Thermal Analysis C. 6. Murphy, Advanced Technology Laboratories, General Electric Co., Schenectady, N. Y.
T 144).
HIS review has a more extended scope than its xedecessors (149, With respect to differential thermal analysis (CITA), the review covers the period from October 1961 to ca. October 1963. Although earlier work is referred to in discussing other techniques, major emphasis essentially is on the same period. The review is not intended to cover exhaustively all t h e published papers in this field, but to indicate signifirant trends and applications of the techr: iques.
THERMOGRAVIMETRY
The history of th3 development of TGA and a review of its applications have been presented by Duval (65) in his recent book, "Inorganic Thermogravimetric .inalysis." Other reviews by D u d (63) and Rocchiccioli (189) have appeared. From these references, it is evident that TG.1 is a technique well established in andytical chemistry. Gordon and Campt)ell (93) presented a n excellent review 3f automatic and recording thermobalances and their operation up to 1960. Since that time a number of equipments have been described (99, 246). Equipments capable of operation over a pressure range of 0 to 3 atm. (105) and 0 to 60 a t m . (25,176) have been developed Publications h a w appeared on various accessories and components. Brefort ( S I ) has described a thernioregulator capable of holding the temperature within +0.01" in the range 200" to 700" C., and a n?w programming system. Garn et ul. (3) have discussed furnace mounting and control for the Ainsworth vacuum automatic thermobalance. .kcessories For the Chevenard thermobalance have been described (151).
Regardless of the t.jpe of balance for TG.\, the operational characteristics of the equipment should be knoiyn. In their evaluation of the Chevenard thermobalance, Simons e' al. (197) established procedures tc determine performance t h a t could be modified to accommodate any system. The work that has heen accomplished with thermobalances has been of two distinct types: iqothermal and dynamic. Although the same equipment has been used for both types of investigations, isothermal TGA has l i d to the development of highly precis. equipment such as the Bakr and McBain balance ( 9 ) , which n m converted into a thermo-
balance by Barret (14). As these special equipments have their principal applications in gas adsorption, etc., they are not considered here. ?\lore extensive information on this aspect of T G h is given in publications resulting from three conferences on vacuum microbalance techniques (17, 123, 223). I n the programmed temperature operation of a thermobalance, a plot of weight loss (or gain) us. temperature is obtained (Figure 1). The d a t a t h a t can be acquired from a single thermogram are extensive. A small initial weight loss, w - W O , generally results from desorption of a solvent. If this occurs within the vicinity of 100" C., it is generally assumed to be associated with the evolution of water. Subsequent calculations based on the thermogram must be corrected for such a loss. The material being exanlined can be assumed to be stable to temperature T I , where it undergoes extensive decomposition with a total weight loss of wo-wl. The percentage of weight lost, (wo-w], zc0)100, particularly with inorganic compounds, can be employed t o determine t h e nature of t h e decomposition-i.e., the loss of COz from a carbonate, etc. At Tz, another stable phase exists. -4s in a n y other analytical technique, there are a number of operational parameters t h a t affect the reliability and reproducibility of thermograms. The major sources of error in T G h appear to be due to temperatiire measurement, heating rate, aerodynamic forces, furnace atmosphere, and heat of reaction studied. Because TG,1 measurements are made to determine thermal stability ranges of materials and temperatures of the onset of thermally induced chemical reactions, the measurement of temperature is important. ,\lthough equipments have
I
Figure 1 .
