside the furnace tube. The shield is grounded, with a platinum wire at some convenient point, in order to eliminate noise of thermionic origin. The furnace is mounted on ball-hearing slides and is moved horizontally to enclose or expose the sample holder. To prepare the furnace for us?, a platinum-platinum-lO~o rhodium thermocouple in a 3/32-in~hinsulating tube is placed in the control thermocouple well and the wires are led to the vertical hole in the core and through the ceramic tube to outside terminals. A differential thermocouple consisting of platinum-lO% rhodium Ivith a joining wire of platinum, palladium-20yo gold, or palladium-lO~o gold is inserted from the side into two of the sample wells and the !vires are led to outside terminals through the four-hole tube. The sample is placed in one of the wells and aluminum oxide as reference material is placed in the other. The furnace is then rolled into place and is ready for use. The furnace thermo-
couple wires are 0.015 inch and the differential thermocouple wires are 0.005 inch in diameter.
A block diagram of the system built by Leeds & Northrup for controlling the temperature and recording the furnace temperature and differential teniperature is shown in Figure 2. The strip chart recorder is a n Xi, Xz Speedomax Model 69955, which gives a continuous plot of the furnace temperature from the control thermocouple and the differential temperature from the differential thermocouple on a single chart. The program controller consists essentially of a motor-driven slide-n ire so designed that full travel takes place in 2.5 hours. A similar slide-wire is mounted on the shaft of the furnace temperature potentiometer. The signals from the two slide-wires are compared by the control unit, L. 6: N. Model 10864. The control unit introduces proportional band, reset, and rate time action to control sensitivity and eliminate overshoot and "hunting." The unbalance signal controls a motor-driven Powerstat, located behind the panel, advancing it or backing it off as required to maintain a heating or cooling rate of 10" C. per minute. The stabilized direct current voltage amplifier, L. & X. Model 9835-B, amplifies the differential temperature signal before it is sent to the recorder. It provides six ranges from 25-0-25 p v . to 1-0-1 mv., corresponding for the nlatinum-lO% rhodium us. palladium20% gold thirmocouple to- ranges of about 0.7°-00-0.7c to 27"-Oo-27O C. The equipment is designed to heat the furnace to a preselected temperature from 0" to 1500' C. a t a constant rate of 10' C. per minute. When the desired tcmperature is reached by the furnace, the program unit operates to maintain that temperature or to cool the furnace VOL. 29, NO. 2, FEBRUARY 1957
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