lezione__analisi termiche.pdf

7/28/2019 lezione__analisi termiche.pdf

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Introduction to Thermal Analysis Methods

Thermal analysis refers to a variety of techniques in which physical property of a sampleis continuously measured as a function of temperature, whist the sample is subjected to

a pre-determined temperature profile.

Thermal Analysis techniques are used in virtually every area of modern science and

technology. The basic information that these techniques provide, such as crystallinity,

specific heat and expansion, are relied on heavily for the research and development of

new products. Thermal analysis techniques also find increasing use in the area of quality

control and assurance, where demanding requirements must be met in an increasingly

competitiveworld.

And of course thermal analysis instruments are used in universities for applications

ranging from basic undergraduate studies to the most sophisticated ostgraduate

research.


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Differential Thermal Analysis (DTA)

the temperature difference between a sample and an inert reference material,T =TS - TR, is measured as both are subjected to identical heat treatments

Differential Scanning Calorimetry (DSC)

the sample and reference are maintained at the same temperature, even during athermal event (in the sample)

the energy required to maintain zero temperature differential between the sample andthe reference, q/t, is measured

Thermogravimetric Analysis (TGA)

the change in mass of a sample on heating is measured

A group of techniques in which a physical property is measured as a function of temperature,while the sample is subjected to a predefined heating or cooling program.


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N2 flow

Ptthermopile

Sample Reference

Ptthermopile

T

1T2

heater heater

W


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Applicat ions Applicat ions:

characteristic temperaturesidentificationglass transitionsmelting and crystallization behaviorheat of melting and crystallizationpuritycompatibilitypolymorphismsolid-liquid ratiospecific heat capacityreaction behaviorheat of reaction

reaction kineticsoxidative stabilitythermal stability

Differential Thermal Analysis (DTA)

Differential Scanning Calorimetry (DSC))

DSC and DTA are techniques by which thedifference in heat flow to or from a sampleand to or from a reference is monitored as afunction of temperature or time, while thesample is subjected to a controlled

temperature program.



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Applications of DTA & DSC (1) Solid State Transitions(2) Solid State Reactions(3) Solid State Decompositions

Crystallinity

DSC can determine the presence & concentration of a crystalline phase in asolid, as well as the melting point of the crystals.

The Glass Transit ion, Tg

DSC can detect the glass transition of an amorphous materials, such aspolymer.

Characterization of Alloys & Composite

Aging and Degradation

Phase Diagram of alloys



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Differential Thermal Analysis

sample holder sample and reference cells (Al)

sensors Pt/Rh or chromel/alumel thermocouples one for the sample and one for the reference joined to differential temperature controller

furnace alumina block containing sample and reference

cells

temperature controller controls for temperature program and furnace

atmosphere

samplepan

inert gasvacuum

referencepan

heatingcoil

alumina block

Pt/Rh or chromel/alumelthermocouples


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Differential Thermal Analysis

advantages:

instruments can be used at very hightemperatures

instruments are highlysensitive flexibility in crucible volume/form Characteristic transition or reaction

temperatures can be accurately determined

disadvantages:

uncertainty of heats of fusion, transition, orreactionestimations is 20-50%

samplepan

inert gasvacuum

referencepan

heatingcoil

alumina block

Pt/Rh or chromel/alumelthermocouples


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DSC differs fundamentally from DTA in that the sample and reference are bothmaintained at the temperature predeterminedbythe program.

during a thermal event in the sample, the system will transfer heat to or from the

sample pan to maintain the same temperature in reference and sample pans two basic types of DSC instruments: powercompensation and heat-flux

Differential Scanning Calorimetry

power compensation DSC heat flux DSC


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Power Compensation DSC

sample holder Al or Pt pans

sensors Pt resistance thermocouples separate sensors and heatersfor the sample and reference

furnace separate blocks for sample and reference cells

temperature controller differential thermal power is supplied to the heaters to maintain the temperature

of the sample and reference atthe programvalue

samplepan

T =0

inert gasvacuum

inert gasvacuum

individualheaters

controller DP

referencepan

thermocouple


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sample holder sampleand reference are connectedby

a low-resistance heat flow path Al or Pt pans placed on constantan disc

sensors chromel-constantan area thermocouples (differential heat flow) chromel-alumel thermocouples (sample temperature)

furnace one block forboth sample and reference cells

temperature controller the temperature difference between the sample and reference is converted to

differential thermal power, q/t, which is supplied to the heaters to maintain thetemperature of the sample and reference atthe programvalue

Heat Flux DSC

samplepan

inert gasvacuum

heatingcoil

referencepan

thermocouples

chromel wafer

constantan

chromel/alumel

wires


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Modulated DSC Heating ProfileModulated DSC (MDSC)

introduced in 1993; heat flux design sinusoidal (or square-wave or sawtooth)

modulation is superimposed on theunderlyingheating ramp

total heat flow signal contains all of thethermal transitions of standardDSC

Fourier Transformation analysis is usedto separate the total heat flow into its twocomponents:

heat capacity (reversing heat flow) kinetic (non-reversing heat flow)glass transition crystallization

melting decompositionevaporation

enthalpic relaxationcure


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Analysis of Heat-Flow in Heat Flux DSCtemperature difference may be deduced by considering the heat flow paths in the DSC

system

thermal resistances of a heat-flux system change with temperature

the measured temperature difference is not equal to the difference in temperaturebetween the sample and the reference