Typical TGA trace
been developed where the temperature of the material is measured directly, usual practice involves insertion of a thermocouple probe in the furnace near the specimen. L-sing such a probe and a thermocouple inserted in a typical TGA crucible, Newkirk (152) determined that the crucible temperature exhibited a 3' to 14" lag, the higher values being obtained with the higher heating rates. I n a similar experiment with CaC204. H20in the crucible, heats of reaction resulted in temperature differences as high as 14" over results obtained with the empty crucible. This latter effect was a function of the mass of material undergoing reaction. Soulen and hlockrin (208) demonbtrated such effects. Sewkirk (162) pointed out that such temperature differences are related through geometry and procedural practice for a given instrument, which permits calibration. to be made. However, he cautioned that differences caused by reactions could cause errors in kinetic constants derived from programmed temperature curves. Other factors-i e., heat capacity, thermal conductivity, fineness of the sample, etc. -should be considered for precise measurements. The temperature will induce a density change in the gaseous environment. This variation will result in an apparent increase in n eight which may be approximated (46)by the expression: Aw = Vd (1 -
7)
where Aw V
= =
d 2'
= =
apparent weight increase volume of sample, crucible, and holder densit>-of air at 273" K. Kelvin temperature
Newkirk (152) has investigated the effect for gaseous atmospheres over the temperature range 25" to 1000" for a load of one KO. 4-0 crucible in a Chevenard balance. I t amounted to 0.1 mg. for HP, 1.4 mg. for air, and 1.9 mg. for .\r. T h e lower the heating rate employed, the lower will be the temperature of the onset of a chemical reaction. This has been demonstrated with pyrolysis of polystyrene (152). dehydration of CaC204 H?O (170), and dehydration of mica (192). Berlin and Robinson (22) have developed a theoretical expression that accounts for the temperatyre a t any stage of the reaction as a function of the VOL. 36, NO. 5, APRIL 1964
347 R
surface area and weight of the specimen, and the rate of heating. The validity of the expression was demonstrated by application to dehydration curves for 5-nitrobarbituric acid and published data (186) for decomposition of CaC03. Effects associated with particle size have been discussed in connection with D T A I (143). Evidence continues to mount that grinding alters the crystalline structure of a material (100, 211). Martinez (140) has shown that grinding will alter the T G h curve, with decreasing temperature of dehydroxylation being observed with increased grinding of minerals. Standardization of sample treatment gave similar results. In TGA, two competing processes occur: those increasing the surface area, such as strains resulting from differences between the original material and its degradation product, and the tendency for sintering to occur. As the temperature becomes more elevated, the sintering process becomes the more important. Dollimore and Xicholson (58)confirmed this by demonstrating that maximum surface area occurred essentially a t the point of maximum jveight loss for the thermal decomposition of a number of metal oxalates. The effect of convection currents resulting from unevenness of heating in the furnace has been considered (46) and the weight increase resulting from this effect was found to follow the expression Aw
=
AebT
T
=
Kelvin temperature
A and b
=
where Aw = weight increase
constants
A and b were found to be 0.14 and respectively, for experi2.6 x ments with a Chevenard balance involving only a crucible and holder (3 cc.). Application of this correction, together with the preceding one for gas density, showed little discrepancy bet\veen the theoretical and experimentally derived curves. Duval (66) suggested that the extent to which air is allowed to escape from the top of a thermobalance is responsible for aerodynamic variations. This could not be confirmed (152). Lukaszewski (132) confirmed the fact that no opening could be found to compensate fully for buoyancy over the entire range, Hoaever, he recommended a crucible and support as!embly to minimize the error to 0.4 f 0.1 mg. More recently, Cahn and Schultz (40) observed that aerodynamic noise is affected by hangdown tube diameter and pressure, is proportional to the horizontal area of the sample container, is only slightly affected by temperature change, and is not affected by sample weight. An interior enclo348 R
ANALYTICAL CHEMISTRY
