1-Thermoanalytical Mehtods in Clay Studies

8/20/2019 1-Thermoanalytical Mehtods in Clay Studies

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Chapter

1

THERMOANALYTICAL METHODS

IN

CLAY STUDIES

Robert C. MACKENZIE

The Macaulay Institute for Soi

1.1 INTRODUCTION

There is nothing new in the

315 BC, Theophrastus refers to

Research,

Craigiebuc l er

,

Aberdeen

,

Scot1 nd

, UK.

thermal study of clays. Indeed, as early as about

the effect of fire

(i.e.

heat) on talc

(as

steatite) and on palygorskite (as mountain wood ) (Eichholz, 1965) and

development in the use

of

heat as a discriminator can be traced from that time

on (Mackenzie, 1981a).

some as a relatively recent technique for clays, has its roots in the eighteenth

century, when the Rev. Stephen Hales

(1727) found that a cubick inch of fresh

un t r i e

earth

(his italics) yielded 43 cubick inches of air on heating and

Josiah Wedgwood

(1782)

detected

only

carbon dioxide

on

firing china clay in a

closed system, the evolved water having condensed and gone unnoticed.

The first thermoanalytical

study of clays was performed in 1887,when Henri

Le Chatelier recorded what were essentially heating-rate curves for halloysite,

allophane, kaolinite, pyrophyllite and montmoril lonite, over the approximate

temperature range 20-llOO°C, in an attempt to use their behaviour on heating as

a classificational criterion.

remarkably authentic - surely

a

tribute to the mineralogists o f the time who had

none of the modern methods of diagnosis available to them.

differences observed, ittle advance, apart from the pub1 ication of some so-called

dehydration curves (Samoilov,

1909

and some heating curves (e.g. Mellor and

Holdcroft,

1911;

Ashley, 1911; Brown and Montgomery, 1912), occurred until Wallach

in

1913

first applied differential thermal analysis (DTA) to clays.

however, seems to have elicited little response and, although the OTA studies

o f

Satoh 1918,1921) aroused more attention, it was not until the early 19405,

subsequent to the detailed studies of Norton

(1939)

and Hendricks and Alexander

(1939),

that DTA blossomed forth as an investigational technique.

simple:

at that period clays excited much interest as the general structure of

the clay minerals had been establisned and the species collected into groups, with

the reLult that methods of identification and estimation additional or complementary

to X-ray diffraction were being sought.

application of DTA to problems that it could not possible solve,and even the use

of

unsuitable equipment and technique, led to the method being discarded by some

as useless in clay mineralogy.

However, by no means all clay mineralogists were

Even evolved gas analysis, which would be considered by

His results suggest that the samples used were

Despite the

Even this,

The reason is

Unfortunately, the indiscriminate


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so d i s i l l u s i o n e d a nd much p a i n s t a k in g wo rk o ve r t h e ye a rs (by e.g. R a lph

E .

G r i m ,

Paul F . Kerr , To sh io Sudo and o t he rs ) g ra d u a l l y demonstra ted t h a t DTA d i d have a

p la ce i n c l a y m i n e r a l o g i c a l s t u di e s .

i n t h e p a rag rap h a b ove )

s h o u ld be s e p a r at e d f r o m p u r e l y t he r m a l s t u d i e s ( r e f e r r e d

t o i n t he

f i r s t

p a rag rap h ) . The re aso n i s t h a t th e rm a l m eth od s h ave t o s a t i s f y

c e r t a i n c r i t e r i a b e f or e t h e y c an be t er me d t h e rm o a n a ly t ic a l .

some o f t h e t h e r m o an a ly t ic a l t ec h n iq u es c u r r e n t l y a v a i l a b l e and t h e i r a p p l i c a t i o n

an d/or a p p l i c a b i l i t y i n c l a y i n v e s t i g a t i o n s a r e th e s u b je c t o f t h e re ma in de r o f

t h i s p a p e r .

A t t h i s p o i n t t h e r ea d er m i g h t w e l l a sk why t h e r m o a n a l y t i c a l s t u d i e s ( d is c u ss e d

T h e s e c r i t e r i a ,

1.2 THERMAL A N A L Y S I S

O ver t h e p a s t f i f t e e n ye a rs m uch a t t e n t i o n h as b een d e vote d t o n o m e n c la ture ,

d e f i n i t i o n and c l a s s i f i c a t i o n o f t he rm o an al yt ic a l te ch niq ue s w i t h t h e r e s u l t t h a t

the methods inc luded can now be c lear ly recogn ized and named.

A c co rd in g t o t h e I n t e r n a t i o n a l C o n f e d e r a t io n f o r T he rm al A n a l y s i s ( IC T A) ,

t h e r m a l a n a l y s i s cov ers (Lombard i , 1980):

A g ro up o f t e ch n iq u es i n w hic h a p h y s i c a l p r o p e r t y o f a s u bs ta nc e a nd /o r i t s

r e a c t i o n p r o du c t s i s mea sured as a f u n c t i o n o f t e m pe ra tu re , w h i l s t t h e s u bs ta nc e

i s su b je c te d t o a c o n t r o l l e d t e m pe ratu re program me .

The t h r e e c r i t e r i a t h a t d i s t i n g u i s h a t h e r m o a n a l y t i c a l m ethod a r e, t h e r e f o r e ,

t h a t a p h y s i c a l p r o p e r t y i s measured as a f u n c t i o n of t e m p e ra t ure under a

cont ro l l ed t empera ture programme. Thus, a s i n g l e i s o t h e rm a l d e t e r m i n a t i o n i s n o t

t h e rm o a n a ly t ic a l b u t ass ess ment o f t h e r e s u l t s o f a s e r i e s o f i s o t h e rm a l

d e t er m i na t io n s a t d i f f e r e n t t em p er at ur es as a f u n c t i o n o f t em p e ra tu r e i s .

S i m i l a r l y , n o n -th erm a l me th od s, such as X- ra y d i f f r a c t i o n , p e rform e d u n de r a

co n t ro l e d t e m p e ra tu re program me become th e rm o a n a l y t i ca l d e te rm in a t i o n s . I n t h e

a ccou n t t h a t f o l l o w s , h ow ever, o n l y t h o se m eth od s n o rm a l l y i n c l u d e d i n t h e rm a l

a n a l y s i s

w i l l

be considered:

i t

s h ou l d be o bs erv ed t h a t c l a s s i c a l c a l o r i m e t r y i s

exc luded , desp i te

i t s

c lo se re l a t i o n s h i p t o some th e rm o a n a l y t i ca l m eth od s.

1.2.1 A v a i l a b l e t h e r m o a n a l y t i c a l t e ch n iq u es .

A

g e ne r al c l a s s i f i c a t i o n o f methods c u r r e n t l y r e co g ni z ed as t h e r m o a n a l y t i c a l

i s g iv e n i n T a bl e 1.1 a lo n g w i th t h e p h ys i ca l p ro p e r t y o n w h i ch t h e y d e p e n d a n d ,

'fo r common metho ds where i t s g e n e r a l l y i n use, t h e a c c ep t ab le a b b r e v i a t i o n

(Lombard i, 1980) . Most o f the techn ique s can be de f in ed

i n

exact ly the same way

as th er m al an a l y s is , t h e p h y s ic a l p r o p er t y i t s e l f - mass f o r the rmograv ime t ry ,

an e l e c t r i c a l c h a r a c t e r i s t i c f o r t he rm o el ec tr om e tr y': e t c .

-

r e p l a c i n g t h e w o r d s

a p h y s ic a l p r o p e r t y i n t h e d e f i n i t i o n . I n some i ns t an c e s,

however, more p re c is e

w o rdin g i s ne cessary . F o r e xa mp le , s i x m eth od s a r e l i s t e d a s b e in g d e pe n de n t o n

change i n mass,

b u t o n l y two a re so d ep en de nt d i r e c t l y : i s o b a r i c mass-change


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TABLE 1.1

Classification of thermoanalytical techniques

Physical property Derived techniques Abbreviation

Mass

Temp ra t r e

Entha py

Dimensions

Mechanical characteristics

Acoustic characteristics

Optical characteristics

Electrical characteristics

Magnetic characteristics

Isobaric mass-change determination

Thermogravimetry

Evolved gas detection

Evolved gas analysis

Emanation thermal analysis

Thermoparticulate analysis

Heating-curve determination*

Differential thermal analysis

Differential scanning calorimetry?

Thermodi atometry

Thermomechanical measurement+

Thermosonimetry5

Thermoacoustimetryg

Thermoptometry

Thermoel ectrometry

Thermomagnetometry

TG

EGD

EGA

DTA

osc

*

I n t h e c o o l i n g mode t h i s beco mes Cooling-curve determination.

t Two types Power-compensation DSC and Heat-flux DSC, c a n b e d i s t i n g u i s h e d .

?

T e s t s u nd er o s c i l l a t o r y l o a d come u n d er t h e h e a d in g Dynamic thermomechanical

5

Thermosonimetry r e f e r s t o so un d e m i t t e d by t h e s a m ple w h er ea s

measurement.

Thermoacoustimetry i nv o l v e s m e a s ur e m e nt o f ch an ges i n t h e c h a r a c t e r i s t i c s o f

im po se d a c o u s t i c w av es p a s s i n g t h r o u g h t h e s a m p l e.

determination, which covers equilibrium techniques, such as the once common

dehydration curves under a constant partial pressure of water vapour, and

thermogravimetry- TG), which uses a dynamic temperature programme. Evolved gas

detection (EGO) and evolved gas analysis (EGA) are secondary techniques whereby

materials evolved during heating are detected or analysed, respectively, and the

remaining two, emanation thermal analysis and thermoparticulate analysis, are

tertiary techniques, being special instances of EGA related to radioactive

emanation and particulate matter, respectively.

listed

i n

Table

1.1

is derivative thermogravimetry

( D T G ) ,

the reason being that

derivative curves can be calculated for most measurements and it would be invidious

to include only one. Attention should also be drawn to the distinction between

derivative

and

differential,

the former applying to the mathematical process and

the latter being used only as the adjectival form of difference (Lombardi,

1980).

Thus, in differential thermal analysis (and differential scanning calorimetry )

the difference in temperature between (and the difference in energy inputs

into ), a substance and a reference material is measured .

A common method that is not

Moreover, heating


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curves - i . e . cu rves f o r sample tempera tu re aga ins t t lm e - g i v e r i s e t o t w o

d e r i v a t i v e s

-

hea t ing - ra te cu rves , where d T / d t i s p l o t t e d a g a in s t t em p er at ur e

T )

o r t im e t), nd inv e rs e hea t ing - ra te cu rves where d t / d T i s p l o t t e d a g a i ns t

T

o r t: b o th t h e se have bee n e x te n s i v e l y used i n t h e p a s t .

