232

CHAPTER – 6

Thermogravimetric (TG/ DTA)

Analysis of SnSeFeX

(X = 0, 0.5, 1.0, 1.5, 2.0 & 2.5)

Nanoparticles

233

6.1 Introduction

Materials sometimes lose its performance due to thermal,

chemical, or mechanical degradation. It has become necessary to

understand the environmental impact of its degradation and also its

effect on material properties. If the rate of degradation can be accurately

measured, then it becomes possible to predict the material properties. In

general, thermal analysis can provide important information on the

temperature dependent properties of materials and on thermally induced

processes (phase transition, decomposition, etc.). Thermal analysis is

advantageous it gives a general view of the thermal behaviour of a

material under various conditions and requires a small amount of

sample. The term thermal analysis can be applied to any technique

which involves the measurement of a physical quantity while the

temperature is changed or maintained in a controlled and measured

fashion. Usually the temperature is, for simplicity, kept constant or

increased linearly with time [1]. The International Confederation for

Thermal Analysis and Calorimetry (ICTAC) defines Thermal Analysis (TA)

as a group of techniques in which a property of the sample is monitored

against time or temperature while the temperature of the sample, in a

specified atmosphere, is programmed. The program may involve heating

or cooling at a fixed (or variable) rate of temperature change, or holding

the temperature constant, or any sequence of these. Since hardly any

measurement worth doing if the temperature is not controlled, almost all

measurements are some type of thermal analysis [1].

6.1.1 Classification of Individual Thermoanalytical Techniques

Thermometry, is the simplest technique of thermal analysis. It

becomes even more useful when time is recorded simultaneously. Such

thermal analyses are called heating or cooling curves. Time is measured

with a clock, temperature with a thermometer.

234

The most basic thermal analysis technique is naturally

calorimetry, the measurement of heat. The needed thermal analysis

instrument is the calorimeter. Intermediate between thermometry and

calorimetry is Differential Thermal Analysis (DTA). In this technique

transition temperature information is derived by the qualitative changes

in heats of transition or heat capacity. As the instrumentation of DTA

advanced, quantitative heat information could be derived from

temperature and time measurements. The DTA has in the last 50 years

increased so much in precision that its applications overlap with

calorimetry.

The basic measurement of length or volume is called dilatometry if

it is carried out at constant pressure or stress. When measuring stress

as well as strain, the technique is called Thermo Mechanical Analysis,

(TMA). Measurements can be made at constant or variable stress or

strain, including periodic changes as in dynamic mechanical analysis.

Finally, Thermogravimetric Analysis or Thermal Gravimetric Analysis

(TGA) is a method of thermal analysis in which changes in physical and

chemical properties of materials are measured as a function of increasing

temperature (with constant heating rate), or as a function of time (with

constant temperature and/or constant mass loss) [2]. TGA can be used

to evaluate the thermal stability of a material. In a desired temperature

range, if a species is thermally stable, there will be no observed mass

change. Negligible mass loss corresponds to little or no slope in the TGA

trace. TGA also gives the upper use temperature of a material. Beyond

this temperature the material will begin to degrade.

Several more complicated thermal analysis techniques are

mentioned from time to time, described in Table 6.1 which involve

additional specialization, also mentioned in addition of time and

temperature. These various techniques can define for future development

in device fabrication by studying stability and to find out thermal

235

parameters by thermal characterization. The arrangement finally adopted

for the defined techniques (Table 6.1) incorporate additional physical

properties and/or techniques as necessary: various modes of certain

techniques can also be distinguished.

The confusion that has arisen about this term is resolved by

separating two modes (Power compensation DSC and Heat-flux DSC) as

described in the definition given in the text.

Table 6.1: Classification of Thermoanalytical Techniques.

Physical Property Derived Technique

Mass

Thermogravimetry (TG)

Isobaric mass change determination

Evolved Gas Detection (EGD)

Evolved Gas Analysis (EGA)

Emanation Thermal Analysis (ETA)

Temperature Heating or cooling curve determination

Differential Thermal Analysis (DTA)

Enthalpy Differential Scanning Calorimetry (DSC)

Dimensions Thermodiliatometry

Mechanical

characteristics

Thermochemical measurements

Dynamic Thermochemical measurements

Acoustic characteristic Thermosonimetry

Thermoacoustimetry

Optical characteristic Thermoptometry

Electrical characteristic Thermoelectrometry

Magnetic characteristic Thermomagnetometry

236

6.1.2 Thermogravimetry

Thermogravimetric analysis covers a wide spectrum of thermo

analytical techniques, which monitor one or more physical properties of a

substance that is undergoing a temperature programmed heating as a

function of time and temperature. It provides a quantative measurement

of any weight changes associated with thermally associated changes.

Thermogravimetric analysers can be called as a thermo balance which is

a combination of a suitable electronic microbalance with a furnace,

which is operated with a computer controlled heating programme. It

allows the sample to be weighed and heated or cooled in a temperature

controlled manner and the mass, time and temperature data to be

recorded under specific atmosphere. The thermogravimetry analyzer

(TGA) system, which combines thermogravimetry (TG) and differential

thermal analyzer (DTA), is widely used in the fields of gas– solid

interactions, fuels, catalysis, polymers and chemical synthesis.

