A N A P P A R A T U S F O R D I F F E R E N T I A L T H E R M A L A N A L Y S I S

By R. A. SCHULZ*

Division of Coal Research, C.S.I.R.O., Sydney, New South Wales, Australia.

[Received 13th August, 1963]

ABSTRACT A description and details of performance are given for a differential thermal apparatus which has been developed at the C.S.[.R.O. Division of Coal Research for use in studies of the mineral matter in coal. An important feature of the design is the block assembly unit, which fits into a vertical furnace, and comprises a stainless steel block in which are the three sample wells and three insulated chromel-alumel thermocouples. The output from the differential thermocoup[es is applied to a simple transistorized D.C. amplifier before being passed to a recorder which traces simultaneously against time the temperature difference and the temperature. Features of the apparatus which have proved especially attractive in research on coal are its automatic operation, ease of main- tenance, and base-line stability. Reproducibility of results is good.

INTRODUCTION A differential thermal apparatus (Fig. 1) has been developed for

studying, inter alia, the high-temperature chemistry of minerals associated with coal. To enable relevant samples to be studied while at the same time meeting the basic requirements of differential thermal equipment (Mackenzie, 1957; Mitchell, 1961; Murphy, 1958, 1960, 1962) several novel features have been incorporated in the design.

As far as possible, consistent with making maximum use of avail- able equipment and facilities, advantage has been taken of recent advances in instrumentation, and against this background, the course of the development work has been largely determined by the following considerations of design:

(a) Conversion of a standard single-point recorder to trace both T and AT variations with time, where T is the temperature of the sample and AT is the difference in temperature between the sample and the inert.

(b) High sensitivity, by introducing a simple, inexpensive amplifier for AT output.

(c) Provision of a block assembly (Fig. 2) and thermocouple system, with easily replaceable thermoeouples, giving highly reproducible results and minimum base-fine drift.

DESIGN OF APPARATUS Experience with equipment of the type described by Grimshaw,

Heaton, and Roberts (1945) revealed certain weaknesses, including

* Present address: School of Chemistry, University of New South Wales.

279

280 R.A. SCHt3LZ

Fro. 1--Differential thermal analysis apparatus: A--recorder; B---control unit; C--programme controller; D--furnace; E--cold junction flask.

~A

K'

FI6. 2 Block assembly and components: A--top section of block; B--base of block; C--locating disc; D--end plug; E--flange; F--connector; G--rod; H--thermocouple insulators; /--well; J---ceramic partition; K--gas escape

ports.

excessive base-line drift (Mitchell and Mackenzie, 1959), difficulty in maintaining the sample-holders and thermocouple wires steady in the assembly, and unsatisfactory calibration and replacement of thermo- couples. Most of these faults have been eliminated in the arrange- ment shown in Fig. 3.

DIFFERENTIAL THERMAL APPARATUS 281

In designing the block the aim has been to provide a symmetrical arrangement, as by so doing many of the thermal problems of differential thermal apparatus can be overcome. The block is large enough to act as a heat reservoir around the sample wells but not so large as to extend beyond the "constant hot zone" of the furnace. The three sample wells are machined into the block at positions which are symmetrical with respect to the furnace axis and which provide maximum thermal insulation (see Fig. 2). Under conditions of hard packing, each well will hold 800 mg of material.

Considerable benefits accrue from the use of the block assembly unit shown in Figs. 2 and 3: for example, it ensures constant and reproducible positioning of the block in the furnace, and accurate

INSULATOR - f

CONTROL T/C

WELL J ' /

24

.GAS ESCAPE PORTS

�9 SAMPLE

. BLOCK

- INSULATOR

/LOCATt~,~G OtSf;

~FURNACE TUBE

END PLUG

SEAL

FLANGE ~ ~ CONNECTOR

~ PtATFORM

FIG. 3--Block assembly in furnace.

location of thermocouple junctions (Grim and Rowland, 1942; de Bruijn and van der Marel, 1954; Mitchell, 1961). Connection into the external circuit is simple and convenient, and the sample can be packed with ease and precision.

