aluminum carbide on the surfaces of the crack as seen in figure 1. The width of observed cooling
cracks observed are generally from 1.6 to 3 mm and may extend the length of the cathode blocks,
~300 cm. The distances between cooling cracks vary widely, but are typically found to occur about
two cathode blocks apart.
The Fracture Behavior of Carbon:
The thermo-electro-mechanical behavior of new cathode
carbon has been described as elastoplastic. [Reference 2] Carbon cathode blocks initially behave
elastically with reversible deformation as stress is applied; however when the stress continues to
increase the carbon material starts to behave in a more plastic manner with irreversible deformation
until fracture occurs. Micro-cracks can be generated during the calcinations and graphitization of
cathode carbon materials; during loading the micro-cracks are gradually closed with volume
contraction. Thereafter, when stresses become high, macro-cracks are initiated in the material and
begin to propagate until failure occurs. The cathode carbon is weakened as it undergoes ductile-
brittle transformation during cell operation due to the cathode lining eventually becomes saturated,
(>3%) with sodium that intercalates and absorbs into the carbon lattice. This causes swelling and
changes the properties of the carbon lining which makes the cathode material less ductile and more
brittle. In addition, the cathode blocks are significantly weakened by micro-cracking caused by the
diffusion of sodium into the carbon lattice.
Thermal Gradients in the Cathode Lining: It is proposed that rapid cooling of cathodes due to
power interruption generates an uneven temperature distribution in the cathode lining which results
in a thermally induced mechanical stress sufficient to cause cracking.
During cooling the top of the
cathode blocks cool faster than the bottom of the cathode blocks resulting in large temperature
gradients in the cathode lining. Sørlie and Øye report that, "due to the very limited elastoplastic
deformation properties of carbon during rapid thermo-mechanical strain, the accumulated stress will
be released in the form of surface energy as the bottom cooling cracks." [Reference 3] Cooling
cracks weaken the carbon lining as some may fill with aluminum upon restart; some cracks continue
to expand and link up and become a basis for failure in the future.
Thermal Modeling Results
Cathode cooling rate:
When a cell loss power, it initially continues to dissipate the same amount of
heat, but there is no more Joule produced, so the cell start to cool down. The average cooling rate
depends on the intensity of the heat loss which itself depends on the operating conditions prior to
the power shutdown and the cell thermal mass. Modern high amperage cells are typically designed
and operated to maximize production so they are operated at very high current density and
corresponding high cell superheat, thin side ledge thickness and high side wall heat flux.
As demonstrated in [Reference 4] it is possible to model cathode cooling. The cell design and cell
operating conditions used in that previous study were typical of early 1990 high amperage
conditions so the resulting cooling rate was correspondingly less that the one recently measured
[Reference 5]. Figure 2 presents the average metal pad cooling rate obtained using a retrofitted cell