The effect of worked material hardness on stone tool wear.

The identification of ancient worked materials is one of the fundamental goals of lithic use wear analysis and one of the most important parts of understanding how stone tools were used in the past. Given the documented overlaps in wear patterns generated by different materials, it is imperative to understand how individual materials' mechanical properties might influence wear formation. Because isolating physical parameters and measuring their change is necessary for such an endeavor, controlled (rather than replicative) experiments combined with objective measurements of surface topography are necessary to better grasp how surface modifications formed on stone tools. Therefore, we used a tribometer to wear natural flint surfaces against five materials (bone, antler, beech wood, spruce wood, and ivory) under the same force, and speed, over one, three, and five hours. The study aimed to test if there is a correlation between surface modifications and the hardness of the worked material. We measured each raw material's hardness using a nano-indentation test, and we compared the surface texture of the flint bits using a 3D optical profilometer. The interfacial detritus powder was analyzed with a scanning electron microscope to look for abraded flint particles. We demonstrate that, contrary to expectation, softer materials, such as wood, create a smoother surface than hard ones, such as ivory.

The mineralogy and structure of use-wear polish on chert.

Polished edges of archaeological stone tools are commonly investigated to obtain information on the tools' uses in prehistory. Yet to this day, it remains unclear what exactly such polishes are and how they form. Answering these questions should allow the elaboration of new interpretative methods based on objective measurements. Two major competing hypotheses of polish formation have been proposed: abrasion and the formation of a thin amorphous film on the chert or flint surface. We employ reflectance infrared spectroscopy, a technique particularly sensitive to thin amorphous films, to investigate these two hypotheses. We found no added amorphous layer that would have formed upon friction against bone, antler, ivory or wood. Our observations suggest polish formation by abrasion, notwithstanding previous claims of added amorphous surface structures. This has implications for our understanding of the physical processes taking place during friction of chert and flint against different materials. Our results also open the possibility to propose new pathways for identifying different use-wear processes, based on the degree of abrasion.

Polymorphism in the 1:1 Charge-Transfer Complex DBTTF-TCNQ and Its Effects on Optical and Electronic Properties.

The organic charge-transfer (CT) complex dibenzotetrathiafulvalene - 7,7,8,8-tetracyanoquinodimethane (DBTTF-TCNQ) is found to crystallize in two polymorphs when grown by physical vapor transport: the known α-polymorph and a new structure, the ß-polymorph. Structural and elemental analysis via selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), and polarized IR spectroscopy reveal that the complexes have the same stoichiometry with a 1:1 donor:acceptor ratio, but exhibit unique unit cells. The structural variations result in significant differences in the optoelectronic properties of the crystals, as observed in our experiments and electronic-structure calculations. Raman spectroscopy shows that the α-polymorph has a degree of charge transfer of about 0.5e, while the ß-polymorph is nearly neutral. Organic field-effect transistors fabricated on these crystals reveal that in the same device structure both polymorphs show ambipolar charge transport, but the α-polymorph exhibits electron-dominant transport while the ß-polymorph is hole-dominant. Together, these measurements imply that the transport features result from differing donor-acceptor overlap and consequential varying in frontier molecular orbital mixing, as suggested theoretically for charge-transfer complexes.

A charge optimized many-body (COMB) potential for titanium and titania.

This work proposes an empirical, variable charge potential for Ti and TiO(2) systems based on the charge-optimized many-body (COMB) potential framework. The parameters of the potential function are fit to the structural and mechanical properties of the Ti hcp phase, the TiO(2) rutile phase, and the energetics of polymorphs of both Ti and TiO(2). The relative stabilities of TiO(2) rutile surfaces are predicted and compared to the results of density functional theory (DFT) and empirical potential calculations. The transferability of the developed potential is demonstrated by determining the adsorption energy of Cu clusters of various sizes on the rutile TiO(2)(1 1 0) surface using molecular dynamics simulations. The results indicate that the adsorption energy is dependent on the number of Cu-Cu bonds and Cu-O bonds formed at the Cu/TiO(2) interface. The adsorption energies of Cu clusters on the reduced and oxidized TiO(2)(1 1 0) surfaces are also investigated, and the COMB potential predicts enhanced bonding between Cu clusters and the oxidized surface, which is consistent with both experimental observations and the results of DFT calculations for other transition metals (Au and Ag) on this oxidized surface.

Atomistic models to investigate thorium dioxide (ThO2).

Thorium dioxide (ThO(2)) is of great interest to energy research as thorium-based nuclear fuel offers the promise of increased proliferation resistance, longer fuel cycles, higher burn-up and improved wasteform characteristics in the generation of nuclear power. However, understanding of ThO(2) as a nuclear fuel is not as comprehensive as UO(2). In order to improve the atomic level understanding of thorium-based fuels, we have developed eight interatomic potential descriptions of ThO(2) by fitting the experimental lattice parameter, elastic constants and static dielectric constants. Using these interatomic potentials, we have calculated the structural and elastic properties, phase stability, defect formation energies, defect binding energies and complexes as well as the energetics of low-index surfaces. A critical assessment of all the potentials is performed by comparing the predicted properties with available experimental and first-principles calculations.

Structure and energetics of 180° domain walls in PbTiO3 by density functional theory.

Density functional theory at the level of the local density approximation with the projector augmented wave method is used to determine the structure of 180° domain walls in tetragonal ferroelectric PbTiO(3). In agreement with previous studies, it is found that PbO-centered {100} walls have lower energies than TiO(2)-centered {100} walls, leading to a Peierls potential barrier for wall motion along <010> of ∼36 mJ m(-2). In addition to the Ising-like polarization along the tetragonal axis, it is found that near the domain wall, there is a small polarization in the wall-normal direction away from the domain wall. These Néel-like contributions to the domain wall are analyzed in terms of the Landau-Ginzburg-Devonshire phenomenological theory for ferroelectrics. Similar characteristics are found for {110} domain walls, where OO-centered walls are energetically more favorable than the PbTiO-centered walls.