Bergaya F. Handbook of Clay Science

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Chapter 12.10: Cation and Anion Exchange1000

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References 1001

Handbook of Clay Science

Edited by F. Bergaya, B.K.G. Theng and G. Lagaly

Developments in Clay Science, Vol. 1

1003

Chapter 12.11

THERMAL ANALYSIS

F. ROUQUEROL, J. ROUQUEROL AND P.L. LLEWELLYN

Laboratoire MADIREL, UMR CNRS-Universite

de Provence Centre de St Je

me,

13397 Marseille Cedex 20, France

12.11.1. BACKGROUND AND DEVELOPMENT

Clays were among the ﬁrst materials to be studied by thermal analysis following

the development of this technique toward the end of the 19th century (Le Chatelier,

1887). In general terms, thermal analysis embraces any technique where a physical

property of the material in question is assessed as a functi on of temperature. This is

illustrated in Fig. 12.11.1 where property ‘‘X’’ (e.g., mass, temperature difference,

length, ﬂow of evolved gas) is plotted against temperature.

In his famous work, entitled ‘‘About the action of heat on clays’’, Henri Le

Chatelier (1887), a professor at the School of Mines in Paris, applied two of his newly

invented thermal analytical methods to the study of clays. The ﬁrst invention was the

Pt/(Pt,10%Rh) thermocouple (still a standard, more than a century later), while the

second was a special photographic recording method. In this method the temperature

reached by the sample (proportional to the distance from the origin of the recording)

and the actual rate of heating (as inﬂuenced by self-cooling of the sample) are si-

multaneously recorded. This is deduced from the appearance of a spark, at 2-second

intervals, in the form of a small bar on a photographic plate (Fig. 12.11.2). When an

endothermic phenomenon occurs, the actual heating rate of the sample decreases

causing the density of the bars to increase. Thus, the physical property assessed by

Le Chatelier (1887) was the self-cooling of the sample vs its temperature. This was the

ﬁrst automatic recording of a thermal analysis experiment on clays.

Equally interesting were the reasons that Le Chatelier put forward to justify his

choice of clays for the experiment: ‘‘the hyd rated aluminium silicates (clays, kaolins,

etc y) y generally form mixtures too complex for chemical analysis to furnish any

precise data on their structure. I have thought that in studying the temperature of

dehydration of these substances, one could perhaps begin to characterise a small

number of chemical species and to distinguish the presence of each of these in

various mixtures. If one rapidly heats a small quantity of clay, there is produced, at

DOI: 10.1016/S1572-4352(05)01037-8

the moment of dehydration, a deceleration in the temperature rise (elevation); this

point of arrest can be used to establish a distinction among the various hydrated

aluminium silicates. The comparison of observations carried out on a very large

number of clays, shows that the complexity of these substances is much less than one

would have originally been led to believe’’ (quoted from the full English translation

by Wendlandt and Collins (1976). From these very ﬁrst experiments Le Chatelier

(1887) concluded: ‘‘The clays could now be classiﬁed, according to their pyrogeneous

decomposition, in ﬁve distinct categor ies’’.

Fig. 12.11.1. Principle of thermal analysis in general (Rouquerol, 2003).

Fig. 12.11.2. Original thermal analysis traces of Le Chatelier for (1) halloysite, (2) allophane,

(3) kaolin, (4) pyrophillite, (5) montmorillonite. The short traces on top were used for tem-

perature calibration after the boiling point of water (Ho), sulphur (S) or selenium (Se) or after

the melting point of gold (Au).

Chapter 12.11: Thermal Analysis1004

The next step, taken at a much later date, was to relate the thermal behaviour of

clays to their crystalline structure. Caille

re and He

nin (1948) discussed the temperatures

of dehydration (an endothermic phenomenon), dehydroxylation (also endothermic)

and recrystallisation (exothermic) in relation to the structures and chemical compo-

sitions of a large set of phyllosilicates and ﬁbrous clay minerals. Given the marked

effect of impurities on the differential thermal analysis (DTA) patterns, Caille

and He

nin (1948) further suggested that DTA may provide a simple means of tracing

the origin of a natural sample. It was not until the late 1950s, however, that DTA

became an important tool for clay mineral identiﬁcation. Structural studies, indicating

the location of water and hydroxyls (as decribed below for simple phyllosilicates)

provided insight into the dehydration behaviour of clay minerals (Mackenzie, 1970).