sure, employed to encompass the pan and sample, also was found to be effective in noise reduction. There are many materials that react with oxygen, water, etc. It is particularly important that such reactions remain under control-Le., appear when desired and vice versa. Decomposition reactions involving the liberation of a gas continuously modify the atmosphere immediately surrounding the specimen. Although such self-generated atmospheres may be useful (89), they have been shown to be a problem in the packed samples of DTA (154). The nature of this effect can be illustrated by decomposition of XaHC03, which decomposed a t 850" in air, but exhibited stability to 1050" in a COj atmosphere (153). The effect of the moisture content in air was shown to have a significant effect on the nature of the products obtained on dehydration of (95). The decomposition of CaC204in a n inert atmosphere (202) was endothermic, but was exothermic in air (152), presumably because of the oxidation of the evolved CO. Small quantities of oxygen, in a supposedly inert atmosphere, could cause other oxidations and confusing results. Operation of a thermobalance in controlled atmospheres has been the subject of many investigations. Synthetic air has been used (83) because of the day to day variations of the H 2 0 content of laboratory air. Controlled atmospheres of HrO ( l o r ) , Con (99), H P ( f o g ) , and NP(217) are representative of many controlled atmospheres employed in T G d . Soulen and Mockrin (202) presented a technique for obtaining controlled atmospheres for which Smith (199) has suggested improvements. The coupling of DTA and TGA was inevitable. The former detects transformations involving energy changes, and the latter shows the extent of loss or gain in weight. The coupling generally permits differentiation between physical and chemical changes. A number of equipments have been proposed to accomplish such measurements simultaneously ( I 19, 164, 172, 174). hlore recently, equipment has been reported (38) to measure DTA, TGA, and radon emanation simultaneously, and applied to the decomposition of thorium salt hydrates. Many thermograms exhibit phenomena occurring so closely in succession that assessment of significant temperatures is difficult. Differential TGA, in which the rate of weight change is plotted against temperature, can be used to solve this problem. Initial applications of this technique involved manual measurement and plotting (147, 173). DeKeyser (55) devised a system based on two furnaces operated with a temperature differential. Erdey, Paulik, and Paulik (74) objected to this approach
because it gave only the ratio of finite increments of the TGA curve. They proposed (73, 74, 168) equipment, called the Derivatograph, for simultaneous registration of TGA, differential TGA, and DTB. This equipment used a permanent magnet on the balance beam to produce a current in a manyturned coil. A review of their work in 1962 (167) listed 20 publications and three patents on this equipment. More recently, Wilburn and Hesford (254) presented a modification to the Stanton thermobalance to provide simultaneous TGA and differential TGA. A problem with torsion-type thermobalances has been condensation of evolved materials on the suspension wire. Waters (227), by controlling the condensation or by selectively adsorbing evolved gases on the same suspension, has developed a technique called fractional TG,1. This approach permits the selective determination of components evolved on decomposition of complex materials-Le., coal. Fluorescence under ultraviolet light was used to identify some of the condensates. Vassallo (217) discussed pyrolysis of polymers and suggested that mass spectrometry and thermal conductivity could be employed for analysis of gaseous products evolved, a method proved to be successful in DTA ( 7 , 90, 229). Infrared analysis (6, 39, 67, 97) and x-ray diffraction have been found particularly useful in identification of stable phases found in TGA. Wendlandt (234) has employed automatically scanned reflectance curves (400 to 700 m r ) from heated and unheated samples to follow the thermal decomposition of inorganic complexes. .I digital computer program was employed by Soulen (201) to accommodate the manual d a t a for calculation of weight loss and rate of weight loss. Wendlandt (228) has discussed the usefulness of a digital recording thermobalance, and has described equipment with such capability. Duval (64, 65) has applied the technique extensively to the study of precipitates and solid reagents of analytical significance. His recent book constitutes a n excellent review of this field. In addition to establishment of proper drying temperatures, the technique can be used to determine the ash content of filter paper and drying characteristics of asbestos filter pads. However. caution must be exerted even in this simple application of the thermobalance. The salicylaldoxine method for determination of zinc (80) was proposed for abandonment (64) because no plateaus were found on the TG.4 curve (54). Subsequent reinvestigation of the chelate (191) showed t h a t the thermogram was highly dependent on the water content of the material employed,