The thermocouple system itself (Fig. 4) is a balanced, independent circuit, very effective in minimizing stray e.m.f.s and base-line drift. It is particularly suitable for use with automatic input switching, which is a feature of the present apparatus. Replacement and checking of thermocouples are simple operations, and adequate precautions have been taken against the production of active thermojunctions in the wiring between the block and the recorder input.

282 R.A. SCHULZ

The output from the thermocouples is applied, via a control unit, to a single-point 0--5 mV recorder* (A, Fig. 1). The control unit (Fig. 5) provides output-input matching from thermocouples to recorder, namely, amplification of the AT signal, attenuation of the T signal, and simultaneous recording of both the AT/t and the T[t traces (where t is time). To avoid thermal e.m.f.s in the resistance

SAMPLE

INERT

TEMPERATURE

SCREW

] AI

] AT c.[j O" INSULATOR Ch

I

CONNECTOR

FIG. 4---Thermocouple system.

Ch r . . . . . "1 CU

AI 1 ~T AI

o ,

ch ', i

Ch ~ Cu e ; fl ~ : Cuo;T

A~ L! . . . . J

COLD JUNCTIONS

,;~ I., I N P U T c > ~ . ~

R~ , RI ATTENUATOR

'~" -"" s~.:~ "~ "k3_ " S."

,, 3,s_~_ ;

-gv S) PI P~ ~ T3

R, tR-

+gV o

R .

l R1s P ' 1"5 v i T~ S~

~ R4

FIG. 5--Circuit diagram of control unit.

networks loading the thermocouples, manganin resistors across the recorder input have been used in place of standard nichrome or carbon types. Similarly, for trace-changing, non-thermal micro- switches (MS1, MS2, Fig. 5) were chosen instead of the relays which had been tried initially.

The transistor amplifier in the /IT circuit has been designed to operate on low current drain, thus ensuring long battery life, constant supply voltage, stable gain, and low noise and drift levels without the * For the 0-1 mV recorder originally used a 0-5 mV recorder was later substituted because of the apparent instability of the former, which was traced to the external

circuit.

J MICRO SWffCHES MS, T -"

RECORDER INPUT

, [ INPUT ~ 5z RELAY TO OPERATE + o I El SWITCHES

DIFFERENTIAL THERMAL APPARATUS 283

expense that a more complicated circuit would entail. However, as the amplifier is subject to temperature instability, its case must be thermostatted inside to a tolerance of +0.1~ for the duration of a run. Besides stabilizing the amplifier this provision ensures that any thermal voltages in the input circuitry are maintained constant. The problem of spurious e.m.f.s (Mitchell, 1961) arose when the ap- paratus was operating at full gain. To prevent signal injection from the furnace winding, the block and both thermocouple circuits are earthed. With the particular circuitry employed, it was found un- necessary to adopt the widely used alternative of introducing an earthed metal sheath inside the furnace (Mitchell and Mackenzie, 1959). Measures taken to clear the instrument of a.c. induction effects (which were traced to sources other than the furnace) included: non-inductive winding of the attenuator resistor, removal of the saturable reactor (normally housed in the programme controller)

ACTUATING MICRO R2 s,s ,.EVER 5W,lC,-,% t ,,o w

ING MECHANISM $4 ( Pton view ) I I

LI L;Z

FIG. ~ i r c u i t diagram of switching mechanism.

from the proximity of other components, immobilization and shielding of lead wires, and connection of condensers (C 4, C s, Fig. 6) across certain switches and relays. Obviously many faults of this type are characteristic of a particular wiring layout, and the remedies do not necessarily have universal application.

DESCRIPTION OF APPARATUS Fig. 1 is a photograph of the complete apparatus, details of which

are given below under four headings: sample block assembly and thermocouple system; furnace and stand; programme controller; recording equipment.