Thermogravimetric analysis is used to determine the material‟s thermal

stability and its fraction of volatile components by monitoring the weight

change that occurs as a sample is heated. This is explained later in

detail. The measurement is normally carried out in an inert atmosphere,

such as Helium or Argon, and the weight is recorded as a function of

temperature. In addition to weight changes, some instruments also

record the temperature difference between the specimen and the

reference pan (differential thermal analysis, or DTA) or the heat flow into

the specimen crucible compared to that of the reference crucible

(differential scanning calorimetry, or DSC). The latter can be used to

monitor the energy released or absorbed via chemical reactions during

the heating process.

A DTA apparatus consists of a sample holder comprising

thermocouples, sample containers and a ceramic or metallic block; a

furnace; a temperature programmer; and a recording system. The key

237

feature is the existence of two thermocouples connected to a voltmeter.

One thermocouple measures the temperature of an inert material such

as Al2O3, while the other is used for measurement for the sample

temperature under study. As the temperature is increased, there will be

a deflection of the voltmeter if the sample is undergoing a phase

transition. This occurs because the input of heat will raise the

temperature of the inert substance, but be incorporated as latent heat in

the material changing phase. In DTA, the differential temperature is

plotted against the time, or against the temperature (DTA curve or

thermogram). Changes in the sample, either exothermic or endothermic,

can be detected relative to the inert reference. Thus, a DTA curve

provides data on the transformations that have occurred, such as glass

transitions, crystallization, melting and sublimation. The area under a

DTA peak is related to the enthalpy change of the sample. Generally a

sharp endothermic (negative peak) DTA peak at particular temperature

indicates the melting point of the sample as the temperature of the

sample at this particular temperature would lag behind the temperature

of reference substance where as an exothermic peak indicate the onset of

decomposition process.

6.1.3 Thermogravimetric Analysis (TGA)

The thermal properties like heat capacities, the glass transition

temperature, melting and degradation of macromolecules can be

analyzed using thermogravimetry and differential thermal analysis along

with differential scanning calorimetry (DSC).

Thermal measurements are based on the measurement of dynamic

relationship between temperature and some property of a system such as

mass, heat of reaction or volume when the material is subjected to a

controlled temperature programme. In thermogravimetric analysis, the

mass of the sample is recorded continuously as a function of

238

temperature as it is heated or cooled at a controlled rate. A plot of mass

as a function of temperature is known as thermogram. The apparatus

required for thermo-gravimetric analysis include a sensitive recording

analytical balance, a furnace, a temperature controller, and a

programmer that provides a plot of the mass as a function of

temperature. Often an auxiliary equipment to provide an inert

atmosphere for the sample is also needed. Changes in the mass of the

sample occurs as a result of rapture and/or formation of various

physical and chemical bonds at elevated temperature that led to the

evolution of volatile products or formation of reaction products. Thus

TGA curve provides information regarding the thermodynamics and

kinetics of various chemical reactions, reaction mechanisms, and

intermediate and final reaction products.

6.1.4 Differential Thermal Analysis (DTA)

A technique in which the temperature difference between a

substance and a reference material is measured as a function of

temperature whilst the substance and reference material are subjected to

the same controlled temperature programme. The record is the

differential thermal or DTA curve; the temperature difference (ΔT) should

be plotted on the ordinate with endothermic reactions downwards and

temperature or time on the abscissa increasing from left to right. The

term quantitative differential thermal analysis i.e. Quantitative DTA,

cover those uses of DTA where the equipment is designed to produce

quantitative results in terms of energy and / or any other physical

parameter.

6.1.5 Thermal Analysis for Physical Significance

Thermal analysis refers to a group of techniques in which the

properties of a substance under study is monitored with respect to time

and temperature in a specified atmosphere. This is often done by heating

239

the precursors (starting materials) in an evaporator isothermally.

Volatization of solid in a broad sense includes any process which result

in conversion of matter from the solid state to the vapor phase.

Volatization process can be accomplished by two ways.

1. Sublimation process in which the gaseous phase composed of the

same type of atoms or molecules supplied by the solid phase i.e.

the composition of the gaseous phase and the solid phase remain

the same - a true vaporization process.

2. A chemical reaction between the solid phase and another species

to form gaseous products. The additional species may be

environmental gases, adsorbed water, or some some solid like

container material. In both cases the composition of the gaseous

phase is always different from the solid phase. This is often termed

as decomposition or pyrolysis.

As solid is heated, the extent of lattice vibrations within the solid are

increased and a temperature would be reached during heating where

following changes can occur:

Melting: the forces of attraction between the constituents decrease

which maintain an orderly arrangement of the solid and comes

down to a more disordered system called liquid state.

Phase transition: a new arrangement of the lattice structure.

Sublimation: direct transformation from the solid state to the gas

phase occurs.

Decomposition: sometimes the molecular rearrangements of bonds

within the solid during heating also result in formation products

chemically different from the solid. These products can be a solid

or gasses. This occurs over a range of temperature.

240

Thus by obtaining gravimetric data of heating a solid sample with

time or temperature in a specified atmosphere, one would easily predict

about the volatility, thermal stability and physical state of the sample at

a particular temperature.

Precisely determined thermodynamic events, such as a change of

state, can indicate the identity and purity of drugs. Compendia

standards have long been established for the melting or boiling

temperatures of substances. These transitions occur at characteristic

temperatures and the compendia standards therefore contribute to the

identification of the substances. Because impurities affect these changes

in predictable ways, the same compendial standards contribute to the

control of the purity of the substances.

Thermal analysis in the broadest sense is the measurement of

physical-chemical properties of materials as a function of temperature.