The i n f o r m a t i o n g iv e n above, t o g e t h e r w i t h t h a t i n T a b le 1.1, i s p ro ba bl y

adequate to a l l o w ap pre c ia t ion o f t he enormous s t r i d e s t h a t have been made over

the past decade o r

so

i n ob t a i n i ng in te rn a t io na l agreement on a genera l nomen-

c l a t u r e and c l a s s i f i c a t i o n s ys te m f o r t h er m oa n al yt ic a l t ec hn iq ue s. T h i s e f f o r t ,

however, has covered no t on l y nomenc la ture o f methods b u t a l so t h a t

o f

components

o f e qu ip me nt, o f a sp e c ts o f e xp e r ime n ta l t e ch n iq ue , o f c r i t i c a l p o i n t s on cu rve s

and o f symbols (Lombard i, 1980) and has been fo r t un at e enough t o re c e i v e the

backing o f na t io na l and in te rn a t io na l s tandards in s t i t u t i o n s , such as AFNOR,

ASTM

and

I S O ,

as we l l as o f ma j o r i n t e r n a t i o n a l b o d ie s such as

I U P A C

(1974, 1980).

Moreover, th e recommenda tions i n Eng l i sh have been conver ted i n t o fo rms accep tab le

i n many ot he r language-speaking areas (Lombardi, 1980; Mackenzie, 1981b), s in ce

d i r e c t t r a n s l a t i o n i s n o t alw ay s p o s s i b l e because

o f

a l ready accep ted conven t ions

i n o the r languages.

1.2.2 Simultaneous tech niqu es

same tim e, le a d in g t o si m ul ta ne ou s de te rm in at io ns such as DTA-EGA, TG-EGA,

DTA-TG-DTG, et c . Th is has advantages and disad vanta ges, and one has t o stud y no t

o n l y t h e bases o f t h e t e ch ni q u es t h e mse lve s b u t a l so t h e n a tu re o f t h e samples

i n vo l v e d b e fo re d e c i d i n g on t h e i r u se.

employed i n co n ju nc t io n wi t h DTA or TG, as one can then r e l a t e the ev o lved

v o l a t i l e s t o s p e c i f i c changes

i n

th e sample ; s im i l a r l y , by comparing s imu l taneous

DTA and DTG curves one can r e a d i l y r e la t e rea ct io ns i nv o l v i ng mass change w i t h

spec i f i c en tha lpy changes.

b o th t i me and ma te r i a l . T he ma j o r d i sa dva n tag e i s t h a t o ptimum co n d i t i o n s f o r

one techn ique may no t nece ss ar i l y be those f o r ano ther .

m i n i m i ze d b y ca re fu l

s e l e c t i o n o f e x p e r im e n ta l c o n d i t i o n s - f o r example, i n

simultaneous DTA-TG, by

using a smal l sample and/or employing a s low heat ing ra te .

I t s o f te n con venient t o make two o r more measurements on one sample a t th e

For example, EGA i s m ost p r o f i t a b l y

And, o f co urse , t h e re i s a co n s i d e ra b l e sa v i n g

i n

However, t h i s can be

1 .2 .3 S tandard iza t ion o f techn iques

S i nc e t h e r m o a n a l y t i c a l r e s u l t s c a n v a r y w i t h e x p e ri m en t al t ec h n iq u e, t h e

Sta nda rd iz a t io n Commit tee

o f

ICTA hav e p u b li s h ed a code o f p r a c t i c e l i s t i n g t h e

i n f o r m a t i o n t h a t s h o ul d be s u p p l i e d w i t h e v er y c u r v e p u bl is h e d : t h e y h ave a l s o

been i n s t ru m e n t a l i n p r o v i d i n g m a t e r i a l s f o r t em p er at ur e c a l i b r a t i o n o f a p pa ra tu s

(Lombardi, 1980).

a p p l i c a t i o n o f t h er m al a n a l y s is .

These aspects should be thoroughly stud ied by anyone consider ing


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1 3 APPLICATIONS TO CLAYS

Emphasis in this article

I s ,

quite deliberately, on the applications or

potential applications of the various techniques now available to clays.

therefore, impossible to deal adequately with Instrumentation, experimental

technique, or even with some basic principles, although all

these are critical in

determining the quality of thermoanalytical results. To overcome this deficiency

the reader is referred to the books of Daniels (1973) and Wendlandt (1974) and to

the excellent reviews that have appeared biennially in A n a l y t i c a l C h e m i s t r y

Fundamental Reviews (e.g. Murphy,

1978)

for a considerable period.

In the account that follows, clay mineralogical applications take priority,

but due consideration must also be given to the wider field of applications to

clays and clay rocks of technological or industrial importance and to accessory

minerals, since the presence or absence of these may well determine the suitability

of a clay for a particular application. With this wide field in mind, it will be

appreciated that the references given are illustrative only: an exhaustive study

of all published work would be inordinately long.

It is,

1 3 1 IIsobaric) mass-change determination

temperature until there is no further mass change and the eq u i l i b r i u m m as s is

plotted against the temperature: the partial pressure

o f

the evolved volatile

(e.g. water or carbon dioxide) is maintained constant throughout the determination

An excellent description of the technique has been given by Weiser and Milligan

(1939).

widely applied to clays in the derivation

of

so-called dehydration curves . An

excellent collection of these was given by Nutting (1943) and the technique was

still

employed for characterization purposes in the 1950s (see e.g., Mackenzie,

1957a).

seems to have fallen into disuse. However, families of isothermal mass-change

curves, particularly in their isobaric mode, can probably yield more reliable

information on the kinetics of reactions than the TG curves so commonly in use

(see below).

In isobaric mass-change determination the sample is heated at each selected

In the past this technique, although not perhaps in an isobaric mode, was

It is rather time-consuming and with the advent of thermogravimetry

1 3 2 Thermogravirnetry (TG)

Although DTA has been the most widely used technique in clay mineralogy, the

use

o f

TG and DTG has grown markedly, particularly since the introduction of the

Derivatograph, which provided simultaneous DTA-TG-DTG curves, and its commercial

production in Hungary in the mid-1950s.

most widely used, and apparently the only one comnercially produced in eastern

Europe, has been upgraded several times and now has various optional additional

This instrument, which is by far the


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a t ta ch m e nts f o r t h e rm o d i l a to m e t r y , EGA, e t c . ( P a u l i k a nd P a u l i k , 1 97 8) . I t s v a l u e

i n c l a y m i ne ra lo gy i s r e a d i l y as se ss ed f r o m t h e s i m u lt an e ou s c u r v e s f o r a l a r g e

number o f c la ys and c l a y m ine ra ls pub l i sh ed by Lang ie r-Kuzn ia rowa (1967) . Ou t -

s i d e e a s te rn E uro pe , t h e rm o g ra v im e t r y was s t im u la t e d b y t h e co m m erc ia l p ro d u c t i o n

o f t h e r o b u s t S ta n to n th erm ob a la n ce i n 1 95 4 a nd s im ul ta n e ou s te ch n iq u e s b y t h e

in t r o d u c t i o n o f t h e M e t t l e r T h e rm o a na lyzer (W iedem ann, 1 9 64 ). A w id e ra n g e o f

thermobalances and s imu l taneous DTA-TG in s t ru me n ts s u i t a b le f o r c l a y s t ud ies can

now be pu rchased (e.g. Dunn, 1980) . th e number o f the l a t t e r tend ing t o incr eas e

as a v a i l a b l e s e n s i t i v i t y has in c re a s ed . O f the va r ious types o f ba lance sys tem

used (Kea t tch and Do l l im ore , 1975), th e nu l l -p o in t e le c t ro ba lan ce now seems the

most common.

between DTA and DTG cu rves, g r ea t ca re must be taken t o ensu re th a t a l l e xpe r im en ta l

v a r i a b l e s a r e i d e n t i c a l f o r b o t h d e t er m i na t io n s

-

f o r exam ple, u se o f a d i f f e r e n t

h e a t i n g r a t e c an d i s p l a c e p ea ks and ev en a l t e r p eak s hape a p p r e c i a b l y ( A l i e t t i ,

B r i g a t t i an d P o pp i, 1 97 9) .

The main uses o f TG add DTG (w hich must be con side red to ge th er ) i n c l a y

mine ra log y have been i n de te rm in ing t he reasons f o r DTA peaks, asses s ing th e range

o v er w h ic h r e a c t i o n s o c cu r an d o b t a i n i n g q u a n t i t a t i v e i n f o r m a t i o n . The m etho ds

a r e n ot p a r t i c u l a r l y s u i t a b l e f o r i d e n t i f i c a t i o n s t ud ie s, a lt ho ug h t h e oc cu rr en ce

o f one o r two peaks on a

DTG

cu rve , and th e r e l a t i v e s i z e s o f t h e two p ea ks when

t h e y a pp ea r, c an a p p a r e n tl y b e em ploy ed i n c h a r a c t e r i z i n g s e r p e n t i n e m i n e r a l s

(Morand i and F e l i c e , 1979) and the d isappearance o f th e hyg rosc op ic mo is tu r e peak

a f t e r K - s a t u ra t i o n c an be used t o d i s t i n g u i s h h y d r o b i o t i t e f r om m o n t m o r i l l o n i t e

i n some s o i l s (Ryzhova, 1980).

and th e m ain u se o f t h e t e ch n iq u e s has u n d o u b te d ly b ee n t o o b t a i n q u a n t i t a t i ve

i n f o r m a t i o n on e v o lv e d v o l a t i l e s , e t c . I n suc h a p p l i c a t i o n s , h ow ever, g r e a t c a r e

must be exe rc is ed, as th e mass change, du r in g, f o r example , a de hy dr ox yl a t io n

r e a c t i o n , c o u l d be s e r i o u s l y a f f e c t e d

if

e r r ou s i r o n i n t h e l a t t i c e were

s im u lt an e ou s ly o x i d i z e d t o f e r r i c . F o r t h i s r ea so n

t oo ,

q u a n t i t a t i v e d e t e r m i n a -

t i o n o f m i n e ra l s by

DTG

(Sma l ley and X idak is , 1979) sh ou ld be under taken on l y

when s u f f i c i e n t c o n fi rm a t o ry e vid enc e t h a t n o th i ng l i k e l y t o i n t e r f e r e w i t h t h e

DTG peak area i s pr es en t and when comparison can be made w i t h a mi ne ra l

t h a t i s

i d e n t ic a l w i t h t h a t i n th e c la y .

It

s n o te w o rt hy i n t h i s r e s p ec t t h a t even t h e

s a t u r a t i n g c a t i o n o f m o n t m o r i l l o n i t e a f f e c t s t h e c h a r a c t e r and t em pe ra tu re o f

th e DTG de hy dr ox yl at io n peak (Schomburg and S tb rr , 1978a).

v a l ua b l e i n e l u c i d a t i n g t h e n a t u re of DTA peaks f o r p a l y g o r s k i t e and s e p i o l i t e

(F erna n de z A l va re z , 1 97 8) and f o r m o n t m o r i l l o n i t e ( I l i u t a , D rim us an d P re da , 1 9 78 )

a nd OTG i n re ve a l i n g m u l t i p l e re a c t i o n s n o t o b v io u s o n th e TG cu rve (e.g. M i fsu d ,

Rau tureau and Forn es, 1978). Changes

i n t h e t e m p e ra tu re ra n g e an d m a g ni tu d e o f

the s te p on the TG, o r peak on th e DTG, cu rv e can p rov ide va lua b le co n f i rm a to ry

ev idence fo r a p a r t i c u l a r phenomenon, such as th e occu r re nce o f NH,+ i n some

When s imul taneous equ ipment i s n o t used and a compar ison i s made

T he se m us t b e re g a rd e d as r a t h e r i s o l a t e d i n s ta n ce s

TG has pro ve d


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1 1

Japanese d ioc tah edr a l micas (H igash i , 1978) : the presence o f th i s i o n no t on ly

moves t h e d e h yd ro xy l a t i o n re a c t i o n t o a l o we r t e mpe ra tu re b u t a l s o i n c re a se s t h e

mass l o s s b ecause o f t h e e vo l u t i o n o f NH,.