Block Assembly and Thermocouple System (Fig. 2). The block is a stainless steel cylinder, 67 mm long and 45 mm diameter, made in two sections. The lower section contains three sample wells, each 15 mm long and 8 mm in diameter, distributed with angular, radial, and longitudinal symmetry in the block as a whole. Each well is shielded over its entire length by a thin ceramic partition (J, Fig. 2). A thermocouple inlet hole is drilled from the base of the block to each well, and is continued through the top section to maintain symmetry and also to act when necessary as a gas escape port. The block sections are held together by a bayonet-type connection.

284 R. A, SCHULZ

The assembly is mounted on a ] in. stainless steel rod which screws into the base of the block. Three screws on the locating disk enable the thermocouples to be fixed rigidly in position. The lower end plug fits between the locating disk and a metal flange, and is pre- vented from rotating by two studs carried on the disk. The con- nector is of ebonite and is provided with six radial slots each large enough to hold two thermocouple wires, one above the other. The screw in each slot enables these wires to be clamped. Spacing along the rod is adjusted so that the block lies wholly within the constant hot zone of the furnace.

The thermocouple system (Fig. 4) comprises three identical chromeI-alumel thermocouples (No. 22 s.w.g.) fitted into 6 in. twin- bore ceramic insulators (2 mm outside diameter) and mounted in the block assembly as shown in Fig. 2, so that the junctions are central

1 2 "

i~* 6 " I

...E ~ ".5. ~ 5 . . . . . 5" 5" 1~1 " i3"

~Ti" Ft t i .... FI)]"

6"T~E WIRE

TUBE:-RECRYSTALL~ED ALUMINA WlNDING:-A.I. REStSTANE-WiRE "KANTHAL~ 18 S.WG. 0"540~ PER FT.

HAXIHUH TEHPERATURE OF WIRE 1350~

FtG. 7--Wiring diagram for furnace.

2" �9

in the wells. Chromel-alumel wires lead from the connector to the cold-junction flask, where they are joined to shielded copper input leads. All junctions are silver-soldered, sheathed in polythene tubing, and immersed in an ice bath.

Furnace and Stand. The furnace is a 1 kVA, 12 in. tube type (50 mm inside diameter) with highly efficient thermal insulation. Fig. 7 gives essential details of the tube and element, while D in Fig. 1 shows the external construction and mounting. Power is supplied f r o m the programme controller via an isolating transformer (180 V/100 V), and a meter monitors the element current, which rises to a maximum of 10 A. End plugs are of moulded refractory cement and are a close fit in the tube.

Beneath the furnace is a sliding platform which carries the block- assembly mounting and centering device. This consists of two eccentric bushes, the upper one being machined to take the centre rod of the assembly, the lower one fitting into the platform (see Fig.

DIFFERENTIAL THERMAL APPARATUS 285

3). The block is placed in the furnace by raising the platform, and centered in the tube by rotating the bushes. In order to maintain a good seal and to fix the block position, the platform is clamped once adjustments have been made.

Programme Controller. Heating of the furnace is programmed by a linear-rate temperature controller similar in basic design to that described by Waters (1958). The present unit has a power output of 1 kVA at 180 V, and utilizes a power transistor as a driver amplifier for the saturable reactor, in place of a magnetic amplifier. Other changes in the circuit are only of a minor nature.

The instrument provides variable control over the following range of heating rates (~ 1, 1-25, 1-5, 2, 2.5, 3, 4, 5, 6, 8, 10 and 12.

Recording Equipment. This consists of a chart recorder and a con- trol unit (Fig. 1). The recorder is a Speedomax single-point type G, with a range of 0-5 mV over a 9�89 in. chart. It is adjusted for centre- zero operation at a chart speed of 6 in./hr.

In Fig. 5 is shown the circuit diagram of the control unit, and in Fig. 6 the associated switching mechanism. The transistorized D.C. amplifier for AT input is a long-tailed pair arrangement with a current drain of 0.19 mA per transistor. It provides amplification factors of • • 5, and • 10, corresponding to a maximum sensitivity of approximately 50/zV per inch of chart, with noise level < 2.5/zV and drift not greater than 2.5/zV per hour.

