December 10, 2014 @ 3:00 pm - 4:00 pm
The Hopkins Extreme Materials Institute in conjunction with the Department of Chemistry and the Department of Materials Science and Engineering are pleased to announce “Relating Hardness, Bonding and Composition in AlLiB14” a seminar Given by Scott Beckman,from the Dept. of Materials Science and Engineering, Iowa State University. The seminar will be held on December 10, 2014 at 3 PM in Maryland Hall, Room 110.
There will be coffee and cookies in Maryland 109 at 2:45 before the talk.
During this period there is great interest in revitalizing manufacturing infrastructure to allow for environmentally clean and sustainable production. The crystal family that has the archetypical composition AlLiB14 has potential application both for its mechanical and functional properties. Drs. Bruce Cook and Alan Russell, from Ames Laboratory (USA), have investigated it near-superhard mechanical properties that may allow it to serve in the extreme conditions necessary for the development of high-performance production methods that can boost efficiency and cut costs.  Dr. Takao Mori, from the National Institute for Material Research (Japan) is investigating this crystal family because of its thermoelectric properties.  Due to its wide band gap, thermal stability, and high Seebeck coefficient it may find application as a high-temperature thermoelectric material. Here we investigate the bonding in this compound by first-principles methods. We relate the hardness of the
material to the electronic structure, which is in turn related to the composition. In addition we predict the thermoelectric properties of the compound.
First-principles, ab initio, methods are used to determine the ideal brittle cleavage strength for several high-symmetry orientations. The elastic tensor and the orientation-dependent Young’s modulus are calculated. From these results the lower bound fracture strength of AlLiB14 is predicted to be between 29 and 31 GPa, which is near the measured hardness reported in the literature.  These results indicate that the intrinsic strength of AlLiB14 is limited by the interatomic B-B bonds that span between the B layers. From detailed analysis of the electronic structure it is predicted that placing the Fermi level just inside the valence band, as it is for Al0.75Mg0.78B14, will maximize the crystal’s strength.
The vibrational modes are calculated for compounds with the AlLiB14, AlMgB14, and Al0.75Mg0.75B14 compositions. The stoichiometric AlMgB14 crystal is found to have three soft phonon modes, which have displacements associat with metal atoms vibrating against the B lattice.  The lattice instability is attributed to the occupation of electronic states in the conduction band. Off-stoichiometric occupation sweeps the Fermi level from the conduction band into the gap, and toward the structure with the maximum crystal strength. In essence the compound naturally tunes itself to have the highest possible strength.  The Boltzmann transport equation is used to investigate the thermoelectric behavior of AlLiB14. It is found to have a high Seebeck coefficient, greater than 200 μV/K, at temperatures near 1000 K.  At this temperature carrier concentrations of around 1×1020 cm-3 are also predicted. Approximating scattering from the bulk elastic behavior predicts at ZT of approximately 0.33 at T=1000 K.