TG and DTG a ls o y i e l d us efu l in fo rm at io n on accessory mine ra ls. Thus, th e

d e h yd ro xy l a t io n o r d e ca rb o n a ti o n o f a cce sso ry h y d ro x i d e o r ca rbo n a te m i n e ra l s i s

u s u a l ly obv ious on TG and DTG curv es (e.g. Iwasa, 1978) and, pro vid ed c a re fu l

c a l i b ra t i o n i s pe r fo rmed i n advance , even sa l t s , such as sodium carbona te and

sod iu m ch l o r i d e ,

can be q u a n t i t a t i v e l y d eterm in ed i n sa l i n e c l a ys (Asomoza

et ai.,

1978).

TG has been e x te n s i v e l y i n ve s t i g a te d as a means f o r s t u d y i n g t h e k i n e t i c s o f

re ac t io ns because (a ) de te rm ina t ions a re le ss t ime-consuming than i so the rma l

i n v e s t i g a t i o n s , ( b ) w i t h i so th erma l methods , some re a c t i o n o ccu rs b e fo re t h e

temp e ra tu re o f i n t e re s t i s r e ach ed an d ( c ) t h e who le t e mp e ra tu re ra n ge

i s

covered

w i t ho ut any mis sing reg ion s (see Sharp, 1972) . Whi le these comments are c or re ct ,

a c t u a l l i m i t a t i o n s on t he d e r i v a t i o n o f k i n e t i c pa ra me te rs a r e f or m id a bl e, n o t

o n l y b ecau se o f t h e o ccu r re n ce of t e mp e ra tu re g ra d i e n t s w i t h i n t h e sample b u t

a l so because o f o t he r more fundamenta l asp ects d e a l t w i t h be low. Most methods

f o r i n t e r p r e t i n g

TG

cu rve s a re b ased on t h e s i mp l e r a te e q u a t i o n

d a / d t k ( 1

-

a)

(where

a

i s t h e f r a c t i o n decomposed i n t i m e t

n

t he o r d e r o f r e a c t i o n and k t h e

ra te co n s ta n t ) combined w i t h t h e A r rh e n i u s e q u a ti o n

k = Aexp (-E/RT) ( 2 )

(where

A

i s t h e p r e- e xp o ne n ti al f a c t o r , E t h e a c t i v a t i o n e ne rg y and R the gas

constan t ) , th e tempera tu re T b e i n g d e f i n e d b y

(where T~ i s t he i n i t i a l t em pe ra tu re and 8

= d T / d t

t he h e a t in g r a t e ) : n ot e t h a t

t h e h e a t i n g r a t e i s assumed t o be c o n s t a i t .

e i t h e r t o f o l l o w t h e d i f f e r e n t i a t i o n m ethod o f Freeman and C a r r o l l ( 19 58 ) o r t h e

i n te g ra t i o n meth od o f Co ats a nd Re dfern (19 63 ). The l a t t e r i s g e n e ra l l y r e ga rd e d

as y i e l d i n g m ost r e l i a b l e r e s u l t s - see, fo r example, th e rece nt study o f Boy and

BMhme (19 79 ) and compare t h e i r r e s u l t s f o r t h e d e h y d ro xy l a t i o n o f k a o l i n i t e w i t h

those determined by a v a r ie t y o f exper im enta l methods (Sharp, 1972) .

inh er en t i n de a l i ng wi t h a dynamic system (see Sharp, 1972) are imnense, o t he r

more fundamental aspects must also be remembered.

The two usual procedures are then

I n a l l t h es e s t u d i es a p p ro x im a ti on s a r e i n v o lv e d and, a lt h ou g h th e d i f f i c u l t i e s

One need only consider the


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1 2

equat ions above.

they wou ld app ly t o a so l i d : moreover,

o n e c o u l d j u s t i f i a b l y q u e r y t h e p h y s i c a l

s i g n i f i c a n c e o f t h e d e r i v e d o r d er o f r e a c t i o n and a c t i v a t i o n e ne rg y when

so

app l ied .

n o t unexp ecte dly, change mechanism du ri ng th e re a c t i o n (Garn, Kawalec and Chang,

1978; Pr ice

e t

al., 1980). Moreover,

many

so l i d - s t a te d e co mpo s it io ns i n powder

system s a r e d i f f u s i o n c o n t r o l l e d ( s e n s u l a t o c o v e r i n g b o t h i n t e r - and

i n t r a -

p a r t i c l e d i f f u s i o n ) , o r f o l l o w some o t h e r l aw,

so

t h a t t h e o rd e r o f r e a c t i o n i s

ra th e r mean ing less (Sharp , 1972) . Consequent ly , Garn (1979) has q u i t e r i g h t l y

suggested t h a t th e te rm ac t i va t i on energy shou ld be rep laced by tempera tu re

c o e f f i c i e n t o f r e a c t io n . W h il e t h e use o f sm a ll

samples and/o r o f a co nst an t

tempera tu re regime over the decompos i t i on in te r v a l (Rouquerol , 1970; Pa u l i k and

P a u l ik , 1972) wo uld r ed uc e t e m pe r at u re g r a d i e n ts t h a t i n t e r f e r e w i t h i n t e r -

p r e t a t io n , these va r ia n t s do no t ob v i a t e th e more fundamenta l ob je c t io ns and much

s tu dy i s s t i l l r e q u i r e d d e s p i t e t h e l a r g e numbers o f p ape rs t o b e f ou nd i n t h e

Journal of Thermal Analysis, Thermochimica Ac t a and Thermal A n a l y s i s A b s t r a c t s

over the past decade.

It

shou ld be noted , however, th a t the bas ic ob je c t io ns

ra i se d a bo ve do n o t n e ce ss a r i l y mean t h a t t h e n ume r i cal va i u e s o b ta i n e d f o r c e r t a i n

kinet ic parameters have no p r a c t i c a l value.

The f i r s t two a r e gas-phase equa t ions and the re i s no guaran tee

Indeed , recen t

EGA

stud ies have shown tha t decompos i t i on react ions can ,

1.3.2 .1 Evolv-d

gas d e t e c t i o n

(EGD)

I n

EGD

one d e te rm i ne s wh ethe r o r n o t gas e v o l u t i o n i s a sso c i a te d w i t h a t h erma l

e f f e c t .

b e i ng ad eq ua te i n d i c a t i o n o f gas e v o l u t i o n i n TG) and t h e s i m p l e s t method i s t o

i n s e r t a th er ma l c o n d u c t i v i t y c e l l i n t he c a r r i e r gas

st ream coming f rom the

equipment ( Ingraham, 1967) , a l tho ugh seve ra l o th er methods ar e a l so a v a i l a b le

(Daniels, 1973).

An

EGD

t e ch n iq u e t h a t h as p ro ve d ve ry u se fu l i n s t u d y i n g p o lyme r d e g ra d a t io n

on h e a t in g i s t he rm a l v o l a t i l i z a t i o n a n a l y s i s ( M c N ei ll , 1 97 7). I n t h i s t h e sam ple

i s heated i n a h ig h vacuum chamber connected t o a vacuum pump though a t r a p co oled

i n l i q u i d n i t r o g e n , t h e p re ssu re be twee n t h e sample a nd t h e t r a p b e i n g measure d

by a P i r a n i gauge. Whenever t h e sample decomposes, th e pr es su re inc rea se s and

the decompos i t i on o f the polymer can thus be fo l l owed . The method i n t h i s fo rm

does n o t g i v e a ny i n d i c a t i o n o f t h e v o l a t i l e p r o d uc t s fo rm ed b u t t h e eq uip me nt

has been modi f ied so t h a t t h e condensed v o l a t i l e s i n t h e c o l d t r a p b o i l o f f as

t h e t r a p i s a l lo w e d t o h e a t up.

t o b e i d e n t i f i e d (McNe i l l , 1 98 0), t h e method i s up gra de d t o EGA

-

always assuming

t h e r e i s no i n t e r a c t i o n betw een condensed v o l a t i l e s .

A

method f o r s imulta neous DTA and EGD by m ea su rin g t h e i n t e n s i t y o f a l a s e r

It i s cus to mary, t h e re fo re , t o use i t i n c o n ju n c ti o n w i t h DTA (mass los s

S i n c e t h e b o i l i n g p o i n t p e r m i t s

each component

beam t r av er s i ng th e sample c e l l above the samp le has re c en t l y been descr ibed i n

Net s u s o k u t e i

(1980).

A l t h o u g h a p p l i e d t o d e r i ve t h e p h a se d i a g ra m o f a known


9/25

1 3

b i n a r y l i q u i d sys tem , t h e method i s c l e a r l y n o n - s p e c i f i c and c o u l d p r o ba b ly be

a d ap te d t o DTA-EGD o f c l a y s o r c l a y -o rg a n i c comple xe s: i t s a d va ntag e l i e s i n t h e

f a c t t h a t i t g iv es a ve ry sharp i n f l e c t i o n immed ia te l y vapour ev o l u t io n commences

e ve n w i t h ve ry s l o w h e a t i n g ra te s .

1 .3 .2 .2 Evolved

g a s

a n a l y s i s

(EGA)

EGA i s much more us ef u l , and indeed usual , than

EGD,

s i n c e i t enables

d e t e r m in a t io n o f t h e i d e n t i t y an d/ or amount o f t h e e v o lv ed v o l a t i l e m a t e r i a l . It

i s norm al ly employed a long w i t h DTA, TG o r DTA-TG-DTG, thu s en ab l i ng q u a n t i t a t i v e

e v a l u a t i o n q f t he e f f e c t s on t h e c ur ve s i n term s o f s p e c i f i c v o l a t i l e s .

Ba s i c i n f o r ma t i o n o n va r i o u s me thods o f EGA, such as mass spectrometry, gas

chroma to grap hy , i n f r a - re d a b s o rp t i o n and se l e c t i v e so rp t i o n , w i l l be fo un d i n t h e

books o f Lodding (1967), Mackenzie (1970-72), Da nie ls (1973) and Wendlandt (1974),

i n t h e P ro cee d in g s o f t h e v a r i o u s I n te rn a t i o n a l Co n fe re nces o n T hermal An a l ys i s

(R ed fer n, 1965; Schwenker and Garn, 1968; Wiedemann, 1972,

1980; B U Z ~ S ,

1975;

Chihara, 1977) , i n the Proceedings o f the F i r s t European Symposium on Thermal

A n a l y s is ( D ol li m or e , 1 97 6) and i n v a ri ou s s c i e n t i f i c j o u r n a l s .