Texp TS TR

temperature

Tfurnace

TRP

TR

TS

TSP

heating block

TR TS

reference

sample

TL

thermocouple is not in physicalcontact with sample


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DSC Calibration

baseline evaluation of the thermal resistance of the

sample and reference sensors measurements over the temperature range of

interest

2-step process the temperature difference of two empty

crucibles is measured the thermal response is then acquired

for a standard material, usuallysapphire, on both the sample andreference platforms

amplified DSC signal is automatically varied with temperature to maintain a constantcalorimetric sensitivity with temperature


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heat flowuse of calibration standards of known heat capacity, such as sapphire, slow accurate heating

rates (0.52.0 C/min), and similar sample and reference pan weights

DSC Calibrationtemperature goal is to match the melting onset temperatures indicated by the furnace thermocouple

readouts to the known melting points of standards analyzedby DSC should be calibrated as close to the desired temperature range as possible

calibrants

high purity accurately known enthalpies thermally stable light stable (hn) nonhygroscopic unreactive (pan, atmosphere)

metals In 156.6 C; 28.45 J /g Sn 231.9 C

inorganics KNO3 128.7 C KClO4 299.4 Corganics polystyrene 105 C benzoic acid 122.3 C; 147.3 J /g


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Sample Preparation

accurately-weigh samples (~3-20 mg) small sample pans (0.1mL) of inert or treated metals (Al, Pt, Ni, etc.) several panconfigurations, e.g., open , pinhole, orhermetically-sealedpans the same material and configuration shouldbe usedfor the sample and the reference material should completely cover the bottom of the pan to ensure good thermal

contact avoid overfilling the pan to minimize thermal lag from the bulk of the material to the

sensor

* small sample masses and lowheating rates increase

resolution, but at the expenseof sensitivity

Al Pt alumina Ni Cu quartz


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Typical Features of a DSC Trace for a Polymorphic System

sulphapyridine

endothermic eventsmelting

sublimationsolid-solid transitions

desolvationchemical reactions

exothermic eventscrystallization

solid-solid transitions

decompositionchemical reactions

baseline shiftsglass transition


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Recognizing Artifacts


mechanicalshock of

measuring cellsample topples

over in pansample pandistortion

shifting ofAl pan

cool air entryinto cell

electrical effects,power spikes, etc.

RT changes intermittantclosing of hole

in pan lid

sensorcontamination

burst ofpan lid


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-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

HeatFlow

(W/g)

0 50 100 150 200 250 300 350

Temperature (C)

FormI FormII Variable Hydrate Dihydrate Acetic acid solvate

Exo Up

Form III

Form IForm II

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

HeatFlow

(W/g)

0 50 100 150 200 250 300 350

Temperature (C)

FormI FormII Variable Hydrate Dihydrate Acetic acid solvate

Exo Up

Form III

Form IForm II

Thermal Methods in the Study of Polymorphs and Solvates

polymorph screening/identification

thermal stability melting crystallization solid-state transformations desolvation

glass transition sublimation decomposition

heat flow heat of fusion heat of transition

heat capacitymixture analysis chemical purity physical purity (crystal forms, crystallinity)

phase diagrams eutectic formation (interactions with other molecules)



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Definition of Transition Temperature

157.81C

156.50C28.87J /g

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

HeatFlow(W/g)

140 145 150 155 160 165 170 175

Temperature (C)

Exo Up Universal V3.3B TA Instruments

extrapolatedonset temperature

peak meltingtemperature



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Melting Processes by DSC

pure substances

linearmelting curve

melting point defined byonset temperature

impure substances

concave meltingcurve

melting characterizedatpeak maxima

eutectic impurities mayproduce a secondpeakmelting with decomposition

exothermic

endothermic

eutecticmelt



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Glass Transitions

second-order transition characterized bychange in heat capacity (no heat absorbed orevolved)

transition from a disordered solid to a liquidsolid

appears as a step (endothermic direction) inthe DSC curve

a gradual volume or enthalpy change may occur, producing an endothermic peaksuperimposed on the glass transition



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Enthalpy of Fusion


157.81C

156.50C28.87J /g

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

HeatFlow(W/g)

140 145 150 155 160 165 170 175

Temperature (C)



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Enthalpy of Fusion by DSCsingle (well-defined) meltingendotherm

area under peak minimal decomposition/sublimation readily measured for high melting polymorph

can be measured for low melting polymorph

multiple thermal events leadingto stable meltsolid-solid transitions (A to B) fromwhich the transition enthalpy (HTR) can be measured*

HfA =HfB - HTR

* assumes negligible heat capacity difference between polymorphs over temperatures of interest

HfA = area under all peaks from B to the stable melt

crystallization of stable form (B) from melt of (A)


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Purity by DSC

eutectic impurities lower the meltingpoint of a eutectic system

purity determination by DSC basedonVant Hoff equation

applies to dilute solutions, i.e., nearlypuresubstances (purity 98%)

1-3 mg samples in hermetically-

sealed pans are recommended polymorphism interferes with puritydetermination, especially when atransition occurs in the middle of themeltingpeak

melting endotherms as a function of purity.

benzoic acid

97%

99%

99.9%

Plato, C.; Glasgow, J r., A.R.Anal. Chem., 1969, 41(2), 330-336.