Instrumental methods have largely supplanted older methods dependent

on visual inspection and on measurements under fixed or arbitrary

conditions, because they are objective, they provide more information,

they afford permanent records, and they are generally more sensitive,

more precise, and more accurate. Furthermore, they may provide

information on crystal perfection, polymorphism, melting temperature,

sublimation, glass transitions, dehydration, evaporation, pyrolysis, solid

solid interactions, and purity. Such data are useful in the

characterization of substances with respect to compatibility, stability,

packaging, and quality control. The measurements used most often in

thermal analysis, i.e., transition temperature, thermogravimetry, and

impurity analysis, are described here.

Transition Temperature - As a specimen is heated, its uptake (or

evolution) of heat can be measured [Differential Scanning Calorimetry

(DSC)] or the resulting difference in temperature from that of an inert

241

reference heated identically [Differential Thermal Analysis (DTA)] can be

measured. Either technique provides a record of the temperature at

which phase changes, glass transitions, or chemical reactions occur. In

the case of melting, both an “onset” and a “peak” temperature can be

determined objectively and reproducibly, often to within a few tenths of a

degree. While these temperatures are useful for characterizing

substances, and the difference between the two temperatures is

indicative of purity, the values cannot be correlated with subjective,

visual “melting range” values or with constants such as the triple point of

the pure material.

6.1.6 Applications

TG Analysis gives us information about the thermal events which

are accompanied by changes in mass. For desorption, decomposition and

oxidation processes, useful information can be collected from TG

analysis. It gives accurate information about drying and the

decomposition of metal hydroxides into oxides in ferrite processing. The

mass losses define the stages and the conditions of temperature and

atmosphere necessary for the preparation of the spinel phase and the

stability. TG curves for complex ternary metal hydroxides like the present

study, may not give the exact reaction occurring even then it can be used

for “finger print‟ purposes [3]. Further it can be utilized for engine oil

volatility measurements, filler content, flammability studies, heat of

transition, oxidative stabilities, thermal stabilities, transition

temperatures and catalyst and coking studies.

6.2 Experimental

Simultaneous Thermogravimetric (TG) and Differential Thermal

(DT) analyses were carried out on a Rigaku thermobalance using about

10 mg of sample. Alumina was used as reference. Generally, these

experiments can be done either under Ar, He or N2 gas atmosphere or

242

without using gas (i.e. in air). Here author has performed TG/ DTA

experiment under air. A vacuum purge of atmospheric air was done

before starting the experiments. This operation induces a systematic

mass loss because, under vacuum, the samples start to lose water from

room temperature. A complete description of the conditions employed

should accompany each thermogram, including make and model of

instrument; record of last calibration; specimen size and identification

(including previous thermal history); container; identity, flow rate, and

pressure of gaseous atmosphere; direction and rate of temperature

change; and instrument and recorder sensitivity.

6.2.1 Construction and Working of Thermal Analyzer

A thermobalance is a combination of an electronic microbalance,

furnace and a temperature programmer. Figure 6.1(a) and Figure 6.1(b)

shows schematic thermobalance. The thermobalance is placed in an

enclosed system to control the atmosphere. The measurements of mass

changes with temperature are carried out with the help of such

thermobalance. The maximum load for thermobalance is 1g and a

sensitivity of 1μg.

The sample should be powdered where possible and should be

spread in a thin and uniform layer in the sample container.

Thermobalance is normally housed in a glass or metal systems to control

the pressure and the atmosphere inside it. A regular gaseous flow may be

maintained in order to remove the evolved gases from the thermobalance

with the care that these the flow gases don not disturb the balance [3].

Temperature sensors are either platinum resistance thermometers or

thermocouples. The temperature controller attached to the instrument

offer heating rates from a fraction of a degree per minute to nearly 100 °C

min-1 with additional characteristic of isothermal heating

243

(a) (b)

Figure 6.1 (a) & (b): A schematic thermobalance

The beam is displaced by change in weight loss with temperature

on sample side. This displacement is detected optically and the drive coil

current is changed to return the displacement to zero. The detected drive

coil current change is proportional to the amount of weight change in

sample and is output as the TG signal. The DTA detects the temperature

difference between the sample holder and the reference holder using

electromotive force of thermocouples, attached to the holders. The

differential is output as the DTA signal.

It is appropriate to make a preliminary examination over a wide

range of temperature (typically room temperature to decomposition

temperature or about 10°to 20 above the melting point) and over a wide

range of heating rates (2 to 20 per minute), which may reveal unexpected

effects; then a single examination or replicate examinations over a

narrow range, bracketing the transition of interest at one or more lower

heating rates, can be made. In examining pure crystalline materials,

rates as low as 1 per minute may be appropriate, whereas rates of up to

10 per minute are more appropriate for polymeric and other semi

crystalline materials. As the reliability of the measurements varies from

244

one substance to another, statements of the number of significant figures

to be used in the reporting of intralaboratory repeatability and of inter

laboratory reproducibility cannot be given here, but should be included

in the individual monograph.

Thermogravimetric analysis involves the determination of the mass

of a specimen as a function of temperature, or time of heating, or both,

and when properly applied, provides more useful information than does

loss on drying at fixed temperature, often for a fixed time and in what is

usually gas atmosphere. Usually, loss of surface absorbed solvent can be

distinguished from solvent in the sample and from degradation losses.

