The experimental solid-state terahertz (THz) spectrum (3-120 cm(-1)) of the beta-crystal form of the high explosive octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) has been analyzed using solid-state density functional theory calculations. Various density functionals (both generalized gradient approximation and local density approximation) are compared in terms of their abilities to reproduce the experimentally observed solid-state structure and low-frequency vibrational motions. Good-to-excellent agreement between solid-state theory and experiment can be achieved in the THz region where isolated-molecule calculations fail to reproduce the observed spectral features, demonstrating a clear limitation of using isolated-molecule calculations for the assignment of THz frequency motions in molecular solids. The deficiency of isolated-molecule calculations is traced to modification of the molecular structure in the solid state through crystal packing effects and the formation of weak C-H...O hydrogen bonds.
A model for the elastic moduli of three-dimensional fiber networks and nanocomposites A model is developed for the elastic moduli of fiber networks composed of elongated particles characterized by aspect ratio polydispersity. The present treatment of elastic fiber networks is integrated with an effective medium model for heterogeneous materials and with percolation theory to provide a framework for describing fiber-reinforced nanocomposites. Model calculations are presented for the dependences of composite moduli on particle aspect ratio, volume fraction, and polydispersity index.
Residential floor dust loading was measured on the smooth floor surface of 488 houses in Syracuse, New York, during the summers of 2003 and 2004. Using U.S. Environmental Protection Agency (EPA) wipe methods, pre-weighed Ghost Wipes, Lead Wipes, or Whatman Filters were employed to collect duplicate samples from (predominantly) kitchens. The collection efficiency of the various media was determined from multiple wipe tests and side-by-side comparisons. The results were normalized and aggregated at the census tract level to determine whether spatial patterns of dust loading could be observed. Loading was found to be log-normally distributed, with a geometric mean value of 0.311 g m(-2) (29 mg of dust per square foot of floor); 95% of the observations fell in the range of 0.042-2.330 g m(-2) (4-216 mg foot(-2)). The sampling for floor dust loading shows some bias for day of the week in which visits to the residential properties were made. After a first-order correction for this effect, results were aggregated by census tract and mapped in a geographic information system (GIS); strong spatial patterns can be identified in an inverse distance weighted mapping. The geographic patterns exhibit a strong correlation with socio-economic/demographic covariates extracted from the 2000 census summaries. Dust mass on the floors is positively correlated with renter-occupied properties and family size; it is negatively correlated with measures of household income.
A model for the elastic coefficients of fiber-reinforced materials is applied toward the analysis of the tensile and shear moduli of nanocomposites reinforced by rod-like cellulose nanoparticles. Our formalism integrates results from percolation theory with micromechanical and effective medium approaches. Polydispersity in the fiber length distribution and anisotropies in the stiffness coefficients of cellulose nanoparticles are taken into account explicitly. Results from calculations employing our model display reasonable agreement with experimental measurements of the moduli as functions of the filler volume fraction for several cellulose nanoparticle-reinforced thermoplastic-based composites.
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