LITERATURE CITED
(1) Ballio, A., Chain, E. B., Dentice
diAccadia. F., Mauri. M.. Rauer, K.. Schlesinger, h. J., ‘ Schlesinger,’ S.;
Nafure 191. 909 (1961). (2) Batchelo;, F. R., Gazeard, D., K’ayler, J. H. C., Ibzd., 191,910 (1961). (3) Feigl, F., “Spot Testa in Organic
Analysis,” 5th ed., p. 279, Elsevier, New York, 1956. (4) Johnson, D. A., Hardcastle Jr., G. A., J. Am. Chem. SOC.83. 3534 (1961). (5) Katritzkv, A. R., J.agowiki, J. M., “Heterocvclic
Chemistrv.” I,
D. r
(7) Mulvaney, J. F., Evans, R . L., Ind. Eng. Chem. 40, 393 (1948).
(8) Streuli, C. A., “Titrimetric Methods,” DD. 97-120. Plenum Press. New York.
_1961. _
227.
Wiley, A$w York, 7960. (6) Kraus, I., J . Biol. Chem. 63, 157 (1925).
RECEIVEDfor review August 22, 1963. Accepted December 16, 1963.
Micro and Semimicro Differential Thermal Analysis (PDTA) CHARLESMAZI~RES Ecole Nafionale SupGrieure de Chide, Universitd de Paris, France
b Through a critical study of the various factors involved in differential thermal analysis, apparatus has been designed which permits examinationin controlled atmosphere and between -180” and +1200° C.-of samples to weighing from 1 to 200 pg. 2 X 10-4 gram) or, alternatively, from 0.1 to 10 mg. (10-4 to 10-2 gram). Thermal effects as small as 1 O+ cal. can be detected. Examples are given showing uses of the apparatus.
the peak, even in the ideal case of an isothermal transformation. (2) The thermocouples generally used as sensors only indicate their own temperature, which may be notably different from the temperature of the sample, especially if the thermocouple is chemically insulated. The approach described herein consists essentially in using a sample as
3 2
C
mineralogists, and solidstate physicists have shown increasing interest in differential thermal analysis (DTA) (7). The advantages and disadvantages of the method, as well as the experimental solutions to various problems, have been the object of numerous articles (4). The present author has given particular attention (6) to the following points: (1) the existence of a thermal gradient between the surface and the core of the sample investigated, resulting in a variable degree of completion of the physicochemical phenomenon giving rise to the thermal effect. This, in turn, results in a broadening of
.lmm L
Figure 1. Micro DTA detecting head
602
I
HEMISTS,
ANALYTICAL CHEMISTRY
difficulties mentioned above, enclosing the sample within the thermocouple was an improvement on conventional design; as pointed out by one of the referees of the present paper, the effect of sample shape and/or sample thermal diffusity on the area of the peak or on its shape is thus minimized. On the other hand, Wittels (9) showed that by using the vacuum apparatus of Whitehead and Breger (8), it was possible to analyze samples of calcite as small as 300 pg. However, the thermal effect involved in the decomposition to CaO is quite large; once more, the sensitivity was probably 0
1
2
3
4
4
:
:
:
:
5cm. i
not to scale
Figure 2. head
Semimicro DTA detecting
1, platinum junction crucible; 2, removable lining; 3, lid
small as possible (down to 1 X gram) to obtain a better definition of its temperature. The micro sample is placed completely inside the junction of the detecting thermocouple which results in optimum use of the thermal effect involved and in excellent identification of the temperature of the sample and of that of the thermocouple. Because of the small mass of the samples involved, a very high sensitivity is essential: this is obtained by foregoing the conventional metal or ceramic block often used to homogenize the temperature. The thermocouple sample cup was first suggested by Herold and Planje ( 8 ) ; the samples they investigated probably weighed about 1 gram (no data are given) ; the thermocouples were embedded in a refractory block. Though this design did not solve all the
11Figure 3.
-
12
High temperature design
1 , 2, water cooled base; 3, refractory sheath; 4, refractory support; 5, 6, platinum base and hood; 7, furnace; 8, refractory six-duct sheath; 1 0 , 1 3 , O-rings; 1 1,12, gas inlet/outlet
3
2
1
E T cold Figure 5.
Figure 4.
Low temperature design
1, Lucite base; 2, thermal insulator; 3,4,blackened copper sheath; 6, Dewar sleeve; 8, heating coil; 9, liquid nitrogen; 10, copper vessel; 1 1, six-duct sheath; 12, Lucite support; 1 3, O-ring
limited by the use of a homogenizing block as a sample holder. EXPERIMENTAL
Apparatus and Operating Procedures. The principles involved in the present approach are embodied in the thermocouple assembly of Figure 1, showing the micro DTA detecting head used for the 1- to 200-fig.
