Thermogravimetric Analysis (Exam 3)

What are 6 possible reactions that can be studied by TGA?

- General thermal stability - Dehydration - Hydride decomposition - Autoxidation - Pyrolysis - Composite analysis

What are the two different ways to measure deltaP in a DSC experiment?

- Power compensation DSC - Heat-flux DSC

Describe characteristics of power compensation DSC

- independent heating units on reference and sample - lower sensitivity than heat-flux method - faster response and higher resolution than heat-flux method

DTA can indicate and monitor what processes and what is it useful for?

- indicates when physical and chemical exo/endothermic processes occur (whether there is a mass change or not) - monitors all chemical changes mentioned for TGA (quizlet card 8), plus physical changes such as glass transition, crystallization, and melting - useful for deriving phase diagrams

Describe characteristics of heat-flux DSC

- single heating unit (allows temp change to occur) - reference and sample sit on thermoelectric platform made of constantan - Chromel disks under platform forms sensing thermocouples - Sweeps T at a constant rate, measures differential heat flow between reference and sample by measuring deltaE and current between thermocouples - monitors T ramp with chromel/alumel thermocouple

Sample pans are ___ in diameter and a few ___ deep.

0.5 cm ; mm

In TGA experiments the temperature is normally increased by ___ *C/min

5-10

differences between Pt and Al (or Al2O3) sample pans

Pt: expensive, inert, can be cleaned and reused Al: essentially disposable

for fire retardancy/combustion analysis, the instruments are configured to operate in ___

air.

The enthalpy change, deltaH, in a DSC graph is equal to

area under peak

How is differential thermal analysis (DTA) done?

by heating a small sample, but not weighing it. Instead, a linear temperature ramp is imposed on both the sample and an inert reference material in its own vessel.

How is Pyrolysis done?

by heating large molecules under inert atmosphere until bonds sever and gaseous fragments evolve.

In high resolution TGA's ___/___ is monitored so as to slow the temperature ramp down whenever ___/___ starts to change rapidly

dW/dt

What is plotted for a DSC graph?

deltaP (Pref-Psample) vs Temperature The deltaP is differential energy flow

Plotting ___ in DSC allows us to do thermodynamic measurements.

deltaP.

What is plotted for a DTA graph?

deltaT (sample vs reference or Ts - Tr) vs ramp T

DSC is used to calculate the ___ of a sample

enthalpy.

studying General thermal stability involves determining...

how hot a system can get before an outgassing reaction occurs.

The oven cavity is typically purged with ___ ___

inert gas

The normal temperature program in a TGA experiment is ___

linear

high resolution TGA's do what to help sharpen the various mass transitions?

monitor dW/dt instead of dT/dt = constant

What is autoxidation?

oxidation resulting from exposure to air.

DTA is often considered a ___ (qualitative/quantitative?) technique

qualitative

Composite analysis is done to determine...

relative amounts of reactive and inert material in a formulation.

deltaP (differential energy flow) of DSC is seldom zero, even if there are no physical or chemical changes going on, because...

sample and reference do not have the same heat capacity.

What is sample size for TGA?

small; a few mg

In differential scanning calorimetry (DSC) what is measured?

the relative energy flow to sample and reference under a temperature ramp.

Dehydration reactions are loss of ___, while hydride decomposition reactions are loss of ___

water ; H2

Describe the basic technique of TGA

weigh a sample as its temperature is increased - any physical or chemical change resulting in a loss of material off the weighing pan will be recorded as a function of temperature.

a graph of mass vs temperature is called a

weight-loss thermogram or just a thermogram.