The measurements can be carried out in atmospheres having controlled

humidity and oxygen concentration to reveal interactions with the drug

substance, between drug substances, and between active substances

and excipients or packaging materials. While the details depend on the

manufacturer, the essential features of the equipment are a recording

balance and a programmable heat source. Equipment differs in the

ability to handle specimens of various sizes, the means of sensing

specimen temperature, and the range of atmosphere control. Calibration

is required with all systems, i.e., the mass scale is calibrated by the use

of standard weights; calibration of the temperature scale, which is more

difficult, involving either variations in positioning of thermocouples and

their calibration; or in other systems, calibration involves the use of

standard materials because it is assumed that the specimen temperature

is the furnace temperature.

Procedural details are specified in order to provide for valid

interlaboratory comparison of results. The specimen weight, source, and

thermal history are noted. The equipment description covers dimensions

and geometry, the materials of the test specimen holder, and the location

of the temperature transducer. Alternatively, the make and model

number of commercial equipment are specified. In all cases,

245

thecalibration record is specified. Data on the temperature environment

include the initial and final temperatures and the rate of change or other

details if nonlinear. The test atmosphere is critical; the volume, pressure,

composition, whether static or dynamic, and if the latter, the flow rate

and temperature are specified.

6.3 Results and Discussion

6.3.1 Broido and Coats Redfern (C – R) Method

Various researchers [4] put forward integral method, which can be

applied to thermogravimetric data assuming order of reaction and from

which the activation energy (Ea) can be estimated. In thermaogravimetric

measurement, the mass conversion is typically calculated as following

Equation 6.1.

0

0 f

m mY

m m

(6.1)

where y is fraction of initial molecules not yet decomposed and

equal to; m represents initial sample mass in an experiment, m denotes

current mass and mf represents final mass. Kinetic studies assume that

the isothermal rate of conversion, dy/dt, is a linear function of the

reactant concentration loss and of temperature independent function of

the conversion, y that is given by Equation 6.2.

( )dy

kf Ydt

(6.2)

where f = (Y) is the reaction model that depends on the mechanism

of degradation. The function ‘k’ is always described by Arrhenius

expression Equation 6.3 can be given by this way.

aE

RTk Ae

(6.3)

246

where A is pre exponential factor (also called as Arrhenius

constant), is assumed to be independent of temperature, E is activation

energy, T is absolute a temperature and R is universal gas constant. The

combination of equation 6.2 and 6.3 gives the following relationship i.e.

Equation 6.4.

( )aE

RTdy

Af Y edt

(6.4)

When temperature of sample is changed under controlled and

constant heating rate (dT/dt), the variation in degree of conversion of

mass can be analyzed as a function of temperature, which depends on

time of heating. Therefore, reaction rate may be written as follows

Equation 6.5.

dy dy dT dy

dt dT dt dt

(6.5)

Thus, change in mass vs temperature can be written as Equation 6.6.

( )aE

RTdy A

e f Ydt

(6.6)

Integral form of from Equation 6.6, initial temperature, to

corresponding to a degree of conversion m o to a peak temperature, Tp

can be written as Equation 6.7.

00 ( )

ap

EY T

RT

T

dY Ae dT

f Y

(6.7)

Using an approximation, Broido rearranged [5] the Equation 6.7

and obtained Equation 6.8 as followed.

2

max1ln ln a

a a

E RAT

Y RT E

(6.8)

247

In Broido’s approximation, the order of thermal degradation is

considered as first order and the calculations are done accordingly.

Assuming the order of equation, Coats and Redferd [6] developed an

integral method for analysis of thermogravimetric data as given below

Equation 6.9.

2

ln(1 ) 2ln ln 1 a

a a

EY RA RT

T E E RT

for n=1 (6.9)

The Horowitz and Metzger [7] modified the equation as below

Equation 6.10.

( )ln(1 )

( )

a p

p

E T TY

R T

for n=1 (6.10)

For both C- R and H- M methods, the correlation coefficient values

among the reaction of different orders are considered. The values of “y”

were determined at different temperature interval from TG curve of

instrument carried out. A plot of ln ln (1/y) in case of Broido’s method,

ln [-(1-y)/T2] in case of C- R method and (1- y) in case of H- M method;

versus 1000/ T for major degradation events yielded straight line and

slope. This slope is equal to – Ea/ 2.303 R [8]. Ea values of all the

samples are presented in Table 6.2 for pure and iron doped SnSe

nanoparticles.

After obtaining the energy of activation i.e. Ea value, from Broido

and Coats- Redfern method, the entropy of activation ΔS can be

calculated by following Equation 6.11.

lnB s

AhS R

k T

(6.11)

where kB is Boltzmann constant, h is Plank’s constant and Ts

peak temperature.

248

The enthalpy of activation (ΔH) and Gibbs free energy (ΔG) can be

calculated from following relationship i.e. Equation 6.12 and Equation

6.13.

H E RT (6.12)

G H T S (6.13)

In Results and discussion part, all these thermal parameters are

investigated by using two thermodynamical model and all the plots were

show good correlation coefficients.

Figure 6.2 (a):TG and DTA thermogram of SnSe nanoparticles.

Temp Cel10008006004002000

DT

A u

V

5.00

0.00

-5.00

-10.00

-15.00

-20.00

-25.00

-30.00

TG

%

100.0

90.0

80.0

70.0

60.0

50.0

40.0

DT

G u

g/m

in

70.00

60.00

50.00

40.00

30.00

20.00

10.00

0.00

43.6%

6.2%

5.7%

55.4%

35Cel

99.7%

247Cel

93.6%

559Cel

51.1%

940Cel

44.3%

85Cel

13.51ug/min

319Cel

24.43ug/min

450Cel

56.95ug/min

469Cel

61.05ug/min

581Cel

13.51ug/min

42Cel

0.00uV

841Cel

-31.63uV

249

Figure6.2 (b): TG and DTA thermogram of SnSeFe0.5 nanoparticles.