Tm =To -.

HoRTo2c 1

f


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Effect of Heating Rate

many transitions (evaporation, crystallization,decomposition, etc.) are kinetic events

they will shift to higher temperature whenheatedata higher rate

the total heat flow increases linearly withheating rate due to the heat capacity of thesample

increasing the scanning rate increasessensitivity, while decreasing the scanning rateincreases resolution

to obtain thermal event temperatures close tothe true thermodynamic value, slow scanningrates (e.g., 15 K/min) shouldbe used DSC traces of a low melting polymorph collected at

four different heating rates. (Burger, 1975)



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Effect of Phase Impurities

Lot A: pure low melting polymorph melting observed Lot B: seeds of high melting polymorph induce solid-state transition below the melting temperature of the

low melting polymorph

2046742FILE#022511DSC.1

2046742FILE#022458 DSC.1 Form II ?

-5

-4

-3

-2

-1

0

HeatFlow(W/g)

80 130 180 230 280

Temperature (C)Exo Up Universal V3.3B TA Instruments

Lot A - pure

Lot B - seeds

lots A and B of lower melting polymorph (identical by XRD) are different by DSC



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Polymorph Characterization: Variable Melting Point

lots A and B of lower melting polymorph (identical by XRD) appear to have a variablemelting point

-1.1

-0.9

-0.7

-0.5

-0.3

-0.1

0.1

HeatFlow(W/g)

110 120 130 140 150 160 170 180

Temperature (C)

DSC010622b.1 483518 HCL (POLYMORPH 1)DSC010622d.1 483518 HCL


Lot A

Lot B

although melting usually happens at a fixed temperature, solid-solid transition temperaturescan vary greatly owing to the sluggishness of solid-state processes



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Measurement of Glass Transition Temperature (Tg) by DSC and Rate Effects



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TGA examines the process of weight changes as a function of time, temperature, and

other environment conditions that may be created within the apparatus.


TG or TGA ---- Thermal Gravimetric Analysis


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thermobalance allows for monitoringsample weight as a function of temperature

weight calibration using calibrated weights

temperature calibration based onferromagnetic transition of Curie pointstandards (e.g., Ni)

larger sample masses, lower temperaturegradients, and higher purge rates minimizeundesirable buoyancy effects


TG or TGA ---- Thermal Gravimetric Analysis


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The range of materials can be studied bythermal analysis Biological materials Building materials Catalysis Ceramics and Glasses

Applications of TGA:

CompositionMoisture contentSolvent contentAdditives

Polymer contentFiller contentDehydrationDecarboxylationOxidationDecomposition

m = mass changedm/dt = rate of mass change/decompositionDTG = derivative thermogravimetryDTG Peak = characteristic decomposition

temperatures identification



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MAJ OR FACTORS AFFECTING THERMOGRAVIMETRY

Mass Temperature

Effect of Atmosphere on Mass Heating rate

Atmospheric turbulence Thermal conductivity

Condensation and reaction Enthalpy of the process

Electrostatic and magnetic forces Sample, furnace, and sensor arrangement

Electronic drift Electronic drift



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Effect of Atmosphere on Mass

the change in density of the gas phase with temperature.

air Wspecimen

example:dry air =1.3 mg/cm

3 , 25 oCdry air =0.3 mg/cm

3 , 1000 oC

For : 20 mg sample ( =1.0g/ cm3 )25 oC ----- 1000 oCa 0.1 wt% loss will be introduced



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(1) AdditivesOxidation Weight gain Temperature ~ time AntiOxidation additive concentration

(2) Extent of CureResidual Weight loss Degree

of cure

(3) Thermal Stability

(4) Reactivity & PhaseEquilibration in Ceramics



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Example of using TGA to identify the composition of a PP/PE blend



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developmentof hyphenatedtechniques for simultaneous analysis

TG-DTA

TG-DSC

TG-FTIR

TG-MS

15.55%(0.9513mg)

24.80C100.0%

179.95C84.45%

-1.8

-0.8

0.2

1.2

2.2

3.2

4.2

TemperatureDifference(V/mg)

-40

0

40

80

120

Weight(%)

20 70 120 170 220 270

Temperature (C)


Hyphenated Techniques

thermal techniques alone are insufficient to prove the existence of polymorphsand solvates

other techniques should be used, e.g., microscopy, diffraction, and spectroscopy

TG-DTA trace of sodium tartrate


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Best Practices of Thermal Analysis

small sample size

good thermal contact between the sample and the temperature-sensingdevice

proper sample encapsulation

starting temperature well belowexpected transition temperature

slow scanning speeds

proper instrument calibration

use purge gas (N2 or He) to remove corrosive off-gases

avoiddecomposition inthe DSC


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