B r i e f l y ,

the methods of

EGA

f a l l i n t o tw o c la s se s : ( a ) t ho s e f o r w hic h

p r i o r k now ledge of t h e n a t u r e o f t h e v o l a t i l i z e d m a t e r i a l i s u nn ec es sa ry and

(b ) those fo r wh ich such knowledge i s ess en t ia l . Wh i le the fo rmer , wh ich

in c lu de mass spect ro metry and gas chromatography, ar e by fa r the most ge ne ra l ly

u se ful , t h e l a t t e r have a d e f i n i t e p l a ce i n s t u d i e s , such as t ho se o n c l a ys , whe re

a l i m i t e d number of v o l a t i l e ma te r i a l s a re t o b e e xp ec te d.

ap p l ie d i n c la y s tud ies : f o r examp le , Mhlle r-Vonmoos and M h l le r (1975) have

demonstrated how mass spe ctr om etr y combined w i t h DTA can revea l the presence o f

organ ic carbon,

p y r i t e and va r ious ca rbona te minera ls i n a c lay , whereas Morgan

(1977) has

used DTA and se pa rat e d et ec to rs t o de ter mi ne when and how much water,

c ar bo n d i o x i d e a nd- s u l ph u r d i o x i d e ( f ro m o x i d a t i o n o f p y r i t e ) a r e e v o lv e d fr o m

clays, sha les and sc h i s t s and Pa u l i k and Pa u l i k (1978) have used t h e i r techn ique

o f th erm al gas t i t r i m e t r y ( i. e. s o r p t i o n o f t h e v o l a t i l e i n a s u i t a b l e s o l u t io n

o r s o lv en t fo l l o we d by t i t r a t i o n ) a long w i th DTA-TG-DTG t o de te rmine the amounts

o f c on ta m in at in g a l u n i t e and c a l c i t e i n b a ux it es .

The opt imum technique depends on ci rcumstances

-

and, no t i n f requen t l y ,on

f inance.

and r e l a t i v e l y c he a pl y p ur ch as ed and a t ta c h e d i n s e r i e s t o a s u i t a b l e t he rm a l

a n a l y s i s i n s t r u me n t , t h e more f l e x i b l e a nd u n i ve rs a l sys te m u s in g mass sp e c t ro -

me t r y i s e xp en si ve .

asp ects must be kept i n mind: fo r example, t h e free, volume around th e sample must

be r e l a t i v e l y sm a ll t o a v o i d undue d i l u t i o n o f e vo lv e d m a t e r i a l w i t h c a r r i e r gas,

th e i n t e r fa ce between a mass spect romete r and t he the rma l a na ly s i s i ns t rument

must be chosen wi th care

so

t h a t o ne v o l a t i l e i s n o t p r e f e r e n t i a l l y e nr ic he d a t

Both have indeed been

W h ile a s e r i e s o f d e t ec t or s s p e c i f i c f o r one v o l a t i l e o n l y c an be r e a d i l y

I n se t t i n g up eq ui pme nt a nd a sse ss in g re s u l t s se ve ra l i mp o r ta n t


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1 4

t h e expense o f a n ot h er , and th e l i k e l i h o o d o f two v o l a t i l e s r e a c t i n g b e f o r e

measurement must be assessed.

A

v a r i a n t o f

EGA

py ro l ys is- gas chromatography-mass s pec trom etry , wh ich was

d ev el op ed m a i n l y f o r t h e s t u d y o f s y n t h e t i c p o ly m er s, h as p ro v ed e x t r e m e l y u s e f u l

i n t h e i n v e s t i g a t i o n and c h a r a c t e r i z a t i o n o f s o i l o r ga n ic m a t t e r ( B ra ce w el l

and

Robertson, 1977)

and s h ou ld a l s o b e a p p l i c a b l e t o o r g a n i c m a t t e r i n c l a y d e p os i ts .

I n t h i s m ethod th e s am ple i s v e r y r a p i d l y p y r o ly s e d i n an i n e r t atm os phere, t h e

p roduc ts be ing sepa ra ted by gas ch romatog raphy and i d e n t i f i e d by mass spect rome t ry .

Such a p ro ce du re y i e l d s f a i r l y l a r g e fr ag m en ts o f t h e o r i g i n a l m o l e cu le s , t h u s

g i v i n g an i n s i g h t i n t o t h e n a t u r e o f t h e or g a ni c p oly me rs p r es e nt .

c ir c um s ta n ce s , and p a r t i c u l a r l y when i n t e r a c t i o n be tw ee n p r o d uc t s

i s

l i k e l y t o

o ccu r, a p r e f e r a b le system i s p y ro l ys i s -m a ss sp e c t ro m e t r y (B ra ce w e l l a nd R o b e rt son ,

1 9 8 0 ) , d e s p i t e

i t s

h i g h e r c o s t b ec au se o f t h e m ore e l a b o r a t e d a t a h a n d l i n g s y s te m

re q u i re d .

I n some

1.3.2.2.1

E m a n a t i o n t h e r m a l a n a l y s i s T h i s i s e s s e n t i a l l y a v a r i a n t

o f

E G A where

t h e r a d i o a c t i v e e ma na tio n e v ol v ed d u r i n g h e a t i n g o f the sample i s measured. While

t h i s d e f i n i t i o n w ou ld n o r m a l l y i n c l u d e o n l y r ad on i so t op e s, i n p r a c t i c e t h e method

has been ex tended t o non - rad ioac t i ve i n e r t gases,

such as argon, kr yp to n o r xenon,

and t h e i r r a d i o a c t i v e i s o to p e s : t h e r e i s th u s a g r a d a t i o n i n t o n or ma l

EGA.

M a t e r i a l s

n o t c o n t a in i n g i n e r t gas c an be l a b e l l e d b y d i f f u s i n g gas i n t o t h e s o l i d a t h i g h

p ressu res and tempera tu res , by inc lu d i ng th e gas du r ing s yn th es is o r by bombard -

i n g t h e s u r f a c e o f t h e s am ple w i t h a c c e l e r a t e d i o n s o f i n e r t gas. Changes i n gas

r e l e a s e r a t e o n h e a t i n g c an t h e n be c o r r e l a t e d w i t h d e h y d r at i on , d e co m p os it io n ,

r e c r y s t a l l i z a t i o n ,

phase t r a n s i t i o n , s o l i d - s t a t e r e a c t i o n s and changes i n s u r f a c e

p r o p e r t i e s . D ev elo pm en t o f em a na ti on th e r m a l a n a l y s i s i n c o n j u n c t i o n w i t h DTA i s

due m a i n ly t o B a le k and h i s c o l l a b o r a t o r s i n C z ec h os lo v ak ia a nd i n f o r m a t i o n o n

t h e or y , i n s t r u m e n t a t i o n and a p p l i c a t i o n s

i s b e s t o b t a in e d f r o m a r e c e n t e x h au s t iv e

rev iew (Ba lek, 1977). A l th oug h no se r i ou s st ud ie s seem y e t to have been made on

c l a y s , a c ce s so ry m i n e r a ls s u ch as i r o n o x i d es , q u a r t z a nd z e o l i t e s hav e r e c e i v e d

a t t e n t i o n . S ev er al p o s s i b l e a p p l i c a t i o n s t o c l a y m i ne ra ls , p a r t i c u l a r l y h o l lo w

f i b r o u s and h i g h l y d i s o r de r e d s p ec i es , s p r i n g t o mind, b u t

i t

h a s y e t t o b e

e s t a b l i s h e d w he th er t h e i n f o r m a t i o n o b t ai n e d w ou ld b e s u p e r i o r t o t h a t f r o m more

convent iona l methods.

So- ca l le d tempera tu re -p rogrammed de so rp t io n cu rves m igh t we1 1 be rega rded

a s r e l a t e d t o t h e abov e, e ve n a l t h o u g h t h e ga ses i n v o l v e d a r e n o t i n e r t .

w ork w i t h t h e s e ( C r i a d o

et

ai., 1980)

has c l e a r l y shown t h e i r v a l ue i n s t u d y i n g

t h e k i n e t i c s

o f

d e s o rp t i o n o f so rb e d g as.

Recent

. _

1.3.2.2.2

T h e m o p a r t i c u l a t e a n a l y s i s

S i n c e t h e d e g r a d a t i o n

o f

s y n t h e t i c p o l y m e r s

y i e l d s c on d e n s a ti o n n u c l e i as w e l l as m o l e c u l a r sp ec ie s , t h e r m o p a r t i c u l a t e a n a l y s i s ,


11/25

1 5

whereby th e amoun t (and sometimes i d e n t i t y ) o f such n uc le i a re measured as a

f u n c t i o n o f tem pe ra ture , i s c l e a r l y c l o s e l y r e l a t e d t o EGA. P a r t i c le s o f t he

o rd e r o f 1 -1 00 nm i n s i z e a re i n vo l ve d a nd an e a r l y re v ie w o f eq uip me nt , t e ch n iq u e

and ap p l i c a t io ns was g iv en by Murphy (1967) . A more r e c e n t v a r i a n t ( g i v e n v a r io u s

names by i t s o r i g i n a t o r s ) i s a c l o se r e l a t i v e o f

EGD,

s in ce n e i t h e r t h e a m o u n t

n or i d e n t i t y o f t h e p a r t i c l e s i s m easu red, b u t when c om bin ed w i t h mass s p e c t r o m e tr y

b e co m e s a g a in e sse n t i a l l y EGA (see Smith , P h i l l i p s and Kaczmarek, 1976; Sm ith ,

M e ie r an d P h i l l i p s , 1 97 7) .

a r e p o s s i b i l j t i e s o f a p p l i c a t i o n i n c o n d i t i o n s where c he mic al t r a n s p o r t o cc urs .

S in c e c h em i ca l t r a n s p o r t ha s r e c e n t l y been ob se rv ed o n h e a t i n g c e r t a i n

i r o n o x id es

(E. Pate rson, pe rson a l commun ica tion ) ,

i t

s , t h e re f o re , m a r g i n a l l y p o s s i b l e t h a t

a p p l ic a t i o ns e x i s t i n t he c l a y f i e l d .

A l t ho u g h t h e t e c h n iq u e h as been a p p l i e d o n l y t o l a r g e o r g a n i c m o le c ul es t h e r e

1.3.3 H e a t i n g cu rve d e te rm in a t i o n

Hea t ing cu rves and

t h e tw o d e r i v a t i v e s , h e a t i n g - r a t e c u r v e s and i n v e r s e h e a t -

i n g - r a t e c ur ve s, w ere a t one t im e w i d e l y used i n c l a y s t u d i e s : i nd ee d, t h e f i r s t

t h e rm o a n a l y t i ca l r e co rd s f o r c l a ys w ere , as m e n t io n e d above, h e a t i n g - ra t e cu rves .

