Thermal stability and runaway testing

A principal goal of thermal hazard evaluation is the accurate determination of the thermal stability and runaway behavior of a substance or mixture in 

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Several safety tests for batteries have been described (96). In the nail penetration test, a LiCo02/mesocarbon microbeads full battery with the branched oligomer showed a significant improvement in the thermal stability and was able to restrain the temperature of the battery at about 85°C. The nail penetration test was conducted according to the standard procedure described in UL1642 (97) and SBA GllOl (98). An internal short circuit occurs upon nail penetration, and the thermal runaway behavior can be observed if there is no mechanism to quench the heat or stop the chain reaction quickly. 

The first aim of a thermal stability screening test (e.g., DSC/DTA) is to obtain data on the potential for exothermic decomposition and on the enthalpy of decomposition (AHd). These data, together with the initial theoretical hazard evaluation, are used in reviewing the energetic properties of the substance (Box 4) and the detonation and deflagration hazards of the substance (Boxes 7 and 8). The screening tests also provide data on the thermal stability of the substance or mixture, on the runaway potential, on the oxidation properties, and to a lesser extent, on the kinetics of the reaction (Box 10). 

In the Bowes and Cameron test [133], the stability of the powder at constant (uniform) ambient temperature is investigated. Cube-shaped baskets, made of wire gauze, are filled with the substance and placed in an oven that is controlled at the desired constant temperature. The temperature in the center of the cube and of the oven are continuously recorded. By testing at different temperatures, and using a number of cube dimensions, the thermal stability of the powder can be established, that is, the determination of the temperature below which the exothermic decomposition of the powder does not result in a runaway. Bowes [133] has given a number of theoretical calculations for scaling up the test results. 

In 55% of the cases, the accidents could have been foreseen by use of risk analysis, and in 35% of the cases by thermal stability testing. Different methods of stability testing were evaluated comparatively during the investigation of a runaway exothermic reaction which occurred during the preparation of a component mixture for a sealing composition in a 1200 1 reactor only DSC was effective in identifying the cause of the hazard. 

Flexometer tests are used to determine thermal stability under dynamic straining conditions. Measurements include temperature rise after a specified period of cycling, set and creep, and in some instances the time or number of cycles to failure in the form of thermal runaway or test piece destruction. In contrast to fatigue cracking tests, heat buildup tests... 

The explosion occurred due to the convergence of several events a dangerous material (o-nitrobenzylnitrate) developed whilst the ONBALD was left in store (the drums of ONBALD had been stored for eight months) the use of different test tubes in the thermal stability test actually invalidated the test results when the runaway did occur, the temperature set point of the automatic quench system was too high and manual override did not work. 

Thermal stability testing showed that the reaction mixture could decompose exothermically with self heating occurring on the plant scale from 145°C (the boiling point of the mixture is about 160°C). Decomposition of the reactant mass would lead to a runaway reaction with the generation of a toxic and irritant gas. This would be vented safely, as far as protecting the reactor from overpressure is concerned, by the emergency relief vent, but would cause a serious toxic and corrosive aerosol emission. 

The results of the hazardous chemical evaluation are used to determine to what extent detailed thermal stability, runaway reaction, and gas evolution testing is needed. The evaluation may include reaction calorimetry, adiabatic calorimetry, and temperature ramp screening using accelerating rate calorimetry, a reactive system screening tool, isoperibolic calorimetry, isothermal storage tests, and adiabatic storage tests. 

The plot shows the response of cells to short circuit where the external circuit resistance is less than 100 mS2. The cell current peaks within less than a minute and the temperature maximum occurs a few minutes later. The current often exhibits a plateau with a slow rise, which is due to increased conductivity of the cell at elevated temperature. Cells typically can withstand an external short circuit, since thermal output is small and the cell is in contact with the test fixture. Thermal management will dictate whether response of cells will be benign, as in this test, or exhibit thermal mnaway. Large cells (i.e., over 10 Ah), cells that can sustain very large short-circuit currents, cells that have higher internal resistance, and cells with low inherent thermal stability are more prone to exhibit thermal runaway. 

At 58 s into the overcharge (after fiill charge), cells 1 and 2 carried less current leaving 3 and 4 to carry more current at 206 s, cells 1 and 2 carried higher current, and all 4 cells stabilized at 320 s at about 590 s, cell 3 appeared to have its CID activated, followed by the other 3 cells over the next minute at about 640 s, the temperature on cell 3 increased from 109 °C to 120 °C in less than 1 s with runaway to >500 °C and, at this point, the other three cells vented flame as well, probably due to thermal propt ation of the initial event. The test was repeated with fresh Sony Li-ion cells obtained from a battery vendor. Figure 17.13 gives the results obtained that showed that all the cells had their CIDs activated as expected. 

Concurrently with analyzing novel materials for increased capacity and rate capability, safety testing of these cell components must be done, first on the raw material and then, in the actual lithium-ion cells. There are several experimental and analytical methods to help assess the reliability, reactivity, and sensitivity of the cell materials to thermal runaway and their stability. Two of the more common methods of identifying effects that thermal abuse will have on the components are differential scanning calorimetry (DSC) and accelerating rate calorimetry (ARC). 

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