Ultrasound-Assisted Liquid-Phase Synthesis and Mechanical Properties of Aluminum Matrix Nanocomposites Incorporating Boride Nanocrystals.

Incorporating boride nanocrystals could significantly impact the mechanical properties of aluminum alloys. Molten salts synthesis offers opportunities to fabricate superhard boride nanoparticles, which can sustain the harsh conditions during the liquid-phase design of metallic nanocomposites. Here hafnium diboride-aluminum nanocomposites are unveiled from molten salt-derived HfB2 nanoparticles sequentially dispersed in aluminum by ultrasound treatment. The structure and size of the nanocrystals are retained in the final nanocomposites, supporting their high chemical stability. Semicoherent interfaces between the nanoparticles and the matrix are then evidenced by TEM, suggesting that the nanocrystals could promote heterogeneous nucleation of Al and then limit the Al grain size to ≈20 µm. Nanoindentation measurements reveal significant grain boundary strengthening and grain refinement effects. It is finally shown that HfB2 nanoparticles also enable a decrease in matrix grain size and an increase in the hardness of the AlSi7 Cu0.5 Mg0.3 alloy. These proof-of-concept materials are paving the way to light-weight Al matrix nanocomposites doped by molten-salt synthesized nanoparticles.

Intricate disorder in defect fluorite/pyrochlore: a concord of chemistry and crystallography.

Intuitively scientists accept that order can emerge from disorder and a significant amount of effort has been devoted over many years to demonstrate this. In metallic alloys and oxides, disorder at the atomic scale is the result of occupation at equivalent atomic positions by different atoms which leads to the material exhibiting a fully random or modulated scattering pattern. This arrangement has a substantial influence on the material's properties, for example ionic conductivity. However it is generally accepted that oxides, such as defect fluorite as used for nuclear waste immobilization matrices and fuel cells, are the result of disorder at the atomic scale. To investigate how order at the atomic scale induces disorder at a larger scale length, we have applied different techniques to study the atomic composition of a homogeneous La 2 Zr 2 O 7 pyrochlore, a textbook example of such a structure. Here we demonstrate that a pyrochlore, which is considered to be defect fluorite, is the result of intricate disorder due to a random distribution of fully ordered nano-domains. Our investigation provides new insight into the order disorder transformations in complex materials with regards to domain formation, resulting in a concord of chemistry with crystallography illustrating that order can induce disorder.

Form birefringence induced in multicomponent glass by femtosecond laser direct writing.

We demonstrate a new kind of form birefringence in lithium niobium silicate glass induced by femtosecond laser direct writing. By combining electron backscatter diffraction and transmission electron microscopy, we reveal a self-assembled nanostructure consisting of periodic phase change: nonlinear optical nanocrystals embedded in a network of "walls" in a vitreous phase. These "walls" are aligned perpendicular to the laser polarization direction. This self-organized nanostructure may successfully explain the origin of the laser-induced birefringence in this multicomponent glass quite differently from pure silica. These findings highlight a spectacular modification of glass, and enable construction of a high contrast three-dimensional refractive index and birefringent structures at the micrometer scale in multicomponent glasses.

Morphology and surface heterogeneities in synthetic goethites.

In the framework on a study of the acido-basic and sorption properties of iron oxides, a thorough characterization of two types of goethite powders was performed in several laboratories joined in a common project. Chemical analysis by ICPAES; high-resolution SEM, TEM, and AFM observations; XRD with line width analysis; and argon and nitrogen sorption isotherms were used for that purpose. The main crystallographic faces of goethite particles could be identified as {001}, {101}, and {121}, and their abundance correlated with the distribution of low-pressure argon adsorption local isotherms. These results will be very useful for further studies on the relationship between surface reactivity in aqueous solution and orientation of solid surfaces.