Figure 6.2 (c): TG and DTA thermogram of SnSeFe1.0 nanoparticles.

Temp Cel800.0600.0400.0200.00.0

DT

A u

V

20.00

10.00

0.00

-10.00

-20.00

-30.00

TG

%

100.0

90.0

80.0

70.0

60.0

50.0

40.0

30.0

DT

G u

g/m

in

200.0

150.0

100.0

50.0

0.0

-50.0

36.8%52.2%

76.9Cel

18.4ug/min

354.1Cel

43.0ug/min

373.7Cel

38.7ug/min

413.2Cel

65.4ug/min

447.8Cel

31.6ug/min

507.9Cel

47.1ug/min

657.8Cel

9.5ug/min

34.2Cel

99.4%

276.5Cel

91.1%

558.8Cel

54.9%936.8Cel

47.2%

41.3Cel

-0.33uV

866.9Cel

-28.45uV

940.3Cel

-27.87uV

Temp Cel800.0600.0400.0200.00.0

DT

A u

V

15.00

10.00

5.00

0.00

-5.00

-10.00

-15.00

TG

%

100.0

90.0

80.0

70.0

60.0

50.0

40.0

DT

G u

g/m

in

150.0

100.0

50.0

0.0

38.8% 53.3%

35.5Cel

99.0%

292.6Cel

91.0%

590.1Cel

53.2%

939.2Cel

45.8%

98.0Cel

16.3ug/min

350.9Cel

38.9ug/min

414.6Cel

58.2ug/min

499.3Cel

62.6ug/min

668.4Cel

13.9ug/min

48.9Cel

-0.09uV

595.5Cel

-12.29uV940.0Cel

-15.95uV

250

Figure 6.2 (d): TG and DTA thermogram of SnSeFe1.5 nanoparticles.

Figure 6.2 (e): TG and DTA thermogram of SnSeFe2.0 nanoparticles.

Temp Cel800.0700.0600.0500.0400.0300.0200.0100.00.0

DT

A u

V

10.00

5.00

0.00

-5.00

-10.00

-15.00

TG

%

100.0

95.0

90.0

85.0

80.0

75.0

70.0

65.0

60.0

55.0

50.0

45.0

DT

G u

g/m

in200.0

150.0

100.0

50.0

0.0

-50.0

-100.0

38.3%

9.5%

2.8%

29.8Cel

99.5%

278.0Cel

90.0%

592.2Cel

52.8%

749.5Cel

48.9%

50.6%

95.2Cel

35.2ug/min

323.2Cel

52.1ug/min

354.0Cel

80.4ug/min

367.8Cel

95.2ug/min

521.8Cel

90.5ug/min36.9Cel

0.33uV

602.3Cel

-14.30uV

Temp Cel600.0500.0400.0300.0200.0100.0

DT

A u

V

-2.00

-4.00

-6.00

-8.00

-10.00

-12.00

TG

mg

3.800

3.600

3.400

3.200

3.000

2.800

2.600

2.400

2.200

DT

G u

g/m

in

90.00

80.00

70.00

60.00

50.00

40.00

30.00

20.00

10.00

251

Figure 6.2 (f): TG and DTA thermogram of SnSeFe2.5 nanoparticles.

6.3.2 Evaluation of Thermal Parameters of SnSeFeX (X = 0, 0.5,

1.0, 1.5, 2.0, 2.5) Nanoparticles

Broido and C - R thermodynamical method / model have been

used for evaluation of thermal parameters. As described earlier, values of

‘y’ were determined at different temperature interval from TG curve.

These thermal parameters of all SnSeFeX (X = 0, 0.5, 1.0, 1.5, 2.0, 2.5)

nanoparticles can be achieved by ploting ln ln (1/y) versus 1/ T graph,

for major degradation events where significant weight or mass loss is

found. Figure 6.3 (a - f) show the Broido plot (i.e plot of ln ln (1/y) versus

1/ T) for SnSeFeX (X = 0, 0.5, 1.0, 1.5, 2.0, 2.5) nanoparticles

respectively. The slope obtained from these plots, equal to – Ea/ 2.303 R

[8]. Thus, by obtaining value of energy of activation using Equation 6.11,

Equation 6.12 and Equation 6.13 one can easily evaluate the entropy of

activation (ΔS), enthalpy of activation (ΔH) and Gibbs free energy (ΔG)

respectively for pure SnSe nanoparticles.

Similarly of ln[-(1-y)/T2] versus 1 / T graph, have been plotted for

all these pure and iron doped SnSe nanoparticles samples (i.e. in C- R

Temp Cel600.0500.0400.0300.0200.0100.0

DT

A u

V

0.00

-2.00

-4.00

-6.00

-8.00

-10.00

-12.00

-14.00

TG

mg

5.000

4.500

4.000

3.500

3.000

DT

G u

g/m

in100.0

90.0

80.0

70.0

60.0

50.0

40.0

30.0

20.0

10.0

252

method). Figure 6.4 (a) to (f) show the C - R plot for SnSe nanoparticles

doped by 0, 0.05, 0.1, 0.15, 2.0 and 2.5 iron concentration level

respectively. By repeating the process as that for the Broido method,

author has reported all the mentioned thermal parameters for all SnSe

nanoparticles sample.

