(DSC) Differential thermal analysis

Report
Materials
Characterisation
- Thermal Analysis
• Introduction to thermal analysis
• Information obtained.
• Interpret & analyse data.
Recommended Texts
Textbooks
• Differential Scanning Calorimetry: An introduction for practitioner, by G. Hohne, Springer (1996).
• Differential Thermal Analysis: Fundamental aspects, Vol 1., edited by R.C. Mackenzie, Academic Press
(1970).
• Differential Thermal Analysis: Applications, Vol. 2, edited by R.C. Mackenzie, Academic Press (1970).
•Differential Thermal Analysis: A guide to techniques and its applications, by M. I. Pope, Heyne (1977).
Journal papers
• “Differential Scanning Calorimetry: applications in drug development”, by S-D Clas, C.R. Dalton, B.C.
Hancock, PSTT, Vol 2(8), pp311-320 (1999).
• “DSC analyses of the precipitation behaviour of two Al-Mg-Si alloys naturally aged for different
times”, by L. Zhen and S.B. Kang, Materials Letters, Vol 37, pp349-353 (1998).
Introduction to thermal analysis
Purpose: to provide quantitative information about
exothermic, endothermic and heat capacity changes as a
function of temperature and time.
Types of thermal analysis techniques:
Differential scanning calorimetry (DSC)
Differential thermal analysis (DTA)
Thermogravimetry (TG)
Thermomechanical analysis (TMA)
Definitions
• A calorimeter measures the heat into or out of a
sample.
• A differential calorimeter measures the heat of a
sample relative to a reference.
• A differential scanning calorimeter does all of the
above and heats the sample with a linear temperature
ramp.
• Endothermic heat flows into the sample.
• Exothermic heat flows out of the sample.
DSC: The Technique
• Differential Scanning Calorimetry (DSC) measures the
temperatures and heat flows associated with transitions in
materials as a function of time and temperature in a
controlled atmosphere.
• These measurements provide quantitative and qualitative
information about physical and chemical changes that
involve endothermic or exothermic processes, or changes
in heat capacity.
Conventional DSC
Empty
Sample
Metal
1
Sample
Temperature
Metal Metal
2
1
Metal
2
Reference
Temperature
Temperature
Difference =
Heat Flow
Netsch DSC 404 F1 Pegasus
•A “linear” heating profile even for isothermal methods
Technical Group Talk
Modes and principles of operation (1)
(b) Power compensated DSC: Temperature differences between the
sample and reference are ‘compensated’ for by varying the heat
required to keep both pans at the same temperature. The energy
difference is plotted as a function of sample temperature.
Modes and principles of operation (2)
(c) Heat flux DSC ultilizes a single furnace. Heat flow into both
sample and reference material via an electrically heated constantan
thermoelectric disk and is proportional to the difference in output
of the two themocouple junctions.
Modes and principles of operation (3)
(a) DTA: difference in temperature between the sample and
reference is plotted against sample temperature.
Influence of Sample Mass
0
DSC Heat Flow (W/g)
Indium at
10°C/minute
Normalized Data
-2
Onset not
influenced
by mass
15mg
10mg
4.0mg
-4
1.7mg
1.0mg
0.6mg
-6
150
6
152
154
156
158
160
Temperature (°C)
162
164
166
Effect of Heating Rate
on Indium Melting Temperature
1
Heat Flow (W/g)
0
-1
-2
heating rates = 2, 5, 10, 20°C/min
-3
-4
-5
154
6
156
158
160
162
164
Temperature (°C)
166
168
170
The temperature difference (DT) between the
reference cell (Trm) and the sample cell (Tsm) in DSC
can be expressed as:
DT = Trm – Tsm = R(dT/dt) (Cs – Cr)
Where
R = thermal resistance
dT/dt = heating rate
Cs = heat capacity of sample
Cr = heat capacity of reference
A change in the heating rate (dT/dt) results in a shift in DT
corresponding to the observed hysteresis shifts in the DSC trace
DSC: Main Sources of Errors
•Calibration
•Contamination
•Sample preparation – how sample is loaded into a pan
•Residual solvents and moisture.