T h e origina 1 rejection has been reconsidered (65). Direct analysis of materials may be made by assignment of weight loss to specific phenomena occurring a t precise temperatures under specified conditions. Typical analyses have been clays for various constituents (16 7 ) ) complex mixtures of hydrates ( 9 4 ) ,determination of pyrites in bauxites (166), and mixtures of nitrates of silver and copper (64). Although normal usage of a thermobalance depends on the direct measurement of gain, or loss, in weight, indirect methods also can be used. CaO would show no weight change on TGA. However, an indirect metk od for analysis has been based on reaction with a n excess of water, converting; it to Ca(OH)2, which will exhibit weight loss of ca. 600" (46). Indirect determination of divalent ions of Ca Sr, and Ba has been made (75) through formation of their insoluble oxalates and subsequent determination of weig;ht loss on TGX of the precipitates. Decomposition of hydrates has been extensively studied by T G A (28, 116, 21 5, 634), and the application to etherates has been reported (181, 215). Thermal decomposition of basic salts also has been investigtted (162,162,163, 225). .\lthough TG.4 has been reported for numerous compounds, the more recent trend of investigators is to investigate series of conipounds-i.e., Pu compounds ( 5 3 ) , sulfamates of nine metals (188), several complex oxalates (59, 237) and several salts of Y, La, and Ce (60). T h e thwmal properties of Werner complexes and chelate compounds have been studied by a number of investigators (30 133, 134, 1711, especially by W e n d l m d t and his COworkers ( 9 1 , 233, 296, 241, 243). Typical of the qolid-state reactions studied by TG.\ habe been compound formation between uranium and yttrium oxides (158), the formation of R N by the reaction of B203with XaNHz ( b f O ) ,and the oxidation of CuS by air ( I 82). Although applicai ions have been made to simple organic compounds (22)) the major organic application has involved polymeric materials. Doyle (61) has summarizzd an extensive investigation of TCr.I on polymers. Vassallo (217) and .indemon ( 2 ) also have investigated the thermal degradation of polymers. Many additional publications have e,ppeared on the kinetics of polymer degradation (1, 62, 108). The shape of the curve is a function of
the reaction kinetics. Accordingly, it is not surprising that TGA curves have been used to acquire kinetic d a t a (133, 134, 161, t6S. 2,'4) Freeman and Carroll (5.5)presented a typical derivation of kinetic constants
from TGX data. Based on a reaction of cC(.), where the type ah(., -+bB(,) . the gaseous product, B , is easily evolved, conventional kinetic expressions were treated mathematically to obtain the expressions
+
E dT - d In( -dX/dt) - x RT2d In X dlnX and
where X E T R t z
= = = = = =
concentration of reactant A energy of activation Kelvin temperature gas constant time order of reaction with respect to A
I t is apparent from these two equations that plots of dT T2d log X
d log (-dX/dt) d log
"'
x
and
(a) x-
In In wo/w A log (-dX/dt) A log
A
+
(a)
- 2.3R A A log W ,
w,= w c - w
w c = weight loss at completion of reaction w = weight loss up to time t
A
(!!-l)
The author: used a simple graphical technique for d a t a aquisition which compared favorably with the more usual tangential method. TGA a t 10" per minute was applied to CaC204.H20, and the reaction, order of reaction, and energy of activation, respectively, were : CaC204.Hz0 CaC204 H20, 1.0, 22 kcal. per mole; C a C 2 0 4 CaCO3 CO2, 0.7, 74 kcal. per molc; and C a C 0 3 CaO CO,, 0.4, 39 kcal. per mole. .Iswas previously noted, the onset of a reaction occurs a t higher temperatures with an increased heating rate. This was shown (152) with C a C 2 0 4 . H 2 0and polystyrene. The latter curves more clearly depicted the marked similarity of the shape of the curves. ;\ccordingly, one could anticipate that the plot of A log d w / d t / A log IT', us. A ( T - I ) / A log TV, would give a family of curves -+
-
+
-+
+
w o = grams of reactant w = grams of reactant at any
time E = energy of activation R = gas constant T , = selected referenre temperature 9 = T - T,, where ' 7 is the temperature at a measurement
DILATOMETRY
Graphs are constructed by plotting log ( d w / d t ) A log w, "'
E9/RTa2
A plot of I n I n w,/w vs. 6 gives a straight line with a slope of E/RT$ The method was applied to dehydration of CaC204.H20 and reasonable agreement with previous results (8.5)was obtained.
- A log ( d w / d t ) A log W ,
where
A
=
where
x
will result in straight lines with a slope of or - E/2.3R and intercepts of - x. Corresponding relationships are given for expressing concentrations in other terms. Employing weight, the derived expression becomes E
with essentially the same slope, E,/2.3R. However, the intercept, x, would vary because T-l would be affected by the rate of heating. Newkirk's (152) plots of the rate of weight loss for l f y l a r a t three different heating rates also illustrate this point. Vassallo (217) has generalized t h a t isothermal pyrolysis of polymers in N P is about 10" higher than in vacuum, while a programmed temperature rise can produce a difference as high as 50" to 60". However, Anderson (1) stated that although TGA measurements may not be as accurate and as precise as isothermal gravimetry for kinetic experiments, they are less time-consuming and probably give more representative values for the pyrolysis of complex substances such as cross-linked epoxide polymers. Horowitz and Metzger (103) have pointed out that graphic methods are tedious and subject to errors when the thermograms are highly precipitous. Assuming evolution of gaseous products, they derived the expression
+
