a compendium of tech stuff

Feb 1, 2013

On 3:42 PM by Lalith Varun   2 comments
   

     Thermogravimetric Analysis is a material characterization technique in which the mass of a substance is monitored as a function of temperature or time under controlled temperature and atmosphere.
This analysis is carried out primarily to determine
1) the composition of materials such as organic and inorganic content in the sample,
2) to predict their thermal stability at high temperatures such as vaporization, sublimation, absorption, adsorption, desorption, chemisorption, reaction kinetics etc.

     A plot of mass change versus temperature, called thermogravimetric (TG) curve is plotted which helps in determining he extent of purity of analytical samples and the mode of their transformations within the specified temperature range.

A Thermogravimetric analyzer makes use of a thermobalance, whose basic components are,
1) Balance
2) Furnace
3) Programmer unit for temperature measurement and control
4) Recording unit for mass and temperature changes

                                                  Block Diagram of a Thermobalance

     The basic requirements of a Balance are accuracy, sensitivity, reproducibility and capacity. There are 2 types of balances
1) Null type balance which consists of a sensor which detects the deviation from the null point and restores the balance to its null point by means of a restoring force.
2) Deflection balance which converts the deflection of balance beam deflection into a suitable mass by means of photographic recording or recording electrical signals or using an electro-chemical device.

     The Furnace provides linear heating over a wide range of operating temperaures, typically -150 deg. Celsius to about 2000 deg. Celsius depending on the requirement.

     Temperature measurement and regulation is done with the help of thermocouples. Usually 2 thermocouples are used, where one records the temperature change, the other actuates the control system.

     The recording unit makes use of a microprocessor which allows for digital data acquisition and processing using a personal computer.

The factors affecting the precision and accuracy of the TG curve are
1) Furnace heating rate
2) Sensitivity of the sensors
3) Recording speed
4) Amount of sample
5) Particle size
6) Heat of reaction etc.

     All the above factors are to be taken into consideration and the instrument should be properly calibrated before performing experiments. It is a very efficient method in material characterization and is widely used in the analysis of polymers, plastics, composites, laminates, pharmaceuticals, rubber, petroleum, food, adhesives etc.