Ozawa-Flynn-Wall Analysis

Ozawa-Flynn-Wall Analysis is the model-free (isoconversional) method of kinetic analysis calculating dependence of activation energy E(α) on degree of conversion α for dynamic experiments with different constant heating rates β.

It is always necessary to check if this model-free method is valid to be used and is applicable because of restrictions of model-free methods.

Ozawa-Flynn-Wall analysis belongs to the group of integral model-free methods where firstly, the integral of main kinetic equation (1) must be found and then the logarithm must be taken.

Integral and then logarithm for Ozawa-Flynn-Wall analysis:

Doyle approximation of Eq2 has linear dependence:

Where:

If the points with the same degree of conversion (isoconversional points) will be taken from experiments performed at different heating rates then values ln[A(α)/F(α)] will be the same for all of them and Eq(3) will look like the straight line

y=b+ax (4)

where:

y=ln(β)
b=5.3305+ ln[A(α)/F(α)]
a=1.052 E/R
x=-1/T .

The Ozawa plot y(x) looks as set of straight lines for different α values, where for each α the activation energy can be found from the slope and pre-exponent from the intercept for known(or assumed) f(α).

More accurate approximation of equation (2) is non-linear and provides more accurate activation energy. (See refined method in ASTM E698).

Advantages and disadvantages of this method and a comparison table with other methods.

Example

Decomposition of La(OH)₃:

Fig.2 Ozawa-Flynn-Wall plot containing straight lines for different conversion values α.

Fig.3 Ozawa-Flynn-Wall Activation energy E(α).

Fig.4 Ozawa-Flynn-Wall pre-exponent A(α) (for the assumption of first-order reaction).

Fig.5 Comparison between experiments(symbols) and simulation(solid lines) for Ozawa-Flynn-Wall dependences E(α) and A(α).

It is seen that model-free method of Ozawa-Flynn-Wall has the problem for multi-step reaction where reaction steps have big difference in activation energy. It is always necessary to simulate the curves by Ozawa-Flynn-Wall method and compare them with experiment. This comparison helps to check if Ozawa-Flynn-Wall method is suitable for analysis of the current reaction.

KInetics Neo

This method is used in Kinetics Neo software as model-free method Ozawa-Flynn-Wall .

Reference

Flynn, J.H.; Wall, L.A. General Treatment of the Thermogravimetry of Polymers. Journal of Research of the National Bureau of Standards – A. Physics and Chemistry70A(6), 487–524 (November–December 1966). Paper 70A6-422.

https://nvlpubs.nist.gov/nistpubs/jres/70A/jresv70An6p487_A1b.pdf