What is Thermogravimetric Analysis...???

   

     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.

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What is the effect of Catalysts on Ignition of Ammonium Perchlorate based propellants...???

     A Catalyst is a substance that is used in small amounts relative to the reactants that modifies the rate of reaction without it being consumed in the reaction and this process is called Catalysis. Catalysts that accelerate the reaction are called positive catalysts while the ones that slow down are called inhibitors. Catalysts react with one or more reactants to form intermediate compounds, which on further reaction gives the final products and regenerating the catalyst during this process.

The action of catalysts have been proposed by G. S. Pearson in 4 models.
1) by accelerating fuel decomposition
2) by accelerating HCLO4 decomposition
3) by accelerating the solid-fuel / HCLO4 reaction on fuel surface
4) by accelerating gaseous fuel reactions in gas phase
In addition catalysts may also enhance AP decomposition.

Catalysts for HCLO4 decomposition can be divided into 3 groups,
1) Highly effective oxides (Cr2O3, NiO, Al2O3, Fe2O3 and CuO)
2) Less reactive oxides (TiO2 and Cu2O)
3) Inactive oxides (CdO, MgO and CaO)
Copper Chromite accelerates the decomposition of HCLO4 but doesn't affect the binder degradation. The relative effectiveness of various catalysts in the ignition process depends on the surface area, particle size and quantity of catalyst used.

     Ignition of propellants is an enormously complex process and a single rate determining step cannot explain it. One or more types of mechanism models such as gas-phase, condensed-phase and catalytic reactions contribute to the ignition process. A single reaction will depend on many factors such as pressure, local temperature, chemical and physical structure, local concentration, etc.

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