Lectrure Thermal Analysis · Thermal Analysis (TA) is a group of techniques that study the...
Transcript of Lectrure Thermal Analysis · Thermal Analysis (TA) is a group of techniques that study the...
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Thermal Analysis
TGA / DTA
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Outline
Definitions
What is thermal analysis?
Instrumentation & origin of the TGA-DTA signal.
TGA
DTA
Basics and applications
Phase diagrams & Thermal analysis • Thermal analysis,an experimental method to determine phase diagrams.
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Definition of the field of Thermal Analysis (TA)
Thermal Analysis (TA) is a group of techniques
that study the properties of materials as they
change with temperature
Nomenclature of Thermal Analysis 00
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Thermal analysis
In practice thermal analysis gives properties like;
enthalpy, thermal capacity, mass changes and the
coefficient of heat expansion.
Solid state chemistry uses thermal analysis for studying
reactions in the solid state, thermal degradation
reactions, phase transitions and phase diagrams.
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Differential scanning calorimetry (DSC): heat difference
Pressurized TGA (PTGA): mass changes as function of pressure.
Thermo mechanical analysis (TMA): deformations and dimension
Dilatometry (DIL): volume
Evolved gas analysis (EGA): gaseous decomposition products
Often different properties may be measured at the same time:
TGA-DTA, TGA-EGA
Thermal analysis ... Includes several different methods. These are distinguished from one another by the property which
is measured.
Thermogravimetric analysis (TGA): mass
Differential thermal analysis (DTA): temperature difference
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Instrumentation
&
origin of the TGA-SDTA signal
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TGA - SDTA Mettler - Toledo
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A modern TGA - DTA
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Balance arm
Operating range: - 200 - 1600 ºC
Heating rate: up to 100 ºC/min
Typical heating rate: 10 – 20 ºC/min
Furnace components
Heating resistor
Rective purge gas inlet
Furnace thermoelement
Heating resistor
Sample crucible
Sample holder
sDTA thermoelement
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Heat transfer from crucible to recording
microbalance & thermo elements
platinum
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Origin of the TGA-DTA signal Schematic diagram showing the different temperatures in the DTA during a thermal process.
Tp, Tr
Ts
Reaction temperature
Peak onset
Time
Tp: program temperature
Tr: reference temperature = Tc furnace temperature
Ts: sample temperature
Tem
pera
ture
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Origin of the TGA-DTA signal
Endothermic peak Tp, Tr
Ts
Reaction temperature
Peak onset
Tp: program temperature
Tr: reference temperature
Ts: sample temperature
Temperature
Tem
pera
ture
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Origin of the TGA-DTA signal
Heating rate 50 K / min
Tc
∆T
∆t
Tp
T
s
∆t = Tau lag
Melting point: Indium
Tp: program temperature
Tc: furnace temperature
Ts: sample temperature
Time / min
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TGA
Thermo Gravimetric Analyis
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TGA, Basics
Measures changes in weight in relation to changes in temperature.
The measured weight loss curve gives information on:
• changes in sample composition
• thermal stability
• kinetic parameters for chemical reactions in the sample
A derivative weight loss curve can be used to tell the point at which
weight loss is most apparent
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TGA; Phenomena causing mass changes
Physical
Gas adsorption
Gas desorption
Phase transitions
Vaporization
Sublimation
Chemical
Decomposition
Break down reactions
Gas reactions
Chemisorption (adsorption by means of chemical
instead of physical forces)
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TGA: Applications
Characterization of Thermal stability
Material purity
Determination of humidity
• Examination of » Corrosion studies (e.g. oxidation or reactions with reactive
gases)
» Gasification processes
» Kinetic processes
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Typical temperature-time programs
Constant heating rate Gradually isothermic Isothermic
T T T
t t t
Experimental Sample size: 1 – 100 mg (typically 5 – 20 mg)
Heating / cooling rate: 1 – 50 ºC / min
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Exhibits three weight losses with temperature in an inert
atmosphere (e.g. N2).
CaC2O4 • H2O → CaC2O4 → CaCO3 → CaO - H2O - CO - CO2
TGA Ex. Decomposition of calcium oxalate monohydrate
Calcium oxalat monohydrat, a standard material often
used to demonstrate TGA performance.
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TGA Ex. Decomposition of calcium oxalat monohydrate
TG
DTG
- H2O
- CO
- CO2
Absolute confirmation of the decomposition process is possible when the gaseous
by products are identified as they evolve, eg. by mass spectrometry (MS).
