Download Thermal Stability and Oxidation Processes

Thermogravimetric analysis (TGA)
Thermogravimetric analysis or thermal gravimetric analysis (TGA)
is a method of thermal analysis in which changes in physical and
chemical properties of materials are measured as a function of increasing
temperature (with constant heating rate), or as a function of time (with
constant temperature and/or constant mass loss).
*Thermogravimetric analysis relies on a high degree of precision in three
measurements: mass change, temperature, and temperature change
. Therefore, the basic instrumental requirements for TGA are a precision
balance with a pan loaded with the sample, and a programmable furnace.
The furnace can be programmed either for a constant heating rate, or for
heating to acquire a constant mass loss with time.
Though a constant heating rate is more common, a constant mass loss rate
can illuminate specific reaction kinetics. For example, the kinetic
parameters of the carbonization of polyvinyl butyral were found using a
constant mass loss rate of 0.2 wt %/min. Regardless of the furnace
programming, the sample is placed in a small, electrically heated furnace
equipped with a thermocouple to monitor accurate measurements of the
temperature. A reference sample may be placed on another balance in a
separate chamber. The atmosphere in the sample chamber may be purged
with an inert gas to prevent oxidation or other undesired reactions.
various components of the sample are decomposed and the weight
percentage of each resulting mass change can be measured. Results are
plotted with temperature on the X-axis and mass loss on the Y-axis. The
data can be adjusted using curve smoothing .
Ceramic Yield
Ceramic yield is defined as the mass percent of starting material found in
the end product. From this, stoichiometry can then be used to calculate the
percent mass of the substance in the sample.
Metal aluminates (MAl2O4) are an important type of mixed-cation oxide
ceramics that have many applications. The metal aluminate CaAl 2O4 is
used in the cement industry as a hydraulic material. The TGA of the
precursor to calcium aluminate CaAl2C18H37O9N3 is shown in Figure 3.
The formation of CaAl2O4 occurs during the thermogravimetric analysis.
This is how the theoretical ceramic yield is calculated for this example:
(1) Calculate molecular weight of CaAl2O4:
(2) Calculate molecular weight of CaAl2C18H37O9N3:
(3) Calculate the percentage that CaAl2O4 is of CaAl2C18H37O9N3:
Therefore, the theoretical ceramic yield for the thermogravimetric
analysis of CaAl2C18H37O9N3 is 29.6%. This correlates well with the
experimentally determined ceramic yield of 28.9%.
As another example of calculating theoretical ceramic yield, the TGA of
calcium oxalate monohydrate. Using the same process detailed above, the
theoretical ceramic yield can be calculated: the formula weight of calcium
oxalate monohydrate is 146 g/mol. The final ceramic product is CaO,
with a formula weight of 56 g/mol. The theoretical ceramic yield is
therefore 38.4%. The actual yield from the TGA was found to be 39.75%.
Some reasons for discrepancies between the theoretical and actual yields
are trapped CO2 and the formation of metal carbides.
In the TGA trace of calcium oxalate monohydrate, the first mass loss
corresponds to loss of water of hydration. The second mass loss
corresponds to decomposition of dehydrated calcium oxalate to calcium
carbonate and carbon monoxide and carbon dioxide. The last mass loss is
due to the decomposition of calcium carbonate to calcium oxide and
carbon dioxide.
Thermal Stability and Oxidation Processes
TGA can be used to evaluate the thermal stability of a material. In a
desired temperature range, if a species is thermally stable, there will be no
observed mass change. Negligible mass loss corresponds to little or no
slope in the TGA trace. TGA also gives the upper use temperature of a
material. Beyond this temperature the material will begin to degrade.
TGA has a wide variety of applications, including analysis of ceramics
and thermally stable polymers. Ceramics usually melt before they
decompose as they are thermally stable over a large temperature range,
thus TGA is mainly used to investigate the thermal stability of polymers.
Oxidative mass losses are the most common observable losses in TGA.
Studying the resistance to oxidation in copper alloys is very important.
For example, oxidative degradation can occur in these alloys as copper
oxides form in atmospheres that are rich in oxygen. TGA can be used to
study the static oxidation of materials such as these for practical use.
Dilatometry
Dilatometry allows dimension changes in a specimen to be read as a
function of temperature. The temperature in range:( 20-1300 °C) and
Working atmosphere: Argon or vacuum. The high level of accuracy
exhibited by the temperature control and the sensitive length measuring
device allow not only the determination of thermal expansion but also
the thermal expansion coefficients, density changes, decomposition,
sintering, phase transition and glass transition temperature of a material.
When carrying out the measurement, the specimens are positioned in a
protective tube and heated in shielding gas or a vacuum atmosphere
respectively. However, the actual specimen temperature is measured with
a thermoelement. A quartz rod on an inductive displacement sensor relays
information on the length change of the specimen. The data thus won are
absolutely essential for the analysis of thermally induced tensions in join
compounds. Especially during the cooling phase of a soldering process
when joining hybrid microsystems, varying expansion coefficients must
be taken into consideration.