Although commercial equipment for dilatometry has been available for some years, the technique is becoming more significant in thermal analysis, particularly with reqpect to the detection of phase changes. Major rmphasis for such measurements has been generated in the field of ceramics, where high temperature meaquring equipment has been required. Equipment for measurements to 1400" was developed in 1952 (129). More recently, equipment has been developed t o perform such measurements to 2000" in an inert atmosphere (194). Other equipments recently have been reported to cover temperature ranges of 0" to 800" (31) and 20" to 1000" (142). A recent report (150) from the Sational Bureau of Standards has reported a dilatometer capable of meaquring linear diy)larcments as small as IO-' r m . Rarford (10) has reported a differential dilatometer which is proposed for the study of kinetics of phase tranqformations. h VOL. 36, NO. 5, APRIL 1964
349 R
lowtemperature (-200" to 0") dilatometer has been reported by Burk (36). Multipurpose equipment has also been reported. Dilatometry and DTA have been coupled by Lehmann and Thormann (127), while Hirota and his coworkers (102) have coupled these techniques and weight loss measurements. The theory of linear expansion measurements is discussed in a recent publication by Wachtman and his coworkers (222). Dilatometry has been used to detect phase changes in h " d h T 0 3 (104) and and NH4Br (175). Coupled with D T h measurements, the technique was used to show a second-order phase transition in N a N 0 3 (179). Argyle and Hummel (4)have applied dilatometry to a number of lead compounds and identified several new compounds. Indications of two new forms of a-UP207 were obtained by such measurements (35). The technique was applied to ZnO-TiOp mixtures (130) to detect compound formation in this system. Subbarao (208) investigated the polymorphism of Bi2Ti4011and detected a phase transition of 250", not only by dilatometry, but by DTA, and plots of the dielectric constant and tan 6 us. temperature. Some years ago, the Volds (221) applied dilatometry to the detection of thermal transitions in alkali palmitates. Work of .this type is still continuing. Egorov and Krylov (69) have applied dilatometry to stearin, where they have shown the following transitions: solid psolid a-stearin -+ liquid stearin +. liquid. Egorov and his coworkers (180) applied the technique to other fats to determine phase changes and melting phenomena. Application of dilatometry to polymeric materials has been particularly useful. Dole (57) reviewed glass transition d a t a from calorimetric and dilatometric investigations to 1960 and found good agreement between the two techniques. Glass transitions in epoxy resins (196) and polyacrylates and polymethacrylates (195) have been investigated by the dilatometric approach. Ito (109)has employed the technique to determine the glass transitions in poly(viny1 alcohol) as a function of molecular weight. The secondorder transition in poly(tetrafluor0ethylene) has heen studied by dilatometry, and the observed change was found to be more distinct in samples having the least crystallinity (193). -+
ELECTRICAL MEASUREMENTS
Thermal analvsis by measurement of electrical resiqtance was first applied to thin metal wires (3, 49). Because of large thermal gradients, a method
350 R
ANALYTICAL CHEMISTRY
developed by Glaser and Moskowitz (92) for refractory metals was not very precise. Reisman and his associates (184) described a technique in which two closely spaced platinium wires (ca. 2 mni.) were embedded in a pressed, fired, ceramic disk. The conductivity of the material was measured as a function of temperature. X transformation in the material being examined is detected by a marked change in slope when the conductivity (or resistance) is plotted against' tempersture of 1 ,'T. When this technique was applied to the KNbO3 K T a 0 3 system (184), the knce appeared on the curve a t temperatures that agreed within =t3" of the solid-liquid temperatures determined by DTA. Equipments for simultaneous DT.\ and conductivity measurements have been reported (20, ,3$). Resistivity measurements made on Pr203 in vacuum were plotted vs. 1 / T . and a comparison of the inflection points, with those detcrmined by vacuum T G h of the same material (48)showed correspondence a t several points. Cypres and Van Ommeslaghe (52) determined the ZrOe transformation temperature to be 1150" by resistance measurements. In another investigation on this same material (101), the electrical resistance showed the tetragonal to monoclinic change to be complete on cooling from 1010" to 980" over a 45-minute period. However, x-ray diffraction showed some tetragonal phase remaining in the specimen. The technique has been used to check solid-state reactions in the SiOs-Pb02 and P02-;il systems (52), and Z r 0 2 binary oxide systems with BeO, CaO, NgO, SrO, and BaO (220). The formation of Ca, Sr, and Ba monozirconates is usually accompanied by a linear shrinkage and a minimum in elect,rical conductivity. The compound, 3Sr0 ,ZrOs, was postulated from the conductance minimum occurring a t this composition. The plot of the log of the specific conductance us. 1/T for t - 0 3 heated in vacuo from 80" to 800", showed four breaks, a t 250" to 300", 350°, 450", and 500" (212). These inflections were associated with transformations in the material-i.e., 6CO3 -+ 2Ly308(up to 450") and the formation of 1 1 0 2 . 9 1 (at 450"). The electrical conductivity of Biz03 has also been studied (254), and it was shown that the transformtttion of a-Rie03 into P-Ri203 results in a conductivity increase by 3 orders of magnitude. On melting, a twofold increase in the conductivity over the &Bid& was detected. Fripiat and Toussaint (87) described a conductivity cell and employed it to study the dehydroxylation of kaolinite. I n the conductivity us. temperature plot, the main features observed were: conductivity increase to 360°, which was associated with increased proton mobility; conductivity
