title: A negative surface energy for alumina.
abstract: The surface energy of a solid measures the energy cost of increasing the surface area. All normal solids therefore have a positive surface energy-if it had been negative, the solid would disintegrate. For this reason it is also generally believed that when certain ceramics can be found in a highly porous form, this is a metastable state, which will eventually sinter into the bulk solid at high temperatures. We present theoretical evidence suggesting that for theta-alumina, the surface energy is strongly dependent on the size of the crystallites, and that for some facets it is negative for thicknesses larger than approximately 1 nm. This suggests a completely new picture of porous alumina in which the high-surface-area, nanocrystalline form is the thermodynamic ground state. The negative surface energy is found to be related to a particularly strongly adsorbed state of dissociated water on some alumina surfaces. We also present new experimental evidence based on infrared spectroscopy, in conjunction with X-ray diffraction and surface-area measurements, that theta-alumina has indeed very stable surface OH groups at high temperatures, and that this form of alumina does not sinter even at temperatures up to 1,300 K.
authors.affiliation: Center for Atomic-scale Materials Physics, Department of Physics, Technical University of Denmark, Building 307, DK-2800 Lyngby, Denmark.
authors.forename: Zbigniew
authors.initials: Z
authors.lastname: Łodziana
authors.forename: Nan-Yu
authors.initials: NY
authors.lastname: Topsøe
authors.forename: Jens K
authors.initials: JK
authors.lastname: Nørskov
mesh_terms: D000537:Aluminum Oxide / Q000737:chemistry*; D003198:Computer Simulation; D003461:Crystallography / Q000379:methods*; D004735:Energy Transfer; D008422:Materials Testing; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D036103:Nanotechnology / Q000379:methods*; D013500:Surface Tension; D013696:Temperature; D014867:Water / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D014867:Water; D000537:Aluminum Oxide