Catherine A. Whitman

Determination of Phase Stability of Elemental Boron

Boron is an important element, used in applications from superhard materials to superconductors. Boron exists in several forms (allotropes) and, surprisingly, it was not known which form (α or β) is stable at ambient conditions. Through experiment, we quantify the relative stability of α-boron and β-boron as a function of temperature. The ground-state energies of α-boron and β-boron are nearly identical. For all temperatures up to 2000 K, the complicated β-boron structure is more stable than the simpler α-boron structure at ambient pressure. Below 1000 K, β-boron is entropically stabilized with respect to α-boron owing to its partially occupied sites, whereas at higher temperatures β-boron is enthalpically stabilized with respect to α-boron. We show that α-boron only becomes stable on application of pressure.

Investigation of factors affecting crystallization of cyclopentane clathrate hydrate

We report the results of systematic investigations of the influence of thermal history and other factors on crystallization of a model clathrate hydrate (cyclopentane hydrate) studied as water-in-oil and oil-in-water emulsions to remove the nucleation influence of substrates other than ice and hydrates. Hydrate and ice seem to form simultaneously under the conditions of these experiments, with ice forming preferentially. Thermal treatment, melting the ice, and leaving only the hydrate, promotes further hydrate formation. Not all the hydrate formed can be accounted for by the recrystallization of water freed by melting ice.

Negative Thermal Expansion (Thermomiotic) Materials

Whereas most materials expand when heated, some shrink, i.e., show negative thermal expansion (NTE, also known as thermomiotic behavior). The excitement about NTE materials is that, in principle, they could compensate for positive thermal expansion, presenting materials that would not experience thermal stress fracture. We summarize the atomic-level mechanisms of NTE, synthetic routes to NTE materials, experimental techniques to investigate NTE, and types of NTE materials and their applications.