However, h e a t i n g c u rv e s f e l l i n t o d is u s e, as t h e lo w s e n s i t i v i t y o f r e c o r d i n g

n ec es sa ry t o c o v e r t h e w h ol e t e m p er a t ur e ra n ge o f i n t e r e s t p r e c l u d e d d e t e c t i o n

o f s m a l l t h e rm a l e f f e c t s , and i n t e r e s t i n t h e t wo d e r i v a t i v e c u r v e s waned as DTA

became e s ta b l i sh e d , s i n c e e ss e n t i a l l y t h e same i n fo rm a t i o n co u ld be o b ta in e d m ore

r e a d i l y .

So

f a r as t h e a u th o r i s a wa re , no h e a t i n g -cu rv e d e te rm in a t i o n s o n c l a ys

are now performed.

1.3.4

D i f f e r e n t i a l th er ma l a n a l y s i s (DTA)

i n c l a y s tu d i e s , w h et he r on

i t s own o r s im u l ta n e o u s l y w i t h o th e r m e th od s su ch as

TG, EGA, e t c .

c e r t a i n a cc es so ry m i n e r a l s and a bn orm al s p ec i es o f c l a y m i n e ra l s , i n d e t e c t i n g

changes i n m i ne r al og y w i t h d e p th o r d i s ta n c e , i n q u a n t i t a t i v e ( o r , more f r e q u e n t l y ,

s e m i - q u a nt i ta t i v e ) s t u d ie s and i n de te r m in in g h e a t s t a b i l i t y o r t h e oc c ur re nc e o f

s o l i d - s t a t e r e a c t i o n s . L i m i t a t i o n s im pos ed by i n s t r u m e n t a t i o n , t e c hn i q ue and

n a t u r a l v a r i a t i o n s i n m i n e r a ls make a c c ur a te q u a n t i t a t i v e w ork d i f f i c u l t and

p re c lu d e i t s u se d i a g n o s t i c a l l y , e xc e pt i n s pe c i a l c ir cu m st an c es . A s t h e s e d e t a i l s

and p o s s i b l e m ethods o f m i n i m i z i n g l i m i t a t i o n s a r e

so

wel1,known (se e Mackenzie,

1957b, 1970-72), t h e method w i l l n o t be d e a l t w i t h h e r e a t a ny g r e a t l e n g t h and

re fe re n ce

w i l l

be made on ly t o some r ec en t deve lopments.

a l tho ugh t he re may n o t be so many i ns t ru me nts on th e mark et now as a few years

ago, those companies t h a t c u r re n t l y manu fac ture equ ipmen t have in t rod uce d many

DTA i s b y f a r t h e b e s t known a nd m os t w id e l y u se d th e rm o a n a l y t i ca l t e ch n iq u e

I t s m ain uses a r e i n f i n g e r - p r i n t i n g specim ens, i n d e t e c t i n g

Commercia l equipment design has

improved enormously over the past decade and,


12/25

16

improvements, Wh i le no ins t r um en t i s un iv e r sa l and th e ins t rum en t must be chosen

h a v i ng r e g a r d t o t h e p ro po se d a p p l i c a t i o n s , t h e g e n e r a l

t r e n d t ow a rd s t h e u se o f

sm a ll sam ple s o f o n l y a fe w m i l l i g ra m s i s t o b e w elcom ed, as

i t

m i n i m i z e s e r r o r s

due t o t h e rm a l g ra d ie n ts a nd,

p r o v id e d r e c o r d i n g s e n s i t i v i t y i s a dequate, e na bl es

s im ul ta n e ou s DTA-TG t o be v a l i d l y u sed . The f a c t t h a t tw o m a rked l y d i f f e r e n t DTA

i n s tr u m e n ts y i e l d e s s e n t i a l l y t h e same c ur v es f o r a d e h y d r o x y l a t i o n r e a c t i o n when

sam ple s i z e i s l e s s t h a n 30 mg (Broersma

et ai.

1978)

c o n f ir m s t h e v a l u e o f u s i n g

ve ry sm a l l samp les. Many manu fac tu re rs a ls o p roduce sev e ra l t ypes o f spec imen

h o l d e r s f o r e a c h i n s t r u m e n t , so t h a t t h e o p t im u m (W i l b u rn ,

1972) can be se lec ted ,

and co n t ro l o f a tmosphere a round the Sample i s now un iv e rs a l. These advances i n

i n s t ru m e n ta t i o n e n a b le DTA t o b e much m ore w id e l y and v a l i d l y u se d i n c l a y s tu d ie s

than eve r be fo re .

An ad va nce i n m e th o do lo g y has b ee n th e i o t r o d u c t i o n o f s te p w ise h e a t i n g (S ta ub

and Perron, 1974; Simonsen and Zaharescu, 1979), by whic h tem pe rat ur e

i s

i n c r e a s e d

i n sm a l l s te p s (0.5-10°C, d e pe n ding o n th e re a c t i o n ) i n s te a d o f co n t i n u o u s l y ,

e q u i l i b r i u m b e i n g a t t a i n e d a t e ach s t e p.

o b t a i n i n g a s e r i e s o f i s o t h e r m a l m eas urem ents d u r i n g o ne d e t e r m i n a t i o n w i t h t h e

r e s u l t t h a t h ea ts o f m e l t i n g , t r a n s i t i o n , e tc ., can be d et er min ed more a c c u r a t e l y .

To t h e a u t h o r ' s kno wledge, t h i s t e c h n iq u e h as n o t y e t b een a p p l i e d t o c l a y s : t h e r e

i s no r e a so n

t o

suppose

i t

s i n a p p li c a b le ,

a l though any apparen t advan tages wou ld

have t o be c r i t i c a l l y checked.

a b l e e f f e c t ( M a r t i n V i v a l d i , G i r e l a V i l c h e z and F e n o l l H a c h- A li ,

1964 ; Do l l imore

and Mason, 1 98 1) : t o p r e v e n t c o n t a i n e r e f f e c t s i n m e t a l l u r g y , e l e c t r o m a g n e t ic

l e v i t a t i o n o f t h e sa mp le has been i n t ro d u ce d ( Jo rd a , F lU k ig e r an d M U l l e r , 1 9 78 )

b u t t h i s w ou ld u n f o r t u n a t e l y b e i m p o s si b le w i t h c l a y s a nd t h e m a t e r i a l o f t h e

s pec im en h o l d e r mu st be c a r e f u l l y s e l e c t e d . The n e c e s s i t y f o r c a r e i n s am ple

pr ep ar at io n has been emphasized by work on q ua rt z (Moore and Rose, 1979), where

smal l

e x ot h er m ic p eaks o bs er ve d a f t e r g r i n d i n g i n s t e e l and ag a t e v i b r a t i o n

m i l l s

have been a t t r i b u t e d t o o x i d a t i o n o f c o n ta m i n at i ng i r o n and r e l e a s e o f s t o r e d

e ne rg y, r e sp e c t i v e l y . On th e o th e r hand, t h e d o u b l i n g o f t h e d e h yd ro x y la t i o n

endotherm o f g o e t h i t e on g r i n d i n g has been a t t r i b u t e d t o p a r t i c l e s i z e e f f e c t s

(Murad, 1979) . The va lu e o f c o n t r o l l e d atmosphere has been con f i rme d by s t ud ies

o n ca rb o n a te m in e ra l s , w h i ch show th a t i n a ca rb o n d i o x id e atm osp he re t h e m i n e r a l s

p res en t can be i d e n t i f i e d and the amounts es t ima ted f o r on ly 0 .125 mg i n a 50 mg

sample (Warne and

M i

c h e l 1 , 1979).

a d j u n c t t o o t h e r te ch niq ue s i n c h a r a c t e r i z a t i o n s t u d ie s .

r e v e a l t h e v a lu e o f t he t e ch n iq u e i n i t s own r i g h t , and

i t

may be appos i te to

c i t e some examples.

Thus, peak tempe ra tu re o f the

low- tem pera tu re endothe rm f o r

H -sa tu ra te d a l l o p h a n e a nd th a t o f t h e h i g h - te m p e ra tu re e xothe rm fo r N a -a ll o ph a n e

i n c re a s e s and d ec re as es , r e s p e c t i v e l y , w i t h i n c r e a s i n g SiO,:Al,O, r a t i o (Henm i,

T h is i s e s s e n t i a l l y e q u i v a l e nt t o

The sample conta iner can somet imes have an undesi r -

T he m a in u se o f DTA i n c l a y s tu d ie s , r e ce n t l y a s i n t h e p a s t , h as b ee n as a n

Y e t so m e i n ve s t i g a t i o n s


13/25

1 7

1 98 0). M oreo ve r, t h e e xo th e rm o f a l l o p h a n e i s l o w ere d i n t e m p e ra tu re and

broadened on admix tu re w i th

i r o n o x i d e g e l s o r h yd ro us i r o n o x i d e m i n e ra l s ,

whereas i t i s u n a f fe c te d b y o x id e m in e ra l s such as h e m a t i t e o r m a gh em ite (S u zu k i

and S ato h, 1 98 0) . F o r d i c k i t e s , i n c re a s in g b re a d th o f t h e d e h yd ro x y la t i o n

endotherm seems

t o

b e a p p r o x i m a t e l y a s s o c i a t e d w i t h

i n c re a s in g d e gre e o f d i s o rd e r ,

a lt h ou g h some e xc e p ti on s s ug ge st t h i s r u l e i s n o t r i g i d ( B r i n d l e y and P o r t e r ,

1978).

t o i n t e r p r e t - presumab ly because dehydroxy la t ion o f o n e l a y e r n e c e s s a r i l y a f f e c t s

t h a t o f n e i g h b o u r in g l a y e r s w h et he r o r n o t t h e y a r e o f t h e same t y p e - b u t

i t

has

re ce n t l y b ee n shown th a t t h e sh ap e a nd s i z e of t h e h yg ro sco p i c m o i s tu re pe ak fo r

S r - sa tu ra te d m ica -m o n tm o r i l l o n i t e s ca n b e u se d t o a sse ss th e p ro p o r t i o n o f expa ns -

i b l e la ye rs p resen t ( Inoue , M ina to and U tada , 1978) .

t h a t o ccu r b e tw e e n ca rb o n a te m in e ra l s , s o l u b l e s a l t s a nd m ic a on h e a t i n g a l s o t a k e

p l a ce w i t h m i c a - m o n t m o r i l l o n i t e b u t n o t w i t h p a l y g o r s k i t e (Mashhady et al., 1980)

and manganese o x id e s r e a c t w i t h k a o l i n i t e i n t h e s o l i d s t a t e a t e l e v a t e d t em p er at ur es

( H o l l a n d and S e g n it , 1 97 6), c a r e m us t be t a ke n i n i n t e r p r e t i n g c u r v e s f o r sy stem s

co n ta in i n g su ch m ix tu re s . I n p o l l u t i o n s tu d i es ,

t h e a r e a o f t h e d e h y d r o x yl a ti o n

e nd othe rm o f c h y so t i l e can a p p a re n t l y b e u se d to a sse ss a sb es tos co n te n t (Me nis ,

Mackey and Garn, 197 8).

f o r c l a y m in e r a ls have been examined by, i n t e r alia, Hl lbner (1927) and E l tantawy

(19 7 9) , w h i l e c l a y -o r g a n i c re a c t i o n p ro d u c ts h ave b ee n i n ve s t i g a t e d b y Kuroda an d

Kato (1979).

