; P c p ~e& t tkc New England Association of Chem
C. 8. Murphy Xerox
Corporation
Webster, New York 14580
Thermal Analysis
N o t too long ago, the expression 'Itherma1 analysis" denoted only thermometric analysis, that is, the familiar time-temperature plot used in phase eauilibria studies. Today, the expression has become merely indicatingthat a method involving heat was applied; also, in general, a higher degree of sophistication has been introduced into the measurement technique. Many of the techniques of thermal analysis have been applied for many years by geologists, mineralogists, metallurgists, and soil scientists; they are relatively new only in the field of chemistry. In the application of thermoanalytical procedures, the general purpose is the determinatiop of temperatures a t which transformations, physical or chemical, occur in matter. The methods that have been employed have been quite diverse. Table 1indicates some of the properties of matter that are measured and the thermal analysis technique associated with each such measurement. Obviously, there are other properties of matter that can be studied as a function of temperature and that could therfore evolve nevmethods of thermal analysis. The present article is principally concerned with differential thermal analysis (DTA); other techniques will be referred to only insofar as they have been used to Based on a lecture presented at the Twenty-Eighth Summer Conference of the New England Association of Chemistry Teachers, Dartmouth College, Hanover, N. H., August, 1966. Revised for publication with the assistance of the editor of the Repml of the NEACT.
complement DTA. Because the principle of the method is well known-biennial reviews are available ( I ) , and both commercial (2) and homemade (3) instruments have been described in THIS J o u F i ~ ~ ~ - a t t e n tion will be centered on certain problems associated with the method and on its actual or potential utility. Let us first consider the interpretation of a peak on a DTA curve. There are three points of interest concerning such a peak: the initiation of the reaction; the significance of the peak apex, and the area under the peak.
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Some Methods o f Thermal Analysis
Measured Property Enthal~iceffects Weight Length Conductivity/ResisE;%;d
,
Thermal Analysis Method Differentialthermal analvsis calorimetry Thermogravnnetric analysis Dllatometry X-ray d~ffraction Electrothermal analysis
material
Effluent gss analysis Pyrolysis gas chromatography Mass s~ectrometricthermal anal$sis Thermobarogravimetricanalysis Thermomanometry Thermousrtieolate analvsis ~orsionilpendulum Mechanical damping Torsional braid analysis Acoustical transmission Acoustic spectrometry Light reflectance Dynamic reflectance epectroscovv
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Volume 46, Number 1 1 , November 1969
/ 721
I t is obvious that the onset of the peak results from the start of the reaction. For example, the initiation of melting reduces the heat transferred to the differential thermocouple junction imbedded in the sample compared to that in the reference, generating a differential emf and the onset of a peak. However, the heat transfer situation is such that the temperature measured either in the reference or in the sample is below the actual melting point of the material, while the temperature of the block is a t least slightly above this point. The onset of a peak, therefore, has a physical significance, but the associated temperature measurement does not. Now let us address ourselves to the peak apex. If we assume that the temperature-measuring thermocouple is centrally positioned in the sample, then the movement of the melting front from the wall of the cell to the thermocouple will occur over some interval of time. When the front contacts the thermocouple, the melting process will be over, heat absorption associated with the process will cease, and the temperature will start to increase rapidly. Under these conditions, the temperature of the peak apex represents the transition temperature. When the temperature-measuring thermocouple is located in the reference material or in the heating block, the temperature recorded a t the peak apex is a function of the thermal characteristics of the sample holder and reference material, as well as the heating rate. I t has no relation to the sample. Ke (4) observed that, with the measuring thermocouple in the heating block, the peak due to the endothermic transformation of Marlex (polyethylene) shifted to higher values with increasing sample size. While this can be explained on the ground that i t should take a longer time to melt the larger samples, it is obvious that there is no physical significance to the peak temperature if it is a function of sample size. Under these conditions, the apex merely indicates the maximum temperature differential encountered during a transition. The portion of the peak beyond the apex represents that period of time where the lagging thermocouple catches up in temperature to the one immersed in the reference material. A casual observation of thermogram peaks will indicate a lack of symmetry. This results because specific heat, rather than heat of transition, becomes the predominant factor once the apex has been passed. It is evident, therefore, that a substantial portion of the peak area is not directly related to the transformation. This last point has an important bearing on the quantitative aspects of DTA. Quantitative analysis by DTA is based on the relation where AH is the heat of reaction; A, the area under the peak; and K, a constant. The derivation of this expression is given by Borchardt and Daniels (6). K can be determined by placing a known quantity of standard material in the equipment and measuring the peak area for a transition with known AH. (As thermocouple response varies with temperature, the constant should be determined at the temperature of application.) Other materials provide a measurable peak area, and, with the constant known, the area measurement will permit the calculation of the heat of reaction. The latter allows the determination of the quantity of material responsible for the transformation. 722
/
Journal o f Chemical Education