decrease to 420", associated with dehydroxylation; and conductivity increase to 490", attributed to nietakaolin lattice defect.. DIFFERENTIAL THERMAL ANALYSIS
Differential thermal analysis has been reviewed in three significant works : "Introduction to Thermography" by Berg (29), and chapters contributed to books by Wendlandt (230)and Kissinger and Sewman (119). Other reviews cover the application of DT.1 to polymers (138, 948) and to solid catalysts ( 2 3 ) . In addition, a popular article (43) discusses the broad applicability of the technique. -1 number of equipments have been described. They include low temperature equipment for temperature ranges from -50°.to 100" (79) and -100" to 500" ( 2 1 ) ; borosilicate glass tube sample holders, etc., for studying propellant systems (187): equipment for use with toxic compounds ( 1 5 6 ) ; apparatus for thermal and differential thermal analysis to 1500" ( 7 8 ) ; and controlled atmosphere and vacuum equipment (231). Apparatus for simultaneous DTA and gas evolution analysis has been developed by n'endlandt (229). The measurement of gas evolution is made by thermal conductivity measurement of a carrier gas (He), a technique previously described ( 7 , 92). Similar equipment has been used by Chamberlain and Green ( 4 2 ) . Thermally evolved H20 and 02 generate heat in H2S04 and pyrogallol solution, respectively. Differential thermocouple iun ctions in such baths and water have permitted recording the evolution of these products (255). Chiu (45) has developed a sample holder to permit visual observation of the specimen undergoing DT-4. Equipment has been developed (203) for DT.1 in which the quantity of electrical energy supplied or nithdrawn, during endothermic or exothermic transformations, respectively, to null t,he differential thermocouple signal is plotted directly. This approach eliminates the usual calibration and the thermogram tailing resulting from one thermocouple catching up to the other. Similar equipment, the differential scanning calorimeter. is being marketed by Perkin-Elmer Corp., and was described by Rrenner et al. ( 1 5 7 ) . These equipments have permitted precise determination of heats of fusion (203). and, therefore. should permit accurate analyses. Parameters affecting the quantitative applirat,ions of DTA have been investigated by Rarrall and Rogers (12, 13) and Yamamoto (250). They reported good results with benzoic acid (250) and salicylic acid (12). Bayliss and Warne (16) have discussed effects of the controllable variables on DT.1.
Veniale (618) compared four methods for quantitative determination of kaolinite in clays: Eierg's (18) graphic method; the area-width method of Carthew (41); Foldvari-Vogl's (82) graphic calculation; and the conventional integrated area technique. Results obtained t y Carthew's and Foldvari-Vogl's methods show linear relationships. H e concluded that there is a reasonable, though not absolute, correlation between some means of DT.4 methods and the amount of material found quantitatively by other methods, regardless of particle size and packing. Mackenzie ( I N ) , hcwever, states that DT;I is the most rapid and accurate single method, provided it is applicable, for clay mineralogq . Together with Robertson, Mackenzie (137) applied the technique to the quantitative determination of halloyeite, goethite, and gibbsite. Kuntxe (124) has applied DTA to the determination of small amounts ( < I % ) of gypsum in calcium sulfate hemihydrate. I n the polymeric field, DTA has been applied to quantit 3tive analysis of epoxy resins and hardeners (6). Clampitt (47) applied the technique to linear polyethylene-high pressure polyethylene blends. The area under the 115" peak was aszociated with the fusion of the high pressure material and t h a t under the 134" peak with the fusion of linear polyethylene. These areas permitted calculation of the amount of linear polymer in the blend. Combining gas chromatography and DTA, Clampitt et al. (by) were able to elucidate the cornonomw distribution in ethylene-acrylate copolymers. DTA was also employed kiy Inoue to determine the extent of crystallinity of high polymers (106). Heats of fusion for Sn, Pb, hl, and d g E 0 3 have been deiermined with considerable accuracy by Speros and Woodhouse (203). Heats of reaction for thermal decomposition of a number of materials have been studied by Wendlandt and his coworkers. Typical examples consist of potassium methyl and ethyl sulfates (243), rare earth sulfates (149), and heats of olation of hydroxoaquotetraamine cobalt(II1) complexes (236). McAdie (135) has determined the heats of clecomposition of a large number of clathrate compounds. DTA has also been used (26) to obtain approximate heats of explosion for several explosives. Eianerjee et al. (24) have employed D T I L to measure the heat of hard rubber reaction. DTA has also been employed t o determine the stored-energy release in metals (244) and from irradiated graphite (178). Ellis and Jfortland ( 7 1 ) have compared the area measurement and ClausiusClapeyron methods of determining heats of reaction with magn3site and kaolinite. The study of ph3se equilibria by
