Engineering topics
Chiang, Y
Publications and source records attributed to Chiang, Y.
Overpotential-Dependent Phase Transformation Pathways
An objective in battery development for higher storage energy density is the design of compounds that can accommodate maximum changes in ion concentration over useful electrochemical windows. Not surprisingly, many storage compounds undergo phase transitions in situ, including production of metastable phases. Unique to this environment is the frequent application of electrical over- and underpotentials, which are the electrical analogs to undercooling and superheating. Surprisingly, overpotential effects on phase stability and transformation mechanisms have not been studied in detail. Here we use synchrotron X-ray diffraction performed in situ during potentiostatic and galvanostatic cycling, combined with phase-field modeling, to reveal a remarkable dependence of phase transition pathway on overpotential in the model olivine Li{sub 1-x}FePO{sub 4}. For a sample of particle size {approx}113 nm, at both low (e.g., <20 mV) and high (>75 mV) overpotentials a crystal-to-crystal olivine transformation dominates, whereas at intermediate overpotentials a crystalline-to-amorphous phase transition is preferred. As particle sizes decrease to the nanoscale, amorphization is further emphasized. Implications for battery use and design are considered.
Single Helix to Double Gyroid in Chiral Block Copolymers
An order-order phase transition of chiral block copolymers (BCPs*) from single helix to double gyroid (H* {yields} G) through a nucleation and growth process was demonstrated. The H* and G phases can be obtained by solution casting from fast and slow solvent evaporation, respectively, suggesting that the H* phase is a metastable phase. Consequently, the coexistence of H* and G phases can be found in the solution-cast samples from intermediate solvent evaporation. To truly examine the transition mechanism of the H* {yields} G, electron tomography was carried out to directly visualize the morphological evolution in real space, in particular, the transition zone at interface. Unlike the mechanisms for the transitions of block copolymers (BCPs) by considering the interdomain spacing matching, a significant mismatch in the lattices for the H* {yields} G was found. Consequently, the transition may require an adjustment on the geometric dimensions to justify corresponding lattice mismatch. As a result, the morphological observations from electron tomography offer new insights into BCP phase transitions.