Differential Thermal Analysis by the Dynamic Gas Technique ROBERT L. STONE Robert 1. Stone Co., 3374 Wesfhill Drive, Austin 4, Tex.
b The differential thermal analysis apparatus used in this study provides means of controlling pressure in the system, composition of the gas which flows through the test powders during the test, and also the temperature of the system. Five test procedures are prescribed where only one of the variables i s dynamic and the other two are held constant. Thermograms are given which illustrate the following types of reactions: oxidation-e.g., manganese oxide, lignite, cotton fiber, coffee, and plastics; thermal decomposition of carbonates (dolomite), hydrates (copper sulfate), and salts (ammonium perchlorate); suppression of reactions by pressure and/or b y gas-e.g., lignite, ammonium perchlorate, Teflon, etc.; crystallographic inversions-e.g., ammonium perchlorate, quartz; evaporation of iiquids b y evacuation; catalytic activity; and melting and freezing of pure substances and mixtures-e.g., paraffin, soap, etc.
T
HE PURPOSE of this paper is to illustrate the use of the dynamic gas method of differential thermal analy.is (DTA) in studying many types of reactions wherein the vnriades, temperature, pressure, and gas-phase composition are controlled and are individually or collectively varied during
the DT.4 run. Excellent reviews of the literature on the historical development of the DTA technique include a paper by Murphy (4) and a book by Smothers and Chiang (9). Some authors, in using DTA results, hand trace the thermograms produced by the recorder to make a smoothlooking curve. Frequently very valuable diagnostic features are lost by doing so. For this reason, the illustrations used in this paper are photos of the original, untouched thermograms because certain very slight inflections are very important. The t,wo principal uses of the DTA apparatus are as an analytical tool for the identification of mineral or chemical species present in t'he test sample, and as a means of observing previously undetermined characteristics of a known substance. This latter use frequently leads to patentable subject matter. The successful use of the method is based on the operator's familiarit,y n-ith and use of the following: (1) Any substance which is stable under fixed values of pressure, temperature, and compositiori wili always be stable under these fixed values but, according to the second law of thermodynamics, when any one of these variables is ten1 must change to rerinm. When any one of these variables exceeds certain limits,
an abrupt redistribution of energy among phases takes place as the system readjusts itself. (2) iiny change-reaction-is theoretically observable by DTA so long as it either absorbs or evolves heat, whether the mass be solid, liquid, or gas. Very sensitive instruments are required for weak reactions. (3) Any observable reaction involves either making or breaking of chemical bonds, absorption bonds. or van der Waals' forces. (4) Changes in energy per unit R-eight (or volume) of a gaqeous liquid, or solid system during a reaction can be the result of a single reaction or the sum of several concurrent or rapidly successive reactions. ( 5 ) DTA does not spell out what net reaction is taking place when an exothermic or endothermic effect is recorded. I t simply records that there is a change in energy content taking place. Knowledge of posqible rractions must he used to develop the meaning of the thermogram. The historical DTA technique and apparatus provided a means of varying 1004~.
200'
Y
STATIC GAS
/
4 Figure 1. Schemotic drawing of pressure chamber and furnace assembly
Figure 2 . Thermogrami cr iiiitic hale showing difference in ~arirlts oetween static ccs and dyncxnlc 3;s vrthods
0111). o w of these 1;ariables. namely, temperature--n.ith no control over the other two variables. pressure and gas~ ~ I : I Svomposition. P Rowland (6, 7 ) dei-(,loped apparatus for control of composition of the gas around the sample holder a t atniosl,lieric (room) pressure. In the prcsent equipment, the gas is forced through the powder during the test. The importance of having the gas flowthrough the powdk rather than around thi. sample holder is illustrated by the following dihcussion where a closed tj-pe of sample cavity is used. At rooni temperature when the powder is plaved in the test cavity, t,he gas is room air. Then, as temperature is raised, the air is replaced by CO,, water vapor, or whatever vapor is being rekaseti by the test material. The gas thus eyolvcd twapcs through the cracks in the seniple holder and a t the same time air is diffusing hack into the powtier. Thus. a t the start of a C0,releasing reaction, the interatitial gas is :iir and a t the height of tlie reaction the gas is CO?; then, as the reaction
x)O
200
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400
500
600
700
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-
30u v
IN NITROGEN
Figure 3. nitrogen
Suppression of oxidation of a lignite by dry
Sample, 2.5% powdered lignite +97.5% a-alumina Both thermograms a t atmospheric pressure
4 Figure 4. Effect of oxygen pressure on oxidation of instant coffee Sample, 5% coffee
+ 95%
alumina
slows down, the CO, is gradually replaced by air. If the reaction is one whose equilibrium temperature is a function of CO, pressure, the shape and temperature of the loop of the therniogram will be affected by the rate of temperature rise, the degree of packing of the sample, the tightness of the qamplr holder, the grain h e . rtc. Experience with th(J dynamic gas holm that for crystallites or grains larger than about 2 microns. these factors have very little effect on tlie thermogram, !mc!ing morr reliability and reproducibility to thi. m u l t > . Grinding may have, pronounced effects on cryst,als a i ,shown by Bradley et nl.