ldvari (1991) gave a comprehensive description of all types of water in minerals

(and especially clay minerals) that can be assessed by thermal analysis. A ﬁrst great

distinction can be made between the water that occurs (i) as molecular water, and (ii)

as hydroxyls capable of cond ensing into water during thermal analysis. ‘Molecular

water’ can exist as interlayer water (in smectites, this water is stabilized by the

presence of exchangeable cations), as zeolitic water (in mordenite and sepiolite) in

cavities or channels, as water adsorbed on the external surface of clay minerals, or as

water formed by capillary condensation in large pores (in chrysotile). Most of these

types of water can be removed by heating at temperatures o100 1C and the initial

amount largely depends on ambient conditions of humidity and temperature. Hy-

droxyls are part of the clay structure and cannot be released without causing more or

less irreversible structural modiﬁcations. Hydroxyls make up what is often called

‘constitutional water’. This term, however, may lead to misunderstanding because

hydroxyls can only become water through condensation. In phyllosilicates, these

hydroxyls are usually bound to the structure by ionic or covalent bonds. Since

hydroxyls have a broad bond energy distribution (due to the different locations in

the structure), and their diffusion between sheets or in channels can be hindered in

different ways, hydroxyls are liberated over a very broad temperature range (between

150 and 1000 1C). This phenomenon is precisely what makes the thermal analysis of

clay minerals so interesting. At the same time, water and hydroxyls can serve as

probes to differentiate clay miner als.

Before discussing some results on the thermal analysis of clay minerals, we will brieﬂy

describe the main techniques available and the two basic modes of operation, namely,

‘temperature-controlled thermal analysis’ and ‘sample-controlled thermal analysis’.

12.11.2. TEMPERATURE-CONTROLLED (CONVENTIONAL) THERMAL

ANALYSIS

More than 95% of the thermal analysis work on clay minerals is carried out using

techniques where the temperature programme of the furnace (or, sometimes, the

sample) is selected and imposed by the experimenter. This ‘temperature-controlled’ or

12.11.2. Temperature-Controlled (Conventional) Thermal Analysis 1005

‘conventional’ thermal analysis is schematically shown in Fig. 12.11.3. Here the heat-

ing control loop makes use of the temperature of the sample or furnace (together with

the measurement of elapsed time, in order to get a temperature programme). More

often than not the programme is linear, that is, a constant heating rate is imposed.

In the case of clay minerals, the property ‘‘X’’ refers, ﬁrst of all, to the temper-

ature difference (DT) between one poin t in/close to the sample and another point in/

close to an inert reference ( Fig. 12.11.4).

Following Le Chatelier (1887), such an experiment measures the self-cooling or self-

heating of the sample but with the added beneﬁt of having a permanent comparison

Fig. 12.11.3. Principle of conventional or ‘‘temperature-controlled’’ thermal analysis

(Rouquerol, 2003).

Fig. 12.11.4. Principle of DTA.

Chapter 12.11: Thermal Analysis1006

between the ‘sample temperature’ and the ‘reference temperature’. This arrangement

was due to Roberts-Austen (1899), a chemist at the Royal Mint, London, and later

named ‘Differential Thermal Analysis’ (DTA). The reference temperature would be

similar to that of the sample if it did not undergo any thermal transformation. Since

the temperature difference is usually smaller than 10 K, this can be the full scale of the

recorder and small thermal events may be detected with great sensitivity. These events

can be endothermic (usually corresponding to water losses) or exothermic (usually

reﬂecting structural rearrangements). Let us stress that maximum sensitivity, in DTA,

is obtained at maximum heating rates (typically between 5 and 20 K/min).

For quantitative thermal analysis, ‘Differential Scanning Calorimetry’ (DSC) is by

principle superior to DTA since the former method not only determines the temper-

ature of transformation but also the corresponding heat. Nevertheless, DSC has two

drawbacks: (i) the upper temperature (typically between 600 and 850 1C) is lower than

that obtainable by DTA (X1000 1C); and (ii) the energy of dehydroxylation is not a

constant (being dependent on the type of bond), and hence, no simple relationship

exists between the amount of heat absorbed and the amount of water released.

The latter limitation is overcome by ‘Thermogravimetry’ (TG) where the quantity

measured is the sample mass. Here the mass of water evolved and thus its amount is

precisely measured irrespective of bonding mode. TG also allows slow heating rates

to be selected but unlike the situation with DTA or DSC, these conditions do not

diminish the sensitivity or accuracy of the experiment.

