Effect Of Material Morphology Research Articles

Chemical, Morphological, and Structural Tuning of Tunnel Manganese Oxides for Intercalation Batteries

In this work, we investigate the effect of tunnel dimension, material morphology, and structural water content on the performance of two tunnel structured manganese oxides in intercalation batteries. Tunnel manganese oxides represent an attractive family of materials for intercalation-based energy storage due to their open crystal structures consisting of well-defined structural tunnels that allow for the reversible insertion/extraction of charge-carrying ions, as well as their low cost, low environmental impact, and high electrochemical activity. Facile hydrothermal methods can be used to synthesize tunnel manganese oxides, which have the general formula AxMnO2, where “A” is a stabilizing cation present within the structural tunnels built from MnO6 octahedra. We focus on comparing the performance of two tunnel manganese oxides with square tunnel structures, α-MnO2 (K0.11MnO2), which contains tunnels of 2x2 octahedra dimension, and todorokite MnO2 (Mg0.20MnO2), which contains tunnels of 3x3 octahedra dimensions. When tested as intercalation electrodes in Na-ion batteries, we find that the material with smaller structural tunnels, α-MnO2, achieved a higher initial capacity of 330 mAh g-1­, while todorokite MnO2, which contains larger 3x3 octahedra tunnels, achieved a capacity of 157 mAh g-1. However, the larger tunnel structure proved to be more stable in a Na-ion battery, demonstrating superior capacity retention, as well as superior rate performance. These results suggest that although smaller tunnels may initially show higher capacity, larger tunnels are preferable for superior electrochemical stability. Further, the effect of material morphology is also shown by synthesizing todorokite MnO2 with both a platelet and a nanowire morphology. In Li-ion batteries, the nanowire morphology exhibited a 30% higher capacity retention after 100 cycles, demonstrating the advantage of a nanowire morphology in terms of enhanced cycling stability. Moreover, the effect of structural water in todorokite MnO2 (0.42 H2O molecules per Mn) is discussed, elucidating the role of structural water on ion insertion of into tunnel structured materials. Lastly, we discuss the effect of controlling the electrochemical potential window used during cycling, the trade off between capacity and capacity retention, and how the stability of these materials can be improved. This work describes the role of tunnel size, ionic content, and material morphology on the behavior of tunnel manganese oxides and thus provides insight into tailoring of tunnel structured materials for intercalation batteries.

Molecular Packing in Organic Solar Cell Materials: Insights from the Emission Line Shapes of P3HT/PCBM Polymer Blend Nanoparticles

Semiconducting polymer devices have seen tremendous progress in development of material and device designs, while device efficiencies have made substantial gains. Still, the effect of material morphology on the optoelectronic properties of semiconducting polymers is not completely understood even though these materials make up the active device layer. In this study we use computational methods to simulate different poly(3-hexylthiophene) (P3HT) morphologies, predict their emission spectra, and compare them to experimentally observed emission spectra for P3HT nanoparticles. We use published X-ray diffraction data on P3HT polymorphs to build the molecular models of nanodomains that differ in the side-chain packing. The atomic and electronic structures of both nanodomains are studied with the force field, Hartree–Fock, CIS, and density functional theory methods. The results confirm the coexistence of type I and II nanodomains, where the shift of the backbones in the same stack is determined by the differences in side-chain packing. Upon excitation, the polymer chains in type II domain are free to slide to their optimal arrangement in the stack, whereas in type I domain this sliding is hindered by the steric repulsion of the side chains and the chains are essentially constrained to keep the ground state geometry. These nanodomains, therefore, differ in their emission spectra: type I emission has a single 0–0 vibronic band, while type II demonstrates pronounced vibronic progression. In agreement with Frenkel exciton theory, splitting of the excited state depends on the longitudinal shift of the π-systems. However, we find that due to the constraints arising from P3HT being confined in nanosized particles, the type I nanodomain increasingly appears as an additional emitter that exhibits J-aggregate character. As a result, a pronounced vibronic structure appears as PCBM blending ratios increase, as opposed to the changes in emission profile due to a different degree of disorder present in weakly coupled H-aggregates. These findings are distinct from those made for bulk P3HT materials.

Fracture toughness of titanium foams for medical applications

The fracture behavior of titanium open foam is characterized and the R-curves of crack propagation from pre-cracks are measured. The crack growth has been optically observed, the measured initiation toughness, J IC, has been analyzed and the effect of material morphology on the J IC is discussed. The fracture toughness was found to be dependent on the expanding crack bridging zone at the back of the crack tip. The compact tension specimens also have some plastic collapse along the ligaments and it has shown that the titanium foam with a higher relative density is tougher. The non-uniform stressing within the plastic zone at the crack tip and the plastic collapse of cell topology behind the tip was found to be the primary cause of the R-curve behavior in low relative density titanium foams.