Upload
letram
View
214
Download
0
Embed Size (px)
Citation preview
definitions
“…. a technique in which the mass of a substance is
measured as a function of temperature, while the
substance is subjected to a controlled temperature
programme.”
“Controlled temperature programme” can mean:
• heating and/or cooling at a linear rate (by far commonest)
• isothermal measurements
• combinations of heating, cooling and isothermal stages
• other, more modern approaches, in which the temperature
profile is modified according to the behaviour of the sample.
instrumentation
GAS-TIGHT
ENCLOSURE
SAMPLE
HEATER
TEMPERATURE PROGRAMMER
BALANCE
CONTROLLER
POWER FURNACE TEMP.
SAMPLE TEMP.
WEIGHTGAS IN
physical limitations on the heating process
CONVECTION
THROUGH
SURROUNDING
ATMOSPHERE
CONDUCTION
THROUGH SAMPLE
PAN AND
INSTRUMENT
RADIATION FROM
FURNACE WALL
EXCHANGE OF GASES:
REACTING GASES IN,
PRODUCTS OUT
INDICATION OF SAMPLE
TEMPERATURE
factors that affect the results
A) INSTRUMENTAL
• heating rate
• furnace atmosphere and flow-rate
• geometry of pan and furnace
• material of pan
B) SAMPLE-RELATED
• mass
• particle size
• sample history/pre-treatment
• packing
• thermal conductivity
• heat of reaction
For a given instrument, careful standardisation of experimental
procedures leads to highly reproducible results.
sources of error
A) MASS
• Classical buoyancy
• Effect temp. on balance
• convection and/or turbulence
• viscous drag on suspension
B) TEMPERATURE
Temperature calibration difficult to carry out accurately.
Many methods exist, but none totally satisfactory.
Best accuracy from simultaneous TG-DTA or TG-DSC instrument.
These are lumped together as the “buoyancy”
correction, and if significant, can be allowed
for by a blank run
NOISY OR ERRATIC RECORDS
CAN ARISE FROM:
• static
• vibration
• pressure pulses in lab.
• uneven gas flow
calibration
MASS - Use standard weights.
Use standard samples to check operation, but unwise to use them
as weight standards.
TEMPERATURE -
Four approaches:
• Observe deflection on Temperature/time curve
• Curie-point standards
• Drop-weight methods
• In simultaneous-type units, use melting standards
• DO NOT use decomposition events to define temperature.
calibration using curie point
weig
ht
ch
an
ge/
mg
temperature /°C
350 370
0.0
0.2
Nickel metal
3°C/min. in nitrogen
summary
PROCESS WEIGHT GAIN WEIGHT LOSS
Ad- or absorption
Desorption, drying
Dehydration, desolvation
Sublimation
Vaporisation
Decomposition
Solid-solid reactions (some)
Solid-gas reactions
Magnetic transitions
recommended reading
D. M. Price, D. J. Hourston & F. Dumont, “Thermogravimetry of Polymers”, R. A.
Meyers (Ed.), Encyclopedia of Analytical Chemistry, John Wiley & Sons Ltd.,
Chichester (2000) pp. 8094-8105.
G. R. Heal,”Thermogravimetry & Derivative Thermogravimetry”, in P.J. Haines (ed.)
Principles of Thermal Analysis & Calorimetry, ch. 4, Royal Society of Chemistry,
Cambridge (2002) pp. 10-54.
C. M. Earnest (Ed.), Compostional Analysis by Thermogravimetry, ASTM STP 97,
American Society for Testing and Materials (1988).