D T X has continued. Kullerud and Yund (123) have employed the method in the construction of the phase diagram for the Ni-S system, while Yund (253) used it in the Ki-.4s system. DTA has also been employed to study the following systems: NaF-BeF2-UF4 (YO), CdO-B203 (98), N a S b O r K T a O s (110), and NH4N0,-Mg(N0& (200). The study of polymorphism in pure materials has been typified by the detection of new phases in .kgNOs (141) and in SrCO3 (8). The cubic forms of BaTi03 (76), alkali metal nitrates (84), and 3CaO.SiOs (252) have been investigated by D T A for detection of physical transformations. Kennedy et al. (111) have continued t o use D T A for detection of melting and phase transitions on subjection of materials to high pressure. Crystallization phenomena in lead borate glasses also have been studied by this technique (21). Sublimation equilibria have been determined by Markowitz and Boryta (139) by determining the D T X solid-vapor characteristics as a function of pressure. Results obtained for XHaCI agreed well with previously obtained isothermal measurements. Krawetz and Tovrog (121) extended this work so that vapor pressures of organic materials could be obtained. Fleming and Johnson (81) used D T A to study the exothermic reaction between h l and GaOs. The intensity of the reaction (1200" to 1900°F.) increased with decreasing u308 particle size and increasing A1 content, and occurred equally as readily in vacuum, argon, and air. T h e reactions of C a C 0 3 and Si02 (177) and CaC03, Si02, and - 1 1 2 0 3 (226) have been studied by DT-4. The Derivatograph has been employed (72) to study high temperature fusion reactions, including the ?rTa2COr.41203 and NazC03-SiOz reactions. Reisman and Berkenblit (184) investigated the formation of CdSe from its elements and were confronted with explosions which were assumed to result from formation of high vapor pressures developed within the particles coated with a product layer. By using small particle sizes, 325-mesh, the synthesis could be achieved without explosion. Other 11-VI compounds also were prepared by these investigators. Cox and Pidgeon (51) used DT;I with large samples to by carbon. study the reduction of .\I2o3 The chlorides of Ta and N b have been investigated by Frere (86),who has shown t h a t in a mixture of NbC1, and TaCls, the former can be converted to a lower chloride by addition of N b a t 400", which permits sublimation of free TaCls. Thermal decompositions of a number of compounds have been studied by DT.1. Among the uranium compounds, for example, hydrated peroxides (50), uranyl and uranium(1V) oxalates (%),
N H I U F ~ (219), and uranyl sulfate (156) have been subjected to DTA. The thermal decomposition of NaAlH4 was shown (68) to decompose into N a H , Al, and HP, with subsequent peaks resulting from decomposition of X a H and melting of A. The decomposition of complex compounds has been extensively studied by Wendlandt and his coworkers (91,233,236,287,240, 241,242). Others (15, 42, 120) have made significant contributions in this facet of DTA. McAdie (135) has used DT.1 to study the thermal decomposition of clathrate compounds. DTA decomposition temperatures mere found to be in good agreement with decomposition temperatures determined by gas evolution techniques, and R mechanism for decomposition of these complexes was proposed. Interest in reaction kinetics has continued. Lueck et al. (131) have developed a method for following reaction kinetics in solution and applied it to the hydrolysis of tetramethyldiamidophosphorochloridate. Kissinger's method (117) was applied by Nathans and Wendlandt (149) to determination of kinetic parameters of decomposition of rare earth sulfates and the results were found to be in general aqreement with isothermal measurements. Wendlandt and Fisher (296) have determined the kinetics of olation by a modification of the method first described by Rorchardt and Daniels (29). The kinetics of fusion have been determined by Speros and M700dhouse (203) from the rate of electrical energy input as a function of time for the melting process. The applicability of DT-4 to organic substances has been discussed by Yamamoto (251). D T A has been applied extensively to aromatic hydrocarbons (128), both as pure materials and as mixtures. The identification of organic compounds from their DTA thermograms has been discussed by Chiu (44). Wendlandt and Hoiberg (238) have applied D T A to 25 organic acids in a n argon atmosphere. In general, endothermic peaks, resulting from such reactions as dehydration, decarboxylation, sublimation, decomposition, and phase transitions, were observed. The nature of the peaks was such t h a t differentiation of the compounds, one from another, could be made easily. Work is continuing (169) on the application of D T A to phase equilibria in fats and their derivatives (165).