A l t h o u g h DTA has bee n w i d e l y u sed i n s t u d y i n g t h e k i n e t i c s o f r e a c t i o n s , t h e r e

i s no do u bt t h a t t h e t e ch n iq u es u sed a r e f r a u g h t w i t h e ven m ore d i f f i c u l t i e s t h a n

mentioned above f o r TG (Sharp, 1972).

t h e r m o a n a l y t ic a l p ap er s do d e a l w i t h t h i s s u b j e c t and a t r e a t i s e on n o n -e q u i l ib r i u m

k i n e t i c s has appeared (Koch , 1977) .

t h a t d ec o m p o s it i on o f s o l i d s f o l l o w s t h e same k i n e t i c s t hr o u g ho u t t h e w h ol e ra ng e

from

a

=

0

t o

a

= 100 (where a i s t h e f r a c t i o n r ea c te d ) a nd th us t h e b e s t t h a t

ca n b e e xp ec ted w ou ld b e to d ed uce va lu e s f o r a l i m i t e d a range.

It i s w e l l k no wn th at DTA cu rv es o f i n t e r s t r a t i f i e d m i n er a ls a r e d i f f i c u l t

S in ce s o l i d - s t a t e r e a c t io n s

T he e f f e c t s o f so rbe d o rg a n i c m a t e r i a l s on DTA cu rve s

H ow ever, a co n s id e ra b le p r o p o r t i o n o f

As mentioned above under TG, i t i s u n l i k e l y

1.3.5

D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y ( D S C )

Two type s o f DSC a re recogn ized, power-compensat ion DSC a n d h e a t - f l u x

DSC

( T a b l e

1.1 .

The re la t ionsh ips be tween these and DTA have recen t ly been d iscussed

i n d e t a i l (Mackenz ie , 1980) and need no t be repea ted he re.

m e n t i o n e d t h a t

DSC

was o r i g i n a l l y l im i t e d t o a maximum te m p e ra tu re o f a b o u t 450°C

because o f t h e i n c r e a s i n g i m po r ta n ce above t h a t t e m pe r at u re o f r a d i a t i v e h e a t

t r an s f e r : however , re ce n t tech no log ica l advances have enab led th e tempera tu re

range t o be extended t o ab out 750-800°C so t h a t a l i m i t e d a p p l i c a t i o n i n c l a y

m in e ra lo g y

i s

now possib le.

It

need only be

The main advantage o f

DSC

o ve r DTA i s t h a t i t i s i n h e r e n t l y q u a n t i t a t i v e f o r


14/25

1 8

cha ng e i n e n th a lp y , h e a t ca p a c i t y , e t c . C o nse qu e nt ly ,

i t

can, f o r example, be

used t o d e t er m in e n o t o n l y t h e e n t ha l p y c hange o c c u r r i n g d u r i n g a r e a c t

a l s o t h e t o t a l e ne rg y r e q u i r e d t o f i r e a c l a y o v e r any s p e c i f i c tem pera tur,

up t o abou t 750°C. The method has, however, n o t been w id e l y used i n th e study 0 ,

c lay s , a l tho ugh in ve s t i ga t i on s on m on tm or i l l o n i te (Homshaw and Chauss idon , 1979) ,

on sm e c t i t e s , z e o l i t e s , h a l l o y s i t e a nd o p a l (Eg er , C ruz -Cu m p li do and F r i p i a t , 1 97 9)

a nd o n sy n t h e t i c g o e t h i t e (P a terso n, 1 98 0)

suggest

i t

can y i e l d v a lu a b le r e s u l t s ,

b o th q u a l i t a t i v e and q u a n t i t a t i v e ,

i n c o n ne c t io n w i t h s o l v a t i o n p ro ble ms . I t has

a l s o been u s e f u l i n d e t e c t i n g d i f f e r e n t t y p es o f s u r f ac e h y d r o x y l g ro up s on s y n t h e t i c

g o e t h i t e ( P at e rs o n and S w a f f i e l d , 1980) and i n r e v e a l i n g m aj or d i f f e r e n c e s i n s o i l

and ro ck q u ar tz samples (Barwood and Hajek, 1979).

I n vie w o f t he s e i n d i c a t i o n s , it wou ld appear tha t

DSC

i s l i k e l y t o be i n cre as -

i n g l y used i n c l a y s t ud i es o v er t h e n e x t few ye ar s, p a r t i c u l a r l y i n q u a n t i t a t i v e

and s u r f ac e i n v e s t i g a t i o n s . An e x t e n s i o n o f t h e c u r r e n t up pe r t e m p e ra t u re l i m i t

w ou ld open ev en g r e a t e r p r o s p ec t s , b u t t h i s do es n o t seem l i k e l y i n t h e n e ar f u t u r e .

1.3 .6

T h e rm o d i l a to m e t r y

I n t h e r m o d i la t o m e t r y t h e v olu me or , m ore u s u a l l y , t h e l i n e a r d im e n s i on s a l

c hange o n h e a t i n g i s s t u d i e d as a f u n c t i o n o f t e m p e r a tu r e un de r n e g l i g i b l e lo a d.

The method i s a n o l d one and i n d ee d t h e o b s e r v a t i o n o f s h r i n k a g e o f c h i n a c l a y

on f i r i n g gave Wedgwood (1782) th e idea f o r h is famous pyrometer , wh ich, a l tho ugh

g r o s s l y i n a c c u r a t e i n a b s o l u t e t er ms , was t h e o n l y means a v a i l a b l e f o r a c c u r a t e l y

comparing h ig h te m p e ratu re s f o r 50 ye a rs o r m ore fr o m t h e 1 78 0s .

knowledge o f t h e f i r i n g s h ri nk a g e o f c l a y i s e s s e n t i a l and where b l e n d i n g ca n be

used to m in im ize such sh r inkage .

It

has a l s o b een a p p l i e d t o c l a y m in e r al s , b u t

t h e d e t e r m i n a t i o n

i s so

s e n s i t i v e

t o

degree o f o r i e n t a t i o n o f p l a t y m in er al s, t h e

n a tu re and am ounts o f a cce sso r i e s , e tc . , t h a t t h e re s u l t s ha ve n o t b ee n co n s id e re d

o f g r e a t v a l ue f o r i d e n t i f i c a t i o n , T h er e hav e been r e c e n t i n d i c a t i o n s , how ever,

t h a t t h e method can ha ve some l i m i t e d d i a g n o s t i c u se s, s i n ce

i t

w o u ld a p p e a r t h a t

s m all amounts o f d i c k i t e o c c u r r i n g i n a k a o l i n c a n be i d e n t i f i e d by a n ex pa ns io n

e f f e c t a t a b ou t 650°C t h a t i s p a r t i c u l a r l y s t r o n g f o r d i c k i t e (Schomberg a nd

Schtlrr, 1978b).

t i o n on t h e mechanism o f i n i t i a t i o n o f t h e d e h y d ro x y la t io n and m u l l i t i z a t i o n

r e a c t i o n s o f k a o l i n i t e ( F la n k, 1979) and t h e r e i s no d o ub t t h a t , bec ause o f i t s

s e n s i t i v i t y t o o r i e n t a t io n e f f e c t s ,

i t

has p o t e n t i a l a p p l i c a t i o n i n a ss e ss in g t he

degree o f o r i e n t a t i o n o f p l a t y p a r t i c l e s i n c l a y s t r a t a .

T he rm o dil at om e te rs and d i f f e r e n t i a l t he r m od il at o m et e rs a r e r e a d i l y a v a i l a b l e

c o m m e r c i al ly an d d e r i v a t i v e t h e r m o d i la t o m e t r i c c u r v e s ca n b e e a s i l y r e c o r d ed f ro m

the rmod i la tomet r i c measuremen ts .

t he r m od il at o m et ry and d e r i v a t i v e t he r m od i l at om e tr y c a n y i e l d r e s u l t s t h a t h e l p t o

Therm od i la tome t ry i s a ve ry common method i n ce ramic techno log y, where a

T h e r m o d i l a t o m e t r y h a s a l s o r e c e n t l y b e e n u s e d t o o b t a i n i n f o r m a -

When perfo rmed si m ul ta ne ou sl y w i t h DTA-TG,


15/25

1 9

ex pl ai n fea tur es on DTA and TG curv es ( P a u l i k and Pa ul ik , 1978). Such simultaneous

measurements may, the r e fo re , be wor thy o f fu r t h e r s tudy by c l ay min era log is ts .

1.3 .7 Thermomechanical measurements

n o n - o s c i l l a t o r y ( i. e . s t a t i c ) o r w i t h an o s c i l l a t o r y ( i . e . dy na mic ) l oa d. The

fo rme r, wh ich a re c u r r e n t l y commonly r e f e r r e d t o as

thermomechanica l ana lysis

o r

TMA

( d e s p i t e t h e e r r o r o f u s in g a n a l y s i s i n suc h a c o n n ot a ti o n) c an hav e

var io us modes depending on whether the st re s s a pp l ie d to th e sample i s compression,

t en s io n , f l e x u r e o r t o r s i o n and o n w he th er d e f o rm a t io n o r p e n e t r a t i o n

i s

measured

(Daniels, 1973).

t h e d i f f e r e n c e b e in g i n th e u se o f a f i n i t e l oa d, w he th er p o s i t i v e o r n e g at iv e:

indeed, th e same equipment can be used f o r b o th measurements (s ee Flank , 1979)

and the re

i s

some argument as t o whether therm odi la t om etry should be separa te ly

recognized. Dynamic thermomechanica l measurements enable a d i f f e r e n t se t o f

parameters, such as the. shear modulus, th e mechanical damping inde x, th e lo ss

t a n g e n t ( t a n 6 , etc., t o be measured as a fu n c t i o n o f temperature .

b e n o te d t h a t i n d i v i d u a l i so th e rma l me a su re me n ts a re n o t i n c l u d e d .

Thermomechanical measurements, both s t a t i c and dynamic, f i n d ex te ns iv e use i n

p oly me r s ci en ce , b u t t h e p o t e n t i a l f o r t h e i r us e i n c l a y m i n er a lo g y appears s l i g h t -

a l though

i t

i s f a s c i n a t i n g t o s pe cu la te on t h e p o s s i b l e r e s u l t o f a p p ly i ng t h e

t o r s i o n a l pendulum ( s o - c a l l e d t o r s i o n a l b r a i d a n a l y s i s

-

see Danie ls, 1973) t o

clay samples.

a re , i n f a c t , a p p l i e d , o f t e n i so th e rm a l l y b u t sometimes, as when t e s t i n g h i g h -

temperatu re r e f ra c t o r ie s , us ing a co n t ro l e d tempera tu re programme. I n ce ramic

and re f r a c t o r y stud ies , measurements may be made o f de form at ion under loa d, shear

modu lus , mo du lu s o f e l a s t i c i t y , c ru s h i n g s t r e n g th , r e f r a c to r i n e ss u nd er lo a d ,

c re ep , s t r e s s re l a x a t i o n an d to ug hn ess a nd o f t h e i r va r i a t i o n w i t h t emp erature.