Application of this technique involves the total area under the curve, part of which is associated with the "catching-up" process and is not related to the reaction. Undoubtedly, this contributes significantly to the fact that quantitative results by DTA usually are good only to * 5 % (6-8). Although evidence has beeu presented to the contrary (9, lo), Barrall and Rogers (11) found that peak areas increased with increased heating rate; they therefore carried out an extrapolation to zero h e a t ingrate in order to find the "true" area. The point just made is one of the fundamental differences between DTA and dynamic calorimetry, since in the latter the two thermocouples are maintained at identical temperatures. Thus, in the case of an endothermic transition, the auxiliary heater in the sample provides the quantity of heat needed to keep the sample a t the same temperature as the reference. The measured quantity is the electrical energy required, there is no "catching-up" process, and the total area under the peak is the direct measure of the heat of reaction. The reference material for DTA studies is usually employed in powdered form and is packed tightly into ths reference well to provide good thermal conductivity. Reference material should have four characteristics 1. Undergo no transitions over the temperature range investigated 2. Be inert with respect to sample holder and thermocouple 3. Have a thermal conductivity matched to that of the sample 4. Haves. heat capacity matched to that of the sample
The first of these has been so impressed on investigators that calcined ALOa has found extensive application as a reference material, despite the fact that it is not always the best choice. Barrall and Rogers (12) have shown that Also3exposed to vapors of methanol, ethanol, and water can produce detectable peaks; this implies that confusing, spurious peaks can be caused by exposure of the reference material to normal laboratory vapors. Carborundum has beeu found to be very useful as a reference material. More recently, partly because of interest in organic and polymeric samples, the other characteristics of reference materials have become more prominent. This has resulted in the use of dioctyl phthalate (IS), silicone oil (14), and similar materials. It is doubtful that any referehce material could be ideal over any reasonable range of temperature. Because of this, the practice of diluting the sample with reference material has been introduced in order to compensate for property differences in the two substances. Other advantages of this procedure are the provision of support for liquid samples (15) and prevention of gassolid reactions from being diffusion-controlled (16). However, the inertness of the diluent becomes of major importance. Alumina, for example, has been shown to react with benzoic acid (17) and 4quinoliuol (12). Barrall and Rogers conducted a series of experiments on the effect of diluents on the melting endotherm of salicyclic acid. Their data are presented in Table 2. The thermal conductivities of Carborundum and iron oxide are of the same order of magnitude, but Carborundum, with the lower specific heat, produced the greater peak area. Iron metal, with the same specific heat as Carbornndum, but with higher thermal conductivity, produced a significantly greater area. However, these investigators cautioned that, with a metal sample
Table 2.
Area Generated by 0.01 g of Salicylic Acid Diluted with Various Materials
D'i1uent Carborundum Iron metal Fa08
Specific Thermal Conduc- Area per Heat of tivity of Diluent, 0.01 g. Salicylio Diluent, cal om-I seo-' Salicylic Acid. % 0&1g - I deg-1 A d d , mm'
5 X
lo-'
6.87 8.82 3.40
0.11 0.10 0.18
2X10-1 3 X lo-'
306 710 280
4.57
0.19
2 X 10.'
322
5.68 8.60 20.0
0.19 0.19 0.47
2 X 10.' 2 X 10.. 4 X 10-I
288 319 92
Glass Beads, 0.029mm
Olhss Beads. 0.29 rnm A1201
Nuiol
holder, a high-thermal-conductivity diluent may decrease the peak area. Although Dannis (18) molded rubber samples into his s a m ~ l eholder. usual ~racticeinvolves compaction of powdered specimens for the sake of thermal couductivity. The manner of packing is important, despite statements to the contrary (19, 20). It has been pointed out (21) that all reactions dependent upon gaseous diffusioneither into or out of the sample will be affected by sample packing. Markowitz and Boryta (22) have shown that gas evolution with loosely-packed samples can result in make-and-break contact with the thermocouple, producing spurious peaks. The variation of packing in DTA (tight) and TGA (loose) was shown to be responsible for marked variation in the results obtained with anhydrous uranyl sulfate (28). Spurious peaks observed in the initial heating of salicylic acid failed to reappear on reheating (IS), an effect attributed to discontinuous passage of the melting front through the material on the initial run. Another controversial facet of DTA has been the relationship of particle size to the resulting thermogram. It has been said that the finer particles release their heat more rapidly (24), and that increasing fineness of samples results in increasing peak area and peak intensity (25). Most work accomplished in this connection has been performed with complex mineralogical and clay specimens. In a classical investigation, Takahashi (26) dry-ground talc for 528 hr and subjected periodically taken samples to DTA and X-ray diffraction. He was able to show that the grinding ultimately destroyed the crystallinity of the specimen and that the differences in the thermograms provided a record of this process. Similar work has been conducted on Indian micas (27). Recent work by Baylis (28) on high purity calcite, where various particle sizes were derived from a single crystal, led him to express doubt concerning the validity of the previously cited relationships between particle size and transformation temperature. More work with simple compounds is required in this area. The sample holder most commonly used for DTA has been a cylinder or block, with holes drilled into it to accommodate sample and reference material. Newer equipment employs glass tubes (29). Materials of construction, selected on the basis of compatibility with the materials to be examined, have ranged from platinum blocks (21) to disposable blocks prepared by drilling holes in insulating brick (SO). A most unusual sample holder has been developed by MaziBres (Sf), who incorporates a small well in the thermocouple junction i b
self. This permits optimization of thermal effects and use of sample sizes as small as lo-' g. Most applications of DTA have involved subjection of the specimen to a steadily increasing temperature. Resistance heating has been most extensively applied; however, induction beating (52) and infrared heating (33) have been used, and preliminary work with an arcimage furnace has been reported (34). The infrared heating method, employing a small ir lamp, offers a very reasonable heating system. Heating rates of 0.5-100°C per minute cam be found in the literature. At low heating rates, the temperature difference between the reference and the samples does not become very large, and this results in broad, flat peaks. The onset of a reaction may be difficult to asseps under these conditions. I n contrast, high heating rates produce intense temperature differences.and can result in lost detail on a thermogram.