The ratio of the potential divider loading the T input (PT, Fig. 5) is adjustable, enabling the chart to be calibrated to read 0-1,000~ for individual chromel-alumel couples. Suitable negative bias is applied across the recorder input by the 1.5 V battery circuit, for zero adjustment of the temperature scale on the left-hand side of the chart. Both AT and T inputs can be short-circuited at any time by push-button switches ($1, $2, Fig. 5), and a 'THRU' position on the attenuator enables the ATinput to be coupled directly to the recorder if desired.

The amplifier and general input circuitry are of printed circuit construction and are contained in a thermostatted case (approx. 5,000 ml) which has a metal shell and foam polymer insulation. A constant temperature is maintained inside by low-power heaters, a small fan, a sensitive bellows thermostat, and a system of heat baffles. A third transistor in the circuit (T3, Fig. 5) compensates for any gradual temperature drift not overcome by the thermostat.

A switching mechanism, included in the control unit, swithecs the recorder input from the AT trace to the T trace every 5 rain. for a period of approximately 5 sec. The mechanism is energized and timed by a small synchronous motor (Fig. 6). Trace-changing is effected by two microswitches (MS l, MS 2, Fig. 5) simultaneously activated by a relay (Lt) which in turn is closed via a microswitch on the mechanism (S 1, Fig. 6). A second microswitch ($2, Fig. 6) is timed to integrate pen lifting with trace changing by activating a relay (L2, Fig. 6) and lever arrangement fitted inside the recorder.

286 R.A. SCnULZ

The two relays (]"1, L2) are energized from a rectified A.C. supply (Fig. 6) and operated automatically in the correct sequence by the switching mechanism. Two push-button switches ($3, S 4, Fig. 6) permit manual operation of either relay at will.

OPERATION

The design and operation of this instrument have been planned in accordance with the standards proposed by Mackenzie and Far- quharson (1953).

All samples are pretreated, and ground to pass a 100 B.S. mesh sieve. The sample for examination is prepared by diluting 200 mg of active material with 600 mg of pre-calcined alumina (the inert). This sample and two 800 mg increments of inert are then packed hard by hand into their respective wells.

The block assembly is fixed centrally in the furnace and the control thermocouple is inserted from the top to fit between the block and the tube (Fig. 3). External connections are made, and zero adjust- ment of both traces is carried out manually before each run. The controller is adjusted to the rate of 10~ and recording is then taken over by the switching mechanism.

During a run the apparatus requires no attention: to eliminate risk of overheating, programming is automatically discontinued at 1,000~ The AT/t trace is recorded as a continuous-line curve and the T/t trace as a dotted-line curve with dots at intervals of 50~ (i.e. every 5 min).

DISCUSSION Fig. 8 is a typical curve produced by the instrument. Replotting

such a curve to obtain a true thermogram is seldom necessary, as peak temperatures can be read off directly. Base-line stability is good: if both wells are filled with inert the base-line shows no deviation greater than 0.25~ up to a furnace temperature of 1,000~ Fig. 9 exemplifies the performance of the programme controller: the heating curve is highly reproducible but the degree of linearity varies accord- ing to whether a metal or a ceramic block is employed.

In Table 1 peak temperatures for various samples are listed. The Wyoming bentonite and kaolinite are the standard minerals used in the international differential thermal analysis survey (Mackenzie and Farquharson, 1953), and the quartz is a pure sample which is also useful for standardization. The peak temperature values obtained with this apparatus agree well with published values. Other samples, namely the A.P.I. reference halloysite, kaolinite, and montmoril- lonite, which have been used for comparative purposes, indicate good reproducibility of peak temperature. For the three shale samples, each from a different band in a coal seam, the peak tem- peratures agree remarkably well.

The following practical details have come to light in the course of day-to-day use of the apparatus.

DIFFERENTIAL THERMAL APPARATUS 287

�9 T vs. T~ME

a T vs. T t ~ r

% -

Sg0*C

Ff6. 8--Differential thermal curve for kaolinite (A.P.I. No. 7, Bath, South Carolina, U.S.A.), traced from actual chart�9 The temperature scale has been inverted, and the AT trace is thus plotted conventionally from left to right.