Thermal parameters of SnSe nanoparticles doped by 0, 0.05, 0.1,

0.15, 2.0 and 2.5 iron concentration level obtained from Broido and

Coats- Redfern (C - R) model have been mentioned in Table 6.2. As the

doping level of iron concentration in SnSe nanoparticles increases (i. e. 0,

0.05, 0.1, 0.15, 2.0 and 2.5), energy of activation first decreases as

73.97, 59.09 and from 0.05, 0.1, 0.15, 2.0 and 2.5 iron concentration it

increase to 64.14 kJ/ mol shown in table 6.2. Similarly ΔS and ΔH

changes by changing the iron concentration level in SnSe nanoparticles.

This can be understand that as we are increasing the doping level of

copper concentration in SnSe nanoparticle, crystallite size or particle size

of SnSe nanoparticles is reduced. This crystallite size or particle size of

as synthesized iron doped SnSe nanoparticles has been calculated from

X- ray diffraction technique, Transmission electron microscopy and UV-

VIS- NIR Spectroscopy (i. e. chapter 3 and chapter 4). It should be noted

that thermal parameters (activation energy, frequency factor, entropy

(ΔS), enthalpy (ΔH) and Gibbs mean free energy (ΔG)) presented here had

been investigated using Broido model.

For this SnSe particles (not nanoparticles) sample, synthesized by

ball milling technique with particle size 200 nm to 500 nm, energy of

activation is found to be 89 kJ/ mol. This value matches to activation

energy obtained for as synthesized iron doped SnSe nanoparticle in this

investigation by using aqueous solution technique [9].

253

Table 6.2: Values of thermal parameters of SnSeFeX (X = 0, 0.5, 1.0, 1.5, 2.0, 2.5) nanoparticles

respectively.

Grown nano powders

Decompition Temperature (°C )

Ts (°C)

Method

Activation energy

Ea

(kJ/ mol)

ΔS

(J/mol

K)

ΔH (kJ/mol)

ΔG (kJ/mol)

correlation

coefficient

Ra

SnSe 300 to 600 450

Broido 73.97 -176.67 67.96 195.69 0.997

Coats – Redfern

72.15 -176.88 66.14 194.02 0.944

SnSeFe0.5 276 to 559 413

Broido 59.09 -178.10 53.39 175.56 0.991

Coats – Redfern

58.06 -178.25 52.36 174.64 0.993

SnSeFe1.0 292 to 560 499

Broido 60.42 -178.90 54.00 192.11 0.995

Coats – Redfern

59.13 -179.08 52.71 190.96 0.975

SnSeFe1.5 276 to 559 367

Broido 60.99 -177.26 55.67 169.11 0.981

Coats – Redfern

60.04 -177.39 54.72 168.25 0.962

SnSeFe2.0 300 to 600 421

Broido 62.46 -177.73 56.69 180.04 0.993

Coats – Redfern

61.96 -177.80 56.19 179.59 0.994

SnSeFe2.5 300 to 600 423

Broido 64.14 -177.54 58.36 181.92 0.993

Coats – Redfern

63.89 -177.57 58.10 181.69 0.983

254

Figure 6.3 (a - f): ln ln(1/y) vs. 1/T (Broido plot) for SnSeFeX (X = 0,

0.5, 1.0, 1.5, 2.0, 2.5) nanoparticles respectively.

y = 8897.3x - 11.856

R² = 0.9969

-2

0

2

4

0.0011 0.0014 0.0017

ln ln(1

/Y

)

(a)

y = 7107.2x - 9.6825

R² = 0.991

-1

0

1

2

3

0.0011 0.0014 0.0017

ln ln(1

/Y

)

1/T(K)-1

(b)

y = 7267x - 9.4861 R² = 0.9992

-1

0.5

2

0.0011 0.0014 0.0017

ln ln

(1/Y

)

1/T (K)-1

( C )

y = 7335.7x - 9.7945

R² = 0.9814

-2

0

2

4

0.0011 0.0014 0.0017

ln ln

(1/Y

)

1/T (K)-1

(d)

y = 7512.8x - 10.107

R² = 0.9927

-2

-0.5

1

2.5

0.0011 0.0014 0.0017 ln ln

(1/Y

)

1/T (K)-1

(e) y = 7715.2x -

10.315 R² = 0.993

-2

0

2

4

0.0011 0.0014 0.0017

ln ln

(1/Y

)

1/T (K)-1

(f)

255

Figure 6.4 (a - f): ln[-ln(1-y)/T2] vs. 1/T (C–R plot) for SnSeFeX (X = 0,

0.5, 1.0, 1.5, 2.0, 2.5) nanoparticles respectively.

y = -8678.3x + 25.792

R² = 0.9443

11

14

17

0.0011 0.0014 0.0017

ln [-l

n(1

-y)/

T2]

1/T (K)-1

(a) y = -6984x +

23.483 R² = 0.9925

11

13

15

0.0012 0.0015 0.0018

ln [-l

n(1

-y)/

T2]

1/T (K)-1

(b)

y = -7112.1x + 23.341

R² = 0.9746

11

13

15

0.0011 0.0014 0.0017

ln [-l

n(1

-y)/

T2]

1/T (K)-1

( c ) y = -7221.1x +

23.789 R² = 0.962

11

14

17

0.0011 0.0014 0.0017

ln [-l

n(1

-y)/

T2]