I n c i v i l e n g i n e e ri n g , l o a d b e a r i n g p ro p e r t i e s , p e n e t ra t i o n measurements , e tc .,

a re us ua l l y ca r r ie d o u t a t room tempera tu re bu t may have t o be made over a l i m i t e d

temperature range.

p r ov i nc e o f t h i s r ev ie w - o r in de ed o f t h e f i e l d o f e xp er ie nc e o f t he a ut ho r -

and w i l l n o t b e f u r t h e r co n s i d e re d .

i n ve s t i g a t i o n o f o i l sh a le s , whe re th ermo mecha ni ca l p ro p e r t i e s , such as d e fo rma t io n

on h e a t i n g , a n i so t ro p y o f comp re ssi ve s t r e n g th , e tc ., a re i mp o r ta n t i n d e te rm i n in g

t h e r e l e a s e

o f

th e o i l p roduct i n the r e t o r t (Rajeshwar, No t tenburg and DuBow,

1979).

A t

present , the re for e , thermomcchanica l measurements ar e o f use ma in ly

i n t e ch no lo gy and i n d u s t r y r a t h e r t ha n i n m in er al og y.

AS n o te d i n T a b le

1.1

thermomechanical measurements can be carr ied out

w i t h

a

C l e a r ly , t h i s t ec hn iq ue i s c l o s e l y r e l a t e d t o t h er m od il at om e tr y,

It shou ld

I n c i v i l e n g i n e e r in g a nd ce ra mic t e ch no l o gy many o f t h e t ech ni qu e s

D es pi t e t h e i r p r a c t i c a l

i mp or ta nc e, t he se a sp ec ts a r e n o t s t r i c t l y w i t h i n th e

A much more r e l e v a n t a p p l i c a t i o n i s i n t h e


16/25

2 0

1.3.8

Thermosonimetry and thermoacoust imetr,y

sound em it te d by a sample on he at in g, whereas i n ther mo aco ust im etry one measures

th e changes i n th e ch ar ac te r i s t i c s o f imposed sound waves caused by passage

through the sample.

As

w i t h t h e t w o t y p e s

o f

thermomechanical measurement, there-

f o re , t h e t y p e o f i n f o r m a t i o n g a in e d

i s

d i f f e r e n t .

Sound emission f rom, o r d e c re p i t a t i on o f , rock s was st ud ied by Smi th and Peach

i n 1949 a nd a sso c i a te d b y Smith ( 1 95 7) w i t h t h e i r p h y l l o s i l i c a te components .

le d to a su rvey o f micas (Hu tch ison , 1966) t h a t de tec ted two tempera tu re ranges

o f

sound emission - a t 305-340°C ass ocia ted w i t h t rap ped wa ter and a t 600-1000°C

a sso c i a te d w i t h p a r t i a l d e h yd ro xy l a t i o n . The a p pa ra tu s used i n t h e se s tu d i e s was

v e ry s i m p le and l i t t l e f u r t h e r e nsued u n t i l L $ n v i k i n 1972 ( se e L l n v i k , 1974)

devised a much more re f in ed sound-measurement ar rangement usin g a s p e c ia l l y designed

wave-guide system: sound emiss ion was record ed e i t h e r as r a t e o r as am pl i tud e

aga ins t tempera tu re.

i t i s r a th e r more d i f f i c u l t t o p er fo rm. The meth od has more re ce n t l y been t a ke n

up by C la rk (1978) and co -workers i n the

UK

who have developed a concurrent (not

simultaneous, as two samples ar e used) thermosonimetry-DTA app ara tus (C la rk and

Gar l ick, 1979) . A l e s s el a b ora te, b u t a l s o l e ss ve rs a t i l e , sys te m ha s been

desc r ibed by Poulou and Bau drac co-G r i t t i (1978) and a S ov ie t inst ru me nt , whereby

pressu re changes i n h i gh vacuum ar e measured as th e sample de cr ep i ta tes ,

i s

co m n e rc i a l l y a v a i l a b l e (Pa w l i ko wsk i , 1 97 9) .

f o r m easurem ent o f r e l e a s e o f m a t e r i a l f r o m i n c l u s i o n s and i t i s d o u b t f u l w he th er

i t would p i c k up o t he r causes o f sound emiss ion such as s t r a i n re lease , microcra ck

propaga t ion , e tc .

LBnvik (1974, 1978) has ap p l ie d th e techn ique to , i n t e r alia, q u a r t z i t e s a n d

b oe h mite a nd h as n o te d ve ry s t r o n g so un d e mi ssi o n a sso c i a te d w i t h p ha se t r a n s i t i o n s

and w i t h de comp o si ti on re a c t i o n s . C l a r k a nd G a r l i c k ( 19 7 9) , w i t h t h e i r co mb in ed

equipment, have come t o th e c onc lusion t h a t maximum sound emission occu rs a t t h e

ext rap o la ted onse t o f t he DTA peak assoc ia ted w i t h such p rocesses - i.e. sound

e m is s io n i s v i r t u a l l y c om p le te a t t h e p o i n t a t w hi ch m ea su ra ble e n t h al p y c hanges

occur

-

and have r e l e a t e d t h e e m i s s i o n o f s ound e s s e n t i a l l y t o t h e r e l e a s e o f s t r a i n :

once the s t r a i n i s re leased, through th e commencement o f a phase change o r a

decomposi tion re ac t io n, sound emiss ion ceases.

A

simi lar mechanism would presumably

app ly

t o

m i c ro c ra c k p ro p ag a ti on b u t o t h e r r e l a t i o n s h i p s c o u l d w e l l h o l d f o r o t h e r

phenomena such as t he re lea se o f i nc lus ion s.

t im e i s now r i p e t o a p pl y

i t

o c l a y m in e r a ls : i n i t s m os t advanced f o rm

i t

mi g h t

we l l h e l p t o e l u c i d a te t h e me ch a n i sms

o f

some r e a c t i o n s s t i l l i l l - u n d e r s t o o d .

Thermoacoust imetry has so f a r been a p p l i e d m a in ly t o p oly me rs a nd, l i k e a l l

measure ments i n vo l v i n g o s c i l l a t i o n o r waves, wh ethe r sou nd o r e l e c t ro ma g n e t ic ,

These techn iques a re q u i t e d i s t i n c t . I n thermoson imet ry one measures th e

T h i s

A l t ho u g h f re q u e n cy a n a l ys i s o f t h e e m i t t e d sou nd i s p o ss i b l e ,

The l a s t i s s p e c i f i c a l l y d es ign ed

The r a p i d de ve lo pm en t i n t h i s f i e l d o v e r t h e l a s t few y e ar s s ug ge sts t h a t t h e


17/25

2 1

g i v e s i n f o r m a t i o n

on

t h e sh ea r mo du lu s, t h e mod ulu s o f e l a s t i c i t y , t h e l o s s ta n g e n t

and such l i k e . An ex c e l le nt de s c r i p t i o n has been g iv en by Perepechko (1975) .

I t s a p p l i c a t i o n t o c l a y s a t t h e moment

i s

u n ce r t ai n: i t s most l i k e l y f i e l d w ould

be i n r e l a t i o n t o f i r e d c l a y p ro d uc t s b u t , l i k e dynamic t h ermo mecha ni ca l measure-

men ts and a l t e rn a t i n g e l e c t r i c a l measurements ,

i t

h a s a l r e a d y f o u n d a p p l i ca t i o n

i n o i l - s h a le tech nolo gy (Mraz, Rajeshwar and DuBow, 1980).

1.3.9 Therrnoptornetry

thermophotometry (measurement o f t o t a l 1 g h t ) , thermospect rometry (measurement o f

l i g h t o f a s p e c i f i c wa ve le ng th ), t h e rm o r e fr a c to m e t ry (m easurem ent o f r e f r a c t i v e

i nd e x ) and th er mo mic ro sc op y f o r e i t h e r e m i t t e d o r r e f l e c t e d l i g h t .

l um in esce nce i s a sp e c i a l case o f t he rmo ph otome tr y whe re e m i t t e d l i g h t o n l y

i s

measured a t temperatures below re d hea t.

Thermoluminescence i s p robab ly th e most w ide l y used lo f the se techn iqu es,

hav ing been w id e ly ap p l ie d t o lu na r and me te or i te samples (Nambi, Bhas in and

Bapat, 1978) as w e l l as t o ch ar ac te r i za t io n o f marb les and l imestones (A fo rdakos,

Alexopoulous and M i l i o t i s , 1974; Nambi and M itr a, 1978; Chistyakova, 1979). It

i s

a l s o used f o r age d e t e r m i n a t i o n i n g eo lo g y

-

f o r e xample, t o d a te t h e b a k in g

o f a sediment (H uxta ble, A i t k e n and Bonhommet, 1978) - and i n a,rchaeology - t o

d a t e c e r a m i c s a n d o t h e r a r t i f a c t s ( C ai rn s, 1 97 6). I t s r e l a t i o n s h i p t o

DTA,

DTG

and ot he r techn ique s has been disc uss ed by Chen (1976). Bas ic equipment fo r

simultaneous thermoluminescence-DTA has been de sc r ibe d by Dav id (1972) and the

more s en s i t i v e and e lab ora te equipment used i n achaeo log ic a l

and chemica l stud ies

by Cai r ns (1976) and Wendlandt (1980), re sp ec t i ve ly . David (1972) reproduces a

g lo w c u r v e f o r a c l a y ( w h ic h f r om i t s DTA c u r v e i s a g i b b s i t i c b a u x i t e ) and

C a ir n s (1 97 6) d is c u ss e s i n d e t a i l t h e o r i g i n o f th er mo lu min es ce i n c lays and i n

ceramic and o the r mate r ia l s .

Thermomicroscopy has been w id e ly a pp l i ed i n s tu d ies on g lasses, re f ra c t o r i e s

Thermoptometry i s a wide term co ve r in g a whole range o f techn iques, such as

Thermo-

and ceramics, and ne at inst rum ents f o r s imul taneous DTA, o r

DSC,

and thermo-

microscopy have been des cr i be d and su cc es sf ul ly used (M i l l e r and Sommer, 1966;

Sommer and Jochens, 1971; K un ihi sa , 1979). Sev eral stan da rd ho t-s tag e microsco pes

w i t h t e m p e ra t ur e c o n t r o l a r e a l s o c o m m e rc i al ly a v a i l a b l e b u t c l a y s as such,

p re su mab ly be ca use o f t h e i r sma l l p a r t i c l e s i ze , have be en ra th e r n e g l e c ted .

found (b ut no t ne ce ss ar i ly under th e same names) i n standard te xt s (e .g . Wendlandt

and Hech t, 1966; We ndlandt, 197 4).

methods have as ye t un tapped po te n t ia l i-n s tudy ing , f o r examp le , changes i n th e

co l o u r o f sm e c t i t e s on sa tu ra t i o n w i t h n o n-ch ro mop ho ri c i o n s a nd mo de ra te h e a t in g .

Equ ip me nt f o r u se w i t h some o f t h e o th e r t e ch ni q u e s re f e r re d t o a bo ve

w i l l

be

I t seems t o th e autho r t h a t some o f thes e


18/25

2 2

1.3 .10 Therrnoelectrometry

Like thermoptometry, thermo electrom etry i s e ss e n ti a ll y a portmanteau term

covering measurement of any electr ical

proper ty as a funct ion of temperature .