(I). 750mm 0, PRESSURE I IOOOC.
2000 3?00
4900
"0"
600°
700°
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,
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9000
DTh thermograms using a constant, temperature anti vnrying pas coniposition were obtained by Crandall and \\'est ( 2 ) in studying the oxidation of cobalt. I n this ( m e , the gas flowed around the sample rather than through it. 1he apparatus used for this paper provides awuratc cont'rol over a11 three variables. T l i ~ cquililiriuni lint, (or surfncc) can hc cr(JSS?d in t h r w w'ayx: by Iloiding tempcmture x i d prrssuw con>tnnt nntl changing the gas environment j 11y holdiiig ten11 conipusition coristwrit a:id var:;ing pres-
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7
VCL. 32, "2. 12, NCVEMBER 1960
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200.
300.
I I 490
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Figure 6. Effect of COZ pressure on decomposition of dolomite, C a C 0 & g C 0 3 \
Sample, 50% NBS No. 88 dolomite +50% Bo* thermograms a t 1 - a m . pressure
200pv.
alumina
I ATMOSPHERES N2
4 Figure 5. Effect of pressure and gas composition on decomposition of NHICIOI 50% C.P. + 50% alumina
Sample,
ClO,
mentation of the apparatus. The present apparatus is so sensitive that certsin reactions causing a temperature of little o . ~ 3 0C. CLln be detected without Outside interference. APPARATUS A N D PROCEDURES
The basic apparatus w described by
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ANALYTICAL CHEMISTRY
NH4-
Rase and Stone (5) consists of the instrumentation, and the pressure chamber and furnace w s e m b l ~ . The cornplete assembly is manufactured by the Robert L. Stone Co. Figure 1 is a schematic of the DTA part of the apparatus which makes control of pressure and atmosphere possible. The furnace and the sample holder (complete with thermocouples) are placed inside a gastight metal cylinder. The desired pres-
sure is created inside the cylinder by using compressed N2 or compressed air, then the desired controlled atmosphere or dynamic gas is streamed via the tubes into and through the powdered samples as indicated by the path of arrows. The gas then escapes into the pressure chamber. The dynamic gas, streaming through the sample, sweeps away any gas being evolved by the decomposition reactions, thereby maintaining a known composition of gas around the particles of test powder at all times. By using an inert gas (A, N I , etc.) as the dynamic gas, one can study the effects of pressure alone on the test substance. This is the Le Chatelier-Braun effect. By using an active g a s - o n e that participates in the reactions being observed-ne can study the reaction according to the Clausius-Clapeyron and van't Hoff equations. Theoretically with this apparatus all three of the variables could be varied simultaneously, but, in practice, most tests involve holding two of the variabies constant while the third is
IfpC. 100.C.
I
200'
200.
300.
400.
300'
I
1
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100mm. N, PRESSURE 750mm. 0, PRESSURE
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PRESSURE
(A)
Figure 7. A. 6.
