Pendant Group Functionalization of Cyclic Olefin for High Temperature and High-Density Energy Storage.

High-temperature flexible polymer dielectrics are critical for high density energy storage and conversion. The need to simultaneously possess a high bandgap, dielectric constant and glass transition temperature forms a substantial design challenge for novel dielectric polymers. Here, by varying halogen substituents of an aromatic pendant hanging off a bicyclic mainchain polymer, a class of high-temperature olefins with adjustable thermal stability are obtained, all with uncompromised large bandgaps. Halogens substitution of the pendant groups at para or ortho position of polyoxanorborneneimides (PONB) imparts it with tunable high glass transition from 220 to 245 °C, while with high breakdown strength of 625-800 MV/m. A high energy density of 7.1 J/cc at 200 °C is achieved with p-POClNB, representing the highest energy density reported among homo-polymers. Molecular dynamic simulations and ultrafast infrared spectroscopy are used to probe the free volume element distribution and chain relaxations pertinent to dielectric thermal properties. An increase in free volume element is observed with the change in the pendant group from fluorine to bromine at the para position; however, smaller free volume element is observed for the same pendant when at the ortho position due to steric hindrance. With the dielectric constant and bandgap remaining stable, properly designing the pendant groups of PONB boosts its thermal stability for high density electrification.

Informatics-Driven Design of Superhard B-C-O Compounds.

Materials containing B, C, and O, due to the advantages of forming strong covalent bonds, may lead to materials that are superhard, i.e., those with a Vicker's hardness larger than 40 GPa. However, the exploration of this vast chemical, compositional, and configurational space is nontrivial. Here, we leverage a combination of machine learning (ML) and first-principles calculations to enable and accelerate such a targeted search. The ML models first screen for potentially superhard B-C-O compositions from a large hypothetical B-C-O candidate space. Atomic-level structure search using density functional theory (DFT) within those identified compositions, followed by further detailed analyses, unravels on four potentially superhard B-C-O phases exhibiting thermodynamic, mechanical, and dynamic stability.

Strong Chemical Bond Hierarchy Leading to Exceptionally High Thermoelectric Figure of Merit in Oxychalcogenide AgBiTeO.

The chemical bond hierarchy (CBH) in prototype cage structures has been considered important for achieving high thermoelectric performance. By performing first-principles calculations and lattice dynamics, we demonstrate CBH hosted distinct rattlers in a noncaged oxychalcogenide AgBiTeO, causing an ultralow κl of 0.9 W/m-K at room temperature. The CBH in this compound leads to a unique structural bonding, where Ag and Te are loosely bonded to the rigid framework of the lattice and form distorted four-centered Ag-Te tetrahedra. These clusters exhibit large atomic vibrational motions in a very shallow potential energy surface, resulting in a rattling motion. The presence of multiple avoided crossing points of low-lying optical mode with longitudinal acoustic mode in phonon dispersion further confirms the rattling-induced thermal damping. Additionally, unique in-plane off-phase collective vibrations of Ag-Te tetrahedra introduce localized flat phonon dispersions that lower the group velocity and significantly reduce the lattice thermal conductivity. Most importantly, it prevents carrier-phonon scattering leading to a high electrical conductivity in AgBiTeO. The combination of intrinsic low lattice thermal conductivity and excellent electronic transport properties gives an unprecedented range of ZT from 1.00 to 1.99 in the large temperature range of 700-1200 K for n-type charge carriers.