This work combines experiments and computer models in order to understand the relationships between electrode microstructure and ionic transport resistances so that one may predict cell performance from fundamental principles. A scanning electron microscope (SEM) with focused ion beam (FIB) was used to image sections of commercially made porous electrodes utilizing LiCoO2 active material. The images reveal the existence of discrete porous carbon domains in the microstructure. Further experiments indicated that these carbon domains are highly tortuous and restrict to a large degree the overall ion transport in the cathode. Two types of 3D models for correlating and predicting the electrode microstructure were explored. The first, known as the dynamic particle packing (DPP) model, is based on aggregates of spheres that move collectively in response to interparticle forces. The second is a stochastic grid (SG) model closely related to Monte Carlo techniques used in statistical physics to study cooperative and competitive phase behavior. The models use a small set of fundamental interdomain and bulk interaction parameters to generate structures from a given electrode mass composition and porosity. Both models were able to semi-quantitatively reproduce experimental tortuosity measurements of cathodes at different porosity values.

No abstract

Reducing the grain size of metals and ceramics can significantly increase strength and hardness, a phenomenon described by the Hall-Petch relationship. The many studies on the Hall-Petch relationship in metals reveal that when the grain size is reduced to tens of nanometers, this relationship breaks down. However, experimental data for nanocrystalline ceramics are scarce, and the existence of a breakdown is controversial. Here we show the Hall-Petch breakdown in nanocrystalline ceramics by performing indentation studies on fully dense nanocrystalline ceramics fabricated with grain sizes ranging from 3.6 to 37.5 nm. A maximum hardness occurs at a grain size of 18.4 nm, and a negative (or inverse) Hall-Petch relationship reduces the hardness as the grain size is decreased to around 5 nm. At the smallest grain sizes, the hardness plateaus and becomes insensitive to grain size change. Strain rate studies show that the primary mechanism behind the breakdown, negative, and plateau behavior is not diffusion-based. We find that a decrease in density and an increase in dissipative energy below the breakdown correlate with increasing grain boundary volume fraction as the grain size is reduced. The behavior below the breakdown is consistent with structural changes, such as increasing triple-junction volume fraction. Grain- and indent-size-dependent fracture behavior further supports local structural changes that corroborate current theories of nanocrack formation at triple junctions. The synergistic grain size dependencies of hardness, elasticity, energy dissipation, and nanostructure of nanocrystalline ceramics point to an opportunity to use the grain size to tune the strength and dissipative properties.

Barium strontium titanate glass-ceramics were successfully produced with one major crystalline phase when Al 2 O 3 was added to the melt. A dielectric constant of 1000 and a breakdown strength of 800 kV/cm was achieved; however the energy density was only measured to be 0.3-0.9 J/cm 3 . These energy density values were lower than anticipated due to the presence of dendrites and pores in the microstructure. Using BaF 2 as a refining agent improved the microstructure and doubled the energy density for BST 80/20 samples. However, no refining agent reduced the increasing amount of hysteresis that developed with increasing applied electric field. This phenomenon is believed to be due to interfacial polarization.

Barium strontium titanate (BST) has been targeted as one potential ferroelectric glass–ceramic for high‐energy density dielectric materials. Previous testing has shown that the dielectric constant of these materials was as high as 1000 and the dielectric breakdown strength up to 800 kV/cm. This did not, however, result in exceptional energy density (∼0.90 J/cc). In order to increase overall energy density refining agents can be added to the melt, but the nucleation and growth of the ceramic particles can also play a role. Therefore, in this study the crystallization kinetics were studied to more fully understand how BST phase forms so that the optimal energy density could be obtained. It was determined that the activation energy of the crystallization of BST 70/30 glass–ceramic is approximately 430 kJ/mol which is close to the dissociation energy of Si–O bonds. The Avrami parameter was found to be ∼3 meaning that three‐dimensional growth is dominant and the mechanism of growth was interface controlled.