Effect of gas pressure
Removal of low temperature water of a Ca-montmorillonite, sample undiluted, dried a t 28' C. and 50% R. H. Dehydration of CuS04.5Hz0. Sample, 1 part C u S 0 4 . 5 H ~ 0-I-5 parts alumina
varied. The same specimen may be subjected to several sets of conditions during the test-for example, a t temperature TI,the gas composition may be changed with the pressure constant, and a t another temperature, Tz, the pressure may be changed with the gas composition constant. There are innumberable combinations, each step involving a simple procedure. There are five basic procedures, (Table I), and of these, 1 and 5 are the simplest and most commonly employed: Procedure 1 is the simplest of the normal DTA variety, and offers a high degree of versatility. The pressure can be anything from a high vacuum to 100 p.s.i.g., and the gas can be any dry gas-N2, COS, OS, He, etc. However, moist gases having a water vapor content of 3% or less can be used. The low water vapor content of the dynamic gas causes the loops of waterreleasing reactions to occur a t lower temperatures than in the static gas technique. For example, in Figure 2, the loop representing loss of mechanical
water reaches its maximum at 80" C. with dynamic dry gas as compared to 110' C. when the test is made under static gas conditions (both tests a t 1 atmosphere pressure). The choice of gas depends on the chemical or mineral being investigated and on the information sought. The thermograms of lignite in Figure 3 illustrate the oxidation reactions in 0 2 atmosphere and distillation reactions in the N2 atmosphere. Figure 4 shows the effects of oxygen pressure on
Table 1.
Procedure Number 1 2
Temperature Rising, fixed rate Rising, fixed rate
the reactions involved in burning an instant coffee. The slight inflections indicated by the arrows and the relative intensities of the large doublet a t 400" C. d 8 e r with the brand of coffee. The significance of the features of these thermograms is being investigated. Figure 5 shows the effects of reactionparticipating gas (NHs) on the decomposition of ammonium perchlorate. Figure 6 also shows the effect of the reaction-participating gas-the effect of COz pressure on the decomposition of the CL~COS portion of dolomite. The choice of pressure for the test is as important as the gas. Figure 7 , A , shows the Le Chatelier-Braun (pressure of inert gas) effect on the removal of interplanar water and cation-hydration water from a Ca-montmorillonitic clay. The strong shoulder in the low pressure run below 100" C. is indicative of vermiculite, and the doublet a t 100" C. shows the presence of H+ and Na+ as exchange cations. Figure 7,B, shows the effect of pressure on the dehydration steps in CuSO,. 5H20. The high-pressure run (the lower thermogram) shows five endotherms and attempts are now being made to determine by thermogravimetric means whether these represent five possible steps in weight loss. Figure 8 shows the effect of oxygen pressure on the reactions of unsintered Teflon. Procedure 2 (change gas composition during the run) is rapidly becoming important; it is employed where the effects of a condensable gas-eg., water vapor-re to be observed. A dry gas is used to a temperature above the condensation temperature of the gas and then the condensable gas is injected. Such runs are classed under Procedure 2 because they involve two dynamic gases during the run. Figure 9 illustrates the application of the procedure on cotton fiber. It shows the difference in reactions under an oxygen atmosphere and in a water vapor a t mosphere. Similar results have been reported by Schwenker (8). Another case where the gas is changed during the run is in studies involving a material that contains organic matter which is best identified in an oxygen
Basic Procedures
Pressure Constant Constant
Gal3
Composition Constant Change at a specific temperature
Constant Change at a specific temperature Cycle or change Constant 4 Constant 5 Constant Constant Cycle Because of difficulty in performing the operations of this procedure, the data are usually obtained by using Procedure 1 at two or more pressures. 3a
Rising, fixed rate
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VOL 32, NO. 12, NOVEMBER 1960
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6 4 Figure 8. Effect of oxygen pressure on decomposition of Teflon Sample, 10% Teflon alumina
+
90%
750rnm. H 2 0 VAPOR
\I
Figure 9 . Effect of gas composition on behavior of cotton fiber Sample, 5% medicinal cotton
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1p.53
.75
.50
MOISTURE
1.0
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CALIBRATION CHART
THERMOGRAMS OF THREE CONSECUTIVE BATCHES Figure 10. Method of determining moisture content of dry powders by evacuation Sample, undiluted; thermogram, room Temperature; instrument range, 20 p v .