Applications to polymer chemistry have continued. Yamamoto (249) has applied the technique to a number of polymers for qualitative identification. Paciorek et al. (160) have shown t h a t qualitative analysis and thermal stability of fluorinated polymers may be obtained from their thermograms. The effect of RlgO, biscinnamylidene, hexaVOL. 36, NO. 5 , APRIL 1964
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methylenediamine, and benzoyl peroxide on fluoroelastomers has been inLow-temperature vestigated (159). transitions in four polymeric materials has been shown by Ke (115). Polyesters (114), urea-formaldehyde (149, and wire enamels (145) have also been studied by DTA. Strella (207) presented a theoretical treatment of melting of polymers, and has derived a method for the precise determination of the melting point and crystalline fraction, which was confirmed with linear polyethylene and crystalline polypropylene. Crystallinity of polymers has also been investigated by Errede and Gregorian (77) and Wunderlich and Poland (247). Burrell (57) has reviewed the methods for determining glass transitions in polymeric materials, and has concluded that DT,I is the simplest routine method for this determination. Work is continuing in this field, with a theory on the manner in which the glass transition is shown on a thermogram (206),and glass transitions have been determined for poly(methy1 methacrylate) and atactic poly(propy1ene) (206), and atactic poly(propylene) as a function of molecular weight (115). DTA has also been used to determine the degree of cure in unsaturated polyester-styrene copolymers (112). The technique has been applied to the determination of the chemical activity of silicate-skeleton catalysts (209), and to determine the extent of coking on silica-alumina cracking catalysts (96). DT.4 has also been used to determine the precipitation of Li in LiF after irradiation with thermal neutrons (125). MISCELLANEOUS METHODS
.4technique called mass spectrometric thermal analysis (MTA) has been proposed by Langer and Gohlke (126). This technique employs a time-of-flight mass spectrometer to detect the gases evolved from a material being subjected to programmed temperate. The technique was applied to the thermal decomposition of R e s o d .4 H 2 0 and the M T A plot was compared with the DTA thermogram. Although the number of peaks obtained in both types of thermograms was identical, exact temperature correspondence mas not obtained. The technique was also applied to the thermal decomposition of the dihydrate dihydrochloride of (ethylenedinitri1o)tetraacetic acid (EDTA), the dihydrate of the EDTA chelate of Ge(TV), CaS04. 2H20,and CuS04.5H20. A s M T 4 provides analysis of evolved gaseous materials, the method would be more akin to TGA. As no d a t a can be acquired on solid-state phase transitions, the method is not a potential substitute for DT.4. A possible disadvantage in the technique would be the necessity to operate under vacuum.
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ANALYTICAL CHEMISTRY
Reese et al. (185) have followed thermal transitions in amobarbital (the 11-1 transition, and melting) by the variation in transmitted polarized light as programmed heating was applied (2” per minute). The authors stated that the technique is capable of detecting all transitions with the exception of the rare isotropic crystal isotropic liquid transformation. As the amount of material was small-Le., less than 5 mg. on a slide-the method would appear particularly useful for studying thermal transformations in organic materials. GE’s condensation nuclei (CK) detector (198) has been applied (146) to t h e thermal degradation of polymeric materials. A furnace was constructed by inserting copper tubing, as a core, into a 100-watt ceramic resistor, and wrapping the composite with asbestos tape. Polymeric specimens (1 X x inch) were inserted in the furnace and programmed heating (50” per hour) was initiated. The gas was passed through a copper coil immersed in water for heat transfer and then into the CN detector. The detection of particulate material emanating from poly(methy1 methacrylate), heated in a Hz atmosphere, occurred at 320”. When filled with silica, decomposition under the same conditions occurred a t 310’. ;ilthough direct detection of particulate matter could be made in this case, the conversion techniques cited by Van Luik and Rippere (216) would be required for detection of degradation products-i.e., HCI from poly(viny1 chloride). This technique requires a carrier gas and is not amenable for vacuum work. A system for the detection of phase changes in incandescent materials has been described by Rupert (190). In this equipment, the sample is cooled and a phototube is employed to follow the luminosity of the sample, which is proportional to temperature. ; i beam splitter is employed to transmit a portion of the light to a calibrated optical pyrometer. The output from the phototube may be recorded or plotted on an oscilloscope, with time (temperature) t w the ordinate. The cooling curves appear to be similar to TG.l thermograms. The technique has been applied to the study of phase equilibria in the S b - C (Sol;), V-T’C (%IF), and Mo-Zr-C (224). Illustrations are given (190) for cooling curves in the 110-C, Zr-C, and I--C systems. LITERATURE CITED
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