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Hydroxyapatite is an important constituent of natural bone, and possesses excellent biocompatibility and bioactivity, but its brittle nature limits its use for bone tissue engineering. Nanohydroxyapatite (nanoHAP) has been used in synthesis of biomimetic composites for more than a decade, yet the mechanics of nanoHAP particles is not fully understood. The present work attempts to advance the current understanding of mechanics of hydroxyapatite at nanoscale, by carrying out systematic nanoindentation experiments on nanoHAP and surface modified nanoHAP [prepared by in situ mineralization in presence of polyacrylic acid (PAAc)]. Quantitative nanomodulus maps of both modified and unmodified HAP nanoparticles indicate that various surface features of HAP nanoparticles can be probed. Dips in values of elastic moduli across the nanoparticle surfaces in modified nanohydroxyapatite are indicative of composite responses from both polymer and mineral phases (PAAc-HAP) on the surface. Nanoindentation experiments were performed at 100, 1,000, 3,000, 5,000, and 8,000 [micro]N loads, respectively, to obtain the indentation response from both shallow and deep penetration depths. Nanoindentation results at shallow penetration depths are influenced by nanoscale surface roughness of irregular-shaped HAP nanoparticles and nonuniform distribution of PAAc in the microstructure. Significant nonbonded interactions between HAP and PAAc, the mechanical properties of individual constituents (HAP and PAAc) lead to superior nanomechanical properties of surface-modified nanoHAP as compared to unmodified HAP. The overall inelastic nanomechanical response (including damage leading to reduced overall elastic modulus) is strongly influenced by the nature of the interfaces between the nanoparticles, especially when indent size is much larger than the particle size. DOI: 10.1061/(ASCE)EM.1943-7889.0000002 CE Database subject headings: Bioengineering; Particle size; Composite materials; Microstructure; Material properties; Interfaces; Biological properties.

We review our theoretical studies of the spin dynamics of ultrathin ferromagnetic films adsorbed on metallic substrates. Of interest are the spin waves with wave vector from the center of the Brillouin zone to its boundary. We utilize a realistic electronic structure for the substrate/adsorbate combination through an empirical tight binding description of the electronic structure of the materials involved, and the spin excitations are explored through the Random Phase Approximation. We can examine very large systems with this scheme, such as multilayer films placed on fully semi-infinite substrates. One principal finding is the adiabatic approximation breaks down qualitatively for these systems. We illustrate for Co films on Cu(100) where we also find excellent agreement with experiment. We conclude by discussing recent data for the Fe bilayer on W(110) where, after an excellent account of the measured exchange stiffness, we find the large wave vector spin waves generated by theory to be far too stiff. Index Terms--SPEELS, spin waves, ultrathin ferromagnets.

We present a systematic investigation of the magnetic properties and magnetization reversal processes in arrays of magnetic nanostructures. We observed in magnetic rings that the transitions between different equilibrium states are strongly dependent on magnetostatic interactions. For magnetic nanowires, a transition from coherent rotation to curling mode of reversal as the thickness of the wire is varied for fixed lateral dimensions was observed. We found in anti-dot nanostructures that the magnetic anisotropy can be controlled by engineering the lattice geometry. Index Terms--Magnetic nanostructures, magnetization reversal, magnetostatic interactions.

Magnesium, the lightest structural metal, is of significance to improve energy efficiency in various applications. Mg / SiC nanocomposites were successfully fabricated by ultrasonic cavitation based dispersion of SiC nanoparticles in Mg melts. As compared to pure magnesium, the mechanical properties including tensile strength and yield strength of the Mg/ SiC nanocomposites were improved significantly, while the good ductility of pure Mg was retained. The grain size of the pure magnesium was refined significantly when SiC nanoparticles were dispersed in the Mg matrix. In the microstructure of Mg/ SiC nanocomposites, while there were still some SiC microclusters, most of the SiC nanoparticles were dispersed very well. Transmission electron microscopy study of the interface between SiC nanoparticles and magnesium matrix indicates that SiC nanoparticles bond well with Mg without forming an intermediate phase. [DOI: 10.1115/1.2823086]