instrumentation of thermogravimetry

1. INSTRUMENTATION OF
THERMOGRAVIMETRY
BY:-
M. Jawad Ahmad

2. •Thermogravimetric analysis (TG) is the study of weight changes
of a specimen as a function of temperature. The technique is
useful strictly for transformations involving the absorption or
evolution of gases from a specimen consisting of a condensed
phase.
•A plot of mass versus temperature (thermogravimetric curves or
TG curves) permits evaluation of thermal stabilities, rate of
reaction, reaction processes, and sample composition.
* Measurements of changes in sample mass with temperature are
made using thermobalance. The balance should be in a suitably
enclosed system so that the atmosphere can be controlled.
Thermogravimetry

3. General considerations
Suitable samples for TG are solids that undergo one of the two
general types of reaction:
Reactant(s)  Product(s)+Gas (a mass loss)
Gas+Reactant(s)  Product(s) (a mass gain)
Processes occurring without change in mass (e.g., the melting of a
sample) obviously cannot be studied by TG.

4. 2.1 Instrumentation
LINSEIS L81

5. Thermogravimetric instrumentation should include several basic
components to provide the flexibility necessary for the
production of useful analytical data:
a) A balance,
b) A heating device,
c) A unit for temperature measurement and control,
d) A means for automatically recording the mass and
temperature changes,
e) A system to control the atmosphere around the sample.

6. Two typical designs of the thermobalance are shown in the following:
The Thermobalance

7. The furnace is normally an electrical resistive heater;
Some basic requirements of the heating chamber are :
be non-inductively wound
be capable of reaching 100 to 200°C above the maximum desired working
temperature
have a uniform hot-zone of reasonable length
reach the required starting temperature as quickly as possible
not affect the balance mechanism through radiation or convection
In order to overcome the problem of possible temperature gradient, infrared
or microwave radiation have been used in some equipment.
infrared heating : use halogen lamp, temperature up to 1400°C, heating rate
can be as high as 1000°C/min, accuracy is about ±0.5°C.
Microwave heating : large sample can be used because uniform heating
generated within sample but temperature measurement and power control are
difficult.
The Heating Chamber

8. Temperature measurement and calibration
Platinum resistance thermometers or
thermocouples are used for temperature
measurement.
Large difference between sample
temperature (Ts) and furnace temperature
(Tf) can exist, sometime as high as 30°C.
Calibration is thus needed.
The difference or lag is more marked
when operating in vacuum or in fast
flowing atmosphere and with high heating
rate.

9. 3). THE FURNACE:-
The furnace and control system should be designed to
produce a linear heating over the whole working
temperature range of furnace. The choice of furnace
heating element and furnace depend upon the
temperature range being studied.

10.  For 11000C; Nichrome wire or ribbon
 For temperature between 1100 0C to 1500 0C; Alloy
of platinum and rhodium
 Using above alloy having Rhodium content of 40
percent use for upto 17500C.
 Above 1750 0C tugnsten or molybdenum may be
employed in reducing atmosphere.

11. 4).TEMPERATURE MEASUREMENT:-
 This can be done in a number of ways. The most
common method is thermocouple.
 For measuring 11000C; chromel or alumel.
 For higher temperature; tungsten or rhenium.

12.  Thermocouple is placed near the sample container
and it has no contact with sample container.
 The sample is kept inside the sample holder but not in
contact with it.
 The thermocouple is placed either in contact with
sample or with the sample container.
 Last one is the best arrangement of sample
temperature detection.

13. 5). RECORDER:-
The recording system are mainly of two types:
1) Time-base potentiometric strip chart recorder
2) X-Y recorders.

14.  Light-beam-galvanometer , photographic paper
recorders or one recorder with two or more pens are
used.
 The heating rate of the furnace for linearity.

15.  In the X,Y,recorders ,we get curves having plot of
weights directly against temperatures.
 Weight change against temperature or time
 %age mass change against temperature or time

16. 6). THERMOBALANCE:-
A good thermobalance should have:
 Capable of recording continuously.
 Cover wide range of temperature.
 Temperature recorder accuracy of ±1%.
 Weight loss recorder accuracy of ±1%.

17.  Heating rate should be linear.
 Radiation & convection currents, and magnetic
effects due to furnace heaters must not effect the
weighting system.
 Sensitivity of balance should be commensurate with
sample size.

18.  Not any chemical attack of volatile products on the
apparatus.
 Crucible located in hot zone.
 Balance protected from the furnace.

19. ADVANTAGES OF TG
 Small sample weight (1-10 mg)
 Cooling time is very short
 Thermobalance can be useful for isothermal studies
 Can hold at 1000C without any balance drift.