Clarke

1. Exceptionally Low Thermal Conductivity in Layered Perovskites: Natural SuperlatticesDavid R. Clarke, University of California-Santa Barbara, DMR 0710523 The focus of our WMN grant this year has been exploring the consequences of crystal structure anisotropy on thermal conductivity. Several anisotropic oxides, such as Sr2Nb2O7 (right) have been found to exhibit: (i) anisotropic thermal conductivity, (ii) temperature independent thermal conductivity and, (iii) exceptionally low thermal conductivity. These provide a challenge to current understanding of phonon transport in oxides and we are led to conclude that highly anisotropic crystal oxides can be considered as naturally occurring superlattice structures in which phonon scattering is affected by the superlattice spacing and the anharmonicity between the blocks in the superlattice. Concurrent simulations show that the thermal conductivity varies with the superlattice spacing and that a minimum exists. The work is important as it indicates that the thermal conductivity of layered oxides can be tuned by control of composition. Thermal conductivity parallel and perpendicular to the layers in La-doped Sr2Nb2O7 an anisotropic oxide. Note: anisotropic conductivity persists to ~ 1000oC. Computed conductivity of layered SrTiO3 based perovskite as a function of number of perovskite layers in the unit cell. Diamond and square symbols refer to conductivity parallel and perpendicular to layers.

2. International Exchange Related to Thermal ConductivityDavid R. Clarke, University of California-Santa Barbara, DMR 0710523 One of the highlights of our World Materials Network, with Profs Pan Wei (Tsinghua University) and Robin Grimes (Imperial, London), has been the continuing exchange of students and the organization of a week long graduate international school on the topic of “Thermal Conductivity and Related Transport Properties of Oxides” held at the University of Florida, Gainesville, from May 17-22nd. Students from 14 states in the US, including nationals from China, Mexico and Philippines, as well as 4 countries (Germany, Mexico, UK and Thailand) participated. They represented a variety of disciplines, ranging from development of improved computational methods to plasma-spraying methods to magnetic materials to the development of improved thermoelectric materials. (Some six attendees who were accepted into the School were unable to obtain visas). An unusual feature of the school was that students were organized in teams of students doing research in different areas in oxides. With initial facilitation from the faculty, members of the teams discussed the lectures in detail to prepare questions of the speakers in special, hour-long question sessions for two of the speakers at a time. The only rule was that the questions could not be questions of clarification. This proved to be very stimulating and popular with both the students and the lecturers. Each team also generated a research proposal which they had to defend to the other teams and were judged by all the students.