definitions“…. a technique in which the mass of a substance ismeasured as a function of temperature, while thesubstance is subjected to a controlled temperatureprogramme.”“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 temperatureprofile is modified according to the behaviour of the sample.
instrumentation GAS IN BALANCE WEIGHT CONTROLLERGAS-TIGHTENCLOSURE SAMPLEHEATER SAMPLE TEMP. POWER FURNACE TEMP. TEMPERATURE PROGRAMMER
example curveMass (%) in green, rate of mass loss (%/°C) in blue.
physical limitations on the heating process EXCHANGE OF GASES: REACTING GASES IN, PRODUCTS OUT CONVECTION THROUGH SURROUNDING ATMOSPHERE RADIATION FROM FURNACE WALL CONDUCTION THROUGH SAMPLE PAN AND INDICATION OF SAMPLE INSTRUMENT TEMPERATURE
factors that affect the resultsA) INSTRUMENTAL B) SAMPLE-RELATED• heating rate • mass• furnace atmosphere and flow-rate • particle size• geometry of pan and furnace • sample history/pre-treatment• material of pan • packing • thermal conductivity • heat of reaction For a given instrument, careful standardisation of experimental procedures leads to highly reproducible results.
effect of heating rate10 mg samples of PTFE, heated at 2.5, 5, 10 and 20 °C/min in nitrogen
isoconversion kinetic treatment
effect of atmosphereCaC 2 CO 4 .H 2 O in air and nitrogen
sources of errorA) MASS NOISY OR ERRATIC RECORDS• Classical buoyancy CAN ARISE FROM:• Effect temp. on balance • static• convection and/or turbulence • vibration• viscous drag on suspension • pressure pulses in lab.These are lumped together as the “buoyancy” • uneven gas flowcorrection, and if significant, can be allowedfor by a blank runB) TEMPERATURETemperature calibration difficult to carry out accurately.Many methods exist, but none totally satisfactory.Best accuracy from simultaneous TG-DTA or TG-DSC instrument.
calibrationMASS - Use standard weights.Use standard samples to check operation, but unwise to use themas 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 sample thermocouple
calibration using drop weight
calibration using curie pointweight change/ mg 0.2 Nickel metal 0.0 3°C/min. in nitrogen 350 370 temperature /°C
calibration using TG-DTA10mg Gold (99.999%), 10°C/min, alumina pan, air.
summary PROCESS WEIGHT GAIN WEIGHT LOSSAd- or absorption Desorption, drying Dehydration, desolvation Sublimation Vaporisation Decomposition Solid-solid reactions (some) Solid-gas reactions Magnetic transitions
recommended readingD. 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).