ICMCTF2014 Session TS1: Mechanical Properties Challenges for Greener Energy Applications and Emissions Reduction

Most lithium ion battery electrodes experience large volume changes caused by concentration changes within the host particles during charging and discharging. Electrode failure, in the form of fracture or decrepitation, can occur as a result of repeated volume changes. In this presentation, we will discuss our recent work on understanding the evolution of concentration, stress, and strain energy in electrodes under various charging-discharging conditions. We show that a dimensionless parameter, the electrochemical Biot number, may be used to characterize stress and strain energy evolution in an electrode. In particular, the electrochemical Biot number determines the maximum stress and strain energy. Based on analytic solutions, we propose tensile stress and strain energy based criteria for the initiation and propagation of cracks in electrodes. We will also discuss the effects of the solid-electrolyte interphase (SEI) that form naturally, as well as “artificial SEIs” by design, on the coupled chemical-mechanical degradation of electrodes. These studies may help understand the degradation mechanisms of electrodes and provide guidelines for developing lithium ion batteries with enhanced durability and performance.

Thin films and coating are mainly used to functionalize the surface of underlying substrates. Generally, the films are not considered as load bearing since the film to substrate thickness ratio is close to zero. However, for some particular applications such as high pressure turbine blades for instance, it is necessary to relieve weight from the mechanical parts in order to limit the deformation by creep. The walls of the load bearing parts are therefore thinned and the coating to substrate thickness ratio increases as a result.

In this context, the influence of an oxide layer on the stiffening of pure nickel has been studied experimentally from 20°C to 600°C, using a resonant method. It is now known that the temperature dependence of Young modulus is not linear in nickel, resulting from magneto-elastic effects. In this study, it is shown that the stiffening of the nickel substrate can be finely tuned by the deposition conditions, up to its Curie temperature found at 370°C. A model using a magneto-mechanical coupling is proposed and discussed in this context. It supports the idea that the internal stresses of the coating play a key role in the observed behavior, even for very low levels of only few MPa.

For the realization of green energy society with hydrogen fuel cycles, microelectronic devices especially such as physical and chemical sensors will have to go into hydrogen environment. There, many kinds of small scale structures with and without deposited thin films are expected to play also mechanically important roles. Since hydrogen is already known for its impact on metallic materials to significantly degrade the strength and fatigue lifetime, it is of urgent importance to acquire fundamental knowledge on possible effects of hydrogen also for the mechanical reliability of those microsystems [1].

When the sensors made with micro electro-mechanical systems (MEMS) technology are concerned, silicon is the most common material to compose fundamental mechanical members. From this point of view, strength and fatigue lifetime of single crystal silicon were examined in this study by applying cyclic stress to the surface while exposing to hydrogen gas atmosphere at room temperature and compared to the case in lab-air. The number of loading cycles to failure was already known to be enormously larger in less humid atmosphere, for example by a factor of 10³ in air with 5% relative humidity (RH) than in lab-air with 40% RH [2]. However, it was newly found to be just slightly larger in hydrogen by a factor of only 10 with less than 5 % RH. In addition, static strength under monotonically increasing stress was also found to be reduced by 10% in hydrogen.

The mechanism for fatigue failure of silicon is not yet clear enough. Because of the effect of humidity mentioned above, it was once believed to be due to the surface oxide layer growing with cyclic stress even though the physical reason for intensive oxidation at room temperature is not explained [3]. Instead, hydrogen could be supplied also from surface water. The effect of hydrogen newly found suggests that fatigue degradation could possibly be caused by accumulation of internal defects such as point defects [4] or dislocations [5] whose activation energy would be reduced with hydrogen similar to the case of metallic materials. More details of fatigue and fracture behavior will be discussed in the presentation to further understand the mechanical reliability of silicon microstructures in hydrogen environment.

[1] Hirakata, H. et al., Engineering fracture mechanics, 77 (2010) 803-818.

[2] Tanemura, T. et al., Journal of micromechanics and microengineering, 23 (2013) 035032.

[3] Muhlstein, C.L. et al., Acta materialia, 50 (2002) 3579-3595.

[4] Kamiya, S. et al., Proc.Transducers (2013), 784-787.

[5] Namazu, T. et al., Journal of microelectromechanical systems, 11 (2002) 125-135.