1/T (K)-1

(d)

y = -7452.8x + 24.034

R² = 0.9941

11

13

15

0.0012 0.0014 0.0016

ln [-l

n(1

-y)/

T2]

1/T (K)-1

(e)

y = -7684.6x + 24.335

R² = 0.9829

11

13

15

0.0012 0.0014 0.0016

ln [-l

n(1

-y)/

T2]

1/T (K)-1

(f)

256

6.3.3 TG & DTA Analysis of SnSeFeX (X = 0, 0.5, 1.0, 1.5, 2.0, 2.5)

Nanoparticles

The SnSeFeX (X = 0, 0.5, 1.0, 1.5, 2.0, 2.5) nanoparticles

synthesized by aqueous solution technique has been subjected to

thermal analysis to study the effect of rise in temperature from room

temperature to 900 °C with a heating rate of 10 K/ min on the weight

loss (TG) and heat flow (DTA). Figure 6.2 (a - f) shows the TG and DTA

curves in air of pure and iron doped SnSe nanoparticles. TG curve of this

sample shows continuous weight losses from room temperature to about

900 °C

The iron (Fe) ion introduces more packing efficiency to the host,

which in turn influences the thermal properties of the material/ host.

The aim of the present work was to study the thermal properties of pure

SnSe and copper doped SnSe nanoparticles using thermogravimetric

analysis and differential scanning calorimetry (TG/ DTA and DSC) [10].

The TG and DTA curves in air of SnSe nanoparticles doped

by 0, 0.5, 1.0, 1.5, 2.0 and 2.5 iron concentration level have been shown

in Figure 6.2 (a - f) [9]. Thermogrammes of all these iron doped

nanoparticles show almost same mass loss events as that of pure or

undoped SnSe nanoparticles.

The results obtained of weight loss from the TGA curve observed

for SnSe nanoparticles doped by 0, 0.5, 1.0, 1.5, 2.0 and 2.5 iron

concentration is tabulated in Table 6.3.

The TG curve exhibits three stages of weight loss: the first one is

less than 12 % occurring from room temperature 35 °C to 275 °C. This

weight loss is described to the removal of water molecules at below 100

°C from the pure and iron doped SnSe nanoparticles sample. The first

weight loss change at low temperatures (i. e. stage I) is due to release of

water/ethanol mixture left in the nanoparticles sample. Although the

257

nanoparticles powder was dried at 100 °C temperature in oven, solvent

molecules from the precipitation procedure remain entrapped in the

nanoparticles. Up to approximately 10 % of the total powder weight could

be lost in this step.

Table 6.3: TG and DTA analysis of SnSeFeX (X = 0, 0.5, 1.0, 1.5, 2.0,

2.5) nanoparticles respectively.

Grown Nano Powders DTG Peak(°c) Weight Loss (%)

SnSe

85 6.2

319 43.6 450

581 5.7

SnSeFe0.5

76.9 8.3

354.1

36.2 413.2

507

657.8 7.7

SnSeFe1.0

98 8

350.9

38.8 414.6

499

668 6.5

SnSeFe1.5

95.2 9.5

323.2

38.3 354

367.8

521.8 2.8

SnSeFe2.0

99 11.25

421 36.83

501

SnSeFe2.5

96 12.18

378

39.45 423

497

The second stage of TGA curve shows weight loss about 40 %

appearing between 275 to 600 °C. There was major change with

compared to other weight loss stages in the weight loss. It indicated that

258

these SnSe nanoparticles synthesized at 200 °C remains almost stable

within the said temperature range in comparison with other weight loss

stages.

The second weight loss is about 40 % between 275 to 600 °C. it is

due to melting of excess Sn and Se in SnSe nanoparticles because Sn

and Se particles starts to melt at about 232 and 221 °C temperature.

Within this temperature range there is not any possibility of melting of

iron, because iron starts to melt about 1535 °C temperature.

The final weight loss is more than 8 % and is between 600 to 900

°C.

Another reason for this is that the stability of SnSe in bulk form is

up to 860 °C temperature because it starts to melt about this

temperature. So it should be noticed that in DTA pattern temperature

below 860 °C, one cannot get any DTA peak due to the melting or

decomposition of SnSe nanoparticles.

The final weight loss is more than 8 % and is between 600 and

about 900 °C. The maximum second stage of the TGA weight loss which

is about 40 % correspondingly, the DTA curve shows peaks at around

400 to 450 °C. X- ray powder diffraction indicates that SnSe

nanoparticles synthesized at 400 °C temperature, does not indicate any

extra reflection peak which proves that these nanoparticles/ compound

is stable up to this temperature. Another reason for this is that the

stability of SnSe in bulk form is up to 861 °C temperature because it

starts to melt about this temperature. So it should be noticed that in

DTA pattern temperature about 400 to 450 °C, one cannot get any DTA

peak due to the melting or decomposition of SnSe nanoparticles. It

should be noted that TG/ DTA of pure SnSe nanoparticles were

performed from room temperature to 900 °C. but here for iron doped

SnSe nanoparticles, For 0, 0.5, 1.0, 1.5, 2.0 and 2.5 iron doping in SnSe

259

nanoparticles, due to less dopant concentration level melting point of

iron doped SnSe nanoparticles could not vary/ changed so much from

that of bulk SnSe compound/ material. DTA curve of this nanoparticles

sample does not show any peak around 860 °C temperature. Around this

temperature in DTA show peak around 550 to 670 °C temperature. When

SnSe has been studied in nanoparticle form, the melting temperature

takes place at other temperature. This change in melting temperature

can be explained on the basis of the large amount of free energy

associated with the grain boundaries of nanoparticles. As material‟s form

has been changed (bulk or nanoparticle) melting point of material has

been changed or by specifically found that in most of the materials

melting point decreases as particle size or crystallite size decrease. Lots

of research on thermal (particularly melting process) characterization

had been done on Sn material. Such type of report on other

semiconductor nanoparticles or group IV - VI compound has not found.