I
240'
range; Figure 2 shows the semimicro detecting head used for the 0.1- to 10-mg. range. One of the junctions holds the sample; the second, identical to the first, is the reference junction; the third, also identical to the other two, is the temperature measuring junction. These detecting heads can be used both with the high-temperature design shown in Figure 3 or with the ow-
245'
250'
-1 I I AT +-o,i'c
AT
junctions
Thermocouple assembly
temDerature design of Figure 4. These figuIes are self-explanatory. Thermocouple wires are BTE/CTE (Trade Mark. AciCries d'Imphy, France, ca. 60 pv./O 'C.) for low temperatures, and Pallaplat (Heraeus, West Germany) or Platinel (Engelhard, USA) (both ca. 40 pv./O C.) for the higher temperatures. DTA runs between - 170" and +80° C. can be obtained with the low-temperature design, while temperatures to 1250" C. can be reached with the hightemperature apparatus. The heating furnace 7 of Figure 2 is characterized by a fairly high power/ weight ratio. This fact, coupled with the cooling brought about by the water circulating within the base, enables rates of temperature variation of 0" C./ minute to A25O C./minute to be attained. Occasionally, rates of up to 80" C./minute have been used with a slightly different arrangement (6). In Figure 4,the heating coil 8 is used to control the rate of evaporation of the liquid nitrogen and, afterwards, the rate of warming up. In the same figure, inlet 7 is used for pressure control. The recording set-up (Figure 5 ) uses a standard galvanometer coupled with a straightforward spot-follower. This setup achieves a noise-free sensitivity of 0.4 pv./cm. (equivalent to O.0lo C./ cm.) for the AT recording.
r>j2J-i bin
240'
-2%
.i.
AT
cm
SCALE
0
1
2
I I !
210'
229' Figure 6.
pDTA of single 27-pg. fragment of N a ~ S 0 4crystal
220'
230'
240'
*-I
250'
I
I I
1
Figure 7. DTA of 1-gram sample of Na2S04 obtained with conventional apparatus VOL. 36, NO. 3, M A R C H 1964
603
--
+O,l'C
f
I
125'
I
Y
1
a
~
Imin
Il-.-.I-l.~ loo' Figure 8.
Tempera ture
1758 I
150'
( 330°)
(550' )
[ 6200)
lA21 I
thermocouples and annealing on their entire length, and by scrupulous shielding from drafts and other thermal disturbances. The second difficulty can only be eliminated by keeping the two halves of the differential thermocouple svmmetrical with resDect to thermal gradients. The placing of the sample in a junction crucible of Figure 1 is most easily carried out by wo&ing within the fielh of a binocular lens which is temporarily swung into position above the microcrucibles for this purpose. The micro sample should, preferably, be in the form of a single piece and can then be handled with short glass or piceine needles. Unfortunately, it is impractical to use the junction crucibles of Figure 1 with powdered samples because of the difficulty in packing the sample and in changing it. Moreover, certain samples badly corrode the thermocouple, especially a t high temperature. The modification shown in Figure 2 meets these difficulties: packing the sample in the lining and weighing it are easier, and changing a corroded lining is no problem. Such an increase by a factor of 100 to 1000 of the mass of the sample certainly lowers the resolution. Certain existing commercial apparatus also work in this range of masses (1); however, an example given below will show the particular features of the results obtained with the present semimicro arrangement. After the sample has been placed in the crucible, the apparatus can be thoroughly purged, if necessary, with an inert gas or evacuated. RESULTS A N D DISCUSSION
( 720.1
I
125'
gDTA of 15-119. silver of tridymite
Two experimental difficulties arise as counterparts of the high sensitivity: "noise" in the thermocouple wires and drift of the base line. The first can be eliminated by careful welding of the
I
Figure 6 is a reproduction of a thermogram obtained on a single fragment of
i 1 i
-90'
I
-80'
I
-70'
Figure 9. (a) First and (b) second heating under argon of 40-gg. sample of irradiated LiF
crystalline NazSOc weighing 27 pg. Roman numerals indicate the various crystal forms. Comparison with Figure 7 showing a classical type thermogram of the same substance obtained on a ca. 1-gram mass shows the considerable increase in the power of resolution: the pDTA thermogram has a truly oscillographic character and shows the successive transformations of each crystalline domain; the irreversibility of the III+I transformation in the absence of crystalline germs can be unambiguously demonstrated. I n the case of the classical thermogram of Figure 7, the overlapping of peaks makes this demonstration more obscure. Figure 8 shows the result of pDTA performed on a thin sliver of tridymite (15 pg.) obtained from a volcanic lava. Though the thermal effect associated with the transformation is quite weakprobably less than 1 cal./gramfragments as small as 1 pg. give peaks which are still quite visible. The successive peaks of Figure 8 correspond
a 1
I
I
Same as figure 10c, 8 pg.
Figure 1 1 .
of LiF
n -10'
0'
10'
I
I I
Figure 10. Successive thermograms, (under argon), of 30-pg. sample of irradiated LiF Figures a t left give highest previous temperature
of treatment.