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ANALYTICAL CHEMISTRY
+ 95%
alumina
atmosphere. In such cases, an atmosphere of 0 2 is maintained up to about 650" C. and is then changed to h', or CO, to prevent oxidation of the sample holder. Such practice is common in studying clays. Procedure 4 (change pressure a t constant temperatures). In this procedure, the pressure may be either reduced or raised one time for the test, or it may be cycled or repeated as many times as desired. The temperature can be any value within the limits of the apparatus. With this procedure the reaction responsible for the thermal effect is caused by crossing the equilibrium line via pressure rather than temperature change. Figure 10 is an example of this procedure as a very rapid method for determining moisture content of a nearly dry powdered substance. The procedure is extremely simple because no furnace is involved and a complete test cycle requires only 2 or 3 minutes. The sample holder is charged a t room temperature and room pressure. The system is then evacuated a t a fixed rate and the loop in the thermogram starts the instant the pressure reaches the 1-apor pressure of the water at room temperature. For rout,ine tests, a calibrnt,ion curve (Figure 10) is first
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PLOT OF PEAK HEIGHTS
IW n
(A) PARAFFIN
- 1 Y
Figure 12. Melting of paraffin and of a commercial soap Rakes in dry nitrogen
If
Temperature, 1 atm.; sample, undiluted; liquid-type sample holder
Figure 1 1. Oxidation-reduction of o manganese oxidecontaining ceramic body by gas cycling with COn and 0 2 Pressure, 1 atm.; instrument range, 50 p. for all peaks
1000
2000
f
I
COOLING, MELTED AT 2 200
1
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162
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16:
REHEATING PREMELTED AT
dEATlNG FRFSH SPLE
2200 I
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( E ) SOAP FLAKES
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Figure .{. Effect o i prcmelting m freezi::g-me!ting thermograms of 4,4'-isoi.rspyl ,icnedipherioi !?P.92%! -nmpler, ur!dili;ted; liquid.tyPc ;ample holder
obtained by running samples of known moisture contents. The decomposition of carbonates, hydrates, hydroxides, etc., can be studied by the same technique. With such compounds, pressures greater than 1 atmosphere are generally employed. The decomposition of CaC03 has been studied by this procedure a t pressures up to 1000 p.s.i.a. and a manuscript describing the results is in preparation. If the decomposition reaction is reversible, the compound can be reconstituted by raising the pressure. Procedure 5 involvcs an abrupt change of gaseous environment so that the equilibrium line is crossed in the most. drastic manner possible. This i s in cont,rast to Procedure 4 where the equilibrium line is crossed slowly. The abrupt change in gas composition causes the test, substance t'o be transferred quickly from an environment in which it is stable to an environment in which it is highly unstable. The result is that the reaction proceeds extremely rapidly. This technique j> so radically different from the usual procedures that very little information has been published. However, the types of reactions that can be studied are well defined. .Imong then; are: oxidation-reduction, including quant'itative determination of low percentages of combustibles; heat of adsorption of gases; determination of surface area of fine powders; and catalytic activit'y. In this procedure, the gases are cycled-from gas -4 to gas B to gas A to gas B, etc.-as many times as desired. When the second gas is injected, the loop reaches its maximum height in 3 or 4 seconds. Therefore, if the data are obtained by simple injection of t!ie second gas., the results are V O i . 32, NO. : 2 , NOVEMBER 1960
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obtained very quickly so that many test samples can be measured per hour. The tests can be made at any temperature and pressure within the limits of design of the apparatus. For example, the test may be a t room pressure and temperature with the two gases being N2and NH,; or at 300" C. and 40 p.s.i.a. using Nz and Ot as the gases, etc. Catalyst testing as described by Rase and Stone (6) is an example of this technique in which the cycling test was done at 330" C. at 1 atmosphere pressure using dry nitrogen and water vapor. Locke and Rase (3)used a similar procedure. Figure 11 is an interesting example of the gas cycling technique applied to an oxidation-reduction problem: The problem was to establish the temperature at which the test substance became u d & by oxygen. A sample of the material was cycle-tested a t several temperatures. The heights of the exothermic and endothermic peaks were plotted us. temperature. The temperature a t which the curves cross the temperature axis is the temperature a t which the substance could not be reduced by Cot. The same data can be obtained by weight gain and weight loss methods but at least 1 week is required as compared to less than 2 hours by DTA. Change of State Data (Inversions, Melting, Vaporization). Crystallographic inversions of powdered materials are observed with powder-type sample holders. The a-quartz to 8-
quartz inversion is the classical example. Melting and freezing experiments can be done without dilution of the sample by using a sample holder designed specifically for handling liquids and molten materials. I n Figure 12, the thermograms of a commercial paraffin and of commercial soap flakes are given as illustrations. Frequently it is desirable to observe first the melting behavior of a compound and then to study its freezing behavior. The thermograms of 4,4'-isopropylidenediphenol, shown in Figure 13, illustrate such a technique.