•Thermal lag
•Heating/Cooling rates
•Sample mass
•Processing errors
Sample preparation
Form of sample: bulk solid, powder (pressed), liquid.
Amount of sample: 3-5mg.
DSC Pan: Al, Pt, stainless steel, Ag, Cu, Al2O3
Sample Preparation : Shape
• Keep sample as thin as possible (to minimise thermal
gradients)
• Cover as much of the pan bottom as possible
• Samples should be cut rather than crushed to obtain a
thin sample (better and more uniform thermal contact
with pan)
99
Operation procedures
• Calibration of instrument
• temperature, heat of reaction, heat capacity scale
using high purity standards (In, Sn, Bi, Pb,Au)
• Baseline correction for a given scan rate (1 - 40K/min).
• Weight samples before (and maybe after) experiment.
Beware of any contamination or reaction with specimen pans.
Enthalpic transition studied by DSC or DTA
Endothermic: Fusion, vaporization, sublimation, desorption, reduction,
decomposition, degradation. Glass transition (e.g. baseline shift).
Exothermic: Crystallization, condensation, solidification, adsorption,
precipitation, oxidation, degradation, curing of resins.
Type of DSC experiments
Dynamic heating - thermodynamic properties
Isothermal heating - kinetic parameters
What can DSC/DTA measure?
•Glass transitions
•Melting and boiling points
•Crystallisation time and
temperature
•Percent crystallinity
•Heats of fusion and reactions
•Specific heat capacity
•Oxidative/thermal stability
•Rate and degree of cure
•Reaction kinetics
•Purity
Dynamic heating
Constant heat rate mode.
(e.g. heat flow vs. temperature).
What can we characterise?
Heat Flow -> exothermic
DSC Thermogram
Cross-Linking
(Cure)
Crystallisation
Glass
Transition
Melting
Temperature
6
Oxidation
Polymer
Glass transition, crystallisation and melting
Endo
Glass transition
Crystallization
Melting
Example DSC - PET
Sample : PET80PC20_MM1 1min
Size : 23.4300 mg
Method: standard dsc heat -cool-heat
Comment : 5/4/06
DSC
File: C:...\DSC\Melt Mixed1\PET80PC20_MM1.001
Operator : SAC
Run Date : 05-Apr-2006 15 :34
Instrument : DSC Q1000 V9.4 Build 287
Tm
1.5
245.24°C
Tc
1.0
Heat Flow (W/g)
Tg
137.58°C
20.30J/g
79.70°C(I)
0.5
228.80°C
22.48J/g
81.80°C
75.41°C
Cycle 1
144.72°C
0.0
-0.5
0
Exo Down
50
100
150
Temperature (°C)
200
250
300
Universal V4.2E TA Instruments
Polymer
Glass transition temperature, Tg.
Endothermic
• Tg is characterized by a change in heat capacity, (i.e. a change in the baseline).
• Relaxation process is quantified as the volume relaxation or enthalpy relaxation.
• It is characterized in DSC by an endothermic peak or enthalpy overshoot at the Tg.
• It is highly dependent on the thermal history of the sample.
Solid to liquid transformation:
Melting or solidification
endothermic
Purity
Ts  Te 
2
RTe X
DH 0 Fi
Where Ts =sample temp
Te = melting temp of pure component
R = gas constant
X =molar fraction of impurity
DHo= enthalpy of fusion at Ts
F = fraction of sample molten.
