Chapter 10 - Gravimetric techniques for characterization of energy materials Flashcards
What are the ideas and basics of thermal analysis? Give examples of typical TGA curves (schematically) in the case of solvent desorption, decomposition, “atmospheric reactions”.
Thermalgravimetric analysis is a technique in which the mass of a substance is monitored as a function of temperature or time as the sample specimen is subjected to a controlled temperature program in a controlled atmosphere.
See slide 13 for example curves.
Basic principles of DTA.
Here the material under study and an inert reference are made to undergo identical thermal cycles. Any temperature difference between sample and reference is recorded. In this technique, the heat will flow to the sample and reference remain the same.
Piezoelectric quartz crystal behavior under an applied bias. What is the definition of the resonance frequency? Why is it possible to monitor changes in mass in the nano-gram range?
The definition of the resonance frequency is the frequency at which there is a standing wave established. It is mathematically defined as:
f_0 = sqrt(µ/rho) / 2t
µ is the shear modulus (a ratio of sheer stress to shear strain), rho is the density and t is the crystal thickness.
The Sauerbrey equation. Range of applicability, variety of contributions affecting the resonance frequency.
The Sauerbrey equation describes the relationship between the resonant frequency shift ∆f and the added mass ∆m.
∆f = -(2f^2∆m)/(Asqrt(rhoµ))
A is the acoustically active surface area.
f is the resonant frequency of unloaded crystal
rho is density
µ is the shear modulus
∆m is the change in mass.
It is valid for small mass changes ∆m
How to overcome problems with selectivity in TGA, DTA and QCM?
Combination of several techniques can often overcome these problems.
Analyze advantages and limitations of TGA and DTA.
Advantages is that these techniques can be used to find:
- melting points and transformation temperatures
- glass transition temperatures
- crystallinity
- moisture / solvent content
- thermal and oxidative stability
- sometimes even purity of materials
It is also a straight-forward and pretty cheap way to characterize some properties.
On major disadvantage is selectivity, especially if several gases are evolved.
What are some processes that we can investigate with thermal analysis?
The change of Nafion polymer from room temperature to after a high temperature treatment.
The intercalation of Li-ions in situ.
The adsorption and functionalization of surfaces or surface reactions.
Name some other thermal analysis methods apart from TGA and DTA. What do these measure?
Differential scanning calorimetry (DSC) - heat difference
Pressurized TGA - mass changes as a function of pressure
Thermo mechanical analysis (TMA) - deformations and dimension
Dilatometry (DIL) - volume
Evolved gas analysis (EGA) - gaseous decomposition products
Who was the first to use thermal transformations occuring in matter as an analytical procedure?
Henry Le Chatelier
What does the measured weight loss curve in TGA give information on?
Changes in sample composition.
Thermal stability
Kinetic parameters for chemical reactions in the sample.
Draw the typical TGA curve of Nafion.
See slide 14
What are factors that affect the respsonse in TGA measurements?
Heating rate and sample size increases the temperature at which the sample decomposition occurs.
Particle size of sample, packing, crucible shape and gas flow rate affects the progress of the reaction.
What can be said of the reproducibility of TGA measurements?
That it is a function of sample preparation. Many parameters can change the outcome.
Draw schematically a setup for DTA.
See slide 17.
Draw a typical DTA curve, and explain what the different features can tell us.
See slide 19.