604
ANALYTICAL CHEMISTRY
I
I-
0' L
10' 8
L
lrnin.
Figure 12.
Low temperature pDTA of 50-pg sample of BaTiOa
'
,SO0
I
925
,
950
975
IO?
'C
..-0.0 5
Trica I c i u in
--o. IT
:
Chart rprcrd
:
SCALE
: & -
0
AT
S i I i ca te
Sample Size
I
5 mg
12
man
2
3cm
AT
B'
I
+0.1%
..
C'
A'
3
Figure 13.
Semimicro DTA of 5-mg. powder sample of tricalcium silicate
once again to the successive transformation of different domains in the microcrystal; they differ from sample to sample but are perfectly reproducible for any one sample. The hysteresis of the reverse transformation during cooling is important and is also evidenced in the figure. An interesting and characteristic example of the use 0: pDTA has been the study of the small lumps of metallic Li that form within a crystal of LiF when it is subjected to thermal-neutron irradiation. X-ray studies had shown that the crystal form of the precipitated metal was not the usual one (bodycentered cubic) but "ather a different and abnormal one (face-centered cubic) which could be transformed into the former by heating. This result was confirmed and elaborrited by pDTA of the melting and crystallization processes of the precipitated phrtse within the LiF matrix ( 3 ) . In Figure 9a, peak B1, (occurring a t 187' C.) shows the melting of the abnormal (face-centered cubic) and rather badly crystallized lithium, while peak AI is due to the melting of bodycentered cubic lithium. The run shown in Figure 9b was performed on the same
sample after all the metallic lithium within it had melted and had resolidified by cooling; the curve only shows a single peak a t the temperature corresponding to the melting of bodycentered cubic lithium. The width of the peak is caused by the fact that the metal is in very small lumps or aggregates with a small degree of organization. Moreover, the variation in the shape of the recorded peaks enabled the author to follow the evolution of the lumps of metallic Li during and after heat treatments at regularly increasing temperatures; in Figures 1Oc and d, peak a: corresponds to the melting of well crystallized lumps, while p is attributed to the melting of lumps differing from the first lot in their size and degree of organization. Figures 1Oc and 11 show the same phenomenon, the only difference in the two analyses being in the masses used in each run: 30 pg. of irradiated LiF in the first case, and 8 pg. in the second, the quantity of metallic Li present in both cases being of the order of a few per cent. The increase in the power of resolution brought about by the use of a small sample is evident. I n Figure 10, the
shape of the thermogram is perfectly characteristic of the previous heat treatment, and it must be emphasized that only the use of a microsample makes differentiation of peaks a and fl clear. The crystallographic transformation of barium titanate shown in Figure 12 provides an example of the use of the apparatus a t low temperatures. Figure 13 shows a thermogram of a 5-mg. finely-powdered sample (particle size less than 1 micron) of tricalcium silicate, obtained with the semimicro detecting head. Determination of the temperature of transformation is still very good. Semiquantitative and relative measurement of the enthalpy changes involved in the various transitions B, C, and D can be obtained by comparison of the areas of the respective peaks. Furthermore, the area of peak B is slightly smaller than that of the peak due to the high-low transition at 574' C. of an equal mass of quartz. It should be pointed out that a very extensive and painstaking x-ray study (IO) a t high temperature failed to show transition B, which is the one most easily seen by DTA. Rapid cooling-possible because of the small thermal inertia-shows the reversibility of transformation A occurring at about 600' C. on heating and a little lower on cooling; this fact, coupled with x-ray studies (10) enabled the authors to prove that A is a true polymorphic transformation and not, as was previously believed, a chemical modification. LITERATURE CITED
( 1 ) Gordon, S., J . Chem. Educ. 40, A87 (1963). ((2) 2 j Herold, P. G., Planje, T. J., J . Am. Ceram. SOC. 31,20 (1948). (3) Lambert, M., Mazibres, C., Guinier, A., J . Phys. Chem. Solids 18, 129 (1961). (4) Mackenzie. (4) Mackenzie, R. C.. C., Mitchell, B. D., Analust 87.420 f 1962). ( 5 ) Ma-ziBres; C., 'Compt. Rend. 248, 2990 (1959).
(6j-MaziPres, C., Ann. Chim. (Paris) 6 , 575 (1961). (7) Murphy, C. B., ANAL.CHEX.34,298R f 1962). (8) Whitehead, W. L., Breger, I. A., Science 111,279 (1950). (9) Wittels, M., Am. Mineralogist 36, 615 (1951). (10) Ytnnaquis, K., Regourd, M., Mazieres, C., Guinier, A., Bull. SOC. Franc. Mineral. Crist. 85, 271 (1962).
RECEIVEDfor review June Accepted October 31, 1963.
10, 1963.
VOL. 36, NO. 3, MARCH 1964
605