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TGA
Common gaseous components originating from inorganic
materials that decompose before the melting point:
H2O, CO, CO2, SOx, NOx, Cl2, F2, CH3OH, etc.
Also some chemical reactions in solid phase result in gaseous
weight loss ex.
Na2CO3 (s) + SiO2 (s) → Na2SiO3 (s) + CO2 (g)
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Factors affecting the TG curve
Heating rate
Sample size
Increases the temperature at which
sample decomposition occurs.
Particle size of sample
Packing
Crucible shape
Gas flow rate
Affects the progress of ther reaction
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DTA
Differential Thermal Analysis
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Any temperature difference between sample and
reference is recorded.
In this technique the heat flow to the sample and reference
remain the same rather than the temperature.
DTA, Basics
The material under study and an inert reference are made
to undergo identical thermal cycles.
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Peak orientation ↑↓ in DTA thermogram
depends on Instrument manufacturer
exothermic
endothermic
Crystallization
Melting
DTA, Basics
The differential temperature is then plotted against time, or
against temperature (DTA curve or thermogram).
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DTA; Phenomena causing changes in heat /
temperature
Physical
Adsorption (exothermic)
Desorption (endothermic)
A change in crystal structure (endo – or exothermic)
Crystallization (exothermic)
Melting (endothermic)
Vaporization (endothermic)
Sublimation (endothermic)
Chemical
Oxidation (exothermic)
Reduction (endothermic)
Break down reactions (endo – or exothermic)
Chemisorption (exothermic)
Solid state reactions (endo – or exothermic)
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Evaluation and interpretation of DTA curves
Onset
Typical data obtained from DTA peak evaluation
• Onset - melting
• Endset
• Integral - enthalpy ∆h
• Peak temp - melting
• Peak height
• Peak width
5 K /min, 1.3 mg
5 K /min, 6, 3 mg
10 K /min, 6, 3 mg
20 K /min, 6, 3 mg
Temperature
Peak temperature is affected by heating rate & sample mass, but not by ∆h (enthalpy) and T onset.
TS
am
ple
–T
Refe
ren
ce
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TGA- DTA Keys for successful experimental practice
Raw materials should be of high purity.
Fine -grained powder should be used to achieve greater
contact area and better equilibrium conditions.
The time at any temperature must be sufficiently long in
order to permit completeness of reactions.
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TGA- DTA Keys for successful experimental practice
Factors affecting the heat transfer, Tau lag & signaling
Crucible
Material
Mass
Volume
Heat capacity
Sample
Mass
Heat capacity
Heat conductivity
Atmosphere
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Phase diagrams & Thermal analysis
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Phase Diagram
A phase diagram show conditions at which thermodynamically
distinct phases can occur at equilibrium.
It is determined experimentally by recording cooling rates over a range of compositions.
Phase transitions occur along lines of equilibrium (=phase
boundaries).
Solidus = Temp. below which the substance is stable in the solid state.
Liquidus = Temp. above which the substance is stable in a liquid state.
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High temperature microscopy
High temperature X-ray diffraction
Measurement of electrical conductivity as function of temperature.
Salt mixtures: solid salts have low conductivity, melts have
high.
Experimental methods for determining
phase diagrams
Thermal analysis
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How to build a phase diagram
Liquid
Melting point
of pure B
Melting point
of pure A
Liquidus
100 % A Composition 100 % B
Eutectic point
Tem
pe
ratu
re
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Constructing phase diagrams by experimental methods:
A) from cooling curves
Phase diagram Cooling curves
time
Method: Temperature
tem
per
ature
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Constructing phase diagrams by experimental methods:
B) from DTA curves
Phase diagram Melting peaks
Temperature
tem
per
ature
TS
am
ple
–T
Refe
ren
ce
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Summary
Thermal analysis gives information about changes in material properties as
function of temperature.
Several different TA methods exist; focus on TGA - DTA
Combining the two techniques (TGA-DTA) - comprehensive study of a
materials thermal behaviour.
► While TG only measures changes caused by mass loss, DTA also
register changes in material where no mass loss occur, e.g. crystal
structure changes, melting, glass transistion, etc.
Carefullenes required with performance of the experimental procedure to
obtain correct weight loss curves and thermograms (e.g. sample
preaparation, choice of crucible, choice of thermal program)
Origin of TG-DTA signal good to know for better understanding of measured
data.