Fracture, electrolyte

Molten sodium and sulphur at 300-400 C are inherently reactive materials, especially when exposed to air or water. The uncontrolled sodixiin/sulphur reaction itself, which would take place in the event of electrolyte fracture, is rapid and highly exothermic. It is therefore necessary to design cells which are intrinsically safe under normal operati j conditions and can withstand likely accident situations ... 

The question of safety is a complex issue and depends very much upon the battery application. ViTe may distinguish between the consequences of spontaneous cell failure, caused by electrolyte fracture, internal short circuiting etc, and externally imposed cell failure as a result of impact damage. For spontaneous cell failure, the principle being adopted in the U.K. is that a burn-out in one cell shall not propagate to adjacent cells. There is good evidence that this can be achieved the features of cell design which minimise the effec l of spontaneous failure have been discussed in a recent paper ... 

If continued to the point of complete sodium depletion, electrolyte fracture will occur. But because of the presence of the metallic aluminum, the cell will remain electrically conductive. This characteristic permits batteries to be configured with long series strings (for more information refer to section 40.4.1 on Electrical Networking ). The quick decrease in cell voltage functions as a reliable indicator for the end of discharge and is used to provide overdischarge protection. 

Electrolyte Fracture Electrolyte fracture can result from rapid or severe temperature and or humidity cycling, including frozen conditions. Electrolyte fracture is not always catastrophic but results in increased hydrogen crossover, leading to failure over time. Freeze-thaw cycling can potentially also result in catalyst layer delamination, as shown in Figure 6.60. 

All metallic materials can suffer electrolytic corrosion. Fractures caused by cathodic hydrogen only occur when the activity of the absorbed hydrogen and the level of the tensile stress, which can be external or internal, reach a critical value. In general, critical hydrogen absorption is achieved only in the presence of promoters. However, under very severe conditions such as at very low pH or very negative potential, critical hydrogen absorption can occur. Steels with a hardness greater than HV 350 are particularly susceptible. 

Ionic conductors, used in electrochemical cells and batteries (Chapter 6), have high point defect populations. Slabs of solid ceramic electrolytes in fuel cells, for instance, often operate under conditions in which one side of the electrolyte is held in oxidizing conditions and the other side in reducing conditions. A signihcant change in the point defect population over the ceramic can be anticipated in these conditions, which may cause the electrolyte to bow or fracture. 

Fig. 14.19 Fracture surface of an anode-supported cell. From left to right, the porous Ni-YSZ anode, the dense 8YSZ electrolyte, and the porous LSM-YSZ cathode.

Approaches to resolving the mismatch caused by different component materials thermal expansion coefficient include increasing the fracture toughness of the electrolyte, controlling the electrolyte processing faults, varying the component thickness, and adding minor constituents to alter the anode properties. 

Based on our observation, a membrane degradation and failure mechanism under the RH cycling, a pure mechanical effect is theorized as the following sequence electrode-microcracking- - crazing initiation at the electrode/electrolyte interface - crack growth under stress cycling- -fast fracture/instability. 

Figure 2. Fracture surface SEM image of porous YSZ electrolyte [4]...

Figure la. Electron microscopic image of a fracture of a multilayer film of a zirconia-based solid electrolyte. 

Cocamidopropyl betaine and cocamidopropyl hydroxysultaine, discussed later, are also used in petroleum production. Their relatively high foaming nature, electrolyte tolerance and hydrolytic stability make them useful for foam acidizing and foam fracturing fluids. 

B.E. Conway, The Solid/Electrolyte Interface, NATO Conf. Ser. 6, Vol. 5 on Atomistics of Fracture, R.M. Latanision, Ed., Plenum (1983) 497. (Review, emphasis on metals double layers and water structure near charged surfaces.)... 

Significant anion effects are observed in the value of scc for certain electrolytes. For example, phosphate inhibits fracture (moves scc to a more positive value) compared with sulfate and chloride solutions [28, 29]. 

Fracture mechanisms and microstructure as well as distribution of chemical elements along it were studied with scanning electron microscopy (SEM) and X-ray microanalysis (XRMA) using Superprobe-733, JEOL, Japan, after deep electrolytic etching with an etchant based on acetic acid. X-ray diffraction phase analysis was done with DRON diffractometer, Burevestnik, the Russian Federation. 

---