Till today according to our knowledge, thermal characterization of

pure SnSe nanoparticles has not reported or rare publications are found

on the thermal characteristics of nanoparticle semiconductors yet. Hence

author has compared obtained results of the variation in thermal

properties of as synthesized nanoparticles have been compared with

earlier report on the thermal properties of elemental nanopartiecles.

In one report of determination of particle size distribution of the

four kinds of Sn nanoparticles has been presented [11]. The data were

fitted with a Gaussian model, and the average particle diameters were

around 82 nm, 39 nm, 36 nm, and 34 nm, respectively. The DSC curves

of the as- synthesized Sn nanoparticles has been presented. The Sn

nanoparticles synthesized in the presence of 0, 0.1, 0.2 and 0.4 g

surfactants were marked as Sn1, Sn2, Sn3 and Sn4, respectively. The

melting temperatures of the Sn1, Sn2, Sn3 and Sn4 nanoparticles were

260

226.1, 221.8, 221.1 and 219.5 °C, and the corresponding latent heats ΔH

of fusion were 35.9, 23.5, 20.1 and 15.6 J/g, respectively [11].

Lots of research on thermal (particularly melting process)

characterization had been done on Sn material. Such type of report on

other semiconductor nanoparticles or group IV- VI compound has not

found. Up to the 1980s, experimental investigations of the size effect on

melting has been performed in a large variety of simple elements of which

the melting point is relatively low, such as Sn, In, Pb, Ge, Bi, Al, Ag, Cu

and Na. Generally, despite the experimental errors in determining the

melting temperature and particle size, evident melting point depression

was observed for small particles, especially when the particle radius is

below 10 nm [12]. In most cases, an approximately linear relationship

between the melting point and the reciprocal particle size is obtained.

The experimental data falls between the lower and upper limits of the

size dependent melting temperature. It is also noted that defects in the

particles, such as dislocations, stacking faults and grain boundaries

and/or twin boundaries may have considerable effects on the size

dependence of melting point, which is believed to be one of the reasons

accounting for scattering of experimental data [13].

It is now known that the thermal characteristics of all low

dimensional crystals, including metals [14 - 18], semiconductors [19, 20]

and organic crystals [21, 22], depends on their sizes. Although there are

relatively extensive investigations on the size dependent melting of

nanocrystals, it has not been accompanied by the necessary investigation

of the size dependent thermodynamics of nanocrystals [14 - 16, 21, 23,

24]. Such an investigation should deepen our understanding of the size

effect of melting. In particular, a complete understanding of the melting

transition in nanocrystals cannot be obtained without a clear

understanding of enthalpy and entropy of melting, which are important

properties of melting. A physical model presented in [25, 26] for the size

261

dependent enthalpy and entropy based on Mott's expression for the

vibrational entropy of melting for metallic crystals at melting temperature

and a model for the size dependent melting [14 - 18]. The theoretical

prediction of the model for the size dependent melting enthalpy and

entropy has been found to be consistent with experimental evidences on

the metallic nanocrystals of Sn and Al, and the organic nanocrystals of

benzene, chlorobenzene, heptane and naphthalene.

In the vibrational entropy of melting of a bulk crystal, has been

represented by Mott [25], and from the bulk, the maximum phonon

wavelength is truncated by the crystallite size and the surface region has

enhanced configurational enthalpy and entropy. With decreasing

crystallite size the relative significance of all these effects increases. They

represented the variation of enthalpy and entropy of melting as a

function of crystallite size by formally extending Mott’s estimation of the

melting entropy of an infinite metallic crystal to metallic and organic

crystals of finite size and by using an expression for the size dependant

melting temperature. Reasonable agreement between the model

prediction and the experimental data of melting enthalpy and entropy for

nanocrystals of metallic Sn and Al and organic benzene, chlorobenzene,

hepten and naphthalene has been found.

6.4 Conclusion

In this chapter author has explained stage wise weight loss events

carried out from TG / DTA set up SnSe nanoparticles doped by iron at

0, 0.5, 1.0, 1.5, 2.0 and 2.5 concentrations.

Effect of synthesis doping of copper concentration level in iron doped

SnSe nanoparticles on different mass loss / weight loss event have

been represented and explained. Similar effect on the DTA peak

262

position has been presented for pure and copper doped SnSe

nanoparticles.

Author has evaluated thermal parameters i.e. energy of activation Ea,

entropy of activation ΔS, enthalpy of activation ΔH and Gibbs free

energy ΔG by using Broido and Coats – Redfern (C- R) model for all

the aqueous solution technique’s as synthesized pure and iron doped

SnSe nanoparticles. Similarly effect of doping of iron concentration

level in SnSe nanoparticles on all the as mentioned thermal

parameters for iron doped SnSe nanoparticles has been investigated.

The current most challenging tasks in research and developments are

property characterization and device fabrication. Here author

successfully investigated and represented size dependent controls of

thermal parameters / thermal characteristics of nanoparticles. There

are the key steps in the development of nanoscience and

nanotechnology: materials preparation, property characterization and

device fabrication.

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