1000

900

800

700

600

5~

400

300

200

100

0

FI(3. 9--Controller performance: /--constant power input; tI---controlled heating.

288 R. A. SCHULZ

T A B L E 1.--Peak temperatures for various samples.

Sample

Quartz heating cooling

Wyoming bentonite?

Kaolinite~

Halloysite No. 29 (A.P.I.)

Kaolinite No. 7 (A.P.L)

Montmorillonite No. 23 (A.P.I.)

3 shale samples from Greta coal seam, N.S.W.

This Apparatus

581 575

724

610

597 589

593 590 591

704 700

592 594 592

Peak temperatures (~

1 This Published Apparatus

mostly 575--580*

680--760 926 (mean 720)1"

570--660 971 (615)t

- - 960 956

957 954 956

860 - - 842

I 966 - - 968

972

Published

860--930 (mean 900)t

* Mackenzie (1962). t Mackenzie and Farquharson (1953).

(a) The stainless steel block gradually becomes encrusted with a powdery scale, with the result that heat distribution within the block is upset, leading to excessive base-line drift. The trouble is avoided by thorough cleaning of the wells after each heating.

(b) Ageing of the thermocouples can produce unexpected humps in the trace, sometimes accompanied by a pronounced displacement of the base line. Naturally, if samples containing some corrosive agent are being tested the thermocouple will be attacked (Fig. 10)

FIG. 10--Thermocouple corrosion.

DIFFERENTIAL THERMAL APPARATUS 289

and a severe mismatch in the AT circuit will result. Frequent in- spection, checking, and replacement of thermocouples has proved to be the most effective remedy.

(c) Discrepancies in published differential thermal analysis data may be attributed in part to differences in the methods of calibration em- ployed (Barshad, 1952; Gordon and Campbell, 1955; Wilburn, 1958). With the apparatus here described good results have been obtained using a method similar to that of Berkowitz (1957), in which the thermocouples are pre-calibrated or matched in a pyrometer furnace prior to use. The recording equipment is then adjusted to suit them.

Acknowledgements.--The author desires to express his grateful thanks to Dr D. J. Swaine for valuable advice and discussions, and to Mr B. Ayling for technicat assistance with the electronic equipment.

REFERENCES

BARSHAD, l., 1952. Amer. Min., 37, 667. BERKOWaTZ, N., 1957. Fuel, Lond., 36, 355.

DE BRUIJN, C. M. A., and VAN DER MAREL, H. W., 1954. Geol. en Mijnb., 16, 69~ GORDON, S., and CAMPBELL, C., 1955. Analyt. Chem., 27, 1102. GRIM, R. E., and ROWLAND, R. A., 1942. Amer. Min., 27, 746, 801.

GRIMSHAW, R. W., HEATON, E., and ROBERTS, A. L., 1945. Trans. Brit. Ceram. Soc., 44, 76.

MACKENZIE, R. C. (editor), 1957. The Differential Thermal Investigation of Clays. Mineralogical Society, London.

MACKENZIE, R. C. (compiler), 1962. Differential Thermal Analysis Data Index. Cleaver-Hume Press, London.

MACKENZIE, R. C., and FARQUHARSON, K. R., 1953. C.R. XIX Congr. GdoL Int., Alger, 1952, 18, 183.

MITCHELL, B. D., 1961. Clay Min. Bull., 4, 246. MITCHELL, B. D., and MACKENZIE, R. C., 1959. Clay Min. Bull., 4, 31. MURPHY, C. B., 1958. Analyt. Chem., 30, 867.

MURPHY, C. B., 1960. Analyt. Chem., 32, 168R. MURPHY, C. B., 1962. Analyt. Chem., 34, 298R.

WATERS, P. L., 1958. J. Sci. Imtrum., 35, 41. WmBtIRN, F. W., 1958. J. Sci. Instrum., 35, 403.