coffee. Melting and decomposition thermograms are given for Teflon, paraffin, and soap. Special techniques involving cycling of the dynamic gas a t constant temperature are illustrated with a manganese oxide-containing ceramic wherein reduction-reoxidation thermograms are obtained a t several temperatures and the peak heights are plotted as a function of temperature. A similar technique is used to determine moisture content of nearly dry powder (less than 1% moisture) wherein the test powder is evacuated and the test is carried out a t room temperature in about 1 minute. Other techniques are illustrated.
GENERAL SUMMARY
The controlled pressure and controlled atmosphere dxerential thermal analysis apparatus can be used for many types of thermal analyses that cannot be performed with older types of equipment. All three of the variables, temperature, pressure, and gas composition, are controlled. The effect of pressure alone is illustrated with thermograms of montmorillonite and of CuSO,. 5Hs0 which show several stages of removal of hydrate water. The effect of pressure of the reactionparticipating gas is illustrated with dolomite (Cot), ammonium perchlorate (NHJ, etc. In the case of the ammonium perchlorate, the 300" C. exotherm is eliminated and the decomposition a t 405" C. becomes violent. The oxidation of compounds is illustrated with cotton fiber, lignin, and
END
LITERATURE CITED
(1) Bradley, W., Burst, J. F., Graf, D. L., Am. Mineralogist 38, 207-17(1953). (2) Crandall, W. B., West, R. R., Am. Cmam. SOC.B d . 35, 66-70 (1956).
(3) Locke, Carl, Rase, Howard, Id. Eng. Chem. 52, 515 (1960). (4) Murphy, C. B., ANAL. CnEta. 30, 867-72 (1958). (5) Rsse, Howard, Stone, R. L., Zbid., 29, 1273-7 (1957). (6) Rowland, R. W., Calif. Dept. Nat. Resources, Diu. of Mines, BuU. No. 169, 151-63 (1955). (7) Rowland, R. A., Jonas, E. C., Am. Mineralogist 34, 550-8 (1949). (8) Schwenker, R. F., Textile Research Institute, personal communication. (9) Smothers, W. J., Chiang, Yao, "Differential Thermal Analysis: Theory and Practice," Chemical Publishing Co., New York, 1958. RECEIVED for review April 19, 1960. Accepted August 1, 1960. Division of Analytical Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960.
OF SYMPOSIUM
A Convenient System of Thermogravimetric Analysis and of Differential Thermal Analysis MNER M. MARKOWITZ and DANIEL A. BORYTA Research and Development Laboratories, Chemicals Division, Foote Mineral Co., Bemyn, Pa.
b A new apparatus suitable for themwgcavimetric and differential thermaF onalyses is described. O f particular interest is the commercially available transducer and detection system used for thermogravimetry. It comprises a linear variable differential transformer and an electromagnetic caiknagnet restoring force assembly. Various thermogravimetric and differential thermal analysis results are reported, and the significance of decomposition and reaction temperatures as
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ANALYTICAL CHEMISTRY
determined by each procedure is discussed briefty.
equipment has manifested considerable reliability, versatility, sensitivity, ruggedness, and ease of manipulat'ion.
H E R M 0 G R AVI Y E T R I C A N A L Y S I S
T ( T G A ) and differential thermal analysis (DTA) are valuable techniques because their application does not usually impose any restrictions on the system being studied. This paper presents the details of construction and arrangement of equipment of moderate cost that has been in use in our laboratory for TGA and DTA studies. This
INSTRUMENTATION
A number of elements of instrumentation and apparatus are common to hoth the TGA and DTA systems. Each system requires the recording of two variables: for TGA, furnace temperature and weight differential; for DTA, sample temperature and the temperature differmtial between the