Thus i t covers va r ia t ion s in r es i s ta nc e , conduc tance , inductance and capac i tance ,

for both d.c.

and

a . c . , a s w ell a s v a r i a t i o n s i n d i e l e c t r i c c o n s t a n t s , t he rm o-

e l e c t r i c i t y , t h e r m a l l y s t i m u l a t e d c u r r e n t , e t c .

r ea c t io ns , so l id - s t a t e t r an s i t io ns , e t c . , was f i r s t apprec ia ted by Berg and

Burmistrova

(1960) ,

wh o

con struc ted an instrum ent f o r s imul taneous d.c. conductance

measurement and DTA.

Developments in :equipment and a p p li c a ti o n s up t o 1974 have

been dis cu sse d by Wendlandt (19 74 ): more r e c e n t ly , a.c . conductance measurements

have been favoured (Wendlandt, 1979). The terms therm ally s tim ulate d cond uctiv ity

and

therma l ly s t imulated c ur re nt have come in to very widespread use, p ar t i cu la r l y

i n r e l a t i o n t o semi-conduc to r r esearch : unfo r tuna te ly , the f a c t th a t bo th a r e

g iven th e abb rev ia t ion TSC has led t o widespread confusion in indexing

-

even

in h e m i c a l A b s t r a c t s .

The value of d.c.

conductance (o r res i s ta nc e) in showing up decomposition

Freund and co-workers, in t h e i r fundamental

study of proton tunneling i n

hydroxides, have rec en tly used d.c. conductance and therm ally s timu lated dep olar-

ization measurements t o provide evidence of th e occurren ce o f pro ton conduc t iv i ty

and t r a n s i t o r y HOH s p e c ie s i n g i b b s i t e ,

b r u c i t e a n d p o r t l a n d i t e b e f o r e t h e

comnencement of dehydroxylation (Freund and Wengeler, 1980; Wengeler, Martens and

Freund, 1980). In th es e s t u di es , they observed th a t the d.c. conductance depends

on the nature of the e lectrodes used and that thermopotent ia l measurements are

s e n s i t i v e t o e l e c t r o n a c c e p t o r s or donors sorbed on p a r t i c l e s u r f a c e s . D c

conductance has a ls o been used t o inv es t iga te wate r so rbed on goe th i t e

(Kaneko

and Inouye, 1979) . S im ilar ap pl ic at io ns ar e l ik el y with c la ys . In o i l s h a l e s ,

the d . c . r es i s ta nc e decreases exponen t ia l ly

w i t h

inc reas ing tempera tu re - an

o b s e rv a t io n a t t r i b u t e d t o c a r b o n a te i o ns b ein g t h e t r a n s p o r t i n g s p e c i e s o r t o

breakdown of hydrocarbon units

i n

the trapped kerogen (Rajeshwar, Nottenburg and

DuBow

1979) .

s e p a r a t e t h e e f f e c t s d u e t o the c lay mat r ix from thos e due to th e kerogen. Attempts

have been made t o solv e th e problem using a.c. measurements inv olving co nductance -

i .e . t h a t i n vo lv e d i e l e c t r i c c o n s ta n t , d i e l e c t r i c l o s s f a c t o r and l o s s ta n g en t

and equipment for s imultaneous and concurrent a.c. capacitance measurement a n d

DTA

has been described

(Rajeshwar, Nottenburg and

DUBOW

1978; Nottenburg e t

ai.,

1979).

DTA and fo r capac i tance measurement i s qu i te d i f f e re n t . I t seems t o the au thor

th at examinat ion of non-oi l -bear ing sha les of s im ila r mineralogical composi tion

c ou ld a s s i s t i n r e so l v in g some o f t h e d i f f i c u l t i e s of i n t e r p r e t a t i o n e n co u nt er ed ,

b u t th er e may be pr ac ti ca l

d i f f i c u l t i e s r e l a t e d t o p a r t i c l e o r i e n t a t i o n , p ore

space , e tc .

Such systems a re extremely complicated and i t

i s

d i f f i c u l t t o

The l a t t e r i s cons idered t he more re l i a b le a s the optimum sample s i z e fo r


19/25

2 3

T h e rm a ll y s t i m u l a t e d c u r r e n t s h ave been r e l a t e d t o v a r i ou s t y pe s o f l a t t i c e

T h e se cu r re n t s ca n a l so b e re l a te d t o t h e rmo l u mi n e sce n ce e f f e c t s (F i e l d s

de fec ts i n i o n ic c ry s ta l s and a rev iew has been g iv en by Radhakr ishna and Har idoss

(1978).

and Moran, 1974) - as, indeed, can th er m al ly st im ul at ed conductance (Chen, 1976;

B r ae u nl ic h , K e l l y and F i l l i a r d , 1 97 9)

-

b u t no a p p l i c a t i o n t o c l a y s c an be t ra c e d .

It may be o f i n t e r e s t t o some to n o te t h a t va l u e s o f t h e t h e rm o e l e c t r i c p ower

( i n uV/ C ag ai ns t l ea d) have been measured by Lee (1973)

for a considerab le number

o f m i n e ra l s , i n c l u d i n g some th a t , l i k e h e ma t it e , ma g n e t i t e and i l me n i t e , can o ccu r

as a c ce s so ri es i n c la y s: i n d i c a t i o n s a r e g i v e n o f t h e s pre ad o f v a lu es f o r t h e

samples t e z te d as w e l l as mean values.

I n c o n c lu s i on , i t would appear that some thermoelect rometr ic techn iques may

have c o n s id e r ab l e p o t e n t i a l i n c l a y s t u d i e s

-

e s p e c i a l ly i n r e l a t i o n t o w ate r-

l o ss r e a c t i o ns and l a t t i c e d e f e c t i n v e s t i g a ti o n s

-

q u i t e a p a r t f ro m t h e i r p o t e n t i a l

va l u e i n t e ch n o l o g i c a l a ssessmen ts .

1.3.11 Thermomagnetometry

t em p er at ur e i s ma gn et ic s u s c e p t i b i l i t y , w i t h co ns eq ue nt d e r i v a t i o n o f t h e C ur i e

p o i n t - a parameter t h a t has proved ext remely us efu l i n some st ud ies on c lay s and

s o i l s . Thus, on t h e b a si s t h a t t h e ve r y l a r g e v a r i a t i o n i n t h e C u r i e p o i n t

o f

i lmen i te (be low 77

K

t o 841 K) c an be c o r r e l a t e d w i t h i t s o r i g i n ,

it

has been

c l a im e d t h a t sam ples w i t h a C u r i e p o i n t i n t h e r e g i o n 1 03-223 K a re l i k e l y t o be

assoc ia te d w i t h k im be r l i t e dep os i t s (Garanin , Kudryavtseva and Soshk ina , 1979).

M oreov er, f o r r e d -y e l lo w d e s e r t i c s o i l s , i t has been p o s s i b l e t o i n t e r p r e t C u r i e

po in ts i n te rms o f the p resence o f h emat i te and maghemite (T imofeev and Smirnov,

1 98 0) : i n t h i s s tu dy ,

non-coincidence

of

the thermomagnet ic curve on a second

h e a t in g has b een i n t e r p r e t e d as i n d i c a t i n g t h e p re se nc e o f f e r r i h y d r i t e - a l though

f o rm a t io n o f i r o n o x i de s f r o m some o t h e r m i n e r a l s p r es e n t c o u l d p o s s i b l y a l s o

c o n t r i b u t e . Be t h a t as i t may,

i t

wou ld seem th a t thermomagnetometry may w e ll

have s i g n i f i c a n t a p p l i c a t i o n s i n c l a y min er alo gy , e s p e c i a l l y i n th e i n v e s t i g a t i o n

o f c e r t a i n a cc es so ry m i n e r a ls .

The most common magnet ic c h a r a c t e r i s t i c to be measured as a f un c t i on o f

1.4

C O N C L U S I O N S

From the above account, i t s ev id en t th a t , a l though TG and DTA a r e b y f a r t h e

mos t w i d e l y used t h e rm o a n a l y t i ca l t e ch n iq u e s i n c l a y m i n e ra lo g y a t p re se n t , many

o t h e r m e t h o d s t h a t c a n e l u c i d a t e s p e c i f i c f e a t u r e s o r b e h a v i o u r a r e a v a i l a b l e .

Thus, EGA and DSC a r e p r e s e n t l y r e c e i v i n g g r e a t e r a t t e n t i o n and t he rm o so ni me tr y

a long wi t h some thermop tometr ic , therm oele ct rom etr ic and thermomagnet ic methods

d es er ve c o n s i d e r a t i o n i n t h e f u t u r e . M ethods u s i ng o s c i l l a t i o n s o r waves t h a t

j i v e i n f o r m a t i o n o n t h e s he ar m odulus , t h e m odulus o f e l a s t i c i t y , e tc ., a r e

r e l e v a n t e s s e n t i a l l y t o t e c hn ol og y and i n du s t r y , a lt ho u gh t h e i r p o s s i b le uses i n


20/25

2 4

more fundamental

studies should not be neglected.

All-in-all

,

however, thermoanalytical methods are not in themselves necessarily

good

in

mineral identification: theirforte is

in

yielding information on

characteristics or detail

that it would be difficult,

if not impossible, to obtain

by any other method.

REFERENCES

Afordakos, G. , Alexopoulos, K. and Miliotis, D., 1974. Using artificial

thermoluminescence to reassemble statues from fragments. Nature, Lond., 250:

Alietti, A., Brigatti, M.F. and Poppi, L . 1979.

An

illite/montmorillonite

interlayer mineral in porphyritic rock 'Alpe di Siusi' (BZ-Italy): The Chemistry

of I/M interlayer minerals. Clay Miner., 14: 39-46.

Ashley, H.E.,

1911.

The decomposition of clays and the utilization of smelter

and

other smoke

in

preparing sulphate from clays.

J.

ind.

Engng

Chem., 3: 91-95.

Asomoza, P.M., Razo,

M.L.,

Chaidez,

T.L. and

Casillas, S.R., 1978. Quantitative

evaluation of Na,CO, and NaCl content of the clays of the ex-lake of Texcoco

(Valley of Mexico) by means of thermogravimetry. J. therm. Analysis, 13: 327-339,

Balek,

V.,

1977. Emanation thermal analysis. Thermochim. Acta, 22: 1-156.

Barwood, H.L. and Hajek, B.F., 1979. Differential thermal characteristics of

soil and reference quartz. J. Soil Sci. SOC. Am., 43: 626-627.

Berg, L.G. and Burmistrova, N.P., 1960. [Thermographic analysis of salts with

simultaneous determination of temperature effects

and

electrical

conductivity.1

Zh.

neorg.

Khim.

5: 676-683.

Boy,

S.

and BMhme,

K.

,

1979.

zur Ermittlunq kinetischer Parameter anhand von Experimentalwerten. Thermochim.

47-48.

GegenUberstel

ung

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Documents

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