Chapter 10 - Gravimetric techniques for characterization of energy materials

Question 1

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”.

Answer 1

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.

Question 2

Basic principles of DTA.

Answer 2

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.

Question 3

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?

Answer 3

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.

Question 4

The Sauerbrey equation. Range of applicability, variety of contributions affecting the resonance frequency.

Answer 4

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

Question 5

How to overcome problems with selectivity in TGA, DTA and QCM?

Answer 5

Combination of several techniques can often overcome these problems.

Question 6

Analyze advantages and limitations of TGA and DTA.

Answer 6

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.

Question 7

What are some processes that we can investigate with thermal analysis?

Answer 7

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.

Question 8

Name some other thermal analysis methods apart from TGA and DTA. What do these measure?

Answer 8

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

Question 9

Who was the first to use thermal transformations occuring in matter as an analytical procedure?

Answer 9

Henry Le Chatelier

Question 10

What does the measured weight loss curve in TGA give information on?

Answer 10

Changes in sample composition.
Thermal stability
Kinetic parameters for chemical reactions in the sample.

Question 11

Draw the typical TGA curve of Nafion.

Answer 11

See slide 14

Question 12

What are factors that affect the respsonse in TGA measurements?

Answer 12

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.

Question 13

What can be said of the reproducibility of TGA measurements?

Answer 13

That it is a function of sample preparation. Many parameters can change the outcome.

Question 14

Draw schematically a setup for DTA.

Answer 14

See slide 17.

Question 15

Draw a typical DTA curve, and explain what the different features can tell us.

Answer 15

See slide 19.

Question 16

What does the area under the curves in DTA tell us?

Answer 16

It tells us the change in enthalpy.

Question 17

What is a quartz crystal micro(nano)balance?

Answer 17

It is a piezoelectric sensing device that consists of an oscillator circuit and a crystal, which is incorporated into the feedback loop of the circuit. The frequency response is sensitive to the nature of the contacting environment.

Basically that means the quartz crystal will have a resonant frequency that changes when molecules are adsorbed on the crystal. We measure this change to say something about the adsorption.

Question 18

What is important to think about when choosing a crystal for QCM?

Answer 18

That it oscillated predictably and reproducibly. This means that we want to choose a cut that has a temperature independent resonant frequency.

Question 19

How can one measure the resonant frequency of a crystal?

Answer 19

By measuring the impedance of the crystal as a function of frequency.

Question 20

What is a Butterworth-van Dyke model?

Answer 20

It is a model of the quartz crystal resonator. It consists of an inductor, capacitor and a resistor in series, parallel to another capacitor.

The inductor corresponds to the intertial component of the oscillation, which is related to the mass displaced during the vibration. The induction is increased when mass is added to the crystal, and thus the resonant frequency goes down.

The capacitor corresponds to the stored energy in the oscillation and is related to the elasticity of the quarts and the surrounding medium.

The resistor corresponds to the dissipation of the oscillation energy from the mounting structures and from the medium in contact with the crystal (i.e. losses induced by a viscous medium).

The capacitor in parallel is the sum of the static capacitances of the electrodes, crystal, holder and connector.

See slide 31 for drawing.

Question 21

What are the possible contributions to the change in resonant frequency in a QCM experiment?

Answer 21

Mass loading
Viscosity and density of the medium in contact with the vibrating crystal
The hydrostatic pressure
The surface roughness
The slippage effect
The temperature

Question 22

How can QCM be used to say something about a solution?

Answer 22

We can do an immersion test. Here we immerse the QCM in a solution, and the decrease in frequency will depend upon the viscosity and density of the solution.

Question 23

What are some advantages and disadvantages of QCM?

Answer 23

+ Sensitivity of the QCM is enough to detect even sub-monolayer amounts of adsorbates at the electrode surfaces.

• Selectivity is an obvious drawback.
• Interpretation of QCM data requires careful account for possible small changes of the temperature, viscosity and the media close to the interface and other effects.