10. TGA, DTA and QCM 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”.
TGA = Thermo-gravimetric analysis.
TGA is 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.
Desorption - declining curve
Atmospheric reaction – increasing curve
Decomposition – single/multi stage curves that are decreasing
Basic principles of DTA.
DTA = Differential thermal analysis
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 flow to the sample and reference will remain the same.
To characterise compounds or element, compared with known DTA sample diagrams.
Moreover, the amount of a substance present in a composite sample will be related to the area under the peaks in the graph, and this amount can be determined by comparing the area of a characteristic peak with areas from a series of standard samples analyzed under identical conditions.
Analyze advantages and limitations of TGA and DTA.
Advantages is that these techniques can be used to find:
• Melting points and transformation temperatures (phase diagrams)
• Glass transition temp
• Crystallinity
• Moisture/solvent content
• Thermal and oxidative stability
• Sometimes even purity of materials
• Straight-forward and pretty cheap way to characterize properties
Major disadvantage: selectivity, especially if several gasses are evolving
Piezoelectric quartz crystal behavior under an applied voltage bias. What is the definition of the resonance frequency?
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.
Why is it possible to monitor changes in mass in the nano-gram range?
This is achieved by measuring the change in resonance frequency of the quartz crystal upon excitation by a driving voltage
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 = acoustically active surface area
f = resonant frequency of unloaded crystal
𝜇 = shear modulus
∆m = change in mass
The equation is valid for small mass changes ∆m< 10%.
Valid for purely elastic materials as quarts or similar
How to overcome problems with selectivity in TGA, DTA and QCM?
Combination of several techniques can often overcome these problems.