Determination of fraction
transformed, x(t)
Peak
onset
x=
subtended area
complete area
Temp
Fig.1. Schematic diagram of a DSC peak, showing the subtended
area and the evaluation of x for a particular Temp
time.
endothermic
Heat flow
Peak
end
Metal: Au alloys
Solid to liquid transformation: Melting or solidification
DTA of Au-Ag-Cu-Zn alloy
1.4
Endo
Heat Flow (W/g)
1.2
1
0.8
0.6
0.4
0.2
0
700
Heating
Cooling
750
800
850
Temperature (C)
900
950
1000
Solid-state transformation
Precipitation in Al alloys in (A) Al-095Mg-0.85Si-0.3Mn-0.1Zr (wt%) alloys
Peaks 1and 3a: clusters of Si and Mg atoms and small ppt formation.
Peak 2: Guinier and Preston (GP) zones formation.
Peaks 3b and 4: Precipitation of b” and b’ phases
Peaks 5 and 6: Formation of Si precipitates and b phase.
Solid-state transformation: Oxidation.
5
32m
Endothermic
Heat flow (mW/mg)
0
-5
70nm
-10
-15
100nm
77nm
Exothermic
-20
500
550
600
Temperature(C)
650
700
Solid State Transformation
Crystallisation from amorphous phase
Exothermic
Amorphous (glassy)
Crystallisation
(Devitrification)
Crystalline (LRO)
Tx increases with increasing Y/Ni ratio.
Tx increases with Cu addition.
Analysis of DSC/DTA data
From dynamic heating mode.
Information:
1. Transformation temperature (e.g. onset, peak).
2. Transformation enthalpy (e.g. area under the peak).
3. Activation energy for transformation (e.g. Kissinger
analysis)
Kissinger analysis
 Tp 2   E
a C

ln
 b  RTp


Where Tp = peak temperature
b = heat rate
Ea = activation energy
R = gas constant (8.314 J/K mole)
C = constant
Plot ln ( Tp2 / b ) vs 1/Tp , gradient = Ea / R.
Kissinger analysis
DSC traces of Ni50.54Ti49.46 thin films at
different heating rates.
Plot of the Kissinger equation for the
crystallization in Ni50.54Ti49.46 thin films.
The activation energy of amorphous
Ni50.54Ti49.46 thin films was 411 kJ/mol,
Isothermal heating mode
Heating at a fixed temperature over a time interval.
(e.g. heat flow vs. time)
exothermic
Isothermal heating
Onset
Dynamic heating of
amorphous melt spun
Fe63Cr18Ti4B15 ribbons
End
Isothermal heating of
amorphous melt spun
Fe63Cr18Ti4B15 ribbons
Determination of fraction
transformed, x(t)
Peak
onset
x=
subtended area
complete area
Time
Fig.1. Schematic diagram of a DSC peak, showing the subtended
area and the evaluation of x for a particular time.
exothermic
Heat flow
Peak
end
Transformation Kinetics
The characteristic of the kinetics (e.g. fraction
transformed versus time plot) is that of the “S-curve”, i.e.
slow at first, then accelerating, then decelerating.
Johnson-Mehl-Avrami Equation
n
x ( t )  1  exp( kt )
Where
x(t) = volume fraction transformed at time t
n = Avrami exponent , which is dependent on
nucleation rate and growth rate.
k = rate constant.
Johnson-Mehl-Avrami Analysis
n
x ( t )  1  exp( kt )
Rewrite in ln form:
ln(-ln(1-x)) = ln k +nln(t)
Plot ln(-ln(1-x)) vs ln(t)
• Straight line with a gradient n.
• Intercept is ln k
Arrhenius equation
Rate constant, k is dependent on temperature
according to the Arrhenius equation given by:
k = A exp (-Ea / RT)
where A = pre- exponential factor, Ea = activation energy,
R = gas constant and T = temperature (K)
Re-write in ln form, gives:
Ln k = ln A - Ea/RT.
Plotting ln k vs 1/T
• Straight line with a slope of Ea/R
• Determine activation energy Ea.
Summary
• Operating principle of DSC, DTA.
• Dynamic and Isothermal heating mode.
• Interpretation of data to extract information
such as transition temperature, activation
energy, kinetics etc.

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