.
Auxiliary Techniques
The assessment of the significance of DTA peaks may be easy or difficult, depending on the nature of the material studied. There are other techniques which may he used in conjunction with DTA to provide additional data to assist in thermogram interpretation. The first of these techniques is thermogravimetric analysis (TGA), the measurement of the gain, or loss, in weight on programmed heating (35). If possible, simultaneous measurement should be made to eliminate anomalous results arising from operational variables. For example, the extent to which water is lost from a hydrate, or carbon dioxide from a carbonate, can easily be assessed by the combined use of TGA and DTA. Atmosphere control is incorporated easily into DTA systems, and the alternate use of inert and oxidizing atmospheres will permit differentiation of thermal decomposition from oxidative degradation (36). Within reasonable limits, pressure will have little effect on a condensed-phase transition, but it will strongly influence reactions where gaseous products are evolved. For example, operation with increasing pressures of water vapor will inhibit dehydration reactions, thus permitting precise identification of the peak associated with such reactions. Stone (37) has employed this technique effectively with kaolinitic soil. Garn (38) has employed self-generated atmospheres to accomplish essentially this same effect. EfRuent gas analysis, a technique devised by Lodding and Hammell (SO), incorporates thermal conductivity detectors in a dynamic atmosphere, before and after the sample. A change in the differential signal indicates evolution of gaseous decomposition products. Gas density measurements also have been used to monitor effluent gas (40) with success. Mass spectrometry (41-43) affords positive identification of the evolved gaseous species. Chiu (44) has described equipment for simultaneous DTA and electrothermal analysis (ETA). ETA consists of the measurement of the resistivity of a specimen during programmed heating. There have been several instances where ETA has been shown to have significantly greater sensitivity than DTA. Applications
DTA is a technique that can be applied to essentially any material. Applications have become so extensive Volume 46, Number
I I, November 1969 / 723
that, as with infrared spectra, encoding systems have been developed for the identification of materials. These include the Scifax DTA Index generated by MacICenzie (published by Cleaver-Hume Press, London, England) and Sadtler's collection of thermograms. Applic$ions of the technique have been particularly widespread in the field of polymer chemistry (1445-51). Biology and botany are highly fruitful fields of endeavor for the future. Thermograms of wood have been shown (59) to be essentially composites of those of its components, cellulose and lignin. Mitchell and Knight (55) have shown that distinction among lower plant forms, algae, and pollen is possible by DTA. Steim's investigation (54) of aqueous solutions of proteins has shown that denaturation of these materials can be detected and that the reactions in protein mixtures occur independently of one another. The recent investigations of Pfeil (55) on cancerous tissue and burns promise to be a most significant application of the technique.
~ nP.,, A N D W~=oarnn,M. H..Ind. E w . Cham., (16) RUDIN,A,. S o ~ n ~ r eH. 53, 137 (1961). (17) Monrw. H.. ~ r r oRICE. H. M.. Anal. Chem.. 27, 36 (1955). (18) D A N N M. ~ . L., J. Appl. Polymer Sci.. 7, 231 (1963). (19) G m v e n . R. M.. J. Am. Ccrom. Soe.. 31,323 (1948). (20) B o e n s m . S. L.. J. Am. Cerom. Soc.. 38, 281 (1955). (21) P A ~ KJ., A,, AND W ~ n ~ n M. n , F., Bull. Am. Ceram. Sao., 33, 168
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~
724 / Journal o f Chemicol Education
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c , c ,Av"",. ,.oco,
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(52) TAN., W. K., AND NEIGL, W. K., J . Pol#-SC~., C6.65 (1964). B. D., A N D K N I ~ X TA., H., J . Ezptl. Botany, 16, l(1965). (53) MITCHEL~, (54) STEIM,J. M., in REDFERN.J. P . (Editor), "PIOO. 1st International Congress on Thermal Analysis, Aberdeen, Scotland." MaoMillan & Co. Ltd., London. 1965. (55) PIEIL. R., p e r s ~ n aeommunioation. i