At ﬁrst sight the combination of DTA with TG, avail able in a number of modern

instruments, is attractive. However, good critical judgment is required for interpre-

ting the results. This is because, the two techniques may not reveal the same thing

although the measurements are done sim ultaneously. For example, a mass loss (due

to the release of water vapour from the upper part of the sample) can be detected few

minutes before the self-cooling wave reached the DTA thermocouple in the bottom

of the crucible. Further, the best sample arrangement for thermogravimetry is a thin

layer of powder, whereas the best one for DTA is a heap around or above the

thermocouple. Thus, both experiments cannot simultaneously be at their best.

A fourth thermal analysis technique, called ‘Thermodilatometry’, is well suited for

investigating the sintering and ﬁring behaviour of clay minerals. Here the quantity

‘‘X’’ refers to the length or volume of the sample. Indeed, therm ogravimetry is not

very useful above 800 1C because the residual water constitutes only a few per cent of

the initial quantity. On the other hand, thermodilatometry can readily detect the

dimensional changes due to sintering that occur at these temperatures.

For a given sample the temperature at which speciﬁc thermal phenomena

occur (as detected by DTA, DSC or TG) is well deﬁned and reproducible, provided

the experimental procedure is very carefully deﬁned. This means that any change

in sample mass/shape/size, crucible cover, location of the two thermocouples,

atmosphere (sample preconditioning), or heating rate is able to modify the resulting

DTA, DSC or TG curve. For this reason, one cannot speak of ‘standard’ temper-

atures for the various DTA peaks or TG steps. Nor can a DTA or TG curve be

12.11.2. Temperature-Controlled (Conventional) Thermal Analysis 1007

reproduced by someone else if the above experimental variables are not speciﬁed. On

the other hand, highly reproducible and well-resolved DTA or TG patterns, can be

obtained speedil y (often within o1 h) if the same experimental procedure is always

used. The latter point is sufﬁcient reason for using these thermal analytical tech-

niques to compare, screen, and check natural clays and clay minerals. We should

also add that although the peak/step temperatures are strongly dependent on he ating

rate (temperature shifts as high as 100 K are observed when the heating rate drops

from 10 to 0.5 K/min), the general shape of a DTA or TG curve, with its peaks or

steps, is quite speciﬁc to, and hence, can serve as a ﬁngerprint of a given clay.

Another notew orthy feature of thermal analysis is that it provides a continuous trace

of the temperature-induced transformation. By contrast, spectroscopic techniques

commonly used to assess and interpret structural and surface modiﬁcations provide

a discontinu ous analysis of selected states of the sample.

12.11.3. SAMPLE-CONTROLLED THERMAL ANALYSIS

The limitations of DTA, DSC and TG, arising from the large dependence of the

recordings on experimental details, also apply to most other conventional, temper-

ature-controlled thermal analysis techniques. The main problem stems from the

existence of (i) temperature gradients enhanced by the self-cooling of the sample on

dehydration, and (ii) pressure gradients produced within the sample by the release of

water vapour. Since these two types of gradients are directly related to the rate of

thermolysis, it is desirable to control and limit this rate.

The left-hand scheme in Fig. 12.11.5 shows the simplest and most straightforward

manner to achieve this end. By exclusively using the physical quantity ‘‘X’’ (sample

mass, length, enthalpy, gas evolved etc.), which is directly related to the rate or extent

of thermal transformation, heating of the sample is controlled in such a way as to

keep the reaction rate at the desired level. The procedure is therefore, referred to as

‘Controlled transformation Rate Thermal Analysis’ (CRTA).

The right-hand scheme in Fig. 12.11.5 represents the generalised principle of what

is now called ‘Sample-Controlled Thermal Analysis’ (SCTA) which includes CRTA,

the most ancient and most developed form. SCTA also opens the possibility of using

a more complex heating control device, where the temperature (T) of the sample can

play a part. In any SCTA experiment, the ‘X loop’ (i.e., the use of a feedback from

the sample) is compulsory whereas the ‘T-loop’ is only supplementary and optional.

As we shall illustrate by a few examples later on, the SCTA approach proved to be

extremely rewarding for analytical purposes (enhanced resolution and separation due to

the control of temperature and pressure gradients), preparative purposes (homogeneity

of the ﬁnal product) and kinetic purposes (sample temperature and its rate of trans-

formation can be accurately measured, while the latter parameter can be modulated).

This principle of SCTA was successfully applied to ‘Evolved Gas Detection’

(Rouquerol, 1964), ‘Thermogravimetry’ (Erdey et al., 1965) and ‘Thermodilatometry’

Chapter 12.11: Thermal Analysis1008